Final Report
Contract CPA 22-69-78
FEASIBILITY STUDY OF NEW SULFUR OXIDE
CONTROL PROCESSES FOR APPLICATION
TO SMELTERS AND POWER PLANTS
Part III: The Monsanto Cat-Ox Process
for Application to Power Plant
Flue Gases
Prepared for:
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
NATIONAL AIR POLLUTION CONTROL ADMINISTRATION
DURHAM, NORTH CAROLINA
STANFORD RESEARCH INSTITUTE
Menlo Park, California 94025 • U.S.A.
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STANFORD RESEARCH INSTITUTE
Menlo Park, California :>--iO25 • U.S.A.
Final Report
Contract CPA 22-69-78
FEASIBILITY STUDY OF NEW SULFUR OXIDE
CONTROL PROCESSES FOR APPLICATION
TO SMELTERS AND POWER PLANTS
Part III: The Monsanto Cat-Ox Process
for Application to Power Plant
Flue Gases
By: KONRAD T. SEMRAU
Prepared for:
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
NATIONAL AIR POLLUTION CONTROL ADMINISTRATION
DURHAM, NORTH CAROLINA
SRI Project PMU-7923
Approved:
N. K. HIESTER, Director
Physical Sciences (Materials)
C. J. COOK, Executive Director
Physical Sciences Division
Copy.~No.
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CONTENTS
FOREWORD v
I INTRODUCTION 1
II OBJECTIVES 5
III SUMMARY 7
IV PROCEDURES 15
A. Formulation of Models 15
B. Cost Factors 16
C. Preparation of Technical Data and Cost Estimates • • 16
V PROCESS DESCRIPTIONS 17
A. Integrated Cat-Ox System for Power Plants 18
B. Cat-Ox System for Application to Existing Power Plant 25
VI PROCESS DATA AND COST ESTIMATES 29
A. Material Flows 29
B. Capital Cost Estimates 34
C. Operating Cost Estimates 38
VII GENERAL DISCUSSION 39
A. Evaluation of the Cat-Ox System 39
B. By-Product Values 42
C. Variation of Bases of Cost Estimates 44
1. Load Factor 44
2. Amortization Period • 45
3. Fixed Charges 46
REFERENCES 47
iii
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CONTENTS (Concluded)
APPENDIXES
A. MODELS FOR HYPOTHETICAL POWER PLANTS A-l
B. COST FACTORS USED IN MODEL STUDIES B-l
C. STACKS FOR USE ON CONTROLLED SULFUR DIOXIDE EMISSION
SOURCES C -1
D. ESTIMATION OF THE VALUES OF SULFUR BY-PRODUCTS D-l
ILLUSTRATIONS
Figure 1 Integrated Cat-Ox System for Power Plant Model B . . . . 19
Figure 2 Cat-Ox Reheat System for Power Plant Model A 20
TABLES
Table I Summary of Estimated Costs — Cat-Ox Reheat System
for Power Plant Model A 11
Table II Summary of Estimated Costs — Integrated Cat-Ox
System for Power Plant Model B 12
Table III Model A — Gas Flows in Cat-Ox Reheat System 30
Table IV Model B — Gas Flows ia Integrated Cat-Ox System .... 31
Table V Model A — Contaminant Removal in Cat-Ox Reheat System . 32
Table VI Model B — Contaminant Removal in Integrated Cat-Ox
System 32
Table VII Power Plant Model A — Summary of Capital and Operating
Costs for Cat-Ox Reheat System 34
Table VIII Power Plant Model B — Capital Investment for Integrated
Cat-Ox System 35
Table IX Power Plant Model B — Summary of Capital and Operating
Costs for Integrated Cat-Ox System 36
Table D-l Estimated Sulfuric Acid Demand in Selected Producing
Areas, 1966 D-6
Table D-2 Phosphoric Acid Production Costs • '• D-7
iv
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FOREWORD
The final report for this study is presented in four separate and
independent parts:
Part I: The Monsanto Cat-Ox Process for Application to
Smelter Gases
Part II: The Wellman-Lord SO Recovery Process for Application
to Smelter Gases
Part III: The Monsanto Cat-Ox Process for Application to Power
Plant Flue Gases
Part IV: The Wellman-Lord SO Recovery Process for Application
to Power Plant Flue Gases
Information for use in this study was supplied to Stanford Research
Institute by Monsanto Company and Wellman-Lord, Inc. under terms of con-
fidentiality agreements between the U.S. Department of Health, Education,
and Welfare, Stanford Research Institute, and each of the cooperating
companies. In accordance with the agreements, Monsanto Company and
Wellman-Lord, Inc. have reviewed and released the parts of the report
dealing with their respective processes. The rights of prior review and
release are designed solely to permit the cooperating companies to assure
themselves that no proprietary or confidential data are being revealed;
they are not intended to restrict Stanford Research Institute's rights
and responsibilities to report its conclusions so long as there is no
incidental disclosure of confidential information. Accordingly, the
release of the reports by Monsanto and Wellman-Lord does not imply that
these companies necessarily concur in all or any of the opinions,
judgments, or interpretations of fact expressed by the author, who
assumes sole responsibility for the report content.
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I INTRODUCTION
Under the Systems Study for Control of Emissions — Primary Non-
ferrous Smelting Industry (Contract No. PH 86-68-85), Arthur G. McKee &
Company and its subcontractor, Stanford Research Institute, carried out
evaluations of a number of sulfur oxide control processes as they might
be applied to offgases from nonferrous smelting. To permit evaluation
of the technical and economic feasibility of these control processes, a
number of models of smelters were created. Stanford Research Institute
carried out the studies necessary to determine the availability of
markets for sulfur by-products open to smelters in various areas, and
the allowable production costs that the smelters would have to attain
in order to break even on the sulfur recovery operations.
The Division of Process Control Engineering of the National Air
Pollution Control Administration (DPCE-NAPCA) desires to extend the use-
fulness of the foregoing study by adding to it technical and economic
evaluations of new and potentially promising sulfur oxide control pro-
cesses. It also wishes to evaluate the same new processes for appli-
cation to power plants. Completion of these preliminary evaluations of
the processes will help determine their potential commercial acceptability.
DPCE-NAPCA has a specific interest in at least two control processes
being offered commercially, the Monsanto Cat-Ox process and the Wellman-
Lord S0_ Recovery process. However, both processes are proprietary, and,
Ct
as a matter of policy, DPCE-NAPCA does not wish to obtain proprietary and
confidential information on the processes. It does, nevertheless, wish
to obtain evaluations in nonconfidential terms. Broadly, DPCE-NAPCA
wishes to obtain estimates of the capital and annual costs of the control
systems for each of the assumed applications, together with appraisals of
the technical constraints on each process and of the current states of
development of the processes.
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Stanford Research Institute was requested by DPCE-NAPCA to carry
out evaluations of the processes under the terms of confidentiality
agreements between the Department of Health, Education, and Welfare,
the owners of the proprietary processes, and Stanford Research Institute.
SRI, acting as a disinterested third party, was to make analyses of the
processes using information obtained from Monsanto Company and Wellman-
Lord, Inc., and to report the results to DPCE-NAPCA without compromising
any of the Monsanto or Wellman-Lord confidential data.
Because of the requirements of confidentiality, it is not per-
missible to describe certain features of the Cat-Ox and Wellman-Lord
processes. The corresponding portions of the systems have had to be
represented only in terms of their general functions, and SRI's eval-
uation of these portions has had to be presented in the form of con-
clusions without supporting data or reasoning. In other instances, the
parts of the systems could be described in general, but specific details
and design parameters could not be revealed.
Within the scope of the present project, it would obviously have
been impossible to inspect and evaluate independently all the company
records and design data even had the cooperating companies been requested
to permit this and had they acceded to the request. The author of this
report, who also conducted the study, evaluated the information provided
at his request, using his own knowledge and relevant data from the
literature and other available sources. Whenever apparent discrepancies
or uncertainties were noted in the information, efforts were made to
secure verification or clarification from the companies. In instances
where resolution of questions was not possible, or the information
required proved to be simply unavailable, the author employed his best
judgment.
Throughout the following sections of this report, information for
which other sources are not specifically cited was generally obtained
from the cooperating companies and accepted by the author either because
it could be verified from other sources or because it appeared reasonable.
In other instances, information or estimates were provided by the com-
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panies that could not be verified independently or judged for reason-
ableness; in such cases, the companies have been specifically cited as
the sources. In still other instances, the author did not accept the
information or estimates provided and in some cases substituted his own;
such cases have also been specifically noted.
The cooperating companies, at their own option and through sub-
stantial efforts, provided the basic capital cost estimates for the model
control systems and the information for estimation of operating and main-
tenance costs. The author in this case acted as a reviewer rather than
as an estimator. The estimates were checked for reasonableness and for
possible errors or omissions. For some components and cost factors, the
author modified the estimates, or substituted others of his own where he
judged them to be more appropriate than those supplied to him. The
author also prepared cost estimates for some auxiliary systems, using
separate data sources.
By specification of the power plant and smelter models, and by re-
view of the results, an effort was made to ensure that the cost estimates
for both control systems were made on strictly comparable bases. Although
it is unlikely that this objective has been met fully, the deviations are
probably within the precision of the estimates themselves.
For convenience, and at the request of DPCE-NAPCA, this final report
is presented in four separate and independent parts. This part deals only
with the Monsanto Cat-Ox process as applied to control of sulfur oxides
in the flue gases from power plants.
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II OBJECTIVES
The objectives of the part of the study covered by this report are
as follows:
1. To prepare block flow diagrams of the Cat-Ox system
showing its configuration and its relation to the
boiler plant in which the sulfur oxides are generated.
2. To present estimated mass and volume flow balances for
the Cat-Ox system.
3. To prepare preliminary engineering estimates of the
capital investment and the total annual cost (including
both fixed and variable charges) for the Cat-Ox system.
From the estimate of total annual cost, secondary
estimates are to be made of the corresponding incre-
mental cost of producing electricity, both on the gross
basis (without allowance for by-product recovery credits)
and on the net basis (with allowance for by-product
recovery credits).
4. To make a qualitative appraisal of technical constraints
on the application and operation of the control system.
5. To appraise (quantitatively, to the extent permitted by
available data) the economic constraints on the appli-
cation of the control system.
6. To assess the current state of development of the Cat-Ox
system, identifying any technological deficiencies whose
elimination might enhance the applicability of the system
to power plants.
The accomplishment of the objectives is subject to any restrictions
that may be imposed under the terms of the confidentiality agreement be-
tween the Government, Monsanto Company, and Stanford Research Institute.
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Ill SUMMARY
The Monsanto Cat-Ox system for sulfur oxides recovery is essen-
tially an adaptation of the well-known contact process for sulfuric
acid manufacture. The gas containing both sulfur dioxide and oxygen
is passed through a fixed bed of catalyst at an appropriate temperature
and most of the sulfur dioxide is oxidized to sulfur trioxide. The gas
is then passed through an absorption tower where the sulfur trioxide is
absorbed in recirculated sulfuric acid. The Cat-Ox system has been
developed for use on gas (primarily power plant flue gases) that contain
dilute concentrations of sulfur dioxide — typically 0.1 to 0.4 percent.
It differs from the conventional contact process plant in three principal
respects:
/
1. The feed gas entering the system either must be already at
a temperature high enough for conversion of the sulfur
dioxide to the trioxide in the catalytic converter, or
else auxiliary heat must be supplied to raise the
temperature. Because of the diluteness of the sulfur
dioxide, the plant is not autothermal; that is, the heat
released by the oxidation of the sulfur dioxide is
insufficient to preheat the feed gas to the reaction
temperature.
