Diagnosis of OH radical by optical emission spectroscopy in a wire-plate bi-directional pulsed corona discharge

ARTICLE IN PRESS

Journal of Electrostatics 65 (2007) 445–451 www.elsevier.com/locate/elstat

Diagnosis of OH radical by optical emission spectroscopy in a wire-plate bi-directional pulsed corona discharge Feng Liu, Wenchun Wang, Su Wang, Wei Zheng, Younian Wang State Key Laboratory of Materials Modification by Laser, Ion and Electron beams, Dalian University of Technology, Dalian 116024, PR China Received 10 March 2006; received in revised form 11 August 2006; accepted 29 October 2006 Available online 30 November 2006

Abstract The emission spectrum of the molecule OH (A2S-X2P, 0–0) during a high-voltage, bi-directional pulsed corona discharge consisting of a gas mixture of N2 and H2O in a wire-plate reactor has been successfully recorded under severe electromagnetic interference at atmospheric pressure. The relative vibrational populations and the vibrational temperature of N2 (C, v0 ) have also been determined. Due to the difficulty of determining the exact overlapping spectral line shape function of the OH (A2S-X2P, 0–0) and the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg), a practicable Gaussian form is used for calculating the emission intensity of OH (A2S-X2P, 0-0) and the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg). The emission intensity of OH (A2S-X2P, 0–0) has been evaluated with a satisfactory accuracy by subtracting the emission intensity of the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg) from the overlapping spectra. The relative population of OH (A2S) has been obtained by the emission intensity of OH (A2S-X2P, 0–0) and Einstein’s transition probability. The influences of peak voltage, pulse repetition rate and O2 flow rate on the relative population of OH (A2S) radicals have also been investigated. We found that the relative population of OH (A2S) rises with an increase in both the peak applied voltage and the pulse repetition rate. When oxygen is added to an N2 and H2O gas mixture, the relative population of OH (A2S) radicals decreases exponentially with an increase in added oxygen. The main physicochemical processes involved are also discussed in this paper. r 2006 Elsevier B.V. All rights reserved. Keywords: OH radical; Emission spectrum; Bi-direction pulsed corona discharge; Relative population

1. Introduction Pulsed corona discharge is a non-thermal plasma characterized by low gas temperature and high-electron temperature. Short-duration pulsed corona discharge has some advantages over other methods, including better energy efficiency compared to other non-thermal plasma techniques. By using pulsed corona discharge, more electrical energy can be put into the energetic electrons generated during discharge compared to gas heating [1]. High-energy electrons generated by short-duration pulsed corona discharge can efficiently dissociate, excite and ionize N2, O2 and H2O into reactive radicals (OH, HO2, O, N, H, etc.) or other active species (O3, H2O2, etc.) [2]. Corresponding author. Tel.: +86 411 84709795; fax: +86 411 84709304. E-mail address: [email protected] (W. Wang).

0304-3886/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2006.10.007

Species such as OH, O, H, N, HO2, N+, N+ 2 and O3 having strong reactivity generated by pulsed corona discharge are considered important for the removal of acid gases from flue gases, removal of organic compounds from water, bacterial decontamination, decomposition of volatile organic compounds and the destruction of other toxic compounds [3–11]. Radials of OH play a central role in this process because of their tendency toward strong oxidation in many physicochemical processes [12,13]. Recently, there have been a number of studies regarding the OH radicals generated during discharge [14–20]. Ono and Oda [14–16] determined the density of OH radicals generated by a pulsed discharge using laser-induced fluorescence with a tunable KrF excimer laser and discussed the O radical’s role in forming OH radicals. Sun et al. [17] did an optical study of radicals (OH, O and H) produced by a pulsed streamer corona discharge in water. Su et al. [18] determined the amount of CO2

