EPA-600/2-75-017
August 1975
Environmental Protection Technology Series
EFFECT OF CHEMICAL COMPOSITION
ON SURFACE RESISTIVITY
OF FLY ASH
p
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U.S. Environmental Protection Agency
Office of Research and Development
Washington, 0. C. 20460
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EPA-600/2-75-017
EFFECT OF CHEMICAL COMPOSITION
ON SURFACE RESISTIVITY
OF FLY ASH
by
R. E. Bickelhaupt
Southern Research Institute
2000 Nintli Avenue South
Birmingham, Alabama 35205
Contract No. 68-02-1303
ROAP No. 21ADJ-029
Program Element No. 1AB012
EPA Project Officer: Leslie E. Sparks
Industrial Environmental Research Laboratory
Office of Energy , Minerals, and Industry
Research Triangle Park, North Carolina 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, B.C. 20460
August 1975
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EPA REVIEW NOTICE
This report has been reviewed by the National Environmental Research
Center - Research Triangle Park, Office of Research and Development,
EPA, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion of environmental technology. Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields. These series are:
1. ENVIRONMENTAL HEALTH EFFECTS RESEARCH
2. ENVIRONMENTAL PROTECTION TECHNOLOGY
3. ECOLOGICAL RESEARCH
4. ENVIRONMENTAL MONITORING
5. SOCIOECONOM1C ENVIRONMENTAL STUDIES
6. SCIENTIFIC AND TECHNICAL ASSESSMENT REPORTS
9. MISCELLANEOUS
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.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 22161.
Publication No. EPA-600/2-75-017
11
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THE EFFECT OF CHEMICAL COMPOSITION ON THE SURFACE
RESISTIVITY OF FLY ASH
ABSTRACT
Resistivity was determined for a group of well characterized
ashes representing both Eastern and Western coals. Data were
taken between 60°C and 250°C in an environment of air con-
taining approximately 9 volume percent water. Chemical
transference experiments were conducted for two ashes having
substantially different chemical compositions.
Chemical analyses of the transference specimens revealed a
pronounced migration of alkali metal ions toward the negative
electrode. It was observed that the surface resistivity was
inversely proportional to the concentration of these ions and
that the iron concentration influenced particularly the
participation of potassium.
Surface resistivity is sensitive to the chemical composition
of fly ash because the alkali metals serve as charge carriers.
The conduction mechanism is probably analogous to that of
glass. This viewpoint is compatible with the usual empirical
observations regarding the effect of certain parameters on
resistivity; for example, the interaction between ash and the
environment.
This report was submitted in partial fulfillment of Contract
No. 68-02-1303 by Southern Research Institute under the
sponsorship of the Environmental Protection Agency. Work
was completed as of October 1974.
111
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CONTENTS
Page
Abstract ii
List of Figures iv
List of Tables v
Acknowledgement vi
j
Sections
I Conclusions 1
II Recommendations 2
III Introduction 3
IV Scope 5
V Experimental Equipment, Procedures and
Calculations 6
VI Results 16
VII Discussion 30
VIII References 36
Appendix A 38
iv
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FIGURES
No. Page
1 Schematic of Apparatus Set-Up for
Resistivity Measurements 8
2 Photograph of General Apparatus Set-Up for
Resistivity Determination 9
3 Photograph of Environmental Chamber and Oven 10
4 Photograph of Environmental Chamber and
Electrode Set 12
5 Typical Resistivity Data as a Function of
Reciprocal Absolute Temperature 22
6 Maximum Surface Resistivity Versus the
Combined Atomic Percentage of Lithium and
Sodium 25
7 Maximum Surface Resistivity Normalized to
0.4 Atomic Percent Lithium Plus Sodium Versus
Potassium Concentration 26
8 Maximum Surface Resistivity Normalized to
0.4 Atomic Percent Lithium Plus Sodium Versus
Iron Concentration 28
9 Maximum Surface Resistivity Normalized to
0.4 Atomic Percent Lithium Plus Sodium Versus
Combined Potassium and Iron Concentrations 29
10 Correlation Between Estimated and Measured
Maximum Surface Resistivity 34
11 Surface Resistivity and Soluble Sulfate as
a Function of Surface Area 41
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TABLES
NO. Paqe
1 Chemical Analyses of Ashes in Weight Percent 17
2 Miscellaneous Ash Characteristics 18
3 Chemical Analyses of Transference Experiments
in Weight Percent 20
4 Characterization of Fractionated Fly Ash 39
VI
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ACKNOWLEDGEMENT
Other Southern Research Institute employees who partici-
pated in this research include Mr. W. R. Dickson and
Mr. Johnny Sutherland - chemical analysis of the ashes,
and Mr. C. A. Reed - physical characterizations and
resistivity measurements.
Vll
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SECTION I
CONCLUSIONS
Surface conduction occurs by an ionic mechanism involving the
interaction between the continuous matrix of glassy particu-
lates and the environment. Transference experiments revealed
that the alkali metal ions serve as charge carriers; however,
the possibility of other ions participating as charge carriers
was not excluded. It was concluded that the attack by certain
environmental species, water and acid, on the surface of the
glassy ash mobilizes the alkali metal ions.
A correlation was established between the magnitude of maximum
surface resistivity and the concentrations of lithium, sodium,
potassium and iron. It was speculated that the role of iron is
related to the structure and/or chemical durability of the ash,
thereby influencing the degree of participation of the alkali
metal ions, particularly potassium. The correlation offers a
possible means of estimating the resistivity of an ash from the
chemical analysis for a given set of environmental conditions.
It was observed that the measured resistance of an ash layer
is inversely proportional to a parameter defining the surface
area of the ash when surface conduction is important. The
magnitude of this effect among ashes is large enough so that
it should be considered in the study of resistivity as a
function of the other factors of influence.
