- Email: [email protected]
Study on density distribution of high-energy electrons in pulsed corona discharge
ARTICLE IN PRESS
Vacuum 73 (2004) 333–339
Study on density distribution of high-energy electrons in pulsed corona discharge Wenchun Wang*, Jialiang Zhang, Feng Liu, Yue Liu, Younian Wang State Key Laboratory of Materials Modification by Laser, Ion and Electron beams, Dalian University of Technology, Dalian 116024, People’s Republic of China
Abstract In this study, the radial profile of the overall emission intensity of the second positive system (C3Pu-B3Pg) emitted from the positive pulse corona discharge of N2 and air in a line-cylinder reactor was successfully recorded against a severe electromagnetic pulse interference coming from the corona discharge at room temperature and 1 atm. The relation between the density distributions of the high-energy electron (whose energy is higher than 11.03 eV and enough to excite N2 to its C3Pu state from the ground state) and the emission profiles is studied with the aid of a reaction radiation rate analysis method. By the relation, it is found that the radial distribution of the high-energy electron decreases nonlinearly with the radial distance from the reactor axis and increases directly with the discharge voltage for any sampling apertures. These experimental results would be helpful to establish the molecule reaction dynamics model of pulsed corona discharge deSO2/deNOx and optimize the power supply and reactor. r 2004 Elsevier Ltd. All rights reserved. Keywords: Electron density; Emission spectrum; Pulsed corona discharge; Plasma
1. Introduction Pulsed corona discharge is one of the nonthermal plasma characterized by low gas temperature and high electron temperature. In the last two decades, high-voltage pulsed corona discharge was extensively studied as a powerful method for pollutant removal from flue gases. Specially, short-duration pulsed corona discharge has some advantages over other methods for pollutant removal, which is, for example, more efficient in energy conversion than other non-thermal plasma techniques. By pulsed corona discharge, more *Corresponding author. Tel.: +86-411-470-9795-16; fax: +86-411-470-7161. E-mail address: [email protected] (W. Wang).
electrical energy input goes into the production of energetic electrons than gas heating [1] because high-energy electrons (whose energy is in the range of 5–20 eV) generated by pulse corona discharge can efficiently dissociate, excite or even ionize SO2, NOx, H2O, O2 and N2 in flue gas into oxygenated active species (O3, H2O2, etc.) or radicals (OH, HO2, etc.) [2]. So, pulsed corona discharge as a new deSO2/deNOx technique will have a promising future for practical application. The consequent chemical reactions caused by these active species can oxidize SO2 and NOx into higher order oxides in the form of H2SO4 and HNO3, which then are converted into aerosols of ammonium sulfate and ammonium nitrate in the form of fine particles when ammonia is injected in the downstream of the discharge reactor. These final
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.047
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products can be collected out by routine methods (ESP or bags) and therefore separated from the flue gas [3]. Recently, there have been a number of published investigations on various electrical discharge processes applied to flue gas [4–11]. Eliasson et al. [4] basically described the electrical discharges and the related plasma chemical processes. Lowke et al. [5,6] investigated the role of various species for the pollutant removal processes and interpreted the removal mechanism of SO2 and nitrogen oxides. In the meanwhile, kinetic simulation studies have also been done on these electrical discharge processes. A kinetic model and the corresponding mathematical model were proposed to describe the removal behavior of nitrogen oxides in a positive pulse corona discharge reactor by Mok et al. [7,8]. They compared their model with some experimental data to estimate the concentration of the radicals produced directly by the dissociation collisions of the energetic electrons and the matrix molecules. Considering the contribution of the excited oxygen atoms to the production of secondary radicals, they derived some expressions for the concentrations of O, H, N atoms and OH radicals as functions of discharge condition parameters. The formation of the active species and free radicals depends directly on the density distribution of the high-energy electrons in the reactor. Although all kinds of active species and free radicals play important roles in deSO2/deNOx from flue gases, the density of the high-energy electrons is the more basic factor for the deSO2/ deNOx efficiencies of corona discharges. In this article, we present a spectroscopic diagnostic scheme to monitor the spatial distribution profiles of the high-energy electron densities in pulsed corona discharge in a line-cylinder reactor, by measuring the optical emission spectra of N2 and air corona against electromagnetic interference caused by the high-voltage pulsed discharge. We applied our diagnostic scheme to investigate the relationship between the high-energy electron density and the discharge conditions, such as the discharge voltage and working gas. O2 species play a unique role in deSO2/deNOx from flue gases. The influences of the oxygen flow on N2 (C3Pu-B3Pg) emission intensity and therefore on the density
spatial distribution of the high-energy electrons are also investigated in this article.
