United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
1
Research and Development
EPA/600/S7-86/010 June 1986
v>EPA Project Summary
Fly Ash Resistivity Prediction
Improvement with Emphasis
on Sulfur Trioxide
Roy E. Bickelhaupt
Research has been conducted to im-
prove and extend the capabilities of a
technique for predicting fly ash resistiv-
ity from the ultimate coal analysis and
the coal ash composition. Emphasis
was placed on determining the quanti-
tative effect of adsorbed suffuric acid
(H2SO4) vapor on resistivity. Ten fly ash
samples were used in order to have a
reasonable spectrum of ash composi-
tion. Resistivity was determined as a
function of temperature in air environ-
ments containing 5% and 10% water.
Isothermal resistivity was measured for
each ash at three temperature levels
using three concentrations of sulfur tri-
oxide (SO3). With respect to resistivity
predictions for conditions not including
SO3, the present data did not suggest
any modification of Model I of the resis-
tivity prediction. However, the much
larger and improved data base recently
acquired with respect to environments
containing SO3 has required a new ap-
proach for predicting this effect. The re-
port presents these data and illustrates
Model II of the resistivity prediction.
This Project Summary was devel-
oped by EPA's Air and Energy Engineer-
ing Research Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Several years ago a model was devel-
oped to predict fly ash resistivity as a
function of temperature and' the con-
centrations of water and S03 in the flue
gas. Input data for the predictions of re-
sistivity are the as-received ultimate
coal analysis and the chemical compo-
sition of the coal ash; i.e., ash produced
by laboratory ignition. Alternate input
data that can be arbitrarily selected are
water and S03 concentrations and fly
ash compositions.
From the beginning, the model for
predicting resistivity has had two fea-
tures that require additional attention.
One, which is not experimentally ad-
dressed in this report, is the technique
that is used to estimate the amount of
H2S04 vapor that will be present in the
flue gas at the inlet to a precipitator
under given conditions. A method is
needed to objectively establish a multi-
plier to convert the stoichiometrically
calculated SO2 value to a SO3 concen-
tration for specific circumstances. The
second feature requiring further effort is
the set of equations used to calculate
the quantitative effect of H2SO4 vapor
on fly ash resistivity. For the original
predictive model, these equations were
based on a small quantity of laboratory
data. Since the predicted resistivity is
very sensitive to the characteristics of
these equations, and since the charac-
teristics of the equations are extremely
sensitive to ash composition, tempera-
ture, and SO3 concentration, additional
research was undertaken to establish an
improved data base for this feature of
the model. These results and a revised
model are discussed in this report.
Experimental Procedure
The fly ashes used were characterized
with respect to chemical composition,
water soluble sulfate, loss on ignition,
Bahco particle size classification, and
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helium pycnometer density. Resistivity
(as a function of temperature, in an air
environment containing nominally 5
and 10 percent water, and using an elec-
tric field intensity of 4 kV/cm) was deter-
mined prior to the experiments with
H2S04 vapor. These tests were done in
accordance with IEEE Standard 548-
1981, descending temperature tech-
nique.
The equipment used for the isother-
mal resistivity tests in environments
containing acid vapor is described in re-
port EPA-600/7-78-035. An ash layer be-
tween two concentric electrodes is equi-
librated with the test environment in an
environmentally controlled chamber
within a thermally controlled oven.
The ash sample was thermally equili-
brated in dry air overnight at the highest
temperature of interest for the experi-
ment being conducted. The environ-
ment was converted from dry to moist
air. When the monitored current result-
ing from a periodically applied 2 kV volt-
age no longer increased, the injection of
SO3 into the system was started. Resis-
tivity was periodically determined until
the 1 mm thick ash layer under test had
equilibrated with the environment. This
equilibration was arbitrarily defined as
a resistivity decrease of <30 percent in
24 hours. After the resistivity data for
the highest temperature had been
recorded and without breaking the con-
tinuity of the test, the equilibration was
repeated at successively lower temper-
atures. At the lowest temperature in the
series and after equilibration had been
achieved, resistivity was determined as
a function of field strength intensity.
The concentration of water and H2SO4
vapor in the environmental exhaust was
evaluated at least daily, and average
values were reported. More than 100
data points were acquired, relating re-
sistivity to temperature for environ-
ments containing SO3.