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SECTION II
RECOMMENDATIONS
Using a research approach generally similar to that reported
herein, other factors that influence the surface resistivity of
fly ash should be investigated with the objective of incorporating
additional parameters into a procedure for predicting the magni-
tude of resistivity. Specifically, a study should be made of the
effect of field strength and variations in the environmental
parameters. Environmental conditions of interest should include
a simulated flue gas with variable concentrations of water,
sulfur trioxide, and sulfur dioxide.
It has been shown that several physical and chemical characteris-
tics of an ash influence resistivity. This knowledge should be
expanded to include a study of the chemical dissolution of ash
as a function of the environmental parameters. Such a study
would help one objectively select conditioning agents.
It is believed that the ability to predict fly ash resistivity
from the chemical composition of commercial ashes for defined
conditions has progressed sufficiently to warrant a study
leading to the prediction of this value from the analyses of
coal and coal ashes produced in a laboratory.
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SECTION III
INTRODUCTION
Electrical resistivity is one of the critical parameters
influencing the collectability of fly ash by electrostatic
precipitation. This report discusses research designed to
acquire additional knowledge about the surface conduction
process. The immediate objective of the work reported herein
was to study surface resistivity of fly ash with respect to
the conduction mechanism and the effect of the chemical
composition. The ultimate pragmatic objective of this
research is to obtain sufficient information so that
resistivity might be predicted from the measurable chemical
factors and conditioning agents might be objectively
selected.
At temperatures of 450°C down to 250°C or for certain condi-
tions as low as 200°C, the volume resistivity of fly ash is
dependent on an ionic conduction mechanism in which sodium
ions serve as the principal charge carriers.1 For a given
ash chemistry, the volume resistivity is also dependent on
ash layer porosity, field strength and temperature.
Over some range of temperature from perhaps as low as 125°C
to as high as 250°C, both volume and surface conduction
mechanisms contribute to the measured value of resistivity.
Below 125 to 150°C, the resistivity is principally controlled
by surface conduction. The factors that are assumed or known
to influence the magnitude of surface resistance include:
the chemical species that make up the environment of the
ash, the temperature, the chemical composition of the ash,
the surface area of the ash, and the field strength. Surface
conduction results from the interaction of the environment
and the ash surface.
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It has been generally accepted2 that surface conduction occurs
by an electrolytic or ionic mechanism dependent principally
on the physical and chemical adsorption of certain species
on the ash surface to produce a conducting film. This
implies that conduction is governed by the electrolysis of
the adsorbed species and that the component ions serve as
charge carriers.
Since the principal microconstituent of fly ash is a glassy
particulate, it is possible that surface conduction occurs in
a manner similar to that of glass. In this case, conduction
would take place by an ionic mechanism in which alkali metal
ions serve as the principal charge carriers. The role of the
environment would not be lessened since the interaction
between the ash surface and environment is required to
mobilize the carrier ions.
This research attempts to obtain information concerning the
surface conduction mechanism and determine the influence
of the chemical composition of fly ash on resistivity.
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SECTION IV
SCOPE
Seventeen commercially produced fly ashes representing a
cross-section of Eastern and Western coals were utilized.
These ashes were chemically and physically characterized with
respect to certain properties known to influence resistivity.
Using an ASME, PTC 28 electrode set, the surface resistivities
were determined at a voltage gradient of ^ 400 volts/cm in
an environment of air containing % 9 volume percent water
over a temperature range of 60°C to 250°C. Chemical trans-
ference experiments were conducted for two ashes having
similar values of resistivity but dissimilar chemical
compositions. The effect of particle size (conducting
surface area) on resistivity was examined for one ash
elutriated to five size fractions.
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SECTION V
EXPERIMENTAL EQUIPMENT, PROCEDURES AND CALCULATIONS
SPECIMEN SELECTION AND ASH CHARACTERIZATION
The ashes were received from commercial steam generating power
plants. With few exceptions, the manner in which the sample
was acquired and the sampling location was not known. After
passing the ash through an eighty mesh screen to remove large
foreign objects, the incoming sample was quartered to yield
eight equivalent, random specimens weighing about 25 grams
each. These specimens were used for resistivity measurements,
chemical transference determinations and ash characterization.
The ashes were characterized for chemical composition, helium
pycnometer density and particle size distribution. Helium
pycnometer density was determined with a Micromeritics Instru-
ment Corporation, Model 1302 helium-air pycnometer using the
manufacturer's suggested procedures. A number 6000 Banco
Micro Particle Classifier was employed to determine the
particle size distribution using the technique outlined by
the manufacturer. Density and particle size data were used
to calculate resistivity-specimen porosity and a factor to
represent the variation in ash surface area. Chemical analyses
were conducted for the elements commonly reported for fly ash.
Three methods of attack are required to prepare fly ash for a
complete chemical analysis. Digestion in HaSOi, - HF-HNOs pro-
vides a sample for the determination of Li, Na, K, Ca, Mg, Fe,
and Ti by atomic absorption spectrophotometry and P by a
colorimetric method. Colorimetric procedures are also used
for the analysis of Al and Si following fusion of fly ash in
sodium hydroxide. The total sulfate content is determined turbi-
dimetrically after fusion of the fly ash in sodium carbonate.
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EXPERIMENTAL SET-UP FOR RESISTIVITY DETERMINATION
Resistivity was determined in the temperature range of
60°C to 250°C in a controlled environment of air and water
vapor. The general appearance of the experimental arrange-
ment is shown schematically and photographically in Figures
1 and 2 respectively. Tank air was metered to provide about
9 volume changes per hour in the environmental chamber. The
air was passed through a glass, fritted-disc bubbler sub-
merged in a constant temperature water bath. Water bath
temperature could be controlled to +_ 0.5°C. Air passing
through the distilled water in the bubbler was approximately
saturated with respect to the temperature of the constant
temperature bath. The air-water mixture was passed through
an externally heated stainless steel tube into the environ-
mental chamber where the gas was diffused by a glass,
fritted-disc. The gas stream was directed toward the bottom
of the environmental chamber and exited out the top through
an externally heated tube that also provided a position for
sampling the gas. Once during every test the gas stream was
sampled to gravimetrically determine the water content.
