- Email: [email protected]
Removal processes of nitric oxide along positive streamers observed by laser-induced fluorescence imaging spectroscopy
23 June 2000
Chemical Physics Letters 323 Ž2000. 542–548 www.elsevier.nlrlocatercplett
Removal processes of nitric oxide along positive streamers observed by laser-induced fluorescence imaging spectroscopy Hisanao Hazama a
a,)
, Masanori Fujiwara b, Mitsumori Tanimoto
b
Department of Electrical Engineering, Faculty of Science and Technology, Science UniÕersity of Tokyo, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan Received 28 December 1999; in final form 5 May 2000
Abstract Removal processes of nitric oxide ŽNO. in pulsed corona discharges were observed by laser-induced fluorescence ŽLIF. imaging spectroscopy in a reactor filled with N2rNO mixtures. Distributions of NO along the streamers and effective energy costs of NO removal Ž ´ rem . obtained from the LIF images were compared with those predicted by a model taking account of the diffusion. For an initial NO concentration wNOx 0 of 1000 ppm ´ rem was 120 " 31 eVrmolecule, while it was increased up to 620 " 160 eVrmolecule for wNOx 0 of 30 ppm, due to a higher recombination loss of the reducing species of the nitrogen atoms. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Air pollution caused by NO x emission from diesel engines is a serious problem. We are developing a deNO x method based on pulsed corona discharges under atmospheric pressure. The deNO x process by oxidation can be regarded as more realistic than that by reduction, because it requires less energy. However, the latter is preferable, especially for mobile engines, since the end products can be directly exhausted to the atmosphere. In order to optimize the discharge conditions and the reaction processes, it is important to understand the characteristics of the N
) Corresponding author. Fax: q81-298-61-5754; e-mail: [email protected]
atom generation as well as the reaction processes in detail. In the present experiment, a simple N2rNO mixture was used as a test gas, though the actual exhausts of the diesel engines contain various other species such as O 2 and H 2 O, which may affect the reaction processes. We have observed the distributions of NO in the reactor by LIF imaging spectroscopy, which has been used in various fields such as combustion engineering w1–3x. Recently, this technique has been applied to observations of NO distributions in pulsed negative w4x and positive w5x corona discharges. However, LIF images accumulated over multiple shots could not resolve the spatial profiles of NO along streamers. In the present Letter, we have observed the removal processes of NO along individual streamers with single-shot LIF images. In addition, the accumulated LIF images have been
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 5 5 7 - 1
H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548
analyzed to quantitatively evaluate the total amount of NO removal.
543
3. Results and discussion 3.1. Distributions of NO along streamers
2. Experimental setup A discharge reactor made of Teflon Ž50 = 50 = 50 mm3 . had an electrode configuration consisting of a tungsten needle electrode and a stainless-steel plane electrode of 30 mm diameter with a gap of 15 mm. The tip radius of the needle electrode was 2 mm. The reactor was statically filled with N2 gas containing a specified fraction of NO at atmospheric pressure and room temperature. After each discharge the reactor was refilled by a fresh mixture. High-voltage pulses were applied to the needle electrode by a MOSFET switching device which enabled us to precisely synchronize the pulses with the diagnostic systems. The pulses typically had a peak amplitude of 23 kV, a duration of 400 ns, a rise time of 40 ns, and a fall time of 600 ns. The g Ž0,0. band of the NO molecule was excited at a laser wavelength of 226 nm to avoid interference with the absorption lines of O 2 molecule w3x. The UV probe laser pulse was generated by second harmonic conversion of the output from a pulsed dye laser which was pumped by a XeCl excimer laser. The energy and the pulse duration of the second harmonic generated by a BBO crystal were typically 2 mJ and about 20 ns, respectively. The second harmonic had a spectral bandwidth of 0.5 pm Ž; 0.1 cmy1 ., and the wavelength was tunable with an accuracy of 0.5 pm. To observe two-dimensional distribution of NO in a planer area including the needle electrode in the reactor, the laser beam was expanded by a cylindrical lens to a cross-section of approximately 15 = 0.5 mm2 . LIF images were recorded by a cooled, intensified, and gated CCD camera with an imaging optics unit through a quartz window of the reactor. The spatial resolution of the imaging system was about 0.2 mm. The gate duration of the CCD cameras was set at 100 ns, since the lifetime of the LIF was typically about 100 ns. An interference filter centered at 254 nm was used to eliminate the stray probing laser light and N2 fluorescence and to pass the NO fluorescence signal of the g Ž0,1. – Ž0,4. band.
