The IGT gasifier (Figure 2.2) is directly heated, which implies that some char and/or biomass are burned to provide the necessary heat for gasification. Direct heating is also the basic principle applied in pressurised reactors for gasifying coal. The higher reactivity of biomass compared to coal permits the use of air instead of pure oxygen.

Steam + oxygen

This could be fortuitous at modest scales because oxygen is relatively costly (Con — sonni and Larson 1994a). However, for the production of methanol from biomass, the use of air increases the volume of inert (N2) gas that would have to be carried through all the downstream reactors. Therefore, the use of oxygen thus improves the economics of synthesis gas processing. Air-fired, directly heated gasifiers are considered not to be suitable before methanol production.

This gasifier produces a CO2 rich gas. The CH4 fraction could be reformed to hydrogen, or be used in a gas turbine. The H2:CO ratio (1.4:1) is attractive to produce methanol, although the large CO2 content lowers the overall yield of methanol. The pressurized gasification allows a large throughput per reactor volume and diminishes the need for pressurization downstream, so less overall power is needed.

The bed is in a fluidized state by injection of steam and oxygen from below, allowing a high degree of mixing. Near the oxidant entrance is a combustion zone with a higher operation temperature, but gasification reactions take place over the whole bed, and the temperature in the bed is relatively uniform (800-1000 °C). The gas exits essentially at bed temperature. Ash, unreacted char, and particulates are entrained within the product gas and are largely removed using a cyclone.

An important characteristic of the IGT synthesis gas is the relatively large CO2 and CH4 fractions. The high methane content is a result of the nonequilibrium nature of biomass gasification and of pressurized operation. Relatively large amounts of CO2 are produced by the direct heating, high pressure, and the high overall O:C ratio (2:1). With conventional gas processing technology, a large CO2 content would mean that overall yields of fluid fuels would be relatively low. The synthesis gas has an attractive H2:CO ratio for methanol production, which

reduces the need for a shift reactor. Since gasification takes place under pressure, less downstream compression is needed.

When operated with higher steam input the IGT gasifier produces a product gas with a higher hydrogen content. This maximum hydrogen mode is especially useful if hydrogen would be the desired product, but the H2:CO ratio is also better for methanol production. However, the gasifier efficiency is lower and much more steam is needed.