Research On The Influence Mechanism Of Chlorobenzenes And PCDD/FS Destruction By Typical Chlorinated Compounds

Chlorinated benzenes(CBz)and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans(PCDD/Fs)have attracted special attention due to their high stability,long-range transport and bioaccumulation.Among the various controlling emission techniques,catalytic oxidation has proved to be a promising technique.Especially,V2O5-WO3/TiO2(VWTi)catalyst has been applied extensively as a catalyst in selective catalytic reduction(SCR)units because of its high deNOx activity,high thermal ability,SO2 poisoning resistance and co-benefit of mercury removal.Besides,it is also proved to realize the thoroughly destruction of chlorinated organic compounds(ClOCs).Previous studies in catalytic oxidation of CBz or PCDD/Fs mainly concentrated on the lab-scale simulated flue gas,only including nitrogen and oxygen.However,the composition is very complicated in actual flue gas.Co-existing of these pollutants will definitely have influence on the catalytic reaction process.This paper presents the results of the laboratory experiments concerning three kinds of chlorine-containing compounds,that is,effects of HCl,NaCl,and various chlorinated benzenes and phenols on the oxidation of 1,2-dichlorobenzene(DCBz)and PCDD/Fs.The related influencing mechanism was also explored.Characterization methods of BET,EDS,XRD,H2-TPR and NH3-TPD were applied to research the surface area,composition,crystalline,oxidation ability and surface acidity of catalysts,to elucidate the catalyst deactivation mechanism.In this study,the potential regeneration-pathways are also explored for practical applications.The main conclusions are summarized as follows:(1)Five commercial SCR(VWTi)catalysts were selected to compare the destruction efficiency(DE)of PCDD/Fs between simulated and real flue gas.The results showed all DEs are higher in former than later,explaining the complex composition of real flue gas will definitely pose negative effects on DE of PCDD/Fs.Through comprehensively analyzing the change rule of 136 chlorinated homologues before and after reaction,VWTi catalysts present higher DE for highly-chlorinated PCDD/Fs than the lowly ones.That is because the highly-chlorinated PCDD/Fs have smaller vapor pressure,thus,tend to be adsorbed on the catalyst surface and further destructed.Under the same reaction conditions,powder catalyst showed higher DE than honeycomb catalyst for their fully interaction between target degradation and catalyst.The correlation coefficient between HRGC/HRMS and CALUX bioassay results is very high(R=0.985),which means CALUX bioassay,as a quick and low-cost detection method,can be applied to screen the high-efficiency catalyst for PCDD/Fs destruction.Meanwhile,the DEs of biological toxicity determined by CALUX bioassay are all higher than the values from HRGC/HRMS,indicating that some intermediates may also cause biotoxicity.Characterization results of five groups of catalyst revealed the advantages which the VWTi catalyst must hold for efficiently destructing PCDD/Fs.(2)HCl,as a kind of acid gas,will competitively adsorb on the catalyst surface,which will suppress the adsorption and destruction process of ClOCs.HCl has a fully contact with surface active sites at a lower GHSV(4,000 h-1 and 6,000 h-1).Thus,volatile metal oxychlorides was formed by chemical reaction,resulting in the decreased vanadium content and irreversible deactivation.Limited HCl was adsorbed on the catalyst and increased the surface acidity at higher GHSV(8,000 h-1 and 10,000 h-1,),resulted in increasing removal efficiency of DCBz.With the increasing reaction temperature,DEs of DCBz were increased;however,DEs of PCDD/Fs were decreased when HCl was existed in the reaction system.Deacon reaction is ascribed to the decreasing DEs of PCDD/Fs.In addition,HCl will further reduce the vanadium content with increasing moisture concentration.There is a good correlation coefficient between the surface vanadium content and DE of DCBz(R=0.93).Activated carbon(AC)was physically mixed with the catalyst in various proportions to promote the conversion efficiency by extending the residence time.(3)With the increasing NaCl loading on catalyst,DEs of DCBz and PCDD/Fs were decreased no matter by impregnation method(marked IM)or diffusion method(marked DI).Even though NaCl tended to block micropore and decrease the specific surface area in IM process,however,obvious decline of surface acid sites and redox ability were observed in DI process.Thus,the diffusion of NaCl exhibited greater poisoning effect compared with impregnation Jmethod during the preparation.By applying sulfuric acid and ultrasonic washing,the NaCl content on the catalyst surface was decreased,however,surface acidity and redox ability was increased,resulting in the regeneration of deactivated catalyst.(4)PCDD/Fs were catalytically oxidised in the presence of benzene(Bz),monochlorobenzene(MCBz),and 1,2-dichlorobenzene(DCBz)and the effect of these additions on the catalytic destruction of PCDD/F increased with more chlorine substituent.The influence of o-CP on PCDD/Fs destruction was decreased with rising reaction temperature.Due to the similar molecular structure of CBz,CPs and PCDD/Fs,competitive adsorption inevitably happened on the catalyst surface,resulted in the decreased DEs of PCDD/Fs.However,some DEs are minus value,indicating some DCBz or o-CP even converted into supplementary PCDD/Fs.Thus,PCDD/F s-formation from DCBz,hexachlorobenzene(HCBz),o-monochlorophenol(o-MCP)and pentachlorophenol(PeCP)was demonstrated and explored.The results showed all the processes present PCDD/Fs formation and a different isomer distribution.Meanwhile,the water vapour will inhibit the PCDD/Fs formation from DCBz on the catalyst surface.Regeneration with oxygen or air allowed to remove the deposited carbon and the original catalytic activity was largely restored after calcination.