Heat Transfer in Fluid beds

Special Topic in ME6204 Convective HeatTransfer:Heat Transfer in Fluidized Beds-An OverviewProfessor A. S. MujumdarME Department, NUS, 2010

ContentsIntroductionParticle CharacterizationFlow past spheres, non-spheresHeat transfer from spheres under large Re rangeFluid Beds – Concept and modificationsGeldart’s ClassificationHeat TransferBed to Immersed Surface Heat Transfer – CorrelationsSample CalculationsFB Drying – Plug flow vs Well-mixedFluid be combustion (Simple Model)Example of Fluid Bed CalculationClosing Remarks

Introduction• fluidized bed describes a finely granulated layer of solid material (referred to as “themass”) that is loosened by fluid flowing through to such an extent that the particlesof solid material are free to move to a certain degree• It is called “fluidized” because the solid material takes on properties similar to thoseof a fluid (liquid)• Fluidized beds are used widely in engineering for applications such as combustion,reactors, drying, gasification of coal/biomass, thermal treatments of metal andpowder coating, granulation, heat transfer etc ……• Gas-solid fluidized systems are characterized by temperature uniformity and highheat transfer coefficients, due to the intense mixture of the solid material by thepresence of gas bubbles• Liquid- solid fluidized beds have also been used for wide applications• Heat transfer in fluid beds can be from Fluid – Particle, Particle – Fluid, Wall – Fluidand to immersed surfaces, if any- care needed in calculation method used!

Particle CharacterizationParticle SizeVolume Diameter (d V )Surface Diameter (d S )Surface-Volume Diameter (d SV )Sieve DiameterStokes Diameter (d st )Free Falling DiameterDrag DiameterProjected Area DiameterFeret DiameterMartin Diameter ….etcd v = 6V 1⁄3pπ d S = S 1⁄2pπ d S v = 6V pS p= d v 3d s2d st =18μU tρ p −ρ s gCommonly used for applications in packed and fluid bedV P – Volume of the particle; S P – Surface area of the particle;U t – terminal settling velocity of the particle

Particle Characterization contd……ShapeSphereCylinder with length (l y ) equal todiameter (d y )Cylinder d y ≠ l yRing with outside diameter of d o , andinside diameter of d iMixed sizesEquivalent particle diameter, d p = d sv = 6/a sd p = diameter of sphered p = d y , the diameter of cylinder6d yd p = 4 + 2 d y ⁄ l yd p = 1.5(d o − d i )1d p =∑ix i ⁄ d pi Irregular shapes with ф = 0.5 to 0.7 d ;p = ф d vd v ≈ d ATable: Suggested equivalent particle diameters for catalysts in catalytic reactor application

Particle Characterization contd……Particle ShapeSphericity (ф)Circularity (⊄)Surface area of volume equivalent sphereф=Surface area of particleCircumference of a circle having samecross − sectional area as the particle⊄=Actual Perim eter of the cross −sectionOperational Sphericity and CircularityHeywood Shape FactorParticle Densityk= V pd a3d a =4A p ⁄ πd a – Projected area diameter; A P – Projected area; V p – Volume of particle

Methods for Direct CharacterizationSieve AnalysisIn U.S. – Taylor sieve size and U.S. Sieve sizeIn Europe – British standard and German DIN sieve sizeMesh Number – number of parallel wires per inchImaging Techniques: Direct measurements using enlarged photographic or electronic imagesof microscopeOptical microscope (1µm to 150 µm)Scanning electron microscope (SEM, 5µm to 0.01 µm)Transmission electron microscope (TEM , 5µm to 0.01 µm)Gravity and Centrifugal SedimentationCharacterization by ElutriationCascade Impaction TechniqueResistivity and Optical Zone Sensing TechniquesCoulter Counter

Particle size ranges for various methodsParticle sizing methodSievingDryWetMicroscopic examinationOpticalElectronicZone SensingResistivityOpticalElutriationLaminar FlowCycloneGravity SedimentationPipette and hydrometerPhotoextinctionX-rayApplicable particlesize (mm)>102 – 5001.0 – 1000.01 – 5000.6 – 12001.0 – 8003 – 758 – 501 – 1000.5 – 1000.1 – 130Measured DimensionSieve diameterLength, Projected AreaStatistical diameterVolumeStokes diameterStokes diameter

Mechanical Properties of ParticlesHardgrove Grindability Index (HGI)Higher the HGI higher the grindability of materialAttrition IndexImportant in fluidization, can affect entrainment and elutriationSolids impaction on plateAbrasiveness Indexfactor used to determine the effective rate of wear of theaforementioned consumablesErosiveness Index

