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Lime Kiln Chemistry and Lime Kiln Chemistry and Effects on Kiln OperationsEffects on Kiln Operations

Honghi TranPulp & Paper Centre University of Toronto

Toronto, Canada

Tappi Kraft Recovery Short CourseSt. Petersburg, Florida, January 7-10, 2008

Presentation OutlinePresentation OutlineBasic chemistry

Calcining reactionLime mud and lime compositions

Effects on kiln operations Lime qualityRing formationTRS and SO2 emissionsRefractory brick performance

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Calcination ReactionCalcination Reaction

Occurs rapidly at about 800oC (1470oF)Rate increases with temperature; decreases with CO2 partial pressureReversible at lower temperatures

CaCO3 CaO + CO2Lime mud Lime

Effect of COEffect of CO22 on CaCOon CaCO3 3 Decomposition TemperatureDecomposition Temperature

20 40 60 80 100CO2 Concentration (%)

600650700750

800850900950

0

Tem

pera

ture

(oC

)

CaCO3

CaO

Typical CO2 rangein lime kiln

1480 F

1400 F

3

0

20

40

60

80

100

Lime Mud

Wei

ght P

erce

nt

CaCO3

Impurities

Composition of Lime MudComposition of Lime Mud

0

20

40

60

80

100

Lime Mud Reburned Lime

Wei

ght P

erce

nt

CaCO3

Impurities

Composition of Reburned LimeComposition of Reburned Lime

CO2

CaO

4

0

20

40

60

80

100

Lime Mud Reburned Lime

Wei

ght P

erce

nt

CaCO3CaO

Impurities

Reburned lime contains Reburned lime contains at leastat least 1.8 times 1.8 times more impurities than lime mud!more impurities than lime mud!

Impurities in Lime MudImpurities in Lime Mud

0.0

0.4

0.8

1.2

1.6

Na2O MgO P2O5 SiO2 SO3 Al2O3 Fe2O3 K2O

Con

cent

ratio

n (W

t%)

Impurities

Na2SO4

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Types of Sodium CompoundsTypes of Sodium Compounds

Water SolubleNa

Water InsolubleNa

GuardedNa 1/3

1/3

1/3

Sodium CompoundsSodium Compounds

Water-soluble sodium (Na)Mainly NaOH and Na2S from residual white liquorBecome Na2CO3 and Na2SO4 in the kilnMelt at about 800oC (1470oF)

Water-insoluble NaFormed by reactions between water-soluble Na and silicate impurities in lime mud and bricksBound within silicates and melt at high temperatures, >1200oC (2190oF)

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Guarded SodiumGuarded SodiumFormed inherently during the causticizing process:

Ca(OH)2 + Na2CO3 = 2 NaOH + CaCO3

(Ca1-xNa2x)CO3

Ca2+

Na+

Ca2+

Ca2+Ca2+

Ca2+

Ca2+

CO32-

CO32-

CO32-

CO32-

CO32-

CO32-

Na+

CO32-

x < 0.01

(Ca1-xNa2x)CO3

Ca2+

Na+

Ca2+

Ca2+Ca2+

Ca2+

Ca2+

CO32-

CO32-

CO32-

CO32-

CO32-

CO32-

Na+

CO32-

Ca2+

Na+

Ca2+

Ca2+Ca2+

Ca2+

Ca2+

CO32-

CO32-

CO32-

CO32-

CO32-

CO32-

Na+

CO32-

x < 0.01

“Guarded” and protected by CaCO3 structure

Guarded SodiumGuarded SodiumInsoluble in water at low temperatures but becomes soluble at high temperatures

Cannot be washedReleased as Na2CO3 at high temperatures in the kiln

(Ca1-xNa2x)CO3 (1-x) CaCO3 + x Na2CO3

Behaves in the same manner as water-soluble sodium

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Sodium Enrichment in Lime DustSodium Enrichment in Lime Dust

MudLime

BurnerDust

Na

Chains

Na

NaNa Na

= ~ 2 (varies from 1 to 3.5)Na/Ca molar ratio in Dust

Na/Ca molar ratio in Mud

Na Enrichment Factor:

Effect of SodiumEffect of SodiumA small amount is good (<0.8 wt% Na in mud)

Promote lime nodulationLower dusting

High Na content may lead toRing formationHigh TRSDead burned limeRefractory damage

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Sodium ControlSodium ControlIncrease mud solids contentImprove mud washing Purge lime dust

Effects on Kiln OperationsEffects on Kiln Operations

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Lime Quality Lime Quality –– Good LimeGood LimeNodule size

<25 mm (1”)

Residual carbonate2% (1.5 - 3%)

Availability90% (85 - 95%)

ReactivitySlaked within 5 min.

Effect of Solids Temperature on Effect of Solids Temperature on Residual CaCOResidual CaCO33 in Limein Lime

Temperature (oC)

Res

idua

l CaC

O3

(wt%

)

0

5

1100700 800 900 1000 1200

10

15

TheoryCaCO3 CaO + CO2

Practice

10

Temperature Temperature vsvs Residual CaCOResidual CaCO33

Bed

Flame

CaCO3

CO2

Heat

CaO

Temperature (oC)

Res

idua

l CaC

O3

(wt%

)

0

5

10

15

1100700 800 900 1000

Theory

1200

Hot End temperature(Daily Average)

Practice

Hot End Temp. vs. Residual CaCO3Hot End Temp. vs. Residual CaCO3

Minimumbenefits

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Lime NodulesLime Nodules

24 mm

Core

Residual CaCOResidual CaCO33 DistributionDistribution

0

20

40

60

80

100

A B C D

Res

idua

l CaC

O3

(%)

55 mm60 mm

Nodule DiameterDBCA

Location in Nodule

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Core Size vs. Nodule Size Core Size vs. Nodule Size

0

20

40

60

80

100

0 20 40 60 80 100 120Nodule Diameter (mm)

Cor

e D

iam

eter

(mm

)

8/21/2002

5/04/2002No Core

Lime Nodules At Kiln Front EndLime Nodules At Kiln Front End

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Effect of Nodule SizeEffect of Nodule Size

00 30 60 90 120 150

Nodule Diameter (mm)

20

40

60

80R

esid

ual C

aCO

3(w

t%)

Assuming: 90% CaCO3 in Core and 3% CaCO3 in Shell

ExampleFor product lime that consists of 75% small nodules and 25% large nodules

Overall residual CaCO3 = 0.75 x 3% + 0.25 x 35% = 11%

An 8% increase inAn 8% increase inresidual CaCOresidual CaCO33 means:means:

10.3% increase in mud load6.4% increase in energy requirementWasting $US 350,000/year in fuel cost for a 1000 ADMT/d kraft pulp mill

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Lime Quality Depends onLime Quality Depends onFront End Temperature

Too low uncooked, high residual CaCO3

Too high dead burned lime, low reactivity

Impurities (mostly Na)Low impurities powdery lime, dustingHigh impurities low availability, low reactivity, dead-burned lime

Lime Quality Depends onLime Quality Depends onRetention time

Too short uncooked, high residual CaCO3

Too long dead burned lime, low lime availability

Burning high-sulfur fuel and/or NCGLow lime availabilityLow reactivity due to CaSO4 formation on lime surfaceVarying residual CaCO3 due to unstable NCG burner flame

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Total Reduced Sulphur (TRS) Total Reduced Sulphur (TRS) EmissionsEmissions

Mainly H2S and CH3SHAlso contain CH3SCH3 and CH3SSCH3

Oxidized to SO2 if burnedH2S + 3/2 O2 SO2 + H2OCH3SH + 3 O2 SO2 + 2 H2O + CO2

Oxidation reactions do not appreciably occur at temperatures below 350oC (660oF)

Main Sources of TRSMain Sources of TRS

Fuel (front end)High S fuels (oil, petcoke)Waste gases (NCG, stripper-off-gas)Poor mixing, incomplete combustion

