United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-018 July 1981
Project Summary
Second Survey of Dry SO2
Control Systems
Mary E. Kel y and S. A. Shareef
This repor: is an update of the first
survey report on the status of dry flue
gas desulfurization(FGD) processes in
the United States published in
February I960. This updated
assessment of dry FGD systems is
based on review of current and
recently completed research, develop-
ment, and commercial activities. Dry
FGD systems covered include spray
dryers with a fabric filter or an electro-
static precip tator (ESP), dry injection
of alkaline material into flue gas
combustion of a pulverized coal/alkali
in an ESP or a fabric filter, and
combustion of coal/alkali fuel
mixtures.
Almost all new systems use a lime
sorbent and include a fabric filter for
particulate collection. Removal
guarantees for SOz range between 62
and 85 percent, depending on coal
sulfur content. Two full-scale indus-
trial spray diying systems are current-
ly in operation, with the first large
utility system scheduled for start-up in
the Spring of 1981.
A number of pilot-scale demonstra-
tion programs funded by vendors
and/or utilities have been completed
in the past year. The Environmental
Protection Agency (EPA) is currently
funding three demonstration pro-
grams (two spray drying and one dry
injection). The Agency is also funding
development of two combustion
processes for SO2 control: combus-
tion of coal/limestone fuel pellets and
combustion of a pulzerized coal/alkali
fuel mixture in a low-NOx burner.
Favored sorbents for continuing pilot
test programs of dry injection include
nahcolite, trona, and upgraded trona
(90 percent NaHCOs).
This Project Summary was develop-
ed by EPA's Industrial Environmental
Research Laboratory, Research Tri-
angle Park. NC. to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
This report is a semi-annual updateon
dry flue gas desulfurization (FGD)
processes in the United States for both
utility and industrial applications. Forthe
purposes of the report, dry FGD is
defined as any process which involves
contacting a sulfur-containing flue gas
with an alkaline material and which
results in a dry waste product for dis-
posal. This includes (1) systems which
use spray dryers for a contractor with
subsequent baghouse or electrostatic
precipitator (ESP) collection of waste
products; (2) systems which involve dry
injection of alkaline material intotheflue
gas with subsequent baghouse or ESP
collection; and (3) other varied dry
systems which are primarily concepts
that involve addition of alkaline material
to a fuel prior to combustion. Also since
the open loop, spray dryer contractor
portion of the Rockwell process had been
adapted for a "throwaway" system, it
has been included here.
The report discusses the commercial
and developmental activities for each
type of process (spray drying, dry irtjec-
-------
tion, and -combustion of a coal/lime-
stone fuel mixture); an assessment of
the state-of-the-art for each type of
process; a brief comparison of the
advantages and disadvantages of dry
and wet FGD systems; and, areas for
further research.
Recent Developments
Spray drying continues to be the only
commercially applied dry FGD process.
Since the last survey was conducted in
the Fall of 1979, eight new utility andtwo
new industrial spray drying systems
have been sold, bringing the total to ten
utility and four industrial commercial
systems (Table 1). All but one of the new
systems use a lime sorbent and all
include a fabric filter for particulate
collection. (One industrial system uses a
sodium carbonate solution in a spray
dryer-based system designed to remove
SOz and HCI from high temperature
gases.) Removal guarantees for S02
range between 60 and 90 percent on the
systemssoldtodate.Thefull-scale utility
systems are designed for relatively low
sulfur coals (0.5 to 1.3 percent sulfur),
while higher sulfur coals (1.5 to 2.5
percent sulfur) are burned at two indus-
trial systems.
Systems designs are similar, except
for variations between spray dryer
designs, atomization and slaking (lime
preparation) techniques, and the use of
gas bypass or solids recycle. The major
spray dryer system vendors offer a basic
system design with recycle or gas
bypass.
Ongoing bench-scale studies aim at
better understanding the mechanisms
of the reaction occurring in the spray
dryer.
Sodium-based sorbents are favored
for dry injection; nahcolite, trona, and
upgraded trona appear to be the most
viable choices. Sorbent availability and
waste disposal, as well as high stoichio-
metric requirements and somewhat
limited S02 removal capability (about 80
percent maximum in tests to date),
continue to restrict the commercial
application of dry injection.