Dimensionless numbersDimensionless numbers of interestSignificanceFor FB Heat Transfer

Flow passed over a single particleParticle Drag CoefficientRatio of force on the particle and the fluid dynamicpressure caused by the fluid times the projectedarea of the particleC D =(1 2Stokes LawDrag force resisting very slow steady relative motion between arigid particle (sphere) of diameter d P and a fluid of infinite extent ofviscosity µ isTotal drag force resisting motionWhere u r is the relative velocityF⁄ )ρ f U r 2 A P F = 1 2 C Dρ f U r 2 A PDrag Coefficient C D is a function of particle’s Reynolds numberThis is known as the stokes law and is valid mainly forU rA PFThis also hold true up towith some error

Flow passed over a single particleThree different regimes based on the magnitude of Re PFor the Stokes Regime0.3 < (Re) PThe Intermediate Regime C D = ff[Re P ] 0.3 < (Re) P < 500Dallavalle (1948)Schiller & Naumann 1933The Newton’s Law Regime (Re) P > 500C D = 0.44

Drag coefficient for different shapesDrag Coefficient against Reynolds Number (Re)

Particle falling under gravity through a fluidThe general force acting on particle are gravity, buoyancy, dragGravity - Buoyancy - Drag = Acceleration FlowFor spherical particle (for acceleration = 0)F bNowSo equation becomesU t is called as terminal settling velocity of the particleOn solving for C D

Particle falling under gravity through a fluidFor Stokes law RegimeHenceIn stokes law regime the terminal settling velocity is proportional to the squere of theparticle diameterIn the Newton’s Law RegimeC D = 0.44In the Newton’s law regime the terminal settling velocity is proportional to the squareroot of particle diameter and independent of viscocity

Multiple particle systemThe motion of each particle is affected by the presence of othersSimple analysis for the fluid particle interaction is not valid but can be adopted tomodel the multiple particle systemFor suspension of particles the stokes law is assumed to apply but an effectivesuspension viscosity and effective average suspension density is assumedEffective suspension viscosity µ e = µ / f(ε)Average suspension density = ρ ave = ε ρ f + (1-ε) ρ Pwhere ε is the voidage or volume fraction occupied by fluidFor suspension of particles the drag coefficient in stokes law regime becomeswhereAnd U r is the relative velocity of particle

Multiple particle systemUnder terminal velocity condition for a particle falling under gravity in a suspension theforce balanceSubstituting µ e and ρ aveU rel,t is known as the particle settling velocity in presence of other particles or HinderedSettling Velocity

Multiple particle systemf(ε) was shown theoretically by Einstein to beFor uniform sphere forming a suspension of solid column fraction less than 0.1Or (1-ε) < 0.1,Richardson and Zaki have given the values of f(ε)for stokes regime to beFor Newton’s Regime

Characterization of Multiple Particle SystemDifferent definitions of average particle diameterArithmetic Meandd̅aaaa = ∑ ii nn iidd pppp∑ ii nn iiSurface Meandd̅ss = ∑ nn 2ii iidd pppp∑ ii nn iiVolume Mean33dd̅vv = ∑ nn iidd pppp∑ ii nn iiiiVolume Surface Meandd̅vvvv = ∑ nn 3ii iidd pppp∑2ii nn ii dd ppppWeight Meandd̅ww = vv ii dd ppppii= ∑ nn 4ii iidd pppp∑3ii nn ii dd ppppWherevv ii =nn 3iidd pppp∑3ii nn ii dd ppppLength Meandd̅ll = ∑ nn 2ii iidd pppp∑iinn ii dd pppp

Fluid Beds - BasicsFluidized beds: Particlesuspended in an upward gasstream

Pressure drop in fixed bedsPressure drop through packed bed / fixed bed of uniformsized solids is correlated by Ergun equationIt has two factors, viscous force and the kinetic energy forceAt low Reynolds number only viscous losses predominateAt high Reynolds number only kinetic energy losses need to be considered

Pressure drop & Minimum Fluidization VelocityAt the onset of fluidization, the gravity force on the particles in the bed must bebalanced by the drag, buoyancy, and pressure forces.

