Large-scale transportation and storage of wood pellets

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Delft University of Technology

Large-scale transportation and storage of wood pellets

Investigation of the change in physical properties

Gilvari, Hamid; van Battum, Coen H.H.; van Dijk, Simon A.; de Jong, Wiebren; Schott, Dingena L.

DOI

10.1016/j.partic.2020.12.006

Publication date

2021

Document Version

Final published version

Published in

Particuology

Citation (APA)

Gilvari, H., van Battum, C. H. H., van Dijk, S. A., de Jong, W., & Schott, D. L. (2021). Large-scale

transportation and storage of wood pellets: Investigation of the change in physical properties. Particuology,

57, 146-156. https://doi.org/10.1016/j.partic.2020.12.006

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ContentslistsavailableatScienceDirect

Particuology

j ou rn a l h om ep ag e :w w w . e l s e v i e r . c o m / l oc a t e / p a r t i c

Large-scale

transportation

and

storage

of

wood

pellets:

Investigation

of

the

change

in

physical

properties

Hamid

Gilvari

a,c,∗

,

Coen

H.H.

van

Battum

a

_,

_Simon

_A.

_van

_Dijk

b

_,

_Wiebren

_de

_Jong

c

_,

Dingena

L. Schott

a

a_Section_of_Transport_Engineering_and_Logistics,_Department_of_Maritime_and_Transport_Technology,_Faculty_of_Mechanical,_Maritime_and_Materials

Engineering,DelftUniversityofTechnology,TheNetherlands

b_Chemical_Services,_Uniper_Benelux_NV,_The_Netherlands

c_Section_of_Large_Scale_Energy_Storage,_Department_of_Process_&_Energy,_Faculty_of_Mechanical,_Maritime_and_Materials_Engineering,_Delft_University_of

Technology,TheNetherlands

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received15October2020 Receivedinrevisedform 27November2020 Accepted8December2020 Availableonline21January2021 Keywords:

Woodpellets

Large-scaletransportation Durability

Finesanddust Breakage

Mechanicaldegradation

a

b

s

t

r

a

c

t

Thechangeinphysicalpropertiesofwoodpellets,withafocusonparticlesizedistributionsduetopellet breakageandattrition,wasstudiedinalarge-scale(∼450ton/h)transportationsystem.Criticallocations withahighprobabilityofbreakagethroughthewholetransportationsystemwerechosenandsampled tostudytheeffectoftransportationsystemdesignandoperationonthemechanicalpropertiesofpellets. Bulkdensity,mechanicaldurability,moisturecontent,andparticlesizedistributionofpelletswere char-acterizedforeachsample.Analysisofvarianceshowedthatthereweresigniﬁcantdifferencesbetween thepercentagesofsmallparticles(<5.6mm)inthesamplestakenatdifferentlocations,especiallyatone withaverticalfreefallof7.8m.Onaverage,thisrelativelylongdropincreasedtheproportionof parti-cles<5.6mminthesamplesfrom8.73%to14.09%,andthatofparticles<3.15mmfrom4.82%to9.01%. Moreover,themeasurementsshowedawidedeviationinthemechanicaldurabilityvalues,betweena minimumof90.8%andamaximumof98.7%,whichwerenotcorrelatedtothesamplingpointsbutrelated topelletproperties.Itcanbeconcludedthatpellettransportationsystemsrequiremorededicateddesign strategiestopreventbreakageandattrition.

©2021ChineseSocietyofParticuologyandInstituteofProcessEngineering,ChineseAcademyof Sciences.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http:// creativecommons.org/licenses/by/4.0/).

Introduction

Worldwideuseofwoodpelletsasarenewableenergycarrier

hasincreased sharply, from∼12millionmetric tonsin 2008to

56millionmetrictonsin2018(Calderón,Gauthier,&Jossart,2019).

Morethan27millionmetrictonsofwoodpelletswereconsumed

inEuropein2018,morethan45%ofthemforindustrialpurposes.

However,notallpelletsconsumedinEuropeareproducedby

Euro-peancountries. Approximatelyone-thirdareimportedfromthe

UnitedStatesandCanada(Calderónetal.,2019).

Duetoitsinherentlyhighmoisturecontentanditslowbulk

andenergydensities,biomassisusuallydensiﬁedtoimprovethese

properties;aprocessthatisadvantageousfortransportationand

handling steps(Tumuluru, Wright,Hess,& Kenney,2011).

Pel-letsarenormallyproducedunderhightemperatureandpressure

∗ Correspondingauthor.

E-mailaddress:h.gilvari@tudelft.nl(H.Gilvari).

conditionsinaso-calledpelletizationprocess,whichinvolvesthe

elasticandplasticdeformationofparticlesandthesofteningof

nat-uralbinderssuchasstarch,protein,lignin,fat,andﬁberstohelp

agglomerateparticles(deSouzaetal.,2020;Kaliyan&VanceMorey,

2009;Mani,Tabil,&Sokhansanj,2008;Nanou,Huijgen,Carbo,&

Kiel,2018).

