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Boilers

The document discusses different types of boilers, including fire tube and water tube boilers. It also describes the basic model of ignition and flame propagation in boilers. Key points covered include the conditions needed to form a stable flame during coal combustion, and the processes of nucleate/convective boiling and film boiling that occur as heat is transferred to water in boiler walls. Departure from nucleate boiling is explained, along with how it can be avoided by increasing pressure, fluid flow rate, or using a lower temperature bulk fluid.

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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 heat of 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 not ignited yet. • Flame propagation is observed if the first burning particle can transfer the flame to the next particle before the volatile matter combustion of the first particle has finished. The distance between particles - d and The 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 heat of 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 not ignited yet. • Flame propagation is observed if the first burning particle can transfer the flame to the next particle before the volatile matter combustion of the first particle has finished. The distance between particles - d and The 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 occurs as 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 the tube on the water layer flowing along the tubewall. •• InIn filmfilm boilingboiling, a thin film of superheated steam covers the inside of the tube wall separating the metal from the liquid water. • Heat transfer through the steam film is much lower than that through water, which means that if film boiling occurs, tube wall temperature will climb and the tube wall is likely to overheat if it is in a high heat input zone. •• TheThe changechange fromfrom nucleatenucleate boilingboiling toto filmfilm boilingboiling isis referredreferred asas thethe CriticalCritical HeatHeat FluxFlux (CHF)(CHF) pointpoint.. At this point in the maximum, considerable vapor is being formed, making it difficult for the liquid to continuously wet the surface to receive heat from the surface. This causes the heat flux to reduce after this point. At extremes, film boiling commonly known as the LeidenfrostLeidenfrost effecteffect. • Depending upon the conditions, it is also referred to as the DepartureDeparture from 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 the tube on the water layer flowing along the tubewall. •• InIn filmfilm boilingboiling, a thin film of superheated steam covers the inside of the tube wall separating the metal from the liquid water. • Heat transfer through the steam film is much lower than that through water, which means that if film boiling occurs, tube wall temperature will climb and the tube wall is likely to overheat if it is in a high heat input zone. •• TheThe changechange fromfrom nucleatenucleate boilingboiling toto filmfilm boilingboiling isis referredreferred asas thethe CriticalCritical HeatHeat FluxFlux (CHF)(CHF) pointpoint.. At this point in the maximum, considerable vapor is being formed, making it difficult for the liquid to continuously wet the surface to receive heat from the surface. This causes the heat flux to reduce after this point. At extremes, film boiling commonly known as the LeidenfrostLeidenfrost effecteffect. • Depending upon the conditions, it is also referred to as the DepartureDeparture from 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 cavities adjacent to the wall if the wall temperature at the heat transfer surface rises above the saturation temperature while the bulk of the liquid (heat exchanger) is subcooled. • The bubbles grow until they reach some critical size, at which point they separate from the wall and are carried into the main fluid stream. • There the bubbles collapse because the temperature of bulk fluid is not as high as at the heat transfer surface, where the bubbles were created. • This collapsing is also responsible for the sound a water kettle produces during heat up but before the temperature at which bulk boiling is reached. • The process of forming steam bubbles within liquid in micro cavities adjacent to the wall if the wall temperature at the heat transfer surface rises above the saturation temperature while the bulk of the liquid (heat exchanger) is subcooled. • The bubbles grow until they reach some critical size, at which point they separate from the wall and are carried into the main fluid stream. • There the bubbles collapse because the temperature of bulk fluid is not as high as at the heat transfer surface, where the bubbles were created. • This collapsing is also responsible for the sound a water kettle produces during heat up but before the temperature at which bulk boiling is reached.
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 heat flux(CHF) of the system, the bulk fluid may boil, or in some cases, regions of the bulk fluid may boil where the fluid travels in small channels. • Thus large bubbles form, sometimes blocking the passage of the fluid. • This results in a departure from nucleate boiling (DNB) in which steam bubbles no longer break away from the solid surface of the channel, bubbles dominate the channel or surface, and the heat flux 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 maintain a high heat flux. • If the heat flux of a boiling system is higher than the critical heat flux(CHF) of the system, the bulk fluid may boil, or in some cases, regions of the bulk fluid may boil where the fluid travels in small channels. • Thus large bubbles form, sometimes blocking the passage of the fluid. • This results in a departure from nucleate boiling (DNB) in which steam bubbles no longer break away from the solid surface of the channel, bubbles dominate the channel or surface, and the heat flux 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 maintain a 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 fluid which has a higher CHF. •• IfIf thethe bulkbulk fluidfluid temperaturetemperature isis tootoo lowlow oror thethe pressurepressure ofof thethe fluidfluid isis tootoo high,high, nucleatenucleate boilingboiling isis howeverhowever notnot possiblepossible.. • DNB is also known as TransitionTransition boiling,boiling, unstableunstable filmfilm boiling,boiling, andand partialpartial filmfilm boilingboiling.. • Transition boiling occurs when the temperature differencedifference betweenbetween thethe surfacesurface andand thethe boilingboiling waterwater isis approximatelyapproximately 3030 °°CC toto 120120 °°CC aboveabove thethe TTSS.. • 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 aa vaporvapor 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 fluid which has a higher CHF. •• IfIf thethe bulkbulk fluidfluid temperaturetemperature isis tootoo lowlow oror thethe pressurepressure ofof thethe fluidfluid isis tootoo high,high, nucleatenucleate boilingboiling isis howeverhowever notnot possiblepossible.. • DNB is also known as TransitionTransition boiling,boiling, unstableunstable filmfilm boiling,boiling, andand partialpartial filmfilm boilingboiling.. • Transition boiling occurs when the temperature differencedifference betweenbetween thethe surfacesurface andand thethe boilingboiling waterwater isis approximatelyapproximately 3030 °°CC toto 120120 °°CC aboveabove thethe TTSS.. • 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 aa vaporvapor filmfilm oror blanketblanket beginsbegins toto formform atat thethe surfacesurface..
