Removal Of Elemental Mercury From Simulated Flue Gas Over Modified Titanium-based Sorbents

Mercury and its compounds,as a highly toxic pollutant,have caused immeasurable harm to the ecological environment and human health worldwide.China is currently the country with the largest anthropogenic mercury emissions in the world.Until now,the largest source of atmospheric mercury emission in China is still coal-fired power plants.Activated carbon injection?ACI?is difficult to be applied in large scale due to its mercury removal efficiency limited by mercury concentration,residence time,reaction temperature,installation condition of air pollution control devices?APCDs?,operation cost and other factors.Therefore,it is necessary to develop efficient and economic non-carbon based sorbents or catalysts for the study of mercury removal from coal-fired flue gas.It has been reported that titanium dioxide has good thermal and chemical stability,large specific surface area and mechanical strength.It can“interact strongly”with the loaded metal and it has good resistance to SO₂ poisioning so it can be modified by transition metal oxides and used in the study of selective catalytic reduction?SCR?for denitrification and mercury removal.In this paper,a series of MnO_x and MMn?M=Fe,Co,Zr,Mo?bimetallic modified TiO₂sorbents were prepared by impregnation method and the study on mercury removal from simulated flue gas?SFG?was carried out in a mercury sorption evaluation set-up.The effects of calcination temperature,loading value,reaction temperature and flue gas components on mercury removal were investigated.Several techniques including TG/DTG,N₂adsorption-desorption,XRD,FT-IR,XPS and Hg-TPD were used to characterize the fresh and spent sorbents and the possible reaction mechanism was explored combined with the results of mercury removal experiments.It is shown that the optimal MnO_x loading value and calcination temperature was 12mol.%and 450 ^oC,respectively.The highest mercury removal efficiency?98.46%?of MnO_x-TiO₂ sorbent was obtained when the temperature was 300 ^oC.O₂ and HCl in simulated flue gas played a positive role in mercury removal.SO₂ had a strong inhibitory effect on mercury removal,which may be due to the competitive adsorption between Hg⁰ and SO₂.At the same time,the manganese sulfate produced during the reaction will cover the surface of the active site,resulting in the decrease in mercury removal efficiency.CO₂ and NO also slightly inhibited mercury removal,while HCl can alleviate the inhibition of SO₂ and NO to some extent.12Mn-Ti-450 has a certain oxidation effect on Hg⁰,and increasing the reaction temperature is favorable for Hg⁰ oxidation,while the addition of SO₂ and NO will inhibit this process.The results of Hg-TPD showed that HgO and amalgam were formed on the surface of the sorbent.The adsorption and oxidation of Hg⁰ on MnO_x-TiO₂ basically follows the Mars-Maessen and Langmuir-Hinshelwood mechanisms.MnO_x-TiO₂ was obviously inhibited by SO₂,so bimetallic?Fe/Co/Zr/Mo?was added to modify it for improving its SO₂ resistance.The experimental results show that when the ratio of Fe:Mn is 3:9 and the reaction temperature is 300 ^oC,the highest mercury removal efficiency is 99.27%;the addition of Fe can significantly improve the SO₂ resistance of the sample,and when 400 ppm SO₂ is introduced,the average mercury removal efficiency for 2 h is still above 60%.Mn⁴⁺in the sorbent participated in the oxidation of Hg⁰,accompanied with the reduction of Mn⁴⁺to Mn³⁺.Besides,the proportion of Mn⁴⁺increased with the introduction of Fe.SO₄^2-was formed after 3Fe9Mn-Ti-400 react in the atmosphere containing SO₂.O₂ and HCl can promote mercury removal,but CO₂ inhibit mercury removal.Low concentration of NO will promote mercury removal slightly,yet high concentration of NO can inhibit mercury removal mildly.