Study of the microstructure and oxidation characteristics of residual coal in deep mines

Residual coal is prone to spontaneous combustion under the influence of high ground temperature, and the risk increases with mining depth. This results in gradual increases in smoke and dust emissions, which seriously affect the ecological and geological environment of the mining area. To avoid spontaneous combustion of the residual coal, protect the resources and ecology of mining areas, and investigate the influence of the deep thermal environment on the microstructure and oxidation properties of coal, the pore evolution characteristics and functional group migration of coal in a thermal environment were studied by N2 adsorption and infrared spectroscopy. A temperature-programmed test system was used to simulate the low-temperature oxidation process occurring in deep coal seams, and the heat release characteristics and oxidation kinetic parameters of the index gas were calculated. The experimental results revealed that the pore oxygen storage capacities and coal oxygen contact areas of residual coal in deep mines are larger than those of coal in shallow seams. As the preheating temperature is increased, the rates of CO and CO2 production and the amount of oxygen consumed in oxidation of heat-treated coal increase. The crossing point temperature (CPT) of heat-treated coal decreases. The deep thermal environment accelerates the rate of condensation decomposition of the aliphatic hydrocarbon groups in coal and fracture of side chain hydrogen bonds connecting strong electron-absorbing groups, which increases the exothermic intensity of the oxidation process. The reaction trigger temperatures for aliphatic hydrocarbon groups, peroxyl radicals, associative hydrogen bonds and other active groups in coal are different. With increasing ambient temperature, the types of active groups in coal increase, and the contents accumulate. With an increase in the deep geothermal temperature, the oxidation activity of coal is enhanced, and the coal is more prone to spontaneous combustion. These research results provide a theoretical reference for deep coal clean mining and prevention of spontaneous combustion disasters and have a potential guiding role in environmental protection and ecological improvement of deep mining areas.