United States
Environmental Protection
Agency
Research and Development
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
EPA/600/SR-94/184 November 1994
sorEPA Project Summary
Demonstration of Sorbent
Injection Technology on a
Tangentially Coal-Fired Utility
Boiler (Yorktown LIMB
Demonstration)
J. P. Clark, R. W. Koucky, M. R. Gogineni, A. F. Kwasnik
Limestone Injection Multistage
Burner (LIMB) technology has been
successfully demonstrated on a tan-
gentially fired coal-burning utility boiler,
Virginia Power's 180 MWe Yorktown
Unit No. 2. This document summarizes
the activities conducted, and results
achieved, under this EPA-sponsored
demonstration program. LIMB com-
bines furnace injection of a calcium-
based sorbent for moderate reductions
of sulfur dioxide (SO2) with a
low-nitrogen-oxide (NOX) firing system
for NOX emissions reduction. The pro-
cess is attractive for retrofit of existing
coal-burning utility boilers, since the
capital equipment requirements and
overall sulfur reduction costs per ton
of sulfur oxide removed are less than
for most other options, such as wet
flue gas desulfurization.
Testing was conducted on an east-
ern bituminous coal with a typical sul-
fur content of 2.3%. Five sorbents were
tested: commercial hydrated lime, with
and without calcium lignosulfonate
treatment, each from two suppliers, and
finely pulverized limestone. Short-term
parametric testing showed full-load SO2
removals of up to 56 and 63% at
calcium-to-sulfur (Ca/S) ratios of 2:1
and 2.5:1, respectively. Intermediate
load performance was higher, with SO2
removals of up to 60 and 67% at Ca/S
ratios of 2:1 and 2.5:1, respectively.
Results varied with specific LIMB oper-
ating conditions, boiler load, and sor-
bent used. Results of both extensive
parametric testing and continuous long-
term operation of the LIMB system are
presented. Results of performance test-
ing of the Low-NOx Concentric Firing
System (LNCFS) Level II firing system
are also presented. Typically, under
comparable boiler operating conditions,
a 42% reduction in NOX at full load and
a 33% reduction in NOX at intermediate
load, relative to baseline levels, were
achieved with the LNCFS Level II Sys-
tem.
The effects of LIMB operation on
boiler, electrostatic precipitator (ESP),
and ash handling system performance
are also discussed. The most signifi-
cant impact on boiler performance was
the collection rate of LIMB solids plus
fly ash on boiler convective surfaces
during continuous operation, resulting
in poorer boiler heat transfer perfor-
mance and higher temperatures leav-
ing the boiler. Continuous operation of
the sootblowing system minimized this
effect. The results of two ESP perfor-
mance tests that were conducted dur-
ing continuous LIMB operation are
discussed and compared to results
from similar testing conducted without
LIMB operation. Ash conditioning and
disposal during LIMB operation at
Yorktown were significantly affected by
the unreacted lime in the ash. These
problems, as well as suggested pre-
cautions to avoid them, are discussed.
Recommendations for LIMB com-
mercialization and an evaluation of
the economics of the technology in
comparison to a conventional flue gas
desulfurization system are dis-
cussed.
Printed on Recycled Paper
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This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
tltlo (sea Project Report ordering Infor-
mation at back).
Introduction
Increasing concern for the environmen-
tal effects of acid rain has led to active
efforts to control emissions of sulfur diox-
ide and oxides of nitrogen, SO2 and NOX,
from the fossil-fuel-burning electric utility
industry. The choice of control techniques
for limiting SO2 and NOX emissions from
utility boilers will most likely include a mix
of technologies to achieve the desired re-
ductions at minimum cost. The primary
method for controlling SO2 from new coal-
burning boilers is flue gas scrubbing. Of-
ten, however, this technique may not be a
viable option for existing power plants.
