U.S. Environmental Protection Agency Industrial Environmental Research EPA-600/7-77-041
Office of Research and Development Laboratory +e\~t^
Research Triangle Park. North Carolina 27711 April 1977
A SURVEY OF SULFATE,
NITRATE, AND ACID AEROSOL
EMISSIONS AND THEIR CONTROL
Interagency
Energy-Environment
Research and Development
Program Report
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EPA-600/7-77-041
April 1977
A SURVEY OF SULFATE,
NITRATE, AND ACID AEROSOL
EMISSIONS AND THEIR CONTROL
by
J.F. Kircher, A.A. Putnam, D.A. Ball,
H.H. Krause, J.M. Genco, R.W. Coutant,
J.O.L. Wendt, and A. Levy
Battelle-Columbus Laboratories
505 King Avenue
Columbus, Ohio 43201
Contract No. 68-02-1323
Task No. 49
Program Element No. EHE624a
EPA Task Officer: W.S. Lanier
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, N.C. 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, D.C. 20460
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ABSTRACT
The objective of this study was to evaluate the effects of fuel and
combustion modifications on the formation of primary acid aerosols and their
significance as combustion generated pollutants from large stationary
sources. The term acid aerosol is used here in its broadest sense to in-
clude all sulfates, nitrates, chlorides, and fluorides in all their forms.
Primary acid aerosols are those aerosols which are emitted directly from a
source or formed, primarily by condensation reactants, in the immediate
vicinity (0.5 mi); secondary aerosols, formed by reactions downstream in
the plume, are not considered. Available field data, which were rather
minimal, were collected and interpreted in view of current knowledge of
mechanisms of formation of potential acid aerosols and their precursors.
Although sulfates, nitrates, chlorides, and fluorides were considered, based
on the available data, only sulfates appear to be significant as primary
acid aerosols. All of the various combustion modifications for NO control
x
are expected to have little effect on emissions of primary acid aerosols.
The exception to this conclusion may be firing with low excess air which has
the potential to abate both NO and acid aerosol emissions. Combustion modi-
X
fications and fuel changes may lead to an increased formation of small
particles which could increase the formation of acid aerosols through heter-
ogeneous reactions. Most of the effects, however, are rather speculative
due to the meager data available. Gaps in our information have been identi-
fied and directions for further research are indicated.
111
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CONTENTS
Abstract ............................. iii
Figures .............................. vi
Tables ..............................
Summary and Conclusions ................... 1
Specific Effects of Combustion Modification on Acid
Aerosol ........................ 5
Effect of Fuel Composition ............... 7
Recommendations ....................... 9
Section I - Introduction, Objective, and Scope ........ 11
Section II - Regimes of Formation .............. 14
Section III - Sampling and Analysis ............. 18
Section IV - Equilibrium Considerations ........... 21
Section V - Mechanisms .................... 44 '
High Temperature Homogeneous Reactions ......... 45
Formation of S02 and SO- .............. 45
Formation of NO- and Nitrate .... ........ 49
Heterogeneous Gas-Solid Reactions ............ 53
Catalytic Oxidation of SO- by Metals and Metal
Oxides (Suspended Phase; ............. 53
Catalytic Oxidation of SO- by Soot and/or Carbon . . 57
Combustion and Particulate Characteristics ..... 58
Catalysis of S02 Oxidation by Ash Deposits ..... 60
Capacity Considerations ............... 63
Early Plume Reactions .................. 63
Physical Processes ................. 64
Chemical Processes ................. 66
Section VI - Pilot and Field Studies ............. 76
SO (H SO.) in Flue Gas ................. 78
Staged Combustion .................. 78
Flue Gas Recirculation ............... 78
Low Excess Air ................... 80
Low Air Preheat ................... 81
Load Reduction ................... 84
Additives ........................ 84
Sulfate in Fly Ash . . . . ............... 88
Sulfates in Coal Fly Ash .............. 88
Particle Size in Coal Firing ............ 92
Oil Combustion ................... 95
Particle Size in Oil Firing ............. 100
Summary ....................... 101
iv
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Sulfate in Deposits 102
Section VII - Influence of Combustion Modification on the
Acid Aerosol Formation Potential of Flue Gas Desulfuri-
zation Processes 108
Potential Acid Aerosols and Flue Gas Desulfurization
Systems 113
Effect of Combustion Modification on Acid Aerosol
Formation in FGD Systems 121
References 129
Appendix 140
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FIGURES
Number Page
1 Regimes of Mechanism Importance .................. 16
2 Equilibrium sulfur products for coal combustion with 10 percent
excess air, 3.27 percent sulfur ................. 27
3 Equilibrium sulfur products for coal combustion with 2 percent
excess air, 3.27 percent sulfur ................. 28
4 Equilibrium flue gas components for coal combustion with 2
percent excess air, 3.27 percent sulfur ............. 29
5 Equilibrium flue gas components for coal combustion with 10
percent excess air, 3.27 percent sulfur ............. 34
6 Equilibrium flue gas components for #6 oil combustion with 2
percent excess air, 2.80 percent sulfur ............. 35
7 Equilibrium flue gas components for #2 oil combustion with 2 oil
combustion with 2 percent excess air, 0.20 percent sulfur ... .36
8 Equilibrium sulfur products for #2 oil combustion with 2 percent
excess air, 0.20 percent sulfur ................. 39
9 Equilibrium sulfur products for #6 oil combustion with 2 percent
excess air, 2.80 percent sulfur ................. 40
10 The variation of the theoretical equilibrium yield and possible
actual yield of SO with time in a boiler ............ 42,
11 Equilibrium distribution of sulfur-containing species in propane
air flames with unburnt gases initially containing 1 percent
S0« - points represent experimental measurements normalized at
60 percent stoichiometric air: o or • gas chromatography, V or f
mass spectrometry ..... . ........ , ........ . -,46^,
12 Effect of excess air on the formation of sulfur trioxide in a
hydrocarbon flame ........................ 48..
13 Effect of equivalence ratio on conversion efficiency for an
ammonia additive (0.5 percent by weight) ............. '52
vi
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14 Catalytic oxidation on SCL to SO by various materials 55
15 Catalytic oxidation of SCL to S0_ by various materials 56
16 Plume dispersion and reaction processes 67
17 Dependence of plume relative humidity on dilution 67
18 Dependence of plume relative humidity on dilution 68
19 Dilution in early plume 69
20 Ratio of sulfur trioxides to total sulfur oxides as a function of
total sulfur oxides measured. , 75
21 Concentration of S0_ in exhaust gas of the first stage combustion
section for air atomization of residual oil with 2.4% sulfur. . . 79
22 Decrease in dewpoint with low excess air in an oil-fired boiler
furnace 83
23 Effect of excess air on SO- level in oil-fired boiler furnace ... 83
24 Chloride/sulfate equilibrium coal 3, 1-8 weight percent S, 0.07
weight percent Cl~. curves: (1) 5 percent Qj excess (2) Stoichio-
metric (3) 2 percent 02 104
25 Deposition of sulfates and chloride from flue gas of 60-MW boiler
fired w'ith^coal of 2.4 percent S and 0.28 percent Cl (3.0 percent
excess 02). ^7 104
vii
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TABLES
Number Page
1 Theoretical Conversion of SO to SO in Flue Gases at Different
Temperatures and Excess Air Levels 22
2 Mean Analytical Values for Constituents in 101 Different Coals
on Whole-Coal Basis 24
3 Product Species Considered in Coal Combustion Equilibrium
Calculations 26
4 Typical Fuel Oil Compositions 33
5 Effect of Excess air at a Temperature of 1800 K (2780 F) 38
6 Coal Ash Analysis Limits as Given by Bureau of Mines. 61
7 Effect of Excess Oxygen on SO- Content and Dewpoint 82
8 Ranges of Major Fly Ash Constituents for a Variety of Coals .... 89
9 Chemical Analysis of Ash From 46 Public Utilities as Given by
Walker, 1974 91
10 Comparison of Average High and Low Ranges of Other Ash Constituents
for High and Low Sulfates Samples by Walker, 1974 93
11 Sulfate Contents in Fly Ash From Coal Firing. 93
12 Trace Elements in Fly Ash With Potential Catalytic Effect on
Oxidizing S02 to S03 94
13 Ranges of Ash Composition in Oil 97
14 Elemental Analyses of Total Particulates 98
15 Typical SOX, NOX, and Particle Loadings in Flue Gases to FGD
Systems 110
16 Qualitative List of Potential Acid Aerosol Formers Emitted from
FGD Systems 117
17 Sulfate Emissions Summary—Reproduced in Part From Richards and
Gerstle (Richards, 1976) 120
viii
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A SURVEY OF SULFATE, NITRATE, AND ACID
AEROSOL EMISSIONS AND THEIR CONTROL
SUMMARY AND CONCLUSIONS
The role of ambient acid aerosols, and in particular sulfates, as
contrasted to SO-, in health and environmental problems is of increasing
concern. As a result of recent studies of the ambient atmosphere, parti-
cularly in northeastern United States, the contribution of primary acid
aerosol or sulfates to the total ambient loading has been questioned.
Historically, it has generally been stated that only about 1 to 3 percent
of the sulfur in a fuel is emitted from the combustion system as SO or
acid. However, since such acid can lead to various sulfates which might be
a; part of the particulate emissions, it is important to consider these as
well as SO as part of the primary acid aerosol. Further, as various com-
bustion modifications become more widely applied to control NO emissions
X
one must be concerned that these previously-held postulations regarding SO
and sulfate emissions are valid. The purpose of this study has been to
evaluate existing data to try to answer these questions.
It should be noted that throughout this study there was considerable
uncertainty in correlating and interpreting such data as was available
because of the difficulties in sampling and analyzing for acid aerosol
components, especially as regards SO , SO , and sulfates. This has led to
£• -J
a great deal of uncertainty in much of the data and has made quantitative
interpretation extremely difficult. A deep-rooted conclusion that underlies
this entire study is that to objectively evaluate the sulfate issue
additional work to enhance sampling and analysis capabilities is urgently
needed.
The results of this study are best summarized by first addressing
seven specific questions raised by the Environmental Protection Agency at
the initiation of this program, then by assessing the specific effects of
combustion modification procedures.
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A. What are the primary acid aerosols being emitted?
The great majority of emissions which may lead to acid aerosols are
sulfur compounds, sulfuric acid, SO , and sulfates although it is
recognized that not all sulfates are acidic. Nitrates have not been
observed nor are they expected in stack particles, but a small amount of
nitrate may be formed in the near plume. The sparse information available
on HC1 or chlorides is in general agreement with basic thermodynamic
considerations that the chlorine in fuel will be emitted primarily as
gaseous HC1 from the stack. Evidence indicates that total primary sulfates
(i.e., those observed within the first half-mile) can be as high as 20
percent of total sulfur emissions or as low as 2 percent.
A significant fraction of the primary sulfates consist of H SO ,
based on field measurements. Equilibrium considerations for coal-firing
indicate that the remaining sulfates are distributed among.CaSO, MgSO,,
and ZnSO . Bisulfates are not formed. Specific sulfates are not identified
but field data suggest Na.SO., K0SO. and FeSO. are also formed. For oil-
2424 4
firing, NiSO and Na SO are major components of the sulfated fly ash.
B. What is the effect of fuel composition?
A significant effect of fuel composition on sulfate emissions should
be expected. Laboratory and field data indicate that as fuel sulfur level
is decreased, the fractional conversion to H SO is increased although total
emission decreases. Moreover, based on fundamental data on catalytic
activity of various metals, it is expected that the effect on total primary
sulfate emissions of trace metal speciation in fuels will be large.
There are three effects related to trace metal composition; these
are catalysis, sulfate capture, and size distribution. V, Mn, Fe, Ni, and
other transition metals have been shown to catalyze the oxidation of S09 to
SO , thus potentially increasing total sulfate emissions, The presence of
other species, i.e., Na, Ca, Mg, etc., affects the distribution of sulfate
in the fly ash.
In addition to affecting total SO, formation, fuel composition is
expected to have a large effect on emitted particle size distribution.
Volatile metals condense to form very small nuclei, giving a high surface
to volume ratio and therefore accelerating catalysis. It should be noted
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that volatile elements may not always form the most volatile combustion
products. Furthermore, small particles may increase the H SO /SO ~ ratio,
especially if large particles are removed.
Field data to support evidence for V, Ni, etc., catalysis of SO
oxidation in fly ash are lacking. There is a scarcity of field data on
speciation of sulfate in fly ash, although a correlation exists between
the Na, Ca, Fe, and Mg content of fly ash and sulfate content.
The effects of fuel composition and the use of additives is summarized
in greater detail on page 7.
C. Are acid aerosols the result of combustion in ^general or
are they the by-product of a limited class of equipment?
In general, the combustion of any sulfur-containing fuel will produce
some primary acid aerosol but there doesn't appear to be, and one would not.
expect, a strong relationship between the type of equipment and acid aerosol
formation. However, to the extent that the equipment influences the for-
mation of fine particles (cyclone and pulverized firing of coal, for
instance, emit finer particles than stoker firing) there may be an effect
on acid aerosol formation resulting from catalytic reactions.
D. What are the mechanisms of acid aerosol formation
and destruction?
Primary acid aerosols are formed by at least the following mechanisms:
(1) High temperature homogeneous SO oxidation
(2) Dry gas-solid reactions converting SO- to SO, .
There is no reason to expect solution chemistry kinetics to in-
fluence primary sulfate emissions.
In general, the acid species are not destroyed once formed. Some
may be removed from the gas stream by adsorption on particles where they
may be in part neutralized and some particles are removed by precipitators,
for instance, but such processes are not completely effective.
E. Are sulfur compounds adsorbed by boiler deposits
and subsequently released during soot blowing?
Yes, there is positive evidence for the adsorption of sulfur oxides,
metal oxides, and chlorides by deposits and subsequent conversion of these
compounds to sulfates. Soot blowing removes some such material and probably
overloads the precipitator and results in emission of sulfates, vanadates,
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and other species. Although field data are lacking in this regard, it is
probable that such deposits catalyze SCL oxidation to some extent but carbon
deposits probably do not contribute to the catalysis at deposit temperatures.
F. What percentage of the total acid aerosol is formed in
the flame zone, in the convective passes, in the stack,
in the plume, and in the atmosphere?
Available data does not allow one to quantify this. The investigators
best estimate is:
Flame zone 10 percent of total S converted
Convective pass 10 percent of total S converted
Stack 0 percent of total S converted
Near plume . 1 percent of total S converted
Atmosphere 80 percent of total S converted
These estimates of conversion refer to that part of the sulfur in the
fuel contributing to aerosol emissions and do not include the sulfur retained
in the ash, slag, etc. That is, it is estimated that of the sulfur in the
stack effluent, up to 10 percent might be converted to acid aerosol con-_
-• .^
stituents in the combustion zone. Similarly, another 10 percent may have
been converted in the convective passes so that up to 20 percent of the
sulfur in the effluent may contribute to primary acid aerosol. Probably 80
percent or more of the sulfur emitted in the stack gases will be SO , which
will be further oxidized in the atmosphere at some later time.
G. What is the effect of CM on primary acid aerosol?
There is no evidence to indicate that CM (combustion modification)
will, in general, be an effective procedure for acid aerosol abatement
although low excess air firing, where practical, may be an exception.
It might be expected, however, that standard CM techniques for NO abate-
X
ment may adversely affect the quantity, speciation, and particle size of
acid aerosol exhaust emissions through increases in the formation of fine
particulates and carbonaceous materials. This is covered in more detail
in the following section.
In brief, no definitive quantitative answers to the above questions
exist at this time. There are some fundamental data and some field data
that allow the conclusion to be drawn that acid aerosol emissions are a
function of fuel composition and combustion conditions. However, quantitive
interpretation is difficult because of three areas of uncertainty:
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• lack of detailed fuel analyses
• undefined combusiton conditions
• uncertainty in particulate emission
sampling and analysis.
SPECIFIC EFFECTS OF COMBUSTION MODIFICATION ON ACID AEROSOL
There are very little pilot or field test data which directly demon-
strate that a particular combustion modification employed to reduce NO and
NO will have an effect, good or bad, on primary acid aerosol. The weight
of the evidence is.that anything which tends to reduce super-equilibrium
oxygen atom concentration in the flame zone will tend to reduce SO . On the
other hand if the production of particulate, especially very small particles,
is increased then the production of acid and sulfate solids might be expect-
ed to increase through heterogeneous processes. In the temperature range
of the convective zone SO might be the result of heterogeneous oxidation
of SO on metallic or metal-containing particles and deposits, whereas at
low temperatures in the stack and near plume it might occur predominantly on
carbonaceous particles. At this time, conclusions regarding the effect of
a particular combustion modification on specific equipment must be highly
speculative.
The one case where the data clearly show that acid aerosol can be
reduced is operation with low excess air. The reduction in NO and NO is
accompanied by reduced SO formation in the flame and the reduced avail-
ability of oxygen pertains throughout the system. This implies extremely
good combustion control and a level of excess air below that generally
attainable in coal combustion at the present time. It must be kept in mind
that in low excess air combustion there is the risk of increased particulate
formation which might lead to an increase in catalytic oxidation of SO..
However, from the standpoint of controlling both NO/NO production and acid
aerosol formation, this would appear to be the most fruitful approach at the
present time.
In the case of staged combustion, there are essentially no data per-
taining to total primary acid aerosol emissions for full-scale systems under
staged conditions. During the first stage of combustion where conditions
are fuel rich, SO formation may be reduced essentially to zero. However
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it has been demonstrated that it is possible to form more SO when air is
added to complete combustion than would have been formed by the same level
of air in a single combustion step. It seems quite likely that metal
speciation and size distribution in particulates and deposits will be
different under staged conditions and vary from one type of staged combus-
tion to another. Although changes in particulate and deposits are expected
to have an effect, so little is known that it is difficult to even speculate
what the effects might be except that in general an increase in particulates
would be expected to lead to increased total sulfate at any given SO level.
For flue gas recirculation there are essentially no data on sulfate
emissions from practical systems. The effect of flue gas recirculation is -
to lower combustion temperature and one study reported a correlation between
flame length and SO production in the combustion zone. But the evidence on
particulate formation was quite contradictory. Certainly the time-tempera-
ture history of material moving through the system will vary with flue gas
recirculation and where and how, and to what extent, the recirculation is
applied. Little is known about the effects of bringing previously formed
SO or particulates into the system early in the combustion process. One
might speculate, also, that lowered temperatures will reduce metal volatili-
zation with a subsequent reduction of very small, high metal content,
catalytically active particles in the convection passes. This would be
expected to reduce the oxidation of S09 to SO in this regime. However,
there is no data to substantiate such speculations.
Reburning, and particularly the use of ammonia, is another case where
there is essentially no data pertinent to this study. Certainly any
ammonium sulfate which might be formed will remain dissociated till low
temperatures are reached. As will be discussed in the section on Early Plume
Reactions, an increase in sulfate formation would be expected if there is
a significant increase in pH.
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EFFECT OF FUEL COMPOSITION
The nature of the fuel obviously will have an effect on acid aerosols
and the increased propensity for sulfates with increasing fuel sulfur hardly
needs to be stated. However, there are essentially no field data that
indicates low concentration of trace species have a major effect on primary
acid aerosols. Although there are apparent correlations between sulfates
and certain metals in fly ash and deposits, it is difficult to state
unequivocally that the sulfate resulted from the presence of the metals.
Based on fundamental data and experiments to show catalytic activity
of fly ash constituents, it is to be expected that trace metals will have
a large effect on sulfate emissions, but existing data from practical
combustion systems do not allow trace metal effects to be quantified.
In the case of coal particulates, the major species are silicon and
aluminum oxides which probably are inert. However, Fe, Na, K, and Ca
oxides are also generally present in lesser quantities and these have been
shown to have some catalytic activity for S0_ oxidation under at least
some conditions. Different metal distributions among coals in conjunction
with different combustion modifications probably lead to variations in the
distribution and speciation of these metals in the particulate, therefore
variation in oxidation of SO. is to be expected.
In general, the sulfate in coal fly ash is less than 2 percent
although occasional higher values are observed. It appears that the sulfur
is largely on the surface and present as SO, rather than adsorbed SO .
Studies of the particulate surfaces suggest the sulfate may be present
largely as iron and/or calcium sulfates. Iron is a major constituent of fly
ash along with sodium and lesser amounts of calcium. Furthermore, since
studies of deposit chemistry have shown that ferric oxide can be an effec-
tive catalyst for S0_ oxidation and sulfate formation, the fragmentary
evidence available suggests iron and its eventual distribution and
speciation may be an important factor in the effects of particulates on
primary acid aerosol emissions.
Particulates from oil combustion, particularly high ash residuals,
are in many respects similar to those from coal except that they are much
more likely to be high in carbonaceous materials and the total amount of
particulate will be smaller. The major difference is the presence of
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vanadium and nickel oxides, which do not generally appear in coal particu-
lates, and are known active catalytic materials for SO oxidation. This
occurence correlates with the generally higher sulfate levels found in
particulate from oil combustion.
Fuel composition in conjunction with combustion conditions is expected
to have a large effect on particle-size distribution. In general, the
evidence suggests that the more volatile metals will be concentrated in the
smaller particles and maximum flame temperature will effect the amount and
species volatilized. These small particles with their relatively large
surface area can be particularly effective catalysts for SCL oxidation and
sulfate formation.
In addition to catalytic effects, differences in size distribution
and speciation are also expected to effect the sorptive properties of fly
ash and depostis for S09 and SO which can effect the eventual formation
of sulfates by noncatalytic mechanisms. However, at present it is virtually
impossible to quantify these effects.
Another aspect of fuel composition is the use of additives, which in
effect change the metal content of the fuel. Generally, materials are used
containing either Mg or Zn, both of which readily form sulfates. The
particles thus formed may be removed by precipitators thereby reducing the
potential for acid aerosol emission. Various forms of the additive have
been shown to be effective, i.e., metals, metal oxides, minerals, or
organometallic compounds. The use of such materials does not reduce
primary acid aerosol formed, as defined in this report, but does reduce the
acid emission, that is SO /H SO,, by forming ZnSO or MgSO . If removing
SO from the flue gas is the goal, then inert materials such as Si02 have
been shown to be effective. These probably act by forming high surface area
particles, like cenospheres, which physically adsorb SO- and result in
particles with sulfuric acid adsorbed on the surface. Overall this may not
be as beneficial as forming magnesium or zinc sulfates.
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RECOMMENDATIONS
Perhaps the most significant finding of this study is the recognition
that clear, quantitative correlations between combustion modifications and
the problem of acid aerosols do not exist. It is not even clear to what
extent primary acid aerosols are a problem. ' Basic and field test data
indicate the fraction of sulfur emitted as acid is generally small but the
uncertainties are large. There is no evidence that nitrates or chlorides
are major contributors to primary acid aerosols. Therefore, to the extent
that there is a problem, it is a problem of formation of sulfuric acid and
sulfates. In light of this general conclusion, the following recommenda-
tions are made. It is not our intent to define specific research tasks but
rather to indicate directions for further research which should prove
fruitful.
• Further research, at least in the form of field
sampling/analysis of partlculate emissions from
CM-operated boiler facilities, is needed to
define the severity of the acid aerosol problem.
• Sampling and analysis procedures need to be improved.
Much of the uncertainty in this report results from the
fact that such data as exists is ambiguous.
• In addition to sulfuric acid (SO-) in the gas phase much
more attention needs to be given to speciation in
solids in order to determine how and to what extent solid
sulfates contribute to the primary acid aerosol problem.
Speciation as a function of particle size distribution is
a particularly vexing problem but, from the standpoint of
acid aerosol effects, it is an important problem area.
• The ratio of primary to secondary sulfate is not well
established. The evidence points to a small value of
this ratio but further work is needed to precisely
establish its value and hence determine the extent to
which primary acid aerosols are a problem.
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« Overall, there is little data on the SO^/SO,, ratio
with and without combustion modification for a given
combustion system. In many situations the available
data suggests that CM is not expected to have a major
effect on primary acid aerosol but there is almost
no documentation for practical systems. Such data is
needed but it is recognized that the experiments will
have to be very carefully defined in terms of choice of
combustion systems, combustion modifications, fuels, etc.,
in order to provide useful data at reasonable cost.
© Laboratory studies on the formation and destruction
of SO- and sulfates in combustion processes, with
and without combustion modification, are needed.
« From the standpoint of mechanisms considerably more work is
needed on heterogeneous SO,, oxidation. The evidence
indicates such mechanisms are a major source of primary acid
aerosol but very little is known about their details. The
potential for carbon particles to act as S02 oxidation
catalyst has been demonstrated but it remains to be shown
how much such a mechanism might contribute to either or
both primary or secondary acid aerosol.
« As a corollary to the above, more information on the formation
of particulates, especially fine, suspended particles, is
needed as a function of combustion modification conditions
and fuel quality as they relate to speciation and particle
size distributions.
o Some further work on homogeneous mechanisms of SO? oxidation
is desirable. Reactions such as S02 + OH under fuel rich
conditions, as would apply to staged combustion, and the
reaction of S02 with vaporized NaCl to form Na2S04 are
examples.
9 Stack-gas cleanup procedures can influence the rate of S02
oxidation in the early plume. This is especially important
for those procedures which may alter the pH of the plume.
Particular attention should be given to those conditions
where excess base, e.g., NH^, might be emitted.
• The question of flue-gas desulfurization systems should
perhaps be considered separately from combustion modification.
That is, what is the potential for FGD to release or remove
acid aerosol regardless of combustion modification?
10
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SECTION I
INTRODUCTION, OBJECTIVE, AND SCOPE
In recent years there has been increasing evidence that sulfates in
the atmosphere may be of more concern as a health and environmental hazard
than sulfur dioxide. Part of this concern is reflected in the fact that
SO- levels in the atmosphere have been on the decline, while sulfate levels
remain unchanged (Altshuller, 1976) (Squires, 1975). For these reasons
this study was undertaken for the Combustion Research Branch of the Environ-
mental Protection Agency. The overall purpose of this study was to survey,
compile, and evaluate data on the generation and emission of acid aerosols
formed within stationary combustion devices. The basic questions being
addressed in this study are:
• What is the effect of combustion modification (CM)
on the formation and emission of primary acid aerosols
in stationary source combustion?
• What is the potential for CM to control the formation
and emission of primary acid aerosol?
• What are the research and development gaps requiring
further effort to answer properly the first two
questions?
It generally is considered that the acidic sulfates, such as sulfuric
acid and ferric sulfate, are of greater concern as health hazards than the
natural or basic sulfates. When speaking of "acid aerosols" in this study
the term is used in the broadest sense. Acid aerosols in this study refer
to any and all liquid and solid particles containing sulfates, nitrates,
chlorides, and fluorides, as well as sulfates and nitrites of sodium,
calcium, ammonia, etc., and all are of equal concern to this study and are
included under the umbrella of "acid aerosols". The study is concerned not
only with total acid aerosol emission but also their speciation and size
11
-------
distribution. It is important to be so all inclusive in terminology at this
point in time (1) because of limitations to specific relationships between
health effects and specific sulfates, nitrates, etc., and (2) because of a
lack of specificity in current chemical characterization of particulate
emissions.
Although an attempt has been made in this study to examine the pro-
duction and emission of sulfates, nitrates, chlorides, and fluorides in
combustion, by far the greatest emphasis is on the sulfates. This comes
about quite naturally due to the instability of nitrates under combustion
temperatures and the dearth of information on chlorides and fluorides in
combustion processes. As regards the formation and emission of sulfates
as primary aerosols, we have arbitrarily defined as primary aerosol any
particulate emitted from the stack and/or produced within the first half-
mile in the plume. In essence, then, any particulate resulting from rapid
reaction (an unanticipated phenomena) on entering the plume or from con-
densation, nucleation, or agglomeration processes as gases and particulate
are mixed and diluted with the ambient environment are considered primary
aerosol. The need for this definition of primary aerosol is apparent since
the chemistry of S07 oxidation in plumes is specifically avoided in this
study. The slow, secondary oxidation of SO in plumes is covered by
numerous other studies. The reader is referred to a recent analysis of S0_
oxidation in plumes carried out for EPA (Levy, 1976) for this aspect of the
acid aerosol problem.
