Disastrous Mechanism and Concentration Distribution of Gas Migration in Fully Mechanized Caving Stope in Wuyang Coal Mine

The overrunning disaster of harmful gas tends to occur in the working face in thick coal seam with high gas concentration, as the fully mechanized caving stope has the characteristics of high mining intensity, high remnant coal, and high gas content. Therefore, the disastrous mechanism and concentration distribution of gas migration in fully mechanized caving stope are the theoretical basis for gas control scheme. Based on the 7607 working face in Wuyang coal mine, the gas emission quantity in working face is comprehensively analyzed by field measurement in this paper. The gas leakage field, oxygen concentration field, and gas concentration field in 7607 working face are simulated by establishing the equal proportional numerical model. Due to the increase of air leakage in working face caused by the high alley pumping drainage, the risk of coal spontaneous combustion is also analyzed, when gas extraction in goaf is carried out. The research results show that the gas drainage technology in high drainage roadway has a remarkable effect on the gas overrunning phenomenon. The gas concentration near the upper corner of the working surface has been reduced from 0.7%-1% to 0.5%. At the same time, it is necessary to pay attention to the risk of coal spontaneous combustion in the goaf for gas drainage in the high drainage roadway. The width of the oxidation zone in the goaf is about 25m deeper than that before the drainage. Research results provide the references for gas control technology and coal spontaneous combustion prevention in similar working faces.


Introduction
Gas overrunning disaster not only directly threatens the safety of coal miners but also restricts the production capacity in coal mine [1,2]. The coal output of fully mechanized caving stope is large, while the gas emission is uneven [3][4][5]. Meanwhile, the gas concentration in fully mechanized caving stope changes greatly in a short time, which is easy to cause gas accidents [6,7]. Long-term production practice shows that gas migration and gas concentration distribution are important parts of gas control in fully mechanized caving stope [8][9][10]. For coal mine with complex production conditions, such as high gas and thick coal seams, it is particularly important to understand the gas migration status in the working face and choose a reasonable gas control scheme [11][12][13]. Only by figuring out the factors affecting the gas concentration in fully mechanized caving stope can the occurrence of gas disaster be prevented, and the high yield and high efficiency of the mine be realized. Therefore, it is necessary to carry out the research on the disastrous mechanism and concentration distribution of gas migration in fully mechanized caving stope.
The gas outburst from the goaf to the working face is one of the main reasons for the gas overrunning disaster [14][15][16]. In high-gas mines, there are more remnant coal in goaf with low-position top coal caving mining technology, and the gap between the remnant coal is large [17,18]. Therefore, after gas is gushing out, it is easy to flow through the goaf and flow under the drive of air pressure. In the "U" type ventilation working face, when the air flow enters the stope from the inlet roadway, the gas in the goaf brings out to the working face, causing the gas concentration in the working face [19][20][21].
The gas drainage technology in high drainage roadway is widely used in China, which plays a good role in preventing the overrunning of gas concentration in the working face [22,23]. For the working face in coal seam with high gas and spontaneous combustion, gas extraction through high drainage roadway alleviates the gas emission disaster but increases the air leakage in the working face to a certain extent and accelerates the process of spontaneous combustion of remnant coal in goaf [24,25]. Xia et al. [26] studied the disaster mechanism of coexistence of gas and coal spontaneous combustion and the action mechanism of coupling effect. Chu et al. [27] analyzed the disturbance effect of roof roadway gas drainage on goaf spontaneous combustion area.
Therefore, it is of practical significance to study the characteristics of air leakage and the distribution of three zones for the prevention and control of coal spontaneous combus-tion in goaf. Therefore, aiming at the problem of gas overrunning in the high gas fully mechanized caving stope, this paper adopts the actual production conditions in the 7607 working   There are thirteen coal seams in the minefield, including two mineable seams, one most mineable seam, and ten locally and occasionally mineable seams, with a total thickness of 13.31 m and a coal content coefficient of 8.17%. Among them, 3# coal seam is located in the middle and lower part of Shanxi formation, with an average thickness of 5.75 m. The whole area is generally stable, which is the coal seam being mined. The ventilation mode in Wuyang coal mine is divided into diagonal type, and the ventilation method is the extraction type. The layout of 7607 working face in Wuyang coal mine is shown in Figure 1. Fully mechanized top coal caving is adopted in the working face, which has the characteristics of high yield and high efficiency. However, due to high mining intensity, fast advancing speed, and large gas emission in the working face, it is easy to cause gas overrunning in the upper corner or mine return air roadway, which brings great potential safety hazard for production.

