EFFECTS OF NATURAL WEATHERING AND LOW-TEMPERATURE OXIDATION ON SOME ASPECTS OF THE COMBUSTION BEHAVIOR OF BITUMINOUS COALS

The influence of natural weathering (in situ) and low-temperature labo ratory oxidation on the combustion behaviour of bituminous coals was i nvestigated. Five sets of coals, each comprising a fresh and crop coal from the same seam, were used. One selected fresh coal sample was oxi dized in the laboratory at 200-degrees-C for 2 and 72 h to elucidate t he effect of low-temperature oxidation (simulated weathering) on the s ubsequent combustion behaviour. The combustion behaviour of the coals was evaluated using char reactivity and burning profiles obtained in a thermogravimetric analyser (TGA) and combustion efficiency determined in a drop-tube reactor (DTR). The reactivities of the chars produced from the crop and laboratory-oxidized coals were higher than those for the chars from the corresponding fresh coals. The reactivities of the crop coal chars were higher than those of the laboratory-oxidized coa ls due to the presence of inorganic species (cations) in the former wh ich could be removed by acid washing. Reactivity changes due to such i norganic species changes did not occur upon laboratory oxidation. The combustion efficiencies of the weathered coals were higher than those of the corresponding fresh coals. A correlation was obtained between t he difference in combustion efficiency between a crop coal and its fre sh counterpart and oxygen uptake during natural weathering. There was an inverse relationship between the combustion efficiencies determined in the DTR and the 'initial temperature' in the TGA burning profiles.