Wet FGD Chemistry andPerformance FactorsGordon Maller – URS CorporationPresented at:2008 Power Gen ConferenceDecember 1, 2008Presentation Outline•FGD Chemistry Overview•Effect of Key Process Variables onFGD Operation and Performance•Chemical Process Problems – HowTo Identify and CorrectWhat Are Chemical Reactions?•Two or more reactants (molecules) combining(reacting) to form different products•Reactants and products can be gas, liquid orsolid•Examples:−SO2(g) + H2O H2SO3(aq)−Ca+++ SO4=+ 2H2O CaSO4*2H2O•The rate of a reaction depends on:−Nature and concentration of the reactants andproducts−TemperatureTypes of SolutionsGas-Liquid(CO2and Water)Liquid-Liquid(Gin and Tonic)Solid-Liquid(Coffee and Sugar)Solid Solution(A crystal whichcontains a mixtureof molecules fromtwo differentsolids)SolubilityDefinition:Amount of substance required toproduce saturated solution.The substance is thesolute. The liquid substance it isdissolving into is thesolvent.TemperatureEffects:Sugar or SaltTypicalGypsumCaSO3Limestone

SolubilityTemperatureAcids and BasespH17 14Definition:Acid = pH < 7 Base = pH > 7

Stomach Acid Sulfurous Acid Vinegar Typical Scrubber Water Limestone Lime

Sulfuric Acid Coca Cola DBA Blood Draino LyeBuffersDefinition:Chemical species that can absorb aH+ion and moderate a pH change:CaCO3+ 2H+Ca+++ HCO3-+ H+Ca+++ H2CO3Overall Chemistry of a Limestone ForcedOxidation FGD SystemSulfurDioxideLimestoneOxygen Water GypsumCarbonDioxide++ ++SO2+ CaCO3+ O2+ H2O CaSO4.2H2O + CO2Gas SolidGas LiquidSolid GasReactions involve gas, liquid, and solid phasesChemical Reaction Steps in FGDProcess•SO2absorption in absorber•Neutralization of the absorber SO2to keep theSO2vapor pressure low so that more SO2canbe absorbed•Dissolution of the limestone or lime reagent toprovide alkalinity for neutralization andcalcium ion for precipitation•Oxidation of absorbed SO2to form sulfate(SO4)•Precipitation of calcium and sulfate or sulfiteto form byproductWet Limestone Forced Oxidized ProcessChemistrySO2Absorption:SO2+ H2O H2SO3H2SO3H++ HSO3Limestone Dissolution:CaCO3+ H2O Ca+++ HCO3+ OHOxidation:HSO3+ O2H++ SO4=Precipitation:Ca+++ SO4=+ 2H2O CaSO4* 2H2OMass Transfer and Reaction Steps Occurat Several Places in FGD System•Absorber•Reaction tank•Solid-liquid separationStackBoilerDustCollectorFuelAirIDFanAirAbsorberWaterAsh RemovalLimestoneSiloBall Mill GrinderSlurry Storage TankWasteWaterGypsumConvertorBelt FilterWashWaterHydrocycloneSO2 Absorber BleedGypsumDewateringReagentReagentPreparationMass Transfer and Reaction Steps Occurat Several Places in FGD System (cont.)Absorber•SO2absorption•Reagent dissolution•CO2evolution or absorption•Sulfite oxidation•Solids precipitation•HCl and HF absorptionMass Transfer and Reaction Steps Occurat Several Places in FGD System (cont.)Reaction Tank•Limestone dissolution•Scrubber solids precipitation•Sulfite oxidationMass Transfer and Reaction Steps Occurat Several Places in FGD System (cont.)Solids-Liquid Separation•Solids concentration−Gravity settling (e.g., thickener)−Hydroclones−Filtering−CentrifugingOverview of Mass Transfer and ReactionSteps by PhaseGas-Liquid Mass TransferCarbon Dioxide Desorption CarbonateSulfur Dioxide AbsorptionOxygen AbsorptionHCl AbsorptionLiquid-Solid Mass TransferCalciumMagnesiumSodiumCarbonateDissolution LimestonePrecipitation Calcium SulfateSulfiteSulfateH+ChlorideFluorideCarbonate + H+BicarbonateSulfite + H+BisulfiteGas PhaseLiquid PhaseSolid PhaseAcid-Base Equilibrium ReactionsSO2(g)+ H2O H2SO3H++ HSO32H++ SO3=CO2(g)+ H2O H2CO3H++ HCO32H++ CO3=SulfurousAcidBisulfite SulfiteCarbonicAcidBicarbonate CarbonateIncreasing pHDecreasing pHpK 1.81 pK 6.91pK 6.37pK 10.25Liquid-Phase Alkalinity•Concentration of liquid species that arealkaline with respect to SO2absorption•Main source of alkalinity for inhibitedoxidation FGD processLiquid-Phase Alkalinity (cont.)Examples:Absorption SO2+ H2O H++ HSO3-GasNeutralization SO3=+ H+HSO3-CO3=+ H+HCO3-AqueousOverall CO3=+ SO2+ H2O HSO3-+ HCO3-Aqueous GasSolid-Phase Alkalinity•Dissolution of limestone in absorberreplenishes alkalinity and allows scrubbing tocontinue•Main source of alkalinity for forced-oxidationFGD processSolid-Phase AlkalinityExamples:Absorption SO2+ H2O H++ HSO3GasDissolution CaCO3Ca+++ CO3=SolidNeutralization CO3=+ H+HCO3-AqueousOverall CaCO3=+ SO2+ H2O Ca+++ HSO3+ HCO3-Solid GasH2OSulfite Oxidation Reactions(Free Radical Chain Reactions)SO3=+ Fe+++SO3* + Fe++where SO3* is a free radicalSO3*.+1/2O2SO4=Ca+++ SO4=+ 2H2O CaSO4.2H2O (gypsum)Sulfite OxidationDepends On:•Ratio of O2:SO2•Solution chemistry−pH−[SO3=]−Trace metals•TemperatureSolid-Liquid Equilibrium•Determines if limestone will dissolve•Determines if gypsum will precipitate•Dissolution refers to formation of ions insolution from a solid•Precipitation refers to formation of solids fromions in solutionWhat is Present in the Liquid Phase?Ions:Ca++, Na+, Mg++, SO4=, Cl-, etc.Ion Pairs:CaCO3, CaHCO3+, MgHSO3-, MgSO3, CaSO4, etc.All liquid phase reactions are at equilibrium:Ca+++ CO3=CaCO3Ca+++ HCO3-CaHCO3+Relative SaturationQ: Will limestone dissolve?R.S. =A:Yes – when R.S. is < 1 (subsaturated)No – when R.S. is > 1 (supersaturated)[Ca++].[CO3=]Ksp[Ca++] = Ca++Activity (Conc.)[CO3=] = CO3=Activity (Conc.)Ksp= CaCO3SolubilityproductconstantQ: Will Gypsum (CaSO4.2H2O)Precipitate?[ Ca++].[SO4=].[ H2O]2KspA:Yes – when R.S. is > 1No – when R.S. is < 1R.S. =Two Types of Gypsum Precipitation•Nucleation (spontaneous)•Crystal growth (controlled)Safe Scaling