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F. Liu et al. / Journal of Electrostatics 65 (2007) 445–451

produced through oxidation of CO by OH radicals for the quantitative measurement of OH. Park et al. [19] analyzed the effects of adding C2H4, H2O, H2O2 to mixtures on both OH emission intensity and NO/NO2/NOx reduction, and discussed the related reaction mechanism in pulsed corona discharge process using a wire-cylinder plasma reactor. Wang et al. [20] investigated the emission spectrum of OH radicals produced in a needle-plate positive pulsed streamer discharge at atmospheric pressure. In this article, we present the emission spectrum of OH (A2S-X2P, 0–0) radicals generated in the high-voltage bidirectional pulsed corona discharge plasma with wire-plate electrode structure at atmospheric pressure. The wire-plate electrode configuration can produce uniform corona discharge near the wires and is suitable for an industrial application. The bi-directional pulsed corona discharge has a unique advantage over other discharges. Both of positive pulse discharge and negative pulse discharge can produce lots of radicals, and the removal of contamination is more effective. Horva´th and Kiss [21] have pointed out that the bi-directional pulsed corona discharge has an advantage over the unidirectional pulsed corona discharge in removing NOx. The emission spectrum of the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg) is simulated assuming the Gauss distribution. The emission spectrum of OH (A2S-X2P, 0–0) has been compared to the simulated spectrum with a satisfactory accuracy. The influences of pulse peak voltage, pulse repetition rate and the O2 flow rate on the relative population of OH (A2S) radicals have been investigated. The main physicochemical processes involved have also been discussed. 2. Experimental setup The experimental setup is illustrated schematically in Fig. 1(a). It includes a bi-directional pulsed power supply, a discharge reactor, an optical detection system and a gasmixing chamber. The bi-directional pulsed power can generate high-voltage pulses with a rising time of about 20 ns, a pulse width of about 40 ns and an adjustable repetition rate ranging from 0 to 350 Hz. The switch of the bi-directional pulsed power supply is a rotary spark gap and the capacitors used in the pulsed power supply include pulsed capacitor (Cp) and storage capacitor (C). The pulsed capacitor is charged by the storage capacitor and delivers pulse power into the discharge reactor. The schematic diagram of the bi-directional pulse power supply circuit is shown in Fig. 1(b). The discharge voltage is measured with an oscilloscope (Tektronix TDS3052B) and a 1:1000 highvoltage probe (Tektronix P6015A 1000  3.0 pF 100 MO). The discharge current is measured with a current probe (Tektronix TCP202). Fig. 2(a) shows the typical waveform of a bi-directional pulse discharge voltage. The waveforms of the positive direction and negative direction of the bidirectional pulse voltage and current are shown in Fig. 2(b) and (c), respectively. The stainless-steel discharge reactor can be evacuated with a rotary pump. The reactor typically

Fig. 1. (a) Schematic of the experimental setup 1. reactor; 2. mixing chamber; 3. water saturator; 4. high-voltage pulse power; 5. grating monochromator; 6. high-voltage power supply; 7. photomultiplier tube; 8. computer. (b) The schematic diagram of the bi-directional pulse power supply circuit. The CP is the pulsed capacitor and the C is the storage capacitor. (c) The detailed drawing of the electrode configuration.

consists of wire-plate electrodes with adjustable gas spacing ranging from 5 to 30 mm. The stainless-steel wire-plate electrodes are placed in the center of the discharge reactor and the discharge plasmas are produced in the gas spacing.

ARTICLE IN PRESS F. Liu et al. / Journal of Electrostatics 65 (2007) 445–451

a 8

Voltage (kV)

4 0 -4 -8

Voltage (kV)

b

-6

-4

-2 0 Time (ms)

2

4

20

20

16

16

12

12

Discharge voltage

8

8 4

Current (A)

-8

447

Fig. 1(c). In order to reduce the interference of discharge pulses to the detection system and other instruments, the high-voltage pulse power supply is placed in a two-layer shielding box. Both the pulsed power supply and the reactor are connected to the ground separately. The optical emission from the discharge region is collected by a MODEL SP-305 grating monochrometer (grating groove is 1200 lines/mm, glancing wavelength is 350 nm). After the diffraction of the grating, the output light signal is converted into an electrical signal by a photo multiplication tube (mode R928). The output of the photo multiplication tube transformed into number signal by the NCL (an interface between grating monochrometer and computer) is recorded by a computer. High-purity N2 (99.999 %) and high-purity O2 (99.999 %) are used as discharge gases under the gas pressure of 1.013  105 Pa. The N2 bubbles the water to control humidity. The temperature of the water is kept at 100 1C. After the N2 bubbles through the water and uniformly diffuses into the discharge space, the gas temperature is about 80 1C. The water vapor concentration is about 10% in discharge reactor.