Development of coal/limestone
pellets and combustion of a coal/alkali
fuel mixture in a low NO, burner are
continuing, under EPA-funding, on
development- and pilot-scales, respec-
tively. The minimal equipment require-
ments of these techniques make them
particularly attractive, but large-scale
demonstrations and full characteriza-
tion of the effects on boiler design and
operation, as well as particulate control,
remain.
Current Status of Dry FGD
Processes
Spray Drying
Two commercial industrial spray
drying systems are operational at this
time. The Rockwell/Wheelabrator-Frye
system at Celanese Fibers Company's
Amcelle plant passed Maryland State
S02 compliance tests in February 1980.
The Mikropul system at Strathmore
Paper has been in operation since July
1979. Although there were some initial
problems with atomization, availability
during the past 11 months of about 90
percent based on boiler demand has
been reported.
Both of the operational industrial
systems burn medium sulfur eastern
coals (1.5 to 3 percent sulfur) and are
designed for 70 to 85 percent S02
removal with lime sorbents. Both
designs include a spray dryer followed
by a fabric filter. The Mikropul system
uses four two-fluid nozzles for
atomization and a pulse-jet fabric filter.
A single rotary atomizer and a pulse-jet
fabric filter are included in the
Rockwell/ Wheelabrator-Frye system.
The first large scale commercial utility
system start-up is scheduled for the
Rockwell/Wheelabrator-Frye system at
Otter Tail Power Company's Coyote
Station. The 110-MW Joy/Niro retrofit
system at Northern States Power is
scheduled to begin operation in the Fall
of 1980, but will be operated initially as
a demonstration unit. Testing at about
half the maximum gas flow rate is
scheduled to begin this fall. Arvexisting
ESP will be used until the baghouse
portion of the system is completed in
early 1981.
The utility systems guarantee S02
removals from 61 to 87 percent. Almost
all of the utility systems use a lime
sorbent and include a fabric filter for
collection of fly ash and the dried
product solids. Exceptions are the
Coyote system which will use a sodium-
based sorbent (initially commercial soda
ash) and a Babcock & Wilcox system .at
Laramie River that will use four ESPs
rather than a fabric filter. Some designs
include hot or warm gas bypass and/or
recycle of spent solids mixture.
The differences between system
design include the use of nozzle or rotary
atomizers and the atomizer configura-
tion in the spray dryer. However, several
vendors claim that there are more plug-
gage or erosion problems with nozzles,
especially for lime slurries. Nozzles,
however, generally have lower capital
and operating costs. Some vendors offer
a "multiple atomizers per dryer" design.
This allows the absorber to remain
operational even when a particular
atomizer has to be taken out of service.
The use of multiple atomizers in non-
FGD applications, however, is not very
common.
Other basic system design differences
include:
1. use of an ESP instead of a fabric
filter for particufate collection
(most use fabric filters),
2. variations in size and shape of
spray dryer (top, side, or bottom
gas entry; single- or two-point
discharge; horizontal dryer or
cylindrical tower with conical
bottom, concurrent, or counter-
current flow), and
3. reagent preparation techniques
for lime (less costly paste slakers
with grit removal or ball mill I
slakers that produce more finely ™
ground product).
In addition to developing a capacity for
supplying a commercial spray drying
system, many firms, as well as EPA, are
involved in large scale demonstrations
and fundamental research on the finer
points of the technology.
There are six major demonstration
programs that have been recently com-
pleted or are underway (Table 2). These
systems range in sjze from 8500 to
120,000 acfm. Many of the studies are
investigating various portions of the
spray-dryer-based process including:
1. fabric filter vs. ESP collection with
regard to collection efficiency and
effect on SOz removal,
2. atomization technique,
3. reagent preparation techniques,
4. reactivity of various sodium- and
calcium-based sorbents, and
5. waste solids disposal.
Key process parameters that are varied I
to characterize and optimize the process
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Table 1. Key Features of Commercial Spray Drying Systems Sold to Date
System
(Vendor)
Coal/SOz Removal
Guarantees
System
Description
Status
Otter Tail Power, Coyote
Station, Unit 1. 410 MW.
