EPA-600/2-76-144
May 1976
Environmental Protection Technology Series
ELECTROSTATIC PRECIPITATORS:
RELATIONSHIP BETWEEN
RESISTIVITY, PARTICLE SIZE, AND SPARKOYER
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
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This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-76-144
May 1976
ELECTROSTATIC PRE CIPITATORS:
RELATIONSHIP BETWEEN
RESISTIVITY, PARTICLE SIZE, AND SPARKOVER
by
HerbertW. Spencer, HI
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
Contract No. 68-02-1303
ROAP No. 21ADL-027
Program Element No. 1AB012
EPA Project Officer: Leslie E. Sparks
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
P repared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20.460
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ABSTRACT
The report gives results of a study of the relationships of the
electrical resistivity of fly ash, its particle size, the occur-
rence of back corona and sparkover, and the electrical character-
istics of electrostatic precipitators (ESP's). The study included
laboratory measurement of the dielectric strengths and resistivity
of five particle-size fractions of a fly ash sample and measure-
ment of the current densities and voltages at which back corona
and sparkover occurred for a 3-mm dust layer covering the plate of
a wire-plate negative-corona discharge device. Results showed
that the peak current density for the formation of back corona
v • t "•v'i
depended on the resistivity of the dust covering the positive
. »
electrode. Operating current densities for full-scale ESP's are
discussed in relation to fly ash resistivity.
11
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CONTENTS
Page
Abstract ii
List of Figures iv
List of Tables vi
Acknowledgments vii
Sections
I Conclusions 1
II Introduction 6
III Historical Review of the Effects of
Dust Layers on Electrical Characteristics
of Corona Discharges 8
IV Apparatus and Procedures 15
V Results of Laboratory Measurements 24
VI Measurements on Full-Scale Precipitators 52
111
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FIGURES
No. Page
1 Allowable current density as a function of resistivity
according to Hall13 13
2 Precipitation rate parameter vs. resistivity, dashed
curve White's field data11* 14
3 Wire plate corona discharge device 16
4 Photographs of the wire plate corona discharge device 17
5 Wire plate corona discharge device circuit schematic 18
6 Electrode configuration for testing dielectric strength
of ash layer 20
7 Resistivity vs. temperature for different particle
size fractions; 1) 74.5% P(Porosity), 0-3 ym;
2) 67.8% P, 3-7 ym; 3) 64.7% P, 7-15 urn; 4) 58.0% P,
15-25 ym; 5) 54.3% P, >25 ym; 9.4% water vapor by
volume 28
8 Photograph of dust surface on the wire plate corona
discharge device 30
9 Additional photographs of dust surfaces on the wire
plate corona discharge device 31
10 Clean plate voltage - current characteristics at
60°C and 120°C 33
11 Voltage - current characteristics for the wire plate
discharge device with 3mm dust layers 34
12 Oscilloscope traces of back corona pulses 36
13 Wire plate corona discharge device voltages for spark-
over and for formation of back corona as a function
of resistivity; wire to plate spacing 6 cm; solid curve
estimate of the average voltage at formation of back
corona; dashed curve potential between dust surface and
wire at formation of back corona 41
14 Current densities for formation of back corona in the
wire plate corona discharge device as a function of
resistivity 42
15 Current densities at sparkover and for formation of
back corona as a function of wire to plate spacing 46
IV
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FIGURES
(Cont'd)
No. Page
16 Sparkover voltages as a function of wire to plate
spacing 47
17 Current density distributions: Wire potential 21 KV,
clean plate spacings of 1) 3 cm and 2) 5 cm 49
18 Current density distributions with and without back
corona 50
19 Current density distributions for clean plate at
several applied potentials and for dust covered plate
with back corona 51
20 Operating current densities as a function of resistivity
for various plants tested by SRI; Numbers refer to data
in Table V; Circles inlet sections, triangles outlet
sections, squares unknown sections, solid symbols either
NH3 or SO3 injection 53
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TABLES
No. Page
I Dielectric strenghths of ash layers 25
II Physical properties of dust layers formed
from Gaston Power Station fly ash particle
size fractions 27
III Sparkover and back corona voltages for five
different particle size fractions 38
IV Chemical analyses of size fractionated fly ash
samples 39
V Precipitator electrical data 54
VI
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SECTION I
CONCLUSIONS
The efficiency of an electrostatic precipitator is a direct
function of the electrical conditions that are obtained in the
precipitator. By increasing the electric field and particulate
charge, the migration velocity of the particles to the collection
plate can be increased, resulting in increased efficiency or
reducing the size of the precipitator needed to meet a given
emission standard. Unfortunately, precipitators are forced to
operate at voltages and current densities significantly below
what can be obtained with a corona discharge system containing
no particulate. The resistivity of the particulate collected on
the plates of an electrostatic precipitator inversely affects
the current density at which the precipitator can operate.
Data are presented in this report for full scale precipitators
and for a laboratory corona discharge device that show the depen-
dence of precipitator electrical operation on particulate
properties. The field data indicate that precipitators collecting
particulate with resistivities below ^1x10:° ft-cm can operate
at current densities on the order of 80 nA/cm2. At this current
density the electric field in the collected dust layer is less
than 0.8 kV/cm2 for particulate with resistivities below 1x1010 fl-cm.
This electric field is less than the dielectric strength of the
collected particulate, which is usually on the order of 20 kV/cm.
Current densities for precipitators collecting particulate with
resistivities above 1x1010 fi-cm were observed to decrease with
resistivity. However, considerable scatter was obtained in
the data. Some units operated at current densities producing
electric fields in the collected particulate layer near the di-
electric strength of the layer, while others operated at current
densities a factor of 10 to 20 times below the limit set by the
dielectric strength and resistivity of the dust layer. It is
estimated that spatial and temporal variations in current density
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can account for a suppression in the operating point a factor
of 2 or more below the limit set by the particulate resistivity
and dielectric strength.
The peak current densities obtained in the laboratory for the
formation of back corona were within approximately 20% of the
point at which the dielectric strength of the layer would have
been exceeded. Field operating points were expected to be set
by the point at which back corona occurs; a comparison of our
field and laboratory measurements indicates a discrepancy between
field operation and small scale corona discharge operation. For
low resistivities, the data indicate that a considerable increase in
operating current densities might be obtained with proper design.
For high resistivity dust the scatter in the field data was such
that a clear interpretation is not possible. However, it
appears as mentioned before that the field units operate a
factor of 10 to 20 times below the expected operating point.
The comparison of the laboratory and field measurements in this
report shows that in the design of precipitators the minimum
and maximum clean plate current density limits must be considered.
For very high resistivity dust, the minimum clean plate stable
operating density must be below the current density for formation
of back corona. For low resistivity ashes (<109 ft-cm), pre-
cipitator electrical design should allow for operating at
current densities in the range of 100 nA/cm2 if maximum
performance is to be obtained.
The objective of the laboratory study was to determine if there
were other particulate properties besides dust resistivity
that affect the operation of a corona discharge when a dust
layer is deposited on, the collection electrode.
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It was determined that the particulate resistivity was the main
factor. Other factors such as particle size, bulk density,
porosity, cross sectional area of voids, and specific surface
area appeared to affect operating points for precipitators only
by their effect on the resistivity of the collected dust layer.
A measurable change in the dielectric strength of the collected
dust layer as a function of the above factors was not observed
for the ash samples studied during this work.
It was observed that ash resistivity varied as a function of
particle size and that if the ash is sufficiently fractionated
by particle size in the precipitator, a decrease in ash
resistivity from the inlet to the outlet can occur. The decrease
can exceed a factor of 2 and result in a corresponding increase
in operating current density from the inlet to the outlet of
the precipitator. The operating electrical data tabulated in this
report for full scale units show an increase in current
density from the inlet to the outlet. The data also showed
that lower operating voltages are normally obtained in the
outlet sections of the precipitator. Precipitator electrical
behavior cannot be entirely explained by dust resistivity alone;
the decrease in space charge effects due to suspended dust
from the inlet to the outlet are probably responsible for the
observed increase in current density for a given voltage.l Space
charge effects due to particulate should be included in any
future laboratory studies. Theoretical calculations of the
voltage-current characteristic for a wire-duct configuration
that include space charge effects should be developed for inter-
preting variations in the voltage-current characteristics of
full scale units.
