Investigation into the evolution of pore structure and fractal characteristics of coal under microwave-assisted acidification conditions

This study employs microwave-assisted acidification to modify coal, offering a potential approach to enhance the production efficiency of coalbed methane. To investigate the evolution characteristics of coal’s pore structure under the synergistic effects of microwave treatment, acidification, and their combination, the optimal acidification ratio was initially determined through acid etching experiments. Subsequently, nitrogen adsorption and high-pressure mercury intrusion experiments were conducted to compare and analyze the pore size distribution and pore structure evolution under various treatment conditions. Based on the nitrogen adsorption results, the impact on fractal characteristics was further evaluated. Additionally, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were performed to examine the surface morphology and internal structural changes of the pore structure. The findings reveal that, in descending order of effectiveness, the degree of pore structure modification from raw coal is as follows: microwave-acidified coal, acidified coal, and microwave-treated coal alone. Under the combined microwave-acidification effect, the pore structure of coal transitions from being predominantly microporous in raw coal to a structure dominated by mesopores, with coexisting macropores and micropores. Furthermore, this synergistic effect leads to a broader distribution of fractures and pores, significantly increases graphitization, and reduces acidification time. Consequently, the combined microwave-acidification process effectively achieves both “pore opening” and “pore expansion”. This research demonstrates that the synergistic microwave-acidification approach holds considerable promise for enhancing coalbed methane extraction.