Study On The Re-emission And Suppression Of Mercury In The Flue Gas

Ontario hydro method (OHM), a mercury speciation method certificated by American Society for Testing and Materials (ASTM), was applied to collect speciated mercury in the flue gas at the four full-scale coal-fired utilities with different electricity capacities (90MW,205MW,190MW and 450MW). Mercury transformation and removal efficiencies through SCR, ESP and WFGD in these four utilities were evaluated. It was found that Hg0 oxidation efficiencies across SCR ranged from 55.2% to 70.4%. When the flue gas passing SCR and no passing SCR, The mercury capture efficiencies by ESP were the 0.9%～2.9% and 5.3%～9.6%, The total mercury removal efficiencies across WFGD were respectively 30.9%-77.4% and 37.8%～89.5%. The Hg0 re-emission phenomena (Hg2+being reduced to Hg0) were observed in two WFGD of utilities, i.e the HgO concentrations at WFGD out were higher than they were at WFGD inlet.A lab-scale simulated WFGD system was set up to investigate the Hg0 re-emission, and effects of the recirculating slurries, waste water collected from a full-scale WFGD and home-made slurries (CaO, CaSO3, CaSO4 and Na2SO3) were performed. There is a significant correlation between Hg0 re-emission and sulfite ion in home-made or natural WFGD slurries. Further, effects of major operational parameters (temperatures, pH values of slurries, oxygen gas concentrations and different carrier gases) and some metal ions on Hg0 re-emission were intensively investigated. The effects of some ions respectively including Sn2+, Pb2+, Fe2+, Ni2+, AsO2-, Cu+, Fe3+and Cu+on Hg0 re-emission were studied. Low chemical valence ions being oxidized by Hg2+led to Hg0 re-emission, and the order of Hg2+reduction capacity was Sn2+>Pb2+>Cu+>Fe2+>As02>Ni2+. In the followed-up Hg0 re-emission suppression tests, the performances of five additives, including H2S, Na2S4, TMT15, NaHS and HI were evaluated. It was found that the produced precipitates HgS and HgI2 can prevent Hg2+from being reduced to Hg0. The cheaper home-made additives also had better effects on suppressing Hg0 re-emission. There also existed the significant correlation between oxidation-reduction potential (ORP) of slurries and Hg0 re-emission. Only the slurries with minus ORP values can result in occurrence of Hg0 re-emission.Figure [67] table [21] reference [151]...