1152
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 19, NO. 6 , DECEMBER 1991
Corona Discharge Processes
Jen-Shih Chang, Member, IEEE, Phil A. Lawless, and Toshiaki Yamamoto
(Invited Review Paper)
Abstract- Corona discharge processes emphasize one of two
aspects of the discharge: the ions produced or the energetic
electrons producing the plasma. In general, in an application
using ions, the corona will occupy a small fraction of the total
volume. However, a process using the electrons will fill most of
the volume with the plasma. Applications of corona discharge
induced plasmas and unipolar ions are reviewed. Current stateof-the-knowledge of ionized environments and the function of
corona discharge processes are also discussed in detail.
Glow
I. INTRODUCTION
A
PPLICATIONS for corona discharge processes have existed for over a hundred years, dating to the first electrostatic precipitator of Lodge [l]. Since then, corona has
been extensively used in several commercial ways and is
gaining attention for use in other applications. The items
that will be discussed here are: electrostatic precipitation,
electrophotography, static control in semiconductor manufacture, ionization instrumentation, control of acid gases from
combustion sources, destruction of toxic compounds, and
generation of ozone.
Corona discharges are relatively low power electrical discharges that take place at or near atmospheric pressure. The
corona is invariably generated by strong electric fields associated with small diameter wires, needles, or sharp edges on an
electrode. Corona takes its name (“crown”) from mariner’s
observation of discharges from their ships’ masts during
electrical storms. The corona appears as a faint filamentary
discharge radiating outward from the discharge electrode.
Because the corona is relatively easy to establish, it has had
wide application in a variety of processes.
Corona process applications emphasize one of two aspects
of the discharge: the ions produced or the energetic electrons
producing the plasma. The ion identities depend on the polarity
of the discharge and the characteristics of the gas mixture,
specifically on the electron attaching species. The electron
energies depend on the gas characteristics and on the method
of generating the corona. In general, in an application using
ions, the corona induced plasma zone will occupy a small
fraction of the total process volume, while a process using the
electrons will fill most of the volume with the plasma.
Manuscript received May 10, 1989; revised August 15, 1991. This work
was supported in part by the Ministry of Energy, Ontario Government, Canada.
J.-S. Chang is with the Department of Engineering Physics, McMaster
University, Hamilton, ON, Canada L8S 4M1.
P.A. Lawless and T. Yamamoto are with the Center of Aerosol Technology,
Research Triangle Institute, Research Triangle Park, NC 27709.
lEEE Log Number 9104433.
Fig. 1. Schematic of type of corona discharges.
PLASMA
CORONA
DISCHARGES
A . Type of Corona Discharge
Corona discharges exist in several forms, depending on
the polarity of the field and the electrode geometrical configurations. For positive corona in the needle-plate electrode
configuration, discharges start with burst pulse corona and
proceed to the streamer corona, glow corona, and spark discharge as the applied voltage increases (Fig. 1). For negative
corona in the same geometry, the initial form will be the
Trichel pulse corona, followed by pulseless corona and spark
discharge as the applied voltage increases. For a wire-pipe
or wire-plate electrode configuration, corona generated at a
positive wire electrode may appear as a tight sheath around the
electrode or as a streamer moving away from the electrode.
Corona generated at negative electrodes may take the form of
a general, rapidly moving glow or it may be concentrated into
small active spots called “tufts” or “beads.”
Negative corona generally propagates by impact ionization of the gas molecules. Positive corona depends more on
photoionization for its propagation: the positive streamer, for
0093-3813/91$01,00
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ENVIRONMENT
WITH
1991 IEEE
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CHANG et al.: CORONA DISCHARGE PROCESSES
example, may advance at as much as one percent of the
speed of light. In either case, the ultraviolet photon flux from
ion-electron recombinations is quite large.
The positive sheath form is known as a Hermstein’s glow [ 2 ]
and is similar to the discharge (at lower pressures) in a Geiger
tube. It is characterized by a steady current at a fixed voltage,
quiet operation, and almost no sparking. The positive streamer
corona is a discharge confined to a narrow channel which
originates at the electrode. It produces an unsteady current
(because the streamer is repetitive), is quite noisy, and is the
direct precursor to a spark: once streamers form at an electrode,
the sparking potential has almost been reached.
The negative glow usually requires clean, smooth electrodes
to form. The glow is made up of individual electron avalanches
which trigger successive avalanches at nearby locations. The
total current from the electrode is relatively steady, but it is
composed of many tiny pulses. The discharge is noisy and
the sparking potential is high compared with the positive
streamer corona. The glow often changes with time into the
tuft form, a process associated with the formation of more
efficient mechanisms of generating successive avalanches. The
tuft corona is also noisy and has a similar sparking potential to
the glow form. The average current is steady, but is composed
of tiny pulses like the glow corona. The tuft corona is more
spatially inhomogeneous than the glow corona. Differences
between negative tuft and glow coronas have been investigated
recently [3], [4].
The corona discharge is usually space-charge limited in
magnitude, since the plasma emits ions of one polarity that
accumulate in the interelectrode space. This gives the corona
a positive resistance characteristic: increases in current require
higher voltages to drive them. If the current in the discharge
is raised sufficiently, additional current-carrying species will
be produced and spark discharges will result. The spark is
usually characterized by a negative resistance characteristic,
but the transition from corona discharge to spark discharge is
not sharply defined.
B. Plasma Environments in a Streamer Corona
The positive streamer forms when the positive ion density is
large enough to extend the region for corona initiation into the
interelectrode gap. The process then builds by photoionization,
with the positive ion head moving in front of a nearly neutral
column. This process gives the positive streamer corona an
active volume much greater than the other forms, which are
confined by their generation mechanisms to the near-electrode
regions.
Schematics of the single streamer charge distribution are
shown in Fig. 2, for a point-to-plane corona in dry atmospheric
air, where the time and size scales of the streamer corona are
listed in Table I. As shown in Fig. 2, the size of the plasma
is approximately 10 cm long and 20 pm wide with relatively
dense plasma (up to lo9 cmP3). Streamer corona is a relatively
low temperature plasma, with iodneutral temperatures of the
Therefore once the streamer corona
order of a few 100 K
discharge is formed in a closed chamber, a low temperature
dense plasma environment can be generated over a large
volume. It is, however, transient in nature.
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Fig. 2. Schematic of the streamer corona charge distributions.
