400 1100 1800 2500 3200

Temperature, °F

FIGURE 5-22. Classification and characteristics of the commercial gasification systems. (From Elliot, M. A., Ed., Chemistry of Coal Utilization, Secondary Suppl. Vol., John Wiley & Sons, New York, 1981. With permission.)

Fixed-Bed Gasifier

In a fixed-bed gasifier, 1/4- to 2-in. coal is supplied countercurrent to the gasifying medium. Coal moves slowly down (sometimes this type of gasifier is called a moving-bed gasifier), ideally in plug flow against an ascending stream of gasifying medium. Reaction zones, shown in Figure 5-23, typically consist of drying and devolatilization, reduction, combustion, and ash zones. In the drying and devolatilization zone, located at the top of the gasifier, the entering coal is heated and dried and devolatilization occurs. In the reduction/ gasification zone, the devolatilized coal is gasified by reactions with steam and carbon dioxide. In the combustion zone, oxygen reacts with the remaining char, and this zone is characteristic of high temperatures. The ash is

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