Why Explore Seaweed Based Biofuels?

Renewable energy will play a fundamental role in transitioning towards a more competitive, secure, and sustainable energy system internationally. Renewable fuels from biomass (biofuels) are expected to contribute especially in hard-to-electrify sectors such as aviation and shipping.  The concept of biologically sourced transportation fuel is not theoretical: more than 98% of U.S. gasoline already contains 10% ethanol (E10) produced from corn biomass. However, usage of terrestrial biofuel feedstocks has several ecological disadvantages. It requires arable land to be diverted from food production, the application of fertilizer and pesticides and the use of irrigation water, which is becoming scarce in many regions (Kite-Powell et al., 2021). In some cases, because of the land-use changes, net carbon emissions actually increase with land-based biofuel production relative to fossil-fuel usage (Searchinger et al., 2008). As shown in the figure below, gross energy yield per hectare from seaweeds is similar to that for maize, the dominant terrestrial feedstock for biomethane. Figure 1: Potential gross energy production per hectare per annum based on a variety of species of seaweed (wwt = wet weight) Source: (State of Technology Review-Algae Bioenergy: IEA Bioenergy, 2017)

How Seaweed Is Converted to Biofuels

Seaweeds are biochemically well-suited for biofuel production. Their cell walls are rich in specialized polysaccharides (laminarin, mannitol, alginate, carrageenan, and ulvan) giving them high carbohydrate content (typically 40–65% of dry weight). Crucially, seaweeds contain little or no lignin, the recalcitrant structural polymer that makes terrestrial biomass costly and energy-intensive to process. This makes macroalgae considerably easier to break down and convert than wood, straw, or grasses. There are three primary pathways for converting seaweed biomass into liquid or gaseous biofuels. Each favors different seaweed species profiles and produces different fuel outputs:

Pathway	Primary output	Key advantage	Main challenge
Anaerobic digestion (AD)	Biomethane / biogas	Uses wet feedstock; no energy-intensive drying required; digestate usable as fertilizer	Real-world yields below theoretical; inhibitory compounds in some species
Fermentation	Bioethanol / ABE (acetone-butanol-ethanol)	Produces higher-energy-density alcohols compatible with existing fuel infrastructure	Requires pre-treatment to release fermentable sugars; no large-scale operational data yet
Thermochemical: Hydrothermal liquefaction (HTL)	Bio-crude → gasoline / diesel / SAF	Handles wet, protein-rich biomass; produces energy-dense hydrocarbon fuel suitable for aviation	Capital-intensive; high catalyst cost; needs process optimization
Thermochemical: Pyrolysis / Gasification + Fischer-Tropsch (F-T)	Bio-oil, syngas → gasoline / diesel / SAF / biochar	Drop-in fuels with no blending limits; biochar co-product has carbon sequestration potential	High ash content in many seaweed species causes reactor slagging; requires pre-washing or ash-tolerant feedstocks

Anaerobic digestion — producing biomethane and liquid fuels

Anaerobic digestion converts the organic matter in seaweed into biogas, a mixture of methane and carbon dioxide, through the action of microorganisms in an oxygen-free environment. This biogas can be used directly for heat and power, upgraded to biomethane (which can be used as a substitute for natural gas for electricity generation), or further processed into liquid transport fuels via established chemical synthesis routes. A key practical advantage is that AD can process wet biomass, avoiding the energy-intensive drying steps required by most other conversion routes. The remaining digestate is rich in nitrogen and phosphorus, making it a potential co-product fertilizer.

Fermentation — producing bioethanol and butanol

Fermentation converts seaweed sugars into bioethanol or acetone-butanol-ethanol (ABE). Unlike terrestrial crops, seaweed polysaccharides must first be broken down by enzymatic or acid hydrolysis before standard or engineered microorganisms can ferment the resulting sugars. The products, ethanol and butanol are compatible with existing fuel infrastructure and internal combustion engines. The MacroFuels EU project produced seaweed-derived fuel and demonstrated it in a road-going vehicle.

Thermochemical conversion — HTL, pyrolysis, gasification, and Fischer-Tropsch synthesis

Hydrothermal liquefaction (HTL) — producing aviation-compatible bio-crude

Hydrothermal liquefaction processes wet seaweed biomass under high temperature and pressure, converting it directly into a bio-crude oil. This bio-crude can be upgraded into hydrocarbon fuels including gasoline, diesel, and critically sustainable aviation fuel (SAF), with energy density comparable to conventional jet fuel. HTL can accept wet feedstock without pre-drying, and an aqueous nutrient-rich co-product can be recycled back to seaweed cultivation. This makes HTL the most promising pathway for producing seaweed-derived aviation fuel, the largest identified strategic market for seaweed biofuels. Pyrolysis and gasification with Fischer-Tropsch (F-T) synthesis thermally decompose dried seaweed to produce bio-oil and syngas (a mixture of CO and H₂), which can then be converted via F-T synthesis into drop-in gasoline, diesel, or SAF with no blending limits and no engine modifications required. A solid biochar co-product is also generated, which has potential value as a soil amendment and carbon sequestration material. The main constraint for seaweed feedstocks is high ash content: species with more than approximately 20% ash by dry weight can cause slagging and corrosion in pyrolysis reactors, requiring either pre-washing or careful species selection. Pelagic biomass such as beach-cast Sargassum is an actively explored feedstock for this route. Seaweed-based biofuels are at an early but pivotal stage. Pilot projects have demonstrated technical feasibility for example laboratory engine tests on seaweed-derived ethanol and acetone-butanol-ethanol (ABE) blends have demonstrated no significant performance degradation relative to fossil gasoline, along with measurable reductions in particulate emissions (MacroFuels EU project). The EU's ReFuelEU Aviation regulation mandates sustainable aviation fuel use, while geopolitical energy security shocks have renewed political urgency around liquid fuel diversification. However, challenges in scaling, cost reduction, and regulatory alignment remain. Sustained investment in innovation, particularly in automation, offshore infrastructure, and in market development and the understanding of environmental impacts of scaling will be essential to unlock macroalgae’s full potential as a sustainable, large-scale bioenergy resource.

Why Explore Seaweed Based Biofuels?

Renewable energy will play a fundamental role in transitioning towards a more competitive, secure, and sustainable energy system internationally. Renewable fuels from biomass (biofuels) are expected to contribute especially in hard-to-electrify sectors such as aviation and shipping.  The concept of biologically sourced transportation fuel is not theoretical: more than 98% of U.S. gasoline already contains 10% ethanol (E10) produced from corn biomass. However, usage of terrestrial biofuel feedstocks has several ecological disadvantages. It requires arable land to be diverted from food production, the application of fertilizer and pesticides and the use of irrigation water, which is becoming scarce in many regions (Kite-Powell et al., 2021). In some cases, because of the land-use changes, net carbon emissions actually increase with land-based biofuel production relative to fossil-fuel usage (Searchinger et al., 2008). As shown in the figure below, gross energy yield per hectare from seaweeds is similar to that for maize, the dominant terrestrial feedstock for biomethane. Figure 1: Potential gross energy production per hectare per annum based on a variety of species of seaweed (wwt = wet weight) Source: (State of Technology Review-Algae Bioenergy: IEA Bioenergy, 2017)

How Seaweed Is Converted to Biofuels

Seaweeds are biochemically well-suited for biofuel production. Their cell walls are rich in specialized polysaccharides (laminarin, mannitol, alginate, carrageenan, and ulvan) giving them high carbohydrate content (typically 40–65% of dry weight). Crucially, seaweeds contain little or no lignin, the recalcitrant structural polymer that makes terrestrial biomass costly and energy-intensive to process. This makes macroalgae considerably easier to break down and convert than wood, straw, or grasses. There are three primary pathways for converting seaweed biomass into liquid or gaseous biofuels. Each favors different seaweed species profiles and produces different fuel outputs:

