Study on the pore structure and radon release characteristics of coal in northern China

Identifying the release characteristics of radon (Rn-222) in coal mines is critical preventing cancer risks for coal miners and coal fires. The present investigates the pore structure characteristics of coal samples from eleven coal mines in northern China, using low-temperature nitrogen adsorption (LTNA) test, combined with the radon exhalation rate in coal. The findings of the study reveal that the N-2 adsorption isotherms of all the coal samples fall under the inverse S type, with micropores dominating in low-rank coals and mesopores dominating in the medium and high-rank coals, due to the separation of organic matter and quartz by shrinkage of micro-components and the orderly arrangement of aromatic rings as a result of ring condensation and thermal cleavage. The pore diameters of coal samples show similar distribution characteristics for sizes >2 nm, represented by a single peak near the pore diameter of 3 nm. Ash yield controls the mesopore and micropore volumes of medium and high-rank coal samples. The radon emission rate displays positive linear correlation (r(2) = 0.87) with micropore volumes of analyzed coal samples due to the infillings of free radon in micropores. The radon element is derived by uranium decay, which causes a greater radon exhalation rate of coal mines in areas near the uranium mines. The results of the present study could be helpful to understand the influence mechanism of radon emission processes in coal, which provides an important basis for reducing cancer risks for coal miners and predicting coal fires.