2. Experimental setup The experimental setup is illustrated schematically in Fig. 1a and b. It is composed of a gas mixing chamber, a pulsed power supply, a discharge reactor and an optical detection system. The pulsed power can supply high-voltage pulse with a rising time of 20 ns, a pulse width of about 800 ns and an adjustable repetition frequency in the range of 10–200 Hz. The reactor has a linecylinder electrode structure. A 4 4 mm asteroid corona wire with an effective length of 710 mm is used as the positive electrode mounted in the center of the cylinder. The outer cylinder is a stainless steel one with an inside diameter of 55 mm. The gas inlet and the outlet of the reactor are controlled with glass vacuum valves. In the two end PMMA flanges there is, respectively, a two-layer shielding gas outlet 5
3
4
1
6
77
Lens 2
20MΩ
Triggering signal
50Ω 9
O2
8
N2
1 He-Ne Laser 2 Mixing Chamber 3 High-Voltage Pulse Power 4 Reactor 5 High-Voltage Power Supply 6 Grating Monochromater 7 Photomultiplier Tube 8 Signal Sampler 9 Computer (a)
(b) Fig. 1. (a) Schematic of the experimental setup for deSO2/ deNOx by positive pulsed corona discharge. (b) The detailed drawing of the line-cylinder corona reactor.
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f 30 mm quartz window for viewing and optical measurements. Four f 3 mm round apertures with distances of 10, 15, 20 and 25 mm from the center axis, respectively, are formed on each quartz window by a blacked four-hole window cover. In order to increase the signal-to-noise ratio of the optical detection system, the faces of both end flanges and the inner wall of the reactor are painted black to absorb stray light and prevent stray light from entering the detection system. In order to reduce the interference of discharge pulses in the detection system and other instruments, the reactor and pulse high-voltage power supply are placed in a two-layer shielding box. Both the shielding box and the reactor are connected to the ground separately. In order to trigger the used BOXCAR (SR265) synchronously with the discharge pulse, the pulse power and the reactor must be securely connected to the ground separately, and the grounding of the trigger for BOXCAR must be connected to the reactor’s grounding. The axis of the reactor and the optical axis of the detection system are collimated with a He–Ne laser beam. The optical emission from the discharge goes through the quartz window into a light pipe (300 mm in length) and is collected by a convergent lens (f ¼ 170 mm) to a MODEL ASI-50SG grating monochromater (grating groove is 1200 lines/ mm, glancing wavelength is 500 nm). After the diffraction of the grating, the output spectral light is converted into an electrical signal by a photomultiplication tube (PMT, mode 9558QB), and the output of the PMT is taken by the used BOXCAR and recorded by a computer. High-purity N2 (99.9%) and normal air are used, respectively, as discharge gases under a gas pressure of 1.013 105 Pa and room temperature.
3. Experimental results and discussion
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+ 1 atm pressure, ions (Nþ 2 ; N ) cannot be speeded up by the short-pulse electric field because of their small migration rates and can be regarded static when electrons with much bigger migration rates can be speeded up to 2–20 eV in kinetic energy [12]. The elastic collisions of the energetic electrons with N2 molecules will increase the kinetic energy of N2 molecules, while the non-elastic collisions will induce N2 molecules to be dissociated, excited or even ionized. Electrons with different energy can excite N2 molecules to different states. Highenergy electrons with energy of more than 11.03 eV can excite N2 molecule to C3Pu state. The radiation lifetime of the excited C3Pu state is about 40 ns [13]. The rate equation related to the temporal variation of the N2 (C3Pu) number density is
d½N2 ðCÞ ¼ Ke ne ½N2 A½N2 ðCÞ dt K½N2 ½N2 ðCÞ:
ð1Þ
On the right-hand side of Eq. (1), the first term stands for the population rate of nitrogen at the C3Pu state by the collisions of high-energy electrons and grounded nitrogen. The second term is for the radiation depopulation rate of nitrogen at the C3Pu state. And the third term is the collision quenching term of nitrogen at the C3Pu state. Here, K is the rate constant of the collision quenching. [N2] expresses the number density of N2 molecule at its ground state. A is the radiation rate constant of the C3Pu state. Ke is the collision excitation rate constant of the C3Pu state by highenergy electrons, and ne stands for the total density of the high-energy electrons in the discharges. By the integration of Eq. (1), the following is obtained to describe the temporal evolution of the density of nitrogen at the C3Pu state: Z t Ke ne ½N2 e½AþK½N2 ðttÞ dt: ð2Þ ½N2 ðCÞ ¼ 0
