Research on evolution law of index gases produced by the coal spontaneous combustion based on wavelet analysis

Determining the law of gas changes in a complex environment and understanding the internal correlations are key to establishing a technical system for the prevention and control of the spontaneous combustion of coal. For changes in the volume fractions of CO and O2 in different areas of the working face, this study determined the evolution process, periodic distribution of the volume fractions of CO and O2 in different areas and the correlation between the two gases by analyzing the multiscale evolution law based on wavelet transform to thereby evaluate the sensitivity of different areas to the spontaneous combustion of coal. The research findings indicate that the volume fractions of CO and O2 on the working face have multiscale effects, with periodic changes in different time scales. In particular, the main period distributions of the two gases in the same area are similar for large time scales and significantly different for small time scales. In addition, the wavelet coefficient oscillation characteristics of the volume fractions of CO and O2 reflect the fluctuation process of the original volume fraction of the gases. The difference in the wavelet coefficient fitting equation in terms of the initial amplitude and the initial phase in the first main period reveals the correlation between different areas and the spontaneous combustion of coal. In particular, information about the gas in mined-out areas is highly correlated with spontaneous combustion, and that in the high drainage roadway are least correlated. The correlation coefficients between spontaneous combustion and the mined-out area, the area inside the bag wall in the upper corner, the area outside the bag wall in the upper corner and the high drainage roadway are 1, 0.16, 0.12 and 0.09, respectively. According to these correlations and the distribution of the "three zones" of spontaneous combustion, accurate division of the dangerous areas of spontaneous combustion is proposed. The study on the multiscale evolution law of the gas produced by the spontaneous combustion of coal provides technical support for the early warning of spontaneous combustion and improves the efficiency of preventing and controlling spontaneous combustion.