Fractal characterization and methane adsorption features of coal particles taken from shallow and deep coalmine layers

• Fractal analyses were performed on shallow and deeper buried coal particles.
• Shallow deposited coals mostly exhibited less Ds and broader PSD.
• Deeper deposited coals mostly showed larger Ds and narrower PSD.
• Methane adsorption capacities had higher-order relations with Ds.

Fractal characterization was conducted on coal samples collected from shallow and deep coal mine layers, to better understand the process required to exploit deep layer coal bed methane (CBM). Data from N2 adsorption/desorption isotherms were used to assess the irregularities of coal pore surfaces (Ds) and the heterogeneities of pore structures (Dp) on the basis of modified Frenkel–Halsey–Hill (FHH) and Neimark equations. Fractal dimensions (D) calculated from the different analysis methods were compared. The results indicate that Ds values are sensitive to coal types that contain different fractal structural features. Shallow-deposited coal usually exhibits low Ds values, while coal collected from deep layers exhibits large Ds values. Carbon content and pore diameter are the main factors in determining the surface fractal dimensions. Deeper-deposited coal is characterized by rougher surfaces and more condensed pore networks. This is because these samples have higher carbon content and more micropores that formed under multiple conditions. Methane adsorption capacity is closely related to the Ds value, and a larger Ds value could indicate an increased capacity of methane adsorption.