Thefragilenatureofpelletscausestheirattritionandbreakage

throughouttheentirelogisticchain(Oveisietal.,2013).Asaresult,

averagepelletlengthmaydecreaseandtheamountofgenerated

smallparticlescanincrease(Boac,Casada,&Maghirang,2008).This

hasconsequencesforpellettransportation,handling,andstorage,

suchasanelevatedriskofdustexplosion,ﬁre,segregation,

arch-ingandequipmentfouling,healthissuesforpeopleinhalingthe

dust,andlosinganotableportionofthematerial(Aarseth,2004;

Ilic,Williams,Farnish,Webb,&Liu,2018;Ramírez-Gómez,2016).

Moreover,pelletbreakageand thegenerationofsmallparticles

underminethepelletizationeffortbydecreasingthebulkdensity

(Sjöström&Blomqvist,2014).

https://doi.org/10.1016/j.partic.2020.12.006

1674-2001/©2021ChineseSocietyofParticuologyandInstituteofProcessEngineering,ChineseAcademyofSciences.PublishedbyElsevierB.V.Thisisanopenaccessarticle undertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).

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Theabilityofbiomasspelletstoremainintactduringloading,

unloading,feeding,andtransportiscalled“mechanicaldurability”

(ISO,2014).Fromthedeﬁnitionitself,themechanical durability

cangiveanindicationoftheextenttowhichthematerialcankeep

itsinitialshape.Inpractice,however,durabilityischaracterized

bymeasuringtheamountofpre-deﬁnedsmall-sizeparticles

cre-atedinlaboratorytests.Thissmallsizemaybedifferentinsomeof

theliterature,butasizeofbetween3and5mmisusually

consid-ered(Grahametal.,2016;ISO,2015).Accordingtotheliterature,

mechanical durabilitydependsonthecharacteristicsoftheraw

material;speciﬁcally,factorssuchasparticlesizeand moisture

content,pelletizationprocessparameters,andstorageconditions

(Gilvari,deJong,&Schott,2019;Shuietal.,2020;Stelteetal.,2012;

Whittaker&Shield,2017).

Undoubtedly,theextentofpelletbreakageandattrition very

muchdependsonthetypeofpellet;thehigherthepellet’sstrength,

theloweritsbreakageandattritionratesare.However,pelletsmay

undergomechanicaldegradationduetohighimpactsand

compres-sionforcescausedby,forinstance,longdrops,increasednumber

of handlingsteps,andpoorequipmentdesign (Ilic etal., 2018;

Ramírez-Gómez,2016).

Different breakage mechanisms have been proposed for

biomasspelletsduringtransportationandhandling,namely

break-age into two or more pellets while practically keeping the

cylindricalshapes,attritionofpelletsurfacesandends,and

crush-ingofthewholepellet(Boacetal.,2008;Thomas,1998).

Inindustry,variousequipmentisusedforthetransportation,

handling,andstorageofpellets.Ofthese,grabs,beltconveyors,

pneumatic conveyors, pipe conveyors,hoppers, transferchutes,

silos, and bins are the most common (Dafnomilis, Lodewijks,

Junginger,&Schott,2018;Larsson,Lestander,Crompton,Melin,&

Sokhansanj,2012).Eachofthesetypesofequipmentmaydegrade

thepelletsmechanically,duetoattrition,compression,orimpact.

Recently,researchhasbeenunderwaytocharacterizethebreakage

andattritionofpellets(Boacetal.,2008;Gilvari,deJong,&Schott,

2020; Murtala,Zigan, Michael, &Torbjörn,2020; Ochkin-König,

Heinrich,&Dosta,2018).Forinstance,researchonpneumatic

con-veyorshasshownthatpelletattritionincreaseswiththeincreasein

particlevelocity,whichisinducedbyincreasingtheairinlet

veloc-ityanddecreasingthemassﬂowofpellets.Moreover,ashorter

bendradiusincreasesthebreakageofpellets(Aarseth,2004;Jägers,

Wirtz,Scherer,&Behr,2020).

Itisknownfrompreviousstudiesthatthemagnitudeofimpact

force, number of handling steps, and amount of bulk material

directlyinﬂuencethebreakageandattritionofpellets(Boacetal.,

2008;Kotzur,Berry,Bradley,Dias,&Silva,2016;Oveisietal.,2013)

investigatedtheeffectofdropheight,pelletmass,andrepeated

handlingstepsonthedegradationofwoodpelletsduringfreefall

while theyplaced thepelletsin abag madeofsynthetic

mate-rials. Theyalsostudiedtheeffectof pelletmassper bagand of

repeatedhandlingsteps,concludingthatgreaterdropheightsand

anelevatednumberofhandlingstepsincreasetheamountofﬁnes.

Moreover,pelletmassshowsalinearcorrelationwiththe

gener-atedmassofﬁnesaslongastheinitialmassiskeptlowerthan

1kg.

