# VGB PowerTech

12

1 2012

FGD 

# upgrade

- 

# Experience

from 

# 52,280

MWe of wet

flue

gas desulphurisation system upgrades Jonas

S

. Klingspor Kurzfassung 30

J ahre Erfahrung

mit der Ertuchtigung n osser Rouchgosentschwefelungsanlogen Seit

der Einfilhrung

der nassen Rauchgasent

· schwefelung

(REA) wurden

die
S0

2 

# -Grenzwerte schrittweise

verscharft. Demzufolge

milssen

al

teren REA-Anlagen

Leistungen

oberhalb

ihrer urspriinglichen

Auslegung erbringen.

Dariiber hinaus

sind

in 

# nassen

REAs


# niche

nurSchwefelemissionen

sondern

auch Halogene,

Staub

und Quecksilber abzuscheiden. Seit mehr

als
30 Jahren beschtiftigt

sich

URS mit



# der Forschung, Entwicklung,

demProbebetrieb

sowie

der Ertiichtigung

nasser

REASysteme

. Bis jetzt wurden

rund 52.280

MW Kraftwerkskapazitiit

ertiichtig.

Dabei wurde die Zuverltissigkeit

durch

die Verminderung von Ablagerungen,

die Herstellung

verwertbarer Nebenprodukte

wie Flugasche

undREAGips, sowie

die sicherer Beherrschung

derEmissionen (Staub, Halogene

, 50

2 

# und Quecksilber) erhOht.

URS

hat gezeigt

, dass unabhtingig

von der urspriinglichen

Auslegung

, jede

REA

so modifiziert

und ertiichtigt

werden

kann,

dass auflerplanmiiflige Nichtverfiigbarkeiten

ausgeschlossen

werden konnen

, und unabhangig

von schwefelreicher

oder schwefelarmer

Kohle

, 50

2

- 

# Abscheidegrade

von iiber 99% erreicht werden. Dabei werden

auch Staub

undQuecksilberemissionen zuverliissig vermindert. In dem vorliegenden

Beitrag
werden

die 

# Aus

legungs-

und Betriebsbedingungen

dargestellt, die

zu sub

- optimaler

Leistung

undVer.fiigbarkeit fiihren

sowie moglicheErtiichtigungsmaflnahmen,

um bestehende REA-System

an den Stand

der Technik anzupassen.



# I 

# Author Dr

. Jonas

S

. Klingspor URS 

# Corporation A usfin/Te

xo s/USA 

# VGE!I

I POW

E R TECH

Introduction Wet

flue

gas desulphurisation

(FGD)

systems
were pioneered

in the late 1960s

to control emissions

of sulphur dioxide

(S0

2

) 

# from

coal

- fired power plants.

Some

of the basic design principles

used

in the original FGD designs

are still used today

. However, early systems showed deficiencies

in many areas

as the basic chemistry,

fluiddynamics and gas/liquid

contact

as well

as mist eliminator technology

were poorly

under

stood

and would

take decades

to refine. Initial

FGD systems

used predominately natural-oxidation

chemistry often lead

to scaling,

build

- up, and poor reliability

.Later, inhibited-oxidation,

based

on addition of thiosulphate

or elemental

sulphur,

was successfully

used

to control

the chemistry to avoid scaling conditions. Eventually, forced-oxidation

was introduced,

which provided

the opportunity

to produce

asalable by-product,

wallboard

- grade gypsum, while providing

a robust chemistry

much less prone

to scaling. Fluid dynamics,

in particulate

gas distri

bution,

was modelled

in the early

days through

scale models, predominately

focussing

on the

gas phase

only while

the interaction

of flue

gas and slurry

was

perceived

as less important.

The impact

of spray nozzles

on the flue

gas distribution was poorly understood

and frequently

ignored. Therefore,

the effect

of nozzle

spray angles;

the spray coverage

as

a function

of the nozzle

size, type, 

# density,

and pressure of the spray nozzles;

and

the impact

of wall sneakage

were typically

not included

in the model. Since

the introduction

of wet FGD systems, emission standards

have gradually

been tightened.