C r y s t a l G r o w t h N u c l e a t i o n

Gypsum Precipitation Rate, r G1.3 – 1.4Gypsum Relative Saturation, RSGFactors that Determine Type of GypsumPrecipitation•Relative saturation of the solution•The presence of sufficient amount of seedsolids•The rate at which the gypsum precipitatesCrystal Size, Shape, and Habit•Affect dewatering and handling properties ofsolids•Determined by:−FGD process or design−Type of crystal (gypsum or calcium sulfite)−Contaminants in the crystal−Crystal habit modifiers−Residence TimeGypsum Crystal: Triclinic System(No Axis of Symmetry)Calcium Sulfite Crystal: OrthorhombicSystem (Three, 2-Fold Axes ofSymmetry)FGD Gypsum Solids From Different Processes**(Same Magnification)Effect of Key ProcessVariables on FGD Operationand PerformancePerformance Variables That RequireControl•SO2removal efficiency•Reagent utilization•Scaling potentialSO2Removal EfficiencyKey Chemical Factors•pH or alkalinity•Excess limestone in recycle slurry (key forforced oxidized process)•Solution chemistry•Inlet SO2loading (combination ofconcentration and load)SO2Removal Efficiency (cont.)Key Mechanical Factors•Liquid-to-gas ratio (L/G)•Mass transfer characteristics of absorber (e.g.,trays, packing)•Gas / liquid distribution•Flue gas bypass (where applicable)Reagent UtilizationKey Factors•pH•Solution chemistry•Solids residence time•Surface area (grind size)•Limestone reactivityCauses of ScalingKey Factors•Oxidation (not usually a factor in forcedoxidation process)•Limestone utilization•Slurry density•Reaction tank volume•ME wash design or operation•ME wash water qualityFactors That Affect FGD Performanceand Reliability•Direct Control−pH**−Slurry density**−L/G•Limited Control−Limestone properties**−Solution chemistry**−Mechanical factors−Water management**To be discussed•No Control−Boiler load cycles−Coal S variations−Inlet flue gasOperators have direct, limited or no control of key parameterspH / Utilization / SO2RemovalRelationshipspH

