F

G

D

S

Y

S

T

E

M

S

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

30

ABB Review 1/1998

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-

F

G

D

S

Y

S

T

E

M

S

70

25

%

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

ABB Review 1/1998

31

F

G

D

S

Y

S

T

E

M

S

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

32

ABB Review 1/1998

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

6

3 4

F

G

D

S

Y

S

T

E

M

S

1

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

ABB Review 1/1998

33

F

G

D

S

Y

S

T

E

M

S

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

34

ABB Review 1/1998

The SO 2 is absorbed through close

F

G

D

S

Y

S

T

E

M

S

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

ABB Review 1/1998

35

F

G

D

S

Y

S

T

E

M

S

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

36

ABB Review 1/1998

F

G

D

S

Y

S

T

E

M

S

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.

ABB Review 1/1998

37

F

G

D

S

Y

S

T

E

M

S

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

9

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

38

ABB Review 1/1998

F

G

D

S

Y

S

T

E

M

S

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