Fates and roles of alkali and alkaline earth metallic species in Loy Yang brown coal during in-situ steam gasification

摘要

Mechanistic investigations were made on steam gasification of nascent chars from pyrolysis of raw Loy Yang brown coal and an acid-washed one that was free from alkali and alkaline-earth metallic (AAEM) species. The pyrolysis and in-situ gasification were performed in a novel drop-tube/fixed-bed reactor, in which an extremely thin fixed-bed of nascent char particles was exposed to steam-containing gas flowing through the bed with high linear velocity and flux. The change with time of the conversion, X, of char from the acid-washed coal was described fairly well by a first-order kinetics, dX/dt = k_a (1 -X), over the entire range of X. For the gasification of nascent char from the raw coal, specific rate of the char conversion, defined by r_(sp) = (dX/dt) / (1 -X), decreased considerably with increasing X up to a certain level, and then became constant at r_(sp) approx = k_a. The decrease in r_(sp) was due to gasification-induced loss of catalysis caused by intra-particle deactivation of AAEM species and their volatilization from the char while the intrinsic reactivity of the char, represented by the rate constant, k_a, was maintained. The time-dependent change in X was described quantitatively by assuming the presence of a single carbonaceous component undergoing non-catalytic gasification and AAEM-species-catalyzed gasification in parallel. The catalytic activity of AAEM species at the initial gasification and the rate of catalytic activity loss were influenced considerably by operating variables such as heating rate for the pyrolysis, period of isothermal heating of the char before exposure to steam, total pressure and/or partial pressure of steam, while k_a was a function of only the temperature and the partial pressure of steam.