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Low-cost FGD systems for emerging markets
for a spray dryer reactor and slurry handling equipment. The seawater technology was pioneered by ABB in the 1970s. This technology has matured slowly and gained increased market acceptance through in-depth, long-term environmental impact assessment. The process uses no reagent, produces no byproducts and is environmentally benign. ABB is the only company to
Emerging markets have been calling for several years for new, lower-
have gained operating experience with
cost flue-gas desulfurization technologies as an alternative to the
this technology in power plants.
mature, proven FGD systems that are currently available. In response to this demand, ABB has developed new FGD technologies that perform as well or better than conventional systems at a much reduced
Novel Integrated Desulfurization
cost. Three main technologies are used to control sulfur dioxide
The key control factor in any dry FGD
emissions: limestone-based wet, lime-based dry or seawater FGD.
process is the relative humidity of the
The optimum FGD solution for a particular installation depends on
flue gas, which can be increased by in-
many site-specific factors, which can be technically and/or financially
jecting water into it. At a relative humid-
driven.
ity of 40–50 %, the hydrated lime becomes activated and absorbs SO 2. In a
T
he combustion of fossil fuels gener-
as a high-velocity, compact absorber,
lime is supplied to the flue gas as a slur-
ates a number of gaseous pollutants,
high-efficiency gas/liquid contact and a
ry (with or without recycled material) with
including sulfur dioxide, which can be
compact reaction tank.
a solids content of 35–50 %.
conventional DFGD process, water and
effectively controlled by air pollution
Dry FGD technologies based on either
The amount of water injected into the
control processes. Three main technol-
rotary atomizers or dual fluid nozzles
flue gas in the NID process is the same
ogies are used to control sulfur dioxide
were introduced in the late 1970s as
as in conventional dry FGD. However, it
emissions:
an alternative to limestone-based wet
is distributed on the surface of the dust
• • •
Limestone-based wet FGD
FGD. The installed base of dry FGD sys-
particles to give a water content of only
Lime-based dry FGD
tems has provided considerable experi-
a few percent. Much more absorbent is
Seawater FGD
ence in areas such as two-phase fluid
therefore recycled than in a standard
Since the introduction of FGD in the late
dynamics and materials, and ultimately
DFGD process, providing a much larger
1960s, global market demand has been
led to the development of a new technol-
surface for the evaporation. The dust
steady at between 5,000 to 10,000
ogy called Novel Integrated Desulfuri-
added to the flue gas thus dries in a very
MWe per year 1 .
zation, or NID. The NID system uses
short time, allowing very small reactor
In 1968 ABB introduced thr first com-
simple but highly advanced moist dust
vessels to be used. The increase in the
mercial utility flue gas desulfurization
recirculation, which eliminates the need
relative humidity of the flue gas is suffi-
system at Kansas Power & Light’s Law-
cient to activate the lime for SO 2 absorp-
rence station. Currently, ABB’s experi-
tion at typical DFGD/NID operating tem-
ence includes 30,000 MWe of lime-
peratures or 10–20 °C above saturation,
stone-based wet FGD systems, 15,000 MWe of dry FGD systems and 4,000
Jonas S. Klingspor ABB Environmental Systems
in practice within the temperature range of 65 to 75 °C 2 .
MWe of seawater FGD systems. The limestone-based wet FGD system has been systematically improved
Arvid Tokerud ABB Miljö
over the years. For example, ABB pioneered forced oxidation for gypsum production, and in 1995 it introduced the LS-2 system, featuring advances such
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NID process description The NID process 3 is based on the ab-
Stefan Åhman
sorption of SO 2 by a dry absorbent con-
ABB Fläkt Industri
taining lime (CaO) or dry hydrated lime, Ca(OH) 2. As an alternative, fly ash con-
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70
25
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103
60
Thin slurry
20 50
MW
Thick slurry
15
40
Conv DFGD
30
10 H2O
20 5
Paste 10
0
5 72 74 76 78 80 82 84 86 88 90 92 94 96
Freeflowing dust
NID AEM
1
Global installed capacity of FGD per year Blue Red
Dry flue gas desulfurization Wet flue gas desulfurization
Moisture history of reagent. In the NID process, water is injected into the flue gas such that the dust particles have a water content of just a few percent. AEM DFGD NID
taining an appropriate amount of alkali can be used.
