| Mercury is one of flue gas pollutants with the high toxicity and bioaccumulation ability. Hence, mercury pollution control has attracted significant concerns. Compared with Hg2+and Hgp, it is difficult to remove Hg0from flue gas by the conventional pollutant control devices. Therefore, Hg0removal is becoming the key point in purification of flue gas containing Hg0. Non-thermal plasma produces highly reactive species, which can effectively oxidize Hg0into mercury compounds, and the oxidized mercury compounds will be further removed by the hind wet flue gas desulfurization system.However, if the flue gas containing Hg0directly passes through the plasma region, the generation of active species will be inhibited because part of the energetic electron produced in electric discharge would collide with the pollutants in flue gas, leading to a decrease in the energy yield. In addition, a plasma reactor with large volume is required when purifying flue gas of high flow rate, giving rise to a significant increase in the construction investment on the flue gas treatment equipment. Considering these limitations and drawbacks of the conventional plasma methods, active species injection by non-thermal surface plasma discharge was proposed to oxidize Hg0in the present study, where a surface plasma discharge reactor was inserted in the simulated flue duct and the generated active species was injected into the flue gas, obtaining high oxidation efficiency of Hg0. The method of active species injection avoided the collision between energetic electrons with pollutants in the flue gas efficiently, thus enhancing the energy yield and meanwhile achieving the purpose of purifying a large flow rate flue gas with a small volume plasma reactor. The main work was conducted in terms of characteristics and mechanisms of Hg0oxidation, and synthetic effects of discharge plasma with catalyst in Hg0oxidation. The detailed work and results were summarized as follows:1. Method and mechanism of oxidizing Hg0in coaled-fired flue gas by active species injection were investigated. The results showed that the active species injection exhibited prominent oxidation ability for Hg0, and oxidation efficiency of Hg0and energy yield reached81.3%and20.7μg kJ-1at3.9J L-1, respectively. Compared with the direct oxidation method, Hg0oxidation efficiency using the active species injection was increased by8.2%and the energy yield was approximately6-7times higher than that in the direct oxidation system. Long-lived active species (such as O3and N2metastable states) played a main role in Hg0 oxidation. Through theoretical and experimental analyses, it was found that the deposits on the surface of simulated flue duct and the mercury compounds in gas phase were mainly HgO.2. The effects of operation parameters on Hg0oxidation were studied. Approximately91.3%of Hg0was oxidized under the conditions of optimized gas parameters and reactor’s configuration parameters. An increase in the discharge length, large electrode diameter, high injection flow rate, and a decrease in Hg0initial concentration and flue gas temperature were favorable for oxidizing Hg0. Water in the flue gas facilitated O2+involving in Hg0oxidation, which enhanced Hg0oxidation; while the presence of NO showed an inhibitory effect on Hg0oxidation because of the competition reaction of NO and Hg0for O3. Chlorine species were formed through active species reacting with HCl that enhanced the oxidation of Hg0. HCl was found to play important role in Hg0oxidation when treating flue gas containing O2, H2O, SO2, NO and HCl. According to the optimized operation parameters, a up-scaled active species injection equipment was built, which was comprised of four surface discharge plasma reactors in parallel, and exhibited good ability for treating flue gas containing Hg0of high flow rate.3. In order to improve the energy yield of active species injection, a combination of active species injection and catalysis system based on CeO2-WO3/TiO2(CeWTi) was proposed to oxidize Hg0in the flue gas. In the combined system, approximately86.6%of Hg0was oxidized at2.6J L-1and energy yield reached32.9μg kJ-1. Compared with the plasma system, Hg0oxidation efficiency was improved by39.9%, and energy yield was increased by approximately14.8μg kJ-1and0.8times. Compared with the single discharge and catalysis system, the combined system had excellent oxidation performance and widened the temperature windows of catalyst (80-350℃). H2O exhibited slight inhibition effect on Hg0oxidation. The combined system showed good resistance to SO2, and also overcame the problem of NO inhibitory effect on Hg0oxidation in the active species injection system. NH3inhibited Hg0oxidation because of the competition of NH3and Hg0for the surface active spot and active oxygen. However, the coexistence of NO with NH3significantly mitigated the inhibition of NH3on Hg0oxidation.In summary, active species injection and the combination with catalysis were effective in oxidizing Hg0and improving the energy yield. Moreover, active species injection could achieve the objective of oxidizing flue gas containing Hg0of large flow rate using a small volume reactor. Therefore, the two systems exhibited promising prospects for practical application. |
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