Changes in activation energy with level of burn-off for bituminous coal chars

The kinetics of oxidation of coal char has been extensively studied during the last decade. A common goal of these studies has been to evaluate reactivity of char under diffusion-free conditions and to identify physico-chemical properties of a char which influence its reactivity. Thermogravimetric analysis has been widely used to obtain rate data for the reaction char oxidation. Isothermal studies have demonstrated that the oxidation rate changes with conversion and exhibits a maximum at a specific level of burn-off. Data have been analyzed to obtain the intrinsic reactivity and activation energy for char oxidation. In most of the previous studies, however, the activation energy has been calculated at a specific conversion, usually at 50% conversion or at a level corresponding to the maximum rate. Since the value of activation energy has a pronounced effect on the intrinsic reaction rate, it seems appropriate to investigate whether activation energy changes with conversion during oxidation of coal char. In this paper the results of a study on the oxidation of three bituminous coal chars are presented. The focus is on the variation of activation energy with level of burn-off for the three chars.