SANDIA REPORTSAND#UURPrinted February 2014
Hybrid-renewable processes for biofuels production: concentrated solar pyrolysis of biomass residues
Anthe George, Manfred Geier, Daniel Dedrick
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SAND200X-XXXXUnlimited Release
February 2014
Hybrid-renewable processes for biofuels production: concentrated solar pyrolysis of biomass residue
Anthe George, Manfred Geier, Daniel Dedrick08367
Sandia National LaboratoriesP.O. Box 5800
Albuquerque, New Mexico
ABSTRACT
The viability of thermochemically-derived biofuels can be greatly enhanced by reducing the process parasitic energy loads. Integrating renewable power into biofuels production is one method by which these efficiency drains can be eliminated. There are a variety of such potentially viable "hybrid-renewable" approaches; one is to integrate concentrated solar power (CSP) to power biomass-to-liquid fuels (BTL) processes. Barriers to CSP integration into BTL processes are predominantly the lack of fundamental kinetic and mass transport data to enable appropriate systems analysis and reactor design. A novel design for the reactor has been created that can allow biomass particles to be suspended in a flow gas, and be irradiated with a simulated solar flux. Pyrolysis conditions were investigated and a comparison between solar and non-solar biomass pyrolysis was conducted in terms of product distributions and pyrolysis oil quality. A novel method was developed to analyse pyrolysis products, and investigate their stability.
4
ACKNOWLEDGMENTS
ThisworkwassupportedbytheLaboratoryDirectedResearchandDevelopmentprogram
at SandiaNational Laboratories.SandiaNational Laboratories is amulti-program laboratory
managedandoperatedbySandiaCorporation,awhollyownedsubsidiaryofLockheedMartin
Corporation,fortheU.S.DepartmentofEnergy'sNationalNuclearSecurityAdministrationunder
contractDE-AC04-94AL85000.
We thank Christopher Shaddix, SNL and Rafael Kandiyoti, Imperial College London, for their
constructive comments during this work and, Trevor Morgan, at the Hawaiian institute of
technologyforhiscontributionstothemethoddevelopmentstudiesinthiswork.
5
CONTENTS
Acknowledgements 04
Contents 05
1.0 Introduction 07
2.0 Simulatedconcentratedsolarpyrolysisreactors andbio-oilproduction 09
2.1 Fluidizedandfixedbedsolarthermochemicalconversionreactorsand
conventionallyheatedbatchpyrolysisreactor 09
2.2 Materials andmethods 14
2.3 GC-FIDanalysis 15
2.4 Productdistribution 15
3.0 Methoddevelopmentforanalyzingheavypyrolysisoils andbio-oilstability
studies 17
3.1 Challengesinbio-oilcharacterization 17
3.2 Materialsandmethods 22
3.2.1 Standardsandsolvents 22
3.2.2 Targeneration 23
3.2.3 Tarrecovery 23
3.2.4 Tarstorage 23
3.2.5 Bulksamplepreparationandtaryieldcalculation 25
3.2.6 Planarchromatography(PC)fractionation 25
3.2.7 Ultimateanalysis(UA) 27
3.2.8 Gas-chromatography(GC-FID) 27
3.2.9 Sizeexclusionchromatography(SEC) 28
3.2.10Laserdesorption/ionisationmassspectrometry(LD-MS) 29
3.2.11UV-fluorescencespectroscopy(UV-F) 30
3.3 Resultsanddiscussion 31
3.3.1 Tar-yields,GC-FIDandultimateanalysis 31
3.3.2 PCfractionationoftheRecoveredDrytars 38
3.3.3 BulkanalysesofRecoveredDrytar 55
6
3.3.4 Precipitatedmaterialsanalyses 58
3.3.5 Summary 62
3.4 Conclusions 64
S3 Supportinginformation 65
S3.1 FuelProperties 66
S3.2 PlanarChromatographyImages 67
S3.3 TarYields,GCandUAFurtherDiscussion 69
S3.4 SECCalibrationandInterpretation 71
S3.5 LD-MSAdditionalInformation 73
S3.6 SynchronousUV-FInterpretation 74
S3.7 PCFractionsSEC,LD-MSandUV-FResults 77
S3.8 Figures,byPCFraction(SEC,LD-MSandUV-F) 87
S3.9 Figures,bySample(SEC,LD-MSandUV-FofPCfractions) 100
S3.10 Figures,LD-MSoftheBulkTars 106
4.0 Pyrolysisofligninresidues fromtransgenic plantmaterial 108
4.1 Transgenicligninsamples 108
4.2 Reactorconfigurationforpyrolysisoftransgenicligninplants 110
4.3 PyrolysisofArabidopsissampleswithreduceddegreeofpolymerization 111
(FCA)
4.4 PyrolysisofArabidopsissampleswithreducedamountoflignin(Qsub2) 116
5.0 References 126
7
1.0 Introduction
The viability of thermochemically-derived biofuels can be greatly enhanced by
reducing the process parasitic energy loads. Gasification processes are globally
endothermic, requiring 20-45% of the feedstocki to be consumed allothermally.
Additionally, in air-blown gasification, high CO2 and N2 dilute the product syngas.
Likewise pyrolysis processes also require external energy to progress. Integrating
renewable power into biofuels production is one method by which these energy
efficiency drains can be eliminated. There are a variety of such potentially viable
“hybrid-renewable” approaches;one is to integrate concentrated solarpower (CSP) to
power biomass-to-liquid fuels (BTL) processes, both during pyrolysis and/or
gasification operations as well as downstream. Barriers to CSP integration into BTL
processesarepredominantlythelackoffundamentalkineticandmasstransportdatato
enableappropriatesystemsanalysisandreactordesign.
There is currently little understanding of the fundamental behavior ofbiomass
particles subjected to solar flux. When considering thermochemical conversion of
carbonaceous feedstocks, many parallel as well as sequential reactions involving
neighboring particles need to be considered. In these complex reactions final product
distribution is a function of the time-temperature-pressure distribution, as well as
history,ofthereactionmixture.Pyrolysisexperimentsarethereforeverysensitivetothe
heatingmethod,flowpatterns,designof reactionzoneandconfigurationofthesample
(shape,size,composition),anddataareoftenanartifactofthemethodsusedtoproduce
them.Thepaucityofdata onsolartreatmentofbiomassisexacerbatedbythefactthat,
to-date,attemptshavenotbeenmadetodecouplethesesecondaryeffectsfromreactor
design.Inthisworkweproposedtodevelopareactorschemethatsimulatessolarinflux
onto biomass particles but minimizes these artifact effects, allowing high quality
fundamental data to be obtained. This will enable rational science-based design of
process scale-up, rather than the post-hoc approaches taken to-date. Part of this
investigation necessitated the development of methods to analyse the bio-oil in its
entirety. Current analytical techniques do not adequately cope with the heavier
components of pyrolysis oils and gasification tars, and it is thought that it is these
components which contribute to the instability of the bio-oils (one of the main
challengesassociatedwith these typesof fuels).Therefore techniquesweredeveloped
8
that could give more information on the higher molecular weight components in
thermochemicallyderived liquids, and the agingand stabilityof gasification tarswere
investigatedviathetechniquesdeveloped,inordertohighlighttheireffectivenessinthis
context.
Otherthanprocessenergy, feedstockisanotherimportantparameterinbiofuels
production.Lignocellulosic biomass typically consistsof20-30%lignin.Utilizing lignin
that is currently a waste stream from the lignocellulosic biofuels process to produce
biofuels would have an enormous impact on the economics of the biorefinery, which
only uses the cellulose portion of the biomass. Feedstock cost is one of the main
economicbarrierstotheuptakeofbiofuelstechnologies,sousingawastestreamasthe
feedstock to the CSP process would make a significant contribution to the process
viability. In this research we investigated the pyrolysis of lignin. Thermochemical
conversionisparticularlychallengingduetothehighquantitiesofcharproduced.Oneof
the great advances in plant science is the ability to manipulate the structure of the
growingplant.Ligninitselfhasbeenshowntobealteredintheplantcellwall,interms
ofbothitsdegreeofpolymerization,theamountofligninthatcanproduceaviableplant
and also the structure of the lignin. In this studywe investigated geneticallymodified
lignins to ascertain if the thermochemical conversion in these plants produces a
differentqualitybio-oilandifthisbio-oilisofasuperiorquality(orhasthepotentialto
besuperior)comparedtotheconversionofthewild-typeplant.
9
2.0 Simulatedconcentratedsolarpyrolysisreactorsandbio-oilproduction
Thermochemicalbehavior,particularlypyrolysisbehaviorishighlydependenton
reactor configuration. Three reactors were developed in this study to enable the
investigation of different aspects of solar thermochemical conversion of biomass. The
firstreactor,(Figure2.1)wasthemostcomplexsystemandwasdesignedtoallowedthe
carefulstudyofthermochemicalbehaviorbyminimizingtoasgreatanextentaspossible
masstransfer limitationswithin fuelparticles that inhibitvolatilereleaseandpromote
char formation. This is particularly important when investigating the conversion of
biomass to fuel as here char conversion should be minimized and liquids yield
maximized. The fluidizingmedium in this instancewas found to lower theachievable
temperature in the system, which whilst higher than required for typical pyrolysis
processes is lower than would be desirable when simulating higher temperature
gasification.Thereforea secondfixed bedreactorwasdevelopedwhichwouldremove
the heat-transfer from the fuel particle to the flowing gas and allow for gasification
studies to be conducted (Figure 2.2). Higher temperature gasification reactions are
somewhat less sensitive to particle size andmass transfer limitations associatedwith
lower temperature biomass pyrolysis reactions attempting to maximize oil yield and
therefore a fixed bedwas deemed to be appropriate for this typeof study. Finally to
allow for comparison between the solar case and conventional pyrolysis systems a
furnacetypepyrolysisreactorwasdesigned(Figure2.3).Thesereactorsaredescribed
below.
2.1 Fluidized and fixed bed solar thermochemical conversion reactors and
conventionallyheatedbatchpyrolysisreactor
Severalassumptions andrequirements weremadeinordertoarriveatthebest
approximation for a concentrated solar powered fuel conversion reactor. All the
reactorsinthisinvestigationwerecustomfabricatedfromquartz,whichalthoughcanbe
fragile, is used in this instance because a) it is transparent to the simulated solar
radiation,b)allowsforaninertatmosphereforthepyrolysisoils(preventingcatalytic
reactionsoccurringwhenpyrolysisproductscomeintocontactwithmetals)andc)also
10
for easy recovery of any condensed material on the reactor. An added benefit is the
possibility of allowing visual monitoring and imaging of the system whilst the
experimentisunderway.
The heat source for this system was intended to simulate concentrated solar
power. Typically argon lamps are often used for this purpose, but due to budget
considerations IR lampswere investigated as suitable alternatives. To enable an even
distributionoflighttothebiomassparticles acolumnatedlightsourcewasrequiredata
narrow wavelength range. The wavelength chosen would need to be transparent to
quartz. ResearchIncStripIR5360lampswereused.Figure2.4depicts thecolumnated
lightproducedfromasinglelampandhowaconfigurationofseverallamps canbeused
to improve the density of the radiance. In this system two lamps were used. The
tungsten emitter in the lamps has an operating temperature of up to 2205 °Cwith a
spectralenergypeakwavelengthof1.15microns(Figure2.5).Thepoweroutletofthese
heaterswas1000Wat240V.Theheaterconsistsofaspecularaluminumreflectorthat
directstheinfraredenergygeneratedbyoneceramicend-seal‘T-3style’quartzhalogen
lamp factory-installed in theheater. Thequartzhalogen lampsheatupandcooldown
instantly inresponsetopowercontrol signals.Theyreach90percentof fulloperating
temperaturewithin three secondsof a cold start.The radiantenergydissipates to ten
percentfivesecondsafterthepowersupplyisdisconnected.
Despitetheresponsivenatureofthelamp,fastpyrolysisneedstobecontrolledto
withinmilliseconds,andwehavethisrequirementhere.Thereforeashuttersystemwas
designed and controlled by LABVIEW, whereby a heat resistant shield made of
refractorymaterialwasdesigned.UsingthisshuttersystemandLABVIEWtheexposure
ofthebiomassparticlestotheheatsourcecanbecontrolledto+/- 5ms.
The operation of the fluidized bed reactor given the design described is as
follows:thefeedingtubeand lampcoolingwatersystemsareactivated,afterwhichthe
hotfluidizinggas(nitrogen,180°C)ispassedthroughthereactor.Thefluidizinggasis
heatedtobelowthepyrolysis temperatureof theparticlesas the short reactiontimes,
and high heating rates required to conduct these fundamental studies could not be
achievedwith the lamp set-up in this reactor scheme, if the experiment was starting
from“cold”.A carriergasat the topof the reactorof low flowrate (Figure2.1) is then
11
introduced into the reactor. The charge of biomass particles are introduced into the
fluidized zone via opening a simple ball valve and are carried through by the small
flowrateoftheaforementionedgas. Theexperimentisnowreadytoproceed.Bymeans
ofLABVIEW,thelampsareswitchedonandafter3secondstheshuttersareopened.The
fluidizedbiomassparticlesareexposedtotheheatsource fortheprescribedtimeafter
whichtheshuttersareautomaticallyclosed.Anysolids(charandunreactedbiomass)is
collectedinthechar-trapafterthefluidizinggasseshavebeenswitchedoff.Thevolatile
pyrolysis products are condensed in a tar-trap, and in theory after the tar trap a gas
analysercouldquantifythepermanentgasesfromtheexperiment.However,thisfacility
wasnotavailablefortheseexperimentsandgasyieldswerequantifiedbydifference. To
comparethepyrolysisproductdistributionfromthesolarheatedandnon-solarheated
caseafurnacebasedpyrolysisreactorwasdesigned(Figure2.3).
12
Batch feeder
Water cooled feeder
tube
Sintered disk to allow
volatiles escape and
prevent particle elutriation
Quartz fluidized
reaction zone
Char/solids catch-pot
Feed carrier gas
Feeder water cooling
Condensable products
outlet
Heated fluidizing gas
Heat resistant
electronically controlled
shutter in front of IR lamp
Thermocouple inlet to
reaction zone
Figure 2.1 Fluidized solar pyrolysis reactor, illustrating key operating components
13
Figure2.2 Fixedbedsolarpyrolysisreactor,showingelectronicallycontrolledshuttercarriergasin-
letandvolatile(bio-oil)outlet.
Figure2.3Columnated solarpyrolysisreactor,showingelectronicallycontrolledshuttercarrier
gasin-letandvolatile(bio-oil)outlet.
14
2.2Materials
Switchgrass (Panicum virgatum) was provided by Dr. Daniel Putnam, University of
CaliforniaatDavis.Switchgrass wasgroundbyaWileyMillthrougha2mmscreenand
separated by a vibratory sieve system (Endecotts, Ponte Vedra, FL). The switchgrass
fractions falling between 60 and 80 mesh were collected for use in this study. The
Figure2.4ColumnatedIRradiationproducedbyIRlampandmulti-lampconfigurationfor
increasingdensityofradiationtotarget.Inthisworkatwolampconfigurationwasused.
Figure 2.5 Emission range of IR lamp used in fluidized solar pyrolysis reactor. Peak
emission is approximately 1 µm which allows for transmittance through quartz and isa
representativewavelengthforsolarradiation.
15
biomasswasdried inaconvectionovenat50degreesCelsius for24hoursbeforeuse.
Analyticalgradereagents(Sigma)wereusedthroughout.
2.3GC-FIDAnalysis
For pyrolysis oil analysis, a Varian GC3800-FID with a Varian ‘FactorFour’ capillary
column (VF-5ms 30M, 0.25mM ID,DF =0.25)was used toquantitatively examine the
tars. 5 μL injection volume (split ratio 50), helium carrier gas (2 mL/min), injector
temperature325°C,columnstartingtemperatureof50°Cfor4minutes,25°C/minramp
to150°C,5°C/minrampto325°C,heldat325°Cfor2minutes.FIDdetectortemperature
was 325°C. For the GC analysis a volume of the isopropanol-tar solution was taken
before the samplewas vacuumdried. Further details of peak identification, standards
andcalibrationareprovidedinS3.
2.4Productdistribution
Switchgrass was pyrolysed at varying conditions (residence time and temperature),
table2.1. Thisdataiscomparedwithexperimentsconductedinthewire-meshreactor
described in section 4.1. Particle heating rate calculations are always challenging in
thermochemical conversion experiments. The challengewith the solar reactor system
designedhereisthattheheatingrateoftheparticlecouldnotbeasaccuratelyinferred
asinthewiremesh,asthetemperaturebeingmeasuredwasthebulktemperatureinthe
reaction zone.Nevertheless heating rates up to 500 °C per second could be achieved
with lamp power at 80% of maximum, with a heated inlet gas of 180° C with the
fluidized systemand1200 °C for the static system. For the char tar and gasproduct
distributions, char isdefinedas the solidsmaterial recoveredafter reaction, tar as the
condensableliquidmaterialrecoveredinthetrapafterreactionandthepermanentgas
yield was calculated by difference. Reactions were repeated in quadruplicate, and
measurement uncertainties were in most cases <5% but always <10% and data are
givenonadry,ashfreebasis.
16
Table2.1pyrolysisofswitchgrassusingsimulatedsolarirradiationviaanIRlamp
Reactor
type
Peak
temperature
(oC)
Holding
time(s)
Char
(wt.%)
Tar
(wt.%)
Gas
(wt.%)
Solar 400 5 24.3 48.34 27.36
Solar 400 30 22.1 47.6 30.3
Solar 550 5 17.8 50.1 32.1
Solar 550 30 17.4 50.4 32.2
Wire-mesh 400 5 19.9 52.8 27.3
Wire-mesh 400 30 19.2 52.6 28.2
Wire-mesh 550 5 18.4 53.5 28.1
Wire-mesh 550 30 18.3 53.2 28.5
In general the solar system showed greater sensitivity to parameter change than the
wire-mesh reactor. With regard tovariations in temperature, it appears for the solar
system that forbothholding times charyieldsdecreasedwith increasing temperature
and whilst both tar and gas yields increased. Pyrolysis product yields are generally
thoughttobemoreaffectedbyheatingratethantemperature,butat400°Cit is likely
that the higher solids yields are due to unreacted material, rather than a greater
productionof char.This isbourneoutby the lower charyields in the solar reactor at
longerresidencetimesatbothtemperaturetests.
Thedataaboveshowthatholdingtimedidnotmakeadifferenceinthecaseofthewire-
meshreactorforcharproductionataparticulartemperaturebutwasamoresignificant
variableinthesolarsystemat400°C.However,particularlyathighertemperatures,the
taryieldswerereducedatlongerholdingtimes Inthesolarreactor.Thewire-meshwas
designed to approximate an ideal system in terms of secondary tar reactions and
cracking. It appears for the solar system, heat transfer within the particle is more
efficientleadingtolowercharyields,however,theevolvedproductsarefurthercracked
to permanent gases. Further investigation of the heat transfer phenomena via this
radiativesystemattheselowertemperatureswouldbemostinteresting.
17
3.0 Methoddevelopmentforanalyzingheavypyrolysisoilsandbio-oilstability
3.1 Challengesinbio-oilcharacterization
Aknownissuewithpyrolysisoilandliquidproductsfromgasificationisaccurate
characterization of themolecular structure, particularly of the heavier components of
the biomass which sample instability. This is particularly challenging if these
componentsaretobeusedforbio-fuelproductionwhich,aswithall fuels,willneedto
havestableattributesthatfallwithincertainstandardvalues.Amethodwasdeveloped
tocharacterizeheaviercomponentsinbiomassbyevaluatingtheheaviesttarsproduced
in the co-gasification of biomasswith coal. A description of the study and results are
showninthissection.
Gasification has been recognised as one of the most efficient thermochemical
processes for converting solid fuels into energy [1]. Despite this high efficiency,
relatively few power stations have adopted this technology, due to economic and
downstream technical drawbacks. The environmental concerns about the levels of
carbon dioxide in the atmosphere have provoked a renewed interest [1-8]. However,
several unsolved problems related to fixed- and fluidised-bed biomass gasification
remainwhich hamper the implementation of the technology.Ofmain concern for any
applicationofthesynthesisgasisthepresenceofimpuritiessuchastar,NH3,HCl,HCN,
H2S and COS [1]. In particular, the amount of tar produced reduces the carbon
conversionandefficiencyandcausessignificantoperationalproblems duetodeposition
on exhaust lines [1, 9]. There are also difficulties with their subsequent storage and
processing[1,5-10].
Gasification tars also contain high levelsof polyaromatic hydrocarbons (PAHs)
thatare recognisedcarcinogensbytheenvironmentalprotectionagency(EPA[11]).A
numberofgascleaningsystemsarecommerciallyavailableandinroutineoperation,but
thesearecostlyandnotcurrentlyoptimised[8-10].
Atpresent,tarsfromtheco-gasificationofbiomassandcoal,aswellasfrompure
biomass,arebeingproducedonasmallbutincreasingscaleaftersynthesisgascleaning.
Recoveredgasification tars are typically re-usedasboiler-fuel. This isnotnecessarily
environmentallyfavourableorcosteffective[12,13].Moreover,theincreasedemphasis
18
on using biomass as a substitute for coal for power generation will lead to greater
amounts of tars being generated in the future. Thesewill need tobe used, stored,or
otherwisedisposed.Gasification tars frombiomass,which containhigherquantitiesof
oxygen than those from coal, will have analogous issues to biomass pyrolysis liquids
withregardtotheirstorage,upgradingand/oruse.
Biomass pyrolysis liquids are composed of a complex mixture of oxygenated
hydrocarbons(e.g.alcohols,aldehydes,carboxylicacids,esters,ethers,ketones,phenols
andsugarderivatives)with15-30wt%waterfromtheoriginalmoistureandasreaction
product [1, 14, 15]. The effects due to ageing on biomass pyrolysis liquids have been
widely studied. Reactionsoccur between oxygenated compounds, volatile components
are lostduetoevaporation leadingtoan increaseofmolecularweight,andchanges in
water content and viscosity can result in phase separation [1, 14-22]. The probable
reactionpathwaysinvolvedintheageingprocesshavebeenreportedtoconferpossible
changes tobulkproperties suchasboiling range,densityandviscosity [14].However,
limitedinformationisavailableregardingchangesonthemolecularlevel.
Ageing studies of biomass pyrolysis liquids have focused on theanalysis of the
trendsinmolecularweight,basedonsizeexclusionchromatographyorviscosity,andon
the determination of the water content [14, 15]. Less attention has been paid to the
studyof chemical composition through techniques suchasGC/GC-MS,FT-IRandNMR
[19,23,24].Theinfluenceofstoragetemperatureandsolventadditiononthestabilityof
oilsandtarshasbeenwidelyexamined,typicallybasedonviscositydatawhichcanonly
provide indicative information on other important properties [14-16, 25, 26]. Several
approaches to improve the stabilityofpyrolysisoilshavebeen reported [15,27].The
additionofapolarsolventisacommonapproachtoimprovethestabilityoftarsduring
storageandcanresultin atwentyfolddecreaseintheageingrate,reductioninviscosity
andacidity,aswellasincreasingheatingvalueandmiscibilitywithfossilfuels [15,18,
23, 26]. The upgrading of biomass pyrolysis liquids by addition of alcohols at room
temperaturehasbeenconductedat the industrialscale since1995 [28]. Similarly, the
storage of gasification and pyrolysis tars in alcohol solution at reduced temperature
(5°C)hasbeenreportedtominimiseageingreactions[14].
