Influence of long-term CH4 and CO2 treatment on the pore structure and mechanical strength characteristics of Baijiao coal

In order to study the influence of adsorptive gas on coal pore structures and mechanical strength, coal specimens were placed in a low-pressure environment of CH4 or CO2 for 30 days to ensure that the specimens were saturated and fully interacted with the gas. Low-pressure nitrogen adsorption, scanning electron microscope, X-ray diffraction, and uniaxial compressive strength tests were employed to study coal's microstructure change and strength characteristics. The results show that after CH4 or CO2 treatment, the proportion of macropores increases by 38.87% and 22.89%, and that of mesopores decrease by 6.34% and 3.73%, respectively, which indicates the evolution of mesopores to macropores. The microcrystalline structure parameters change obviously after long-term gas adsorption treatment. The values of d002 and d100 increase, while Lc, La, and Mc's values both decrease, resulting in a loose coal structure. Besides, a conceptual model is proposed to explain the evolution mechanism of pore structures under the effect of long-term gas adsorption.