Space may not be available for installing
scrubbers and disposing of waste from
the scrubbers. In addition, the installation
of scrubbers may not be a viable eco-
nomic choice for older units with less than
10-20 years of remaining useful life. The
need for a method of providing a moder-
ate degree of emission control at lower
cost and with minimal retrofit to an exist-
ing boiler and minimal space has led to
the development of the LIMB process. In
this process for simultaneous SO2 and
NOX control, a calcium-containing mate-
rial, such as limestone (CaCO3) or hy-
drated lime [Ca(OH)A, is injected into the
furnace at appropriate temperatures. The
material, called the sorbent, breaks down
to form lime (CaO). The CaO reacts with
SO2 in trie gas to form calcium sulfate
(CaSO4). CaSO4 is collected in the boiler's
partfeulate removal device, along with the
coal ash, and transferred to a landfill site.
Concurrent with SO2 control by injection
of the calcium-containing material, NOX is
controlled by installation of low-NOx burn-
ers that provide staged introduction of air
to control flame temperature and gas
chemistry in the flame region.
The LIMB process had its origin in the
observation that significant sulfur is re-
tained during the burning of coals that are
high in alkali-containing ash and low in
sulfur. This observation led to several in-
vestigations (in Europe, Japan, and the
U.S. during the 1960s) of the feasibility of
injecting calcium-based sorbents directly
Into the furnace of a coal-fired boiler as
the primary SO2 control technique. Early
field demonstrations of furnace injection,
using limestone, produced disappointing
levels of SO2 reduction. During the late
1970s, interest in direct sorbent injection
for SO2 control waned as emphasis shifted
to scrubber development. However, dur-
ing the early 1980s, development of im-
proved firing systems with lowered
combustion zone temperatures, provided
the impetus for the development of LIMB
as a low-cost retrofittable system for the
simultaneous reduction of SO2 and NOX.
The EPA established a major research
and development program, both at their
laboratories and with contractors, to es-
tablish LIMB as a viable, low-cost, alter-
native to conventional flue gas
desulfurization (FGD) systems for attain-
ing moderate reductions in SO2 on ex-
isting coal-burning boilers. Through early
research efforts, LIMB eventually evolved
into the more generic "furnace sorbent
injection" as sorbents that were more
reactive than limestone, such as Ca(OH)2,
and injection locations that were more
consistent with the optimum SO2 capture
"window" of 2300 to 1650°F (1260 to
890°C), such as into the upper furnace,
were identified. Because tangential firing
introduces special considerations for the
design of sorbent injection systems due to
the unique aerodynamics in the sorbent
mixing region of tangentially fired boilers,
the EPA established parallel development
programs for LIMB application to both wall-
fired and tangentially fired boilers. The
Yorktown demonstration project was the
culmination of a multiphase program, which
included both pilot- and prototype-scale
testing, to develop LIMB for tangentially
fired boiler applications. The EPA selected
Combustion Engineering (C-E) as prime
contractor for the demonstration project in
1987. Virginia Power's Yorktown Unit No.
2, a 180-MWe coal-fired boiler located at
the Yorktown Power Station in Yorktown,
Virginia, was selected as the host unit.
C-E, Virginia Power, Stone & Webster,
and the U.S. Department of Energy all
helped fund the project.
The demonstration project objectives
were established as at least 50% reduc-
tion in SO2 at a Ca/S of 2.5:1 and a
reduction in NOX emissions to 0.4 lb/106
Btu (0.17 kg/GJ) or below. These emis-
sions would be reduced with no significant
adverse impact on boiler pperability and
reliability. An eastern bituminous coal with
a sulfur content of 2.0 to 2.5% was
selected as the demonstration coal.
The overall program was conducted in
two phases: site-specific studies to sup-
• port the system design, and the field dem-
onstration itself. The site-specific studies
included a boiler characterization test to
identify the location of the ideal sulfation
zone through extensive mapping of the
boiler temperatures profile, identification
of promising injection schemes via cold-
flow testing on a plastic model of the
Yorktown boiler, and pilot-scale combus-
tion testing to verify performance of both
the most promising injection schemes and
the proposed low-NOx firing system. Those
injection schemes that performed best
during pilot-scale testing were incorporated
into the LIMB system design. Computa-
tional fluid dynamics computer modeling
of the ESP inlet ducts, where the flue gas
humidification system would be located,
showed that there was insufficient resi-
dence time in the ducts to achieve a close
approach to the adiabatic saturation tem-
perature for enhanced SO2 capture.