One final comment by way of introduction: sulfates, nitrates, acid
precipitation products and the like observed in the ambient are generally
referred to as "second generation pollutants", i.e., pollutants formed by
secondary reaction of SO and NO as referred to above. These reactions are
well recognized, and it is important in the context of the study at hand to
recognize at the outset that the emission of primary sulfates, nitrates,
etc., is small relative to other pollutants or relative to second generation
acid aerosols. To repeat, the objective of this study is to evaluate the
significance of primary acid aerosols as combustion-generated pollutants
and to consider how these may be effected by fuel and combustion modifi-
cations.
12
-------
Due to the lack of definitive field test data, it has been necessary
to approach the problem from a fundamental point of view and make appropriate
speculative extrapolations. First considered are equilibria and its impli-
cations for speciation, then the fundamental mechanisms of acid aerosol
formation are reviewed. Available field data have been collected and
interpreted in light of these principals and research gaps identified.
13
-------
SECTION II
REGIMES OF FORMATION
In evaluating the effect of various combustion modifications on acid
aerosol formation, it must be kept in mind that various contributors to the
aerosol may be formed in different parts of the total system under quite
different conditions. For this reason, we have divided the combustion system
into regimes starting with the flame or combustion region and proceeding
through the system to the near plume. In the combustion zone, temperatures
are maximum and most of the pertinent reactions are expected to be homo-
geneous gas phase and fairly rapid. A rapid temperature quench may allow
a frozen equilibrium to be attained among sulfur species. Perhaps as much
as half the primary acid could be formed in this region.
A second regime, at somewhat lower or intermediate temperatures, is the
region of the convective passes. Here the rapid decrease in temperature
slows many of the homogeneous reactions to the point that they contribute
little to the formation of acids or their precursors. In this and following
regions the system is not at thermodynamic equilibrium. In this regime,
heterogeneous reactions may be most important. Oxidation of SO., for
instance, may occur by adsorption and reaction on suspended particulate
/
matter or by adsorption and reaction on heat transfer surfaces or the
deposits building up on those surfaces. Since the deposits remain in the
boiler environment for a long time compared to a quantity of gas passing
through the region, there is time for considerable chemistry to occur within
the deposit. When such deposits become dislodged they carry reaction
products into the atmosphere. Most of the rest of the primary acid aerosol
material is probably formed in this region.
14
-------
The third regime is the region of the stack where the temperature has
dropped significantly. As a practical matter, temperatures are kept as low
as possible to maximize economy and high enough that moisture and acid do
not condense. Due to the lowered temperature, not much chemistry leading to
acid formation occurs here although particulates may be agglomerating and
some gas-solid reactions could occur.
The final regime considered in this report is the near plume at close
to ambient temperature. Little acid formation is expected in this region
although moisture will be condensing and picking up already formed acid. In
the case where something changes the plume pH significantly, i.e., NH»
injection, more reaction may occur.
For the most part, there is insufficient information to quantify or
even rigorously define the chemistry occurring in these various regimes.
However, it is possible for combustion modification to influence the reac-
tions in all regimes since what occurs in the combustion zone can modify
reaction conditions in all succeeding regimes. The data shown in Figure 1
denotes, in a qualitative manner, which reaction mechanisms are likely to be
important in each of the temperature regimes discussed above. The impor-
tance of high temperature homogeneous reactions is shown in Curve A; that
of gas-solid reactions in Curves B and B'; that of reactions involving
solution chemistry in Curve C; and the importance of photochemical reactions
in Curve D. The area under these curves represents the possible total
sulfates formed by the pertinent reaction. The role of gas-solid reactions
is shown two ways, B and B', since the extent and temperature range is
uncertain. It is clear that solution chemistry (Curve C) and photochemical
mechanisms (Curve D) are not likely to be applicable to primary acid
sulfate formation, although they are important as far as secondary sulfates
are concerned.
In the following sections of this report possible reactions in the
first four temperature regimes are discussed, relating field test data to
them where such data exist. Equilibrium considerations at temperatures
characteristic of the first two regimes are discussed first. At lower
temperature frozen equilibrium is assumed. The following section considers
basic mechanisms. High temperature homogeneous reactions (the first regime)
15
-------
Time-Seconds
10
3600
o
Crt
(-1
CO
t-l
0)
O
c
CO
4J
1-1
O
s
ca
•i-4
c
to
Flame Convection
*" Zone '*" Section
"* fStackl
Primary
Sulfates"
Immediate Plume"
Far Plume_
"Ambient
Air
I Secondary
^Nulfates
rHigh T,/homogeneous
Photochemical
Intermediate
Temp,
Dry Gas-Solid Solution Chemy
\ Heterogeneous
3500 uoo 350
Temperature, F
60
Figure 1. Regimes of Mechanism Importance
16
-------
are considered first followed by heterogeneous reactions which, probably
occur at somewhat lower temperatures characteristic of the second regime.
Possible reactions in the near plume (the fourth regime) are considered
last.
The next section of the report summarizes findings from pilot and field
studies. This data cannot be directly tied to the regimes, as defined, but
where possible inferences are drawn based on basic data developed in the
various regimes.
The next section discusses the possible influence of combustion modifi-
cation on flue gas desulfurization processes. Although such processes are
not a primary subject of this report, they can contribute to acid aerosol
emissions and therefore some discussion is warranted.
17
-------
SECTION III
SAMPLING AND ANALYSIS
Before examining the available field sulfate data some discussion of
sampling and analytical practices is in order. Possibly the most serious
gap in attacking this problem of acid aerosols lies in the data itself and
this deficiency permeates this entire study. It has generally been recog-
nized that sulfate particulate analyses are strongly affected by sampling
procedures, i.e., the effect of substrate, SO , humidity, etc. As a
consequence, most sulfate (and similarly nitrate) particulate data reported
must be considered suspect to some degree. The various analytical procedures
have recently been critically reviewed by Tanner and Newman (1976).
Throughout the course of this study difficulty was encountered not
only because of a general lack of data on primary acid aerosol emissions
but also because of uncertainty arising in the methods of analysis and
reporting data. It is not always clear whether sulfate refers just to the
SO (i.e., H?SO.) from flue gas or SO? in the solids or, as in a few cases,
the sum of the two. Total SO? in a particle is not necessarily indicative
of the acidity of the particle. However, as mentioned at the beginning of
this report we are including all sulfate in drops and particles under the
term acid aerosol. Speciation of the sulfate is difficult to determine and
almost never defined. One is left to infer, for instance, that if a particle
is high in sodium and magnesium and also high in sulfate, then the likely
species present are sodium sulfate and magnesium sulfate.
Accurate specification of acid aerosol speciation and concentrations
in combustion products is critically dependent on a number of parameters
that can involve both combustion system variables and sampling and analysis
methodology. One commonly used procedure is to attempt to achieve a mass
balance between the input rates of components of the fuel/ air mixture and
apparent output rates of the individual components as a means for specifying
18
-------
sampling and analysis efficiency. Often, the inherent range of the input
variables is not recognized explicity, and erroneous conclusions concerning
the significance of the sampling and analysis results can easily be made.
For example, the expected output rate of sulfur species is determined not
only by the input rate of sulfur in the fuel but by the detailed (C, H, N,
S, 0, ash, HO) composition of the fuel. The net range expected in the
output rate of sulfur species is thus a composite of the effects of the
uncertainties of 6-7 separate analyses plus the variations due to changes
in firing rate. This aspect of the overall problem is compounded in a coal-
fired system by the inherently heterogeneous nature of the fuel, and the
absence of continuous representative sampling of the fuel.
Sources of error also exist in the commonly used sampling methods.
Ideally, the sampling method should achieve separation and stabilization
of each of the species of interest, while maintaining a consistently high
level of collection efficiency. Acid aerosols, and their precursors, are
characteristically reactive species. Hence care must be taken to avoid
contact of the sample with surfaces that may react in such a way as to ob-
scure differences between the identities of the species of interest. For
example, the use of unlined metal probes can result in the formation of
solid sulfates because of wall reactions with sulfuric acid in the flue gas.
Glass fiber filter media commonly used in sampling trains frequently con-
tains appreciable amounts of basic substances. These substances can react
with acidic components, especially SO^, of the flue gas. In stack gas
sampling, the amount of sulfur trapped by this type of reaction is probably
insignificant with respect to the total solid sulfate trapped on the filter.
However, removal of sulfuric acid from the gas stream by this type of
reaction can interfere with accurate collection of sulfuric acid at a later
point in the sampling train. In the case of sampling of the plume, or even
the near plume, filter interactions can be a major source of error. Under
plume conditions, the absorption of S0_ by the basic filter components can
lead to apparently high levels of sulfate in the trapped particulate.
Impingers used in stack gas sampling trains also can be source of error in
specification of flue gas sulfate. Absorption of SO in impinger solutions
and subsequent oxidation of the resulting sulfite to sulfate during sampling
19
-------
and storage prior to analysis has been demonstrated and will, of course,
lead to erroneously high acid values.
These chemical sources of error inherent in the sampling methodology
have been characterized, and alternative procedures have been recommended
(Hillenbrand, 1972) (Coutant, 1976). In brief, these recommendations include
the use of fused-silica or Vycor-lined probes, use of high purity fused-
silica filters, and the use of the Goks^yr-Ross procedure for sulfuric acid.
In spite of the errors involved, however, the Method 6 and Method 8 proce-
dures continue to be used on a wide basis.
20
-------
SECTION IV
EQUILIBRIUM CONSIDERATIONS
On investigating reactions and mechanisms which can lead to oxides of
sulfur and those of nitrogen, and thence to acid aerosols, it appears that
the same nonequilibrium factors, particularly superequilibrium oxygen atom
concentrations, need to be considered for both groups of compounds in the
high temperature, flame region. The reaction products, as determined by
many investigators of practical systems, are indicative of a chemical
system which has not reached thermodynamic equilibrium. That is, the ob-
served species concentrations are characteristic of the system at equilibrium
at a temperature different from that measured. In industrial combustion
systems this generally means that the product distribution of pollutants
is characteristic of the systems at relatively high temperatures, e.g., at
the fire box exit, even though the measurements were made at relatively low
temperatures, e.g., in the stack. Thus, the composition is kinetically
controlled. This is most striking in the case of SO /SO where SO is
generally observed to be only a few percent of SO in the flue gas. At
high temperatures most of the sulfur would be in the form of SO. at equilib-
rium. But as the equilibrium system cools SO becomes the predominant
gaseous sulfur compound. The predicted equilibrium distribution between
SO and SO , and considering only these species, is shown in Table 1 for
various levels of excess air at a number of temperatures(Schwartz and Dietz,
1975). That the predicted result is not observed means simply that as the
temperature drops the rate of SO oxidation slows and the gases pass through
the system before they reach equilibrium, i.e., the chemistry is kinetically
controlled. This is a gross oversimplification of the many factors involved,
of course.
21
-------
TABLE 1. THEORETICAL CONVERSION OF SO TO SO,
AT DIFFERENT TEMPERATURES AND EXCESS
IN FLUE GASES
AIR LEVELS
Pso3
- — - x 100 for excess air
P + P
SO- SO.
Temperature *• -*
°F
800
980
1460
1340
1520
1700
1880
2060
2240
°C
427
527
627
727
827
927
1027
1127
1227
10%
97.1
81.2
47.0
20.0
8.2
3.7
1.34
1.01
0.60
5%
96.1
75.8
39.1
15.3
6.1
2.7
1.33
0.73
0.43
17.
91.7
58.7
22.6
7.6
2.87
1.25
0.61
0.33
0.20
0.1%
77.9
31.1
8.5
2.55
0.93
0.40
0.20
0.11
0.06
P = partial pressure of indicated species.
22
-------
Consideration of the thermodynamic equilibrium chemistry can be instruc-
tive since it describes the direction the reactions are moving. Figures 2
and 3 are the result of calculations based on combustion of coal of an
average composition (Ruch, 1974), containing 3.2% sulfur. The elemental
distribution is shown in Table 2. The formation of both gaseous and con-
densed species are included in the calculations. They are based on burning
with 2% and 10% excess air. The figures show the distribution of sulfur
among a variety of compounds. The various compounds considered in this
analysis are given in Table 3. The calculated values for each computation
are given in the Appendix. At high temperatures the sulfur appears almost
entirely as SO.. As the temperature decreases, the system at equilibrium,
SCL is converted to SO- and the two become almost equal at about 800 K
(980 F). However, the formation of solid sulfates also increases and as the
temperature continues to decrease the solids become the predominant sulfur
species and sulfur in the gas phase falls to very low values. It will be
noted that there is little difference, at equilibrium, between 2% and 10%
excess air. That is, almost any excess oxygen determines the main direction
of the reactions from the standpoint of equilibrium. There are small dif-
ferences, of course, as in the SO /SO ratio.
Various gaseous species may contribute to acid aerosol formation, parti-
cularly sulfates, and these are indicated in Figures 4 and 5, again for the
same coal composition with 10% and 2% excess combustion air. These are in-
dicative of the expected equilibrium flue gas compositions. It will be
noted that sulfur, compounds dominate the situation from the standpoint of
acid or acid precursors until the temperature drops below about 700 K
(801 F). Below this HC1 becomes the predominant species because at equilib-
rium the formation of solid sulfates removes almost all sulfur from the gas
stream. That is, at low temperature equilibrium, one would expect most of the
sulfur in the system in the solid phase (ash) and most of the chlorine in the
gas phase.
'23
-------
TABLE 2. MEAN ANALYTICAL VALUES FOR CONSTITUENTS IN
101 DIFFERENT COALS ON WHOLE-COAL BASIS
Constituent Mean Standard Deviation Minimum
As
B
Be
Br
Cd
Co
Cr
Cu
F
Ga
Ge
Hg
Mn
Mo
Ni
P
Pb
Sb
Se
Sn
V
Zn
Zr
Al
Ca
Cl
Fe
K
Mg
Na
Si
Ti
Org. S(a)
Pyr. S
Sul. S
Tot. S
SXRF
Ash
Btu
C
H
N
0
14.02 ppm
102.21 ppm
1.61 ppm
15.42 ppm
2.52 ppm
.57
13.75 ppm
15.16 ppm
60.94 ppm
3.12 ppm
6.59 ppm
0.20 ppm
49.40 ppm
7.54 ppm
21.07 ppm
71.10 ppm
34.78 ppm
1.26 ppm
2.08 ppm
4.79 ppm
32.71 ppm
272.29 ppm
72.46 ppm
1.29%
0.77%
0.14%
1.92%
0.16%
0.05%
0.05%
2.49%
0.07%
1.41%
1.76%
0.10%
3.27%
2.91%
11.44%
21,748.91
70.28%
4.95%
1.30%
8.68%
17.70
54.65
0.82
5.92
7.60
7.26
7.26
8.16
20.99
1.06
6.71
0.20
40.15
5.96
12.35
72.81
43.69
1.32
1.10
6.15
12.03
694.23
57.78
0.45
0.55
0.14
0.79
0.06
0.04
0.04
0.80
0.02
0.65
0.86
0.19
1.35
1.24
2.89
464.50
3.87
0.31
0.22
2.44
0.50
5.00
0.20
4.00
0.10
1.00
4.00
5.00
25.00
1.10
1.00
0.02
6.00
1.00
3.00
5.00
4:oo
0.20
0.45
1.00
11.00
6.00
8.00
0.43
0.05
0.01
0.34
0.02
0.01
0.00
0.58
0.02
0.31
0.06
0.01
0.42
0.54
2.20
11,562.00
55.23
4.03
0.78
4.15
Maximum
93.00
224.00
4.00
52.00
65.00
43.00
54.00
61.00
143.00
7.50
43.00
1.60
181.00
30.00
80.00
400.00
218.00
8.90
7.70
51.00
78.00
5,350.00
133.00
3.04
2.67
0.54
4.32
0.43
0.25
0.20
6.09
0.15
3.09
3.78
1.06
6.47
5.40
25.80
14,362.00
80.14
5.79
1.84
16.03
Footnote appear on the following page
24
-------
(a) Abbreviations other than standard chemical symbols: organic sulfur
(Pyr. S), sulfate sulfur (Sul. S), total sulfur (Tot. S), sulfur by
X-ray fluorescence (SXRF), air-dry loss (ADL), moisture (Mois.)»
volatile matter (Vol.), fixed carbon (Fix. C), high-temperature (HTA),
low-temperature ash (LTA).
25
-------
TABLE 3. PRODUCT SPECIES CONSIDERED IN COAL COMBUSTION
EQUILIBRIUM CALCULATIONS
A1F3
A1C1
M(N\)3
B2°3
BF
3
BC1
3
BeO
Be(N03)2
BeF2
BeCl2
BeS04
F0
2
HF
CaC03
CaO
Ca(OH)2
C a (NO )
2 2
C a (NO )
3 2
CaF
2
CaCl
2
CaSO.
4
C12
HC1
Fe2°3
Fe(NO )
FeCl2
Fed
3
MgO
Mg(N03)2
MgS04
Na20
Na CO
2 3
NaN00
2
NaN03
NaF
NaCl
Na2S°3
Na2S04
NaHSO.
4
NaOH
PC13
PCI
5
POC13
HPO
3
H_PO.
3 4
P 0
4 10
Pb (NO )
3 2
PbO
PbCl2
PbSO.
4
Si02
VO
V2°4
V00C
2 5
ZnO
Zn(N03)2
zlco'
ZnSO.
4
S0
2
so.
2
so3
H2S°3
H2S°4
CO
co2
H 0
2
H2
CH4
COS
°2
NO
2
NO
NO
2
N00,
2 4
N 0
2
N2°5
HNO
3
NOC1
FeSO,
VC1
26
-------
439 710 980
2780
10'
500
700
900
1500
1700
1100 1300
°K
Figure 2. Equilibrium sulfur products for coal combustion with 10 percent
excess air, 3.27 percent sulfur
1900
27
-------
439 710
980
2780
10'
500
700
900
1500
1700
1100 1300
°K
Figure 3. Equilibrium sulfur products for coal combustion with 2 percent
excess air, 3.27 percent sulfur
1900
28
-------
439 710
980
2780
500
700
900
1500
1700
1900
1100 1300
°K
Figure 4. Equilibrium flue gas components for coal combustion with 2 percent
excess air, 3.27 percent sulfur
29
-------
-, 439
2780
500
700
900
1500
1700
1100 1300
°K
Figure 5. Equilibrium flue gas components for coal combustion with 10
percent excess air, 3.27 percent sulfur
1900
30
-------
In the solid at equilibrium at about 800 K (980 F) the distribution of
sulfates, nitrates, chlorides, etc., is dominated by CaSO,. On the basis of
mole fraction in the solid the distribution of S0~ is
CaS04 1.4 x 10"1
MgS04 1.4 x 10"2
Na.SO. 6.9 x 10~3
2 4
ZnSO. 2.6 x 10~3
4
. PbS04 6.2 x ID"5
All other sulfates and sulfites are at the level of 10~ . The sum of all
such species would be about 2 x 10~5 and include
FeSO. > Na0S00 > NaHSO^ > A10(SO.)0 > BeSO. .
4 . z j 3 243 4
A variety of chlorides also would be expected and together they would.
total about 4 x 10 mole fraction. The compounds, all in the range of 10 ,
include Aid , Fed , ZnCl2, NaCl, VC1 , MgCl , CaCl , and BeCl , in descend-
ing order.
It is interesting to note that very little nitrate or nitrite appear as
products in these calculations. They total about 2 x 10~^ mole fraction and
include
NaNO. > Mg(N00)2 > NaNO,, > A1(NO_). > Ca(NOj_ = Ca(NO.K > Be(NO_)0.
23 3 33 32 22 32
Fluorides also are present in the same concentration range and the total
of various compounds is about 2 x 10 mole fraction. The compounds expected
to be present include A1F > CaF > MgF > NaF > BeF .
Two phosphorous acids also are expected, phosphoric (H PO ) and meta-
—6
phosphoric (HPO ), and their total concentration would be about 9 x 10 mole
fraction.
There is essentially no difference between the 10% and 2% excess
combustion air cases for all of these various compounds except some
slight general decrease with decreasing excess air, but the change is
hardly significant. At still lower temperatures than considered above
(< 800 K), there is little change among the species except for A12(S04>3
which at equilibrium would be the major species. This would occur at the
expense of gas phase sulfur (SO and SO ), but may not happen in practice
because the kinetics become too slow. The rest of the species mentioned
above are essentially unchanged.
31
-------
Based on many observations in a wide variety of combustion systems,
the composition observed at the stack is more characteristic of equilibrium
at higher temperature than the exit temperature. That is, the composition
appears to be "frozen" at the higher temperature distribution. If it is
assumed that above 1300 K (1800 F) the reaction system is at equilibrium
and below it the composition is unchanging, then flue gas compositions can be
estimated. On a molar basis for coal with 3.27% S and 10% excess combustion
air, the distribution of potential acid aerosol components would be:
SO ^ 2000 ppm
SO ^ 200 ppm
HC1 ^ 100 ppm
NO ^ 50 ppm
0.3 ppm
"I
HF
^^i
If the same coal burned with only 2% excess combustion air, then the expect-
ed distribution would show an increase in SO^/SO,. ratio and a decrease in
both NO and NO . The expected distribution would be:
SO ^ 2400 ppm
SO i
HC1 n
NO i
HF/
N00 i
" 70 ppm
•" 100 ppm
u 10 ppm
' 0.3 ppm
u <0 . 1 ppm
If we made the same assumptions about rates for the solid components
and further, that the fly ash would contain about the same distribution of
compounds as the total solids, then we would expect in the fly ash, on a
molar basis
CaSO. ^ 1%
4
MgSO ^ 0.1%
ZnSO % 100 ppm
The various other sulfates, chlorides, etc., mentioned previously would
appear at about the ppm level.
32
-------
In a similar manner, the equilibrium predictions can be compared for
fuel oil combustion. Since we are still burning a hydrocarbon fuel, the
basic combustion parameters, particularly the temperature range, remain the
same. The major difference results from the changed distribution of im-
purities which can lead to acid aerosols. A distillate fuel will generally
have relatively low levels of sulfur, for instance, and very low levels of
ash, i.e., the metallic constituents which can lead to a wide variety of
inorganic sulfates as in the coal case. A residual oil, on the other hand,
will have comparable levels of sulfur, fuel nitrogen, and possibly ash.
Therefore the results of equilibrium calculations are similar to those for
coal.
Figures 6 through 9 summarize the results for a typical No. 6 and
No. 2 fuel oil composition (Levy, 1971) (Cato, 1974). The distribution
of elements for the two oils is given in Table 4 and the products included
in the computations are the same as those used in the coal computation
(Table 2) except, of course, that those compounds involving elements not
included in the fuel are not included in the calculation. The complete
results of the calculations are given in the'Appendix. It should be pointed
out that chlorine is not included although it is recognized it may be
present in some oils. However, the oil analyses available did not include
chlorine. The No. 6 oil contains about an order of magnitude more sulfur
than the No. 2 fuel and relatively large amounts of vanadium, iron, and
aluminum, as well as smaller amounts of a number of other metals. Although
calculations were performed for both 10% and 2% excess air, only the 2%
cases are shown since the results were nearly the same.
The distribution of potential acid aerosol species in the flue gas is
shown in Figures 6 and 7. The distribution of products in the flue gas for
the two oils is about the same, the major difference being the magnitude of
the various species. The magnitude of the sulfur species is, of course,
directly proportional to the amount of sulfur initially in the fuel. The
major difference between the fuel oil cases and that of coal, for equili-
brium conditions, is that more of the initial sulfur in the fuel oils remains
in the flue gas at low temperature. This results from the lesser ash and
hence less formation of solid sulfates. As a result of more sulfur in the
flue gas at low temperatures oxidation proceeds further and at the lowest
33
-------
TABLE 4. TYPICAL FUEL OIL COMPOSITIONS
Species
C
H0
2
N?
/
S0
2
V
Ni
Na
Fe
Al
Si
Mg
Cr
Ca
Co
Ti
No. 6
85.1
11.2
0.44
2.80
ppm
225
40
18
262
251
188
20
16
13
12
6
No. 2
87.0
12.8
0.025
0.20
< 0.1
< 0.1
< 0.8
34
-------
439 710 980
2780
10
500
700
900
1500
1700
1100 1300
°K
Figure 6. Equilibrium flue gas components for #6 oil combustion with
2 percent excess air, 2.80 percent sulfur
1900
35
-------
.,439 710
980
2780
10
500
Figure 7.
700
900
1100
1300
1500
1700
1900
Equilibrium flue gas components for #2 oil combustion with 2
oil combustion with 2 percent excess air, 0.20 percent sulfur
36
-------
temperature sulfuric acid becomes the major gas phase sulfur species. For
the No. 6 oil this could amount Co about 0.1 percent of the flue gas at
equilibrium, Figure 6. As mentioned previously, however, the system does
not come to equilibrium and the calculation represents the potential for
sulfuric acid formation, not a prediction of the amount actually formed.
As noted in the coal case discussed previously, there is little effect
due to differences in excess air. From the standpoint of combustion modifi-
cations, the maximum difference would be expected from the situation in
various types of staged combustion wherein less than stoichiometric air is
initially utilized. A summary of effects on potential contributors to acid
aerosols is shown in Table 5 where 2% less than stoichiometric combustion air
is compared to 10% excess. It will be seen that at equilibrium at maximum
temperature almost all of the sulfur is in the form of SO . There is a
large increase in SO in all cases when the air is increased but even so the
SO /SO ratio remains approximately 1000. The main effect in going from
less than to more than stoichiometric air is the increase in NO and NO .
However, the ratio of NO/NO is again about 1000. In staged combustion,
additional air is added in some manner to complete combustion at a reduced
temperature. For equilibrium calculations, the results at the reduced
temperature and increased air are the same regardless of the distribution of
products at the higher temperature so that the calculated combustion system
effluent is unaffected by considerations of staged combustion.
The distribution of sulfur in the solid phase at low temperature is
what one would expect from the combustion of oil. For residual fuel oils
where the total metal content and the number of different metals are both
relatively high, the solids will contain a variety of sulfates. For the
No. 6 fuel oil used in this example, at low temperature equilibrium, one
would find aluminum, iron, magnesium, and nickel sulfates and sodium bi-
sulfate. However, in contrast to the coal case where total ash was much
higher, the solids in this example contain only a couple of percent of the
total sulfur available in the fuel as shown in Figure 9. Almost all of the
sulfur is present as uncondensed sulfuric acid. In the case of the No. 2
oil the contrast with coal is more striking because there is even less ash.
-4
As shown in Figure 8 only a very small fraction, 10 , of the fuel sulfur
37
-------
TABLE 5. EFFECT OF EXCESS AIR AT A
TEMPERATURE OF 1800 K (2780 F)
U)
00
Percent of
Stoichiometric
Mr
-2
+10
-2
+10
SO,
SO,
NO
NO,
1.7 x 10
-3
1.5 x 10
-3
No. 6 Oil (2.80% S)
7.3 x 10~8 3.9 x 10~5
2.1 x 10~6 1.2 x 10~3
No. 2 Oil (0.20% S)
&. 2 x 10
-7
1.1 x 10
9.8 x 10
-4
-5
4.7 x 10
1.3 x 10
-7
3.8 x 10
1.2 x 10
-5
-3
7.6 x 10
-7
HC1
Coal
-2
+10
2.
2.
7 x 10 3
4 x 10
7
2
.0
.8
x 10
x 10
(3.27% S)
-8
-6
2.
1.
4 x
0 x
io~5 — *
10~3 5.8 x 10~?
1.0
9.7
x 10 4
x 10~5
* These values are all very low.
-------
439 710
980
2780
10
500
Figure 8.
700
900
1500
1700
MOO 1300
°K
Equilibrium sulfur products for #2 oil combustion with 2
percent excess air, 0.20 percent sulfur
1900
39
-------
.439 710
980
2780
10
NaHS04\ No2S04
CaS04
1100 1300 1500 1700
°K
Equilibrium sulfur products for #6 oil combustion with 2
percent excess air, 2.80 percent sulfur
1900
40
-------
would appear as free sulfur and sodium bisulfate in the small amount of
ash expected from a distillate fuel.
