Experimental study on damage law of liquid CO2 cyclic freeze–thaw coal

In order to study the damage characteristics of coal under liquid CO2 (LCO2) cyclic freeze-thaw (F-T), ultrasonic detector and infrared thermal imaging technology were used to discuss the ultrasonic wave propagation of coal and the surface temperature evolution law of coal samples during LCO2 cyclic F-T. The nuclear magnetic resonance technology was used to study the pore structure evolution of coal body. The results show that the velocity of primary wave and secondary wave is reduced. The more cycles, the lower the minimum temperature of coal side and end face. The temperature difference at different points on the same measuring line is large, indicates that the heterogeneity of coal sample under the same temperature causes anisotropy of heat transfer inside the matrix. The pore structure changes greatly under the LCO2 cyclic F–T action. With the increase of the number of cycles, the proportion of adsorption pores decreased, the proportion of seepage pores increased, and the pore volume increased，the total porosity and effective porosity showed an exponential upward trend, the bound porosity showed an exponential downward trend, indicating that LCO2 cyclic F–T is conducive to the increase of crack volume and the improvement of crack connectivity.