,
J
FUELCHARACTERISTICSOFSEWAGESLUDGEANDOTHER
COMBUSTIONPROCESSWITHCOAL
SUPPLEMENTALFUELSREGARDINGTHEIREFFECTONTHECO-
Th.Gerhardt,R.Cenni,V.Siegle,H.Spliethoff,K.R.G.Hein
UniversityofStuttgart,IVD,Pfaffenwaldring23,70569Stuttgart,Germany
Tel./fax:#49-711-685-3395/#49-711-685-3491
ABSTRACT
IntheEuropeancountries,andespeciallyinGermany,theco-combustionofbiomassandwaste
materialstogetherwithcoalinthepowerplantsisexpectedtofindwideapplicationinthenear
future.AttheIVDseveralkindsofsupplemental.fuelsaretestedtofindouttheircombustion
behaviourindifferentfiringsystemstogetherwiththeregularfuelshardcoalandlignitecoal.The
investigationsweredoneinbenchscalefacilitiesforbasicresearchbutalsouptopilotscale
combustionrigs.Inordertogetinformationaboutdestructionandformationofhazardousmatter
multiplevariationsofthecombustionparameterswereappliedunderconditionslikethosein
industrialfurnaces.Inthispapercharacteristicsofthefuelsarecomparedaccordingtoimmediate
analysis,elementaryanalysisandanalysisoftheashcomponents.Combustionexperiments
werecarriedoutwithvariousportionsofthermallydriedmunicipalsewagesludge.Theby-
productsofthecombustionprocesswerecollectedandbalanced.
INTRODUCTION
Theconversionofenergyfromfossilfuelsintoheatandelectricityinvolvesunavoidablythe
emissionofC02whichisknownasagreenhousegas.Theintentiontoreducetheamount
emittedtotheatmosphereleadsfirstofalltothereductionofenergyconsumption.Thenext
objectiveistoincreasetheefficiencyoftheenergyconversionprocesswhichissuccessfully
donebypowerplantdevelopmentforalongtimeanduptoahighstandardintodays
combustionsystems.Wecanusenowthishighefficientfacilitiesandsubstitutepartofthecoal
inputbyC02-neutralbiomasslikestrawandwood.
Alsowithco-fireingwastematerialsimilareffectscanbeobtained.Wasteincineratorplantshave
tobepreparedforvarioushazardousmattercomingalongwiththeinhomogeneousmixedwaste
material.Theexpensestocoverallpossiblecompositionsofwasteinthecombustionsystem
andespeciallyinthefluegascleaningsystemlowertheefficiencyfarbelowthestandardofthe
powerplants.Forspecialkindsofwastewhichoccurseparatelyandshowconstantand
homogeneouspropertiestheco-combustionwithcoalequivalenttobiomasscanachievehigher
yieldofenergycomparedtotheusualwasteincinerationsystems.Toensureadisposalwithout
higherrisksforenvironmentthecompositionofthiswastematerialshastobecarefullychecked
regardingthecontentsofhazardousmatter.Anotherreasontotreatwastematerialinexisting
combustionfacilitiesforpowergenerationisthecostsavingaspect.Additionalequipmenttoco-
firethesupplementalfuelscanbelimitedtothestorage,transportanddosingdevices.
FUELCHARACTERISTICS
AttheIVDseveralresearchprojectswithawidevarietyofexperimentswerecarriedoutonthis
topicinthelastyears.Havingstartedtheco-combustionwithbiomasslikestrawandfreshcut
woodthesupplementalfuelschangedtowastemateriallikewastewood,municipalsewage
sludgeandplasticgranulateetc.ThemainfuelswereinprincipalGermanhardcoalsandin
severalplacesalsoGermanlowrankcoals(browncoal)havebeenused.Table1showsan
overviewontheaveragefuelpropertiesofsupplementalfuelscomparedtothoseoftheregular
fuelsandthemixedwaste.Theresultsofimmediateanalysisandelementaryanalysisare
calculatedtothebaseofdrysubstanceinordertobecomparable.
