Analysis of impulse electric field effect on organic exhaust gas decomposition

环境科学学报990108环境科学学报旧万万数据资源系统 ENVIRONMENTAL数字化期刊WANFANG DATA (CHINAINFO)SCIENCESDIGITIZED PERIODICAL1999年第1期No.11999Analysis of impulse electric field effect on organic exhaust gasdecomposition*Zhang Yu, Jin Xin-yu**, Jian Xuan-zhen, Wang RuiDepartment of Information and Electronic Engineering, Zhejiang University, Hangzhou310027 ChinaAbstract--The technique of organic exhaust gas decomposition with impulse corono-dischrgeplasma has been investigated in this study. It has been discovered that the impulse electricfieldaffected the decomposition efficiency with the secondary electron emission coefficient8 of the corona electrode as an intermediate: when w was fixed the corona electrodematerial with higher 8 could induce higher decomposition efficiency. In these experimentswolfram electrode which has the highests has really induced the highest decompositioneFficiency.Keywords: impulse electric field power; impulse corona discharge; secondary electronemissioncoefficient; organic exhaust gas decomposition1 Introductionand The organic exhaust gases mainly contain adipose, aromatic hydrocarbon, alcohol, etheramine and so on. They have greatly polluted the atmospheric environment, and eventhreatened humanbeings existence. At present, the commonly used methods of the organic exhaust gas disposalcontainchemical catalytic oxidation (Su, 1990), electron beam irradiation (Xu, 1986), and microbialdecomposition(Tanaka, 1986)and soon. But these methods often needed the high cost ofproduction and complicated equipments. The technique of plasma decomposition has beenstudied in detail in the decomposition of some inorganic waste gases(Matsumoto, 1983), butthere has been few researches on the organic exhaust gas decomposition using this techniqueThough there have been some experimental reports(Huang, 1997Zheng, 1997), there has been very few reports about the detailed effCNMHeelectric fieldfle∥/Eyqk/ jese/jes99je9901/990108hm(第1/11页)2010323033:46环境科学学报990108on the organic exhaust gas decomposition In this paper, based on lots of experiments,according to thestreamer theory of high-voltage impulse corona discharge, the electric field energy exchangemechanismin the plasma chemistry, and the reaction equipment applicability in the experiments, themechanism of the discharge electric field effects on the decomposition and the actual electricfield structure of the impulsecorona discharge have been analyzed2 ExperimentalThe schematic diagram of experimental system is shown in Fig. 1. It was a wire-cylinderreactor of impulse corona plasma. The reaction tube shell was made of stainless steel. TheInnerpin-like corona electrode was made of wolfram wire, and also could be made of copper orstainlesssteel wire, which was connected with the plasma electric source. The plasma electric sourceconsistedof dc source, high-voltage impulse source, and waveform shaper where the impulse width ofhigh-voltage impulse was about 5 ms(to be improved further). Organic exhaust gases flowedoutof their vessels and into the reactor through pin valves and flowmeters. In this experiment,toluenexylene or dichloromethane flowed into the reactor respectively, and the high-voltage impulseSourcewas stitched on to excite corona. Experimental gases were the steel-bottle gases containing900 ppmtoluene or xylene or containing 959 ppm dichloromethane, whose flow rates were 45 ml/minHitachi 163 GC and IRS were used to analyze the gas ingredients before and after thereaction3 Results and discussionsUnder the controls of directional reaction conditions, toluene, xylene or dichloromethanewasdecomposed into low-toxic matters, such as CO2, H20(gas )and NH3 and so on. In Table 1theexperiment results of toluene and xylene were given. In Table 2, thent resultsdichloromethane decomposition with different electrode were given中国煤化CNMHGcan beseen that there were different decomposition effects with the three kinds of electrodesfle∥/Eyqk/ jese/jes99je9901/990108hm(第2/11页)2010323033:46环境科学学报990108wolframinduced the highest decomposition efficiency, copper electrode was the second high, andsteel was the lowest. Decomposition organic exhaust gases by high-voltage impulse coronaplasmatechnique, means that the gas molecules were bombarded by high-energy electrons and ionsfromimpulse corona discharge, the inner bonds of molecules were cut off, and then thedecompositionoccurred. The energies of free electrons from impulse corona were in the range of 2-20 e Vandmost of the corresponding