Investigation of the fractal characteristics of adsorption-pores and their impact on the methane adsorption capacity of various rank coals via N2 and H2O adsorption methods

Coal seam is a sedimentary body with complex pore system. The gas adsorption property of coal is greatly determined by the adsorption-pores (<100 nm), and the fractal dimension can be used an index to estimate the impact of the adsorption-pores on the methane adsorption capacity of various rank coals. In this paper, low-temperature N-2 adsorption and H2O adsorption methods were used to study the adsorption-pores structure and its fractal features. The results show that the development of adsorption-pores closely depends on the coalification, and the degree of pore development exhibits a U-shaped trend with increasing coal rank. Additionally, the pore size distributions of coal samples from N-2 adsorption analysis and H2O adsorption analysis are similar, especially for samples LH7 and WLH8. Fractal analysis indicates that D-2(N-2) (0.5 P/P-0 < 1) can more accurately characterize the fractal features of adsorption-pores than D-2(H2O), which may be a result of the significant coal-H2O interaction. Moreover, D-2(N-2) has stronger correlations with the coal pore parameters. With the increase in D-2(N-2), the Langmuir volume first decreases and then increases, which is probably associated with the competition effect of the pore structure and surface irregularity of coal. When D-2(N-2) < 2.7-2.8, the pore structure plays a key role, while for D-2(N-2) > 2.7-2.8, the influence of the specific surface area is more prominent. The equilibrium moisture content of the coal samples also has a positive correlation with D-2, except for low-rank coal sample YZG2 due to the presence of a large amount of oxygen-containing functional groups, which increases its water-holding capacity.