Geothermal-Induced Spontaneous Combustion of Deep-Mined Coals A Systematic Investigation of Functional Group Reaction Networks and Stage-Resolved Kinetics Across Metamorphic Grades

Deep coal mining operations are subject to elevated geothermal gradients that fundamentally alter coal oxidation behavior, yet the coupling mechanisms between the ground temperature and coal metamorphic grade remain poorly understood. In this study, we establish a comprehensive framework integrating multiscale structural characterization, functional group reaction network analysis, and stage-resolved kinetics to elucidate geothermal-induced spontaneous combustion mechanisms. Four coals spanning a wide range of metamorphic grades (Ro,max = 0.58%-1.12%) were subjected to simulated geothermal conditions (30, 40, and 50 °C) and characterized by using TG-DTG and in situ FTIR techniques. We propose a novel "Geothermal Activation-Oxidation Acceleration" (GAOA) mechanism wherein ground temperature pretreatment activates oxygen-containing functional groups, creating reactive sites that substantially lower oxidation barriers. A functional group reaction network model was developed, revealing a hierarchical reactivity sequence: -OH > CO > C-O-C > aliphatic C-H > aromatic CC. Multimethod kinetic analysis (Coats-Redfern, FWO, KAS, and Starink) demonstrated that activation energy decreased by 15.3%-28.7% under 50 °C pretreatment, with lower-rank coals exhibiting greater sensitivity (ΔEa = 28.7 kJ/mol for long-flame coal vs 18.2 kJ/mol for coking coal). Based on these findings, we developed a Spontaneous Combustion Risk Index incorporating metamorphic grade, geothermal gradient, and functional group reactivity, providing a quantitative tool for fire hazard assessment in deep mining operations. This work advances the mechanistic understanding of geothermal effects on Coal Spontaneous Combustion and offers practical guidance for risk management in China's increasingly deeper mines.