Typically the gravimetric determination showed a water
concentration of 9.5 volume percent when the equipment was
set to deliver 10.0 volume percent.
The stainless steel environmental chamber was housed in a
conventional laboratory oven as shown in Figure 3. All feed-
throughs and connecting surfaces were sealed with high tempera-
ture 0-rings. The simple on-off thermal control that was
standard for the oven was disconnected, and a three-mode,
solid state device was used to control the temperature of the
oven. A control thermocouple in contact with the base of the
chamber was maintained within + 1°C.
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CO
Environmental sampling port
PTC 28 apparatus
N
Externally heated piping
Power
source
for oven
Externally heated exit piping
Calibrated C/A thermpetSuple
mV Potentiometer
Cold junction
Fritted disc
Environmental chamber
Fritted disc air bubbler
Make-up water reservoir
Heater
\
Pump
^
Constant temperature bath
Pressure regulator
'Air flowmeter
'Air tank
Figure I . Schematic of Apparatus Set-up for Resistivity Measurements
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Figure 2. Photograph of general apparatus
set-up for resistivity determination
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Figure 3. Photograph of environmental chamber
and oven
10
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Resistivity was measured using the ASME PTC-28 apparatus3
shown placed in the environmental chamber in Figure 4. Clip-on
electrical leads are shown with the electrode set resting on
alumina supports. The air-water vapor mixture was dispersed
downward beneath the lower, stainless steel, fritted disc
electrode. Specimen temperature was determined with a
calibrated, sheathed thermocouple that was positioned a few
mm above the upper surface of the ash. The test circuit
included a Keithley Model 240A high voltage supply and a
Keithley Model 610 electrometer.
TEST PROCEDURE FOR RESISTIVITY MEASUREMENT
Description
The weight and volume of the dish-shaped lower electrode was
determined so that specimen porosity could be calculated using
the helium pycnometer density of the ash. Electrode sets
were ultrasonically cleaned in a commercial cleaning solution,
rinsed in water, rinsed in alcohol and dried before each test.
The ash specimen was poured into the dish-shaped electrode
with occasional tapping of the edge and bottom of the
electrode. Excess ash was leveled to the upper lip of the
dish using a straight edge. The tapping action inhibited
bridging of the particles and yielded a specimen into which
the upper electrode did not readily sink. After weighing the
lower electrode containing ash, the upper electrode was placed
in position, and the assembly was set on the alumina supports
in the environmental chamber. The following steps were taken:
internal connections were made, the top sealed in position,
the chamber placed in the oven, external connections were made
and the oven was closed for test. The environmental chamber
was allowed to thermally equilibrate overnight at 60°C. Ex-
ternally heated inlet and outlet tubing and the constant
11
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Figure 4. Photograph of environmental
chamber and electrode set
12
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temperature water bath were automatically started about
two hours prior to the beginning of the test.
Two hundred volts dc were applied to the resistivity
specimens, and current versus time was monitored. When the
current was decreasing at a rate less than 10% in 10 minutes,
it was assumed to be stable. The time required to reach
current stability varied from 10 to 30 minutes among ashes.
After the current had stabilized,the applied voltage was
removediand the air-water environment was introduced. Every
ten minutes thereafter the voltage was reapplied and the
current read after one minute. The current increased about
3 or 4 orders of magnitude due to the introduction of the
air-water mixture and was stable" after 20 to 40 minutes.
The major portion of the current increase took place in the
first ten minute increment. The stabilized current reading
with the air-water mixture flowing was used to calculate
resistivity at 60°C.
The temperature set point was then advanced for the next
data point, 80°C. Temperature increase at the position
immediately above the ash specimen was observed. When the
rate of temperature rise was less than l°C/5 minutes, the
voltage was applied and current read one minute later. The
application of voltage and reading of current was repeated
after an additional 15 and 30 minutes: The temperature set
point was then increased to the next level. In this manner
resistivity was determined for six temperatures: about
60, 80, 110, 140, 180, 220 and 260°C.
It was believed that the procedure was satisfactory for the
main objective of this work; i.e., the evaluation of surface
resistivity as a function of ash composition.
13
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TEST PROCEDURE FOR CHEMICAL TRANSFERENCE
Chemical transference experiments were run using the resistivity
test equipment described above. The ash specimens were
allowed to equilibrate in an environment of air-10 volume
percent water at 60°C. Current was continuously monitored
versus time under an applied voltage of 2000 volts dc.
The tests were operated continuously for 300 to 400 hours.
The temperature, voltage and environment utilized were
selected to minimize the length of time required to establish
a chemical concentration gradient of sufficient magnitude
for detection using a simple technique.
After the specimen had been subjected to the above procedure,
the ash delineated by the area of the upper electrode was
removed in layers about 1 mm thick using a miniature vacuum
cleaning device. Each of these layers of ash as well as an
equivalent specimen that had not been tested were chemically
analyzed. The data were interpreted in terms of changes in
chemical concentrations as a function of distance between
the positive and negative electrodes. Inherent characteristics
of the ash and the difficulty encountered in separating the
ash into uniform layers allow only a semi-quantitative
evaluation of the data.
Resistivity
p = V/I • A/1 (1)
p = resistivity in ohm-cm
V = applied voltage in volts
I = measured current in amperes
A = effective electrode area in cm2
1 = specimen thickness in cm.
