EPA-650/2-74-079
AUGUST 1974
Environmental Protection Technology Series
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EPA-650/2-74-079
TECHNIQUES
FOR
MEASURING FLY ASH
RESISTIVITY
by
Grady B. Nichols
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
Contract No. 68-02-1303
ROAP No. 21ADJ-029
Program Element No. 1AB012
EPA Project Officer: Leslie E. Sparks
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D . C . 20460
August 1974
LIBRARY
U. S. LNVI,tC,'Wii:K7AL PROTECTS, '^ENCY
EDISON, N. J. 08817
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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ABSTRACT
This report discusses the methods used for the measurement of
the electrical resistivity of particulate matter suspended in
an effluent gas stream. Factors affecting the resistivity of
the particulate matter and its measurement are discussed. A
description of the operating characteristics of a variety of
currently available devices is given, together with a discussion
of the significant differences between the various devices.
A discussion is also included of in-situ versus laboratory
techniques for measuring resistivity and the interpretation
of data measured by these methods. Comparative values of
resistivity as determined by the various measurement techniques
are given for the limited conditions under which simultaneous
measurements were made with two or more methods.
111
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CONTENTS
Page
Abstract iii
List of Figures v
Acknowledgements vi
Sections
I Introduction 1
II Significance of Particulate Resistivity to
Electrostatic Precipitator Operation 2
III Factors Influencing Resistivity 5
IV Methods for Measuring Resistivity 15
V References 42
IV
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FIGURES
No. Page
1 Voltage vs current for a precipitator with
2.3-cm plate spacing and 0.29-cm corona wire 3
2 Typical temperature-resistivity relationship
for fly ash 6
3 Resistivity vs temperature for two fly ash
samples illustrating influence of sodium content 8
4 Resistivity as a function of moisture content
for a fly ash sample at various temperatures 10
5 Variation in particulate in-situ resistivity
with electric field for tHe" parallel disc
measurement method with the point-to-plane
probe 13
6 Bulk electrical resistivity apparatus, general
arrangement 18
7 Schematic of apparatus set-up for resistivity
measurements 19
8 Point-to-plane resistivity probes equipped for
thickness measurement 24
9 Typical voltage-current relationships for
point-to-plane resistivity probe 26
10 Resistivity apparatus using mechanical cyclone
dust collector 29
11 Cyclone probe inserted in duct 31
12 Kevatron resistivity probe 32
13 Lurgi in-situ resistivity probe 35
14 Comparison of Kevatron and cyclone resistivities
with point-plane resistivities at an electric field
of 2.5 kV/cm 37
15 Comparison of Kevatron and cyclone resistivities
with point-plane resistivities at an electric field
of 2.5 kV/cm. 38
v
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ACKNOWLEDGEMENTS
Information for the preparation of this report was obtained
from Dr. R. E. Bickelhaupt, Mr. Sabert Oglesby, Jr., and
Dr. Herbert W. Spencer. Dr. C. E. Feazel provided editorial
review.
In addition to the above members of the Southern Research
Institute staff, assistance was provided by several electric
power companies distributed across the United States in
allowing measurement programs to be conducted at their
plants.
The assistance of Dr. Harry J. White is gratefully acknowl-
edged, especially for discussions and analysis of the work
which led to this document.
vx
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SECTION I
INTRODUCTION
This report describes the operation, advantages and disadvan-
tages of utilizing a number of different types of resistivity
measurement instruments and techniques that are currently
available for determining the electrical resistivity of partic-
ulate suspended in effluent gas streams. The data presented
and the conclusions drawn are the result of work conducted by
Southern Research Institute personnel and by others that have
been active in this field. As a result of these studies,
Southern Research now utilizes the point-to-plane in-situ
resistivity probe for field measurement and a modified A.S.M.E.
Power test code #28 device for laboratory measurements.
The behavior of an electrostatic precipitator can be related
to the value of the resistivity of the suspended particulate
in a theoretical manner. The electrical sparking or back
corona conditions are related to the electrical breakdown of
the dust layer as described later in this report. Therefore,
any instrument selected as a measurement tool should provide
values that are consistent with those that electrostatic
precipitator theory would predict for limiting conditions.
The point-ot-plane probe provides data that are more clearly
consistent with observed behavior of operating electrostatic
precipitators than the other alternative devices. This factor,
together with the similarities between the operation of a
precipitator and the instrument are the basis for the selection
of the point-to-plane device as the preferred measurement
instrument.
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SECTION II
SIGNIFICANCE OF PARTICULATE RESISTIVITY
TO ELECTROSTATIC PRECIPITATOR OPERATION
The electrical resistivity of particulate matter present in
the effluent gas stream is one of the primary factors that
determine the operating characteristics of an electrostatic
precipitator. In a conventional single-stage, dry-electrode
electrostatic precipitator, the total corona current flows
through the previously collected dust layer to reach the
grounded collection electrode. This flow of current establishes
an electric field (E) in the dust layer proportional to the
corona current density (j) and the particulate resistivity (p)
as given by
E - jp. (1)
The electric field in the dust layer yields a voltage drop
(VV) across the dust layer proportional to the dust layer
thickness (t) of
VV = Et. (2)
If the resistivity of the dust layer is increased while the
current density is held constant, the electric field in the
layer increases proportionately (Eq. 1). If the electric
field in the dust layer exceeds the field strength for corona
initiation (electrical breakdown), an electron avalanche will
occur in the dust layer similar to that which occurs adjacent
to the corona wire. This electrical breakdown acts as a
limit on the allowable electrical conditions in the precipi-
tator as is discussed below.
The manner in which this breakdown limits the precipitator
performance is dependent upon the value of the resistivity
of the dust and the thickness of the layer. If the resistivity
is in the moderately high range (^10ll ohm-cm) the breakdown
will generally initiate electrical sparkover between the
precipitator electrodes; whereas if the resistivity is very
high (^lO12 ohm-cm) breakdown of the dust layer will occur at
a voltage too low to propagate a spark across the interelectrode
region. This gives rise to a condition of reverse ionization
or back corona. Figure 1 illustrates these two conditions.
The figure shows the current density as a function of applied
voltage for an electrostatic precipitator with a 0.28-cm
(0.109-in.) diameter corona wire, a 23-cm (9-in.) plate spacing,
and a dust layer thickness of 1 cm. If the dust layer resis-
tivity is in the moderately high range, e.g., 2x10 ohm-cm,
electrical breakdown in the dust layer will occur at an applied
voltage greater than that required for sparking between clean
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APPLIED VOLT AGE, KILO VOLTS
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Figure 1. Voltage vs current for a precipitator with
23-cm plate spacing and 0.29-cm corona wire.
Solid curve is for a clean electrode. The two
dashed curves represent conditions for a 0.5-cm
layer of dust with the resistivities indicated.