Looking atcorn-basedanimalfeedpellets,Boacetal.(2008)

studiedtheeffectofthenumberofhandlingstepsonthequantity

ofbrokenpellets(particlespassingasievewitha5.6mmmesh)

anddustparticles(<0.125mm)generated.Thefeedpellets

con-tained13.2%moistureandhadanominaldiameterof6.4mmand

adurabilityof92.9%,accordingtothetumblingboxmethod.Their

testsetupincludedabucketelevatorwithaheightof54.9m,which

cycledthepelletsfromthebottomofstoragebinstotheirtopand

fromaﬁrstbintoasecondone,thenviceversa,withanaverage

ﬂowrateof59.4metrictonsperhour.Theyrepeatedthedischarge

andloadingofthebinseighttimesandobservedanotableincrease

intheproportionofparticles<5.6mm,from17.5%to50.2%.The

averagedustgenerationwas0.069%pertransfer.

Asshownabove,existingliteratureregardingthemechanical

strength,breakagebehavior,andgenerationofﬁnesanddustof

biomasspelletsislimitedmainlytolaboratoryorpilot-scale

stud-ies.The objective of this paper, therefore, is to quantify small

particlesatdifferentpositionsinalarge-scale“realworld”

trans-portationsystem(∼450ton/h).Thisshouldallowustodetermine

themajortransportstepsinwhichpelletbreakageandattrition

occurand one can useit as a benchmarkfor investigating the

changesinpelletpropertiesinanyotherpellettransportsystem.In

addition,thechangesinotherpelletproperties—suchas

mechani-caldurability,bulkdensity,andmoisturecontent—duetomultiple

transportationstepsarestudied.For thatpurpose,apellet-ﬁred

powerplantintheNetherlandswaschosenasacasestudyto

inves-tigatethebreakageandattritionofwoodpellets.Thetransportation

systeminthispowerplantconsistsofagrabunloadingsystem,belt

conveyors,andstorageinasilo.

Materialsandmethods

Thematerialforthiscasestudyoriginatedwithananonymous

companyintheUSA.Awiderangeofwoodyfeedstockwasused

toproducethepellets,includingboth softand hard woods.No

informationaboutthedensiﬁcationprocesswasdisclosed.After

theirproductionand localstorage inthe USA,thepelletswere

transportedinbulktoalocalportandloadedintoanocean-going

vesselwithacapacityof28,000metrictons,thenshippedacross

theAtlanticOceantotheportofAntwerp,Belgium.Heretheywere

transshippedtobargesholding∼2,500metrictonseach.Twelve

bargeswerethusrequiredfor theentiretransatlantic cargo.No

informationwasdisclosedregardingchangestotheparticlesize

distributionofthepelletsuptothisstage.Thetwelvebarges

pro-ceededtotheportofRotterdam,wheretheyberthednexttothe

enduser’splantatarateofonebargeperday.Thecargocomprised

amixtureofsevendifferenttypesofwoodpellets,whichdiffered

bycolor,diameter,andlengthasshowninTable1.

Physicalpropertiesofthepellets,includingtheirmoisture

con-tent, bulk density, diameter,length, and mechanical durability,

weremeasuredasfollows.Moisturecontentwasmeasured

accord-ingtoENstandard14774-2(EN,2009),using300gofpelletsthat

wereplacedinanovenat105◦Cfor24h.Themoisturecontent

wasthencalculatedbasedonthedifferencebetweenthemassof

thepelletsbeforeandafterthetest.Bulkdensitywasmeasured

accordingtoENstandard15103(EN,2010)bymeansofa5Lsteel

cylinderusing thetapmethod.Thebulk densitywascalculated

fromthemassofpelletsdividedbythevolumeofthecylinder.

Pel-letdiametersweremeasuredaccordingtoENstandard16127(EN,

2012)bymeansofadigitalcaliper.Twenty-eightpelletswere

mea-suredinthisway:fourperpellettype,chosenatrandom(Table1).

PelletlengthdistributionsweremeasuredaccordingtoEN

stan-dard16127(EN,2012),usinganin-houseimageprocessingtool;

detailsofthiscanbefoundinourpreviouspublication(Gilvari,De

Jong,&Schott,2020).Mechanicaldurabilitywasmeasured

accord-ingtoISOstandard17831-1(ISO,2015),whichisacommonglobal

methodfordurabilitymeasurementofbiomasspelletsandisused

extensivelybyresearchersinthisﬁeld(Brunerová,Müller, ˇSleger,

Ambarita,&Valáˇsek,2018;Dyjakon&Noszczyk,2019;Gilvari,Cutz

etal.,2020;Larsson&Samuelsson,2017;Stelteetal.,2012;Thrän

etal.,2016).Themechanicaldurabilitywasdeterminedby

plac-ing500±10gofsievedpellets(round-holedsievewithascreen

sizeof3.15mm)ina tumblingcan.Thiswasrotatedata speed

of50rpmfor10min.Afterthetest,thesamplewassievedagain

usingthesamesieve,thenweighed.Itsmechanicaldurabilitywas

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Fig.1.A2-dimensionaloverviewofthetransportationsystem.Thenumbersshowthesamplinglocations.AdetailedoverviewoftheautomaticsamplerisshowninFig.2. Thesizeoftheequipmentmaynotrepresenttherealcase.

Table1

DifferenttypesofwoodpelletsshippedfromtheUSAtotheportofRotterdam.