Humidification for ESP performance main-
tenance during LIMB operation had been
demonstrated to be effective at two other
EPA-sponsored LIMB projects: the
Edgewater wall-fired demonstration project
and the tangentially fired prototype test
program at Richmond Power & Light. En-
hanced SO2 capture with a close approach
to saturation had already been demon-
strated at Edgewater. To avoid incurring
additional costs to extensively modify the
Yorktown ducts for enhanced SO2 cap-
ture, the Yorktown humidification system
was designed only to maintain ESP per-
formance.
While the final system design was es-
sentially complete by June 1990, construc-
tion did not begin until May 1991 due, in
large part, to a 1-year delay in the Unit
No. 2 outage, which was rescheduled to
allow installation of a new steam turbine/
generator set.
A 1-month baseline test to establish the
performance of Unit No. 2 with its original
firing system was conducted in March
1991, prior to installation of the LIMB sys-
tem. An extensive ESP performance test
was also conducted. The medium-sulfur
eastern bituminous demonstration coal was
used for this test.
The boiler was returned to service in
December 1991, with the LNCFS Level II
low-NOx firing system in place and instal-
lation of the sorbent injection equipment
nearly complete. Performance testing of
the LNCFS system was conducted in
March 1992. LIMB system start-up/shake-
down testing started in May 1992 and
continued through August 1992. Perfor-
mance testing of the LIMB system, includ-
ing three long-term demonstration tests of
over 22 days of continuous operation each,
extended from September 1992 to Octo-
ber 1993. The results of the LIMB and
low-NOx testing, plus the effects of LIMB
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on Unit No. 2 operation, are presented
below.
NOX Emission Reduction
A LNCFS Level II system was installed
to meet the project objective of 0.4 lb/106
Btu (0.17 kg/GJ) NOX emissions. The
LNCFS Level II system features sepa-
rated overfire air (SOFA) for staged com-
bustion, flame attachment tips for early
fuel ignition, and offset air nozzles to fur-
ther minimize NOX formation. During early
testing of the LNCFS Level II system, it
was determined that the new Unit No. 2
steam turbine had introduced substantial
changes in the boiler waterwall and
reheater heat absorption requirements.
The boiler controls responded by raising
burner tilts, which, in turn, adversely af-
fected the effectiveness of the LNCFS fir-
ing system in reducing NOX. To make the
LNCFS performance data fully comparable
to baseline data with the original firing
system, all NOX emissions were adjusted
to the 0° burner tilt used during baseline
testing and to a common fuel nitrogen
content. The LNCFS system without
overfire air performed at essentially the
same level as the original baseline sys-
tem at full boiler load, suggesting that the
concentric firing system, in combination
with the flame attachment tips, did not
contribute to overall NOX reduction. A 15%
reduction in NOX was measured at inter-
mediate boiler load. Adding SOFA pro-
duced a 42% reduction in NOX at full load
and a 33% reduction at intermediate load.
Carbon-in-ash levels for the baseline sys-
tem and the LNCFS system with SOFA
nozzles closed were comparable at ap-
proximately 7%, under similar test condi-
tions. The effect of fully opening all three
SOFA nozzles was to increase carbon-in-
ash by about 85%. Comparable effects
have been noted on other LNCFS Level
ll-equipped units burning similar coals.
LIMB System for Yorktown
The Yorktown sorbent injection system
was designed to provide a great deal of
operational flexibility to permit operation
of the system over a wide range of boiler
and sorbent injection conditions. Three el-
evations for injection into the boiler were
provided. The elevations were selected
based on the results of site-specific stud-
ies, including field and pilot-scale testing.
The middle and lower elevations each pro-
vided for tangential injection of the sor-
bent through 16 injectors located at the
eight corners of the divided furnace. The
lower elevation was designated as the
primary injection location for reduced boiler
load operation. The middle elevation, lo-
cated approximately at the "nose" of the
furnace, was designated as the primary
injection elevation for full boiler load op-
eration. The third elevation, with 16 injec-
tors across the front wall of the boiler,
above the tangential injector elevations,
was an alternate full-load injection loca-
tion.
