EPA-650/2-74-092
OCTOBER 1974
Environmental Protection Technology Series
.v.v.v
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EPA-650/2-74-092
SODIUM CONDITIONING
TO REDUCE FLY ASH RESISTIVITY
by
R. E. Bickelhaupt
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
Contract No. 68-02-0284
ROAPNo. 21ADJ-029
Program Element No. 1AB012
EPA Project Officer: L. 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
October 1974
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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.
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ABSTRACT
The report gives results of a review of recent research,
including the results of two field tests of sodium
conditioning, on the effects of sodium content on the
electrical resistivity of coal fly ash. It presents a
procedure for calculating the amount of sodium that
must be added to reduce fly ash electrical resistivity
to a desired value. It discusses advantages and
disadvantages of sodium conditioning for reducing fly
ash resistivity.
This report was submitted in partial fulfillment of
Contract No. 68-02-0284 by Southern Research Institute
under the sponsorship of the Environmental Protection
Agency. Work was completed as of July 1974.
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iv
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TABLE OF CONTENTS
Page
Abstract iii
Sections
I Introduction 1
II Background 2
III An Approach to Sodium Conditioning 5
IV Advantages and Disadvantages 7
V References 9
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SECTION I
INTRODUCTION
The resistivity of fly ash is often too high under a
particular set of operating conditions to permit the
most efficient use of an electrostatic precipitator.
A variety of substances, generally referred to as
conditioning agents, can be added to the boiler or to
the effluent gas downstream to attenuate the high
resistivity/
Research has shown that sodium is the principal charge
carrier in the electrical conduction process through an
ash layer at temperatures above 250°C. This observa-
tion suggests that sodium might be utilized as a
conditioning agent. This report briefly reviews this
point in a pragmatic manner.
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SECTION II
BACKGROUND
For a given ash layer porosity and temperature, it has
been observed that the resistivity is inversely propor-
tional to the amount of sodium and lithium present in
the ash.l A log-log plot of resistivity versus the
atomic concentration of lithium plus sodium shows
approximately a two order of magnitude decrease in
resistivity for a one order of magnitude increase in
lithium-sodium concentration. This relationship is
shown in Figure 1 for a specific temperature and ash
layer porosity.
Transference experiments gave direct evidence that these
two elements were the principal charge carriers in the
volume conduction process. This information combined
with ash characterization studies that indicated the
major constituent in fly ash is amorphous lead to the
conclusion that conduction in fly ash is analogous to
that of silicate glass. Additional research2 has verified
the above results and expanded the knowledge pertaining
to the compositional factors that influence the electrical
resistivity of fly ash.
The analysis of very recent data3 has shown that surface
resistivity also is governed at least in part by the
aforementioned chemical effects. This was the expected
observation. If fly ash possesses a conduction mechanism
analogous to silicate glass, the surface resistivity should
be sensitive to the alkali metal content.
Two field tests were conducted in which sodium conditioning
was attempted. Operational temperatures were between
140°C and 175°C, the range in which surface conduction
should dominate. Both were technically successful in
that the resistivity was reduced the predicted amount by
the addition of sodium. However, neither of the two
power stations are using sodium conditioning. One
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concluded it was not an economical approach for them
and are continuing their search for a conditioning
agent. The other company elected to build a hot side
precipitator in an effort to be independent of
conditioning agents. A portion of these field tests
conducted by Southern Research Institute and Calgary
Power Limited of Calgary, Alberta, Canada were
recently reviewed. "*
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SECTION III
AN APPROACH TO SODIUM CONDITIONING
AMOUNT OF Na20 ADDITION
The combined atomic percentage of lithium and sodium was
used to establish a correlation between resistivity and
these elements for a great number of ashes. In applying
this information for the attenuation of resistivity for
a given ash, the weight percent of sodium oxide can be
used satisfactorily. Since there always is some disparity
among resistivity data and since a wide range of operational
temperatures are employed, the correlation between resis-
tivity and sodium oxide concentration is shown below
algebraically rather than graphically.
PI/P2 = (W2)2/(W1)2 (1)
pi = measured resistivity in ohm-cm
P2 = desired resistivity in ohm-cm
wi = measured weight percent Na20 in ash
wz = required weight percent Na20 in ash
Example: Assume a measured resistivity of 1x10lz ohm-cm
at a given set of operating conditions for an ash containing
0.30 weight percent Na20. Assume it is desired to reduce
the resistivity to 5x1010 ohm-cm under the same operating
conditions.
Then:
(w2)2 = m x (0.30)2 and w2 = 1.34 weight percent
2 _ 1x1012
If the value of w2 reaches 3.0 weight percent, there would
be reason to believe that the measured values for resistivity
and/or Na20 are in error.
This example calculation indicates that the sodium oxide
concentration in the ash should be increased by 1.04 weight
percent. If the hypothetical plant is producing 10,000
pounds of ash per hour and selects Na2CO3 (58.5% Na2O) as
a source of Na20, the sodium carbonate would be required at
a rate of 178 pounds per hour.
