Experimental Study On The Mercury Re-emission Characteristics In Wet FGD Systems Of Power Plant

Mercury emission from power plants gets more attention, because of its air pollution.The ways using existing pollution control equipment to remove mercury get moreapplications. Wet FGD system can effectively remove divalent mercury in the flue gas, butelemental mercury can hardly be removed. However, even part of mercury absorbed by thescrubber would be reduced to elemental mercury and release to the flue gas again, resultingin lower total mercury removal efficient. This is the second release of mercury. Oneobjective of this work is to explore mercury re-emission characteristics by changing theparameters of wet FGD slurry, to seek the best conditions for reducing mercury release.Another objective is to study the White Mud’s desulfurization effects and the situations ofmercury re-emission in White Mud slurry.Laboratory-scale experiments on the reduction of Hg2+were performed in a simulatedscrubber through adding Hg2+in the slurry. The effectsof the initial Hg2+concentration,slurry temperature, pH value, different ions including SO32-, SO42-and Cl-, and additivessuch as Na2S and H2O2on the re-emission of Hg2+were investigated. The results showsthat:①Initial mercury concentration has little effect on the mercury re-emission ratio.②Slurry temperature promotes the reduction of mercury.③pH value has significant impacton mercury release. Low or high pH value would result in lowerre-emission levels.④Oxidized mercury is reduced by aqueous S(IV). Higher concentrations of sulfites ionsinhibit oxidized mercury transformation to elemental mercury.⑤Sulfate ion inhibitsmercury re-emission by the formation of HgSO4. But its concentration has little effect onmercury re-emission level.⑥The complexes are formed when adding chloride ion to theslurry and result in very low mercury re-emission rate.⑦Na2S inhibit the reduction ofmercury greatly by generating HgS. Hydrogen peroxide also lead to lower re-emissionlevel.Calcium-based White Mud can be used as wet FGD system desulfurizer because of itslarge amount of production and strong alkaline. Experiments on White Mud slurry’sdesulfurization were carried out compared with limestone in order to confirm the feasibility of using White Mud. Slurry temperature and inlet SO2concentration were investigated. Theresult shows: White Mud can achieve even better desulfurization effect than limestone atthe beginning. However, White Mud’s effective desulfurization time is shorter thanlimestone’s. Mercury re-emission can be decreased by changing slurry temperature and pH,or adding Na2S and chloride ion to slurry.