STUDIES ON MODELING OF COUNTERCURRENT COAL GASIFICATION REACTORS: ITERATIVE SOLUTION METHODS

A general steady state model for a countercurrent moving-bed coal gasifier has been developed. The model accounts for reactions within the gas and solid streams, for mass and heat transfer between the streams, and for heat loss and feed injection at the wall. Two iterative solution methods, namely a variant of Picard's method and what can be called the forward-backward (F-B) method, were used to solve the multidimensional two-point-boundary-value problem (TPBVP) which corresponds to the equations of the model. Only the F-B method provided satisfactory results. Using the F-B method, in many cases relatively rapid convergence was obtained to solutions that satisfied overall energy and element balances on the reactor. In cases where the combustion zone was displaced from the bottom of the reactor, however, convergence was often slow, indicating that the steady state solution was not well-determined.; A countercurrent reactor model has been used to simulate the steady state profiles in a moving-bed slagging gasifier which gasifies a briquetted mixture of crushed coal and cellulosic waste using oxygen-steam blast as the gasifying agent.; A 2:1 ratio of coal to cellulosic waste (in the briquetted mixture) has been used for the present simulation study. Satisfactory agreement was obtained between the simulated and the experimental results. Steam to oxygen ratio close to 1.3 and a blast temperature around 475 K was observed to be the desirable operating condition for the slagging gasifier.; A lumped parameter model has been set up to study the dynamics of a countercurrent reactor, especially to investigate the nature of poorly determined steady states observed while simulating the steady state of a dry bottom countercurrent moving-bed coal gasification reactor.; A simple reaction has been used to simulate steady states similar to the coal gasification reactor. Modal analysis has been used to study the steady states using the linearized equations, around the steady state, describing the dynamics of the reactor system. Two slowly decaying modes have been identified which influence the dynamics of the (unforced) reactor. The second slowest mode, called here the drift mode, has been found to be responsible for the drifting high temperature zone (wandering combustion zone in dry bottom coal gasifier), and also has been associated with the poorly determined steady states.