Heat loss from shell radiationHeat loss inproductHeat loss inPreheaterExit gasCooler exit airWhere does the heat go ?
For perpetual pyro process in a kiln the heatrequired is only heat of clinker mineral formation,ie., 380 - 400 Kcal/kg clinker. 280 – 350 Kcal/kgclinker is wasted which is about 40 - 45 % .The dream of design engineer is to make heatlosses to minimum and how to optimize the heatconsumption.
The preheater heat lossesPre heater gas temperatureDesign of cyclonesNo of stagesDust lossInlet / out let velocity ratioLocation of meal distribution boxesPreheater( Surface) heat losses through radiationCalciner gas retention timeCombustion effciencyCoal residue & raw meal residueLocation of firing nozzles
Different flamesNormal flameFlame with lowSecondary air tempDistorted nozzleFlame –poorhood geometryOr distorted nozzleFlame at the centerFlame downwardFlame upward
Flame lengthLong flame, unstable coating,High back end tempLow shell temperatureShort intense divergent flameGood for burningLow back end temperaturePoor refractory life, highShell temperatureConvergent flameGood for burningGood for refractoryStable coatingLow shell temperature
The Ideal Flamehot !short !stable !T"long" flame"short" flameComplete combustion:- CO = 0- SO2, NOX ↓Homogeneous:- no temperature peaks- no local CO on the clinker bedLonger flame increase the back end temperature resulting inHeat loss at kiln exit and hot meal clogging
Objectives“It is desirable to operate the kiln at thelowest fuel consumption. This must beconsistent with the highest practical outputat an acceptable market quality.”
WHY TO DO IT• To get a detailed view of the kiln line performance• Evaluate exact data for heat consumption,production,...• Basis for comparison– impact of investment or modifications carriedout– comparison to other plants• Detect weak points - Action Plan• Detect optimization potential• Check of sensors, weigh feeder,...
Σ massin = Σ massoutMASS BALANCEΣ heatin = Σ heatoutHEAT BALANCE“Energy cannot be created or destroyed but may beconverted from one form to another”Energy in = Energy out
Boundary selection• Any boundary shape can be chosen.• Every stream that crosses the envelope must be taken intoaccount.• The boundary line is chosen so that the boundary pointsare:– useful for the balance goals– easily accessible for reliable measurements
Dust Exists System Dust Does Not Exit BoundaryKilnSystemDustKilnFeedMeasuringpoint (t/h)KilnSystemDustKilnFeedClinker ClinkerBoundary selection and streams
Kiln SystemPrimary airClinkerFuelcoolingairFalse airKiln feedKiln exitgasesWatersprayExit dustReturndustS heatin = S heatoutWalllossesHeat balanceCooler exhaust gasBypass gas and dust
Heat Transfer MechanismsHeat Transfer Mechanisms• Conduction• Convection• Radiation200°C 50°Cheat
ConductionConduction• Transfer of heat from the hotter to colderpart of a body• By direct molecular contactFurnace wallHot gas1200°CCold air25°CLQ kAT TLh c=−Q
ConvectionConvection• Natural convection:– fluid moving from difference of density due to differenttemperatures• Forced convection:– fluid is moved by the action of an external devicehot airhot aircold air cold airnatural convection forced convection( )Q hA T Tw f= −
RadiationRadiation• Energy transferred by electro-magneticradiation( )Q A T T= −σ 14242000°C 50°CQ
HEAT TRANSFER• Radiant heat transfer• Free convection(occurs by naturalthermal draft, at lowwind velocities)• Forced convection(occurs at high windvelocities)ConvectionRadiationAir
Chemical Reaction• Endothermic reaction - heat is consumed– Calcium Carbonate breaks down to CaO (lime)and CO2 when heated– it takes heat in Þ the reaction is endothermic.• Exothermic reaction - heat is released– CaO (lime) reacts with Silica and the cementminerals are formed– the process gives out heat Þ the chemicalreactions is exothermic.
