How does incineration plant work?

Answer 1

The way the incinerator works is pretty simple. Trucks bring the waste from domestic and commercial sources to the tipping hall where they dump it into bunkers which are maintained at a lower pressure than the outside to prevent odours escaping. Grapples from overhead cranes will then mix the waste to produce a more uniform fuel that will help maintain a steady combustion process. The waste is then transferred into the furnace hopper; this is a fully automated process. 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 odorous 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 wastes facilities in the county, 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.

There are various flue gas cleaning stages: The Electrostatic Precipitators will initially remove 99% or more of particulates. These are primarily dust and ash particles, but may also include minute quantities of heavy metals, dioxins and furans. Acid Gas Scrubbing consists of a lime mixture being injected into the gas stream. This reacts to neutralise acid gases such as sulphur dioxide, hydrogen fluoride and hydrogen chloride. Activated Carbon Activated carbon injection is used to remove organic compounds such as dioxins and also volatile metals such as Mercury or cadmium. The activated carbon provides a surface onto which the heavy metals can adhere and these will then be filtered out at a later stage. The final filtration of particulate matter typically uses a bag house filter. At this stage of the cleaning process, particulate matter is primarily made up of spent activated carbon and spent hydrated. This material is usually recycled back into the combustion chamber to ensure that dioxins are properly destroyed in the high temperatures.

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. 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.