xvEPA
United States Industrial Environmental Research EPA-600/7-80-077
Environmental Protection Laboratory April 1980
Agency Research Triangle Park NC 27711
Charge Measurements
of Particles Exiting
Electrostatic Precipitators
Interagency
Energy/Environment
R&D Program Report
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
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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
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9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
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essary environmental data and control technology. Investigations include analy-
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This report has been reviewed by the participating Federal Agencies, and approved
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tion Service, Springfield, Virginia 22161.
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EPA-600/7-80-077
April 1980
Charge Measurements of Particles
Exiting Electrostatic Precipitators
by
J.R. McDonald, M.H. Anderson, and R.B. Mosley
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
Contract No. 68-02-2610
Program Element No. EHE624
EPA Project Officer: Leslie E. Sparks
Industrial Environmental Research Laboratory
Office of Environmental Engineering and Technology
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
-------
ABSTRACT
The objective of this research was to investigate particle
charging in positive and negative corona discharge as a function
of temperature from 38°C to 343°C in order to establish, especial-
ly at hot-side precipitator temperatures, the relative effective-
ness of the two possible methods of charging.
In positive corona discharge, only positive ions can partic-
ipate in the particle charging process. In negative corona
discharge, there is the potential for both negative ions and free
electrons to participate in the particle charging process. Thus,
since different charging mechanisms are possible for positive and
negative corona discharge, the values of particle charge obtained
under similar electrical operating conditions for the two cases
might differ.
The values of charge on individual particles exiting two
different laboratory precipitators have been measured in an exper-
imental apparatus which utilizes a Millikan cell. The measurement
system was capable of obtaining data on particles with radii down
to approximately 0.2 ym. Measurements were directed at fine
particles with radii approximately between 0.3 ym and 1.5 ym.
Measurements were obtained for redispersed fly ash particles
carried in air at temperatures from 38°C to 343°C. The electrode
geometries and electrical operating conditions utilized in the
experiments are typical of full-scale precipitators.
At comparable voltages and currents for positive and negative
corona discharges, the data show that the ratio of the values of
negative to positive charge for radii in the range 0.6-1.3 ym
increases from a value of approximately 1 to a value of approxi-
mately 2 as the temperature increases from 37°C to 343°C. The
predictions of a mathematical model of electrostatic precipitation
which employs an ionic charging theory show good agreement with
all the positive charging data, but show good agreement with the
negative charging data only at temperatures below 37°C. The
differences in the measurements and the model predictions are con-
sistent with the postulation of free electron charging at elevated
temperatures in negative corona discharge.
111
-------
This report is submitted in partial fulfillment of Contract
68-02-2610, Task 10, by Southern Research Institute under sponsor-
ship of the U.S. Environmental Protection Agency. This report
covers the period October 1, 1978, to March 10, 1979, when it was
completed.
IV
-------
CONTENTS
Abstract ii
Figures v
Tables vii
Acknowledgments viii
Nomenclature ix
Metric Conversion Factors.... x
1. Introduction 1
2. Summary and Conclusions 3
3. Recommendations 4
4. Measurement Technique and Apparatus 5
5. Experimental Program, Data, and Results 11
6 . References 45
-------
FIGURES
Number Page
1 Experimental apparatus for measuring the radius
and charge of particles 8
2 Theoretical and measured particle charge as a
function of particle radius at the outlet of
laboratory Precipitator A at ambient tempera-
ture 12
3 Clean air, wire, and plate voltage-current curves
for Precipitator C at ambient temperature 17
4 Clean air, wire, and plate voltage-current curves
for Precipitator C at 148°C (300°F) 18
5 Clean air, wire, and plate voltage-current curves
for Precipitator C at 343°C (650°F) 19
6 Average voltage-current curves with particles for
Precipitator B at 38°C (100°F) 20
7 Voltage-current curves for the different electri-
cal sections with particles for Precipitator C
at 148°C (300°F) 21
8 Average voltage-current curves with particles
for Precipitator C at 343°C (650°F) 22
9 Measured positive and negative particle charge
versus measured radius and comparison with
theory at 38°C (100°F) and 15 nA/cm2 (13.9
yA/f t"5 ) 23
10 Measured positive and negative particle charge
versus measured radius and comparison with
theory at 38°C (100°F) and 35 kV 24
VI
-------
Number
11 Measured negative particle charge versus measured
radius and comparison with theory at 38°C
(100°F) with 15 nA/cm2 (13.9 yA/ft2) and
36 nA/cm2 (33.4 yA/ft2) 25
12 Measured positive and negative particle charge
versus measured radius and comparison with
theory at 148°C (300°F) 31
13 Measured positive and negative particle charge
versus measured radius and comparison with
theory at 232°C (450°F) 34
14 Measured positive and negative particle charge
versus measured radius and comparison with
theory at 343°C (650°F) and 30 nA/cm2
(27.9 yA/ft2) 38
15 Measured positive particle charge versus measured
radius and comparison with theory at 343°C
(650°F) with 15 nA/cm2 (13.9 yA/ft2) and
30 nA/cm2 (27.9 yA/ft2) 39
16 Measured negative particle charge versus measured
radius and comparison with theory at 343°C
(650°F) with 30 nA/cm2 (27.9 yA/ft2) and
85 nA/cm2 (79.0 yA/ft2) 40
VII
-------
TABLES
Number Page
1 Measured and Predicted Values of Particle Charge
at the Outlet of a Laboratory Precipitator 13
2 Particle Charge for Positive Corona at 38°C,
37-39 kV, 15 nA/cm2 26
3 Particle Charge for Negative Corona at 38°C,
34-35 kV, 15 nA/cm2 27
4 Particle Charge for Positive Corona at 38°C,
35 kV, 9 nA/cm2 28
5 Particle Charge for Negative Corona at 38°C,
38 kV, 36 nA/cm2 29
6 Particle Charge for Positive Corona at 148°C,
33.0 kV, 18 nA/cm2 32
7 Particle Charge for Negative Corona at 148°C,
25.7 kV, 31 nA/cm2 33
8 Particle Charge for Positive Corona at 232°C,
31.5 kV, 17 nA/cm2 35
9 Particle Charge for Negative Corona at 232°C,
28.1 kV, 28 nA/cm2 36
10 Particle Charge for Positive Corona at 343°C,
26-27 kV, 30 nA/cm2 41
11 Particle Charge for Negative Corona at 343°C,
27-28 kV, 30 nA/cm2 42
12 Particle Charge for Positive Corona at 343°C,
25 kV, 15 nA/cm2 43
13 Particle Charge for Negative Corona at 343°C,
30-31 kV, 85 nA/cm2 44
Vlll
-------
ACKNOWLEDGMENTS
Precipitator C referred to in this report was provided for
use during this project by the U.S. Environmental Protection
Agency, Industrial Environmental Research Laboratory, Particulate
Technology Branch (PATB). The authors would like to thank EPA
personnel, G. Ramsey, B. Daniel, R. Valentine, and R. Ogan, for
their assistance.