Two types of heatLatent HeatLinked to modification by chemical reaction,change in state, change in structureSensible HeatAbsorbed or released by asubstance
The heat to remove from a material to cool it down to thereference temperature (usually 0ºC).Q = M × Cp (T) × (T - T0)M = specific massCp (T) = specific heat of a material at temperature TT = temperature of MSensible heat
Qf = mf × ( LHVf + Cpmean f (Tf) × Tf )Qf : heat from fuel (kcal/h)mf : fuel flow rate (kg/h)LHVf : fuel low heat value (kcal/kg)Cpmean f : mean specific heat of fuel (kcal/kg.ºC)Tf : fuel temperature (ºC)ORh = m • CVh : heat from fuel (kcal/kg clk)m : specific fuel consumption (kg/kg clk)CV : calorific value of fuel (kcal/kg fuel)Heat from fuel
Incomplete combustion• The kiln exit gases might contain some un burnt gases (CO,H2, CH4)• The combustion heat from those fuels must be included as aout streamQic = mCO ×LHVCO + mH2 × LHVH2 + mCH4 ×LHVCH4The heat loss through the gas can be calculated to:h = m•(CO%•12640+H2%•10800+ CH4% • 35 840)m = specific gas quantity (Nm3/kg clk)
Heat of Reaction• Heat of reaction is the difference between the heatabsorbed in decarbonating the limestone and the heatreleased in forming the clinker minerals• It should be noted that raw meal chemistry affects thereaction heat, the heat absorbed by the process getsbigger as the LSF of the materials rises• 420 kcals/kg clinker is used if little else is known
Clinker theoretical heat of formation• The heat required to form clinker from dry raw mix• ZKG formula (German formula):Qt = 4.11 Al2O3 + 6.47 MgO + 7.64 CaO - 5.11 SiO2 - 0.60 Fe2O3• If no clinker analysis: assume Qt = 420 kcal/kg ck• Must be added to the clinker heat content as latent heat or as a separateoutput heat stream.CaF2 addition reduce the heat of reaction considerably butIt has the other implications.
Heat of formation• Heat of dehydration of clay (endothermic)• Heat of decarbonation of CaCO3 + MgCO3 (endothermic)• Heat of formation of clinker (exothermic!)• General assumption for the three: 1750 kJ / kg clk 0r• 400 Kcal/kg cl
Qw : heat loss through wall (W)atot : total heat transfer coefficient (W/m².C)A : shell area (m²)T : shell temperature (ºC)Ta : ambient temperature (ºC)( )Q A T Tw tot a= −αHeat loss through kiln shell
05101520253035404550556065100 200 300 400 500 600T - T° (C)W/M2Cv = 14 m/s wind13121110987654321v = 0 m/s (free convection)SS = 0.9Ambient T° - 20°CGlobal heat transfer coefficientRadiation and convectionheat transfer coeffcient( Total)Radiation and Convection
How is cooling accomplishedHow is cooling accomplishedHeat transferby radiationand convectionHeat movesto clinker edgeby conductionAir flows overclinker coolingsurface
Heat Transfer in ClinkerHeat Transfer in Clinker• Convection - Surface to Air• Conduction - Inside to Surface• Heat transfer is driven by temperature difference• Takes place at the clinker surface• To maximize it:– Increase the air/material contact time with:• Deeper bed ( ⇒ more power)• Slower air flow (⇒ larger cooler)
Old conventional grate platescreate sand blasting effect or fluidizationThis creates poor heat exchangeModern cooler plates flow resistancebranch off the air , createsless fluidization , better heat exchangeCross flowCounter currentMechanical flow regulator
Heat Exchange Between Clinker and AirHeat Exchange Between Clinker and AirTemperatureBedthicknessclinkerairFixed bedFluidized bedAir inAir outClinkerAir inAir outClinkerTemperatureBedthicknessclinkerairMore efficient recovery with fixedbed
General Truths (All Coolers)General Truths (All Coolers)1. The hotter the inlet temperature the hotter the clinkeroutlet temperature.2. The hotter the cooling air temperature the hotter theclinker outlet temperature.3. The longer the air/material contact time the cooler theclinker outlet temperature.