2. The system operates on wet gas. The feed gas is not dried
before it enters the converter.
3. The heat in the exit gas is used to a greater or lesser
degree to concentrate the sulfuric acid formed in the
final absorption step.
For power plant applications the Cat-Ox process is offered in two
forms, one for use in new plants and one for previously existing plants.
The Cat-Ox system for new plants is constructed as an integral part of
the boiler and steam generating system. It includes some of the normal
components of a boiler plant, including the economizer, the electro-
static precipitator, and the air heater, but not all the costs of these
components are properly chargeable against sulfur oxides recovery. The
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Cat-Ox system for application to existing plants, which is termed a
"Cat-Ox reheat system, " is added to the power plant at the point where
the flue gas is normally delivered to the stack, and the entire cost is
chargeable to sulfur oxides recovery. The flue gas must be reheated to
the reaction temperature by burning supplementary fuel, and it is neces-
sary 'to provide a preheater to transfer the heat to the flue gas entering
the converter and a heat exchanger subsequently to transfer part of the
heat in the converter exit gas to the flue gas entering the system.
Because the gas is not dried before conversion of the sulfur dioxide
to sulfur trioxide, the amount of sulfuric acid mist formed during the
gas cooling and absorption steps is relatively much higher than in the
conventional contact process. Hence, a high-efficiency mist collector
must be used to recover the mist from the tail gas. In the Cat-Ox
process, a fiber-bed mist eliminator is employed.
The concentration of the sulfuric acid produced by the system is
78 percent. In the integrated Cat-Ox system, stronger acid can be
produced only if heat is diverted from the power plant cycle for use
in concentrating the acid. A more complicated system and additional
equipment are also required. In the Cat-Ox reheat system, stronger
acid could be produced by adding the extra equipment and burning a
greater amount of supplementary fuel. However, it should be possible to
concentrate the acid much more economically with a separate, conventional
acid concentrator.
All the basic concepts of the Cat-Ox process have been demonstrated
previously, although technical problems remain with respect to specific
applications of the process. The principal problems (actual or potential)
in the specific Cat-Ox system for power plant applications are associated
with fly ash. The fly ash must be removed from the flue gas with very
high efficiency in a hot, dry process. Dust not removed from the entering
flue gas tends to plug the catalyst bed in the converter, contaminate the
product acid, and plug the fiber-bed mist eliminator. Monsanto uses an
electrostatic precipitator (or a combination of cyclone separators followed
by a precipitator) for the hot, dry gas cleaning operation. The rate at
which the catalyst bed plugs depends, of course, upon the quantity of dust
8
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passing through the precipitator. When the catalyst bed becomes plugged,
it is necessary to remove the catalyst for cleaning. The cleaning
operation requires a shutdown of two to three days and involves the loss
of about 2.5 percent of the catalyst by attrition and breakage (Monsanto
estimates). Monsanto estimates that it will be necessary to clean the
catalyst at three-month intervals if the dust passing to the catalyst
bed does not exceed the quantity allowed for in the Monsanto design.
The precise quantity of dust permissible in the Cat-Ox system is con-
sidered confidential by Monsanto. However, the required dust collector
efficiency on fly ash from firing of pulverized coal is greater than
99.6 percent, which is higher than is now being provided commercially in
fly ash precipitators.
Of the dust that passes through the converter, a portion is collected
in the acid in the absorption tower, and the remainder is collected with
very high efficiency by the fiber-bed mist eliminator, where the buildup
of solids produces an increase in the resistance to gas flow. Monsanto
reports that it has developed a mist eliminator that can be washed
periodically, while on stream, to remove the accumulated solids. However,
they declined to furnish information on its design and operation; the
author is therefore unable to judge what the actual status of this develop-
ment may be, but questions whether there has yet been an adequate demon-
stration of the ability to keep the mist eliminator free of solids buildup
over long periods of continuous operation.
The amount of maintenance required for the Cat-Ox system will be
acutely dependent on the performance of the electrostatic precipitator,
even in the absence of outright precipitator failure. Even a small dete-
rioration of the precipitator performance may increase the quantity of
dust entering the converter by a large factor, with a corresponding
reduction in the length of the intervals between shutdowns for cleaning.
It will therefore be necessary to attain high standards of maintenance
and operation of the precipitators — probably much higher than the
standards normally reached in the U.S. power industry.
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Cat-Ox systems are provided in multiple, parallel independent
trains, so that part of the power plant can continue to operate even
if one train should fail. The Cat-Ox reheat system can be bypassed in
the event of complete failure. If, however, part or all of the inte-
grated Cat-Ox system should fail, it is necessary to reduce the load on
the boiler or shut it down. Consequently, the converter in each train
is further divided into parallel modular sections that can be dampered
off to permit catalyst cleaning if this should be necessary between
scheduled shutdowns.
It appears possible to cope with the actual or potential technical
problems of the Cat-Ox system should components of the existing system
not prove adequate. However, the alternative solutions will tend to
increase both capital costs and those operating costs other than main-
tenance.
The by-product produced by the Cat-Ox system, 78 percent sulfuric
acid contaminated with a small amount (less than 0.1 percent) of fly ash,
is not a generally favorable one from an economic standpoint. Unless it
can be transported by barge, its movement to markets is severely limited
by the cost of shipment. This is also true, though in lesser degree, of
the more concentrated acids, but the Cat-Ox product acid is additionally
limited in applications by its lower concentration and its impurities.
The manufacture of phosphate fertilizers is virtually the only large and
growing market that might absorb a large output of the Cat-Ox acid, and
the availability of even this market is highly dependent upon specific
local conditions.
To permit estimates of capital and annual costs of the Cat-Ox and
Wellman-Lord SO_ Recovery systems, two models of hypothetical power
A
plants were created. (Details are presented in Appendix A.) The first
(Model A) is an existing 500-megawatt plant located in central Pennsylvania
(Altoona). The second (Model B) is a new 1000-megawatt plant located on a
navigable river in the Midwest (Cairo, Illinois). Cost estimates were made
for a Cat-Ox reheat system to be applied to the Model A plant (Table I) and
for an integrated Cat-Ox plant to be applied to the Model B Plant (Table II)
10
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Table I
SUMMARY OF ESTIMATED COSTS
CAT-OX REHEAT SYSTEM FOR POWER PLANT
MODEL A
Item
Cost or Credit
Capital Investment
$16,200,000
$32.40/kw
Gross Costs before Credits
$4,853,950/yr
1.39 mills/kwh
13.9£/million Btu
$3.70/ton of coal
Credits for Product Acid
A. Acid price $11.00/ton
B. Acid price $ 6.00/ton
$l,181,000/yr
$ 644,300/yr
Net Costs after Credits
A. Acid price $11.00/ton
B. Acid price $6.00/ton
$3,672,750/yr
1.05 mills/kwh
10.5£/million Btu
$2.80/ton of coal
$4,209,650
1.20 mills/kwh
12.0£/million Btu
$3.20/ton of coal
Prices on 100-percent acid basis.
11
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Table II
SUMMARY OF ESTIMATED COSTS
INTEGRATED CAT-OX SYSTEM FOR POWER PLANT MODEL B
Item-
Cost or Credit
Gross Capital Investment
Capital Credit
Incremental Capital Investment
$50,800,000
$16,896,000
$33,904,000
$33.90 Aw
Gross Costs before Credits
Credit for Product Acid1
Acid price $10.00/ton
$ 7,610,940/yr
1.09 mills/kwh
11.3£/million Btu
$3.02/ton of coal
$2,084,600/yr
Net Costs after Credits
$5,526,340/yr
0.789 mill/kwh
8.22
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Estimates of annual cost were based on a plant life of 20 years and on
an annual operating time of 7000 hours (load factor 80 percent). The
estimated prices for by-product sulfuric acid are based on an assumed
Gulf Coast price for sulfur of $30/long ton, which is probably as high
as can be anticipated in the period up to 1975.
For the Cat-Ox reheat system used in the Model A plant, fixed charges
account for 48 percent of the gross total annual cost before allowance for
by-product credits. The next largest item of cost (18 percent) is that
for the No. 2 fuel oil used in reheating the flue gas. Two acid prices
are presented in estimating the by-product credit for the acid produced;
the lower of the two is applicable in the event that it should be neces-
sary to displace existing captive acid producers in the Pittsburgh area,
where the acid from the Altoona plant would have to be sold.
For the integrated Cat-Ox system used in the Model B plant, a
capital credit is due because a large part of the gross capital invest-
ment is for items that would be part of the conventional power plant
without sulfur dioxide emission controls. Fixed charges account for 65
percent of the gross total annual cost before allowance for by-product
credits. Maintenance (13 percent) and electrical power (10 percent) are
the next largest cost items.
13
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IV PROCEDURES
A. Formulation of Models
The formulation of the models of the hypothetical power plants was
based first on conditions specified by DPCE-NAPCA. Part of the remaining
model conditions were derived from the NAPCA specifications so as to be
consistent with the latter. Other conditions had to be selected on a
relatively arbitrary basis, and where this was necessary, guidance was
obtained from the literature or from discussions with informed individuals,
In other cases, conditions had to be selected on completely arbitrary
bases, and the author used his own experience and best judgment. The
complete models are presented in Appendix A.
The basic premise employed throughout was that all estimates of the
costs of sulfur oxides control should be made in relation to a base level
represented by the equivalent conventional plant without sulfur oxides
emission controls. Emission control was to be charged with whatever
costs were incurred above those of the conventional, or base-level, plant.
Similarly, it was to be credited with any savings from the cost of the
base-level plant, or with any income derived from sale of by-products.
If the sulfur oxide control system included components common to
the base-level plant, it was to be charged only with the increment in the
costs of these components over those of the corresponding items in the
base-level plant (or credited with the difference if the items in the
conventional plant were more expensive).
The sulfur oxide control system was to be charged with the cost of
power required to move the flue gas through any parts of the system
specific to emission control. It was also to be charged with a pro-
portionate share of the capital cost of the fan and motor if the latter
also supplied draft for the rest of the boiler system.
15
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The model formulations were circulated to DPCE-NAPCA and to Monsanto
Company and Wellman-Lord, Inc. for review before adoption.
B. Cost Factors
The factors used in making cost estimates, and the bases on which
they were adopted, are presented in Appendix B. Salaries, payroll bene-
fits, and overhead were taken to be the same as those used in the previous
o
study of the nonferrous smelting industry.
Estimates of the prices that might be obtained for sulfur by-products
were made by Stanford Research Institute, and are presented in Appendix D.
The choices of the specific sites for the hypothetical plants (within the
general areas specified by DPCE-NAPCA) were made to facilitate estimation
of definite prices.
C. Preparation of Technical Data and Cost Estimates
The Monsanto Company prepared the technical designs for the model
control systems, based on the conditions formulated by SRI, and esti-
mated the capital investments and the utilities and maintenance require-
ments. The author reviewed these estimates and accepted most of them.
Since the design data and specific component configurations were in most
cases not revealed by Monsanto, it was generally impractical to make a
critical, independent analysis of the capital cost estimates. The cost
breakdowns for individual components of the systems were supplied by
Monsanto, however, and were surveyed for general reasonableness and con-
?
sistency with the model specifications.
Some of the original estimates of system requirements or costs made
by Monsanto were not accepted, and the author supplied his own estimates.
Most of the differences were resolved with Monsanto; the operating labor
requirement was the only substantial exception.