(Rockwell/ Wheelabrator-
Frye)
Basin Electric Power
Coop, Antelope Valley
Station, Unit 1. 440 MW.
(Joy/Niro)
Basin Electric Power
Coop, Laramie River
Station, Unit 3, 500 MW.
(Babcock & WHcox)
Northern States Power,
Riverside Station, Units
6 & 7, 110 MW. Retrofit.
(Joy/Niro)
Tucson Electric, Springer-
ville Station, Units 1 & 2
350 MW each. {Joy/Niro)
United Power Association,
Stanton Station, 65 MW.
(Research-Cottrell)
Plan River Power Author-
ity, Rawhide Station, Unit
1, 250 MW. (Joy/Niro)
Colorado Ute Association,
Craig Station, Unit 3, 450
MW. (Babcock & WHcox)
North Dakota lignite, 0.78%
S average, 7050 Btu/lb, 7%
ash, 70% SOi removal for
all fuels.
North Dakota lignite, 0.68%
S average, 1.22% S maxi-
mum. 62% SOs removal for
average coal, 78% for
maximum S coal.
Wyoming subbituminous,
0.54% S average, 0.81% S
maximum, 8140 Btu/lb, 8%
ash. 82% SOz removal for
average S coal, 90% for
maximum S coal.
1% S Montana Coal, 3.0 to
3.5% S Illinois coal. SO2
removal varying between
70 and 90% during demon-
stration tests.
New Mexico coal, 0.69% S.
61% SOz removal.
Low and intermediate sulfur
Montana subbituminous
coal.
Western subbituminous-
coal. 1.3% S 80% SOt J/.
removal.
0.70% S, 8950 Btu/lb, 14%
ash design coal; 0.40% S,
10250 Btu/lb, 8% ash per-
formance coal. 87% SOa
removal for design coal.
Four parallel spray dryers
with 3 centrifugal atomizers
each, followed by fabric filter
with Dacron bags. Will initially
use commercial soda ash.
Sorbent utilization guarantee
of 80%.
Five parallel spray dryers
(one spare), single rotary
atomizer per dryer, followed
by fabric filter with Teflon-
coated fiberglass bags. Lime
sorbent with partial recycle
of solids. Ball mill slaker.
Four parallel reactors (one
spare) with 12 fluid nozzles
each. Each reactor followed
by as ESP. Lime sorbent, no
solids recycle.
One spray dryer with rotary
atomizer. Will initially be
demonstrated at 300,000
acfm with ESP. Full flow
with fabric filter. Ball mill
and attrition slaker for lime
sorbent.
Spray dryer/fabric filter
design. Lime sorbent.
Rotary atomization.
Spray dryer/fabric filter
rotary atomizers, possibly
multiple atomizers per dryer.
Lime sorbent.
Spray dryer/fabric filter
design. Rotary atomizers
Lime sorbent.
Horizontal spray dryers with
nozzle atomizers, followed
by fabric filter. Solids recycle.
Ball mill slaker for lime
sorbent.
Start-up scheduled for mid-
1981.
Start-up scheduled for April
1982.
Start-up scheduled for
Spring 1982.
Testing with existing ESP
scheduled to start Fall 1980.
Fabric filter on-line in early
1981.
Unit 1 scheduled to start up
in late 1984; Unit 2 in 1986.
Start-up scheduled for 1981.
Start-up scheduled for 1983.
Initial operation in November
1982. Commercial operation
in April 1983.
Sunflower Electric Coop.
'olcombe Station, Unit 1,
'31O MW. (Joy/Niro)
Western subbituminous
coal 80% SOi removal.
Spray dryer/fabric filter.
Rotary atomization. Lime
sorbent.
Start-up scheduled for 1983.
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Table 1. (continued)
System
(Vendor)
Coal/SOz Removal
Guarantees
System
Description
Status
Industrial
Celanese Fibers Com.,
Amcelle Plant, 65000
acfm. (Rockwell/Wheela-
brator-Frye)
Strathmore Paper,
Woronco, MA, 40000
acfm. (Mikropuf)
University of Minnesota,
2 units at 120,000 acfm
each. (Kennecott-
Development Co., Environ-
mental Products Division)
Calgon, KY, 57000 acfm.
