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Recovery Boiler Deposit Formation Recovery Boiler Deposit Formation andand Plugging PreventionPlugging Prevention

Honghi TranPulp & Paper Centre University of Toronto

Toronto, Canada

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

Problems with DepositsProblems with DepositsReduce boiler thermal efficiencyCreate a corrosive environment at tube surfacesLead to unscheduled boiler shutdowns for deposit removal or tube repair

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Presentation OutlinePresentation OutlineEntrained ParticlesDeposit PropertiesPlugging in Different RegionsDeposit CleaningPlugging Prevention

Entrained ParticlesEntrained Particles

Particles

Deposits

Dust

Black Liquor

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Particle TypesParticle Types

Carryover500 μm

(100 μm - 3 mm)

ISP20 μm

(1 - 100 μm)

Fume0.5 μm

(0.1 - 1 μm)

CarryoverCarryover FumeFume

ISPISPDeposits onDeposits onAir Cooled ProbeAir Cooled Probe

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EconomizerEconomizer

SuperheaterSuperheater

Boiler BankBoiler Bank

Presentation OutlinePresentation OutlineEntrained ParticlesDeposit PropertiesPlugging in Different RegionsDeposit CleaningPlugging Prevention

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Particle Composition (Wt%)Particle Composition (Wt%)

Carryover

NaCl 2%K-salts 2.5% Other 1%

Na2SO445%

Na2CO350%

NaCl 4.0%K-salts 3.5%

Other 0.2%

Na2SO487%

Na2CO3 6%

Fume

Superheater

Precipitator DustEconomizer

Deposit CompositionDeposit Composition

Na2CO3

Na2SO4

NaCl

K-saltsOther

Boiler Bank

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Appearance of a Deposit ConeAppearance of a Deposit Cone

1200

1400

1600

800

1000Tem

pera

ture

(oF)

600 700 800 900500

Firstmelting

Flowing

Completemelting

Temperature (oC)

Sticky0%

70%15%

~1%

100%

LiquidPhase

First melting temperature is important for corrosionSticky and flowing (slagging) temperatures are important for fouling/pluggingThese temperatures are related to liquid content, which is a function of:

deposit composition (mainly chloride)temperature

Melting TemperaturesMelting Temperatures

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Deposit Sticky TemperatureDeposit Sticky Temperature

0 mole% K/(Na+K)

mole% Cl/(Na+K)0 102 124 146 168 18 20

850

700750800

500550600650

Tem

pera

ture

(oC

)

0 mole% Cl/(Na+K)

1

5

10

20

ChlorideChloride

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Deposit Sticky TemperatureDeposit Sticky Temperature

mole% K/(Na+K)

mole% Cl/(Na+K)0 102 124 146 168 18 20

0

10 20

5

850

700750800

500550600650

Tem

pera

ture

(oC

)

SurfaceDiffusion

BulkDiffusion

Materialtransfer

Particle SinteringParticle Sintering

Temperature

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Dust Sintering Dust Sintering Test (1 hour)Test (1 hour)

Before After 480oC (900oF)

Vol. Shrinkage 58%

After380oC (720oF)

Vol. Shrinkage 18%

Compressive Strength Compressive Strength ComparisonComparison

GlassFired brickCementSintered dustInsulating brickUnsintered dust

MPa800 - 2000

7030 - 60

507

<2

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Presentation OutlinePresentation OutlineEntrained ParticlesDeposit PropertiesPlugging in Different RegionsDeposit CleaningPlugging Prevention

Plugging at the Superheater InletPlugging at the Superheater Inlet

Screen

Superheater

Screen

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Deposit Accumulation PatternDeposit Accumulation Pattern

ScreenSuperheater

FurnaceWall

Carryover

Plugging in the SuperheaterPlugging in the SuperheaterCaused by carryover deposition

High gas temperatureLarge amount of sticky carryover

Not by fume depositionSmall amount of fume

Deposits

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Plugging at the GB InletPlugging at the GB Inlet

