Research On Catalytic Oxidation Of Elemental Mercury Over SCR Catalyst

Coal-fired mercury polution has received tremendousattentions as an air pollutantfollowing particulates, NO_x and SO₂.Currently there aren't any mature technology that is available for mercury control, and it is of great significance to control the mercury emission using the existing air pollution control devices?APCDs? in coal-fired power plants. Mercury may be presented in flue gas as elemental mercury?Hg⁰?, oxidized mercury(Hg²⁺) or particle-bound mercury?Hg^p?.The proportion of the different mercury species varies throughout the system, which influence the mercuryremoval efficiency bythetypical APCDs.Generally, mostparticle-bound mercury can be captured by the APCDs such as electrostatic precipitators and baghouses. The oxidizedmercury is water-soluble and can be efficiently removed in the wet flue gas desulfurization?WFGD? processes of coal combustion. However, the elemental mercury is difficult to capture with typical APCDs due to its volatility and chemical stability. As such, to improve the mercuryremoval efficiency by the APCDs, it is necessary to oxide those elemental mercury to water-soluble oxidizedmercuryas much as possible to decrease the rate of elemental mercury. SCR catalyst is capable of oxidizing elemental mercury to oxidizedmercury. Unfortunately,the oxidation efficiency ofelemental mercury by the conventional SCR catalysts is limited because of the factors such as different fluegascomposition and so on. It is necessary toresearchthe catalytic oxidation characteristics of elemental mercury over SCR catalysts and strengthen the catalytic oxidation efficiency of the elemental mercury for mercury pollution controlusing the APCDsin coal-fired power plants.Firstly, the test-bed forthe catalytic oxidation of elemental mercury over SCR catalyst was built up and a sequence of experiments were carried out in order toevaluate the influence of different operational parameterson catalytic oxidation of elemental mercury over conventional SCR catalyst, including the reaction temperature, theammonia nitrogen ratio, the content of SO₂, etc.Experimental results showed that, the increase in temperature can promote the oxidation of elemental mercuryto a certain extent, but higher temperature have a negative effect that will reduce the elemental mercury conversion; The amount of injected ammonia would inhibit the oxidation of elemental mercury, and theexit concentration of Hg²⁺ was almost zero at theammonia nitrogen ratio of 1.5; Underconditions of higherSO₂ concentrations, the oxidation of elemental mercury efficiency increased slightly, but the effect was notobvious.Based on above experiments, the conventional SCR catalyst was modified by impregnation method and experiments were performed toevaluate the influence of different operational parameterson catalytic oxidation Hg⁰ over modifiedSCR catalystcompared with the conventional SCR catalyst, including the reaction temperature, theammonia nitrogen ratio, the concentration of SO₂, etc.Experimental results showed that, the Hg⁰ oxidation efficiency of two modified SCR catalysts have improved significantly than conventional SCR catalyst, the oxidation efficiency wasabove 90% and the temperature window of modified SCR catalyst was wide; The ammonia content had a little impact on Hg⁰ oxidation efficiency over two modified SCR catalysts andthe Hg⁰ oxidation efficiency significantly increased compared with conventional SCR catalystreached to above 90%; The efficiency of the modified SCR catalytic oxidation of elemental mercury could remain at around 90% under differentconcentrations of SO₂. The results provided a theoretical basis for the collaborative mercury removal by using the existing APCDs.