(Joy/Niro)
1.5 to 2.0% S eastern coals.
SOZ removal. 70% for 1.0%
S coal and 87% for 2% S
coal.
2.3 to 3% S eastern coal.
75% SOz removal.
Subbituminous coal, 0.6 to
0.7% S. 70% S02 removal.
6000-8000 ppm SOa 8000
ppm halides. 75% S02
removal, 90% HCI removal.
Spray dryer with single
rotary atomizer followed by
fabric filter with felt/fiber-
glass bags. Paste slaker for
lime sorbent. No solids
recycle.
Spray dryer with four two-
fluid nozzles, followed by
fabric filter with specially
finished acrylic bags.
Spray dryer with single
rotary atomizer followed by
fabric filter with fiberglass
bags. Lime sorbent.
Spray dryer/fabric filter.
Rotary atomizer. Soda ash
sorbent. Removing SOi HCI
from 1700°F gases. Solids
recycle.
Operational. Passed
Maryland State compliance
tests in February 1980.
Has achieved guaranteed
removal.
Operational. Now achieving
removal guarantee.
Commercial operation in
Fall 1981.
Under construction.
Table 2. Major Spray Drying Demonstration Activities
Vendor
Location
Size
Comments
Babcock & Wilcox
Buell Envjrotech/
Anhydro
Combustion Engineering
Combustion Engineering
Ecolaire Systems, Inc.
Research-Cottrell
(Cottrell Environmental
Sciences)
Pacific Power & Light
Jim Bridget Station
Colorado Springs-Martin
Drake Station
Northern States Power
Sherburne County Unit #1
Alabama Power-Gadsden
Station (under construction}
Nebraska Power - Gerald
Gentlemen Station
Public Service of Colorado
Comanche Station
120,000 acfm Testing in progress.
8,500 acfm Also EPA-funded dry injection
program at same location.
20,000 acfm Testing complete.
100,000 acfm Testing to start in September
1980 soon after construction
is completed.
10,000 cfm Testing in progress.
mobile pilot plant
10,000 acfm EPA-funded, test in progress
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Include SOz inlet concentration, sorbent
'stoichiometry, flue gas temperature
drop over the spray dryer, gas residence
time in dryer, approach to saturation,
atomizer disc speed (rotary atomizers),
sorbent slurry or solution concentration,
and source (spray dryer fallout or col-
lected solids) and amount of solids
recycle. An additional objective of the
programs is todemonstratethe system's
capability to achieve the desired SOz
removal on a sustained basis.
Objectives of smaller scale funda-
mental research also being conducted
include obtaining a better understand-
ing of the reaction mechanisms and
definition of the most important vari-
ables affecting the rate and degree of
completion, characterizing the effects of
atomized droplet size, sorbent particle
size, and fly ash alkalinity, and charac-
terizing the chemical and physical
stability properties of sodium- and
calcium-based waste solids. These
activities are geared toward increasing
the applicability of spray drying to high
sulfur coal-fired units.
Dry Injection
Dry injection is a very attractive alter-
native for combined removal of SOz and
'ly ash with minimized equipment
requirements. But the commercial
development of thistechnology has been
constrained due to the lack of the pre-
ferred sorbent (nahcolite) and accept-
able disposal practices for the sodium-
based waste solids.
Studies have shown that dry injection
of nahcolite intothedirtyfluegasstream
and collection of the solids in a fabric
filter results in 60 to 80 percent SOz
removal at moderate gas temperatures
(300 to 350 °F) and inlet SOzConcentra-
tions «2000 ppm). Nahcolite utilization
is generally less than 80 percent but it is
not available in the quantities required
for large scale commercial operations.
Trona is less reactive than nahcolite and
thus higher stoichiometries are
required to achieve the same SO2 re-
moval. Lime and limestone achieve
significant SOz removal only at much
higher gas temperatures (<600°F).