SYSTEM DATA COLLECTION• Process• Type of kiln• Nominal capacity• Supplier• Fuel and firing system• Type of burner nozzle• Dust reintroduction system• Dimensions of main equipment• Data on fans, drives, etc.
OPERATING DATA• Various operating data (rpm, kW, temperature andpressure profiles along kiln system, grate speed,undergrate pressures, etc.)• Electric power readings (before / after test)• Chemical analysis of raw meal, dust(s) and clinker,LSF, SR, AR, etc.
Cooler heat lossesOptimised air flow &air distribution & sealingClinker nodulesFiner fraction &big balls causesbad heat exchangeRadiation losses from wallsExit air temperature
Measurement Plan• Duration of an audit ?• What to measure? How to measure?– Material balance– Gas flows– Heat Balance• Frequency of sampling and measurements?• Which analyses have to be carried out ?• Which further data are to be collected ?
• All referred to 1 kg clinker Production = .... t/h• Reference temperature = 0°C Specific• Ambient temperature = ...°C Heat cons. = .... kJ/kg clkSpecification Heat(kg/kg clk), Temp.(Nm3/kg clk)(kW etc.) °C (kJ/kg clk) (%)Fuel combustion - primary firing -- secondary firing -Burnable matter in kiln feed -Raw meal: sensible heatFuel: sensible heatPrimary air: sensible heatCooler air: sensible heat -Total of inputs -100%INPUT DATA SUMMARY
Specification Temp. Heat(kg/kg clk),(Nm3/kg clk) °C (kJ/kg clk) (%)(kW etc.)Heat of formationWater evaporation: - kiln feed- water spray (s)Exhaust gas: - sensible heat- dust sensible heat- dust CaO-loss- unburnt gases (CO, etc)Cooler: - waste air sensible heat- middle air sensible heat- clinker exit sensible heatBypass losses: - sensible heat- dust sensible heat- dust CaO-loss- unburnt gases (CO, etc)Radiation and Convection: - Preheater- Rotary kiln- Cooler- Tert air ductRest (difference)Total of outputs100%OUTPUT DATA SUMMARY
1. INPUT from sensible heatFUEL from combustionRAW MEAL from sensible heatfrom sensible heat of waterCOMBUSTION AIR from sensible heat of all theair supplied (prim. sec.)Total input2. OUTPUTHeat of formationEvaporation of water from raw mealExhaust gas sensible heatDust sensible heatIncomplete combustion (CO)Clinker exit temperatureCooler exhaust gasesLosses due to radiation and convectionWater cooling (Recupol inlet chute)DifferenceTotal outputkJ/kg clk25556025716757501750237075425—59100540—1525750%0.4126.96.36.199.210030.4188.8.131.52—1.01.79.4—2.6100Wet ProcesskJ/kg clk1533433017203425175050631421—5027645242143425%0.4184.108.40.206.610051.114.89.20.6—220.127.116.11.20.4100Semi-Dry (Lepol)kJ/kg clk13315054—6322317501363618—63423297—233223%0.497.61.7—0.210054.30.419.70.6—2.013.19.2—0.7100Dry Preheater (4-Stage)HEAT BALANCE EXAMPLES
Cooler BalanceTertiary air Vent air0,65 Nm³/kgck 1,14 Nm³/kgckSecondary air 719°C 293°C0,27 Nm³/kgck Coal mill Raw mill1029°C 0,00 Nm³/kgck 0,00 Nm³/kgckClinker95.6001465°CGrate surface 52,20 m²Standard load 44,0 t/d/m²Cooling air Clinker2,07 Nm³/kgck 95.6004°C 107°C
