Study on Influencing Factors of Low Oxygen Formation in Return Air Corner of Shallow Buried Thick Coal Seam Working Face

Low oxygen concentrations frequently occur at the return-air corner during the mining of shallow-buried thick coal seams, which poses a safety and health hazard to underground workers. This study investigated the causes of low oxygen concentrations and potential control measures at Working Face 5011 of Madiliang Coal Mine. Low-temperature coal oxidation experiments, tracer-gas tests, and numerical simulations were conducted to investigate the effects of surface air leakage, ventilation rate, and nitrogen injection rate on the oxygen concentration distribution in the return air corner. The results showed that the oxygen concentration at the return-air corner decreased as the surface air leakage rate increased. The higher the surface air leakage the lower the oxygen concentration, 18.2% and 17.0% in the minimum and maximum leakage situations, respectively. Increase in the volume of air supplied and N2 injection rate leads to a rise in the concentration of oxygen followed by a decrease. From the results obtained, the optimum air supply volume of the working face is determined as 2200 m3/min and the optimum nitrogen injection rate is 1000 m3/h. In this case, the highest oxygen level in the return air corner is approached and the spontaneous combustion risk zone in the goaf is relatively small. The results show that the mechanism which leads to low oxygen concentration in the return air corner under combined action of the various factors is clarified, and they also provide a theoretical basis to prevent and control the low oxygen condition in working face.