Removal of NO and SO2 in Corona Discharge Plasma Reactor with Water Film

Plasma Science & Technology Vol.6, No.2, Apr. 2004Removal of NO and SO2 in Corona Discharge PlasmaReactor with Water FilmHeYuanji(贺元吉)',DongLiming(董丽敏),YangJiaxiang(杨嘉祥)21 The Fourth Institute of the Second ATrillery, Bejjing 100085, China2 Harbin University of Science and Technology, Harbin 150040, ChinaAbstract In this paper, a novel type of a corona discharge plasma reactor was designed, whichconsists of needle-plate-combined electrodes, in which a series of needle electrodes are placed in aglass container flled with fue gas, and a plate electrode is immersed in the water. Based on thismodel, the removal of NO and SO2 was tested experimentally. In addition, the effect of streamerpolarity on the reduction of SO2 and NO was investigated in detail. The experimental resultsshow that the corona wind formed between the high-voltage needle electrode and the water bycorona discharge enhances the cleaning eficiency of the fue gas because of the presence of water,and the cleaning eficiency will increase with the increase of applied de voltage within a definiterange. The removal fficiency of SO2 up to 98%, and about 85% of NOx removal under suitableconditions is obtained in our experiments.Keywords: corona discharge, plasma reactor, water filmPACS: 52.751Introductionvestigated for the removal of NO2, but its conversionefficiency is limited approximately to 60%6. Also,Nitrogen oxides (NOx) and sulfur oxides (SOx)the use of water vapor and a NaZSM-5 catalyst hasemitted from thermal power plants, factories, andshowed a promising NOx conversion eficiency in theautomobiles cause acid rain which is detrimen-range of 70%[7].tal to the enviroment.At the present time,For most of these applications, the plasma reac-ammonia-catalytic De-NOx and calcium-gypsum De-tor was of a conventional dry type and it may beSOx methods have been used for the treament (not adequate to remove certain stable NOx and SO2combustion flue gases emitted from thermal powerpresent in the fue gas. In this work, a novel type .plants. Although these processes are effective andof a corona discharge plasma reactor was designed.reliable, both the initial and the running costs areThe reactor is composed of needle-plate -combinedvery high.electrodes, in which a series of needle electrodes areNonthermal plasma technology characterized byplaced in a glass container flled with fue gas, and alow temperature and high electron temperature is ofplate electrode is immersed in the water. And the re-moval of NO and SO2 was tested in experiments, Ingreat significance in the control of gaseous pollutantsin recent years[1~3]. Many researchers have been in-addition, the fundamental phenomenon on the inter-vestigating the removal of NOx and SO2 with thisface between the needle electron and the water wastechnology. However, a complete removal of NOxinvestigated in detail.by nonthermal plasma alone is very dificult and re-searchers recently have been looking into the use of2 Experimental systemadditives, chemicals, and catalysts in combination中国煤化工with the nonthermal plasma process (4,5[. The addi-xperimental set-uption of hydrocarbons such as ethylene has been in-for4.YHCNMHGwninFig1. The .He Yuanji et al. : Removal of NO and SO2 in Corona Discharge Plasma Reactor with Water FilmDC high olltage]Needle'electrode100-●-With waterMixer]Gas VVWVGas out,。80-■- Without water,WaterGrounded electrode8SO2 or NO20b510”2Fig.1 Experimental set-upApplied positite DC vltage 1kV10■-Without water。98●-With waterexperimental set-up is comprised of a simulated flue97gas unit, a high voltage power supply, a plasma re-卷96actor and analyzers for SO2 and NO.In the plasma reactor, the needle electrodes areplaced over the water surface at a distance of 20 mm,B 12|and the plate cathode is immersed in the water.8tThe experimental gases are the standard gases ofb)|SO2+N2 and NO+N2 whose temperature and pres-Appied pstitite DC vlage 1KVsure are 20+2 °C and 1.013 X 105 Pa, respectively.Concentrations of SO2 and NO were measured at theinlet and the outlet of the reactor by SOA-7000 SO2(a). Removal fficiency of NO versus applied positiveanalyzer and NOA-7000 NO analyzer, respectively.voltage with