Fractal characteristics of pore structures in 13 coal specimens: Relationship among fractal dimension, pore structure parameter, and slurry ability of coal

• D1 describes the surface roughness of meso- and macro-pores in coal.
• D2 describes the volumetric roughness of fine mesopores in coal.
• D2 increases with increasing specific surface area and total pore volume of coal.
• The increase in D2 has a negative effect on the slurry ability of coal.
• With increasing coal rank, D2 has a decreased trend.

The fractal characteristics of pore structures in 13 different coal specimens were investigated. Insights into the relationship among fractal dimension, pore structure parameter, and slurry ability of coal were provided. N2 adsorption/desorption at 77 K was applied to analyze the pore structure of coal. Two fractal dimensions, D1 and D2, at relative pressures of 0 to 0.45 and 0.45 to 1, respectively, were calculated with the fractal Frenkel–Halsey–Hill model. Results reveal that the value of D1 is mainly affected by the influence of meso- and macro-pores with an average pore size range of 10 nm to 220 nm on the specific surface area; therefore, D1 can be utilized to quantitatively describe the surface roughness of these meso- and macro-pores in coal. Meanwhile, the value of D2 is mainly related to the effects of fine mesopores with an average pore size range of 2 nm to 10 nm on the total pore volume; therefore, D2 can be utilized to quantitatively describe the volumetric roughness of these mesopores in coal. D1 has no apparent linear correlation with the pore structure parameters and maximum solid loading of coal, and D2 has a positive linear correlation with the specific surface area and total pore volume of coal. The increase in specific surface area, total pore volume, and D2 has negative effects on the slurry ability of coal. High-rank coals with high ash content and low volatile matter relatively have higher D1 and lower D2. Meanwhile, with increasing coal rank, D2 has a decreased trend. The fine mesopores with an average pore size range of 2 nm to 10 nm in coal have direct effects on the pore structure parameters and D2 of coal; thus, the slurry ability of coal may be improved if the number of these mesopores in coal is reduced by modification processes, such as microwave irradiation, hydrothermal treatment and so on.