Comparative analysis of thermokinetic behavior and gaseous products between first and second coal spontaneous combustion

To investigate the entire process of first and second coal spontaneous combustion, a 15-t experimental furnace for coal spontaneous combustion and synchronous thermal analyzer coupled with Fourier transform infrared spectroscopy were employed. The process of experimental tests was as follows: First, coal temperature was increased from 31.0 °C (room temperature) to 452.7 °C without intervention. Second, the coal sample was cooled to approximately 90.0 °C in an anaerobic atmosphere. Third, air was supplied until the sample reached 418.0 °C. Finally, the coal was cooled again to 100.0 °C in an anaerobic atmosphere. The variations of temperature, mass, heat energy intensity, and gaseous products were investigated. The results indicated that the temperature change rate first increased and then decreased during the first and second coal spontaneous combustion. Compared with the first coal spontaneous combustion, the rates of temperature and mass loss change with a heating rate of 2.5 °C/min were higher at the combustion stage of the second coal spontaneous combustion, but the variations of the exothermic reaction rate and mass loss with other heating rates (5.0, 10.0, and 15.0 °C/min) were lower. The differences among CO, CO2, alkanes, and alkenes during the first and second coal spontaneous combustions were insignificant until 300.0 °C. However, the emission of CO and CO2 during the second coal spontaneous combustion was significantly higher than during the first coal spontaneous combustion, whereas the release of alkanes and alkenes was substantially weaker when the temperature was higher than 300.0 °C. Moreover, the amount of H2O during the first coal spontaneous combustion was higher than that during the second coal spontaneous combustion.