CO2 and steam-gasification in a high-pressure wire-mesh reactor:: the reactivity of Daw Mill coal and combustion reactivity of its chars

Interrelationships between extents of coal gasification, char gasification and combustion reactivities have been examined as a function of CO2 and steam pressure and holding time. Experiments have been carried out in a high-pressure wire-mesh reactor equipped with a steam injection facility. Evidence has been presented linking minima in weight loss vs, reactive gas pressure curves with deactivation as a result of secondary char deposition. At longer times, extents of gasification were about 2-3 times higher in steam compared to CO2. With increasing reactivity of the ambient gas, the minimum in the weight loss vs. pressure curve appears only at the shortest reaction times. At higher pressures, reactivities between zero and 10 s were lower than those between 10 and 20 s. These data are novel and support the suggestion that a relatively unreactive layer of re-polymerised tar tends to slow down the gasification of the main body of the char in the initial stages - until it is itself consumed. Combustion reactivities of pyrolysis and gasification chars decrease with increasing pressure. When exposed to a temperature of 1000 degrees C, combustion reactivities of chars were found drop rapidly within about 10 s to relatively low and stable values. The end values were independent of pressure and composition of the reactive gas. The results are relevant to the design of pilot and commercial scale reactors. On the basis of limited available evidence, it appears that secondary char deposition caused by tar re-polymerisation plays an increasingly significant effect with increasing particle size. (C) 1999 Elsevier Science Ltd. All rights reserved.