Ultrasonic-Induced Changes in Nanopores: Molecular Insights into Effects on CH₄/CO₂ Adsorption in Coal

The nanometer-sized pores within coal are the primary sites for CH₄ adsorption and competitive adsorption with CO₂. Reasonable modification of the nanopore structure to enhance CH₄ desorption, diffusion rates, and CO₂ competitive adsorption effects can enhance significantly coalbed methane (CBM) production. However, ultrasonic synchronous modification of multiple features of nanopores leads to complex and variable gas adsorption behaviors in coal. To reveal the effect of ultrasonic modification of coal nanopores on gas adsorption, pore measurement experiments and molecular simulation studies were conducted. The results showed that the volume ratio of diffusion pores to adsorption pores (V₂/V₁) decreased significantly after ultrasonic excitation. In the original coal sample, V₂/V₁ was 3.05, while in the coal sample after ultrasonic treatment, V₂/V₁ ranged from 0 to 2.54. With decrease in the proportion of the volume of diffusion pores, the proportion of CH₄ migration from the pore walls of the adsorption pores increased continuously. The proportion of CH₄ migration from the pore walls of the diffusion pores to the pore space of the diffusion pores decreased continuously. The results of gas–solid interaction energy calculation showed that ultrasonic treatment of coal decreases the V₂/V₁ ratio, leading to 7.1–23.3% increase in CO₂ competitive adsorption effect. It also resulted in 4–49% improvement in competitive adsorption efficiency. Additionally, based on gas–solid interaction energy data, an adsorption capacity evaluation model for coal under different gas compositions and pore volume ratios was constructed. The findings can guide ultrasonic-enhanced CBM.