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electrodes, and sparking will occur at a reduced current density.
For a very high resistivity dust with the conditions shown,
electrical breakdown will occur in the dust layer at a much
lower current density and applied voltage. Under these condi-
tions, the dust layer will be continuously broken down electri-
cally and will interject ions of an opposite electrical polarity
from those produced by the corona into the interelectrode space.
The precipitator electrical operating conditions are thus limited
by a high resistivity dust layer and the precipitator is con-
strained to operate at lower currents and voltages than one
collecting a lower-resistivity dust. The magnitude of the
reduction in electrical operating conditions is a direct func-
tion of the dust resistivity.
In view of the importance of the resistivity of the dust layer
as a primary factor in limiting the performance of a precipi-
tator, it becomes necessary to determine the resistivity of the
material to be collected in order to estimate the conditions
to be expected in a precipitator. *
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SECTION III
FACTORS INFLUENCING RESISTIVITY
The electrical resistivity of a collected layer of fly ash
varies with temperature in a manner illustrated in Figure 2.
Above about 225°C, resistivity decreases with increasing
temperature and is independent of flue gas composition. Be-
low about 140°C, resistivity decreases with decreasing tempera-
ture and is dependent upon moisture and other constituents
of the flue gases.
In analyzing the conduction process, it is convenient to
consider the resistivity as involving two independent con-
duction paths, one through the bulk of the material (volume
conduction) and the other along the surface of the individual
particles, associated with an adsorbed surface layer of some
gaseous or condensed material. Either of these paths may
become the dominant conduction mode under conditions that
exist in operating precipitators, or, as is the general case,
both mechanisms may be important. The volume conduction is
dependent upon the chemical composition of the particulate
material, whereas surface conduction is controlled by the
chemical compositions of both the particulate and the
effluent gas stream.
FACTORS INFLUENCING VOLUME RESISTIVITY
Volume conduction in fly ash is an ionic process resulting
from the migration of alkali metal ions, especially sodium.
Whether the conduction takes place through the particles or
along the particle surface has not been definitely established.
The important distinction is that volume conduction, or volume
resistivity, is governed only by the character and composition
of the dust and is independent of gas composition.2
Volume conduction in all dusts encountered in industrial gas
cleaning is temperature dependent. In the case of ionic
conduction, increased temperature imparts greater thermal
energy to the structure of the material, allowing carrier
ions to overcome adjacent energy barriers and to migrate
under the influence of an electric field. Thus, for volume
conduction, an increase in the temperature produces an increase
in the number of carriers available to contribute to the
conduction of the particulate layer.
Figure 3 shows the relationship between volume resistivity and
temperature for two fly ash samples produced by combustion of
coal. The change of resistivity with temperature can be
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SURFACE
RESISTIVITY
VOLUME RESISTIVITY
COMPOSITE OF
SURFACE AND
VOLUME
RESISTIVITY
I09
J I I I 1 I 1 I 1 I 1 I I I I I I I I
70 100 150
200 250 300 400 600 800 1000
TEMPERATURE,°F
Figure 2. Typical temperature-resistivity relationship
for fly ash.
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expressed in the form of an Arrhenius equation
p = po exp (Q/kT) (3)
where p is the resistivity, PO is a material constant, Q is
an experimentally determined activation energy, k is Boltzmann's
constant, and T is the absolute temperature. For the fly ash
example shown in Figure 3, the material constant po is different
for fly ash with different sodium ion contents. Graphically,
a shift in po causes a parallel shift in the temperature-
resistivity curve. The experimental activation energy Q is a
rate phenomenon and represents the slope of the temperature-
resistivity curve. The quantities p0 and Q are useful in
defining electrical conduction properties of solid or granular
materials as a function of temperature.
In some types of dusts, conduction may be electronic instead
of ionic. Nevertheless, the Arrhenius equation applies,
whether the conduction is electronic or ionic, and the
temperature-resistivity relationships are similar, differing
only in the values of the constants in the Arrhenius equation.
Volume resistivity of a dust sample is also related to its
porosity. Intuitively, one would expect a higher resistivity
to be associated with a more porous dust layer due to the
smaller quantity of material in a given volume.
For fly ash samples, a 25% change in specimen porosity causes
a change of one decade in resistivity. A generalized relation-
ship between specimen porosity and resistivity was found2 for
fly ash to be
where
log pc = log pm + S(Pc-Pm) (4)
p = resistivity at porosity P
c c
p = resistivity at porosity P
m m
S = Vlog p/V%P = 0.04.
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200 300
TEMPERATURE,°C
200
Figure 3.
300 400 500 600
TEMPERATURE , °F
700
800
Resistivity vs temperature for two fly ash
samples illustrating influence of sodium
content.
8
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FACTORS INFLUENCING SURFACE RESISTIVITY
Surface conduction requires the establishment of an adsorbed
layer of some material either to provide an independent con-
duction path or to interact with some component of the
particulate material to provide a surface conduction pathway.
If the effluent gas stream contains condensable material
(e.g., water or sulfuric acid) and if the temperature is low
enough that an adsorbed layer can form, then the surface
conduction will become significant.
For temperatures below about 150°C (300°F), surface conduction
occurs via the lower resistance path created by the absorbed
moisture or chemical components which occurs at these lower
temperatures. Both moisture and chemically reactive substances
such as sulfur oxides and ammonia are commonly present in many
industrial gases.
Physical adsorption as well as condensation can be involved in
surface conduction. At temperatures below the dew point, the
rate of deposition on the surface of a dust would be high.
However, for most circumstances the adsorbate is deposited on
the dust surface and can provide a surface conduction pathway
even at temperatures considerably above the dew point, as is
shown in Figure 4.
In surface conduction, the mechanism of charge transport
appears to be ionic; however, the migrating species have not
been identified. They could be ions extracted from or carried
on the dust surface or those deposited from the gas stream.
An example of how surface resistivity of fly ash depends on
the composition of the flue gas is the case of fly ash from
coal-fired boilers burning sulfur-containing coals. The
burning of coal containing sulfur produces sulfur dioxide (S02)
in quantities dependent on the sulfur content. Under normal
conditions, about 0.5 to 1% of the S02 present is oxidized to
SO3, which serves to reduce the resistivity of the fly ash,
if the temperature is low enough for the SO3 to be adsorbed
on the ash. Thus, high-sulfur coals tend to produce ash
with lower resistivities than coals with lower sulfur contents.
In general, lowering the flue-gas temperature increases the
SO3 absorption, so that the resistivity of the fly ash can be
controlled to some extent by changes in flue gas temperature.
The influence of electric field on conduction in insulating
materials has been well documented. In solid materials,
increasing electric field permits a greater number of migrating
ions to participate in the conduction process. In granular
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Figure 4 .
