Experimental study on the mechanism of radon exhalation during coal spontaneous combustion in goaf

Abstract There is a good positive correlation between the high-temperature environment formed by coal spontaneous combustion in goaf and the radon exhalation in coal. The higher the temperature is, the greater the radon exhalation will be. Increasing concentrations of released radon gas will accumulate in an enclosed goaf and ultimately migrate upwards to form a radon anomaly on the ground surface. Based on this principle, the surface-based radon detection method can determine the location of underground hidden heat sources by detecting high-concentration radon areas on the ground. At present, the surface-based radon detection method has been applied in multiple fire zones as an effective technique. However, the research on the mechanism of radon exhalation during coal spontaneous combustion is insufficient, which severely restricts the detection accuracy of heat source locations of coal spontaneous combustion. In this study, radon exhalation from coal samples with different degrees of metamorphism (lignite, long-flame coal, coking coal and lean coal) at different temperatures was measured, and six main factors affecting radon exhalation (moisture, specific surface area, crack development, minerals, temperature, and gas production) were further explored. The experimental results show that with the increase of temperature, the radon exhalation from coal samples increases first and then decreases. The dissolution of radon in water and the adsorption and closure of radon in capillary pores are the main existing ways of radon in coal. When the temperature rises, the radon dispersion caused by pore water evaporation and the radon migration in cracks and pores caused by coal pyrolysis are the main reasons for the significant increase of radon exhalation.