Fractal characterization of adsorption-pores of coals from North China: An investigation on CH4 adsorption capacity of coals

To better understand the characteristics of adsorption-pores (pore diameter < 100 nanometers) and their influence on CH4 adsorption capacity of coals, we have conducted fractal analysis for 13 fresh coal samples (Ro from 0.79 to 4.24%) in North China. Isotherms of N2 gas adsorption/desorption analyses indicate that coals have different adsorption characteristics at relative pressure of 0–0.5 and 0.5–1. On this basis, two fractal dimensions D1 and D2 (at relative pressure of 0–0.5 and 0.5–1, respectively) were obtained using the fractal Frenkel–Halsey–Hill (FHH) method, in which both proposed fractal exponents, ‘(D − 3) / 3’ and ‘(D − 3)’ were investigated. The results show that the fractal exponent ‘(D − 3)’ provides more realistic results than fractal dimensions calculated from (D − 3) / 3. The two fractal dimensions, D1 and D2, have different correlations with CH4 adsorption capacity of coals. The CH4 adsorption capacity does not vary with increasing fractal dimension D1 up to about 2.5, but thereafter increases with D1. In contrast, the CH4 adsorption capacity varies negatively with D2 within the entire data range. Further investigation indicates that D1 represents fractals from pore surface area generated by surface irregularity of coals, while D2 characterizes fractals related to pore structures that are controlled by the composition (e.g., ash, moisture, carbon) and pore parameter (e.g., pore diameter, micropores content) of coals. Higher fractal dimension D1 correlates to more irregular surfaces that provide more space for CH4 adsorption. Higher fractal dimension D2 represents higher heterogeneity of pore structure and higher liquid/gas surface tension that reduce CH4 adsorption capacity. Therefore, more irregular coal surface and more homogeneous pore structure indicate higher CH4 adsorption capacity of coals.