• However, at any point on the surface, the conditions may oscillate between film and nucleate boiling, but thethe fractionfraction ofof thethe totaltotal surfacesurface coveredcovered byby thethe filmfilm increasesincreases withwith increasingincreasing temperaturetemperature differencedifference.. •• AsAs thethe thermalthermal conductivityconductivity ofof thethe vaporvapor isis muchmuch lessless thanthan thatthat ofof thethe liquid,liquid, thethe convectiveconvective heatheat transfertransfer coefficientcoefficient andand thethe heatheat fluxflux reducesreduces withwith increasingincreasing temperaturetemperature differencedifference.. • In recirculating boiler designs, it is important to limit heat release in furnace and to provide enoughenough waterwater flowflow thatthat thethe pointpoint ofof CHFCHF isis notnot reachedreached.. •• IfIf CHFCHF occurs,occurs, seriousserious damagedamage toto thethe tubestubes isis likelylikely.. • In sub-critical – pressure once through boilers, it is important that the CHF point be permitted to occur only in areas of fewfew lowlow heatheat inputinput ratesrates andand highhigh flowflow ratesrates toto avoidavoid tubetube wallwall overheatingoverheating. • However, at any point on the surface, the conditions may oscillate between film and nucleate boiling, but thethe fractionfraction ofof thethe totaltotal surfacesurface coveredcovered byby thethe filmfilm increasesincreases withwith increasingincreasing temperaturetemperature differencedifference.. •• AsAs thethe thermalthermal conductivityconductivity ofof thethe vaporvapor isis muchmuch lessless thanthan thatthat ofof thethe liquid,liquid, thethe convectiveconvective heatheat transfertransfer coefficientcoefficient andand thethe heatheat fluxflux reducesreduces withwith increasingincreasing temperaturetemperature differencedifference.. • In recirculating boiler designs, it is important to limit heat release in furnace and to provide enoughenough waterwater flowflow thatthat thethe pointpoint ofof CHFCHF isis notnot reachedreached.. •• IfIf CHFCHF occurs,occurs, seriousserious damagedamage toto thethe tubestubes isis likelylikely.. • In sub-critical – pressure once through boilers, it is important that the CHF point be permitted to occur only in areas of fewfew lowlow heatheat inputinput ratesrates andand highhigh 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, the coal burns before dropping off at the end as ash. •Some degree of skill is required, particularly when setting up the grate, air dampers and baffles, to ensureensure cleanclean combustioncombustion leavingleaving thethe minimumminimum ofof unburntunburnt carboncarbon inin thethe ashash. •The coal-feed hopper runs along the entire coal-feed end of the furnace. AA coalcoal gategate isis usedused toto controlcontrol thethe raterate atat whichwhich coalcoal isis fedfed intointo thethe furnacefurnace byby controllingcontrolling thethe thicknessthickness ofof thethe fuelfuel bedbed. • CoalCoal mustmust bebe uniformuniform inin sizesize as large lumps will not burn out completely by the time they reach the end of the grate. •Coal is fed onto one end of a moving steel grate. •As grate moves along the length of the furnace, the coal burns before dropping off at the end as ash. •Some degree of skill is required, particularly when setting up the grate, air dampers and baffles, to ensureensure cleanclean combustioncombustion leavingleaving thethe minimumminimum ofof unburntunburnt carboncarbon inin thethe ashash. •The coal-feed hopper runs along the entire coal-feed end of the furnace. AA coalcoal gategate isis usedused toto controlcontrol thethe raterate atat whichwhich coalcoal isis fedfed intointo thethe furnacefurnace byby controllingcontrolling thethe thicknessthickness ofof thethe fuelfuel bedbed. • CoalCoal mustmust bebe uniformuniform inin sizesize as large lumps will not burn out completely by the time they reach the end of the 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 grategrate burningburning.. • 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 thethe grate,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 in many industrial applications. Spread Stoker Fired BoilerSpread Stoker Fired BoilerSpread Stoker Fired BoilerSpread Stoker Fired Boiler • Spreader stokers utilizeutilize aa combinationcombination ofof suspensionsuspension burningburning andand grategrate burningburning.. • 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 thethe grate,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 in many 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 of combustion in which the new coal is heated by radiation in the presence of air and located under ignited fuel bed. The heating of coal is running less rapidly and release volatile matter combine with air, so generate low smoke • Overfeed Stoker Firing Overfeed stoker firing is the process of combustion in which thethe unignitedunignited fuelfuel oror incomingincoming coalcoal isis locatedlocated aboveabove ignitedignited fuelfuel bedbed.. The ignited fuel transfer heat to the incoming coal by radiation. Moreover coal is heated by convection from hot gases that has been through the combustion. Secondary air is added to perform complete combustion unless steam boiler will produce more smoke because the hot gases contain little oxygen. • Underfeed Stoker Firing Underfeed stoker firing is the process of combustion in which the new coal is heated by radiation in the presence of air and located under ignited fuel bed. The heating of coal is running less rapidly and release volatile matter combine with air, so generate low smoke • Overfeed Stoker Firing Overfeed stoker firing is the process of combustion in which thethe unignitedunignited fuelfuel oror incomingincoming coalcoal isis locatedlocated aboveabove ignitedignited fuelfuel bedbed.. The ignited fuel transfer heat to the incoming coal by radiation. Moreover coal is heated by convection from hot gases that has been through the combustion. Secondary air is added to perform complete combustion unless steam boiler will produce more smoke because the hot gases contain little oxygen.
Pulverized Coal BoilerPulverized Coal BoilerPulverized Coal BoilerPulverized Coal Boiler A pulverized coal-fired boiler is an industrial or utility boiler that generates thermal energy by burning pulverized coal (also known as powdered coal or coal dust). This type of boiler dominates the electric power industry, providing steam to drive large turbines. Pulverized coal provides the thermal energy which produces about 50% of the world's electric supply. A pulverized coal-fired boiler is an industrial or utility boiler that generates thermal energy by burning pulverized coal (also known as powdered coal or coal dust). This type of boiler dominates the electric power industry, providing steam to drive large turbines. Pulverized coal provides the thermal energy which produces about 50% of the world's electric 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 supercritical power plants have been developed for improvement of power efficiency. Oxy-fuel combustion technology is being pursued for carbon capture and storage. Further reduction 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 and has significant advantages over conventional firing system and offers multiple benefits :  Compact boiler design  Fuel flexibility: Coal, biomass, rice husk, bagasse & other agricultural wastes.  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, the particles are undisturbed at low velocity. • As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized”. • Fluidized bed combustion (FBC ) has emerged as a viable alternative and has significant advantages over conventional firing system and offers multiple benefits :  Compact boiler design  Fuel flexibility: Coal, biomass, rice husk, bagasse & other agricultural wastes.  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, the particles are undisturbed at low velocity. • As air velocity is gradually increased, a stage is reached when the individual 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 very slowly upwards through the mesh, it percolates between the sand particles without disturbing them. • When the velocity of the air stream is gradually increased, a point is reached when individual sand particles are forced upwards; they become supported by the air stream and begin to move about within a bed with a fairly well defined surface. • At still higher upward air velocities, an important change occurs; the bed becomes very turbulent with rapid mixing of the particles. • Bubbles, similar to those in a briskly boiling liquid, pass through the bed and 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 has not only the appearance, but also some of the properties, of a boiling fluid. • Imagine a box containing sand resting on a mesh. If air is blown very slowly upwards through the mesh, it percolates between the sand particles without disturbing them. • When the velocity of the air stream is gradually increased, a point is reached when individual sand particles are forced upwards; they become supported by the air stream and begin to move about within a bed with a fairly well defined surface. • At still higher upward air velocities, an important change occurs; the bed becomes very turbulent with rapid mixing of the particles. • Bubbles, similar to those in a briskly boiling liquid, pass through the bed and 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 has not only the appearance, but also some of the properties, of a boiling fluid.
