Long-term smoldering of underground coal fires is a major source of land subsidence, ecological degradation, and persistent greenhouse gas emissions, with its evolutionary process exhibiting typical disaster chain characteristics. However, the internal linkage between microstructural evolution during coal smoldering and re-ignition risk remains poorly understood. To address this gap, this study comprehensively employed thermal analysis and high-temperature in-situ Fourier transform infrared spectroscopy (FTIR) to investigate the entire smoldering process of coal under oxygen concentrations ranging from 21% to 1%. The results indicate that as oxygen concentration decreases, the coal smoldering process undergoes a transition from surface oxidation-dominated ignition to a mechanism dominated by volatile release participation near approximately 3% O₂. High-temperature in-situ FTIR reveals that the evolution of aliphatic hydrocarbons, oxygen-containing functional groups, and aromatic structures can effectively characterize the structural reconstruction of coal. Aliphatic hydrocarbons govern the exothermic capacity at the early combustion stage, while aromatic structures and oxygen-containing functional groups dominate combustion sustainability and burnout. Further comparison shows that higher coal rank significantly delays the evolution of functional groups. The re-ignition risk assessment of smoldering coal residues indicates that oxygen-limited conditions lead to the retention of more volatile components in the residues, endowing them with higher re-ignition potential, which serves as a critical risk source for the sustained spread and disaster chain expansion of underground coal fires. The findings establish a novel approach for hazard identification and risk assessment under oxygen-lean smoldering conditions, providing theoretical support for precise monitoring, early warning of disaster risks, and resilient prevention and control of underground coal fires.

Coal fire,Smoldering,Functional group evolution,Re-ignition risk,Disaster prevention and control