The Fischer-Tropsch (FT) synthesis was developed in the 1930s, by which a gas con­taining carbon monoxide (CO) and hydrogen (H2) can be converted into long-chain hydrocarbons which have properties similar to crude oil products. A gas containing as its main components H2 and CO can be produced by the high-temperature gasification of coal, biomass and waste, and is known as syngas. The gasification process produces a mixture of CO, H2, methane (CH4), carbon dioxide (CO2), nitrogen (N2) and water (H2O). Natural gas can also be used in the FT process. The FT synthesis was used to produce diesel and petrol in World War II using coal as the starting material (Prins et al., 2004). At present, syngas is mainly used by the chemical industry (Fig. 7.2) for ammonia production and only 8% is being used to produce hydrocarbon-based fuels called ‘gas to liquid’ (GTL) fuels where natural gas is used. In order to make the pro­cess sustainable, coal and gas should be replaced with biomass and waste materials. However, the process is costly and so this fuel is still under development.

Syngas production

At present, there are two industrial methods of producing syngas from biomass: a fluidized bed gasifier and entrained flow gasifier. The fluidized bed gasifier converts

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Fig. 7.2. Present industrial uses of syngas: ammonia production, hydrogen for refineries, methanol production, electricity generation and GTL which is the conversion of gas to a liquid fuel. (From van der Drift and Boerringter, 2006.)

biomass using an air-blown circulating fluidized bed operating at 900°C, but as the gas formed is not clean the system requires a catalytic reformer to remove many of the contaminants (Fig. 7.3). The gas from the fluidized bed gasifier contains H2, CO, CO2, H2O and considerable amounts of hydrocarbons such as CH4 benzene and tars (Table 7.1). The second option is entrained flow gasification where higher tempera­tures (1300°C) are used. This system requires a supply of very small particles to burn

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correctly so that any material used has to be milled, which is energy-intensive and makes handling difficult.

No matter which method is used to produce the gas, extensive syngas cleaning and conditioning are required before the FT process can be used to produce liquid fuels as the contaminates inhibit the catalyst. The syngas also needs to have a H2/CO ratio of 2:1. The concentration of CO and H2 can be adjusted in the water shift reactor which converts CO to H2 and CO2. The reverse can also be carried out as the syngas com­position varies depending on the feedstock.