and without waterb). Removal eficiency of SO2 versus applied positive3 Experimental resultsFig.2 Effect of water on the removal rate of NO andFig.2 shows the effect of water on the removalSOzrate of NO and SO2. The important features are theremoval eficiency of NO and SO2 is higher with wa-4 Discussionter than that of without water. In the absence ofwater, however, the removal rate of SO2 is far lessthan that in the presence of water.The experimental results confirm that the removalrate of NO and SO2 is related with the existence oFig. 3 shows the effect of corona dischatge on theremoval rate of NO and SO2. It can be seen that thewater, and the polarity of applied voltage as well.removal rate increases with increasing the positive orThese results can be explained by interface effect ofnegative voltage.gas and water, plasma chemistry and space chargeeffect.Fig. 4 shows the efect of voltage polarity on re-moval rate of NO and SO2. It has been found that4.1 Plasma chemistry reaction onthe removal rate with a positive voltage is substan-tially greater than that with a negative voltage.中国煤化工gas and waterYHCNMHG2251Plasma Science & Technology, Vol.6, No.2, Apr.200400厂60厂●Positive9o- - Negative5-●-Positivee。50|+ -Negative鱼870f40是6235-3oK50-_:=:25-a)20(a).51.0 1.53Applied DC vllage 1kVResidence time/s100[-●- Positive--Positive9f--Negative5F公信98f30f85t/96|b)(b)95-10150.5 10 15.0 25 3.0Applie voltage 1kV(a) NO removal rate versus applied voltage in the pres-(a) NO removal rate versus residence time in the absenceence of water (NO:500x 10-5 %，residence time: 2 s)of water (NO: 500x 10-“% , applied de voltage: 18 kV)(b) SO2 removal rate versus applied voltage in the pres-(b) SO2 removal rate versus residence time in the pres-ence of water (SO2:1000 x 10 0%, residence time: 2 s)ence of water (SO2: 1000 x 10- 6 % , applied de voltage:Fig.3 Effect of corona discharge on removal rate of NO18 kV)and SO2Fig.4 Effect of voltage polarity on removal rate of NOWhen a high DC voltage is applied to the needleselectrode, a high electric field takes place between theEgas brings about an intense corona discharge in theneedle electrodes and the water surface, so that theAue gas and creates high energy electrons. Thesewater is polarized and some tapered water columnshigh energy electrons are collided with neutral gascome into being, thus making the distance betweenmolecules (N2, O2, H2O), leading to the formationthe needles electrode and the plate electrode shorterof a variety of new species including ions, metasta-and the electric field stronger to produce ions windbles, atoms, and free radicals:in between, as shown in Fig.5.O2+e*-→0+O+e,Moreover, because ofewater >> Egus ,and Egas >>Ewater on the interface of water and fue gas, the high中国煤化工e2)MHCNMHG2252He Yuanji et al. : Removal of NO and SO2 in Corona Discharge Plasma Reactor with Water FilmNeedel electrodes↓↓↓↓ ↓Cororma dshage egonAffect gas separation-Polarized water(a) positive needle to(b) negative needle tonegative platepositive plateAAAAAWaterFig.6 The representation of space charge generated byneedle-to-plate electrodePlate electrodeHSO3+ HO -→H2SO4,(15)Fig.5 The schematic diagram of corona discharge onSO3 + H20- + H2SO4.(16)the interface of fue gas and waterIt would be reasonable to consider that more 0H inthe presence of water is produced than in the absenceof water. Therefore, the transformed yield from NO2N2+e*→N+N+e,(3)into HNO3 increases and makes SO3 entirely trans-Oz+e*- +0+0('D)+e,(4)formed into H2SO4. So, as shown in Fig. 2, the re-moval rate of NO and SO2 is higher with the waterwhere e* is the high energy electron, e is the ground-than that without the water.state electron.The increased applied de voltage enhanced coronaThese products are all chemically active and candischarge and the coronas, in turn, can reduce the ef-create more secondary radicals by electron-ion andfective thickness of the gas/ water boundary layer toenhance the mixture between radicals and fAue gas.ion- ion reactions as well as electron detachments:As shown in Fig. 3, the removal rate of NO and SO2H+O2- + HO2, .(5increases with the augmentation of positive or neg-ative dc voltage. Moreover, the corona can increase02+0→O3.