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1
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
2
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
5
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)
6
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
7
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
8
Sodium ControlSodium ControlIncrease mud solids contentImprove mud washing Purge lime dust
Effects on Kiln OperationsEffects on Kiln Operations
9
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
11
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
12
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
13
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
14
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
15
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
16
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
17
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
18
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
19
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
20
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
21
MidMid--kiln Rings Always Form!kiln Rings Always Form!
Calcination800oC (1470oF)
MudLime
Temperature Variation Temperature Variation Makes Rings GrowMakes Rings Grow
MudLime
22
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
23
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
24
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