Comparative study on energy sustainability of biofuel production chains

Comparativestudyonenergysustainabilityofbiofuelproductionchains

DCocco

DepartmentofMechanicalEngineering,UniversityofCagliari,Piazzad’Armi,Cagliari09123,Italy.email:cocco@dimeca.unica.it

Themanuscriptwasreceivedon8September2006andwasacceptedafterrevisionforpublicationon17April2007.DOI:10.1243/09576509JPE365

Abstract:Thispaperisconcernedwithacomparativestudyoftheenergysustainabilityoftheproductionofbiodieselfromoleaginousplants(rapeandsunflower),ofbioethanolfromsugarcrops(sugarbeetandsweetsorghum)andofelectricityfromlignocellulosematerials(miscanthusandshortrotationforestrypoplar).Theresultsshowthelignocellulosefeedstocktoperformbestintermsofbothnetenergyproducedperunitareaofcultivatedland,fromaround184GJ/hatomorethan434GJ/ha,andofenergyratiobetweenenergyproducedandenergyconsumed,intheorderof12–19.Biodieselandbioethanolproductionwerefoundtobelessadvantageousintermsofenergysustainability,especiallywhenresiduesandby-productsarenotusedasfeedstock.Forbioethanolproduction,sweetsorghumexhibitedthehigherenergyratioofaround5.2,duemainlytotheheatrecoveredfromresidueincogenerationplants.Asforbiodieselproduction,neithertherapenorthesunflowerprovedtobeparticularlysustainablewithanenergyratioofaround1.3–1.4,butperformancecanbeimprovedusingtheagriculturalandindustrialprocessingresiduestoproduceenergy,increasingenergyratiosupto3.4–4.2.

Keywords:biodiesel,bioethanol,biomasspowerproduction,energysustainability

1

INTRODUCTION

Thelatestenergypolicydevelopmentsreflectatendencytoencourageincreasedenergyproduc-tionfromrenewablesources,forenvironmental,socio-economic,andstrategicreasons.Amongtherenewableenergysources,biomassplaysakeyrole,somuchsothatinthewhitepaperonrenewableenergyresources,theEUforecastsanincreaseinthecon-tributionofbiomasstogrossinternalconsumptionwithintheEUfromaninitial45Mtoe(milliontonsofoilequivalent)toaround135Mtoeby2010[1–3].Thoughtheuseofresidualbiomasscancontributesignificantlytomeetenergyrequirements,clearlythiscanonlybeachievedthroughthelargescalecul-tivationofdedicatedbiomasscrops.PromotingtheproductionofenergycropsontheotherhandcouldwellhelptoalleviatethecurrentcrisisintheEuropeanagriculturalsector.

Anumberofplantspeciesaregrownworldwidetoproduceliquid,solid,orgaseousfuels.Lignocellulosebiomasscanbeburneddirectlytoproducethermalenergy,orasfeedstockinpowergenerationplants.The

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vegetableoilextractedfromoleaginousplantspecies(suchassoybean,sunflower,rapeseed,etc.)canberefinedasbiodieselandutilizedforpoweringmotorvehiclesorasareplacementforheatingfuels.Othersugar-(sugarcane,beet,sweetsorghum)orstarch-rich(corn,potatoes,wheat,barley)plantscanbeusedtoproduceethanol,thatcanbeusedasapartialreplace-mentforpetrolasmotorfuel.Thesamerawvegetableoilsandtheethanolcanalsobeutilizedtoproduceheatand/orpower[4–6].

However,simplyreplacingfossilfuelswithbiofuelsdoesnotnecessarilyguaranteetheenergyandenvi-ronmentalsustainability.Indeed,thoughitistruethatthechemicalenergyproducedbybiomassisasophis-ticatedformofsolarenergystorage,itisalsotruethatlargeamountsofenergyandmaterialsareoftenrequiredtocultivate,harvestandconvertthebiomassintoaformsuitablefortheenduser(fuelformotorvehicles,heat,orpower),thusemittingpollutantsintotheatmosphere[7–9].

Inthissense,thepresentstudyexaminesbio-fuelsustainabilitythroughacomparativeanaly-sisofthethreemostpromisingbiofuelenergy

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638DCocco

chains,namelypowerproductionfromlignocellulosebiomass,biodieselproductionfromoilcrops,andbioethanolproductionfromsugarrichplants.Energyrequirementsforthecultivation,transport,andcon-versionoftheenergycropsareevaluatedforeachbiofuelproductionchainaswellastheprimaryenergysavingsachievedthroughbiofuelutilization,includ-ingtheutilizationofanyprocessby-productsandresidues.2

ENERGYCROPPRODUCTIONCHAINS

(largestamountofenergyproducedperunitofculti-vatedareaorhighestoutput–inputenergyratio).Asapositiveenergybalanceisanindispensableelementforensuringsustainabilityoftheproductionchain,thisstudyfocusestheattentiononlyontheenergyaspects.Forthethreeproductionchainsthefollowinginputsarehereconsidered:

(a)fuelconsumptionoffarmmachinery,trans-portvehicles,andbiomassprocessing/conversionplantsintheformofprimaryenergy(i.e.includingenergycostsofextractionfromprimarysources,transportationandconversionintomarketablefuels);

(b)electricpowerrequirementsofirrigationpumping

systemsandcropprocessing/conversionplants,reportedastheprimaryenergyrequiredbyther-moelectricpowerstations;

(c)primaryenergyrequiredtoproducefertilizers,

seeds,insecticides,andanyothermeansofproduction(includingenergyrequiredtoman-ufactureandinstallagriculturalandindustrialmachinery).Similarly,theenergyoutputsmadeavailablebythebiofuelproductionchainarerepresentedby:

(a)usefulenergyoutput,equivalenttotheprimary

energyofthefossilfuelreplaced(thatthusalsoincludesextraction,transportation,andconver-sionlosses);

(b)energycreditforusefulresiduesandby-products.Forthethreeproductionchainsspecificprimaryenergyconsumptionforfertilizerswastakenas65MJ/kgfornitrogen-based(N)fertilizers,15MJ/kgforphosphorus-based(P)fertilizers,and10MJ/kgforpotassium-based(K)fertilizers.Primaryenergycon-tentoffossilfuelswastakenas50MJ/kg(includingconsumptionforoilextraction,refining,andtrans-portation),primaryenergyconsumptionforsupplyingirrigationwateras1MJ/m3.Primaryenergyembod-iedinmachinerywastakenas10percentoftotalenergyconsumptionforthecultivationphase.Energyrequirementsfortransportationoftheagriculturalproductshasbeencalculatedtakingas50kmtheaver-agedistancetotheprocessingplantandaspecific

Dependingontheintrinsicpropertiesoftheplantspecies(composition,moisturecontent,density,etc.)andonthebiofuelenergyenduse(motorfuel,heat,orpowergeneration),configurationoftheenergycropproductionchaincandiffersubstantially.Thepresentstudyexaminesthemostpromising,largescalepro-ductionchains,consideringaspectssuchascropyieldandadaptability,levelofindustrialmaturityofbio-fuelproductiontechnologiesandtheenergyenduseoptionsfortheseproducts.Asfarasenduseiscon-cerned,biofuelsforthetransportationsectorandforpowergenerationwouldappeartopromisehigherprofitmargins,alsobearinginmindtherisingcostoffossilfuels,especiallycrudeoilandnaturalgas.Thusthethreemostimportantproductionchainsaresummarizedasfollows:

(a)productionofbioethanolfromsugarcrops;(b)productionofbiodieselfromoleaginouscrops;(c)powergenerationthroughsteampowerplants

fuelledbylignocellulosebiomass.AsshowninFig.1,eachproductionchaincanbedividedintothreedifferentstages:cropcultivationandharvesting,transportationtotheprocessingplant,andconversionintodirectlyusableformsofenergy(liquidfuelsorelectricpower).Foreachstageenergyiscon-sumedandvariousmeansofproductionarerequired,buttheprocessesalsoproduceresiduesandpollutantemissions.Optionappraisalcouldbebasedonpurelyeconomic(lowestenergyproductioncostorhighestcropprofitability),environmental(lowestemissionsofpollutantsorgreenhousegases),orenergycriteria

Fig.1Simplifiedschemeofthebiofuelproductionchain

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Comparativestudyonenergysustainability639

energyconsumptionof2.0MJ/tkm.Primaryenergyrelatedtoelectricpowerwascalculatedtakingas35percenttheaverageefficiencyofthereferencepowergenerationplant.Theusefulenergyofethanolandbiodieselcorrespondstotheprimaryenergysavingsintheformofpetrolanddieselfuel,takenas1.1timestheirlowerheatingvaluesforconsideringtheenergyusedtoproducefossilfuels.Theotherassump-tionsandtheenergybalancesforthethreeproductionchainshereconsideredarediscussedbelow.3

BIOETHANOLPRODUCTIONCHAIN

Ethanol(ethylalcohol,C2H5OH)isacolourless,non-toxicoxygenatedliquidfuel(lowerheatingvalue26.8MJ/kg),thatisproducedviafermentationofsug-arsmediatedbysingle-cellorganisms(yeasts).Alter-natively,itcanbechemicallysynthesized.In2004,worldproductionofethanolamountedtosome40billionlitres(almostdoublethe2000production).Roughly95percentisderivedfromagriculturalproducts,mainlysugarcane,sugarbeet,andsweetsorghum.Ethanolcanalsobederivedfromstarch-bearingcropssuchascorn,wheat,barley,potatoes,etc.,viahydrolysisofthestarchandfermentationoftheglucosetherebyproduced.Ethanolproductionfromlingo-cellulosebiomassviahydrolysisisstillintheexperimentalstage.Seventy-fivepercentoftheworld’sethanolsupplyisproducedbyBrazil(around15billionlitresin2004,fromsugarcanealone)andUSA(around13billionlitresin2004nearlyallderivedfromcorn)[10].