Row Type1 Type2 Type3 Type4 Type5 Type6 Type7

1

2

3

4

sievebythemassoftheinitialsamplemultipliedby100.The mois-turecontent,bulkdensity,andmechanicaldurabilityexperiments wererepeatedtwice;thereportedvaluesarethusthemeanvalues ofduplicatemeasurements.

To determinethechangeinparticlesizedistributions ofthe samples,threedifferentsieveswithscreensizesof5.6mm(square holes),3.15mm(roundholes),and1mm(squareholes)wereused. Thesievewiththesmallestscreen sizedeterminedtheamount

of dust (< 1mm),the medium one determined the amount of fines(1mm<fines<3.15mm),andthelargestonewaschosento determinetheamountoflumps(3.15mm<lumps<5.6mm).All particleslongerthan5.6mmwereconsideredtobe“whole pel-lets”.Hereafter,allparticlessmallerthan3.15mmarereferredto as“finesanddust”,andallthosesmallerthan5.6mmas“small particles”.

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Tocharacterizetheclassifiedparticles,everysamplewasfirst weighedandthensievedmanuallyusingthethreescreensizes.The massesofdust,fines,andlumpswerethenweighedandrecorded. ThepercentageofeachcategorywascalculatedusingEq.(1):

Pi=

mi

mt ×

100% (1)

whereP_iisthepercentageofcategoryiinasample,m_iisthemass

ofcategoryiinthesample,andmtisthetotalmassofthesample.

Thepercentageof“wholepellets”wascalculatedbysubtractingthe

percentageofsmallparticles(<5.6mm)from100.

Samplinglocationsandmethods

OncethepelletshadarrivedinRotterdam,theywereunloaded

byaclamshellgrabintoahopper(seeFig.1).Thisfedtheincoming

materialontoacoveredbeltconveyor(hereafterreferredtoasthe

ﬁrstconveyor).Thegrab’scapacityis10metrictonsandtheheight

ofthehopperis4m.Adustﬁlterisinstalledjustafterthe

hop-pertocollectairbornedustbeforethematerialisloadedontothe

ﬁrstconveyor.Thistransportedthepelletsonanupwardinclineto

thetopofatransfertowerwithaheightof24.0m,ataspeedof

1.7m/s.Thelengthandwidthoftheﬁrstconveyorare150.0and

1.4m,respectively.It,therefore,hasanangleofinclinationof9.2◦.

Atthetopofthetransfertower,thepelletsweredroppedfromthe

ﬁrstconveyorontoasecondonebymeansofafreefallwitha

ver-ticaldropof7.8m.Betweenthesetwoconveyorsisachainbucket

sampler,whichcollectedsamplesataconsistentcross-sectionof

thematerial’sstreamduringitsfreefall.Thisisanautomatic

sam-pler,alreadyinstalledinthesystemforon-sitesamplingpurposes.

Thesamplesittookweretransferredtoasmalldosingconveyor

(length3.0m,width0.2m),whichinturnfedarotarytubedivider

asshowninFig.2.Theroleoftherotarytubedividerwastodivide

asmallportionofthesamplesbyrotatingthematerialsinsucha

waythat,eventually,2kgofsampleswerecollectedevery11min.

Thesecondconveyorhasalengthof500mandtransferredthe

pelletsataspeedof2.6m/stothetopofthesilo,intowhichthey

weredroppedviaafreefall.Wheneverrequired,pelletsare

dis-chargedfromthebottomof thesilo forfurtherprocessing.The

samplesforthisstudyweretakenonthreeconsecutivenon-rainy

daysinthewinterof2019.Everyday,abargecarrying2500metric

tonsofpelletswasunloadedintothesystem.Sixdifferentlocations

throughouttheentiretransportationsystem,fromthebargetothe

silo,wereidentiﬁedforthesamplingusedinthisstudy.Samples

weretakenatdifferentincrementsfromdifferentsampling

loca-tions,andallwereanalyzedintermsofparticlesizedistribution,

mechanicaldurability,bulkdensity,andmoisturecontent.A

sum-maryofthesamplinglocations,daysofsampling,andnumberof

samplestakenateachlocationisgiveninTable2.Intotal,77

sam-pleswerecollectedforthisstudy.Allwereplacedinsealedplastic

Fig.2.Detailedoverviewoftheautomaticmechanicalsamplersysteminthe trans-fertower.

bagstopreventmoistureuptakeandanyfurtherchangestopellet

propertiesduetovaryingenvironmentalconditions.Thesamples

werethentransportedcarefullytoalaboratoryattheDelft

Univer-sityofTechnologyforfurtheranalysis.Thesamplinglocationsare

explainedindetailbelowandtheyareshowninFig.1.

Samplinglocation 1wassituated insidethebarge.Samples

werecollectedfromtheuppersurfaceofthepelletcargoat

dif-ferenttimeintervals.Atthislocation,thephysicalpropertiesofthe

woodpelletscanbecharacterizedjustbeforetheyentertheend

user’stransportationsystem.Asmentionedbefore,thepelletswere

unloadedfromthebargeusingaclamshellgrabwithacapacityof

10metrictons.Eachgrabcycletookapproximatelyoneminute,so

emptyingtheentirebarge(∼2500metrictons)tookmorethan4h.