2
Adsorbed equilibrium moisture Dry flue gas desulfurization Novel integrated desulfurization
The NID process is thus characterized
fly ash only, and had no desulfurization
by a very high recycle rate and maximum
capability. These filter systems handle all
Water is added to the absorbent in a
utilization of the reagent. The large sur-
of the flue gas from the boilers and are
humidifier prior to its introduction into
face area allows rapid evaporation of
capable of treating gas at a nominal rate
the flue gas. A unique feature of the NID
the injected water, thus enabling the
of 2 × 518,000 Nm 3/h.
technology is that all the recycled ab-
volume of the reactor/dryer in the NID
An agreement was signed between
sorbent is wetted in the humidifier, which
process to be an order of magnitude
Elektrownia Laziska and ABB, whereby
optimizes the utilization of the recycled
smaller than the corresponding equip-
ABB would install and test the new NID
absorbent. After activation/drying, the
ment in a conventional dry flue gas
concept on one of the compartments of
dried recycle dust is separated from the
cleaning system based on spray dryer
the new fabric filter.
flue gas in a high-efficiency dust collec-
technology.
The NID demonstration unit 4 was
tor, preferably a fabric filter. From here,
installed on a compartment of the fabric
the dust is again fed to the humidifier,
filter of unit 2. Initial supportive testing in
with make-up lime also added. Water is
Experience with NID
the ABB R&D laboratory at Växjö,
fed to the humidifier in a quantity suffi-
In June 1994, the Polish power company
Sweden, focused on dust wetting and
cient to maintain a constant outlet flue
‘Elektrownia Laziska’ placed an order
the operating performance of the full-
gas temperature. The control system
with ABB for a high-efficiency fabric filter
scale unit. Efficient and homogeneous
uses a feed forward signal with back
system downstream of unit 2, where it
dust wetting is important for the success
trim, based on the inlet and outlet flue
would collect the fly ash. The boilers of
of the NID process. The wetting aspects
gas temperatures, supplemented by a
units 1 and 2 at Laziska burn pulverized
were studied separately in a semi-com-
signal indicating the gas flow. The outlet
coal and each has a rated output of
mercial-scale humidifier, utilizing a mix-
SO 2 concentrations plus the flue gas
120 MWe. The fuel is domestic hard coal
ture of fly ash and lime. On conclusion of
flow determine the lime flow to the
from nearby mines. Initially, the fabric
this study, the humidifier was added to
system.
filter systems were designed to collect
an aerodynamic flow model, which
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programmes have been run in the meantime. Based on the results from the demonstration Boiler
plant,
Elektrownia
Laziska
placed orders with ABB for the extension Fabric filter
of the NID technology to their two 120-MW boilers (units 1 and 2). Both full-scale units 5 were commissioned during 1996. For the full-scale plant, it was decided
Fuel
to install a commercial dry lime hydrator.
Water
Although operation with quicklime alone was proven, it was felt that this would add unnecessary risk to the project. Humidifier
Quicklime is fed from a silo into the dry
CaO
hydrator, from which the dry hydrated lime is transported by pneumatic means to the two FGD units.
