ISGEC John Thompson

DESIGN ASPECTS OF CIRCULATING FLUIDIZED BED COMBUSTION BOILER

ISGEC John Thompson

Basics of FluidisationFluidisation is originated from the word “FLUID” or ability to flow.Fluidisation is originated from the word “FLUID” or ability to flow.

In a fluidised bed all properties of fluids are incorporated like :In a fluidised bed all properties of fluids are incorporated like :-Fluid flows due to elevation difference.Fluid flows due to elevation difference.-It maintains horizontal levelIt maintains horizontal level-It exerts pressure at bottom proportional to the height of columnIt exerts pressure at bottom proportional to the height of column-It attains shape of vessel in which it is stored.It attains shape of vessel in which it is stored.-If two vessels are connected, a common level is maintained.If two vessels are connected, a common level is maintained.-The molecules can move without requirement of any external energy The molecules can move without requirement of any external energy randomly inside the boundary of the liquid ( Brownian motion).randomly inside the boundary of the liquid ( Brownian motion).-The molecules can not leave the boundary unless they reach the The molecules can not leave the boundary unless they reach the saturation temperature.saturation temperature.

In a fluidised bed, the particles of bed material attains all the above In a fluidised bed, the particles of bed material attains all the above properties of the fluids.properties of the fluids.

ISGEC John Thompson

Basics of FluidisationIn a fluidised bed, the particles of bed material attains all the above In a fluidised bed, the particles of bed material attains all the above properties of the fluids.properties of the fluids.

Consider a particle of bed material in non fluidised state. It exerts a Consider a particle of bed material in non fluidised state. It exerts a downwards force equal to its weight and hence can not move. If a stream downwards force equal to its weight and hence can not move. If a stream of air is allowed to flow below this particle, it experiences a upward drag of air is allowed to flow below this particle, it experiences a upward drag force equal to pressure of air x its cross section area. If we go on force equal to pressure of air x its cross section area. If we go on increasing the air pressure, a time will come when the drag force balances increasing the air pressure, a time will come when the drag force balances the weight of the particle and it will come in weightless condition and now it the weight of the particle and it will come in weightless condition and now it is free to move anywhere inside the bed without any additional energy. is free to move anywhere inside the bed without any additional energy. Now the bed attains all the properties of a fluid and experiences a very Now the bed attains all the properties of a fluid and experiences a very high turbulence and mixing. high turbulence and mixing.

This indicates that fluidisation system is highly sensitive to particle weight/ This indicates that fluidisation system is highly sensitive to particle weight/ size. Every system has its unique fluidisation velocity and the performance size. Every system has its unique fluidisation velocity and the performance changes with change in particle size.changes with change in particle size.

ISGEC John Thompson

Evenly distributed air is passed Evenly distributed air is passed upward through a finely divided bed upward through a finely divided bed of sand supported on a fine mesh, of sand supported on a fine mesh, the particles are undisturbed at low the particles are undisturbed at low velocity.As air velocity is gradually velocity.As air velocity is gradually increased, a stage is reached when increased, a stage is reached when the individual particles are the individual particles are suspended in the air stream. Further, suspended in the air stream. Further, increase in velocity gives rise to increase in velocity gives rise to bubble formation, vigorous bubble formation, vigorous turbulence and rapid mixing.turbulence and rapid mixing.

The bed of solid particles exhibits The bed of solid particles exhibits the properties of a boiling liquid. the properties of a boiling liquid. This state is called be fluidized.This state is called be fluidized.

Combustion takes place at about Combustion takes place at about 840840OOC to 950C to 950OOC.C.

Principle of fluidization

ISGEC John ThompsonCirculating fluidized bed.

Relation between gas velocity and solid velocity

With higher air velocities, the bed particles leave the combustion with the flue gases so that solids recirculation is necessary to maintain circulating fluidized bed.

The mean solids velocity increases at a slower rate than does the gas velocity, as illustrated in Figure

Therefore, a maximum slip velocity between the solids and the gas can be achieved resulting in good heat transfer and contact time with the limestone, for sulfur dioxide removal.