The intensity distribution of the second positive Ž0,0. emission ŽSPE. of N2 Ž337.1 nm., shown in Fig. 1, provides a useful measure of the local yield of the N atoms generated by energetic electrons. The upper level of this transition is 11.0 eV above the ground state, while the dissociation energy of the N2 molecule is 12.0 eV. Bright emissions were observed along the streamers, especially in the region surrounding the tip of the positive needle electrode and on the negative plane electrode surface, where cathode fall regions were established. Although a lot of streamers propagated between the electrodes, they followed different ways in each shot of the discharges. Therefore, only those of the streamers captured in the probing laser sheet were recorded as the LIF images. The raw LIF images taken after the discharges were normalized by that taken without the discharge in order to eliminate an effect of lateral nonuniformity of the probing laser intensity. The dark tree extending from the tip of the needle electrode is a projection of the depressed density profile of NO ŽFig. 2a,c.. For an initial NO concentration wNOx 0 of 30 ppm, a spatial extent of NO depletion expands in a time range of 100 ms to 6 ms. On the other hand, when wNOx 0 s 1000 ppm, the corre-
Fig. 1. A typical image of the SPE of N2 integrated for 1.7 ms from the discharge onset, where the NO concentration was 30 ppm. The profile of the emission intensity along the center line of the gap is plotted on the right-hand side of the image.
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H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548
Fig. 2. Ža,c. Distributions of NO concentration in temporal sequence measured by LIF imaging spectroscopy in a series of independent discharges, where the initial NO concentrations wNOx 0 were 30 ppm Ža. and 1000 ppm Žc., respectively, and the injected discharge energy was about 9 mJ. Žb,d. Distributions of NO concentration along single streamer calculated with the model introduced in Section Section 3.3, where wNOx 0 were 30 ppm Žb. and 1000 ppm Žd., respectively and fd s 8.6% Žsee Section 3.2..
sponding region starts to shrink after 100 ms and is smeared in several milliseconds. The temperature rise led to a decrease in the LIF intensity or an apparent increase in the amount of
NO removal under the present experimental conditions w1–3x. Therefore, the temperature rise due to discharge heating and the yield of the N atoms were approximately estimated.
H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548 Table 1 Dominant reactions in the N2 rNO mixture and their rate coefficients at 295 K w15x Reaction
Rate coefficient Žcm3 rs.
NOqN N2 qO NOqOqM NO 2 qM NO 2 qO NOqO 2 NO 2 qOqM NO 3 qM NO 2 qN 2NO NO 2 qN N2 OqO NOqNO 3 2NO 2 2NqM N2 qM 2OqM O 2 qM NqOqM NOqM NqO 2 NOqO OqO 2 qM O 3 qM NOqO 3 NO 2 qO 2
3.1=10y1 1 2.5=10y12 a 9.8= 10y12 2.3= 10y12 a 6.0=10y13 2.4= 10y12 2.7=10y11 9.7=10y14 a 2.3=10y14 a 2.3=10y13 a 8.0=10y1 7 1.6=10y14 a 1.7= 10y14
™ ™ ™ ™ ™ ™ ™ ™ ™ ™ ™ ™ ™
a
The rate coefficients for three-body reactions are multiplied by the number density of the background N2 gas for comparison with the coefficients for two-body reactions.
