Quantitative characterization of pore structure and gas adsorption and diffusion properties of low-rank coal

Low-rank coal is widely distributed in the northwest, north and northeast of China and has large reserves, but some serious gas outburst disasters have occurred in recent years in some low-rank coal coalfields. In order to study the pore structure of low-rank coal and the characteristics of gas adsorption and diffusion, and its influence on gas disaster, the N2/CO2 adsorption, small angle X-ray scattering (SAXS), gas adsorption/desorption experiments and fractal theory were used to study six groups of coal samples. According to the relationship between coal sample pore structure parameters and gas adsorption/desorption characteristic parameters, the correlation between the micropore structure of low-rank coal and the macro gas adsorption and diffusion characteristics was obtained. Results show that the micropore (≤2 nm) volume of low-rank coal is 0.055-0.064 cm3/g, which is generally higher than that of medium-rank coal (0.026-0.060 cm3/g). The mesopores of low-rank coal are mainly distributed in 2-8 nm, while the most developed pore diameters of medium-rank coal are 2-4 nm. The mesoporous pore volume and specific surface area of low-rank coal are generally larger than those of medium-rank coal. SAXS results show that most developed pore size of low-rank coal is about 10 nm, while the most developed pore size of medium-rank coal is near 35 nm. This differs significantly from the most developed pore size of about 3 nm on the coal sample measured by N2 adsorption method, which may be the presence of closed pores in the coal or pores that cannot be measured by gas adsorption method. The results of fractal dimension calculation show that the pores of low-rank coal are more complex than those of medium-rank coal. Combined with Fourier transform infrared spectroscopy analysis, low-rank coal has a loose molecular structure and contains more functional groups with long side chains, which leads to the development of micropores and mesopores. There is a positive correlation between the pore volume of micropores and Langmuir volume (VL), and the pore volume of mesopores also has a positive correlation with gas diffusion coefficient (D). VL and D reflect the adsorption and diffusion characteristics of coal, so low-rank coal has relatively strong gas adsorption and release capabilities, and is prone to abnormal gas gushing and gas outburst. © 2021, Editorial Office of Journal of China Coal Society. All right reserved.