Parallel sorbent feed systems supplied
sorbent to both sides of the divided fur-
nace. Feed rates of from 4 to 15 tons/hr
(3.6 to 13.6 tonnes/hr) (total) allowed sub-
stantial flexibility in evaluating LIMB sys-
tem performance over a range of boiler
loads and Ca/S molar ratios. In addition,
either sorbent feed-system was capable
of feeding both sides of the furnace, if the
other feed system was not available. Each
feed system consisted of a day bin (65
tons—58.9 tonnes—of hydrate), plus feed-
ers and transport blowers. A large fan
provided additional air to achieve sorbent
injection velocities into the furnace of up
to 450 ft/sec (137 m/sec).
Sorbent for LIMB testing at Yorktown
was delivered by rail to an unloading site
approximately 9 miles (14.5 km) from the
Yorktown Station. At this site, the sorbent
was off-loaded to pneumatic trucks, typi-
cally holding 20 tons (18 tonnes) of hy-
drated lime, for delivery to the Station
where the sorbent was pumped into a
long-term storage bin designed to hold
390 tons (354 tonnes) (roughly 2 days
supply) of hydrate. The truck unloading
system worked well throughout the opti-
mization and demonstration test periods.
As many as 13 trucks were unloaded in 1
day during demonstration testing.
SO2 Removal Performance:
Optimizing Testing
A parametric test program was con-
ducted to evaluate the effectiveness of
the LIMB System over a wide range of
configuration and operating variables. The
primary objective of the program was to
identify the LIMB configuration and oper-
ating variables that provided the greatest
reduction in SO2, consistent with good
boiler performance, and identify the opti-
mum sorbent system operation for long-
term demonstration testing.
A total of 147 tests were conducted
from September 1992 through September
1993. Full-load tests were conducted at
all three injection elevations. One alter-
nate injection yaw configuration was
tested. Intermediate load tests were con-
ducted at two injection elevations. The
effect of Ca/S on SO2 capture was evalu-
ated. The effect of sorbent mixing on SO2
capture was evaluated by varying injec-
tion air plus sorbent discharge velocity
from 150 to 450 ft/sec (45.7 to 137.2 m/
sec) (approximately 2.5 to 7.5% of total
airflow to the boiler) and injector tilt from
-35 to +35° relative to horizontal (0° tilt)
injection. Commercial hydrated lime from
two suppliers was tested. Tests were also
conducted using calcium-lignosul-
fonate-treated hydrate from the same two
suppliers. In addition, limited testing was
conducted using finely pulverized
limestone as the sorbent.
SO2 capture did not improve significantly
when testing with calcium-lignosulfonate-
treated hydrate. Data for pulverized lime-
stone (95% through 325 mesh) were
limited and showed considerable variabil-
ity. The limestone tests, which included
low- (down to 90 MWe), intermediate- (128
MWe), and full-load (169 MWe) operation,
showed a clear advantage for the in-
creased residence time available at low
load. In general, the SO2 removal perfor-
mance for pulverized limestone was about
60% of the performance for hydrated lime.
Optimized full-load SO2 removal effi-
ciency on hydrated lime was 56% at Ca/S
= 2:1 and 63% at Ca/S = 2.5:1. At inter-
mediate load, SO2 removal was 60% at
Ca/S = 2:1 and 67% at Ca/S = 2.5:1. The
increase in SO2 capture at intermediate
boiler load is attributed to an increase in
residence time in the 2300 to 1650°F (1260
to 890°C) sulfation "window" at intermedi-
ate load. SO2 capture improved with in-
creasing injector discharge velocity up to
300 ft/sec (91 m/sec). Tests conducted at
450 ft/sec (137 m/sec) showed no addi-
tional improvement in SOZ capture. No
significant effect of injector tilt on SO2 cap-
ture was measured during this optimiza-
tion testing. Boiler operation, however, as
measured by the ability to. maintain super-
heat and reheat outlet temperatures, was
improved during injection through the
middle elevation when the injectors were
tilted downward. Full load boiler operation
was further improved by injecting through
the lowest elevation with the injectors tilted
upward.
