BoilersBoilers

Dr. Rohit Singh Lather, Ph.D.Dr. Rohit Singh Lather, Ph.D.

IntroductionIntroductionIntroductionIntroduction

Different Types of BoilersDifferent Types of BoilersDifferent Types of BoilersDifferent Types of Boilers

Fire Tube and Water TubeFire Tube and Water Tube

Straight Tube, Bent Tube, Horizontal, Vertical and Inclined BoilersStraight Tube, Bent Tube, Horizontal, Vertical and Inclined Boilers

Waste Heat Recovery Boilers (WHRB)Waste Heat Recovery Boilers (WHRB)Waste Heat Recovery Boilers (WHRB)

Subcritical and Supercritical BoilersSubcritical and Supercritical Boilers

Fuel Fired Boilers : Oil, Gas, CoalFuel Fired Boilers : Oil, Gas, Coal

BASIC MODEL FOR IGNITION AND FLAME PROPAGATIONBASIC MODEL FOR IGNITION AND FLAME PROPAGATIONBASIC MODEL FOR IGNITION AND FLAME PROPAGATIONBASIC MODEL FOR IGNITION AND FLAME PROPAGATION

• One of two particles burns first, then, the other particle is ignited by the heatof combustion of the one burning particle.

• When the first particle ignites, volatile matter is pyrolized.• A volatile matter flame is formed around the first particle. The flame grows

due to volatilization, and the flame heats the next particle which has notignited yet.

• Flame propagation is observed if the first burning particle can transfer theflame to the next particle before the volatile matter combustion of the firstparticle has finished.The distance between particles - d andThe time of flame propagation - s.Flame propagation velocity Sb was defined as the value of d divided by s.

• One of two particles burns first, then, the other particle is ignited by the heatof combustion of the one burning particle.

• When the first particle ignites, volatile matter is pyrolized.• A volatile matter flame is formed around the first particle. The flame grows

due to volatilization, and the flame heats the next particle which has notignited yet.

• Flame propagation is observed if the first burning particle can transfer theflame to the next particle before the volatile matter combustion of the firstparticle has finished.The distance between particles - d andThe time of flame propagation - s.Flame propagation velocity Sb was defined as the value of d divided by s.

Combustion of CoalCombustion of CoalCombustion of CoalCombustion of Coal

It is necessary to meet following three conditions to form a stable flame.1. The coal + air mixture at the ignition point is flammable.2. Coal particles are heated by high temperature gas in the recirculation flow.3. Coal particles are also heated by radiant heat from the surroundings.

Furnace Wall Heat TransferFurnace Wall Heat TransferFurnace Wall Heat TransferFurnace Wall Heat Transfer

Heat Transfer to water in the boiler water wall is complicated by the fact that it occursas water changes phase to steam. This takes place in two different ways:

Heat Transfer in the boilerHeat Transfer in the boiler

Nucleate /Convective BoilingNucleate /Convective Boiling Film Boiling

• The boiler tube surface remains effectively covered by water all the times.•• InIn nucleatenucleate boilingboiling, steam is generated in individual bubbles which are

continuously swept away or is generated in the steam filled centre of thetube on the water layer flowing along the tubewall.

•• InIn filmfilm boilingboiling, a thin film of superheated steam covers the inside of thetube wall separating the metal from the liquid water.

• Heat transfer through the steam film is much lower than that throughwater, which means that if film boiling occurs, tube wall temperature willclimb and the tube wall is likely to overheat if it is in a high heat inputzone.

•• TheThe changechange fromfrom nucleatenucleate boilingboiling toto filmfilm boilingboiling isis referredreferred asas thethe CriticalCriticalHeatHeat FluxFlux (CHF)(CHF) pointpoint.. At this point in the maximum, considerable vaporis being formed, making it difficult for the liquid to continuously wet thesurface to receive heat from the surface. This causes the heat flux toreduce after this point. At extremes, film boiling commonly known as theLeidenfrostLeidenfrost effecteffect.

• Depending upon the conditions, it is also referred to as the DepartureDeparturefrom Nucleate Boiling (DNB) or Dry Out (DO).from Nucleate Boiling (DNB) or Dry Out (DO).

• The boiler tube surface remains effectively covered by water all the times.•• InIn nucleatenucleate boilingboiling, steam is generated in individual bubbles which are

continuously swept away or is generated in the steam filled centre of thetube on the water layer flowing along the tubewall.

•• InIn filmfilm boilingboiling, a thin film of superheated steam covers the inside of thetube wall separating the metal from the liquid water.

• Heat transfer through the steam film is much lower than that throughwater, which means that if film boiling occurs, tube wall temperature willclimb and the tube wall is likely to overheat if it is in a high heat inputzone.

•• TheThe changechange fromfrom nucleatenucleate boilingboiling toto filmfilm boilingboiling isis referredreferred asas thethe CriticalCriticalHeatHeat FluxFlux (CHF)(CHF) pointpoint.. At this point in the maximum, considerable vaporis being formed, making it difficult for the liquid to continuously wet thesurface to receive heat from the surface. This causes the heat flux toreduce after this point. At extremes, film boiling commonly known as theLeidenfrostLeidenfrost effecteffect.

• Depending upon the conditions, it is also referred to as the DepartureDeparturefrom Nucleate Boiling (DNB) or Dry Out (DO).from Nucleate Boiling (DNB) or Dry Out (DO).

• The process of forming steam bubbles within liquid in micro cavitiesadjacent to the wall if the wall temperature at the heat transfer surfacerises above the saturation temperature while the bulk of the liquid (heatexchanger) is subcooled.

• The bubbles grow until they reach some critical size, at which point theyseparate from the wall and are carried into the main fluid stream.

• There the bubbles collapse because the temperature of bulk fluid is not ashigh as at the heat transfer surface, where the bubbles were created.

