PARTICULATE
CONTROL
HIGHLIGHTS
PARTICULATE
TECHNOLOGY
BRANCH
U.S. Environmental Protection Agency
Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, North Carolina 27711
EPA-600/8-77-020c
January 1978
ADVANCED CONCEPTS IN
ELECTROSTATIC PRECIPITATORS:
PARTICLE CHARGING
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EPA-600/8-77-020C
December 1977
PARTICULATE CONTROL HIGHLIGHTS:
ADVANCED CONCEPTS
IN ELECTROSTATIC PRECIPITATORS:
PARTICLE CHARGING
by
D. Pontius and W. Smith
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
Contract No. 68-02-2114
Program Element No. EHE624
EPA Project Officer: Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, N.C. 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, D.C. 20460
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ABSTRACT
The objective of this research program was to develop and verify an accurate theory of
particle charging for conditions that are typically found in industrial electrostatic precipitators.
A new theory was developed, in which the thermal motion of ions is assumed to dominate
the charging process. The theory was shown to agree with published experimental data to
within 15%. In order to further verify the new theory, experimental determinations of particle
charging were made using a mobility analyzer to find the end points of particle trajectories in
an electric field. For particles of 0.32 to 7 micrometers diameter, the agreement between
theory and experiment was found to be within 20%.
THE COVER:
The Lichtenberg figure shown in the illustration was
produced by placing a photographic emulsion in the
path of an electrical breakdown streamer occurring
during high intensity corona conduction. The split-
ting into very fine branches is characteristic of
breakdown conduction paths, especially near the
positive electrode. In many instances the perfor-
mance limits of an electrical corona system are
defined by its breakdown characteristic.
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CONTENTS
Abstract ii
Particle Charging Process 1
Laboratory Particle Charging Measurements 4
Applications 6
FIGURES
Figure 1. The basic precipitation process 2
Figure 2. Particle charging process in a corona discharge system 3
Figure 3. Laboratory apparatus for particle charging experiments 4
Figure 4. Schematic representation of laboratory apparatus for particle
charging experiments 5
Figure 5. Number of electrical charges per particle vs. charging field strength 5
Figure 6. Number of electrical charges per particle vs. ion density-residence
time product 6
Figure 7. Two-stage electrostatic precipitator concept 7
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ADVANCED CONCEPTS IN ELECTROSTATIC PRECIPITATION: PARTICLE CHARGING
The first successful application of the principles
of electrostatic precipitation to the removal of sus-
pended particulate material from industrial exhaust
gas streams occurred in the first decade of this cen-
tury. Since that time a technology has emerged
which now serves as an important element in the
control of air pollution from industrial sources.
The results of recent studies indicate that further
developments in electrostatic precipitation technol-
ogy may be derived from an improved understand-
ing of the fundamental processes involved.
The Particulate Technology Branch of the U. S.
Environmental Protection Agency Utilities and
Industrial Power Division is responsible for an on-
going program of research and development for the
purpose of exploring possible advances in the tech-
nology. The scope of this program ranges from
theoretical and laboratory studies of basic processes
to field demonstrations of new systems and tech-
niques.
The operation of electrostatic precipitators is
based upon the fact that a charged particle located
in an electric field will experience a force which is
proportional to the product of the strength of the
electric field and the quantity of electric charge
on the particle. In conventional electrostatic pre-
cipitators an electric field is established in such a
manner that the electrostatic force acting upon the
suspended particles is perpendicular to the direction
of flow of the flue gas through the precipitator.
Particles are thus driven out of the gas stream and
collected on large metal surfaces. Because the rate
of particle collection depends on the magnitude of
the electrostatic forces on the particles, any increase
in the electric field strength or in the particle
charging rate will result in an improved particle col-
lection efficiency (see Figure 1).
This report concerns theoretical and experimental
investigations into the particle charging process, and
the implications of the results of these studies on
the further development of the electrostatic precip-
itator technology.
PARTICLE CHARGING PROCESS
In an electrostatic precipitator, particles become
charged through collisions with ions produced from
gas molecules by an electrical corona discharge. A
segment of a conventional precipitator might be
viewed, in a simplified form, as a pair of vertical
parallel plates with evenly spaced vertical wires in
the plane midway between the plates (Figure 2).
