vvEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Technology Transfer
Summary Report
Sulfur Oxides Control
Technology Series:
Flue Gas Desulfurization
Spray Dry0r Process
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Technology Transfer
EPA 625/8-82-009
Summary Report
Sulfur Oxides Control
Technology Series:
Flue Gas Desulfurization
Spray Dryer Process
September 1982
This report was developed by the
Industrial Environmental Research Laboratory
Research Triangle Park NC 27711
-------
This summary report was prepared jointly by the Radian Corporation of
Austin TX and the Centec Corporation of Reston VA. E. D. Gibson, M. A.
Palazzolo, and M.; E. Kelly of Radian are the principal contributors. T. G. Brna
is the EPA Project Officer. Photographs taken at Argonne National
Laboratory are by Al Meyers and Ron Skidmore of the Argonne Graphic
Arts Division. ;
Comments on or ^questions about this report or requests for information
regarding EPA flue gas desulfurization programs should be addressed to:
Emissions/Effluent Technology Branch
Utilities and Industrial Processes Division
IERL, USEPA (MD-61)
Research Triangle Park NC 27711
This report has been reviewed by the Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency, Research Triangle
Park NC, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
COVER PHOTOGRAPH: Argonne National Laboratory power plant,
500,000-lb/h steam capacity
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Introduction
The Environmental Protection
Agency (EPA) is studying spray dryer
flue gas desulfurization (FGD) as
part of an extensive program of FGD
technology development. In this
throwaway process (Figure 1),
sulfur dioxide (SO2) is removed from
the flue gas by an atomized lime
slurry [Ca(OH)2] or a solution
of sodium carbonate (Na2CO3). The
hot flue gas dries the droplets to
form a dry waste product while the
absorbent reacts with sulfur
dioxide in the fliie gas. Dry waste
solids—consisting of sulfite (SO3)
and sulfate (S04) salts, unreacted ab-
sorbent, and fly ash—are col-
lected in a fabric filter (baghouse) or
electrostatic precipitator (ESP) and
are typically disposed of by landfill.
By mid-1 981, a total of 12 utility and
11 industrial spray dryer systems had
been sold in the. United States.
All but three of these systems will use
lime as a reagent. One 100-MW
utility demonstration (at North-
ern States Power Company's River-
side Station, Minneapolis,
Minnesota) and two industrial sys-
tems (atStrathmore Paper, Woronco,
Massachusetts; and Celanese
Fibers Company's Amcelle Plant,
Cumberland, Maryland) were
operational as of mid-1981.1
The EPA has been active in sponsor-
ing the demonstration and test-
ing of spray dryer FGD: From
December 1979 to July 1980, pilot
tests were performed with various
absorbents on a unit treating
20,000 actual ft3/min (570 actual
m3/min) at the City of Colorado
Springs' Martin Drake Station.2 From
May 1980 to December 1980,
additional pilot testing was con-
ducted on a unit treating 10,000 ac-
tual ft3/min (285 actual m3/min)
at Public Service of Colorado's
Key
H^H Flue gas/off-gas
H^Bll Cleaned flue gas
1HHJI Absorption feed
[P-'-~- '\ Air pollutants
H^^H Other systems
Cleaned
flue gas
Lime or sodium
carbonate
Flue
gas
Disposal
Figure 1. ;
Major Components of Spray Dryer FGD Process
-------
Comanche Station.3 These tests pro-
vided data on key process vari-
ables that affect SO2 removal.
Emission testing funded by EPA was
carried out at Celanese Fibers'
Amcelle Plant from May 1980 to
December 1980. The Amcelle testing
included 23 days of continuous
SC>2 monitoring in support of the New
Source Performance Standards
(NSPS) being developed for indus-
trial boilers.4
Spray dryer FGD is currently the only
commercially applied dry FGD
process. Other dry FGD processes
under development include
dry injection and combustion of coal-
alkali fuel mixtures. Several fac-
tors, including mandated and
voluntary increases in coal use and
the 1979 NSPS for utility boilers, have
promoted increased research
and development and commercial
application of the dry FGD
technology.
Interest in spray dryer FGD has been
spurred primarily by the poten-
tial cost savings dry FGD offers over
conventional wet FGD, particu-
larly for low-sulfur-coal (less than 1.5
percent) applications. Advantages
of spray dryer FGD over wet
FGD systems include dry waste pro-
duction, lower initial capital
investment, lower projected operat-
ing costs for fuels of moderate
sulfur content (up to 3 percent), and
less process complexity, which
may lead to greater system
reliability.
Higher absorbent cost is the major
disadvantage of spray dryer
FGD relative to wet FGD systems.
This higher cost results from the
higher priced absorbent (lime versus
limestone) and the higher
Spray dryer exit. Coyote Electric Generating Station, Beulah, North Dakota
stoichiometric ratios necessary.
Current limits on the applicability of
spray dryer FGD stem from a lack
of data on installations firing high-
sulfur coal, although vendors report
high SO2 removal capabilities.
This summary report provides a basic
description of the spray dryer
FGD process. Both sodium carbonate
and lime spray drying are dis-
cussed, although lime is emphasized
because of the higher reagent
costs and waste disposal problems
associated with sodium carbon-
ate and the greater commercial
acceptance of lime for dry FGD.
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Process Description
Spray dryer FGD consists of four
major steps:
• Absorbent preparation
• Absorption and drying
• Solids collection
• Solids disposal
Solids disposal is not an integral part
of the process but is associated
with spray dryer FGD. Figure 2 illus-
trates the process flow for a
typical spray dryer FGD system.
Flue gas exiting the combustion air
preheater comes in contact
with an alkaline solution or slurry in a
spray dryer. The flue gas passes
through a contact chamber, and the
solution or slurry is sprayed into
the chamber with a rotary or nozzle
atomizer. The heat of the flue
gas dries the atomized droplets while
the droplets absorb sulfur dioxide
from the flue gas. The sulfur
dioxide reacts with the alkaline re-
agent to form solid phase sul-
fite and sulfate salts.
Most of the solids (and any fly ash
present) are carried out of the
dryer in the exiting flue gas. The rest
fall to a hopper at the bottom of
the dryer. With spray drying, in con-
trast to wet FGD, the flue gas is
not saturated with moisture after the
absorption step. The gas approaches
20° F to 50° F (11 ° C to 28° C) of
the saturation temperature.
The solution or slurry sprayed into
the dryer is pumped from
an absorbent holding tank. Fresh
absorbent and dilution water are
added to the tank and the con-
tents are stirred as needed. In some
systems, dilution water is also
added to the absorbent feed just up-
stream of the spray dryer to
improve control of the outlet flue gas
temperature. When lime is the
absorbent, recycle solids from the
spray dryer hopper or downstream
solids collection equipment contain
unreacted absorbent and are often
used to supplement the fresh
absorbent feed. Either recycle
solids are slurried separately and
added to the absorbent feed just up-
stream of the spray dryer, or they
are added directly to the fresh
absorbent in the;holding tank.
Flue gas may be reheated after it
leaves the spray dryer to prevent con-
densation in downstream solids
collection equipment. To accomplish
reheat, the flue gas from the
spray dryer is mixed either with hot
flue gas from upstream of the
combustion air preheater or with
warm flue gas from upstream of the
spray dryer. The reheated flue
gas then flows to the solids collec-
tion device where the dry solids
(reaction products, unreacted
absorbent, and fly ash) are collected.
A fabric filter (baghouse) is the
most common solids collection de-
vice, but ESP's are also used.
When a baghouse is used, significant
absorption of sulfur dioxide
may occur during solids collection.5
Absorbent in the solids collected
on the surface of the bags reacts with
sulfur dioxide remaining in the
flue gas.
The cleaned flue gas leaves the
collection device and is exhausted
to the atmosphere through a stack.
Dry waste solids collected in the
spray dryer and the collection device
are typically disposed of by landfill.
