Handbook of Combustion, Volume 4 (Solid fuels)

608 Pages·2010·77.49 MB·English

by  Maximilian Lackner

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1
1
Overview of Solid Fuels, Characteristics and Origin
TobyG.Bridgeman,JennyM.Jones,andAlanWilliams
1.1
OriginoftheSolidBiomassandFossilFuels
1.1.1
FormationofCoal
Millionsofyearsago,giantfernsandtreesflourishedinvastswampareas.Thedecay
of dead plant matter laid down peat beds. These peat beds were later covered by
sedimentation of minerals and the organic matter experienced the geochemical
effectsoftemperatureandpressurethatledtocoalificationofthedepositedmaterial.
Duringthepast300millionyearsthisprocessofpeatformationandcoalificationhas
occurredseveraltimes,leadingtocoalseamscoveredbylayersofsedimentaryrocks.
Coalification is the conversion and metamorphosis of the organic matter from
wood ! peat ! coal due to geothermal temperatures and pressures. As the
coalificationproceedsthecoalbecomesmorematureanditspropertieschange,for
example oxygen content decreases, and the percentage of carbon increases. This
differenceinmaturityofcoalsisknownascoalrank.Table1.1givesthechangesin
properties of coal with rank. These properties as well as the ash content largely
determinethetypeofcombustionequipmentemployedforpowergenerationfrom
coal.ThedevelopmentoftheuseofsolidfuelsisoutlinedinVol.1Ch.21onthe
HistoryofCombustion.
Coal reserves are distributed throughout the world and their properties vary
dependingontheirlocation.Therearetwobasicvariationsseenamongcoals,coal
rankandcoaltype.Coaltypeisdeterminedbytheplantmaterial,algaeremains,
spores,andsoonfromwhichthecoalwasformed.Coalrankisameasureofthe
degree of alteration of the deposited organic matter by chemical and physical
processesovervastgeologicaltimeperiods.Coalpetrographyisthestudyofcoal
type. Coal contains different microscopically recognizable organic constituents
called macerals. These macerals are coalified remnants of the organic starting
biomass materials. Three groups of macerals are recognized in the higher rank
coals (bituminous and anthracite) – vitrinite (from coalified cell wall material),
liptinite (or exinite) (from coalified spores, resins, algae, etc.), and inertinite
HandbookofCombustionVol.4:SolidFuels
EditedbyMaximilianLackner,FranzWinter,andAvinashK.Agarwal
Copyright(cid:1)2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
ISBN:978-3-527-32449-1
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2 1 OverviewofSolidFuels,CharacteristicsandOrigin
Table1.1 Changeincoalpropertywithrank.
Rankstage Coalproperty
! !
Peat Moisture Carboncontent
Lignite Volatilematter Calorificvalue
Bituminouscoal Oxygencontent Aromaticity
Anthracite Hydrogencontent Reflectance
(e.g.,fromcoalifiedcarbonizedwood).Coalsfromdifferentpartsoftheworldhave
differentamountsofthemaceralgroups–forexample,NorthAmericanandUK
coalsarehighinvitrinite.Australiancoalshavehigherinertinitecontent.Table1.2
listsexamplesofthepetrographiccompositionsofcoalsfromdifferentpartsofthe
world [1]. The Hardgrove grindability index (HGI) is included, where available,
andthisphysicalpropertyisdiscussedbelow.Thepropertiesarethusdependent
on the nature of the biomass that they were formed from, their thermal and
chemical history, and also the way the inorganic material from mud and rock
debrisisincorporated.
Themaceralgroupscanbedistinguishedfromeachotherbytheirreflectance(i.e.,
amountoflightreflectedfromapolishedcoalsample).Thedifferentmaceralgroups
havedifferentchemicalanddifferentreactiveproperties,forexampleliptinitehas
relatively high H content, inertinite has relatively high C content, vitrinite is
intermediate;allmaceralgroupsapproachsimilarcompositionsathighrank.
