Pore Morphology and Oxidation Behaviour of Deep-Loaded Granular Coal at High Initial Temperatures

As coal mining proceeds deeper, fragmented coal is subjected to high geostress and elevated thermal environments, causing changes in its micro-physical properties and spontaneous combustion (SC) risks. Deep mining operations entail significant risks of coal fires. In order to investigate the effects of high stress and high thermal environments on the microstructural and physicochemical damage of fragmented coal, and to elucidate the mechanisms influencing oxidation behavior, we conducted experiments including low-temperature nitrogen adsorption, synchronous thermal analysis, and in-situ diffuse reflectance spectroscopy. These experiments aimed to explore the pore morphology and oxidation characteristics of high initial temperature unloading granular coal (HITU-GC) unloading conditions. The results indicate that with increasing initial temperature, characteristic point temperatures initially rise and then decrease, while both the mass loss and heat release during the low-temperature oxidation stage decrease. After pressure-unloading composite treatment, the oxidation process accelerates with more intense reactions, resulting in an overall increase in heat release. The pore shapes of HITU-GC show no significant difference compared to the original coal, mainly comprising non-rigid aggregates of micropores, platy or layered matrix particles, including numerous slit-shaped pores. The coal surface also exhibits ink bottle-shaped pores and cone-shaped pores. Higher pre-oxidation temperatures correlate with higher specific surface areas. Higher stress levels from pressurized unloading correlate with lower specific surface areas. The specific surface area of sample O60-P16 is reduced by 0.879 m2/g compared to sample O60-P0. Pressurized unloading treatment disrupts mesoporous pores. Following thermal environment and pressure-unloading, the coal's aromatic hydrocarbon content decreases, while the contents of hydroxyl, aliphatic hydrocarbons, and oxygen-containing functional groups (FG) all increase. Sample O60-P4 shows the highest content of active FG. While thermal environments enhance coal oxidation activity, pressure-unloading treatment increases the extent of coal surface fragmentation. The synergistic effect of both significantly increases the SC risk of coal. This study provides a theoretical basis for controlling thermal hazards associated with high-initial-temperature unloading fragmented coal in deep goaf areas.