16
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V PROCESS DESCRIPTION
The Monsanto Cat-Ox process is essentially a variation on the con-
tact process for sulfuric acid manufacture, which is extensively treated
in the literature.'* The process was developed primarily for removal of
sulfur dioxide from the flue gases of coal- or oil-fired power plants,14'15
in which the sulfur dioxide concentration usually falls within the range
from 0.1 to 0.4 percent by volume. It differs from the conventional
contact process in three principal respects:
1. Because of the diluteness of the sulfur dioxide, the plant
is not autothermal; that is, the heat released by the
oxidation of the sulfur dioxide is insufficient to preheat
the feed gas to the reaction temperature (generally above
800°F). Hence, if the gas is not already near that
temperature, auxiliary heat must be employed.
2. The system operates on wet gas. The feed gas is not dried
before it enters the converter (catalytic reactor). Hence,
the amount of sulfuric acid mist formed is relatively much
greater than that formed in the conventional contact
process, and a collector must be employed to recover the
mist from the tail gas.
3. The heat in the exit gas from the converter is used to a
greater or lesser degree to concentrate the sulfuric acid
formed in the final absorption step. The concentration
of the acid produced depends upon the temperature to which
the flue gas stream is reduced during contact with the acid
in the absorber.14
All of the component basic concepts of the Cat-Ox system have been
demonstrated previously under some circumstances. Catalytic oxidation
of sulfur dioxide at low concentrations has been demonstrated previously
in the laboratory and in pilot plant studies that were preliminary
parts of the development of the Cat-Ox system.-^ A similar process
(SNPA-Topsoe) has been in large-scale commercial operation in Lacq,
France for several years on the incinerated tail gases from a Claus sul-
C C *T
fur plant; '' there the concentration of sulfur dioxide is of the
order of 1.0 percent.
17
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Some relatively conventional contact plants for manufacture of
sulfuric acid from hydrogen sulfide have been built for use on wet
gases.4 The hot gas from the combustion of hydrogen sulfide (partly
cooled in a waste heat boiler and diluted with air to give a sulfur
dioxide concentration of about 7.5 percent) enters the converter without
being dried. The hot exit gas from the converter is not cooled, as is
customary in contact plants, but goes directly to an absorber. Because
of the relatively high water content of the gas stream, it is not
feasible to produce acid stronger than 93 to 94 percent. Also, rela-
tively large quantities of sulfuric acid mist are formed that must be
removed from the tail gas with some type of mist collector. The prin-
cipal advantage sought in this type of plant is a reduction in capital
cost.
In this report, two variations of the Cat-Ox system are described:
1. An integrated system to be constructed as part of a
new power plant (Fig. 1).
2. A system to be added to an existing power plant
(Fig. 2).
A. Integrated Cat-Ox System for Power Plants
The integrated Cat-Ox system, which is conceptually applied to the
hypothetical power plant Model B, cannot be applied to an existing power
plant without an extended shutdown and major alterations of the plant.
Consequently, it will generally be feasible to install it only where it
can be incorporated as an integral part of a new plant.
The system as applied in the demonstration plant at the Portland
station of the Metropolitan Edison Company is described in a paper by
Stites et al.^2 The arrangement as proposed for use in a large power
plant typified by Model B differs in some details from that employed in
the demonstration plant. Parts of the system, as shown in Fig. 1, are
integral components of the power plant and would be in use even if the
Cat-Ox system were not employed. These are the electrostatic pre-
cipitator, the fly ash handling system, the economizer, the air heater,
18
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Courtesy of Monsanto Enviro-Chem Systems, Inc.
TA-7923-28
FIGURE 2 CAT-OX REHEAT SYSTEM FOR POWER PLANT MODEL A
-------
the steam/air heater, the induced draft fan, and the forced draft fan.
Some of these units are, however, more and some less costly than the
corresponding ones in the conventional power plant. The electrostatic
precipitator handles the flue gas at a higher temperature than in the
conventional installation, and hence must be correspondingly larger.
(Fly ash precipitation is reportedly easier at the higher temperature,
however, so that the increase in the required precipitator size may not
be directly proportional to the increase in gas volume.) The efficiency
of the precipitator must also be higher than is currently being provided
in any commercial fly ash precipitators.
The air heater recovers less of the heat from the flue gas than
does that in a conventional plant, and hence is smaller than the latter.
The induced draft fan must be sized and powered to overcome the gas flow
resistance of the converter, absorbing tower, and mist eliminator as well
as that of the elements of the conventional system.
The air heater proposed by Monsanto for the Model B plant is a
tubular recuperative type. Alternatively, a regenerative air heater of
the Ljungstrom type might be used. The Ljungstrom air heater, which is
widely used in conventional power plants, is less costly than the tubular
type. However, there is leakage of air into the flue gas. Hence, with
the Ljungstrom unit the system downstream of the air heater would have to
be designed to handle a slightly greater gas flow.
The outlet flue gas temperature at the air heater, and hence the
amount of heat recoverable in the latter, is determined by the necessity
for remaining above the acid dew point to avoid corrosion. In addition,
it is necessary to avoid "cold-end" corrosion in the air heater itself.
If air at ambient temperature is admitted directly into the air heater,
the metal wall temperature may be low enough in localized areas for acid
condensation to take place on the flue-gas side. To avoid this, the
combustion air is heated first in the fluid/air heater, and second in the
steam/air heater. Such tempering of combustion air with a steam/air
heater is commonly practiced in conventional power plants using high-
sulfur fuels, also in order to avoid air heater corrosion.
21
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The fluid-air heater is employed to recover additional heat from
the circulated acid, which is cooled after each pass through the absorb-
ing tower. Heat is transferred from the acid circulation cooler to the
fluid/air heater through an intermediate heat transfer fluid. Direct
transfer of heat from the acid to the air in the fluid/air heater is
avoided because a leak in the tubing could introduce acid into the com-
bustion air system.
Additional heat is recovered from the acid by heating of condensate
as feed water for the boiler. The possibility of acid leaking into the
boiler feed water is avoided by circulation of an intermediate heat trans-
fer fluid between the acid circulation cooler and the condensate heat
exchanger.
The heat recovered from the acid by the preheating of the boiler
feed water and combustion air compensates for the reduced heat recovery
in the air heater. The overall heat recovery in the power plant cycle
is essentially the same as in the corresponding conventional power plant,
although part of the heat is recovered at lower temperature levels at a
corresponding increase in capital expenditure for heat transfer equip-
ment. The detailed breakdown of the low-level heat recovery is consid-
ered confidential by Monsanto, and stream temperatures in this part of
the system are therefore not indicated in Fig. 1. The minimum outlet gas
temperature at the absorbing tower is set by the concentration of the acid
produced (78 percent).
In a conventional power plant, the preheating of the boiler feed
water is accomplished with steam extracted from the turbines driving the
generators. In the Cat-Ox system, where the boiler feed water is preheated
in the acid cooler, the equivalent amount of steam can be used directly to
drive the turbines, providing a credit for additional power generation.
However, additional cooling water must be supplied to the condensers to
condense the additional amount of steam exhausted from the turbines. The
cost of the extra cooling water is taken as a debit against the Cat-Ox
system.
22
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The absorption of the sulfur trioxide in the cooled, recirculated
acid is accomplished in a packed tower. The mist of sulfuric acid
formed during the cooling of the gas, plus any entrained droplets of
the circulating acid carried out of the tower, are recovered in a fiber-
bed mist eliminator. Fine particles of fly ash that have escaped collec-
tion by the electrostatic precipitator or the catalyst bed in the con-
verter are collected in part in the absorber, and remain in the product
acid. The particles not removed from the gas in the absorber are collec-
ted with very high efficiency by the mist eliminator. Other fly ash
particles present in the circulating acid may be carried in entrained
droplets to the surfaces of the mist eliminator. The gradual buildup of
solid particles in the eliminator produces an increase in the gas pressure
drop, which must be counteracted by periodic washing of the fiber beds.
The maximum permissible solids buildup is determined by the allowable gas
pressure drop, which is in turn set by the fan and motor capacity provided
for in the system design. In the design used, the pressure drop across
the mist eliminator ranges from approximately 13 inches of water in the
clean condition to 20 inches of water with the maximum permissible solids
buildup.
The original mist eliminator used in the Portland demonstration plant
was a Brink mist eliminator.2,12 It was necessary to shut down the unit
in order to clean the mist eliminator elements when they had become
plugged with fly ash particles. However, Monsanto now has under develop-
ment a new type of fiber-bed eliminator that is designated as the Cat-Ox
mist eliminator.9'10 Monsanto reports that this unit can be washed while
on stream, eliminating the need for shutdowns except at scheduled main-
tenance periods. As provided for in the systems shown in Figs. 1 and 2,
the fiber beds are washed with a portion of the product acid that is con-
tinuously filtered and accumulated in the process acid tank. Monsanto
states that the fiber beds may be washed with water instead of acid, but
has presented no estimates of the probable frequency of washing required
under average operating conditions. If the frequency of washing is not
too great and the quantity of flushing fluid required is moderate, it
should be possible to use water without excessively diluting the product
acid.
23
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Monsanto considered the design and construction of the Cat-Ox mist
eliminator to be particularly sensitive information falling outside the
provisions of the confidentiality agreement, and did not reveal the
information for evaluation in this study.
The Cat-Ox system as proposed for the Model B plant consists of
four parallel trains, each of which can be operated independently. Modu-
lar construction is employed; each converter is divided into three sec-
tions which can be closed off independently by dampers. Thus, one-twelfth
of the total converter flow area can be cut out of the system if it should
be necessary to clean the catalyst between the regular plant shutdown
periods.
The gradual buildup of fly ash in the catalyst bed results in an
increase in the gas pressure drop, although there is reportedly little
effect on the conversion efficiency of the catalyst. The system there-
fore includes equipment to withdraw, convey, clean, and return the
catalyst to the converter. Monsanto estimates the total downtime for a
converter unit as two or three days, including time for cool-off, clean-
ing, and heat-up of the catalyst. The loss of catalyst due to attrition
and breakage during screening and mechanical handling is estimated by
Monsanto to be about 2.5 percent by volume (and weight) for each cleaning.
Monsanto also anticipates that it will be necessary to clean the catalyst
beds after each three months of operation, or a total of four times per
year. The corresponding loss of catalyst is about 10 percent per year.
The vanadium pentoxide catalyst is a special type designed specifically
for use in the Cat-Ox system.
In the converter, a single pass through a catalyst bed is employed
to attain 90-percent conversion of the sulfur dioxide to the trioxide
with the boiler furnace operating at full load. Because power consump-
tion represents such a large item in the operating cost, the catalyst bed
depth and configuration are designed for low gas pressure drop (1 inch of
water when clean). As with the mist eliminator, the maximum permissible
solids buildup in the catalyst bed is set by the fan and motor capacity,
and the Monsanto design provides for a normal maximum pressure drop
24
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through the dirty bed of 4 inches of water. The required precipitator
efficiency and the permissible dust concentration in the gas stream
entering the converter are considered by Monsanto to be confidential
information. It can be stated only that the precipitator efficiency is
Q
in excess of 99.6 percent.
The principal incremental gas pressure drop chargeable against the
Cat-Ox system is that through the converter, absorbing tower, and mist
eliminator. The pressure drops across the economizer, air heater, and
electrostatic precipitator would be incurred in the conventional system.
The pressure drop across the absorbing tower is approximately 5 inches of
water, and there are some losses in the additional ductwork of the Cat-Ox
system. The total incremental pressure drop is taken as 22 inches of
water for the clean system and 32 inches of water with the designed max-
imum allowable solids buildup.