As in the previous discussion of coal combustion, it is instructive
to assume the system is at equilibrium above some temperature where kinetics
are fast, and that composition is unchanging below that temperature in order
to get some idea of what flue gas and ash .compositions might be. Making the
same assumptions of frozen compositions below 1300 K (1800 F) as in the coal
case, the expected concentration of acid aerosol precursors in the flue gas
for the No. 6 oil with 2 percent excess air and containing 2.8 weight per-
cent sulfur are:
SO ^ 1500 ppm
SO ^ 100 ppm
NO ^ 20 ppm
The fly ash would be expected to contain about equal amounts of magnesium,
nickel, sodium, and calcium sulfates amounting to a total of a few mole
percent.
The No. 2 oil, containing 0.2 weight percent sulfur would be expected
to produce in the flue gas, based on the same assumptions:
SO ^ 90 ppm
SO ^ 4 ppm
NO % 20 ppm
In this case there would be much less fly ash but what there was could
contain as much as 10 to 20 mole percent Na^SO, plus Na^SO .
A comparison of equilibrium considerations as they relate to observed
SO production in practical boiler systems has been given by Hedley (Hedley,
1967) and is shown in Figure 10. The solid lines are theoretical equili-
brium calculations for the conversion of S09 to SO with either 10% or 0.1%
£- -J
excess air. The dotted line indicates typical actual values at temperatures
characteristic of various parts of the boiler system. The horizontal lines
indicate expected temperature ranges for various parts of the system. This
is consistent with the overall views of mechanisms and kinetics. At high
temperatures, point Y, the formation of SO , is controlled by superequilib-
rium oxygen atom concentration. As the gas moves away from the flame and
begins to cool the oxygen atoms recombine or react and SO concentration
41
-------
INCREASED TIME
1600
1400
1200
1000
800
400
TEMPERATURE. K.
Figure 10. The variation of the theoretical equilibrium yield and possible
actual yield of SO with time in a boiler.
'42
-------
falls toward true equilibrium. When the temperature reaches about 1200 K
the kinetics become too slow for SO concentration to follow equilibrium
as the gases pass rapidly through the boiler system. Though the concen-
tration of SO may rise somewhat it will not reach true equilibrium.
43
-------
SECTION V
MECHANISMS
From the standpoint of the effect of combustion modification on
pollutant formation in general and the potential acid aerosol constituents
in particular, most CM schemes are attempts to reduce maximum flame temper-
atures and minimize the excess oxygen, particularly during the period when
temperatures are near maximum. These procedures influence the kinetics of
the combustion processes primarily through control of the oxygen atom
concentration. Although the mechanisms of NO and SO formation are not
X X
known completely, it appears that the 0-atom concentration and that of
closely related species, such as OH and HO- are critical in oxidizing ni-
trogen to NO and NO^ as well as sulfur to S0? and SO . Since these same
radicals are important in the oxidation of carbon, the kinetics of combus-
tion and pollutant formation are inseparable. In addition, CM can influence
particulate formation which can further modify possible acid aerosol for-
mation through such processes as adsorption, heterogeneous reactions, and
catalysis. Furthermore, these latter reactions will take place at lower
temperatures and in parts of the combustion system quite distinct from the
aforementioned combustion reactions. The high temperature homogeneous
reactions will occur for the most part in the first regime described pre-
viously. The heterogeneous processes are expected in the second regime.
The final part of this section of the report is concerned with reactions in
the forth regime, the near plume. Although the detailed mechanisms are
not known well enough to make precise predictions, some generalizations can
be made which are useful in considering CM approaches.
44
-------
HIGH TEMPERATURE HOMOGENEOUS REACTIONS
Formation of SO and SO
On combustion of fuels containing sulfur essentially all of the sulfur
that leaves the combustion system through the stack will be in the form of
SO accompanied by a few percent of SO . This situation always pertains
in oxidizing systems, which include all industrial boilers, regardless of
the initial form of the sulfur. Mechanisms of conversion of sulfur in the
fuel to SO and SO have been described (Levy, 1970), (Hedley, 1967),
(Cullis, 1972). In the flame, the sulfur is mainly in the form of SO and
SO even in a fuel-rich situation. The reactions are in general fast and
equilibrium calculations provide a reasonable picture. Results by Johnson
et al., (Johnson, 1970) for a propane/air flame containing sulfur are shown
in Figure 11 where it is seen that species other than SO and SO do not be-
come significant until the air drops below 80 percent of stoichiometric.
The SO results from oxidation of a variety of sulfur species such as S, H.S,
COS, etc., by various processes which for H S probably proceeds as
0 + H2S -»• OH + SH (1)
0 + SH ->• SO + H (2)
H + H2S -> H2 -I- SH . , (3)
0 atoms and SO are then chain carriers for further oxidation. Although the
details of the process are not fully elucidated, the mechanism probably
includes
SO + 0 ->• SO + 0 (4)
0 + SO + M -»• SO + M (5)
Oxygen atoms also are the key to SO formation and, as pointed out by
Hedley (Hedley, 1967), the fact that SO, produced in flames is generally
greater than equilibrium amounts at flame temperature is consistent with
superequilibrium oxygen atom concentrations in flames. The pertinent
reaction controlling the SO, and approach to steady state are thought to
be (Merryman, 1971)
SO. + 0 + M ->SO + M (6)
45
-------
60 7C BO SC 100 HO 120
FUEL-AIR RATIO IV. OF SJOICH AiB )
J
l«67 20'3 Jit'
AOliBATlC
221.0 2?tO 2172 2071
Figure 11. Equilibrium distribution of sulfur-containing species
in propane air flames with unburnt gases initially
containing 1 percent SO - points represent experimental
measurements normalized at 60 percent stoichiometric air:
o or • gas chromatography, V or T mass spectrometry
46
-------
The importance of oxygen to the formation of SO in practical combustion
systems has been recognized for many years and was well illustrated by the
work of Barrett, et al., (Barrett, 1966) summarized in Figure 12. When
oxygen is at or below stoichiometric there is virtually no SO formed but
the first percent increase in excess oxygen greatly increases the formation
of the trioxide.
Particularly important to CM considerations, and possible effects of
staged combustion, is the pilot scale work of Hedley (Hedley, 1967). He
observed that adding oxygen to hot incompletely combusted fuel containing
sulfur produced more SO than burning with the same amount of oxygen added
to the original combustion air. This effect is attributed to the relatively
high concentration of CO in the incomplete combustion mixture. When com-
bustion was completed by adding oxygen, the 0 atom'concentration was greatly
increased, presumably by
CO + OH = CO + H (8)
H + 0 = 0 + OH (9)
which in turn significantly increased SO production.
The homogeneous catalysis of SO to SO by NO is well known and has
been discussed by Hedley (Hedley, 1967) and Cullis and Mulcahy (Cullis,
1972). The overall reaction can be described as
NO + 1/2 02 = N02 (10)
NO + SO = NO + S03 . - (11)
The rate of Reaction 11 has been measured (Armltage, 1971) up to about 920 C
(1688 F). It appears that the rate of SO oxidation is limited by the rate
of formation of NO , i.e., the former reaction above is rate limiting. The
significance of such reactions is not clear in practical combustion situa-
tions but the homogeneous catalysis could contribute to the formation of SO
at low temperature where the reaction of SO with oxygen is quite slow. On
the other hand, such reactions may play a part in flames as shown by the
work of Levy and Merryman (Levy, 1965). In developing the microstructure
of H S flames, they demonstrated an increase in SO when the oxidant was
switched from 0_ - Ar to 0 - N .
-------
25
20
»ANG( OF VHUES
50-j CQUIVUEM TO 1%
CONVERSION OF S TO SO;
txputiG POSITION:
2-t inch FROM BURNER PL«TE
9S 100 IDS 110 US 120 I2S 130
TOTAL AIR. PERCENTAGE OF SlOlCHlOMETRiC AIR
Figure 12. Effect of excess air on the formation of sulfur
trioxide in a hydrocarbon flame
48
-------
The effect of NO at high temperatures has been investigated by Wendt
and Ekmann (Wendt, 1975) and found to be quite different. At flame
temperatures they found the presence of SO decreased the formation of
NO in a methane/air flame. H S as a fuel additive also decreased NO '
formation. In each case, the decrease was as much as 36 percent. The
mechanism of these high temperature reactions is largely unresolved but
SO is known to promote the recombination of free radicals such as H and
OH (Webster, 1965) which are important to the formation of NO in flame
environments. In any event, the implication of these results is important
to the practical utilization of sulfur bearing fuels. As pointed out by
Wendt and Ekmann, the results suggest that fuel desulfurization may lead to
increased thermal NO production; but the effect on fuel NO , if any,
X X
remains unresolved.
Formation of NO and Nitrate
Even though most of the available data indicate that nitric acid and
nitrates do not contribute significantly to primary acid aerosol, it is
thought that some brief review of the mechanisms of formation of the
important precursors, NO and NO , should be included in this report. This is
particularly so since most combustion modifications being used or considered
are for the purpose of controlling NO and NO formation. Further, it is in-
herent in the combustion processes that when sulfur and nitrogen are both
present, factors influencing formation of oxides of nitrogen and oxides of
sulfur cannot be treated entirely independently.
The formation of oxides of nitrogen results from oxidation during com-
bustion of nitrogen from two sources; nitrogen chemically bound in the fuel,
and nitrogen from the combustion air. The mechanisms by which nitrogen from
these two sources is oxidized are different. Most attempts to explain NO
formation from thermal fixation of combustion air generally start with the
Zeldovitch mechanism
0 + N = NO + N (12)
N + 0 = NO + 0 . (13)
This mechanism, however, fails to predict NO concentration in and close to
the flame front where the concentration is higher than equilibrium. The
high NO concentration in this region is generally attributed to the super-
equilibrium concentration of H, OH, ar\* 0 all of which reach maximum
49
-------
concentration in the vicinity of the f;lame front. The hydroxyl radical,
through the reaction,
OH + N = NO + H (14)
is thought to be particularly important in fuel rich situations (Ay, 1976),
Bowman, 1972), (Williams, 1972).
Fenimore (Fenimore, 1971) suggested that other routes to NO which
whould be considered to account for the initial "quickly formed'1 NO, which
he called "prompt NO", include
1/2 C2 + N2 = CH + N (15)
CH + N2 = CHN + N (16)
which would be followed by (13). Still other routes have been suggested,
for instance, Merryman and Levy (Merryman, 1975). Such reactions provide
an alternate path for breaking the strong N bond.
Knowledge of the kinetics of these reactions and of combustion radical
concentrations allow a qualitative understanding of NO observations in and
close to flames and the thermal formation of NO, at least for fuel-lean
situations. Under equilibrium conditions, the Zeldovich mechanism predicts
NO fairly well. However, there are still many unanswered questions, parti-
cularly for fuel rich conditions and our ability to predict NO and N02
formation under these conditions needs to be improved.
In the case of fuel-bound nitrogen knowledge of mechanisms and kinetics
is incomplete. The mechanisms of oxidation of fuel nitrogen and molecular
nitrogen are distinctly different but not necessarily independent. For com-
bustion modifications then, the implication is that the effectiveness of a
given CM in reducing NO formation will depend on the fuel. It has been well
established that CM can reduce NO in the case of natural gas, propane, or
distillate oil combustion but is less effective in the case of coal or
residual oil combustion because of the fuel-bound nitrogen in the latter
fuels. Although the mechanisms are not clear, several general features are
established:
• A large part of the fuel-bound nitrogen can be converted to
NO during combustion
• The fraction converted to NO increases with excess air and
decreases as the fuel-N .concentration increases
50
-------
o Conversion of fuel-N is not as sensitive to temperature
as the thermal nitrogen mechanism
o The conversion to NO is not very dependent on the nature
of the N-compound as shown by various basic studies.
The mechanisms which have been proffered for the conversion of fuel-N
to NO are all rather speculative. Early studies of ammonia/oxygen flames
(Maclean, 1967) (Wolfhard, 1952) demonstrated that pyrolysis led to NH and
then NH radicals before reaching the flame front and that NO began to
increase rapidly when NH reached its peak concentration. Fine (Fine, 1974)
and Sarofim (Sarofim, 1975) and coworkers have suggested a sequence of events
which may be represented as
RNH2 < _? NH2 < —> NH < — > N
V V V V
NO N NO N
Ammonia, a pyrolysis product of coal, can be converted essentially
quantitatively to NO (Sarofim, 1975). However, the conversion drops dramat-
ically when the reaction system is fuel rich, Figure 13, where the product
is presumably N . Such results have clear implications for CM such as staged
combustion. Initial combustion with less than stoichiometric air would be
expected to lead to N? rather than NO. The removal of some heat prior to
completing combustion with some excess air would tend to lower the con-
version of N,, to NO by the "thermal" mechanism discussed earlier. Thus,
the total conversion to NO would be decreased.
Pyrolysis studies of model compounds for fuel-N, e.g., pyridines,
quinolines, etc., have shown that HCN can be the major pyrolysis product
at temperatures above 1000 C (1832 F) in the absence of oxygen (Axworthy,
1975). Pyrolysis of No. 6 fuel oil, a crude, and a coal sample under
similar conditions produced fuel-N conversions to HCN in the range 30 to 50
percent and much less ammonia. This suggests HCN could be an important
intermediate to NO formation than fuel-N.
Axworthy, in premixed flat flam experiments, compared the rate of
formation of NO from NH and HCN (Axworhty, 1975). The results indicate
that NO is formed mostly just downstream from the luminous flame zone. The
rate of conversion to NO from
is suppressed in rich flames.
rate of conversion to NO from NH appears greater than for HCN and conversion
51
-------
O
2
O
h-
2
O
I/)
tt
2
O
u
100
9O -
80
70 -
60 u
50 -
30 U
20
TO
0
! !ii L__i_.._j..__..: i
.80 .84
.88
.92
Figure 13. Effect of equivalence ratio on conversion efficiency for
an ammonia additive (0.5 percent by weight)
52
-------
It is recognized that heterogeneous reactions could als.o play a
significant part in conversion of fuel-N to NO. Pyrolyses experiments with
fuels and model compounds demonstrate the tendency to form nitrogen con-
taining carbonaceous residues (Axworthy, 1973). Even the volatile N-hetero-
cyclics have a strong tendency to form solids which at lower temperatures
may contain most of the original nitrogen. Thus, one could expect some
nitrogen containing soot which could lead to heterogeneous formation of NO
during burn-out. Wendt and Schultz (Wendt, 1976) have molded coal char burn-
out and their results are in agreement with the observed weak temperature
dependence of fuel-NO formation and predict an effect of particle size.
Also, the results suggest that free stream CO may be more important than 0
in determining the effect of excess air on heterogeneous formation of fuel-
NO. This is an area which warrants further investigation particularly since
an effect of some CM techniques such as low excess air firing and staged
combustion may lead to increases in particulate formation and hence increas-
ed heterogeneous oxidation of nitrogen.
HETEROGENEOUS GAS-SOLID REACTIONS
Catalytic Oxidation of SO by Metals
and Metal Oxides (Suspended Phase)
The question often has been raised as to how much of the SO,, produced
in boilers is the result of the catalytic oxidation of SO in contrast to
the previously discussed homogeneous reactions. In the temperature range
characteristic of the second regime of the combustion system, heterogeneous
reactions leading to increased oxidation of SO can occur and are the
subject of this section of the report. Several investigations have been
made of the catalytic activity of fly ash components in this regard.
Fletcher and Gibson (Fletcher, 1954) showed that for temperatures above
600 C (1112 F) , Fe_0 greatly increased the formation of sodium sulfate from
sodium chloride and SO , while at temperatures below 600 C, the sulfate was
formed only from SO already present. Thus Fe 0 is a strong catalyst in
the oxidation of SO.. These findings were confirmed in the fundamental work
of Vogel, et al, who found that high relative humidity helped promote this
fly ash catalysis.
53
-------
That materials other than Fe~0 in a boiler system can be effective
catalysts for the oxidation of SO to S0_ was demonstrated by Wickert
(Wickert, 1957). He found that although Fe90 was the most active of the
materials that he tried, a sample of fly ash brought about a maximum of
36 percent conversion of SO to SO at about 760 C (1400 F). On the other
hand, SiO and Al 0 were only weak catalysts in this system. What is most
significant, however, was the observation that these catalytic reactions
were highly temperature dependent as shown in Figure 14. The catalytic
effect of the Fe?0 is greatest at superheater metal temperatures while that
of the fly ash goes through a maximum at a slightly higher temperature. The
broader peak with Fe_0 indicates the greater importance of this compound as
a catalyst for the formation of SO , as it operates through a wider temper-
ature range. Obviously the surface area available and length of time for
fly ash to spend inside the catalytically active temperature window are of
paramount importance in determining the importance of fly ash catalysis.
Manganese dioxide is a powerful converter of S0« to sulfate over a wide
range of temperatures, from room temperature (Corn and Cheng, 1972) to at
least 340 C (644 F) (Vogel, 1974) and so may play a role throughout the
entire convection and stack zone of a power plant. High relative humidities
(without condensation) are necessary for the catalysis to proceed.
One of the best catalysts for the conversion of SO,, to SO is V 0 .
In the contact process for the manufacture of sulfuric acid SO is passed
over V 0 catalysts at a temperature of 427 to 621 C (800 to 1150 F). With
a contact time of 2 to 4 seconds, the conversion of SO to SO is 90 percent
or greater. Residual fuel oils from the Middle East and from Venezuela '
contain significant amounts of vanadium and in the combustion process this
is converted to V_0 . As a consequence, the potential for SO formation by
heterogeneous catalytic reaction with V^O in the combustion of these oils
is great. Wickert also investigated the effect of VjO and mixtures con-
taining V 0 and other boiler deposit components on the oxidation of S0«
(Wickert, 1959). His results are shown in Figure 15 in which the temperature
dependence of the catalytic reaction is again apparent. In this case V2°5
was a better catalyst for the reaction than was Fe-O.,. A mixture containing
90 percent V 0 and 10 percent Na SO was just as good a catalyst as was the
54
-------
c
QJ
(J
flj
a
•a
OJ
X
o
CM
O
CO
20
10
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
v,oo Temperature, F
Figure 1A. Catalytic oxidation on SCL to SO by
various materials
55
-------
Ln
80
80
70
60
760
940
Temperature , F
1120
1300
1650
/
Legend
V205
90 V205/IONo2SO«...)
[lO V205/90No2S04.)
--n— Boiler deposits : 60 % V205, 8 % No20
3.5 % NiO , 5% Fe203, 15 % S03
-O-47V205/37Na2S04/l6Si02 ) Weight
37 V205/28 NazSC>4 /35 CaS04 -2H20 ) basis
40O
500
700
800
600
Temperature , C
Figure 15. Catalytic oxidation of SO to SO by various materials
900
A-44S25
-------
Fe 0 . However, the boiler deposit from the burning of residual oil con-
taining vanadium was the best catalyst of all, causing a 90 percent con-
version of SO to SO . Several materials were at their greatest effective-
ness in the 900 to 1100 F temperature range of typical boiler tubes.
Catalysis by V 0 also was examined by Napier and Stone using short contact
times with typical flue gas compositions (Napier and Stone, 1958). The
catalyst consisted of V.O and K_SO, on a silica support. With 1000 ppm S09
in the gas stream and the catalyst at 430 C (806 F), from 94 to 98 percent
conversiton of the SO to SO was achieved with contact times ranging from
90 to 430 milliseconds. When the contact time was held constant at 170
milliseconds, S0_ conversions ranging from 92 to 98 percent were obtained
when the S09 content in the gas stream was varied from 340 to 2700 ppm. It
was concluded that the required contact time for catalytic oxidation at low
SO concentrations is much lower than that used in the contact process for
sulfuric acid. In addition it was noted that trhe presence of water vapor
and CO in the gases had no adverse effect on the catalyst. Hence, heter-
ogeneous catalysis by V 0 in flue gases can well be an important source of
SO-. However, V_0 is effective only in a high temperature "window", and
this again poses the question of how combustion conditions affect the avail-
able surface area and time that fly ash particles spend in this catalytical-
ly effective high temperature window. Furthermore, V 0 is ineffective at
room temperatures, and so probably does not contribute to secondary sulfates.
Unoxidized vanadium on an Al.O support is not effective, at least at
340 C (644 F) as shown by Vogel, et al.
Catalytic Oxidation of SO., by Soot
and/or Carbon
The role of carbon as a catalyst to oxidize SO to SO is reported
primarily by Novokov (Novokov, 1974). He found that freshly generated soot
or graphite convert SO to SO which is bound to the particles. Evidence
that soot was also catalytically active, especially in the present of water
vapor was also found. Furthermore, soot was active at only a specified
distance down stream from a flame. This distance may imply the existence of
a temperature "window" or limitations of capacity to adsorb and react SO .
The data shows that the decrease of SO across a sooty filter is independent
57
-------
of SO. inlet concentration, a fact which Novokov explains by hypotesizing
that the number of active sites on soot particles is controlling rather than
inlet species concentrations. Novokov also states that increases in S0«
oxidation occur at higher 0_/C,H0 ratios, although none of these data lie
2. jo
in the fuel lean regime. He attributes this to an increase in the number of
"ultrafine, high surface area particles" although it may be due simply to
increased 0 availability. In all his experiments there was a pronounced
saturation effect, implying a finite capacity for this process.
However, Novokov's work is not quantitative and so it is not known how
much conversion is possible through this mechanism. It is not clear whether
particle age or temperature is the determining factor for catalytic activity.
There is at present no demonstration that this carbon mechanism is not
important, and so it seems reasonable to conclude that the mechanism might
account for significant primary S0,= formation, especially when fresh carbon
particles or soot are formed. Clearly further work is required to quantify
this effect and to determine its practical significant.
Combustion and Particulate Characteristics
There is considerable evidence that particulates from coal combustion
may contain catalytically active materials. Therefore, the particle size
and surface area provided by fly ash will affect the overall uptake and con-
version of S0~ to SO,. In this section we address the problem of how com-
bustion conditions affect the size distribution of potentially catalytic fly
ash particles.
An extensive analysis of a variety of major and minor elemental
constituents in 101 different coals from around the United States is
illustrated in Table 2 (Equilibrium Considerations, page 24). The various
elements as such are shown in Table 2; they in fact occur in complex arrays
of molecules which to date have defied complete chemical description.
The most important coal constituents determining the amount and
composition of particulates formed in combustion are those contained in
the mineral matter. Generally, mineral matter can be classed as inherent
and extraneous although exact definitions are difficult because the same
ash constituent may be present in both classes. Inherent mineral matter is
made up of chemical;elements such as iron, calcium, magnesium, phosphorus,
58
-------
potassium, and sulfur that were combined organically with the plant tissues
when the coal was formed. Inherent mineral matter seldom exceeds 2 percent
of the weight of the coal (Reid, 1971) and the rest of the a^h component is
comprised of extraneous mineral matter which consists of minerals mixed in
with the coal seams during formation but not actually part of the coal itself.
About 95 percent of the mineral matter is coal is made up of kaolinite
(A120 -2Si02-2H 0), pyrites (FeS2), and calcite (CaCO ) (Reid, 1971). Other
elements listed in Table 2 exist in a variety of forms th'at have not been
definitely determined at this time. /
The fate of the various coal constituents during combustion is a deeply *—r.
involved process depending on the original chemical form of the elements and |
the time-temperature history of the combus.tion process. For instance, pyrite I
(FeS) begins to oxidize at only 260 J2^(500 F) but rapid heating in a pulver-
ized coal fired furnace can cause/pyrite to melt before the sulfur has been
driven off or the iron oxidized to Fe?0 . Hence, moderate to high heating
rates will result in the formation of a liquid phase of FeS, whereas slower
heating rates will result in the formation of SO and Fe 0 which will also
flux the silicates in the ash.
The formation of ash or particulate matter in the flue gases resulting
from coal combustion occurs generally by two mechanisms. The first involves
formation of larger particles comprised of the low volatile mineral consti-
tuents in the coal. These particles consist of cenosphere or hollow, blown-
out sections resulting from ashing of the co-?.l. The second mechanism
involves the formation of very small particles resulting from the conden-
sation of the more volatile mineral constituents (e.g., Se, As, Pb, Cd, and
V).
Abel and Rancitelli have shown that certain selected refractory
constituents (Eu, Hf, Sc, Ta, Th, La) are converted nearly quantitatively
from the coal to fly ash during the combustion process (Abel, 1975), whereas
for the more volatile Se and Sb only 30 percent and 20 percent of each
element, respectively, could be detected in the fly ash. Schultz, et al
have indicated that the amount of various trace elements ending up in the
fly ash is a function of the vapor pressure of that element at the combustion
temperature of the process with the higher vapor pressure constituents being
retained to a lesser extent (Schultz, 1975).
59
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Elements with higher vapor pressure or high volatility have shown
a tendency to preferentially concentrate in the smaller fly ash particles
due to condensation. In airborne coal fly ash the elements Pb, Tl, Sb, Cd,
Se, Zn, As, Ni, Cr, S, and Be showed higher concentrations in finer particles
(Natusch, 1974). Rancitelli has also shown this trend for three fly ash
samples from different coal-fired boilers (Rancitelli, 1974). In this case
there were significantly higher concentrations of As, Mn, Sb, and V in
particles less than 10 micrometers whereas there was a trend to higher con-
centrations of Fe in particles over 10 micrometers. About the same con-
centrations of Al, Se, and La were present in particles of all sizes. Cow-
herd, et al (1975) have also shown that more volatile constituents concen-
trate in the finer particles.
The major constituents in coal fly ash consist of the major mineral
elements in the coal itself: Si, Al, Na, Mg, K, Fe, and Ca. Fly ash
analyses generally report ash constituents as the oxide form or in the
elemental form although it is usually assumed that in all cases the com-
pounds are in the oxide form.
Analyses of ranges of the major fly ash components in a variety of
coal samples as compiled by the Bureau of Mines (Fieldner, 1942) are pre-
sented in Table 6. Large variations exist in all components but generally
SiO is the major constituent making up about 1/3 to over 1/2 of the ash
sample. Aluminum oxide (Al 0 ) is usually the second most prominent com-
pound comprising about 1/10 to just less than 1/2 of the sample. Iron
compounds, which are reported in a variety of forms (Fe, FeO, Fe^O , Fe_0,),
are about equal in order of magnitude with aluminum. The alkali metals
(Na, K) and alkaline earth metals (Mg, Ca) along with Ti comprise the
remainder of the primary mineral constituents in the ash.
Catalysis of SO Oxidation by Ash Deposits
Fly ash is deposited on boiler tubes and is available for catalysis
of SCL to SO, over a long period of time. One would expect that the metals
and metal oxides shown to be catalytically active would contribute to
catalysis of SO oxidation, when deposited on boiler tubes, provided the
temperatures remained in the correct range. In this section we highlight
some of the differences between fly ash catalysis in the dispersed mode and
in the deposit mode.
60
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TABLE 6. COAL ASH ANALYSIS LIMITS AS GIVEN
BY BUREAU OF MINES
Constituent
Percent
Silica
Aluminum Oxide
Ferric Oxide
Calcium Oxide
Magnesium Oxide
Titanium Oxide
Alkalies
30-60
10-40
5-30
2-20
0.5-4
0.5-3
61
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The potential catalytic effect of deposits has been shown by the work
of Glebov, et al (1973). He demonstrated that SO could be oxidized to SO
by molecular oxygen in the presence of boiler deposits over the temperature
range of 900 to 400 C (1670 to 752 F), the range of a boiler's convective
heating surfaces. The most active catalyst powder was found to be a deposit
from the convective bundles of oil-fired boilers. The deposit showed
catalytic activity comparable to a vanadium pentoxide powder in the experi-
ments. Maximum conversion occurred at a catalyst temperature of 560 C
(1040 F). Further, effective catalysts were prepared from mixtures of V?0,.
+ Na-SO, + Fe 0 which had a maximum conversion at 640 C (1184 F).
Using empirical coefficients in conjunction with a simple model of a
boiler system, Glebov predicted SO in flue gas as a function of deposit
thickness, icoiivective bundle surface area and temperature, and excess air.
The predicted values were compared to measured values from a boiler burning
a high sulfur oil and good agreement was found. These results indicate
that when deposits which may contain vanadium pentoxide are allowed to build
up, it is possible that catalysis by the deposits could control the SO, ,
effluent.
