Evolution of the spontaneous combustion characteristics of thermally decomposed coal and its underlying mechanism

Thermal decomposition is ubiquitous in underground coal mining, but its influence on spontaneous combustion characteristics (SCC) remains controversial, and the underlying mechanism has yet to be fully elucidated. This study investigated the SCC and underlying evolutionary mechanism of thermally decomposed coal (TDC) through low-temperature oxidation, TG-DSC, kinetic analysis, FTIR and ESR experiments. The results show that the TDC presented lower crossing point temperature and produced more CO and CO 2 during the initial stage of low-temperature oxidation, while in the later stage it is the opposite. The TG-DSC analysis indicates that the initial mass loss of TDC is greater, and the characteristic temperature is lower. The kinetic analysis shows that the activation energy of TDC in the initial stage is 33.18 kJ·mol −1 , which is lower than raw coal. But it rapidly increases to 231.97 kJ·mol −1 and remains higher than raw coal. The ESR experiments indicated that the concentration of TDC radicals after thermal decomposition at 200℃ reaches 3.27 times than raw coal. Combined with the FTIR analysis, it further indicated that the thermal effect causes the unstable functional groups decomposed and produced active sites, resulting in enhanced SCC in the early stage; while the premature decomposition of the functional groups leads to the weakened SCC in the later stage. Therefore, TDC exhibited enhanced SCC at the early stage of oxidation, but weakened later on. These findings provide possible causes and prevention strategies for re-ignition of fire area after its unsealing.