| The pollution of mercury has attracted greate concern at home and abroad, and the release of mercury from flue gas is one of the main resources.It was known that 50-95% of the flue gas Hg2+ could be removed synergistically by calcium-based wet flue gas desulfurization system( Wet Flue Gas Desulfurization, WFGD). But under some circumstances, a portion of the oxidized mercury captured by WFGD system could be reduced to Hg0, causing secondary pollution and resulting in lower mercury removal efficiency. Considering the fact that Hg2+ in calcium-based wet flue gas desulfurization systems could be easily reduced to Hg0 and then released into the atmosphere, we conducted a series of experimental researches to explore the impact of various factors on mercury reduction under laboratory conditions, and the reduction mechanism of Hg2+ in desulfurization system also explored in this paper, so as to obtain valuable reference for process and operation optimization to control mercury pollution synergistically with SO2 by calcium based wet flue desuifurization system.With a simulating slurry system of wet flue gas desulfurization in the laboratory, the effects of p H, temperature, Cl-, SO32-, Ca2+ and other factors on Hg2+ reduction were investigated. The results indicated that increasing the temperature and lowering the p H could promote mercury reduction. Excess SO32- with Hg2+ could form Hg(SO3)22-,which is a more stable complex than Hg SO3 and thus more difficult to release Hg0. Cl- would inhibit Hg2+ reduction obviously, while Ca2+ and SO32- formed insoluble Ca SO3 which would consume SO32-, thus promoting mercury reduction.On the basis of the single factor experimental research, Box-Behnken response surface methodology was used to design 43 factorial experiments and Design Expert 8.0.6 software was used to fit the results so as to obtain divalent mercury reduction under different conditions in wet FGD systems. By fitting the experimental data, we got a multiple regression prediction model of Hg2+ reduction rate as a function of temperature(A), p H(B), Cl-(C), SO32-(D).The response surface model analysis showed that higher temperature accelerated Hg2+â†'Hg0 reaction and promoted Hg2+ reduction;while the interaction of Cland SO32- was significant, with decreased Hg2+ reduction rate when the concentrations of Cl- and SO32- increased.This study also studied the impact of the contact time, p H, reaction temperature, initial mercury concentration, and gypsum dosage on the concentration of mercury in solution. Through the analysisof the intake of mercury by gypsum, the adsorption isotherm, andadsorption kinetics, the affinity between gypsum and mercury was discussed. The results showed that: gypsum crystallization had a certain adsortion, coprecipitation effect on mercury. The intake amoute of mercury to gypsum increased with time, but tended to be stable after 18 h of reaction, which was in line with the law of second-order kinetic model. Under alkaline conditions, it could be observed that a higher amount of mercury was taken in, with the maxmum intake amount up to 8.72ug/g when p H = 13.The uptake amaunt was found to decrease with gypsum content. Moreover the affinity of gypsum and mercury could be well described with Langmuir EXT1 model in a Hg2+ concentration range of 60-1000 ug/L when p H = 6.Combined with XPS analysis, we discussed the main mechanism of mercury reduction in desulfurization slurry. The main pathways were through mercury-sulfur complexes formed by Hg2+ and SO32-, which eventually released Hg0. By thermodynamic calculation of the main reactions of divalent mercury reduction process in desulfurization system, we found that the reduction could proceed with deep limits. |
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