Despite these constraints, several dry
injection development studies are being
conducted (Table 3). Objectives of these
programs include improvement of
sorbent reactivity (utilization), particu-
larly for the more available sorbents and
characterization of the waste solids for
>acceptable disposal techniques. The
following process parameters were
varied during the te
method (continuous
combination of thos
ing cycle time, inje
sorbent particle size
tration, and sorb
Sorbents being inve
lite, trona, upgradec
sodium bicarbonate
bonate.
Combustion of
Fuel Mixtures
Variations of comb
include test work or
coal/limestone pe
stokers, which is b
Battelle Laboratorie;
3.5:1 Ca:S (mole
scheduled to begin ir
a 60,000 Ib steam/I
Motors' Indianapoli
scale tests havesho
retention of the avail
the 3.5:1 Ca:S pellet
Energy and Em/in
Inc., (EERC) is alsov
tion of a pulverize
mixture in a low-NO,
Btu/hr scale. Limes
both been tested, wi
ing higher sulfur r
percent retention i
ratio of 2 to 3). The
been demonstrated
cant SOa removal. F
be carried out on the
Btu/hr scales.
Research and dev
with both processes
characterizing effec
operation, and maim
strating the degre<
achievable, and (3
effects of the resulti
ulate loading.
State-of-the-Ar
Dry FGD isanattr
st: sorbent feeding
batch, precoat, or a
3 methods), clean-
;tion temperature,
S02 inlet concen-
nt stoichiometry.
itigated are nahco-
trona, commercial
, and sodium car-
?oal/Alkali
ustion modification
the combustion of
lets in spreader
3ing conducted by
i. A 1 4-day test of a
ratio) pellet was
November 1980on
ir boiler at General
j plant. Laboratory
wn 50 to 75 percent
ablefuel sulfur with
inmental Research
orkingoncombus-
d coal/alkali fuel
burner on a 70,000
one and trona have
h limestone show-
stention (50 to 70
t a stoichiometric
technique has also
to achieve signifi-
jture test work will
1,10, and 50x106
elopment activities
are focused on (1)
s on boiler design.
enance, (2)demon-
of SOz removal
determining the
g increased partic-
Assessment
ctive alternative to
conventional wet scrubbing because it
produces a dry, easy-to-handle waste
rather than a wet-sludge. There are also
potential capital and operating costs
savings resulting from reduced
equipment requirements, lower energy
and water requirements, and relative
process simplicity.
Advantages/Disadvantages of
Dry FGD vs Conventional
Wet Scrubbing
The advantages and disadvantages of
the three technologies discussed here
are based on pilot plant data, limited
reported operating experience, and
conceptual studies.
Technically, spray drying and wet FGD
can be compared in four areas: reagent
requirements, energy requirements,
operation and maintenance require-
ments, and waste disposal requirements.
Dry systems require a higher stoichi-
ometric ratio of sorbent on the basis of
moles of sorbent required per mole of
SOz removed. Stoichiometric ratios for
dry systems, are based on moles of
sorbent required per mole of SOz in the
inlet flue gas. Thus, a reported stoichio-
metric ratio (SR) of 1.22 for a dry system
achieving 80 percent SOz removal
would translate into a SR of 1.5 under
the conventional definition for wet
systems. Dry systems also require an
increased SR to achieve a given SOz
removal at increased inlet SOz
concentrations. This factor may pose a
technical limit to application of spray
drying to high sulfur coals, since the
amount of liquid (and therefore,
sorbent) that can be sprayed into the gas
is limited by the available flue gas
temperature drop over the spray dryer.
This temperature drop is in turn fixed by
the inlet gas temperature, the margin of
safety (in terms of degrees above the-
adiabatic saturation temperature) that
must be maintained, and the overall SOz
removal efficiency required (which
limits warm or hot gas bypass). The
maximum attainable solution concen-
tration or weight percent solids in the
sorbent slurry also limits the amount of
sorbent that can be added per unit time.
The energy consumption of the dry
system should be less than for the wet
scrubbing because of lower pumping
requirements (lower L/G) and reduction
or elimination of the need for flue gas
reheat.