Wet FGD systems

are now being used

to control

in addition

to S0

2 

# halogens, particulate

matter,

and mercury. URS

has been involved

in research,

development

, testing,

and upgrade

of wet FGD systems

for more

than

30 years;

and

to date we have modified

52,280

MWe

of wet FGD systems

to improve reliability

byeliminating scaling; produce

a salable

by

- product namely

fly ash and gypsum;

and control emissions

of particulate

matter, halogens

, S0

2

, 

# and mercury.

URS

has demonstrated that every

wet FGD system, regardless

of design

and configuration,

can

bemodified

to achieve uninterrupted

operation

between scheduled

outages,

achieve

S0

2 

# removal efficiencies

beyond

99

% 

# onlowto high-sulphur

coals,

and

to efficient

pro

vide multi-pollutant

control.

This paper provides

an overview

of the design

and operating

issues resulting

in less than

opti

mum performance

and available
upgrade options

to improve

system performance · to the performance

level provided

by new state-of-the-art

FGD systems. 

# Operating

p

r oblems Ba

c kground 

# Wet FGD systems

were

first introduced

in the late 1960s

and through

a multitude

of design iterations

have converged

t owards bottom

· supported

absorber

towers,

round or rectangular,

with

a standard

set

of

ab

· sorber internals

to optimise performance and reliability.

However,

the road

to

areliable design

has involved multitude

of design options,

which

in retrospect

were implemented

without

full understand

ing

of FGD chemistry

and fluid dynamics

. Wet FGD systems

that have been provided commercially,

on utility coal-fired

boilers include: - Plump

bob absorber, - Rectangular

hung absorbers, - Cross-flow horizontal absorbers, - Co

-c urrent packed absorbers, - Disc

and donut absorbers, - Double

loop absorbers, - Venturi absorbers, - Rod absorbers

, - Open spray tower absorbers, - Open

spray tower

and

tray absorbers, and - Single

and twin tower fountain

absorbers Some

of the more common absorber

designs

are illus

t rated

in Figure

1. This paper details typical operating

problems

with

wet FGD systems,

as well

as available

upgrade solutions associated with different absorber

designs. Typica

l operating
problems 

# The
root cause

of the absorber

opera

t ing problems

, listed above,

can typically

be

di

vided into

nine categories: 1 -Unba

l anced

flow between modules

, 2 -Poor

gas distribution

in FGD system,

101

-

# -

FGD upgrade



# VGB PowerTech



# 12

1 2012 - inhibited oxidation - avoidance

of

s ulphite blinding - avo

i dance

of aluminium

fluoride blinding -avoidance

of crystal modifiers -avoidance

of foaming

agents High-efficiency



# nozzles and spray headers Fig.

1

. 

# Common absorber

design including

plump

bob,

rod, packed

and tray/spray

tower absorbers.



# Nozzles

used

in wet FGD systems include hollow

cone nozzles,

full cone nozzles,

and pig

t ail nozzles.

For

a limestone

/ gypsum type chemistry,

t he nozzle performance is mostly determined

by the masstransfer characteristics

across

the interface

between

the gas and slurry

. The mass transfer 

# is often expressed

in terms

of gas film

and liquid

film thickness

across

the flue

gas and slurry interface.

The thickness

of the

gas film

is

a function

of the relative velocity

between

the slurry

and

the flue gas.

A higher gas velocity

will yield

a thinner

gas film and potentially

a higher

mass transfer

rate. For mass transfer

into

the slurry,

it



# is often more useful

to

u se the surface renewal

concept instead

of

a liquid-film
approach.

The rate

of surface renewal

is

a function

of the internal

mixing

or

tu

r bulence

in the slurry. A higher degree

of tur

b ulence results

i n

a higher

rate

of surface renewal

, therefore potentially

a higher

ma

ss transfer

rate.