Water
End product
The flue gas to the fabric filter is transported in two main flue gas ducts, each
Hydrator Ca(OH)2
Operating principle of the NID demonstration unit
3
NID process flow diagram
allowed testing of the combined systems
compressed air, which dislodges the
for wetting and dust dispersion into the
dust. The filter hopper catches the dust,
flue gas. These activities were supported
to which make-up absorbent is added
by computational fluid dynamics (CFD)
before it is sent to the humidifier imme-
modelling.
diately below the hopper for recycling. In
The flue gas is taken from a common
the humidifier, a controlled amount of
inlet flue gas duct into a vertical duct
water is added to the recycled material
acting as inlet to the NID reactor. In
to maintain the desired outlet flue gas
the reactor, the flue gas is thoroughly
temperature.
mixed with wetted dust consisting of a
The lime powder is stored in a silo,
mixture of absorbent and recycled ma-
from which it is transported pneu-
terial, ie reaction products of SO 2 and
matically to the filter hopper, thus being
absorbent, mixed with fly ash. The NID
introduced into the flow of recycled
reactor is connected directly to the
material. The amount of lime added is
fabric filter such that the gas flows
controlled via a signal from an SO 2 meter
horizontally into the filter bags, where
at the outlet duct which sets the speed
the particles are separated from the flue
of a rotary feeder at the silo discharge.
gas.
The demonstration plant was started up
The filter is cleaned by pulses of
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in February 1995, and a number of test
1 2 3 4 5 6
4
Outlet plenum Filter bags Rotary feeder Humidifier Outgoing flue gas/damper Incoming flue gas
1
5
2
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Full-scale NID installation at Laziska
1
Water
2
2
5
Lime
with its own induced-draft flue gas fan.
Performance of the full-scale
Start up of unit 2 followed later in the
The main flue gas ducts branch off into
NID plant
autumn of 1996. The performance data
two filter compartments, both of which
Commissioning and testing of the full-
for the NID installation at Laziska are
can be isolated by inlet and outlet
scale plant took place in steps. The first
summarized in Table 1.
dampers. Thus, there are four separate
full-scale humidifier was installed in the
FF compartments, each equipped with
NID system of unit 1 in February 1996.
a hopper for recycled dust from the
Aerodynamic tests and optimization of
Comparison with conventional
filter bags. The recycled dust from
the plant followed. It was concluded
DFGD
these four hoppers is fed to the humid-
that
hopper/rotary
The NID process requires considerably
ifiers by means of rotary screws. Finally,
feeder/humidifier operated as expected.
less sophisticated equipment than con-
after wetting in the humidifiers, the re-
It was further confirmed that the pres-
ventional dry FGD. Neither a rotary
cycled dust is fed back into the inlet
sure drop over the unit is fully within the
atomizer with its high-speed machinery
ducts via short air slides. This arrange-
range that could be expected on the
nor dual fluid nozzles with their need for
ment allows for part-load operation with
basis of the laboratory tests and CFD
compressed air are required. The power
individual compartments and mixers off
modelling.
needed to mix the recycled material and
line.
the
fabric
filter
In August 1996, all the gas paths of
reagent in the humidifiers is much lower
unit 1 were successfully brought on line.
than for the corresponding items in a
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contact between the seawater and flue gas in a counter-current flow. Low-den-
Table 1: Performance data
sity packing is used to optimize the gas/
Parameter
Data/results
Size Flow rate Inlet temperature SO2 concentration SO2 removal guaranteed measured Reagent Dust loading Particulate emission guaranteed measured
2 × 120 MW 2 × 518,000 Nm3/h 165 °C 1,500–4,000 mg/Nm3
liquid contact. No reagent is added or needed 7 . Seawater is naturally alkaline, and therefore has a large neutralizing capacity. The acidic effluent flows to the Sea Water Treatment Plant (SWTP), where
80 % 95 % CaO 22,000 mg/Nm3
the absorbed SO 2 is oxidized by aeration to form harmless SO 42–, which is discharged into the sea. The sulfate is totally dissolved in the
50 mg/Nm3 15 mg/Nm3
seawater. Since sulfate is a natural constituent of seawater, the seawater returned to the sea has only a slightly higher sulfate content. This increase lies well within variations occurring naturally
conventional dry FGD system. An im-
sludge, and finally ended up as coal or
in seawater, and the difference from the
portant consequence of using humid-
oil 6 .