Resistivity as a function of moisture content
for a fly ash sample at various temperatures.
10
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materials additional influences of electric field may become
important.3 Possible effects are:
• an increase in temperature at the contact points
between particles caused by joule heating
• an electric discharge in the dust layer due to
the enhanced field near adjacent particles.
FACTORS INFLUENCING MEASUREMENT OF RESISTIVITY
Resistivity of a dust layer is determined experimentally by
collecting a sample of the dust from a gas stream and measuring
the current and voltage characteristics of a defined geometrical
configuration of the dust. The method of collecting the dust
from the gas stream, the method of forming the dust layer,
and the conditions of measurement all influence the resistivity
measurement.
Particle Size
For determination of the true particle size distribution, the
sample should be taken from the gas stream in a manner (e.g.,
isokinetically) that insures that the sample is representative
of the particle size distribution of the dust in the gas
stream. However, due to problems of probe design, most of
the resistivity probes either do not sample isokinetically or
do not collect all the particles sampled. In either instance
the sample is not representative of the size distribution of
the dust in the gas stream.
Even if isokinetic sampling were used, the particle size of the
dust layer deposited in each field of a precipitator differs
due to the variation in collection efficiency as a function of
particle size. Consequently, in determining resistivity to
correspond to that of each field of a precipitator, the particle
size distribution associated with each field would have to be
simulated. In general, such a procedure would be impractical,
and some means of obtaining a reasonably representative sample
is employed.
It has been shown that the resistivity of fly ash varies with
the size fraction, the smaller size fractions having lower
resistivities. 4 The extent of variation in resistivity with
particle size differs with the conduction mode. In the surface
conductivity region, the surface area to mass ratio is higher
and a larger percentage of adsorbed gases are present. Also
the smaller particle size gives a lower porosity sample.
11
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Source Variability
A second factor influencing resistivity measurement is source
variability. In spite of attempts to obtain a uniform boiler
fuel by blending the coal supply, the chemical composition
of the coal will vary enough to be reflected in observable
changes in the S02 level of the flue gases and in the chemical
composition of the fly ash. Thus, to minimize errors due to
source variability, resistivity measurements should be made
on samples taken over a sufficiently long period of time, and
the results should be averaged to obtain a representative
value.
Electric Field
Since the resistivity of a dust varies with electric field,
it is important that measurements be made at an electric field
corresponding to that in the precipitator and/or that the value
of the field at which the measurement is made be specified.
In some resistivity probes the voltage is increased until the
dust layer breaks down, and the resistivity reported is that
corresponding to the condition just prior to breakdown. Other
probes impose a fixed voltage across a pair of electrodes to
establish a field. Generally the magnitude of the field is
very low, of the order of 1 kV/cm for this latter type of
instrument.
Figure 5 shows a typical relationship between resistivity and
electric field for fly ash (from coals with low and moderate
sulfur contents). The reported values would of course be
different depending upon whether the measurement was made at
a low field or near breakdown.
Method of Depositing Dust Layer
In an electrostatic precipitator, the dust layer is deposited
electrostatically and the particles are> aligned somewhat as
the dust layer is built up. In some sampling probes the dust
layer is deposited electrostatically, whereas in other probes
the dust is collected by other means and allowed to fall into
the measurement cell.
The significance of the method of deposition has not been
quantitatively determined. However, to the eye, dust layers
deposited electrostatically appear denser than those established
by free fall of the dust. In probes in which the dust is
allowed to fall into the measurement cell, some attempt is made
to vibrate the cell or otherwise establish a reproducible
density of the deposited dust. In other probes, measurement
12
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6 8 10 12
ELECTRIC FIELD, KV/CM
14
16
18
20
Variation in particulate in-situ resistivity
with electric field for the parallel disc
measurement method with the point-to-plane
probe. Temperature - 265°C(330°F);dust layer
thickness - 1.0 mm.
13
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technique involves a disc placed on the dust surface. This
disc provides some compaction of the dust layer.
Time of Current Flow
When voltage is applied to a dust layer, the magnitude of the
current will initially be high and it will then fall off,
rapidly at first and slowly thereafter. The initial current
surge is due to absorption current, which charges the capacitance
associated with the dust layer. The subsequent decrease in
current is due to depletion of the charge carriers or polariza-
tion at the dust-electrode interface.
If current is allowed to flow for a considerable time prior to
making resistivity measurements, the value will be lower than a
measurement made immediately following application of a voltage.
All the above factors aid in complicating what is basically
a very simple measurement. Since so many factors affect
resistivity, field measurements invariably show a certain
amount of scatter - quite often ten-fold. It is thus nec-
essary that several measurements be made to obtain meaningful
results.
14
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SECTION IV
METHODS FOR MEASURING RESISTIVITY
The determination of the electrical resistivity of particulate
material is made indirectly. None of the available devices is
capable of making this measurement directly. The resistivity
is computed from the resistance of a sample of the material
with a known geometrical configuration. Typically, the
geometry of the sample will be either a rectangular or cylindri-
cal solid, or the volume of space between concentric cylindri-
cal electrodes. In each instance, the relationship between
the resistivity and resistance of what is considered to be a
homogeneous material is given by
P = RA/1 (5)
where
p = resistivity (ohm-cm)
R = resistance (ohm)
A = cross sectional area (cm2)
1 = length (cm)
In each measurement device, the amount of material actually
utilized for the measurement is on the order of one cubic
centimeter or less. Layer thickness from one-half to three
millimeters is common. Using this minute sample of material
selected from the large quantities of fly ash generated during
a measurement period raises serious questions as to just how
representative of the total fly ash material this sample can
be. This factor may, in part, explain the wide range of
scatter actually observed in a resistivity measurement program.
Several techniques can be used for measuring the resistivity,
and several types of equipment are available for this purpose,
with no general agreement as to their relative merits.
However, the choice of method and equipment can be aided by
the following considerations.
One consideration is whether an absolute resistivity is to be
made for scientific or engineering purposes or whether a
relative or rank ordering type of measurement is sufficient.
If one is attempting to relate the behavior of an electro-
static precipitator to theoretically derived relationships,
15
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then it is important to attempt to evaluate the absolute
resistivity of the dust. However, if one has accumulated
a considerable quantity of resistivity data over a period
of time with one type of device and in addition has similarly
accumulated experience as to how a particular type of electro-
static precipitator behaves with the related particulate re-
sistivity, then other methods may be equally applicable for
these particular investigators.