• This collapsing is also responsible for the sound a water kettle producesduring heat up but before the temperature at which bulk boiling isreached.

• The process of forming steam bubbles within liquid in micro cavitiesadjacent to the wall if the wall temperature at the heat transfer surfacerises above the saturation temperature while the bulk of the liquid (heatexchanger) is subcooled.

• The bubbles grow until they reach some critical size, at which point theyseparate from the wall and are carried into the main fluid stream.

• There the bubbles collapse because the temperature of bulk fluid is not ashigh as at the heat transfer surface, where the bubbles were created.

• This collapsing is also responsible for the sound a water kettle producesduring heat up but before the temperature at which bulk boiling isreached.

Departure from Nucleate BoilingDeparture from Nucleate BoilingDeparture from Nucleate BoilingDeparture from Nucleate Boiling

• If the heat flux of a boiling system is higher than the critical heatflux(CHF) of the system, the bulk fluid may boil, or in somecases, regions of the bulk fluid may boil where the fluid travels insmall channels.

• Thus large bubbles form, sometimes blocking the passage of thefluid.

• This results in a departure from nucleate boiling (DNB) in whichsteam bubbles no longer break away from the solid surface of thechannel, bubbles dominate the channel or surface, and the heatflux dramatically decreases.

• Vapor essentially insulates the bulk liquid from the hot surface.• During DNB, the surface temperature must therefore increase

substantially above the bulk fluid temperature in order to maintaina high heat flux.

• If the heat flux of a boiling system is higher than the critical heatflux(CHF) of the system, the bulk fluid may boil, or in somecases, regions of the bulk fluid may boil where the fluid travels insmall channels.

• Thus large bubbles form, sometimes blocking the passage of thefluid.

• This results in a departure from nucleate boiling (DNB) in whichsteam bubbles no longer break away from the solid surface of thechannel, bubbles dominate the channel or surface, and the heatflux dramatically decreases.

• Vapor essentially insulates the bulk liquid from the hot surface.• During DNB, the surface temperature must therefore increase

substantially above the bulk fluid temperature in order to maintaina high heat flux.

Parts of BoilerParts of BoilerParts of BoilerParts of Boiler

• DNB may be avoided in practice by increasing the pressure of the fluid,increasing its flow rate, or by utilizing a lower temperature bulk fluidwhich has a higher CHF.

•• IfIf thethe bulkbulk fluidfluid temperaturetemperature isis tootoo lowlow oror thethe pressurepressure ofof thethe fluidfluid isis tootoohigh,high, nucleatenucleate boilingboiling isis howeverhowever notnot possiblepossible..

• DNB is also known as TransitionTransition boiling,boiling, unstableunstable filmfilm boiling,boiling, andand partialpartialfilmfilm boilingboiling..

• Transition boiling occurs when the temperature differencedifference betweenbetween thethesurfacesurface andand thethe boilingboiling waterwater isis approximatelyapproximately 3030 °°CC toto 120120 °°CC aboveabove thetheTTSS..

• This corresponds to the high peak and the low peak on the boiling curve.

• During transition boiling of water, thethe bubblebubble formationformation isis soso rapidrapid thatthat aavaporvapor filmfilm oror blanketblanket beginsbegins toto formform atat thethe surfacesurface..

• DNB may be avoided in practice by increasing the pressure of the fluid,increasing its flow rate, or by utilizing a lower temperature bulk fluidwhich has a higher CHF.

•• IfIf thethe bulkbulk fluidfluid temperaturetemperature isis tootoo lowlow oror thethe pressurepressure ofof thethe fluidfluid isis tootoohigh,high, nucleatenucleate boilingboiling isis howeverhowever notnot possiblepossible..

• DNB is also known as TransitionTransition boiling,boiling, unstableunstable filmfilm boiling,boiling, andand partialpartialfilmfilm boilingboiling..

• Transition boiling occurs when the temperature differencedifference betweenbetween thethesurfacesurface andand thethe boilingboiling waterwater isis approximatelyapproximately 3030 °°CC toto 120120 °°CC aboveabove thetheTTSS..

• This corresponds to the high peak and the low peak on the boiling curve.

• During transition boiling of water, thethe bubblebubble formationformation isis soso rapidrapid thatthat aavaporvapor filmfilm oror blanketblanket beginsbegins toto formform atat thethe surfacesurface..

• However, at any point on the surface, the conditions may oscillatebetween film and nucleate boiling, but thethe fractionfraction ofof thethe totaltotal surfacesurfacecoveredcovered byby thethe filmfilm increasesincreases withwith increasingincreasing temperaturetemperature differencedifference..

•• AsAs thethe thermalthermal conductivityconductivity ofof thethe vaporvapor isis muchmuch lessless thanthan thatthat ofof thetheliquid,liquid, thethe convectiveconvective heatheat transfertransfer coefficientcoefficient andand thethe heatheat fluxflux reducesreduceswithwith increasingincreasing temperaturetemperature differencedifference..

• In recirculating boiler designs, it is important to limit heat release infurnace and to provide enoughenough waterwater flowflow thatthat thethe pointpoint ofof CHFCHF isis notnotreachedreached..

•• IfIf CHFCHF occurs,occurs, seriousserious damagedamage toto thethe tubestubes isis likelylikely..

• In sub-critical – pressure once through boilers, it is important that the CHFpoint be permitted to occur only in areas of fewfew lowlow heatheat inputinput ratesrates andandhighhigh flowflow ratesrates toto avoidavoid tubetube wallwall overheatingoverheating.

• However, at any point on the surface, the conditions may oscillatebetween film and nucleate boiling, but thethe fractionfraction ofof thethe totaltotal surfacesurfacecoveredcovered byby thethe filmfilm increasesincreases withwith increasingincreasing temperaturetemperature differencedifference..