TABLE I
TIMEAND
SPACE SCALES FOR THE STREAMER CORONA FOR
THE GAP FROM 1 TO 100 CM
Streamer rise time
Streamer duration
Repetition period
Streamer diameter
Electric field
Ion temperature
2:
2:
2:
1 ns
1
1 ns
20 p m
lo-’’
lo3 K
E/-V
Vm2
C . Plasma Environment in a Spark Discharge
The spark discharge propagates ions more intensely than
does the streamer corona discharge. The core of the spark
channel usually is a dense, high temperature plasma, and the
total propagation channel of spark discharges usually changes
with time. Therefore, a spark discharge induced plasma will
occupy a smaller fraction of the total volume than that of the
streamer corona induced plasma.
111.
CORONA AS ION SOURCE
The plasma zone of the corona extends only a few millimeters in the direction of the electric field, except for streamer
corona and sparks. Outside this region, unipolar ions produced
in the plasma are transported by the electric field. In the
positive corona, the ions produced by electron impacts retain
their identities or form the core of cluster ions that carry charge
to the counter electrode.
In the negative corona, significant fractions of the current
can be carried by low-energy electrons for several centimeters
beyond the plasma zone. However, depending on gas compositions, most electrons attach to form negative molecular ions by
attachment reactions and create the space charge that stabilizes
1154
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IE{EE TRANSACTIONS ON PLASMA SCIENCE, VOL. 19, NO. 6, DECEMBER 1991
I
o
~
o
~
o
i
o
Fig. 3: Schematic of wire-plate type ESP and electrode arrangements
the corona due to the reduction of mobility. The precise ion
identities depend strongly on the species present in the gas.
Often, the most acidic species present becomes the dominant
charge carrier.
in negative corona to avoid the formation of streamers and
the lower sparking potential associated with them. When the
particle diameter is in the field charging regime
10 pm), the
collection efficiency of the ESP is proportional to the square
of the electric field between electrodes [l]. Consequently,
maximizing the field strength is a major goal of designers.
The ions from the corona serve to charge individual dust
particles as the gas carries them through the ESP. At the
same time, the electric field “nudges” the particles toward the
counter electrodes, which are flat collecting plates. We use
the word “nudge” since turbulent velocity fluctuations in the
gas flow are often larger than the velocity the particles would
achieve in still air. The overall particle collection process is
then statistical in nature and is often described by an equation
that is a negative exponential in length or treatment time.
The charging of the particulates creates a nearly immobile
space charge, at least compared with the high mobilities of the
ionic carriers. The space charge always influences the corona
generation,
to
~ often
o causing
~ the voltage
~
~rise several kilovolts
above the value with no particles present at the same current.
For extremely fine particulate fumes, the particulate density
may be sufficient to absorb all ions before they reach the
counter electrode. The ESP appears to operate with no current
in such a case, even though many particles are being charged
and removed.
The indoor air cleaner operates with positive corona to
reduce the production of ozone. The charging of particles
is usually separated from the collection. This allows both
processes to be optimized and results in somewhat better
performance than would be achieved in a standard one-stage
design. The small volume of gas treated also allows the indoor
cleaner to be sized more conservatively than the industrial unit.
The full description of an ESP requires many more considerations than can be given here [l], but without the corona, as
the ion source, the ESP will not operate at all.
B. Electrophotography
Electrophotography and electrographic printing make use of
small corona devices as surface chargers. The photoconductive
drum of a copier/printer must be given a uniform charge prior
to exposure to the light image. Exposure to light discharges
A . Electrostatic Precipitators
the surface, leaving patterns of charge on the drum which can
The electrostatic precipitator (ESP) is the most prevalent attract oppositely charged toner particles. The toner is then
application for corona as an ion source. Precipitators operate transferred to the paper with the influence of a charger on
at an industrial scales for collecting the particulate emissions the back side of the paper; the charger produces an electric
in the utility, iron/steel, paper manufacturing, and cement field that enhances transfer. Corona chargers also remove the
and ore-processing industries. Precipitators are also used in residual charges on the photoconductive drum so that it may
building and home ventilation systems for control of particles be cleaned prior to subsequent exposures [6].
The corona polarity for the drum charging depends on the
in the indoor environment.
Corona in the ESP is generated at high voltage electrodes, toner characteristics used, but is often negative. The transfer
commonly round wires, which are centered between flat col- corona must be of the same polarity to attract the toner
lecting plates. The corona plasma occupies only a small particles away from the drum. The cleaning corona polarity
volume near the wire; the rest of the interelectrode space is must be the opposite in order to neutralize remnant charges on
filled with ions from the corona. In this region, charging and the drum. The corona charger dimensions are small, usually a
movement of particles takes place. These are shown in Fig. 3. few centimeters in cross section and operating at only a few
For the best dust particle charging environment with the hundred volts since the operating voltages are proportional
least power consumption, the industrial precipitator operates to the dimensions. Because the function of the charger is to
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CHANG et al.: CORONA DISCHARGE PROCESSES
deliver ions, the delivery is done at as low a potential as
possible.
One of the more desirable characteristics of the corona
charger is the ability to control the surface charging level by
adjusting the voltages on the corona electrodes. The electrode
voltages also affect the uniformity of the deposited charge to
some extent. Polarity is important because negative tuft formation must be avoided in most electrophotographic applications.
C. Clean Room Ionizers for Static Control
In the manufacture of semiconductor chips, control of static
charges in the handling of the wafers is important for two
reasons. The first is that electrostatic attraction of particles
in the air may enhance their deposition on the wafers, where
they cause defects in subsequent operations. The second is
that electrical breakdown of thin oxide layers may occur at
low voltages induced by small surface charges because of the
thinness of the layers. The corona systems are designed to
neutralize surface charges by deliberately introducing ions of
one or both polarities near the ceilings of the clean rooms and
allowing the air flow to carry them to the surfaces below.
The ions are produced by needles set into bars just below
the ceiling that are separated by distances of several inches
to several feet. The counter electrodes are also at the ceiling
level. This means that the principal flow of current is parallel
to the ceiling and that any useful ion concentrations at the
work level come from convective transport by the downward
flow of air. Some systems work by monitoring the balance of
positive and negative ions at a witness plate near the work
are being controlled. If the net charges on the plate deviates
too far in either direction, then the ion production rate of the
opposite polarity is increased to compensate for it.
Other systems simply switch polarities at slow rates. This
causes a continuous cycling of the surface charges between
positive and negative extremes, where neither polarity is
allowed to become too large. The ion concentration is set high
enough to quickly overcome the static charges resulting from
handling the wafers. The last type of system uses coronas
of both polarities at higher current levels to ensure a bipolar
ionic concentration of sufficient level to neutralize any surface
charge quickly.
