PB 212 607
A STUDY OF RESISTIVITY AND CONDITIONING OF
FLY ASH
Southern Research Institute
Birmingham, Alabama
February 1972
DISTRIBUTED BY:
KTDi
National Technical Information Service
U. S. DEPARTMENT OF COMMERCE
5285 Port Royal Road, Springfield Va. 22151
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A STUDY OF RESISTIVITY AND CONDITIONING OF FLY ASH
Final Report To
ENVIRONMENTAL PROTECTION AGENCY
Office of Air Programs
Contract CPA 70-149
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BIBLIOGRAPHIC DATA 1- Report No. ' 2. •
SHEET . EPA-R2-72-087 . *
4. Tit e and Subtitle
A Study of Resistivity and Conditioning of Fly Ash
7. Author(s)
9. Performing Organization Name and Address
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
12. Sponsoring Organization Name and Address
ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Monitoring
Research Triangle Park, North Carolina 27711
3. Recipient's Accession No.
5. Report Date
February 1972
6.
8. Performing Organization Rept.
No.
10. Projcct/Task/Work Unit
IK Contract /Grant No.
CPA 70-149
No.
13. Type of Report & Period
Covered
Final
14.
15. Supplementary Notes
16. Abstracts An experimental study was made of the injection of S03 or H2S04 gaseous condi-
tioning agent to alleviate the problem of high electrical resistivity of fly ash. This
>roblem interferes with the electrostatic precipitation of fly ash in power stations
>urning low-sulfur coals and thus providing very low concentrations of H2S04 as a natur-
ally occurring conditioning agent. Experimental measurements included in the study con-
sisted of determinations of fly-ash resistivity in situ and analyses of coal, flue gases,
md fly ash with injected S03 and H2S04 concentrations up to 50 ppm. For comparison with
conditioning by injected S03 and H2S04, a parallel study was made of conditioning by
12S04 as produced naturally during the burning of coal. To compare the economic aspec'ts
af different processes of S03 and H2S04 injection, an analysis was made of the various
capital and operating costs in existing injection facilities. Further experimental study
was made of the use of NH3 and H20 conditioning agents. NH3 was studied as a means of
oping with poor precipitator efficiency in a power station burning a high-sulfur coal
and thus producing an undesirably high concentration of H2S04. The effect of H20 injec-
tion was observed in one plant with the concentration increased to 14% from the normal
value, of 7%.
17. Key Words and Document Analysis. 17o. Descriptors
Air pollution
Electrical resistance
Fly ash
Flue gases
Coal
Sulfur compounds
Electric power plants
Injection
Gas injection
Sulfur trioxide
17b. Identifiers/Open-Ended Terms
Resistivity
Air pollution control
17c. COSATI Field/Group
Sulfuric acid
Ammonia
Water injection
Conditioning (treating)
Thermodynamics
18. Availability -Statement
Unlimited
19. Si-i-urity Class (This
20. Security ( l.iss ('Ihi.-
I'a.tc
t'N( I.ASSIKIFn
21. -\0. ,>l I'.l.KC:
15£
22.
N V tS-.iS I R tl V .
THIS FORM MAN MK
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A STUDY OF RESISTIVITY AND CONDITIONING OF FLY ASH
Final Report to
ENVIRONMENTAL PROTECTION AGENCY
Office of Air Programs
Contract CPA 70-149
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
February, 1972
A839-2504-XIII
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ABSTRACT
An experimental study was made of the injection of SO 3 or
gaseous conditioning agent to alleviate the problem of high
electrical resistivity of fly ash. This problem interferes with
the electrostatic precipitation of fly ash in power stations
burning low-sulfur coals and thus providing very low concentra-
tions of HaSCK as a naturally occurring conditioning agent.
Experimental measurements included in the study consisted of
determinations of fly-ash resistivity in, situ and analyses of
coal, flue gases, and fly ash with injected SOs and HaSOi* concen-
trations up to 50 ppm. For comparison with conditioning by
injected SOs or HaSCK, a parallel study was made of conditioning
by HaSOit as produced naturally during the burning of coal. To
compare the economic aspects of different processes of SO 3 and
HaSOit injection, an analysis was made of the various capital and
operating costs in existing injection facilities.
Further experimental study was made of the use of NHs and
HaO conditioning agents. Usually, NHs is used at low concentra-
tions comparable to those of SOs or HaSOi* for the same purpose —
that is, to lower electrical resistivity to a desirable range.
However, during this investigation,., NH 3 was studied as a means of
coping with poor precipitator efficiency in a power station burn-
ing a high-sulfur coal and thus producing an undesirably high
concentration of HaSOi*.- At customary precipitator temperatures,
the HzO vapor produced duringxthe combustion of coal normally
acts in some degree as a naturally occurring conditioning agent,
usually at concentrations between 5 and 10% by volume in the flue
gases. As part of this investigation, ' the effect of HzO injec-
tion was observed in one plant with the "concentration increased
to 14% from the normal value of 7%.
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TABLE OF CONTENTS
Page
SUMMARY xiii
I. INTRODUCTION 1
A. Technical Background 1
B. Scope of This Investigation 4
II. EXPERIMENTAL FACILITIES, APPARATUS, AND PROCEDURES 6
A. Power-Plant Facilities 6
B. Electrical-Resistivity Determinations 6
C. Coal Analyses 14
D. Fly-Ash Analyses 14
E. Flue-Gas Analyses 20
III. STUDIES OF POWER PLANTS EQUIPPED WITH FACILITIES FOR
THE INJECTION OF ANHYDROUS S03 VAPOR 29
A. Cherokee Station, Unit 2 29
B. X Station, Unit 4 37
IV. STUDIES OF POWER PLANTS EQUIPPED WITH FACILITIES FOR
THE INJECTION OF VAPORS FROM LIQUID H2SOi» 39
A. Cherokee Station, Unit 3 39
B. Arapahoe Station, Unit 4 47
C. Y Station, Unit 6 53
V. STUDY OF A POWER PLANT EQUIPPED WITH FACILITIES FOR
INJECTION OF ANHYDROUS NH3 VAPOR 60
A. Description of the Widows Creek Station, Unit 7... 60
B. Analyses of Coal Samples 61
C. Investigation of Conditioning in the Absence of
NHs Conditioning Agent 61
D. Investigation of Conditioning with Injected NHs... 64
VI. STUDIES OF POWER PLANTS WITHOUT PERMANENT FACILITIES
FOR INJECTION OF CONDITIONING AGENTS 72
A. Kingston Station, Unit 5 72
B. Gallatin Station, Unit 4 79
C. Bull Run Station 81
D. Z Station, Unit 1 82
E. Shawnee Station, Unit 10 84
Preceding page blank
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TABLE OF CONTENTS (Continued)
Page
VII. DISCUSSION OF THE RESULTS OF THE CONDITIONING STUDIES. 97
A. Interpretation of Resistivity Data 97
B. Discussion of Conditioning by Injected SOs or
H2SO^ 98
C. Discussion of Conditioning by Naturally Produced
S03 112
D. Discussion of Conditioning by Injected HaO or NH3. 117
VIII. DISCUSSION OF BASIC THERMODYNAMIC, ENGINEERING, AND
ECONOMIC ASPECTS OF CONDITIONING WITH SO3 OR H2SO* 119
A. Optional Processes for Generating Vapors of the
Conditioning Agents 119
B. Vaporization of Stabilized SOs 119
C. Vaporization of H2SO.» 121
D. Economic Aspects of S03 and H2SOii Conditioning.... 124
IX. CONCLUSIONS AND RECOMMENDATIONS 133
X. ACKNOWLEDGMENTS 134
REFERENCES 135
VI
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TABLE OF CONTENTS (Continued)
TABLE Page
1. Power-Plant Stations Investigated in the Comparison of
Conditioning by Injected and Naturally Produced SOs or
H2S04 ................................................. 7
2. Comparison of Sampled and Observed SOs Concentrations
by the Condensation Method ............................ 24
3. Comparison of Sampled and Observed SOs Concentrations
by the Absorption Method .............................. 25
4 . Sulfur and Ash Contents of Coal Burned at Cherokee
Unit 2 ................................................ 30
5. Electrical Resistivity of Fly Ash at Cherokee Unit 2.. 32
6. Chemical Properties of Fly-Ash Samples Collected at
Cherokee Unit 2 .................. . .................... 34
7. Concentrations of Flue Gases at Cherokee Unit 2 ....... 36
8. Effects of SOs Conditioning Agent on the Resistivity
and Sulfate Content of Fly Ash at X Station ........... 38
9. Sulfur and Ash Contents of Coal Burned at Cherokee
Unit 3 ................................................ 40
10. Components of the Ash in the Coal Burned at Cherokee
Unit 3 ................................................ 41
11. Electrical Resistivity of Fly Ash at Cherokee Unit 3.. 42
12. Chemical Properties of Fly-Ash Samples Collected from
Ducts at Cherokee Unit 3 .............. ................ 43
13. Concentrations of Flue Gases at Cherokee Unit 3 ....... 45
14. Chemical Properties of Fly-Ash Samples from Hoppers at
Cherokee Unit 3 ....................................... 46
15. Sulfur and Ash Contents of Coal Burned at Arapahoe
Unit 4 ................................................ 49
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TABLE OF CONTENTS (Continued)
TABLE
16. Components of the Ash in the Coal Burned at Arapahoe
Unit 4 49
17. Electrical Resistivity of Fly Ash at Arapahoe Unit 4.. 50
18. Chemical Properties of Fly-Ash Samples Collected at
Arapahoe Unit 4 51
19. Concentrations of Flue Gases at Arapahoe Unit 4 52
20. Reported Concentrations of Injected H2SOi, and Equiva-
lent Observed Concentrations in the Fly Ash and Flue
Gases at Arapahoe Unit 4 53
21,. Sulfur and Ash Contents of Coal Burned at Y Station... 55
22. Electrical Resistivity of Fly Ash at Y Station j. 56
23. Chemical Properties of Fly-Ash Samples Collected at Y
Station 57
24. Concentrations of Flue Gases at Y Station 59
25. Electrical Resistivity of Fly Ash at Widows Creek
Unit 7 without NH3 Conditioning 62
26. Chemical Properties of Fly-Ash Samples Collected Ahead
of the Electrostatic Precipitator at Widows Creek
Unit 7 without NH3 Conditioning 63
27. Concentrations of Flue Gases at Widows Creek Unit 7
without NHs Conditioning 64
28. Electrical Resistivity of Fly Ash at Widows Creek
Unit 7 with NHa Conditioning 65
29. Chemical Properties of Fly-Ash Samples Collected Ahead
of the Electrostatic Precipitator at Widows Creek
Unit 7 with NHs Conditioning 67
30. Concentrations of Flue Gases at Widows Creek Unit 7
with NHs Conditioning 70
Vlll
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TABLE OF CONTENTS (Continued)
TABLE Page
31. Electrical Resistivity of Fly Ash at Kingston Unit 5.. 73
32. Chemical Properties of Fly-Ash Samples Collected at
Kingston Unit 5 ....................................... 76
33. Concentrations of Flue Gases at Kingston Unit 5 ....... 78
34. Electrical Resistivity and Chemical Properties of Fly
Ash at Gallatin Unit 4 ................................ 81
35. Electrical Resistivity and Chemical Properties of Fly
Ash at Bull Run Station ............................... 82
36. Electrical Resistivity and Chemical Properties of Fly
Ash at Z Station ...................................... 83
37 . Concentrations of Flue Gases at Z Station ............. 84
38. Sulfur and Ash Contents of Coal Burned at Shawnee
Unit 10 ............................................... 85
39. Electrical Resistivity of Fly Ash at Shawnee Unit 10
without Limestone Injection ........................... 87
40. Chemical Properties of Fly-Ash Samples Collected at
Shawnee Unit -10 without Limestone Injection ........... 88
41. Concentrations of Flue Gases at Shawnee Unit 10 with-
out Limestone Injection ............................... 89
42. Electrical Resistivity of Fly-Ash and CaO Particles at
Shawnee Unit 10 during Limestone Injection ............ 91
43. Chemical Properties of Samples of Fly Ash and CaO
Collected at Shawnee Unit 10 during Limestone Injec-
tion .................................................. 94
44. Concentrations of Flue Gases at Shawnee Unit 10 during
Limestone In j ection ................................... 95
45. Summary of Observed Concentrations of SOz and 80s in
Power Plants Burning Coal of Various Sulfur Contents.. 115
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TABLE OF CONTENTS (Continued)
TABLE
46. Concentrations of Acid Vapors Theoretically Obtained
from 66°B^ HaSOi* in a Low-Temperature Injection
System 122
47. Percentages of SOa Vapor Converted to SOz Vapor at
Equilibrium at Various Temperatures and 02 Concentra-
tions in a High-Temperature Injection Process 123
48. Capital Costs of S03 and H2SOi, Conditioning Systems
Installed for the Public Service Company of Colorado.. 124
49. Estimate of Capital Cost of an SOa Injection System
for a Single 250-MW Power Unit 125
50. Estimates of Capital Costs of Conditioning Systems for
Various Sizes of Single-Unit Power Stations 127
51. Itemized Energy and Raw Material Costs for Three Types
of Acid Injection Systems for a Single 250-MW Power
Unit 128
52. Estimated Total Operating Costs of Acid Injection in a
Single 125-MW Power Unit 130
53. Estimated Total Operating Costs of Acid Injection in a
Single 250-MW Power Unit 131
54. Estimated Total Operating Costs of Acid Injection in a
Single 500-MW Power Unit 132
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TABLE OF CONTENTS (Continued)
FIGURE Page
1. Schematic Diagram Showing the Major Components of the
Various Power Plants Investigated during this Program. 8
2. Observed Variation of Current with Time with Fly Ash
Collected in a Cyclone Sampling Apparatus... .......... 10
3. Observed Variation of the Resistivity of Fly Ash with
Electric Field in One Power Plant (Kingston Unit 5) ... 13
4. Schematic Diagram of Apparatus for Collection of SOs
by the Condensation Method ............................ 23
5. Schematic Diagram of Apparatus for Collection of SOa
by the Method of Absorption in an Isopropanol-Water
Solution .............. . . .............................. 26
6. Resistivity of Fly Ash at Widows Creek Unit 7 with and
without NHs Injection ................................. 66
7. Concentration of S0u~2 in Fly Ash at Widows Creek
Unit 7 with and without NHa Injection ................. 68
8. Concentrations of S03 Observed at Widows Creek Unit 7
with and without NHs Injection ........................ 71
9. Electrical Resistivity of Fly Ash at Kingston Unit 5
with and without HaO Injection ........................ 74
10. Sulfate Content and pH Value of Fly Ash at Kingston
Unit 5 with and without HaO Injection ................. 77
11. Concentrations of SOa at Kingston Unit 5 .............. 80
12. Resistivity of Fly Ash at Shawnee Unit 10 with and
without Limestone Injection ........................... 92
13. Resistivity of Fly Ash as a Function of the Injected
Concentration of Conditioning Agent ................... 100
14. Resistivity of Fly Ash as a Function of the Relative
Concentrations of Conditioning Agent and Fly Ash ...... 103
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TABLE OF CONTENTS (Concluded)
FIGURE Page
15. Resistivity of Fly Ash as a Function of the Concentra-
tion of Conditioning Agent and the Surface Area of
the Fly Ash 104
16. Resistivity of Fly Ash as a Function of the Sulfate
Content of the Ash 106
17. Dew Points Predicted from the Data of Greenewalt and
Abel and Gmitro and Vermeulen for Flue Gases Contain-
ing HaSOn at Various Concentrations in the Presence of
H20 at Concentrations of 7 and 10% 109
18. Concentration of SOz in Flue Gases as a Function of
the Sulfur Percentage in Coal 113
19. Percentage of Sulfur Oxides Found as SOa as a Function
of Temperature in Various Plants 116
Xll
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SUMMARY
This report summarizes an experimental study that was per-
formed for the Environmental Protection Agency, Office of Air
Programs, under Contract CPA 70-149, entitled "A Study of Resis-
tivity and Conditioning of Fly Ash." The study was started in
July, 1970, and concluded in August, 1971.
The electrical resistivity of fly ash is one of the most
important parameters that controls the efficiency of fly-ash
removal in coal-burning power stations by the use of electro-
static precipitators. If the resistivity is too high, as it is
normally when a low-sulfur coal is burned, the collection effi-
ciency is poor because the electric field between the corona
wires and the collecting electrodes falls to a low value, exces-
sive sparking occurs in the interelectrode space, or back corona
occurs in the deposited fly ash. Alternatively, if the resis-
tivity is too low, as it may be when a high-sulfur coal is burned,
collection efficiency can again be poor because the electrical
force holding deposited ash on the collecting electrodes is not
high enough to prevent excessive reentrainment during rapping of
the electrodes. The upper limit of the acceptable resistivity
range is approximately 2 x 1010 ohm cm, and the corresponding
lower limit is about 1 x 107 ohm cm. Various gaseous agents—
mainly SOs, HzSOif, NHs, and HaO—can be injected into the stream
of flue gases to overcome the problem of high resistivity. One
of the agents—NHs—can also be used to cope with the problem of
low resistivity.
The initial task in this experimental study was to develop
or select apparatus that would be appropriate for making deter-
minations of fly-ash resistivity and flue-gas concentrations.
Several devices for in situ measurements of resistivity were
fabricated and used Tn field experiments. A gas sampling train
for collecting sulfur oxides was the principal apparatus for gas
analysis; it consisted of a condenser for SO3 (always present as
H2SOi» vapor in the customary range of precipitator temperatures)
and an aqueous H202 bubbler for SO2.
The central task in the field studies was to evaluate the
effects on fly-ash resistivity that are produced by injecting SOa
or H2SOit vapor in the flue gases of power stations burning coals
with low sulfur contents (approximately 0.5% by weight). Most of
these studies were carried out in the Cherokee and Arapahoe
plants of the Public Service Company of Colorado, where provision
had been made to inject SO3 by the evaporation of the stabilized
liquid oxide in dry air or to inject H2SOn by the evaporation of
the concentrated liquid acid, either at a low temperature in
ambient air or at a high temperature in the combustion products
Xlll
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from natural gas. Regardless of the injection system used, the
injected agent occurs in the flue gases as HaSOi*, and it acts as
a supplement to the low concentration of HaSOii produced in the
combustion of low-sulfur coal. An additional study was carried
out in another power station with a low-sulfur fuel and a low-
temperature H2SCH injection system; the results obtained by other
investigators in still another power station with a low-sulfur
coal and an SO3 injection system were made available for compari-
son with the results of the field studies in this program.
Conditioning parameters that varied in the different power sta-
tions included the gas temperature; the fly-ash concentration,
particle size, and chemical constitution; and the site of SOs or
H2SOi» injection (upstream from both mechanical and electrostatic
collectors, between the two types of collectors, or upstream from
an electrostatic precipitator in the absence of a mechanical
collector).
All but one of the injection systems produced the desired
lowering of the fly-ash resistivity. The one exception was of
the high-temperature HaSOi* type; the ineffectiveness of this
system may have been attributable to an inadequate distribution
system, and the conclusion that the system is inherently unsound
is not warranted by the results obtained in this program. In
general, the experimental findings indicated that each of the
conditioning parameters enumerated above had some bearing on the
injected concentration of SOa or H2SCK that was needed for opti-
mum results. However, the experimental results can be general-
ized as follows: an injected concentration of 20 ppm was
sufficient to produce the desired lowering of resistivity in each
instance. To the degree that fly-ash basicity or flue-gas tem-
perature was lowered, the required concentration was lowered.
Conclusions about the mechanisms of HaSOu conditioning are given
later in this summary, inasmuch as they are based in part on data
obtained in plants without conditioning facilities, where only
naturally occurring H2SOi» was present.
A study of NHs conditioning was made in the Widows Creek
power station of the Tennessee Valley Authority. The working
hypotheses to explain the role of NHa in this plant were that
this basic conditioning agent neutralizes the excessively high
concentration of naturally produced RzSOi* from high-sulfur coal
(about 4% sulfur), raises the resistivity of the fly ash, and
minimizes reentrainment losses in the precipitator. From the
results obtained in this program, none of these hypotheses can be
unequivocally disputed. However, in repeated experiments, the
expected raising of resistivity was not observed, and this
hypothesis seems at present to be erroneous. If it is erroneous,
the mechanism by which NH3 increases the precipitator efficiency
xiv
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must be different in some respects from that described. Under a
future research program, further efforts will be made to explain
the role of NHs at the Widows Creek plant.
A very brief study of H20 conditioning was made at the
Kingston station in the TVA system. It was found that with 2%
sulfur as the fuel, the flue gases contained about 10 ppm of
and 7% of HaO as vapors at the temperature investigated, about
330°F, but the fly-ash resistivity was higher than desired under
these conditions. Subsequently, however, it was found that
injecting a stream of water into the boiler to increase the con-
centration of HaO vapor to 15% gave an .appreciable reduction in
resistivity and, simultaneously, a decrease in the concentration
of HaSOi, to 3 ppm and a corresponding increase in the sulfate
content of the fly ash. These results illustrate one of the
tenets reached in explaining the mechanism of HzSO* conditioning:
H2SOi4 and HaO vapors act in conjunction to control the resistivity
of fly ash.
Among the studies made in plants without conditioning facil-
ities, the results obtained at the Shawnee plant of the TVA
system were among the most meaningful data. Tests were performed
with and without injection of dry limestone into the boiler as a
means of controlling the emission of S02. The highly basic
particles of lime formed during the calcining of the limestone
virtually eliminated the naturally produced HaSCK from the flue
gases and produced a mixture of lime and fly-ash particles with a
resistivity much above the level considered practical for effi-
cient precipitator operation.
One of the final tasks in this program was to explain the
mechanism of fly-ash conditioning with HaSOit and to establish
certain guidelines as to the expected efficiency of conditioning
with a variation in the parameters considered relevant. The
following conclusions were reached:
and H20 vapors are concurrently collected on
the surface of fly-ash particles, producing a low-
resistance path for the flow of electricity on the
surface rather than through the interior of the
particles.
Joint collection of HaSOi* and HaO vapors can occur
by the mechanism of adsorption at temperatures above
the dew point of the mixture of vapors. Clearcut
evidence to support this conclusion was obtained in
one of the plants with a flue-gas temperature
unequivocally above the dew point. Unequivocal evi-
dence pertinent to this conclusion could not gener-
ally be obtained in the other power stations with
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SOUTHERN RESEARCH INSTITUTE
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lower temperatures because of the present uncertainty
about the exact location of the dew point at speci-
fied HaSOi* and HaO concentrations.
• Joint collection of HaSCK and HaO by condensation
undoubtedly will occur at sufficiently high vapor
concentrations and low temperatures. Thus, condensa-
tion is a possible mechanism of conditioning, but it
is not a necessary mechanism.
• Low temperature or low fly-ash basicity favor condi-
tioning by a given concentration of H2SOit. It
appears that a surface layer containing HaSOi, per se_
is essential for effective conditioning. If the fly
ash is chemically neutral, such a layer is readily
produced. On the contrary, if the fly ash is
markedly basic, as it is with a high lime content,
the HaSOit-containing layer is less readily produced,
owing to the apparent need to neutralize the surface
constituents of the ash before HaSCK as such can
remain. The experimental data show that the concen-
tration of HaSCK needed for conditioning increases
as the fly-ash basicity increases and, in addition,
the total amount of HaSCK that must be collected on
the ash for adequate conditioning also increases as
the basicity increases.
The concluding task was to compare the costs of HzSCK condi-
tioning by injection of SOs or HzSCK vapor in the different injec-
tion systems operated by the Public Service Company of Colorado.
From the cost data supplied by this utility company, the follow-
ing estimates about capital and operating costs were reached for
individual power units with different production capacities and
injection rates:
Operating cost,
Unit Capital _ mils/kWh __
Injection system
size, MW cost, dollars 10 ppm 20 ppm
SO3 evaporation
evaporation
Low temperature
High temperature
125
250
500
125
250
500
125
250
500
169,000
169,000
256,000
288,000
375,000
570,000
300,000
387,000
583,000
0.0515 0.0641
0.0327 0.0453
0.0281 0.0407
0.0770 0.0888
0.0542 0.0662
0.0440 0.0559
0.0759 0.0840
0.0518 0.0599
0.0408 0.0490
xvi
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In summary, the SO3 evaporation system appears least expensive,
primarily as a result of the greater capital cost experienced for
the HaSCK systems in the installations of the Public Service
Company. However, the relative costs of the different systems
are subject to change, especially if HzSOi, injection systems
should be installed in greater numbers than they have to date,
resulting in reduced capital costs.
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A STUDY OF RESISTIVITY AND CONDITIONING OF FLY ASH
I. INTRODUCTION
This is the final report of an investigation by Southern
Research Institute of the use of chemical conditioning agents to
control the electrical resistivity of electrostatically precipi-
tated fly ash in coal-burning electric power stations. This
investigation was carried out under Contract CPA 70-149 during the
period from July 1, 1970, to August 31, 1971.
A. Technical Background
For the efficient removal of fly ash from flue gases in an
electrostatic precipitator, several conditions must be satisfied.
One of the most important conditions is that the electrical resis-
tivity of the ash deposited on collector electrodes shall lie
within an appropriate range of values, usually believed to have a
lower limit of about 1 x 107 ohm cm and an upper limit of about
2 x 1010 ohm cm.1'2 Other conditions, however, are of comparable
importance and must be satisfied simultaneously. For example, the
distribution of gases must be reasonably uniform, the velocity of
the gases passing between the corona wires and the collector elec-
trodes must not be excessive, and the power supply must be capable
of maintaining an adequate current density at the electrodes. '2
If the resistivity of the collected ash is too low, only a
small voltage drop can be maintained across the collected layer.
Hence, there will be only a small electrical force holding the ash
to the collector electrodes, and the extent of reintrainment of
ash particles in the adjacent gas stream may be severe. On the
other hand, if the resistivity of the collected ash is too high,
either of the following phenomena may occur: (1) The resistance
through the collected layer of ash will lower the corona current
that can be produced with the normal operating voltage and, as a
consequence, the electric field in the gas stream and the resulting
migration velocities of negatively-charged fly-ash particles toward
the collecting electrodes will be markedly reduced. Usually, an
attempt to overcome the effect of excessive resistance through
the collected ash with increased voltage and correspondingly
increased electric field will meet with failure as the result of
the occurrence of rapid sparking (the maximum voltage that can
be produced between the corona wires and the collecting electrodes
is usually about the same with or without collected dust).3
(2) The resistance through the collected layer of ash may be suffi-
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SOUTHERN RESEARCH INSTITUTE
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-2-
cient to cause electrical breakdown in the layer with attendant
formation of positive gaseous ions ("back corona" or "reverse
ionization"), neutralization of negative charges on the ash par-
ticles, and reintrainment of the ash particles in the gas stream.
It appears from the literature that problems stemming from
low resistivity are rare but those stemming from high resistivity
are a fairly common occurrence, especially in power plants burning
low-sulfur coals. High resistivity of ash from low-sulfur coal is
attributed to the low concentration of SO3 that is produced in the
combustion process and the resulting failure of the ash to collect
sufficient amounts of S03 and H20 vapors from the gas stream to
produce a conductive surface film. Owing to the increasing
emphasis on the use of low-sulfur coals to minimize emission of
sulfur oxides and the simultaneous demands for improvements in
fly-ash collection, increasing efforts are being made in the power
industry to find satisfactory methods to overcome the problem of
high resistivity.
There are, at present, essentially three methods of over-
coming the problem of high resistivity of fly ash. One method is
to operate electrostatic precipitators at temperatures well below
the normal range (e.g., below 250°F), at which enhanced collection
of H20 vapor can produce a sufficiently conductive surface film to
make up for the shortage in S03. ** Still another method is to
operate a so-called "hot precipitator" ahead of the air heater with
temperature in excess of 600°F, where the volume conductivity of
fly ash (in contrast to the surface conductivity referred to above)
i^Jiigh enough to permit efficient precipitation.5 Still another
method, permitting operating of precipitators at customary
temperatures (275 to 325°F) is to inject chemical conditioning
agents to make up for the shortage of naturally produced S03.
Trials of a variety of conditioning agents have been
reported in the literature during the past 30 years. The types of
agents investigated have included: (1) HaO vapor (used as a sup-
plement to the H20 vapor produced during the burning of coal);
(2) various acidic agents, typified by S03 or its analog HaSOi*
(also used as a supplement to the product of coal combustion);
(3) various basic compounds, typified by NH3; and (4) various com-
pounds derived from S03 and NH3, such as (NH^)2S04. Some of the
earliest and most extensive work on conditioning was conducted
by Chittum.7 This work was done principally with laboratory equip-
ment rather than with full-scale industrial installations (which
would have been more desirable from a practical point of view);
even so, the work is still worthy of attention, for it included a
wide variety of conditioning agents and a number of different types
-------
-3-
of particulate substances. Chittum recognized clearly the impor-
tance of conditioning agents for increasing the surface conduc-
tivities of precipitated solids. He reached the conclusion that,
in general, acidic conditioning agents were most effective for
treatment of basic solids whereas basic agents were most effective
for treatment of acidic solids. In conjunction with this conclu-
sion, he theorized that the adsorption of conditioning agents of
opposite acid-base behavior from suspended solids enhanced the
adsorption of H20 vapor and the resulting formation of a conduc-
tive surface layer.
