The diagram below gives a simplified overview of what happens to municipal solid waste in an incinerator. While the exact design of each incinerator may be slightly different, they will generally follow the diagram above and will consist of the following areas:
On entering the incineration facility, waste trucks make their way to the tipping hall. Here the waste is offloaded into large bunkers for storage. The airin the reception area and in the bunkers is maintained at a lower pressurethan outside (negative pressure) and this prevents odours escaping.
Waste will be received from both household and commercial sources. Some of the waste will be bulky in nature, so it may need to be broken down into smaller pieces. Overhead traverse cranes fitted with grapples mix the waste before feeding it into the furnace hoppers. The mixing of the waste is useful in producing a more uniform fuel that will help maintaining a steady combustion process within the desired operating conditions
The cranes and grabs transfer the mixed waste from the bunker to the furnace 'hopper'. This process can be fully automatic in modern incineration plants. At the bottom of the hopper a metering ram pushes the waste onto the combustion grate, which agitates and transports the waste through the combustion chamber. Combustion takes place at temperatures of 850 - 1100oC, the temperature at which odourous gases and all dioxins will be destroyed.
Fly ash is the particulate removed during the gas cleaning phase. It is generally about 1-3% by weight of the original waste. Fly ash is considered hazardous and so it must be disposed of in a specially designed facility. At present there are no hazardous waste facilities in Ireland, so hazardous material will be exported for safe disposal.
The combustion process produces flue gas containing water vapour, nitrogen, carbon dioxide, nitrogen oxides, oxygen and particulate matter. Some of these compounds are harmful to health and therefore the flue gas is thoroughly cleaned before it is discharged to the air. The flue gas cleaning equipment of a modern incineration plant is complex and can take up about half of the space within the plant. There are various flue gas cleaning designs, but modern plants generally include the following stages:
A boiler converts the energy from the combustion into high pressure steam. The combustion chamber is surrounded by water tube walls, which are heated by radiation from the combustion. The hot flue gases release additional heat in additional tube panels in the boiler. The steam goes into a turbine, which drives an electric generator. Generally, about 10% of the electricity is used on site and the remainder is fed into the national grid. The incineration of 400,000 tonnes of waste can supply the annual electricity consumption of more than 30,000 homes. The heat remaining after the electricity production can be used to heat water, which can be directly piped to people's homes in a district heating system. This can supply the annual requirements for heating and domestic hot water for approximately 25,000 homes.
At the end of the grate the solid waste has been completely burned out. The remaining residue is called bottom ash, which is ejected at the bottom of the combustion chamber. The bottom ash corresponds to about 15 - 20% by weight or 4-6% by volume of the original waste. After storage the bottom ash may be screened into fine and coarse fractions and the ferrous metals (iron or steel) in the ash will be extracted using large magnets. The metals represent 5 - 10% of the bottom ash and are sent to the steel works for recycling. The remaining bottom ash is non-hazardous and is typically used in other applications such as an aggregate in concrete or for road building.