Continued….Continued….Continued….Continued…. • There are lower and upper limits of air velocity between which satisfactory fluidization of sand, or any other granular substance, will take place. • The velocity of the air stream causing fluidization is termed 'fluidizing velocity'. • For a bed of any material, the larger the particles, the greater the velocity of 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 fluidizingfluidizing velocityvelocity 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 / gas velocity needed to keep the particles fluidized and, on the other hand, by the maximum velocity that can be used before an excessive quantity of bed particles are blown out of the bed containment box. • There are lower and upper limits of air velocity between which satisfactory fluidization of sand, or any other granular substance, will take place. • The velocity of the air stream causing fluidization is termed 'fluidizing velocity'. • For a bed of any material, the larger the particles, the greater the velocity of 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 fluidizingfluidizing velocityvelocity 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 / gas velocity needed to keep the particles fluidized and, on the other hand, by the maximum velocity that can be used before an excessive quantity of bed 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 has important 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 has important characteristicsimportant characteristics • The bed finds its own level. If the vessel containing the fluidized bed of solids is tilted from a horizontal position, the surface of the bed remains level. • Provided the fluidized state can be maintained, the bed can be transferred from one container to another as though it were a liquid. • Solid particles in a fluidized bed are violently churned about; rapid mixing occurs and any added particles are quickly distributed throughout the bed. • Objects can float or sink in a fluidised bed according to their density, as in a liquid. • When a fluidized bed is heated, the thorough mixing enables heat to be rapidly transferred from one part to another, ensuring near uniformity of temperature, as in a boiling liquid. . • Mixing in a fluidized bed causes heat to be rapidly transferred to a cooler surface (for example, a water tube) immersed in it. The constant movement brings a continuous supply of hot particles to this heat transfer surface. • The bed finds its own level. If the vessel containing the fluidized bed of solids is tilted from a horizontal position, the surface of the bed remains level. • Provided the fluidized state can be maintained, the bed can be transferred from one container to another as though it were a liquid. • Solid particles in a fluidized bed are violently churned about; rapid mixing occurs and any added particles are quickly distributed throughout the bed. • Objects can float or sink in a fluidised bed according to their density, as in a liquid. • When a fluidized bed is heated, the thorough mixing enables heat to be rapidly transferred from one part to another, ensuring near uniformity of temperature, as in a boiling liquid. . • Mixing in a fluidized bed causes heat to be rapidly transferred to a cooler surface (for example, a water tube) immersed in it. The constant movement brings a continuous supply of hot particles to this heat transfer surface.
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 boiling fluid. b) Granular solids remain in layers when one is but rapid mixing occurs on fluidization (right). c) A bed of stationary particles supports objects (left). On fluidization, an object of lower density (the green while the higher density (red ball) sinks. d) In a bed of stationary particles (left), heat there are big differences in temperature. In a fluidized bed mixing ensures uniformity of temperature. a) Sand particles resting become fluidized when (right) and take on the some of the properties of a boiling fluid. b) Granular solids remain in layers when one is but rapid mixing occurs on fluidization (right). c) A bed of stationary particles supports objects (left). On fluidization, an object of lower density (the green while the higher density (red ball) sinks. d) In a bed of stationary particles (left), heat there are big differences in temperature. In a fluidized bed mixing ensures uniformity of temperature.
• With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. •• TheThe bedbed ofof solidsolid particlesparticles exhibitsexhibits thethe propertiesproperties ofof aa boilingboiling liquidliquid andand assumesassumes thethe 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 bed attains a uniform temperature. • The fluidized bed combustion (FBC ) takes place at about 840°C to 950°C. Since this temperaturetemperature isis muchmuch belowbelow thethe ashash fusionfusion temperaturetemperature, melting of ash and associated problems are avoided. •• TheThe lowerlower combustioncombustion temperaturetemperature isis achievedachieved becausebecause ofof highhigh coefficientcoefficient ofof heatheat transfertransfer due to rapid mixing in the fluidized bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. •• TheThe gasgas velocityvelocity isis maintainedmaintained betweenbetween minimumminimum fluidisationfluidisation velocityvelocity andand particleparticle entrainmententrainment velocityvelocity. This ensures stable operation of the bed and avoids particle entrainment in the gas stream. • With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. •• TheThe bedbed ofof solidsolid particlesparticles exhibitsexhibits thethe propertiesproperties ofof aa boilingboiling liquidliquid andand assumesassumes thethe 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 bed attains a uniform temperature. • The fluidized bed combustion (FBC ) takes place at about 840°C to 950°C. Since this temperaturetemperature isis muchmuch belowbelow thethe ashash fusionfusion temperaturetemperature, melting of ash and associated problems are avoided. •• TheThe lowerlower combustioncombustion temperaturetemperature isis achievedachieved becausebecause ofof highhigh coefficientcoefficient ofof heatheat transfertransfer due to rapid mixing in the fluidized bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. •• TheThe gasgas velocityvelocity isis maintainedmaintained betweenbetween minimumminimum fluidisationfluidisation velocityvelocity andand particleparticle entrainmententrainment velocityvelocity. This ensures stable operation of the bed and avoids particle entrainment in the gas stream.
FluidizationFluidizationFluidizationFluidization • Fluidization is a two-phase process in which dispersed solid material is suspended in a stream of gas flowing upstream through the fluidized grate. The layer of solid body particles suspended in flowing gas forms fluidized bed. • The fluidized bed is in the quasi-equilibrium state only in some range of the velocity of the flowing upstream gas, depending on the size of particles of bed. • The fluidized bed of a boiler contains mainly particles of an inert material, 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 is suspended in a stream of gas flowing upstream through the fluidized grate. The layer of solid body particles suspended in flowing gas forms fluidized bed. • The fluidized bed is in the quasi-equilibrium state only in some range of the velocity of the flowing upstream gas, depending on the size of particles of bed. • The fluidized bed of a boiler contains mainly particles of an inert material, 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 bed material. • For high Sulphur coals Limestone addition to the bed material reduces SO2 emissions. • CFBC uses crushed coal of 3 to 6 mm size. This requires only a crusher not a pulverizer. • From storage hoppers Conveyer and feeders transport the coal to feed chutes in the furnace. • Start up is by oil burners in the furnace. Ash spouts in the furnace remove the ash 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 bed material. • For high Sulphur coals Limestone addition to the bed material reduces SO2 emissions. • CFBC uses crushed coal of 3 to 6 mm size. This requires only a crusher not a pulverizer. • From storage hoppers Conveyer and feeders transport the coal to feed chutes in the furnace. • Start up is by oil burners in the furnace. Ash spouts in the furnace remove the ash 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 bed material and the coal particles and keeps it in suspension. • The coal combustion takes place in this suspended condition. This is the Fluidized bed. • Special design of the air nozzles at the bottom of the bed allows air flow without 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 air in 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 bed material and the coal particles and keeps it in suspension. • The coal combustion takes place in this suspended condition. This is the Fluidized bed. • Special design of the air nozzles at the bottom of the bed allows air flow without 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 air in 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 the Forced 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 a deep bed is used to extract large amount of heat. • This will improve the combustion efficiency and sulphur dioxide absorption in the bed. • 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 combined cycle power generation. • The combined cycle operation (gas turbine & steam turbine) improves the overall conversion efficiency by 5 to 8%. • In Pressurized Fluidized Bed Combustion (PFBC ) type, a compressor supplies the Forced 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 a deep bed is used to extract large amount of heat. • This will improve the combustion efficiency and sulphur dioxide absorption in the bed. • 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 combined cycle power generation. • The combined cycle operation (gas turbine & steam turbine) improves the overall conversion 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 Bed Combustion. (AFBC ). • This involves little more than adding a fluidized bed combustor to a conventional shell boiler. • Such systems have similarly being installed in conjunction with conventional water tube boiler. • Coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type of fuel fed to the combustion chamber. • The atmospheric air, which acts as both the fluidization and combustion air, is delivered 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 of the boiler flow past the economizer, the dust collectors and the air preheater before being exhausted to atmosphere. • Most operational boiler of this type is of the Atmospheric Fluidized Bed Combustion. (AFBC ). • This involves little more than adding a fluidized bed combustor to a conventional shell boiler. • Such systems have similarly being installed in conjunction with conventional water tube boiler. • Coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type of fuel fed to the combustion chamber. • The atmospheric air, which acts as both the fluidization and combustion air, is delivered 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 of the boiler flow past the economizer, the dust collectors and the air preheater before 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 bed parameters are so maintained as to promote solids elutriation from the bed. • They are lifted in a relatively dilute phase in a solids riser, and a down- comer with a cyclone provides a return path for the solids. • There are no steam generation tubes immersed in the bed. • Generation and super heating of steam takes place in the convection section, water walls, at the exit of the riser. • In a circulating system the bed parameters are so maintained as to promote solids elutriation from the bed. • They are lifted in a relatively dilute phase in a solids riser, and a down- comer with a cyclone provides a return path for the solids. • There are no steam generation tubes immersed in the bed. • Generation and super heating of steam takes place in the convection section, water walls, at the exit of the riser.