ISGEC John ThompsonAFBC or Bubbling Bed

Features of bubbling bed boilerFeatures of bubbling bed boiler   AFBC use the in-bed evaporator tubes AFBC use the in-bed evaporator tubes

for extracting the heat from the bed to for extracting the heat from the bed to maintain the bed temperature. maintain the bed temperature.

The bubbling bed has heat transfer tubes The bubbling bed has heat transfer tubes in the bed of limestone, sand and fuel. in the bed of limestone, sand and fuel. The velocity of fluidising air is in the The velocity of fluidising air is in the range of 1.2 to 3.7 m /sec. Very little range of 1.2 to 3.7 m /sec. Very little material leaves the bubbling bed – about material leaves the bubbling bed – about 2 to 4 kgs of solids are recycled per kg 2 to 4 kgs of solids are recycled per kg of fuel burned.of fuel burned.

The bed depth is usually 0.9 m to 1.5 The bed depth is usually 0.9 m to 1.5 meter deep and the pressure drop meter deep and the pressure drop averages about 1 inch of water per inch averages about 1 inch of water per inch of bed depth. of bed depth.

ISGEC John ThompsonFeatures of bubbling bed boiler

Coal is crushed to a size of 1 – 6 mm (depending on coal Coal is crushed to a size of 1 – 6 mm (depending on coal characteristics) and fed to the combustion chamber.characteristics) and fed to the combustion chamber.

The atmospheric air, which acts as both the fluidization air and The atmospheric air, which acts as both the fluidization air and combustion air, is delivered at a pressure, and flows through the combustion air, is delivered at a pressure, and flows through the bed after being preheated by the exhaust flue gases. The in-bed bed after being preheated by the exhaust flue gases. The in-bed tubes carrying water generally act as the evaporator.tubes carrying water generally act as the evaporator.

If temperature exceeds 950If temperature exceeds 950ooC , there is risk of clinker formation in C , there is risk of clinker formation in the bed and combustion efficiency declines below 800 the bed and combustion efficiency declines below 800ooC.C.

For efficient sulphur retention the temperature should be in the For efficient sulphur retention the temperature should be in the range of 800range of 800ooC to 850C to 850ooC.C.

ISGEC John Thompson

CFBC boiler is a device for generating steam by CFBC boiler is a device for generating steam by burning fossil fuel in a furnace operated under a burning fossil fuel in a furnace operated under a special hydrodynamic conditions where fine special hydrodynamic conditions where fine solids are transported through a furnace at a solids are transported through a furnace at a velocity exceeding free fall velocity of individual velocity exceeding free fall velocity of individual particles.particles.

The major fraction of the solids entrained out of The major fraction of the solids entrained out of the vessel is captured by a cyclone separator the vessel is captured by a cyclone separator and is circulated back to a point near the base and is circulated back to a point near the base of furnace at sufficiently high rate to cause of furnace at sufficiently high rate to cause refluxing of solid in furnace. Creation of special refluxing of solid in furnace. Creation of special hydrodynamic condition is key to this process.hydrodynamic condition is key to this process.

FUNDAMENTALS OF CIRCULATING FUNDAMENTALS OF CIRCULATING FLUIDIZED BED BOILER (CFBC)FLUIDIZED BED BOILER (CFBC)

ISGEC John Thompson

A certain combination of gas velocity, recirculation rate, A certain combination of gas velocity, recirculation rate, solid characteristics, volume of the solids, geometry of the solid characteristics, volume of the solids, geometry of the system gives rise to this special hydrodynamic condition system gives rise to this special hydrodynamic condition where solid particles are subjected to a upward velocity where solid particles are subjected to a upward velocity greater than free falling velocity of most individual particles. greater than free falling velocity of most individual particles. Solids are moved up & down in groups called clusters. Solids are moved up & down in groups called clusters.

Clusters move vertically, sideways, as well as down wards. Clusters move vertically, sideways, as well as down wards. They are continuously formed dissolved & reformed again.They are continuously formed dissolved & reformed again.

Fuel particle burn generating heat. Part of heat is extracted Fuel particle burn generating heat. Part of heat is extracted by water/steam surfaces located in the furnace at very high by water/steam surfaces located in the furnace at very high

heat transfer rateheat transfer rate..