The chemical properties of solid fuels are defined by two important sets of
laboratory measurements, the proximate analysis and the ultimate analysis, and
thesignificanceofthesearediscussedlaterandexamplesgiven.Inaddition,several
othertestsandanalysesareuseful,forexample,thecokingpropertiesofcoal,particle
sizes,hardness,andthetracemetalspresent.
Twocarbonaceousfuelby-productsarealsoavailablefromcoal.Thefirstiscoke,
producedbyslowpyrolysisofintermediaterankcoals,whichpassthroughaplastic
mesophaseduringheating,andusedintheironandsteelindustries.Cokecanalsobe
derivedfrompetroleumpitch,thatis,petroleumcoke.Lowerrankcoalsdonotpass
throughamesophaseduringheating,andslowpyrolysisproducesachar,whichcan
beactivatedtomakeactivatedcarbons.
Table1.2 Maceralanalysisofvariouscoals(%byvolume,ash-free)[1].
Kellingley Pittsburgh(#8) HunterValley Reitspruit Yanowice Hambach
Vitrinite 69 89 74 70 67 82a)
Liptinite(exinite) 19 1 3 3 10 17
Inertinite 12 10 23 27 23 1
Hardgroveindex 34 — 53 — — —
a) Huminite.
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1.1 OriginoftheSolidBiomassandFossilFuels 3
1.1.2
OriginofBiomassFuels
Thetermbiomasscoversawiderangeoffuelssuchaswood,agriculturalcrops,forest
andagriculturalwastesorresidues.Muchofmunicipalsolidwasteisalsocomposed
oflignocellulosicbiomassmaterialintheformofpaper,cardboardandwastewoods,
andsoon.Biomass,whethergrownasforestsorgrass,orproducedspecificallyasan
energycrop,orasanagriculturalwasteresultingfromfoodproduction,isamajor
sourceofenergy.Itiswidelydistributedaroundtheworldbuttheyieldandrateof
growthisdependentonwateravailability,thatis,rainfallandsoilquality.Theterm
biomassalsoincludestheaquaticandmarinebiomass,(micro)algaeandmacroalgae
(seaweed).Biomass,ifgrownsustainably,isarenewableenergyresourceatthepoint
of cultivation and largely carbon neutral, but this is not the case after it has been
harvested,transportedandpreparedforutilizationbecauseoftheinherentenergy
requirements.Becauseoftheneedtomaintainthebiomassresources,sustainability
isamajorconsideration.
Biomass contains significant amounts of oxygen and thus has a lower calorific
valuethancoal.Itis,however,veryreactive,producinglargeramountsofvolatiles
thancoals,theamountproduceddependingontheheatingrate.Thecomposition
determinesthereactivityandtheamountofvolatilesproduced.Themaincompo-
nents of any land-based biomass material (known as lignocellulosic biomass) are
cellulose,hemicellulose,andlignin,withligninhavingthegreateraromaticcontent,
thatis,thehighestcarboncontent.Cellulose,alinearpolysaccharideofD-glucose,is
the main constituent of dry wood, comprising approximately 40–50 wt%. Hemi-
cellulosesarebranchedpolysaccharidesthatconsistofseveralmonomericcompo-
nents;typicalweightsindrywoodare20–30%.Thehemicellulosesinsoftwoodshave
different characteristics to those in hardwoods, and this difference arises from a
differentmixofmixofmonomers.Hemicellulosesarerelativelyeasilyhydrolyzedin
acidtotheirmonomericcomponents(D-glucose,D-mannose,D-galactose,D-xylose,
L-arabinose,andD-glucuronicacidbeingthemainones).Theremainingdryweight
ofbiomassisthelignin,acrosslinkedpolymerofphenylpropaneunits.Lignincanbe
isolatedasaninsolubleresidueafterremovalofthepolysaccharidesbyhydrolysis.