• Fine particles of partly burned coal, ash and bed material are carried along with the flue gases to the upper areas of the furnace and then into a cyclone. • In the cyclone the heavier particles separate from the gas and falls to the hopper of the cyclone. • This returns to the furnace for recirculation. Hence the name Circulating Fluidized Bed combustion. The hot gases from the cyclone pass to the heat transfer surfaces and go out of the boiler. • For large units, the taller furnace characteristics of CFBC boilers offers better space utilization, greater fuel particle and sorbent residence time for efficient combustion and SO2 capture, and easier application of staged combustion techniques for NOx control than AFBC steam generators. • • CFBC boilers are generally more economical than AFBC boilers for industrial application requiring more than 75 – 100 T/hr of steam. • Fine particles of partly burned coal, ash and bed material are carried along with the flue gases to the upper areas of the furnace and then into a cyclone. • In the cyclone the heavier particles separate from the gas and falls to the hopper of the cyclone. • This returns to the furnace for recirculation. Hence the name Circulating Fluidized Bed combustion. The hot gases from the cyclone pass to the heat transfer surfaces and go out of the boiler. • For large units, the taller furnace characteristics of CFBC boilers offers better space utilization, greater fuel particle and sorbent residence time for efficient combustion and SO2 capture, and easier application of staged combustion techniques for NOx control than AFBC steam generators. • • CFBC boilers are generally more economical than AFBC boilers for industrial 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. Ignition temperature, flammability limit concentration (explosion limit concentration) and burning velocity (flame propagation velocity) are important ignition performance parameters. • The ignition temperature sometimes decreases when the particle diameter is increased. Such results would lead to the idea that flame stabilization becomes easy when particle diameter is increased. However, decreasing the particle diameter 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 furnace wall temperature is several hundred degrees Celsius for a water wall, and it is larger than one thousand degrees Celsius for a caster wall. • Ignition properties are fundamental combustion performance parameters for engineering design of combustion systems. • Fundamental ignition Performance parameters : coal ignition. Ignition temperature, flammability limit concentration (explosion limit concentration) and burning velocity (flame propagation velocity) are important ignition performance parameters. • The ignition temperature sometimes decreases when the particle diameter is increased. Such results would lead to the idea that flame stabilization becomes easy when particle diameter is increased. However, decreasing the particle diameter 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 furnace wall temperature is several hundred degrees Celsius for a water wall, and it is larger than one thousand degrees Celsius for a caster wall.
Relationship between coal concentration and flame propagation velocity. Relationship between coal concentration and flame propagation velocity. • Coal concentrations and flame propagation velocities are shown as normalized values. • The coal concentration was in inverse proportion to the third power of the distance d. • When the coal concentration increased, flame propagation velocity increased. • But there was an upper limit value (Sb-max) to the flame propagation velocity. • The flame propagation velocities were almost zero at the lean flammability limit. • Absolute values of L and Sb-max vary with coal properties and burning conditions. Relationship between L and Sb-max is shown in Fig. 8. Lean flammability limit; L was inversely proportional to maximum flame propagation velocity; Sb-max. • Coal concentrations and flame propagation velocities are shown as normalized values. • The coal concentration was in inverse proportion to the third power of the distance d. • When the coal concentration increased, flame propagation velocity increased. • But there was an upper limit value (Sb-max) to the flame propagation velocity. • The flame propagation velocities were almost zero at the lean flammability limit. • Absolute values of L and Sb-max vary with coal properties and burning conditions. Relationship between L and Sb-max is shown in Fig. 8. Lean flammability limit; L was inversely proportional to maximum flame propagation velocity; Sb-max.

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Boilers

  • 1.
    BoilersBoilers Dr. Rohit SinghLather, Ph.D.Dr. Rohit Singh Lather, Ph.D.
  • 2.
  • 3.
    Different Types ofBoilersDifferent 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
  • 4.
    BASIC MODEL FORIGNITION 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 heat of 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 not ignited yet. • Flame propagation is observed if the first burning particle can transfer the flame to the next particle before the volatile matter combustion of the first particle has finished. The distance between particles - d and The 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 heat of 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 not ignited yet. • Flame propagation is observed if the first burning particle can transfer the flame to the next particle before the volatile matter combustion of the first particle has finished. The distance between particles - d and The time of flame propagation - s. Flame propagation velocity Sb was defined as the value of d divided by s.
  • 5.
    Combustion of CoalCombustionof CoalCombustion of CoalCombustion of Coal
  • 6.
    It is necessaryto 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.
  • 8.
    Furnace Wall HeatTransferFurnace 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 occurs as 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
  • 9.
    • The boilertube 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 the tube on the water layer flowing along the tubewall. •• InIn filmfilm boilingboiling, a thin film of superheated steam covers the inside of the tube wall separating the metal from the liquid water. • Heat transfer through the steam film is much lower than that through water, which means that if film boiling occurs, tube wall temperature will climb and the tube wall is likely to overheat if it is in a high heat input zone. •• TheThe changechange fromfrom nucleatenucleate boilingboiling toto filmfilm boilingboiling isis referredreferred asas thethe CriticalCritical HeatHeat FluxFlux (CHF)(CHF) pointpoint.. At this point in the maximum, considerable vapor is being formed, making it difficult for the liquid to continuously wet the surface to receive heat from the surface. This causes the heat flux to reduce after this point. At extremes, film boiling commonly known as the LeidenfrostLeidenfrost effecteffect. • Depending upon the conditions, it is also referred to as the DepartureDeparture from 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 the tube on the water layer flowing along the tubewall. •• InIn filmfilm boilingboiling, a thin film of superheated steam covers the inside of the tube wall separating the metal from the liquid water. • Heat transfer through the steam film is much lower than that through water, which means that if film boiling occurs, tube wall temperature will climb and the tube wall is likely to overheat if it is in a high heat input zone. •• TheThe changechange fromfrom nucleatenucleate boilingboiling toto filmfilm boilingboiling isis referredreferred asas thethe CriticalCritical HeatHeat FluxFlux (CHF)(CHF) pointpoint.. At this point in the maximum, considerable vapor is being formed, making it difficult for the liquid to continuously wet the surface to receive heat from the surface. This causes the heat flux to reduce after this point. At extremes, film boiling commonly known as the LeidenfrostLeidenfrost effecteffect. • Depending upon the conditions, it is also referred to as the DepartureDeparture from Nucleate Boiling (DNB) or Dry Out (DO).from Nucleate Boiling (DNB) or Dry Out (DO).
  • 10.