Ethanolisusedininternalcombustionenginesasapartialreplacementforpetrol:itcanbeblendedwithpetrolinvaryingproportionsorusedinrefineriesforproducingethyl-tertiary-butyl-ether,anadditiveusedintheproductionofreformulatedpetrol.Petrol–ethanolblendshavehigherknockresistanceandlowerCOandunburnthydrocarbonemissions[4].

Clearlythechoiceofmost,theconvenientenergycropwillalsohavetotakeintoaccounttheclimateandagronomicfeaturesoftheregionconcerned.AsfarassouthernEuropeisconcerned,andItalyinparticular,cornandwheatarenotconsideredaspotentialenergy

Table1

cropsalsobecauseoftheirlowyields.Ontheotherhand,sugarcaneisatropicalplantandthereforelittlesuitedtothetemperateclimateofMediter-raneancountries.Forthesereasons,thepresentstudyexaminessugarbeetandsweetsorghum.

Inspiteofthegenerallylessfavourableenergyper-formanceofitsproductionchain,sugarbeet(Betavulgaris),isawidelygrowncropinEuropeandItaly.TheaverageyieldforsugarbeetinItalyrangesfrom45to55t/ha,whereasinsomeotherEuropeancoun-tries(France,Belgium,Austria)theaverageyieldsareabout60–70t/ha.Furthermore,uptothejuiceextrac-tionstagetheethanolandsugarproductionchainsfrombeetarethesame.Thus,theinterestinthiscropliesessentiallyinthefactthattheproductionchaincanbesetupwithrelativeeaseandfairlyrapidly,bymodi-fyingexistingsugarbeetrefineries,manyofwhicharethreatenedwithclosureinItalybecauseofchangestothecommonagriculturalpolicyconcerningthesugarbeetsector.Ontheotherhand,similarlytosugarcanesweetsorghum(Sorghumbicolor)hasamuchmorefavourableenergyperformance,butasitisarelativelynewenergycrop,cultivationonalargescalewillcer-tainlyrequirelongertimes.Sweetsorghumisatropicalplantbutadaptswelltotemperateclimateswhereitgrowsduringthespring-summer,evenifitusuallyrequiresirrigation.Theexperimentscarriedoutoverthelast10–15yearsinItaly,Spain,andGreeceshowverypromisingresults,withdrymatterproductionsupto50t/ha[8,11–13].

Anenergyanalysisoftheethanolproductionchainhasbeencarriedoutusingdatareportedintheliter-ature[8–15].Thedatawereappropriatelymediatedandadapted,astheywereofteninconsistentandsometimesnotdirectlyapplicabletothedifferentagri-culturalcontexts.Forsugarbeet,anaverageyieldof50t/hawithasugarcontentof15.5percenthasbeenconsidered;forsorghumatotalbiomass(stem,leaves,andapicalpart)yieldof120t/ha,correspondingto90t/haofstemwitha11.2percentsugarcontentavailableforprocessinghasbeenassumed.

Table1givesprimaryenergyconsumptionforsugarbeetandsweetsorghumcultivation.Thetableshowsthatoverall,theenergyrequirementsforthetwocropsdonotdiffersubstantially(22.9GJ/haforsugarbeet

Primaryenergyconsumptionforsugarbeetandsweetsorghumcultivation

Fertilizers

N

P10015001001500

K88020200

Pesticidesandseeds–600–650

Fuel11055001909500

Irrigation4000m3/ha40005000m3/ha5000

Machines–2078–2335

Total–

22858–

25685

Sugarbeet

Amount(kg/ha)Energy(MJ/ha)SweetsorghumAmount(kg/ha)Energy(MJ/ha)

14091001006500

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Table2Energybalancefortheethanolproductionchain

Energyoutput

Conversion59.272.45

Ethanol95.92111.58

Residues23.0880.37

Netbalance31.881.80

Output/inputenergyratio1.375.17

Energyconsumption

Cultivation

Sugarbeet(GJ/ha)

Sweetsorghum(GJ/ha)

22.8625.69

Transportation5.09.0

against25.7GJ/haforsweetsorghum),theuseoffertilizersandfuelaccountsforroughly70percentofenergyconsumptionandthatsubstantialamountsofenergyarealsousedforirrigation(18–20percent).Energybalanceofthecultivationphasebysimplycon-sideringthelowerheatingvalueoftheagriculturalproducts(1MJ/kgforsugarbeetand2.13MJ/kgforsorghum),showsthattheavailableenergyoutputis2.3timestheenergyconsumedforsugarbeetand7.5timesforsweetsorghum.Inthelattercasetheratioincreasesupto9.9iftheenergycontentofthebiomassasawhole(thatisinadditiontothestem,boththeleavesandtheapicalpartthatusuallyremaininthefieldafterharvesting)isconsidered.

