§
V
ro
LU
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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-600/8-77-020a
December 1977
RESEARCH ON ELECTROSTATIC
PRECIPITATOR TECHNOLOGY
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental Protec-
tion Agency, have been grouped into nine series. These nine broad categories were
established to facilitate further development and application of environmental tech-
nology. Elimination of traditional grouping was consciously planned to foster technology
transfer and a maximum interface in related fields. The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECHNOLOGY
series. This series describes research performed to develop and demonstrate instrumen-
tation, equipment, and methodology to repair or prevent environmental degradation from
point and non-point sources of pollution. This work provides the new or improved tech-
nology required for the control and treatment of pollution sources to meet environmental
quality standards.
REVIEW NOTICE
This report has been reviewed by the participating Federal Agencies, and approved for
publication. Approval does not signify that the contents necessarily reflect the views and
policies of the Government, nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
This document is available to the public through the National Technical Information
Service, Springfield, Virginia 22161.
THE COVER:
The cover photograph shows a large electrical
power plant with electrostatic precipitators
installed for gas cleanup. Sixteen precipitators
clean the gas from each unit before it goes
into the atmosphere.
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EPA-600/8-77-020a
December 1977
PARTICULATE CONTROL HIGHLIGHTS:
RESEARCH ON ELECTROSTATIC
PRECIPITATOR TECHNOLOGY
by
S. Oglesby, Jr. and G. Nichols
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
A major research program on electrostatic precipitator technology is directed toward
improving the performance of precipitators in controlling industrial participate emissions,
notably fly ash from coal combustion in electric power plants. Techniques have been
developed for sampling stack gas, measuring particle size distribution of fly ash, and
determining collection efficiencies for particles 0.01 - 10 urn in diameter. Techniques for
measuring electrical resistivity of fly ash have been assessed. Relationships between
electrical effects, such as reverse corona, caused by high resistivity of the deposited fly ash,
have been investigated. The influence of particle size and chemical composition of the fly ash
on both the resistivity and dielectric strength of the deposited fly ash has also been studied.
Relationships have been established between resistivity and chemical composition of fly ash,
especially its alkali metal content, for precipitator operating temperatures below about
250 °C. On the basis of these relationships, a mechanism for ionic surface conduction has been
proposed that complements the ionic mechanism in bulk conduction in fly ash particles at
higher operating temperatures. The efficacy of conditioning fly ash by adding sulfur trioxide
to flue gas in order to lower fly ash resistivity was established in trials at electric power
plants. Reentrainment of particles from deposited fly ash has been investigated in relation to
precipitator rapping procedures and gas flow distribution. A mathematical model of the elec-
trostatic precipitation process has been developed which uses fundamental relationships to-
gether with measurements of precipitator geometry, electrical conditions, and particle size
distributions to calculate collection efficiency.
11
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CONTENTS
The Precipitation Process 1
Measurement of Collection Efficiency 3
Techniques for Measuring Particle-Size Distribution 3
Electrical Effects, Particle Size, and Resistivity 6
Resistivity and Chemical Composition of Fly Ash 7
Techniques for Measuring Electrical Resistivity 9
Conditioning of Fly Ash 10
Rapping Re-entrainment of Fly Ash 11
A Mathematical Model of Electrostatic Precipitation 12
References 13
Other Reports Available 14
FIGURES
Figure
1 Schematic diagram of an electrostatic precipitator collecting
dust [[[ iv
2 Measured and computed collection efficiency as a function of
particle size for electrostatic precipitator, Plant 1 .................... 4
3 Measured and computed collection efficiency as a function of
particle size for hot-side electrostatic precipitator, Plant 2
4 Schematic diagram , operation of cascade impactor ........................ 5
5 Electrical resistivity of fly ash as a function of temperature ............... 8
6 Point-to-plane resistivity probe equipped for thickness
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HIGH-VOLTAGE
SUPPLY
PLATE
RAPPING
SYSTEM
DUST
LADEN
AIR
GROUNDED
COLLECTION
ELECTRODES
CLEANED
AIR
CORONA WIRES
DUST COLLECTION
HOPPERS
Figure 1. Schematic diagram of an electrostatic precipitator collecting dust.
IV
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RESEARCH ON ELECTROSTATIC PRECIPITATOR TECHNOLOGY
A comprehensive research program on
electrostatic precipitator technology is be-
ing conducted for the U.S. Environmental
Protection Agency by Southern Research
Institute. It emphasizes the control of
airborne particulate emissions from station-
ary sources.
The program, which was begun in 1969,
developed from the Agency's recognition
that air pollution by particulate matter from
industrial sources is an increasingly serious
problem. One of the major pollutants is fly
ash from the combustion of coal. A total of
more than 3 million tons per year of fly ash
is discharged into the atmosphere from coal-
burning electric utility power plants, about
one-sixth of the industrial particulate
emissions in the United States.
Electrostatic precipitators are the most
widely used of the high-efficiency devices
for the control of fly ash emissions from
electric utility power plants. The EPA re-
search was undertaken to explore ways of
improving the performance of electrostatic
precipitators, specifically by providing data
that can be used for precipitator design on a
sound engineering basis. This report pre-
sents some of the results that have been
obtained.
The general structure of the EPA re-
search program was outlined in a systems
study carried out early in the program. The
program structure takes into account the
steps in the precipitation process and the
corresponding parameters which thus can
influence overall performance in collecting
industrial particulate emissions. 1~1*
THE PRECIPITATION PROCESS
In an electrostatic precipitator, the
force that is used to separate a particle
from the gas stream in which it is suspended
results from an electric charge on the
particle in the presence of an electric field
(see Figure 1). The particle is charged by the
attachment of gas ions which are generated
by an electrical corona discharge. In most
precipitators used for collecting industrial
dusts such as fly ash, a negatively charged
corona is produced by applying a rectified
high voltage to rows of wires or metal strips
suspended vertically in a horizontal gas
flow. An electric field is established be-
tween the wires and grounded metal plates,
which are also suspended vertically, parallel
to the rows of wires and parallel to the
direction of gas flow. In the electric field
between the wires and plates, the charged
particles move to the plates, where they are
deposited. As the layer of deposited par-
ticles builds up on the collection electrodes,
it is periodically knocked off by a rapper or
vibrator on the electrodes, and the parti-
culate material falls into a hopper for
removal.