Ionic neutralizers using corona have been called into question since they produce significant numbers of small particles
during their operation
To a large extent, the production of
particles is a property of the electrode materials and polarity
used [8], [9]. Moreover, the size of particles seems to be well
below the damaging size at current levels of technology. The
trade-offs between the beneficial effects of static neutralization
and the deleterious particle production have to be carefully
considered at present.
D.Atmospheric Pressure Ionization Sources
A specialized gas analysis technique, atmospheric pressure
ionization mass spectrometry (API-MS), uses corona as the
principal source of ions in a chain of ionization events [lo].
The API-MS is a mass spectrometer with two distinctly
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1155
different operations: ionization at atmospheric pressure and
analysis in vacuum.
Ionization at atmospheric pressure tends to be a gentle
process, because ion exchange reactions seldom transfer more
than 3-5 eV of energy. In contrast, the impact ionization
by electrons in conventional mass spectrometry involves
40-60 eV, typically. As a result, the API system is more
likely to preserve molecular structures after ionization and
allow them to be measured.
The atmospheric pressure section is coupled to the high
vacuum section through a small orifice, about 10 ,um in
diameter, with a high capacity vacuum pump maintaining an
adequately low pressure (-lop6 torr). Some designs pass the
gas and ions through a skimmer (second orifice) backed up to
a second pump into the mass analyzer, because the differential
pumping requirements are less severe.
The ion molecule reactions that occur between source ions
and the target molecules are dependent on the relative binding
energies of the species. For negative ions from corona, the extra electron is exchanged with more electronegative molecules
until the most acidic molecules (highest electronegativity)
become the final ions that reach the analysis section. It has
been estimated that ionization efficiencies for strongly acidic
molecules may reach 100%.
With positive ions from corona, the situation is different.
The positive ion may capture electrons from less electronegative molecules, but the ion transfer is highly mediated by the
presence of neutral molecules clustering around the positive
core, particularly water molecules. The clustered molecules
shield the core from direct charge exchanges and thus stabilize
the ion. The positive API-MS spectrum contains many more
molecular species than does the negative spectrum, for this
and other reasons.
I v . CORONA INDUCED PLASMA REACTORS
AND
POWERSUPPLIES
Plasma chemical processes have been known to be highly
effective in promoting oxidation, enhancing molecular dissociation, or producing free radicals to enhance chemical reaction,
and, therefore, have many applications. Until recently, plasma
processes were carried out in either the high-temperature
environment of transferred and nontransferred arc plasmas or
at pressures low enough to give large active volumes with
higher electron and lower gas temperature glow discharge
plasmas. With the advent of corona reactors, low temperature
and lower power plasma processing has been extended to
atmospheric pressure applications. The reactions are generally
limited to low concentrations by the plasma density, restricting
them to expensive materials or those untreatable by more
conventional processes, such as hazardous wastes.
Reactors can be classified by their physical construction and
method of energization. Each physical arrangement has certain
advantages for reactions, while the energization is closely
coupled with the reactor design. Reactor designs that have
been studied are: point-to-plane, point-to-point, packed bed,
and coaxial or wire duct. The reactors can be driven by direct
current, (dc) alternating current (ac), or pulsed dc.
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Fig. 4.
Glow discharge flow channel
A. Point-Plane and Point-Point Reactors
Several types of dc energized point-electrode reactors have
been developed and tested for gas chemistry applications.
Although designed with different purposes in mind, the configurations could be put to other uses.
The first device was a narrow gap, multipoint-to-plane
geometry device in which the gas passed through a corona
discharge at high velocity (approximately 100 m/s). The upper
multipoint electrode (cathode) was separated from the lower
flat electrode (anode) by acrylic spacers that isolated the
electrodes electrically and allowed visual observations of the
corona, as shown in Fig. 4.The narrow gap spacing ensured
that the interelectrode space was filled with corona induced
plasma [ l l ] ; however, the lateral spacing of the pins allowed
major fractions of the gas flow to bypass the corona zones. A
dc was applied to the multipoint pins through current limiting
resistors. This type of reactor showed a high average field and
current density (18 kV/cm and 1.0 mA/point, respectively).
A second device was constructed in the form of a narrow
gap, triangle-shaped, corona discharge device as shown in
Fig. 5. This design was intended to reduce the electrical
sneakage (bypassing of the corona induced plasma) which
was the major problem for the multipoint-to-plane geometry
device [12], [13]. Infrared thermography of negative pointto-plane corona discharge also showed a large sneakage zone
[14]. The point-to-plate distance was approximately 0.7 cm.
The volumetric filling factor of the corona induced plasma in
this device was much higher than in the multipoint device.
For these corona reactors, it is apparent that the discharge
region is the area where useful reactions are most likely
to occur. The highest electric fields, ion concentrations, and
electron energies exist in this region, and it is expected that
the free radicals and highly excited molecules have the greatest
concentrations there. However, the useful plasma conditions
are achieved in the small volume defined by the discharge.
Another device, the corona torch, was developed to use the
streamer corona and enlarge the active volume. This plasma
device consists of two small diameter hollow electrodes as
shown in Fig. 6 [15], [16]. The gas flow enters the upstream
cylindrical hollow electrode and exits at a downstream cylindrical hollow electrode. Therefore, all the reactive gas will be
passing through the active corona-induced plasma zone. Highspeed gas flow near the exit of the electrode will be cooling
electrodes; hence, the chemical reactions and the stability of
discharge will be enhanced.
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Fig. 5.
Schematic diagram of inner electrode configuration of glow discharge
channel.
Fig. 6. Schematics of corona torch plasma reactor.
A similar device using capillary discharge tubes consists of
a small diameter cylindrical tube with needle-type electrodes
placed at the end of a cylindrical tube [17]. A high voltage
corona is applied to form streamer coronas or repeated spark
discharges. The schematic diagram for this arrangement is
shown in Fig. 7. The capillary walls confine the streamer and
forces the gas to pass through the streamer region with minimal
sneakage. The pressure drop of the corona torch and the capillary discharge tube are large compared with the point-plane
type reactor; however, the pressure drop of the reactor may
be reduced by corona discharge with optimum conditions
[16]. Since the residential time requirement for chemical
processings for this type of reactor is small, multiflow channel
concepts will be used for a larger gas flow rate application.
B. Bed Reactors
The packed bed reactor is a tubular reactor packed with
a ferroelectric (high dielectric ceramic) pellet layer such as
BaTi03, SrTi03, or PbTi03 [MI. The dielectric is held within
the tube arrangement by two metal mesh electrodes connected
to a high voltage ac power supply as shown in Fig. 8. When
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CHANG et al.: CORONA DISCHARGE PROCESSES
I-
L
Fig. 7. Schematic of capillary tube plasma reactor
Fig. 9. Schematic of pulsed corona plasma reactor.
