MICROSTRUCTURE CHANGE OF COAL SEAMS UNDER LOW TEMPERATURE: IMPLICATIONS FOR UNDERGROUND COAL GASIFICATION

Underground coal gasification (UCG) is prospectively an economical and environmentally sustainable technology, which can in principle significantly contribute to tackle the world's energy demand while mitigating pollution and global warming. The UCG process is mainly divided into three areas, the temperature from high to low is: oxidation zone above 1200 °C, reduction zone at 600-1200 °C, and dry distillation zone at 200-600 °C. There are many previous studies that have proved the changes of coal in the oxidation zone and the reduction zone, including thermal expansion, dehydration, pyrolysis and gasification. However, the coal seam changes in the dry distillation zone are rarely studied. Dry distillation zone is the most important way for the gas production process, studying the changes of coal cleats in the dry distillation zone can better optimize UCG. Therefore, Collie coal in Collie Basin, Australia was selected as the research object and this work investigated how coal cleat changes during heating through scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that changes in cleats porosity, equivalent diameter and aperture width can be divided into three times, based on the heating temperature, which was observed that coal porosity decreased at 80 °C (caused by thermal coal expansion), increased at 150 °C (due to volatile matter release), and then decreased slightly at 200 °C. The variation of cleat porosity, equivalent diameter and aperture width related to water evaporation and coal matrix expansion was revealed, furthermore this study also pointed out that at a temperature of 150 °C, the coal seam cleat volume is the largest, which is the best gas recovery point. The research results can provide effective guidance for UCG process design and optimization.