Fischer-Tropsch diesel

The Fischer-Tropsch process is one of the advanced biofuel conversion technologies that comprise gasification of biomass feedstocks, cleaning and conditioning of the produced synthesis gas, and subsequent synthesis to liquid (or gaseous) biofuels. The Fischer-Tropsch process has been known since the 1920s in Germany, but in the past it was mainly used for the production of liquid fuels from coal or natural gas. However, the process using biomass as feedstock is still under development. Any type of biomass can be used as a feedstock, including woody and grassy materials and agricultural and forestry residues. The biomass is gasified to produce synthesis gas, which is a mixture of carbon monoxide (CO) and hydrogen (H₂). Prior to synthesis, this gas can be conditioned using the water gas shift to achieve the required H₂/CO ratio for the synthesis. The liquids produced from the syngas, which comprise various hydrocarbon fractions, are very clean (sulphur free) straight-chain hydrocarbons, and can be converted further to automotive fuels. Fischer-Tropsch diesel can be produced directly, but a higher yield is achieved if first Fischer-Tropsch wax is produced, followed by hydrocracking. Fischer-Tropsch diesel is similar to fossil diesel with regard to a.o. its energy content, density and viscosity and it can be blended with fossil diesel in any proportion without the need for engine or infrastructure modifications. Regarding some fuel characteristics, Fischer-Tropsch diesel is even more favourable, i.e. a higher cetane number (better auto-ignition qualities) and lower aromatic content, which results in lower NOx and particle emissions.

For the production of Fischer-Tropsch diesel the main technological challenges are in the production of the synthesis gas (entrained flow gasifier). These barriers also apply to other gasification-derived biofuels, i.e. bio-methanol, bio-DME and biohydrogen. The synthesis gas is produced by a high-temperature gasification, which is already used for coal gasification. Biomass has different properties than coal and, therefore, several process changes are necessary. First, the biomass pre-treatment and feeding need a different process, because milling biomass to small particles is too energy-intensive.

Moreover, small biomass particles can also aggregate and plug feeding lines. Pre-treatment processes like torrefaction or pyrolysis (which produces a liquid oil) could be developed to overcome these problems. Second, due to the higher reactivity of biomass (compared to coal) the gasification temperature might be decreased, resulting in higher efficiencies, but this will require different gasification and burner design. Third, the ash composition in biomass is different from that in coal, which results in different ash and slag behaviour, which is an important factor in the gasifier and still needs to be studied thoroughly. This ash and slag behaviour is also important for the cooling of the syngas, for which innovative development is desired. Other research topics are the cleaning and conditioning of synthesis gas, development of several types of catalysts, and the utilisation of by-products such as electricity, heat and steam. In Germany, a pilot production facility for Fischer-Tropsch liquids from biomass is currently in operation.