19.3.1 Gasifiers

Gasification converts biomass into a gaseous mixture of syngas consisting of hydrogen, carbon monoxide, methane and carbon dioxide. The gasification of biomass is a crucial matter for the application of the BTL process, as BTL-FT technology has not been established mainly due to difficulties in syngas production/ cleaning-up from biomass. Moreover, almost 75% of the investment costs in a B TL plant are in the pre-treatment, gasification and gas cleaning section; therefore, the gasification pressure and medium greatly influence the economy of both gasifier and downstream equipment (Hamelinck et al, 2004). There are many technologies available for syngas production, as presented in Fig. 19.2 (Balat et al., 2009). Biomass gasifiers can be classified as air-blown, oxygen-blown or steam-blown, as atmospheric or pressurized, as slagging or non-slagging, as fixed bed updraft/downdraft, fluidized bed or entrained flow, and as allothermal (indirect heating) or autothermal (direct heating by combustion of part of the feedstock). A detailed description of the biomass gasification technology and the different types of gasifiers is given in Chapter 16 of the present book, which is dedicated to the production of bio-syngas via gasification. Therefore, attention in the present paragraph is paid to the gasification technology suitable for integration in a BTL-FT plant for the production of liquid fuels.

Fixed bed gasifiers have a relatively low throughput and therefore for large-scale applications, as in the case of BTL, with very strict requirements concerning the purity of the syngas, are considered unsuitable (Wang et al, 2008). On the basis of

throughput, complexity, cost and efficiency issues, circulating fluidised bed (CFB) (Hamelinck et al, 2004; Tijmensen et al, 2002; Wang et al., 2008; Zhang, in press) and entrained flow gasifiers (van der Drift et al., 2004) are very suitable for large — scale syngas production. Examples of CFB gasifiers employed for the gasification of biomass that have reached a certain degree of commercialization are the Lurgi CFB process, the Foster Wheeler gasifier, the VVBGC gasifier constructed under the EU-funded project Chrisgas, the UCG (Ultra Clean Gas) programmed by VTT, etc. (Higman and van der Burgt, 2008). Slagging entrained flow gasifier manufacturers are Shell, Texaco, Krupp-Uhde, Future-Energy (formerly Babcock Borsig Power and Noell), E-gas (formerly Destec and Dow), MHI (Mitsubishi Heavy Industries), Hitachi and CHOREN (formerly UET) (van der Drift et al, 2004).

The biomass gasification technology most close to commercialization for syngas production in a BTL-FT plant is the CHOREN Carbo-V patented biomass gasification process (Fig. 19.3). The process is a good example of the application of entrained flow gasifiers in the BTL process and is being used in the first demonstration, that is 15 000 tons per year BTL plant in Freiberg, Germany, coupled with the Shell SMDS FT process (Rudloff, 2005). The CHOREN Carbo-V patented gasification process consists of three stages: low-temperature, high-temperature and endothermic entrained-bed gasification (Rudloff, 2005). During the first stage, the biomass is continuously carbonized through partial oxidation with oxygen at temperatures between 400°C and 500°C, that is, it is broken down to a tar-containing gas (volatile parts) and solid carbon (char). The tar-containing gas is then fed to the high-temperature gasifier, where it is partially oxidized using oxygen as the gasification agent. The heat, which is released as a

result of the oxidation process, warms up the carbonization gas to temperatures that exceed the ash melting point of the fuels that have been used, that is 1300°C-1500°C. At these temperatures, any unwanted longer-chain hydrocarbons, for example tar and even methane, are broken down. The gas that is produced primarily consists of carbon monoxide, hydrogen, carbon dioxide and steam. The char from the low-temperature gasifier is cooled, ground down to pulverized fuel and is then blown into the stream of hot gas coming from the combustion chamber in the entrained flow gasifier. A huge amount of heat is absorbed when gasifying the char, and this allows lowering the temperature of the gas to 800°C-900°C in a matter of seconds. This ‘chemical quenching’ process produces a tar-free gas with a low methane content and high proportions of combustible carbon monoxide and hydrogen.