4

Discharge current

3. Experimental results and discussion 0

0

3.1. The measurement of the OH radicals emission spectrum 0

300

600 Time (ns)

900

1200

4

4

0

0 Discharge current

-4

-4 -8

-8 -12

Current (A)

-12

Discharge voltage

-16

-16 0

300

600 Time (ns)

900

1200

Fig. 2. (a) Typical waveform of the bi-directional pulse discharge voltage in N2 and water vapor mixture gas. (b) The discharge voltage and discharge current waveforms of the positive pulse of the bi-directional pulse. (c) The discharge voltage and discharge current waveforms of the negative pulse of the bi-directional pulse.

3.0x104

Δv = 0

Δv = -1

1.8x104

1x10

3

0 304

Δv = +1

1.2x104

0.0 300

OH (A-X)

2x103

2.4x104

306

308

310

Δ v = -2 Δ v = -3

6.0x103

A stainless-steel plane (56 mm  56 mm) is used as the grounded electrode. The corona wires, 48 mm long, 0.2 mm in diameter and 20 mm in space between each wire, are fixed 8 mm apart from the grounded electrode. A quartz optical fiber head is placed parallel to the wire electrode. The details of the electrode configuration are shown in

3x103

N2 (C→B)

3.6x104 Emission intensity (a.u.)

Voltage (kV)

c

In the non-thermal plasma generated by the bi-directional pulse corona discharge at atmospheric pressure, electrons with high migration rates can be accelerated and possess high kinetic energy. The non-elastic collisions between the energetic electrons and H2O, N2 and O2 molecules can induce H2O, N2 and O2 molecules to be dissociated, excited and ionized. Radicals and activated + species such as OH, HO2, O, H, N, O3, H2O2, O and 2, N + N2 can be produced. Fig. 3 shows a typical emission spectrum produced during a bi-directional pulsed corona discharge of an N2 and H2O gas mixture at atmospheric

Δ v = -4 330

360 390 Wavelength (nm)

420

Fig. 3. Typical emission spectrum generated by bi-directional pulse corona discharge at the pulse peak voltage of 22 kV and the pulse repetition rate of 80 Hz.

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pressure with 22 kV pulse peak voltage and 80 Hz pulse repetition rate. The flow rate of N2 in the reactor is kept at 200 ml/min. The emission spectrum mainly consists of OH (A2S-X2P, 0–0) and N2 (C3Pu-B3Pg). 3.2. The relative vibrational population and the vibrational temperature of N2 (C, v0 ) From Fig. 3, we can see that the emission spectrum of OH (A2S-X2P, 0–0) is seriously interfered by the emission spectrum of the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg). Therefore, it is necessary to subtract the emission intensity of the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg) from the overlapping spectra to get the emission intensity of OH (A2S-X2P, 0–0). The relative vibrational population of N2 (C, v0 ) can be obtained through the equation S v0 v00 / N v0 ðFCÞv0 v00 R2e ðrv0 v00 Þn3v0 v00 ,

(1)

where Sv0 v00 is the integral emission intensity, Nv0 the population of the vibrational energy level v0 at electronic excitation state, (FC)v0 v00 the corresponding Frank–Condon factor (quoted from Ref. [22]), nv0 v00 the corresponding transition frequency and Re(rv0 v00 ) the electronic transition moment. Re(rv0 v00 ) is nearly constant. By calculating the integral emission intensity Sv0 v00 of the Dv ¼ 3 and Dv ¼ 4 vibrational transition bands of N2 (C3PuB3Pg), the relative vibrational population of N2 (C, v0 ) under the current experimental conditions is N0: N1: N2: N3: N4 ¼ 1.00: 0.340: 0.095: 0.032: 0.011, as calculated from Eq. (1). The vibrational temperature of N2 (C) can be obtained by the following equation: N 1 =N 0 ¼ eDE=kT v ,

