The Thermodynamics. of Gasification
In Chapter 1 we defined gasification as the production of gases with a useable heating value from carbonaceous fuels. The range of potential fuels, from coal and oil to biomass and wastes, would appear to make the task of presenting a theory valid for all these feeds relatively complex.
Nonetheless, the predominant phenomena of pyrolysis or devolatilization and gasification of the remaining char are similar for the full range of feedstocks. In developing gasification theory it is therefore allowable to concentrate on the “simple” case of gasification of pure carbon, as most authors do, and discuss the influence of specific feed characteristics separately. In this work we will be adopting this approach, which can also be used for the partial oxidation of gases such as natural gas.
In the discussion of the theoretical background to any chemical process, it is necessary to examine both the thermodynamics (i. e., the state to which the process will move under specific conditions of pressure and temperature, given sufficient time) and the kinetics (i. e., what route will it take and how fast will it get there).
The gasification process takes place at temperatures in the range of 800°C to 1800°C. The exact temperature depends on the characteristics of the feedstock, in particular the softening and melting temperatures of the ash as is explained in more detail in Chapter 5. However, over the whole temperature range described above, the reaction rates are sufficiently high that modeling on the basis of the thermodynamic equilibrium of the main gaseous components and carbon (which we will assume for the present to be graphite) gives results that are close enough to reality that they form the basis of most commercial reactor designs. This applies unconditionally for all entrained slagging gasifiers and may also be applied to most fluid-bed gasifiers and even to moving-bed gasifiers, provided the latter use coke as a feedstock.
One exception to the above assumption that one can model with thermodynamic equilibria alone is the moving-bed gasifier where coal is used as a feedstock and where the blast (oxygen and steam) moves counter-currently to the coal as in, for example, the Lurgi gasifier that is described in further detail in Section 5.1. In such gasifiers pyrolysis reactions are prevalent in the colder upper part of the reactor, and therefore a simple description of the process by assuming thermodynamic equilibrium is not allowable for that region of the reactor. The reactions in the hot bottom section in which coke reacts with the blast can, however, be described well by thermodynamic equilibrium. A second exception is the gasification of biomass at temperatures of about 850°C (Kersten 2002).
In this discussion of the theory of gasification the emphasis will be limited to gasifiers that operate at temperatures of 850°C and higher. Below 850°C is, on the one hand, the realm of pyrolysis reactions that are extremely complex to model, whereas, on the other hand, the partial oxidation reactions proceed at so slow a rate that they become of little practical value.