19
Standard analytical methods are available for the recovery [29] and
characterisation [30] of tars from gasification, which enable comparisons to bemade
betweendifferentreactorconfigurationsandfuels.Thestandardmethodshowever,are
basedongaschromatography(GC)andthegravimetricweightofthetar.Thislimitsthe
analysistomaterialsthatarevolatileatGCcolumnconditions.Ingeneral,littleisknown
about the composition of materials labelled as ‘tars’ in terms of molecular mass
distribution,averagemolecularmassorstructure,beyondinformationavailablefromGC
methods [1, 8, 12, 13, 31]. For compounds present in gasification tars the typical GC
upper limit is ~350 u for aromatic compounds and ~600 u for aliphatic compounds,
C60-C70 or a little higher (C100), using high temperature GC [32-34]. The smaller
molecular weight components typically constitute between 30-60 wt% of the entire
sample(dependingonreactionconditionsandfeedstock),sothereexistsalargegapin
understandingoftheimportantremainingportionofthetar[12].
Studies in the literature reveal that more advanced analytical techniques have
rarely been applied to gasification tars and in particular those from biomass. This is
despite the fact thatbiomass and coal gasification tars havebeen of studied since the
1990sandhavebeen indentifiedasakey issue tobeaddressed for improvingenergy
conversion [35-37]. This point has also been reiterated more recently in a
comprehensivereviewofthechemistryofthermochemicalprocessesusedtosynthesise
transportationfuelsfrombiomass[1].Muchoftheearlyworkusingadvancedanalytical
methods focused on coal tars and pitch [36, 37], petroleummaltene and asphaltenes
[38], coal [39], partial oxidized aromatic hydrocarbons [40] and products from wood
pyrolysis[41-43].Mostof thesestudies focusonthe identificationofsmall tomedium
sizedmolecules(<800u)[39-42],insomecasesusingGCbasedmethods[39,41] orin
combinationwithliquidchromatography[40,42].
Some of these early studies looked at the highermassmolecules (>1000 u) in
attempts tomore fully characterise the samples [36-38,43,44] mostlybasedon laser
desorption-MS(LD-MS).Thesestudiesshowedevidenceformolecularmassgreaterthan
2000uinawoodtarpitch[44],woodpyrolysisliquids[43],coaltars[36,37] petroleum
asphaltenes [38, 45] and pyrolysis products from waste plastics [46]. However, no
studiesusingLD-MSonbiomassgasificationtars,ortars fromco-gasificationwithcoal,
couldbefoundintheliterature.Inaddition,greatprogresshasbeenmadeinoptimizing
20
the experimental conditions for LD-MS and interpretation of the results, in particular
whenappliedtocomplexsamplessuchasthosementionedabove,asdetailedinarecent
reviewarticle[47].
Morerecentlyioncyclotronresonancemassspectrometry(ICR-MS)hasreceived
attentionforstudyingcomplexsamplessuchasoilsandtarsduetounprecedentedlevel
ofmassresolution,typicallyto0.1ppmofmass,atmassesofafewhundredmassunits.
Themethod has been applied to coal tar pitch [48], carbonblack [49], products from
pyrolysis of shale oils [50], heavy bitumen derived samples [51], and petroleum
asphaltenes [52]. The method was able to provide extremely valuable and detailed
information formolecularmass up to amaximumof 1200u (typically< 800 u) inall
these samples. However, the method is known to suffer from selective sampling
(incomplete sampling) issues, which is not fully understood [47]. In addition, all the
samplesmentionedaboveareknowntocontainhighermassmoleculesthandetectedby
ICR-MS. It cannot therefore be used to determine average mass estimates or mass
distributionsforcomplexmixturesofhydrocarbons.Thesepointsandthewiderbenefits
andlimitationsofICR-MShavebeenaddressedinarecentreviewarticle[47].
Similar problems persist when trying to fully characterise pyrolysis derived
liquidsandtarsfrombiomass,coalandpetroleumresidues,intermsofmolecularmass
distribution, averagemolecularmassor structural compositionbyotheradvancedMS
methods[12,13,47].Typically,thestandardmethodforgasificationtaranalysisisalso
usedonpyrolysisliquids[8,29].Inmanyofthestudiesmentionedabovethatlookedat
materials beyond the GC range, size exclusion chromatography (SEC) was used for
molecular weight determination, with tetrahydrofuran (THF) as eluent and solvent,
oftenwith a refractive index detector [14, 15, 27, 43]. These SEC systems have been
showntobeincapableofresolvingsmalltomediumsizedpoly-aromatichydrocarbons
intermsofmolecularsizeormass[53-55].ThisisthoughttobeduetoTHFactingasa
weaksolventforthesamplesinquestionandenablinginteractionswiththeSECcolumn,
and also the usefulness of refractive index for detection is questionable. A number of
studies have addressed these, and the wider issues, over the last decade for coal,
petroleum,biomassandbitumenderivedoils,tarsandpitches[12,13,31,53-57] anda
reviewofthesestudieshasbeenreported[47].
21
TheaforementionedinvestigationsdemonstratedthatN-methyl-2-pyrrollidinone
(NMP)wasasuperiorSECeluentandsolventfortarsthatcontainhigharomaticityand
fewerhetero-atomssuchascoalorbiomassderivedgasificationoils/tars[47].Despite
theirhighoxygencontent,pyrolysisliquidsfrombiomasscanalsobeanalysedwithNMP
astheSECeluent[58,59].
ItisimportanttonotethatusingSECwithNMPeluentdoesnotprovidecomplete
informationorquantificationofmolecularmassdistributions,although ithasproveda
valuable tool for making relative comparisons between similar samples and is better
understoodthanotherSECsystems[47,53,55].Itisimportantthatthelimitationsofthe
technique are understood and accounted for when interpreting SEC results to avoid
over-interpretationofdata.
Therefore, there remains a great need in the gasification and pyrolysis
communitiestoobtainmoreaccurateinformationonmolecularweightdistributionsand
structuralfeaturesofliquid/solidproductsbeyondthe1000umassrange[12,13,47].
Tothisend,arecentlyreportedanalyticalapproachwasassessed[31,47,56,60].This
methodologyhas been successfully applied to thedetermination of averagemolecular
mass number estimates, molecular mass distributions and detailed structural
informationonpetroleumandcoal tarpitchderivedmaltenesandasphaltenes[47,56,
60,61] aswellasbitumen(oil/tarsand)samplesandsolubilitysub-fractions[47,62].
The study at hand assesses the validity of the above mentioned analytical
approach [31,47,56, 60] for tars from the co-gasificationof a softwood (pine)anda
Polish black (sub-bituminous) coal. To study the validity of the above methods, we
attemptedto identifychanges incomposition(intermsofmolecularmassdistribution
and aromatic structural features) after ageing the tars under different storage
conditions.Findings fromthescopingstudyarereported. It is intendedthatdata from
this scoping study would first determine if using this methodology is justified, and
secondly,assessifthisnewmethodologycanspurfurtherresearchinthefuture.
Changesdue toaging in the compositionof gasification tarsareexpected tobe
lesssignificantcomparedtochangesimpartedontarsfromtheuseofcatalysts,thermal
upgradingprocesses,orseenduringtheageingofbiomasspyrolysisliquids.Therefore, if
themethodsusedherearesensitiveenoughtoidentifychangesingasificationtarsdue
22
to ageing then they would likely also be suitable for studying all the processes
mentionedabove.
3.2 Materialsandmethods
The tars used for this study were recovered from a 20 kWth internal circulating
fluidised bed gasification and combustion reactor [63, 64] using the tar protocol (TP)
method [29]. The collected tars were then subjected to different storage conditions
where temperature, ageing timeandexposure to lightwerevaried; these samplesare
defined in Section 2.4. The results from analysing these samples are presented as
follows:
1. Amassbalance,showingtaryields,GC-FIDanalysisandultimateanalysisof the
bulksamples.
2. Planar chromatography (PC) [31,56] wasused to fractionate thebulk samples,
anddataobservedfromthePCfractions(withoutfurtherchemicalanalysis)are
presented.
3. PCfractionsfromthedifferenttarswerethensubjectedtofurtheranalysisusing
SECandLD-MStodeterminemolecularmassdistributionestimates.Synchronous
fluorescence spectroscopy (UV-F) was used to compare relative extents of
conjugation.Finally,thedatawerecombinedtomakepossibleinterpretations,in
terms of chemical reactivity of the tars based on known chemistry from the
literature.Thesemethodswerebasedonthosedevelopedinpreviousstudies[31,
47,56,60].
4. LD-MSwasthenusedtoanalysethebulktars,anddatacomparedwiththePCfor
amorerobustinterpretationofthewholematerial.
5. Finally, the aged samples exhibited precipitation. The final component of this
studywas to analyse this precipitated solid tarmaterial to gain further insight
intothetarageingprocess.
3.2.1 Standardsandsolvents
Polystyrene, amongst other polymer and PAH standards, was used to calibrate
the SEC system as previously reported [53, 54]. The solvents were from VWR,
23
(chloroform, heptane and acetone as HPLC grade), Rathburn Chemicals Ltd.,
Walkerburn, Scotland, UK (NMP – peptide synthesis grade) and used without further
purification.
3.2.2 Targeneration
The tarwasgeneratedusinga20kWth internal circulating fluidisedbed (ICFB)
gasifierwhichhasbeendescribedelsewhere[63,64].Gasificationtestswereperformed
usingpelletsmade from ablendofpinewood (ovendried,~12wt%moisture) anda
black Polish sub-bituminous coal (~8 wt% moisture), at a ratio of 7:3 wt%. The
propertiesofthefuelaregiveninSupportingInformation(S)SectionS1.Thecylindrical
pelletsizewas2mmindiameterand6mminlength.Thesewerefedatarateof4.0kg
perhour.
The tarwas collectedunder steamgasification conditionsat800°C,witha flow
rateof3.75Nm3/hofsteamand3.0Nm3/hofairtothegasifier;airtofuelequivalence
ratio0.15;steamtofuelratio0.75.Thecombustorwasoperatedwithanairflowrateof
11 Nm3/h. The fluidised bed contained approximately 12 kg of inert silica sand of
particlesize0.2-0.4mm.Gasresidencetimeinthebedwas1-2secondsand3-5seconds
inthefreeboard.
3.2.3 Tarrecovery
Thetar samplewas recoveredusingthetarprotocol(TP)method[29,30].The
methodusesanimpingertrainwhereaslipstreamoftheproducergas ispassedthrough
a series of seven bottles containing isopropanol, some at -20°C and others at 40°C.
Approximately0.1Nm3 oftheproducergaswaspassedthroughtheimpingertrainover
aperiodofonehourduringstableoperation.The isopropanol tarsolutionwaspooled
from the seven impingerbottles and filteredwithin twohoursof collection to remove
particulates(i.e.charandbedmaterial). A1µmglassfibrefilterwasused;thefiltrate
wasasinglephasesolution.
3.2.4 Tarstorage
To study the effect of storage conditions on the composition of tar, the tar
solutionswerestoredunderfourdifferentconditions,wherethetime,temperatureand
exposuretolightwerevaried,asdescribedbelowforeachsample:
24
TN2-0h:Thetarsample recovered in isopropanolwas frozen in liquidnitrogenwithin2
hoursofrecoveryandstoredinthisway.Immediatelypriortotheanalysis, thesample
wasthawedinthedarkat5°Candthenvacuumdried.Thisisdeemedtobe“freshtar”.
T5C-6m:The isopropanol tar solutionwas stored for6monthsat5°C, in theabsenceof
light,priortobeingvacuumdriedandanalysed.
T20C-6m: The isopropanol tar solution was kept at room temperate (~20°C), in the
absenceoflightfor6months,priortobeingvacuumdriedandanalysed.
T20C-6m-L: The remainder of the tar solutionwas left at room temperature in the fume
cabinet and thereby exposed to indirect sunlight for 6months prior to being vacuum
driedandanalysed.
Whilst these conditions were chosen in order to be systematic in varying
temperature,timeandexposuretolight,resultsfromsampleT20C-6m showedsimilardata
to thoseobtained fromT5C-6m. Therefore, forbrevityandclarity, thepresentedwork
will focus on the samples that showed the most significant differences between one-
another and highlight the benefits of using this analytical approach: TN2-0h, T5C-6m and
T20C-6m-L.TheresultsfromT20C-6m arenot shownordiscussedexplicitlysincethefindings
aresimilartothoseforT5C-6m.
Additionally, to study the effect of time during storage, aliquots of all the tar
solutionswere taken for GC analysis after 20 hrs, 3 days,20 days and 6months, and
compared to those from the fresh tar solution immediately after it was thawed from
liquid nitrogen (TN2-0h). The results obtained after 3 and 20 days storage provided
limited information and therefore will not be discussed herein, although they are
reported in S3. The GC analysis of the fresh tar (TN2-0h) was performed before any
precipitatewasobservedto haveformed,whileforthe20hoursoldsamplesprecipitate
had already formed and was removed before performing the GC analysis. Therefore,
comparingresults forTN2-0h withthesamplesagedfor20hoursgivesacomparisonof
thetarcompositionbeforeandafterprecipitation.
As an additional check, the thawedTN2-0h sample,whichwas observed to form
precipitatewithin 14 hours of standing (at 5°C in the dark),was also analysed byGC
afterremovingtheprecipitate.Itshouldbenotedthattheexactpointintimewhenthe
25
precipitate formedwas not determined.Moreover, this 14 hour old sample was only
usedtoexaminechangesinthetar’sGCcompositionbeforeandafterprecipitationand
wasnot subjectedtofurtheranalyses.
Themain focus of this study is the application of LD-MS, SEC and UV-F to the
analysisof thetarsamples.Onlythe freshtar (TN2-0h)andthe6monthagedtarswere
analysed by all of these techniques, not the intermediate samples mentioned above
whichwere analysed byGC only. A summary of the nomenclature, storage conditions
andanalyticaltechniquesthatwereappliedtoeachsamplearelistedinTable1.
Table 1, Nomenclature of the tar samples, their storage conditions and the analytical
techniquesapplied.
Label
Storage AnalyticalTechnique
Temp
(°C)Sun-Light Age GC
Recovered
Dry
weight
UA UV-F SEC LD-MS
TN2-0h -196 No <5hrs X X X X X X
TN2-14h -196 No 14hrs* X - - - - -
T5C-20h 5 No 20hrs X X - - - -
T5C-6m 5 No 6months X X X X X X
T20C-20h-L 20 Yes 20hrs X X - - - -
T20C-6m-L 20 Yes 6months X X X X X X
*14hrsreferstothetimethesamplewasstoredinafridge(5°C)afterbeingthawedfromliquidnitrogen
3.2.5Bulksamplepreparationandtaryieldcalculation
Gravimetric analysis and tar yield calculations were performed based on the
standardmethod described in detail elsewhere [29, 30] and only briefly summarized
here,usingthefollowingstepstoprovidefourfractions:
1. GCTotal:The tar-isopropanol solutionwas directly analysed byGC to quantify
thematerialintheGCrange.
26
2. Precipitate:The tar-isopropanol solutionswere filteredusinga glass-fibre filter
of1µmtorecoveranyprecipitate.Theprecipitatewaswashedwith five lotsof
freshisopropanol(10mLeach)beforebeingdriedinafanassistedovenat105°C
untilnosolventremained,approximately2hours.This fractionwasweighedto
yieldtheprecipitateweightinthesolution
3. Recovered Dry: The filtrate after the precipitated material was removed was
vacuumdriedfor4hoursatatemperatureof75°Cand10-3bar.Thismaterialwas
weighedtodeterminethemassofthedried,recoveredmatter.Anylightmaterial
wouldbelostduetovacuumdrying.
4. GC Recovered Dry: The Recovered Dry fraction was then redissolved and
analysedbyGC.ThedifferencebetweentheGCTotalandtheGCRecoveredDry
fractionisthemassofvolatileslostduringvacuumdrying.
TheTotalTarmaybecalculatedasfollows:
TotalTar=Volatiles+Precipitate+RecoveredDry
Where:Volatiles=GCTotal– GCRecoveredDry
Thebulksampleusedinallfurtheranalysiswasthe“RecoveredDry”fraction,i.e.
thetarwithoutprecipitateorlightvolatiles.Vacuumdryingwasusedhereasthesolvent
cannot be completely removed by other means. Additionally, vacuum drying was
employed to remove themost volatile components from the samples so that a better
comparison could be made between SEC and LD-MS mass estimates. Without this
vacuumtreatmentthe samples forSECandLD-MSwouldbequitedifferentduetothe
highvacuumintheLD-MSsamplechamberleadingtothelossofmoleculeswithmassof
lessthan~200u.Itisimportanttonote,however,thatthedryingprocesscouldresultin
changes in the sample composition (due to reactions) and it is not possible to isolate
theseeffectsfromthosethatoccurredduringstoragealone.
3.2.6Planarchromatography(PC)fractionation
27
Thebulk(dried)tarsamplesdescribedabovewerefractionatedbyusingplanar
chromatography to aid in their analysis. Aluminium backed PC plates of 20 cm2 with
silica gel thickness of 250 µm (Whatman, UK) were used. The plates washed with
acetone and then chloroform before use. The tar was dosed onto the PC plate as a
solution inchloroform;multiplesampledoseswereaddedat theoriginof theplate to
increasetheamountofsample.Allsampleswerecompletelysolubleinchloroform.The
PCplatewassuccessivelydevelopedwithchloroform,acetone,andheptane,beingdried
every time before the application of a new solvent. Figure 1 gives an example of the
mobility-fractions forthetarTN2-0h,FigureS2.1shows imagesof thePCplates fromall
three tars and provides further information. Each sample was eluted with the same
solvent three times consecutively, to the same height, before moving on to the next
solvent. No attempt was made to quantify the PC fractionation due to the inherent
difficulties involved; although the apparent abundances from visual inspection are
describedinSection3.2.
Inordertooptimizethesamplefractionation,aseriesoftestswereperformedto
select the solvents and order of elution. The aim of the PC separationwas to provide
relatively few discrete mobility-fractions and to isolate some material at the origin,
whichinpreviousstudieswascomposedofthehighestmassspecies[31,56].
This material, which was immobile in all of the eluents, was denoted as PC
fractionF1.Subsequentfractionsofmaterialwithincreasingmobility(higherupthePC
plate)weretakenatregularintervalsandgroupedbynumberbasedonthesolventthey
weremobilein.FractionF2wasacetonemobilebutchloroformandheptaneimmobile;
FractionF3waschloroformmobile,butheptaneimmobile.Thematerialatthefurthest
solventfrontwaslabelledfractionF4(heptanemobile);cf.Figure1andS2.1forimages
oftheplateswiththenumberingschemeoverlaid.
ForLD-MSanalysisthedifferentmobility-fractionswerecutoutfromthePCplate
anddirectlyadheredtotheLD-MStarget.
For SEC and UV-F analysis the sampleswere recovered by removing the silica
fromtheplatesandextractingthiswithNMP.TheresultingNMPsolutionwas filtered
(1µm).
3.2.7Ultimateanalysis(UA)
28
The carbon, hydrogen, sulphur and nitrogen contents of the dried bulk tar
sampleswere determinedwith a LECO-CHNS-932microanalyzer. The oxygen content
was obtained directly using a LECO-VTF-900 furnace coupled to the microanalyzer.
Thesesamplescontainedno moistureandno ash.Duplicateanalysesshoweddeviation
fromthemeanwerelessthan+/- 0.5%oftheabsolutevalue.
3.2.8Gas-chromatography(GC-FID)
GCwasusedtostudythesmallermoleculesinthesamples.TheGCanalysisofthe
fresh tar(TN2-0h)wasperformedimmediatelyafter thawing.Thesamplewas inspected
after14hours(at5°C intheabsenceoflight)anditwasobservedthatprecipitatehad
formed. Therefore the precipitate was removed by filtration and GC analysis was
performedonthesupernatant.ComparingtheGCresultsfromTN2-0h andafter14hours
storagegivesacomparisonofthetarcompositionbeforeandafterprecipitation.Asan
additional check, the samples storedat5°C in thedarkandat20°C in thepresenceof
indirectsunlight,whichwereobservedtoformprecipitatewithin20hoursofstanding,
wasalsoanalysedbyGCafterremovingtheprecipitate(Table3).Itshouldbenotedthat
these samples were not checked between the ages of 5-20 hours due to practical
experimentalconstraints.
AVarianGC3800-FIDwithaVarian‘FactorFour’capillarycolumn(VF-5ms30M,
0.25mM ID, DF = 0.25) was used to quantitatively examine the tars. 5 μL injection
volume (split ratio 50), helium carrier gas (2 mL/min), injector temperature 325°C,
columnstartingtemperatureof50°Cfor4minutes,25°C/minrampto150°C,5°C/min
ramp to325°C,heldat325°C for2minutes.FIDdetector temperaturewas325°C.For
theGCanalysis avolumeof the isopropanol-tar solutionwas takenbefore the sample
wasvacuumdried.Furtherdetailsofpeak identification,standardsandcalibrationare
providedinS3.
3.2.9Sizeexclusionchromatography (SEC)
The operating conditions andmethodology have been reported elsewhere [53,
54].Briefly,aMixed-Dcolumn(5µmparticlesize,300mmx7.5mmi.d.)packedwith
polystyrene/ polydivinylbenzene beads, was operated at 80°C with a Knauer M100
isocraticHPLCpump. NMPwasusedaseluent(0.5mLmin-1)andsolvent.
29
Material eluting from the columnwas detected by UV-absorbance at 270, 300,
350,and370nm.Theresultsobtainedat300nmareconsideredrepresentativeofthe
maintrendsobservedatallwavelengths;onlythoseresultswillbeshownanddiscussed.
The SEC system was calibrated using standard polystyrene (PS),
polymethylmethacrylate (PMMA) andpolysaccharide (PSAC) samples, aswell as small
standardPAH,O-PAHandN-PAHcompounds[53-55].
In the conversion of elution time tomass estimate, thematerials eluting early
from the column (<15 minutes) which is excluded from column porosity cannot be
accuratelyaccountedfor.Thisisbecausethenatureofthisearlyeluting(apparentlyhigh
mass)materialremainsuncertainandisthoughttohaveahydro-dynamicvolumewhich
departs from those of the calibration materials, possibly due to three-dimensional
conformationwheresizeisnoteasilyrelatedtomolecularmass[53,55].Previouswork
onsimilarmaterialshasshownthisearlyelutingmaterial(<15minutes)tobeofhigher
averagemolecularmass(>2500u) thanthe laterelutingmaterials [55].Theseaspects
andtheapplicationofthiscalibrationaredescribedinsaidpublicationsandoutlinedin
S4.
3.2.10Laserdesorption/ionisationmassspectrometry(LD-MS)
ABrukerDaltonicsReflexIVMALDI-TOFmassspectrometerwasusedforLD-MS.
Nomatriceswereusedbecause the samples in this studybehaveas self-matrices [31,
56].Nomatrixdeflectionvoltagewasused;anitrogen laserof337nmwasemployed.
Themethodappliedinthisstudyhasalsobeendescribedindetailelsewhere[31,56].
Linear-modewasusedwithadelayedionextraction(DIE)timeof0,and600ns.
The mass range was m/z 0-300,000; Ion source 2 = 16.5 kV and Lens = 9.5 kV. The
digital gain (DG)was set to its lowest level (1x). In all cases shown, 10 spectrawere
addedusingthepulsed ionextractionmethodonthesamepoint.50spectrawerealso
addedforsomeofthesamples;thesespectrawerefoundtomatchthosewhereonly10
spectra were summed, with an increased signal-to-noise ratio. Due to time
considerations theadditionof10 spectraof eachanalysiswas consideredsatisfactory
forthepurposeofthisstudy.
