Study on the effect of mixed acid treatment on the microstructure and gas desorption kinetics of lignite

Research on physical and chemical structure, pore morphology, and desorption characteristics of coal before and after acidification is crucial for advancing coalbed methane development. Changes in coal morphology, pore structure, mineral composition, and gas desorption behavior before and after acidification were systematically analyzed using techniques such as scanning electron microscopy (SEM), low temperature liquid nitrogen adsorption (LTNA), x-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). SEM images reveal that acidification treatments substantially alter the surface morphology of lignite. The combined application of hydrofluoric acid, hydrochloric acid, and acetic acid effectively removes inorganic minerals from coal while enhancing the connectivity of surface pores. LTNA experiments indicated that the specific surface area, pore volume, and the maximum adsorption capacity of lignite significantly increased following mixed acid treatment. The XRD and FTIR analyses demonstrated that the acidification treatment altered the surface chemical properties and enhanced the reactivity of lignite. Gas desorption characteristics were measured before and after acidification. The combined treatment of the three acids significantly improved the gas desorption performance, and the initial desorption velocity and gas release amount increased, and showed more obvious advantages in the initial stage of gas desorption. A comprehensive quantitative index (φ) was established to relate the microscopic parameters of lignite with the gas desorption parameters, allowing for the evaluation of acidification treatment impact on gas desorption performance. The φ value for the three-acid combined treatment group was 3.83, significantly exceeding 1, which highlights the superior effect of acidification treatment in enhancing desorption performance.