Experimental Investigation on Coal Deterioration Deformation Mechanism and Energy Evolution Law after High Temperature Pre-damage

Deep coals are generally in a high geothermal environment, and when UCG (underground coal gasification) is carried out, the long-term high temperature effect will cause different degrees of thermal stress damage to coals and rocks, which will change the original stress balance inside coals and rocks, and bring potential threats to safe and efficient mining. For this reason, taking the deep coal with impact tendency as the research object, the experiments on uniaxial compression and AE (acoustic emission) characteristics of the deep coal with impact tendency after high temperature are carried out, and the internal relationship between the overall deformation characteristics and AE characteristics of coal samples are described. The interaction mechanism of energy accumulation, dissipation and release in different stages of coal and its correlation with temperature are analyzed. The test results show that the higher the temperature, the greater the thermal stress damage, and the more obvious the deterioration of compressive strength of coal samples. When in a high temperature environment (≥ 300 °C), the coal samples have almost no compaction stage, the plastic deformation increases rapidly, and the failure mode transitions from brittle failure to “tough-brittle” failure. The AE energy reaches the maximum at 100 °C, and decreases linearly with the increase of temperature; the elastic modulus of coal is greatly affected by temperature, at 400 °C, compared with intact coal samples, the decrease rate is 62.17%; the peak strain of coal samples decreases continuously with the increase of temperature, but the average strain rate increases continuously; the energy accumulation and release are closely related to temperature, with the increase of temperature, the energy storage capacity of coal samples decreases gradually, and the percentage of dissipated energy increases significantly, and the kinetic energy of coal samples is small when they are destroyed, and the destruction speed is relatively slow; the energy conversion of coal samples can be divided into the initial energy accumulation stage I, the accelerated energy accumulation stage II and the rapid energy dissipated stage III; the percentage of elastic energy in the three stages before the destruction of coal samples went through the process of low–high-low, while the dissipated energy is the opposite, and the dissipated energy increased significantly before the destruction, indicating that the coal samples entered the accelerated destruction stage.