The plates are electrically grounded and a high vol-
tage is applied to the wires. Because of the shape
and placement of the wire and plate electrodes, the
applied voltage produces an electric field between
them which is strongest near the wire. Some of
the gas molecules close to the wires are ionized by
the effects of the intense electric field. If the vol-
tage on the wire is negative, as in most industrial
precipitators, positive ions will be attracted toward
the wire, and negative ions will be repelled toward
the grounded plates. Since the ionization occurs
very near the wires, most of the space between the
plates will contain negative ions. The gas from
which dust particles are to be removed is forced to
flow horizontally between the plates of the precipi-
tator. Upon encountering the negative ions, the
particles become charged, and may then be driven
by the electric field toward the grounded plates.
It is easy to see how an ion may reach the sur-
face on an electrically neutral dust particle and be
deposited on it by a simple collision process. But
in order to provide for efficient electrostatic precip-
itation, it is generally necessary for each particle to
accumulate many times the charge of a single ion.
The addition of charges of the same polarity to a
charged particle requires the action of forces to
overcome the electrostatic repulsion between the
charged particle and the ions. In an electrostatic
precipitator, energy is available in two specific
forms to provide for particle charging. One source
of energy is the applied electric field, and the other
is the random thermal motion of the gas molecules
and ions.
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HIGH VOLTAGE
CORONA WIRES
ELECTRICALLY
GROUNDED PLATE
Figure 1. The Basic Precipitation Process. Particles charged by electrical
corona discharge current are forced toward the plates by a
strong electric field near each plate.
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Early in the development of particle charging
theories, electric field effects and thermal, or dif-
fusion, effects were dealt with separately. It was
found that the particle charging process may be
described adequately for particles less than about
0.2 /zm in diameter by a theory based entirely on
the thermal motion of the ions, without regard to
the presence of an electric field. Such an approach
is justified on the grounds that the thermal energy
of an ion is much greater than the energy which
can be derived from even a very strong electric field
acting on an ion in a gas. But the random thermal
motion involves frequent collisions between ions
and gas molecules, so that the average distance be-
tween collisions is only about 0.1 jum. For a
charged particle several micrometers in diameter
the effective range of the repulsive force on an ion
is many times greater than the mean free path of
the ion. On this larger scale, the longer-range,
directed force due to the applied electric field
provides the more effective mechanism for par-
ticle charging.
Separate theories for particle charging, each
limited in applicability, are useful in delineating
the nature of the operative charging mechanisms,
but for particle diameters between approximately
0.2 and 2.0 jum neither the field charging theory
the diffusion charging theory, nor a sum of the
two, provides satisfactory agreement with experi-
mental results.
A more comprehensive theory has been derived,
taking into account simultaneously the effects of
ion diffusion and the applied electric field. Al-
though the electric field makes a small contribution
to the kinetic energy of an ion, the most important
effect of the field is upon the distribution of ions
in the system. In the vicinity of a particle the
electric field consists of that resulting from the
applied voltage plus a contribution due to the
charge on the particle, polarized by the applied
field. The theoretical approach employed is to
determine the ion density distribution near a
particle in terms of the local electric field, and
then calculate statistically the rate at which ions
reach the particle due to their thermal velocities.
The theoretical charging rate cannot, in general,
be expressed in a closed, algebraic form, but re-
quires the use of a computer to carry out the
calculation.
The charging theory indicates that the total
charge accumulated by a particle is strongly
dependent upon the electric field strength, the
FREE
ELECTRONS
REGION OF CORONA GLOW
NEGATIVE IONS
Z
o
o
< a
O Ul
— Q
CC
\-
UJ
UJ O O.
Figure 2. Particle charging process in a corona discharge system.
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Figure 3. Laboratory apparatus for particle charging experiments.
diameter of the particle, the numerical density of
ions, and the residence time of the particle in
the charging region. The latter two variables in-
variably occur as a product in charging theories.
Thus, the ion density-residence time product is
usually treated as a single parameter representing
the total exposure of a particle to ions as it passes
through the charging region. Other variables
which have a significant effect on the particle
charging process include the gas temperature, the
electrical mobility of the ions in the gas, and the
dielectric constant of the paniculate material.
LABORATORY PARTICLE CHARGING
MEASUREMENTS
In order to provide sufficient data for validation
of the theory, a series of charge measurements
must be made on particles which have been sub-
jected to accurately controlled charging conditions.
