Gas migration and spontaneous combustion hazard zone division in goaf of roof-cutting entry retaining working face under “U+L” ventilation

To systematically investigate the gas migration laws in the roof-cutting and pressure relief gob-side entry retaining (abbreviated as root-cutting entry retaining) face and goaf under the “U+L” ventilation system, the 011206 roof-cutting entry retaining face of Wuhushan Mine, CHN Energy Wuhai Energy, was taken as the research object. Using a combination of theoretical analysis, experimental research, numerical simulation and engineering verification, the gas migration laws, volume fraction distribution characteristics and spontaneous combustion prevention technology in the goaf of the roof-cutting entry retaining face were comprehensively analyzed. Key oxygen consumption parameters of No.12 coal samples were obtained through closed oxygen consumption experiments. On this basis, a numerical model of the goaf was established to simulate and analyze the evolution laws of gas migration and volume fraction distribution under different working conditions. The results show that increasing the air distribution ratio of the entry retaining intake airway or raising the total air supply volume of the working face will significantly expand the coverage of the high-oxygen (a relatively high volume fraction) zone in the goaf, resulting in an obvious increase in the width of the oxidation zone (when the air distribution ratio increases, the width of the oxidation zone on the entry retaining side increases by up to 116.7%; when the air supply volume increases, the oxidation zone width increases by 32.3%); effectively reducing the air leakage rate from the roof-cutting entry retaining into the goaf can significantly narrow the high-oxygen zone and reduce the oxidation zone width, thereby effectively inhibiting the formation of spontaneous combustion hazard zones. The simulation reveals a critical threshold: when the air leakage rate drops below 5%, further reducing the air leakage rate has a very limited effect on narrowing the oxidation zone and improving the spontaneous combustion control effect, while technical difficulty and cost surge with little benefit. Numerical simulation and engineering verification show that the high-risk zones of spontaneous combustion in the goaf are mainly distributed in three areas: the affected zone of the entry retaining intake airway, the 5-15 m section along the roof-cutting entry retaining, and the area behind the working face. With oxygen volume fraction of 15.53% as the critical value, the “three zones” of spontaneous combustion were divided. It is determined that the suffocation zone on the intake side is 119 m away from the working face, and the maximum width of the oxidation zone is 96 m.