The performance of an electrostatic pre-
cipitator is affected by the concentration
and size distribution of the particles being
collected, the electrical resistivity and co-
hesivity of the layer of collected particles,
and the uniformity of the gas flow.
1
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The mechanism by which the fly ash
particles are electrically charged depends on
the particle size, in addition to other factors
such as the magnitude of the electric field
and the ion density. Large particles (those
larger than a few micrometers in diameter)
are charged by the impact of gas ions
travelling along electric field lines, and the
charging ceases when the particle reaches a
saturation charge that provides a sufficient
repelling force. For normal operating cur-
rents, the saturation charge is reached
rapidly, within a fraction of a second. Small
particles are charged more slowly, at a rate
dependent on diffusion of the gas ions to the
vicinity of the particle.
The electric field in a precipitator is
determined by the geometry of the elec-
trode system (the corona field being high
near the corona electrode and diminishing
with distance toward the collection elec-
trode) and the space charge in the inter-
electrode region. The magnitude of the
space charge is established by the number of
elemental charges in this region. Both gas
ions and charged particles contribute to the
charge.
The efficiency with which the particles
are collected also depends on the particle
size. Even though an electrostatic pre-
cipitator can achieve a high overall col-
lection efficiency, it will typically have a
lower efficiency for some fractions of the
smaller particles in the flue gas. As an
example, the measured fractional collection
efficiency of a precipitator can decrease
from 99.9% for particles above 1 Mm in
diameter to 95% for particles 0.5-1 ym in
diameter, and then increase to 99% for
particles less than 0.3 ym in diameter.
The minimum in collection efficiency for
particles 0.5 - 1 ym in diameter can be
explained in terms of the relative contri-
butions of particle charging by different
processes.
The collection efficiency is also influ-
enced by the electrical resistivity of the fly
ash. The electrical current from the corona
discharge passes through the layer of depos-
ited fly ash on the collection electrode, and
the voltage drop across the layer depends on
the resistivity of the fly ash, the current
density, and the thickness of the layer. If
the resistance of the dust layer is too high
(about 1011 ohm-cm), the electric field in
the layer can become high enough to exceed
the field strength for electrical breakdown
or corona discharge. These occurrences limit
the voltage and current that can be used,
and hence limit performance of the precipi-
tator.
The electrical resistivity of fly ash de-
pends principally on its chemical composi-
tion, the flue gas composition, and the
temperature at which the precipitator
operates. The chemical composition of the
fly ash in turn depends on the composition of
the coal and accessory minerals from which
it is produced in combustion, the type of
boiler used, and the combustion tempera-
ture. Most of the sulfur in the coal appears
as sulfur dioxide in the flue gas, and a small
amount of the sulfur dioxide is oxidized
further to sulfur trioxide, which reacts with
water vapor in the flue gas to produce
sulfuric acid vapor. At the temperatures at
which most precipitators operate, 150°C or
below, some of the sulfuric acid is adsorbed
on the surface of the fly ash particles. The
adsorbed acid lowers the electrical resistiv-
ity of the fly ash, and consequently lowers
the resistivity of the layer deposited on the
collection electrode. As a rule, coals with
sulfur contents of 1.5% or more produce fly
ash with a sufficiently low resistivity for
good collection; those with less than 1% will
present problems in satisfactory collection.
Problems in collecting fly ash with high
resistivity can be alleviated in some instan-
ces by adding sulfuric acid, sulfur trioxide,
or other chemical compounds to the flue
gas. Alternately, the electrostatic precipita-
tor may be installed in the flue gas duct
upstream of the air heater, where it will
operate at 250-^00°C. At these tempera-
tures, the resistivity of most fly ashes is
sufficiently low that the electric current in
the precipitator is not limited by fly ash
resistivity.
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The collection efficiency of an electro-
static precipitator can also be degraded by
losses in rapping the collection electrodes.
The transfer of the fly ash deposit to the
hopper provides the chance for loss of
particles by entrainment in the gas stream.
Although the extent of the loss is moderated
by the cohesivity of the fly ash, entrainment
can be responsible for half of the total
emissions from a precipitator.
Other sources of loss and degraded per-
formance are uneven gas flow distribution
and passage of particles through the non-
electrified space around the electrodes of
the precipitator.
MEASUREMENT OF COLLECTION EF-
FICIENCY
To obtain performance data on operating
precipitators and to establish the extent to
which selected sampling and analytical tech-
niques can be used for this purpose, tests
were made on full-scale industrial electro-
static precipitators used for the control of
fly ash emissions from electric power
plants.5 Two of the installations selected
were: (1) a precipitator collecting fly ash at
an electric utility power plant burning an
eastern coal (Gorgas Power Station, Ala-
bama Power Company); (2) a precipitator for
fly ash collection installed on the hot gas
side of the air heater in an electric utility
power plant burning a low-sulfur western
coal.
Concentrations of fly ash were measured
at the inlet and outlet of each electrostatic
precipitator. The average overall mass col-
lection efficiencies were about 99.6% at
Plant 1 and about 99.2% at Plant 2. Parti-
cle-size distributions were also measured at
each inlet and outlet and used for calculat-
ing fractional collection efficiencies. The
results are shown in Figures 2 and 3.
