Characteristic Diagnosis Of Atmospheric Discharge Plasma And Kinetics Study Of Reactive Species

Low-temperature plasma has received extensive attention due to its promising application prospects in the field of air pollutants degradation and energy conversion.To fulfill the need for particular application,constructing stable plasma source and investigating the interaction mechanism between plasma and substances,have been a hot research topic of plasma application.Based on application for VOCS degradation and CO2/CH4 conversion,this thesis reports the diagnosis and improvement of plasma sources,diagnosis of the active species in plasma and modeling study of chemical kinetics processes.The main research contents are as follows:1.A diffuse Sine AC dielectric barrier discharge(DBD)is successfully obtained by optimizing the electrode structure.ICCD images,waveforms of voltage and discharge current are measured to study the discharge mode and dynamic process.The effects of electrode structure,applied voltage,electrode gap distance and gas composition on the discharge uniformity,optical emission intensity and plasma temperature are investigated.It is found that the discharge channels show obvious shrinkage characteristics in discharge with one single dielectric plate.The ICCD images show that the discharge in the positive half cycle is a typical streamer mode,and the breakdown of gas gap is not synchronized.While the discharge in the negative half cycle is a glow-like mode with a better synchronization.The discharge current in the negative half cycle is higher 2 or 3 times than in the positive half cycle.Using double-layer dielectric plates can limit the discharge current intensity and significantly improve discharge uniformity.The discharge channels in both positive and negative half cycle are more diffuse.The electrical characteristics and gas temperatures with different operating time show that the stability of discharge is improved by using double-layer dielectric plates.2.Both unipolar(positive and negative)and bipolar pulse are employed to generate diffuse DBD plasma,and a comparative study,in aspects of the discharge images,electrical characteristics,optical emission spectra(OES),and plasma gas temperatures,is carried on.The discharge evolution dynamic processes and energy transfer of nanosecond pulsed DBD with a needle-plate electrode configuration are analyzed by ICCD images and temporal-spatial resolution OES.By calculating the generation and quenching rate of N2(C3Πu)、N2+(B2∑u+),the reduced electric field(E/N)are calculated using the temporal-spatial emission spectra.It is found that there are two obvious discharge current peaks in pulse voltage rising edge and falling edge,respectively,for unipolar high-voltage pulse,while there is only one relatively strong breakdown in a single pulse time for bipolar high-voltage pulse.Three main stages in NPDBD are distinguished,i.e.,the streamer breakdown from needle tip to plate electrode,the regime transition from streamer to diffuse,and the propagation of surface discharge on the plate electrode surface.At the beginning of the discharge,the E/N near the needle tip is about 590 Td,which is high enough to excite the initial breakdown as a positive streamer regime.The streamer builds up a new electric field with radial direction and pre-photoionizes the surrounding air to drive the subsequent breakdown.Hence,an abundance of fine secondary streamer channels around the initial streamer,contributing to the transition to diffuse regime.Once the steamer reached the surface of dielectric plate,the E/N near the surface of dielectric plate increases,driving the surface discharge propagates along the radial,while the volume discharge extinguished.The vibration temperature of N2 increased with the duration of the discharge,which means electron energy was transferred into the vibrational level.However,the rotation temperature was mainly constant with increasing discharge time,which means the gas heating was not obvious during the discharge pulse.3.Both sine AC DBD and nanosecond pulsed DBD(NPDBD)are used in formaldehyde degradation,and the removal efficiency with different power density and different oxygen mole fraction(OMF)are investigated.To understand the mechanisms of iommaldehyde degradation by nanosecond pulsed discharge plasma,a zero-dimensional(0D)chemical kinetics model is developed for formaldehyde degradation by NPDBD.It is found that the energy consumption of NPDBD is still much lower than that of ACDBD.Modeling results show that OMF has a clear effect on the number density and life of the reactive species.When the OMF is low,both the N2(A)radicals,OH radicals and H atoms exhibit a high number density.With OMF rising,as N2(A)radicals,OH radicals and H atoms were quenched by O2,O atoms become the dominant species.As a result,OMF has a clear effect on the relative contributions of the active species in the degradation of formaldehyde.In the pure nitrogen discharge,the important active species in formaldehyde degradation are OH radicals,H atoms,and metastable N2 molecules,in which OH has the highest contribution(about 69%),and contribution of H,and metastable N2 molecules is 20%and 11%respectively.When the OMF increases from 0 to 2%,the contribution of OH radicals increases,up to 98%,while the contribution of H atoms and metastable N2 molecules decreases.When the OMF increase to 3%,O atoms begin to play a role in the degradation,and its contribution increases with the increasing of OMF,gradually occupying a dominant position.At the same time,as OH radicals are largely quenched by oxygen molecules,their contribution gradually decreases.Although the density of metastable nitrogen molecules is much lower than that of OH radicals and O atoms,they also play a nonnegligible role role,with a contribution rate of about 10-17%.4.A 0D chemical kinetics model is devolved to investigate the underlying plasma chemistry of methane dry reforming in nanosecond pulsed discharge.The time evolution of the most important species densities are calculated.Furthermore,the most important loss and formation processes of CO2 and CH4,as well as the most important product formation processes,are explored.The model reveals that the most important dissociation reactions of CO2 and CH4 are electron-impact dissociation reaction,dissociated CO2 into CO and O,and dissociated CH4 into CH3 and H,respectively.The major products are CO,H2,C2 and C3 hydrocarbons.The dominant formation reaction of CO is electron impact dissociation of CO2,but dissociation of CO2 upon collision with H radicals is also quite important.The most important production process of H2 is the reaction of CH4 with H radicals.C2H2 is the most abundant hydrocarbon product,which is mainly produced by the recombination of two CH2 radicals and dissociation of C2H4.The CH3 radicals are also important intermediate radicals,and play a crucial role in the formation of hydrocarbon products.Indeed,the recombination of CH3 with C2H5 is the most important formation reaction of C2H4,while the most important formation reactions of C2H6 and C3H8 are the three-body recombination of two CH3 radicals,and the three-body recombination of CH3 with C2H5 radicals,respectively.It is found that most of CO2 molecules are populated into vibrational state from the ground state in the pulse duration time.Hence,the vibrational states of CO2 play important role in the dissociation of CO2.When looking at the overall CO2 dissociation,the ground state only contributes for 15%,while the vibrational levels contribute for 85%.