Since the work of Chittum was concluded, reports have been
published of various trials of conditioning agents in both labora-
tory facilities and full-scale industrial plants, notably those in
the coal-burning power industry. Examples of reported applications
of S03 or the chemically-equivalent substance HaSO*, have been
described by White;1'8 Busby, Darby, and Whitehead3'6 (who have
discussed the early and highly successful application of S03 in
the Kincardine plant in Scotland); and Watson and Blecher.9
Examples of reported trials of NH3 and NHa-SOs salts have been
described by Watson and Blecher,9 Baxter,10 Reese and Greco,11 and
Dalmon and Tidy.12 The reports cited, here do not include all of
the publications relevant to the subject of gas conditioning, but
they are representative of the recent literature on the subject.
They discuss both highly successful results with conditioning
agents, as indicated above for the application of S03 at Kincardine,
and some relatively unsuccessful results, as reported from trials
of S03 in a plant in Australia.8
Reference was made above to the chemical equivalence of the
chemical substances indicated by the formulas S03 and H2S04.
There is experimental evidence that S03 and H20 react very rapidly
to form H2SOn in the vapor state,13 and that the extent of the
reaction is governed by the concentration of excess H20 and the
temperature.'1* Although there are no apparent rate data for the
dissociation of H2SO,, to S03 and H20 under conditions where H2SOi,
is unstable, it is reasonable to assume that the dissociation
process will also occur rapidly until equilibrium is reached. If
a large excess of H20 is present as in the flue gases produced
in coal-burning power plants, the data of Bodenstein and
Katayama1" show that at a representative concentration of H20,
10% by volume, more than 90% of the total of S03 and H2SOi, will
occur as S03 above 700°F and more than 90% will occur as
below 475°F.
SOUTHERN RESEARCH INSTITUTE
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-4-
B. Scope of This Investigation
The investigation of conditioning carried out by this
Institute under Contract CPA 70-149 was devoted exclusively to
the use of conditioning agents in the coal-burning power indus-
try. Furthermore, it was devoted primarily to the use of the
agents S03 and H2SCU, but it included relatively brief studies
of the agents H20 and NH3. All of the agents except NH3 were
used to cope with problems of high resistivity; the agent NH3
was investigated under the relatively rare conditions of low
resistivity.
There were two principal reasons for undertaking this
research:
• First, several scheduled installations of S03 and
H2SOi» injection facilities in this country, such
as those of the Public Service Company of Colorado,
afforded an opportunity to investigate the effec-
tiveness and cost of conditioning by either of these
agents under a variety of conditions—(1) coal
composition; (2) fly-ash composition, concentration,
and particle size; (3) conditioning temperature; and
.(4) conditioning-agent concentration and injection
method. The results of the investigation were
expected to provide general guidelines of value to
utility companies considering the use of these con-
ditioning agents to cope with high-resistivity
problems, permitting prediction of the results to be
obtained in new installations and prediction of the
costs to be incurred in the plants involved.
• Second, the planned utilization of apparatus
designed for iri situ determinations of electrical
resistivity was expected to show the effectiveness
of conditioning in a realistic context. So-called
in situ determinations are those made with the
appropriate experimental apparatus inserted in the
entrance duct of a precipitator both in the absence
and the presence of conditioning agent. This
approach of making iii situ determinations of resis-
tivity is far superior to the approach of removing
samples to a laboratory, an approach often used in
part despite the known hazard of changes in the
surface properties of fly ash following removal
from the flue-gas environment. Moreover, consis-
tent use of changes in resistivity as the criterion
-------
-5-
of effectiveness is much more preferable than the
alternative used by some investigators of deter-
mining changes in the efficiency of precipitator
operation. The basic premise of using S03 or
HaSOu as a conditioning agent is that a reduction
in surface resistivity is the only direct result to
be expected. An improvement in precipitator effi-
ciency will not necessarily follow; certainly it
will follow only to a limited degree at best if,
for example, the precipitator operates with an
excessively high flow rate of flue gases or with
electrodes of inadequate surface area or power
density. Information obtained by one utility and
reported to us on a confidential basis showed that
S03 improved efficiency when the power plant was
operated at a power level well below the rated
value but with a flue-gas velocity consistent with
the precipitator design, whereas SO 3 did not
improve efficiency during power production at the
rated level and gas flow through the precipitator
at an excessive rate.
The specific objectives of this investigation may be sum-
marized as follows:
• To determine in general whether fly ash of varying
composition derived from coals of varying compo-
sition can be effectively conditioned with S03 or
• To show in specific detail how the effectiveness of
conditioning is governed by parameters such as the
properties of the fly ash, the concentration of the
conditioning agent, and the mode of injection of the
conditioning agent
• To formulate theories to explain the mechanisms of
conditioning
u
• To analyze the costs incurred during the use of
conditioning agents
SOUTHERN RESEARCH INSTITUTE
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-6-
II. EXPERIMENTAL FACILITIES, APPARATUS,
AND PROCEDURES
A. Power-Plant Facilities
The principal tasks of this investigation were carried out
on the basis of field studies at various power-generating stations,
all burning pulverized coal, and on the basis of supplemental
laboratory analyses of samples collected at these plants. As
listed in Table 1, five power units with S03 orH2SOi» injection
systems were investigated, and six other units without S03 or H2SOU
injection systems were investigated for comparative purposes. Most
of the power stations are identified by names, locations, and owner
companies; however, three of the stations are identified only by
code letters, in view of requests by the owner companies not to
identify these plants explicitly.
Details about the conditions under which each plant was
operated are given later in this report as an introduction to the
discussion of the experimental studies of that plant. However,
Table 1 gives information about some of the most important features
of the various plants. Moreover, Figure 1 shows the schematic
arrangement of the major elements found in the various plants and
referred to in the table air heater, mechanical collector and
electrostatic precipitator for fly-ash removal, and site of injec-
tion of SO3 or H2SO^ conditioning agent. Information in the table
shows that mechanical collectors were not present in several of the
plants; it also indicates the different sites at which conditioning
agent^was injected ahead of the mechanical collector and ahead of
the electrostatic precipitator.
B. Electrical-Resistivity Determinations
During the investigation, use was made of two basic types of
apparatus, differing in the mode of sample collection and the design
of electrodes. The two types of apparatus are referred to, for
convenience, as cyclone and point-plane resistivity probes. They
differed primarily as follows: (1) in probes of the cyclone type,
centrifugal force is used for sample collection and concentric
cylindrical electrodes are used for resistance measurements;
(2) in a probe of the point-plane type, electrostatic precipitation
is used for sample collection and parallel discs are used as the
electrodes for resistance measurements. The design and operation of
each type of probe are described briefly in the following paragraphs
and more completely in a published article by other investigators 5
and in the final report from the Institute under Contract
CPA 70-166.16
-------
Table 1. Power-Plant Stations Investigated in the Comparison of
Conditioning by Injected and Naturally Produced SOs or H2SOi,
Facilities
for fly-ash
Station name
Cherokee
X*3
Cherokee
Arapahoe
yb
Widows Creek
Kingston
Gallatin
Bull Run
Zb
Shawnee
Power
unit
2
4
3
4
6
7
5
4
_d
1
10
a. MC = mechanical
b. Identified only
in
0
c
-1
J
n
z
a
in
ni
>
n
i
z
u>
-t
c
H
m
c. "None" signifies
Owner company
Public Service Company of Colorado
—
Public Service Company of Colorado
Public Service Company of Colorado
—
Tennessee Valley Authority
Tennessee Valley Authority
Tennessee Valley Authority
Tennessee Valley Authority
—
Tennessee Valley Authority
Location
Denver , Colo .
—
Denver, Colo.
Denver, Colo.
—
Stevenson, Ala.
Kingston, Tenn.
Gallatin, Tenn.
Oak Ridge, Tenn.
—
Paducah, Ky.
Facilities
for fly-ash
removal3
MC, EP
EP
MC, EP
MC, EP
EP
EP
MC, EP
MC, EP -
EP
EP
MC, EP —
conditioning
' Injection
Agent
SO 3
SO 3
H2SO.»
H2SO,»
H2SO>
None0
None0
None0
None0
None0
None0
site3
Before
Before
Before
Before
Before
—
—
—
—
—
~
EP
EP
MC
EP
EP
collector, EP = electrostatic precipitator .
by code letter at the request of the owner company.
no facility for injection of either
has permanent facilities for NHs injection, and the
injection
d. The Bull
during
one series of field tests.
Run Station consists of only one very large
SOs or H2SOi». The Widows Creek unit,
Kingston unit had
power unit.
temporary facilities
-
however ,
for H2O
-------
Boiler
Air
heater
«\
Mechanical
collector
/\
Electro-
static
precipitator
Stack
Injection lines
(alternates)
SO 3 or
H2SO^
evaporator
i
CD
Figure 1. Schematic Diagram Showing the Major Components
of the Various Power Plants Investigated
during this Program
-------
-9-
i. Cyclone probes
a. Design developed by Cohen and Dickinson
A cyclone probe developed and described by Cohen and
Dickinson15 was made available for use during this investigation
through the courtesy of Lodge-Cottrell Limited. From a literal
point qf view, this device is a "probe" only in the sense that
it includes a pipe for insertion into the duct from which a
fly-ash sample is to be taken. Other important components of the
device——the cyclone in which the sample is collected by centri-
fugation from a high-velocity gas stream, and the cylindrical
resistivity cell to which the sample is transferred by a rapping
procedure are located in an insulated chamber placed outside
the flue but maintained at the duct temperature with two internal
space heaters.
During this investigation, it was found that several modi-
fications were desirable to ensure satisfactory use of the Cohen-
Dickinson probe. One modification consisted of wrapping heating
tape and insulation around the sampling pipe to avoid a decrease
in temperature of the sample between the duct and the cyclone.
Other modifications consisted of placing thermocouples in the
sampling line and on the wall of the resistivity cell to ensure
that temperatures in these locations were controlled at the
temperature in the duct.
Resistances of collected samples were determined by applying
a voltage of 1.0 kV across the electrodes and measuring the flow
of current thus produced. The electrodes had a common height of
3.8 cm and diameters of 0.4 and 1.4 cm about a common axis. The
average value of the electric field produced in the annular sample
between the electrodes was 2.0 kV/cm. Typically, with constant
voltage applied, the current gradually decreased with time and
appeared to approach a final value about one order of magnitude
lower than the initial value, as illustrated by typical data in
Figure 2.* The decrease in current was attributed to slow changes
in the surface chemistry of the sample, and the initial current
reading was therefore taken as the best value for calculating
resistance. Sample resistivities were computed from the initial
resistances, the diameters of the electrodes, and the height of
the electrodes.
This and other phenomena that complicate the task of obtaining
reliable iri situ resistivity values are discussed later in this
report and in another report from this laboratory.16
SOUTHERN RESEARCH INSTITUTE
-------
1.00
0.75
•H
-P
(0
0.50
c
0)
o
0.25
o
I
0
10
12
Time, min
Figure 2. Observed Variation in Current with
Time with Fly Ash Collected in
a Cyclone Sampling Apparatus
-------
-11-
With the procedure outlined above, the Cohen-Dickinson
probe was used with fair success on several occasions.
Ultimately, however, it was replaced with an Institute-designed
probe that offered as its principal advantage a shortening of
the time required for temperature equilibration prior to
sampling.
b. Design originated in this laboratory
The essential components of a second cyclone probe that was
designed and fabricated in this laboratory were a cyclone sample
collector and a cylindrical resistivity cell with dimensions
similar to those in the Cohen-Dickinson apparatus; The height
of the electrodes was 4.4 cm and the diameters were 0.47 and 1.27
cm. In contrast to the cyclone and cell in the Cohen-Dickinson
device, these components of the Institute apparatus were mounted
at the end of a pipe that was inserted directly approximately
1 m into the duct where samples were collected. The samples were
pumped into the cyclone through a rectangular slot and the effluent
gases were then discharged through the pipe, which served as a
connection to a high-volume pump as well as a mount for the cyclone
and the cell. A thermocouple was placed at the extremity of the
assembly (the base of the cell) to indicate the time when tempera-
ture equilibration in the duct had been reached and sampling could
be started.
Electrical measurements and calculations were made with the
modified cyclone probe by the same procedures as with the Cohen-
Dickinson probe. With the usual applied voltage of 1.0 kV the
average electric field in the sample during resistivity measure-
ments was 2.5 kV/cm, only slightly higher than that in the Cohen-
Dickinson apparatus. The current through the samples in both
cyclone probes showed a similar time dependence; thus, the initial
current was used for calculating resistivity in the Institute probe
as in the Cohen-Dickinson probe.
2. Point-plane probe
The essential components of a point-plane probe constructed
in this laboratory were a tapered rod of stainless steel to serve
as a source of negative corona, a copper disc with an area of
5 cm2 located centrally about 3.8 cm from the corona source, and
a second disc with about the same area but with a small opening
in the center to permit movement along the axis of the corona
source. The electrodes (the corona source and the two disc elec-
trodes) were mounted on the end of an aluminum pipe, which per-
mitted insertion of the electrodes into the duct for a distance of
SOUTHERN RESEARCH INSTITUTE
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-12-
about 1m. A perforated shield was placed around the electrodes
to avoid excessive gas turbulence near the electrodes but, at
the same time, to permit gas migration into the electrode area.
A thermocouple was placed near the electrodes to indicate the
time when temperature equilibration in the duct occurred.
The procedure of collecting a sample consisted of applying
a voltage of about 15 kV between the corona source and the first
disc electrode described above, with the second disc electrode
retracted from the corona region. After sample collection was
completed, the power supply providing the current was discon-
nected, and the second disc electrode was moved into contact with
the deposited ash on the first electrode. A spring was used to
produce a compressive force on the deposited ash, and the thick-
ness of the deposit usually, about 0.1 cm was then determined
by means of a gage that indicated the separation between the two
disc electrodes. Except in unusual circumstances, such as those
encountered in one power station that is discussed in Section
III.A.I, the resistance of the sample was determined by applying
various voltages across the deposit and measuring the corresponding
currents at electric fields in the range from about 1 to 20 kV/cm
(the upper limit with each sample was the field required for elec-
trical breakdown to occur). The resistivity of the sample was
then calculated by multiplying the resistance of the sample by the
factor A/h, where A was the electrode area (5 cm2) and h was the
sample thickness.
Negligible time-dependence of resistivity data obtained with
the point-plane apparatus was observed in contrast with the marked
time-dependence observed with the two cyclone probes. However,
marked electric-fieId dependence of the data recorded with the
point-plane apparatus was usually encountered. Figure 3 sum-
marizes data that were obtained in one of the field studies (at the
Kingston Station, which is discussed in Section VI.A). The curve
plotted in this figure shows the relative values of resistivity, in
the range of electric fields up to 12 kV/cm. The data points at
2, 4, 8, and 12 kV/cm compare the calculated average ratios of
resistivities observed at these fields to the resistivity observed
at 1 kV/cm; the bars show the maximum range in individual experi-
ments. Extrapolation of the curve was made to the vertical axis
to show the approximate relative value of resistivity at fields
lower than the minimum of 1 kV/cm that was employed experimentally.
Presumably, a field dependence of resistivity data obtained with
the cyclone probes would have been observed if it had been possible
to discern the effects from field variation in the presence of
marked changes in resistivity with the time of measurement.
-------
in
O
c
H
ni
o
I
Z
in
H
H
H
PI
1.0
0.9
0.8
0.7
>i
+ 0.6
-H
W
0)
M
*4-l
O
0)
0)
•H
0)
0.4
0.3
0.2
0.1
i
(-•
u>
I
6 8
Electric field, kV/cm
10
12
14
Figure 3. Observed Variation of the Resistivity of Fly Ash
with Electric Field in One Power Plant (Kingston Unit 5)
-------
-14-
C. Coal Analyses
Samples of pulverized coal being fed to the boiler were
collected in most of the power stations discussed in this report.
For all samples, determinations were made of the total percentages
of sulfur and ash constituents. For some of the samples, deter-
minations were made of sulfur in each of the three major forms:
sulfate, pyritic sulfur, and organic sulfur. Also on occasion,
determinations were made of the relative concentrations of the
major components of the ash.
Sulfur and ash concentrations in coal were determined by
established ASTM methods. Each sample was ground to pass a 60-
mesh sieve (provided that it had not previously been ground
sufficiently before collection), and it was then dried at 110°C
and processed chemically by ASTM Methods D-271 and D-2492.
D. Fly-Ash Analyses
1. Collection of samples
The majority of the samples of fly ash used for chemical
analyses were collected with the cyclone and point-plane resis-
tivity probes at sampling ports near the entrances of electro-
static precipitators, as discussed in Section II.B. Following
determinations of resistivity, the samples were promptly removed
from the sampling probes, stored in polyethylene bottles, and
transported to the laboratory for analytical work.
Most of the remaining samples used for chemical analyses
were^collected with cyclone sampling probes inserted in ports
located ahead of air heaters. The Cohen-Dickinson resistivity
apparatus was used in this manner simply as a sample collector
in some of the initial field work, but it was replaced later with
an all-metal apparatus similar to the cyclone probe constructed
at the Institute for resistivity determinations. The few
remaining samples were collected by methods briefly described
later in this report.
2. Analyses by wet-chemical methods
This section discusses the procedures that were used most
frequently for determination of the chemical properties of fly
ash. Some of the procedures were followed with all samples;
others were not followed with all samples but nevertheless were
used with sufficient frequency to represent a substantial part
of the overall analytical effort. On occasions cited later in
this report, variations in these procedures were made in special
circumstances, and still other procedures were followed to obtain
supplemental information.
-------
-15-
a. Determination of pH produced in an aqueous slurry
One of the procedures routinely used in the analyses of
fly-ash samples was to prepare a slurry of ash with distilled
water and determine the equilibrium value of the pH reached
after prolonged stirring. Each sample of ash was mixed with
water in the ratio of 0.1 g of ash to 30 ml of water. Usually,
the equilibrium pH was reached in about 30 min, but sometimes
the slurry was stirred for 60 min to ensure that equilibrium had
been established. During the time required for equilibration,
each slurry was protected from ambient air to avoid absorption
of atmospheric C02. Approximately the same value of pH was
recorded during the final period of stirring or after stirring
had been stopped and the suspended ash particles had been removed
from the liquid phase by settling or centrifugation.
The procedure used for determining pH was similar to a
procedure reported in the past by Lee, Friedrich, and Mitchell.17
From the standpoint of convenience, it was preferred to an alter-
native procedure described by White,8 which involved extraction
with hot water in Soxhlet apparatus.
b. Determination of the excess of base or acid soluble
in water.
During the initial phase of this investigation, the fly-ash
samples consistently showed that they contained an excess of solu-
ble base that is, they produced pH values in aqueous slurries
that were substantially above the neutral value of pH 7. For
several of these samples, the liquid phase from the slurry was
separated from the residual ash by centrifugation; then the liquid
was titrated to pH 7 with a solution of HC1, with a pH meter for
recording pH versus added HC1. From the results of the titration,
the quantity of water-soluble base in the original ash was cal-
culated as the equivalent weight percentage of CaO in the ash,
assuming that only CaO had dissolved as Ca+2 and OH~ ions.
Several of the samples contained basic substances other than
compounds dissolving to produce OH~ ion. This conclusion was
based on a comparison of the titration curves obtained with the
pH meter in experiments with extracts of fly ash and with solu-
tions containing OH~ ion as the only base present. The soluble
basic substances ion may have included silicate and aluminate ions;
the titration curves of several fly-ash extracts resembled the
curves obtained with synthetic mixtures containing silicate and
aluminate ions. Clear-cut identification of bases other than OH~
ion by the titration procedure was not attempted, however, because
SOUTHERN RESEARCH INSTITUTE
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-16-
the locations of inflections in the titration curves of fly-ash
extracts were usually not distinct and could have been caused
by a variety of other bases than those suggested as possible com-
ponents.
In later work, several fly-ash samples were found to contain
excess acid rather than excess base, in view of the fact that they
produced pH values in aqueous slurries considerably lower than
pH 7. Extracts of these samples were titrated to pH 7 with NaOH,
and attempts were made to interpret the titration curves thus
obtained. It was clearly shown that the acids neutralized below
pH 7 were largely substances other than H+, probably including
hydrated A1+3 and Fe+3 ions.
c. Determination of total soluble sulfate
Knowledge of the quantity qf sulfate present in fly ash was
of particular interest, in view of the fact that this property was
indicative of the amount of S03 or H2SOi, deposited on the ash
during conditioning in the presence of flue gases. Determination
of sulfate in forms soluble in water was, therefore, a procedure
routinely used in fly-ash analyses, as in previous work described
by White.8
Water-soluble sulfate was assumed to be present primarily on
the fly-ash surface, as concluded by White1'8 and confirmed in
other work discussed subsequently in this report. This component
of the ash was determined by the following steps: (1) treatment
of an aqueous extract prepared during the pH determination with a
sulfonate ion-exchange resin to replace interfering cations with
H+ ion, as described by Fritz and Yamamura;18 (2) dilution of the
aqueous H2SOU solution resulting from the ion-exchange process with
excess isopropanol, producing a waterrisopropanol ratio of 1:4;
and (3) titration of the resulting solution of H2SOit in the mixed
solvent with Ba(C10^)2 as the titrant and thorin as the indicator,
as described by Fritz and Yamamura18 and by Fielder and Morgan.19
d. Determination of soluble SO^"2 as H2SO^
In determining the pH of slurries of fly ash from several
sources, an interesting dependence of pH on the time of stirring
was noted. Immediately following addition of fly ash to dis-
tilled water with a usual pH of about 5.5 (showing the presence of
absorbed atmospheric C02), a rapid decrease in pH was observed.
Then, with increased time of contact between the ash and the water,
an increase in pH occurred until finally an equilibrium value was
reached sometimes below 7 and sometimes substantially above 7.
-------
-17-
A minimum in pH with increasing time of contact was assumed to
indicate that free acid probably I^SO,, on conditioned ash
occurred in the outermost surface layer and thus dissolved first.-
but that basic substances occurred in the underlying surface
layers and ultimately partly or completely neutralized the acid
dissolved initially.
Near the conclusion of the contract, efforts were made to
find a means of confirming the above hypothesis. Several pure
alcohols and alcohol-water mixtures were used for extracting fly-
ash samples, and a pH meter was used to indicate whether free acid
could be extracted selectively, even in the presence of excess
base. The results of these experiments appeared to be quite
promising, as indicated in the following paragraph.
In an ethanol-water mixture consisting of 95% of ethanol
and 5% of water by volume, several fly-ash samples produced
steadily decreasing values of pH until an equilibrium value well
below that of the solvent was reached, whereas in water the same
fly-ash samples produced an initial decrease in pH and then a
sharp rise in pH to values far above pH 7, even up to pH 10 or 11.
On the other hand, in the mixed solvent, still other fly-ash
samples caused essentially no change or a slight increase in pH.
It was concluded, therefore, that some samples contained free acid,
whereas others did not. Finally, it was discovered that if an
acidic extract was separated from the residual ash and titrated
with a solution of NaOH in the ethanol-water mixture, the titra-
tion curve observed with the pH meter was practically identical
to the titration curve obtained with a synthetic mixture of HjSOij,
ethanol, and water. (To facilitate rapid equilibration of the pH
meter in work with the mixed solvents, it was found expedient to
add KC1 to the mixture at a concentration of 1 x 10~3M.)
On the basis of the observations described above, the follow-
ing procedure was adopted as a means of determining the amount of
free I^SO^ on fly ash: A sample of 0.1 g of fly ash was stirred
with 15 ml of the mixed solvent for 10 to 15 min until the pH
reached a stable value. If the pH was lower than that of the
original solvent, the liquid phase was then separated from the
residual ash by centrifugation and titrated with an ethanol-water
solution of NaOH with the pH meter as an indicator of the endpoint.
The apparent concentration of H2S04 as a weight percentage of the
ash was then calculated.
In an effort to confirm that H2S04 was, indeed, selectively
extracted, attempts were made to show that the amount of S04~2
ion extracted was equivalent to the amount of acid extracted.
SOUTHERN RESEARCH INSTITUTE
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-18-
Generally speaking, these attempts were not successful. Although
S0n~2 was found, it was not found in amounts precisely equivalent
to the acid. However, the method used for determining S0i,~2 —
based on titration with Ba(ClOi»)2 with thorin as indicator—lacked
the sensitivity and precision needed to obtain accurate determina-
tions of S0i»~2 at concentrations equivalent to the concentrations
of acid found.
Despite the generally poor agreement between concentrations
of acid and SCK"2 found in the ethanol-water extracts, it is felt
that the results of the acid determinations show with fair cer-
tainty that free HaSOit did, indeed, exist on a number of the fly-
ash samples—even on several samples that had been stored in the
laboratory for several months before they were analyzed.
3. Analyses by optical methods
Studies of the chemical compositions of a few fly-ash sam-
ples were made by the methods of X-ray diffraction and electron-
microprobe analysis. It was assumed that the method of X-ray dif-
fraction might be of value for identifying specific compounds in
the ash, provided they exist in crystalline form, and that the
method of electron-microprobe analysis might be useful for deter-
mining specific elements on the surfaces of ash particles.
X-Ray diffraction. Analysis of fly-ash particles by the
method of X-ray diffraction indicated that the ash consisted
largely of amorphous rather than crystalline substances. Analysis
of soluble components of the ash—isolated by dissolution in water
and^subsequent drying—failed to give an identifiable diffraction
pattern. Thus, X-ray diffraction appeared to be of limited value
as an analytical method in this investigation.
Electron-microprobe analysis. In view of the paramount
interest in the presence of sulfur as S0i*~2 ion on the surfaces of
fly-ash particles, electron-microprobe analysis was investigated
as an alternative method for determining sulfur. The method, not
heretofore employed in studies of fly-ash conditioning, proved to
be of value for studies of the distribution of sulfur on various
particles and for semiquantitative determinations of sulfur concen-
trations.
One matter of interest was to determine the relative distri-
butions of sulfur on the original surfaces of fly-ash particles
and on cross sections of particles that had been exposed by polish-
ing. Sulfur was readily detected on the original surfaces but was
not found in the interior material.
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-19-
Another question investigated was how the sulfur was
'Distributed among a large group of particles. Nearly all of
:he particles analyzed contained detectable amounts of sulfur;
nowever, a small number of particles contained unusually large
amounts of the element. No correlation was found between sul-
fur concentration and parameters such as particle color, mag-
netism, and other elemental components predominantly iron and
silicon in some instances and calcium, silicon, and aluminum in
others. However, all of the particles containing exceptionally
high concentrations of sulfur were, for some unexplained reason,
agglomerated with other smaller particles.
In one of the samples analyzed quantitatively, it was
found that approximately 99% of the particles contained about 0.1%
of sulfur, and the remaining 1% contained up to 5% of sulfur.
Analysis of this sample by the wet-chemical method previously
described showed that the sample contained 0.72% of water-
soluble SO^"2, the equivalent of about 0.24% of sulfur. The
agreement between the two results seems reasonably satisfactory.
In two more samples, quantitative analyses were made of
aggregates of particles with an enlarged electron beam approxi-
mately 10 ym in diameter, compared with the number-median particle
diameter of about.1.5 ym. One of the samples had been collected*
in the absence of S03 conditioning agent; the microprobe analysis
showed that it contained 0.4% of sulfur, compared with the calcu-
lated value of 0.5% of sulfur based on the experimental finding of
1.5% of soluble S04~2. The second sample had been collected with
SO3 conditioning agent added; the microprobe analysis showed that
it contained 0.7% of sulfur, compared with the calculated value
of 1.0% based on the experimental finding of 3.0% of soluble SO,/
As in the analyses of individual particles, the results of micro-
probe analyses of aggregates of particles seem to agree satis-
factorily with the results obtained by the wet-chemical procedure. .
For routine use, determination of sulfur as SO,,"2 ion by
the wet-chemical procedure appeared to be preferable to determin-
ation by electron-microprobe analysis. However, the validity of
the chemical method for showing the amounts of S04~2 collected on
the surfaces of the particles after injections of SOs conditioning
agent was strengthened by the results of the electron-microprobe
analyses, especially the findings that sulfur was located primarily
on the particle surfaces and distributed with sufficient uniformity
on all particles to have a significant effect on electrical resis-
tivity.
-2
SOUTHERN RESEARCH INSTITUTE
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4. Determinations of particle-size distribution
Particle-size distributions in fly-ash samples were deter-
mined by a microscopic technique with visible radiation. Approxi-
mately 25 mg of each sample was suspended in 2 ml of glycerine;
approximately 50 yl of the suspension was then spread on a micro-
scope slide and protected with a cover. Particles in different
size ranges (<2, 2-5, 5-10, 10-25 and >25 ym) were counted by
use of a microscope equipped with a MSA Dust View Microprojector
(product of Mine Safety Appliances Company). The microscope and
projector allowed viewing of the fly-ash particles at a magnifi-
cation of 1000 diameters on a 200- by 250-mm screen with a cali-
brated grid. Usually, a total of 300 to 400 particles was counted
in six to ten fields on the slide, each approximately 0.20 mm
high and 0.25 mm long.
In each sample, most of the particles smaller than 10 ym
were translucent spheres. Most of the particles larger than
10 ym were also spherical but a substantial fraction were opaque
rather than translucent. There were relatively few agglomerates
of spherical particles and other particles of irregular shape.
The agglomerates and irregular particles in various size ranges
were counted on the basis of their maximum dimensions.
Percentages of each sample in the various size ranges were
calculated on the number basis and plotted on logarithmic proba-
bility paper. As a rule, the plotted data fell near a straight
line, signifying a log-normal particle-size distribution. The
intersection of the coordinate for 50% probability with the
straight line representing the data points was recorded as the
number-median particle diameter.