Utilization 

# SO

2 

# Removal(lower)(higher)Slurry Density (Slurry SolidsConcentration)•Minimum slurry density ensures adequatecrystal surface area for precipitation. This isimportant to prevent scaling•Maintaining a higher slurry density canimprove limestone utilization•Slurry density affects solids residence timewhich, in turn, affects limestone utilization andscaling potentialExplanation of Effect of Slurry Densityon Limestone Utilization•Maintaining pH and SO2removal requires aspecific limestone loading (g-limestone/Lslurry) level for the system.•At low density (low concentration of solids inthe slurry), a higher concentration of thesolids will be limestone. Therefore limestoneutilization will be lower.Solution Chemistry•Dissolved Cl-(Ca++)Concentration−Inhibits dissolution due to common ion effect•Forced oxidation vs. natural oxidation−Stripping of CO2tends to enhance dissolution•Aluminum fluoride blinding•Sulfite blinding−Problem during periods of incomplete oxidation•All can affect the ease with which reagentdissolves and removal performance ofscrubberEffect of Dissolved CalciumConcentration0 5 10 15 20 25 30 35 40Limestone Loading (g/L)

SO 2 Removal (%)7580859095High Dissolved CaLow Dissolved Ca(Different Shaped Symbols Represent DifferentLimestone Tested)Effect of Oxidation Mode0 5 10 15 20 25 30 35 40Limestone Loading (g/L)

SO 2 Removal (%)7580859095Forced Oxidation ModeNatural Oxidation Mode(Different Shaped Symbols Represent DifferentLimestone Tested)Effect of Soluble Aluminum onLimestone Utilization0Aluminum (mg/L)

Utilization (%)2010 20 30 4030405060708090100Limestone Properties Affecting ScrubberPerformanceProperties will affect pH-Utilization-Removalrelationship•Grind•Composition•Reactivity (dolomitic fraction)Effect of Limestone Grind0 5 10 15 20 25 30 35 40Limestone Loading (g/L)

SO 2 Removal (%)708085909575Course Ground Limestone (70% Pass 325 mesh)Fine Ground Limestone(90% Pass 325 mesh)(Different Shaped Symbols Represent DifferentLimestone Tested)Effect of Limestone Dolomitic FractionLimestone With Low Dolomitic FractionLimestone With High Dolomitic Fraction0

