Thermal Methods Flashcards
What is thermal treatment of Waste?
Combustion of MSW under controlled conditions, recovering renewable energy in the form of electricity and/or heat.
What are the main methods of Thermal treatment of waste?
Incineration, Gasification & Pyrolysis.
What is Thermal Degradation?
The molecular deterioration of the materials because of over heating.
It occurs at a temperature at which some components of the material separate or react with one another or with the atmosphere to modify the macro or microstructure.
What is the difference between Incineration, Gasification and Pyrolysis?
1) Incineration - combustion in excess air
2) Gasification - Partial Oxidation
3) Pyrolysis - Combustion in the absence of air
What is the objective of thermal technology?
1) To reduce volume and Weight
2) Recover conversion products and energy
Why is Pollution Control Necessary?
Because of:
1) Dioxins and furans
2) Heavy Metals
3) Toxic and corrosive gases
4) Polycyclic aromatic hydrocarbons (PAHs)
5) Wastewater / fly ash / bottom ash
Why is the Calorific value Important?
1) The higher the calorific value the more energy is produced
2) There is a minimum value plants use - 9MJ/kg
Briefly explain the Incineration Process?
- Converts all C and H to C02 and H20
- Two stage process
1) Organic substances turned into gases ( <30 mins)
2) Gas and particles burnt ( 2-3 seconds)
Advantages of Incineration (+)
1) Can occur near point of collection
2) Result is a biologically sterile ash product
3) Large volume reduction
4) Produces no methane
5) Sources of energy
6) Reusable bottom ash residues
7) Best Practical Environmental Option for certain hazardous wastes
Disadvantages of incineration (-)
1) Higher costs and high capital investment compared to landfill.
2) Perception of hungry Burner - to stay efficient - restricting other waste options
3) Lack flexibility
4) Specific Calorific Value design of system
5) Perception that emissions adversely affect health
6) Production of residue
What does the application Incineration Require?
1) Well functioning waste management system
2) stable supply - 100,000 tonnes/year
3) Lowest CV at least &MJ/kg
4) Long contracts in place
5) Planning and public opposition resolved.
Controlling factors of combustion
1) Sufficient Temperature
2) Sufficient Residence Time at temperature
3) Turbulence
Plus an excess of oxygen
What is a fluidised bed incinerator?
A type of furnace where the fuel particles are combusted while suspended in a stream of hot gas.
Bed consists of sand particles
Gases and bed continuously mix to ensure complete combustion
Can be used to incinerate liquids, sludges, solids or gases.
What are the Advantages of a Fluidised bed (+)?
1) Perception of being more environmentally responsible
2) High combustion efficiency
3) Residence time calculations are reliable
4) good emissions control
5) Less ash
6) Relatively low Maintenance - no moving parts
7) Better combustion conditions - sand provides continuos attrition of burning material and exposes fresh material for combustion
Disadvantages of a fluidised bed (-) ?
1) requires treatment of waste
2) Lower throughputs - 35% less
3) Limited commercial use UK
4) High capital costs
5) dioxin emissions
6) Minimum tonnage - hungry
7) ash produced - but less
8) negative perception of incinerators in general
What is Gasification?
1) between pyrolysis and combustion (partial oxidation)
2) doesnt burn, breaks down molecules apart with heat and a small amount of oxygen
3) recombining molecules to produce syngas
4) Efficiency <87%
5) uses processed waste or RDF as fuel
What is syngas?
A fuel produced from gasification
fuel, chemicals, fertiliser, to generate electricity & for consumer products.
Advantages of Gasification (+)
1) syngas can be used for a variety of purposes - can be purified or
2) Production of hydrogen
3) Has a wider range of operating conditions compared with pyrolysis.
Disadvantages of Gasification (-)
1) Increased capital and operational costs compared to incineration
2) Increased technical experience required to maintain facility compared to incineration.
What is Refuse Derived Fuel?
- High calorific fraction of processed MSW
- Substitute material fuel for thermal technology, particularly gasification and pyrolysis
- produced by shredding and dehydrating solid waste
What are the main steps in any Thermal Treatment of Waste Process?
1) The Tipping Floor
- Household waste is tipped into a concrete bunker
- Overhead cranes load the waste fuel into the bunkers
2) The Boiler
- In under 2 hours the waste is reduced to 10% of its original volume
- only recyclable metals are separated from the remaining inert ash
3) Combustion Chamber
- the water in steel tubes is heated and turned into steam
4) The Turbine Generator
- powered by the steam to generate clean renewable electricity, which is sent to the power grid
5) The Scrubbers
- utilises reagent addition for removal of acid gases
6) The Baghouse
- Sophisticated fabric bag filters capture any remaining particles within the flue exhaust
- The throughly cleaned gas stream is continuously monitored for strict environmental compliance.
What is CHP?
Combined Heat & Power (CHP) is the use of a heat engine or a power station simultaneously to generate both electricity and useful heat.
What is the difference between incineration and Advanced Thermal Treatment (ATT)?
ATT treats waste to produce secondary products whereas incineration directly combusts waste.
What is Incineration Bottom Ash (IBA) ?
- A form of ash produced in incineration facilities
- discharged from moving grate
- can be used as an aggregate once contaminants have been removed
What are ROCs?
- Policies to support large scale renewable energy in UK
- Legal obligation on electrical suppliers to purchase a steadily rising proportion of renewable energy.
Where does Energy From Waste (EFW) fit into the waste hierarchy?
- Energy recovery is above Landfill
- Reduction and Recycling are higher
What is a gate fee?
A charge for disposing of waste (per tonne) by waste plants.
What is the difference between Proximate and Ultimate analysis?
Proximate aims to determine:
- Moisture
- Volatile matter
- Ash
- Fixed Carbon
Ultimate aims to analyse:
- Carbon
- Hydrogen
- Oxygen
- Sulphur
- Ash
