The Experimental Study On The Simultaneous Removal Of Elemental Mercury（Hg⁰）and NO From Flue Gas By CeO₂ Modified SCR Catalysts

Mercury has been listed as one of the most hazardous and toxic pollutants by the laws and regulations of more and more countries or regions due to its heavy metal characteristics,volatility,and bioaccumulation. According to the reports,coal-fired utilities are currently the largest single anthropogenic emission source of mercury around the world. Meanwhile,Nitrogen oxides（NO,NO₂,and N2O） are major causes of photochemical smog,acid rain ozone depletion and greenhouse effects. Strategies for better control of multi-pollutants emmtited form coal-fired plants thus earn the greatest attention and are highly demanded.The paper introduces the sources, characters and harm of mercury and nitrogen oxides,discusses the recent research advances in both mercury and nitrogen oxides control technologies. The major role and advantages on flue gas denitration and mercury oxidation by the selected catalytic reduction of NO method have been synthetically illuminated. As to the modification of the SCR catalyst, it not only cloud improve the capability of denitration, but also promote mercury removal from the flue gas.In this study, V₂O₅-WO3/TiO₂ catalysts were prepared by ultrasound-assisted impregnation method and used as reference sample. Both fresh SCR catalysts and used for more than 26 000 h ones are obtained from coal-fired plant. Mercuy oxidation activity experiments were conducted at 150-400℃. Besides, scanning electron microscopy（SEM）,Brunauer-Emmett-Teller（BET） and X-ray diffractogram（XRD） analyses were carried out and analyzed comprehensively. The experimental results showed that less than 50% mercury oxidation efficiency was achieved by the SCR catalysts that from power plants at the optimal temperature, while the V₂O₅-WO3/TiO₂ catalysts could remove 68.18% elemental mercury（Hg⁰） from the simulated flue gas under the same condition. It implied that SCR catalysts could promote the mercury oxidation process to some extent. Then the characteristics of the samples were analyzed,and the possible reaction mechanisms involved in the Hg⁰ oxidation and NO conversion reactions over the SCR catalysts were proposed as well. To further improve the capacibility of SCR catalysts, a series of Ce-doped on V₂O₅/TiO₂ catalysts synthesized by an ultrasound assisted impregnation method were employed to investigate simultaneous removal of Hg⁰ and NO from the simulated flue gas in a fixed bed reactor. Both NH₃-SCR of NO and Hg⁰ oxidation behavior over V₂O₅-CeO₂/TiO₂ catalyst has been studied.V1Ce10 Ti exhibited the highest Hg⁰ oxidation efficiency of 81.55% at 250℃ with a desired NO removal efficiency under the same condition. Furthermore, Hg⁰ removal behavior under both oxidation and selective catalytic reduction（SCR） condition over V1Ce10 Ti were wellinvestigated to further probe into the feasibility of one single unit for multi-pollutants control in industry application. The efficiency of Hg⁰ oxidation under oxidation condition is better than that SCR condition,which was mainly attributed to the inhibiting effect of NH₃. O₂ could be a promoter for the catalyst’s activity,while the addition of SO₂ played an irreversible inhibiting role on both NO and Hg⁰ removal. When 8% H₂O（g） was added into the flue gas,both the NO conversion and Hg⁰ oxidation efficiencies declined quikly, with an approximately 33% and 21% decrease respectively. Moreover, SEM, BET, XRD, and X-ray photoelectron spectroscopy（XPS） analyses were used to characterize the samples.Consequently, based on the above experimental study, we proposed one single step process for multi-pollutants including Hg⁰ and NO emission control from coal-fired flue gas.