UTILIZATION OF EXHAUST STEAM OF WASTE TO ENERGY (WTE ...

UTILIZATION OF EXHAUST STEAM OF WASTE TO

ENERGY (WTE) POWER PLANTS FOR WATER DESALINATION

JingyaWeiAdvisor:ProfessorNickolasJ.Themelis

SubmittedinpartialfulfillmentoftherequirementsforthedegreeofMasterofScienceinEarthResourcesEngineering

DepartmentofEarthandEnvironmentalEngineering

TheFuFoundationSchoolofEngineeringandAppliedScienceColumbiaUniversity

December2015

Researchco-sponsoredby:

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UTILIZATION OF EXHAUST STEAM OF WASTE TO ENERGY (WTE) POWER PLANT FOR WATER DESALINATION

EXECUTIVESUMMARYThereareoveronethousandwastetoenergy(WTE)plantsoperationalaround

the world to recovery energy and generate electricity from 250 million tons ofmunicipal solid waste (MSW) per year. The most common WTE plants areelectricity-only producers and use the heat of combustion to produce steam thatpowersaturbinegenerator.Tomaximizeelectricityproductivity,theexhauststeamfromturbineissenttoanairorwatercooledcondenser,thenrecycledtotheboiler.ForatypicalWTEplantof10t/hcapacity,thegrosselectricitygeneratingcapacityisabout6MWh/hour,whichintheU.S.representsabout20%oftheenergycontainedintheMSWcombusted.Suchplantsdonotmakeuseoftheenergycontainedintheexhauststeamfromturbine.

ToimprovetheenergyefficiencyofWTEplants,manyEuropeanWTEplantsarecombinedheatandpower(CHP)facilities.Thesefacilitiesselltheextractorexhauststeam from turbine for district heating (DH) system. A CHP WTE plant of 10 t/hcapacityand50%steamextractioncanproduceupto5MWh/hourofelectricityplus 31,000-62,500MJ/hofheat.Although20%ofelectricpowerissacrificed,theoverallefficiency increases from 20% to at least 65% compared to the condensing WTEplants. heat recovery system, the thermal efficiency for these facilities can reach80-90%.

However, the use of theWTE exhaust steam for district heating is limited toclimates with cold winter and also by the cost of DH transmission systemconstruction..This thesisexaminesanotheroption forutilizing the turbineexhauststeamasanenergysourceforthermalwaterdesalination.

Desalinationreferstoprocessesthatremovesaltandothermineralsfromsalinewater to provide freshwater. Almost 16,000 desalination facilities have been builtworldwide, producing over 70 million cubic meters of potable water. Waterdesalination uses either thermal or membrane technologies. Main technologyincludes electricity powered reverse osmosis (RO), steam and electricity poweredmultiple-effect distillation (MED) and multi-stage flash evaporation (MSF). Withlower energy consumptionof 1-7 kWh/m3 anddecreasing cost of 0.1-1.0USD/m3,membrane(RO)desalinationisapreferablechoiceformostdesalinationplantsandrepresents60%oftheinstalledcapacityworldwide.TheMSFdesalinationunitshavea relative high temperature demand, 100-130°C for feed steam and 90-120°C foroperation,resultinginanhigherenergyconsumptionof13.5-25.5kWh/m3electricalequivalent and cost of 0.5-1.75 USD/m3. The MED process can operate with lowpressure(0.3-0.5bar),lowtemperature(70-90°C)steambynearlyvacuumpressurecondition.Withlowerelectricconsumptionof1.5-2.5kWh/m3thanMSF,thetypicalproductioncostofMEDisaround0.7-1USD/m3,competitivetoROtechnology..

Inspired from CHP plants for DH, two power-desalination cogeneration plantswereexamined in this study:TheSt.BarthWTE-desalinationcogenerationplant in

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theCaribbeanandtheHebeiHuanghuapower-desalinationplantinChina.Withonly1.5t/hMSWcapacity,theSt.BarthWTEplantsells67%ofrecoveredenergytotheMED desalination plant which produces 1350 m3 freshwater per day. The heatconsumption forpotablewaterproduction is8.6MJ/m3 (40.6kWh/m3). Inaddition,about 8.5% of the chemical energy in the MSW is transformed to electricity,sufficient for theelectric consumptionofboth the WTEand theMEDprocess. Inbrief, this plant helps St. Barth to dealwith itsMSWwhile also solving thewatersupplyissueoftheisland.

TheHuanghuapowerplant isacoal-poweredplantwith2,520MWgenerationcapacity, locatedonthePohaiGulfofChina.Anestimated3,200,000-4,400,000m3offreshwaterisconsumedannuallyasfeed-waterfortheboilerunits,aswellasfordesulfurization and other processes. Huanghua solved this massive waterconsumptionthroughseveralcoupledMEDdesalinationunitswhichusetheturbines’exhaust steamas theheat source.TheseMEDunitsproduce57,500m3/dof freshwaterandconsume40%oftheenergyinthecoalinput.Inadditionto meetingitsown needs, the plant also produces up to 10 million tons of freshwater for use(14,000-28,000m3/d)bythePortofHuanghua.Withthiscombinationofelectricityandfreshwaterproduction,90%oftotalenergyinthecoaltotheplantistransferredtovaluableproducts,resultinginanestimatedsavingof500tonsperdayofcoal.

This study showed that the cogeneration of power andwater is an ecologicaland economical solution, both for WTE and fossil fuel-fired power plants.ConsideringthebenefitsandrequirementsofWTE-desalinationcogenerationplants,themostpromisingplacesforthistechniquearethosetropicalislandsorgulfcitieswith little access to land, fuels and waters. Suitable regions include theMediterranean Sea (Cyprus, Crete, etc.), Persian Gulf countries (Kuwait, Bahrain,Oman,etc.),CaribbeanSeaislands,theRedSea,etc.ThecasesofCyprusandUnionTerritoryofLakshadweep(India)wereanalyzed.Thisstudyalsoincludedtheenergyandresourcesidebenefits.However,additionalcost-benefitanalysisisnecessaryforaspecificgeographicarea.

AcknowledgementsFirst, I would like to express my appreciation to my advisor, Professor Nickolas J.Themelis, forhisvaluableandopportuneguidanceuponmythesiscompletion.Hispatience,motivationandabundantexperiencegavemealotofencouragementandinspiration during my work. I also want to show my thanks to my dear LilianaThemelisforallherkindnessandsupportduringmystudyatColumbiaUniversity.

Inaddition, I amgrateful to thankSIDEMandShenhuaGroupe for sharingme thedataof theirdesalinationequipmentandplants. Lastbutnot least, Iwould like tothankmyparentsandfriends.It’stheirsupport,companyandloveencouragingmeovercomethedifficultiesanddisappointmentsthroughoutmyacademiccareer.

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TableofContents

ExecutiveSummary

Acknowledgements

1.Background............................................................................................7

2.EnergyFlowAnalysisofTypicalExhaustSteamUtilization.................122.1.WithoutExhaustSteamTreatmentandUtilization.....................................12

2.1.1.DescriptionofProcess......................................................................122.1.2.ThermodynamicSimulatedModelofWTEProcess.........................132.1.3.EnergyFlowofSimulatedWTEModel.............................................15

2.2.ExhaustSteamCondensation......................................................................202.2.1.Condensingsteamturbine...............................................................202.2.2.ThermodynamicSimulatedModelofCondensingWTEPlant..........222.2.3.DiscussionofCondensationSimulationModelofWTEProcess......22

2.3.ExhaustSteamforDistrictHeating..............................................................252.3.1.DistrictHeatingandCogenerationWTEPlants................................252.3.2.ThermodynamicSimulationofCHPWTEPlant................................262.3.3.DiscussionofCondensationSimulationModelofWTEProcess......27

3.MainDesalinationTechnologies..........................................................323.1.RODesalinationTechniques........................................................................333.2.ThermalDesalinationTechniques................................................................36

3.2.1.MultipleEffectDistillation(MED).....................................................363.2.2.MED-TVCandMED-MVC..................................................................383.2.3.Multi-StageFlash(MSF)Distillation.................................................40

3.3.ComparisonofPopularDesalinationTechnologies.....................................42

4.CaseStudyofCogenerationDesalinationPlant..................................434.1.St.BarthWTEDesalinationCogenerationPlant..........................................43

4.1.1.BackgroundofSt.Barth....................................................................434.1.2.WasteManagementandWaterProductioninSt.Barth..................444.1.3.WTE-DesalinationPlantEnergyFlowEstimation.............................45

4.2.HebeiHuanghuaPower-DesalinationCogenerationPlant..........................484.2.1.IntroductionofHuanghuaPowerPlant............................................484.2.2Coal-FiredPower-WaterCogenerationEnergyFlowEstimation.......49

5.LocationsforWTE-DesalinationPlantsDevelopment.........................535.1.Cyprus..........................................................................................................545.2.UTofLakshadweep.....................................................................................56

6.Conclusion...........................................................................................61

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REFERENCES............................................................................................64

ListofFigures

Figure1WTEProcessandApplication.........................................................................7Figure2Cumulativecontractedandcommissioneddesalinationcapacity1965-20119Figure3Totalworldwideinstalledcapacitybyfeedwatercategory............................9Figure4Top10countriesbytotalinstalledcapacitysince1945.................................9Figure5Installedmembraneandthermalcapacity,1980-2010(cumulative)..........10Figure6Top10countriesbytotalinstalledthermalcapacitysince1945.................10Figure7SchematicDescriptionofWTEPlant............................................................13Figure8EnergyFlowofBasicSimulatedModel–WithoutExhaustSteamTreatment

andUtilization(Unit:MJ/h,BasedonMSWincinerationcapacityof10t/h)...17Figure9Water-cooledsurfacecondenser..................................................................21Figure10Air-cooledsurfacecondenser.....................................................................21Figure11ExtractionSteamTurbine...........................................................................26Figure 12 Energy Output for Cogeneration WTE Plants with Different Steam

ExtractionRatio.................................................................................................29Figure13Totalworldwideinstalledcapacitybytechnology.....................................32Figure14MechanismofOsmosisandReverseOsmosis............................................33Figure15RODesalinationPlants...............................................................................34Figure16SpiralWoundROMembrane......................................................................34Figure17AnnualizedSWROCostTrends...................................................................35Figure18SchematicofMEDEvaporatorandSingleEffectUnit.................................37Figure19SchematicofMED-TVCProcess..................................................................39Figure20SchematicofThermocompression.............................................................39Figure21SchematicofMED-MVCProcess................................................................40Figure22MSFDesalinationProcessandMSFCell.....................................................41Figure23MapofSt.Barth.........................................................................................43Figure24WTE-DesalinationPlantofSt.Barth...........................................................44Figure25CombustibleWasteDriedandMoved........................................................44Figure26GuohuaCangdongPowerPlantDesalinationFacility(25,000t/d)..............48Figure27SchematicofShenhuaGuohuaLarge-scaleMEDProcess(12,000t/d).......49Figure28MapofCyprus............................................................................................54Figure29MSWCompositionofCyprus......................................................................55Figure30MapofLakshadweep.................................................................................57Figure31SchematicofLTTDprocessforKaravatti.....................................................59

ListofTables

Table1EnergyFlowResultofBasicSimulationModel..............................................16Table2DesignparametersofsomeWTEplantsandsimulatedmodels....................18Table3EnergyFlowAccountingofWTEPlants(ForChapter2.1)..............................19

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Table4EnergyFlowResultofCondensingSimulationModel....................................23Table5EnergyFlowAccountingofWTEPlants(ForChapter2.2)..............................24Table6EnergyFlowResultofThreeDHSimulationModel.......................................28Table7ConsumptionandCostofFossilFuelsDH......................................................28Table8EnergyFlowAccountingofWTEPlants(ForChapter2.3)..............................30Table9WaterBoilingPointTable...............................................................................36Table10ComparisonofCharacteristicsofMainDesalinationTechnologies.............42Table11OperationDataofSt.BarthWTEPlant........................................................45Table12ParameterEstimationofSt.BarthWTEPlant..............................................45Table13OperationParameterofSt.BarthDesalinationPlant..................................46Table14EnergyFlowResultofSt.BarthWTE-DesalinationPlant(1hScale).............46Table15EnergyFlowAccountingofSt.BarthWTEPlants.........................................47Table16IntroductionofHuanghuaPowerPlant.......................................................48Table17HuaghuaPowerPlantTurbineOperationData............................................50Table18OperationParameterofHuanghuaDesalinationUnits................................50Table19EnergyFlowResultofHunaghuaPower-DesalinationPlant(1hScale)........51Table20EnergyFlowAccountingofHuanghuaPower-DesalinationsPlants.............52Table 21 Distribution of MSW in Cyprus Can Be Utilized for Energy Recovery per

Region...............................................................................................................55Table22OperationParameterofCyprusDesalinationPlants....................................56

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1.BackgroundThermalenergycanberecoveredfrommunicipalsolidwaste(MSW)byincinerationbefore it transports to landfilling, typically being output in the form of steam,electricityorhotwater.Simultaneouslythevolumeofwastecanbereducedbyupto80%-90% with a mass reduction of 50%. According to a 2012 market reportpublishedbyGermanenvironmentalconsultancy,ecoprog, thereare2150 thermalwastetoenergy(WTE)plantsoperationalaroundtheworldwithalmost250milliontons waste treatment capacity per year. In addition, 250 new waste to energyfacilitieswith a total capacityof 70million tonsper yearwill be commissionedby2016,mainlyleadingbytheinvestmentofChinaandEurope[1].ThemostprovenanddominantWTEtechnologyismassburning,orgratecombustion,duetoitssimplicityofoperation,highavailabilityandrelativelylowcapitalcost.Ofover800mainWTEplantsintheworld,about600ofthemchoosemassburntechnology.Varioustypesof grate are available depending on the way that the waste is fed on to thecombustion grate, including horizontal grate, forward-moving grate, reverse-actinggrateandrollergrate[2].

Figure1WTEProcessandApplication

Withsimilartechnologies,theutilizationofenergyrecoveredfromMSWisofseveraldifferenttype.InthemajorityofcountrieslikeUSandChina,themostcommonWTEplantsareelectricity-onlybusiness.Theyusethesteamproducedbycombustiontodrivetheelectricitygenerationturbine,theexhauststeamfromturbineissenttoacondenserforcooling,thenrecycledtoboilerorinjectedintonaturalwatersystem.Itshouldbenotedthatsuchprocessmakesverylittleuseoftheenergycontainedintheexhauststeamfromturbineandlossesover60%oftheenergyreleasedfromthecontrolledcombustionofMSW.TakingUnitedStatesfor instance,62of its84WTE

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plantsgenerateelectricityastheironlyenergyproduct.Basedonitsshareofmarket,about11millionmegawatthoursof electricity is generated in 2012whileover 20millionmegawatthoursofenergyfromMSWlost[3].Possibleapplicationofthispartof energy, however, is finite due to the relative low pressure and temperaturecharactersofexhauststeam.Toreducetheenergy lossesofelectricityfrompowergeneration, countries like Europe and South Korea uses energy recovered indifferentways.Manyof theirWTEplantsareheat-only facilitiesorcombinedheatandpower(CHP)facilities.Thesefacilitiesprovideheatenergyastheirproductintheformofsteamorhotwaterandsells it fordistrictheatingandcooling(DHC)orforother related industrial production processes. The thermal efficiency for thesefacilities are typically around 80%, much higher than the electric business plants.Besides,tangibleeconomicbenefitscanbeachievedatthesametime.Basedonthesuccess in Europe, DH is a great choice for many places to fully utilize energy inexhauststeam.However,thisapplicationrestrictedbythehighrequestandcostofDHtransmissionsystemconstructionaswellasthesuitableclimateconditionofthearea.Currentlyanewchoiceisposedbytheworldwidefreshwaterscarcity--usingtheexhauststeamasanenergysourceforthermalwaterdesalination.

Waterdesalinationreferstoprocessesthatremovesomeamountofsaltandothermineralsfromsalinewatertoprovidepeoplewithneededfreshwater.Withlimitedfreshwaterresources(1%)andcontinuousgrowingpopulation,thescarcityoffreshwaterandtheneedforadditionalwatersuppliesarealreadyposingmajorproblemsformorethanabillionofpeoplearoundtheworld.Thisissueissignificantlycriticalinmanyaridregionsoftheworldandmanyofthemevendonothavesurfacefreshwaterresourcessuchasriversand lakes,andmanyonlyhave limitedundergroundwaterresources.TheWorldHealthOrganizationpredicts thatbymid-century, fourbillionofus—nearly two-thirdsof theworld’spresentpopulation—will faceseverefresh water shortages. Recent advances in technology have made seawaterdesalinationarealisticandmostpromisingsolutiontosupplyfreshwater.