14
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Porosity of Resistivity Specimen
P = t(Ve - W/dh)/Ve] x 100 (2)
P = percent porosity
Ve = volume of lower electrode in cm3
W = weight of ash occupying Ve in grams
dn = helium pycnometer density of ash in grams/cm3
Surface Area Factor for a Unit Volume of Ash
S = s • N (3)
S = area in cm2 of 1 cm3 of ash
s = surface area of a single ash particle having
the mass-median diameter of the ash
N = 1a" .3 (Reference 4) (4)
N = total number of particles occupying 1 cm3
if all are equal to the mass median diameter
P = percent porosity
a - shape factor; ir/6 for spheres
dm = mass median diameter of the ash.
Atomic Percentage of Elements
After deleting the loss on ignition value, the chemical analyses
reported as oxides were normalized to 100 weight percent.
Each normalized weight percentage of oxide was divided by the
respective molecular weight to obtain molecular decimal
fractions . From these values the molecular percentage of each
oxide was calculated. The atomic percentage of cation was
calculated by multiplying the molecular percentage of the oxide
by the number fraction of cations in the given oxide .
15
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SECTION VI
RESULTS
CHARACTERIZATION OF ASHES
The results of microscopic, X-ray and microprobe analyses
can be collectively summarized as follows. Over 80 percent
of the ash was made up of spherically shaped, glassy
particles. Within particles and among particles, chemical
heterogeneity can be exhibited. From 5 to 20 percent of
the ash consisted of unburned carbon and crystalline com-
pounds.
Table 1 shows the chemical analyses for the various ashes
used in this study. The sufficient variation for each
reported element suggests that as a group the ashes represent
a reasonable cross-section of compositions suitable for
this study. Other important ash characteristics are shown in
Table 2. Helium pycnometer density values were used to
calculate resistivity specimen porosities. Porosity values
and the mass median diameter values were used to calculate
the surface area factors as shown in the previous section.
While not a precise characteristic, the surface area
parameter proved useful in analyzing resistivity data as a
function of ash composition.
16
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Table 1. CHEMICAL ANALYSES OF ASHES IN WEIGHT PERCENT
Li20 Na2O K20
MgO
CaO Fe2O3 A12O3 SiO2
TiO
P2O
1
2
3
4
5
6
7
8
9
11
12
14
15
17
18
19
20
0.02
0.01
0.03
0.03
0.01
0.01
0.04
0.04
0.01
0.02
0.01
0.05
0.03
0.04
0.02
0.02
0.07
0.32
0.35
0.38
0.20
1.84
0.25
0.33
0.29
2.31
1.77
0.25
0.38
0.43
0.28
1.28
1.83
0.27
3.10
2.36
3.34
0.26
0.20
0.89
3.88
2.69
0.91
1.13
0.81
2.99
2.58
2.25
0.78
1.19
2.72
1.04
1.66
1.29
5.76
12.75
1.88
1.57
0.98
1.04
1.93
2.57
1.42
1.00
1.04
3.22
0.94
0.76
2.62
3.72
1.04
22.60
31.00
11.10
0.77
0.64
12.10
6.36
13.30
1.00
0.62
4.55
9.14
5.18
0.38
20.50
16.10
9.70
4.25
11.20
3.71
10.01
9.12
4.23
4.61
4.66
10.91
5.63
24.70
4.85
3.81
3.88
19.60
17.80
25.90
21.00
14.80
23.60
27.50
29.10
25.10
24.60
23.60
26.70
28.10
21.20
19.50
27.16
29.78
46.40
43.30
49.90
38.80
22.00
55.60
51.40
52.00
49.60
53.70
53.60
49.10
49.60
39.80
55.10
57.27
52.74
1.69
1.27
1.98
1.19
0.60
1.56
1.79
2.55
1.39
1.49
0.88
1.87
2.29
1.24
1.79
1.05
1.88
0.60
0.36
0.32
0.32
0.39
0.14
0.32
0.42
0.19
1.06
0.19
0.41
0.39
0.31
0.35
0.17
0.18
1.56
0.93
0.29
0.97
2.19
0.22
0.25
0.15
0.21
0.75
0.19
0.29
0.44
0.43
0.72
0.40
0.28
2.10
10.30
4.40
0.33
0.41
0.74
1.50
1.40
0.45
1.49
0.00
3.20
0.58
5.49
0.93
0.60
7.08
Soluble sulfate.
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Table 2. MISCELLANEOUS ASH CHARACTERISTICS
Ash
NO.
1
2
3
4
5
6
7
8
9
11
12
14
15
17
18
19
20
Helium
Pycnometer
Density
in gms/cc
2.65
2.56
2.33
2.70
3.00
2.48
2.29
2.41
2.34
2.19
2.39
2.25
2.47
2.35
2.51
2.06
2.50
Resistivity
Specimen
Porosity in %
56.5
63.4
59.1
59.5
69.8
57.6
52.5
52.7
57.4
54,5
47.4
61.9
71.5
44.1
56.8
57.0
74.8
Mass
Median
Diameter
in ym
19
12
24
14
12
12
20
17
10
10
20
20
6
67
8
22
13
Surface
Area
Factor
in cm" l
1380
1840
1020
1740
1510
2120
1430
1670
2450
2740
1580
1170
2850
500
3240
1200
1180
18
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TRANSFERENCE DATA
The objective of this facet of the program was to determine
whether an alteration in chemical composition occurs in the
direction of the voltage gradient, thereby suggesting the
participation of certain ions in the conduction process as
charge carriers. Two ashes having exceedingly different
chemical compositions but similar values for resistivity
were selected for the transference study. A portion of the
ash specimen was chemically analyzed before the test.
After the specimen had been subjected to a 2000 V/cm voltage
gradient for several hundred hours at 60°C in an atmos-
phere of air containing 9 volume % water, the ash was
removed in several layers parallel to each other and
perpendicular to the voltage gradient. These individual
layers of ash were also chemically analyzed.
