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Environmental BenefitsOf Thermochemical Conversion

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content Definition Types

(1) Biochemical

(2) Physiochemical

(3) Thermochemical Environmental and Other Benefits Carbon and Energy Considerations

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What are conversion technologies?

Technologies used to convert solid waste into useful products, chemicals and fuels are referred as conversion technology.

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Conversion technology

These technologies can be defined by three types of conversion processes:

1) Biochemical

2) Physiochemical

3) Thermochemical

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Biochemical conversion

It include …

Anaerobic digestion (occurs in controlled reactors or digesters and also in a less controlled environment in landfills)

Anaerobic fermentation (for example, the conversion of sugars from cellulose to ethanol)

Biochemical conversion proceeds at lower temperatures and lower reaction rates than other conversion processes.

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Physiochemical conversion

It include …

Transesterification (biodiesel production)Physical and chemical synthesis of products from

feedstock

It is primarily associated with the transformation of fresh or used vegetable oils, animal fats, greases, tallow and other suitable feedstocks into liquid fuels or biodiesel.

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Thermochemical conversion

It include…

Incineration

Gasification

Pyrolysis

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Gasification

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

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

Temperature > 700oCMore flexible than incineration, more technologically

complex than incineration or pyrolysis, more public acceptance

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PyrolysisThermal degradation of carbonaceous materials.

Lower temperature than gasification (400 – 700oC)Absence or limited oxygenProducts are pyrolitic oils and gas, solid charDistribution of products depends on temperaturePyrolysis 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

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Waste Incineration

Energy recovery through complete oxidation

• 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

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Thermochemical conversion

It is characterized by higher temperatures and faster conversion rates.

Best suited for lower moisture feedstock.Thermochemical routes can convert the entire

organic (carbon) portion of suitable feedstocks. Inorganic fraction (ash) of a feedstock does not

contribute to the energy products but may increased nutrient loading in wastewater treatment and disposal facilities.

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Environmental and Other Benefits

Products and benefits from conversion technologies will differ based on the technology used and the feedstock converted.

The following discusses products and benefits derived by type of conversion process.

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Environmental and Other Benefits

Thermochemical conversion processes such as gasification, pyrolysis and incineration can remove materials from the solid waste stream and can also create:

1) liquid fuels such as biodiesel, ethanol and oil

2) electricity, heat and steam from combustible gases such as methane

3) chemicals and consumer products from oils and syngas

4) activated carbon for the food processing industry

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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.