The process
The operation of the plant consists of five main elements:
The Process (Click to enlarge in a new tab/window)
Waste reception
Pre-treated waste with recyclable materials removed will be delivered to the reception hall within the building. This area will be operated under negative air pressure ensuring that air is drawn into the building preventing the escape of dust and odours. Air drawn in and through the hall will be used in the combustion process.
Waste will be transferred into a combustion chamber via a feed chute and airlock section.
Combustion
Combustion takes place in two stages. Primary combustion takes place on a moving mechanical grate to mix burning and unburnt waste to ensure efficient and complete combustion. This proven technology is already successfully used in many operational energy recovery plants across the world.
The combustion gasses from the primary stage are then heated further in a secondary chamber where they reach a minimum temperature of 850 degrees Centigrade for at least two seconds. This complies with the requirements of the Waste Incineration Directive and ensures the destruction of harmful particulates.
The burnt waste from primary combustion is called 'bottom ash'. This non-hazardous material amounts to approximately 25% of the input waste and will be recycled for use as an aggregate in the construction industry. Ferrous metals recovered from the bottom ash will also be recycled.
Energy generation
Heat produced from the combustion process will be fed into heat recovery boilers where it will form high-pressure steam. This will be used to drive a turbine to generate electricity. A proportion of this would be used to power the facility itself but the majority (around 21MW) would be exported to the National Grid.
Flue gas treatment
The emissions from the plant will be strictly regulated by the Environment Agency to ensure that they meet the requirements not only of local air quality standards but also of the European Waste Incineration Directive (WID). Energy recovery technology has improved significantly over recent years and modern facilities not only meet these exacting standards but consistently achieve much lower emission limits.
Compliance with these standards will be achieved through a combination of rigorous combustion control and the provision of an air pollution control system that will treat all flue gas prior to emission. In combination, these measures will minimise Nitrogen Oxide (NOx) emissions, neutralise acidic gasses and facilitate the removal of dust, metal compounds and dioxins. Following treatment, the vast majority of the emissions from the chimney will be made up of carbon dioxide and water.
Residue handling
Three types of residue will be collected in the flue gas treatment process:
- The first is 'bottom ash' which is burnt waste from the primary combustion process. This non-hazardous material will be collected and recycled for use as an aggregate in the construction industry. Ferrous metals recovered from the bottom ash will also be recycled.
- The second residue is 'fly ash', which is ash that has literally 'flown' through the combustion chamber and has subsequently been filtered out. This will be taken off site for disposal.
- The third is called 'APC residue' or 'spent lime'. This is collected separately to the fly ash in bag filters and will be suitable for further use in the chemical treatment industry.
The combined quantities of these treatment residues would be in the region of 6% to 10% of the annual waste throughput (by weight).
It would take a modern day incinerator 120 years to emit as much dioxin as the millennium firework display in London.
The dioxins emitted from bonfires on November 5th are greater than the total annual dioxin emissions for all EfW plants in the UK.
Source: SEPA (Scottish Environmental Protection Agency)