As the simple sulfates remain in the deposits for extended periods of
time, they are gradually converted to complex sulfates by the action of the
sulfur oxides in the flue gas stream. The formation of alkali-iron tri-
sulfates such as Na Fe(SO,)« from the reactions of sulfur oxides with Na^SO,
and Fe-O- was studied using radioactive tracer techniques by Krause, et al
(1968). The experiments were performed at 600 C (1112 F) and with 2500 ppm
SO- and 30 ppm SCL in a gas stream containing 3 percent 0 . It was demon-
strated that the reaction rate of S0_ to form the trisulfate was 970 times
as that with S0». A similar compound is formed with potassium and in this
case the reaction rate of SO. exceeded that of S02 plus oxygen by a factor
of 1260. Experience with trisulfate formation in an operating boiler was
reported by Anderson and Diehel, who placed a probe in front of the super-
heater tubes (Anderson, 1955). Gas temperatures in the region of the probe
were 982 to 1094 C (1800 to 2000 F) and the metal surface temperature of the
specimens was maintained at 566 C (1050 F). In this case the fly ash
collected from the boiler was found to contain 10.3 percent of sulfate ex-
pressed as SO . The initial deposit collected on the probe after a week's
62
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exposure contained 15.7 percent SO , and after several weeks time, the SO
concentration reached 35 percent.
The significance of the formation of sulfates in deposits and the build-
up of high concentrations of sulfates stems from the fact that portions of
these deposits are removed periodically from the boiler tubes by soot blow-
ing. This operation is carried out at least once a shift and by its nature
creates a large amount of particulate in the boiler in a short period of
time. As a consequence the capacity of the electrostatic precipitators is
taxed during this period and it is quite likely that a significant portion
of particulate sulfate passes through the precipitator and is emitted from
the stack. Unfortunately, no data are available yet on these overload
conditions which are potential sources of particulate sulfate in the
atmosphere.
Capacity Considerations
Of paramount importance in determining the overall conversion of SO to
SO, by fly ash catalysis is to address the question of capacity of avail-
able surface. Fundamental studies on the catalytic activity of V 0 indicate
conversions of 90 percent, yet these have never been observed in combustion
units. The difference is probably due to the fact that the catalytic activ-
ity of V 0 in combustion units is constrained by capacity limitations
brought on by available active surface area constraints. Combustion modifi-
cations, by altering size distributions of certain metallic species, may alter
the capacity of these species to convert SO to SO, . Capacity considerations
may be applicable both to fly ash catalysis in the disperse phase, fly ash
deposit catalysis and catalysis by carbon. Capacity considerations will also
affect the amount of H SO , formed prior to contacting metal, which reacts
to form metal sulfates.
EARLY PLUME REACTIONS
The final regime where chemical reactions might be expected to in-
fluence primary acid aerosol emissions is the near plume. For the purpose
of this study we have arbitrarily defined aerosol observed in the early
plume as primary aerosol. Technically this is an arguable point. The
primary basis of this definition is to separate the roles of particulate,
i.e., SO and H SO condensation, formed very rapidly in leaving the stack,
63
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from the typically slow plume chemistry, i.e., SCL oxidation process over
periods of minutes to hours downwind in a plume.
In the early stages of plume development, say in the first half mile
of travel, pollutants are subjected to a rapidly changing environment. Both
the temperature and composition of the plume change rapidly in this area
because of mixing of the hot flue gas with the relatively cool ambient air.
Accurate measurements of reactions in this early plume are nonexistent
because of the difficulties of aerial sampling in the narrow early plume.
Extrapolations of rates of chemical reactions occurring well downstream of
the plume origin can be made, but these are not expected to be very accurate
unless some accounting is made of the effects of the early cooling and mix-
ing processes.
There are many processes that can contribute to the overall pollutant
changes in the early plume. These processes are indicated in Figure 16. In
this figure, the physical processes of cooling, dilution, and condensation
(the rapid processes) are segregated from the possible (and slower) chemical
interactions for purposes of presentation; ultimately physical processes must
be considered in concert with the chemical processes. The chemical inter-
actions are then further broken down into homogeneous and heterogeneous
categories.
Physical Processes
A detailed discussion of plume modeling is beyond the scope of this
study. Rather, it will suffice here to consider the qualitative manner of
change of parameters important to the chemical interactions, namely,
temperature and relative humidity, as a function of dilution.
Typical stack gas temperatures for coal- and oil-fired power plants
are 135 C (275 F) and 437 C (819 F), respectively. Choice of these operat-
ing temperatures reflect differences.of sulfuric acid dew points in the two
types of systems. As a first approximation, it can be assumed that the
ambient air mixes uniformly with the stack gas, and that the heat capacity
of the stack gas is about the same as that of air. The temperature of the
plume of any point T , then is related linearly to dilution, i.e.,
T = fT + (l-f)T (17)
p s a
64
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CHEMICAL PROCESSES
Photochemical
Reactions
b. Hydrocarbon
Reactions
Metal Ion
Catalysis
b. Ozone Oxidation
H. .Homogeneous Reactions
2. Cooling
3. Condensation
5. Droplet Reactions
Figure 16. Plume dispersion and reaction
processes
-------
where T is the stack gas temperature, T is the ambient temperature, and
f is the fraction of stack gas in a unit volume of plume at any point.
The relative humidity of the plume depends on the plume temperature
as well as on the fuel composition and the relative humidity and temperature
of the ambient air. At the stack exit, the water content of the stack gas
is high, but the temperature is also high. The result is a low relative
humitity. As dilution proceeds, both temperatures and average water con-
centration decrease. However, the relative humidity is linearly dependent
on the water concentration and exponentially dependent on the inverse of the
absolute temperature. The relative humidity can be given by
RHp = RHa [(fN Q/N Q) + (1-f) (ID' e~5174 (| - f )1
P a
where Nu and N ,. are the mole fractions of water in the flue gas and
n u H u
ambient air, respectively. Results of application of Equation 18 to several
operating conditions with coal and oil firing are shown in Figures 17 and 18.
It is obvious from these figures that appreciable dilution must occur before
the plume RH begins to rise significantly. Under some conditions, the plume
RH can go through a maximum. These maxima occur when the ambient air is
relatively cool arid at values of (1-f) in the neighborhood of 0.9. Beyond
this point, the RH of the plume approaches that of the ambient air. It is
generally agreed that the oxidation of S09 to sulfate occurs most readily
at RH values greater than 50-70 percent. The approximate relationships
between f and distance for a 10 mph wind is shown in Figure 19. Figures 17-
19 thus serve as a guide to estimation of the lag time that may be required
for chemical reactions that are dependent on the relative humidity.
Chemical Processes
Sulfate Formation. The oxidation of SO to sulfate in plumes has been
studied extensively in recent years because of the impact of this process
on environmental health questions (Gartrell, 1963) (Dennis, 1969) (Coutant,
1972) (Stevens, 1971) (Huser, 1976) (Weber, 1970) (Newman, 1975) (Utah, 1975).
In spite of this extensive study, conclusions have only been in qualitative
agreement, at best. The basic problem of specification of plume chemistry
and kinetics has been tied to the difficulties of aerial sampling combined
with the variability of meteorological conditions (Beilke, 1975). Only
recently has an attempt been made to coordinate well-characterized chemistry
66
-------
100
90
80
70
60
50
S! 40
OJ
0 30
10
COAL
T =273, RH = 90%
A A
T =294, RH =90%
A A
T =273, RH =50%
A A
T =294, RH =50%
A A
9 10 20
30 40 50 60
100 (1-t:)
70
90 100
Figure 17. Dependence of plume relative humidity on dilution
67
-------
100
90
80
70
60
50
40
3
« 30
n
3
iH
* 2.0
10
0
OIL
TA=273, RHA=90%
T =294, RH =90%
A A
TA=273, RHA=50%
T =294, RH =50%
A A
0 10 20 30 40 50 60 70 80 . 90 100
100 (1-f)
Figure 18. Dependence of plume relative humidity on dilution
68
-------
1.0
o.i U
10 JOO
Distance, m
Figure 19. Dilution in early plume
JO 00
-------
with the variables inherent in plume dispersion to develop a reasonable
model of SO oxidation (Freiberg, 1976). Even so, this model has incor-
porated only one aspect of the possible plume chemistry, namely the iron-
catalyzed oxidation of SO in aqueous droplets, and it ignores some
aspects of early plume mixing.
It has been argued that many other prpcesses including ozone oxidation,
active hydrocarbon reactions, homogeneous oxidant reaction, and nonaqueous
heterogeneous catalysis can contribute to the oxidation of SO to sulfate in
a plume (Davis, 1975) (Cox, 1974, (Cox, 1972) (Smith, 1974) (Wilson, 1970)
(Esponson, 1965) (Penkett, 1972) (Novokov, 1974).
The general status of knowledge on plume oxidation of S0~ has been
reviewed recently by Levy, Drewes, and Hales (Levy, 1976). The preliminary
conclusions of Levy, et al, can be summarized as follows:
(1) Plume measurements indicate a range of half-lives for SO,,
of the order of 0.5-10 hrs, depending on meteorological
conditions. Ambient temperatures and relative humidity
appear to be the most significant variables.
(2) Ozone oxidation of SO in droplets may be important in the
downstream portion of the plume, but is probably not important
in the early plume where the principal reaction of ozone is
with NO.
(3) A lag time in SO oxidation may occur near the stack, but this
has not yet been well characterized, there being data both con-
firming and denying this hypothesis.
(4) Hydrocarbons may promote SO oxidation via photochemical smog
reactions.
(5) Considerations of homogeneous gas phase reactions indicates
that oxidation by OH could be fast enough to account for some
observations, but OH concentrations in a plume are not well-
characterized.
(6) Aqueous phase reactions with catalysis by various metal-ion
flyash components and NH« are known to be fast enough to
account for plume observations, and observed relative humid-
ity dependencies suggest these reactions as the principal
source of secondary sulfate.
70
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(7) The sorption and oxidation of SO on dry iron oxide or
carbon is a capacity-limited phenomenon. Inasmuch as this
interaction is said not to be dependent on relative humidity,
it may be significant in the early plume, but there exist no
data to confirm this hypothesis.
The conclusion that the aqueous phase reactions are most important in
the plume oxidation of SO. implies that the time lag suggested in Figures
17-19 may be an important factor in limiting the oxidation in the early
plume. Also, it should be noted that studies of aqueous phase oxidation of
SO indicate a strong inverse relationship between the rate of oxidation and
the hydrogen ion concentration in the solution. Various studies yield rates
proportional to [H+].
Droplets formed in the earliest portion of the plume, i.e., near the
stack exit are likely to consist primarily of sulfuric acid. This may be
partly neutralized by basic components of the fly ash, but it is expected
that neutralization of the bulk of the sulfuric acid will occur via reaction
with ammonia from the ambient air. Typical concentrations of sulfuric acid
in flue gas are of the order of 30 ppm; typical ambient levels of ammonia are
of the order of 6 ppb. Thus, a considerable amount of mixing and dilution
of the plume with the ambient air is required for neutralization of the
sulfuric acid. This too implies the existence of a lag time before appreci-
able oxidation of SO. can occur in the plume. For example, droplets formed
near the plume origin are estimated to have a pH of about 3. At a range of
about 2.5 km, enough ammonia will have mixed with the plume to form ammonium
bisulfate, but the pH at this point will have risen only to about 3.3. Thus,
the pH is effectively 3 throughout the first 0.8 km (0.5 mile) of plume
travel. The rate of absorption of SO in droplets having this low pH will
be very low.
At wind velocities of 5 to 10 mph, the time required for a plume travel
of 0.5 mile is 3 to 6 min. Assuming for the moment that there is no time
lag, and that the results of previously referenced studies can be extrapo-
lated back to the stack exit, it is estimated that the conversion of SO. to
sulfate in the early plume is no more than 4 to 7 percent under high humidity
conditions. If, on the other hand, one considers the time lag associated
with the development of high relative humidity within the plume and neutra-
lization of the sulfuric acid, the apparent overall rate of conversion is
71
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much less. Using the model of Beilke, et al., (Beilke, 1975), we estimate
that these effects would confine conversion of 0.3 to 0,7 percent in the
first half mile. The recent data of Wilson, et al., indicate about 1
percent conversion after 40 min. and suggest a maximum of only 0.2 percent
conversion within 6 min. Of course, little or no conversion is expected
in the early plume if the relative humidity of the ambient air is low
(Wilson, 1976).
Nitrate Formation. In contrast to the case of sulfate formation,
there has been very little attention paid to the formation of nitrates in
power plant plumes. In general, the formation of nitrates in the atmosphere
is tied to the photochemical reaction chain. In particular, the reaction of
ozone with NO to produce NO followed by reactions of the NO is generally
believed to be the major route for nitrate formation. In recent studies
under the MISTT program, Wilson, et al., have verified the consumption of
ozone by NO in the early plume (Wilson, 1976). Their results show that the
mass flow of NO- in the plume is (within an experimental error of 20 percent)
equal to the loss of ozone in the air mixed with the plume. Again this
reaction depends on mixing of air with the plume. At ambient levels, of say
80 ppb ozone and flue gas concentration of say 200 ppm NO, large volumes of
air will be required for appreciable conversion of the NO to NO . Assuming
instantaneous reaction, only about 3 percent of the NO is converted to NO
after one-half mile travel.
Heterogeneous reactions of NO also are possible in the plume. For
example, the sorption of NO on various metal oxides including Fe 0 has been
documented by a number of investigators (Otto, 1970). In reducing systems,
this sorption can be the first step in catalytic reduction of the NO to NO
and/or N_. In oxidizing atmospheres, NO is converted largely to nitrates on
these surfaces. Unfortunately,.there has been little done to determine the
significance of these reactions with respect to fly ash surfaces in plume
like atmospheres.
The kinetics of NO and NO reactions in the early plume are even less
well characterized than in the case of sulfate formation. The work cited
above suggests the possibility of formation of nitrates in the early plume
in both daylight and nighttime conditions, but current information does not
allow estimation of the extent of nitrate formation. Based on thermodynamic
72
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considerations and the low nitrate values of ambient particulate, primary
nitrate aerosols and/or early plume nitrates are probably negligible.
SUMMARY
In the high temperature regime, the reactions involving nitrogen and
sulfur are largely homogeneous gas phase reactions leading to the formation
of NO and SO . As the temperature falls, it would be expected based on
thermodynamic equilibrium, that further oxidation would occur. However,
various kinetic contraints allow this to occur to only a limited extent.
At lower temperatures, on the other hand, catalysis of SO oxidation
by certain metal and metal oxide species may be a significant mechanism in
the formation of primary sulfate emission. Combustion conditions and metal
species volatility will determine the resulting fly ash surface area avail-
ble for catalytic action. Capacity considerations, determined by active
surface area, may constrain the total amount of SO converted to sulfate.
Temperature is an important consideration, and well defined temperature
"windows" exist, outside whose range catalytic reactions of SO do not occur
rapidly. This is especially true of vanadium which is very active at high
temperatures but quite inactive in the near plume or ambient temperature
zone.
There is some qualitative evidence that soot and carbon particles may
also catalyze SO oxidation. The extent of oxidation possible and the
relationship between activity and surface area, temperature and other
variables have not been established. However, it seems prudent at this time
to assume that carbon may contribute to primary sulfate formation.
Boiler deposits also may form complex sulfates, which, when removed by
soot blowing, may overload the precipitator and contribute significantly to
primary sulfate emissions.
Based on the foregoing, it seems prudent to speculate that formation of
sulfate through fly ash/soot catalysis is at least as important a mechanism
for SO, formation as the high temperature homogeneous mechanism. Its magni-
tude is likely to be a strong function of combustion conditions, and fuel
composition.
The formation of sulfates and nitrates in the early plume is strongly
limited by mixing and dilution processes. In the case of sulfates, the time
73
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associated with cooling sufficiently to gain high relative humidities, and
the relatively long time required for neutralization of the existing sulfuric
acid combine to limit the further oxidation of SCL to <0.3-0.7 percent in
the early plume. In the case of nitrate formation, the reaction between
ozone in the ambient air and NO in the plume can be taken as instantaneous,
but the mixing rate is slow enough to limit NO conversion to only about 3
percent in the first half mile of plume travel.
However, it is important to keep speculation about mechanisms in
perspective with regard to the formation of primary acid aerosols. In
general, N0« and SO , precursors to acid, are found to be only several
percent of the NO and S09 in the combustion system effluent. Sulfate is
found, occasionally at high levels, in fly ash and when gaseous and parti-
culate sulfates are combined , the total may be of the order of 10 percent
(Homolya, et. al., 1976). Nitrate is almost never observed. Chlorine in
the fuels would be expected to produce some HCL in the effluent but there is
virtually no data on this point. However, HC1 has been observed (Piper,
1958) and in one case for a power plant burning pulverized coal, it was
determined that virtually all the fuel chlorine appeared as HCl in the flue
gas (lapalucci, et. al., 1969) but the fuel chlorine is generally low for
most US coals. This observation is in general agreement with the thermo-
dynamic considerations discussed previously. Thus, based on data presently
available to us, to the extent that there is a primary acid aerosol problem,
it is a problem of SO and sulfate. The situation may be summarized by
considering the results of Cato (Cato, 1974) and Sommer (Sommer, 1975) shown
in Figure 20. Where the ratio of SO to (SO + SO ) is shown for a variety
of fuels with a range of sulfur contents in a number of different boiler
systems. The apparent increase in conversion to SO at low sulfur con-
centration may be real or it may reflect the uncertainty of current analyses
at low concentrations.
-------
0)
en
o
"c
0)
_
o
a.
•n
O
to
o
o
x
O
en
"o
o
~o
a:
18.0
16.0
14.0
12.0
10.0
t
8.0
G.O
4.0
2.0
nn
/s
V
/» ^>
e
CD o
o O
X
O /rs
O
© O
X *
X
X
A
6CD
^
^ 0
v O
& X
A
i
e@
Fuel and Atomization Type
a
0
e
^^
fij^
X
O
X
^
Natural gas
Oil No. 2
Oil No. 5
Oil No. 6
Coal spread
Cocl pulveri
Coal underf
Coal cyclon
Coal
Oil
er
zer
ed
2
400
SCO 1200 1600 2000 2400
Total Sulfur Oxides Concentration, dry at 3%02, ppm
2800
3200
Figure 20. Ratio of Sulfur trioxides to total sulfur oxides as a
function of total sulfur oxides measured
-------
SECTION VI
PILOT AND FIELD STUDIES
In this section, and those that follow, the data from field tests
and pilot studies of large stationary sources with respect to acid aerosols
emitted and the effect of combustion modification (CM) on those aerosols are
summarized. Types of combustion modification, which change conditions in
the first regime, are described first, followed by a discussion of available
data on flue gases. This is followed by a discussion of available pilot and
field data on particulates and deposits in light of possible heterogeneous
reactions in the second regime. At the outset, it is important to recognize
that such data are sparse and, in general, it is difficult to say that a
given change in acid aerosol is the result of a specific combustion modifi-
cation. The following sections, therefore, contain considerable speculation
wherein we attempt to predict the effect, if any, of CM on acid aerosol
based on meager field data and extrapolation from basic studies.
For the purposes of this program, only a few general types of CM are
considered, all of which are currently considered for control of NO/NO . At
the present time, CM definitions are rather loose and not all authors agree
as to.what should be considered as CM. The basic approaches and their many
variations have been described in detail by others, e.g., Brown (Brown,
1974), Shimizu (Shimizu, 1975), Salooja (Salooja, 1972), and Lachapelle
(Lachapelle, 1974). This study is concerned primarily with staged combus-
tion, flue gas recirculation, low excess air and the effects of fuel com-
position including additives.
In staged combustion, the fuel is burned in what amounts to two steps.
In the first stage combustion proceeds with only 90 percent or less air
relative to stoichiometric. Too great a deficiency will result in smoke.
Heat then is removed from the products. After removal of sufficient heat,
the remaining air is added and the combustion process completed in the
76
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second stage with an overall excess of air. There are several terms that
are used to designate variations on this method; the terms are not mutually
exclusive and are not well defined. However, in general, two-stage com-
bustion (Barnhart, 1960) (Beer, 1973) (Dykema, 1975) (Atkins, 1975) and
overfire air jets (Crawford, 1975) are similar. The terms off-stoichiometric
firing (Bagwell, 1971) (Breen, 1973) (Jain, 1972), biased firing (Cato, 1974),
and BOOS firing (Dykema, 1975) (Burner Out of Service) are applied to the
more general cases where, in an array of burners, some will be fired fuel
rich and the remainder will be fired fuel lean. In practice, the latter
generally seems to mean with no fuel.
Reburning is similar in concept to staged combustion, and is actually
a type of staged combustion. The fuel-air mixture is initially burned with
large excess air to keep the thermal NO production down, some heat is
X
removed, and then additional fuel is added and burned to return the ultimate
mixture ratio to an acceptable value.
In flue-gas recirculation, part of the products of combustion, taken
from some point in the furnace system after a significant amount of heat has
been extracted, are added to the furnace air supply. Thermal NO is reduced
X
because of the reduction in peak temperature. For an as yet unexplained
reason, particulate production is also decreased. From a thermodynamic
point of view, the products at any temperature for a given overall fuel-air
ratio are the same for staged combustion and flue-gas recirculation. Flue
gas recirculation can be used both in retrofit and new designs but it is
more effective in NOV reduction for gases and light oils than for heavy oils
A.
and coal (Shimizu, 1975).
It is well known that the use of as low excess air as possible leads to
more effective use of the thermal energy in the gases. Furthermore, NOX and
SOo production are reduced because of the lowering of 0_ in the primary
flame zone. On the other hand, CO production will increase. Usually, the
minimum value of excess air is determined when an acceptable smoke number is
exceeded. Better balancing of multiple burner fuel and air supply, and
better mixing in each burner aids in reducing the excess air, which is
currently routine practice in utility boilers.
77
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S03 (H2S04) IN FLUE GAS
Staged Combustion
There is little experimental information from practical-sized equip-
ment on the effect of staged combustion on SO . The pilot scale work of
Archer, et al., (Archer, 1970) investigated two-stage combustion of a
high vanadium residual oil with 2.4% sulfur. They presented the data shown
in Figure 21 for the SO concentration leaving the first combustion chamber
portion of the furnace. These results demonstrate that SO can be reduced
essentially to zero when the first stage is slightly fuel rich. They
explain their results by noting from previous work that carbonaceous
particles inhibit SO formation, react with SO , and physically adsorb it.
Such changes do not mean SO is completely eliminated from the boiler,
however. When air is added at the second stage to complete combustion SO.,
may well be formed, as demonstrated by Hedley (Hedley, 1967), in excess of
that which would have been formed in single-step combustion with the same
total air. Also, heterogeneous reactions in the boiler section of the
system may produce as much SO in spite of staged combustion. This position
was summed up by Schwieger (Schwieger, 1974) "...catalytic oxidation of SO^
to SO, in the superheater and reheater section generally is considered to
contribute most of the SO . Thus, there might be an unacceptably high level
of SO at the air-heater inlet despite an acceptable SO level at the fur-
nace outlet." Pilot scale and more basic studies tend to confirm this
expectation, for instance, the previously discussed work of Glebov, et al.,
(Glebov, 1973), (page 62). However, there is not data from practical
systems which substantiate these heterogeneous effects when staged combus-
tion is used.
Flue Gas Recirculation
The situation is quite similar when flue gas recirculation is used.
It is well known that thermal NO and N02 are reduced by this CM (Tomany,
1971)(Turner, 1972)(Rawdon, 1973)(Muzio, 1974)(Armento, 1974) but there is
little direct evidence on SO and SO . In one investigation Koizumi, et al.,
(Koizumi, (1969), in studying the combustion of a 2-1/2 percent sulfur heavy
fuel oil in a compact combustor (about 10^ W/m^), noted that the variable
flame length, for the excess air conditions used, decreased as recirculation
78
-------
CO
cd
O
M
cd
•s
w
CO
0
ex
a
60 r-
20
0.8
1.0
1.2
1.4
(Air/Fuel)/(Air/Fuel) Stoich.
Figure 21. Concentration of SO in exhaust gas of the first stage
combustion section for air atomization of residual oil
with 2.4% sulfur
79
-------
increased up to 20 percent, then increased slightly up to 40% recirculation,
before starting a final decrease. Furthermore, the acid dewpoint (measured
just beyond the combustor) showed a parallel trend. In fact, the acid dew-
point correlated quite well with the flame length. The authors ascribe this
effect to improved mixing. Whether this acid decrease would be maintained
in view of possible heterogeneous reactions in other parts of the system is
questionable. However, at this time there is essentially no data regarding
SO- from practical systems employing this CM.
Low Excess Air
It is well known that low excess air is effective for reducing NO and
N02 (Barrett, 1973) (Brown, 1973)(Brown, 1974) and limiting acid in boilers.
Basic studies indicates that as excess air approaches zero the ratio SO /SO
also approaches zero. Csaba (Csaba, 1974) and Macfarland(Macfarland, 1962)
compute theoretically the values for various mixture ratios, for specific
fuel compositions and a range of product temperatures. They demonstrate the
expected increase in ratio of SO to SO as the excess air increases. It
should be noted that the effect of mixture ratio on the SO /SO ratio
persists throughout the furnace in their calculations. These results are
consistent with "normal" conversion of SO to SO at this point which Gills
reported as 0.2 percent to 2.5 percent. (Gills, 1973)
Koizumi, et al. (Koizumi, 1969), showed for their compact heavy fuel
oil combustor that the acid dewpoint, for 2.5 percent sulfur fuel, rose
very rapidly from 50 C at 1/2 percent 0 (the same value as with no sulfur)
to about 110 C (230 F) at 1 percent 0 , and 125 C (257 F) at 4 percent 0 .
Experience with oil-fired systems, where low excess air operation is
most practical at the present time, has demonstrated that this mode of
operation minimizes the formation of sulfates in deposits in the high
temperature portion of the boiler, reduces the amount of sulfuric acid
formed, and eliminates the emission of acid smuts. Successful operation
with low excess air requires that the oxygen in the flue gas be maintained
at levels below 0.2 percent. Such operation requires precise control of
the fuel-air ratio in all parts of the combustion system to prevent thermal
cracking of hydrocarbons and the emission of smoke. Consequently, low
excess air operation has been limited to oil-fired systems, because the
technology for burning pulverized coal with such little oxygen does not exist.
80
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Normal operation with 12 to 20 percent excess air results in the formation
of 25 to 30 ppm SO in the flame with fuels containing 2 to 3 percent sulfur.
The excess air must be less than 2 percent to decrease the SO by about
half. Further lowering of the excess air results in a rapid drop of the
SO level, and at about 0.1 percent oxygen in the flue gas the SO con-
centration will be reduced essentially to zero.
The first experience with reducing the excess air in boilers occurred
at several power stations in England. Crossley reported that at the
Marchwood station where the SO content had been about 20 ppm and the
sulfuric acid dewpoint was near 150 C (300 F), a reduction in excess oxygen
from 3 percent to about 0.5 percent resulted in a reduction in SO to about
3 ppm and a drop in the dewpoint to 121 C (250 F). Similar results were
obtained in three other stations as shown in Table 7(Crossley, 1959).
By redesigning the oil burners and exercising very close control on the
fuel-air ratio, Glaubitz in Germany was able to lower excess oxygen to 0.2
percent for routine operations. Under these conditions, the surfuric acid
was reduced to such an extent that the dewpoint approached that of water, as
shown in Figure 22. Glaubitz stated that after 12,000 hours of operation,
the boiler still did not have to be shut down for cleaning, indicating that
the strongly bonded deposits which build up as a result of the formation of
large amounts of sulfates had not developed in this boiler (Glaubitz, 1963).
In later work Glaubitz measured the SO concentration as a function of the
oxygen in the flue gas in the oil-fired boiler. As shown in Figure 23, the
SO concentration drops rapidly as the oxygen in the flue gas becomes less
than 1 percent (Glaubitz, 1963).