Pathway	Primary output	Key advantage	Main challenge
Anaerobic digestion (AD)	Biomethane / biogas	Uses wet feedstock; no energy-intensive drying required; digestate usable as fertilizer	Real-world yields below theoretical; inhibitory compounds in some species
Fermentation	Bioethanol / ABE (acetone-butanol-ethanol)	Produces higher-energy-density alcohols compatible with existing fuel infrastructure	Requires pre-treatment to release fermentable sugars; no large-scale operational data yet
Hydrothermal liquefaction (HTL)	Bio-crude → gasoline / diesel / SAF	Handles wet, protein-rich biomass; produces energy-dense hydrocarbon fuel suitable for aviation	Capital-intensive; high catalyst cost; needs process optimization

Anaerobic digestion — producing biomethane and liquid fuels

Anaerobic digestion converts the organic matter in seaweed into biogas, a mixture of methane and carbon dioxide, through the action of microorganisms in an oxygen-free environment. This biogas can be used directly for heat and power, upgraded to biomethane (which can be used as a substitute for natural gas for electricity generation), or further processed into liquid transport fuels via established chemical synthesis routes. A key practical advantage is that AD can process wet biomass, avoiding the energy-intensive drying steps required by most other conversion routes. The remaining digestate is rich in nitrogen and phosphorus, making it a potential co-product fertilizer.

Fermentation — producing bioethanol and butanol

Fermentation converts seaweed sugars into bioethanol or acetone-butanol-ethanol (ABE). Unlike terrestrial crops, seaweed polysaccharides must first be broken down by enzymatic or acid hydrolysis before standard or engineered microorganisms can ferment the resulting sugars. The products, ethanol and butanol are compatible with existing fuel infrastructure and internal combustion engines. The MacroFuels EU project produced seaweed-derived fuel and demonstrated it in a road-going vehicle.

Hydrothermal liquefaction (HTL) — producing aviation-compatible bio-crude

Hydrothermal liquefaction processes wet seaweed biomass under high temperature and pressure, converting it directly into a bio-crude oil. This bio-crude can be upgraded into hydrocarbon fuels including gasoline, diesel, and critically sustainable aviation fuel (SAF), with energy density comparable to conventional jet fuel. HTL can accept wet feedstock without pre-drying, and an aqueous nutrient-rich co-product can be recycled back to seaweed cultivation. This makes HTL the most promising pathway for producing seaweed-derived aviation fuel, the largest identified strategic market for seaweed biofuels. Seaweed-based biofuels are at an early but pivotal stage. Pilot projects have demonstrated technical feasibility  for example laboratory engine tests on seaweed-derived ethanol and acetone-butanol-ethanol (ABE) blends have demonstrated no significant performance degradation relative to fossil gasoline, along with measurable reductions in particulate emissions (MacroFuels EU project). The EU's ReFuelEU Aviation regulation mandates sustainable aviation fuel use, while geopolitical energy security shocks have renewed political urgency around liquid fuel diversification. However, challenges in scaling, cost reduction, and regulatory alignment remain. Sustained investment in innovation, particularly in automation, offshore infrastructure, and in market development and the understanding of environmental impacts of scaling will be essential to unlock macroalgae’s full potential as a sustainable, large-scale bioenergy resource.

Why Explore Seaweed Based Biofuels?

Renewable energy will play a fundamental role in transitioning towards a more competitive, secure, and sustainable energy system internationally. Renewable fuels from biomass (biofuels) are expected to contribute especially in hard-to-electrify sectors such as aviation and shipping.  The concept of biologically sourced transportation fuel is not theoretical: more than 98% of U.S. gasoline already contains 10% ethanol (E10) produced from corn biomass. However, usage of terrestrial biofuel feedstocks has several ecological disadvantages. It requires arable land to be diverted from food production, the application of fertilizer and pesticides and the use of irrigation water, which is becoming scarce in many regions (Kite-Powell et al., 2021). In some cases, because of the land-use changes, net carbon emissions actually increase with land-based biofuel production relative to fossil-fuel usage (Searchinger et al., 2008). As shown in the figure below, gross energy yield per hectare from seaweeds is similar to that for maize, the dominant terrestrial feedstock for biomethane. Figure 1: Potential gross energy production per hectare per annum based on a variety of species of seaweed (wwt = wet weight) Source: (State of Technology Review-Algae Bioenergy: IEA Bioenergy, 2017)

How Seaweed Is Converted to Biofuels

Seaweeds are biochemically well-suited for biofuel production. Their cell walls are rich in specialized polysaccharides (laminarin, mannitol, alginate, carrageenan, and ulvan) giving them high carbohydrate content (typically 40–65% of dry weight). Crucially, seaweeds contain little or no lignin, the recalcitrant structural polymer that makes terrestrial biomass costly and energy-intensive to process. This makes macroalgae considerably easier to break down and convert than wood, straw, or grasses. There are three primary pathways for converting seaweed biomass into liquid or gaseous biofuels. Each favors different seaweed species profiles and produces different fuel outputs:

Pathway	Primary output	Key advantage	Main challenge
Anaerobic digestion (AD)	Biomethane / biogas	Uses wet feedstock; no energy-intensive drying required; digestate usable as fertilizer	Real-world yields below theoretical; inhibitory compounds in some species
Fermentation	Bioethanol / ABE (acetone-butanol-ethanol)	Produces higher-energy-density alcohols compatible with existing fuel infrastructure	Requires pre-treatment to release fermentable sugars; no large-scale operational data yet
Hydrothermal liquefaction (HTL)	Bio-crude → gasoline / diesel / SAF	Handles wet, protein-rich biomass; produces energy-dense hydrocarbon fuel suitable for aviation	Capital-intensive; high catalyst cost; needs process optimization

Anaerobic digestion — producing biomethane and liquid fuels

Anaerobic digestion converts the organic matter in seaweed into biogas, a mixture of methane and carbon dioxide, through the action of microorganisms in an oxygen-free environment. This biogas can be used directly for heat and power, upgraded to biomethane (which can be used as a substitute for natural gas for electricity generation), or further processed into liquid transport fuels via established chemical synthesis routes. A key practical advantage is that AD can process wet biomass, avoiding the energy-intensive drying steps required by most other conversion routes. The remaining digestate is rich in nitrogen and phosphorus, making it a potential co-product fertilizer.

Fermentation — producing bioethanol and butanol

Fermentation converts seaweed sugars into bioethanol or acetone-butanol-ethanol (ABE). Unlike terrestrial crops, seaweed polysaccharides must first be broken down by enzymatic or acid hydrolysis before standard or engineered microorganisms can ferment the resulting sugars. The products, ethanol and butanol are compatible with existing fuel infrastructure and internal combustion engines. The MacroFuels EU project produced seaweed-derived fuel and demonstrated it in a road-going vehicle.