•• AsAs thethe thermalthermal conductivityconductivity ofof thethe vaporvapor isis muchmuch lessless thanthan thatthat ofof thetheliquid,liquid, thethe convectiveconvective heatheat transfertransfer coefficientcoefficient andand thethe heatheat fluxflux reducesreduceswithwith increasingincreasing temperaturetemperature differencedifference..

• In recirculating boiler designs, it is important to limit heat release infurnace and to provide enoughenough waterwater flowflow thatthat thethe pointpoint ofof CHFCHF isis notnotreachedreached..

•• IfIf CHFCHF occurs,occurs, seriousserious damagedamage toto thethe tubestubes isis likelylikely..

• In sub-critical – pressure once through boilers, it is important that the CHFpoint be permitted to occur only in areas of fewfew lowlow heatheat inputinput ratesrates andandhighhigh flowflow ratesrates toto avoidavoid tubetube wallwall overheatingoverheating.

Travelling Grate Fired BoilerTravelling Grate Fired BoilerTravelling Grate Fired BoilerTravelling Grate Fired Boiler

•Coal is fed onto one end of a moving steel grate.

•As grate moves along the length of the furnace, thecoal burns before dropping off at the end as ash.

•Some degree of skill is required, particularly whensetting up the grate, air dampers and baffles, toensureensure cleanclean combustioncombustion leavingleaving thethe minimumminimum ofofunburntunburnt carboncarbon inin thethe ashash.

•The coal-feed hopper runs along the entire coal-feedend of the furnace. AA coalcoal gategate isis usedused toto controlcontrol thetheraterate atat whichwhich coalcoal isis fedfed intointo thethe furnacefurnace byby controllingcontrollingthethe thicknessthickness ofof thethe fuelfuel bedbed.

• CoalCoal mustmust bebe uniformuniform inin sizesize as large lumps will notburn out completely by the time they reach the end ofthe grate.

•Coal is fed onto one end of a moving steel grate.

•As grate moves along the length of the furnace, thecoal burns before dropping off at the end as ash.

•Some degree of skill is required, particularly whensetting up the grate, air dampers and baffles, toensureensure cleanclean combustioncombustion leavingleaving thethe minimumminimum ofofunburntunburnt carboncarbon inin thethe ashash.

•The coal-feed hopper runs along the entire coal-feedend of the furnace. AA coalcoal gategate isis usedused toto controlcontrol thetheraterate atat whichwhich coalcoal isis fedfed intointo thethe furnacefurnace byby controllingcontrollingthethe thicknessthickness ofof thethe fuelfuel bedbed.

• CoalCoal mustmust bebe uniformuniform inin sizesize as large lumps will notburn out completely by the time they reach the end ofthe grate.

A Typical Travelling GrateA Typical Travelling GrateA Typical Travelling GrateA Typical Travelling Grate

Schematic Travelling Grate Fired BoilersSchematic Travelling Grate Fired Boilers

• Spreader stokers utilizeutilize aa combinationcombination ofof suspensionsuspension burningburning andand grategrateburningburning..

• The coal is continually fed into the furnace above a burning bed of coal.

• The coalcoal finesfines areare burnedburned inin suspensionsuspension;; thethe largerlarger particlesparticles fallfall toto thethegrate,grate, wherewhere theythey areare burnedburned inin aa thin,thin, fastfast--burningburning coalcoal bedbed.

• This method of firing provides good flexibility to meet load fluctuations,since ignition is almost instantaneous when firing rate is increased.

• Due to this, the spreader stoker is favored over other types of stokers inmany industrial applications.

Spread Stoker Fired BoilerSpread Stoker Fired BoilerSpread Stoker Fired BoilerSpread Stoker Fired Boiler

• Spreader stokers utilizeutilize aa combinationcombination ofof suspensionsuspension burningburning andand grategrateburningburning..

• The coal is continually fed into the furnace above a burning bed of coal.

• The coalcoal finesfines areare burnedburned inin suspensionsuspension;; thethe largerlarger particlesparticles fallfall toto thethegrate,grate, wherewhere theythey areare burnedburned inin aa thin,thin, fastfast--burningburning coalcoal bedbed.

• This method of firing provides good flexibility to meet load fluctuations,since ignition is almost instantaneous when firing rate is increased.

• Due to this, the spreader stoker is favored over other types of stokers inmany industrial applications.

Coal and Gas Spread Stoker Fired BoilerCoal and Gas Spread Stoker Fired BoilerCoal and Gas Spread Stoker Fired BoilerCoal and Gas Spread Stoker Fired Boiler

COALCOAL

GASGAS

• Underfeed Stoker Firing

Underfeed stoker firing is the process ofcombustion in which the new coal is heated byradiation in the presence of air and located underignited fuel bed. The heating of coal is running lessrapidly and release volatile matter combine withair, so generate low smoke

• Overfeed Stoker Firing

Overfeed stoker firing is the process of combustionin which thethe unignitedunignited fuelfuel oror incomingincoming coalcoal isislocatedlocated aboveabove ignitedignited fuelfuel bedbed.. The ignited fueltransfer heat to the incoming coal by radiation.Moreover coal is heated by convection from hotgases that has been through the combustion.Secondary air is added to perform completecombustion unless steam boiler will produce moresmoke because the hot gases contain little oxygen.

• Underfeed Stoker Firing

Underfeed stoker firing is the process ofcombustion in which the new coal is heated byradiation in the presence of air and located underignited fuel bed. The heating of coal is running lessrapidly and release volatile matter combine withair, so generate low smoke

• Overfeed Stoker Firing

Overfeed stoker firing is the process of combustionin which thethe unignitedunignited fuelfuel oror incomingincoming coalcoal isislocatedlocated aboveabove ignitedignited fuelfuel bedbed.. The ignited fueltransfer heat to the incoming coal by radiation.Moreover coal is heated by convection from hotgases that has been through the combustion.Secondary air is added to perform completecombustion unless steam boiler will produce moresmoke because the hot gases contain little oxygen.