(2)

where N1 and N0 are the relative vibrational populations of the 1 and 0 vibrational states, respectively. DE is the energy difference between the 1 and 0 vibrational states, k the Boltzman’s constant, and Tv the vibrational temperature. The vibrational temperature between the 1 and 0 vibrational states can be calculated. The vibrational temperatures between the 2 and 1, 3 and 2, 4 and 3 vibrational states can also be obtained in the same way. Under the current experimental conditions, the mean vibrational temperature of N2 (C) is found to be about 2445 K. The emission intensity of the Dv ¼ +1 (1–0, 2–1, 3–2, 4–3) vibrational transition band of N2 (C3Pu-B3Pg) can be simulated by Eq. (1), where the relative vibrational population of N2 (C, v0 ), Frank–Condon factor and the spectrum response of photomultiplier tube are known. Since the experimental pressure is an atmosphere, gas molecule collision frequency is about 1010 times per second calculated according to gas dynamics. The radiation lifetime of the N2 (C) state is about 40 ns, and the N2 (C) molecule collides with other gas molecules for about 400 times before radiative emission. The rotational equilibrium is reached at each vibrational state of the N2 (C) state. The vibrational distribution can be described by the same

1.6x104 Emission intensity (a.u.)

448

N2 (C→B)

1.2x104

8.0x103

4.0x103

0.0 300

OH (A→X)

305

310

315

320

Wavelength (nm) Fig. 4. Typical emission spectrum of OH (A2S-X2P 0–0) and the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg). The dot line is obtained through the simulation of the emission spectrum of the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg) by the Gauss form and the relative populations of N2 (C, v0 ).

equation. Due to the difficulty of determining the exact emission spectrum line shape function, a Gaussian form is used for the deconvolution of the overlapping emission spectra (Fig. 4). The Gauss distribution is used to simulate the emission spectrum data of the Dv ¼ +1 (1–0, 2–1, 3–2, 4–3) vibrational transition band of N2 (C3PuB3Pg). And the emission intensity of OH (A2S-X2P, 0–0) may be obtained with a satisfactory accuracy by subtracting the emission intensity of the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg) from the overlapping spectra. The relation between the emission intensity S and the excited state relative population N is given as follows: S ¼ NAhv,

(3)

where A denotes the Einstein’s transition probability, h the Planck constant and n the transition frequency. Therefore, the relative population of OH (A2S) can be calculated from Eq. (3) when the emission intensity of OH (A2S-X2P, 0–0) and the corresponding Einstein’s transition probability (A(OH) ¼ 0.0145  108 s1) are known. 3.3. The effect of pulse peak voltage on the relative population of OH (A2S) radicals The effect of pulse peak voltage on the relative population of OH (A2S) radicals is shown in Fig. 5. The pulse repetition rate is 80 Hz and is kept constant during the measurements. The flow rate of N2 in the reactor is kept at 200 ml/min. Fig. 3 clearly shows that the relative population of OH (A2S) radicals increases approximately linearly with an increase in pulse peak voltage. OH radicals are mainly produced by electron–molecule collisions. The reaction process is expressed as e þ H2 O ! e þ H þ OH;

k1 ¼ 2:6  1012 .

(4)

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1.6x106 Relative population (a.u.)

Relative population (a.u.)

1.5x106

449

1.2x106

9.0x105

6.0x105

1.2x106

8.0x105

4.0x105 19

20

21 22 23 Pulse peak voltage (kV)

40

24

Fig. 5. The relative population of OH (A2S) radicals as a function of pulse peak voltage at the pulse repetition rate of 80 Hz.

Increasing pulse peak voltage can lead to an increase in the high-energy electron density and the electron mean energy, and therefore high density of OH (A2S) radicals produced by during the electron–molecule interactions (4). Thus, the populations of OH (A2S) radicals increase with an increase in pulse peak voltage. 3.4. The effect of pulse repetition rate on the relative population of OH (A2S) radicals The effect of pulse repetition rate on the relative population of OH (A2S) radicals is shown in Fig. 6. The pulse peak voltage is 22 kV and is kept constant during the measurements. The flow rate of N2 in the reactor is kept at 200 ml/min. The relative population of OH (A2S-X2P, 0–0) increases approximately linearly with an increase in the repetition rate of pulsed discharge, as shown in Fig. 6. When the pulse peak voltage is kept constant, each pulse discharge produces the nearly same amount of radicals and active species. Thus, the relative population of OH (A2SX2P, 0–0) increases with an increase in the pulse repetition rate. 3.5. The effect of O2 addition on the relative population of OH (A2S) radicals In order to investigate the effect of oxygen on the relative population of OH (A2S) radicals, we added oxygen to the N2 and H2O mixture during the bi-directional pulsed corona discharge. The added O2 capture a lot of free electrons and form O 2 ions, and the humidity further leads to an increase in the attachment of electrons to O2. Thus, the adding O2 to the mixture results in a decrease in the density of free electrons and the electron mean energy [23–25]. The effect of the concentration of O2 on the relative population of OH (A2S) is shown in Fig. 7. Pulse peak voltage and pulse repetition rate are 21 kV and 100 Hz,

60 80 100 Pulse repetition frequency (Hz)

120

Fig. 6. The relative population of OH (A2S) radicals as a function of pulse repetition rate at the pulse peak voltage of 22 kV.