Hydrothermal liquefaction (HTL) — producing aviation-compatible bio-crude

Hydrothermal liquefaction processes wet seaweed biomass under high temperature and pressure, converting it directly into a bio-crude oil. This bio-crude can be upgraded into hydrocarbon fuels including gasoline, diesel, and critically sustainable aviation fuel (SAF), with energy density comparable to conventional jet fuel. HTL can accept wet feedstock without pre-drying, and an aqueous nutrient-rich co-product can be recycled back to seaweed cultivation. This makes HTL the most promising pathway for producing seaweed-derived aviation fuel, the largest identified strategic market for seaweed biofuels. Seaweed-based biofuels are at an early but pivotal stage. Pilot projects have demonstrated technical feasibility  for example laboratory engine tests on seaweed-derived ethanol and acetone-butanol-ethanol (ABE) blends have demonstrated no significant performance degradation relative to fossil gasoline, along with measurable reductions in particulate emissions (MacroFuels EU project). The EU's ReFuelEU Aviation regulation mandates sustainable aviation fuel use, while geopolitical energy security shocks have renewed political urgency around liquid fuel diversification. However, challenges in scaling, cost reduction, and regulatory alignment remain. Sustained investment in innovation, particularly in automation, offshore infrastructure, and in market development and the understanding of environmental impacts of scaling will be essential to unlock macroalgae’s full potential as a sustainable, large-scale bioenergy resource.

Why Explore Seaweed Based Biofuels?

Renewable energy will play a fundamental role in transitioning towards a more competitive, secure, and sustainable energy system internationally. Renewable fuels from biomass (biofuels) are expected to contribute especially in hard-to-electrify sectors such as aviation and shipping.  The concept of biologically sourced transportation fuel is not theoretical: more than 98% of U.S. gasoline already contains 10% ethanol (E10) produced from corn biomass. However, usage of terrestrial biofuel feedstocks has several ecological disadvantages. It requires arable land to be diverted from food production, the application of fertilizer and pesticides and the use of irrigation water, which is becoming scarce in many regions (Kite-Powell et al., 2021). In some cases, because of the land-use changes, net carbon emissions actually increase with land-based biofuel production relative to fossil-fuel usage (Searchinger et al., 2008). As shown in the figure below, gross energy yield per hectare from seaweeds is similar to that for maize, the dominant terrestrial feedstock for biomethane. Figure 1: Potential gross energy production per hectare per annum based on a variety of species of  seaweed (wwt = wet weight) Source: (State of Technology Review-Algae Bioenergy: IEA Bioenergy, 2017)

How Seaweed Is Converted to Biofuels

Seaweeds are biochemically well-suited for biofuel production. Their cell walls are rich in specialized polysaccharides (laminarin, mannitol, alginate, carrageenan, and ulvan) giving them high carbohydrate content (typically 40–65% of dry weight). Crucially, seaweeds contain little or no lignin, the recalcitrant structural polymer that makes terrestrial biomass costly and energy-intensive to process. This makes macroalgae considerably easier to break down and convert than wood, straw, or grasses. There are three primary pathways for converting seaweed biomass into liquid or gaseous biofuels. Each favors different seaweed species profiles and produces different fuel outputs:

Pathway	Primary output	Key advantage	Main challenge
Anaerobic digestion (AD)	Biomethane / biogas	Uses wet feedstock; no energy-intensive drying required; digestate usable as fertilizer	Real-world yields below theoretical; inhibitory compounds in some species
Fermentation	Bioethanol / ABE (acetone-butanol-ethanol)	Produces higher-energy-density alcohols compatible with existing fuel infrastructure	Requires pre-treatment to release fermentable sugars; no large-scale operational data yet
Hydrothermal liquefaction (HTL)	Bio-crude → gasoline / diesel / SAF	Handles wet, protein-rich biomass; produces energy-dense hydrocarbon fuel suitable for aviation	Capital-intensive; high catalyst cost; needs process optimization

Anaerobic digestion — producing biomethane and liquid fuels

Anaerobic digestion converts the organic matter in seaweed into biogas, a mixture of methane and carbon dioxide, through the action of microorganisms in an oxygen-free environment. This biogas can be used directly for heat and power, upgraded to biomethane (which can be used as a substitute for natural gas for electricity generation), or further processed into liquid transport fuels via established chemical synthesis routes. A key practical advantage is that AD can process wet biomass, avoiding the energy-intensive drying steps required by most other conversion routes. The remaining digestate is rich in nitrogen and phosphorus, making it a potential co-product fertilizer.

Fermentation — producing bioethanol and butanol

Fermentation converts seaweed sugars into bioethanol or acetone-butanol-ethanol (ABE). Unlike terrestrial crops, seaweed polysaccharides must first be broken down by enzymatic or acid hydrolysis before standard or engineered microorganisms can ferment the resulting sugars. The products, ethanol and butanol are compatible with existing fuel infrastructure and internal combustion engines. The MacroFuels EU project produced seaweed-derived fuel and demonstrated it in a road-going vehicle.

Hydrothermal liquefaction (HTL) — producing aviation-compatible bio-crude

Hydrothermal liquefaction processes wet seaweed biomass under high temperature and pressure, converting it directly into a bio-crude oil. This bio-crude can be upgraded into hydrocarbon fuels including gasoline, diesel, and critically sustainable aviation fuel (SAF), with energy density comparable to conventional jet fuel. HTL can accept wet feedstock without pre-drying, and an aqueous nutrient-rich co-product can be recycled back to seaweed cultivation. This makes HTL the most promising pathway for producing seaweed-derived aviation fuel, the largest identified strategic market for seaweed biofuels. Seaweed-based biofuels are at an early but pivotal stage. Pilot projects have demonstrated technical feasibility  for example laboratory engine tests on seaweed-derived ethanol and acetone-butanol-ethanol (ABE) blends have demonstrated no significant performance degradation relative to fossil gasoline, along with measurable reductions in particulate emissions (MacroFuels EU project). The EU's ReFuelEU Aviation regulation mandates sustainable aviation fuel use, while geopolitical energy security shocks have renewed political urgency around liquid fuel diversification. However, challenges in scaling, cost reduction, and regulatory alignment remain. Sustained investment in innovation, particularly in automation, offshore infrastructure, and in market development and the understanding of environmental impacts of scaling will be essential to unlock macroalgae’s full potential as a sustainable, large-scale bioenergy resource.

Why Explore Seaweed Based Biofuels?

Renewable energy will play a fundamental role in transitioning towards a more competitive, secure, and sustainable energy system internationally. Renewable fuels from biomass (biofuels) are expected to contribute especially in hard-to-electrify sectors such as aviation and shipping.  The concept of biologically sourced transportation fuel is not theoretical: more than 98% of U.S. gasoline already contains 10% ethanol (E10) produced from corn biomass. However, usage of terrestrial biofuel feedstocks has several ecological disadvantages. It requires arable land to be diverted from food production, the application of fertilizer and pesticides and the use of irrigation water, which is becoming scarce in many regions (Kite-Powell et al., 2021). In some cases, because of the land-use changes, net carbon emissions actually increase with land-based biofuel production relative to fossil-fuel usage (Searchinger et al., 2008). As shown in the figure below, gross energy yield per hectare from seaweeds is similar to that for maize, the dominant terrestrial feedstock for biomethane. Figure 1: Potential gross energy production per hectare per annum based on a variety of species of  seaweed (wwt = wet weight) Source: (State of Technology Review-Algae Bioenergy: IEA Bioenergy, 2017)  

How Seaweed Is Converted to Biofuels

Seaweeds are biochemically well-suited for biofuel production. Their cell walls are rich in specialized polysaccharides (laminarin, mannitol, alginate, carrageenan, and ulvan) giving them high carbohydrate content (typically 40–65% of dry weight). Crucially, seaweeds contain little or no lignin, the recalcitrant structural polymer that makes terrestrial biomass costly and energy-intensive to process. This makes macroalgae considerably easier to break down and convert than wood, straw, or grasses. There are three primary pathways for converting seaweed biomass into liquid or gaseous biofuels. Each favors different seaweed species profiles and produces different fuel outputs:  

