Effect of Thermal Memory in Acoustic Emission in Fossil Coal after Pre-Disintegration by Cryogenic Treatment

Acoustic emission response of fossil coals being at different stages of metamorphism to cyclic variation of effective thermal stresses is experimentally investigated. The equipment and procedure used in the experiments are described. The features of the response are revealed and analyzed in the samples of anthracite, lignite and bituminous coal with different damage extent governed by the preliminary cyclic freezing and thawing, as well as by water saturation. It is shown that the signature of such features is a thermal analog of the Felicity effect which appears in each cycle of temperature action. The regularities of this effect are found, and their physical explanation is given based on the analysis of defect formation in coals at different stages of thermal treatment. The methodical approaches are proposed and substantiated, which allow structural damage, thermal resistance, oxidation and proneness to frost weathering of coal to be estimated by the Felicity effect in the acoustic emission response of coal to cyclic thermal forces. Possibility of using the found features to predict structural changes in coal products which are in long-term storage under specific climatic conditions, as well as for forecasting risk of self-heating and spontaneous combustion of coal products is discussed.