545
reasonable at a reduced electric field higher than about 10y1 6 V P cm2 w8x, DT is estimated to be 30 K at a very early stage of the streamer formation Ž- 1 ms.. Then DT is relaxed to about 7.7 K within 100 ms by thermal conduction. Several measurements of the gas temperature of positive streamers in atmospheric air have been reported. Rotational emission spectroscopy of the SPE has shown that DT is moderate Ž- 20 K. in the primary streamer, while it is higher Ž- 500 K. in the secondary streamer w9–11x. On the other hand, DT has been measured to be 1.5–47 K by the schlieren method at about 100 ms after the streamer formation w11,12x. Therefore, the effects of the temperature rise seem to have no significant influence on the present observation of the LIF signal as well as theoretical modeling of the
3.2. Discharge heating and generation of the N atoms With reference to Fig. 1, it is assumed that the injected discharge energy of 9 mJ is distributed to 15 streamers of a radius of 100 mm Ža typical streamer radius w6x. and a length of 15 mm Žthe gap length., where the energy density is estimated to be 1.3 Jrcm3. The corresponding temperature rise DT for the injected energy density u is given by DT s
fh u Cv
,
Ž 1.
where C v is the heat capacity per unit volume of N2 at constant volume and f h is a fraction of the electron energy transferred to the thermal energy. Generally, in electron–molecule collisions at a reduced electric field in the streamer Ž) 10y1 6 V P cm2 ., the electron energy cannot be readily transferred to the thermal energy of the neutral molecules, but the main part of the electron energy goes into the excitation of metastable vibrational levels. Only a small fraction of the energy stored in rotationally excited levels is quickly converted to the translational energy of the molecules. It has been proposed that f h s 0.5–2% based on an approximate estimation in modeling of the streamer w7x. Under the assumption that f h s 2%, which is considered to be
Fig. 3. Total amounts of the species calculated with the present model Ž fd s8.6%.. The points are the total amounts of NO removal by the single discharge Ž DNO. evaluated from the LIF images, where the injected discharge energy was about 9 mJ.
H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548
546
generation is insensitive to the applied field w14x. Therefore, the assumption that fd s 2.5–8.6% leads to an estimated concentration of wNx yield , 0.14–0.47%, at the above-mentioned energy density of 1.3 Jrcm3. 3.3. Model calculation considering the diffusion In order to verify the above estimations, a simple model calculation has been made taking account of the reactions listed in Table 1 as well as the effect of the diffusion. It is assumed in this model that the N atoms are produced at a constant rate of wNx yieldrtd during the discharge duration of td Žs 400 ns. and that all the species distribute uniformly in the streamers with a radius of r Ž t ., which increases with time t due to the diffusion as
Fig. 4. Effective energy cost of NO removal ´rem . The points are evaluated from the LIF images, where the injected discharge energy was about 9 mJ. The curves are calculated with the present model.
(
r Ž t . s r 02 q 4 Dt ,
reaction processes. It is necessary to directly measure the gas temperature for verification of the estimation and understanding of the dynamics. The local concentration of the N atoms generated for the injected energy density u in the background N2 gas of a number density of n is estimated as
w N x yield s
fd u n´
,
Ž 3.
where r 0 is the initial streamer radius and D is the diffusion coefficient, which is approximated to be 0.2 cm2rs Žthe self-diffusion coefficient of the N2 molecule. for all the species. The effect of the diffusion on the concentration wMx of a species M is evaluated with a rate of variation in wMx due to the diffusion
Ž 2.
where ´ Žs 6 eV. is the energy required to produce a single N atom and fd is a fraction of the input energy used to dissociate the N2 molecules. The value of ´rfd is therefore an effective energy cost of the N atom generation, which has been theoretically and experimentally estimated to be 70–240 eVratom, i.e., fd s 2.5–8.6% w13,14x. It has been suggested in modeling of the streamer that the electric field inside the streamer is space-charge shielded and that the effective energy cost of the N atom
ž
d wMx dt
/
s y Ž wMx y wMx 0 . diff
2 dr r dt
,
which is combined with the rate equations, where wMx 0 is the concentration of M before the discharge. The solutions shown in Fig. 2b,d qualitatively reproduce the features of the distributions of NO in Fig. 2a,c. When wNOx 0 s 30 ppm, the N atoms excessively produced along the streamers rapidly reduce the local NO molecules on a timescale of
Table 2 Fractions of consumption of the N atoms predicted by the present model Reaction
Fraction wNOx 0 s 30 ppm
™ ™
Ž2a. NO q N N2 q O Ž2b. 2N q M N2 q M Ž2c. N q O q M NO q M
™
Ž 4.
