Diagnostic Studies Of Atmospheric Pressure Pulsed Packed Bed And Microwave Torch Discharge Plasma

Plasma catalysis are gaining more and more attention in recent years for various environmental applications like CO₂ conversion,ozone generation,H₂ production,and gas pollutant removal,for its unique advantages in synergy effects of plasma and catalysis.The nanosecond pulse packed bed dielectric barrier discharge is the most popular structure to generate low temperature plasma,and the dielectric materials which are packed in the reactor are often used as the support of the catalyst.Because the dielectric effects,originating from the dielectric constants of the dielectric materials,can influence the generation and propagation of the discharge plasma,and then influence the application of the plasma catalysis.Hence,it is necessary to study the dielectric effects of the packing on interaction mechanism between plasma and the dielectric material first,the main research content are as follows:1.In this paper,dielectric columns with different dielectric constants are employed as dielectric materials in the packed bed reactor to investigate the dynamic behaviors of plasma interaction processes.The effects of the dielectric constants(zirconia:?=25 and PTFE:?=2.5)on the production of reactive species are studied for plasma catalysis applications.Comparison studies of discharge images,electrical characteristics,discharge dynamic evolution and spatial-temporal resolved optical emission spectroscopy are carried on when zirconia and PTFE columns are employed.The results show that there are four discharge processes existing in the packed bed reactor:surface streamer on the dielectric column,local discharge at the contact point,surface discharge on the grounded dielectric plate,and the volume discharge.The production of reactive species such as N₂(C³?_u),N₂⁺(B²?_u⁺)and O(3p⁵P)depend on the discharge processes to a great extent.The production of the N₂⁺(B²?_u⁺)always accompanies the formation of the streamer by electrons direct impact process to excite the ground state nitrogen molecules to N₂⁺(B²?_u⁺).And the O(3p⁵P)is generated in two different ways,which plays major role during the voltage pulse raising and falling time,respectively.The first way is the direct and fast one-step ionization and excitation by high energy electrons with O₂.The second way is the energy transfer from the nitrogen metastable N₂(A³?_u⁺)and energetic electrons,in which way the O is first ionized from O₂ and then be excited to O(3p⁵P).Furthermore,compared with zirconia column,PTFE column is more conductive to the generation of reactive species.2.The effect of pulse peak voltage on discharge evolution process and the distribution of reactive species are investigated in PTFE packed bed reactor.And the effects of pulse peak voltage and dielectric materials on discharge modes conversion,reactive species and relative vibration population are mainly studied.The results show that,when a polytetrafluoroethylene(PTFE)column is employed,with an increase in the peak voltages,the form and modes of the discharge both change.First,the discharge channel becomes“wider”and the discharge volume becomes larger,which results in more reactive species.Second,the discharge mode changes from surface discharge to diffuse discharge during the falling edge of the voltage pulse.Under the same experimental conditions,the number of nitrogen molecules distributed at N₂(C,v=1)is larger and the reactive species produced in the alumina packed bed reactor were fewer than those in the PTFE packed bed reactor.Finally,the emission intensities of the emission spectrum corresponding to the reactive species increased with increasing peak voltage.3.The preliminary characteristic of microwave plasma torch are studied,and the effect of microwave absorption power and oxygen content on the spatial distribution of active species and gas temperature are focused on.The results show that,with the increase of microwave absorption power,the corresponding emission spectrum intensity of N₂⁺,OH(A)and NO(A)increase.However,due to the differences in the production mechanism of the active particles,the corresponding spatial distributions are different.The gas temperature at different position of the microwave plasma torch increases with the increase of the microwave absorption power,and the greater the distance from the nozzle,the lower the gas temperature.Moreover,the intensity of the emission spectrum of OH(A)and NO(A)increases with the increase of oxygen content,but the oxygen content has little effect on the gas temperature.