Catalytic Destruction Of PCDD/Fs Over Mn-based Catalysts And Performance Of Catalytic Filters

Municipal solid waste incineration（MSWI）emits a large amount of flue gas,which contains particulate matter,acidic gases,organic pollutants and heavy metals.The waste gas that includes solid particles and toxic gases will not only destroy the ecological environment,but also have a serious harm to human’s health.In the air pollution control devices（APCDs）,a bag filter for dust removal is an important unit.Catalytic filter integrates catalytic oxidation with dust removal,simultaneously removing dioxin and particles in the flue gas.The application of catalytic filtration technology only need to replace the dust removal filter in the existed baghouse with composite catalytic filter,requiring no new equipment.Polytetrafluoroethylene（PTFE）filter is the best option for the bag filter,the adsorbability of PTFE filter is so poor that it is difficult to fix catalysts.The current research has the problem:the working temperature of catalyst is higher（>250℃）than that of PTFE filter（180℃）.The key points of this paper include:（1）develop catalysts for PCDD/Fs removal;（2）composite technology of PTFE filter and catalyst;（3）catalytic destruction of PCDD/Fs on PTFE catalytic filter;（4）the reaction paths of PCDD/Fs catalytic destruction.In this paper,1,2-dichlorobenzene（1,2-DCB）and furan are chosen as model compound of PCDD/PCDFs,to evaluate the activity of the catalysts in the oxidation of the chlorinated aromatic ring and oxygenated aromatic ring.Manganese based catalysts were prepared by solvothermal and co-precipitation methods.A MCFe catalyst with high efficiency at low temperatures was selected.The PTEE filter and MCFe catalyst were further assembled to prepare the integrated catalytic filter material MCFe@PTEE.The performance,stability and toxicity resistance of catalytic filter media in synergistic removal of 1,2-dichlorobenzene and furan were tested.Then,the catalytic performance of catalytic filter for PCDD/Fs was tested and reaction mechanism were analyzed.The main conclusions include the following aspects:（1）The Mn-based catalysts prepared by solvothermal and co-precipitation method,respectively.The effects of metal components,metal ratio and prepare method on the catalytic activity of 1,2-DCB and furan.Then the catalyst MCFe was selected,which showed high activity,selectivity and stability.The removal efficiency of 1,2-dichlorobenzene was higher than that of VWTi at 120-210℃,The results showed that at 240℃,the removal efficiencies of 1,2-dichlorobenzene and furan by MCFe are 73.19%and 100%,respectively.MCFe has better stability than other catalysts,with 48.66%removal efficiency after 180 min.The selectivity of CO₂/CO for MCFe reached 96%at 270℃.It is easier for furan to be adsorbed on the surface of the catalyst to achieve preferential removal,when 1,2-dichlorobenzene and furan coexist in the flue gas.After the complete removal of furan on the surface of the catalyst,1,2-dichlorobenzene adsorbed on the active site of the catalyst surface for catalytic oxidation to occur.The removal efficiencies of 1,2-dichlorobenzene and furan were 67.35%and 100%at 240℃.（2）The MCFe powder was loaded on the PTFE filter by ultrasonic impregnation coating method and coated with foam coating.The optimal conditions for preparing catalytic filter material were screened by analyzing the loading and catalyst particle size.This method is simple and feasible,which not only make a bond of high strength between the catalyst and PTFE filter,but also ensure the permeability and catalytic activity.The effects of temperature,O₂ content,SO₂、NO on the catalytic activity of 1,2-DCB and furan were studied.The effect of gas hourly space velocity on the catalytic activity of PCDD/Fs was studied.The catalytic filter material was used to remove 1,2-DCB and furan simultaneously.When the temperature was 210℃,the removal efficiencies for 1,2-DCB and furan were 42.68%and 100%,respectively.The removal efficiency of MCFe@PTFE for furan was higher than that of Remedia filter.Finally,the catalytic filter was used for the removal of PCDD/Fs.At 180℃,11%O₂,GHSV 35000 h^-1,and loading capacity of300 g/m²,the removal efficiency of MCFe@PTFE for 17 kinds of PCDD/Fs was 74.0%and that of 136 kinds of PCDD/Fs was 82.3%.The removal efficiency of PCDD/Fs homologues varies with the degree of chlorination.In addition,the dust removal efficiency of MCFe@PTFE catalytic filter reached 99.9%.（3）GC-MS and Gasmet were used to detect the intermediate products and final products in the reaction process.The possible reaction paths were proposed according to the reaction products.When the 1,2-DCB and furan coexist in the flue gas,furan was first absorbed on the catalyst and C-O bond was abstracted and formed aldehydes or ketones,then further oxidization to the final products,CO₂,H₂O.The C–Cl bond of 1,2-DCB was abstracted by nucleophilic substitution of the oxygen from OH groups and the formation of the chlorophenoxy species,aldehydes or ketones,long chain hydrocarbon.A fragment of 1,2-DCB was oxidized to chlorinated hydrocarbons via electrophilic substitution.These byproducts could undergo further oxidization to the final products CO₂,H₂O,HCl.The reaction products in the PCDD/Fs catalytic destruction include aromatic hydrocarbon（C₆H₆,C₇H₈,C₆H₅Cl）,long chain hydrocarbon(C₆H₁₄,C₆H₁₂,C₇H₁₆,C10H16)and aldehydes or ketones（C6H12O）.First,PCDD/Fs was absorbed on the activated sites of catalyst,the C-O bond was weaker than C-Cl band,the C-O bond was abstracted and formed aromatic hydrocarbon,then nucleophilic substitution of the oxygen from OH groups and the formation of the chlorophenoxy species,aldehydes or ketones,long chain hydrocarbon,then oxidized to chlorinated hydrocarbons via electrophilic substitution.The byproducts would undergo further oxidization to the final products CO₂,H₂O,HCl and so on.