Law of Gas Occurrence in Working
Face. Fitting the gas content measured in Wuyang coal mine to the buried depth of the coal seam, the relationship between the gas content of the coal seam and the buried depth of the coal seam is obtained. The corresponding relationship between the gas content in the 3# coal seam and the buried depth is shown in Figure 2.

Geofluids
The buried depth of coal seam at 7607 working face in Wuyang Coal Mine is 512 m-593 m. According to the formula of relation between gas content and buried depth, the coal seam gas content at 7607 working face is calculated to be 10.63 m 3 /t-13.95 m 3 /t.

Gas Migration Law and Coal Spontaneous
Combustion Risk under U-Type Ventilation The average depth of coal seam in the working face is 600 m, and the thickness is 6.06 m, and the mining height is 3 m, and the coal discharge is 3 m, and the raw coal gas content in the working face is 9.1 m 3 /t, and the air distribution volume is 4000 m 3 /min. On the basis of the U-shaped ventilation scheme, the 7607 working face adopts gas drainage technology in high drainage roadway. Taking the intersection of the return airway and the working face as the origin of the coordinates in the COMSOL numerical analysis software, the direction of mined-out area is the x -axis direction, and the inclination is the y-axis direction, and the vertical direction is the z-axis direction to establish the geometric model, as shown in Figure 3. The section size of intake airway and return airway is 17.5 m 2 (5 m × 3:5 m).
According to the actual situation in the 7607 working face, the strike length of the model is 200 m, and the inclination length is 227 m. The mesh of the working face area in the numerical calculation model is encrypted, shown in Figure 4. Tetrahedral mesh is used for mesh generation, the total number of meshes is 6 × 10 5 , and mesh encryption is carried out at the working face.
The intake airflow roadway is the entrance of speed, and the return air roadway is the free flow, and the high suction roadway is the mass exit, and the four sides which contact the high suction roadway and the goaf are the interface. The gas emission law in working face, the air leakage flow field in goaf, the distribution law of oxygen concentration field, and gas concentration field are calculated. The numerical simulation parameters are shown in Table 1.
According to the mass conservation equations, the momentum conservation equations, and the diffusion motion equations, the mathematical model of oxygen and gas transport in the working face and the mined-out area is established, combined with the actual conditions in fully mechanized caving stope. The numerical simulation software COMSOL is employed to solve the model.

Distribution of Seepage Parameters in Goaf.
Under the effect of air leakage pressure difference in goaf, the air flows between these pores. The surface of coal adsorbs the oxygen in the air leakage and reacts with it to form spontaneous combustion, which is the main reason for spontaneous combustion of residual coal in goaf. The distribution of porosity in goaf is closely related to the lithology of coal roof, compressive strength of coal seam, and distribution of mine pressure in goaf. The distribution of seepage parameters in goaf is shown in Figure 5.
Due to the collapse of the overlying strata in the goaf, the stress distribution in the goaf meets the "O" ring distribution theory. At the end of the working face and two roadway positions, the coefficient of coal and rock crushing expansion and porosity and permeability are larger, while the coefficient of coal and rock fragmentation and porosity and permeability are smaller in the middle of the goaf due to stress compaction. In order to further analyze the gas seepage environment in the goaf, the seepage parameters at the position of z = 1:5 m at the floor of the goaf are further extracted, shown in Figure 6.
There are some differences in the distribution of seepage parameters in different positions of goaf. From the goaf near the end of the working face and two roadways, the porosity is larger, and the porosity in the middle and deep is smaller. Due to the collapse of overlying strata, the crushing expansion coefficient, porosity, and permeability at the inlet and return air roadway near the working face and the upper and lower angle of the deep solid wall boundary of the goaf are all large; the influence range is about 20 m. The porosity near the end of working face reaches the maximum value of

