Choosing waste to energy technologies
There are three main waste to energy processes: incineration, anaerobic digestion and gasification and pyrolysis.
Incineration involves burning waste at high temperatures and generating and exporting energy. The volume of waste is reduced by around 90 per cent, significantly reducing the need for landfill.
Gasification and pyrolysis
These are both types of thermal treatment and their use for the treatment of waste is a relatively recent development. The organic portion of waste is heated either in the complete absence of oxygen or with limited oxygen to produce a chemical reaction. The gas produced (known as syngas) can be used as a fuel to manufacture steam or electricity. Pyrolysis involves a less complex reaction and takes place at a lower temperature range than gasification. Other thermal treatment technologies do exist but are less developed and their take up is very limited in the UK.
Anaerobic digestion is a process that breaks down organic matter into simpler materials. The main product of the digestion process is a methane rich bio-gas which is suitable as gas engine feedstock with subsequent energy recovery. Alternatively, the bio-gas can be further refined as a vehicle fuel. There are approximately 12 anaerobic digestion plants operating in Europe, using municipal solid waste (MSW) as their primary organics feedstock.
This technology has significant carbon and energy benefits over other options for managing food waste.
A good reference example is the plant installed by Valorga at La Coruna in Northern Spain, which has a design capacity of 140,000 tonnes per annum. In addition, the Western Isles Council in Scotland has specified anaerobic digestion for the treatment of its organic waste from MSW, and has one plant now operating at Comhaile nan Eilean Siar.
Regional or city-wide waste strategies and energy strategies can explore the appropriateness and role of waste to energy technologies in a given region or city. They can identify the potential technologies and broad locations of waste to energy facilities. The benefits will be maximised where facilities produce both energy and heat.
At the site scale, the development of incineration, gasification and pyrolysis involve similar characteristics. Sites are typically between 2 and 5 hectares and require a large industrial building to house operations. These plants have capacity for between 50,000 and 250,000 tonnes of waste per annum. There is no definitive guidance on how far such developments should be located from sensitive receptors. To maximise opportunities for effective combined heat and power or district heating schemes, however, waste to energy plants should be located as close as possible to residential or commercial areas.
The development of anaerobic digestion plants generally requires sites between 1 and 1.5 hectares. Typically the plants have capacity to manage 20,000 to 50,000 tonnes of waste per annum. Guidance suggests these facilities should generally not be developed within 250m of sensitive land uses.
Examples of energy-from-waste plants
Tyseley energy from waste plant
This plant was built in 1996 to replace an existing 1970s mass burn incinerator and to allow the council to reduce its reliance on landfill. It burned 313,000 tonnes of rubbish in 2006/07, producing enough electricity to power 40,000 local homes.
The plant runs 24 hours a day, 365 days of the week and is well within UK and EU standards for emissions; all that is seen coming from the chimney stack is steam.
Tyseley recovers several thousand tonnes of metals each year after the incineration process. The bottom ash, which is what remains in the furnace, is used in road building programmes.
CABE and Urban Practitioners
with the cities of Birmingham, Bristol, Leeds, Liverpool, Manchester, Newcastle, Nottingham and Sheffield