22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Ozone and by-products generation characteristics by novel air-fed ozone
generator which combines homogeneous discharge and filamentary discharge
N. Osawa and Y. Yoshioka
Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa, Japan
Abstract: Ozone and by-products generation characteristics from an air-fed ozone
generator by filamentary discharge (FD), homogeneous discharge (APTD), and alternate
(FD and APTD) discharge modes were investigated. The ozone yields by the alternate
mode and the APTD mode were 7.8% and 35.2% lower than that of by FD mode. N 2 O
generation rates by the alternate mode and the APTD mode were 30.4% and 50.2 % lower
than that of by FD mode. The lowest N 2 O concentration at the same ozone concentration
was achieved by the alternate mode.
Keywords: Ozone, low by-products emission, atmospheric pressure Townsend discharge.
1. Introduction
Ozone is a strong oxidizing agent and it can be applied
to water treatment, gaseous pollution control, etc [1].
Usually, ozone is produced by Dielectric Barrier
Discharge (DBD), which is composed of many
Filamentary micro-Discharges (FDs). The reduced
electric field strength (E/n) at the streamer head of FD
was calculated as 800 Td in atmospheric pressure dry air
by Komuro et al [2]. Electrons accelerated by this high
E/n induce not only O 2 dissociation but also N 2 excitation
and dissociation. Therefore, it is considered that if we use
air as a source gas of ozone generation, by-products like
N 2 O, NO, NO 2 , HNO 3 , N 2 O 5 would be generated [1].
So far, we succeeded in generating the Atmospheric
Pressure Townsend Discharge (APTD) in dry-air [3] and
investigated the application of the APTD to an air-fed
ozone generator [4]. The experimental results showed that
(1) the maximum ozone yield was obtained by the FD
mode, however, the ozone yield decreased drastically at
higher Specific Input Energy (SIE), (2) in case of APTD
mode, the yield decreased slightly with the increase of
SIE. Recently, we investigated by-products from two
kinds of air-fed ozone generators using a Fourier
Transform Infrared (FTIR) spectrometer with a multi path
gas cell [5]. In the both types of ozone generators, HNO 3 ,
N 2 O 5 , and N 2 O were detected as by-products. However,
intensities of the absorbance spectra of HNO 3 , N 2 O 5 and
N 2 O in ozone gas from APTD mode were lower than
those of from FD mode at the same ozone concentration.
Therefore, we concluded that the APTD can suppress byproducts generation from the air-fed ozone generator. The
reason seems to be as follows. Because the E/n of the
APTD is lower than that of the streamer head, the
dissociation of N 2 molecule and water vapour and
excitation of N 2 molecule by electron impact are weak.
This leads to the suppression of subsequent NOx
generation. Recently, we succeeded in generating the
alternate mode of APTD and FD by a simple DBD device
in dry-air at atmospheric pressure [6]. We considered if
O-3-4
we apply this mode to the air-fed ozone generator, ozone
with low by-products can be generated.
In this paper, we investigated ozone and by-products
generation characteristics by alternate mode DBD.
2. Experimental setup and discharge appearance
2.1. Experimental setup
Fig. 1 shows experimental setup. This system consists
of an AC high voltage power source, a DBD device set in
a chamber and various measurement devices.
Atmospheric dry-air (absolute humidity: 119.3 mg/m3)
was used as source gas of ozone generation. The flow rate
was fixed to 2.0 L/min (25 °C, 1013 Pa) using a mass
flow
controller
(SEC-400mk3,
Horiba,
Ltd.).
Concentrations of ozone and N 2 O were measured by an
UV absorption type ozone monitor (EG-3000B/01, Ebara
Jitsugyo Co., Ltd.) and the FTIR spectrometer (IR
Affinity-1, Shimadzu corp.) with the multi path gas cell (3
m, Gemini Scientific Instruments) respectively. Gas
temperature in a plasma zone was measured directory by
a fibre optic thermometer (FL-2000, Anritsu Meter Co.,
Ltd.).
