Emission And Migration Of Mercury In Wet Flue Gas Desulfurization And Processing Of Byproducts

Mercury have been identified as a potential hazard to both human health and environment. Coal-fired power plant is one of the largest sources of anthropogenic mercury emission. Removal of elemental mercury(Hg0) is the the biggest impediment of reducing mercury emission in coal-fired power plant. Wet Flue Gas Desulfurization system(WFGD) can remove SO2 and Hg2+ effectively, but the phenomenon that Hg2+ is reduced to Hg0 decreases the mercury removal efficiency of coal-fired power plant. In order to improve the mercury removal efficiency of coal-fired power plant and WFGD, this paper experimentally studied the emission and migration of mercury in WFGD and processing of byproducts.Univariate methods were used to study the effect of p H on Hg0 release. When p H was between 3.0 and 8.0, the lower the p H, the more Hg2+ reduced by S(IV), resulting in the increase of Hg0 release. When p H is below 4.0, S(IV) is mainly in the form of HSO3-, then HSO3- and Hg2+ will react with Hg2+ to form Hg SO3 and Hg HSO3+. Hg SO3 and Hg HSO3+ are easy to decompose, causing massively Hg0 release. As the p H rises, the concentration of SO32- increases, then SO32- will react with Hg SO3 to form Hg(SO3)22-. Hg(SO3)22- is more stable, reducing the rate of Hg0 release. The rate of Hg0 release could be predicted using the quantitative relationship of S(IV) concentration, p H and time. The quantitative relationship was supported by the mathematical fitting methods and experiments.In order to study the effects of metal ions on Hg0 release, this study used thermodynamic theory to study the mechanism that metal ions and S(IV) synergistic affected on Hg0 release. At alkaline environment, Fe2+, Mn2+ and Co2+ caused Hg0 release. At acidic environment, Fe2+ caused Hg0 release. Mn2+ and S(IV) had a synergistic effect on promoting Hg0 release. Co2+ and S(IV) had the same effects. Ni2+ and S(IV) also had the same effects.This paper explored the migration characteristics of mercury in the three-phase, then proved the roles of additives in mercury release in gypsum calcination processes. CO2, O2 and NO3- increased the mercury proportion of vapor phase, meanwhile reduced the mercury proportion of solid and liquid phase. Ca SO3 and Cl- significantly inhibited mercury transporting to the vapor phase, and Cl- substantially increased the mercury proportion of liquid. TMT, DTCR and Na HS significantly reduced the mercury proportion of vapor and liquid phase, and increased the mercury proportion of solid phase. The effect of mercury sequestration by TMT was the best, and Na HS had the worst effect on mercury sequestration in gypsum slurry. The remaining mercury content of the calcined gypsums with additives were higher than those without additives. Temperature programmed experiments showed that when Na HS, TMT and DTCR were add to gypsum slurry, Hg compounds in gypsum were mainly Hg S, Hg3(TMT)2 and Hg(DTCR)2, respectively. The peak of decomposition temperature and the range of decomposition temperature were gradually reduced in the order of Hg S, Hg3(TMT)2, Hg(DTCR)2 and Hg Cl2. The decomposition temperature of Hg3(TMT)2 ranged from 170 ℃ to 290 ℃, The decomposition temperature of Hg(DTCR)2 ranged from 110℃ to 270℃.