Transportaccountsforasubstantialpartofthetotalenergyconsumedbytheproductionchain:around5.0GJ/haforbeetand9.0GJ/haforsorghum,i.e.22and35percentoftheenergyrequiredforthecropcultivation,respectively.

Theprocessforproducingethanolfromsugarcropsconsistsessentiallyinextractingthesugartopro-duceajuice,fermentationofthesugarbyyeastsfollowedbyethanoldistillingandfinallydewateringofthealcohol–watersolution.Thesugarextractionprocessproducesasolidresidue,namelybeetpulpandsorghumbagasse.Thebeetpulp,whichhasveryhighmoisturecontent(97percentinthecaseexam-inedhere),isdriedbymechanicalprocessingandheattreatmentandusedasanimalfeed.Thesorghumbagasse,whichhasalowermoisturecontent(50percentinthecaseexamined),isrecoveredforuseasfeedstockincogenerationplants.

Ethanolproductionhasbeenhereevaluatedcon-sideringasugarextractionrateof95percentforsweetsorghumand97percentforsugarbeet,afermen-tationyieldof40percentbymassand1.5percentethanollossesduringdistillation.Onthebasisoftheseassumptions,sugarbeetproducesaround3.29t/haofpureethanol(roughly4170l/ha),against3.83t/ha(roughly4850l/ha)forsorghum.

Theethanolproductionprocessrequireslowtem-peratureheatforthefermentationanddistillationphasesaswellaselectricenergyforpoweringthedifferentplantdevices.Powerandheatarebothpro-ducedbyacogenerationplant(basedonasteamextractionturbine),thatisfuelledbythebagasseforsweetsorghumandfossilfuelsforsugarbeet.Forthisreason,theindustrialprocessingofsugar

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beetconsumesasubstantialamountofprimaryenergy(around59.27GJ/ha),whereastheuseofsweetsorghumactuallyyieldsanetelectricityproduction(equaltoabout80.37GJ/haintermsofprimaryenergy)andonlyrequiresaninternalconsumptionoffossilfuelsof2.45GJ/ha.However,anenergycreditof23.08GJ/ha,evaluatedsimplyonthebasisofitslowerheatingvalue(12MJ/kg),canbeassignedtothebeetpulp.

Table2summarizestheenergybalancefortheentireproductionchain,indicatingenergyconsumptionandusefulenergyproduced.Theusefulenergyoftheresiduescorrespondstotheelectricpowerproducedbythecogenerationplantandtotheenergycreditofthebeetpulp.Overall,thesorghumproductionchainexhibitbetterenergyperformance,intermsofbothagriculturallanduseandconversionefficiency.Infact,sorghumproducesroughly16percentmoreethanol(111.6GJ/haagainst95.9GJ/ha),buttheoverallnetenergybalance(alsoconsideringenergyderivedfromtheresidues)isalmostfivetimesgreater(1.8GJ/haagainst31.9GJ/ha).Furthermore,theratioofusefulenergyproducedtooverallenergyconsumedisabout5.2forsorghum,comparedtojust1.37forsugarbeet.Theoverallenergybalanceforsugarbeetworsensfur-theriftheenergycreditofthebeetpulpisnotincluded(givinganetbalanceofjust8.8GJ/haandanout-put/inputratioofaround1.1),whileforthesweetsorghumenergybalanceremainspositiveevenwhenconsideringzeronetelectricityproductionfromthebagasse(netbalanceof74.4GJ/haandoutput/inputratioof3.0).Obviously,theoverallenergybalanceoftheethanolproductionfromsugarbeetcanimprovewithreferencetoCentralEuropecountries(Germany,France,etc.)wherehighersugarbeetandlowersweetsorghumyieldsareachieved.

4BIODIESELPRODUCTIONCHAIN

Thetermbiodieseldenotesliquidfuelsproducedviatransesterificationofrawoilderivedfromoil-bearingcrops.Thechemical/physicalpropertiesofbiodieselaresimilartodieselfuelandconsequentlycanread-ilyreplaceitindieselengines.Theuseofbiodieselleadstopoorerengineperformance(intheorderof5percent),areductioninparticulatematter,CO,SOX,

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andvolatileorganiccompoundsemissionsbuthigherNOXemissions[16].

Biodieselcanbeproducedeitherfromrawveg-etableoilssuchasrapeseed,sunflower,soybean,andpalmoil,orfromanimalfats,seaweed,andusedcookingoils.In2004,worldproductionofbiodieselamountedtoalmost3milliontons,2milliontonsintheEUalone(anincreaseof35percentover2003)wheretheleadingproducersareGermany,France,andItaly.Themostpromisingandwidelygrowncrops,especiallyinEurope,arerapeseedandsun-flower,around84percentofbiodieselbeingderivedfromrapeseedoiland13percentfromsunfloweroil[4,16,17].