A typical

spray pattern

for two adjacent

nozzles

is shown

in Figure

4. 3- Poor spray coverage, 4-

No droplet-to-droplet

interaction, 5 - Sneakage, 6- Mist eliminator

wash system, 7- Reaction

tank mixing

and introduction of forced oxidation

air, 8- Scrubber chemistry,

and 9- Materials

of construction. Some

of the common

flue

gas distribution problems

are illustrated

in Figure

2. With older

wet FGD systems,

gas sneakage around nozzles

inside

the absorber

as well as sneakage

along

the absorber

walls

was common

due

to narrow-angle

nozzles

and a low-nozzle

density leading

to poor spray coverage

as shown

in Figure

2. Poor spray coverage

can also produce

skewed

distribution
across

the absorber.

These common problems

can occasionally

be found

in new absorber installations.

Poor spray coverage and poor

gas distribution

impacts

theperformance

of the absorbers

and frequently limits

the maximum achievable

S0

2

# removal efficiency

to less than

95

%. 

# Common

flue

gas maldistribution

problems associated

with

the absorber

inlet and outlet

ducts

are shown

in Figure

3. Skewed distribution

in the absorber

inlet duct

will often generate

flue

gasrecirculation

zones leading

to build-up

of solids in the vicinity

of the wet/dry interface. Likewise

, a skewed

flue

gas distribution

at the absorber

outlet

duct

will often

lead

to excess

mist eliminator
carryover,

scaling of the absorber

outlet

duct,

and stack

and possibly

stack

rain. 

# Available

upgrade

options Most

wet FGD systems, regardless

of initial physical

design

or chemistry,

can

be

up

graded

to achieve

above

99%

$0

2 

# removal efficiency

and

to operate uninterruptedly between scheduled

outage

cycles.

Upgrades typically

have

to address fluid-dy

namic related

issues

such

as uneven

distribution between absorber modules

, unev

e n gas flow

in the absorber

inlet duct,

poor gas

/ liquid

con tact

, skewed distribution

in the absorber

outlet

duct,

and poor mixing in the reaction

tank. These issues typically are materialised

through deposits

andscaling

in the absorber

inlet

and outlet ducts, and poor

S0

2 

# removal efficiency. 

# -

102

Older

wet
FGD systems

were typically equipped

with

poor gas/liquid

contact devices resulting

in S0

2 

# removal

below acceptable

levels

. Frequently,

the poor gas/liquid

co ntact limited

S0

2 

# removal to around

95

o/o, 

# regardless

of measures taken.

Occas

iona

ll y, inefficient gas/liquid contact devices

are also found

in modern wet FGD systems. A number

of upgrades solutions

areavailable

to improve

the performance

ofexisti ng FGD systems: 1 -Flow distribution improvements - turning

vanes -perforated

plates -bull

nose -wall

rings 2 - Gas liquid contact devices - sieve
trays -spray
nozzle layout -double

hollow

cone nozzles 3 -Chemist

ry - performance

additives - for ced oxidation Coverage



# 0 1 2 

# 3 4 

# 5 6+



# Hollow

cone nozzles

gene

rate

a hollow cone spray pattern

by giving

the slurry

a centrifugal

rotation

before exiting

the

no

z

zle. Pigtail nozzle generates

spiral hollow cone spray patterns

by use

of the pigtail but

the spray pattern

can more accuratly Fig.

2. Comm

on absorber problems
including

spray coverage

, flue

ga

s distribution

and

flue
gas sneakage

. F ig.

3.

