Mechanisms of Long-Term Ultraviolet Radiation on the Low-Temperature Oxidation Characteristics of Coal with Varying Degrees of Metamorphism

To investigate the impact of prolonged ultraviolet (UV) radiation on the low-temperature oxidation characteristics of coals with varying degrees of metamorphism, this study employed temperature-programmed oxidation and in-situ Fourier-transform infrared (In-situ FTIR) spectroscopy. The research focused on brown coal, long-flame coal, and coking coal, examining both untreated samples and those subjected to UV radiation for 3, 6, 9, and 12 months. The results demonstrated that long-term UV radiation consistently promotes low-temperature oxidation in coals, with this effect peaking at 6 months. At this peak, compared to untreated coals, brown coal exhibited increases in CO and CO2 yields of 2.06 and 2.25 times, respectively; long-flame coal showed increases of 1.93 and 2.06 times, respectively; and coking coal demonstrated increases of 1.85 and 2.00 times, respectively. Further analysis revealed that the impact of prolonged UV radiation on coal’s low-temperature oxidation mechanism varies across different coal types and oxidation stages. This variation is reflected in coal-specific and stage-dependent changes in key active functional groups and the groups contributing most to structural changes when comparing untreated and UV-irradiated coals. These findings provide a theoretical foundation for evaluating and mitigating the self-ignition risks of coals subjected to long-term UV radiation exposure.