    • The processof forming steam bubbles within liquid in micro cavities adjacent to the wall if the wall temperature at the heat transfer surface rises above the saturation temperature while the bulk of the liquid (heat exchanger) is subcooled. • The bubbles grow until they reach some critical size, at which point they separate from the wall and are carried into the main fluid stream. • There the bubbles collapse because the temperature of bulk fluid is not as high as at the heat transfer surface, where the bubbles were created. • This collapsing is also responsible for the sound a water kettle produces during heat up but before the temperature at which bulk boiling is reached. • The process of forming steam bubbles within liquid in micro cavities adjacent to the wall if the wall temperature at the heat transfer surface rises above the saturation temperature while the bulk of the liquid (heat exchanger) is subcooled. • The bubbles grow until they reach some critical size, at which point they separate from the wall and are carried into the main fluid stream. • There the bubbles collapse because the temperature of bulk fluid is not as high as at the heat transfer surface, where the bubbles were created. • This collapsing is also responsible for the sound a water kettle produces during heat up but before the temperature at which bulk boiling is reached.
  • 14.
    Departure from NucleateBoilingDeparture from Nucleate BoilingDeparture from Nucleate BoilingDeparture from Nucleate Boiling • If the heat flux of a boiling system is higher than the critical heat flux(CHF) of the system, the bulk fluid may boil, or in some cases, regions of the bulk fluid may boil where the fluid travels in small channels. • Thus large bubbles form, sometimes blocking the passage of the fluid. • This results in a departure from nucleate boiling (DNB) in which steam bubbles no longer break away from the solid surface of the channel, bubbles dominate the channel or surface, and the heat flux 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 maintain a high heat flux. • If the heat flux of a boiling system is higher than the critical heat flux(CHF) of the system, the bulk fluid may boil, or in some cases, regions of the bulk fluid may boil where the fluid travels in small channels. • Thus large bubbles form, sometimes blocking the passage of the fluid. • This results in a departure from nucleate boiling (DNB) in which steam bubbles no longer break away from the solid surface of the channel, bubbles dominate the channel or surface, and the heat flux 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 maintain a high heat flux.
  • 15.
    Parts of BoilerPartsof BoilerParts of BoilerParts of Boiler
  • 16.
    • DNB maybe avoided in practice by increasing the pressure of the fluid, increasing its flow rate, or by utilizing a lower temperature bulk fluid which has a higher CHF. •• IfIf thethe bulkbulk fluidfluid temperaturetemperature isis tootoo lowlow oror thethe pressurepressure ofof thethe fluidfluid isis tootoo high,high, nucleatenucleate boilingboiling isis howeverhowever notnot possiblepossible.. • DNB is also known as TransitionTransition boiling,boiling, unstableunstable filmfilm boiling,boiling, andand partialpartial filmfilm boilingboiling.. • Transition boiling occurs when the temperature differencedifference betweenbetween thethe surfacesurface andand thethe boilingboiling waterwater isis approximatelyapproximately 3030 °°CC toto 120120 °°CC aboveabove thethe TTSS.. • 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 aa vaporvapor 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 fluid which has a higher CHF. •• IfIf thethe bulkbulk fluidfluid temperaturetemperature isis tootoo lowlow oror thethe pressurepressure ofof thethe fluidfluid isis tootoo high,high, nucleatenucleate boilingboiling isis howeverhowever notnot possiblepossible.. • DNB is also known as TransitionTransition boiling,boiling, unstableunstable filmfilm boiling,boiling, andand partialpartial filmfilm boilingboiling.. • Transition boiling occurs when the temperature differencedifference betweenbetween thethe surfacesurface andand thethe boilingboiling waterwater isis approximatelyapproximately 3030 °°CC toto 120120 °°CC aboveabove thethe TTSS.. • 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 aa vaporvapor filmfilm oror blanketblanket beginsbegins toto formform atat thethe surfacesurface..
  • 17.
    • However, atany point on the surface, the conditions may oscillate between film and nucleate boiling, but thethe fractionfraction ofof thethe totaltotal surfacesurface coveredcovered byby thethe filmfilm increasesincreases withwith increasingincreasing temperaturetemperature differencedifference.. •• AsAs thethe thermalthermal conductivityconductivity ofof thethe vaporvapor isis muchmuch lessless thanthan thatthat ofof thethe liquid,liquid, thethe convectiveconvective heatheat transfertransfer coefficientcoefficient andand thethe heatheat fluxflux reducesreduces withwith increasingincreasing temperaturetemperature differencedifference.. • In recirculating boiler designs, it is important to limit heat release in furnace and to provide enoughenough waterwater flowflow thatthat thethe pointpoint ofof CHFCHF isis notnot reachedreached.. •• IfIf CHFCHF occurs,occurs, seriousserious damagedamage toto thethe tubestubes isis likelylikely.. • In sub-critical – pressure once through boilers, it is important that the CHF point be permitted to occur only in areas of fewfew lowlow heatheat inputinput ratesrates andand highhigh flowflow ratesrates toto avoidavoid tubetube wallwall overheatingoverheating. • However, at any point on the surface, the conditions may oscillate between film and nucleate boiling, but thethe fractionfraction ofof thethe totaltotal surfacesurface coveredcovered byby thethe filmfilm increasesincreases withwith increasingincreasing temperaturetemperature differencedifference.. •• AsAs thethe thermalthermal conductivityconductivity ofof thethe vaporvapor isis muchmuch lessless thanthan thatthat ofof thethe liquid,liquid, thethe convectiveconvective heatheat transfertransfer coefficientcoefficient andand thethe heatheat fluxflux reducesreduces withwith increasingincreasing temperaturetemperature differencedifference.. • In recirculating boiler designs, it is important to limit heat release in furnace and to provide enoughenough waterwater flowflow thatthat thethe pointpoint ofof CHFCHF isis notnot reachedreached.. •• IfIf CHFCHF occurs,occurs, seriousserious damagedamage toto thethe tubestubes isis likelylikely.. • In sub-critical – pressure once through boilers, it is important that the CHF point be permitted to occur only in areas of fewfew lowlow heatheat inputinput ratesrates andand highhigh flowflow ratesrates toto avoidavoid tubetube wallwall overheatingoverheating.
  • 18.
    Travelling Grate FiredBoilerTravelling 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, the coal burns before dropping off at the end as ash. •Some degree of skill is required, particularly when setting up the grate, air dampers and baffles, to ensureensure cleanclean combustioncombustion leavingleaving thethe minimumminimum ofof unburntunburnt carboncarbon inin thethe ashash. •The coal-feed hopper runs along the entire coal-feed end of the furnace. AA coalcoal gategate isis usedused toto controlcontrol thethe raterate atat whichwhich coalcoal isis fedfed intointo thethe furnacefurnace byby controllingcontrolling thethe thicknessthickness ofof thethe fuelfuel bedbed. • CoalCoal mustmust bebe uniformuniform inin sizesize as large lumps will not burn out completely by the time they reach the end of the grate. •Coal is fed onto one end of a moving steel grate. •As grate moves along the length of the furnace, the coal burns before dropping off at the end as ash. •Some degree of skill is required, particularly when setting up the grate, air dampers and baffles, to ensureensure cleanclean combustioncombustion leavingleaving thethe minimumminimum ofof unburntunburnt carboncarbon inin thethe ashash. •The coal-feed hopper runs along the entire coal-feed end of the furnace. AA coalcoal gategate isis usedused toto controlcontrol thethe raterate atat whichwhich coalcoal isis fedfed intointo thethe furnacefurnace byby controllingcontrolling thethe thicknessthickness ofof thethe fuelfuel bedbed. • CoalCoal mustmust bebe uniformuniform inin sizesize as large lumps will not burn out completely by the time they reach the end of the grate.
