1 Lime Kiln Chemistry and Lime Kiln Chemistry and Effects on Kiln Operations Effects on Kiln Operations Honghi Tran Pulp & Paper Centre University of Toronto Toronto, Canada Tappi Kraft Recovery Short Course St. Petersburg, Florida, January 7-10, 2008 Presentation Outline Presentation Outline Basic chemistry Calcining reaction Lime mud and lime compositions Effects on kiln operations Lime quality Ring formation TRS and SO 2 emissions Refractory brick performance
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2-3 Lime Kiln Chemistry & Effects on Kiln Operation1 Lime Kiln Chemistry and Effects on Kiln Operations Honghi Tran Pulp & Paper Centre University of Toronto Toronto, Canada Tappi
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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
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
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)