Absorbent Preparation
Absorbent solutions or slurries are
prepared on site for spray dryer
processes. Lime is the most popular
reagent, although sodium car-
bonate may be used. Other sodium-
based reagents, such as nahcolite
and trona ore, have also been
shown to be effective absorbents for
spray drying.2
Sodium carbonate absorbent is pre-
pared as a concentrated solution
and stirred in a tank. Sodium carbon-
ate is more soluble and more reac-
tive than lime; however, the
leachability of sodium reaction prod-
ucts can result in waste disposal
problems.
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Flue gas/off-gas
Cleaned flue gas
Absorption feed
Air pollutants
Other systems
To disposal
Figure 2.
Spray Dryer FGD Process
Sodium carbonate dissolves in water
to produce sodium ions:
Na2C03 Za 2Na+ + COg2 (1)
In lime systems, pebble lime (CaO)
must be slaked to produce a
reactive lime slurry (Figure 3a). The
slaking reaction can be repre-
sented as:
Ca(OH)2
(2)
Slaked lime dissolves in waterto pro-
duce calcium ions:
Ca(OH)2 ^ Ca+2 + 20H~
(3)
Absorbent utilization can often be
improved by recycle of the waste
solids, particularly in lime systems,
where the unreacted absorbent
remaining in the waste solids can
be used. Waste solids recycle
may not be advantageous for
sodium-based systems, however,
because the more reactive sodium-
based reagent is usually com-
pletely consumed during the first
pass through the spray dryer.
Absorption and Drying
Gaseous sulfur dioxide is absorbed
from the flue gas in the spray
dryer (Figure 3b). As flue gas enters
the dryer, it disperses and imme-
diately mixes with atomized
solution or slurry. Gas phase sulfur
dioxide rapidly dissolves into
the liquid phase of the droplets and
reacts with the absorbent to form
solid phase salts; simulta-
neously, the solid particles are
dried by the heat of the flue gas. Flue
gas carrying solid particles exits
the spray dryer and flows to a solids
collection device. Some S02 absorp-
tion also takes place downstream
of the spray dryer.6
During absorption, the flue gas in the
spray dryer is adiabatically
humidified by water evaporated from
the solution or slurry. The amount
of water injected into the spray
atomizer with the absorbent solution
or slurry therefore controls the gas
temperature approach to the
adiabatic saturation temperature.
Process variables that affect SO2
removal in the spray dryer include ap-
proach to adiabatic saturation
temperature, the flue gas residence
time in the spray dryer, and the
absorbent stoichiometry. These vari-
-------
Flue gas/off-gas
Cleaned flue gas
Absorption feed
Air pollutants
Other systems
»\\ \
/ ' i i«»
ii'1, ' \
Spray dryer
Fresh
absorbent
holding
tank
(a)
(d)
f
To landfill
Figure 3. '.
Lime Spray Dryer FGD with Baghouse: (a) Absorbent Preparation, (b) Absorption and Drying, (c) Solids Collection, and
(d) Solids Disposal
ables are discussed in the Design
Considerations section.
The overall SO2 absorption reactions
for lime spray drying can be repre-
sented as:
Ca(OH)2(s)
CaSO3 •
S02(g) +.H2O(I) ^
+ %H2O(I)
• 1/2H2O(s)
3/2H2O(l) ^±
2H20(s)
(4)
(5)
For the sodium carbonate process,
the overall SO2 absorption reac-
tions are:
Na2C03{s) + S02,(g) J±
Na2SO3(s) + CQ2(g)
Na2S03(s)+1/202(g)^±
Na2SO4(s)
(6)
(7)
Reaction mechanisms and mathe-
matical models have been pos-
tulated for the lime spray dryer
process.7-8 Equations 8 through 1 3
show the series of reactions that
lead to the overall reactions of
slaked lime and sulfur dioxide
(Equations 4 and 5).8
The sulfur dioxide is absorbed in
water and reacts with water to form
sulfurous acid (H2SO3) before disso-
ciating to form sulfite ions, accord-
ing to the following sequence
of reactions:
S02(g) ;± S02(aq)
SO2(aq) + H2O i± H2SO3
H2S03 i± H+ + HS0 J±
S03-2
(8)
(9)
(10)
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Argonne fly ash hopper system
Dissolved lime (see Equation 3) and
other alkaline species from re-
cycled solids orfly ash neutralize the
absorbed sulfur dioxide and
thereby drive the reactions in Equa-
tions 8, 9, and 10 to completion.
Some of the sulfite ions are oxidized
by flue gas oxygen to form sul-
fate ions:
SOT2 + 1/202
(11]
The sulfite and sulfate ions then pre-
cipitate as calcium salts:
Qa+2 -(. SCT2 + M-t-UO ^
CaSO3-VaHzCHs) (12)
Ca+2 + SO"2 + 2H20 j±
CaS04 • 2H20(s) (13)
The quantity of calcium ions avail-
able in the liquid phase of the
slurry to form the calcium salts is lim-
ited by the solubility of slaked
lime in water. Theicalcium ions react
in the liquid phase of the slurry
droplet, and they are replaced with
fresh ions from the dissolution
of additional solid phase slaked lime
(Equation 3).
The reactions leading to the overall
reactions of sodium carbonate
and sulfur dioxide (Equations 6 and 7)
are the same as those given for
the overall reactions of slaked lime
-------
and sulfur dioxide (Equations 4
and 5). The sulfite and sulfate ions
precipitate as sodium salts:
2Na+ + SO"2
2Na+ + SO"2
: Na2S03(s)
Na2SO4(s)
(14)
(15)
In lime spray dryers, carbon dioxide
(CO2) absorbed from the flue gas
can react with the slurry to form cal-
cium carbonate (CaCO3), thus
reducing the availability of calcium
ions.
C02(g) J± C02(aq)
CO2(aq) + H2O J± H2CO3
H2C03
2H+
Ca+2
C032
C032 ±;
: CaCO3(s)
(16)
(17)
(18)
(19)
The importance of carbon dioxide
absorption in lime spray drying
has not been fully investigated. Pilot
plant tests have indicated,
however, that the absorbent lost to
reaction with carbon dioxide may
be recovered by solids product
recycle.2 ;
Solids Collection
Solid particles in flue gas exiting the
spray dryer are collected by a
baghouse or ESP (Figure 3c). The
solid particles consist of reac-
tion products, unreacted absorbent,
and fly ash. The cleaned flue gas
leaves the collection device and is
exhausted through a stack.
When baghouses are used, SO2 ab-
sorption continues during the
solids collection step as unreacted
absorbent in the solid waste
reacts with the sulfur dioxide remain-
ing in the flue gas.6 This additional
SO2 absorption! can only occur
when residual rrioisture remains in
the solid particles.9
Solids Disposal
Disposal methods for solids collected
from spray dryer processes vary
with the type of absorbent used.
Spray dryer FGD has an advantage
over conventional wet FGD in
that sludge handling equipment
(such as clarifiers, thickeners,
vacuum filters, and centrifuges) is not
required. Waste solids from spray
dryer processes have handling
properties similar to dry fly ash and
are usually conveyed pneumatic-
ally to storage bins and then trucked
to landfill sites for disposal
(Figure 3d).
Integrated System
The foregoing steps are part of the
integrated system. Figure 3
shows how the four steps—absorb-
ent preparation, absorption and
drying, solids collection, and solids
disposal—relate to form a com-
plete lime spray dryer FGD process.
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Design Considerations
Several design elements must be
considered in the selection
and operation of a spray dryer FGD
system:
• Absorbent selection and
preparation
• Spray dryer design and operation
• Solids collection
• Solids recycle
Absorbent Selection and Preparation
The selection of an absorbent de-
pends mainly on reagent cost
and availability, ease of waste dis-
posal, and SO2 removal require-
ments. Most spray dryers use lime in-
stead of sodium-based reagents
for three primary reasons:
• Lime is generally the less expen-
sive reagent.
• Lime is the more readily available
in most areas.
• Lime spray dryer wastes can be
disposed of more easily in an
environmentally acceptable
manner.
Sodium carbonate is quite soluble in
water. In the spray dryer environ-
ment it is highly reactive, which
leads to good material utili-
zation. The high solubility of the
resultant sodium salts, however, cre-
ates a waste disposal problem
because of the potential for leaching.
