Correlation analysis of the functional groups and exothermic characteristics of bituminous coal molecules during high-temperature oxidation

Coal spontaneous combustion is characterized by high-temperature oxidation and microscopic mechanisms. These are essential aspects to understand when attempting to control or prevent spontaneous combustion. Three fresh bituminous coal samples were collected for experimentation from Huainan, China. X-ray diffraction and Fourier-transform infrared spectrometry were conducted to determine the microscopic characteristics of coal during high-temperature oxidation. The mineral structures and functional groups were ascertained to ensure that the original structural characteristics of the coals were obtained. Thermogravimetry–differential scanning calorimetry was used to divide the high-temperature oxidation into four substages and obtain detailed exothermic characteristics. The heat energy release exhibited a positive correlation with an increase in temperature. Moreover, to investigate the relationship between the functional groups and their exothermic characteristics, correlation analysis was conducted for describing the quantitative phenomena within the four oxidation stages. The results revealed that the oxygen-containing groups were initially the most active and reactive among the 14 types of functional groups. The most rapid stage of spontaneous combustion, in which should be the most cautious and aware of risks, was the oxygen adsorption and mass gain stage because the heat energy output increased rapidly and passed this stage, thus leading to dangerous and possibly unrecoverable situations.