Influence of coal secondary oxidation on the concentration field of composite goaf

The secondary oxidation of oxidized residual coal in composite goaf is one of the important causes of coal mine fire in recent years. A quantitative analysis of the impact of secondary oxidation on the spontaneous combustion in the composite goaf from the perspective of gas concentration field is of great significance to the safe production of coal mine. It is conducive to putting forward the fire prevention scheme when the condition of re-mining is considered. In this study,the SIIN₂6-9 working face of the Liujia coal mine in Inner Mongolia Pingzhuang Energy Co.,Ltd. was taken as the engineering background. The constant temperature difference guidance method was employed to measure the macroscopic spontaneous combustion characteristics of original and oxidized residual coal that were both collected from the Liujia coal mine. The experimental results showed that the secondary oxidation process of the collected residual coal was more active. Under the conditions of the same temperature and oxygen concentration,the rate of oxygen consumption and CO generation was higher for oxidized residual coal. According to the measured macroscopic spontaneous combustion characteristics,a method to determine the coal-oxygen reaction source term was developed which considered various kinds of residual coals in the goaf. Subsequently,a numerical model to predict the spontaneous combustion in goaf under dynamic advance condition was established. The developed model was validated by comparing the numerical results and field measurements. The numerical results showed that the secondary oxidation had a significant effect on increasing the rates of oxygen consumption and CO generation. In comparison with the goaf with a single coal seam,the oxidizing zone of the composite goaf with secondary oxidation process moved toward to the working face at a distance of about 27 m,and the maximum CO concentration increased 30%. The secondary oxidation of residual coal is a potential threat of coalmine fire that cannot be ignored.