An updated study on CH4 isothermal adsorption and isosteric adsorption heat behaviors of variable rank coals

Abstract

Abstract: Understanding the gas sorption behavior of coals not only benefits for enhancing coalbed methane (CBM) recovery but also provides important insights for simultaneous coal and gas extraction. This study aims to quantitatively evaluate the CH4 adsorption behaviors by a series of experiments and the applicability of different adsorption dynamic models in different rank coals (0.48-2.59% of Ro,m). Moreover, the isosteric heat and ultimate heat of CH4 adsorption in different rank coals were also analyzed. The results indicate that the fitting accuracy of different adsorption models is presented as Dubinin-Astakhov (D-A) model > Langmuir model > Dubinin-Radushkevich (D-R) model > Freundlich model for different rank coal samples, and the maximum adsorption volume reaches the lowest value at the Ro,m of ∼1.4%. This phenomenon indicates that the CH4 adsorption in coals is a micropore filling or multi-layer adsorption process (especially in anthracites at high pressure stage) rather than a simple Langmuir monolayer adsorption process. Moreover, the CH4 adsorption process in coal samples can be divided into three stages: the low relative pressure stage ( 0.30 MPa). As the relative pressure continuously increases, the adsorption mechanism changes from the collision stage between pore surface and CH4 molecules to the intermediate stage of monomolecular adsorption and the multilayer adsorption stage. Through calculating the isosteric heat and ultimate heat of CH4 adsorption, the adsorption heat generally increases with the adsorption capacity increasing, and shows a positive correlation with the micropore surface area. The effects of micropore structure and surface area on the adsorption heat should be crucial in coals. These results may be significant for understanding the process of CH4 adsorption.