Physicochemical Mechanism and Staged Fire Prevention Strategy of Coal Spontaneous Combustion with Different Metamorphic Degrees: Based on Molecular Dynamics and Microscopic Experimental Characterization

Coal spontaneous combustion (CSC) seriously restricts mine safety production and effective environmental protection. Its physical and chemical disaster-causing mechanism is complex and the reaction stage is changeable, which significantly limits effective prevention and control. Combined with SEM-EDS, DSC, FTIR, and molecular dynamics simulation, physical and chemical mechanism and gas generation law of coal in low temperature oxidation stage are analyzed. By comparing reaction differences of coal with different metamorphic degrees in the heating, reaction law of low-temperature oxidation of coal is further clarified, and the staged fire prevention and extinguishing strategy is proposed accordingly. The results show that flocculent sediments appear on the surface of coal at the initial stage. As the temperature increases, the pores and cracks of coal further expand. Among them, oxidation reaction of HM with low metamorphic degree is the most intense, and the surface O/C increased from 0.108 to 0.304, while the C content decreased from 89.22% to 75.15%. In the initial stage of oxidation, the reaction is dominated by -COOH and -CH2/CH3, and the decarboxylation reaction is the key sequence of chain initiation. The simulation results show that CO2 and CO are mainly derived from -COOH and -CHO, while -OH and -CH2/CH3 indirectly promote gas formation by forming peroxy radical intermediates. Based on the physical and chemical mechanisms, fire prevention and extinguishing in the early stage should focus on -COOH oxidation resistance and moisture retention; in the later stage, it should focus on free radical scavenging and physical oxygen barrier plugging.