The overall performance of the sorbent
injection system was good throughout the
optimization test period. The primary op-
erational problem during this period was
the unreliability of the solids pumps in
both the long term and day bin areas. In
the long-term bin area, the solids pump
was able to supply only 11 tons/hr (10
tonnes/hr) of sorbent to the day bins ver-
sus the design rate of 20 tons/hr (18.1
tonnes/hr). By removing the solids pump
and discharging from the existing rotary
feeder directly into a "pick-up tee," the
required capacity was achieved and main-
tained through the remainder of the test
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program. The solids pumps in the day bin
area also proved troublesome. While the
pumps were able to achieve the required
maximum capacity of 7.5 tons (6.8 tonnes)/
hr/pump, ft was necessary to operate the
pumps at the unusually high screw speed
of 1500 rpm to attain this feed rate. The
pumps were not able to operate continu-
ously at this speed. Since this was unac-
ceptable for long-term continuous
demonstration testing, the solids pumps
were removed and replaced with rotary
airlock feeders which, in turn, discharged
into "pick-up tees." This arrangement op-
erated well during both optimization and
continuous demonstrating testing, although
the rotary feeders repeatedly experienced
an unanticipated problem: hard deposits
on the feeder drum surfaces. This required
occasional cleaning but did not interrupt
the demonstration tests.
At the discharge of the injectors, hard
deposits formed inside the nozzle tips
which restricted, but did not completely
close, the nozzles. This necessitated peri-
odic removal and cleaning of the injectors
to permit design air/sorbent flow rates.
The mechanism for formation of these de-
posits is not yet understood.
SO2 Removal Performance:
Demonstration Testing
A primary objective of the Tangentially
Fired LIMB Demonstration Project was to
evaluate the long-term effectiveness of the
integrated LIMB system in achieving the
SO2 and NOX emissions reduction objec-
tives and establish the long-term effects
on boiler, ESP, and ash handling system
operation. Three long-term demonstration
tests were conducted. Test duration and
gaseous-emission data collection objec-
tives for these tests were established in
accordance with Performance Testing re-
quirements contained in Vol. 44, No. 113
of the Federal Register. These require-
ments may be satisfied in as few as 22
days (out of 30) if at least 18 hours of
acceptable continuous emissions monitor-
ing (GEM) data are produced each day.
Because of the high quality of GEM data,
H was possible to complete each of the
demonstration tests after only 22 to 24
days, having satisfied test duration require-
ments.
During the demonstration tests, the
boiler was operated in its normal duty
cycle. Sootblowing and ash handling sys-
tem operation were adjusted as required
by LIMB operation. ESP operation and
humidification of the gas and solids enter-
ing the ESP were adjusted to conform to
opacity requirements, except during ESP
performance tests when additional require-
ments were specified.
The sorbent injection configurations and
operating conditions were selected for the
demonstration tests to maintain good boiler
performance and operability as well as
achieve good SO2 capture performance.
Thus, when injection was through the
middle elevation, the injectors were tilted
downward to improve boiler temperature
control. The first demonstration was con-
ducted with lignosulfonate-treated hydrate
as the sorbent. Untreated commercial hy-
drate was used for the second and third
demonstration tests. The SO2 removal ef-
ficiencies at the Ca/S molar ratios of com-
mercial interest (Ca/S = 2:1 and 2.5:1)
were essentially the same for all three
tests, averaging 44% at Ca/S = 2:1 and
50% at Ca/S = 2.5:1. These levels are
lower than those measured during optimi-
zation testing. Two factors that adversely
impacted the SO2 removal performance
were sootblowing requirements and data
interval selection. During continuous in-
jection of sorbent, the Unit No. 2
sootblowing system was operated almost
continuously to maintain critical tempera-
tures within limits. Continuous cleaning
may reduce secondary SO2 capture by
deposited sorbent material, thereby reduc-
ing overall. SO2 capture. Sootblowers were
not operated during individual optimiza-
tion tests. The second factor-data interval
selection-was also affected by sootblower
operation. While it was possible to deter-
mine maximum SO2 capture (minimum
CEM SO2) for individual optimization test
points without sootblower operation, it was
not possible to obtain continuous demon-
stration test periods of sufficient duration
without including at least one sootblowing
cycle. This resulted in the requirements to
use average, rather than minimum, CEM
SO2 levels to define SO2 capture. There-
fore, the impact of the continuous
sootblowing requirement tends to be con-
siderable.