Fig. 8. Schematic of ac packed bed plasma reactor.
an ac field is applied, an intense electric field is formed around
each dielectric pellet contact point, producing high energy free
electrons as well as air molecular ions throughout the cross
section of the reactor.
The spherical pellets enhance the electric fields in the
contact regions between adjacent pellets and lead to microbreakdowns in the gaps. The high dielectric constant of the
ceramic enhances the effect; when glass pellets, with much
lower dielectric constant, are substituted, sparking between the
contact electrodes occurs before significant microspark activity. Sputtering of the pellet material is significantly influenced
by the discharge conditions, where the spark discharge normally generates relatively larger sputtering of materials 191.
Another silent discharge plasma reactor has been tested for
oxidative destruction of various agent simulants. In this case,
the packed bed consisted of metallic catalysts on alumina
pellets. The plasma under these conditions is not localized
as with the ferroelectric pellets, and the catalytic action is
an important factor in the performance. The plasma strength
depends on coupling the electric field through dielectriccoated electrodes, rather than metallic electrodes. This leads
to a strong frequency dependence, but seems to produce an
operation that is not sensitive to relative humidity and flow
rate [20].
In these reactors, frequencies of operation range from power
line values to a few thousand hertz, and the voltages are
primarily determined by the reactor dimensions and electric
fields desired. The pressure drop of the bed reactors is much
larger compared with other reactors discussed. However, if we
were to operate under fluidized conditions, this problem might
be substantially reduced.
C. Pulsed Reactor Operations
Pulsed dc with pulse durations less than 1 ,us has the
advantage of generating electrons with limited movement
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of ions. As a result, much higher electric fields can be
applied during the duration of the pulse without causing spark
breakdown in the reactor. The nanosecond pulser generally
employs a dc power supply that is used to charge a low
inductance capacitor. The discharge of the capacitor through a
spark gap gives the short rise time required. The pulselength
can be determined by the load resistance and coupled to the
reactor, such as the one shown schematically in Fig. 9. The
spark gap discharge is currently required to produce the short
rise times. Conventional thyristor switching cannot produce
such a short pulse. The fast rise time of the pulse also allows
the electric field at corona onset to become much higher than
in the dc corona, since statistical time lags govern the initial
formation of the corona. The electron energies in the pulse
corona are correspondingly higher than for dc.
Additional advantages of nanosecond pulsing over dc energization are several. The brief application of voltage allows
the production of electrons without raising the ion temperature
(which forms various radicals) at atmospheric pressure. This
means that less energy goes into heating the gas for a given
electron concentration. The electron concentration can be
orders of magnitude larger than under dc conditions because of
the higher electric field at corona onset. Because of the higher
concentration, space charge effects disperse the electrons more
uniformly throughout the reactor volume. The reactors can be
designed with larger volumes because of the greater range
of the energetic electrons and filling efficiency. This allows
the mechanical tolerances to be relaxed, for most geometries.
Finally, the high overvoltage of the pulse for corona onset
means that many reactors can be driven in parallel with the
differences in their individual onset thresholds not affecting
the operation.
The disadvantage of pulse operations is the electromagnetic
compatibility (EMC) design. Normally, all designs must overcome relatively large electromagnetic pulse (EHP) impacts,
since EMP increases with increasing current pulse rising times
[211.
The same reactor types as used for dc energization can be
used with the pulsed energization, but the dimensions can be
altered for optimum results.
D. Polarity Effects
The dc and pulsed dc reactors can be operated with either
polarity, while the ac reactors intrinsically show no polarity
effects. Unfortunately, positive corona operation with streamer
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 19, NO. 6, DECEMBER 1991
corona is extremely difficult to control, with sparking occurring frequently. The negative dc corona is more stable, but the
operating voltage range is small. Without current limiting, the
heating of the gas by the discharge quickly leads to a negative
resistance characteristic and sparking. The corona torch and
the capillary tube plasma reactors,use a high-speed gas flow
(superficial gas velocity 10 to 100 m/s) to stabilize and cool
the corona discharge and the electrodes.
The short pulse reactor controls these problems to a much
better degree. As a result, extensive work has been done with
positive streamer corona because of its much longer range.
The space charge enhancement of the electric field at the head
of the streamer allows it to propagate into regions where the
Laplacian field is too low for plasma generation. Consequently,
in a diverging geometry, the positive streamer may have a
range 5-10 times longer than the negative corona. The range
of the streamer is partially offset by its confinement to a narrow
channel. At distances from the electrode, there are significant
gaps in the volumetric coverage, leading to potential sneakage
problems.
Negative corona has been less utilized in these reactors
because of its shorter range. However, within the useful range
of the electrons, the volumetric coverage should be better
than for streamer corona because of the higher diffusivity and
mobility of the electrons.
The pulsed nature of the discharge introduces a duty cycle
factor into considerations of efficiency. At the very least, the
effective residence time for treatment may be raised because
of the duty cycle. This may not be important in some cases,
since the plasma is merely used to initiate reactions that occur
according to their own kinetics; in others, the plasma reactions
may be the rate determining step.
V. OZONESYNTHESIS
The ozone generation system by corona discharges was
made by Simens [22] approximately 100 years ago. A silent
discharge, as shown schematically in Fig. 10, is generally used
where the implementation of silent discharge corresponds to
a capacitor that has two dielectrics between its plates: 1) the
insulator and, connected in series with it, 2) air or oxygen.
In a silent discharge, the discharge generated ions traverse
the space in a pulse, and are stored in the surface of the
dielectric materials. Since these accumulated space charges
generate a reverse electric field, the corona discharge will
be terminated. Therefore, we can produce a sparkless high
electron temperature/low gas temperature reactive plasma to
enhance chemical reactions. At present, commercial ozonizers
have an energy yield of 90 and 180 g/kWh for dry air and
pure oxygen gas, respectively. Since the theoretical limit of
energy yield is 1200 g/kWh, 92.5% of the energy in a silent
discharge is lost as heat. More recently, some preliminary
investigations have been conducted to operate a ceramic made
reaction chamber under near liquid nitrogen temperatures [23].
In these cases, the energy yield is observed to be in the range
of 400-600 g/kWh [24].
It is generally believed [22], [24] that the ozone generated
in a corona discharge is a two step process as follows:
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Fig. 10. Schematic of silent discharge system.
i) Generation of oxygen free radicals by ionic processes:
0 2 + e - + 0:+2e:
direct ionization
dissociative ionization
(1)
(2)
O+O+e:
dissociation
(3)
0-+o:
dissociative attachment
(4)
0’
4
0
2e:
ii) Generation of ozone by free radical reactions:
0
0 2
M
M;
M
0 2
or N2
(5)
where details of other secondary reactions can be found
in [22], [24] and are summarized in Fig. 11. Except for
dissociative attachments, the reactions in the first step are
not significantly influenced by gas temperature [22], [24].
