Variation of coal and rock adsorption and specific surface area under stress loading and unloading

Based on the theory of adsorption and desorption of coal and rock, the adsorption test system of coal and rock under stress loading and unloading was developed, CO₂ gas was used as adsorbent to carry out the experiment on the change of coal and rock adsorption properties during the process of stress loading and unloading. In addition, isothermal adsorption tests were carried out at different stress points, and the specific surface area of different stress points was calculated by Langmuir method and BET method respectively. The results show that: The gas adsorption rate of coal is related to the adsorption accumulation times of the coal during loading or unloading, and is independent of the stress. The more cumulative adsorption times, the faster adsorption rate of coal and rock. The stress load obviously affects the adsorption capacity of coal and rock, and the loading will restrain the adsorption capacity of coal and rock. During the whole loading process, the adsorption capacity decreases by 16.2%. The adsorption capacity of CO₂ gas on coal is highly sensitive to the stress in a certain range. Unloading will promote the adsorption capacity of coal and rock. During stress unloading, the adsorption capacity increased by 8.3%, and the initial unloading (5.09−4.08 MPa) contributed 63.8% to the change of adsorption capacity, the initial unloading process plays a decisive role in the enhancement of the adsorption capacity of coal and rock, and the adsorption capacity increases gradually with the decrease of stress after the initial unloading, and the adsorption capacity change curve during the unloading process is always higher than that during the loading process. Loading reduces the specific surface area, unloading increases the specific surface area. Under loading conditions, the specific surface area calculated by Langmuir method and BET method decreased by 22.2% and 21.3% respectively, and increased by 21.2% and 20.5% respectively during unloading. The specific surface area under unloading condition is always larger than that under the same stress level under loading condition.