FUNDAMENTALS OF CIRCULATING FUNDAMENTALS OF CIRCULATING FLUIDIZED BED BOILER (CFBC)FLUIDIZED BED BOILER (CFBC)

ISGEC John Thompson

BASICS OF CIRCULATING FLUIDIZED BED COMBUSTION CFB, uses greater combustor superficial gas velocities than

the BFB. CFB operates under a special fluid dynamic condition, in

which the fine solids particles are transported and mixed through the furnace at a gas velocity exceeding the average terminal velocity of the particles. The major fraction of solids leaving the furnace is captured by a solids separator and recirculated back to the base of the furnace. The high recycle rate intensifies solids mixing and evens out combustion temperatures in the furnace.

CFB systems operate in a fluid dynamic region between that of BFB and a transport reactor (pulverized combustion). This fluidization regime is characterized by high turbulence, solid mixing and the absence of a defined bed level. Instead of a well‑defined solids bed depth, the solids are distributed throughout the furnace with a steadily decreasing density from the bottom to the top of the furnace.

ISGEC John Thompson

CFB is characterized by:

** High fluidizing velocity of 4.0 -6.0 m/s (13 – 20 ft/s).High fluidizing velocity of 4.0 -6.0 m/s (13 – 20 ft/s).** Dense bed region in lower furnace without a distinct Dense bed region in lower furnace without a distinct

bed levelbed level** Water-cooled membrane walls.Water-cooled membrane walls.** Optional in-furnace heat transfer surfaces located Optional in-furnace heat transfer surfaces located

above the dense lower bed above the dense lower bed ** Solids separator to separate entrained particles from the Solids separator to separate entrained particles from the

flue gas stream and recycle them to the lower furnace.flue gas stream and recycle them to the lower furnace.** Aerated sealing device, loop seal, which permits return Aerated sealing device, loop seal, which permits return

of collected solids back to the furnaceof collected solids back to the furnace

ISGEC John Thompson

CFB Separator A solids separator located at the outlet of the combustion chamber A solids separator located at the outlet of the combustion chamber

separates entrained particles from the flue gas stream. The separator separates entrained particles from the flue gas stream. The separator is designed for a very high solid collection efficiency with nearly 100 is designed for a very high solid collection efficiency with nearly 100 per cent efficiency for particles greater than 60 microns in diameter. per cent efficiency for particles greater than 60 microns in diameter.

The collected solids are returned to the combustion chamber via the The collected solids are returned to the combustion chamber via the loop seal, which provides a pressure seal between the positive loop seal, which provides a pressure seal between the positive pressure in the lower furnace and the negative draft in the solids pressure in the lower furnace and the negative draft in the solids separator. This prevents the furnace flue gas from short circuiting up separator. This prevents the furnace flue gas from short circuiting up the separator dipleg and collapsing the separator collection efficiency. the separator dipleg and collapsing the separator collection efficiency. The recirculation system has no moving parts and its operation has The recirculation system has no moving parts and its operation has proven to be simple and reliable. By injecting small amounts of high proven to be simple and reliable. By injecting small amounts of high pressure fluidizing air into the loop seal, the solids movement back to pressure fluidizing air into the loop seal, the solids movement back to the lower furnace is maintained.the lower furnace is maintained.

ISGEC John ThompsonCirculating Fluidised Bed Combustion (CFBC)

(1- 6 mm size) fuel and limestone are injected into the (1- 6 mm size) fuel and limestone are injected into the furnace furnace

The particles are suspended in a stream of upwardly The particles are suspended in a stream of upwardly flowing air (60-70% of the total air), which enters the flowing air (60-70% of the total air), which enters the bottom of the furnace through air distribution nozzles. bottom of the furnace through air distribution nozzles. The balance of combustion air is admitted above the The balance of combustion air is admitted above the bottom of the furnace as secondary air.bottom of the furnace as secondary air.

While combustion takes place at 840-980While combustion takes place at 840-980ooC, the fine C, the fine particles (<450 microns) are elutriated out of the particles (<450 microns) are elutriated out of the furnace with flue gas velocity of 4-6 m/s. The particles furnace with flue gas velocity of 4-6 m/s. The particles are then collected by the solids separators and are then collected by the solids separators and circulated back into the furnace. This combustion circulated back into the furnace. This combustion process is called circulating fluidized bed (CFB). process is called circulating fluidized bed (CFB).