30
AHIMASdetector (Bruker’s ‘high-mass detector’) operating in the linearmode
wasusedto investigatethehighermolecularmass region.Detectionofhighmass ions
canbeenhancedthroughtheuseofavariablehighmassaccelerator(HMA)voltage.
Whenthebulksampleswereanalysed,theionintensityofthesmallermassions
was reduced (to avoid overloading the HIMAS detector) by reducing the high mass
acceleratorvoltagefromthemaximumvalueof10kVto6kV.Atthesametime,thelaser
powerwasincreasedbeyondthatnecessaryforionisationofthesmallmoleculesofthe
samples.Thisistoexaminetheinfluenceoflaserpoweronthemassdistribution,which
is generally significant. A laser power of 30-50% (of the maximum available) was
typicallyfoundtobeadequatetoionisethesamples.Thebulktarsampleswereapplied
totheLD-MStargetneatfollowingtheproceduredescribedelsewhere[31,56].Mainly
theresultsobtainedwithaDIEof600nswillbereportedanddiscussed inrelationto
the‘bulk’samples,forthereasons outlinedinS5.
DuringtheexaminationofthePCmobility-fractions,theHMAvoltagewassetto
itsmaximumvalue(10kV)inallcases.Bydesorbingthesampledirectlyfromthesilica
surfaceofthePCplate,highlaserpowerscouldbeappliedwithnoresultingsignificant
changeofthemassspectra,apart fromproducingflat-toppedpeaksduetooverloading
ofthedetector.ForthePCfractions,onlyLD-MSspectraacquiredinlinear-modewithno
DIEareshown.AcleansilicasurfacefromthePCplatewasfoundtogivenoobservable
ioncurrentunderanyoftheLD-MSoperatingconditionsused[56].
AmajorobstacletotheuseofLD-MSisthevariabilityofthe datadependingon
theconditionsandtheoperator[31,56].Theapproachusedinthisstudywasfoundto
giveconsistentfindings independentoftheoperator.Fouroperatorsrecordedthedata
reported. This provides adegree of confidence in the reproducibilityof the approach.
TheresultsfromLD-MSarenotquantitative.
3.2.11UV-fluorescencespectroscopy(UV-F)
ThePerkin-ElmerLS55bluminescencespectrometerwassetwithaslitwidthof5
nm,toscanat500nmmin-1;synchronousmodefluorescencespectrawereacquiredata
constantwavelengthdifferenceof20nm.Aquartzcellwith1cmpathlengthwasused.
Theprocedure hasbeendescribedelsewhere[31,47,65].
31
Thespectrometer featuredautomatic correction for changes in source intensity
as a function of wavelength. Emission, excitation, and synchronous spectra of the
sampleswereobtainedinNMPsolutionforallofthesamples; onlysynchronousspectra
are shown. Solutionswere dilutedwith NMP to avoid self-absorption effects: dilution
wasincreaseduntilthefluorescencesignalintensitybegantobothdecreaseinintensity
andtherelativeintensitiesofthedifferentmaximainthespectraceasedtochange.The
UV-F spectra are displayed as peak normalised because this enables relative
comparisonsbetweenthesamples;theresultsarenotquantitative.
To ease the discussion of the UV-F data the results will be described with
referencetotheapproximatenumberofconjugatedaromaticringsthatwouldfluoresce
at the equivalent wavelength to the sample in question. This is based on a recently
noticed correlationbetween thepeakmaximum fromUV-F in synchronousmode, and
thenumberofconjugatedaromaticringsasdeterminedbyNMRandaveragestructural
parametercalculationsforcoalandpetroleumderivedsamples[47,60].Themaximum
intensity of fluorescence shifts steadily to longer wavelengths by about 30 nm per
additionalaromaticringinaconjugatedaromaticsystem,where1ring=270nm,2rings
=300nm,andsoon.
It shouldbenotedhowever, that someof thetar samples contain slightlymore
oxygen than the coal andpetroleumderived samples studied previously,which could
affect the UV-F results. The influence of oxygen on the UV-F spectrum of large PAH
molecules(>500u),however,isnotwellenoughunderstoodtobeabletocommenton
its effects in detail. The definitions outlined above are not a literal description of
chromophores such as those present in the tars but as ameans to discuss qualitative
differencesbetweensamplesandtheirdominantfeatures.Furtherdetailsareprovided
inS6regardingtheinterpretationoftheUV-Fresultsandtheinfluenceofoxygen.
3.3 Resultsanddiscussion
3.3.1 Tar-yields,GC-FIDandultimateanalysis
32
The tar TN2-0h and the aged samples were analyzed by means of the standard
approach (GC-FID and gravimetric yield) [30] as described in Section 2.5. TN2-0h was
analysedimmediatelyafterbeingthawedfromliquidnitrogenstorage;T5C andT20Cwere
both analysed after 20 hours and 6months of storage; the results are summarized in
Table2.TN2-0h wastheonlysamplethatwasanalysedbeforeanyprecipitationoccurred,
whereasprecipitatewasobservedforallothertarsolutionswithin20hoursofresting.
Theprecipitatewasremovedfromthesolutionsbeforetheywereanalysed.
The yields are presented in Table 2 for each tar as determined after different
periods of storage, results are normalised to grams per normal cubic meter of the
producergas(g/Nm3).ErrordeterminationisdescribedinS3.
Table2:Taryieldsafterdifferentdurationsofstorage(g/Nm3producergas)
SampleAgeTN2 T5C T20C
g/Nm3
GCTotal
0hours 5.2
20hours 4.6 4.9
6months 2.3 2.9
Precipitate
0hours 0.0
20hours 0.06 0.09
6months 0.03 0.07
Total 0.09 0.16
RecoveredDry
0hours 1.3
20hours 1.4 1.4
6months 1.8 1.9
GCRecoveredDry
0hours 0.6
20hours 0.6 0.6
6months 0.2 0.2
Volatiles 0hours 3.9
33
20hours 3.2 3.5
6months 0.5 1.0
TotalTar
0hours 5.9
20hours 5.5 5.8
6months 4.0 4.8
Relativedeviations+/- 5%forGC,15%forRecoveredDry,and30%forprecipitate.
Blankdenotesnotapplicable.
The mainfindingsfromstudyingthetaryields(Table2)werethatTN2-0h contains
thegreatestproportionofmolecules intheGCrangeandthe lowest inthegravimetric
range,withnooccurrenceofprecipitation.After6months storageT5C-6m andT20C-6m-L
showadecreaseinthequantityofGCrangemolecules(from~5g/Nm3 to~2.5g/Nm3)
andanincreaseinRecoveredDrytars(from1.3g/Nm3to1.8-1.9g/Nm3),comparedto
TN2-0h(errorsaregiven in the footnotetoTable2andS3).Precipitatewasobserved in
these samples. These results are an indication of a change inmass distributionof the
storedsamplestowardshigheraveragemolecularmasses,which is inaccordancewith
reportedstudiesofpyrolysisliquids[15,16,18,19,21,22].
ThedifferencesbetweentheGC results fromT5C-6m andT20C-6m-L wererelatively
minor after 20 hours of storage (Table 3). Both samples also exhibit the presence of
precipitate,toaslightlylargerextentinT20C-6m-L,althoughtheerrorinthedetermination
hastobeconsidered(cf.Table2andS3).Thetarsolutionswereregularlycheckedover
a 3 month period and no further precipitationwas observed; however, when the tar
solutionswerecollectedafter6monthsstorageprecipitatewasapparentinbothcases.
Todetermineiftheprecipitate(after20hoursand6months)containedanyGC
range species its dichloromethane soluble fraction was examined; no evidence of GC
rangemoleculeswasfound.Therefore,aggregation ofsmallmoleculescanberuledout
asamechanismfortheprecipitateformation.
Bycomparingthetaryieldsforthestoredtars(T5C andT20C-L)after20hoursand
6months,alarge reductionintheGCrangecomponentsisevident,by~2.0-2.5grams
34
per Nm3 forbothsamples(Table2).Ifthosemoleculeshadreactedtoformlargerones
they would be observed by an equivalent increase in the ‘Recovered Dry’ and
‘precipitate’yields.However,thegravimetricyieldsonlyincreasedby<1gramperNm3
andtheprecipitateswerelessthan200mgperNm3.Itislikelythatthediscrepancyis
duetosomeoftheGCrangecompoundsbeinglosttotheglasswareduringstorage.This
is thought to be due to attractionand adhesion ofmolecules to the glass surface, not
necessarilyduetoreactionsfollowedbylosetothesurfaces.Otherfactorsaffectingthe
quantification include evaporation of solvent and/or volatile compounds over the 6
monthstorageperiodandwaterformationduringtheageingreactions(notquantifiable
byGC).Infuturestudiesitwouldbebeneficialtodeterminethewatercontentusinge.g.
aKarl-Fischertitration[66].
The tar solutions were stored in ground glass stoppered bottles and sealed with
parafilm; however, some loses due to evaporation are still thought to have occurred
after 6months of storage and during sampling. This is thought to be the case as the
greatest changeswere in the naphthalene concentrationwhich is quitevolatile and is
unlikelytobereactiveunderthestorageconditions.
ToseeifGCcanprovideinformationontheprecipitationprocess,thethawedTN2-0h
solution was examined before any precipitation occurred, and then again after
precipitation hadoccurred (after filtration to remove the precipitate).Theprecipitate
formed within 14 hours of standing in the dark at 5°C after thawing. No significant
differenceswere observed for GC tars in TN2 before and after precipitation (Table 3).
Specifically, comparing these resultswith those from the20hoursold samples,which
hadprecipitatedmaterial,showsthatonlysmallchangesoccurredintheGCtars(Table
3). There is a slight decrease in acenaphthylene and phenanthrene, the decrease in
naphthalene is only slightly greater than the deviation. The only other statistically
significant changes are a slight increase in the amounts of benzo[a]pyrene,
indeno[1,2,3]pyreneandbenzo[g,h,i]pyrene.
Of the molecules detected in the GC range acenaphthylene and phenanthrene are
likely tobethemostreactive,but therewas littleevidence forthesehaving reacted in
the14houroldsample(Table3).Thissuggeststhedifferenceobservedinthe20hours
35
old sampleswere not related to the formationof precipitate andweremost probably
duetolossesofmaterialtotheglasswareandotherexperimentalerrors.Itwouldseem
that the precipitate is primarily formed by reactions between molecules that are not
detectedbyGC.
Theseresults andthetaryieldsarediscussed further inS3,asarethesourcesand
scalesoftheerrors.TheweightoftheprecipitatefromtarTN2 after14hourscouldnot
bedeterminedduetothesmallvolumeofsolutionfrozeninliquidnitrogen;hence,not
enoughmaterialcouldberecoveredtoweigh.
Table3,GC-FIDresultsforthetarsamplesbeforevacuumdrying,displayedasmgper
cubicmeterofproducergas.Relativeerrorswerelessthan+/- 5%.
SampleName
TN2 T5C T20C
mg/Nm3
0h 14h 20h 20h
Naphthalene 3000 3000 2600 2700
Acenaphthylene 710 700 640 670
Acenaphthene 10 10 10 10
Fluorene 50 60 40 50
Phenanthrene 460 450 380 410
Anthracene 90 90 80 80
Fluoranthene 230 240 220 240
Pyrene 240 250 220 240
Chrysene 30 30 20 30
Benzo[a]anthracene 30 40 30 30
Benzo[k]fluoranthene 30 40 30 30
36
Benzo[b]fluoranthene 10 10 10 10
Benzo[a]pyrene 40 60 40 40
Indeno[1,2,3-cd]pyrene 30 40 20 20
Dibenz[a,h]anthracene 5 10 5 5
Benzo[g,h,i]perylene 5 30 5 20
TotalEPA16 5000 5100 4400 4600
Unknowns 280 280 230 270
GCTotal 5300 5400 4600 4900
Ultimateanalysis(UA)wasusedtogain further informationabout thetars.The
resultsobtainedforsamplesTN2-0h,T5C-6mandT20C-6m-L (Table4)showanincreaseinthe
amount of oxygen in T20C-6m-L compared to T5C-6m and a corresponding decrease in
hydrogen.TN2-0h showsthehighestC/Hratio.TheamountofoxygeninTN2-0h wasgreater
thaninthecoldstoredtar,but lessthantheroomtemperaturesample.Thetrendsinthe
UAresultswererepeatable.Table5representsthesameresultsintermsofthenumber
ofatomsinamoleculewithamassof500u,itcanbeseenthatthenumberofoxygen
atomsrangesfrom0.2to1.4permolecule,whereasthereare36to38carbonatoms.
Taken together these results indicate that in TN2-0h (no precipitate) there are
significant quantities of highly aromatic PAH and oxygen containing PAH compounds.
Afterprecipitation(i.e.theagedsamples),lesscarbonisdetectedinthemoleculesthat
remained in solution andmore hydrogen, and the C/H decreased. Therefore itwould
seemthat themosthighlyaromaticcompoundshadprecipitated. InT5C-6m lessoxygen
wasfoundthaninthefreshtarwhichsuggestsmanyoftheO-PAHcompoundshavealso
precipitated.ThematerialthatremainedinsolutionforT5C-6m hasalowerC/Hratiothan
thedissolvedtarsinT20C-6m-L whichsuggeststhemoleculesarelessaromatic.InT20C-6m-L
moreoxygenwasobservedthan inTN2-0h which indicatesthissamplehasprecipitated
37
mainlyPAHsandpossibleundergoneadditional reactionswithoxygen from theairor
thesolventduringitsstorage.
The increase in nitrogen and sulphur content of dissolved tars observed with
ageingsuggests thatmoleculescontainingNorSdonotprecipitateasreadilyas those
that do not contain them; therefore N and S become concentrated in the solution.
However,theabsolutedifferencesinNandSbetweenthesamplesareminor,Table5.As
these findings are somewhat unexpected and only duplicate analyses of each tarwas
possible (due to limited amounts of sample), future studies should investigate these
aspectsmore closely to confirm these results. Inaddition, theprecipitate couldnotbe
examinedbyUA in this studybecause theamountof sample recovered from the filter
was insufficient for theanalysis. In future studies, adifferent filtrationmethodwill be
employed and larger sample volumes treated so that UA can be performed on the
precipitate.
Table4,Ultimateanalysisofthevacuumdriedtars,TN2-0h andafter6monthsstorage.
Element TN2-0h T5C-6m T20C-6m-L
Wt.% Wt.% Wt.%
C 91.5 86.6 86.9
H 5.5 11.6 7.3
N 0.3 0.6 0.6
S 0.5 0.8 0.7
O 2.2 0.5 4.5
Deviationestimatedat+/- 0.5%oftheabsolutevalue,basedontworepeats.
Table5,Numberofatomsinatarmoleculewithamassof500u,forTN2-0h andafter6
monthsstorage.
Element TN2-0h T5C-6m T20C-6m-L
38
Numberofatoms
C 38 36 36
H 28 58 37
N 0.1 0.2 0.2
S 0.08 0.13 0.11
O 0.7 0.2 1.4
C/H 1.4 0.6 1.0
Summarizing the results from the standard/commonly applied techniques (GC,
gravimetricweight andUA) for studying tars and oils fromgasification and pyrolysis,
changes to higher mass materials in the aged samples can be implied. However, the
differencesobservedcouldinprinciplealsobeduetootherreasons,suchaschangesin
polarity/solubilitywithageing.Thiscouldreducetheamountofcompoundsdetectedby
GCandproduceprecipitate.The information obtained from these techniquescan thus
only be regarded as indicative. In addition, there was no evidence for a significant
changeintheGC-FIDresultsintarTN2 beforeandafterprecipitationofmaterial,Table3.
To obtainmoredetailed information about changes inmolecularmass distribution or
structuralfeatures(conjugation)duetoageing,techniquessuchasSEC,LD-MSandUV-F
canproveuseful, aswillbedemonstratedbelow.
3.3.2 PCfractionationoftheRecoveredDrytars
Toaid the investigationof thevacuumdried tarsand thematerial they contain
beyond the range of GC the Recovered Dry material as defined in Section 2.5 was
fractionated via planar chromatography (PC). Themethodology behind this analytical
approachhas beenreportedelsewhere[31,47,56].ImagesofthePCplatesbearingthe
tarsamplesafterdevelopmentareshowninS2,FigureS2.1.Anexampleisshownbelow
inFigure1forthetarTN2-0h:
39
Figure1, PCplatebearingthevacuumdriedtarTN2-0h afterbeingelutedwith chloroform(F3)
followedby acetone (F2) and finallyheptane (F4).The imageon the left-hand sidewas taken
underwhitelight,andtheright-handsideunderUV-light(260nm).
ThePCfractionationisnotquantitative;nonetheless,itwaspossibletomakearelative
comparison between the apparent amounts of material in each mobility-fraction by
visual inspection. The apparent order of abundance of the PC mobility-fractions was
determined internally for each tar sample and is listed below (the main fluorescent
bandsareshowninbold):
TN2-0h F3c≥F4a≥F4b ≥F3b >F3a ≥F2>F1
T5C-6m F1>F2≥F4b ≥F4a>F3c(noF3b,noF3a)
T20C-6m-L F2≥F1>F3b >F3c>F4a(traceF4b,noF3a)
Where F1 = immobile, F2 = acetone mobile / chloroform and heptane immobile, F3
chloroformmobile/heptaneimmobile,F4heptanemobile;thelabelsc,bandareferto
materialwithdecreasingmobilityinthesamesolventasdenotedbythecorresponding
number– i.e.theyarenot associatedtorepeatedelutionwiththesamesolvent.
BycomparingtheapparentorderofabundanceforTN2-0h withthoseoftheaged
tars indicative information regarding changes in composition can be obtained. Only
significantdifferencesarementioned.TN2-0h containslessoftheimmobilematerial(F1)
F4bF4aF3c
F3b
F3a
F2
F1
40
relativetoitsotherfractionsthanwasthecaseforthe6monthagedsamples.Itwasalso
evidentthattherewassomefluorescentmaterialassociatedwiththeimmobilematerial
(F1)inTN2-0h thatwasnotseenfortheothersamples.
Theagedsamplesallappearedtocontainsimilaramountsofimmobilematerial,
althoughT5C-6m wastheonlytar thatappearedtocontainmorematerial in fractionF1
than inF2.Another cleardifference wasobservedbetweenT5C-6m andT20C-6m-L,where
themainfluorescentbandfromT5C-6m wasF4b(notobservedforT20C-6m-L)whileforT20C-
6m-L it was F3b (not observed for T5C-6m). In addition, the highly fluorescent material
observed inF3aofthe fresh tarwasnotseen inthe6monthagedtars.These findings
aredifficult to interpreton theirown (i.e.without thedataLD-MS, SEC andUV-Fwill
provide).However,theydoprovideclearevidenceofachangeincompositionofthetar
uponageingthatis dependentonthestorageconditions.Furtherdetailsregardingthe
planarchromatographyresultsaregiveninS2.
3.3.2.1SEC,LD-MSandUV-FanalysesofPCfractions
ToexaminetheRecoveredDrytarscomprehensivelytheirPCmobility-fractions
wereanalysedbySEC,LD-MSandUV-F.Thefindingsfromtheseanalyseswereusedto
determinethemassrangeandextentofaromaticconjugationforeachtar.Astheseare
notstandardreferencetechniquesforanalyzingtars(theyareinthevalidationphaseof
development) the completedata sets for each samplearepresented in the supporting
information(S7-S9).Forbrevity,asummaryofthekeyfindingsfromtheanalysisofthe
PC fractions is presented below with a complete account provided in S7. To aid the
comparisonoftheequivalentPCfractionfromdifferentsamplestheLD-MS,SECandUV-
F results are displayed by PC fraction in S8. To observe trends between PC fractions
fromasinglesampletheresultsarealsoshownbysample inS9.Someofthesefigures
arealsoshowninthearticletohighlightthemainfindings.
Table6 summarizes themass estimates from SECandLD-MS for thePC fractions; the
methodology for estimating average mass and mass ranges/distributions has been
described previously [47, 55, 56]. To discuss the results and draw conclusions the
following assumptions are used, but this does not however discount other possible
41
assumptions and interpretations. Further work is needed to confirm the assertions
outlinedbelow.
1. The results fromTN2-0h are considered to represent themoleculesoriginally
presentinthetar.
2. WhenUV-FsignalisobservedinaPCfractionfromTN2-0h butthesamesignal
isnotobservedintheequivalentfractionfromtheagedtars(forexamplePC
fraction F3c, Figure 2a), the molecules containing the chromophore
responsible for the fluorescence are assumed to have reacted (either with
another tarmolecule,orwith the solventoroxygen in theair).Hence these
molecules have different mobility during PC and are found in different
locationsorasprecipitate.
3. Themost probable ageing reactions are those involving tarmoleculeswith
highly conjugated aromatic systems eitherwith themselves or with oxygen
thatispresentinthesolventortheair.Thesereactionsareunlikelytocausea
significant change in the chromophore (conjugation) present in the original
tarmoleculewhenobservedbyUV-F if theyproceedas showninFigure3a.
WenotethattheadditionofacarboxylgrouptoaromaticsasshowninFigure
3b (in the presence of sunlight) can have two effects, depending on the
positiontheyhaveontheringsystem.Anaddedcarboxylicgroupcaneither
destroyconjugation,or increaseconjugation(causingaredshift) [67],cf. S6
forfurtherdetail.However,duetothelowoxygencontentsofthesetars,these
effectsareunlikelytosignificantlyinfluencetheresults.
4. Some of the UV-F results show that molecules with significantly greater
conjugationareproducedduringageing(suchasthoseintheprecipitateorF1
fromT5C-6m,discussed below).Inthesecases,itispossiblethatPAHmolecules
havereactedwithoneanotherresultinginlarger,highermassmoleculeswith
increased aromatic conjugation (the role of oxygen in this process is
42
unknown).Therearenoreportsof the formationofaromaticcarbon-carbon
bonds, or of deoxygenation as shown in Figure 3a, happening at room
temperature.Thereisalsolimitedinformationforoxygenactinginawaythat
increases conjugation in large aromatic molecules (>500 u) at room
temperature.However, the shift of fluorescence to higherwavelengthswith
ageinghastobearesultofoneoftheseinterpretations,orfromacombined
effect.
5. Fragmentation of some molecules occurs during LD-MS analysis. This is
probablyduetonon-conjugatedbonds(suchasaliphaticoroxygencontaining
bridges)betweenaromaticcores.Structuressuchasthesewereidentifiedin
recentstudiesofpetroleumsamplesasbeingthemostdifficulttoobserveby
LD-MSduetoexcessivefragmentation[47,56,61].
6. WhenSECshows evidence for largermolecules thanwitnessed fromLD-MS
this is an indication that the larger molecules fragment during ionization,
hence the mass of the parent molecule is not detected or is greatly
underestimated byLD-MS.
43
Figure2a,SynchronousUV-Fspectra(peaknormalised)ofPCfractionF3c.
Figure 2b, LD-MS spectra of PC fraction F3c at low and high laser power (left and right
respectively),noDIE,HMAvoltagewas10kV.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600
Wavelength / nm
Norm
alis
ed Inte
nsity
T-N2-0h F3c
T-5C-6m F4a + 3c
T-20C-6m-L F3c
Solvent
10 100 1000 100000
200
400
600
800
1000
1200
Ion C
ount
m/z
T-20C-6m-L LP35% T-5C-6m LP60% T-N2-0h LP65%
10 100 1000 100000
400
800
1200
1600
2000
Ion
Cou
nt
m/z
T-20C-6m-L LP45% T-5C-6m LP70% T-N2-0h LP80%
44
Figure2c,SECchromatograms(areanormalised)ofPCfractionF3cat300nm.