background level can no longer be de-
ifiers rather than nozzles or rotary atom-
Since sulfur bacteria were active in
izers is that all the equipment the oper-
the same anaerobic environment, nat-
ator has to attend to is situated near
urally abundant seawater sulfate sub-
As the Flakt-Hydro seawater process
ground level in an enclosure shared with
sequently converted to sulfides, which
is a once-through system, the flue gas is
the fabric filter, lowering costs and mak-
were deposited in the organic material.
sub-cooled as it passes through the ab-
ing maintenance easier.
Sulfur thus became a constituent of fos-
sorber. The flue gas typically requires
sil fuels.
reheating before being discharged to the
Finally, since water is added directly
tected just a very short distance from the point of discharge.
to the NID humidifier there is no slurry
The Flakt-Hydro process absorbs
handling, which would require special
SO 2 from flue gas in seawater and oxi-
pumps, etc. The high recycle rate also
dizes it to sulfate prior to discharge; in
means that only dry material is handled
other words it returns sulfur to the sea in
by the system. This ensures that the gas
the form in which it originally appeared –
Performance of seawater
ducts, etc, are free of build-up, for
as dissolved sulfate.
FGD
example due to wet slurry impacting on the surfaces of the installation.
Flakt-Hydro seawater
Laboratory tests, long-term bioassay
atmosphere; however, wet stacks can also be employed.
Chemically,
the
Flakt-Hydro
system
testing and recipient follow-up have
is very similar to the established wet
detected no significant effects of the
limestone/gypsum process, with the
effluent discharged by the Flakt-Hydro
difference that no solid reagent is
system.
required and precipitation of solids
FGD
is not needed. Hence, the system
Coal has its origin mainly in the large
exhibits a similar performance and is
marine and submarine forests that
Seawater FGD process
capable of meeting stringent emission
existed during the Carboniferous Period
The Flakt-Hydro process utilizes sea-
requirements for low to medium sulfur
some 350 to 280 million years ago. Oil
water’s inherent capability for absorbing
coals.
originates from sea organisms that
and neutralizing sulfur dioxide. Seawater
The effluent discharge from the Flakt-
settled on the bottom of prehistoric
is used in large amounts at coastal
Hydro seawater process has been
seas.
power plants as the cooling medium in
extensively studied by independent environmental agencies.
The organic material was later cov-
condensers. This seawater can be re-
ered by silt, preserved and fossilized in
used downstream of the condensers to
an anaerobic environment (with little or
control SO 2 emissions.
no free oxygen) within the seabed
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The SO 2 is absorbed through close
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Long-term bioassay testing
fur recovery unit. The purpose of the
US EPA
A long-term bioassay test programme
investigations was to determine the in-
The EPA region II of New York evalu-
was carried out several years ago at
fluence of the seawater FGD on the local
ated the process in connection with
the Cabras power station on the island
flora, fauna and marine sediments. Sam-
a 2 × 150-MWe coal fired IPP power
of Guam. Plankton, shellfish and other
pling was conducted before and after
project in Puerto Rico. In their draft per-
marine organisms were kept in aquari-
installation of the new seawater FGD
mit they stated: ‘Our review of the
ums filled with effluent water from a
outlet.
proposed seawater FGD technology
Flakt-Hydro seawater treatment plant.
The survey was carried out by the
indicates that the seawater scrub-
The same organisms were also kept in
Department of Fisheries and Marine
bing
fresh seawater aquariums for compari-
Biology at the University of Bergen, Nor-
successfully since 1933. In particular,
son.
way.
it has been applied at over a dozen
technology
has
been
in
use
No harmful effects on the marine life
The first samples were taken in March
facilities in Europe and Asia. Accordingly,
could be determined over the test peri-
1989, prior to start-up of the flue-gas
EPA consider this a proven technology.
od of one year. The test was carried out
desulfurization system. Repeated test-
EPA determine that the seawater scrub-
by marine biologists from R.W. Beck and
ing took place in March 1990, after ap-
bing would be appropriate for this pro-
associates and was monitored by the
proximately six months of operation.
ject.’
