Initial Characteristics of Gas Desorption in High-Rank Soft and Hard Coals and the Causes of the Differences

To study the initial characteristics of gas desorption in high-rank soft and hard coals, a purpose-built constant-temperature gas desorption device was used to conduct gas desorption experiments on three groups of collected high-rank soft and hard coal samples. The results show that the gas desorption index (<i>K</i> ₁) of drill cuttings from soft and hard coals follows a power function relationship with the adsorption equilibrium pressure. Under the same adsorption equilibrium pressure, the gas desorption index (<i>K</i> ₁) of drill cuttings from soft coal is 1.39-2.66 times that of hard coal. When the adsorption equilibrium pressure is 1 MPa, in the first 1 min, the gas desorption rate of soft coal is 1.38-2.56 times that of hard coal, and in the first 10 min, the gas desorption rate of soft coal is 1.35-2.49 times that of hard coal. The initial law of gas desorption in soft and hard coals is suitable for classifying Sun Zhongxu's form of the governing equation. To reveal the causes of the differences in the initial characteristics of high-rank soft and hard coal gas desorption from a microscopic perspective, scanning electron microscopy (SEM) was used to observe the surface pore structure of soft and hard coals. Additionally, mercury intrusion porosimetry (MIP) and low-temperature N₂ adsorption/desorption (LTN₂GA) experiments were undertaken to ascertain the full pore size distribution of soft and hard coal samples. The results show that (1) the breccia pores on the surface of soft coal are more developed, and the pore connectivity is better, which provides favorable conditions for the rapid desorption of gas at the initial stage. Furthermore, the crushed grain pores and ink bottle pores on the surface of soft coal are more developed, providing a place for the adsorption of more gas; (2) the total pore capacity of soft coal is 4.12-10.57 times that of hard coal, and the proportion of large and medium pore capacity in soft coal is 4.47-10.70 times that of hard coal, which provides a favorable channel for the rapid desorption of gas in the initial stage. The ratio of medium and micropore specific surface area of soft coal is 2.26-2.77 times that of hard coal, offering more positions for gas adsorption. The results offer a reference for those seeking to understand the differences in gas desorption in the early stage of high-rank soft and hard coals and provide technical support for improving the accuracy of determining coalbed gas content and gas desorption index (<i>K</i> ₁) of drill cuttings.