Release and the interaction mechanism of uranium and alkaline/alkaline-earth metals during coal combustion

•The release and speciation transformation of uranium during coal combustion were studied.•Alkaline and alkaline-earth metal play an important role in the release of uranium.•The interaction mechanism of uranium and alkaline/alkaline-earth metals was proposed.

Uranium is one of the typical naturally occurring radioactive materials in coal. The release and speciation transformation of uranium was investigated at various combustion temperatures, and the thermodynamic modelling was performed to complement the experimental work. The results showed that the uranium release ratio did not increase consistently with the combustion temperature increasing, where the highest release ratio occurred at 500 °C. At the temperature range of 500-900 °C, the uranium release ratio obviously decreased, which could be attributed to the formation of uranate with the interaction of alkaline/alkaline-earth metal compounds in coal. However, some of the thermal unstable uranate was decomposed and released at the temperature above 1000 °C, while part of them remains stable in the combustion product even when the sample was heated at 1200 °C. Further, the interaction mechanism of uranium and alkaline/alkaline-earth metals during coal combustion was proposed based on the experimental and modelling results. This study will provide valuable information for understanding the primary factors and processes that affect the release of uranium during coal combustion.