Differences in competitive adsorption mechanisms of CH4 and CO2 in mylonitic and primary structure coal

Abstract

Tectonically deformed coal exhibits significantly different gas adsorption-desorption characteristics from primary structure coal due to intense coal mass deformation, making it a research hotspot in coalbed methane development and gas disaster prevention and control. To explore the differences in the competitive adsorption mechanisms of CH 4 and CO 2 between mylonitic coal and primary structure coal, this study took No. 8 coal from the Zhuxianzhuang Mining Area as the research object. Based on experimental results, molecular models of mylonitic coal (C 214 H 159 N 3 O 20 ) and primary structure coal (C 208 H 152 N 2 O 18) were constructed respectively. Isothermal adsorption simulations of single-component and mixed gases of CH 4 andCO 2 at different ratios were carried out using molecular dynamics methods. The results show that the adsorption of CH 4 and CO 2 in both types of coal belongs to physical adsorption, with stronger adsorption capacity under low temperature and high pressure. Under the same conditions, the adsorption capacity and adsorption heat of CO 2 are higher than those of CH 4 , and the adsorption advantage of CO 2 in mylonitic coal is more prominent. This is closely related to the more complete aromatic ring conjugated π-electron system, abundant oxygen-containing functional groups and a higher proportion of ultramicropores in mylonitic coal. With strong polarity and small kinetic diameter, CO 2 molecules are more prone to interact with π-electrons and produce the “pore-filling effect”. In mixed gas adsorption, the total adsorption capacity and interaction energy of both coal types increase with the rise of CO 2 molar fraction. Although the adsorption selectivity coefficient of CO 2 to CH 4 decreases, it is always greater than 1, and the selectivity coefficient of mylonitic coal is higher. The energy distribution curve of CO 2 shifts to adsorption sites with low interaction energy, while that of CH 4 shows the opposite trend, and the shift amplitude of gas molecular curves in mylonitic coal is larger. In addition, the interaction energy of mylonitic coal is dominated by van der Waals force, accounting for a higher proportion than that of primary structure coal. Its CH 4 adsorption heat also has a larger fluctuation range, reflecting a stronger competitive adsorption capacity for CO 2 . The research findings provide a theoretical basis for mine gas prevention and control and efficient coalbed methane development.