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A misaligned corona wire was shown to increase the chance for
formation of back corona and sparkover. It was observed that
by decreasing the wire to plate spacing the ratio of the peak
current density to average current density was increased. It
was also observed that the peak current density at the formation
of back corona is independent of the wire to plate spacing.
The result of these two observations is that if a wire is
misaligned, back corona will form at an average current density
less than the expected value. Misalignment may account for some
of the discrepancy between the operating electrical characteris-
tics of field units and laboratory devices. The magnitude of
the effect for precipitators would need to be determined with
a multi-wire system.
Other factors such as the variations in dust layer thickness and
errors in in-situ resistivity measurements may also play a part
in the discrepancy between field units and laboratory measure-
ments. An investigation of the in-situ resistivity measurement
procedure is needed to determine if the procedure is affecting
the results. The variations in dust layer thickness produced
by nonuniform rapping probably produce significant variations in
current density, increasing the probability of back corona for
a given average current density, and should be investigated.
The effects on current density distribution could be determined
by measuring the current densities to various points on a
precipitator plate covered with a non-uniform dust layer. The
non-uniform dust layer would be formed by rapping the plate
to remove some of the dust, and then precipitating a new dust
layer on the surface.
Measurements of the current density distributions for the following
conditions: no dust layer, dust layer without back corona, and
dust layer with back corona showed a drastic change in current
density distribution with the formation of back corona. This
had been previously observed by Kercher.2 Back corona had the
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effect of increasing the average current density while leaving
the current densities in some places unchanged.
FUTURE RESEARCH
Further investigation will be needed to clearly delineate the
difference between full scale units and a small scale corona
discharge when a particulate is present. A determination of
the exact current density at which a precipitator can operate
for a given set of dust properties will depend on further
investigations. Such studies are needed if accurate performance
characteristics are to be theoretically determined for
electrostatic precipitators.
Specific areas for future research are as follows:
1. Gathering of additional data correlating operating points
of field units and dust resistivity.
2. Further development of the correlation between laboratory
resistivity measurements and in-situ measurements.
3. Test in a dry pilot scale precipitator to study the effect
the following factors have on the electrical behavior of
electrostatic precipitators:
a. Design of electrodes (wires and plates)
b. Dust properties
c. Uniformity of current density distribution with
and without dust layers
d. Thickness and uniformity of dust layers
e. Space charge
f. Gas composition
4. Theoretical calculations of V-I characteristics for a
parallel plate precipitator including space charge effects.
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SECTION II
INTRODUCTION
The objective of this research is to provide information for
interpreting the electrical behavior of electrostatic pre-
cipitators when that behavior is governed by the characteristics
of the collected dust layer.
Dust layers affect precipitator electrical behavior in several
ways. For one, they introduce a resistance element into the
electrical circuit, which, except for the nonlinear character-
istics of the resistivity of dust layers, behaves in the same
manner as the incorporation of an ohmic resistor in the circuit.
Second, the electrical breakdown of the dust layer and the
resulting formation of point corona in the dust layer, called
by the descriptive terms "back corona", "back ionization",
"back sprays", and "back discharge", drastically affect the
electrical behavior of a precipitator. Back corona occurs when
a highly conductive point is formed in the dust layer, usually
by electrical breakdown of the dust at a point where the
current density is such that- the ohmic buildup of voltage exceeds
the dielectric strength of the dust. These visible discharges
in negative corona affect precipitator operation by decreasing
sparkover potential, increasing the average current density, and
producing positive ions that neutralize the negative space
charge in the corona gap.
The sparking and back corona conditions for fly ash were investi-
gated using a negative corona wire-plate discharge device in
the laboratory. In particular, the variation of corona
characteristics as a function of the properties of dust layers
formed from different particle size fractions of fly ash was
studied. The resistivities, dielectric strengths, porosities,
mass median diameters, and chemical compositions of 5 particle
size fractions of a fly ash were determined.
6
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Different particle size fractions were studied because precipi-
tators fractionate the inlet dust, the particles collected in
the inlet sections being larger than those in the outlet sections,
Previous attempts to relate sparkover to particle size have not
been successful.3
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SECTION III
HISTORICAL REVIEW OF STUDIES ON THE EFFECTS OF DUST LAYERS ON
ELECTRICAL CHARACTERISTICS OF CORONA DISCHARGES
In 1918, Wolcott4 found that a dielectric sheet of discontinuous
surface placed over the plate electrode of a point-plane electrode
system lowered the sparkover voltage by about 50% for negative
corona, although it affected positive corona only slightly. In
1933, Franck5 observed that dust layers also influence the charac-
teristics of discharges, especially in an asymmetric electrode
arrangement. For a negative corona discharge, he found that the
sparking voltage decreased to about 1/3 of that without a dust
layer. The sparkover voltages increased with thicker dust layers
from a minimum at approximately 1.5 mm, presumably because of the
increased potential drop across the dust layer needed to reach
breakdown.
Franck interpreted the phenomenon of breakdown in terms of
the relative dielectric constants and conductivities of dust
and air. The dust layer and air at the dust layer boundary were
assumed to be homogeneous dielectrics with homogeneous fields.
The ratio of the electric field in the gas (Ei) to the electric
field in the dust layer (E2) is given by
Ei _ £2
E2 £i
when there is no current and
E_j_ _ 0_2_
E2 (Ji
when there is a current, where ei,e2 are the dielectric constants
and ai,a2 are the conductivities of the air and the dust layer,
respectively. Because the dielectric constant of the dust is greater
8
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than the dielectric constant of air, the electric field in
the dust layer is smaller than the electric field in the
air above the dust layer when the potential is first applied.
At steady state, the electric field in the dust layer is
larger than the field in the air above the layer if the
conductivity of the layer is low.
When the break-down voltage of the dust layer is reached, thin
conductive channels appear as spots in the dust layer. These
spots redistribute the field and behave like corona points (back
corona). For negative corona, this produces a strong anodic
field, lowering the potential for sparkover. For positive corona,
a strong anodic field exists prior to the breakdown of the dust,
and thus the effect on sparkover is not as great.
In 1948, White6 ascribed the detrimental effects of back corona
in reducing the collection efficiency of an electrostatic pre-
cipitator to a lowering of the sparkover potential and to the
production of positive ions which decrease the efficiency of
charging of suspended dust particles in a negative corona. He
noted that the electrical field (E) in the dust is determined
by the product of the current density and the dust resistivity,
according to Ohm's Law. Therefore, in an electrostatic precipi-
tator collecting a typical dust having a resistivity of 101° fi-cm
and a dielectric strength of 10 kV/cm, back corona would set in
at an operating current density of 1 yA/cm2.
Using voltage pulses of 1-2 microsecond duration, White obtained
peak current densities which were far greater than the direct
current densities for the formation of back corona. This can
be explained in part by the estimated charging time of 30 micro-
seconds for a layer of dust with 10:1 ^-cm resistivity. The
voltage across the dust layer for a pulsed current is less than
the voltage for a direct current when the pulses are shorter
than the charging time and when the time between pulses is longer
than the discharge time.
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In a series of papers, Penney7"10, reported on the
effects of resistivity layers on both the electrodes
of corona discharges. The term "flare" was used to denote a
discharge consisting of repetitive current pulses or streamers
which originate from a fixed location on the anode when the
anode is partially covered by a high resistivity material.
Penney's experiments showed a reduction in sparkover potential
for negative corona with increased resistivity and with de-
creased wire size. Although not reported, the reduction as a
function of wire size was probably related to increased current
densities obtained with the smaller wires, for a given potential.
By using a negatively charged ball, Penney was able to show that
the flares were a source of positive ions. The flares consisted
of current pulses of 0.05 y-sec rise time and 0.5 y-sec decay
times with a period of approximately 0.33 y-sec between peak
current pulses of 30-80 mA. Penney's experiments indicated that
with an imperfect insulating coating on the anode and a source
of ionization,current pulses followed by sparkover can occur
if the average gradient is of the order of 5 kV/cm.
This gradient is significantly below the gradient of 30 kV/cm
for breakdown of gases between clean metallic electrodes.
Penney suggested that the space charges resulting from the flare
produce a gradient which is favorable to the development of a
spark or at least alter the field, so that the initial voltage
distribution is rather unimportant.
An investigation of back corona in high-resistivity dust layers
o
by Kercher showed that there was a slight variation in the current
density at which back corona formed for a layer thickness < 3mm
but that Ohm's Law held for a thickness > 3mm. He also showed
that when back corona formed the discharge current was concen-
trated at the breakdown sites.