Toensurethatthesamplesrepresentthepelletpropertiesat

dif-ferentlayersinsidethebarge,samplesweretakenevery30min.

Ninewerecollectedontheﬁrstdayandeightonthesecond,using

aplasticscooptogatherapproximately6kgofmaterialpersample.

Sampling location 2 was situated in an extraempty barge

(hereafter referred to as the second barge)positioned next to

thepellet-loadedbarge.Notethatnosuchsecondbargeisused

innormalplantoperations.However,thissamplinglocationwas

introducedinordertoinvestigatetheeffectonthepelletproperties

ofthemechanicalforcesexertedduringgrabbing.Atthislocation,

onegrabofthematerialwasdischargedeveryhourtocollectﬁve

individualgrabsintotal.Toobtainarepresentativesamplefrom

everydischargedgrab,thegrabcontentsweredumpedontoaﬂat

surfaceinthesecondbarge,creatingapileofwoodpellets.

Sam-plingwasthenperformedaccordingtoEN/TSstandard 14778-1

(EN/TS,2005)bytakingninesamplesofapproximately0.5kgeach

fromdifferentlocationswithinthepile(Fig.3),usingaplasticscoop.

Fig.3. Schematicrepresentationofthepileofpelletsinthesecondbargeandofthesamplestakenit(cyanmarkers).Notethattherealpiledeviatedfromaperfectcone shape.

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Table2

Summaryofsamplinglocations,daysofsampling,andnumberofincrementsperlocation.

Samplinglocation Locationnumber Dayofsampling Numberofincrements

Barge 1 1₂ 8₉ Secondbarge 2 1 5 Firstconveyor 3 1 8 2 9 Rotarydivider 4 3 17 Secondconveyor 5 1₂ 8₉ Silo 6 3 4 Total – – 77

Fig.4.Isolationofthesamplesontheconveyorusingthestopped-beltmethod.

Theseninesampleswerecollectedfromthetop(onesample), mid-dle(threesamples),andbottomofthepile(ﬁvesamples),andwere combinedtocreateonesinglesample.Samplingatthislocationwas undertakenonlyontheﬁrstday.

Samplinglocation3wassituatedinsidethetransfertowerat

theendoftheﬁrstconveyor,whichbringsthepelletsfromthe hop-pertothetransfertower.Thissamplinglocationissituatedjust beforethepelletsweredischargedontothesecondconveyor(see

Fig.1).Thislocationwasusedtoestimatetheeffectonthematerial’s

propertiesofgrabbingandofaverticalfreefallof4m(thehopper’s

height).Notethatineachgrabbingcycle,thepelletsweredumped

atthecenterofthehopper,andsotheeffectonthemofhopperwall

impactscanbeconsiderednegligible.Thissamplinglocationis

use-fulasabenchmarktocheckthereliabilityofthepropertiesofthe

samplesdividedbytheautomaticsampleratlocation4.Atthis

loca-tion,samplesweretakenusingthestopped-beltmethodprovided

forinEN/TSstandard14778-1(EN/TS,2005).Inotherwords,the

entiretransportationsystemwashaltedatthesamplingtimesand

thesamplesweretakenfromthebeltconveyorafterbeingisolated

fromtheothermaterialonitbymeansoftwosteelplates(Fig.4).

Consequently,allthematerialonacross-sectionoftheconveyor

beltwascollectedandlabeledascomingfromsamplinglocation

Fig.5. Thesiloandsamplingports.

3attherelevanttime.Eachsamplecontainedapproximately6kg

ofmaterial.Aswithlocation1,samplingatthislocationwas

per-formedondaysoneandtwo.Moreover,allthesamplesherewere

collectedatthesametimeasthosefromlocation1(thebarge).In

all,17sampleswerethuscollectedatthislocation.

Samplinglocation4wasapartoftheautomaticmechanical

samplerwithinthetransfertower.Samplestakenatthislocation

canbeusedtoinvestigatethereliabilityofthematerialproperties

providedbytherotarydividerwithregardtotheotherlocations

inthetransportationsystem,e.g.attheendoftheﬁrstconveyor.

Eachsamplewascollectedautomatically,ﬁllingaplasticbagwith

approximately2kgofmaterialwithin11min.Sampleswereonly

takenatthislocationonthethirdday,in17increments.

Sampling location 5was situated atthe end of thesecond

conveyor,justbeforethematerialwasdischarged intothesilo.

Samplingatthis locationcandemonstrate theeffectofa 7.8m

verticalfreefall(betweenthetwoconveyors)onpellet

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Table3

Pelletpropertiesatdifferentlocationsondifferentdaysofsampling.Numbersin parenthesesshowthestandarddeviations.

