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Lime Kiln Principles And OperationsLime Kiln Principles And Operations

Terry N. AdamsTerry N. Adams

Technical ConsultantTechnical Consultant

Seattle WASeattle WA

Slide 2OutlineOutline

•• Lime kiln equipmentLime kiln equipment

–– flames, chains, refractoriesflames, chains, refractories

–– product coolers, external mud dryersproduct coolers, external mud dryers

•• Lime kiln energy efficiencyLime kiln energy efficiency

•• Alternative fuels for lime kilnsAlternative fuels for lime kilns

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Slide 3Exterior Of A Lime Reburning KilnExterior Of A Lime Reburning Kiln

BurnerBurner

Flue gasFlue gasto the ID fanto the ID fan

Reburn limeReburn limedischargedischarge

LimeLimemudmudslurryslurry

Slide 4Interior Of A Lime Reburning KilnInterior Of A Lime Reburning Kiln

DamDam

FlameFlame

RefractoryRefractory

LiftersLifters ChainsChains

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Slide 5Rotary Kiln Flame ShapesRotary Kiln Flame Shapes

Long lazy Long lazy flameflame

Short bushy Short bushy flameflame

Compact Compact mediummedium--length flamelength flame

Slide 6Rotary Kiln Chain SystemsRotary Kiln Chain Systems

LowLow--density density curtain chaincurtain chain

HighHigh--density density curtain chaincurtain chain

Garland Garland chainchain

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Slide 7Lime Kiln Chain SystemsLime Kiln Chain Systems

Slide 8Rotary Kiln Refractory SystemsRotary Kiln Refractory Systems

SingleSingle--Brick SystemBrick System TwoTwo--Brick SystemBrick System

Refractory bricks

Refractory bricks

Insulating bricks

Steel ShellSteel Shell

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Slide 9Refractory WastageRefractory Wastage

•• Most refractory damage due to wastageMost refractory damage due to wastage

–– Smooth, Smooth, ““bird bathbird bath”” refractory thinningrefractory thinning

•• Due to high temperature chemical attackDue to high temperature chemical attack

•• Protect refractory with a coating of limeProtect refractory with a coating of lime

•• Operate kiln for lower refractory face tempOperate kiln for lower refractory face temp

–– Low primary air flow, avoid flame impingementLow primary air flow, avoid flame impingement

–– ColdCold--end excess Oend excess O22 > 1> 1½½%%

Slide 10Refractory Damage PhotosRefractory Damage Photos

Refractory Wastage Refractory Collapse

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Slide 11Lifters And Discharge DamsLifters And Discharge Dams

Lifters

Cold end of kiln

LiftersLime bed

Hot endof kilnDam

Discharge Dam

Slide 12Rotary Kiln Product CoolersRotary Kiln Product Coolers

Satellite Product CoolersSatellite Product Coolers Induced Induced secondary airsecondary air

Reburned limeReburned lime

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Slide 13External Lime Mud DryingExternal Lime Mud Drying

Electrostaticprecipitator

PrecipitatorPrecipitatorcatchcatch

Kiln

CycloneCyclone

GasGas

Mud screwMud screw

Slide 14Fans, Draft, And OFans, Draft, And O22

•• PA fan only for flame shapingPA fan only for flame shaping

•• ID fan is main air moving fanID fan is main air moving fan

–– Use ID fan to control OUse ID fan to control O22

–– Often limits production capacityOften limits production capacity

–– Wet scrubber is main flow restrictionWet scrubber is main flow restriction

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Slide 15Lime Kiln Heat RateLime Kiln Heat Rate

•• Heat rate is a measure of energy efficiencyHeat rate is a measure of energy efficiency–– Units are MM Btu/ton CaO or GJ/Units are MM Btu/ton CaO or GJ/tonnetonne CaOCaO

–– Often stated as MM Btu/ton Often stated as MM Btu/ton ““productproduct””

•• Typical rangeTypical range–– 5 to 9 MM Btu/ton CaO5 to 9 MM Btu/ton CaO

–– 5.8 to 10.5 GJ/5.8 to 10.5 GJ/tonnetonne CaOCaO

–– Lower is betterLower is better

Slide 16Lime Kiln Energy Balance ComponentsLime Kiln Energy Balance Components

•• DryingDrying

•• Calcining : CaCOCalcining : CaCO33 + heat+ heat CaO + COCaO + CO22

•• LossesLosses

–– Heat loss through shellHeat loss through shell

–– Heat loss in hot lime productHeat loss in hot lime product

–– Heat loss with gas and dust exiting at cold endHeat loss with gas and dust exiting at cold end

•• A portion of fuel higher heating value is unavailableA portion of fuel higher heating value is unavailable

–– HHV is measured and reportedHHV is measured and reported

–– Only Lower Heating Value (LHV) is actually availableOnly Lower Heating Value (LHV) is actually available

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Slide 17Example for Kiln Heat RateExample for Kiln Heat Rate

KILN PARAMETERSFuel Nat GasProduction rate 250 TPDMud dry solids 78%Kiln exit O2 3%Lime availability 85%Dust loss 18%Product temp 600°FCold end gas temp 600°FShell Heat Loss 11.4 MM Btu/hr