As described in Section III, the measured value of resistivity
is dependent upon a number of factors. If the measurements
are contemplated for rank ordering or relative behavior, then
wide latitude is allowed in the selection of a method. For
the relative measurement type of investigation, it becomes
important to merely assure that the measurement conditions
are reasonably well duplicated for each condition, and the
selection of method becomes of secondary importance. Either
in-situ or laboratory methods may be applicable to a study
of this nature if the sample collection conditions, including
temperature, are identical. However, if the purpose of the
study is to evaluate how an electrostatic precipitator will
behave with a new or significantly different type of dust
under a given set of conditions, in-situ measurements will
probably be necessary.
For comparative evaluations, in-situ measurements must be made
with the same instrumentation. As discussed later in this
report, extreme care must be exercised in attempting to compare
resistivity values obtained with one type of device with those
obtained by another type.
LABORATORY VS IN-SITU MEASUREMENTS
The determination of whether the particulate resistivity should
be measured in the laboratory or in-situ is based on an evalua-
tion of the significance of the surface conduction component.
If the surface conduction is negligible because of high
temperature (>200°C) or because of the absence of any reactive
or condensable material (H20, SO3, etc.) in the effluent gas
stream, then laboratory measurements are appropriate.
However, if reactive constituents are present and if the
temperature is in the vicinity of the dew point of the con-
densables, such that there is a reasonable probability that
an adsorbed surface layer will exist, then it is imperative
that the resistivity be measured in-situ.
It is important to make measurements in the effluent gas
stream rather than in the laboratory even though the chemical
composition of the gas stream can be duplicated in the labora-
tory. The reason for this distinction is that as the particulate
16
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sample is collected, cooled and transported to the laboratory,
there is a reasonable probability for chemical reactions to
occur that would modify the particulate matter prior to
measurement.
LABORATORY MEASUREMENTS
The standard technique for conducting laboratory resistivity
measurements is described in the American Society of Mechanical
Engineers Power Test Code 28, Determining the Properties of
Fine Particulate Matter. This code was adopted by the Society
in 1965 as a standard practice for the determination of all
the properties of fine particulate matter which are involved
in the design and evaluation of dust-separating apparatus.
The tests include such properties as terminal settling velocity
distribution, particle size, bulk electrical resistivity, water-
soluble sulfate content, bulk density, and specific surface.
The document defines bulk electrical resistivity as the
resistance to current flow, expressed in ohm-centimeters,
through a dust sample contained in a cubic volume one centi-
meter on a side when exposed to an electrical voltage equiva-
lent to 90% of the breakdown voltage of the sample, applied
uniformly across two opposite faces of the cube. The code
specifies that the property is to be determined at 150°C (300°F)
and at a humidity of 5% by volume, unless otherwise specified.
Apparatus
The basic conductivity cell is shown in Figure 6. It consists
of a cup which contains the ash sample and which also serves
as an electrode, and an upper electrode with a guard ring.
To conform with the code, the high-voltage conductivity cell
must have the same dimensions as shown, and must use electrodes
constructed from 25-micron porosity sintered stainless steel.
The controlled environmental conditions required for the
measurement of resistivity in the laboratory can be achieved
by an electric oven with thermostatic temperature control
and with good thermal insulation to maintain uniform internal
temperature, and a means to control humidity. Humidity may
be controlled by any one of several conventional means,
including circulation of pre-conditioned gas through the
oven, injection of a controlled amount of steam, use of a
temperature-controlled circulating water bath, or the use
of chemical solutions which control water vapor pressure.
It is desirable to circulate the humidified gas directly
through the dust layer; hence the reason for the porous
electrodes. Figure 7 illustrates a suitable set-up for
resistivity measurements.
17
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Mechonicol
Guide
(Insulated)
Guard Ring
l-l/8in.Dia.by
1/8 in. thick
Movable Electrode
3/4 to I in. Dia.
by 1/8 in. thick
Dust Cap
3 in. ID, 5mm deep
•To High Voltage Supply
Figure 6.
Bulk Electrical Resistivity Apparatus,
General Arrangement.
The movable disk electrode is weighted so that the pressure
on the dust layer due to gravitational force is 10 grams per
square centimeter. The nominal thickness of the dust layer
is 5 millimeters. The actual thickness is to be deter-
mined with the movable electrode resting on the surface of the
dust. All electrode surfaces in the region of the dust
layer are to be well rounded to eliminate high electric
field stresses.
18
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Procedure for Laboratory Resistivity Measurements
The first problem encountered in making any resistivity measure-
ment is obtaining an appropriate dust sample. The prescribed
procedure for PTC 28 Code assumes that samples of gas-borne
dust are taken from a duct in accordance with the Test Code
for Determining Dust Concentration in a Gas Stream (PTC 27-1957),
The PTC 27 Code involves isokinetic dust sampling at various
points in the duct. It is recommended that samples should
not be obtained from a large bulk of material in a hopper,
silo, or similar location. If it is necessary that samples
be obtained from such a location, procedures which will in-
sure that the sample is representative of the whole must be
used. For any resistivity test to be performed on a bulk
sample, it is necessary that a random sample be obtained.
This can be done by quartering the bulk sample to obtain the
test sample.
To break up agglomerates and to remove foreign matter, e.g.,
collection plate scale, the specimen cam be passed through
an 80-mesh screen.
The procedure for making the resistivity measurement according
to Power Test Code 28 follows:
1) The sample is placed in the cup of the
conductivity cell by means of a spatula.
Then it is leveled by drawing a straight
edge blade vertically, across the top of
the cup.
2) The disc electrode is gently lowered onto the
surface. It should rest freely on the sample
surface without binding on any supports.
3) The conductivity cell is mounted in the environ-
mental chamber and equilibrium temperature and
humidity are established. The Code specifies
that a temperature of 150°C (300°F) and a humidity
of 5% by volume are to be used for the test, unless
otherwise specified.
4) A low voltage is applied to the cell and then
gradually raised in a series of steps up to the
point of electrical breakdown of the sample layer.
Current transients will occur when the voltage
is first applied or increased across the cell.
It is necessary that these die away before re-
cording current and voltage readings (approxi-
mately one minute). A record of the current-
20
-------
voltage characteristic of the dust is obtained.
Preferably using another sample, the above is
repeated; when another sample is not available,
the sample layer should be remixed and releveled
after each run in order to break up any spark
channels that may have been formed in the dust
layer. A total of three runs should be made.
The average breakdown voltage is then calculated.
Before taking the samples to breakdown, it is
necessary to determine whether the temperature
and moisture content of the sample are in equilib-
rium with temperature and humidity of the con-
trolled environment. A test for equilibrium
is that the voltage-current measurements are
reproducible to within 10% when determined by
two successive measurements made 15 minutes
apart .
5) The resistivity of the samples is then calculated
in the range of 85 to 95% of the average break-
down voltage, using the corresponding currents
from the previously recorded voltage -current
characteristics .