Softwood lignin (guaiacyl lignin) is derived mainly from coniferyl alcohol units,
whilsttypicalhardwoodlignin(guaiacyl-syringyllignin)isacopolymerofconiferyl
andsinapylalcohols.Generally,hardwoodscontainslightlylowerlignincontentthan
softwoods.Table1.3givesexamplebiochemicalcompositions[2,3].Notably,biomass
alsocontainsaminorfractionofcomponentsthataresolubleinwaterororganic
solvents such as hexane, ethanol, acetone, and so on. These are known as the
extractives, are usually less than 10 wt%, and consist of fats, waxes, terpenoids,
steroids, as well as phenolic constituents. [Obviously, in oilseed-crops such as
soybean,rape,etc.,lipids(triglycerides)makeupalargeweightfractionoftheseeds.]
Algaedonotcontainthesamelignocellulosepolymersasterrestrialbiomass,but
are mainly characterized by protein, carbohydrate, simple lipids and glycolipid
contents.Thelipidfraction,whichisofahydrocarbonnature,canbehighinsome
species,andisthusofinterestinbiodieselproductionfromalgae.
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4 1 OverviewofSolidFuels,CharacteristicsandOrigin
Table1.3 Biochemicalcomposition(%)ofvariousbiomassfuelsonadryash-freebasis(daf)[2,3].
Fuelsample Hemicelluloses Celluloses Lignin Extractives
Hazelnutshell[2] 30.4 26.8 42.9 3.3
Wheatstraw[2] 39.4 28.8 18.6 —
Olivehusk[2] 23.6 24.0 48.4 9.4
Beechwood[2] 31.2 45.3 21.9 1.6
Sprucewood[2] 20.7 49.8 27.0 2.5
Corncob[2] 31.0 50.5 15.0 3.5
Teawaste[2] 19.9 30.2 40.0 9.9
Walnutshell[2] 22.7 25.6 52.3 2.8
Almondshell[2] 28.9 50.7 20.4 2.5
Sunflowershell[2] 34.6 48.4 17.0 2.7
Softwood[3] 25–30 35–40 27–30 —
Hardwood[3] 20–25 45–50 20–25 —
Wheatstraw[3] 20–25 33–40 15–20 —
Switchgrass[3] 10–40 30–50 5–20 —
1.1.3
Peat
Peat represents the first stage of conversion of vegetable matter into coal and is
an intermediate product, which chemically has a composition and properties
betweenbiomassandlowrankcoal.Itaccumulatesinswampsatarateofabout
3min 2500years.Becauseofthenatureofformation itisassociatedwithhigh
levels of moisture content and usually some drying has to be undertaken prior
to use. Consequently, the classification largely employed for its use as a fuel is
based on the water content, and the degree of plant decomposition, namely, the
VonPostHumificationScale.ThisisascaleextendingfromH1wheretheplant
material is not decomposed through to H10 where the plant is completely
decomposed and there is no free water. Peats with a value of H4 or greater are
used as a fuel. Such peats can be characterized by conventional solid fuel test
methods (such as those of ASTM) to determine the key parameters such as
moisture content, ash content, and percent organic matter in soil. Peat is not
widely used now, although it is used as a milled power station fuel in some
countries,orindomesticheatingaspeatbriquettes.
1.1.4
DerivedFuelsandWasteorOpportunityFuels
Derived,wasteoropportunityfuelscomemainlyfromdomesticrefuse(municipal
solid waste, MSW), wood products from industrial processes such as furniture
manufacture that cannot be recycled wood, reclaimed wood from demolition,
rubber tires, and plastic wastes. It also includes sewage sludges and animal
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1.2 AvailabilityandResourceBaseoftheFossilandBiomassFuels 5
manures,suchaschickenlitter.Therefuse-derivedfuels(RDFs)areproducedfrom
thelignocellulosiccomponent(andsometimesplastic)intheMSWandcanhave
variablecomposition.Atpresent,theseformasmallpartofthetotalinventoryof
solidfuelsalthoughafterprocessingtheycanbeusedforalltypesofcombustion
units.Amajorissueisthatastheextentofrecyclingincreasesthecalorificvalueof
thesefuelsdecreases.