The conversion of sulfur dioxide to the trioxide in the contact
process depends on both the temperature and the contact time in the
catalyst bed. In the integrated Cat-Ox process, variations in boiler
load will affect both the gas temperature and the contact time. As the
boiler load is reduced, the flue gas temperature tends to drop^ but
the contact time increases, so that the two effects tend within limits
to counteract one another. The Monsanto design is intended to provide
a high level of conversion over a reasonable range of variation in the
boiler load. However, it is anticipated that a reduction of the boiler
load to 50 percent of the rated capacitity may reduce the flue gas
temperature to about 750° and reduce the conversion of the sulfur
dioxide to about 80 percent. In such a case, the quantity of sulfur
dioxide emitted would remain the same as at full load, because the
reduction in the quantity of flue gas would compensate for the reduced
conversion efficiency.
B. Cat-Ox System for Application to Existing Power Plant
The Cat-Ox system proposed for application to the Model A power
plant is additive instead of integral. It could be bypassed entirely
without affecting the operation of the power plant except during a rela-
25
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tively short period in which the final tie-ins are made; this period
could coincide with the annual shutdown of the power plant for overhaul.
The additive Cat-Ox system (Fig. 2), termed by Monsanto a "reheat
system," takes the gas from the discharge of the existing induced draft
fan. A new precipitator is added to reduce the quantity of dust to a
tolerable level, which is the same as in the integrated Cat-Ox system.
A new induced draft fan is added to move the gas through the Cat-Ox
system.
The relatively cool (325 F) flue gas must be heated to a temperature
high enough to permit catalytic oxidation of the sulfur dioxide. Part of
this preheating is accomplished by exchange of heat from the converter
exit gas in the gas heat exchanger. The rest of the preheating is
accomplished by direct injection of hot combustion gases from the reheat
furnace, which is fired with No. 2 fuel oil of low ash content. The hot
gas leaving the converter passes through the gas heat exchanger where it
heats the incoming cool feed gas and is itself cooled to 450°F before it
enters the absorbing tower. From this point on, the arrangement and
operation are essentially the same as in the integrated Cat-Ox system.
The temperature of the gas leaving the heat exchanger, 450°F, is set as
in the integrated system by the necessity of staying above the sulfuric
acid dew point.
In the gas heat exchanger, as in the air heater of the integrated
system, it is necessary to avoid cold-end corrosion. The temperature of
the incoming cool flue gas must therefore be raised somewhat, and this
is accomplished by the recycle of a portion of the hot exit gas to the
inlet of the exchanger. The sizes of the new induced draft fan and the
gas heat exchanger must be increased accordingly to handle the greater
total flow of gas.
The acid circulated to the absorbing tower is cooled directly with
cooling water, and there is no recovery of the low-level heat. The
product acid is of 78 percent concentration, as in the integrated system.
The gas heat exchanger is of the Ljungstrom regenerative type
instead of the recuperative type employed as the air heater in the inte-
26
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grated system. In the regenerative heater, seal leakage permits some of
the cool incoming gas to bypass the heater and converter and infiltrate
the hot gas stream going to the absorbing tower. The sulfur dioxide in
the leakage gas is not oxidized and recovered. Consequently, the frac-
tional conversion of sulfur dioxide in the gas that does pass through
the converter must be increased to compensate. Monsanto estimates that
the leakage will be 5 percent of the input gas flow, and that 94 percent
conversion of the sulfur dioxide can be maintained. Overall recovery of
sulfur dioxide as sulfuric acid will be 89 percent.
In the reheat system as contemplated for general application to
power plants,10 Monsanto proposes to design for 90 percent conversion,
so that with a leakage of 5 percent of the gas through the heat exchanger
the overall removal of sulfur dioxide will be 85 percent. However, for
the Model A plant an overall removal of 90 percent of the sulfur dioxide
was specified. In the Cat-Ox converter design to be used for power plant
applications, 94 percent conversion is a practical upper limit; hence,
the overall removal of sulfur dioxide falls, slightly short of the speci-
fied level, although not very significantly so. The converter could be
designed to achieve a greater conversion, but presumably it would require
a greater quantity of catalyst and have a higher gas pressure drop. A
preferable alternative approach might be to use a tubular heat exchanger,
which would be more costly but would avoid the gas leakage problem assoc-
iated with the Ljungstrom exchanger.
In the Cat-Ox plant proposed by Monsanto for application to the
Model A power plant, provisions are made to divert the entire flow of
flue gas directly to the stack through a dampering system should it be
necessary to take the recovery system off the line for unscheduled main-
tenance or catalyst cleaning. Hence, no provisions are made in the con-
verter for taking modular sections out of service for catalyst cleaning
while the gas flow is continued through the remaining sections, as can be
done in the integrated system. Scheduled catalyst cleaning will take
place at three-month intervals, and periodic flushing of the mist elim-
inator can be carried out, as in the integrated system.
27
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Two complete and independent parallel trains are used, so that one-
half of the normal full-load gas flow can be accommodated if one train
is out of service.
The equipment and its arrangement for reheating the flue gas and
subsequently recovering heat from it10 were chosen by Monsanto to
achieve desired economies in capital investment and operating costs.
The use of No. 2 fuel oil for reheating was specified by SRI for the
Model A plant. Natural gas might be substituted where it is available.
Coal or residual oil could be used in a reheat system, but the equipment
and its arrangement would necessarily be different than are portrayed in
Fig. 2 and Reference 10.
The entire capital and operating costs of the additive, or reheat,
Cat-Ox system are, of course, chargeable to sulfur dioxide recovery.
The total gas pressure drop through the system ranges from 31 inches of
water in the clean condition to 41 inches of water with the designed
maximum permissible dust buildup in the converter and mist eliminator.
In the reheat Cat-Ox system, the temperature of the flue gas enter-
ing the converter is independent of the boiler load and can be held at a
constant level even while the boiler load and the flow of flue gas are
varying. Hence, the conversion can be maintained at a high level at all
times. A reduction in flue gas flow should permit an increase in con-
version because of the increased contact time in the catalyst bed.
28
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VI PROCESS DATA AND COST ESTIMATES
A. Material Flows
The estimated quantities of coal burned in the two hypothetical
power plants — and hence the calculated quantities of flue gas, sulfur
compounds, and fly ash to be handled —• were fixed by the assumed heat
rates (see Appendix A). The heat rates assumed for the Model A and
Model B power plants were 10,000 and 9,600 Btu/kwh, respectively. These
values are probably between 5 and 10 percent higher than those that might
reasonably be obtained at comparable actual plants. Although the assumed
heat rates are perhaps excessively conservative, they are well within the
probable precision of the other estimates made for the study.
Although the calculated volume of flue gas generated was increased
by use of the high values of heat rate,, no allowance was made for any
increase in gas volume resulting from air infiltration into the flue gas
handling system, which occurs in actual practice.
The estimated rates and conditions of gas flow through the Model A
and Model B Cat-Ox systems are presented in Tables III and IV, respec-
tively. The flow rates of the main flue gas stream are given both at
standard conditions (70°F, 1 atm) and at the actual conditions existing
at the indicated points in the system. Allowances were made for the small
quantities of air that bleed into the system after being used to purge the
insulator compartments of the electrostatic precipitators. In the Cat-Ox
reheat system (Model A), the flue gas also receives a small additional
quantity of gas from the direct-fired reheater furnace.
Some water vapor is removed from the flue gas in the absorbing tower,
where it reacts with sulfur trioxide to form sulfuric acid and also dilutes
the product.
The estimated distributions of the contaminants (sulfur oxides and fly
ash) in the control systems are presented in Tables V and VI. In the
29
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Table III
MODEL A
GAS FLOWS IN CAT-OX REHEAT SYSTEM
Gas Flows Entering System
Flue gas entering precipitator
Purge air to precipitator
insulator compartments
Flue gas from reheater furnace
Total
1,564,360 CFM (325 F, 1 atm)
1,056,200 SCFM (70°F, 1 atm)
4,510 SCFM (70 F, 1 atm)
35,280 SCFM (70°F, 1 atm)
1,095,990 SCFM (70 F, 1 atm)
Gas Flows Leaving System
Water vapor in product acid
Tail gas to stack
5,110 SCFM (70 F, 1 atm)
1,432,550 CFM (236°F, 1 atm)
1,090,880 SCFM (70°F, 1 atm)
Total
1,095,990 SCFM (70 F, 1 atm)
30
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Table IV
MODEL B
GAS FLOWS IN INTEGRATED CAT-OX SYSTEM
Gas Flows Entering System
Flue gas entering precipitator
5,011,840 CFM (850 F, 1 atm)
2,027,690 SCFM (70°F, 1 atm)
Purge air to precipitator
insulator compartments
15,500 SCFM (70 F, 1 atm)
Total
2,043,190 SCFM (70 F, 1 atm)
Gas Flows Leaving System
Water vapor in product acid
Tail gas to stack
9,930 SCFM (70 F, 1 atm)
2,677,760 CFM (238°F, 1 atm)
2,033,260 SCFM (70°F, 1 atm)
Total
2,043,190 SCFM (70 F, 1 atm)
31
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Table V
MODEL A
CONTAMINANT REMOVAL IN CAT-OX REHEAT SYSTEM
Contaminants Entering in Flue Gas
Fly ash 1,405 Ib/hr
SO 22,200 Ib/hr
ft
S03 391 Ib/hr
Contaminants Emitted in Tail Gas
Fly ash Negligible
H SO vapor and mist 142 Ib/hr
£t *x
S00 2,380 Ib/hr
£i
By-Product Recovered
78-percent H SO 39,330 Ib/hr
£i *.
Equivalent 100-percent H SO 30,680 Ib/hr
Table VI
MODEL B
CONTAMINANT REMOVAL IN INTEGRATED CAT-OX SYSTEM
Contaminants Entering in Flue Gas
Fly ash 53,960 Ib/hr
SO 42,620 Ib/hr
£t
SO 751 Ib/hr
O
Contaminants Emitted in Tail Gas
Fly ash Negligible
H SO vapor and mist 252 Ib/hr
^ 4
SO 4,160 Ib/hr
£t
By-Product Recovered
78-percent H SO 76,360 Ib/hr
& i
Equivalent 100-percent H SO 59,560 Ib/hr
£i 4
32
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estimates for the Model A system (Table V), the very small quantity of
sulfur present in the No. 2 fuel oil used for reheating has been ignored.
For the amount of sulfuric acid emitted in the tail gases from the
systems, Monsanto did not specify the distribution between vapor and mist.
However, most of the escaping acid is in the vapor state; probably no
more than 10 percent of it is mist.
The quantities of fly ash emitted in the tail gases are character-
ized as negligible. Estimates of the actual quantities were not pro-
vided by Monsanto; however, the author believes that the emissions of
fly ash will not exceed three to five Ib/hr.
B. Capital Cost Estimates
The estimates of capital investment presented in Tables VII, VIII,
and IX were prepared by Monsanto with only two minor exceptions. The
author modified the original credit for the electrostatic precipitators
of the Model B plant (Table VIII) for better conformity with the spec-
ifications set for the models. The credit represents the cost of the
precipitator of 99.5 percent efficiency, operating at 325°F, that would
be used on the power plant if no sulfur oxide emission controls were
employed (see Appendix A).
The author also provided the credit for the stack for the Model B
plant, which represents the difference between the cost of an 800-foot
stack used with no sulfur oxide emission controls, and that of a 500-
foot stack used when emission controls are applied (see Appendix C). The
same stack credit was assigned arbitrarily to both the Cat-Ox and Wellman-
Lord control systems for the Model B plant. The precision of the estimate
is low, and the credit is provided mostly for purposes of illustration.