(6)the transferred SO2 to the water by difusion.In fue gas applications, these radicals may reactedPolarity effect of applied voltagewith them to form products:In corona discharge processes, the high-voltageelectrodes are immersed in the atmospheric-pressureNO+O→NO2,(7)gas with either a positive or a negative polarity, butNO+O3- +NO2+O2,(8)the voltage polarity has been shown to afect gas sep-aration.NO+OH→HNO2,(9Fig. 6 shows a representation of space charge gen-erated by needle-to-plate electrode, E is a denotationHNO2+0- + NO2 + OH,(10)of the electric field between the needle electrode andHNO2+OH-+NO2+H2O,(11)the plate electrode and Ead is a denotation of the ap-pendant electric field generated by space charge. AsNO2 + OH→HNO3,(12)shown in Fig. 6, the positive de produces the spaceSO2+0-→SO3,charge bridging across the entire gap, whereas the"中国煤化ilarge located nearSO2 +0H- + HSO3,the!_tric field increases:TYHCNMHG2253Plasma Science & Technology, Vol.6, No.2, Apr. 2004in region II and the electric field decreases in region Iobtained with a positive corona discharge in the re-in Fig.6 (a), but the electric field increases in regionactor with water. According to this efect, a water-I and the electric field decreases in region II in Fig.film electrical corona reactor designed has been used6 (b). However, the electric field increases to induceto simulate fue gas, the result of which shows thata greater ionization cofficient, and the number ofnot only the removal eficiency of SO2 up to 98%，positive ions increase significantly, too.but also about 85% of NOx removal under suitableAs time goes on, positive coronas can produceconditions are obtained.longer streamers and a greater corona volume fullyflled with the gas ionizing a larger volume, henceresulting in a larger active volume and higher en-Referencesergy, but negative coronas appear only in a smallregion around the discharge needlepoint. The avail-Giorgio D, Luigi C, Massimo R. Industry exper-iments on pulse corona simultaneous removal ofable corona space in positive discharges is about 10NOx and SO2 from fue gas. IEEE Trans. Ind.times more than that of negative coronas.Application, 1990, 26: 535~541It is considered that the application of a posi-Masuda S, Nakao H. Control of NOx by posi-tive voltage leads to the formation of many positivetive and negative pulsed corona discharges. IEEEstreamers, which have a high electric field region atTrans. Ind. Application, 1990, 26: 374~383their heads, and more energetic electrons producingLowke J J, Morrow R. Therecical analysis of re-morefree radicals for the removal of NO and SO2.moval of oxides of sulphur and nitrogen in pulsedAs shown in Fig.4, the removal rate of NO and SO2operation of electrostatic precipitators. IEEEby a positive voltage are substantially greater thanTrans. Plasma Sci, 1995, 23: 661~669that by a negative voltage.Oda T, Kato T, Takahashi T, et al. Nitric oxidedecomposition in air by using nonthermal plasma5 Conclusionsprocessing with additives and catalyst. IEEETrans. Ind. Application, 1998, 34: 268~272Shimizu K，Oda T. De-NOx process in fAuThe experimental results show that the coronaas combined with nontbermal plasma ancwind formed between the high-voltage needle elec-catalyst. IEEE Trans. Ind. Application, 1999,trode and the water by corona discharge enhances35: 1311~1317the cleaning eficiency of the fue gas because of the6Mizuno A, Shimizu K, Yanagihara K, et al. Effectpresence of water, and the cleaning eficiency will in-of additives and catalysts on removal of nitrogenoxides using pulsed discharge, in Conf. Rec. 1996crease with the increase of applied dc voltage withinIEEE-IAS Annu. Meeting, vol. 3, San Diego, CA,a definite range.Oct. 6~10, p1808~1812, 1996The NO removal rate significantly increases withShimizu K, Hirano T, Oda T. Effect of water va-increasing the applied dc voltage, but the SO2 re-por and hydrocarbons in removing NOx by us-moval rate was not significantly affected by the ap-ing nonthermal plasma and catalyst. IEEE Trans.plied dc voltage and slightly improved in the presenceInd. Application, 2001, 37: 464~471of water film.(Manuscript received 19 September 2003)In general, a higher removal eficiency has beenE-mail address of He Yuanj: uwbhyj@sina.com中国煤化工MYHCNMHG2254