Effect of additives on mercury partitioning in wet-limestone flue-gas desulfurization

The wet-flue-gas desulfurization(FGD)process plays an important role in removing water-soluble flue-gas components such as sulphur dioxide(SO_(2))and oxidized mercury compounds.Under the reducing environment of the FGD,there is the possibility of re-emission of the already absorbed mercury(Hg)to the gas phase,which may be diminished by the utilization of specific additives.In this study,the effect of two different additives on Hg re-emission from the aqueous phase and Hg partitioning in gypsum and filtrate of a lab-scale wet-limestone FGD is investigated.Furthermore,the behaviour of additives in the presence of different halides is studied.The studied additives are TMT 15■as a sulphidic precipitating agent,which forms non-soluble mercury compounds,and activated lignite(AL)as a carbon-based sorbent,which adsorbs Hg compounds from the aqueous phase.TMT 15■has no significant effect on SO_(2) absorption;on the other hand,addition of AL improves the SO_(2)-removal efficiency by up to 30%.Using both additives,Hg re-emission is suppressed in all the experimented cases except for AL in the absence of halides,in which Hg re-emission shows no change.Thus,the need to form nucleophilic oxidized mercury compounds in the slurry for the adsorption of oxidized mercury on AL can be concluded.Usage of both additives improves Hg retention in the slurry to different extents.It is shown that,for the additive-free slurries,the Hg-adsorption capacity of the solid fraction of the slurry is the limiting parameter.Moreover,the utilization of both additives results in a significant increase in the Hg concentration of solid fraction.The correlation between redox potential and partitioning of Hg in the slurry is presented by comparing the change in the redox potential of slurries when additives are used.

Sulphite measurement and its influence on Hg behaviour in wet-limestone flue-gas desulphurization

The most abundant and typical reducing agent for oxidized mercury in the slurry of wet flue-gas desulphurization(FGD)is the absorbed sulphur dioxide(SO_(2)),which is present as different species of bisulphite or sulphite,depending on the pH of the slurry.In this study,two different measurement principles for continuous sulphite measurement in the slurry of lab-scale FGD were investigated to check their feasibility to be implemented in a wet FGD.The first method is based on light absorbance at the characteristic wavelength of sulphite measurement using a spectrophotometer and,in the second method,sulphite is measured as sulphur dioxide using a gas sensor.In addition,the correlation of sulphite concentration and mercury(Hg)in the slurry can be shown by measuring sulphite semi-continuously.It was concluded that using a spectrophotometer leads to distorted results.In contrast,measuring sulphite as SO2 in the gas phase proved to be more selective.The implementation of the measurement technique in the lab-scale FGD showed promising results for sulphite measurement.Thus,the correlation of Hg and sulphite concentration could be shown at different synthetic slurries containing different halides.Using a slurry without halides demonstrated the ambivalent influence of sulphite in reactions involving Hg,in which sulphite acts as a ligand for Hg complexes as well as a reducing agent,depending on the existing concentration.However,in the presence of halides,the role of sulphite was less significant.