Characteristics of heat production and key structure evolution in the initial stage of coal spontaneous combustion

The heat production during the initial stage of coal spontaneous combustion is the root cause of coal spontaneous ignition and serves as the most direct indicator for assessing the risk of coal spontaneous combustion. To determine the oxidation heat produced per unit of oxygen consumed by coal in the initial stage of spontaneous combustion., an innovative method was employed, utilizing a six-port valve combined with a quantitative loop to precisely control the amount of air introduced into the coal sample tank Based on the theory of coal oxidation heat, a calculation model was constructed, enabling the successful determination of the unit oxygen consumption oxidation heat of coal under different latent period conditions. Experimental results revealed that under constant temperature oxidation at 50℃, the oxidation heat generated by the four coal samples, namely LJ, WD, ZZ, and XT, was 66.94 kJ/mol 40.76 kJ/mol, 42.82 kJ/mol, and 23.31 kJ/mol, respectively. This discovery indicates that the oxidation heat produced during the initial stage of coal spontaneous combustion is significantly lower than the commonly used oxidation heat value of 300 kJ/mol in numerical simulations, and that LJ coal, with the lowest degree of metamorphism, produced the highest heat. Four coal samples all reached the maximum heat production rate when the oxidation time proceeded to about 40 s. To analyze the reasons for the differences in heat production under various temperature conditions during the initial stage of coal spontaneous combustion, the pore structure of the coal samples was tested through low-temperature nitrogen adsorption experiments. The results found that the pore structure of coal is not the dominant factor determining the differences in coal oxidation heat release; rather, a more developed pore structure merely effectively promotes the coal-oxygen complex reaction. Electron paramagnetic resonance (EPR) experimental results demonstrate that the oxidation heat production during the initial stage of coal spontaneous combustion is a kinetic behavior controlled by reaction sites. An increase in temperature leads to an increase in the number of primary reaction sites in coal and accelerates the rate of secondary reaction site generation during the coal oxidation process. The essence of these reaction sites is free radicals, and the rate of increase in free radical concentration determines the rate of coal oxidation reactions.