25MWStraw-Fired,HighEfficiencyPowerPlant1.AimTheaimoftheproposedprojecthasbeentodesign,construct,ownandoperateanewelectricitygenerationcapacitybasedonrenewableenergysources,inthiscaseonstraw,withthepossibilityofaddingwoodchips,withasoundtechnologicalandeconomicalbasis.Theowner,EHN,expectsthisprojecttopromotetheimplementationofitsfuturestrategiesandplansinitsfieldsofactivity.Bio-energycropsarealreadybeinginvestigatedatdemonstrationscaleasafuelalternative,also.Fromthepointofviewoftechnologicaldevelopment,theaimhasbeentodemonstratetheinnovativetechnicalsolutions,highefficiencysteamboilerwithhighsteamtemperatureforwoodandstrawfiringwithnewsuperheaterconstruction,tosolveslaggingandcorrosionproblemsuptonowrelatedtocombustionofstraw.Thesuccessfuldemonstrationhascreated(andwillcreate)newprojectandjobopportunitiesbyofferingaviablesolutiontoexploitdifficultbiomassfuelswhichcurrentlyaretreatedmostlyaswaste.2.IntroductionPhoto1:ViewoftheSangesaPowerPlantwithstrawbarnontheright,andboilerbuildingontheleftThepowerplantisanelectricitygenerationfacilitybasedonrenewableenergy,whichsuppliesanetamountof25MWofelectricitytothegrid.Anadditionalpowerproductionofabout2.5MWofelectricityisgeneratedforconsumptionintheownoperationsystemsoftheplant,andheatproductionisnowadaysreleasedatthecondensingsystem,whichiscooledbyawaterintakefromanirrigationchanneloftheIratiriver.Theplantoperationavailabilityisexpectedtobe8.000hours/year,whichleadstoanannualelectricityproductionof200GWh.TheplantislocatedattheIndustrialAreaofRocaforte,nearthelocalityofSangesa,intheregionofNavarra,Spain.Thetechnologyisbasedonaninnovativebiomassboiler,togetherwithaconventionalsteamcircuitandsteamturbineprocess.Thecoretechnologyislocatedintheboiler,whichincludesnovelhangingplatensuperheatersforthesteam,specificallydesignedwithspecialmaterialsandshapesforminimizingcorrosionontheirsurface.Italsoincludesavibratinghydrogratemadeoftwodifferentsections,andaninnovativefeedingsystemdesign,includingsafetydevicesforfireprevention.Thewholeboilerandfeedingsystemisanimproveddesignofthetechnologicalpartner,theDanishcompanyFLSmilj.Asfuel,theplantisinitiallydesignedforusingonlystraw100%,andalsomixturesofwoodchipsandstrawupto50%(inheatvalues).Atthemoment,onlytheinvestmentsinfacilitiesandlogisticsforstrawhavebeencarriedout,butenoughspaceisavailablefortheconstructionofanadditionalbarnandfeedingsystemsforwoodchips.Annualfuelconsumptionoftheplantis160000tons/yearofstraw,mainlyofwheat,barleyandcorn,allofwhichiscollectedallaroundtheregion.Logisticsforfuelcollectioninthefields,handling,transportandstorageincludelong-termcontractswithfarmersandservicecompaniesinordertoprovidethewholeannualsupply.Theplantfirstconnectiontothegridwasachievedon25thJune2002.Afterseveraloperationteststheplanthasreachedfullloadoperation,andresultsarecompletelysuccessful.3.TechnicaldescriptionBoilerThestrawandwoodfiredboilerisanimprovedFLSmiljdesign.Itincludesavibratinghydrogratespeciallydefinedforhandlingthespecificqualityofthestrawtobeburnt.Thegrateismadeintwosectionsthatenabletheuseofstraworamixtureofwoodandstrawasfuel,havinglowashmeltingtemperatureanddifficultsinteringorslaggingproperties.Thestrawisfedtotheboilerthroughfourmechanicalfeedopenings.Thetransportationoftheburningmassandtheashthroughthegrateismadebytheashremovalsystem,whichperiodicallyvibrates,duringafewsecondsevery5minutes.Thecycleofvibrationandtheintervalsareadjustedduringthecommissioningoftheboilerdependingonthequalityofthestraw.Thegrateitselfismadeasaboilerpanelwall,includingperforationsforinputofthecombustionair.Thistypeofgratehasverylowmaintenancecostsandhighreliability.Photo2:Boiler,turbineandofficesbuildingsAshthatfallsdownfromthegrateisthentransportedtoacontainer,andtheflyashiscollectedinthebagfilter.Theremainingparticlecontentinthefluegasesislessthan50mg/Nm3.Fromthebagfilterashhoppers,theashisconveyedtoaflyashcontainer.Boththebottomandflyashesarebeingreusedasorganicfertilisers.Thehotfluegasesareledtothehangingplatensuperheatersinsidetheboilerandthentothesecondpass,wherethefeedwatereconomizerandcombustionairpreheatersarelocated.Economizerandairpre-heatingareusedtocoolthefluegasesdowntoapproximately130Cbeforereleasingthemthroughthestackintotheatmosphere.SuperheatersThenewstrawfiredhangingplatensuperheatersincreasethesteamtemperatureupto540Cdirectlywiththestrawcombustiongasesinsidetheboiler.Duetotherapidhightemperaturecorrosioncausedbythealkalinecontentsofthestrawash,thesuperheatershavebeencarefullydesigned,usingresistantmaterials,anexactdesign(includingtheappropriateadditionalsurfaceareaforheatexchange),andcompletelyeliminatingtheuseofnaturalgasorotherfuelsforsuperheatingthesteam.AresearchprojectwiththePublicUniversityofNavarraisalsoundergoing,aimingatanalysingnewmaterialsforthesesuperheaters.SteamcycleThepowerplantprocessisdesignedforhighelectricalefficiency.Thisisaccomplishedbyusingthecondensateandfeedwaterpre-heating,“internalco-generation”.Thehighpressureandtemperaturesteam(100bar,540C)isfedfromtheboilertotheextraction-condensingsteamturbine.Steamisextractedfromthesteamturbineforpre-heatinginfouruncontrolledextractions.Theexhauststeamfromtheturbineiscondensedinacondenserusingriverwaterascoolingmedium.Fromthecondenserthecondensateisreturnedthroughthepre-heatingstagestothefeedwatertankandfurthertotheboilerforsteamgeneration.Figure1:ProcessdiagramTurbine-GeneratorThesteamturbineisofextraction-condensingtype.Ithasacapacityof30MWattheterminalsofthealternator.Withanauxiliarypowerconsumptionofnearly2.5MW,theaveragenetelectricaloutputis25MW.Thealternatoroutputvoltageis10kV.Theplantisconnectedtothe66kVgridthroughablocktransformer.Separateauxiliarytransformersareusedtofeedtheauxiliarypowerforplantsownpowerconsumption.4.PerformanceoftheplantBeforestartingtheplantoperation,strawcollectionandstorageinthefieldstartedinsummer2000.Campaignsofyears2001and2002wereusedtofinetuningthelogisticsactivitiesandincreasingtheamountofstoredstraw.Theplantfirstconnectiontothegridwasachievedon25thJune2002,whichmeantthestartofthecommissioningandmonitoringphase.Sincethen,severaltestsandadjustmentshavebeenmade,includingthegradualpowerincreaseuntilfullloadwasreachedatthebeginningof2003.InJuly2003theplantisstilladjustingitsproductionparameters,andfullcommercialoperationwillstartaftersummer.Duringthefirstwholeyearofoperationattestinglevel(fromJune2002toMay2003),theplanthasconsumedanamountequivalentto99124tonsofstrawwith11%ofmoisture,andthecorrespondingelectricitysuppliedattheutilitysubstationhasbeen118.94GWh.Theefficiencywillbeimprovedto200GWh/awith160000tonsofstrawoncecommercialoperationwillbereached.Theproposedrenewableenergyprojecthasaclearlypositiveimpactontheenvironmentintheregion,asitreplaceselectricitywhichwouldotherwisebeproducedwithfossilfuels,anditdoesnotaddthecurrentemissionsofstrawcombustion,becausethestrawisoftenburnedonthefields.Onthecontrary,becauseofthecontrolledcombustionandfluegascleaning,theCOandparticleemissionsfromthepowerplantareclearlylower.Itiscalculatedthatwiththebiomassfiredpowerplantof25MW,some210000tons/yearofCO2and4600tonsofSO2-emissionareavoidedincomparisontopowerproductionwithcoal.5.EconomicPerformanceTheoperationalcostconsistsofthefuelsupplycostsandtheoperation&maintenancecostsoftheplant.Costofstrawsupplyisaround36/t,includingeverylogisticactivityuntilfuelreachestheplant.Astheannualconsumptionis160000tons,fuelsupplycostsarearound5760000/year.Ontheotherhand,theordinaryO&Mcostsareestimatedaround12/MWh,plusotherfinancial,amortization,sparepartsandextraordinarycosts.Theexpectedincomesoftheplantwilldependontherateforelectricitysupplytothegridbybiomassmeans,whichisannuallyrevised.Asanaverage,theutilitypurchasestheelectricityatarateof65/MWh,sothattheannualelectricitysaleswillbearound13million.Asfinancialsources,theEuropeanCommissionhascontributedwith3million,andtheSpanish“InstitutoparalaDiversificacinyAhorrodelaEnerga”(IDAE)hasfinancedanother2.4million,plusa