Municipal Solid Waste Incineration

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

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

Flexibility of Gasification

Pyrolysis Thermal degradation of carbonaceous materials Lower temperature than gasification Absence or limited oxygen Products are gas, liquid, solid char 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 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 Considerations Tonne 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 Time Temperature Turbulence

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

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

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

Particulates Solid Condensable Causes Control 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 components Good Combustion Practices Emission Control Devices

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

Role of Excess Air – Control Three Ts Insufficient O2 Stoichiometric Excess Air T Amount of Air Added

Role of Excess Air – Cont’d Insufficient O2 Stoichiometric Excess Air Increasing Moisture Amount of Air Added

Role of Excess Air – Cont’d Stoichiometric NOx T Optimum T Range (1500 – 1800 oF) PICs/Particulates Insufficient O2 Excess Air 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 fill Road construction Landfill daily cover Cement block production Treatment of acid mine drainage

Refuse Boiler Fabric Filter Stack Spray Dryer Tipping Floor Ash Conveyer Metal Recovery Mass Burn Facility – Pinellas County

Overhead Crane

Turbine Generator

Fabric Filter

Return to Home page Updated August 2005