New insight into CH4 adsorption mechanism in coal based on modeling analysis of different adsorption theories

Abstract

Based on the principle of Gibbs excess adsorption measurement, high-pressure isothermal adsorption and desorption experiments of CH4 were performed on fractured coal samples, and CH4 adsorption phase density was obtained by the intercept method according to the experimental data. The Langmuir model, BET model, DA model, and their corrected model, the supercritical adsorption models were used for the fitting analysis of CH4 excess adsorption capacity. A hybrid model of different adsorption theories was developed based on CH4 adsorption phase density and theoretical adsorption capacity predicted by the intercept method. The transition in the occurrence pattern and control mechanism of CH4 in coal were investigated. The fitting of the Langmuir model, BET model, DA model, and their corrected models, the SBET model, the SDA model, and the SDA-L model to the CH4 excess adsorption capacity in coal shows that all multilayer adsorption theory model is not suitable for fitting the CH4 excess adsorption capacity in coal. The uncorrected Langmuir model and DA model are only suitable for fitting the CH4 excess adsorption capacity in coal in the low-pressure stage (pressure below 10 MPa). The corrected Langmuir model, the corrected DA model, the SDA model, and the SDA-L model can be used to fit the CH4 excess adsorption capacity in coal, but they show some bias in predicting the theoretical CH4 adsorption capacity and the CH4 adsorption phase density. The fitting of the three main adsorption theory models suggests that there may be simultaneous monolayer adsorption and microporous filling adsorption. The fitting results of the hybrid model based on monolayer adsorption and microporous filling adsorption theory depend on showing that with the increase of temperature, the occurrence of CH4 in coal gradually changes from mainly monolayer adsorption to mainly microporous filling adsorption, and the transformation of occurrence mode is related to temperature and the development of pores in coal.