circulating fluidized bed technology

7/31/2019 Circulating Fluidized Bed Technology

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B A B C O C K & W I L C O X

C I R C U L A T I N G F L U I D IZ E D - B E D B O I L E R

An O v e r v i e w o f B & W C F B B o i l e r T e c h n o l o g y

The Babcock & Wilcox circulating fluidize d-bed (CFB) b oiler is designe dfo r high reliabili ty and availabilitywith low maintenance, while complying with stringent emission regulations B& W's CFB techno logy is

unique and includes a simple U-beam particle sep arator design. This is the result o f extensive research

and develop ment an d com mercial operating experience.

B& W and it s jo in t ventur e co mpa nies have sold more than 38 fl uid iz ed-bed proje cts o f whic h 10 ar e

atmospheric circulating fluidiz ed-b ed boilers.

CFB PROCESS

Ho w the B& W CFB Internal Circulat ion Boi ler Works

In a circulat ing fluidized-bed boiler, a port ion of com bustio n air is introduced throug h the botto m o f the

bed. The bed mater ial normal ly consis ts of fue l, l imestone and ash . The b ot tom of the bed i s suppor ted by

water-cooled m em bra ne walls with special ly designe d air nozzles which distribute the air uniformly . Th e

fuel and l im eston e (for sulfur capture) are fed into the lower bed. In the presence o f f luidizing air, the fuel

and l imestone quickly and uni formly mix under the turbulent envi ronment and behave l ike a f luid . Carbon

part ic les in the fuel are expo sed to the combu stion air . The balance of com bust ion air is introduced at the

top of the lower , dense bed . This s taged combust ion l imi ts the format ion of n i t rogen oxides (N O) .

The bed fluidizing air velocity is greater than the terminal velocity of mo st of the part ic les in the bed and

thus fluidizing air elutria tes the part ic les through the comb ustion cham ber to the U- bea m separators a t the

furnace exit . The captured solids, including any unburned carbon and unuti l ized calcium oxide (CaO),

are reinjected directly b ack into the combu stion chamb er without passin g through an external recirculation.

This internal solids circulat ion provides longer residence t ime for fuel and l imeston e, result ing in good

combust ion and improved su l fur capture .

CFB Steam Generator

The CFB boiler is arranged with a single furnace havin g full-height/part ia l depth s traight-tube division

wails with or without steam-cooled wing wails. The furnace and part ic le separator enclosure walls are

com pos ed of water-cooled m em bra ne tubes. The superheater enclosure is a comb ination of steam-co oled

and water-cooled mem bran e tubes.

Feedwater enters the unit a t the econo mizer inlet, f lows through the econom izer banks in the convecti on

pass to the outlet header, and then to the steam dru m feedwater inlet . Water in the dru m pass es through


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l a rge downcom ers an d mul t ip le supply tubes to feed the enclosure wal ls and divis ion wal l s. Steam-water

mixtu res from the va rious circui ts flow throu gh headers and r iser tr ibes back to the drum.

Saturated ste am is routed from the dru m to the superheater enclosure side wails ( i f supplied) and then to

the pr imary superheater located in the convect ion pass . Steam then t ravels to the in le t headers of the

superheater wing wal l s ( i f suppl ied) in the furnace . Steam pass ing through the wing wal l s i s col lec ted and

routed back to the secon dary superheater through spray a t temperators . The s tea m passes throug h the

secon dary superheater a nd d ischarges to the out let terminal adjacent to the boi ler .

CFB BOILER MAJOR SYSTEMS

Circulat ing Fluidized-Bed Furnace

The furnace des ign has been developed f rom B& W's 30 years of exper ience wi th f lu idized-bed technology.

The me cha nics of fuel a nd l imes tone feed, air distr ibution, star t-up syste m, refractory, bed drains, water-cooled wal l s , e tc . , a re based on research and dev elopment and comm ercia l opera ting uni t s .

The CFB furnac e operates as an extend ed f luidized bed o f sol id part icles. M ost o f these entrained sol ids

recirculate within the furn ace or are captured by the primary impact separator (U-be ams ) at the furna ce

exit and are returned internal ly to the bot to m of the furnace.

