| Coal-fired power plant flue gas is considered as one of the main anthropogenic sources. In China, nearly 40% of the annual total emitted mercury has been from coal-fired utilities. Mercury in flue gas is present in three forms:elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate-bound mercury (Hgp). Unlike oxidized mercury (Hg2+) and particulate-bound mercury (Hgp), elemental mercury (Hg0) is more difficult to be removed by the existing air pollution devices (APCDs) than others because of its insoluble and highly volatile. Therefore, how to effectively remove Hg0 is the key to control the mercury emissions. The removal of elemental mercury from coal-fired plant flue gas by electrostatic precipitators (ESP) electric field was explored in this paper. In order to enhance the oxidation efficiency of Hg0 by ESP electric filed, oxidants were introduced into the system.Two simulated ESP reactors were used in this research. One simulated ESP reactor (static reactor) was used to study the reaction kinetics between Hg0 and oxidants. Meanwhile, another ESP reactor (dynamic reactor) was used to study the Hg0 removal in fluidized flue gas by oxidants. When the reaction time, temperature and discharge voltage was 1.5s,413K and 9kV, respectively, Hg0 oxidation rate can be achieved 85.6% in air balance. That indicates the oxidation of Hg0 by using simulated ESP electric field is effective.03 and O from the ionization of O2 from air play an important role in elemental mercury removal. With the increase of oxygen concentration and the discharge voltage, Hg0 removal efficiency will increase accordingly. H2O and NO show great inhibitory effect in the removal of Hg0 while SO2 not. The reaction time and reaction temperature have important influence on Hg0 conversion.HC1 was introduced into reaction system to enhance the removal of Hg0 in ESP electric field. In the discharge voltage of 9kV, when the concentration of HC1 and the reaction temperature is 80ppm and 413K, Hg0 conversion rate was up to about 98%. During UV static experiment, in the discharge process, there is a significant peak at the place of 330nm. It proves to be chlorine exists in the reaction system. We conclude that Cl2 and Cl were generated in the process of the discharge which played a major role in Hg0 removal.O2 or H2O have a role in promoting while SO2 or NO inhibiting the conversion of Hg0. In simulated flue gas conditions, when the energy density of 753J/L (voltage 9kV), Hg0 conversion rate closes to about 80%.S2Br2 in simulated ESP electric field can greatly reduce the use of oxidants, and the conversion rate of elemental mercury has also been significant risen. When the discharge voltage is 9kv, only 2ppm of S2Br2, Hg0 oxidation rate was up to 70%; improving S2Br2 concentration to lOppm, Hg0 oxidation rate increases up to 90%. The half-life of Hg0 during static experiment was about 100s, compared to HC1, it has been greatly improved. By studying the impact of other components from flue gas on mercury removal, it founds that higher concentrations of O2 inhibited Hg0 removal efficiency obviously; low concentrations of NO did not inhibit Hg0 removal efficiency significantly, while high concentrations of NO inhibit Hg0 removal greatly. The existing of H2O did not inhibit Hg0 removal efficiency significantly, and it is beneficial for Hg0 oxidation and stabilization to some extent. Considering that WFGDs were widely used in coal-fired power plants. Co-oxidation of Hg0 by ESP electric field- S2Br2 is favorable and is appropriate for existing treatment devices.ESP has been widely used in the air pollution control. It seems to be a good choice to remove mercury by ESP. Sulfur halides could enhance the removal of Hg0, and most oxidation products are stable mercuric sulfide. Using sulfur halides as oxidants can also reduce the use of halogens and the possibility of a secondary pollution. Therefore, it appeared to be a promising technique to enhance the removal of Hg0 by ESP and sulfur halides. |
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