Putting the Sun into your Fuel Tank

Synthetic fuel made from biomass, so-called SunFuel, can be produced with two different basic methods. Firstly, there is the familiar process with gasification and subsequent liquefying of the biomass. Fischer-Tropsch synthesis is mainly used to liquefy the resulting gas. In the MtSynfuel® process developed by Frankfurt-based company Lurgi, methanol synthesis follows the production of synthesis gas first. The methanol is used for olefin production and then for oligomerisation.

Gasification and synthesis

In terms of producing synthetic fuels from biomass, the Choren process is the most advanced. In the process developed by the company of the same name, the gasification of the biomass is followed by Fischer-Tropsch synthesis. The final product is a highly pure fuel – depending on the type of synthesis, more diesel, more petrol or more kerosene is produced. The company from Freiberg in Saxony currently owns the world’s only facility that can produce a biomass-to-liquid fuel (BtL). The mineral oil company Shell is now collaborating with Choren to further develop and expand this facility.

Shell is one of the experts in mastering the Fischer-Tropsch process. Shell has been producing synthetic fuels from natural gas since 1992 at its GtL facility in Bintulu (Malaysia). The same Fischer-Tropsch process has been used by South African company Sasol since the fifties to produce fuel from coal. The special part of the Choren process is that the starting material required for the Fischer-Tropsch, the synthesis gas, is not produced from natural gas or coal.

Other processes are currently in the laboratory stages or have not progressed past the planning phase. The Karlsruhe research centre (FZK), for example, is working on a similar concept to the Choren process. A basic substance made from bio oil and coke, called slurry, is gained from biomass in a fast pyrolysis and is then converted into synthesis gas in a second step in an entrained-flow gasifier. The synthesis, i.e. the conversion of the synthesis gas into fuel would also be carried out in a Fischer-Tropsch reactor. The slurry has a high energy density and is very suitable for decentral creation with subsequent transport to a central Fischer-Tropsch system. This can substantially reduce the logistical costs for supplying biomass.

At the FZK, slurry is already being produced in a so-called pyrolysis reactor in trials. In addition to the high energy density, the intermediate product still has the advantage that it can be fed into the corresponding entrained-flow gasifier at high pressure (25 bar and more) more easily than biogenic solids. A complete system, in which the finished synthetic fuel is produced, has not yet been built, however.

Another process for producing fuel using synthesis gas was developed by Prof. Bernd Meyer at the Institute for Energy Process Technology and Chemical Engineering IEC) at the TU Bergakademie Freiberg. It is based on fluidised bed gasification with subsequent fuel synthesis. The MtSynfuel® process developed by Lurgi is used. However, both the gasification and the synthesis are still in the planning stages. At the moment, the operational plans are being drawn up as part of one of the FNR-sponsored projects (FNR = Agency of Renewable Resources, associated with the German Federal Ministry of Consumer Protection, Food and Agriculture).  

Direct liquefaction

In the direct liquefaction, the biomass is converted into a liquid fuel without a synthetic gas being produced in-between. This promises a considerable improvement in thermal efficiency compared with the synthesis gas process. This development is comparable with the direct liquefaction of coal (developed by IG Farben in the thirties), which also has greater thermal efficiency (up to 63%) than gasification of coal followed by FT synthesis (about 44 % with coal, but about 70% with natural gas). Both processes, the direct liquefaction and the Fischer-Tropsch technology, are currently going through a renaissance in China because the low raw material and wage costs allow economic coal liquefaction. The processes for direct liquefaction of biomass, however, are only at the start of their development and are currently only in the planning phase or laboratory stages at the most.

A corresponding process for the direct conversion of organic substances (DoS) is being developed on a laboratory scale, for example, by Prof. Thomas Willner at the Hamburg University of Applied Sciences. However, continuous operation, which could guarantee a technically useful scale, and a self-sufficient hydrogen supply needs to be provided or developed. Further variants of direct liquefaction are, for example, depolimerisation, also known as catalytic breaking of hydrocarbon chains – e.g. biomass, plastics or old oils. The catalytic breaking is already used successfully with old oils and plastics. As the product quality is not yet in line with the requirements of modern diesel car engines, there is a considerable need for development in this area, in particular with the use of biomass.

The technical details of the most important processes are dealt with in the following articles.

Fischer-Tropsch Synthesis

Choren Process

MtSynfuels Process