EPA-650/2-74-114
OCTOBER 1974
Environmental Protection Technology Series
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EPA-650/2-74-114
CONDITIONING
OF FLY ASH
WITH SULFAMIC ACID,
AMMONIUM SUIFATE,
AND AMMONIUM BISULFATE
by
Edward B. Dismukes
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
Contract No. 68-02-1303
ROAP No. 21ADJ-029
Program Element No. 1AB012
EPA Project Officer: Leslie E. Sparks
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
October 1974
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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ABSTRACT
The report summarizes recent experience with three agents—
sulfamic acid, ammonium sulfate, and ammonium bisulfate—used
to regulate the electrical resistivity of fly ash in electric
generating stations to ensure satisfactory collection of fly
ash in electrostatic precipitators (ESPs). It presents infor-
mation about the effectiveness of these agents in pilot- and
full-scale ESPs. It also presents the limited information
available from practical trials of these agents concerning
their conditioning mechanisms. It discusses in detail the
fundamental physical and chemical properties of the agents
that are relevant to fly-ash conditioning. From this informa-
tion and the results of ESP tests/ the report offers tentative
conclusions about conditioning mechanisms. Finally, the
report briefly discusses the economic aspects of using each
of the agents as a conditioning substitute for sulfur triox-
ide.
This report was submitted in partial fulfillment of the
research performed by Southern Research Institute under Con-
tract 68-02-1303 with the Environmental Protection Agency.
The work discussed was completed in June 1974.
111
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CONTENTS
Page
Abstract iii
List of Tables vi
Acknowledgments viii
Sections
I Conclusions 1
II Recommendations 2
III Introduction 4
IV Technical Background 7
V Precipitator Tests of Sulfamic Acid as a Condi- 11
tioning Agent
VI Precipitator Tests as Ammonium Sulfate and 20
Ammonium Bisulfate as Conditioning Agents
VII Fundamental Properties and Conditioning Mech- 23
anisms of Sulfamic Acid
VIII Fundamental Properties and Conditioning Mech- 31
anisms of Ammonium Sulfate and Ammonium
Bisulfate
IX Economic Aspects of Sulfamic Acid, Ammonium 41
Sulfate, and Ammonium Bisulfate as Condition-
ing Agents
X References 44
XI Appendix. Composition of a Commercial Condi- 49
tioning Agent Based on Sulfamic Acid
v
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TABLES
Page
1. Results of Precipitator Tests at Cabin Creek 14
Station with Sulfamic Acid as a Flue-Gas
Additive
2. Results of Fly-Ash Analyses at Cabin Creek 16
Station with and without Sulfamic Acid as a
Flue-Gas Additive
3. Results of Gas Analyses at Cabin Creek Station 17
with and without Sulfamic Acid as a Flue-Gas
Additive
4. Results of Precipitator Tests at Mercer Station 18
with Sulfamic Acid as a Flue-Gas Additive
5. Results of Fly-Ash Analysis at Station A 19
6. Results of Precipitator Tests by the Koppers 21
Company with Ammonium Sulfate or Ammonium
Bisulfate as the Conditioning Agent
7. Results of a Volatilization Study of a Commer- 27
cial Form of Sulfamic Acid
8. Equilibrium Partial Pressures of Ammonia in 33
Equilibrium with Ammonium Sulfate and Ammo-
nium Bisulfate at Various Temperatures
9. Equations for the Equilibrium Constants Asso- 35
ciated with the Decomposition Reactions of
Ammonium Sulfate and Ammonium Bisulfate
10. Equilibrium Partial Pressures of Ammonia and 35
Sulfuric Acid in Equilibrium with Ammonium
Salts at Various Temperatures
11. Weight Losses by Volatilization of Ammonium 37
Sulfate or Ammonium Bisulfate at a Heating
Rate of 4.0°C/Min
12. Relative Costs of Sulfur Trioxide, Sulfamic 41
Acid, and Related Conditioning Agents
VI
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TABLES (Concluded)
No.
13. Relative Costs of Conditioning by Sulfur 42
Trioxide and a Commercial Form of Sulfamic
Acid
vii
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ACKNOWLEDGMENT S
Grateful acknowledgment is made of the assistance rendered by
various industrial firms in providing some of the information
included in this report. These companies are as follows:
• Apollo Chemical Corporation
• American Electric Power Service Corporation
• Buell Division of Envirotech Corporation
• Environmental Elements Corporation (a subsidiary
of Koppers Company)
Gratitude is also expressed for private communications from
investigators at the Central Electricity Research Labora-
tories in Great Britain (W. D. Halstead and J. Dalmon), which
aided the interpretation of publications from that institu-
tion.
viii
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SECTION I
CONCLUSIONS
On the basis of the results of precipitator tests discussed
in this report, the three conditioning agents reviewed—sul-
famic acid, ammonium sulfate, and ammonium bisulfate—appear
to be acceptable substitutes for sulfur trioxide if consid-
ered only on the basis of their effectiveness in conditioning
high-resistivity fly ash. The three agents offer advantages
over sulfur trioxide in the ease and freedom from hazard in
handling. Ammonium bisulfate, however, does not appear to be
worthy of serious consideration as a conditioning agent in
the United States because it is apparently not an inexpensive
standard commodity. Sulfamic acid seems to be competitive in
cost with sulfur trioxide only under limited conditions.
Ammonium sulfate appears to be the one of the three agents
that is most nearly competitive economically with sulfur tri-
oxide.
Confidential information from industrial sources other than
those named in this report makes it desirable to exercise
caution in the selection of sulfamic acid or ammonium sulfate
over sulfur trioxide as a conditioning agent. Precipitator
tests with these agents have not been uniformly successful.
As a matter of record, tests with sulfur trioxide have not
been uniformly successful either. However, the comparatively
limited industrial experience with the newer agents is a fac-
tor that should lead to a greater degree of caution in the
selection of one of these compounds.
The mechanisms of fly-ash conditioning by sulfamic acid, ammo-
nium sulfate, and ammonium bisulfate include the desired
effectiveness in lowering the electrical resistivity of fly
ash from low-sulfur coal. However, the physical and chemical
processes by which the lowering of resistivity occurs cannot
be conclusively identified. The process of conditioning
undoubtedly varies as the type of agent, the injection temper-
ature, and the precipitator temperature vary. Even so, the
chemical conversion of sulfamic acid or ammonium sulfate to
liquid ammonium bisulfate and the coating of fly-ash particles
with this conductive substance appear to be important pro-
cesses in certain instances. The same processes may increase
the cohesiveness of fly ash from moderate- to high-sulfur coal
and thus aid in coping with the excessive reentrainment of the
low-resistivity ash from these fuels.
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SECTION II
RECOMMENDATIONS
The research described in this report was limited to a survey
of existing knowledge on fly-ash conditioning with sulfamic
acid, ammonium sulfate, and ammonium bisulfate. It did not
include any experimental work toward evaluating the effective-
ness of these conditioning agents or elucidating the mecha-
nisms by which conditioning occurs. From an academic point
of view/ experimental work in these areas would be desirable.
However, from a more practical point of view, the value of
further work in these areas must be weighed against the needs
for continuing research on other aspects of conditioning
including the better known conditioning agents, sulfur triox-
ide and ammonia.
With consideration having been given to present needs for
additional research on the use of conditioning agents, the
following recommendations are offered:
• Continuing efforts should be made by the Environ-
mental Protection Agency through its contractors
to compile and evaluate new information from the
electric power industry on novel conditioning
agents including sulfamic acid, ammonium sulfate,
and ammonium bisulfate. These efforts should
follow the approach described in this report,
including an assessment of the practical value
of the conditioning agents in specific power
plants and an assessment of their value on an
industry-wide basis. Specifically, an effort
should be made to determine the mechanisms by
which conditioning occurs and the range of power-
plant conditions where the operation of these
conditioning mechanisms will be helpful.
• Experimental work should be undertaken at some
future time to resolve questions unanswered by
utility trials of novel conditioning agents, in
the event that the practical importance of these
agents reaches a higher level than that attained
thus far.
• Experimental work should be undertaken in the
immediate future to resolve unanswered questions
about the role in fly-ash conditioning by the
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more conventional compounds, sulfur trioxide and
ammonia. A broad discussion of new work in this
area will be incorporated in a summary report on
sulfur trioxide and ammonia conditioning now being
prepared for the Environmental Protection Agency
by Southern Research Institute. However, an in-
sight to some of the new work needed is given by
these questions: (1) Is ammonia an acceptable
substitute for sulfur trioxide in treating high-
resistivity fly ash from low-sulfur Western coals?
(If so, it offers the distinct advantages of lower
cost and less difficulty in handling.) (2) Does
sulfur trioxide conditioning cause objectionable
stack emissions of excess conditioning agent as
noxious sulfuric acid mist or sulfate-containing
fine particles of fly ash? If so, what are the
circumstances under which stack emissions are
large enough to be of serious consequence? (The
claim is sometimes made that no stack emission of
sulfur trioxide vapor occurs, because the fly ash
being treated removes all of the material from
the flue gas. This claim, however, is disputed
by unpublished data recently obtained by Southern
Research Institute.) (3) How can sulfur trioxide
conditioning be made more generally successful?
How can the injection system be designed to avoid
the loss of vapor by condensation to an acid mist,
which is ineffective for ash conditioning? (There
is evidence that deficient designs of injection
systems account for some unsuccessful trials of
sulfur trioxide conditioning.)
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SECTION III
INTRODUCTION
The prevention of excessive emissions of fly ash from coal-
burning electric power stations is one of the major current
problems in controlling air pollution. This task may be
accomplished by the use of high efficiency electrostatic pre-
cipitators, liquid scrubbers, and fabric filters. For many
years, however, the control of fly-ash emissions has depended
mainly on the use of electrostatic precipitators. Currently,
the importance of electrostatic precipitators for gas clean-
ing is continuing; at the same time, the effectiveness of
precipitators is being severely challenged by increasingly
stringent requirements for the control of fine particles,
which are relatively difficult to collect by electrostatic
precipitation.
EFFECT OF THE ELECTRICAL RESISTIVITY OF FLY ASH ON THE PER-
FORMANCE OF AN ELECTROSTATIC PRECIPITATOR
The electrical resistivity of fly ash is one of the most
important parameters that control the efficiency of fly-ash
removal in coal-burning power stations by the use of electro-
static precipitators. If the resistivity is too high, as it
is normally when a low-sulfur coal is burned, the collection
efficiency is poor because the electric field between the
corona wires and the collecting electrodes falls to a low
value, excessive sparking occurs in the interelectrode space,
or back corona occurs in the deposited fly ash. Alternatively,
if the resistivity is too low, as it may be when a high-sulfur
coal is burned, collection efficiency can again be poor
because the electrical force holding deposited ash on the
collecting electrodes is not high enough to prevent excessive
reentrainment during rapping of the electrodes. The upper
limit of the acceptable resistivity range is approximately
1 x 1010 ohm cm.1/2 The lower limit has been defined as
values ranging from 1 x 107 to 1 x 10* ohm cm.2'3
The problem of excessively high electrical resistivity is a
more widely occurring problem than that of excessively low
resistivity. The problem of high resistivity is becoming
increasingly important as the electric utility industry is
turning to the use of low-sulfur coal to meet limitations on
the emission of sulfur dioxide. The burning of low-sulfur
coal lowers the production of sulfur dioxide as desired, but
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it also lowers the production of sulfur trioxide or its chemi-
cal equivalent, sulfuric acid, with unfavorable consequences
on the performance of an electrostatic precipitator in the
normal range of operating temperatures, around 150°C.
The conduction of electricity through fly ash at normal pre-
cipitator temperatures, around 150°C, occurs primarily through
adsorbed gases such as water vapor and sulfur trioxide (or
sulfuric acid) that are present on the surfaces of individual
fly-ash particles. If the availability of sulfur trioxide is
lowered as the result of the combustion of low-sulfur coal,
the electrical resistivity of the fly ash will be raised.