E. Flue-Gas Analyses
1. Collection of SOa and S02 samples
The components of flue gases that were of paramount impor-
tance during this investigation of gas conditioning were S03 and
H2SOi». As indicated previously in this report, the relative con-
centrations of S03 and H2SOi4 vapors in flue gases depend upon
both the temperature and the concentration of H20 vapor, assuming
that chemical equilibrium involving S03, H2SOi», and H20 vapors is
continously maintained. However, SOs and H2SOif are not distin-
guishable by the analytical methods employed during this investi-
gation; thus, for convenience in discussions of flue-gas analyses,
the formula S03 is used to indicate both chemical substances.
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Another component of flue gases that was of considerable
interest was SOa- This compound is ineffective as a condi-
tioning agent. However, S02 is a potential source of SOs
through reaction with Oz, and it is a practical indicator of
the percentage of sulfur in the fuel. Thus, the concentration
of S02 was routinely determined along with the concentration of
S03.
a. Survey of available collection methods
Collection of the S03 in flue gases is a difficult problem
because (1) S03 is easily lost as the result of condensation with
excess HaO to form liquid HaSCH-HaO mixtures if high temperatures
are not maintained in the sampling line between the source and
the collector and (2) SO3 normally occurs in flue gases at low
concentrations in the presence of a large excess of a probable
interferant, SOz, which can be oxidized to S03 by Oa. Examples
of methods that have been used by various investigators in efforts
to overcome these sources of difficulty are described briefly
below. All of the methods use a heated sampling probe fitted with
a filter to remove the fly-ash particles and any liquid particles
of a condensed E2SO^-H20 mixture that may be present; however,
they use different types of apparatus to collect samples as a
SOi^"2 salt or as E2SOk, as follows:
• The flue gases are pumped through a bubbler filled
with a solution of NaOH and an oxidation inhibitor,
such as benzyl alcohol. The S03 and S02 are col-
lected in the solution as S(\~2 and S03~2 salts,
respectively. The conversion of S03~2 to SO^"2 is
repressed by the presence of the oxidation inhibi-
tor, and each ion may be determined in the presence
of the other.20'21
• The flue gases are pumped through a bubbler filled
with a 4:1 mixture of isopropanol and water, which
collects S03 with much higher efficiency than SOa.
Any SOa collected with the S03 is removed with a
purge of inert gas, such as N2, to avoid inter-
ference in the analysis of the sample.22'23
• The flue gases are pumped through a condenser coil
maintained below the dew points of S03-H20 mixtures
but above the dew points of other vapor mixtures
or individual vapors, such as an S02-H20 mixture or
H20 vapor alone. A satisfactory range of condenser
temperatures is 140 to 195°F.2i|'25
SOUTHERN RESEARCH INSTITUTE
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In contrast to the collection of S03 , the collection of S02
is a reasonably simple task. In a sampling train that contains
either a condenser or an isopropanol -water mixture as the collec-
tor for S03 , a bubbler filled with a 3% solution of H202 in water
may be used downstream from the S03 collector to collect S02 as
H2SCV
Both condensation and absorption methods for collecting S03
were thoroughly evaluated under laboratory conditions before
they were used in field studies, as described in the following
paragraphs. The absorption method for collection S02 , on the
other hand, appeared clearly suitable for field use and was not
subjected to preliminary evaluation under laboratory conditions.
b. Evaluation of the condensation method for collecting S03
The apparatus used for collecting SOs by the condensation
method is similar in design to that described by Lisle and
Sensenbaugh. 2>* It is shown schematically in Figure 4 as part of a
sampling train that includes an absorber for
The sampling probe includes two concentric tubes with lengths
of 1.2 m; the inner tube or sampling line is made of Pyrex with an
internal diameter of about 7 mm, and the outer tube used for support
and insulation is made of stainless steel with an external diameter
of about 25 mm. The annulus between the two tubes contains an
electrical heating tape around the wall of the Pyrex tube and an
insulating asbestos tape around the heating tape. The end of the
Pyrex tube that is inserted in the flue is packed with quartz wool
to prevent particles of fly ash and H2SO^-H20 condensate from
entering the collection system; the other end of the Pyrex tube is
fitted with a ball-and-socket joint for connection to the condenser.
The condenser consists of a helical condensation tube made from
Pyrex tubing with an internal diameter of about 7 mm and an overall
length of about 1 m; a spray trap consisting of a fritted-glass
filter (sealed to the helix near the exit) ; a heated bath of
ethylene glycol and water around the helix and filter; and a steel
pipe fitted with an external heating tape for containing and heat-
ing the water-glycol mixture. The S02 scrubber is a bubbler
filled with a 3% solution of H202 in water. The flow-rate indica-
tor is a Charcoal Test Meter (product of American Meter Company)
with an inlet filter of Drierite or, as an alternative, a vapor
trap immersed in ice water. The Charcoal Test Meter registers the
integral of flow rate and time and, thus, shows the total volume
of dry flue gases sampled except for the relatively small volumes
of S03 and S02 collected upstream. A small vacuum pump (Model
1031-V102-351 of Cast Manufacturing Corporation) is used for
sampling flue gases at an approximate rate of 2 1/min for a period
of about 20 min.
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Flue-gas
sample I
Wall of flue
Heated
sampling probe
condenser
Vent
Pump
absorber
(peroxide-water
solution)
Flow
meter
Drierite
or
cold trap
Figure 4. Schematic Diagram of Apparatus
for Collection of SOa by the
Condensation Method
SOUTHERN RESEARCH INSTITUTE
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The performance of the above apparatus was evaluated in the
laboratory by injecting aqueous H2S0lf at known rates in the sam-
pling probe (heated to a sufficient temperature to volatilize
both the S03 and the H20 added) and determining the amounts of S03
collected in the condenser by the methods described subsequently
in Section II.E.2. Room air was pumped through the sampling
probe and the condenser at an approximate rate of 2 1/min to
sweep the S03 from the point of injection to the condenser. In
some of the experiments, S02 was injected in the airstream before
it entered the sampling probe; the average concentration of S02
in these experiments was 1700 ppm.
Table 2 lists the experimental data comparing the concen-
trations of SOa found with the concentrations sampled, in the
range from 0 to 20 ppm with or without SO2 present. In general,
the agreement between observed and injected concentrations is
within acceptable limits. The results obtained when S02 was
present but no S03 was injected indicate that slight degrees of
oxidation of S02 to S03 occurred either in the airstream or the
condenser; thus, with low S03 concentrations in flue gases, small
positive errors may occur in the observed SO3.concentrations as
the result of the oxidation of SO,.
Table 2. Comparison of Sampled and
Observed SO3 Concentrations by
the Condensation Method
Concn of gas sampled, ppm Concn of SO3
S02 found, ppma
1700 0.8-0-9
0 1.7-2.0
0 13.3-15.9
1700 15.9
1700 18.3
19.8 1700 15.2-18.1
a. Based on sample analyses by the methods
described in Section II.E.2. Results
from multiple determinations are shown
as a range of values.
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c. Experimental study of the isopropanol-water absorption
method for collecting SO3
Components of the apparatus used for collecting SO3 by
absorption in a 4:1 mixture of isopropanol and water are shown
schematically in Figure 5. Except for the S03 absorber, the com-
ponents of this apparatus are the same as those described above
in connection with the condensation method. The absorber for S03
and the auxiliary absorber for S02 are made of Pyrex as described
in a publication of Shell Development Company.23 They consist,
in essence, of a bubbler filled with the isopropanol-water mix-
ture for collecting SO3 and a second bubbler filled with a
peroxide-water mixture for collecting S02. Both bubblers were
immersed in ice-water mixtures to optimize collection efficiencies.
Following a period of flue-gas sampling, a stream of nitrogen is
passed through the two bubblers to transfer any SOz in the
isopropanol-water mixture to the peroxide-water mixture.
The effectiveness of the apparatus for collecting SO3 was
evaluated in the laboratory by the procedures previously described
in the discussion of the condensation method. The experimental
data are listed in Table 3. These data, like those obtained with
the condensation method, indicate that the efficiency of S03 col-
lection was, in general, satisfactory. The results obtained with
SOa present but with SO3 absent, however, indicate that signifi-
cant positive errors in SO3 concentrations are to be expected in
the range of low values, as the result of the oxidation of S02 to
S03.
Table 3. Comparison of Sampled and
Observed S03 Concentrations by
the Absorption Method
Concn of gas sampled, ppm Concn of SOa
>3
SO, SO2 found, ppma
0 1200 3.1
2.0 0 1.8-2.0
17.0 1200 16.5-18.4
25.0 0 24.6
a. Based on sample analyses by the methods
described in Section II.E.2. Results
from multiple determinations are shown
as a range of values.
SOUTHERN RESEARCH INSTITUTE
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Flue-gas
sample
1
Wall of flue
Heated
sampling probe
absorber
(isopropanol-water
solution)
Vent
Pump
SOa absorber
(peroxide-water
solution)
Flow
meter
Drierite
or
cold trap
Figure 5. Schematic Diagram of Apparatus for Collection
of SO3 by the Method of Absorption in an
Isopropanol-Water Solution
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d. Field studies
During the initial field studies, use was made of both of
the collection methods discussed above for collecting S03 and
SO2 samples. Good agreement was obtained by the two methods and
thus, in time, either of the methods was considered satisfactory.
However, on the basis of convenience, the condensation method was
considered preferable and was adopted for routine use.
2. Analyses of S03 and S02 samples
Two methods were investigated under laboratory conditions
for determining the S03 and SOa collected as H2SCK by either of
the two collection procedures. One method was based on titration
of HaSOt with Ba(Cl(H)2, with a 4:1 mixture of isopropanol and'
water as the solvent and the organic dye thorin as the indicator
of the endpoint.18 'l9 The second method was based on treatment
of the collected H2SOi, with excess solid barium chloroanilate and
spectrophotometric determination of the chloroanilate ion released
in the solution during formation of BaS04 in place of the original
barium chloroanilate.'6 The intensity of ultraviolet absorption
at 310 nm was used for calculating the original concentration of
H2SO.,.
Our experience showed that the titration method was substan-
tially less sensitive, requiring roughly 25 times as much sample
as the spectrophotometric method. The titration method was, on
the other hand, far more convenient in terms of the time and the
apparatus required for a determination. Moreover, it was suffi-
ciently sensitive for use in determining S03 in flue gases at
concentrations down to 1 ppm with samples of reasonable volumes
(e.g_. , 40 liters), and it was sufficiently sensitive in determining
tEe characteristically much higher concentrations of S02. In
numerous sample analyses by the two methods, the results obtained
by the two methods were in good agreement. Thus, we considered
the titration method preferable, especially in field work where
simplicity was of considerable importance.
3. Determination of H20 vapor
In addition to SO3 and S02 , H20 was one of the components
of flue gases that was of vital importance in the investigation
of gas conditioning. In contrast to S03 and S02, however, H20
did not vary widely in concentration. Thus, concentrations of
H20 were determined consistently in every plant but on a less
frequent basis than those of S05 and S02.
SOUTHERN RESEARCH INSTITUTE
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The technical literature is not clear about the methods that
may be used most successfully for determining the concentration of
H20 vapor in flue gases. The detailed analytical information of
Cuffe et al27'28 about flue-gas compositions, for example, includes
data on H20 concentrations but gives no description of the method
used to obtain these data. On occasion, publications have referred
to the use of wet-and-dry bulbs for H20 determinations,29 but our
impression of such devices is that the results obtainable would be
of dubious validity. We felt that, as an alternative, use of an
efficient drying agent in solid form, such as Drierite or Mg(C10,t)2,
would be preferable.
In accordance with the above viewpoint, we evaluated
Drierite as a collector for water vapor. A tube packed with a
weighed quantity of Drierite, approximately 50 g, was placed
between the heated probe and the flow meter shown in either Figure
4 or 5, with all of the other components of the apparatus omitted.
Experimental data obtained with simulated flue-gas mixtures showed
high efficiency of water-vapor recovery and indicated that accu-
rate determinations of water-vapor concentration could be made in
the presence of other flue-gas components. Determinations of H20
vapor in field studies were, therefore, made on the basis of sam-
ple collection with Drierite.
4. Determination of NO and N02 in combination
Still other components of flue gases that may be indirectly
involved in gas conditioning are the oxides of nitrogen, NO and
NO2. Occasional determinations were made of the sum of concentra-
tions of these two oxides, referred to as NOX. The procedure con-
sisted of sample collection as HN03 in aqueous HgSOi,, neutraliza-
tion of the acidic sample with KOH, addition of phenoldisulfonic
acid to produce nitrophenoldisulfonic acid, and spectrophotometric
determination of the nitro compound.21
The results of all determinations of NOX are included in
this report as a matter of record. No attempt was made, however,
to draw any conclusions about the influence of NOX on fly-ash
conditioning.
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III. STUDIES OF POWER PLANTS EQUIPPED WITH FACILITIES
FOR THE INJECTION OF ANHYDROUS SO3 VAPOR
A. Cherokee Station, Unit 2
1. Description of the plant facilities
The Cherokee Station is operated by the Public Service
Company of Colorado in Denver, Colorado. It consists of four
units ranging in production capacity from 110 to 360 MW. Units 1,
2, and 4 are equipped with facilities for the injection of S03
from a central installation in which commercial stabilized S03
(obtained from Allied Chemical Corporation under the tradename
"Sulfan") is converted from the liquid to the vapor in a stream of
dry air.30 The S03 injection facilities were designed and in-
stalled by Lodge-Cottrell Limited during the first part of 1971.
As discussed in the following section, Unit 3 is equipped with an
independent system for the injection of H2SO,, by evaporation of
the concentrated acid.30 Installation of the injection facili-
ties at the four Cherokee units was made to cope with high resis-
tivities—in excess of 1 x 10ll ohm cm at temperatures of 290°F
and above—that are typical of fly ash from the coal normally
burned in the Cherokee Station (mined in Routt County, Colorado).
During this program, studies of the Cherokee system for SO3
injection were made only at Unit 2. This unit, which has a rated
capacity of 110 MW, includes both a mechanical collector and an
electrostatic precipitator for fly-ash removal at temperatures
averaging about 290°F. The S03 is pumped to the injection site
at a concentration of about 9% by volume in air at an elevated
temperature of undetermined value and injected through a manifold
between the mechanical and electrostatic collectors.* This mani-
fold consists of 42 injection lines inserted into the wall of the
duct in a rectangular grid roughly 20 ft in the vertical direction
(the direction of flow of the flue gases) and roughly 25 ft in
the horizontal direction. Presumably, each injection line leads
to a number of ports in a horizontal plane across the path of the
flue gases, but the geometry of the injection system within the
duct cannot be stated with certainty. Each of the injection lines
is fitted with a valve that should permit uniform injection of the
SOa if the opening of the valve is properly adjusted or, in other
words, permit compensation for non-uniform flow of flue gases in
various cross-sections of the duct.
The general characteristics of injection systems for SO3 or
Sulfan are discussed in detail in Section VIII.
SOUTHERN RESEARCH INSTITUTE
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Although the conditioning agent passes through the injection
lines in dry air as S03 vapor, it must be rapidly and quantitative-
ly converted to H2S04 vapor on entering the duct, where it reacts
with excess H20 vapor at a comparatively low temperature of 290°F.
The injection system has sufficient capacity to produce an H2S(\
concentration in the duct in excess of 20 ppm by volume with the
unit operating at full load, with flue gases being produced at the
rate of about 500,000 ft3/min (here, the concentration and the
flow rate are expressed on an actual basis—:L.e., with H20 vapor
present at the nominal temperature of 290°F and" the ambient
pressure of 0.83 atm).
Our studies were made at Cherokee Unit 2 on July 6 and 7,
1971, soon after the injection system had been placed in operation
and before the owner company had performed enough tests to realize
optimum performance. At the time of our visit, the optimum rate
of S03 injection was believed to be that used to produce 15 to
20 ppm of H2S(\ in the duct. However, this range now appears to
be too high on the basis of the experimental data given in the
following sections.
2. Results of conditioning studies
a. Analyses of the coal
Composite samples of coal being fed to the boiler of Cherokee
Unit 2 on July 6 and 7 were analyzed for total sulfur and ash. The
results are listed in Table 4. The results for the two dates show
good agreement in sulfur percentages but a substantial difference
in ash percentages. The discrepancy in ash concentrations is of
little concern, however, because most of the data relative to gas
conditioning were taken on July 7.
The data for the coal sample collected on July 7 are repre-
sentative values for coal from the source being used (Routt County,
Colorado), based on analyses reported by the Bureau of Mines 3 J•3 2
and on our analysis from an investigation.at Cherokee Unit 3 in
October, 1970 (see Section IV.A.I).
Table 4. Sulfur and Ash Contents of Coal Burned at
Cherokee Unit 2
Concentration, %
Component July 6July 7
Sulfur 0.60 0.62
Ash 12.2 7.9
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Most of the sulfur in Routt County coal is distributed between the
pyritic and organic forms, as shown by the analyses given later.
Data published by the Bureau of Mines and cited in Section IV.A.I
of this report show that strongly basic oxides, such as CaO, repre-
sent about 10% of the ash in Routt County coal, whereas the three
most common weakly basic and acidic substances—A1203, Fe203/ and
Si02—represent about 80% of the ash in this coal.
b. Resistivity of the fly ash
All of the resistivity data for Cherokee Unit 2 were taken
on July 7 with the point-plane probe inserted in a port immediate-
ly ahead of the electrostatic precipitator.
Attempts to determine the resistivities of the initial series
of collected samples with a high-voltage power supply, as described
in Section II.B, were completely unsuccessful, in view of evident
electrical breakdown in the collected samples at electric fields
lower than 1.0 kV/cm. It was then learned that faulty combustion
conditions in the boiler were producing an unusually high percentage
of unburned carbon in the ash, of the order of 5% by weight rather
than the usual 0.5% by weight or less. It was assumed that an
even higher volume percentage of carbon in the ash (expected on
the basis of reported differences in density of unburned carbon
and fly ash)33 must have existed and the large volume percentage
of highly conductive carbon particles caused electrical breakdown
at unusually low applied fields.
The only available means of overcoming the above difficulty
was to use an electrometer (Keithley Model 6IOC) as both the
voltage source and the current-measuring device. With this appara-
tus, the maximum applied voltage was only 0.01 kV and the maximum
electric field produced in the samples was only of the order of
0.1 kV/cm or less. The resistivity data, therefore, cannot be
directly compared with data from other plants, where resistivities
are reported for substantially higher fields.
The resistivity data obtained without S03 injection and with
S03 injection to produce estimated concentrations of 13 and 27 ppm
in the duct are listed in Table 5. The concentrations of injected
S03 were calculated from the evaporation rates of S03 and the flow
rate of flue gases determined by the utility company during our
study. Calculations from the available data led to results that
were approximately 30% higher than the concentrations of 10 and
20 ppm originally requested before our experiments were started.
SOUTHERN RESEARCH INSTITUTE
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Despite the difficulties encountered in obtaining resistivity
values under the desired conditions, the data nevertheless show
that either concentration of injected S03 lowered the resistivity
of the fly ash from a value in excess of 1 x 10ll ohm to a value
around 1 x 108 or less. There was no apparent gain in effectiveness
of the conditioning agent on raising the concentration from 13 to
27 ppm. It is probable that even less than 5 ppm would have been
entirely adequate for conditioning the ash to the extent required.
It would obviously have been desirable to determine resistivity
values at S03 concentrations below 13 ppm; however, it was not
possible to make these determinations because a failure in the S03
injection system caused an extended interruption in its use.
Table 5. Electrical Resistivity
of Fly Ash at Cherokee Unit 2
Estimated
concn of injected Resistivity,3
SO 3, ppm ohm cm
0 1.6 x 1011
13 1.4 x 107
13 5.6 x 107
27 2.5 x 107
27 3.8 x 107
27 5.0 x 107
a. Determined with the point-plane probe with an
electric field of 0.1 kV/cm or less at a temper-
ature of 290°F (July 7, 1971).
The effectiveness of the S03 for conditioning the ash was
confirmed by readings of the secondary voltages and currents of
the precipitator power supply. It was confirmed especially by a
comparison of the voltages and currents in the power unit supply-
ing the inlet section of the precipitator. With no injection,
observed electrical values were 37 kV and 80 mA. With 27 ppm of
injected S03, the corresponding set of electrical values was 41 kV
and 720 mA. On injection of the conditioning agent, the spark
rate in the inlet section decreased from 25 to 5 sparks/min.
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c. Chemical properties of the ash
Table 6 gives a summary of analytical data for individual
fly-ash samples collected at the entrance of the electrostatic
precipitator with the point-plane probe. This table also gives
information about analytical properties of other ash samples col-
lected with a cyclone device, one collected at the entrance of the
precipitator and one collected near the entrance to the air heater
(at a much higher temperature, upstream from the point of S03
injection). The column headings for the chemical properties are
to be interpreted on the basis of information given in Section
II.D. and in the footnote referring to the percentage of unburned
carbon.
The following points are the most significant observations
to be made:
• In the absence of any effect from injected S03 con-
ditioning agent, the flow of the ash through the air
heater and the mechanical collector caused
—a decrease in pH from a moderately basic value
(10.1) to an approximately neutral value (ca. 7)
—an increase in the weight percentage of total
sulfate from approximately 0.3 to 1.0%
—an increase in weight percentage of free HjSO^
from about 0.01 to 0.05%
—a reduction in the weight percentage of unburned
carbon by about 11 to 4%.
These effects must have resulted principally from
the change in particle-size distribution that occurred
during the loss of the larger particles in the mechani-
cal collector. However, they may have resulted, in
part, from interaction of the fly ash with the small
concentration of naturally available S03, which was
found at a concentration of 2 ppm ahead of the air
heater (see the following section).
•With SO3 injected at increasing levels, changes in
the chemical properties of the ash consisted of
SOUTHERN RESEARCH INSTITUTE
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Table 6. Chemical Properties of Fly-Ash Samples
Collected at Cherokee Unit 2
Estimated concn
Sampling
location3
AH
EP
EP
EP
EP
EP
EP
EP
EP
Temp,
oF
695
290
290
290
290
290
290
290
290
of injected
SO 3 / ppm
0
0
0
13
13
13
27
27
27
Sample
collector*
P
P
P
P
c
P
P
P
Concn of
fly ash,c
gr/ft3
2.60
0.96
0.96
0.96
0.96
0.96
0.96
0.96
0.96
Chemical properties of fly ash
Soluble SO.,
10.1
7.3
6.6
6.8
6.9
6.9
7.1
6.7
7.4
Total
0.29
0.97
1.28
1.42
1.51
1.53
1.85
1.76
1.89
H2SO,»
0.01
0.05
0.05
0.10
0.10
0.19
0.13
0.11
C,
11.1
4.3
4.4
5.2
3.5
a. AH signifies a location before the air heater, and EP signifies a location before
the precipitator. The date of sampling was July 7, 1971.
b. C signifies a cyclone collector, and P signifies the point-plane probe.
c. Expressed for "standard" conditions—dry gases at 32°F and 1 atm. Values at the
entrance to the precipitator are based on data obtained by the utility company on
July 7. The value at the entrance to the air heater is estimated from a published
correlation of ash concentrations in the coal and the dust,31* although it is
inconsistent with the utility company's estimate of an efficiency of 80% in ash
removal in the mechanical collector
d. Determined on the basis of the quantity of CO2 produced from unburned carbon
particles in a C-H-N elemental analyzer.
i
OJ
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—consistently increasing concentrations of total
so,,-2
—generally increasing concentrations of free E2SOlt,
The increases of about 0.4 and 0.8% in the SO,,"2
content of fly ash at the ash concentration of
0.96 gr/ft3 correspond to losses of about 1 and 2
ppm, respectively, in the injected concentrations
of S03/ which are small in comparison with original
concentrations of 13 and 27 ppm, respectively.
d. Concentrations of flue gases
Table 7 gives a summary of the observed concentrations of
flue gases on two dates, July 6 and July 7. The principal obser-
vations to be made from the analytical data given in this table
are as follows:
• The concentration of H20 was approximately constant,
averaging 7.4% by volume.
• The concentration of S03 in the absence of injected
conditioning agent ranged from 1 to 3 ppm. The lowest
value, 1 ppm, was observed at the precipitator on July
6 when, unfortunately, no resistivity values could be
obtained because of the previous described problem
from unburned carbon in the ash. A somewhat higher
value, 3 ppm, was observed at this location the next
day; this value may not be as representative of the
value normally to be found at this location, since it
was not absolutely certain that the S03 injection line
had cleared before sample was collected. It was neces-
sary to collect the sample without injection on July 7
after all-day injection of S03; the time of sampling
was deliberately delayed for nearly 1 hr after the
injection equipment had been shut off. However, con-
tinued flow of air through the injection lines may
have caused continued injection of S03 at a low level
during the sampling, even though the plant personnel
considered a delay of 1 hr sufficient. The value of
2 ppm ahead of the air heater on July 7 would not have
been influenced by the injection of S03, which occurred
at a location downstream from the air heater.
• Two results for the concentration of S03 during in-
jection of the agent at the lower concentration (11 and
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Table 7. Concentrations of Flue Gases
at Cherokee Unit 2
Estimated concn
Sampling^ Temp, of injected
Concentrations of
flue gases
Date
July 6
July 7
location
EP
EP
EP
AH
AH
EP
EP
EP
EP
EP
EP
EP
oF
290
290
290
695
695
290
290
290
290
290
290
290
SO 3 , ppm
0
0
0
0
0
0
13
13
13
27
27
27
H20, % SO 3, ppm
7.3 1
1
1
2
2
_ o
7.4
12
11
7.5
26
23
SO 2 , ppm
432
422
406
507
512
473
_
452
442
—
460
420
a. AH signifies a location before the air heater, and EP
signifies a location before the precipitator.
b. The concentrations listed are the results of individual
experiments and are expressed on the "wet" or actual
basis. The concentration of 02 recorded by the utility
company was in the range from 4 to 5%.
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12 ppm) were slightly below the estimated concentra-
tion of injected agent (13 ppm), as expected from the
small amount of agent collected on the fly ash. The
concentrations found during injection at the higher
concentration (23 and 26 ppm) were also only slightly
lower than the estimated concentration of injected
agent (27 ppm), again as expected from the fly-ash
analyses.
• The observed concentrations of S02 fluctuated through
a range of nearly 100 ppm. The variations in SOz
concentration are believed to reflect actual varia-
tions, primarily as a result of periodic variations
in the composition of the coal being burned in the
boiler. (Simultaneous determinations of SOz by our
method and by either a similar method employed by
other investigators or a method consisting of con-
tinuous instrumental monitoring leads to the con-
clusion that errors in our method would not be as
large as 100 ppm in the 400 to 500 ppm range.)
As noted above, our data for S03 concentrations indicated
that most of the injected SO3 remained in the gas phase. Determi-
nations of SO3 at the precipitator outlet by chemists of the
utility company confirmed that this phenomenon occurred. A concen-
tration of 23 to 24 ppm was found at the outlet during injection
of SO3 at an estimated concentration of 27 ppm, confirming that an
unnecessarily high concentration of SO3 was injected. A lower,
more acceptable emission of SO3 would be expected with the injected
concentration lowered, as seems desirable.
B. X Station, Unit 4
1. Description of the power unit
The power station referred to by the code letter X cannot be
identified by actual name or location in view of an agreement
reached with the owner company. All of the information pertaining
to this plant was provided by a representative of the company;
none of the data given subsequently in this report was obtained
by our staff and, thus, the reliability of the data cannot be
vouched for. Of necessity, a full description of the plant cannot
be given, but the most important features are described below.
The power unit of the X station discussed here employs only
an electrostatic precipitator for the removal of fly ash. The
operating temperature of the precipitator was reported to be 230°P
during the tests with SO3 injection. This temperature would
normally be considered low enough for the HzO vapor in the flue
gases to serve adequately by itself as a conditioning agent;
however, the reported fly-ash resistivity of unconditioned ash at
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230°F was 4.5 x 1011 ohm cm and thus above the desired maximum
value of 2 x 1010 ohm cm.
Details of the S03 injection system were described only as
follows: Sulfan was evaporated in a stream of preheated dried
air with a temperature of 700°F and a dew point below -40°F, but
it was injected in the duct at a much lower temperature of 300°F.
Unquestionably, the SO, reacted with H20 vapor to produce H2S(\
vapor on entering the duct, and a large part of the HjSO^ may
have condensed with additional H20 vapor to produce a mist of
H2SC\ and H20 unless acid collection on the ash occurred at a
faster rate than the condensation process (see Section VII.B.2).
2. Results of conditioning tests
Some of the results produced during the use of S03 as a
conditioning agent are shown in Table 8. These are the only
reported data showing the effects of SO3; no information indi-
cating changes in flue-gas composition was made available.
However, it can be stated that the ash was treated with S03 at an
unusually high concentration, 7.5 gr/ft3 (expressed in terms of
gas volume under standard rather than actual conditions of temper-
ature and pressure) and, furthermore, the untreated ash was highly
basic, producing a slurry pH of about 11 in an aqueous slurry.
(The pH produced by a fly-ash sample from X station was deter-
mined in our laboratory by the usual procedure and found to be
11.7; we do not know, however, whether this sample was repre-
sentative of the ash that was subjected to SO3 conditioning.)
Table 8. Effects of SO3 Conditioning
Agent on the Resistivity and Sulfate
Content of Fly Ash at X Station
Injected concn Resistivity,a SO^"2,
of SQ3, ppm ohm cm %
0 4.5 x 1011 0.17
10 2.3 x 1010 0.31
18-20 7.0 x 109 0.38
a. Determined with a point-plane apparatus at an unspeci-
fied electric field.
