Thermochemical biomass conversion: Techniques

Gasifiers, pyrolysis reactors, hot pressurised water and electricity-making bacteria

What this tab covers The methods used to capture this energy: how each works, and how they compare on efficiency, cost, life-cycle emissions, maturity and scale.

Five techniques are covered. Gasification for heat and power and fast pyrolysis for heating oil are early commercial; hydrothermal liquefaction and torrefaction are at pilot or demonstration scale, and microbial fuel cells remain experimental.

Comparison

Method	Efficiency	Cost	Life-cycle emissions	Maturity	Scale
Gasification to syngas⁴⁵⁶⁷	25-30% electricity with engines; 30-40% in IGCC; 14-18% mass yield to Fischer-Tropsch distillate	USD 3,000-4,000/kW for gasifier-engine CHP, USD 2,500-5,500/kW and USD 110-130/MWh for IGCC (2007 estimates); Fischer-Tropsch plants over USD 1 billion	EU typical saving 83% for waste-wood Fischer-Tropsch diesel (about 16 g CO2eq/MJ, calculated); ICAO 7.7-8.3 g CO2eq/MJ for residue-based Fischer-Tropsch jet	early commercial	Engine CHP units 0.2-1 MWe; IGCC 10-30 MWe; 20 Fischer-Tropsch fuel plants announced
Pyrolysis to bio-oil and biochar¹²³	Up to 65-75% bio-oil by mass (fast); about 35% char (slow)	Not quantified	Not quantified	early commercial	Bio-oil for heating from Ensyn (Canada, 2008), Fortum Joensuu (Finland, 2013) and Empyro (Netherlands, 2015); KIT bioliq pilot (Germany, 2014)
Hydrothermal liquefaction (HTL)⁵⁸⁹	Not quantified	Not quantified	Not quantified	pilot	Pilot plants such as Alder Renewables; not ASTM-approved for jet fuel
Torrefaction¹³	About 82% of dry mass kept as solid	Not quantified	Not quantified	pilot	Demonstration plants such as Topell Energy, Duiven (Netherlands), from 2010
Microbial fuel cells (experimental)¹¹	Coulombic efficiency up to 55.6% (laboratory)	Capital cost up to 30 times that of conventional wastewater treatment	Not quantified	research	Laboratory and small pilot devices; reported power densities 2.44-3.31 W per square metre of electrode

Gasification makes a versatile gas and some of the lowest-emission fuels (about 5-8 g CO2eq/MJ from residues and waste), but tar cleaning and synthesis are costly: over USD 1 billion for a Fischer-Tropsch refinery. Fast pyrolysis is cheaper and keeps most of the mass as liquid, but the oil is watery and acidic, so it is mainly burned for heat. HTL suits sludges and algae but needs pressure vessels and hydrogen. Torrefaction is a pre-treatment for shipping and co-firing; microbial fuel cells are far from competitive.

How each method works

Gasification to syngas

Biomass is heated with a limited supply of air, oxygen or steam so it partly oxidises into syngas (carbon monoxide, hydrogen and methane). After tar and dust removal the gas fuels engines or turbines, or is converted to liquid fuel by Fischer-Tropsch synthesis.⁴⁵⁶⁷

Pyrolysis to bio-oil and biochar

Dry, ground biomass is heated rapidly without oxygen and the vapours are quickly cooled into bio-oil; the char and gas heat the reactor. Slower heating maximises char, usable as fuel or as soil biochar.¹²³

Hydrothermal liquefaction (HTL)

Wet biomass such as sewage sludge, algae or food waste is held in hot, pressurised water, which breaks it into a heavy biocrude without drying the feed; the biocrude still needs hydrogen treatment.⁵⁸⁹

Torrefaction

Biomass is roasted at 200-350 C without oxygen, driving off water and some volatiles. The dry, water-resistant product can be pelleted into 'bio-coal' and handled like coal.¹³

Microbial fuel cells (experimental)

Bacteria growing on an electrode break down organic matter, often in wastewater, and pass electrons to the electrode, giving a small current without combustion.¹¹

Sources

Torrefaction and pyrolysis (European Industrial Bioenergy Initiative factsheet EIBI-4) ETIP Bioenergy
IEA Bioenergy Update 29: Task 34 technology report on fast pyrolysis IEA Bioenergy
Fast pyrolysis: key points (table of product yields by pyrolysis mode) IEA Bioenergy Task 34 (Direct Thermochemical Liquefaction)
Energy Technology Essentials ETE03: Biomass for Power Generation and CHP (2007 cost estimates) International Energy Agency (copy hosted by nachhaltigwirtschaften.at), 2007
Progress in commercialisation of biojet / SAF (van Dyk, IEA Bioenergy workshop presentation) IEA Bioenergy Task 39 (hosted by DBFZ), 2023
Directive (EU) 2018/2001 (Renewable Energy Directive II), Annex V Part B: estimated typical and default savings for future biofuels. Default values not changed by amending Directive (EU) 2023/2413 (RED III); a Commission revision was in draft in 2026 European Union (EUR-Lex), 2018
CORSIA Default Life Cycle Emissions Values for CORSIA Eligible Fuels (ICAO document 06) International Civil Aviation Organization, 2025
Biojet fuels: technologies and status of commercialisation (van Dyk and Saddler) IEA Bioenergy Task 39, 2020
Reaching Zero with Renewables: Biojet fuels IRENA, 2021
Recent Applications, Challenges, and Future Prospects of Microbial Fuel Cells: A Review (Chakma et al., Global Challenges 9(5) 2500004) Wiley (open access, PubMed Central), 2025