In addition to the assumptions above, information from the literature can also
giveinsightintopossiblemechanismsofreaction,andthisisdiscussedherebeforethe
dataareinterpretedbelow.Fromthelittlethatisknownaboutreactionmechanismsof
large PAHs, alkyl aromatics and oxygen containing aromatic molecules, it is the
molecules containing the largest conjugated chromophores that are thought to be the
mostreactive[47,60,68-70].Oxygencanreactwithmoleculessuchasthese,undermild
conditions (260-290°C [69]), and is thought to cause cross-linking through carboxylic
(Figure3a)andpossiblyetherbridgesbetweenaromaticcompounds[14,68].
It has been reported that PAHs with aliphatic hydrogen are themost reactive
during oxidative thermal treatments of oils and pitches [70, 71]; i.e. polymerization
reactionsbetweenPAHsappeartoproceedviacross-linkingwheretheactivesitesare
mainly located on the aliphatic groups (the radical chainmechanism). The amount of
oxygeninthereactionproductsincreasesbyafewweightpercent;whereasthechanges
inmassdistributionandaromaticityaremoreprofoundthanwouldbeexpecteddueto
theincorporationofoxygenalone[60,69,71].Theexactroleofoxygenremainsunclear.
Itisknownhowever,thatfurtherthermaltreatment(>440-460°C)ofthereaction
productsfromoxidativethermaltreatmentcanleadtoremovalofoxygenandresultsin
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
7.0E-03
8.0E-03
9.0E-03
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Rela
tive Inte
nsity
T-N2-0h F3c
T-5C-6m F4a + 3c
T-20C-6m-L F3c
45
anincreaseinaromaticityasshowninFigure 3a,andthesizeoffusedaromaticcarbon
ringsystems[68-71].Thereisnoreportedevidencehowever,ofthishappeningatroom
temperature. It shouldbe noted that the incorporation of oxygen into alkyl aromatics
and their decomposition as described above, and shown in Figure 3a, would not
significantly alter the conjugation of the molecules and is unlikely to significantly
influence their UV-F spectrum (in synchronousmode). This is because previous NMR
andUV-Fstudiesofsamplessimilar tothosebeingexaminedhere(anthraceneoiland
their air-blown reactionproducts [31, 60,69,71]) found the aromatic coreswere the
leasteffectedduringthefirstphaseofair-blowingreactions;thereactionsoccurmainly
at aliphatic sites in themolecules that contain the largest aromatic chromophores, as
mentionedabove.Thesetypesofreactionsproceedintheabsenceoflight.
In the presence of sunlight a different mechanism can occur leading to
incorporationofoxygenintoPAHmoleculesthroughreactionswithoxygenfromtheair
[72-74].Themechanismproposedisthroughsingletoxygenformedbyenergytransfer
fromaPAHmoleculeinitstripletstate(formedviaUV-VISphoto-excitationofthePAH
groundstatefollowedbyintersystemcrossing).TheoxygencanbondtothePAHgiving
peroxideswhichareinturnphotolysetogivecarbonylsandeventuallyhydroxyls[72].
Figure 3b displays a simplified example of the photo-oxidation of phenanthrene to
phenanthrenequinone[72].Despitethedepictedapparentsimplicityitisimportantto
recall that the formation and breaking of peroxides implies the possibility of a large
rangeof(radical)reactionsincludingfurtheroxidationwithgroundstateoxygentoform
e.g.moreperoxides,radicals,carbonylsandhydroxylgroups,butalsofragmentationand
polymerisationofmolecules.
Hydroxyl groupsonPAHmolecules can react further leading topolymerization
processes occurring as described in reference [14]; this provides a comprehensive
reviewofpossiblereactionsofoxygenatedhydrocarbonsthatoccurinbiomasspyrolysis
oils. However, most of the reactions described therein are unlikely to occur in the
gasificationtarsbeingexaminedduetotheirmuch loweroxygencontents(lessthan5
wt%)thanbiomasspyrolysisoils(15-30wt%).Therefore,theinfluenceofoxygenonthe
resultsfromthesetarswillberelativelyminor. Inaddition,withoutinformationonthe
oxygencontentoftheprecipitateitisfutiletospeculateonoxygen’sroleintheageingof
46
this tar, or without NMR, FT-ICR-MS and FT-IR data. It is intended to address these
aspectsinfuturestudies.
Ingasificationandpyrolysistarsandoilsfreeradicalsarealsolikelytoplayarole
inageingreactions;however,noinformationisavailableontheiryieldinthesetarsor
theirroleduringageing.Additionally,thetarmoleculesrecoveredfromgasificationare
not thermodynamically stable, as their residence time at the high temperature in the
gasifierisinsufficientforequilibriumtobereached[1];itisthereforealsopossiblethat
part of the aging involves molecules re-arranging slowly to more energetically
favourable configurations, that they would have reached more quickly at high
temperature in the gasifier. However, detailed information on these processes is not
available forthesizesandstructuresofmoleculesbeing investigatedhere. Inaddition,
tracesofashorcharcouldremaininthetarsolutionevenafterfiltrationandifpresent
couldwellplayaroleintheageingprocesses.
Figure 3a, Simplified schematic example of the incorporation of oxygen into alkyl
aromaticsanddecompositionleadingtocross-linkingorincreasedlevelsofaromaticity
[68].
CH3
COOH
O
O
O
O
O
+ O2
+ O2
-CO2
-CO2
-CO2
-CO
47
Figure 3b, Example of the photo-oxidation of phenanthrene to phenanthrene quinone
[72].
Table6,Molecularmassestimates forthePCfractionsof theRecoveredDrytars from
SECandLD-MS.Theresultsarepresentedaspeakmaximumvaluesforthemainbands
ofsignal inorderofabundance; for theLD-MSresults therangeoverwhich ionswere
observedisalsogiven.
TN2-0h T5C-6m T20C-6m-L
SEC LD-MS SEC LD-MS SEC LD-MS
Fraction Definition Mass/u Massm/z Mass/u Massm/z Mass/u Massm/z
F4bPeakMax 150 210 140 200
Range <200-400 <200-300
F4a
PeakMax 190 260 180+ 250 160 325
2nd Peak 1100 500 1000+ 600 1000 650
Range <200-600 220-1500 250-1250
F3c
PeakMax 220 275 180+ 330 180 350
2nd Peak 900 1000+ 1000 650
Range 250-550 200-1100 220-1100
F3b
PeakMax 275 <200 460
2nd Peak 900 >2000 <200
3rd Peak >2000
Range <200-400 <200-750
F3aPeakMax 220 <200
2nd Peak 900
O
O
O2
UV-light
48
3rd Peak >2000
Range <200-800
F2
PeakMax 210 550 200 700 650 600
2nd Peak 650 600 >2000
3rd Peak >2000 >2000
Range 200-1300 220-1500 220-1500
F1
PeakMax >2000 650 >2000 725 >2000 725
2nd Peak 1000 900 1300 1100
Range 200-2000 250-2000 300-1900
+SECresultsforF3candF4afromT5C-6m arefromthecombinedF3candF4afractionsas
theycouldnotberecoveredseparately.
Combining SEC, LD-MS, and UV-F analysis: When results for equivalent PC
mobility-fractionswhere compared from the different samples it was generally found
theydidnotcontainidenticalmolecularmassesorstructures(aswasanticipated).Often
they shared common features but show significant differences in either mass
distribution or extents of conjugation (or occasionally both) due to the ageing
conditions. It is thus an important finding that it is not correct to simply assume that
equivalent PC mobility-fractions represent the same component(s) in different tar
samples.
Forexample,Figures2a-2cshowtheUV-F,LD-MSandSECspectra,respectively,
forPCfractionF3cfromthefreshandagedtars.AsignificantlydifferentUV-Fspectrum
was obtained from F3c from the fresh tar than the aged tars. Larger aromatic
chromophoreswere observed in TN2-0h than in the aged tars. Differences between the
LD-MSspectra are alsoapparent,where F3c fromTN2-0h showed the lowestm/z peak
maximum, a narrower distribution at low laser power and extensive fragmentation
when a higher laser power was used as well as some new ions at m/z >400. T5C-6m
showed a similar m/z peak maximum to TN2-0h but has a much greater tendency to
fragment during ionization and produce awiderm/z range of ions.No fragment ions
49
wereobservedfromPCfractionF3cofT20C-6m-L andagreaterabundanceofhighermass
ions(m/z>400)weredetectedherethanintheothertars,Figure2b.
SEC shows that fraction F3c from the fresh tar contains more larger-sized
moleculesthanthesamefractionfromtheagedtars(Figure2c).Thetwoagedtarsgave
almost identical SEC chromatograms; whereas their LD-MS spectra are noticeable
different. This is partly due to the lower resolution of SEC than LD-MS and may be
evidenceofthelargestmoleculesobservedbySEC(inTN2-0h)fragmentingduringLD-MS
analysis.Furtherworkisneededtobetterunderstandthedifferencesobservedbetween
theSECandLD-MSresults.
It is possible that themolecules containing the largest aromatic chromophores
present in F3c of the fresh tar react upon ageing and are no longer found in the F3c
fractionof theaged tars.Although, thetwoaged tarsappeartobeverysimilarbySEC
andUV-F,LD-MSindentifiescleardifferences.ThemoleculesinT20C-6m-L aremorestable
during ionisationwhilst those in T5C-6m are less stable. Thematerial observed in this
fraction in the aged samples contains molecules with less-conjugated chromophores
thanwerepresentinsamefractionfromTN2-0h.
It shouldbenoted that the trendsobserved forPC fractionF3cof thedifferent
tarsarenotrepresentativeofthebehaviouroftheotherfractions;eachfractionshowed
subtle anduniquedifferences.However, forbrevityonlya summaryof the findings is
presentedbelowwiththefullaccountprovidedinS7andthefigurespresentedinS8.
InspectionofthecompletesetofLD-MSresultsrevealsfurtherdifferencesinthe
tendency for fragmentation or susceptibility towards laser ionisation (i.e. strength of
laser power needed to observe satisfactory signal) for the different PC fractions, and
different tars. T20C-6m-L was least prone to fragmentation and its ions were easiest to
observe.T5C-6m showedthegreatesttendencytofragmentandwasthemostdifficultto
observe.Ingeneral,themoremobilefractionsweremostlikelytofragment.TheLD-MS
resultsalsoprovidesomeconfirmationofthemassestimatesderivedfromSEC(cf.Table
6andS7,TablesS7.1– S7.7).
Allthetarsamplesshowedevidenceofcontainingmoleculeswithmasses<200uto
>2000u.Thereweredistinctdifferenceshowever,intheirmassdistributionbeforeand
after ageingdepending also on the storage conditions. In general, therewas a shift to
50
highermassesintheagedtarscomparedtothefreshtar,andforT20C-6m-L relativetoT5C-
6m i.e.heaviermaterial inwarmerconditionsandthepresenceof light.Manyof thePC
fractionswerecomposedentirelyofmaterialsbeyondtherangeofGC,suchasfractions
F1andF2,cf.Table6,FiguresS8.5aandS8.6a.
Examining the extents of conjugation present in the tars (via UV-F) shows that
conjugated4-6aromatic ring systemsdominate inalmostall cases. It shouldbenoted
however, that fluorescence quantum yields are dramatically lower for conjugated
aromaticringsystemsthatcontainmorethan~8ringsasdiscussedinS6andelsewhere
[47,55],whichmeans,ifpresent,theyarealwaysunderestimated.Thefreshtarcontains
morearomaticmoleculeswithhighdegreeofconjugationthantheaged tarswhenthe
mobilePC fractions (F3 and F4) are compared. Correspondingly,when comparing the
leastmobilefractions(F1andF2),thereverseistypicallyfoundfortheagedtars,where
theycontainmorearomaticmoleculeswithhighdegreeofconjugationthaninthefresh
tar (there are exceptions, cf. S7). PC fraction F1 of T5C-6m contained the largest
conjugated systems of all the tar samples (equivalent to an average of 8 conjugated
aromaticrings).
When the UV-F spectra for all the PC mobility-fractions from all the tars are
consideredthefollowingobservationscanbemade:
i) The chromophores responsible for the fluorescence observed at 340-360 nm in
TN2-0h F4b move to F3b in T20C-6m-L, and possibly to F1; whereas for T5C-6m they
remaininF4b.
ii) The chromophores responsible for the fluorescence observed at 440-550 nm in
TN2-0h F3cappearstomovetoF3bandF1inT20C-6m-L;andtoF1forT5C-6m.
iii) Thechromophores identified in theprecipitate (cf. Section3.4,Figures6a-d)and
T5C-6m F1 (Figure 4a) showed maximum fluorescence intensity at >450 nm (>6
conjugated aromatic rings); these chromophores seem to originate from PC
51
fractions F4a, F3c, F3a and F2 in TN2-0h, (Figure 4b); the evidence for this is
discussedfurtherinS7andinthefinalsummary (Section3.5).
Figure 4a, Synchronous UV-F spectra (peak normalised) of PC fraction F1; for TN2-0h
therewasweaksignalduetosamplelowabundance.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600
Wavelength / nm
No
rmalis
ed I
nte
nsi
ty
T-N2-0h F1
T-5C-6m F1
T-20C-6m-L F1Solvent
52
Figure4b,SynchronousUV-Fspectra(peaknormalised)ofthePCfractionsfromTN2-0h.
AlthoughtheUV-Fresultsaredifficult tointerprettheydoshowthatchangesin
conjugationoccurduringageinginadditiontotherebeingachangeinthedistributionof
thetarsonthePCplate.However,itisnotpossibletodeterminewhetherthisissolely
due to changes in aromaticity or a result of the incorporation of oxygen into the
molecules as mentioned previously. It was often the case that the biggest differences
betweenanequivalentPCfractionfromthefreshandagedtarswereobservedbyUV-F.
Information on relative extents of conjugation is difficult to obtain by othermethods.
NMRcouldprovidemoredetailedinformationandwouldgreatlyaidthe interpretation
oftheUV-Fresults;however,thedifficultyofrecoveringtherequisitequantityofsample
fromplanarchromatographyhampersstudiesviaNMR.
In future studies, column chromatography fractionation could be performed to
obtainquantitative information;however, thiswillnotbedirectly comparable toaPC
fractionation as the mechanisms of separation differ. It would be best to do this in
parallel to a PC fractionation as it is more difficult to perform LD-MS analysis on
fractionsrecoveredfromcolumnchromatographythanfromPC[31].
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600
Wavelength / nm
Norm
alis
ed I
nte
nsi
tyT-N2-0h F4b
T-N2-0h F4a
T-N2-0h F3c
T-N2-0h F3b
T-N2-0h F3a
T-N2-0h F2
T-N2-0h F1
F3c
F3a
F4b
F2
F3b
F1
Solvent
53
From a visual comparison of the PC plates bearing the fractionated tar some
initial assertions were drawn. TN2-0h had very little immobile material (F1), larger
quantitiesofF3candF2,andnoprecipitatewasobservedbeforethesamplewasdried.
Therefore, it ispossible that itwasthedarkbandofmaterialdenotedasF3cthatwas
involvedinreactionswithF4bandF3btogeneratethematerialthatislabelledF1and
precipitate in the aged samples. The interpretation of the UV-F results as described
above(andinS7)concurswiththeseassertionswhencombinedwiththemassestimates
(from SEC and LD-MS, S9), i.e. these are the PC fractions that showed the biggest
differencesafterageing.ItissuggestedthatitwasmainlyPCfractionsF4bandF3cthat
react with one another to generate F1 and the precipitated materials during ageing;
fractionsF3b,F3aandperhapsF2alsoseem tobe involved inthesereactionsbut toa
lesserextent.
Theresultsalsoimplythatdifferentprocessesoccurredwhenthetarwasstored
atroomtemperatureinthepresenceofindirectsunlightthanwhenstoredat5°Cinthe
absenceoflight.Thisisasopposedtothesamechangesoccurringinbothsamples,but
slower in the caseof the sample stored at5°C.Whenstored at5°Call of thematerial
seen asF3b inTN2-0h disappears, probablydue to reactions formingmaterial that can
mobilize to another fraction or precipitate. Meanwhile, storing the tar at room
temperatureandexposedtoindirectsunlightresultedintheabsenceoffractionF4band
achangedincomposition ofF3b;significantdifferenceswerealsoobservedbetweenthe
respective F1 and F2 fractions. These differences are probably related to the altered
oxygencontentsoftheagedtars(Table4)andtheinfluenceofsunlight,oralackofit,on
the reaction mechanism available to the tarmolecules, as discussed earlier in Section
3.2.1.
Itisinterestingherealsotospeculateifthephotochemistryofbiomassmaygive
cluestothephotochemistryofitstars.Thephotochemistryofbiomassandinparticular
ligninhasbeenwidelystudied,duetoitssusceptibilitytoabsorbbothvisibleandnear
UVlight[75].Theresultingexcitedstateshaveshowntoenablelignindepolymerisation
inthepresenceofoxygen.Afterlightabsorptionthemoleculesintheirexcitedstatecan
formradicalspecies,usuallybycleavageofthecommonβ-O-4aryletherlinkage,which
thenreadilyreactwithoxygentoformnewchromophores.Atthetemperatureatwhich
54
the gasification tarswereproduced here, the lignin in the pine is highlymodified but
maycontainsomeβ-O-4aryletherlinkagespresentintheoriginalbiomass.
No oxygenated species were identified from the GC analysis of the tars. This
impliesoxygen is concentrated in themoleculesbeyondthe rangeofGC.On theother
hand,oxygenatedhydrocarbonsareknowntoco-elutewithpurehydrocarbonsduring
GC and this may explain their absence [76]. These aspects warrant further
investigations;andarediscussedfurtherinS7.
An unexpected finding was that tar T20C-6 m-L, which has the highest oxygen
content of the three tars and a greater C/H ratio than T5C-6 m, contains aromatic
moleculeswithlowdegreeofconjugation.EvenmoresurprisingisthatT20C-6m-L wasthe
moststabletowardsLD-MSanalysis(leastpronetofragmentation,withthemosteasily
observable ions). In contrast, T5C-6m has the least oxygen (although the absolute
difference is small, Tables 4 and 5) and the lowest C/H ratio but contains the most
aromatic molecules with the highest degrees of conjugation of the three tars; it also
showedthegreatesttendencytowardsfragmentationduringLD-MSanalysis.
Theresultsdescribedaboveappeartobeevidenceofphoto-oxidationreactions
havingoccurredduringstorageofthetar.Thepresenceofindirectsunlightresultedin
partialphoto-oxidation of tarT20C-6m-L, via themechanismdescribed earlier in Section
3.2.1anddisplayedinFigure3b.Itwouldseemthattheincorporationofoxygengroups
into these predominantly aromatic molecules improves there susceptibility towards
laser desorption and ionization, and destroys (or reduces) aromatic conjugation.
WhereastarT5C-6m wasnotexposedto light, therefore itreactedmainlyviaadifferent
mechanismleadingtomoleculeswithgreatermolecularmassesandlessoxygencontent
(remaining in solution). These molecules also contain aromatic chromophores which
havehighdegreesofconjugation,buttheywerenotstabletowardslaserdesorptionand
ionisation.
Theseobservations show that theC/H ratiowhich is typicallyused toestimate
the aromaticity of tar samples does not give information regarding the extents of
aromaticconjugation.Forexample, theC/HratioshowsatrendofTN2-0h>T20C-6m-L>T5C-
6m, however, UV-F showed the extent of conjugation to be T5C-6m>TN2-0h>T20C-6m-L. In
addition, therelationshipbetweenextentof aromaticconjugationandsusceptibility to
55
LD-MSanalysisiscontrarytothatwhichwouldbeexpectedbasedonpreviousstudiesof
lignin (in preparation for publication), petroleum, bitumen and coal derived samples
[31,47, 55,56,60].Where lignin is themost difficult toanalyse followedbybitumen,
with petroleum and coal-derived samples the easiest. However, in those previous
studies all the samples were stable under the same conditions as used to age the
gasificationtarsinthepresentstudy.Additionalstudiesarerequiredtounderstandthe
relevanceoftheseobservations.
3.3.3 BulkanalysesofRecoveredDrytar
To enable a significantly more robust interpretation of LD-MS data, the PC
fractiondatamustbeconsidered intandemwithbulksampleLD-MSdata,ratherthan
eitherof these approachesalone.Thishasbeendiscussed indetail elsewhere [31,47,
56]; briefly, whilst information gained from the bulk tars is considered more
representativethanresultsfromPCfractionationalone,theLD-MSanalysisofthebulk
tarsgiveshighlyvariableresultsdependingontheconditionsused[31,47,56].This is
demonstrated in Figures S10.1 and S10.2 for T5C-6m and T20C-6m-L. Therefore combined
informationfrombothdatasetsisusedininterpretationofresults.
Figures5a-cshowthechromatogramsandspectrafromtheanalysisofthebulk
RecoveredDrytarsTN2-0h,T5C-6m andT20C-6m-L bySEC,LD-MS andUV-F,respectively.The
LD-MS spectra displayed in Figure 5bwere selected after analysing low to high laser
powersandtheinfluenceofdifferent‘delayedionisationextraction’timesontheresults,
anexampleofthesetestsandtheirresultsisgiveninS10.Thechoiceofwhichspectrato
usetorepresentthesamplewasbasedontheinformationgainedfromtheexamination
ofthePCfractions,cf.S7toS9,asdescribedpreviously[56].
SEC reveals that TN2-0h contains smaller sized molecules, on average, than the
aged tars (Figure 5a). The aged tars (T5C-6m and T20C-6m-L) show very similar SEC
chromatograms to one-another and give almost identical mass spectra from LD-MS
(Figure 5b), with T20C-6m-L possibly containing more higher-mass (m/z) ions. The
differencesobservedaresubtlebutnoticeable.Withtheinformationobtainedfromthe
analysis of the PC fractions, it can be safely concluded that there is a trend towards
higheraveragemassmaterials:TN2-0h <T5C-6m <T20C-6m-L.
56
UV-F spectra of the different bulk tars are similar (main peak ~390 nm,
equivalent to 5 aromatic rings – Figure 5c); the main difference is that TN2-0h shows
slightlymorefluorescenceatwavelengthsgreaterthan400nm(>5rings),followedby
T5C-6m withT20C-6m-L givingtheweakestsignalinthatregion.Thiscouldberelatedtothe
aromaticmoleculeswiththehighestdegreesofconjugation,originallypresentinTN2-0h,
having reacted and precipitated from the aged samples. This explains thedecrease in
fluorescence at >400 nm in those samples (this confirms the findings from the PC
fractions– S7).
WhentheUV-FspectrumfromthebulksampleiscomparedtoitsPCfractionsit
can be seen that the bulk samples resemble theirF3c and F4a PC fractions (themost
mobile,lowmassmaterials).Thisisconsistentwithpreviousobservationswheresignal
from larger aromatic chromophores is difficult to observe in the presence of less
conjugatedaromatics.Thisisthoughttoberelatedtoareductioninthequantumyields
asthearomaticsystemsbecomemoreconjugated[55,56,61,77].
The analysis of the bulk tars reveals that only limited information could be
obtained about the changes that have occurred during ageing and highlights the
importanceof fractionatingthetarspriortotheiranalysis. It is important tonotethat
fractionation of the tars by PC cannot readily be done quantitatively; therefore, it is
beneficialtoanalyzethebulksamplesaswellastheirPCfractions.
57
Figure5a,Areanormalsizeexclusionchromatogramsofthebulk tarsTN2-0h,T5C-6m,andT20C-6m-L,
detectionat300nm.
