Key elements of a modern Energy-from-Waste plant
Energy from Waste (EfW) is a highly developed technology which allows the transformation of every day household waste into a renewable source of electricity.
Waste is collected from the households and businesses within the local area and delivered to the plant where it is temporarily stored in an enclosed bunker. The bunker area is maintained under a slight negative pressure to ensure that any odour is drawn into the process and does not breakout from the facility. The waste fuel is transferred by crane and loaded into the main thermal combustion chamber. The thermal combustion chamber consists of a moving grate. The action of the grate slowly tumbles the waste down the grate ensuring complete burn-out of the material. A controlled flow of air, extracted from the reception hall and bunker, is injected under the grate to ensure optimal combustion conditions are maintained and flue gas production is minimised. This air flow also assists the process by cooling the grate to maximise the life of the facility - EfW facilities will typically have a design life in excess of 30 years.
The solid ash produced is passed through a water quench which provides a seal between the atmosphere and the combustion chamber. Following the extraction of metals, the remaining bottom ash is stored in a separate ash bunker until it can be taken away and re-used by aggregate recyclers or in road construction. In this way, EfW facilities will typically divert over 90% of the waste received away from landfill.
The hot gases produced pass upwards through the combustion chamber into the Boiler chamber. The gas is passed over multiple banks of tubes containing water. The heat is transferred into the water, converting it into steam. The steam is then heated through one final stage of energy recovery which creates high pressure superheated steam.
The high pressure steam is then transferred to a steam turbine and generator set, where the steam is transformed back into water, converting the energy into electricity. A small proportion of this electricity (normally less than 12%) is then used internally to power all the EfW plant equipment and the remaining 88% is exported to the local electricity grid. The condensed steam is then returned to the boiler to repeat the heat transfer process and to maximise the energy efficiency of this thermodynamic cycle.
The cooled flue gases are discharged into Flue Gas Treatment equipment where they are treated to ensure they are safe for discharge into the atmosphere. Lime and activated carbon are injected into the gas to remove acid gases, heavy metals and dioxins and meet the strict emission limits imposed by the Waste Incineration Directive. Any residues from the Flue Gas Treatment Process are then collected and disposed of in a specialised facility. The gas is drawn though an induced draught fan and then passed up a stack which has continuous online measurement of the gas composition. This online measurement demonstrates that all gas released to atmosphere comply with all necessary legislative requirements.
If suitable outlets can be secured, the EfW facility can also be used to provide a source of heat for local businesses and houses in a district heating scheme. The addition of district heating maximises the efficiency of the energy recovered from the waste fuel.
The overall design of EfW facilities has changed significantly over the years and modern facilities ensure that they are a safe, environmentally friendly way of tackling climate change and reusing waste to generate sustainable electricity.
Below is a schematic of the overall process (click to view as a PDF)