IX
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NOMENCLATURE
F net downward force, nt
m mass of the particle, kg
a- downward acceleration, m/sec2
q charge on the particle, coul
E electric field, V/m
g acceleration due to gravity, m/sec2
F
a arag rorce acting on tne part.
,3
:<_>uo 11 ic u J. uuif rwy/ ii1
particle radius, m
viscosity of the gaseous medium, nt-sec/m2
drag force acting on the particle, nt
p density of the particle, kg/m;
p density of the gaseous medium, kg/m:
a
V downward velocity of the particle, m/sec
(l+A£/a) Cunningham correction factor,
I molecular mean free path, m
A = 1.257 + 0.400 exp (-1.10 a/£)
F net upward force, nt
a upward acceleration, m/sec2
v upward velocity of the particle, m/sec
S distance over which the particle moves, m
t time to fall the distance S, sec
t time to rise the distance S, sec
E V/D electric field between two parallel plates, V/m
V voltage applied across two parallel plates, V
D spacing between the parallel plates, m
-------
METRIC CONVERSION FACTORS
To convert from
ft
in .
MA/ft2
ft/sec
gr/acf
Tr> r-onwor-f- -f-rom °T? 1
TO
m
cm
nA/cm2
m/sec
gm/m3
°F -
-n op. op _ _i
Multiply by
0.3048
2.54
1.075
0.3048
2.28
I- 459 „, ,
1.8
XI
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SECTION 1
INTRODUCTION
In the electrostatic precipitation process, particles sus-
pended in a moving gas stream are charged as the gas is passed
through a corona discharge. The particles are charged due to
collisions with molecular ions and, possibly, free (unattached)
electrons in the case of negative corona discharge. Since the
force which drives a charged particle to a collection electrode
is proportional to the charge on the particle, the mechanisms
involved in the particle charging process and the attainable
values of particle charge under various operating conditions are
of fundamental importance with respect to understanding and uti-
lizing the electrostatic precipitation process.
In the positive corona discharge, the electrons created in
the avalanche process near the discharge electrode migrate to the
discharge electrode leaving behind positive molecular ions which
migrate through the interelectrode space toward the collection
electrode. In this case, the interelectrode space, exclusive of
the relatively small region near the discharge electrode where
active gas breakdown occurs, consists of neutral gas molecules
and positive molecular ions. Therefore, for the positive corona
case, only positive ions can participate in the particle charging
process.
Evidence and conjecture of the presence of free electron
charging has been documented in the literature for laboratory
charging experiments with small-scale charging devices at room
temperature.1'2 The mechanism by which free electrons charge
particles could be quite different than that for ionic charging.
Intuitively, the extent of free electron charging should depend
on the electrode geometry (especially the spacing between the
discharge and collection electrodes), applied voltage, gas tempera-
ture, and gas pressure. At the present, the mechanisms by which
electrons would charge particles have not been described theoret-
ically, and no experimental data are available that truly isolate
electron charging from ionic charging. In applying fundamental
principles to full-scale precipitators, effects due to free elec-
trons are generally neglected.
-------
The purpose of the work presented here was to make charge
and radius measurements on individual fly ash particles in a
typical full-scale, electrode geometry at various temperatures
up to that which is typical in a hot-side precipitator. The
values of charge and radius of individual particles exiting two
different laboratory precipitators have been measured with an
experimental apparatus which utilizes a Millikan cell. Measure-
ments were obtained for redispersed fly ash particles at tempera-
tures from 38 to 343°C at various voltages and current densities
typical of full-scale precipitator operating conditions.
It was anticipated that by making the above measurements
two questions of importance could be answered. The first ques-
tion relates to how positive and negative coronas compare in
effectiveness for charging particles. Higher voltages and cur-
rents before sparkover may result with one type of corona
discharge and may lead to significantly higher values of particle
charge. The possibly different charging mechanisms in the cases
of positive and negative coronas may result in significantly
different values of particle charge under similar operating con-
ditions. The second question relates to whether free electrons
in the case of negative corona significantly enhance particle
charging at higher temperatures (and/or reduced pressures).
Recent field data from a hot-side precipitator collecting fly ash
indicate enhanced collection efficiencies for particles with
radii between 0.5 and 1.5 ym3. These collection efficiencies
were predicted by using the measured operating voltages and cur-
rent densities in conjunction with an ionic charging theory and
were significantly lower than those measured. In this case, free
electron charging may have been prominent in the electrostatic
precipitation process.
-------
SECTION 2
SUMMARY AND CONCLUSIONS
The apparatus and technique described for measuring particle
radius and charge are capable of providing reliable data on indi-
vidual particles charged in an electrostatic precipitation device.
For temperatures up to 343°C (650°F), the measured values of
charge acquired in positive corona by particles with radii in the
range between 0.3 ym and 1.5 ym are in good agreement with those
predicted by an ionic charging theory. For temperatures less
than 37°C (100°F), the measured values of charge acquired in nega-
tive corona by particles with radii in the range between 0.3 ym
and 1.5 ym are in good agreement with those predicted by an ionic
charging theory. For temperatures of 232°C (450°F) and 343°C
(650°F), the measured values of charge acquired in negative corona
by particles with radii between 0.6 ym and 1.3 ym are significant-
ly higher than those measured for positive corona under similar
conditions and those predicted from a completely ionic charging
theory. These data indicate that the charging mechanisms for
positive and negative corona differ as temperature increases.
The enhanced values of particle charge for negative corona at
elevated temperatures are consistent with the postulation of a
free electron contribution in the particle charging mechanism.
Thus, a general theory for particle charging in negative corona
must contain ionic and electronic charging mechanisms whose rela-
tive contributions are temperature dependent. The data indicate
that, even in a full-scale precipitator utilizing negative corona,
the effect of free electrons on the charging process can not be
ignored for typical temperatures between 148°C (300°F) and 371°C
(700°F).
-------
SECTION 3
RECOMMENDATIONS
Since the results of this work indicate that at temperatures
greater than 148°C (300°F) particle charging in full-scale elec-
trode geometries with negative corona is more effective than with
positive corona and that the values of particle charge obtained
in negative corona at these temperatures can not be predicted by
an ionic charging theory, it is recommended that further work be
done to better describe these results. Since possible free elec-
tron charging in negative corona offers the only significant
physical difference between positive and negative corona, it is
recommended that particle charging due to free electrons be
investigated in the future. Presently, no data are available that
truly isolate free electron charging from ionic charging. There-
fore, it is recommended that experiments be designed and performed
to study isolated free electron charging of particles as a func-
tion of electron density, particle residence time, electric field
strength, particle diameter, gas pressure, and gas temperature.
