J. Inst. Electrostat. Jpn.

, 31, 2 (2007) 72 77

Removal of Gaseous Acetaldehyde and Fine Particles Using Corona
Discharge with Wet Electrostatic Flocking ElectrodeBong-Jo SUNG,* Tomo NAITO,* Ahmed ALY,* Sung-Hwa LEE,** Kazunori TAKASHIMA,*
Shinji KATSURA***and Akira MIZUNO*, 1(Received May 31, 2006; Accepted December 20, 2006)

Electrostatic precipitator (ESP) has been widely utilized to collect airborne particles in industrial and indoor
ventilation systems. However, conventional ESP has not been used to remove gaseous pollutants from a gas flow.
Recently, a wet-type ESP has been used for removing them. In this study, to improve the removal performanceof gaseous pollutants and particulate matters, a wet-type ESP with a wet electrostatic flocking electrode wasexperimentally studied by measuring the removal efficiency of acetaldehyde and fine particles. And a carbon
fabric plate as a collecting electrode was suggested to effectively remove ozone and fine particles in ESPs.
Removal efficiency of acetaldehyde (CH3CHO: about 30 ppm) in one-pass test was about 68.2% at 8.4 W of
input power with residence time of 2.1 s and high collection efficiency was obtained to submicron particles(0.3-0.5 Pm in diameter) with residence time of 1.05 s. The combined system of the wet type ESP and ESP witha carbon fabric plate electrode also was examined for removing them. These results suggest that the wet-type
ESP has a potential of the simultaneous removal of gaseous and particulate pollutants.

1. Introduction

Therefore, it is very important to control not only particulate

Electrostatic precipitator has been widely utilized to control

matters but also these gaseous pollutants in the atmosphereparticulate matter in industrial and indoor ventilation systems

environment.

1-4). The collection efficiency is usuallyPrevious work using an rectangular ESP equipped with anmore than 99%, and fine particles causing adverse effects onelectrostatic flocking electrode (E.F.Electrode) demonstrated

for cleaning gas flow3)

human health also can be collected effectively . However,high collection performance on capturing airborne particles

ESP has not been used to remove gaseous pollutants from asuch as tobacco smoke 6-7). Purpose of this study is to test thegas flow4). Recently, a wet-type ESP had been reported tofeasibility of simultaneous removal of gaseous pollutants andremove soluble and non-soluble pollutants such as NO2, SO2,

airborne particles using a wet-type ESP equipped with a wet

3-5). Especially, it is very useful for treating

E.F.Electrode. Voltage and corona current characteristics

them because odor pollutants existing in indoor air such as(V-I characteristics), removal efficiencies of acetaldehyde

acetaldehyde

andand fine particles were measured. Furthermore, in order toorganic

remove the ozone generated by dc corona discharge, an ESP

compounds (VOCs) from numerous industries are alsowith a carbon fabric plate electrode (C.F.P.Electrode) as a

typical gaseous pollutants, and their presence in ourcollecting plate was experimentally examined.

HCl, and dioxins

ammonia(CH3CHO),(NH3)

environment

has

are

formaldehydewater-soluble.

been

considered(HCHO)

Volatiletobeseriousenvironmental challenge because of causing health hazard.
Key words electrostatic precipitator, acetaldehyde, ozone
electrostatic flocking, carbon fabric*Dept. Ecological Eng., Toyohashi Universityof Technology,
1-1 Tempaku-cho, Toyohashi 441-8580 Japan**LG Electronics, Research Laboratory, Digital Appliance
Company, 391-2 Gaeumjeong-dong, Changwon City,
Gyeongnam, 641-711, Korea***
Dept. Biological and Chemical Eng., Gunma University,
Kiryu, Tenjin-cho 1-5-1, 376-8518, Japan1
mizuno@eco.tut.ac.jp

2. Experimental
2.1 Experimental reactor
Figure 1 shows the details of a wet-type ESP. It consists ofan E.F.Electrode backed with swelled polymer (wet
E.F.Electrode) as a collecting electrode and a stainless steel
wire discharge electrode of 0.1 mm diameter. The
E.F.Electrode was made of a metal mesh (20 mesh) using
electrostatic flocking of nylon fibers with following
characteristics: volume resistivity of less than 108 :m,

Removal of Gaseous Acetaldehyde and Fine Particles Bong-Jo SUNG et al.