Lime, on the other hand, has a rela-
tively low solubility and is less
reactive in the spray dryer. Material
utilization is lower :as a result,
particularly when high SO2 removal
is required. Low utilization can
be partly overcome by modifications
in the process design; for exam-
ple, solids recycle can be used. Lime
has an advantage in that the
wastes produced can usually be dis-
posed of in landfills without
special treatment.
Absorbent preparation techniques
are particularly important in lime sys-
tems. Slaking water used to pro-
duce lime slurries for spray drying
must be of good quality.2 Low-
solids water produces small,
highly porous, reactive particles.
High-solids wastewater, such as
cooling tower blowdown, produces
large slurry particles that are less
reactive and decrease lime
utilization.
Two types of slakers can be used to
prepare lime slurries: ball mill slak-
ers and paste slakers.10 Ball mill
slakers are more popular because
they generally produce a more
finely ground absorbent and contrib-
ute to a more reactive slurry. Paste
slakers offer a potential benefit,
however, in that the resulting slurry is
somewhat less abrasive than the
slurry from ball mills and may
reduce wear on pipes and pumps.
Slaking temperature, amount of
water, and lime purity also affect the
quality of the slaked lime slurry.2
Spray Dryer Design and Operation
Primary considerations in spray dryer
design and operation include the
quantity of flue gas to be cleaned,
the inlet flue gas temperature
and moisture content, the flue gas
SO2 content, and the desired SO2
removal efficiency. These fac-
tors determine the physical size
of the dryer and the amount of
absorbent required. Thus, they affect
capital and operating costs
significantly.
The design of the spray dryer contact
chamber, the flue gas disperser,
and the solution or slurry atomizers
is important to efficient removal
of sulfur dioxide. Contact chamber
volume must provide a flue
gas residence time that maximizes
SO2 removal and permits ade-
quate drying of the absorbent
particles. Most lime spray dryers
have a flue gas residence time of 10
to 12s.
8
-------
The flue gas disperser and the spray
atomizer should provide for inti-
mate mixing of the flue gas and the
atomized droplets. Complete
mixing aids the mass transfer of sul-
fur dioxide to the droplets. There-
fore, the flue gas disperser should
ensure that the gas flow pat-
terns within the contact chamber
bring all the flue gas in contact with
the atomized spray. The atomizer
should also produce small
droplets to maximize the droplet
surface area available for SO2 absorp-
tion. If the droplets are too small,
however, they may dry out
before sufficient S02 absorption
has occurred.
Atomizer types used in spray dryer
FGD processes include rotary
atomizers and two-fluid nozzles.
Each type has its advantages and
disadvantages, although most
vendors prefer rotary atomizers.1
Rotary atomizers use a rapidly spin-
ning disk (up to 20,000 r/min) to
produce a fine droplet mist in
the spray dryer. The size of the drop-
lets .varies, with the velocity and
diameter of the rotating disk and is
reasonably independent of the
liquid feed rate to the atomizer. Thus,
rotary atomizers have one impor-
tant advantage in that droplet size
can be kept the same regardless
of the feed rate of absorbent solution
or slurry. This advantage is impor-
tant because the required absorbent
feed rate varies with fluctuations
in the flue gas flow rate and the
inlet SO2 concentration. The pri-
mary disadvantage is that rotary
atomizers are mechanically complex
compared with nozzles. A second,
potential disadvantage is plugging of
the atomizer orifices; many ven-
dors minimize this problem through
improved disk design.11
Two-fluid nozzles use high-pressure
air (or steam) to break up the ab-
sorbent solution or slurry into a fine
mist of small droplets. These
nozzles have two main advantages:
they have no moving parts, and
large passages for liquid can be used
to minimize plugging. Their pri-
mary disadvantage is that the change
in absorbent liquid feed rate al-
ters the droplet size, which varies the
SO2 removal efficiency. This
problem is somewhat alleviated by
multiple nozzles; but more operat-
ing problems seem to occur
with nozzle atomizers than with
rotary atomizers;11
Further commercial experience with
spray dryer FGD systems is
needed for a full assessment of the
drawbacks and benefits unique to
each type of atomizer.
Design variations in the placement of
flue gas discharge points in the
spray dryer, the type of gas disperser,
and the numberof atomizers are
based on vendor experience or
preference. Because commercial
experience with these varia-
tions is lacking,itechnical compari-
son is difficult. For example, flue
gas discharge points are usually near
either trie.top or the bottom of the.
spray dryer hopper. Placing the
gas discharge point near the top may
be advantageous because plugging
of the spray dryer hopper would
not interfere with gas flow unless
the hopper were to become com-
pletely filled. More operating experi-
ence is needed at full-scale utility
FGD units to determine the
likelihood of hopper plugging.
Several important process or operat-
ing variables affect the design
and performance of the spray dryer,
including: flue gas approach to
saturation temperature at the dryer
outlet, absorbent stoichiometry,
and inlet SO2 concentration.The out-
let flue gas temperature is con-
trolled by the amount of water
injected into the spray atomizer
with the absorbent solution or slurry.
As this temperature approaches
the adiabatic saturation temperature,
the residual moisture level in the
spray-dried solids increases. The
residual moisture aids the mass trans-
fer of unreacted absorbent from
the center of the particle toward the
surface, where it.can react with
the absorbed sulfur dioxide. Absorb-
ent is utilized more readily at the
particle's surface because of
the greater area available for SO2 ab-
sorption. Thus, SO2 removal rates
and absorbent utilization increase as
the approach to saturation is
narrowed. Most spray dryers are
operated with an approach between
20° F and 50° F (11 ° C and 28° C)
above the adiabatic saturation
temperature.1
The closeness of the approach to sat-
uration is restricted by the need to
avoid condensation in downstream
solids collection equipment. The
restriction can be overcome in part if
warm or hot gas is bypassed
around the spray, dryer to reheat the
dryer outlet gas, but the amount of
untreated gas used for this purpose
may limit removal of sulfur di-
oxide in the spray dryer process.
Also, an energy penalty is associated
with the use of hot gas from up-
stream of the combustion air pre-
heater because less energy is
available for air preheat.
Absorbent stoichiometry directly
affects SO2 removal in the spray
dryer. Stoichiometry for dry scrub-
bing has been defined as moles of
fresh sorbent introduced to the
system divided by moles theoretically
required for complete reaction
with all the sulfur dioxide entering
the system, whether or not all
the sulfur dioxide is removed. This
definition results in a lower
stoichiometric ratio than does the
conventional definition for wet
scrubbing, which is based on moles
of sulfur dioxide removed by the
system. For example, a reported stoi-
chiometric ratio of 1.2 for a dry sys-
tem achieving 80 percent SO2
removal would be equivalent to
1.5 for a wet scrubbing system.
The absorbent stoichiometry may be
raised by an increase in the
amount of absorbent fed to the
spray dryer. A higher absorbent stoi-
chiometry enhances removal of
-------
Coyote soda ash holding tank (left) and control panel (right)
sulfur dioxide. This method is limited
by two factors, however. First,
absorbent utilization decreases with
increased stoichiometry, raising
absorbent and disposal costs.
Second, an upper limit is reached on
the solubility of the absorbent or
on the percent by weight of absorb-
ent solids in the slurry. Decreased
absorbent utilization can be
partly overcome by operation of the
spray dryer at a closer approach to
the saturation temperature and
by recycle of the collected
solids. Preliminary tests at the
Northern States Riverside demon-
stration system have shown
that overall S02 removals of 80 per-
cent can be achieved (spray dryer
and baghouse) with an absorb-
ent stoichiometry of less than 1.2 at
an inlet S02 concentration of
about 900 ppm.12
The effect of inlet SO2 concentration
on spray dryer performance has
not been fully investigated; however,
over the range of concentrations
studied (500-1,500 ppm), increases
in concentration only moderately
decrease SO2 removal.2'5 The range
studied corresponds to the
concentrations of sulfur dioxide in
flue gas from boilers using low-
to moderate-sulfur coals (gen-
erally less than 3 percent sulfur by
weight). Additional information on
the effects of higher SOZ con-
centrations will soon be available
from testing at one Utility demonstra-
tion (Northern States' Riverside
Station) and at three industrial sys-
tems [Department of Energy's
(DOE) Argonne National Laboratory,
Argonne, Illinois; Strathmore
Paper; and General Motors' Buick
Division, Flint, Michigan].
