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 treatment of coals during pyrolysis and/or early stages of carbon oxidation on the reactivity of the resulting char towards further gasification. 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 prior to thermogravimetric analysis. Parallel microstructural characterization of the samples was accomplished by XRD analysis of untreated coal and heat treated char. Differences in gasification reactivities of chars prepared at a given temperature for various carbonization times are observed, larger in the early stage of gasification, levelling off at 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 temperature results in substantial char reactivity reduction, associated with major changes of both the turbostratic carbon structure and the mineralogical constitution of the inorganic matter. Experimental results are analysed in the light of a phenomenological model of combined annealing and char gasification. The model is based on a simple reaction scheme according to which char deactivation due to thermal annealing occurs in series-parallel to gasification. The two processes take place over comparable time scales under the experimental conditions tested. The ability of the model to fairly reproduce the observed phenomenology suggests that annealing can be one key to the explanation of the evolution of char gasification rate with burn-off. The discussion on the relative time scales of annealing versus gasification is further extended by examining a body of literature data on annealing of a variety of carbons at different temperatures. Copyright (C) 1997 Elsevier Science Ltd