Maintaining a sufficiently low electrical resistivity is
important without regard to the size of fly-ash particles
being collected. However, it is extremely important in limit-
ing the emission of small particles, having diameters less
than 2 ym. To charge small particles effectively, high cur-
rent densities in a precipitator are essential, and they can
only be maintained effectively if the resistivity of the fly
ash is not excessive.
CONTROL OF ELECTRICAL RESISTIVITY PROBLEMS BY CHEMICAL CONDI-
TIONING
The technology referred to as "conditioning" provides a means
of coping with the high electrical resistivities found with
fly ash from low-sulfur coal. Usually, a conditioning agent
is used specifically for this purpose. In some instances,
however, a conditioning agent may be used to cope with
unusually low resistivities. In still other instances, more-
over, the phenomenon produced by a conditioning agent may not
involve the resistivity to any degree. As pointed out subse-
quently, a conditioning agent may affect the cohesiveness of
fly-ash particles rather than the resistivity of the parti-
cles, and it may also produce a space-charge effect in which
there is a change in the electrical properties of the flue
gas rather than the fly ash. In this report, the term condi-
tioning is used to embrace the use of all chemical additives
for improving the collectibility of fly ash in an electro-
static precipitator, regardless of the mechanism of action of
the additives.
The best known method of conditioning fly ash is to inject
either sulfur trioxide or sulfuric acid vapor into the flue-
gas stream ahead of a precipitator.1"9 Injection of sulfur
trioxide or sulfuric acid vapor supplements the quantity of
this substance that is produced naturally, and the most likely
explanation of this conditioning method is that the flue-gas
additive lowers the resistivity of the fly ash.
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Ammonia is a conditioning agent that is sometimes used in
place of sulfur trioxide or sulfuric acid.10"12 Injection of
this compound may, under certain circumstances, lower the
resistivity of fly ash from low-sulfur coal. It appears
doubtful that this mechanism of conditioning by ammonia has
ever been conclusively demonstrated; certainly, all efforts
by Southern Research Institute to demonstrate this effect
have been unsuccessful.13 Even so, there is adequate evidence
that ammonia sometimes improves the collectibility of high-
resistivity ash by some mechanism. Injection of ammonia into
the flue gas from a high-sulfur coal may help in overcoming
the problems associated with low-resistivity fly ash. Mecha-
nisms of conditioning under these circumstances apparently do
not involve any significant change in resistivity, but they
do seem to involve an enhancement of the electric field in a
precipitator through a space-charge effect and an enhancement
of the forces of cohesion binding together individual fly-ash
particles, thus lowering reentrainment losses.13
Flue-gas additives in addition to sulfur trioxide and ammonia
that improve the electrostatic precipitation of various types
of particulate substances include organic amines, inorganic
acids, and inorganic salts.11*'15 Specific examples of com-
pounds in these categories are (1) ethyl, diethyl, and tri-
ethyl amines; (2) phosphoric and sulfamic acids; and
(3) ammonium sulfate, ammonium bisulfate, calcium chloride,
sodium chloride, and sodium sulfate. At present, there is'an
appreciable interest in the utility industry in three of
these compounds: sulfamic acid, ammonium sulfate, and ammo-
nium bisulfate. The following sections of this report give
the results of a survey to determine the utility of these
three compounds as flue-gas additives in competition with
sulfur trioxide or sulfuric acid.
Conditioning of fly ash may be accomplished not only by the
use of flue-gas additives but by the use of boiler additives.
Recent publications have indicated that the addition of com-*
pounds of sodium and iron to the coal fed to a boiler pro-
vides a practical basis for lowering the resistivity of the
fly ash produced.16/17 The greatest value of these boiler
additives may occur in power stations using so-called "hot"
precipitators, which collect fly ash from flue gas around 300
to 350°C. At these temperatures, the conduction of electric-
ity through fly ash takes place by ion migration through the
interior rather than along the surface of individual parti-
cles. There is the possibility, however, that boiler addi-
tives may be beneficial at lower precipitator temperatures
where surface rather than volume conduction is predominant.
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SECTION IV
TECHNICAL BACKGROUND
Much of the current interest in sulfamic acid, ammonium sul-
fate, and ammonium bisulfate as conditioning agents for fly
ash apparently originated with the thought that these com-
pounds would be conveniently handled sources of sulfur triox-
ide. Theoretically, these compounds can be thermally decom-
posed to sulfur trioxide in hot flue gas, as indicated by the
following chemical equations:
H2NS03H —» NH3 + SO3 (1)
(NHi»)2SOi, —» 2NH3 + H2O + S03 (2)
NH,»HSOi» —» NH3 + H20 + SO3 (3)
The handling of sulfur trioxide and its chemical equivalent,
sulfuric acid, is difficult in a power plant because both
compounds are highly corrosive and toxic liquids that must be
vaporized before injection into flue gas. The handling of
sulfamic acid, ammonium sulfate, or ammonium bisulfate, on
the other hand, is not a source of significant difficulty.
Both of these compounds exist as relatively noncorrosive and
nontoxic solids at ambient temperatures; they can be injected
into a flue-gas stream as a powdered solid or, alternatively,
they can be dissolved in water and injected as an aqueous
spray.
One trend of thought about the use of sulfamic acid, ammonium
sulfate, and ammonium bisulfate has been described by Lowe of
the Central Electricity Research Laboratories in Great
Britain.18 Lowe's publication indicates that interest was
initially focused on sulfamic acid in the belief that this
compound would serve as a convenient source of sulfur trioxide,
as shown in Equation 1. However, Lowe reports the discovery
that sulfur trioxide is only a minor product of the decomposi-
tion of sulfamic acid and that ammonium bisulfate is a far
more significant product. The decomposition process yielding
ammonium bisulfate is much more complex than that yielding
sulfur trioxide as shown by Equation 1. In any event, Lowe
indicates that promising results obtained with sulfamic acid
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led to direct trials of ammonium bisulfate as an alternative
conditioning agent and later to trials of ammonium sulfate.
This compound may be thermally decomposed to the bisulfate'as
shown by this chemical equation:
(NHOaSOi, —» NH3 + NH^HSOi, (4)
The decomposition of ammonium sulfate shown by Equation 4 is
obviously less extensive than that previously shown by Equa-
tion 2 and would be expected to occur at lower temperatures.
A rather different trend of thought led the Koppers Company
to investigate ammonium sulfate and ammonium bisulfate as con-
ditioning agents for fly ash.19 Prior to its consideration
of these two compounds, the Koppers Company had extensive
experience with ammonia as a conditioning agent.10 This
experience showed that ammonia was effective in conditioning
fly ash from moderate- to high-sulfur coal but rarely effec-
tive in conditioning fly ash from low-sulfur coal. The con-
clusion was that ammonia was effective only when naturally
produced sulfur trioxide was present at a sufficient concen-
tration to react with ammonia in the presence of water vapor
to produce either ammonium sulfate or ammonium bisulfate.
Either of these compounds could be formed by the reversal of
Equation 2 or 3 at sufficiently low flue-gas temperatures.
Interest in sulfamic acid, ammonium sulfate, and ammonium
bisulfate as conditioning agents has undoubtedly been stimu-
lated by a recent publication by Dalmon and Tidy of the
Central Electricity Research Laboratories in Great Britain l5
This paper compares these and other novel conditioning agents
with sulfur trioxide and ammonia. Dalmon and Tidy used a
laboratory-scale precipitator for investigating various types
of conditioning agents, which are classified below as to
physical state at ambient temperatures:
• Gaseous compounds—sulfur trioxide, hydrogen
chloride, and ammonia
• Liquid sulfuric acid (injected at a concentration
of 0.11 to 9.4 M in water)
• Solid compounds derived from sulfur trioxide and
ammonia—sulfamic acid, ammonium sulfate, and
ammonium bisulfate (injected as solids or solutes
in aqueous solutions at concentrations of 0.23 to
1.8 M)
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• Solid inorganic salts—sodium chloride, sodium
sulfate, calcium chloride, and lithium iodide
(injected as solutes in aqueous solutions at con-
centrations of 0.45 to 1.8 M)
The particulate conditioned with these agents was water-washed
fly ash from a coal-fired power station. The gas stream con-
taining the fly ash was produced by combustion of a low-sulfur
paraffin and adjusted in composition by the addition of water
vapor to a humidity level of about 8% by volume. The gas
stream was essentially free of sulfur dioxide except when
this gas was added deliberately to obtain a better approxima-
tion of the composition of flue gas from a low-sulfur coal.
Dalmon and Tidy reported that each additive to the gas stream
improved the efficiency of fly ash collection in the absence
of added sulfur dioxide, with the degree of improvement
usually being controlled by the efficiency of uptake of the
conditioning agent by the fly ash. They reported, however,
that only a limited number of these additives improved the
collection efficiency in the presence of added sulfur dioxide,
which itself caused a significant improvement if added at con-
centrations of 100 to 300 ppm by volume, as found in flue gas
from low-sulfur coal. The only additives effective in the
presence of sulfur dioxide—that is, under the conditions
better simulating a flue-gas environment—were sulfur triox-
ide, sulfuric acid, sulfamic acid, ammonium sulfate, ammonium
bisulfate, and ammonia. Among these compounds, ammonia was
effective only if hydrogen chloride (one of the normal con-
stituents of flue gas) was also added to the gas stream
investigated under laboratory conditions. (It is possible
that ammonia might have been effective if sulfur trioxide had
been added in place of hydrogen chloride but ammonia was not
evaluated with sulfur trioxide as a secondary additive.)
Successful trials of sulfamic acid and ammonium sulfate have
been made in full-scale power plants in both Great Britain
and the United States. The trials in Great Britain have been
made under the auspices of the Central Electricity Generating
Board, which was responsible for the previously described
work of Dalmon and Tidy15 and Lowe.18 The trials in the
United States have been made by several utility companies
with the aid of two companies supplying the two conditioning
agents: (1) Apollo Chemical Corporation, which supplies a
proprietary blend of sulfamic acid with other chemicals that
is known as "Coaltrol PPA-30" (a powder) or "Coaltrol LPA-40"
(an aqueous solution); and (2) Environmental Elements Corpora-
tion (subsidiary of the Koppers Company), which supplies
ammonium sulfate as the product "Koppers K." There appears
to have been no evaluation of ammonium bisulfate in any full-
9
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scale power plant either abroad or in this country, perhaps
because of the greater cost of this chemical and the lack of
availability of the chemical as a commercial item.
In this report, sulfamic acid is regarded as the essential
constituent of the agents supplied by Apollo Chemical Corpora-
tion. This compound is clearly the primary constituent in
terms of weight percentage, as described in the Appendix of
this report. The minor constituents may play some role in
the conditioning of fly ash, but they have not been character-
ized sufficiently to warrant any speculation about their role
in conditioning.
Insofar as Koppers K is concerned, there is no reason for
believing that the conditioning agent contains any active com-
pound except ammonium sulfate. Presumably, any other com-
pounds present are normal impurities in the commercial grade
of ammonium sulfate.
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SECTION V
PRECIPITATOR TESTS OF SULFAMIC ACID AS
A CONDITIONING AGENT
PILOT-PLANT TESTS
A brief discussion of Damon and Tidy's pilot-scale precipita-
tor tests including sulfamic acid and other conditioning
agents15 has already been given in Section IV. A more com-
plete discussion of those tests involving sulfamic acid is
given in the following paragraphs.
In the pilot-plant precipitator tests performed by Dalmon and
Tidy/ sulfamic acid was injected upstream from the precipita-
tor at a temperature of 400 or 165°C. At the higher tempera-
ture, the agent was injected either as a powder or as an
aqueous solution (0.45 M) ; at the lower temperature, the agent
was injected only in the aqueous form. The conditioned fly
ash was then collected in the precipitator at a temperature
of 145°C.