USA Environmental Protection Agency
Since then, sampling has been carried
(EPA).
out on a yearly basis. No harmful impact on the benthos
European Union
was observed after the plant was started
Unión Eléctrica de Canarias, S.A (UNEL-
University of Bergen
up. The amount of organic material,
CO) has two 2 × 80-MWe power stations
The decision was taken at the Statoil
sulfates and trace metals remains within
in the Canary Islands, Spain. The plants
refinery in Norway to start a recipient
the natural range for marine sediment.
are equipped with Flakt-Hydro seawater
follow-up programme around its sea-
No distinguishable difference in the en-
FGD and are designed to comply with all
water outlet. The refinery installed the
vironmental conditions in the area be-
environmental regulations for air and
Flakt-Hydro process to absorb SO 2 in
fore and after the FGD system was de-
water quality valid in Spain and within
the flue gas from their residual catalytic
ployed is evident after 52 months of con-
the European Union. The power stations
cracker and in the off-gases from a sul-
tinuous operation.
started up in 1995/1996.
6
The natural sulfur cycle
7
Seawater FGD process flow diagram
Seawater
SO2
Flakt Hydro SO4
SO4
Coal / oil with sulfur
H2S
Flue gas
Dust collector
SO 2 absorber
Reheat equipment
Seawater
Seawater treatment plant
Air
Clean flue gas
Treated seawater
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The absorber inlet includes a flue gas quencher to protect the packing.
Table 2: Basic design data for recent seawater systems
The
gas/liquid
contact
is
counter-
current; the flue gas enters the ab-
Plant
UNELCO
Paiton
Shenzhen
Location
Canary Islands, Spain
East Java, Indonesia
She Kou, China
Size
4 × 80 MWe
2 × 670 MWe
300 MWe
Fuel
2.7 % S oil 1.5 % S coal
0.4 % S coal
0.7 % S coal
SO2 removal
91 %
91 %
90 %
used when conditions are disturbed and
Special features
GGH
Wet stack, concrete absorber
GGH, axial booster fan
when carrying out maintenance on the
sorbers through the bottom and leaves through the outlet at the top. The treated flue gas is discharged through a wet stack. The FGD system includes an absorber bypass duct, designed to be
FGD plant. Seawater from the process downstream of the condensers is introduced at the top of the absorber packing. The acidified absorber ef-
Scottish Power also selected this sys-
A total of 4,400 MW of equivalent
fluent collects in the absorber sump
tem for its 2,400-MW Longannet plant,
electrical capacity has been put into
at a sufficient level to ensure gra-
the
operation or is under construction
vity flow to the SWTP and avoid
(Table 2).
having to pump this highly corrosive
third
largest
power
station
in
Europe, in the event that FGD is installed. In the meantime, the authorities
liquid.
have approved the Seawater FGD sys-
A portion of the cooling-water dis-
tem for use at Longannet and ABB’s
Paiton, Indonesia
charge is routed directly to the SWTP to
Flakt-Hydro process is considered the
In September 1995, ABB received an
provide optimum conditions for the
best
order to deliver a Flakt-Hydro seawater
chemical reactions. In addition, ambient
scrubbing process for the 1,340 MW
air is blown into the SWTP basins for the
Paiton Private Power Project (phase 1) in
oxidation and oxygen saturation. The
practical
environmental
option
(BPEO).