Also, since there is no adequate free electron charging theory
presently available, it is recommended that work be performed to
develop such a theory.
-------
SECTION 4
MEASUREMENT TECHNIQUE AND APPARATUS
The technique utilized for simultaneously determining the
charge and radius of a particle is based on the downward and
upward motion of a charged particle in an insulating gas under
the influence of a uniform, reversible electric field. When the
gravitational force and the force due to the electric field act
on the charged particle in the same direction, then the charged
particle will experience a net downward force given by
FD = maD = - qE - mg +
3
= - qE - (p - pa)(Tra3)g + (6iTnavD)/(l + Afc/a) , (1)
where
F_ = net downward force (nt) ,
m = mass of the particle (kg) ,
aD = downward acceleration (m/sec2) ,
q = charge on the particle (coul) ,
E = electric field (V/m) ,
g = acceleration due to gravity (m/sec2),
F, = drag force acting on the particle (nt) ,
p = density of the particle (kg/m3) ,
p = density of the gaseous medium (kg/m3),
c*
a = particle radius (m) ,
n = viscosity of the gaseous medium (nt-sec/m2) ,
v_ = downward velocity of the particle (m/sec) ,
-------
(1+Ai/a) = Cunningham correction factor,
i = molecular mean free path, (m) , and
A = 1.257 + 0.400 exp (-1.10
Similarly, the force acting on the charged particle due to the
electric field can be made opposite to that of the gravitational
force in such a manner that the charged particle will experience
a net upward force given by
F = ma = qE - mg - F ,
u u M ^ d
= qE - tp^ - Pj(iwa3)g - (6irnv )/(! + Afc/a) , (2)
p a j u
where
F = net upward force (nt) ,
a = upward acceleration (m/sec2),
v = upward velocity of the particle Cm/sec) ,
and all other symbols are as previously defined.
Assuming that the terminal velocity of the particle is
reached instantaneously, then a = a = 0 and
and
= S/tD (3)
v = S/t , ...
u u (4)
where
S = distance over which the particle moves (m),
t = time to fall the distance S (sec), and
t = time to rise the distance S (sec).
Adding equations (1) and (2) yields
- (|7ra3)(p^ - p )g + [(6nr\aS) / (I + AA/a) ] (£- - £-) = 0. (5)
-3 P a tD u
-------
Subtracting equations (1) and (2) yields
- 2 qE + [(6TrnaS)/(l + A£/a) J t^- + £-) =0. (6)
D u
Equations (5) and (6) can be solved simultaneously to obtain
expressions for a and q.
Equation (5) can be rewritten in the form
Solving for a yields
9nS
a = -^ - ^ - — . (8)
Solving equation (6) for q yields
q = § [(37rnaS)/(l + A£/a) ] (£- + J-) , (9)
D u
where
E = V/D,
V = voltage applied across two parallel plates (V), and
D = spacing between the parallel plates (m).
A technique and apparatus have been developed for measuring
the radius and charge on individual particles which have been
treated in an electrostatic precipitation device. The technique
consists of extracting a sample of gas from the precipitation
device and directing part of this extracted sample into a modi-
fied Millikan measurement cell. A single particle can then be
isolated, and its transit times in a uniform electric field can
be utilized in equations (8) and (9) to determine its radius and
charge.
A drawing of the measurement apparatus with its insertion
into a precipitator is shown in Figure 1. The gas sample is
extracted, through two-inch diameter tubing. At temperatures less
than 204°C, the tubing can be made of teflon or metal with equiv-
alent results. This tubing contains two bends and is electrically
grounded through a spiral wire which runs along the inner surface
of the tubing. The tubing runs upward from the measurement cell
-------
DISCHARGE ELECTRODES
DUCT
CONTROL PAD
VOLTS
POWER SUPPLY
Figure 1. Experimental apparatus for measuring the radius and charge
of particles.
-------
and makes a right angle bend so that the tubing can enter a sam-
pling port. The second bend in the tubing is made so that the
tubing will be antiparallel to the gas flow and so that the
entrance to the tubing can be located inside the electrified reg-
ions although the port is located downstream from these regions.
The tubing leading down to the measurement cell contains a butter-
fly valve which can be opened to allow a sample to be drawn into
the measurement cell and can be closed to prevent the gas sample
from being drawn back into a precipitation device which has nega-
tive pressure inside and to prevent air flow disturbances during
the measurements. Below this valve and below the measurement
cell there is a connection to a pump. With the valve open, the
pump is operated and particles are drawn into the measurement
cell. When a sufficient number of particles is obtained in the
measurement cell, the pump is turned off with simultaneous clos-
ing of the valve. Once the pump is turned off and the valve is
closed, the particles very quickly cease to have any motion due
to gas flow. At this point, only the gravitational field, vis-
cous drag, and electric field which can be imposed across the
parallel plates of the measurement cell have any influence on the
motion of the particles.
In those cases where the particle concentrations are too
large for easy isolation of individual particles, the extracted
gas sample can be diluted. Filtered, outside air can be mixed
with the extracted sample by means of a bleed valve preceded by
a filter.
The Millikan measurement cell is cylindrical in shape with
a diameter of 3.8 cm and a plate spacing (or approximate height)
of 0.5 cm. Gas enters the cell through a small hole in a conical
depression in the top plate. The particles are illuminated by a
high intensity microscope lamp and are viewed through a micro-
scope attached to the measurement cell. The distance traveled
by the particles is determined by a graticule which is mounted
near the focal plane of the obiective lens of the microscope.
Measurement of the particle transit times was performed with a
stopwatch. Voltages on the order of 3 to 15 volts were applied
across the plates to produce the electric fields necessary to
generate the data presented in this report. The temperature of
the gas in the measurement cell was determined by means of a
thermocouple.
In the system described here, particles with radii down to
approximately 0.2 ym can be observed. Measurements were con-
cerned primarily with fine particles with diameters approximately
between 0.3 ym and 1.5 ym since (1) they are the most difficult
to collect in a precipitator, (2) their behavior in a precipitator
is the most difficult to model, and (3) their escape into the
atmosphere offers the greatest health hazard. Measurements on
smaller particles can be readily obtained by allowing the larger
-------
particles to settle out. By waiting approximately three minutes
after a new sample is introduced into the measurement cell, only
particles of approximately 0.5 ym or less in radius will remain
in the field of view. Measurements on the larger particles can
be readily made by choosing those particles which fall the fast-
est under the influence of gravity. The measurement system also
has the capability of determining the magnitude and sign of the
charge on a particle. Thus, the system has the potential to be
used to analyze the effects on particle charge of back corona
and rapping reentrainment. In addition, particles can be suc-
cessfully extracted and analyzed from electrified regions at
different locations in a precipitation device.
