Decomposition of NO2 in Oxygen-free NO2: N2 Gas Mixture by Back-Corona Generated Plasma

The back-corona discharge has been successfully applied as a plasma source for decomposition of NO₂ in the oxygen-free gas mixture of N₂:NO₂. The paper reports a first attempt to use back-corona discharge for noxious gas conversion. The preliminary results of laboratory experiments in a needle-to-plate reactor show that the De-NO_x processes in back-corona discharge are similar to the dc streamer corona discharges generated in the same geometry. Both types of discharges convert NO₂ to nitrogen, oxygen and also to N₂O and NO. However, back-corona discharge produces less NO, and is more efficient energetically in NO_x decomposition than the dc streamer corona discharge.     

Comparison of electrical discharge techniques for nonthermal plasmaprocessing of NO in N2

This paper presents a comparative assessment of three types of electrical discharge reactors: 1) pulsed corona, 2) dielectric-barrier discharge, and 3) dielectric-pellet bed reactor. The emphasis is on the efficiency for electron-impact dissociation of N₂(e+N₂ →e+N+N) and the subsequent chemical reduction of NO by nitrogen atoms (N+NO→N₂+O). By measuring the concentration of NO as a function of input energy density in dilute mixtures of NO in N₂, it is possible to determine the specific energy cost for the dissociation of N₂. Our experimental results show that the specific energy consumption (eV per NO molecule reduced) of different types of electrical discharge reactors are all similar. These results imply that, during radical production in electrical discharge reactors, the electric field experienced by the plasma is space-charge shielded to approximately the same value. The specific energy consumption for the dissociation of N₂ using electrical discharge processing is measured to be around 240 eV per nitrogen atom produced. In the NO-N₂ mixture, this corresponds to a specific energy consumption of around 240 eV per NO molecule reduced     

氮气对电晕放电等离子体降解甲基蓝的影响

电晕放电等离子体技术是近年发展起来的一种新型高级氧化工艺,因其处理效果好、操作简单、占地面积小的特点在印染废水处理领域得到了广泛应用。目前因大部分有机污染物的降解机理不详,该技术尚处于探索阶段。因此,为了尽早将电晕放电等离子体技术应用于工业印染废水的处理,不同污染物降解机理的研究对该技术的工业化和产业化应用具有重要意义。至今,电晕放电等离子体技术对研究较多的染料的降解效果均较好,然而,是否适合所有染料的降解有待进一步研究。采用电晕放电等离子体技术处理三苯甲烷类染料甲基蓝,研究了溶液的初始浓度对甲基蓝紫外-可见光谱中芳香环的降解率、发色基团吸光度变化的影响,测定了溶液的浓度、总有机碳（TOC）、总氮（TN）、pH值等指标随着放电时间的变化,并对其相关性进行了分析。结合紫外-可见光谱（UV-Vis）、三维荧光光谱（3D-fluorescence）和傅里叶变换红外光谱（FTIR）三种光谱学手段分析了电晕放电降解甲基蓝过程溶液的颜色、荧光物质和官能团变化,分析了电晕放电降解甲基蓝30 min后生成的中间产物。结果表明：电晕放电等离子体降解甲基蓝过程,溶液的浓度随着放电时间的延长逐渐减小,表明该技术对甲基蓝溶液有一定的降解能力;降解过程高压电极放电击穿含有大量氮气的空气产生N,NO·,N+₂等含氮高活性粒子,这些粒子通过扩散作用迁移至液相,使得溶液中TN含量在整个降解过程逐渐升高;另有部分含氮高活性粒子与钨钢针电极溶出的C元素键合生成发色的CN双键,使得溶液中的总有机碳在放电5 min时有所升高。延长反应时间产生的高活性粒子与溶液中的有机物（甲基蓝及中间产物）继续作用,部分有机物矿化生成CO₂,引起溶液中TOC含量的下降。电晕放电相同时间内产生的活性粒子数量相当,增大甲基蓝浓度,未被降解的甲基蓝分子越多,导致甲基蓝降解率的减小。电晕放电过程甲基蓝分子之间的聚合与发色CN双键的生成共同促使甲基蓝发色基团吸光度在放电5 min时达到最大;且甲基蓝溶液的初始浓度越高,吸光度（A₅-A₀）升高的越多。概括来说,甲基蓝结构中发色CN双键的存在是电晕放电等离子体降解甲基蓝过程溶液颜色加深再变浅的主要原因。反应过程羟基自由基的消耗导致放电5 min时溶液的pH值升高;随着反应的进行溶液中生成的硝酸及小分子酸增强了溶液的酸性,导致pH值降低。三维荧光光谱结果表明,甲基蓝降解过程出现了三类明显的荧光峰,位于E_X/E_M=310~320/430~450,E_X/E_M=240~250/320~340和E_X/E_M=280/340,分别代表腐殖酸类物质、芳香族蛋白质和溶解性微生物代谢副产物。甲基蓝溶液降解前的荧光物质主要为腐殖酸类,随着降解时间的延长,腐殖酸类物质首先降解生成了芳香族蛋白质,进一步降解产生可溶性微生物代谢副产物。比较电晕放电前后甲基蓝溶液的红外光谱图和红外分峰图发现,甲基蓝结构中N-H键3 432.8 cm-1处不对称伸缩振动峰红移了0.3 cm-1,烯烃和苯环上C-H键2 975.9 cm-1处的伸缩振动峰向高波数偏移了0.5 cm-1,1 638.7 cm-1处RCHCHR的双键伸缩振动位置蓝移了3.2 cm-1,芳仲胺的C-N伸缩振动峰1 341.6 cm-1向高波数偏移了1.3 cm-1,磺酸基SO的伸缩振动峰1 121.1和1 034.3 cm-1分别红移了3.8和13 cm-1,甲基蓝结构中的环外CC双键与CN双键吸收峰消失,在1 692.4和1 400.4 cm-1处分别出现了CO和NO的伸缩振动吸收峰,产生了2,5-环己二烯-1,4-二酮、对硝基苯磺酸钠和芳香酮类等中间产物。该结果对于利用电晕放电等离子体技术处理甲基蓝废水具有重要的理论意义和实用价值。     