Elements of Mass BalanceElements of Mass BalanceTertiary airVentairComp 1 Comp 2 Comp 3 Comp 4 Comp 5 Comp 6 Comp 7 Comp 8SecondaryairclinkerclinkermCK1mCK2mSA mTA mCMmVAmF1 mF2 mF3 mF4 mF5 mF6 mF7 mF8Coalmill airRawmill airmRM
Mass BalanceMass BalanceIn Outclinker (mCK1) clinker (mCK2)cooling air (SmFi) secondary air (mSA)tertiary air (mTA)coal mill air (mCM)raw mill air (mRM)vent air (mVA)In = Outsecondary air flow: calculated by differenceshould be validated against a kiln balance
Heat BalanceHeat BalanceIn Outclinker (HCK1) clinker (HCK2)cooling air (SHFi) secondary air (HSA)tertiary air (HTA)coal mill air (HCM)raw mill air (HRM)vent air (HVA)wall losses (HWL)In = Outsecondary air heat : calculated by differencegood to validate it against kiln heat balancesecondary air temperature: calculated from secondary air heat
Temperature Stratification of air above Clinker BedTemperature Stratification of air above Clinker BedSecondary &Tertiary airAir tocoal millVentair1400°C300°C 250°C200°C175°C 125°C100°C1000°C700° C500°C400°C 150°C
Measuring Actual Bed DepthMeasuring Actual Bed Depthfloor
Cooling EfficiencyCooling Efficiencyclinkerininputheatclinkerbylostheat=ηinckoutckinckoutckinckhh1hhh−=−=ηQualifies the cooling of the clinker but not the clinker cooler.More cooling is possible with more air but that does notimprove the cooler efficiency.An efficient cooler would give same cooling with less air.
Cooler lossCooler losscooler loss = all heat not recovered by combustion air(secondary or tertiary)cooler loss = heat content of clinker leaving cooler (hck out)+ heat content of vent air+ heat content of coal mill air+ heat content of raw mill air+ wall heat lossesOften a specification in supplier process performance guarantee
Operating results coolerCoolerefficiency[%]Combustion air Nm³/kgKl.0,75 0,8 0,85 0,9 0,95606570758085Standard - cooler New - competition REPOL RSNew-type coolersOld-type coolers
Heat saving1.Run the plant stable , continously with consistent production2.Minimise the false air ingress by giving efficient seals3.Optimise the flame as per our requirement4.Minimise the variation in airflow, meal flow and fuel flow rate5.Reduce the radiation losses by giving proper insulation6.Optimise the cooler operation and cooler efficiency7.Optimise the cyclone efficiency in the preheater8.Minimise the variation in chemistry of raw meal and ash in fuelby efficient blending.9. Addition of mineralisers reduce the heat of reaction by 20 – 30 Kcal/ kg.cl afterthorough study on the rheological properties of cement. CaF2, Dolomite andslag are good mineralisers.Set pointNaturaland acceptablevariationhigh variationNot acceptable Variation is a devil in any process
1.31Kg/kg cl0.1Dust in tertiary duct70.180.16%1Excess air in kiln12-0.1-0.17Deg C10Temperature of primaryair110.220.22%1Excess air in calciner13-0.25-0.17Kcal/kgcoal100Heat content in coal91615.7Kg/kgcl0.1False air through hood141918.7Kg / kg cl0.1False air through inletseal150.680.66%1Amount of primary air100.470.45%1Moisture in coal81.30.12Kg/kg cl0.1Dust from kiln61.11.11Kcal/kgcl1Heat of reaction5-73-74.37%1Carbon in rawmeal45.65.6%1Hydrate water31.81.8%1Feed moisture2-0.5- 0.57Deg c10Feed temperature1Heat, kcal/kgclILCHeat, kcal/kgclSLCunitbyA change ins.noHeat calculation