10
-------
SECTION 5
EXPERIMENTAL PROGRAM, DATA, AND RESULTS
The capability of the technique and apparatus to make
accurate measurements of the radius and charge of particles
treated in an electrostatic precipitator has been demonstrated
in previous work. Measurements were performed under essentially
idealized conditions in laboratory Precipitator A. This precip-
itator has one gas passage, four electrical sections which are
0.76 m (2.5 ft) long, plates which are 38.1 cm (15 in.) high, a
25.4 cm (10 in.) plate-to-plate spacing, a 12.7 cm (5 in.) wire-
to-wire spacing, and a 0.24 cm (0.094 in.) discharge electrode
(wire) diameter. The normalized standard deviation of the gas
velosity distribution and the gas sneakage per baffled electri-
cal section were both measured to be less than 10%. For the
experiments in laboratory Precipitator A, low mass concentrations
of dioctylphthalate (OOP) droplets were generated by an aerosol
sprayer and were carried through the precipitator by air at ambi-
ent conditions. Since (1) the particles were spherical, (2)
particle reentrainment could not exist, and (4) gas sneakage was
minimal, the results of the measurements could be interpreted
with little ambiguity.
Measurements were made at precipitator operating conditions
consisting of negative corona, an average applied voltage of
44.2 kV, an average current density of 21.5 nA/cm2 (20.0 yA/ft2),
and a gas velocity of 1.5 m/sec (4.9 ft/sec). The data obtained
at the precipitator outlet from the measurements on 486 individual
particles are shown graphically in Figure 2 and are tabulated in
Table 1. The data on each individual particle were obtained from
the average values of three measurements each of the time required
for the particle to travel downward a distance of 0.129 cm under
the influence of an electric field and of the time required to
travel the same distance upward. The bars on the average data
points are one standard deviation and are not necessarily repre-
sentative of the error in the measurement technique but, instead,
they are more representative of the expected spread in charge on
a particle with a given radius due to the fact that different
particles travel different paths through the precipitator and
experience different charging electric field strengths and ion
densities. Also, particle radii in a narrow band have been
grouped together with a radius given by the midpoint of the band.
11
-------
10
8
UJ*
_J
o
p
tr
UJ
CL
UJ
O
-------
TABLE 1. MEASURED AND PREDICTED VALUES OF
PARTICLE CHARGE AT THE OUTLET OF
A LABORATORY PRECIPITATOR
Radi us
Size Range
(10-6m)
0.25-0. 35
0. 35-0.45
0.45-0.55
0.55-0.65
0.65-0.75
0.75-0.85
0.85-0.95
0.95-1.05
1.05-1.15
1.15-1.25
1.25-1.35
1.35-1.45
1.45-1.55
No.
Part
3
35
57
46
68
80
66
56
42
20
5
6
2
Mean
Charge
(coul)
1.28xlO~17
1.98xlO~17
2. 88xlO~ 17
4. 54xlO~ 17
6.71xlO~17
9.10xlo"17
1.21xlO~16
1.43xlO"16
1.64xlO~16
1.82xlO~16
3.32xlO~16
2.61xlO~16
4.30xlO~16
Min
Charge
(coul)
4.20xlO~18
5.22xlO~18
1. 96x10" 17
2. 76x10" 17
3.74xlO~17
4. 16xlO~ 17
6.41xlO~17
6.08xlO~17
8.09xlO~17
1.16xlO~16
2.39xlO~16
1.48xlO~16
3.95xlO~16
Max
Charge
(coul)
1. 82x10" 17
3. 82x10" 17
6.03xlQ-17
1.17xlO"16
1.04xlO~16
1.58xlO~16
1. 87x10" 16
2.68xlO~16
2.94xlO~16
3.17xlO~16
4.39xlO~16
4.93xlO~16
4.59xlO~16
Normalized
Standard Predicted
Deviation Charge
0.481
0.270
0.265
0.346
0.234
0.271
0.280
0.330
0.306
0.289
0.237
0.438
0.748
(coul)
1. 61x10" l7
2. 62x10"' 7
3.81xlO"17
5. 16x10" 17
6.75xlo"17
8. 49x10" 17
1. 05x10" 16
1.26x10" IS
1. 48x10* 16
1. 75x10" 16
2.02x!0"16
2.31x!0"16
2.61xlo"16
13
-------
The solid curve is a model1* prediction of charge as a function of
particle radius at the outlet of the precipitator. The model
employs an ionic charging theory.5
In a wire-plate geometry, the electric field does not vary
significantly over a major portion of the interelectrode space
and it varies rapidly only over a small high field region near
the discharge electrode.6 Thus, one would expect that the
majority of the particles of a given radius exiting the precipi-
tator would have essentially the same charge but there would be
a spread in charge lying in a range determined by the minimum and
maximum values of the electric field. These expectations are
evidenced in the data in that the normalized standard deviations
are small. One might also expect that a certain percentage
(possibly 5-10%) of the smaller particles (less than 0.5 ym in
radius) would pass through high electric field regions near the
discharge electrodes and would acquire charges which are signifi-
cantly higher than those predicted by the average electric field
used in the model. Although values of the electric field in the
high field region should be up to 8.5 times the average value of
electric field used in the model, the ratios of measured charge
to predicted charge did not exceed a value of 2.8. It was also
observed that, while it was relatively easy to find particle
charges significantly larger than that predicted by the model for
radii greater than approximately 0.7 ym, very few particles were
found to have a significantly higher charge than predicted by the
model for radii less than approximately 0.7 ym. In fact, in the
range of radius between 0.2 and 0.8 ym, only one particle out of
289 measured had a charge which was over a factor of 1.6 times
the predicted charge. In addition many samples were observed in
which the particles larger than 0.5 ym in radius were allowed to
settle out in order to determine if highly charged submicron
particles were present. In all these samples, the remaining par-
ticles all moved with essentially the same velocity in an applied
electric field. Thus, they all had approximately the same value
of charge. Measurements made on gas samples taken from the
middle of the inlet electrical section also produced the same
results.
The limited data shown in Figure 2 and Table 1 indicate that
the measurement technique is reliable and that the model predic-
tions are in good agreement with the average measured value of
particle charge. The higher theoretical predictions for particle
radii less than 0.6 ym are inherent in the approximate theory at
the values of electric field which were utilized in the experi-
ment. The use of an average electric field determined by divid-
ing the applied voltage by the wire-to-plate spacing and of a
particle residence time determined by dividing the precipitator
length by the average gas velocity appears to be adequate for
14
-------
predicting particle charge. Also, an ionic charging theory
appears to be adequate for describing the data.
