Nanopore Characteristics of Coal and Quantitative Analysis of Closed Holes in Coal

Methane gas is mainly present in coal in two forms: free and adsorbed. There are a large number of closed pores inside the coal, which makes it difficult to measure the gas content of the coal. Therefore, studying the nanoscale closed pores of coal is of great importance for gas control. To study the pore structure characteristics of coal with different deformation degrees and to analyze the volume fraction of closed pores in coal, various coal samples were analyzed by the low-temperature liquid nitrogen adsorption method (LT-N₂GA), the carbon dioxide adsorption method, and small-angle X-ray scattering (SAXS). The variation of parameters such as the pore size, pore volume, specific surface area, and degree of metamorphism was compared by using different methods to obtain the proportion of the closed pore volume of different coal samples. The results show that with the increase of the degree of coal metamorphism, the total pore volume and specific surface area of coal samples show a decreasing trend first and then an increasing trend, while the average pore diameter of coal samples gradually increases first and then decreases sharply. When the degree of deterioration of coal is low (volatile content > 20%), the closed pores of coal account for more than 48% of the open pores. When the degree of deterioration of coal samples is relatively high (volatile content <20%), the proportion of large pores in coal bodies decreased from 59.47 to 29.07%, and the proportion of pores in mesopores decreased from 12.15 to 11.09% and finally increased to 11.65%, and the proportion of micropore diameter increased from 28.38 to 59.28%. The volume fraction of the coal sample measured by the SAXS experiment shows that when the coal quality is high, the volume of the mesopores is large, which is consistent with the results of the low-temperature liquid nitrogen and CO₂ adsorption experiments. Judging from the number of holes, the number of closed holes is 1 to 3 orders of magnitude greater than the number of open holes, and the number of closed holes of coal samples accounts for more than 94% of the total number of holes. It shows that the number of closed holes in the coal is far greater than the number of open holes, so the gas in the coal is mainly concentrated in the closed holes, and the formation of closed pores is partly because of the collapse of the internal structure and partly because of the volatilization of unstable substances. The research combined with LT-N₂GA, the carbon dioxide adsorption method, and SAXS test methods can better analyze the number of closed pores of coal and characterize the nanopore fracture structure of coal. The novelties of this article are that this is a quantitative analysis performed using a scientific method of SAXS. The findings of this study can lead to a better understanding of the coal and gas outburst mechanism and the existence of gas to adopt better prevention measures.