Kinetic mechanism of surfactant−enhanced CH₄ desorption and diffusion in coal

The impact of surfactant modification on the kinetic properties of methane (CH₄) in coal is examined, with Xutong coal samples taken as the primary research object. Five surfactants — dodecyltrimethylammonium bromide (DTAB), alkyl polyglucoside (APG), methyltrimethoxysilane (MTMS), sodium dodecyl sulfate (SDS), and polyethylene oxide (PEO) — were employed to treat the coal samples. The performance of CH₄ desorption under varying pressure settings was evaluated, and diffusion coefficients were obtained based on fitting single pore diffusion models and time-varying diffusion models. The alterations in the microscopic pore architecture and oxygenated functional groups of coal samples pre- and post-surfactant treatment were analyzed using gas (N₂ or CO₂) adsorption and Fourier-transform infrared spectroscopy (FT−IR). Macromolecular structural models of the coal samples were created before to and during modification, based on elemental analysis, ¹³C nuclear magnetic resonance (¹³C−NMR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) data. Molecular dynamics (MD) simulations were utilized to investigate the impact of tiny chemical structural alterations on the kinetic characteristics of CH₄. Results show that all five surfactants significantly altered the micropore structure of the coal. Relative to the unmodified MC sample, the MC-PEO sample exhibited a 30% decrease in micropore-specific surface area and a 25.6% reduction in micropore volume. FT−IR analysis revealed that the CH₂/CH₃ ratio of the MC-PEO sample increased from 0.31 to 0.39, while the proportion of polar functional groups (e.g., carboxyl and carbonyl) decreased from 18.1% to 16.9%. In contrast to MC-PEO, treatments with the other four surfactants induced opposite changes in the surface properties of coal. Under adsorption equilibrium pressures ranging from 0.74 to 2.0 MPa, CH₄ desorption from MC-PEO increased by 7.0%−24.1%, with the diffusion coefficient rising by 3.0%−19.8%. The chemical formulas of the molecular models for MC and MC-PEO coal were determined to be C₁₈₅H₁₃₂O₁₅N₃ and C₁₈₅H₁₄₁O₁₃N₃, respectively. MD simulation results indicated that the theoretical diffusion coefficient increased from 0.3×10⁻⁹ m²/s (MC) to 1.2×10⁻⁹ m²/s (MC-PEO), consistent with the experimental trend. Collectively, these findings reveal at both macroscopic and microscopic levels that the nonionic surfactant PEO promotes CH₄ desorption and diffusion by reducing micropore adsorption sites and the number of surface polar oxygen-containing groups in bituminous coal. A theoretical basis for improving coalbed methane extraction efficiency is provided.