ions were in the active-ion energy range of 20-50 eV(Matsumoto1983)As the electrons have lighter masses and greater speeds, the energy range of electrons, whichcouldbombard the molecules and induce their ionization or decomposition, is 5-15 eVTail gasFig. I Schematic diagram of experiment system 1. Plasma electric source; 2. pulse parametergauge; 3. reaction body; 4. insulator; 5. corona electrode; 6. organic exhaust gas; 7flowmeter;8 pin valve; 9. tail gas testerTable 1 High-voltage pulse corona plasma decomposition experiments of toluene and xylenewithdifferent corona electrodesGas T,C R.H., Vp, kv W,w, n, of wolfram n of copper n, of stainless steelToluene 27792.1中国煤化工CNMHGNotes: T is the average temperature; R H. is the relative humidity; Vp is the amplitude offle∥/Eyqk/ jese/jes99je9901/990108hm(第3/11页)2010323033:46环境科学学报990108plvoltage; W is the output power; is the decomposition efficiencyTable 2 Decomposition results of dichloromethane with different corona electrodes4×4mm4×4mmstainless steelcopper wreWireVp W,n, Vp W,nKW|%K∨W(%4039824038762372.871.427674352264291.856291.854Notes: Vp is the amplitude of pulse voltage; w is the output power; is the decompositionefficiencyAccording to the streamer theory of high-voltage corona discharge, the main form ofcoronadischarge is streamer discharge(Meek, 1940: Loeb, 1940). At the tip of the streamer,excitationssplitting and ionization are very intense, and free radicals are created here. The active freeradicalsuseful for the decomposition of gas molecules mainly occurred in the initial streamer. Theshorter therising time of impulse was, the easier the establishment of initial streamer was; the narrowImpuisecorona has many advantages(Wu, 1988); and for the reaction system of narrow impulsecoronadischarge can maintain normal temperature, which is suitable for industrialization, so thenarrowimpulse corona is appropriate to the gas decomposition. Because the positive impulse coronahn effects than the negative impulse corona(Mizuno, 1986; Bai, 1993), thepositiveimpulse corona has been applied in these decomposition experiments中国煤化工When the decomposition experiment was carried out in normalCNMHGsomefle∥/Eyqk/ jese/jes99je9901/990108hm(第4/11页)2010323033:46环境科学学报990108gases in it(always air). In these gases smaller O2, HO(gas) and n2 were easier to decomposeinto O*, H*, N free radicals(as shown in Formula(1)than bigger organic molecules underthbombardments of many high-energy electrons in the plasma fields. This is because thesesmallmolecules i.e. O2 had low activation energies and low binding energies while those biggerorganIcmolecules had complex and tight combinations→)20*,H2O∈H*+OH米,M2()2M*where ev is the bombardment of the electrons Then these free radicals reacted with themolecules, and made the latter into active groups. Further these radicals continuously reactedthe organic active groups, and made the latter decompose thoroughly. The inner reaction wasa chemical equilibrium process,free radical(O*, etc. )+organic compounds 9organic groups+small inorganic molecules(CO2,etc.(2)in which there were the right directional "decomposition of organic compounds and theadverse"recombination "of organic groups. Only under the continuous bombardments ofhigh-energy electronscould o* and so on. Free radicals were created continuously(the right directional reaction ofthetwo-way creation Reaction(2)is superior, owing to the continuous electron impacts ), and thereactants density on the left of the formula increased so the equilibrium declined to the rightdirectionand the organic compounds decomposed thoroughly It should be stated that there are ions intheplasma, which can react directly with the organic active groups, to reduce the number of theand promote the reaction (2) to decline to the right direction, or whi中国煤化工organIcrHaCNMHGcompound molecules and make it compose directly. But as the excitation activation energy ofthfle∥/Eyqk/ jese/jes99je9901/990108hm(第5/11页)2010323033:46环境科学学报990108normal gas molecule is lower than the ionization energy, which leads to less active ions thantheexcited free radicals; and as the collisions between ions and neutral molecules are mainlymomentumexchange or charge transfer, and the direct composition collision is difficult to occur( there is afairly, small cross-section or probability for this kind of collision), so the effects of the activeions are very little in the whole processAccording to the plasma and gas discharge theories, because some gas particles areIonizedowing to the universe radiation or other outer