Pressure drop & Minimum Fluidization VelocityThe u mf , the superficial velocity at minimum fluidizing condition is found by usingexpression for ∆p/LIn a simplified formFor small particles (Re P < 20)Or

Pressure drop & Minimum Fluidization Velocityfor larger particles the simplified form isorWen and Yu have found for variety of systemsfor small particles the simplified form isandFor larger particles (Re P > 20)For whole range of Reynolds number

Geldart’s ClassificationA: Aeratable (U mb > U mf ) Material has significant deaeration time (FCC Catalyst)B: Bubbles Above (U mb = U mf ) 500 micron sandC: Cohesive (Flour, Fly Ash)D: Spoutable (wheat, 2000 micron polyethylene pellets)

Geldart’s Classification contd…GroupABCDCharacteristics and Properties• Good fluidization quality, aeratable, easily fluidized, smooth at lowvelocity and bubbling at higher velocity and slug at high velocity, bedexpands, Good solids mixing• Small mean particle size, low density, typically 30< d p

Geldart’s Classification contd…

Geldart’s Classification contd…

Fluidization RegimesVelocity Increasing

Fluidization Regimes: DescriptionVelocity range Regime Fluidization Features and Appearance0 ≤ U ≤ U mf Fixed Bed Particles are quiescent; gas flows through intersticesU mf ≤ U ≤ U mbParticulateRegimeBed expands smoothly and homogeneously with small-scale particlemotion; bed surface is well definedU mb ≤ U ≤ U ms Bubbling Regime Gas bubbles form above distributor, coalesce and grow; gas bubblespromote solids mixing during rise to surface and breakthroughU ms ≤ U ≤ U C Slug flow Regime Bubble size approaches bed cross section; bed surface rises and fallswith regular frequency with corresponding pressure fluctuationU C ≤ U ≤ U kTransition toturbulentFluidizationPressure fluctuation decrease gradually until turbulent fluidizationregime is reachedU k ≤ U ≤ U tr Turbulent Regime Small gas voids and particle clusters and streamers dart to and fro;bed surface is diffused and difficult to distinguishU ≥ U tr Fast Fluidization Particles are transported out of the bed and need to be replaced andrecycled; normally has a dense phase region at bottom coexistingwith a dilute phase region on top; no bed surfaceU » U trPneumaticConveyingUsually a once-through operation; all particles fed are transportedout in dilute phase with concentration varying along the columnheight; no bed surface

Fluidization Regimes: DescriptionTransition between regimesEquations have been published for transition linesbetween various regimes. This map can be used toidentify the type of flow regime that will exist for agiven particle under specific flow conditions.Smooth or particulate fluidizationBubbling or aggregative fluidizationSlugging criteria

Design of distributor in fluidized beds• Uniform gas sparging is govern by the effective design of gas distributionsystem and is very important to have uniform heat transfer• Parameter affecting performance of Gas distribution system comprise of;Gas Sparger geometryGas chamber geometryPressure drop across the gas distribution system• A good distributor should;Obtain a spatially uniform gas distribution, without stagnant zonesPrevent solids loss by leakageMinimize solid erosionAvoid choking of the distributorHave a definite and non-changing (with time) pressure drop for the gas

Design of distributor in fluidized bedsPressure Drop across the DistributorThe pressure drop across the distributor ΔPd is used as the criterion fordesign, and ΔPd, values varying from 0.01 to 1.0 times the pressure dropacross the bed ΔPb have been suggested.Siegel (1986):ΔPd/ΔPb = 0.14 - 0.22 [Galileo number (1 - 10,000)]Kunii & Levenspiel (1991):ΔPd/ΔPb = 0.1 – 0.3Siegel, M. H., Merchuk, J.C., Schugerl, K., 1986. Air-Lift Reactor Analysis: Interrelationships between Riser,Downcomer and Gas-Liquid Separator Behavior, including Gas Recirculation Effects. AIChE Journal 32(10),1585-1595Kunni, D., Levenspiel, O., 1991. Fluidization Engineering, 2 nd Ed., Butterworth-Heinemann, Boston.

Design of distributor in fluidized bedsNozzle Position (Litz, 1972)Side entry:H = 0.2D + 0.5D noz, when D noz> D 100H = , when D noz< D 100Bottom entry:18D noz( D − )H = 3D noz> D 36D noz, whenH = 100D noz, when D noz< D 36Where, D - diameter of gas distribution chamberD noz - diameter of the nozzleH - distance between the nozzle centerline and thedistributor plate Litz, W. J., 1972. Design of gas distributors. Chemical Engineering 13, 162-166.

