Abstract
In order to grasp the effect of soft and hard coal pore structure on gas adsorption characteristics, based on fractal geometry theory, low-temperature nitrogen adsorption and constant temperature adsorption test methods are used to test the pore structure characteristics of soft coal and its influence on gas adsorption characteristics. We used box dimension algorithm to measure the fractal dimension and distribution of coal sample microstructure. The research results show that the initial nitrogen adsorption capacity of soft coal is greater than that of hard coal, and the adsorption hysteresis loop of soft coal is more obvious than that of hard coal. And the adsorption curve rises faster in the high relative pressure section. The specific surface area and pore volume of soft coal are larger than those of hard coal. The number of pores is much larger than that of hard coal. In particular, the superposition of the adsorption force field in the micropores and the diffusion in the mesopores enhance the adsorption potential of soft coal. Introducing the concept of adsorption residence time, it is concluded that more adsorption sites on the surface of soft coal make the adsorption and residence time of gas on the surface of soft coal longer. Fractal characteristics of the soft coal surface are more obvious. The saturated adsorption capacity of soft coal and the rate of reaching saturation adsorption are both greater than those of hard coal. The research results of this manuscript will provide a theoretical basis for in-depth analysis of the adsorption/desorption mechanism of coalbed methane in soft coal seams and the formulation of practical coalbed methane control measures.
Highlights
Coal is a kind of porous solid medium with developed pore structure. e abundant pore structure in the coal body is a place for gas storage, and a channel for gas production. e adsorption state of gas in coal makes the mechanical properties of gas-containing coal different from non-adsorption media
A self-developed adsorption/desorption device is used to test the gas adsorption and desorption performance of soft coal and hard coal under low-temperature conditions, combined with scanning electron microscope and low-temperature nitrogen adsorption experiment to test the pore characteristics of experimental soft coal and hard coal
Analyze the microscopic pore and fracture structure and connectivity of typical soft coal, use the box dimension algorithm to measure the fractal dimension of the microstructure of the coal sample, and compare and analyze the relevant characteristics of hard coal
Summary
Coal is a kind of porous solid medium with developed pore structure. e abundant pore structure in the coal body is a place for gas storage, and a channel for gas production. e adsorption state of gas in coal makes the mechanical properties of gas-containing coal different from non-adsorption media. E natural pore and fissure structural characteristics of coal determine that coal has good gas adsorption capacity and storage performance. Bing et al [1] studied the pore structure and adsorption characteristics of outburst coal and found that micropores with a pore size of 3–5 nm in outburst coal are the main gas adsorption space. Jiang et al [9] used the mercury intrusion method to determine the characteristic parameters of the ultrafine pore structure of coal and found that as the hardness of the coal increases, the fractal dimension of pores continues to decrease. Klimenko et al [11] used the conditional moment model to study the fractal characteristics of the transportation, storage, and adsorption processes in the pores of CO2 porous media. Harpalant et al [14,15,16,17,18] showed that the coal body will produce expansion stress and deformation after adsorbing gas. ere are few systematic research results on the influence of soft coal pore structure on gas adsorption performance
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