Thermochemical biomass conversion involves processes at much higher temperatures and generally higher pressures than those found in biochemical conversion systems. Key intrinsic characteristics distinguishing thermochemical from biochemical biofuels are the flexibility in feedstocks that can be accommodated with thermochemical processing and the diversity of finished fuels that can be produced.
Thermochemical production of biofuels begins with gasification or pyrolysis. The former is generally more capital-intensive and requires larger scale for best economics, but the final product is a clean finished fuel that can be used directly in engines. The discussion here focuses on gasificationbased processing, by which a variety of different biofuels can be produced, including Fisher-Tropsch liquids (FTL), dimethyl ether (DME), and various alcohols.
During gasification, biomass (with 10 – 20 per cent moisture content) is heated (typically by combusting a portion of the biomass in oxygen) to cause it to be converted into a mixture of combustible and non-combustible gases. Contaminants in the gas are removed, followed in some cases by adjustments (using the “water-gas shift” reaction) of the composition of the gas (also called synthesis gas, or syngas) to prepare it for further downstream processing. Carbon dioxide (CO2) is a diluent in the syngas and so is then removed to facilitate subsequent reactions downstream. The major components of the now-clean and concentrated syngas are carbon monoxide (CO) and hydrogen (H2), usually with a small amount of methane (CH4). The CO and H2 react when passed over a catalyst (the CH4 is inert) to produce liquid fuel. The design of the catalyst determines what biofuel is produced. In most plant designs, not all of the syngas passing over the catalyst will be converted to liquid fuel. The unconverted syngas typically would be burned to make electricity to provide some or all of the power needed to run the facility and in some cases to export electricity to the grid. A second option for converting syngas to liquid fuel – one that is less well-developed commercially than the catalytic process just described. With this option, specially-designed micro-organisms ferment the syngas to ethanol or butanol.