Inthepast45years,thedesalinationindustryhasgrownfromvirtuallyzero,toover70million cubic meters of treated water per day and almost 16,000 desalinationfacilitieshavebeenbuiltworldwide.Uptill2012,seawaterhasalreadyaccountedforalmosttwo-thirdsofallfeedwater[4].Stimulatedbythethirstofwaterandabundantoilsupply,thedesalinationmarketisledbyMiddleEast,theGulfStates,withashareof 53.4%, followed by North America (17%) and Europe (10%)[5 ]. Technologyadvancement and wide adoption of desalination also causes a significant costcut-downwhichinturnsfurtheracceleratethedevelopmentofdesalinationmarket.Thecostofdesalinatedwaterhasdownedto0.45~1US$/m3 in2013comparedtothe 9 US$/m3 in 1970, nearly compatible to the price of tap water in US (~0.53US$/m3). Consequently, transportation, energy and environmental costs have nowreplacedtechnologyastheprimary impedimentsto large-scaledesalinationandasthemost dominated part, energy cost accounts to 30~45% of the total operativecost[6][7][8].

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Figure2Cumulativecontractedandcommissioneddesalinationcapacity1965-2011[4]

Figure3Totalworldwideinstalledcapacitybyfeedwatercategory[4]

Figure4Top10countriesbytotalinstalledcapacitysince1945[4]

Water desalination uses either thermal or membrane technologies. Membranetechnologies include reverse osmosis (RO), electrodeionisation (EDI) andelectrodialysis (ED) and generally powered by electricity. Thermal technologiesinclude multiple-effect distillation (MED) and multi-stage flash evaporation (MSF)

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and are powered by steam together with some electricity. As for sea waterpurification, all of the approaches mentioned is well developed and proved bypractical production. With lower energy consumption and decreasing cost ofmembrane, membrane desalination is a preferable cost-effective choice for mostdesalinationplants. It has grown rapidly since2003,occupied60%of the installeddesalination capacity worldwide. Being more energy-intensive, the comparativelyhigh price of steam is exceptionally believed to be the key blocker of thermaltreatment development. Despite expectations of decline, however, thermaldesalination also continues to grow. It may cause by the development of solarthermal technologies and its new bond to power plants. Saudia Arabia and theUnitedArabEmirateshaveledthethermaldesalinationmarketsincetheGulfStatesaccesstoabundantoilsupplies.

Figure5Installedmembraneandthermalcapacity,1980-2010(cumulative)

Figure6Top10countriesbytotalinstalledthermalcapacitysince1945

Asismentioned,somebig-scalepower-desalinationcogenerationplantshavebeenbuiltrecentyears,mostlyinSaudiArabiaarea,tomakeuseofthewasteenergyfromnearby power plants cut down the energy cost of desalination. Similar to manytraditional power plants, WTE plants also have considerable potential to be an

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energysuppliertothermaldesalinationfacilities.ThismayhighlyincreasetheenergyefficiencyofWTEplantswhilecreateaccountablefreshwatertomitigatethewaterscarcity.Thisstudyaimsatmakingan initialassessmentofthepossiblebenefitsofusingexhauststeamfromWTEforwaterdesalinationby:

- Analyze and compare the energy flow and efficiency of existing exhaust steamtreatmentorapplication,includingcondensinganddistrictheating;

- Analyzeandcomparetheoperationrequirement,energyconsumptionandcostofmostpopularthermaldesalinationtechniques;

- CasestudyofexistingWTE-desalinationplants;- Basedontheanalysisabove,suggestsomeplacessuitableforthedevelopment

ofwaste-to-watercogeneration.

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2.EnergyFlowAnalysisofTypicalExhaustSteamUtilization

2.1.WithoutExhaustSteamTreatmentandUtilization

2.1.1.DescriptionofProcess

WTE plant is designed to combust unrecyclable MSW and simultaneouslyrecuperates the energy and cleans the gases generated from combustion[9]. Bydefinition,wasteincinerationiscarriedoutwithsurplusofair.Thisprocessreleasesenergy and produces solid residues as well as a flue gas emitted into theatmosphere[10].

AschematicdescriptionofgeneralincinerationprocessisrepresentedasFigure7.Asdepicted,MSWisfirstdischargedintoandthenextractedfromawastebunkerbyanoverheadclawcrane,thenloadedintoafeedhopperofWTEfurnaceandprocessedon a moving grate in order to achieve a correct combustion in combustionchamber[11].

Thehigh temperatureoxidation in thechamber reduces largeparticleMSWtoashand discharged at the lower end of the grate. Some of the ash is transported forlandfillingwhilesomecanbereusedinapplicationssuchasfillinginthebuildingandconstructionindustries.Toguaranteethehighperformanceofcombustion,theairistypically fed into thecombustion twice.Theprimaryair is injected frombottomofthegrate,dryingtheMSWwhileprovideoxygenforcombustion;thesecondaryairisinjectedfromtopofthechamber,providingacirculateconditionwithinthechamberas well as providing extra oxygen for unburned residuals. Additional fuelsmay beneededtoensurethecombustiondependingonthecompositionandheatvalueofMSW.Thecombustionproduct,calledfluegases,thenexchangesitsenergywiththeboiler. The water in tubes within the boiler becomes superheated steam (highpressure) as heat transferred, and is sent to drive the turbine that generateselectricity. Low pressure steam from the generator then exhausted for furthertreatmentoruse.

Intermsofthesteamturbine,itcanberoughlydividedintocondensingturbineandnon-condensing (back pressure) turbine. For a condensing steam turbine, it isoperatedwithanexhaustpressure lessthanatmospherictomaximizethepressuredrop through the turbine, thus greater the energy extracted from steam input(approx.30-40%efficiency).ThetechnologywithcondensingturbinewillfurtherbeexplainedandanalyzedinChapter2.2.Thenon-condensingturbine,onthecontrary,isoperatedwithanexhauststeamequaltoorinexcessofatmosphericpressureandismostwidelyusedforprocesssteamapplications(refineries,districtheatingunits,paper plants, and desalination facilities). The exhaust steam pressure can be

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controlledbyregulatingvalvetosuittheneedsoftheprocesses.

The flue gas cleaning system, or air pollution control (APC) system, ensurescontrolled emissions fromWTE plants. A certain percentage of pollutant particles,however,willstillbeemittedintotheatmosphereaftertreatment,dependingonthecompositionofMSWandthetypeofAPCsystemchose.ThecommonpollutantsareCO2,N2O,NOx,SOxandNH3.

Figure7SchematicDescriptionofWTEPlant

2.1.2.ThermodynamicSimulatedModelofWTEProcess

To assess the energy efficiency of the WTE processes mentioned above, athermodynamicmodelisneeded.Accordingtothefirstlawofthermodynamics,foranisolatedsystemthetotalamountofenergyisconstant,that istosaytheenergyinputequals to theenergyoutputandenergy losses. For this study,wedefine theWTEplantandtheelectricitygeneratorunitasourstudysystemandourmaintargetis the flowof energy generated andused from theMSW. In this case, despite theextra fuels added tohelp the combustionandenergy forpreheating,otherenergylike electricity consumed by equipment operation will not be included in thecalculation.Thisisbecauseinmostcasestheelectricityconsumedissuppliedbytheplantsthemselvesbydetouringpartofpowergeneratedforself-usebeforeinjecttogrid.Andtobetteranalysistheenergytransformationofthissystem,wedividethematerialflowintothreeparts:

1) Waste flow: this flow mainly indicates the flow of solid particles within thesystem,fromMSWtobottomashviaincineration.

2) Fluegasflow:thisflowmainlyindicatestheflowofgasphasewithinthesystem,

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from the air preheated, injected into the combustion chamber, heated byincineration,exchangedwithboilerandthenexhaustedaftertreatment.

3) Water flow: this flowmainly indicates the flow ofwater and steamwithin thesystem,injectedwaterfromoutsource(municipaltapwater),heatedbyboilertosteam,drivetheturbineandthenexhausted.

Certain percentage of energy will be lost due to radiation during the masstransportationandheatexchangingprocesses.Fromthethreemass flow, itcanbeclearlyclarifiedtheinputandoutputofenergyofthesimulatedsystem:

EnergyInput:1) MSWfeed;2) Extrafuelsassistingincineration(ifneeded);3) Energyusedforairpreheating;4) Energyusedforfeedwaterpreheating.

EnergyOutput:1) Electricitygenerated;2) Energyremainedinexhauststeam;3) Energyremainedinbottomash;4) Energyremainedinexhaustedfluegas;5) Other energy losses during the process due to the radiation or efficiency of

equipment.

To set a justifiedWTE system to calculate the energy flow baseline for this study,technologicalrestrictionsaswellasemissionandsafetyconcernsmustbetakenintoconsideration.Steamtemperatureshouldbe limitedtoavoidcorrosionrisks. Inthecase ofmixedwaste combustion, the average temperature rangeof combustion isbetween1,000°Cto1,200°C[2]andthefurnacetemperatureis1,050°C[11].ForChina,thetemperatureistypicallyalowerrangefrom850~950°Cduetothelowheatvalueofwaste.Besides, formostAPCsystem,thefluegasesshouldnotbecooledbelow200 °C to avoid the risk of condensation of aggressive compounds[12]. MultipleresearchesandWTE facilitieshasbeen studied toensure theparameter chosearelogicalandsensible.OperationparametersofsomeexistingstudymodelsandWTEplantsissummarizedinTable2.

Accordingtotheworkingdatafromstudies,thesimulatedsystemrunsasbelow:1) Combustion System: The MSW (12MJ/kg, 10 t/h) is burned with grate-fired

furnaceandnoextrafuelsisadded.Dryairisimposedasprimaryandsecondaryairfeedandisheatedto120°Ctoobtainacombustiontemperatureof1100°C.Assume the air is preheated respectively without using heat from combustionsystem.Theamountofairisestimatedtoensurearesidualoxygenconcentrationofthefluegasequalto7%,accordingtotheEuropeanregulations[13].Accordingto the “CEWEP Energy Efficiency Report (Status 2007-2010)”[14], the meanprimaryairfedof314WTEplantsis3m3/kgMSWwhilethesecondaryairfedis

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1.5m3/kgMSWwithoutfluegasrecirculation.Setthefedrateas4m3/kgand2m3/kg respectively for this model. With plenty supply of oxygen, assume theconcentrationofCOisnegligibleandtheunburnedcarboninthebottomashis0.5%ofLHV.Thethermodynamicequilibriumofcombustionproductsisimposedaftersecondaryairinjection.

2) HeatRecoverySystem(BoilerandSuper-heater):Thewaterfeedstotheboilerispreheated to 120 °C, 4.5 MPa to ensure certain heat efficiency. The workingconditionsoftheRankinecyclewithinboilerwerefixedatasteamtemperatureof400°Candpressureof4.0MPa.Aftertheradiationheatexchangewithboiler,severalunitsofsuper-heaterandaneconomizerwereconsideredcorrespondingto the water-wall of combustion chamber, the convective section of theevaporator, feed-water preheater as well as adjuster of flue gas temperaturerequiredbythepracticeofWTEfacilities.Assumethatthefluegasattheendoftheradiantsectionisabout850°C,enteringtheeconomizerat430°C,exhaustedfor treatment at about 200 °C according to the temperature requirement bysemi-drycleaningsystem[15].Assumetheheattransferefficiencybetweenwaterandfluegasis95%.Thetreatedfluegas(150°C)isnotrecirculated.

3) Power Generation System: The multi-stage back-pressure steam turbine isconsideredforthemostbasicWTEmodel.Nobleedingsteamsforotherusewiththemodel.Accordingtothestudies,anisentropicefficiencyof70%isimposed[16],electrical and mechanical efficiency are together set to be 95%[17]. The steamexhausts from turbine with pressure of 1MPa. Considering the electricityproductionof83electricity-onlyWTEplant inEurope[14],adoptagenerate levelof0.45MWh/tMSW,fortheturbine.

2.1.3.EnergyFlowofSimulatedWTEModel

Usingthemodelbuilt in2.1.2,theenergyconsumptionandlossforeachprocessiscalculated.Theprinciplesandmainformulasare:

1)TheenergyinputfromMSW:E(MSW)=IncinerationCapacityofMSW*LHV

2)Theenergytransactioncalculationofairandwater/steamisbasedontheirheatvalueorenthalpyunderprocessstatus;similarly,theoperationstatusofflowcanbeestimatedfromtheirheatvalueorenthalpy: �E(Process)=MassFlowRate*Time*[enthalpy(status1)–enthalpy(status2)]

E(Transfer)=�E(Process)*Efficiency

3)Electricityproductionfromsteamturbine: ElectricityProduceCapacity=Productivity*IncinerationCapacityofMSW

=�E(SteamPassTurbine)*ElectricalandMechanicalEfficiency*IsentropicEfficiency

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The step-by-step calculation is listed in Table 3 and the result can be brieflysummarized as Table 1. Figure 8 represents the overall energy flow of simulatedsystembasedononehour scale (10 tonsofMSWcombusted).Witha combustioncapacityof10t/h,theplantgenerates16,200MJelectricityperhourwhileproducing34.6tonsofsteam(1MPa,180Celsius)whichunderlying58%oftotalenergyinput.Despiteutilizationortreatmentofexhauststeam,only15%oftheenergyfromMSWis sufficiently used (electricity and flue gas cleaning). This portionmay increase inpart by recirculating part of the treated flue gas to down the consumption of airpre-heating. In terms of the unavoidable energy losses, the combustion and heattransformation efficiency matters the most, attributing to nearly half of the totallosses.Besides,theisentropicefficiencyofsteamturbineisakeyfactortoreducingthe energy losses. Despite these energy losses, such process consumes 24,410MJextraenergyforairandwaterpre-heating.Alternativeenergysourcescanbewood,building materials or fossil fuels (coal, diesel, etc.). The large water consumption,approximately 3.5 ton per ton of MSW, also stresses the importance of exhauststeamtreatmentwhichwillbediscussedinlaterchapters.

Table1EnergyFlowResultofBasicSimulationModel

EnergyInflow Amount(MJ) Percentage EnergyOutflow Amount(MJ) Percentage

MSW 120,000 83.10% GrossElectricity 16,200 11.21%

Airpre-heating 8,333 5.77% ExhaustSteam 83,681.40 57.90%

Waterpre-heating 16,076.9 11.13% FlueGasCleaning 5,187 3.59%

ExhaustFlueGas 10,776 7.46%

BottomAsh 150 0.10%

OtherLosses 28,537 19.74%

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Figure8EnergyFlowofBasicSimulatedModel–WithoutExhaustSteamTreatmentandUtilization(Unit:MJ/h,BasedonMSWincinerationcapacityof10t/h)

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Table2DesignparametersofsomeWTEplantsandsimulatedmodels

Type Description

WasteFlow FlueGasFlow Waterflow

ReferenceCapacity LHVofMSW

ExtraFuelAdded

LossDueto

UnburntCarbon

Primary/Secondar

yairTemp.

FlowRate

FlueGasTemp.at

Super-heaterInlet

FlueGasTemp.Outlet

FeedWater

SteamTemp.