Distribution of Flow Field and Concentration Field in
Goaf. The change of gas composition in goaf, especially the content of oxygen and gas, depends on the distribution of air leakage and airflow. Therefore, the study of goaf seepage flow is an important theoretical basis for residual coal spontaneous combustion and gas emission and accumulation. The study of goaf seepage distribution provides an important basis for the study of goaf gas distribution and residual coal spontaneous combustion. When the inlet air volume is 4000 m 3 /min, the distribution of air leakage velocity at the floor z = 1:5 m of goaf is shown in Figure 7.
The air leakage rate in the goaf is relatively large at about 20 m near the end of the working face on the strike in Figure 7(a). When it reaches 50 m deep into the goaf, the air leakage rate gradually decreases and is less than 1 m/min. The reason for this phenomenon is the "O" circle distribution of coal mass in goaf. With the development of mining, the roof of goaf collapses. Therefore, there are a lot of air leakage into the goaf in the upper and lower corners in the goaf. The oxygen concentration distribution in the goaf is uneven in Figure 7(b). Due to the velocity inertia of the air leakage flow on the inlet side, the oxidation depth on the inlet side is greater than that on the return side.
In order to analyze the law of gas distribution in the stope and effectively control the gas level in the working face, the three-dimensional gas concentration distributions in goaf and floor are shown in Figures 7(c) and 7(d). Due to the dilution effect of the air leakage flow on the gas in the goaf, the gas concentration in the air inlet side is significantly lower than that in the air return side. Because the influence scope of air leakage is limited, the dilution effect of air flow on gas is relatively weak, when it reaches the middle and deep part of goaf. Therefore, the gas concentration in the middle and deep part of goaf is higher.

Division of Spontaneous Combustion Oxidation Zone in
Goaf. In order to further analyze the location of spontaneous combustion oxidation zone in goaf, the "three zones" in the goaf are divided by 0.004 m/s (0.24 m/min) and 0.001 m/s (0.1 m/min). The distribution range of spontaneous combustion oxidation zone based on air leakage wind speed is shown in Figure 8(a). The spontaneous combustion oxidation zone in goaf is distributed in the range of 50 m-150 m behind the goaf. On the inlet and return wind side, the isoline of 0.004 m/s is deep to about 120 m in the goaf strike. Based on the analysis of the distribution law of oxygen concentration in goaf, the oxygen concentration contour with z = 1:5 m at the floor is extracted for analysis, in order to clearly understand the distribution pattern of spontaneous combustion oxidation zone in goaf.
The distribution of oxidation zone in goaf with 18%-10% oxygen concentration as the division index is shown in Figure 8(b). The white area near the working face is the tropical zone, the colored area between 18% and 10% oxygen concentration contour is the spontaneous combustion zone, and the area with oxygen concentration less than 10% is the asphyxiation zone. When the spontaneous combustion zone extends from the air inlet side of the goaf to the air return side of the goaf, its width gradually decreases. The width of the oxidation zone on the air return side of the goaf is relatively narrow, compared with the central part of the goaf and the air inlet side. In one side of the inlet roadway, the range of spontaneous combustion zone is 75 m-160 m, and the width of spontaneous combustion zone is 85 m, and the width of spontaneous combustion zone is 85 m. The spontaneous combustion zone in the middle of goaf is 85 m-150 m, and the width of spontaneous combustion zone is 65 m. The range of spontaneous combustion zone on one side of the

The Distribution of Flow Field and Concentration Field in
Goaf. In order to further control the gas level in the goaf, gas extraction is carried out in the high pumping roadway above the roof of the goaf, and the extraction rate is 260 m 3 /min.
Firstly, the air leakage velocity distribution at z = 1:5 m in the goaf floor under the condition of high pumping roadway is shown in Figure 9(a). In order to further analyze the change rule of oxygen concentration on the inlet and outlet side under the action of high pumping roadway, the changes of oxygen concentration at 30 m away from the inlet and outlet side are extracted, shown in Figure 9(b).
The oxygen concentration at the air inlet side drops to 18% after entering the goaf about 110 m, and the oxygen concentration at the air inlet side about 170 m is 10%. The three-dimensional gas concentration distribution in goaf under the effect of high pumping roadway is shown in Figure 9(c). In order to clearly understand the gas concentration distribution in working face and goaf, the contour map of gas concentration at the goaf floor z = 1:5 m is extracted, shown in Figure 9(d).