A compact high-voltage pulse generator using a rotary airhole sparkgap

A pulsed-corona-discharge nonthermal plasma technique offers the advantages of energy efficiency and capability for the simultaneous removal of coexisting pollutants. The key to success in a nonthermal plasma approach is to produce a corona discharge in which the bulk of the electrical energy goes into the production of energetic electrons, rather than into gas molecules heating. By driving the nonthermal plasma reactor with very short pulses of high-voltage, short-lived corona discharge plasmas are created that consist of energetic electrons, which in turn produce the radicals and ozone responsible for the decomposition of the toxic molecules. Because of the short lifetime of the pulsed corona discharge, little electrical power is dissipated in movement of heavy molecule ions in the processing region, thus avoiding heating the gas and providing good electrical energy efficiency.A high-voltage pulse generator with a rotary airhole sparkgap, instead of a conventional rotary ball sparkgap, has been proposed. Its rise and fall time characteristics to be used as a smart pulse generator for pulsed corona discharge-type nonthermal plasma reactor were investigated. Parametric studies showed that the proposed airhole sparkgap had a very fast rise time, very short fall time, very low-and-stable breakdown voltage, and reliable pulse repetition characteristics, compared with the conventional rotary ball sparkgap.     

A novel design of electrodes system for gas treatment integrating ceramic filter and SPCP (surface corona discharge induced plasma chemical process) method

A novel configuration of electrodes system for non-thermal plasma gas treatment was proposed, in which surface corona discharge induced plasma chemical process (SPCP) and a ceramic filter were integrated. The system was characterized by a very simple configuration and a high contact efficiency between gas and discharge plasma. A feasibility of the newly developed apparatus was studied for NO_X and soot treatment. NO_X as gaseous pollutant was decomposed effectively. Soot suspended in gas as particulate pollutant was captured on the ceramic filter and disappeared with the surface corona discharge treatment on the filter surface. Furthermore, soot and NO_X were treated simultaneously. Because soot is removed by oxidization and NO_X by reduction, therefore, there is a possibility that they are treated simultaneously and complementarily.     

Formation of reactive species in gliding arc discharges with liquid water

The effects of gas composition on gliding arc (glidarc) electrical discharge reactors with pure water have been studied. The glidarc reactors utilized AC electrical discharges with two different electrode configurations. In one case a set of two stainless steel electrodes connected to a single power supply was placed in the gas phase over the liquid surface (power=250–300 W, maximum voltage=12 kV). The second experimental arrangement utilized a reactor with a set of three stainless steel electrodes supplied by two identical high-voltage transformers, where the electrodes were placed over the water surface or with the water sprayed directly in the plasma formed between the electrodes (power=500–600 W, maximum voltage=12 kV). The variation of pH and conductivity and the formation of hydrogen peroxide, ozone, nitrate, and hydrogen were measured. The effects of the type of gas, including pure oxygen, pure nitrogen, and dry air, were determined.     