Pulverized Coal BoilerPulverized Coal BoilerPulverized Coal BoilerPulverized Coal Boiler

A pulverized coal-fired boiler is an industrial orutility boiler that generates thermal energy byburning pulverized coal (also known aspowdered coal or coal dust).

This type of boiler dominates the electricpower industry, providing steam to drive largeturbines.

Pulverized coal provides the thermal energywhich produces about 50% of the world'selectric supply.

A pulverized coal-fired boiler is an industrial orutility boiler that generates thermal energy byburning pulverized coal (also known aspowdered coal or coal dust).

This type of boiler dominates the electricpower industry, providing steam to drive largeturbines.

Pulverized coal provides the thermal energywhich produces about 50% of the world'selectric supply.

A Typical Pulverized Coal PlantA Typical Pulverized Coal PlantA Typical Pulverized Coal PlantA Typical Pulverized Coal Plant

Pulverized Power Plant SystemPulverized Power Plant SystemPulverized Power Plant SystemPulverized Power Plant System

Air System

Primary Air (PA Fan)

Secondary Air (FD Fan)

Seal Air System

Pressure Parts

Water Circuit: Economizer, Water wall panels.

Steam Circuit: Primary Superheater, Final Superheater, Reheater.

Coal Feeding System

Coal Feeder: Rotary Volumetric, Gravimetric

Coal Mill (Pulverizer): Ball Mill or Drum Mill, Contact Mill

Coal Burner: Coal burner, Auxiliary oil Burner

Ash Handling System

Air System

Primary Air (PA Fan)

Secondary Air (FD Fan)

Seal Air System

Pressure Parts

Water Circuit: Economizer, Water wall panels.

Steam Circuit: Primary Superheater, Final Superheater, Reheater.

Coal Feeding System

Coal Feeder: Rotary Volumetric, Gravimetric

Coal Mill (Pulverizer): Ball Mill or Drum Mill, Contact Mill

Coal Burner: Coal burner, Auxiliary oil Burner

Ash Handling System

Tangential Firing of CoalTangential Firing of CoalTangential Firing of CoalTangential Firing of Coal

Thermal efficiencies and CO₂ EmissionsThermal efficiencies and CO₂ EmissionsThermal efficiencies and CO₂ EmissionsThermal efficiencies and CO₂ Emissions

Source: IEA 2008• Recently, reduction of CO2 emissions is required for coal fired thermal power

plants. To achieve this, various approaches have been taken. Ultra supercriticalpower plants have been developed for improvement of power efficiency. Oxy-fuelcombustion technology is being pursued for carbon capture and storage. Furtherreduction of the environmental load, such as NOx reduction, is also still required.

Fluidized Bed CombustionFluidized Bed CombustionFluidized Bed CombustionFluidized Bed Combustion

• Fluidized bed combustion (FBC ) has emerged as a viable alternative andhas significant advantages over conventional firing system and offersmultiple benefits :

Compact boiler design

Fuel flexibility: Coal, biomass, rice husk, bagasse & other agriculturalwastes.

Higher combustion efficiency

Reduced emission of noxious pollutants such as SOx and NOx.

Wide capacity range- 0.5 T/hr to over 100 T/hr.• When an evenly distributed air or gas is passed upward through a finely

divided bed of solid particles such as sand supported on a fine mesh, theparticles are undisturbed at low velocity.

• As air velocity is gradually increased, a stage is reached when theindividual particles are suspended in the air stream – the bed is called“fluidized”.

• Fluidized bed combustion (FBC ) has emerged as a viable alternative andhas significant advantages over conventional firing system and offersmultiple benefits :

Compact boiler design

Fuel flexibility: Coal, biomass, rice husk, bagasse & other agriculturalwastes.

Higher combustion efficiency

Reduced emission of noxious pollutants such as SOx and NOx.

Wide capacity range- 0.5 T/hr to over 100 T/hr.• When an evenly distributed air or gas is passed upward through a finely

divided bed of solid particles such as sand supported on a fine mesh, theparticles are undisturbed at low velocity.

• As air velocity is gradually increased, a stage is reached when theindividual particles are suspended in the air stream – the bed is called“fluidized”.

Continued….Continued….Continued….Continued….

• Imagine a box containing sand resting on a mesh. If air is blown veryslowly upwards through the mesh, it percolates between the sandparticles without disturbing them.

• When the velocity of the air stream is gradually increased, a point isreached when individual sand particles are forced upwards; they becomesupported by the air stream and begin to move about within a bed with afairly well defined surface.

• At still higher upward air velocities, an important change occurs; the bedbecomes very turbulent with rapid mixing of the particles.

• Bubbles, similar to those in a briskly boiling liquid, pass through the bedand the surface is no longer well defined but becomes diffused.

• A bed of solid particles in this state is said to be 'fluidised', because it hasnot only the appearance, but also some of the properties, of a boilingfluid.

• Imagine a box containing sand resting on a mesh. If air is blown veryslowly upwards through the mesh, it percolates between the sandparticles without disturbing them.

• When the velocity of the air stream is gradually increased, a point isreached when individual sand particles are forced upwards; they becomesupported by the air stream and begin to move about within a bed with afairly well defined surface.

• At still higher upward air velocities, an important change occurs; the bedbecomes very turbulent with rapid mixing of the particles.

• Bubbles, similar to those in a briskly boiling liquid, pass through the bedand the surface is no longer well defined but becomes diffused.

• A bed of solid particles in this state is said to be 'fluidised', because it hasnot only the appearance, but also some of the properties, of a boilingfluid.