1.2
AvailabilityandResourceBaseoftheFossilandBiomassFuels
ThemajorsourcesofenergyintonnesoilequivalentaresummarizedinTable1.4
basedondatagiveninReferences[4,5];dataforthetotalusageofbiomassisbasedon
Reference[6].Coalthereforeprovides29%ofallcommercialenergysources,which
reducesto25%ifallenergysourcesareincluded.
1.2.1
Coal
Coalhasbeenusedasamajorsourceofenergyandchemicalsformanycenturies,
anduntilthenineteenthcenturytheresourcewasconsideredtobesolargethatthe
question of lifetime was not considered. However, the considerable exponential
growthrateinusageatthattimeraisedquestionsaboutthecontinuedavailabilityand
supply of coal. This was later developed into the concept of coal reserves and the
lifetimesofthevariousreserves,whichbecameamajorissue,althoughcouplednow
withconcernsregardingenvironmentalimpact.Thetotalresourcebaseisverylarge,
about 18000 Gtoe [7, 8], but not all of this is recoverable. Several statistics are
available, for example [4, 5], listing the proved (or “economically recoverable”)
reservesandtheratesofusageofcoalformostcountries,geographicregionsand
oftheworldasawhole.Theprovedreservesofanthraciteandbituminouscoalare431
Gtoe,andofsub-bituminousare417Gtoe,givingareservestoproductionratio(R/P),
the lifetime under prevailing economics and usage, of 133 years. In terms of
distribution,theUSAholds28.6%ofthereserveswithaR/Pratioof234;overhalf
Table1.4 Annualusageofdifferentenergysourcesintonnesoilequivalent(toe)peryear.
Fuel Usage(Gtoeyr(cid:1)1)
Coal 3.2
Oil 4.0
Naturalgas 2.6
Nuclear 0.6
Renewable(hydroelectricity) 0.7
Biomass(estimated,mainlynon-commercial) 1–2
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6 1 OverviewofSolidFuels,CharacteristicsandOrigin
thereservesareintheformoflignites.ThetotalforEuropeandEurasiais32.1%
(R/P¼224), with the Russian Federation possessing 18.5% (R/P¼500), China
13.5%(R/P¼45),andAustralia9%(R/P¼194).Coalistradedasaninternational
commodityandthuslifetimesforlocalregionshavelittlemeaning.
Estimatesofthelifetimefortheavailabilityofcoalaremorecomplicatedthanthe
R/Pvaluesgivenabovebecauseofthechangeofratesofproductionwithtimeand
because of different estimates of the ultimate recoverable resource available. For
example,theEnergyWatchGroupstatesthat
“Globalcoalreservedataareofpoorquality,butseemtobebiasedtowardsthehigh
side. Production profile projections suggest the global peak of coal production to
occuraround2025at30%abovecurrentproductioninthebestcase.Thereshouldbe
a wide discussion on this subject leading to better data to provide a reliable and
transparentbasisforlongtermdecisionsregardingthefuturestructureofourenergy
system.”
1.2.2
Biomass
Theresourcebaseofbiomassisdependentontheinstantintimewhentheestimate
is made since it is dependent on the rate of usage and the rate of planting and
growth.TheworldbiomassconsumptionlistedinTable1.4is1–2Gtoeyr(cid:1)1and
isdominatedby thenon-commercialcomponent.Thelistedcommercialactivity
is small on a global basis although it is significant in certain countries such
as Sweden, Germany, and Brazil. The resources base is large and, at least in
principle, renewable; the resource available annually is approximately equal to
thatofcoal.
1.2.3
Peat
TherearelargedepositsofpeatintheNorthernhemisphere,inparticularinRussia,
Scandinavia, Germany, and Ireland. The resource base of 461300 Mt peat [9] is
approximately equal to the oil reserves in Saudi Arabia. Consumption is small,
although it is also used for horticultural purposes, but generally the usage is
declining.