Laboratory use of the PTC 28 apparatus to study character-
istics of dust resistivity involves a slightly different
procedure than that described above. Usually it is not
necessary to determine the breakdown voltage of the dust
layer; hence, a fixed potential is applied across the cell,
and then the factors under investigation such as ash chemistry
and bulk density are varied.
Calculations
Resistivity can be calculated in the following way. First,
calculate the resistance of the dust layer R.
r, / -u \ V(volts)
R(ohms) = I (amps) (6)
Then calculate the resistivity p.
p (ohm-cm) - R(ohms) } (7)
The moisture content of the air in the environmental chamber
can be determined by weighing a tube filled with calcium sulfate
(Drierite) before and after passage of a measured volume of air
through it. The volume of dry air passed through the tube is
determined from the flow rate and the sampling time.
21
-------
Other laboratory techniques may be desirable to determine
certain electrical characteristics of the ash, for example,
the method being used in research on the resistivity of fly
ash at elevated temperature. The technique utilizes a self-
supporting sintered disc of fly ash, rather than a loose powder,
This technique is commonly used in the electrical evaluation
of ceramic insulators. It was selected for the study of
volume resistivity because it allows certain post-test analyt-
ical work to be done. The details of specimen preparation
and measurement technique are given by Bickelhaupt. 5
Simulated Flue Gas
Another refinement to the usual laboratory method is based on
more nearly duplicating the gaseous environment to which the
dust is exposed. Either the PTC 28 device or one of the
in-situ devices described below could be utilized in a con-
trolled environment that is used to simulate, e.g., the flue
gas conditions of the effluent gas stream from a boiler.
In the case of the in-situ devices the particulate material
may be redispersed in this controlled environment to simulate
the in-situ conditions. The principal drawback to this
technique is the strong possibility that chemically active
compounds present on the particles may modify their properties
in the time between collection and measurement.
IN-SITU MEASUREMENTS
Several decisions must be made in setting up and conducting
in-situ resistivity measurements. These decisions involve
device selection and operation, site selection, determination
of the number of samples required to characterize the dusts,
and the auxiliary data required, as well as safety precautions.
The selection of the device is dependent upon a number of
factors, including the availability of each device and one's
past experience. The operating characteristics of each device,
which are discussed below, will supply the rationale for
device selection and operation.
In-situ Probes
A number of different instruments are available for making
resistivity measurements. These instruments differ funda-
mentally in the method of sample collection, degree of com-
paction of the dust sample, and the values of the electric
field and current density utilized for the measurement, as
well as the method of maintaining thermal equilibrium and
the method of deposition in the measurement cell. These
22
-------
differences in operation lead to differences in the character-
istics of the sample and in the values obtained for the resis-
tivity. Each device is discussed individually with some dis-
cussion of the advantages and disadvantages for actual use.
Instruments utilizing electrostatic collection and measure-
ments on the undisturbed dust layer measure the resistance of
a dust layer that was formed by collecting individual particles
aligned by the electric field identical to the conditions in a
standard precipitator. This procedure leads to a compact dust
layer with good interparticle contact. Those devices that
utilize dust layers collected and redeposited will be operating
on a disturbed and recompacted layer. This difference in oper-
ation may lead to differences in contact potential between the
adjacent particles and to different porosity in the sample that
may influence the value obtained for the resistivity.
Point-to-Plane Probe
The point-to-plane probe for measuring resistivity has been
in use since the early 1940's in this country. 1 Two models
of this device are shown in Figure 8. The probe is inserted
directly into the dust-laden gas stream and allowed to come
to thermal equilibrium. The particulate sample is deposited
electrically onto the measurement cell through the electro-
static action of the corona point and plate electrode. A
high voltage is impressed across the point and plate electrode
system such that a corona is formed in the vicinity of the
point. The dust particles are charged by the ions and perhaps
by free electrons from this corona in a manner analogous to
that occurring in a precipitator.
The dust layer is formed through the interaction of the
charged particulate with the electrostatic field adjacent
to the collection plate. Thus, this device is intended to
simulate the behavior of a full-scale electrostatic precipi-
tator and to provide a realistic value for the resistivity
of the dust that would be comparable to that in the actual
device.
In the point-to-plane technique, two methods of making measure-
ments on the same sample may be used. The first is the "V-I"
method. In this method, a voltage-current curve is obtained
before the electrostatic deposition of the dust, while the
collecting disc is clean. A second voltage-current curve is
obtained after the dust layer has been collected. After the
layer has been collected and the clean and dirty voltage-
current curves obtained, the second method of making a measure-
ment may be used. In the second method, a disc the same size
23
-------
PICOAMMETER
CONNECTION
HIGH VOLTAGE
CONNECTION
STATIONARY POINT
DIAL INDICATOR
PICOAMMETER
CONNECTION
MOVABLE
SHAFT
STATIONARY
POINT
GROUNDED
RING
Figure 8. Point-to-plane resistivity probes equipped for
thickness measurement.
24
-------
as the collecting disc is lowered on the collected sample.
Increasing voltages are then applied to the dust layer and
the current obtained is recorded until the dust layer breaks
down electrically and sparkover occurs. The geometry of the
dust sample, together with the applied voltage and current,
provide sufficient information for determination of the dust
resistivity.
In the point-to-plane method, the voltage drop across the
dust layer is determined by the shift in the voltage-vs-
current characteristics along the voltage axis as shown in
Figure 9. The situation shown is for resistivity values
ranging from 109 to 101l ohm-cm.
If the parallel disc method is used, dust resistance is
determined from the voltage measured just prior to sparkover.
In both methods the resistivity is calculated as the ratio
of the electric field to the current density.
The practice of measuring the resistivity with increasing
voltage has evolved because the dust layer behaves as a
nonlinear resistor. As the applied voltage is increased,
the current increases greater than that attributable to
the increase in voltage. Therefore, as described in the
A.S.M.E. Power Test Code Number 28 procedure, the value
just prior to sparkover is reported as the resistivity.
There is considerable justification for using the value of
resistivity prior to electrical breakdown as the resistivity,
since it is precisely at electrical breakdown that the resis-
tivity causes problems within the precipitator. The electri-
cal breakdown in the dust layer in the operating precipitator
either initiates electrical sparkover or reverse ionization
(back corona) when the resistivity is the factor limiting
precipitator behavior. If neither of these events occur, the
dust layer merely represents an additional voltage drop to the
precipitator power supply.
Even though there are many similarities between the operation
of the point-to-plane device and a full-scale precipitator,
several problems also exist. The first problem encountered
is the determination of the thickness of the dust layer.
Some devices make use of a thickness measurement system built
into the probe. In other devices, the instrument is with-
drawn from the duct and the thickness of the layer is esti-
mated visually by inspecting the dust layer. However, the
dust layer is almost always disturbed by the air flow through
the sampling port and extreme care is required to preserve
the layer intact.