SteamPressure

SteamFlowtoTurbine

Exhauststeam

IsentropicEfficiencyofTurbine

TypeofTurbine

GrossElectricPower

SimulatedModelStudy

Representativeofanumberofplantsin

NorthernItaly

65,000ton/year

10.11MJ/kg

BuildingMaterials 0.8%LHV 120°C 15.55Nm3/s

(dry,11%O2)

Max650°C(Controlledbyeconomizer)

160°C / 400°C 45bar 8.70kg/s 2.6bar / Condensing 588kWh/t[18]

390,000ton/year

10.11MJ/kg

BuildingMaterials 0.8%LHV 120°C 91.93Nm3/s Max650°C 140°C / 440°C 65bar 51.87kg/s 2.6bar / Condensing 807kWh/t

SimulatedModelStudy

Respectstotechnologicalrestrictionsrelatedtoelectricityproduction

100,000ton/year

9.61MJ/kg Biomass / / / / / / 400°C 4MPa 40t/h

1.1MPa(Bleed)

70%Condensing 600-700

kWh/t[19]

1.1MPa Non-Condensing

200-270kWh/t

SimulatedModelStudy

Fluidizedbedcombustor,parametersareestimatedfromanumberofplants

usingFBC

56.52t/h 10.5MJ/kg / / / / 820-920°C

(Typically) 200°C / 380°C 40bar 377°C,37bar / 90% Condensing 740kWh/t [12]

WTEPlantMVAZistersdorfWTE

plantinAustria,withgratefurnace

17.3t/h 12MJ/kg Null / / / / 195°C / 405°C 42bar / / / / 116,000MWh

(795kWh/t)

[20]

WTEPlantIVMWTEplantin

Belgium,withmoveablefurnace

14t/h 10MJ/kg

Oil(404,259liters/y)

/ / / / 180°C / 400°C 35bar / / / / 38,811MWh(425kWh/t)

WTEPlant L90AffaldsforbrændingWTEplantinDenmark

24t/h(180,000

t/y)

11.5MJ/kg

Oil(315,000liter/y)

/ / / / / 130°C 400°C 42bar 25.13kg/s / / / 136,393MWh(633kWh/t)

WTEPlant AarsFjernvarmeværkWTEplantinDanmark

5t/h(Line2) / Null / / / /

45-55°C(After

cleaning)

/ 430°C 47bar / / / / 17,277MWh(340kWh/t)

WTEPlant RenoNordline4inAalborg,Denmark

20t/h(160,000

t/y)

12MJ/kg / <0.23% 145°C/

125°C / 620°C 180°C / 425°C 50bar 80t/h(22.42kg/s) 220°C 17.9MW [21]

StudyonWTEPlant

StudyofefficientparametersofmiddletolargesizedWTEplants

/ / / / / / 850-950°C / / / / 6kg/kWh0.005-0.009MPa

70% Condensing / [22]

StudyonWTEPlant

Studyontheeffectofsteamparametertowardselectricitygeneration

750t/d 6.22MJ/kg / / / / / / /

400°C 4MPa 83.9t/h / 82% / 341kWh/t[23]

450°C 6.4MPa 76.5t/h / 82% / 363kWh/t

WTEPlant WTEplantdesignedinYuxi,China 200t/d / / 0.5-1.5% 220°C / 850°C 200°C 140°C,5.0

MPa 415°C 4.0MPa 15.3t/h / / / / [24]

SimulatedModelStudy

Theworkingconditionswereimposedbasedonprocessdataofmany

recentincinerationplantsinItalyandEurope.

185t/d 15.38MJ/kg / neglected 115°C 17.5Nm3/s,

10.6Nm3/s

700°C(1150°C,1atmfor

combustion)

170°C 120°C,7.0MPa 450°C 6.0MPa

6.0MPa,22.8kg/s,449°C

0.1MPa

79%(highpressure)81%(lowpressure)

Condensing 19.2MW [15]

*1bar=0.1MPa

19

Table3EnergyFlowAccountingofWTEPlants(ForChapter2.1)

Process Formula EfficientAmount(MJ/h) Losses(MJ/h) Efficiency(%) Remark

TotalchemicalenergyinMSW 12MJ/kg*10t/h=120,000MJ/h 120,000 - - LHV=12MJ/kgCapacity:10t/h

Heating up of primarycombustionair

1.25kJ/m3/°C * 4m3/kg MSW * 10t/h * (120°C - 20°C) /90%=5,555.6MJ/h 5,000 555.6 90%

Heatvalueofair:1.25kJ/m3/°CHeattransferefficiency:90%Airfed:4m3/kgMSW

Heating up of Secondarycombustionair

1.25kJ/m3/°C * 2m3/kg MSW * 10t/h * (120°C - 20°C) /90%=2,777.8MJ/h 2,500 277.8 90%

Heatvalueofair:1.25kJ/m3/°CHeattransferefficiency:90%Airfed:2m3/kgMSW

Energyinbottomash 0.5%*12MJ/kg*25%*10t/h=150MJ/h - 150 - Unburnedcarbon:0.5%LHVMassreductionofMSW:75%

Energy input in flue gas inchamber

1.4kJ/m3/°C*1.3*6m3/kg*10t/h* (1100°C -120°C)=107,016MJ/h 107,016 12,834 89.3%

Heatvalueoffluegas:1.4kJ/m3/°CVolumeoffluegas:1.3*V(airfed)1415Loss=chemicalenergyinMSW–energyinbottomash–energyinfluegas

Heatemitfromfluegasinradiantsection

1.4kJ/m3/°C*1.3*6m3/kg*10t/h* (1100°C -850°C)=27,300MJ/h 27,300 - - Fluegastemperatureoutofradiantsection:850°C

Heat emit from flue gas insuper-heaters

1.4 kJ/m3/°C * 1.3 * 6m3/kg * 10t/h * (850°C - 430°C) =45,864MJ/h 45,864 - - Fluegastemperatureoutofsuper-heaters:430°C

Heat emit from flue gas ineconomizer

1.4 kJ/m3/°C * 1.3 * 6m3/kg * 10t/h * (430°C - 200°C) =25,116MJ/h 25,116 - - Fluegastemperatureoutofsuper-heaters:430°C

Energyforfluegascleaning 1.4 kJ/m3/°C * 1.3 * 6m3/kg * 10t/h * (200°C - 150°C) =5,460MJ/h 5,187 273 95% Usedforsemi-dryprocesswaterevaporation

Efficiency:95%

Energyinexhaustfluegas 107,016+5,000+2,500–27,300–45,864–25,116–5,460=10,776MJ/h - 10,776 Calculatebyseparatetheparttransferredfromenergyinput

Feedwaterpreheating (506.813kJ/kg – 88.144kJ/kg) * 9.6kg/s * 3600s / 1000 /90%=16,076.88MJ/h 14,469.2 1,607.7 90% Enthalpyofwater:506.813kJ/kg(120°C,4.5MPa),88.144kJ/kg(20°C,4.5MPa)

Heattransferefficiency:90%

Heating up of water ineconomizer 25,116MJ/h*95%=23,860.2MJ/h 23,860.2 1,255.8 95%

Energyinwater:506.813kJ/kg*9.6kg/s*3600s/1000+23,860.2MJ/h=41,375.7MJ/hEnthalpyofwater/steam:41,375.7MJ/h/(9.6kg/s*3600s/1000)=1197.212kJ/kgStatusofwater/steam:257.4°C,4.5MPa,evaporationfractionof4.48%

Heating up of water in radiantsection 27,300MJ/h*95%=25,935MJ/h 25,935 1,365 95%

Energyinwater/steam:41,375.7MJ/h+25935MJ/h=67,310.7MJ/hEnthalpyofwater/steam:67,310.7MJ/h/(9.6kg/s*3600s/1000)=1947.646kJ/kgStatusofwater/steam:257.4°C,4.5MPa,evaporationfractionof49.26%

Heating up of water insuper-heaters

(3210.87kJ/kg–1947.65kJ/kg)*9.6kg/s*3600s/1000 -67,310.7MJ/h=43,657MJ/h 43,657 2,207 95.2% Enthalpyofwater:1947.646kJ/kg(257.4°C,4.5MPa),3210.87kJ/kg(400°C,4.2MPa)

Loss=45,864MJ/h–43,657MJ/h

Electricitygenerated 0.45MWh/t*10t/h*3600=16,200MJ/h 16,200 852.6 95% Productivity:0.45MWhelpertonofMSWElectricalandmechanicalefficiency:95%

Energy from steam to drive theturbine (16200MJ/h+852.6MJ/h)/70%=24,360.9MJ/h 17,052.6 7,308.3 70% Isentropicefficiencyofturbine:70%

Energyinexhauststeam 3214.37kJ/kg * 9.6kg/s * 3600s / 1000 – 24,360.9MJ/h =86,727.7MJ/h 86,727.7

Enthalpyofwater:3214.37kJ/kg(400°C,4.0MPa)Enthalpyofexhauststeam:86,727.7MJ/h/(9.6kg/s*3600s/1000)=2509.483kJ/kgExhauststeamstatus:179.9°C,1MPa,evaporationfractionof76.53%

Energyinexhauststeamfromthesystem

86,727.7 – (88.144kJ/kg * 9.6kg/s * 3600s / 1000) =83,681.4MJ/h 83,681.4 Calculatedbyseparatethepartofenergyinfeedwater(20°C,4.5MPa)

20

2.2.ExhaustSteamCondensation

2.2.1.Condensingsteamturbine

For the conventional WTE utilities, the primary type of turbine adopted is thecondensing turbine. The exhaust steam from power generation turbine is directlyrejectedintosurfacecondenserswhichmaintainsanalmostvacuumconditionatthedischarge of the turbine. Operated with an exhaust pressure much lower thanatmosphericone, themaximizedpressuredropenlarges theenergyextracted fromsteaminput.Accordingtothestudy,theeffectivenessofcondensingsystemcanbequantified through theprinciple: the lower thepressure thegreater theeffects[25].From the previous calculation, we notice that there still 60%-80% of the energyremainingintheexhauststeam.Therefore,thecoolingsystemusuallyextractsheatat2to4timestherateofelectricpowergeneratedandanysmallimprovementcanthen lead to large fuel saving and efficiency enhancement for the plants[26 ].Simultaneously,theexhauststeamisconvertedintopurewaterbycondensationandcanbefurtherreusedinboilerasfeedwatertoachieveacloseloopofwateraswellascuttingdownthefreshwaterconsumption.Thesurfacecondenserhasshellsandanarrayoftubesandcanbeclassifiedaswater-cooledcondenserandair-cooledcondenseraccordingtothecoolingmedium.

Forthewater-cooledcondenser,itcanbecooledbyriver,lakeorcoolingtowerwater.Commonly, the coolingwater flows through the tubeswhile the steam enters theshellsideandthecondensationoccursontheoutsideoftubes(Figure9).Thenthecondensate drips down and are collected at the bottom pan of condenser calledhotwell.Asa smallamountofair isknownto leak into theshell sideundernearlyvacuumpressure, a relatively small air ejector isused to remove thosegases fromthe condenser. Typically the coolantwater temperature has a 10 degree rise frominlet tooutletunder full load[27].Water fromnaturalwaterbody (sea, lakes, rivers,etc.) is adopted for chilling inmany power plants. However, the scarcity of watersupplyandanexcessivelyhigherenvironmental impactatoverheatingofthewatertablesusedasheatsinks[28]withdrawtheadoptionofwater-cooledcondensers.

21

Figure9Water-cooledsurfacecondenser

Fortheair-cooledcondenser,itusuallyconsistsofanarrayoffannedtubemodulesarranged in parallel rows[29]. This kind of condenser feeds the steam through thetubeswhilethecoolantairflowingaroundthetubesoutside,forcedbyaxialflowfanunits located above or below (Figure 10). As a result, heat from the condensingsteam is rejected to theenvironment via the finned tubes. Theproblem is that anair-cooledcondenser,however,issignificantlymoreexpensiveduetothehighenergyconsumption to drive the fans. Also it cannot achieve as low the steam turbineexhausttemperatureandpressureaswater-cooledcondenserduetothe limitationof inlet air temperature,which is greatly infectedby the climate conditionof localenvironment[ 30 ][ 31 ]. The condensation temperature within the condenser isconsidered to be 6°C-20°C above air inlet temperature for general purposes andvaries according to the ambient temperature[32]. The air temperature reaches a20°C-30°Criseasitpassesthroughthecoils[33].

Figure10Air-cooledsurfacecondenser

22

2.2.2.ThermodynamicSimulatedModelofCondensingWTEPlant

The assumption of combustion and boiler system operating condition keeps thesameasthesimulationmodelofchapter2.1.2.Boththewater-cooledandair-cooledcondensers will be studied. The differences and changes for a condensing powergenerationprocessarelistedasfollows:

(1) Theheatremovedfromtheexhauststeam(containsinoutletcoolingwaterorair)willnotbefurtherused,buttreatedasenergylosses.Assume95%oftheenergyremovedfromexhauststeamistransformedintocoolant.

(2) The cooled steam from the condenser would be recycled to the boiler, extraenergymayneededforwaterpre-heatingaccordingtothecalculationresult.

(3) Accordingtotheformerstudies,assumethecondensingpressure(exhauststeampressure)ofwater-cooledcondenseris0.1MPawhile0.25MPafortheair-cooledcondenser. The input coolant water (air) has a temperature of 20°C. A 10°Ctemperature rise is considered for the coolingwater, a20°C for the coolingair.The output temperature of cooled exhaust steam (liquid) is 60°C.No steamorwaterleakageisconsideredforthismodel.

(4) Considering theelectricityproduction capacity rangeof83electricity-onlyWTEplant in Europe (0.075-0.873MWh/tMSW)[14], adopt a generate level of 0.65MWh/tMSWforthewater-cooledturbine,0.60MWh/tfortheair-cooledturbine.Themulti-stagecondensingsteamturbineisconsideredandnobleedingsteamsforotherusewiththemodel.Accordingtothestudies,anisentropicefficiencyof80% is imposed[16], electrical andmechanical efficiency are together set to be95%[17].

2.2.3.DiscussionofCondensationSimulationModelofWTEProcess

According to the modified model we built in 2.2.2, the calculation for bothwater-cooled and air-cooled condensationWTE system is listed in Table 5. For thecondensingsteamturbinesystem,thegrosselectricitygeneratingcapacityisaround22,000MJ to 23,000MJ, contributes to 15-17% of total energy input, 20% of theenergycontaininginMSWcombusted.Thisportioncanbefurtherincreasedto30%withlowerextractsteampressureof5-8kPawhichishundredtimeslowerthanourmodel. Compared to thebasic non-condensing cycle in Chapter 2.1, 7000MJmoreelectricityisproduced,increasingtheenergyefficiencybyatleast5%.

No extra feed water is needed by cooling down and recycling the exhaust steam(water), thus cutting down 50% of the energy consumed by water pre-heatingcomparedtothebasicmodel.However,thewaterdemandforchillersisconsiderable.With10ton/hMSWcombustedcapacity,theestimatedwaterflowratefor10degreerise in condenser is as much as 1,629 ton per hour. Affected by the size, coolingcapacityandambienttemperatureofwater-cooledcondenser,somechillerunits inmarketevenshowslargerwaterconsumptionupto9,000tonperhour[34]underthe

23

working capacity of simulatedWTEplantmodel. For large scale powerplants, thispart of water is always taken from and emitted to nearby water bodies, causingseries heat pollution effects towards the ecosystem. The injection of heat fromcoolingwatertonaturalwaterbodysignificantlyincreasesitstemperature,leadingtothereductionofdissolvedoxygenanddeathofaquaticanimals.Also,ahigherwatertemperature promotes the reproduction of disease-causing bacteria and virus andincreasesthetoxicityofsomechemicals(cyanideandheavymetalions)[35].

Fortheair-cooledchillers,anestimationof2,800,000m3/hairisneededtocarryouttheheatfromexhauststeam.Whencalculatewitha162.25kW(~585MJ/h)eight-fanair-coolingmodelfromcondenserproducerFRITERMA.S.[32],120unitswithtotalairflow rate of 4,757,000m3/h is consumed for this combustion scale, togetherwith1109MJ/helectricpowertodrivethefans.

From the discussion above, the traditional electricity-only WTE plants withcondensing steam turbine more sufficiently generate power from MSW than thebasicmodelandwellused20-30%ofenergy fromtotal input.However,abouttwothirdsofenergyfromMSWlossesduringcondensationprocesses.Therelativehighenvironmental impacts and energy consumption also attract consideration againstthisinefficientoperationmode.