Division of Spontaneous Combustion Oxidation Zone in
Goaf. Compared with the leakage field under U-shaped ventilation, the air leakage velocity range at the return air corner becomes larger due to the influence of gas extraction in the high pumping roadway; especially, the air leakage velocity at the position of the return air roadway is significantly increased. The three-zone distribution of goaf under the condition of gas extraction in high pumping roadway is shown in Figure 10(a), which is divided by air leakage speed as an indi-cator. The spontaneous combustion oxidation zone in goaf is distributed in the range of 60 m-160 m behind goaf. The red area is the tropic zone, the blue area is the suffocation zone, and the gradient area between them is the spontaneous combustion oxidation zone. Compared with the distribution of air leakage velocity under U-type ventilation, the depth of oxidation zone in goaf under high extraction roadway is larger. On the inlet and return wind side, the 0.24 m/min isoline is deep to about 140 m of the goaf strike. In Figure 11(b),    the white area near the working face is the scattered tropical zone, the colored area between the 18% and 10% oxygen concentration contour lines is the spontaneous combustion zone, and the area with oxygen concentration less than 10% is the asphyxiation zone. The range of spontaneous combustion zone on one side of the air inlet roadway is 90 m-175 m in Figure 10  The volume fraction of oxygen and gas on both sides of air intake airway and air return airway in goaf before and after gas extraction in high drainage roadway is compared, as shown in Figures 11 and 12. It can be seen from Figure 11 that the air leakage in goaf is increased after gas extraction in high drainage roadway, and the oxygen volume fraction in the inlet and return air side is increased significantly. When the depth of goaf is 50 m, the oxygen volume fraction at the inlet airway and return airway side increases by 2% and 5%, respectively. Gas drainage in high drainage roadway    Figure 11: Volume fraction of oxygen in intake (a) and return (b) airway before and after high drainage. 13 Geofluids obviously reduces the gas volume fraction in goaf, and the gas volume fraction on both sides of inlet and return airway decreases obviously, as shown in Figure 12. When the depth of goaf is 50 m, the gas volume fraction at the air inlet and air return side decreases by 10% and 20%, respectively. Especially, the gas concentration near the upper corner of the working face is obviously reduced, which effectively inhibits the gas emission from the goaf to the working face.

Conclusions
(1) The buried depth of coal seam is the key factor affecting gas occurrence. The relationship between coal seam gas content and coal seam buried depth is obtained, the growth gradient of gas content is 4.2 m 3 /t/hm, and the coal seam gas content in the 7607 working face is predicted to be 10.63 m 3 /t-13.95 m 3 /t. Through the measurement of gas content, it is proved that the gas occurrence law is consistent with the actual situation (2) The measured gas emission from the coal wall and falling coal mass in the working face is 26.09 m 3 /min, the gas entering the goaf with air leakage is 1.12 m 3 /min, and the air leakage in the working face is 581 m 3 /min. The simulation results show that the range of spontaneous combustion oxidation zone in goaf in 7607 working face is 50 m-160 m at the inlet roadway side, 50 m-150 m at the central goaf, and 20 m-150 m at the return air roadway side (3) By measuring the gas accumulation concentration in different areas in the working face and determining the natural oxidation zone (seepage area) in goaf in numerical simulation, the gas accident area is comprehensively determined. Through the gas control effect monitoring, it is confirmed that the gas concentration in the upper corner of the fully mechanized top coal caving stope in Wuyang coal mine is not more than 0.8%, and the gas concentration in the return air roadway is not more than 0.8%.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.