Thermochemical Conversion Technologies

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, hydrogen, or steam Produces electricity, fules (methane, hydrogen, ethanol, synthetic diesel), and chemical products Temperature > 1300oF More flexible than incineration, more technologically complex than incineration or pyrolysis, more public acceptance

Flexibility of Gasification

Pyrolysis Thermal degradation of carbonaceous materials Lower temperature than gasification (750 – 1500oF) Absence or limited oxygen Products are pyrolitic oils and gas, solid char Distribution of products depends on temperature Pyrolysis oil used for (after appropriate post-treatment): liquid fuels, chemicals, adhesives, and other products. A number of processes directly combust pyrolysis gases, oils, and char

Pyrolyzer—Mitsui R21

Thermoselect (Gasification and Pyrolysis) Recovers a synthesis gas, utilizable glass-like minerals, metals rich in iron and sulfur from municipal solid waste, commercial waste, industrial waste and hazardous waste High temperature gasification of the organic waste constituents and direct fusion of the inorganic components. Water, salt and zinc concentrate are produced as usable raw materials during the process water treatment. No ashes, slag or filter dusts 100,000 tpd plant in Japan operating since 1999

Thermoselect (http://www.thermoselect.com/index.cfm)

Fulcrum Bioenergy MSW to Ethanol Plant Construction on Fulcrum Bioenergy municipal solid waste to ethanol plant, Sierra BioFuels, is set to begin in the 4th quarter of 2008. Located in the Tahoe-Reno Industrial Center, in the City of McCarran, Storey County, Nevada, the plant will convert 90,000 tons of MSW into 10.5 million gallons of ethanol per year.

Plasma Arc Heating Technique using electrical arc Used for combustion, pyrolysis, gasification, metals processing Originally developed by SKF Steel in Sweden for reducing gas foriron manufacturing Plasma direct melting reactor developed by Westinghouse Plasma Corp. Further developed for treating hazardous feedstocks (Contaminated soils, Low-level radioactive waste, Medical waste) Temperatures (> 1400oC) sufficient to slag ash Plasma power consumption 200-400 kWh/ton Commercial scale facilities for treating MSW in Japan

Plasma Arc Technology in Florida Green Power Systems is proposing to build and operate a plasma arc facility to process 1,000 tons per day of municipal solid waste (garbage) in Tallahassee, Florida. Geoplasma is proposing to build a similar facility for up to 3,000 tons of solid waste per day in St. Lucie County, claims 120 MW will be produced Health risks, economics, and technical issues still remain

Process Heated using direct current arc plasma for high T organic waste destruction and gasification and Alternating current powered, resistance hearing to maintain more even T distribution in molten bath

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

Waste Incineration - Disadvantages High capital cost Skilled operators are required (particularly for boiler operations) Some materials are noncombustible Some material require supplemental fuel

Waste Incineration - Disadvantages Air contaminant potential (MACT standards have substantially reduced dioxin, WTE 19% of Hg emissions in 1995 – 90% reduction since then) Volume of gas from incineration is 10 x as great as other thermochemical conversion processes, greater cost for gas cleanup/pollution control Public disapproval Risk imposed rather than voluntary Incineration will decrease property value (perceived not necessarily true) Distrust of government/industry ability to regulate

Carbon and Energy Considerations Tonne of waste creates 3.5 MW of energy during incineration (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) Unlike biochemical conversion processes, nonbiogenic CO2 is generated Should not displace recycling

WTE Process

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

Conclusions Combustion remains predominant thermal technology for MSW conversion with realized improvements in emissions Gasification and pyrolysis systems now in commercial scale operation but industry still emerging Improved environmental data needed on operating systems Comprehensive environmental or life cycle assessments should be completed

Return to Home page Updated August 2008