25
-------
3.0r
2,5
2.0
o
1.5
LU
O
LU
DC
tr
3
1.0
0.5
0
0
8 12
VOLTAGE , KV
SPARK
NO DUST
DEPOSIT ON
PLATE
VOLTAGE DROP ACROSS
DUST LAYER (Vd) FOR
DUST THICKNESS
(xd) = 0.001 METER
16
20
Figure 9. Typical voltage-current relationships for
point-to-plane resistivity probe.
26
-------
Advantages and Disadvantages T-
The advantages of utilizing the point-to-plane probe for
in~situ measurements are:
1. The particulate collection mechanism is the same
as that in an electrostatic precipitator,
2. The dust-gas and dust-electrode interfaces are
the same as those in an electrostatic precipitator.
3. The measurement electric field and current densities
are comparable to those in the precipitator.
4. Flue gas conditions are preserved.
5. The values obtained for the resistivity are in
general consistent with the electrical behavior
observed in the precipitator.
6. Measurements can often be made by two different
methods.
The disadvantages are:
1. The measurement of the dust layer thickness can
be difficult.
2. High voltages are required for collection.
3. Considerable time is required for each test.
4. Experienced personnel are required for testing.
5. A number of measurements are required for gaining
confidence in the measured value (there is con-
siderable scatter in the data).
6. Particle size of the collected dust is not repre-
sentative.
7. Sample size is small.
8. Carbon in the ash can hamper resistivity measurements.
Cyclone Resistivity Probes
The cyclone resistivity probe measures the resistivity of a
parciculate sample that is extracted from the effluent gas
stream by an inertial cyclone collector. The dust sample is
27
-------
deposited between two concentric cylindrical measurement
electrodes. The dust-laden gas sample is extracted through a
sampling nozzle by a pump into the cyclone separator where the
collected dust falls into the measurement cell. The gas flow
rate is adjusted to provide an isokinetic sample if desired.
As mentioned previously, the collection characteristics of the
cyclone are such that even though the sampling system is opera-
ting isokinetically, the dust sample collected is not identical
with that in the gas stream. Notwithstanding this, it is often
desirable to use isokinetic conditions,.
By applying a voltage across the cell and monitoring the current
flowing through the cell, the filling of the cell can be observed
by the increase in current through the cell. When the current
levels off, the cell is full and the sampling is stopped. The
current is then monitored until it stabilizes.
The resistivity of the sample is calculated from
P = KR (8)
where R is the resistance of the dust layer (ohm) and K is
a constant for any particular cell (cm). The constant K is
defined by
K = In (r2/ri) (9)
where
L = length of cell (cm)
ri = radius of inner electrode (cm)
iC2 = radius of outer electrode (cm)
The Simon-Carves cyclone resistivity instrument, as described
by Cohen and Dickinson6, is one of the more widely used cyclonic
devices. The sample collection and measurement cell is located
in a temperature-controlled chamber as shown in Figure 10, ex-
ternal to the duct, with the sample extracted through a sample
probe. The sampling line must be thermally controlled to pre-
serve the flue gas condition. The dust sample is compacted
into the measurement cell by the action of a vibrator.
A somewhat different design of this device is made to be in-
serted directly in the flue. The dust is collected and measured
while the device is retained in the flue gas environment.
The probe is operated in the following manner: It is inserted
into the flue and permitted to come to thermal equilibrium with
the flue gas. A sample is then drawn through the apparatus by
a pump, and the gas flow measured. Isokinetic sampling can be
achieved by adjusting the flow so that the inlet velocity of the
gas to the probe and the flue gas velocity are the same. A
28
-------
Exhaust from
cyclone
Thermometer
Inlet from
sampling probe
Cyclone
Heater
Resistivity cell
•Connection to
megohmmeter
Figure 10.
Resistivity apparatus using mechanical cyclone
dust collector [from Cohen and Dickinson 6]
29
-------
vibrator attached to the probe is used to keep dust from col-
lecting on the walls of the probe and to give uniform com-
paction. Figure 11 shows a schematic of this instrument.
Advantages:
1. Low voltage instrumentation may be used.
2. Dust layer thickness is fixed by cell geometry.
3. The electric field is easily duplicated from
test to test.
Disadvantages:
1. The cylindrical cell yields a non-uniform
electric field.
2. The electrical noise is unusually high.
3. It is difficult to determine when the sample
cell is full.
4. Compaction of the dust layer is not reproducible.
5. The thermal control of the external model is
difficult.
6. The values of resistivity obtained are unreal-
istically high for electrostatic precipitator
applications.
7. Particle size of dust is not representative.
8. The dust layer in the cell is not electrostatically
deposited.
Kevatron Electrostatic Precipitator Analyzer
The Kevatron resistivity device7 is designed to simulate
in-situ measurements in an external thermally-controlled cell.
The sampling probe is inserted directly into the flue gas for
extracting an isokinetic sample. The sampling line leads to
a miniature wire-pipe type of electrostatic precipitator,
where the particulate material is collected on the surface of
the pipe. The collected dust layer is removed from the pipe
and deposited in a concentric cylindrical measurement cell
by removing the electrical energization and applying an
acceleration to the pipe. A schematic drawing of the system
is shown in Figure 12.
30
-------
o>
o
"5
u
3
Ti
(U
-P
CO
G
•H
(U
X!
O
(U
c
o
rH
u
rH
rH
Q)
tn
-H
Pn
31
-------
Thermocouple
Temperature
controller
Isothermal
meter
r^r^vo^t
EHT 25 kV
DC power
supplies
• Resistivity
» chamber
Current mA
Precipitation
chamber
L_
e Plotter
Temperature
controller
DC power
supply
Electromagnetic
compactor
Figure 12. Kevatron resistivity probe [from Tassicker,
e_t al7]
32
-------
The particulate is in the flue gas environment throughout
the entire measurement period. The flue gas flows through the
sampling lines and wire-pipe precipitator and exhausts to the
atmosphere. Provisions must be made to preserve the thermal
conditions in the flue duct through the sampling line to avoid
upsetting the chemical equilibrium conditions in the flue.
Without this precaution, a temperature drop in the sampling
line may lead to an increased absorption for any naturally
occurring conditioning agents such as sulfur trioxide and
moisture in the effluent gas stream.
The instrument is designed to internally compute the resistivity
of the dust in the measurement cell, when used with the graph
paper supplied. The system projects a spot of light on the
graph grid, thus eliminating the computation of resistivity
that is required for other instruments. The measurement
is conducted with applied voltage of 3, 30 or 300 volts across
an electrode spacing of 0.2 cm for electric fields of 15, 150
or 1500 volts per centimeter, respectively.