SO 2 Removal (%)75808590955 10 15 20 25Limestone Loading (g/L)Chemical Process Problems –How To Identify and CorrectProblem Areas To Be Discussed•Scaling and solids buildup•Mist eliminator pluggage•Limestone blinding•Poor SO2removal•Poor reagent utilization•Poor gypsum byproductqualityScaling and Solids BuildupCommon areas for scaling:•Scrubber inlet•Absorber internal walls andsupport structure•Absorber spray headers•Inside spray piping•Trays•Reaction tank wallsCauses of Scaling and Solids Buildup•Chemical (high gypsum relative saturation)−Oxidation−Insufficient seed solids−Solid residence time that is too short−Poor wash water quality•Wet-dry interface and recirculation of slurry•Poor reagent utilizationCauses of Scaling in FGD SystemsHigh Gypsum Relative Saturation:•Scaling can occur in all areas of absorber andon mist eliminator (ME) as result of solidsnucleation due to high relative saturation•High relative saturation usually result ofinsufficient seed material or high precipitationrates•Scaling in ME may result from poor qualitywash water (water saturated or nearlysaturated with gypsum)Causes of Scaling in FGD SystemsWet/dry Interface:•Occurs in area of absorber where hot flue gasis quenched•Scaling results from fluctuations in gas flowand gas distribution. Causes areas along ductwalls or other internal structures to vary fromwet to dry•If slurry is carried back into this area, it maydry and accumulate as a hard scale composedof calcium sulfate, and/or fly ashCauses of Scaling in FGD SystemsWet/dry Interface:•Scaling as result of a wet/dry interface iseliminated by:−Modifying the physical design of the absorber inletto eliminate the flow disturbance−Controlling the location of the wet/dry interface bymodifying the way flue gas is quenchedScaling and Solids BuildupSteps to Correct:•Control process chemistry within designlimits•Ensure that inlet flue gas is quickly andcompletely quenched•Modify absorber inlet design to eliminateflow disturbances and gas recirculation•Maintain reagent utilization within designrangeMist Eliminator Pluggage•Chemical causes−Poor reagent utilization−Poor quality wash water•Mechanical causes−Poor design of wash system−Broken or plugged wash system−High gas velocity−Non-uniform gas flow and liquid loading distributionMist Eliminator Pluggage•Indication−Increase in pressure drop−Visual observation during inspection•ME pluggage can:−Result in increased gas velocity throughME and liquid carryover into outlet ductand stack−Cause stack rainout and particulateemissions−Damage ME due to weight of solidsConsiderations For ME Wash System•ME faces to wash•Wash coverage (150 to 200%)•Wash intensity and pressure (1.5 gpm/sqft, 30 to 45 psi)•Wash duration and frequency•Recommended wash nozzlesInhibited Dissolution of Limestone•Limestone must dissolve in scrubber toprovide alkalinity•Certain dissolved chemical species cansignificantly slow or stop the dissolution oflimestone•Inhibition - Slowing of dissolution•Blinding - Significant slowing or stopping ofdissolution•High concentrations of dissolved chloride andmagnesium will inhibit dissolution•The mechanism for this inhibition is called“common-ion” effectLimestone Blinding•In forced oxidation process, limestoneblinding can result from high concentrationsof dissolved sulfite or aluminum-fluoridecomplex•Either sulfite or aluminum-fluoride complexwill react on surface of limestone particle toblock dissolving site•Aluminum-fluoride blinding often initiated byhigh concentration of inlet fly ash•Sulfite blinding initiated by incompleteoxidationPoor SO2Removal as Result of ChemicalProblemCauseAnalyticalIndicationCorrective ActionInsufficient Limestone InScrubberLow carbonate measured inscrubber solidsIncrease pH set-pointMalfunctioning pH MonitorPoor agreement during pHcalibration checkMake repairs to pH monitorsystemSulfite BlindingPoor reagent utilizationalong with elevated levelsof soluble sulfiteVerify operation of oxidationair system. Correct asrequiredAluminum Fluoride BlindingPoor reagent utilizationalong with elevated levelsof aluminum and fluorideImprove particulate removalupstream of scrubberPoor Limestone Quality orOff-Spec Grind SizeSieve analysis, compositionanalysisModify grind circuit and/orobtain better limestonePoor SO2Removal as Result ofMechanical ProblemCauseIndicationCorrective ActionPlugged Spray Header(s)or Spray Nozzle(s)Low recycle flow Asindicated by flow meter ormotor ampsClean during outageBroken Spray HeaderIndication from flow meteror motor amps. Observedduring inspectionRepair during outageLoss of Recycle PumpCapacityLow recycle flow asindicated by flow meter ormotor ampsRepair during outagePoor Reagent UtilizationCauseAnalyticalIndicationCorrective ActionMalfunctioning pH MonitorPoor agreement during pHcalibration checkMake repairs to pH monitorsystemSulfite BlindingPoor reagent utilizationalong with elevated levelsof soluble sulfiteVerify operation of oxidationair system. Correct asrequiredAluminum Fluoride BlindingPoor reagent utilizationalong with elevated levelsof aluminum and fluorideImprove particulate removalupstream of scrubberPoor Limestone Quality orOff-Spec Grind SizeSieve analysis, compositionanalysisModify grind circuit and/orobtain better limestonePoor Gypsum Byproduct Quality•High acid insoluble (inerts) fraction−Verify operation of particulate control device upstream ofscrubber−Optimize operation of process hydroclones to maximizeconcentration of inerts in overflow−Increase liquid purge rate•Poor reagent utilization•Low sulfite oxidation−Verify operation of oxidation air systemPoor Gypsum Byproduct Quality(cont.)•High moisture−Verify operation of primary and secondary dewateringsystem−Purge fines from process−Check filter cloth for blinding. Clean or replace asrequired−Increase temperature of filtercake wash water−Verify system chemistry•High level of limestone•High level of acid insolubles•Presence of crystal modifier