静電気学会誌，33, 1 (2009) 8-13
J. Inst. Electrostat. Jpn.

Study of NOx Removal Processes by Microplasma Generation
Marius BLAJAN*,1, Masaki KANAMORI*, Hidenori MIMURA**
and Kazuo SHIMIZU*(Received August 20, 2008 ; Accepted November 26, 2008)
Microplasma discharge at relatively low voltages and very small discharge gaps is analyzed. The electricalcharacteristics of the microplasma electrodes and the small size of the supply circuit show that thistechnology can be applied for NOx removal processes especially for mobile exhaust gas sources. The
effect of the discharge gas temperature and O2 amount in the gas composition are investigated. The
electrode arrangement using a parallel connection allows the treatment of different gas composition at
different flow rates.
The experimental results show that with the increase of the discharge gas temperature the NOx removal
efficiency increases for the gas composition without O2. At higher gas temperatures the energy efficiencyof the NOx removal process is lower. By increasing the amount of O2 in the discharge gas the NOxremoval efficiency will decrease. Due to the decomposition of O3 at higher temperatures with the increaseof gas temperature the NO removal efficiency in the presence of O2 is decreasing and also fewer quantities
of NO were oxidized to NO2. Higher NOx removal efficiencies were obtained at lower concentrations ofNO and lower gas flow rates.
1.

Introduction

Electrical characteristics of microplasma discharge were

In the last years the problems concerning the atmosphericanalyzed. The experimental works that have implied more thanpollution due to the NOx emissions by stationary sources

one pair of electrodes were performed using a parallel(factories) and mobile sources (automobiles, ships, airplanes)

connection between electrodes pairs.

imposed a search for solutions to treat this exhaust gases. Non
thermal plasma has been researched as a technology for NOx
removal in decades.1-5) Due to the very effective ways forgenerating ozone also the discharges at very small gaps were
investigated.6) The “pd” values (pressure x electrode separation)of atmospheric pressure microplasmas are similar to those forlarge volume, low pressure plasmas but with much higher
energy densities.7) Microplasma generation is a technology that
due to its relatively low voltage discharge can be easily applied
for NOx removal on small mobile sources such as automobiles
and motorcycles.8)
In previous works we have already studied NOx removalusing multiple microplasma electrodes linked in seriesconnection.9,10) Different gas temperatures and different
concentration of O2 and NO were used in this study to analyze theNOx removal processes by atmosphericpressure microplasma.

Keywords: microplasma, NOx removal, dielectric barrierdischarge* Innovation and Joint Research Center, Shizuoka University,
3-5-1 Jyouhoku Hamamatsu, 432-8561, Japan.
** Research Institute of Electronics, Shizuoka University, 3-5-1
Jyouhoku Hamamatsu, 432-8561, Japan.
1

blajanmarius@yahoo.com

Fig. 1 Experimental setup. Gases with different specified
amounts of NO, N2 and O2 in composition were treated by the
microplasma electrodes covered with a dielectric layer andplaced inside a gas tube heated by an electric furnace. The
treated gas is analyzed by a NOx and O2 gas analyzer.

Study of NOx Removal Processes by Microplasma Generation (Marius BLAJAN et al.)

9

Fig. 3 Volt-Ampere tendency curve for estimation of thecapacitive current of electrode with a discharge gap of 0 µm.

Fig. 2 A microplasma reactor. The electrodes connected inparallel are supplied with an AC voltage up to 1.2 kV peak. The
high voltage supplied to the electrodes and corresponding
currents and discharge power are measured by a high voltage
probe, a current transformer and a digital oscilloscope.

2.

Experimental setup

Microplasma generation with dielectric barrier discharge isrealized with perforated metal plates electrodes covered with adielectric layer. The electrodes have an aperture ratio of 34%
and the discharge gap is set at about 0 to 10 µm. An electric
furnace was used to change the gas temperature inside thereactor (Fig. 1). The experiments were performed with a gas

Fig. 4 Discharge voltage, total measured current and discharge
current for one pair of electrodes with a discharge gap of 0 µm.

containing NO, O2 and N2 in different amounts. The results wereanalyzed with a NOx O2 gas analyzer (Shimadzu, NOA 7000).

3.

