Study for the Effect of Temperature on Methane Desorption Based on Thermodynamics and Kinetics.

Abstract

Desorption hysteresis is important for primary gas production. Temperature may cause serious changes in the methane adsorption/desorption behaviors. In order to study the mechanism of methane desorption and desorption hysteresis, three sets of samples of long-flame coal, coking coal, and anthracite were collected, and experiments such as microscopic composition determination, liquid nitrogen adsorption, and isothermal adsorption/desorption were performed. From the perspectives of desorption kinetics, desorption thermodynamics, and methane occurrence state, the differences in methane and methane desorption characteristics and the desorption hysteresis mechanism are discussed. The results show that at the same temperature, anthracite (SH3#) has the largest saturated adsorption capacity and residual adsorption capacity, followed by coking coal (SGZ11#), and long-flame coal (DFS4#) has the smallest. As the temperature increases, the theoretical desorption rate and residual adsorption capacity of anthracite (SH3#) and coking coal (SGZ11#) will increase first and then decrease. Temperature and methane desorption do have positive effects, but temperature may have a threshold for promoting methane desorption. It is necessary to comprehensively consider the influence of temperature on the activation of gas molecules and the pore structure of coal. Under the premise of a certain temperature, as the pressure increases, the desorption hysteresis rate changes in a logarithmic downward trend, the methane desorption hysteresis rate in the low-pressure stage (P < 4 MPa) is large, and the methane desorption hysteresis rate in the high-pressure stage (P > 4 MPa) is lower; during the isobaric adsorption process, the adsorption capacity of anthracite (SH3#) increases the fastest, followed by SGZ11#, and that of DFS4# is the smallest. In the low-pressure stage (P < 4 MPa), the adsorption capacity increases significantly with the increase in pressure, but in the high-pressure stage (P > 4 MPa), the adsorption capacity does not change significantly with pressure, instead gradually stabilizes. Under the same pressure, the molecular free path of methane increases with temperature. Under the premise of constant temperature, in the low-pressure stage (0 < P < 4 MPa), when the pressure continues to decrease, the free path of methane molecules increases significantly, resulting in a decrease in diffusion capacity. In the high-pressure stage (4 < P < 8 MPa), when the pressure continues to decrease, the free path of methane molecules does not change significantly; the sample desorption process of three sets of samples DFS4#, SGZ11#, and SH3# occurs, and the intermediate adsorption heat is greater than the isometric adsorption heat during the adsorption process, indicating that the desorption process needs to continuously absorb heat from outside the system. The energy difference produced in the process of adsorption and desorption causes the desorption hysteresis effect. The greater the difference in the isometric heat value of adsorption, the more significant the hysteresis is.