The objective of the present experimental program was to
make charge and radius measurements in a typical full-scale,
electrode geometry under both positive and negative corona at
various temperatures up to that which is typical in a hot-side
precipitator. In these experiments, the gas stream entering the
precipitator was laboratory air containing low resistivity,
redispersed fly ash particles. In principle, although sparkover
may occur at a different applied voltage for positive corona than
negative corona in laboratory air at atmospheric pressure near
sea level, the clean air, clean plate, voltage-current curves for
the two cases should be nearly the same up to sparkover for typi-
cal full-scale plate spacings since the starting voltages and
effective ion mobilities do not differ appreciably. Thus, for
low mass loadings of low resistivity fly ash particles, compari-
sons of particle charging capabilities of positive and negative
corona might be made where essentially the same applied voltages
and currents are utilized in both cases. Any significant differ-
ences in particle charging capabilities under these conditions
would indicate different charging mechanisms for the different
types of corona.
At higher temperatures (and/or reduced pressures), the mean-
free-paths of ions and electrons increase. Thus, in the case of
negative corona, free electrons can penetrate further into the
interelectrode space and possibly can have an increased effect on
voltage-current characteristics and particle charging. Compari-
son of voltage-current characteristics and particle charging for
both positive and negative corona at higher temperatures should
provide further insight into the effect, if any, of free elec-
trons. If significant penetration of free electrons occurs in
the interelectrode space, this might be evidenced in a larger
difference in positive and negative voltage-current character-
istics than would be obtained if only ions carried the current.
However, even if differences in voltage-current characteristics
can not be firmly established, differences in particle charging
capabilities may still exist.
The effect of temperature on particle charging was examined
in laboratory Precipitators B and C. Precipitator B has one gas
passage, four electrical sections which are 91.4 cm (36 in.) long,
plates which are 91.4 cm (36 in.) high, a 25.4 cm (10 in.) plate-
to-plate spacing, a 22.9 cm (9 in.) wire-to-wire spacing, a
0.32 cm (0.125 in.) discharge electrode (wire) diameter. Precip-
itator C has one gas passage, four electrical sections which are
1.22 m (4 ft) long, plates which are 1.22 m (4 ft) high, a
25.4 cm (10 in.) plate-to-plate spacing, a 22.9 cm (9 in.) wire-
to-wire spacing and a 0.32 cm (0.125 in.) discharge electrode
(wire) diameter. Measurements at 38°C (100°F) were performed at
the outlet of Precipitator B, while measurements at 148°C (300°F),
15
-------
232°C (450°F), and 343°C (650°F) were performed at the outlet of
Precipitator C.
Figures 3-5 show typical real time voltage-current traces
obtained from Precipitator C for clean air, wires, and plates at
three different temperatures. All these curves were obtained
sequentially over a relatively short time period. Under these
conditions and with good electrode alignment, it can be seen that
voltage-current curves which are nearly coincident are obtainable,
Figures 6-8 show some typical average voltage-current curves
obtained when the air stream contained fly ash particles and when
the wires and plates were somewhat dirty. The data in Figure 6
represent an average over Precipitator B. The data in Figures 7
and 8 were obtained from Precipitator C with curves from all the
electrical sections shown in Figure 7 and an average over the
entire precipitator shown in Figure 8. All data with particles
in the air stream obtained at 343°C (650°F) from Precipitator C
were acquired approximately four months prior to that at 148°C
(300°F) and 232 C (450°F). During the later measurement period,
the precipitator was not operating as well as before, and the
voltage-current curves with particles for positive and negative
corona were widely separated. Since the inlet mass loading was
nominally on the order of 1.14 gm/m3 (0.5 gr/acf) and the parti-
cle size distribution was rather large, the effect of particles
on the voltage-current curves could not be attributed entirely
to a particulate space charge effect in the gas. The observed
behavior of the voltage-current curves might have been due to
deposits of a high resistivity ash on the wires and plates
acquired during other experiments performed just prior to these
measurements. In any event, it was often observed that the
addition of particles caused the positive and negative voltage-
current curves to separate to a larger extent than anticipated.
Figures 9-11 show particle radius and charge measurements
made at the outlet of Precipitator B at 38°C (100°F) with an
average gas velocity of approximately 1.5 m/sec (5.0 ft/sec).
These and additional data are tabulated in Tables 2-5. Model
predictions for the different conditions are also shown for
comparison with the data. The data in Figure 9 were obtained
with a constant average current density of 15 nA/cm2 (13.9
yA/ft2) and average applied voltages for positive and negative
corona of 38.0 kV and 34.5 kV, respectively. The data in
Figure 10 were obtained with a constant average applied voltage
of 35.0 kV and average current densities for positive and
negative corona of 9 nA/cm2 (8.4 yA/ft2) and 15 nA/cm2 (13.9
yA/ft2), respectively. The data in Figure 11 were obtained for
negative corona only at average applied voltages of 34.5 kV and
38.0 kV, corresponding to average current densities of 15 nA/cm2
(13.9 yA/ft2) and 36 nA/cm2 (33.4 yA/ft2), respectively. For
all the data at 38°C (100°F), the theory and data show good
16
-------
4.0
3.0
<
E
cc
QC
D
O
2.0
1.0 -
NOTE: CLEAN WIRES/CLEAN PLATES
NEGATIVE CORONA
PLATE SPACING = 25.4 cm
WIRE SPACING = 22.9cm
SWIRES/SECTION
0.32 cm DIAMETER WIRES
POSITIVE CORONA
PLATE SPACING =25.4 c
WIRE SPACING = 22.9cm
SWIRES/SECTION
0.32 cm DIAMETER WIRES
10
20
30 40
VOLTAGE, kV
Figure 3. Clean air, wire, and plate voltage-current curves for Precipitator C
at ambient temperature.
17
-------
5.0,
I
4.0
NOTE: CLEAN WIRES/CLEAN PLATES
NEGATIVE CORONA—^
PLATE SPACING = 25.4 cm
WIRE SPACING = 22.9 cm
SWIRES/SECTION
0.32 cm DIAMETER WIRES
POSITIVE CORONA v^
PLATE SPACING =25.4 cm
WIRE SPACING = 22.9cm
SWIRES/SECTION
0.32cm DIAMETER WIRES
3.0
cc
tc.
o
2.0
1.0
30
VOLTAGE, kV
50
Figure 4. Clean air, wire, and plate voltage-current curves for Precipitator C
at 148°C (300°F).
18
-------
1
-------
1250
1200
1150
1100
1050
1000
950
900
850
800
750
a 700
£ 650
g 600
5 550
0 500
450
400
350
300
250
200
150
100
50
_ TEMPERATURE 38°C
FEEDER ON
— • NEGATIVE CORONA
' • POSITIVE CORONA
I I
I I
I I I I I
10 14
18
22 26 30 34
VOLTAGE. kV
38
46
Figure 6. Average voltage-current curves with particles for
Precipitator B at 38°C (100°F).
