A Method for Removal of CO from Exhaust Gas Using Pulsed Corona Discharge

Journal of the Air & Waste Management Association
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A Method for Removal of CO from Exhaust Gas
Using Pulsed Corona Discharge
Xiaohong Li , Lin Yang , Yuyong Lei , Jiansheng Wang & Yiyu Lu
To cite this article: Xiaohong Li , Lin Yang , Yuyong Lei , Jiansheng Wang & Yiyu Lu (2000) A
Method for Removal of CO from Exhaust Gas Using Pulsed Corona Discharge, Journal of the Air &
Waste Management Association, 50:10, 1734-1738, DOI: 10.1080/10473289.2000.10464215
To link to this article: https://doi.org/10.1080/10473289.2000.10464215
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Li et al.
TECHNICAL
PAPER
ISSN 1047-3289 J. Air & Waste Manage. Assoc. 50:1734-1738
Copyright 2000 Air & Waste Management Association
A Method for Removal of CO from Exhaust Gas Using Pulsed
Corona Discharge
Xiaohong Li, Lin Yang, Yuyong Lei, Jiansheng Wang, and Yiyu Lu
College of Mechanical Engineering, Chongqing University, Chongqing City, People’s Republic of China
ABSTRACT
An experimental study of the oxidation of CO in exhaust
gas from a motorcycle has been carried out using plasma
chemical reactions in a pulsed corona discharge. In the
process, some main parameters, such as the initial CO
concentration, amplitude and frequency of pulses, residence time, reactor volume, and relative humidity (RH),
as well as their effects on CO removal characteristics, were
investigated. O3, which is beneficial to reducing CO, was
produced during CO removal . When the exhaust gas was
at ambient temperature, more than 80% CO removal efficiency was realized at an initial concentration of 288
ppm in a suitable range of the parameters.
NOMENCLATURE
C0 =
initial concentration of the CO inside the reactor
D=
diameter of the reactor
f=
frequency of the pulsed voltage
RH = relative humidity of the air inside the reactor
L=
length of the reactor
T=
residence time of the exhaust gas in the reactor
Vf =
flow rate of the exhaust gas inside the reactor
Vp = voltage of the pulse
INTRODUCTION
CO is one of the most abundant of the gaseous pollutants. More than 4 × 1012 kg CO is emitted worldwide per
IMPLICATIONS
Continued research, development, and demonstration of
the chemical process in pulsed corona discharge reactors will present an alternative technology for use in achieving CO removal. Combined with other technologies, this
air cleaner has a promising future in reducing pollutants
from many sources, such as road tunnels, coal-burning
power plants, factories, internal-combustion engines,
animal farms, and so on. Successful technologies emphasize the fast rising time of the pulsed power as well as
its matching with the reactor. A modeling study and field
test are required to improve the performance of the existing system.
1734 Journal of the Air & Waste Management Association
year.1 CO emission sources include coal-burning power
plants, factories, internal-combustion engines, animal
farms, and so on; the main source is the exhaust gas from
various kinds of automobile engines. Urban air may have
an unacceptable density of CO, especially in highway tunnels, due to the heavy concentration of automobiles and
factories. CO has no unpleasant odor but is harmful to
health; therefore, as with other pollutants such as NOx,
SOx, dust particles, and so on, efforts to remove CO emissions are important.
Various pollution control devices are used by industry and meant for these pollutants. For example, coal-fired
power stations utilize selective catalytic reduction processes for NOx removal, wet or dry scrubbers for SO2 removal, and electrostatic precipitators for particulate matter
removal.1,2 Recently, a more efficient method, which utilizes pulsed corona discharge, has been developed.1-6 However, to our knowledge, little study has been done on the
removal of CO using pulsed corona discharge. Due to
greater public awareness of the danger associated with
inhaling polluted air, there may be a great demand for
such simple pollution control devices.
In this work, using a positive peak pulse produced by a
rotating pulse generator, the performance of oxidizing CO
using a corona-discharge plasma reactor, which consists of a
coaxial wire-to-cylinder electrode geometry, was studied. The
parameters in this experiment have been set as follows:
C0 = 150 ~ 350 ppm
L=
1200 mm
D = φ50 ~ φ100 mm
RH = 30 ~ 95%
T = 0.4 ~ 4 sec
Vf = 0.3 ~ 0.8 m/sec
Vp = 0 ~ 60 kV
f=
20 ~ 100 Hz
Prior work has shown better performance in the simultaneous removal of NOx, SOx, and particles using
pulsed corona discharge.1-6 Here, a different version of its
application has been carried out, and its performance has
been measured.