Dalmon and Tidy made determinations and calculations of the
following:
• Precipitator efficiency
• Precipitation rate parameter wp (cm/sec)
wp = -lnl-E
where E = precipitator efficiency
k = 0.315 sec/cm (a constant based on the precipi
tator geometry and the flow rate of the gas
stream)
• Uptake of sulfamic acid by the fly ash
• Sparkover voltage of the precipitator
• Electrical resistivity of the fly ash at the
precipitated bulk density and at a constant bulk
density of 0.975 g/cm3
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The investigators found that injection of sulfamic acid in
solution was much more effective than injection of the agent
as a solid. Using the solution method of injection, they
found that the uptake of about 0.8% by weight produced the
following desirable changes:
• Precipitation rate parameter—increased from
9.5 to 14 cm/sec
• Sparkover voltage—increased from 42 to 49 kV
• Resistivity—lowered from about 1 x 10llf to
1 x 1013 ohm cm at a bulk density of 0.975 g/cm3
Uptake of sulfamic acid by the fly ash was judged on the
basis of the quantity of soluble sulfate ion found in the
conditioned ash. The analysis of soluble constituents of the
ash indicated that the sulfamic acid was decomposed to a sul-
fate, perhaps ammonium bisulfate as the initial product. How-
ever, little ammonium ion was found with the sulfate ion, and
the explanation offered was that strong bases in the fly ash
reacted with ammonium ion and allowed ammonia to be lost by
volatilization. (Although the fly ash had been water-washed
prior to use, it was highly basic, producing a pH of 11 in a
10%-by-weight slurry in water. No further information about
the chemical properties of the ash was published.)
FULL-SCALE TESTS
The following paragraphs summarize information about trials
of sulfamic acid in one British power station and in three
American power stations. In the British station, the condi-
tioning agent was presumably the ordinary commercial form of
sulfamic acid. In each of the American stations, on the other
hand, the agent was the blend with other chemicals that is
supplied by Apollo Chemical Corporation as a powdered solid.
Rugeley Power Station (Great Britain)
Dalmon and Tidy have given a brief summary of precipitator
tests with sulfamic acid at the Rugeley Power Station in Great
Britain.15 They state that the conditioning agent was added
to the flue gas ahead of the air preheater at a temperature
of 500°C by spraying an aqueous solution having a concentra-
tion of 1.3 M. The concentration thus injected in the flue
gas was 20 ppm by volume, based on the assumption that com-
plete volatilization occurred without decomposition (this
assumption is used only as a convenient basis for comparing
12
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the rate of injection with the rates of injection of gaseous
conditioning agents such as sulfur trioxide and ammonia).
The emission of fly ash was reportedly lowered by 55%.
Cabin Creek Power Station (Appalachian Power Company)
The most extensive investigation of sulfamic acid in the
American utility industry apparently has been made at the
Cabin Creek Station of Appalachian Power Company, a subsid-
iary of American Electric Power Company. Cabin Creek Station
is located in West Virginia, where state law requires a maxi-
mum dust emission rate of 0.09 mg/kcal (0.05 lb/106 Btu) but
prohibits the use of sulfur trioxide as a conditioning agent
to reach this emission rate when low-sulfur coal is burned to
minimize sulfur dioxide emission. Thus, Appalachian Power
Company undertook an investigation of sulfamic acid to deter-
mine whether use of this agent would permit compliance with
the state requirements.
The utility company investigated sulfamic acid as a flue-gas
additive when coals normally available were burned. The
company also investigated this additive when certain "metal-
lurgical" coals (low in ash content) were burned on an experi-
mental basis, even though these coals were not available in
sufficient amount for sustained use. Sulfur percentages in
the various coals were in the range of 0.8 to 1.0% except in
one instance where the value was 1.4%. Some of the results
of the precipitator tests at Cabin Creek Station were pub-
lished in a technical bulletin by the supplier of the sulfamic
acid.20 More extensive data from these tests were provided
for publication in this report by a representative of the
American Electric Power Service Corporation.21
At Cabin Creek Station, sulfamic acid was injected as a pow-
der into the flue gas produced in each of two boilers with
rated loads of 50 MW (each boiler was one of a pair compris-
ing one unit with a rated load of 100 MW). The site of injec-
tion was the duct upstream from the economizer, where the
flue-gas temperature was approximately 625°C. The treated
gas stream then passed through a precipitator where the tem-
perature averaged about 185°C in one instance or about 160°C
in another instance. Coals from three different sources with
ash contents of about 10% by weight were burned in one boiler,
whereas coals from four other sources with ash contents of
only 6% were burned in the second boiler.
Table 1 summarizes the conditions under which sulfamic acid
was investigated and gives comparative precipitator efficien-
cies with and without this additive. This table lists rates
of sulfamic acid injection as a weight fraction of the rate
of coal consumption. Calculations indicate that 1 part of
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Table 1. RESULTS OP PRECIPITATOR TESTS AT CABIN CREEK
STATION WITH SULFAMIC ACID AS A FLUE-GAS ADDITIVE3
Coal
Type
CI
C
E
ON
1C
AS
A
% S
1.4
0.9
0.9
1.0
1.0
0.8
0.8
% Ash
9.7
10.4
10.3
6.0
5.7
5.7
5.1
Boiler
load,
MW
48
36
33
32
39
40
44
36
42
45*
42
5ld
50
52
52
45<*
45
47d
44
45
Additive rate,b
parts per
2000 parts
of coal
1.1
1.4
2.0
0
1.3
0
1.1
1.3
1.6
0
1.3
0
0.5
1.1
2.0
0
1.2
0
1.0
1.8
Precipitator
efficiency, c
%
98.6
98.7
99.1
91.0
98.2
98.1
97.2
98.8
99.0
95.3
95.1
68.9
88.2
87.6
94.0
95.3
95.1
78.6
89.7
84.6
Most of the data presented are averages computed from the
^results of several tests.
To calculate the approximate concentration in the gas stream
in parts per million by volume, multiply the values listed
cby the factor 10.
Average gas temperatures were 185°C with Coals CI, C, and E
nd 160°C with Coals ON, 1C, AS, and A.
ot reported but estimated from comparative values of the
rate of steam production.
14
-------
sulfamic acid to 2000 parts of coal corresponds to roughly
10 ppm by volume of the conditioning agent in the flue gas
(based on the assumption of complete volatilization of the
agent without thermal decomposition). The efficiency data
indicate that conditioning with sulfamic acid produced favor-
able results with all of the coals except two (Coals ON and
AS). However, an analysis performed by the Appalachian Power
Company led to the conclusion that the only statistically
significant improvement occurred with Coal 1C.
For each of the coals investigated, a precipitator efficiency
of 99% or greater is needed to satisfy the maximum dust emis-
sion level of 0.09 mg/kcal. From Table 1, it appears that
the maximum additive rates used permitted this requirement to
be approximated with Coals CI and E but not with the other
fuels. The statistical study cited above led to the conclu-
sion that the improvement in precipitator performance attained
with sulfamic acid was not enough to meet the emission stan-
dard with coals of high emission levels and that the improve-
ment was inconsequential with coals of low emission levels.
Table 2 summarizes the results of fly ash analyses that were
performed at the Cabin Creek Station. One of the most strik-
ing aspects of the analytical information is the loss-on-
ignition data. These data indicate that the combustion effi-
ciency in the boilers was always low but especially low when
the low-ash fuels (Coals ON, 1C, AS, and A) were burned. The
oxide percentages show this primary difference: the low-ash
coals had lower soda and lime contents but higher sulfur tri-
oxide contents. Determinations of total sulfur trioxide,
water-soluble sulfur trioxide, and pH were not always per-
formed. However, the apparent effect of the conditioning
agent on these ash properties was that the agent lowered the
pH of ash from four of five coals and increased the soluble
sulfur trioxide on the ash from three of four coals for which
the data are available. These effects suggest that sulfur
trioxide from decomposed sulfamic acid was deposited on the
ash, although occurrence of this effect is not consistently
confirmed by the available data.
Table 3 shows the results of determinations of the concentra-
tions of sulfur dioxide, sulfur trioxide, and ammonia at the
outlet of the precipitator during the tests with Coals CI, C,
and E. The concentrations of sulfur dioxide are about as
expected from coals containing 0.9 to 1.4% sulfur considering
the large excess of air that was present in the gas stream.
The concentrations of sulfur trioxide were all reported as
"nil." The minimum concentration of sulfur trioxide that was
detectable by the experimental method was not reported; how-
ever, it can be concluded that the injection of the condition-
ing agent caused no perceptible increase in the concentration
15
-------
Table 2. RESULTS OF FLY-ASH ANALYSES AT CABIN CREEK STATION
WITH AND WITHOUT SULFAMIC ACID AS A FLUE-GAS ADDITIVE
Coal
burned
Ci
C
E
ON
1C
AS
A
LOI,
%a
10.3
12.7
35.5
39.4
39.7
49.3
40.5
Na20,
%
0.7
0.6
0.7
Tr
Nil
0.2
Nil
Constituent of fly asha
K20,
%
0.3
0.3
0.3
0.6
0.6
0.4
0.3
CaO,
%
3.8
3.9
4.9
Nil
Nil
Nil
Nil
MgO,
%
-
0.6
0.3
Tr
0.5
0.2
0.3
A1203/
%
Fe203 ,
%
(total, 38.2)
22.0
26.2
29.2
36.5
38.9
35.1
13.2
13.9
18.1
7.8
4.9
3.7
SiO2,
%
56.4
58.0
57.0
51.4
51.1
50.5
59.3
Total
S03, %b
w/o
-
-
-
-
1.7
1.6
1.3
aTr._ , , , . . """ '
w/
-
-
-
-
1.4
1.9
1.0
Soluble
S03, %b
w/o
—
-
-
0.3
0.7
1.2
0.8
w/
_
-
—
0.6
0.9
1.5
0.8
pHb
w/n
8.8
—
7.8
4.3
5.4
4.7
w/
9.0
_
5.8
4.1
4.6
4.4
LOI indicates loss on injection. Oxides percentages are for the ash after
ignition. Tr indicates that only a trace was detected.
The symbol w/ indicates that the additive was present, and the symbol w/o
indicates its absence. Soluble S03 is the material dissolved in a 1% aqueous
slurry. The pH value is for this slurry after 15 min of stirring.
-------
Table 3. RESULTS OF GAS ANALYSES AT CABIN CREEK
STATION WITH AND WITHOUT SULFAMIC ACID
AS A FLUE-GAS ADDITIVE3
Coal
CI
C
E
Additive rate ,
parts per
2000 parts of coal
1.1
1.3
0
1.3
0
1.1
1.6
Gas concentrations,
ppm
SO 2
420
414
332
430
305
316
343
SO 3
Nil
Nil
Nil
Nil
NH3
2.7
2.8
Nil
Nil
Nil
0.3
1.0
The data presented are averages computed from the
results of several determinations.
of effluent sulfur trioxide, despite the expected formation
of this compound from the decomposition of sulfamic acid.
Perhaps the most noteworthy aspect of the gas determinations
is that ammonia was usually found during conditioning, show-
ing that at least partial decomposition of the sulfamic acid
did occur.
Mercer Station (Public Service Electric and Gas Company, New
Jersey)
Apollo Chemical Corporation has reported favorable results
with sulfamic acid at the Mercer Station of the Public Service
Electric and Gas Company in New Jersey.22'23 The data pub-
lished from this power station are summarized in Table 4.
These data support the claim that the injection of 1 part of
conditioning agent per 2000 parts of coal brought the emission
to a level below the maximum allowed by state law;22 however,
other data published for this station indicate that the pre-
ferred additive rate is 2.5 parts of conditioning agent per
2000 parts of coal.23
A representative of the utility company has confirmed that
the conditioning agent gives a distinct improvement in precip-
itator performance but has not agreed to release information
beyond that summarized in Table 4.2** This source of informa-
tion has indicated that the use of sulfamic acid is continu-
ing at the Mercer Station, whereas it has been discontinued
at the Cabin Creek Station.