ISGEC John ThompsonCirculating Fluidised Bed Combustion (CFBC)

There are no steam generations tube immersed There are no steam generations tube immersed in the bed. Generation and super heating of in the bed. Generation and super heating of steam takes place in the convection section, steam takes place in the convection section, water walls, at the exit of the riser.water walls, at the exit of the riser.

CFBC boilers are generally more economical CFBC boilers are generally more economical than AFBC boilers for industrial application than AFBC boilers for industrial application requiring more than 75 – 100 T/hr of steam. requiring more than 75 – 100 T/hr of steam.

ISGEC John Thompsonperformance of CFBC

The temperature of about 870The temperature of about 870ooC is reasonably constant throughout the process C is reasonably constant throughout the process because of the high turbulence and circulation of solids. The low combustion because of the high turbulence and circulation of solids. The low combustion temperature also results in minimal NOx formation.temperature also results in minimal NOx formation.

  Sulfur present in the fuel is retained in the circulating solids in the form of Sulfur present in the fuel is retained in the circulating solids in the form of calcium sulphate is removed in solid form. The use of limestone or dolomite calcium sulphate is removed in solid form. The use of limestone or dolomite sorbents allows a higher sulfur retention rate, and limestone requirements have sorbents allows a higher sulfur retention rate, and limestone requirements have been demonstrated to be substantially less than with bubbling bed combustor.been demonstrated to be substantially less than with bubbling bed combustor.

  The combustion air is supplied at 1.5 to 2 psig rather than 3-5 psig as required The combustion air is supplied at 1.5 to 2 psig rather than 3-5 psig as required by bubbling bed combustors. by bubbling bed combustors.

It has high combustion efficiency.It has high combustion efficiency. It has a better turndown ratio than bubbling bed systems.It has a better turndown ratio than bubbling bed systems.

ISGEC John ThompsonAdvantages of Cirulating Fluidised Bed Combustion Boilers

High Efficiency: High Efficiency:

Reduction in Boiler Size:Reduction in Boiler Size:..

Fuel Flexiblity: FFuel Flexiblity: Fuels like washer rejects, agro waste can be burnt efficiently.uels like washer rejects, agro waste can be burnt efficiently.

BBoilers can fire coals with ash content as high as 62% and having calorific value oilers can fire coals with ash content as high as 62% and having calorific value as low as 2,500 kcal/kg.as low as 2,500 kcal/kg.

Pollution Control;Pollution Control;SOSO22 formation can be greatly minimised by addition of formation can be greatly minimised by addition of

limestone or dolomite for high sulphur coals. Eliminates NOlimestone or dolomite for high sulphur coals. Eliminates NOxx formation.  formation. 

Low Corrosion and Erosion:Low Corrosion and Erosion:are less due to lower combustion temperature, are less due to lower combustion temperature, softness of ash and low particle velocity (of the order of 1 m/sec). softness of ash and low particle velocity (of the order of 1 m/sec). 

No Clinker Formation :No Clinker Formation : temperature of the furnace is in the range of 750–900 temperature of the furnace is in the range of 750–900oo C. C.

Less Excess Air – Higher COLess Excess Air – Higher CO22 in Flue Gas: in Flue Gas:20 – 25% excess air only. 20 – 25% excess air only. 

No Slagging in the Furnace-No Soot Blowing:No Slagging in the Furnace-No Soot Blowing:volatilisation of alkali volatilisation of alkali components in ash does not take place and the ash is non stocky. components in ash does not take place and the ash is non stocky. 