Continued….Continued….Continued….Continued….

• There are lower and upper limits of air velocity between whichsatisfactory fluidization of sand, or any other granular substance, will takeplace.

• The velocity of the air stream causing fluidization is termed 'fluidizingvelocity'.

• For a bed of any material, the larger the particles, the greater the velocityof the air (or other gas) that is required to fluidize it;

•• ForFor particlesparticles ofof aa givengiven size,size, thethe heavierheavier theythey are,are, thethe greatergreater thethe fluidizingfluidizingvelocityvelocity needsneeds toto bebe..

• In practice, a fluidized bed will contain particles of different sizes.

• The operating limits are set, on the one hand, by the minimum air / gasvelocity needed to keep the particles fluidized and, on the other hand, bythe maximum velocity that can be used before an excessive quantity ofbed particles are blown out of the bed containment box.

• There are lower and upper limits of air velocity between whichsatisfactory fluidization of sand, or any other granular substance, will takeplace.

• The velocity of the air stream causing fluidization is termed 'fluidizingvelocity'.

• For a bed of any material, the larger the particles, the greater the velocityof the air (or other gas) that is required to fluidize it;

•• ForFor particlesparticles ofof aa givengiven size,size, thethe heavierheavier theythey are,are, thethe greatergreater thethe fluidizingfluidizingvelocityvelocity needsneeds toto bebe..

• In practice, a fluidized bed will contain particles of different sizes.

• The operating limits are set, on the one hand, by the minimum air / gasvelocity needed to keep the particles fluidized and, on the other hand, bythe maximum velocity that can be used before an excessive quantity ofbed particles are blown out of the bed containment box.

A fluidized bed of solids behaves in many ways like a liquid and hasA fluidized bed of solids behaves in many ways like a liquid and hasimportant characteristicsimportant characteristics

A fluidized bed of solids behaves in many ways like a liquid and hasA fluidized bed of solids behaves in many ways like a liquid and hasimportant characteristicsimportant characteristics

• The bed finds its own level. If the vessel containing the fluidized bed ofsolids is tilted from a horizontal position, the surface of the bed remainslevel.

• Provided the fluidized state can be maintained, the bed can be transferredfrom one container to another as though it were a liquid.

• Solid particles in a fluidized bed are violently churned about; rapid mixingoccurs and any added particles are quickly distributed throughout the bed.

• Objects can float or sink in a fluidised bed according to their density, as ina liquid.

• When a fluidized bed is heated, the thorough mixing enables heat to berapidly transferred from one part to another, ensuring near uniformity oftemperature, as in a boiling liquid. .

• Mixing in a fluidized bed causes heat to be rapidly transferred to a coolersurface (for example, a water tube) immersed in it. The constantmovement brings a continuous supply of hot particles to this heat transfersurface.

• The bed finds its own level. If the vessel containing the fluidized bed ofsolids is tilted from a horizontal position, the surface of the bed remainslevel.

• Provided the fluidized state can be maintained, the bed can be transferredfrom one container to another as though it were a liquid.

• Solid particles in a fluidized bed are violently churned about; rapid mixingoccurs and any added particles are quickly distributed throughout the bed.

• Objects can float or sink in a fluidised bed according to their density, as ina liquid.

• When a fluidized bed is heated, the thorough mixing enables heat to berapidly transferred from one part to another, ensuring near uniformity oftemperature, as in a boiling liquid. .

• Mixing in a fluidized bed causes heat to be rapidly transferred to a coolersurface (for example, a water tube) immersed in it. The constantmovement brings a continuous supply of hot particles to this heat transfersurface.

Fluidization of solidsFluidization of solidsFluidization of solidsFluidization of solids

a) Sand particles resting become fluidized when (right)and take on the some of the properties of a boilingfluid.

b) Granular solids remain in layers when one is but rapidmixing occurs on fluidization (right).

c) A bed of stationary particles supports objects (left).On fluidization, an object of lower density (the greenwhile the higher density (red ball) sinks.

d) In a bed of stationary particles (left), heat there arebig differences in temperature. In a fluidized bedmixing ensures uniformity of temperature.

a) Sand particles resting become fluidized when (right)and take on the some of the properties of a boilingfluid.

b) Granular solids remain in layers when one is but rapidmixing occurs on fluidization (right).

c) A bed of stationary particles supports objects (left).On fluidization, an object of lower density (the greenwhile the higher density (red ball) sinks.

d) In a bed of stationary particles (left), heat there arebig differences in temperature. In a fluidized bedmixing ensures uniformity of temperature.

• With further increase in air velocity, there is bubble formation, vigorousturbulence, rapid mixing and formation of dense defined bed surface.

•• TheThe bedbed ofof solidsolid particlesparticles exhibitsexhibits thethe propertiesproperties ofof aa boilingboiling liquidliquid andand assumesassumesthethe appearanceappearance ofof aa fluidfluid –– “bubbling“bubbling fluidizedfluidized bed”bed”.

• If sand particles in a fluidized state is heated to the ignition temperatures of coal,and coal is injected continuously into the bed, the coal will burn rapidly and bedattains a uniform temperature.

• The fluidized bed combustion (FBC ) takes place at about 840°C to 950°C. Since thistemperaturetemperature isis muchmuch belowbelow thethe ashash fusionfusion temperaturetemperature, melting of ash andassociated problems are avoided.

•• TheThe lowerlower combustioncombustion temperaturetemperature isis achievedachieved becausebecause ofof highhigh coefficientcoefficient ofof heatheattransfertransfer due to rapid mixing in the fluidized bed and effective extraction of heatfrom the bed through in-bed heat transfer tubes and walls of the bed.