17
-------
Table 4. RESULTS OF PRECIPITATOR TESTS AT MERCER STATION
WITH SULFAMIC ACID AS A FLUE-GAS ADDITIVE3
Sulfur
percentage
in coal
0.96
1.47
Additive rate,
parts per
2000 parts of coal
0
0.50
0.75
1.00
4.00
0
1.20
Emission rate,
mg/kcal
(lb/106 Btu)b
0.39 (0.22)
0.11 (0.06)
0.17 (0.09)
0.09 (0.05)
0.08 (0.05)
0.32 (0.18)
0.13 (0.07)
Mercer Station boiler rating, 300 MW. Additive
binjected at gas temperature of about 425 to 480°C.
Emission allowed, 0.18 mg/kcal (0.1 lb/106 Btu).
Station A (Unidentified Utility Company)
The Buell Division of Envirotech Corporation investigated
sulfamic acid as a conditioning agent in two units of a power
generating station designated as Station A.25 These two units
have power-generating capacities of about 25 MW and are
equipped with electrostatic precipitators having design effi-
ciencies of 95.0% in cleaning flue gas at temperatures of
about 190°C. Neither precipitator operates at the design
efficiency, however, when collecting fly ash from the coal
that is now customarily used as a fuel at Station A.
The coal normally burned at Station A contains about 1% of
sulfur and 9% of ash constituents. The fly ash produced from
this coal typically has the composition given in Table 5.
Laboratory measurements of the electrical resistivity of the
ash indicate that the value is typically about 1 x 1012 ohm
cm at the gas temperature where the ash is collected in the
precipitators.
Sulfamic acid was investigated as a conditioning agent to
determine whether it would allow the precipitators to reach
or exceed their design efficiencies. In the two units of
Station A, the conditioning agent was injected as a powder at
locations upstream from the air preheaters where the esti-
mated flue-gas temperatures exceed 600°C. The injection
rates ranged from 0.5 to 1.5 parts by weight of the condition
ing agent for each 2000 parts by weight of the coal being
burned. The performance of the precipitators with the
18
-------
Table 5. RESULTS OF FLY-ASH ANALYSIS
AT STATION A
Constituent3
Na20
K2O
MgO
CaO
A1203
Fe203
Si02
TiO2
P205
SO 3
Percentage by weight
0.4
2.5
0.7
1.4
27.0
6.2
48.5
0.7
0.3
0.8
Loss on ignition, 7.6%.
conditioning agent present was based on determinations of
efficiency in some of the tests and determinations of precipi-
tator currents and observations of stack emission in other
tests.
Efficiency data from four tests of one unit were made avail-
able by Buell for citation in this report. With the feed
rate of sulfamic acid set as 1 part of the agent to 2000 parts
of coal, measured efficiencies were 96.1, 98.5, 98.8, and
99.1%. All of these efficiencies exceeded the design value
of 95.0% as desired, and they were well above the values aver-
aging about 90% without conditioning. Characteristically,
injection of sulfamic acid increased the precipitator currents
and improved the appearance of the stack emission. The change
in precipitator currents implies that the electrical resistiv-
ity of the fly ash was lowered by the conditioning agent.
Buell appears to be satisfied with the performance of sulfamic
acid as a conditioning agent except for one problem that it
created: plugging of the air preheaters, which are of the
tubular type at Station A. This problem is regarded as severe
enough to make sustained use of sulfamic acid impractical at
this power station. Another reported disadvantage of sulfamic
acid is its relatively high cost as a conditioning agent in
comparison with sulfur trioxide or sulfuric acid.
19
-------
SECTION VI
PRECIPITATOR TESTS OF AMMONIUM SULFATE AND AMMONIUM
BISULFATE AS CONDITIONING AGENTS
PILOT-PLANT TESTS
Pilot-plant evaluations of ammonium sulfate and ammonium
bisulfate fall into two categories. One investigation was
carried out by Dalmon and Tidy in a laboratory-scale precipi-
tator,15 as discussed previously in this report. Other inves-
tigations were conducted by the Koppers Company with a sub-
stantially larger pilot-scale precipitator, 9 which was trans-
ported to various power stations and used to collect fly ash
from the flue gas generated in these stations.
Investigation by Dalmon and Tidy
1 5
This study of ammonium sulfate and ammonium bisulfate was
very similar to the study of sulfamic acid by the same inves-
tigators, as described in Section V. Each compound was
injected as a powder at a gas temperature of 400°C or as an
aqueous solution at the same temperature or a lower tempera-
ture, 165°C. Each compound was much more effective when
injected in a solution. Ammonium sulfate was injected at
various concentrations ranging from 0.23 to 1.80 M, whereas
ammonium bisulfate was injected only at a concentration of
0.45 M.
Each compound increased the precipitation rate parameter and
the sparkover voltage and lowered the electrical resistivity
of the fly ash to a degree dependent on the uptake of the com-
pound by the ash. The effectiveness of each compound in mak-
ing any of these changes in the precipitation parameter was
comparable to the effectiveness of sulfamic acid if compared
on the basis of the uptake in moles of conditioning agent per
unit weight of fly ash.
Like sulfamic acid, ammonium sulfate and ammonium bisulfate
were found in the fly ash mainly as sulfate ion. The quantity
of ammonium ion found was never as high relative to the quan-
tity of sulfate to match the stoichiometry of 2:1 for ammonium
sulfate or 1:1 for ammonium bisulfate. Loss of ammonium ion
as ammonia gas was assumed to occur as the result of the basic
nature of the fly ash.
20
-------
Investigations by the Koppers Company
1 9
Pilot-plant precipitator data have been obtained by the
Koppers Company with ammonium sulfate as the conditioning
agent in three power stations and with ammonium bisulfate as
an alternative agent in one of these plants. These three
plants are referred to in this report as Stations B, C, and D.
The test conditions and the results are summarized in Table 6.
It will be noted that the electrical resistivities of uncon-
ditioned fly ash in the three plants varied widely. The
resistivity at Station B was only 1 x 107 ohm cm. The resis-
tivity at Station C was 1 x 1010 ohm cm, which is cited as
the upper limit for satisfactory operation of an electrostatic
precipitator.1'2 Finally, the value at Station D was quite
high, 1 x 1013 ohm cm. Despite the wide range in resistivi-
ties, the performance of the precipitator was improved in each
instance by the injection of the conditioning agents at a
concentration equivalent to 15 ppm by volume (a hypothetical
value assuming volatilization without decomposition).
Table 6. RESULTS OF PRECIPITATOR TESTS BY THE KOPPERS
COMPANY WITH AMMONIUM SULFATE OR AMMONIUM
BISULFATE AS THE CONDITIONING AGENT
Station
B
C
D
Sulfur
in coal,
2.5
1.0
0.5
Gas
temp,
°C
150
150
130
Fly-ash
resistivity, a
ohm cm
1 x 107
1 x 1010
1 x 1013
Conditioning
agent*3
None
(NHi* ) 2 SO i,
NH^HSO^
None
-------
The results obtained at Stations B and C are the only data
cited in this report where the collection of fly ash of low
to moderate resistivity was improved by a conditioning agent.
It must be assumed that in these instances the mechanism of
conditioning was not a lowering of resistivity but some other
effect, perhaps a space-charge effect or an increase in the
cohesiveness of the ash. These latter mechanisms of condi-
tioning are discussed in Section VIII of this report.
The results obtained at Station D almost certainly reflect
conditioning through the mechanism of lowered resistivity.
This conclusion is .supported by measurements of electrical
parameters of the precipitator. Without conditioning, the
secondary voltage and current were 38.0 kV and 10.4 mA. With
conditioning, the values of these parameters increased to
41.4 kV and 25.7 mA. (No electrical data for Stations B and
C are available for inclusion in this report. Also, no resis-
tivity data for conditioned ash in any of the plants are
available for presentation.)
FULL-SCALE TESTS
Dalmon and Tidy have reported trials of ammonium sulfate at
two power stations in Great Britain where the electrical
resistivity of the fly ash was presumably excessive.15 At
Rugeley Station, the compound was injected as an aqueous
spray ahead of the air preheater at a temperature of 500°C to
produce an effective concentration in the gas stream of
20 ppm; it lowered the emission of fly ash by 55%, as did sul-
famic acid under similar conditions (Section V). At the Aber-
thaw Station, the compound was injected as an aqueous spray
upstream from the air preheater at 300°C or downstream from
the preheater at 150°C; the rate of injection at either tem-
perature was controlled to produce an effective concentration
in the flue gas of 25 ppm. At Aberthaw, the conditioning
agent lowered the emission of fly ash by 65% when injected at
the higher temperature or 41% when injected at the lower tem-
perature .
The Koppers Company has reported the results of conditioning
with ammonium sulfate at one full-scale power station in the
United States.19 The agent was injected downstream from the
air preheater at a temperature of 140°C to produce a concen-
tration of 15 ppm in the flue gas. The fly ash in this plant
was produced from a low-sulfur coal and had a high electrical
resistivity, 1 x 1012 ohm cm as measured under laboratory
conditions. The conditioning agent lowered the emission of
fly ash by 50%, presumably as a result of its lowering of the
resistivity of the ash.
22
-------
SECTION VII
FUNDAMENTAL PROPERTIES AND CONDITIONING MECHANISMS
OF SULFAMIC ACID
PHYSICAL AND CHEMICAL PROPERTIES
The fundamental physical and chemical properties of sulfamic
acid at both ambient and elevated temperatures are important
in the use of this compound as a conditioning agent for fly
ash. The properties at ambient temperatures are relevant in
a practical sense to the methods used for handling the com-
pound and injecting it into flue gas, which involve either
the dry solid or an aqueous solution. The properties at ele-
vated temperatures, on the other hand, are important theoreti-
cally in explaining the mechanisms by which the compound acts
as a conditioning agent.
Ambient Temperatures
26-31
Pure sulfamic acid exists as a white crystalline solid at
ambient temperatures. It is quite stable through a wide range
of temperatures, even up to its melting point of 205°C. In
contrast to sulfuric acid, to which it is structurally
related, it is not a hygroscopic compound.
The structural similarity of sulfamic and sulfuric acids is
indicated by the formulas showing their functional groups:
H2N-SO2-OH and HO-S02-OH, respectively. Perhaps a more appro-
priate formula for sulfamic acid in the crystalline state is
that of the bipolar ion, +H3N-S02-O~. Undoubtedly, the sta-
bility of this bipolar ion relative to that of the uncharged
molecule accounts for the fact that sulfamic acid normally
occurs as a solid, whereas sulfuric acid occurs as a liquid.
Sulfamic acid is readily soluble in water, dissolving to the
extent of approximately 25 g in 100 g of water at 25°C. It
behaves as a moderately strong electrolyte in water, thus con-
ducting electricity readily in aqueous solutions. For the
process of ionization in water, shown by Equation 5, the
H2NS03H —» H2NS03~ + H+ (5)
equilibrium constant at 25°C is 0.101. At a concentration of
0.1 M in water, sulfamic acid produces a pH of 1.25, only
23
-------
slightly higher than the pH of 1.00 that is produced by sul-
fur ic acid at the same concentration. Evidently, the bipolar
ion mentioned above is far less important for sulfamic acid
in solution than for the compound in the crystalline state.
Elevated Temperatures
At the elevated temperatures where sulfamic acid is used as a
conditioning agent, the processes of melting, hydrolysis, and
thermal decomposition may all be relevant to the mechanism of
fly-ash conditioning. The importance of physical and chemi-
cal changes in the compound during the conditioning process
has been stressed by various investigators. Lowe, for exam-
ple, pointed out the original hypothesis that sulfamic acid
serves as a source of sulfur trioxide through the reaction
given previously in this report:18
H2NS03H —» NH3 + S03 (1)
Representatives of Apollo Chemical Corporation stress the
importance of injecting their formulation of sulfamic acid at
high temperatures to achieve volatilization, although they do
not describe the nature of the volatilization process.2°i* 3
The normal melting point of sulfamic acid is 205°C. Up to
this temperature, dry sulfamic acid appears to be stable.31
At elevated temperatures in the presence of water, however,
the compound may undergo hydrolysis, yielding ammonium bisul-
fate as the product:
H2NSO3H + ,H2O —» NHi^HSO., (6)
It is known that this process is accelerated as the tempera-
ture of sulfamic acid in aqueous solutions is increased.29
It is possible that this process may also occur between dry
sulfamic acid and water vapor at temperatures between the
boiling point of water and the melting point of sulfamic acid
(that is, between 100 and 205°C).