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IV. STUDIES OF POWER PLANTS EQUIPPED WITH FACILITIES
FOR THE INJECTION OF VAPORS FROM LIQUID H2SQi»
A. Cherokee Station, Unit 3
1. Description of the plant facilities*
As previously stated, Unit 3 of the Cherokee Station, with a
rated production capacity of 165 MW, has facilities for the injec-
tion of vapors from liquid HaSOi,, whereas Units 1, 2, and 4 have a
common SOs injection system.30 The injection system was installed
by Western Precipitator Division of Joy Manufacturing Company and
placed in operation in August or September of 1970. As originally
installed (and also as now operated), the system injected the acid
vapors immediately ahead of the mechanical collector and thus sub-
jected all of the fly ash from the boiler to conditioning, unlike
the SO3 system at Cherokee Unit 2. During November, 1970, however,
the system was temporarily modified to inject the conditioning
agent between the mechanical collector and the precipitator and
thus treat only the relatively small fly-ash particles that cannot
be removed mechanically. The temperature of flue gases through the
mechanical and electrostatic collectors ranges from about 300 to
310°F.
The source of acid vapors in the Cherokee Unit 3 conditioning
system is commercial 66°Be H2SO., (containing 93.2% of HzSOi* and
6.8% of HaO by weight35). The acid is evaporated in a stream of
hot combustion gases that are produced by burning natural gas with
sufficient excess air to produce the following approximate gas con-
centrations: 3 ° C02, 2%; H20, 5%; 02, 16%; and N2, 77%. Evapora-
tion of the acid reportedly occurs within the temperature range of
700 to 1000°F,30 which is high enough to decompose the HzSOt, exten-
sively to SOa and H20 but to avoid decomposition of the SOs to SOz
(see Section VIII.C).
The injection system includes a manifold of about 8 injection
lines inserted into the duct at regular intervals in a line across
the direction of flow of flue gases. The nature of the distribu-
tion system for injected vapors inside the duct is not known. The
volume ratio of carrier gases for the conditioning agent to flue
gases in the duct, however, is reportedly about 1:770.30 Because
Most of the descriptive information given here is based on pub-
lished literature as cited, not on information disclosed by the
utility company or the contractor for the injection facilities.
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the agent can be injected to produce a concentration of condition-
ing agent in the duct in excess of 40 ppra, the combined concentra-
tions of SOs and HaSCK in the injection lines must exceed 30,000
ppm on occasion. From considerations of the dew point of the HaO-
SOs-HaSOit mixture and the maximum absolute pressure in the injec-
tion line (approximately 0.98 atm) the temperature of the injection
must be at least 400°F to avoid loss of conditioning agent by
condensation before injection occurs (see Section VIII.C). Conver-
sion of the SOs in the injection lines to HzSOt, presumably occurs
rapidly following entrance into the duct, with an excess of HaO
vapor and at a temperature of only 300 to 310°F.
Our work on the site of Cherokee Unit 3 was performed during
the period from October 13 to 19, 1970, when the acid vapors were
injected ahead of the mechanical collector as at present. However,
supplemental studies were conducted with samples collected by
Public Service Company personnel both before and after the site of
injection was moved temporarily to the duct between the mechanical
and electrostatic collectors during the following month.
2. Results of conditioning studies
a. Analyses of the coal
Table 9 gives the results of analyses of samples of the coal
being fed to the boiler of Cherokee Unit 3 on October 17 and 19
when two series of determinations of fly-ash resistivity were made.
This table indicates that the coal contained approximately 0.5% of
sulfur and 8.6% of ash on both dates. The composition was only
slightly different from the composition of coal from the same
source (Routt County, Colorado) found at Cherokee Unit 2 on another
occasion, as previously discussed in Section III.A.I.
Table 9. Sulfur and Ash Contents
of Coal Burned at Cherokee Unit 3
Concentration, %
Component October 17 October 19
Sulfur
as sulfate 0.02
as pyritic sulfur 0.12
as organic sulfur 0.33
Total 0.47
Ash 8.72
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Table 10 gives the concentrations of individual components
of the ash sample prepared during the determination of the concen-
tration of ash in the coal collected on October 19 and shows the
high ratio of weakly basic and acid substances to strongly basic
substances that is characteristic of Routt County coal. The data
in Table 10 were determined by fusing the ash sample with NaC03;
cooling the melt and neutralizing it with HC1; and analyzing the
residue for Si02 gravimetrically,36 Fe203 and CaO by titration
procedures with EDTA,37 MgO and A1203 by atomic absorption spec-
troscopy,38 and K20 by flame photometry.39
Table 10. Components of the Ash
in the Coal Burned at Cherokee Unit 3
Component Concentration, %
Si02
A1203
Fe203
CaO
MgO
K20
Subtotal3 91.8
a. Other components would include S03 and numerous rela-
tively minor components such as Ti02, P2°5» and Na20.
b. Resistivity of the fly ash
Resistivities of fly ash at the entrance of the precipitator
at Cherokee Unit 3 were determined on two dates, October 17 and
19. In the first series of measurements, a mixture of coal and
natural gas was used as the fuel in the boiler, with the natural
gas representing about 25% of the fuel value. In the second series
of measurements, only coal was used as the fuel.
Determinations of resistivity were made with the two types
of cyclone probes described in Section II.B with an average
electric field of 2.0 to 2.5 kV/cm in each sample. Data were
obtained both with and without H2S04 injection, with concentrations
of the conditioning agent reportedly as high as 33 and 44 ppm dur-
ina the two series of determinations.
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Table 11 summarizes the results of the resistivity deter-
minations. This table shows that surprisingly little change in
resistivity was produced during the injection of H2SOn. ,In either
series of measurements, the largest reduction observed was less
than one order of magnitude, despite the high concentrations of
H2SOi» that were reportedly injected.
Table 11. Electrical Resistivity
of Fly Ash at Cherokee Unit 3
(Validity questionable as discussed in text)
Temp, Reported concn of Resistivity,b
Datea °F injected HzSOn, ppm ohm cm
Oct. 17 300 0 2.0 x 1012
6 9.3 x 101l
17 7.8 x 1011
26 5.6 x 101l
33 5.0 x 101l
Oct. 19 310 0 1.0 x 1012
13 1.0 x 1012
26 6.0 x 101l
33 6.3 x 101 l
44 6.6 x 101l
a. On October 17, a mixture of coal and natural
gas was used as the fuel. On October 19, only
coal was used as the fuel.
b. Determined with cyclone probes with an average
electric field of 2.0 to 2.5 kV/cm.
c. Chemical properties of ash samples collected from the
ducts ahead of the air heater and the precipitator
Table 12 lists the results of chemical analyses of fly-ash
samples collected with cyclone probes in sampling ports ahead of
the air heater and ahead of the electrostatic precipitator during
conditioning tests with coal only as the fuel.
The data obtained in the absence of conditioning agent show
evidence of a reduction in the basicity of the ash and an increase
in the sulfate content of the ash as the sampling point was moved
from the location ahead of the air heater to the location ahead
of the precipitator. As at Cherokee Unit 2, these changes must
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Table 12. Chemical Properties of Fly-Ash Samples
Collected from Ducts at Cherokee Unit 3
Reported concn Concn of Median
Chemical properties of fly ash
c
-i
71
m
in
Sampling
Temp, of injected
location3 °F
AH
EP
EP
EP
EP
EP
740
310
310
310
310
310
H2SO.» , ppm
0
0
13
26
33
44
fly
ash,b
gr/ft»
2
0
0
0
0
0
.6
.4
.4
.4
.4
.4
particle
size, ym
2.0
1.5
1.5
1.5
1.5
1.5
Soluble base
pH
10.5
10.0
9.9
9.9
9.8
9.5
as CaO, %
0.
0.
0.
0.
0.
0.
65
35
34
31
28
25
Soluble SO.,"2, %
Total . H2SO.,c
0.
0.
0.
- 1.
1.
1.
57
77 0.01
90
09
00
12 0.02
I
to
1
a. AH signifies a location before the air heater, and EP signifies a location before the
precipitator. The date of sampling was October 19, 1970, with coal alone as the fuel.
Cyclone probes were used as the sample collectors.
b. Expressed for "standard" conditions. Estimated at the air heater from the ash content
of the coal and at the precipitator from data supplied by the utility company.
c. Determined after several months of sample storage in the laboratory and thus perhaps
lower than those immediately after collection.
O
I
a>
H
-I
C
-I
PI
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have been the result principally of a change in particle-size
distribution across the mechanical collector; however, they may
have been caused, in part, by interaction of the fly ash with S03
to convert part of the basic components of the ash to the corre-
sponding SO^"2 salts.
The data obtained at the precipitator with and without
injection of the conditioning agent show further decreases in the
basicity of the ash and increases in the S04~2 content as the
concentration of the conditioning agent was increased, indicating
that the ash collected at least part of the conditioning agent,
even though only small reductions in resistivity were observed as
a result.
d. Concentrations of flue gases
Table 13 gives the results of flue-gas analyses with and
without injection of conditioning agent during the tests with both
coal and natural gas as fuels and with only coal as the fuel.
The data in Table 13 that are of primary interest are the
concentrations of S03. With up to 33 or 44 ppm of the condition-
ing agent added, increases of only 1 to 2 ppm of S03 were observed.
These very small changes are in marked contrast to the large
changes observed at Cherokee Unit 2 during S03 injection.
The other data in Table 13 require only brief comment. The
S02 concentration was only about 220 during burning of both coal
and gas and about 350 ppm during burning of coal alone. This
difference is attributable to the low concentration of S02 produced
from natural gas. The value of 350 ppm is about 100 ppm lower than
the typical value observed at Cherokee Unit 2 with Routt County
coal, containing a slightly higher percentage of sulfur.
It is of interest to compare the amounts of conditioning
agent reportedly injected with the amounts accounted for by the
fly-ash and flue-gas analyses. Under the conditions with 44 ppm
reportedly injected, the quantity of SO,,'2 gained by the ash at
the entrance to the precipitator was 0.35% (Table 12). If it is
assumed, that all of the ash exposed to the conditioning agent (ca.
2.6 gr/ft3) gained the same amount of sulfate, the quantity of
conditioning agent accounted for on the ash by a gain of 0.35%
is approximately 5 ppm. Combination of this value with the gain
in S03 found in the gas phase, approximately 2 ppm, gives a total
of only 7 ppm, less than 20% of the concentration reportedly in-
jected. In contrast, the results obtained at Cherokee Unit 2 ac-
counted for essentially 100% of each concentration of S03 injected.
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at
O
Table 13. Concentrations of Flue Gases
at Cherokee Unit 3
Sampling Temp,
Datea location13 °F
Oct. 13 AH 740
Oct. 17 EP 300
Reported concn
of injected
HzSOit, ppm.
0
0
6
17
26
33
Concentrations of flue cfasesc
HaO, % SO3/ ppm
2
9.5 <1
- <1
<1
- <1
2
SO 2 i ppm NOx / ppm
226
222 415
222
210
217
216
Oct. 16 AH 740 0 <1 314
Oct. 19 EP 310 0-1 358
13 7.7 1 357 392
26 - <1 349
33-2 349
44 8.4 3 357
a. On October 13 and 17, a mixture of coal and natural gas was burned in the
5 boiler. On October 16 and 19, only coal was used as the fuel.
X
n
z b. AH signifies a location before the air heater, and EP signifies a location
before the precipitator.
o c. Expressed on the "wet" or actual basis. The concentration of O2 recorded
1 by the utility company was in the range from 3 to 4%.
in
H
H
C
H
n
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e. Chemical properties of ash samples collected from the
hoppers of the mechanical collector and the electro-
static precipitator
In view of the discrepancies between injected concentrations
of the conditioning agent and calculated concentrations based on
changes in fly-ash and flue-gas compositions, it was of interest
to determine whether the fly ash deposited in the mechanical col-
lector collected a disproportionately high concentration of the
conditioning agent. Unfortunately, no sample of ash deposited in
the mechanical collector was collected during our visit to Cherokee
Unit 3 during October. In the absence of these samples, we
requested personnel of the utility company to remove samples from
the hoppers of the mechanical collector at a later date and to
submit these samples for analysis along with samples collected
simultaneously from the hoppers of the electrostatic precipitator.
Two groups of samples were thus obtained; two sets were collected
during continued injection of H2SO^ ahead of the mechanical col-
lector and one set was collected during injection between the
mechanical and electrostatic collectors. The results of the analy-
ses are listed in Table 14.
Table 14. Chemical Properties of Fly-Ash Samples
from Hoppers at Cherokee Unit 3
Datea
Nov. 17
Nov. 18
Nov. 23
Reported concn
of injected
H2SOU, ppm
0
40
0
40
0
50
0
50
0
15
0
15
Hopper
location
MC
MC
EP
EP
MC
MC
EP
EP
MC
MC
EP
EP
Chemical properties of fly ash
Soluble base Soluble SOi,-2
pH as CaO.% (total), %
10.4
1.51
0.73
0.51
0.17
0.79
0.59
0.20
0.31
1.17
1.45
0.48
0.56
0.33
0.72
1.58
1.96
0.20
0.41
,04
,00
1,
1,
0.16
0.23
0.87
0.87
a. On November 17 and 18, acid injection occurred ahead of the
mechanical collector. On November 23, it occurred between
the mechanical and electrostatic collectors.
b. MC signifies the mechanical collector; EP signifies the
electrostatic precipitator.
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The data for samples exposed to the conditioning agent ahead
of the mechanical collector on November 17 and 18 lead to conflict-
ing observations. On the first occasion, the ash deposited mech-
anically gained less sulfate during H2SCK injection than that
deposited electrostatically; on the second occasion, the ash in
the hoppers of the mechanical collector showed a slight gain in
sulfate during injection, whereas that in the hoppers of the elec-
trostatic precipitator showed a small decrease or, practically1
speaking, no change of real significance. If it is assumed that
the ash deposited in the mechanical collector on November 17 and
18 corresponded to a concentration in the inlet duct of 2.2 gr/ft3
(an estimate corresponding to the concentrations previously listed) ,
the gains in S0u~2 correspond to concentrations of conditioning
agent of only 1 to 2 ppm, a negligible fraction of the 50 ppm that
was reportedly injected.
The data for samples collected on November 23 with injection
between the mechanical and electrostatic collectors show, practi-
cally speaking, no gain of sulfate in the ash deposited mechani-
cally (as expected) or in the ash deposited electrostatically
(contrary to the expected result) , despite the fact that 15 ppm
was reportedly injected to condition fly ash at an estimated con-
centration of only 0.4 gr/ft3.
Unfortunately, no data on fly-ash resistivity or flue-gas
composition are available for the dates on which the hopper sam-
ples were collected. The gain in S0u~2 content of ash collected
electrostatically on November 17 suggests that little change in
resistivity was effected, because this gain was about the same as
that observed with little change in resistivity on October 19
(Table 12) . The absence of measurable increases in the SOi*"2 con-
tent on either November 18 or 23 implies that no change in resis-
tivity was effected on these dates. It seems reasonable to assume
that negligible increases in the concentration of SOa in the gas
phase occurred on any of the dates in November — especially on
either of the latter two occasions — in view of the fact that our
work in various plants has never shown a measurable increase in
the concentration of SO? during injection of conditioning agent
without a measurable increase in the SOi*"2 content of fly ash.
B. Arapahoe Station, Unit 4
1 . Description of the plant facilities
The Arapahoe Station is the second of two stations in Denver
that are operated with SO? or HaSOi, injection systems. Originally,
only Unit 4 was equipped with acid injection facilities, and this
unit was investigated by our staff on October 21 and 22, 1970,
shortly after the installation of the injection facilities was
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completed by the contractor, Universal Oil Products Company. In
the first half of 1971, however, the acid evaporation system was
enlarged and additional injection lines were installed to permit
acid injection in Units 2 and 3. The remaining unit at Arapahoe,
Unit 1, is now consistently operated with natural gas as the fuel
and thus is not serviced by the present injection system.
Low-sulfur coals from Weld County, Colorado, and Hanna
County, Wyoming, are usually used as fuels in Arapahoe Units 2, 3,
and 4. They produce fly ash of high resistivity, as typified by
our observed value of 3.8 x 1012 ohm cm with unconditioned ash
from Weld County coal at a temperature of 275°F. Acid condition-
ing was considered to be the most expedient solution to the high-
resistivity problem at Arapahoe as at Cherokee.
Owing to a secrecy agreement between the Public Service Com-
pany and Universal Oil Products Company, a complete description of
the acid injection system used at Arapahoe Unit 4 during our field
studies cannot be given in this report. The following, however,
can be stated: The source of the acid vapors was 66°Be HaSOi,, the
carrier gas for the acid vapors from the evaporator was air with
ambient relative humidity, the acid was evaporated at a lower
temperature than at Cherokee Unit 3, and the site of injection was
the duct between the mechanical and electrostatic precipitators.
The acid vapors could be injected in the duct at a sufficient rate
to produce concentrations in the duct above 20 ppm (as HzSOi*) with
the unit at full load, 110 MW, and producing flue gases at a total
rate of about 525,000 ft3/min at 275°F and ambient pressure,
approximately 0-. 83 atm.
2. Results of conditioning studies
a. Analyses of the coal
Analytical results for the coal from Weld County, Colorado,
that was burned during the conditioning studies at Arapahoe Unit 4,
are shown in Tables 15 and 16. The sulfur and ash contents given
in Table 15 are similar to those of Routt County coal, which were
previously given in Tables 4 and 8 in connection with studies at
Cherokee Units 2 and 3. The composition of the ash in the Weld
County coal given in Table 16 is substantially different from that
in the Routt County coal, which was previously given in Table 10.
The most significant differences are that the Weld County coal
contains higher percentages of strongly basic oxides, such as
CaO, and lower percentages of weakly basic or acidic oxides, such
as SiOa and Al20s. The observed differences are consistent with
differences in the compositions reported by the U. S. Bureau of
Mines.31'32
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Table 15. Sulfur and Ash Contents
of Coal Burned at Arapahoe Unit 4
Component Concentration, %
Sulfur
as sulfate
as pyritic sulfur
as organic sulfur
Total
Ash
Table 16. Components of the Ash in
the Coal Burned at Arapahoe Unit 4
Component Concentration, %
Si02
A1203
F62C-3
CaO
MgO
K20
Subtotal3 83.0
a. Other components would include S03
and numerous relatively minor com-
ponents such as Ti02, PaOs, and
b. Resistivity of the fly ash
Data showing resistivity values of the fly ash at Arapahoe
Unit 4 are listed in Table 17. These data were obtained with a
cyclone probe in which an average electric field of 2.0 to 2.5
kV/cm was imposed on each collected sample. The data indicate
that the resistivity was lowered by nearly two orders of magnitude
with only 6 ppm of injected HaSOi* and by only slightly higher
degrees with 12 or 18 ppm of the conditioning agent.
c. Chemical properties of the fly ash
Observed properties of fly-ash samples collected in cyclone
probes in the ducts ahead of the mechanical collector and the
SOUTHERN RESEARCH INSTITUTE
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Table 17. Electrical Resistivity of Fly Ash
at Arapahoe Unit 4
Reported concn of Resistivity,a
injected H2SOU/ ppm ohm cm
0 3.8 x 1012
6 5.7 x 1010
12 3.2 x 1010
18 1.9 x 1010
a. Determined with a cyclone probe with an average
electric field of 2.0 to 2.5 kV/cm at a temperature
of about 275°F (October 21 or 22, 1970).
electrostatic precipitator are listed in Table 18. In comparison
with data previously given for Cherokee Units 2 and 3, the data
in Table 18 indicate that the fly ash at Arapahoe was appreciably
more basic than that at the Cherokee Units and was more comparable
in basicity to the ash at X station. The only evidence that the
available base was neutralized by the conditioning agent was found
with the highest injected concentration of H2SO^, 18 ppm. Yet, on
the other hand, the SO,,"2 content of the ash increased consistent-
ly as the injected concentration of H2SO^ increased.
A possible explanation for relatively constant values of pH
and concentrations of soluble base, despite increasing concen-
trations of S0^~2, is that the fly ash contained enough base in
excess of the collected H2SO^ to saturate the aqueous phase of
the slurry at the concentration of 3%. In other words, it is possi-
ble that all of the base did not dissolve with the proportions of
ash and water used.
The high basicity of the Arapahoe ash compared with the
properties of the Cherokee ash are consistent with the marked
differences in CaO concentrations of the ash fractions noted previ-
ously in discussions of the different coals burned at the two plants,
d. Concentrations of flue gases
Concentrations of flue gases found at Arapahoe Unit 4 are
listed in Table 19. The data of primary interest—the concen-
trations of S03—show a maximum increase of 2 ppm with 18 ppm of
-------
c
X
ni
a
z
a
m
in
m
n
z
10
-I
H
C
H
m
Table 18. Chemical Properties of Fly-Ash Samples
Collected at Arapahoe Unit 4
Reported concn Concn of Median
Chemical properties of fly ash
Sampling Temp, of injected
location3 °F H2SOi,, ppm
AH
EP
EP
EP
EP
740
275
275
275
275
0
0
6
12
18
fly ash,13
gr/ft3
2
0
0
0
0
.3
.5
.5
.5
.5
particle
size, ym
1.7
1.5
1.5
1.5
1.5
Soluble base
. pH as CaO, %
11.1
11.1
11.1
11.0
10.8
1
2
2
2
1
.93
.10
.10
.14
.62
Soluble SO., ~2, %
Total H2SOi,
1
1
2
2
2
.00
.50 nilc
.23
.50
.97 nilc
i
en
M
1
a. AH signifies a location before the air heater, and EP signifies a location before the
precipitator. The dates of sampling were October 21 and 22, 1970. Cyclone probes were
used as the sample collectors.
b. Expressed for "standard" conditions. Estimated at the air heater from the ash content
of the coal and at the precipitator from data supplied by the utility company.
c. Reported as "nil" in view of the absence of a pH change in an ethanol-water slurry.
Determined after several months of storage in the laboratory and thus not necessarily
representative of the value at the time of sample collection.
-------
Table 19. Concentrations of Flue Gases
at Arapahoe Unit 4
Reported concn
Sampling Temp, of injected Concentrations of flue gases"
% SO 3 / ppm SO 2 , ppm NOX , ppm
location3
AH
EP
EP
EP
EP
op
740
275
275
275
275
H 2 SO i* , ppm
0
0
6
12
18
<1 416
8.9 <1 387
1 446
8.8 2 413
2 430 626
a. AH signifies a location before the air heater, and EP signifies a
location before the precipitator. The dates of sampling were
October 21 and 22, 1970.
b. Expressed on the "wet" or actual basis. The concentration of 02
recorded by the utility company was usually in the range from
4 to 5%.
,
en
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H2SC\ injected. This increase is comparable to that found at
Cherokee Unit 3 with 44 ppm of H20 reportedly injected but much
lower than that to be expected at Cherokee Unit 2 with 18 ppm of
S03 injected, based on data at higher and lower concentrations of
SO 3.
It is of interest to compare the concentrations of H2S(\
reported injected at Arapahoe Unit 4 with the sums of the concen-
trations found as increases in the SOi,"2 content of the fly ash
and increases in the S03 content of the gas phase. Table 20 lists
data permitting this comparison, assuming that fly ash entering
the precipitator at an estimated concentration of 0.5 gr/ft3 uni-
formly increased in SO^'2 content by the percentages indicated in
Table 18. The comparison shows that, of the reported concentration
of injected H2SO^, the percentage accounted for ranged from 33 to
50%, substantially higher than the value calculated for Cherokee
Unit 3 (20%) but lower than the values observed at Cherokee Unit 2
(essentially 100%).
Table 20. Reported Concentrations of Injected E^O^ and
Equivalent Observed Concentrations in the Fly Ash
and Flue Gases at Arapahoe Unit 4
Calculated concn based on
Reported concn of sample analyses, ppm
injected H2SOit, ppm Fly ash Flue Gases Total
12 325
18 426
C. .Y Station, Unit 6
1. Description of the plant facilities
The Y Station, like the X Station previously discussed,
cannot be identified by actual name, location, or owner company.
However, the information presented here about the Y Station—unlike
that pertaining to the X Station—was obtained during an on-site
investigation by our staff and during subsequent laboratory analy-
ses of samples collected at the plant.
The Y Station has a rated power-production capacity of about
140 MW. It is equipped only with an electrostatic precipitator
for the removal of fly ash, and the operating temperature of the
SOUTHERN RESEARCH INSTITUTE
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precipitator is approximately 320°F. The vapors from concentrated
H2S(\ are injected in the duct entering the precipitator through
a grid of 24 uniformly-spaced injection ports that are located in
a plane across the path of flow of flue gases; each of the in-
jection ports is an open 0.125-in. pipe and is fed with condition-
ing agent through a series of branched injection lines that origi-
nate at a central acid evaporator.
The contractor that installed the injection system cannot
be identified by name in this report. However, the basic features
of the injection system can be described in fairly specific terms
as follows: The source of the acid vapors is 66°Be HjSO^, and
the production of the vapors occurs with co-current flows of the
liquid acid and preheated ambient air through a bed in which the
maximum inlet-air temperature is about 500°F and the minimum
outlet-gas temperature is about 400°F. Thus, the injection system
operates at substantially lower temperatures than that at Cherokee
Unit 3; it is of the type referred to as a "low-temperature" H2S(\
system and discussed in Section VIII. The acid vapors pass
through the injection lines principally as H2SO,,; the balance
present as S03 presumably reacts rapidly with excess H20 vapor to
produce additional H2SO^ on entering the duct and cooling to 320°F.
The fairly simple geometry of the duct between the injection
ports and the sampling ports for fly-ash and flue-gas samples at
the Y Station permitted a reasonably accurate estimate of the
contact time of the conditioning agent and the fly ash between in-
jection and sampling. In some of the experiments with the power
unit operating a full load, the contact time was about 1 sec; in
other experiments with the unit at half load, the contact time
was approximately 2 sec. The contact time of 1 sec was probably
the minimum in all of the plants investigated, although the contact
times in the other plants could not, as a rule, be estimated with
comparable accuracy.
The capacity of the HjSO^ evaporator limited the injected
concentration of conditioning agent to a value of 8 ppm with the
boiler at full load, producing a flow of flue gas of approximate-
ly 500,000 ft3/min (expressed on the basis of water present for
the temperature of 320°F and the ambient pressure of approximate-
ly 1 atm) . With the H2S0lf operating at full capacity and the boiler
at half load, however, an injected concentration of 16 ppm was
reached during some of our experiments.
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2. Results of conditioning studies
a* Analyses of the coal
The results of analyses of coal samples collected on the
two dates of our investigation at the Y station (March 1 and 2,
1971) are given in Table 21. The data in this table indicate that
the sulfur percentage averaged about 0.6% and the ash percentage
averaged about 12% and did not differ appreciably on the two dates.
Table 21. Sulfur and Ash Contents
of Coal Burned at Y Station
Concentration! %
Component March 1 March 2
Sulfur 0.64 0.59
Ash 12.34 11.34
No effort was made to determine the distribution of sulfur
in various forms or to determine the composition of the ash. Other
data obtained during analyses of the fly ash produced in the
absence of conditioning agent indicated, however, that the ash
was virtually neutral in terms of the relative concentrations of
soluble bases and acids.
b. Resistivity of the fly ash
Fly-ash samples to be used for determinations of electrical
resistivity were collected at the entrance of the electrostatic
precipitator with two devices, the cyclone and point-plane probes
described in Section II.B. The electric field during all of the
measurements with the cyclone sampler was 2.5 kV/cm. The electric
field during various measurements with the point-plane sampler,
on the other hand, varied from 2.1 to 3.7 kV/cm. This variation
was the result of applying a voltage of either 0.1 or 0.5 kV across
samples that ranged in thickness from 0.04 to 0.23 cm in thickness.
The data given in Table 22 indicate that the injection of
either 4 or 8 ppm of I^SOi, vapor lowered the resistivity from
2 x 1012 ohm cm to about 1 x 10ll ohm cm and that injection of
16 ppm caused further lowering to values in the approximate range
from 1 x 109 to 1 x 10l° ohm cm. It is not known why 4 and 8 ppm
of the conditioning agent produced about the same resistivity
change; it is possible that the explanation lies in the fact that
experiments with 4 ppm were carried out one day and the other
experiments with 0, 8, 16 ppm were carried out the following day
SOUTHERN RESEARCH INSTITUTE
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(the coal analyses for the two days of experimentation do not give
much basis for this hypothesis, however) . The data obtained with
the cyclone and point-plane probes were in good agreement for 4
and 8 ppm of conditioning agent but differed by roughly a factor
of 10 with 16 ppm injected. The disparity in the data at 16 ppm
may have been the result, in part, of an average higher electric
field in the point-plane samples (3.4 kV/cm, compared with 2.5
kV/cm in the cyclone samples) .
Table 22. Electrical Resistivity of Fly Ash
at Y Station
Reported concn
of injected
HaSOi, ppm
0
4
8
16
Sampling
device^
C
C
C
P
C
P
C
P
C
P
C
P
C
P
C
Electric
field, kV/cm
2.5
2.5
2.5
2.9
2.5
2.4
2.5
2.8
2.5
2.1
2.5
3.1C
2.5
3.7C
2.3
Resistivity
ohm cm
2.0 x
2.0 x
1012
1012
1.2 x 1011
0.7 x 1011
1.8 x 1011
2.0 x 1011
1.5 x 1011
1.4 x 1011
1.3 x 101l
2.1 x 1011
1.6 x 1011
1.1 x 1010
0.1 x 1010
2.5 x 1010
0.2 x 101°
C and P indicate cyclone and point-plane sampling
probes, respectively.