Com bust io n.air is admit ted to the furna ce as follows:

Primar y air throug h a bubble cap air distr ibutor in the furna ce bot tom.

Seco ndary air throug h nozzles and material inject ion points at two levels in

the lower furnace.

The region of the furnace below the lower secondary a i r l evel i s ca lled the pr imary zone.

The circulat ing f luidized bed form s two dist inct regions:

Den se bed in the primary zone { 14 f t /s (4.25 m/s)}.

Dilute bed in the middle and uppe r furnace { 19.7 f t /s (6 m/s) }.

The t ransi t ion between these two regions is gradual , and operat ing experience indicates low erosion

potential.

The sol ids densi t ies in the di lute bed and transi t ional regions of a circulat ing-bed com bus tor are relat ively

high. This resul ts in high er rates of ga s-sol ids react ions for combu st ion, sulfur capture and heat t ransfer

between the bed and the furnace wal l s. The furnace height i s selec ted to maxim ize carbon burnou t and

sulfur capture. B&W operates higher sol ids densi t ies compared to other suppliers to opt imize sulfur

capture and heat t ransfer .


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The material separated by the U-beam primary separator at the furnace exit is returned to the lower

furnace by gravity, falling as a curtain along the rear furnace wall. In the lower furnace, these solids are re-

entrained by primary and secondary air and carried back to the furnace exit. This intensive furnace back-

mixing provides uniform distribution and optimum residence time. Finer solids not collected by the

primary separator are carded by gases through the convection pass, are collected by the secondary separator,

and are recirculated to the lower furnace.

D e n s e B e d i n t h e P r i m a r y Z o n e

The primary zone design provides for uniform distribution and intensive mixing of primary combustion

air and bed solids supplied by material feed systems and recirculated from the primary and secondary

separators.

The cross section of the primary zone is reduced relative to the upper furnace to promote good mixing and

turbulence and to ensure solids entrainment throughout the boiler load range. The high rate of mass

transfer in the primary zone provides intense combustion and calcination/sulfation reactions.

The substoichiometric conditions in the primary zone promote conversion of fuel nitrogen compounds to

elemental nitrogen, thus reducing NO x formation.

D iL u te B e d i n t h e M i d d l e a n d U p p e r F u r n a c e

The middle and upper sections of the CFB furnace are designed for the following:

sufficient residence time for fuel burnout and sulfur capture,

high solids inventory for improved heat transfer rates and sorbent reaction surface, heat transfer surface of the enclosure walls, in-furnace division, and wing wails, and

good mixing of secondary air and combustion products.

A I R A N D G A S S Y S T E M S

Air from the primary air fan or forced draft fan (single fan option) is heated by a steam coil air heater and

flows across a partitioned section of the tubular air heater. It is then directed to a water-cooled windbox at

the bottom of the furnace. This windbox is divided into many compartments across the width of the unit,

with dampers to control the flow of primary air to each compartment. A portion of the primary is also

admitted to the furnace through the fuel chutes. A duct burner is installed in each main pr imary air feed

duct to facilitate boiler start-up.

Air from the secondary air fan or forced draft fan is heated by a steam coil air heater and passes through

the secondary section of the tubular air heater. This secondary air flows to the distributing nozzles located

across the width of the furnace on both the front and rear wails. Dampers vary the proportions of the

secondary air to the front and rear distributing nozzles.


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A port ion o f the seconda ry air is admit ted to the furnace through overb ed burners.

The f lue gas wi th ent ra ined sol ids leaves the furnace through the U-bea m pr imary par t icle separa tor , and

passe s through th e convect ive heat recovery pass to the secondary separator . The f lue gas with an y remain ing

fine part icles cont in ue thro ugh the air heater . Most o f these remain ing f ine part icles are remo ved at the

bag hou se or electrostat ic precipitator (ESP).