73 (33)

diameter 19.5 ঙ and length 1 mm 6-7). Wet E.F.Electrodegenerated submicron particles were mixed in a mixingof which length was 50 mm was placed on the inner wall ofchamber with particle-free clean air (N2: 80% and O2: 20%,an acryl cylinder (O.D.: 50 mm, length: 200 mm). The

0.5 L/min) to obtain desired flow rates and introduced intopolymer layer was swelled with CaCl2 solution (5%, 50cc).

the ESP. Total gas flow rate was set at 2L/min resulting in

Figure 1(b) shows the details of an ESP equipped with athe residence time about 1.05 s in the wet type ESP or 1.8 s

carbon fabric plate as a collecting electrode. The ESP

in the ESP with C.F.P.Electrode. The collection efficiency of

consists of a cylindrical carbon fabric plate, which has

the particles from 0.3 Pm to 0.5 Pm in a geometric mean

thickness of 1.5 mm and length of 80 mm, and discharge

diameter was measured by using a dust counter (Dan-scienceelectrode of 0.1 mm stainless steel wire at the center. The

82-1800). The average number concentration of inlet aerosol

collecting plate was placed in an acryl cylinder with innerwas 1.37u104 count/L. The collection efficiency K is given

diameter of 34 mm and length of 150 mm.

by Eq.(1).
K (㧑) 㧩N inlet  N outlet
100
N inletWhere, K

collection efficiency of the tested ESP(1)Particle number concentration of inlet of the tested

NinletESP
NoutletParticle number concentration of outlet of the tested

ESP
Dust Counte
Mass Flow
(a)
N2

O2

Flow
meter

Micro ammeter

Heating dryerHigh Voltage
SourceNebulizer
Particle generator

Gas flowԘ QT ԙ

Mixing Chambe
(a)

(b)

High voltage High voltage
sourcesourceFig. 1 Details of a wet-type ESP with a wet E.F.Electrode(a)Micro ammeterand an ESP with a C.F.P.Electrode (b)Gas flow
2.2 Experimental setup
Figure 2(a) shows a schematic diagram of experimental
setup for measuring collection efficiency for submicron

ԘGC-FIDparticles.
Fluorescent particle (Fluoresbrite Multfluorescent 0.5 Pm
Microspheres) was used as tested submicron particles. The
fluorescent particle stock solution of concentration of
11

3.9u10 /mL was diluted 4000 times with pure water and the
diluted solution was sprayed by a nebulizer and introduced
into the duct after drying by using a heating dryer. Theԙ

Mass Flow

N2 O2

Mixing Chamber
Ԙ wet type ESP
CH3CHOԙ ESP with C.F.P.Electrod(b)Fig. 2 Schematic diagram of the experiment setup:(a) particle treatment system (b) gas treatment system using the
combined system

74 (34)

31

2

2007

Figure 2(b) shows a schematic diagram of experimental

This is due to lower electrical resistance of the wet flocking

setup for treatment of gaseous pollutants using the combined

fibers compared with dry flocking fibers. In case of the wet

system of the wet type ESP using wet E.F.Electrode and ESP

ESP, the back corona can be also effectively suppressed thanwith C.F.P.Electrode. Test gases were prepared from gas

the ESP with dry flocking fibers irrespective of humidity

cylinders (CH3CHO: 285 ppm (N2 base), N2: 99.9%, O2:

condition of the air.

99.9%). Total gas flow rate was set at 1L/min using mass
0.6

with a wet E.F.Electrode and ESP with a carbon fabric plate

0.5

electrode (C.F.P.Electrode) was about 2.1 and 3.6 s,
respectively. Acetaldehyde diluted with nitrogen and oxygen(N2: base, O2: 20%) was selected as a simulated gas. 30 ppm
of acetaldehyde was used in this experiment. Inlet and outlet
concentrations were analyzed using a gas chromatographCorona Current (mA)flow controllers, and the gas residence time in wet-type ESP(Shimadzu Corp., GC-14B) equipped with a flame ionizationwetflocking
dryflocking

0.4
0.3
0.2
0.1
0

detector (FID). The operating temperature of GC-FID was0

set at 150qC.