喷嘴-平板直流电晕放电中的OH(A2S+→X2P, 0-0)光谱研究

 为更深入地认识电晕放电低温等离子体中自由基的生成机理，以发射光谱测量为基础并结合背景气体淬灭率影响，研究了常压下喷嘴-平板电晕自由基簇射过程中放电参数、背景气体、电极气成分等因素对OH(A²S⁺→X²p, 0-0)发光的影响。结果表明：在放电参数影响中，放电电压及放电电流都会影响OH生成量，OH发光随功率增加而大大增强；在加湿氮气直流电晕放电中有明显的OH(A²S⁺→X²p, 0-0)光谱存在，但加湿空气条件下OH生成较少；载气增湿后OH生成量明显增多，而Ar和O­2的存在分别增强和减弱了OH(A²S⁺→X²p, 0-0)发光，可能的原因是这两种物质影响了放电和OH(A²S⁺)的淬灭。     

Diagnosis of electron temperature in Ar/O2 mixed gas and destruction of toluene/benzene by positive dc discharge plasma

The electron temperature (T_e) of positive dc corona plasma in Ar/O₂ atmosphere was diagnosed, and plasma decomposition of toluene/benzene was studied in a razor–plate reactor. Experimental results revealed that T_e would increase with corona current until it reached a peak value, and then decrease; the volume fraction of Ar (φ_Ar) in Ar/O₂ mixed gas also influenced T_e, the higher φ_Ar, the lower T_e. Though the decomposed volume fraction of toluene/benzene was positively related to the input power, the decomposition efficiency did not monotonically increase with the specific energy density. The highest energy yield reached 3.8 g-toluene/kWh and 2.4 g-benzene/kWh, respectively.     

Decomposition of benzene using wire-tube AC/DC discharge reactors

Decomposition of benzene was investigated using single and two-stage stainless steel reactors with application of high-voltage AC and DC currents. Benzene removal, CO, and CO₂ were measured as a function of reactor diameter, applied voltage, and stage of reactor configuration. The results showed that, benzene removal and CO₂ selectivity improved in two-stage configuration and reached up to 59% and 97.8% respectively, where the gap size was 8.5 mm. Formation of OH, N-H and N=O function groups were seen in the exhaust gas. Subsequently, organic acids, phenols, and alcohols were identified as the main byproduct of benzene decomposition considering GC-MS analysis.     

Modelling of an improved positive corona thruster and actuator

In this work we are going to perform a simulation of a positive corona discharge in nitrogen gas, using two different asymmetric capacitor geometries. We intend to increase the highest ion wind velocities and electrostatic propulsion forces on the considered structures. In our model, the used positive ion source is a small diameter wire, which generates a positive corona discharge directed to the ground electrodes. By applying the fluid dynamic and electrostatic theories, all hydrodynamic and electrostatic forces will be computed in an attempt to demonstrate the greater performance of the new developed geometries.     

A wire-to-wire type nonthermal plasma reactor with ferroelectric pellet barrier

A new wire-to-wire type of discharge plasma reactor with ferroelectric pellet barrier has been proposed and investigated experimentally. It was found that there were intense microdischarge currents taking place on the AC corona-charged surfaces of the ferroelectric pellet barrier. These surfaces were placed on the bottom wire electrode in the reactor. This type of nonthermal plasma reactor was found to generate a corona discharge twice per half cycle of the applied AC high voltage, once from the upper corona wire and again from the surface of the pellet barrier.As a result, the proposed plasma reactor has the potential to greatly increase ozone production and may find use as an effective means for removing pollutant gases.     

Application of a non-thermal surface plasma discharge in wet condition for gas exhaust treatment: NOx removal

This paper deals with the NOx removal with the help of a non-thermal surface plasma discharge in wet conditions. The gas treatment device consisting of a surface discharge and a wet-type reactor, was characterized through FTIR and electrical measurements. The ability of the proposed system for the cleaning of gas exhaust was studied. NOx as gaseous pollutant was decomposed effectively. To improve the chemical conversion, a coil was inserted in the electric circuit then a catalyst was placed in the plasma area. Results showed an improvement of NOx removal by an increase in radical species produced and synergistic effect, respectively.     

