Study on cause mechanism of corner oxygen deficiency in mining face of extra-thick spontaneous combustion coal seam

Taking the 8105 working face of Madaotou Mine as the engineering background, the numerical calculation model of the flow field of the mining face in the extra-thick and spontaneous coal seam was established to analyze the oxygen consumption rate in the goaf, the air intake volume of the working face, and the nitrogen injection(nitrogen injection amount and nitrogen injection position) of the buried pipe in the goaf on the distribution of oxygen concentration in the stope. The research results show that: the low-oxygen environment produced by low-temperature oxidation of the residual coal in the goaf is the precondition of oxygen deficiency in upper corner of working face; with the oxygen consumption rate increases, the oxygen concentration near the working face and the upper corner decreases, and the upper corner oxygen-poor area gradually expands. With the increase of the air intake of the working face, the oxygen concentration of the working face gradually increases, the oxygen concentration of the upper corner gradually decreases, and the oxygen-poor area gradually increases. When the air inlet is greater than 2 778 m3/min, the oxygen-poor area in the upper corner tends to be stable, but the oxygen concentration is still gradually decreasing. With the increase of nitrogen injection amount, the oxygen concentration of the working face and the upper corner drops rapidly. When the nitrogen injection amount is greater than 2 825 m3/h, the pressure of the nitrogen injection port and the working face is out of balance, and a large amount of nitrogen flows into the working face to form low oxygen environment, and at the same time, the degree of oxygen depletion near the upper corner is aggravated; with the depth of the nitrogen injection position, the oxygen concentration of the working face and the upper corner changes little.