2

4

6

8

10

12

Applied Voltage(- dc kV)

The removal efficiency of acetaldehyde defined in Eq.(2)was calculated by measuring the concentration at the inlet

Fig. 3 V-I characteristics: 2L/min of total gas flow withand outlet in each reactor of the combined system.

particle loading

Removal efficiency (㧑) 㧩C in  C out
100
C in

(2)

Figure 4 shows the collection efficiencies for particles
between 0.3-0.5 Pm in diameter using the wet-type and dry-

Where, Cin (ppm)㧩Inlet concentration of tested ESP

type ESP. Without any voltage application, collection

Cout (ppm)㧩Outlet concentration of tested ESP

efficiency of the wet-type ESP and the dry-type ESP wereabout 31.4%, 15.7%, resulting from spontaneous particles

In both tests, a high-voltage power supply (Pulse Electronic

deposition on the collecting plate and inner wall of theEngineering Co., Ltd, Model-502) was used to apply areactor. Collection efficiency rapidly increased with applied

various dc negative high voltage. A micro ammeter and a

voltage and then reached a plateau of about 94% and 87-89%

high voltage probe were used to measure corona current

respectively at –7 kV. The time-lapse change of theaccording to applied voltages. Ozone concentration was alsocollection efficiency was measured at –7 kV as shown in Fig.

measured with gas detection tubes (GASTEC Co., Ltd). All

5. During operation for 90minute collection efficiency

the experiments were carried out at room temperature andshowed almost constant value. Judging from Figs. 4 and 5,

atmospheric pressure.

overall collection performance of the wet-type ESP for
submicron particles was higher than that of the dry-type one.

3. Results and discussion

These results can be attributed to the following reasons: 1) a

3.1 Removal of fine particles

wet-type

3.1.1 Wet-type ESP equipped with a wet E.F.Electrode

E.F.Electrodemore

effectively

suppressed

re-entrainment of precipitated particles by a water film on

In order to examine the corona discharge in the wet-type ESP,

the surface of the collecting plate in addition to suppression

voltage and corona current characteristics were measuredof re-entrainment due to gradient force produced byand compared to those of a dry-type ESP with dry flocking

convergence of electric field at the tip of fibers as well as thefibers. The results were plotted in Fig. 3. In the wet-type ESP,

suppression of backward gas stream 6,7), 9) 2) as shown in Fig.

corona onset voltage was -5 kV and flashover occurred at -11

3, the higher corona current in case of wet-type ESP

kV. In case of the dry type ESP onset and flashover voltage

enhanced the amount of the particle charge resulting in thewere -5.5 kV and over -9 kV, respectively. Corona current ofhigher migration velocity of the particles to the collecting

the wet-type ESP was higher than that of the dry-type one.

plate.

also

100
90
80
70
60
50
40
30
20
10
0

Corona Current (mA)

Collection efficiency (%Removal of Gaseous Acetaldehyde and Fine Particles Bong-Jo SUNG et al.

dryflocking
w etflocking

0 1 2 3 4 5 6 7 8 9

0.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0

a luminum
plate electrodecarbon fabricplate electrode0

Applied Voltage (- dc kV)

75 (35)

1

2

3

4

5

6

7

8

Applied Voltage (dc,- kV)

Fig. 4 Collection efficiency for submicron particles of
0.30.5 Pm in diameter: gas flow of 2 L/min, residence timeof 1.05 s

Fig. 6 V-I characteristics of ESP equipped with a
C.F.P.Electrode: gas flow-2 L/min (O2: 20%, N2: base)100

Collection efficiency (%)

Corona tufts
80
60

dry flocking
wet flocking

40

Discharge off

(a)

