Fractal characteristics of spontaneous combustion of gas-bearing coal under different temperature and adsorption pressure conditions

The composite disaster of gas and coal spontaneous combustion in the goaf is increasingly becoming the main disaster pattern that restricts the safe production of mines. To study the disaster mechanism of the composite disaster of gas and coal spontaneous combustion, the experiments of low-temperature oxidation microstructure and thermal analysis of coal under different CH4 adsorption pressures were carried out in the paper by low-temperature liquid nitrogen adsorption, scanning electron microscopy and TG-FTIR testing methods, and a pore fractal calculation model was set up based on the fractal theory to investigate the influence of adsorbed CH4 on the pore structure and thermal stability of coal. The results showed that the adsorbed CH4 inhibited the thermal damage destruction of micropores by oxidation, thus a large number of micropores existed in gas-bearing coal and the pore morphology changed compared with the raw coal which was mainly medium- and large-porous; with the increase of adsorption pressure, the mass loss of coal samples in the stage of water loss and desorption was 4%, 2.9%, 3.2% and 3.2%, respectively, and CH4 occupied the adsorption sites of the coal oxygen reaction leading to the reduction of compounds involved in the oxidation reaction, and the active substances involved in the oxidation reaction decreased in the stage of oxidative weight gain, and the oxidative weight gain of the coal samples was 0.67%, 0.41% and 0.31%, respectively, and the spontaneous combustion of the gas-bearing coal in all stages of the characteristic temperature points lagged with the increase of adsorption pressure. A fractal model based on pore size distribution and CH4 adsorption/desorption process was established. The morphological inhomogeneity of the coal surface increased during the pre-oxidation period of low CH4 pressure, the gas adsorption capacity of the coal surface was elevated, the possibility of oxidation reaction increased, the risk of coal spontaneous combustion increased, and the ability of low gas to inhibit the spontaneous combustion of coal was weaker, and was correlated with the specific surface area of the primary pores. The research results investigated the influence of different residual gas contents on the spontaneous combustion characteristics of coal, further verified the accuracy of the continuous physical simulation platform for the whole process of adsorption-desorption-oxidation of gas-bearing coal competition under different adsorption pressures and temperatures, and provided basic support for the prevention and control of gas-fire coupling disaster in the composite disaster environment of the goaf.