SOLI DS SEP ARATI ON SYSTEM

The sol ids separa t ion sys tem s i s a key e lemen t of any CFB boi ler des ign, inf luencing both capi ta l and

opera t ing cos t s . The separa t ion sy s tem af fec t s the sol id inventory in the furnace , which impacts furnace

temperature cont rol ( furnace heat t ransfer) , carbon burnout and l imestone uti li zat ion. T he B &W CFB

boiler uses a two-stage sol ids separat ion system :

Prim ary part icle separators - U- bea ms

Seco ndary part icle separator system - mult i -cyclo ne dust col lector (MD C) or ESP

The Primary Particle Separator (U-Beams)

B& W uses an impact for pr imary par t ic le collec tion, which i s d if ferent f rom hot cyclones comm on ly used

for CFB boi lers. B &W 's imp act separa tor i s unique in CFB boi ler des ign.

Impact separa tors have b een used for severa l decades to separa te par t ic les grea ter than 100 microns . The

dust laden gas s t ream impinges on the s taggered ver tical a r rangement of U-bea m channels . B& W has

conducted considerable research and developme nt on impact separa tors on the Cold Mod el T es t Faci li ty

a t B& W' s Al l iance Research Center. Geomet r ica l cor re la tions have been developed based on o pera t ing

variables such as gas veloci ty; sol ids loading, num ber of chann el rows, and part icle size. Thes e relat ionships

have been appl ied succ essful ly to B& W's com mercia l CFB boi ler des igns .

B & W pr imary sol ids col lec tor cons ist s of two (2) rows of U-bea ms located wi thin the furnace at the gas

exi t and four (4) addi tional rows of U-b eams located immed ia te ly down st ream of the in- furnace U-beams.

Solids col lected by the front two rows discharge downward direct ly to the furnace along the rear wall .

Sol ids col lected by the rear rows o f U- be am s discharge into a hopp er integral to the furnace rear wall and

return by grav i ty to the furnace throu gh openin gs distr ibuted across the width o f the uni t.

These U -beam s are mad e of s ta inless s tee l . Individual U-beam s are in the form o f channels s ix inches

(152 ram) wide by seven inches (178 m m) deep. Two bol ts through the water cooled roof susp end each

beam, protected by an enclosure. Dynamic (gas and sol ids) st resses, s tat ic (dead load) st resses, design

temperatures and m ater ia l creep s t rength are used to des ign the U-beam s.

A pan a t the lower end o f each U-beam holds the U-beam in a l ignment and accom moda tes hor izonta l and

ver tical thermal expansion. These pa ns a l so form a gas bar rier a t the bot tom discharge end of the bea ms

to prevent gas bypass ing and improve par t ic le collect ion. B& W's opera t ing exper ience wi th U-b eam s has


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been very successful . The U-bea ms have m ainta ined geomet ry and s t ructura l in tegr ity wi th no eros ion.

Erosion potent ial is low du e to the chro miu m oxide scale that forms on sta inless steel at furnac e operat ing

temperatures . Lower gas veloci ty through the U-bea m and des ign wi th a l l impact angles a t 90 d egrees are

a l so favourable . The U-b eam suppor t s have mainta ined thei r original condi t ion o ver t ime.

B&W CFB Boiler Impact Separator Offers Several Advantages U- be am separators, an integral part of the top- supported boi ler , are easy to instal l and repair .

No high tempera ture f lue gas expa nsion joints or refractory-l ined ducts are required.

Bui lding v olume i s reduced.

The uni form, low veloci ty gas f low across the width o f the boiler a t the furnace exi t reduces

the erosion potent ial in the upp er furnace an d the col lectors.

There is no thick refractory to l imit star t -up and load cha nge rates.

Mai ntena nce costs are lower beca use of less refractory.

Secondary Particle Separator System

The seco ndary sep arator is a conventional mult i -cyclon e dust col lector or the f i rst pass o f the ESP. The

small diameter low temperature col lect ing cans in the MDC al low for higher f ine part icle col lect ion

eff iciencies. The two stage part icle separat ion syste m with high eff iciency secon dary separat ion provid es

overa ll col lect ion ef f ic iencies wel l in excess of 99.7%. T his a l lows the B&W CFB to achieve the high er

furnace dens i t ies and uni form ver t ica l t emperature prof ile in the furnace . B&W 's co mme rcia l uni t s wi th

in- furnace U-Bea ms typica lly have furnace temperature var ia t ions a long the furnace heig ht of only abou t

25F (14C) at full load.