Different types of fluid beds / modifications

ExampleA packed bed is composed of cubes 0.02 m on a side. The bulk densityof the packed bed, with air, is 980 kg/m 3 . The density of the solid cubes is1500 kg/m 3 .• Calculate the void fraction (ε) of the bed.• Calculate the effective diameter (D p ) where D p is the diameter of a spherehaving the equivalent volume.• Determine the sphericity of the cubes.• Estimate the minimum fluidization velocity using water at 38 C and a towerdiameter of 0.15 m.VoidFractionWe know:Vbed= Vfluid+ VsolidsandWbed= Wfluid+ WsolidsρbedVbed= ρfluidVfluid+ ρsolidsVsolidsρsolidsVsolids>> ρfluidVfluid

Examplecontd…..0.3515009801133=−=−=−=≅∴mkgmkgVVVandVVsolidsbedbedsolidsbedbedbedsolidssolidsbedbedρρερρερρ( ) mDDDadiameterEffectiveppp0.02560.0263333=∴==ππ( )0.816666 3131=⎟⎠⎞⎜⎝⎛==ΦππaaSphericitys( )( )2223249599.803994315001.75150 1smkgsmmkgmkguDDugMimimum Fluidization Velocityfmfpsmfmfpsmfffp=⎟∗⎠⎞⎜⎝⎛−⎥⎥⎦⎤⎢⎢⎣⎡+Φ−Φ=−ρµεερρρ

Examplecontd…..( )2225323323109.7480.4450.0250.819941.751.750.445141mfmfmfpsmffmfmfsusmkgumkgDu∗×=∗∗∗∗=Φ∗=∴=Φερεε( )( ) ( )( ) ( ) ( )smusmkgusmkgusmkgusmkgmum skgcpDumfmfmfmfmfmfpsmfmf0.07149591597109.748015970.4450.0250.810.0010.6930.445115011502222222522322322=−∗+∗×=∗=∗∗⎟∗⎠⎞⎜⎝⎛∗∗−∗=Φ∗−∗εµε

Heat Transfer in fixed and Fluidized beds• Main advantage of fluidized beds is the extremely large area of solid surfaceexposed to the fluidizing media• High solid surface area greatly facilitates solid-to-gas heat transfer• Because of the solids mixing generated within the bulk of a bubbling gasfluidized bed, temperature gradients are reduced to negligible proportions• High rates of heat transfer are obtainable between the fluidizing solids andthe immersed transfer surfaceParticle-to-gas heat transferBed-to-surface heat transferUse of immersed surfaces

Heat Transfer in Fixed BedsHeat transfer in fixed bed consists of following mechanisms(1) conduction heat transfer between particles,(2) convective heat transfer between particles and fluid,(3) interaction of both (1) and (2),(4) Radiative heat transfer between particles and the flowing gas(5) Heat transfer between bed wall and bed particles(1) Particle to fluid heat transferHeat transfer to single particle can be expressed asHeat transfer coefficient can be evaluated as(2) Heat transfer through wall (one dimensional model)For homogeneous model, temperature of fluid and of bed are assumed identicalforand

Heat Transfer in Fixed Beds(3) Heat transfer through wall (two dimensional model)For homogeneous model,temperature of fluid and of bed are assumed identicalforand(4) Effective radial thermal conductivityforand

Heat Transfer in Fluid bedsHeat transfer in a bubbling fluidized bed• Gas to particle heat transfer coefficients are typically small, of the order of 5 - 20 W/m 2 K• However, because of the very large heat transfer surface area provided by a mass of smallparticles, the heat transfer between gas and particles is rarely limiting in fluid bed heattransfer• One of the most commonly used correlations for gas-particle heat transfer coefficient is thatof Kunii and Levenspiel (only for low particle Reynolds numbers)for• Gas to particle heat transfer is relevant where a hot fluidized bed is fluidized by cold gas• While following are the correlation suggested based on experimental data *forfor*Chen, J.C., Heat Transfer in handbook of fluidization and fluid systems, 2003

Analysis of Gas-particle heat transferThe energy balance across the element givesIntegrating with boundary conditions T g = T g0 at L = 0

Analysis of Gas-particle heat transferThe distance L n , in which the gas-to-particle temperature falls by a factorIs given by,The distance over which the temperature distance is reduced to half its initial value,L 0.5 is then

