Molecular Insights into Microstructure and CH4/CO2 Adsorption of Mylonitic Coal

Here, macromolecular representation and CO2-CH4 competitive adsorption properties of mylonitic coal (MC) were revealed based on 13C nuclear magnetic resonance, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, and grand canonical Monte Carlo simulation. Results suggested that MC has a higher content of aromatic carbon, non-protonated aromatic carbon, and alkyl-substituted aromatic carbon compared with primary coal (PC). There were much longer aliphatic chains and lower branching degrees for MC than PC. The basic structural units of MC were characterized by a higher percentage of longer aromatic fringes (>1 nm) with higher structural alignment (38–48%, 41% in average) than PC (31–38%, 35% in average). Molecular probe results suggested that MC has more free volumes than PC at a given occupied volume. Single adsorption of CO2 and CH4 revealed a higher saturation adsorption capacity of CO2 and CH4 and more predominant energy reduction of MC. Competitive selectivity of CO2 over CH4 (SCO2/CH4) was >1, indicating the adsorption advantages of CO2 over CH4. At pressures of <3.0 MPa, the SCO2/CH4 of MC was higher than PC. For pressures of 3~4 MPa, they were close, indicating that the competitive adsorption advantages of CO2 over CH4 become weak at higher pressures for MC reservoirs. For temperature dependence, it decreases with the increasing temperature, suggesting that high temperature could exert a more significantly unfavorable impact on CO2 adsorption than for CH4 adsorption. Both the swelling ratios decrease with the increasing temperature for PC and MC; however, that of MC was higher than that of PC, suggesting that MC possess stronger swelling deformation at a given temperature. The outcomes of this paper indicated that compared with PC, the higher tendency of gas outburst for the MC zone was jointly controlled by a higher proportion of large aromatic fringes, more extensive development of structural defects, higher specific surface area of micropores, and stronger adsorption affinity of CH4. Competitive adsorption results suggested that MC reservoirs are more favorable for CO2-ECBM engineering than those of PC.