1.11.1Kg / kg cl0.1Raw meal271.2Kg/ kg cl0.1False air cyclone C12219.1Kg/ kg cl0.1False air cyclone C5261.21.1Kcal/kg cl1standard Cooler loss287.2Kg/ kg cl0.1False air cyclone C324Cyclone efficiency29- 0.1%1Cyclone- K12912.4Kg/ kg cl0.1False air cyclone C4253.4Kg/ kg cl0.1False air cyclone C2231918.8Kg/ kg cl0.1False air cyclone K5211210.9Kg/ kg cl0.1False air cyclone K4206.95.8Kg/ kg cl0.1False air cyclone K3193.32.6Kg/ kg cl0.1False air cyclone K2181.10.82Kg/ kg cl0.1False air cyclone K1171615.7Kg / kg cl0.1False air calciner16Heat, kcal/kgclILCHeat, kcal/kg clSLCunitbyA change ins.no
-0.26%1Cyclone- K533-0.38-0.28%1Cyclone- C436-23-21.8Change from 4 to 5 stage411.61.6%1By pass of kiln gases40-11-10.0Change from 5 to 6stages420.13%1Recarbonation, KS3843440.510.37%1Recarbonation, CS390.82-0.74%1Cyclone- C537-0.34- 0.24%1Cyclone- C335-0.27-0.18%1Cyclone- C234-0.21-0.14%1Cyclone- C134-0.12%1Cyclone- K432-0.10%1Cyclone- K331-0.08%1Cyclone- K230Heat, kcal/kgclILCHeat, kcal/kg clSLCunitbyA change ins.no
Heat loss from shell radiationInsulation effect of refractoriesoptimised coating ,300mm thkFlame Shape & flame lengthRing formation shootsthe gas velocitytakes the heat fartherinto the kiln, increasesthe back end temperatureExit gas velocity at kiln inlet= 10 m/sv=15 -16 m/sSteadyFeed rateWith lessFluctuationOfcalcinationHeat losses from kilnParasite air ( ingress of falseair entry) at inlet , outlet hood&preheater.Primary air & coal transportair are all false entry.FluctuationsIn processHood take offV=5 m/sWell controlled air flowFuel flow
769.34.40Total Output6.0Radiation Loss from Cooler20.2Radiation Loss from Kiln37.1Radiation Loss from Preheater4.8Heat of Evaporation of Moisture20.91110.1881.0Heat Through Clinker91.62930.2521.2Heat Through Cooler Vent410.0Heat of Clinkerisation7.93360.2360.1Heat of PH Exit Dust170.83360.2472.1Heat of PH Exit GasesKcal/kg clinkerdeg CKcal/kg degCkg/kg clinkerHeatTemperatureSp.heat capacityMass flowHeat Output relative to 0 deg C769.34.40Total Input412.3Heat of Coal Combustion in Calciner295.7Heat of Coal Combustion in Kiln1.1560.2380.1Sensible heat of Coal and Conveying Air2.8600.2870.2Sensible heat of Coal1.3460.2380.1Sensible Heat of PH Leak Air27.1460.2472.4Sensible heat of Cooling air8.1Heat through combustibles in raw meal21.0600.2121.6Sensible heat of Kiln Feedkcal/kg Clinkerdeg CKcal/kg degCkg/kg clinkerHeatTemperatureSp.heat capacityMass flowHeat Input relative to 0 deg CSpecific heatConsumption=Total heat output –Total sensible heat769.3-61.3 = 709Kcal/kg cl
Automation further helps to run the plant more stableby reducing the meal, fuel flow and air flow.Running the kiln continuously withconsistent production is the best way toreduce the fuel and power bills.For consistent production we must have short ,Convergent and intense flame, less chemistry variationof raw meal , less variation in ash content of fueland stable cooler operation. Automation further helpsto run