These figures also show the fractional
collection efficiencies of the precipitators
calculated with the use of a theory-based
computer model of the electrostatic precipi-
tation process. The use of this model, which
is discussed in more detail in a later section
of this report, involved the preparation of
graphs of computed overall mass collection
efficiency as a function of specific collect-
ing area (area of the collection electrode
per unit volume of gas flow) with the
precipitator geometry, electrical conditions,
and the measured inlet particle-size distri-
butions used as input data to the computer
program for each installation. In addition to
these predictions from theory, the effect of
variation in gas velocity distribution over
the cross-section of the gas stream was
taken into account by including values for
the standard deviation ag in the gas velocity
distribution. The computer program also
included procedures for estimating losses in
collection efficiency due to gas by-passage
of the electrified regions of the precipitator
and for re-entrainment of particles into the
effluent gas stream from the deposited fly
ash.
The curves in Figures 2 and 3 represent
fractional collection efficiency values that
are predicted by the computer model with
OP assumed to be 25%, a value that is
typical for normal operation of an elec-
trostatic precipitator, but with no allowance
for loss of collection efficiency from gas by-
passage or fly ash re-entrainment.
Figure 2 indicates reasonably good
agreement between the predicted and ob-
served values characterizing the perform-
ance of the precipitator in Plant 1. In
contrast, most of the fractional collection
efficiency values measured for the preci-
pitator in Plant 2 were considerably lower
than the calculated theoretical values. The
difference can be interpreted as the result
of a 10-20% loss of collection due to gas by-
passage and re-entrainment over each of
three stages in the precipitator.
TECHNIQUES FOR MEASURING
PARTICLE-SIZE DISTRIBUTION
In evaluating the performance of these
and other electrostatic precipitators, par-
ticle-size distributions were measured with
inertia! cascade impactors and optical coun-
ters for particle diameters greater than
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o
z
LU
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99.99
99.98
99.9
99.8
99
95
90
10
1 I I I
0.1
• I I i I I
THEORETICAL -
A IMPACTOR
OPTICAL PARTICLE
° COUNTER
... DIFFUSION
BATTERY
0.5 1
PARTICLE DIAMETER,
10
Figure 2. Measured and computed efficiency as a function of particle
size for precipitator installation at Plant 1.
o
UJ
o
99.99
99.98
99.9
99.8
99
95
UJ
z
o
o 90
o
o
10
0.1
A IMPACTOR
O OPTICAL PARTICLE COUNTER
D DIFFUSION BATTERY
0.5 1
PARTICLE DIAMETER, urn
10
Figure 3. Measured and computed collection efficiency as a function
of particle size for hot-side electrostatic precipitation
installation, Plant 2.
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about 0.3 vim- F°r particle diameters of
0.005 ym to 0.3 urn, diffusion batteries were
used in conjunction with condensation nuclei
counters.
The mechanism by which a cascade
impactor operates is illustrated in Figure 4.
The measurement of particle size is based
on the inertia! properties of the particle
suspended in the gas stream. For each stage
in a cascade impactor, the gas stream passes
through an orifice and forms a jet which is
directed toward an impaction plate. For
each stage there is a characteristic particle
size, the cut diameter, which has a 50%
probability of impaction. Most of the
smaller particles will follow the gas stream
to the subsequent stages where the jet
velocities are progressively higher. After
operation for the appropriate length of time,
the catch on each stage of the impactor is
weighed and the particle-size distribution is
calculated.
In evaluating an electrostatic precipi-
tator, a low-volume flow impactor is used at
the inlet of the precipitator, where the dust
concentration is high, and a high-volume
impactor is used at the outlet, where the
dust concentration is low. Inlet and outlet
measurements are usually made at the same
time.
The optical counters used were designed
to measure the scattered light from single
particles in highly diluted gas samples. The
particle size is determined from the ampli-
tude of the scattered light pulses and the
concentration of the particles (by number,
not mass) from the pulse rate.
The use of diffusion batteries for sizing
particles is based on the circumstance that
as the gas flows through a narrow channel
the suspended particles will diffuse to the
channel walls at a predictable rate, the
magnitude of which depends on the particle
size, for a given battery configuration. The
particle concentrations (by number) in the
gas at the inlets and outlets of the batteries
are measured with condensation nuclei coun-
ters. The action of these counters is based
SMALL
PARTICLE
Figure 4. Schematic diagram, operation of
cascade impactor.
on the condensation of supersaturated water
vapor on the particles and measurement of
the attenuation of a light beam by the
resulting fog.
Concurrently with the demonstration
that cascade impactors can be used in
evaluating the performance of electrostatic
precipitators, improved techniques for their
use were being developed. 6'7
An experimental study was made of the
precision, or reproducibility, of measuring
particle-size distribution by impactors.
Since the amount of dust that can be
collected within a reasonable test time
usually weighs less than 10 milligrams per
stage, it is impractical to determine the
weight of the catch by weighing the rela-
tively heavy metal collection plate before
and after sampling. Therefore, a glass fiber
mat was placed on the collection plate to
serve as the collection substrate.
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Before this substrate was recommended
for use in field tests, it was tested in the
laboratory under conditions simulating those
encountered in sampling stack gases. When
clean air (temperature 120°C) was drawn
through an impactor containing glass fiber
mats for 6 hours, the mats lost 0.1 - 0.2 mg
each, a value that was considered tolerable
for most tests in which the impactor would
be used. A more serious change in weight
was a gain of up to 5 mg per mat that was
observed on exposure of the glass fiber
substrates to flue gas from coal combustion.
Chemical analysis showed that most of the
weight gain was soluble sulfate salts,
evidently formed by reaction of sulfur
oxides in the flue gas with alkaline compo-
nents of the glass fiber. Exposure of the
glass fiber mat for several hours to flue gas
from which the particulate matter had been
removed by filtration reduced the weight
gain during subsequent use by a factor of
about ten.
