Desorption Effects and Laws of Multiscale Gas-Bearing Coal with Different Degrees of Metamorphism

Understanding gas desorption effects and laws of coal mass under different conditions is essential for the effective exploration of gas emission in underground coal mines, prediction and prevention of coal and gas outburst, accurate detection of gas [coal methane (CBM)] content in coal seams, and prediction of CBM productivity. Using a self-developed test platform, we simulated gas adsorption and desorption and performed physical simulation tests. Based on these tests, we investigated the differences in the total amount of gas desorbed, desorption rate, and initial amount of gas desorbed by long-flame coal, coking coal, meager-lean coal, and anthracite on different scales under different gas pressures. Two methods are used for compensating gas loss, namely, the method and the power function method, as stipulated in the current Standards for Determination of Gas Content in Coal Seams in China. By combining these two methods, we analyzed the applicability of these two compensation methods in coal on different scales with varying degrees of metamorphism under gas pressures. The results demonstrated that (1) under the same gas adsorption pressure, the cumulative total amount of gas desorbed per unit mass within 90 min for the four kinds of coal samples increases with the degree of metamorphism. Changes in the cumulative amount of gas desorbed per unit mass and the desorption rate with the degree of metamorphism vary with stages. Notably, a higher adsorption pressure leads to a more obvious stage change. (2) Under the same gas adsorption pressure, the cumulative total amount of gas desorbed per unit mass and the desorption rate of coal with the same degree of metamorphism are inversely proportional to the size of the coal sample. This indicates significant scale effects. The larger the degree of metamorphism and gas adsorption pressure, the more significant are the scale effects of gas desorption. (3) For coal with the same degree of metamorphism, the higher gas adsorption pressure leads to a larger cumulative total amount of gas desorbed and a higher desorption rate throughout the desorption process and a larger proportion of the cumulative amount of gas desorbed in the initial stage. The smaller the size of the coal sample, the more obvious the pressure effects of gas desorption are. (4) For coal samples with the same degree of metamorphism, when the gas content in coal seams is kept constant, the larger the size of the coal sample, the smaller the actual gas loss is. Moreover, a higher gas content in coal seams results in a greater gas loss and a larger calculation error for gas loss. Compared with the method, the power function method reveals a smaller deviation between the calculated gas loss and the actual gas loss, which is found to be more accurate. A larger size coal sample results in higher accuracy in the calculated gas loss.