ISGEC John Thompson

ADVANTAGES OF CFBCADVANTAGES OF CFBC FUEL FLEXIBILITY - CFBC BOILER CAN BURN A WIDE FUEL FLEXIBILITY - CFBC BOILER CAN BURN A WIDE

RANGE OF FUELS WITH GOOD EFFICIENCY WITH OUT RANGE OF FUELS WITH GOOD EFFICIENCY WITH OUT MUCH CHANGE IN HARDWARES OF THE FURNACEMUCH CHANGE IN HARDWARES OF THE FURNACE

HIGH COMBUSTION EFFICIENCY - IT CAN BURN FUEL HIGH COMBUSTION EFFICIENCY - IT CAN BURN FUEL PARTICLES OF WIDE SIZE WITH VERY HIGH COMBUSTION PARTICLES OF WIDE SIZE WITH VERY HIGH COMBUSTION EFFICIENCY OF 98% EFFICIENCY OF 98%

LOW AIR POLLUTION - NOLOW AIR POLLUTION - NOXX EMISSION IS BELOW EMISSION IS BELOW

ACCEPTABLE LIMIT. IT CAN REDUCE SOACCEPTABLE LIMIT. IT CAN REDUCE SOXX EMISSION BY EMISSION BY

95% WITH MINIMUM USE OF LIMESTONE95% WITH MINIMUM USE OF LIMESTONE

GOOD TURNDOWN CAPABILITY - IT CAN RESPOND GOOD TURNDOWN CAPABILITY - IT CAN RESPOND FLUCTUATING LOAD DEMAND MUCH EASILYFLUCTUATING LOAD DEMAND MUCH EASILY

SMALLER FEED POINTS - IT REQUIRES ONLY A FRACTION SMALLER FEED POINTS - IT REQUIRES ONLY A FRACTION OF FEED POINTS OF AN AFBC BOILER OF SIMILAR SIZE OF FEED POINTS OF AN AFBC BOILER OF SIMILAR SIZE WOULD REQUIRE.WOULD REQUIRE.

ISGEC John Thompson

SPECIAL COMPONENTS OF CFBC BOILERS

- COMBUSTOR

- COMPACT SEPARATOR

- STEAM COOLED CONVECTION CAGE

- BOTTOM ASH REMOVAL SYSTEM – STRIPPER COOLER

- FUEL FEEDING SYSTEM

- AIR DISTRIBUTION SYSTEM

ISGEC John Thompson

COMBUSTOR -- CONSISTS OF MEMBRANE WALL GAS TIGHT CONSISTS OF MEMBRANE WALL GAS TIGHT

ENCLOSURE DESIGNED TO CONTAIN A SLIGHT ENCLOSURE DESIGNED TO CONTAIN A SLIGHT POSITIVE PRESSUREPOSITIVE PRESSURE

-- LOWER COMBUSTION CHAMBER FLOOR HAS AN AIR LOWER COMBUSTION CHAMBER FLOOR HAS AN AIR DISTRIBUTION GRID FOR INTRODUCING PRIMARY AIRDISTRIBUTION GRID FOR INTRODUCING PRIMARY AIR

-- THERE IS NO HEAT TRANSFER TUBES IMMERSED IN THERE IS NO HEAT TRANSFER TUBES IMMERSED IN THE BEDTHE BED

-- IN LOWER PORTION WHERE RAPID CHANGE FOR SOLD IN LOWER PORTION WHERE RAPID CHANGE FOR SOLD FLOW PATTERN OCCURS THERE WALL TUBING ARE FLOW PATTERN OCCURS THERE WALL TUBING ARE PROTECTED BY THIN LAYER OF REFRACTORY.PROTECTED BY THIN LAYER OF REFRACTORY.

-- LOWER COMBUSTOR HAS OPENINGS FOR PRIMARY & LOWER COMBUSTOR HAS OPENINGS FOR PRIMARY & SECONDARY AIR PARTS, BURNERS, FUEL, LIMESTONE SECONDARY AIR PARTS, BURNERS, FUEL, LIMESTONE AND BED MATERIAL FEED POINTS ETC.AND BED MATERIAL FEED POINTS ETC.

ISGEC John Thompson

COMBUSTOR -- FUEL FED INTO LOWER COMBUSTOR MIX QUICKLY FUEL FED INTO LOWER COMBUSTOR MIX QUICKLY

AND UNIFORMLY WITH BED MATERIAL, HENCE THERE AND UNIFORMLY WITH BED MATERIAL, HENCE THERE IS NO VISIBLE BED LEVEL. BED DENSITY DECREASES IS NO VISIBLE BED LEVEL. BED DENSITY DECREASES PROGRESSIVELY WITH HEIGHT.PROGRESSIVELY WITH HEIGHT.