Thetypicalmunicipalhouseholdwastedeliveredtotheincinerationplanthasabout30%
moisture.ThedrysubstanceconsistsofnearlySO%volatileswhichisalmostthecomplete
amountofcombustibles.Besidesasmallamountoffixedcarbontherestofthewastematerialis
mineralsubstancedeterminedasash.ThefuelcalledRDFisanabbreviationforrefusederived
fuelandmeansafractionofnormalhouseholdwastewhererecyclablematerialissortedoutand
whichisgroundtoahomogeneousparticlesizeforbetterhandlingindifferentcombustion
systems.Theportionofashdecreasedcomparedtothenormalwastetoalevelof14%and
thereforethelowerheatingvalue(LHV)isnearlydouble.Thisdifferenceisevenenlargedby
lowermoisturecontentsofabout15%oftheRDF.
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Amaterialnormallyincludedinthemunicipalwasteisplastic.Duetoseparatewastecollection
orinindustrialproductionprocessesthismaterialsometimesoccursseparatelyKnowntohave
bigenergycontentcombustioncanbeareasonablepossibilitytodisposemixedorminorquality
fractionswhicharenotworthtorecycleintonewproducts.Thedifferentkindsofplasticare
verysimilarregardingtheircombustionproperties.Thehighdensitypolyethyleneistakenasa
sampletoshowthem.Noticeableisthatthecompletedrysubstanceconsistsofonlyvolatile
matter.Ashismissingtotallyexceptsomereinforcedmaterialswithfibreglass[I].
Asaby-productofthecokeovenprocessthetaroilofhardcoalswastestedintheIVDfurnace
concerningitsabilitytoreducenitrogeneoxidesasareductionfuelbyfuelstaging.Beingaliquid
fuelitofferedbestpossibilitiestooptimizethemixingconditionsinthereactionzones.SOit
showedverygoodresultsinminimizationofhazardousmatterwiththeprimarymeasure,fuel
staging[I].Ontheotherhandcombustionunderhightemperatureisasuitablemethodto
disposethiscarcinogenicorganicsubstance.
Comingtothesewagesludgeweseeasupplementalfuelwhichisverysimilarinthefuel
characteristicstostrawandwoodregardingonlytheorganicshare.Theanalysisshowsthe
typicaldataofsludgefrommunicipalwastewatertreatment.Anobviousdifferencetostrawand
woodisthehighashcontentalsoresponsibleforthereducedLHV.Asaproductofacleaning
processthevariationofsinglepropertiescanbehighandsothevaluesshownintablearean
averagefrom15differentsludges.Evenifextremedeviationsarepossiblethestandarddeviation
ismostlyinbetweenkIO%.Inthewastewatertreatmentthesewagesludgeisseparatedwitha
contentofdrysubstanceabout5%.Mechanicaldewateringbycentrifugeorfilterpressare
increasingthatuptorangebetween20and45%,accordingtoareductionofvolumeandweight
of80to90%.Foralongerstorageandabettersuitabilitytohandlethesludgeanadditional
thermaldryingupto90%ofdrysubstanceiscarriedoutinmoreandmorecases.
Theanalysisforstrawandwoodareonbehalfofadatabasewithmorethan100differentkinds
ofbiomasswhichcanberoughlydevidedinthistwogroups.Acloserdescriptionisgivenina
publicationofV.Siegleinthisconference[2].Aswellthesematerialsaretheoriginsoftheregular
fossilfuelsdiscussedasthemainfuelsinthiscontext.Duringthecoalificationthebiomass
turnedfirsttopeatthantoligniteandbrowncoalbeforehardcoalandanthraciteareformed.Due
tothisprocessthebigcontentofvolatilematterinbiomassistransformedmoreandmoreinto
fixedcarbonandthewatercontent,about50%inthebrowncoal,isreducedbyhighpressure
andtempcratureinthemines.
