Multiscale thermal behavioral characterization of spontaneous combustion of pre-oxidized coal with different air exposure time

To comprehensively investigate the multi-scale characteristics of coal spontaneous combustion, in situ infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were utilized. The samples were pre-oxidized at 70 °C for 2, 4, 6, and 8 h (the raw coal and above samples were named R0, R2, R4, R6, and R8), then the secondary oxidation experiments were carried out. The results showed that the contents of –OH, –C=O, and –C=C- increased with the pre-oxidation time. The aromatic structure of pre-oxidized coal is more susceptible to oxygen erosion during secondary oxidation. The migration and transformation of –OH, –C=O are the underlying causes of water evaporation and gas desorption during low-temperature oxidation. The spontaneous combustion behaviors in stage I and stage II have different reaction mechanisms and kinetic characteristics. In light of Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS), the apparent activation energy of stage I is R0>R2>R4>R6>R8, and stage II is R6>R8>R2>R4>R0. Long pre-oxidation time in stage I will inhibit coal spontaneous combustion, while the opposite is true in stage II. By the Pearson correlation analysis, –OH is the key structure affecting the low-temperature endothermic stage of coal, revealing the correlation between the multi-scale thermal behavior of pre-oxidized coal spontaneous combustion.