-- ON FUEL LOAD OPERATION ABOUT 40 - 50% OF HEAT ON FUEL LOAD OPERATION ABOUT 40 - 50% OF HEAT GENERATED BY COMBUSTION IS ABSORBED BY WALLGENERATED BY COMBUSTION IS ABSORBED BY WALL

-- HIGH CIRCULATING SOLIDS AND BACK MIXING HIGH CIRCULATING SOLIDS AND BACK MIXING INTENSITY PROVIDE HIGH HEAT TRANSFER RATE.INTENSITY PROVIDE HIGH HEAT TRANSFER RATE.

-- AMOUNT OF PRIMARY AIR NEEDED INITIAL AMOUNT OF PRIMARY AIR NEEDED INITIAL FLUIDISATION OF BED MATERIAL HAS TO BE FLUIDISATION OF BED MATERIAL HAS TO BE MAINTAINED UNDER ALL CONDITIONS.MAINTAINED UNDER ALL CONDITIONS.

ISGEC John Thompson

COMBUSTOR

-- DISTRIBUTION OF AIR BETWEEN PRIMARY AND DISTRIBUTION OF AIR BETWEEN PRIMARY AND

SECONDARY AIR IS IMPORTANT TO AVOID SECONDARY AIR IS IMPORTANT TO AVOID

EXCESSIVELY HIGH TEMPERATURE IN THE LOWER EXCESSIVELY HIGH TEMPERATURE IN THE LOWER

COMBUSTION CHAMBER, TO ENSURE GOOD COMBUSTION CHAMBER, TO ENSURE GOOD

COMBUSTION EFFICIENCY AS WELL AS LOW NOCOMBUSTION EFFICIENCY AS WELL AS LOW NOXX

PRODUCTION.PRODUCTION.

ISGEC John Thompson

Kick Out ArrangementThe transition zone in the lower furnace is provided with a special arrangement called Kick-out to eliminate erosion of the furnace wall tubes due to impingement of bed material.

ISGEC John Thompson

COMPACT CYCLONE

-- CYCLONE IS A VITAL PART OF CFBC BOILER CYCLONE IS A VITAL PART OF CFBC BOILER

-- CYCLONE IS DESIGNED TO PROVIDE EFFICIENT CYCLONE IS DESIGNED TO PROVIDE EFFICIENT SEPARATION OF ENTRANCED SOLIDS FROM THE HOT SEPARATION OF ENTRANCED SOLIDS FROM THE HOT FLUE GAS AND RETURN MOST OF THE UNBURNED FLUE GAS AND RETURN MOST OF THE UNBURNED CARBON AND CALCINED LIMESTONE FOR BETTER USE CARBON AND CALCINED LIMESTONE FOR BETTER USE ALONG WITH INERT ASH PARTICLES.ALONG WITH INERT ASH PARTICLES.

-- CYCLONE SEPARATES PARTICLES GREATER THAN 60 CYCLONE SEPARATES PARTICLES GREATER THAN 60 MICRONS WITH 99.5% EFFICIENCY.MICRONS WITH 99.5% EFFICIENCY.

-- SOLID CAPTURED IN CYCLONE ARE RECIRCULATED SOLID CAPTURED IN CYCLONE ARE RECIRCULATED BACK TO COMBUSTION CHAMBER THROUGH A BACK TO COMBUSTION CHAMBER THROUGH A SPECIAL NON MECHANICAL LOOP SEAL.SPECIAL NON MECHANICAL LOOP SEAL.

-- SOLID FLOW IN THE LOOP SEAL IS MAINTAINED SOLID FLOW IN THE LOOP SEAL IS MAINTAINED RELIABLY BY FLUIDISATION WITH A SMALL AMOUNT RELIABLY BY FLUIDISATION WITH A SMALL AMOUNT OF HIGH PRESSURE AIR.OF HIGH PRESSURE AIR.

-- THERE IS NO MOVING MECHANICAL PARTS IN LOOP THERE IS NO MOVING MECHANICAL PARTS IN LOOP SEAL.SEAL.