Much of the first demonstration test was
conducted at minimum sorbent feed in
response to concerns over indications of
deposit buildup on the floors and turning
vanes of the ESP inlet ducts. The ducts
were cleaned during a subsequent non-
LIMB-related outage. The second and third
demonstration tests were conducted at
design sorbent feed rates with minimal
deposit problems, reflecting the effective-
ness of improved humidification lances that
were installed for these tests.
ESP and Humidifier
Performance
The Yorktown Unit No. 2 ESP has a
specific collection area (SCA) of 720
ft2/1000 acfm (14 m2/1000 m3/sec) of
treated gas. This exceptionally large ESP
provided an opportunity to evaluate the
impact of LIMB on ESP performance over
a wide range of conditions. Prior to the
start of optimization testing, a test was
conducted with design sorbent feed rates
but without humidification. The ESP per-
formance deteriorated rapidly after only a
few hours of operation. This was consis-
tent with results obtained at Edgewater.
With the humidification system in opera-
tion, it was possible to maintain ESP per-
formance at or near its pre-LIMB levels.
Throughout the optimization and first dem-
onstration test periods, water distribution
was poor, resulting in repeated problems
with deposits in the ESP ducts, on the
ESP inlet turning vanes, and on the
humidification lances themselves. Destruc-
tive examination of two lances revealed
weld cracks that allowed the air and water
to mix within the lances, producing severe
maldistributions of water. Lances fabricated
with revised procedures significantly im-
proved water distribution with a dramatic
reduction in duct and turning vane depos-
its. Opacity was always maintained within
compliance limits throughout the three con-
tinuous demonstration tests.
ESP performance tests were conducted
during the baseline test and the first two
demonstration tests. Total particulates en-
tering and leaving the ESP, particle size
distributions, and particle resistivities were
measured under various ESP operating
conditions. Fields were taken out of ser-
vice to simulate smaller ESPs, with the
objective of identifying the smallest ESP
that could operate with LIMB without
modification. Performance test results
showed that essentially no performance
losses were seen during ESP operation
With SCAs of 720 to 480 ft2/1000 acfm (14
to 9 mz/100 m3/sec) of treated gas, under
the same LIMB and humidifier operating
conditions. Further SCA reductions re-
sulted in increases in opacity. Collection
efficiencies were comparable during the
three performance tests. The average col-
lection efficiency with LIMB was 99.21%,
versus 99.42% without LIMB. Flyash re-
sistivity varied over a narrow range.
Baseline resistivity without LIMB was 3.7
x 1010 ohm-cm. Resistivity during the first
performance test with LIMB was 1.4 x 10"
ohm-cm. During the second performance
test with LIMB, resistivity was slightly lower
at 1.4 x 1010 ohm-cm, suggesting improved
humidification effectiveness with the modi-
fied lances.
An unexpected ESP performance effect
was noted during LIMB testing with pul-
verized limestone. The humidifier was
turned off during the final 2 days of test-
ing. Opacity did not increase above
non-LIMB levels. No ESP performance
-------
measurements, including flyash resistiv-
ity, were made during limestone testing.
Ash Handling Effects
The Yorktown Ash Handling Facility fea-
tures a single ash silo that receives flyash
from all three units. When Units No. 1 and
2 were both in operation, LIMB ash from
Unit No. 2 was mixed with, and diluted by,
ash from Unit No. 1. Under these
conditions, the mixed ash was easier to
handle and produced less steaming than
in previous LIMB field demonstrations. It
was, nevertheless, more difficult to entirely
remove the ash from ash truck beds than
when no LIMB ash was present. When
Unit No. 1 was not in operation, the LIMB
ash was somewhat more difficult to
process through the rotary conditioner,
which received the ash frorri the silo, added
water while mixing, and discharged the
mixture into trucks. Steaming was greater
for LIMB-only ash than for the mixed Unit
No.1/Unit No. 2 ash.
Long-term continuous testing of the
LIMB system placed increased strain on
the Yorktown ash handling system. The
concentration of LIMB ash in the ash silo
increased over that experienced during
optimization testing. This resulted in a
greater degree of steaming of the condi-
tioned LIMB ash in the ash trucks and at
the disposal site. The LIMB ash exhibited
a much greater tendency to set up in the
truck beds than did non-LIMB ash or a
mixture of Unit No. 1 ash with no LIMB
and Unit No. 2 ash with LIMB. Frequent
problems of material setting up in the truck
beds were experienced. Decreasing the
water to the rotary conditioner (mixer) ap-
peared to reduce problems of setting up
and steaming.
