babcock & wilcox circulating fluidized-bed boiler - Thermax
babcock & wilcox circulating fluidized-bed boiler - Thermax
babcock & wilcox circulating fluidized-bed boiler - Thermax
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BABCOCK & WILCOX<br />
CIRCULATING FLUIDIZED-BED BOILER<br />
An Overview of B& W CFB Boiler Technology<br />
The Babcock & Wilcox <strong>circulating</strong> <strong>fluidized</strong>-<strong>bed</strong> (CFB) <strong>boiler</strong> is designed for high reliability and availability<br />
with low maintenance, while complying with stringent emission regulations B&W's CFB technology is<br />
unique and includes a simple U-beam particle separator design. This is the result of extensive research<br />
and development and commercial operating experience.<br />
B& W and its joint venture companies have sold more than 38 <strong>fluidized</strong>-<strong>bed</strong> projects of which 10 are<br />
atmospheric <strong>circulating</strong> <strong>fluidized</strong>-<strong>bed</strong> <strong>boiler</strong>s.<br />
CFB PROCESS<br />
• How the B&W CFB Internal Circulation Boiler Works<br />
In a <strong>circulating</strong> <strong>fluidized</strong>-<strong>bed</strong> <strong>boiler</strong>, a portion of combustion air is introduced through the bottom of the<br />
<strong>bed</strong>. The <strong>bed</strong> material normally consists of fuel, limestone and ash. The bottom of the <strong>bed</strong> is supported by<br />
water-cooled membrane walls with specially designed air nozzles which distribute the air uniformly. The<br />
fuel and limestone (for sulfur capture) are fed into the lower <strong>bed</strong>. In the presence of fluidizing air, the fuel<br />
and limestone quickly and uniformly mix under the turbulent environment and behave like a fluid. Carbon<br />
particles in the fuel are exposed to the combustion air. The balance of combustion air is introduced at the<br />
top of the lower, dense <strong>bed</strong>. This staged combustion limits the formation of nitrogen oxides (NO).<br />
The <strong>bed</strong> fluidizing air velocity is greater than the terminal velocity of most of the particles in the <strong>bed</strong> and<br />
thus fluidizing air elutriates the particles through the combustion chamber to the U-beam separators at the<br />
furnace exit. The captured solids, including any unburned carbon and unutilized calcium oxide (CaO),<br />
are reinjected directly back into the combustion chamber without passing through an external recirculation.<br />
This internal solids circulation provides longer residence time for fuel and limestone, resulting in good<br />
combustion and improved sulfur capture.<br />
• CFB Steam Generator<br />
The CFB <strong>boiler</strong> is arranged with a single furnace having full-height/partial depth straight-tube division<br />
wails with or without steam-cooled wing wails. The furnace and particle separator enclosure walls are<br />
composed of water-cooled membrane tubes. The superheater enclosure is a combination of steam-cooled<br />
and water-cooled membrane tubes.<br />
Feedwater enters the unit at the economizer inlet, flows through the economizer banks in the convection<br />
pass to the outlet header, and then to the steam drum feedwater inlet. Water in the drum passes through
large downcomers and multiple supply tubes to feed the enclosure walls and division walls. Steam-water<br />
mixtures from the various circuits flow through headers and riser tribes back to the drum.<br />
Saturated steam is routed from the drum to the superheater enclosure side wails (if supplied) and then to<br />
the primary superheater located in the convection pass. Steam then travels to the inlet headers of the<br />
superheater wing walls (if supplied) in the furnace. Steam passing through the wing walls is collected and<br />
routed back to the secondary superheater through spray attemperators. The steam passes through the<br />
secondary superheater and discharges to the outlet terminal adjacent to the <strong>boiler</strong>.<br />
CFB BOILER MAJOR SYSTEMS<br />
• Circulating Fluidized-Bed Furnace<br />
The furnace design has been developed from B&W's 30 years of experience with <strong>fluidized</strong>-<strong>bed</strong> technology.<br />
The mechanics of fuel and limestone feed, air distribution, start-up system, refractory, <strong>bed</strong> drains, watercooled<br />
walls, etc., are based on research and development and commercial operating units.<br />
