Effects of natural weathering and low-temperature oxidation on some aspects of the combustion behaviour of bituminous coals

Abstract

The influence of natural weathering ( in situ) and low-temperature laboratory oxidation on the combustion behaviour of bituminous coals was investigated. Five sets of coals, each comprising a fresh and crop coal from the same seam, were used. One selected fresh coal sample was oxidized in the laboratory at 200 °C for 2 and 72 h to elucidate the effect of low-temperature oxidation (simulated weathering) on the subsequent 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 coals due to the presence of inorganic species (cations) in the former which could be removed by acid washing. Reactivity changes due to such inorganic 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 the difference in combustion efficiency between a crop coal and its fresh 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.