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SOURCE OF Na20 ADDITION
Since it is believed that for sodium to function properly
it must be an integral part of the fly ash, the substance
used to supply the NaaO must be one that decomposes,
vaporizes, or melts at boiler temperatures. The source
material should not nocuously affect equipment or the
environment. Beyond these two points, the selection of
a material as a source of NazO becomes an economic compro-
mise searching for the lowest cost per pound of sodium
oxide added. Factors that must be considered include:
availability of low grade-low cost materials, transporta-
tion, percentage of NaaO supplied by the substance, etc.
Potential compounds that can be considered as sources of
NaaO include: NazCOa, Na2SOi», NaOH, NaNOs, etc. Because
of the deleterious effects due to chlorine, NaCl cannot
be used; however, one might consider salt as a raw
material used to produce a suitable additive. With respect
to cost, the plant operator may wish to consult with
chemical houses to determine if a usable substance might
be obtained in an impure state at some stage of production.
METHOD AND LOCATION OF ADDITION
To insure that the sodium becomes an integral part of all
the ash, the source of sodium should be uniformly dis-
tributed throughout all of the coal feed. If the source
material is water soluble, the addition can be made either
dry or as a water spray. Wet or dry feeders having
suitable capacity are commercially available.
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SECTION IV
ADVANTAGES AND DISADVANTAGES
The advantages can be summarized as: small capital invest-
ment, low labor cost for operation and maintenance, no
obvious safety or materials handling problems, and the
amount of source material fed can be easily adjusted to
compensate for the changes in sodium level of incoming
coal.
The disadvantages are source material cost and potential
boiler fouling. Whether or not material cost inhibits
the use of sodium conditioning depends on several factors
including: the amount required, the availability of low-
cost (impure or partially processed) forms of source
materials, and the distance the material must be trans-
ported .
Ash fouling particularly related to lignites has been
reported5 and related to sodium concentration. This is
generally associated with ashes containing either a very
high sodium concentration or a moderate sodium concentration
and low fusion temperature. If sodium conditioning is
utilized without discretion, fouling could be a problem.
The following comments are directed toward this potential
problem.
1) From the results of the examination of twenty-
five ashes produced from sub-bituminous,
bituminous and anthracite coals from the
Eastern and Western parts of the U.S.A.,
the ash having the highest resistivity
should require an addition of approximately
2.5% NaaO to make the resistivity acceptable.
2) Coals yielding ashes inherently containing
2 to 3 percent NaaO are used without fouling
problems.
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3) Fouling problems can also be related to the
softening and fusion characteristics of the
ash. While sodium can depress the fusion
temperature of an ash, this characteristic
is also dependent upon the amount and ratios
of other constituents. Some of these con-
stituents also lower resistivity, reference
2. It would seem unlikely that an ash
requiring an attenuation of resistivity
would also be subject to fusion problems.
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SECTION V
REFERENCES
1. Bickelhaupt, Roy E. Electrical Volume Conduction in
Fly Ash. J. Air Pol. Con. Assoc. 24(3):251-255, 1974.
2. Bickelhaupt, Roy E. Influence of Fly Ash Compositional
Factors on Electrical Volume Resistivity. Environmental
Protection Agency Technology Series, EPA-650/2-74-074,
July 1974.
3. Bickelhaupt, Roy E. Surface Resistivity and the Chemical
Composition of Fly Ash. Paper presented at Symposium on
Electrostatic Precipitators for Control of Fine Particles.
EPA Contract No. 68-02-1303, Pensacola Beach, Florida.
September 30 - October 2, 1974.
4. White, Harry J. Resistivity Problems in Electrostatic
Precipitation. J. Air Pol. Con. Assoc. 24(4):314-338,
1974.
5. Gronhovd, G. H., A. E. Harak, and P. H. Tufte. Ash
Fouling and Air Pollution Studies Using a Pilot Plant
Test Furnace. Paper presented at the 1969 Lignite
Symposium, Grand Forks, North Dakota. May 1-2, 1969.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-650/2-74-092
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Sodium Conditioning to Reduce Fly Ash Resistivity
. REPORT DATE
October 1974
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
R. E. Bickelhaupt
B. PERFORMING ORGANIZATION REPORT NO.
SORI-EAS-74-320
2938-X
9. PERFORMING ORG -\NIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
1AB012; ROAP 2LADJ-029
11. CONTRACT/GRANT NO.
68-02-0284
12. SPONSORING AGENCY NAME AND 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: Through July 1974
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The report gives results of a review of recent research, including the results of two
field tests of sodium conditioning, on the effects of sodium content on the electrical
resistivity of coal fly ash. It presents a procedure for calculating the amount of
sodium that must be added to reduce fly ash electrical resistivity to a desired value.
It discusses advantages and disadvantages of sodium conditioning for reducing fly ash
resistivity.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Sodium
Treatment
Electrical Resistivity
Coal
Fly Ash
Air Pollution Control
Stationary Sources
13B
07B
20C
08G
21B
18. DISTRIBUTION STATEMENT
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
15
Unlimited
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
10
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