Another error in sampling flue gas can be
produced by electrostatic forces due to
charges on the suspended particles. Pre-
liminary experiments on neutralizing the
particle charges indicated that this is more
of a problem for collection on bare metal
plates in an impactor than for collection on
glass fiber substrates.
When an impactor is operated at a flow
rate higher than some critical value, errors
can result from particle bounce or jet
scouring, with subsequent collection of par-
ticles on the wrong stages. Laboratory tests
were conducted with monodisperse aerosols
to determine the values of jet velocity that
allowed satisfactory deposition on impactor
stages.
Cyclones have been used less than im-
pactors for making particle size distribution
measurements; they are bulky and give less
complete separation than impactors. How-
ever, they are useful when larger samples
are needed, e.g., for chemical analysis. An
example is a system of three cyclones that
were designed for collection of samples of
fly ash having average particle sizes in the
respirable range (below 3 urn). The cyclones,
with cut points of 0.5 um, 0.95 ym, and 2.6
um, were mounted in series with a back-up
filter. The system was designed for a sample
flow rate of 28 liters/mm and will fit
through a 15-cm diameter port.
ELECTRICAL EFFECTS, PARTICLE SIZE,
AND RESISTIVITY
Electrical effects in an electrostatic
precipitator were studied in a laboratory
wire-plate apparatus that simulated some of
the behavior of a precipitator when its
performance is governed by the electrical
characteristics of the layer of collected
dust.8
The dust layer affects the electrical
behavior of the precipitator by introducing a
resistance element with non-linear char-
acteristics into the electrical circuit. When
electrical breakdown of the dust layer
occurs, the resulting back corona discharge
limits precipitator performance: it reduces
the voltage and current at which the pre-
cipitator can operate without sparkover.
The laboratory experiments included the
measurement of the corona characteristics
of layers formed from different particle-
size fractions of fly ash, such as those
produced in the fractionation of fly ash that
occurs as the flue gas passes through the
precipitator.
Particle size of the fly ash and the
porosity of the collected layer of fly ash
affected precipitator operation indirectly
through changes they caused in electrical
resistivity of the collected layer. No rela-
tion was found between dielectric strength
and resistivity.
Voltage-current curves measured in the
wire-plate apparatus showed that the fine
particle size had the highest sparkover
voltage and the lowest resistivity.
The variation in resistivity with particle
size was large enough that the resistivity of
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the fly ash would be decreased by a factor
of two, with a corresponding increase in
operating current density, as the fly ash is
fractionated on its passage through a preci-
pitator.
In the laboratory apparatus, peak current
densities for the formation of back corona
were within about 20% of the value at which
the electric field in the collected dust would
have exceeded the dielectric strength of the
dust layer. A survey of full-scale electro-
static precipitator installations collecting
fly ash showed operating current densities
varying over a wide range. The corre-
sponding values of electric field in the
collected fly ash layer in some installations
would have exceeded the dielectric strength
of the layer.
RESISTIVITY AND CHEMICAL COMPOSI-
TION OF FLY ASH
Since the electrical resistivity of fly ash
is one of the most critical parameters
affecting its collection in an electrostatic
precipitator, considerable effort has been
devoted to exploring the basic mechanisms
involved in electrical conduction in fly ash
and specifically the relationships between
conductivity and chemical composition of
the ash.
It had previously been shown that in
precipitators operating at flue gas tempera-
tures above about 250 °C, electrical conduc-
tion in fly ash takes place through the bulk
of the particles and depends on an ionic
mechanism.9 Sodium ions are the principal
charge carriers, with lithium ions carrying a
small amount of the current. At flue gas
temperatures below about 150°C, the path-
way of electrical conduction appears to be
along the ash surface and the conduction is
influenced by the alkali metal concentration
in the ash and the flue gas species adsorbed
on the surface. At intermediate tempera-
tures of 150° to 250° C, both surface and
volume mechanisms contribute to conduc-
tion. With increasing temperature, the
surface resistivity increases and the volume
resistivity decreases. These effects are
shown in Figure 5.
In order to elucidate the mechanism of
surface conduction, the effect of the chemi-
cal composition of fly ash on its electrical
resistivity at temperatures below 250°C was
studied. l °
Samples of fly ash collected from flue
gas from the combustion of representative
eastern and western coals were examined.
Resistivities were measured at voltage grad-
ients of about 400 V/cm at 60 to 250°C in a
controlled atmosphere of air containing
about 9 vol-% water. A correlation was
established between the resistivity and the
combined contents of sodium and lithium in
the ash. Transference experiments on fly ash
samples subjected to a 2000 V/cm voltage
gradient for several hundred hours at 60° C
in air containing about 9 vol-% of water
showed appreciable migration of lithium,
sodium, and in some instances, potassium
ions to the negative electrode. The major
portion of the charge transported could be
accounted for by the change in composition.
Cross correlations of the resistivity data
indicated that iron in the ash accelerated
the release of potassium ions, perhaps by
promoting the dissolution of the ash surface.
The results suggest a mechanism for
surface conduction in which certain chem-
ical species react on the surface of the fly
ash particle with an agent such as water in
the surrounding atmosphere to release alkali
metal ions from the surface to serve as
charge carriers. The number of ions released
can be expected to depend on the concentra-
tion of ions available, the chemical dura-
bility of the ash in the hostile environment,
the temperature, and the types and
concentrations of environmental species
brought into contact with the ash particle
surface.
The conducting pathway can be estab-
lished even at temperatures considerably
above the dew point of the flue gas, _i.e.,
above the temperature at which water or
other components of the gas would begin
condensing to liquids.