Discussion
A computer program, described in re-
port EPA-600/7-79-204 and designated
Model I, was developed to predict fly
ash resistivity as a function of tempera-
ture and the important flue gas con-
stituents, water and S03. The method
used the as-received ultimate coal anal-
ysis and the coal ash chemical composi-
tion to make the calculations. About 40
fly ash samples were used to obtain the
experimental data necessary to produce
the correlations required for the predic-
tive technique. It was believed that the
use of a large number of fly ash samples
produced from all ranks of coal in sev-
eral types of commercial furnaces ade-
quately defined the influence of ash
composition on resistivity while mini-
mizing the role of ash particle size distri-
bution and ash layer porosity with re-
spect to resistivity. The effect on
resistivity of environmental water con-
centration and electric field intensity
was extensively examined. Due to limi-
tations related to funding and time, min-
imum data were obtained to quantify
the effect of environmental H2S04 vapor
on resistivity. Consequently, it was rec-
ommended that additional research be
conducted to enlarge and improve the
data base correlating resistivity with
H2SO4 vapor. Also, it was pointed out
that improvement was needed in the
method of estimating the amount of
SO3 (H2S04 vapor) that will be measure-
able at the inlet to a cold-side precipita-
te r from the stoichiometrically calcu-
lated S02 concentration.
Between the publication of Model I
and the research reported here, labora-
tory data relevant to the resistivity pre-
diction model were periodically ob-
tained. Usually these data simply
verified previous observations. How-
ever, tests were conducted using fly
ashes having high concentrations of
calcium and magnesium that showed
extra sensitivity to environmental water
concentration with respect to resistivity.
This deviation from the previous resis-
tivity/water vapor correlation used in
the Model I resistivity prediction was in-
corporated in the computer program,
and the program was designated Model
IA.
The principal objective of the experi-
mental work reported here was to ex-
pand the data base upon which the pre-
dicted effect of H2SO4 vapor on fly ash
resistivity is based. Experiments were
conducted to evaluate the relationships
between resistivity, fly ash composi-
tion, temperature, and S03 concentra-
tion in an air and water vapor environ-
ment. To keep the test matrix within the
limits of the research program, 10 fly
ashes were selected, representing the
spectrum of ash composition. An at-
tempt was made to have available
ashes with: 1) low concentrations of
sodium and calcium, with iron concen-
trations varying from low to medium to
high; 2) low concentrations of sodium
and iron, with calcium concentrations
varying from low to medium to high;
and 3) low iron concentration, moderate
sodium concentration, and calcium con-
centrations varying from low to high.
Between the publication of Model I
and the research reported here, an IEEE
standard was issued for resistivity test-
ing in air environments containing
water vapor. Also in this time frame,
new test equipment was installed in the
laboratory. To ensure that the new
equipment and procedures would yield
results identical to those produced pre-
viously, the resistivities of the 10 ashes
described above were evaluated as a
function of temperature in test environ-
ments consisting of air containing 5 and
10 percent water vapor using the de-
scending temperature technique of the
IEEE standard.
The resistivity data acquired with air
environments containing either 5 or 10
percent water vapor were reviewed
with respect to:
• The slope of the high temperature
segment of the resistivity/tempera-
ture curve.
• The difference between measured
and predicted (Model I/Model IA)
data at 150 and 350°C.
• The effect of water concentration
on resistivity as a function of tem-
perature.
A review of these points concluded that
there was no justification for modifying
Model I/Model IA with respect to the
prediction of resistivity for environ-
ments containing no SO3 .
Each of the 10 ashes was evaluated
using a minimum of three S03 concen-
trations at each of three temperatures in
an air environment containing 10 per-
cent water vapor. In a given test series
using a constant S03 concentration, re-
sistivity was determined at three suc-
cessively lower temperatures. About
100 data points were generated in this
fashion, and these data were reviewed
with respect to:
• Resistivity attenuation under a
fixed set of conditions: 144°C,
E = 4kV/cm, an air environment
containing 10 percent water and 4
ppm of SO3.
• Resistivity as a function of S03 con-
centration under the above condi-
tions.
• Resistivity as a function of temper-
ature for three concentrations of
S03.
• Resistivity as a function of electric
field intensity for three levels of
SO3 concentration and several tem-
peratures.