wNOx 0 s 1000 ppm
fd s 2.5%
fd s 8.6%
fd s 2.5%
fd s 8.6%
0.40 0.52 0.07
0.16 0.80 0.04
0.95 0.01 0.03
0.59 0.24 0.16
H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548
547
Table 3 Fractions of the consumption of O atoms predicted by the present model Reaction
Fraction wNOx 0 s 30 ppm
™ ™ ™ ™
Ž3a. NO q O q M NO 2 q M Ž3b. NO 2 q O NO q O 2 Ž3c. N q O q M NO q M Ž3d. NO 2 q O q M NO 3 q M
wNOx 0 s 1000 ppm
fd s 2.5%
fd s 8.6%
fd s 2.5%
fd s 8.6%
0.41 0.31 0.18 0.07
0.36 0.28 0.26 0.07
0.44 0.40 0.03 0.10
0.31 0.28 0.27 0.07
microseconds and then gradually spread the spatial extent of NO depletion through the diffusion of the N atoms. On the other hand, when wNOx 0 s 1000 ppm the reduction of NO is terminated at about 100 ms after the discharge, and then the local depression of NO is filled in by inward diffusion of the ambient untreated NO molecules. The temporal evolution of the total amounts of different species predicted by this model is shown in Fig. 3, where DNO is the total amount of NO removal by the single discharge. The experimental value of DNO has been evaluated by a spatial integration of the depression profile of NO concentration obtained from the LIF image averaged over 100 shots of the discharges, where a cylindrical symmetry around the axis of the needle electrode is assumed. The effective energy cost of NO removal, ´rem , evaluated from the asymptotic amount of DNO at 32 ms after the discharge is compared with that predicted by the model in Fig. 4. A good agreement suggests that the above assumptions are reasonable. In this model, the N atoms are principally consumed in three reactions listed in Table 2. As the combined reactions of Ž2a. and Ž2c. result in an effective recombination loss of the N atoms, the fractions of the N atoms used for the net reduction of NO are estimated to be 12–33% and 43–92% for wNOx 0 of 30 and 1000 ppm, respectively. When wNOx 0 s 30 ppm, the higher loss of the N atoms due to the recombination increases ´ rem . On the other hand, after the depletion of the N atoms the oxygen atoms released in NO reduction Ž2a. are depleted mainly via reactions Ž3a. and Ž3b. in Table 3. No significant increase in the amount of NO x is seen in Fig. 3, where the total yields of NO 2 are 16% and
10% of DNO for wNOx0 of 30 and 1000 ppm, respectively. In summary, it is suggested that a decrease in the recombination loss of the N atoms, especially in an initial high-concentration stage, should result in the direct improvement in the efficiency of NO x removal.
Acknowledgements The authors thank Mr. T. Sone for his contribution to setting up the experiments, and Mr. M. Kosuge for his assistance in the experiments and analyses. We also thank Profs. M. Ishida, K. Onda, and S. Kogoshi for their helpful comments and discussions.
References w1x J.M. Seitzman, G. Kychakoff, R.K. Hanson, Opt. Lett. 10 Ž1985. 439. w2x P. Andresen, G. Meijer, H. Schluter, ¨ H. Voges, A. Koch, W. Hentschel, W. Oppermann, E. Rothe, Appl. Opt. 29 Ž1990. 2392. w3x M.G. Allen, K.R. McManus, D.M. Sonnenfroh, AIAA 950172 Ž1995.. w4x G.J. Roth, M.A. Gundersen, IEEE Trans. Plasma Sci. 27 Ž1999. 28. w5x H. Hazama, M. Fujiwara, T. Sone, H. Hashimoto, M. Ishida, M. Tanimoto, Proc. Asia-Pacific Workshop on Water and Air Treatment by Advanced Oxidation Technologies, 1998, p. 70. w6x A.A. Kulikovsky, J. Phys. D: Appl. Phys. 28 Ž1995. 2483. w7x E. Marode, F. Bastien, M. Bakker, J. Appl. Phys. 50 Ž1979. 140. w8x J.C. Wren, Whiteshell Nuclear Research Establishment RC248-1 Ž1989..