Rapeseed(Brassicanapus)isgrownwithoutirri-gationwithautumn–springcycles,iscoldresistantbutalsorathersensitivetospringdrought.Sunflower(HelianthusannuusL.)issowninspringtomatureinsummerandthriveswellunderrelativelyhightemperatures.Despitebeingdroughtresistant,itisusuallygrowninirrigatedareas,especiallyinsouth-ernEurope.AverageyieldofrapeseedcultivationinItalyisaround1.5–1.8t/haagainstanaverageyieldincentralEuropeancountriesofaround3–4t/ha.InItaly,averagesunflowerseedyieldofroughly2.0–2.2t/ha,issubstantiallysimilarwiththoseofAustriaandFrance[18].

Table3givesprimaryenergyrequirementsforculti-vationandharvestofrapeandsunflower.Inadditiontotheassumptionsdiscussedinthepreviouspara-graph,theresultsshowninTable3arebasedonsuitablyadaptedanddetailedevaluationscarriedoutwithreferencetoItalyandotherEuropeancoun-tries[9,16,17,19–22].Inparticular,anaverageannualproductionof1.8t/haforrapeseedand2.2t/haforsunflowerseedshasbeenconsidered.

AscanbeseenfromTable3,energyrequirementsforcultivatingrapearelowerthanforsunflower(around16.8GJ/haagainst22.9GJ/ha),themainreasonbeingthatrapeisgrownwithoutirrigation.Fertilizersandfuelaccountforaround65percentoftotalenergyconsumptionforsunflowerandalmost90percentforrape.Evaluationofthecropssimplyonthebasisofthelowerheatingvalueofproducts(26.3MJ/kgforsunflowerseedand26.8MJ/kgforrapeseed),showsthatthecultivationphasemakesavailable57.9GJ/haforsunflower(2.5timesmoreenergythanthatcon-sumedforcultivation)and48.2GJ/haforrapeseed(2.9timestheenergyconsumed).Inadditiontotheseeds,whicharethemainproduct,rapeandsunfloweralsoyieldlargeamountsofstraw(takenas3.6t/haforrapeand4.4t/haforsunflower),whichcouldbeuti-lizedforenergyproduction(thelowerheatingvalueofstrawhasbeenhereassumedas14MJ/kg).Inthecaseexamined,thesunflowerstrawhasanenergycontentintheorderof61.6GJ/haagainst50.4GJ/haforrapestraw,increasingtheoveralloutput/inputenergyratiotoaround5.2forsunflowerand5.9forrape.

Theindustrialprocessingphaseforbiodieselpro-ductionconsistsessentiallyinextractingtheoilanditstransesterification.Oilisextractedsimplybymeansofamechanicalpress,witharecoveryof80–85percentoftheoilcontent.Thisisgenerallyfollowedbysol-ventextractionusinghexanetorecoveranother15–18percentofoil.Theresidueofmechanicalprocessingisaprotein-richpresscakewhilesolventextractionproducesalessusefulresidue.Theseresidues(about60–70percentoftheoriginalseedmass),especiallythepresscake,areusedinanimalfeedandarehenceaby-productwithanenergycredit.Inthetranses-terificationprocesstheoilreactswithmethanolinthepresenceofbasiccatalysts(potassiumhydrox-ide,sodiumhydroxide,etc.)producingglycerineasamarketableby-product(roughly10percentofbiodieselmass).Energyconsumptionforoilextrac-tionandtransesterificationisrepresentedessentiallybyelectricpower.

Inthepresentanalysisa40percentrapeseedandsunflowerseedoilcontent,98percentoilextractionefficiency,methanol/oilratioof10percentand98percentoil-to-biodieselconversionratehasbeencon-sidered.Onthebasisoftheseassumptions,rapeseedchainproducesaround706kg/haofrawvegetableoil,correspondingtoroughly691kg/haofbiodiesel(around786l/ha),whilethesunflowerchainpro-ducesaround862kg/haofrawoilcorrespondingto845kg/haofbiodieselfuel(960l/ha).

