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Technical Sheet: COMBUSTIONCombustion of waste involves the total conversion of organic solids to oxidised end products, primarily carbon dioxide, water and ash. Combustion can be purely for waste reduction, as with incineration, but an added benefit is the recovery of energy in the form of heat and steam. In the case of municipal or medical wastes, efficient combustion destroys pathogens and toxic compounds that may have been difficult to deal with otherwise.

Pulp mills in NZ have existing energy boilers of various types and ages, which take a mixture of wood waste and other fuels. In some cases sludge is fed to the boiler but whether this is a fuel source or a disposal route depends on the water content. Other fuel sources such as plastics and waste oil may exist on site but use of these depends on consent conditions and ease of integration with existing fuel feed mechanisms.

Discharges (emissions to air and ash) from the boilers depend on fuel inputs and combustion control. Monitoring for NOx, SOx, VOCs and particulates may be required and regular maintenance to reduce the build-up of corrosive or scaling materials will be necessary.

Materials Accepted • Greenwastes(Fuelvalue)

• Woodwaste(Fuelvalue)

• Sludge(fuelvalueordestructionoptiondependingonwatercontent)

• Organicwastewithhighcalorificvalue(eg:wasteoil)

Forsludgecombustionthereisaneedtoreducewatercontent(variousphysicaldewateringoptions)andhaveappropriatefeedsystemsinplace.Sludgefromdifferentprocessstageshavevaryingproperties,thereispotentialtoblendtoimprovehandlingordewateringcharacteristics.

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Examples in Current Use NZ

SawmillssuchasRedStagPulpmillssuchasKinleith;Tasman;WPI&PanPac

SilverFernFarms(fluidisedbedboiler–dewateredmeatprocessingsludgeandwoodwaste)

Overseas

Multipleexamplesofpulpmillsandsawmillsrecoveringenergyfromwaste.

Infrastructure and Space Requirements

Mayrequireseparateplantwithsomeancillaryequipment.Including,materialpre-processing(sizereduction)andfeeding,heatrecovery(superheaters,economisers),watertreatment(demineralisedwater),powergeneration(steamturbine,generators),airpollutioncontrol(cyclones,wetscrubbers,ashdewatering),bottomashdisposalandthecontrolsystem.Thisplantrequiressignificantspace.

Capital Cost Veryhighcapitalinvestmentforestablishmentorretrofitting.HoweverawoodwastefiredboilerplantisacommonandstandardcomponentofapulpmillinNZ.

Operating Cost Significantcostsforoperationandmaintenanceofplant.Ifanauxiliaryfuelisused(e.g.coal)orthewoodwasteisdamp,thisisanaddedcost.Verywetwastematerialsuchaspoorlydewateredsludgewouldleadtolowerheatproductionandincompletecombustion.Theoperationcostsarebalancedbytheneedforpowerandsteamgenerationtoruntheprocessingplant.

End Product Thermalandelectrical(forCHPplant)energy

Ash-quantityandcompositionwilldependonmaterialscombustedandthecombustionprocess.Thedefaultsolutionforashfromwood/biomasscombustioninNewZealandisdisposaltolandfill(normallyadedicatedsite).Anumberofalternativesarenotedbelow.

Ashforconstruction

• Flyashcanbeusedinsomebrickandcementapplications.Woodashhasexcellentbondingagenttouseinconcreteandcement.

• Asstructuralfill,foundationsupport,pavementbaseandconduitbedding

Ashforagriculture/forestry

• Woodashappliedtolandcanincreasegrowthandyieldofagriculturalcrops,increasepH,andincreasepotassiumandphosphorusavailability(dependentonashsource/characteristics)

• Asacompostfeedstock

• Potentialforusewithfertiliser

Ashforstabilizationoradsorbency

• Woodashneutralizestheacidicwastematerialfromminingoperationstopreventleachingofcontaminantsandtobindcontaminantswithinthewaste

• Soilstabilization-canbeusedaspottingorlimingagent

• WhenmixedwithWWTPresiduals,ashcanprovidedailycoverforlandfills

• Flyashwithunburnedcarboninrangesof27-32%,hasbeenfoundtobeaneffectiveadsorbentofcertainodoursandcolour.

Note:Acid-rinsedashcanbemoreuseableasaproductastheheavymetalcontentcanbereduced.

Operating capacity e.g. viable at low tonnage vs. high tonnage

Viableathightonnage

ThisPulp&PaperSolidWasteTechnologyAssessmentwassupportedbytheMinistryfortheEnvironment’sWasteMinimisationFund.

CONTACT

KimMcGrouther073435772kim.mcgrouther@scionresearch.com

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Potential consenting issues • Needconsentforfluegasandashdisposal

• Highpressureandhightemperatureoperations–safetyclearancerequired

• Onsitepowergeneration

Technology Risk Aqualitativeassessmentofthelikelihoodoffailureoftheoptionorscenarioduetoissuesrelatedtothetechnologicalsolutione.g.equipmentfailure,unabletoachieveoutputstandards

3=lowrisk,technologywellprovencommerciallyinNewZealand

Commercial Risk Noofsuppliers

Rangeofinputmaterials

Aqualitativeassessmentofthelikelihoodoffailureoftheoptionorscenarioduetoissuesrelatedtothecommercialarrangementse.g.supplierunabletomaintainoperations,increaseincostofprocess,transportoron-goingsitemanagementexceedthoseabletobereasonablerecoveredorthoseforcomparableoptions.

3=lowriskbutreturnsaffectedbymoisturecontentoffuel,supplierwellprovencommerciallyinNewZealand

Market Risk Aqualitativeassessmentofthelikelihoodoffailureoftheoptionorscenarioduetoissuesrelatedtothe‘market’fortheproducte.g.ausefortheproductcannotbefoundduetoconcernsabouttracecontaminants.

Forpower:

3=lowrisk,multiplesecuremarketsforproduct

Fordiversionofashfromlandfill:

1=highrisk,potentialforproducttohavenomarket