Coal–water interactions have a prominent impact on the prediction of coal mine gas disasters and coalbed methane extraction. The change of characteristics in the microscopic pores of coal caused by the existence of water is an important factor affecting the diffusion and migration of gas in coal. The low-pressure nitrogen adsorption experiments and gas desorption experiments of a low-rank coal with different equilibrium moisture contents were conducted. The results show that both the specific surface area and pore volume decrease significantly as the moisture content increases, and the micropores (pore diameter <10 nm) are most affected by the water adsorbed by coal. In particular, for a water-equilibrated coal sample at 98% relative humidity, micropores with pore sizes smaller than 4 nm as determined by the density functional theory model almost disappear, probably due to the blocking effects of water clusters and capillary water. In this case, micropores with a diameter less than 10 nm still contribute most of the specific surface area for gas adsorption in coal. Furthermore, the fractal dimensions at relative pressures of 0–0.5 (D 1) and 0.5–1 (D 2) calculated by the Frenkel–Halsey–Hill model indicate that when the moisture content is less than 4.74%, D 1 decreases rapidly, whereas D 2 shows a slight reduction as the moisture content increased. In contrast, when the moisture content exceeds 4.74%, further increases in the moisture content cause D 2 to decrease significantly, while there is nearly no change for D 1. The correlation analyses show that the ultimate desorption volume and initial desorption rate are closely related to the fractal dimension D 1, while the desorption constant (K t) mainly depends on the fractal dimension D 2. Therefore, the gas desorption performances of coal have a close association with the pore properties of coal under water-containing conditions, which indicate that the fluctuation in moisture content should be carefully considered in the evaluation of gas diffusion and migration performances of in situ coal seams.

Geo-temperature is a key geological parameter for coalbed methane (CBM) recovery. Especially for deep CBM, the impact of temperature on methane adsorption of coal should receive more emphasis. In this study, the methane adsorption performances of three anthracite coal samples from Shanxi Province, China, under different temperature and gas pressure conditions (20–60 °C, 1–10 MPa) are simulated based on the grand canonical Monte Carlo (GCMC) method. The results show that the Dubinin-Astakhov (DA) model has a higher applicability for characterizing CH4 adsorption than the Langmuir model and Dubinin–Radushkevich (DR) model, which can be attributed to the heterogeneous micropore surface of coal with great specific surface area, and the unfixed value of the structural heterogeneity parameter s. Further analysis indicates that the CH4 adsorption capacities of the studied coals are reduced by approximately 3.3 cm3/g for each 10 °C increase in temperature. This finding is mainly related to the great kinetic energy of gas molecules and the easier transformation of the adsorbed gas into free gas under high temperature conditions. Moreover, the net heat of CH4 adsorption in coal increases with temperature, because of the enhanced thermal movement of gas molecules. Furthermore, a new equation for the adsorbed gas amount of coal that considers temperature effect is established, where both the DA adsorption model and a negative power-type equation n = 0.011P –0.39 are introduced. Compared to the classical equation, the newly established model provides a better predication of the adsorbed gas amount of anthracite. This indirect calculation method is a good supplement to the field tests of the CBM content.