The combustion of waste results in the production of a mixture of gases containing pollutants such as carbon dioxide, sulphur dioxide, dust and soot, as well as nitrogen oxides, heavy metal bearing fumes and unburned hydrocarbons.
The pollutants can be removed from the flue gases by using the latest flue gas cleaning systems, so that harmful organic and inorganic substances are no longer discharged into the atmosphere. These substances are partly incorporated in the reaction-neutral slag or concentrated in the filter dust as end product of the flue gas cleaning process, allowing them to be deposited safely underground.
Pollutants are therefore continuously removed from the environmental cycle - pollutants that were formerly emitted to the environment from waste dumps.
Modern flue gas cleaning systems guarantee a targeted degradation of pollutants by means of different system chains. Which system chain is used again depends on the particular conditions of the site location and the composition of the waste.
None of these system components purifies one exclusive flue gas component. It can for instance contribute to separation of solids as well as to cleaning from organic trace impurities and heavy metals.
Various manufacturer-specific flue gas cleaning methods exist at present, which are composed of variable system components. Irrespective of the procedure, solids, contaminated gases and organic trace impurities are separated, and heavy metal components are removed by filtration.
The waste incineration process produces dioxins and furans (from combustion) as well as gaseous forms of heavy metals such as mercury and cadmium. These substances are already contained in the waste and are liberated by the combustion process.
In old landfill sites, these pollutants were allowed to enter the environmental cycle untreated. Even the redevelopment of old and the construction of new waste dumps did not prevent the continued existence of these pollutants in spite of environmental barriers.
Flue gas cleaning however removes these pollutants from the environment permanently.
An important part of flue gas purification is the removal of particulates from the exhaust gas. These dusts are partly produced during the incineration process, but also by the individual components of the flue gas cleaning process - such as the spray absorber, by spraying activated carbon and lime milk.
Different methods are available for this, such as filtration of particles through porous materials or separation by means of centrifugal force. Another option is to charge the dust particles electrostatically, which can also be combined with wet cleaning.
A number of system components can be made use of for the solids separation process. To a certain degree, they are also effective when used for the separation of gaseous pollutants.
Modern waste incineration facilities make use of various combustion technology systems. A well controllable incineration efficiency chiefly depends on a coordination of grate system, furnace chamber, reburning zone and steam generator type.
A high degree of decomposition of all hydrocarbon compounds is achieved if good flue gas turbulence is achieved by the incineration air and gas feed-through.
This is not the case for nitrogen oxide compounds (NOx), i.e. the gaseous oxides of nitrogen. These are produced in the course of every combustion process. In combination with solar irradiation, harmful ozone can be formed when these compounds enter the environment. Nitrogen oxides are also responsible for the development of smog and acid rain.
Two steps prevent the discharge of harmful nitrogen oxide compounds to the atmosphere during waste incineration. The first step involves incineration temperature optimisation: this reduces NOx production by up to 30 percent.
The second step involves two processes which achieve a reduction of up to 90 percent. EEW Energy from Waste plants use the SNCR process for this.
Dioxins and furans include over 200 compounds that can basically be described as chlorine-containing hydrocarbons of varying toxicity. They do not occur in nature, but are unwanted side products of all kinds of combustion processes.
The emission of dioxin has been reduced continuously over the past ten years. Reasons for this achievement include the optimisation of operating conditions, the development of more effective flue gas cleaning methods in conjunction with higher incineration temperatures. Dioxins and furans are destroyed at temperatures above 900°C. Complete burn-up of flue gases is therefore a considerable contribution to minimisation of dioxin/furan emissions.
Emissions can also be reduced significantly by favourable management of the flue gases. This means getting the flue gases beyond the critical temperature of 900°C as quickly as possible without the gases cooling off in the process, because cooling off could be associated with new formation of pollutants.
A residence time of at least two seconds at 850°C is prescribed by law for the gases. Thermoelements at the end of the first pass monitor adherence to this value. Further flue gas cleaning components downstream avoid the emission of any newly formed dioxins.