The data given in Table 3 illustrate the results of these
experiments. An obvious migration of alkali metal ions
can be seen. The alkali metal concentration has formed a
gradient consisting of a value less than the original con-
centration at the positive electrode to a value greater than
the original at the negative electrode. This suggests that
the positive alkali metal ions served as charge carriers
and were transported to the negative electrode. Such a
concentration gradient was not found for the other species
analyzed.
19
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Table 3. CHEMICAL ANALYSES OF TRANSFERENCE EXPERIMENTS IN WEIGHT PERCENT
Before test, Ash 1
At positive electrode
Elevation 1
Elevation 2
At negative electrode
Before test, Ash 9
At positive electrode
Elevation 1
Elevation 2
At negative electrode
Li2O
0.017
0.016
0.017
0.017
0.045
0.01
0.01
0.01
0.01
0.01
Na2O
0.31
0.26
0.29
0.38
0.54
2.54
2.53b
2.39
2.47
2.70
K20
3.00
2.88
2.96
3.11
3.80
0.94
0.88
0.90
0.90
0.97
Fe203
21.0
21.4
21.2
21.4
21.6
4.2
4.2
4.2
4.2
4.2
CaO
2.6
2.5
2.4
2.6
2.5
12.6
12.3
12.4
12.8
12.6
MgO
0.91
0.87
0.88
0.86
0.87
0.97
0.95
0.93
0.97
0.98
SO.,3
1.7
1.7
1.7
1.7
1.5
0.24
0.25
0.26
0.24
0.24
NJ
O
aSoluble sulfate.
^Probable error.
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Utilizing the relative changes in alkali metal concentration
and the quantitity of electricity passed during the experi-
ment, it was estimated that a major portion of the charge
could be accounted for by the change in composition. These
data do not define completely the overall conduction process
nor do they prove that no other ions participate. However,
the data offer excellent evidence that alkali metal ions
are important in surface conduction and that an attempt to
analyze surface resistivity as a function of ash chemistry
is justified.
RESISTIVITY DATA
The type of resistivity data taken in this research is
illustrated in Figure 5. The open circles represent the
measured resistivity data from this research. The closed
circles were determined from a computer, program5 designed
to predict volume resistivity as a function of ash chemistry,
temperature, and porosity. v These two ashes were selected
to illustrate the typical data because they are almost
identical in every characterization except that ash 9
contains considerably more sodium. Therefore, it can be
suggested that the two order of magnitude difference in
surface and volume resistivity was due to a one order of
magnitude difference in sodium concentration alone.
To analyze the resistivity data in terms of fly ash com-
position, the maximum surface resistivity of each ash was
selected from curves like those in Figure 5. This particular
resistivity value was chosen to avoid the effect of the
variation in sensitivity to water vapor pressure among the
ashes. The use of resistivity values taken at a particular
temperature or water vapor pressure would not alter the result
of this research.
21
-------
I0>2
E
o
i
E
x:
o
CO
CO
LJ
CC
10'°
I08
O MEASURED VALUES
COMPUTER VALUES
£ 10"
/ /- ~\ -\
ASH NO.6
ASH N0.9 —
3.0 2.6 2.2 1.8 1.4 1000
T*ir
60 112 182 282 442 °C
140 233 360 540 827 °F
TEMPERATURE
Figure 5. Typical resistivity data as a function of
reciprocal absolute temperature
22
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The as-measured maximum or peak surface resistivity was
corrected to a common specimen surface area prior to analysis
as a function of composition. Ancillary experiments showed
that the measured resistance of an ash was inversely pro-
portional to the surface area of the specimen provided
ash chemistry was invariant with respect to particle size
fraction. All resistivity data were corrected to a surface
area factor of 2000 cm2/cm^. The surface area factor was
calculated as shown by equations3 and 4 in Section V. The
basis for applying this correction is detailed in Appendix A.
Other investigators have observed the relationship between
surface resistivity and a parameter defining available
conducting surface. Dalmon and Tidy6 related decreasing
surface resistivity to increasing specimen bulk density
for a specific ash in which case the ash chemistry,
theoretical or true density, and particle size distri-
bution were constant.
Since the transference experiments at 60°C indicate the
alkali metal ions serve as charge carriers, one would
intuitively expect an inverse correlation between surface
resistivity corrected to a common surface area and the
summation of the three alkali metal atomic concentrations.
This correlation failed, indicating the influence of an
additional parameter. The situation was clarified when
the factors involved were independently examined.
23
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The relationship between surface resistivities normalized
to a given surface area and the combined lithium and sodium
concentrations is shown in Figure 6. The result was similar
to that obtained in a study of volume resistivity.5 The
constructed curve ties together the data for groups of
Western ashes having low and moderate amounts of lithium
plus sodium. Since the transference experiment showed only
a minor migration of potassium for the Western ash and
Figure 5 showed the pronounced influence of sodium on
resistivity, it is suggested that Figure 6 reasonably
illustrates the effect of the combined lithium and sodium.
An attempt to rationalize the lower resistivities for the
Eastern ashes based on high potassium content failed. When
all the resistivity data in Figure 6 are normalized to a
constant value of 0.4 atomic percent lithium plus sodium,
and plotted against potassium concentration, the lack of
correlation was obvious. This is shown in Figure 7 where
the resistivity data for the Western ashes are clustered
at one level of resistivity and the data for the Eastern ashes
are spread over two orders of magnitude for a narrow band
of potassium concentrations. Since the transference data
for the Eastern ash indicated a substantial migration of
potassium, Figure 7 presents an incongruous result. If one
compares the general order of resistivity values for the
Eastern ashes in Figure 7 with the chemical compositions of
Table 1, it becomes apparent that the iron concentration may
be important.
24
-------
E
o
E
o
H;
>
1-
(O
-------
\
MAXIMUM
10"
0
1 ' 1
D 20
4 • O9
>5° 180 °6 ,.