Advantages:
1. The resistivity is internally computed, obviating
field calculation.
2. Clean electrode and dust-covered electrode voltage-
current curves can be obtained.
3. Some variation in electric field is allowable in
the measurement.
Disadvantages:
1. The equipment is very heavy and bulky, difficult
for field work.
2. Sampling lines require temperature control.
3. Mirror alignment in resistivity computation is
critical.
4. Particle size of the dust is not representative.
5. Density of dust in the cell is not reproducible.
6. Dust is not deposited in the cell electro-
statically.
7. Resistivity values can be unreasonably high.
33
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Lurgi Electrostatic Collection Resistivity Device
The Lurgi Apparatebau-Gesellschaft mbh in Frankfurt, West
Germany, developed an in-situ resistivity probe described by
Eishold, 8 consisting of two corona wire electrodes equally
spaced from an interlocking comb arrangement as shown in
Figure 13. This device is inserted either directly into the
flue duct for in-situ measurements or into a thermally and
environmentally controlled chamber for simulated in-situ
laboratory measurements.
The dust is collected on the interlocking comb structure by
electrostatic forces. The dust layer forms on the surface of
the comb structure and fills the region between the two comb
segments. After the sample is collected, a potential is
applied across the dust layer. The configuration of the cell
(the cross-sectional area and spacing between the electrodes)
is such that the resistivity of the sample is ten times the
measured resistance. This factor of ten is based on neglect
of any electrical fringing through the adjacent fly ash. The
measurements are made using an ohm-meter without specifying
the electric field at which the measurements are made.
COMPARISON OF RESISTIVITY PROBES
The resistivity probes described in Section IV differ primarily
in the manner of collection of the dust particles from the gas
stream, the manner of dust deposition in the measuring cell,
the cell geometry, and the electrical conditions during
measurement.
Reiterating from earlier in this report, because of the nature
of the collection devices, the size distributions of the
particles in the samples are not representative of the size
distribution of the dust particles in the duct. Neither the
cyclone nor the electrostatic devices are efficient collectors
of fine particles, so the particle size distribution in the
resistivity sample is biased toward the larger particles.
This condition can cause some variation in the results obtained
with different devices.
A second difference in the resistivity probes is the manner
of depositing the dust in the measuring cell. The point-plane
probes and the Lurgi probe deposit the dust electrostatically
onto the surface of the measuring cell. Consequently, some
alignment of the dust particles occurs and in general the
deposited dust layer is more dense than that in the other
types of measurement apparatus.
34
-------
Figure 13. Lurgi in-situ resistivity probe
35
-------
The effect of alignment on dust resistivity has not been
quantitatively determined. However, variations in density
can influence resistivity values by as much as 10-fold,
as reported by Cohen and Dickinson.
A third difference in the resistivity probes is the value
of the electric field at which resistivity is measured.
Standard procedures for the Kevatron and Simon-Carves probes
are to measure resistivity at relatively low electric fields.
By contrast, the procedure for the point-plane probe is to
measure the resistivity at a field near breakdown. As a
consequence, the values of resistivity as measured by the
different methods vary by as much as a decade due to electric
field differences.
The combined effect of these variables is that the resistivity
values reported by investigators using different techniques
vary widely. Upper values of resistivity measured by a point-
plane probe in the vicinity of 1012 to 10 : 3 ohm-cm have been
reported, whereas upper values of 10l ** to 1015 ohm-cm have
been reported by other techniques.
There have been no definitive studies to compare results of
resistivity measurements by the various devices. However,
limited studies have been conducted at electric power gener-
ating plants using the in-stack cyclone, Kevatron, and
point-plane probes.9 Resistivity values measured by these
probes are compared in Figures 14 and 15. Figure 14 shows
the settled-out cyclone data plotted against the point-plane
data, using the point-plane data at 2.5 kV/cm, which corre-
sponds to the field in the cyclone apparatus.
Figure 15 shows the peak values of resistivity from the
Kevatron and cyclone probes plotted against point-plane data
from the same (2.5 kV/cm) field. In this case, much better
agreement is obtained between the cyclone and point-plane
data. The Kevatron data are still higher than the average
of the cyclone or point-plane data, although there are sta-
tistically insufficient data to draw firm conclusions re-
garding the Kevatron values.
The logic of comparing the peak values of resistivity from the
cyclone with the point-plane data can be rationalized to some
extent by the fact that fresh dust is being deposited on the
surface during the precipitation process.
In view of the scatter of the data obtained with any one probe,
as noted earlier in Section III, the discrepancies shown in
Figures 14 and 15 are not unexpected.
36
-------
10
13
o
9. IO'2
CO
LU
CO ..
OT 10"
LU
cr
O
cr
LU
LU
z
O
_l
o
O
10s
10
8 _
IO1
10'
O
PERFECT CORRELATION LINE
O -CYCLONE
O —KEVATRON
10
8
10
10
10'
10
12
10'
,13
POINT-PLANE RESISTIVITY,OHM-CM
Figure 14. Comparison of Kevatron and cyclone resistivities
with point-plane resistivities at an electric
field of 2.5 kV/cm. Settled values for cyclone
peak values for Kevatron.
37
-------
o
I
s
I
o
co
UJ
t-
CO
CO
UJ
a:
O
o:
UJ
UJ
z
o
_l
o
o
10
14
10
13
I0'2
> 10' I
I09
io8-
10
10'
Figure 15
O
PERFECT
CORRELATION LINE
O
O
O -CYCLONE
O - KEVATRON
10
8
10-
10
10
10'
10
12
10
13
POINT-PLANE RESISTIVITY,OHM-CM
Comparison of Kevatron and cyclone resistivities
with point-plane resistivities at an electric
field of 2.5 kV/cm. Peak current values used
for Cyclone and Kevatron.
38
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PRACTICAL FACTORS IN RESISTIVITY MEASUREMENTS
Selection of Sampling Sites
The first priority in selection of a sampling site is the
location of a point in the operating system where the
conditions of the gas and the gas-borne dust particles are
representative of the environment for which resistivity is
being determined. That is, the gas temperature, gas composi-
tion, and particle history must be the same as that found,
for example, in the precipitator. Usually the inlet of the
precipitator is selected as the point for making resistivity
measurements. However, sampling at several points across the
duct may be required to obtain a representative measurement
where there are variations in temperature across the duct.
Variations in gas flow velocity and dust loading in the duct
must also be taken into account, since these conditions can
result in non-representative dust samples with some types of
resistivity apparatus.
When selecting a site for the measurements, practical consid-
erations must also be remembered. At the site location,
sampling ports must exist or be installed. The normal practice
is to use 4-inch pipe for the ports. Electrical power
(117-120 VAC, 60 cycle), must be available at the site location
for the operation of the measuring equipment. In many locations,
adapters will be required for mating of plant electrical out-
lets with the standard three-prong plugs found on most labo-
ratory equipment.