•• TheThe gasgas velocityvelocity isis maintainedmaintained betweenbetween minimumminimum fluidisationfluidisation velocityvelocity andand particleparticleentrainmententrainment velocityvelocity. This ensures stable operation of the bed and avoids particleentrainment in the gas stream.

• With further increase in air velocity, there is bubble formation, vigorousturbulence, rapid mixing and formation of dense defined bed surface.

•• TheThe bedbed ofof solidsolid particlesparticles exhibitsexhibits thethe propertiesproperties ofof aa boilingboiling liquidliquid andand assumesassumesthethe appearanceappearance ofof aa fluidfluid –– “bubbling“bubbling fluidizedfluidized bed”bed”.

• If sand particles in a fluidized state is heated to the ignition temperatures of coal,and coal is injected continuously into the bed, the coal will burn rapidly and bedattains a uniform temperature.

• The fluidized bed combustion (FBC ) takes place at about 840°C to 950°C. Since thistemperaturetemperature isis muchmuch belowbelow thethe ashash fusionfusion temperaturetemperature, melting of ash andassociated problems are avoided.

•• TheThe lowerlower combustioncombustion temperaturetemperature isis achievedachieved becausebecause ofof highhigh coefficientcoefficient ofof heatheattransfertransfer due to rapid mixing in the fluidized bed and effective extraction of heatfrom the bed through in-bed heat transfer tubes and walls of the bed.

•• TheThe gasgas velocityvelocity isis maintainedmaintained betweenbetween minimumminimum fluidisationfluidisation velocityvelocity andand particleparticleentrainmententrainment velocityvelocity. This ensures stable operation of the bed and avoids particleentrainment in the gas stream.

FluidizationFluidizationFluidizationFluidization

• Fluidization is a two-phase process in which dispersed solid material issuspended in a stream of gas flowing upstream through the fluidizedgrate. The layer of solid body particles suspended in flowing gas formsfluidized bed.

• The fluidized bed is in the quasi-equilibrium state only in some range ofthe velocity of the flowing upstream gas, depending on the size ofparticles of bed.

• The fluidized bed of a boiler contains mainly particles of an inertmaterial, like sand and ash, including particles of SO2 sorbent.

• The coal content in a fluidized bed is not considerable, it is only from 3%to 5% of the whole mass of the bed.

• Fluidization is a two-phase process in which dispersed solid material issuspended in a stream of gas flowing upstream through the fluidizedgrate. The layer of solid body particles suspended in flowing gas formsfluidized bed.

• The fluidized bed is in the quasi-equilibrium state only in some range ofthe velocity of the flowing upstream gas, depending on the size ofparticles of bed.

• The fluidized bed of a boiler contains mainly particles of an inertmaterial, like sand and ash, including particles of SO2 sorbent.

• The coal content in a fluidized bed is not considerable, it is only from 3%to 5% of the whole mass of the bed.

Bed MaterialBed MaterialBed MaterialBed Material

• To start with the bed material is sand.

• Some portion is lost in the ash during the operation and this has to be made-up.

• In coal fired boilers the ash from the coal itself will be the makeup material.

• When firing bio fuels with very low ash content sand will be the makeup bedmaterial.

• For high Sulphur coals Limestone addition to the bed material reducesSO2 emissions.

• CFBC uses crushed coal of 3 to 6 mm size. This requires only a crusher not apulverizer.

• From storage hoppers Conveyer and feeders transport the coal to feed chutesin the furnace.

• Start up is by oil burners in the furnace. Ash spouts in the furnace remove theash from the bottom of the furnace.

• To start with the bed material is sand.

• Some portion is lost in the ash during the operation and this has to be made-up.

• In coal fired boilers the ash from the coal itself will be the makeup material.

• When firing bio fuels with very low ash content sand will be the makeup bedmaterial.

• For high Sulphur coals Limestone addition to the bed material reducesSO2 emissions.

• CFBC uses crushed coal of 3 to 6 mm size. This requires only a crusher not apulverizer.

• From storage hoppers Conveyer and feeders transport the coal to feed chutesin the furnace.

• Start up is by oil burners in the furnace. Ash spouts in the furnace remove theash from the bottom of the furnace.

Fluidized BedFluidized BedFluidized BedFluidized Bed

• At the bottom of the boiler furnace there is a bed of inert material.

• Bed is where the coal or fuel spreads.

• Air supply is from under the bed at high pressure. This lifts the bedmaterial and the coal particles and keeps it in suspension.

• The coal combustion takes place in this suspended condition. This is theFluidized bed.

• Special design of the air nozzles at the bottom of the bed allows air flowwithout clogging.

• Primary air fans provide the preheated Fluidizing air.

• Secondary air fans provide pre-heated Combustion air.

• Nozzles in the furnace walls at various levels distribute the Combustion airin the furnace.

• At the bottom of the boiler furnace there is a bed of inert material.

• Bed is where the coal or fuel spreads.

• Air supply is from under the bed at high pressure. This lifts the bedmaterial and the coal particles and keeps it in suspension.

• The coal combustion takes place in this suspended condition. This is theFluidized bed.

• Special design of the air nozzles at the bottom of the bed allows air flowwithout clogging.

• Primary air fans provide the preheated Fluidizing air.

• Secondary air fans provide pre-heated Combustion air.

• Nozzles in the furnace walls at various levels distribute the Combustion airin the furnace.

