Study on the radon gas transmission rate in overburden rock under the influence of coal spontaneous combustion

In the field of coal mine safety, radon detection technology is mainly used to identify the sources of coal spontaneous combustion (CSC) fires. Here, a method for calculating the rate of transmission of radon gas in the rock layer overlying the CSC region is proposed. The radon transmission rate in rock as a function of temperature was investigated through laboratory-scale experiments. In addition, numerical simulations were carried out in COMSOL to calculate the changes in the radon concentration and radon transmission rate in the overlying rock layer after 30, 60, and 85 days of CSC. Finally, field experiments were conducted by drilling samples at different locations in the mine site and analyzing the levels of radon daughter nuclei present. The results show that radon release peaks at 250 °C, reaching 3.1-3.9 times the value at 30 °C. Simulated peak radon transmission rates after 60 and 85 days of CSC are 52.7% and 40.8% higher, respectively, than after 30 days. Field-measured average transmission rates in fire zones are 42.8%-50.5% higher than in the non-fire zone. A previously unreported nonlinear profile of radon transmission rate with distance is revealed: the rate increases sharply to a peak from the fire source, then gradually declines and stabilizes. The enhancement effect of CSC weakens with increasing distance. Porosity variations at layer interfaces induce only minor fluctuations in transmission rate. The results of this study can help to improve the current understanding of the mechanism of long-distance radon transmission in rock formations and guide the accurate application of radon detection technology in the field.