The process

 The operation of the plant consists of five main elements:

Westmill EfW Process Diagram

The Process (Click to enlarge in a new tab/window)

Waste reception

The residual waste will be delivered to the reception hall within the building and tipped into the bunker. This reception area will be operated under negative air pressure ensuring that air is drawn into the building preventing the escape of dust and odours. Air 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 takes place in two stages. Primary combustion takes place on a moving mechanical grate, to help the mixing of 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°C for at least two seconds. This complies with the requirements of the European Waste Incineration Directive (WID).

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 34 megawatts) would be exported to the National Grid.

Emission treatment

The emissions from the plant will be strictly regulated by the Environment Agency to ensure that they meet local air quality standards and the requirements of the WID.

Energy recovery technology has improved significantly over recent years and modern facilities not only meet these exacting standards but also consistently achieve much lower emission limits.

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. These measures will minimise Nitrogen Oxide (NOx) emissions, neutralise acidic gasses and facilitate the removal of dust, metal compounds and dioxins. The vast majority of the emissions from the flue stacks will be made up of carbon dioxide and water.

According to the Scottish Environmental Protection Agency, "It would take a modern day incinerator 120 years to emit as much dioxin as the millennium firework display in London"

Information issued on September 3rd this year by the Health Protection Agency states that, "Incinerators that are well run and regulated do not pose a significant threat to public health" and that potential damage to health is "likely to be so small that it would be undetectable."

Residue handling

Two types of residue will be collected in the flue gas treatment process. The first is ‘fly ash’- ash that has literally ‘flown’ through the combustion chamber and has subsequently been filtered out. This will be taken off site for disposal or treatment.

The second is called 'APC residue' or 'spent lime'. This APC residue is collected separately to the fly ash in bag filters (see diagram above) 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 5% of the annual waste throughput (by weight).