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w9x K. Kondo, N. Ikuta, J. Phys. Soc. Jpn. 59 Ž1990. 3203. w10x N. Spyrou, B. Held, R. Peyrous, C. Manasis, P. Pignolet, J. Phys. D: Appl. Phys. 25 Ž1992. 211. w11x Y.L.M. Creyghton, E.M. van Veldhuizen, W.R. Rutgers, in: B.M. Penetrante, S.E. Schultheis ŽEds.., NATO ASI series, vol. G 34, part A, Non-Thermal Plasma Techniques for Pollution Control, Springer, Berlin, 1993, p. 205.
w12x G.A. Woolsey, S.J. MacGregor, O. Farish, Proc. R. Soc. Lond. A 405 Ž1986. 355. w13x K. Yan, S. Kanazawa, T. Ohkubo, Y. Nomoto, Trans. IEE Jpn. 119-A Ž1999. 731. w14x B.M. Penetrante, M.C. Hsiao, B.T. Merritt, G.E. Vogtlin, P.H. Wallman, IEEE Trans. Plasma Sci. 23 Ž1995. 679. w15x A.C. Gentile, M.J. Kushner, J. Appl. Phys. 78 Ž1995. 2074.
Chemical Physics Letters 323 Ž2000. 542–548 www.elsevier.nlrlocatercplett
Removal processes of nitric oxide along positive streamers observed by laser-induced fluorescence imaging spectroscopy Hisanao Hazama a
a,)
, Masanori Fujiwara b, Mitsumori Tanimoto
b
Department of Electrical Engineering, Faculty of Science and Technology, Science UniÕersity of Tokyo, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan Received 28 December 1999; in final form 5 May 2000
Abstract Removal processes of nitric oxide ŽNO. in pulsed corona discharges were observed by laser-induced fluorescence ŽLIF. imaging spectroscopy in a reactor filled with N2rNO mixtures. Distributions of NO along the streamers and effective energy costs of NO removal Ž ´ rem . obtained from the LIF images were compared with those predicted by a model taking account of the diffusion. For an initial NO concentration wNOx 0 of 1000 ppm ´ rem was 120 " 31 eVrmolecule, while it was increased up to 620 " 160 eVrmolecule for wNOx 0 of 30 ppm, due to a higher recombination loss of the reducing species of the nitrogen atoms. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Air pollution caused by NO x emission from diesel engines is a serious problem. We are developing a deNO x method based on pulsed corona discharges under atmospheric pressure. The deNO x process by oxidation can be regarded as more realistic than that by reduction, because it requires less energy. However, the latter is preferable, especially for mobile engines, since the end products can be directly exhausted to the atmosphere. In order to optimize the discharge conditions and the reaction processes, it is important to understand the characteristics of the N
) Corresponding author. Fax: q81-298-61-5754; e-mail: [email protected]
atom generation as well as the reaction processes in detail. In the present experiment, a simple N2rNO mixture was used as a test gas, though the actual exhausts of the diesel engines contain various other species such as O 2 and H 2 O, which may affect the reaction processes. We have observed the distributions of NO in the reactor by LIF imaging spectroscopy, which has been used in various fields such as combustion engineering w1–3x. Recently, this technique has been applied to observations of NO distributions in pulsed negative w4x and positive w5x corona discharges. However, LIF images accumulated over multiple shots could not resolve the spatial profiles of NO along streamers. In the present Letter, we have observed the removal processes of NO along individual streamers with single-shot LIF images. In addition, the accumulated LIF images have been
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 5 5 7 - 1
H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548
analyzed to quantitatively evaluate the total amount of NO removal.
