Incinerators work by combusting mixed refuse in a furnace, at extremely high temperatures of 850°C or more.
Before it is operational, the incinerator first has to be brought up to the running temperature. This is done by burning oil for a period of over 24 hours. For this reason the process is only suited to constant operation – any plant shutdown will be uneconomic and environmentally unsound.
The incineration process reduces the waste to about a third of its original weight and converts it into heat, flue gases, particulates, bottom ash and fly ash.
The heat produced can be used to generate electricity. In CHP (Combined Heat and Power) incinerators heat may also used to heat homes and factories nearby.
Gases from the combustion process such as sulphur dioxide are passed through air pollution control systems before being vented to the atmosphere via a tall chimney stack. These filtering systems should also prevent particulates produced in the furnace from escaping into the atmosphere. However, even with modern systems, only the biggest particles are caught, while ultra-fine and nano-particles escape. These particles may be toxic in concentrations.
Bottom ash is described as inert but contains a concentration of toxins, including heavy metals such as cadmium and mercury. Should this material be used for other purposes such as for road building, these toxins can leach into the environment.
Fly ash, that is the ash that is caught by pollution abatement equipment and prevented from escaping from the chimney, is highly toxic, containing heavy metals and dioxins. This has to be disposed of at ‘specially designated’ toxic waste sites.