## 3 Results and Discussion
### Formation Mechanism of Low Oxygen in Return Air Corner of Working Face  
The results show that the low oxygen in the return air corner of 5106 working face is caused by multiple factors, as shown in Figure 12. The sources of low oxygen gas and the low oxygen problem in the corner of return air have the following reasons: (1) The geological report of the well field and the observation of goaf gas in working face 5106 show that the gas in the lower 5 coal seam is in the CO\({}_{2}\)-N\({}_{2}\) zone, and the main components of the gas in the goaf are N\({}_{2}\) and CO\({}_{2}\); (2) Under the influence of mining, the goaf of working face 5106 and its overlying goaf 1112 formed a crack in the communication ground. Under the action of atmospheric pressure, the low oxygen body in the goaf 1112 will accelerate to discharge to the goaf 5106. (3) In the process of mining, continuous and open nitrogen injection is adopted to prevent fire in the goaf. With the continuous nitrogen injection, there is a large amount of nitrogen with high concentration in the goaf; (4) Under the action of air flow and periodic pressure, low oxygen gas in the goaf moves towards the working face and gathers in the upper corner, resulting in low oxygen phenomenon. Coordinated Prevention and Control Technology System of Low Oxygen in Return Air Corner and Coal Spontaneous Combustion  
The reason for the low oxygen in the corner of return air is that the surface air leakage will continuously transport the low oxygen gas to the working face through the goaf, resulting in the low oxygen concentration in the corner of the working face. But low oxygen gas in preventing coal spontaneous combustion in goaf has played a positive role in Influence of methane on the prediction index gases of coal spontaneous combustion: A case study in Xishan coalfield, China. If the oxygen concentration in the goaf is blindly increased, the problem of low oxygen in the corner of return air can be solved, but this will cause a sharp increase in the risk of spontaneous combustion of coal in the goaf. In order to solve this contradiction, this paper creatively uses the pressure air belt to deal with the low oxygen in the corner of the return air. At the same time, comprehensive fire prevention measures such as plugging in the corner, nitrogen injection and grouting are used to restrain the oxidation of the left coal in the goaf and the low oxygen in the corner of the return air and the coal spontaneous combustion. 1. Build a pressure zone to increase oxygen concentration in the upper cornerw  
In order to solve the problem of local low oxygen in the return air corner, the concept of pressure air belt was proposed from the perspective of increasing fresh air, that is, by introducing compressed air in a certain area (such as the return air corner) to form a three-dimensional pressure air strip. This method can solve the problem of local hypoxia caused by eddy current phenomenon in the area where low oxygen gas is concentrated out (such as the corner of return air). The specific method is as follows: Use the compressed air supplied to the mine by the ground air compressor, install the end control device at the end of the pressure air pipe (as shown in Figure 13), and use the high-pressure hose to guide the downhole pressure air to form a pressure air belt in the upper corner (as shown in Figure 14), so as to increase the oxygen concentration in the upper corner. Arrange four vertical pressure air ducts in the upper corner (as shown in Figure 15), and set 20 to 30 pressure air outlet holes on each pressure air duct to form a pressure air belt in the upper corner, thereby increasing the oxygen concentration in the upper corner. Figure 12: Formation mechanism of low oxygen in return air corner. Figure 13: Connecting the pressure air belt device. On 4 June 2021, the equipment of return air corner pressure belt was processed and debugged.