Solids Collection
The choice of the paniculate collec-
tion device is influenced by SO2
removal during particulate collection
and by vendoror customer pref-
erence. Fabric filters (or baghouses)
are the most widely used collec-
tion devices in commercial
spray dryer systems sold as of mid-
1 981. Of 23 utility and industrial
systems sold, only 1 specifies
an ESP. The primary advantage of,
baghouses over ESP's is that
unreacted alkalinity in the solids
10
-------
collected on the bag surface can re-
act with sulfur dioxide in the
flue gas. Studies have shown that
baghouses can account for 10 per-
cent of the total sulfur dioxide
removed by a system.2 Baghouses
also may be more economical
and effective in collecting highly
resistive fly ash; they can achieve
high particulate removal efficiencies
regardless of the type of coal
burned, whereas the highly resistive
fly ash from low-sulfur western
coals may be more difficult and costly
to collect using ESP's.11
The ESP's have a potential advan-
tage over baghouses because
they are less sensitive to condensa-
tion and, therefore, the spray
dryer can be operated closer to the
saturation temperature, which
results in higher S02 removals across
the spray dryer. The primary dis-
advantage of ESP's is that they may
not be able to meet stringent
particulate emission regulations if
the inlet particulate concentrations
become too high.13 Future testing
of full-scale systems should pro-
vide more information on the
maximum inlet concentrations that
can be expected.
Baghouses collecting spray drying
solids usually use fiberglass
filter bags with pulse-jet cleaning
mechanisms in industrial applica-
tions and reverse-air cleaning
mechanisms in utility applications.1
Solids Recycle \
Recycling waste solids to the spray
dryer reduces raw material re-
quirements because unreacted
absorbent is consumed during
recycle. Moreover, if the fly ash is
high in available alkalinity, the
available alkaline species may aid SO2
absorption by reacting with sulfur
dioxide in the flue gas.9'11 The
amount of alkalinity available in the
fly ash depends; primarily on the
type of coal fired.
The type of absorbent and the
amount of alkalinity available in the
fly ash are important in determin-
ing the benefits and economics of
solids recycle for a particular
spray dryer application. Solids re-
cycle is particularly beneficial
for lime systems because utilization
is low for once-through absorb-
ent. Solids recycle can increase SO2
removal in 1ime:spray dryers as
much as 10 percent at a given
absorbent stoichiometry.4 Thus
solids recycle can achieve a required
SO2 removal with less raw material.
As a rule, solids recycle is not bene-
ficial in sodium carbonate sys-
tems because utilization is relatively
high for once-through absorbent.
The potential reduction in raw mate-
rial requirements may not justify
the cost of a waste solids re-
cycle system.
Maximum benefit from available fly
ash alkalinity is pbtained by slurrying
the waste recycle solids separately
from the fresh absorbent feed.
The recycle slurry is then mixed with
the fresh absorbent at the point of
injection into the spray dryer.
If the recycle solids are slurried sep-
arately, the alkaline species
dissolve more readily and may then
react with sulfur dioxide in the
flue gas to reduce absorbent
requirements.2
As a further benefit, the fly ash in the
waste solids recycle may also
act as a surface catalyst for absorb-
ent utilization by providing an
alternative site for precipitation of
the absorption reaction products,2'6
thereby decreasing the solid
deposits on the absorbent parti-
cles. Absorbent utilization is improved
because more absorbent sur-
face area is available for reaction
with sulfur dioxide.
11
-------
Environmental
Considerations
Important environmental considera-
tions in spray dryer FGD include
SO2 and particulate removal capabil-
ities and dry waste product dis-
posal properties. In both pilot- and
demonstration-scale testing
with low-sulfur fuels, the spray dryer
FGD process has consistently
shown the ability to meet the NSPS
for removal of sulfur dioxide from flue
gas exiting coal-fired boilers.
Most available data are from pilot-
scale applications of-the process
because only one utility-size
demonstration system and two
industrial-size commercial systems
are operational.
The EPA funded pilot-scale testing at
Colorado'Springs' Martin Drake
station. These tests [20,000 actual
ft3/min (570 actual m3/min)]
showed overall SO2 removal effi-
ciencies above 75 percent for a lime
stoichiometry of less than 1.6 mol
lime permol inletsulfurdioxide.2The
tests were conducted at inlet SO2
concentrations of 1,500 ppm,
without solids recycle, and at an ap-
proach to saturation temperature
of 20° F (11 ° C). In other tests,
fly ash from high-sulfur eastern coals
was added to the flue gas along
with sulfur dioxide to simulate high-
sulfur (about 4 percent) coal appli-
cation; SO2 removals greater
than 90 percent were measured
with a lime stoichiometry of 1.6 mol
lime per mol inlet sulfur dioxide.14
These tests were conducted at
inlet SO2 concentrations of
4,000 ppm, with 50 percent solids
recycle, and at an approach to satur-
ation temperature of 20° F (11 ° C).
Preliminary tests oh the lime utility-
scale (100-MW) demonstration
system at Northern States' Riverside
Station showed an overall S02 re-
moval efficiency of 94 percent at an
absorbent stoichiometry of less
than 1.4. These tests were
conducted at an inlet S02 con-
centration of about 840 ppm, at an
approach to saturation temper-
ature of 18° F (10° C), and with solids
recycle.12 ;
Test results are also available from
two commercial lime spray
dryer installations. Compliance
test results from the Strathmore
Paper system, which treated flue gas
produced by combustion of
moderate-sulfur coal, show an aver-
age S02 removal efficiency of
92 percent.15 With coal averaging
2 percent sulfur,4 an average
S02 removal of 70 percent was
achieved at the Celanese Fibers sys-
stem, based on 23 days of accept-
able continuous SO2 monitoring
data obtained over a 33-day
test period. These tests were con-
ducted without solids recycle.
Further operating experience will be
required to reach optimal S02
removal efficiencies in full-scale
spray dryer systems. The available
test results, however, have provided
data sufficient for the design and
sale of 12 utility and 11 indus-
trial spray dryer systems.
All the spray dryer systems sold to
utilities specify low-sulfur west-
ern coals or lignite (average
sulfur content of 1.5 percent or less).
The SO2 removals guaranteed for
these systems range from 61 to
87 percent, depending on the coal
sulfur content.1
So far, for fuels with more than
1.5 percent sulfur, spray dryer FGD
applications have been for industrial
boilers. The overall cost effect of
the extra absorbent needed for high-
sulfur coal application is less
severe in these smaller systems than
in utilities, which must treat larger
quantities of flue gas. Many of
the planned and operating industrial
spray dryer systems are applied to
higher sulfur (up to 3.5 percent)
eastern coals. These systems have
SO2 removal guarantees from
70 to 90 percent.3
12
-------
High participate removal efficiencies
(99 percent or greater) can usually
be obtained by ESP's or fabric
filter baghouses !n coal-fired applica-
tions. In spray dryer systems, both
ESP's and baghouses should be
capable of achieving high par-
ticulate removal. The baghouse may
be advantageous, however, in
locations where environmental con-
siderations require low outlet par-
ticulate concentrations, such as
0.01 gr/ft3 (0.02 g/m3). Inlet dust
loadings as high as 25 gr/ft3
(57 g/m3) require an ESP efficiency
of 99.96 percent—a value that
may be difficult to achieve consist-
ently with an ESP in a power plant.13
Properly designed baghouses,
on the other hand, should be able to
meet an outlet emission of 0.01 gr/ft3
(0.02 g/m3), regardless of inlet
loading. In addition, the baghouse
will usually be more reliable in
achieving low opacity.9
Spray dryer FGD systems collect the
fly ash together with the FGD
reaction products. The composi-
tion of the waste solids varies
depending on the coal type and the
operating conditions of the proc-
ess. Waste product from a spray
dryer system applied to a boilerfiring
low-sulfur coal consists of approx-
imately 75 percent fly ash, 1 5 to
20 percent calcium sulfite and sul-
fate salts, 5 percent moisture,
and less than 5 percent unreacted
sorbent.16
Waste solids from spray dryer sys-
tems have handling properties
similar to fly ash, and are therefore
more easily disposed of than
wastes from conventional wet FGD
systems. The dry waste from lime
spray drying exhibits physical
properties suitable for on-site land-
fill disposal. The waste solids
are nonhazardous under current EPA
Resource Conservation and
Recovery Act guidelines—their
leaching characteristics have been
determined andlare well within
the guidelines set by EPA for
definition of a nonhazardous
waste.16-17
Fly ash from low-sulfur western coal
exhibits cementltious (pozzo-
lanic) properties;. Fixation reactions
associated with|these properties
have been studied to determine the
landfill characteristics of wastes
from spray dryer systems applied to
boilers firing low-sulfur west-
ern coals. The addition of about 20
percent water is:reported to give the
most desirable landfill disposal
properties for solid wastes contain-
ing fly ash from low-sulfur western
coals.16'17
Another method for disposing of lime
spray drying wastes has been de-
veloped by Niro, Atomizer, Inc.