radiants, there are still a small quantity of initialfreecharges in normal gases( Zhao, 1993), and the quantity is independent upon the types ofdischargegases and electrodes. During the transient acting period of impulse voltage which could notInducethe gas breakdown and spark discharge, the few initial electrons were accelerated by thestrongelectric field, then bombarded gas molecules into free radicals, and then the electrons headedthe positive corona electrode, bombarded this electrode to induce secondary electronemission. itwas these secondary electrons who greatly increased the quantity of the free electronsbombarding gas molecules. According to tue streamer theory, in the action process of thepositiveimpulse corona streamer many electrons bombarded into the positive corona electrode andproducedthe secondary electrons. With the development of the electron avalanche, a great quantity ofpositivecharges gathered in the head part of the streamer, which would partly counteract the electricfield betweenthe head streamer and the positive corona electrode therefore the restraint forces on theelectrons would decrease in this region. So with fairly high energies, the secondary electronscouldeffectively participate in the activation reactions of the neutral gas molecules in their movingpMeanwhile, their isotropic diffusion would be in favor of the proces n中国煤化工merwithCNMHGpositive charges diffused in the direction perpendicular to the axis of th electron avalancheand infle∥/Eyqk/ jese/jes99je9901/990108hm(第6/11页)2010323033:46环境科学学报990108favor of the formation and growth of the streamer. More widely spread charged particlescould takepart in the activation process of the molecules between the two electrodes. Thus the secondaryelectronscould promote the production of the active radicals, and make the organic exhaust gasdecomposition more efficiently. In the decomposition experiments with a fixed impulsepower, for the cylinder electrode wasthe same, the decomposition efficiency was dependent on the differences in the secondaryelectronemission coefficients of the corona electrode to a great extent. Actually in the experiments,thedecomposition efficiency was the highest when wolfram was the corona electrode. Byaringthe secondary electron emission coefficients of the three metal corona electrodes, it can befoundthat wolfram has the greatest value so the above conclusion was proved right Maximumelectron emission coefficients of the three corona electrodes are compared in table 3 Liu1980)Table 3 Maximum secondary electron emission coefficients(8 m) of three corona electrodesElectrodeWolframCStainless steel1.4291.24On the other hand, the secondary electron emission coefficient (? of metal has thefollowingrelation with the original energy of incident electrons( Wpo; Liu, 1980)AnWhere中国煤化工CNMHG()=eap()e(y3)fle∥/Eyqk/ jese/jes99je9901/990108hm(第7/11页)2010323033:46环境科学学报990108This relation is shown in Fig. 2(Liu, 1980). When Wpo is relatively low, the quantity ofexcitedelectrons in the emitter is also low, so the secondary electron emission is relatively weakwhenWpo is very high, although the quantity of excited electrons is high, those excited electronsmostly deep in the emitter and have relatively low escape probability, so the secondaryelectronemission is weak too. When Wpo is very low(Wpo=10-100e V), Formula 3)becomes(5)which expresses the direct ratio relationship between? and wpo in the beginning period; whenWis very high(Wpo>10 ke V), Formula(3) becomesBIA 1which expresses that? will descend while Wpo ascends. Obviously, because the energies ofincidentoriginal electrons onto the two electrodes almost fully come from the acceleration effect ofthehigh-voltage impulse electric field, there is nearly linear relation between Wpo and thesource output power W: the greater W is, the greater Wpo is. And from above qualitativeanalysesit can be seen that has very close relation with the decomposition efficiency () In fact, whentheelectrode material is fixed the maximum secondary electron emissi中国煤化 Ts alsofixedHCNMHGwhile the actual? will vary with the original electron energy wpo according to Fig. 2, andwpo willfle∥/Eyqk/ jese/jes99je9901/990108hm(第8/11页)2010323033:46环境科学学报990108vary with the impulse electric field power W. Thus, when W changes, ?will changeaccordingly, andwill change too. As shown in Fig 3, in the experiments, when the impulse voltage power Wincreasedwould also increase, but when w came to a certain high value, the ascending tendency od thedecomposition efficiency would begin to slow down, and tended to a stable value; if theincrease ofWcontinued, the electric breakdown between the electrodes would easily occur, the plasmastatewould be destroyed, so does not get low at the later period in Fig 3. By comparing Fig. 2 withFig. 