Various investigators have studied the thermal decomposition
of sulfamic acid to fragments such as ammonia and sulfur tri-
oxide. Halstead31 performed one of the most comprehensive
studies. This study was carried out with the purpose of
explaining how sulfamic acid acts as a conditioning agent for
fly ash. Halstead reported that heating sulfamic acid in a
stream of dry nitrogen causes little volatilization below the
melting point or above the melting point up to a temperature
24
-------
of 323°C but then produces considerable volatilization between
323 and 467°C. The observed yield of sulfur trioxide is only
about 0.2 mole per mole of sulfamic acid, rather than 1.0 mole
as shown by Equation 1. The remaining sulfur in the starting
material, about 0.8 mole, is released as sulfur dioxide as a
result of an internal oxidation-reduction process in the
residue that releases elemental nitrogen (N2) along with the
sulfur dioxide.
For a constant decomposition temperature of 397°C, Halstead
concluded that molten sulfamic acid volatilizes through a
series of decomposition reactions. The information given by
Halstead is expressed by the reaction sequence given on the
following page, which is based on one mole of molten sulfamic
acid as the starting material. The first reaction (Equa-
tion 7) is the conversion of the melt to a molten mixture of
two new compounds: a cyclic sulfur-nitrogen compound and
ammonium bisulfate. The next reaction (Equation 8) evolves
gaseous sulfur trioxide and again alters the composition of
the melt, producing ammonium pyrosulfate and different sulfur-
nitrogen compounds. Subsequently, a further reaction (Equa-
tion 9) evolves gaseous nitrogen, water, and sulfur dioxide
and once more alters the composition of the melt. Finally,
the residual melt remains constant in composition but evolves
gaseous ammonia, nitrogen dioxide, water, and sulfur dioxide
(Equation 10). When volatilization is complete, the net
reaction is shown by the sum of Equations 7 through 10:
NH2S03H(1) —» 0.167 S03(g) + 0.833 SO2(g) (ID
+ 0.500 H2O(g) + 0.083 N2 (g) + 0.6-67 NH3 (g) + 0.167 NO2 (g)
Halstead did not report any information about the rates of
the several reactions given above. However, rate data were
obtained in the laboratory of American Electric Power Service
Corporation for the volatilization of sulfamic acid in the
commercial form used at the Cabin Creek Station. A sample of
this material was held for 1 hr at various temperatures and
the cumulative losses in weight were determined, with the
results shown in Table 7.
The above information bearing on the stoichiometry and the
rate of decomposition of sulfamic acid may or may not be
applicable to the decomposition of this compound when it is
dispersed as a powder in hot flue gas. First, with regard to
stoichiometry, the presence of significant concentrations of
water vapor and oxygen in flue gas may suppress the formation
of ammonium pyrosultate (the anhydride of ammonium bisulfate,
as shown in Equation 8), and the reduction of the sulfur
25
-------
ro
1.000 NH2S03H(1)
{0.083 (NHS02)6 + 0.500 NH^HSO,, } (1)
0.167 S03 (g)
{0.083 N2 + 0.167 H20
+ 0.250 SO2> (g)
+ {0.167 NH4HSCH + 0.0167 (NH^)2S207
+ 0.083 (NH2)2S02 + 0.083 NH2 (S02NH) 2S02NH2 } (1)
V
+ {0.167 (NH4)2SO^ + 0.167 NH,
+ 0.083 NH2(S02NH)2S02NH2}(1)
I
{0.667 NH3 + 0.167 N02 + 0.333 H20 + 0.583 SO2}(g)
(7)
(8)
(9)
(10)
REACTION SEQUENCE FOR THE DECOMPOSITION OF
SULFAMIC ACID ABOVE ITS MELTING POINT31
-------
Table 7. RESULTS OF A VOLATILIZATION
STUDY OF A COMMERCIAL FORM
OF SULFAMIC ACID
Temperature ,
°C
100
200
400
500
600
700
900
Cumulative
weight loss, %
0
0
6
10
10(+)
11
30
trioxide constituent of sulfamic acid to sulfur dioxide, as
shown in Equations 9 and 10. Second, with regard to rate,
the dispersion of a fine powder might lead to much more rapid
decomposition than that indicated by the above loss-in-weight
data, which presumably were obtained with bulk powder resting
in some type of vessel in a laboratory furnace.
CONDITIONING MECHANISMS
It must be assumed that in the pilot-plant and full-scale
investigations of sulfamic acid described earlier in this
paper the lowering of the electrical resistivity of the fly
ash was one probable mechanism of conditioning. It is also
necessary, however, to consider a space-charge effect and an
increase in the conesiveness of the ash as alternative mecha-
nisms, as observed with ammonia conditioning.13
Lowering of the Electrical Resistivity of Fly Ash
The laboratory experiments of Dalmon and Tidy15 included
measurements of the resistivity of fly ash, which indicated
that the resistivity was lowered by sulfamic acid condition-
ing. On the other hand, the full-scale precipitator studies
in this country did not, unfortunately, include any measure-
ments of resistivity. These studies did, however, yield data
on fly-ash composition and precipitator electrical parameters
that suggest conditioning by a lowering of resistivity. A
question of both practical and theoretical interest is what
chemical substances could have been responsible for this
effect at the widely varying temperatures at which the sul-
famic acid was injected and the treated fly ash was collected.
27
-------
Successful results have been reported with sulfamic acid
injected as an aqueous spray at temperatures of 165, 400, and
500°C. Successful trials of sulfamic acid have also been
reported with the injection of the powdered solid at tempera-
tures ranging from about 400 to 625°C. In view of the fact
that the stability of sulfamic acid changes markedly through
the range of temperatures that have been used for injection,
different chemical substances undoubtedly participate in the
conditioning of fly ash as the injection temperature is
altered.
Low-Temperature Injection—
The lowest injection temperature that has been cited in this
report, 165°C, is below the melting point of sulfamic acid
and also below the range of temperatures that is required for
molecular fragmentation of the compound. At this temperature,
therefore, injection of an aqueous solution of sulfamic acid
may leave a residue of small particles of the sulfamic acid
to be coprecipitated with the fly ash after the liquid water
of the solution has evaporated. Alternatively, during the
process of injection, the sulfamic acid may undergo hydrolysis
to ammonium bisulfate as illustrated in Equation 6.
The coprecipitation of discrete particles of sulfamic acid
and fly ash conceivably could have lowered the effective
resistivity of the ash. The basis for this statement is
Dalmon and Tidy's finding that the coprecipitation of discrete
particles of fly ash and another conditioning agent, sodium
chloride, lowered the resistivity of the ash. Their publica-
tion includes the reproduction of a photomicrograph showing a
large particle of fly ash partially coated with crystals of
sodium chloride, all smaller than 1 ym in size.15
The formation of ammonium bisulfate and the coprecipitation
of this compound with fly ash would undoubtedly have been a
more effective process for lowering the resistivity than the
coprecipitation of the original sulfamic acid. Above 144°C,
ammonium bisulfate is a liquid that can spread over the sur-
faces of individual fly-ash particles, producing a continuous
film of a conductive compound rather than a discontinuous
coating of discrete crystals. (Different melting temperatures
of the bisulfate have been reported by various investigators;
here, the value cited is that reported by Kelley et al.,32
which appears to be the most reliable.)
The low-temperature conditioning of fly ash with sulfamic
acid through the action of liquid ammonium bisulfate has been
postulated as an explanation for the effectiveness of sul-
famic acid in the work of Dalmon and Tidy.33 A critical
aspect of the circumstances under which sulfamic acid was
28
-------
employed by Dalmon and Tidy was the temperature of the pre-
cipitator, 145°C—just above the reported melting point of
ammonium bisulfate.
High-Temperature Injection—
All of the injection temperatures for sulfamic acid above
165°C were above the melting point of the compound and high
enough to decompose the compound through the sequence of reac-
tions shown on page 26. Injection of sulfamic acid as a
powdered solid undoubtedly was followed to some degree by
this sequence of reactions. Injection of the conditioning
agent in an aqueous solution, on the other hand, conceivably
could have produced ammonium bisulfate by hydrolysis (Equa-
tion 6) as the water evaporated from the solution droplets.
The formation of some gaseous sulfur trioxide but, more impor-
tantly, the formation of a relatively large quantity of ammo-
nium bisulfate at high temperatures (Equations 7 and 8) has
been proposed as an explanation of sulfamic-acid conditioning
with high-temperature injection.33 Thus, the formation of
ammonium bisulfate has been proposed as the key to the condi-
tioning process at high temperatures as well as low tempera-
tures of injection.
A matter of some concern that is related to the above conclu-
sions is Dalmon and Tidy's failure to find ammonium and bisul-
fate ions on their conditioned fly ash in the 1:1 molar ratio
that occurs in ammonium bisulfate.15 As previously pointed
out in this report, Dalmon and Tidy suggested that most of
the ammonium ion was lost as ammonia gas as a result of the
reaction between the surface film of ammonium bisulfate and
basic constituents of the fly ash. Perhaps ammonium ion
remained in the outermost layer of the deposit, which was not
in physical contact with the basic constituents of the ash.
Conditioning through the Space-Charge Mechanism
It seems likely that conditioning by sulfamic acid through
the space-charge mechanism will occur only if two temperature
conditions are satisfied: (1) the compound is injected at a
temperature that is high enough to cause molecular fragmenta-
tion to ammonia and sulfur trioxide as gaseous products and
(2) the decomposition products subsequently reach a consider-
ably lower temperature that permits the products to recombine
in the presence of water vapor as fine particles of ammonium
sulfate or ammonium bisulfate. If the injection temperature
is low, molecular fragmentation will not occur, and only
large particles with a low concentration by number will be
introduced. On the other hand, if the temperature of the
flue gas is not lowered sufficiently as the gas stream
29
-------
approaches the precipitator, recombination of the molecular
fragments will not be thermodynamically possible. The kinetic
studies of Halstead3 * indicate that the injection temperature
would probably have to be above 300°C. The thermodynamic data
of Kelley e_t a_l.32 indicate that the precipitator temperature
would have to be as low as 195°C if ammonia and sulfur triox-
ide at concentrations of the order of 10 ppm are to react and
produce ammonium sulfate or the temperature would have to be
even lower, 173°C, if they are to produce ammonium bisulfate
as an alternative product.
Both temperature conditions were satisfied in most of the
investigations of sulfamic acid described in this report. The
most outstanding exception occurred in the pilot-plant work
of Dalmon and Tidy with the lower of two injection tempera-
tures, 165°C.15 Unfortunately, the available data do not pro-
vide any basis for determining whether the space-charge effect
was a significant conditioning mechanism. However, the prob-
lem of air preheater plugging that was encountered by Buell2 5
is comparable to the problem encountered in some studies of
ammonia conditioning when the site of ammonia injection was
upstream from the air preheater.10'11 The similarity of these
problems with the two conditioning agents suggests a possible
similarity of conditioning mechanisms.
Conditioning through the Mechanism of Increasing the Cohesive-
ness of Fly As"E
Ammonium bisulfate, one of the decomposition products of sul-
famic acid, may act as a conditioning agent by increasing the
cohesiveness of fly ash,31* particularly at precipitator tem-
perature above 144°C where the compound exists as a liquid.
The importance of this process, however, cannot be assessed
because of the lack of pertinent data.
30
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SECTION VIII
FUNDAMENTAL PROPERTIES AND CONDITIONING MECHANISMS
OF AMMONIUM SULFATE AND AMMONIUM BISULFATE
PHYSICAL AND CHEMICAL PROPERTIES
The content of this section of the report parallels the con-
tent of the preceding section dealing with sulfamic acid. It
consists of, first, a discussion of the properties of ammonium
sulfate and ammonium bisulfate (particularly those related to
fly-ash conditioning) and, second, a discussion of the mecha-
nisms of conditioning by the two ammonium salts.