3.1. Relationship between the high-energy electron density and the emission intensity of N2 second positive system (C3Pu-B3Pg) in positive pulse corona discharges at 1 atm In the non-equilibrium plasma generated by pulsed corona discharge at room temperature and
Ke should be regarded as something of a constant because it is only dependent on the energy distribution of the high-energy electrons, which that is almost peaked at the excitation and ionization potentials of nitrogen, and K, [N2] and A are all constants during the discharge period under steady discharge conditions. Expression (2)
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can be rewritten as: Z t ne e½AþK½N2 ðttÞ dt: ½N2 ðCÞ ¼ Ke ½N2
ð3Þ
0
Eq. (4) says that the number density of N2 (C3Pu) is temporally proportional to the highenergy electron density in the discharge period. The emission intensity of the second positive band system of N2, I, which comes from the transition of C3Pu-B3Pg, is expressed as the following radiation rate equation Ke ½N2 ne : I ¼ A½N2 ðCÞ ¼ A ð5Þ A þ K½N2 In Eq. (5), the factor [N2(C)] has been substituted by Eq. (4). The equation means that the emission intensity varies synchronously to ne, the highenergy electron density. 3.2. The emission intensity of the second positive band system of N2 (C3Pu-B3Pg) in positive pulse corona discharge in air and pure nitrogen Fig. 2 shows a sample of emission spectra emitted from a positive pulsed corona discharge of nitrogen at 1 atm and room temperature in the wavelength range of 290–440 nm. The spectra consist mainly of the second positive system of nitrogen. The radial distribution of the intensity of the 0–0 vibrational transition band of the second positive system detected at different corona discharge conditions is shown in Fig. 3. As shown in Fig. 3, for the first aperture near the corona wire, the emission intensity is the strongest, which indicates that the density of the high-energy
Fig. 2. Sample emission spectrum of N2 (C3Pu-B3Pg) from N2 positive pulsed corona discharge.
2500
Emission intensity I (arb)
Considering that the high-energy electron density, ne, varies temporally with the discharge current pulses and the width of one discharge current pulse is about 800 ns, a so-called adiabatic approximation can be introduced into the calculation of the integration of Eq. (3), because the 800 ns width of the discharge pulses is much more longer than the time constant, 1/A+K[N2], which is shorter than 40 ns. As a result of the integration of Eq. (3) with the adiabatic approximation, we obtain Ke ½N2 ðCÞ ¼ ½N2 ne : ð4Þ A þ K½N2
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1000
500
0 0
5
10
15
20
25
30
35
Radial distance (mm) Fig. 3. Measured emission intensity radial distribution of N2 (C3Pu-B3Pg) from positive pulsed corona discharges of N2 at different voltages.
electron (X11.03 eV) is the biggest at this place. However, for the fourth aperture near the inner wall of the reactor, the emission intensity is the weakest and the density of high-energy electrons is the lowest. In Fig. 4, the solid line corresponding to the linear relationship between the emission intensity of N2 (C3Pu-B3Pg) and the discharge voltage (30–45 kV) is obtained by the least-squares fitting of those data
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Fig. 4. Linear relation between the emission intensity of N2 and discharge voltage from the first aperture (10 mm from reactor central axis).
measured under the same discharge condition as those for Fig. 3. The solid straight line indicates that the high-energy electron density in the discharges linearly increases with the discharge voltage V. Fig. 5 shows the emission spectrum of the positive pulsed corona discharge in air, in the range of 290–420 nm. From the intensity of the 0–0 vibrational transition band of the nitrogen second positive system (C3Pu-B3Pg), the radial profiles of the emission intensity of N2 from the positive pulsed corona discharge in air are presented in Fig. 6. This figure reveals the following. (1) From the first aperture to the fourth aperture, the emission intensity I differs greatly from each other because of the different distances away from the reactor center axis. The ratios of the emission intensities at different apertures are evaluated to be 1.0 : 0.67 : 0.50 : 0.33, which is similar to the spatial distribution of the discharge electric field intensity and therefore reasonably reflects the density distribution of the high-energy electrons (X11.03 eV). (2) The highest voltage in Fig. 6 is 43 kV, because the breakdown voltage of air at 1 atm is not higher than 43 kV, while the breakdown voltage of N2 at 1 atm is higher than 50 kV. Based on Eq. (5) and Fig. 6, the distribution profiles of the normalized high-energy electron density in the air positive corona discharge at different discharge voltages are shown in Fig. 7.
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Fig. 5. Sample emission spectrum of N2 (C3Pu-B3Pg) from air positive pulsed corona discharge.