10
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The effect of resistivity on the formation of back corona has
also been studied by Simm.ll He substantiated previous work,
but observed that the sharp increase in current with the for-
mation of back corona was at a somewhat higher current density
than Ohm's Law predicted. This indicates that back corona may
occur before a distinct change in the voltage-current charac-
teristics is visible.
Apparently the effect of particle size of the dust on back corona
has not been adequately studied. In experiments with different
particle-size fractions of fly ash, Penney3found that the larger
particle-size fraction he was using had a lower sparkover poten-
tial than a smaller particle-size fraction, even though the re-
sistivity of the large particle-size fraction was lower. However,
the larger particle-size fraction contained highly conductive
carbon particles.
Electrical operating conditions in industrial electrostatic pre-
cipitators have been simulated by Herceg"12 using a point-to-plane
corona device and layers of porous insulating materials to simu-
late precipitated dust layers. In this work he attempted to physi-
cally model the corona discharge device by a lumped-element circuit
based on a systematization of corona phenomena into four regions
described as the primary ionization, accumulation, transport, and
secondary-ionization regions. A solid-state analog was designed
on the basis of the lumped-element circuit model. However, no
data were presented with the design to show that it did model
a point-to-plane corona discharge.
Hall13 has indicated that the practical limits of operating
current density are calculated from Ohm's Law, in which the
breakdown voltage (E) has a value of 1-2 kV/cm, approximately
an order of magnitude smaller than the measured dielectric
11
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strengths of dust layers. White1"* has indicated that the
precipitation rate parameter of field units (an empirical
parameter that characterizes the performance of a given
precipitator under a specified set of operating conditions)
varies inversely with the resistivity of the dust being
collected by the unit. The practical effects of dust layers
on precipitation are summed up in graphs by Hall13 (Figure 1)
and by Whitelk (Figure 2).
12
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500
RESISTIVITY , ohm-cm
Figure 1. Allowable current density as a function of
resistivity according to Hall.13
13
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RESISTIVITY, Ohm-cm
Precipitation rate parameter vs resistivity;
Dashed curve White's field data.1"
14
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SECTION IV
APPARATUS AND PROCEDURES
This section describes the equipment and the procedures used
to study the influence of dust layers on the electrical be-
havior of a wire-plate corona discharge.
CORONA DISCHARGE DEVICE
The wire-plate corona discharge device used in the laboratory
experiments is shown schematically in Figures 3 and 4. It
consists of a 0.089 cm (0.035 in.)-diameter stainless steel
corona wire supported 6.0 cm above a 10-cm square stainless
steel plate. The plate electrode is recessed 3 mm in a Teflon
block. Five insulated segments are incorporated for measuring
the current density at five separate points on the plate.
A Hipotronics DC power supply capable of providing 100 kilovolts
maintains high negative potentials on the corona wire of the
corona discharge device (ripple is less than 0.5% of output
voltage, when operating the device). The corona discharge
device is installed in an environmental chamber so that tempera-
ture and humidity can be controlled during the measurements.
A Teflon-insulated copper tube extending to the device at the
bottom of the chamber is connected at the top of the chamber to
a voltage divider (a Hipotronics Model 100 high voltage meter).
The corona current to any one of the five insulated plate seg-
ments or the current to the outer plate is measured by selective-
ly connecting the desired element to the current-measuring circuit
displayed in Figure 5 while shorting the other elements of the
flat-plate electrode to ground. The current measuring circuit
consists of a spark protector, a 100 n resistor, and a Keithley
414A picoammeter. The voltage across the 100 fi resistor in series
with the electrometer is amplified five times by a differential
amplifier and displayed on a 10 MHz oscilloscope for recognition
of back corona onset.
15
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TEFLON
INSULATION
1/4 IN.
BRASS ROD
CORONA BALL
2 IN. OIA
BRASS ROD
8-32 THREADS
5/8 IN.DIA.
PLATE I , 1-1/2 IN. x 3/4 IN.
4 PLATES 1-1/2 IN. x 3/8 IN.
Figure 3. Wire plate corona discharge device
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Figure 4. Photographs of the wire plate corona
discharge device.
17
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OSCILLOSCOPE
DIFFERENTIAL
AMPLIFIER
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Figure 5. Wire plate corona discharge device circuit schematic
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By rounding the edges of the flat collecting plate and by mount-
ing corona balls on the ends of the corona wire support, the
maximum clean-plate voltages were increased to above 70 kV and
the maximum clean-plate current densities were increased to
~3 pA/cm2 (8 mA/ft2) . This current density limit is about a
factor of 80 larger than peak current densities in full-scale
precipitators.
RESISTIVITY AND ELECTRICAL BREAKDOWN APPARATUS
The electrical resistivity of the fly ash was measured in a
conductivity cell built to the specifications of the American
Society of Mechanical Engineers Power Test Code 28. The cell
consists of an electrode cup for holding the ash sample and a
flat electrode with a guard ring that is placed lightly on the
dust, layer. Voltage is supplied to the electrode by a Keithley
240 A high-voltage supply. The current through the cell is
measured with an electrometer assembly similar to that used
with the corona discharge device.
Dielectric strength measurements on the fly ash samples were
made in a specially designed cell (Figure 6). The cell in-
corporates a Power Test Code 28 conductivity cell cup for holding
the sample and an upper electrode with rounded edges machined
from stainless steel. A Peschel Instruments 24-'cV high-voltage
supply furnishes a negative potential. The overload protection
indicator on the power supply is used for detecting breakdown.
Dielectric breakdown of air was measured with the same equipment.
These pieces of equipment were installed in an environmental
chamber so that temperature and humidity could be controlled
during the measurements.
ENVIRONMENTAL CHAMBER
Temperature and humidity are controlled and monitored in an in-
sulated chamber housing the wire-plate corona discharge device,
19
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3mm DUST LAYER
Figure 6. Electrode configuration for testing
dielectric strength of ash layer
20
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resistivity cells, and ash and air breakdown cells. System air
is bubbled through a temperature-controlled water bath for humidity
regulation. This air is heated by two thermostatically-controlled
heating elements mounted directly in front of a fan which circu-
lates air through the chamber. Two other heaters, rheostatically
controlled, heat the air before it passes through the fan and as
it is blown into the chamber. This combination of heaters allows
continuous control of temperature to 180°C. The humidity in the
chamber is controllable from 1% to 15% water vapor by volume with
an approximate accuracy of 1%.
Temperature inside the chamber is monitored by chromel-alumel
thermocouples at various locations in the chamber. A 150°C
mercury thermometer is visible through a glass window in the
chamber.
Moisture content of the chamber atmosphere is determined by
weighing the water absorbed by a cylinder of Drierite (CaSCK)
after a known volume of air is drawn through the cylinder from
the chamber. An Atkins dew point indicator continuously moni-
tors the relative moisture content.
PROCEDURES
Ash from the Gaston Power Station, Wilsonville, Alabama, was
separated into five particle-size fractions by the Donaldson
Company of Minneapolis, Minnesota. From these size fractions,
samples were quartered out and screened through an 80-mesh screen,
to break up large agglomerates and to remove any trash present
in the sample. Smaller samples were quartered out from the
above sample for chemical analysis, resistivity measurements,
dielectric strength measurements, and back corona measurements.
The sample for the back corona measurement is placed on the
plate of the corona discharge device and is smoothed to a depth
of 3 mm while shaking and vibrating the device to compact the
21
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dust layer. The same procedure is used to fill the two resistivity
cells and the dielectric breakdown cell.
These devices are then placed in the environmental chamber. The
electrodes are put in place on the dust layers and electrical
connections are made to each device. A check of electrical connec-
tions is made before closing the chamber. After the chamber is
closed the temperature and humidity controls are adjusted to give
the desired conditions by increasing temperature and moisture from
ambient conditions.
Once the environmental conditions are stabilized, electrical
readings are made.
The plate current distribution in the corona discharge device is
measured as a function of voltage for determining the voltage-
current characteristics, back corona, and sparkover data. The
voltage is adjusted and read from a Hipotronics Model 100 high
voltage meter. At constant voltage, inputs from the plate segments
are consecutively switched to a picoammeter and these currents re-
corded. As current readings are seldom constant, only as many
significant figures as indicated by the current range are recorded,
or the range of current fluctuation is recorded with the most
consistent value of current. Current readings are taken starting
at applied voltages of 10 kV and then at applied voltages up to
60-70 kV in increments of 2 to 4 kV. The size of the increment
depends on the stability of the voltage-current characteristics.