  • 19.
    A Typical TravellingGrateA Typical Travelling GrateA Typical Travelling GrateA Typical Travelling Grate
  • 20.
    Schematic Travelling GrateFired BoilersSchematic Travelling Grate Fired Boilers
  • 21.
    • Spreader stokersutilizeutilize aa combinationcombination ofof suspensionsuspension burningburning andand grategrate burningburning.. • 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 thethe grate,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 in many industrial applications. Spread Stoker Fired BoilerSpread Stoker Fired BoilerSpread Stoker Fired BoilerSpread Stoker Fired Boiler • Spreader stokers utilizeutilize aa combinationcombination ofof suspensionsuspension burningburning andand grategrate burningburning.. • 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 thethe grate,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 in many industrial applications.
  • 22.
    Coal and GasSpread Stoker Fired BoilerCoal and Gas Spread Stoker Fired BoilerCoal and Gas Spread Stoker Fired BoilerCoal and Gas Spread Stoker Fired Boiler COALCOAL GASGAS
  • 23.
    • Underfeed StokerFiring Underfeed stoker firing is the process of combustion in which the new coal is heated by radiation in the presence of air and located under ignited fuel bed. The heating of coal is running less rapidly and release volatile matter combine with air, so generate low smoke • Overfeed Stoker Firing Overfeed stoker firing is the process of combustion in which thethe unignitedunignited fuelfuel oror incomingincoming coalcoal isis locatedlocated aboveabove ignitedignited fuelfuel bedbed.. The ignited fuel transfer heat to the incoming coal by radiation. Moreover coal is heated by convection from hot gases that has been through the combustion. Secondary air is added to perform complete combustion unless steam boiler will produce more smoke because the hot gases contain little oxygen. • Underfeed Stoker Firing Underfeed stoker firing is the process of combustion in which the new coal is heated by radiation in the presence of air and located under ignited fuel bed. The heating of coal is running less rapidly and release volatile matter combine with air, so generate low smoke • Overfeed Stoker Firing Overfeed stoker firing is the process of combustion in which thethe unignitedunignited fuelfuel oror incomingincoming coalcoal isis locatedlocated aboveabove ignitedignited fuelfuel bedbed.. The ignited fuel transfer heat to the incoming coal by radiation. Moreover coal is heated by convection from hot gases that has been through the combustion. Secondary air is added to perform complete combustion unless steam boiler will produce more smoke because the hot gases contain little oxygen.
  • 24.
    Pulverized Coal BoilerPulverizedCoal BoilerPulverized Coal BoilerPulverized Coal Boiler A pulverized coal-fired boiler is an industrial or utility boiler that generates thermal energy by burning pulverized coal (also known as powdered coal or coal dust). This type of boiler dominates the electric power industry, providing steam to drive large turbines. Pulverized coal provides the thermal energy which produces about 50% of the world's electric supply. A pulverized coal-fired boiler is an industrial or utility boiler that generates thermal energy by burning pulverized coal (also known as powdered coal or coal dust). This type of boiler dominates the electric power industry, providing steam to drive large turbines. Pulverized coal provides the thermal energy which produces about 50% of the world's electric supply.
  • 25.
    A Typical PulverizedCoal PlantA Typical Pulverized Coal PlantA Typical Pulverized Coal PlantA Typical Pulverized Coal Plant
  • 26.
    Pulverized Power PlantSystemPulverized 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
  • 27.
    Tangential Firing ofCoalTangential Firing of CoalTangential Firing of CoalTangential Firing of Coal
  • 29.
    Thermal efficiencies andCO₂ 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 supercritical power plants have been developed for improvement of power efficiency. Oxy-fuel combustion technology is being pursued for carbon capture and storage. Further reduction of the environmental load, such as NOx reduction, is also still required.
  • 30.
    Fluidized Bed CombustionFluidizedBed CombustionFluidized Bed CombustionFluidized Bed Combustion • Fluidized bed combustion (FBC ) has emerged as a viable alternative and has significant advantages over conventional firing system and offers multiple benefits :  Compact boiler design  Fuel flexibility: Coal, biomass, rice husk, bagasse & other agricultural wastes.  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, the particles are undisturbed at low velocity. • As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized”. • Fluidized bed combustion (FBC ) has emerged as a viable alternative and has significant advantages over conventional firing system and offers multiple benefits :  Compact boiler design  Fuel flexibility: Coal, biomass, rice husk, bagasse & other agricultural wastes.  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, the particles are undisturbed at low velocity. • As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized”.
  • 31.
    Continued….Continued….Continued….Continued…. • Imagine abox containing sand resting on a mesh. If air is blown very slowly upwards through the mesh, it percolates between the sand particles without disturbing them. • When the velocity of the air stream is gradually increased, a point is reached when individual sand particles are forced upwards; they become supported by the air stream and begin to move about within a bed with a fairly well defined surface. • At still higher upward air velocities, an important change occurs; the bed becomes very turbulent with rapid mixing of the particles. • Bubbles, similar to those in a briskly boiling liquid, pass through the bed and 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 has not only the appearance, but also some of the properties, of a boiling fluid. • Imagine a box containing sand resting on a mesh. If air is blown very slowly upwards through the mesh, it percolates between the sand particles without disturbing them. • When the velocity of the air stream is gradually increased, a point is reached when individual sand particles are forced upwards; they become supported by the air stream and begin to move about within a bed with a fairly well defined surface. • At still higher upward air velocities, an important change occurs; the bed becomes very turbulent with rapid mixing of the particles. • Bubbles, similar to those in a briskly boiling liquid, pass through the bed and 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 has not only the appearance, but also some of the properties, of a boiling fluid.
  • 32.
    Continued….Continued….Continued….Continued…. • There arelower and upper limits of air velocity between which satisfactory fluidization of sand, or any other granular substance, will take place. • The velocity of the air stream causing fluidization is termed 'fluidizing velocity'. • For a bed of any material, the larger the particles, the greater the velocity of 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 fluidizingfluidizing velocityvelocity 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 / gas velocity needed to keep the particles fluidized and, on the other hand, by the maximum velocity that can be used before an excessive quantity of bed particles are blown out of the bed containment box. • There are lower and upper limits of air velocity between which satisfactory fluidization of sand, or any other granular substance, will take place. • The velocity of the air stream causing fluidization is termed 'fluidizing velocity'. • For a bed of any material, the larger the particles, the greater the velocity of 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 fluidizingfluidizing velocityvelocity 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 / gas velocity needed to keep the particles fluidized and, on the other hand, by the maximum velocity that can be used before an excessive quantity of bed particles are blown out of the bed containment box.
  • 33.
