Study On Mechanisms Of PCDD/Fs Formation By De Novo Synthesis And Inhibition By S-inhibitor

Accelerated economic development in China has led to the generation of more municipal solid waste (MSW). Because of high volume reduction, MSW incineration treatment was developed rapidly. However, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) emission from MSW incineration constrains the application of incineration treatment for MSW in China.This dissertation launched a series of fundamental experimental study, with aim to investigate the formation mechanism of PCDD/Fs and inhibition mechanism of SO2. The objectives of the current research involved as follows:1) A 35-reaction core kinetic model for the formation of PCDFs from the oxidation of 2,4,6-trichlorophenol (2,4,6-TrCP) in the presence of hexane was developed based on experimental data and a model that was presented in the literature. A new pathway to 2,4-diclorophenoxyl radical (2,4-DiCP·) which is precursor of PCDFs was presented in this paper.2,4-dichlorophenol-6-ly (2,4-DiCPL-) is the key intermediate in this pathway. Ab initio calculations were utilized to calculate some thermochemical data of related reactions by using Gaussian 03 program at the B3LYP/6-31(d) theoretical level. The rate constants were analyzed and discussed using transition state theory (TST), by which the pathway was proven to be reasonable. By adding new reactions, correcting of the reaction rate of 2,4-DiCP·self-decomposition and modifying, the core, the updated model provided good agreement between experimental and calculated yields of PCDFs using the REACT kinetic program over the temperature range of 875-1100 K.2) O and H2O are two of the key parameters for PCDD/Fs formation. The effects of oxygen speciation and H2O on the de novo synthesis of PCDD/Fs were studied in a tubular oven with model fly ashes. The experimental results show that the O from H2O and CuO do not take part in de novo reactions, but the addition of O2 promotes formation of PCDD/Fs strongly. The principal transportation pathway of O2 to solid phase is as follows:Cu2Cl2 reacts with O2 to form a kind of complex. The complex oxidizes the carbon and is reduced to Cu2Cl2.The effects of H2O vapor on de novo synthesis of PCDD/Fs, polychlorinated biphenyls (PCBs) and benzene (PCBz) were investigated at two levels (none and 10 vol.%) in various model systems containing five different carbons, CuCl2, and quartz, exposed to a flow of 10%O2/N2 at 300℃. The influence of H2O was studied on (1) speciation and behavior of copper compounds, (2) catalytic oxidation of carbons of distinct reactivity, and (3) formation of organic chlorine compounds, with the aim to investigate any effects on de novo synthesis. It is found that H2O converts CuCl2 to CuCl2·CuO, and finally to CuO in a flow of 10%O2/N2 at 300℃and that it decreases of organic chlorine (C-Cl) formation. When CuCl2 is supported on carbon, the addition of H2O promotes the catalytic oxidation of this carbon. When CuCl2 is supported on quartz, however, H2O inhibits carbon oxidation. A decrease in chlorination level of PCDD/Fs and PCBs with water addition is observed for all (six) model ashes; yet this addition affects the yields of PCDD/Fs and PCBs differently. Under the experimental conditions tested H2O does not react with Cu2Cl2, which is the catalyst of carbon oxidation. On the basis of the experimental results, the following mechanism is proposed:conversion of CuCl2 into CuO which is less reactive in de novo synthesis and promotion of catalytic oxidation of carbon by Cu2Cl2.3) The effects of SO2, SO3 on de novo synthesis of PCDD/Fs were studied by using model fly ashes incorporating copper oxide and activated carbon. It was found that the inhibitive effect of SO2 on PCDD/Fs formation is similar to that of SO3. To investigate the inhibition mechanism, CUSO4 formations from both CuO and CuCl2 were examined. The ability of SO3 to convert CuCl2 and CuO on a silica support into sulfate is much stronger than that of SO2. However, replacing silica by activated carbon leads to a much high conversion of CuCl2 to CUSO4 in the presence of SO2. The promotion by activated carbon is explained by the reduction of CuCl2 to Cu2Cl2 and eventual conversion of Cu2Cl2 into CuSO4 is the main inhibition mechanism of SO2 on de novo synthesis of PCDD/Fs. 4) The effect of sulfur dioxide (SO2) on de novo synthesis of polychlorinated dibenzo-p-dioxins, dibenzofurans (PCDD/F) and biphenyls (PCB) was investigated in model systems containing carbon, activated by cupric chloride (CuCl2). The influence of SO2 on speciation and behavior of copper compounds, catalytic oxidation of carbons, and formation of organic chlorine (extractable organic chlorine and non-extractable organic chlorine) was also studied to investigate the inhibition mechanism of SO2 on de novo synthesis. It was found that SO2 can sulfate CuCl2, but does not react with CuO and CuCl2·CuO in a 10%O2/N2 atmosphere at 300℃The suppression by SO2 of organic chlorine (C-Cl) formation also confirmed these findings. It was also found that catalytic oxidation of carbons was strongly suppressed by SO2. A dramatic decrease in PCDD/Fs formation was observed for all five carbons, while adding SO2; at the same time the average chlorination level also decreased. However, the inhibiting effect on PCB was less apparent compared with that on PCDD/F. On the basis of the experimental results, the conversion of both cupric and cuprous chloride into the non-reactive sulfate was proposed as inhibition mechanism.5) Knowledge of congener pattern and homologue profiles of PCDD/Fs, PCBs and PCBz formed by de novo synthesis is help for elaborating the mechanism of formation and influence of the toxic compounds formed by de novo synthesis. By the help of the component analysis (CA) the best correlated congeners are effectively detected. Various good correlated pairs between PCDD/Fs, PCBs and PCBz have been elaborated by using linear regression between the 15 samples. It was found that a good correlation between PCBz and PCDFs.