2.3 Lignocellulosic-based biofuels from synthesis gas
The book on Lignocellulosic-based biofuels from synthesis gas explores the conversion of lignocellulosic biomass into biofuels.
2. Fischer-Tropsch synthesis
Fischer-Tropsch synthesis
The Fischer-Tropsch (FT) process synthesizes a mixture of different paraffin, olefin and oxygen containing compounds and water from hydrogen and carbon monoxide in the syngas. Final products are gasoline, diesel or fuel oil.
The so called synthol process was invented by Franz Fischer and Hans Tropsch in the early 1920s. The process was then continously improved and the influence of process parameters like temperature, pressure, and catalysts where analysed and optimized.
The basic reactions can be described by:
The exact reaction mechanism is not fully understood until today because of its complexity. During FT synthesis various different reactions take place. See more details of the reaction mechanism in Rauch et al. (2018 “Biokerosene Book”).
The length of the obtained hydrocarbons ranges from C1 (methane) to C20+ (waxes). Mainly unbranched molecules (i.e. n-alkanes) are produced. The chain length distribution in the final product is determined by the chain growth probability (usually between 0.7 and 0.9). Higher values increase the share of waxes (long chained hydrocarbons). The chain growth probability is influenced by temperature, pressure, catalysts, reactor type and H2/CO ratio in the syngas.
The reaction takes already place at temperatures between 160°C and 200°C. Higher temperatures promote the formation of short chained and more branched products and increases the amount of secondary products. But also carbon deposition on the catalyst surface is increasing.
The reaction runs already at atmospheric pressure. With increasing pressure the processes run better.
Iron, cobalt, nickel or ruthenium are used as catalysts. The selection of the catalyst influences strongly the spectrum of synthesis products. For example cobalt promotes the formation of alkanes. Ruthenium is the most active catalyst for FT synthesis, but it is rarely used because of its high price.
Fixed bed, fluidized bed or slurry reactors can be used for the FT process. The fixed bed reactors are often designed as multi-tubular reactor (see figure below). The tubes enable cooling because the synthisis is a strongly exotherm process. The tubes are filled with catalysts. The smaller the catalyst particles the better the conversion rate. The cooling agent is surrounding the tubes.
The process can be designed in a way that unconverted gases are redirected into the reactor. This increases the conversion efficiency (full conversion mode). But the process can also be desigend without this difficult recycling step.
Since commercially available reactors have been developed for large-scale application in coal-to-liquide industry they might be too large for biomass-based FT plants. Smaller reactors based on the fixed bed technology are developed currently. The single modules can also be combined to realize larger plants.
The final products mostly consist of alkanes and are free of sulphur and aromatics. The last step is to upgrade and to separate the FT synthesis products, which is done by hydrocracking and distillation or rectification. During hydrocracking chained hydrocarbons are split in products with lower boiling points (e.g. naphta, kerosene or diesel) by adding hydrogen. Cracking is also facilitated by catalysts (e.g. nickel). At the same time isomerization is happening; i.e. long-chained hydrocarbons are converted into branched hydrocarbons. During distillation the desired fractions are separated on the basis of their boiling points.