The Kinetics of Gasification and Reactor Theory
The kinetics of gasification is as yet not as developed as is its thermodynamics. Homogeneous reactions occurring, for example, in the gas phase can often be described by a simple equation, but heterogeneous reactions are intrinsically more complicated. This is certainly the case with the gasification of solid particles such as coal, (pet) coke, or biomass because of their porous structure. The latter complication causes mass transfer phenomena to play an important role in the gasification of solids.
3.1 KINETICS
The kinetics of coal gasification has been and still is a subject of intensive investigation. Despite this, the results of such investigations have to date flowed into the design procedures for commercial gasification reactors to only a limited extent. In contrast to Chapter 2, therefore, the presentation of kinetic theory in this chapter is restricted to the basic ideas and an indication of how and where the appropriate application of kinetic theory could help in the design of future reactors.
A simplified reaction sequence for coal or biomass gasification can be described, as in Figure З-1.
For coke gasification, where the volatiles have already been removed in a separate process step, only the char reactions apply. As is discussed in other parts of this book, counter-current moving-bed gasifiers such as the Lurgi gasifier are an exception to this model, since the oxygen reacts with the coke and the devolatilization takes place using the hot synthesis gas. The volatiles in such a process do not come into contact with free oxygen.
For oil gasification, where the feedstock consists almost entirely of pure volatiles, the pattern is also slightly different.
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Figure 3-1. Reaction Sequence for Gasification of Coal or Biomass (Source: Adapted from Reimert and Schaub 1989)