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
7.0E-03
8.0E-03
9.0E-03
1.0E-02
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Re
lativ
e Inte
nsi
ty
T-N2-0h
T-5C-6m
T-20C-6m-L
10 100 1000 100000
200
400
600
800
Ion
Co
unt
m/z
T-N2-0h LP40% H8 kV T-5C-6m LP35% H9 kV T-20C-6m-L LP40% H8 kV
58
Figure5b,LD-MS spectraofbulktarsTN2-0h,T5C-6m,andT20C-6m-L withnoDIE, laserpower (LP)
andhighmassaccelerator(H)voltageareshowninthelegend.
Figure5c,PeaknormalisedsynchronousUV-FspectraofthebulktarsTN2-0h,T5C-6m,andT20C-6m-L.
3.3.4 Precipitatedmaterialsanalyses
SECandUV-Fresultsfromtheanalysisoftheprecipitatesthatformedwithin20hours,
andafter6monthsofstorageofthetarsaredisplayedinFigures6a-d.Itshouldbenotedthat
theprecipitatedmaterialswere fullysoluble inNMPat theconcentrationsused.ForTN2-0h the
precipitate was recovered after 14 hours storage in a fridge after being thawed from liquid
nitrogen – this sample is labelled TN2-14h-ppt. LD-MS was not applied to the analysis of the
precipitates as part of this scoping study; however, it would provide additional useful
informationandwillbeconsideredinfuture.
Figure6ashowstheSECchromatogramsfromtheprecipitaterecoveredafter20hours
storageofT5C (T5C-20h-ppt)andT20C (T20C-20h-ppt)and14hoursforTN2 (TN2-14h-ppt),alongsidetarT20C-
6m-L tohighlightthedifferencesbetweenthespectrafromthetarsandtheprecipitates.Evidence
ofasmallbutsteadyincreaseinthesizeofthemoleculespresentintheprecipitatecanbeseen
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600 650Wavelength / nm
No
rma
lise
d I
nte
nsity
T-N2-0h
T-5C-6m
T-20C-6m-L
Solvent
59
fromTN2-14h-ppt <T5C-20h-ppt <T20C-20h-ppt.Alltheprecipitatescontainmoleculesoflargersizethan
thosepresentinthetars.
Comparingtheprecipitatefrom20hoursstoragetothatrecoveredafter6monthsshows
virtuallynochangeinsizedistributionforT5C-6m;however,forT20C-6m-L there isashifttolarger
moleculesintheprecipitaterecoveredafter6months,cf.Figures6aand6b.
UV-F reveals that all the 20 hour precipitates gave spectra almost identical to one
another,andtheprecipitatefromTN2 (14hourprecipitate)showedslightlylessfluorescenceat
wavelengthsgreaterthan470nm(>7conjugatedaromaticrings),Figure6c.Thespectrafrom
theprecipitatesweresignificantlydifferenttothosefromthebulktarsortheirPCfractions,with
theexceptionofT5C-6m PCfractionF1whichgaveanalmostidenticalspectrum(Figure4a).
ComparingtheUV-Fspectraobtainedfromtheprecipitateafter6monthsstoragetothat
after20hours shows that forT5C-6m there isnodiscernabledifference; forT20C-6m-L there is an
increaseinfluorescenceat~500nm,cf.Figures6cand6d.Theanalysisoftheprecipitateshows
thatthismaterialwasnotoriginallypresentinthefreshtarandmustthereforebetheproductof
reactions between the tar molecules (or with the solvent, or dissolved oxygen) during their
storage.Itislikelythatgreaterdifferencescouldbedetectedbetweentheprecipitatedmaterials
ifplanarchromatographyandLD-MShadalsobeenused,asfortheliquidtarsamples;however,
that was beyond the scope of the present investigation. In addition, ultimate analysis would
providevaluableinformationandwillbeusedinfuturestudies.
60
Figure6a,Areanormalsizeexclusionchromatogramsoftheprecipitateafter20hoursfrom,T5C
andT20C, andTN2 recoveredafter14hoursafterthawingfromliquidnitrogen,thebulktarT20C-6m-
Lisshownforcomparison,detectionat300nm.
Figure6b,AreanormalsizeexclusionchromatogramsoftheprecipitatefromT20C-6m-L,T5C-6m
after6monthsstorage,andthePPTfromTN2-14h (after14hoursstorage).
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
7.0E-03
8.0E-03
8 10 12 14 16 18 20 22 24 26 28Time / mintues
Rela
tive I
nte
nsi
tyT-N2-14h-ppt
T-5C-20h-ppt
T-20C-20h-L-ppt
T-20C-6m-L
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
8 10 12 14 16 18 20 22 24 26 28Time / minutes
Re
lative I
nte
nst
iy
T-N2-14h-ppt
T-5C-6m-ppt
T-20C-6m-L-ppt
61
Figure6c,PeaknormalisedsynchronousUV-Fspectraoftheprecipitateafter20hoursstorageof
the tar solutions T5C and T20C, and after 14 hours for TN2, alongwith the fresh tar T1N2-0h for
comparison.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600 650Wavelength / nm
No
rmalis
ed I
nte
nsity
T-N2-0h
T-N2-14h-ppt
T-5C-20h-ppt
T-20C-20h-L-ppt
Solvent
62
Figure6d,PeaknormalisedsynchronousUV-Fspectraoftheprecipitateafter6monthsstorage
ofthetarsolutionsT20C-6m-L,andT5C-6m; and14hrsprecipitatefromTN2-14h.
3.3.5Summary
When all the findings from the analyses of the tars, their PC fractions and the
precipitatedmaterials are considered together inferences canbemade regarding the changes
thatoccurredduringageingunderdifferentstorageconditions.
TheresultsstronglysuggestevidenceofagingreactionstakingplaceandthatPCfraction
F1intheagedtarsiscomposedmostlyofreactionproductsofaging.Thisisbecausetherewasa
verylowabundanceofF1inthefreshtar,solittleso,that itwasdifficulttoobtainsatisfactory
SEC and UV-F results. There is evidence that F1 of the aged tars contains higher molecular
masses and different chromophores than the fresh tar (Figures 4a and S8.6a-c); this is most
evidentforPCfractionF1fromT5C-6m.Therewasnoprecipitationnotedinthefreshtarandthe
precipitatefromtheagedsampleswasfoundtocontainmoleculesofhighermass(largersize)
andwithgreaterconjugationthanthematerialinthefreshtar(Figures5a-cand6a-d).Thisisall
evidenceofaseriesofreactionsoccurringduringageingwhichresultinanincreaseinmolecular
massandconjugationinthetars.Theprecipitatesrecoveredafter20hoursfromtheT5C-6m and
T20C-6m-L samples contained molecules of similar size distribution and extents of conjugation.
After6monthstheprecipitatefromT5C-6m showedlittlechangeby SECorUV-F,whereasT20C-6m-L
had increased in size (mass) and contained more aromatic molecules with high degree of
conjugation(Figures6a-d).
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600 650Wavelength / nm
Norm
alis
ed Inte
nsi
tyT-N2-14h-ppt
T-5C-6m-ppt
T-20C-6m-L-ppt
Solvent
63
Itwasdeterminedthatthelargestaromaticchromophores(8conjugatedaromaticrings,
UV-Fpeakmax475nm) andlargestmoleculeswerefoundinPCfractionF1ofT5C-6m(Figures4a,
6b,6dandS8.6b).PCfractionF1fromT20C-6m-L containedmoleculesapproximatelythesamesize
asF1fromT5C-6m,howeverthechromophoresaremuchlessconjugated,onaverage(5aromatic
ringsversus8).TheprecipitaterecoveredfromT20C-6m-Lcontainedthenextlargestmoleculesand
sizes of conjugated chromophores (7-8 rings, 460 nm),whichwere slightly larger than those
fromtheT5C-6m precipitate.AtthesametimeT20C-6m-L contained less conjugatedchromophores
thanT5C-6m orthefreshtar.
LD-MS analysis of the PC fractions shows that fraction F1 from the fresh tar has an
average mass (peak max. m/z) of ~650 and the aged tars m/z ~725, with a shift in the
distributiontowardshighermasses in theagedtars,extendingtoat leastm/z2000(Table6).
ComparingthesemassestotheaveragenumberofaromaticringsasdeterminedbyUV-F(TN2-0h
5aromaticrings,T5C-6m 8ringsandT20C-6m-L 5rings)itcanbedeterminedthatthesemolecules
probablycontainmorethanonearomaticcore(as5rings=~250uand8rings=~400u), i.e.
more probably archipelago- and island-like structural configurations rather than continental
[61]. However, NMR and ultimate analysis data are needed to determine the amounts of
aromaticandaliphaticmaterial,ideallyforeachPCfraction,butthereweresample limitationsto
conductingNMR.
WhentheresultsforbulktarsandtheirPCfractionsareconsideredtogether,itcanbe
determinedthatTN2-0h containssignificantquantitiesofaromaticmoleculeswithhighdegreeof
conjugationandafterprecipitation(i.e.theagedsamples)theC/Hratiodecreased.Thereforeit
wouldseemthatthemosthighlyaromaticcompoundshadprecipitated.InT20C-6m-L moreoxygen
was observed than in TN2-0h which indicates mainly PAHs had precipitated and possibly
undergoneadditionalreactionswithoxygenfromtheairorthesolventduringitsstorage.InT5C-
6m lessoxygenwasfoundthaninthefreshtar.ThissuggestsmanyoftheoxygencontainingPAH
compoundshavealsoprecipitated.TheC/HratioofT5C-6m (0.62)andT20C-6m-L (0.99)impliesthat
the latter is more aromatic; however, UV-F reveals it is T5C-6m that contains molecules with
greater extents of conjugation (Figure S9.1c, S9.2c and S9.3c). T20C-6m-L shows evidence of
containing slightly highermolecularmasses and largermolecules. This sample ismuchmore
stableduringLD-MSanalysisthanT5C-6m(FigureS9.2a-bandS9.3a-b).
The results suggest that the ageing process for T20C-6m-L proceeded via a different
mechanismthanthatforT5C-6m. Indeed, it isprobablethat thepresenceof indirectsunlighton
T20C-6m-L inducesphoto-oxidationreactionswhichcannotoccurinT5C-6m storedcoldinthedark.It
islessprobablethattheresultsaresimplyduetokineticeffectswheretarT20C-6m-L hasreacheda
64
thermodynamically more stable state than T5C-6m due to enhanced kinetics from higher
temperatures. In other words, it is unlikely that the ageing / polymerization reactions are
progressingthroughthesamemechanismbuthaveprogressedtoalesserextentinT5C-6m thanin
T20C-6m-L. It is rather the case that there are a number of reaction mechanisms occurring
simultaneously with competing influences from kinetic, thermodynamic, and photochemical
effectswhensamplesarestoredexposedtolight.
T5C-6m and T20C-6m-L did not show statistically significant differences in degree of
precipitation.Itisnotpossibletoderiveconclusiveinformationregardingtheageingmechanism
fromthisstudyduetothelackofkeyinformationsuchastheoxygencontentoftheprecipitate
andideallyforthePCfractions.Inaddition,NMRstudiesarerequiredtoaidtheinterpretationof
the UV-F results and to reveal mechanistic information. However, to perform all of these
analysescomprehensivelyalargeamountoftar(>10g)wouldberequired.
However,theinvestigationsheredemonstratethatdetailedinformationcanbeobtained
bythecombineduseofSEC,LD-MSandUV-Fmethods,whichcanidentifydifferencesbetween
tars that are difficult to obtain by other approaches. UV-F and LD-MS are very sensitive
techniquesthatcanprovideevidence for thepresenceof largemolecules(>1000u)andthose
with highly-conjugated chromophores (>7 conjugated aromatic rings equivalent) that are
formed during ageing. The methods are able to identify these materials, even when they are
presentinlowabundance,ifusedinconjunctionwithplanarchromatography.Knowledgeofthe
presence of these high mass molecules and/or aromatic molecules with high degrees of
conjugation in tars and oils, even in low quantities, is essential when considering their
properties and furtheruse.These tars andoilshave applications inmanyprocesses including
combustion, reforming and upgrading but these large molecules can cause problems of char
formation/cokinginengines/boilersorpoisoningofcatalysts.
3.4 Conclusions
The tars recovered in this study using the tar protocol method rapidly exhibited
instability,withprecipitateformingwithin14hoursofstanding;thiswasindependentofstorage
at5°Corat20°C.Onlystoringthesolutioninliquidnitrogenhaltedthisprocess.
The study revealed that the molecules that contained the largest sizes of conjugated
aromatic ring systems, rather than the molecules with the greatest masses, were primarily
involvedinageingreactions,resultinginprecipitationoccurring.
Storingthetarisopropanolsolutioninthedarkatreducedtemperature(5°C)appearsto
result in a different ageing reaction mechanism to that when the sample is left at room
65
temperature and exposed to indirect sunlight. There are probably numerous mechanisms
occurring,andforthesampleexposedtoindirectsunlightthereisevidenceofadditionalphoto-
oxidationreactions.Fromthispreliminaryinvestigationthesedetailscannotbedeterminedwith
certaintyandtheinfluenceofsunlightandoxygenshouldbeinvestigatedfurther.
The differences observed between these biomass/coal mixture samples were greater
thananticipated frompreviousstudiesofcoal,petroleumandbitumenderivedmaterials [47].
However,itremainsdifficulttodefinitivelyinterprettheresults,whichhighlightsthecomplexity
ofattemptingtoelucidatethemolecularpropertiesoftarsortheirageingmechanisms.
An important finding of this scoping study is that the combined analytical approach
includingSEC,LD-MSandUV-F,whichwasoriginallydevelopedforstudyingheavysamplessuch
aspitchesandasphaltenesrather thanoilsandtars [47],canbeequallywellappliedto these
(lower molecular weight) biomass tars. In fact, the methodology could provide clearer
information for tarsandoils thanpitchesandasphaltenes,as themoleculesherehaveamore
suitable mass and size range for study via SEC, LD-MS and UV-F (mass range <5,000 u and
chromophores<10rings).However,thehigheroxygencontentofbiomasstarscomparedtothat
ofheaviersamplessuchaspitchesandasphaltenesmakesresultsinterpretationlessexact.
Future studies should include NMR, FT-ICR-MS and FT-IR analyses to aid the
interpretationoftheUV-Fresultsandtoclarifythefindingsingeneral.Ultimateanalysisofthe
bulk precipitate, and ideally also the PC fractions, would provide additional valuable
information.
Throughtheapplicationofthecombinedanalyticalapproachoutlinedhereitispossible
tobuildupadetailedunderstandingoftarsamplesandothersimilarbiomass,coalorpetroleum
derived liquids in terms of molecular mass range, average mass estimates and extents of
aromaticconjugation.For amorethoroughunderstandingof theagingprocess thesemethods
shouldbecombinedwiththosecurrentlybeingdevelopedforbio-oils.
One of the more troublesome aspects of getting useful energy from tars is their high
viscosityand their tendencytoage intoevenmoreviscousmaterial, suchthat learningdetails
aboutthechemicalroutesofageingandhowtopreventthatchemistryfromhappeningcanhave
importantimplicationsforutilizingtars.
3. S: Supportinginformation
S1;FuelProperties
66
S2;PlanarChromatographyImages
S3;TarYields,GCandUAFurtherDiscussion
S4;SECCalibrationandInterpretation
S5;LD-MSAdditionalInformation
S6;SynchronousUV-FInterpretation
S7;PCFractionsSEC,LD-MSandUV-FResults
S8;Figures,byPCFraction(SEC,LD-MSandUV-F)
S9;Figures,bySample(SEC,LD-MSandUV-FofPCfractions)
S10;Figures,LD-MSoftheBulkTars
S3.1;Fuelproperties
ThefuelpropertiesreportedinTableS1.1belowweredeterminedinpreviousstudies
[1].Approximatecontentsofcellulose,hemicelluloseandlignininatypicalpinewood
sampleis48.0,23.5and28.5wt%respectively(excludingextractives,3.9wt%)[2].
TableS1.1,Propertiesofthefuelsusedtogeneratethetarsamples
Fuel PolishcoalPine
woodchips
Origin Poland Portugal
Typebituminous
coalsoftwood
67
Proximateanalysis
FixedC,
%54.9 13.6
Volatiles,
%28.8 74.5
Ash,% 8.6 0.3
Moisture,
%7.7 11.6
Ultimateanalysis
C,%daf 79.1 51.6
H,%daf 4.5 4.9
S,%daf 0.5 0.2
N,%daf 1.3 0.9
Cl,%daf 0.4 0.07
O,%daf 14.2 42.4
Deviationsdeterminedaswithin+/-0.5%,absolute.
S3.2;PCfractionation
Imagesoftheplanarchromatographyplatesbearingthethreetarsamplesareshownin
FigureS2.1.
68
TN2-0h T5C-6m T20C-6m-L
Figure S2.1, PC plates bearing the vacuum dried tar samples after being eluted with
chloroform(F3) followed by acetone (F2) and finally heptane (F4). The image on the
left-handsidewastakenunderwhitelight,andtheright-handsideunderUV-light(260
nm).
Themainbandsofmaterialarelabelledasfollows:
F1=immobile(darkband)
F2=acetonemobile/chloroformandheptaneimmobile(darkband)
F3=chloroformmobile/heptaneimmobile(darkandfluorescentbands)
F4=heptanemobile(yellowandfluorescentbands)
Note:InFigureS2.1thesmallsilvercirclevisibleforT20C-6m-L F1,under‘whitelight’isdue
to the aluminium backing-plate showing through due to loss of the silica coating. In
addition, the lighter colour of T20C-6m-L observed under UV when compared to the other
imageswasdue to the photo being takenduring theday time,whereas the otherswere
takenatnight.Cautionisrequiredwhentryingtodrawconclusionsfromtheimagesofthe
F4b
F4a
F3c
F3b
F4b
F4a
F3c
F4a
F3c
F3b
69
PCplatesdisplayedinFigureS2.1asthephotosarenotofhighenoughresolutiontoconvey
thefullextentofthevisualinformationthatwasobservedanddescribedinthemanuscript
(whichwasbasedontheexaminationofagreaternumberofPCplates).
Chloroformwasthefirsteluentusedwhichresultedinonemainbandofdarkmaterial
atthesolventfront(F3c)andnumerousweakerbandsfurtherdowntheplate(F3band
F3abeingthemainsones),manyofwhichwerehighlyfluorescentinthegreen,yellow,
orangeandblue(thiscannot beclearlyseenfromthephotographsdisplayed inFigure
S2.1). Less emphasis was placed on the separation and identification of these mobile
components,theaimwasto isolatethehighermasscomponentsandtogenerateafew
representativebandstoaidthecharacterisationoftheparentsamples.
S3.3;Taryields,GCandUAfurtherdiscussion
Errorinthetaryields: ByexaminingthetaryieldsinTable2itisapparentthereare
errors inthemassbalancespresented.This isnoticeablewhencomparingthe20hour
GCresultstothoseafter6monthsofstorage,asdiscussedinthemaintext(Section3.1).
Thedeviationsinthedeterminationsofthe‘recovereddry’tarweightandtheweightof
the precipitate were also fairly significant (+/- ~15 and 30 % respectively). For the
recovereddrytaryield,theerrorswereduetothelimitedvolumeandlowconcentration
of thesolutionaswellas thehighvolatilecontentof thetars; theactualweightsbeing
determined were in the tens of milligrams range at best. Similarly, the quantity of
precipitaterecoveredwasinthemilligramrange.
GC-FIDexperimental – additionalinformation:Beforeeachsetofanalysesastandard
solution(containingthe16PAHsindentifiedascarcinogensbytheEnvironmentProtect
Agency, EPA-16)was used for peak identificationand area calibration. Dodecanewas
usedasaninternal quantitativestandard.Alltheresultswerenormalisedtogramsoftar
pernormalcubicmeterofexhaustproducergasfromthegasifier(g/Nm3).Tocheckthe
repeatabilityof the results triplicate sampleswere analysed from each stock solution
andthescatter foundtobe lessthan+/- 2%(relativestandarddeviation- RSD).Table
70
S3.1 presents the concentrations of the quantified compounds (EPA-16) in the tar.
Unknown peaks were included in the calculation of the total GC tar yield. This was
achievedbycomparingthecombinedareacountsoftheunknownstonaphthalene,the
calculated g/Nm3 was then halved and that amount reported. Benzene, toluene and
xylenewerenot accountedforbytheGCmethodused.
GC results: Some information can be inferred from the analysis of the relative peak
intensities, to seehow the tars changewithageing.There is a strong trendshowing a
decrease intheamountofGCrangemoleculeswithageing,TableS3.1;however, there
wasnoevidenceforanequivalentincreaseinthecorrespondinggravimetrictaryields
orprecipitate.Therefore,itseemsunlikelythattheselosesareduetoreactionsbetween
GC rangesmolecules resulting in higher mass species; it is more probably a result of
evaporationorlossestotheglasswareasdiscussedinthemanuscript.Thisisthoughtto
bethecasebecausethegreatestchangeswereinthenaphthaleneconcentrationwhichit
isunlikelytobereactiveunderthestorageconditions.
ExaminationoftheGCresultsfortarTN2-0h beforeandafterprecipitateconfirmsthatno
significant change can be detected (cf. the 0 and 14 hour samples, Table S3.1). The
biggestchangeswereaslightdecrease inacenaphthyleneandphenanthrene,although,
thesearewithinthescatteroftheresults.Theonlychangeslargerthanthescatterwere
a slight increase in the amounts of benzo(a)pyrene, indeno(1,2,3)pyrene and
benzo(g,h,i)pyrene,andasmalldecreaseintheamountofnaphthalene,asdescribedin
themanuscript.
Tarcomponentsmorevolatile thannaphthalenewerenot examined; it ispossible that
the apparent increase in the amount of benzo(a)pyrene, indeno(1,2,3)pyrene and
benzo(g,h,i)pyrenewasduetosomeofthemorevolatilecompounds(thannaphthalene)
reacting, possibly with naphthalene to form these species, and/or due to solvent
evaporation or loss of other volatile components. However, it would be surprising if
naphthalenewasreactiveundertheseconditions.
TableS3.1,GC-FIDresultsforthetarsamplesbeforevacuumdrying,displayedasmgper
cubicmeterofproducergas
71
SampleName TN2 T5C T20C
Units mg/Nm3
SampleAge 0h 14h 20d 20h 3d 20d 6m 20h 3d 20d 6m
Naphthalene 3000 3000 2800 2600 2700 2800 1000 2700 2900 2700 1500
Acenaphthylene 710 700 680 640 630 680 400 670 680 660 420
Acenaphthene 10 10 10 10 10 10 10 10 10 10 5
Fluorene 50 60 50 40 50 50 20 50 50 50 20
Phenanthrene 460 450 470 380 400 440 230 410 420 420 230
Anthracene 90 90 100 80 80 90 40 80 90 90 40
Fluoranthene 230 240 250 220 220 230 140 240 230 230 150
Pyrene 240 250 250 220 220 240 150 240 230 230 150
Chrysene 30 30 30 20 30 30 20 30 30 30 20
Benzo(a)anthracene 30 40 40 30 40 40 20 30 40 40 20
Benzo(k)fluoranthene 30 40 40 30 40 30 20 30 40 30 20
Benzo(b)fluoranthene 10 10 10 10 10 10 5 10 10 10 10
Benzo(a)pyrene 40 60 50 40 50 50 40 40 50 50 40
Indeno(1,2,3)pyrene 30 40 30 20 30 30 20 20 30 30 20
Dibenzo(a,h)anthracene 5 10 5 5 5 5 5 5 5 5 5
Benzo(g,h,i)perlyene 5 30 30 5 30 5 20 20 30 30 20
TotalEPA16 5000 5100 4800 4400 4500 4700 2100 4600 4900 4600 2700
Unknowns 280 280 270 230 250 240 170 270 250 220 180
GCTotal 5300 5400 5100 4600 4750 4900 2300 4900 5150 4800 2900
Relativeerrorswerelessthan+/- 5%
S3.4;SECcalibrationandinterpretation
72
TheSECelutiontimewasconvertedtomassusingafivepointPScalibrationperformed
onthedayofanalysis.Thisequation(Eq1)wasappliedfrom15.0– 20.7minutes.After
this timeacalibrationbasedonsmallpolyaromatichydrocarbon(PAH)standards(Eq
2)wasused,basedonprevious studies[3-5].Theequation fromthePAHstandards(Eq
2)wasappliedtothe20.7– 24.0minregion.24.0minutesrelatestoamassofabout100
u;anysignalintheregion24-25minuteswasgivenavalueof100u.25minutes is the
permeation limit for this column. Materials eluting earlier than 15.0 minutes were
assignedanestimatedmassof2500u.Thereasonsforthisarebrieflydescribedinthe
followingsection.