The CFB furnace operates as an extended <strong>fluidized</strong> <strong>bed</strong> of solid particles. Most of these entrained solids<br />
recirculate within the furnace or are captured by the primary impact separator (U-beams) at the furnace<br />
exit and are returned internally to the bottom of the furnace.<br />
Combustion.air is admitted to the furnace as follows:<br />
• Primary air through a bubble cap air distributor in the furnace bottom.<br />
• Secondary air through nozzles and material injection points at two levels in<br />
the lower furnace.<br />
The region of the furnace below the lower secondary air level is called the primary zone.<br />
The <strong>circulating</strong> <strong>fluidized</strong> <strong>bed</strong> forms two distinct regions:<br />
• Dense <strong>bed</strong> in the primary zone { 14 ft/s (4.25 m/s)}.<br />
• Dilute <strong>bed</strong> in the middle and upper furnace { 19.7 ft/s (6 m/s) }.<br />
The transition between these two regions is gradual, and operating experience indicates low erosion<br />
potential.<br />
The solids densities in the dilute <strong>bed</strong> and transitional regions of a <strong>circulating</strong>-<strong>bed</strong> combustor are relatively<br />
high. This results in higher rates of gas-solids reactions for combustion, sulfur capture and heat transfer<br />
between the <strong>bed</strong> and the furnace walls. The furnace height is selected to maximize carbon burnout and<br />
sulfur capture. B&W operates higher solids densities compared to other suppliers to optimize sulfur<br />
capture and heat transfer.
The material separated by the U-beam primary separator at the furnace exit is returned to the lower<br />
furnace by gravity, falling as a curtain along the rear furnace wall. In the lower furnace, these solids are re-<br />
entrained by primary and secondary air and carried back to the furnace exit. This intensive furnace back-<br />
mixing provides uniform distribution and optimum residence time. Finer solids not collected by the<br />
primary separator are carded by gases through the convection pass, are collected by the secondary separator,<br />
and are recirculated to the lower furnace.<br />
• Dense Bed in the Primary Zone<br />
The primary zone design provides for uniform distribution and intensive mixing of primary combustion<br />
air and <strong>bed</strong> solids supplied by material feed systems and recirculated from the primary and secondary<br />
separators.<br />
The cross section of the primary zone is reduced relative to the upper furnace to promote good mixing and<br />
turbulence and to ensure solids entrainment throughout the <strong>boiler</strong> load range. The high rate of mass<br />
transfer in the primary zone provides intense combustion and calcination/sulfation reactions.<br />
The substoichiometric conditions in the primary zone promote conversionof fuel nitrogen compounds to<br />
elemental nitrogen, thus reducing NO x formation.<br />
• DiLute Bed in the Middle and Upper Furnace<br />
The middle and upper sections of the CFB furnace are designed for the following:<br />
• sufficient residence time for fuel burnout and sulfur capture,<br />
• high solids inventory for improved heat transfer rates and sorbent reaction surface,<br />
• heat transfer surface of the enclosure walls, in-furnace division, and wing wails, and<br />
• good mixing of secondary air and combustion products.<br />
AIR AND GAS SYSTEMS<br />
Air from the primary air fan or forced draft fan (single fan option) is heated by a steam coil air heater and<br />
flows across a partitioned section of the tubular air heater. It is then directed to a water-cooled windbox at<br />
the bottom of the furnace. This windbox is divided into many compartments across the width of the unit,<br />
with dampers to control the flow of primary air to each compartment. A portion of the primary is also<br />
admitted to the furnace through the fuel chutes. A duct burner is installed in each main primary air feed<br />
duct to facilitate <strong>boiler</strong> start-up.<br />
Air from the secondary air fan or forced draft fan is heated by a steam coil air heater and passes through<br />
the secondary section of the tubular air heater. This secondary air flows to the distributing nozzles located<br />
across the width of the furnace on both the front and rear wails. Dampers vary the proportions of the<br />
secondary air to the front and rear distributing nozzles.
