Recovery - The process of Energy from waste (efW)
The technology of mass-burn energy-from-waste is a tried and tested method of capturing the inherent energy value of residual municipal and similar wastes. There are some 50 plants in operation or under construction in the UK, recovering energy from around 7 million tonnes of residual and commercial and industrial waste per year.
A conventional EfW plant such as EnviRecover will comprise the following elements:
Waste reception hall and bunker
Waste delivered to site is directed to the ‘front end’ of the main building where it is discharged into a storage bunker, mixed, then loaded by a grab crane into a feed hopper for the main combustion grate. The tipping hall draws air into the building to support the combustion process, which means that any odour arising from the waste is kept inside the plant. Mixing the waste in the bunker helps to even out any variances in the calorific content of the mixed waste.
The combustion grate
This can take a number of forms, but generally comprises inclined rollers or plates that move the waste material through the combustion zone whilst aerating it to provide optimum combustion conditions. The grate will always be started up using a secondary fuel so that all the waste is combusted continuously at high temperature for a set time. These parameters are set by the permit to operate the plant issued by the Environment Agency.
A conventional steam boiler and economiser are used to convert the high temperature gaseous products of combustion into high temperature/high pressure steam. The process water for the boiler is demineralised so that it does not clog up the boiler tubes and can be reused through the process to maximise efficiency.
Using the energy
Nearly all EfW plants install a conventional steam turbine to generate electricity that can be supplied to the National Grid, through a substation and transformers. It is also possible to supply steam and/or hot water to nearby buildings as a direct energy use.
Behind the boiler/economiser, the flue gases from combustion have to be filtered and ‘scrubbed’ of particulates and potentially hazardous elements before being released to the atmosphere via the stack. The most common processes involve injecting a form of lime into the gas stream to reduce acidity, as well as activated carbon to capture potential pollutants such as heavy metals and dioxins. The gases pass through a series of filters to sieve out particulates. The exhaust gases are monitored continually prior to emission to ensure that the emissions fall within permitted limits set under EU legislation.