As is discussed in Appendix C, some reduction of stack height in this
model case appears justified. It appears to the author, as to some
others, that the problems of securing dispersion of scrubbed stack gases
have been overstressed.
33
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Table VII
POWER PLANT MODEL A
SUMMARY OF CAPITAL AND OPERATING COSTS FOR CAT-OX REHEAT SYSTEM
Item
Capital Investment
Annual Costs
A. Fixed Charges
B. Direct Operating Costs
1. Operating labor
2. Supervision
3. Payroll benefits
4. Maintenance
5. Electricity
6. Fuel oil
7. Cooling water
C. Indirect Costs
1 . Overhead
Total Annunl Cost
(Before credits)
Credit for Acid (100%)
Net Annual Cost (After credits)
A. Acid Price $11 .00/ton
B. Acid Price $6. 00/ton
Quantity
9333 hrs/yr
1750 hrs/yr
15,100 kw
1200 gal/hr
32,000 gpm
107,380 tons/yr
Cost Basis
$
14,5% of capital investment
$3.75/hr
$4.75/hr
25% of labor and supervision
3% of capital investment
0.55£/kwh
$4.25/barrel
$0,02/1000 gal
50% of labor, supervision, and
maintenance
$
mills/kwh
^/million Btu
$/ton coal
$11.00 ton 100% acid
$ 6,00 ton 100% acid
$
mills/kwh
^/million Btu
$/ton coal
$
mills/kwh
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Table VIII
POWER PLANT MODEL B
CAPITAL INVESTMENT FOR INTEGRATED CAT-OX SYSTEM
Total Capital Investment $50,800,000
(Before Credits)
Credits
Induced Draft Fans and Drives 963,000
Forced Draft Fans and Drives 738,000
Precipitators and Supports 3,100,000
Air Heaters 3,195,000
Economizers 716,000
Fly Ash System 253,000
Flues, Ducts and Supports 3,007,000
Steam Coil Air Heaters 67,000
Electrical Switchgear and Wiring 598,000
Stack 850,000
General Field Costs 2,100,000
Subtotal $15,587,000
Engineering and Engineering Overhead 530,000
Contingencies @ 5% 779,000
Total Credits $16,896,000
Incremental Cost of Cat-Ox System $33,904,000
35
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Table IX
POWER PLANT MODEL B
SUMMARY OF CAPITAL AND OPERATING COSTS FOR INTEGRATED CAT-OX SYSTEM
Item
Capital Investment
Annual Costs
A. Fixed Charges
B. Direct Operating Costs
1. Operating labor
2, Supervision
3. Payroll benefits
4. Maintenance
5. Electricity
6. Cooling water
7. Condenser cooling water
C. Indirect Costs
1 . Overhead
Total Annual Cost
(Before credits)
Credit for Acid (100%)
Net Annual Cost
(After credits)
Quantity
18,666 hrs/yr
3,500 hrs/yr
20,700 kw
466 gpro
25,800 gpm
208,460 tons/yr
Cost Basis
$
14.5% of capital investment
$3.75/hr
$4.75/hr
25% of labor and supervision
3% of capital investment
0 . 55
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C. Operating Cost Estimates
The estimates of fuel, water, and power requirements for the two
Cat-Ox control systems (Tables VII and IX) were developed by Monsanto,
and the corresponding operating costs were calculated by application of
the appropriate cost factors.
The estimate of the annual maintenance cost as three percent of the
Cat-Ox plant capital investment was provided by Monsanto; it includes
the cost of catalyst makeup. However, this estimate is predicated upon
maintenance of specified standards of fly ash collector performance, and
if the latter are not consistently attained, the costs for maintenance of
the equipment may be substantially higher, as is discussed in Sections V
and VII.
Monsanto proposes that the power plant operators should also operate
the Cat-Ox system, and that the only extra labor required should be that
for handling product acid on the day shift. The author believes that at
least one full-time operator will be required on each shift for the Model
A Cat-Ox plant, and that at least two will be required for the Model B
Cat-Ox plant. These operating labor allowances have therefore been
included in the cost estimates presented in Tables VII and IX. In addi-
tion, one day laborer is provided for the Model A plant and two for the
Model B plant. The author considers that this operating labor allowance
is a minimum, and that in view of the critical nature of the fly ash
collector performance, the actual requirement might be twice as great.
37
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VII GENERAL DISCUSSION
A. Evaluation of the Cat-Ox System
There are no reasons to doubt the technical feasibility of the
basic Cat-Ox process. As is noted above, the basic process (essen-
tially, the contact process) is fully established. Technical problems
remain with respect to specific aspects of application of the process,
but these do not constitute essential limitations on the process itself.
It is still uncertain whether some of these technical problems have as
yet been resolved in a fully satisfactory manner, or whether the
approaches taken are in all cases the most appropriate. However, none
of these problems appears to be insoluble. The principal question con-
cerns the possible additional costs that may be necessary to provide
satisfactory solutions.
There are three principal technical problems (actual or potential)
in the Cat-Ox process; these are listed below in their order of
importance:
1. Dust collection
2. Acid mist formation and collection
3. Corrosion.
Of these three, corrosion is dealt with most readily. It presents
its greatest threat in heat exchangers, where it can be avoided, but only
at the expense of reduced heat recovery and of increased capital and
operating costs. Dust collection is by far the greatest problem. Acid
mist collection presents a problem primarily because of its inter-
relationship with the dust collection problem.
It is clear that a large part of the difficulties involved in the
development of the Cat-Ox process — probably the great majority of all
the difficulties — have been associated with fly ash. The permissible
39
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quantity of dust in the gas entering the converter represents a. balance
between the cost of gas cleaning and the cost of maintenance — catalyst
cleaning and makeup, and cleaning of the mist eliminator — plus costs
or losses associated with contamination of the product acid. The fly
ash precipitator must operate not only with very high efficiency but also
with a very high degree of reliability. A serious precipitator failure
could speedily cause the catalysts beds to be loaded with fly ash to the
point where the system would be inoperable. The problem of reliability
is particularly acute with the integrated Cat-Ox system, which cannot be
bypassed. Multiple trains and modular construction are necessary to
ensure that most of the available boiler capacity will remain available
even if a portion of the Cat-Ox system should fail. Nevertheless, even
a deterioration of precipitator performance without outright failure may
seriously increase the problems of converter maintenance. It will there-
fore be necessary to attain high standards of maintenance and operation
of the precipitators — probably much higher than the standards normally
reached in the U.S. power industry. Providing adequate monitoring of
precipitator performance may prove demanding, since even a slight dete-
rioration in efficiency that would not be readily apparent might double
the amount of dust entering the converter.
The collection of the acid mist itself is not likely to present any
serious problems, since fibrous mist eliminators of the Brink type have
been amply demonstrated to give both efficiency and reliability in mist
collection. However, the deposition of residual fine fly ash particles
in the packed fiber bed is a serious problem. Dislodging and removing
solid particles deposited in depth in such packed beds is difficult, and
the history of past attempts to do so is not encouraging. Solids buildup
in the Brink eliminator proved to be a problem in the demonstration
12
plant, and it appears that no satisfactory solution has been developed
until very recently, if indeed it has been attained even now.
Although Monsanto representatives declined to reveal any design
information on the new Cat-Ox mist eliminator, they did permit the writer
to make a brief inspection of some operating records on the unit being
tested at the Portland demonstration plant. The writer believes that he
40
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has been able to deduce the basic approach being taken by Monsanto. If
this deduction is correct, the method being used is neither novel nor
original, but it does possess merit. However, the degree of success
attainable with this type of approach is acutely dependent upon the
quantity and particle size distribution of the dust as well as upon the
detailed design of the eliminator. These same factors can also have
major effects upon the capital cost of the eliminator unit. Since in-
formation of this type has not been made available for study, it is not
possible to evaluate the actual status of this development. It appears
that substantial progress has been made with the new eliminator, but the
writer is not convinced that there has yet been an adequate demonstration
of the ability to keep the eliminator free of solids buildup over long
periods of continuous operation.
If the approach being taken by Monsanto should not in fact be suf-
ficiently effective, there are other techniques available for approaching
the problem of dealing with the combination of acid mist and residual fine
solid particles. It should be possible to develop adequate alternative
solutions, although some additional capital and operating costs may be
incurred.
From an economic standpoint, the Cat-Ox system presents some evident
problems. The capital investment appears relatively high even when it is
considered that cost estimates for some potential alternative systems may
be highly optimistic. However, a greater limitation may be the nature of
the by-product. The relatively dilute (78 percent) sulfuric acid is a
material with a value acutely sensitive to plant location. It has few
uses capable of absorbing a large and growing volume; phosphate fertilizer
manufacture is the most important if not the only one. The acid cannot be
economically shipped for substantial distances except by water. (See below
and Appendix D.) The presence of residual fly ash particles in the acid
may be objectionable in some possible applications, but should not be a
significant limitation on use of the acid in phosphate fertilizer manu-
facture.
The problem of finding markets for the sulfuric acid is shared by
any other processes for recovering acid — even concentrated acid —
41
-------
from power plant flue gases, but with the weak acid it is proportionately
more serious. Sites for possible installations of the system will have
to be chosen carefully; if several large power plants in a given area do
install Cat-Ox systems, they could easily flood any feasible acid markets.
On the basis of this study, the technical improvements in the Cat-Ox
system that appear most needed are ones that might increase the effic-
iency and reliability of the fly ash collection system. Potential improve-
ments of this nature would probably add further to the capital cost, but
might well be justified by reduction in the maintenance costs. As an
example, cyclone collectors might be installed downstream of the electro-
static precipitator. Stanford Research Institute and other investigators
have noted that a large fraction (half or more) of the dust penetrating
dry precipitators may consist of reentrained floes that are of relatively
large particle size even though composed of smaller discrete particles.
Properly designed secondary cyclones might cut the normal dust load enter-
ing the converter by as much as 50 to 75 percent. The material collected
by the cyclones would be that which would deposit predominantly in the
catalyst bed. Very fine discrete particles, which would tend to pass
through the catalyst bed, would be little affected. The secondary
cyclones would be of little benefit in the event of precipitator failure.
B. By-Product Values
Over the period of about one year (1969), sulfur supplies have
passed abruptly from shortage to surplus, and it is now difficult, if
not impossible, to determine whether there is an established price for
sulfur. The Gulf Coast price of sulfur produced by the Frasch process
basking been the base level for world sulfur prices, but it is apparently
not so at present, and it is uncertain whether it will be again. In the
previous study of the nonferrous smelting industry® it was estimated that
the average Gulf Coast price of sulfur over the period to 1975 might be
$30/long ton. When the estimate was made, the Gulf Coast price was over
$40/long ton; currently, sulfur is reported to be selling in some areas
for less than $15/long ton. Although sulfur prices could rise again
during the next five years, the figure of $30 now appears to be at the
42
-------
optimistic end of the probable range of prices that may prevail to 1975
(see Appendix D). Nevertheless, it has been used as the basis for by-
product price estimates in this study because there appears to be none
that can be used with significantly greater confidence, and the results
still have some usefulness if their limitations are understood.