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v io11
•§
I-
>
1010
10*
108
2.8
84
183
2.4
144
291
2.0
227
440
1.6
352
666
1.2
560
1040
1000/T(°K)
°C
°F
TEMPERATURE
Figure 5. Electrical resistivity of fly ash as a function
of temperature.
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TECHNIQUES FOR MEASURING ELECTRI-
CAL RESISTIVITY
As part of the supporting work that had
to be done for the tests of precipitator
performance, it was necessary to make sure
that suitable equipment and techniques were
available for measuring the electrical resis-
tivity of collected dust (primarily fly ash).
An assessment of equipment and techniques
for this use was made.] 1
The disk-electrode apparatus specified in
the American Society of Mechanical
Engineers Power Test Code 28 was judged to
be satisfactory for laboratory measurement
of electrical resistivity.
Several probe designs are available for
measuring the electrical resistivity of fly
ash in installed electrostatic precipitators,
the measurements being made either on fly
ash collected in the flue duct or on fly ash
collected immediately outside the flue duct
from a sampled gas stream.
A point-to-plane probe designed at
Southern Research Institute is illustrated in
Figure 6. This probe can be inserted in the
flue for collection of fly ash and measure-
ment of its electrical resistivity in the duct.
The fly ash is deposited in the cell electro-
statically.
Other instruments that are available are:
-The Simon-Carves instrument. The dust
is collected in a small cyclone and com-
pacted into a measurement cell by a
vibrator. Designs are available for use in or
out of the stack.
-The Kevatron apparatus. The dust is
collected in a small wire-pipe electrostatic
precipitator, operated out of stack, and
dumped into a measurement cell.
-The Lurgi probe. The dust is collected
electrostatically and its resistivity measured
in place. It can be operated in or out of the
stack.
HIGH VOLTAGE
CONNECTION
DIAL INDICATOR
PICOAMMETER
CONNECTION
MOVABLE
SHAFT
STATIONARY
POINT
GROUNDED
RING
Figure 6. Point-to-plane resistivity probes
equipped for thickness measurement.
None of the collection devices gives a
sample with a particle-size distribution
representative of the distribution of the
dust particles in the flue duct. Neither
the cyclone nor the electrostatic preci-
pitator is an efficient collector of fine
particles, so the particle size distribution
in the sample is biased toward the larger
particles.
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Electrostatic collection of the dust in
the Southern Research Institute probe and
the Lurgi probe can be expected to produce
some alignment of the dust particles and a
denser deposit than those in the other
instruments.
The measurements in the Kevatron and
Simon-Carves instruments are made at rela-
tively low electric fields, whereas the mea-
surements in the Southern Research
Institute probe are made at fields near
electrical breakdown.
These differences are sufficient to
explain the 10-fold variation in resistivity
values reported by different investigators.
CONDITIONING OF FLY ASH
Treating, or conditioning, fly ash with
chemicals in the flue duct to increase the
extent of its collection in an electrostatic
precipitator was investigated on full-scale
power-plant installations.12 The chemical
agents used were sulfur trioxide (or sulfuric
acid) and ammonia. An attempt was made to
find circumstances under which the chemi-
cals were most effective.
The results of the tests indicated that
various methods for injecting concentrated
sulfuric acid, anhydrous sulfur trioxide, or
sulfur trioxide from the catalytic oxidation
of sulfur dioxide, were equally effective
when the equipment was properly engineered
and maintained. The injection site in the
flue gas duct could be upstream from the
electrostatic precipitator, upstream from
the combination of a precipitator and a
mechanical fly ash collector, or between the
precipitator and the mechanical collector.
The minimum concentration of sulfur
trioxide required was 5-20 ppm of the flue
gas, depending on the flue gas temperature
and the chemical composition of the fly ash.
The resistivity of the fly ash was decreased
from 1012 ohm-cm to an acceptable value of
1010 ohm-cm. Fly ash of widely varying
chemical compositions could be successfully
conditioned, larger quantities of sulfur trio-
xide being required for highly alkaline fly
ash. The conditioning was effective at flue
gas temperatures from 110°C to at least
160 °C and perhaps near 200 °C.
Chemical analysis of the treated fly ash
indicated that the lowering of the fly ash
resistivity by sulfur trioxide resulted from
deposition of sulfuric acid on the surfaces of
the fly ash particles, either by adsorption or
condensation. The electrical conduction of
the fly ash particles may involve hydrogen
ion transport in a surface film of sulfuric
acid or transport by alkali metal ions from
the fly ash dissolved in a surface film.
The sulfur trioxide also increased the
cohesiveness of the collected fly ash par-
ticles and thus reduced the extent of their
re-entrainment in the flue gas when the
precipitator electrodes were rapped to dis-
lodge the collected ash. Such an effect
might be of practical value in the use of
sulfur trioxide as a conditioning agent for
low-resistivity fly ash. If the resistivity is
too far below the normal range of values
(1010-10n ohm-cm), the electrical force in
the deposited fly ash layer will not provide
sufficient restraint against reentrainment.
The effectiveness of sulfur trioxide was
also reflected in improved operation of the
electrostatic precipitators. Injection of sul-
fur trioxide generally permitted both higher
current densities and higher voltages to be
reached without the occurrence of excessive
sparking.
The results of a few plant tests with
ammonia as a conditioning agent indicated
that it is not as widely applicable as sulfur
trioxide. However, it improved the collec-
tion efficiency of fly ash from some coals
with a range of sulfur contents. These coals
produced fly ashes with resistivities that
were not excessively high, and the injection
of ammonia had little if any effect on the
resistivity. The effect of ammonia appeared
to be an increase in the space charge in the
precipitator, with a resulting enhancement
of the electric field and increased collection
efficiency. This mechanism could involve
the formation of fine particles of ammonium
10
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suJfate or bisulfate, which become electri-
cally charged in the precipitator and which
lower the average mobility of the charge
carriers. With fly ash of abnormally low
resistivity, ammonia also appeared to in-
crease the cohesiveness of the ash.