Slide 18Lime Kiln Heat RateLime Kiln Heat Rate

•• Overall Heat Rate = 8.4 MM Btu/ton CaOOverall Heat Rate = 8.4 MM Btu/ton CaO

Contribution Heat rate Fraction RemedyCalcining reaction 2.82 34% No change possibleHeat to evap water 1.75 21% Increase mud dry solidsHeat in exit gas 1.50 18% Decrease exit gas tempShell heat loss 1.10 13% Insulating refractoryLHV/HHV loss 0.82 10% Change fuelHeat in product 0.29 3% Product coolersHeat in dust 0.11 1% Improve chain system

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Slide 19Caution With Heat Rate CalculationCaution With Heat Rate Calculation

•• Gives instantaneous valueGives instantaneous value

–– DoesnDoesn’’t include down time, upsetst include down time, upsets

•• Good to assess where improvements neededGood to assess where improvements needed

•• Some changes affect more than one parameterSome changes affect more than one parameter

–– Refractory changes shell loss & exit tempRefractory changes shell loss & exit temp

–– Mud solids affects evaporation loss and exit tempMud solids affects evaporation loss and exit temp

–– Fuel changes LHV/HHV, exit gas temp, and dust lossFuel changes LHV/HHV, exit gas temp, and dust loss

Slide 20Impact of Mud Dry Solids on Heat RateImpact of Mud Dry Solids on Heat Rate

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60 65 70 75 80 85 90Lime Mud Dry Solids, %

Hea

t Rat

e, G

J/to

nne

CaO Mill data

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Slide 21Major Heat Rate ImprovementsMajor Heat Rate Improvements

KILN PARAMETERS Base Refractory Fuel Type ChainsFuel Nat Gas Nat Gas Fuel Oil Nat GasProduction rate, TPD 250 250 250 250Mud dry solids 78% 78% 78% 78%Kiln exit O2 3% 3% 3% 3%Lime availability 85% 85% 85% 85%Dust loss 18% 12% 18% 18%Product temp, °F 600 600 600 600Cold end gas temp, °F 600 475 500 525Shell Heat Loss, MM Btu/hr 11.4 6.3 11.4 11.4Heat Rate, MM Btu/ton 8.4 7.0 7.4 8.0Change -17% -11% -4%

Slide 22Minor Heat Rate ImprovementsMinor Heat Rate Improvements

KILN PARAMETERS Base Excess air Dust InertsFuel Nat Gas Nat Gas Nat Gas Nat GasProduction rate, TPD 250 250 250 250Mud dry solids 78% 78% 78% 78%Kiln exit O2 3% 2% 3% 3%Lime availability 85% 85% 85% 92%Dust loss 18% 18% 12% 18%Product temp, °F 600 600 600 600Cold end gas temp, °F 600 575 600 600Shell Heat Loss, MM Btu/hr 11.4 11.4 11.4 11.4Heat Rate, MM Btu/ton 8.4 8.1 8.2 8.2Change -3% -3% -2%

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Slide 23Common Lime Reburning Kiln FuelsCommon Lime Reburning Kiln Fuels

•• Nat gas and fuel oil are most commonNat gas and fuel oil are most common

–– Fuel oil is more efficient, gives higher capacityFuel oil is more efficient, gives higher capacity

•• Petroleum cokePetroleum coke

–– Many applications, low costMany applications, low cost

–– Sulfur and metals can be highSulfur and metals can be high

–– Thermal NOx can be highThermal NOx can be high

–– Improves heat rateImproves heat rate

Slide 24Other Solid/Liquid Kiln FuelsOther Solid/Liquid Kiln Fuels

•• Wood and bark powderWood and bark powder

–– NPEs can be highNPEs can be high

–– Lime purge & makeup needed to control NPEsLime purge & makeup needed to control NPEs

•• LigninLignin

–– Sulfur can be highSulfur can be high

•• Pyrolysis oilsPyrolysis oils

–– Not currently used, pH & Not currently used, pH & μμ possible issuespossible issues

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Slide 25Gasification FuelsGasification Fuels

•• Coal, wood and other fuels can be gasifiedCoal, wood and other fuels can be gasified

•• FuelFuel--gas can be used to fire kilngas can be used to fire kiln

–– Similar to natural gasSimilar to natural gas

–– Lower inert level, well established technologyLower inert level, well established technology

–– OOnn--line availability ~ 85%line availability ~ 85%

–– Wet gasifier feedstocks Wet gasifier feedstocks deratederate kilnkiln

–– Burner and chains must be designed for fuelBurner and chains must be designed for fuel--gasgas

Slide 26Fuels From the Pulp MillFuels From the Pulp Mill

•• Turpentine or methanol liquidTurpentine or methanol liquid

•• Stripper OffStripper Off--Gas (SOG)Gas (SOG)

•• NonNon--condensable gas (NCG)condensable gas (NCG)

–– Can be wet and sulfur level can be highCan be wet and sulfur level can be high

•• Tall oil and tall oil pitchTall oil and tall oil pitch

–– Can fire 100% tall oil, similar to fuel oilCan fire 100% tall oil, similar to fuel oil

–– 16,000 Btu/lb and 16,000 Btu/lb and low sulfurlow sulfur

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Slide 27How To Improve Lime Kiln PerformanceHow To Improve Lime Kiln Performance

•• Improve refractory systemImprove refractory system

–– thicker bricks, twothicker bricks, two--brick systembrick system

•• Switch fuelsSwitch fuels

•• Improve chain systemImprove chain system

–– amount for heat transfer, layout for dustamount for heat transfer, layout for dust

•• Improve ID fan/scrubberImprove ID fan/scrubber

•• Dry mud externallyDry mud externally