Pathway	Primary output	Key advantage	Main challenge
Anaerobic digestion (AD)	Biomethane / biogas	Uses wet feedstock; no energy-intensive drying required; digestate usable as fertilizer	Real-world yields below theoretical; inhibitory compounds in some species
Fermentation	Bioethanol / ABE (acetone-butanol-ethanol)	Produces higher-energy-density alcohols compatible with existing fuel infrastructure	Requires pre-treatment to release fermentable sugars; no large-scale operational data yet
Hydrothermal liquefaction (HTL)	Bio-crude → gasoline / diesel / SAF	Handles wet, protein-rich biomass; produces energy-dense hydrocarbon fuel suitable for aviation	Capital-intensive; high catalyst cost; needs process optimization

 

Anaerobic digestion — producing biomethane and liquid fuels

Anaerobic digestion converts the organic matter in seaweed into biogas, a mixture of methane and carbon dioxide, through the action of microorganisms in an oxygen-free environment. This biogas can be used directly for heat and power, upgraded to biomethane (which can be used as a substitute for natural gas for electricity generation), or further processed into liquid transport fuels via established chemical synthesis routes. A key practical advantage is that AD can process wet biomass, avoiding the energy-intensive drying steps required by most other conversion routes. The remaining digestate is rich in nitrogen and phosphorus, making it a potential co-product fertilizer.

Fermentation — producing bioethanol and butanol

Fermentation converts seaweed sugars into bioethanol or acetone-butanol-ethanol (ABE). Unlike terrestrial crops, seaweed polysaccharides must first be broken down by enzymatic or acid hydrolysis before standard or engineered microorganisms can ferment the resulting sugars. The products, ethanol and butanol are compatible with existing fuel infrastructure and internal combustion engines. The MacroFuels EU project produced seaweed-derived fuel and demonstrated it in a road-going vehicle.

Hydrothermal liquefaction (HTL) — producing aviation-compatible bio-crude

Hydrothermal liquefaction processes wet seaweed biomass under high temperature and pressure, converting it directly into a bio-crude oil. This bio-crude can be upgraded into hydrocarbon fuels including gasoline, diesel, and critically sustainable aviation fuel (SAF), with energy density comparable to conventional jet fuel. HTL can accept wet feedstock without pre-drying, and an aqueous nutrient-rich co-product can be recycled back to seaweed cultivation. This makes HTL the most promising pathway for producing seaweed-derived aviation fuel, the largest identified strategic market for seaweed biofuels.   Seaweed-based biofuels are at an early but pivotal stage. Pilot projects have demonstrated technical feasibility  for example laboratory engine tests on seaweed-derived ethanol and acetone-butanol-ethanol (ABE) blends have demonstrated no significant performance degradation relative to fossil gasoline, along with measurable reductions in particulate emissions (MacroFuels EU project). The EU's ReFuelEU Aviation regulation mandates sustainable aviation fuel use, while geopolitical energy security shocks have renewed political urgency around liquid fuel diversification. However, challenges in scaling, cost reduction, and regulatory alignment remain. Sustained investment in innovation, particularly in automation, offshore infrastructure, and in market development and the understanding of environmental impacts of scaling will be essential to unlock macroalgae’s full potential as a sustainable, large-scale bioenergy resource.

Why Explore Seaweed Based Biofuels?

Renewable energy will play a fundamental role in transitioning towards a more competitive, secure, and sustainable energy system internationally. Renewable fuels from biomass (biofuels) are expected to contribute especially in hard-to-electrify sectors such as aviation and shipping.  The concept of biologically sourced transportation fuel is not theoretical: more than 98% of U.S. gasoline already contains 10% ethanol (E10) produced from corn biomass. However, usage of terrestrial biofuel feedstocks has several ecological disadvantages. It requires arable land to be diverted from food production, the application of fertilizer and pesticides and the use of irrigation water, which is becoming scarce in many regions (Kite-Powell et al., 2021). In some cases, because of the land-use changes, net carbon emissions actually increase with land-based biofuel production relative to fossil-fuel usage (Searchinger et al., 2008). As shown in the figure below, gross energy yield per hectare from seaweeds could be similar to that for maize, the dominant terrestrial feedstock for biomethane.

How Seaweed Is Converted to Biofuels

Seaweeds are biochemically well-suited for biofuel production. Their cell walls are rich in specialized polysaccharides (laminarin, mannitol, alginate, carrageenan, and ulvan) giving them high carbohydrate content (typically 40–65% of dry weight). Crucially, seaweeds contain little or no lignin, the recalcitrant structural polymer that makes terrestrial biomass costly and energy-intensive to process. This makes macroalgae considerably easier to break down and convert than wood, straw, or grasses. There are three primary pathways for converting seaweed biomass into liquid or gaseous biofuels. Each favors different seaweed species profiles and produces different fuel outputs:  

Pathway	Primary output	Key advantage	Main challenge
Anaerobic digestion (AD)	Biomethane / biogas	Uses wet feedstock; no energy-intensive drying required; digestate usable as fertilizer	Real-world yields below theoretical; inhibitory compounds in some species
Fermentation	Bioethanol / ABE (acetone-butanol-ethanol)	Produces higher-energy-density alcohols compatible with existing fuel infrastructure	Requires pre-treatment to release fermentable sugars; no large-scale operational data yet
Hydrothermal liquefaction (HTL)	Bio-crude → gasoline / diesel / SAF	Handles wet, protein-rich biomass; produces energy-dense hydrocarbon fuel suitable for aviation	Capital-intensive; high catalyst cost; needs process optimization

 

Anaerobic digestion — producing biomethane and liquid fuels

Anaerobic digestion converts the organic matter in seaweed into biogas, a mixture of methane and carbon dioxide, through the action of microorganisms in an oxygen-free environment. This biogas can be used directly for heat and power, upgraded to biomethane (which can be used as a substitute for natural gas for electricity generation), or further processed into liquid transport fuels via established chemical synthesis routes. A key practical advantage is that AD can process wet biomass, avoiding the energy-intensive drying steps required by most other conversion routes. The remaining digestate is rich in nitrogen and phosphorus, making it a potential co-product fertilizer.

Fermentation — producing bioethanol and butanol

Fermentation converts seaweed sugars into bioethanol or acetone-butanol-ethanol (ABE). Unlike terrestrial crops, seaweed polysaccharides must first be broken down by enzymatic or acid hydrolysis before standard or engineered microorganisms can ferment the resulting sugars. The products, ethanol and butanol are compatible with existing fuel infrastructure and internal combustion engines. The MacroFuels EU project produced seaweed-derived fuel and demonstrated it in a road-going vehicle.

Hydrothermal liquefaction (HTL) — producing aviation-compatible bio-crude

Hydrothermal liquefaction processes wet seaweed biomass under high temperature and pressure, converting it directly into a bio-crude oil. This bio-crude can be upgraded into hydrocarbon fuels including gasoline, diesel, and critically sustainable aviation fuel (SAF), with energy density comparable to conventional jet fuel. HTL can accept wet feedstock without pre-drying, and an aqueous nutrient-rich co-product can be recycled back to seaweed cultivation. This makes HTL the most promising pathway for producing seaweed-derived aviation fuel, the largest identified strategic market for seaweed biofuels.   Seaweed-based biofuels are at an early but pivotal stage. Pilot projects have demonstrated technical feasibility  for example laboratory engine tests on seaweed-derived ethanol and acetone-butanol-ethanol (ABE) blends have demonstrated no significant performance degradation relative to fossil gasoline, along with measurable reductions in particulate emissions (MacroFuels EU project). The EU's ReFuelEU Aviation regulation mandates sustainable aviation fuel use, while geopolitical energy security shocks have renewed political urgency around liquid fuel diversification. However, challenges in scaling, cost reduction, and regulatory alignment remain. Sustained investment in innovation, particularly in automation, offshore infrastructure, and in market development and the understanding of environmental impacts of scaling will be essential to unlock macroalgae’s full potential as a sustainable, large-scale bioenergy resource.

