Experimental Study On Flue Gas Mercury Oxidized By Molybdenum Modified Catalysts

As the largest artificial source of mercury emission, Hg emission from coal-fired power plants attracts more and more attention. Using the air pollutants control devices to reduce the mercury emission from plants is a potential economical method. The main idea is utilizing the wet flue gas desulfurization device (WFGD) to remove Hg2+. The premise of using this method is to oxidize Hg0to improve the concentration of Hg2+in the flue gas. Exploration an efficient mercury oxidation catalyst for reducing Hg emission from coal-fired power plants has the value of engineering application and scientific research.Research of denitrification suggested that the addition of MoO3to titanium-vanadium catalyst could enhance the ability of resistance to sulfur and arsenic of the catalyst. Therefore, the role of MoO3in V2O5-MoO3/TiO2for mercury oxidation was firstly determined. The results showed that the addition of MoO3to V2O5/TiO2significantly enhance the mercury oxidation ability of this catalyst in6%O2/N2atmosphere. Additionally MoO3/TiO2performed no mercury oxidation ability, so the role of Mo in V2O5-MoO3/TiO2for mercury oxidation is not directly involving in the Hg0oxidizing process, but assisting O2oxidized V3+to V5+. O2plays an important role in mercury oxidized by V2O5-MoO3/TiO2and this process follows Mars-Maessen mechanism.Then the influence of flue gas component on mercury oxidation by V2O5-MoO3/TiO2and the mercury ability of this catalyst in actual flue gas were introduced. The results suggested that HCl promoted the mercury oxidation by this catalyst in the presence of O2. In this process O2could apply the lattice oxygen for Hg0oxidized by HCl over V205-MoO3/TiO2. NO could promote the mercury oxidation by this catalyst in both the presence and absence of O2. However, SO2, NH3and H2O exhibited negative impact on mercury oxidation by this catalyst, especially the H2O, it has the strongest inhibition and could cause irreversible effect on the catalyst surface. The V2O5-MoO3/TiO2showed strong mercury oxidation ability in the actual flue gas, and the mercury oxidation efficiency could reach90%. However, the actual selective catalytic reduction (SCR) system in this plant exhibited poor mercury oxidation ability, the main reason is that the SCR system has running for35000h, and the catalytic activity of oxidizing Hg0is suppressed.Considering of the drawbacks of SCR catalyst application in300-430℃, the mercury oxidation ability of a kind of low temperature catalyst, Mn/CNT and Mn-Mo/CNT, were introduced. The results suggested that in the absence of HC1, Mn/CNT could perfectly catalytic O2oxidizing Hg0at150-250℃, and even with little HCl existing, this catalyst could reach a high mercury oxidation efficiency. However, this catalyst almost lost the mercury oxidation ability in the presence of500ppm SO2because SO2could react with MnO2in the catalyst to form manganese sulfate. Tests of mercury oxidized by Mn-Mo/CNT indicated that SO2could enhance the mercury oxidation by this catalyst. The main reason is that Mo could promote the SO2transformed to SO3which is beneficial for mercury oxidation, and Mo could also protect the catalytic activation of MnO2on the catalyst. Long time test showed that Mn-Mo/CNT was an effective available low temperature for mercury oxidation and Hg0oxidized by HCl over Mn-Mo/CNT following the Langmuir-Hinshelwood mechanism.Since Mn-Mo/CNT could exhibit strong mercury oxidation ability in the presence of SO2, the last part investigated the mercury oxidation ability of Mn-Mo/CNT in the oxy-fuel combustion flue gas which contains high concentration of SO2. As comparison, the mercury removal ability of a commercial activated carbon in this atmosphere was also explored. The results suggested that no influence of CO2on this activated carbon was found, but SO2and H2O could strongly inhibit the mercury removal by this AC. This activated carbon is not suitable for mercury removal in oxy-coal combustion flue gas. As for Mn-Mo/CNT, high concentration of CO2could strongly suppress the mercury oxidation by this catalyst, but high concentration of SO2weakened the inhibition of CO2, and HCl completely eliminated the inpact of CO2. The main reason was that HCl could more easily reacted with the active Mn on the catalyst surface than CO2. The test in complex oxy-fuel combustion flue gas suggested that Mn-Mo/CNT could be used for mercury removal in oxy-coal combustion flue gas at low temperature.