Table4EnergyFlowResultofCondensingSimulationModel

Energy

Inflow

Amount

(MJ)Percentage

Energy

Outflow

(W)

Amount

(MJ)Percentage

Energy

Outflow

(A)

Amount

(MJ)Percentage

MSW 120,000 87.57%Gross

Electricity23,400 16.93%

Gross

Electricity21,600 15.62%

Air

pre-heating8,333 6.08%

Refrigerant

Loss68,036 49.21%

Refrigerant

Loss70,288 50.84%

Water

pre-heating8,706 6.35%

Flue Gas

Cleaning5,187 3.75%

Flue Gas

Cleaning5,187 3.75%

Exhaust

FlueGas10,776 7.79%

Exhaust

FlueGas10,776 7.79%

Bottom

Ash150 0.11%

Bottom

Ash150 0.11%

Other

Losses30,706 22.21%

Other

Losses30,256 21.88%

24


Process FormulaEfficientAmount

(MJ/h)Losses(MJ/h) Efficiency(%) Remark

Feed water

preheating

(506.813kJ/kg– 254.915kJ/kg)

* 9.6kg/s * 3600s / 1000 /

90%=9,672.9MJ/h

8,705.6 967.3 90%

Enthalpy of water: 506.813kJ/kg (120°C, 4.5MPa),

254.915kJ/kg(60°C,4.5MPa)


Electricity generated

fromturbine(W)

0.65MWh/t * 10t/h * 3600 =

23,400MJ/h23,400 1,231.6 95%

Productivity:0.65MWhelpertonofMSW

Electricalandmechanicalefficiency:95%

Electricity generated

fromturbine(A)

0.60MWh/t * 10t/h * 3600 =

21,600MJ/h21,600 1,136.8 95%



Energyfromsteamto

drivetheturbine(W)

23,400MJ/h /95%/80% =

30,789.5MJ/h24,631.6 6,157.9 80% Isentropicefficiencyofturbine:80%

Energyfromsteamto

drivetheturbine(A)

21,600MJ/h /95%/80% =

28,421.0MJ/h22,736.8 5,684.2 80% Isentropicefficiencyofturbine:80%

Energy in steam from

turbine(W)

3214.37kJ/kg * 9.6kg/s *

3600s /1000–30,789.5MJ/h

=80,299.1MJ/h

80,299.1

Enthalpyofwater:3214.37kJ/kg(400°C,4.0MPa)

Enthalpyofexhauststeam:2321.444kJ/kg

Exhauststeamstatus:99.61°C,0.1MPa

Energy in steam from

turbine(A)

3214.37kJ/kg * 9.6kg/s *

3600s /1000–28,421.0MJ/h

=82,667.6MJ/h

82,667.6


Enthalpyofexhauststeam:2392.002kJ/kg

Exhauststeamstatus:127.41°C,0.25MPa

Energy in exhaust

steam from

condenser(W)

251.222kJ/kg * 9.6kg/s *

3600s/1000=8682.2MJ/h8,682.2


Calculatedbyseparatethepartofenergyinfeedwater

(20°C,4.5MPa)

Energy in water from

condenser(A)

251.348kJ/kg * 9.6kg/s *

3600s/1000=8686.6MJ/h8,686.6 Enthalpyofwater:251.35kJ/kg(60°C,0.25MPa)

Energytocoolant(W)(80,299.1 MJ – 8682.2 MJ) *

95%=68,036.1MJ68,036.1 3580.8 95%

Water condition: Inlet 20°C, 2 MPa; Outlet 10°C

increase. Water flow rate: 68,036.1MJ/h / (127.564

kJ/kg-85.7984kJ/kg)=1629.0ton/h

Energytocoolant(A)(82,667.6 MJ – 8686.6 MJ) *

95%=70,287.7MJ70,287.7 3699.4 95%

Air condition: 1.5MPa, T(inlet) = 20°C, 20°C increase.

Heat value of air: 1.25 kJ/m3/°C. Air flow rate:

70287.7MJ/h/1.25kJ/m3/°C/20°C=2,811,508m3/h

*W:water-cooledcondensationmodule;A:aircooledcondensationmodule.

25

2.3.ExhaustSteamforDistrictHeating

2.3.1.DistrictHeatingandCogenerationWTEPlants

DistrictHeating(DH)isdefinedasthedistributionofthermalenergyfromacentral

heatsourcetoitssurroundingresidentialbysteamorhotwaterthroughaninsulated

pipenetwork.Thesteamorhotwateristhendirectedintobuildingsandcirculated

through heating equipment. In the case of hot water systems or newer steam

systems, heat exchangers are frequently used. This successfully isolates the users

from the thermal system, thus preserving the integrity of both DH system and

customers. DH best suits to those areas with high population density and cold

climatewhichhelpensuring stableand competitivepricing[36]

. Typically the central

heatsupplyforDHcanbefromcoal-firedplants,oil-firedplantsorWTEplants(heat

only).Anotherapproach,called“cogeneration”orCombinedHeatandPower(CHP),

is applying the by-product steam of some other utilities to achieve high energy

utilizationefficiency.ForaCHPplant,theenergyefficiencycanreachashighas80%

to90%.

Currently,221of314EuropeanWTEplantssellheatasenergyproduct;184ofthese

are CHPproduction plants. These plants generate from700-899 kWhof electricity

andfrom400toupto2,000kWhofheatpertonofMSWincinerated[14]

.TheDanish

WTEfacilitiesobtainanaverageenergybalanceofupto0.6MWhelectricityplusup

to2MWhheatpertonofMSW,thustriplingtheamountoftotalenergyobtained.In

United States, twenty eight of the 88 WTE plants sell steam as product and 21

cogenerate a total of about 470MW of heat (corresponding to 1.6 million lbs of

steam per hour) and 272 MW of electricity[37]

. Despite the increasing of energy

efficiency,therearestillsignificantadvantagestobegainedfromacogenerationWTE

plant fordistrictheating: LowerGHGemission levelsofWTE facilities compared to

othertechnologiesandoverallfuelconservation.

Asmentionedabove,districtheatingcandividedintotwodifferentmodesaccording

to themedium: steam system and hot water system. To feed a steam carried DH

system, the steam bleeds has to be extracted with higher-pressure to fulfill the

requiredpressuredropinthepipingnetwork.Extractionturbineisusedinthiscase

whichhasopenings for extractionof a portionof the steamat some intermediate

pressure, the rest of the steam is condensed, as illustrated in Figure 11. Part of

electricitygeneratingcapacity is lostduringcogenerationprocessandwelldepends

onthepressure,flowrateandnumberofextractionsofextractedsteam.Theratioof

electricity lost toheatgenerationordinarily range0.1 to0.2kWhofelectricityper

kWh of thermal energy obtained[38]

. A steam pipeline system normally services

consumerswithin2-3milesfromthecentralheatsourcewhichisshortandlimited.

Consideringexistingproblems for steamDH system likepipeline corrosion, storage

26

capacity,transmissiondistanceandheatloss,itisnowreplacedbymoreefficientand

economicalhotwatersystem.

Figure11ExtractionSteamTurbine

Tosupplyahotwaterthermalsystem,theWTEplantsnormallyproducehotwaterby

placing a heat exchanger at the back pressure turbine exhaust, transforming the

energy in low pressure steam from turbine bleeds to DH system’s water supply.

Extraction steam turbine is adopted by some CHP facilities to heat the water in

multiplestages[39]

.Inthiscase,thewaterflowofWTEfacilityiskeptseparatingfrom

the water network for thermal market. Simultaneously, the dropped energy level

conveyedbyexhauststeamremarkablyreducestheloadsandcostforcondensation,

especially for those whose coolant is not returned. The maximum supply

temperaturesofhotwatersystemsaredesignedbetween110-130°Candreturnat

50-70 °C. The recommended temperatures forhotwaterDH supply is between93

and 121 °C, the ratio of thermal energy extraction to electricity generation in

medium scale is 2.0 to 2.5MW thermal perMW electricity[40]

. Hot water system

allowsthetransmissionofheatover longerdistancesupto20mileswithrelatively

lowheatlossbetween5%-10%.Thepressurewithinpipelinesdependsonthesystem

size and ambient operating temperature, mostly varying from 9-17 bar in winter

seasons.SteelisthemostfrequentlyutilizedpipematerialinDHsystemsinorderto

preventgroundwaterdamageonexternalpipesurfaces.

2.3.2.ThermodynamicSimulationofCHPWTEPlant

The assumption of combustion and boiler system operating condition keeps the

same as the simulationmodel of chapter 2.1.2. According to the application ratio,

choose hotwater DH system for the simulatedmodel. The supply temperature of

110 °C and return temperature of 60 °C is considered for the model, with a

transmissionheat lossof5%.Thepressureofhotwater system is set as10bar (1

MPa).Threedifferentheatexchangemodeswillbestudied:

27

(1) The firstmodel usingmultiple stage back pressure steam turbinewithout any

bleeds in themiddle.All theexhaust steam from turbine is directed to aheat

exchanger for DHwaterwarming,with an assumed heat transfer efficiency of

95%.Assume theoutflow steam (water) status fromheatexchanger is 120 °C,

0.8MPa.TheoutflowisrecirculatedinWTEplantviapumpasfeedwaterandin

this case there is no need for extra feedwater pre-heating.Other parameters

assumptionsofturbinekeepthesameasbasemodelin2.1.2.

(2) Thesecondmodelusingextraction/condensing steamturbine. In thismodel, a

water-cooledcondenser (95%efficiency) isconsideredafter thesteamturbine.

Assume50%ofsteamisextractedat1MPaatextractionturbinestage.Adopta

generate level of 0.43MWh/tMSW (70% isentropic efficiency) for the turbine

beforeextraction.Assumetheexhauststeamfromcondensingsteamturbineis

undersamestatusasChapter2.2(99.61°C,0.1MPa,Enthalpyof2321.444kJ/kg,

80% isentropic efficiency). The exhaust steam (60°C, 0.1 MPa) chilled is

recirculatedasfeedwater.A10°Ctemperatureriseisconsideredforthecooling

water.

(3) The thirdmodelusingextraction/backpressure steamturbine.Assume50%of

steam is extracted, 60°C and 1.3 MPa, at extraction turbine stage. Both the

bleedsandexhauststeam(179.9°C,1MPa)areusedfortwo-stagewaterheating

toDHsystem:thefirststageisdonebyexhauststeamwhiletheextractedsteam

is used for the 2nd stage, bring the DH temperature to 110°C. The condensed

steamfromsecondstageheatexchangercascadesintothefirstexchanger,then

returnedtotheplantsystemasfeedwater.Adopta70%isentropicefficiencyfor

bothstageofturbine.Toestimatethemassflowcanbeheatedbywastesteam,

suppose the mixed steam (water) from exchanger has the same amount of

energy as water at 1MPa, 120°C. In this occasion, no extra feed water

pre-heatingisconsideredformodelthree.

2.3.3.DiscussionofCondensationSimulationModelofWTEProcess

Accordingtothemodifiedmodelwebuiltin2.3.2,thecalculationforthreeexhaust

steamtreatmentmodelislistedinTable8.Whenapplyingalltheexhauststeamfrom

back pressure turbine for DH (Model I), the WTE still generates 16,200 MJ gross

electricityperhourtogetherwithapproximately62,500MJthermalenergy.Although

7,000 MJ of electric power is sacrificed, the overall efficiency of the WTE plants

increasesfrom21%toover65%,thusover70%oftheenergyfromcombustedMSW

is sufficiently used. In some advanced cogeneration WTE plants of Norway and

Germany,theefficiencycanbeevenashighas80%-90%.

28

Table6EnergyFlowResultofThreeDHSimulationModel

AllforDH(I) DH+Condensing(II) TwoStageExtraction(III)

Amount

(MJ)Percentage

Amount

(MJ)Percentage

Amount

(MJ)Percentage

GrossElectricity 16,200 12.60% 18,669 13.99% 12,907 10.00%

DH 62,545 48.66% 31,273 23.44% 67,010 51.94%

WaterPre-heating 0 - 4,353 - 0 -

RefrigerantLoss - - 33,985 25.48% - -

FlueGasCleaning 5,187 4.03% 5,187 3.89% 5,187 4.02%

ExhaustFlueGas 10,776 8.38% 10,776 8.08% 10,776 8.35%

BottomAsh 150 0.12% 150 0.11% 150 0.12%

OtherLosses 33,686 26.21% 33,360 25.01% 32,993 25.57%

Useful 83,932 65.29% 55,129 41.32% 85,104 65.96%

Compared to traditional fossil fuels fired cogeneration power plants, considerable

economic and environmental benefits can be achieved. According to European

environmental policy, European power plants have to annually demonstrate a CO2

emission allowance corresponding to their actual CO2 emissions. The price for the

second commitment period (2008 to 2012) is 22 € (23.61USD) per ton of CO2

[41].

Assumingthat50%oftheenergyfromfossilresources isusedtosupply62,500MJ

DHdemandinCHPplants,thefuelconsumption,economicandenvironmentalcost

ofsomerepresentativesourcesarelistedinTable7.Dependingonthefueltype,2-15

million dollars can be saved each year, 30 thousand tons of coal (or 5.6 million

gallons of oil) consumption and more than 40 thousand tons of carbon dioxide

emissioncanbepreventedbyadoptingexhauststeamfromaWTEplantforDH.

Table7ConsumptionandCostofFossilFuelsDH

Fuels

Calorific

Value

(kJ/kg)[42]

Amount

(ton/h)

UnitPrice

(USD/t)

FuelCost

(USD/h)

CO2 EF

(g/kWh)[43]

CO2

Emission

(ton/y)

CO2

Emission

Allowance

(USD/y)

HardCoal 27,431 4.56 50.08 228.21 270 41,062.24 969,479

Oil147,708

(kJ/gal)

846

(gal)

2.08

(USD/gal)1,759.68 360 54,749.65 1,292,639

Waste 12,000 10 0 0 20 3,041.65 71,813

FortheCondensing-DHmodel,itgenerate18,700MJgrosselectricitywith31,300MJ

heat.ComparedtothewholecondensingWTEplants,itsacrifices20%ofelectricity

capacitytoearna20%additionofoverallefficiency.Forthecondensingturbinepart,

theproductivitydecreasedfrom0.65MWhelto0.14MWhelduetothedecreasing

steampressureandamount.Halfoftherefrigerantdemand,800tonsofwaterper

hour, is saved as well as the energy loss during condensation. Changing the

proportionofsteamextractedaftertheturbineforDHandkeepingtheothersteam

29

parametersthesame,theenergyoutputandefficiencyofplants isshowninFigure

12.ItisclearthatenergyconveyedbysteamforDHismuchmoreefficientlyutilized

than that fordrivinga condensing turbine.The totalenergyefficiencyofCHPWTE

plantclimbsstably,from16%to65%,astheextractedproportionofsteamincreases.

Simultaneously, the water consumption and energy loss for exhaust steam

condensationkeepscuttingdown.Toreachthehighestworkingefficiency,theplant

isunderthesameoperationstatusasmodelIwebuilt.

Forthethirdmodel,theDHsystemisheatedbyexhauststeamintwostages.Among

these threemodels,model III generatepowerwithhighestheat-electricity ratioof

5.2. The efficiency under this operation condition is approximately 65%, nearly

equalstomodelI.Similarly,theefficiencyandpowerratiomayvaryunderdifferent

steamextractproportionandpressure.

Figure12EnergyOutputforCogenerationWTEPlantswithDifferentSteamExtractionRatio

From the discussion above, applying the exhaust steam as a source of DH is an

energy efficient, environmental friendly and economical solution for both WTE

facilities and heat consumers. More than 70% of power from MSW can be well

utilized. Furthermore, the energy consumption and losses of pre-heating and

condensationisavoided,andcuttingdowntheoperatingexpensesoftheWTEplants

atthesametime.

30


Process FormulaEfficient

(MJ/h)

Losses

(MJ/h)

Efficiency

(%)Remark

Energy from

steam to DH

water(I)

(2509.483kJ/kg–504.207kJ/kg)*9.6kg/s*3600s/1000*95%=65,837.2MJ/h 65,837.2 3,465.1 95%

Enthalpy of steam (hot water):

2509.483 kJ/kg (179.9°C, 1 MPa),

504.207 kJ/kg (120°C, 0.8 MPa),

461.987 kJ/kg (110°C, 1 MPa),

251.977 kJ/kg (60°C, 1 MPa); Water

massflow:87.1kg/s

EnergyforDH(I) 65837.2MJ/h*95%=62545.3MJ/h 62,545.3 3,291.9 95% Transmissionloss:5%

Electricity

generated from

extractionturbine

(II)

0.45MWh/t*10t/h*3600=16,200MJ/h 16,200 852.6 95%

Productivity: 0.45MWhel per ton of

MSW

Electrical and mechanical efficiency:

95%

Energy from

steam to drive

the extraction

turbine(II)

(16200MJ/h+852.6MJ/h)/70%=24,360.9MJ/h 17,052.6 7,308.3 70% Isentropicefficiencyofturbine:70%

Energy from

steam to drive

the condensing

turbine(II)

(2509.483kJ/kg–2321.444kJ/kg)*4.8kg/s*3600s/1000=3,249.3MJ/h 2,599.5 649.8 80% Isentropicefficiencyofturbine:80%

Electricity

generated from

condensing

turbine(II)

2599.5MJ/h*95%=2,469.5MJ/h 2,469.5 130.0 95%


MSW (5 ton MSW base considering

steamextraction)

Energy from

steam to DH

water(II)

(2509.483kJ/kg–504.207kJ/kg)*4.8kg/s*3600s/1000*95%=32,918.6MJ/h 32,918.6 1,732.6 95% Watermassflow:43.6kg/s

EnergyforDH(II) 32918.6MJ/h*95%=31272.7MJ/h 31,272.7 1,645.9 95% Transmissionloss:5%

Energy in exhaust

steam from

condenser(II)

251.222kJ/kg*4.8kg/s*3600s/1000=4341.1MJ/h 4,341.1 Enthalpyofwater:251.22kJ/kg(60°C,

0.1MPa)

Energy to coolant

(II)(40114.6MJ–4341.1MJ)*95%=33,984.8MJ 33,984.8 1,788.7 95%

Water condition: Inlet 20°C, 2 MPa;

Outlet10°Cincrease.Waterflowrate:

33,984.8MJ/h / (127.564 kJ/kg -

85.7984kJ/kg)=813.7ton/h

Energy for feed (506.813kJ/kg–254.915kJ/kg)*4.8kg/s*3600s/1000/90%=4836.4MJ/h 4,352.8 483.6 90% Enthalpy of water: 506.813kJ/kg

31

water preheating

(II)

(120°C, 4.5MPa), 254.915kJ/kg (60°C,

4.5MPa); Heat transfer efficiency:

90%

Energy from

steam to drive

the extraction

turbine(III)

(3214.37kJ/kg–2796.04kJ/kg)*9.6kg/s*3600s/1000=14,457.5MJ/h 10,120.2 4,337.3 70%

Enthalpy of steam: 3214.37 kJ/kg

(400°C,4MPa),2796.04kJ/kg (195°C,

1.3 MPa); Isentropic efficiency of

turbine:70%

Electricity

generated from

extractionturbine

(III)

10120.2MJ/h*95%=9614.2MJ/h 9,614.2 506.0 95%


MSW

Electrical and mechanical efficiency:

95%

Energy from

steam to drive

the 2nd stage

turbine(III)

(2796.04kJ/kg–2509.483kJ/kg)*4.8kg/s*3600s/1000=4,951.7MJ/h 3,466.2 1,485.5 70% Isentropicefficiencyofturbine:80%

Electricity

generated from

2nd stage turbine

(III)

3466.2MJ/h*95%=2,469.5MJ/h 3,292.9 173.3 95%


MSW (5 tonMSWbased considering

steamextraction)

Energy from

steam to DH

water(III)

[(2509.483kJ/kg+2796.04kJ/kg)*4.8kg/s-504.348kJ/kg*9.6kg/s]*3600s/1000*95%

=70,536.7MJ/h70,536.7 3,712.5 95% Watermassflow:93.3kg/s

EnergyforDH(III) 70536.7MJ/h*95%=67,009.9MJ/h 67,009.9 3,526.8 95% Transmissionloss:5%

*I,II,IIIstandsformodelone,twoandthree.