(patent pending). The dry waste
product is combined with 10
to 20 percent water and pelletized
to between 2.5 and 3 times its
bulk density. The resulting syn-
thetic gravel (Nirok) is reported to
have a cured density of about
120 Ib/ft3 (1,900 kg/m3) and a
compressive strength of over 10,000
Ib/in2 (69,000 kPa). It can be used
in concrete mixes, cement founda-
tions, and roadbase compositions.16
Solids from sodium-based spray
dryer systems are not consid-
ered suitable for disposal by conven-
tional landfill methods. The
sodium salts produced by the proc-
ess are highly Water soluble, and
a lined landfill is necessary to
prevent them from leaching into the
ground water.11-18 The Sinterna®
Process has been developed to
stabilize these wastes by converting
the untreated waste material to
pellets with reduced leaching poten-
tial, but the process.does not
appear to be economically desir-
able.17 Wastes from lime-based
systems are economically and envi-
ronmentally preferable, therefore,
because they can be landfilled
without special treatment.
Current and planned disposal
methods for spray drying wastes
parallel established practices for wet
FGD systems. Wastes from the
only commercial systems in opera-
tion, at Celanese Fibers' Amcelle
Plant and Strathmore Paper, are
being trucked to landfill, as are the
wastes from the demonstration
at Northern States' Riverside
Station. Tentative waste dispo-
sal plans of utilities with con-
tracts for commercial systems range
from dry landfill to clay-lined
ponds for wetted solids. Eight util-
ities have reported disposal plans as
follows:1
• Natural clay-lined landfill
• Landfill lined with PVCa
• Landfill with fixation
• Wetted transport to landfills (two
utilities)
• Wetted transport to clay-lined
pond
• Unspecified landfills (two utilities)
Studies by EPA, DOE, the Electric
Power Research Institute (EPRI), and
many vendors are being con-
ducted to characterize further the
physical and chemical properties of
the dry waste. As more commer-
cial systems begin operation, these
efforts should gain momentum
and result in a broader data base.
aPolyvinyl chloride.
13
-------
Status of Development
The application of spray drying to
FGD developed from earlier commer-
cial uses of spray dryers. These
devices had been used for many
years in a wide range of drying, reac-
tion, and purification processes
in the chemical industry. Most FGD
spray dryer designs are direct
adaptations of the standard designs
used in other industries, such as
the food processing industry.11
Spray dryer FGD also evolved in part
from early dry injection FGD
studies initiated in the 1 960's. In
the systems studied, dry absorbents
were injected into the flue gas,
and the reaction products were col-
lected in fabric filter baghouses
or ESP's. Results from many of these
early studies were disappointing,
but interest in the technology
continued because of the potential
benefits of such uncomplicated
approaches to FGD.
Unlike efforts in the development of
wet FGD, where the Government
has predominated, early efforts in
spray dryer FGD were supported pri-
marily by industry.^Several com-
mercial dry scrubbing systems had
been sold before the Govern-
ment started to support research
and development in this area.18 Early
test work included that of Atomics
International with its aqueous
carbonate process at Southern
California Edison's Mohave Station,
Laughlin, Nevada, in 1972; testing
involved a pilot-scale demonstration
of the spray dryer for S02 removal.
In 1 974, Niro Atomizer initiated tests
of spray drying for FGD applica-
tion at its Copenhagen facility, inves-
tigating various alkaline sorbents
such as lime, limestone, and sodium
carbonate.
A major impetus to the development
of spray dryer FGD occurred in
1977 when the Basin Electric Power
Cooperative expanded dry injection
studies at its Leland Olds Station
to include spray drying. Basin Elec-
tric requested bids' for new plants to
be fueled with western coal and
lignite. Several vendors built and
operated small pilot units to prequal-
ify as bidders. Test units were
operated at Basin Electric's Neal
and Leland Olds Stations in
North Dakota and at Otter Tail Power
Company's Hoot Lake Station in
Minnesota. These were the first U.S.
applications of the spray dryer for
a nonregenerable FGD system,
and three contracts were awarded in
1 978 on the basis of the tests.
The recipients were Babcock &
Wilcox and two joint ventures: Joy
Manufacturing Company and Niro
Atomizer (Joy-Niro), and Rock-
well International and Wheelabrator-
Frye, Inc. (Rockwell-WF).
In the past few years several com-
panies and consortia have become
active in dry FGD. Thirteen
firms currently offer commercial
spray dryer systems, and several
have contracted to build commercial
units. All the utility units are for
lignite or low-sulfur western coal
applications, where removal
efficiencies and absorbent consump-
tion are usually lower than with
high-sulfur coals and, in some cases,
the alkalinity of the fly ash can
supplement the absorbent.11
As of mid-1981, 12 utility systems
(totaling over 3,800 MW) and
11 industrial systems had been sold
(Tables 1 and 2).1 These systems
are mostly lime spray dryer and bag-
house combinations. Only two
commercial systems, both applied to
industrial boilers, were fully oper-
ational: the Rockwell-WF system
at Celanese Fibers' Amcelle
Plant and Mikropul Corporation's
system at Strathmore Paper. The
first utility system to become opera-
tional will likely be another
Rockwell-WF system, this one at
Otter Tail Power's Coyote Station,
Beulah, North Dakota. Startup
procedures at Coyote were initiated
in January 1 981.1 Two of the
most recent industrial systems sold
will be applied to boilers firing high-
sulfur coal. At least five utilities
have specified dry FGD only or are
considering either dry or wet FGD for
14
-------
Table 1.
Commercial Utility Spray Dryer FGD
Process, utility, and station
Lime:
Basin Electric Power Coop.:
Antelope Valley, Beulah ND:
Unit 1
Unit 2
Laramie River, Wheatland WY:
Unit 3
Colorado Ute Electric Association: Craig
Station, Craig CO Unit 3
Marquette Board of Light & Power: Shiras
Station, Marquette Ml, Unit 3
Platte River Power Authority: Rawhide
Station, Fort Collins CO, Unit 1
Sierra Pacific Power & Light: North Valmy
Station, Valmy NV
Sunflower Electric Coop.: Holcombe Sta-
tion, Hays KS Unit 1
Tucson Electric: Springerville Station,
Tucson AZ:
Unit 1
Unit 2
United Power Association: Stanton Sta-
tion, Stanton ND
Sodium carbonate:
Otter Tail Power: Coyote Station, Beulah,
ND Unit 1
Systems Sold as of
FGD units ;
Spray dryers;
(MW) Atomizers
per dryer
430 5° 1 rotary
430 5C 1 rotary'
500 4° 1 2 nozzles
450 NA Nozzlesd
44 1 1 rotary
250 NA Rotaryd :
270 3 3 rotary
310 NA Rotaryd
350 NA Rotaryd
350 NA Rotaryd
65 NA Rotaryd •
41 0 4 3 rotary •
Mid-1981
Gas
treated
actual
ft3/min)
2 200
• 2 200
2 810
NA
226
NA
1,200
NA
NA
NA
NA
1 890
Coal
Type % Sa
subbitu-
minous
nous
Western 0 5
subbitu-
minous
Western 1 .3
subbitu-
minous
Subbitumi- 04-1 0
nous
Western 1 3
subbitu-
minous
subbitu-
minous
subbitu-
minous
Lignite (a)
% SO2 Solids
removal collection .
