Determination of ignition temperature and kinetics and thermodynamics analysis of high-volatile coal based on differential derivative thermogravimetry

In order to accurately determine the ignition temperature (Ti) of coal, Coal spontaneous combustion (CSC) process was tested by a simultaneous thermal analyzer. Next, the abrupt change point of the differential derivative thermogravimetric curve (DDTG) of coal after the high adsorption temperature was taken as the Ti of CSC. Besides, variations of kinetics and thermodynamics during the CSC before and after the Ti were calculated. It was found that the mass loss, heat releases and gaseous products of coal changed slowly before the Ti and surged sharply after the Ti. The coal at ignition temperature was in thermodynamic equilibrium with low activity. However, when the temperature was greater than Ti, the aromatic hydrocarbons in coal begin to decompose, resulting in the increased of active sites and the release of volatiles, and the coal enters an irreversible combustion stage. At this time, upward trends were obtained for the apparent activation energy, pre-exponential factor, enthalpy change, and entropy change, while a gradual decrease was observed for the Gibbs free energy change. Moreover, reaction intensity between oxygen and coal would be increased due to increased of active sites in the coal and enhanced of volatiles release under a slower heating rate.