E xamples

of common

inlet
and

ou tlet duct

flue

gas rna/distribution

pr ob

l e m.VGBPowerTech121201270%ofS02removalFiut0.5m•••••••••••••30%ofS02removal•Remaining1.0m•••0•••••••••'•••••••••••••••••••••FGDupgrade-cessful,whileusingagreaterdistancecanbedeceptive.Dropletturbulencecanalsobegeneratedbydroplet-to-drorletinteractionandcollision.Thiscanbeachievedbyselectinganappropriatenozzle-to-nozzledistanceaswellasbyusingdoublehollowconenozzles,ahigh-efficiencysprayheader,showninFigure5.Ahigh-efficiencysprayheaderisobtainedbymaintainingahighnozzledensityandbyusingwideanglenozzles.ThewhitishareasbetweennozzlesinFigure5areareasofhighdroplet-to-dropletcollisionandcreateahighcapacityforS02removal.Doublehollowconenozzlesalsogeneratedropletto-dropletcollisionwithinthespraycone.ThesearevisibleinFigure5asawhitisharearightbelowthenozzletip,andalsocreateahighcapacityforS02removal.Fig.4.Typicaldropletgenerationanddroplet-to-dropletcollisionbyhollowconenozzles.Ahighefficiencysprayheaderincombinationwithdoublehollowconenozzlesprovidessubstantialimprovementsinperformance,asshowninFigure6.Onaverage,ahighefficiencysprayheaderanddoublehollowconenozzlesprovidesa40percentincreaseinperformance,asdeterminedbyside-by-sidetestingofanupgradedabsorbermoduleandanunmodifiedabsorbermodule.becharacterisedasafullconenozzle.Also,pigtailnozzlegeneratesanunevenslurrydistributioninthecrosssectionbelowthenozzle,whicheffectsgasdistributioninsidetheabsorbertower.BoththepigtailandfullconenozzlesarelessoftenusedinwetFGDsystemsbecausetheygenerateapressurecentrebelowthenozzles,whichforcesthefluegastotravelaroundthesprayconegeneratingapoorfluegas/slurrymixingenvironment.Ahollowconenozzle,ontheotherhand,generatesalowpressureareabelowthetipofthenozzle,whichattractsthefluegasandforcesthefluegastotravelthroughthesprayconeratherthanarounditand,therefore,providinganexcellentfluegas/slurrymixingenvironment.SlurrynozzlesusedinwetFGDsystemsinitiallygeneratealiquidsheetastheslurrySlurrynozzlesforwetFGDsystemsareoftencharacterisedbytheSoutermeandropletsize.Amoreusefulcharacteristicistheexitvelocityoftheslurry.Useofthespraycoverageatthe0.5mdistancefromthenozzlestocapturethemostreactiveregionofthespraycoveragehasprovenverysueleavesthenozzle.Theliquidsheetbreaksfig.s.Highefficiencysprayheaderandspraynozzles.upintosmallerligaments,whicheventuallyproducedroplets,asshowninFigure4.$02hassuccessfullybeenmeasuredinsidewetFGDsystemsbymultiple organisationsandithasbeenconcludedthatthemajorityoftheS02removaloccurswithinadistanceofO.Smfromthenozzletip,inanareathatonlyhasthinslurrysheets,andbeforeanydropletsareformed.Astheslurrybreaksupintoligamentsanddroplets,theslurrybecomeslessandJessturbulentand,therefore,lessreactive.Althoughtherelativevelocitybetweenthefluegasandtheslurrydropletsdecreasesomeastheslurrytravelsawayfromthenozzle,theassociatedincreaseingasfilmthicknessdoesnotexplainthedramaticreductionin502removalwithdistancefromthenozzletip.Asthenozzleinjectstheslurry,theslurryexhibitsaforcefulinternalturbulence,which,overtime,isconsumedbygenerationofslurryligamentsandeventuallydroplets.Bythetimedropletsareformed,theyareclosetostagnant;providingasubstantialbarrierjJ§!00IS10aseo11)..,40100LJG..,.s!Mac:f!.0forliquid-sidemasstransferinternally.fig.6.Performanceofhighefficiencysprayheaderanddovblehollowconenozzles.If>.)VGBIP0WER T ECHl03--FGDupgradeFig.7.Typicaltraylayoutandroundandhexagonalholepatterndesign.Figure6showstheperformancewiththreeandfourrecyclepumpsinoperation.Theupgradedmodulecanachievethesameremovalefficiencywiththreepumpsinoperationwhencomparedtotheunmodifiedmodulewithfourpumpsinoperation.Hence,substantialpowersavingscanberealisedbyupgradingtoahigh-efficiencysprayheaderanddoublehollowconenozzle,orasubstantiallyhigherS02removalefficiency.Sie'lclroyAsievetrayprovidesalow-costandhighlyefficientapproachtoimprovegas/liquidcontactinwetFGDsystems.Anabsorbertowercanbeequippedwithoneortwosievetrays.Typicallydesignedtoprovideabout50mmofw.g. pressuredrop,asievetrayprovidesaboutthesameremovalefficiencyasonesprayheaderwithoutrequiringanyofthecomplexityofarecyclepumpandsprayheadersystem.Inadditiontoprovidingimprovedso2removal,asievetrayalsoprovidesthefollowingbenefits:-Increasesparticulateremoval,-Correctsgasdistributionproblemsinthelowerpartoftheabsorbersprayzone,-Protectstheinletductfromscaling,-ProvidesthelowestcostapproachtoaFGDupgrade,and-Providesamaintenanceplatformthatavoidsdrainingthereactiontanktogainaccesstothesprayheadersandmisteliminators.TypicaltraylayoutswithroundandhexagonalholepatternsareshowninFigure7.Theopenareaofasievetrayisafunctionofgasvelocityandliquidloading.Roundholesarepredominatelyused.However,athighgasvelocitiessuchas6m/s,theopenarearequirementissolargethatthestructuralintegrityofasievetrayiscompromised.Thestructuralintegritycanbemaintainedbyswitchingtoahexagonalhole-pattern.Thetrayistypicallypartitionedtoassurethatthedesiredslurry/WallBecausetheliquidspraydensityisinherentlylessincloseproximitytotheabsorberwalls,thepressuredropinthisareaislessthanatanypointinthesprayzone.Thefluegastakesadvantageofthelowerpressuredropalongtheabsorberwallsandthuswallsneakageisgenerated.Dependingonthesizeoftheabsorbertower,wallsneakagecanbesignificantorimmaterial.WallsneakagecanoftenlimitFGDsystemperformancetolessthan97%so2removal,insmallerabsorbertowers.Wallsneakagecanbeavoidedbyinstallingwallrings,aslopingplateextendingfromtheabsorberwallasshowninFigure8.Thespraynozzleslocatedadjacenttotheabsorberwallmustbearrangedatadistancefromtheabsorberwallsuchthatthefluegasisdirectedinthemiddleofthenozzlespraycone.Awallringpositionedinthisfashionalsoprovidesprotectionfromwallerosion.Theimpactofthewallsneakagezoneismuchmoreprominentissmallabsorbersbelow10mindiameterthaninlargerabsorbertowers.Largeabsorbermodulesalsoexperiencewallsneakage,butthesneakagezoneisrelative smallwhencomparedtothetotalabsorbercrosssection.VGBPowerTech1212012ProiectexamplesThreerecentFGDupgradeprojectarepresentedtoillustratetheapproachtoupgradingwetFGDsystemincluding:1-Venturiscrubber,2-Rodscrubber,and3-Co-currentscrubber.Venturi(plumpbob)scrubberVenturiscrubberswereoriginallyintra·ducedtoprovideutilitiesanopportunitytocombineflyashremoval,andS02removalinawetFGDsystem,and,thus,avoidtheinstallationofadryparticulatecollectorsuchasanelectrostaticprecipitator(ESP)orfabricfilter.Further,theremovedflyash,wasinmanycases,usedtoprovidethealkalinityforS02removalavoidingthecostofalimestonegrindingsystemandalimestonereagent.Inaplumpbobventurisystem,theventuriportionoftheabsorberdesignisplacedabovetheabsorberandafterthefluegashaspassedthroughtheventurithroat,theflueenterstheabsorberinadowncomerlocatedinthecentreofthemodule,asshowninFigu r e9.Atthebottomofthedowncomer,thefluegasmakesa180degreeturn,passesthroughthesprayzone,andexitstheabsorberthroughanoutletductlocatedatthesideoftheabsorber.The180degreeturnafterthedowncomerskewsthefluegasheavilytowardstheabsorberouterwalls,andthesideexitfurtherskewsthefluegastowardthesideoftheoutletduct.Asievetraywasinstalledattheelevationofthe180-degreefluegasturnatthebottomofthedowncomer.Thepressuredropprovidedbythesievetray(-sommofw.g.)providesanevengasdistributionprofiledownstreamofthesievetray.Thesingleexistingsprayheaderswerereplacedwithfluegasmixingoccurs.Fig.8.Installationofwallringstoavoidwallsneakage.-104VGBPowerTech1212012Fig.9.PlumpbobabsorberbeforeandafterupgradeandassociatedCFDmodel.twohighefficiencysprayheadersequippedwithdoublehollowconenozzles.Tomakeroomforthesecondsprayheader,thetwostagemisteliminatorwasreplacedwithanintegrateddiamondmisteliminator,whichrequiresonlyonelevelofsupportbeams.Toovercomethelateralskewedgasdistributiongeneratedbythesideexit,aperforatedplatewaslocatedabovethemisteliminatorcoveringa80degreecrosssectiononeithersideoftheoutletductcentreline.Toaccomplishthebestpossiblegasdistribution,theperforatedplatehasavariableopenareabecomingincreasinglymoreopeninadirectionmovingawayfromthecentrelineoftheoutletduct.Severaloftheseplumpbobabsorbershavebeenupgradedusingmagnesiumenhancedlime,limestone,andflyashasreagent.Withtheupgradeapproachdescribedabove,itispossibletoobtainthesameremovalefficiencyandreliabilityasanewstate-of-the-artsystem.RodscrubberRodscrubbersweredesignedtoprovidegas/liquidcontactthroughanumberofroddecksconsistingofl-inchpipesdenselypacked,andafloodingsprayheaderabovetheupperroddeck,asshowninFigure10.Therodscrubberissimilartoawayitachievesgas/liquidcontacting.Theaspectratioofarodscrubberisunusualwitha 