Ambient Temperatures3 5 ' 3 6
At ambient temperatures, ammonium sulfate and ammonium bisul-
fate exhibit some of the same properties as sulfamic acid.
Both compounds occur as white crystalline solids, and they
are readily soluble in water, where they are strongly ionized
and readily capable of conducting electricity.
In an aqueous solution, ammonium sulfate behaves as a weak
acid as the result of the ionization of the ammonium ion:
NH^+ — » NH3 + H+ (12)
When dissolved in water, ammonium bisulfate is a significantly
stronger acid than ammonium sulfate because of the ionization
of the bisulfate ion:
HSO.T — » H+ + SOiT2 (13)
Approximate pH values of 0.1 M solutions of ammonium sulfate
and ammonium bisulfate are 5.5 and 1.6, respectively.
Elevated Temperatures
Melting Points —
At elevated temperatures, ammonium sulfate melts without
decomposition only in a closed system, and its melting point
is reportedly to be around 510°C.3S The compound decomposes
31
-------
prior to melting in an open system, producing ammonia and
ammonium bisulfate.35 Ammonium bisulfate, on the other hand,
melts without significant decomposition at a temperature of
about 144°C.32
Equilibrium between Ammonium Sulfate and Ammonium Bisulfate—
The decomposition of ammonium sulfate to ammonia and ammonium
bisulfate was shown earlier by Equation 4. The reaction is
shown below with the physical states of the reactant and the
products indicated (s = solid, 1 = liquid, and g = gas):
(NlMaSOi, (s) —> NH3 (g) + NH^HSO., (s,l) (4)
The dual notation for the physical states of ammonium bisul-
fate signifies that this reaction product is a solid below
its melting point and a liquid above this temperature.
Kelley et al. have published thermodynamic data for the reac-
tion shown by Equation 4;32 their information on the equilb-
rium constants can be approximated as follows:
• Below the melting point of NH^HSO.,, 144°C
log K* = -5,659/T + 5.630
• Above the melting point of NH^HSOi,
log K* = -6,519/T + 7.693
Use of these equations yields the values in Table 8 for the
partial pressure of ammonia in equilibrium with the two pure
ammonium salts at selected temperatures ranging from 120 to
500°C.
Kijoura and Urano37 and Rozenknop and Sedov38 have published
data from recent determinations of the partial pressure of
ammonia in equilibrium with the two salts above the melting
point of the bisulfate. The partial-pressure data from both
of these sources are substantially higher than the data in
Table 8 for the upper range of temperature. A possible
explanation for the discrepancies is that neither of the later
investigations actually yielded results to be compared with
the predictions based on the paper of Kelley et al., which
deals with a hypothetical system in which pure solid ammonium
32
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Table 8. EQUILIBRIUM PARTIAL PRESSURES OF AMMONIA IN
EQUILIBRIUM WITH AMMONIUM SULFATE AND AMMONIUM
BISULFATE AT VARIOUS TEMPERATURES
Temperature ,
°C
120
144
175
200
225
300
400
500
Pressure of
NH3 , atm
1.70
1.15
1.39
8.15
4.01
2.07
1.02
1.82
x 10-9
x 10"8
x 10-7
x 10~7
x 10"6
x 10-"
x 10~2
x lO"1
sulfate and pure liquid ammonium bisulfate are both present
(that is, with a thermodynamic activity of unity for both).
The mixtures used in the later investigations probably were
a mixture of pure solid sulfate and a binary melt of the two
salts in which the activity of the bisulfate was less than
unity.
Equilibria between the Salts and Molecular Fragments—
Theoretically, it is possible for ammonium sulfate and ammo-
nium bisulfate to decompose to the molecular fragments:
ammonia, water, and sulfur trioxide. The formation of these
constituent compounds from the salts has been shown previ-
ously by Equations 2 and 3, which are repeated below with the
physical states of the reactants and the products indicated:
--» 2NH3(g) + H20(g) + S03(g)
(2)
—» NH3 (g) + H20(g) + SO3 (g)
(3)
If the temperature of flue gas does not exceed 300°C, the
presence of approximately 10%-by-volume of water vapor will
convert sulfur trioxide to sulfuric acid by this reaction:39
H20(g) + S03(g) —
(14)
33
-------
Hence, at temperatures below 300 °C in a flue-gas environment
it is appropriate to write Equations 15 and 16 (below) in '
place of Equations 2 and 3.
2SO,,(s) — » 2NH3(g) + HaSCMg) (15)
s,l) — » NH3(g) + H2SO4 (g) (16)
In the temperature range from 300 to 400 °C, reactions leading
to the production of both sulfur trioxide and sulfuric acid
must be taken into account. However, if the temperature of
flue gas exceeds 400°C, the water vapor in the flue gas will
not significantly react with sulfur trioxide to form sulfuric
acid. Hence, above 400°C, Equations 2 and 3 are more appro-
priate than Equations 15 and 16.
The above discussion ignores the possibility of reactions in
which ammonia reduces sulfuric acid or sulfur trioxide to
sulfur dioxide with the concomitant formation of elemental
nitrogen:
2NH3(g) + 3H2SCMg) ~ » N2 (g) + 6H20(g) + 3SO2 (g) (17)
2NH3 (g) + 3S02 (g) — > N2 (g) + 3H2O(g) + 3SO2 (g) (18)
Both of these reactions are favored by thermodynamic factors
through a wide range of temperatures;32 however, they appear
to be inconsequential for kinetic reasons except perhaps in
the upper part of the range of temperatures discussed in the
preceding paragraph.
Approximate equations that can be used for calculating the
equilibrium constants of the reactions shown in Equations 2,
3, 14, 15, and 16 are given in Table 9. These equations are
based on data published by Kelley et al. for the decomposi-
tions of ammonium sulfate and ammonium bisulfate32 and pub-
lished in JANAF Tables for the reaction of sulfur trioxide
with water to produce sulfuric acid.39 The principal basis
for uncertainty in the use of these equations is the fact
that Kelley et al. found it necessary to use approximate
values of the thermodynamic properties of ammonium bisulfate
because of the absence of experimental data for this compound
34
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Table 9. EQUATIONS FOR THE EQUILIBRIUM CONSTANTS
ASSOCIATED WITH THE DECOMPOSITION REACTIONS OF
AMMONIUM SULFATE AND AMMONIUM BISULFATEa
Temp,
log K2
log K3
log Km
log Kas
log Kie
< 144°C (T < 417°K)
= -23,353/T + 31.530
= -17,694/T + 25.900
5,368/T - 8.139
= -17,985/T + 23.391
= -12,326/T + 17.761
Temp,
log K2
log K3
log Km
log KI 5
log KI e
> 144°C (T > 417°K)
= -23,353/T + 31.530
= -16,834/T + 23.837
5,368/T - 8.139
= -17,985/T + 23.391
= -11,466/T + 15.698
aThese are only approximations because terms involving func-
tions of temperature other than 1/T have been ignored.
The equations in Table 9 have been used to calculate the par-
tial pressures of ammonia and sulfuric acid that would theo-
retically be produced by the decomposition of ammonium sulfate
or ammonium bisulfate in a hypothetical flue-gas environment
containing neither of these gaseous products prior to the
introduction of the ammonium salt. (The flue-gas environment
is described as hypothetical for, in actuality, it would con-
tain a finite background concentration of sulfuric acid as a
result of the partial combustion of sulfur in the coal to
this compound.) The calculations were made with the assump-
tion that at least a trace of excess ammonium salt would be
present. The results of the calculations, covering the
temperature range from 120 to 300°C, are given in Table 10.
Table 10. EQUILIBRIUM PARTIAL PRESSURES OF AMMONIA
AND SULFURIC ACID IN EQUILIBRIUM WITH AMMONIUM
SALTS AT VARIOUS TEMPERATURES
Temp,
°C
120
144
175
200
225
300
(NH.Ji
[NH3], atm
4.40 x 10-8
3.32 x 10~7
3.28 x 10~6
1.67 x 10"5
7.22 x 10-5
2.72 x 10-3
SO ^
[H2SOi»] , atm
2.20 x 10"8
1.66 x 10~7
1.64 x 10~6
8.36 x 10~6
3.61 x 10~5
1.36 x 10~3
NHtHSO*
[NH3] = [H2SO»], atm
1.58 x 10~7
1.26 x 10-6
1.27 x ID"5
5.33 x 10-5
2.17 x 10~"
6.98 x 10-1*
35
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Mechanisms of Decomposition of the Salts as Observed Experi-
mentally-"-
The thermal decompositions of ammonium sulfate and ammonium
bisulfate have been studied by a number of investigators.
However/ the results of two investigations by Kiyoura and
Urano37 and by Halstead1*0 appear to be most appropriate for
discussion in this report.
Kiyoura and Urano determined differential thermal analysis
(DTA) and thermogravimetric analysis (TGA) curves as ammonium
sulfate or ammonium bisulfate was heated in a stream of air,
sometimes with ammonia or water vapor added. They concluded
that the first step in the decomposition was the release of
ammonia gas and the formation of the compound triammonium
hydrogen sulfate as a solid residue, which is stoichiometri-
cally equivalent to the double salt of ammonium sulfate and
ammonium bisulfate:
2(NHi»)aSOi,(s) --» (NHO3H(SOO2 (s) + NH3 (g) (19)
This reaction was said to be predominant below 170 °C. The
reaction usually given for the deammoniation of ammonium sul-
fate (previously shown by Equation 4) , however, was said to
be predominant above 170°C:
(NH,J2SO., (s) — » NHijHSCMl) + NH3 (g) (4)
Kiyoura and Urano found that liquid ammonium bisulfate
released water vapor and produced ammonium pyrosulfate solid
as the main reaction during heating at temperatures above
170°C:
2NHi,HSCMl) — > (NHH)2S2O7 (s) +H2O(g) (20)
They also reported the formation of small amounts of sulfamic
acid in the solid residue and the formation of unspecified
amounts of ammonia, sulfur trioxide, sulfur dioxide, sulfuric
acid, hydrogen, and nitrogen in the gas phase.
Each of the reactions written above (shown by Equations 4, 19 t
and 20) was found to be reversible. That is, the addition of'
ammonia to the airstream passing over ammonium sulfate sup-
pressed the deammoniation of this compound and the addition of
water vapor suppressed the dehydration of ammonium bisulfate.
36
-------
Each of the ammonium salts was heated at the rate of 4.0°C/min
or sometimes at a slower rate. All of the TGA curves show
that loss of weight by volatilization occurred slowly, regard-
less of the rate of heating. In air containing no added ammo-
nia or water vapor, the heating rate of 4.0°C/min produced
the losses in weight listed in Table 11 as the temperature
reached values ranging from 200 to 400°C. In air containing
one of the reaction products as an initial component, lower
losses of weight were observed as the specified temperatures
were reached.
Table 11. WEIGHT LOSSES BY VOLATILIZATION OF
AMMONIUM SULFATE OR AMMONIUM BISULFATE
AT A HEATING RATE OF 4.0°C/MIN
Compound
(NHiJaSOu
NH^HSO^
Temperature ,
°C
200
250
300
350
200
250
300
350
400
Cumulative
weight loss, %
1
2
15
100
1
3
8
25
100
Halstead's experiments with ammonium sulfate in argon at a
constant temperature of 400°C also showed the deammoniation
of this compound (Equation 4) and the dehydration of the
residue (Equation 20). These experiments also permitted a
description of the reaction ultimately leading to complete
volatilization of the residue from the reaction shown by
Equation 20. Halstead concluded the overall reaction at
400°C could be expressed as follows:
(s) —> 4NH3(g) + 3S02(g) + 6H20(g) + N2(g) (21)
It is to be noted that no sulfur trioxide or sulfuric acid is
shown as a reaction product.