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Li et al.
EXPERIMENTAL SETUP
Reactor
A schematic diagram of the experimental apparatus is
shown in Figure 1. The reactor consists of a coaxial wireto-cylinder electrode system. A copper wire (φ2.5-mm
diameter) was suspended along the axis of a stainless steel
cylinder (inner diameter φ50–φ100 mm, thickness 2 mm).
The effective length of the reactor is 1200 mm. Positive
polarity of high-voltage pulses was applied to the inner
copper wire electrode.
Power Apparatus
Pulse voltage was generated using a capacitor bank and a
rotating spark gap. The capacitor bank was charged using a
dc high-voltage source and discharged through rotating
spark gaps, producing a fast-rising (~50 nsec rise time), highvoltage pulse of ~300 nsec duration. The output pulses had
a maximum peak amplitude of ~60 kV and base amplitude
of 10 kV, and 0 ~ 100 Hz frequency was obtainable.
Gas Flow System
The gas flow system, shown in Figure 1, is composed of a
gas source fan, valve, and reactor. Exhaust gas with CO
and particles is drawn into the electrostatic precipitator
by the fan and then fed into the reactor, where a pulse
discharge is initiated. By changing the speed of the fan
and adjusting the valve, we could vary the flow rate inside the reactor or residence time of exhaust gas (i.e., CO).
Note that the gas pressure inside the reactor is slightly
higher than the atmospheric pressure and no attempt has
been made to heat or cool the reactor externally. The room
temperature was varied in the range of 20 ~ 25 °C.
Measurements
Voltage pulses were recorded using a P6015 Tektronix probe
and digital oscilloscope. Figure 2 shows the typical voltage
and current wave forms of the pulse power. The CO
concentration can be measured by a KG9201 CO analyzer,
and the flow rate inside the reactor can be measured by a
QDF-2A hot-wire probe. However, in these experiments,
we actually measured the air flow rate inside the reactor by
flow meter (see Figure 1) and assumed the flow rate of exhaust gas to be equal to the measured air flow rate.
RESULTS
Features of CO Reduction
In the process of CO oxidation, the input power was measured by the power meter; the measured power is the sum
of the power delivered to the discharge, the transformer
losses, and the power dissipated in the rotary spark gap.
The latter two parts are assumed to be constant in this
work; therefore, the variation of power into the discharge
can be reflected by the whole input power. In the experiments, we can change the input power by changing the
applied voltage from 0 to ~60 kV.
The model of discharge chemistry by the activated
radicals in a discharge may be as follows:1
O2 + e → 2O + e
(1)
H2O + e → OH* + H* + e
(2)
O2 + O → O3
(3)
Radicals and ozone are thus formed:
CO + O3 → CO + O2 + O
(4)
CO + O → CO2
(5)
CO + OH* → CO2 + H*
(6)
In cases such as this, significant O3 generation has been
observed. Ozone generation in a corona discharge is
Figure 1. Schematic diagram of the experimental arrangement.
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Journal of the Air & Waste Management Association 1735
Pulse Current (A)
Pulse Voltage (kv)
Li et al.
Time (nsec)
Time (nsec)
Figure 2. Typical pulse waveform of rising part.
O + O2 + M → O2 + M; M = O2 or N2
(7)
In the present work, positive-polarity peak pulses were
mainly used, in view of an earlier work by Mizuno et al.,8
which showed high efficiency for other gaseous pollutant removal. The time of propagation of the streamers in
positive pulsed corona discharge is longer, ionizing a larger
active volume than that of negative pulsed discharges.
These result in larger active volume and higher energy
and, therefore, better performance.
Figure 3 shows the variation of CO concentration
versus pulse voltage amplitude. The initial CO concentration in this case was 280 ppm; the other parameters
are also presented in the figure caption. It can be seen
that CO reduction started at about 30 kV (the inception
of the discharge), and it appears that the reduction rate is
very sharp with increasing voltage. The quantities of radicals were increased due to the increased power delivered
into the discharge. However, the CO concentration did
not further decrease with the voltage; instead, it increased
by ~55 kV, as a result of the balance of oxidizing process.
For this reason, the tendency of the curve becomes gentle
at high voltage, as depicted in the figure.