The quantity of material produced dur-
ing continuous testing taxed the Yorktown
ash handling personnel and equipment. A
substantial increase in the amount of time
Unit No. 2 was operated at full load, fol-
lowing the substantial heat rate improve-
ment as a result of the steam turbine
replacement, contributed to this problem.
All of the LIMB ash was placed in one
area of a single ash cell at the Yorktown
ash disposal site. No significant problems
were experienced due to the cementitious
properties of the LIMB ash. It was pos-
sible to spread the LIMB ash by bulldozer
with only occasional steaming problems.
Boiler Performance and
Operability
During LIMB operation, lime, ash, and
LIMB products accumulated on boiler heat
transfer surfaces at a greater rate than
during normal non-LIMB operation. This
reduced the heat absorption effectiveness,
shifted the gas temperature profile in the
boiler downstream, and reduced overall
boiler efficiency. Reheat outlet tempera-
ture, for example, typically dropped about
30 F° (16.7 C°) during continuous full-load
sorbent injection at Ca/S = 2:1. Frequent
sootblowing recovered essentially all of
the temperature loss, suggesting friable,
easily removable deposits. The increase
in the temperature of the gas entering the
ESP inlet ducts (and the humidifier) as a
result of the shift in heat absorption pat-
tern during sorbent injection was typically
75 to 80°F (23.9 to 26.7°C). This shift in
absorption and the concurrent increase in
gas heat loss resulting from the increase
in gas flow and deposits on heat transfer
surfaces typically resulted in a reduction
of 1.5 points in boiler efficiency during
continuous sorbent injection relative to the
efficiency level during continuous testing
without sorbent injection. The net boiler
efficiency reduction was about 1 point
after adjusting for carbon-in-ash and cal-
cination/sulfation effects. Optimized
sootblower coverage and operation
would likely reduce this penalty.
During injection of finely pulverized lime-
stone, boiler effects were significantly dif-
ferent than those observed during injection
of hydrated lime. Superheater and reheater
outlet temperatures did not drop as much
as during hydrate injection, suggesting a
decreased tendency of deposits to build
up on the heat transfer surface. As a
consequence, sootblowing requirements
were significantly reduced.
No LIMB-related boiler outages were
experienced during any part of the LIMB
test program at Yorktown, including the
three demonstration tests. Boiler operabil-
ity was, however, adversely affected by
the demonstration coal and operation of
the LIMB system, in addition to the effects
of the new Unit No. 2 steam turbine. In-
creased slagging at the burner corners
was noted throughout the LIMB test pro-
gram. However, slag formation on the
waterwalls was, generally, not severe and
did not require operating the wall
sootblowers. Ash/sorbent deposits built up
around many of the sorbent injection ports.
These deposits would break away and fall
into the boiler ash hopper and were ulti-
mately removed from the boiler via the
wet bottom ash removal system. Occa-
sional problems were experienced in the
bottom ash removal system due to the
high lime content of this material.
Economics and
Commercialization
Capital and operating costs associated
with a 180-MWe "base case" LIMB sys-
tem were generated, based on actual
Yorktown costs, and used as a basis for a
sensitivity study on the effects of unit size
and various process parameters on LIMB
economics. The Yorktown system, plus a
new dedicated ash silo, had a total capital
requirement of $81/kW, with 2.5% S coal
at a Ca/S of 2.5:1. Levelized cost for the
base case was $781/ton ($861/tonne) SO2
removed. These costs are lower than those
reported for previous LIMB demonstrations,
indicating a maturing of this SO2 control
technology.
Capital investment increased 46%, to
$118/kW, for a 100-MWe unit but de-
creased 23%, to $62/kW, for a 300-MWe
unit, clearly showing the economy of scale
for a retrofit LIMB system.