    A fluidized bedof solids behaves in many ways like a liquid and hasA fluidized bed of solids behaves in many ways like a liquid and has important 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 has important characteristicsimportant characteristics • The bed finds its own level. If the vessel containing the fluidized bed of solids is tilted from a horizontal position, the surface of the bed remains level. • Provided the fluidized state can be maintained, the bed can be transferred from one container to another as though it were a liquid. • Solid particles in a fluidized bed are violently churned about; rapid mixing occurs and any added particles are quickly distributed throughout the bed. • Objects can float or sink in a fluidised bed according to their density, as in a liquid. • When a fluidized bed is heated, the thorough mixing enables heat to be rapidly transferred from one part to another, ensuring near uniformity of temperature, as in a boiling liquid. . • Mixing in a fluidized bed causes heat to be rapidly transferred to a cooler surface (for example, a water tube) immersed in it. The constant movement brings a continuous supply of hot particles to this heat transfer surface. • The bed finds its own level. If the vessel containing the fluidized bed of solids is tilted from a horizontal position, the surface of the bed remains level. • Provided the fluidized state can be maintained, the bed can be transferred from one container to another as though it were a liquid. • Solid particles in a fluidized bed are violently churned about; rapid mixing occurs and any added particles are quickly distributed throughout the bed. • Objects can float or sink in a fluidised bed according to their density, as in a liquid. • When a fluidized bed is heated, the thorough mixing enables heat to be rapidly transferred from one part to another, ensuring near uniformity of temperature, as in a boiling liquid. . • Mixing in a fluidized bed causes heat to be rapidly transferred to a cooler surface (for example, a water tube) immersed in it. The constant movement brings a continuous supply of hot particles to this heat transfer surface.
  • 34.
    Fluidization of solidsFluidizationof solidsFluidization of solidsFluidization of solids a) Sand particles resting become fluidized when (right) and take on the some of the properties of a boiling fluid. b) Granular solids remain in layers when one is but rapid mixing occurs on fluidization (right). c) A bed of stationary particles supports objects (left). On fluidization, an object of lower density (the green while the higher density (red ball) sinks. d) In a bed of stationary particles (left), heat there are big differences in temperature. In a fluidized bed mixing ensures uniformity of temperature. a) Sand particles resting become fluidized when (right) and take on the some of the properties of a boiling fluid. b) Granular solids remain in layers when one is but rapid mixing occurs on fluidization (right). c) A bed of stationary particles supports objects (left). On fluidization, an object of lower density (the green while the higher density (red ball) sinks. d) In a bed of stationary particles (left), heat there are big differences in temperature. In a fluidized bed mixing ensures uniformity of temperature.
  • 35.
    • With furtherincrease in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. •• TheThe bedbed ofof solidsolid particlesparticles exhibitsexhibits thethe propertiesproperties ofof aa boilingboiling liquidliquid andand assumesassumes thethe 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 bed attains a uniform temperature. • The fluidized bed combustion (FBC ) takes place at about 840°C to 950°C. Since this temperaturetemperature isis muchmuch belowbelow thethe ashash fusionfusion temperaturetemperature, melting of ash and associated problems are avoided. •• TheThe lowerlower combustioncombustion temperaturetemperature isis achievedachieved becausebecause ofof highhigh coefficientcoefficient ofof heatheat transfertransfer due to rapid mixing in the fluidized bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. •• TheThe gasgas velocityvelocity isis maintainedmaintained betweenbetween minimumminimum fluidisationfluidisation velocityvelocity andand particleparticle entrainmententrainment velocityvelocity. This ensures stable operation of the bed and avoids particle entrainment in the gas stream. • With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. •• TheThe bedbed ofof solidsolid particlesparticles exhibitsexhibits thethe propertiesproperties ofof aa boilingboiling liquidliquid andand assumesassumes thethe 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 bed attains a uniform temperature. • The fluidized bed combustion (FBC ) takes place at about 840°C to 950°C. Since this temperaturetemperature isis muchmuch belowbelow thethe ashash fusionfusion temperaturetemperature, melting of ash and associated problems are avoided. •• TheThe lowerlower combustioncombustion temperaturetemperature isis achievedachieved becausebecause ofof highhigh coefficientcoefficient ofof heatheat transfertransfer due to rapid mixing in the fluidized bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. •• TheThe gasgas velocityvelocity isis maintainedmaintained betweenbetween minimumminimum fluidisationfluidisation velocityvelocity andand particleparticle entrainmententrainment velocityvelocity. This ensures stable operation of the bed and avoids particle entrainment in the gas stream.
  • 36.
    FluidizationFluidizationFluidizationFluidization • Fluidization isa two-phase process in which dispersed solid material is suspended in a stream of gas flowing upstream through the fluidized grate. The layer of solid body particles suspended in flowing gas forms fluidized bed. • The fluidized bed is in the quasi-equilibrium state only in some range of the velocity of the flowing upstream gas, depending on the size of particles of bed. • The fluidized bed of a boiler contains mainly particles of an inert material, 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 is suspended in a stream of gas flowing upstream through the fluidized grate. The layer of solid body particles suspended in flowing gas forms fluidized bed. • The fluidized bed is in the quasi-equilibrium state only in some range of the velocity of the flowing upstream gas, depending on the size of particles of bed. • The fluidized bed of a boiler contains mainly particles of an inert material, 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.
  • 37.
    Bed MaterialBed MaterialBedMaterialBed 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 bed material. • For high Sulphur coals Limestone addition to the bed material reduces SO2 emissions. • CFBC uses crushed coal of 3 to 6 mm size. This requires only a crusher not a pulverizer. • From storage hoppers Conveyer and feeders transport the coal to feed chutes in the furnace. • Start up is by oil burners in the furnace. Ash spouts in the furnace remove the ash 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 bed material. • For high Sulphur coals Limestone addition to the bed material reduces SO2 emissions. • CFBC uses crushed coal of 3 to 6 mm size. This requires only a crusher not a pulverizer. • From storage hoppers Conveyer and feeders transport the coal to feed chutes in the furnace. • Start up is by oil burners in the furnace. Ash spouts in the furnace remove the ash from the bottom of the furnace.
  • 38.
    Fluidized BedFluidized BedFluidizedBedFluidized 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 bed material and the coal particles and keeps it in suspension. • The coal combustion takes place in this suspended condition. This is the Fluidized bed. • Special design of the air nozzles at the bottom of the bed allows air flow without 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 air in 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 bed material and the coal particles and keeps it in suspension. • The coal combustion takes place in this suspended condition. This is the Fluidized bed. • Special design of the air nozzles at the bottom of the bed allows air flow without 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 air in the furnace.
  • 39.
  • 40.
  • 41.
    Air flow velocityin 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
  • 42.
    FBC featuresFBC featuresFBCfeaturesFBC 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
  • 43.
    Structure of fluidizedbedStructure 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)
  • 45.
    Bubbling CirculatingBubbling CirculatingBubblingCirculatingBubbling Circulating
  • 46.
    Concerning the Pressurein 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).
  • 47.
    Pressurized Fluidized BedCombustionPressurized Fluidized Bed CombustionPressurized Fluidized Bed CombustionPressurized Fluidized Bed Combustion • In Pressurized Fluidized Bed Combustion (PFBC ) type, a compressor supplies the Forced 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 a deep bed is used to extract large amount of heat. • This will improve the combustion efficiency and sulphur dioxide absorption in the bed. • 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 combined cycle power generation. • The combined cycle operation (gas turbine & steam turbine) improves the overall conversion efficiency by 5 to 8%. • In Pressurized Fluidized Bed Combustion (PFBC ) type, a compressor supplies the Forced 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 a deep bed is used to extract large amount of heat. • This will improve the combustion efficiency and sulphur dioxide absorption in the bed. • 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 combined cycle power generation. • The combined cycle operation (gas turbine & steam turbine) improves the overall conversion efficiency by 5 to 8%.
  • 48.
