Abstract
The suction negative pressure is an important factor affecting the spontaneous combustion of coal around a borehole. Because the mechanism of suction negative pressure in the gas extraction process remains unclear, a constant suction negative pressure is often used in coal mines, leading to a low efficiency of gas extraction in deep coal seams. Moreover, the coal body easily undergoes spontaneous combustion during the extraction process, which is not conducive to safe mining. To study the effect of the suction negative pressure near the end sealing section, a numerical model of the combustion process around a borehole under the influence of suction negative pressure was established using COMSOL. The variation laws of the gas seepage velocity, oxygen concentration, and coal temperature in the borehole cycle were analyzed, and the gas suction negative pressure under different sealing parameters was optimized to ensure efficient gas extraction and prevent the spontaneous combustion of coal. The results showed that the negative pressure of extraction provides the power required for gas seepage into the borehole, and the gas flow rate increases with increasing negative pressure of extraction, exhibiting a linear growth trend. The range of the coal suffocation zone around the sealing section decreases with the increase in the negative pressure. With the extension of the gas extraction time, the oxygen concentration decreases rapidly, and the inflection point advances with the increase in the negative pressure. When the negative pressure of gas extraction is <40 kPa, the range of the high-temperature area around the block increases with the negative pressure of gas extraction. Based on the present situation of the spontaneous combustion induced by gas drainage in the Pingdingshan No. 10 Coal Mine, different sealing parameters should be set with different negative pressures of extraction, and the negative pressure of extraction should not exceed −18 kPa when the sealing depth is 20 m and the sealing length is 8 m in the 24130 working face. These parameter settings can help effectively prevent spontaneous combustion during the extraction process.
Highlights
The negative pressure of gas extraction is one of the main indices affecting the progress of gas extraction in coal mines [1,2,3,4,5]
In the process of gas drainage along the bed, the negative pressure provides the power for gas extraction, and the gas and surrounding gas continuously flow into the extraction pipe through the fissured coal around the plugging section, realizing gas extraction
In the distance range of 0–7 m in the plugging section, the seepage air flow velocity is low. In this area, combined with the distribution law of the oxygen volume fraction, the oxygen volume fraction is greater than 8%, and when the wind velocity is low, the coal body is prone to spontaneous combustion
Summary
The negative pressure of gas extraction is one of the main indices affecting the progress of gas extraction in coal mines [1,2,3,4,5]. Based on the actual sealing situation of the 24130 working face of Pingdingshan No 10 Mine and considering gas diffusion, permeability variation, coal oxidation reaction, and porous media heat transfer, this study established a gas drainage coupling model under different drainage negative pressure conditions. The simulation results reveal the variations in the gas concentration, seepage distribution, and coal temperature around the borehole; help analyze the negative pressure of drainage under different sealing parameters; and lay a theoretical foundation for ensuring safe and efficient gas drainage. The combustion of air in the fractured coal body around a borehole is a complicated process, and the volume fraction of the gas in the porous medium is affected by multiphysical parameters.
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