The M DC or f i rs t pass ESP helps manage inventory on especia lly low ash input fuels such as a low a sh

and low sulp hur coal . I t also enhan ces calcium ut i l izat ion and carbon burno ut of f ine material .

The co nvect ion pass i s des igned to accom modate the sol ids rec i rcula tion around the M DC or ESP loop.

The U-B eam s capture subs tant ia l ly al l mater ia l above 300 microns and a lmo st a ll mater ia l above 200

microns . This resul ts in dus t loadings through the convect ion pass which range f rom as low as 0 .05 lb(0.023 kg ) to 0.025 lb (0.1 lkg) per poun d (0.4536 kg) of f lue gas. B& W' s experience w ith conve ct ive

heat ing sur face per formance has been excellent. Convect ion pass tube eros ion i s m inimize d due to lower

flue gas veloci ty [30 to 40 f t /s (9.1 to 12.2 m/s)] and in-l ine tube arrangem ents.

The M DC or ESP sol ids hopper i s located a t a h igh elevation, which a l lows use of an a i r ass is ted, gravi ty

re turn recycle s ys te m f rom the secondary col lec tor . This sy s tem uses rotary valves wi th low pressure drop

to control sol ids f low. A small volu me of f luidizing air from the primary air fans al low the material to

mov e back into the furnace through mul t ip le re turn points across the width of the furnace rear wal l .


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Bed temperature cont rol i s enhan ced by us in g sol ids inventory located underne ath the mul t icyclone dus t

col lec tor or as a separa te hopp er in the case of f i r s t pass ESP col lec tion. Wh en the furnace temperature

increases above the target, bed material from the part icle storage is transferred to the furnace by increasin g

the recycle f low rate from the mu lt i -cyclone or the hopper. The increased invento ry of circulat ing material

enhanc es furnace heat t ransfer , thus reducing bed temperature.

Inversely, when th e bed tem perature decreases, the inventory of circulat ing sol ids in the furnac e is reduc ed

by s lowing dow n the recycle ra te f rom the MD C or the ESP hopper , and c i rcula t ing mater ia l is t ransfer redto s torage . This cont rol metho d i s used both d ur ing constant load opera tion and dur ing load change to

impr ove the load fol low ing capabil i ty and provide a wider turn-dow n rat io.

The cur rent des ign of the B&W two-s tage par tic le sePara t ion sys te m exceeds the per forman ce o f a s tand-

a lone cyclon e-based CFB sys tem, providing higher overa l l col lec t ion ef f ic iency. Design fea tures o f the

convect ion pass , mul t i -cyclone dus t col lec tor and dus t col lec tor recycle provide an econ omical sys te m

which a l so reduces eros ion potentia l and auxi l iary power consumpt ion.

Bed Drain and Cooling System

Bed ash i s purged f rom the furnace to control bed sol ids inventory

and remove oversized material that may enter the fuel . Material

exi t s the furnace through bed dra ins . These sol ids are a t bed

temperature and must be cooled prior to handling. Water-cooled

screws or f luidized-bed coolers are used to cool the material and

control the rate of m aterial drained.

I t i s desi rable to minim ize the amo unt of materia l dra ined f rom the

furnace because the high temperature a t wh ich i t i s drained resul ts

in a sensible heat loss. Str ict control of fuel size decreases the

amou nt of mater ia l tha t mu st be dra ined through the bed dra ins by

reducing the amo unt of overs ized mater ia l tha t enters wi th the fuel.

Sol ids exit ing the water -cooled screws pass through a screen which

remo ves mater ia l grea ter than 2000 microns . The screened sol ids

then enter the ash remov al sys tem.

With f luidized-bed coolers, part icles less than 350 microns are

injected back into the furnace.

Convective Heat Recove ry System

The ver tica l pendant type superheater des igned for a CFB boi ler i s unique and non-dra inable . The des ign

provides m etal temp erature protect ion during star t -up. A vert ical pendant su perheate r is located after the

four (4) rows o f external U-beams . Superheater eros ion potentia l i s cons iderably reduced due to very low

gas veloci t ies. Unifo rm gas distr ibution is ensured to the superheater for bet ter performance. Th e superheater

sect ions are encas ed with ei ther steam-co oled or water -cooled walls .