Analysis of Gas-particle heat transferA bed of 450µm particles is operating at 150˚C. The temperature and superficial velocityof the incoming gas are 550˚C and 0.4 m/s, respectively. Approximately how far willthe incoming gas have penetrated into the bed before it is cooled to 350˚CGas physical properties can be estimated at average temperature over the specifiedrange. In this particular case the average temperature is 450 ˚C. Hence the physicalproperties of airGas velocity at 450˚C for an inlet velocity of 0.4m/s at 550˚C will be 0.35m/s

Heat Transfer in Fluid bedsBed to surface heat transferIn a bubbling fluidized bed the coefficient of heat transfer between bed and immersedsurfaces (vertical bed walls or tubes) can be considered to be made up of three additivecomponentsThe particle convective component h cp , which is dependent upon heat transferthrough particle exchange between the bulk of the bed and the region adjacent tothe transfer surface (heat transfer due to the motion of packets of solids carrying heatto and from the surface)The interphase gas convective component h gc , by which heat transfer betweenparticle and surface is augmented by interphase gas convective heat transferThe radiant component of heat transfer h rThus,Approximate rangeof significance40µm 1mm > 800 µm and athigher staticpressureHigher temperatures(> 900 K) anddifference

Heat Transfer in Fluid bedsParticle convective heat transferOn a volumetric basis the solids in the fluidized bed have about one thousand times theheat capacity of the gas and so, since the solids are continuously circulating within thebed, they transport the heat around the bed. For heat transfer between the bed and asurface the limiting factor is the gas conductivity, since all the heat must be transferredthrough a gas film between the particles and the surfaceHeat transfer from bed particles to an immersed surface

Heat Transfer in Fluid bedsParticle convective heat transferThe particle-to-surface contact area is too small to allow significant heat transfer.Factors affecting the gas film thickness or the gas conductivity will therefore influencethe heat transfer under particle convective conditions.Decreasing particle size, for example, decreases the mean gas film thickness and soimproves h pc . However, reducing particle size into the Group C range will reduceparticle mobility and so reduce particle convective heat transfer. Increasing gastemperature increases gas conductivity and so improves h pc .Particle convective heat transfer is dominant in Group A and B powders. Increasinggas velocity beyond minimum fluidization improves particle circulation and soincreases particle convective heat transfer.The heat transfer coefficient increases with fluidizing velocity up to a broad maximumh max and then declines as the heat transfer surface becomes blanketed by bubbles.

Heat Transfer in Fluid bedsBed to surface heat transferRange of fluidized bed-to-surface heat transfer coefficients

Heat Transfer in Fluid bedsBed to surface heat transferEffect of fluidizing gas velocity on bed – surface heat transfer coefficient

Heat Transfer in Fluid bedsParticle convective heat transferThe maximum in h pc occurs relatively closer to U mf for Group B and D powderssince these powders give rise to bubbles at U mf and the size of these bubblesincrease with increasing gas velocityGroup A powders exhibit a non-bubbling fluidization between U mf and U mb andachieve a maximum stable bubble size.Zabrodsky (1966) has given correlation for h max for group B particlesKhan (1978) has given correlation for h max for group A particles

Heat Transfer in Fluid bedsGas convective heat transferGas convective heat transfer is not important in Group A and B powders where the flow ofinterstitial gas is laminar but becomes significant in Group D powders, which fluidize athigher velocities and give rise to transitional or turbulent flow of interstitial gasIn gas convective heat transfer the gas specific heat capacity is important as the gastransports the heat around.Gas specific heat capacity increases with increasing pressure and in conditions where gasconvective heat transfer is dominant, increasing operating pressure gives rise to an improvedheat transfer coefficient h gc .Baskakov and Suprun (1972) has given correlation for h gcwhere U m is the superficial velocity corresponding to the maximum overall bed heat transfercoefficient

Heat Transfer in Fluid bedsGas convective heat transfer to immersed surfacesSeveral approaches have been used to estimate h cThe most common approach assigns thermal resistance to a gaseous boundary layer at theheat transfer surface, the enhancement of heat transfer is then attributed to the scouringaction of the solid particles on the gas film, decreasing the effective film thicknessLava’s correlation (1952) for vertical surfaces, for larger particles, isWender and Cooper’s correlation (1958) for vertical tubes,forWhereWhere r is the radial position of the heat transfer tube and R b is the radius of the bed

Heat Transfer in Fluid bedsGas convective heat transfer to immersed surfacesVreedenberg’s correlation (1958) for horizontal tubes,andforforWhere

