CH4 adsorption and diffusion characteristics in stress-loaded coal based on molecular simulation. (1st February 2023)

• Record Type: Journal Article
• Authors: Lin, Hai-fei; Long, Hang; Li, Shu-gang; Bai, Yang; Xiao, Tong; Qin, Ao-li
• Is Part Of: Fuel. Volume 333(2023)Part 2
• Publication Date: 2023-02-01
• Subjects: Molecular simulation -- Micro- and meso-porous slit models -- Stress-strain characteristics -- Adsorption characteristics of CH4 -- Diffusion coefficient and type; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2022.126478
• Languages: English
• ISSNs: 0016-2361

Abstract

Highlights: The micropore and mesopore models of bituminous coal were established. The micromechanical properties of coal molecule unit cells with different pore sizes were studied. The adsorption and diffusion characteristics of coal cells under different deformation characteristics were different. Diffusion types of CH4 in coal slits under different stress were clarified. Abstract: The Variety of roof stress during mining was one of the main factors affecting the adsorption and diffusion characteristics of coalbed methane. CH4 adsorption and diffusion characteristics in micro- and meso-pores of coal under different stress loading were simulated in this study by means of molecular mechanics and molecular dynamics. Firstly, the micro- and meso-pores of coal with four different slit widths (10 Å, 20 Å, 50 Å, and 80 Å, respectively) were established. Then, the stress-strain characteristics of the frameworks under different stress loading were studied by applying uniaxial stress. Finally, the CH4 adsorption and diffusion characteristics in different stress-deformed frameworks were investigated. Under the action of stress loading, the mesoporous framework showed a stronger deformation tendency. This process can be divided into the deformation occurring in the coal molecule and the slit. The energy evolution of coal framework under stress presented an exponential decay trend, which resulted in a decreasing trend in the adsorption amount of CH4 in the framework. Under the loading of 10 GPa external stress, the CH4 absolute adsorption amount in the four frameworks decreased by 65.21 %, 65.21 %, 68.07 %, and 70.98 %, and the excess adsorption amount decreased by 54.33 %, 69.07 %, 69.75 %, and 73.85 %, respectively, compared with the initial framework ( σ = 0 GPa). The absolute adsorption amount and self-diffusion coefficient of CH4 both increased with the increasing slit width. In the compression stage of the framework, the heat released by CH4 adsorption showed a decreasing trend, and the self-diffusion coefficient of CH4 showed an increasing trend. However, in the stage of slit compression, the adsorption heat and self-diffusion coefficient tended to increase and decrease, respectively. By analyzing the diffusion types of CH4 in micro- and meso-pores, it was found that, in micropores, transitional and Knudsen diffusion were dominant, while in mesopores, transitional and Fick diffusion were dominant. This study can provide theoretical reference for coal seam gas pre-extraction engineering.