20
0

0

20

40

60

80

Time (min)Fig. 5 Time-lapse change of collection efficiency: applied
voltage -7 kV, gas flow of 2 L/min, residence time of 1.05 s
3.1.2 ESP equipped with a C.F.P.ElectrodeVI characteristics of the ESP equipped with a C.F.P.
Electrode was shown in Fig. 6. The VI characteristics of analuminum plate electrode is also shown in the figure. In caseof the C.F.P. Electrode, when negative dc high voltages wereapplied to the wire electrode, the corona onset voltage wasabout –3 kV and flashover was observed at –7 kV or 7.5 kV.
On the other hand, in case of the aluminum plate electrode,the corona onset voltage and the flashover voltage wereabout –4.5 kV and at 12 kV, respectively. Corona current of
the C.F.P. Electrode was much higher than that of thealuminum plate electrode. This is partly because of thedifference of the discharge configurations as shown in figure
7.

(b)

100

(c)

Fig. 7 Discharge image: (a) aluminum plate -4.5 kV (b)carbon fabric plate -3.5 kV (c) carbon fabric plate -4.5
kV
Figure 7 shows the photographs of the discharge image inboth cases. In case of aluminum plate, a small number ofbright spots were observed on the discharge wire as shown in(a). This is quite similar to typical negative corona discharge.
In case of the C.F.P.Electrode shown in (b) (c), a large
number of bright spots were observed. In this case, somecarbon fiber particles were attracted to the high voltage
electrode and stuck on its surface. These particles triggered
the corona discharge. Actually, there were two types ofbright spots observed when the C.F.P.Electrode was used.
One was the normal corona spots along the wire similar to
those in case of aluminum plate, and the other was found at
the attracted carbon fiber particles on the discharge wire. The

76 (36)

31

2

2007

number of corona spots can be a good explanation for the
100

Ozone generation in ESPs is a serious problem in the realapplications 8), 10-11). In this experiment, to suppress the ozone
generated by corona discharge using the C.F.P.Eelectrodewas examined.
Figure 8 shows the comparison of ozone generation
between C.F.P.Electrode and aluminum plate electrodetypically used in conventional ESP. As a result, ozone

Collection Efficiency (%)

higher corona current in case of C.F.P.Elelectrode.

concentration of the C.F.P.Electrode was much lower thanozone concentration in case of the C.F.P.Electrode was 0.2
ppm but in case of aluminum plate electrode, it was about 60
ppm. These results suggest that ozone was adsorbed or
decomposed on the C.F.P.Electrode.
Collection efficiency for the submicron particles using the

60
40
20
0

that of the aluminum plate electrode. For example,
comparing in case of the same corona current of 20 PA, the

80

F

0

1

2

3

4

5

6

7

Applied Voltage (- dc, kV)

Fig. 9 Collection efficiency for submicron particles
of 0.3-0.5 Pm in diameter: gas flow of 2 L/min,

3 residence time of 1.8 s.

C.F.P.Electrode was examined. Figure 9 shows collection
efficiency as a function of applied voltage. As shown in Fig.

3.2 Removal of acetaldehyde and ozone

9, when the applied voltage was only 3 kV, collection

The combined system shown in Fig. 2(b) was tested for

efficiency was more than 90%. We achieved collection

removal of acetaldehyde and ozone. The results were shown

efficiency of about 92-95% when the applied voltage

in Table 1. Figure 10 shows the removal efficiencies of

exceeded 3 kV, and at the same time ozone concentration

acetaldehyde for one-pass test in the wet-type ESP as a

was about 0.2-6 ppm (corona current was about 20130 PA).

function of input power. And the results were compared with

These results suggest that the process using the combination

those of the wet-type ESP and the dry-type ESP with andof corona discharge and C.F.P.Electrode is effectively

without flocking fibers. Without any voltage application,

applicable not only to suppress ozone but also to capture fineacetaldehyde removal efficiency was about 10% in theparticles as well as low voltage operation.

wet-type ESP and the combined system due to adsorption.
The removal rate increased with input power and when the
input power exceeded 6 W the removal efficiency was

250

saturated and maximum removal efficiency of the wet-type
ESP alone was about 68.2% at 8.4 W of input power.