20
-------
5.5
5.0
4.5
4.0
* 3.5
E
LU
-------
5.5
5.0
4.5
4.0
3.5
H 3.0
in
oc
2- 2.5
O
2.0
1.5
1.0
0.5
I I I I I I I I I I I I I I I I I
N
I I I I
TEMPERATURE 343°C —
FEEDER ON
-------
•5 IO-
o
16
O NEC. COR. V ° 34-35 kV
THEORY LINE 1
O POS. COR. V ° 37-39 kV
THEORY LINE 2
10-7
PARTICLE RADIUS, meter*
Figure 9. Measured positive and negative particle charge versus measured
radius and comparison with theory at 38°C (WO°F) and 15 nA/cm?
(13.9
23
-------
_o
8
*
Ul
I
u
10-
16
VOLTAGE: 35 kV
NEGATIVE CORONA
POSITIVE CORONA
15 NANOAMPS/cm2
9 NANOAMPS/cm2
ERROR BARS FOR
NEGATIVE CASE
10-7
1Q-6
RADIUS, meters
Figure 10. Measured positive and negative particle charge versus measured radius
and comparison with theory at 38°C (100°F) and 35 kV.
24
-------
- 10-
111
O
K
<
O
16
10
NEGATIVE CORONA
15 nA/cm2, 34-35 kV I]
2 O 36 nA/cm2 38 kV
,-17
10
,-7
ID'6
PARTICLE RADIUS, meters
Figure 11. Measured negative particle charge versus measured radius and
comparison with theory at 38°C (100°F) with 15 nA/cm2
(13.9 nA/ft2) and 36 nA/cm2 (33.4
25
-------
TABLE 2
PARTICLE CHARGE FOR POSITIVE CORONA
AT 38°C, 37-39 kV, 15 nA/cm2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Number of
Particles
in
Size Band
15
23
23
28
14
16
7
7
5
1
2
Average
Charge/
Particle
do-17
coulombs)
2.155
2.733
3.705
5.419
6.777
9.745
12.00
13.71
16.91
16.85
30.56
Standard
Deviation
do-17
coulombs)
0.7507
0.8730
0.8348
1.750
1.692
3.122
4.430
1.423
2.786
2.295
Normalized
Standard
Deviation
0.35
0.32
0.23
0.32
0.25
0.32
0.37
0.10
0.16
0.08
Average
Number
of
Charges/
Particle
135
171
232
339
424
609
750
857
1057
1053
1910
26
-------
TABLE 3
PARTICLE CHARGE FOR NEGATIVE CORONA
AT 38°C, 34-35 kV, 15 nA/cm2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Number of
Particles
in
Size Band
2
15
39
25
31
17
12
3
3
— -
1
Average
Charge/
Particle
do'17
coulombs)
1.777
2.644
3.314
4.662
6.551
8.765
12.28
13.85
19.62
24.78
Standard
Deviation
do'17
coulombs)
0.3136
0.7142
0.6943
1.755
1.758
1.888
5.462
3.261
2.307
— — V*
Normalized
Standard
Deviation
0.18
0.27
0.21
0.38
0.27
0.22
0.44
0.24
0.12
___
Average
Number
of
Charges/
Particle
111
165
207
291
409
548
768
866
1226
1549
27
-------
TABLE 4
PARTICLE CHARGE FOR POSITIVE CORONA
AT 38°C, 35 kV, 9 nA/cm2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Average
Number of Charge/
Particles Particle
in (10~17
Standard
Deviation Normalized
(10~17 Standard
Size Band coulombs) coulombs) Deviation
0.5945
0.8297
1.116
1.018
3.071
0.5088
34
16
15
19
10
4
3
1.761
2.639
3.556
4.268
7.860
7.926
8.799
1.056
0.34
0.31
0.31
0.24
0.39
0.06
0.12
Average
Number
of
Charges/
Particle
110
165
222
267
491
495
550
28
-------
TABLE 5
PARTICLE CHARGE FOR NEGATIVE CORONA
AT 38°C, 38 kV, 36 nA/cm2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Number of
Particles
in
Size Band
20
32
38
28
17
9
4
3
Average
Charge/
Particle
do-17
coulombs)
1.278
2.219
3.623
4.808
7.110
9.251
15.25
14.30
Standard
Deviation
do-17
coulombs)
0.3608
0.5699
1.048
1.125
2.450
2.890
4.337
1.345
Normalized
Standard
Deviation
0.28
0.26
0.29
0.23
0.34
0.31
0.28
0.09
Average
Number
of
Charges/
Particle
80
139
226
301
444
578
953
894
29
-------
agreement. Since the least amount of data was acquired for the
smallest and largest radius bands, the agreement with theory
should be expected to be less for these particle radii.
Figure 12 shows particle radius and charge measurements made
at the outlet of Precipitator C at 148°C (300°F) with an average
gas velocity of approximately 1.4 m/sec (4.5 ft/sec). These and
additional data are tabulated in Tables 6 and 7. The average
applied voltage and current density for negative corona were 25.7
kV and 31.0 nA/cm2 (28.8 yA/ft2), respectively. The average
applied voltage and current density for positive corona were 33.0
kV and 18.0 nA/cm2 (16.7 yA/ft2), respectively. During measure-
ments with both positive and negative corona, it was difficult to
maintain constant electrical operating conditions. Wide fluctua-
tions in applied voltage and current occurred with intermittent
sparking. Thus, these data are not useful for comparing positive
and negative particle charging under comparable conditions. How-
ever, the data give an illustration of the capability of the
measurement system to detect adverse charging conditions.
Figure 13 shows particle radius and charge measurements
made at the outlet of Precipitator C at 232°C (450°F) with an
average gas velocity of approximately 1.4 m/sec (4.5 ft/sec).
These and additional data are tabulated in Tables 8 and 9. The
average applied voltage and current density for negative corona
were 28.1 kV and 28.0 nA/cm2 (26.0 yA/ft2), respectively. The
average applied voltage and current density for positive corona
were 31.5 kV and 17 nA/cm2 (15.8 yA/ft2), respectively. After
the measurements at 148°C (300°F), the wires and plates of the
precipitator were cleaned by brushing. This resulted in more
favorable and stable electrical operating conditions. For the
operating conditions during the measurements, the theory predicts
essentially the same charge versus radius relationship for the
positive and negative corona conditions. While the theory agrees
well with the data obtained with positive corona for all particle
radii, it clearly underpredicts negative particle charge for radii
between approximately 0.7 ym and 1.2 ym. In this range of radius,
the ratio of the average measured negative charge to the predicted
charge varies from 1.25 to 1.59, increasing with increasing diameter,
These data indicate that negative corona is more effective than
positive corona in charging particles with radii between 0.7 ym
and 1.2 ym at 232°C (450°F). Comparison of these data with the
theoretical predictions and the information in Figures 2-5
suggests that the use of a completely ionic charging theory to
describe negative particle charging at 232°C (450°F) is inade-
quate. Since the only major physical difference between the
positive and negative corona process is the free electron pene-
tration into the interelectrode space in the negative corona
case, the enhanced particle charge with negative corona at 232°C
(450°F) might be due to increased free electron charging.
