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

Abstract

To better understand the characteristics of adsorption-pores (pore diameter <100 nanometers) and their influence on CH 4 adsorption capacity of coals, we have conducted fractal analysis for 13 fresh coal samples (R o from 0.79 to 4.24%) in North China. Isotherms of N 2 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 D 1 and D 2 (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, '(Z)-3)/3' and '(D-3)' were investigated. The results show that the fractal exponent '(Z)-3)' provides more realistic results than fractal dimensions calculated from (D-3)/3. The two fractal dimensions, D 1 and D 2 , have different correlations with CH 4 adsorption capacity of coals. The CH 4 adsorption capacity does not vary with increasing fractal dimension D 1 up to about 2.5, but thereafter increases with D 1 In contrast, the CH 4 adsorption capacity varies negatively with D 2 within the entire data range. Further investigation indicates that D 1 represents fractals from pore surface area generated by surface irregularity of coals, while D 2 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 D 1 correlates to more irregular surfaces that provide more space for CH 4 adsorption. Higher fractal dimension D 2 represents higher heterogeneity of pore structure and higher liquid/gas surface tension that reduce CH 4 adsorption capacity. Therefore, more irregular coal surface and more homogeneous pore structure indicate higher CH 4 adsorption capacity of coals.