The Study On Migration And Transformation Of Uranium During Coal Combustion

Uranium in coal is usually migrated to fly ash and bottom ash in the process of coal combustion, or enrichment in fine particles surface and then emissions to the atmosphere in forms of aerosol. The uranium in ash and atmosphere can be transformed into the water environment and biosphere through raining or leaching process, and this is harmful to human health and ecological environment. In southwest of China,uranium is enrichment greatly in some lignite, and the radioactive contamination caused by uranium can not be ignored in coal using process. Therefore a detailed study of uranium migration and transformation characteristics during coal combustion aims to reduce uranium release rate is great important.With expect to indicate the direction for further research, this paper studied the migration and transformation characters of uranium in coals which come from Xiaolongtan coal mine and Diandong power plant.Firstly, the occurrence of uranium in coal in different temperature and pressure is simulated by thermodynamics equilibrium software. Uranium is mainly combined with elements such as alkali metal(Na, K)and alkaline-earth metal(Mg, Ca, Sr, Ba) to generate the corresponding compounds during coal combustion. With the increase of temperature,uranium is mainly exists in forms of oxide(UO2.33(?)?UO2?UO2(uraninite)),and the valence of uranium showed a downward trend. a small amount of UO3(g) is generate when the temperature above 1600?.The release characters of uranium in coal is studied in the fixed bed experiment under the conditions of pyrolysis, gasification and combustion. The volatile rate of uranium is small in the above experiments. In the pyrolysis experiment,the release rate of uranium in coal increases with the increases of temperature. In the experiments of gasification and combustion, uranium release rate is decreased with the increase of temperature in the range of 700? to 1000?. The distribution and enrichment characteristics of uranium in the bottom and fly ash of Shangdu or Xilin power plant were studied. The results show that there is a slight enrichment behavior of uranium in fine particles.In the experiments of gasification and combustion, the release rate of uranium is decrease with the increase of the addition amount of CaCO3/CaO. A detailed analysis andexploration has been carry out aiming to find the reasons of causing the above phenomenon.The results shows that the uranium did compounds with alkali and alkaline earth metals at high temperature(1000?),and calcium uranium(CaUO4) has been found in coal ash with x-ray diffractometer(XRD) and environmental scanning electron microscopy(ESEM).Uranium is mainly combined with Ca to form calcium uranium to inhibit the volatilization of uranium in the process of coal combustion and CO2 gasification, and chemical reaction equation is: 6CaCO3+2U3O8+O2(g)?6CaUO4+6CO2(g).