A portion of the secondary air is admitted to the furnace through over<strong>bed</strong> burners.<br />
The flue gas with entrained solids leaves the furnace through the U-beam primary particle separator, and<br />
passes through the convective heat recovery pass to the secondary separator. The flue gas with any remaining<br />
fine particles continue through the air heater. Most of these remaining fine particles are removed at the<br />
baghouse or electrostatic precipitator (ESP).<br />
SOLIDS SEPARATION SYSTEM<br />
The solids separation systems is a key element of any CFB <strong>boiler</strong> design, influencing both capital and<br />
operating costs. The separation system affects the solid inventory in the furnace, which impacts furnace<br />
temperature control (furnace heat transfer), carbon burnout and limestone utilization. The B&W CFB<br />
<strong>boiler</strong> uses a two-stage solids separation system:<br />
Primary particle separators - U- beams<br />
• Secondary particle separator system- multi-cyclone dust collector (MDC) or ESP<br />
• The Primary Particle Separator (U-Beams)<br />
B&W uses an impact for primary particle collection, which is different from hot cyclones commonly used<br />
for CFB <strong>boiler</strong>s. B&W's impact separator is unique in CFB <strong>boiler</strong> design.<br />
Impact separators have been used for several decades to separate particles greater than 100 microns. The<br />
dust laden gas stream impinges on the staggered vertical arrangement of U-beam channels. B&W has<br />
conducted considerable research and development on impact separators on the Cold Model Test Facility<br />
at B&W's Alliance Research Center. Geometrical correlations have been developed based on operating<br />
variables such as gas velocity; solids loading, number of channel rows, and particle size. These relationships<br />
have been applied successfully to B&W's commercial CFB <strong>boiler</strong> designs.<br />
B&W primary solids collector consists of two (2) rows of U-beams located within the furnace at the gas<br />
exit and four (4) additional rows of U-beams located immediately downstream of the in-furnace U-beams.<br />
Solids collected by the front two rows discharge downward directly to the furnace along the rear wall.<br />
Solids collected by the rear rows of U- beams discharge into a hopper integral to the furnace rear wall and<br />
return by gravity to the furnace through openings distributed across the width of the unit.<br />
These U-beams are made of stainless steel. Individual U-beams are in the form of channels six inches<br />
(152 ram) wide by seven inches (178 mm) deep. Two bolts through the water cooled roof suspend each<br />
beam, protected by an enclosure. Dynamic (gas and solids) stresses, static (dead load) stresses, design<br />
temperatures and material creep strength are used to design the U-beams.<br />
A pan at the lower end of each U-beam holds the U-beam in alignment and accommodates horizontal and<br />
vertical thermal expansion. These pans also form a gas barrier at the bottom discharge end of the beams<br />
to prevent gas bypassing and improve particle collection. B&W's operating experience with U-beams has
een very successful. The U-beams have maintained geometry and structural integrity with no erosion.<br />
Erosion potential is low due to the chromium oxide scale that forms on stainless steel at furnace operating<br />
temperatures. Lower gas velocity through the U-beam and design with all impact angles at 90 degrees are<br />
also favourable. The U-beam supports have maintained their original condition over time.<br />
B&W CFB Boiler Impact Separator Offers Several Advantages<br />
• U-beam separators, an integral part of the top- supported <strong>boiler</strong>, are easy to install and repair.<br />
• No high temperature flue gas expansion joints or refractory-lined ducts are required.<br />
Building volume is reduced.<br />
The uniform, low velocity gas flow across the width of the <strong>boiler</strong> at the furnace exit reduces<br />
the erosion potential in the upper furnace and the collectors.<br />
• There is no thick refractory to limit start-up and load change rates.<br />
• Maintenance costs are lower because of less refractory.<br />
• Secondary Particle Separator System<br />
The secondary separator is a conventional multi-cyclone dust collector or the first pass of the ESP. The<br />
small diameter low temperature collecting cans in the MDC allow for higher fine particle collection<br />
efficiencies. The two stage particle separation system with high efficiency secondary separation provides<br />
overall collection efficiencies well in excess of 99.7%. This allows the B&W CFB to achieve the higher<br />
furnace densities and uniform vertical temperature profile in the furnace. B&W's commercial units with<br />
in-furnace U-Beams typically have furnace temperature variations along the furnace height of only about<br />
25°F (14°C) at full load. •<br />
The MDC or first pass ESP helps manage inventory on especially low ash input fuels such as a low ash<br />
and low sulphur coal. It also enhances calcium utilization and carbon burnout of fine material.<br />
The convection pass is designed to accommodate the solids recirculation around the MDC or ESP loop.<br />
The U-Beams capture substantially all material above 300 microns and almost all material above 200<br />
microns. This results in dust loadings through the convection pass which range from as low as 0.05 lb<br />
(0.023 kg) to 0.025 lb (0.1 lkg) per pound (0.4536 kg) of flue gas. B&W's experience with convective<br />
heating surface performance has been excellent. Convection pass tube erosion is minimized due to lower<br />
flue gas velocity [30 to 40 ft/s (9.1 to 12.2 m/s)] and in-line tube arrangements.<br />
The MDC or ESP solids hopper is located at a high elevation, which allows use of an air assisted, gravity<br />
return recycle system from the secondary collector. This system uses rotary valves with low pressure drop<br />
to control solids flow. A small volume of fluidizing air from the primary air fans allow the material to<br />
move back into the furnace through multiple return points across the width of the furnace rear wall.