Why Explore Seaweed Based Biofuels?

Renewable energy will play a fundamental role in transitioning towards a more competitive, secure, and sustainable energy system internationally. Renewable fuels from biomass (biofuels) are expected to contribute especially in hard-to-electrify sectors such as aviation and shipping.  The concept of biologically sourced transportation fuel is not theoretical: more than 98% of U.S. gasoline already contains 10% ethanol (E10) produced from corn biomass. However, usage of terrestrial biofuel feedstocks has several ecological disadvantages. It requires arable land to be diverted from food production, the application of fertilizer and pesticides and the use of irrigation water, which is becoming scarce in many regions (Kite-Powell et al., 2021). In some cases, because of the land-use changes, net carbon emissions actually increase with land-based biofuel production relative to fossil-fuel usage (Searchinger et al., 2008). As shown in the figure below, gross energy yield per hectare from seaweeds could be similar to that for maize, the dominant terrestrial feedstock for biomethane.

How Seaweed Is Converted to Biofuels

Seaweeds are biochemically well-suited for biofuel production. Their cell walls are rich in specialized polysaccharides (laminarin, mannitol, alginate, carrageenan, and ulvan) giving them high carbohydrate content (typically 40–65% of dry weight). Crucially, seaweeds contain little or no lignin, the recalcitrant structural polymer that makes terrestrial biomass costly and energy-intensive to process. This makes macroalgae considerably easier to break down and convert than wood, straw, or grasses. There are three primary pathways for converting seaweed biomass into liquid or gaseous biofuels. Each favors different seaweed species profiles and produces different fuel outputs:  

Pathway	Primary output	Key advantage	Main challenge
Anaerobic digestion (AD)	Biomethane / biogas	Uses wet feedstock; no energy-intensive drying required; digestate usable as fertilizer	Real-world yields below theoretical; inhibitory compounds in some species
Fermentation	Bioethanol / ABE (acetone-butanol-ethanol)	Produces higher-energy-density alcohols compatible with existing fuel infrastructure	Requires pre-treatment to release fermentable sugars; no large-scale operational data yet
Hydrothermal liquefaction (HTL)	Bio-crude → gasoline / diesel / SAF	Handles wet, protein-rich biomass; produces energy-dense hydrocarbon fuel suitable for aviation	Capital-intensive; high catalyst cost; needs process optimization

 

Anaerobic digestion — producing biomethane and liquid fuels

Anaerobic digestion converts the organic matter in seaweed into biogas, a mixture of methane and carbon dioxide, through the action of microorganisms in an oxygen-free environment. This biogas can be used directly for heat and power, upgraded to biomethane (which can be used as a substitute for natural gas for electricity generation), or further processed into liquid transport fuels via established chemical synthesis routes. A key practical advantage is that AD can process wet biomass, avoiding the energy-intensive drying steps required by most other conversion routes. The remaining digestate is rich in nitrogen and phosphorus, making it a potential co-product fertilizer.

Fermentation — producing bioethanol and butanol

Fermentation converts seaweed sugars into bioethanol or acetone-butanol-ethanol (ABE). Unlike terrestrial crops, seaweed polysaccharides must first be broken down by enzymatic or acid hydrolysis before standard or engineered microorganisms can ferment the resulting sugars. The products, ethanol and butanol are compatible with existing fuel infrastructure and internal combustion engines. The MacroFuels EU project produced seaweed-derived fuel and demonstrated it in a road-going vehicle.

Hydrothermal liquefaction (HTL) — producing aviation-compatible bio-crude

Hydrothermal liquefaction processes wet seaweed biomass under high temperature and pressure, converting it directly into a bio-crude oil. This bio-crude can be upgraded into hydrocarbon fuels including gasoline, diesel, and critically sustainable aviation fuel (SAF), with energy density comparable to conventional jet fuel. HTL can accept wet feedstock without pre-drying, and an aqueous nutrient-rich co-product can be recycled back to seaweed cultivation. This makes HTL the most promising pathway for producing seaweed-derived aviation fuel, the largest identified strategic market for seaweed biofuels.   Seaweed-based biofuels are at an early but pivotal stage. Pilot projects have demonstrated technical feasibility  for example laboratory engine tests on seaweed-derived ethanol and acetone-butanol-ethanol (ABE) blends have demonstrated no significant performance degradation relative to fossil gasoline, along with measurable reductions in particulate emissions (MacroFuels EU project). The EU's ReFuelEU Aviation regulation mandates sustainable aviation fuel use, while geopolitical energy security shocks have renewed political urgency around liquid fuel diversification. However, challenges in scaling, cost reduction, and regulatory alignment remain. Sustained investment in innovation, particularly in automation, offshore infrastructure, and in market development and the understanding of environmental impacts of scaling will be essential to unlock macroalgae’s full potential as a sustainable, large-scale bioenergy resource.

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Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the chapter, "Cultivation and Drying Considerations".

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the chapter, "Cultivation and Drying Considerations".

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the chapter, "Cultivation and Drying Considerations". When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

 

Figure 3: Stylized View of the three major conversion pathways clockwise from left Anaerobic Digestion, Fermentation and Thermochemical process (represented by Hydrothermal Liquefaction)  Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Depending on the conversion process, we can obtain a number of intermediate and final products from seaweed sources. See the table below for some of the key products and their uses.

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuel.

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

 

Figure 3: Stylized View of the three major conversion pathways clockwise from left Anaerobic Digestion, Fermentation and Thermochemical process (represented by Hydrothermal Liquefaction)  Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Depending on the conversion process, we can obtain a number of intermediate and final products from seaweed sources. See the table below for some of the key products and their uses.

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuel.

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

 

Figure 3: Stylized View of the three major conversion pathways clockwise from left Anaerobic Digestion, Fermentation and Thermochemical process (represented by Hydrothermal Liquefaction)  Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Depending on the conversion process, we can obtain a number of intermediate and final products from seaweed sources. See the table below for some of the key products and their uses.

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuel.

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

 

Figure 3: Stylized View of the three major conversion pathways clockwise from left Anaerobic Digestion, Fermentation and Thermochemical process (represented by Hydrothermal Liquefaction)  Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Depending on the conversion process, we can obtain a number of intermediate and final products from seaweed sources. See the table below for some of the key products and their uses.

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuel.

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

 

Figure 3: Stylized View of the three major conversion pathways clockwise from left Anaerobic Digestion, Fermentation and Thermochemical process (represented by Hydrothermal Liquefaction)  Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Depending on the conversion process, we can obtain a number of intermediate and final products from seaweed sources. See the table below for some of the key products and their uses.

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels  

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

 

Figure 3: Stylized View of the three major conversion pathways clockwise from left Anaerobic Digestion, Fermentation and Thermochemical process (represented by Hydrothermal Liquefaction)  Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Depending on the conversion process, we can obtain a number of intermediate and final products from seaweed sources. See the table below for some of the key products and their uses.

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels  

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

 

Figure 3: Stylized View of the three major conversion pathways clockwise from left Anaerobic Digestion, Fermentation and Thermochemical process (represented by Hydrothermal Liquefaction)  Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Depending on the conversion process, we can obtain a number of intermediate and final products from seaweed sources. See the table below for some of the key products and their uses.

