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

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• 作者：Yanbin Yao ; Dameng Liu ; Dazhen Tang ; Shuheng Tang ; Wenhui Huang
• 关键词：Coalbed methane (CBM) ; Fractal dimension ; Adsorption capacity ; N₂ gas adsorption method ; North China
• 刊名：International Journal of Coal Geology
• 出版年：2008
• 出版时间：7 January 2008
• 年：2008
• 卷：73
• 期：1
• 页码：27-42
• 全文大小：1239 K

文摘

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 (R_o 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.