Release And Transportation Of Mercury In Solid Wastes From Coal Fired Power Plants

Huge amounts of mercury are emitted from coal fired power plants every year.Research on estimating the regional mercury emission from coal fired power plants becomes very necessary. However, the estimation results using various models are always disagreed with each other because there are different factors needed for different models. It is very necessary for the study of mercury emission to compare the results from different models. In addition, it is also very critical to analyze the contents and environmental behaviors of mercury in FGD gypsum for the emission estimation. In our study, three estimation models were applied and compared. The species of mercury in gypsum were analyzed using sequential extraction procedure.Firstly, The concentrations of mercury in coal, fly ash, slag and gypsum were determined by atomic fluorescence spectrometry after wet digestion. The distribution characteristics of mercury in various waste samples and coals were studied and discussed. The transportation of mercury in solid wastes was studied. Mercury in slag accounted for about 0.13%-0.74% of total mercury. Mercury in fly ash and gypsumaccounted for 0.30%-63.80% and 2.95%-41.56% of total mercury,respectively. Three estimation models including Mass Balance(MB) model, Emission Modification Factors(EMF) model, and Flue Calculation(FC) model were applied to calculate the emission amount of mercury from the coal fired power plants. The calculated emission amounts generated from three models were compared and discussed. The results indicated that the values from different models for the same plant were largely discriminated.Secondly, the three models were applied to calculate mercury emission amounts from power plants in Hebei province. The distribution of mercury in the power plants(A-G) was similar. The elemental fraction(F4) ranging from 47.52% to 78.9%was the highest one, while the ion-exchangeable fraction(F2) and sulfide fraction(F5)were much lower. In FGD gypsum samples from H, I, J power plants, mercury in the water soluble fraction and ion-exchangeable fraction accounted for 82.87%, 70.39%,54.83% of the total amounts, respectively.Thirdly, the transformation of mercury species under ultraviolet irradiation(UV)and dark conditions was studied. The results demonstrated that the proportion of mercury in different fractions was in the following order: elemental fraction>water soluble fraction>soluble fraction>ion-exchangeable fraction>sulfide fraction.Under ultraviolet irradiation, the percentage of water soluble Hg(F1) decreased gradually with the aging time and ultraviolet irradiation intensity, while mercury inelemental fractions increased.At room temperature, we found that mercury escaped from desulfurization gypsum and the release of mercury in gypsum was promoted by ultraviolet irradiation.Meanwhile,the mercury speciation in gypsum was changed under UV irradiation.