u • • i -i • b date
(design) device
62 FF 1 982
62 FF 1 985
82 ESP 1 982
87 FF 1983
80 FF 1 982
80 | FF 1983
76 FF 1984
80 FF 1 983
61 FF 1 984
61 ' FF 1984
NA FF 1 981
70 FF 1 981
8Average. ' ' !
bFF = fabric filter; ESP = electrostatic precipitator. :
clncludes 1 spare.
dNumber not available. i
eLow to medium. . . . : •
Note.—NA = Data not available. i ;
SOURCES: Kelly, M. E., and S. A. Shareef, Third Survey of Dry SO 2Contrbl Systems, EPA 600/7-81-097, NTIS No. PB 81-21 8976, June 1981.Gehri,
Dennis, Rockwell International, personal communication, Sept. 25, 1981.
15
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Table 2.
Commercial Industrial Spray Dryer FGD Systems Sold as of Mid-1981
FGD units
Process, company, and station
Limo:
Argonne National Laboratory, Argonne II
ColaneSQ Fibers: Amcelle Plant, Cumber-
land MD
Fairchild Air Force Base, Spokane WA:
Unit 1
Unit 2
Unit 3
General Motors: Buick Division, Flint,
Ml
University of Minnesota: Unit 1 , Minneap-
olis MM
Size
(1 ,000
Ib/h
steam)
170
110
170
110
11O
110
450
85
NA
Spray dryers
No.
1
1
1
1
1
1
1
1
2
Atomizers
per dryer
1 rotary^
rotary,
rotary
rotary '
rotary'
rotary,
1 rotary
4 nozzles
1 single
nozzle.
Gas
volume
treated
(1 ,000
actual
ft3/min)
75
65
NA
46.5
46.5
46.5
167
40
120
Coal
Type
Illinois
bitu-
minous
Eastern
NA
NA
NA
NA
Indiana
bitu-
minous
Eastern
Subbitu-
minous
%sa
3.5
1.5-2.5
1.0
1.0
1.0
1.0
1 .0-3.0
2.0-3.0
0.6-0.7
%S02
removal
(design)
79
70-85
NA
85
85
85
70-90
75
70
Solids
collection
device6
FF
FF
FF
FF
FF
FF
FF
FF
FF
Startup
date
1981
1980
1981
1983
1983
1983
1982
1979
1981
Sodium carbonate:
Calgon, Catlettsburg KY NA
University of Minnesota: Unit 2, Minneap-
olis MN NA
1 multi-
ple
nozzle
1 1 rotary, 57
1 1 rotary 120
NA
Subbitu-
minous
1.0-2.0
0.6-0.7
75
75
FF
FF
1981
1981
'Average.
bFF - fabric filter.
Now.—NA = not available. !
SOURCES: Kelly, M. E.,and S. A. Shareef, Third Survey of Dry SO2 Control Systems, EPA 600/7-81-097, NTIS No. PB 81-218976, June 1 981. Steele,
Gone, Niro Atomizer, personal communication, July 1 6, 1981.
new units that will go on line
before the end of the decade.1 There
are also two important full-scale
(100-MW) utility demonstrations:
theJoy-Niro lime system at Northern
States' Riverside Station (now
operational) and the Flakt, Inc.,
lime system adapted for lime or
sodium carbonate at Pacific Power
and Light Company's Jim Bridger
Station, Rock Springs, Wyoming
(planned for startup in 1982).
Current research and development
efforts are examining process
variables affecting SO2 removal, as
well as application of the tech-
nology to high-sulfur coal. Studies by
EPA, DOE, EPRI, vendors, and
others include assessments of the
following:7
• Importance of lime stoichiometry
• Effect of flue gas approach to
saturation temperature
• Effect of waste solids recycle from
the spray dryer and baghouse
• Refinements of atomization and
sorbent preparation techniques
• Role of alkalinity in fly ash removal
• Need for reheat
The technical and economic feasibil-
ity of the vendors' approach to
design features such as type
of atomizer, degree of approach to
saturation, particulate collection
method, and waste recycle remain to
be demonstrated. Current trends
16
-------
suggest that many new systems will
use a lime slurry with rotary atom-
izers, partial flue gas bypass
for reheat, and fabric filter collection.
Most of these spray dryer FGD
units will likely be limited to low-
sulfur-coal conditions, because the
cost of lime spray drying relative
to wet limestone scrubbing for high-
sulfur-coal applications is still
uncertain.
The rapid growth of spray dryer FGD
results partly frorn its potential
technical and economic benefits
and from the increased use of west-
ern coals by utilities, for which
spray drying is especially suited.
Also important has been the broad
base of spray dryer and particu-
late collection technology from
which spray dryer FGD directly
evolved. The April 1981 Dis-
trict Court decision upholding the
NSPS for utility boilers has been a
recent impetus to the technology.
The NSPS include a variable
percentage SO2 removal provi-
sion that requires at least 70 percent
S02 removal for boilers firing coal.
Spray drying provides a potentially
lower cost alternative to wet FGD
for low-sulfur-coal applications.
17
-------
Raw Materials and Utilities • Electric energy for auxiliary equip-
ment such as agitators/ con.
The raw material requirements of veyors, and feed preparation
spray dryer FGD processes are equipment
for lime or sodium carbonate re- prQcess wgter irements include
agent, and the ut.lity needs are dj|ution wgter for absorbent prep.
for energy and process water. Energy ^.^ and for sprgy dryer Qut|et f|(je
requirements include: ggs temperature control. In lime
• Pumping energy to move absorb- spray dryer systems, slaking
ent solution or slurry to the water accounts for 10 to 30 per-
spray dryer atomizer cent of the total process water
• Electric energy forflue gas booster requirements.
blowers (forced- or induced-
draft fans) Table 3 gives the estimated annual
• Electric energy to rotate the raw material and utility requirements
atomizers (if rotary type) of sodium carbonate and lime
Table 3. !
Estimated Annual Raw Material and Utility Requirements for Lime and
Sodium Carbonate Spray Dryer FGD Processes: New 500-MW Coal-Fired
Power-Generating Unit
, Component Requirement
Low-sulfur coal: ;
Lime spray dryer system:a'b
Raw materials (lime, 1,000 tons) 1 °->
Utilities: '.
Fuel (1,000 gal) •• 1 63'8
Process water (106 gal) • • 82-2
Electricity (106 kWh) • •,• • 39-6
Sodium carbonate spray dryer systern:b'°
Raw materials (sodium carbonate, 1,000 tons) 18.4
Utilities:
Fuel (1,000 gal) 1 65-7
Process Water (10s gal) 70-2
Electricity (106 kWh) • • 41 -2
High-sulfur coal: Lime spray dryer system:d
Raw materials (lime, 1,000 tons) • • 112.4
Utilities:
Fuel (1.000 gal). ^
Process water (106 gal) 143-6
Electricity (1Q6 kWh) 42'7
Energy for reheat (106 Btu) 137-4
"Warm gas bypass reheat. Stoichiometry of 1.2 mol lime per mol S02 absorbed. Landfill disposal
1 mi from FGD facilities.
bCoal with 0.7% S. Meets emission regulation of 1.2 Ib S02/106 Btu with 70% S02 removal.
°No gas bypass. Stoichiometry of 1 mol sodium carbonate per mol SO2 absorbed. Lined-
pond-disposal 1 mi from FGD facilities.
dHot gas bypass reheat. Coal with 3.5% S. Stoichiometry of 1.8 mol lime per mol SO2 absorbed.
