Dynamic competition mechanism of organic–inorganic acid synergistic transformation on low–rank coal pore structure and its impact on coalbed methane desorption efficiency

• Verify ternary acid synergy, providing a new CBM extraction technical path. • Reveal organic-inorganic acid dynamic competition in lignite modification. • Establish a quantitative index for lignite gas desorption pre-/post-acidification. To address the inefficient coalbed methane (CBM) desorption caused by dense pore structures in low-rank coal, this study proposes a novel research framework of “organic-inorganic acid dynamic competition mechanism-driven targeted pore reconstruction”. Through systematic acidification treatments using hydrofluoric acid (HF), hydrochloric acid (HCl), and acetic acid (CH 3 COOH) in various combinations on lignite, we comprehensively investigated the evolution of pore architecture, mineral phase transformation, and surface chemical characteristics through advanced characterization techniques including scanning electron microscopy (SEM), low-temperature nitrogen adsorption (LTNA), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The results demonstrate that ternary acid synergy significantly enhances pore complexity and specific surface area, optimizes microcrystalline structure, and promotes desorption kinetics reinforcement. XRD and FTIR analyses reveal a three-stage chain reaction mechanism: HF preferentially dissolves silicates, HCl selectively eliminates carbonates, and CH 3 COOH modulates oxygen-containing functional groups, collectively achieving “mineral dissolution - pore expansion - surface wettability optimization”. Methane desorption experiments confirm remarkable performance enhancement in acidified samples, showing 3.99 mL/g ultimate desorption capacity and 85% improvement in diffusion rate constant. A normalized comprehensive performance index ρ integrating critical pore parameters and desorption kinetics was established, quantitatively verifying the efficiency amplification mechanism of the organic-inorganic acid hybrid system. This work provides fundamental insights into pore structure regulation and CBM recovery enhancement in low-rank coal reservoirs, offering theoretical guidance for developing efficient acid based stimulation technologies in unconventional gas extraction.