## 2 Overview of the Working Face and Depressurized Gas Extraction Process Methods  
### Overview of the Working Face  
Dalong Coal Mine is located in the central and western part of Tiefa Coal Field, with a strike length of 4.43 km and a tendency width of 3.58 km. Its 1504 working face in East 2 is located in the northern part of the 15 mining area in East 2, which is located within the protection range of the pressure relief of the 1501 and 1502 working faces and the 1503 working face, with an elevation of \(-\)516.5 to \(-\)554.2 m. Its mining type is traditional underground mining. The north side of this working face is bounded by the protective coal pillar of the second level 1504 Transportation Shun in East 2, the south side is bounded by the protective coal pillar of the second level 1504 Return Shun in East 2, the east side is bounded by the protective coal pillar of the East 2 lane, and the west side is bounded by the F6 fault. The location of the 1504 working face in East 2 is shown in Figure 1.  
The coal seams in the working face of East 2 1504 are stable. The overlying coal seam is the 14\(\mu\) coal seam, covering an average height of 38.65 m, and the height of the coal seams ranges from 1.45 to 0.82 m, with an average of 1.14 m. The overlying coal seam is the 16\(\mu\) coal seam, covering an average height of 37.78 m, and its height ranges from 1.50 to 0.82 m, with an average of 1.13 m. The thickness of this coal seam ranges from 1.60 to 2.20 m, with an average of 1.90 m. This coal seam is a medium-thickness coal seam with low variation and a simple structure. The main types of coal mined in the area are non-stick coal and long-flame coal, with good quality and a stable coal seam. The length and width of the working face are 2131 m and 235 m, respectively; the area is 482,390 m\({}^{2}\), the covering height is 37.7 m, and the coal storage capacity is 1,770,000 t.  
Figure 1: The location of the 1504 working face in East 2.  
The inclined long wall backward coal mining method is adopted for the 1504 working face in East 2, and collapsing methods are used to manage the roof of the mining air zone. The working face is designed considering the use of the U-shape ventilation method, in which the transporting chute brings in air and the returning chute carries air away. According to the actual situation of Dalong Coal Mine, the actual air volume required during the mining of the 1504 working face in East 2 is a minimum 1000 m\({}^{3}\)/min and cannot exceed 3884 m\({}^{3}\)/min. The absolute gas outflow during normal mining is 20.66 m\({}^{3}\)/min; the gas outflow from falling coal and the coal wall is 5.7 m\({}^{3}\)/min.  
The old top of the 1504 working face in East 2 of the Dalong Coal Mine is coarse sandstone, the direct top is siltstone, the pseudo-top is charcoal mudstone, and the direct bottom is siltstone. The compressive strength of the direct top is 22.52 kPa; therefore, the top plate of the 1504 working face in East 2 is a medium-hard top plate. The respective formulas for calculating the maximum heights of the bubbling down zone and the fissure zone are as follows:  
\[H_{m}=\frac{100M}{4.7M+19}\pm 2.2 \tag{1}\]  
\[H_{l}=\frac{100M}{1.6M+3.6}\pm 5.6 \tag{2}\]  
where \(H_{m}\) is height of the fallout zone (m), \(H_{l}\) is the height of the fracture zone (m), and \(M\) is the coal seam mining height, taken as 2.35 m.  
According to Equations (1) and (2), the maximum height of the fallout zone is 4.6-9.0 m and the maximum height of the fracture zone is 23.01-34.21 m.