Pulse corona induced plasma chemical process: a horizon of new plasma chemical technologies

Transcription

1 Pure & Appl. Chem., Vol. 6, No. 5, pp , Printed in Gret Britin IPAC Pulse coron induced plsm chemicl process: horion of new plsm chemicl technologies Senichi Msud Fukui Institute of Technology, Gkuen, Fukui 91, Jpn Abstrct A very intense coron dischrge cn be produced by pplying shrp rising, nrrow pulse high voltge between coron nd counter electrodes of coron system. Becuse of n extremely incresed sprk voltge electrons re highly ccelerted even under ordinry gs pressure, while ions with much lrger mss re not effectively ccelerted s result of too short pulse durtion time. Hence, strongly nonequilibrium plsm with high electron temperture nd low ion nd moleculr tempertures is produced. A number of gs phse rections, such s DeNOx, DeSOx nd control of hrdous nd toxic gseous pollutnts like iercury vpour from incinertion plnts, cn be effectively relied, where the pollutnts re converted into solid erosol prticles, with or without dditive gents like mmoni, to be esily removed in downstrem precipittors. This lso opens new wy of producing ultrfine prticles of vrious exhotic mterils. Tretment nd dry processing of vrious surfces under ordinry pressure (nd lso reduced or incresed pressures) re lso possible. 1. INTRODCTION The uthor investigted with his coworker possibility of enhncing DeSOx nd DeNOx in Bbem method from thought tht copious electrons, produced by ioniing collision of primry high energy electrons nd remining with low energy fter performing useful chemicl rections, could be regenerted in energy by pplying n electric field. The results of experiments were very positive (11, but only under the condition tht coron dischrge took plce. Further more, the use of DC voltge produced comprtively lrge ionic current to cuse quite high electric loss. This suggested tht very nrrow pulse high voltge or microwve high voltge must be used for exploittion of such effect in order to void energy loss due to ccelertion of ions. A gret technicl difficulty to proceed with this ide ws the lck of power supplies which must be high in efficiency, low in cost, nd durble enough to llow long term continuous opertion. The microwve ws discrded becuse of its too high cost nd too low efficiency in power supply. A shrp rising, very nrrow pulse voltge ws only one possibility in view of its cost. The uthor nd his coworkers succeeded to develop very efficient, low cost, nd relible nnosecond or submicrosecond pulse power supply using rotry sprk gp for fst switching of condenser in combintion with its phseselective AC chrging (2). Fig. 1 indictes its circuit digrm, nd Fig. 2 photogrph of one of the pulse power supply which cn energie 2, m in terms of totl counter electrode re. This pulser cn be directly connected to the lod when it is smll, but in the cse when lod cpcity is lrge, the use of the coron trnsmission line (3) should be most effective in both plsm chemicl performnce nd energy efficiency. This uses long coron wire, igg formed, on which such nrrow pulse voltge proceeds in form of trvelling wve without losing its originl rise nd durtion times, thus producing its essentil ctivity nd uniformity long the entire length of the wire. After completing the nnosecond pulse power supply nd the circuit technologies for n effective use of the pulse, the uthor nd coworkers strted (1) DeNOx nd DeSOx tests in lbortory nd n incinertion plnt, (2) tests of mercury vpour control t the incinertion plnt, nd (3) oone production test in lbortory. Bsed on these test results theoreticl considertions were lso mde on the mechnism of the pulse coron induced plsm chemicl process (PPCP). The uthor is convinced t this stge tht PPCP could provide number of costeffective mens for oxidiing nd other pplictions. In the following our test results re presented together with the discussion on the results, 727