Ozone (ppm)

200

Comparing at the same input power of about 6 W, the150

aluminum plateelectrodedry-type ESP without flocking fibers. Using the combined100

carbon fabricplate electrodewet-type ESP achieved removal efficiency 28% higher thansystem, the maximum removal efficiency was 71.1%. The
increase in the removal efficiency with applied voltage can50

be attributed to the following reasons: 1) ionic wind bycorona discharge accelerated gas flow toward collecting

0plate resulting in enhanced absorption, 2) wet collecting
0

50

100

150

Corona Current (µA)

Fig. 8 Ozone generation with two different types ofcollecting plates as a function of corona current: gas
flow-2 L/min (O2: 20%, N2: base)plate promoted absorption of acetaldehyde because of its
water-soluble characteristic, 3) OH radical by corona
discharge in humid condition might have played a role todecompose acetaldehyde to CO2 and H2O 4).

Removal of Gaseous Acetaldehyde and Fine Particles Bong-Jo SUNG et al.

Table 1 Acetaldehyde and ozone concentration in the
combined system: Ca1 (outlet of the wet type ESP), Ca2(outlet of the combined system), Coin (generated ozone in the
wet type ESP), Co1 (without voltage application in the ESP
with C.F.P.Electrode), Co2 (with –dc 5 kV, 80 PA in the ESP
with C.F.P.Electrode)
AppliedAcetaldehyde
Ozone
voltage(ppm)(ppm)(-dc, kV)
Ca2
Coin
Co1
Co2
Ca1

26.1
16.8
14.3
12.4
9.7
9.2

Removal efficiency (%)

0
8
9
10
11
12

80

25.6
16.0
13.8
12.2
9.0
8.4

60
100
105
100
100

55
90
100
100
100

8
12
12
13

wet flocking
dry flocking
dry non- flocking
com bined system

77 (37)

performance of simultaneous removal of gaseous pollutants
and particulate matters. And carbon fabric plate as acollecting electrode was experimentally studied to suppressozone generation by dc corona discharge in ESPs. The
results are summarized as follows:
1) Removal efficiency of acetaldehyde using the wet-type
ESP alone was about 68.2% at 8.4 W of input power withresidence time of 2.1 s.
2) The collection performance of wet E.F.Electrode forsubmicron particles (0.3-0.5 Pm in diameter) was higher thanthat of dry type one. In case of wet E.F.Electrode collection
efficiency was achieved 94.2% at –dc 7kV. On the other
hand in case of dry E.F.Electrode was 87.1% at the same
voltage.
3) The process using the combination of corona discharge
and carbon fabric plate is effectively applicable not only tosuppress ozone but also to achieve high collection efficiency
for submicron particles.
Consequently, the wet-type ESP and the combined systemhave a potential of the simultaneous removal of gaseous and60

particulate pollutants and can cope with the well-known

40

problems of ESPs, such as abnormal re-entrainment ofparticles with low resistivity and back corona for high20

resistivity particles.

0

0 1 2 3 4 5 6 7 8 9
Input power(W)Fig. 10 Removal efficiency of acetaldehyde as a function of
input power: gas flow of 1 L/min, residence time of 2.1 s (thewet type ESP) and 3.6 s (ESP with C.F.P.Electrode).
Wet-type ESP showed high removal efficiency forsubmicron particles and acetaldehyde but high concentrationozone was also generated. To cope with this problem, ozone
removal using the combined system was examined. The
results are summarized in Table 1. For various applied
voltages to the wet-type ESP, ozone concentration at down-stream of the wet-type ESP and ESP with C.F.P.Electrode
was measured in the combined system. Ozone of 100 ppm,
which was generated in the wet-type ESP, was reduced to
12-13 ppm by passing through the ESP with C.F.P.Electrodefixed at –5 kV (80 PA) during operating the combined
system.
4. Conclusions
The dc corona discharge with a wet electrostatic flocking
electrode has been employed in order to examine theThis study was partially supported by smoking researchfoundation and the 21st Century COE Program “EcologicalEngineering for Homeostatic Human Activities”, from theministry of Education, Culture, Sports, Science andTechnology. And authors are grateful to research laboratoryof LG Electronics Digital Appliance Company for valuable
discussions.
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