Property Location Dayofsampling Averagevalue

Moisturecontent(%) All All 5.9(0.3)

Bulkdensity(kg/m3₎ _All _All ₆₄₀₍₂₅₎

Mechanicaldurability(%) All All 97.6(1.3)

Diameter(mm) 3,4 2,3 7.11(0.59) Lengthdistributiona (mm) 1, 3, 4 1, 2, 3 50%<13.40 99%<32.70 100%<42.10

a_Based_on_the_pellet_length_{distributions}_of_samples_shown_in_Fig.₇_.

wasassumedtobenegligible.Thestopped-beltmethodwiththe

sametypeofsteel-plateisolatorsasatsamplinglocation3(Fig.4)

wasusedhere,too.Samplesweretakenatthislocationondays

oneandtwoonly,eachweighingapproximately6kg.Onaverage,

thesewerecollectedthreeminutesaftereachofthoseatlocation

3,sincethatwashowlongittookforthepelletstobetransferred

fromtheretolocation5onthesecondconveyor.

Sampling location6wassituatedatthestoragesilo(witha

heightof31mandadiameterof9m)andwasusedonthethirdday.

Foursampleshavebeencollectedfromthefourhatchesinstalled

atthewallbecausesamplinginsidethesilowasimpossibleatthe

timeoftheexperiments.Thesehatcheswerelocatedattwolevels,

andtwosamplesweretakenateachlevel(seeFig.5).

Resultsanddiscussion

Mechanicaldurability,bulkdensity,andmoisturecontent

Fig.6showsthepelletpropertieswithregardtothesampling

locationsanddays.Analysisofvariance(ANOVA)ofall77samples

revealedthattherewasnobiasintheresultsforpelletpropertiesat

differentlocationsorondifferentdays(˛=0.05,i.e.95%conﬁdence

interval).FromFig.6,itcanbeseenthatmechanical durability,

bulkdensity,andmoisturecontentareindependentofthedayof

samplingand ofthesamplinglocation.Moreover,therewasno

correlationbetweenbulkdensityandmoisturecontent.However,

themechanicaldurabilityvariedbetween90.8%and98.7%inall

locations,andthisexceededthe2%repeatabilitylimitsetbythe

standard(ISO,2015).Theaveragevaluesofpelletproperties

mea-suredatdifferentlocationsonthreedifferentdaysofsamplingcan

befoundinTable3.Thistableimpliesthatalthoughalarge

devi-Fig.7. Pelletlengthdistributionsofdifferentbatchesofpelletsusedfor mechani-caldurability(DU)test(legend:digitsfromlefttoright=samplingday-sampling location-numberofincrement-repetitionnumber).

Fig.8.Size-classiﬁedparticlesaftersieving.

ationindurability,onlya fewsampleswidelydivergefromthe

averagevalueandthattheaveragedurabilitycanbeconsideredas

reliabledatainourstudy.

Nocorrelationwasfoundbetweenthemechanicaldurability

ofthepelletsandthenumberofhandlingstepstheyunderwent.

Nevertheless,therearetwoexplanationsforthevariationwefound

inmechanicaldurability.First,inourpreviousstudy(Gilvari,De

Jongetal.,2020)weshowedthatpelletlengthdistributions(PLDs)

canbeacontributingfactorinthemeasuredmechanicaldurability

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Fig.9.Sievinganalysisatlocations1(ﬁrstbarge),3(endoftheﬁrstconveyor),and5(endofthesecondconveyor).Thesamplenumberscorrespondtothesamplingorder.

value,whichincreaseswithgreaterpelletlengths.Tostudythis

effect,thePLDsoftensamples(ﬁveoutof77samples,withtwo

replicationseach)weredeterminedbeforedurabilitytests(Fig.7)

usingourself-developedin-houseimageprocessingtool(Gilvari,

DeJongetal.,2020).AscanbeseeninFig.7,asamplecontainingthe

highestquantityofsmallparticles(<5.6mm)showedthelowest

mechanical durabilityvalue.Take,forexample,samples2-3-2-1

and2-3-2-2inFig.7.Thesesamplesweretworandomlyselected

500gfromtheinitial6kgsamplethatwastakenonday2atlocation

3asthethirdincrement.However,theirdurabilityvaluesdifferby

4.7%.Thisrelativelyhighdeviationcanconﬁrmtheeffectoflength

distributions.Second,astheshareofeachpellettypeineachsample

wasnotcountedhere,itissuspectedthatotherfactorssuchasthe

shareofpellettypemaycauseabiasintheresults.

Particlesizedistributions

Fig.8showsasampleimageoftheclassiﬁedparticles.Inthis,it

canbeseenthatallparticleslongerthan5.6mmarewholepellets

retainingtheircylindricalshape.

Asexplainedinsection2,samplesatlocations1and3were

col-lectedatthesametimeandeachdiscretesampleatlocation5was

collectedonaveragethreeminutesaftereachcollectionatlocations

1and3.Therefore,itislikelythatallcorrespondingsamples

origi-natedfromthesamelocationinthebarge.Hence,theeffectonthe

particlesizedistributionsattheselocationsofdifferent

transporta-tionunitscanbeinvestigatedwithregardtotheiroriginalparticle

size.

Fig.9showstheresultsofsievinganalysisatlocations1,3,and

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Fig.10.Percentagesofsmallparticles(<5.6mm)atdifferentlocationsontheﬁrstandseconddaysofsampling.

Table4

Averageshareofcategorizedparticlesatlocations1,3,and5ontheﬁrstandseconddaysofsampling.