Oscilloscope waveforms of the current are noted concurrently
with the current measurements. Visual observations of the
device are also recorded.
The resistivity-cell current measurements are taken one minute
after voltage is applied. Current values from both cells are
consecutively read at one voltage by switching the picoammeter
input from one to the other. The data are taken so that each
22
-------
cell is read at two applied voltages, 200 V and 1000 V.
Determination of breakdown potentials in dust and air follows
an identical procedure. Voltage to the cell is slowly and
steadily increased at about 500 V/sec until the supply over-
load indicator shows sparkover. The potential at that point
is recorded.
CALCULATIONS
Current densities for the wire-plate and resistivity cells are
calculated as the current divided by the appropriate electrode
area. Resistivity, p, from the resistivity cell data is
calculated by Ohm's Law and the definition of resistivity:
RA
P = D-
V
R ~ I
so that p = l" " D
where
I = current from cell
V = voltage applied (200 or 1000 V)
R = resistance of ash layer
D = depth of ash layer
A = area of electrode
23
-------
SECTION V
RESULTS OF LABORATORY MEASUREMENTS
The current-voltage characteristics of the test device were
measured with a clean and with a dust-covered plate at various
temperatures from 60°C to 180°C and at humidities in the range
of one percent to fifteen percent water vapor by volume. Simul-
taneously, the resistivity and dielectric strengths of the ash
layers were measured. Layers of fly ash having mass median
diameters of 2.3 jam, 3.6 ym, 8.2 ym, 14.5 ym, and 40 ym were used
for the study. Particle size fractions having these mass median
diameters were obtained by mechanically separating fly ash from
the Gaston Power Station into the following size fractions:
0-3 ym, 3-5 ym, 5-7 ym, 7-15 ym, and >25 ym. The mass median
diameter of these particle size fractions were determined by
Bahco analysis of each size fraction.
The current density at which back corona forms theoretically
depends on the ratio of the dielectric strength and the resis-
tivity of the ash. The dielectric strengths of the different
particle size fractions of the Gaston ash are tabulated in Table
I. Corresponding values for air are given in this table for a
3 mm gap between electrodes similar to the ones used for the ash
electrical breakdown measurements.
The average dielectric strength of the 3 mm dust layers is
22.7 kV/cm, with a standard deviation of 2.3 kV/cm. The average
dielectric strength of the 3 mm air gap was 36.9 kV/cm* with a
standard deviation of 0.5 kV/cm. The dielectric strengths of
the different particle-size fractions bear no definite relation
to the particle size. The data also indicate no correlation of
the dust dielectric strength with either temperature or resistivity,
*According to Paschen's Law for a 0.3 mm gap with a uniform
field the dielectric strength of air at p = 720 Torr and T = 293°C
is 35.23 kV/cm.1S
24
-------
TABLE I
DIELECTRIC STRENGTHS OF ASH LAYERS
NJ
Ash
Particle
Size
15-25 ym
0-3 ym
3-7 ym
7-15 ym
>25 ym
T(°C)
-
60
70
80
100
120
140
160
180
100
160
100
160
100
160
100
160
H20(%)
1.0
2.0
1.6
2.4
3.0
3.4
3.9
4.7
3.0
4.0
2.7
4.3
3.0
4.0
2.4
3.6
Ash
Breakdown
(kV)
6.8
6.1
6.6
6.8
7.2
6.5
8.0
7.9
7.3
7.5
6.8
7.3
5.4
6.5
6.8
6.1
Dielectric
Strength
Ash Resistivity
kV/cm p(n-cm)
22.7
20.3
22.0
22.8
24.0
21.7
26.8
26.3
24.3
25.0
22.8
24.2
18.0
21.7
22.8
20.2
4.4xl010
2. 9x10 12
1.3xl012
5. 6x10 12
2. 0x10 1 3
5. 6x10 13
1.3xl013
6.4xlO)2
2.9xl010
8.3X101 i
1.2xl012
2. 2x10 12
8. 1x10 1 1
2.0xl01£
2.4xl012
2.6xl012
Dielectric Strength
Air
kV/cm
•TV v / Will
36.3
36.7
37.3
37.7
37.3
36.5
36.7
37.0
Average ash breakdown 6.8 kV
D. C. voltage increased at the rate of 0.5 kV/sec.
Dust layer thickness was 3 mm.
-------
Some investigators studying thin glass plates as well as 50 jjm
hollow glass spheres have observed that breakdown strengths
change from approximately 30 kV/cm to approximately 4 kV/cm with
a temperature change from 20°C to 200°C, while other investigators
found no temperature dependence.16 Results from this investigation
with Gaston Power Station ash showed no temperature dependence.
The physical properties of the dust layers studied in this in-
vestigation are tabulated in Table II. The resistivities of the
different particle size fractions are shown in Figure 7. The re-
sistivities in the surface conduction region shown in this figure
increased with increased particle size'. Bickelhaupt18
discusses the relationship of resistivity and particle size and
surface area. For the different particle size fractions, the
estimated diameter of voids in the dust layer ranged from 3 to
29 mm. The surface area per unit volume likewise changed an order
of magnitude.
These results indicate that changes in the physical properties
of the collected ash layer in a precipitator will influence pre-
cipitator operation by changing the ash resistivity and not by
significantly changing the dielectric strength of the ash layer.
EROSION AND DUST LAYER SURFACES
The dust layer surfaces of large particles at low resistivities
are eroded when current producing voltages are applied. A com-
bination of electrostatic forces and mechanical forces due to
"electric wind" scatters the dust, thins the layer and creates
surface irregularities. This phenomenon is especially detrimental
to close study of dust layers.
Gaston Power Station dust resistivities at ambient temperature
and humidity (23°C and 1-2% H20 by vol) are two or three orders
26
-------
TABLE II
PHYSICAL PROPERTIES OF DUST LAYERS FORMED
FROM THE GASTON ASH PARTICLE SIZE FRACTIONS
Size Range Medium Average
of Fraction, Diameter, Measured
ym ym Porosity,*
0-3
3-7
7-15
15-25
+ 25
2.3
3.6
8.2
14.5
40
76.7
69.9
62.4
57.3
54.7
Cross sectional Diameter, of Surface of the
average area circle particle with
of void formed by with equivalent No. of median
contact of spheres,** pore area, Particles/ diameter,++
ym2 ym cm" ym2
6
10
39
98
676
.15
.6
.2
.9
.5
2.
3.
7.
11.
29.
8
7
1
2
3
3
1
1
2
1
. 66x10 10
.232xl010
.30xl09
.675x10"
-35xl07
1
4
2
6
5
.66x10'
.072x10*
.llxlO2
.60xl02
.03xl03
Surface per
cm3 of dust, +
m2
.608
.502
.274
.177
.068
Determined by Bahco analysis
*Porosity = P = 100% x (1 - Apparent^densitv^f material,
**Cross sectional area of
void formed by contact of spheres = A = .36d2 [—'-—„— ]
1-
Too
+number of particles per unit vol. = N = (1 —— }/ov d3
++surface area of particle = Ag = ird2
+++surface are per cubic centimeter of dust layer = NAg = ir(l -
d = diameter based on assuming all the
particles are of the same diameter.
av = 7 for spheres1 7
for spheres = 6(1 -
/d
-------
10
o
2
O
CO
CO
UJ
tr
10
IOOO/T(°K)
°C
TEMPERATURE
Figure 7. Resistivity vs. temperature for different particle
size fractions; 1) 74.5% P(Porosity), 0-3 ym;
2) 67.8% P, 3-7 ym; 3) 64.7% P, 7-15 ym;
4) .58.0% P, 15-25 ym; 5) 54.3% P, >25 ym;
9.4% water vapor by volume.
28
-------
of magnitude lower than those at higher temperatures (180°C).
Lower resistivities cause correspondingly smaller voltage drops
across the dust layer for a given current density. This
decreases the tensile strength of the layer. The total cohesive
effects of van der Waals' forces are more pronounced for
smaller particles, making them more difficult to free from a
surface than larger ones.
The largest particle fraction studied, with particle diameters
greater than 25 microns, was blown badly under all moisture
conditions studied. A 3 millimeter layer was cleared from the
plate in about three minutes at room conditions with 30 kV
applied. Higher temperatures and moisture contents reduced this
effect; 0-3 ym and 3-7 ym size fractions were not significantly
affected by corona wind under any of the conditions investigated.