There are apparent discrepancies between the above experi-
mental findings and the previously discussed predictions from
thermodynamic equations. At the present, it is not certain
37
-------
how these discrepancies can be resolved, partly because the
experimental conditions were substantially different from
those prevailing in a flue-gas environment for which the
thermodynamic predictions were made.
CONDITIONING MECHANISMS
Section VI of this report cited several instances where
ammonium sulfate and ammonium bisulfate improved the collect-
ibility of fly ash with high electrical resistivities. It
also cited two instances where the ammonium salts improved
the collectibility of fly ash with low to moderate resistiv-
ities. Thus, different mechanisms of conditioning were
clearly in operation. Undoubtedly, lowering the resistivity
was the primary mechanism of conditioning the high-resistivity
ash. Tentatively, a space-charge effect and an alteration of
the cohesiveness of fly ash are assumed to be important
mechanisms with low- to moderate-resistivity ash.
Lowering the Electrical Resistivity of Fly Ash
The aspect of this conditioning mechanism that is of primary
interest is the identification of the chemical substances
that lower the resistivity of fly ash when ammonium sulfate
or ammonium bisulfate is used as a flue-gas additive. The
identification of the active substances is a matter of impor-
tance as in the instance of sulfamic-acid conditioning because
of the wide range of injection temperatures that have been
used for the two ammonium salts. As stated in Section VI, gas
streams containing high-resistivity ash have been treated with
the ammonium salts at temperatures ranging from 130 to 400°C.
Low-Temperature Injection—
Ammonium sulfate and ammonium bisulfate have been injected
into the flue gas entering two pilot-plant precipitators and
two full-scale precipitators in the form of aqueous solutions
at comparatively low temperatures—a minimum of 130°C and a
maximum of 165°C. Reference is made to the data given previ-
ously in Tables 8, 10, and 11 showing theoretical equilibrium
partial pressures and rates of formation of the decomposition
products of the ammonium salts (ammonia and sulfuric acid in
the gaseous state). The data in these tables leave little
reason to postulate that appreciable decomposition of either
ammonium salt occurred at injection temperatures in the range
of concern, 130 to 165°C. It seems probable, therefore, that
each flue-gas additive was coprecipitated with the fly ash.
It is possible, of course, that some chemical process
38
-------
occurred within the coprecipitate such as the release of
ammonia as a result of the action of basic constituents of
the ash, as described by Dalmon and Tidy.15
A coprecipitate of ammonium sulfate and fly ash can be visual-
ized as an intimate mixture of discrete particles of each sub-
stance (comparable to the coprecipitate of sodium chloride
and fly ash discussed in Section VII). It is somewhat sur-
prising that such a mixture could have the same resistivity
as fly ash conditioned with sulfur trioxide, where the sur-
face of the ash should be much more uniformly coated with the
conditioning agent. Yet, according to Dalmon and Tidy, ash
conditioned with the two agents has the same resistivity if
the ratio of moles of conditioning agent to the weight of ash
is the same.15
A mixture of ammonium bisulfate and fly ash precipitated at a
temperature above the melting point of the bisulfate might
easily consist of ash particles uniformly coated with the con-
ditioning agent. It is a matter of record that in the only
instances where the bisulfate was used as a flue-gas additive
in the low-temperature range the injection temperature and
the precipitator temperature were still above the melting
point of the bisulfate.l5•l9
High-Temperature Injection—
The highest injection temperatures cited in Section VII were
300 and 400°C for ammonium sulfate and 400°C for ammonium
bisulfate. Reference to Tables 10 and 11 will show that com-
plete decomposition of both compounds was thermodynamically
feasible at injected concentrations of the order of 20 ppm by
volume and that rapid attainment of equilibrium was also
possible. The primary question about the mechanism of condi-
tioning is what phenomena subsequently occurred as the flue
gas was cooled before entering the precipitator.
One possibility is that the decomposition products recombined
through reversal of the high-temperature reactions, yielding
a fume of ammonium sulfate or ammonium bisulfate. Electrical
charging of the fume in the precipitator could have enhanced
the space-charge component of the electric field and thus pro-
duced an undesirable increase in the rate of sparking if no
change occurred in the high-resistivity fly ash.
Another phenomenon seems more probable in view of the fact
that improvements in precipitator efficiency were achieved.
That is, a sufficient quantity of the sulfuric acid vapor pro-
duced at the injection temperature was collected by the fly
ash and thus lowered the resistivity.
39
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Other Mechanisms of Conditioning
Other mechanisms of conditioning by ammonium sulfate and
ammonium bisulfate probably included, on occasion, a space-
charge effect and an alteration of the cohesiveness of fly
ash. These mechanisms have already been discussed in connec-
tion with conditioning by sulfamic acid (Section VII), and
only a few of the previous comments need be stressed in
regard to the ammonium salts. First, as suggested on the
previous page, the space-charge effect was of likely impor-
tance only with high-temperature injection (capable of frag-
menting the salts to their gaseous constituents) and with
relatively low-temperature precipitation (capable of allowing
the gases to be present after recombination as particulates
at high concentrations on a number basis). Second, the
effect on cohesiveness was of most likely importance with
low- or moderate-resistivity ash, for which reentrainment was
not restrained sufficiently by electric force.
40
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SECTION IX
ECONOMIC ASPECTS OF SULFAMIC ACID, AMMONIUM SULFATE,
AND AMMONIUM BISULFATE AS CONDITIONING AGENTS
Dalmon and Tidy made a comparison of the relative costs of
sulfur trioxide, sulfuric acid, sulfamic acid, ammonium sul-
fate, and ammonium bisulfate as conditioning agents.15 Their
first basis of comparison was the purchase price per mole of
sulfur trioxide theoretically available from one mole of each
compound. Their second basis of comparison took into account
the efficiency of pickup of each agent by fly ash as observed
experimentally. The results of the comparisons are given in
Table 12. The relative costs given in this table indicate
that sulfamic acid and ammonium bisulfate have a decidedly
adverse competitive position resulting from both purchase
price and effectiveness.
Table 12. RELATIVE COSTS OF SULFUR TRIOXIDE,
SULFAMIC ACID, AND RELATED
CONDITIONING AGENTS15
Conditioning
agent
Sulfur trioxide
Sulfuric acid
Sulfamic acid
Ammonium sulfate
Ammonium bisulfate
Cost per
mole purchased
1.0
0.6
5.7
1.1
5.5
Cost for
effective use
1.0
1.1
11.4
2.4
11.1
Capital and operating costs were not taken into account in
the above comparisons. Dalmon and Tidy pointed out that these
items of cost would be much lower for any of the alternatives
to sulfur trioxide that can be injected as sprays of dilute
aqueous solutions or as powdered solids. Even so, these
investigators regarded the direct costs of sulfamic acid and
ammonium bisulfate as too great to be offset by lower capital
costs; thus, they concluded that only sulfuric acid and ammo-
nium sulfate would be competitive with sulfur trioxide.
41
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The current relative prices in the United States of all of
the conditioning agents except ammonium bisulfate appear to
be about the same as those cited by Dalmon and Tidy.1*1'1*2 NO
price information for ammonium bisulfate as a chemical commod-
ity appears to be available currently; indeed, the availabil-
ity of this compound is questionable, for no supplier of the
compound is cited in several current chemical purchasing
guides.*3-*5
Despite the unfavorable cost of sulfamic acid, the Apollo
Chemical Corporation apparently is having significant success
in marketing blends of this conditioning agent with other com-
pounds. For comparable rates of injection on a molar basis,
the relative costs of Apollo's dry chemical and sulfur tri-
oxide in a 200-MW power station have been estimated by the
supplier23 as shown in Table 13. The figures in this table
indicate that the lower capital cost of injecting the Apollo
agent offsets the higher cost of the agent and make the total
annual cost of using this agent lower than that of using sul-
fur trioxide. These figures are based, however, on low injec-
tion rates producing additive concentrations in flue gas of
only about 7.5 ppm by volume. It seems likely that if higher
injection rates were needed the capital costs would increase
less sharply than the additive costs and the injection of
sulfur trioxide would become more economical.
Table 13. RELATIVE COSTS OF CONDITIONING BY
SULFUR TRIOXIDE AND A COMMERCIAL FORM
OF SULFAMIC ACID2 3
Source of cost
Capital
Conditioning agent
Maintenance
Cost per year3
Sulfamic acid
$ 4,000
75,600
10,000
?89,600
sui±ur trioxide
$100,000
11,880
25,000
?136,880
aPower-plant size, 200 MW.
A representative of the utility industry21 with experience in
conditioning with both sulfur trioxide and sulfamic acid has
challenged the validity of the comparison given in Table 13
on the basis of reagent costs. He has asserted that the
assumed rate of addition of sulfamic acid is impractically
low and that the assumed unit cost of sulfur trioxide is
higher than the cost paid by his company. The point regarding
the assumed rate of addition of sulfamic acid is essentially
42
-------
the same as the point raised in the preceding paragraph; it
implies that the indicated cost advantage of sulfamic acid
vanishes if, for example, the rate of addition is necessarily
twice the value assumed.
The figure given in Table 13 for the capital cost of condi-
tioning with sulfur trioxide ($100,000) is about four times
the capital cost estimated by Southern Research Institute
($24,500) for conditioning with 10 ppm of sulfur trioxide in
a slightly larger plant (250 MW rather than 200 MW).9 The
estimate by the Institute was made in 1971, whereas that given
in Table 13 was made in 1974. The effect of inflation over
the 3-yr period would account for part of the difference in
estimated capital costs. Still, the difference in estimates
appears to be too large to be reconciled in terms of the
effect of inflation. Unfortunately, the background data used
in computing the figure in Table 13 are not available, and
the difference between this figure and the earlier estimate
cannot be satisfactorily explained.
No comparative cost data for conditioning with ammonium sul-
fate and sulfur trioxide appear to have been published. It
seems likely, however, that ammonium sulfate would be an
economical alternative to sulfur trioxide as a result of the
lower capital cost of an injection system for the ammonium
salt and the slight difference in costs of purchasing the
salt and the sulfur trioxide.
43
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SECTION X
REFERENCES
1. White, H. J. Industrial Electrostatic Precipitation.
Reading (Mass.), Addison-Wesley Publishing Company
Inc., 1963. p. 294-330.
2. Oglesby, S., and G. B. Nichols. A Manual of Electro-
static Precipitator Technology: Part I—Fundamentals
Southern Research Institute, Birmingham, Ala. Contract
CPA 22-69-73. The National Air Pollution Control
Administration, Cincinnati, Ohio. August 25 1970
p. 166-186.
3. Archer, W. E. Electrostatic Precipitator Conditioning
Techniques. Power Eng. 76:50-53, December 1972.
4. Busby, H. G. T., and K. Darby. Efficiency of Electro-
static Precipitators as Affected by the Properties and
Combustion of Coal. J. Inst. Fuel (London). 36:184-
197, May 1963.
5. Darby, K., and D. 0. Heinrich. Conditioning of Boiler
Flue Gases for Improving Efficiency of Electrofilters
Staub Reinhaltung Luft (English edition). 26:12-17
November 1966.
6. Coutaller, J., and C. Richard. Amelioration du
Depoussierage Electrostatique par Injection de S03
[Improvement of Electrostatic Precipitators by Injec-
tion of S03]. Pollut. Atmos. (Paris). 9:9-15,
January-March 1967.
7. Schrader, K. Improvement of the Efficiency of Electro-
static Precipitation by Injecting SO3 into the Flue Gas
Combustion. 42 (4):22-28, October 1970.
8. Busby, H. G. T., C. Whitehead, and K. Darby. High
Efficiency Precipitator Performance on Modern Power
Stations Firing Fuel Oil and Low Sulphur Coals. Lodge-
Cottrell, Ltd. (Presented at Second International
Clean Air Congress of the International Union of Air
Pollution Control Association. Washington. December G
11, 1970.) 56 p.
44
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9. Dismukes, E. B. A Study of Resistivity and Condition-
ing of Fly Ash. Southern Research Institute,
Birmingham, Ala. Contract CPA 70-149. Environmental
Protection Agency, Research Triangle Park, N. C.