The econom iser i s des igned wi th bare tubes enclosed in a carbon s tee l cas ing. Economise r sur face i s

ar ranged in- line to avoid ash bui ld-up between the tube banks . Econom iser sur faces are des igned very

conservat ively due to varying convect ion pass dus t loadings and to accomm odate a range of fuel ash

content . B& W' s operat ing experien ce indicates that sootblowers are not required.


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Key Features / Benefits of B & W CFB Boilers

Techn ology is sui table to bu m a wide range of fuels, or opportuni ty fuels with less exp ensi ve fuel

preparat ion. (Fuels burned in B & W CFB boilers are high ash-waste coal, high su lfur coal , l igni te,pet . coke, anthraci te culm, wo od waste, etc. )

Use state-of- the-art CFB techno logy to achieve lower em issio ns levels. (Sulfur capture is >90 %

and NO x emiss ion i s


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FUEL, LIMESTONE, AND SAND HANDLING SYSTEMS

Fuel Feed System

The fuel flows from fuel storage bunkers located in front of the boiler to gravimetric feeders. Each

gravimetric feeder dischargers into a gravity feed chute. Air is injected at the base of each feed chute toensure a positive flow of fuel into the furnace.

Fuel is fed into the pr imary zone of the CFB furnace. The injection points are distributed across the width

of the furnace for uniform fuel feed. Furnace depth is designed for proper fuel mixing within the bed.

Limeston e Feed System

The limestone flows from the storage silo located adjacent to the boiler into gravimetric or volumetric

feeders which meter the quantity of limestone entering the unit. The feeders discharge via the feed chutes

into the primary zone of the furnace.

Inert Bed Material Feed System

When a fuel with a low ash and low sulfur content is used, it may be necessary to provide supplemental

inert solid bed material such as sand to maintain inventory in the circulating bed. Increased limestone

feed rate in most cases is not economical because, without substantial sulfation, limestone consumption

is high. Excess limestone, when calcined produces soft lime which breaks down quickly to very fine

particles that passes to the baghouse with little effect on the bed inventory.

EMISSIONS

Sulfu r Capture

Sulfur capture in the CFB process is achieved by adding limestone. The limestone is normally in the f orm

of calcium carbonate (CaCO3) with impurities of magnesium carbonate (MgCO3), plus aluminium and

iron oxide. When the limestone is added into the circulating fluidised bed at high temperature [1550 to

1650 F (843 to 899 C)], the CaCO 3 undergoes endothermic reactions to become CaO and CO2. Fuel

sulfur oxidizes to become SO 2. In the presence of oxygen, the CaO reacts exothermically with SO2 to

form CaSO4 (calcium sulfate), thus capturing the sulfur. The calcium sulfate is in the form of solid material,

which can be drained from the bed. The reactions are :

CaCO3 -- - - --> CaO + CO2(endothermic reaction)

CaO + 1/2 0 2+ SO2 --> CaSO 4 (exothermic reaction)

Sulfur capture is influenced by various factors such as fuel properties. Sulfur content, calcium to sulfur

molar ratio, limestone reactivity, furnace temperature, gas and solids residence time, and limestone particle

size.


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N O R e d u c t i o nx

Low NO emiss ions are a major benefit of CFB boilers. When burning fuel in CFB boilers, approximate lyx

50 to 70% of the combustion air flow enters through the grid as primary air. The substoichiometric

amount of air suppresses volatile nitrogen oxidation to NOx by creating a fuel-rich zone in the fuel

devolatilization region. The secondary air is added further above the lower reducing zone. Since the fuel

nitrogen is already transformed into molecular nitrogen, formation of NOx above this zone is controlled.

The relatively low combustion temperature [1550 to 1650 F (843 to 899 C)] also helps reduce NO x

formation.

NO emissions in CFB boilers are influenced by various factors including nitrogen and volatile matter inx

the fuel, furnace temperature, excess air, bed stoichiometry and limestone feed rate.