East Java, Indonesia. The process will
treated seawater is finally discharged
Experience
be used in conjunction with the plant’s
through the existing cooling-water outlet
The seawater process was first intro-
two 670-MW coal-fired boilers.
system.
duced in the early 1970s in Norway,
Each 670-MW boiler is equipped with
where it was used for desulfurization
two induced draft fans. The plant is
of flue gases from oil-fired boilers,
designed to operate without the assist-
Shenzhen, China
smelters and refineries. The process
ance of booster fans. Each of the boilers
ABB recently received the order for a
was employed for the first time in a
is fitted with two concrete absorber
Flakt-Hydro seawater system for the
coal-fired power plant in India in 1988
modules employing low-density pack-
300-MWe Shenzhen power plant in
(TATA’s Trombay Power Station, Unit 5).
ing.
Shenzhen, South China. A single absorber will be used to treat the flue gas from the boiler complex. The flue gas train will include a booster fan, a single absorber tower and a regener-
Table 3: LS-2 size reduction
ative gas-to-gas heat-exchanger. The absorber employs low-density packing
Size reduction Absorber diameter Overall height Plate area Plate weight Liquid-to-gas ratio Power consumption
Saving 15–25 % 20–30 % 25–35 % 35–45 % 20–40 % 10–20 %
and is designed for 90 percent SO 2 removal. Before being discharged to the stack, the flue gas passes through the regenerative
gas-to-gas
heat-exchanger.
Effluent seawater is treated in the
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SWTP before being discharged into the ocean.
LS-2 limestone wet FGD ABB’s newest limestone-based wet FGD system has been in operation at Ohio Edison’s Niles Plant 9 since September 1995. Drawing on nearly 30,000 MW of worldwide wet FGD experience, ABB has incorporated several innovations into the Niles system which are designed to reduce the overall
cost
of
compliance
with
SO 2
emissions requirements. Collectively, these improvements are referred to as LS-2 [1]. The turnkey system installed at the Niles plant is rated at 130 MWe and was designed and erected over a 22-month period. The system also produces wallboard-grade gypsum, all of which is sold to a local wallboard manufacturer.
LS-2 process description The LS-2 includes a number of innovative process improvements. The spray tower has the ability to run at velocities as high as 5.5 m/s, and features a compact spray zone with ABB’s patented nozzle arrangement, plus a compact reaction tank. The reagent system is based on an ABB Raymond roller mill 8 which features a completely dry grinding
ABB dry roller mill. This limestone grinding mill forms the basis of the reagent system used in the LS-2 wet FGD process.
8
circuit. This limestone grinding system is less costly both to construct and operate, yet produces a significantly
technology and employs a high super-
absorber, compared with a current 400-
finer grind. The primary dewatering sys-
ficial gas velocity and fine-grind lime-
MWe state-of-the-art absorber, is indi-
tem features fully integrated high-effi-
stone for a significant reduction in
cated in Table 3.
ciency hydrocyclones followed by centri-
absorber size and recycle tank volume.
fuges for secondary and final dewatering.
The spray zone is followed by a
In addition, the LS-2 absorber em-
proprietary two-stage mist eliminator
ploys a new patented spray header
system with vertical flow bulk entrain-
design which allows a higher spray den-
ment separator which is followed by a
sity to be used, resulting in a smaller
two-pass, horizontal-flow, chevron-type
Absorber
number of
spray levels and a cor-
mist eliminator. The mist eliminator
The design of the LS-2 absorber has
responding reduction in absorber height.
system is capable of operating at up to
evolved from ABB’s open spray tower
The potential saving with the LS-2
10 m/s.
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Flue gas to unit 1 flue
Ljungstrom GGH
Flue gas from units 1 and 2
Mill gas duct Absorber
Centrifuges
Gypsum bin
Limestone hopper
Pulverizer
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LS-2 installation at the coal-fired, 130-MW Niles generating station of Ohio Edison, USA
Grinding System
leaving the roller mill is collected in a
to fully utilize their separation capabil-
The additive preparation system features
cyclone and transported pneumatically
ities.
an ABB Raymond roller mill 8 . The mill
to a storage silo.
