The efficiency of gas extraction from the soft coal seam with ultralow permeability is low. Gas extraction with large-diameter borehole is proposed to deplete gas content for preventing gas outburst disaster in this study. The fractures around the large borehole will enhance the permeability in the damage area to promote gas extraction. We established a damage-stress-seepage coupling model for large-diameter borehole gas extraction in soft coal seam. This mathematical model contains governing equations of gases sorption and transport, coal deformation, and damage, reflecting the coupling responses between gas and coal seam. The model is solved by the finite element method to simulate the gas drainage large-diameter borehole through roadway. Distributions of elastic modulus, damage area, and maximum principal stress in soft coal seam with different borehole diameters including 94 mm, 133 mm, 200 mm, and 300 mm are analyzed. The gas pressure, gas content, and effective extraction area in soft coal seam are discussed. Results show that the shear failure zone appears around the large-diameter borehole, and its permeability rises sharply. This opens up the gas transport channel and is conducive to the rapid extraction. It is confirmed that gas extraction using large-diameter borehole (300 mm) can greatly improve the efficiency of the gas preextraction in soft coal seam by increasing gas extraction rate. These provide a foundation for guiding the operation of gas extraction with large borehole from the soft coal seam in the field.

To further explore the mechanism of coal spontaneous combustion and better grasp the laws of spontaneous combustion, this article studied the adsorption behavior of O2 in coal. Materials studio software was applied to study the adsorption of oxygen under different water content, different pore sizes, and different oxygen-containing functional groups by means of grand canonical Monte Carlo and molecular dynamics simulation methods. The results show that the adsorption capacity of O2 decreases with the increase in water content. With the increase of molecular pore size of coal, the adsorption capacity of O2 increases, and the tight adsorption amounts decrease. The equivalent adsorption heat is less than 42 kJ/mol, indicating that the adsorption of O2 in coal pores is physical adsorption. The smaller the physical adsorption energy and charge transfer value of the hydroxyl group for O2, it indicates that the hydroxyl group is the active group for the physical adsorption of O2.

Large diameter borehole drilling in fractured soft coal seam may lead to problems such as drilling blockage, perforation, and hole collapse. The protective screen pipe in the drilling hole can support the damaged coal, prevent the drilling blockage, and improve the coal permeability. The mathematical model of gas drainage by large diameter borehole in fractured soft coal seam was established. Taking the 3205 working face in Shanxi Fenxi Zhongxing Coal Mine as the background, the numerical simulation was carried out by importing the mathematical model into COMSOL Multi-physics software to study the gas migration law around large boreholes with protective screen pipe in fractured soft coal seam. The results show that the gas flow velocity in section A of the borehole and pipe is between 0 and 0.25 m/s, and the gas flow velocity in the borehole is smaller than that in the pipe and screen hole. The gas velocity in the borehole is lower than that in the protective screen pipe. In the large diameter borehole, the protective screen pipe can support the collapsed coal and prevent the borehole blockage. The free gas in the borehole can be quickly extracted out through the screen holes, which ensures the gas extraction efficiency. The results provide a guidance for promotion and application of the technology of large diameter borehole with protective screen pipe in field.