However, the reaction rate of second step is observed to be
significantly influenced by gas temperatures, where reaction
rate, hence, ozone generation rate (reaction
decreases with
increasing gas temperature; i.e., the reaction rate IC
2.5
exp(970/Tg) [22], [24]. Gas temperature dependence of
other radical reactions and reaction rates related to ozone loss
and generation processes are shown in Table
The ozone
generation processes are substantially reduced by increasing
gas temperature, while the ozone loss processes are significantly enhanced by increasing gas temperature. Therefore,
if we operate an ozonizer in lower temperature conditions,
substantial enhancement of ozone generation can be expected.
Typical results obtained by Masuda et al. are shown in Fig. 12
[23]. No investigators have successfully modeled the ozone
concentration generated by the corona discharge in air, since
the ionic and neutral species involved and the number of
chemical reactions needed to be considered are not clearly
understood at present.
Normally, commercial ozonizers use pure oxygen or nitrogen removed air to avoid generation of N,O,. More recently,
the effect of gas compositions on the ozone formation and loss
processes under the silent discharge were investigated [22].
For example, the mixture of Ar, He, CO2, etc., have been
investigated [22]. In some conditions, the ozone energy yield
is substantially enhanced (up to 400 kg/kWh). However, the
effect of gas composition may enhance formations of the toxic
gas such as NzOy or acid gases such as HmC,O,,NH4N03,
etc.
VI. COMBUSTION
GAS TREATMENT
Pulse energization was developed in pollution control to
overcome operating problems with ESP’s. During development of the nanosecond pulse and electron beam technologies
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CHANG et al. CORONA DISCHARGE PROCESSES
TEMPERATURE
1159
TABLE I1
DEPENDENCES
OF OZONE-RELATED
RADICAL SPECIES REACTIONRATES
Process
Reactions
Temperature Dependence
2.5
6.9
6.2
1.5
1.2
1.5
1.2
exp(970/T)
10-34(T/300)-2
10-34(T/300)-2
7 10-34 300 K
lo-" exp( -2218/T)
lo-" exp(-2400/T)
lo-" at 300 K
IO-'* exp( -1300/T)
exp( -245O/T)
I00
10,
on
-100 - 0 0
-60
-..O
Surface Teilpcrnturc
Ts
Fig. 12. Ozone energy yield as a function of reactor surface wall temperature
(after [29]).
PjO +M Xi 0
Two body reaction rate k
[cm3/sl.
Three body reaction rate k
Fig. 11. Formation and loss processes of ozone in air.
for ESP's, it was found that significant reductions of nitrous
and sulfur oxides were taking place as well. The close relationship between particulate control by ESP's and the use of
pulsed energization to achieve control of the acid gases has
led to several investigations for these applications.
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NO, and SO, emissions are the major cause of acid rain.
C02, CH4, and N20 emissions cause a greenhouse effect
which leads to abnormal global heating of the atmosphere, and
can create a temperature inversion layer that traps pollutants.
There is a vital interest in controlling these emissions-in the
near term for acid rain and in the long term for the greenhouse
effect.
Typical flue gas emitted from coal burning power station
contains 10 to 15% of C02, 400-1000 ppm of NO,, and
50-500 ppm of SO,. It has been known that SO2 can be
removed by a wet scrubber through the film of a liquid solvent
such as water, water solutions of NaOH, Na2C03, limestone,
etc. [25]. More recently, similar wet processes are achieved
for NO, if a wet ESP can be operated under spark discharge
conditions [25]. The problem with this type of wet gas cleaning
system is generating a large quantity of liquid waste as well
as the transport and storage of large quantities of inexpensive
neutralizing materials such as limestone, etc. The size of ESP
becomes much larger, and the life becomes much shorter
compared with ordinary dry systems. Beside these problems,
in order to treat NO,, approximately 25% of electrical power
generated by a coal burning boiler must be used in this system
[25]. For CO2 gas, this process has not been investigated;
1160
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 19, NO. 6, DECEMBER 1991
however, we do not expect any mechanisms can reduce CO2
Classical models of discharge chemistry proposed by variin this wet process.
ous authors [31], [33] are based on the fact that the electron
Several laboratory scale experiments have been conducted is the main source of radical particle generation as follows:
in Japan and West Germany by using electron beam
+e
0 0+e
De-NO,/De-SO,
processes, and this dry process has been
N
2
t
e
t
N+N+e
commercialized by Ebara Corporation of Japan. However, due
to the nature of the system to use a high energy electron beam,
NO e
N 0+e
the radiation protection system and the electron beam sources
NHS+e - + N H + H z + e
occupy a large space for installations.
NH2 H e.
Although present technologies seem capable of controlling
of the nitrous and sulfur oxides, there is at present no proven Then radical particles will react with each other to generate
technology for controlling CO2 emissions, other than the active species such as 0 3 , OH, etc. to eliminate NO,. For
planting of trees. Indeed, most processes for removing CO2 example:
from flue gas are so energy intensive that most of a utility’s
NO NH
N2 OH
output would be required to effect direct cleaning. The three
0 2
NH2
HNO OH
CO2 chemical or physical separation systems are proposed
NO, + O H
De-NO,.
[26]-[28] as follows:
1) wet absorption method-absorption of CO2 by liquid More recently, Chang et
[15], [22] suggested that radical
amine- or alkaline-type solvents;
species can be generated through the dissociative recombina2) dry absorption method-absorption of CO2 by Zeolite tion of the positive and negative species via formations of
packed bed filters;
various kinds of ions from ion molecular reactions. Because
3) membrane separation method-CO2 separation by poly- the order of two body reaction rates for electron-ion related
mer membrane.
reactions is 10 to lo8 times larger compared with those for
All the CO2 separated will be liquefied and stored in a deep neutral species (as shown in Table III), the effective production
sea. However, the construction cost incurred by these proposed or loss of each species becomes comparable with neutral
systems, will become a 70% increase in the cost of the plant reactions despite the fact that the density of ions is much less
and the overall power plant efficiency will be reduced to 34% than the density of neutral species (as shown in Table IV).
from the current 42% [26]. Approximately 90% of the emitted Therefore, ionic reactions may play an important role in the
CO2 is expected to be recovered by these processes.
discharge chemistry.
Higashi et
[29], Maezono and Chang [15], and Xie et al.