Figure1showstheenergycontentoftheorganicpartofthefuels.Atfirstsightanincreasing
fixedcarboncontent(Waf)correspondingtoadecreasingshareofvolatilematterinthe
combustiblesasshowninfigure2increasesalsothelowerheatingvalue.Thisiscorrectforthe
groupofbiomassbasedfuelsstartingfromstrawgoingfurthertobrowncoal,hardcoal,
anthraciteandchar.Iftheoriginoftheorganicsubstancehoweverisdifferentfromthebiomass
deviationsarenoticeable.Forthesewagesludgeforexamplethehighercontentofvolatilematter
comparedtothebiomassiscomingalongwithahigherenergycontent.Theplasticmaterialis
totallydifferentinitsbehaviourconsisting100%ofvolatilematteritshowsthemaximum
heatingvalueofmorethan40MJkg.
EXPERIMENTAL
\
\
.
TheIVDoperatesa500kWpilotscaletestfacilityforpulverizedcoalcombustion.Itisa
verticalfurnacewithaninternaldiameterof0.7mandanactivelengthof7m.Thechamberis
completelywatercooledandthefirst4mbeginningwiththepositionoftheburnerontopare
refractorylined.Intheteststobedescribedinthispaperespeciallytheashremovalsystemisof
interest.Accordingtotheindustrialplantsthereisabottomashhopperforcoarseparticlesand
slagdrops.Theairpreheaterwiththeneedofsmallflowratestorealizetheheattransferisthe
nextstepwhereparticlesareseparatedfromthefluegas.Therangeofparticlesizecollectedhere
isstartingfrom10pnupto1mm.Operationtemperaturethereinisabout500°Conthesideof
thefluegas.Thefirstseparationofflyashisdoneinacyclonecollectingparticlesinthemge
between5pmand100pmatthetemperatureabout350°C.Theflyashinherehassimilar
propertiestothatofelectrostaticprecipitators(ESP)inthepowerplant.Finallythefluegas
passesabagfilterwithanadjustabletemperatureupto200°C.Thefinedustfoundinthis
deviceisinseveralwayscomparabletoscrubberresiduesoflargescaleplants.
Thepurposeofthetestswastoobtainknowledgeaboutthechangesinoperationoftheplant
andinqualityofthesolidcombustionresiduesbyaddingthermallydriedmunicipalsewage
sludgeintothepulverizedcoalcombustionsystem.Startingfromthepurecoalcombustion
sewagesludgewasaddedinincreasingshareof5,IO,15,20,and25%ofthethermalinput.The
experimentswerecarriedoutforadurationofIOto20hoursateachadjustmentandash
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\
I
b
i
balanceswereperformedevery4hours.Duetothehighashcontent,whichisnearly5timesthat
Ofthecoal,andonlyonethirdoftheenergycontent,everyMWproducedbysewagesludge
causesIStimestheashofthecoalcombustion.Figure3showstherelationbetweentheshare
Offuelmassflowandashmassflowindependenceontheshareofthermalpowerproducedby
thesewagesludgeinthegivencombinationoffuels.
RESULTSANDDISCUSSION
Figure4showsthe10mainelementsintheashofthecoalincomparisonwiththecontentsof
theminthesewagesludgeash.Tofindaninfluenceoftheco-combustionitisreasonabletolook
atthoseelementswithahigherconcentrationandwhatisevenmoreimportantwithadifference
inconcentrationbetweenthetwofuels.Asafirstexampletheironwaschosenbecauseitis
expectedtoshowonlysmalldeviationfromthetheoreticallycalculatedaverageconcentrationsin
theashfractions.Evennoenrichmentofironspeciesindependenceonparticlesizeor
separationtemperatureinthecollectingdevicesareassumed.Theresultsdrawninfigure5
demonstratethis.Theincreasinglinerepresentsthetheoreticalaverageconcentrationofironthe
combustionresidueshouldhaveandthescatteredpointsareshowingthemeasured
concentrations.Thesymbolsdistinguishbetweenbottomash,airpreheaterresidueandflyash
outofthecycloneandthebagfilter.Itisobviousthatthereisnosignificantenrichmentor
volatilizationofthiselementandthedeviationcharacterizesthereliabilityofthemeasureddata.
