Structural Evolution during Key Stages of Coal Spontaneous Combustion and Its Impact on Oxidation Characteristics

The temperature gradients within the spontaneous combustion zones lead to the coexistence of coal samples at different oxidation stages. To elucidate the critical role of each spontaneous combustion stage in subsequent oxidative reactivity, this study systematically characterized the oxidation properties, functional group evolution, and pore structure development of preoxidized coal samples and raw coal using a combination of thermogravimetric analysis (TG-DTG), Fourier transform infrared spectroscopy (FTIR), and N₂ adsorption analysis. The main results are as follows. The sample in the dynamic equilibrium stage (Dshun1) exhibited macroscopically "passivated" coal bodies (slightly elevated <i>T</i> ₁), yet its microstructural thermal stability began to decline (<i>T</i> ₂ reduced to 103 °C). The coal sample in the oxygen uptake and weight gain stage (Dshun2) completed the formation and accumulation of active spontaneous combustion centers during preoxidation (<i>S</i> _BET: 0.22 m²/g), lowering the reaction energy barrier (<i>T</i> ₁ significantly decreased to 60 °C) and extending the dynamic equilibrium stage (60 °C→294 °C). These properties significantly enhanced the sample's spontaneous combustion tendency and concealment. For samples in the early thermal decomposition stage (Dshun3), a remarkable thermal decomposition weight loss rate of 31.6% indicated the secondary oxidation in advance. Samples in the mid-to-late thermal decomposition and combustion stages exhibit "ignition source" characteristics. Based on the physicochemical structural evolution patterns and oxidative property differences of coal samples during critical spontaneous combustion stages, this study provides theoretical support for developing differentiated spontaneous combustion prevention strategies.