The centrifuges are designed to
system accepts a limestone feed stock
The supply of ground limestone to the
dewater the gypsum byproduct down
sized at less than 40 mm (1.6 inches).
absorber reaction tank is based on de-
to a moisture content of 8 percent or
Untreated flue gas is used to dry and
mand. The limestone grind will typically
less. Also, they are located directly
convey the limestone during mill oper-
be 99.5 percent less than 44 µm (325
above a gypsum storage bunker. Hence,
ation. The flue gas leaving the milling
mesh), although coarser grinds will be
the centrifuges discharge directly into
system is returned to the absorber for
tested.
the storage bunker, eliminating the
processing.
need
The limestone preparation and hand-
for
a
costly
solids
handling
system.
ling system is completely dry and in-
Dewatering
cludes a wetting system just before the
The dewatering system consists of
directly from the gypsum bunker into
injection into the reaction tank.
Wallboard-grade gypsum is loaded
hydrocyclones for primary dewatering
trucks for transportation to the wall-
The grinding system comprises a
and centrifuges for secondary de-
board manufacturer.
limestone loading area and a storage silo
watering. The hydrocyclones and the
for pebble limestone. Ground limestone
absorber loop are closely integrated
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Experience at Niles The LS-2 system was installed at the
Table 4: Typical LS-2 performance results
Niles station 9 in September 1995. Although the system is designed to pro-
Parameter Gypsum purity Gypsum moisture Gypsum chloride Sulfite oxidation Gypsum MMD SO2 removal Limestone grind Gas velocity Reheat
cess all the flue gas from one boiler, it is cross-connected to unit 1 and unit 2 boilers in order to maximize the flue gas availability and maintain a high FGD system capacity factor. Each boiler is rated at 108 MWe net, the absorber system being rated at 130 MWe.
Goal >95 % 30 µm >90 % 99 %4.5 m/s >93 °C
Test results 97–98 % 6–8 % 30–50 ppm 99.9 % >50 µm >97.5 % 85.99 %< 44 µm 5.5 m/s >99 °C
Treated flue gas is discharged into the existing unit 1 stack, which is carbon steel lined. In order to protect this lining, an ABB Air Preheater Ljung-
Performance
The LS-2 system was started up at a
strom type gas-to-gas heat-exchanger
All of the subsystems are operational
velocity of 3 m/s for initial checking and
(GGH) was installed. The GGH features
and have met their design requirements.
tuning. The velocity was quickly in-
ABB’s patented horizontal shaft orien-
As of January 1, 1998, the system had
creased to 4.5 m/s and later to 5.5 m/s.
tation, which greatly reduces the amount
been on-line for approximately 18,000
The higher velocity provides a much
and cost of expensive ducting. All duct-
hours and produced 80,000 tonnes of
improved gas/liquid contact, resulting
ing and plate surfaces on the cool
gypsum to the given specifications. The
in a reduced liquid-to-gas ratio. Thanks
side are lined with flake glass, while sur-
gypsum purity has consistently ex-
to the wall rings, there is superior gas/
faces on the hot side are unlined carbon
ceeded requirements in terms of purity,
liquid contact close to the walls. The
steel.
moisture, and chloride content. The
high velocity mist eliminator is working
crystal size has lent itself to easy de-
well.
watering, and residual moisture levels
The GGH has operated above ex-
down to 6 percent are easily achievable.
pected heat transfer rates and the
Table 5: Comparison of dry, seawater-based, and limestone-based wet FGD technologies Dry FGD
Seawater WFGD
Limestone WFGD
Name
NID
Flakt-Hydro
LS-2
Related experience
15,000 MWe
4,000 MWe
30,000 MWe
Type
Moist dust injection
Packed bed tower Compact absorber No chemicals No byproduct
Open spray tower Compact absorber High velocity Advanced spray header design Fine grind
• Low moisture level • No spray dryer required • Fabric filter version • ESP version
• • •
• • • •
Reagent
Lime
Seawater
Limestone
Byproduct
Fly ash/calcium sulfite/lime
None
Sulfur