Determined with different resistivity probes and
slightly varying fields, as indicated by second and
third columns, at a temperature of 320°F. Listed
in the sequence as actually determined with 8 ppm
of H2SOi» on March 1 and with 0, 4, or 16 ppm of
H2SOi, on March 2.
In excess of 3.0kV/cm and thus in excess of any of
the other electric fields, simply as a result of
the thickness of the sample collected and the volt-
age applied across the sample.
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c. Chemical properties of the fly ash
Table 23 summarizes the data showing the chemical properties
of the fly ash collected by different methods both in the absence
and presence of conditioning agent.
Table 23. Chemical Properties of Fly-Ash Samples
Collected at Y Station
Reported
concn of
injected
Sampling Temp H^O^,
locationa °F ppm
AH >600 0
EP 320 0
4
8
16
Chemical properties0 of
Sampling
device"
F
C
C
P
C
P
C
P
PH
Min.
-d
-d
4.7
4.6
4.5
4.6
4.4
4.1
Eq'm.
6.9
8.1
5.7
6.5
4.6
5.6
4.6
4.2
Soluble
Total
0.27
0.24
0.35
0.29
0.41
0.34
0.36
0.51
fly ash
scu-2, %
HaSCK
-
<0.01
_
—
_
-
0.01
AH signifies a location before the air heater, and EP signi-
fies a location before the precipitator. The dates of sam-
pling were March 1 and 2, 1971. Both cyclone and point-
plane probes were used for collecting samples, as indicated
by the fourth column.
F designates a filtration unit employed by the personnel of
the station, whereas C and P indicate cyclone and point-plane
resistivity probes.
Estimated concentration was 3.3 gr/ft3 (standard conditions).
Number median diameter was ca. 2.0 ym.
No minimum was observed; the recorded value increased con-
tinuously toward equilibrium value.
SOUTHERN RESEARCH INSTITUTE
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One of the most notable features of the analytical data is
that the pH values indicate that the fly ash was essentially
neutral in the absence of conditioning agent but became distinctly
acidic in the presence of the conditioning agent. Minima in the
pH values observed during equilibration of samples with water gave
evidence of a strong-acid layer overlying less acidic or slightly
basic substances in the water-soluble surface layers. Determi-
nations of H2SO^ on two samples approximately 3 months after the
samples were collected gave further evidence that the conditioning
agent produced an H2SO^-containing outer layer on the fly-ash
particles.
Another feature of the analytical data is that the maximum
gain in SOi,"2 caused by the conditioning agent was only about 0.25%
of the fly-ash weight. Unfortunately, the SO,,"2 data do not show
a consistent trend toward higher values as the concentration of
conditioning agent was increased. Our failure to observe the ex-
pected trend is attributed, in part, to variations in the SO,^2
percentage in unconditioned ash and, in part, to differences in
sampling properties of the cyclone and point-plane probes.
d. Concentrations of flue gases
Table 24 summarizes the results of determinations of the
concentrations of H20, S03, and S02 in the flue gases. The H20 and
SO2 values are all in the expected range and show significant vari-
ations only in the S02 concentrations, which indicate that a possi-
ble hour-to-hour variation of significant magnitude occurred in the
sulfur content of the coal. The S03 data show that less than 1 ppm
of-SO-3 was present until conditioning agent was injected and then
the concentration of S03 increased sharply and represented most of
the injected agent at each level of injection. The discrepancy in
the two concentrations, 6 and 11 ppm, found with 8 ppm injected was
larger than expected; it is not known whether the discrepancy was
the result of experimental error or poor control of the injection
rate.
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Table 24. Concentrations of Flue Gases
at Y Station
Reported concn
of injected Concentrations of flue gasesa
H 2 SO I* , ppm
0
4
8
16
H20, %
7.7
7.3
7.7
7.9
SO 3, ppm
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V. STUDY OF A POWER PLANT EQUIPPED WITH FACILITIES
FOR INJECTION OF ANHYDROUS NHs VAPOR
A. Description of the Widows Creek Station/ Unit 7
The Widows Creek Station is one of several power stations
in the Tennessee Valley Authority System that burns coal of
unusually high sulfur content—about 3.5%, on the average. In
such power plants, problems in precipitator performance from
high-resistivity ash is not to be expected because of the
abundance of naturally produced SO3; in several of the plants,
nevertheless, poor precipitator performance is encountered.
Data published by Reese and Greco11 indicate that the low-
resistivity of the ash and accompanying excessive reentrainment
is the principal cause of difficulty.
The injection of NH3 has been found a promising means of
improving precipitator performance at Unit 7 of the Widows Creek
plant. Widows Creek Unit 7 was built with a production capacity
of about 575 MW and only an electrostatic precipitator for the
removal of fly ash. Several years ago, however, the plant was
modified by installation of facilities to permit NH3 injection
ahead of the air heater on one side of the dual system of ducts
leading from the boiler to the precipitator; later, because of
difficulties from air-heater clogging, the injection facilities
were modified to provide injection between the heater and the
entrance of the precipitator.11 The injection system is quite
simple; it consists only of a lance with several openings into
the-duct; the simplicity of design permits periodic removal of
the injection line from the duct and cleaning of solid deposits
from the lance as necessary. The modified system was operated
intermittently during several visits by our staff to the Widows
Creek plant, including a visit on April 5 and 6, 1971, that is
discussed in detail in this report.
One of our objectives in visits to the Widows Creek plant
was to study conditioning by the abundant naturally occurring
SO3 in a range of temperatures. Unit 7 is designed in such a
manner that the temperature in one of the ducts leading to the
precipitator can be raised or lowered from the normal tempera-
ture of about 270°F with a corresponding temperature change in
the opposite direction in the second duct. The second objective
was to gain an understanding of the mechanism of NHs conditioning
under the conditions that prevail at Widows Creek Unit 7.
Several hypotheses to explain the effect have been advanced; in
general, these hypotheses are based on assumptions that the NH3
acts as a base to neutralize part of the S03, thus raising the
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inherently low fly-ash resistivity and minimizing reentrainment,
especially that occurring during the rapping of collector
electrodes to deposit the collected ash in the precipitator
hoppers.11 Thus, the mechanism of NHs conditioning at Widows
Creek is distinctly different from that in other plants referred
to in Section I, where the function of NH3 is to overcome high
resistivity.
B. Analyses of Coal Samples
On April 5 and 6, 1971, studies were made at Widows Creek
Unit 7 both with and without NH3 injection. A number of coal
samples were collected at intervals on these two dates in April
and analyzed separately for sulfur and ash. Both sulfur and
ash concentrations were reasonably constant throughout the
sampling period but were found to range from 3.31 to 3.74% and
from 16.0 to 19.0%, respectively. The average concentrations of
3.59% of sulfur and 17.5% of ash appeared to be fairly represen-
tative of the fuel throughout the period of conditioning studies
as well as during normal periods of operation of the plant, in
view of coal analyses supplied by TVA personnel.
No attempt was made to determine the different forms of
sulfur and the components of the ash during the work in April.
However, determinations on another occasion indicate that approxi-
mately 75% of the sulfur is pyritic and most of the balance is
organic. Moreover, pH determinations of aqueous slurries of the
fly ash collected on different occasions indicate that the ash
is normally rich in soluble basic substances.
C. Investigation of Conditioning in the Absence of
Conditioning Agent
1. Resistivity of the fly ash
Table 25 lists the results of determinations of the fly-ash
resistivity at recorded temperatures ranging from 254 to 288°F—
below and above the normal value of 270°F. These data in this
table were obtained with both cyclone and point-plane probes with
the usual electric field of 2.5 kV/cm in the cyclone samples and
various electric fields in the range from 0.6 to 15.0 kV/cm in the
point-plane samples. At least two determinations were made with
different electric fields in each point-plane sample, and up to
nine determinations were made with different fields in selected
samples. The data obtained for the point-plane samples at various
fields were plotted in a graph; interpolations and extrapolations
from this graph were made to permit reporting of the values of
resistivity at electric fields of 1.0, 2.5, 5.0, and 10 kV/cm.
SOUTHERN RESEARCH INSTITUTE
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Table 25. Electrical Resistivity of Fly Ash at
Widows Creek Unit 7 without NH3 Conditioning3
Resistivity, ohm cm, found
with different sample collectors
and various electric fields
Cyclone
Temp, collector, ^__
°F 2.5 kV/cm 1.0 kV/cnT2.5 kV/cnT5.0 kV/cnT10.0 kV/cm
Point-plane collector
7.0 x 1010 2.6 x 1010 1.0 x 1010 0.5 x 1010
1.7 x 10:0 0.3 x 1010
288
286 7.5 x 109 1.0 x 1011 4.5 x 1010
284 1.0 x 1010
258
254 1.0 x 109
5.0 x 108 3.7 x 108 2.8 x 10'
2.2 x 10'
a. Determined on April 5 and 6, 1971.
Based on a comparison of data at an electric field of
2.5 kV/cm, the resistivity of fly ash at the Widows Creek plant
appeared to decrease from about 1 x 1010 ohm cm or more at
temperatures near 285°F to about 1 x 109 ohm cm or less at
temperatures near 255°F. Thus, the resistivity at the normal
operating temperature of about 270°F would be well below the
assumed maximum value of 2 x 1010 ohm cm for efficient precipi-
tator operation. The decrease in resistivity of about one order
of magnitude during a reduction of temperature of 30°F in the
range below 300°F is at least qualitatively in agreement with
the trend in resistivity described by White;: it signifies the
increasing surface conduction by collected vapors—assumed to be
principally H2O—-as the temperature is lowered .
2.
Chemical composition of the fly ash
Table 26 shows the results of chemical analyses of four
of the six fly-ash samples for which resistivity data were given
in the preceding section. No result is given for the remaining
two samples, which were lost on removing the point-plane probe
from the duct. Also, no comparative data for samples collected
ahead of the air heater can be reported; during the work at the
Widows Creek plant in April, no apparatus was available for
collection of fly ash through the small sampling ports that were
available ahead of the air heater.
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Table 26. Chemical Properties of Fly-Ash Samples
Collected Ahead of the Electrostatic Precipitator
at Widows Creek Unit 7 without NH3 Conditioning3
Chemical properties0 of fly ash
Sampling Soluble SO**"2
device" pH Total
C 11.0 0.73 0.04
P 10.8 0.79
C 11.0 0.77
C 10.7 0.88
a. Determined on April 5 and 6, 1971.
b. C and P indicate cyclone and point-plane sampling
probes, respectively.
c. Estimated concentration was 4.1 gr/ft3 (standard
conditions).
The analyses of fly-ash samples collected at temperatures
near 285 and 255°F at the entrance of the precipitator indicate
that a slight increase in the S0^~2 content of the ash—approxi-
mately 0.1%—occurred as the temperature was lowered. It appears,
therefore, that the lowering of resistivity was caused not only
by increased collection of HaO vapor but also by increased
collection of SO3 vapor or, more exactly, H2SOi» vapor in view of
the range of temperatures studied.
3. Concentrations of flue gases
Concentrations of Had, SO3, and S02 found in the ducts ahead
of the air heater and the precipitator are listed in Table 27.
These concentrations show much higher values of both of the sulfur
oxides than the values found in other plants previously discussed,
in which coals of much lower sulfur contents were burned.
The SO3 concentrations, in particular, show that substantial
concentrations of this conditioning agent are naturally available
when a high-sulfur coal is burned as at the Widows Creek plant.
The SO3 concentrations also show the decrease expected with
decreasing temperature and increased collection on the fly ash.
SOUTHERN RESEARCH INSTITUTE
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Table 27. Concentrations of Flue Gases
at Widows Creek Unit 7 without
NHa Conditioning
Sampling Temp, Concentrations of flue gases
location5 °F HaO, % SOs, ppm SO2, ppm
AH 625 - 15 2860
20 2460
EP 288 - 15 2660
286 7.9 - 2640
284 - 18 2640
254 - 8 2450
a. AH signifies a location before the air heater, and
EP signifies a location before the precipitator.
The dates of sampling were April 5 and 6, 1971.
b. Expressed on the "wet" or actual basis. The concen-
tration of Oz recorded by the utility company was
approximately 3%.
D. Investigation of Conditioning with Injected NHa
1. Resistivity of the fly ash
During the studies on April 5 and 6, NH3 was injected at
rates calculated to produce concentrations of 7 and 15 ppm of
NHs vapor in the duct between the air heater and the electro-
static precipitator. During injections of NHa at the 7-ppm
level, duct temperatures were near 275°F; during injections at
the 15-ppm level, duct temperatures were near 285°F during one
series of experiments and near 255°F during a second series of
experiments. The results of resistivity determinations during
injections of NHa, determined by the same procedures as those
used in the absence of NHa, are listed in Table 28.
All of the resistivity values obtained at an electric
field of 2.5 kV/cm, both with and without NH3 injection, are
plotted in Figure 6 to show possible effects of NHa injection.
Graphs A and B in this figure show cyclone and point-plane
data separately; however, in both graphs the same line is used
to show approximate averages from the two sets of data. In
general, the plotted data show little evidence of an effect of
NHs on resistivity. The strongest indication of an effect from
NH3 is given by the point-plane data at temperatures in the
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range from 280 to 290°F. Here, the apparent effect is a slight
lowering of resistivity, and this effect is contrary to the
expected increase in resistivity. It is our opinion that the
available resistivity data cannot explain the improvement in
precipitator efficiency that injected NH3 reportedly produces
at the Widows Creek plant.
Table 28. Electrical Resistivity of Fly Ash at
Widows Creek Unit 7 with NH3 Conditioning3
Temp
278
274
273
290
289
286
283
256
Resistivity, ohm cm, found with different sample
collectors and various electric fields
Cyclone
collector,
2.5 kV/cm
2.3 x 109
3.0 x 109
7.5 x 109
Point-plane collector
1
4
4
.0
.5
.5
kV/cm
x 109
x 109
2
2
2
.5
.8
.8
kV/cm
x 109
x 109
5
1
1
.0
.6
.6
kV/cm
x 109
x 109
10.
0.8
0.8
0
x
x
kV/cm
109
109
5.0 x 109
2.5 x 109
1.3 x 1010
3.3 x 1010
6.0 x 109
2.8 x 108
5.8 x 109
1.4 x 1010
2.2 x 109
2.1 x 108
3.0 x 109
7.5 x 109
0.9 x 109
1.6 x 108
1.0 x 10 9
2.5 x 109
0.5 x 109
1.3 x 108
a. Determined on Anril 5 and 6, 1971, with 7 ppm injected at
temperatures of 273-278°F and with 15 ppm injected at
temperatures of 256 and 283-290°F.
2. Chemical properties of the fly ash
Table 29 summarizes the results of chemical analyses of
some of the samples collected for resistivity determinations.
Unfortunately, data cannot be given for several samples collected
with the point-plane apparatus, because loss of these samples
occurred as the collecting device was being removed from the
duct. Table 29 includes the results not only of pH and SO^'2
determinations but the results of NH3 determinations for three
samples. Determinations of the NH3 extracted in aqueous slurries
were made by use of Nessler's reagent. l*° Control experiments
with known amounts of NH3 added, to extracts of unconditioned ash
showed that at least 60 to 80% of the NH3 to be expected on
conditioned ash could be found, assuming quantitative collection
of the conditioning agent. With 15 ppm of NH3 injected and
collected on the ash, the concentration of NHs in a sample of
the ash would be 0.12% by weight; with 7 ppm injected, the
concentration would be 0.05%.
SOUTHERN RESEARCH INSTITUTE
-------
10
i i
10
i o
-p
-H
•H
-P
10
3 !09
«
10f
_ A. Cyclone samples
ONo NH3
Q 7 ppm of NH3
®15 ppm of NH3
250
B. Point-plane samples
ONo NH3
B 7 ppm of NH3
B15 ppm of NH3
cr«
i
260
270
280 290 300 250
Temperature,
260
270
280
290
300
Figure 6.
Resistivity of Fly Ash at Widows Creek Unit 7
with and without NH3 Injection
-------
-67-
Table 29. Chemical Properties of Fly-Ash Samples
Collected Ahead of the Electrostatic Precipitator
at Widows Creek Unit 7 with NH3 Conditioning3
Calculated
concn of
NH 3, ppm
15
Temp,
"
278
278
274
273
290
283
256
256
Sampling
device
C
P
P
C
C
C
C
P
Chemical properties of fly ash
11.4
10.5
11.0
11.4
11.6
11.6
10.9
9.5
Soluble SO-*-', %
Total H 2 SO i*
0.94 0.04
1.15
1.29
0.87
0.94
1.03 0.04
0.86
1.21
Soluble
NH3, %
<0.02
<0.02
<0.02
-
a. Determined on April 5 and 6, 1971.
b. C and P indicate cyclone and point-plane sampling probes,
respectively.
c. Estimated concentration was 4.1 gr/ft3 (standard conditions).
The data in Table 29 show marked disparities in the pH and
SCK-2 data for cyclone and point-plane samples at either rate of
NHs injection or at any of the experimental temperatures. In
general, the data indicate that more SO3 was collected by the
point-plane apparatus than by the cyclone collector, a phenomenon
apparent in studies at some of the other plants.
The data showing S0t~2 concentrations in the ash with and
without NHa injection (Tables 26 and 29) are plotted for compar-
ison in Figure 7. The line constructed in this figure shows the
expected concentrations of S0t~2 at various temperatures, based
on the estimated fly-ash concentration (4.1 gr/ft3) and on the
observed SO3 concentrations at various temperatures (summarized
subsequently in Figure 8). The line in Figure 7 has a negative
slope corresponding to the positive slope of the relationship
between the concentration of SO3 and the temperature. There is
unfortunate scatter in the data about the line in Figure 7.
However, despite the scatter, it appears reasonable to draw the
conclusion that a. change in temperature had a greater influence
than the injection of NHs on the SOi»-2 concentration in the ash.
SOUTHERN RESEARCH INSTITUTE
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1.3
1.2
-- 1-1
x;
W
d
CM
' 1.0
J-
o
w
o
o 0.9
c
o
u
0.8
0.7
r ' Q ' ' .
3 DNo NH3
\ S> 63 7 ppm of NH3
?; IS 15 ppm of NH3
B
\ ^
\
\
\
\
0 B
\
O
-------
-69-
The results of NH3 determinations showed no evidence of
the presence of NH3 on the ash. The sensitivity of the
analytical method, however, was such that each result can be
expressed only as a value less than 0.02% of the weight of the
sample analyzed. On this basis, the results signify that less
than 40% of the NHa injected at 7 ppm was found and less than
20% injected at 15 ppm was found.
In addition to the determinations of NH3 in the fly-ash
samples collected for resistivity determinations, still other
determinations were made with samples collected on the quartz-
wool particulate filter of the SOa-SOa sampling train. The
latter determinations led to the finding of the equivalent of
about 4 ppm of NH3 as the vapor at either level of injection.
It appears that the NHs may have been present in the form of
very small particles that were inefficiently collected by the
cyclone and point-plane probes. The particulate matter may
have been small fly-ash particles with a surface deposit of
NHs or small particles of a reactipn product between NH3 and
H2S04 vapors, such as the (NHij^SOi, postulated by Reese and
Greco.11 It is also possible that NH3 may have been sampled
from the duct as a gas but retained on the glass wool or
collected fly-ash particles by a process of adsorption.
3. Concentrations of flue gases
Concentrations of HaO, S03, S02 / and NH3 in the flue gases
during periods of NH3 injection were determined, as shown by the
experimental results in Table 30. For collecting NH3 from the
gas phase, a sampling train consisting of a heated probe and
particulate filter (as described in Section II. E.) and a bubbler
filled with 0.1 N aqueous HaSOi* was used; for determining the
NHs collected in the HzSOi, solution, a procedure based on
Nessler's reagent1*0 was followed.
The concentrations of HaO, SOa, and SOa are similar to
those found without NH3 injection (Table 27). The concentrations
of NH3 represent only small fractions of the concentrations
reportedly injected and are not large enough to account for the
total concentrations reported when combined with the equivalent
concentrations found in samples of the fly ash.
SOUTHERN RESEARCH INSTITUTE
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Table 30. Concentrations of Flue Gases at Widows Creek
Unit 7 with NH3 Conditioning3
Calculated ,
concn of Temp, Concentrations of flue gases
injected NHa, ppm °F H20, % SO3, ppm SO2, ppm NHa, ppm
7 278 8.0 13 2620
276 - - 0.2
274 - 15 2720
273 - 13 2670
15 290 - 18 2850
283 8.4 - - 0.4
254 - 10 2790
a. Collected at the entrance of the electrostatic precipitator
on April 5 and 6, 1971.
b. Expressed on the "wet" or actual basis. The concentration
of Oa recorded by the utility company was approximately 3%.
The concentrations of S03 found with and without NH3
injection are plotted in Figure 8. There is no apparent evidence
of a significant change in the SO3 during NH3 injection, because
the data points lie near Line A in the figure whether they were
obtained with injection or without. If the injected NH3 had
reacted with S03 at concentrations indicated by Line A to produce
particles of (NHOaSOij, the injection of 7 ppm of NH3 would have
lowered the S03 concentrations to values along Line B, and the
injection of 15 ppm would have lowered the S03 concentrations to
values along Line C. The inadequacy of either Line B or Line C
for showing the observed concentrations of S03 with NH3 injected
indicates that reaction to produce (NHi»)2s°if did not occur to a
significant degree.
-------
-71-
I
35
30
25
20
g 15
O
u
10
250
QNO NH3
07 ppm of NHs
®15 ppm of NH3
260
270 280
Temperature, °F
290
300
Figure 8. Concentrations of SOa Observed
at Widows Creek Unit 7 with and
without NHs Injection
{Lines A, B, and C are discussed in the text.)
SOUTHERN RESEARCH INSTITUTE
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VI. STUDIES OF POWER PLANTS WITHOUT PERMANENT FACILITIES
FOR INJECTION OF CONDITIONING AGENTS
A. Kingston Station, Unit 5
1. Description of the plant facilities
The Kingston Station in the TVA system is normally operated
with coal of intermediate sulfur content, roughly 2%. Unit 5 of
this station is a facility with a rated production capacity of
200 MW. It is equipped with both mechanical and electrostatic
collectors for fly-ash removal, and the usual operating tempera-
ture of the precipitator is about 300°F. Even with a moderate
concentration of naturally produced SOs available as a condition-
ing agent, the temperature is high enough to cause the fly ash to
have an undesirably high resistivity (above 1 x 1011 ohm cm),
which lowers the efficiency of the precipitator.
Field studies at Kingston Unit 5 were conducted on two occa-
sions during this investigation. The results of work on May 4, 5,
and 6, 1971, are summarized in this report in detail, for they
show the variation of fly-ash resistivity over a wide range in
temperatures and corresponding SOs concentrations. The results
of the work during May are of further interest in that they show
the lowering of resistivity that can be effected by the use of
H20 vapor as a conditioning agent. Kingston Unit 5 has no perma-
nent facilities for injecting HaO; however, for a brief period in
May, a stream of water was pumped into the boiler to raise the
concentration of HaO in the flue gases from the normal value of
7% to about twice the normal value or 14%.
2. Results of conditioning studies
a. Analyses of the coal
Percentages of sulfur and ash were determined for eight
coal samples collected at intervals during the studies on May 4,
5, and 6. The results appear to vary randomly with sampling time
and may be summarized as follows:
Sulfur: Range, 1.85-2.34%
Average, 2.12%
Ash: Range, 16.4-25.5%
Average, 19.6%
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b. Resistivity of the fly ash
Samples of fly ash to be used for determinations of electri-
cal resistivity were collected at the entrance of the precipitator
at Kingston Unit 5 with both cyclone and point-plane sampling
probes. Results of the determinations of resistivity with an
electric field of 2.5 kV/cm in the cyclone samples and electric
fields of 1.0, 2.5, 5.0, and 10.0 kV/cm in the point-plane samples
are listed in Table 31. Data obtained at a field of 2.5 kV/cm in
both types of samples are plotted in Figure 9.
Table 31. Electrical Resistivity of
Fly Ash at Kingston Unit 5a
Resistivity, ohm cm, found with different sample
collectors and various electric fields
Temp,
OF
372
370
365
350
347
346
341
329
328
325
323
323
319
308
302
301
296
335
331
329
323
Cyclone
collector,
2
3
1
2
3
3
1
5
2
1
.5
.6
.7
.0
.0
.3
.7
.5
.0
.5
kV/cm
_
X
_
X
—
X
_
_
X
—
_
X
X
_
—
X
_
_
X
X
1012
1013
1013
1012
1012
1012
1011
1011
1011
Point-plane
1.0 kV/cm
1.7
2.7
2.0
6.6
1.3
7.8
2.8
2.1
2.6
2.9
4.0
1.8
X
X
_
X
—
X
_
X
X
—
X
X
—
_
X
X
-
X
X
_
—
1012
1012
1012
1011
1012
1011
1012
1012
1011
1011
1011
1011
1
1
1
5
9
6
„
1
1
2
2
3
1
2.5 kV/cm
.4
.8
.3
.8
.0
.0
.6
.7
.1
.3
.2
.5
X
X
_
X
—
X
—
X
X
—
X
X
—
_
X
X
-
X
X
_
—
101
101
101
101
101
101
101
101
101
101
101
101
2
2
2
1
1
1
2
2
1
1
1
1
collector
5.0 kV/cm
1.0
1.3
1.0
4.6
6.0
4.2
1.1
1.5
1.7
2.3
1.1
X
X
_
X
—
X
—
X
X
—
—
X
—
_
X
X
-
X
X
_
—
1012
1012
1012
1011
1011
1011
1012
1011
1011
1011
1011
10.0 kV/cm
7
1
7
3
4
3
1
1
8
.0
.0
.0
.6
.0
.0
.1
.2
.8
X
X
_
X
-
X
—
X
X
—
-
—
—
—
X
X
-
^
X
_
~™
1011
1012
1011
1011
1011
1011
1011
1011
1010
a. Determined on May 4, 5, and 6 without injection of HzO (first
group of data) or with injection of H2O (second group of data)
SOUTHERN RESEARCH INSTITUTE
-------
101
10
1 3
e
o
10
a 2
M
•H
(0
-------
-75-
The principal observations to be made from the data as
plotted in Figure 9 are as follows:
• Resistivity values obtained in the absence of
injected HaO with the cyclone apparatus are, on
the average, nearly one order of magnitude higher
than the values obtained with the point-plane
apparatus, as indicated by Curves A and B. How-
ever, the data obtained during EzO injection are
virtually the same with both devices.
• As the temperature was increased from the normal
value of about 300°F, the resistivity increased to
a maximum near 350°F and then decreased, as a
result first of decreasing surface conduction and
then increasing volume conduction, as described by
White.1 Obviously, lowering the temperature from
300°F.would have produced a desirable reduction in
resistivity, as a result of increased surface con-
duction.
• Injection of H20 to increase the vapor concentra-
tion from 7 to 14% lowered the resistivity at
temperatures in the 320-340°F range by one to two
orders of magnitude, depending upon the type of
resistivity apparatus.
c. Chemical properties of the fly ash
Table 32 lists the results of chemical analyses of fly-ash
samples collected at Kingston Unit 5. Figure 10 shows graphs of
the pH and SOi,"2 values as functions of temperature.
One of the noteworthy aspects of the experimental results
is that in the absence of injected H20 the fly, ash was virtually
neutral as collected at the high temperature of 645°F ahead of
the air heater and then increasingly acidic as collected at
decreasing temperatures in the range from 372 to 296°F ahead of
the precipitator. A second noteworthy aspect of the data is that
three of the four samples collected in the presence of injected
HaO showed marked decreases in pH and increases in S04~2 content
in comparison with samples collected in the absence of injected
HaO in the same temperature range. It is evident, therefore,
that the effects of HaO injection on the ash included a marked
increase in the collection of the available SO3 by the fly ash.
SOUTHERN RESEARCH INSTITUTE
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Table 32. Chemical Properties of Fly-Ash Samples
Collected at Kingston Unit 5a
Estimated concn
of injected
H20, %
Sampling
location13
AH
AH
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
EP
Temp,
°F
645
645
372
370
365
350
347
346
341
329
328
325
323
323
319
308
302
301
296
335
331
329
323
Sampling
device0
C
C
P
P
C
P
C
P
C
P
P
C
P
P
C
C
P
P
C
P
P
C
C
Chemical properties
Soluble S0u~2
pH •, (total) , %
6.35
6.40
6.38
5.77
6.60
6.57
6.09
6.07
5,
4,
,70
,87
5.11
6.20
5.11
4.70
5.50
4.95
4.66
5.80
4.57
4.89
0.24
0.31
0.32
0.34
0.33
0.35
0.40
0.38
0.34
0.39
0.38
0.39
0.36
0.54
0.36
0.47
0.60
0.43
0.60
0.53
a. Determined on May 4, 5, and 6, 1971. Estimated fly-ash con-
centration was 4.5 gr/ft3 (standard conditions).
b. AH signifies a location ahead of the air heater, whereas EP
signifies a location ahead of the precipitator.
c. C and P indicate cyclone and point-plane sampling probes,
respectively.