Electrical characteristic of microplasma discharge

The microplasma reactor (Fig. 2) is inside a glass tube with

The microplasma current both contains the capacitive

a diameter Ø 44 mm and the electrodes have a diameter Ø 42

current and the discharge component. The capacitive current

mm. The temperature inside the reactor was set to 25°C, 100°C,

was estimated using the Volt-Ampere tendency curve for the

200°C, 250°C and 300°C and it was measure with a probe at

higher values of the discharge voltage (Fig. 3).

the exterior of the glass tube. The value of the gas temperature

The estimated capacitive current was subtracted from theis considered as being equal to that measured at the exterior of

total current in order to obtain the discharge current.11)

the glass tube.

Capacitive current was measured to 7.2 mA at 0.6 kV. We have

Due to the relatively low discharge voltages (around 1 kV)

estimated to 11 mA for 1.2 kV according to Fig. 3. Thus it

two small high voltage power supplies (LECIP, M-1H)

results a multiplication factor of 1.5 for estimated capacitive

connected in parallel were used to supply the microplasma

current at 1.2 kV. The discharge voltage, the total measured

reactor with alternative voltages with frequencies of about 25

current and the discharge current are presented in Fig. 4.

kHz. The peak values of the discharge voltages are measured by

A high electric field (E=107–108 V/m) could be obtained

a high voltage probe (Tektronix, P6051), and the discharge

with applied voltage of 1 kV. Due to the dielectric constant of

currents by an AC current transformer (Tektronix, P6021). The

dielectric layer εr=104 and the narrow discharge gap (10 µm)

discharge power is measured using a digital oscilloscopeassures the formation of non-thermal plasma and discharge(Tektronix, TDS 3014) and is calculated by a computer fromcurrent.10) Their high electron temperature ensures the

Lissajous figures. The figures are obtained by measuring the

production of very high concentrations of energetic andcapacitance charge of capacitor C=10 nF connected between

chemically active species.12)

electrode and ground.

The discharge power versus discharge voltage curves for 5

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第 33 巻静電気学会誌

35

1 pair2 pairs
3 pairs
4 pairs
5 pairs

30
Discharge Power (W)

第 1 号（2009）

25
20
15
10
5
0
0.4

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2
Discharge Voltage (kV)

1.3

Fig. 6 Discharge power versus gas temperature for theexperimental setup using 4 pairs of electrodes with a discharge
gap of 0 µm at 1.1 kV discharge voltage.

Fig. 5 Discharge power of microplasma electrodes. Up to 5
electrode pairs were used to measure the discharge power at
different discharge voltages in the gas with the composition NO
6,000 ppm and N2 balance.
pairs of electrodes are presented in Fig. 5. The measurements
were performed with 5, 4, 3, 2 and 1 pair of electrodes. As thedischarge gas was used NO 6,000 ppm, N2 balance and the gas
flow rate was set at 0.2 L/min. The discharge gap was set at 0

Fig. 7 Lissajous figures representing the discharge power of 4pairs of electrodes at 1.1 kV discharge voltage with a discharge
gap of 0 µm at (a) 25°C and (b) 300°C.

µm. The discharge power was calculated from the obtained
Lissajous figures.
The discharge power increases with the increase of thedischarge voltage thus for 5 pairs of electrodes at the discharge
voltage of 1.2 kV it was measured a discharge power of 32.6 W.
For 1 pair and 2 pairs of electrodes the discharge power at 1.2
kV was 10.6 W and 23.7 W respectively. Due to the individual
difference of each electrode pair, the difference between the
discharge power of 1 pair and 2 pairs of electrodes is notsimilar.
4.

NOx removal by microplasma

The effects of the O2 content and gas temperature on NOx

Fig. 8. NOx removal efficiency versus discharge voltage at
different discharge gas temperatures.

removal process have been studied.
4.1

NOx removal process in absence of O2

In order to determine the characteristics of the NOx removal
process in absence of O2, the experimental setups presented in
Figs. 1 and 2 were used. In this paper NOx represents the sumof NO and NO2.
The gas with the composition NO 505 ppm, C3H8 500 ppm

discharge power of 4 pairs of electrodes at 25°C and 300°C at
the same discharge voltage of 1.1 kV.
The characteristics of NOx removal efficiencies versus
discharge voltage for the gas composition NO 505 ppm, C3H8
500 ppm and N2 balance and gas flow rate of 1 L/min arepresented in Fig. 8.

and N2 balance was used as a discharge gas and gas flow rate

NOx removal efficiency of 70% was measured at 300°C

was set at 1 L/min. Four pairs of electrodes with a discharge

with 1.1 kV. In comparison at the same discharge voltage forgap of 0 µm were attached in the reactor. The temperature was

the gas temperature of 25°C the NOx removal efficiency was

set to 25°C, 100°C, 200°C and 300°C.

