An Experimental And Mechanistic Research On Removal Of VOCs And Hg~0 Over Modified Activated Coke

As the main energy consumption structure in China,coal not only promotes the rapid development of economy,but also causes the characteristics of coal smoke air pollution in China.With the implementation of“Ultra low emission”and“Air Pollution Prevention and Control Action Plan”,the emission of SO₂,NO_x and PM has been controlled effectively,and the emission of volatile organic compounds（VOCs）and Hg⁰ has gradually attracted wide attention of the society.Ozone is the key issues of air pollution control in China.As one of the main precursors of ozone pollution,how to achieve the effective control of VOCs emission is curial to the ozone control.Hg⁰ possesses high volatility,strong toxicity,persistence,long-distance migration and bioaccumulation,and has strong biological toxicity,which has been listed as one of the most toxic global atmospheric pollutants.Therefore,how to effectively control the emission of Hg⁰ and VOCs has become a research topic widely concerned by researchers in various countries.As an effective adsorbent-catalyst with high mechanical strength and good regeneration performance,activated coke（AC）has been widely used in industrial desulfurization.It is a promising technology to adopt AC for Hg⁰ and VOCs removal on the basis of the existing air pollution control device.However,the pore structure and surface functional groups of AC cannot always meet the needs of efficient removal of VOCs and Hg⁰,and the removal temperature window of VOCs and Hg⁰ is quite different.Therefore,how to develop AC with high removal efficiency of VOCs and Hg⁰ has very important practical significance.In this paper,the removal of single pollutant was firstly studied,and the modification,characterization and the performance for VOCs and Hg⁰ removal of were researched with active coke as the research object.Mn Fe/AC,Mn Fe/NAC,Mn Co/AC and Mn Co/Zr-AC were prepared by impregnation method and hydrothermal method,respectively.The activity of modified AC for VOCs（HCHO and toluene）and Hg⁰ removal was investigated,and the mechanism of modification method on the physical and chemical properties of AC was characterized.On this basis,the structure-activity relationship between the physicochemical properties of modified AC and the removal activity was explored,and the interaction mechanism of VOCs and Hg⁰ on the surface of modified activated coke was revealed.Moreover,the mechanism of VOCs and Hg⁰ removal by modified activated coke was clarified,and the modified AC with high removal efficiency of VOCs and Hg⁰ was developed.The main research results obtained in this paper are as follows:（1）A series of Mn_xFe_y/AC samples were prepared by impregnation method for HCHO removal.At the broad temperature window of 180-330°C,the optimal Mn_0.75Fe_6.02/AC exhibited excellent HCHO removal activity,superior stability and selectivity,and possessed the potential in the simultaneous control of HCHO and Hg⁰.There existed competitive adsorption between SO₂,NO and HCHO,and the addition of O₂ weakened the inhibition of SO₂,and NO₂ produced by NO oxidation possesses stronger oxidation capacity than O₂,which promoted the removal of HCHO.H₂O further facilitated the oxidation of HCHO by supplementing the consumed-OH.The synergy of Mn O_x-FeO_x,as well as strong interaction between metal oxides and AC support,endowed sample with more developed porous structure and abundant active oxygen species,and thus promoted the adsorption and oxidation of HCHO.Moreover,based on FT-IR results,HCHO was adsorbed on the sample through active sites such as-OH,and then oxidized to formate and carbonate intermediates,which finally was further oxidized to CO₂ and H₂O.（Chapter 3）（2）To improve the dispersion of active components,hydrothermal method was adopted to prepare a series of Mn O_x-Co O_x modified activated coke,which was applied for toluene removal from flue gas.The co-doping of Mn O_x-Co O_x significantly improved the removal activity of sample for toluene,and the optimal 9%Mn₁Co₁/AC possessed T₉₀ of 238℃for toluene removal.The addition of O₂ accelerated the further oxidation of toluene intermediates,especially the transformation of benzyl alcohol→benzaldehyde and benzoic acid→maleic acid.Besides,an irreversible inhibition on toluene removal was observed for SO₂,while the weakened removal of toluene due to the competitive adsorption between H₂O and toluene would recovery when cutting off H₂O.The removal of toluene on Mn Co/AC proceeded as toluene→benzyl alcohol→benzaldehyde→benzoic acid→maleic acid→carbonate→CO₂+H₂O.Moreover,the conversion of toluene into benzoic acid could be carried out at lower temperature,while the further oxidation of benzaldehyde→maleic acid→carbonate→CO₂+H₂O only occurred at higher temperatures.The cracking of benzene ring is the rate-limiting step in the removal process of toluene.It proved that modified AC is effective for different type of VOC.（Chapter 4）（3）Based on Chapter 3,Mn O_x-FeO_x was loaded on（NH₄）₂S₂O₈ modified activated coke to prepare Mn Fe/NAC for Hg⁰ removal.（NH₄）₂S₂O₈ treatment on AC led to the generation of abundant micropores and oxygen-containing functional groups,which endowed Mn Fe/NAC with larger specific surface area,higher dispersion of active components,stronger reducibility and more high valence metal ions.Therefore,Mn Fe/NAC possessed higher Hg⁰ removal efficiency and stronger SO₂-resistance.Hg-TPD results showed that mercury mainly existed in the form of phy-Hg,C=O-Hg,COO-Hg and O_L-Hg O on Mn Fe/NAC.The priority of active oxygen species for Hg⁰ removal was C=O＞COO-＞O_L,and some Hg O was desorbed from sample and released into the flue gas.The inhibition of SO₂ was mainly due to the sulfation of Mn O_x and its occupation of active oxygen species.And the doping of FeO_x could effectively weaken the adsorption of SO₂ on C=O and COO-,thereby enhancing the sulfur resistance of Mn Fe/NAC.Besides,the promoting effect of O₂ was mainly through the regeneration of the consumed lattice oxygen species.The excellent thermal regeneration performance further confirmed the application potential of Mn Fe/NAC in the removal of Hg⁰ from coal-fired flue gas.（Chapter 5）（4）The feasibility of removal of VOCs or Hg⁰ by modified AC has been fully verified in the above research.Based on the above,Mn_xCo_y/Zr_z-AC was prepared by stepwise impregnation method for the simultaneous removal of HCHO and Hg⁰.The optimal sample Mn_2/3Co₈/Zr₁₀-AC achieved 99.87%and 82.41%removal efficiency for HCHO and Hg⁰ at240℃,respectively.The dopant of Zr O₂ enhanced the specific surface and pore volume of support,which was beneficial for the dispersion of metal oxides.Chemisorption was proved to play the curial role in Hg⁰ removal,and oxidation also worked.Moreover,there existed competition of active sites between HCHO and Hg⁰.And HCHO predominated in the competition of active oxygen species,especially for lattice oxygen,thus suppressed Hg⁰removal.Furthermore,after five thermal regeneration,Mn_2/3Co₈/Zr₁₀-AC still exhibited superior activity for simultaneous removal of HCHO and Hg⁰,indicating that modified AC possessed an excellent application prospect for the simultaneous removal of VOCs and Hg⁰.The research could provide the theoretical guidance for the simultaneous control of air pollutants.（Chapter 6）...