Municipal Solid Waste Incineration Combustion Types Incineration energy

Municipal Solid Waste Incineration

Combustion Types Ø Incineration (energy recovery through complete oxidation) – Mass Burn – Refuse Derived Fuel Ø Pyrolysis Ø Gasification Ø Plasma arc (advanced thermal conversion)

Gasification Ø Partial oxidation process using air, pure oxygen, oxygen enriched air, or steam Ø Carbon converted into syngas Ø More flexible than incineration Ø More public acceptance

Flexibility of Gasification

Pyrolysis Ø Thermal degradation of carbonaceous materials Ø Lower temperature than gasification Ø Absence or limited oxygen Ø Products are gas, liquid, solid char Ø Distribution 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 100 s 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 Considerations Ø Tonne of waste creates 3. 5 MW of energy (eq. to 300 kg of fuel oil) powers 70 homes Ø Biogenic portion of waste is considered CO 2 neutral (tree uses more CO 2 during its lifecycle than released during combustion) Ø 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 Ø Risk imposed rather than voluntary Ø Incineration will decrease property value (perceived not necessarily true) Ø Distrust of government/industry ability to regulate

Three Ts Ø Time Ø Temperature Ø Turbulence

System Components Ø Refuse receipt/storage Ø Refuse feeding Ø Grate system Ø Air supply Ø Furnace Ø Boiler

Energy/Mass Balance Energy Loss (Radiation) Waste Flue Gas Mass Loss (unburned C in Ash)

Flue Gas Pollutants Ø Particulates Ø Acid Gases Ø NOx Ø CO Ø Organic Hazardous Air Pollutants Ø Metal Hazardous Air Pollutants

Particulates Ø Solid Ø Condensable Ø 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 Ø Control – Cyclones - not effective for removal of small particulates – Electrostatic precipitator – Fabric Filters (baghouses)

Metals Ø Removed with particulates Ø Mercury remains volatilized Ø Tough to remove from flue gas Ø Remove source or use activated carbon (along with dioxins)

Acid Gases Ø From Cl, S, N, Fl in refuse (in plastics, textiles, rubber, yd waste, paper) Ø Uncontrolled incineration - 18 -20% HCl with p. H 2 Ø Acid gas scrubber (SO 2, HCl, HFl) usually ahead of ESP or baghouse – Wet scrubber – Spray dryer – Dry scrubber injectors

Nitrogen removal Ø Source removal to avoid fuel NOx production Ø T < 1500 F to avoid thermal NOx Ø Denox sytems - selective catalytic reaction via injection of ammonia

Air Pollution Control Ø Remove certain waste components Ø Good Combustion Practices Ø Emission Control Devices

Devices Ø Electrostatic Precipitator Ø Baghouses Ø Acid Gas Scrubbers – Wet scrubber – Dry scrubber – Chemicals added in slurry to neutralize acids Ø Activated Carbon Ø Selective Non-catalytic Reduction

Role of Excess Air – Control Three Ts Stoichiometric T Insufficient O 2 Excess Air Amount of Air Added

Role of Excess Air – Cont’d Stoichiometric Increasing Moisture Insufficient O 2 Excess Air Amount of Air Added

Role of Excess Air – Cont’d Stoichiometric NOx T Optimum T Range (1500 – 1800 o. F) PICs/Particulates Insufficient O 2 Excess Air Amount of Air Added

Ash Ø Bottom Ash – recovered from combustion chamber Ø Heat Recovery Ash – collected in the heat recovery system (boiler, economizer, superheater) Ø Fly Ash – Particulate matter removed prior to sorbents Ø Air Pollution Control Residues – usually combined with fly ash Ø Combined Ash – most US facilities combine all ashes