3mabsorbersideanda10mabsorberwidthtoaccommodatetheroddeckwithoutrequiringanexcessofstructuralsupports.Therodscrubberexhibitsanumberofissuesincludingpoorgas/liquidmixingasgasandliquidmixingisinherentlyunstableunlessconfinedbythetypeofpartitionsusedwithasievetrayandwalleffectsduetounusualaspectratio.Onlytworodscrubberswereeverbuilt.originaldesignfluegasvelocitywasabout3m/s,thus,bringingthefluegasvelocitytoabout6rn/swiththeby-passeliminated.Ahighvelocity(hexagonalholes)traywasinstalledimmediatelyabovetheinletducttodistributethefluegasevenlyacrosstheabsorbercrosssection.Threelevelsofhighefficiencysprayheaders,equippedwithdoublehollowconenozzles,werealsoinstalled.Newlargerrecyclepumpswererequiredtoincreasetheliquid-to-gasratio.Duetotheunusualabsorberaspectratio,andthehighS02removalrequirement,eachspraylevelwasequippedwithwallrings.Thehorizontalflowmisteliminatorisslantedbackwardstowardsthegasflowtoimprovedrainageandthedropletcollectionefficiency.Initially,theabsorberhada90-degreeturn,withoutanyflowcorrectiondevices,fromthesprayzonetothemisteliminator.Thatarrangementworkedwellattheoriginaldesignfluegasvelocityof3m/s.However,at6m/s,thesharp90-degreeturngeneratesanoticeableunevenfluegasdistributionandalsoanunevendropletdensitydistribution.Toaccommodatethehigherfluegasvelocityanddropletloading,abullnosewasinstalledintheuppersprayzonetoguidethefluegasaroundthe90-degreeturn.Finally,aforcedoxidationconversionwasprovidedalongwithanewballmillgrindingsystem.......FGDupgrade-Co-currentscrubberPackedabsorbertowersareoftenconfiguredasadown-flowabsorberwithfluegasenteringatthetopoftheabsorberandexitingatthebottom.Theabsorbertypicallyincludesmultiplelevelsofpackingwithspacinginbetweenlayers,asshowninFigure11.Themaindrawbackofthistypeofdesignisscalingandpluggingofthepacking.Scalinghasparticularlybeenaproblemwithunitsburninghighsulphurcoals.Thissystemwasoriginallydesignedtoprovide90%S02removalon4.5%sulphurcoal.Aftertheupgrade,thesystemachievedS02removalefficiencywellabove99%.Toprovideastablechemistry,theFGDsystemwasconvertedfrominhibitedoxidationtoforcedoxidation,requiringupgradesofthe dewateringsystem.Anewlimestonegrindingsystemwasalsoprovided.Theabsorbertowerwasconvertedfromapackedtowertoaco-currentopenspraytowerandnewhighercapacityandhigher-pressurerecyclepumpswereinstalled.Thehighefficiencysprayheaderswereequippedwithdoublehollowconenozzles.Ahigher-pressurenozzlewasselectedtoimprovetheperformanceoftheco-currentheader.Withco-currentsprayheaders,thepressuredropacrosstheabsorbermodulewasreducedfrom150mmofw.gto0mmofw.g.SummaryFromtheinstallationofthefirstFGDsystemsinthelate1960stotheendofthe1980s,theindustryhadapoorunderstandingoffluiddynamics,theimportantroleofgas/liquidcontactandtheslurrychemistry,inparticularhowtoavoidscalingcon·ditions.ManyoftheFGDsystemsinstalledduringthatperiodhadseriousdeficienciesthataffectedperformanceandreliability.ThewetFGDcommunityhas,overtime,gravitatedtointegratedopensprayandtraytowersthatcanprovideS02removalefficienciesabove99%anduninterruptedoperationbetweenscheduledoutages.Unfortunately,therearestillwetFGDsystemsinstalledtodaythatdonottakeadvantageThisparticularinstallationwasoriginallydesignedwith40%fluegasby-passaroundtheFGDsystem.ThestationS02removalwasoriginalabout50%andaftertheupgrade,thefluegasbypasswascompletelyeliminatedandastationS02removalefficiencyabove98%wasachieved.TheFig.10.RoddeckabsorberbeforeandafterupgradeandassociatedCFDmodel.VGE3IP0WE R T E C H105