1.2.4
WasteMaterials
Thesevaryfromcountrytocountrydependingonthewastecollectionandcompeting
recycling facilities available. Worldwide it is estimated that the total resource of
combustible waste usable as a fuel is about 2.2Gt [10], of which about 1.2Gt is
municipalwasteandtherestindustrial;onlyabout10%isusedasafuelatpresentbut
itshouldbenotedthatthesefiguresareveryapproximate.
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1.3 MethodsofCharacterizingSolidFuels 7
1.3
MethodsofCharacterizingSolidFuels
The chemical and physical properties of solid fuels may be characterized by
laboratory analytical methods largely developed for coals and cokes [11, 12] and
extendedinapplicationtobiomassandwastematerials.Thesehavebeenadoptedas
1) 2)
thestandardtestmethodsinmostcountriesandinparticulartheUSA ,UK ,and
Germany3), and have been incorporated into EU and international standards4),5).
MorerecentlargecoalproducerssuchasAustralia,India,andChinahaveproduced
analogousstandards.Someofthestandardsforcoalandbiomassaregiveninthe
Appendix.
1.3.1
ProximateandUltimateAnalysesofCoalsorBiomass
Proximateandultimateanalysesarethemainstandardlaboratorytestmethodsfor
solidfuels,coal,coke,andbiomassfuels.Theanalysescarriedoutonthesefuelswill
only be representative when the sample received is truly representative of the
material sampled. There are various standard methods available on preparing
samples of coal and for analysis. The major difficulty arises with biomass and
especially waste materials because of the lack of homogeneity of the samples;
consequently,therearespecialstandardsforbiomassandwaste.
Proximateanalysisarisesfromtheterm“approximateanalysis”andisrelatedto
thefactthattheywerenotexact.Atpresentthetestsareempiricalbutareprecisein
that they are carried out using a standard method. Determinations are made of
moisture, volatile matter, fixed carbon, and ash on a mass percent basis. These
analysesareundertakenbyindividualtests.Moistureisdeterminedbyheatingthe
sampleat104–110(cid:2)Cfor1hour,butthemethodappliedwilldependonthenatureof
thesample.Therearealsotechniquesavailabletodetermineequilibriummoisture.
Ash is defined as the residue remaining after combustion under strictly defined
conditions.Mineralmatterreferstothewt%oforiginalmineralsinthecoalpriorto
combustion.Itisdeterminedfromwt%ashusingempiricalrelationships.Inthecase
ofcoalsthecarbonatesandsulfurcompoundsarelargelydecomposed,andinthe
caseofbiomasscarehastobetakentopreventlossofvolatilepotassiumcompounds.
The determination of volatile matter was developed to be relevant to the coking
industryandisbasedonaslowheatingratetoafinaltemperatureto850(cid:2)C.Fixed
carbonisthematerialremainingafterthedeterminationofvolatilematter,moisture,
1) ASTMInternational,100BarrHarborDrive,POBoxC700,WestConshohocken,PA,19428-2959
USA.
2) BSIBritishStandards,389ChiswickHighRoad,London,W44AL,UnitedKingdom.
3) DIN,DeutschesInstitutfu€rNormunge.V.,Burggrafenstraße6,10787Berlin,Germany.
4) CEN,AvenueMarnix17,B-1000Brussels,Belgium.
5) InternationalOrganizationforStandardization(ISO),1,ch.delaVoie-Creuse,Casepostale56,1211
Geneva20,Switzerland.
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8 1 OverviewofSolidFuels,CharacteristicsandOrigin
andash–thatis,itisameasureofthesolidcombustiblematerialinthefuelafterthe
expulsionofvolatilematter:
wt%fixedcarbon¼100(cid:1)wt%volatile matter(cid:1)wt%moisture(cid:1)wt%ash
ð1:1Þ
It is possible to undertake all the analyses instrumentally using TGA (thermo-
gravimetric analysis), where all the parameters are determined in one operation.