Table3Primaryenergyconsumptionforrapeandsunflowercultivation

Fertilizers

N

Rape

Amount(kg/ha)Energy(MJ/ha)Sunflower

Amount(kg/ha)Energy(MJ/ha)

15097501006500

P50750901350

K707001301300

Pesticidesandseeds–300–950

Fuel7537501155750

Irrigation––

5000m3/ha5000

Machines–1525–2085

Total–

16775–

22935

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Table4

Energybalanceforthebiodieselproductionchainwithoutcomputingtheenergyofresiduesandby-products

Energyconsumption

Cultivation

Transportation0.180.22

Conversion6.457.88

Biodieselenergy33.7141.20

Netbalance10.3110.17

Output/inputenergyratio1.441.33

Rape(GJ/ha)

Sunflower(GJ/ha)16.7822.94

Table5

Energybalanceforthebiodieselproductionchaincomputingtheenergyofresiduesandby-products

Biodiesel+presscake+glycerine

Netbalance

Output/input2.071.90

Biodiesel+straw

Netbalance66.2978.59

Output/input3.653.38

Biodiesel+straw+press

cake+glycerineNetbalance80.9496.50

Output/input4.233.92

Rape(GJ/ha)

Sunflower(GJ/ha)24.9628.08

Table4summarizestheenergybalancefortheentirebiodieselproductionchainfromrapeandsunflowerseedsconsideringbiofueltobetheonlyusefulprod-uct.Overall,verysimilarresultsareobtainedforrapeandsunflower.Infact,forbothchainsonlymarginalamountsofenergyarerequiredtotransporttheseeds,whereasindustrialprocessingaccountsforaround25–28percentoftotalenergyconsumption.Electricpoweraccountsforover55percentofenergyrequire-mentsforprocessing(shownintheformofprimaryenergy),theremaining45percentbeingtheenergyofmethanol(againintermsofprimaryenergy).Netenergyproductionisintheorderof10GJ/ha,withanoutput/inputratiointheorderof1.3–1.4.

However,fromanenergystandpoint,thepresscakeandtheglycerineareimportantby-products.Onthebasisoftheirlowerheatingvalues(12MJ/kgforpresscakeand18MJ/kgforglycerine)theenergycreditofpresscakeismorethan40percentofbiodieselenergy(13.1GJ/haforrapeseedand16.1GJ/haforsunflower),comparedtothe5percentenergycreditoftheglyc-erine(1.5–1.9GJ/ha).Includingtheenergycreditforthepresscakeandglycerinesubstantiallyimprovestheenergybalanceoftheproductionchain.AscanbeseenfromTable5,thisgreatlyimprovesboththenetenergyoutput(uptoaround25–28GJ/ha)andtheoutput/inputenergyratio(uptovaluesofaround2.0).Extensivecultivationofrapeand/orsunflower(10000–20000ha)overarelativelysmallareacouldwellmakestrawharvestingforuseasfeedstockinthermoelectricpowerstationsapromisingoption.Becauseoftheseasonalnatureofthecrops,thebestsolutionwouldbetousestrawasfeedstockforlargethermoelectricpowerplants(300–400MW),forshortperiodsoftimeasareplacementforasmallproportion(5–10percent)ofconventionalfossilfuels(coal),ratherthanforsmalldedicatedpowerplants(10–20MW).Theadvantagewouldbethehigher

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conversionefficiency(40–44percentforlargepowerstationsagainst22–28percentforsmallplants),aswellasthecapitalcostsavingofadedicatedpowerplant.Moreover,thiscouldpotentiallyalleviatemanyoftheproblemsassociatedwithstorage(needforlargestoragecapacity,lossofdrymatter,etc.)offeedstockfordedicatedpowerplants.

Whenusedasfeedstockforlargesizepowersta-tionswith40percentnetefficiency,strawcanattainprimaryenergysavingsintheorderof70.4GJ/haforsunflowerand57.6GJ/haforrape.AsshowninTable5,thisresultsinamarkedincreaseinthenetenergybalanceofthebiodieselproductionchainuptoval-uesofaround78.6GJ/haforsunflowerand66.3GJ/haforrape,andanequallylargeincreaseintheenergyoutput/inputratio,being3.38forsunflowerand3.65forrape.Clearly,takingintoaccounttheenergycreditforthepresscakeandglycerinefurtherimprovesperformance.

Finally,itshouldbeobservedthattheoverallenergybalanceofbiodieselproductionfromrapecouldimproveremarkablywithreferencetoCentralEuropecountries(Germany,France,etc.)wheretherapeyieldisabouttwicethatofItaly.

5ELECTRICPOWERPRODUCTIONCHAINThethermochemicalconversionofbiomassforelectricitygenerationcanbeachievedessentiallyusingexternalcombustionpowergenerationsystems(steamplants,Stirlingengines,ororganicRankinecyclesystems)orgasificationprocessesfollowedbysyngassupplytointernalcombustionenginesandgasturbines.Currently,theonlycommerciallyavail-abletechnologiesformedium-to-largesizeplants(morethan10MW)aresteampowerplants,whereasforsmallsizeplants(10–50kW)Stirlingenginescan

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Table6Primaryenergyconsumptionformiscanthusandpoplarcultivation

FertilizersN

P5075030450

K200200065650

Pesticidesandseeds–300–200

Fuel9750703500

Irrigation5000m3/ha50003000m3/ha3000

Machines–1930–1235

Total–

21230–

13585

Miscanthus

Amount(kg/ha)Energy(MJ/ha)PoplarSRF

Amount(kg/ha)Energy(MJ/ha)

1006500704550

beused.Thoughpromisingintermsofefficiency,gasificationbasedtechnologiesdonotyetqualifyasindustriallymature[23–25].

