Research progress of spontaneous combustion of coal containing gas under the compound disaster environment in the goaf

The compound disasters of gas and coal spontaneous combustion have become the main disaster mode that restricts safe mine production. As the “ignition source” of gas combustion and gas explosion accidents in goaf, coal spontaneous combustion is undoubtedly the key influence factor for preventing compound disasters. At present, there is still insufficient understanding on the influence characteristics and mechanism of both the methane gas (free state) of the airflow filed and the residual methane gas (mainly adsorbed state) of the fractured coals on the coal spontaneous combustion. Therefore, the latest research progress in the field of the spontaneous combustion under gas-containing conditions was explored. It was found that the rich research results have been obtained on the characteristics of coal spontaneous combustion under the gas-containing airflow/environment. However, there is a lack of experimental platforms and the research results for simulating the impacts of the adsorbed gas on the coal spontaneous combustion. It is pointed out that the free gas can leads to a significant “hysteresis effect” in the generation on the oxidation gas products of coal, the weakening of the heat release intensity of coal oxidation and the overall increase in the activation energy, all of which reflect the inhibitory effect of the methane in the free state on coal spontaneous combustion. Correspondingly, the microscopic characteristic parameters of functional groups, free radicals, and coal microcrystalline structure exhibit the regular change with methane concentration during coal spontaneous combustion process. Furthermore, considering the difference in the occurrence state of the residual methane inside the fractured coal and the free methane in the goaf environment, the influence mechanisms of methane on the coal spontaneous combustion are investigated including the displacement and dilution effects of methane, the effect of competition adsorption/desorption between methane and oxygen, as well as the chemical adsorption of oxygen molecules and the coal-oxygen reaction effects under the high temperature environment. At last, it is proposed that the research urgently needs to breakthrough the bottleneck issues of the evolution characteristics of coal spontaneous combustion under the impact of adsorbed gas, the multi-filed characteristics of gas-containing coal spontaneous combustion, and the monitoring and warning of gas-containing coal spontaneous combustion.