2 728 S. MASDA lllgh Voltge ronous Motor,Coup1 Ing Condenser Yo1 tge Pulrc Fonnlng Condenser Fig.1 Circuit Digrm of Nnosecond Pulser with Rotry Sprk Gp Li c, NO2 without Bem F: + 2lini il p(kv/ u Vp(kV) Pek Voltge Vp nd Pek Field p wire (12mm di) tocylinder (loonun di) dry irnoammoni (18ppm) Bem(5uA;1.5MV;3H scn.) Pulse(5ns rise; 311s hlf ti1:soh) Fig.3 Correltion between Bem nd PPCP (Negtive Polrity) 2 N A, $ ; tr = 5 ns, T = 2 C. : humid ir(r.h.=65%) B : dry ir Fig.2 Industril Nnosecond Pulser Pek Voltge Vp nd Pek Field D Fig.4 ffect of Pulse Field Intensity (conditions: see Fiq.3) lolm Fig. 5 ffect of Humidity (Positive) 3 Pulse 4 voltge 5 6 ( kv ) 7 positive coron Tr=5nS s o 2. TST RSLTS 2.1 Correltion between bem nd PPCP (pulse coron induced plsm chemicl process) At first tests were mde to clrify if crosscorreltion exists, s expected before, in the overll effect of rections between Bem nd PPCP, or if they re independent in their effects. Fig. 3 indictes typicl result which were obtined for NO gs in dry ir with negtive pulse coron, where mmoni ws dded. The curves with Bem only do not indicte ny chnge up to 11 kv/cm, where shrp chnges occur in both NO nd NO1 concentrtions. However, these chnges re exctly those which hppen with PPCP without the id of Bem. Hence, it cn be concluded tht no crosscorreltion exists between Bem nd PPCP. Hence Bem process nd PPCP must be considered seprtely.

3 Pulse coron induced plsm chemicl process: horion of new plsm chemicl technologies ffect of pulse pek field intensity The effect of PPCP begins to pper only fter coron onset s shown in Fig. 4 in which the coron strts t p=5 kv/cm ( p: pek voltge Vp/(gp)). This figure lso includes the effect of Bem which produced initil drops of NO nd NOx. ith incresing field NO indictes grdul drop to become ero t p=12 kv/cm, while NO2 rises concurrently, s result of NO oxidtion, up to 11.5 kv/cm, beyond which it suddenly drops. In other words, the dominnt ctive species producing NO? removl is different from those for NO oxidtion, nd the formers re produced beyond much higher threshold vlue of p. It is nticipted from Fig. 5 indicting the effect of humidity (4), nd Fig. 6 indicting the effect of oxygen (5), tht nd 3 re likely to be dominnt for NO oxidtion while OH rdicl is likely to be essentil for NO2 removl. 2.3 ffect of polrity of pulse voltge This is very complicted, s shown in Figs. 7 nd 8. At room temperture positive pulse lwys produces much better performnce thn negtive one, lthough energy bsed efficiencies re the gme ( 5). However, with the 1.o () NO Vb=18kV fp=soh 'c c C u c NOAir:, dded.,. ' = o Gs Residence Time Tg (5) Gs Residence Time Tg (s) 4 Fig.6 ffect of Oxygen (Negtive; Dry) 1 n v N 4 2 ( 5 C O L 3 4 ' k" b 1 1 I kv gs': NO+room ir; Tr=5ns Fig.7 ffect of Polrity on NO Removl qs: MOfroon ir; Tr=5ns kv Fig.8 ffect of Polrity on NO2 Removl

4 73 S. MASDA increse in temperture the tendency reverses, s shown in the bove figures (4), t lest in clen gs (room ir + NO) conditions. However, in combustion gs from n incinertor boiler plnt these differences between positive nd negtive polrities in the performnce disppers, while energy efficiency is lwys better in positive pulsing PPCP ( 6). The difference in gs lies in much higher concentrtions of wter (17 X), Con (1 %), CO (3 ppm), nd HC1 (35 ppm) nd existence of fine prticulte in combustion gs. The superiority of using negtive pulsing for PPCP my open wy to pplying PPCP inside n electrosttic precipittor, where both prticultes nd gseous pollutnts (NOx, SOX, mercury vpour, etc.) could be simultneously removed. All these results tell tht creful preliminry test must be mde on cse by cse bsis to void risk due to drstic chnge of PPCP chrcteristics cused by smll chnge in gs composition. 2.4 SO, removl The lbortory tests indicte tht SO2 cn only removed fter NO nd NO2 hve been removed by PPCP. In the prcticl combustion gses, however, SO2 is the first to be removed, followed by NO nd NO2 removl. It is nticipted tht, in combustion gs which includes prticulte, SO3 converted from SO2 will be rpidly bsorbed on the prticulte surfce to void its reverse decomposition into SO. This scvenging effect of SO3 is likely to be n essentil fctor which enhnced SOX removl by PPCP in combustion gses. The ctive species dominnt for DeSOx by PPCP is likely to be OH rdicl. 2.5 ffect of coron power, pulse frequency nd gs residence time The PPCP effect is determined by prmeter (P/Q)/Tg'/P, where P = coron power, Q = gs flow rte, nd Tt = gs residence time (Fig. 9) (5). At room temperture the PPCP effect is liner function of pulse frequency, s shown in Fig. 1, while t n elevted temperture devition occurs from the stright line. A number of test dt indicte tht the pollutnt concentrtion shows n exponentil decy with the gs residence time nd pulse frequency (Fig. 11) (4). 2.6 ffect of mmoni Addition of mmoni produces substntil improvement in the removl of NO, while it hs much less effect on NO removl. NO2 fst indictes rise owing to NO oxidtion, rriving t pek, nd drops. This pek vlue is reduced to the hlf of tht without mmoni ddition t room temperture (5). However, t n elevted temperture this dvntge of mmoni is lost, nd the performnce becomes deteriorted by mmoni ddition in DeNOx. Its effect on DeSOx t n elevted temperture is not yet cler. 2.7 ffect of pulse rise time The pulse rise time hs substntil effect on the performnce of PPCP. Shorter the rise time the more the intensive nd elongted re the pulsed stremers, nd better is the PPCP performnce. In the ppliction of PPCP in very lrge plnt hving very lrge interelectrode cpcity, it is difficult to reduce the rise time below 5 1 ns, even if the high frequency oscill.tion prt of the pulse leding edge be used. The only wy of overcoming this difficulty is to use the trvelling wve mode, s described bove. h c ill.c.r r m > C l p: I.. Fig.9 ffect of Power (negtive; dry) (P/Q)/JTq ki.lh/i.in~~s~/~)