Heat Transfer in Fluid bedsRadiative heat transferFor temperatures beyond 600 o C radiative heat transfer plays an increasing roleand must be accounted for in calculationsFor rule of thumb estimate, the radiative heat transfer component can beestimated using absolute temperatures and an adaptation of the stefen -Boltzman equation in the formwhere ε r is the reduced emissivity to take into account the different emissivity properties ofsurface ε s and bed ε b and is given byAn alternative correlation given by Panov et al. (1978) for approximate estimate is

Application of Fluidized BedsPhysicalIndustrial ProcessesChemicalDominatingMechanismHeat and/ormass transferbetweengas/particlesHeat and/or masstransfer betweenparticle/particleor particle/surfaceHeat transferbetweenbed/surfaceGas/gas reactionsin which solid actsas catalyst or aheat sinkGas/solid reactionsin which solids aretransformedApplications• Solids Drying• Absorption ofsolvents• Cooling offertilizer prills• Food Freezing• Plastic coating ofsurfaces• Coating ofpharmaceutical tablets• Granulation• Mixing of solids• Dust Filtration• Heat treatment oftextile fibres,wires, rubber,glass, metalcomponents• Constanttemperature baths• Oil cracking,reformingmanufacturing of• Acrylonitrile• Phthalic Anhydride• Polyethylene• Chlorinatedhydrocarbons• Coal combustion• Coal gasification• Roasting of nickel andzinc sulphides• Incineration of liquidand solid waste• Catalyst Regeneration• Decomposition oflimestone

ApplicationsFluidized bed dryingFluidizing with hot air is an attractive means for drying many moist powders andgranular productsThe technique has been used industrially for drying crushed minerals, sand,polymers, fertilizers, pharmaceuticals, crystalline materials and many otherproducts.The main reason for its popularity isEfficient gas-solids contacting leads to compact units and relatively low capital costcombined with high thermal efficiencyVery high heat and mass transfer and hence reduced drying timesThe absense of moving parts, low maintenance cost and possibility of usingcontinuous modeThe main limitation is the material to be dried should be fluidizable and shouldhave narrow particle size distribution

Fluidized Bed DryingWell mixed fluid bed dryer• common FBD used in industry.• bed temperature uniform, equal to theproduct and exhaust gas temperatures.• particle residence time distribution iswide• wide range of product moisture content.• feed is continuously charged into FB ofrelatively dry particles, this enhancesfluidization quality.• a series of well-mixed continuous dryersmay be used with variable operatingparameters.

Fluidized Bed DryingPlug flow fluid bed dryer• vertical baffles are inserted tocreate a narrow particle flow path.• narrow particle residence timedistribution.• nearly equal residence time for allparticles regardless of their size• uniform product moisture content.• length-to-width ratio from 5:1 to30:1.• inlet region may be agitated orapply back-mixing, or use a flashdryer to remove the surfacemoisture.

Fluidized Bed DryingDifferent designs of fluid bed dryers

Fluidized bed combustion boilersAtmospheric Fluidized Bed Combustion (AFBC) Boiler• Most common FBC boiler that uses preheated atmospheric air as fluidization andcombustion air

Fluidized bed combustion boilersPressurized Fluidized Bed Combustion (PFBC) Boiler• Compressor supplies the forced draft and combustor is a pressure vessel• Used for cogeneration or combined cycle power generation

Fluidized bed combustion boilersCirculating Fluidized Bed Combustion units• A “Circulating Fluidized Bed Boiler” commonlyabbreviated as CFB is a device for generatingsteam by burning fossil fuels (coal) in a furnaceoperated under a special hydrodynamicsconditions.• Bed material is heated upto the ignitiontemperature of the coal with the help of naturalgas burner. Coal and limestone are injected at thebottom of the combustor.• Total air required for combustion is split in toprimary air and secondary air.• Circulating solids are transported in the combustorat a velocity exceeding the terminal velocity ofaverage particles.• Recirculation of solids creating uniformity in thetemperature makes the combustor as an efficientcombustion system

Key References• Yang, W.C.; Handbook of Fluidization and Fluid –Particle Systems, Marcel Dekker, USA,2003• Kunni, D.; Levenspiel, O. Fluidization Engineering, 1969• Geldrt, D. Gas Fluidization Technology, John Willey and sons, USA, 1969• Mujumdar, A.S. Handbook of Industrial Drying, 3rd Ed; CRC Press: Boca Raton, FL,2006• Botterill, J.S.M., Fluid-Bed heat transfer, 1975.• http://www.erpt.org/012Q/rhod-00.htm• http://www.dct.tudelft.nl/race/research/staff/vanommen/index.html