Research On Degradation Of Dioxins Via Microwave-metal Discharge Plasma

With rapid worldwide industrial development,standards of living have continuously improved,but those improvements too often come with a steep environmental cost.The problem of air pollution is the most serious,especially the pollution of Persistent Organic Pollutants(POPs)such as dioxins,which are the most difficult to handle and control.Although these traditional dioxin control measures reduce the total emission of dioxins,these methods still have drawbacks such as absorbent treatment,harsh application environment and high production cost,which limit the application of these methods.Therefore,seeking to develop new and efficient methods to control dioxin emissions was imperative.In this paper,a new method of microwave-metal discharge excitation plasma was studied.And applied to the degradation of a dioxin-like substance—TCB.It also investigated the effect of oxygen and water vapor by volume in simulated gas as well as the effect of microwave power,total flow of gas and initial concentration of TCB on the degradation of TCB.In addition,the formation and transformation pathways of TCB degradation products were analyzed by Gasboard 3100,Gas chromatograph-mass spectrometer and Ion chromatograph.Moreover,the mechanism of degradation of dioxin in plasma environment was reasonably deduced.Following conclusions were obtained:(1)Tungsten metal has long-term stability of microwave plasma jet stability and good high temperature resistance,it can be used as a discharge metal for subsequent experiments.(2)The symmetric placement of the metal was greater than that of the asymmetrically placed plasma,and the jet length was relatively long.And comparing with the nitrogen,oxygen as well as air in the laboratory research level,argon was more suitable as a discharge gas for subsequent dioxin degradation experiments.(3)After 7min of the experiment,TCB evaporation could reach the equilibrium of gas-liquid two phase,and the absorbent could completely absorb TCB in the first 10min.(4)The volume content of oxygen and water vapor on the degradation effect of gas phase TCB(η(max)=5vol.%O2,η(max)=4vol.%H2O),In an argon atmosphere containing 5vol.%O2 and 4vol.%H2O(g),3kW microwave input power completely degraded TCB(concentration 7.05μg/mL).And the effect of both was synergy in the plasma environment.(5)The effect of microwave input power,total flow of gas and initial concentration of TCB on the degradation effect was:the lower the initial concentration and the total flow of gas,the larger the microwave input power,and the better the degradation effect was.(6)The final products of gas phase TCB degradation were mainly CO2,HC1 and CH4.Moreover,the conversion rates of CO2 and HC1 reached 89%and 99.8%under conventional microwave power(1kW),respectively.During this period active free radicals(O and OH)play an important role in the formation of the final product.(7)According to the degradation behavior of gas phase TCB in plasma environment,the general degradation behavior of dioxin in plasma environment was obtained:when dioxin entered the plasma region,de-chlorination,de-hydrogenation and benzene ring fracture occurred first,followed by formation of HCl,CnHm,CH4 and unstable fragment particles.After the initial reactions occured,fragment particles and chain hydrocarbons continue to have complex physical and chemical reactions with the active radicals;almost all of them eventually bacome CO2,CH4 and HC1.