AC high voltage was applied to the DBD device by a
step-up transformer. The maximum applied voltage and
frequency were 20.2 kVp (zero-to-peak voltage) and 600
Hz respectively. The applied voltage (V) and the current
were measured by an oscilloscope (TDS-2024B,
Tektronix, Inc.) using a high voltage probe (EP-50K,
Step-up transformer
(1:150)
Pressure gauge
H.V. probe
(2000:1)
AC
Power
Source
Dry
air
Integral
capacitor
(0.1 μF)
~200V
~1.1kHz
Ch.1
Ch.2
Oscilloscope
(200 MHz, 2.0 GS/s)
Ch.3
Electrode
Barrier
Image
intensifire
MFC
Chamber Digital
camera
Shunt
resistor
(10 kΩ)
FTIR
with gas cell
Differential
probe
(100 MHz)
fibre optic
thermometer
Ozone
monitor
Gas out
Fig. 1. Experimental system.
1
20
applied voltage
Spacer
(2 mm)
R15
Voltage (kV)
80
100
L.V.
Discharge zone
H.V.
gap voltage
0
2
1
0
current
-10
-1
Solder
2
1
2
Gap length
10
H.V.
80
P = 0.1 MPa
f = 50 Hz
HV: A440
LV: A473
Current (mA)
100
Electrode [tungsten]
(Thickness: 0.01 mm)
Exhaust hole φ10
L.V.
φ20
Barrier [Al2O3: 92%]
(A440 or A473)
Fig. 2. DBD device
Table 1. Features of barrier material.
Material code
A473
A440
Discharge mode
APTD
FD
Main material
Al2O3
Purity
92%
Relative permittivity (1 MHz)
9.1
9.8
Color
White
Black
Surface roughness Ra
0.390 µm
0.410 µm
Table 2. Configurations of DBD device.
Discharge mode
FD
APTD
alternate mode (APTD and FD)
Barrier material
H.V. side
L.V. side
A440
A440
A473
A473
A473
A440
Pulse Electronic Engineering Co., Ltd.) and a differential
probe (700924, Yokogawa Electric Corporation)
respectively. An integral of the current (charge q) was
measured from the voltage drop across an integral
capacitor (0.1 μF). Besides, the discharge power was
calculated by multiplying the area of V–q Lissajous figure
by power frequency. Discharge photographs were taken
by a digital camera (D800E, Nikon Imaging Japan Inc.)
with an image intensifier (C5100, Hamamatsu Photonics
K. K.).
Fig. 2 shows a DBD device. We used two kinds of
alumina barriers (Material code: A473 and A440,
Kyocera Corporation), one of which can generate APTD
and the other cannot. Features of barrier materials are
summarized in Table 1. The gap length was fixed to 2 mm
using spacers. The size and thickness of the barrier are
100 cm2 and 2 mm respectively. The electrode material is
tungsten, and its effective area and thickness are 58.9 cm2
and 0.01 mm respectively. The electrode was implanted
into alumina barrier in order to avoid generation of
abnormal discharges from the edges of the electrodes.
Therefore, the barrier thickness from the tungsten film
electrode to the barrier surface is 1 mm.
Configurations of DBD device were summarized in
Table 2. When the A473 alumina and A440 alumina were
used as H.V. side and L.V. side barrier material
respectively, APTD and FD generates alternately in every
half cycle in the discharge volume.
2
-20
-2
0
2
4
-2
6 8 10 12 14 16 18
Time (ms)
Fig. 3. Current, gap voltage, and applied voltage
waveform of alternate mode of APTD and FD.
(a) positive polarity
(b) negative polarity
Fig. 4. Discharge photographs.
2.2. Discharge appearance of alternate mode
Since discharge appearance of APTD in dry air was
reported in our paper [3], we will introduce the discharge
photographs and current waveforms of the alternate mode
of APTD and FD [6].
Fig. 3 shows the waveforms of the current, the applied
voltage and the gap voltage. If A473 alumina barrier
becomes cathode (0 – 10 ms), the current waveform is
smooth without pulses. However, if A440 alumina barrier
becomes cathode (10 – 15 ms), the current waveform has
many pulses as in typical filamentary DBD.
Fig. 4 shows discharge photographs of the alternate
mode. If A473 alumina barrier becomes cathode, FDs are
not recognized in the gap. The luminosity gradually
increased from cathode to anode, which is a feature of
APTD. However, if A440 alumina barrier becomes
cathode, many FDs were recognized in the gap. From
these photographs, it is apparent that APTD mode and FD
mode were generated alternately by the simple DBD
device. Next, the ozone and N 2 O generation
characteristics by FD mode, APTD mode, and alternate
mode were investigated.