5 o"
.20 019 Q,5
1 A _^_o»«
D14
Q D3
D EASTERN ASH Qg
O WESTERN ASH
SURFACE AREA -2000cm-' D7
~ Li + Na-0.4% D '7 ~~
D 1
1 1 1
.1 1.0 10
ATOMIC PERCENTAGE POTASSIUM
Figure 7. Maximum surface resistivity normalized to 0.4
atomic percent lithium plus sodium versus
potassium concentration
26
-------
In Figure 8 the resistivity data normalized to 0.4 atomic
percent lithium plus sodium was plotted against the concen-
tration of iron. A good pseudo-correlation is shown in
that iron itself does not participate in the conduction
process. It was deduced that iron affects the chemical solu-
bility of the ash, thereby particularly influencing the
release of potassium to serve in the conduction process. It
was interesting to note that the Eastern ashes 20, 15, 14, 2
and 17 form a straight line. These ashes have similar
compositions with the exception of iron content. Also, it
was observed that ashes 5 and 7, deviating the most from
the constructed line, have respectively the lowest and
highest potassium concentrations.
In Figure 9 the relationship is shown for resistivities at
constant lithium plus sodium concentration as a function of
combined potassium and iron concentration. The correlation
is reasonably good as shown; however, if one considers the
individual data points with the proposed role of iron and
ash solubility as a frame of reference, the correlation
is excellent. First, the data for ashes 7 and 14 may
represent the expected spread for this type of data. On
the other hand, ash 7 contains the greatest amount of total
alkali which may also affect ash solubility as well as
supply charge carriers. Ash 1 contains by far the greatest
amount of soluble sulfate for the high iron ashes and thereby
may have lower resistivity due to increased reaction between
ash and the environment. Ashes 15 and 20 contain a signi-
ficant quantity of potassium but are low in iron, thereby
limiting the release of potassium. Ash 5 containing a
moderate amount of iron possesses very little potassium to
support conduction.
27
-------
6
o
I
e
JZ
o
co
en
UJ
cc.
UJ
o
g
cc
z>
CO
I0'2
05
D EASTERN ASH
O WESTERN ASH
10"
O.I
SURFACE AREA -2000 cm'1
Li* No -0.4 a/o
1.0
ATOMIC PERCENTAGE IRON
10.0
Figure 8. Maximum surface resistivity normalized to 0.4
atomic percent lithium plus sodium versus
iron concentration
28
-------
E
u
E
.c
o
V)
CO
UJ
DC
UJ
O
If
a:
CO
§
X
10
12
10"
D EASTERN ASH
O WESTERN ASH
SURFACE AREA-2000cm-
Li + No -0.4Q/0
20
O.I 1.0
ATOMIC PERCENTAGE POTASSIUM + IRON
10.0
Figure 9. Maximum surface resistivity normalized to 0.4
atomic percent lithium plus sodium versus
combined potassium and iron concentrations
29
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SECTION VII
DISCUSSION
SURFACE CONDUCTION
From the foregoing results, it has been observed that alkali
metal ions migrate through the fly ash toward the negative
electrode under the influence of an applied moderate voltage
gradient and conditions suitable for surface conduction. This
suggests that these ions have the role of charge carriers
and therefore have a pronounced effect on the surface
resistivity of the ash. Figure 5 illustrated the magnitude
of the effect. Experiments7 with fused accessory coal minerals
have also illustrated the attenuation of resistivity due to
the presence of alkali metals. In addition, Selle et al6
have demonstrated the decreasing surface resistivity of fly
ash with increasing sodium concentration.
If the alkali metal ions serve as charge carriers, an inverse
correlation should exist between resistivity and some measure-
ment of the available carriers. Without knowledge about the
degree of availability for the various carriers, the correla-
tions were attempted using the total atomic concentrations
of the alkali metal ions. A correlation between resistivity
and the combined concentration of lithium and sodium was
readily demonstrated. However, the effect of potassium alone
or in combination with the other alkali metals was not detected.
Since the transference experiments revealed a minor migration
of potassium for one ash and a major contribution by this
element for another ash, additional factors of potential
influence were considered. It was deduced that the partici-
pation of the potassium as a charge carrier was related to
the iron concentration of the ash. It is suggested that
the presence of iron influences the reactivity between the
ash and the environment promoting the dissolution of ash
30
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surface thereby releasing available potassium. Ancillary
experiments have shown that the release of the alkali ions
in the presence of water is enhanced for ashes of high iron
content. Correlations between resistivity and iron con-
centration or iron plus potassium concentrations were
obtained.
The following potential mechanism for the surface conduction
of fly ash is suggested by this research. Under appropriate
environmental conditions, the ash surface reacts with some
agent (for example, water) capable of releasing alkali metal
ions from the surface to act as charge carriers. The number
of ions released is dependent on: the concentration of
available alkali metal ions, the chemical durability of the
ash in the hostile environment, the temperature, and the type
and concentration of environmental species brought in contact
with the ash surface. Tentatively, it would seem that the
sodium is more readily released than the potassium. The
latter apparently requires more vigorous interaction between
the ash and the environment. Although this could be brought
about by several of the above mentioned factors, the presence
of iron probably by lowering the chemical durability of the
ash has facilitated the participation of potassium.
The above discussion principally parallels the explanation
of surface conduction for silicate glass. Since the main
microconstituent in the ashes examined is a fused or glassy
substance, this approach seems justified. Surface conduction
for glass is usually explained9 as an ionic migration of
inherent alkali metal ions under the influence of an electric
field. Clean, dry glasses containing monovalent cations have
exceedingly high surface resistivity. In the presence of a
31
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humid environment, the resistivity decreases slowly at low
partial pressures of water, and then rather rapidly as high
relative humidity is approached. On the other hand, a very
pure, clean siliceous glass maintains a high surface resis-
tivity in a humid environment, indicating the importance of
the alkali metals to the conduction process.