    Atmospheric Fluidized BedCombustion (AFBC) Boiler Atmospheric Fluidized Bed Combustion (AFBC) Boiler • Most operational boiler of this type is of the Atmospheric Fluidized Bed Combustion. (AFBC ). • This involves little more than adding a fluidized bed combustor to a conventional shell boiler. • Such systems have similarly being installed in conjunction with conventional water tube boiler. • Coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type of fuel fed to the combustion chamber. • The atmospheric air, which acts as both the fluidization and combustion air, is delivered 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 of the boiler flow past the economizer, the dust collectors and the air preheater before being exhausted to atmosphere. • Most operational boiler of this type is of the Atmospheric Fluidized Bed Combustion. (AFBC ). • This involves little more than adding a fluidized bed combustor to a conventional shell boiler. • Such systems have similarly being installed in conjunction with conventional water tube boiler. • Coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type of fuel fed to the combustion chamber. • The atmospheric air, which acts as both the fluidization and combustion air, is delivered 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 of the boiler flow past the economizer, the dust collectors and the air preheater before being exhausted to atmosphere.
  • 49.
    Circulating Fluidized BedCombustion 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 bed parameters are so maintained as to promote solids elutriation from the bed. • They are lifted in a relatively dilute phase in a solids riser, and a down- comer with a cyclone provides a return path for the solids. • There are no steam generation tubes immersed in the bed. • Generation and super heating of steam takes place in the convection section, water walls, at the exit of the riser. • In a circulating system the bed parameters are so maintained as to promote solids elutriation from the bed. • They are lifted in a relatively dilute phase in a solids riser, and a down- comer with a cyclone provides a return path for the solids. • There are no steam generation tubes immersed in the bed. • Generation and super heating of steam takes place in the convection section, water walls, at the exit of the riser.
  • 50.
    • Fine particlesof partly burned coal, ash and bed material are carried along with the flue gases to the upper areas of the furnace and then into a cyclone. • In the cyclone the heavier particles separate from the gas and falls to the hopper of the cyclone. • This returns to the furnace for recirculation. Hence the name Circulating Fluidized Bed combustion. The hot gases from the cyclone pass to the heat transfer surfaces and go out of the boiler. • For large units, the taller furnace characteristics of CFBC boilers offers better space utilization, greater fuel particle and sorbent residence time for efficient combustion and SO2 capture, and easier application of staged combustion techniques for NOx control than AFBC steam generators. • • CFBC boilers are generally more economical than AFBC boilers for industrial application requiring more than 75 – 100 T/hr of steam. • Fine particles of partly burned coal, ash and bed material are carried along with the flue gases to the upper areas of the furnace and then into a cyclone. • In the cyclone the heavier particles separate from the gas and falls to the hopper of the cyclone. • This returns to the furnace for recirculation. Hence the name Circulating Fluidized Bed combustion. The hot gases from the cyclone pass to the heat transfer surfaces and go out of the boiler. • For large units, the taller furnace characteristics of CFBC boilers offers better space utilization, greater fuel particle and sorbent residence time for efficient combustion and SO2 capture, and easier application of staged combustion techniques for NOx control than AFBC steam generators. • • CFBC boilers are generally more economical than AFBC boilers for industrial application requiring more than 75 – 100 T/hr of steam.
  • 51.
    Types of CirculatingFB BoilersTypes of Circulating FB BoilersTypes of Circulating FB BoilersTypes of Circulating FB Boilers
  • 52.
    CFB Solid WasteSeparatorCFB Solid Waste SeparatorCFB Solid Waste SeparatorCFB Solid Waste Separator
  • 53.
    Internals of CFBInternalsof CFB -- 670670Internals of CFBInternals of CFB -- 670670
  • 54.
    Details of theBedDetails of the BedDetails of the BedDetails of the Bed
  • 55.
    Structure of BFBStructureof BFBStructure of BFBStructure of BFB Burring of coal particles in FB
  • 56.
    Air NozzlesAir NozzlesAirNozzlesAir Nozzles
  • 57.
    Sources of UnburnedCarbon 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
  • 58.
    Fuel HandlingFuel HandlingFuelHandlingFuel Handling Worm - Gear Coal Handling Pneumatic Handling
  • 60.
  • 61.
    Advantages of FBfiring 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.
  • 62.
  • 63.
    INTREX (Integrated HeatExchanger Technology)INTREX (Integrated Heat Exchanger Technology)INTREX (Integrated Heat Exchanger Technology)INTREX (Integrated Heat Exchanger Technology)
  • 64.
    11stst Generation PFBC,GenerationPFBC, KaritaKarita, Japan, Japan11stst Generation PFBC,Generation PFBC, KaritaKarita, Japan, Japan
  • 65.
    22ndnd Generation PCFBGenerationPCFB22ndnd Generation PCFBGeneration PCFB
  • 66.
    22ndnd Generation PFCB,Cottbus GermanyGeneration PFCB, Cottbus Germany22ndnd Generation PFCB, Cottbus GermanyGeneration PFCB, Cottbus Germany
  • 67.
    Super Critical Oncethrough CFB BoilerSuper Critical Once through CFB BoilerSuper Critical Once through CFB BoilerSuper Critical Once through CFB Boiler
  • 75.
    Ignition • Ignition propertiesare fundamental combustion performance parameters for engineering design of combustion systems. • Fundamental ignition Performance parameters : coal ignition. Ignition temperature, flammability limit concentration (explosion limit concentration) and burning velocity (flame propagation velocity) are important ignition performance parameters. • The ignition temperature sometimes decreases when the particle diameter is increased. Such results would lead to the idea that flame stabilization becomes easy when particle diameter is increased. However, decreasing the particle diameter 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 furnace wall temperature is several hundred degrees Celsius for a water wall, and it is larger than one thousand degrees Celsius for a caster wall. • Ignition properties are fundamental combustion performance parameters for engineering design of combustion systems. • Fundamental ignition Performance parameters : coal ignition. Ignition temperature, flammability limit concentration (explosion limit concentration) and burning velocity (flame propagation velocity) are important ignition performance parameters. • The ignition temperature sometimes decreases when the particle diameter is increased. Such results would lead to the idea that flame stabilization becomes easy when particle diameter is increased. However, decreasing the particle diameter 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 furnace wall temperature is several hundred degrees Celsius for a water wall, and it is larger than one thousand degrees Celsius for a caster wall.
  • 76.
    Relationship between coalconcentration and flame propagation velocity. Relationship between coal concentration and flame propagation velocity. • Coal concentrations and flame propagation velocities are shown as normalized values. • The coal concentration was in inverse proportion to the third power of the distance d. • When the coal concentration increased, flame propagation velocity increased. • But there was an upper limit value (Sb-max) to the flame propagation velocity. • The flame propagation velocities were almost zero at the lean flammability limit. • Absolute values of L and Sb-max vary with coal properties and burning conditions. Relationship between L and Sb-max is shown in Fig. 8. Lean flammability limit; L was inversely proportional to maximum flame propagation velocity; Sb-max. • Coal concentrations and flame propagation velocities are shown as normalized values. • The coal concentration was in inverse proportion to the third power of the distance d. • When the coal concentration increased, flame propagation velocity increased. • But there was an upper limit value (Sb-max) to the flame propagation velocity. • The flame propagation velocities were almost zero at the lean flammability limit. • Absolute values of L and Sb-max vary with coal properties and burning conditions. Relationship between L and Sb-max is shown in Fig. 8. Lean flammability limit; L was inversely proportional to maximum flame propagation velocity; Sb-max.