Classification warning for spontaneous combustion of residual coal in goaf under air leakage conditions - A case study

• Quantify the threshold of CO in goaf and return corner at different stages of coal spontaneous combustion under the impact of air leakage. • Propose a collaborative classification method for predicting coal spontaneous combustion using CO composite indicators. To enhance the accuracy of index gases in predicting coal spontaneous combustion severity in goaf under air leakage conditions, this study conducted thermogravimetric and temperature-programmed experiments to investigate the characteristics of coal mass variation, types of index gases, and their concentration evolution during the coal spontaneous combustion process. The relationships between coal spontaneous combustion stages, coal temperature, and index gases were established. Combined with on-site measurements of gas distribution characteristics in the goaf, the early warning thresholds of carbon monoxide in both the goaf and return air corner during different coal spontaneous combustion stages were quantified. The results indicate that coal spontaneous combustion can be divided into six stages: latent, slow oxidation, fast oxidation, critical, pyrolysis fission, and combustion. The emergence temperatures of carbon monoxide, ethylene, and acetylene are 30°C, 120°C, and 231°C, respectively. The inflection point temperatures of the ignition carbon oxidation coefficient are 70°C and 160°C. The oxygen concentration in the goaf ranges from 21%-17%, 17%-13%, and 13%-9%, while the carbon monoxide threshold concentrations during the latent stage of coal spontaneous combustion are 180 ppm, 300 ppm, and 500 ppm. In the latent and slow oxidation stages of coal spontaneous combustion, the carbon monoxide thresholds in the return air corner are 57 ppm and 293 ppm, respectively. Based on different indicator gases and their thresholds, coal spontaneous combustion is classified into five warning levels: grey, blue, yellow, orange, and red. The research results provide a scientific basis for the early warning of coal spontaneous combustion in similar working faces.