Even the original estimate of $30/long ton f.o.b. the Gulf Coast
was based on the assumption that no very substantial quantity of sulfur
would be recovered at power plants before 1975. The amounts of sulfur
and sulfur by-products potentially recoverable from the coal consumed by
•
power plants (whether from the coal itself or from the flue gases) are
so large that recovery of a substantial part of them could completely
disrupt existing local sulfur markets. The estimates made of sulfur and
sulfuric acid prices that might be obtained by the Model A and Model B
power plants (see Appendix D) embody the tacit assumption that no other
large power plants near the assumed locations will also be recovering
sulfur by-products.
In summary, it appears that the credits assigned to sulfur by-
products from the Model A and Model B plants are the highest that can be
reasonably expected in the period up to 1975. The actual credits might
be much lower, and the situation that may exist after 1975 is unpredict-
able on the basis of currently available information. Conservatively,
the long-term assessment of the economics of recovery processes should
probably be based on the gross costs without allowance for by-product
credits.
It is sometimes suggested that recovery processes can be more eco-
nomically applied to power plant flue gases if coals with extra high
sulfur contents are used. Since the sulfur dioxide concentration in the
flue gas will be increased, the unit cost of production of the sulfur
by-product should indeed be reduced. However, if the by-products must
enter markets that are already glutted, the additional quantity of
material may merely exert an increased downward pressure on prices. The
actual advantage, if any, of using the high sulfur coal would probably
be related to lower purchase prices for the coal.
43
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C. Variations of Bases of Cost Estimates
The choice of some of the economic factors assumed in making the
cost estimates was to a considerable degree arbitrary, as it has been in
similar studies made by other workers. There is no general agreement
among different workers on what constitutes a realistic set of bases.
The result is that estimates for different control systems have been made
on a wide variety of bases — frequently only vaguely defined ~ so that
the results cannot be directly compared. In this study, it has been
assumed that valid comparison of different control systems is more im-
portant than absolute accuracy, and hence that it is more important that
the bases used should be uniform than that they should be the "correct"
ones — whatever the latter might be. An effort has been made to describe
fully all bases used, so that the results can be recalculated to any
different bases that other persons may wish to use. Although the bases
chosen for the models (Appendixes A and B) are believed to be generally
reasonable, it must be noted that other possible assumptions regarding
some factors might be equally reasonable but lead to markedly different
cost estimates.
1. Load Factor
The assumed annual load factor of 80 percent (equivalent to 7,000
hours per year of operation at full load) was specified by DPCE-NAPCA.
It appears to be fairly reasonable for large, new, or relatively new
baseload plants such as those typified by Models A and B. It is less
certain whether the 80 percent factor can be assumed to hold over the
whole of the assumed life (20 years) of the emission control systems.
The Federal Power Commission assumes (Hydroelectric Power Evaluation,
Report P-35, March 1968) that the annual load factor will be relatively
high over the first half of the estimated 30- to 35-year service life of
a power plant, then will diminish, giving a lifetine average of 55 to 60
percent. Dennis and Bernstein^ have assumed a load factor of 60 percent
over an assumed 11-year life for the emission control systems. Although
their assumptions may be somewhat too conservative, the author believes
that the average factor of 80 percent assumed in the present study is
probably too optimistic.
44
-------
Because of the high fixed charges associated with the emission
control systems, the load factor exerts great leverage in determining
the incremental cost of producing electricity. The estimates of incre-
mental unit costs presented in Tables I and II are probably as low as
they can be expected to be with respect to load factor. The actual
costs would probably be higher.
2. Amortization Period
Depreciation for the emission control systems was based on an
assumed useful life of 20 years. It was considered that any control
system that might go into service at the present time will be obsolete
within 20 years, if not in a shorter period. It was assumed that tax
laws and the regulations of public utility commissions will permit the
use of this period in determining taxes and rates for electrical power.
At this time there appear to be no precedents to indicate how the exist-
ing depreciation schedules of the tax laws might be interpreted in some
o
specific cases. Dennis and Bernstein assumed a depreciation period of
11 years, as specified for chemical plants by the Internal Revenue
Service. Their choice of depreciation period apparently was influenced
by a conviction that the recovery processes would become obsolete within
the 11-year period as well as by the IRS regulations.
Some components of sulfur dioxide recovery systems are relatively
conventional items of chemical plant equipment. For example, a basically
standard contact process plant would be used to convert concentrated
sulfur dioxide from a recovery system to sulfuric acid. Allowances for
depreciation of the acid plant might then depend upon whether the plant
was owned by the power company and considered to be part of the utility,
or owned by an adjacent chemical company and considered to be part of a
chemical plant.
For the purposes of the present study, such considerations were
ignored and it was assumed that all components of the control and
recovery systems would be depreciated over the 20-year period. Never-
theless, it must be recognized that the assigned useful life of the
45
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control system, like the load factor of the power plant, can exert a
very great weight in determining the incremental unit cost of producing
electricity. The author believes that, on the whole, the assumption of
a useful life of 20 years is as optimistic as can now be made for any
control system, and that the actual useful lives might well prove to be
shorter. In such case, the incremental unit costs of producing elec-
tricity given in Tables I and III could be substantially increased.
3. Fixed Charges
The total fixed charge of 14.5 percent of the capital investment
per year was adopted from a report by the Tennessee Valley Authority,13
which was in turn adapted from guidelines suggested by the Federal Power
Commission, and embodies the assumption of a 20-year depreciation period.
Recent increases in both taxes and the cost of capital are producing rises
in the fixed charges associated with electric power generation. The
figure of 14.5 percent is therefore lower than may be appropriate at the
present time. It has been used in the present study primarily for the
sake of consistency with previous studies, and because the uncertainties
in other factors assumed appear to make greater elaboration of the capital
charge estimates of limited utility. The trend in taxes and capital costs
will, however, tend to increase the incremental unit costs of electrical
power generation estimated in Tables I and II.
46
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REFERENCES
1. Boyer, A. E., F. B. Kaylor, T.V. Ward, and F. J. Gottlich,
Atmospheric Dispersion of Saturated Stack Plumes, Des. Oper.
Air Pollut. Contr., Pap. MECAR Symp. 1968 (Pub. 1969), 32-40.
2. Brink, J. A., Jr., W. F. Burggrabe, and L.E. Greenwell, Mist
Eliminators for Sulfuric Acid Plants, Chem. Eng. Progr. 64_ (11),
82-86 (Nov. 1968).
3. Dennis, R., and R. H. Bernstein, Engineering Study of Removal of
Sulfur Oxides from Stack Gases, Report No. GCA-TR-68-15-G,
American Petroleum Institute, New York, N. Y. (1968).
4. Duecker, W. W., and J. K. West (Eds.). "The Manufacture of Sulfuric
Acid," Reinhold Publishing Co., New York (1959).
5. Guyot, G. , SNPA Process for the Treatment of Residual Gases with
Low Sulfur Dioxide Concentration, Chim. Ind., Genie Chim. 101 (1),
31-34 (Jan. 1969).
6. Guyot, G. , Production of Concentrated Sulfuric Acid from Sulfur-
Containing Gases with High Water Vapor Content, Chim. Ind., Genie
Chim. 101 (6), 813-816 (Mar. 1969).
7. Guyot, G., and J. P. Zwilling, SNPA's Process for H2S04 Production
Developed with Eye on Air Pollution, Oil Gas J. 64 (47), 198-200
(Nov. 21, 1966).
8. McKee & Company, Arthur G., Systems Study for Control of Emissions —
Primary Nonferrous Smelting Industry, Final Report to National Air
Pollution Control Administration, June 1969, Contract No. PH 86-68-85.
9. Monsanto Company, St. Louis, Missouri, Air Pollution Control for
Electric Utilities, Bulletin (Undated — issued 1970).
10. Monsanto Enviro-Chem Systems, Inc., St. Louis, Missouri, Cat-Ox
System for Existing Power Generating Stations, Bulletin (Undated —
issued 1970).
11. Napier, D. H., and M. H. Stone, Catalytic Oxidation of Sulfur
Dioxide at Low Concentrations, J. Appl. Chem. 8 (12), 781-786
(Dec. 1958).
12. Stites, J. G., Jr., W.R. Horlacher, Jr., J. L. Bachofer, Jr., and
J.S. Bartman, Removing SO2 from Flue Gas, Chem. Eng. Progr. 6J> (10),
74-79 (Oct. 1969).
47
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13. Tennessee Valley Authority, Sulfur Oxide Removal from Power Plant
Stack Gas — Use of Limestone in Wet-Scrubbing Process, Conceptual
Design and Cost Study No. 2 for National Air Pollution Control
Administration (1969).
14. Tigges, A. J., Recovery of Values from Sulfur-Dioxide-Containing
Flue Gases, Brit. Pat. No. 1,074,937 (July 5, 1967).
15. Zawadzki, E. A., Removal of Sulfur Dioxide from Flue Gases: The
BCR Catalytic Gas Phase Oxidation Process, Trans. Soc. Mining
Engrs. AIME 232, 241-246 (Sept. 1965).
48
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Appendix A
MODELS FOR HYPOTHETICAL POWER PLANTS
For the hypothetical power plant models, DPCE-NAPCA specified the
following conditions:
1. The two model plants shall be an existing 500-MW plant
located in central Pennsylvania and a new 1000-MW plant
located in the Midwest on a large navigable river.
2, Both plants shall have dry-bottom boiler furnaces and
shall use coal containing 3 percent of sulfur and 9 percent
of ash. The flue gases shall contain 0.21 percent of
sulfur oxides.
3. The base load shall be 7,000 hours per year (load factor
80 percent).
4. The existing plant shall have an economizer with an exit
gas temperature of 750°F and an electrostatic precipitator
operating at 325°F. The new plant shall have no temperature
constraints on gas cleaning equipment. The height of the
stack for the existing plant shall be 500 feet. Sufficient
space shall be available to permit installation of any
equipment needed.
5. The efficiency of removal of the sulfur dioxide shall be
90 percent. Regional air and water pollution regulations
shall be applicable.
DPCE-NAPCA also supplied an analysis of the coal assumed to be
burned and a calculated analysis of the flue gas formed:
A-l
-------
Analysis of Coal, As Fired
Constituent
C (burned)
Ho
2
No
2
S
00
2
H2°
Ash
lb/100 Ib, Coal as Fired
72.7
5.1
1.4
3.0
6.9
1.0
9.0
100.0
Flue Gas Analysis
Component Percent by Volume
CO2 13.89
SO0 and SO 0.2148
£ 3
0_ 3.301
•A
N2 74.514
H~O 8.08
100.0
Excess air used = 20 percent. Moisture
content of air = 0.013 Ib/lb dry air.
A-2
-------
Quantity of Flue Gas Formed
Basis Lb Moles Gas/100 Ib Coal Burned
Wet 43.67
Dry 40.14
The remainder of the model conditions were derived by SRI from the
foregoing conditions, supplemented by assumptions based on data in the
literature, or on the experience or judgment of the author.