# -

FGD upgrade



# .... .... .... Fig.

II. 

# Ca<urrent

absorber

before

and after upgrade

and associated

CFD model. F

i g.

12

. High eHiciency



# wet FGD system design. 

# of high efficiency

and high reliability

de-

The early

FGD systems

are frequently

asked sign concepts

.

to provide performance

levels

notoriginal

# VGB PowerTech

12 2012 ly intended.

Utilities

are faced

with making prudent financial

decis

i ons

to comply

with new more stringent emission standards

re

- placing

older

FGD systems

with

new mod- ern designs

can rarely

be economically

jus- tified.

URS

has proven

over

and over again that older

FGD systems

can

be upgraded to achieve

the same performance

and
reli- ability

as

a new modern

FGD system

for

a fraction

of the cost.

The core elements

that should

be include

in upgrades

of older

FGD systems
include

all

or some

of the follow

- ing elements: - Comple

t e understanding

of the fluid

dynamics

from

the absorber

inlet duct

to the stack breach, - Even

flue

gas distribution

between

ab

sorber modules

and

at the absorber

inlet duct, - Even

flue

gas distribution

through

the spray

zone

and

the mist eliminator, - A sieve

tray, - High efficiency

spray headers

, - Double
hollow

cone nozzles, -Wall

rings

, - Stable chemistry
(inhibited

or forced oxidised), - Limestone

quality excluding

excess

content

of crystal modifiers,

and - Mist eliminator

wash systems

using

service water

as source

for mist eliminator wash water,

ample

wash header
nozzle pressure

and spray density. A high-efficiency,

wet FGD system

includ

ing

a sieve

tray

, high-efficiency

sprayheader, double hollow

cone nozzles,

and wall rings

are shown

in Figure

12. Absorber tow

e

r s utili

si ng this design approach

can readily achieve

S0

2 

# removal efficiencies above

99



# % on low

to high sulphur

coals. This design approach

also provides

particulate removal efficiencies

above

70



# %

# percent

and removal efficiencies

of oxidised mercury

above

90



# o/o.



# I