3. Experimental results
Fig. 5 shows the ozone concentration as a function of
SIE in 3 different discharge modes. The ozone
concentrations by FD mode, APTD mode and alternate
mode increased with the increase of SIE. However, in
cases of FD mode and alternate mode, saturation tendency
appeared at high SIE region of around 500 J/L. In the
region of SIE below 430 J/L, highest ozone concentration
was obtained by FD mode at the same SIE.
Fig. 6 shows the ozone yields as a function of SIE in
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50
2500
N2 O concentration (ppm)
Ozone concentration (ppm)
3000
2000
1500
1000
500
0
FD mode
APTD mode
Alternate mode
100 200 300 400 500
Specific input energy (J/L)
40
30
20
10
0
600
1.0
80
Generation rate of N2 O (g/kWh)
Ozone yield (g/kWh)
FD mode
APTD mode
Alternate mode
60
40
20
100 200 300 400 500
Specific input energy (J/L)
600
0.6
0.4
0.2
100 200 300 400 500
Specific input energy (J/L)
600
Fig. 8. Generation rate of N2O.
60
50
N2 O concentration (ppm)
different discharge modes. The highest ozone yield was
obtained by FD mode at the SIE of around 50 J/L.
However, it decreased drastically with the further increase
of SIE. In case of APTD mode, the maximum ozone yield
was obtained less than the SIE of 50 J/L, and the yield
was 1.8 times lower than that by FD mode. However, the
ozone yield did not decrease with the increase of SIE. In
case of alternate mode, the highest ozone yield was
obtained at the SIE of around 50 J/L. However, the yield
was 1.2 times lower than that by FD mode. The ozone
yield did not decrease up to 200 J/L of SIE as in the
APTD mode. However, the yield decreased drastically
with the further increase of SIE as in the FD mode.
Fig. 7 shows the N 2 O concentration as a function of
SIE in 3 different discharge modes. The N 2 O
concentrations by FD mode, APTD mode and alternate
mode increased with the increase of SIE. The lowest N 2 O
concentration at the same SIE was obtained by APTD
mode.
Fig. 8 shows the generation rate of N 2 O as a function of
SIE in 3 different discharge modes. In case of FD mode,
the N 2 O generation rate increased with the increase of
SIE and it reached the maximum value of around 200 J/L.
Then, the generation rate decreased with the further
increase of SIE. In case of APTD mode, the generation
rate increased with the increase of SIE up to 50 J/L.
However in the region between 50 J/L and 600 J/L, the
increase of generation rate was small. In case of alternate
mode, generation rate was 1.3 – 1.4 times higher than that
FD mode
APTD mode
Alternate mode
0.8
0
600
Fig. 6. Ozone yield.
O-3-4
100 200 300 400 500
Specific input energy (J/L)
Fig. 7. N2O concentration.
Fig. 5. Ozone concentration.
0
FD mode
APTD mode
Alternate mode
FD mode
APTD mode
Alternate mode
40
30
20
10
0
500
1000
1500
2000
Ozone concentration (ppm)
2500
Fig. 9. Ozone concentration vs. N2O concentration
by APTD mode. However, the tendency was alike as in
APTD mode.
Fig. 9 shows the relation between N 2 O concentration
and ozone concentration in 3 different discharge modes. It
can be seen that the lowest N 2 O concentration at the same
ozone concentration was achieved by the alternate mode.
4. Discussions
Here, we discuss why ozone yield were influenced by
the discharge mode. Generally, the thermal decomposition
of ozone starts at a gas temperature of above 120 °C [7]
and this reaction is represented as follows,
O 3 + thermal →O 2 + O ························ (R1)
In order to clarify the influence of the heat by
3
Plasma zone gas temperature (OC)
100
80
FD mode
APTD mode
Alternate mode
60
40
20
0
100 200 300 400 500
Specific input energy (J/L)
600
Decrease of N2 O
generation rate (%)
Decrease of
ozone yield (%)
Fig. 10. Plasma zone temperature
60
50
40
30 Ave. = 35.2%
20
10
60
50
40
30
20
10
0
APTD mode
Alternate mode
Ave. = 7.8%
Ave. = 50.2%
Ave. = 30.4%
APTD mode
Alternate mode
100
200
300
400
Specific input energy (J/L)
500
Fig. 11. Decrease of ozone yield and generation rate of
N2O (Reference value: FD mode)
discharge, we measured plasma zone gas temperature
using the fibre optic thermometer.