A. Conditions Common to BothModel Plants
1. Heating Value of Coal
13,325 Btu/lb (calculated from DuLong Formula)
2. Flue Gas Generated
Volume of gas per 100 Ib of coal burned (wet basis)
= 16,880 SCF (70°F, 1 atm)
= 25,013 CF (325°F, 1 atm)
= 43,335 CF (900°F, 1 atm)
3. Air Inleakage into Flue Gas Handling System
Assumed nil
4. Fly Ash
Ash = 9.0 lb/100 Ib coal burned
Unburned carbon = 0.35 lb/100 Ib coal burned
Total ash and unburned carbon =9.35 lb/100 Ib coal burned
Fly ash (ash + carbon) leaving furnace in flue gas
= 80 percent of total
=7.49 lb/100 Ib coal burned
Fly ash concentration in flue gas = 3.105 grains/SCF (70°F,
1 atm, wet basis)
5. Sulfur Oxides Concentrations
SO + SO = 0.2148 percent by volume, wet basis
£ O
SO = 30 ppm = 0.003 percent by volume, wet basis
•3
SO_ = 0.2118 percent by volume, wet basis
2»
A-3
-------
6. Sulfur Emission
All of sulfur assumed to go into flue gas
Total sulfur emitted =3.0 lb/100 Ib coal burned
Sulfur emitted as SO
tj
= 0.0417 Ib S/100 Ib coal burned
= 0.1042 Ib SO3/100 Ib coal burned
Sulfur emitted as SO_
= 2.958 Ib S/100 Ib coal burned
= 5.916 Ib S02/100 Ib coal burned
7. Efficiency of Emission Control
Sulfur oxide removal efficiency = 90 percent
Allowable discharge of sulfur (as SO2, SO , or H SO.)
not to exceed 10 percent of the input sulfur to the
collection system, or 0.3 Ib S/100 Ib coal burned
8. Fuel Available for Reheating Flue Gas
No. 2 fuel oil (sulfur content 0.35 percent or lower)
9. Reductant Available for Reducing Sulfur Dioxide
Natural gas (heating value 1000 Btu/CF)
B. Model A Plant
1. Location
Altoona, Pennsylvania
2. Generating Capacity
500 MW
3. Temperature of Flue Gas
o
At exit from economizer: 750 F
At electrostatic precipitator: 325°F
4. Electrostatic Precipitator Efficiency
95 percent
A-4
-------
5. Stack Height
500 feet
6. Drives for Fans and Pumps
Electric
7. Heat Rate
10,000 Btu/kwh
8. Coal Consumption
0.7505 Ib/kwh
9. Coal Firing Rate
375,250 Ib/hr
187.63 tons/hr
10. Flue Gas Flow Rate
1,056,204 SCFM (70°F, 1 atm, wet basis)
1,564,355 CFM (325°F, 1 atm, wet basis)
11. Fly Ash Leaving Furnace in Flue Gas
28,106 Ib/hr
14.053 tons/hr
12. Fly Ash in Flue Gas Leaving Precipitator
Concentration = 0.1553 grain/SCF (70°F, 1 atm, wet basis)
Mass emission rate = 1405.3 Ib/hr
13. Sulfur Oxide Emissions
S00 = 22,200 Ib/hr
A
= 11.100 tons/hr
= 266.40 tons/day
S03 = 391.01 Ib/hr
= 9384 Ib/day
A-5
-------
C. Model B Plant
1. Location
Cairo, Illinois
2. Generating Capacity
1000 MW
3. Gas Cleaning System
No temperature limits apply. Unless some other gas
cleaning system is specified as an integral part of the
sulfur oxide control process, the power plant will be
assumed to be equipped with an electrostatic precipitator
operating on flue gas at a temperature of 325 F and having
an efficiency of 99.5 percent.
4. Stack Height
The plant will be assumed to have an 800-foot stack when
not equipped with a sulfur dioxide control system, and a
500-foot stack when equipped with a sulfur dioxide control
system of 90-percent efficiency.
5. Drives for Fans and Pumps
Either electric or steam turbine
6. Heat Rate
9600 Btu/kwh
7. Coal Consumption
0.7204 Ib/kwh
8. Coal Firing Rate
720,400 Ib/hr
360.20 tons/hr
A-6
-------
9. Flue Gas Flow Rate
2,027,690 SCFM(70°F, 1 atm, wet basis)
3,003,230 CFM (325°F, 1 atm, wet basis)
5,203,090 CFM (900°F, 1 atm, wet basis)
10. Fly Ash Leaving Furnace in Flue Gas
53,958 Ib/hr
26.979 tons/lu-
ll. Fly Ash in Flue Gas Leaving Precipitator of 99.5 Percent
Efficiency
Concentration = 0.01553 grain/SCF (70 F, 1 atm, wet basis)
Mass emission rate = 269.79 Ib/hr
12. Sulfur Oxide Emissions
S00 = 42,619 Ib/hr
^
= 21.309 tons/hr
= 511.43 tons/day
SO = 750.66 Ib/hr
*5
= 18,016 Ib/day
A-7
-------
Appendix B
COST FACTORS USED IN MODEL STUDIES
In an actual case, the busbar cost of electricity and the cost of
steam generated at a power plant are, of course, related to the cost of
fuel and other plant operating costs as well as to the fixed costs, which
are in turn related to the capital investment and the plant load factor.
In practice, it appears that the complexity of cost accounting makes it
difficult for even a utility company to assign costs precisely for an
individual power plant in its system.
In the present study, reasonable costs were assumed for individual
items such as fuel, steam, and electrical power. No attempt was made to
reconcile these costs, but it was appreciated that the values adopted may
actually not be entirely consistent with one another.
A. Fuel Costs
Coal
The cost of coal was estimated from data in "Steam-Electric Plant
Factors (1967)," published by the National Coal Association. The costs
(^/million Btu) for coal as burned in Pennsylvania power plants were
averaged for coals having heating values greater than 13,000 Btu/lb, which
had higher unit costs than the lower-grade coals. The average cost per
million Btu was applied to the hypothetical coal assumed in the present
study (heating value 13,325 Btu/lb) to give the following costs:
31.2£/million Btu
$8.30/ton
The average costs of coal used in Illinois, Indiana, Ohio, and
Pennsylvania were lower, but applied to coals having heating values in
the range of 10,000 to 12,000 Btu/lb. Presumably these coals had generally
higher ash contents than the hypothetical coal.
B-l
-------
Natural Gas
An estimate of the cost of natural gas was taken as the rounded
average of the average costs of natural gas burned at power plants in
Illinois, Indiana, Ohio, and Pennsylvania. The data were taken from
"Steam-Electric Plant Factors (1967)." The value adopted was:
30£/million Btu
The heating value of the gas was assumed to be 1,000 Btu/CF.
No. 2 Fuel Oil
The cost of No. 2 fuel oil in central Pennsylvania was estimated
at SRI to be:
$4.25/barrel
B. Water
The costs of water for various uses were taken from lists of
standard factors drawn from the literature or were figures used for esti-
mating purposes at SRI.
Cost
Raw makeup water 2£/1000 gal
Once-through cooling water (temperature
80°F) 2£/1000 gal
Process water 20£/1000 gal
Boiler feed water 40£/1000 gal
Retreatment of process steam
condensates for boiler use 38£/1000 gal
C. Steam
The cost of high-pressure steam was very roughly estimated at SRI
as:
65C/1000 lb (2400 psig, 1050°F)
The cost of the steam includes the cost of boiler feed water
treatment.
B-2
-------
It was assumed that low-pressure steam was available as needed,
obtained by extraction from the power plant turbines. The value of low-
pressure steam extracted from turbines, or exhausted from a backpressure
turbine, was assumed to be proportional to the value of the high-pressure
steam in the ratio of the enthalpies of the steam at the two conditions.
D. Electrical Power
The busbar cost of electrical power generated and used in the power
plant itself (including the emission control system), after inspection of
data published by the Federal Power Commission and after discussions with
staff of the Pacific Gas & Electric Co., was assumed to be as follows:
Generating cost 3.2 mills/kwh
Fixed charges 2.3 mills/kwh
Total cost 5.5 mills/kwh
E. Salaries and Payroll Benefits
Salaries
Operating labor $3.75/hour
Supervision $4.75/hour
Payroll benefits
25 percent of labor and supervision
F. Maintenance
Taken as an appropriate percentage of capital investment to cover
both labor and materials.
G. Indirect Costs
Overhead
50 percent of labor, supervision, and maintenance
B-3
-------
H. Fixed Charges
The total fixed charge was adopted from the Tennessee Valley
Authority's Conceptual Design and Cost Study No. 2 (1969) for National
Air Pollution Control Administration, "Sulfur Oxide Removal from Power
Plant Stack Gas—Use of Limestone in Wet-Scrubbing Process." The break-
down of fixed charges (adapted from Federal Power Commission guidelines)
is given as follows:
Annual Percent of
Capital Investment
Depreciation (20-year life) 5.00
Insurance 0.25
Cost of capital (capital structure
assumed to be 50% debt and 50%
equity)
Bonds at 6% of average
undepreciated investment 1.50
Equity at 11% of average
undepreciated investment 2.75
Taxes
Federal 2.80
State 2.20
Total fixed charges 14.50
B-4
-------
Appendix C
STACKS FOR USE ON CONTROLLED
SULFUR DIOXIDE EMISSION SOURCES
Evidently, no clear policy has yet been developed with respect to
choice of the heights of stacks to be used on power plants equipped with
sulfur dioxide emission controls. The use of a high stack on an uncon-
trolled emission source reduces the maximum ground-level concentration
of sulfur dioxide at the point where the plume reaches the ground, but
it does not, of course, reduce the total emission of sulfur dioxide that
may affect areas beyond the range of influence of the stack. If emission
controls are used as well, not only is the maximum ground-level concen-
tration further reduced, but the quantity of sulfur dioxide affecting
the area beyond the range of influence of the stack is reduced also.
If, on the other hand, the use of emission controls is accompanied
by a reduction of the stack height, the reduction of the maximum ground-
level concentration of sulfur dioxide may be partly lost, depending upon
the balance between the reduction in stack height and the reduction in
emission. The point of maximum ground-level concentration will be
brought closer to the emission source.
There is reluctance to make major reductions in stack height, even
with installation of sulfur dioxide emission controls, because of concern
with other contaminants that may still remain in the flue gas and with
C4
the possibility of failures of the emission control system.
If the sulfur dioxide emission control system reduces the temper-
ature of the flue gas, the effect is to reduce the effective stack
height because the density of the gas is increased and hence its buoyancy
C3 C4
is reduced. ' When the hot gas is scrubbed, the effect of gas cool-
ing is counteracted to some degree by the lowering of the gas density due
to the increase in the water vapor content of the gas stream.
C-l
-------
Particular concern has been expressed about "negative buoyancy."
If the plume emerging from the stack is saturated and contains water
droplets, the subsequent evaporation of the drops in the atmosphere may
Cl C2
cool the gas sufficiently to cause the plume to descend rapidly. '
The presence of the droplets in the plume may directly produce such a
descent by increasing the effective density of the gas in the plume.
However, the author believes that the presence in stack gas of any such
relatively gross quantities of liquid would most likely be due to
entrainment from the scrubber, which can be prevented. It has been
Cl
suggested that at least some of the reported instances of rapid descent
of plumes of scrubbed gas may have resulted from downwash produced by
adjacent structures. Plumes of scrubbed gas that is saturated but still
substantially above ambient temperature have been commonly observed to
rise and disperse in the same general manner as do warm, dry plumes.
Careful observations of such plume behavior have been reported by Boyer
et al.C1
The necessity for reheating scrubbed stack gases to ensure adequate
dispersion is not entirely clear, at least if the efficiency of the
scrubber is high and some minimum stack height is used. Using a hypo-
thetical 200-MW power plant unit with a 300-foot stack as an example,
C4
TVA calculated the effects of cooling (by scrubbing) and of reheating
the flue gas on the maximum ground-level concentration of sulfur dioxide,
assuming varying scrubber efficiencies. The results indicated that if
the scrubber had an efficiency of at least 9O percent and the stack
height was at least 300 feet, failure to reheat the gas would have only
a small effect upon the percentage reduction in the maximum ground-level
concentration of sulfur dioxide achieved by scrubbing. Applying a
commonly used rule of thumb that a stack should be at least 2.5 times
the height of adjacent structures, a typical power plant should in any
case use a stack at least 300 feet in height in order to avoid possible
downwash of the gas.