32

3.MainDesalinationTechnologies

Thoughourearthisa“bluemarble”,awaterworld,butabout97.5percentofthatisundrinkablehighsalinitywaterandunfortunatelytwo-thirdsoffreshwateristuckedawayinfrozenglaciersorotherwiseunavailableforouruse.Asaresult,morethanonesixthpeople(1.1billion)aroundworldarewaterstressed,ordonothaveaccessto potable water. Under the current fresh water consumption rate, two-thirds ofpopulationmay face water shortages by 2025. The good news is that technologyadvances and cost reduction in seawater desalinationhavemade it a realistic andpromising solution to fresh water supply. By 2012, desalination have alreadycontributefortwo-thirdsofworldfeedwater,morethan60millioncubicmetersperday,mainlyforMiddleEastandGulfStates.Theproductioncostofdesalinationhavedowned to approximately 0.45-1USD/m3 in 2013 compared to9USD/m3 in 1970.Butforsomedevelopingareas,theexpensesisstillaround3-5USD/m3.

Salinewaterdesalinationcanberealizedessentiallybyeither thermalprocessesormembrane processes. Effective integration or cogeneration of desalination andpowerfacilitiescanreducethecostofdesalinationandelectricpowerproduction.Inmany countries, power demand may fluctuate from 100% in summer to 30% inwinterduetoairconditioningsystemswhereaswaterdemandremainsstablealltheyear round. Thermal andmembrane processes are coupled in some cogenerationplantstosolvethelargevariationbetweensummerandwinter.Currentdesalinationmarket is major occupied bymembrane technologies because of its lower energyconsumption and decreasing cost. The most widely used is reverse osmosis (RO),accounting for 60% of the total industry. Other membrane techniques likeelectrodeionisation (EDI) and electrodialysis (ED) only occupy approximately 4%oftotal desalination capacity. With new technology keeps developing, the energyconsumptionofdesalinationhavesignificantlyreducedtoaslowas3kWhpercubicmeter.However,comparedtolocalfreshwatersupplytechniqueswhichconsumes0.1-1 kWh/m3, desalination is still energy-intensive and need further sustainablesolutions.

Figure13Totalworldwideinstalledcapacitybytechnology[4]

33

3.1.RODesalinationTechniques

1) TechnicalPrinciples

The coreprincipleofROdesalination is a natural phenomenon,osmosis, bywhichwaterautomaticallypassesthroughasemi-permeablemembranefromlowersalinitysolutionintoamoreconcentratedsolution.Noheatingorphasechangetakesplacein this process.Whenpressure is applied to thehigher salinity solution, thewaterwillbeforcedtoflowinareversedirectionthroughthesemi-permeablemembranewhileleavingthesaltbehind.

Figure14MechanismofOsmosisandReverseOsmosis

2) TechnicalDescription

For an RO desalination plants, it essentially consists of four parts: a pretreatmentsystem, a high pressure pump, a core membrane system and the post treatmentsystem. In some of the plants, there is an energy recovery system between thepumpsandmembranetocutdowntheenergydemandofdesalinationplants.

ThefeedsalinewaterisfirstpretreatedtopromisethecleannessofROmembranessurface. Therefore, suspended solids contained in feedwatermust be removed toprevent salt precipitation or microbial growth. Pre-treatment may involveconventional water treatment methods such as screen separation, chemicalsedimentation and filtration, or advancedmembrane processes likemicrofiltration(MF)orultrafiltration (UF)[44].Also,pHadjustment is sometimes requiredbasedonwatercharacteristics.

Highpressurepumpsectionsuppliesthepressureneededtoencourageclarifiedfeedwater flowing throughmembrane, typically between 55-85 bar depending on thetemperature and salinity of water[45]. The normally operation pressure is 2-17 bar(30-250psi)forslightlybrackishwater,17-28bar(250-400psi)forbrackishwaterand55-82 bar (800-1200 psi) for seawater[46][47]. The high pressure pumps involve themajor energy requirement of the whole RO processes, around 5-7 kWh per cubicmeter treated. The recent development ofmore efficient energy recovery devices,pressureexchangerandenergyrecoverypump,realizetherecoveryofenergyfromconcentrate brine flow via piston system, thus greatly reducing the overall energyconsumptiontoapproximately1-3kWh/m3.

34

Figure15RODesalinationPlants

The membrane assembly consists of a pressure vessel and a semi-permeablemembrane elements inside, varying from one to eight per vessel. Themembraneinhibitsthepassageofdissolvedsaltswhilepermittingwatertopassby,thusdividingthe feedwater flow into a freshproduct streamand a concentratedbrine stream.However,nomembraneworksperfect so that theremaybea small percentageofsalt remaining in the product water. The most widely accepted RO membraneconfigurationtypeisSpiralwoundwhichgenerallycomposedofcelluloseacetateorof other composite polymers. Spiral wound module is actually a flat sheet ofmembrane wrapped around a central collecting tube. The pressurized feed waterflowsinspaceswithinthemembraneenvelope,purifiedandcollectedinthetube.Aswaterpurified, the remaining salinewaterbecomemoreconcentrated, resulting inover-saturation of salts and ever-boosting energy input to overcome the naturalincreasedosmoticpressure. In this case, certainportionof feedwater iswithdrawfromthemembraneelements.Thedischargedamounttypicallyrangesfromaround20%forbrackishwaterto50-80%forseawater.Theconcentrateflowisnormallyjust1.5-3.5bar (20-50psi) less than the feedpressurewhile theproductwater runsatatmosphericpressure.

Figure16SpiralWoundROMembrane45

Themainpurposeofdesalinatedwaterpost-treatment istostabilizethewaterandprepare it for distribution. This mainly includes re-mineralization, pH adjustment,disinfectionanddegasificationbeforedeliveredtodistributionsystem.

3) ProductionCost

35

The production cost for RO desalination plants is influenced by massive factorsincludingelectricityprice, laborcost, feedwaterquality, treatmentcapacityandsoon[48]. According to “Courtesy of Water Desalination Report”, the sea water ROdesalination cost is generally within the variation of 700-1200 USD/acre-foot(0.58-1.00 USD/m3). In United States, the brackish water RO desalination cost isabout0.10-1.05USD/m3.Asidefromfixedcapitalcost,themostsignificantcostsaretypicallythecostsofelectricity(40-55%),membranereplacement(5-10%),andlabor(~5%)[49].

Figure17AnnualizedSWROCostTrends

4) ProductivityandEffectiveness

Currently the RO technology can separate over 99% of salt from seawater with aproductivityof15-24liters/m2membraneperday(1.7-3.3m3/d/membrane[50]).Madeofcompositepolyamide,themembranefornowissaidtobeguaranteeda5yearslife without replacement. Besides, the woundedmembrane elements greatly savethespaceofdesalinationplants, resulting inaveryhighspace/productioncapacityratio,rangingfrom25,000to60,000liters/day/m2[47].

5) Disadvantages

- Inpractice,ROtechnologyishighlydependsonareliableenergysource;

- Limitedbythedesignofwaterintakeandpretreatment,ROdesalinationplantsusing seawater may be interrupted by stormy weathers because of there-suspensionandincreasingsuspendedparticulateconcentration;

- Even new materials is applied to strength the RO membrane, it is still verysensitive to abuse so that extra construction for feed water pretreatment isalwaysinneed;

- Therearebacterialcontaminationrisksonmembranesurface,causingtastesand

36

odorsproblemsonproductwater;

- ThehighlyconcentratedbrinefromROtechnologymustbecarefullydisposedtoavoid hazardous environmental impacts on oceanic and underground watersystem.

3.2.ThermalDesalinationTechniques

As itsname implies, thermaldesalination techniquesheatup the salinewater andgather pure product water mainly by distillation. Distillation is defined as apurification processes of fluid via evaporation and condensation. The condensedproduct of distillation is usually of single compound and free from salt due to thedifferent evaporation point. This simple principle based thermal technologies iswidelyusedinseawaterdesalinationbutrarelyusedinbrackishwaterdesalination,mainlybecauseof itshighcosts involved.Solarpowerarenowoftencoupledwiththermal desalination facilities to cut down their cost and unrenewable energyconsumptions. The two major thermal ways of desalination are Multiple EffectDistillation (MED) and Multi-Stage Flash Distillation (MSF). Depending on theavailabilityandqualityofenergyon-site,MEDplantssometimesfitwiththermalormechanical compressor to enhance their performance and optimize their energyrequirements, called MED-TVC (Thermal Vapor Compression) and MED-MVC(MechanicalVaporCompression).Theyaretypicallypoweredbysteamtogetherwithsome electricity. For thermal desalination processes, no special pre-treatment isrequired. But to protect the evaporators and pipelines, filtration is sometimesappliedtoremovelargesolidparticles.

3.2.1.MultipleEffectDistillation(MED)


TheMEDmethod is said to be themost efficient, economical and easy operatingthermaldesalinationprocess,occupiedapproximately28%ofdesalinationmarket.Itis a low temperature thermal process that collect fresh water by distillation in asequenceofvesselscalledeffects.Eachoftheeffectmaintainsalowertemperatureandpressurethanthepreviousone.Sincetheboilingpointofwaterdecreaseswithreduced pressure (Table 9), the salinewater can efficiently keep evaporating in allvesselswithpressurecontrol,evenunderlowtemperatureof40°C.

Table9WaterBoilingPointTablePressure 1bar 0.47bar 0.32bar 0.25bar 0.1bar

Boilingpoint 100°C 80°C 70°C 65°C 45°C

2) ProcessDescription

The core of MED process is the MED evaporator. It consists of a serious ofconsecutiveeffects,2-16typically,remainedatdecreasingleveloftemperatureandpressure.Figure18showstheschematicthree-stageMEDevaporatorandazoomed

37

pictureofsingleMEDcell.Eacheffectcontainsabundleofhorizontalheattransfertubeswhereinheatingsteamisintroducedandcondensed.Feedseawaterissprayedfromtopofthebundleandflowsdownwardtube-by-tubewithgravity.Theseawatercools the tube externally, condensing the steam inside into purified water.Simultaneously, the heat released from the inner steam flow warms up the feedwater and partly evaporates it. The seawater gradually concentrates withevaporationandgivesbrineatthecellbottomwhichwillthenbetransporttonextstage. The vapor formedoutside the tube is directed to flow inside tubes for nextstageandusedasheatingsourcewhere theprocess repeats. In the lasteffect, theproducedsteamcondensesinaconventionalshellandtubesheatexchangercooledbyfeedseawater.Extracondensingseawaterotherthansprayedfeedflowisrejectedbacktothesea.Theconcentratedbrineiscollectedcelltocelltillthelastone,thenextractedbycentrifugalpumps.

ForMED desalination, only the first effect with highest pressure requires externalheat source and greatly withdraw the energy consumption of this process. Theheatingsteamofthefirsteffectisgenerallylowpressurecondensingsteam(0.3-0.5bar,70-90°C).Otherheatingmedialikehotwater,wasteenergyfrompowerplantsorsolar heat can also be applied for thismethod. Tomaintain the effect cells at lowpressure,theseawatertemperaturestypicallyremainsbelow65-70°C,thusavoidingunnecessary heating and allowing a good control of scaling. Typical pressure dropacrossthesystemis5-50kPa(lessthan5kPa/stage).Themoreeffectsaprocesshas,thehigherperformanceratioitgenerallyreaches.

(a)

(b)

Figure18SchematicofMEDEvaporatorandSingleEffectUnit[51]

3) EnergyConsumptionandProductionCost

38

TheenergyconsumptionofMEDprocessisresultfromvariesoperationfactors:TopBrineTemperature(TBT)knownasthemaximumtemperatureofthebrinesolution;Number of stages; Gain Output Ratio (GOR) known as the ratio between productwater amount per unit mass of dry saturated steam supplied to the system. ThetypicalelectricpowerconsumptionofMEDis1.5-2.5kWh/m3,thethermalenergyis60-110 kWh/m3 corresponding to GOR value from 10 to 6. Electrical equivalentdefinedastheamountofelectricalenergycannotbeproducedbecauseofextractionofheating steam is alsoused toevaluate theenergy consumption for theprocess.ForMED technique, the electrical equivalent for thermal energy is 5-8.5 kWh/m3,6.5-11kWh/m3inall.TheproductioncostforMEDtechniqueis0.7-1USD/m3whilethecapitalcostis3.5-4USDperinstalledgallonperday.

4) Productivity

For recent years, the unit treatment capacity of MED technology is significantlyincreased from4,500m3/d todramatic45,000-68,000m3/d[52].The typicalworkingcapacityofanMEDunitisabout5,000-15,000m3/d[53].

3.2.2.MED-TVCandMED-MVC

As an enhancement of conventional low-temperature MED process, MED-TVC(Thermal Vapor Compression) and MED-MVC (Mechanical Vapor Compression) isinvolvedinmanythermaldesalinationfacilitiestooptimizetheperformanceofMEDevaporators,particularlyinmanycogenerationplants.

The MED-TVC evaporator fitted a thermal compressor, called thermocompressor,withbasicMEDunitstotakeadvantageofpressureofavailablesteam(usually2.5-3bar,120-150°C)extractedfromback-pressureorextractionsteamturbine.Thesupplysteam, called motive steam, is fed into thermocompressor through a nozzle. Themotive steam expansion within compressor body forces the sucking out of lowpressurevaporfromevaporator.Bothmotiveandsuctionsteamsarethenmixedbydiffusionandfinallydischargedwiththepressuresuitableforfirstvessel.Thelatentheat in last stage vapor is recycled to evaporator at the same time and becomesavailableagainforMEDdesalinationprocesses,leadingtoenergysavings.

AhigherGORcanbeobtainedwithMED-TVCunits,usually12-15andcanevenupto17. The thermal energy can reduced to 40 kWh/m3 with a GOR of 16 while theelectricpowerconsumptionstaysthesame(1.5-2.5kWh/m3).Therearealsostudiessuggestalowerelectricityconsumptionvariationof1-1.7kWh/m3fortheMED-TVCprocess[52].Butwithrelativehighextractsteampressure,theelectricalequivalentforMED-TVCprocessishigh,9.5-25.5kWh/m3forthermalenergy,and11-28kWh/m3inall.Moreover,therecyclingofpartofvaporpermitstheevaporatorrunninginlargerscale.ThetypicalunitproductivityofMED-TVCis10,000-35,000m3/d,doubledthansimpleMEDunits.