The Effect of Pulse Voltage Frequencies
Figure 4 shows test results at different frequencies of voltage for a fixed residence time of 4 sec and for an initial CO
concentration of 280 ppm. From these curves, it can be
seen that the removal efficiency of CO at high voltage and
relatively high power generally increases with an increase
in frequency. Obviously, this may be due to the presence
of more streamers and, consequently, the generation of
1736 Journal of the Air & Waste Management Association
more radicals within the residence time in the discharge at
higher frequencies, and thus an increase of energy in the
reactor. According to the results, we suppose that only the
first output pulse, having the fast rising time (≤ 50 nsec), as
seen in Figure 2, contributed to the oxidizing CO; that is,
only a fraction of input power was effective in removal of
CO. When compared at the same input power (e.g., 75 W),
the higher the frequency of the pulse voltage, the more
effectively power was utilized. Consequently, a higher CO
removal rate was obtained. These results explain in another
way why ordinary pulse voltage (rising time within the
millisecond range, e.g., 50 msec) had no effect on CO removal, even when high power (e.g., 1 kW) was supplied to
the reactor in our prior experiment.
The Effect of the Internal
Diameter of the Reactor
Figure 5 shows the results of tests using different internal
diameters of the reactor. CO removal efficiency decreased
Concentration (ppm)
believed to be a two-step process as follows:5,6 (1) generation
of free oxygen radicals by direct and dissociative ionization,
as well as by dissociation and dissociative attachment; and
(2) generation of ozone by free radical reactions:
Applied Voltage (kV)
Figure 3. Variation of CO concentration with the pulsed voltage. D =
50 mm, L = 1200 mm, f = 60 Hz, T = 4 sec, C0 = 280 ppm, RH = 64%,
and vf = 0.3 m/sec.
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90 Hz
50 Hz
20 Hz
C0 = 150 ppm
C0 = 280 ppm
C0 = 350 ppm
Input Power (W)
Figure 4. CO removal efficiency vs. input power for different
frequencies of pulse voltage. D = 50 mm, L = 1200 mm, T = 4 sec,
C0 = 280 ppm, RH = 64%, and vf = 0.3 m/sec.
significantly in the case of larger diameters. Obviously,
the load of power input into a discharge was higher than
that for small diameters for a fixed length of the reactor.
Hence, the quantities and density of the activated electrons and radicals were lower; on the other hand, the
amount of CO to be treated was greater in the reactor of
larger volume.
The Effect of Initial Concentration
Figure 6 presents removal efficiencies at different initial
concentrations of CO flowing into the reactor. It appears
that when the input power was very low, CO removal
was not influenced significantly. However, when the voltage was increased to a certain value, at a fixed value of
input power, higher CO removal efficiency was obtained
at a lower initial concentration. It is clear that a certain
number of electrons and radicals produced by a fixed input power can oxidize only a certain amount of CO.
D = 50 mm
D= 76 mm
D = 100 mm
Input Power (W)
Figure 5. CO removal efficiency vs. input power for different internal
diameters of the reactor. D = 50 mm, L = 1200 mm, T = 4 sec, C0 =
280 ppm, RH = 64%, and vf = 0.3 m/sec.
Volume 50 October 2000
Figure 6. CO removal efficiency vs. input power for different values of
initial concentration of CO. D = 50 mm, L = 1200 mm, f = 60 Hz, T = 4
sec, RH = 64%, and vf = 0.3 m/sec.
The Effect of Residence Time
From Figure 7, it can be seen that the residence time
had a significant influence on CO removal. After the
onset of the discharge, at a fixed value of input power,
higher CO removal efficiency was obtained at longer
residence times.
The Effect of Relative Humidity
Figure 8 shows CO removal efficiency versus input power
at different levels of RH. In this case, better results were
found when the proper high RH was set. This may have
been due to the increase of radicals (OH*) in a discharge
at a higher RH.2 However, other experiments11 have shown
that the presence of water increases OH* inside the reactor. On the other hand, the onset of discharge is delayed
because of the high RH; as a result, the O and O3 are decreased, the oxidation of CO is lower, and, therefore, CO
reduction efficiency decreases. In this test, the best results are obtained at an RH of ~88%. As illustrated by the
curves in Figure 8, the CO removal at RH = 95% was
slightly lower than at RH = 88%.
CO Removal Efficiency (%)
Input Power (W)
CO Removal Efficiency (%)
CO Removal Efficiency (%)
CO Removal Efficiency (%)
Li et al.