Fig. 6. Measured emission intensity radial distribution of N2 (C3Pu-B3Pg) from positive pulsed corona discharges of air at different voltages.
Comparing the profiles in Figs. 3 and 6, both of which are obtained at the same experimental conditions, the emission intensities of N2 (C3Pu-B3Pg) in N2 discharges are much stronger than those in air discharges. Some mechanisms for the intensity differences are discussed in the following: (1) In air discharges, the concentrations of N2 and O2 are 78% and 21%, respectively. Because
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Fig. 8. Emission intensity of N2 (C3Pu-B3Pg) as a function of O2 flow rate into N2 positive pulsed corona discharge. Fig. 7. Radial distribution of normalized high-energy electron density in air positive pulsed corona discharge.
the concentration of N2 in air is lower than pure N2, the emission intensity of N2 in air discharges is certainly weaker than that in nitrogen discharges. (2) The O2 in air discharges captures a large number of electrons and becomes negative ions, so the densities of the electrons in air discharge are much lower than that in nitrogen discharges under same discharge conditions. The emission intensity of N2 is remarkably weakened by the oxygen component of air discharge. 3.3. Effects of O2 addition on the emission intensity of N2 (C3Pu-B3Pg) in nitrogen discharge The addition of an oxygen flow into a nitrogen positive pulse corona discharge depletes the free electrons in the discharge by the following two processes: e þ O2 -O 2;
ð6Þ
e þ O2 þ M-O 2 þ M:
ð7Þ
Here, M may be O2, N2, and other molecules. The added O2 decreases the density of free electrons greatly. When the flow rate of O2 increases, the number density of excited N2 (C3Pu) generated in
the discharge should decrease rapidly because O2 is also a strong quenching agent of the excited N2 (C3Pu) in the discharge. By mixing oxygen flow with different rates into the discharge system of N2, the effect of the concentration of O2 on the emission intensity of the N2 transition of C3Pu-B3Pg is studied. The power supply voltage is 40 kV and is kept constant during the measurements. The flow rate of N2 into the reactor is kept at 0.18 m3/h. The emission spectra of the N2 transition of C3Pu-B3Pg are clearly recorded through the fourth aperture from the pulsed corona discharges at eight different oxygen input flow rates of 10, 20, 30, up to 80 ml/ min. Fig. 8 shows the emission intensity of N2 (C3Pu-B3Pg) as a function of O2 flow rate. It can be seen that the emission intensity decreases almost linearly with the increase of O2 flow rate.
4. Conclusion In this paper, we record successfully the radial profile of the emission intensity of the second positive system (C3Pu-B3Pg) emitted from the positive pulse corona discharge of N2 and air in a line-cylinder reactor. Moreover, we prove the direct relation between the density of the highenergy electron and the emission intensity of the second positive system. For the positive pulsed
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corona discharge reactor, the high-energy electron densities generated in air and nitrogen corona discharge always present a radial non-linear decreasing tendency, which is coincident with the electric field distribution in the line-cylinder reactor. In addition, it is also shown that the high-energy electron density ne takes a similar radial distribution profile at different discharge voltages. From the first aperture near the corona wire, the emission intensity is the strongest, which indicates that the density of the high-energy electron is the biggest at this place. But, from the fourth aperture, the emission intensity is the weakest. For any apertures, there is a linear relationship between the high-energy electron density and the discharge voltage. Otherwise, when the O2 flow is added to the discharge reactor, the overall emission intensity of N2 is remarkably reduced and thus the high-energy electron density is correspondingly reduced in the reactor because O2 captures a great deal of electrons and thus the density of the high-energy electron decrease.
Acknowledgements The author would like to thank professor Xuechu Li for friendly discussion and suggestion.
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This work is supported by the united fund of the National Natural Science Foundation Committee of China and Engineering Physical Institute of China under Grant No. 10276008 and the fund of Liaoning Province Natural Science Foundation Committee under Grant No. 20022138.