Dust was first disturbed at the outer edge of the layer,
particularly at corners. Higher voltages caused blowing at
all points on the surface (Figure 8 upper photograph). Craters
were formed at :the sites of back corona and sparkover (Figure
8 lower photograph). Large particle sizes showed peculiar
patterns in the dust layer at sites of back corona and spark-
over. Particles appeared to be redeposited and realigned on
the surface (Figure 9 upper photograph).
With a dust layer, sparkover normally occurred:on the inner plates,
forming craters. It often occurred at several points on the
surface. In high temperature and low moisture content conditions
cracks in the surface formed between plates. Sparkover and back
corona tended to occur in these cracks (Figure 9 lower photograph).
29
-------
Blown dust layer, large particles
25 )_im (Temp 86°C, Moisture 12V/O)
Craters formed by sparkover to un-
disturbed dust layer
Figure 8. Photographs of dust surface on the wire plate
corona discharge device.
ii
-------
Patterns formed on dust layer composed
of large particles by back-corona and
sparkover
Cracks formed in dust layer of 15-25
particles at 180°C
Figure 9. Additional photographs of dust surfaces on the wire
plate corona discharge device.
31
-------
THE WIRE PLATE CORONA DISCHARGE CHARACTERISTICS
The voltage-current (V-I) characteristics for the wire-plate
negative corona discharge device* discussed previously are
plotted in Figure 10 for temperatures of 12,0'°G1,and 60°C. The
shift in the V-I characteristic with increasing temperature re-
sults from the decrease in gas density with increasing tempera-
ture. The series of graphs in Figure 11 show the changes in the
voltage-current characteristics* that occur.with .an increase in
ash resistivity"'from 3x109 fl-cm to 1x1012 fl-cm.-- The solid line
in each of the'se plots is the characteristic obtained without
an ash layer..;'; The voltage-current characteristicvwith an ash
layer is indicated by the circles. The dashed., line is the clean
plate characteristic shifted according to the theoretical voltage
\ " '
drop across the ash layer. The voltage drop is calculated using
Ohm's Law, the measured resistivity, and the measured thickness
of the dust layer.
AV = jpl
AV = potential across the dust layer
j = current ..density ,• . ...
f- ";/. . •. . ' '•£
p = resistivity "..:
1 = depth of the ash layer. ;;
The shift in the V-I characteristic with a 109 ^ft-cm resistivity
ash is shown in graph 1 of Figure 11. The difference between
these two curves is masked in practice by variations in moisture,
temperature, and corona wire characteristics between measurements
with and without ash layers.
For a 1010 ft-cm,resistivity ash, the shift is several kilovolts
and is easily observed as shown in graph 2 of Figure 11. The
data points are near the shifted curve until back corona occurs.
*Center segment of the wire plate discharge device. Note that the
V-I characteristics are plotted on semi-log scales. The curves
have a normal appearance when plotted on linear scales.
32
-------
z
UJ
cr
ct
o
UJ
o
tr
<
o
10
-4
10
-5
IO'6
10
-7
I20°C
10 20 30 40 50
POTENTIAL , kV
60
70
80
Figure 10. Clean plate voltage-current characteristics
33
-------
I0-4i—I GRAPH I h
ic
5
oc
UJ
I-
UJ
o
I I0~4
o:
o
UJ
P~6.6xl09 Q -cm
H20
CLEAN PLATE .
— - SHIFTED CLEAN
PLATE
O ASH PLATE
(GRAPH 2 }-
, IOO°C,3v/o
o
-------
With the formation of back corona, there is a sharp increase in
current, which is clearly observed on a semi-log plot. This break
in the characteristic determines the start of back corona.
For higher} current'S^r glpfaswere(observed at various locations on
the dust layer; in addition, bursts 6;f current occurred at random
intervals/of approximately 0^:1 to'l^sec. The burst lasted for
,>'\ •'. -~ , , '•.,, . { -... • . - •• • . i jjj'^'xi-,'
approximately 2 visec'with peak curr4|ijfes of 2mA. Typical pulses
'.•f :'V<- -.-•' "' -•• -1 • '•••'"^•:;s-£,h^*
are displayed in Figure 12. The\in'di%Ldual pulses occurred less
frequently than the flares observed by; Penney-1 1 and lasted four
times longer'*- The.,peak currents-were a factor of 10 smaller than
the 30-80 *lnAc peak: current that Penney?'reported.. Two separate
types of'pulses were observed both of'which occurred only when back
corona spots appeared on the dust layer surfaces. The positive
pulses had a high frequency component superimposed on the pulse
and appear to be related to changes in the characteristics of the
brushes on the corona wire. Some of the brushes appeared to change
from blue to,yellow when back corona formed, in agreement with an
•-•*.. ' ^'l~-,- ~~ > f - "^"' _ - ^
earlier observation by White. The pulses indicated that the
back corona sp'bt was pulsing. These purrent pulses are much larger
*>•"-" - .-. . •' '. . .[ •*
than the'^Trichel pulses19 that are observed when negative corona
is firstiMnitiated. *
' '
The calculated voltage drop across th^ dust with a resistivity of
3x1011 J|reji|is "indicated by the dasjled! curve in graph 3 of Figure 11.
$ •',t!-?f?-'--:'•: •_•';'.: • •" '• '-'"' •-- "" i ?^y
The measured V-i did not follow this :curve. The V-I characteristics
with a 10:1 fi-cm dust layer initially had higher currents than the
clean plate characteristic and paralleled the shifted clean plate
characteristic until back corona developed. Starting currents
higher than clean plate starting currents were observed several times
for dust layers with 1011 J^-cm resistivities. A satisfactory ex-
planation for this behavior does not exist at this time.
35
-------
to
LU
to
LL)
(T
CC
73
O
£
o
LJ
Q:
a:
o
OSCILLOSCOPE
OUTPUT
ELECTROMETER
OUTPUT
TIME, lOms/cm
OSCILLOSCOPE
OUTPUT
TIME , O.Susec/cnn
Figure 12. Oscilloscope traces of back corona pulses
36
-------
In graph 4, the V-I characteristic for a 1012 fi-cm ash layer is
plotted. Back corona formed near the corona onset. The current
increased sharply.
A comparison of the V-I characteristics of the different particle
size fractions indicates that ash resistivity is the important
parameter determining where back corona and where sparkover occur.
The results of the measurements are shown in Table III. The fines
have the highest sparkover potential and the lowest resistivity.
Penney2 also found higher sparkover voltages with his fine particle
size fraction, which had higher resistivities than his larger par-
ticle sizes. However, Penney's large particle size fraction con-
tained large carbon particles, which reduced the resistivity of
the ash.
The chemical compositions of the ash fractions used in this study
are tabulated in Table IV. The data indicate that the carbon
content-loss on ignition (LOI) of these size fractions was largest
in the small particle size fraction and least in the large par-
ticle size fraction. Measurements by Bickelhaupt2° show that re-
sistivity of fly ash is unaffected by a carbon content of 10% or
less. Except for the carbon content, the compositions of the
size fractions are similar. However, as shown by Bickelhaupt18
the resistivity of the ash decreased with decreasing particle size
in the surface conduction region.
As mentioned previously, the surface characteristics of ash layers
formed from the different particle size fractions varied signifi-
cantly because of the decrease in the cohesiveness of the dust
with increasing particle size. The small particle size fraction
produced dust layers that cracked when the Teflon base of the
wire plate discharge base expanded. The large particle size
fractions produced dust layers that were easily blown by the corona
37
-------
TABLE III
SPARKOVER AND BACK CORONA VOLTAGES FOR FIVE DIFFERENT PARTICLE SIZE FRACTIONS
co
CD
Back
Particle Back Corona
Size Moisture* Sparkover Corona Current
Fraction, Temperature* Volume, Voltage, Voltage. Density.
_ S _ ' . o'
ym
0-3
3-7
7-15
15-25
>25
°C
90
90
90
90
90
%
13.
11.
14
12
12
5
8
Resistivity
kv
52
48
46
42
42
kV
52
44
42
42
34
yA/cm
>1.7
.99
.76
.71
.25
1
2
3
1
ft- cm
IxlO9
.9xl010
.4xl010
.6xl010
.SxlO1 '
* Chosen to obtain reasonable resistivities.