T = 4.0 sec
T = 2.7 sec
T = 1.5 sec
Input Power (W)
Figure 7. CO removal efficiency vs. input power for different residence
times. D = 50 mm, L = 1200 mm, f = 60 Hz, C0 = 280 ppm, RH = 64%,
and vf = 0.3~0.8 m/sec.
Journal of the Air & Waste Management Association 1737
Li et al.
REFERENCES
CO Removal Efficiency (%)
1.
RH = 95%
RH = 88%
RH = 64%
RH = 30%
Input Power (W)
Figure 8. CO removal efficiency vs. input power for different values of
RH. D = 50 mm, L = 1200 mm, f = 60 Hz, T = 4 sec, C0 = 280 ppm,
RH = 64%, and vf = 0.3 m/sec.
Matching Reactor and Input Power
We also considered the matching of input power with
the diameters of the reactor and the lengths of the reactor. In our experiments, the diameter of the reactor was
φ50 mm, φ76 mm, and φ100 mm at lengths of 1200 mm,
1800 mm, and 2400 mm, respectively. Experimental results12 show that, under the same conditions, the shorter
the reactor, the higher the CO removal efficiency. This
was because our pulse voltage power was limited; therefore, we give here only results obtained at a constant reactor length of 1200 mm. Still, it is apparent that the
supplied power must match the parameters (i.e., diameter and length) of the reactor in order to obtain a significant CO removal efficiency.
CONCLUSIONS
The removal of CO by pulsed corona discharge with a
reactor of a coaxial wire-to-cylinder electrode system was
experimentally investigated. The following conclusions
were obtained: (1) CO removal can be achieved by pulsed
corona discharge; (2) CO removal efficiency is affected
significantly by the parameters of input power and reactor as well as by their matching; (3) better CO removal
performance was obtained at high input power and high
frequency of pulsed voltage; (4) a longer residence time
and lower initial CO concentration are advantageous to
CO removal in the tested type of discharge; and (5) a certain high RH was also beneficial in reducing CO.
1738 Journal of the Air & Waste Management Association
Pen, D.Y.; Lin, S.N. The Control of Air Pollution; Environment Science
Press: Beijing, China, 1994; pp 7-8.
2. Clements, J.S.; Mizuno, A.; Finney, W.C.; Daries, R.H. Combined Removal of SO2, NOx and Fly Ash from Simulated Flue Gas Using Pulsed
Streamer Corona; IEEE Trans. Ind. Applicat. 1989, 25 (1), 46-53.
3. Chakrabarti, A.; Mizuno, A.; Shimizu, K.; Matsuoka, T.; Furuta, S. Gas
Cleaning with Semi-Wet Type Plasma Reactor; IEEE Trans. Ind. Applicat.
1995, 31 (3), 500-506.
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Chemical Process for Removal of NOx and SO2 from Combustion
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5. Oda, T.; Kato, T.; Takahashi, T.; Shimizu, K. Nitric Oxide Decomposition in Air by Using Nonthermal Plasma Processing with Additives
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6. Chang, J.S.; Lawless, P.A.; Yamamoto, T. Corona Discharge Process;
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8. Mizuno, A.; Shimizu, K.; Chakrabarti, A.; Descalescu, L.; Furuta, S.
NOx Removal Process Using Pulsed Discharge Plasma; IEEE/IAS Conf.
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About the Authors
Xiaohong Li (corresponding author) received B.S., M.S., and
Ph.D. degrees in Resource and Environmental Engineering
from Chongqing University, Chongqing, China. He has also
studied at California University at Berkeley and at the University of Queensland in Australia. He is currently a professor in the College of Mechanical Engineering and vice president of Chongqing University. He is mainly engaged in research and education in the area of water jet and exhaust
gas treatment using nonthermal plasma. Dr. Li may be
reached at [email protected]. Lin Yang received a B.S. degree in Electrical Engineering and an M.S. degree in Resources and Environmental Engineering from Chongqing
University, where he is currently pursuing a Ph.D degree in
Mechanical Engineering. His current research interest is
environmental control of polluted gas using plasma discharge. Yuyong Lei received B.S., M.S., and Ph.D. degrees
in Resources and Environmental Engineering from
Chongqing University, where he is currently an associate
professor in the College of Mechanical Engineering. He is
engaged in research on the mechanical design and manufacture of pulsed corona discharge apparatuses and polluted gas treatment using pulsed corona discharge.
Volume 50 October 2000