Influence and inerting mechanism of inert gas atmospheres on the characteristics of oxidative spontaneous combustion in coal

To study the influence and inerting mechanism of inert gas atmospheres on the characteristics of oxidative spontaneous combustion in coal, a temperature-programmed experiment was conducted to quantitatively characterize the inhibition efficiency of N2, CO2, and mixed inert gas on the coal spontaneous combustion process. Quantum chemical methods were applied to disclose the weak interaction mechanism of CO2 and N2 with the active groups in SY bituminous coal. The results showed that during low-temperature oxidation of coal, the presence of both N2 and CO2 reduced the release of CO, increased the crossing point temperature of coal, and inhibited the oxidative coal spontaneous combustion process. The inerting effect of CO2 was always more significant than that of N2 on coal spontaneous combustion, and the mixed inert gas had an inerting effect between that of CO2 and that of N2. The differences in inerting of coal spontaneous combustion between CO2 and N2 were obvious in the low-temperature region, while differences were less notable in the high-temperature region. Moreover, the inerting effect of the mixed inert gas was consistent with that of CO2 on coal spontaneous combustion. In the absence of oxygen, the gas atmosphere had little influence on coal pyrolysis. However, the inerting effect of CO2 and N2 on coal oxidation gradually became noticeably different as the O2 concentration increased. Therefore, the selection of an inert gas atmosphere for fire prevention should be based on a comprehensive evaluation of the field temperature, O2 concentration, and type of inert gas. Both CO2 and N2 interacted with the six active groups of coal via VDW interactions. The interaction intensity, mutual penetration distance, and interaction energy produced by the interaction of CO2 with these active groups were always larger than those of N2. When O2 attacked dimers formed by these active groups and CO2, it was repulsed by oxygen atoms on both sides of the CO2 molecule and attracted to the central carbon atom of the CO2 molecule. This could prevent O2 from combining with the active groups in coal and effectively inhibit coal spontaneous combustion. These research results will lay a theoretical foundation for inert gas in underground coal mine fire prevention applications.