Mud (feed end)Na2S in lime mudPoor mud washing

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TRS Resulting fromTRS Resulting fromPoor Mud WashingPoor Mud Washing

Na2S + H2O + CO2 H2S + Na2CO3

Temperature is too low for H2S to oxidize

MudLime

Burner CO2H2S

Na2S, H2O

A simple test to determine the A simple test to determine the source of TRS emissionssource of TRS emissions

Increase kiln excess O2

If TRS is significantly reduced, it likely originates from the burnersIf NOT, it likely comes from the feed end

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TRS ControlTRS ControlFuel (Front end)

Optimize burner performanceIncrease excess O2

Mud (Feed end)Decrease Na2S content in mud through◆ improved mud washing◆ high mud solids content◆ sulphide oxidation (by air passing through mud on

filter)

SOSO22 EmissionsEmissionsSO2

Formed as a result of TRS oxidationCaptured by lime as CaSO4, which is converted into Na2SO4 in green liquor

Emissions occur only when S input is highhigh sulphur fuel oil, petcokeNCG/SOG

Controlled byGas scrubbingFresh lime

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Ring Formation and RemovalRing Formation and Removal

Causes of Ring BuildCauses of Ring Build--upupSticky particles

wet mud after chain sectionmelting of sodium compounds

Hardening of ring deposits

withstand tumbling/ sliding motion of bed

Particles

Bed

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Ring Deposits Become Hard Ring Deposits Become Hard Through:Through:

Chemical reactionsrecarbonation of CaO: CaCO3

sulphation of CaO: CaSO4

High temperature sinteringRecarbonation is the most important

CaO CaO

CaCO3CO2

CaOCaO

< 800oC(1470oF)

Hardening via RecarbonationHardening via Recarbonation

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Normal CaO (soft)Kiln Wall

Effect of Temperature Effect of Temperature Fluctuations on Ring GrowthFluctuations on Ring Growth

Low Temperature CaCO3 (hard)

Low TemperatureCaCO3 (hard)

Normal CaCO3 (hard)CaO (soft)

Normal CaCO3 (hard)CaO (soft)

CaO (soft)Normal Kiln Wall

Effect of High Sodium ExcursionsEffect of High Sodium Excursions

CaO (soft)High Soda

CaCO3 (hard) CaO (soft)

High Soda

CaCO3 (hard)CaO (soft)

Normal

CaCO3 (hard)CaO (soft)

Normal

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MidMid--kiln Rings Always Form!kiln Rings Always Form!

Calcination800oC (1470oF)

MudLime

Temperature Variation Temperature Variation Makes Rings GrowMakes Rings Grow

MudLime

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Aggravates ringing problems byaltering fuel and water inputsaltering flame patternsforming hard CaSO4 deposits

Effect of sulphation is less important compared to recarbonation

sulphation occurs in a narrow temperature rangeSO2 conc. in kiln gas << CO2 conc.

Effect of NCG BurningEffect of NCG Burning

Refractory Brick PerformanceRefractory Brick PerformanceNo reaction between brick and CaO or CaCO3

Brick damage mainly caused by reactions between SiO2 in brick and impurities in lime at high temperaturesProper selection of brick is important

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Refractory Brick ManufacturingRefractory Brick Manufacturing

Depending on how they were made, bricks with same composition may have different properties!

AluminaSilica

Mixture

Aluminosilicate

Brick

> 1200oC(2190oF)

Good BricksGood BricksLow porosity and good chemical resistanceGood thermal shockAlumina content: 40 - 70%

< 40%: high porosity, poor resistance to chemical attack> 70%: susceptible to thermal shock and spalling

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Brick Service Life May Be Brick Service Life May Be Extended ByExtended By

Better mud washingReducing dregs carryoverAvoiding burner flame impingementLowering front end temperature (accepting higher residual carbonate)

SummarySummaryMany kiln operating problems are related to chemistry

Calcination reaction Recarbonation reaction Sulphation reactionNa compounds and behaviorsTRS emissionsSO2 capture and emissionsRefractory damage