Fig. 10 shows plasma zone gas temperature as a
function of SIE in 3 different discharge modes. The
temperatures increased with the increase of SIE. However,
the temperatures did not influenced by discharge modes.
Therefore, the gas temperature is not the cause of the
difference of ozone yield by discharge mode.
Next, we discuss why lowest N 2 O concentration at the
same ozone concentration was obtained by the alternate
mode. Fig. 11 shows the decrease of ozone yield and the
decrease of N 2 O generation rate, which were calculated
by using the experimental data from FD mode as
reference value. Average values of the decrease of ozone
yield by alternate mode and by APTD mode were 7.8%
and 35.2% respectively. On the other hand, average
values of the decrease of N 2 O generation rate by alternate
mode and by APTD mode were 30.4% and 50.2 %
respectively. From these calculation results, we concluded
that, since decrease of ozone yield was lower than that of
N 2 O generation rate, lowest N 2 O concentration at the
same ozone concentration was obtained by the alternate
mode.
Finally, we discuss why the decrease of N 2 O generation
rate was higher than that of ozone yield in case of
alternate mode. It is well known that ozone is formed by
these reactions,
e + O 2 → e + O 2 (A3∑ u +) → e + O(3P) + O(3P) ··(R2)
e + O 2 → e + O 2 (B3∑ u -) → e + O(3D) + O(3P)···(R3)
4
O + O 2 + M → O 3 + M ····························· (R4)
Here, the average electron energy of about 6–9 eV would
be ideal for the dissociation of O 2 by electron impact [1],
because the energy thresholds for reactions of (R1) and
(R2) are 6.0 eV and 8.4 eV, respectively.
On the other hand, it is reported that N 2 O is formed by
N 2 metastable reaction [8],
e + N 2 → e + N 2 (A3∑ u +) ··························· (R5)
e + N 2 → e + N 2 (B3Π g ) ···························· (R6)
N 2 (A, B) + O 2 → N 2 O + O ······················· (R7)
Here, the energy thresholds for reactions of (R5) and (R6)
are 6.17 eV and 7.35 eV, respectively [9]. Form these
reactions, it is apparent that the energy threshold of
N 2 (A3∑ u +) generation is bit higher than that of O 2 (A3∑ u +)
generation. Therefore, decrease of N 2 O generation rate is
higher than that of ozone yield.
5. Conclusions
The effects of discharge mode to ozone and by-products
generation were investigated. The conclusions are as
follows;
(1) Average ozone yields by alternate mode and by APTD
mode were 7.8% and 35.2% lower than that of by FD
mode
(2) Average N 2 O generation rates by alternate mode and
by APTD mode were 30.4% and 50.2 % lower than that
of by FD mode.
(3) In case of alternate mode, since the decrease of N 2 O
generation rate is higher than that of ozone yield, the low
N 2 O emission performance at the same ozone
concentration was achieved.
6. Acknowledgements
This work was supported by a MEXT (Ministry of
Education, Culture, Sports, Science and Technology) –
Supported Program for the Strategic Research Foundation
at Private Universities 2011 – 2016, a Grant–in–Aid for
Young Scientists (B), 25820111, and Takahashi Industrial
and Economic Foundation. We would like to thank
Kyocera Corporation for providing alumina barrier plates.
7. References
[1] B. Eliasson, U. Kogelschatz, IEEE Trans. Plasma Sci.,
19, 6 (1991).
[2] A. Komuro, R. Ono, J. Phys. D: Appl. Phys., 47
(2014).
[3] N. Osawa, Y. Yoshioka, IEEE Trans. Plasma Sci., 40,
1 (2012).
[4] N. Osawa, H. Kaga, Y. Fukuda, S. Harada, Y.
Yoshioka, R. Hanaoka, Eur. Phy. J Appl. Phys., 55 (2011).
[5] N. Osawa, and Y. Yoshioka, J. Adv. Oxid. Technol.,
17, 2 (2014)
[6] N. Osawa, Y. Yoshiok, in Proc. of the 14th Int. Symp.
on High Pressure Low Temperature Plasma Chem.
(HAKONE XIV) (2014)
[7] M. Taguchi, IEEJ Trans. FM, 134, 11 (2014)
[8] B. Eliasson, U. Kogelschatz, IEEE Trans. Plasma Sci.,
19, 2 (1991)
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[9] Y. Itikata, J. Chem. Ref. Data, 35, 1 (2006)
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