Effects of cyclic freeze-thawing of coals on their ability to oxidize

This paper presents the results of experimental studies of the effect of a relatively small number of freeze-thaw cycles (FTC) of coal on their oxidative reactivity at different temperatures. For this purpose, the methods of thermal analysis of coal in air atmosphere were used, such as thermogravimetric analysis (in the temperature range 30–900 °C) and isothermal calorimetry at 40 °C, as well as optical microscopy of the coal metallographic specimens before and after the FTC. It has been established that the ability of lignite to oxidation at high temperatures and at 40 °C remains almost unchanged after FTC. Under conditions of a limited number of freeze-thaw cycles, the studied middle-rank hard coals exhibit a higher ability to change of the organic matter structure as compared with the lignite. This is manifested in an increase in the thermal resistivity of these coals during oxidative thermal destruction under the elevated temperatures (350–900 °C), which results in a decrease in the maximum thermaldecomposition rate by more than 1.5 times as compared to the original coal and the appearance of an additional interval of thermal oxidative destruction in the range of 750–900 °C. Another indicator is an increase in the oxidative stability of the studied middle-rank hard coals under conditions of lowtemperature oxidation (40 °C), which is reflected in a decrease in the intensity of heat release after a single FTC and the absence of significant changes after two- and threefold cyclic low-temperature effects. At the same time, for these hard coals, after a fourfold cycle of freezing-thawing, an increase in the intensity of oxidation is observed in comparison with the initial ones. It has been found that the above effects are presumably associated with the formation of an oxides film on the surface of the studied coals under the influence of cyclic low-temperature effects. The increase in the number of FTC effects on middle-rank hard coals allows for a partial destruction of such films, which leads to an intensification of oxidation processes.The study has been supported by the Russian Foundation for B asic Research (project No. 18–05–70002).