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These data made it apparent that fly
ash composition plays a major role in
the relationship between resistivity and
adsorbed H2SO4 vapor. The resistivities
of ashes that contain moderate
amounts of either iron (>1.0 atomic per-
cent) or magnesium plus calcium (>5.0
atomic percent) are readily affected by
H2S04 vapor. Those ashes that contain
lesser amounts of both elemental
groups are more difficult to condition. It
was also observed that resistivity
values determined under the influence
of adsorbed H2S04 is more sensitive to
electric field intensity than the resistiv-
ity values determined in environments
containing no S03.
With respect to the prediction of resis-
tivity as influenced by H2S04 vapor,
Model II (based on the experimental
data described here) takes a different
approach than Model I/Model IA. Each
step or correlation used to calculate re-
sistivity (except electric field intensity)
is tempered or influenced by the ash
composition. It is believed that the resis-
tivity prediction method, Model II, is a
significant improvement over the previ-
ous models. Use of Model II has pro-
vided results that are in good agree-
ment with additional laboratory
resistivity data, precipitator electrical
characteristics, and in situ resistivity
data.
In addition to the new equations used
for predicting resistivity, the technique
for estimating the concentration of S03
to be expected in the flue gas entering
the precipitator based on a given coal
analysis has been altered for Model II.
An effort has been made to take into
account the amount and composition of
the fly ash. First, an expression has
been inserted to attenuate the esti-
mated amount of SO3 if the amount of
ash in the coal exceeds 10 percent. Sec-
ond, the estimated amount of SO3 is de-
termined as a percentage of the S02
value. The S02 is calculated from the
stoichiometric combustion of the as-
received coal analysis using a specific
amount of excess air. In the case of
Models I and IA, a single multiplier was
used to convert calculated SO2 values to
S03 for all coals. A recent review of field
test data suggests that two multipliers
should be used. Consequently, Model II
selects the desired multiplier based on
fly ash composition. The uncertainty re-
garding this estimate of the SO3 con-
centration is clearly the weakest link in
the prediction of resistivity based on
coal analysis.
The user should be alerted to two
other aspects of Model II. First, the char-
acteristics and amount of laboratory
data available for the development of
Model II require that resistivity can be
predicted on a step-function basis with
respect to fly ash composition. Conse-
quently, it is possible that specific fly
ashes differing only slightly in composi-
tion can have substantially different
predicted resistivity values for a specific
combination of temperature and acid
concentration. Second, there has been
little experience using Model II with
coals having large concentrations of
ash and/or water; e.g., 50 percent. For a
given sulfur content, fuels of this type
should produce disproportionately
large amounts of S03. It is not known
whether this excess S03 will appear in
the precipitator flue gas.
Extensive use of Model II, as it applies
to S03, will define the merit of the pro-
cedure. It is not a replacement for either
laboratory or in situ measurements of
resistivity. In fact it is highly recom-
mended that all three sources of resis-
tivity data should be considered for
comparisons with the precipitator elec-
trical characteristics. Possible uses of
Model II include:
• Anticipating the effect of coal
cleaning on fly ash resistivity.
• Substantiating either laboratory or
in situ resistivity data.
• Selection of coals or coal blends for
specific situations.
• Developing a topographic map of a
coal field with respect to ash resis-
tivity.
• Modeling precipitators for design
fuel composition.
• Troubleshooting poorly perform-
ing precipitators.
• Determining the applicability of
flue gas conditioning with respect
to the amount of S03 required for a
given temperature and ash compo-
sition.
• Determining the required sodium
addition for sodium-conditioning a
given coal.
Recommendations
An improved computer program is
available to calculate fly ash resistivity
as a function of temperature for flue gas
compositions containing SO3. Addi-
tional research would be of value: 1) to
determine the H2SO4 vapor conduction
mechanism, 2) to estimate the concen-
tration of H2S04 vapor in flue gas with
greater confidence, and 3) to under-
stand trie variation in the effectiveness
of H2SO4 conditioning with different fly
ash compositions.
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R. E. Bickelhaupt is with Southern Research Institute. P. 0. Box 55305,
Birmingham, AL 35255-5305.
Leslie E. Sparks is the EPA Project Officer (see below).
The complete report, entitled "Fly Ash Resistivity Prediction Improvement with
Emphasis on Sulfur Trioxide." (Order No. PB 86-178 126/AS; Cost: $11.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use 5300
EPA/600/S7-86/010
0000329 PS
U S> ENVIR PROTECTION AGENCY
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