Investigating the Coupling Mechanism Between Coal Quality Evolution and Surface Morphology During Low Temperature Oxidation of Coals with Different Metamorphic Grades

Low temperature oxidation is the critical process triggering coal spontaneous combustion. This study systematically investigates the multiscale evolution of lignite, bituminous coal, and anthracite during low temperature oxidation (30–150°C) via programmed heating oxidation, proximate/ultimate analyses, and scanning electron microscopy. Results demonstrate that as coal rank increases, moisture and volatile content decrease alongside a transition from porous to dense surface morphologies, resulting in significantly lower indicator gas (CO, CO2, C2H4, and C2H6) release. After oxidation from 30°C to 150°C, lignite loses 13.07% moisture and 4.05% volatile matter, and its CO concentration reaches 3454.9 ppm at 150°C, far exceeding the values of bituminous coal (2133.73 ppm) and anthracite (443.63 ppm). During oxidation, moisture evaporation and devolatilization drive a progressive morphological shift from micropore initiation to structural collapse, with lignite exhibiting the most pronounced mass loss. Crucially, we elucidate a dynamic coupling mechanism: the escape of volatiles and oxidation products promotes pore interconnection and cracking, while the relative enrichment of ash and fixed carbon partially mitigates structural disintegration. This interaction forms a positive feedback loop consisting of physicochemical evolution, pore development and enhanced gas generation, which controls the oxygen diffusion efficiency. By bridging the gap between macroscopic coal-quality degradation and microscopic structural transformation from a multiscale perspective, this work clarifies the intrinsic mechanisms governing low temperature oxidation and provides a robust theoretical foundation for the early detection and prevention of coal spontaneous combustion hazards.