WhileTGAproximateanalysisisquitewellvalidatedforcoals,thisisnotthecasefor
biomass, where TGA tends to give a higher volatile yield and lower ash than the
standardmethodsforbiomass.
Theproximateanalysismethodisnotrelevanttotheuseoffuelsunderthehigh
heatingratesandhigherfinaltemperaturesfoundinfurnaceflames.Forsuchcases
“hightemperaturevolatilemattercontent”hasfoundapplication,andismeasured
onnon-standardequipment,suchasaheatedwiremeshreactororahightemper-
aturedroptubereactor.However,nostandardtestmethodisavailable.
In ultimate analysis the amounts of C, H, S, and N are determined in the
combustion products resulting from complete combustion; oxygen can be deter-
mineddirectly,butismorecommonlycalculatedbydifference:
wt%O¼100(cid:1)ðwt%Cþwt%Hþwt%Sþwt%NÞ(cid:1)wt%moisture(cid:1)wt%ash
ð1:2Þ
Theapplicationofultimateanalysisforcoalandcokeiswellestablished.Biomass
has much higher O and smaller N and S contents (in general) and so different,
relevantstandardsareneededfortheanalysis.MeasurementcanbemadeofCland
thetracemetals.
Resultscanbepresentedonthebasisofan“asreceived”(ar),drymineralmatter
free(dmmf)basisorcorrectedtoa“dryashfree”(daf)basis,forexample:
100(cid:3)measured; wt%C
wt%CðdmmfÞ¼ ð1:3Þ
100(cid:1)moisture; wt%(cid:1)wt%MM
100(cid:3)measured; wt%C
wt%CðdafÞ¼ ð1:4Þ
100(cid:1)moisture; wt%(cid:1)ash;wt%
1.3.2
CalorificValue
Another important test is the calorific value, which is usually undertaken using a
bombcalorimeter.Thegrosscalorificvalue(GCV)(highheatingvalue,HHV)isthe
quantity of heat evolved when a unit mass of fuel is completely burned and the
combustion products cooled to 288K. The net calorific value (NCV) (low heating
value,LHV)isthequantityofheatevolvedwhenaunitmassoffueliscompletely
burned.Manyempiricalformulaefortheestimationofthecalorificvalueofcoalfrom
theultimateorproximateanalyseshavebeenproposed.Frequently,itisestimatedon
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1.3 MethodsofCharacterizingSolidFuels 9
thebasisofacorrelationbasedontheultimateanalysis[13];forexample,theDulong
formulaforcoalfromdryash-freeweightpercents:
GCVðkJkg(cid:1)1Þ¼338:55ð%CÞþ1443:9ð%HÞ(cid:1)180:6ð%OÞþ105ð%SÞ
ð1:5Þ
andforbiomassfromas-receivedultimateanalysisweightpercents[14]:
GCVðkJkg(cid:1)1Þ¼3:55ð%CÞ2(cid:1)232ð%CÞ(cid:1)2230ð%HÞþ51:2ð%C(cid:3)%HÞ
þ131ð%NÞþ20600
ð1:6Þ
Thenetcalorificvalue(MJkg(cid:1)1,drybasis)canbecalculatedfromthegrosscalorific
value (dry basis) if the hydrogen fraction (H on an “as received” basis) and the
moistureareknown,accordingto:
NCVðMJkg(cid:1)1Þ¼GCV(cid:1)2:45½moistureþ9H(cid:4) ð1:7Þ
wherethelasttermcorrespondstothelatentheatofevaporationofwater.
ExamplecalorificvaluesaregiveninSection1.4.