543
3. Results and discussion 3.1. Distributions of NO along streamers
2. Experimental setup A discharge reactor made of Teflon Ž50 = 50 = 50 mm3 . had an electrode configuration consisting of a tungsten needle electrode and a stainless-steel plane electrode of 30 mm diameter with a gap of 15 mm. The tip radius of the needle electrode was 2 mm. The reactor was statically filled with N2 gas containing a specified fraction of NO at atmospheric pressure and room temperature. After each discharge the reactor was refilled by a fresh mixture. High-voltage pulses were applied to the needle electrode by a MOSFET switching device which enabled us to precisely synchronize the pulses with the diagnostic systems. The pulses typically had a peak amplitude of 23 kV, a duration of 400 ns, a rise time of 40 ns, and a fall time of 600 ns. The g Ž0,0. band of the NO molecule was excited at a laser wavelength of 226 nm to avoid interference with the absorption lines of O 2 molecule w3x. The UV probe laser pulse was generated by second harmonic conversion of the output from a pulsed dye laser which was pumped by a XeCl excimer laser. The energy and the pulse duration of the second harmonic generated by a BBO crystal were typically 2 mJ and about 20 ns, respectively. The second harmonic had a spectral bandwidth of 0.5 pm Ž; 0.1 cmy1 ., and the wavelength was tunable with an accuracy of 0.5 pm. To observe two-dimensional distribution of NO in a planer area including the needle electrode in the reactor, the laser beam was expanded by a cylindrical lens to a cross-section of approximately 15 = 0.5 mm2 . LIF images were recorded by a cooled, intensified, and gated CCD camera with an imaging optics unit through a quartz window of the reactor. The spatial resolution of the imaging system was about 0.2 mm. The gate duration of the CCD cameras was set at 100 ns, since the lifetime of the LIF was typically about 100 ns. An interference filter centered at 254 nm was used to eliminate the stray probing laser light and N2 fluorescence and to pass the NO fluorescence signal of the g Ž0,1. – Ž0,4. band.
The intensity distribution of the second positive Ž0,0. emission ŽSPE. of N2 Ž337.1 nm., shown in Fig. 1, provides a useful measure of the local yield of the N atoms generated by energetic electrons. The upper level of this transition is 11.0 eV above the ground state, while the dissociation energy of the N2 molecule is 12.0 eV. Bright emissions were observed along the streamers, especially in the region surrounding the tip of the positive needle electrode and on the negative plane electrode surface, where cathode fall regions were established. Although a lot of streamers propagated between the electrodes, they followed different ways in each shot of the discharges. Therefore, only those of the streamers captured in the probing laser sheet were recorded as the LIF images. The raw LIF images taken after the discharges were normalized by that taken without the discharge in order to eliminate an effect of lateral nonuniformity of the probing laser intensity. The dark tree extending from the tip of the needle electrode is a projection of the depressed density profile of NO ŽFig. 2a,c.. For an initial NO concentration wNOx 0 of 30 ppm, a spatial extent of NO depletion expands in a time range of 100 ms to 6 ms. On the other hand, when wNOx 0 s 1000 ppm, the corre-
Fig. 1. A typical image of the SPE of N2 integrated for 1.7 ms from the discharge onset, where the NO concentration was 30 ppm. The profile of the emission intensity along the center line of the gap is plotted on the right-hand side of the image.
544
H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548
Fig. 2. Ža,c. Distributions of NO concentration in temporal sequence measured by LIF imaging spectroscopy in a series of independent discharges, where the initial NO concentrations wNOx 0 were 30 ppm Ža. and 1000 ppm Žc., respectively, and the injected discharge energy was about 9 mJ. Žb,d. Distributions of NO concentration along single streamer calculated with the model introduced in Section Section 3.3, where wNOx 0 were 30 ppm Žb. and 1000 ppm Žd., respectively and fd s 8.6% Žsee Section 3.2..
sponding region starts to shrink after 100 ms and is smeared in several milliseconds. The temperature rise led to a decrease in the LIF intensity or an apparent increase in the amount of
NO removal under the present experimental conditions w1–3x. Therefore, the temperature rise due to discharge heating and the yield of the N atoms were approximately estimated.
H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548 Table 1 Dominant reactions in the N2 rNO mixture and their rate coefficients at 295 K w15x Reaction
Rate coefficient Žcm3 rs.