References [1] Mizuno A, Clements JS, Davis RH. IEEE Trans Ind Appl 1986;22:516. [2] Dahiya RP, Mishra SK, Veefkind A. IEEE Trans Plasma Sci 1993;21:346. [3] Liu Shuhai. Dev Environ Sci 1994;2:75. [4] Eliasson B, Kogelschatz U. IEEE Trans Plasma Sci 1991;19:309. [5] Lowke JJ, Kogelschatz U. Pure Appl Chem 1994;66:1287. [6] Lowke JJ, Kogelschatz U. IEEE Trans Plasma Sci 1995;23:661. [7] Mok YS, Ham SW. Chem Eng Sci 1998;53:1667. [8] Mok YS, et al. IEEE Trans Plasma Sci 1998;26:1566. [9] Kim DJ, Choi Y, Kim KS. Plasma Chem Plasma Process 2001;21:625. [10] Young Sun Mod, In-Sik Nam. Chem Eng J 2002;85:87. [11] Takao Namihira, et al. IEEE Trans Plasma Sci 1998;29:592. [12] Eliasson B, Kogelschatz U. J Phys B 1986;19:1241. [13] Suchard SN. Spectroscopic Data, Heteronuclar Diatomic Molecules, Part A. California: The Aerospace Corporation Los Angeles; 1975. p. 1724.
Vacuum 73 (2004) 333–339
Study on density distribution of high-energy electrons in pulsed corona discharge Wenchun Wang*, Jialiang Zhang, Feng Liu, Yue Liu, Younian Wang State Key Laboratory of Materials Modification by Laser, Ion and Electron beams, Dalian University of Technology, Dalian 116024, People’s Republic of China
Abstract In this study, the radial profile of the overall emission intensity of the second positive system (C3Pu-B3Pg) emitted from the positive pulse corona discharge of N2 and air in a line-cylinder reactor was successfully recorded against a severe electromagnetic pulse interference coming from the corona discharge at room temperature and 1 atm. The relation between the density distributions of the high-energy electron (whose energy is higher than 11.03 eV and enough to excite N2 to its C3Pu state from the ground state) and the emission profiles is studied with the aid of a reaction radiation rate analysis method. By the relation, it is found that the radial distribution of the high-energy electron decreases nonlinearly with the radial distance from the reactor axis and increases directly with the discharge voltage for any sampling apertures. These experimental results would be helpful to establish the molecule reaction dynamics model of pulsed corona discharge deSO2/deNOx and optimize the power supply and reactor. r 2004 Elsevier Ltd. All rights reserved. Keywords: Electron density; Emission spectrum; Pulsed corona discharge; Plasma
1. Introduction Pulsed corona discharge is one of the nonthermal plasma characterized by low gas temperature and high electron temperature. In the last two decades, high-voltage pulsed corona discharge was extensively studied as a powerful method for pollutant removal from flue gases. Specially, short-duration pulsed corona discharge has some advantages over other methods for pollutant removal, which is, for example, more efficient in energy conversion than other non-thermal plasma techniques. By pulsed corona discharge, more *Corresponding author. Tel.: +86-411-470-9795-16; fax: +86-411-470-7161. E-mail address: [email protected] (W. Wang).
electrical energy input goes into the production of energetic electrons than gas heating [1] because high-energy electrons (whose energy is in the range of 5–20 eV) generated by pulse corona discharge can efficiently dissociate, excite or even ionize SO2, NOx, H2O, O2 and N2 in flue gas into oxygenated active species (O3, H2O2, etc.) or radicals (OH, HO2, etc.) [2]. So, pulsed corona discharge as a new deSO2/deNOx technique will have a promising future for practical application. The consequent chemical reactions caused by these active species can oxidize SO2 and NOx into higher order oxides in the form of H2SO4 and HNO3, which then are converted into aerosols of ammonium sulfate and ammonium nitrate in the form of fine particles when ammonia is injected in the downstream of the discharge reactor. These final
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.047
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products can be collected out by routine methods (ESP or bags) and therefore separated from the flue gas [3]. Recently, there have been a number of published investigations on various electrical discharge processes applied to flue gas [4–11]. Eliasson et al. [4] basically described the electrical discharges and the related plasma chemical processes. Lowke et al. [5,6] investigated the role of various species for the pollutant removal processes and interpreted the removal mechanism of SO2 and nitrogen oxides. In the meanwhile, kinetic simulation studies have also been done on these electrical discharge processes. A kinetic model and the corresponding mathematical model were proposed to describe the removal behavior of nitrogen oxides in a positive pulse corona discharge reactor by Mok et al. [7,8]. They compared their model with some experimental data to estimate the concentration of the radicals produced directly by the dissociation collisions of the energetic electrons and the matrix molecules. Considering the contribution of the excited oxygen atoms to the production of secondary radicals, they derived some expressions for the concentrations of O, H, N atoms and OH radicals as functions of discharge condition parameters. The formation of the active species and free radicals depends directly on the density distribution of the high-energy electrons in the reactor. Although all kinds of active species and free radicals play important roles in deSO2/deNOx from flue gases, the density of the high-energy electrons is the more basic factor for the deSO2/ deNOx efficiencies of corona discharges. In this article, we present a spectroscopic diagnostic scheme to monitor the spatial distribution profiles of the high-energy electron densities in pulsed corona discharge in a line-cylinder reactor, by measuring the optical emission spectra of N2 and air corona against electromagnetic interference caused by the high-voltage pulsed discharge. We applied our diagnostic scheme to investigate the relationship between the high-energy electron density and the discharge conditions, such as the discharge voltage and working gas. O2 species play a unique role in deSO2/deNOx from flue gases. The influences of the oxygen flow on N2 (C3Pu-B3Pg) emission intensity and therefore on the density
spatial distribution of the high-energy electrons are also investigated in this article.