-------
TABLE IV
CHEMICAL ANALYSES OF SIZE FRACTIONATED FLY ASH SAMPLES
Particle Size Range
Chemical
Constituent
Li20
Na20
K20
MgO
CaO
Fe203
A1203
Si02
Ti02
P205
SO 3
LOI
0-3y
.08
1.16
2.86
1.13
1.59
6.62
28.136
45.04
3.19
0.705
0.81
8.47
98.981
+ 0.81
3-7y
.08
1.07
2.63
1.08
1.84
5.93
29.52
45.35
2.74
0.482
0.54
8.71
99.432
0.54
7-15y
.08
.544
2.63
1.17
1.54
6.71
30.09
47.39
2.80
0.321
0.29
6.16
99.435
0.29
15-25y
.08
1.34
2.67
1.21
1.41
7.72
27.23
49.74
2.87
0.356
0.22
3.67
98.30
0.22
+25y
.08
1.23
2.67
1.28
1.50
12.84
24.66
51.14
2.74
0.496
0.09
0.85
99.48
0.09
Avg.
.08
1.07
2.69
1.17
1.58
7.96
27.92
47.73
2.87
0.47
0.39
5.57
99.50
SUM 99.79 99.97 99.725 98.52 99.57
39
-------
wind when using conditions which produced low resistivities.
Penney2 also experienced this problem. These conditions affected
our data. For example, sparking normally occurred at locations on
the anode plate where a crack was present in the dust layer.
However, the effects appeared to be slight compared to changes in
resistivity.
The influence of ash resistivity on the formation of back corona
is displayed in Figures 13 and 14. In Figure 13 the voltages at
which back corona occurred and at which sparkover occurred as
determined from the measured V-I characteristics are plotted as
function of resistivity. The horizontal lines at the top of the
graph indicate the voltages at which sparkover occurred for
various temperatures and show a decrease in sparkover potential
with increasing temperature. The dashed curve indicates the
potential between the outer surface of the dust layer and the
corona wire at the formation of back corona or sparkover for a
3 mm dust layer with a dielectric strength of 24 kV/cm. The
solid curve is an estimate of the average voltage at the formation
of back corona. The data points for the voltages at which back
corona occurred are denoted by circles and the voltages at which
sparkover occurred are denoted by triangles.
For ash with a resistivity greater than 2x1010 ft-cm, the current
density at sparkover is large enough that the ohmic voltage drop
in the dust layer exceeds the dielectric strength of the ash layer
and back corona is observed at voltages significantly below the
sparkover voltage.
The dashed curve in Figure 13 was obtained by subtracting the
theoretical voltage drops in the ash layer at sparkover (or at
the formation of back corona if back corona existed before spark-
over) from the average voltage curve for the formation of back
40
-------
68
64
60
56
52
48
UJ
o
CLEAN PLATE SPARKOVER VOLTAGE AT IOO°C
CLEAN PLATE SPARKOVER VOLTAGE AT 160°C
O VOLTAGE AT FORMATION OF BACK CORNA
A VOLTAGE AT SPARKOVER
10
15
RESISTIVITY ,ohm-cm
Figure 13. Wire plate corona discharge device voltages for sparkover
and for formation of back corona as a function of
resistivity; wire to plate spacing 6 cm. Solid curve
estimate of the average voltage at formation of back
corona, dashed curve potential between dust surface and
wire at formation of back corona.
-------
10
r5
CJ
o
\
<
V)
LJ
O
LU
or
oc
o
IO'6
10
-7
10"
D MEASURED CURRENT DENSITY
AT START OF BACK CORONA
\ X MAXIMUM CLEAN PLATE CURRENT DENSITY WITHOUT SPARKOVER
\ >y ^^- CALCULATED MAXIMUM
V ^^"^ PEAK CURRENT DENSITY
\ \s WITHOUT BACK CORONA
\ o o
— o \ cPn
N o o
• ^
CALCULATED v
MAXIMUM AVERAGE \
CURRENT DENSITY
_WITHOUT BACK CORONA
\ o
MINIMUM STABLE STARTING CURRENT DENSlV^
FOR CLEAN PLATE
10
10
10'
10
12
10
13
ASH RESISTIVITY, ohm-cm
Figure 14.
Current densities for formation of back corona in
the wire plate corona discharge device as a function
of resistivity
10
14
-------
corona. The voltage drop across the ash layer after the for-
mation of back corona is assumed to remain fixed at a voltage
equal to the product of the layer thickness and the dielectric
strength of the ash.
For ash with a resistivity below 2x1010 fl-cm, the current density
at sparkover produced voltage drops in the dust layer that were
less than the measured breakdown voltage of the dust layer.
However, as shown in Figure 13, the voltages at sparkover were
substantially below the clean plate sparkover voltages and back
corona was observed just before sparkover. A full explanation
is not available. Cracks and distortion? in the dust layers
can produce high electric field regions that probably account
for some of the reduction in sparkover voltages. The reduction
in the sparkover voltage is also partially the result of the
smaller spacing between the wire and dust layer when compared
to the separation between the wire and plate.
In a precipitator, the voltage drop across the dust layer will
vary in time as the thickness of the dust varies and, depending
on the dust resistivity and operating current densities, the
changing voltage drop can account for changes in precipitator
performance with time, especially when a precipitator is first
turned on.
Although there is considerable scatter, the data in Figure 13
show that the potential difference between sparkover and the
formation of back corona increases with increasing resistivity.
Theoretically, back corona occurs when the electric field pro-
duced in the dust layer is greater than the dielectric strength
of the ash. The average electric field in the dust layer is
given by the product of the current density and the resistivity
43
-------
of the layer. Thus, current density at the formation of back
corona is given by
. _ breakdown
p at breakdown
where E^^^^^ is the dielectric strength of the dust layer and
pbreakdown is the resistivity of the ash layer at breakdown.
The solid diagonal line in Figure 14 is a graph of the above
expression for E, , , =24 kV/cm. The two dashed horizontal
breakdown
lines represent the respective maximum and the minimum current
densities achievable with the wire plate corona device used in
this series of experiments. These horizontal lines set the
operating limits for the device.
The squares in Figure 14 represent either the minimum current
density obtained if back corona occurred as soon as the corona
discharge was initiated or the current density at which the
back corona occurred as determined from the V-I curves and back
corona current pulses.
A comparison of the solid diagonal line and the squares indi-
cates reasonable agreement between the theory of the formation
of back corona as function of resistivity and the actual processes
The two data points near 10 *1 fi-cm that lie above the diagonal
line correspond to data from V-I curves similar to the one shown
in graph 3 of Figure 11.
The average current density for the total plate area of the wire
plate discharge device was 0.3 to 0.5 times the peak current
density.
44
-------
Hence, the dashed diagonal line in Figure 14 represents the maxi-
mum average current density at which the device could be operated
without the formation of back corona. Likewise, the average
operating current density of precipitators is below the peak
theoretical predicted values.
VARIATION IN SPARKOVER VOLTAGE AS A FUNCTION OF THE WIRE TO
PLATE SPACING
The current densities at which back corona developed as a
function of the wire to plate spacing for the corona discharge de-
vice are displayed in Figure 15. The current densities at which
sparkover occurred are indicated by squares for the clean plate.
The sparkover voltages as a function of wire to plate spacing for
both the clean plate and the ash covered plate are plotted in
Figure 16.
The current density plot suggests that the current density for
the formation of back corona is independent of the wire to
plate spacing as predicted by theory. Using E = jp to calculate
the electric field in the dust layer when back corona forms, we
obtain a value of 16.5 kV/cm*, which is approximately 6 kV/cm
less than the measured dielectric strength of the dust layer.
The estimated error in the resistivity measurement can account
for the difference between this calculated dielectric strength
and the measured dielectric strength.
Although changing the wire to plate spacing does not affect the
current density at which back corona forms, it does decrease the
voltage at which back corona and sparkover occurs as illustrated
by Figure 16. The decrease in the sparkover voltage when a dust
layer is present results from the increase in peak current density
that is obtained for a given voltage when the wire to plate spacing
is decreased. The significant increase .in current density with
* j = 1.1 yA/cm2
p - l.SxlO10 ft-cm
45
-------
o
(O
Z
UJ
o
I-
UJ
a:
a:
o
102
10'
10
-I
10
-2
Q
O CURRENT DENSITY AT FORMATION
OF BACKCORONA, p/vl.5xlOlon-cm
D CURRENT DENSITY AT SPARKOVER
FOR CLEAN PLATE
2345
WIRE TO PLATE SPACING, cm
Figure 15. Current densities for sparkover and for formation
of back corona as a function of wire to plate spacing,
46
-------
UJ
cr
ui
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80
70
60
50
40
30
20
10 —
ACLEAN PLATE
O3mm ASH LAYER
P=lxlOl°fi-cm
0 I 2 3 4 56
WIRE TO PLATE SPACING.cm
Figure 16. Sparkover voltage as a function of wire to plate spacing,
47
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decreasing wire plate spacing is illustrated in Figure 17 by
the current density distributions obtained at a wire potential
of 21 kV with wire to clean plate spacings of 3 cm and 5 cm.