39

Figure19SchematicofMED-TVCProcess[51]

Figure20SchematicofThermocompression

Unlike mainly thermal powered MED and MED-TVC processes, the MED-MVCevaporator isaMEDevaporatoronlypoweredbyelectricalenergy.All the featuresdescribedforMEDevaporatorapplyforMED-MVCprocess,butthevaporproducedby the coldest effect is recovered to the first bundle through a mechanicalcompressor. The electric powered compressor brings the extract vapor to thepressure condition prevailing inside the first vessel, enabling the latent heatre-availablefordistillation.Theheatfromdistillatedproductanddischargedbrineisrecovered to preheat the seawater feed by two heat exchangers. Due to this highefficiency operationmode, theMED-MVC process does not calls for extra coolingstuffsotherthanfeedseawaterwhichmakesitattractiveincoolingsourceintensiveplaces.

Theenergy inputofMED-MVC typically ranges fromabout18kWh/m3 fora singleeffectevaporatortoaround8kWh/m3forafour-cellevaporator.Thisisquietsimilarto simple MVC process which use 7-12 kWh/m3 electric energy generally. Thisprocess is attractive in terms of energy consumption compared to MED-TVC.However,due to thehighcostofmechanical compressor, the investment is alwayshigher than foraMEDorMED-TVCplant.Anotherproblem for thisprocess is thatthe plant size greatly restricted by the capacity of available compressor on themarket. For the time being, the common unit size is 100-2,500 m3/d while themaximumpossiblesizeproducefreshwaterattherateofapproximately5,000m3/d.

40

Figure21SchematicofMED-MVCProcess[51]

3.2.3.Multi-StageFlash(MSF)Distillation


Multi-Stage Flash (MSF) distillation is based on distillation through several ormulti-stagechambersoperatingatprogressivelylowerpressures.Thefeedseawaterispre-heatedunderhighpressure.Whenthesalinewaterisledintothefirstchamber,thepressureisreleasingcausesarapidboilingorsuddenevaporationoffeedwater,known as ‘flashing’. This ‘flashing’ continues happening in each successive stagebecauseofthereducingpressurewithinchambers.

2) ProcessDescription

TheMSF process delivers feed seawater within a closed pipe passing through theflash chambers and is heated by vapor generated from flashing. The low-pressuresteam(2-3.5bar,100-130°C)istreatedasanadditionalheatsourcetowarmuptheseawater from tubes to the initial high temperature, typically around 110°C. TBTshould be limitedwithin 90-120°C to avoid precipitationof salt. The flashed vaporexchanges heat with feed tubes, condensed and collected as product water. Thevelocity of flashed vapor shouldnot exceed6m/sdue to the entrainmentof brinedropletintovaporsystem.

According to the brine treatment,MSF distillation has a division of ‘once-through’mode or ‘brine recycled’ mode. For once-through design, the feed water passthroughchambersonceand the leftbrine isdirectlydisposed.For recycleddesign,partofthebrineisrecycledandmixedwithfeedwatertoimprovetherecoveryratio.MSF plants are subject to corrosion caused by the turbulence of saltwater unlessstainlesssteelisusedextensively.

41

(a)One-ThroughMSFProcess

(b)BrineRecycledMSFProcess

(c)MSFCell

Figure22MSFDesalinationProcessandMSFCell

3) EnergyConsumptionandProductionCost

The electricity mainly consumed by pumps for MSF procedure is 4-6 kWh/m3 ofdistillate.Theheatconsumptionisabout55-110kWh/m3withGORvaluefrom12.2to6.Withaheating steampressureof2-3.5bar, theequivalentelectricpower forthermalsupply is0.5-19.5kWh/m3,13.5-25.5kWh/m3 intotal.TheproductioncostforMSF technique is 0.5-1.75 USD/m3[54] while the capital cost is 5.5-10 USD perinstalledgallonperday[55].

42

4) Productivity

AMSFplantsevaporatorunittypicallycontains15-25stageswithunitcapacityfrom50,000 m3/d to 70,000 m3/d. The featured GOR of MSF process is around 8. Toensure the efficient operation of MSF process, the system should not run below70-80%ofdesignedcapacity[56].

3.3.ComparisonofPopularDesalinationTechnologies

Theoperatingconditionofdescribeddesalination technologies canbe summarizedasTable10.

Table10ComparisonofCharacteristicsofMainDesalinationTechnologies[53][57] RO MSF MED MED-TVC MED-MVCTypical Unit Treatment Capacity(m3/d)

24,000 50,000-70,000 5,000-15,000 10,000-35,000 100-2,500

FeedWaterQuality Specificpre-treated

Notcritical Notcritical Notcritical Notcritical

DistillateQuality(ppmTDS)1stage:3002 stage:10-50

1-10 1-10 1-10 1-10

SteamTemperature(°C) / 100-130 70-90 120-150 /SteamPressure(bar) / 2-3.5 0.3-0.5 2.5-3 /

OperatingpressureorTemperature55-82 bar(seawater) 90-120°C 65-70°C 65-70°C 65-70°C

Thermal Power Consumption(kWh/m3)

/ 55-110 60-110 40-110 /

Electric Power Consumption(kWh/m3)

1-7 4-6 1.5-2.5 1.5-2.5 8-18

ElectricalEquivalent(kWh/m3) 1-7 13.5-25.5 6.5-11 11-28 8-18ProductionCost(USD/m3) 0.45-0.58 0.5-1.75 0.7-1 ~1*CapitalCost(USD/gal/d) 0.35-1.18 5.5-10 3.5-4 4.5-9.0 MaintenanceCost Medium Low Low Low Low

43

4.CaseStudyofCogenerationDesalinationPlant

Inmostcases,particularlyfor largedesalinationplants,the lowestcostandhighestefficiency is obtained by coupling water production with power generation,optimizing the exhaust conditions of turbines to feed the desalination units. Thelarge desalination plants around Arabian Sea are usually developed on suchcogeneration concept. Sometimes RO units are associated to these thermaldesalination processes to further optimize the practicability and feasibility ofcogenerationdesalinationfacilities,calledhybridplant[58].Thispartaimstogiveoutseveral case studies of existing cogeneration desalination plants to betterunderstandingtheirprocess,productivityandenergyflowdetails.

4.1.St.BarthWTEDesalinationCogenerationPlant

4.1.1.BackgroundofSt.Barth

SaintBarthélemy(alsoknownasSt.BarthorSt.Barts)islocatedintheFrenchWestIndies,approximately160mileseastofPuertoRicoandthenearerVirginIslandsand15miles southeast of St.Martin[59]. St Barth belongs to France and has EuropeanambienceunlikeanyotherislandintheCaribbean,withatotalpopulationof7,237.This eight-square-mile islandhas hilly landscape and aboundwith beaches, almostcompletely surrounded by shallow-water reefs. Gustavia is the capital city, built atthewestislandaroundtheharbor[60].Withfewnaturalresources,theeconomyofStBarthisfosteredbyhigh-endtourismandduty-freeluxurycommerce,servingmorethan200,000visitorsprimarily fromNorthAmerica.TheclimatetypeofStBarth istropicalclimate,practicallynovariation intemperaturewithtwoclearseasons:dryand humid. With no natural rivers and streams, fresh water resources is in shortsupply, especially in tourist summer season from May to November. Fresh watersupplyonSt.Barthismainlyprovidedbyseawaterdesalination,rainwatercollectionand via water tanker import. Besides, nearly all food, energy resources and mostmanufacturedgoodsaresuppliedbyimport.

Figure23MapofSt.Barth[61]

44

4.1.2.WasteManagementandWaterProductioninSt.Barth

Torelievetheconflictbetweenresourceshortagerealityand improvingenergyandfreshwaterdemandsaswellasto improvethewastemanagesystemonisland,St.Barth institute a recycling program for household trash in 1998[ 62 ]. AWTE-desalinationfacility,ownedbyGroupeTIRU(EDF),isthenunderconstructedatGustaviaandputintooperationin2001.

Figure24WTE-DesalinationPlantofSt.Barth

AccordingtotheMSWmanagementsystemofSt.Barth,thewasteisfirstcollectedatthe island’s waste disposal plant, located just outside Gustavia, for recyclablesseparation.Components likebatteriesandmetalsaregenerallystockpiledandsentoff-island toGuadeloupe,Miami,Francebybarge for recycling;glass is repurposedlocallytocreatesub-strata;trash,paper/cardboardandplasticcontainersaresenttotheWTE plant for incineration[61]. This open-air classification process increase thecombustible component percentile and partially dried the waste by sunlight,resultinginarelativehighcalorificvalueoffeedMSW.

Figure25CombustibleWasteDriedandMoved[61]

The WTE plant combusts MSW collected using a Cyclerige oscillating kiln with acapacity of 1.5 tons per hour, 9000 tons annually[2]. With 80% thermal recovery

45

efficiency, this incinerationprocesscanproduce3 tonsofsteamper tonofburnedMSW[ 63 ]. The electricity generated from turbine is used for the operationconsumptionofWTEandneighboreddesalinationplant,therestofsteamissoldinformofheattopowerthethermalMEDunitsprovidedbySIDEM[64].SteamdeliveredfromWTE can produce 1200-1720[2] cubicmeters of potable water per day, 1350cubicmeterperdaygenerally[63],providing40%oftheisland’swaterdemand[65].Therest of freshwater is producedby aROdesalinationplant. In 2009, themaximumproductionofpotablewaterinSt.Barthis4,300cubicmetersperdaycomparingtoanaverageconsumptionof3,000cubicmetersperday[66].In2013,EDFinstalledtwoextrapowergenerationunitstotheWTEfacilitywith16MWoutputinall[67].

Table11OperationDataofSt.BarthWTEPlant 2008 2009MSWCombusted/ton 9,762 9,038HeatSold/MWh 20,666 19,876ProductionFactor/MWh/tonMSW 2.117 2.199

AverageWorkingHour 6508h17.8h/d

6025h16.5h/d

4.1.3.WTE-DesalinationPlantEnergyFlowEstimation

Since only very limited information is offered by St. Barth local government anddevice suppliers, parameter estimation is introduced for brief energy flow study.AccordingtotheWTEplantoperationdata in2008and2009,theestimationresultcanbeshownasTable12.Thecalorificvalueof feedwaste isestimated fromsoldheat,with60%-80%energy transferefficiency fromMSW.The fluegas fromkiln isused for combustion feed air pre-heating. According to the water temperature inCaribbeanSea(23-31°C),anaverageof27°Cisconsideredasfeedwatertemperature.Lossesforsteamtransportbetweentwounitsisignored.Otheroperatingparametersareestimatedbasedonpreviousmodelsandheatbalanceequations.

Table12ParameterEstimationofSt.BarthWTEPlant EstimatedRange ParameterAdoptedLHVofMSW(MJ/kg) 9.5-13.2 11.5MSW Incineration Capacity(t/h)

1.5 1.5

SteamGeneration(kg/s) 1.25 1.25HeatSold(MWh/tMSW) 2.1-2.2 2.15HeatSold(kJ/kgSteam) 2556-2628 2580StatusofSoldSteam 246°C, 15bar 246°C, 15barStatusofBackSteam/Water 80°C,15bar 80°C, 15barAverageWorkingHour(h/d) 16.5-17.8 17

46

Table13OperationParameterofSt.BarthDesalinationPlant[51]DesalinationTechnique MED ProcessFreshWaterGenerationCapacity(m3/h) 913Effect1:Temperature,Pressure(BoilingPoint) 60°C, 0.20bar (58.9°C)FreshWaterTemperature(°C) 32FeedRawWaterMassFlow(kg/s) 81.25RawWaterTemperature(°C) 27SalineTemperature(°C) 38SalinityofRawWater 3.6%SalinityofExhaustSaline 5.0%DensityofRawSeaWater(t/m3) 1.0238HeatCapacityofRawWater(kJ/kg�°C) 4.096

Fromtheaccountingresult,theSt.BarthWTEplantisaheat-focusedfacilitythat67%ofheatissoldforfreshwaterproductionandonly8.5%istransformedtoelectricity.Theover-allenergyefficiencyofSt.Barth’sWTEunitisaround75%,stillhavespacefor improvement. Based on the boiler productivity data (3 ton steam per ton ofMSW),thesteamtemperatureandpressurewouldbereallyhigh(4.5-5MPa,480+°C).If steam supplied at 2.58MJ/ton (steam) status, then the feed steam would alsounderveryhighstatus,possiblyover150°Candreturnedaround80°C.ComparingtogeneralMEDunitswith0.3-0.5bar,70-90°Csteamfeed, thishighenergy levelandobvious temperature gap greatly improves the raw sea water dealing rate andacceleratetheheattransferbetweenfeedsteamandrawwater,whilethereturnedhotwaterdoesnotneedtobepre-heated.Therelativehighheattransferefficiency(~92%) also ensure the productivity ofMED unit. Regardless of the electric powerconsumption,theheatconsumptionforpotablewaterproductionis8.6MJ/m3(40.6kWh/m3). In all, the coupled WTE and thermal desalination plants reached animpressive GOR of 17.8, higher than the world’s average 8-10. Looking at severalMED desalination project operated by SIDEM, this GOR data is acceptable from atypicalGORlevelof14.

Table14EnergyFlowResultofSt.BarthWTE-DesalinationPlant(1hScale)

Plant EnergyInflowAmount(MJ)

Percentage EnergyOutflowAmount(MJ)

Percentage

WTEPlant

MSW 17,250 100% GrossElectricity 1,456.4 8.44% Desalination 11,610 67.30% Airpre-heating 1,125 6.52% WTELoss 4,296.1 24.90%

DesalinationPlant

Steam 11,610 100%Fresh WaterProduction 1,711.7 14.74%

ExhaustSaline 9,498.1 81.81% OtherLosses 940.2 8.10%

47

Table15EnergyFlowAccountingofSt.BarthWTEPlants

Process FormulaEfficientAmount

(MJ/h)Losses(MJ/h) Efficiency(%) Remark

Totalchemicalenergy

inMSW

11.5MJ/kg * 1.5t/h = 17,250

MJ/h17,250 - -

LHV=11.5MJ/kg

Capacity:1.5t/h

Heatingupofprimary

combustionair

1.25kJ/m3/°C*4m3/kgMSW

* 1.5t/h * (120°C - 30°C) /

90%=750MJ/h

675 75 90%

Heatvalueofair:1.25kJ/m3/°C


Airfed:4m3/kgMSW

Heating up of

Secondary

combustionair

1.25kJ/m3/°C*2m3/kgMSW

* 1.5t/h * (120°C - 30°C) /

90%=375MJ/h

337.5 37.5 90%

Heatvalueofair:1.25kJ/m3/°C


Airfed:2m3/kgMSW

Energy to boiler

steam

17,250MJ/h * 80% =

13,800MJ/h13,800 3450 80%

HeatEfficiencyofChambertoBoiler:80%

Lossincludeheattransferloss,bottomash,fluegasloss

EnergySoldinHeat2.15MWh/t MSW * 1.5t/h =

11,610MJ/h11,610

Energyfromsteamto

drivetheturbine13,800-11,610=2,190MJ/h 1,533 657 70% Isentropicefficiencyofturbine:70%

Electricitygenerated1,533MJ/h * 95% = 1,456.35

MJ/h1,456.4 76.6 95%



Efficient energy for

fresh water

production

22.75kg/s * 4.18 kJ/kg/°C *

3600s * (32°C - 27°C) =

1,711.71MJ/h

1,711.7

Energy for raw water

heating

(81.25-22.75)kg/s * 4.10

kJ/kg/°C * 3600s * (38°C -

27°C)=9,498.06MJ/h

9,498.1

EnergyLossforMED11610-1711.7-9498.1 =

940.2MJ/h 940.2 91.9%

48

4.2.HebeiHuanghuaPower-DesalinationCogenerationPlant

4.2.1.IntroductionofHuanghuaPowerPlant

Huanghua coal-fired power plant is the first power plant in China that coupledelectricity generationwithdesalinationunits. It isownedand investedbyShenhuaGrouptogetherwithHebeiandCangzhoulocalgovernment,operatedbyShenhua’ssubside Guohua Cangdong Power. The designed power generation capacity ofHuanghuapowerplant is6520MW,2520MWamongthemisnowunderoperation.Locatednear PohaiGulf, direct seawater cooling is used for all the condensers ofHuanghua power plant. Approximately 3,200,000 - 4,400,000 m3 of freshwater isconsumed annually as feed-water for the four boiler units, as well as fordesulfurizationandotherprocesses.Thismassivewaterconsumptionrequirementiscompletely filledthroughcoupleddesalinationunitsofHuanghuapowerplant.Thepower-water cogeneration process achieves zero fresh water consumption andeffectively converts conventional water-intensive power plant into a fresh watersupplier to surrounding regions.With the completionof 3rd-stagedesalinationunitconstruction,thecurrentproductioncapacityofHuanghuapowerplanthasreached57,500 ton per day. Despitemeeting the needs of self-utilization, the desalinationunits produce 5-10 million external tons of fresh water annually (14,000-28,000ton/d) for the Port of Huanghua in Cangzhou, Hebei, aswell as industrial users innearbysteelplants[68].

