United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-917009 Jan. 1992
Project Summary
Measurement and
Prediction of the Resistivity of
Ash/Sorbent Mixtures
Produced by Sulfur Oxide
Control Processes
Ronald P. Young
The report gives results of research
into the measurement and prediction of
the resistivity of ash/sorbent mixtures
resulting from processes intended to
remove sulfur oxides from the flue gas
produced when burning medium- to
high-sulfur coal. It was found that the
usual method for measuring the resis-
tivity of coal fly ash samples must be
modified to obtain accurate and repeat-
able values for the resistivity of these
mixtures. The high resistivities of the
mixtures appear to be due to the quite
high resistivities of the sorbent com-
pounds they contain. This research in-
dicated that, if analytical difficulties
can be overcome, it should be possible
to predict the resistivity of an ash/sor-
bent mixture based on a knowledge of
the amounts of calcium compounds
contained in the dust.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Furnace and cold-side sorbent injec-
tion processes under development for con-
trolling sulfur oxide (SOX) emissions at
coal-fired power plants result in a dust
which consists both of coal fly ash and
partially spent sorbent material. This means
that the dusts produced with these tech-
nologies will be physically different from
ordinary fly ash. The presence of the sor-
bent material also causes the resistivity of
the mixture to be much higher than that of
the fly ash alone. Since most sites being
considered for retrofit installation of these
control processes have small electrostatic
precipitators, such high resistivity dusts
will probably be quite difficult to collect.
Resistivity Measurements
The usual procedure for measuring the
resistivity of coal fly ash yields inconsistent
resistivity values and an overall lack of
repeatability when applied to ash/sorbent
mixtures. This was found to be due to
decomposition of calcium hydroxide at tem-
peratures above 300°C. But by limiting the
maximum temperature to 250°C and mak-
ing the measurements using descending
temperatures, consistent and repeatable
resistivity values were obtained.
Modeling Resistivity
The computer model for predicting the
resistivity of fly ash developed by EPA and
Southern Research Institute did not work
for these mixtures. This was attributed to
the physical difference of ash/sorbent mix-
tures from fly ash. In particular, the existing
computer model predicts resistivity prima-
rily on the basis of the amount of alkali
metal ions (primarily those of sodium and
lithium) in the fly ash. But charge transfer-
ence tests did not identify any migrating
species in the ash/sorbent mixtures, indi-
cating that the alkali metal ions do not play
a primary role in charge conduction in
these mixtures.
Printed on Recycled Paper
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Examining the resistivities of reagent
grade calcium compounds thought to be
present in the mixtures showed that these
resistivities were quite high. This indicated
that the resistivity of an ash/sorbent dust
might be determined by the resistivity of
one or more calcium compounds contained
in the dust. Multiple regression analyses
found that the resistivity of a mixture was
strongly correlated with the resistivity of
one or more sorbent compounds. Attempts
to correlate the resistivity of these complex
mixtures with the amount of sorbent com-
pounds determined by chemical analysis
of the sample were generally unsuccess-
ful. However, similar attempts with less
complex samples consisting only of sor-
bent exposed to SOX were much more
successful. This indicates that the proce-
dure used to determine the amounts of
calcium compounds within the ash/sorbent
mixtures may not have been adequate.
Therefore, the research described here
indicates that the approach used to predict
the resistivity of ash/sorbent samples
should work if an accurate method of de-
termining the chemical composition of the
mixtures can developed.
Ronald'PrYoungiswfth Southern Research-Institute, Birmingham^Al~35255-5305. —
Louis S. Hovls is the EPA Project Officer, (see below).
Tha complete report, entitled "Measurement and Prediction of the Resistivity of Ash/
Sorbent Mixtures Produced by Sulfur Oxide Control Processes," (Order No. PB92-
126812/AS; Cost: $19.00, 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
BULK RATE
POSTAGE & FEES PAID
EPA PERMIT NO. G-35
Official Business
Penalty for Private Use $300
EPA/600/S7-91/009
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