Low Air Preheat
Lower air preheat is another change of input conditions which lowers
the formation of NO and NO . There is considerable information regarding
the lower preheat effect on the SO./SO ratio. However, as Glebov(Glebov,
1973) points out, "data on the influence of flame temperature on process of
formation of SO is very inconsistent. It has been firmly established that
in pulverized-fuel-fired boilers, the content of SO in the gases decreases
—with increasing temperature in the furnace. However, Crumley, et al.,
(Crumley, 1956) on the basis of experimental data they obtained..." using
81
-------
TABLE 7. EFFECT OF EXCESS OXYGEN ON SO
CONTENT AND DEWPOINT
Excess O;>, 803, Dewpoint,
% ppm F
Marchwood 3 20 270-320
Marchwood 0.5 2-7 240-255
Poole <0.6 5 180
Poole 4 45 320
S. Denes 1.7 -- 260
Ince 4.5 18
Ince 1.0 7
82
-------
100
0123
0»ygcn in Flue Gas , percent
Figure 22. Decrease in dewpoint with low excess
air in an oil-fired boiler furnace
80
60
'E
I"
K>
O
I
I
0.5 10 1-5
Oxygen in Flue Cos , percent
2 0
Figure 23. Effect of excess air on SO level in
oil-fired boiler furnace
83
-------
kerosene and distillate, show an increase in SO to a flame temperature of
1750 C (3182 F) followed by a leveling off. The difference in the results
from the two fuels is considerably less than the difference in 2 percent
sulfur in the kerosene and 3 percent in the distillate. At 70 percent
excess air with kerosene, about 7 percent of the sulfur was in the form
of SO ; at 28 percent excess air, about 5 percent.
Glebov (Glebov, 1973), in agreement with Macfarland (Macfarland, 1962),
Csaba (Csaba, 1974), and Gudzyuk, et al., (Gudzyuk, 1972) shows with
thermodynamic calculations that the SO./SO^ ratio decreases with increasing
preheat temperature. But Glebov's data on the combustion of high sulfur
fuel oil show a constant value of SO from 2100 to 2500 C (3800-4530 F) for
two values of excess air. Gudzyuk, et al., (Gudzyuk, 1972) indicate possible
effects on SO of high excess air regions near cool walls which might explain
some of the contradictory results. As discussed in the section on fly-ash
chemistry, SO can be removed from the flue gases by reaction with metal
oxides to form solid sulfates, thus reducing the SO concentration in some
regions. Or heterogeneous reactions might increase SO under some con-
ditions. Therefore, it is virtually impossible without additional data to
predict what the effect of lower air preheat on SO might be in a given
system.
Load Reduction
Based on very meager data, it appears that load reduction has no
significant effect on SO emissions. Glebov (Glebov, 1973) found no effect
of load on S0_ over a range of 20 percent to 80 percent design load in his
study of high sulfur, heavy oil in an experimental furnace. In his theo-
retical computations he found no change in going from 100 percent to 70
percent load, assuming a catalytic activity of deposits equivalent to that
produced by Fe?0 , but some increase in SO. with decreasing load, assuming
catalysis by V 0 .
ADDITIVES
Although additives are not generally included in the class title
"combustion modification", it was felt that in the special context of this
study, referring as it does to acid aerosol production, some comment should
be made. Schwieger (Schwieger, 1974) sums up his review of additives by
84
-------
noting that to control SCL the addition should minimize catalytic oxidation
of SCL to SO , react with or absorb SO , or neutralize H SO,. The tradi-
tional additives for this purpose are MgO and MgO/Al 0 . Krause, et al.,
(Krause, 1976) state that "Significant control of SO or total sulfur
emissions by additives does not appear to be possible, although emissions of
SO can be controlled."
An early investigation of additives for SO control was conducted by
Rendle and Wilsdon who used an experimental furnace which burned 6 to 10
pounds per hour of residual fuel oil (Rendle, 1956). These investi-
gators tried a variety of materials which were fed into the oil-fired
furnace at a combustion chamber temperature of 1000 C (1830 F) and 25 per-
cent excess air. The results of their experiments can be summarized as
follows:
(1) Powdered SiO gave a maximum of 50 percent reduction in
SO concentration, presumably by physical absorption of
the sulfuric acid formed.
(2) Carbon blacks did not change SO concentration signifi-
cantly.
(3) Tars containing pyridine and other organic nitrogen
compounds did not change SO levels appreciably.
(4) Oil soluble magnesium and zinc naphthenates completely
eliminated SO from the gas stream. The magnesium
compound was slightly more effective than the zinc
derivative because less material was required for the
same effect.
(5) MgO, zinc dust, and dolomite also were capable of
removing all of the SO but relatively large amounts
of these materials were required.
In each case the amount of additive required was equal to or greater than
the stoichiometric amount required to react with the SO in the gas stream.
For compounds of the same metal, the oil-soluble materials proved to be more
efficient.
Similar success with magnesium and zinc compounds has been reported by
other investigators in the laboratory and by actual practice in full-scale
boilers. Flint and coworkers, using a small refractory furnace fired with
residual oil, also showed that zinc naphthenates could be used to eliminate
85
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SO from the gas stream (Flint, 1953). In their furnace system, 0.07 weight
percent of the zinc naphthenate reduced the SO by 75 percent, while twice
this amount of the zinc compound completely eliminated the SO . In another
laboratory-scale study with residual oil, Lewis observed that SO. could be
completely eliminated by the addition of 0.07 weight percent magnesium in
the form of magnesium naphthenate (Lewis, 1955). He also tried an oil
soluble zinc compound, but found it to be less effective than the magnesium
naphthenate.
In full-scale boiler applications of additives, the Florida Power
Corporation found dolomite to be partially effective. Huge and Plotter have
reported that at the Inglis Station the addition of 0.1 weight percent of
dolomite as a slurry in the fuel oil resulted in about 33 percent reduction
in the SO concentration in the flue gas. At the Higgins station, the dolo-
mite was introduced in the combustion air as a dry powder. In this case,
0.1 weight percent of dolomite reduced the SO by 42 percent. However,
doubling the amount of dolomite resulted in only a 50 percent reduction in
S03 (Huge, 1955).
Experience in England at the Marchwood station was reported by
Wilkinson and Clarke (Wilkinson, 1959). In this case residual oils con-
taining up to 4.5 percent sulfur constituted the fuel. The addition of
dolomite, which was introduced in the vicinity of the burners at a rate of
3 to 4 pounds per ton of fuel, brought about an 80 percent reduction in the
SO levels. Magnesium carbonate also was tried, and the 75 percent reduc-
tion in SO was achieved with the addition of 1 to 2 pounds of additives per
ton of fuel burned. An effort was made to use magnesium oxide similarly as
an additive but the fouling and clogging problems which resulted made its
use ineffectual in this case.
A combination of the magnesium additive with low excess air operation
was reported by Reese, et al (Reese, 1965), who added magnesium metal to
the furnace at various locations in a large oil-fired unit which had b'een
experiencing problems with boiler tube corrosion as well as with emissions
of acid smuts from the stack. The addition of granular magnesium metal
(20 to 59 mesh) to this oil-fired furnace reduced SO in the gases by 60
to 75 percent in different parts of the boiler system. In addition, the
emission of acid smuts was stopped. Although low excess air contributed
86
-------
part of these beneficial results, important benefits were attributed to the
MgO which resulted from the combustion of the metallic magnesium. It was
suggested that the MgO coated the catalytic tube surfaces and eliminated
this mode of formation of SO . Evidence that this mechanism was indeed
operative was obtained from the fact that 12 pounds of magnesium will
neutralize 40 pounds of SO , whereas the total SO in the flue gases in
this furnace were reduced by 57 pounds per hour when 12 pounds per hour
magnesium was added.
The combination of a magnesium oxide additive with low excess air
operation also has been reported favorably by Exley of the Long Island
Lighting company (Exley, 1966). Four power stations of this company used
the additive treatment to reduce acid stack emissions. In addition,
magnesium vanadate was recovered from the deposits in the furnace in
sufficient quantity to make it worthwhile selling the residue for its
vanadium value.
In working with a research boiler fueled with residual oil, Lee and
his associates developed an additive which consisted of a mixture of
hydrated MgO and Al«0. having a magnesium-to-aluminum ratio of 9 to 1
(Lee, 1969). It was demonstrated that SO can be completely eliminated
from the flue gases by the use of this additive in a concentration of 0.1
volume percent in the oil.
There have also been some attempts to use magnesium compounds as fuel
or combustion zone additives in coal-fired systems. Michel and Wilcoxson
(Michel, 1955) observed that dolomite used at a rate of 7.5 pounds per ton
of coal reduced the SO concentration by 50 percent. Doubling the dolomite
addition reduced the amount of SO by 90 percent.
Corbett and Fling (Corbett, 1953) added zinc ore concentrate to coal
used in stoker-fired boilers at the Brimsdown Power Station. The zinc oxide
smoke generated in this fashion eliminated the SO from the flue gases when
the additive was used at a concentration of 0.25 weight percent.
The net result of the use of an additive is to form a metal sulfate
such as MgSO, or ZnSO. instead of sulfuric acid. The formation of these
0 4 4
solid particulate compounds will be beneficial to the extent that they are
removed by the electrostatic precipitator. Even if they are not removed
from the flue gas stream completely and some particulate sulfates are
87
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emitted to the atmosphere, it may still be of benefit to have them in the
form of the matal sulfate rather than as a sulfuric acid aerosol. Similar
considerations can be applied to the situation with the vanadium in fuel
oil because the magnesium vanadate formed by the additive is a high melting
compound which will not be held in the tube deposits and passes through the
boiler system. As in the case with the sulfates, that portion which is not
removed in the electrostatic precipitators will appear in the particulate
emitted from the stack. In this case it becomes a question of whether it
is better to have vanadium oxides or magnesium vanadate as the particulate
material which is distributed through the atmosphere.
SULFATE IN FLY ASH
The composition and characteristics of particulate matter generated
in the combustion process depend on a wide variety of variables including
fuel composition, firing method, flame temperature, and amount of oxidant
or excess air. Coal and No. 5 and No. 6 fuel oil are the two general types
of fuels of most interest in generation of particulate emissions because of
their significant ash content and also potential for containing significant
amounts of sulfur. This results in their being possible generators of
acidic particles or particles rich in sulfates. They also contain signifi-
cant bound nitrogen, but as explained earlier, nitrates are too unstable and
it is not surprising that we have uncovered no nitrate particulate informa-
tion in this study. This part of the report addresses the composition and
characteristics of sulfate particulates generated during the combustion
process.
Sulfates in Coal Fly Ash
As discussed previously (page 58) most of the mineral matter in coal
is made up of kaolinite, calcite, and pyrites. The major elements found in
the fly ash are from these minerals and consist of mostly Si, Al, Na, Mg, K,
Fe, and Ca. Table 8 illustrates concentration of the major ash constitutents
for a variety of coals taken from the general literature. These analyses
are in general agreement with those of Table 6 (Catalytic Oxidation, page
61). Table 8 also illustrates some typical measured concentrations of fly
ash sulfur compounds, reported as SO , which are of major interest in
analyzing the impact of fly ash as a primary acid aerosol or sulfate. This
88
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TABLE 8. RANGES OF MAJOR FLY ASH CONSTITUENTS FOR A VARIETY OF COALS
Reference
Davis, 1938 Max
Kin
Average
Walker. 1974 Max
CO
vo
Bolton, 1975
Bickelhaupc. 1974 Max
SiO
49.
30.
40.
60.
17.
10.
Aft
ou.
40.
2 FeO Fe,03
0 4.3 19.0
3 1.3 3.9
9 2.8 0.2
0 18.1
3 3.3
0 23.5
9e i
Jt J
1 3.4
A1203
30.6
16.4
24.1
28.1
11.2
3.5
17.8
Ti02
1.5
0.8
1.1
1.6
0.2
0.25
2n
• u
1.1
CaO
10.6
1.2
4.3
28.2
8.0
0.5
6.8
KgO
1.9
0.5
1.0
8.1
1.5
0.9
0.9
Na20
2.2
0.4
0.9
6.1
0.2
0.3
0.1
KjO S
2.6 2
1.1 0
1.6 1
2.5 24
0 0
— 10
0.3 0
Ignition
0 Loss
.3 27.9
.4 1.5
.2 11.5
.2 —
.05 —
.5 —
~~
.3
Other Coanents
0.8 Results of 13 different
0.1 ash samples
0.4
— 13 different fly ashes
— with low sulfur western
coal
— 1 test on nedlun sulfur
coal (5.1 percent)
~*" 6 different fly ashes a
with low sulfur western
fly
all
I -I
LL
coal
-------
data is supplemented by Table 9 showing ranges of fly ash composition given
by Walker for 46 coals (Walker, 1974).
Generally, the sulfate level (reported as SO ) is on the order of less
than 1 percent up to about 2 percent. Two cases studied by Walker, however,
yielded sulfate contents of over 20 percent with the higher case being 24.2
percent (Walker, 1974). Bolton also recorded an unusually high sulfur con-
tent (reported in this case as sulfur instead of SO ) in one particular coal
fly ash sample (Bolton, 1975). It is difficult to say why these high fly
ash sulfur contents were recorded. They apparently have no relation to
coal sulfur content as the two high readings obtained by Walker were on coal
of less than 1 percent sulfur while coal used in the Bolton test has a sul-
fur level of over 5 percent. Also, of all the 13 coal/boiler combinations
analyzed by Walker only 3 had over 6 percent sulfate in the fly ash as S0_
despite the fact that in all cases coal sulfur content was less than 1
percent. Table 10 shows variations in the ranges of concentrations of Na,
Ca, Mg, K, and Fe in Walker's data for the 3 cases with high sulfate com-
pared to average high and low ranges for 7 cases with low sulfate. Na, Ca,
and Mg were consistently higher in the high sulfate cases whereas K and Fe
exhibited no clear trend.
Additional data on sulfate content in fly ash is shown in Table 11.
Three of these cases show sulfate contents for two different locations in
the boiler. Where SO is added ahead of the electrostatic precipitator
(Oglesby, 1975) to enhance ESP performance, a clear increase in sulfate
content is shown at the ESP outlet. Three out of four readings taken at the
air heater and stack (Burton, 1973) show a clear increase in sulfate con-
centration in the stack. A fifth reading taken with soot blowing illustrates
a clear increase in sulfate concentration at the air heater but a low read-
ing in the stack.
Photomicrographs of particles collected in the stack of the coal-
fired Thomas A. Allen Steam Plant of TVA revealed a "fuzzy" coating on the
surface (Bolton, 1974). This fuzzy coating was determined to be a sulfur
compound by scanning electron microscope fluorescence analysis combined
with ion etching by bombardment with argon ions. This would indicate that
sulfur in fly ash is primarily present as a coating on the surface of the
fly ash particles.
90
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TABLE 9. CHEMICAL ANALYSIS OF ASH FROM 46
PUBLIC UTILITIES AS GIVEN BY
WALKER, 1974
Con s 11 tu en t Percent:
Carbon as C 0.37 - 36.2
Iron as Fe00_ or Fe.O. 2.0 -26.8
2. 3 34
Magnesium as MgO 0.06 - 4.77
Calcium as CaO 0.12-14.73
Aluminum as A^O-, 9.81 - 53.4
Sulfur as SO, 0.12 - 24.33
Titanium as Ti()2 0.50 - 2.8
Carbonate as C03 0.05 - 2.6
Silicon as Si02 17.3 - 63.6
Phosphorus as P20_ 0.07 - 47.2
Undetermined 0.08 - .18.9
91
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This same sulfate coating effect was also observed for ten fly ash
samples. (Burton, 1973). The investigators found that the sulfur was
present as a sulfate and that the degree of surface coverage by the sulfate
coating varies from 5 to 40 monolayers. A less firm conclusion was that the
sulfates may be iron or calcium compounds because a close match was found
2
between the binding energies of the S P electrons to those for sulfates of
+2 +3 +2
polyvalent cation such as Fe , Fe , and Ca
In addition to the major ash constitutents discussed, fly ash also
contains a long list of minor or trace constituents. The accuracy of
determining the concentration of these elements varies considerably and
techniques of measurement are still being developed. Nevertheless, certain
of these trace elements may be of importance in determining the potential
for particulates formed in combustion to affect the emission of acidic
aerosols.
The six elements identified as having the most potential for catalytic
effects for converting SO to SO in the flue gas are V, Fe, Ni, Pt, Na, Cr,
and Cu. The approximate concentrations of these elements as reported in 3
different studies are given in Table 10. The recent results of both Sheibely
(Sheibley, 1975) and Abel and Rancitelli (Abel, 1975) are related to an NBS/
EPA Standard fly ash sample. The results of Bolton are based on results of
fly ash samples taken from the Thomas A. Allen Steam Plant in Memphis,
Tennessee (Bolton, 1975). Two numbers are given for the Bolton results
indicating measurement by neutron activation analysis (NAA) and by spark
source mass spectrometry (SSMS).
As reported earlier in Tables 7-9, iron and sodium are major con-
stituents in fly ash and this also shows in Table 12. Vanadium, chromium,
and copper also occur in measurable quantities on the order of several
hundred ppm. Platinum and nickel do not appear in these analyses as
significant components.
Particle Size in Coal Firing
The size range of particles resulting from coal combustion is an
important factor in estimating their pollutant potential. Particle size
is important in that it determines the particle surface area available for
contact with the flue gases, hence affecting adsorption rates with various
92
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TABLE 10. COMPARISON OF AVERAGE HIGH AND LOW RANGES OF OTHER
ASH CONSTITUENTS FOR HIGH AND LOW SULFATES SAMPLES
BY WALKER, 1974
Description
Average Concentration Range (percent)
Sulfate (SO-)
CaO
MgO
K?0
FeC
3 cases hig;
in sulfate
17.6-20.3
0.84-2.82 21.4-23.8 6.0-7.1 0.3 -0.49 7.5 -9.3
/ cases low
in sulfate
1.3- 2.6
0.52-0.90 15.2-19.9 2.9-4.4 0.93-1.75 6.76-11.4
U>
TABLE 11. SULFATE CONTENTS IN FLY ASH FROM COAL FIRING
Reference Test Location Sulfate Concentration in Fly Ash, percent Comment
Cuffee,
Cowherd
Oglesby
Burton,
1964
, 1975
, 1975
1973
ESP
ESP
ESP
ESP
ESP
inlet
inlet
outlet
inlet
outlet
Air heater
Stack
0.
7.
5.
0.
0.
5.
7.
2
0
4
26
28
8
6
0.
1.
9.
24.
46(a)
49
(b)
3 0.7 2.2 13. 7V '
1 15.8 1.6 3.8
Measured
Measured
Measured
Measured
as sulfur
as sulfate
as sulfate
as SO-
(a) With S03 injection ahead of ESP.
(b) With soot blowing.
-------
TABLE 12. TRACE ELEMENTS IN FLY ASH WITH POTENTIAL CATALYTIC
EFFECT ON OXIDIZING S02 TO SO
Constituent, ppm
Reference
Sheibley, 1975(a)
-P- (si
Abel, 1975 v'
Bolton, 1975 (c)
Coutant, 1975
Burton, 1973
V
230
220
200/350
91-396
120-440
Fe Ni
52780
65000
93000/iooooo
(b) 45-338
(b) 500-1000
Pt
—
—
(b)
(b)
(b)
Na
2658
3700
7000/3000
(b)
(b)
Cr
122
131
356/70
143-712
300-500
Cu
142
<300
^00
50-182
100-400
Mn
466
489
323/700
97-200
300-500
(a) NBS/EPA Standard fly ash.
(b) Values not received.
(c) NAA/SSMS.
-------
gas components, and it also affects the relative ease and efficiency
with which the particles can be collected. As described earlier, there
is evidence that sulfates occur as a particle coating (Bolton, 1974).
The results of particle size measurements made at the boiler exit on
fly ashes from 69 pulverized coal fired boilers (IGCI/ABMA) indicate that
for the most probable distribution the mass median diameter of the particles
is about 10 micrometers. This agrees well with data by Walker which indi-
cates that for 30 tests on pulverized coal fired boilers the mass median
diameter of the particles was from 5 to 15 micrometers (Walker, 1974).
Walker also presented results on 2 cyclone fired units which yielded
particle size ranges within the limits for pulverized firing.
Stoker fired boilers tend to produce fewer small particles than pul-
verized or cyclone fired units resulting in an overall larger size distri-
bution. This is due to the lower combustion intensity or heat release rate
per unit volume and also to the fact that the coal is burned in larger
lumps or pieces. Data on a particular traveling grate stoker fired unit
indicates that in 21 tests the mass median diameter ranged from 12.5 to 37
micrometers (Bradway, 1975).
Oil Combustion
Particulate emissions from oil fired units result in much the same
manner as those from coal combustion. Ash or non-combustible constituents
in the oil form particulates both by ashing of refractory components and by
condensation of more volatile constituents. Also, there is the possibility
of forming condensed carbon particles or soot depending on the combustion
conditions and degree of fuel atomization.
Just as in coal, ash in crude oil exists in basically two forms in-
herent and extraneous. Inherent ash is that bound up in the structure of
the oil itself and consists of complex organic compounds. Extraneous ash
constituents are compounds such as salt and other chlorides, fine particles
of sand, corrosion products from ships bunkers and pipelines and, in re-
sidual fuel oil, wastes from refinery processes.
Whereas coal ash typically can be at least 5 percent or more by weight
of fuel, ash in oil seldom exceeds 0.1 percent. Sulfur contents of oil
are also usually lower than those in coal but in the case of residual oil
95
-------
can range up to 5 percent by weight of fuel. Based on analysis of up to 150
samples of residual oil over the 3 year period from 1955 to 1957 (Orr, 1960)
sulfur ranged from a low of 0.29 to a high of 5.25 percent and ash ranged in
content from 0.004 to 1.9 percent (only one-tenth of the samples contained
more than 0.1 percent).
Typical ranges of compositions of oil ashes for oils from different
regions of the United States and from overseas are shown in Table 13. Also
shown is the range of concentration of ash constituent in residual fuel oil.
As can be seen, ash constituents vary widely even more so than those report-
ed for coal in Tables 7-9. Sulfates occur in oil ash in much higher con-
centrations than in coal ash (values of over 40 percent shown in Table 13).
It is noteworthy that oils can contain significant amounts of V and Ni,
elements that have been identified as having a potential catalytic effect on
oxidizing SO to SO thus having a potential effect on sulfate emissions.
This differs from coal ash where V and Ni are present in ash at only a few
hundred parts per million. Also the nature of vanadium compounds in crude
oil is such that they are stable up to 800 F. Hence they are not destroyed
by refinery operations and as a result they concentrate in the residual
(Reid, 1971).
A complete analysis of two different oil ashes after combustion
(Negherbon, 1966) is presented in Table 14. This analysis yielded sulfate
concentrations (as SO ) of about 17 and 25 percent. Small amounts of
nitrate (NO ) and chloride were also detected in one sample.
Carbon is also shown as a significant constituent in these two samples
accounting for over half of the ash content in one case. According to
Novakov carbon could be an important constituent in allowing fly ash
particles to adsorb SO,, from the flue gas forming acid constituents
(Novakov, 1974).
Carbon is usually reported as percent combustible in oil fly ash which
also would include small amounts of other combustible constituents such as
hydrogen. The amount of combustible constituents or carbon in the fly ash
depends on combustion conditions and firing method. McGarry reports percent
combustibles in oil ash as 99.3 percent for mechanical atomization, 80
percent for steam atomization, and 40 percent for air atomization (McGarry,
1972).
96
-------
TABLE 13. RANGES OF ASH COMPOSITION IN OIL
Constituent, percent
Reference
Ellis, 1937(a)
Thomas, 1938(b)
Huge, 1955(c)
Orr, 1960(d)
Max
Min
Max
Min
Max
Min
Max
Min
Si02
60. 0
5.0
38.3
0.8
7.4
19.0
86
6
Fe,0,
44.0
8.0
97. 5(
8.9
5.1
16.8
57
C.9
A12°3
39.0
8,0
e)
0.3
0.1
76
3
CaO
11.0
1.0
12.6
0.7
1.2
4.8
10
1.4
MgO
4.0
1.0
4.2
0.2
2.6
1.7
1.7
1.0
MnO V 0
0.5 5.0
0 0
0.4 5.1
0.2 0
15.0
0.1 1.6
740
14
NiO
4.0
0
4.4
0.5
3.2
0.9
25
1.3
Na20 K20
12.0 2.0
1.0 0
30.8 1.0
0.1
26.4(f)
19.8
35 1.2
5 0.2
so3 ci
21.0 0.2
1.0 0
42.1 4.6
0.9
40.3
34.8
—
__ __
Percent in oil ash
Percent in oil ash
Percent in oil ash
Parts per million
in oil
(a) Fourteen Hungarian and one California oil.
(b) Crudes froa) across the United States.
(c) Values froa Florida Power (Top) and Tampa Electric (Bottom) .
(d) P.esidual oil analysis.
(e) Combined
and
.
(f) Combined Na20 and K20 as NaJX
-------
TABLE 14. ELEMENTAL ANALYSES OF TOTAL PARTICULATES
(Data in percent, Negherbon, 1966)
Elements
Carbon
F.ther, soluble
Hydrogen
Ash (900° C)
Sulfates as SO,
Chlorides as Cl
Nitrogen as l\Q~
Iron as fcJ^-,
Chromium £3 CrO.
Nickel as NiO
Vanadium as V_0.j
Cobalt as Co^O,,
Silicon as Si02
Aluminum as AljO,
Barium as BaO
Magnesium as MgO
Lead as I'bO
Calcium as CaO
Sodium as N'a.O
Copper as CuO
Titanium as TiO.;
Test A
Total solids from burning
PSa 400 oil (collected in
n laboratory electrical
precipitator at 230° 1')
V,
58. 1D
2.3
-
17.4
17.5
-
-
3.1
.06
1.8
2.5
.08
.6
1.6
.4
.2
.1
.2
.9
.01
-
Test B
Tot.nl. solids from burning,
4° API oil (collected in a
f.lasF, filter sock at
300° I-)
K
18. lb
4.4
-
51.2
25.0
.5
.3
3.7
.3
13.2
4.7
.3
9.7
14.9
.1
.7
.2
.4
3.0
.25
.OO'i
a Pacific Standard.
b Value, probably includes minor amount of hydrogen.
98
-------
TABLE 14. (Continued)
Test A Test B
Total solids from burning Total solids from burning
TS 400 oil (collected in 4° API oil (collected in a
a laboratory electrical glass filter sock at
Elements pre.cipitn tor at 230° F) 300° F)
Molybdenum as MoO. .02
.03
Boron as B-O, .01 .1
Manganese as MnO,, .04 .04
Zinc as ?.nO - .06
Phospborus as I'^O,. .9 -
Strontium as SrO .04
Titanium as TiO .03
99
-------
Negherbon indicates that the amount of particulate emissions from oil
firing depend mostly on combustion efficiency and rate of deposits buildup
in the furnace. Combustion efficiency in this case refers to the amount
of combustible or unburnt carbon remaining in the ash after combustion. No
correlation was found between particulate emissions and the ash content of
the oil. Particulate loadings from oil combustion from a variety of sources
were found to range from 0.005 to 0.205 gr/scf with the bulk of the values
from 0.025 to 0.06 gr/scf (Negherbon, 1966).
Negherbon also found that through high pressure atomization particu-
late emissions could be reduced by 66 percent over low pressure atomiza-
tion. He also found that increasing the temperature of residual oil to
the burner from 116 C (240 F) to around 315 C (600 F) could reduce parti-
culate emissions by 15 to 17 percent. These reductions in emissions
presumably result from increased combustion efficiency or reduced carbon
level in the ash.
Particle Size in Oil Firing
Particle size is also an important parameter in evaluating particle
characteristics. As was the case with coal, there is evidence that the
more volatile constituents in oil ash tend to concentrate in smaller
particles. Tests by Engdahl et al (Engdahl, 1969) indicate that vanadium
tended to concentrate on particles greater than 4 micrometers.
The particle size distribution was found to vary considerably among
various literature sources (Negherbon, 1966) (McGarry, 1972) (Goldfarb,
1972). Mass mediam diameters from 0.3 to 10 micrometers were reported -
somewhat lower than coal ash particle sizes.
Type of atomization is important in determining the particle size range
Test of steam, air, and mechanical atomized burners (McGarry, 1972) operat-
ing on a utility boiler with heavy fuel oil showed that air atomized burners
produced the greatest number of fine particles (28 percent less than 3.3
micrometers) while mechanical atomized burners produced the least on a
percentage basis (9 percent less than 3.3 micrometers). Steam atomization
was about in the middle with 13.5 percent less than 3.3 micrometers. How-
ever, this may be misleading because, due to the higher overall amount of
particulate generated in the mechanical atomized unit over air atomized, the
100
-------
total amount of particulate under 3.3 micrometers generated by the mechani-
cal case was about 8 times that of the air atomized case per megawatt of
power produced.
