Study on suppression characteristics of coal spontaneous combustion risk by nitrogen injection in extra-thick and high top coal caving

Aiming at the problems of large amount of residual coal and serious air leakage in the goaf of extra-thick and high top coal caving mining face, the gas products, oxygen consumption and heat release characteristics of coal oxidation spontaneous combustion process under the conditions of without nitrogen injection and nitrogen injection were analyzed by temperature programmed oxidation experiment and heat release test experiment. By using numerical simulation software, the inhibition of nitrogen injection on the risk zone of coal spontaneous combustion in goaf under different nitrogen injection positions was analyzed and the nitrogen injection plan are optimized. The results show that the process of oxidation and spontaneous combustion of coal can be inhibited by nitrogen injection. The temperature point of each gas product is delayed by 25-30 ℃, and the oxygen consumption rate and heat release are greatly reduced. The heat release without nitrogen injection is 5.67 times that of nitrogen injection. When nitrogen is injected at different positions, the risk area of coal spontaneous combustion in goaf is different and contracted. When nitrogen is injected at ports 2, 3, 4 and 5, that is, the intake side is 60 m, 80 m, 100 m, 120 m away from the working face when nitrogen is injected, the inhibition area of risk area accounts for 12.28%, 32.71%, 39.71% and 44.09% respectively. The change of temperature inhibition in goaf is mainly concentrated in the zone near the nitrogen injection port. Nitrogen injection ports 2 and 3 joint optimized plan can effectively inhibit 47.25 % of the risk area of coal spontaneous combustion in goaf.