Glass containing alkali metal ions can react with water by
ion exchange, forming metallic hydroxides (for example,
NaOH) which further react with water forming mobile ions on
the surface. This process is also in part responsible for
the dissolution of glass in water. In addition to the ion
exchange mechanism, the glass network itself can be destroyed
by reaction with water as well as other agents. The dissolution
of the network can permit the accelerated diffusion and migra-
tion of ions larger than sodium, for example potassium.
If the suggested surface conduction mechanism for fly ash
is essentially correct, it places the role of ash chemistry
in the proper perspective and allows one to more objectively
consider problems regarding resistivity.
ESTIMATION OF RESISTIVITY
It is conceivable that the figures shown above can be used
to estimate maximum surface resistivity from the chemical
analysis of ash to anticipate problems for precipitators
operating in the region of 150°C. Since the research
represents an initial result, the utilization of these data
should be approached with caution. Certainly the information
will be improved upon with acquisition of additional labora-
tory data and field information.
32
-------
A perusal of the individual data points, with respect to
the correlations established, leads to the empirical
suggestion that about 1.5 atomic percent iron is required
to facilitate the participation of potassium in the con-
duction process. Using this observation, a correlation was
developed between estimated resistivity values and those
determined in the laboratory. If the ash contained less
than 1.5 atomic percent iron, the resistivity value from
the curve in Figure 8 for the given iron concentration was
determined. This resistivity value was located in Figure 6
at 0.4 atomic percent lithium plus sodium. The intersection
of a line, through this point and parallel to the curve in
Figure 6, with the ordinate of the known combined lithium
and sodium concentration yielded the estimated maximum surface
resistivity. For ashes containing more than 1.5 atonic
percent iron, the same procedure was used except the starting
point was Figure 9 using the combined iron and potassium
concentration. The resultant relationship between estimated
maximum surface resistivity and the laboratory measured
values is shown in Figure 10.
The satisfactory correlation shown in Figure 10 was to be
expected since the correlations illustrated in Figures 6, 8,
and 9 were reasonably good. Keeping in mind that the resistivity
numbers displayed are approximately an order of magnitude
greater than those determined in the field, it was interesting
to compare the available plant operating experiences with the
estimated maximum resistivities. At the plants producing ash
having an estimated resistivity >1 x 1012 ohm-cm, all had a
high resistivity problem. No problem with high resistivity
occurred at plants producing ash with an estimated resistivity
of <5 x 1011 ohm-cm. Between these two resistivity values,
problems have occurred.
33
-------
O BASED ON Li, No AND Fe
D BASED ON Li , No , Fe AND K
SURFACE AREA-2000cm
MEASURED RESISTIVITY, ohm-cm
Figure 10. Correlation between estimated and measured
maximum surface resistivity
34
-------
On the positive side, this technique is tailored to a maxi-
mum value and ashes possessing wide variations in chemistry,
including soluble sulfate, were used. Negatively, the
method does not take into account: (1) the magnitude of the
effect of water vapor pressure (which at the temperature
considered is known to vary with the specific ash), (2) the
variation in resistance related to surface area—however, this
can be included, (3) the effect of field strength on resis-
tivity, and (4) the effect of variations in environmental or
flue gas chemistry. It is hoped that this technique of
resistivity estimation will be improved upon and that the
influence of the above factors will be investigated.
35
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SECTION VIII
REFERENCES
1. Bickelhaupt, R. E. Electrical Volume Conduction in
Fly Ash. J. Amer. Pollut. Contr. Assoc. 2A_: 251-255,
March 1974.
2. White, H. J. Resistivity Problems in Electrostatic
Precipitation. J. Amer. Pollut. Contr. Assoc. 24:314-
338, April 1974.
3. Determining the Properties of Fine Particulate Matter.
Power Test Code 28, American Society of Mechanical
Engineers, New York, 1965.
4. Dallavalle, J. M. Micromeritics. New York, Pitman,
1948. p. 133.
5. Bickelhaupt, R. E. Volume Resistivity - Fly Ash Compo-
sition Relationship. Environmental Science and Technology.
2(4):336-342, April 1975.
6. Dalmon, J. and D. Tidy. A Comparison of Chemical Additives
as Aids to the Electrostatic Precipitation of Fly Ash.
Atmos. Environ. (Oxford, England). 6_:721-734, October
1972.
7. Dalmon, J. and E. Raask. Resistivity of Particulate Coal
Minerals. J. Inst. Fuel (London). 46_: 201-205, April 1972.
8. Selle, S. J., P. H. Tufte, and G. H. Gronhovd. A Study of
the Electrical Resistivity of Fly Ashes from Low-Sulfur
Western Coals Using Various Methods. U.S. Bureau of Mines.
(Paper 72-107, presented at Air Pollution Control Associa-
tion 65th Annual Meeting. Miami Beach. 1972).
36
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9. Doremus, R. H. Glass Science. New York, Wiley, 1973
Chapters 12 and 13.
10. Shale, C. D., J. H. Holden, and G. E. Fasching.
Electrical Resistivity of Fly Ash at Temperatures to
1500°F. Report R17041, Bureau of Mines, U.S.
Department of the Interior, 1968.
37
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APPENDIX A
A BASIS FOR NORMALIZING SURFACE RESISTIVITY TO
A COMMON SURFACE AREA PARAMETER
In attempting to establish relationships between resistivity
and the chemical composition of fly ash, one must consider
other factors that can contribute to the magnitude of
resistivity. Intuitively the measured surface resistance
should be inversely proportional to some measure of the
conducting surface area. Since this parameter is inherent
for the particular ash, one cannot eliminate the effect but
can attempt to take it into account when analyzing resistivity
data as a function of ash composition.
