Abstract
The presence of gas content in medium- and high-rank coal poses a threat to safety production. Safe gas extraction is based on a correct understanding of the pore structure of coal. This work investigates the pore structure characteristics of medium- and high-rank coal and evaluates their fractal structure. The coal samples were collected from Huainan Coalfield and Qinshui Coalfield, and divided into four types, according to the difference in surface bright characteristics. Through adopting low-temperature liquid nitrogen adsorption and desorption, and applying Kelvin equation, we obtain the main pore structure types and main pore size distribution characteristics of various coal briquettes. Electron microscope scanning structure and scientific analysis were used for special adsorption and desorption curves and hysteresis to find the dynamic reason. According to the different adsorption mechanism and Frenkel–Halsey–Hill-based model, with P/ P0 = 0.4 as the dividing point of fractal dimension analysis, the pore structure of coal samples is classified into five grades. The fractal evaluation results are consistent with the results of curve analysis and pore size analysis.
Highlights
Medium-rank coal refers to the coal whose maximum vitrinite reflectance is in the range of 0.50%–2.50%, such as bitumite
The specific surface area of CZ coal sample per unit mass is only 54.54% (= Specific surface area of CZ / Specific surface area of LZ) of that of LZ coal sample per unit mass, which is consistent with the research results of many scholars such as Zou and Yang (1998) comparing the relative changes of the surface area and the degree of metamorphism
Bright type III coal in middle- and high-rank coal pore volume 0.001872 cm3/g and specific surface area of 0.4807 m2/g belong to the low category, the pore area and realize bimodal pore area, pore volume curve peak is located in the micropore area aperture and aperture surface curve peak is consistent, but the specific surface area rate of change in the area is greater than the pore volume change rate and explain about 2 nm porous occupies a more and more complex pore structure, joint adsorption stripping did not form a closed curve and hysteresis loop is evidence that point
Summary
Medium-rank coal refers to the coal whose maximum vitrinite reflectance (denote by Romax) is in the range of 0.50%–2.50%, such as bitumite.
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