Twoelementswhicharetypicallyhigherconcentratedinthesewagesludgeashthaninthecoal
arethecalciumandthephosphorus.Inordertoavoidahigherriskofslaggingandfoulingthese
newcomponentsfortheplantareofmajorinterest.Thccalciumdescribedinfigure6followsas
wellthelineoftheoreticalavengeconcentration.Thetrianglesrepresentingthebagfilter
concentrationsareclearlybelowtheaverageandthebottomashtogetherwiththeairpreheater
retainsmostofthecalciuminthefrontpartofthefluegaspath.Intheorythecalciumisknown
tolowertheashmeltingpointwhichcangiveanexplanationforagglomerationofparticleswith
enrichedCa-contentsinthehotpartofthefacility.Adifferentbehaviourisdeterminedfor
phosphoruswhichoccursonlyinsewagesludge.Aswecanseeinfigure7themeasurementof
thefiltersamplesclearlyshowanenrichmentinthecolderendofthefluegasway.Furthermore
thecalculatedaverageconcentrationisnotachieved.Apossibleexplanationforthemissing
phosphoruscanbegivenbyspecieswhicharevolatilizingduringthecombustionandcondensate
inanypartofthepipesystem.Incaseslikethatthetimetoreachasteadyconditionregarding
inputandoutputmaybemuchlongerthanthe20hoursmaximumofthetests.Toconfirmthis
assumptionconcentrationsofphosphoruswillbemeasuredindependenceonthedurationof
oneexperimentaladjustment.
Thestrongesteffectofenrichmentinthecolderpartofthefluegaspathisobserved,as
expected,withthemercury.Alsotheeffectofvolatilizingisclearlyproofedbythisexample
showninfigure8.About50%ofthemercuryfedintotheplantwithfuelsisleavingitas
elementaryHginthefluegas.TheboilingpointofHgat358°Cishigherthenthefluegas
temperatureinthestack,butintherangedownto-39°Citisliquidandthereforeitvaporizes
partlyintothefluegasatmosphere.Amaximumsaturationofmercuryinairisgivenat100g/m3
ifthetemperatureis200°C.Sothereisnolimitationcausedbythiseffect.TheSO%ofmercury
capturedintheflyasharemainlyboundinHgSandHgCI.Theenrichmentonthesurfaceofthe
smallparticlesinthebagfilterishigherthanthatofanyothermeasuredcomponent.Evensothe
biggestamountoftheHgintheresidueswascapturedinthecycloneashbecausetherewas
foundthebiggestshareoftheashmassflow.
Figure9showsanoverviewoftheenrichmentbehaviourinthebagfilteroftheIVDplantforall
substancesmeasuredinthesolidresidues.Theyaresortedinorderofthecalculatedenrichment
factorwhichcomparestheelementconcentrationinthebagfiltertotheaverageconcentrationof
everyflameadjustment.Theconcentrationinthebagfilterisdevidedbytheaverage
concentrationand1issubstracted.So0meansnoenrichment,positivenumbersarestandingfor
ahigherconcentrationinthefilterandnegativeforalowerone.Finallythemeanvaluesoverall
oftheadjustmentsarecalculatedanddrawninthediagram.Therangeoff20%(-0.2to0.2)can
notproofasignificantenrichmentbecauseofthescatteringofthemeasurements.Butespecially
theheavymetalsfoundastraceelementsinthefuelsaredeterminedtobefoundinhigher
concentrationsinthefilter.Someofthemainashcomponentsdoalsoshowanyenrichment
howevernotsodistinct.Potassium,phosphorusandsodiumconcentrateinthefilterandthe
calciumasmentionedisfoundmoreinthefrontofthefluegaspath.
Thedistributionofashbetweenthevarioushopperswasabout20%inthebottomashhopper,
9%intheairpreheaterand16%inthebagfilter.Thebiggestamountwasfoundinthecyclone
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with54%ofthewholeash.Thisdistributionwasconstanteveniftheashflowwiththehighest
shareofsewagesludgewasalmostfivetimesthatofthecoalcombustion.
