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To study the distribution law of coal spontaneous combustion zone in goaf of two intake and one return ventilation working face, the oxygen concentration under different working conditions during the mining of the 8301 working face of Tashan Coal Mine was simulated, the danger zone of spontaneous combustion of coal was determined and corresponding fire prevention measures were proposed. The spontaneous combustion danger zone of coal in the goaf of two intake and one return ventilation working face is large, and the spontaneous combustion zone reaches 96 m when the mining distance is 150 m; nitrogen injection can greatly change the distribution of spontaneous combustion zones and reduce the spontaneous combustion of coal in the goaf. For the two intake and one return ventilation working face, nitrogen injection should be considered on both sides of the goaf; increasing air volume allows the boundary of the spontaneous combustion zone to extend deep into the goaf and increase the distance of the front end from the working face.

For the special case above the working face of Tashan Coal Mine, the numerical simulation software was used to study the distribution of oxygen molar concentration in the goaf under different mining distances and different amounts of nitrogen injection and determine the coal spontaneous combustion danger zone in the goaf. Results show that long-distance extraction of gas makes the coal spontaneous combustion danger zone larger. As the mining distance increases, the spontaneous combustion danger zone gradually widens. When the mining distance is 50 m, the rate of widening the spontaneous combustion zone suddenly becomes larger. The ratio of the width of the spontaneous combustion zone on the inlet side to the length of mining distance is 180% higher than that of the mining distance of 30 m to 50 m, and the return side is 140% higher, causing larger risk of spontaneous combustion of coal; nitrogen injection can greatly reduce the coal spontaneous combustion danger zone in the goaf.

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The numerical simulation is used to research the influence of nitrogen injection on spontaneous combustion in goaf. The spontaneous combustion mathematical model on the coupling of air flow field, oxygen concentration field, and residual coal temperature field was established with nitrogen injection in goaf. Then the software of numerical computation was programmed by Finite Volume Method. Combined with the example, the distributions of air flow field, oxygen concentration field and residual coal temperature field at different nitrogen injection volume were obtained by the software. The results show that the nitrogen injection could effectively prevent the spontaneous combustion fire in goaf and the highest temperature in goaf decreased with the nitrogen injection volume increasing. Finally, the accuracy of the numerical simulation was verified by the temperature observation in field. The achievement of this research is of theoretical and practical significance for the prevention of coal spontaneous combustion in goaf. [ABSTRACT FROM AUTHOR]

Energy migration theory of spontaneous combustion in goaf was advanced basis for energy conservation principle, then the energy balance equation of goaf was established to obtain the highest temperature equation of goaf and the equation of the minimum safe speed of coal workface. Combined with the field condition, the influence factors of the highest temperature and the minimum safe speed were analyzed, such as advancing speed of workface, air supply a-mount and nitrogen injection. The results show that accelerating speed, reducing air supply amount and increasing nitrogen injection quantity all can effectively reduce the spontaneous combustion dangerous in goaf, but the fire prevention effect of nitrogen injection will decrease with the increase of the advancing speed. In addition, the minimum safe speed becomes smaller for the air volume reduced or nitrogen injection quantity increased, which is better for preventing the spontaneous combustion in goaf. The achievement of this research is of theoretical and practical significance for pre-judgment of the highest temperature of goaf and formulation of the reasonable fire prevention measures in goaf. [ABSTRACT FROM AUTHOR]