Exploration On High Efficient Technology For VOCs Degradation By Non-Thermal Plasma

With the continuous development of industrialized production and agricultural intensification,and the over-exploitation and utilization of natural resources by human beings,the natural environment has been increasingly seriously damaged and affected,which causes more and more challenges to environmental protection and pollution control.Volatile organic compounds?VOCs?are the main pollutants that bring great harm to the natural environment and human health after NOx,SO_x and H₂S.Therefore,reducing,converting and decomposing VOCs is an important field in environmental protection research.Non-thermal plasma?NTP?is a new technology developed in recent decades for VOCs debating.Because of its simple equipment,wide applicability,fast processing time and other advantages,it has been received extensive attention and studied,and gradually applied in some industrial VOCs treatment.However,NTP technology still has some drawbacks such as incomplete oxidation,low mineralization,low energy yield?EY?,unclear mechanism and low utilization of high-energy electrons.In order to improve the degradation efficiency of VOCs,EY,CO₂ selectivity and other parameters,in this thesis aiming at resolving the issue of high energy consumption and other problems which hinder the practical application of NTP VOCs degradation technology to industry,the exploration on degradation of VOCs represented by toluene and benzene through two methods,namely pulse modulated dielectric barrier discharge?DBD?and corona discharge combined with external magnetic field,had been carried out for improving the degradation efficiency,EY and exploring the degradation mechanism.The study and results are summarized as follows:?1?The electric field strength and the electron mean kinetic energy in co-axial DBD were estimated based on static electric filed model.It was found the electric field strength and electron mean kinetic energy in double dielectric layer DBD are greater than that with single layer under the same discharge conditions.The calculation results can be successfully applied to explain the toluene degradation efficiency in double dielectric layer DBD is larger than that in single layer DBD under the same discharge parameters of pulse modulation DBD plasma.?2?The effect of pulse modulation DBD plasma on benzene degradation was studied.In pulse modulation DBD plasma with constant applied voltage,it was found that the discharge power and reactor temperature increased with duty cycle,suggesting pulse modulation power supply can not only reduce the power output and Joule heat,but also reduce the temperature of the reactor owing to the cooling during the non-discharge stage,which is beneficial to the safe operation and protection of the reactor;the degradation efficiency of benzene decreases with reducing duty cycle,but the EY and CO₂ selectivity increase.It suggests that the degradation of VOCs by pulse modulated plasma can not only improve EY but also CO₂ selectivity.?3?Using toluene as VOC agency,the key factors affecting the degradation of VOCs by pulsed DBD plasma were studied.Experimental results demonstrated that the toluene removal efficiency and CO₂ selectivity increased with specific input energy?SIE?,and decreased with initial concentration and flow rate.As the SIE was increased from 98 J/L to 287 J/L,the concentrations of O₃ and NO₂ increased and reached the maximum at SIE of 287 J/L,but decreased dramatically as the SIE is greater than 287 J/L.It was also found the removal efficiency of toluene in different background gas decreases in the order:O₂>air>N₂,at the same value of SIE.?4?The effects of relative humidity?RH?on benzene degradation were studied.It was found that there is an optimum humidity value for the benzene removal efficiency and EY.Around relative humidity of 60%,the optimum benzene removal efficiency and EY were achieved.As RH was less than 60%,benzene removal efficiency increased with RH,but decreased with RH as RH is greater than 60%.This variation trend was also found for EY.Furthermore,it was also found in experiments that the increasing of RH inhibited the formation of ozone and NO₂ remarkably.Both of the concentrations of ozone and NO₂ decreased with RH at same value of SIE.Therefore in future industrial application,degradation effect can be optimized and harmful components can be reduced by controlling RH in VOCs.?5?Comparative study on the benzene,toluene and their mixture under atmospheric pressure with pulse modulation plasma was carried out.The experimental results showed that the pulse modulated DBD plasma has a great advantage in VOCs treatment.Compared with the literature data,it was found that the EY and energy constant?^?E?values obtained from degradation of benzene or toluene are higher than those obtained from general DBD plasma,indicating that both the degradation efficiency and energy yield of VOCs were significantly improved by pulse-modulated DBD plasma,which showed good application advantages of it.In terms of the same initial concentration of benzene and toluene,under the same value of SIE,it was found the degradation efficiency of toluene,EY and energy constant are higher than those in the case of degradation of benzene,which means toluene is more likely to be degraded.Compared with the degradation of toluene with the same concentration,NTP produces higher ozone concentration in the process of degradation of benzene,which means that the amount of ozone produced by NTP in degrading VOCs is related to the VOCs species.The experimental results also showed that the relationship between CO₂ selectivity and SIE,and that between carbon balance and SIE are not only related to the discharge model,but also related to the type of VOCs and its initial concentration.In the degradation of the mixed VOCs of benzene and toluene,it was found that the degradation efficiency obtained after mixing,EY,O₃,CO₂ selectivity and carbon balance were lower than in the degradation of single component.?6?FT-IR?Fourier Transform-Infrared Spectroscopy?and GC-MS?Gas Chromatograph-Mass Spectrometry?measurements were employed to analyze product of benzene/toluene degradation with pulse modulation DBD plasma.The analysis of the outlet gases through FT-IR in the process of degrading benzene and toluene in air plasma,indicated that the main components of the outlet gases were CO₂,CO,O₃,H₂O,HCOOH and benzene ring derivatives.According to the GC-MS results,it is suggested that the byproducts from benzene degradation primarily consisted of phenol and nitrobenzene.The main components of the residues formed on the inside wall of plasma reactor are 2,4-dinitro-phenol and 4-nitro-phenol.The outlet gas after toluene degradation was also analyzed by GC-MS,suggesting that the main byproducts included ethylbenzene and benzonitrile in N₂ plasma.For both O₂and air plasma,the following byproducts were detected:formic acid,acetic acid,maleic anhydride and benzaldehyde.By analyzing the degradation product and degradation efficiency with atmosphere,nitrogen or oxygen as background gas,it was found that O free radical plays a very important role in the degradation process.Based on the analysis results,possible pathways for benzene and toluene degradation were proposed.?7?Because of nonuniformity and low electron temperature,corona discharge plasmas has low efficiency of VOCs degradation.In order to further improve the removal efficiency,energy efficiency and other parameters,the promotion of toluene removal by combination of NTP and magnetic field was investigated.Experimental results indicated that the external magnetic field could efficiently enhance the removal efficiency of toluene,EY,CO₂ selectivity and carbon balance.During the discharges with the same applied voltage and energy density,they increased with magnetic induction strength or magnet length.As to the mechanism,it is considered that the high-energy electrons moved with a much longer helical path-length under Lorentz force in magnetic field and consequently had longer retention time in the plasma than that in the discharges without external magnetic field,leading to larger collision probability with background gas and consequently increase of electron density and free radical in the discharge area,as result the removal efficiency,EY,CO₂ selectivity and carbon balance were increased.The greatest advantages of using magnetic field are,the method is simple,the experiment operation is easy and no external energy is needed.