Includes electrical requirements of fabric filter baghouse. Meets emission regulation of
1.2 Ib S02/106 Btu with 90% S02 removal. Landfill disposal 1 mi from FGD facilities.
Notes.—Midwest plant operating 5,500 h/yr.
SOURCE- Burnett T X., and K. D.Anderson, Technical Review of Dry FGD Systems and Economic
Evaluation of Spray Dryer FGD Systems, EPA 600/7-81-014, TVA EDT-1 27, NTIS No. PB 81 -
206476, Feb. 1981.
18
-------
spray dryer processes for a new
500-MW power-generating unit
firing low-sulfur coal.11 System de-
sign and operating conditions
affect the requirements and must
be considered for each specific
installation. Absorbent needs vary
with the desired SO2 removal, the
coal sulfur content, the use of
recycle, and (if recycle is used) the
available alkalinity in the fly ash.
Fuel requirements given are for
trucking dry waste solids to the
landfill site. The table includes elec-
trical needs of the fabric filter
baghouse.
Spray dryer systems using hot gas
bypass (from upstream of the
combustion air preheater) to reheat
the flue gas incur an energy penalty
because of the decrease in energy
available to heat air in the preheater.
Hot gas bypass will usually be
necessary only when high SO2 re-
movals (approaching 90 percent) are
required, as in higher sulfur coal
applications. Hot gas bypass min-
imizes the amount of untreated
gas that must be bypassed around
the spray dryer and thus increases
the SO2 removal that can be
obtained with a given spray dryer
system.
Table 3 also gives the estimated raw
material and utility requirements of
a lime spray dryer process for a
new 500-MW boiler firing high-
sulfur coal,11 and includes the
estimated energy penalty associated
with hot gas bypass for this sys-
tem. Because data are lacking for
full-size spray dryer systems applied
to boilers firing high-sulfur coal,
these estimates may change
considerably after ongoing
demonstration-scale studies are
completed.1
Reagent requirements are the major
cost component of the annual
material and utility needs of the spray
dryer FGD process. Comparison by
weight of the raw material require-
ments of lime and sodium carbon-
ate spray dryer systems can be
somewhat misleading because the
molecular weight of sodium car-
bonate (106) is nearly twice that
of lime (56.1). In molar terms, lime
systems usually require more reagent
than do sodium carbonate sys-
tems because lime is less reactive
and therefore has lower material
utilization. In terms of weight, how-
ever, sodium carbonate sys-
tems require more raw material, and
the higher cost per unit weight
of sodium carbonate usually
makes these systems more expen-
sive to operate. ;Substituting other
sodium compounds for sodium car-
bonate may make sodium sys-
tems more competitive with lime
systems. In particular, trona ore
appears to be an attractive alter-
native in Western States where it is
readily available.19 No commercial
spray dryer systems sold as of
mid-1981, however, plan to use
trona ore as an absorbent.
The total utility needs of lime spray
dryer systems are generally lower
than those of limestone wet
scrubbing systems.11 These needs
vary, however, with system de-
sign and the type of coal burned,
which determine the absorbent feed
rate requirements and the amount
of solids to be disposed of.
In low-sulfur-coal applications, lime
spray dryer systems have slightly
lower energy requirements
than wet limestone systems because
less energy is needed for pumping.
Considerably less solution or slurry
must be moved through spray
dryer systems. A large part of the
associated energy savings, however,
is lost to the energy needed for
atomizing the solution or slurry.
In high-sulfur-cpal applications, the
total energy requirements for lime
spray dryer systems are much
lower than for wet systems because
pumping energy needs are re-
duced and less ;energy is needed
for flue gas reheat.11 Although spray
dryer systems on boilers firing
high-sulfur coal incur an energy
penalty for hot gas bypass, the pen-
alty is much less than the energy
needed to reheat exit gases from wet
scrubbing systems.
Installation Space and Land
Installation space and total land
requirements for spray dryer FGD
systems vary depending on the plant
size, the spray dryer design, and the
type of absorbent used. Installa-
tion space requirements for a lime
spray dryer system have been
estimated from recent Tennessee
Valley Authority (TVA) and EPA
studies.11-20
The total estimated installation
space requirement for a 500-MW
lime spray dryer FGD system is about
2.3 acres (0.9 ha), including space
for a recycle waste solids slurry
system and a process control area
(Figure 4). The total area needed for
process control, solids recycle, and
absorbent preparation and stor-
age is estimated at 0.3 acre (0.1 ha).
Total installation space requirements
for a 500-MW sodium carbonate
spray dryer system should not
differ significantly from those of
the lime system shown in Figure 4.
Space requirements for the
spray dryer and solids collection
equipment would be similar for both
systems. Absorbent preparation
would require less space in the
sodium carbonate system, and space
would probably hot be needed for
solids recycle storage. Absorb-
ent storage, however, would
require more space; sodium car-
bonate reagent is stored as a
saturated aqueous solution, and
would need approximately twice the
storage space needed for the lime
system feed.11
A large additional area is needed for
waste solids disposal, on or off
site. Assuming a 30-yr lifetime (or
165,000 h of operation), a lifetime
19
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Key
260ft
Flue gas/off-gas
Cleaned flue gas
Absorption feed
Air pollutants
Other "systems
ID fan
Recycle
storage
bins
Fabric filter
baghouse
125ft
product
tanks
' 340ft
i 110ft i
Figure 4.
Lime Spray Dryer Installation Space Requirements
landfill for a 500-MW lime spray
dryer system applied to low-sulfur
coal (0.7 percent sulfur) would
require an area of about 154 acres
(62 ha) with an initial depth of 30 ft
(9.1 m). A sodium carbonate
system would require a clay-lined
pond 30 ft (9.1 m) deep with an
area of 195 acres (79 ha). These dis-
posal area requirements include
space for fly ash disposal as well as
waste FGD solids. The sodium
carbonate system requires more
solids disposal area than does the
lime system because a greater total
volume of waste is; produced.
Spray dryer systems need a larger
total area for waste disposal than do
wet scrubbing systems. For exam-
ple, a 500-MW wet limestone
scrubbing system would need about
134 acres (54 ha) of disposal land
with an initial depth .of 30 ft (9.1 m).11
Spray dryer systems produce more
waste because their raw material
utilizations are lower. Any disadvan-
tages and costs associated with the
greater volume of waste are
usually outweighed by the advan-
tage of a dry waste product, which
eliminates the need for dewater-
ing equipment and may allow earlier
reclamation of land used for FGD
waste disposal.
20
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Costs
Reasonably accurate cost estimates
for spray dryer systems can be
based on vendor-supplied capital
cost information and on operating
data from pilot- and demonstration-
scale studies. Capital and oper-
ating cost estimates for lime and
sodium carbonate spray dryer FGD
systems have been prepared by TVA
for comparison with wet lime-
stone scrubbing FGD systems.11 The
actual costs of commercial-size
spray dryer systems may vary
widely from these estimates, de-
pending on design and operating
developments resulting from full-
scale operation in the near future.
Tables 4 and 5 give specific compo-
nents of annual operating costs
Argonne spray dryer, 75,000 actual ft3/min
21
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Table 4.
Annual Operating Costs for Lime Spray Dryer FGD System on a New 500-MW Coal-Fired Power-Generating Unit
Costs
Component
Annual quantity
Unit ($)
Annual
operating
($1,000)
Mills/kWh
Direct costs, first yean
Conversion costs: '
Operating labor and supervision: i
Flue gas desulfurization ;... 25,400 man-hours 15.00/man-hour 381 0.14
Solids disposal ;... 28,152 man-hours 21.00/man-hour 591 0.21
Utilities: !
