Thermodynamic Characteristics of Methane Adsorption and Desorption on Varied Rank Coals: A Systematic Study

Coalbed methane (CBM) primarily exists in the adsorbed state within coalbeds. Understanding adsorption/desorption of coal to CH4 is highly important for estimating CBM reserves, predicting productivity, and enhancing CBM extraction. In this work, the properties of coal, with R-o,R-m = 0.50-2.54%, adsorbing/desorbing CH4, were quantitatively evaluated. Moreover, the variation characteristics were analyzed utilizing the theories of isosteric heat of adsorption, adsorption potential, and surface free energy. The results show that the D-A and Weibull functions have better fitting accuracy, indicating that there is multilayer adsorption or micropore filling; while considering the physical significance of the formula parameters, the Langmuir and desorption equations are more practical, and as the R-o,R-m increases, the V (L )of samples to CH4 presents a "U-shape" change. The isosteric heat of adsorption is proportional to the adsorption amount, but the change rates vary due to the different coverage of CH4, showing that the low- and medium-metamorphic-degree coals change faster. The adsorption potential exhibits a negative correlation with the adsorption volume, and the adsorption potential of high-metamorphic coal is higher, owing to the higher proportion of micropores. As the adsorption progresses, the cumulative decrease in surface free energy of samples rises, but the rate of change gradually decreases. The adsorption heat and potential of the desorption are higher than those of the adsorption process, and the variation of surface free energy is lower; also, the differences become greater with more CH4 desorption, revealing the mechanism of the adsorption hysteresis.