30
-------
o
oc
10-"
Ill
1 o NEGATIVE CORONA,
25.7 kV. 31 nA/cm2
2 O POSITIVE CORONA,
33 kV, 18 nA/cm2
•!>/ 1
10-7
10-6
PARTICLE RADIUS, m
Figure 12. Measured positive and negative particle charge versus measured
radius and comparison with theory at 148°C (30G°F).
31
-------
TABLE 6
PARTICLE CHARGE FOR POSITIVE CORONA
AT 148°C, 33.0 kV, 18 nA/cra2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Number of
Particles
in
Size Band
5
15
19
18
27
19
23
13
6
6
Average
Charge/
Particle
do-17
coulombs)
1.428
2.342
3.243
4.743
6.115
8.470
8.498
11.31
12.32
15.02
Standard
Deviation
(10'17
coulombs)
0.1032
0.5156
0.8471
0.8260
1.840
1.994
2.058
3.769
0.6139
1.938
Normalized
Standard
Deviation
0.081
0.2202
0.2612
0.1741
0.3009
0.2354
0.2421
0.3333
0.0498
0.1291
Average
Number
of
Charges/
Particle
89
146
203
296
382
529
531
707
770
939
32
-------
TABLE 7
PARTICLE CHARGE FOR NEGATIVE CORONA
AT 148°Cf 25.7 kV, 31 nA/cm2
Median
Radius of
Size Band
(microns)
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.3
Number of
Particles
in
Size Band
14
28
36
30
10
16
7
3
4
Average
Charge/
Particle
do'17
coulombs)
1.447
2.426
3.799
4.627
6.710
7.773
8.862
23.09
25.62
Standard
Deviation
(10~17
coulombs)
0.6014
0.6695
1.544
1.513
2.195
1.381
1.827
11.043
7.145
Normalized
Standard
Deviation
0.4156
0.2759
0.4064
0.3270
0.3271
0.1777
0.2061
0.4516
0.2788
Average
Number
of
Charges/
Particle
90
152
237
289
419
486
554
1443
1601
33
-------
§
o
-------
TABLE 8
PARTICLE CHARGE FOR POSITIVE CORONA
AT 232°C, 31.5 kV, 17 nA/cm2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.3
1.4
Number of
Particles
in
Size Band
9
10
19
29
17
23
17
13
4
2
2
Average
Charge/
Particle
do-17
coulombs)
1.405
2.461
2.938
3.886
5.201
7.027
9.857
9.925
15.36
20.78
17.00
Standard
Deviation
do'17
coulombs)
0.2762
0.5891
0.8750
0.9217
1.118
2.891
3.555
2.020
4.012
6.675
8.368
Normalized
Standard
Deviation
0.1966
0.2393
0.2978
0.2372
0.2150
0.4114
0.3607
0.2036
0.2612
0.3213
0.4922
Average
Number
of
Charges/
Particle
88
154
183
243
325
439
616
620
960
1299
1063
35
-------
TABLE 9
PARTICLE CHARGE FOR NEGATIVE CORONA
AT 232°C, 28.1 kV, 28 nA/cra2
Median
Radius of
Size Band
(microns)
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Number of
Particles
in
Size Band
5
32
32
24
21
14
4
9
5
3
Average
Charge/
Particle
do-17
coulombs)
2.568
3.959
5.413
7.989
10.62
12.85
17.64
20.57
22.89
20.88
Standard
Deviation
do-17
coulombs)
0.3808
0.9971
1.470
2.372
3.506
4.321
4.570
6.723
5.570
4.038
Normalized
Standard
Deviation
0.1483
0.2518
0.2715
0.2967
0.3303
0.3362
0.2591
0.3268
0.2434
0.1934
Average
Number
of
Charges/
Particle
161
247
338
499
664
803
1102
1286
1431
1305
36
-------
Figures 14-16 show particle radius and charge measurements
made at the outlet of Precipitator C at 343°C (650°F) with an
average gas velocity of approximately 1.4 m/sec (4.5 ft/sec).
These and additional data are tabulated in Tables 10-13. The
precipitator had been cleaned thoroughly prior to these measure-
ments. During the measurements, the electrical operating condi-
tions were extremely stable. The data shown in Figure 14 were
obtained for a current density of 30 nA/cm2 (27.9 yA/ft ) for
both positive and negative corona with essentially the same aver-
age applied voltages of 26.5 kV and 27.5 kV, respectively.
Similar to the results obtained at 232°C (450°F), the theory
agrees reasonably well with the data for positive corona but
clearly underpredicts particle charge obtained with negative
corona for radii between approximately 0.6 ym and 1.3 ym. In this
range of radius, the ratio of the average measured negative charge
to the predicted charge ranges from 1.27 to 1.94, generally
increasing with increasing diameter. These data again suggest
the possibility of free electrons participating in the charging
process with negative corona.
Figures 15 and 16 contain data showing the effect of elec-
trical conditions on particle charging with positive and negative
corona at 343°C (650°F). Again, the theory agrees relatively
well with the data for positive corona but underpredicts particle
charge obtained with negative corona for radii between approxi-
mately 0.6 ym and 1.3 ym. The data show the same trend as the
theory in that larger applied voltages and current densities
result in higher values of particle charge. Also, the ability
for negative corona to acquire a significantly higher voltage
and current density prior to sparkover resulted in a further
increase in particle charge as compared to the positive corona.
37
-------
£
o
o
10
,-17
CURRENT DENSITY: 30 nanoampj/cm2
D NEGATIVE CORONA V 27-28 kV
O POSITIVE CORONA V 26-27 kV
THEORY
10-7
ID"6
RADIUS, m«tan
Figure 14. Measured positive and negative particle charge versus measured
radius and comparison with theory at 343°C (65CPF) and
30 nA/cm2 (27.9
38
-------
O
POSITIVE CORONA
O CURRENT DENSITY
30 nA/cm2 - THEORY LINE 1
O CURRENT DENSITY
15 nA/cm2 - THEORY LINE 2
10-7
10'6
PARTICLE RADIUS, meters
Figure 15. Measured positive particle charge versus measured radius and
comparison with theory at 343°C (650°F) with 15 nA/cm?