NOqN N2 qO NOqOqM NO 2 qM NO 2 qO NOqO 2 NO 2 qOqM NO 3 qM NO 2 qN 2NO NO 2 qN N2 OqO NOqNO 3 2NO 2 2NqM N2 qM 2OqM O 2 qM NqOqM NOqM NqO 2 NOqO OqO 2 qM O 3 qM NOqO 3 NO 2 qO 2
3.1=10y1 1 2.5=10y12 a 9.8= 10y12 2.3= 10y12 a 6.0=10y13 2.4= 10y12 2.7=10y11 9.7=10y14 a 2.3=10y14 a 2.3=10y13 a 8.0=10y1 7 1.6=10y14 a 1.7= 10y14
™ ™ ™ ™ ™ ™ ™ ™ ™ ™ ™ ™ ™
a
The rate coefficients for three-body reactions are multiplied by the number density of the background N2 gas for comparison with the coefficients for two-body reactions.
545
reasonable at a reduced electric field higher than about 10y1 6 V P cm2 w8x, DT is estimated to be 30 K at a very early stage of the streamer formation Ž- 1 ms.. Then DT is relaxed to about 7.7 K within 100 ms by thermal conduction. Several measurements of the gas temperature of positive streamers in atmospheric air have been reported. Rotational emission spectroscopy of the SPE has shown that DT is moderate Ž- 20 K. in the primary streamer, while it is higher Ž- 500 K. in the secondary streamer w9–11x. On the other hand, DT has been measured to be 1.5–47 K by the schlieren method at about 100 ms after the streamer formation w11,12x. Therefore, the effects of the temperature rise seem to have no significant influence on the present observation of the LIF signal as well as theoretical modeling of the
3.2. Discharge heating and generation of the N atoms With reference to Fig. 1, it is assumed that the injected discharge energy of 9 mJ is distributed to 15 streamers of a radius of 100 mm Ža typical streamer radius w6x. and a length of 15 mm Žthe gap length., where the energy density is estimated to be 1.3 Jrcm3. The corresponding temperature rise DT for the injected energy density u is given by DT s
fh u Cv
,
Ž 1.
where C v is the heat capacity per unit volume of N2 at constant volume and f h is a fraction of the electron energy transferred to the thermal energy. Generally, in electron–molecule collisions at a reduced electric field in the streamer Ž) 10y1 6 V P cm2 ., the electron energy cannot be readily transferred to the thermal energy of the neutral molecules, but the main part of the electron energy goes into the excitation of metastable vibrational levels. Only a small fraction of the energy stored in rotationally excited levels is quickly converted to the translational energy of the molecules. It has been proposed that f h s 0.5–2% based on an approximate estimation in modeling of the streamer w7x. Under the assumption that f h s 2%, which is considered to be
Fig. 3. Total amounts of the species calculated with the present model Ž fd s8.6%.. The points are the total amounts of NO removal by the single discharge Ž DNO. evaluated from the LIF images, where the injected discharge energy was about 9 mJ.
H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548
546
generation is insensitive to the applied field w14x. Therefore, the assumption that fd s 2.5–8.6% leads to an estimated concentration of wNx yield , 0.14–0.47%, at the above-mentioned energy density of 1.3 Jrcm3. 3.3. Model calculation considering the diffusion In order to verify the above estimations, a simple model calculation has been made taking account of the reactions listed in Table 1 as well as the effect of the diffusion. It is assumed in this model that the N atoms are produced at a constant rate of wNx yieldrtd during the discharge duration of td Žs 400 ns. and that all the species distribute uniformly in the streamers with a radius of r Ž t ., which increases with time t due to the diffusion as
Fig. 4. Effective energy cost of NO removal ´rem . The points are evaluated from the LIF images, where the injected discharge energy was about 9 mJ. The curves are calculated with the present model.
(
r Ž t . s r 02 q 4 Dt ,
reaction processes. It is necessary to directly measure the gas temperature for verification of the estimation and understanding of the dynamics. The local concentration of the N atoms generated for the injected energy density u in the background N2 gas of a number density of n is estimated as
w N x yield s
fd u n´
,
Ž 3.
where r 0 is the initial streamer radius and D is the diffusion coefficient, which is approximated to be 0.2 cm2rs Žthe self-diffusion coefficient of the N2 molecule. for all the species. The effect of the diffusion on the concentration wMx of a species M is evaluated with a rate of variation in wMx due to the diffusion
Ž 2.