Another important role of ions in combustion gas treatment
[27] have investigated reduction of CO2 from combustion flue is ion induced aerosol formations. Based on the modified
gas using a corona discharge in some time with a reduction Thomson-Gibbs equation for the homogeneous nucleation
of SO2 and NO,.
induced by ions [34], as follows:
The corona discharge process is one of the most effective
e2
methods of removing S02, NO,, and CO2 from industrial
In(&)
R, f h R $
A4
flue gases. This flue gas treatment consists of adding a small
amount of ammonia and argon to the flue gas, and irradiating
R
the gas by means of an electron shower, thereby causing reacactions which convert the S02, NO,, and CO2 to ammonium sulfate [30], ammonium-sulfate nitrate [30], and carboxylic acid The action of the ions is expected to be a lowering of
the threshold or an increase in the rate of nucleation due
aerosols. These salts may then be collected from the flue gas by
to the influence of the central force field, where
is the
means of such conventional collectors as an ESP or baghouse.
density, P/P,
S , is the saturation ratio; R, is the aerosol
This process has numerous advantages over currently used
radius; is the dielectric constant; is the surface tension;
conventional processes as follows:
A4 is the molecular weight;
is the vapor pressure; P,
The process simultaneously removes SOz, and NO,, and is the plane surface vapor pressure; and subscripts a, g and
part of CO2 from flue gas at high efficiency level.
c refer to aerosol, gas, and critical, respectively. Here, we
It is a dry process which is easily controlled and has must note that the gas phase saturation ratio is a function
excellent load following capability.
of time in the present case, and the number of surface
Stack gas reheat is not required.
charges due to the ion deposition to the surface of the
The pollutants are converted into a salable agricultural aerosol particle will be influenced by the growth of aerosol
particles. Heterogeneous nucleation may also occur in the
fertilizer.
The process has low capital and operating cost require- present system, since a clustering reaction will be generated
ments, since the process may retrofit to the existing in a core particle that consists of up to 30 H20 or NH3
molecules; e.g., X+(H20)30 or Y+(NH3)30 [35]. Once large
system.
Part of the energy losses (radiation heat loss via vibra- cluster ions or ultrafine particles are formed, the growth of
tional and rotational excitations of molecules) in a power the aerosol particle will be controlled by ion density and the
electron temperature as has been proposed by Chang [36].
plant will be recovered.
d)
[ZO(€i€31
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CHANG et al.: CORONA DISCHARGE PROCESSES
TABLE I11
TYPICAL
ORDER OF MAGNITUDE
OF VARIOUS CHEMICAL REACTION
RATES
1161
A . De-No, and De-SO,
The nanosecond pulsing technology has been further applied to control of NO, and SO,. Masuda and Nakao [25],
[40] reported that De-NO, is possible by both positive and
ION-MOLECULE
(40 0.4) 10-10
negative
pulsing and De-SO, only by positive pulsing. The
20 10-'0
H; + H ~ H: + H
reaction speed is greatly enhanced by raising the peak field
HzO'
H20
H30+ CH
13
intensity, and increasing the pulse frequency, lowering gas
Ar+
Hz
ArH+
H
3.5 IO-''
0.004 10-10
temperature, and using sharp corona wires. Increasing the
KH;
H~
NH,~ H
o+
0
0.4 10-10
moisture and oxygen enhances this conversion efficiency. It
ATOMIC OR RADICAL
was also reported that the combination of pulsing and electron
C Z H ~ C Z H B C4HIO
2.7 lo-"
beam technologies does not significantly enhance the removal
O + N O z -xo+o2
2 . 3 10-12
efficiency [41], [42]. Masuda and Nakao [25] explain that the
0 03
202
2 . 5 10-14
process is possible because, under normal pressure, it can raise
H+Hz-+Hz+H
7 10-17
C H ? CzHs
CH4 C 2 H j
the electron temperature for production of active chemical
lo-''
Br+H2 - + H B r + H
lopZ4
species such as OH, 0, 0 3 , N, etc., without raising ion
MOLECULE
temperature, which had been possible only by high frequency
No 0 3 x02
glow discharge under low pressure conditions.
HI C z H j I
C Z H G 11
The reactor has the geometry of a wire-pipe ESP, but the
2x02
2x0 0 2
power supply must be capable of consistently producing high
IONS IN LIQUIDS
voltage pulses less than 1 in duration. In a pulsed electron
e-(aq)
H+
H
4 10-1'
reactor, the reactions are dominated more by the concentration
7 10-19
CO2 O H HCO,
of energetic electrons and consequently will perform well at
1.6 lo-=
CH3COOC2Hj CHCHzCOOCzHjOH
low gas concentrations. The pulsed electron technology has
ION-ELECTRON
been shown to cause reactions between oxidizing radicals such
1.3 10-6
e IiHZ
h72
NHz
as OH, 0, and 0 3 and NO, and SO,, at the concentrations
e+Hs-+Hz+H
2.3 lo-'
found
in flue gases to form several acid aerosol particles with
ELECTRON-MOLECULE
NH3 or H20 injections [40], [42], [43]. These can be readily
1 10-11
e+03+0-+02
neutralized by the injection of limestone, and the final products
e H 1 - +IH
9.6 l o r x
removed from the flue gas in an ESP.
POSITIVE-NEGATIVE ION
H+
HHZ o
1 10-7
Clements et al.
have demonstrated that by superimposNO+
products
10-6
ing a dc bias on the pulsed voltage, SO2 removal efficiencies
in excess of 99% may be achieved for humidified gas streams.
Although NO, removal efficiencies were not reported, the
TABLE IV
collection efficiency of high resistivity fly ash particles was
REACTION
RATEEQUATION
FOR NEUTRAL
AND IONIC EQUATIONS
significantly higher than with direct current alone.
an. k 1 I4[BI k2 [c+l
Generally, the conditions necessary for effective removal
[BI
d+
of SO2 and NO, include high pulse frequency, peak field
-4 B
Y
intensity, lower gas temperatures, and sharp corona wires.
c+ B
L
k]
kl
Imposition of a dc bias serves to enhance ion molecule
kl[=1]M kz
reactions, thus improving the observed removal effectiveness.
As with electron beam technologies, the addition of a reducing
A typical deposition speed has been observed to be a few gas such as ammonia greatly increases the removal of NO2
times to an order of magnitude faster than with neutral species. from the flue gas while not significantly affecting the rate of
Here we must note that the aerosol particle surface reactions NO removal. These scavenger gases also serve to enhance SO2
in the present cases must be considered to be additional to removal. The ammonia should be considered an integral part
the homogeneous nucleation model for detailed modeling. of the process because it effectively removes the oxidizing
Robbins and Cadle [39] show that the surface velocity constant gases from the reaction and prevents reverse reactions from
of an aerosol particle of size 0.3-1 pm, with NH3, is on the occurring [40], [43].