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels  

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

 

Figure 3: Stylized View of the three major conversion pathways clockwise from left Anaerobic Digestion, Fermentation and Thermochemical process (represented by Hydrothermal Liquefaction)  Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Depending on the conversion process, we can obtain a number of intermediate and final products from seaweed sources. See the table below for some of the key products and their uses.

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels  

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels            

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)        Figure 4: Stylized View of the Fermentation Process Adapted from (Roesijadi et al., 2010)

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels  

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels            

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)        Figure 4: Stylized View of the Fermentation Process Adapted from (Roesijadi et al., 2010)

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 3: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

 

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels            

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)        Figure 4: Stylized View of the Fermentation Process Adapted from (Roesijadi et al., 2010)

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from (Roesijadi et al., 2010)

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

         

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)         Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Thermochemical Processes Hydrothermal liquefaction (HTL) and Gasification via pyrolysis	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels

Companies and Key Research

Process type	Key initiatives/Companies	Notable Research Findings
Anaerobic digestion (AD)	Solrød Biogas, Denmark (operational since 2016) Co-digests beach-cast seaweed with CPKelco processing residues, manure, and food by-products. Upgraded to biomethane for grid injection c. 2020. Seaweed ~0.5% of feedstock by volume — proof of commercial-scale integration.   Barbados — Sargassum-to-biogas (early commercial stage) Small-scale digester demonstrated biogas from beach-stranded Sargassum, including a test drive of a biogas-powered Nissan Leaf. Estimated 20,000 m³/day biogas from 500 t/day Sargassum at full scale — equivalent to daily fuel sales of a standard filling station.	Core challenge: seaweed cell walls resist microbial breakdown, keeping real-world yields well below theoretical maxima. Pre-treatment (hydrothermal or enzymatic) has been a major lever. Pre-treatment R&D for improved yields Lin et al. (2019): Hydrothermal pre-treatment of S. latissima at 140°C increased biomethane yield 22.6% (281 → 345 mL CH₄/g VS). Lamb et al. (2019): Enzyme pre-treatment of S. latissima increased biomethane yield 23.5%.   Ulva AD yield improved 56% by maceration pre-treatment (171 → 271 L CH₄/kg VS).
Fermentation	No major companies exist	MacroFuels EU project (2016–2019) Six-partner consortium; most complete end-to-end fermentation demonstration to date. Key results: • Ethanol: laminarin extraction + hydrolysis + yeast fermentation achieved 4.9% ethanol by volume; 9.4 L distilled ethanol delivered for engine testing. • Butanol: 9.6 L purified n-butanol from S. latissima using an engineered Clostridium beijerinckii strain capable of direct conversion to ABE. • Furanics: high furfural and 5-methylfurfural yields from seaweed hydrolysates; furanic diesel additive from Palmaria palmata tested in compression-ignition engine. • Ensiling: biological ensiling with lactic acid bacteria validated for long-term storage, preserving fermentable sugars and reducing heavy metals (Cd, Hg).   Wargacki et al. (2012) — Science Engineered E. coli to simultaneously degrade, transport, and metabolize alginate — the polysaccharide standard organisms cannot ferment. Produced ethanol at ~80% of theoretical maximum yield directly from brown macroalgae. Biological proof-of-concept for enhanced seaweed sugar utilization.
Thermochemical Conversions (Hydrothermal Liquefaction)		Anastasakis & Ross (2011) Foundational batch HTL of Laminaria saccharina (brown kelp): maximum bio-crude yield 19.3 wt% at 350°C; higher heating value 36.5 MJ/kg — comparable to heavy crude or bitumen.   Allen and Pearce (2024)— continuous-flow HTL of Sargassum First continuous-flow reactor applied to seaweed: 12% biocrude yield (vs ~20% in batch configuration). Transition to continuous flow is essential for commercial viability and is the active R&D frontier.   PNNL / DOE Bioenergy Technologies Office — algae HTL program (ongoing since 2014) Sustained US national lab program focused on microalgae; 2023 reactor design update addresses fouling and clogging — applicable to high-ash macroalgae feedstocks.   UCLA / Schmidt Sciences — alkaline thermal treatment + REE co-recovery Choi et al., (2025) Novel HTL variant using wet Sargassum: alkaline thermal treatment (NaOH + Ni catalyst + N₂) produces hydrogen fuel rather than bio-crude, with rare earth elements extracted from residual biomass. Avoids energy-intensive drying; uses problem biomass (invasive blooms) as feedstock.
Thermochemical Reactions: Gasification via Pyrolysis and Fischer-Tropsch (F-T)	Macrocarbon- Gran Canaria based startup aiming to grow Sargassum to convert to SAF, biochar and carbon black.	Pyrolysis produces bio-oil (liquid fuel precursor) and biochar (soil amendment / carbon sequestration co-product). Ash content is the critical constraint: >20% DW causes slagging and corrosion in pyrolysis reactors, ruling out most brown and many green algae without pre-washing. Key advantage over fermentation: F-T products are drop-in fuels compatible with existing aviation and diesel infrastructure — no blending limits or engine modifications required. Del Río et al. (2022) Thermogravimetric and pyrolysis characterization of red macroalgae including Porphyra. Established ash content thresholds for reactor slagging. High protein in Porphyra (~40% DW) means food/nutraceutical use dominates commercially; pyrolysis most viable for post-extraction residue. Allen et al. (2024) Updated pathway comparison including F-T for macroalgae; assessed suitability of different species and cultivation systems for thermochemical routes.

Table 2: Summary of initiatives and foundational research into the major conversion pathways for seaweed biofuels

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Products Generated by Conversion Processes and their Use

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

Table 3: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen. Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed. Novel approach: alkaline thermal treatment for hydrogen Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)  

	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen. Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed. Novel approach: alkaline thermal treatment for hydrogen Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)  

	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen. Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed. Novel approach: alkaline thermal treatment for hydrogen Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)  

	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen. Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed. Novel approach: alkaline thermal treatment for hydrogen Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)  

	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).  

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen. Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed. Novel approach: alkaline thermal treatment for hydrogen Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)  

	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).  

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen. Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed. Novel approach: alkaline thermal treatment for hydrogen Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)  

	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).  

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen. Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed. Novel approach: alkaline thermal treatment for hydrogen Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)  

	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).  

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen. Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed. Novel approach: alkaline thermal treatment for hydrogen Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)  

	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).  

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen. Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed. Novel approach: alkaline thermal treatment for hydrogen Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)  

	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).  

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

Key Projects and Research Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Key Projects and Research MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Key Projects and Research

Foundational seaweed HTL work

Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen.   Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed.

Novel approach: alkaline thermal treatment for hydrogen

Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)

Other potential methods:

Supercritical CO₂ Extraction:  This is an experimental method that has been explored for producing biodiesel from green macroalgae (Chaetomorpha linum). It yielded less oil than HTL in one study (Roesijadi et al., 2010). Pyrolysis is another thermochemical method that can convert macroalgae into bioenergy. However, pyrolysis requires a dry feedstock with a moisture content of less than 10%. (Stefania Rocca et al., 2015) This necessitates considerable energy for drying the macroalgae biomass before pyrolysis.  

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.  

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).  

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

The following projects represent the main advances and active R&D. Commercial and pilot-scale plants Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Some key projects and research to advance the technology include MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Some key projects include

Foundational seaweed HTL work

Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen.   Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed.

