Study on the Effect and Mechanism of Water Immersion on the Characteristic Temperature during Coal Low-Temperature Oxidation

Spontaneous combustion of dried water-immersed coal (DWIC) wastes many coal resources, and so the effect of water immersion on the characteristic temperature during coal low-temperature oxidation and its possible reasons were investigated. The characteristic temperatures of coal oxidation were obtained by thermogravimetry. The possible causes for the changes in characteristic temperatures were analyzed from the pore structure and microscopic function groups by means of liquid nitrogen adsorption/desorption experiment and in-situ Fourier transform infrared spectroscopy. The results show that, in the early stages of coal low-temperature oxidation, the characteristic temperatures of DWIC samples were lower compared to those of raw coal samples. The differences between the characteristic temperatures of the DWIC and raw coal first increased but then decreased as the temperature rose. When the coal’s temperature rose to the maximum weight gain temperature, the differences had almost disappeared. Water-immersion reduced the coal’s specific surface area but increased the average diameter of its pores and the total pore volume. These changes in the coal pores were beneficial for oxygen migration and transport. During the whole low-temperature oxidation stage, the DWIC hydroxyl concentration is higher compared to that of raw coal, and this made the initial reaction of coal and oxygen more likely to occur. In the late low-temperature oxidation stages, the methyl concentration had a considerable influence on the coal–oxygen reaction.