RAPPING RE-ENTRAINMENT OF FLY ASH
In a continuously operating electrostatic
precipitator, the collected fly ash is re-
moved periodically from the collecting plate
electrode by rapping it. The dislodged dust
falls in agglomerates into a hopper below
the electrodes. A mechanical conveyer
removes the fly ash from the bottom of the
hopper for disposal. The rap is made auto-
matically by a hammer or vibrator according
to a time schedule that can be set to give
rapping cycles of a few minutes to a few
hours, depending on the fly ash properties. In
this way, the rapping process can be ad-
justed for maximum effectiveness on a
specific fly ash. Also, complicated rapping
sequences can be established that involve
applying schedules with different rapping
intervals to different plates in the precipi-
tator.
Two general approaches to rapping are
used. One is to rap often and to provide a
high intensity of rapping, in an attempt to
minimize the thickness of the residual dust
layer. The other is to vary the intensity and
frequency of rapping in an attempt to
minimize the quantity of material re-
entrained in the gas stream.
An appreciable amount (up to 5%) of the
collected fly ash can be re-entrained in the
gas and lost in the gas emitted from the
precipitator and from the power plant stack.
This re-entrained fly ash can amount to half
of the total particulate emissions.
An experimental study of rapping tech-
niques was carried out to explore the effects
of operating parameters on the extent of re^
entrainment.13 The measurements were
made on a large experimental wire-plate
electrostatic precipitator that represented
one electrical section of a full-scale unit
with horizontal gas flow. The experiments
involved the collection of a redispersed fly
ash, the particles of which had (before
redispersion) a mass median diameter of 16
um, with a geometric standard deviation of
5 um. (This particle-size distribution is
typical of fly ash from pulverized coal-fired
boilers.) The gas flowing through the preci-
pitator and in which the fly ash was
suspended was a simulated flue gas, obtained
from an oil burner. The gas temperature
was 125-140°C, typical of flue gas from
coal-fired boilers in power plants (on the
downstream side of the air preheater).
The concentrations of fly ash in the flue
gas at the inlet and outlet of the elec-
trostatic precipitator were measured. The
particle size distribution of the fly ash was
determined by the use of impactors inserted
in the flue at the inlet and outlet of the
precipitator. For both types of measure-
ment, traverses across the cross section of
the duct were made to ensure representative
sampling.
When the time interval between the raps
was increased from 12 to 52 minutes, the
following effects were observed:
(1) The total rate of fly ash emissions
from the precipitator decreased from 12
kg/hr to 8 kg/hr.
(2) At the 12-minute interval, the
emissions due to rapping were 6 kg/hr (50%
of the total). At the 52-minute interval, the
emissions due to rapping were 1 kg/hr (12%
of the total).
(3) The average size of the fly ash
particles emitted from the precipitator in-
creased as the result of the rapping re-
entrainment, and the percentage contribu-
tion of the fine particles, _i.e., those < 3 um
in diameter, to the total mass of fly ash
emitted from the precipitator decreased
from 25% to 10%.
(4) The overall collection efficiency of
the precipitator increased from 88.6% to
93.9%.
11
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These effects of lengthening the time
between raps may be explained as being due
to an increase in the size and number of
aggregates of fly ash particles which, as
they are dislodged from the collecting plate,
are coherent enough to reach the hopper
without being broken up and re-entrained in
the gas stream. The layer of fly ash on the
collecting plate is apparently compacted by
electrical or mechanical forces after it is
deposited.
Not all the deposit was removed by the
rapping. On some of the plates, a residual
dust layer was built up that was not removed
from the plate at values of rapping intensity
sufficient for removing freshly deposited fly
ash, calculated to be 1-3 kg/m2 for 100%
removal. These results suggest that
auxiliary, heavier rappers might profitably
be used on a regular, perhaps daily, basis.
The fly ash emitted from the lower third
of the precipitator was 60-80% of the total
mass, due to a vertical gradient in dust
concentration in the flue gas. This in turn
could be ascribed to uneven distribution of
the suspended fly ash resulting from gravita-
tional settling and re-entrainnment of fly
ash from the hoppers. It was evident from
photographs that a large part of the re-
entrainment was due to a boil-up or rebound
of particles from the hoppers.
A MATHEMATICAL MODEL OF
ELECTROSTATIC PRECIPITATION
The first successful electrostatic preci-
pitators for controlling industrial dust emis-
sions were installed in 1910. Within a few
years, it was recognized that the efficiency
of dust collection was exponentially related
to parameters such as gas velocity and
collecting plate area.
In 1922 W. Deutsch put this relationship
into a more comprehensive form that in-
corporated concepts from electrical theory.
The Deutsch equation gives the efficiency of
collecting dust from a gas stream as a
function of the collecting plate area, the
flow rate of the gas, and the size of the dust
particle and its migration velocity. The
migration velocity is the net velocity to the
collecting plate resulting from the opposi-
tion of two forces, the force of electrostatic
attraction and the viscous drag of the gas
that retards the movement of the particle.
A mathematical model for electrostatic
precipitation has been developed that is
based on the Deutsch equation.11* Mathe-
matical expressions for the components of
the equation can be formulated from theo-
retical relationships which have been con-
firmed by experimental data. These
expressions are used to calculate the elec-
tric field, particle charging rates, and the
space charge resulting from the presence of
charged particles, and the results are used in
the computer program from which particle
collection efficiency is computed.
The Deutsch equation is idealized in that
it assumes thorough mixing of the gas due to
turbulent flow, a uniform concentration of
dust particles, and a constant migration
velocity for all particles. These assumptions
or conditions hold true only for particles of
nearly the same size and only for short
lengths through the precipitator.