The apparatus requirements include a source of
uniformly-sized particles, a device for charging the
particles, and a system for measuring the average
charge per particle. The experimental equip-
ment is shown in Figure 3, and a schematic
diagram is given in Figure 4 that identifies
the main components.
In the experiments done at Southern Research
Institute, particles were generated by two methods.
One was a dispersal of pre-sized plastic spheres in
a liquid suspension by means of an atomizer. The
second method was based on a device in which a
fine liquid stream containing dissolved aerosol
material was broken up into uniform droplets by
passage through an orifice vibrating at an ultra-
sonic frequency. A drying chamber was used with
each of these systems to evaporate the excess
liquid, leaving an aerosol of dry, nearly uniform
particles.
The charging device was a specially designed
cylindrical corona system, in which provisions,
were made for separate control of ion density
and electric field strength in the particle charging
volume.
Particle charge measurements were made by
means of a mobility analyzer, an instrument which
permits accurate determination of the end points
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PARTICLE
CHARGER
Figure 4. Schematic representation of
laboratory apparatus for
particle charging experiments.
of the path of a charged particle moving through a
smoothly flowing gas stream under the influence
of an electric field perpendicular to the direction
of gas flow. The trajectory of a particle depends
on both the aerodynamic properties of the par-
ticle and the electrostatic force on the particle.
Given the gas flow rate, the voltage applied to the
mobility analyzer electrodes, the diameter of the
particle and the end points of the trajectory, the
charge on the particle can be determined by a
straightforward calculation.
In a series of experiments, particles were
charged under various conditions, and the resulting
charge measurements were compared with theoret-
ical calculations for the same conditions. Values
of electric field strength, E, and the product of
ion density and residence time, Nt, were selected
to span the range of conditions encountered in
actual electrostatic precipitators. Measurements
were carried out on panicles ranging from 0.32 to
7.0 micrometers (Aim) in diameter.
The effects of some of the more significant
charging parameters are illustrated in Figures 5 and
6. The total charge per particle as a function of
electric field strength for polystyrene latex particles
of four different diameters is presented in Figure 5.
Although many of the experimentally determined
values of particle charge, indicated by the plotted
symbols, fall slightly short of the theoretical curves
for the larger particles, a steady rise in the average
charge per particle as the electric field strength in-
creases is evident. By comparing the four curves in
this graph at fixed values of electric field strength,
it may also be seen that the average charge per par-
ticle is approximately proportional to the square of
the particle diameter.
In Figure 6 the particle charge is plotted against
Nt for four values of the electric field strength.
The particles used for experimental verification were
dioctyl phthalate droplets, 1.4 /urn in diameter. The
theory appears to predict values of particle charge
which are too low where the electric field is large,
500
100
o
-------
1000
800
600
S 500
c
f 400
I
I 300
O)
111"
cc
I 200
100
80
60
50
i i i r i
4 6
Mt. sec/m3 x 1012
10
Figure 6. Number of electrical charges per
particle vs. ion density-residence
time product.
and too high where a small value of electric field
is used. But for both experimental and theoretical
results there is a distinct saturation charging effect,
evidenced by a reduced slope in the curves as Nt
increases. Thus, if the current density, and hence
the ion density, in an electrostatic precipitator is
large, the time required for a particle to become
charged to an essentially maximum value may be
quite small. In many full-scale precipitators the
charging process may be considered complete within
the first few inches of the active length of the
system.
APPLICATIONS
An accurate and complete particle charging
theory, backed by thorough experimental verifi-
cation, can be useful as a diagnostic tool to deter-
mine areas of possible improvement in existing
electrostatic precipitators and as a foundation for
innovations in the design of new systems.
Some types of dusts and fumes have proven to
be especially difficult to collect by precipitation.
Among these are particulate materials exhibiting
high electrical resistivity and gaseous effluents
carrying very large numerical densities of fine par-
ticles. Both problems are concerned to some ex-
tent with difficulties in maintaining adequate
charging conditions.
Materials of high electrical resistivity, when col-
lected on the plates of a precipitator, form localized
regions of high electric field strength and corona
discharge near the surface of the plate. The ions
injected by this back corona are opposite in polar-
ity from those generated at the corona wire. The
presence of ions of both positive and negative
polarities in the space between the corona wires
and the plates results in extreme degradation of
particle charging effectiveness.