Photocatalytic performance of TiO2/V2O5 nanocomposite powder prepared by DC arc plasma

TiO₂/V₂O₅ nanocomposite powder was synthesized by the DC arc plasma, and its photocatalytic activity was examined by decompositions of Rhodamine B solution and toluene gas. In the synthesis of TiO₂/V₂O₅ nanocomposite powder, TiCl₄ and VOCl₃ precursors were introduced into thermal plasma flame with argon carrier gases through separated two gas bubblers. They were decomposed by Ar–N₂ thermal plasma generating Ti and V vapors, followed by the formation of oxides with the injection of additional oxygen into a plasma reactor. Nanocomposite composed of relatively small size V₂O₅ nanoparticles on a spherical TiO₂ nanoparticle which was about 250 nm in diameter was identified by X-ray diffractometry, electronic microscopy, and energy dispersive spectroscopy when the ratio of carrier gas flow rates for TiCl₄ to VOCl₃ was 1:4 or 1:5. In ultraviolet–visible absorption spectroscopy, the absorbed wavelength of light for synthesized TiO₂/V₂O₅ nanocomposite powder was wider than that for commercially available TiO₂ nanopowder. Therefore, Rhodamine B solution exposed to visible light was decomposed by TiO₂/V₂O₅ nanocomposite, whereas it was not decomposed by TiO₂ nanopowder. In addition, toluene decomposition in a dielectric barrier discharge reactor was carried out with nano-sized photocatalysts of TiO₂ nanopowder and TiO₂/V₂O₅ nanocomposite. Relatively higher removal rate of toluene was found in the case of TiO₂/V₂O₅ nanocomposite in virtue of improved photocatalytic performance.     

Jerks-and-jumps type electrode diagnostics in a DBD reactor by OES

This work reports on the successful measurement of the second-positive N₂(B ? C) and the first-negative N ₂ ⁺ (B ? X) band system emissions intensities for the transitions observed in the 337.1-and 391.4-nm wavelengths, respectively, by optical emission spectroscopy (OES). By determining the electron energy distribution function and the spectroscopy results, it is possible to obtain the average electron energy of the system. Two dielectric barrier discharge reactors of rectangular geometry have been used for this purpose: a first reactor, endowed with a single dielectric and a modified electrode, in which discontinuities of the jerks-and-jumps type were manufactured in order to confine the discharge, and a second reactor of conventional flat electrodes and a double dielectric. The final objective of this study is to ascertain which of the reactors provides a higher efficiency in the NO _x removal from a gas mixture. The results indicate that, for the first reactor, energies on the order of 11 eV were reached, while lower energies up to 8 eV were obtained in the second reactor. This indicates clearly that the first reactor is close to the corona discharge behavior. As to the removal efficiency, the first reactor showed a better performance with inferior concentrations around a 50-μmol/mol mixture.     

Removal of NOx from NO2∶NO∶N2 mixture by a pulsed and dc streamer corona in a needle-to-plate reactor

The results of laboratory experiments on reduction of NO _x in the oxygen free gas mixture NO₂NON₂ simulating exhaust gas, by means of pulsed and dc streamer corona discharges generated in a needle-to-plate reactor have been presented. The results show that the dc corona discharge is more efficient in De-NO _x process than the pulsed corona discharge. This is in contrast to the results obtained in the wire-to-cylinder reactors where the pulsed corona discharge removes NO _x more efficiently. The results also lead to the conclusion that in the dc streamer corona discharge the short pulses and long interelectrode distances are recommended in order to increase the NO _x conversion rate.Presented at 17^th Symposium Plasma Physics and Technology, Prague, June 13–16, 1995.This work was financially supported by the Polish Academy of Sciences (projects IMP 3.1 and 3.3) and by the Polish Committee for Scientific Research (KBN Grant No. P40103304).     

气体介质对多间隙气体开关电晕均压与自击穿特性的影响

快脉冲直线变压器型驱动源(FLTD)是近年来快速发展的新型脉冲功率源技术,多采用多间隙气体开关作为开关器件。电晕均压措施有利于提升开关击穿性能,但不同气体中电晕放电有显著区别。本文首先研究了空气中针电极对单间隙电晕放电特性的影响,确定了电晕针电极的尺寸,之后研究了N2,CO2,SF6/N2混合气体、C4F7N/N2混合气体中的电晕放电特性,研究了电晕均压6间隙气体开关击穿电压及其稳定性随气体种类和气压的变化规律。实验结果表明,N2中电晕电流较大且不稳定,空气中电晕电流比N2中低,且电晕放电较为稳定,微量强电负性气体加入会极大降低电晕放电电流。当采用空气和N2作绝缘介质时,气体开关击穿电压随气压升高线性增加,但存在低值击穿,微量强电负性气体混合N2可显著提升击穿电压的稳定性。1%SF6/99%N2混合气体在0.18 MPa时,击穿电压约为197.33 kV,标准偏差占击穿电压比例为1.50%,1%C4F7N/99%N2混合气体在0.15 MPa时,击穿电压约为190.42 kV,标准偏差为0.55%。这表明,微量环保替代气体C4F7N与N2的混合气体对于提升多间隙气体开关击穿电压稳定性有显著作用。     