Particle growth by natural agglomeration through an oil fired boiler
was assumed negligable by Engdahl where theoretical considerations found
that about 10 seconds residence time would be required for particles to
grow from an assumed initial size of 0.02 micrometers to a final size of
0.065 micrometers (Engdahl, 1969).
Combustion modification techniques for low NO emissions can also
X
affect particulate emissions in oil firing. Muzio, et al (1974) showed
NO reductions were limited by increased smoke; the limit being a function
of the fuel. Best results for a No. 6 fuel oil were obtained with
combined FGR and staging which gave up to 45 percent decrease in NO
with a two-unit increase in Bacharach number.
Summary
The characteristics of particles generated during coal combustion are
primarily a function of ash composition and combustion conditions. Sulfate
contents in coal fly ash are mostly in the range of 1 to 5 percent by weight
of ash although some data show values as high as 24 percent. Evidence indi-
cates that the sulfates are present as coatings on the surface of the
particle and may be present as mostly iron or calcium sulfate. No signifi-
cant contents of nitrate or chlorides in fly ash were reported in the
literature reviewed. Combustion conditions mostly affect particle size and
emission rate. Pulverized and cyclone firing create the smallest particles
(mass median diameters from 5 to 15 micrometers) while stoker firing pro-
duces larger particles (mass median diameters from 12 to 40 micrometers).
Particulates emitted from oil firing are mostly a function of com-
bustion conditions rather than ash composition. The most important
variable appears to be degree of atomization. This has a direct effect on
the size and carbon content of particles emitted. The size range of
particle emitted in oil combustion varies considerably and mass median
diameters from 0.3 to 10 micrometers were found. Carbon content of the fly
ash also varies considerably and recorded values of over 50 percent by
weight of ash were found. In addition to having higher carbon content than
101
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coal ash, oil ash also typically has higher vanadium, nickel, and magnesium
content. This may in part account for oil ash also having higher sulfate
levels (over 40 percent in some recorded cases) than coal ash. Small
amounts of chlorides and nitrates were found in some ash compositions.
SULFATES IN DEPOSITS
In this section we are concerned with deposits formed on various
surfaces of the boiler system as distinguished from suspended particulate
described in the previous section. Chemical reactions in the deposits
which are formed in fossil fuel combustion play an important role in the
formation of sulfates and other aerosols which are emitted from the stack.
The metal oxides and in some cases, metal chlorides, which are formed in the
flame and the immediate post-flame zone are converted to sulfates by the
action of the SCL, SO , and the oxygen in the flue gas stream. In addition,
some components of both the fly ash and the boiler deposits formed by it can
function as heterogeneous catalysts for the oxidation of SO. to SO . The
SO will then be formed from sulfuric acid by reaction with the moisture in
the flue gases or solid sulfates by reaction with the metal oxides. The
metal sulfates which accumulate in the deposits on the boiler tubes can also
be released into the gas stream by the periodic soot blowing which is requir-
ed to prevent fouling of the boiler passes. All of these factors contribute
to the total sulfate burden of the gas stream which leaves the stack.
There is very little data available in the literature on the rates at
which sulfates are formed in boilers but in the boiler deposits it is a
relatively slow process.
Most of the sulfur in fossil fuels is volatilized during the com-
bustion process, and very little of the sulfur remains in the ash unless the
combustion conditions are poor. The sulfur oxides thus formed then react
with the metal oxides and chlorides to form the metal sulfates. The re-
actions also involve water and oxygen and undoubtedly are very complex,
probably proceeding in a series of bimolecular steps to reach the final
sulfate product. The overall reaction may be described by the typical
equation:
2NaCl + H0 + SQ + 1/2 0 = NaS0 + 2HC1. (19)
102
-------
Halstead and Raask made equilibrium calculations for the sodium chloride-
sodium sulfate system, and the results for a coal containing 1.8 weight
percent sulfur and 0.07 weight percent chlorine are shown in Figure 24
(Halstead, 1969). In this figure, the variations of the equilibrium
partial pressures of NaCl and Na^SO in the flue gases are plotted against
temperature for three different oxygen levels. The broken lines in the
figure show the saturated vapor pressures of NaCl and Na?SO, and the
temperatures at which these lines intersect the equilibrium pressure lines
are those at which the particular compounds will condense from the flue
gases. The important conclusion from this plot is that when coal is burned
under normal operating conditions with 3 to 5 percent excess oxygen, Na_SO
and not NaCl will condense from the flue gas if the gas is at chemical
equilibrium within the superheaters where the flue gas temperature lies
between 927 C (1700 F) and 1125 C (2060 F). In order to test this conclus-
ion, probes were inserted into a 60 megawatt boiler fired with coal con-
taining 2.4 percent sulfur and 0.28 percent chlorine. Analysis of the
deposits showed that whereas the buildup of the sulfates on the probe
continued steadily, chloride was never present in the deposits in more than
trace quantities at all temperatures between 527 C (980 F) and 927 C (1700
F). The results are shown in Figure 25. The traces of chloride which were
observed in the deposits were attributed to the absorption of vapor phase
NaCl by the condensed sulfates. Similar results were obtained in a 200
megawatt boiler fired with coal having 1.1 percent sulfur and 0.97 percent
chlorine. However, when the same coal was burned in a 15 megawatt boiler,
it was found that NaCl did condense together with greater amounts of Na SO,
and K SO,. By way of explanation, it was suggested that in the smaller
boiler the mixing of the fuel and the combustion air was less efficient
and led to a fuel-rich condition at the point where the probe was inserted.
It was also suggested that the residence time of the flue gas, which was
only 1 to 2 seconds between the burners and the probe, may have been too
short for the complete conversion of NaCl to Na?SO,. Once NaCl does con-
dense in the deposit, the chloride may well persist for a relatively long
period of time.
103
-------
-4 _
f- -6 -
-7 -
•e -
1500
1303 1100 900
TEMPERATURE. °K
50f
Figure 24. Chloride/sulfate equilibrium coal 3, 1-8
weight percent S, 0.07 weight percent Cl^.
curves: (1) 5 percent 0- excess (2) Stoichio-
metric (3) 2 percent 0
Saturated vapor pressure NaCl
Saturated vapor pressure Na~SO,
I 5
25
"= 0
K,SO,
NiCI
800 1000 1700
PROBE METAL TEMPERATURE °K
Figure 25. Deposition of sulfates and chloride from flue
gas of 60-MW boiler fired with coal of 2.4
percent S and 0.28 percent Cl (3.0 percent
excess CL)
104
-------
The rate at which NaCl in the deposit is converted into Na_SO was
investigated by Bishop, using a laboratory combustor burning pulverized
coal (Bishop, 1968). In the early stages of deposition the chlorine con-
tent of the material collected on the probe was substantially greater than
the sulfur concentrations. As high as 30 weight percent chlorine were
observed while the sulfur expressed as sulfate comprised about 15 percent
of the deposit. As the time of exposure to the flue gases continued, the
chloride content decreased while the sulfur content increased. It appears
that after about 6 hours, a steady state condition was reached at which
the chloride concentration was 3 percent while the sulfate amounted to
50 percent of the deposit. X-ray diffraction confirmed the fact that the
compounds in the deposit were NaCl and Na.SO,. The coal used for these
studies was relatively low in sulfur and high in chlorine, there being
0.9 percent of each of these elements in the coal. Consequently with
typical American coals in which the chlorine content is relatively low
compared to the sulfur, the small amount of chloride formed would be very
quickly consumed and converted to sulfate.
The relative effectiveness of SO and SO in transforming NaCl to
Na7SO, was investigated by Fletcher and Gibson using radioactive sulfur as
a tracer to follow the reactions (Fletcher, 1954). This work demonstrated
that on the boiler tube surfaces, which are always at temperatures of 600 C
(1112 F) or less, the SO in the flue gases is the primary agent in sulfate
formation. However, in the gas-phase reactions occurring in the postflame
zone, the sulfate formation would result primarily from the reaction of SO
and oxygen with the fly ash particle. The effect of iron oxide on the con-
version process was shown to be significant. The rate of sulfate formation
from SO was not affected by the presence of the iron oxide catalyst, the
rate of sulfate formation from SO , on the other hand, was greatly increased
(see Figure 14, page 55). Thus, at 500 C (932 F) without Fe20 in the
mixture about one fifth of the Na_SO, was formed from the SO . However, in
the presence of the Fe70 , 80 percent of the much larger total quantity of
sulfate was derived from the SOj. Even at temperatures as low as 300 C
(527 F) 65 percent of the sulfate came from the SO^. In this case SO was
the predominant factor over the whole temperature range.
105
-------
Because the inorganic content of fuel oil is low, the fly ash formed
in oil burning systems is substantially less than that from coal burning
systems. At the same time carbonaceous material makes up a greater portion
of the fly ash particle resulting from the combustion of oil. Measurements
by DuBois and Monkeman on the combustion products of high sulfur residual
fuel oil showed that the soot particles which contained a large amount of
sulfate were also strongly acidic, indicating the presence of adsorbed
sulfuric acid (Dubois, 1973).
Since two-stage combustion requires that the first stage be a fuel-
rich zone, combustion is not complete; thus, the oxidation of the sulfur
and the carbon will not be complete and the fly ash and deposit chemistry
is different from that which occurs in an oxidizing atmosphere.
In an oxygen-deficient atmosphere all of the sulfur from the decom-
position of pyrites, FeS_, will not be oxidized to sulfur dioxide. The
first step in the combustion of FeS^ is a dissociation reaction in which
an atom of sulfur is released. An experimental study of the reaction under
conditions that would exist in a furnace showed that this sulfur was re-
leased in 0.5 seconds at a temperature of 1093 C (2000 F) (Halstead, 1969).
The pressure of the sulfur vapor thus formed reaches one atmosphere at 779 C
(1435 F). The experiments demonstrated that with the short residence time
in the flame, both sulfur and FeS could be deposited on furnace wall tubes.
The presence of FeS in deposits found on badly corroded boiler tubes
was reported by W. T. Reid and his colleagues in the early 1940's (Reid,
1945). In seven of the furnaces that they examined, sulfide deposits were
found in some areas. Up to 5 percent carbon also was noted in some of
these deposits, indicating that the normal oxidation process had not occurr-
ed. The cause for these unusual conditions was found to be poor distribu-
tion of the coal stream leaving the burners, coupled with coarse pulveriza-
tion which resulted in deposition of incompletely burned coal on the
furnace walls. The problems were corrected by adjustments in the coal
distributor pipes and the grinding mill, thus insuring that the carbon and
the sulfur would be completely oxidized before reaching the wall tubes.
Although this situation was an extreme case, it demonstrates the possibility
that with reduced oxygen in the first stage of combustion there will not be
sulfate formation on the heat receiving surfaces in this portion of the
' i • ' '
106
-------
boiler. As the fuel mixture is carried into the oxdizing zone, the remainder
of the sulfur will be oxidized to SO and SO and sulfate deposits can be
formed in the usual manner. However, the deposits in the reducing zone will
contain sulfur in the form of FeS. When this FeS is dislodged during the
soot blowing process, it is likely that the mass of material being carried
through the oxidizing zone will prevent the complete oxidation of the sulfur
and particles of FeS will be carried through the boiler with some escaping
the precipitator and being emitted from the stack.
107
-------
SECTION VII
INFLUENCE OF COMBUSTION MODIFICATION
ON THE ACID AEROSOL FORMATION POTENTIAL
OF FLUE GAS DESULFURIZATION PROCESSES
By their nature, flue gas desulfurization (FGD) systems inevitably emit
limited quantities of acid aerosols even without considering combustion
modification techniques. Although our primary concern is the effect of CM
on acid aerosol emission, since FGD systems can contribute to acid aerosols
a brief discussion is included. These emissions occur from FGD systems
principally as (1) unremoved molecular and particulate matter, (2) chemicals
entrained in scrubbing liquors when wet desulfurization processes are used,
and as (3) entrained solid sorbents in the case of dry processes. When com-
bustion modification is used in conjunction with flue gas desulfurization
processes, the acid aerosol forming tendencies of the flue gas obviously can
be changed. The degree to which the acid aerosol forming tendencies of flue
gas are changed by combustion modification techniques depend greatly on the
type of flue gas desulfurization process being used. Since there is a
myriad of processes currently being developed, the approach used in this
study was to:
(1) Review existing flue gas desulfurization technology
(2) Select representative systems from the numerous avail-
able processes
(3) Speculate on the acid aerosol forming potential of the
flue gas desulfurization process operating normally,
and
(4) Speculate on changes in the acid aerosol forming
potential of FGD processes when combustion modifi-
cation techniques are employed.
108
-------
The path taken by the flue gas to the environment when a stationary
power source is equipped with a flue gas desulfurization process is from
the flame and convective zones where the combustion modification is
occurring, through the air preheater and then through an electrostatic
precipitator for ash removal. From the electrostatic precipitator the gas
then passes through a desulfurization unit (usually a wet scrubber) where
the flue gas is adiabatically saturated to the water dew point. In the
desulfurization unit, chemicals are added to promote SO and SO removal.
From the desulfurization unit, the gases usually pass to a mist eliminator
to remove entrained mist containing the desulfurization chemicals. From
there, the gas usually proceeds to a stack gas reheater where heat is
added to the gas to give the plume buoyancy and protect the stack against
acid corrosion. Following reheat, the gases are emitted to the ambient
atmosphere. In the case of oil fired boilers, quite often an electrostatic
precipitator is not used. If the desulfurization process utilizes dry
sorpiton of S0~ as the main removal technique, there will be no reheat or
mist eliminator sections in the flow train. Likewise, in new plants burn-
ing coal, sometimes the ESP will be eliminated from the gas flow train if
enough confidence is placed in the wet scrubber to remove fly ash from the
gas as well as SO-/SO. in the desulfurization process. Also in industrial
boilers, the air preheater and/or the particle collection device may not be
found. This section of the report is concerned with acid aerosol formation
in the region beginning with the ESP and ending at the reheater.
Typical values of SO (SO and SO ), NO (NO and NO ), and particulate
X i. .3 X ^
loadings in flue gases going to FGD systems are given in Table 15. The
pollutant load will, of course, depend upon the fuel used, i.e., coal, oil,
or gas. The SO content of the gas will depend upon the sulfur content of
X
the fuel and can vary between wide limits, about 200 to 3000 ppm, depending
upon whether low or high sulfur fuel is burned and whether it is oil or
coal. The NO contents of the flue gas is determined by the method of com-
X
bustion, the fuel used, and whether combustion modification techniques are
used to control the amount of NO and NO produced. Particle loading can
range from 3 to 6 grains/scf for coal and about 0.04 to 0.02 grains/scf for
oil. In addition, numerous other minor constituents are generated in the
combustion process.
109
-------
TABLE 15. TYPICAL SO , NO , AND PARTICLE LOADINGS
IN FLUE GA&ES T§ FGD SYSTEMS
Boiler
Fuel
Coal
Oil
Gas
so2/so3,
ppm
600-3000 (a)
200-2000 (a)
Nil
N0/N02,
ppm
225-800
100-600
50-375
Particulate,
grains/scf
3-6(b>
0.04-0.2(c)
Nil
(a) Depends on the sulfur content of the fuel.
(b) Mainly fly ash with some unburned carbon.
(c) Mainly unburned carbon.
110
-------
The scrubbing of SO with reactant liquors requires the intimate
contact of SO with the absorbing media. To achieve this gas/liquid contact,
numerous types of scrubbers have been developed. For example, in lime/
limestone FGD systems, (1) mobile bed, (2) marble bed, (3) spray tower, and
(4) venturi scrubbers are used. In most cases, the scrubbing liquor is
atomized or sprayed from nozzles countercurrent (mobile bed, marble bed,
and spray tower scrubbers) or cocurrent (venturi scrubbers) to the gas
stream. Mists of solid and liquid particulates are generated in both the
atomization and the turbulent mixing processes of scrubbing. Most mist
droplets return to the scrubbing solution because of either particle
coalescence (small particle combination to form larger particles) and the
force of gravity, or particle collisions with scrubber internals, adhesion,
and drainage back into the liquor reservoir. Those not collected are en-
trained within the scrubber and are directed to the mist eliminator. Several
factors, including gas velocity, pressure drop, nozzle design, liquid to gas
(L/G) ratio, mixing turbulence, scrubber design, and scrubbing liquor den- .
sity and viscosity, affect both the number and the size of entrained
droplets.
Mist eliminators (ME) were first used in the electric power industry
for removing the fine droplets entrained in (or carried with) the humidified
air exiting cooling towers. With scrubbers for example, in lime or limestone
scrubbing applications, a mist eliminator is a device employed to collect,
remove, and return to the scrubbing liquor the liquid and solid particulates
which are entrained with desulfurized flue gas exiting the FGD scrubber or
absorber. Most ME's employed in flue gas desulfurization applications are
based on the principle of inertial impaction. The particles entrained in
the gas stream pass through the ME where they are forced to make sudden
changes in flow direction. The particles collide and adhere to the ME
surfaces where they are collected while the remaining gas stream passes on
through the ME. The collected particles combine with other particles until
they form a mass of sufficient weight to drain off by gravity. In high
solids applications, especially limestone scrubbing, the collected solids
tend to form a mud-like deposit over a period of time which must be washed
from the collector to keep it free-flowing and operational.
Ill
-------
The collection of fine particulates entrained with a gas stream can be
accomplished by one or more of the following mechanisms.
(1) Settling,
(2) Inertial impaction,
(3) Diffusion, and
(4) Electrostatic deposition.
Two mist characteristics, the loading and the particle-size distribu-
tion, are important in determining the removal efficiency of a ME. At
present neither is measured by FGD system operators or vendors. Without
these measurements, collection efficiencies cannot be calculated. In
addition, the effect of gas velocity, L/G ratio, liquor composition, and
other operational variables cannot be adequately evaluated in terms of ME
operation. Design features such as scrubber design, degree of turbulence,
use of bulk separation devices, and the ME design itself also lack proper
evaluation.
A major problem in operating wet flue gas desulfurization systems is
the need for stack gas reheat. Reheat is required to avoid downstream
condensation and corrosion, to avoid a visible plume, and to enhance plume
rise and dispersion of pollutants. With regard to the general effect stack
gas reheat has on acid aerosol formation, reheating will cause acid aerosols
formed in the FGC system to be dried out, i.e., due to the change in
temperature the water dew point will change. This will cause evaporation of
water from the mist droplets and hence cause a change in the particle size
distrubution of acid aerosols formed in the FGD system. Also due to changes
in temperature, chemical changes could take place within the mist particles.
For example, increased temperature could lead to the decomposition of stable
species present at the normal operating temperature of the wet FGD system
(125 to 130 F). This would depend of course on the FGD system in question.
Combustion modification techniques per se would not be expected to cause
significant changes in the physical-chemical processes occurring normally
when a boiler is equipped with an FGD system and the stack gas is being
reheated.
112
-------
POTENTIAL ACID AEROSOLS AND FLUE
GAS DESULFURIZATION SYSTEMS
Several FGD processes representative of available technology were
selected for analysis and included both wet processes and dry processes.
Wet processes all use a mist eliminator to remove entrained chemicals
added to the process to promote SO,, removal. Thus unremoved mist would
seem to be a potential source of acid aerosol emissions. Similarly, dry
processes would in general have entrained absorbent fines eluted from the
flue gas/solid contacting device used for desulfurization. In general,
dry processes do not have a particle removal device analogous to a mist
eliminator following the reactor to remove entrained solids. Usually, the
contacting device is designed to minimize this problem. Entrained solids
from dry processes would consist of sulfates and perhaps nitrates of the
sorbent.
Wet processes could involve either slurry scrubbing (for example,
the lime/limestone and MgO processes) or clear liquor scrubbing. In the
latter case, the scrubbing liquor could have a wide range of pH values—
from dilute alkali (either sodium or ammonium ion), to slightly acidic
buffered solutions (BOM citrate and Stauffers' phosphates processes), to
low pH solution such as the Chiyoda process.
In dry processes, the temperature sequence experienced by the flue gas
would be considerably different compared to wet processes because of the
adiabatic quenching operation used in the latter type of systems. In both
cases, the residence time the gas spends proceeding through the FGD system
would be about the same. This should be true even if the boiler were
operated using combustion modificaiton techniques.
Boilers equipped with FGD systems have the potential to emit to the
environment materials considered as acid aerosols. In a very general sense,
potential acid aerosol sources may be classified according to the following
"loose" scheme:
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Species Source
1. Sulfur Species
a. Metal sulfates/sulfites Entrainment in mist and FGD sorbents
b. Metal bisulfates/bisulfates Entrainment in mist
c. H.SO, U)/H_SO («,) Entrainment in scrubber mist and
unabsorbed SO^/SO. partition to
mist
d. S02 Residual SO- leaving FGD systems
e. SO Residual SO leaving FGD systems
f. (NH,) SO, Generated in NH scrubbing
g. CuSO. Generated in CuO desulfurization
h. Other molecular species such
(f) and (g) which are intrin-
sic to specific FGD systems
2. Particulate Species
a. Particulate ash Ash with sorbed acidic molecular species
b. Carbon particles Carbon with sorbed acidic molecular
species
3. Nitrogen Compounds
a. Metal nitrates/nitrites Entrained in mist and on FGD solid
sorbents
b. NO Residual leaving FGD systems
c. N0? Residual leaving FGD systems
4. Chlorides
a. HC1 Residual leaving FGD systems and
partitioned to mist
b. Metal chlorides Entrained in mist
c. NH.C1 Generated in NH~ scrubbing
5. Fluorides (same as chlordies described above)
As seen in the above classification the main sources of acid aerosols are:
(1) Unremoved molecular species—SO , S0», NO, NO , HC1, HF, etc.
(2) Particulate species formed in the combustion process which
can act as sorbents for acidic molecular species
(3) Entrained mist from wet FGD systems
(4) Entrained solids from dry FGD systems
114
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(5) H2S04 USed as a Scrubbin8 a§ent> for example in the
Cniyoda process
(6) H2S03 and H2S°4 formed by partition of unreacted SO
and SO to mist particles.
With few exceptions, FGD systems tend to mitigate and diminish the
acid aerosol problem relative to the case of no control. This occurs
because there is a finite efficiency for removal of the molecular species
and particulate matter contained in the flue gas by the desulfurization
processes. For example, SO removal can vary between 70 and 90 percent
depending upon the FGD system in question and the sulfur content of the
fuel. Likewise some degree of NO removal occurs in FGD systems although
X
this is admittedly small, perhaps 10 to 40 percent depending on the FGD
systems. For each of the other pollutants, similar numbers for removal
efficiencies can be speculated.
Ignoring the inefficiencies in FGD systems which lead to unremoved
molecular species and particulate matter, the main source of potential acid
aerosols directly attributable to the FGD systems are:
(1) Entrained mist containing chemicals used in wet FGD
systems (including mist generated during flue gas
quenching)
(2) Entrained solids eluted from dry FGD systems which
consist of partially reacted sorbents containing
sulfates, sulfites, nitrites, nitrates, etc.
Very few quantitative data exist as to the importance of these sources to the
acid aerosol problem and neither the process developers, system operators nor
regulatory agencies seem to be collecting such information.
The difficulty of the situation is compounded by the fact that acid
aerosol emissions in the form of entrained mist and eluted solid particles
depend greatly upon the FGD system in question, i.e., the specific process
being used on the combustion source. For example, potential acid aerosol
emissions from a boiler equipped with the lime/limestone process would be
expected to be considerably different from the Chiyoda CT-101 process
which would be different from the Shell CuO process and so on. Additionally,
mist carryover would be expected to vary considerably with scrubber type and
mist eliminator design. For example spray towers and venturi scurbbers
produce heavier mist loadings than a marble bed scrubber which tends to abate
115
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mist formation by the nature of its design. Also droplet size becomes
finer and mist loading increases as the pressure drop through the
scrubber increases, i.e., for a given liquid flow rate, more mist at a
smaller diameter is produced because the gas velocity is increased. Alter-
nately, at a given gas velocity through the scrubber, more mist at a finer
size is produced as the liquid rate is increased (higher L/G ratio).
Bascially the same phenomena occurs in dry FGD processes in which the
gas is contacted at elevated temperatures in adsorbers of different designs.
For example, the grain loading of partially reacted CuO sorbent eluted from
the Shell process would be expected to be lower than the grain loading of
partially reacted char eluted in the Foster Wheeler-Bergbau Forchung process.
This occurs because of difference in reactor design configurations in the
two processes and sorbent chemicals and their abrasive properties.
The crux of the above discussion is that it is very difficult to
generalize and that each FGD system must be considered in its own right.
Although admittedly a compromise when one considers that there are over 50
processes under development, an attempt was made to "qualitatively
characterize" the entrained materials eluted from several systems. Such
materials would have to be considered as potential acid aerosol formers and
directly attributable to the operation of the FGD system. This "best guess"
speculative analysis is summarized in Table 16 where it is seen a wide
variety of chemical substances can be emitted depending upon the FGD process.
Richards and Gerstle attempted to summarize available quantitative
information (actually "best guesses") on the contribution FGD systems make
to the acid aerosol problem (Richards, 1974). All of their work was concern-
ed only with "sulfate" emissions and wet FGD processes. No mention was made
of dry processes, since in all probability no data were available to them.
The data of Richards and Gerstle together with estimates for Unit 6 of
Paddy's Run (LG&E) and Unit 1 of Will County (Commonwelath Edison Company)
are presented in Table 17. The latter two estimates were obtained from data
presented by Rosenberg, et al. (Rosenberg, 1976) All data pertain to mist
eliminator entrainment of scrubbing liquors. As seen in Table 18, the data
scatter considerably. , Consider for instance, estimates for Unit 6 of the
Paddy's Run Station of LG&E which uses a lime scrubber. Estimates range
from between 0.25 to 3.5 percent of the input sulfur in the coal, i.e.,
116
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TABLE 16. QUALITATIVE LIST OF POTENTIAL ACID AEROSOL FORMERS EMITTED FROM FGD SYSTEMS
Process
Magnesium Oxide Process
Double Alkali Process
Entrained Particles
Wet Process Liquids
(Mist)
Dry Process
Solids
Comments
Lice/Limestone Process
CaS04
CaSO
CaHS03
CaS03 • 1/2 H20
CaSO,-2H20
CaC03
Slurry mist with dissolved
solids.
Also Ca(OH)2 would be present in
lime scrubbing.
MgO
NaC03
Na2S03
NaHS03**
Potential dry sulfate and sulfite
emissions occur from, calcination
step in the process. Also acid
mist from H-SO, production.
Trace quantities of CaSO./CaSO,/
Ca(OH). recirculated from the
reaction tank to the scrubber
are undoubtedly also emitted.
NaOH
Wellniati-Lord Process
Na2S03
NaHS03**
NaOH
Side reactions occur in the
evaporator/crystallizer which
•lead to po'lythionate compounds,
such as N7a2S207, being present in
liquor.
-------
TABLE 16. (Continued)
Process
Catalytic-IFP Process
BOM Citrate Process
oo
Entrained Particles
Wet Process Liquids Dry Process
(Mist) Solids
(NH )HSO
4 4
NaH2Cit
NaH2Cit
NaHSO **
NaHSO,
Cotzr.ents
Fine particles of (NH.KSO, are
formed between the gas phase
reaction of S0_
flue gas.
and NH, in the
pH4 to 5 solution containing a wide
variety of sodium citrate/bicitrate
and sulfate compounds.
Stauffer Phosphate Process
NaHS03**
Very similar to BOM citrate process,
pH 4 to 5 scrubbing liquor. Also
polythionates due to oxidative side
reactions.
Chiyoda CT-101 Process
V°3
H2S°4
Strongly acidic (pH ~ 0.3) scrubbing
liquor with dissolved ferric
sulfate catalyst.
Shell SFGD' Process
CuO
Eluted solids from acceptor reactors.
-------
TABLE 16. (Continued)
Entrained Particles
Process
Wet Process Liquids
(Mist)
Dry Process
Solids
Comments
Foster Wheeler - Bergbau
Forschung Process
Char
Char/SCL
Char
Solids eluted from char adsorber.