Civitano et al. [42], [44] have shown promising results
order of lo5 cm4/s. Based on the charge-free aerosol particle
gas reaction model of Huntzicker et al. 1371 and Rube1 and at a small pilot scale (800 ft3/min) using ammonia gas as
Gentry [38], the reaction is controlled by gas phase diffusions the reducing reagent. In addition, they have been studying
and internal particle diffusions. For the corona environments, the reactions that occur and have developed a preliminary
the electric field induced drift motion of ionic species near model for the formation of corona streamers within which the
the charged aerosol particle must be considered physically energetic electrons create the free radicals that are responsible
on top of the gas phase diffusions. Therefore, a substantial for the oxidation of the NO and SO2 molecules. The pulsed
enhancement of the gas-particle surface reaction rate will be electron process has been shown to operate over a wide
range of concentration conditions. Civitano et al. (421, [44] are
expected.
Reactions
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k (cm3//S)
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1162
IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 19, NO. 6, DECEMBER 1991
ol
02
Fig.
Mechanism of De-SO,NOs and the formation of NH4NO3
aerosol particles (after [40]).
(c)
Fig. 13. Schematic of CO,, KOz, and SO, removal processes. (a) CO2
reduction process.
KO, oxidation processes. (c) SO, oxidation process.
moving ahead with a 2-MW pilot evaluation that is expected
to be followed by a full scale demonstration.
Azuma et al. [53] conducted NO, reduction experiments
by a coaxial ac corona discharge reactor in real diesel engine
exhaust gas. The results showed that 100% of the soot and
80% of the NO, were eliminated by a corona reactor.
The chemical reactions under the plasma environment are
very complicated; hundreds of chemical reactions may be
involved. Among many reaction processes, the major role of
corona induced chemical reaction processes for C02, SO,, and
NO, gases is like those schematically shown in Fig. 13.
For the case of NO, conversion, the presence of NH3 will
cause the formation of free radical ammonia and reduction of
NO, by ionic and free radical reactions, which will convert to
NH4N03 (or N02-H20) molecules, as shown in Fig. 14. Also
NH3 will form NH4N03 aerosol in corona induced plasma.
For SO2 removal, direct dissociation by electrons is less
likely to occur than dissociation via free radical and ionic
reactions, which form highly oxidized molecules, as illustrated
in Fig. 12. In the presence of ammonia or moisture under
the ion induced atmosphere, SO2 is converted into H2S04 (or
S02-H20) aerosols [40]. Typical results are shown in Fig. 15
~ 5 1 [401.
,
B. De-CO,
The principle of the corona induced plasma decomposition
of the CO2 is shown in the following reactions:
CO,(v) + e
CO(v')
C &(VI')
0+e
+e
where v, v', and v" are the different vibrational states of
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500F_-
0
the molecules. Since electron impact decomposition via the
acceleration of the electron needs less energy, due to light
mass by external electric field, compared with thermal decompositions, it is possible to develop more efficient processes
compared with the combustion processes. Since electrons are
initiated reactions, classical thermodynamics based on the
dissociation energy concept cannot apply to the process due
to the quantum mechanics effects. The dissociation of the
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CHANG et al.: CORONA DISCHARGE PROCESSES
1163
e
1
0
(56)
Fig. 16. 'Reduced CO2 concentration as a function of CO2 percent mixture
in air before corona treatment [CO,] 0 at discharge current 0.5 mA and gas
flow rates 0.7 (l/min) (after [12]).
molecules by electrons depends on the rotational, vibrational,
and electronical excitation states of the molecules or molecule
ions and the energy distribution of the electron wave motions
[31], [32]. The vibrational and rotational excitations depends
on both gas and electron temperatures, and the other two
factors above depend on the electron temperatures. Hence,
with the vibrational and rotational excitations thermal loss
of molecules in a combustion gas, a part of the energy loss
normally becomes thermal radiations and can be recovered in
this electronic process. The dissociated product CO, 0, and
C will be react with H2, NH,, and OH to form a reforming
gas (Fig. 13). This reforming gas can be used in chemical
and metallurgical processes, or can be recirculated to the
combustion zone as a fuel or stored for later burning. The
former strategy may have some value in reducing sulfur and
nitrogen oxides produced by the boiler, while the latter strategy
would have value as a method for load-leveling-one way of
the store energy methods. In the presence of H20, and NH3,
Fig. 17. Formation and loss processes of CO2 in air with Ar mixtures
ion induced CO,,-H20 aerosols will be formed. They will be
collected by electrostatic precipitations or bag filters.
Higashi al. [29] and Weiss [46] have shown that CO2 zone as a fuel or stored for later burning. The former strategy
concentration in a N2-CO2 or pure CO2 gas can be reduced may have some value in reducing sulfur and nitrogen oxides
by dc and pulsed streamer corona discharges, respectively. produced by the boiler, while the latter strategy would have
Boukhalfa et al. [47] and Chang [48] observed that the reduc- value as a method for load leveling. In the presence of H20,
tion of CO2 was on the order of 10' ppm up to 100 p A corona ion induced CO,-H20 aerosols will be formed. They will be
discharge current, and a theoretical model was also proposed. collected by ESP or filters with a neutralizing agent such as
Maezono and Chang [15] have investigated the reduction of limestone.
CO2 gas at concentrations up to 10000 ppm in a laboratory
scale dc corona torch, as shown in Fig. 16. The mechanisms
Hazardous Emissions
of CO2 reduction in the presence of noble gases were also
studied.
Control of hazardous emissions from incinerators is an
The role played by argon is essential to enhance the for- important field. Control of heavy metals is a most critical
mation of electrons and the dissociation of CO,, as shown issue. Masuda et al. (491 and Urabe et al. [SO] have recently
in Fig. 13. Argon is initially ionized in the plasma and reacts conducted an experiment at an incineration plant and have
with COz. Direct dissociation by electrons will dominate in shown a very interesting result for the removal of Hg and
the CO producing process [15]. Also, dissociation via free HCl vapors, using dc and nanosecond pulsing energization.
radical and ionic reactions will take place to form CO, C, The reaction products form fine particles from the gas phase
0, and highly oxide or nitride molecules as shown in Fig. 17 reaction with ammonia, which is collected in a precipitator or
[15]. The converted CO can be recirculated to the combustion fabric filters.