Fuel 163,750 gal 1.60/gal 262 0.10
Process water ;... 82.2 X106 gal 0.14/1,000 gal 12 0.004
Electricity i... 39.6 X 106 kWh 0.037/kWh 1,464 0.53
Maintenance, labor and materials 2,136 0.78
Analyses 4,191 man-hours 21.00/man-hour 88 0.03
Total conversion costs j... 4,934 1.794
Delivered raw materials (lime) '.... 10,100 tons 102.00/ton 1,030 0.37
Total direct costs J... 5,964 2.164
Indirect costs: Overhead, first year, plant, and administrative .'... 1,717 0.62
Total first-year operating and maintenance costs '-.... 7,681 2.784
Lovehzod capital charges (14.7% of total capital investment) :... 11,336 4.12
Total first-year revenue requirements 19,017 6.904
Lovclizod operating and maintenance costs (1.886 X first-year operating;and
maintenance) ;... 14,486 5.27
Levelized capital charges (14.7% of total capital investment) '... 11,336 4.12
Total levelized annual operating costs ',... 25,822 9.39
Notes.—Upper Midwest plant operating 5,500 h/yr. 1984 revenue requirements. 30-yr plant life. 1,346,700 tons/yr western coal burned, 9,500
Btu/kWh. 0.7% sulfur. Warm gas bypass reheat and waste solids recycle. Stoichiometry of 1.2 mol lime per mol S02 absorbed. Meets emission
regulation of 1.2 Ib S02/106 Btu with 70%S02 removal. Maintenance costs estimated at 6% of nonlandfill capital investment plus 3% of landfill invest-
ment. Landfill disposal 1 mi from plant. Includes investment and revenue requirements for fly ash removal and disposal. Total direct investment,
$38,587,000; total fixed investment, $59,369,000; total capital investment, $77,113,000.
SOURCE: Burnett, T. A., and K. D. Anderson, Technical Review of Dry FGD Systems and Economic Evaluation of Spray Dryer FGD Systems, EPA 600/
7-81-014. TVA EDT-127, NTIS No. PB 81-206476, Feb. 1981.
and capital charges for a lime and
a sodium carbonate spray dryer FGD
system. The tabulations assume
installation on a new 500-MW
power-generating unit burning low-
sulfur coal (0.7 percent sulfur). Both
FGD systems are designed for 70
percent SC"2 removal. The lime sys-
tem, however, uses both untreated
warm gas reheat and waste solids
recycle, and the sodium carbonate
system uses neither. Because all the
flue gas must be treated in the
sodium carbonate system, this
system requires four operating spray
dryers. The lime system operates
with three spray dryers because by-
pass reheating reduces the flow of
flue gas to be treated.
The annual operating costs for the
sodium carbonate system are about
9 percent higher than those for the
lime system, primarily because raw
material costs and total capital
investment are higher for the sodium
carbonate system. The higher cap-
ital investment results mainly
from the costs associated with con-
structing an environmentally safe
pond for sodium waste disposal.
22
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Table 5.
Annual Operating Costs for Sodium Carbonate Spray Dryer FGD System on a
Generating Unit
New 500-MW Coal-Fired Power-
Costs
Component
Annual quantity
Unit ($)
'. Annual
operating
($1,000)
Mills/kWh
Direct costs, first year:
Conversion costs: • ;
Operating labor and supervision: ]
Flue gas desulfurization ;..... 16,640 man-hours 15.00/man-hour 250 0.09
Solids disposal 28,362 man-hours 21.00/man-hour 596 0.22
Utilities: : • :
Fuel 165,700 gal 1.60/gal 265 0.10
Process water !.... 70.2 X106 gal 0.14/1,000 gal ' 10 0.004
Electricity 41.15 X106kWh 0.037/kWh 1,523 0.55
Maintenance, labor and materials ,.... ; 1,863 0.70
Analyses ;• • • • 4,191 man-hours 21.00/man-hour 88 0.03
Total conversion costs I : 4.595 1.694
Delivered raw materials (sodium.carbonate) ; 1 8,350 tons 145.00/ton 2,661 0.97
Total direct costs : I 7,256 2.664
Indirect costs: Overhead, first year, plant, and administrative j. . . . : : 1,475 0.54
Total first-year operating and maintenance costs ;.... 8,731 3.204
Levelized capital charges (14.7% of total capital investment) I. . . . 11,679 4.25
Total first-year revenue requirements .!.... ; 20,410 7.454
Levelized operating and maintenance costs (1.886 X first-year operating and
maintenance) .;.-.. 16,467 5.98
Levelized capital charges (14.7% of total capital investment) 11,679 4.25
Total levelized annual operating costs i.... , 28,146 10.23
Notes.—Upper Midwest plant operating 5,500 h/yr. 1984 revenue requirements. 30-yr plant life. 1,346,700 tons/yr western coal burned, 9,500
Btu/kWh, 0.7% sulfur. No gas bypass, no waste solids recycle. Stoichiometry of 1.0 mol sodium carbonate per mol SO2 absorbed. Meets emission
regulation of 1.2 Ib SO2/106 Btu with 70% S02 removal. Maintenance costs estimated at 5% of nonlandfill capital investment plus 3% of pond invest-
ment. Pond disposal 1 mi from plant. Total direct investment, $40,941,000; total fixed investment, $61,408,000; total capital investment,
$79,448,000.
SOURCE: Burnett, T. A., and K. D.Anderson, Technical Review of Dry FGD Systems and Economic Evaluation of Spray Dryer FGD Systems. EPA 600/
7-81-014, TVA EDT-127, NTIS No. PB 81-206476, Feb. 1981.
Table 6 gives estimated capital and
annual operating costs of 500-MW
lime spray dryer systems for four
different coals. The costs depend on
a number of site-specific factors.
Any specific situation should
be evaluated for availability and
cost of raw materials, utilities, and
area for waste disposal. Annual oper-
ating costs for spray dryer systems
will be most sensitive to the costs of
raw materials and electricity.
The cost estimates given for lime
spray dryer systems were compared
with cost estimates for wet lime-
stone scrubbing systems.11 For
all three low-sulfur-coal applications,
the comparison showed that both
the capital and operating costs
of lime spray dryer systems are lower
than those for limestone scrub-
bing systems.
In the high-sulfur-coal applications,
the capital and operating costs for
the spray dryer and limestone scrub-
bing systems were essentially
the same within the accuracy of the
cost estimates.11 Further study of
spray dryer processes on boilers
firing high-sulfur coal should provide
data for improved estimates of the
relative costs of the two FGD
systems in high-sulfur-coal applica-
tions.
23
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Table 6. j
Estimated Capital and Operating Costs for Lime Spray Dryer FGD Process on a New 500-MW
Generating Unit
„. . . . Total capital
System characteristics . b
investment
C°altyP9° %S _,,- Absorbent n .
in %S% stoichi- Operating 6
coal removal ometry* varlables
Lignite 07 70 12 Warm gas 826 165
! bypass
reheat,
i waste
solids
recycle
Western 07 70 12 Warm gas 771 1 54
; bypass
reheat,
waste
solids
recycle
Eastern , 07 70 1 3 Warm gas 75 3 151
', bypass
; reheat,
| no waste
solids
recycle
i pass
! reheat,
waste
i solids
; recycle ,
Coal-Fired Power-
Annual
operating costs0
/ $106 Mills/kWh
28 7 1043
25 8 9 39
25 2 918
47 1 1713
'Coal heating values: lignite = 6,600 Btu/lb; western = 9,700 Btu/lb; eastern = 11,700 Btu/lb.
Project beginning early 1981, ending late 1983. Average cost for sea ling, mid-1 982. Minimum in-process storage, redundant spray-drying train, pumps
are spared. FGD process investment begins at boiler heat exit. Excludes stack plenum and stack; includes only nominal construction overtime.
C1984 revenue requirements. ''
dMoets emission regulation of 1.2 Ib S02/106 Btu. :
*mol S02 absorbed. ;
Notes,—Midwest plant operating 5,500 h/yr. 30-yr plant life. Landfill disposal 1 mi from plant. Includes investment and revenue requirements for fly ash
removal and disposal.
SOURCE: Burnett, T. A., and K. D. Anderson, Technical Review of Dry FGD Systems and Economic Evaluation of Spray Dryer FGD Systems, EPA 600/7-
81-014. TVA EDT-127. NTIS No. PB 81-206476, Feb. 1981. ;
24
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26
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