2. Experimental setup The experimental setup is illustrated schematically in Fig. 1a and b. It is composed of a gas mixing chamber, a pulsed power supply, a discharge reactor and an optical detection system. The pulsed power can supply high-voltage pulse with a rising time of 20 ns, a pulse width of about 800 ns and an adjustable repetition frequency in the range of 10–200 Hz. The reactor has a linecylinder electrode structure. A 4 4 mm asteroid corona wire with an effective length of 710 mm is used as the positive electrode mounted in the center of the cylinder. The outer cylinder is a stainless steel one with an inside diameter of 55 mm. The gas inlet and the outlet of the reactor are controlled with glass vacuum valves. In the two end PMMA flanges there is, respectively, a two-layer shielding gas outlet 5
3
4
1
6
77
Lens 2
20MΩ
Triggering signal
50Ω 9
O2
8
N2
1 He-Ne Laser 2 Mixing Chamber 3 High-Voltage Pulse Power 4 Reactor 5 High-Voltage Power Supply 6 Grating Monochromater 7 Photomultiplier Tube 8 Signal Sampler 9 Computer (a)
(b) Fig. 1. (a) Schematic of the experimental setup for deSO2/ deNOx by positive pulsed corona discharge. (b) The detailed drawing of the line-cylinder corona reactor.
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f 30 mm quartz window for viewing and optical measurements. Four f 3 mm round apertures with distances of 10, 15, 20 and 25 mm from the center axis, respectively, are formed on each quartz window by a blacked four-hole window cover. In order to increase the signal-to-noise ratio of the optical detection system, the faces of both end flanges and the inner wall of the reactor are painted black to absorb stray light and prevent stray light from entering the detection system. In order to reduce the interference of discharge pulses in the detection system and other instruments, the reactor and pulse high-voltage power supply are placed in a two-layer shielding box. Both the shielding box and the reactor are connected to the ground separately. In order to trigger the used BOXCAR (SR265) synchronously with the discharge pulse, the pulse power and the reactor must be securely connected to the ground separately, and the grounding of the trigger for BOXCAR must be connected to the reactor’s grounding. The axis of the reactor and the optical axis of the detection system are collimated with a He–Ne laser beam. The optical emission from the discharge goes through the quartz window into a light pipe (300 mm in length) and is collected by a convergent lens (f ¼ 170 mm) to a MODEL ASI-50SG grating monochromater (grating groove is 1200 lines/ mm, glancing wavelength is 500 nm). After the diffraction of the grating, the output spectral light is converted into an electrical signal by a photomultiplication tube (PMT, mode 9558QB), and the output of the PMT is taken by the used BOXCAR and recorded by a computer. High-purity N2 (99.9%) and normal air are used, respectively, as discharge gases under a gas pressure of 1.013 105 Pa and room temperature.
