Recognition of adsorption phase density, adsorption model, and adsorption mechanism of CH₄ in coal

The determination of CH₄ adsorption phase density in coal is crucial for accurately measuring methane adsorption capacity in coal, evaluating the potential of coalbed methane resources, and understanding the control mechanisms and model characterization of methane adsorption in coal. High-pressure isothermal adsorption-desorption experiments were conducted at different temperatures on some fractured coal samples (R_o,max about 0.7%) from the Huainan-Huaibei coalfield. The pore structure of coal samples were quantitatively characterized by mercury injection and low-temperature liquid nitrogen adsorption. The high-pressure adsorption-desorption characteristics of methane in coal were analyzed, and the relationship between CH₄ adsorption phase density, adsorption phase volume, theoretical adsorption capacity, and temperature was analyzed based on the Gibbs excess adsorption measurement principle and the intercept method. The Langmuir model based on the monolayer adsorption theory, the BET model based on multimolecular layer adsorption theory, and the DA model based on the microporous filling theory and its optimized model were used for fitting and analysis. A mixed model of different adsorption theories was established based on the predicted CH₄ adsorption phase density and theoretical adsorption capacity obtained from the intercept method. The occurrence and control mechanism of methane in coal were preliminarily explored. The results show that the CH₄ excess adsorption reaches the peak and decreases at the inflection point in both the adsorption and desorption stages. The equilibrium pressure corresponding to the peak value of the excess adsorption capacity is related to the experimental temperature and the pore structure of the coal sample. The CH₄ adsorption phase density decreases with increasing temperature and the two are in a power function relationship. The CH₄ adsorption phase density is controlled by the temperature and pore structure of the coal, with higher the temperature and developed pores leading to, lower CH₄ adsorption phase density. The fitting of the Langmuir model, BET model, DA model, and their calibration models for CH₄ excess adsorption in coal shows that the uncalibrated adsorption theory models are only suitable for fitting the excess adsorption capacity of methane in coal at the pressures below 10 MPa. For the entire pressure stage, the uncalibrated adsorption theory model and the calibrated BET model are not suitable. However, the Langmuir model and the DA model calibrated based on CH₄ adsorption phase density optimization have certain deviations when fitting the CH₄ excess adsorption capacity in coal, but they could still be used for fitting. The mixed model based on the monolayer adsorption and microporous filling adsorption theory shows that as the temperature increases, the occurrence form of CH₄ in coal gradually changes from monolayer adsorption to mainly microporous filling, and the transformation of the occurrence form is related to the temperature and pore development in coal.