* No attempt was made to list the chemical analogs of absorbed NO, N02> HC1, HF, etc., which are contained in
small quantities in the flue gas and partially removed by the FGD process.
** Other more complex polythionates are undoubtedly present, for example Na^S-O sodium pyposulfate.
-------
EMISSIONS SXJUmiff—RBPRODVCED IN PART FROM
GEBSTLE (Richards, 1976)
WestUn Westlin Westlin
York Van NesP'
4.
s
s ns
ILJLras—
stonte
Lawrence L G & E Will Cousty
Lines tone Line Limes tone
Injection scrubber scrubber
S
Ctoal,
S 3,«& S
Coal,
3.52t S
70
0.11
ia
Coal,
•J'^**** O
165
0.078
o.i
-------
feed to the boiler, is emitted to the environment as CaSO. and CaSO,, in the
4 3
form of mist particles.* Likewise at the Will County Station where lime-
stone wet scrubbing is practiced, approximately 0.1 percent of the sulfur
input in the fuel is thought to be emitted as aqueous mist entrainment in
the form of CaSO./CaSO slurry (5 to 8 percent suspended solids and 5000 ppm
dissolved solids). On Unit 4 of the Lawrence Station of Kansas Power and
Light Company, where tail end limestone injection wet scrubbing is practiced,
a similar estimate is that about 1.1 percent of the sulfur in the coal is
emitted as entrained mist (agina CaSO,/CaSO solution). For the Mystic
Station of Boston Edison Company on a 150 MW oil filter boiler employing MgO
scrubbing, about 0.4 percent of the sulfur is emitted as a MgO/MgSO MgSO
slurry mist.
It is very difficult to draw conclusions from fragmentary numbers such
as those in Table 17. The best that can be done is to conclude that mist
eliminator entrainment is probably a small problem compared with the
inefficiencies in the FGD systems for sulfur removal, i.e., where perhaps
10 to 30 percent of the input sulfur to the boiler is emitted as SO which
has the potential to be an acid aerosol former. Even this conclusion is
suspect however, and would depend upon the health effects associated with
the relative sources and the eventual composition the acid aerosols assume.
Although no data can be cited, the same conclusion probably holds for the
dry FGD systems also.
EFFECT OF COMBUSTION MODIFICATION ON
ACID AEROSOL FORMATION IN FGD SYSTEMS
All of the combustion modification techniques described in this report
will lead to changes in the physical-chemical characteristics of the flue
gas. These modifications in character will then manifest themselves in
changes in the acid aerosol formation potential of the flue gas as it pro-
ceeds through the FGD system relative to the case where no combustion
* Using the percent sulfur input to the boiler being emitted as sulfate may
not be a valid basis for making qualtative judgements since this will de-
pend on the relative health effects of the emitted sulfates.
121
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modification was employed on the combustion source. Succinctly stated,
all combustion modification techniques will lead to at least one of the
following changes in characteristics of the flue gas:
(1) Decreased gas volume due to either low excess air
and/or low firing rate. This would amount to perhaps
a 10 to 20 percent change in volume relative to the
case of no combustion modification.
(2) Decreased total NO and NO concentration in the flue
gas. This would amount to a 25 to 65 percent reduc-
tion in the NO and NO content of the flue gas depend-
ing upon the type of fuel burned, for example, from
about 225-800 ppm to about 100-500 ppra for the case of
coal firing.
(3) The total concentration of SO and SO in the flue gas
should remain about the same. For example, for coal
this would be anywhere from 600 to 3000 ppm total de-
pending upon the coal source (western or eastern bitu-
minous) and its sulfur content.
(4) The ratio of SO to SO, should increase slightly due to a
slight decrease in SO-} concentration. This occurs because
the formation of SO , although thermodynamically favored
at low temperatue, will be limited by the kinetic rate of
formation which is diminished at low temperature.
(5) Particle formation rates may increase due to combustion
modification, especially for carbonaceous materials.
Since these materials will probably be submicron in size,
this will effectively increase the particle number load on
the ESP and flue gas contacting device used in the FGD
system. The mass load on the particle removal devices
would not be expected to increase significantly.
(6) Fuel additives, especially compounds containing metal
and/or metal oxides will increase the particulate
load on the ESP and FGD systems. Since particles
formed from additives should also be submicron in
size, the same comments made under (5) are applicable
here.
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In a very qualitative sense, introducing combustion modification on an
existing boiler equipped with an FGD system leads to several changes. Since
the gas volume decreases, less mist will be entrained off the back of the
scrubber. However, since the gas velocity through the scrubber will decrease
the mist eliminator efficiency will also be decreased. Since any FGD
system must be designed to cope with fluctuations in boiler load which re-
sult in the wet scrubber or dry adsorber seeing decreased gas flow rate,
the net effect will probably be insignificant. FGD systems are usually
designed for the maximum flow rate of flue gas generated by the boiler, i.e.,
when the boiler operates under maximum load and maximum excess air. Also,
FGD systems are designed to have good turndown capabilities, i.e., perhaps
by a factor of two, or be able to operate at half load on the boiler. Since
combustion modifications would result in perhaps a 10 to 20 percent decrease
in flow rate, this should be within the turndown capabilities of most FGD
systems. For boilers where no FGD system exists, introduction of combustion
modification should present no serious problem for the FGD system. This
conclusion is reached because designers of the FGD system would simply
factor into the design the effect combusiton modification would have on the
decreased volumetric flow rate.
Decreased NO and NO concentration in the flue gas would decrease the
potential for acid aerosol formation. This results from the obvious de-
creased emission of NO and NO to the enviornment and the change for parti-
tion to mist and/or solid particles escaping the desulfurization unit. Also,
since the concentration of NO and NO is decreased in the flue gas, the con-
centration of nitrates and nitrites in scrubber liquors should be lower.
Hence, the concentration in mist particles entrained off the back of mist
eliminators will be decreased. In dry desulfurization processes, based
upon adsorption theory and typical adsorption isotherms, NO and NO- sorbed
on entrained solids would also be expected to be less since the gas phase
NO and NO concentrations are reduced.
With regard to Item 3, since the S0_ and SO concentration in the flue
gas remains essentially the same under combustion modification, the acid
aerosol potential of the flue gas will be essentially the same as for a
boiler normally operating with an FGD system. The potential for FGD systems
to emit and contribute to the acid aerosol problem has been previously
123
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discussed and those comments apply here. Likewise under Item 4, since the
ratio of SO to SO remains essentially constant or increases slightly due
£m J
to combustion modification, this will have a negligible effect on the
operation of the FGD system and the acid aerosol formation of the gas. In
actual practice, FGD systems are designed to handle a range of S0_ concen-
trations., with an upper limit set on the maximum SO concentration (hence
sulfur input) to the process.
The most significant effect on acid aerosol formation results from
Items 5 and 6, increased small particle formation due to carbon formation
or the addition of fuel additives. Both result in the same synergistic
effects when interacting with FGD systems and will be discussed together.
The formation of small particles of carbon, active metals, and metal oxides
will lead to increased particle surface area. These materials can serve as
adsorption sites for potential acid aerosol formers, notably S0?, S0_, NO,
NO , HC1, HF, and other minor constituents of flue gas. Also, these small
particles could serve as sites to catalyze oxidative reactions. Since the
temperature is elevated to about 150 C (300 to 325 F) at the point in the
duct work where the gases leave the ESP, the predominant sorption mechanism
would in all probability be chemisorption although some amount of physical
adsorption would undoubtedly occur. Some sorption would also occur on the
larger fly ash particles. However, since the surface area increases greatly
with the small submicron particles, and because of the greater chemical
reactivity of carbon, elemental metals, and metal oxides, one would expect
greater sorption on the carbonaceous and active metal surfaces formed during
combustion than on ash particles. As the flue gases normally pass through
the electrostatic precipitator, ash removal of particles down to about 1
micron would occur. Collection efficiency would depend on the age and
physical condition of the ESP and would range from about 90 percent for old
units on retrofit boilers to 99 percent on new units. However, virtually no
removal is afforded submicron particles. Thus, the increased number of
small particles formed in combustion'modification would aggrevate the acid
aerosol problem.
In the demister and reheat sections of the FGD system, virtually no
small particle removal will occur. Additionally, particles of a larger size
will be added to the gas due to reentrainment from the mist eliminator so
124
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the particle size distribution will change. Also, the reheat section, since
mist drying is occurring, contributes to a further change in the particle
size distribution. Additionally, the added residence time of the gas in the
scrubber removal section, mist eliminator and reheat sections of the FGD
system will lead to further adsorption of unremoved molecular species on
small submicron particles.
Dry removal processes would give rise to essentially the same scenario
described above except that no quenching, mist elimination, or reheat occurs,
Little if any removal of submicron particles would occur in dry FGD systems.
They would also suffer from the added disadvantage of putting entrained
solids into the flue gas. These solids, such as partially reacted char in
the Foster Wheeler-BF process or partially reacted CuO in the Shell FGD
process, would serve as additional sites for sorption of potential acid
aerosols. Solid char particles would also be expected to catalyze the
reaciton of SO to SO .
SUMMARY
SO,, removal technology has been reviewed with regard to the formation
of acid aerosols under combustion modification conditions. A myriad of
FGD processes exist and currently over 50 are in various stages of develop-
ment. FGD technology was categorized to permit a wide variety of processes
to be considered. Ten (10) processes were selected for analysis which
represent a cross-section of the current and emerging state of FGD technol-
ogy. To perform any type of meaningful analysis, two situations must be
considered. First the situation where acid aerosols are emitted from
boilers equipped with FGD systems but not performing combustion modifica-
tion. Second, assessing changes is acid aerosol emission from boilers
equipped with FGD systems and where combustion modification is being per-
formed. Based upon this analysis, the following conclusions are drawn:
(1) As a general rule, it is.felt that FGD system mitigate
the acid aerosol problem. The conclusion is based on
the fact that FGD processes have finite removal effi-
ciencies for many of the species considered as potential
acid aerosol formers. Hence, FGD processes should lead
to reduced masses of emissions which could lead to
secondary acid aerosols in the .environment. However,
inefficiencies exist in FGD systems and molecular and
125
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particulate species capable of forming acid aerosols are
emitted even when FGD systems are operating normally.
(2) There exists, however, certain emissions which can be
considered as potential acid aerosol formers which are
directly attributable to the FGD process.
There are
(a) Entrained mist containing chemicals used in the
wet FGD processes—including mist generated
during the FGD quenching operation
(b) Entrained solids eluted from dry FGD processes
which consist of partially reacted sorbents
which contain sulfates, sulfites, nitrates,
nitrites, chlorides, etc.
(3) Very few data, either qualitative or quantitative, exist
as to the importance of these FGD derived sources to the
acid aerosol problem. Neither the process vendors, FGD
process operators (boiler operators), or regulatory
agencies appear to be taking measurements and performing
analytical studies to elucidate the importance of these
sources.
(4) Acid aerosols emitted as particulate matter (both as
liquids in mist and entrained solids) depend greatly
on the FGD system in question and each FGD process
must be evaluated separately as to its potential for
aggravating the acid aerosol problem. Thus, it is
difficult, if not impossible, to generalize among FGD
systems.
(5) The physical and chemical nature of the particulate
emissions from FGD processes will depend for the most
part on the process and the hardware used in its
operation. A wide range of particle size, pH values,
and chemical composition are possible. The particles
emitted should have the physical-chemical characteristics
of the reactive liquors used in the wet FGD systems and
the partially reacted sorbents used in the dry processes.
Certain FGD processes by their very nature, for example,
the Chiyoda CT-101 and the Foster Wheeler-Bergbau
Forschung processes, would appear to have the potential
to aggravate the acid aerosol problem more than other
processes.
(6) Major variables thought to determine FGD process
emissions of acid aerosol particles are
(a) Wet processes - reactor design, chemical
nature of the scrubber liquors, removal
126
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efficiencies, scrubber gas velocity, pressure
drop, liquid to gas ratio, and mist eliminators
efficiency
(b) Dry processes - reactor design or configuration,
chemical nature of the solid sorbent, removal
efficiencies, gas velocity through the reactor,
and pressure drop.
(7) Considering sulfur emissions only (because what few data
exist are in this area), quantitative estimates as to the
fraction of input sulfur to the boiler emitted as entrained
particles vary widely. For coal-fired boilers equipped
with FGD processes, it appears that between 0.25 to 3.5
percent of the sulfur input to the boiler is emitted as
entrained particles. For data on one oil-fired boiler
equipped with a wet FGD system, a similar number would be
about 0.4 percent of the available sulfur is emitted as
entrained particles. At best, such fragmentary numbers
must be considered as speculative guesses.
(8) Because of the scarcity of available data in this area,
it is difficult and probably unwise to draw conclusions.
The best that appears possible is to conclude that en-
trained FGD particles are probably a minor contributor to
the acid aerosol problem, as compared to the inefficiencies
in the FGD processes where perhaps 10 to 30 percent of the
input SO /SO to the scrubber and much of the NO/NO are
emitted to tne environment. Even this conclusion is suspect
because it would depend on the relative health effects
associated with the particles emitted from FGD systems and
those of the unabsorbed species which have the potential to
form secondary acid aerosols.
(9) As far as FGD processes are concerned, all combustion
modification techniques can be represented for purposes
of analysis in terms of one or more of the following
effects:
(a) Decreased gas volume due to low excess air
and/or low firing rate
(b) Decreased total NO and NO concentration in
the flue gas
(c) The total concentration of SO and SO in
the flue gas remains the same, i.e., the sulfur
load on the FGD process remains about the same
(d) The ratio of S02 to SO increases slightly due
to small decreases in the SO concentration
127
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(e) Increased rates of formation of submicron
particles.
(10) As a general rule, combustion modification would not
be expected to greatly aggravate the acid aerosol
problem associated with a boiler equipped with an FGD
process, i.e., relative to the case where the boiler is
operrated with an FGD process but no combustion modifi-
cation. In other words, going to combustion modification
on a boiler already equipped with an FGD process should
not change greatly the existing acid aerosol emissions.
(11) The most significant effect that combustion modification
might have on FGD processes appears to be in the increas-
ed number of small submicron particles potentially formed
during combustion modification relative to normal boiler
operation. These particles could serve as sorption sites
for acid gases present in the flue gas. Addiitonally,
they could serve as active sites to promote oxidative
reactions. The efficiency for removal of submicron
particles by the gas/liquid or gas/solid contacting de-
vices used in FGD processes would be minimal. The same
conclusion would be true for the electrostatic precipi-
tators found on most large boilers preceding the FGD
systems. Consequently, to the degree that combustion
modification increases the rate and number of submicron
particles, it would aggravate the acid aerosol problem.
128
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139
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APPENDIX
The tabulated results of the computed equilibrium product concen-
trations for combustion of coal and two fuel oils are collected in this
Appendix. The elemental composition of the coal used in these calcu-
lations is given in Table 2, page 24. The compositions of the fuel oils is
given in Table 4, page 33. The product species considered in these
calculations are given in Table 3, page 26 and the same product species were
considered for all the coal and oil calculations except for those situations
where the lack of an element, in the case of the oils, precludes the
existence of a compound.
140
-------
COAL RUN AT 1800K, 1 ATM, 2<>/ EXCESS AIR
.itECIIH
—EQUILIBRIUM-
CONCENTRATION
JLMDLES).
VECTOR.
EQUILIBRIUM _
CONCENTRATION
..(MOLES)
0 6;6-
0 h\-}_p.i
OCo-0
8 MaO
0 Y-zCq'
k vo
U> rJo. 9 O ^iJ
^ V_CX- ^ I "
JKU^O;
I» ^r-e S0>4
V }v|(\-C\
If N\a Pi
UM" "
{ ^°H
k PbO
J ^S,oS
% 6 3
i» "^t^n
l» (V\aC\ 2_
«» Pv,
• tSiu1
.79998423c-01
•19996590E-01
•19975115E-02
.(«9&11375E-Oi*
.99665234E-06
•98582292E-06
.97619824E-06
9, •96<+'*3119£-06
.9WU13770E-Q6
•915527H7E-06
.86703502E-06
•85806733E-05
.85067910E-06
.80269965E-06
.78679595E-06
.782188bdE-G6
.78137500E-06
.78125781E-06
• 7812500L1E-06
.77986829E-06
a, .7"*92i»817£-06
.732i*3335E-06
.7291352=E-06
.72567773E-06
. 5 5 8 1 8 0 <+ 5 E - 0 5
.5250000GE-06
4 BePs. .12537500E-06
k &-L *" .b2500000E-07
^ K/\3(NOa^« .lOOOOOOOE-Od
It F- (OC3\T .10000000E-08
=» V^e^t . 10000000 E- 08
k ^c^)Oo.«10000000E-08
•» 2-nFr .10000000E-08
<» C.'VNO.""'-. 10000000E-08
l» Pto^J Cff*L~* 10000000E-08
IffOiLHfl^ .10000000E-08
k CA^NO^^. 100000GOE-08
^>Jix\lC- .10000000E-08
tfe?&n§MioooooooE-oS
<» 2.nC\2.'J • 10000 0 0 OE-06
TOTAL CO>JDENSID ,1<»20 7078E + 00
V A 3 PC ^ .62506250E-06
^ V . , .S250000Qt-06
VCouOHJT. .51035181E-06
k ©e-SO^ . 1888750 OE-06
l> tVOlo .12600000E-06
141
-------
COAL RUN AT 800K, 1 ATM, 2% EXCESS AIR
c
VEDTOR
0 S.'U-z.
0 PM^O-i
0 C{k30a
0-_ 1
F^r, Dl
0 {A^Sc/q
0 I^SCM
0 t» OT.
0 PiTfi>4
0 \U Her
1 S^
«» vW
tST-1^
*» V^^Oi^
*» ^e Sc>4
J^
k VJ^ \0o-j
. *» \;o
l»V)«t>A5CH
** VOo^Cl
t InCU
tln''L
Ui?^?3
** V/C\2^
*» ?hO
u v
i» f^b
*» l^\C4\-«i.
IVJ»PO
J?i
k C
E3UILI9RIJM
CONCENTRATION
(MOLES)
o79993<+19E-Gl
.19995192E-G1
.l'*99d736E-31
.°993^!9&Elc3
.!*9611375E-3i+
".SlIzbotlE-OS
o 9 7 6 5 6 6 i» 1 E - G 6
.9758736GE-C6
.9727U072E-36
•973139G3E-G6
.93548898E-C6
.92957669E-36
«88588t92E-G6
.B6331196E-06
,85'+97009E-G6
. 8 5 1 4 1 9 3 3 E - 3 6
.35C17871E-G6
. 8'*5'f86't6E-u6
,839'»6636E-36
o781253GGE-C6
o7731037iiE-36
.77269235E-06
o76831371E-G6
.7b662550E-3o
o75120007E-06 •
o72321289E-36
o66l39Q69E-36
,6578336'+E-06
.62500000E-36
o525000GOE-06
.62371397E-06
o51818750E-C6
.61128833E-06
.61033573E-06
,6091'*612E-36
o b31>*56l6E-G6
EQUILIBRIUM
C CONCENTRATION
VECTOR (MOLES)
k W£,\-L »59732339E-06
^f\\'^^~, o572»*7B63E-G6
^ C^VJC^ o 56199333 E-36
*»Co.iKlC)/-, o561.99281E-G6
*» eo^pv\Y~»56199257E-06
•» PbC\i o52911255E-G6
0 fctSsa o26662530E-36
** be (jOOiiz. « 25 130003 E- 06
** &tO .18G13750E-C6
t ge. • 3125GO 3G E-07
U 2o^OO^,ol30330GGE-C8
*» Fb^O^V« 1 C 0 0 0 0 GO E- 3 6
b^e f'Uo^r.lOOGOOOOE-06
k tv\Cjb " tJ «
«* to v? 0 o
<* ft\ - 0.
«* tv\a Oo
TOTAL 20NOEMSE3 d o 1<»3<»7»»22E«- 0 G
. • '
59756050E-06
142
-------
COAL RUN AT 650K, 1 ATM, 2°/o EXCESS AIR
_Y_£.C.LO*_
3D.MCENTRATION
0 ~CO-_ .79998777E-01
19995121E-01
t
. 19961315E-02
0 _KbJ& 5 0u~«. .19883333 E_r 0 2_
.-•-39.2._9_9Z.O_3.EMU....
1*9611375 £-0.7.i7.b.Er.O_6_
.90915301E-Ob
.37783353E-06
. 33(*5331b£-0b
_._6JJL&3 5.3.6E-Q6_
. 32«f917S5£-06
77163357E-06
. 75531925E-06
k ?t>
•7b200379£-0b
-t_7_?.1.9.6.2.d.9tr.0.6_
. 7<*3b73tbE-06
k Co-O .S8625916E-06
(f \JC\2_ .b255b250£-0b
J»_A/ LO 2 5 0 0 0.0 0 En0.6.
It NaLS'O->, . 51i»»»99'«5£-06
ol033E>73£-06
OF.-06..
<» Co.
d
(»
.a091«*3l2E-
_c 6.0.8.105.^.3 L-
. 5053071 3E-
_i3.97.2.5.9 05£•
t\Jo-C-\
^2.. 58blbGlb£'
._57.58233.9£
. 55820373E
-------
COAL RUN AT 650K, 1 ATM, 2°/o EXCESS AIR
EQUILIBRIUM
VECTOR
0 ^2-
0 14 jjb
0 \Ac.\
0 60-2.
0.>,
1 VAL
ob!03515b£-09
. 501b0262£-Q9
. 26519G52£-09
,9765b250£-12
. 32183552£-12
.15095071£-12
. 1-+7V7169£-12
1 CO " » b59t9159£-13
1V)^Q . 57521*37 9£-13
0 t^-eC-l^;
0 Pi^ Olo
1 HM&2,
0 5 r» i o
0 poc^3
1 C-O^
1 K)iOH
lP^\n
o 59970 '230t-l<*
,i»7479o26c-lb
. 10382101E-20
.779bl:>25£-22
«_7.2.8.i*.37.81£r31
. 19282572£-31
TJTAL GftS£3
b^729302£-39
1.0030 ATM.
144
-------
COAL RUN AT
1 ATM, 20/0 EXCESS AIR
EQUILI8RIJM
C
VECTOR
0 VI -i.
0 r.Oo
0 vArO
0 <,n/.
0 t>0^
o r>i
0 vvc\
0 Uo^OM
o V\P
op^n.0
0 NO
0 Cl-2.
0 NJOL
o MLn
0 hjOC\
i pa^
0 v\t
0(LO
0 \-\\OQ2,
1P4
0 vA^SO^
o per.i-,
0 B^'
0 T^ci\3
o e>c\v,
o ^^i-l.D^
0 ^,05
0 co^b
0 9C\rL
0 V= T.'
0 5,
o?c\s
TOTAL SftSEO'JS
COMCtNTRATION
(MOLES)
,3019999<»E+02
.59999971E*01
,2if97897aE«-Gl
.i»2578898E-Cl
.33925i*01£-01
,20^6799E-G1
.398791*+3£-02
.99233569E-0'*
.13598132E-Oi*
.96C390«*1£-05
,|»<»932<*70Z-C5
.29158354E-C6
.69136868E-07
.38S99738E-09
.877213^t5E-lG
.17282J12E-10
• SSgoS'+'tgE-ll
.31671197E-11
.16^63992E-11
.1«*'»59'»28E-11
.65577312E-12
.29618026E-12
.13036372E-13
.52988441E-18
.96568373E-19
.bi»26620<*£-21
.10096320E-25
.69097803E-29
.18389653E-29
.18785523E-3'*
.37607557E-37
.12123215E-'*!
,3879895'»E<-C2
EQUILIBRIUM
3 CONCENTRATION
VECTOR _tMOL£S)
D Hi »_3_q 192637E102_
. 5 9 •> 8 9 7 2 !+ E «• 01
,2J»_9 3 6.3o 5.EJLQ.1
No.
*.3Jt2.8.«»Jt.3JLEi02
.18132719E-02
_t33.77_0 3.0.5.E=.0.3
.17673517E-03
3J»2_e^LQ.
-------
COAL RUN AT 1800K, 1 ATM, 98o/o OF STOICHIOMETRlC AIR
VECTOR
0 ^Ox.
00-^0
0 Fp-O-
0 $-\c^G
o e>2_o*,
0 \I-£"OL^
"» \/O
t ^^
» £3SA»
t fcts°H
!» Maf-i^
J ^^
% K?fn
* loSOu
EQUILIBRIUM
CONCENTRATION
(MOLES)
o79998883£-01
ol999b570£-01
,'+9bll375E-0'+
o30388027E-05
.996b523<*£-0b
o9761982'+£-Ob
o9iti»31327£-0b
o9'»'*13770E-Ob
,915527'+7E-Ob
. 88171+ !»21E-Ob
,8bbb5753E-Ob
.8<+037109£-Ob
.803b'+037E-Ob
.79081423E-06
«78533dlOE-Ob
,782bl295£-0b
,78137500E-Ob
,78125781£-06
**
VECTOR
•»'fie.
i» PeC
<»Fe.(k
I *f£
. }i M1^
<» PbtJJC
"» J3c(v:
^B^^o
*Cn/\V
«» CotCOi
l» VW. JvJ
•» Pi \CVOI
TDTftL CONDENSED "
EQUILIBRIUM
CONCENTRATION
(MOLES)
i. ol25375JOE-Ob
o62500030£-07
1-2. .100000 00 E- 08
^iVlOOOOOOOc-08
\T_ .ldbOOOOOE-08
J^;*". 10 00 00 00 £-08
r, ".10000000E-08
)0g>jtl0000030t-08
l^^_ olOOOOOO OE-08
O^^olOOOOOOOE-08
OjS'J.10000000c-08
O^ .10000000E-08
•3.1'»207022£ + 00
Ph
_LZ-8425_0_0_pE^06_
. 7 7~8 9\ o 8 2 E - 0 b
,7ifll2b72£-06
,72921207£-0b
&68786
-------
COAL RUN AT 1800K, 1 ATM, 98o/o OF STOICHIOMETRlC AIR
EQUILIBRIUM
- VECTOR
0 Vi-j
CONCENTRATION
(MOLES)
0 U^O ,2<»'*<+85<*3c>01
0 CO
0 S&o
0 ^2.
o ac\
D VJa.OlA
0 kiO
o k)<^
0 ^f\
*
•
t
.
.
99986595E-01
52**27315ii-01
397 6<+i*90£-02
90361207E-03
55060<*70E-03
39770305E-03
0 0*2. • 2358'+5 S'+c.-OS
0 P^OiO
0 •pbCX'i
.
•
97065331E-05
79<+32717E-05
0 6>t?2> *" . 2B52 3 D 1 ^E- 0 5
0 ^10S «15067'+i+2t-05
0 M^
0 ^>^
0 Wi_O
0 KJOo
0 CA. "
0 dl^.
0 y^££ |
B WZ.SOM
D 1-^7^93
o 6013
0 CL^ ii
av^voo^
0 VOdA^
0 E>^-^s
a pc.\-b
0 ft \ ^-,
0 f^ "
0 VO^Oij
0 V^-z."
0 Pc.xcr"'
TOTHL GA3EOU5
•
.
.
•
•
.
.
.
.
•
•
•
•
.
•
t
.
•
.
i*8506261£-06
27010727E-06
45979989E-07
11688390E-07
22186532E-08
22659726E-09
26821798E-10
226f»it761E-10
3233^^ 16E-11
373t»2558E-12
39171302E-11*
27816925E-1<»
12590951E-15
59332360E-16
75076736E-19
2«*992239£-2**
12872567E-29
169&3283E-33
3760<«355Et02
TOTAL PRESSURE = 1.0000 ATM.