Bed temperature control is enhanced by using solids inventory located underneath the multicyclone dust<br />
collector or as a separate hopper in the case of first pass ESP collection. When the furnace temperature<br />
increases above the target, <strong>bed</strong> material from the particle storage is transferred to the furnace by increasing<br />
the recycle flow rate from the multi-cyclone or the hopper. The increased inventory of <strong>circulating</strong> material<br />
enhances furnace heat transfer, thus reducing <strong>bed</strong> temperature.<br />
Inversely, when the <strong>bed</strong> temperature decreases, the inventory of <strong>circulating</strong> solids in the furnace is reduced<br />
by slowing down the recycle rate from the MDC or the ESP hopper, and <strong>circulating</strong> material is transferred<br />
to storage. This control method is used both during constant load operation and during load change to<br />
improve the load following capability and provide a wider turn-down ratio.<br />
The current design of the B&W two-stage particle seParation system exceeds the performance of a standalone<br />
cyclone-based CFB system, providing higher overall collection efficiency. Design features of the<br />
convection pass, multi-cyclone dust collector and dust collector recycle provide an economical system<br />
which also reduces erosion potential and auxiliary power consumption.<br />
• Bed Drain and Cooling System<br />
Bed ash is purged from the furnace to control <strong>bed</strong> solids inventory<br />
and remove oversized material that may enter the fuel. Material<br />
exits the furnace through <strong>bed</strong> drains. These solids are at <strong>bed</strong><br />
temperature and must be cooled prior to handling. Water-cooled<br />
screws or <strong>fluidized</strong>-<strong>bed</strong> coolers are used to cool the material and<br />
control the rate of material drained.<br />
It is desirable to minimize the amount of material drained from the<br />
furnace because the high temperature at which it is drained results<br />
in a sensible heat loss. Strict control of fuel size decreases the<br />
amount of material that must be drained through the <strong>bed</strong> drains by<br />
reducing the amount of oversized material that enters with the fuel.<br />
Solids exiting the water-cooled screws pass through a screen which<br />
removes material greater than 2000 microns. The screened solids<br />
then enter the ash removal system.<br />
With <strong>fluidized</strong>-<strong>bed</strong> coolers, particles less than 350 microns are<br />
injected back into the furnace.<br />
• Convective Heat Recovery System<br />
The vertical pendant type superheater designed for a CFB <strong>boiler</strong> is unique and non-drainable. The design<br />
provides metal temperature protection during start-up. A vertical pendant superheater is located after the<br />
four (4) rows of external U-beams. Superheater erosion potential is considerably reduced due to very low<br />
gas velocities. Uniform gas distribution is ensured to the superheater for better performance. The superheater<br />
sections are encased with either steam-cooled or water -cooled walls.<br />
The economiser is designed with bare tubes enclosed in a carbon steel casing. Economiser surface is<br />
arranged in-line to avoid ash build-up between the tube banks. Economiser surfaces are designed very<br />
conservatively due to varying convection pass dust loadings and to accommodate a range of fuel ash<br />
content. B&W's operating experience indicates that sootblowers are not required.