Location Samplingday %ofsmallparticles(<5.6mm) %ofﬁnesanddust(<3.15mm) %ofdust(<1mm)

1(ﬁrstbarge) 1 10.28 6.11 2.50 2 10.40 5.93 2.27 3(endofﬁrst conveyor) 1 8.61 4.91 1.95 2 8.87 4.73 1.74 5(endofsecond conveyor) 1 14.24 9.05 3.81 2 13.93 8.98 3.77

sampling.Accordingtotheseresults,oneachdaythebargescontain anotableamountofsmallparticlesatdifferentsamplingtimesprior tounloading.Thisalsoshowsthatdifferentlayersofthebarge’s cargo(fromtoptobottom)containdifferentparticlesize distribu-tions.However,noevidencewasfoundregardingtheaccumulation ofsmallparticlesinonespeciﬁclayeroratonespeciﬁclocationin thebarge.Theresults(Table4)showthat,onaverage,theshareof

eachsizecategoryondayoneisconsistentwiththeresultsonday

two.

Thestatisticalanalysisshowsasigniﬁcantdifferencebetween

theamountofﬁnesanddustinthesamplescollectedat

differ-entlocations(˛=0.05,i.e.95%conﬁdenceinterval).Themeasured

p-valueforone-wayANOVAanalysisbetweenthesampling

loca-tionsandtheamountofﬁnesanddust(<3.15mm)waszero,which

meansthattherewasasigniﬁcantdifferenceintheaveragevalues.

Onaverage,theproportionofﬁnesanddustwas6.02%atlocation

1(ﬁrstbarge),4.82%atlocation3(endoftheﬁrstconveyor),and

9.01%atlocation5(endofthesecondconveyor)ontheﬁrstand

seconddays.

Asimilartrendtothatforﬁnesanddust(<3.15mm)isobserved

forallsmallparticles(<5.6mm)aswell,sincetheANOVA

analy-sisshowssigniﬁcantdifferencesbetweenthepercentagesofthese

by samplinglocation.The averageproportion ofsmall particles

(< 5.6mm)ondaysoneand twowas10.33%atlocation1(ﬁrst

barge),8.73%atlocation3(endoftheﬁrstconveyor),and14.09%at

location5(endofthesecondconveyor).Thisindicatesthata

signif-icantquantityofsmallparticles(<5.6mm)isgeneratedthroughout

theentiretransportationsystem,fromthebargetotheendofthe

secondconveyorjustbeforethematerialentersthesilo.

Oneinterestingresultisthereductioninthenumberofsmall

particles(<5.6mm)afterthepelletsaregrabbedanddischarged

ontotheﬁrstconveyor,i.e.betweenlocations1and3(Fig.10).The

proportionofsmallparticlesdecreasedrelativelybetweenthese

twolocationsbyupto16.24%ontheﬁrstdayand14.71%onthe

secondday.Therearetwoexplanationsforthis.Firstly,anumber

ofsmallparticlesescapedfromthegrabintotheairwhilethegrab

wasloadinginthebargeandshiftingupwardstowardsthe

hop-per.Secondly,thedustﬁltersatthebeginningoftheﬁrstconveyor

removedsomedust;however,theaccumulateddustinthoseﬁlters

wasnotmeasuredinthisstudy.Ontheotherhand,bycomparison

withlocation3(endoftheﬁrstconveyor),theaverageproportion

ofsmallparticles(<5.6mm)atlocation5(endofthesecond

con-veyor)increasedfrom8.61%to14.24%ontheﬁrstdayandfrom

8.87%to13.93%onthesecondday,showingarelativeincreaseof

65.74%and57.04%,respectively.Assumingthattheeffectof

con-veyorvibrationsonthegenerationofsmallparticles(<5.6mm)is

negligible,alloftheseextraonesmusthavebeengenerateddue

totheverticalfreefallof7.8matthetransferpointbetweenthe

twoconveyors.Boacetal.(Boacetal.(2008))observedanaverage

increaseof3.83%inthenumberofparticlessmallerthan5.6mmin

onetransportationcycleoffeedpelletsinabucketelevatorandsilo

feedingsystemforpelletswiththedurabilityof92.9%anda

nom-inalpelletdiameterof6.4mm.Inthisstudy,however,anaverage

5.36%increaseinthenumberofparticlessmallerthan5.6mmwas

observedsolelyduetoafreefallof7.8m.Thisisprobablyrelated

tothecompressionandimpactforcesassociatedwiththeamount

ofmaterialbeingtransported,whichwasaround450ton/hinthe

presentstudyand54.9ton/hintheworkofBoacetal.(Boacetal.

(2008)).

Theresultsshowthattheaverageproportionofsmallparticles

atlocation2(secondbarge)was6.56%,witha1.51%standard

devia-tion.Thiswaslowerthantheﬁgureforlocation1(10.34%).During

grabbingfromtheﬁrstbargetothesecondone,itwasobserved

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Fig.11.Shareofsmallparticlesatlocations2,4,and6.

leakage.Becauseofthis,themeasuredsmallparticlesarenot

rep-resentativeforthewholeparticlesandtheeffectofgrabbingonthe

generationofsmallparticlesremainsunclear.