Plugging at the GB inletPlugging at the GB inletNarrow tube spacing (2”)Carryover deposition upstream (superheater)

High gas temperatureSticky carryover

Fume deposition and sintering

High gas temperatureLarge amount of fume

Deposits

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Cause of Plugging at the GB InletCause of Plugging at the GB Inlet

TimeMassive depositbuildup zone

Clean PluggedFouled

Plugging in the Generating BankPlugging in the Generating Bank

Fume deposition and sintering

High gas temperatureLarge amount of fume

Inadequate sootblowing

DepositsDeposits

Deposits

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Flue Gas

Steam6200 KPa(900 psi)

Deposit

Tube

550oC

450oC (Hard)

270300oC (Soft)

Temperature across a GB Tube and Temperature across a GB Tube and Deposit LayerDeposit Layer

Deposits sinter rapidly if they are:Deposits sinter rapidly if they are:jammed between tubeson stabilizing bars and baffleson sealed tubes (no steam inside)on mud drum (for bi-drum boilers)

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Plugging in the Generating BankPlugging in the Generating Bank

Plugging in the Generating Bank of a Plugging in the Generating Bank of a Single Drum BoilerSingle Drum Boiler

Flue Gas

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Plugging in the EconomizerPlugging in the Economizer

Fume deposition and sinteringSticky fume (acidic sulphate formation)

Low bed temperatureHigh sulphidity

Inadequate sootblowing

Na2SO4 + SO2 +½ O2 +H2O = 2 NaHSO4

Deposits

Baffle

Deposits

Baffle

Presentation OutlinePresentation OutlineEntrained ParticlesDeposit PropertiesPlugging in Different RegionsDeposit CleaningPlugging Prevention

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Nozzle Throat JetPIP

Lance Tube

Sootblower LanceSootblower Lance

350 psig steam60 to 100 sootblowers per boilerConsume 3 to 12% total boiler stream production

Factors Affecting SootblowingFactors Affecting SootblowingEfficiency Efficiency -- Jet Jet

Peak Impact Pressure (PIP)

Nozzle designSteam flow rate and pressure

Distance to depositBlowing angleBlowing sequence and frequency

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Factors Affecting SootblowingFactors Affecting SootblowingEfficiency Efficiency -- DepositDeposit

Deposit adhesion strengthDeposit composition (Cl, K)Deposit temperature

Deposit size and thicknessDeposit and tube temperature

Tube Temperature and PIP Tube Temperature and PIP RequirementRequirement

Economizer160-270oC: LOW(320-520oF)

Superheater300-500oC: HIGH(570-930oF)

Generating Bank270-300oC: MEDIUM(520-570oF)

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Presentation OutlinePresentation OutlineEntrained ParticlesDeposit PropertiesPlugging in Different RegionsDeposit CleaningPlugging Prevention

Plugging due to Sticky CarryoverPlugging due to Sticky Carryover

Main CausesExcessive carryoverHigh flue gas temperature

Preventive MeasuresMinimizing carryover quantityReducing carryover stickinessLowering flue gas temperaturesIncreasing sootblowing efficiency

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Plugging due to Fume SinteringPlugging due to Fume Sintering

Main CausesExcessive fume High flue gas temperatureIneffective sootblowing

Preventive MeasuresMinimizing fume quantity (low bed temperature, coarse spray)Lowering flue gas temperaturesIncreasing sootblowing efficiency

Plugging due to Sticky FumePlugging due to Sticky FumeMain Causes

Low carbonate content in the fumeHigh SO2/SO3 conc. in the flue gasIneffective sootblowing

Preventive MeasuresIncreasing bed temperatureLowering liquor sulphidityIncreasing sootblowing efficiency

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SummarySummaryDeposit formation and characteristics vary greatly with location within a boiler, and differ from boiler to boiler.It is important to identify the exact cause in order to devise appropriate solutions