The influence of NH3 on NO2 conversion in a dc corona discharge in N2:O2:CO2:NO2:NH3 mixture

The aim of this paper is to investigate the influence of NH₃ additive (540–1470 ppm) on the conversion of NO₂ and the creation of NO and N₂O in a mixture of N₂:O₂:CO₂: NO₂:NH₃ subjected to the so-called direct current (dc) corona discharge. The dc corona discharge was generated in a needle-to-plate reactor. Seven positively polarized needles were used as one electrode and a stainless steel plate as the other. The time-averaged discharge current was varied from 0 to 7 mA. It was found that the dc corona discharge decomposed NO₂ and produced NO and N₂O. The reduction of NO₂ was higher without NH₃ additive if the residence time of the operating gas was relatively short. However, in a longer corona discharge processing the NH₃ additive may be useful for reduction of NO₂.Supports from the Research and Development Commitee (KBN) under Programme KBN 0889/P4/93/04 and the Polish Academy of Sciences within IMP 3.1 project are gratefully acknowledged.     

Ferro-electric pellet shape effect on C/sub 2/F/sub 6/ removal by a packed-bed-type nonthermal plasma reactor

Ceramic dielectric material pellet shape effects on the performance of perfluoroethane (C/sub 2/F/sub 6/) gas removal from simulated semiconductor process gas using packed-bed reactor are experimentally investigated. The bench-scale cylindrical shaped (plasma part: 30-mm inner diameter and 20-mm length) plasma reactor consists of two metal mesh electrodes packed with spherical, cylindrical, or hollow cylindrical shaped ferro-electric pellets with various dielectric constants. The 60-Hz ac high voltage was applied to the mesh electrode. The 3000 ppm C/sub 2/F/sub 6/ gas diluted with nitrogen was used as simulated gas with flow rate of 30 mL/min. The C/sub 2/F/sub 6/ concentration was monitored using Fourier transform Infrared absorption spectroscopy measurements. The results show that the packed-bed plasma reactor with the hollow cylindrical-shaped pellets removed the C/sub 2/F/sub 6/ gas with energy efficiency of 3.7 g/kWh. This value was almost 1.5 times higher than the efficiency 2.5 g/kWh in case of the spherical pellets. The discharge characteristics in the reactor were also changed with the pellet shape. The discharge onset voltage decreases by changing the pellets shape from sphere to hollow cylinder. The quantity of charges accumulated with the microdischarge currents increases by changing the pellet shape from sphere to hollow cylinder in spite of fact that the energy consumed in the reactor decreases.     

Corona discharge as a temperature probe of atmospheric air microwave plasma jet

We developed and tested a new method for temperature measurements of near-LTE air plasmas at atmospheric pressure. This method is specifically suitable for plasmas at relatively low gas temperature (800–1700 K) with no appropriate radiation for direct spectroscopic temperature measurements. Corona discharge producing cold non-equilibrium plasma is employed as a source of excitation and is placed into the microwave plasma jet. The gas temperature of the microwave plasma jet is determined as the rotational temperature of N₂? produced in the corona discharge. The corona probe temperature measurement was tested by the use of a thermocouple. We found a fairly good agreement between the two methods after correcting the thermocouple measured temperatures for radiative losses. The corona probe method can be generally applied to determine the temperature of the near-LTE plasmas and contrary to the thermocouple it can be used for higher plasma temperatures and is not affected by radiative losses and problems of interaction with the microwave plasma and electromagnetic fields.     

Investigations of the spatio-temporal build-up of atomic oxygen inside the micro-scaled atmospheric pressure plasma jet

The ascent of atomic oxygen densities created inside the micro-scaled atmospheric pressure plasma jet has been investigated spatially resolved under parameter variations such as applied power, gas mixture and gas velocity using two-photon absorption laser induced fluorescence spectroscopy. Along the discharge channel an increase of the atomic oxygen density within the plasma is observed. The density shows an exponentially asymptotic convergence into an equilibrium close to the effluent. In the post-discharge effluent an exponential spatial decrease can be found. Typical ascent distances of a few hundreds of μm decrease with the applied power and increase with gas velocity and oxygen admixture. The maximum atomic oxygen density increases with applied power and admixed molecular oxygen up to more than 10¹⁶ cm^-3. An increase of the maximum atomic oxygen density with increasing gas velocity has been found. Optical emission spectroscopy measurements indicate a strong increase of the nitrogen emission at low gas flow rates along the channel.