Municipal Solid Waste Incineration

Combustion Types

Incineration (energy recovery through complete oxidation)– Mass Burn– Refuse Derived Fuel

PyrolysisGasificationPlasma arc (advanced thermal

conversion)

Gasification

Partial oxidation process using air, pure oxygen, oxygen enriched air, or steam

Carbon converted into syngasMore flexible than incineration More public acceptance

Flexibility of Gasification

Pyrolysis

Thermal degradation of carbonaceous materials

Lower temperature than gasificationAbsence or limited oxygenProducts are gas, liquid, solid charDistribution of products depends on

temperature

Waste Incineration - Advantages• Volume and weight reduced (approx. 90% vol. and

75% wt reduction)• Waste reduction is immediate, no long term

residency required• Destruction in seconds where LF requires 100s of

years• Incineration can be done at generation site • Air discharges can be controlled • Ash residue is usually non-putrescible, sterile,

inert• Small disposal area required• Cost can be offset by heat recovery/ sale of energy

Environmental ConsiderationsTonne of waste creates 3.5 MW of

energy (eq. to 300 kg of fuel oil) powers 70 homes

Biogenic portion of waste is considered CO2 neutral (tree uses more CO2 during its lifecycle than released during combustion)

Should not displace recycling

Waste Incineration - Disadvantages• High capital cost• Skilled operators are required (particularly for

boiler operations)• Some materials are noncombustible • Some material require supplemental fuel• Public disapproval

Risk imposed rather than voluntary Incineration will decrease property value (perceived

not necessarily true) Distrust of government/industry ability to regulate

Three Ts

TimeTemperatureTurbulence

System Components

Refuse receipt/storageRefuse feedingGrate systemAir supplyFurnaceBoiler

Energy/Mass Balance

Waste Flue Gas

Energy Loss (Radiation)

Mass Loss (unburnedC in Ash)

Flue Gas Pollutants

ParticulatesAcid GasesNOx

COOrganic Hazardous Air PollutantsMetal Hazardous Air Pollutants

Particulates

Solid Condensable Causes

– Too low of a comb T (incomplete comb) – Insufficient oxygen or overabundant EA (too high T) – Insufficient mixing or residence time – Too much turbulence, entrainment of particulates

Control– Cyclones - not effective for removal of small

particulates – Electrostatic precipitator  – Fabric Filters (baghouses) 

Metals

Removed with particulates Mercury remains volatilized Tough to remove from flue gas Remove source or use activated

carbon (along with dioxins)

Acid Gases

From Cl, S, N, Fl in refuse (in plastics, textiles, rubber, yd waste, paper)

Uncontrolled incineration - 18-20% HCl with pH 2

Acid gas scrubber (SO2, HCl, HFl) usually ahead of ESP or baghouse – Wet scrubber – Spray dryer – Dry scrubber injectors

Nitrogen removal

Source removal to avoid fuel NOx production

T < 1500 F to avoid thermal NOx

Denox sytems - selective catalytic reaction via injection of ammonia

Air Pollution Control

Remove certain waste componentsGood Combustion PracticesEmission Control Devices

Devices

Electrostatic PrecipitatorBaghousesAcid Gas Scrubbers

– Wet scrubber– Dry scrubber– Chemicals added in slurry to neutralize

acids

Activated CarbonSelective Non-catalytic Reduction

Role of Excess Air – Control Three Ts

Amount of Air Added

Insufficient O2

Stoichiometric

Excess Air

T

Role of Excess Air – Cont’d

Insufficient O2

Stoichiometric

Excess Air

Increasing Moisture

Amount of Air Added

Role of Excess Air – Cont’d

Insufficient O2

Stoichiometric

Excess Air

PICs/Particulates

NOxT

Optimum T Range

(1500 – 1800 oF)

Amount of Air Added

Ash

Bottom Ash – recovered from combustion chamber

Heat Recovery Ash – collected in the heat recovery system (boiler, economizer, superheater)

Fly Ash – Particulate matter removed prior to sorbents

Air Pollution Control Residues – usually combined with fly ash

Combined Ash – most US facilities combine all ashes

Schematic Presentation of Bottom Ash Treatment

Ash Reuse Options

Construction fillRoad constructionLandfill daily cover Cement block productionTreatment of acid mine drainage

StackRefuse Boiler

TippingFloor

Fabric FilterSpray Dryer

Metal Recovery

Ash Conveyer

Mass Burn Facility – Pinellas County

Overhead Crane

Turbine Generator

Fabric Filter

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Updated August 2005