Themostsuitableplantsforthermochemicalcon-versionareligninandcelluloserichspecies,likefibresorghum,corn,kenaf,miscanthus,andfastgrowingtreessuchaspoplar,robinia,eucalyptus,andwil-low[4].ForclimatesinsouthernEuropeofmajorinterestaremiscanthusandpoplarshortrotationforestry(SRF),thoughtheresultsobtainedcanalsobereadilyextendedtosimilarcrops(fibresorghumandeucalyptusforexample).

Miscanthus(Miscantuss.giganteus),aperennialoriginatinginSEAsia,isrichinligninandcelluloseandhasaC4metabolism.ExperiencehasshownthatitadaptswellalsotosouthernEurope’stemperatecli-mates.Atfullmaturitythemiscanthusstemsreachaheightof3–4m(asmuchas7–10mintropicalregions)withdiametersofaround10mm.Vegetativegrowthperiodbeginsinspringandceasesatthebeginningofautumn.Thegrasscanbeharvestedattheendofwinter(FebruarytoMarch)soastoallowreductionofthemoisturecontenttoaround20–25percent.Thisplantisfairlydroughtresistant,thoughirrigatedcropsproducebetteryields,withmoderatenutrientrequire-mentsandstandlifeofabout10years[4,26–29].Thepoplar(Populusspp.)hasexcellentyieldandharvestcyclesof1–2years.Poplartreescanbegrowninanunirrigatedareas,thoughproductivitybenefitssignifi-cantlyfromevenlowvolumeirrigation(25–30percentofwaterrequiredbycropssuchascorn).Withannualharvestcycles,thepoplarreachesaheightofaround6mwithadiameterofabout8–10cm.HarvestingisdonebetweenNovemberandApril,whenvegetativegrowthceases,withcuttingandchippingharvestingmachines.Annualyieldsareabout35–45t/ha,withamoisturecontentof50–55percent.Afternaturaldry-ing,moisturecontentofpoplarwoodchipsisusuallylower(20–30percent)[4,9,29].

Theelectricpowerproductionchainherecon-sideredsimplyconsistsofthebiomasscultivation,harvesting,andtransportationstages,followedbycombustioninapowergenerationplant.Anannualproductionof25t/haofdrymatterformiscanthus

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and18t/haforpoplar,havingamoisturecontentatharvestof20percentand50percent,respectively,hasbeenconsidered.Table6givestheaverageannualpri-maryenergyrequirementsformiscanthusandpoplarcultivationandharvesting.AsTable6shows,thepoplarrequires36percentlessenergythanmiscant-hus(13.6GJ/haagainst21.2GJ/ha),inrelationtoitslowerfertilizer,fuel,andwaterrequirements.

Similarlytotheethanolandbiodieselproductionchains,theenergyassessmentofthecultivationphasecanbecarriedoutonthebasisofthelowerheat-ingvalueofmiscanthusandpoplar(12.9MJ/kgformiscanthusand7.8MJ/kgforpoplar,accordingtolowerheatingvalueofthedrymatter,16.7MJ/kgformiscanthus,and18.0MJ/kgforpoplar,andmoisturecontentatharvest).Overall,primaryenergymadeavailablebythecultivationphaseisaround402GJ/haformiscanthusagainstabout279GJ/haforpoplar,withanoutput/inputenergyratioof18.9and20.5,respectively.Bearinginmindthelargequantitiesofbiomassinvolved(roughly30t/ha),energyconsump-tionfortransportationtothethermoelectricpowerplantisfarfromnegligible.Infact,consideringanaver-agedistanceof50km,transportaccountsforaround3.1–3.6GJ/ha(15percentofenergyconsumedformiscanthuscultivationand26percentforpoplar).Conventionalsteampowerplantsspecificallydesignedtoburnbiomassgenerallyoperatewithnetelectricoutputsintheorderof10–20MWandeffi-cienciesof25–28percent[24].Higherefficienciescomparablewithcoalfiredsteampowerplants(40–44percent),couldonlybeachievedwithmuchlargerplants(300–400MW),butthisisnotafeasibleoptionasitwouldbealmostimpossibletoobtainasuffi-cientsupplyofbiomass.Asalreadymentioned,wherepossible,thebestoptioniscofiringofbiomassandconventionalfossilfuels(coal)inlargepowerstations.Thisoptionoffersseveraladvantagesineconomic,energy,andlogisticterms.