1.3.3
AshComposition
Asuiteoftenelementsisnormallyincludedintheanalysisofmetalsofsolidfuels,
namely,silicon,aluminum,titanium,iron,calcium,magnesium,sodium,potassium,
phosphorus,andsulfur.Inaddition,chlorineandcarbonateandmanganeseareuseful
inclusionsintheanalysis.Thereareseveralstandardmethodsforthedeterminationof
ashcomposition,includingX-rayfluorescence,ordigestionoftheashanddetermi-
nationbyatomicadsorptionspectroscopy(AAS)or,morerecently,inductively-coupled
plasma(ICP)witheithermassspectraloratomicemissiondetection.Typicalanalyses
forvarioussolidfuelsaregiveninSection1.4.Forcertainfuels,forexample,MSWor
manures/sludgesitisappropriatetomeasuretheheavy/tracemetalcontent.Themain
targetsarearsenic,barium,cadmium,chromium,lead,mercury,selenium,andsilver.
SomeofthemetalscanbedeterminedbyICPbysimilarmethodstothemainash
components.However,thereareseparatestandardsforthemorevolatilemetalssoasto
minimizelossesduringtheashpreparationandanalysis.
1.3.4
AshFusibility
Ashfusibilityexaminesthesofteninganddeformationbehaviorofapyramidofthe
ash at high temperature in a controlled furnace by a standard test method (5 or
8Kmin(cid:1)1)(Appendix).Measuredaretheinitialdeformationtemperature(pointat
whichthepyramiddisappears),hemisphericaltemperature(heightofspecimen¼1/
2
thebaselength)andflowtemperature(heightofspecimen¼1/ oftheheightithadat
3
thehemisphericaltemperature).
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10 1 OverviewofSolidFuels,CharacteristicsandOrigin
1.3.5
PhysicalProperties
The important physical properties are density, both real (intrinsic) and apparent,
porosity, and hardness in relation to transport and to milling. Other physical
properties can be important depending on the form of combustion. Forexample,
coal swelling is important in relation to PF (pulverized fuel) and fluidized bed
combustion.
Bulkdensityismeasuredbyweighingaknownvolumeofthesolidfuel.Inthe
caseofcoalthismightbelumpcoal,whileinthecaseofbiomassitcanbebaled,
chipped,orpelletized–whateverformisusedfortransportingthefuel.Thebulk
density is a useful parameter for determining the transportation costs and fuel
storageneeded,andisalsoneededtocalculatethevolumetricenergydensity(i.e.,
heatingvalueperm(cid:1)3).
Intrinsicdensityisthedensitycalculatedwhenanyporesandvoidsareignored;it
canbemeasuredfromthevolumeofheliumpenetrationintoaknownmassoffuel,
and is sometimes referred to as the helium density. For coals and carbons it is
consideredtobethedensityofthecarbonstructureandforbiomassitisthedensityof
thecellwallmaterial.
The porosity of solid fuels is the volume percentage occupied by the pores,
which, for most coals and carbonaceous solids, can be calculated from density
measurements using helium (penetrates all pores <0.3nm) and mercury (no
penetration into the pores at low pressure). This latter point is true if pores are
smaller than 10mm diameter, which is generally not the case for plant cell wall
material!
TheHardgrovegrindabilityindex(HGI)(ASTMD409-08)measurestherelative
hardnessofacoalrelativetoastandardcoalthathasanindexof100.Alowerindex
valueindicatesamoredifficultcoal.TheHGIisastandardmeasureoftheeaseof
millingofcoaltothefineparticlesizesusedinpulverizedcoalpowerstations.Itis
not,however,agoodmeasurementforbiomass,sincemostbiomasswillnotmill
completely in coal crushing mills. This is an area where further development of
standardtestsforbiomassisneeded.
Inadditiontothelaboratorymethods,othertechniquessuchastheuseofsmall-
scale drop tube furnaces and pilot test furnaces may be employed to simulate
conditionsinalarge-scalemulti-burnerboilerfurnace.
1.4
PhysicalandChemicalPropertiesoftheSolidFuels
Dataoncoalareavailableinseveralcompilations,inparticulartheArgonneNational
Laboratory [15] Premium Coal Bank and the British Coal Utilisation Research
Association(BCURA)CoalSampleBank[1].Dataonbiomassareavailableinthe
ECNPhyllisbiomasscompositiondatabase[16].Typicalexamplesofthesolidfuels
aregiveninthetablesbelow.