-------
(0
O
c
H
m
3)
Z
3J
n
in
n
i
z
in
H
4.5
5.0
5.5
6.0
6.5
\
7.0
With H2O
\ injection
Without
H20 injection^
O
00
280 300 320 340 360
Temperature, °F
380
u . /
0.6
<*>
..
fi 0.5
fi\
w
-P
c
o
o
«< o-4
1
J-
o
co
0.3
0.2
1 I 1 1
-^ 8 •
^ ^ With H20
® \ injection
^
O
v^^^
^"V^^ 0
^^••v^^
^S~v>-*.^ m ^
c^--*^^ O
O O — -^^^^
Without H2O
injection
i i i i
i
**,
**.
1
280 300 320 340 360 380
Temperature,
Figure 10. Sulfate Content and pH Value of Fly Ash
at Kingston Unit 5 with and without H2O Injection
-------
-78-
d. Concentrations of flue gases
Table 33 lists the concentrations of H20, SOa, and SO2
found at Kingston Unit 5 with and without H20 injection. The
observed concentrations of H20 and SOs are of primary interest.
First, the observed concentrations of H20 show that the expected
increase of about 7% increased during H20 injection. Second, the
observed concentrations of SOa show wide variations with changes
in temperature in the absence of H20 injection and with the
change in H20 concentration during injection in the narrow temper-
ature range between 320 and 330°F.
Table 33. Concentrations of Flue Gases at Kingston Unit 5a
Calculated concn
Sampling Temp,
location*5 °F
of injected
H20, %
AH
EP
EP
645
640
640
635
635
365
363
345
341
324
324
319
319
314
296
296
334
330
329
323
0
0
0
0
0
0
0
0
0 i
0
0
0
0
0
0
0
7
7
7
7
Concentrations of flue gasesc
H20, % SO3, ppm S02, ppm
5.7
6.9
7.5
5.9
6.2
13.6
13.6
13
14
11
13
9
15
18
10
16
10
9
9
12
9
5
4
3
3
3
1470
1730
1730
1660
1650
1500
1580
1460
1400
1240
1320
1630
1700
1770
1450
1440
1300
1410
1410
a. Determined on May 4, 5, and 6, 1971.
b. AH signifies a location ahead of the air heater, whereas
EP signifies a location ahead of the precipitator.
c. Expressed on the "wet" or actual basis. The concentration
of 02 recorded by the utility company was approximately 3%,
-------
-79-
To show the variations in SO 3 concentration with temperature
and HaO concentration, the observed SOa concentrations are plotted
in Figure 11. From this figure, it is apparent that lowering the
temperature from the values near 640°F ahead of the air heater to
values near 360°F ahead of the precipitator caused an increase in
the concentration of SOa , which was probably the result of partial
oxidation of the large excess of S02 by Oa; on the other hand, it
is apparent that lowering the temperature from about 360 to 300°F
ahead of the precipitator and increasing the collection of SOa by
the fly ash (as observed) caused a sharp decrease in the concen-
tration of SOa. From the figure, it is to be noted, in conclusion,
that H?.0 injection lowered the SOa concentration appreciably at
temperatures between 320 and 330°F. This concluding observation
signifies that the conditioning of fly ash that occurs during
injection may be largely the result of increased collection of
SOa, rather than of
B. Gallatin Station, Unit 4
At Unit 4 of the Gallatin Station in the TVA system, the
conditions of plant operation and the problems of precipitator
operation are similar to those at the Widows Creek Station with-
out NHa injection. Specifically, coal with a high sulfur content
is burned, and the temperature within the precipitator is usually
about 270°F — low enough to produce a fly-ash resistivity approach-
ing the lower limit of acceptable values, where excessive rein-
trainment is to be expected.
Gallatin Unit 4 has a production capacity of about 330 MW,
and it is operated with both a mechanical collector and an elec-
trostatic precipitator for fly-ash removal. Unlike Widows Creek
Unit 7, it does not have facilities for NH3 injection.
A brief investigation was made of Gallatin Unit 4 on May 26
and 27, 1971. The experiments performed consisted only of deter-
minations of the resistivity of the fly ash and chemical analysis
of the fly ash. No effort was made to analyze coal' samples or to
determine flue-gas concentrations, because of the reported simi-
larity of these parameters to those at Widows Creek Unit 7.
The results of the experiments are summarized in Table 34.
The resistivity data show the expected trend of decreasing value
with decreasing temperature and indicate the low value antici-
pated at the normal temperature of 270°F, about 1 x 10 9 ohm cm.
The pH values do not show the expected trend toward lower values
with decreasing temperatures, perhaps as a result of minor varia-
tions in coal composition during our studies; however, the S0<*~2
data show the expected increases in total SOi»~2 with decreases in
temperature and also indicate that free H2SOi» represented a sub-
stantial function of the total S0i»~2.
SOUTHERN RESEARCH INSTITUTE
-------
-80-
20
15
a
a
10
c
CJ
c
o
u
0
280
300
O
O
o
o
o o
o
? Without HaO injection
9.- With H20 injection
320
380
630
340 360
Temperature, °F
Figure 11. Concentrations of SOs at Kingston Unit 5
650
-------
-81-
Table 34. Electrical Resistivity and Chemical Properties
of Fly Ash at Gallatin Unit 4
Range of observed Chemical properties
Temp, resistivity values, Soluble S0u~2, %
°F _ ohm cma _ pH Total
329 (2-3) x 1010 9.35 0.87 0.07
287 (3-4) x 109 9.80 1.15 0.09
260 (2-6) x 108 10.15 1.60 0.07
a. Determined on May 26 and 27, 1971, with the point-
plane apparatus at an electric field of 2.5 kV/cm.
C. Bull Run Station*
At the Bull Run Station in the TVA system, in contrast to
Widows Creek and Gallatin Stations, coal of low sulfur content
(about 1%) is normally used as the fuel. Thus, the fly ash at
Bull Run should have a resistivity higher than that at Widows
Creek or Gallatin, perhaps high enough not only to overcome the
problem of reintrainment at either of those plants but to intro-
duce the alternative problem of excessive sparking or back corona,
A brief study, comparable to that described in the preced-
ing section for Gallatin Unit 4, was made at Bull Run Station on
several dates in March, 1971, when two different coals were
burned. The Bull Run Station is a 950-MW installation with only
an electrostatic precipitator for collecting fly ash. Our work
consisted only of determinations of fly-ash resistivity and chemi-
cal properties. Concentrations of SO2 were determined by TVA
personnel and found to be consistently in the range from 830 to
950 ppm, signifying that no appreciable difference existed in the
sulfur content of the coal and that the percentage of sulfur was
about 1%, the typical value.
The results of the fly-ash studies are given in Table 35.
Resistivity values at a temperature of 270°F were, as expected,
higher than those at the Widows Creek and Gallatin Stations, and
high enough to lead to the usual problems in precipitator perfor-
mance with high resistivity. The chemical analyses showed that
the ash from one coal was acidic whereas the other was nearly
* Consists of only one large power-production unit.
SOUTHERN RESEARCH INSTITUTE
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neutral; in addition, these analyses showed lower concentrations
of S0i»~2 in the ash, as expected with the low concentrations of
S03 predictable in flue gases from coal with only 1% sulfur.
Table 35. Electrical Resistivity and Chemical Properties
of Fly Ash at Bull Run Station
Range of observed Chemical properties
Temp, resistivity values, S0i»~2, %
Coala °F ohm cmb pH (total)
A 270 (2-10) x 1011 4.4 0.28
B 270 (2-10) x 101l 6.0 0.35
a. The coals identified as A and B are referred to as
"Haddox" and "Bull Run" coals, respectively.
b. Determined on May 16 and 23, 1971, with a cyclone
probe at an electric field of 2.5 kV/cm.
D. Z Station, Unit 1
Z Station cannot be identified by name, location, or owner.
However, it is located in a region where coals of various composi-
tions are available for use as fuel. On the basis of both
analyses made in this laboratory and analyses reported by the
Bureau of Mines,31'32 sulfur contents of the coal can be expected
to vary in the intermediate range of 1 to 2%.
Experiments were conducted during January, 1971, at Unit 1
of the Z Station, an installation designed with a 220-MW capacity
and equipped with only an electrostatic precipitator for fly-ash
removal. On two dates, January 6 and 7, the most extensive work
was conducted and, thus, only the results obtained on these dates
are given in this report. On these dates, percentages of sulfur
in the coal were 0.95 and 1.90%, respectively, and the correspond-
ing percentages of ash were 15.8 and 16.0%. The difference in
sulfur concentrations on January 6 and 7 probably represent the
extremes to be found in different coals burned from time to time
during extended periods of plant operation.
Considerable difficulty was encountered in obtaining consis-
tent values of resistivity in the range of temperatures investi-
gated, 256 to 319°F. The results of the resistivity determina-
tions, which are listed in Table 36, fail to show the expected
-------
-83-
upward trend with increasing temperature in the range studied.
The data are, nevertheless, noteworthy in that they indicate that
the resistivity was also higher than the maximum value of
2 x 1010 ohm cm desired for efficient precipitator operation, not-
withstanding the difference in SOs that should have been available
for conditioning of the ash during combustion of the different
coals. The results of the chemical analyses of the fly ash, which
are included in Table 36, show that the ash was basic and con-
tained only moderate amounts of SOi,"2.
Table 36. Electrical Resistivity and Chemical Properties
of Fly Ash at Z Station
Temp, Resistivity,3 Chemical properties
"~pH SOi, 2, % (total)
Date
Jan. 6
Jan. 7
°F ohm cm
256
269
283
305
319
269
270
278
283
287
292
296
310
3
7
3
4
1
1
1
1
2
7
6
3
1
X
X
X
X
X
X
X
X
X
X
X
X
X
101
101
101
101
101
101
101
101
101
101
101
101
101
2
2
2
2
3
2
2
2
1
1
1
1
2
10.7 0.41
10.9 0.32
9.7 0.45
9.8 0.42
9.8 0.41
9.7 0.47
a. Determined with a cyclone probe at an electric field
of 2.5 kV/cm.
The results of flue-gas analyses are given in Table 37.
The maximum concentration of SO3 that appeared to be available
for conditioning of the fly ash was about 4 ppm, based on the
analyses of samples taken ahead of the air heater on the date
when the coal had the higher content of sulfur. This concentra-
tion seems surprisingly low in comparison with the concentration
of about 20 ppm found at Kingston Unit 5 under similar conditions,
One distinction in compositions of fly ash at the two plants—the
higher basicity of the ash at the Z Station—may be the principal
reason for the difference in observed SOa concentrations.
SOUTHERN RESEARCH INSTITUTE
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-84-
Table 37. Concentrations of Flue Gases at Z Station
Sampling Temp, Concentrations of flue gasesk
Date location5 °F H?.0, % SO3, ppm SO2, ppm
Jan. 6 EP 256 7.4 1 700
269 - 1 600
Jan. 7 AH 600 - 4 820
600 - 5 860
EP 292 6.9 1 910
300 - 2 1020
a. AH signifies a location before the air heater, and
EP signifies a location before the precipitator.
b. Expressed on the "wet" or actual basis.
E. Shawnee Station, Unit 10
1. Description of the power unit
Unit 10 of the Shawnee Station in the TVA system was unique
among the plants investigated during this program in that it was
equipped with facilities to control the emission of S02 by injec-
tion of dry limestone into the boiler. Dry-limestone injection
is one of several processes being evaluated for control of S02
emission; it is based on calcination of limestone (CaCOs) in the
boiler to produce lime (CaO) and on reaction of the CaO with SC>2
and Oz in the flue gases to produce CaSOi* on the surface of the
CaO particles. **l As reaction with the combination of S02 and 02
occurs, simultaneous reactions of the CaO with S02 alone to pro-
duce CaSOs and with the small naturally occurring concentration
of SOs to produce CaSOi, may also occur. **2
Our staff carried out studies at Shawnee during times of
plant operation with and without CaCOs injection on July 15, 16,
21, and 22, 1971. Our principal objective in these studies was
to determine the effect of CaO particles on the resistivity of
the fly ash; however, we were also interested to learn the degree
to which CaCOa would lower the concentration of SO2 and SO3. The
studies were planned to include experiments when coals of differ-
ent sulfur contents were used as fuels and thus a range of concen-
trations of SO2 were subject to removal by reaction with CaO.
-------
-85-
Shawnee Unit 10 has a production capacity of about 175 MW
and produces flue gases at the rate of about 540,000 ft3/min at
310°F and ambient pressure (about 315,000 ft3/rain under standard
conditions). It is equipped with both a mechanical collector and
an electrostatic precipitator to remove fly ash. It was investi-
gated during CaCOa injection at rates of 167 and 333 Ib/min;
assuming complete calcination of the CaCOs occurred in the boiler,
it thus produced CaO particulate concentrations of about 2.1 and
4.2 gr/ft3, compared to estimated fly-ash concentrations in the
range from 3.9 to 4.5 gr/ft3 (standard conditions) with the ash
percentage in the coal in the range from 16.3 to 19.5%.
2. Analyses of the coal and the limestone
The composition of the coal was deliberately varied during
our field work at Shawnee Unit 10, especially with respect to the
sulfur content. Compositions of coal samples collected at differ-
ent times on different dates and analyzed in our laboratory are
shown by the data in Table 38. The average daily percentage of
sulfur ranged from 1.81 to 3.89%, and the percentage of ash
ranged from 16.3 to 19.5%. On two days, especially July 21, the
coal apparently varied in composition significantly when constant
composition was desired.
Table 38. Sulfur and Ash Contents of Coal
Burned at Shawnee Unit 10
Time of Concentration, %
Date sampling Sulfur Ash
July 15 Morning 1.79 18.2
Midday 1.78 18.1
Afternoon 1.87 17.8
Average 1.81 18.0
July 16 Morning 3.82 16.1
Morning 3.96 16.5
Average 3.89 16.3
July 21 Morning 3.71 22.0
Morning 2.97 19.3
Midday 2.54 17.3
Average 3.07 19.5
July 22 Morning 1.57
Midday 1.96
Afternoon 2.40
Average 1.98
SOUTHERN RESEARCH INSTITUTE
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-86-
A sample of the CaCOa was analyzed for purity on the basis
of the amount of HC1 neutralized by a weighed sample. The appar-
ent purity was about 98%, assuming that the impurities did not
react as acid or base. The CaCOa injected in the boiler during
all of our studies was described as a "coarse" grade, signifying
that 50% of the particles by weight would pass a 400-mesh sieve
(in contrast to a "fine" grade, of which 80% would pass a 400-
mesh screen).
3. Results of experiments without limestone injection
a. Resistivity of the fly ash
Each of the determinations of the resistivity of fly ash
was made in a duct leading into the precipitator with the point-
plane apparatus, operating with a wide range of electric fields
in the collected sample. Because of the sampling requirements of
another organization with the primary responsibility for operating
and evaluating the CaCOa injection equipment, it was necessary in
most of the experiments to determine resistivity in a high-temper-
ature duct, where the range of observed temperatures was 330 to
417°F and thus well above the range at which electrostatic precip-
itators are normally operated. However, it was possible on one
date to make the determinations at more appropriate temperatures
in the range from 266 to 305°F.
The results of the determinations of resistivity without
CaCOa injection are given in Table 39. In general, the data
include values at electric fields ranging from 1 to 20 kV/cm.
For some of the samples, however, data are omitted at several
values of field, either because the range of fields used experi-
mentally was restricted or because electrical breakdown occurred,
as indicated for one sample at fields of 15 and 20 kV/cm.
The data in Table 39 will be discussed later in greater
detail in a comparison with the data obtained when CaCOa was
injected. One noteworthy feature of the data obtained, however,
is the dependence of resistivity on electric field was diminished
as the temperature increased. This finding—contrary to observa-
tions at Kingston Unit 5—may be illustrated by the data recorded
for a temperature of 266°F and 305°F on July 16, for example, or
more emphatically by the data at 266°F on July 16 and various
temperatures up to 385°F on other dates, when the composition of
the coal was different.
-------
Table 39. Electrical Resistivity of Fly Ash at
Shawnee Unit 10 without Limestone Injection
Temp,
Date °F
July 15 375
376
July 16 266
273
305
July 21 330
SOUTHERN
3
PI
PI
n
in
H
ITUTE
July 22 375
385
Resistivity,
1.
2.3
7.0
5.5
5.8
8.0
0 kV/cm
x 1010
x 1010
x 1010
x 1010
x 1010
2.5
1.4 x
8.0 x
4.0 x
5.0 x
kV/cm
1010
1010
1010
1010
ohm cm, at various electric fields3
5
8.
3.
3.
2.
1.
5.
.0 kV/cm
0 x 109
5 x 1010
5 x 1010
3 x 1010
8 x 1010
0 x 1010
5.0 x 101 ° 4.0 x 101 °
7.0 x 1010 6.0 x 1010
a. Determined with a point-plane resistivity probe
of electric fields was not extended throughout
b. BD indicates that electrical breakdown occurred
electric fields shown.
10
5.
2.
1.
8.
1.
4.
.
0
0
2
0
0
2
0 kV/cm
x 109
x 1010
x 108
x 107
x 1010
x 1010
15.0 kV/cm 20.0 kV/cm
1.8 x 101 ° 1.8 x 101 °
6.0 x 107 5.0 x 107
i
BO*5 ED*3 ^
I
3.6 x 101 ° -
3.0 x 1010 2.5 x 101 °
4.6 x 1010 3.8 x 1010 3.0 x 1010
No value is listed if the range
the maximum range usually covered.
in the collected sample at the
-------
-88-
b. Chemical properties of the fly ash
Table 40 lists the observed values of chemical properties
of fly-ash samples collected in the absence of CaCOa injection.
This table includes the values of properties not generally deter-
mined in studies of plants other than Shawnee. It includes the
total percentage of water-soluble material in each sample and the
percentage of each sample dissolved as Ca+2 ion. Determinations
of these properties were made by comparing sample weights before
and after extraction with water and by determining dissolved Ca+2
by atomic absorption spectroscopy.
Table 40. Chemical Properties of Fly-Ash Samples Collected
at Shawnee Unit 10 without Limestone Injection
Concn of Chemical properties of the fly ashb
Temp, fly ash,a Water-soluble components, %
Date °F gr/ft3 pH Total ^j-z """ Ca+2
July 15 375 4.2 10.2 6 1.7 1.0
376 4.2 9.0 5 1.6 0.6
July 16 266 3.9 6.2 6 1.1 0.4
273 3.9 4.6 7 1.2 0.4
305 3.9 4.2 2 1.4 0.4
July 21 330 4.5 11.7 9 2.7C -C
July 22 375 4.0 11.0 8 1.5 1.0
385 4.0 11.0 8 1.5 0.8
a. Estimated for standard conditions from the ash percentage
in the coal.
b. All of the samples were collected in the point-plane
resistivity probe.
c. Electron microprobe analyses for sulfur and calcium yielded
percentage concentrations of 0.7% and 3.3%, respectively.
The sulfur percentage is believed to be a reasonable value
for the surface material; it is in reasonable agreement
with the value of 0.9% calculated from the concentration
of soluble S0i»~2. However, the calcium percentage may
include material below the surface (see Table 43).
-------
-89-
The properties of the ash show significant variations with
the different dates of collection, as expected from the differ-
ences in temperatures used for sampling and the differences in
coal used as the fuel. The properties of the ash are discussed
later in greater detail in comparison with the properties of ash
found during CaCOs injection.
c. Concentrations of flue gases
Concentrations of HzO, SOs, and S02 observed in the absence
of CaCOa injection are listed in Table 41. The data for the sul-
fur oxides are of primary interest. They show, for example, the
expected variations in the concentration of SOa as the result of
variations in the sulfur content of the coal. They also show
variations in the concentration of SOa as expected, in view of
differences in coal and fly-ash compositions and differences in
sampling temperatures.
i
Table 41. Concentrations of Flue Gases at Shawnee Unit 10
without Limestone Injection
Sampling Temp, Concentrations of flue gases*3
Date location3 °F H20, % SO3, ppm S02> ppm
July 15 EP 375 9.2 3 1580
EP 376 - 5 1530
July 16 EP 266 10.0 1 3030
EP 305 9.4 5 2890
EP 305 - 5 2880
July 21 AH 750 - 1 2070
EP 330 7.6 <1 1740
July 22 AH 720 - 1 1150
a. AH signifies a location ahead of the air heater, and
EP signifies a location ahead of the electrostatic
precipitator.
b. Expressed on the "wet" actual basis. In the absence
of a determination of the concentration of H20 on
July 22, a value of 8% was assumed for calculating
the concentrations of SOa and
SOUTHERN RESEARCH INSTITUTE
-------
-90-
However, it is desirable to make certain observations about the
variations in the concentration of SOa, as follows:
• The data at temperatures above 350°F might be
expected, on initial consideration, to show differ-
ences corresponding to differences in the sulfur
percentage of the coal. However, they do not show
the differences expected on this basis. The high-
est concentrations of SO3 observed at temperatures
above 350°F were found during combustion of the
coal with the lowest percentage of sulfur (on
July 15). The lack of correlation of observed SO3
concentrations with percentages of sulfur in the
coal must be a result of differences in the basicity
of the ash and resulting differences in the extent
of SOs collection by the ash. The highest concen-
trations of SOs above 350°F occurred in the presence
of ash of relatively lower basicity and presumably
low effectiveness for collecting SO3.
• The data obtained at relatively low temperatures of
266 and 305°F on July 16, when the coal had the high-
est percentage of sulfur, indicate that significant
losses of the available S03 occurred as a result of
collection on the fly ash, especially at the lower
of the two temperatures.
4. Results of experiments with limestone injection
a. Resistivity of the dust
Table 42 lists the results of determinations of the resis-
tivity of dust collected during periods of CaCOs injection. This
dust consisted of mixtures of fly-ash and CaO particles, includ-
ing the SOs and SOa collected as CaSOi,. One noteworthy feature
of the data obtained at high temperatures is that the resistivity
increased with increasing electric field, in distinction to the
behavior of fly ash in previous studies with CaO particles absent,
-Figure 12 compares the resistivity data obtained at a con-
stant field, 5.0 kV/cm, with and without CaCOs injection through-
out the study at Shawnee Unit 10. One graph is used to show all
of the data obtained on different dates, despite differences in
the properties of the coal and fly ash. The resistivity of the
fly ash alone, as observed when CaCOs was not injected, is por-
trayed by one curve, whereas the resistivity of the fly ash-CaO
mixtures, as produced when CaCOs was injected, is represented by
another curve.
-------
Table 42. Electrical Resistivity of Fly-:Ash and CaO Particles
at Shawnee Unit 10 during Limestone Injection
Reported
injection
SOUTHERN RESEARCH INSTITUTE
•rate of CaCOa
Date Ib/min
July 15 333
July 16 333
333
July 21 167
333
July 27 333
, Temp, Resistivity,
°F 1.0 kV/cm 2.5 KV/cm
340
255
273
407
360
417 4.0 x 1011 5.0 x 1011
a. Determined with a point -plane resistivity- probe. No
fields was not extended throughout the maximum range
ohm cm, at
5.0 kV/cm
various electric fieldsa
10.0 kV/cm 15.0 kV/cm 20.0 kV/cm
3.0 x 1010 2.7 x 1010
4.0 x 10J1 2.4 x 1011 1.5 x 1011 9.0 x 1010
4.5 x 1011 4.5 x 1011 4.5 x 1011 4.5 x 1011 ^
1.3 x 1013 1.7 x 1013 2.3 x 1013 2.6 x 1013 1
1.1 x 1013 1.0 x 1013 9.0 x 1012 8.0 x 1012
8.0 x 1011 1.2 x 1012 1.6 x 1012 2.0 x 1012
value is listed if the range of electr.ic
usually covered.
-------
10:
1013
-92-
With limestone
10
1 2
4J
•H
>
-H
-P
W
1
Without limestone
10
1 0
109
250 270 290 310 330 350 370
Temperature, °F
390
Figure 12. Resistivity of Fly Ash at Shawnee Unit 10
with and without Limestone Injection
(Based on data in Tables 39 and 42 for an
electric field of 5 kV/cm)
430
-------
-93-
Figure 12 shows beyond reasonable doubt that injection of
CaCOs raised the resistivity of collected dust by about two
orders of magnitude. In the normal temperature range of precipi-
tator operation, 275 to 325°F, the presence of CaO in the dust
raised the resistivity to a range well above the acceptable upper
limit of 2 x 1010 ohm cm. As will be shown in the following sec-
tion, the effect of CaO on resistivity occurred despite the
presence of a large quantity of SOi*"2 in the ash.
b. Chemical properties of fly ash-CaO samples
Results of chemical studies of dust samples collected dur-
ing times of CaCOs injection are listed in Table 43. Comparison
of these results with the results of experiments without CaCOj
injection (Table 40) leads to the following observations:
• The pH values of aqueous slurries of fly ash and CaO
were uniformly high, ranging from 12.4 to 12.8 (com-
pared with an observed value of 12.8 for an aqueous
slurry of pure CaO), whereas the pH values of
slurries of fly ash alone were consistently lower,
ranging from a minimum of 4.2 to a maximum of 11.7.
• The percentages of water-soluble materials were con-
sistently higher with CaO present, ranging from 22
to 48%. The corresponding percentages ranged from
2 to 8% with CaO absent. (Note: To facilitate
dissolution of soluble materials, 0.1 g of each
sample of particulate matter was first mixed with
30 ml of water to determine the pH value and then
mixed with an additional 270 ml of water prior to
determinations of the total amount of water-soluble
components or of S0i*~2 or Ca+2.)
• The percentage of soluble SO^"2 increased markedly
with CaO present. The minimum increase was 4.5%
on July 21, and the maximum was 10.6% on July 16.
• The percentage of soluble Ca+2 also increased
markedly with CaO present, as expected. No compari-
son for July 21 is possible, but increases of about
4 to 9% occurred on the other dates.
c. Concentrations of flue gases
Concentrations of flue gases found during periods of CaCOa
injection are listed in Table 44. The principal observations to
be made from these data are as follows:
SOUTHERN RESEARCH INSTITUTE
-------
Table 43. Chemical Properties of Samples of Fly Ash and CaO
Collected at Shawnee Unit 10 during Limestone Injection
Date
Limestone
injection
rate, Ib/min
Temp,
Concn of particulate
matter, gr/ft3a
Chemical properties of the fly ash*3
July 15
July 16
July 21
July 22
a. Expressed
333
333
333
333
167
333
333
340 4.2
350 4.2
255 3.9
273 3.9
407 4.5
360 4.5
417 4.0
4.2
4.2
4.2
4.2
2.1
4.2
4.2
12.4
12.5
12.7
12.8
12.4
12.4
12.4
31
32
48
48
23
22
34
6.2
5.5
11.7
11.9
7.2
_c
6.1
for standard conditions.
b. All of the samples
c. Electron
were collected in
the point-plane
microprobe analyses indicated that the perce
resistivity
probe .
sntages of sulfur a
5.9
were 3.3 and 31.2%, respectively. It is believed that a fairly reliable estimate
of soluble SOu~2 (9.9%) can be made from the sulfur determination. However, it is
obvious that an estimate of the soluble Ca+2 cannot be made from the calcium deter-
mination, since the total percentage of soluble material was less than the percent-
age of calcium found. Evidently, the microprobe detected insoluble forms of calcium
in the interior of the fly-ash particles.
-------
in
O
c
H
I
pi
a
z
PI
Ul
PI
O
X
z
Ul
-I
Table 44. Concentrations of Flue Gases at Shawnee Unit 10
during Limestone Injection
Limestone injection
Date
July
July
July
July
15
16
21
22
Rate,
Ib/min
333
333
333
167
333
333
Mole
CaO
2
2
1
0
1
3
ratio,
:SOsa
.2
.2
.2
.9
.8
.0
Sampling
location*
EP
EP
AH
EP
AH
AH
Temp, Concentrations of flue gasesc
J °F H2O, %
400
340
765
420 8.2
750
740
S03,
1
1
1
1
0
0
ppm
.2
.1
.0
.1
.9
.5
SO 2, ppm
962
962
1860
2072
1621
800
i
VD
Ul
1
a. Calculated from the rate of CaCOa injection and the rate of SOa production
in the absence of CaCOs injection, based on the observed SO2 concentration
and the estimated total flow rate of flue gases, approximately
540,000 ft3/min at 310°F and 1 atm.
b. AH signifies a location ahead of the air heater, and EP signifies a loca-
tion ahead of the electrostatic precipitator.
c. Expressed on the "wet" or actual basis. In the absence of a determination
of HaO, values listed in Table 41 were used for calculating the concentra-
tions of SOa and
C
•H
-------
-96-
• On three of the four days of experimental work, the
concentration of S02 was lowered by CaC03 injection
by approximately 30%. On July 21, however, no clear-
cut effect on the concentration of S02 was apparent
at either of the two rates of CaCOs injection. On
this date, furthermore, no clearcut effect was evi-
dent from the concentrations of S02 determined by an
instrumental method used by the plant personnel.
The apparent explanation for the apparent ineffective-
ness of CaCOa on July 21 is that the composition of
the coal varied throughout the day and thus the con-
centration of SOz produced in the boiler varied
during the period of sampling.
• Under circumstances where the concentration of SO3
was observed to be above 1 ppm in the absence of
CaCOs injection, the concentration was lowered to
about 1 ppm during injection.
• Even with excess amounts of CaO present, as indicated
by calculated CaOrSOa mole ratios as high as 3.0,
the removal of SOa was far from complete. On July 22,
for example, when the CaO:SOa ratio was 3.0, somewhat
less than 30% of the SOa was removed; on this occa-
sion, therefore, only about 10% of the available CaO
was used. It is to be expected that utilization of
the CaO will be incomplete, because a CaSOi, surface
coating masks a substantial part of the CaO and pre-
vents reaction in the contact time allowed.
