Thermochemical Conversion Technologies

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, hydrogen, or steam

Produces electricity, fules (methane, hydrogen, ethanol, synthetic diesel), and chemical products

Temperature > 1300oFMore 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

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 FloridaGreen 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 - DisadvantagesHigh capital costSkilled 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 disapprovalRisk imposed rather than voluntaryIncineration 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

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

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

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Updated August 2008