The Research On The Removal Of Element Mercury From Coal-fired Flue Gas By SCR Catalysts At Low Temperature

Mercury can be released into the atmosphere easily because of its extremely highvolatility. It will then deposit into rivers, lakes, and oceans, producing the most toxic speciesof mercury, methylmercury, which can bioaccumulate within living organisms by food chainand cause adverse effects on human health. The emission of mercury from coal-firedutilities has become the main anthropogenic air pollution source for countriesutilizing coal as their principal energy input. Consequently, development oftechnologies governing/controlling mercury emission from coal-fired power plantshas become an urgent issue for the countries using coal as their principal energy input.Based on the situation of our country and existing pollution control devices incoal-fired power plants, it is the most economical way to remove Hg from the flue gasthrough the currently available pollution control devices. It has been proven that a lotof commercial catalysts used in selective catalytic reduction （SCR） are favorable forthe oxidation of Hg⁰along with NO emission control. However, the predictability ofthe oxidation extent is unreliable. Moreover, the required operating temperature forthese commercial catalysts is higher （>300oC）. Unfortunately, the life span ofcatalysts is shortened because of the chronic exposure to high concentrations of SO2and ashes in the flue gas. Therefore, it is of great significance to develop the lowtemperature （<300oC） SCR catalysts which not only avoids those problems, but alsopromotes the removal of NO and Hg at low temperatures.This paper introduces the physicochemical property, use and harm of mercury,and discuss the source of mercury released from coal-fired power plants. It alsopresents the progress of mercury control technologies for coal-fired power plants.Furthermore, the theory and technological process for SCR of NO and thedevelopment of SCR catalyst are discussed. Based on these, modified activatedcarbon fiber （ACF） and zeolite （HZSM-5）, were developed and used to evaluate theeffects on NO and Hg⁰removal to produce an effective mercury emission controlstrategy for coal power plants.In our experiment, modified ACF and HZSM-5was developed and some relatedexperiments parameters including preparation method, reaction temperature, loading value and flue gas component for evaluating the effects on Hg⁰removal efficiencieswere conducted in a lab-scale SCR system. Some physicochemical techniques wereemployed to characterize the samples. The mass transfer rules of Hg⁰and thephysic-chemical behavior of Hg⁰on the catalyst surface was studied in order to clarifythe removal mechanism of Hg⁰.On that basis, the effects of the optimum catalyst on Hg⁰removal and theselective catalytic reduction of NO with NH3were investigated under differentexperiment conditions, including preparation method, reaction temperature, loadingvalue and flue gas component. In order to clarify the removal mechanism of Hg⁰andNO, some physicochemical techniques were employed to characterize the samples.The experimental results showed:（1） the Hg⁰removal efficiencies of CeO₂/ACFwere increased and then decreased with the increase of reaction temperature, and theoptimum temperature was150oC. Furthermore, the loading of CeO₂significantlyinfluence the Hg⁰removal ability of CeO₂/ACF. Especially when the loading value ofCeO₂was6%, the Hg⁰removal efficiency of CeO₂/ACF reached about90%. Moreover,in the presence of O₂in the flue gas, promotional effects of NO and SO₂on the Hg⁰removal were found, while the presence of H2O inhibited the Hg⁰removal.（2） Incomparison to other SCR catalysts, CeO₂/ACF possessed a strong ability for removalof Hg⁰and NO. Especially when the reaction temperature was200oC and the loadingvalue of CeO₂was11%, the NO and Hg⁰removal efficiency of CeO₂/ACF reachedabout82%and90%, respectively. Therefore, it indicated that CeO₂/ACF was suitablefor removing Hg⁰and NOxefficiently from the flue gas in the existing SCR device.（3）The Hg⁰removal efficiency of CeO₂/HZSM-5offer excellent Hg⁰removal efficiencyin the temperature range of80-320oC, which indicated that it had the widetemperature window for Hg⁰removal. However, the temperature window ofCuO/HZSM-5for Hg⁰removal was narrow. Only when the temperature reached240oC, CuO/HZSM-5achieved higher Hg⁰removal efficiency. So, it indicated that CeO₂possessed a strong ability for removal of Hg⁰in comparison to CeO₂/HZSM-5.（4） theexperimental results of CuO/HZSM-5on combined removal of Hg⁰and NO from fluegas showed that when the reaction temperature was250oC and the loading value ofCuO was6%, the NO and Hg⁰removal efficiency of CuO/HZSM-5reached about90%and90%, respectively. Moreover, the regeneration of CuO/HZSM-5was easy. Thus, it indicated that CuO/HZSM-5was suitable for removing Hg⁰and NOxefficiently fromthe flue gas in the existing SCR device.