EquationsusedtoconvertSECelutiontimetomass:
Eq1: 15.0– 20.5minutesregion: y=9.683-0.346.x PScalibration
Eq2: 20.5– 24.0minutesregion: y=6.902-0.210.x PAHcalibration
Wherey=log10 (MM);andx=elutiontimeinminutes
12.0– 15.0minutesregion: >2,500uaveragemassLD-MScalibration.
24.0– 25.0minutesregion: Valuekeptconstantat100u
CommentonthedisparitybetweentheSECandLD-MSmassestimates: Thereisan
apparentdisparitybetweenthemassestimatesderivedfromaSECPScalibrationwith
thosefromLD-MSmeasurementswhenanalysingcomplexhydrocarbonmixtures[3-6].
73
This is thought toariseforanumberofreasonsasdescribedinarecentreviewarticle
[6].Inbrief,ithaspreviouslybeenshownthatthematerialwhichelutesintheexcluded
region(<15minutes)of thisSECsystemhasahigheraveragemassthanthematerials
eluting in the retained region (15-25 minutes), for similar materials. However, it is
difficult to accurately determine the mass of the excluded material, as discussed
elsewhere [3-6]. The PS calibration appears to greatly overestimate the mass of the
excluded material observed by SEC, an LD-MS study found average masses (m/z) of
2500 - 3500 when the excluded SEC material was examined in isolation of retained
material; whereas, the SEC PS calibration indicated masses >100,000 u. In addition,
whenthereissomeretained SECmaterialalsopresentinthesamplebeinganalysedby
LD-MS, as for these samples, it tends to dominate the LD-MS spectra evenwhen it is
presentinlowerabundance(massdiscriminationandotherfactors[3,5-9]).
S3.5;LD-MSadditionalinformation
For theLD-MSanalysis all the sampleswere investigatedusinga0 (zero) and600ns
delayedionextraction(DIE)time.Spectrawerealsorecordedinreflector-mode(results
not shown). It isprudent toexamine the samplesbyaswide a rangeof conditionsas
possible to aid interpretation. The LD-MS results presented in the manuscript were
thoseobtainedunderthefollowingconditions:
ForthePCfractions: onlyspectraobtainedinlinear-modeoperationwithaHMAvoltage
of 10 kV, no DIE time and with varying levels of laser power applied, are displayed.
Theseconditionsarebasedonpreviousstudies[3,7].Briefly,thepurposeofthe analysis
ofthePCfractionsistoobtaininformationonthemassrangeofthesample.Therefore,
theconditionsareselectedtoaidtheobservationof the fragment ionsandthehighest
masscomponents(ions).
Forthebulktars: theaimwastoobtainmassspectrathatmoreaccuratelyrepresentthe
massdistributionofthesampleinitsentirety.Therefore,onlyspectraobtainedinlinear-
mode operation with the following conditions are reported, DIE time of 600 ns and
wheretheHMAvoltagewasreducedasthelaserpowerwasincreased.Theseconditions
areusedtoaidthedetectionofthehigherm/zionsandavoidoverloadingthedetectors
withlow m/zions.
74
A few LD-MS spectra are presented that were obtained under different conditions to
thosedescribedabovetohighlightcertainpoints;inthese instancestheconditionswill
bestatedinthetext.
Theuseofthepeakofmaximumintensity(peakmax.)toestimateaveragemassvalues
fromSECandLD-MSwasconsideredsufficientforthepurposesofthisscopingstudy(i.e.
to draw relatively comparisons) based on previous experience [3, 6,7]. Details of the
methods used to determine molecular mass estimates are also provided in said
publications.
S3.6;SynchronousUV-Finterpretation
The approach used to interpret the UV-fluorescence results is based on previous
investigations.AnumberofstudieshavereportedthatUV-Fspectrashowbathochromic
shifts (to longer wavelengths, red-shift) and emit lower fluorescence intensities with
increasing sizes of conjugated aromatic ring systems [5, 8, 10, 11].More recentwork
alsosupportstheseconclusions.Aqualitativerelationshiphasbeennotedbetweenthe
wavelength of maximum fluorescence and number of conjugated aromatic rings in a
polynucleararomatic(PNA)system,asdeterminedbysynchronousmodeUV-FandNMR
spectroscopy, respectively [6]. Thiswas for a number of coal,petroleum andbitumen
derivedoils, tars,pitchesandasphaltenes,andtheirsolubilitysub-fractions[3,10,12-
14]. ThiscorrelationwasdrawnfromacomprehensivereviewofUV-Fspectroscopyand
otheranalyticaltechniquesusefulforanalysiscomplexhydrocarbonmixtures[6].
Figure S6.1 and Table S6.1 demonstrate the relationship between synchronous UV-
fluorescence spectra for a series of coal and petroleum derived samples with their
numberofconjugatedaromaticrings(determinedbyNMR)superimposedonthepeak
withmaximumintensityof fluorescence.Themaximumintensityof fluorescence shifts
steadilytolongerwavelengthsbyapproximately30nmperadditionalaromaticringina
conjugated aromatic system, based on NMR results [3, 10, 12-14]. Fluorescence
intensitiesintheUV-fluorescencespectrumofthesamples4P1and5P1wereverylow
75
andshowedabroaddistribution,rangingfrom350tomorethan650nm.Thesefindings
suggest the presence of molecules containing a wide range of chromophores, some
correspondingtopolycyclicaromaticringsystemswithmorethan10conjugatedrings–
asdeterminedby13C-nmr[6,14].
It should be noted that oxygen and other substituents on aromatic ring systems also
influence fluorescence characteristics. Limited information is available however,
regarding their influence on the fluorescence of poly aromatic hydrocarbons with
molecular masses greater than ~400 u. A summary of some relevant fundamental
aspectsofUV- fluorescencearegivenbelow,furtherdetailscanbefoundelsewhere[15].
When substituent groups are introduced to an aromatic molecule the effect on
fluorescencecharacteristics dependsonposition ontheringtheyoccupyaswellastheir
functionality.Electron-acceptingunsaturated functionalgroups,suchas -COOH, -NO2, -
C=O, NH2, or -C=S-R can influence fluorescence characteristics of PAH’s. Such groups
have low lying vacant π* orbitalswhich can become occupied by an excited electron
fromthearomaticring.Electrondensity is transferredfromtheringtothesubstituent
(intramolecularchargetransfer).Anothercomplicationis the introducedofC=O,C=S, -
NO2,orhetero-atoms(N,S).Eachofthesegroupsprocessesalonepairofelectronsinan
orbitalparalleltotheplaneofthearomaticring,whichcanbepromoted.Thiscanhave
profoundeffectonfluorescence,dramaticallyreducingfluorescenceintensity[15].
When groups containing ‘n’ electrons (n = non-bonding electrons, as in C=O) are
conjugated with a π electron group, the effect is the same as increasing aromatic
conjugation onfluorescencecharacteristics.Therefore,additionofC=OgroupstoPAHs
and alkyl-aromatics results can result in a red shift of the fluorescence (to longer
wavelengths),dependingonthepositiontheyoccupy.Aromaticcarbonylcompoundsare
stronglyinfluencedbythelow-lying(n,π*)singletexcitedstates(asn-to-π*transitionis
100xlessintensethanπ-π *);therefore,thissignificantlyreducesfluorescencequantum
yield[15].
Auxochromes,whichgenerallydonotabsorbsignificantlyinthe200-800nmregionwill
affect the fluorescence spectrumof the chromophore towhich it is attached.OH is an
auxochrome; however, it typically has a minor influence on the chromophore it is
76
attached to [15]. Ether groups typically have no influence on conjugation or
fluorescence.
DuetothecomplexnatureoffluorescencecharacteristicsinlargePAHmoleculesitwas
notpossibletoaccountfortheinfluenceofoxygenontheUV-Fresultsofthetarsbeing
studiedorinthecorrelationdescribedaboveandshowninFigureS6.1andTableS6.1.
Tosimplifythediscussionofthe UV-Fresultstheterm‘numberofconjugatedaromatic
rings’or‘numberofrings’willbeusedinreferencetothewavelengthwherethepeakof
maximum fluorescence intensity is observed, using the correlation provided in Table
S6.1. In should be noted that this is not meant as a literal description of the
chromophore, it is only used to make relative comparisons between samples. The
changesin fluorescencecharacteristicsreferredtocouldalsobeduetothe influenceof
oxygenorothersubstituents,notnecessarilychangesinaromaticconjugationalone.
Figure S6.1, Synchronous UV-F spectra (absolute intensity - mV) of sample fractions
showing correlation with conjugated aromatic rings as determined by NMR. CO is
creosote oil, AO1 anthracene oil, PPS pyridine soluble fraction of coal tar pitch, P-1 a
syntheticpitch-likematerial,3P1,4P1and5P1aresolubilityfractionsofP-1.[3,10,12-
14].
0
50
100
150
200
250
300
350
250 300 350 400 450 500 550 600 650 700
Wavelength / nm
Ab
so
lute
In
ten
sit
y /
mV
CO = 2 rings
AO1 = 2-5 rings
PPS = 7-9 ring
P-1 = 4-7 rings
3p1 = 3-12 rings
4p1 >10 rings
5p1 = >>10 rings
Normal Response >250 mV
Weak Response <250 mV
1 2 3 4 5 6 7 >8
Number of Conjugated Aromatic Rings (based on NMR results)
CO AO1
P-1
3P1
PPS
4P1
5P1
77
TableS6.1,Correlationsbetweentheaveragenumbersofringsinpolynuclear aromatic
ring systems (determined by NMR) and the wavelengths of maximum fluorescence
intensityintheUV-fluorescencespectra[6]
Wavelengthof
peakwith
maximum
intensity
Approximate
Numberof
AromaticRings
Wavelengthof
peakwith
maximum
intensity
Approximate
Numberof
Aromatic Rings
270nm 1ring 390nm 5ring
300nm 2ring 420nm 6ring
330nm 3ring 450nm 7ring
360nm 4ring ≥480nm ≥8ring
S3.7;PCfractionsSEC,LD-MSandUV-Fresults
InthissectionadetailedaccountoftheSEC,UV-FandLD-MSresultsfromtheanalysisof
the PCmobility-fractions is given. The PC fractions thatwere not observed in all the
samples are discussed first followed by those common to all the samples. This is
beneficialbecausethefractionsthatwerenot observedinallthesamplesareofgreatest
interestastheyarethematerialsthathavereactedoraretheproductsofreactions.
Fluorescentmaterial,PCfractionF3a(SISectionS9,FiguresS9.1a-c)
This material was only observed in TN2-0h. It contains some material with molecular
massesapproaching1000u,andalsocontainssignificantamountsofsmallermolecules
(~200u).UV-Fshows5-9aromaticringsequivalentchromophores,onaverage(Table
S7.1). It isnot surprising that thematerial in F3awas not found in the aged samples
consideringpreviousstudies[3,14,16].WhereithasbeenfoundthatthePAHmolecules
containing the most highly-conjugated chromophores (largest poly nuclear aromatic
groups) are the most reactive, described further in the manuscript. PC fraction F3a
contains aromaticmolecules with some of the most highly-conjugated chromophores
78
thatwereobservedforthefreshtar;thereforeitisnotsurprisingthesemoleculeshave
reacted,probably toproduce larger,highermassmoleculeswithdifferentmobilityon
thePCplateafterageingthetar(orhaveprecipitated).Hence,PCfractionF3awasnot
observedintheagedsamples.
TableS7.1,SummaryofSECandLD-MSmassestimates,andUV-FresultsforPCfraction
F3afromTN2-0h
F3a Definition Units TN2-0h
LD-MSPeakMax./m/z
Uppermassm/z
<200
800
SECPeakMax.
2nd Peaku
220
900
UV-F
PeakMax.,(Ar.
Rings+)
2nd Peak,(Ar.
Rings+)
3rd Peak,(Ar.
Rings+)
nm
(rings)
390(5)
440(7)
470(8)
+ Ar.Ringsreferstotheequivalentnumberofconjugatedaromaticringsthatwould
fluoresceatthesamewavelength,asdescribedinSISectionS6.
Fluorescentmaterial,PCfractionF4b: (FiguresS8.1a-c,SISectionS8).
TN2-0h andT5C-6m weretheonlysamplestonoticeablycontainthisPCfraction.Theresults
indicatethatitwasroughlythesamematerialinbothsamples.UV-Fshowsthespectra
from TN2-0h and T5C-6m were very similar (Table S7.2, Figure S8.1c). There was one
significant differencehowever;TN2-0h containedachromophorethatfluorescedstrongly
at360nm(~4ringseq.)thatwasnotpresentinT5C-6m.Inaddition,thefreshtarcontains
agreaterabundanceoftheleast-conjugatedchromophores(3-4rings)andfewer5ring
79
species, relatively. The ring sizes predicted from UV-F correspond with the mass
estimatesfromLD-MSandSEC.
Despitethe lowaveragemassof this fraction itwas stillpossible toobtainreasonably
strongionscounts,andrepeatablespectra,duringLD-MSanalysis.Thiswasnotthecase
forfractionF3b(discussednext).
TableS7.2,SummaryofSECandLD-MSmassestimates,andUV-FresultsforPCfraction
F4bfromTN2-0h andT5C-6m
F4b Definition Units TN2-0h T5C-6m
LD-MSPeakMax.
Uppermassm/z
210
400
200
300
SEC PeakMax. u 150 140
UV-F
PeakMax.,(Ar.
Rings+)
2nd Peak,(Ar.
Rings+)
3rd Peak,(Ar.
Rings+)
4th Peak,(Ar.
Rings+)
nm
(rings)
390(5)
355(4)
360(4)
345(3)
390(5)
355(4)
345(3)
-
+ Ar.Ringsreferstotheequivalentnumberofconjugatedaromaticringsthatwould
fluoresceatthesamewavelength,asdescribedinSISectionS6.
Fluorescentmaterial,PCfractionF3b: (FiguresS8.4a-c,SISectionS8).
PC fractionF3bwas onlyobserved for TN2-0h andT20C-6m-L. The combined information
from SEC, LD-MS and UV-F reveals that fraction F3b from the two samples is not
composed of the same materials (Table S7.3). TN2-0h contains molecules of lower
80
molecular weight (~300 u) and with less-conjugated aromatic systems (4-5 rings
equivalent),onaverage, thanT20C-6m-Lwhichhasanaveragemassof~450u fromSEC.
However,LD-MSanalysisresultedinmainlyfragmentionsbeingobservedwithfewions
detectedabovem/z450.UV-FconfirmedthatT20C-6m-L islikelytocontainmore,higher
massmoleculesthanTN2-0h asitprovidesevidenceforlarger-sizesofconjugatedsystems
(5-8 ringsequivalent inT20C-6m-L compared to4-5 rings inTN2-0h). Fraction3bwasnot
observedinT5C-6m.
UV-F revealsmarkeddifferences in theextentsof conjugationof the chromophores in
theF3bfractions.Thefreshtar(TN2-0h)mainlycontainschromophoresequivalentto4-5
fused aromatic rings,while the aged sample (T20C-6m-L) containmainly 5-8 fused rings
equivalents. ItshouldberelativelyeasytoobservethesematerialsviaLD-MS;however,
forT20C-6m-L therewaslittleevidenceofmoleculeslargeenoughtocontainthenumberof
ringssuggestedbyUV-F. It is likely that themolecules inT20C-6m-L fractionF3bcontain
bridges between 3-4 ring aromatic units in a way that maintains conjugation; this is
possibly related to the incorporation of oxygen into the samples (Table 4).Moreover,
this may also explain why the UV-F and SEC results indicate the presence of larger
molecules than could be observed by LD-MS, and why fragment ions were mainly
observed. In addition, as T20C-6m-L was exposed to indirect sunlight the sample has
probably undergone photo-oxidation reactions during ageing; this could be related to
theobservationoutlinedabove.
Of the three techniques UV-F provided the clearest evidence for their being different
moleculespresent in theF3bPC fractionofTN2-0h andT20C-6m-L (notpresent inT5C-6m).
Therewasasurprisingrangeofdiversity inextentsof conjugationandmassrange for
the two samples. It is possible that the bright green fluorescence observed from this
material,whenobservedunderUVforbothTN2-0h andT20C-6m-L,wasfromasingletypeof
chromophorethatiscommontodifferentmoleculesinthetwosamples.
Comparing fractions F3b and F4b shows that F3b appears to contain higher mass
materials,onaverage;however,thehighermassmoleculeswerenotstabletowardlaser
ionisation.BothfractionsF3bandF4bpredominantlycontain5aromaticringequivalent
81
chromophores however, F3b contains chromophores that were more conjugated, on
average.
TableS7.3,SummaryofSECandLD-MSmassestimates,andUV-FresultsforPCfraction
F3bfromTN2-0h andT20C-6m-L
F3b Definition Units TN2-0h T20C-6m-L
LD-MSPeakMax.
Uppermassm/z
<200
400
<200
750
SEC PeakMax. u 275 460
UV-F
PeakMax.,(Ar.
Rings+)
2nd Peak,(Ar.
Rings+)
3rd Peak,(Ar.
Rings+)
nm
(rings)
400(5)
370(4)
-
415(5)
445(7)
490(8)
+ Ar.Ringsreferstotheequivalentnumberofconjugatedaromaticringsthatwould
fluoresceatthesamewavelength,asdescribedinSISectionS6.
PCfractionscommontoallthesamples:
PCfractionF4a (FiguresS8.2a-c,SISectionS8)
Theanalysisof theF4aPCfractions fromthe threetar samples revealssomecommon
features (TableS7.4).TheSECresultswerealmost identical asweretheUV-Fspectra;
however, LD-MS revealed differences. TN2-0h and T5C-6m have similar ion distributions,
centred aroundm/z250;however,T5C-6m also containsa significant amountofhigher
m/zions,~650,tailingtom/z~1500.T20C-6m-L showsamainbandofionsaroundm/z
82
325 and a second band at m/z ~650; these higher mass (m/z) ions were present in
lowerabundancethanseenforT5C-6m.
ThehighestmassmaterialpredictedinTN2-0h fromSECappearstofragmentduringLD-
MS as few ionswere observed with m/z >500. Whereas, in the aged samples a new
group of higher mass molecules were detected that were more amenable to LD-MS
analysis.UV-FshowedthatTN2-0h andT5C-6m containmoleculeswithverysimilarextents
ofconjugation(5-7rings);T20C-6m-L showsaslightincreasein4ringchromophoresanda
decreasein7ringequivalents.
The combined information suggests a subtle change in the materials present in PC
fractionF4adependingonthestorageconditions.Thiscouldbeinterpretedasevidence
for changes in structure rather than just a change in mass distribution. The sample
storedatroomtemperaturehaslostthematerialthatcontainedthearomaticmolecules
withthehighestdegreesofconjugationandthehighestmassspeciesasseeninTN2-0h.It
ispossiblethatmaterialhasreactedwithoxygenand/orothertarmoleculesresultingin
itbecomingpartofanotherPCfractionorformedprecipitate.ThelackofoxygeninT5C-
6m (compared to T20C-6m-L, Table 4) could be related to it containing more low-mass
moleculesthatarenotstabletowardslaserionization.
TableS7.4,SummaryofSECandLD-MSmassestimates,andUV-FresultsforPCfraction
F4afromTN2-0h,T5C-6m andT20C-6m-L
F4a Definition Units TN2-0h T5C-6m T20C-6m-L
LD-MSPeakMax.
Uppermassm/z
260
500
250
1500
325
1250
SECPeakMax.
2nd Peaku
190
1100
180&
1000&
160
1000
UV-F
PeakMax.,(Ar.
Rings+)
2nd Peak,(Ar.
Rings+)
3rd Peak,(Ar.
nm
(rings)
390(5)
420(6)
440(7)
390(5)&
420(6)&
440(7)&
390(5)
420(6)
370(4)
83
Rings+)
+ Ar.Ringsreferstotheequivalentnumberofconjugatedaromaticringsthatwould
fluoresceatthesamewavelength,asdescribedinSISectionS6.
&FractionsF4aandF3ccombined;thesamplescouldnotberecoveredseparately.
PCfractionF3c (FiguresS8.3a-c,SISectionS8)
SECshowsallthreesamplescontainmoleculeswithasimilarsizedistributionwhichis
composedmainlyofsmallmolecules(~200u)anda lowintensityshoulder,relatingto
largermolecules (~1000u).LD-MSrevealsa trendtowardshighermasses(m/z) from
TN2-0h <T5C-6m <T20C-6m-L.
UV-F reveals a dramatic difference in the extents of conjugation between the F3c
fractions.TN2-0h containsmainly7-8ringequivalentchromophoreswhereasT5C-6m and
T20C-6m-L contain5-6ringchromophores(almostidenticalspectra).
The results reveal that thematerial present in PC fraction F3c in the aged samples is
very different when compared to TN2-0h (Table S7.5). It seems that in TN2-0h this PC
fractioncomprisesofsmall- tomedium-sizedmolecules(200-500u)thatcontainhighly-
conjugated aromatic chromophores (7-8 rings eq.). Upon storage the majority of this
material disappears, presumable due to reactions to produce larger-sized molecules
withdifferentmobilityon thePCplate, or formsprecipitate.Thematerialobserved in
thisfractionintheagedsamplesappearstocontainmoleculesofgreatermassandless
conjugatedchromophoresthanwerepresentinTN2-0h.
84
TableS7.5,SummaryofSECandLD-MSmassestimates,andUV-FresultsforPCfraction
F3cfromTN2-0h,T5C-6m andT20C-6m-L
F3c Definition Units TN2-0h T5C-6m T20C-6m-L
LD-
MS
PeakMax.
Uppermassm/z
275
550
330
1100
350
1100
SECPeakMax.
2nd Peaku
220
900
180&
1000&
180
1000
UV-F
PeakMax.,(Ar.
Rings+)
2nd Peak,(Ar.
Rings+)
3rd Peak,(Ar.
Rings+)
nm
(rings)
440(7)
460(8)
410(6)
390(5)&
420(6)&
440(7)&
390(5)
420(6)
440(7)
+ Ar.Ringsreferstotheequivalentnumberofconjugatedaromaticringsthatwould
fluoresceatthesamewavelength,asdescribedinSISectionS6.
&FractionsF4aandF3ccombined;thesamplescouldnotberecoveredseparately.
PCfractionF2 (FiguresS8.5a-c,SISectionS8)
SEC results show that PC fraction F2 from TN2-0h and T5C-6m have a similar size
distributionwithapeakmaximumat~200uanda significant shoulderat~600u. In
contrastT20C-6m-L hasapeakmax.at~600uandmorematerialelutingintheexcluded
SECregion.MaterialelutingintheexcludedSECregionhaspreviouslybeenshowntobe
ofaveragemass~2500uorgreaterforsimilarmaterial[5];cf.SI S4fordetails.