February 1972. 138 p.
10. Baxter, W. A. Recent Electrostatic Precipitator Experi-
ence with Ammonia Conditioning of Power Boiler Flue
Gases. J. Air Pollut. Contr. Assoc. 18:817-820,
December 1968.
11. Reese, J. T., and J. Greco. Experience with Electro-
static Fly-Ash Collection Equipment Serving Steam-
Electric Generating Plants. J. Air Pollut. Contr.
Assoc. 18:523-528, August 1968.
12. Watson, K. S., and K. J. Blecher. Further Investiga-
tion of Electrostatic Precipitators for Large Pulver-
ized Fuel-Fired Boilers. Air Water Pollut. Int. J.
(Oxford, England). 10:573-583, September 1966.
13. Dismukes, E. B. Southern Research Institute,
Birmingham, Ala. Unpublished data from a current inves-
tigation supported by the Tennessee Valley Authority.
14. Chittum, J. F. Western Precipitation Corporation, Los
Angeles, Calif. Unpublished data from studies in 1942-
1945. (For excerpts, see Reference 1).
15. Dalmon, J., and D. Tidy. A Comparison of Chemical Addi-
tives as Aids to the Electrostatic Precipitation of
Fly-Ash. Atmos. Environ. (Oxford, England). 6:721-
734, October 1972.
16. Dalmon, J., and E. Raask. Resistivity of Particulate
Coal Minerals. J. Inst. Fuel (London). 45:201-205,
April 1972.
17. Bickelhaupt, R. E. Influence of Fly Ash Compositional
Factors on Electrical Volume Resistivity. Southern
Research Institute, Birmingham, Ala. Report No. EPA-
650/2-74-074. U. S. Environmental Protection Agency,
Washington, D. C. July 1974. 41 p.
18. Lowe, H. J. Reduction of Emission of Pollutants—
Recent Advances in Electrostatic Precipitators for Dust
Removal. Phil. Trans. Roy. Soc. Ser. A. 265(1161):301-
307, 1969.
45
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19. Zarfoss, J. R. Environmental Elements Corporation
(subsidiary of the Koppers Company), Baltimore, Md.
Private communication, May 1974.
20. The Increase in Precipitator Efficiency by Use of
Apollo PPA-30 with Low Sulfur Coals. Apollo Chemical
Corporation, Whippany, N. J. Bulletin U-556. 1973.
21. Morris, E. B. American Electric Power Service Corpora-
tion, New York, N. Y. Private communications, November
1973 and June 1974.
22. Kukin, I. The Practical Applications of Chemical Addi-
tives to Increase Electrostatic Precipitator Efficiency
of Coal Fired Units. Apollo Chemical Corp. (Presented
at Fall Meeting of Pennsylvania Electric Association.
October 18-19, 1973.) 9 p.
23. Chemical Helps Trap Flyash from Low-Sulfur Coal. Elect.
Light and Power. 52(7):20-21, April 1974.
24. Billings, J. Public Service Electric and Gas Company,
Newark, N. J. Private communication, April 1974.
25. Gallaer, C. A. Buell Division of Envirotech Corpora-
tion, Lebanon, Pa. Private communication, January 1974.
26. Sautmyers, D., and R. Aarons. Sulfamic Acid and Sul-
famates. In: Kirk-Othmer Encyclopedia of Chemical
Technology (Volume 19), Standen, A. (ed.). New York,
John Wiley and Sons, Inc., 1969. p. 242-249.
27. Stecher, P. G. (ed.). The Merck Index. Rahway (N. J.),
Merck and Co., Inc., 1968. p. 997.
28. Linke, W. F. Solubilities—Inorganic and Metal Organic
Compounds (Volume I). Princeton (N. J.), D. Van
Nostrand Company, Inc., 1958. p. 1167.
29. Brasted, R. C. Comprehensive Inorganic Chemistry
(Volume VIII). Princeton (N. J.), D. Van Nostrand
Company, Inc., 1971. p. 215-227.
30. King, E. J., and G. W. King. The lonization Constant
of Sulfamic Acid from Electromotive Force Measurements.
J. Amer. Chem. Soc. 74:1212-1215, March 1952.
31. Halstead, W. D. Vaporisation of Sulphamic Acid. J.
Appl. Chem. Biotechnol. (London). 21:22-26, January
1971.
46
-------
32. Kelley, K. K. , C. H. Shomate, F. E. Young, B. F. Nay lor,
A. E. Salo, and E. H. Huffman. Thermodynamic Proper-
ties of Ammonium and Potassium Alums and Related Sub-
stances, with Reference to Extraction of Alumina from
Clay and Alunite. Bureau of Mines, Washington, D. C.
Technical Paper 688. 1946. p. 66-69.
33. Halstead, W. D. Central Electricity Research Labora-
tories, Leatherhead, England. Private communication,
January 1974.
34. Dalmon, J. , and D. Tidy. The Cohesive Properties of
Fly Ash in Electrostatic Precipitation. Atmos . Environ.
(Oxford, England). 6:81-92, February 1972.
35. Holmes, W. C. Ammonium Compounds. In: Kirk-Othmer
Encyclopedia of Chemical Technology (Volume 2),
Standen, A. (ed.). New York, John Wiley and Sons, Inc.,
1963. p. 329-330.
36. Stecher, P. G. (ed.). The Merck Index. Rahway (N. J.),
Merck and Co., Inc., 1968. p. 65, 71.
37. Kiyoura, R. , and K. Urano. Mechanism, Kinetics, and
Equilibrium of Thermal Decomposition of Ammonium Sul-
fate. Ind. Eng. Chem. Process Des. Develop, 9:489-494,
October 1970.
38. Rozenknop, Z. P., and N. V. Sedov. Thermal Dissociation
of Ammonium Sulfate. Zhur. Neorg. Khim. 2:2543-2552,
1957 [Chemical Abstracts 52:8700f, 1957].
39. Stull, D. R. , and H. Prophet (ed.). JANAF Thermochemi-
cal Tables. Washington, National Bureau of Standards,
1971. Unnumbered pages listed in this alphabetical
order: H20, HaO.tS, and 03S.
40. Halstead, W. D. Thermal Decomposition of Ammonium Sul-
phate. J. Appl. Chem. (London). 20:129-132, April
1970.
41. Current Prices of Chemicals and Related Materials.
Chemical Marketing Reporter. 205 (23) :30-41, June 1974.
42. Tillen, G. Allied Chemical Corp., Industrial Chemicals
Division, Houston, Texas. Private communication, June
1974.
43. Chemicals, Raw Materials, and Specialties. Chem. Week.
113(18): 368, October 1973.
47
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44. OPD Chemical Buyers Directory. New York, Schnell Pub-
lishing Company, Inc., 1973. p. 103.
45. Thomas Register of American Manufacturers (Volume 1).
New York, Thomas Publishing Company, 1974. p. 163.
48
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SECTION XI
APPENDIX. COMPOSITION OF A COMMERCIAL CONDITIONING
AGENT BASED ON SULFAMIC ACID
Although representatives of Apollo Chemical Corporation
acknowledge that their dry and liquid conditioning agents
known as Coaltrol PPA-30 and Coaltrol LPA-40 contain sulfamic
acid as a major constituent, they prefer to describe their
agents as activated blends of chemical agents.20 On one
occasion, they described the additive used as a dry solid or
in an aqueous solution as a blend of "amines and nitrogen-
containing inorganic compounds combined with specific cations
that are synergistic with the N-H cations." On the same
occasion, they asserted that the additive is custom-blended
for individual boilers and that variables determining the
blend are the amounts of sulfur, sodium, and iron in the coal,
the overall ratio of basic to acidic constituents in the coal,
and the total ash content of the coal.23
Analyses of samples of Coaltrol PPA-30 have been made in the
laboratories of the American Electric Power Service Corpora-
tion, the Buell Division of Envirotech Corporation, and
Southern Research Institute. The results of these analyses
are in essential agreement. They do not reflect any signifi-
cant variation in the compositions of the samples analyzed,
although the supplier of the agent asserts that the composi-
tion is changed to satisfy specific boiler requirements.
Analyses of samples of Coaltrol PPA-30 by the American Elec-
tric Power Service Corporation indicate that the agent con-
sists of about 90%-by-weight sulfamic acid and 10%-by-weight
manganous sulfate monohydrate (MnSOt, 'H20) .21 These analyses
consisted of determinations of a large number of elements,
especially metals; a determination of the infrared spectrum;
and a comparison of the physical properties of the agent with
a blend of sulfamic acid and the hydrate of manganous sulfate.
Analysis in the Buell laboratory confirmed that sulfamic acid
accounted for 90% of the conditioning agent.25 The balance
of the agent was reported as manganese dioxide (MnOz).
Although the presence of a compound of manganese seems likely,
the presence of the dioxide would be inconsistent with the
water-solubility of the agent.
49
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Additional analyses at Southern Research Institute led to the
following results: (1) the agent was found to be almost com-
pletely water-soluble in the weight ratio of 1 part of the
solid to 10 parts of water (only about 0.8% of the solid
failed to dissolve); (2) infrared and nuclear magnetic reso-
nance spectra of the agent showed that sulfamic acid was the
predominant component; (3) weight percentages of hydrogen and
nitrogen were consistent with the occurrence of sulfamic acid
as the primary component; and (4) titration of an aqueous
solution with sodium hydroxide confirmed that sulfamic acid
was the principal component (based on the calculated equiva-
lent weight of the agent as an acid), but it also indicated
that a relatively small amount of metal ion forming a reddish-
brown precipitate in an acidic pH region (perhaps ferric or
manganic ion) was also present.
50
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TECHNICAL REPORT DATA
{Please read Jmuructiont on the reverse before completing)
1. REPORT NO. 2.
EPA-650/2-74-114
4. TITLE AND SUBTITLE
Conditioning of Fly Ash with Sulfamic Acid, Ammonium
Sulf ate , and Ammonium Bisulfate
7. AUTHOR(S)
Edward B. Dismukes
B. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue , South
Birmingham, Alabama 35205
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
NERC-RTP, Control Systems Laboratory
Research Triangle Park, NC 27711
3. RECIPIENT'S ACCESSIOItNO.
S. REPORT DATE
October 1974
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
SORI-EAS-74-396
Project 3134-XIII
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADJ-029
11. CONTRACT/GRANT NO.
68-02-1303
13. TYPE OF REPORT AND PERIOD COVERED
Through June 1974
14. SPONSORING AGENCY CODE
18. SUPPLEMENTARY NOTES
ie. ABSTRACT ,pne repO1it summarizes recent experience with three agents --sulfamic
cid, ammonium sulfate, and ammonium bis ulf ate --used to regulate the electrical
esistivity of fly ash in electric generating stations to ensure satisfactory collection
I fly ash in electrostatic precipitators (ESPs). It presents information about the
ffectiveness of these agents in pilot- and full-scale ESPs. It also presents the
imited information available from practical trials of these agents concerning
heir conditioning mechanisms. It discusses in detail the fundamental physical and
chemical properties of the agents that are relevant to fly-ash conditioning. From this
information and the results of ESP tests, the report offers tentative conclusions about
conditioning mechanisms. Finally, the report briefly discusses the economic aspects
of using each of the agents as a conditioning substitute for sulfur trioxide.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Air Pollution Electrostatic Pre-
Fly Ash cipitators
Treatment Electric Power
Electrical Resistivity Plants
Sulfamic Acids Physical Properties
Ammonium Sulfate Chemical Properties
Air Pollution Control
Stationary Sources
Particulate
Ammonium Bisulfate
Conditioning Mechanisms
13B
21B
20C, 10B
07C
07B, 07D
78. DISTRIBUTION STATEMENT
19. SECURITY CLASS (ThisReport/'
Unclassified
Unlimited
21. NO. OF PAGES
58
20. SE
Unc
URITY CLASS (TMspage)
-"sified
22. PRICE
EPA Form 2220-1 (9-73)
-51-
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