Determination of Number of Measurements
The determination of the number of individual measurements re-
quired to characterize the resistivity of the dust is related
to the range of operating conditions anticipated and the vari-
ability in the coal. It is desirable when designing a new
precipitator installation that the worst operating conditions
be covered in the test schedule.
The variability in plant operating conditions that is of the
greatest concern is the variation in flue gas temperature
throughout the year. The change in the ambient air temperature
from winter to summer can cause the flue gas temperature to
vary as much as 15°C (SOT) while the temperature variation
across the duct downstream from a rotating (Ljundstrom) air
heater may be 25°C (50°F). This combined temperature spread
may cause a significant variation in the dust resistivity and
care must be exercised to assure that the widest variation is
covered.
39
-------
As mentioned in Section III, the day-to-day variations in
characteristics of the coal supply may also cause significant
variations in the particulate resistivity. This variability
will show up as a considerable scatter in the measured value
of resistivity over the measurement period. When this varia-
tion occurs, it becomes imperative to make a sufficient num-
ber of measurements at each temperature to obtain a statis-
tically significant value for the resistivity at each of the
ranges of conditions encountered.
The precipitator acts to smooth out short-term variations in
particulate resistivity. Dust layers ranging from perhaps
one centimeter on the inlet plates to some lower value, per-
haps only a millimeter, on the outlet plates build up during
several hours of collection time. The average buildup rate
on the precipitator plates is on the order of one millimeter
per hour, exponentially distributed through the precipitator,
such that the dust layer on the plates may represent an aver-
aging of the instantaneous dust conditions of many hours of
operation. Therefore, there is a rationale for averaging
the measured values of resistivity for each temperature
condition to arrive at the resistivity representative of the
particular installation.
The determination of how many measurement points are required is
therefore based on the variability of the source and the exper-
ience of the technician making the measurements. Typically,
six to ten measurements at intervals of 10°C (20°F) are suffi-
cient if plant conditions are reasonably constant.
The auxiliary data required when conducting tests on an operating
precipitator include:
• coal samples for proximate and ultimate analysis
• flue gas temperature and composition (including
concentration of SO3)
• precipitator voltage-current relationships
• fly ash samples for laboratory analysis.
Safety Precautions
Extreme caution must be exercised when conducting measurements
in ducts containing flue gas. Typically, the flue gas at
temperatures exceeding 150°C (300°F) will contain a significant
quantity of sulfur oxides and fly ash particles. If the access
40
-------
port has been covered for a period of time, significant amounts
of fly ash will accumulate in the port. Some ducts will be
under a positive pressure of a few inches of water; in others,
there exists the probability of "puffing". Therefore, extreme
care must be exercised when opening ports and when inserting or
extracting probes because of this presence of particulate and
sulfur oxides in the gas.
Additional care must be exercised when utilizing resistivity
probes with high voltages. Sufficient electrical grounds must
be attached prior to handling any probe connected to an electri-
cal supply.
A shock hazard also exists when inserting or extracting any
ungrounded probe. An ungrounded probe inserted into a particu-
late-laden gas stream may become electrically charged by a
triboelectric mechanism. Therefore, probes should be grounded
prior to insertion into a flue duct.
A hazard also exists because of the location of the sampling
ports. Often, the ports were installed after the construction
of the plant at locations remote from standard walkways. All
scaffolds and walkways should be tested prior to use and all
hazards that can be reasonably detected should be corrected.
41
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SECTION V
REFERENCES
1. White, H. J. Resistivity Problems In Electrostatic
Precipitation. J. Air Pol. Contr. Ass. 24(4) ,
April 1974.
2. Bickelhaupt, R. E. Volume Resistivity - Fly Ash Compo-
sition Relationship. Environ. Science and Technology.
9_(4) :336-342, April 1975.
3. Masuda, S. The Influence of Temperature and Moisture
on the Electrical Conductivity of High-Resistivity Dusts.
Staub Reinhaltung der Luft in English (Dusseldorf).
25_(5):l-7, 1965.
4. Bickelhaupt, R. E. Surface Resistivity and the Chemical
Composition of Fly Ash. J. Air Pol. Contr. Ass. 25(2),
February 1975.
5. Bickelhaupt, R. E. Electrical Volume Conduction in Fly Ash.
J. Air Pol. Contr. Ass. 2_4(3) , March 1974.
6. Cohen, L. and R. W. Dickinson. The Measurement of the
Resistivity of Power Station Fine Dust. J. Sci. Instrum.
(London). 4:0:72-75, 1963.
7. Tassicker, O. J., Z. Herceg, and K. J. McLean. A New
Method and Apparatus to Assist the Prediction of Electro-
static Precipitator Performance. Institution of Engineers,
Australia. Electrical Engineering Transactions (Sydney).
EE5(2):277-278, September 1969.
8. Eishold, H. G. A Measuring Device for Determining the
Specific Electrical Resistance of Dust. Staub Reinhaltung
der Luft in English (Dusseldorf). 26^(1) : 14-18, January
1966.
9. Nichols, G. B. and J. P. Gooch. .An Electrostatic Precipi-
tator Performance Model. Report to Environmental Protection
Agency on Contract No. CPA 70-166 by Southern Research
Institute, Birmingham, Alabama. July 1972. 171 p.
42
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TECHNICAL REPORT DATA
(Please read Inttmctions on the reverse before complctuif;}
1. REPORT NO.
EPA-650/2 -74-079
3. RECIPIENT'S ACCESSIor*NO.
4. TITLE AND SUBTITLE
Techniques for Measuring Fly Ash Resistivity
5. REPORT DATE
August 1974
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Grady B. Nichols
8. PERFORMING ORGANIZATION REPORT NO
SORI-EAS-75-366
3134-XIV
9. PERFORMING ORdANIZATION NAME ANO 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 AGENCY NAME ANO ADDRESS
EPA, Office of Research and Development
NERC-RTP, Control Systems Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The report summarizes significant factors related to the measurement of electrical
resistivity of the particulate matter suspended in a gas stream. It describes some of
the mechanisms of electrical conduction in fly ash from coal combustion as it
influences resistivity and its measurement. The report also reviews techniques for
measuring resistivity and the problems associated with each. It presents some data
comparing the values of resistivity obtained by different techniques.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Measurement
Electrical
Resistivity
Fly Ash
Coal
Combustion
Air Pollution Control
Stationary Sources
Particulates
Gas Stream
13B
14B
20C
21B
21D
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report/
Unclassified
21. NO. OF PAGES
49
20. SECURITY CLASS (This page I
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
43
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