Large numerical densities of very fine particles
interfere with the charging process by modifying
the electric field in the charging region. The
motion of a charged particle in an electric field is
very sluggish in comparison with the drift velocities
of the ions. If the number of particles per unit
volume is large enough that a substantial part of
the electrical charge between the corona wires and
plates resides on the particles, then the slow move-
ment of the particles will result in a buildup of
space charge in areas where ions would be swept
out quickly by the action of the electric field. As
the nearly stationary charge distribution increases
in the vicinity of the corona wires, the electric
field is diminished, resulting in reduced ion gener-
ation. In severe cases the corona discharge may be
quenched entirely.
The existence of problems of the sort described
in the foregoing paragraphs complicates the engi-
neering work involved in electrostatic precipitator
design. Predictions of performance based simply
on total effluent gas volume and precipitator plate
area may fail completely if the specific character-
istics of the particulate material are not taken into
account.
In coal-fired power plants, the increased use of
low-sulfur western coal is accompanied by problems
in the collection of high-resistivity fly ash.
Metallurgical processes produce a wide variety
of dusts and fumes. It is therefore necessary,
in designing an electrostatic precipitator for a
new application, to characterize the effluent as
completely as possible. Conventional approaches
to the design problem may be inappropriate for
some sources of pollution.
In some applications a relatively simple modifi-
cation of the conventional technology may be
sufficient. For example, the use of specially
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designed corona discharge electrodes in place
of straight wires may provide an increased corona
current and concomitantly higher ion density. For
especially troublesome dusts, a two-stage precipi-
tator (Figure 7) may be effective. In such a device
the charging and collecting processes are separate.
By using a high current density in the charging
stage, the particles can be brought to a high state
of charge in a very short part of their paths through
the system. The charging stage could, therefore,
be relatively small, and extraordinary measures
could be taken to control particle charging prob-
lems which cannot be dealt with economically in
a conventional single-stage precipitator. The col-
lecting stage of such a system could be operated
at a low current density and high electric field
strength, since charging is already accomplished.
A concept of this type offers flexibility in design,
particularly in the methods which may be used to
deal with problem dusts in the charging stage.
As the scope of electrostatic precipitator appli-
cations is broadened the results of fundamental
investigations into the particle charging and col-
lection processes will form a basis for the develop-
ment of the necessary new techniques. To this
end the Particulate Technology Branch of the
U. S. Environmental Protection Agency is contin-
uing to identify problem areas and to pursue the
research necessary to deal effectively with all
aspects of particulate emission control from station-
ary sources.
* A more detailed description of the charging studies is
given in: "Fine Particle Charging Experiments", D. H.
Pontius, Larry G. Felix, J. R. McDonald, and W. B.
Smith. EPA-600/2-77-173. August 1977.
PRECHARGER COLLECTOR
(HIGH CURRENT DENSITY) (HIGH ELECTRIC FIELD STRENGTH)
DIRTY
AIR
\
\
\
CHARGED
PARTICLES
__
\
\
\
CLEAN
AIR
Figure 7. Two-stage electrostatic precipitator concept.
1
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TECHNICAL REPORT DATA
(Please read launictions on the reverse before completing)
i. REPORT NO.
EPA-600/8-77-020c
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Control Highlights '.
Advanced Concepts in Electrostatic Precipitators:
Particle Charging
5. REPORT DATE
December 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
D. Pontius and W. Smith
8. PERFORMING ORGANIZATION REPORT NO.
SORI-EAS-77-676
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
68-02-2114
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
-WOVERED
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES
919/541-2925.
5 IERL-RTP project officer is Dennis C. Drehmel, Mail Drop 61,
16. ABSTRACT
rep0rt; giv6s highlights o.f an EPA research program aimed at devel-
oping and verifying an accurate theory of particle charging for conditions that are
typically found in industrial electrostatic precipitators. A new theory was developed,
in which the thermal motion of ions is assumed to dominate the charging process. The
theory was shown to agree to within 15 percent of published experimental data. To
further verify the new theory, experimental determinations of particle charging were
made, using a mobility analyzer to find the end points of particle trajectories in an
electric field. For particles of 0.32 to 7 micrometers diameter, the agreement
between theory and experiment was within 20 percent.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Charged Particles
Electrostatic Precip-
itators
tons
Ionic Mobility
Particles
Charging
Dust
Air Pollution Control
Stationary Sources
Particle Charging
Thermal Motion
Particulates
13B
20H
07D
20L
11G
8. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
11
20. SECURITY CLASS {Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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