Energy from waste

Energy from waste (EfW) (also referred to as energy recovery) facilities treat waste under controlled conditions, to reduce the volume and recover value from the waste through the generation of electricity and/or heat. Electricity generated from a typical 200,000tpa facility would be approximately equal to the consumption in 20,000 houses and heat produced by the plant can be used in industrial or district heating schemes where the connection costs are proportionate; this generally means that the facility needs to be located in an urban area or immediately adjacent to a heat user.

Energy from waste facilities incorporate control measures for the flue gas emissions from the combustion process. The configuration of the flue gas cleaning equipment will be specific to the type of plant, and possibly to the supplier, but normally includes an acid scrubbing plant, carbon injection system, and electrostatic precipitators or fabric 'type' filters. These systems are designed to meet the emission limits set down in the EU Waste Incineration Directive which have been set to control the risks posed to the environment and human health, and is implemented through the Waste Incineration (England and Wales) Regulations 2002 (there is similar legislation implementing the Directive in Scotland and Northern Ireland).

Regulation of energy from waste facilities is by the Environment Agency in England and Wales, the Scottish Environmental Protection Agency in Scotland, and the DoENI in Northern Ireland. These organizations regulate all incinerators and waste to energy plants that burn more than 1 tonne of waste per hour. A permit to operate a facility will only be issued if the regulator is satisfied that the plant will be designed, built, operated and maintained in such a way that the requirements of the directive are met and human health and the environment is protected. Following the issue of a permit, an assessment of the plant operations and environmental performance is conducted, including continuous and periodic monitoring and inspections.

Waste delivered to an energy from waste plant is received into a pit where an overhead crane mixes it and feeds a charging hopper. The waste handling area is inside the building and is kept under negative air pressure to avoid the escape of dust and odour. The majority of incinerators currently in operation include a moving grate which is designed to handle large volumes of residual wastes with no requirement for pre-treatment. The waste is slowly propelled through the furnace by the grate so that the waste continuously enters at one end and the ash is discharged at the other. The combustion process, assisted by gravity, creates a 3-stage process: drying, combustion and burnout, with the emphasis on ensuring both optimum and complete combustion of the wastes so that the bottom ash contains less than 3% total organic carbon.

The temperature in the combustion process is maintained at 850°C or above, for at least two seconds, which ensures complete combustion of the waste. Energy is recovered from the hot flue gases by a boiler system, creating steam to turn a turbo-generator and the electricity is fed to the grid.

Combustion of most forms of household, commercial and non-hazardous industrial wastes results in a volume reduction of approximately 75 to 80%, leaving 20 to 25% bottom ash and a much smaller quantity of residues from the flue gas cleaning system and fly ash from the combustion process. The bottom ash contains any metals; steel, aluminium, copper etc that were present in the incoming waste streams. These can be recovered from the ash and recycled. The remaining bottom ash is used in building products and in construction as a substitute aggregate. The use of bottom ash in this way increases the quantity of waste recovered to around 95%. The flue gas treatment (or air pollution control) residues are classified as hazardous waste (due to the alkalinity of the material). These materials are first treated and then landfilled.

Energy from waste (EfW) plants

The company have announced plans to develop energy from waste plants at: