Fractal character of coal nanopore and effect of deviation corrected, coal rank, and gas adsorption

Pore structure in coal is fundamental to determining the flowability properties and mechanical functions. Various insights into pore structure, including pore size distribution, specific area, and pore volume, have been obtained in experiments with many methods. Systematic research is lacking on how to characterize pore structure with more detailed information using fractal theory. Here, using the small-angle X-ray scattering (SAXS) method, we studied the fractal characters of four coal samples and showed how the deviation corrected, coal rank, and gas adsorption affected the fractal dimension of the pore structure. The average pore diameters of four samples obtained by the gyration radius using Guinier curves were 50.25–52.95 nm. The surface fractal of coal samples was the dominant feature, and the fractal features altered with the q value. Scattering information of simple bodies, such as pores and fractures, could superimpose the non-integer nature of the fractal dimension. The Porod and Debye laws guided the deviation correction process in the high q region. Pore fractals might be changed into surface fractals through the fractal dimension after the deviation correction, and the SAXS data obtained by Debye law correction maintained the originality and validity better than Porod law correction. The irregular shape of M was observed on the curve of fractal dimension as the maximum reflectance of vitrinite increases, resulting in three stages. More adsorption sites per unit projection area could be found in a larger fractal dimension of the pore surface, which results in an increase in Langmuir adsorption volume. The increase in the adsorption pressure reduced the fractal dimension. Our findings enhance the experimental support for comprehending the structure of coal nanopores. [Display omitted] •Surface fractals dominated the coal nanopores with the sizes of 2.21–70.24 nm.•The deviation correctness may be changed surface fractal into pore fractal by Porod law and Debye law.•Surface fractal dimensions show an irregular M-curve trend with an increase in maximum vitrinite reflectance.•Gas adsorption increase the smoothness of coal pore surface and decrease the fractal dimension.