Additional NO x reduction (say 40 to 60% of the CFB process NOx) can be achieved by injecting ammonia

(NH3) either before or after the U-beams. The factors influencing additional NOx reduction are NH3/NOx

molar ratio, initial NO x Concentration, furnace temperature, degree of NH3 mixing and gas residencetime.

C O E m i s s i o n s

CO emissions from a CFB boiler are generally very low. The formation of CO is due to incomplete

combustion and is a function of many parameters such as bed temperature, excess air, type of fuel, non-

uniform fuel distribution, overfire air/gas mixing, and gas residence time in the furnace.


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T B W C F B B o ile r

O perab i li ty Strength

Key Features Key Benefits1. Excellent turndown withou t auxiliary

fuel (up to 5:1).1. Reduced opera ting costs at low load

operation.

. All in te rnal prim ary solids recycle andgravity feed second ary solids recyclewith FD fan or PA fan air.

. Reduced auxiliary pow er consumptioncompare d to using high press ureblowers.

,

4.

High solids co llection efficiency withtwo stages (>99.8%).

The en tire CFB unit has thin refractoryinstalled.

3. Increased com bustion efficiency andreduced operating cost.

4. Reduced start up time and reducedoperating costs. (Hot cyclone withthick refractory has prolonged start up).

Key Features

Lower Maintenance/Higher Reliabil ity

Key Benefits.

.

.

All -internal p rim ary solids recyclesystem (U-beam s) within a furnace.

Low er velocities in the furnace, furnaceexit, U-beams, and supe rheater

No s ootblowers required in theconvection pass

1. Avoid high maintenance thick refractory(ex :hot cyclone) Avoid forced outage concerns due

to thick refractory failure, andspecial teams required to reinstallrefractory.

2. Reduced erosion potential due to lowgas velocities. (Highv elocity gas/solids

entering cyclone leads to erosion). TBW's CFB had no erosion

maintenance on U-beams afterseveral years of operation.

Reduced superheater erosionpotential.

3. Avoid maintenance and forced outageon convective surface failures causedby sootblowers


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Circulating Fluidized-Bed Boiler Experience(B&W, AE&E, B&W JV)

C u s t o m e r

P l a n t C a p a c i t y S t a r t - U pLoc at ion Ib /hr ( t /hr ) Fuel Da te

Ultrapower

Ul t rapower

Sithe Energy

Lauhoff Grain Co.

Ebensburg Pow er Co.

Pusan Dyeing Co.

Tha i Petrochemical Ind.

Kanor ia Chemicals &Industries Ltd.

Southern I l linoisUniversity

Los Angeles CountySan itation District(3 boilers)

W est Enfield, 220,000 Wood wa stes 1986Maine USA (100) & wood chips

Jonesboro, 220,000 Wood was tes 1986Maine US A (100) & wood chips

Ma rysvil le, 164,000 Wood wa ste s 19"86Ca lifornia US A (74.3)

Danvi l le, 225,800 Bituminou s 1989

Ill inios US A (102.4) coal

Ebensburg, 465,00 0 High ash 1990Penn sylvania USA (211) was te coal

Pusa n 176,370 Coal & hea vy 1991Republic of Korea - (80) oil

Rayong, 286,6 00 Coal l ignite, 1994Th ailand (130) oil & gas

Renukoot, 231,48 0 High ash coal 1995India (105)

Carbo ndale, 120,000 Coal, petroleum 1996Ill inois, US A (54.4) coke & natural gas

Carson, 48,000 Sew age sludge --California, US A (21.8)

Our Branch Offices :

MUMBAI

Dhanra j M ahal, 2nd floor, ChhatrapatiShivaji Maharaj Marg, Near Gatewayof India, C olaba, Mumbai 400 03 9Tel. : 91-22-2045391, 2045324

Fax. : 91-22-2040859

CHENNAI

610, Anna Salai,Chennai 600 006Tel; : 91-44-8271891, 8272007

Telex : 041-7886 TMAX INFax. : 91-44-8277401

NEW DELHI

9, Commu nity Centre, Basant Lok

New Delhi 110 057Tel. : 91-11-6145319, 614532 6Telex : 031-72013 TMA X INFax. : 91-11-6148679


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Circulating Fluidized-Bed Boiler Experience


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circulating fluidized bed technology

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