Figure 7.3 gives an overall view of the methods that can be used to produce biofuels from coal and biomass, and Table 7.2 gives the maximum concentration of impurities that syngas should have in order to be suitable for FT synthesis. Too high a concen­tration of impurities will poison the cobalt catalyst in the process.

The exothermic FT synthesis combines H2 and CO when passed over a cobalt catalyst at a temperature of around 260°C producing a mixture of hydrocarbons including petrol (C8—Cn) with an average of C8H18 and diesel (Cn—C21) with an overall hydrocarbon average of C16H34.

2H2 + CO = CH2 + H2O (7.3)

The FT synthesis unit operations are given in Fig. 7.4 when using dried biomass. The dried biomass is gasified in an entrained flow gasifier at 900-1300°C in the presence

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of steam and oxygen. In some cases, the biomass may be pretreated by pyrolysis or torrefaction (Fig. 7.3) or even taken from a fluidized bed gasifier. The ash is removed and the gas is cleaned of sulfur-containing compounds, and then the CO/H2 ratio is adjusted by the water shift reaction. The cleaned gas is then passed over a cobalt cata­lyst in the FT reactor producing a range of hydrocarbons from CH4 to waxes. The alpha factor shown in Fig. 7.5 describes the proportion of the various products formed, and this is affected by the catalyst used and process conditions. Maximum diesel production is around 30% of the total products at an alpha value of 0.85-0.9. The lower-temperature conditions which favour diesel production are 260°C, with cobalt-based catalyst at a pressure of 15-40 bar.

The process of gasification, gas cleaning and FT synthesis is a complex chemical process where the larger the scale, the more economic the process (Fig. 7.6). As the size increases, the conversion costs reduce, levelling out at around 1800 MWth (mega­watts thermal) while the other costs remain static.

Thus, the production plant, using biomass to produce syngas and FT products, will be much larger compared to other biomass processes because of the increased efficiency

Fig. 7.5. The effect on the products formed in the Fischer-Tropsch process of the alpha factor, the probability of chain growth.

Fig. 7.6. The effect of scale on the economics of the Fischer-Tropsch process. (Redrawn from van der Drift and Boerrigter, 2006.)

of the FT process at the larger scales. The fossil fuel-based FT plants are huge, above 1000 MWth. With biomass, there may be a problem in supplying such a large process without extensive transport of biomass from distant sources. This may mean that any biomass-based plant is likely to be smaller at 100 MWth. However, there are ways to treat biomass to reduce its volume, so that it can be transported easily to the large central FT plant. The first is torrefaction, where biomass is heated at 250-300°C, which turns it into a brittle, solid mass that can be treated like coal. The second option is pyrolysis at 500°C that converts the biomass into oil/char slurry (Fig. 7.3).

At present syngas is mainly used by the chemical industry (Fig. 7.2), but some 8% (500 PJ per year) is used to produce fuels called GTL. FT processes are operated by Sasol in South Africa, and Shell in Bintulu, Malaysia. These are large plants of 1000 MWth due to the economies of scale and in one case use natural gas (CH4).

To supply the EU-25, ten large plants of 1000 MWth would be required. At present, small — to medium-scale gasification systems of biomass are used for distrib­uted heat and power (CHP) production. The larger scale of the GTL production also allows for the possibility for CO2 capture and storage.