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 19, NO. 6, DECEMBER 1991
1164
In the absence of moisture, the use of nanosecond negative
pulsing gives one order of magnitude lower power consumption for obtaining the same efficiency, compared to the dc
energization. The use of negative pulsing consumes much
less power for a wide range of temperature variations. It has
also been found that the removal efficiency is approximately
proportional to the production of ozone. When the air is
replaced with nitrogen gas, the efficiency is nil, indicating
that oxidation could be taking place due to 0 3 or 0 radicals
in the air. The removal efficiency is decreased by increasing
the moisture content and increasing the gas temperature, but
it is significantly improved by the addition of HC1 gas in the
gas stream regardless of pulse polarities and temperature. It is
speculated that Hg vapor is easily converted to form chloride
compounds under pulsing energization. This technology was
tested on the actual gas stream from the incinerator and a
collection efficiency as high as 80% was obtained, even within
a higher temperature environment (ca. 15OoC).
D. Toxic Gas Destruction
The single-stage, triangle-shaped, dc corona device referred to in Section IV was developed as an approach to
detoxify chemical warfare gases [12]. The destruction efficiency for a simulant gas was determined. Two levels of
dimethyl methyl-phosphate (DMMP) concentration (5 and
10 pg/L) and moisture content (approximately 1 and 10%)
were examined. The decomposition efficiency did not show a
significant difference at these levels. The destruction efficiency
did depend on the electrical sneakage and the residence
time. The maximum efficiency achieved for a single stage
device was 72%, whereas the multipoint-to-plane reactor only
had 15%. From the model study, it was concluded that the
optimum shape of the triangular corona device was with an
apex angle of about 60’ and a point-to-plate distance of
approximately 0.7 cm; these dimensions provide minimum
electrical sneakage [12].
Then a four-stage, triangle-shaped corona device was constructed, consisting of individual stages in series with flow
restricting devices placed between them to improve mixing.
The destruction efficiency of this device was 90%. However,
the efficiency tended to decrease as the DMMP deposits
increased. It was speculated that the decomposition product
was conductive and that current leakage along the dielectric
wall resulted in poor plasma distribution.
E. Volatile Organic Compounds
The conventional technologies for controlling VOC’s are
thermal incineration, satalytic oxidation, and carbon adsorption. These approaches are widely accepted but they have
associated limitations with respect to a range of applications
such as concentration and compound treatments, cost and
energy requirements, as emerging technologies.
Two laboratory scale atmospheric pressure plasma reactors,
ac energies ferroelectric (high dielectric ceramic) packed bed
reactor and a nanosecond pulsed corona reactor, were developed to evaluate the destruction efficiency of various VOC’s
at parts per million levels.
Complete destruction was obtained for toluene. Conversion
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of methylene chloride at 95% and trichlorotrifluoroethane
(known as CFS-113) at 67% was achieved for the plasma
reactors used. The conversion was dependent on the mean
electron energy (or peak pulse voltage and pulse repetition
rate) in the reactor and was also related to how strongly
halogen species were bonded with carbon [19], [51].
The corona processes may be ideal with application to
a wide range of industrial VOC’s that are present in low
concentrations in streams of variable flow rate [52].
VII. CONCLUDING
REMARKS
Applications of corona discharge induced plasmas can be
summarized as follows:
Corona discharge processes emphasize one of two aspects of the discharge: the ions produced or the energetic
electrons producing the plasma.
For corona as an ion source, applications exist in electrostatic precipitation, electrophotography, static control
in semiconductor manufacture, and atmospheric pressure
ionization-mass spectrometry.
For the corona-induced plasma reactor applications, control of NO,, SO,, CO,, toxic gases, volatile organic
compounds, hazardous emissions, and ozone synthesis
are among the applications being investigated.
For ozone synthesis by silent discharges, the operation
of silent discharge reactors at low temperatures or the
mixing of suitable noble gases are the subjects of current
investigations. Studies of modified reactor configurations
also show promise.
The mechanisms of plasma treatment of combustion
gases and ozone synthesis are not well understood, and
the potential for generating toxic and greenhouse gases
during the corona-induced plasma processes is quite real.
Reactive species in the plasma need to be identified
and their dependence on electron energy should be
determined if improved understanding of the reactions
is to be obtained.
The mechanisms of ion- or plasma-induced aerosol
particle formations and their growth rates are also poorly
understood and would bear further investigation.
ACKNOWLEDGMENT
The authors wish to thank Dr. D. S. Ensor, Prof. S. Masuda,
P. Mohant, and Prof. M. Rea for valuable comments and
discussions.
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Jen-Shih Chang (M’90) received the B Eng and
M Eng degrees from the Musashi Institute of
Technology, Japan, and the Ph D degree from York
University, Canada
During 1973-1974 he was Researcher at Centre de Recherches en Physique de I’Environment,
CNRS, France From 1975 to 1977, he was a Project
ScientistiAssistant Professor with the Department of
Physics, York University From 1978, he was an Assistant Professor at the Department of Engineering
Physics, McMaster University During 1985- 1986,
he was a Visiting Professor at the Department of Electrical Engineering at the
University of Tokyo and Musashi Institute of Technology. He is currently a
Professor at McMaster University
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 19, NO. 6, DECEMBER 1991
1166
Phil A. Lawless received the Ph.D. degree in
physics from Duke University in 1974.
In 1974, he joined the RTI, where he has worked
on and led research in many fields. For the past
seventeen years he has worked on aerosol research
projects, with particular emphasis on electrostatic
precipitator research. He has made significant contributions toward the development of mathematical
models for the precipitation process, including
prediction of the voltage-current characteristics of
wire-date and wire-oiDe Dreciuitators. He is the
authors of numerous publications on modeling of ESP’s.
Dr. Lawless is a member of the AAAR.
Authorized licensd use limted to: IE Xplore. Downlade on May 10,2 at 19:02 UTC from IE Xplore. Restricon aply.
Toshiaki Yamamoto received the M.S. degree in
energy engineering from the University of Illinois
in 1972 and the Ph.D. degree in mechanical engineering from the Ohio State University in 1979.
He is a Research Mechanical Engineer in the
center of aerosol technology of Research Triangle Institute (RTI) in Research Triangle Park, NC,
where he leads various projects relating to the
application of atmospheric pressure plasma technology, microcontamination control, computational
fluid dynamics, electrohydrodynamics, and indoor
air pollution research. Prior to joining RTI, he was an adjunct professor in the
Engineering Department as well as a research engineer at Denver Research
Institute at the University of Denver. He is the author of over 100 publications.
Dr. Yamamoto is a member of Sigma Xi, the ASME, IES, AAAR, IEJ, and
JUST.