147
-------
COAL RUN AT 500K, 1 ATM, 10°/o EXCESS AIR
?QUTL T3RIUM
5 CONCENTRATION
BS.tf? o799987<*3£-01
OCxSfy '\1999501bE-01
Df^fu Bl!*89b095E-01
0 HcySoa ol99b0038£-02
OkWiASO^ o!9882803£-02
0 LrvSOii 839582253£-03
O^e^O^ .20521233E-03
DMPOo .39<*52237E-0<*
0 fbSO»\ o58390398 £-05
OMtO^ o3t*3990b3£-05
l» PbMn <,9769'tl<*lE-06
*> Mi-KitJ^ o9765b541£-06
5>/^^O^r,' .9751*3783 £-06
fe '•i.'nCb^ ~ o 9 5 9 b 1 0 0 5 £- 0 b
b P^D o 9378<»375£-Ob
!» CuMND3\ ,93229009£-0b
%Co.(viO,>,« .93228911E-06
l»KU-,(n <,8933<+575E-Ob
h Ha6 o891187i*0£-0b
% fM/yJO^^ o3bb21329£-0b
<» Z-nO" " <,85070207E-06 T3Tft
^ 7«%Clz. c8i*922i»58E-06
t KWj.So4 .8!*590520£-06
k fAg o80<*38539E-06
^ tr/.. «79b83917E-Ob
*» MgCl'L o7826078b£-0b
tlnfUO^o . 78125781 E- 06
fc 6 "*"o78125000£-0b
l» IJb^P o765b586b£-0b
i A 11,0 3 .7i»897360£-06
t T^n o7(*2b7578E-Ob
<» T£C\S o730b75bl£-0b
'* AI .7231553'*£-06
«» NJft.UUa, o57513080E-06
IjWrt^Do, «=>72i»801i*E-Ob
*> W^^Si'^3 obi*881189£-0b
fe fJfi^ «bi*781189E-Ob
•tN/C-li ob25"»3750E-06
iM/oD., «b2531250E-Ob
«» \JO ' .32512500E-06
t^PV.rvW.V .b2512500E-Ob
EQUILIBRIUM
3 CONCENTRATION
VAPT0' (HOLES)
^ V .&2500000E-06
l» ?b ,o2500000E-Ob
I* 5i ,6103<*lb8£-0b
l> Ca .i091'«=)12E-06. _
l»Co.(pU'V. .60128bl7£-0b
"» VJ^COVA o5b82313b£-06
^ C, *" o5663233a£-06
(>rt-aP8a « 556721+blE-Ob
^ fAa^i. o5i*255970£-0b
bP^C\o o507i*3121£-0b
0 E>cSoti »i*0371875E-Ob
l» Bf-O «3b2b<*Q63E-Oi>
(6/V\F3 o3398059l£-0b
ue^f-L ^98781250£-07
'* €>ec\^ »53500000£-07
!» 1**>_ (VJ 0^-j. « "» 5 1 093 7_5 E - 0 .7
l» 6e-" "'~o31250000E-07
t» .<, .28 16552 7 £-09
«»OaO fll
fc "i(^^~1. 0 o
«» CrtCl« 0.
L CONDENSED " ol<*«*60856E + 00
1A8
-------
COAL RUN AT 650K, 1 ATM, 10°/o EXCESS AIR
EQUILI3RIUM_
JONCENFRATION
(MOLES)
7999Bb3dE-01
2<*998203t-Ql
j_l<»998-»
-------
COAL RUN AT 800K, 1 ATM, 10°/o EXCESS AIR
EQUILIBRIUM
Z :DNSENTRATION
VrCTOD (HOLES)
0 5*07. .79996419E-01
(j fli^/n^ o 2^99817 3E.-01
0 C.a.S>t>?2531&'^3 , ^ 7 2 <* 7 ? 0 1 - - 0 (=•
<» CiLltJ^'lo pol9937p£-0b
C^\i_ »m99?7:50E-On
It 6eF?. .12b00000d-0b
if R^ •31250000E-07
^ ?.r\(»Ji)^)2. 10000300E-08
tt Pto AJO -,^0 , 11)0000 QOc-09
it ^ejjc^^. 1000000UE-08
LL ?-.-, Po 0,
i* At 0.
it PbD (j.
^ Mfl 0»
TDTftL CONOENS£0 . It*3it7376£*00
150
-------
COAL RUN AT 1800K, 1ATM, 10% EXCESS AIR
EQUILI8RIUM-.
C CONCENTRATION
i/ECTOR 4MOLE-S4
0 SlOi -79998533E-01
n ai-^p.3-^2^-9 9 9.5.a.aE=JXi-
C
UEC-TOR-
EQUILIBRIUM
CONCENTRATION
(MOLLS)
0
__ 0
0
___ D ___
0
Co_O .1999658
-------
COAL RUN AT
1 ATM, 10°/o EXCESS AIR
SCO K
Ln
r-o
JLELCICLX
0
0
0 UCI
^r
KI n
_£QJILIBRIUM
CONCENTRATION
IMOLES1
o31999993E«-02
_«_5_93-9.9 9_6_5_E J-.01
iJiOJQ
o35770989£-02
.39 9_9A3_0_9 £ji03
_ft^.9.6_6_0_8j2_5 £ --CL6
o85087085E-07
o596'+0288E-09
fl^Z.5^.1 ISL3. £j^0^
0 ViOCV" .10116688E-09
t PH a.9.5 3 6Z'».3.2 £.=LU.
o232797WE-ll
.17069D93E-11
0 VJtO oll5f 8.93 32.!* £^13
o30517578E-13
o30517578£-13
.20770177E-16
«120'»53i»OE-18
_,31006622£-21
o5560100i*E-23
l5258519E-2<»
pn
TOTftL GASEOUS
o<*1556331£-28
tfECTOR
a
o
0 V-V
lONCENTRMTION
_IMOLCS_)
59999733£*01
i»(*919065E-i-00
39602o63E-02
o 576067d3£-03
o!5189318E-0'*
_«_11.3_97.3_6J E-0
-------
COAL RUN AT «•*•- 1 ATM, 10°/o EXCESS AIR
PROP
\&oo
-J/.-.CT-
0
SL
0
—EQUILIBRIUM-
CONCENTRATION
(MOL£S)
-EQUILIBRIUM.—
C CONCENTRATION
—VECTOR (.MOLES)
..do?
.31999934£fQZ
_..!*573-8373.£f.aa_-
.50051512E-01
_..lo387_377£-Q
COT.
•0.2—
.598S7925E+T1
_SQg._..
^.998-7JLLa3E^O-L_
,i»2988767E-01
0 VACI .39755730E-02
JD—tJcuOB ..ifl-7-7.3 0 71F r.fl 2
.12351561E-02
0 M2.O
-fl-iooa
1 vAVJO-j
J~*>«L-
. 18858 + 55E-08
-*_8S010990£-09-
.75890920E-10
,19073436c-10
1 CO
-O-Fe-Clj--
1 ^z.503
-.239=*3996£-i2
.23861545E-12
.12692916E-13
1 MzC^ .30462106E-18
-0—6^9, ,.93733763£-l
I KJi.05 . 22008931E-22
-t-42C4e .-3835012 IE-?
1 COS .25350653E-31
*iL2A«».9fiiE^.O-3_
.2<*096306E-0<»
.968^8«f26E-05
.219701«*OE-05
.96136215E-03
0
0—Pr
0
-D-
o C\£
_D_
0 HL504 .13068832E-09
; « 55-97.63 <* 2£ -.10_.
0 HUOs .36^76755E-10
0 COS"' .18259921E-10
.D (i.?>SCb>_*JJ-'*.a
0 5z .69937733E-13
-^k-
-T.QJ.AL GASEOUS-
-i*4.a357-'
0 P^
_J.OT.AI GA.SEOUS-
.32106879E-35
153
-------
0,7, 2%
I At
*.
I6OO
c
vi-:c TOP
CL
EQUILIBRIUM
CONCENTRATION
(MOLES)
-06
.? 5? 63 77 fit -Oh
.1302493^^-0^
.93'i63647F-'07
.SlOUOOOOt-07
* A//
4 A//
4
4
4
"4
MO
\J
NO*'
.25000000E-07
.1937hnoO£-07
.1 9000000FT-07
.1 H7SOOOOE.-07
* A//
TOTAL CONDtMStn
.1 OOOOOOOF-OH
.1OOOOOOOE-OH
A/_06_
*• /I/A /V ,5 £<£• 3 8 3 7 0 313 E- 0 6
Jl-Xft //<3. v ? 7 6 0 0 0 0 OE - 0 6
lo-t 5bJ.21225000E-06
.53812500E-07
_5 1 5 oog o_q R>O 7^
.39250000E-07
07
p Z
07
.19375000E
1 o o E
.18750000E-
._17_625p_OOE-
.i27500bOE'
. 1250'ogqOEj
."ii"'
0 7
O 7
CONDENSED
.A/0. .11.I.1J
/y/T/VOjX.iooo;
.28P.30703E-05
.72<+9c:921E*-01
•_6_3999558E_*_qi
".ib'oboo'oo'E + oi"
.1000COOOE+31
i AJ-2.O
JL/tALGL^.
0 //2.
0 rf/O
.66930511E-06
.t4_7750700E-07_
."l«t 01761 BE-08
.55903577E-11
.33308B53E-13
,392510'»7E-19
.•5"5423i?F*0?TQTAL_GAsEOyi
.98239301E-32
• 59 52^_5 IJiEj- 0 2_
154
-------
Fuel Oil t 2 °/o
Air . I At*
650
ION
(MOLE.S)
_ _ t';:0.*
" Sz * 7^A0106BE-06"
4 /VA //$•*-• 37*000001>06_
" "' 3 '
i ^3 -
4
14
i*
vo
M1^'
TOTAL CONbEMSh11
.si son uo oh.-07
,!H75000nt-o7
.l?SOOOOnL-07"
.1 1 73u37SK-()7
.117 197bOt -07
. 1 nooocoot-o«
500 K
FOUILIHHIUM
CONCEUTPAT ION
0 fa K50if'^^47l3ooofr-o6
,5 2. ,rt?367 Ib^b -06
d '.fi
Ma AJO
NOLI C6
4 A/«^3^
0 ///<^
._y __•
4
4
4
4
4
4
* A//
TOTAL COMDE'NSED "
Vb
ff 0
I ^?bGOOt-06
fc-07
P1H7SOOOE-07
?trO()OOOOE-()7
lri7SOOOOfc-07
,1 1734375h-07
.117lrt7SGE-07
•h^SOOOOOb-OH
Oz
1 ^
TOTAL
*01
.inoonoooE+oi
. loohn'oOoE'+o'i"
. 1 o o o n o o o K + o 1
.H09?707Rfr-10
"-1 0
do
J0^
COS
MO*
HM03
A/2 ^)
4
2.
C?
. 1 0 0 0 n 0 0 0 K + 0 1
". 1'0'000"OOGE + OV
.ionoooooE*ol
l^n<*47t -0?.
UK-07
.141 4hb3^E-c;()
TOTAL (i
155
-------
10 % £>c w A<
EQUILIBRIUM
C CONCf.'NTKATION
VECTOR (MOLL'?)
!ZO .3
0 _ J(liO .51000000E-07
,?t>0 0000 Ot-0 7
o^b'OOOGOOF. -07
.19000000E-07
T *^
4 v
'4
_ .1 oo oo no OF;-nft"
4 ///Y0//A. . i on oo oo ot-oft
TOTAL COMfJt'MSUn „ 27^ 1 3 1 n7d-0b
0
n
0 i t. —
0 " ^Oi. . 1 01 5S366f' + 0 1
0 ^,0 .10000000t-«-Ol
0"" frr- . TOO 0000 OE+ 01
o ft e, . i o o o o o c o t: + o i
o MO
o
0
0
0
0
0 'ftzO" . 3A74b300t-OS
0 //^5^ . 157 3ft 44 «K-10'
, ' .360h09fa]f-.'-13"
'^ ^
0
TOTAL GASEOUS x ., ft 37 1 7 1 4 Ht+0 ?
156
-------
O\\) 10% Eiders Air I
QOOK
C
_V.E CI O.R.
EQUILI3RIUH
CONCENTRATION
(MOLES)
,7620630bE-06
5&I.718 G7J 31E-06
«» A/rt H SOf. .38370313E-06
3760 000 OE-06.
212250UOE-06
1 <* 2 5 1 5 6 3 cMJ 6
53818500E-07
«3 15 O 0 QOE-Q7
.39250OOOE-07
.2625000 0 E- P 7_
.19375000E-07
.190000DOE-07
.18750000C-07
Q E-Q7
///•'
.12750000^-07
. 12 5_0_Q_0 0 P E.^07_
TOTAL CONDENSED
« 1 1 7 1 aj; 5 Q c^p_7_
.1000000 OE-08
. 26896727c>05
0
0
0
2
0
1
TOTAL GASEOUS
-at
f
fi
t/L
A/
C H
Hi O .63999<»7i>E*01
. 10^297^2^+01
,10000000c>01
.10000000E+01
_._1QPJQQ_O..QQE*.01_
.55877658E-02
.B7078213E-03
.39612055£-0ii
H4- .26115257E-Q1*
.33971930E-05
ffy.O .32860917E-08
fffifO 3.23 8 ft 9 6 3 0 E H3 3_
//, .257i9345E-il
CO .5865<*906E-12
_5 0 3_._161190 11E -.13.
T _ ,_8. 2_7 3 .t» 3 6 9 L^2_
.
-------
Fuel O;/J 10% Excess /4/r/ /A+n , 6SO
C CONCtNTPAT ION
VtCTO" (MOLES)
0 Afo
t> (2.
"
0 j\]j $0^ .S1SOOOOOE-07"
n - «34?5ooone -o7
7. .1Q375000K-07
\/ 0 .I9oonnont-o7
^ .lS7SOOOt.-07
'
4
4' / .] !7lH7bOE-07
4
'TOTAL co'^
n
0 - .7?49<^71 IF. +01
n _ -
o" */£ .1 nonooooF-t-o
0 fj£ . loooooont+ni
o /?r . 1 » o o o o o o E + o
0
o
0
0
0 CO .
1 A/7 05- -^7U
GAStOUS . b37 OhPB 1 1 +0 ?
158
-------
/A+K,
iUM
f. "" CONCtNTkAT ION
VECTOP (MOLES)
o No. HSC% .9ft4750onr-oft~
:-o6
F-n6"
,37r,00000t-06
^A^ /i/^_ .32*M7500K-Oh
!-Ob
0 M; c/l,/~7:i9^9g9i/4|-I-"o7~
IV / o(-"r .
4 Ni^Q,)-^ .P6o00000f-07"
4 M . 1 87"50 OOnF -0 7
4 VO . 14b6250 nFl-07
'* Nll'O . I27'b0000t-07"
11 7 34 37 St. -07
1 171H750F.-07"
4 flj j .6^^0000Ot-OR
TOTAL" coN'OtNsED ".i7s?i'68S>.-o5"
0 /Vt .46bOBOOOF>0?"
n C_0t,
0 H7-O
n Q-^ . 104 ?6i 3K +o i
n' /V^ .iooon'ooot>oT'
0 H£ .1 0000000h+01
o Ar '
0 "5A, .1 So 30 6191--03
l"C
0
0 A/0 ,1U1^770E-07-
0
-------
Fuel Oil -2% Lxcess Air. t A+
y»
• J
TOTAL
c
VfiCTOW
CONCEMPAiTION
(MOLtS)
o
A/a.
C
t 0
u 5c
Ala OH
15S47bb3fc"-06
.?6000000t-07
4 Al/0
.18750000E-07
.17093750E-07"
".69785156E-08
4 Mi
0 f
0 d
0 ^
0 fs/
0
0 " He
0 CO
*•* A/J.
0 50i2.
n
.10000000E-OR
.inoooooof- -08
.1 OOOOOOOfT-Ofl
.409B790SE-I-0?
1 OOOOOOOL-t-01
1 00060GOF>01
. 1
" NO*
0
5-
.94Q90355F.-10
h -13"
.1 H471443F-1 3
0 A
TOTAL GAStous ' ^
,S764733?fc'*0?
160
-------
FUELOIL*fe
excess air
C
VECTOW
EQUlLTBP'i
CONCENTHAT
"(MOLLS)
UM
ION
TOTAL GASEOUS
K , (MOLLS)
5l 0.2=^. •J?." 1.3 P.7 3 8 E_- 0 3__
Offh-' ^° '^ ^ 7 ^ £~~°'*
50781250F-0_6
—0 S
.iooonooot-08
f, _._looj)noonF-o6_
rfT .iToo'obooE-dfe
looonoont-08
• 1 onooooot -08
OOOOOOOE-08
TOTAL CONDENSED
161
.1 7745761E-0?
-------
FUELOIL%
\07oexcess Qir
800 K
"EQUILIBRIUM"
CONCENTRATION
TflOLTST
TO'TAL GASEOUS
.46322866E-0~3
• 2 33 62 12 6 E - 0 3
07W6b-04
,3b952507E-04
-78151250E
.95937500E
.89484 766E
084445726E
;T3T8T257IE
.78137500E
_
TT»T66T3~6bE
.65Q7B125E
•60238991E
."S^b'BTOTeE
.5653RP27E-
75"5F3TU55r=
.54630029E-
?:** fi'2"SO'OE=
S 08 06 25 OE -0 6
. 37 1 875OE-6
Q .30449 2 l_g E - 0 6_
TOTAL CONDENSED
162
- 0 6
z_. o o on o o o t - o 6
o ° °. ° 0_°J - o.8
. 1 000000 OE-OH
1 ftl 04020E-02
-------
FUELC1LH
107o
excess air
K
.2rs>5309b>:
.81KKibb'QK
4
4
,_664
Ci^Oa .294S909bf
— _._15.349.7.77_E
.148713T3E;
.781_51250K_
"79TlT9b"3Tt~
,95937bOOE
WA
1°.3_
•03
•04
•04
'J?.4_
•~0~4
•05
~0^~
-OiS
""EHUlUih^l'UM
CONCENTK^TION
4
,91601bfa3E-0(S
(MOLES)"
.I4-3-9.1? 9JL4 F- ±P_?_
.708~99689'EV01
A.^19_18 849E-I-01
.95267617E+00
.7
-------
FUEL OIL
|0%€
EQUILIBRIUM
CONCENTRATION
(MOLES)
.6162448HE-03
Qir
500 K
0 Ti"
o29459095E-04
.14871313E-Q4
~. i4403b73t.-04
.78151250E-05
,98304b50E-0|C)
EOUILIhMIUM
CONCENTKATION
VECTOH
TOTAL GASEOUS
(MOLES)
.43919994E+02
.70899245E+01
.89484766E-06
.66328125E-06
.B4612500E-06
.81017b41E-OfS
78~JJ2~6060E-01
,_21483515E-02
. 6~9Tn?bB9~<7F=0~4~
.26332044E-05
A^L
.78137500E-OIS
.64361689F.-07
±80219b93E-09
.l"946"l9"OTF-iT
,3t5228454E-14
,5576tt824E-21
7?T7?o?3TE^.?r'
^52037133E-23
7ST5"6"444"3r+0"2~
.78125000E-06
.72977515E-06
.70320359E-06
,69«31445E-Oft
.65078125E-06_
76T4^2bTTOT^"06
,ft297A563E-06
'25~315"?T)"9'E^'0"fir~
.507B7500E-06
,S078l?50E-n^
TOTAL CONDENSED
164
.50690466E-06
73 0 4' 4~9 2 F9 K - (T
-------
U OIL #1*
xcess Q/r
) K
C
VECTOW
o /W
0 Lj Q
o r>i
0 5
" vo
*» /^ f
*• ^/ 1 (^ n) z
** Jji60^
"•Cfts^-Q:?
J ^An
* Til
«• V
<» e.
''Wo? 50>s
J Tip
Jc cn^
:d^a
tafcrtb-
J^zO
*» Aj^.SO-f
H^^H'
** ^?v
^
*i Mj f A/0
^V/^'
•• CnCfwl
EQUILIBRIUM
"TDNCENTR'ATION"
(MOLES)
.58130738E-03
.*46385290E-03
.233585f»8r-03
; .21953696E-03
.dTOSTTSCCHOTT
3.300'43799E-0'»
. 1«»871313E-0'»
,lt»320763E-0«»
.78151250E-05
i .99767920E-06
.97971692E-06
.9711.9531E-06
.95937500E-06
.9««770996E-06
.93638867E-06
.91601563E-06
.91117975E-06
.90820313E-06
.86328125E-06
.8U385353E-06
.83366Qi»2E-06
1 .78181250E-06
.78137500E-06
.78125000E-06
.7<*U05005E-06
L .66<*27930E-06
.6507B125E-06
.61377197E-06
St .60318153E-06
.58978516E-06
.5U630029E-06
; .52187500E-06
| .5080625QE-06
^7.50 793750E-06 .
.507R1250E-06
,212<4255£«E-06
!d.21003906E-06
)1.10000000E-OB
2_ .10000000E-08
gj2o°0°S22SE-008
I3Z.10000000E-08
)t..10000000E-08
),. 1000000 OE -08
TOTAL CONDENSED
7r77¥576"2E^O"?~
-------
ruELoim
-2%
800 K
EQUILIBHIUM
CONCENTRATION
VECTOR
o.di'.0_
0&.Q
o Ffi
V
*
Z-54
(MOLES)
.581Q3B65E-03
~.46~3724B4E-03
-l^lPA8 S48E-03
,21953~464E-03
.82061434E-04
,6bS15371E-04
CtfuO T3W43T9ST-b"4~
i"S~Tr4T36"9fiTr-6"4~
.78151250E-05
EQUlUlBHIUM
CONCENTRATION
4
"4~/
4
.96949884K-06
795?3Y5'(fOE^r
VECTOR
0
GO-2.
(MOLES)
.3 91 27 99 IE+02
".•6T5BOT9TETOT
4
4
79"4TTO^T6"F^O"(
93638867E-06
.91602344E-06
.91297302E-06
0
0
«875879?8E-Ol
,72493T9"5F='0"T
T87030476E-6T
o3799Q5B4F-Q5
"72^622¥riT^O"6r
.10143804E.-0&
T4T^'4742Cyt:-OT"
o2l007064E-07
"7T585"0~95r9E^n~
.53S74300E-12
4
4
.86328125E-06
TBTT8"5^r2 E^6"5~
_»_83 504861 Ej-0 6_
T8"2"898"92"9^"06~"
4 T/' 0
-T-;
TOTAL GASEOUS
. 2^297373F-28
7519084
.78137500E-06
T7"8T2irFO"Ot"^Tr6~
/^ .7793?4b8E-Q(S
' . 7"6301T62"OT^O6~
76"9"2'61T4T5E^Oi5~
.6fe427930E-Qfi
o 65078 I"2?E-06
.63 4_8_7 5 p__0 E -06
.6r869900r-06
°^).C).22/*075El06
;5"H9T8"5i"6T-66
'."52T8'7~SO"0(T-Ob~
e!50806250E-06_
".507~9375bE'-o"6
.50781250E-06
.10QOnOOOF-08
, 1000'odOOfT^OH
TOTAL CONDENSED
166
W 00 -rl ° ° °" °6 "E-~'° 8
A/d/Upy. lOOOOOOOE-08
OaC. i o o~o oinrfi E^DIT
,roo"o6oOor-"(Ta~
.1 7743U1E-02
-------
— L_ Wl L. \J
> excess air
)OK
C
VECTOR
o 4k ft-
o fe2 O
0 V/a.O-5
0 AA<} 5Oi,
o ISJ i' TOg
EQUILIBRIUM
CONCENTRATION
(MOLES)
.=>162<»4. 294590 95E-01*
C
VECTOH.
o MT.
o Cfi-z.
o Ha.0
0 <">-7
o 603
0 ^flz:
i NO
1 C«*
0 A/0Z.
i Si
t/fteS,
i H» 504'
i AtOr
i COS
TOTAL GASEOUS
CONCENTRATION
(MOLES)
,
it Ti
«* Co3 04
it C.
-------
FUEL OIL *k
(.50 K
EHUILIHPJUM
CONCENTRATION
VECTOW
(MOLES)
.40725 99 5E+02
rn)8~99~69"?E+Ol
.55914658E+Q1
7T5501 7.l2F+6~Cr
075672226E-01
.13219644E-02
.1308564 7E-05
7534~HB21 IE-Ob
.6245337HE-06
I3"5T)6L-Ot>
70644301E-10
3"3676WOE~-M~T
.74360756E-12
TO~TAL GASEOUS
.85643265E-16
.1 78T9T48E-19
.54655465E-24
"75"36"4T9"53
tUUILIBWIUM
CONCENTRATION
(MOLES)
.6_162 4 4 88 E_-^0^_
.4659"l243E-03
.21953896E-03
.81106559E-04
0/^^f!5CX| .71183593E-04
0 A/I 5Qti »664Q7096E-04
n1!
-------
ma OIL
2%excess air
500 K
CONCENTRATION
VECTOR.
(MOLES)
o
o
tr
o
/)
,7089y865E*01
.bbr647l4T + OT~
.15435673E+00
.?0028144E-02
.26332044E-05
.T4r3T8-77<7E-'06
.28B43668E-07
.66894965E-10
o /v?O
1 C
TOTAL GASEOUS
.133646b6E-20
.12476"39'~3E^2T
,S357229bE-»-02
C.
CONCENTRATION
VECTOR
(MOLES)
•6)^?.4_4f1^.-°JL
V4659F24~3E-o3
.21965019F-03
.RUnh5b9F:-0*
0 Co. 504 .29459095F.-04
" ' "a._'_15tlIl313ErfL4_
£0\A . l"4"403"573"E-04
* ,7H15l2bOE-05
4
.96304bbOK.-06
^ Nkagflft'94995117E_-_06_
4 C^s 2".9160l5b3E-06
M»-
V
. 9082031 3E-06
;Try-4B4~7"bbE^6~6~
. 86328125E-06
4 Co» C>4 .ff^crnrscroEXIT'S"
4 \Jr\ .81017841E-06
-^ -——-"—)r=IT6~
7&'24-3-9-209-E^O~?5-
.6HOOOOOE-06
75-0238-99-Tr-OTr-
.50787500F.-06
.507812SOF.:-06
.5.0081152E-06
169
0 •
-------
TECHNICAL REPORT DATA
(Please read Inaructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-77-041
2.
3. RECIPIENT'S ACCESSION- NO.
4. TITLE ANDSUBTITLE
A Survey of Sulfate, Nitrate, and Acid Aerosol
Emissions and Their Control
5. REPORT DATE
April 1977
6. PERFORMING ORGANIZATION CODE
7 AUTHOR(S)J F Kircher,A. A. Putnam,D.A.Bali,H. H.
Krause, J. M. Genco,R. W.Coutant,J.O. L.Wendt, and
A. Levy
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Battelle-Columbus Laboratories
505 King Avenue
Columbus, Ohio 43201
10. PROGRAM ELEMENT NO.
EHE624a
11. CONTRACT/GRANT NO.
68-02-1323, Task 49
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 3-12/76
14. SPONSORING AGENCY CODE
EPA/600/13
is. SUPPLEMENTARY NOTES T£RL-RTP Task Officer for this report is W.S.Lanier, Mail Drop
65, 919/549-8411 Ext 2432.
is. ABSTRACT
report gives results of an evaluation of the effects of fuel and combustion
modifications on the formation of primary acid aerosols (used broadly to include all
sulfates, nitrates, chlorides, and fluorides in all their forms) and their significance
as combustion-generated pollutants from large stationary sources. Primary acid aer-
osols are emitted directly from a source or formed (primarily by condensation reac-
tants) in the immediate vicinity (0. 5 mile); secondary aerosols, formed downstream in
the plume, are not considered. Available, rather meager field data were collected and
interpreted in view of current knowledge of mechanisms of formation of potential acid
aerosols and their precursors. Although sulfates, nitrates, chlorides, and fluorides
were considered, based on available data, only sulfates appear to be significant as
primary acid aerosols. All of the various combustion modifications for NOx control
are expected to have little effect on primary acid aerosol emissions , except perhaps
firing with low excess air which has a potential to abate both NOx and acid aerosol
emissions. Combustion modifications and fuel changes may lead to an increased for-
mation of small particles which could increase the formation of acid aerosols through
heterogeneous reactions. Most effects, however, are speculative due to the meager
data available. Information gaps have been identified; further research is indicated.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATl Field/Group
Air Pollution
Aerosols
Fuels
Combustion
Boilers
Furnaces
Coal
Fuel Oil
Sulfates
Chlorides
Fluorides
Nitrogen Oxides
Air Pollution Control
Stationary Sources
Acid Aerosols
Combustion Modification
No. 6 Oil
Nitrates
13B
07D
2 ID
21B
ISA
07B
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
177
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
170
------- |