Study 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 change 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#) is the smallest. As the temperature rises, 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 are not completely positive effects, and 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, and the methane desorption hysteresis rate in the low pressure stage (P 4MPa) is large, and the methane desorption hysteresis rate in the high-pressure stage (P>4MPa) is lower; During the isobaric adsorption process, the adsorption capacity of anthracite (SH3#) increases the fastest, followed by SGZ11#, and DFS4# is the smallest. In the low-pressure stage (P 4MPa), the adsorption capacity increases significantly with the increase of pressure, but in the high pressure stage (P 4MPa), the adsorption capacity does not change significantly with pressure, but 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<4MPa), when the pressure continues to decrease, the free path of methane molecules increases significantly, resulting in a decrease in the diffusion capacity. In the high-pressure stage (4<P<8MPa), when the pressure continues to decrease, the free path of methane molecules does not change significantly; DFS4#, SGZ11#, SH3# sample desorption process of three sets of samples, 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.

A New Empirical Model to Predict Methane Adsorption Amount of Anthracite Considering Temperature Effect

“…Many scholars believed that there is a critical temperature point (CTP) that makes the maximum adsorption capacity or the adsorption amount at a certain gas pressure show a trend of a rapid decrease at first and then basically unchanged. 8,19 It should be noted that the maximum test temperatures are 70 and 100 °C in the studies of Guan et al 8 and Zheng et al, 19 respectively, which are significantly greater than the maximum temperatures in the studies of this paper and Gao et al 60 The two different results may have a lot to do with the maximum test temperature. The amount of gas adsorbed by coal is related to the two kinds of molecular interactions: the solid/gas interaction and the gas/gas interaction.…”

“…Similarly, Gao et al 60 also found that the saturated methane adsorption capacity of coal has a linear negative correlation with the temperature range of 25−45 °C. However, the weakening extent of temperature increase on the gas adsorption capacity of coal is not invariable.…”

Study on gas content variation and main controlling of low-rank coal in Huanglong Jurassic coalfield

“…When the pressure increases to a certain extent, the increase rate of adsorption amount gradually slows down. The adsorption capacity of coal to methane decreases with the increase of temperature under the same pressure(Chen et al, 2021; Gao et al, 2021; Zheng et al, 2021). Comparing the relationship between DFS and GJH temperature and adsorption capacity, it is found that the adsorption capacity increased by 4–4.7m 3 /t, when the pressure increased from 1 MPa to 4 MPa (Figure 16).…”

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

“…24,25 Through analysis of the change laws of the Langmuir effective diffusion coefficient with the coal rank and their influencing factors, Yan 26 found that a rise in the coal rank increased the development degree of micropores and gas desorption capacity. Wang, 27 Gao, 28 and Lin 29 concluded a higher adsorption pressure of coal mass is related to the stronger movement of methane molecules, more favorable for gas desorption, and accelerates gas diffusion. Additionally, the high pressure increases the diffusion coefficient and the cumulative amount of gas desorption and diffusion.…”