Table7summarizestheoverallenergybalancesforthetwoelectricpowerproductionlines,forburningbiomassindedicatedpowerstations(netefficiencyof25percent)andcofiringwithcoal(netefficiencyof40percent).Inbothcasesbiomassfedtotheboilerhasa

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4DCocco

Table7Energybalancefortheelectricalpowerproductionchain

Energynetbalance

Biomassenergy401.88279.00

η=25%262.70183.87

η=40%434.93304.50

Output/inputratioη=25%11.7911.70

η=40%18.8618.72

EnergyconsumptionCultivation

Miscanthus(GJ/ha)PoplarSRF(GJ/ha)

21.2313.59

Transportation3.133.60

Table8Synthesisoftheenergybalancesforthethreebiofuelproductionchains

Ethanol

Sugarbeet

Sweetsorghum1.805.17

Biodiesela

Rapeseed24.962.07

Sunflower28.081.90

ElectricalpowerbMiscanthus262.7011.79

PoplarSRF183.8711.70

Netbalance(GJ/ha)Output/inputratio31.881.37

aWiththeenergycreditsofpresscakeandglycerine.bDedicatedpowerplantwith25percentefficiency.

moisturecontentof20percent.Forthepoplarwoodchips,moisturecontenthasbeenreducedbynaturaldrying,asaresultofwhicha10percentdrymatterlosshasbeenalsoconsidered[24].Forcalculatingthenetenergybalance,theelectricpowerproducedwastakentobeequaltotheprimaryenergysavingsachievedwithrespecttoafossilfuelfiredpowerplant.Efficiencyforthistypeofplantwastakenas35percent,includingprimaryenergyconsumptionforfossilfuelextractionandtransport.

Asalreadymentioned,energybalanceofthecul-tivationphaseisstronglyinfavourofmiscanthuswithanetoutputofabout402GJ/haagainstabout279GJ/haforpoplar.Netenergybalanceoftheentirechainisalsomorepositiveformiscanthus(produc-ing44percentmoreelectricity),whilethelowerenergyrequirementsforcultivation,yieldsimilarout-put/inputenergyratios(roughly12),againslightlyhigherformiscanthus.Table7clearlyshowsthemajoradvantagesinenergytermsofcofiringbiomasswithfossilfuels.Thegreaterenergyefficiency(40versus25percent)leadstoaroughly66percentincreaseinthenetenergybalance(withnetbalancesintheorderof435GJ/haformiscanthusand305GJ/haforpoplar)andenergyoutput/inputratios(about19againstabout12).

Onceagain,itshouldbeobservedthatthepresentstudyreferstotheclimateconditionsofItaly,andtheoverallenergybalanceofthebiofuelchainbasedonSRFcouldimprovewithreferencetoCentralandNorthEuropecountries.6

CONCLUDINGREMARKS

Table8summarizestheoverallenergybalancesforthethreebiofuelproductionchainsanalysed.Inparticular,thebiodieselenergyperformancereferstothecasewiththecomputationofenergycreditsofboth

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presscakeandglycerine,whereastheelectricalpowerproductionchainreferstotheconversionofbiomassthroughdedicatedpowergenerationplants.Table8showstheelectricalpowerproductionchaintoper-formbestintermsofbothnetenergyproducedperunitareaofcultivatedland(from184GJ/haofpoplartomorethan262GJ/haofmiscanthus)andofenergyratiobetweenenergyproducedandenergyconsumed(roughly12).Asalreadymentioned,theenergyper-formanceofthischaincanbeimprovedincaseofcofiringthebiomasswithcoalinlargesizepowergenerationplantswithnetenergyoutputsofabout435GJ/haformiscanthusand305GJ/haforpoplarandoutput/inputenergyratiosofabout19.

Afavourableenergyperformancecanbealsoachievedbytheethanolproductionchainbasedonthesweetsorghumcultivation(netenergyout-putofabout155GJ/haandoutput/inputratioofabout5.2),mainlybecauseoftheinternalenergyrecoveryofthebagasseinthecogenerationplant.Biodieselproductionfromrapeseedandfromsun-flowerandbioethanolproductionfromsugarbeetwerefoundtobelessadvantageousintermsofenergysustainability(output/inputenergyratiosofabout1.3–1.4).Forbiodieselproductionperformanceimprovesiftheenergycreditsofindustrialprocessingresiduesarecomputedandespeciallyiftheculti-vationresidues(straw)areusedforpowergenera-tion(output/inputenergyratioscouldincreaseupto3.4–4.2).

Asalreadymentioned,thechoiceofthebestbiofuelproductionchainisremarkablyinfluencedbythecli-mateconditions.Forthisreason,theenergybalanceofthebiofuelproductionchainsforCentralorNorthEuropecountriescouldleadtodifferentconclusions.Forexample,rapeisexpectedtoperformbetterthansunflowerinCentralEuropecountries,aswellasSRFisexpectedtoperformbetterthanmiscanthusinNorthEuropecountries.

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Comparativestudyonenergysustainability5

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