Novel approach: alkaline thermal treatment for hydrogen

Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)

Other potential methods:

Supercritical CO₂ Extraction:  This is an experimental method that has been explored for producing biodiesel from green macroalgae (Chaetomorpha linum). It yielded less oil than HTL in one study (Roesijadi et al., 2010). Pyrolysis is another thermochemical method that can convert macroalgae into bioenergy. However, pyrolysis requires a dry feedstock with a moisture content of less than 10%. (Stefania Rocca et al., 2015) This necessitates considerable energy for drying the macroalgae biomass before pyrolysis.  

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.  

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).  

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale.

Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010)

The following projects represent the main advances and active R&D. Commercial and pilot-scale plants Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists.

Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010

Some key projects and research to advance the technology include MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging.

Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010

Some key projects include

Foundational seaweed HTL work

Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen.   Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed.

Novel approach: alkaline thermal treatment for hydrogen

Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)

Other potential methods:

Supercritical CO₂ Extraction:  This is an experimental method that has been explored for producing biodiesel from green macroalgae (Chaetomorpha linum). It yielded less oil than HTL in one study (Roesijadi et al., 2010). Pyrolysis is another thermochemical method that can convert macroalgae into bioenergy. However, pyrolysis requires a dry feedstock with a moisture content of less than 10%. (Stefania Rocca et al., 2015) This necessitates considerable energy for drying the macroalgae biomass before pyrolysis.  

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries

Species Selection, Cultivation and Harvesting

Species selection:

Seaweeds possess complex cell wall architecture that serves crucial functions including mechanical support, cell-cell adhesion, and environmental protection. The cell wall is primarily composed of specialized polysaccharides-it is these polysaccharides that give seaweeds their high carbohydrate content, which combined with low lignin make them suitable for biofuel pathways. Several seaweed species have been studied for use in biofuels, but brown seaweeds such as Saccharina latissima (sugar kelp) have been the most widely researched for biofuel uses due to their high carbohydrate content and for their comparatively large biomass yields when using traditional cultivation approaches.  Below are some popular species that are being explored for seaweed-based biofuels and their attributes.

Figure 2: Seaweed Species being explored for biofuels

Hatchery/Nursery:

The two primary ways of propagating seaweeds are vegetative (using fragments of the seaweed) and spore-based methods.  For more information on the state of approach of modern hatcheries nurseries, see the cross-cutting Cultivation chapter.

Cultivation:

Adoption of seaweed-based biofuels will require a scale-up of several orders of magnitude in seaweed farming. According to an ARPA-E estimate, the United States has suitable conditions to grow 500 million dry metric tons of macroalgae per year, which is approximately 150 times current global production. Such production volumes would yield 10% of the US annual transportation energy demand. Given spatial constraints and competing uses in near-shore areas, much of the expansion will need to occur further offshore. (Definitions of offshore vary but according to ARPA-E’s HAEJO program, offshore refers to “deep water locations or waters beyond state jurisdiction but not including areas beyond national jurisdiction.” In the US, this typically means farming beyond approximately 3 nautical miles). For more details on offshore farming, refer to the section on offshore farming in the Cross Cutting Cultivation chapter.  

Harvesting:  

Once mature, seaweed is removed from artificial substrates. Several tropical seaweed species such as Kappaphycus alvarezii and Sargassum spp.  are capable of producing multiple harvests a year, while most temperate seaweeds have one optimal harvest time a year. The decision to harvest is based on current water temperature (measured via in-situ thermometers or regional oceanographic services), visual assessment of seaweed canopy density and color and historical seasonal patterns.  The current state of seaweed harvesting involves a mix of traditional manual techniques and developing mechanical methods. For large-scale offshore farms, cost-effective mechanized harvesting will be essential and prototype systems are being developed overseas. The most common systems include: -rotating blades that can be suitable for species growing attached to supporting structures; and -suction systems followed by cutting that can be used for floating seaweeds species (e.g. Sargassum and Gracilaria).For more information, refer to the harvesting section in the Cross Cutting Cultivation chapter. When to harvest; Timing largely depends on local water temperature and species so can vary significantly by latitude.  Through periodic destructive sampling, the BioMara project identified autumn as the optimal harvest season for biofuel production, due to the peak sugar content in seaweeds during this period (Šuopys et al., 2021).  To extend the harvesting window, they recommended cultivating multiple seaweed species, as different species reach peak sugar content at different times. Pre-treatment for Processing: After harvesting, macroalgae is washed to remove foreign objects and chopped or milled to increase surface area. Many conversion processes require dry feedstock; reducing water content from ~80% to 20–30% also increases shelf-life and reduces transportation costs (Stefania Rocca et al., 2015). Sun-drying is the main method of drying seaweed, but it is weather dependent (Milledge & Harvey, 2018). Conventional dryers such as drum dryers or conveyor dryers are also common, but they require energy and are therefore a source of emissions in areas without a clean electricity grid. Ensiling — preserving seaweed by controlled anaerobic fermentation with lactic acid bacteria — is an alternative that avoids drying entirely. The MacroFuels project demonstrated that ensiling sugar kelp significantly reduces heavy metal content (cadmium, mercury) while maintaining sugar and protein levels, enabling year-round continuous feedstock supply (Šuopys et al., 2021).

Harvesting Wild Seaweed

Wild seaweed harvesting remains commercially significant at global scale: for example, a significant portion of the world's alginate supply is derived from wild-harvested Laminaria hyperborea along Norway's Atlantic coastline.  In addition to cultivated seaweed, there is growing interest in harvesting wild seaweed blooms, particularly Sargassum, which has proliferated across the Atlantic since 2011. These blooms, spanning over 5,000 miles from West Africa to the Gulf of Mexico, often wash ashore and decompose on beaches, releasing hydrogen sulfide and damaging local tourism—especially in the Caribbean. Initiatives to valorize this beach-cast seaweed for products, including as biofuels are underway. Similarly, in China, recurring Yellow Sea “green tides” of Ulva prolifera have led to research on tracking, harvesting and converting the biomass (Xing et al., 2019, Sun et al., 2022).

Conversion to Biofuels

There are three primary pathways for converting macroalgae to bioenergy. Table 1 below summarizes key parameters across all three pathways including current Technology Readiness Levels (TRL).  

Pathway	Key steps	Final product	Co-products	TRL	Main advantages	Main challenges
Anaerobic digestion (AD)	Milling/slurrying → anaerobic digestion → gas cleaning → methanol synthesis → MTG or F-T process	Gasoline / diesel / biomethane	AD digestate as fertilizer	7	Uses wet feedstock; digestate as fertilizer; mature core technology	H₂S removal required; real-world methane yields below theoretical
Fermentation	Milling/slurrying → pretreatment (enzymatic/acid hydrolysis) → fermentation → distillation	Ethanol, butanol	Glycerol	5	Produces higher-energy-density alcohols; drop-in compatible	Requires pre-treatment; no large-scale operational data
Hydrothermal liquefaction (HTL)	Slurry prep → HTL (300–350°C, 120–180 bar) → hydrotreating → hydrocracking	Gasoline, diesel, SAF	Biochar, aqueous nutrient phase	4	Handles wet, protein-rich biomass; aviation-compatible output	Capital-intensive; high catalyst cost; batch process to date

Table 1: Summary of major conversion pathways for seaweed biofuels  

Anaerobic Digestion (Technological Readiness Level 7)

