Simulation of the influences of different minerals contained in acidified coal on CH4 adsorption

Abstract

To reveal the microscopic mechanism of CH4 adsorption by the main minerals in acidified coal, the depth of acid ion intrusion, the amount of CH4 adsorbed using isothermal adsorption experiments, and heat of adsorption were simulated by Grand Canonical Monte Carlo (GCMC) method for the composite molecular model composed of kaolinite, calcite, dolomite, and quartz with coal, respectively. The results show that the depth of acid ion intrusion directly affects the amount of methane adsorption. At pressures of 0–8 MPa, the F− intrusion depth directly affected the CH4 adsorption of the composite model of coal–kaolinite (quartz); the depths of Cl− and CH3COO− intrusion directly affected the methane adsorption of the composite model of coal–dolomite; the mixed acid containing Cl− and CH3COO− had the greatest effect on promoting methane adsorption. When acidifying the coal–calcite (quartz) compound structure, Cl− and CH3COO− increased the depth of F− intrusion, and the increase caused by CH3COO− was greater than that of Cl−. After acidification of the coal–calcite composite, the relationship between methane adsorption and heat of adsorption gradually changed from negative to positive with the increase of the hydrofluoric acid (HF) concentration; after acidification of composites of coal and other minerals, CH4 adsorption and heat of adsorption were positively correlated. The amount of CH4 adsorption after coal–kaolinite and coal–quartz acidification exceeded that of coal–calcite and coal–dolomite, mainly because the acidification of dolomite and calcite would generate CO2 to compete with CH4 for adsorption.