Removal Of Elemental Mercury From Flue Gas Using The Magnetic Fe-containing Carbon Prepared From The Sludge Flocculated With Ferrous Sulfate

In recent years,mercury pollution in the atmospheric environment has received increased attention.As the biggest anthropogenic sources of Hg emissions,it is of great significance to control mercury pollution in coal-fired power plants.Through the coal combustion,most of the mercury in coal would exist in the form of elemental mercury（Hg⁰）in flue gas.Hg⁰ could not be effectively removed via existing air pollution control devices（APCDs）.At present,activated carbon injection（ACI）is considered to be a practical and effective technology for controlling mercury emmision from power plants and has become the research focus in the field of mercury removal.However,its material cost is high,and it brings negative influence to the utilization of fly ash,which limits its its practical applications.Most importantly,the technology fails to realize the final centralized control of mercury pollution in coal-fired flue gas.The mercury compounds transfer from the flue gas to fly ash,desulfurization slurry,desulfurization gypsum and other by-products.This will increase the risk of secondary mercury pollution.Therefore,it is extremely attractive to research and develop regenerable magnetic adsorbent to overcome these limitiatons.On the other hand,the production of municipal sludge is very large in our country.As a typical flocculant,ferrous sulfate（Fe SO₄）has been widely used in the flocculation and dewatering of excess sludge in sewage treatment plants.Ferrous sulfate flocculated sludge has the potential to prepare magnetic biochar for mercury adsorption.If the sludge can be prepared as adsorbent for mercury emission control in coal-fired power plants,it will produce great environmental and economic benefits.The sewage sludge flocculated with ferrous sulfate（SFS）was first prepared by one-step pyrolysis to obtain magnetic Fe-containing carbon for mercury removal.Results showed that only a small amount of Fe_xO_yas well as extremely weak magnetism were observed at pyrolysis temperatures of less than 500℃.SFS tended to exhibit intensive agglomeration,leading to the drastic increase of the crystalline-phase particle size at high pyrolysis temperature（>700℃）,which depressed the Hg⁰ removal efficiency.The optimal pyrolysis temperature was 700℃,corresponding to the production of some sulfides,an optimal content of Fe_xO_y and a suitable BET surface.Hg⁰removal efficiency of SFS700（SFS pyrolyzed at 700℃）reached 80.7%at the reaction temperature of 125℃.The presence of O₂and low concentration of SO₂enhanced the Hg⁰ removal,while the H₂O vapor and high SO₂concentration inhibited it.Meanwhile,good resistance for the adsorbent to moderate concentrations of SO₂ and H₂O was observed.Moreover,the good magnetism performance is conducive to the recovery and utilization of the SFS700 in flue gas.In order to improve the Hg⁰removal capacity of ferrous sulfate sludge-based adsorbent,a novel regenerable material of magnetic adsorbent（Mn-SFS）based on ferrous sulfate sludge modified by manganese oxide was developed.Results showed that the prepared Mn-SFS adsorbent has good mercury removal ability and regeneration performance under simulated flue gas conditions.The thermal decomposition of Mn（NO₃）₂at 400℃can enlarge the pore size of the adsorbent.The adsorbent(Mn₁₀-SFS)with optimal 10%of Mn attained approximately 86.1%of Hg⁰ removal efficiency at the reaction temperature of 150℃under simulated flue gas atmosphere.The existence of O₂,NO and HCl facilitated the removal of Hg⁰,while SO₂ and water vapor inhibited the removal of mercury as a result of competitive occupation.The five cycles of capture-regeneration demonstrated that the Mn₁₀-SFS presented good Hg⁰ removal efficiency and regeneration performance.To improve the sulfur resistance and applicability of the ferrous sulfate sludge-based adsorbent,the magnetic Fe-containing carbon-based adsorbent prepared from sludge flocculated with ferrous sulfate by different activators activation was further investigated.Results showed that when 30%Zn Cl₂ was added to SFS(names as Zn₃₀-SFS),sulfides,chloride,magnetic Fe_xO_y,as well as a suitable BET surface area were produced during pyrolysis.In addition,the generated magnetism during the pyrolysis process was helpful for the recovery and utilization of Zn₃₀-SFS in flue gas.Zn₃₀-SFS showed a maximum mercury removal efficiency of 96.1%.Owing to the strong affinity of multiple active sites(including Cl^-,S^2-,₂^2-and Fe_xO_y)to Hg⁰,the mercury removal over the chloride and sulfide adsorbents was not easily interfered by SO₂concentration,thus exhibited excellent sulfur resistance.Finally,the regeneration method and mechanism as well as its application and regeneration process over Zn₃₀-SFS were studied pertinently.Results showed that mercury and its compounds can be completely desorbed in N₂ atmosphere at 400℃for1h,and then the surface-active sites and magnetic properties of the adsorbent can be recovered in 4%O₂+100ppm HCl+400ppm H₂S atmosphere.As Zn₃₀-SFS is prepared from sewage sludge,its material cost is obviously much less than such artificial magnetic sorbents.Moreover,the Zn₃₀-SFS prepared by one-step activation and chemical modification by Zn Cl₂ was much simpler than sorbents which require two or more preparation steps（including the preparation of a carrier,the loading of metal sulfides,and magnetic preparation）.Therefore,the cost of using Zn₃₀-SFS to remove Hg⁰ from flue gas was much lower than previously reported magnetic sorbents.Moreover,its wide operating temperature window and good magnetism performance makes it adaptable for working as an effective mercury sorbent.This kind of sorbent can be injected into the flue gas and collected by the ESP and WESP,which broadens the application range of the adsorbent used for mercury removal from coal-fired flue gas.Hence,the results of this study have guiding significance for the reuse of sludge after flocculation by adding ferrous sulfate.