3. Experimental results and discussion
335
+ 1 atm pressure, ions (Nþ 2 ; N ) cannot be speeded up by the short-pulse electric field because of their small migration rates and can be regarded static when electrons with much bigger migration rates can be speeded up to 2–20 eV in kinetic energy [12]. The elastic collisions of the energetic electrons with N2 molecules will increase the kinetic energy of N2 molecules, while the non-elastic collisions will induce N2 molecules to be dissociated, excited or even ionized. Electrons with different energy can excite N2 molecules to different states. Highenergy electrons with energy of more than 11.03 eV can excite N2 molecule to C3Pu state. The radiation lifetime of the excited C3Pu state is about 40 ns [13]. The rate equation related to the temporal variation of the N2 (C3Pu) number density is
d½N2 ðCÞ ¼ Ke ne ½N2 A½N2 ðCÞ dt K½N2 ½N2 ðCÞ:
ð1Þ
On the right-hand side of Eq. (1), the first term stands for the population rate of nitrogen at the C3Pu state by the collisions of high-energy electrons and grounded nitrogen. The second term is for the radiation depopulation rate of nitrogen at the C3Pu state. And the third term is the collision quenching term of nitrogen at the C3Pu state. Here, K is the rate constant of the collision quenching. [N2] expresses the number density of N2 molecule at its ground state. A is the radiation rate constant of the C3Pu state. Ke is the collision excitation rate constant of the C3Pu state by highenergy electrons, and ne stands for the total density of the high-energy electrons in the discharges. By the integration of Eq. (1), the following is obtained to describe the temporal evolution of the density of nitrogen at the C3Pu state: Z t Ke ne ½N2 e½AþK½N2 ðttÞ dt: ð2Þ ½N2 ðCÞ ¼ 0
3.1. Relationship between the high-energy electron density and the emission intensity of N2 second positive system (C3Pu-B3Pg) in positive pulse corona discharges at 1 atm In the non-equilibrium plasma generated by pulsed corona discharge at room temperature and
Ke should be regarded as something of a constant because it is only dependent on the energy distribution of the high-energy electrons, which that is almost peaked at the excitation and ionization potentials of nitrogen, and K, [N2] and A are all constants during the discharge period under steady discharge conditions. Expression (2)
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can be rewritten as: Z t ne e½AþK½N2 ðttÞ dt: ½N2 ðCÞ ¼ Ke ½N2
ð3Þ
0
Eq. (4) says that the number density of N2 (C3Pu) is temporally proportional to the highenergy electron density in the discharge period. The emission intensity of the second positive band system of N2, I, which comes from the transition of C3Pu-B3Pg, is expressed as the following radiation rate equation Ke ½N2 ne : I ¼ A½N2 ðCÞ ¼ A ð5Þ A þ K½N2 In Eq. (5), the factor [N2(C)] has been substituted by Eq. (4). The equation means that the emission intensity varies synchronously to ne, the highenergy electron density. 3.2. The emission intensity of the second positive band system of N2 (C3Pu-B3Pg) in positive pulse corona discharge in air and pure nitrogen Fig. 2 shows a sample of emission spectra emitted from a positive pulsed corona discharge of nitrogen at 1 atm and room temperature in the wavelength range of 290–440 nm. The spectra consist mainly of the second positive system of nitrogen. The radial distribution of the intensity of the 0–0 vibrational transition band of the second positive system detected at different corona discharge conditions is shown in Fig. 3. As shown in Fig. 3, for the first aperture near the corona wire, the emission intensity is the strongest, which indicates that the density of the high-energy
Fig. 2. Sample emission spectrum of N2 (C3Pu-B3Pg) from N2 positive pulsed corona discharge.
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Emission intensity I (arb)
Considering that the high-energy electron density, ne, varies temporally with the discharge current pulses and the width of one discharge current pulse is about 800 ns, a so-called adiabatic approximation can be introduced into the calculation of the integration of Eq. (3), because the 800 ns width of the discharge pulses is much more longer than the time constant, 1/A+K[N2], which is shorter than 40 ns. As a result of the integration of Eq. (3) with the adiabatic approximation, we obtain Ke ½N2 ðCÞ ¼ ½N2 ne : ð4Þ A þ K½N2
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1500
1000
500
0 0
5
10
15
20
25
30
35
Radial distance (mm) Fig. 3. Measured emission intensity radial distribution of N2 (C3Pu-B3Pg) from positive pulsed corona discharges of N2 at different voltages.
electron (X11.03 eV) is the biggest at this place. However, for the fourth aperture near the inner wall of the reactor, the emission intensity is the weakest and the density of high-energy electrons is the lowest. In Fig. 4, the solid line corresponding to the linear relationship between the emission intensity of N2 (C3Pu-B3Pg) and the discharge voltage (30–45 kV) is obtained by the least-squares fitting of those data
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Fig. 4. Linear relation between the emission intensity of N2 and discharge voltage from the first aperture (10 mm from reactor central axis).
measured under the same discharge condition as those for Fig. 3. The solid straight line indicates that the high-energy electron density in the discharges linearly increases with the discharge voltage V. Fig. 5 shows the emission spectrum of the positive pulsed corona discharge in air, in the range of 290–420 nm. From the intensity of the 0–0 vibrational transition band of the nitrogen second positive system (C3Pu-B3Pg), the radial profiles of the emission intensity of N2 from the positive pulsed corona discharge in air are presented in Fig. 6. This figure reveals the following. (1) From the first aperture to the fourth aperture, the emission intensity I differs greatly from each other because of the different distances away from the reactor center axis. The ratios of the emission intensities at different apertures are evaluated to be 1.0 : 0.67 : 0.50 : 0.33, which is similar to the spatial distribution of the discharge electric field intensity and therefore reasonably reflects the density distribution of the high-energy electrons (X11.03 eV). (2) The highest voltage in Fig. 6 is 43 kV, because the breakdown voltage of air at 1 atm is not higher than 43 kV, while the breakdown voltage of N2 at 1 atm is higher than 50 kV. Based on Eq. (5) and Fig. 6, the distribution profiles of the normalized high-energy electron density in the air positive corona discharge at different discharge voltages are shown in Fig. 7.
