Various gasifier technologies have been developed over many decades and tailored to suit specific needs. These processes operate at pressures from atmospheric to >20 bars and at temperatures between ~700-1500°C. According to the way the feedstock is brought in contact with the gasifying agent, the gasifier is classified into the following types.

16.4.1 Fixed bed gasifier

Fixed bed gasifiers, which consist of a fixed bed of biomass through which the oxidation medium flows in updraft or downdraft configuration, are simple and reliable designs and can be used to gasify wet biomass economically on a small scale for CHP applications (Wang et al., 2008). However, they produce syngas with large quantities of tar or/and char, due to the low and non-uniform heat and mass transfer between the solid biomass and the gasifying agent
(Wang et al., 2008). The product gas must be extensively cleaned before use. Moreover, the throughput for this type of gasifier is relatively low and, therefore, for large-scale applications, as in the case of biomass to liquid (BTL), with very strict requirements concerning the purity of the syngas, fixed bed gasifiers are considered unsuitable.

There are three types of fixed bed gasifiers, as shown in Fig. 16.5: updraft (counter-current), downdraft (co-current), and cross draft gasifier. In case of the updraft gasifier, the fuel is supplied at the top and the air at the bottom so that fuel moves against the air flow, while in the case of the downdraft, air is introduced above the oxidation zone and the product gas is removed from the bottom. In the case of the cross draft, feedstock moves downward while the product gas leaves in a sideways direction. Figure 16.6 shows temperature distribution along the height of the gasifier (see Table 16.2).

In the case of the updraft gasifier, the tar content of the product gas is high, thus it cannot be used directly for the engine applications. As in an updraft gasifier, the pyrolysis zone lies above the combustion zone; the tar formed does not pass through the combustion zone, thus resulting in higher tar content in the product

Moving-bed gasifier (dry ash)

16.6 Temperature distribution along the height of the gasifier (Higman and Burgt, 2003).

gas. This is the opposite of the downdraft gasifiers, where all the pyrolyzed product passes through the oxidation zone, thus product gas has lower tar content. As in the case of the updraft gasifier, the hot gases pass upward so their energy is available to vaporize the moisture. Due to this property, updraft gasifiers can gasify relatively higher moisture content fuel than downdraft gasifiers. Constriction in the oxidation zone of the downdraft gasifier, however, makes its design more complicated and difficult to scale up. Cross draft gasifiers are used mainly for charcoal gasification. However, during the process, the temperature could reach 1500°C, which could lead to material problems (Stassen and Knoef, 2001).

Figure 16.7 shows different designs of downdraft gasifiers. The Imbert type has a narrow constriction near the oxidization zone for efficient combustion of the fuel. Conversely, the stratified type does not have any narrow constriction, making it easier to design and scale up. Another design is the multi-stage downdraft gasifier, which was developed and tested at the Asian Institute of Technology biomass research laboratory. In this type, air is supplied at two stages. Similarly, in the case of the two-stage gasifier, the biomass is first pyrolyzed in a separate zone and then the tar formed is combusted in another gasification zone to supply the heat required for gasification. This type of gasifier can produce product gas with a tar content well below 50 mg/Nm3. In the case of the vortex gasifier, the air is supplied so as to create a vortex that causes the volatile pyrolysis product to move up and in the presence of air become combusted. Although gasification and pyrolysis in the vortex gasifier take place in a single reactor, the tar content in a product gas is similar to that of the multi-stage gasifier (Fock and Thompson, 2001). The multi-fuel downdraft gasifier commercialized in China can be operated with wood, corncobs, hard nut shells, sawdust, and hard coal (see Table 16.3).

Source: Chopra and Jain, 2007.