where ´ Žs 6 eV. is the energy required to produce a single N atom and fd is a fraction of the input energy used to dissociate the N2 molecules. The value of ´rfd is therefore an effective energy cost of the N atom generation, which has been theoretically and experimentally estimated to be 70–240 eVratom, i.e., fd s 2.5–8.6% w13,14x. It has been suggested in modeling of the streamer that the electric field inside the streamer is space-charge shielded and that the effective energy cost of the N atom
ž
d wMx dt
/
s y Ž wMx y wMx 0 . diff
2 dr r dt
,
which is combined with the rate equations, where wMx 0 is the concentration of M before the discharge. The solutions shown in Fig. 2b,d qualitatively reproduce the features of the distributions of NO in Fig. 2a,c. When wNOx 0 s 30 ppm, the N atoms excessively produced along the streamers rapidly reduce the local NO molecules on a timescale of
Table 2 Fractions of consumption of the N atoms predicted by the present model Reaction
Fraction wNOx 0 s 30 ppm
™ ™
Ž2a. NO q N N2 q O Ž2b. 2N q M N2 q M Ž2c. N q O q M NO q M
™
Ž 4.
wNOx 0 s 1000 ppm
fd s 2.5%
fd s 8.6%
fd s 2.5%
fd s 8.6%
0.40 0.52 0.07
0.16 0.80 0.04
0.95 0.01 0.03
0.59 0.24 0.16
H. Hazama et al.r Chemical Physics Letters 323 (2000) 542–548
547
Table 3 Fractions of the consumption of O atoms predicted by the present model Reaction
Fraction wNOx 0 s 30 ppm
™ ™ ™ ™
Ž3a. NO q O q M NO 2 q M Ž3b. NO 2 q O NO q O 2 Ž3c. N q O q M NO q M Ž3d. NO 2 q O q M NO 3 q M
wNOx 0 s 1000 ppm
fd s 2.5%
fd s 8.6%
fd s 2.5%
fd s 8.6%
0.41 0.31 0.18 0.07
0.36 0.28 0.26 0.07
0.44 0.40 0.03 0.10
0.31 0.28 0.27 0.07
microseconds and then gradually spread the spatial extent of NO depletion through the diffusion of the N atoms. On the other hand, when wNOx 0 s 1000 ppm the reduction of NO is terminated at about 100 ms after the discharge, and then the local depression of NO is filled in by inward diffusion of the ambient untreated NO molecules. The temporal evolution of the total amounts of different species predicted by this model is shown in Fig. 3, where DNO is the total amount of NO removal by the single discharge. The experimental value of DNO has been evaluated by a spatial integration of the depression profile of NO concentration obtained from the LIF image averaged over 100 shots of the discharges, where a cylindrical symmetry around the axis of the needle electrode is assumed. The effective energy cost of NO removal, ´rem , evaluated from the asymptotic amount of DNO at 32 ms after the discharge is compared with that predicted by the model in Fig. 4. A good agreement suggests that the above assumptions are reasonable. In this model, the N atoms are principally consumed in three reactions listed in Table 2. As the combined reactions of Ž2a. and Ž2c. result in an effective recombination loss of the N atoms, the fractions of the N atoms used for the net reduction of NO are estimated to be 12–33% and 43–92% for wNOx 0 of 30 and 1000 ppm, respectively. When wNOx 0 s 30 ppm, the higher loss of the N atoms due to the recombination increases ´ rem . On the other hand, after the depletion of the N atoms the oxygen atoms released in NO reduction Ž2a. are depleted mainly via reactions Ž3a. and Ž3b. in Table 3. No significant increase in the amount of NO x is seen in Fig. 3, where the total yields of NO 2 are 16% and
10% of DNO for wNOx0 of 30 and 1000 ppm, respectively. In summary, it is suggested that a decrease in the recombination loss of the N atoms, especially in an initial high-concentration stage, should result in the direct improvement in the efficiency of NO x removal.
Acknowledgements The authors thank Mr. T. Sone for his contribution to setting up the experiments, and Mr. M. Kosuge for his assistance in the experiments and analyses. We also thank Profs. M. Ishida, K. Onda, and S. Kogoshi for their helpful comments and discussions.
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