Fluidization PhenomenonFluidization PhenomenonFluidization PhenomenonFluidization Phenomenon

Fixed Bed

Air flow velocity in boiler furnace vs. combustion patternAir flow velocity in boiler furnace vs. combustion patternAir flow velocity in boiler furnace vs. combustion patternAir flow velocity in boiler furnace vs. combustion pattern

FBC featuresFBC featuresFBC featuresFBC features

• Direct contact of particles with intensive mass and heat exchange,• Uniform temperature in the fluidized bed• High heat capacity of fluidized bed making it possible to burn fuels of low

quality, wet and with high content of ash• Effectiveness of bed temperature control by supply of fuel, air and heat

extraction

• Direct contact of particles with intensive mass and heat exchange,• Uniform temperature in the fluidized bed• High heat capacity of fluidized bed making it possible to burn fuels of low

quality, wet and with high content of ash• Effectiveness of bed temperature control by supply of fuel, air and heat

extraction

Structure of fluidized bedStructure of fluidized bedStructure of fluidized bedStructure of fluidized bed• Boilers with Bubbling (stationary) Fluidized Bed (BFB)

• Boilers with Circulating Fluidized Bed (CFB)

Bubbling Fluidized Bed Circulating Fluidized Bed (CFB)

Bubbling CirculatingBubbling CirculatingBubbling CirculatingBubbling Circulating

Concerning the Pressure in a FurnaceConcerning the Pressure in a FurnaceConcerning the Pressure in a FurnaceConcerning the Pressure in a Furnace• Atmospheric fluidized bed boilers (pressure approximately atmospheric) (ACFB).

• Pressure fluidized bed boilers (pressure much higher that atmospheric) (PCFB).

Pressurized Fluidized Bed CombustionPressurized Fluidized Bed CombustionPressurized Fluidized Bed CombustionPressurized Fluidized Bed Combustion

• In Pressurized Fluidized Bed Combustion (PFBC ) type, a compressor supplies theForced Draft (FD) air and the combustor is a pressure vessel.

• The heat release rate in the bed is proportional to the bed pressure and hence adeep bed is used to extract large amount of heat.

• This will improve the combustion efficiency and sulphur dioxide absorption in thebed.

• The steam is generated in the two tube bundles, one in the bed and one above it.Hot flue gases drive a power generating gas turbine.

• The PFBC system can be used for cogeneration (steam and electricity) or combinedcycle power generation.

• The combined cycle operation (gas turbine & steam turbine) improves the overallconversion efficiency by 5 to 8%.

• In Pressurized Fluidized Bed Combustion (PFBC ) type, a compressor supplies theForced Draft (FD) air and the combustor is a pressure vessel.

• The heat release rate in the bed is proportional to the bed pressure and hence adeep bed is used to extract large amount of heat.

• This will improve the combustion efficiency and sulphur dioxide absorption in thebed.

• The steam is generated in the two tube bundles, one in the bed and one above it.Hot flue gases drive a power generating gas turbine.

• The PFBC system can be used for cogeneration (steam and electricity) or combinedcycle power generation.

• The combined cycle operation (gas turbine & steam turbine) improves the overallconversion efficiency by 5 to 8%.

Atmospheric Fluidized Bed Combustion (AFBC)Boiler

Atmospheric Fluidized Bed Combustion (AFBC)Boiler

• Most operational boiler of this type is of the Atmospheric Fluidized BedCombustion. (AFBC ).

• This involves little more than adding a fluidized bed combustor to aconventional shell boiler.

• Such systems have similarly being installed in conjunction with conventionalwater tube boiler.

• Coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type offuel fed to the combustion chamber.

• The atmospheric air, which acts as both the fluidization and combustion air, isdelivered at a pressure, after being preheated by the exhaust fuel gases.

• The in-bed tubes carrying water generally act as the evaporator.

• The gaseous products of combustion pass over the super heater sections ofthe boiler flow past the economizer, the dust collectors and the air preheaterbefore being exhausted to atmosphere.

• Most operational boiler of this type is of the Atmospheric Fluidized BedCombustion. (AFBC ).

• This involves little more than adding a fluidized bed combustor to aconventional shell boiler.

• Such systems have similarly being installed in conjunction with conventionalwater tube boiler.

• Coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type offuel fed to the combustion chamber.

• The atmospheric air, which acts as both the fluidization and combustion air, isdelivered at a pressure, after being preheated by the exhaust fuel gases.

• The in-bed tubes carrying water generally act as the evaporator.

• The gaseous products of combustion pass over the super heater sections ofthe boiler flow past the economizer, the dust collectors and the air preheaterbefore being exhausted to atmosphere.

Circulating Fluidized Bed Combustion Boilers (CFBC)Circulating Fluidized Bed Combustion Boilers (CFBC)Circulating Fluidized Bed Combustion Boilers (CFBC)Circulating Fluidized Bed Combustion Boilers (CFBC)

• In a circulating system the bedparameters are so maintained as topromote solids elutriation from thebed.

• They are lifted in a relatively dilutephase in a solids riser, and a down-comer with a cyclone provides areturn path for the solids.

• There are no steam generationtubes immersed in the bed.

• Generation and super heating ofsteam takes place in the convectionsection, water walls, at the exit ofthe riser.

• In a circulating system the bedparameters are so maintained as topromote solids elutriation from thebed.

• They are lifted in a relatively dilutephase in a solids riser, and a down-comer with a cyclone provides areturn path for the solids.

• There are no steam generationtubes immersed in the bed.

• Generation and super heating ofsteam takes place in the convectionsection, water walls, at the exit ofthe riser.

• Fine particles of partly burned coal, ash and bed material are carried alongwith the flue gases to the upper areas of the furnace and then into acyclone.

• In the cyclone the heavier particles separate from the gas and falls to thehopper of the cyclone.

• This returns to the furnace for recirculation. Hence the name CirculatingFluidized Bed combustion. The hot gases from the cyclone pass to the heattransfer surfaces and go out of the boiler.

• For large units, the taller furnace characteristics of CFBC boilers offersbetter space utilization, greater fuel particle and sorbent residence timefor efficient combustion and SO2 capture, and easier application of stagedcombustion techniques for NOx control than AFBC steam generators.•

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

• Fine particles of partly burned coal, ash and bed material are carried alongwith the flue gases to the upper areas of the furnace and then into acyclone.

