Study on the formation mechanism of a radon source during coal spontaneous combustion in a goaf

Abstract

Surface isotope radon detection technology (SIRDT) is an effective physical exploration technology that provides a solution for detecting hidden underground fire sources. However, because of the inaccessibility of deep underground spaces, current research on the formation mechanism of radon sources during coal spontaneous combustion in a goaf is insufficient, which severely restricts the use of SIRDT to detect such fires. In this study, the quantitative relationship between the coal temperature and radon exhalation rate was revealed through radon exhalation experiments during the low-temperature oxidation of coal, which provides measurement data for the radon source term of coal spontaneous combustion in a goaf. A multi-physical, coupled numerical model of coal spontaneous combustion in a goaf was constructed, and the intrinsic dynamics of radon migration and the formation mechanism of the radon source in the process of coal oxidation were thoroughly investigated. The results show that the radon distribution in a goaf is because of the competition between air leakage and the “porous chimney effect.” When the temperature of the coal was low, air leakage dominated the radon transportation in the goaf, causing radon to accumulate in the area close to the air-return side. When the temperature of the coal increased, the thermal buoyancy and vortex phenomena appeared in the high-temperature area of the goaf, at this point, the “porous chimney effect” took the leading role. The updraft generated by the thermal buoyancy effect rapidly carried the radon to the top of the goaf. The vortex promoted airflow from the surrounding area of coal spontaneous combustion into the interior of the “porous chimney,” which accelerated the process of coal spontaneous combustion and increased the amount of radon transported upward; thus, further intensifying the accumulation of radon at the top of the goaf. This accumulation becomes a source of radon that continues to migrate toward the surface.