Behavior of Inherent Metallic Species as a Crucial Factor for Kinetics of Steam Gasification of Char from Coal Pyrolysis

Abstract

Four pairs of raw and acid-washed brown coals were subjected to pyrolysis and in-situ steam gasification at 900 °C in fixed-bed reactors with or without forced gas flow through the bed of char particles. The chars from the acid-washed coals were gasified obeying a first-order kinetics with respect to the fraction of unconverted char regardless of the presence/absence of forced gas flow. Its presence/absence considerably influenced the gasification kinetics of the chars from the raw coals, even altering the manner of change in the rate of gasification with time and the char conversion, such as the apparent order of reaction. It was found that alkali and alkaline earth metallic (AAEM) species were allowed to stay in/on the char in the absence of the forced gas flow but volatilized extensively in its presence. Thus, the catalysis of AAEM species diminished in the presence of forced gas flow much quicker than in its absence, in which the catalysis was lost exclusively by intraparticle deactivation of AAEM species. The char conversion was quantitatively described as a function of time by a kinetic model that considered the progress of catalytic and noncatalytic gasification in parallel and also the loss of catalysis along with the progress of gasification.