Evolution mechanism of methane adsorption capacity in vitrinite-rich coal during coalification

Abstract

In this study, the evolution of methane adsorption capacity was investigated by analyzing both naturally matured coal samples and samples that matured by pyrolysis to different ranks. Methane adsorption isotherms, mercury injection, and small-angle X-ray scattering experiments were employed to characterize the methane adsorption capacity and pore structures. Simulation methods were used to study the methane adsorption capacities of different functional groups in the coal matrix by constructing slit pores whose surfaces were modified by carboxyl, hydroxyl, aromatic ring, or methyl groups. In the simulations, the methane adsorption capacity per surface at 10 MPa were very similar: 0.31 cm3/m2, 0.29 cm3/m2, 0.28 cm3/m2 and 0.26 cm3/m2, respectively, indicating different functional groups do not significantly affect the variability of methane adsorption capacities. The evolution of methane adsorption capacity was similar to that of micropore volume, implying methane molecules were mainly adsorbed in micropores in coals. It was also observed that the evolution of methane adsorption capacity in artificially matured samples was similar to that of the naturally matured coals, implying differences in the coalification pathways had minimal influence on the evolution of methane adsorption capacity. With Rr increasing from 0.5% to 4.2%, the methane adsorption capacity evolved in four stages: 0.5%–1.4% (decreasing adsorption), 1.4%–2.0% (significantly increasing adsorption), 2.0%–3.7% (slightly increasing adsorption) and 3.7%–4.2% (decreasing adsorption). The results of this study show that during the coalification process, the molecular structure essentially determines the evolution of the methane adsorption capacity by controlling evolution of micropores.