Summary

The term "Biomass-to-Liquid (BtL)", in general, includes all processes or process chains that convert biomass to liquid fuels. The actual usage, however, is more specific, and comprises processes for the gasification of biomass and for the synthesis of liquid fuels from synthesis gas. A general scheme for the production of BtL-fuels is given in Figure 1.

Figure 1: BtL-Production – General Scheme

The process of gasification and the necessary biomass preparation is explained in a different section of this work. Synthesis gas cleaning and conditioning processes depend on the gasification process and the following synthesis processes, as well as the following liquid fuel conditioning, and thus cannot be generalized. The most important synthesis processes for liquid fuels from synthesis gas are Fischer-Tropsch synthesis and methanol synthesis, the latter of which might be further followed by the Methanol-to-Gasoline process (MtG) or by the production of MTBE.

Fischer-Tropsch synthesis is process catalysed by iron or cobalt, which forms paraffins, olefins and alcohols from synthesis gas. It was developed in the 1920s in Germany by F. Fischer and his co-worker H. Tropsch. There are different versions of the process that might lead to different product distributions. The Fischer-Tropsch synthesis process had been mainly used for the production of synthetic fuels from coal-based synthesis gas in Germany during World War II and in South Africa during the Apartheid regime.

Synthetic methanol has been produced first by the company BASF in Germany in 1923. At the beginning, a "high pressure" process was introduced, operating at pressures of 250 - 350 bars. Later, pressure could be reduced to 50 - 100 bars in the "low pressure" process, using different catalysts. Since the overall reaction is strongly exothermic, different reactor types have been developed. Mobil Oil Corp. developed the so-called ZSM-5, an acidic zeolite catalyst. This catalyst is capable of converting methanol to hydrocarbons in the range of C₅ to C₁₀ in the methanol-to-gasoline process (MTG). The process has been commercialized in New Zealand in the 1980s. With some changes in process design, it can also be used to produce olefins or aromatics.

Methyl tert-butyl ether (MTBE) can be produced by the addition of methanol to (usually fossil) isobutylene. Thus, it can be considered partial renewable if the methanol is synthesized from biomass derived synthesis gas. Different MTBE processes have been developed, e.g. by ARCO, Phillips, IFP, CD Tech and Snamprogretti. Fossil MTBE is already used for the increase of octane numbers in gasoline.

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