ISGEC John Thompson

Steam Cooled Convection Cage

Acts as an enclosure for Super heaters

ISGEC John Thompson

BOTTOM ASH REMOVAL SYSTEM

-- TO MAINTAIN THE CONSTANT BED MATERIAL TO MAINTAIN THE CONSTANT BED MATERIAL INVENTORY IN THE FURNACE, BOTTOM ASH REMOVAL INVENTORY IN THE FURNACE, BOTTOM ASH REMOVAL RATE SHALL BE CONTROLLED BY SUITABLE SYSTEMRATE SHALL BE CONTROLLED BY SUITABLE SYSTEM

-- WE HAVE PROVIDED STRIPPER ASH COOLERS FOR WE HAVE PROVIDED STRIPPER ASH COOLERS FOR ABOVE PURPOSE.ABOVE PURPOSE.

-- IN ADDITION TO BOTTOM ASH REMOVAL THIS SYSTEM IN ADDITION TO BOTTOM ASH REMOVAL THIS SYSTEM PERFORMS FOLLOWING FUNCTIONS.PERFORMS FOLLOWING FUNCTIONS.

PROVIDES COOLING OF BOTTOM ASH MATERIALPROVIDES COOLING OF BOTTOM ASH MATERIAL

CLASSIFIES THE BOTTOM ASH MATERIAL AND CLASSIFIES THE BOTTOM ASH MATERIAL AND RETURN LIGHT PARTICLES TO MAINTAIN FURNACE RETURN LIGHT PARTICLES TO MAINTAIN FURNACE BED QUALITYBED QUALITY

RECOVERS HEAT FROM ASHRECOVERS HEAT FROM ASH

RECOVERS FINE UNBURNT CARBONRECOVERS FINE UNBURNT CARBON

ISGEC John Thompson

AIR DISTRIBUTION

-- IN ORDER TO ENSURE THE COMBUSTION AS IN ORDER TO ENSURE THE COMBUSTION AS COMPLETE AS POSSIBLE 20% EXCESS AIR AT 100% COMPLETE AS POSSIBLE 20% EXCESS AIR AT 100% MCR SHALL BE PROVIDED.MCR SHALL BE PROVIDED.

-- IN CFBC BOILERS THE AIR SUPPLIED ARE DIVIDED IN CFBC BOILERS THE AIR SUPPLIED ARE DIVIDED INTOINTO

PRIMARY AIRPRIMARY AIR -- SUPPLIED THROUGH FLOOR GRIDSUPPLIED THROUGH FLOOR GRIDAND PARTLY THROUGH AND PARTLY THROUGH

OSM OSM ABOVE ABOVE GRIDGRID

SECONDARY AIRSECONDARY AIR -- COMPRISES OF AIR SUPPLIED COMPRISES OF AIR SUPPLIED THROUGH THROUGH

SECONDARY AIR SECONDARY AIR NOZZLES, AIR FOR FUEL FEED, NOZZLES, AIR FOR FUEL FEED,

LOOP SEAL LOOP SEAL FLUIDISING, BURNER FLUIDISING, BURNER

COOLING ETC.COOLING ETC.

ISGEC John Thompson

AIR DISTRIBUTION

-- HIGHER VOLATILE FUEL BURN HIGHER IN THE HIGHER VOLATILE FUEL BURN HIGHER IN THE FURNACE AND REQUIRES LESS PRIMARY AIR TO KEEP FURNACE AND REQUIRES LESS PRIMARY AIR TO KEEP THE LOWER BED COOL.THE LOWER BED COOL.

-- LOW VOLATILE FUELS REQUIRE MORE PRIMARY AIR LOW VOLATILE FUELS REQUIRE MORE PRIMARY AIR AS COMBUSTION TAKES PLACE IN LOWER BED.AS COMBUSTION TAKES PLACE IN LOWER BED.

-- IN MULTI FUEL APPLICATIONS BOTH EXTREME CASES IN MULTI FUEL APPLICATIONS BOTH EXTREME CASES ARE TO BE CONSIDERED FOR DESIGN.ARE TO BE CONSIDERED FOR DESIGN.

ISGEC John Thompson

Air Distribution for Different

Fuels