The ratio between the peak current density and the average
current density is also increased by decreasing the wire to
plate spacing.
The effects discussed above can be expected to occur in full
scale units where there is poor alignment of the wire and
plate. Back corona will form at a lower average current density
than the resistivity of the ash would indicate and the operating
voltage throughout the section with poor alignment will be greatly
reduced.
CURRENT DENSITY VARIATIONS
The current densities to small rectangles on the plate parallel
to the corona wire are plotted in Figure 18 and 19. The solid
lines in Figure 18 represent the clean plate current density
distributions perpendicular to the corona wire for voltages of
22 kV, 28 kV, 40 kV,. and 52 kV.
The data points represent the current density distributions for
a dust layer with and without back corona. This data shows that
the effect of back corona is localized in the region in which
the electrical breakdown occurs. In Figure 19, current density
distributions for a 6x1012 fi-cm resistivity dust layer are plotted,
The area covered by back corona is much larger than in the pre-
vious case. The outer two segments appear to be free of back
corona.
48
-------
10
-5
10
-6
CVJ
E
o
>-
K
lf>
LJ
O
UJ
CCL
a:
10"
10
-8
Figure 17,
01 2345
DISTANCE FROM CENTER,cm
Current density distributions: Wire potential of
21 kV, clean plate spacings of 1) 3 cm and 2) 5 cm
49
-------
2x10
-5
10
,-5
CVJ
-------
2x10'
10
-5
CM
E
o
- CLEAN PLATE
DUST COVERED PLATES
WITH BACK, CORONA
P=5.6xlOl2Q-cm
WIRE TO PLATE
O 28 kV
O24kV
A 22 kV
UJ
Q
UJ
CC
cr
3
o
10-6
O
A
52 kV
40 kV
28 kV _
10
-7
-8
22 kV
10
Figure 19
0
I
2345
DISTANCE FROM CENTER LINE,cm
Current density distributions for clean plate at
several applied potentials and for dust covered
plate with back corona
51
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SECTION VI
MEASUREMENTS ON FULL-SCALE PRECIPITATORS
The operating current densities versus resistivity for several
full-scale precipitators are plotted in Figure 20. The resistivi-
ties were measured using a point-to-plane resistivity probe.21
The operating current densities are the typical spatial and time
average current densities that existed during efficiency tests.
A large portion of the data in this plot came from an ash con-
ditioning study by Dismukes.1
An analysis of the data plotted in Figure 20 is difficult for
several reasons. There is no well defined point for the most
efficient current density setting when sparking and back corona
occur. Thus, the operating current densities are set to some
degree by the operator's opinion as to what the best setting is.
A second problem is that in several cases the operating current
densities are set by the power or current capability of the power
supply and not by the electrical characteristics of the precipi-
tator. A third problem is that the average current density is
plotted; and back corona and sparking depend on peak current
densities, which vary from one unit to another. In one case,
plant personnel stated that the plates had buckled and that the
wire to plate spacing varied throughout the unit (data point 6).
The operating average current densities were substantially below
those that would normally be expected for the resistivity measured
at that installation. But, when consideration is given to the
effect that misalignment has on peak current densities, the ob-
served current densities are not surprising. Another difficulty
is in the variation in resistivity across the width of the pre-
cipitator when a Lundstrom preheater precedes the unit.* Laboratory
measurements also suggest that variations in resistivity along the
length of the precipitator may occur for temperatures at which
*Documentation showing the location of the resistivity measure-
ments with respect to the precipitator sections was not avail-
able for much of the data plotted in Figure 20.
52
-------
U)
10'
10'
RESISTIVITY, ohm-cm
Figure 20.
Operating current densities as a function of resistivity
for various plants tested by Southern Research Institute.
Numbers refer to data in Table V. Circles, inlet sections;
triangles, outlet sections; squares, unknown sections;
solid symbols, either NH3 or SO3 injection.
-------
TABLE V
PRECIPITATOR ELECTRICAL DATA
Plant
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
Resistivity,
ft- cm
2x10"
5.3x10"
2x10"
7.3xl010
7.3xl010
2.8xl010
1.3xl07
1.2xlO:o
3.0x10'
2.0xl011
4.0xl011
3. 0x10 M
2.2x10"
3.0x10"
3.8xl010
2.8x10°
2.0xl08
2. 0x10 e
8. 0x10 7
1.6x10"
3.0xl010
Electrical
Section
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
inlet
Outlet
Inlet
Outlet
Inlet
Inlet
Inlet
Outlet
Inlet
Outlet
Inlet
Inlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Inlet
Inlet
Outlet
Operating
Voltage, kV
40
36
48
44
50
22
40
46
41
44
39
35
38
36
24
30
30
36
38
41
39
_
42
40
_
35
45
40
47
41
38
29
46
30
37
41
31
29
Operating
Current
Density,
nA/cnr
33
33
49
33
2
25
25
95
5
95
20
33
35
45
8
14
11
13
25
22
40
70
22
60
31
29
30
62
15
60
45
45
20
45
4
38
83
95
Injection
NH3
NH3
32 ppm SOs
32 ppm SO 3
14 ppm SO 3
14 ppm SO 3
14 ppm NH3
14 ppm NH3
13 ppm NHs
13 ppm NH3
5 ppm NHj
20 ppm NH3
20 ppm NH3
10 ppm NH3
Comment
All plants coal
fired boilers
except as noted
Cement plant
Electrode
misalignment
Electrode
misalignment
54
-------
surface conduction is important (< 230°C).
For resistivities less than 109 ft-cm, direct current densities
larger than 1 yA/cm2 are feasible without exceeding the di-
electric strength of the collected dust layer. However, current
densities in field units did not exceed 0.1 yA/cm2 even for these
low resistivities.
There are several reasons for this, one of which is that several
of the units are equipped with power supplies that are inadequate.
A second reason is the large size of field units. It is not un-
usual for a power supply to handle a plate area of 300 m2 (3000 ft2)
which is much larger than the 100 cm2 that our laboratory
corona discharge device had. This increased area and the imprac-
ticality of machining rounded edges on all parts of a field unit
greatly increase the probability for sparkover. Our laboratory
measurements also indicated that clean plate sparkover voltage
and current density are reduced to some degree even by low re-
sistivity dust layers.
In some installations the current appeared to be limited by space
charge, especially when ammonia was injected as an agent.
This point is discussed by Dismukes in a recent report.1 He
suggests that NHs combines with S03 to form a fine particulate
which results in a greatly increased space charge. This in turn
results in increased operating voltages, decreased current den-
sities, and increased sparkover rates. These occur without a
significant increase in resistivity and show that resistivity is
not the only factor that can increase sparkover rates. Calcu-
lations of the effects of particle space charge for cylindrical
corona indicate that the space charge results in a higher electric
field at the passive electrode.22 For negative corona, the field
strengths at the anode would be increased and this could lead to
55
-------
lower sparkover potentials than those obtained without a dust
loading.
A comparison of the inlet and outlet data shown in Figure 20 in-
dicates that in most cases the outlet current density exceeds
the inlet current density, while the inlet voltage exceeds the
outlet voltage. There are two possible explanations for the
variations in the voltage-current characteristics from the inlet
to outlet. One attributes the variations to changes in dust load-
ing and the other to changes in the thickness and resistivity
of the collected dust layer.
The effect of a variation in resistivity from the inlet to the
outlet is illustrated by the following considerations. If a
resistivity of 10J1 ft-cm is obtained in the inlet and 5x1010 fi-cm
in the outlet, the inlet peak current density could be as high as
200 nA/cm2 without back corona and the outlet could be as high
as 400 nA/cm2 for a breakdown strength of 20 kV/cm.