(13.9 iiA/ft2) and 30 nA/cm2
39
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3 10'18
iu
O
ec
<
o
I I I I I I I I I
11
NEGATIVE CORONA
1 O 30 nA/cm2. 27-28 kV
2 a 85 nA/cm2, 30-31 kV
10-7
10-6
PARTICLE RADIUS, meters
Figure 16. Measured negative particle charge versus measured radius and
comparison with theory at 343°C (650°F) with 30 nA/cm2
(27.9 nA/ft2) and 85 nA/cm2 (79.0
40
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TABLE 10
PARTICLE CHARGE FOR POSITIVE CORONA
AT 3439C, 26-27 kV, 30 nA/cm2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
Number of
Particles
in
Size Band
9
16
21
19
28
9
14
12
7
1
7
1
Average
Charge/
Particle
do-17
coulombs)
1.598
2.642
3.582
4.186
5.385
7.255
7.613
10.91
12.30
25.20
20.42
29.38
Standard
Deviation
do-17
coulombs)
0.5438
1.083
1.094
1.349
1.342
2.037
1.130
2.364
2.212
7.253
_ __
Normalized
Standard
Deviation
0.34
0.41
0.31
0.32
0.25
0.28
0.15
0.22
0.18
0.35
— _._
Average
Number
of
Charges/
Particle
99
165
224
262
337
453
476
682
769
1575
1276
1836
41
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TABLE 11
PARTICLE CHARGE FOR NEGATIVE CORONA
AT 343°C, 27-28 kV, 30 nA/cm2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Number of
Particles
in
Size Band
1
6
19
27
32
16
17
12
7
5
4
2
1
Average
Charge/
Particle
do'17
coulombs)
1.717
3.004
4.640
6.836
9.193
13.69
14.60
17.60
20.76
31.46
32.16
33.71
48.38
Standard
Deviation
do-17
coulombs)
0.5930
0.9379
1.550
3.131
3.570
4.116
3.632
3.274
4.783
4.225
0.085
___
Normalized
Standard
Deviation
0.20
0.20
0.23
0.34
0.26
0.28
0.21
0.16
0.15
0.13
0.002
___
Average
Number
of
Charges/
Particle
107
188
290
427
575
856
913
1100
1298
1966
2006
2107
3024
42
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TABLE 12
PARTICLE CHARGE FOR POSITIVE CORONA
AT 343°C, 25 kV, 15 nA/cm2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Number of
Particles
in
Size Band
4
4
9
6
10
4
3
3
3
Average
Charge/
Particle
coulombs)
1.463
2.235
2.787
4.018
5.572
6.610
7.465
9.768
11.09
Standard
Deviation
(10~ l 7
coulombs)
0.3552
0.5683
0.7414
0.6848
1.213
0.9339
0.7264
0.0576
1.097
Normalized
Standard
Deviation
0.24
0.25
0.27
0.17
0.22
0.14
0.10
0.006
0.10
Average
Number
of
Charges/
Particle
91
140
174
251
348
413
467
611
693
43
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TABLE 13
PARTICLE CHARGE FOR NEGATIVE CORONA
AT 343°C, 30-31 kV, 85 nA/cm2
Median
Radius of
Size Band
(microns)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Number of
Particles
in
Size Band
1
12
20
36
26
27
11
7
2
2
2
1
Average
Charge/
Particle
do-17
coulombs)
3.943
5.533
9.137
12.04
17.62
19.84
27.18
33.08
37.75
49.71
47.94
53.41
Standard
Deviation
do-17
coulombs)
0.7909
1.578
2.256
3.212
3.590
4.022
5.761
1.810
7.622
7.660
___
Normalized
Standard
Deviation
0.14
0.17
0.19
0.18
0.18
0.15
0.17
0.05
0.15
0.16
__«
Average
Number
of
Charges/
Particle
246
346
571
753
1101
1240
1699
2068
2359
3107
2996
3338
44
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SECTION 6
REFERENCES
1. Penney, G. W., and R. D. Lynch. Measurements of Charge
Imparted to Fine Particles by a Corona Discharge. AIEE, 76:
294-299 (July, 1957).
2. Pontius, D. H., L. G. Felix, J. R. McDonald, and W. B. Smith.
Fine Particle Charging Development. EPA-600/2-77-173, NTIS
PB271-727/AS, U.S. Environmental Protection Agency, Research
Triangle Park, North Carolina (1977).
3. Marchant, G. H., Jr., and J. P. Gooch. Performance and
Economic Evaluation of a Hot-Side Electrostatic Precipitator.
EPA-600/7-78-214, U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina (1978).
4. McDonald, J. R. A Mathematical Model of Electrostatic Pre-
cipitation (Revision 1): Volume 1. Modeling and Programming.
EPA-600/7-78-llla, NTIS PB284-614, U.S. Environmental Protec-
tion Agency, Research Triangle Park, North Carolina (1978) .
5. Smith, W. B., and J. R. McDonald. Development of a Theory
for the Charging of Particles by Unipolar Ions. J. Aerosol
Sci., 7:151-166 (1976).
6. McDonald, J. R., W. B. Smith, H. W. Spencer, and L. E. Sparks.
A Mathematical Model for Calculating Electrical Conditions in
Wire-Duct Electrostatic Precipitation Devices. J. Appl. Phys.,
48(6) -.2231-2246 (1977) .
45
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TECHNICAL REPORT DATA
(Please read Inuructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-80-077
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Charge Measurements of Particles Exiting
Electrostatic Precipitators
5. REPORT DATE
April 1980
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
J.R. McDonald, M.H.Anderson, and R. B.Mosley
8. PERFORMING ORGANIZATION REPORT NO
3858-10
SORI-EAS-80-332
9. PERFORMING OROANIZATI ON NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
68-02-2610
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 PERIOD COVERED
Final; 10/78-10/79
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES IERL_RTP project officer is Les lie E. Sparks, Mail Drop 61,
919/541-2925.
. ABSTRACT
repOrt. gives results of an mves tigation of particle charging in positive
and negative corona discharge as a function of temperature from 38 to 343 C in order
to establish, especially at hot-side electrostatic precipitator (ESP) temperatures,
the relative effectiveness of the two possible methods of charging. Charge values on
individual particles exiting two laboratory ESPs were measured in an experimental
apparatus utilizing a Millikan cell. Measurements were directed at fine particles
with radii between 0. 3 and 1, 5 micrometers. Measurements were obtained for redis-
persed fly ash particles carried in air at temperatures from 38 to 343 C. The elec-
trode geometries and electrical operating conditions utilized were typical of full-
scale ESPs. At comparable voltages and currents for positive and negative corona
discharges, the ratio of the values of negative to positive charge for radii between
0. 6 and 1. 3 micrometers were shown to increase from about 1 to 2 as temperatures
increased from 37 to 343 C. Predictions of a mathematical model of ESP, employing
an ionic charging theory, showed good agreement with all positive charging data; but
good agreement was shown with negative charging data only at temperatures below
37 C. Differences between measurements and model predictions are consistent with
the postulation of free electron charging at high temperature (negative corona).
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution Electric Corona
Electrostatic Precipitators
Fly Ash Temperature
Dust
Electrical Charge
Measurement
Pollution Control
Stationary Sources
Particulate
13B
131
21B
11G
20C
14B
3. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report/
Unclassified
21. NO. OF PAGES
57
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
46
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