49%.

Increase of discharge power was confirmed with increase of
the gas temperature for discharge voltage of 1.1 kV (Fig. 6).
Figure 7 presents the obtained Lissajous figures for the

The highest NOx removal efficiency of 72% was obtained
for a gas temperature of 200°C at a discharge voltage of 1.2 kV.
The highest values of removal efficiency for the gas

Study of NOx Removal Processes by Microplasma Generation (Marius BLAJAN et al.)11

temperatures of 300°C and 325°C were 70% and 58% at 1.1 kVand 1 kV respectively. No discharge occurred over these valuesof the discharge voltages at 300°C and 325°C due to themodification of electrical properties of the dielectric layer athigh temperatures. The curve for the temperature of gas of
325°C shows similar characteristics with the curve for the gas
temperature of 300°C.
In the absence of O2, NO could be chemically reduced to N2
by atomic nitrogen species, produced by electron impact:13)

e  N 2  e  N ( 4S )  N ( 4S )(1)

N ( 4 S )  NO  N 2  O(2)Fig. 9 NOx removal efficiency / discharge power ratio versus
discharge gas temperature at a discharge voltage of 1.1 kV.

where N(4S) is a ground state nitrogen atom.
NOx removal efficiency increases with the increase of thegas temperature. Discharge power is higher for higher gas
temperature. The NOx removal efficiency per discharge power
ratio is presented in Fig. 9. At a gas temperature of 25°C thevalues is 3.3 %/W in comparison with 2.8 %/W at 300°C. That
could be explained by the changes in the waveform of thedischarge currents. At 25°C the currents have higher peakvalues but with the increase of the temperature the charged
currents are higher.10)
4.2

O2 concentration effect on NOx removal

In order to simulate the condition of a exhaust gas O2 wasadded in composition of the discharge gas in percentages 1%,
2% and 5%.
The characteristics of NOx removal efficiencies versus O2

Fig. 10 NOx removal efficiency versus O2 concentration in the
discharge gas at a discharge voltage of 1 kV, 2 L/min gas flowrate and 1,010 ppm NO concentration.

concentration for 4 pairs of electrodes are presented in Fig. 10.
The composition of the gas was NO 1,010 ppm, O2 0%, 1%,
2% and 5% and N2 balance.
The gas flow rate was set at 2 L/min. The highest NOx
removal efficiency of 13.9% was measured at 300°C for the gas
composition with 0% O2 and the lowest of 3.5% was measured
at 100°C for the gas composition with 5% O2. The discharge
voltage was fixed to 1 kV. At this discharge voltage the electricfield is high enough to dissociate nitrogen and form NO. The
dissociation energy for nitrogen is around 9 eV and thedissociation energy for oxygen is around 5-6 eV. Thus the mainchemical reactions that are occurring in the microplasma are:6)*

N  O3  NO  O 2(3)

NO  O3  NO 2  O 2(4)

Fig. 11 NOx removal efficiency versus O2 concentration in the
discharge gas at a discharge voltage of 1 kV, 1 L/min gas flowrate and 1,010 ppm NO concentration.

a discharge gap of 0 µm.
The results presented in Fig. 11 shows similar tendencies ofNOx removal efficiencies are higher at 1 L/min gas flow

the NOx removal efficiency characteristics versus O2

rate comparatively with the values obtained at 2 L/min gas flow

concentration in the discharge gas. The gas composition wasrate. The highest efficiency of 30.3% was measured at 200°C

NO 1010 ppm, O2 0%, 1%, 2% and 5% and N2 balance. The gas

for the gas with 0% O2 and the lowest of 8.3% at 100°C for theflow rate was set at 1 L/min. Four pairs of electrodes were used withgas with 5% O2.