Key Features / Benefits of B & W CFB Boilers<br />
• Technology is suitable to bum a wide range of fuels, or opportunity fuels with less expensive fuel<br />
preparation. (Fuels burned in B & W CFB <strong>boiler</strong>s are high ash-waste coal, high sulfur coal, lignite,<br />
pet. coke, anthracite culm, wood waste, etc.)<br />
• Use state-of-the-art CFB technology to achieve lower emissions levels. (Sulfur capture is >90 %<br />
and NO x emission is
FUEL, LIMESTONE, AND SAND HANDLING SYSTEMS<br />
• Fuel Feed System<br />
The fuel flows from fuel storage bunkers located in front of the <strong>boiler</strong> to gravimetric feeders. Each<br />
gravimetric feeder dischargers into a gravity feed chute. Air is injected at the base of each feed chute to<br />
ensure a positive flow of fuel into the furnace.<br />
Fuel is fed into the primary zone of the CFB furnace. The injection points are distributed across the width<br />
of the furnace for uniform fuel feed. Furnace depth is designed for proper fuel mixing within the <strong>bed</strong>.<br />
• Limestone Feed System<br />
The limestone flows from the storage silo located adjacent to the <strong>boiler</strong> into gravimetric or volumetric<br />
feeders which meter the quantity of limestone entering the unit. The feeders discharge via the feed chutes<br />
into the primary zone of the furnace.<br />
• Inert Bed Material Feed System<br />
When a fuel with a low ash and low sulfur content is used, it may be necessary to provide supplemental<br />
inert solid <strong>bed</strong> material such as sand to maintain inventory in the <strong>circulating</strong> <strong>bed</strong>. Increased limestone<br />
feed rate in most cases is not economical because, without substantial sulfation, limestone consumption<br />
is high. Excess limestone, when calcined produces soft lime which breaks down quickly to very fine<br />
particles that passes to the baghouse with little effect on the <strong>bed</strong> inventory.<br />
• Sulfur Capture<br />
EMISSIONS<br />
Sulfur capture in the CFB process is achieved by adding limestone. The limestone is normally in the form<br />
of calcium carbonate (CaCO3) with impurities of magnesium carbonate (MgCO3), plus aluminium and<br />
iron oxide. When the limestone is added into the <strong>circulating</strong> fluidised <strong>bed</strong> at high temperature [1550 to<br />
1650 °F (843 to 899 °C)], the CaCO 3 undergoes endothermic reactions to become CaO and CO 2. Fuel<br />
sulfur oxidizes to become SO 2. In the presence of oxygen, the CaO reacts exothermically with SO 2 to<br />
form CaSO 4 (calcium sulfate), thus capturing the sulfur. The calcium sulfate is in the form of solid material,<br />
which can be drained from the <strong>bed</strong>. The reactions are :<br />
CaCO 3 -- -- --> CaO + CO 2 (endothermic reaction)<br />
CaO + 1/2 0 2 + SO 2<br />
--> CaSO 4 (exothermic reaction)<br />
Sulfur capture is influenced by various factors such as fuel properties. Sulfur content, calcium to sulfur<br />
molar ratio, limestone reactivity, furnace temperature, gas and solids residence time, and limestone particle<br />
size.
• NO Reduction<br />
x<br />
Low NO emissions are a major benefit of CFB <strong>boiler</strong>s. When burning fuel in CFB <strong>boiler</strong>s, approximately<br />
x<br />
50 to 70% of the combustion air flow enters through the grid as primary air. The substoichiometric<br />
amount of air suppresses volatile nitrogen oxidation to NO x by creating a fuel-rich zone in the fuel<br />
devolatilization region. The secondary air is added further above the lower reducing zone. Since the fuel<br />
nitrogen is already transformed into molecular nitrogen, formation of NO x above this zone is controlled.<br />
The relatively low combustion temperature [1550 to 1650 °F (843 to 899 °C)] also helps reduce NO x<br />
formation.<br />
NO emissions in CFB <strong>boiler</strong>s are influenced by various factors including nitrogen and volatile matter in<br />
x<br />
the fuel, furnace temperature, excess air, <strong>bed</strong> stoichiometry and limestone feed rate.<br />
Additional NO x reduction (say 40 to 60% of the CFB process NOx) can be achieved by injecting ammonia<br />
(NH3) either before or after the U-beams. The factors influencing additional NO x reduction are NH3/NO x<br />
molar ratio, initial NO x Concentration, furnace temperature, degree of NH 3 mixing and gas residence<br />
time.<br />
• CO Emissions<br />
CO emissions from a CFB <strong>boiler</strong> are generally very low. The formation of CO is due to incomplete<br />
combustion and is a function of many parameters such as <strong>bed</strong> temperature, excess air, type of fuel, nonuniform<br />
fuel distribution, overfire air/gas mixing, and gas residence time in the furnace.
