Research On Effect And Reaction Mechanisms Of Typical Transition Metal Compounds On Heterogeneous Catalytic Formation Of Dioxins

Ever since the discovery of polychlorinated dibenzo-p-dioxins and dibenzofurans（PCDD/F）in the effluents from Municipal Solid Waste Incineration（MSWI）in late 1970s,numerous endeavours were launched to identify their major pathways of formation and characteristics of emissions.Until now,however,there are still many points remaining unclear about the formation mechanisms of dioxins.Though several transition metal compounds have been compared for their activities in forming PCDD/F,their impacts on isomer distribution patterns were seldom studied,leading to a lack of view on the essence of metal catalysed formation mechanisms.Reaction models of carbon chlorination and dioxin formation promoted by some transition metal compounds have been proposed by using X-ray absorption fine structure（XAFS）spectroscopy,while the models remain incomplete.This thesis concentrates on typical transition metals（e.g.,Cu,Fe,Cr,Ni,Zn,etc.）,investigating their influence and reaction mechanisms on formation of dioxins.Both the reactants and the products are focused on,aiming to futher extend and complete the research field of heterogenous catalytic formation of dioxins.On this basis,this thesis applies the conclusions from the mechanism study to two types of thermal treatment of waste-open burning of waste electrical and electronic equipment（WEEE）and municipal solid waste incineration（MSWI）-with the aim of revealing the formation and distribution characteristics of dioxins catalysed by transition metals in real situations.The target of this thesis is leading to more insight into the formation mechanisms of dioxins and serving as a basis for minimising their emission.The main conclusions of this thesis are listed as below:（1）Chlorides of tramsition metals combine the function of chlorinating agent with that of catalyst.Typically,they are two orders of magnitude more productive than the corresponding oxides or the blank sample,yet this figure varies from case to case（from 2 to 700）,as well as with reaction atmosphere and the target group（PCDD or PCDF）considered.CuCl₂ is by far the most active PCDD/F catalyst,followed by CrCl₃,NiCl₂ and ZnCl₂.Rising oxygen content in the carrier gas significantly promotes the total PCDD/F yield.On average,oxygen in the gas phase raises PCDD linearly and PCDF approximately according to a?-order law.Statistical analysis of 136 PCDD/F isomer distribution patterns is established and used as a tool for discriminating dioxins formation pathways.The signatures from the different catalytic systems are remarkably distinct,suggesting differences in PCDD/F-formation pathways when catalysed by distinct catalysts.The existence of chlorophenols（CP）-route congeners in de novo formation is confirmed;their contributions at different conditions are then discussed.Remarkably,common pathways of formation for seventeen 2,3,7,8-substituted congeners are absent at all levels of chlorination.（2）Model fly ash（MFA）incorporating FeCl₃ was tested for PCDD/F formation,at different temperatures,oxygen contents in the reaction atmosphere,and reaction time.A maximum output is reached at 380°C.The PCDD/F formation in the absence of atmospheric oxygen is largely restricted,limited to ca.7.5%of that in 5%of oxygen.On average,oxygen in the gas phase raises PCDD approximately according to a reaction order of about?;while the reaction order is nil for PCDF.A single test run（350°C,10%O₂）was devised to differentiate between the gaseous PCDD/F-output evolved during a first and during a second?h,and the remaining residue,with the residue representing the bulk（92%）of the output.In all cases a complete analysis on isomer distribution pattern was produced.Still,temperature（250-550℃）exhibits a more significant influence on isomer profiles than oxygen content ranging from 5 to 20%.The most typical PCDD/F isomer distribution pattern from FeCl₃-catalysed system is represented by the solid residue tested at 350°C,10%O₂.The selectivities of CP-route congeners tend to decrease with rising temperature,with 420°C as a pivotal point.Selectivities of the seventeen 2,3,7,8-substituted PCDD/F congeners react variably towards the influencing factors tested.The analyses confirm the absence of parallel formation routes of 2,3,7,8-PCDD/F.（3）A systematic and in-depth investigation on effect and reaction mechanisms of Cr compounds on PCDD/F formation was conducted.A series of de novo tests,conducted over a wide range of temperature（from 250 to 550°C）and for four different oxygen contents（0,5,10,20%）,confirm the promoting effect of CrCl₃ on the PCDD/F formation.Both PCDD and PCDF reach their summit at 350°C and also peak at 10%O₂.In situ XAFS spectroscopy was applied to investigate the behavior of Cr and Cl during heating,describing the entire pathways picture of Cr-promoted formation of dioxins.The effect of oxygen was studied by measuring XAFS spectra on samples heated at three different oxygen concentrations（0,10,20%）.According to these spectra,chromium compounds play two key roles during dioxins formation:（a）chlorinating carbon,using chlorine derived from conversion of CrCl₃ into Cr₂O₃,and further oxidation to Cr（VI）,and（b）facilitating oxidative destruction of the carbon matrix,while reducing Cr（VI）to Cr₂O₃.The oxychlorination reaction mechnims of typical transition metal compounds on de novo synthesis of dioxins are summarised,based on the XAFS investigation of Cr in this thesis and the reseach on other transition metals conducted by other researchers.（4）The effect and reaction mechanism of Cu on open burning of WEEE is investigated,based on the combination of thermogravimetric analysis,in situ XAFS spectrosipy and PCDD/F formation tests.The addition of Cu into WEEE suppresses the volatilisation of Cl from the system,lowers the temperature required for formation of poly-condensed aromatic intermediates and largely promotes carbon-matrix decomposition.Besides,the presence of copper show effects on promoting the transformation of the Cl in PVC into other species and stimulating the formation of PCDD/F largely.In situ XAFS spectroscopy was applied for recording the dynamic changes in forms of Cu and Cl during the temperature rising process,revealing the mechanisms of the whole process of PCDD/F formation.The effects of Cu during MSWI and open bruning are compared.The reaction pathways at lower oxygen content with unburned carbon are similar,but affected by the“delayed effect”.When the chlorine source was changed from PVC to NaCl,only a very small amount of Cl could be transferred by Cu to the aromatic carbon.（5）A systematic study was conducted on the formation and distribution characteristics of PCDD/F and polychlorinated biphenyls（PCB）at different stages in the post-combustion zone of typical domestic circulating fluidised bed municipal solid waste incinerators.The ashes collected from different locations were also characterised for their physical and chemical properties.A continuous rising trend is observed in the PCDD/F-,PCB-and TEQ-concentrations in the effluent downstream the gas cooling process,until the inlet of air pollution control system（APCS）.The majority of them are then removed after the APCS.The main formation pathways of PCDD/F are discussed based on the analysis of fingerprints.The gas/solid distribution pattern is influenced by both formation pathways and adsorption/desorption effect.The load of PCDD/F in ashes amplifies steadily with the decreasing temperature;PCB presents a fluctuating tendency,but the dioxin-like PCB（dl-PCB）behaves similarly to PCDD/F.The greatest rise in PCDD/F TEQ is noticed among the economiser.As temperature decreases,a slightly rising tendency is observed in chlorine content,as well as in the Cl/S ratio;and some enrichment of copper,zinc and lead is found in low-temperature ash.Considering the operating temperature of the sampled locations,all these factors stimulate the formation of organochlorinated pollutants.The speciation of carbon and chlorine on ash surfaces was established by XPS,showing a positive correlation between organic chlorine and oxygen-containing carbon functional groups.Principal component analysis（PCA）is applied to study the links between the ash characteristics and distribution of PCDD/F and PCB.