Hydrothermal Liquefaction for Bioenergy and Biofuel Production

Hydrothermal liquefaction (HTL) is a thermochemical conversion process that transforms wet biomass into an energy-dense biocrude under high-pressure, moderate-to-high-temperature aqueous conditions. Operating typically between 250 °C and 375 °C and at pressures of 10–25 MPa, subcritical water acts as both solvent and reactant, breaking down lignocellulosic polymers, proteins and lipids into smaller hydrocarbons. Feedstocks range from microalgae and agricultural residues to sewage sludge and forestry by-products, offering a feedstock-agnostic pathway that bypasses energy-intensive drying. The primary product—a viscous biocrude—can be upgraded via catalytic hydrotreating, distillation and solvent extraction into transportation fuels such as gasoline, diesel and kerosene. Side streams include an aqueous phase rich in organics and a solid char fraction, each of which can be valorised to improve overall process economics and sustainability. Key challenges involve reducing oxygen and nitrogen heteroatoms in biocrude, achieving continuous-flow operation at scale, integrating heat recovery, and demonstrating favourable techno-economic and life-cycle greenhouse-gas metrics. Advances in reactor design, catalyst development and process integration continue to drive HTL towards commercial viability, with particular emphasis on energy return on investment, carbon reduction and circular-economy considerations.

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Hydrothermal Liquefaction for Bioenergy and Biofuel Production publication trend

The graph below shows the total number of articles in hydrothermal liquefaction for bioenergy and biofuel production across all publications each year (not limited to Nature Index journals).

Technical terms

Hydrothermal Liquefaction: A process using hot, pressurised water to depolymerise wet biomass into biocrude and by-products.

Biocrude: A complex, viscous oil produced by HTL, rich in hydrocarbons, oxygenates and nitrogen-containing compounds.

Catalytic Hydrotreating: A refining step that removes oxygen and nitrogen heteroatoms from biocrude using hydrogen and metal catalysts.

Lignocellulosic Biomass: Plant-derived matter composed of cellulose, hemicellulose and lignin, commonly used as a feedstock for thermochemical conversion.

References

1. Upgrading of hydrothermal liquefaction biocrudes from mono- and co-liquefaction of cow manure and wheat straw through hydrotreating and distillation. Chemical Engineering Journal (2023).
2. An assessment of sustainability metrics for waste-to-liquid fuel pathways for a low carbon circular economy. Energy Nexus (2023).
3. A review on hydrothermal liquefaction of biomass. Renewable and Sustainable Energy Reviews (2018).
4. A Review of Hydrothermal Liquefaction Bio-Crude Properties and Prospects for Upgrading to Transportation Fuels. Energies (2015).
5. Continuous Hydrothermal Liquefaction of Biomass in a Novel Pilot Plant with Heat Recovery and Hydraulic Oscillation. Energies (2018).