Thermal radiation-driven coal spontaneous combustion and its risk classification and early warning

Abstract

Coal spontaneous combustion is one of the major mine disasters and also a primary trigger of secondary disasters such as mine gas and coal dust explosions. To reveal the combustion characteristics and risk evolution mechanisms of thermal radiation-driven coal spontaneous combustion, a cone calorimeter system was employed to conduct combustion tests on coal samples with four particle sizes (0.28, 0.158, 0.111, and 0.086 mm) under three heat fluxes (30, 50, and 75 kW·m −2 ). The dynamic evolution patterns of heat release, mass loss, gas products, smoke production, and residue structure was analyzed. Furthermore, by integrating the second-order response surface methodology with a dynamic radiation absorption efficiency model, a two-dimensional classification and evaluation system for coal spontaneous combustion risk was established, centered on combustion driving force and energy utilization efficiency. The results indicate that heat flux is the dominant factor influencing coal combustion behavior. Both the heat release rate and total heat release increased critically with increasing heat flux. At a heat flux of 75 kW·m −2 , the peak of heat release rate reached 31.05 kW·m −2 , approximately 91.6% higher than the 16.2 kW·m −2 recorded at heat flux of 30 kW·m −2 . The total heat release increased from 9.7 MJ·m −2 to 18.1 MJ·m −2 , corresponding to an increase of 86.6%. The combustion mode transitioned from a heat transfer-limited regime under low heat flux to an oxygen diffusion-controlled regime under high heat flux. Particle size affected the combustion process through specific surface area effects, packing thermal resistance, and char layer formation, with finer particles effectively enhancing the thermal response rate and combustion intensity. A heat flux of 50 kW·m −2 was identified as the critical range for incomplete combustion and heavy smoke generation, whereas 75 kW·m −2 promoted complete combustion of volatiles and secondary oxidation of soot particles, thereby significantly reducing smoke production. The second-order response surface model exhibited good predictive performance, and the predicted peak heat release rate, serving as the combustion driving force, increased monotonically with heat flux, whereas the dynamic radiation absorption efficiency decreased from approximately 0.28 at 30 kW·m −2 to 0.13 at 75 kW·m −2 , revealing that intense combustion under high heat flux is accompanied by severe energy utilization loss. The four-quadrant hazard classification model, constructed based on multi-factor coupling, can effectively distinguish among real fire risk, potential fire risk, relative safety, and inflated fire risk scenarios, enabling a quantitative and refined determination of coal spontaneous combustion risk.