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静電気学会誌

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第 1 号（2009）

Table 1 Removed NOx quantities at various gas temperaturesand flow rates without O2.
Gas Temperature

Removal quantity

Removal quantity

at 1 L/min

at 2 L/min(°C)(µg/min)(µg/min)

25

357

267

100

314

237

200

411

339

Table 2Treated efficiencies of NO at various gas
temperatures without O2.
Gas Temperature (°C)

25

100

200

250

Treated Efficiency (g[NO]/kWh)0.921.10 1.22

1.41

Fig. 12 NOx removal efficiency versus O2 concentration in
the discharge gas at a discharge voltage of 1 kV, 1 L/min gas
flow rate and 350 ppm NO concentration.

Fig. 13 NO removal efficiency versus O2 concentration in the
discharge gas at a discharge voltage of 1 kV, 1 L/min gas flowrate and 350 ppm NO concentration.

Fig. 14 NO2 concentration versus O2 concentration in the
discharge gas at a discharge voltage of 1 kV, 1 L/min gas flowrate and 350 ppm NO concentration.

The removed NOx quantity is decreasing from 411 µg/min

The NO removal efficiency is increasing with the increaseat a gas flow rate of 1 L/min to 339 µg/min at a gas flow rate ofof temperature in absence of O2 (Fig. 13). The NO removal

2 L/min (Table 1).

efficiency is decreasing with the increase of temperature in

The

gas

composition

was

NO

1,010

ppm

initialconcentration, 0% O2 and N2 balance at 200°C.
4.3

Heat effect on NOx removal in the presence of O2

presence of O2.
That could be explained by the destruction of the O3 at hightemperatures and according with (3) fewer quantities of NO

The NOx removal efficiency versus O2 concentration for thewere produced. Also fewer quantities of NO were oxidized togas composition NO 350 ppm, O2 0%, 1%, 2% and 5% and N2

NO2 according to (4) in comparison with lower temperaturesbalance are shown in Fig. 12. The gas flow rate was set at 1

(Fig. 14).

L/min. Four pairs of electrodes were used with a discharge gapof 0 µm.

5.

Conclusion

The highest NOx removal efficiency of 58.4% was

Microplasma generation is a technology that could bemeasured at 250°C for the gas composition with 0% O2 and theapplied for NOx removal processes. The following conclusions

lowest of 8.4% at 100°C for the gas composition with 5% O2.

were obtained:

According to Fig. 5, the measured discharge power for 4(1) The discharge power increases with the increase of thepairs of electrodes is 13.1 W. Thus it result the treated NO

discharge voltage. For 5 pairs of electrodes, linked inefficiencies of the microplasma reactor for the gas composition

parallel connection, the highest value of 32.6 W was

without O2 presented in Table 2. The highest value of 1.41 g

measured at 1.2 kV discharge voltage.

(NO)/kWh was measured at 250°C.

(2) NOx removal efficiency increases with the increase of theStudy of NOx Removal Processes by Microplasma Generation (Marius BLAJAN et al.)gas temperature at the same discharge voltage in absenceof O2. The highest efficiency of 72% was obtained at
200°C and at 1.2 kV.
(3) With decrease of the initial concentration of NO from
1,010 ppm to 350 ppm, NOx removal efficiency increases
from 30.3% to 51.1% at 200°C without O2 at 1 kV.

2)
3)
4)
5)

(4) At higher gas flow rates, NOx removal quantity isdecreasing from 411 µg/min (1 L/min) to 339 µg/min (26)

L/min) for 200°C without O2.

7)(5) The highest value of NO treated efficiency of 1.41 g(NO)/kWh was measured at 250°C without O2.
(6) NO removal efficiency increases at high gas temperature

8)9)

without O2. From 40.4% at 25°C it was increased to60.5% at 250°C for the same gas condition.
(7) Due to the destruction of O3 at high temperatures, NO

10)removal efficiency is higher at lower temperatures withO2.
At higher temperatures, the oxidation process could bedecreased and produced fewer quantities of NO2. By
increasing the amount of O2 at 250°C, the NOx removal
efficiency decreased from 58.4% (without O2) to 10.5%(5% O2).
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