LIMB has been demonstrated as a vi-
able low-cost option for achieving moder-
ate levels of SO2 reduction with capital
and operating costs that are substantially
below those of conventional flue gas
desulfurization systems. The 1990 Clean
Air Act Amendments (CAAAs) focus on
high SO2 removal efficiency technologies.
They also make SO2 allowances (credits)
available at a fraction of the cost of FGD
systems. Consequently, the domestic mar-
ket for LIMB, with its moderate SO2 re-
moval, is currently best suited to older,
small to intermediate units, which might
otherwise be retired, and also to combina-
tions of LIMB with back-end SO2 removal
processes. An example would be LIMB
combined with the ADVACATE process to
achieve high overall -SO2 removals. Thus,
in the contemporary domestic utility mar-
ket, LIMB alone is postured as a "niche"
technology for units requiring moderate
levels of SO2 control. However, if com-
bined with a complementary back-end sys-
tem, LIMB can offer the potential for
competitive (i.e., 90 +%) SO2 removals at
favorable capital and operating costs.
Summary and Conclusions
The viability of LIMB as a low-cost op-
tion for achieving moderate levels of SO2
reduction on tangentially coal-fired utility
boilers was established under the
EPA-sponsored LIMB Demonstration Pro-
gram at Virginia Power's 180-MWe
Yorktown Unit No. 2. Extensive paramet-
ric testing to evaluate various sorbent in-
jection configurations and process
variables was conducted over a range of
boiler loads. Full-load SO2 removals of up
to 56 and 63% at Ca/S molar ratios of 2:1
and 2.5:1, respectively, were achieved,
substantially above the project objective
of 50% SO2 removal at a Ca/S molar ratio
of 2.5:1. Higher SO2 removals were
achieved at intermediate boiler load. Test-
ing was conducted on calcitic hydrated
lime, with and without calcium lignosul-
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fonata treatment, each from two suppli-
ers. SO2 removal did not increase signifi-
cantly during testing with the treated
hydrates. Three continuous long-term tests
of the LIMB system, each of from 22 to 24
days in duration, were conducted over the
normal boiler operating range. Lower SO2
removal levels were seen during this con-
tinuous testing, due, largely, to the need
to continuously operate the sootblowers
to remove accumulated LIMB solids from
boiler heat transfer surfaces. Commercial
LIMB installations would require increased
sootblower coverage to maintain boiler
surface cleanliness. No boiler outages at-
tributable to LIMB were experienced dur-
ing the entire LIMB test program.
ESP performance was maintained dur-
ing LIMB operation by humidification of
the gas stream at the inlet to the ESP. No
deep humidification for enhanced SO2 re-
moval was attempted. Early problems with
deposition of LIMB solids on duct sur-
faces were resolved by improvements in
humidifier lance fabrication procedures.
Opacity was maintained within compliance
requirements throughout the test program.
LIMB ash was collected in, and pro-
cessed from, a common ash silo that
received material from both coal-burning
units. Problems were experienced with
material setting up in ash truck beds.
Steaming of material in the ash trucks
varied with the concentration of LIMB ash,
decreasing as dilution from non-LIMB Unit
No. 1 ash increased. A dedicated ash silo
would be a necessary component of a
commercial LIMB installation.
The LNCFS Level II firing system
achieved a 42% reduction in NOX at full
boiler load and 33% reduction in NOX at
intermediate boiler load, relative to baseline
levels with the original tangential firing sys-
tem, under comparable boiler operating
conditions. These levels were below the
program NOX objective of 0.4 lb/106 Btu
(172 mg/J).
Capital and levelized costs associated
with the 180 MWe Yorktown LIMB system
were determined to be $81/kW and $7817
ton ($859/tonne) SO2 removed, respec-
tively, based on actual equipment and op-
erating costs for the Yorktown system,
plus the cost of a new ash silo. Sensitivity
studies showed the cost effectiveness of
LIMB on tangentially fired units from 100
to 300 MWe operating on coals with 2 to
3% sulfur.
The commercial applicability of a stand-
alone LIMB system is currently consid-
ered to be that of a "niche" technology for
certain older units as a result of the 1990
CAAAs. In combination with a comple-
mentary "back-end" SO2 removal process,
however, LIMB offers the potential of
achieving compliance levels of SO2 re-
moval at costs that are significantly below
those of conventional FGD systems.
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