5 Pulse coron induced plsm chemicl process: horion of new plsm chemicl technologies o 2 tr = 5 ns, T = 2 C Residence time ( S ) Pulse Frequency fp (H) ~ ; ~ 4mmx4mm squre wire Fig.11 ffect of Residence Time (positive oolrity) Fig.1 ffect of Frequency (Negtive) 2.8 Control of mercury vpour in combustion gs The test mde t n incinertion plnt for PPCP removl of Hg vpour from combustion gs indicted drstic result (6). ithin 2 seconds gs residence time mercury vpour with.5 mg/nm3 (.6 ppm) could be 1 % converted into solid prticultes (oxides nd chlorides) in wide rnge of gs temperture (3 3 deg.c) with pulse frequency of 5 H. The mercury removl ws enhnced by the ddition of HC1 or Cl2. The lrger residence time did not produce substntil improve (t reduced voltge), while lowering of residence time cused substntil drop in performnce. This tells tht certin quntity of rdicls must be produced to fully oxidie Hg vpour, while beyond this quntity no improvement occurs. In other words, the rection speed is likely to be very high. 3. FNDAMNTAL MCHANISM OF PPCP The mechnism is divided into electricl one nd chemicl one. The electricl one id tht PPCp enbles highly nonequilibrium plsm with high electron temperture nd low ionic nd moleculr tempertures to occur under norml pressure, s described previously. This is unique feture resulted by shrp rising, nrrow pulse high voltge pplied to coron system which hs strong electron source (coron electrode). The chemicl mechnism is not yet fully understood, lthough number of nlysis hve been presented (7, 8). It is certin tht electrons hving n dequtely high energy (upper til of Dryvestin distribution) would produce copious ctive species:, OZ*, N, 2, OH rdicl, etc. Some of the theoreticl olcultions indicte resonbly good pproximtions of the test results, while others very different results. 4. CONCLSION The unique feture of PPCP for vrious pplictions under ordinry gs pressure is indicted with mny exmples. It is expected tht PPCP would provide new re of plsm chemicl process in mny pplictions. RFRNCS (1) S. Msud nd M. Hirno, Rdition Phys. Chem., 17, p.223 (1981). (2) 5. Msud nd S. Hosokw, Proc. I/IAS 1984 Ann. Conf. p (3) S. Msud, H. Nktni nd 5. Hosokw, ibid Ann. Conf. p.966. (4) S. Msud nd Y. u, lectrosttics '87, Inst. Phys. London. (5) S. Msud nd H. Nko, Proc. I/IAS 1986 Ann. Conf. p (6) S. Msud, Y. u, T. rbe nd M. Ono, Proc. ISPC/Tokyo (7) L. Civitno et l, Proc. Into. Atom. n. Ag. Consult. Meet. (1986, Krlsruhe). (8) J. Person et l, PSITR259/542 (D86362) (Oct. 1985).