• In the cyclone the heavier particles separate from the gas and falls to thehopper of the cyclone.

• This returns to the furnace for recirculation. Hence the name CirculatingFluidized Bed combustion. The hot gases from the cyclone pass to the heattransfer surfaces and go out of the boiler.

• For large units, the taller furnace characteristics of CFBC boilers offersbetter space utilization, greater fuel particle and sorbent residence timefor efficient combustion and SO2 capture, and easier application of stagedcombustion techniques for NOx control than AFBC steam generators.•

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

Types of Circulating FB BoilersTypes of Circulating FB BoilersTypes of Circulating FB BoilersTypes of Circulating FB Boilers

CFB Solid Waste SeparatorCFB Solid Waste SeparatorCFB Solid Waste SeparatorCFB Solid Waste Separator

Internals of CFBInternals of CFB -- 670670Internals of CFBInternals of CFB -- 670670

Details of the BedDetails of the BedDetails of the BedDetails of the Bed

Structure of BFBStructure of BFBStructure of BFBStructure of BFB

Burring of coal particles in FB

Air NozzlesAir NozzlesAir NozzlesAir Nozzles

Sources of Unburned Carbon in FBCSources of Unburned Carbon in FBCSources of Unburned Carbon in FBCSources of Unburned Carbon in FBC

• Very small particles of coal are blow-up from the bed due to:

• Increase of porosity of coal particles due to oxidation

• Particles decomposition as a result of thermal thermal tension in particles

• Collision of particles in a bed

• Friction of particles in a bed

• Very small particles of coal are blow-up from the bed due to:

• Increase of porosity of coal particles due to oxidation

• Particles decomposition as a result of thermal thermal tension in particles

• Collision of particles in a bed

• Friction of particles in a bed

Fuel HandlingFuel HandlingFuel HandlingFuel Handling

Worm - Gear Coal Handling

Pneumatic Handling

ParametersParametersParametersParameters

Advantages of FB firing over PB firing systemsAdvantages of FB firing over PB firing systems

• Simplification of the fuel supply system.• Possibility of burning low-caloric fuels.• Possibility of flue gas desulfurization in the bed.•Reduction of NOx emission due to the lower temperature of burning.

Development TrendsDevelopment Trends

INTREX (Integrated Heat Exchanger Technology)INTREX (Integrated Heat Exchanger Technology)INTREX (Integrated Heat Exchanger Technology)INTREX (Integrated Heat Exchanger Technology)

11stst Generation PFBC,Generation PFBC, KaritaKarita, Japan, Japan11stst Generation PFBC,Generation PFBC, KaritaKarita, Japan, Japan

22ndnd Generation PCFBGeneration PCFB22ndnd Generation PCFBGeneration PCFB

22ndnd Generation PFCB, Cottbus GermanyGeneration PFCB, Cottbus Germany22ndnd Generation PFCB, Cottbus GermanyGeneration PFCB, Cottbus Germany

Super Critical Once through CFB BoilerSuper Critical Once through CFB BoilerSuper Critical Once through CFB BoilerSuper Critical Once through CFB Boiler

Ignition• Ignition properties are fundamental combustion performance parameters for

engineering design of combustion systems.

• Fundamental ignition Performance parameters : coal ignition. Ignitiontemperature, flammability limit concentration (explosion limit concentration) andburning velocity (flame propagation velocity) are important ignition performanceparameters.

• The ignition temperature sometimes decreases when the particle diameter isincreased. Such results would lead to the idea that flame stabilization becomeseasy when particle diameter is increased. However, decreasing the particlediameter is very important to obtain a stable flame for actual burner systems.

• For actual boilers, the coal flames are surrounded by the furnace wall. The furnacewall temperature is several hundred degrees Celsius for a water wall, and it islarger than one thousand degrees Celsius for a caster wall.

• Ignition properties are fundamental combustion performance parameters forengineering design of combustion systems.

• Fundamental ignition Performance parameters : coal ignition. Ignitiontemperature, flammability limit concentration (explosion limit concentration) andburning velocity (flame propagation velocity) are important ignition performanceparameters.

• The ignition temperature sometimes decreases when the particle diameter isincreased. Such results would lead to the idea that flame stabilization becomeseasy when particle diameter is increased. However, decreasing the particlediameter is very important to obtain a stable flame for actual burner systems.

• For actual boilers, the coal flames are surrounded by the furnace wall. The furnacewall temperature is several hundred degrees Celsius for a water wall, and it islarger than one thousand degrees Celsius for a caster wall.

Relationship between coal concentration andflame propagation velocity.

Relationship between coal concentration andflame propagation velocity.

• Coal concentrations and flame propagation velocities are shown asnormalized values.

• The coal concentration was in inverse proportion to the third powerof the distance d.

• When the coal concentration increased, flame propagation velocityincreased.

• But there was an upper limit value (Sb-max) to the flamepropagation velocity.

• The flame propagation velocities were almost zero at the leanflammability limit.

• Absolute values of L and Sb-max vary with coal properties andburning conditions. Relationship between L and Sb-max is shownin Fig. 8. Lean flammability limit; L was inversely proportional tomaximum flame propagation velocity; Sb-max.

• Coal concentrations and flame propagation velocities are shown asnormalized values.

• The coal concentration was in inverse proportion to the third powerof the distance d.

• When the coal concentration increased, flame propagation velocityincreased.

• But there was an upper limit value (Sb-max) to the flamepropagation velocity.

• The flame propagation velocities were almost zero at the leanflammability limit.

• Absolute values of L and Sb-max vary with coal properties andburning conditions. Relationship between L and Sb-max is shownin Fig. 8. Lean flammability limit; L was inversely proportional tomaximum flame propagation velocity; Sb-max.