CONCLUSIONS
Theanalysisofvariousmaterialsintendedtobeusedordisposedassupplementalfuelsincoal
firedpowerplantshasshownthatthereisalwaysarangeoftheresultssometimeswithabig
gapbetweenminimumandmaximum.Thisisconsistentespeciallywiththenaturalproductslike
biomassoranymixedwastematerial.Neverthelesstheinvestigatedfuels,biomassandmunicipal
sewagesludgeshowedquiteconstantandhomogeneouspropertiesexceptingsomeloadscoming
fromanyspecialtreatment.Adecreasingcontentofvolatilematterintheorganicpartwas
comingalongwithanincreasingheatingvalueatleastforthegroupofbiomassoriginatedfuels,
peatandcoal.
Mostnoticeableforthesewagesludgewasthehighestshareofash,nearly50%ofthedry
substance,comparedtoalltheotherfuels.Inthatscoreattentionwasturnedtothebehaviourof
thecompoundsinashduringthecombustionprocess.Theincreasingshareofsewagesludgeup
toalevelof25%ofthethermalinput,correspondingto80%sewagesludgeashinthewhole
ash,hadnosignificanteffectonthedistributionbetweenthedifferentashremovalsystems.
Eveniftheashamountis5timesbiggerthanthatofthecoalcombustion.TheheavymetalsHg,
Zn,Pb,Ni,CuandCdshowedanenrichmentinthebagfilterattheendofthefluegaspath
whichwasonlyforthemercuryclearlyproportionaltothesewagesludgeshare.The
concentrationsofthemainash-componentsaremoreconsistent.Thebiggestdifferencebetween
theashesofsewagesludgeandcoalaretheelementscalciumandphosphoruswhicharefound
onlyoratleastinahighershareinthesewagesludge.Onlypotassium,phosphorusandsodium
areenrichedinthefineashofthefilter.Thecalciumhoweverisfoundinhigherconcentrationsin
thehoppersofthehotpartofthefacility.
Forsomeelementssignificantamountscouldnotbemeasuredinthesolidresidues.Incaseofthe
highvolatiletraceelementmercuryitisobviousthatabout50%oftheinputisleavingtheplant
inanelementaryformviathestackwiththefluegas.Incaseofthephosphorusvaporizationand
condensationofsomespeciesaresuspectedtoholdbackthiselementinthepipesystemuntila
steadyconditionisachieved.Furthermeasurementswillhavetoconfirmthat,
REFERENCES
[l]
Th.Gerhardt,R.Cenni,H.Spliethoff,K.R.G.Hein,UniversityofStuttgart,IVD:
CombustionBehaviourofCoal-WasteFlamesinPulverizedFuelFiringSystems.Inflame-
MeasurementsinaPilotScaleFacilitywithHardCoalandDriedSewageSludge;
InternationalTechnicalConferenceonCoalUtilizationandFuelSystems;March,1997,
Clearwater,Florida,USA
[2]V.Siegle,H.Spliethoff,K.R.G.Hein,UniversityofStuttgart,IVD:Characterisationand
PreparationofBiomassforCo-CombustionwithCoal;AmericanChemicalSociety
DivisionofFuelChemistry,Spring1998MeetingMarch29-April2,Dallas,Texas,USA
Figure1:Energycontentoffuels(waf)
masssludgeRDFcoalcoalcarbonmaterial
Figure2:Contentofvolatilematterintheorganicsubstance
100
7
.-80
E
-f70
''060
s?
50
340
330
e
9
-.-
3-20
310
0
carboncoalcoalmassRDFsludgematerlal
Figure3:Relationbetweenshareoffuelmass,Figure4:Elementsintheashof
ashamountandshareofthermal
power
9090
8080
7070
6060
5050
4040
3030
2020
10
sha{$
00
sewagesludgeshare
ofthermalinputin%
051015202530
sewagesludgeandhardcoal.
Kpb
!NaKthardcoal
Rmsewaeslude
AI-
-.
05101520
shareintheash[%]
201
Figure5:Ironconcentrationintheash
Fig1
Zn
Pb
Ni
cu
K
Cd
P
Na
AI
ire9:Enrichmentofelementsin
thebagfilter
shareofthesewagesludgeashinX
Figure7:Phosphorusconcentrationintheash
shareofthesewagesludgeashin96
Figure8:Mercuryconcentrationintheash
18
16
14
12
10
8
6
4
2
0
0102030405060708090100
shareofthesewagesludgeashinX
202
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