In the present study, it was assumed that in the Model A plant, the
treated flue gas would be discharged through the existing 500-foot stack.
C-2
-------
In the case of the Model B plant, it was assumed that the plant would be
equipped with an 800-foot stack if it had no sulfur dioxide emission
controls, and with a 500-foot stack if it were equipped with an emission
control system of 90 percent efficiency. Such a reduction in stack
height does not appear to be unreasonable. In the light of present
knowledge, a minimum stack height of 500 feet for the controlled emission
source seems appropriate.
The Model B control systems (both Cat-Ox and Wellman-Lord) were each
given credit for the differential between the costs of the 800- and 500-
foot stacks. A rough estimate of the cost differential for stacks 30
feet in inside diameter ($850,000) was made from Fig. 6-1 of "Control
•C2
Techniques for Sulfur Oxide Air Pollutants.'
REFERENCES
Cl. Boyer, A.E., F. B. Kaylor, T. V. Ward, and F. J. Gottlich,
Atmospheric Dispersion of Saturated Stack Plumes, Des. Oper.
Air Pollut. Contr., Pap. MECAR Symp. 1968 (Pub. 1969), 32-40.
C2. National Air Pollution Control Administration, Control
Techniques for Sulfur Oxide Air Pollutants, Publication No.
AP-52 (Jan. 1969)
C3. Scorer, R. S., "Air Pollution," Pergamon Press, New York (1968)
C4. Tennessee Valley Authority, Sulfur Oxide Removal from Power
Plant Stack Gas—Use of Limestone in Wet-Scrubbing Process,
Conceptual Design and Cost Study No. 2 for National Air
Pollution Control Administration (1969)
C-3
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Appendix D
ESTIMATION OF THE VALUES OF SULFUR BY-PRODUCTS
As applied to power plant flue gases, the Monsanto Cat-Ox system
can produce only 78-percent sulfuric acid. The Wellman-Lord system
produces concentrated sulfur dioxide, which has relatively insignificant
markets as such but can be converted to elemental sulfur or to either
93-percent or 98-percent sulfuric acid.
The estimates of the sulfur by-product prices at the two hypothetical
power plant sites (Altoona, Pennsylvania for the Model A plant, and Cairo,
Illinois for the Model B plant) were based on an assumed Gulf Coast price
for sulfur of $30/long ton. This Gulf Coast price was estimated during
the previous study as a likely average over the period to 1975.
Currently, sulfur prices have become chaotic as sulfur supplies have
shifted abruptly from shortages to surpluses. Apparently it is scarcely
possible to define an "established" price for sulfur. The development of
sulfur surpluses has followed a continuing period of low activity in the
market for fertilizers, which provides the largest outlet for sulfur.
In the next five years, the withdrawal of marginal producers and a
possible revival of the fertilizer market may cause sulfur prices to rise
from their present lows. However, the figure of $30/iong ton now appears
to be an optimistic one. It perhaps represents the upper end of the
probable range of Gulf Coast prices to be encountered in the period to
1975. It cannot even be assumed that the Gulf Coast sulfur price will
continue to maintain its previous status as the base line for world sulfur
prices. Nevertheless, if the above limitations are recognized, the
assumption of the $30/long ton price is probably as good as any that can
be made at this time. Prices at other locations can be estimated by
adding to the Gulf Coast price the cost of transportation from the Gulf
This material was prepared by F. Alan Ferguson, Industrial Economist, SRI.
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Coast. This procedure assumes, of course, that the incursion of the
sulfur from a new source will not be large enough to disrupt the local
markets.
Model A Plant - Altoona, Pennsylvania
The closest and most ready markets for sulfur by-products produced
at the hypothetical power plant Model A would be located in the Pittsburgh
area, about 120 miles away. The acid users in the Pittsburgh region cur-
rently consume over 500,000 tons of acid per year, a large portion of
which is used to clean (pickle) iron and steel products and to produce
ammonium sulfate from the ammonia in coke oven gas. Both of these uses
could employ 78-percent sulfuric acid as well as 93-percent or 98-percent
acid, and the total market is more than large enough to absorb all the
acid that could be produced at the Altoona plant. The Pittsburgh area
could also absorb elemental sulfur that might alternatively be produced
at Altoona.
The prices that could be obtained for the various possible by-
products producible at Altoona will depend upon a number of factors, a
few of which can be allowed for at the present time. On the basis of a
Gulf Coast sulfur price of $30/long ton, the delivered price of elemental
sulfur in the Pittsburgh area will be about $36.50/long ton. Since the
elemental sulfur now being sold in the Pittsburgh area is merchant sulfur,
that produced at Altoona could probably capture about half the existing
market if sold at the same price of $36.50/long ton. Since the cost of
transportation from Altoona to Pittsburgh would be about $3.50/long ton,
the f.o.b. price of the sulfur at Altoona might be $33/long ton.
Estimating the price of Altoona sulfuric acid is more difficult
because some of the Pittsburgh area acid is made for captive consumption
and the rest is made and sold as merchant acid. The prices of captive and
merchant acid can differ by as much as a factor o.f two. The price of
merchant acid will vary according to whether the acid is sold on long
term contracts or when and as needed (spot sales). However, relatively
little acid is sold at the higher spot prices. The highest price the
plant at Altoona could expect to receive for its acid at Pittsburgh
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would be the same as that which would return a reasonable profit to an
acid producer operating a contact acid plant and using Gulf Coast sulfur
as raw material. The cost of producing acid from sulfur costing $36.50/
long ton would be about $13/short ton (100 percent acid basis). With the
allowance of about 15 percent as a return on investment, the corresponding
price of acid would be $15/short ton (100 percent acid basis). The
corresponding f.o.b. prices at Altoona can be estimated by subtracting
the transportation charges of $4/ton (100 percent acid basis) for
93-percent acid and $5/ton (100 percent acid basis) for 78-percent acid,
giving:
93-percent acid: $12/ton of 100 percent acid
78-percent acid: $ll/ton of 100 percent acid
If it should be necessary to displace existing captive producers to
capture adequate markets, the Altoona acid might have to sell at Pittsburgh
for about $2/ton less than the cost of production from sulfur, or $ll/ton.
The corresponding f.o.b. prices at Altoona would then be:
93-percent acid: $7/ton of 100-percent acid
78-percent acid: $6/ton of 100-percent acid
A detailed market analysis would be required to determine whether the
price of the Altoona acid would be nearer the upper end ($11 to $12/ton)
or the lower end ($6 to $7/ton) of the price range estimated.
Model B Plant - Cairo, Illinois
Finding markets for elemental sulfur produced at a plant at Cairo
should not be difficult. In 1965, over 1.2 million tons of elemental
sulfur were barged along the Ohio River and the upper Mississippi River.
Assuming a Gulf Coast sulfur price of $30/long ton, sulfur produced along
the rivers should sell for $30 plus the cost of barging sulfur from the
Gulf Coast to the actual plant site. The cost of barging elemental sulfur
from the Gulf to Cairo, Illinois is about $2.90/long ton. Hence, sulfur
produced at the Cairo plant should sell for about £33/long ton.
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If sulfuric acid, rather than elemental sulfur, should be produced,
finding markets for large quantities of acid might be substantially more
difficult. In 1966, only about 130,000 tons of acid were barged along
the Ohio River and 30,000 tons along the upper Mississippi River. From
the summary (Table D-l) of uses of sulfuric acid near three sites on
these two rivers, it is apparent that the only end use large enough to
consume the quantities of acid that could be supplied from the Cairo
plant would be the production of phosphate fertilizers.
A recovery system producing sulfuric acid at Cairo might supply
either existing phosphate fertilizer manufacturers or a new plant located
at or near Cairo to take advantage of the local source of acid. Supplying
such a new fertilizer manufacturer would probably yield the highest price
for the acid, although a more detailed market analysis would be required
to determine whether this would actually be the case. Nevertheless, the
sulfuric acid would have to be priced to provide the new fertilizer
producer with a competitive advantage over the existing suppliers. The
existing producers of phosphate fertilizers that are capable of supplying
fertilizer to the midwestern market at the lowest prices are probably
those who manufacture their product at plant sites in Louisiana and ship
it to the markets by barge. The price of the by-product acid at Cairo
must therefore permit the new producer to make his fertilizer for less
than the Louisiana producers can make and ship theirs.
In Table D-2 a comparison is made of the costs of producing 54-
percent wet process phosphoric acid in Louisiana and at the hypothetical
Cairo site. The data indicate that the Cairo producer could pay about
£32.20 for the amount of 100-percent sulfuric acid necessary to make 1.85
tons of 54-percent (P 0 ) phosphoric acid, which is equivalent to 1.0 ton
£t 5
of P O . Since 2.7 tons of sulfuric acid (100-percent basis) are required
« O
to produce the 1.85 tons of phosphoric acid, the hypothetical producer
could pay $11.94/ton (100-percent acid basis) for the sulfuric acid and
be able to make phosphoric acid to sell for the same price as that made
in Louisiana and shipped to the Cairo area. However, the sulfuric acid
would probably have to be offered at somewhat less than $11.94/ton in
order to provide incentive to the new phosphoric acid producer. Under
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these circumstances, the sulfuric acid could probably be sold for $10
to $ll/ton (100-percent acid basis). Since little or no transportation
would be required, 78-percent and 93-percent acid should sell for the
same price per unit quantity of 100-percent acid.
REFERENCES
Dl. McKee & Company, Arthur G., Systems Study for Control of
Emissions — Primary Nonferrous Smelting Industry, Final
Report to National Air Pollution Control Administration,
June 1969, Contract No. PH 86-68-85.
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Table D-l
ESTIMATED SULFURIC ACID DEMAND IN SELECTED
PRODUCING AREAS, 1966
(thousands of short tons 100% H2S04)
Cairo-Paducah St. Louis Chicago
Phosphate fertilizers 107 789 1,375
Ammonium sulfate 13 5 113
Iron and steel pickling — 3 169
Rayon and cellulose films and fibers — — 74
Petroleum refineries 10 81 78
Sulfonated detergents 15 57 33
Ammonium sulfate — 28 12
Titanium dioxide — 410
Hydrofluoric acid 45 — 38
Total Major End Uses 190 1,373 1,892
Source: A. D. Little, Inc.
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Table D-2
PHOSPHORIC ACID PRODUCTION COSTS
I
Plant Location
(per ton 100% P2°5>
Phosphate rock (72 bpl)
$6.00/ton + $3.25 frt
$6.00/ton + $5.85 frt
Elemental sulfur
$30/long ton + $1.50 frt
H SO. manufacture $3.40
per ton
(excluding cost of
sulfur but including
15% return on investment)
Phosphoric acid manufacture
(excluding raw material
cost and profit)
Barge freight $3.00/ton
(phosphoric acid from
Louisiana to Cairo, 111.)
Subtotal
Louisiana
Cairo, Illinois
Quantity
Required
3.2 tons
0.82
2.7
1.85
H2SO4 Purchase
$11.94/ton
(to match cost of producing
phosphoric acid at Cairo, 111.
and in Louisiana)
Total
Cost/Ton
100% P2o5_
$29.60
25.83
9.18
14.00
5.55
.16
0
Quantity Cost/Ton
Required 100% P0O
•2-5-
3.2 tons $37.92
0
0
2.7
0
0
$84.IP
14.00
0
$51.92
32.24
$84.16
Source: SRI
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