-------
-97-
VII. DISCUSSION OF THE RESULTS OF THE
CONDITIONING STUDIES "
A. Interpretation of Resistivity Data
To preface the following discussion of the results of the
conditioning studies, the several difficulties encountered in
making and interpreting iri situ measurements of electrical resis-
tivity must be recalled. These difficulties can be enumerated as
follows: (1) the uncertainty in the significance of the decreas-
ing resistivity values obtained with the cyclone resistivity
probes at increasing times of measurement; (2) the lack of a con-
sistent relationship between resistivity values and the different
electric fields applied in the point-plane resistivity probe
(usually resistivity decreased with increasing electric field
but sometimes it increased; sometimes the dependence of resistiv-
ity on electric field appeared to vary with temperature and
sometimes it did not); (3) the lack of a consistent relationship
between the reported data for the same electric field in the
cyclone and point-plane samples (usually the values obtained with
the cyclone samples were higher but sometimes they were about the
same as those obtained with the point-plane samples—or even
lower than the latter).
To the difficulties listed above, there can be added the
problem of knowing how an experimentally observed value of resis-
tivity compares to the value that is significant in the precipita-
tor. It has been recommended by one author that electrical
resistivity always be measured at the electric field required for
electrical breakdown in the sample.1 It is reasonable to question
this approach, however, in view of the variance to be expected in
the required electric field as a result of a wide variance in
(1) the compaction of the fly ash; (2) the extent of contamination
of the ash with unburned, highly conductive carbon particles;
(3) the composition and the resistivity of the ash;* or (4) the
composition and the resistivity of the flue gases.
Even in the face of the experimental and theoretical uncer-
tainties listed above, we believe that we are on fairly firm
grounds in discussing the experimental results on the usual basis
of average values obtained at the same electric field (usually
The resistivity may be low enough to make sparking through the
gases rather than breakdown in the deposited ash the power-
limiting factor in a precipitator.
SOUTHERN RESEARCH INSTITUTE
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-98-
2.5 kV/cm) in both cyclone and point-plane samples, with time-
zero values selected from the time-dependent cyclone data. The
validity of this basis for discussing the resistivity data is
discussed in considerable detail in another report from this
laboratory.16 Because of circumstances beyond our control, how-
ever, we are unable to use this basis for the data obtained at
Cherokee Unit 2, where carbon contamination led to breakdown at
electric fields lower than 2.5 kV/cm, or at the X Station, where
the electric field was unreported (in the absence of specific
information, the assumption is made that the field was that
required for breakdown, but this assumption cannot be used to
estimate the field quantitatively for reasons listed above).
B. Discussion of Conditioning by Injected SO3 or H2SOi»
1. General conclusions
a. Type of conditioning agent
This research program dealt with two sources of 803 and
HaSOi, conditioning agents—(1) stabilized anhydrous SOa available
from Allied Chemical Corporation under the trade name Sulfan and
(2) concentrated aqueous H2SOi» (available from various commercial
sources but consistently characterized in composition by either
the density, 66°Be, or the weight percentage of H2SOi», 93.2%).
With respect to the conditions under which the concentrated
aqueous acid was evaporated and injected, this program included
work with two types of HaSOit injection processes, which are
referred to in this report as low- and high-temperature processes.
The two types of HaSOi* injection processes are different not only
on the basis of temperature but on the basis of the chemical form
to which the acid is converted by evaporation for transport
through the injection lines; in the low-temperature process the
vapor is predominantly HaSCK, and in the high-temperature process
it is predominantly SOa°.
The results of our investigation showed an apparent failure
to achieve the desired conditioning in only one of the five plants
equipped with SOa and HaSOn injection systems. This failure
occurred in an installation that has a high-temperature HaSO^
system (Cherokee Unit 3). The reason for the failure was not
identified, although several attempts were made to explain the
failure. Perhaps a faulty or unreliable injection system caused
the acid to be injected in an inactive form such as a HaSOif-HaO
mist, in view of the possible exposure of the SOs-HaO vapor mix-
ture in the injection lines to a duct temperature below the vapor
dew point. Because of the limited scope of .our studies of the
high-temperature HaSOi* system at Cherokee Unit 3, our findings
should not be construed as an indication that such a system is
basically unworkable.
-------
-99-
In general, our experimental findings lead to the funda-
mental conclusion that either an S03 or a H2SOit injection system
may be used to condition high-resistivity ash with roughly the
same likelihood of success. Such findings are not surprising
in view of the fact that injection of either SOs or HaSOi, will
theoretically lead to existence of only one of the two substances,
HaSOn, in the usual range of precipitator temperatures. The
choice between an SO3 or a HaSOi* system apparently should be
based not on the predicted efficiency of conditioning but on
other factors, such as costs or other factors as discussed
later in Section VIII.
b. Concentration of conditioning agent
The results of this investigation indicate that injection
of either SOs or HzSOi* at a rate sufficient to produce a concen-
tration between 5 and 20 ppm by volume in the flue gases will
usually be adequate for conditioning fly ash from low-sulfur coal.
This conclusion is based on studies with fly ash of various com-
positions in a wide range of temperatures.
The data from four plants showing resistivity as a function
of the concentration of injected agent are plotted in Figure 13.*
It appears that one curve is reasonably appropriate for showing
the resistivity data obtained at three of the plants but that a
distinctly different and lower curve is needed to represent the
data at the fourth plant (Cherokee Unit 2).
No clearcut explanation can be offered for the lower curve
showing the data at Cherokee Unit 2. At least a possible explana-
tion is that the curve was located below the expected range
because of the influence of an abnormal concentration of carbon
particles in the ash. However, this explanation does not seem
altogether plausible, for the influence of the carbon should have
been strongest in the absence of conditioning agent; thus, if it
were possible to correct the data for the effects of the carbon,
the data would still be abnormally low with the conditioning
agent present.
The apparent location of data points along one curve for
the other three plants in the Arapahoe, X, and Y Stations may be
somewhat fortuitous, in view of the differences in fly-ash
The data from a fifth plant, Cherokee Unit 3, are omitted
because of the uncertainty that the reported concentrations of
agent in this plant were actually reached.
SOUTHERN RESEARCH INSTITUTE
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-loo-
-H
>
•H
.p
W
•H
CO
0)
Arapahoe Station, Unit 4
Y Station, Unit 6
X Station, Unit 4
Cherokee Station, Unit 2
107 -
5 10 15 20 25 30
Injected concn of conditioning agent, ppm
Figure 13. Resistivity of Fly Ash as a Function
of the Injected Concentration
of Conditioning Agent
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composition, temperature, and other parameters. The data follow
separate curves if plotted as functions of parameters other than
the injected concentration of conditioning agent, as shown in the
following sections.
It is worthy of emphasis that the concentration probably must,
as a rule, be kept below some critical value, which is related to
the capacity of the fly ash to collect the agent quantitatively.
In two of the installations investigated, Unit 2 of the Cherokee
Station and Unit 6 of the Y Station, the injection of excessive
amounts of conditioning agent apparently led to stack emissions of
unacceptably high HaSCK concentrations.
c. Location of the injection of conditioning agent
It is evident that SO3 or HaSCU can be used effectively if
it is used to treat fly ash before or after partial removal of the
ash occurs in a mechanical collector. Successful conditioning was
observed in two plants without mechanical collectors (X and Y Sta-
tions) and in two plants with injection sites between mechanical
and electrostatic collectors (Cherokee Unit 2 and Arapahoe Unit 4).
Poor results were obtained in the one plant with injection ahead
of both mechanical and electrostatic collectors (Cherokee Unit 3),
but these results were evidently not characteristic of the results
to be expected with dual collectors.
Observations made with different injection sites lead to the
following conclusions:
• If fly ash is subjected to conditioning ahead of a mechani-
cal collector, the large fraction of ash removed mechani-
cally does not cause an unacceptable loss in the available
concentration of conditioning agent. This conclusion is
probably the result of the fact that the average size of
particles collected mechanically is large compared with
those at the entrance of the precipitator, thus the surface
area to mass ratio is low. The total amount of the agent
deposited on mechanically collected ash is therefore smaller
than that deposited on ash entering the precipitator.
• If fly ash is in contact with the conditioning agent for
only a brief time, as short as 1 sec in one plant inves-
tigated with injection immediately ahead of the electro-
static precipitator (the Y Station), an adequate degree of
conditioning can still occur.
No work was done during this program with SO3 or H2SOi» injec-
tion ahead of an air heater, although acid injection at this site
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has been reported. Because of the thermodynamically predicted de-
composition of SOs or HaSOi* to SOa and Oz at the high temperatures
that prevail in locations ahead of air heaters, as discussed later
in Section VIII-C, injection at these locations appears unwise.
do Concentration and particle size of the fly ash
Information about the influence of concentration and particle
size of fly ash on the effectiveness of conditioning was obtained
during studies in plants supplied with coals of widely varying ash
contents and operated with and without mechanical collectors„
Data showing resistivity as a function of the concentration
of injected conditioning agent relative to fly-ash concentration
and particle size are shown in Figures 14 and 15„ In Figure 14,
the concentration parameter is the ratio of the concentration of
conditioning agent to the concentration of fly ash at the point of
injection. In Figure 15, the concentration parameter is the ratio
of the concentration of conditioning agent to the surface area of
the fly ash (expressed in relative units) at the point of injection.
The factor expressing surface area in relative units was calculated
by multiplying the fly-ash concentration by the mean ratio of sur-
face area to mass. The value of the area-to-mass ratio^ was calcu-
lated by the principles described by Irani and Callis, with the
use of particle-size distributions obtained on the number-median
basis by microscopic counting. For fly ash that had not passed
through a mechanical collector, the value of the area-to-mass ratio
was proportional to the approximate factor 1/9.4, where 9.4 repre-
sents the pertinent particle size expressed in micrometers. For
fly ash that had passed through a mechanical collector, the cor-
responding value of the area-to-mass ratio was proportional to the
larger factor 1/2.9.
As plotted in Figure 14 or 15, the data for Cherokee Unit 2
do not show an unusual degree of divergence from the data for the
other plants, in contrast to the behavior previously noted. In
fact, the data for each plant appear to follow a distinct curve in
Figure 14 or 15. Possible explanations for the different curves
are offered in the following sections that discuss the influence
of fly-ash composition and temperature.
e. Composition of the fly ash
In connection with the influence of fly-ash composition, the
conclusion of overriding importance is that no fly ash was found
with a composition that prevented effective conditioning. Even so,
with various compositions, it is to be expected that the concentra-
tion of conditioning agent required will vary significantly as a
result of changes in fly-ash composition.
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-103-
10
1 3
Arapahoe Station, Unit 4
Y Station, Unit 6
X Station, Unit 4
Cherokee Station, Unit 2
106
Ratio of the concentration of conditioning agent (ppm)
to the concentration of fly ash (gr/ft3)
Figure 14. Resistivity of Fly Ash as a Function
of the Relative Concentrations of
Conditioning Agent .and Fly Ash
SOUTHERN RESEARCH INSTITUTE
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Arapahoe Station, Unit 4
Y Station, Unit 6
X Station, Unit 4
Cherokee Station. Unit 2
0
100
120
140
Ratio of the concentration of conditioning agent (ppm)
to the surface area of the fly ash (relative units)
Figure 15. Resistivity of Fly Ash as a Function of
the Concentration of Conditioning Agent
and the Surface Area of the Fly ash
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The unfavorable results reported for fly-ash conditioning at
Unit 3 of the Cherokee Station might lead to the conclusion that
fly ash produced from Routt County coal is difficult to condition.
However, the quite favorable results reported for Unit 2 of this
station with fly ash from the same coal show quite clearly that
this conclusion would be distinctly erroneous. Moreover, as pre-
viously discussed, the reported concentrations of conditioning
agent at Unit 3 are of doubtful validity.
The fact that fly-ash composition has a bearing on the effec-
tiveness of conditioning is suggested by data shown previously in
Figures 14 and 15, although the relative influences of composition
and temperature cannot be clearly separated. However, consider as
examples the data obtained at Cherokee Unit 2 and Arapahoe Unit 4
with fly ash of markedly different compositions but nearly equal
temperatures (290 and 275°F, respectively). The Cherokee ash re-
quired much less conditioning agent than the Arapahoe ash on either
basis shown in Figure 14 or 15, despite its somewhat higher tempera-
ture.
The importance of fly-ash composition is suggested further
by the data plotted in Figure 16, in which resistivity is shown as
a function of the SO.*"2 content of the ash. If consideration is
again given to the results at Cherokee Unit 2 and Arapahoe Unit 4,
it is seen that the Cherokee ash required less gain in S0i*~2 than
the Arapahoe ash.
The difference in the behavior of the fly ash at the Cherokee
and Araphaoe Stations may be attributed to the higher basicity of
the Cherokee ash and the resulting requirement of more acidic con-
ditioning agent for a given change in resistivity. Possible rea-
sons for this phenomenon are discussed later in Section VII.B.2.
Briefly stated, however, the reasons are that much of the condi-
tioning agent is wasted in neutralizing basic components of the ash
and that only a small part is used effectively to produce a surface
layer of free acid.
f. Temperature of the ash
The temperature of the fly ash during conditioning can be ex-
pected to influence the effectiveness of SO3 or H2S04 conditioning
agent by exerting an influence on the adsorptivity of the ash for
HaSOi* vapor or permitting the occurrence of acid condensation on
fly-ash nuclei. It is difficult to separate the effects of tempera-
ture variations from the effects of composition changes in the dif-
ferent plants investigated. However, on the basis of recorded data
and theoretical arguments given later in Section VII.B.2, we believe
that it is reasonable to conclude that, in general, lowering the
temperature will increase the effectiveness of conditioning. Lower-
ing the temperature will increase the affinity of a neutral or
weakly basic ash for H2SOH vapor. Lowering the temperature will
SOUTHERN RESEARCH INSTITUTE
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•4J
•H
•H
W
10
10
1 3
1 2
10
1 1
1. D Arapahoe Station,
Unit 4
2. O Y Station, Unit 6
3. A X Station, Unit 4
4. O Cherokee Station,
Unit 2
10'
107
0.5 1.0 1.5 2.0 2.5 3.0
Concentration of S0i»~2 in ash, %
3.5
Figure 16. Resistivity of Fly Ash as a
Function of the Sulfate Content
of the Ash
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-107-
probably have a smaller influence on the affinity of a strongly ba-
sic ash for H2SOi» but it will probably enhance the probability of
formation of a free-acid layer by lowering the rate of acid-base
reactions and minimizing the wastage of collected agent. If the
temperature is lowered to such an extent that condensation of acid
occurs on fly-ash nuclei, a very pronounced lowering of resistivity
must be expected.
The above arguments may provide explanations for the behavior
of the fly ash at the X Station, which appeared to be anomalous in
several respects. This ash was comparable in basicity to the fly
ash at the Arapahoe Station, but it appeared to be much more easily
conditioned than the ash at Arapahoe and appeared to behave in a
manner similar to the nearly neutral ash at the Y Station. The
temperature in the X Station, 230°F, was below the acid dew point,
whereas it was above the acid dew point in the other plants. Thus,
the mechanism of acid collection may have been different at the X
Station and the rate of reaction of collected acid with the fly ash
was probably slower at this station.
2. Theoretical considerations
a. Mechanisms of collection of HaSCK on fly-ash particles
There are at least two distinctly different mechanisms by which
I^SOi, in flue gas can be collected on the surfaces of fly-ash parti-
cles. One mechanism is the condensation of a mixture of HaSCH and
HaO vapors with fly-ash particles serving as condensation nuclei.
This phenomenon results in the formation of a liquid layer on the
particles; however, it can occur only if the temperature is below
the dew point of the vapors existing in the duct. Other mechanisms
involve the adsorption of HaSOt and probably the concurrent adsorp-
tion of H20 as well. As discussed subsequently, different mechanisms
of adsorption may exist, depending upon the sequence in which H2SOi»
and HzO are adsorbed.
(1) Acid condensation
This phenomena of acid condensation can be critically analyzed
only if reliable thermodynamic data exist for predicting dew points
of HzSOij-HaO vapor mixtures or if reliable experimental data exist
for showing dew points of adequately analyzed vapor mixtures.
Two notable attempts to secure the necessary thermodynamic
data for predicting dew points have lt been made. The first of these
two efforts was reported by Miiller in 1959 and was based on data
previously compiled by Greenewalt"4 5 and Abel**6; the second effort,
SOUTHERN RESEARCH INSTITUTE
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reportedly with the advantage of^more accurate background data, was
reportedly Gmitro and Vermeulen in 1963 and summarized by Snowdon
and Ryan in 1969. Dew points predicted from the original work of
Greenewalt and Abel and the later work of Gmitro and Vermeulen are
shown in Figure 17 for various concentrations of HaSOi* at two dif-
ferent concentrations of H20, 7 and 10% (covering the usual range
found in flue gases produced in coal-burning plants) for an assumed
total pressure of 1 atm.* The curves in Figure 17 show that at a
given set of Ha SO it and H20 concentrations the dew point predicted by
the data of Greenewalt and Abel is approximately 30°F higher than
the value predicted by the data of Gmitro and Vermeulen. They also
show that at a given HaSOi* concentration the dew point predicted by
either source of data decreases about 10°F as the concentration of
H20 is lowered from 10 to 7%.
Numerous efforts have been made experimentally to determine dew
points of H2SOi»-H20 vapor mixtures of known composition. One of the
most recent and perhaps most reliable study was reported by Lisle and
Sensenbaugh.2** The results of this study indicate that the dew point
curves based on the work of Greenewalt and Abel are more accurate than
those based on the work of Gmitro and Vermeulen, despite the access
of the latter authors to more up-to-date reference data.
To predict the possibility that acid condensation may have oc-
curred in the several plants where SOs and HaSOi* were injected as
conditioning agents, the range of reported concentrations of the
agent in each plant is shown by data points along a horizontal dashed
line in Figure 17. The conclusion reached by comparing the locations
of the several horizontal lines with the locations of the dew-point
curves is that only the temperature at the X Station was clearly be-
low the dew point at all injected concentrations of conditioning
agent. The possibility that the temperature at Arapahoe Unit 4 was
below the dew point at the two higher concentrations of conditioning
agent cannot be excluded; however, the temperature was definitely
above the dew point at the lowest concentration of the agent, which
gave evidence of a marked reduction in resistivity. Finally, the
possibility that the temperature in either of the other two plants
Recalculation of dewpoints from the data of Greenewalt and Abel
was made, since the results of Miiller's calculations from
these data cannot be easily seen from Muller's graphical sum-
mary.
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U)
o
c
X
tn
a
z
-P
m
*
340
320
300
280
260
240
220
200
Y Station, Unit 6
Cherokee Station, Unit 2
Arapahoe Station, Unit 4
— A A
I
5 10
Concn of
20
ppm
1. G, A (10%)
(7%)
o
vo
I
50
100
Figure 17. Dew Points Predicted from the Data of Greenewalt and
Abel (G, A) and Gmitro and Vermeulen (V, M) for Flue Gases
Containing H2SOi» at Various Concentrations in the
Presence of H20 at Concentrations of 7 and 10%
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was ever below the dew point can be completely excluded, assuming
that the possible locations of the dew points cannot be outside
the range indicated by the curves in the figure.*
The above conclusions indicate, at least, that condensation
of acid is not necessary for the conditioning of fly ash by HaSOi*.
Although condensation may have been involved in the conditioning
process at X Station, it did not necessarily occur; if chemisorp-
tion of the H2SOi» on the highly basic ash occurred more rapidly
than the condensation process, the concentration of H2S01, vapor
could have been lowered to levels below the minimum required for
condensation.
(2) Acid adsorption
The above consideration of the condensation mechanism leads
us to the conclusion that acid adsorption is at least a sufficient
mechanism, and, thus, not the necessary mechanism, for fly-ash
conditioning in the presence of HaSOii vapor.
The pioneering work by Chittum7 led to the conclusion that
conditioning of fly ash by an acidic vapor involves, first, the
chemisorption of this vapor on the fly ash and then enhanced
adsorption of HaO vapor on the chemically altered surface of the
fly ash. Chittum argued that a strongly basic ash would have a
greater affinity for an acidic vapor, such as HaSCK, than a
neutral or acidic ash and, thus, a strongly basic ash should be
most easily conditioned.
The results of our work appear to be inconsistent with
Chittum 's hypothesis. This point can be supported by the data
obtained in the studies at Arapahoe Unit 4 and Cherokee Unit 2.
The Arapahoe ash was more basic than the Cherokee ash and, more-
over, was conditioned at a slightly lower temperature and a
slightly lower concentration than the Cherokee ash. In view of
the differences in the conditioning parameters, the Chittum
hypothesis would predict more efficient conditioning of the
Arapahoe ash. The experimental data show that more efficient
conditioning occurred with the Cherokee ash, whether the criterion
In the above discussion, the fact that the total pressure was
not precisely 1 atm in all of the plants is ignored. In the
Arapahoe and Cherpkee plants in the Denver area, the total
pressure was only about 0.83 atm. However, the conclusions
reached above would not be significantly altered by making a
correction for the difference in pressures.
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of effectiveness is based on the lowering of resistivity observed
with a given rate of injection of conditioning agent or the lower-
ing of resistivity observed with a given amount of agent collected
as S0i»~2 on the ash (Figures 13 and 16) .
Additional data contrary to the Chittum hypothesis were
obtained at the Y Station, where the resistivity was lowered
markedly with either a low rate of injection of conditioning agent
or a small amount of agent collected on the ash, despite the near
neutrality and high temperature of the unconditioned ash (two fac-
tors that were unfavorable for chemisorption).
Still other data contrary to the Chittum hypothesis were
obtained at Shawnee Unit 10 with and without CaCOs injection in
the boiler. With injection, the amount of SOa collected on the
dust particles as SCK"2 was greatly increased, but the effective-
ness of the H20 for conditioning the ash was markedly reduced.
It is our belief that a highly basic ash will require more
acid conditioning agent than a neutral ash, in terms of either
the concentration injected in the duct or the concentration
collected in the ash, for the following reasons: (1) the acid
initially collected is not simply adsorbed but it undergoes reac-
tion with basic components of the ash, such as CaO, to form SCK"2
salts, such as CaSCK, that may have little affinity for HzO;
(2) the amount of acid collected must increase to the point that
an adequately thick protective layer of SO*"2 salts covers the
remaining basic components of the ash and the amount of acid
collected thereafter can exist on the surface of the ash in the
form of HaSCK rather than as SCK~2 salts; (3) the layer of H2SOi»
ultimately produced exhibits a strong affinity for HzO and, thus,
a layer consisting of both HjSCK and HaO with the necessary elec-
trical conductivity is finally produced on the surface of the ash.
The fact that only a small amount of collected S0i»~2 was
apparently required to condition the highly basic fly ash in the
X Station appears, on initial consideration, to be inconsistent
with the theoretical concepts outlined in the preceding paragraph.
One possible rationale for the apparent discrepancy was that con-
ditioning in the X Station occurred at an unusually low tempera-
ture, where the rate of diffusion of H2SCU through the initially
produced layer of SCK~2 salts would be relatively low and, thus,
the required thickness of the protective layer of S0i»~2 salts
would be reduced.
b. Mechanism of surface conduction on conditioned ash
It is our conviction that the predominant conductive mate-
rial on conditioned fly ash is free HaSOn. At least semiquan-
tative evidence for the presence of the acid has been repeatedly
SOUTHERN RESEARCH INSTITUTE
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obtained during analyses of fly ash from various sources. Cer-
tainly the existence of HjSOi,, probably in a mixture with H20,
would provide a much higher conductivity than the existence of
HzO alone, assuming that the behavior of the two substances in
liquid form gives a reasonable basis for predicting their behavior
in thin adsorbed layers on the surface of fly-ash particles.
C. Discussion of Conditioning by Naturally Produced SO 3
As stated in Section I of this report, high resistivity of
fly ash does not cause problems in the operation of electrostatic
precipitators in power stations that burn coal with an adequate
percentage of sulfur, in excess of 1% or perhaps even 2%. It is
a matter of theoretical interest that if it were possible to main-
tain chemical equilibrium between the two products of sulfur com-
bustion, SOz and SOa, practically all of the S02 would be con-
verted to S03 at temperatures below 1000°F with 02 in the usual
range of concentrations and, thus, even coal with a very low
percentage of sulfur would produce an adequate concentration of
SOs.1*9 It is a matter of record, however, that chemical equilib-
rium is not maintained, because SOa is always the predominant
oxide of sulfur emitted from a coal-burning power station, as
shown during this investigation and by other investigators.27'28
White1 has indicated that, as a rule, about 1% of the sul-
fur oxides exists as SO 3 in the flue gases of a coal-burning
power station. Other investigators27 have reported that the
percentage of SO 3 may be as high as 10%; our work has shown per-
centages usually below 1%, on the other hand. The fact that the
percentage of SO3 varies widely is not surprising. Although the
gas-phase conversion of SOa to S03 is slow, comparatively rapid
conversion may occur as the result of catalytic effects of various
components of the fly ash and thus may lead to an unusually high
percentage of SO3. On the other hand, adsorption of the SO3 on
fly-ash particles may occur to an extensive degree and, thus,
lead to an unusually low residual percentage of SOs. Furthermore,
it has been shown that variations in the amount of excess air
supplied to the boiler can lead to significant changes in the con-
centration of 80s-50 In view of the varying data in the litera-
ture and the opposing phenomena that control the net conversion
of SOa to SOs, it is of interest to review the relative concentra-
tions of SO2 and SO3 found in various power stations during this
investigation and to determine what generalizations may be made
about the adequacy of naturally produced SO3 for conditioning fly
ash.
The concentrations of SOa found in various power stations
with various percentages of sulfur in the coal are plotted in
Figure 18. With a few exceptions, the data indicate that the SOa
-------
in
O
H
X
m
a
z
a
m
ui
m
O
X
H
H
m
04
CM
o
en
M-l
o
3000
2500
2000
1500
o
c
3 1000
500
Sulfur in coal, %
Figure 18. Concentration of S02 in Flue Gases
as a Function of the Sulfur
Percentage in Coal
(Each data point shows the SOz concentration
in the middle of the observed range.)
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concentration can be predicted with reasonable accuracy by multi-
plying the percentage of sulfur in the coal by the factor 750.*
The predicted SOa concentration pertains to a location either up-
stream or downstream from the air heater in view of the fact that,
in general, no significant variation across the air heater has
been discovered.
The relative concentrations of naturally produced S02 and
SOa found in various power stations, including the different con-
centrations of SOa found upstream and downstream from the air
heater, are listed in Table 45. Calculated percentages of SOa are
included in this table and also plotted in Figure 19. No attempt
was made to construct a curve through the plotted data points, in
view of both the scatter in the data and the variations in the
properties of the fly ash in the various plants represented. Even
so, from an overall point of view, the data in Figure 19 suggest
that a peak value of the SOa percentage will usually occur within
the range of temperatures normally existing in the air heater,
with an upper limit of 600°F or more and a lower limit usually of
325°F or less, depending upon operating conditions. It is note-
worthy that the highest percentage of SOa found, around 1%,
occurred in the Kingston plant at a temperature within the range
described, 365°F.
It is not unreasonable to assume that the maximum concentra-
tion of naturally produced SO3 that will be effective for fly-ash
conditioning will be the value found at a temperature around 350°F,
where the peak in resistivity normally occurs as a result of the
relative contributions of surface and volume conductivities.
Higher concentrations may occur at higher temperatures, but part
of the SOa that occurs at high temperatures may be collected by
the fly ash as a low-conductivity S0i»~2 salt when the temperature
approaches the critical region around 350°F. Collection of the
SOa remaining around 350°F, however, can lead to the formation of
a high-conductivity acid layer, especially as the temperature
approaches the usual range of precipitator operation of 325 to
275°F.
From the widely scattered data plotted in Figure 19, it is
not possible to cite the percentage of sulfur oxides to be
expected as SOa in all power plants at any given temperature.
However, if it is assumed that the upper limit will, as a general
rule, be around 0.7% in the critical region near 350°F—an
Some degree of variance is to be expected from variations in
the excess air used for combustion and the heating value of
the coal.
-------
Table 45. Summary of Observed Concentrations of SOa and
in Power Plants Burning Coal of Various Sulfur Contents
Predicted Observed concn
Observed concn of SO3
Power plant
Cherokee Unit 2
Cherokee Unit 3
Arapahoe Unit 4
Y Station, Unit 6
Widows Creek Unit 7
Kingston Unit 5
Z Station, Unit 1
Shawnee Unit 10
in
0
c
H
FI
z a. Based on Figure
a
Z b. Based on an SO2
n
a
n
i
z
in
H
c
-1
n
concn of
SO 2 , ppma
458
375
367
465
2690
1590
712
1460
1360
2900
2300
1480
range of
SO 2 , ppm
406-512
314-358
387-446
373-488
2450-2860
1300-1770
600-700
820-1020
1530-1580
2880-3030
1740-2070
1150
Upstream from air heater
Temp, °F SO3, ppm S03 , %b
695 2 0.4
740 <1 <0.3
740 <1 <0.3
_
625 15-20 0.6-0.8
640 9-13 0.6-0.9
_ _ _
600 4-5 0.4-0.5
_ _ _
- - -
750 1 0.05
720 1 0.1
Downstream from air
Temp, °F
290
310
275
320
285
255
365
325
295
260
295
375
305
266
330
-
18, showing the concentration of SO2 as a function of the sulfur
concentration
at the middle
of the observed range shown
SO 3 , ppm
1-3
1
<1
<1
15-18
8-10
15-18
9-10
4-5
1
1-2
3-5
5
1
<1
-
percentage
heater
S03, %*>
0.2-0.6
0.3
<0.3
<0.3
0.6-0.7
0.3-0.4
0.9-1.2
0.6-0.7
0.3-0.4
0.2
0.1-0.2
0.2-0.3
0.2
0.04
<0.05
-
in coal.
in the third column.
