• A critical temperature threshold of 500 °C triggers severe degradation of coal pillar bearing capacity, with strength and elastic modulus reductions up to 45.53% and 61.34%, respectively.
• Above 500 °C, coal undergoes significant pyrolysis under N2 and CO2 atmospheres, leading to decreased oxygen-containing functional groups and increased aromaticity and graphitization.
• High-temperature exposure induces microstructural changes, including dislocation and slip within coal crystal nuclei, resulting in increased porosity and deformation.
• The failure mode of coal transforms from brittle to ductile with rising temperature, impacting the stability of underground coal gasification gasifiers.