Several vendors claim that the spray-
dryer-based systems will have lower
maintenance requirements and more
operational flexibility than .comparable
wet systems. Spray dryer system
designs do not include sludge handling
equipment or large slurry recirculation
equipment. There is no wet/dry
interface in the spray dryer system other
than that in the gas suspension, making
the process operation more flexible with
respect to variations in boiler load and
inlet SOz concentration.
Economics appear to be one of the
major driving forces behind selection of
spray drying over conventional wet
systems for low sulfur coal applications.
Basin Electric evaluated the cost of a dry
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Table 3. Current Dry Injection Programs
Vendor
Location
Size
Comments
EPA/Buell-Envirotech
Colorado Springs -
Martin Drake Station
DOE/Grand Forks Energy GFETC Labs
Technology Center
DOE/Pittsburgh Energy
Technology Center
EPRI/KVB
PETC Labs
Public Service Company
of Colorado - Cameo Station
4500 acfm Testing completed in May 1980,
EPA funded.
200 acfm Testing complete. Report expected
in Fall 1980.
500 Ib coal/hr Testing in progress.
furnace
20MWe
Testing in progress.
system to be 15 to 25 percent less over a
35-year plant life than a wet system for
Laramie River and Antelope Valley,
respectively. The Tennessee Valley.
Authority (TVA) found the cost of a lime-
based spray drying system to be con-
siderably less than for a wet system. The
basis for this estimate was a new 500-
MV plant burning a low sulfur western
coal with a 70 percent S02 removal
requirement. The higher reagent re-
quirements and the reagent cost differ-
ential between lime and limestone may
result in a significant economic disad-
vantage for spray drying for high sulfur
applications.
The same advantages with respect to
energy, and operation and maintenance
requirements should apply for a dry
injection system. The equipment re-
quirements for dry injection are lessthan
for conventional wet scrubbing or spray
drying. However, the dry injection
system has a distinct disadvantage with
regard to reagent utilization and
reagent-related operating costs. Nahco-
lite utilization has been relatively low in
tests conducted to date (60 to SOpercent),
leading to fairly high reagent require-
ments to achieve high 862 removal.
Also, sodium-based sorbents are much
more reactive than lime or limestonebut
are considerably more expensive. The
present dry injection technique would
then be limited to relatively low sulfur
coals. Sodium-based wastes, being
readily soluble in water, also entail high
disposal costs relative to the stabilized
lime and limestone-based wastes.
The combustion of coal/alkali fuel
mixtures to control SO2 has obvious
economic potential because of minimal
equipment requirements and the fact
that significant S02 removal has been
demonstrated with limestone. However,
both processes (combustion of coal/
limestone pellets and firing a pulverized
coal/limestone mixture in a low-NO*
burner) are still in the early stages of
development, and the effects on boiler
design, operation, and maintenance
have yet to be fully characterized. Also,
these technologies are currently limited
to specific boiler types; i.e., spreader
stokers for the pellets and dry bottom
pulverized coal boilers for thecoal/lime-
stone fuel mixture. Fuel sulfur content
may be limited by the sheer volume of
reagent required for higher sulfur
applications and the resulting effects on
boiler and particulate control device
operation.
Areas for Further Research
There are several areas requiring
further research if dry FGDtechnology is
to become a widely applicable alterna-
tive to conventional SO2 control
techniques.
Concerning spray drying, research
effort in the following areas could serve
to increase process applicability for
units firing high sulfur coal:
1. Improved understanding of the
absorber and downstream SO2/
sorbent reaction mechanisms,
2. improved reagent preparation
techniques,
3. improved understanding of fly ash
alkalinity utilization and investiga-
4.
tion of various sorbent recycle
schemes, and
development of a limestone spray
drying process.
In addition to research in areas (1)
through (3) above, dry injection work
may also need to focus on improving the
reactivity of sorbents that are more
readily available than nahcolite and do
not pose the same waste disposal prob-
lems as those with sodium compounds.
Research in combustion of coal/alkali
fuel mixtures will need to define the
important process variables, suchasbed
temperature, and their effect on SOz
retention; and further evaluate the long-
term effects of firingfuel/alkali mixtures
on boiler operation.
4 U.S GOVERNMENT PRINTINO OFFICE. 1801-757.012/7166
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Environmental Protection
Agency
Center for Environmental Research
Information
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