Table16IntroductionofHuanghuaPowerPlant[69]

StageofConstruction I II III

Year 2004-2007 2007-2009 2013

PowerGeneration 600MW�2 660MW�2 1,000MW�4

DesalinationUnit 10,000ton/d�2 12,500ton/d 25,000ton/d

OperationStatus Under Operation Under Operation Incomplete

Investment(USD) 794,500,000 44,000,000

Figure26GuohuaCangdongPowerPlantDesalinationFacility(25,000t/d)

AlthoughtheequipmentinvestmentandwaterproductioncostsforSWROtechnique

49

is lower, but its higher requirement in terms of seawater quality reduce itsadaptability. It has no obvious advantages in investment as well as operationalenergy consumption when applied in northern China with relative low sea waterquality[70]. Driven by the company’s resource-saving strategic development needs,ShenhuaGuohuaselectMED-TVCtechnologyforitscogenerationofpowerandwater.Through research, development, and application, ShenhuaGuohuahas proprietaryof large-scaleMEDequipment andbuilt up a 25,000 ton/dMEDunit atHuanghuapowerplant.

The MED unit developed by Shenhua Guohua uses a horizontal-pipe falling filmevaporator (Figure27)[70]. Theexhaust steam frompowerplant turbineenters theheatexchangeroffirsteffectandcondensesafterreleasingsensibleandlatentheat.The feed seawater is partially evaporatedby absorbing this heat, then guided intothe secondeffect. Such step is repeated foreach sequential evaporator.When theexhauststeamhasahighertemperatureandpressurelevel,partoftheevaporatedvapor can be recirculated via a thermal vapor compressor to increase its pressureand transfer it to the first effect heating source, thus improving the efficiency andreducingthecostsofwatergeneration.Thevaporfromlasteffect isdirectedintoacondenser for feed raw water pre-heating, the condensed fresh water is alsocollected as product. To implement efficient heat transfer, Shenhua GuohuamasteredthemechanismsofvacuumheattransferandflowwithsmalltemperaturedifferencesandmultiphasethroughR&D.

Figure27SchematicofShenhuaGuohuaLarge-scaleMEDProcess(12,000t/d)

4.2.2Coal-FiredPower-WaterCogenerationEnergyFlowEstimation

TheoperationparameterofHuanghuapower-desalinationplantisshowninTable17andTable18. In2009,Huanghuapowerplantgenerate9,410,000MWh(8,950,000MWhsold)with2520MWinstalledcapacity.Thecoalconsumptionleveloffinanceisaround 331g/kWh[71]. Take this generation level into considered, the effectiveoperationtimeof theturbine is3734h,10.23h/d.Thereturnedheatsteam(water)temperature from 1st effect is typically around 140°C. Since the third stage powerunit is stillunderconstructionandthe2ndstage isundereco-adjustment,only firstconstructionstage’srecentoperationparameterisofferedbythepowerplant.Inthiscase,theestimatedenergyflowaccountingofHuanghuapowerplantonlytakesthe

50

firstconstructionstageintocalculation.

Table17HuaghuaPowerPlantTurbineOperationData

StageofConstruction I II[72]

III[73]

Equipment

ManufacturerShanghai Electric Shanghai Electric Shanghai Electric

NumberofTurbine 2 2 4

GenerationCapacity 600MW 660MW 1,000MW (1,030max)

Feed Steam

Temperature(°C)538 566 600 (610max)

Feed Steam Pressure

(bar)167 242 250 (270max)

FeedSteamFlowRate 1800t/h 1800t/h ~2800t/h

SteamExtraction 4 stage 4 stage 4 stage

ThermalEfficiency 45.4%

CoalConsumption 330-350g/kWh ~300g/kWh ~275g/kWh

Table18OperationParameterofHuanghuaDesalinationUnits

StageofConstruction I[74]

II[75]

III[76]

Equipment

ManufacturerSIDEM Shenhua Guohua Shenhua Guohua

DesalinationTechnique MED-TVC MED-TVC MED-TVC

NumberofEffect 4 6 10

Fresh Water

Generation Capacity

(t/d)

10,000 12,500 25,000

SteamSupply

4th extraction from

600MW×2 steam

turbine

4th extraction from

660MW×2 steam

turbine

4th extraction from

600MW×2 and

660MW×2 steam

turbine

VaporExtractiontoTVC 4th Effect 4

thEffect 7

th Effect

FeedSteamFlow(t/h) 50

52

(48.5 exhaust steam

and 3.5 extraction

vaporfromeffect)[77]

77

Feed Steam Pressure

(bar)3.7-5.5(<8.0) 5.5(>3) 5.5

WorkLoadFeasibility 50-100% 40-110% 40-100%

Feed Steam

Temperature(°C)320 (<380) 320 320

GOR 8.33 9.67-10.2 13.5(�13)

Designed Raw Water

Temperature(°C)-

25(-1.5-30)

42(from condenser)20

42(fromcondenser)

SalinityofRawWater 3.6% 3.6% 3.6%

ConcentrationTimes 1.4-1.5 - -

Salinity of Exhaust

Saline- - 5.11%

ElectricityConsumption 0.82-1.0 kWh/t 1.0-1.2kWh/t

To improve theheatefficiencyofpowerplant, there is a steam re-heatingprocessbetween the high pressure turbine and middle pressure turbine for the 600MWsteamturbine.Italsohasfour-levelsteamextractionforfeedwaterpre-heatingfrom35°Cto273.7°C,theextractedsteamisthenmixedwithwatersupplyasfeedwater.

51

Seawaterisappliedascondensateinthesteamcondenserwhichcanbealsotreatedas pre-heating process of sea water desalination, especially in low temperaturewinter seasons. The overall mass flow of raw sea water is 260,000m3/d withtemperatureof25°C.ThesteppedaccountingprocessescanbeshowninTable20.

Accordingtotheaccountingresult(Table19),withtheheatingrecoverysystemandcoupleddesalinationsystem,only10%oftotalenergyinputlostduringtheprocess,theelectricityconsumptioncanbecoveredbytheelectricitygeneratedfromturbine.About 40% of the heat generated is removed into sea water pumping from thesurrounding PohaiGulf, cutting theplant’s condensingwater consumption to zero.However,merely20%oftheheatisefficientlyutilizedfordesalinationprocess,evenincluding part of heat consumed by the 25,000m3/dMED unit. During the winterseason,theefficientproportionincreasestoaround40%duetothelowerseawatertemperature. Assume an overall thermal efficiency of 80% to 20,000 m3/dcoal-powered MED desalination unit, approximately 480-500 tons of coal (47,500USD[78]) is saved every day. The reducedwater production cost also guarantee thewatersupplyandwaterpriceofsurroundinglocalarea.

Table19EnergyFlowResultofHunaghuaPower-DesalinationPlant(1hScale)

PlantEnergy

InflowAmount(MJ) Percentage

Energy

Outflow

Amount

(MJ)Percentage

PowerPlant

Coal 11,630,016.0 77.45%Gross

Electricity4,320,000.0 28.77%

Electricity 216,000 1.44% Desalination 6,290,831.5 41.89%

Ancillary

Fuel3,169,600.8 21.11% AirPre-heating 1,562,623.1 10.41%

Feed Water

Pre-heating1,289,584.5 8.59%

OtherLoss 1,552,577.7 10.34%

Desalination

Plant

Exhaust

Steam6,040,352.5 96.02% FreshWater 32,687.6 0.52%

Extract

Steam250,479.0 3.98% ExhaustSaline 154,685.4 2.46%

Raw Sea

Water

Pre-heating

920,186.9 14.63%

Losses 5,183,271.6 82.39%

52

Table20EnergyFlowAccountingofHuanghuaPower-DesalinationsPlants[79]Process Formula Efficient(MJ/h) Losses(MJ/h) Efficiency(%) Remark

Coal combusted inpowerplant

29.28MJ/kg * 331kg/MWh * 1200MW =11,630,016MJ/h

11,630,016 - -LHV=29.28MJ/kgCapacity:331g/kWh

Energytosteam(3398.62-1200.26)*1800*2+(3539.42-3013.81)*1520*2=9,511,950.4MJ/h

9,511,950.4 78.74%Enthalpy of Steam: 3398.62kJ/kg (538°C, 167bar), 1800t/h;1200.26kJ/kg (273.7°C, 190bar); 3013.81kJ/kg (314.4°C,35.8bar);3539.42kJ/kg(538°C,33.2bar)

Energy from flue gasforairpre-heating

2118065.6*0.5=1,059,032.8MJ/h 1,059,032.8 1,059,032.8 50%

Energy from extractsteam for feed waterpre-heating

(3125.1*130+3013.81*150+3325.16*70+3114.49*75+2917.08*60+2753.22*40+600*50-1200.26*575)*2=1,899,373.1MJ/h

1,289,584.5

125,897.6 93.37%

Enthalpy of Steam: 3125.1kJ/kg (378.5°C, 58.6bar), 130t/h;3398.62kJ/kg (314.4°C, 35.8bar), 150t/h; 3325.16kJ/kg (432.5°C,16.1bar), 70t/h; 3114.49kJ/kg (326.7°C, 7.4bar), 125t/h; 2917.08kJ/kg(225°C, 3 bar), 60t/h; 2753.22kJ/kg (139.5°C, 1.3bar), 40t/h;147.538kJ/kg(35°C,10bar)

Energy from extractsteam for airpre-heating

629,487.9*0.8=503,590.3MJ/h 503,590.3

Energy from exhauststeamtorawseawater

(2612.1-146.65) * 1225 *2 =6,040,352.5MJ/h

6,040,352.5 Enthalpy of Steam: 2612.1kJ/kg (60°C, 5.88kPa), 146.65kJ/kg(35°C,5.88kPa)

Energy to desalinationunits

(3104.79-600)MJ/t*50t/h*2=250,479MJ/h 250,479 970 99.4%Enthalpy of Steam: 3114.49kJ/kg (326.7°C, 7.4bar); 3104.79kJ/kg(320°C, 5.5bar); The Loss indicate the transmission loss from powerplanttodesalinationunits.

Energy from steam to generateelectricity

[3398.62*1800+(3539.42-3013.81)*1520-(3125.1*130+398.62*150+3325.16*70+3114.49*125+2917.08*60+2753.22*40)-2612.1*1225]*2=4,686,677.3MJ/h

4,686,677.3 Enthalpy of Steam: 3398.62kJ/kg (538°C, 167bar), 1800t/h;2612.1kJ/kg(60°C,5.88kPa)

Electricitygenerated 600MW*2=4,320,000MJ/h 4,320,000 366,677.3 92.18%

Energy for raw waterpre-heating

(7350+5865)*4.096*(42-25)=920,186.88MJ/h

920,186.9 5,120,165.6 15.23%

(7350+5865)*4.096*(42-0)=2,273,402.9MJ/h 2,273,402.9 3,766,949.6 37.64%

Energy left in freshwaterandsaline

5395*4.096*(32-25)+1955*4.18*(29-25) =187,373MJ/h

187,373 63,106 62.24%Raw SeaWater Supply: 75000t/h; Heat Capacity of SeaWater:

4.096kJ/(kg˙°C)

53

5.LocationsforWTE-DesalinationPlantsDevelopment

Fromtheprevious study, thecogenerationofpowerandwater is anewecologicalandeconomicalsolution.Forpowergenerationplant,itmaintainsthecertainpowergenerationlevelbyadoptionofcondensingsteamturbine,cutsdownthecondensingwater/air consumptionby adopting seawater as condensate, improves the energyefficiency and alleviates the local water scarcity by utilizing exhaust heat fordesalination process. For desalination process, it greatly cuts down the fuelconsumptioncomparedtosimpledesalinationplantsandinturnreducesthecostoffresh water production. According to the study[76], for a coal powered MEDdesalinationprocess, thecapitalcost isaboutUS$0.87percubicmeter freshwaterproduced. The steam cost (mainly coal cost) covered nearly 50%of the total cost.Whenextractsteamorexhauststeamfrompowerplant isadoptedasdesalinationheat sources, the total cost of capital water production reduced to US$0.40 andUS$0.19 respectively. Suchenvironmental andeconomicalbeneficial effectswill befurtherenlargedwithWTEfacilities,thesustainablewastemanagementmethod.

ConsideringthebenefitsandrequirementsofWTE-desalinationcogenerationplant,themostpromisingplacesforthistechniqueshouldbe:

1)Alongtheseaside;2)Limitedfreshwatersupply;3)Limitedaccesstofuelsources;4)LimitedlandsourceforefficientMSWmanagement;5)Relativewarmorhightemperaturewithmildseasonalfluctuation.

In this case, those gulf cities and small islands is the prioritized places for thedevelopment of WTE-desalination cogeneration plants. Feasible options includingareaslike:

� Mediterranean:Cyprus;Crete;� PersianGulf-Kuwait,Bahrain,Oman,Qatar,theUnitedArabEmirates(UAE);� CaribbeanSea:US-VirginIslands,PuertoRico;Mexico-Cancun,Yucatan;� NorthAfrica:Egypt-RedSeaorSinai;� Otherregions:Kenya-Mombasaregion;SouthEastAsia–Malaysia,etc.

In accordance with listed conditions, Cyprus and Union Territory of Lakshadweep(India) are selected to do brief feasibility analysis for WTE-desalination facilitydevelopment.

54

5.1.Cyprus

Figure28MapofCyprus

Cyprus is a former British colony and became independent in 1960, capitaled inNicosia.ItisanislandlocatedinMediterraneanSea,southofTurkey,withanoverallland area of 9,241 km2 and population of 1,189,197 (2015 est.) [59]. Cyprus has aMediterraneanclimatewithhot,drysummers(26-29°Cinaverage)andcoolwinters(10-13°C inaverage).With total renewablewater resourceof0.78km3 (2011), thechronicwatershortageworriesCyprusalot.ThecapitawaterwithdrawsofCyprusis213.5m3perpersonperyear(2009)and180,000m3inall.Thisnumbercontinuouslyincreasedwithboostpopulationandtourists.

TheelectricityconsumptionofCyprus is4.296billionkWh(2012est.),allproducedbylocalpowerplants.90%oftotalinstalledcapacityisfromfossilfuelswhiletheleft10% generated from renewable sources including MSW. No hydro-electric powerplantisimpliedinCyprus.Thoughhas141.6billioncubicmetersprovednaturalgasreserved, the continental fuel consumption of Cyprus is completely provided byimport, mainly oil product and some coal and biomass[80]. The amount of refinedpetroleumproductsimportedis52,480bbl/day(2012est.).ThetotalcarbondioxideemissionfromenergyconsumptioninCyprusreaches8.801milliontonsin2012.

AccordingtoWtERT[81],Cyprusgenerated754kgofMSWperpersonperyear:13%recycled,87%landfilled,0%compostedor incinerated.ThecompositionofMSWinCyprus is shown in Figure 29[ 82 ], 51.6% of which is recyclable and 65.6%biodegradable.Therefore there is still large for the improvementofCyprus’swastemanagementsystemandenergyrecoveryfromwaste.AstudyofEPEM[83]estimatedtheamountofMSWofCyprusthatcanbefurtherutilizedforenergyrecovery(Table21) and analyzed the development possibility of several main energy recoverytechnology. Compared to pyrolysis, gasification and plasma techniques, the WTEplants with waste incineration is the most promising one, lower cost with moremature and developed technology.With new constructed incinerationWTE plant,424,400MWhenergycanberecoveredperyear,surplus339,500MWhannually.

55

Figure29MSWCompositionofCyprus

Table21DistributionofMSWinCyprusCanBeUtilizedforEnergyRecoveryperRegion[83]

IntermsofthedesalinationindustryofCyprus,thegovernmenthasinvestedheavilyinthecreationofwaterdesalinationplantswhichcurrentlyhavesuppliedalmost12%oftotalwaterconsumptionand40-50%ofdomesticwater.Fivedesalinationplantsisunderoperationwhilethe6thonelocatedatVassilikoswasinauguratedrecently[84].TheoperationaldataoffiveexistingplantscanbesummarizedasTable22.Allofthedesalination plants in Cyprus applies the RO technology for its water productionprocessconsideringthe limitedaccess to fuels.Theaveragepowerconsumptionofthese RO desalination plants is 4.50kWh/m3 or three liters of petrol per ton.Therefore,atleast800MWh(6.8%oftotalelectricityconsumption)or3,354litersofpetrol per day (6.5% of total consumption) is consumed for the fresh waterproduction.