A commercial ash was elutriated into five size fractions to
examine the effect of a surface area parameter on measured
resistivity. All pertinent information for the ash fractions
is given in Table 4. Although the chemical compositions for
the five size fractions are not invariant, it was believed
that the uniformity was sufficient to derive an approximate
relationship with which one could normalize surface resis-
tivity to a constant surface area factor. It is obvious
that the loss on ignition values are greater for the finer
fractions. Shale's work and additional tests in this laboratory
indicate that the variation in LOI for a total LOI of < 10%
should not influence the results. Comment on the sulfur
concentrations will be made below.
Each ash fraction was subjected to the standard surface
resistivity test outlined in Section V. A family of surface
resistivity versus reciprocal absolute temperature curves
were produced similar to those shown in Figure 5. At all
temperatures the resistivity was greatest for the coarsest
38
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Table 4. CHARACTERIZATION OF FRACTIONATED FLY ASH
Fraction (ym) <3 3-7 7-15 15-25 >25
Chemical Analysis
Li20
Na20
K20
MgO
CaO
Fe203
A1203
Si02
Ti02
P205
SO 3 (total)
SOiT (soluble)
LOI
LOD
Helium Pycnometer
Density in gms/cc
Resistivity Speci-
men Porosity in %
Mass Median
Diameter in ym
Surface Area
Factor in cm"1
0.08
0.48
2.81
0.97
1.19
6.89
25.90
44.90
1.79
0.79
0.64
0.59
8.71
0.43
2.75
74.5
2.3
6520
0.08
0.47
2.63
0.94
1.33
5.90
26.20
46.70
1.45
0.51
0.51
0.41
9.05
0.18
2.63
67.8
3.6
5360
0.08
0.51
2.81
0.98
1.12
8.34
26.80
48.50
1.52
0.46
0.28
0.26
6.00
0.13
2.56
64.7
8.0
2640
0.08
0.47
2.91
1.13
1.16
7.69
27.60
50.60
1.52
0.48
0.19
0.16
3.60
0.07
2.39
58.0
15.0
1680
0.09
0.45
2.84
1.15
1.06
11.32
27.90
51.90
1.63
0.38
0.12
0.07
0.87
0.07
2.31
54.3
40.0
690
39
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fraction and lowest for the finest fraction. The three other
fractions produced curves between these limits. The only
questionable and unexplained data were produced by the 7-15
ym fraction which yielded lower than anticipated resistivity
values at temperatures above 125°C. From these curves,
the resistivity values at the peak (^150°C) and at 100°C
were taken for analysis.
Surface resistivities as a function of a surface area parameter
are shown in Figure 11. The circles represent the maximum
surface resistivity, and the triangles represent the surface
resistivity at 100°C. It is believed that the linear inter-
pretation of these data points is correct. These data show
the pronounced effect particle size (conducting area) can
have on resistivity. The effect is greater for lower
temperatures. The slope of the curve for data taken at the
maximum resistivity position (about 150°C) is ^ 1.0, for
100°C ^ 1.5, and for 60°C ^ 2.0. This increase can be
intuitively rationalized since at the lower temperatures
more water molecules should spend more time on a given unit
of surface.
The data used in this research to evaluate resistivity as a
function of ash chemistry were normalized utilizing the above
information. Using a slope of one, peak resistivities were
adjusted from the measured value at some measured surface
area factor to the resistivity the ash would have registered
if the surface area factor had been 2000 cm" . This value
was chosen to minimize the magnitude of correction for any
one ash.
40
-------
10
13
10
12
E
o
e
o
CO
CO
UJ
OH
10'
O MAXIMUM SURFACE RESISTIVITY «* I50°C
A SURFACE RESISTIVITY AT IOO°C
D PERCENT SOLUBLE SULFATE
10
10
10'
,0
ID-'
UJ
Z)
CO
UJ
-J
CO
d
CO
t-
LU
o
cr
UJ
0.
JO-2
10'
SURFACE AREA PARAMETER,cm-'
Figure 11,
Surface resistivity and soluble sulfate
as a function of surface area
41
-------
In Table IV a disproportionate weight percent of sulfur is
found with the finer fractions. This is comewhat misleading,
A plot of soluble sulfate versus surface area factor is
shown in Figure 11 with a slope of one. This demonstrates
that the sulfur which is principally associated with the
ash surface is rather uniformly distributed with respect to
unit surface area.
42
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. RtFORT NC.
EPA-600/2-75-017
A. TITLE AND SUBnTl T
Effect of Chemical Composition on Surface Resistivity
of Fly Ash
3. RECIPIENT'S ACCESSION-NO.
5. REPORT DATE
August 1975
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
R.E. Bickelhaupt
8. PERFORMING ORGANIZATION REPORT NO
SORI-EAS-75-397
3134-XV
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADJ-029
11. CONTRACT/GRANT NO.
68-02-1303
12. SPONSORING AGENCV 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; 4/74 - 4/75
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT,
The report gives results of a study of the effect of chemical composition on
the surface resistivity of fly ash. The resistivity of a group of well characterized
fly ashes was determined. Resistivity data were taken between 60 and 250 C in an
environment of air containing approximately 9 vol. % water. Chemical transference
experiments were conducted for two ashes with substantially different chemical com-
positions. Chemical analyses of the transference specimens revealed a pronounced
migration of alkali metal ions toward the negative electrode: the surface resistivity
was inversely proportional to the concentration of these ions. Ion concentration infl-
uenced the participation of potassium in the current conduction process. Surface
resistivity is sensitive to the chemical composition of fly ash because the alkali
metals serve as charge carriers. The conduction mechanism is probably analogous
to that of glass. This viewpoint is compatible with the usual empirical observations
regarding the effect of certain parameters on resistivity; for example, the interac-
tion between ash and the environment,
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Air Pollution
Fly Ash
Surface Resistivity
Chemical Composition
b.lDENTIFIERS/OPEN ENDED TERMS
Air Pollution Control
Stationary Sources
c. cos AT I Field/Group
13B
21B
20C
07D
8 DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
50
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
43
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