Therefore, in constructing the mathe-
matical model, the precipitator is divided
into one-foot segments down its length, and
each successive segment is considered as an
incremental length in calculating the
amount of fly ash collected from the gas
stream as it moves through the precipitator.
In this way, the Deutsch equation can be
used more accurately than for the precipi-
tator as a whole.
In an analogous manner, the variation in
collection efficiency for different particle
sizes can be taken into account by con-
sidering the suspended fly ash to be com-
posed of a mixture of particles in a set of
size ranges that fits the curve of particle
size distribution. A value for migration
velocity is calculated for each particle-size
range in each increment of length.
12
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After these calculations have been per-
formed for all the increments of length, the
overall collection efficiency (by mass) is
calculated.
Corrections are made, on the basis of
operating experience, for non-ideal perform-
ance: the gas flow is not uniform over the
cross-section of the precipitator, some of
the collected fly ash is entrained in the gas
flow leaving the precipitator, and some of
the dust-laden gas by-passes the electrified
regions of the precipitator.
The mathematical model that was ob-
tained by these methods has proven to be
useful in predicting the performance of full-
scale industrial electrostatic precipitators
operating on flue gas from coal-fired elec-
tric power boilers.
REFERENCES
1. Oglesby, Sabert, Jr., and G.B. Nichols. An Electrostatic Precipitator Systems Study. A
Manual of Electrostatic Precipitator Technology: Part I, Fundamentals. APTD 0610,
National Air Pollution Control Administration, Cincinnati, OH, 1970. 322 pp. PB 196380.
2. Oglesby, Sabert, Jr., and G.B. Nichols. A Manual of Electrostatic Precipitator Tech-
nology: Part II, Application Areas. APTD 0611, National Air Pollution Control
Administration, Cincinnati, OH, 1970. 875 pp. PB 196381.
3. Oglesby, Sabert, Jr., and G.B. Nichols. Selected Bibliography of Electrostatic Precipitator
Literature. APTD 0612, National Air Pollution Control Administration, Cincinnati, OH,
1970. PB 196379.
4. Oglesby, Sabert, Jr., and G.B. Nichols. An Electrostatic Precipitator Systems Study.
Final Report. APTD 0657, National Air Pollution Control Administration, Cincinnati,
OH, 1970. PB 198150.
5. Nichols, G.B., and J.D. McCain. Particulate Collection Efficiency Measurements on Three
Electrostatic Precipitators. EPA-600/2-75-056, U.S. Environmental Protection Agency,
Research Triangle Park, NC, 1975. 84 pp. PB 248220/AS.
6. Smith, W.B., K.M. Gushing, and J.D. McCain. Particulate Sizing Techniques for Control
Device Evaluation. EPA-650/2-74-102, U.S. Environmental Protection Agency, Research
Triangle Park, NC, 1974. 127 pp. PB 240670/AS.
7. Smith, W.B., K.M. Gushing, G.E. Lacey, and J.D. McCain. Particulate Sizing Techniques
for Control Device Evaluation. EPA-650/2-74-102a, U.S. Environmental Protection
Agency, Research Triangle Park, NC, 1975. 132 pp. PB 245184/AS.
8. Spencer, Herbert W., III. Electrostatic Precipitators: Relationship Between Resistivity,
Particle Size, and Sparkover. EPA-600/2-76-144, U.S. Environmental Protection Agency,
Research Triangle Park, NC, 1976. 68 pp. PB 257130/AS.
9. Bickelhaupt, R.E. Influence of Fly Ash Compositional Factors on Electrical Volume
Resistivity. EPA-650/2-74-074, U.S. Environmental Protection Agency, Research Triangle
Park, NC, 1974. 49 pp. PB 237698/AS.
13
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10. Bickelhaupt, R.E. Effect of Chemical Composition on Surface Resistivity of Fly Ash.
EPA-600/2-75-017, U.S. Environmental Protection Agency, Research Triangle Park, NC,
1975. 50 pp. PB 244885/AS.
11. Nichols, G.B. Techniques for Measuring Fly Ash Resistivity. EPA-650/2-74-079, U.S.
Environmental Protection Agency, Research Triangle Park, NC, 1974. 49 pp. PB
244140/AS.
12 Dismukes, E.B. Conditioning of Fly Ash with Sulfur Trioxide and Ammonia. EPA-600/2-
75-015, U.S. Environmental Protection Agency, Research Triangle Park, NC. 169 pp. PB
247231/AS.
13. Spencer, Herbert W., III. Rapping Reentrainment in a Nearly Full-Scale Pilot Electro-
static Precipitator. EPA-600/2-76-140, U.S. Environmental Protection Agency, Research
Triangle Park, NC, 1976. 178 pp. PB 255984/AS.
14. Gooch, J.P., J.R. McDonald, and Sabert Oglesby, Jr. A Mathematical Model of Electro-
static Precipitation. EPA-650/2-75-037, U.S. Environmental Protection Agency, Research
Triangle Park, NC, 1975. 162 pp. PB 246188/AS.
OTHER REPORTS AVAILABLE
In addition to the reports listed above, the following have recently been issued by the
EPA. Copies of all EPA reports can be obtained from the National Technical Information
Service, U.S. Department of Commerce, Springfield, VA 22161.
Fly ash resistivity and conditioning:
Bickelhaupt, R.E. Sodium Conditioning to Reduce Fly Ash Resistivity. EPA-650/2-74-092,
U.S. Environmental Protection Agency, Research Triangle Park, NC, 1974. 15 pp. PB
236922/AS.
