THE RELEVANCE OF THERMAL ANNEALING TO THE EVOLUTION OF COAL CHAR GASIFICATION REACTIVITY

The present work is focused on the influence of the severity of heat t reatment of coals during pyrolysis and/or early stages of carbon oxida tion on the reactivity of the resulting char towards further gasificat ion. Thermogravimetric analysis has been directed to characterize the reactivity of heat treated samples of a bituminous coal in atmospheres containing different carbon dioxide partial pressures. Carbonization was carried out at different temperatures and for different times prio r to thermogravimetric analysis. Parallel microstructural characteriza tion of the samples was accomplished by XRD analysis of untreated coal and heat treated char. Differences in gasification reactivities of ch ars prepared at a given temperature for various carbonization times ar e observed, larger in the early stage of gasification, levelling off a t larger burn-off. The apparent reaction order with respect to carbon dioxide partial pressure decreases along burn-off for the gasification of equally heat treated samples. Increasing the heat treatment temper ature results in substantial char reactivity reduction, associated wit h major changes of both the turbostratic carbon structure and the mine ralogical constitution of the inorganic matter. Experimental results a re analysed in the light of a phenomenological model of combined annea ling and char gasification. The model is based on a simple reaction sc heme according to which char deactivation due to thermal annealing occ urs in series-parallel to gasification. The two processes take place o ver comparable time scales under the experimental conditions tested. T he ability of the model to fairly reproduce the observed phenomenology suggests that annealing can be one key to the explanation of the evol ution of char gasification rate with burn-off. The discussion on the r elative time scales of annealing versus gasification is further extend ed by examining a body of literature data on annealing of a variety of carbons at different temperatures. Copyright (C) 1997 Elsevier Scienc e Ltd.