## 2 Experiment and Methods
### Low Oxygen Causes Analysis  
When the pressure of the goaf is greater than that of the working face, the low oxygen gas in the goaf will push out to the working face. The negative pressure ventilation mode causes the lowest pressure at the return air corner, so the low oxygen phenomenon at the return air corner is the most serious. Therefore, the change in the pressure difference between the working face and the goaf is the main reason for the low oxygen gas emission. The mine ventilation system delivers fresh air flow to the working face from the surface. The change of static pressure of the working face synchronizes with the change of atmospheric pressure of the surface. The temperature difference between day and night in Shendong mining area is large. The atmospheric pressure changes obviously in one day. When the surface atmospheric pressure decreases, the static pressure of the working face also decreases. This results in a large pressure difference between the working face and the goaf. The abnormal emission of low oxygen gas in the goaf leads to the change of O2 concentration in the return air corner. The change rule of oxygen concentration in daily return air corner of 22104 working face affected by surface atmospheric pressure and temperature is analyzed, as shown in Figure 3.  
According to Figure 3, from 0 to 13 o'clock on the same day, oxygen concentration varied from 17.3-18.4%, while atmospheric pressure and temperature showed an overall upward trend. From 13 o'clock to 17 o'clock, oxygen concentration gradually decreased and low oxygen appeared. At 16 o'clock, oxygen concentration reached the lowest value of  
Figure 3: Diurnal variation data of oxygen concentration in the return airway corner. 15.8%. At the same time, atmospheric pressure showed a downward trend and temperature showed a rising trend. From 17 o'clock to 24 o'clock, oxygen gradually increased to more than 17%, while atmospheric pressure showed a rising trend and temperature showed a declining trend. The pressure difference on that day was 400 Pa and the temperature difference was 8.2 \({}^{\circ}\)C. The results show that the change of surface atmospheric pressure and temperature will lead to the change of oxygen concentration in the corner of return air. It is greatly affected by the numerical changes of pressure difference and temperature difference in a short time. The mining speed of 22104 working face in Shangwan Coal Mine is 16 m/d, the speed of the shearer is fast, the average thickness of the coal seam is 6.6 m, and the full-height mining method is adopted. When mining is too fast, it is easy to cause the goaf coal seam roof to not collapse in time. Then, the periodic pressure step distance will be too long. When mining to a certain distance, and a large area of overhanging roof suddenly collapses, low oxygen gas in the goaf will be squeezed into the mining space, resulting in abnormal gas concentration at the return airway corner. The oxygen concentration in the return air corner of different pressure step distance regions was statistically compared, as shown in Figure 4. When the 22104 face is advanced from 1332 m to 1524 m, the average periodic pressure step distance is 18.45 m. When advancing from 1785 m to 2200 m, the average periodic pressure step distance was 12.69. The lowest oxygen concentration value was 13.6% and the variance was 1.16 in the area from 1332 m to 1524 m; the lowest oxygen concentration value was 15.9% and the variance was 0.76 in the area from 1785 m to 2200 m. The results show that the smaller the periodic pressure step, the smaller the dispersion degree of oxygen concentration, the higher the data stability, and the smaller the abnormal gas emission degree of goaf. Therefore, the periodic pressure step distance is also one of the factors affecting the oxygen concentration in the corner of the return air.