Evolution and mechanism of oxidation characteristics of coal with different degrees of thermal damage in coalfield fire zones based on multi-method joint characterization

Coalfield fires prevail in major coal‑producing countries worldwide, causing resource waste and environmental pollution. Thermal damage is a critical factor affecting the evolution of fire zones. However, the evolution of coal oxidation characteristics under different degrees of thermal damage remains unclear. This study systematically investigates the differences in oxidation characteristic temperatures, gaseous products, and activation energy of thermally damaged coal. It further investigates the evolution of functional groups and interprets oxidation pathways of key active groups, revealing the evolution mechanism governing coal oxidation under varying thermal damage conditions. The results show that thermal damage at 100 ℃ lowers characteristic temperatures, accelerates gas release, and reduces oxidation activation energy. Thermal damage at 200 ℃ suppresses low-temperature oxidation reactivity while imposing negligible effects on high-temperature behaviour. Severe thermal damage at 400 ℃ and 600 ℃ weakens coal spontaneous combustion propensity but increases its high-temperature oxidation susceptibility. As thermal damage temperature rises, the evolution of coal functional groups is retarded, with lower reaction rates and improved thermal stability. Aliphatic C–H and C=O are identified as the primary active groups for oxidation. Quantum chemical simulations indicate thermal damage shortens aliphatic side chains, raising energy barriers and endothermicity for oxidation of small molecular coal models. The enhanced spontaneous combustion propensity of 100 ℃-treated coal originates from water evaporation and newly exposed active sites, whereas the loss of highly reactive chemical structures accounts for the weakened propensity under high-temperature thermal damage. These outcomes offer theoretical support for coalfield fire‑zone prevention and mitigation.