Coal gasification process descriptions

A computer model has been developed to provide numerical simulations of fluidized bed coal gasification reactors and to yield detailed descriptions, in space and time, of the coupled chemistry, particle dynamics and gas flows within the reactor vessels. Time histories and spatial distributions of the important process variables are explicitly described by the model. With this simulation one is able to predict the formation and rise of gas bubbles, the transient and quasi-steady temperature and gas composition, and the conversion of carbon throughout the reactor. 

The computer model is based upon a continuum description of fluidization in coal gasification reactors. In general, fluidized flows are dominated by specific physicochemical processes and, hence, require particular theoretical representations. For example, in the heavily loaded gas-particle regime appropriate to fluidization, the solid particles dominate the transport of momentum and energy. This aspect of fluidization is reflected in the mathematical descriptions which have been used in the fluidized bed model. 

In any text about coal (but more specifically in any chapter about coal gasification), it is appropriate to include a listing of the types of processes available as well as a description of the different processes. Thus, it is the intent here to give selected examples of specific processes. This is in direct contrast to the inclusion of an appendix to the chapter on coal liquefaction (Chapters 18 and 19) in fact, the situation is quite different with respect to liquefaction processes. 

As it is nearly impossible to make any general kinetic description of the gasification processes as a practical design basis, constrained equilibrium calculations offer a useful tool for comparative studies [51]. If gasification reaction rates are considered as a rate of approach to chemical equilibrium, increasing residence times lead to gasification products near equilibrium [52]. This is especially true for fluidized-bed and entrained-flow processes. Because the equilibrium state does not depend on the path used to achieve it, a process simulator such as Aspen Plus [49] can use a hypothetical reactor to decompose coal into its elements. The subsequent equilibrium calculation can be carried out, including other feed streams. 

The description given apphes to DR processes that are based on the use of gaseous reductants ia shaft furnaces, batch retorts, and fluidized beds. In the processes that use sohd reductants, eg, coal (qv), the reduction is accomphshed to a minor extent first by volatiles and reduciag gases that are released as the coal is heated and then by CO that is formed by gasification of fixed carbon contained ia the coal char with CO2. Reductioa by sohd carboa and coal volatiles ia kilns is insignificant. 

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