Spontaneous Combustion Characteristics of High−Sulfur Coal Subjected to Different Water Immersion Durations

To clarify the effect of water immersion duration on the spontaneous combustion behavior of high−sulfur coal, coal samples with a sulfur content greater than 3% were immersed for 15, 30, and 45 d. Mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), Fourier−transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) were used to characterize the pore−fracture structure, surface micromorphology, functional−group distribution, and thermal response of the samples. The results show that, with increasing immersion duration, the pore−fracture system gradually evolved from local opening to enhanced connectivity, while the coal surface became rougher and more porous. The 45 d sample exhibited the most pronounced pore−fracture openness. FTIR analysis indicated staged changes in oxygen−containing functional groups after immersion, with the strongest hydroxyl (−OH) response occurring in the 45 d sample. TGA results showed that the main reaction stage of the immersed samples shifted toward a higher temperature region; the 30 d sample showed relatively prominent mass−loss and heat−release intensities, whereas the 45 d sample exhibited more evident pore−fracture openness, functional−group activation, and a stronger tendency for heat accumulation. Overall, prolonged water immersion strengthened coal–oxygen contact conditions and self−heating sensitivity in high−sulfur coal, and the 45 d sample showed the highest potential spontaneous combustion propensity.