3it can be seen that the two figures are similar at the first greater part of the whole process, andthereason for their difference is the unfavorable situations mentioned above. in which theexperimentcan not continue normally. The experiment with80Stainless steel60Wno evFig 2 Relationship between secondary Fig3 Relationship betweenoriginal energy of incident electron woo decomposition efficiency and source outputelectron emission coefficient s andpower W(dichloromethane)higher w has not been done. but when carefully observed it can be seen in Fig. 3 that therisingtendency slowed down at the later period and the later part varied gently so it can be inferredthat the impulse electric filed has great influences on the organic exhaust gas decompositionsuchtoluene, xylene and dichloromethane and it indirectly affects the final decompositionen)中国煤化工by affecting the secondary electron emission coefficient(s of the coCNMHGOn theimpulse electric field power (w) is relatively low for& of the corona electrode is lowaccordinglyfle∥/Eyqk/ jese/jes99je9901/990108hm(第9/11页)2010323033:46环境科学学报990108there are not many high-energy electrons and active free radicals in the reactor, so is relativelywith the increase of W, 8 increases quickly, the quantity of high energy electrons and activeradicalsincrease accordingly, thus the decomposition is depended and in increases continuously; butwhen wincreases to a certain value, the corresponding 8 value tends to saturate gradually because ofthmaterial property, and it varies very gently near the maximum value 8, which results in thenearly stablequantity of high energy electrons and active radicals in the reaction and the situation that theascendingtendency of would begin to slow down; if the increase of W continued, 8 will decreaseslowly, andunder the condition that the experimental equipments could bear, would begin to decreaseslowly toobut in the actual corona discharge reactor the electric breakdown between the electrodeswould theneasily occur, the plasma state produced by the corona discharge would be destroyed, and theexperimental equipments would also suffer some damage, so the experiment could notcontinueand the corresponding measurement could not be made. That is to say, in fact, the situation atthistime will have gone beyond the area of the technique of organic exhaust gas decomposition bydischarge plasma discussed in this paper4 ConclusionThe impulse electric field has a great influence on the total decomposition efficiency( nexperiments of decomposing organic exhaust gases such as toluene, xylene, anddichloromethaneusing the technique of high-voltage impulse corona discharge plasma. The mechanism of thisinfluenceis that the impulse electric field power(W) changes indirectly, with the correspondinvariation of thesecondary electron emission coefficient(8 as an intermediary. When the material of thecoronaelectrode is fixed, varies with the impulse power according to the above regular pattern, inwhich it中国煤化工increases relatively quickly at the first greater part of the whole prodCNMHG asaturatedstate and varies very gently, and at last it decreases very slowly if the experimental equipmentfle∥/Eyqk/ jese/jes99je9901/990108hm(第10/11页)20103230:3:46环境科学学报990108couldbear. When W is fixed, the corona electrode material which has a greater?value will lead to ahigher valueReferencesBai xY, yic w, zhang zt, bai m d, shao z y, zhang y l, 1993. Environment science, 14(1):37-40Huang L W, Tan TE, 1997. China Enviornmental Science, 17(5): 443Liu X Q, 1980. Cathode electronics. Beijing: Science Press, 389, 394, 397, 420, 423Loeb l b, Meek JM, 1940. J Appl Phys, 11(6): 438-447Matsumoto O, 1983. chemistry Industirial Science, 47(7): 418MeekJM, 1940. Phys rev, 57(8): 722Mizuno A, Clements J S, Davis h, 1986. IEEE Trans ind Appl, IA-(3): 516Su JH, 1990. environmental Engineering, 8(4): 52Tanaka K.1986. Industrial water. 332. 2WuY, 1988. Journal of North-east Teachers University, (1): 6Xu ZZ,1986. Environmental Engineering, 4(2): 18Zhao H Q, 1993. Plasma Chemistry and processing. Hefei: China Science and TechnologyUniversity Press, 108Zhen l,jiang X Z, 1997. Environmental Science, 18(5): 62-64(Received for review December 8, 1997. Accepted April 10, 1998)中国煤化工CNMHGfle∥/Eyqk/ jese/jes99je9901/990108hm(第11/11页)20103230:3346