27.30%

5.40%

7.70%

11.20%3.20%

38.30%

6.90% Paper

Glass

Metals

Plastics

Textiles

Organics

Other

56

Table22OperationParameterofCyprusDesalinationPlants[85][86]

NameDhekeliaDesalination

PlantLarnacaDesalination

Plant

LimassolDesalinationPlant[87]

PaphosDesalinationPlant

MoniMobilDesalinationPlant[88]

Start ofProduction

April 1997 May 2001 2004 2006 2008

FreshWaterGenerationCapacity(m3/d)

54,000-60,000 55,800-62,000 20,000-40,000

30,000-40,000

18,000-20,000

ServiceArea

Nicosia-Larnaca-Famagusta Water SupplySystem

Nicosia-Larnaca-Famagusta Water SupplySystem

Limassol Paphos Limassol

DesalinationTechnique

RO RO RO RO RO

Recovery ~50% ~50% 45% / 44%SellingPrice(€/m3)

0.82 1.08 / / 1.39

Withboostpopulationandtourists,theCyprusgovernmentisstillplannedtoenlargeitsdesalinationcapacity.SimultaneouslythereducingspaceforwastelandfillingandintensiveenergyaccessibilitycallsforefficientenergyrecoveryfromMSW.Sincetherawseawatertemperaturegreatlymatterstheproductivityofthermaldesalinationprocesses, the relative warm and dry climate condition also potentially ensure astableproductivityof coupled thermaldesalination facilities. From thedata above,WTE-thermaldesalinationcogenerationplantcanpossiblybringCyprus51,000MWhextraelectricityand85million tonsof freshwaterperyearwhichnearlyequals tothe installedROdesalinationcapacity.Besides, thewaterandelectricitygenerationcostwillsignificantlyreducefromthecurrentstatusduetonearlyzerofuelcost.Andthis sustainable way will mitigate the GHG emission of the power and watergenerationprocesses,helpthegovernmenttomeetthecarbonreductionregulationsofEU.

In short, coupled WTE-desalination technology is highly recommended and wellsuitable for thesustainabledevelopmentofCyprusandexactlysolvesseveralmostsevere and troubled environmental issues at the same time. Of course, cities orcountries aroundMediterranean Seawith similar climate andembarrassments cantakethissolutionintotheirconsiderationforthefutureconstruction.

5.2.UTofLakshadweep

TheUnionTerritory(UT)ofLakshadweepisagroupofislandsgovernedbytheUnion

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GovernmentofIndiaintheLaccadiveSea,200to440kmoffthesouthwesterncoastofIndia.Thearchipelagoisformedof12atolls,3reefsand5submergedbanks,withatotalofaboutthirty-nine islandsandislets.Themain islandsareKavaratti,Agatti,Minicoy,andAminiandKavaratti servesas thecapitalof theUT.TheLakshadweepregion enjoys tropical climate, 25-30°C all the year round. The annual rainfall is2,500mm,mainlyinMaytoAugust[89].ThesurfaceareaofthewholeKavarattiislandsis just 32 square kilometerswith population of 64,429 (2011). It has 10 inhabitedislands, 17 uninhabited islands[90]. The population density in the inhabited islands(2,013 persons per square km) is much above the national average of 324 whileAminihasthehighestdensityof2,839persons/km2[91].

Figure30MapofLakshadweep

ThepowersupplyoftheUTismainlythroughdieselpoweredgeneratingsetslocatedin eight inhabited islands. During 2002, the installed capacity reaches 9,837kW,generated 19,856 MWh electricity in all[92]. About 6,600,000 liters of high speeddiesel (HSD) is transported fromBeypore annually toprovideuninterruptedpowersupplyintheislands[93].Asanareaofsunandsea,Lakshadweepambitiouslybuildupits renewableenergy system. So far four solarphotovoltaic (SPV)powerplants areinstalled in four islands, generating 185kW of energy while a 100kW one underinstallation atMinicoy. According to theUnion Territory Administration, 20%of itsdemand(1MW)isaimedtobemetthroughsolarenergy[93].Besides,an80kWwindpowered generator is installed at Kavaratti and Agatti and one 200kW capacitybiomassgasifierplantwasproposedtobeinstalledatKavaratti[94].

The absence of systematic waste management and treatment is one major issuefaced Lakshadweep. The untreated domestic sewage are discharged into the seadirectlywithoutanytreatment.Thenon-degradablesolidwastesaredumpedonthenarrowshore lineatoneendofeach islandby the localbodies[95],or rightbehindeachhouse-hold. It isestimatedthatabout5 tonsofMSWperday isgeneratedat

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Lakshadweep in 2011 (0.382kg per day per capita[96]). In addition, bulks of plasticwastecontainingsand,andbuildingmaterialsarebroughttotheislandfromKerala.Therefore, there is an urgent need for proper solid waste management in thesetourismbasedislands.

Noneof theLakshadweep islandshaveriversorcreaksbutonebetweentwo largelagoonsofKavarattiisabrackishone.Tomakethingsevenworse,itisrevealedthatthescarcefreshwaterresourcesinLakshadweeparethreatenedbysalinityintrusionandsewagecontamination.Intheabsenceofsewagetreatmentplants,about50,000to100,000 litersofsewagewaste is let intoseptic tanksorcesspoolseachday[91].Around 50% of the island's population suffered from stomach, dental and skindiseases due to biological contamination of water. Considering a basic residentialwater consumption levelof55 litersper capitaperday,3,543,595 litersofpotablewater is required every day. However, the natural fresh water resource ofLakshadweepcanmerelyoffer303,550litersperday,lefta3,240,045litersdeficit,letalongtheextrawaterconsumptionoftourismandotherindustries.

A combined system consisting of ground water collecting wells/ponds, rainwaterharvesting(RWH)tanksanddesalinationplantsisadoptedintheseislandstosupplythemunicipalfreshwater.Atotalof436freshwaterpondswererecordedinwholeteninhabitedislands,90inKalpeniIsland,75inAmini,54inAgathiand53inKiltan.Most of these ponds areman-made ecosystemswith a dimension of 5-10 squaremeters and there are two big ponds of about 25 square meters at Amini andKavaratti islands[97]. There are two community rain water harvesting system, oneeachatKavarattiandMinicoy.Inaddition,therearemorethan1,700individualRWHtanks constructed on the roof top. Currently speaking, Lakshadweep have createdinfrastructuresforRWHtoanextentofabout11millionlitersperyear(~30,150L/d).Ten RO desalination units are installed at different islands of Lakshadweep[98],providingapproximately400,000(est.)litersoffreshwaterperday[99].Besides,threedesalination plants, based on the Low Temperature Thermal Desalination (LTTD)technology developed and demonstrated by the National Institute of OceanTechnology(NIOT),havebeensuccessfullybuiltoneeachatKavaratti(2005),Minicoy(2011),andAgatti (2011)[100].ThecapacityofeachLTTDplants is100,000 litersperday.SixmoreprojectsareunderprogressatAndroth,Amini,Kalpeni,Chetlat,KadmatandKalpeni, cost 1.25billion IndianRupees (18.6millionUSdollars) in all[101]. TheLTTD desalination plantwas the sole source of drinkingwater free piped to everyhouseholdonLakshadweepislands.

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Figure31SchematicofLTTDprocessforKaravatti

(WW,Warmwater;CW,Coldwater)

LTTD technology is a thermal desalination method which based on pressure andtemperaturedifferentialbetweensurfaceseawateranddeepseawater.Thewarmersurface sea water is evaporated at low pressures, the obtained vapor is thencondensed by the colder deep sea water (Figure 31). In terms of the energyconsumptionofpumps,theLTTDplantconsumesaround10kWhofelectricalenergypercubicmeteroffreshwaterproduced,higherthanotherdesalinationtechniques.AccordingtothecostestimatesmadebyanindependentagencyforLTTDtechnology,thecostper literofdesalinatedpotablewater isabout0.97Rupee(0.01USdollar)for island based plants[102]. Though the cost is said to be reduced with enlargedtreatmentscale,itstillneedtobefurtherverified.

From the description above, the Lakshadweep administration need to evolve anefficient anddetailedMSWmanagement plan to protect and conserving its fragilecoastalmarineecosystems.Tobeginwith,apilotprojectonsolidwastedisposalandcollection is required. Then the WTE incineration plant or pyrolysis plant can beintroduced to Lakshadweep islands to improve theMSWmanagement and reducefuel import. Since the desalination industry improvement and enlargement is stillnecessary in islands,awasteheatrecoveryfreshwatergeneratorprojectof10,000liters is under installation at Kavaratti Power House to provide portion of potablewater and optimize diesel utilization efficiency[93]. Under such background, thecoupled diesel/waste power plant and desalination plant is a promising andgovernmentfavorablesolutiontoLakshadweep.Consideringthere’snobigindustrialbusiness on the island, the majority of MSW is combustible for the WTE plant.Assume80%ofthesolidwaste(12MJ/kg)iscollectedforcombustion,approximately490,000kWh of electricity and 73,000m3 (202,250 L/d) of fresh water can begeneratedeachyear,tosomeextentreleasetheenergyandwaterintensity.

In short, coupledWTE-thermal desalination technology is recommended and wellsuitableforthesustainabledevelopmentofLakshadweep.Iteconomicallysolvesthemostseverewaterandwasteissuesatthesametime.Italsosavesthespaceforsolidwaste treatment and cut the fuel import. Considering the relative low solidwastegenerationrate,thisfacilityisespeciallysuitedtobeconstructedindensepopulated

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islandlikeKarawattiandAmini.

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6.Conclusion

There are 2150 waste to energy (WTE) plants operational around the world torecovery energy and generate electricity from 250 million tons of municipal solidwaste (MSW)per year. Themost commonWTEplants are electricity-onlybusinessand that use heat to produce steam and drive turbines. To maximize electricityproductivity, the exhaust steam from turbine is sent to an air or water cooledcondenser,thenrecycledtoboilerorinjectedintonaturalwater.ForatypicalWTEplant of 10 t/h capacity, the gross electricity generating capacity is around 22,000MJ/hto23,000MJ/h,about20%oftheenergycontaininginMSWcombusted.Suchprocessmakes very little use of the energy contained in the exhaust steam fromturbineandlossesover50%oftheenergyreleasedfromthecontrolledcombustionofMSW.Inaddition,anestimationof2,800,000m3/hairor4,757,000m3/hwaterisconsumed under this combustion scale, together with 1,109 MJ/h electric powerconsumption to drive the fans. The exhaust condensate air and water also causesomethermalpollutiontosurroundingecosystem.

To improve the energy efficiency ofWTE plants, countries like Europe and SouthKorea uses energy recovered in different ways. Many of their WTE plants arecombined heat and power (CHP) facilities. These facilities sacrifice part of theirelectricityproductivityandselltheextractorexhauststeamfromturbinefordistrictheating (DH) system. Three type of turbine systemmay adopted for CHP plant: 1)backpressuresteamturbine,alloftheexhauststeamfromturbineisusedforDH;2)condensingextractionsteamturbine,partofsteamexhaustfromturbineforDH,theleft exhaust steam is condensed; 3) back pressure extraction turbine, both theextract steamandexhaust steam isused forDH.Under this threesituationsaCHPWTEplantof10t/hcapacityand50%steamextractioncangenerates12,900-18,700MJ/hgrosselectricityandapproximately31,000-62,500MJ/hheat.Although20-40%ofelectricpowerissacrificed,theoverallefficiencyincreasesfrom20%toatleast65%compared to the condensing WTE plants, thus sufficiently using over 70% of theenergy from MSW. According to European environmental policy’s CO2 emissionallowance price and fuel price, 2-15 million dollars can be saved each year, 30thousandtonsofcoal (or5.6milliongallonsofoil)consumptionandmorethan40thousand tonsof carbondioxideemissioncanbepreventedperyear.With furtherimprovementofheatrecoverysystem,thethermalefficiencyforthesefacilitiescanreach 80-90%. However, use the exhaust steam for DH is restricted by the highrequest and cost of DH transmission system construction as well as the suitableclimate condition of the area, especially for those torrid zones. New solutions ofexhaust steam utilization still need further exploration. Currently a new choice isposedbytheworldwidefreshwaterscarcity--usingtheexhauststeamasanenergysourceforthermalwaterdesalination.

Desalination refers to processes that remove salt and other minerals from salinewater to provide freshwater. Almost 16,000 desalination facilities have been builtworldwide,producingover70millioncubicmetersofpotablewaterandaccountingfor two-thirdsof feedwater.Waterdesalinationuseseither thermalormembrane

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technologies. Main technology includes electricity powered reverse osmosis (RO),steamandelectricitypoweredmultiple-effectdistillation(MED)andmulti-stageflashevaporation(MSF).With lowerenergyconsumptionof1-7kWh/m3anddecreasingcostof0.1-1.0USD/m3,membrane(RO)desalinationisapreferablechoiceformostdesalination plants occupied 60% of the installed capacity worldwide. The MSFdesalinationunitshasarelativehightemperaturedemand,100-130°Cforfeedsteamand90-120°Cforoperation,resultinginanhigherenergyconsumptionof13.5-25.5kWh/m3 electrical equivalent and cost of 0.5-1.75 USD/m3. TheMED process canoperatewithlowpressure(0.3-0.5bar),lowtemperature(70-90°C)steambynearlyvacuumpressurecondition.Withlowerelectricconsumptionof1.5-2.5kWh/m3thanMSF,thetypicalproductioncostofMEDisaround0.7-1USD/m3,competitivetoROtechnology.Sinceenergycostaccountsto30-50%oftotaldesalinationcost,findanalternative thermal source lies thekeyof current thermaldesalination techniques’development.

Inspired from CHP plants for DH, two power-desalination cogeneration plants isstudied: St. Barth WTE-desalination cogeneration plant and Hebei Huanghuapower-desalinationcogenerationplant.With1.5t/hMSWincinerationcapacity,theSt. BarthWTE plant sells 67% of recovered energy forMED desalination process,generating1350m3freshwaterperdayfordomesticuse.Theheatconsumptionforpotablewaterproductionis8.6MJ/m3(40.6kWh/m3)andreachedanimpressiveGORof 17.8. Besides, about 8.5% of energy from MSW is transformed to electricity,completelycoveredtheelectricconsumptionofbothWTEandMEDprocess.Inbrief,thisplanthelpsSt.BarthefficientlydealsitsMSWwhilesolvethewatersupplyissueoftheislandwithnegativeenergyinput.

Huanghuapowerplant isa coal-poweredplantwith2,520MWgenerationcapacitylocated along Pohai Gulf of China. Approximately 3,200,000-4,400,000m3 offreshwater is consumed annually as feed-water for the boiler units, aswell as fordesulfurization and other processes. Huanghua solved this massive waterconsumption through several coupled MED desalination units which applied theextractionandexhauststeamfromturbineasheatsource.TheseMEDunitsproduce57,500 m3/d of fresh water, consumed 40% of energy from coal input. Despitemeeting the needs of self-utilization, the plant also produce 5-10million externaltons of freshwater annually (14,000-28,000m3/d) for the Port of Huanghua.Withsuchcombinedsystem,90%of totalenergy input transferred tovaluableproducts,approximately 480-500 ton/d of coal (47,500 USD/d) is saved. The reduced waterproductioncostalsoguaranteethewatersupplyandwaterpriceofsurroundinglocalarea.

From the study, it can be noted that the cogeneration of power and water is anecological and economical solution. When extract steam or exhaust steam fromcoal-fired power plant is adopted as desalination heat sources, the total cost ofcapital water production reduced from 0.87 USD to 0.40 USD and 0.19USDrespectively.SuchadvantageswillbefurtherenlargedwithWTEfacilitiesduetothewaste management profit. Considering the benefits and requirements ofWTE-desalinationcogenerationplant, themostpromisingplaces for this techniqueare those tropical islands or gulf citieswith little access to land, fuels andwaters.

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SuitableareaincludesMediterraneanSeacountries(Cyprus,Crete,etc.),PersianGulfcountries(Kuwait,Bahrain,Oman,etc.),CaribbeanSeaislands,RedSeaareaandsoon. In specification, the situation of Cyprus and Union Territory of Lakshadweep(India) are analyzed and improved the immediacy and necessity of such integratesolution to this area. This study only carried out the energy and resource sidebeneficialanalysis,furthercost-benefitanalysisisneededtoapprovetheeconomicalaccessibilityofthissolution.

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