Effects of cyclic freezing and thawing of coals at their behavior at low- and high-temperature oxidation

Abstract

Effects of cyclic freezing and thawing (FTC) on coals properties, such as technological, structural and sorption, has recently attracted wide interest. This article is dedicated to studying of FTC on behavior of coals of different types at low- and high-temperature oxidation. Scanning electron microscopy revealed that coals particles morphology after FTC alters similarly for coals of different types. These changes relate to formation of partially oxidized coal matter films. Thermogravimetric studies allowed to observe that, after FTC, the combustion interval for bituminous coals was widened by additional high-temperature stage (750–900 °C). The rate of combustion and its activation energy were decreased significantly. As for lignite, FTC does not lead to any pronounced alterations of the combustion pattern. Isothermal calorimetry (at 40 °C) allowed revealing that FTC at the lignite led to increase of heat generation during low-temperature oxidation. But no significant changes were observed for bituminous coals. The obtained results on changes of coals structure and properties after FTC are similar to given in the literature regarding the behavior of coals after mild oxidation. This allows to presume that FTC of the studied coals led to their mild oxidation. The interrelation was found between the rate of deactivation of the active sites of the first type and the combustion rate constant. It was shown that this relation determines the decrease of the combustion rate and its activation energy for coals after FTC. Active sites of the second type presumably determine the intensity of the processes of coals low temperature oxidation.