Dismukes, E.B. Conditioning of Fly Ash with Sulfamic Acid, Ammonium Sulfate, and
Ammonium Bisulfate. EPA-650/2-74-114, U.S. Environmental Protection Agency, Research
Triangle Park, NC, 1974. 58 pp. PB 238922/AS.
Dismukes, E.B. A Study of Resistivity and Conditioning of Fly Ash. EPA-R2-72-087, U.S.
Environmental Protection Agency, Research Triangle Park, NC, 1972. 134 pp. PB 212607.
Wet electrostatic precipitators:
Gooch, J.P., and J.D. McCain. Particulate Collection Efficiency Measurements on a Wet
Electrostatic Precipitator. EPA-650/2-75-033, U.S. Environmental Protection Agency, Re-
search Triangle Park, NC, 1975. 60 pp. PB 244173/AS.
Gooch, J.P., and A.H. Dean. Wet Electrostatic Precipitator System Study. EPA-600/2-76-142,
U.S. Environmental Protection Agency, Research Triangle Park, NC, 1976. 204 pp. PB
257128/AS.
14
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Electrostatic precipitator applications:
Gooch, J.P., G.H. Marchant, Jr., and L.G. Felix. Particulate Collection Efficiency Measure-
ments on an Electrostatic Precipitator Installed on a Paper Mill Recovery Boiler. EPA-600/2-
76-141, U.S. Environmental Protection Agency, Research Triangle Park, NC, 1976. 121 pp. PB
255297/AS.
Electrostatic precipitator performance model:
Gooch, J.P., and G.B. Nichols. An Electrostatic Precipitator Performance Model. EPA-650/2-
74-132, U.S. Environmental Protection Agency, Research Triangle Park, NC, 1972. 171 pp. PB
238923/AS.
Measurement techniques:
McCain, J.D., K.M. Cushing, and A.N. Bird, Jr. Field Measurements of Particle Size Dis-
tribution with Inertial Sizing Devices. EPA-650/2-73-035, U.S. Environmental Protection
Agency, Research Triangle Park, NC, 1973. 52 pp. PB 226292/AS.
Cushing, K.M., G.E. Lacey, J.D. McCain, and W.B. Smith. Particulate Sizing Techniques for
Control Device Evaluation: Cascade Impactor Calibrations. EPA-600/2-76-280, U.S.
Environmental Protection Agency, Research Triangle Park, NC, 1976. 94 pp. PB 262849.
Smith, W.B., K.M. Cushing, and J.D. McCain. Procedures Manual for Electrostatic Precipi-
tator Evaluation. EPA-600/7-77-059, U.S. Environmental Protection Agency, Research
Triangle Park, NC, 1977. 430 pp. PB
Felix, L.G., G.I. Clinard, G.E. Lacey, and J.D. McCain. Inertial Cascade Impactor Substrate
Media for Flue Gas Sampling. EPA-600/7-77-060, U.S. Environmental Protection Agency,
Research Triangle Park, NC, 1977. 89 pp. PB
Smith, W.B., K.M. Cushing, and G.E. Lacey. Particulate Sizing Techniques for Control Device
Evaluation. Special Report. Andersen Filter Substrate Weight Loss Study. EPA-650/2-75-022,
U.S. Environmental Protection Agency, Research Triangle Park, NC, 1975. 25 pp. PB
240720/AS.
15
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
\. REPORT NO.
EPA- 600/8- 77- 02 Oa
2.
4. TITLE AND SUBTITLE
Particulate Control Highlights: Research on Electro-
static Precipitator Technology
7. AUTHOR(S)
S.Oglesby, Jr. , and G.Nichols
9. PERFORMING ORGANIZATION NAME Ah
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 352
JO ADDRESS
05
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
3. RECIPIENT'S ACCESSION NO.
5. REPORT DATE
December 1977
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
SORI-EAS-77-677
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
68-02-2114
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 11/76-11/77
14. SPONSORING AGENCY CODE
EPA/600/13
IB. SUPPLEMENTARY NOTES JERL-RTP project officer is Dennis C. Drehmel,
919/541-2925.
Mail Drop 61,
IB. ABSTRACT The rep0rt gives highlights of a major EPA research program on electro-
static precipitator (ESP) technology, directed toward improving the performance of
ESPs in controlling industrial particulate emissions, notably fly ash from coal com-
bustion in electric power plants. Relationships between electrical effects, such as
reverse corona, caused by high resistivity of the deposited fly ash, have been investi-
gated. The influence of fly ash particle size and chemical composition on the resisti-
vity and dielectric strength of the deposited fly ash has also been studied. Relation-
ships have been established between fly ash resistivity and chemical composition,
especially its alkali metal content, for ESP operating temperatures below about 250 C.
Based on these relationships, a mechanism for ionic surface conduction has been pro-
posed that complements the ionic mechanism in bulk conduction in fly ash particles
at higher operating temperatures. The efficacy of conditioning fly ash by adding SOS
to flue gas (to lower fly ash resistivity) was established in trials at electric power
plants. Reentrainment of particles from deposited fly ash has also been investigated
In relation to ESP rapping procedures and gas flow distribution. A mathematical model
of the ESP process has been developed, using fundamental relationships together with
measurements of ESP geometry, electrical conditions, and particle size distribution.
17.
a. DESCRIPTORS
KEY WORDS AND DOCUMENT ANALYSIS
b.lDENTIFIERS/OPEN ENDED TERMS
Air Pollution Fly Ash Air Pollution Control
Electrostatic Pre- Coal Stationary Sources
cipitators Combustion Particulates
Oust Electric Power Reverse Corona
Emission Plants Collection Efficiency
Industrial Processes Electric Corona
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
20. SECURITY CLASS (This page)
Unclassified
c. COSATI Field/Group
13B 21B
21D
11G
10B
13H 20C
21. NO. OF PAGES
20
22. PRICE
EPA Form 2220-1 (9-73)
16
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