AD is the most mature seaweed-to-fuel pathway. The core technology is commercially proven for seaweed as a minor co-feedstock ; the outstanding question is whether seaweed can serve as a viable primary or significant co-feedstock at commercial scale. Figure 3: Stylized View of the Anaerobic Digestion Process Adapted from (Roesijadi et al., 2010) The following projects represent the main advances and active R&D. Commercial and pilot-scale plants Solrød Biogas — Denmark (operational since 2016) The most-cited example of seaweed in a commercial-scale AD facility. Commissioned in 2016 outside Copenhagen, the plant co-digests beach-cast seaweed collected from Køge Bay alongside residues from the CPKelco pectin/carrageenan processing plant, agricultural manure, and food industry by-products. Seaweed represents approximately 0.5% of the total feedstock by volume — not a primary feedstock, but a proof of concept for integration at commercial scale. The plant has since been upgraded with a biogas upgrading unit (supplied by Malmberg, commissioned c. 2020) to produce biomethane for injection into the gas grid. Barbados — Sargassum-to-biogas (early commercial stage) A Caribbean team has demonstrated biogas production from beach-stranded Sargassum using a small-scale digester in Barbados, culminating in a test drive of a biogas-powered Nissan Leaf. The team has estimated that processing 500 tonnes of Sargassum per day at a full-scale facility could generate 20,000 m³ of biogas which is equivalent in energy to the daily fuel sales of a standard filling station. This represents the most advanced attempt to commercialize Sargassum for AD. Pre-treatment research to improve AD yields Real-world methane yields from seaweed AD remain well below theoretical values, primarily because seaweed cell walls are difficult for microorganisms to break down into simpler carbohydrates. Most recent R&D has focused on pre-treatment methods to address this: Hydrothermal pre-treatment of Saccharina latissima mild heat treatment (140°C) increased biomethane yield by 22.6%, from 281 to 345 mL CH₄/g of volatile solids (Lin et al., 2019).   Similarly, pre-treatment of S. latissima with enzymes before AD increased the biomethane yield by 23.5% (Lamb et al., 2019).

Fermentation to Alcohols (e.g., Ethanol and Butanol)

Fermentation of seaweed to ethanol and butanol has advanced primarily through the EU MacroFuels project (2016–2019), which remains the single most significant body of work in this area. The core challenge that seaweed polysaccharides cannot be fermented by conventional yeast without prior hydrolysis  has been partially addressed, but no commercial-scale fermentation using seaweed as a primary feedstock yet exists. Figure 4: Stylized View of the Fermentation Process Adapted from Roesijadi et al., 2010 Some key projects and research to advance the technology include MacroFuels EU Horizon 2020 project (2016–2019) MacroFuels was a six-partner European research consortium funded under Horizon 2020, targeting a range of products (bioethanol, biobutanol, furanics, and biogas) from macroalgae. It represents the most complete end-to-end demonstration of seaweed fermentation to date Key Results:

• Ethanol production: A new fermentation strategy combining laminarin extraction, hydrolysis, and yeast fermentation achieved 4.9% ethanol by volume. 9.4 liters of distilled ethanol were delivered for engine testing.
• Butanol production: 9.6 litres of purified n-butanol were produced from Saccharina latissima. The MacroFuels team engineered a novel strain of Clostridium beijerinckii capable of directly converting algal biomass to ABE (acetone-butanol-ethanol).
• Furanics (thermochemical route): High yields of furfural and 5-methylfurfural were obtained from seaweed hydrolysates. Furfural was upgraded to furanic fuel blends by reaction with ethanol or butanol. A furanic diesel additive derived from red seaweed (Palmaria palmata) was produced at kg-scale and tested in a compression-ignition engine.
• Ensiling advance: Biological ensiling with lactic acid bacteria was validated as a cost-effective long-term storage method that preserves fermentable sugars and reduces heavy metals (cadmium, mercury) in the biomass — a key step toward year-round processing.

 Engineered bacteria for alginate fermentation Wargacki et al.,2012 demonstrated that Escherichia coli can be engineered to simultaneously degrade, transport, and metabolize alginate, the polysaccharide that standard microorganisms cannot ferment. The engineered strain produced ethanol directly from brown macroalgae at ~80% of the theoretical maximum yield. While not a commercial demonstration, this paper established the biological proof of concept for enhanced seaweed sugar utilization.  

Hydrothermal liquefaction (HTL) — TRL 4

HTL of seaweed has been demonstrated in laboratory-scale batch reactors. The primary R&D challenge is transitioning to continuous-flow operation, which is essential for any commercially viable plant. This transition is actively being pursued in academia and for general biomass feedstocks, with some seaweed-specific work emerging. Figure 5: Stylized View of the Hydrothermal Liquefaction Process. Adapted from Roesijadi et al., 2010 Some key projects include

Foundational seaweed HTL work

Anastasakis & Ross, 2011 demonstrated batch HTL of the brown kelp Laminaria saccharina, achieving a maximum bio-crude yield of 19.3 wt% at 350°C, with a higher heating value of 36.5 MJ/kg that is comparable to heavy crude oil or bitumen.   Continuous HTL of Sargassum Shifting to continuous-flow is essential for commercial viability, Allen and Pearce, 2024 used a continuous-flow reactor on Sargassum. This first of a kind system achieved biocrude yield of 12%, compared to yield for batch systems with the same configuration at 20%. PNNL DOE algae HTL program The Pacific Northwest National Laboratory, funded by the US DOE Bioenergy Technologies Office, has run a sustained algae HTL program since 2014.. Work focuses on microalgae but the continuous-flow reactor design is applicable to macroalgae. The most recent design update (2023) to address fouling and clogging can benefit high-ash feedstocks like seaweed.

Novel approach: alkaline thermal treatment for hydrogen

Researchers at UCLA, funded by Schmidt Sciences' Virtual Institute of Feedstocks of the Future, have developed an alternative to standard HTL using Sargassum as feedstock. Wet Sargassum is treated with sodium hydroxide, a nickel catalyst, and nitrogen gas via alkaline thermal treatment to produce hydrogen fuel, with rare earth elements extracted from the residual biomass as a co-product. Unlike standard HTL this route produces hydrogen (a zero-emission fuel) rather than bio-crude. Source: C&EN news coverage (March 2025)

Other potential methods:

Supercritical CO₂ Extraction:  This is an experimental method that has been explored for producing biodiesel from green macroalgae (Chaetomorpha linum). It yielded less oil than HTL in one study (Roesijadi et al., 2010). Pyrolysis is another thermochemical method that can convert macroalgae into bioenergy. However, pyrolysis requires a dry feedstock with a moisture content of less than 10%. (Stefania Rocca et al., 2015) This necessitates considerable energy for drying the macroalgae biomass before pyrolysis.  

Product	Definition	Relevant Use
Methanol	A simple alcohol (CH₃OH), colorless, volatile, and flammable.	Precursor to fuels, Transportation Fuel, Chemical Feedstock
Ethanol	A two-carbon alcohol (C₂H₅OH) which is flammable, colorless and volatile.	Transportation Fuel, Chemical Feedstock
Butanol	A four-carbon alcohol (C₄H₉OH), more energy-dense and less volatile than ethanol.	Transportation Fuel, Solvent
Gasoline	Typically, a petroleum-derived liquid hydrocarbon blend is used as transport fuel.	Transportation Fuel
Syngas	A mixture of CO, H₂, and CO₂ produced by gasification.	Methanol Synthesis, Electricity
Volatile Fatty Acids (VFAs)	Short-chain organic acids (e.g., acetic, propionic, butyric).	Chemical Precursors, Biofuel Intermediates
Bio-Oil	A synthetic tar-like substance made from pyrolysis and HTL with high levels of oxygen and nitrogen (when made from algae)	Biofuel Intermediates, Carbon Sequestration

  Table 2: Definition of Seaweed Based Biofuel Products and Intermediaries