A half order of magnitude change in the resistivity of the
collected dust layer from the inlet section to the outlet section
of a precipitator is reasonable. Our laboratory measurements
show a decrease in ash resistivity of nearly one order of magni-
tude for particle size fractions with mass median diameters of
40 to 2.3 ym. The resistivity decreases with decreased particle
size for temperatures at which surface conduction is important.
Thus, a decrease in resistivity from the inlet section to the
outlet section results from the fractionation of the ash into a
larger particle size fraction in the inlet section and smaller
particle size fractions throughout the rest of the unit. In
addition, a change in resistivity can result from a temperature
variation from the inlet to the outlet. For hot precipitators,
the resistivity would be expected to increase from inlet to outlet
and for cold precipitators, the resistivity would decrease from
the inlet to outlet.
56
-------
Average current densities depend on the spatial variation and
time variation of the current. For full wave rectification
and no filtering, the time average current density is 70.7% of
the peak current. Typical spatial variations result in spatial
average currents equal to 60% of the peak currents.
Thus, the time and spatial average current equals approximately
50% of the peak current. For our hypothetical problem, the
maximum possible average currents without back corona are
estimated to be 100 nA/cm2 for the inlet and 200 nA/cm2 for the
outlet. If the depth of the collected dust layer is the same
in the inlet and outlet, for the above conditions, the voltage
drop across the dust layers is the same in both sections. For
the same configuration in both sections, the operating voltage
across the discharge should be higher in the outlet since the
currents are higher. However, this does not agree with most of
the data tabulated in Table V. Theoretically, the thickness of
the dust layer decreases exponentially throughout the precipi-
tator if rapping variations are neglected. This results in
larger voltage drops across the inlet dust layers when both
units are operated with current densities near the critical
value for the electrical breakdown of the dust layer. This can
lead to higher voltages in the inlet sections than in the
outlet sections.
The above explanation for the variations in electrical charac^
teristics from the inlet to the outlet is unsatisfactory for
low resistivities (<5xl09 fi-cm), where voltage drops less than
250 V occur for 0.5 cm thick dust layers and a current density
of 0.1 yA/cm2. The decrease in the space charge produced by
the charged suspended particulate from the inlet sections to the
outlet sections appears to be the dominant reason for larger
currents and lower voltages in the outlet sections.
57
-------
The time required for changes in the voltage-current character-
istics gives some indication of the importance of the above
mechanism. Dismukes1 observed that when ammonia was injected
the time lag between injection and the effect on precipitator
operation was very short. In contrast, resistivity effects take
longer to occur, since they depend on the accumulation of dust
on the collecting plates.
58
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REFERENCES
1. Dismukes, E. B. Conditioning of Fly Ash with Ammonia.
In: Proceedings, Symposium on Electrostatic Precipitators
for the Control of Fine Particles. Pensacola Beach.
Environmental Protection Agency, Washington, D. C. Publi-
cation Number EPA-650/2-75-016. January, 1975. NTIS PB
240440/8WP. p. 257-287.
2. Kercher, H. Electric Wind, Back Discharge and Dust Re-
sistance as Parameters in Electrostatic Precipitators.
Staub-Reinhalt. Luft (in English) . 2_9_(8) : 14-20, 1969.
3. Penney, G. W., and T. E. Alverson. Influence of Mechanical
Collection on Electrostatic Precipitator Sparkover Voltage—
A Laboratory Simulation. IEEE Trans. Ind. Gen. Appl. 7^(3):
433-438, May-June, 1971.
4. Wolcott, E. R. Effects of Dielectrics on the Sparking
Voltage. Phys. Rev. 12:284-292, 1918.
5. Franck, S. Funkenentladungen in Luft-Staubgemischen [Spark
Discharges in Air Dust Mixtures]. Z. Phys. (Berlin).
8^7:323-229, 1933.
6. White, H. J. Characteristics and Fundamentals of the
'Back Corona1 Discharge. (Presented at Gas Discharge
Conference, Brookhaven National Laboratory, Upton, N. Y.,
1948.)
7. Penney, G. W. Electrostatic Precipitation of High-Resis-
tivity Dust. Trans. Am. Inst. Electr. Eng., Part 2.
713:1192-1196, 1951.
8. Penney, G. W., and J. G. Hewitt. Some Measurements of
Abnormal Corona. Trans. Am. Inst. Electr. Eng., Part 1.
77:319-327, July, 1958.
59
-------
9. Penney, G. W., and S. E. Craig. Sparkover as Influenced
by Surface Conditions in D-C Corona. Trans, Am. Inst.
Electr. Eng., Part 1. 21:112~118' MaY- I960.
10. Penney, G. W., and S. Craig. Pulse Discharges Preceding
Sparkover at Low Voltage Gradients. Trans. Am. Inst.
Electr. Eng., Part 1. 80^:156-162, May. 1961.
11. Simm, W. Untersuchungen xiber des Riickspruhen bei der
elektrischen Staubabscheidung.[Studies of the Back Spray
in Electrical Dust Removal] Chem. Ing. Tech. (Weinheim).
2:43-49, 1959.
12. Herceg, Z., and R. M. Huey. Model for Corona Modes in
Point-to-Plane Device with Coated Electrodes. Proc. Inst.
Electr. Eng. (London). 12_0:394-399, 1973.
13. Hall, H. J. Trends in Electrical Precipitation of Elec-
trostatic Precipitators. In: Proceedings of Electrostatic
Precipitator Symposium, Birmingham, Alabama. February
23-25, 1971. p. 75-116.
14. White, H. J. Industrial Electrostatic Precipitation.
Reading, Massachusetts, Addison-Wesley, 1963. p. 327.
15. Alston, L. L. High Voltage Technology, Oxford University
Press, London, 1968. p. 48.
16. Morey, George W. The Properties of Glass. 2nd Edition.
Reinhold Publishing Corp., New York, 1954. p. 532.
17. Dallavalle, J. M. Micromeritics. 2nd Edition. Pitman
Publishing Corp., New York, 1948. p. 131.
18. Bickelhaupt, R. E. Surface Resistivity and the Chemical
Composition of Fly Ash. J. Air Pollut. Contr. Assoc.
i25_:148-152, February, 1975.
19. Loeb, L. B. Fundamental Processes of Electrical Discharges
in Gases. John Wiley and Sons, Inc., New York, 1939. p. 517
60
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20. Bickelhaupt, R. E. Personal Communication.
21. Nichols, G. B. and H. W. Spencer. Test Methods and
Apparatus for Conducting Resistivity Measurements.
Southern Research Institute, Birmingham, Alabama,
Contract No. 68-02-1083. Environmental Protection
Agency, Research Triangle Park, N.C., 1975.
22. Lowe, H. J. and D. H. Lucas. The Physics of Electro-
static Precipitation. Brit. J. Appl. Physics, (London),
Suppl. _2:540-47, 1953.
61
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TECHNICAL REPORT DATA
(Please read Innmctions on the reverse before completing)
1. REPORT NO.
EPA-600/2-76-144
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Electrostatic Precipitators: Relationship Between
Resistivity, Particle Size, and Sparkover
5. REPORT DATE
May 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
HerbertW. Spencer, HI
8. PERFORMING ORGANIZATION REPORT NO.
SORI-EAS-75-629
3134-XVI
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-027
11. CONTRACT/GRANT NO.
68-02-1303
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND I
Final; 4/74-12/75
PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA-ORD
^.SUPPLEMENTARY NOTES JERL-RTP Project Officer for this report is L.E. Sparks, Mail
Drop 61, Ext 2925.
. ABSTRACT
rep0rt gives results of a study of the relationships of the electrical
resistivity of fly ash, its particle size, the occurrence of back corona and sparkover,
and the electrical characteristics of electrostatic precipitators (ESP's). The study
included laboratory measurement of the dielectric strengths and resistivity of five
particle-size fractions of a fly ash sample and measurement of the current densities
and voltages at which back corona and sparkover occurred for a 3 -mm dust layer
covering the plate of a wire-plate negative -corona discharge device. Results showed
that the peak current density for the formation of back corona depended on the resis-
tivity of the dust covering the positive electrode. Operating current densities for
full-scale ESP's are discussed in relation to fly ash resistivity.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Electrostatic
Precipitators
Fly Ash
Electric Corona
Dust
Electric Sparks
Flashover
Air Pollution Control
Stationary Sources
Back Corona
Particulate
Electrical Resistivity
Measurement
13B
2 IB
20C
14B
11G
B. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
68
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
62
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