Fig.11showsthatthepercentageofsmallparticlesatlocation

4 (automaticsampler)onthethirddaywas6.05%,witha

stan-dard deviationof 2.04%.Asdiscussed earlierinthissection,the

resultsfromtheﬁrsttwodaysshowahighrepeatabilityinterms

ofgeneratingsmallparticlesateachsamplinglocation.Therefore,

theshareofsmallparticlesateachlocationondaysoneandtwo

canbepostulatedtobesimilartothatonthethirdday.Basedon

thisassumption,theresultsfortheﬁrstandseconddaysatlocation

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Fig.12.Correlationsbetweenclassiﬁedsmallparticles.

thirddayatlocation4(automaticsampler).Theaverage

propor-tionofsmallparticlesatlocation3ontheﬁrstandseconddayswas

8.73%,whileatlocation4itwas6.05%.Thus,onaverage,thenumber

ofsmallparticlesdecreasedbetweenthesetwolocationsby2.68%,

showingthattheautomaticsampler(location4)doesnotproduce

similarparticlesizedistributionswhencomparedtolocation3.A

possiblereasonforthisisthesegregationofbiggerparticlesdue

totherotationofparticlesintherotarydivider,whichseparates

largerparticlesfromthesmalleronesinsuchawaythatthelatter

arerejectedasresidue(seeFig.2).

Theaveragepercentageofsmallparticles(<5.6mm)atlocation

6(silo)isshowninFig.11.Onaverage,thesemadeup4.00%of

thesamplestakenfromthesilohatches.Comparingtheproportion

ofsmallparticlesherewiththatatlocation5(endofthesecond

conveyor),itcanbeconcludedthatthewholepelletsaccumulated

atthesilowalls.Thiscanbeexplainedbypercolationsegregation,

wherebythesmallerparticlesarecapturedinthevoidsandbigger

onesmovedowntheslopetowardsthewalls.

Thomas(Thomas(1998))showedthatpelletfragmentationand

attritionarethetwomajormechanismsforphysicaldegradation

ofpellets.Fig.12showstheshareoflumps,ﬁnes,anddustinall

thesamplestakenfromdifferentlocationsandincludesasurface

ﬁtthatisthebestﬁtforallthesamples.Althoughthebreakage

mechanismwasnotstudiedinthiswork,Fig.12showsthatthe

sharesofdust,ﬁnes,andlumpsatdifferentlocationsandon

differ-entsamplingdaysarecorrelatedtooneanotherwithanR2_value

of0.866.Inotherwords,inallsamples,thequantityofeach

size-classiﬁedparticlewasincreasedbyincreasingtheamountofother

size-classiﬁedparticles.

Conclusions

Inthisstudy,wehaveinvestigatedthechangeinthephysical

propertiesofcommercialwoodpelletswithafocusonbreakage

and attrition behaviorduring large-scale(∼450ton/h)transport

andstorageinapellet-ﬁredpowerplantintheNetherlands.Our

mainconclusionisthattransferringthewoodpelletsviaafreefall

fromaheightof7.8m(thetransferpointbetweentwo

convey-ors) increasestheproportionofsmallparticles(<5.6mm)from

8.74%to14.09%,onaverage,andtheamountofﬁnesanddustfrom

4.82%to9.01%forpelletswithanaveragemechanicaldurabilityof

97.6%(basedonISOstandard17831-1).Thisemphasizesthe

impor-tanceofequipmentdesignandoperationwithrespecttomaterial

degradation.Themechanicaldurabilityofthesamplestakenat

dif-ferentlocationsdifferedbyupto7.9%,whilenocorrelationwas

observedbetweenmoisturecontent,bulkdensity,sampling

loca-tion,ordayofsamplingandmechanicaldurability.Thissuggests

thatotherproperties,mostprobablypelletlengthdistribution,play

asigniﬁcantroleinthemeasuredmechanicaldurabilityvalue.

Declarationofinterests

Theauthorsdeclarethattheyhavenoknowncompeting

ﬁnan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto

inﬂuencetheworkreportedinthispaper.

Acknowledgements

The authors would like to thank Jurgen Visser and Ronald

KoomansofUniperBeneluxfortheircarefulplanningand

coor-dination,aswellasforsharingtheirsamplingexpertise.Wethank

EovarvanDijk,KoenJongste,andSjoerdSmidofPeterson

Rotter-damforcoordinatingandexecutingthesamplingonsite.Wealso

thankSjaakvanHurck(ESI/EurosiloBV),FrankvanderStoep(EBS),

LeoLokker(EMO),MarkBouwmeester,andEmielvanDorp(RWE)

forusefuldiscussionsinpreparingthemeasurements.Wethank

TjibbeBronandJorisBerendschot(DelftUniversityofTechnology)

fortheirsupportinanalyzingthesamples.

ThisstudyhasreceivedfundingfromtheTopConsortiumfor

KnowledgeandInnovationfortheBiobasedEconomy(TKI-BBE),

undergrantnumberBBE-1713(Biomassapellets:Degradatie

tij-denstransportenhandling).

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