Ultrasonic modification of coal nanopore physicochemical Structure: Investigating CH4/CO2 adsorption behavior

Optimizing nanopore structures in coal to reduce methane adsorption intensity is crucial for enhancing CH₄ production. This study investigates the effects of ultrasonic modification on methane adsorption by assessing changes in coal’s physicochemical structure post-treatment. Next, a new pore structure model was developed to represent the microporous and mesoporous physicochemical properties and connectivity patterns of the coal sample, and it was used to simulate the gas adsorption behavior. Results indicate that while ultrasonic treatment minimally impacts on the size of adsorption and diffusion pores, it alters their volume ratio. Chemical characterization shows that while the overall carbon skeleton remains stable, the elemental composition of the pore walls changes significantly, with oxygen content reduced by 45.28 % to 57.09 %. A composite pore model based on series connection theory of adsorption and diffusion pores reveals a strong correlation between diffusion pore proportions and the adsorption capacities of CH₄ and CO₂, with lower diffusion pore ratios enhancing gas adsorption. In a binary gas competitive adsorption system, gas–solid interaction energy analysis reveals that the oxygen content of pore walls significantly influences competitive adsorption efficiency. When oxygen content is comparable, the diffusion pore ratio becomes the primary factor, ranking competitive adsorption efficiencies of four coal samples as follows: A1 (action 10 min) < A2 (action 40 min) < A3 (action 60 min) < Y (original coal sample). Additionally, gas density distribution curves indicate that CO₂ displacement of CH₄ at primary adsorption sites in proportion to their relative quantities, indicating no absolute preference in competitive adsorption. Methane trajectory analysis reveals that, in the original coal sample (Y), methane occupied strong adsorption areas (adsorption pores and pore walls) for 97 % of the time. After ultrasonic treatment, this occupancy decreased to 85 % (A2) and 81 % (A3), suggesting that physicochemical changes in coal after treatment favor CO₂ competitive adsorption over CH₄. Finally, a model was developed to predict the gas–solid interaction energy in the competitive adsorption system. The errors between the predicted and actual values were 3.57 % (for coal-CH₄) and 6.68 % (for coal-CO₂), respectively.