| During the activities and production process of human beings, lots kinds of wastewater have been produced, especially for the nitrogen-containing wastewater. It includes many toxic and hazardous materials, and has great negative effect on the environment. The nitrogen-containing pollutants in the wastewater are mostly organics containing amino groups and cyano group, which are difficult to degrade effectively by the traditional wastewater treatment methods. Supercritical water oxidation (SCWO) is a new and effective technique for organic wastewater treatment. At temperatures and pressures above the critical point of water (374℃and22.1MPa), organic compounds, dielectric constant and ion products decrease, and the number of hydrogen bonds increases, water behaves like a nonpolar solvent with high ability of diffusion and transfer, water and oxygen or other oxidants are completely mixed in a single homogeneous aqueous phase, free of mass transfer limitations. SCWO has proved to have high destruction efficiencies within short period for a wide range of organic pollutants wastewater. The organics can be ultimately decomposed to CO2, H2O and other inorganic compounds.Meanwhile, N2O was found to be one of the major pollutants in the SCWO of organic wastewater. As the important greenhouse gas, N2O has strong global warming potential, and it has contributed to7.9%of the global warming effect. More environmental benefit will be aquired if N2O is reduced or avoided during the process of SCWO. Thus, it is necessary to understand the nitrogen transfer details during SCWO and grasp the mechanism of N2O occurance. The relative study is very limited.The thesis summarized the special properties of SCWO (including hydrogen bonds, density, thermal conductivity, diffusion coefficients, dielectric constants, solubility and ionization degree et al.), as well as the typical characteristics of SCWO. A complete review was given on the topic of kinetics and mechanism of organics decomposition in SCWO.To explore the kinetics and mechanism of nitrogen-containing organics in SCWO, we select Ethylene diamine (EDA) and N-phenylglycinonitrile--the organics containing groups of-NH2-CN as the typical pollutants. The experiments were conducted on a set of continuous flow SCWO equipment under different performance parameters. The decomposition efficiency and affecting factors of two kinds of pollutants; the intermediates of EDA and N-phenylglycinonitrile by multiple methods such as GC-MS were analyzed; the reaction kinetics and COD removal kinetics were investigated; the reaction pathways and mechanisms were inferred by experimental data and quantum chemical method. The gaseous products of EDA and N-phenylglycinonitrile in SCWO, were identified and quantified by GC and GC-MS. Based on the analysis results, the oxidation and decomposition pathways were conjectured. None linear regression were conducted on the experimental data with gaussian-newton method by Matlab, and the kinetic parameters were obtained.The conclusions obtained were listed as following:(1) SCWO can decompose the nitrogen-containing organics effectively. Compared to traditional methods such as wet air oxidation (WAO) and incineration, SCWO has the features of wide use, high converse efficiency, rapid oxidation reaction, small reactor and low second pollution level. SCWO is the effective green technology for the organic pollutants. Under certain conditions, the COD removal of nitrogen-containing organics can be over99%.(2) When temperatures, residence times, pressure and Stoichiometric Ratio (SR) of Oxygen increased, the decomposition effeiciency (or COD removal) of the organic pollutants in SCWO increased. Generally, among these affecting factors, temperatures and residence times have larger effect on the oxidation reaction than the other factors. The effect of oxidant dose on the oxidation reaction in supercritical water, depended on the reaction process, and greater effect in the early period than in the late period was found. Pressure has light effect on the organic oxidation reaction, and the effect can be attributed to the various reactant concentrations and residence times induced by pressure change. When temperatures were high (above450℃), after the removal reached a relative high level, EDA residue and COD concentration in the wastewater tended to change steadily. Furthermore, if the same removal was obtained, the increase of temperature can shorten the residence time for reactants in the reactor, but the problem of strict requirement for the reactor design will be also proposed.(3) The gas products and intermediates analysis results of EDA oxidation in supercritical water showed, the major components of gas products were N2, N2O, CO and CO2. The intermediates for EDA oxidation were ethane, ethend etc, which supply information for the determination of EDA reaction pathways in SCWO. Based on the identification of products obtained in EDA in SCWO, the relation between the features and products of EDA in SCWO was analysed. It can be seen that the direct oxidation products included N2, CO, CO2, straight chain saturated alkanes, as well as a certain amount of ethylene and acetylene et al. The above products are not the results of direct oxidation. Thus, when EDA was decomposed in SCWO, many side reactions such as coupling, hydrolysis, pyrolysis and isomerization occurred at the same time. By analysis of the intermediates of EDA in SCWO with GC-MS, and combined of the structure features of EDA, the oxidation pathways for EDA oxidation in supercritical water were explored. The EDA decomposition in SCWO were via radical reactions, and followed the general principles of radical reactions, which includes three periods:chain initiation, chain propagation and chain termination. The chain initiation was accomplished by-OH, the pyrolysis can also produce some radicals of high activity. In the process of EDA decomposition in SCWO, ethylene, ethane, acetylene and methane were also determined, and these compounds were also produced by a series of radical reactions.(4) According the kinetics of EDA and in SCWO and COD removal kinetics, the kinetic model for EDA and COD removal in SCWO at25MPa.673-823K were:The kinetic models for COD removal of N-phenylglycinonitrile in SCWO at400-550℃,25MPa was:(5) The products and mechanism of2-chlorophenol (2-CP) in supercritical water was studied with quantum chemical method. The primary single-ring products of the2-CP oxidation in supercritical water were determined to be chlorohydroquinone,2,4-dichlorophenol and2,6-dichlorophenol and4-chlorophenol, among which chlorohydroquinone had the highest molar yields. The theoretical results were in good agreement with the experimental findings, and help to better understand the detailed mechanism of2-CP decomposition initiated by·OH radical in supercritical water.(6) With quantum chemical method, the intermediates and reaction pathways of N2O and CO reaction in supercritical water were studied. The reaction was found to be the major approach for N2O decomposition in supercritical water. The results can supply basic information and theoretical reference for the N2O control in the SCWO.SCWO is a new technique for organic wastewater treatment and has attracted wide attention internationally. The studies concerned the basic theory and application reports have also been conducted widely and made some progress. In order to promote the application of SCWO to practical industrial application, many aspects need to be enhanced, such as (1) Catalytic oxidation and relative problems solve,(2) the properties of supercritical water and function,(3) engineering implement and amplified experiments. |
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