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 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, '(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, 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.