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1.4
AA-
dp
1.2
CO
01
•o
fi
CO
o>
H3
-H
X
O
M
3
<4-l
rH
3
W
0.8
0.6
0
•
0
•
I
250
275
300
325
&
350 550 600
Temperature, °F
650
700
750
800
Figure 19. Percentage of Sulfur Oxides Found as SOs
as a Function of Temperature in Various Plants
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assumption reasonably consistent with the data plotted in
Figure 19, despite some notable exceptions—calculations can be
made of the minimum percentages of sulfur in coal that will pro-
duce various concentrations of SOa in the range up to 20 ppm,
which appear adequate on the basis of studies with injected SOa
or HaSOi*. Calculations based on the relationship shown in Fig-
ure 18 between the SO2 concentration in flue gases and the sulfur
percentage in coal gave the following results:
SO3, ppm Sulfur, %
5 0.95
10 1.90
15 2.85
20 3.80
These calculations are generally consistent with industrial
experience indicating that coal with a minimum sulfur percentage
of 1 to 2% will be required if adequate conditioning of the fly
ash by naturally produced S03 is to occur. They are also gener-
ally consistent with our resistivity data for fly ash that had
been conditioned only with naturally occurring 863. Our data in
this context may be summarized as follows:
• With coals containing around 0.5% of sulfur, the
resistivity of the fly ash entering the precipitator
was always excessive.
• With coals containing about 1% of sulfur, the resis-
tivity of the fly ash was still above the maximum
desired value.
• With coals containing about 2% of sulfur, the resis-
tivity was slightly above or below the maximum
desired value, depending upon the temperature.
• With coals containing 3% or more of sulfur, the
resistivity was in the desired range or below this
range, again depending upon the temperature.
D. Discussion of Conditioning by Injected H20 or NHa
Only a limited study of conditioning by injected HaO was
included in this program. From the data obtained, it appears that
a major effect of HgO is the more effective utilization of the SOs
that is naturally produced. Certainly, at temperatures of 320 to
330°F, as studied at Kingston Unit 5, this effect is indicated by
the experimental data. Perhaps at lower temperatures, which are
more commonly found in electrostatic precipitators, the effect of
H20 injection can be relatively independent of
SOUTHERN RESEARCH INSTITUTE
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The relatively extensive study of NHa conditioning failed
to explain the mechanism by which this agent overcomes the problem
of low resistivity encountered with fly ash from high-sulfur coal.
A major question not satisfactorily answered during this study is
the fate of the injected NHs. Thus, conclusions cannot be given
in regard to possible mechanisms by which the NHa functions.
Conditioning of fly ash by NHa as a remedy for either low
resistivity at the Widows Creek Station1 l or high resistivity at
various Australian stations9 is a subject to which intensive
future research should be devoted.
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VIII. DISCUSSION OF BASIC THERMODYNAMIC ,
ENGINEERING, AND ECONOMIC ASPECTS OF
CONDITIONING WITH S03 OR H2SOU
A. Optional Processes for Generating Vapors of the Conditioning
Agents
The various processes that can be used for injecting S03 or
have been described by Whitehead.6 They are as follows:
(1) Vaporization of stabilized SOa
(2) Vaporization of
(3) Stripping of SOa from oleum (a solution of S03 in
anhydrous H2SOi»)
(4) Catalytically converting part of the S02 in the
flue gases to SOa
(5) Burning sulfur and catalytically converting the
S02 thus produced to SOa .
Studies of the first two of the processes have been included
during this investigation. The other three processes have various
shortcomings that limit their applicability. Stripping S03 from
oleum has the disadvantage that the residue of H2SOi» must be
returned to the manufacturer for recharging with SOa . Conversion
of part of the S02 in the flue gases would require removal of the
fly-ash particles from a stream of gases to avoid fouling of the
V205 oxidation catalyst. Whitehead has estimated that in a plant
burning coal with 0.4% sulfur the removal of fly-ash particles
would require a hot-side precipitator capable of removing 99.9%
of the fly ash in 8% of the total gas volume, and he has estimated
further that the size of the precipitator would be about one-fifth
of the size of the main precipitator.6 Burning of sulfur suffers
from the disadvantage of difficulties in controlling the burning
process as required.
B. Vaporization of Stabilized SOs
1. Properties of SOa
The commercial form of stabilized SOa that is available
from Allied Chemical Corporation is described as a water-white
liquid containing at least 99.5% of SOa by weight and minor
amounts of H2S04 and the stabilizing agent. Some of the physical
properties of Sulfan reported by the manufacturer pertinent to
its use as a conditioning agent are the following:
SOUTHERN RESEARCH INSTITUTE
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Specific gravity, 1.85 at 95°F
Viscosity, 1.5 cp at 90°F
1.3 cp at 100°F
Freezing point, 62.5°F
Boiling point, 113°F
Heat of vaporization, 230 Btu/lb
The fact that ambient temperature is frequently below the freezing
point requires heating and insulation of storage and other hand-
ling facilities. A temperature range of 90 to 100°F is recom-
mended during handling of the liquid.
The chemical properties of Sulfan require exceptional care
in its handling. Liquid SOa and water react with explosive vio-
lence to produce I^SO^. Thus, when the SOa is evaporated in a
stream of air, the air must be thoroughly dried; however, if com-
mercial driers capable of lowering the water dew point to -40°F
at pressures up to about 8 atm are used, the problem of drying
the air can be satisfactorily overcome. Any liquid SOa spilled
must be neutralized with soda ash (or some other suitable weak
base) before it can be flushed away with water. Safety showers
must be installed in the handling area for the protection of
personnel.
The reactivity of liquid SOa also necessitates care in the
selection of the materials used for storing and handling. For
different components of the handling equipment, the following
materials are recommended:
Storage tank—flange-quality steel (ASTM A285 Grade C),
assembled in accordance with Part UW of ASME Code
for Unfired Pressure Vessels.
Pipe and fittings—Schedule 80 black steel pipe with
all-welded construction; stainless steel braided
hose or reinforced Teflon hose for flexible assemblies
Valves and pumps—FA-20 alloy and Teflon components.
2. Processing equipment
A flow diagram showing the principal items of equipment
needed in a system injecting evaporated SOa has been presented by
Whitehead.6 These items include the following:
• System for supplying air to the SOa evaporator
—compressor
—dryers
—heater
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• Heated storage tank for SOa
• Evaporator for SOa, of the steam-heated vertical
falling-film type (A surface area of 4 ft2 is
sufficient to vaporize SOa at the rate of 150 Ib/hr
and supply a 250-MW power unit with approximately
20 ppm of conditioning agent.)
• Heated lines leading from the evaporator to the
injection manifold.
Although the equipment must be used with the indicated pre-
cautions, the injection of SOa offers some important process-
engineering advantages over the injection of vapors from H2SOi».
The higher volatility of SOa allows lower system operating temper-
atures and a higher ratio of SOa to carrier gas in the injection
lines.
C. Vaporization of H2SO^
1. Thermodynamic considerations
The thermodynamic aspects of the evaporation of concentrated
aqueous H2SOi* at the usual commercial strength (66°B£ or 93.2% of
H2SOi* by weight) are far more complex than those of the evapora-
tion of SOa, as a result of the fact that the HzSOit-HaO binary
system departs markedly from ideal behavior at all relevant tem-
peratures. The temperature and relative amounts of acid vapors
and carrier gas used for the evaporation of H2SOi» must take into
account the complex liquid-vapor equilibria in the binary H2SOi*-H20
system that prevails at relatively low temperatures (up to about
400°F) and the additional complexities of vapor equilibria involv-
ing H2SOit, SOa, H20, S02, and 02 that occur at higher temperatures.
One of the types of H2SOi» evaporation and injection systems
investigated during this program has been referred to as a "low-
temperature" system. It is based on evaporation of the acid at
temperatures of 400 to 500°F in air of ambient humidity levels.
The maximum concentration of acid vapors that can be produced in
this range of temperatures is limited by the low volatility of
H2SOi» in the presence of H20 vapor at ambient concentrations, for
which the value of 1% by volume may be used as a representative
concentration (this value corresponds to a relative humidity of
about 30% at 75°F). With the vapor-pressure data of Gmitro and
Vermeulen, **7 calculations have been made of the maximum concentra-
tions of acid vapors that are theoretically 'obtainable at tempera-
tures of 400, 450, and 500°F in an injection system with optional
total pressures of 1.0 atm and 2.0 atm. The results are listed
in Table 46; they show that H2SOi* is sufficiently volatile to
SOUTHERN RESEARCH INSTITUTE
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permit the injection lines to carry concentrations in the range
from 12,000 to 146,000 ppm (1.2 to 14.6% by volume). In practice,
however, a concentration much lower than the range theoretically
obtainable—about 5,000 to 10,000 ppm—is maintained in the
injection lines. Other data included in Table 46 are the calcu-
lated percentages of the total concentrations of 80s and H2SOi»
that would be present as SOs under the conditions of evaporation
described; these show that only 4 to 15% of the conditioning
agent would be transported through the injection lines as SOa,
whereas 85 to 96% would be transported as H2SOi,.
Table 46. Concentrations of Acid Vapors Theoretically
Obtained from 66°Be H2SOi» in a Low-
Temperature Injection System
Total Total concn Percentage of
pressure, Temp, of 80s and total vapors
atm °F H2SOi,, ppm present as SOa
1.0 400 26,000 6
450 66,000 10
500 146,000 15
2.0 400 12,000 4
450 32,000 9
500 72,000 14
One practical reason for maintaining a concentration of
only about 5000 ppm in the injection lines of the low-temperature
HaSOi, system is that this concentration can be conveniently main-
tained with a co-current evaporator, in which 660Be" HzSOi, is
admitted at a temperature of about 75°F and preheated air is
admitted at a temperature of about 490°F. We have estimated that
the heat required to evaporate the acid and reach a final vapor
temperature of about 450°F is approximately 590 Btu/lb. Air
supplied at 490°F in sufficient quantity to produce a final tem-
perature of 450°F will provide the heat required to produce a
vapor concentration of 5000 ppm in the exit stream from the
evaporator.
The second type of HzSOi, evaporation and injection system
investigated during this program has been referred to as a "high-
temperature" system. In the installation of this type at Cherokee
Unit 3, the hot gases produced by burning natural gas are used to
evaporate 660Be" H2SO., in the range of temperature from 700 to
1000°F. There is no problem in reaching very high vapor concen-
trations in this range, in view of the fact that the maximum
boiling point of the HaSOit-HaO liquid system is exceeded.1*7 It
is of interest that in the range from 700 to 1000°F the vapor of
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H2SOi» is almost completely dissociated as S03 and HaO.1*7 It
must be kept in mind, however, that on injection through pipes
that are not adequately insulated, the mixture of SOs and H20
may be cooled sufficiently to condense as an H2SOi,-H20 liquid
aerosol. The possibility of condensation in insufficiently
insulated injection lines is enhanced by the high concentration
of H20 vapor produced during the evaporation process.
Another interesting aspect of the high-temperature H2SOi,.
system is that partial decomposition of 80s to SOa and Oz may
occur if the concentration of 02 is not maintained at a high
level. Table 47 shows calculated values of the percentage of
the SO3 that may be converted to S02 at various assumed 02 con-
centrations (the calculations were based on the data cited by
Hedley*9) .
Table 47. Percentages of SOa Vapor Converted to S02 Vapor
at Equilibrium at Various Temperatures and Q2
Concentrations in a High-Temperature
Injection Process
Concn of Oa/ %
0.1 1.0 2.0 3.0 10.0 20.0
700 31110 0
800 11 4 3 3 1 1
900 78 11 8 7 4 3
1000 53 26 20 17 10 7
1100 73 46 38 33 22 16
1200 86 66 58 53 38 30
1300 92 79 73 69 55 46
1400 96 88 84 81 70 62
2. Process-engineering considerations
The principal items of equipment now used in both low- and
high-temperature HaSOi* injection systems are listed below:
Acid-storage tank
Metering pump
Acid vaporizer
Heat-traced ducts and injection manifolds.
SOUTHERN RESEARCH INSTITUTE
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In the high-temperature system, a low pressure blower producing
an absolute pressure of about 1.1 atm (2 psig) is used to supply
air for burning the natural gas and pumping the effluent vapors
from the acid vaporizer to the injection ports. In the low-
temperature system, a turbo-compressor is used to supply air at
an absolute pressure of about 1.8 atm (12 psig) to an air heater
and then to the acid vaporizer.
D. Economic Aspects of SO 3 and
1. Capital costs
Conditioning
Information secured from the Public Service Company of
Colorado about the installed costs of various conditioning
systems is summarized in Table 48. This information is for the
conditioning systems now in use, which differ in some respects
from those in use during our field studies at the Arapahoe and
Cherokee Stations. The cost data differ substantially in
several respects from the anticipated costs that were previously
published in the literature.30
Table 48. Capital Costs of S03 and
Conditioning Systems Installed for the
Public Service Company of Colorado
of System
Power units
supplied
Power
rating,
MW
SO 3 evaporation
H 2 SO i» evaporation,
high temperature
Number
2
1
evaporation,
low temperature
2
1
evaporation,
low temperature
110
360
165
44
110
44
Total power
rating, MW,
of units
supplied
580
165
198
44
Approximate
total
cost, $
500,000
300,000
400,000
250,000
An independent estimate was made of the capital cost of an
SO3 conditioning system to supply a single 250-MW power plant,
as shown in Table 49. The procedure used for estimating costs
was based on ratios of total costs to direct equipment costs for
process plants built in the last 15 years at an existing plant
site.51 Application of the same procedure to the SO3 system
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instailed to serve the multiple power units operated by the
Public Service Company resulted in an estimate of about
$450,000—only 10% below the actual cost, showing the validity
of estimation procedure.
Table 49. Estimate of Capital Cost of an S03 Injection
System for a Single 250-MW Power Unit
Item
Storage tank, horizontal, 10,000 gal
Custom-fabricated SO3 evaporator,
stainless steel, ~4-ft area
Compressor, ~100 ft3/min (standard conditions),
i25-psig delivery pressure (includes com-
pressor, motor, air filter, after cooler,
and receiver)
Rotameters
Metering pump
Injection manifold
Air heater
Air dryer
Steam boiler
SUBTOTAL - equipment on the F.O.B. basis
Equipment freight cost, estimated as
5% of F.O.B. cost
SUBTOTAL - equipment as delivered
Installation cost, estimated as 47% of (E)
Building, assumed to be 1000 ft2 at $20/ft2
Piping, estimated as 66% of (E)
Instrumentation, estimated as 18% of
Electrical auxiliaries, estimated as
11% of (E)
SUBTOTAL, DIRECT COSTS
Engineering & supervision costs,
estimated as 33% of (E)
SUBTOTAL, DIRECT + INDIRECT COSTS
Contractor's fee, estimated as 5% of (D,I)
Contintency, estimated as 10% of (D,I)
TOTAL
(E)
Cost*
$ 7,350
12,000
14,000
420
940
2,000
600
1,800
5,000
$ 44,WO
2,200
$ 46,200 (E)
21,700
20,000
30,500
8,300
5,100
$131,800 (D)
15,200 (I)
$147,000 (D,I)
7,400
14,700
$169,100
a. Listed on the F.O.B. basis for each item of equipment on
the first nine items.
SOUTHERN RESEARCH INSTITUTE
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The estimate given in Table 49 is considerably higher than
recent estimates given in the literature. Darby and Heinrich52
estimated the cost of an S03 injection system for a 300-MW plant
to be about $63,000 in 1966, while Coutaller and Richard53 gave
an estimate for a 250-MW unit of $60,000 to $80,000 in 1967 (both
of these estimates pertained to European installations). Actual
experience of the Public Service Company and our independent
estimate indicate that the literature estimates are unrealisti-
cally low for the installation of facilities in the United States
under current economic conditions, certainly with the extent of
instrumentation used in the Public Service Company plants.
An estimate of the cost of an SC-3 system to supply a single
500-MW power plant was made by use of the power rule, given by
Cn = C(R)X
where Cn and C are the capital costs for expanded and existing
facilities, respectively; R is the ratio of expanded to existing
capacity; and x is a size exponent, which has been found to range
between 0.6 and 0.7 in many instances.51 With an assumed value
of 0.6 for the size exponent, the estimated cost for a 500-MW
power unit is $256,000. It is assumed, on the other hand, that
for a 125-MW unit, the cost would be the same as estimated for a
250-MW unit, because this cost ($169,000) appears to be about the
minimum that can be reached in constructing an SOa injection
facility. It is to be noted that the estimated cost of $256,000
to supply a single-unit 500-MW plant is much lower than the actual
cost of $500,000 experienced by the Public Service Company in
supplying a three-unit plant of only slight greater total capa-
city, 580 MW. The discrepancy in costs is the result of the
multiplicity of facilities such as evaporators that must be pro-
vided in a system designed to supply two or more power units
either concurrently or independently as the need arises.
Estimates of the costs of .HzSOi* injection systems to supply
individual power units of various sizes were based on the data
supplied by the Public Service Company. For a high-temperature
system supplying a 165-MW plant, the actual cost of $300,000 was
used as the base figure. For a low-temperature HaSCK system
supplying a single 198-MW plant, it was assumed that the cost
would be about $325,000* rather than the $400,000 spent to supply
This figure was reached arbitrarily by averaging the costs of
facilities supplying three,units of 198-MW total capacity and
one unit of 44-MW capacity. The figure should clearly be less
than that experienced for the 198-MW capacity, in view of the
triplicate components needed to supply three units concurrent-
ly or independently.
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three units with the same total production capacity. For the two
types of systems serving power units of 250- and 500-MW capaci-
ties, capital costs were calculated from the power rule. For a
power plant of 125 MW, it was assumed that the cost of a high-
temperature system would be $300,000, the same as that experienced
for a 165-MW plant. For a plant of the same capacity, the cost of
a low-temperature system was estimated by averaging the actual
cost of a 44-MW installation ($250,000) with the estimated cost
for a single-unit 198-MW plant ($325,000), thus obtaining $288,000,
The above-described estimates of capital costs of SOs and
HaSOit injection systems for single-unit 125-,' 250- , and 500-MW
power stations are summarized in Table 50.
Table 50. Estimates of Capital Costs of Conditioning Systems
for Various Sizes of Single-Unit Power Stations
Type of conditioning plant
Stabilized SOa evaporation
High-temperature
evaporation
Low- temperature H2SOit
evaporation
2. Operating costs
Size of unit, MW
125
250
500
$169,000 $169,000 $256,000
300,000
288,000
387,000
375,000
583,000
570,000
A listing of energy and raw materials costs for three types
of injection systems serving a single 250-MW power unit is given
in Table 51. These costs are based on 7,000 operating hours at
full load per year (load factor = 0.80). The following assump-
tions were used in obtaining the estimates.
• Liquid SOs cost = $65/ton + $21/ton shipping
(400 miles)
• 66°Be H2SOi, cost = $31.60/ton + $10/ton shipping
(less than 200 miles)*
• Steam cost for SOs evaporation = $1/1,000 Ib
It is assumed that
than SO 3 .
will, as a rule, be more accessible
SOUTHERN RESEARCH INSTITUTE
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Table 51. Itemized Energy and Raw Material Costs for
"Three Types of Acid Injection Systems for
a Single 250-MW Power Unit
Type of system
SO3 evaporation
Item
Injected concn,
10 ppm
$/Year Mils/kWh $/Year Mils/kWh
Injected concn,
20 ppm
SO 3
Steam
Power :
Total
22
Trace heating
Air
Air
compression
heating
1
24
,000
125
110
780
,100
,100
0.
0.
0.
0.
0.
0.
0126
0001
0001
0004
0006
0138
44
1
Te
,000
250
110
780
,100
,200
0.
0.
0.
0.
0.
0.
0252
0001
0001
0004
0006
0264
H2SOi, evaporation,
high temperature
H2SOi» evaporation,
low temperature
Natural gas
Power: Trace heating
Air blower
Total
H2SOi»
Power: Trace heating
Air compression
Air heating
Total
12,900
1,100
800
200
0.0074
0.0006
0.0005
0.0001
25,800
2,200
800
400
0.0147
0.0013
0.0005
0.0002
15,000
0.0086
29,200
0.0167
12,900
500
2,350
5,750
21,500
0.0074
0.0003
0.0013
0.0033
0.0123
25,800
500
4,700
11,500
42,500
0.0147
0.0003
0.0027
0.0066
0.0243
to
00
I
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• Natural gas cost = $0.40/1,000 ft3 (standard conditions)
• Low-pressure blower and turbo-compressor efficiency =
40%
• High-pressure compressor efficiency = 50%
Airflow was assumed to be held constant with the SOa evapo-
ration system, but a variation in airflow proportional to changes
in injection level was assumed for the acid evaporation processes.
A low pressure blower (2 psig) provides the air used for combus-
tion in the high-temperature evaporation system; the required
flow rate of air was based on an assumed concentration of SO3 of
15,000 to 16,000 ppm in the injection line. A turbo-blower
(10-15 psig) was assumed to provide the air in the low-temperature
system; the flow rate of air was based on an assumed concentration
of 5,500 ppm in the injection line.
The cost calculations indicate that the low-temperature
acid evaporation system has total energy and raw material costs
approximately equal to those of the SO3 evaporation system, but
they show that the high-temperature acid evaporation system is
considerably cheaper to operate because of lower air-compression
requirements and the lower cost of natural gas as a heat source.
Other system designs are possible, of course, such as an indi-
rectly fired gas heater in a low-temperature acid evaporation
system.
3. Total annual operating costs
Tables 52, 53, and 54 were prepared to provide a comparison
of the total estimated annual costs of operating the three types
of injection processes for power units of 125-, 250-, and 500-MW
generating capacities. Annual capital charges were computed as
14.5% of the estimated capital costs in Table 51, and annual
labor maintenance costs were estimated at 5% of the capital
costs.51 Comparisons of the three systems indicate that the two
H2SOif evaporation processes would require similar expenditures
but that the SOs process would be considerably less expensive.
The lower annual operating cost of the SO3 system arises primarily
from the lower capital investment required. If I^SOit evaporation
systems should be installed in greater numbers than they have been
to date, the cost per installation could decline and become com-
parable to that of the SOs system.
In view of present-day requirements for reduced stack par-
ticulate emissions, the differences in costs of the three condi-
tioning processes are not likely to be significant. Instead of
cost, the reliability and past performance of a system offered by
a given vendor should be the primary basis upon which a condition-
ing process is selected.
SOUTHERN RESEARCH INSTITUTE
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Table 52,
Type of system
SO3 evaporation
Estimated Total Operating Costs of Acid Injection
in a Single 125-MW Power Unit
Ha SO i» evaporation,
high temperature
Ha SO i, evaporation,
low temperature
Item
SO 3
Steam
Electricity
Capital charges
Labor and maintenance
Total
H2SOi»
Fuel
Electricity
Capital charges
Labor and maintenance
Total
H2SOi,
Electricity
Capital charges
Labor and maintenance
Total
Injected concn
10 ppm
S/Yeair
11,000
60
1,000
24,500
8,500
45,.100
6,450
550
900
43,500
15,000
66,400
6,450
4,600
41,800
14,400
67,200
Mils/kWh
0.0126
0.0001
0.0011
0.0280
0.0097
0.0515
0.0074
0.0006
0.0010
0.0497
0.0172
0.0759
0.0074
0.0053
0.0478
0.0165
0.0770
Injected concn,
20 ppm
$/Year
22,000
125
1,000
24,500
8,500
56,100
12,900
1,100
1,000
43,500
15,000
73,500
12,900
8,600
41,800
14,400
77,700
Mils/kWh
0.0252
0.0001
0.0011
0.0280
0.0097
0.0641
0.0147
0.0013
0.0011
0.0497
0.0172
0.0840
0.0147
0.0098
0.0478
0.0165
0.0888
o
I
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Table 53,
Estimated Total Operating Costs of Acid Injection
in a Single 250-MW Power Unit
Type of system
S03 evaporation
H2SOi» evaporation,
high temperature
evaporation,
low temperature
o
H
X
m
31
Z
33
n
0)
m
Item
SO 3
Steam
Electricity
Capital charges
Labor and maintenance
Total
H2SOi,
Fuel
Electricity
Capital charges
Labor and maintenance
Total
H2SOi»
Electricity
Capital charges
Labor and maintenance
Total
Injected concn,
10 ppm
5/Year
22,000
125
2,000
24,500
8,500
57,100
12,900
1,100
1,000
56,100
19,400
90,500
12,900
8,600
54,400
18,800
94,700
Mils/kWh
0.0126
0.0001
0.0011
0.0140
0.0049
0.0327
0.0074
0.0006
0.0006
0.0321
0.0111
0.0518
0.0074
0.0049
0.0311
0.0108
0.0542
Injected concn,
20 ppm
$/Year
44,000
250
2,000
24,500
8,500
79,200
25,800
2,200
1,200
56,100
19,400
104,700
25,800
16,700
54,400
18,800
115,700
Mils/kWh
0.0252
0.0001
0.0011
0.0140
0.0049
0.0453
0.0147
0.0013
0.0007
0.0321
0.0111
0.0599
0.0147
0.0096
0.0311
0.0108
0.0662
I
t-1
to
M
I
H
n
-------
Table 54.
Type of system
SOa evaporation
evaporation,
high temperature
HjSOi, evaporation,
low temperature
Estimated Total Operating Costs of Acid Injection
in a Single 500-MW Power Unit
Item
S03
Steam
Electricity
Capital charges
Labor and maintenance
Total
H2SOi,
Fuel
Electricity
Capital charges
Labor and maintenance
Total
H2SO,,
Electricity
Capital charges
Labor and maintenance
Total
Injected concn,
10 ppm
5/Yeap
44,000
250
3,800
37,100
12,800
98,000
25,800
2,200
1,200
84,600
29,200
143,000
25,800
16,700
82,600
28,500
153,600
Mils/kWh
0.0126
0.0001
0.0011
0.0106
0.0037
0.0281
0.0074
0.0006
0.0003
0.0242
0.0083
0.0408
0.0074
0.0048
0.0236
0.0082
0.0440
Injected concn,
20 ppm
?/Year
88,000
500
3,800
37,100
12,800
142,200
51,600
4,400
1,600
84,600
29,200
171,400
51,600
32,900
82,600
28,500
195,600
Mils/kWh
0.0252
0.0001
0.0011
0.0106
0.0037
0.0407
0.0147
0.0013
0.0005
0.0242
0.0083
0.0490
0.0147
0.0094
0.0236
0.0082
0.0559
I
M
U)
N3
I
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IX. CONCLUSIONS AND RECOMMENDATIONS*
This investigation has shown conclusively that different
systems for injecting SOa or HaSOi» vapor in the flue gases of a
coal-burning power station provide an effective remedy for the
high electrical resistivity of fly ash produced from low-sulfur
coal basically without regard to such parameters as fly-ash compo-
sition, concentration, or particle size. If high resistivity is
the major factor limiting the performance of an electrostatic
precipitator in the usual range of operating temperatures, injec-
tion of SO 3 or H2SOi, should markedly improve the efficiency of
the precipitator for removing fly ash from the flue gases.
In the area of economics, this investigation has produced
estimates of the costs of operating 80s and HaSOi* injection
systems. Depending upon the type of injection system selected
and the concentration of conditioning agent needed, the estimated
annual operating costs are in the range from 0.03 to 0.07 mils/kWh
in a plant of representative capacity, 250 MW. These costs are
to be compared with the current average cost of power production
of about 5.0 mils/kWh on a national basis. Again depending upon
the plant operating parameters for the alternative methods of
alleviating the problem of high resistivity—(1) reduction in the
temperature of precipitator operation and (2) installation of a
hot-side precipitator (located ahead of the air heater)—the incre-
mental annual costs are in the ranges of (1) 0.03 to 0.05 mils/kWh
and (2) 0.07 to 0.09 mils/kWh, respectively,16 in a 250-MW plant.
Further research needs to be done (1) to establish the
effectiveness of NH3 and related compounds, such as (NHi»)2SOi»,
for coping with the high-resistivity problem associated with low-
sulfur coal or the low-resistivity problem encountered with high-
sulfur coal, (2) to establish the mechanisms by which NHs and the
related compounds behave as conditioning agents, (3) to estimate
the costs to be incurred in the use of these agents, and (4) to
keep abreast of new developments in the use of SOz and HaSOi, con-
ditioning. Continued work on conditioning in these areas will
probably be resumed during the latter part of 1972.
A more extended discussion of the technical conclusions from
this investigation is given in the Summary of this report
(pages xiii through xvii).
SOUTHERN RESEARCH INSTITUTE
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X. ACKNOWLEDGMENTS
Grateful acknowledgment is made of assistance by several
utility companies, especially the Public Service Company of
Colorado and the Tennessee Valley Authority, in making power-plant
facilities available for our use and for supplying some of the
experimental and economic data included in this report.
Recognition is also made of the important contributions by
various members of the Institute staff in completing the research.
The major contributions were made by the following personnel:
Grady B. Nichols, Head of the Particulate Control
Section
Walter R. Dickson, Supervisor of the Analytical Ser-
vices Section
John P. Gooch, Research Chemical Engineer
Steven L. Estes, Associate Chemist
David E. Crawford, Associate Chemist
Edward J. Muglach, Engineering Technician
Submitted by:
/-'
y
/
'..
Edward B. Dismukes, Head
Physical Chemistry Section
Approved by:
Sabert Oglesby, Jr>, Director
^Engineering Research
Birmingham, Alabama
February, 1972
A839-2504-XIII
(1:7:100:50) pc, gf, mt, cw, cf
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