## 2 Experiment and Methods
### Low Oxygen Source Analysis  
The gas in the return air corner and inside the goat was sampled and analyzed. The main components of the gas were determined by gas chromatography. As shown in Table 1, the O2 concentration in the air return corner is 17.004%, the N2 concentration is 82.685%, and the CO2 concentration is 0.289%. The highest nitrogen concentration in the goat reaches 95.538%. Therefore, it can be determined that the low oxygen in the air return corner is mainly caused by the high nitrogen concentration. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Place** & **O2 (\%)** & **N2 (\%)** & **CO (\%)** & **CO2 (\%)** & **CH4 (\%)** \\ \hline Return air corner & 17.004 & 82.685 & 0.0056 & 0.289 & 0.0171 \\ Goaf (25 m) & 16.599 & 83.058 & 0.0072 & 0.336 & 0 \\ Goaf (219 m) & 8.2686 & 91.103 & 0.005 & 0.6039 & 0.0193 \\ Goaf (489 m) & 3.0502 & 95.538 & 0.0009 & 1.4036 & 0.0071 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Gas analysis of goat. Figure 1: Three-dimensional schematic diagram of 22104 working face. The gas at the return air corner comes from the fresh air flow of the working face and the gas emitted from the goat. The sources of the gas inside the goat include the gas produced by the desorption of the broken coal after the coal seam is mined, and the gas consumed and produced during the chemical reaction of the left coal inside the goat. Samples were taken from 12 # coal and 22 # coal respectively. Gas components were determined through coal desorption experiment and gas chromatography analysis. The results are shown in Table 2. The average proportion of N\({}_{2}\), CH\({}_{4}\) and CO\({}_{2}\) in the desorption gas is 92.76%, 5.98% and 1.21% respectively. It can be determined that the main form of coal seam gas emission is N\({}_{2}\) emission. The N\({}_{2}\) extracted from broken coal is one of the sources of low oxygen gas in the goat. The Shendong mining area belongs to shallow buried deep, easy to spontaneous combustion, near coal seam group multiple disturbance mining [45], and the remaining coal in goaf is easy to be oxidized. A certain amount of residual coal samples was taken from 22104 goats of the Shangwan Mine, and the coal oxidation experiment was carried out by means of oxidation heating experiment device, and the corresponding laws of oxygen consumption and gas production were analyzed. The change results of gas concentration in the process of heating and oxidation of coal samples are shown in Figure 2. In the test temperature range, the gases produced by oxidation include CO, CH\({}_{4}\), CO\({}_{2}\), C\({}_{2}\)H\({}_{4}\), C\({}_{2}\)H\({}_{6}\), and C\({}_{3}\)H\({}_{8}\). Meanwhile, the change of O\({}_{2}\) concentration during the oxidation process of residual coal was recorded. During the whole experiment, the oxygen concentration decreased continuously and the production of low oxygen gas increased continuously. As the residual coal in the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Sampling Locations**} & \multicolumn{3}{c}{**Gas Composition**} \\ \cline{3-5}  & & **CH\({}_{4}\) (\%)** & **CO\({}_{2}\) (\%)** & **N\({}_{2}\) (\%)** \\ \hline
1 & Auxiliary transport roadway (1500 m) & 4.22 & 1.63 & 94.01 \\
3 & 12 \# coal seam & Air return roadway (1570 m) & 6.56 & 0.60 & 92.80 \\
5 & Auxiliary transport roadway (1750 m) & 5.72 & 2.25 & 92.03 \\
7 & Air return roadway (1750 m) & 7.34 & 0.80 & 91.81 \\
8 & 22 \# coal seam & Air return roadway (1850 m) & 6.25 & 1.70 & 92.05 \\
9 & & Auxiliary transport roadway (2850 m) & 5.79 & 0.29 & 93.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Analysis results of coal seam gas components. Figure 2: Regular of gas generation by oxidation heating of residual coal. goaf is in the broken state, the contact area between coal and oxygen increases, leading to a more sufficient oxidation reaction. The coal absorbs and consumes oxygen, and the oxygen proportion in the goat decreases continuously, forming a high nitrogen and low oxygen environment.