TBW CFB Boiler<br />
Key Features<br />
Operability Strength<br />
Key Benefits<br />
1. Excellent turndown without auxiliary<br />
fuel (up to 5:1).<br />
. All internal primary solids recycle and<br />
gravity feed secondary solids recycle<br />
with FD fan or PA fan air.<br />
,<br />
4.<br />
High solids collection efficiency with<br />
two stages (>99.8%).<br />
The entire CFB unit has thin refractory<br />
installed.<br />
1. Reduced operating costs at low load<br />
operation.<br />
. Reduced auxiliary power consumption<br />
compared to using high pressure<br />
blowers.<br />
3. Increased combustion efficiency and<br />
reduced operating cost.<br />
4. Reduced start up time and reduced<br />
operating costs. (Hot cyclone with<br />
thick refractory has prolonged start up).<br />
Key Features<br />
Lower Maintenance/<br />
Higher Reliability<br />
Key Benefits<br />
.<br />
.<br />
.<br />
All-internal primary solids recycle<br />
system (U-beams) within a furnace.<br />
Lower velocities in the furnace, furnace<br />
exit, U-beams, and superheater<br />
No sootblowers required in the<br />
convection pass<br />
1. Avoid high maintenance thick refractory<br />
(ex :hot cyclone)<br />
• Avoid forced outage concerns due<br />
to thick refractory failure, and<br />
special teams required to reinstall<br />
refractory.<br />
2. Reduced erosion potential due to low<br />
gas velocities. (Highvelocity gas/solids<br />
entering cyclone leads to erosion).<br />
• TBW's CFB had no erosion<br />
maintenance on U-beams after<br />
• several years of operation.<br />
• Reduced superheater erosion<br />
potential.<br />
3. Avoid maintenance and forced outage<br />
on convective surface failures caused<br />
by sootblowers
Circulating Fluidized-Bed Boiler Experience<br />
(B&W, AE&E, B&W JV)<br />
Customer<br />
Ultrapower<br />
Ultrapower<br />
Sithe Energy<br />
Lauhoff Grain Co.<br />
Ebensburg Power Co.<br />
Pusan Dyeing Co.<br />
Thai Petrochemical Ind.<br />
Kanoria Chemicals &<br />
Industries Ltd.<br />
Southern Illinois<br />
University<br />
Los Angeles County<br />
Sanitation District<br />
(3 <strong>boiler</strong>s)<br />
Plant Capacity Start-Up<br />
Location Ib/hr (t/hr) Fuel Date<br />
West Enfield, 220,000 Wood wastes 1986<br />
Maine USA (100) & wood chips<br />
Jonesboro, 220,000 Wood wastes 1986<br />
Maine USA (100) & wood chips<br />
Marysville, 164,000 Wood wastes 19"86<br />
California USA (74.3)<br />
Danville, 225,800 Bituminous 1989<br />
Illinios USA (102.4) coal<br />
Ebensburg, 465,000 High ash 1990<br />
Pennsylvania USA (211) waste coal<br />
Pusan 176,370 Coal & heavy 1991<br />
Republic of Korea -(80) oil<br />
Rayong, 286,600 Coal lignite, 1994<br />
Thailand (130) oil & gas<br />
Renukoot, 231,480 High ash coal 1995<br />
India (105)<br />
Carbondale, 120,000 Coal, petroleum 1996<br />
Illinois, USA (54.4) coke & natural gas<br />
Carson, 48,000 Sewage sludge --<br />
California, USA (21.8)<br />
Our Branch Offices :<br />
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Shivaji Maharaj Marg, Near Gateway<br />
of India, Colaba, Mumbai 400 039<br />
Tel. : 91-22-2045391, 2045324<br />
Fax. : 91-22-2040859<br />
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610, Anna Salai,<br />
Chennai 600 006<br />
Tel; : 91-44-8271891, 8272007<br />
Telex : 041-7886 TMAX IN<br />
Fax. : 91-44-8277401<br />
NEW DELHI<br />
9, Community Centre, Basant Lok<br />
New Delhi 110 057<br />
Tel. : 91-11-6145319, 6145326<br />
Telex : 031-72013 TMAX IN<br />
Fax. : 91-11-6148679
Circulating Fluidized-Bed Boiler Experience