Fractal characteristics of pulverized high volatile bituminous coals with different particle size using gas adsorption

To better understand the effect of particle size on fractal characteristics of pores in coal, three high volatile bituminous coals with mean maximum vitrinite reflectance of 0.82-0.90% collected from the NarykskoOstashinskaya coalbed methane field in Kuznetskiy Basin in Russia were crushed and sieved to five progressively decreasing particle size fractions: 1.0-0.5 mm, 0.50-0.25 mm, 0.250-0.125 mm, 0.125-0.063 mm and 0.063-0.032 mm. For all particle size fractions, particle size distribution measurement, proximate analysis, maceral analysis and low-pressure N-2 adsorption and CO2 adsorption analyses were conducted. Fractal dimensions D-1 (pore surface roughness) and D-2 (pore structural irregularity) were determined from relative pressure intervals of 0-0.45 and 0.45-1 of N-2 adsorption isotherm with the application of the fractal FrenkelHalsey-Hill theory, respectively. The results demonstrate significant differences in fractal dimensions in different particle size fractions. As particle size of coal samples decreases, D-1 decreases, while D-2 increases. Some constricted mesopores are transformed into non-constricted mesopores during crushing. The non-constricted mesopores increasingly concentrate in smaller pore sizes, which results in smoother pore surface (decreasing D-1) and more heterogeneous pore structure (increasing D-2). Mesopores with pore width between 2 nm and 5 nm are the key to fractal dimensions. Individual mesopore structure parameters closely associated with particle size have universal correlations with fractal dimensions. The increasing mesopore specific surface area and volume and decreasing average mesopore width lead to a decrease in D-1 and an increase in D-2, which further confirms the effect of particle size on fractal dimensions.

Automated summary

This study investigates the effect of particle size on the fractal characteristics of coal pores. It was found that as the particle size decreases, the roughness of the pore surface decreases and the irregularity of the pore structure increases. Mesopores with a width between 2 nm and 5 nm play a key role in determining the fractal dimensions.