The LD-MS results show a mono distribution for all the tars. TN2-0h had the lowest
averagemass(m/z)~550; theagedsampleshadroughlythesamedistributionasone
another,with an averagem/z of 600-700 and ions tailing off tom/z ~1100. The low
massmaterial(200u)predictedbySECforTN2-0h andT5C-6m wasnotobserved;thiswas
85
probably due to it being lost in the high vacuum of the MS sample chamber. The
contradiction between the SEC and LD-MS results for T5C-6m and T20C-6m-L could be
evidenceofT20C-6m-L containmorelarger-sizedmoleculesthanT5C-6m buttheyfragment
duringLD-MS,hencetheirmassspectraappearsimilar.
UV-FshowsTN2-0h containsless-conjugatedchromophores(4-5rings)thanT5C-6m orT20C-
6m-L whichgaveidenticalspectratooneanother(5-6rings).
These results show that themolecules inPC fractionF2ofTN2-0h areof loweraverage
mass and contain less-conjugated chromophores than the aged samples (Table S7.6).
T5C-6m contains more, low mass molecules than T20C-6m-L; although, the extents of
conjugationwere almost identical.This suggests thatongoing fromTN2-0h toT5C-6m or
T20C-6m-L thereisacommonmechanismwhichresultsinchromophoresofsimilarextents
of conjugation remaining in F2 (or being formed). For T20C-6m-L there appears to be a
secondprocesswhich results inhighermolecularmassmolecules remaining inF2 (or
being produced) which were not stable towards LD-MS analysis, with no additional
change in theextentof conjugation.This couldbe related to thegreaterabundanceof
oxygen in T20C-6m-L and may be evidence for oxygen bridges between tar molecules
resulting in no change in the extent of conjugationwhich are cleaved relatively easily
during LD-MS analysis. This may be related to the possibility of photo-oxidation
reactionsoccurringduring storageof tarT20C-6m-L.However, furtherwork isneededto
understandtheseaspects.
TableS7.6,SummaryofSECandLD-MSmassestimates,andUV-FresultsforPCfraction
F2fromTN2-0h,T5C-6m andT20C-6m-L
F2 Definition Units TN2-0h T5C-6m T20C-6m-L
LD-MSPeakMax.
Uppermassm/z
550
1300
700
1500
600
1500
SECPeakMax.
2nd Peaku
210
650
200
600
650
>2000
86
UV-F
PeakMax.,(Ar.
Rings+)
2nd Peak,(Ar.
Rings+)
3rd Peak,(Ar.
Rings+)
nm
(rings)
390(5)
350(4)
445(7)
415(6)
395(5)
470(8)
415(6)
395(5)
470(8)
+ Ar.Ringsreferstotheequivalentnumberofconjugatedaromaticringsthatwould
fluoresceatthesamewavelength,asdescribedinSISectionS6.
PCfractionF1 (FiguresS8.6a-c,SISectionS8)
The SEC and LD-MS results for PC fraction F1 were generally similar from all the
samples.ThemostsignificantdifferencewasseenforTN2-0h whichgaveveryweaksignal
whenanalysedbySECandUV-F.However;thiswasmainlyduetothelowabundanceof
fraction F1 in TN2-0h. It was possible to obtain satisfactory LD-MS results from all the
samples. All the F1 samples had similar mass distributions with a peak maximum
between m/z 600-750, and ions tailing off to aroundm/z 2000,with a shift towards
highermassesintheagedtars.AllthesamplesshowedaSECchromatogramwithapeak
max.intheexcludedregion(<15minutes,>2500averagemass)withsomematerialalso
eluting in the retained region (15-25minutes).Considering the limitationsof SEC and
LD-MSderivedmassestimates(cf.SIS4andS5)theseresultsshowgoodconsistency.
UV-F revealed the largest differences between the F1 samples. TN2-0h gave a very low
intensity spectrum;however, the signal that couldbedetectedhada similarprofile as
T20C-6m-L (4-6ringeq.).Ontheotherhand,T5C-6m showedaverydifferentspectrumwith
chromophoresof6-10aromaticringsequivalent,onaverage.
The results from the immobile fraction (F1) show therewasa low abundance of this
material in the freshtarand itwascomposedofhighmassmolecules(>500uaverage
mass)withchromophoresequivalentto4-6rings.T5C-6m hadamuchgreaterabundance
of thismaterial; itwasalsostructurallyverydifferenttotheequivalent fromTN2-0h,or
T20C-6m-L,despite havingasimilarmolecularsizeandmassdistribution.Themoleculesin
87
T5C-6m containaromaticmoleculeswithsignificantlyhigherdegreesofconjugationthan
anyoftheothertarsorPCfractions(TableS7.7,FiguresS9.1c,S9.2candS9.3C).
Thedifferencesoutlinedabovearepossiblyrelatedtothedifferentoxygencontentsof
the samples, where T5C-6m had less than the fresh tar or T20C-6m-L (Table 4). The
implicationsofthesefindingsarediscussedfurtherinthemanuscript(Section3.5).
TableS7.7,SummaryofSECandLD-MSmassestimates,andUV-FresultsforPCfraction
F1fromTN2-0h,T5C-6m,andT20C-6m-L
F1 Definition Units TN2-0h T5C-6m T20C-6m-L
LD-
MS
PeakMax.
Uppermassm/z
650
2000
725
2000
725
1900
SECPeakMax.
2nd Peaku
>2500*
1000*
>2500
1300
>2500
1100
UV-F
PeakMax.,(Ar.
Rings+)
2nd Peak,(Ar.
Rings+)
3rd Peak,(Ar.
Rings+)
nm
(rings)
400*(5)
470*(8)
-
475(8)
410(5-6)
535(10)
400(5)
410(5-6)
460(7)
+ Ar.Ringsreferstotheequivalentnumberofconjugatedaromaticringsthatwould
fluoresceatthesamewavelength,asdescribedinSISectionS6.
*Veryweaksignal.
3.8;Figures,byPCfraction(SEC,LD-MSandUV-F)
PCfractionF4b
88
Figure S8.1a, LD-MS spectra of PC fraction F4b at low and high laser power (top and
bottomrespectively),noDIE,HMAvoltagewas10kV.
10 100 1000 100000
100
200
300
400
500
Ion C
ount
m/z
T-5C-6m LP50% T-N2-0h LP70%
10 100 1000 100000
200
400
600
800
Ion
Cou
nt
m/z
T-5C-6m LP60% T-N2-0h LP80%
89
FigureS8.1b,SECchromatograms(areanormalised)ofPCfractionF4bat300nm.
FigureS8.1c,SynchronousUV-Fspectra(peaknormalised)ofPCfractionF4b.
PCfractionF4a
0.0E+00
2.0E-03
4.0E-03
6.0E-03
8.0E-03
1.0E-02
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Rela
tive Inte
nsity
T-N2-0h F4b
T-5C-6m F4b
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600
Wavelength / nm
Norm
alis
ed Inte
nsity T-N2-0h F4b
T-5C-6m F4b
Solvent
90
Figure S8.2a, LD-MS spectra of PC fraction F4a at low and high laser power (top and
bottomandrightrespectively),noDIE,HMAvoltage10kV.
10 100 1000 100000
100
200
300
400
500
Ion C
ou
nt
m/z
T-20C-6m-L LP45% T-5C-6m LP30% T-N2-0h LP60%
10 100 1000 100000
200
400
600
800
1000
1200
1400
1600
Ion
Co
un
t
m/z
T-20C-6m-L LP65% T-5C-6m LP70% T-N2-0h LP70%
91
FigureS8.2b, SECchromatograms(areanormalised)ofPCfractionF4aat300nm.
FigureS8.2c,SynchronousUV-Fspectra(peaknormalised)ofPCfractionF4a.
PCfractionF3c
0.0E+00
2.0E-03
4.0E-03
6.0E-03
8.0E-03
1.0E-02
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Rela
tive
Inte
nsi
ty
T-N2-0h F4a
T-5C-6m F4a + 3c
T-20C-6m-L F4a
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600
Wavelength / nm
No
rmla
ise
d In
ten
sity
T-N2-0h F4a
T-5C-6m F4a + 3c
T-20C-6m-L F4a
Solvent
92
Figure S8.3a, LD-MS spectra of PC fraction F3c at low and high laser power (top and
bottomrespectively),noDIE,HMAvoltagewas10kV.
10 100 1000 100000
200
400
600
800
1000
1200
Ion C
ount
m/z
T-20C-6m-L LP35% T-5C-6m LP60% T-N2-0h LP65%
10 100 1000 100000
400
800
1200
1600
2000
Ion C
ount
m/z
T-20C-6m-L LP45% T-5C-6m LP70% T-N2-0h LP80%
93
FigureS8.3b,SECchromatograms(areanormalised)ofPCfractionF3cat300nm.
FigureS8.3c,SynchronousUV-Fspectra(peaknormalised)ofPCfractionF3c.
PCfraction F3b
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
7.0E-03
8.0E-03
9.0E-03
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Rela
tive Inte
nsi
ty
T-N2-0h F3c
T-5C-6m F4a + 3c
T-20C-6m-L F3c
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600
Wavelength / nm
Norm
alis
ed Inte
nsity
T-N2-0h F3c
T-5C-6m F4a + 3c
T-20C-6m-L F3c
Solvent
94
Figure S8.4a, LD-MS spectra of PC fraction F3b at low and high laser power (top and
bottomrespectively),noDIE,HMAvoltagewas10kV.
10 100 1000 100000
100
200
300
400Io
n C
ou
nt
m/z
T-20C-6m-L LP40% T-N2-0h LP50%
10 100 1000 100000
200
400
600
800
1000
Ion C
ount
m/z
T-20C-6m-L LP50% T-N2-0h LP60%
95
FigureS8.4b,SECchromatograms(areanormalised)ofPCfractionF3bat300nm.
FigureS8.4c,SynchronousUV-Fspectra(peaknormalised)ofPCfractionF3b.
PCfraction F3a(notshownasonlypresentinTN2-0h,seeFiguresS9.1a-c)
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Rela
tive I
nte
nsi
ty
T-N2-0h F3b
T-20C-6m-L F3b
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600
Wavelength / nm
Norm
alis
ed Inte
nsi
ty
T-N2-0h F3b
T-20C-6m F3b
Solvent
96
PCfraction F2
Figure S8.5a, LD-MS spectra of PC fraction F2 at low and high laser power (top and
bottomrespectively),noDIE,HMAvoltagewas10kV.
10 100 1000 100000
100
200
300
400
500
600
700
Ion C
ou
nt
m/z
T-20C-6m-L LP30% T-5C-6m LP20% T-N2-0h LP40%
10 100 1000 100000
400
800
1200
1600
Ion C
oun
t
m/z
T-20C-6m-L LP35% T-5C-6m LP35% T-N2-0h LP70%
97
FigureS8.5b,SECchromatograms(areanormalised)ofPCfractionF2at300nm.
FigureS8.5c,SynchronousUV-Fspectra(peaknormalised)ofPCfractionF2.
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Rela
tive Inte
nsity
T-N2-0h F2
T-5C-6m F2
T-20C-6m-L F2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600
Wavelength / nm
No
rma
lise
d I
nte
nsi
ty
T-N2-0h F2
T-5C-6m F2
T-20C-6m-L F2
Solvent
98
PCfraction F1
FigureS8.6a,LD-MSspectraofPCfractionF1atlowandhighlaserpower(leftandright
respectively),noDIE,HMAvoltagewas10kV.
10 100 1000 100000
100
200
300
400
500
600
700
Ion
Co
un
t
m/z
T-20C-6m-L LP15% T-5C-6m LP10% T-N2-0h LP30%
10 100 1000 100000
400
800
1200
1600
Ion
Co
un
t
m/z
T-20C-6m-L LP35% T-5C-6m LP25% T-N2-0h LP40%
99
FigureS8.6b,SECchromatograms(areanormalised)ofPCfractionF1at300nm;forTN2-
0h therewasweaksignalduetolowabundance.
FigureS8.6c,SynchronousUV-Fspectra(peaknormalised)ofPCfractionF1; forTN2-0h
therewasweaksignalduetosamplelowabundance
0.0E+00
5.0E-04
1.0E-03
1.5E-03
2.0E-03
2.5E-03
3.0E-03
3.5E-03
4.0E-03
4.5E-03
5.0E-03
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Rela
tive
In
ten
sity
T-N2-0h F1
T-5C-6m F1
T-20C-6m-L F1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600
Wavelength / nm
No
rmalis
ed I
nte
nsi
ty
T-N2-0h F1
T-5C-6m F1
T-20C-6m-L F1Solvent
100
S3.9;Figures,bysample(SEC,LD-MSandUV-FofthePCfractions)
TarSampleTN2-0h
Figure S9.1a, SEC chromatograms (area normalised) of PC fractions from TN2-0h, at
300nm.
0.0E+00
2.0E-03
4.0E-03
6.0E-03
8.0E-03
1.0E-02
8 10 12 14 16 18 20 22 24 26 28Time / minutes
Rela
tive Inte
nsi
ty
T-N2-0h F4b
T-N2-0h F4a
T-N2-0h F3c
T-N2-0h F3b
T-N2-0h F3a
T-N2-0h F2
T-N2-0h F1
F4b
F1
F2
F3b
F3a
F3c
F4a
101
Figure S9.1b, LD-MS spectra of the PC fractions from TN2-0h when low and high laser
powerwasused(leftandrightrespectively),noDIE,HMAvoltagewas10kV.
10 100 1000 100000
200
400
600
800
1000
1200
1400
Ion C
ount
m/z
F4b LP70% F4a LP60% F3c LP70%
10 100 1000 100000
400
800
1200
1600
2000
Ion C
ount
m/z
F4b LP80% F4a LP70% F3c LP80%
10 100 1000 100000
200
400
600
800
1000
1200
1400
Ion
Co
un
t
m/z
F3b LP50% F3a LP70% F2 LP40% F1 LP30%
10 100 1000 100000
400
800
1200
1600
2000
2400
Ion
Cou
nt
m/z
F3b LP60% F3a LP80% F2 LP70% F1 LP40%
102
FigureS9.1c,SynchronousUV-Fspectra(peaknormalised)ofthePCfractionsfromTN2-
0h.
TarSampleT5C-6m
Figure S9.2a, SEC chromatograms (area normalised) of PC fractions from T5C-6m, at
300nm.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600 650
Wavelength / nm
Norm
alis
ed I
nte
nsi
tyT-N2-0h F4b
T-N2-0h F4a
T-N2-0h F3c
T-N2-0h F3b
T-N2-0h F3a
T-N2-0h F2
T-N2-0h F1
F3c
F3a
F4b
F2
F3b
F1
Solvent
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
7.0E-03
8.0E-03
9.0E-03
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Rela
tive I
nte
nsity
T-5C-6m F4b
T-5C-6m F4a + 3c
T-5C-6m F2a
T-5C-6m F1
F4b
F1
F2
F4a + 3c
103
Figure S9.2b, LD-MS spectra of the PC fractions from T5C-6m when low and high laser
powerwasused(leftandrightrespectively),noDIE,HMAvoltagewas10kV.
FigureS9.2c,SynchronousUV-Fspectra(peaknormalised)ofthePCfractionsfromT5C-
6m.
10 100 1000 10000
0
100
200
300
400
500
Ion
Co
un
t
m/z
F4b LP50% F4a LP40% F3c LP40%
F3b LP45% F2 LP15% F1 LP10%
10 100 1000 100000
400
800
1200
1600
2000
2400
Ion C
ount
m/z
F4b LP60% F4a LP70% F3c LP65% F3b LP60% F2 LP40% F1 LP40%
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600 650
Wavelength / nm
No
rmalis
ed I
nte
nsi
ty
T-5C-6m F4b
T-5C-6m F4a + 3c
T-5C-6m F2
T-5C-6m F1
F1F2F4a + 3c
F4b
Solvent
104
TarSampleT20C-6m-L
Figure S9.3a, SEC chromatograms (area normalised) of PC fractions from T20C-6m-L, at
300nm.
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
7.0E-03
8.0E-03
9.0E-03
8 10 12 14 16 18 20 22 24 26 28
Time / minutes
Rela
tive I
nte
nsity
T-20C-6m-L F4a
T-20C-6m-L F3c
T-20C-6m-L F3b
T-20C-6m-L F2
T-20C-6m-L F1
F4a
F1
F2F3b
F3c
105
FigureS9.3b, LD-MSspectraof thePCfractions fromT20C-6m-L whenlowandhigh laser
powerwasused (left and right respectively),noDIE,HMAvoltagewas10kV.Note, it
wasverydifficulttoobtainaspectrumfromF3.
FigureS9.3c,SynchronousUV-Fspectra(peaknormalised)ofthePCfractionsfromT20C-
6m-L.
10 100 1000 100000
100
200
300
400
500
600
700
800
Ion C
ount
m/z
F4a LP45% F3c LP30% F3b LP40% F2 LP20% F1 LP10%
10 100 1000 100000
400
800
1200
1600
2000
2400
2800
Ion
Co
un
t
m/z
F4a LP60% F3c LP50% F3b LP50% F2 LP40% F1 LP45%
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 300 350 400 450 500 550 600 650
Wavelength / nm
No
rmalis
ed I
nte
nsi
ty
T-20C-6m-L F4a
T-20C-6m-L F3c
T-20C-6m-L F3b
T-20C-6m-L F2
T-20C-6m-L F1
F3b
F4a
F1F3c F2
Solvent
106
S3.10;Figures,LD-MSofthebulktars
ExamplesoftheinfluenceofthelaserpowerstrengthanduseofdifferentDIEtimes,on
the LD-MS spectra obtained from the bulk tar T5C-6m are shown in Figures S10.1 –
S10.2.
FigureS10.1,LD-MSspectraofthebulkT5C-6m whenzeroand600nsDIEtimewereused
(leftandrightrespectively);laserpowerstrengthandHMAvoltage(kV)aregiveninthe
legend.
10 100 1000 100000
400
800
1200
1600
Ion C
ount
m/z
LP55% H6.5 LP45% H7.5 LP40% H7.5 LP35% H9.0
10 100 1000 100000
400
800
1200
1600Io
n C
oun
t
m/z
LP70% H6.0
LP50% H7.5 LP40% H10.0 LP30% H10.0
107
Figure S10.2, LD-MS spectra of the bulk tar T20C-6m-L when zero DIE time was used,
showingthe influenceof laserpower strengthandHMAvoltage(kV)onthespectrum;
laserpowerandHMAaregiveninthelegend.
10 100 1000 100000
200
400
600
800
1000
1200Io
n C
ount
m/z
LP70% H6.5 LP45% H7.5 LP40% H8.0 LP35% H10.0 LP30% H10.0 LP20% H10.0
108
4.0Pyrolysisofbiomassresidues fromtransgenicplantmaterial
Inmodels of future bio-refineries it is deemed that useof geneticallymodified
plantsthatareeasiertodegradebyenzymeswillbebeneficial.Theresiduesfromthese
biorefinerieswillneedtobeutilizedandthermochemicalconversion isconsideredone
of the most technically advanced methods by which to achieve valorization of these
residues. Recently significant advances have been made in modifying biomass
composition in planta. This includes manipulating the type, amount and degree of
polymerizationof lignin,whichcausesmanyof thepyrolysisproblemsassociatedwith
thethermochemicalconversionofbiomass. Thepossibilityexiststoremovesomeorall
of these R&D challenges by conducting pyrolysis experiments on the genetically
modifiedplants and understanding how thesemodificationsaffect pyrolysisbehavior.
Theoretically,forexample,reducingthelignincontentinplantsshouldreducethefinal
charcontentandquality,andenhancetheyieldsofthelightercomponentsinthebio-oil.
Investigations need to be carried out to ascertain the effects of altering the biomass
compositioninthelignin.
The Joint BioEnergy Institute (JBEI) has developed plants which contain less
lignin and lignin of a smaller degree of polymerization aswell as lignin of a different
composition. We propose to conduct pyrolysis on these plants and ascertain how
modificationsintheplantaffectfundamentalpyrolysisbehavior.Inthispartofthestudy
weaimed tounderstandthedifferencesintheproportionofchar,tarandgasproduced
by the genetically modified plants versus the wild type, and also to understand
differences in the composition of the bio-oil, and char reactivity as a function of the
geneticmodification.
4.1Transgeniclignin samples
Twosamplessetshavethusfarbeeninvestigated,fromplantsengineeredatJBEI.
The first set used an alternative strategy to reduce lignin recalcitrance. A dominant
approachwasdevelopedthatdivertsprecursors fromtheligninpathwayandenhances
productionof C6C1 aromaticsthatareknownasnon-conventionallignin monomersafter
exporttotheapoplast.Comparedwithregular C6C3 monolignols, theseC6C1 monomers
have reduced polymerization properties as they lack propanoid side-chain and its
conjugated double bond, disabling them from undergoing condensation at their b-
109
position. We also demonstrated that C6C1 monomersaccumulate as end-groups in the
ligninofFCA transgenics,resultinginreducedligninDPandcellwallslessrecalcitrantto
enzymatichydrolysis. Thereduceddegreeofpolymerizationisshownforthecellulolytic
enzymaticlignin(CEL)ofArabidopsisplantsinFigure4.1aandbbelow. Duetothelack
of calibrations standards for lignin and difficulty in understanding precise lignin
structurepolydispersityvaluesarealwaysrelativeratherthanabsolute.Neverthelessa
reduceddegreeofpolymerizationwasshowninthesamplesbelow.
Figure 4.Polydispersity of cellulolytic enzymatic lignin (CEL) in thewild type (solid
black line) FCA transgenics (grey line) as analysed by size exclusion chromatography
usingUVabsorbancedetection(Figure4a)andUVfluorescencedetection(b).
110
Thesecondsetof samples,socalledQsubsamples, weredevelopedtohaveareduced
lignincontent,ratherthanareduceddegreeofpolymerization. Twonewsampleswere
createdtohave11%and7%lignincomparedto19%lignininthewildtype.
4.2 Reactorconfigurationforpyrolysisoftransgenicligninplants
Transgenic plantswere available invery small quantities (100’smg), therefore itwas
deemed themost appropriate reactor toanalyses these sampleswas theatmospheric
pressurewire-mess,availableatthelaboratoriesofourcollaboratorsatImperialCollege
London. The reactor is shown in Figure 4.2. The design and principle of wire-mesh
reactorsisdescribedindetailelsewhere.Thebasicdesignconceptoftheseinstruments
is straightforward. Milligram quantities of sample particles are placed between two
layers of folded wire-mesh. This assembly is weighed and stretched between two
electrodes. Finewire thermocouples are attached. A controlled current is then passed
throughthemesh,whichalsoservesasa resistanceheater.After the samplehasbeen
exposed to a pre-programmed time-temperature profile, the weight change of the
assemblyisdetermined.Dependingonthepurposeoftheexperiment,volatilesand/or
tarsmayberecoveredandcharacterised.Thisreactorconfigurationallowsexperiments
tobecarriedoutusingwiderangesofheatingrates(1– 20,000°Cs-1),temperatures(to
2,000°C)andpressures(to160bar).Thisreactorconfigurationprovidesthecurrently
mostoptimizedmethodbywhichtopyrolysesmallquantitiesof transgenicplantsand
theirlignintounderstandsmallchangesinpyrolysisbehaviorwhich mightoccur.
111
4.3 PyrolysisofArabidopsissampleswithreduceddegreeofpolymerization(FCA)
Thepyrolysis oftwosamplesFCAandthecorrespondingwildtype(WT)wascarriedout
intheatmosphericwiremeshreactoratapeaktemperatureof400oC,5sofholdingtime
andaheatingrateof1000oC/s.Theseoperatingconditionswereselectedwiththeaim
ofobservingcleardifferencesbetweenthesamplesduetoincompletepyrolysis.Before
being used, the two sampleswere crushed and sieved between 106 and150µm and
Figure4.1 Theatmosphericpressurewire-meshreactorwithtartrap,maximumheatingrate10,000°Cs-1,batch.Legend:[1]CopperCurrentCarrier;[2]LiveElectrode;[3]BrassClamping Bar; [4] Sample Holder Support Plate; [5] Mica Strip; [6] Wire-mesh SampleHolder; [7] Electrode; [8] Stainless Steel Tubes; [9]Mica Layer; [10] Brass Pillars; [11]SinteredPyrexGlassDisk;[12]BasePlate;[13]PyrexBell;[14]O-ringSeal;[15]Off-takeColumn;[16]O-ring;[17]CarrierGasEntryPort;[18]ConnectionforVacuumPump.
112
driedovernight. ThenomenclatureFCA3andWT3isgivenasthreeseparatesamples
wereused.
Table2showstheproductyieldsobtainedfromthepyrolysisofbothsamples.Duplicate
experimentswerecarriedoutforeachdatapoint.FCA3yieldedslightlylargeramountof
charthanWT3.FCA3hasanincreasedlevelofcondensation(C-Cbonds)whichexplains
the increasedcharyield.On theotherhand,FCA3gaveriseto lesstaryieldthanWT3.
Tarproportion inwithinthetotalvolatile fractionalsodiminished inthecaseofFCA3
comparedtothatinthecaseofWT3.
Table 4.1. Experimental conditions and product yields from Wire Mesh Reactor
experiments of biomasswith reduced degreeof polymerization and its corresponding
wildtype.Heatingrate– 1,000oC/s,Pressure– 1bar,Atmosphere- He
Sample Peak Temperature (oC)Holding time
(s)
Char
(wt.%)
Tar
(wt.%)
Gas
(wt.%)
WT3 400 5 27.8±0.6 37.4±0.2 34.9±0.8
FCA3 400 5 27.1±0.4 34.9±0.1 38.1±0.5
Tars frompyrolysisofWT3andFCA3wereanalysedbygaschromatography(GC)and
sizeexclusionchromatography(SEC).
GCanalyseswereperformedinaPerkinElmer“Clarus500”chromatographwith flame
ionizationdetector(FID).TheGCwasequippedwithanon-polarcapillarycolumnHT5
(25 mx0.32mmand0.1μmfilmthickness).Aflowrateof10mL/minofHewasusedas
carriergaswithasplitratioof5.4:1.Theinitialoventemperaturewas40oC,whichwas
held for 1.0 min. It was then programmed to rise to 380 oC at 10 oC/min with an
isothermheld for10min.Theprogrammedtemperature injectorwasrampedfrom80
oC(held for0.5min)to380oCat100oC/min.TheflamewasmaintainedwithH2 flow
rateof45mL/minandairflowrateof450mL/min.Calibrationswithasetofn-alkanes
(C8toC30)andastandardPAHwereusedtoevaluatethepercentageofelutionof the
material.
Figure 1 shows the chromatograms obtained for the analyses of tars from FCA3 and
WT3. As can be seen, no significant differences are observed between the two tar
samples.
113
Figure4.2. GC chromatograms of tars obtained from thepyrolysis of FCA3 andWT3.
WMRat400oC,5sand1000oC/s.
In addition, Table 4.2 summarizes the data calculated from the n-alkanes and PAH
calibrations. Approximately 80% of the area under the evolved peaks was identified
based on the n-alkanes and PAH calibrations. However, the area identified only
corresponds to around 1 % of the area expected based on the calibration. In other
words,99%oftheinjectedsamplesdidnotevolvethroughtheGCcolumn(materialwith
boilingpointhigherthan550 oC). Thisisbeinginvestigatedfuthre.
Table 4.2.Results from GC analyses of tars obtained from the pyrolysis of FCA3 and
WT3.WMRat400oC,5sand1000oC/s
CompoundBoiling
point(oC)
Tarfrom
WT3(wt.%)
Tarfrom
FCA3(wt.%)
Acenaphthylene 280 0.12 0.14
C29 441 0.63 0.47
Benzo(A)Pyrene 496 0.30 0.24
Indeno(1,2,3,CD)Pyrene&
Dibenz(A,H)Anthracene524&536 0.09 0.00
2.5 7.5 12.5 17.5 22.5 27.5 32.5 37.5 42.5
Time (min)
Tar from FCA3
Tar from WT3
114
Benzo(G,H,I)Perylene 550 0.07 0.00
Percentageidentifiedfromtheareaobtained 74.65 84.58
A 300mm long, 7.5mm i.d. polystyrene/polydivinylbenzene-packedMixed-D column
with5μmparticleswasusedforSECanalyses.Thecolumnwasoperatedat80°Canda
flowrateof0.5mL/min.NMPwasusedasthemobilephase.Detectionwascarriedout
usingaKnauerSmartlinediodearrayUV-absorbancedetector.As NMPisopaqueat254
nm, detection of standard compounds and sampleswas performed at 300nm,where
NMPispartiallytransparent.
A calibration of the Mixed-D column was carried out using two sets of standards,
polystyrene (PS) and polyaromatic hydrocarbons (PAH). The PS-based calibration is
appliedtothe11- 20mintimerange,whilethePAH-basedcalibrationisusedin20-24
minregion,resultinginthefollowingcalibrationequations:
11-20minregion:log10[MM]=10.6320– 0.4038[elutiontime(min)]
20-24minregion:log10[MM]=6.8495– 0.2095[elutiontime(min)]
Figure4.3 showsmolecularweightdistributionbysizeexclusionchromatographyofthe
tarsfromthepyrolysisofFCA3andWT3.Inbothcases,bimodaldistributionsofsignal
areobserved.Theearlyelutingpeakcorrespondstomaterialofmolecularsizeunableto
penetratetheporosityofthecolumnpacking,andisreferredtoas“excluded”fromthe
columnporosity.Theexclusionlimitofthecolumn,definedaccordingtothebehaviour
of polystyrene standards, is about 200,000 u (although molecular conformation is
consideredtobethefactorthatcausesmoleculestobecomeexcludedfromthecolumn
porosity rather than molecular weight). The second eluting peak corresponds to the
materialabletopenetratetheporosityofthecolumnpacking.
Thetwotarsshowsimilarmolecularweightdistributions,withnosignificantdifferences
between them. Both chromatograms show a lift-off in the baseline around 16.7 min,
whichcorrespondstoapolystyrenemassofabout7,740u.Thisisconsideredtheupper
limit in the molecular weight distribution of these samples. Both tars present a
maximumpeakat19.7min,which corresponds toapolystyrenemassof about475u.
115
Tar fromWT3seems tohaveamolecular weightdistribution slightly shifted to lower
valuesthanthetarfromFCA3.However,thisdifferenceisnotsignificant.
Figure4.3. SECchromatogramsof tarsobtainedfromthepyrolysisofFCA3andWT3.
WMRat400oC,5sand1000oC/s.
APerkin-ElmerLS50luminescencespectrometerwasusedforUV-Fanalyses.Thedevice
wassetwithaslitwidthof5nm, toscanat240nmmin-1;synchronousspectrawere
acquired at a constant wavelength difference of 20 nm. A quartz cell with 1 cm path
lengthwasused.Thespectrometerfeaturedautomaticcorrectionforchangesinsource
intensityasafunctionofwavelength.Synchronousspectraofthesampleswereobtained
inNMP.SolutionsweredilutedwithNMP toavoidself-absorptioneffects:dilutionwas
increased until the fluorescence signal intensity began to decrease. All shown spectra
have been peak-normalized to account for the different fluorescence yields of the
differentsamplesandhighlighttheshiftsinthepeakmaximum.
Figure4.4 showstheUV-FspectraofthetarsfromthepyrolysisofFCA3andWT3.The
two tars exhibit similar fluorescence. Themost intense fluorescence for both samples
was centered around 315 nm. Additionally, both tars presented a second peak at the
longerwavelengthof350nm,thetar fromWT3showingaslightlymoreintensesignal.
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
8 13 18 23 28
No
rmal
ised
Are
a
Time (min)
Tar from FCA3
Tar from WT3
116
Figure4.4. UV-FspectraoftarsobtainedfromthepyrolysisofFCA3andWT3.WMRat
400oC,5sand1000oC/s.
ThemainconclusionsobtainedfromthepyrolysisofWT3andFCA3thusfarare:
Charyield fromFCA3washigher than that fromWT3.On the contrary,FCA3gave
risetolesstaryieldthanWT3.
BasedonGC,SECandUV-Fanalyses,therearenosignificantdifferencesbetweenthe
tarsobtainedfromFCA3andWT3.
The change in the degree of polymerization of the samples seems to affect the
pyrolysis product yields but not the composition of the tars produced. More
experimentsareneededtoprovethisstatement.
Sincepyrolysisofthebiomasssamplesisincompleteundertheoperatingconditions
studied,clearerdifferenceswereexpected.Asfurtherconditionsneedtobeevaluate.
4.4 PyrolysisofArabidopsissampleswithreducedamountoflignin(Qsub2)
The pyrolysis of two samples from Set 3 (WT2 and Qsub2) was carried out in the
atmosphericwiremeshreactoratdifferentpeaktemperaturesandholdingtimes,andat
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 350 450 550 650 750
Sig
nal (N
orm
.)
Wavelength (nm)
Tar from WT3
Tar from FCA3
117
aheatingrateof1000oC/s.Beforebeingused,thetwosampleswerecrushedandsieved
between106and150µmanddriedovernight.
Table 4.3 shows the product yields obtained from the pyrolysis of both samples.
Duplicateexperimentswerecarriedoutforeachdatapoint.Qsub2yieldedslightlylarger
amountofcharthanthecorrespondingwildtypeWT2at400oCand5sofholdingtime.
Ontheotherhand,Qsub2gaverisetolessamountoftarfractionthanWT2.Therefore,
tar proportion in within the total volatile fraction also diminished in the case of the
modifiedbiomassQsub2compared to that in the caseofWT2.On theotherhand, the
increase in temperature from400 to 550 oC gave rise to a slight higher yield of total
volatiles.Nevertheless, change in tar yieldwas negligible. Additionally, product yields
werenotaffectedbytheincreaseinholdingtimefrom5to30s.
Table 4.3. Experimental conditions and product yields from Wire Mesh Reactor
experimentsofbiomasswithlesslignincontentanditscorrespondingwildtype.Heating
rate– 1,000oC/s,Pressure– 1bar,Atmosphere– He
Sample Peak Temperature (oC)Holding time
(s)
Char
(wt.%)
Tar
(wt.%)
Gas
(wt.%)
WT2 (S3) 400 5 18.3±0.5 41.7±0.4 40.1±0.1
Qsub2 (S3)
400 5 18.8±1.2 39.6±0.1 41.6±1.3
Qsub2 (S3)
550 5 17.1±0.4 39.9±0.2 43.1±0.6
Qsub2* (S3)
550 30 16.6 40.0 43.4
*furtherreplicatesrequired
TarsfrompyrolysisofWT2andQsub2wereanalysedbyGC,SECandUV-F.
GCanalyses
GCanalyseswereperformedinaPerkinElmer“Clarus 500”chromatographwith flame
ionizationdetector(FID).TheGCwasequippedwithanon-polarcapillarycolumnHT5
(25 mx0.32mmand0.1μmfilmthickness).Aflowrateof10mL/minofHewasusedas
carriergaswithasplitratioof5.4:1.Theinitialoventemperaturewas40oC,whichwas
held for 1.0 min. It was then programmed to rise to 380 oC at 10 oC/min with an
isothermheld for10min.Theprogrammedtemperature injectorwasrampedfrom80
oC(held for0.5min)to380oCat100oC/min.TheflamewasmaintainedwithH2 flow
118
rateof45mL/minandairflowrateof450mL/min.Calibrationswithasetofn-alkanes
(C8toC30)andastandardPAHwereusedtoevaluatethepercentageofelutionof the
material.
Figure 4.5 shows the chromatograms obtained for theanalysesof tars fromWT2 and
Qsub2. As can be seen, no remarkable results were obtained from these analyses.
AlmostnopeakwasdetectedforthetarfromQsub2.Somepeakswereobservedinthe
caseoftar fromWT2,whichshowsthepresenceofsomecompoundelutingwithinthe
temperature range relative to GC analyses. In general, it can be said thatmost of the
componentsinthetarsfromWT2andQsub2presentaMWdistributionthatexceedthe
operatingwindowof theGC(materialwithboilingpointlowerthan570oC).Thus, the
area identified corresponds to less than0.1%of theareaexpectedbasedon thePAH
calibration.
Figure4.5. GCchromatogramsof tarsobtainedfromthepyrolysisofWT2andQsub2.
WMRat400oC,5sand1000oC/s.
Table2 shows the resultsobtained from theGCanalysesofWT2andQsub2biomass.
Resultsaregivenastheratioofthemassobtainedindifferentboilingpointrangeswith
theinitialmassofbiomasschargedontheWMR.Resultsoftars fromSeries1biomass
areincludedforcomparison.
500000
520000
540000
560000
580000
600000
620000
640000
2.5 7.5 12.5 17.5 22.5 27.5 32.5 37.5 42.5
Inte
nsi
ty (
a.u
.)
Time (min)
Tar from WT2
Tar from Qsub2
119
Table4.4.ResultsfromGCanalysesoftars obtainedfromthepyrolysisofFCA3andWT3(Series1)andWT2andQsub2(Series2)
Mass in the BP range/Initial Mass of Biomass (µg/µg)
Boiling point range (oC)
Tar from WT3 (S1)
Tar from FCA3 (S1)
Tar from WT2 (S3)
Tar from Qsub2 (S3)
Tar from Qsub2 @550oC, 5s
Tar from Qsub2 @550oC, 30s
147 - 266 0.027 0.041 0.037 0.005 0.046 0.027
267 - 339 0.080 0.090 0.034 0.010 0.011 0.005
340 - 399 0 0.002 0.020 0.005 0.003 0
400 - 449 0.202 0.152 0.017 0.002 0.033 0.001
450 - 491 0.197 0.078 0.002 0.002 0.016 0
592 - 572 0.050 0 0.004 0.001 0 0.001
573 - 587 0 0 0 0 0 0.021
Total amount analysed (wt.%)
1.53 1.11 0.28 0.07 0.36 0.15
120
Ascanbeseen,theamountanalysed byGCrespecttotheinitialmassofbiomasschargeon
theWMRwasslightlylargerinthecaseofthesamplesfromSeries1.Theamountevolved
inthecaseofQsub2at400oCand5swasnegligible.RegardingtarsfromSeries1,mostof
thedetectedcompoundsevolvedintherangeofequivalentboilingpointof400-490oC.In
the caseof tar fromWT2 (Series2), the small amountofdetectedmaterial eluted in the
rangebetween150and400oC.InthecaseoftarfromQsub2(Series2)at550oCand5s,
the small amountofdetectedmaterial eluted in the rangesbetween150and340 oCand
400 and 491 oC. Regarding the tar from Qsub2 (Series 2) at 550 oC and 30s, the small
amount of detectedmaterial eluted in the ranges between 150 and 270 oC and 575 and
590 oC.
SECanalyses
A300mmlong,7.5mmi.d.polystyrene/polydivinylbenzene-packedMixed-Dcolumnwith
5μmparticleswasusedforSECanalyses.Thecolumnwasoperatedat80°Candaflowrate
of 0.5 mL/min. NMP was used as the mobile phase. Detection was carried out using a
Knauer Smartline diode array UV-absorbance detector. As NMP is opaque at 254 nm,
detectionof standardcompoundsandsampleswasperformedat300nm,whereNMP is
partiallytransparent.
A calibration of the Mixed-D column was carried out using two sets of standards,
polystyrene (PS) and polyaromatic hydrocarbons (PAH). The PS-based calibration is
appliedtothe11- 20mintimerange,whilethePAH-basedcalibrationisusedin20-24min
region,resultinginthefollowingcalibrationequations:
11-20minregion:log10[MM]=10.6320– 0.4038[elutiontime(min)]
20-24minregion:log10[MM]=6.8495– 0.2095[elutiontime(min)]
Figures4.6and4.7 showmolecularweightdistributionbysizeexclusionchromatography
ofthetarsfromthepyrolysisofWT2andQsub2.Inallcases,bimodaldistributionsofsignal
areobserved.Theearlyelutingpeakcorrespondstomaterialofmolecular sizeunable to
penetrate the porosity of the column packing, and is referred to as “excluded” from the
columnporosity.Theexclusionlimitofthecolumn,definedaccordingtothebehaviourof
polystyrenestandards,isabout200,000u(althoughmolecularconformationisconsidered
121
tobethefactorthatcausesmoleculestobecomeexcludedfromthecolumnporosityrather
than molecular weight). The second eluting peak corresponds to the material able to
penetratetheporosityofthecolumnpacking.
Figure4.6 showsmolecularweightdistributionby sizeexclusion chromatographyof the
tars from thepyrolysisofWT2and Qsub2at 400 oCand5 s.The two tars show similar
molecularweightdistributions.Bothchromatogramsshowalift-offinthebaselinearound
16.7 min,whichcorrespondstoapolystyrenemassofabout7,740 u.Thisisconsideredthe
upper limit in the molecular weight distribution of these samples. Both tars present a
maximumpeakaround19.65min,whichcorrespondstoapolystyrenemassofabout475
u.Tar fromQsub2seems tohavea slightlynarrowermolecularweightdistribution than
thetarfromWT2,whichisshiftedtolowerMWvalues.
Figure 4.6. SEC chromatograms of tars obtained from the pyrolysis ofWT2 and Qsub2.
WMRat400oC,5sand1000oC/s.
0.0000
0.0010
0.0020
0.0030
0.0040
0.0050
0.0060
8 13 18 23 28
No
rmal
ised
Are
a (a
.u.)
Time (min)
Tar from WT2
Tar from Qsub2
122
Figure4.7 showsmolecularweightdistributionbysizeexclusionchromatographyof the
tars from the pyrolysis of Qsub2 at different final temperatures and holding times. The
three tars show similar molecular weight distributions. Thus, the three chromatograms
showalift-offinthebaselinearound16.7 min,whichcorrespondstoapolystyrenemassof
about 7,740 u.This is considered the upper limit in themolecularweight distribution of
these samples. Tar from pyrolysis at 550 oC and 5 s presents a maximum peak around
19.78 min, which corresponds to a polystyrene mass of about 440 u. Although the
maximumpeakwasslightlydisplacedtowardslongerretentiontimethanthatoftarfrom
pyrolysis at400 oCand5 s, tar from550 oCand 5sexhibits abroaderMWdistribution.
Thus,thechromatogramcoversbothlighterandheavierMW.Tarfrompyrolysisat550oC
and 30 s presents a MW distribution clearly displaced towards lighter values. It has a
maximumpeakaround19.97min,whichcorrespondstoapolystyrenemassofabout370
u.Therefore,theincreaseininholdingtimeseemstogiverisetolightermolecularweight
distribution.
0.0000
0.0010
0.0020
0.0030
0.0040
0.0050
0.0060
8 13 18 23 28
No
rmal
ised
Are
a (a
.u.)
Time (min)
Tar from Qsub2 @ 400oC,5s
Tar from Qsub2 @ 550oC,5s
Tar from Qsub2 @ 550oC,30s
123
Figure4.7. SECchromatogramsof tarsobtainedfromthepyrolysisofQsub2atdifferent
temperaturesandholdingtimes.
UV-Fspectra
APerkin-ElmerLS50 luminescence spectrometerwasusedforUV-Fanalyses.Thedevice
was setwith a slit width of 5 nm, to scan at 240 nmmin-1; synchronous spectra were
acquiredataconstantwavelengthdifferenceof20nm.Aquartzcellwith1cmpathlength
wasused.Thespectrometerfeaturedautomaticcorrectionforchangesinsourceintensity
as a functionofwavelength. Synchronous spectra of the sampleswere obtained inNMP.
SolutionsweredilutedwithNMP toavoid self-absorptioneffects:dilutionwas increased
until the fluorescence signal intensity began to decrease. All shown spectra have been
peak-normalized toaccount for thedifferent fluorescenceyieldsof thedifferent samples
andhighlighttheshiftsinthe peakmaximum.
Figure4.8 showstheUV-FspectraofthetarsfromthepyrolysisofWT2andQsub2at400
oC and 5 s. The two tars exhibit fluorescence in the same region of the spectra. Similar
profiles are observed; however, Qsub2 present a spectrum slightly displaced towards
shorter wavelengths compared to that from WT2. This result points to the presence of
smaller polynuclear aromatic groups in the case of Qsub2. Thus, the most intense
fluorescence was centred around 314 nm forWT2 and 307nm for Qsub2. Additionally,
both tars presented a second peak at longer wavelength. WT2 showed a more intense
signalatthislongestwavelength,withapeakat360nm.Thesecondpeakwasaround355
nminthecaseofQsub2.The less intense florescenceat shorterwavelengthsuggests the
presence of smaller conjugated aromatic systems in the tar obtained from the modified
biomassQsub2,with reduced amount of lignin. These results are consistentwith earlier
findingsfromSEC,bothofwhichpointtowardhigherconcentrationsoflargerMWgroups
beingproducedinthetarobtainedfromthewildtypebiomass.
124
Figure4.8 UV-FspectraoftarsobtainedfromthepyrolysisofWT2andQsub2.WMRat400oC,5s
and1000oC/s.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 350 450 550 650 750
Sig
nal
(No
rm.)
Wavelength (nm)
Tar from WT2
Tar from Qsub2
125
Figure 4.9. UV-F spectra of tars obtained from the pyrolysis Qsub2 at different temperatures and
holding times.
Figure4.9 showstheUV-FspectraofthetarsfromthepyrolysisofQsub2atdifferentpeak
temperatures (400 and 550 oC) and holding times (5 and 30 s). The three tars exhibit
similarfluorescenceprofiles.Thus,themostintensefluorescencewascentredaround307
nmforallthecases.Additionally,thethreetarspresentedasecondpeakaround354nm.
Conclusions
ThemainconclusionsobtainedfromthepyrolysisofWT2andQsub2inSeries3are:
Tar yield from Qsub2 at 400 oC and 5 s was smaller than that from WT2. On the
contrary,Qsub2gaverisetomorecharyieldthanWT2.
GC did not gave rise to significant results, since less than 0.3 wt.% of the injected
materialseemstohaveelutedfromthecolumn. Thisneedstoberevisited.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 350 450 550 650 750
Sig
nal
(No
rm.)
Wavelength (nm)
Tar from Qsub2 @400oC,5s
Tar from Qsub2 @550oC,5s
Tar from Qsub2 @550oC,30s
126
SECandUV-F results showed slightdifferencesbetween the tarsobtained fromWT2
andQsub2.FindingsfromSECandUV-Fpointtowardhigherconcentrationsofsmaller
aromaticgroupsinthetarobtainedfromthemodifiedbiomassQsub2.
Increasing final temperature and holding time had a slight influence on the products
yields.
SECresultsshoweddifferencesintheMWdistributionofthetarsobtained.
127
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2. Koppatz, S., et al.,H2richproductgasby steamgasificationofbiomasswith in situ
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3. Pan,Y.G.,etal.,Fluidized-bedco-gasificationofresidualbiomass/poor coalblendsfor
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5. Pinto,F.,etal.,Effectofcatalystsinthequalityofsyngasandby-productsobtainedby
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