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Fig. 5. Sample emission spectrum of N2 (C3Pu-B3Pg) from air positive pulsed corona discharge.
Fig. 6. Measured emission intensity radial distribution of N2 (C3Pu-B3Pg) from positive pulsed corona discharges of air at different voltages.
Comparing the profiles in Figs. 3 and 6, both of which are obtained at the same experimental conditions, the emission intensities of N2 (C3Pu-B3Pg) in N2 discharges are much stronger than those in air discharges. Some mechanisms for the intensity differences are discussed in the following: (1) In air discharges, the concentrations of N2 and O2 are 78% and 21%, respectively. Because
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Fig. 8. Emission intensity of N2 (C3Pu-B3Pg) as a function of O2 flow rate into N2 positive pulsed corona discharge. Fig. 7. Radial distribution of normalized high-energy electron density in air positive pulsed corona discharge.
the concentration of N2 in air is lower than pure N2, the emission intensity of N2 in air discharges is certainly weaker than that in nitrogen discharges. (2) The O2 in air discharges captures a large number of electrons and becomes negative ions, so the densities of the electrons in air discharge are much lower than that in nitrogen discharges under same discharge conditions. The emission intensity of N2 is remarkably weakened by the oxygen component of air discharge. 3.3. Effects of O2 addition on the emission intensity of N2 (C3Pu-B3Pg) in nitrogen discharge The addition of an oxygen flow into a nitrogen positive pulse corona discharge depletes the free electrons in the discharge by the following two processes: e þ O2 -O 2;
ð6Þ
e þ O2 þ M-O 2 þ M:
ð7Þ
Here, M may be O2, N2, and other molecules. The added O2 decreases the density of free electrons greatly. When the flow rate of O2 increases, the number density of excited N2 (C3Pu) generated in
the discharge should decrease rapidly because O2 is also a strong quenching agent of the excited N2 (C3Pu) in the discharge. By mixing oxygen flow with different rates into the discharge system of N2, the effect of the concentration of O2 on the emission intensity of the N2 transition of C3Pu-B3Pg is studied. The power supply voltage is 40 kV and is kept constant during the measurements. The flow rate of N2 into the reactor is kept at 0.18 m3/h. The emission spectra of the N2 transition of C3Pu-B3Pg are clearly recorded through the fourth aperture from the pulsed corona discharges at eight different oxygen input flow rates of 10, 20, 30, up to 80 ml/ min. Fig. 8 shows the emission intensity of N2 (C3Pu-B3Pg) as a function of O2 flow rate. It can be seen that the emission intensity decreases almost linearly with the increase of O2 flow rate.
4. Conclusion In this paper, we record successfully the radial profile of the emission intensity of the second positive system (C3Pu-B3Pg) emitted from the positive pulse corona discharge of N2 and air in a line-cylinder reactor. Moreover, we prove the direct relation between the density of the highenergy electron and the emission intensity of the second positive system. For the positive pulsed
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corona discharge reactor, the high-energy electron densities generated in air and nitrogen corona discharge always present a radial non-linear decreasing tendency, which is coincident with the electric field distribution in the line-cylinder reactor. In addition, it is also shown that the high-energy electron density ne takes a similar radial distribution profile at different discharge voltages. From the first aperture near the corona wire, the emission intensity is the strongest, which indicates that the density of the high-energy electron is the biggest at this place. But, from the fourth aperture, the emission intensity is the weakest. For any apertures, there is a linear relationship between the high-energy electron density and the discharge voltage. Otherwise, when the O2 flow is added to the discharge reactor, the overall emission intensity of N2 is remarkably reduced and thus the high-energy electron density is correspondingly reduced in the reactor because O2 captures a great deal of electrons and thus the density of the high-energy electron decrease.
Acknowledgements The author would like to thank professor Xuechu Li for friendly discussion and suggestion.
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This work is supported by the united fund of the National Natural Science Foundation Committee of China and Engineering Physical Institute of China under Grant No. 10276008 and the fund of Liaoning Province Natural Science Foundation Committee under Grant No. 20022138.
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