Heat treatment-induced loss of combustion reactivity of a coal char: the effect of exposure to oxygen

A South African bituminous coal was heat treated at different temperatures in the range 500-1300 degreesC under inert conditions with or without exposure to pulses of air. The effect of the heat treatment conditions on the reactivity of the resulting char towards oxygen under chemical kinetic controlled regime was assessed. The oxygen content of chars subjected to different heat treatment conditions was also measured by elemental analysis. Results indicated that exposure to pulses of air during the early stages of heat treatment at temperatures up to 1200 degreesC mitigates the effects of thermal annealing as far as the loss of combustion reactivity of the resulting char is concerned. Beyond about 1200 degreesC, exposure to air is ineffective to mitigate the effects of thermal annealing on combustion reactivity. Altogether, it appears that mutual interactions exist between modifications of the carbon structure induced by thermal annealing, uptake of oxygen during heat treatment and the oxy-reactivity of the resulting char. The preliminary findings of this work would suggest that the loss of carbon combustion reactivity cannot be assessed on the basis of its time-temperature history only, but modes and extent of carbon interaction with oxygen come into play. A speculative explanation of the phenomenology is presented and discussed. (C) 2004 Elsevier Inc. All rights reserved.