3000 Relative population (a.u.)

3.0x105

2500 2000 1500 1000 500 0

10

20 30 40 Oxygen flow rate (ml/min)

50

Fig. 7. The relative population of OH (A2S) radicals as a function of O2 flow rate at the pulse peak voltage of 21 kV and the pulse repetition rate of 100 Hz. The solid line is the experimental curve. The dot line is the exponential curve fitted.

respectively. The flow rate of N2 in the reactor is kept at 200 ml/min. From Fig. 7, we can see the relative population of OH (A2S) decreases exponentially with an increase in the oxygen flow rate. In order to explain the observed experimental phenomena, we further discuss the physicochemical processes of interaction under the condition of added oxygen. The added O2 can be dissociated through the reactions (5)–(7) during the electron–O2 molecule interactions: e þ O2 ¼ O þ O;

k2 ¼ 1:2  1012 ,


e þ O2 ¼ e þ O þ O 1 D ; e þ O2 ¼ Oþ þ O þ 2e;

k3 ¼ 7:2  1011 , k4 ¼ 4:6  1016 .

1

(5) (6) (7)

A lot of O atoms and O ( D) can be produced when oxygen is added. Ono and Oda [14] have pointed out that in a humid air, most of OH radicals can be produced during the

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interaction of O (1D) and H2O:
O 1 D þ H2 O ¼ OH þ OH; k5 ¼ 2:3  1010 .

(8)

Penetrante [25] also points out that O2 may forms water + cluster ions (O+ 2 (H2O) and H3O (H2O)), and they play a central role in forming OH radicals through the dissociative reactions: þ Oþ 2 ðH2 OÞ þ H2 O ! H3 O þ OH þ O2 ,

H3 Oþ ðOHÞ þ H2 O ! H3 Oþ þ OH þ H2 O:

(9) (10)

Thus, a lot of OH radicals and excited state OH (A2S) radicals can be produced. At the same time, a lot of O atoms and O3 molecules produced from the added O2 can react with OH radicals: OH þ O ¼ O2 þ H;

k6 ¼ 3:8  1011 ,

OH þ O3 ¼ HO2 þ O2 ;

k7 ¼ 6:5  1014 .

(11) (12)

Because of the depletion of OH radicals and free electrons, the relative population of OH (A2S) radicals decreases with an increase in the O2 flow rate. The rate constants k1–k7 (cm3/s) in reactions (4)–(8) and (11)–(12) are the values obtained at the mean electronic energy kTe of 3.3 eV and Tion (ETneutr) of 300 K [26]. 4. Conclusions In this study, we have successfully recorded the emission spectrum of the molecule OH (A2S-X2P, 0–0) during the bi-directional pulse corona discharge with wire-plate electrode at atmospheric pressure. The relative vibrational population and the vibrational temperature of N2 (C, v0 ) have been determined. Gauss distribution have been used to simulate the emission spectrum data of the Dv ¼ +1 (1–0, 2–1, 3–2, 4–3) vibrational transition band of N2 (C3Pu-B3Pg). The emission intensity of OH (A2S-X2P, 0–0) has been evaluated with a satisfactory accuracy by subtracting the emission intensity of the Dv ¼ +1 vibrational transition band of N2 (C3Pu-B3Pg) from the overlapping spectra. The relative population of OH (A2S) radicals has been obtained by the emission intensity of OH (A2S-X2P, 0–0) and Einstein’s transition probability. We found that the relative population of OH (A2S) radicals increases approximately linearly with an increase in the pulse peak voltage and the pulse repetition rate; however, the population decreases exponentially with an increase in the flow rate of the added oxygen. Acknowledgments The author would like to thank professor Xuechu Li for friendly discussion and suggestion. This work is supported by the united fund of the National Natural Science Foundation Committee of China and Engineering Physical Institute of China granted under No. 10276008 and the

fund of Liaoning Province Natural Science Foundation Committee granted under No. 20022138.

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