Organic pollutants are completely oxidized in the afterburner chamber through high temperatures and sufficient residence time. The thermal combustion reactions that take place destroy pollutants that were converted into a gaseous state in the upstream rotary kiln, before the cleaned exhaust gas passes through further purification stages. The following sections explain the most important technical relationships surrounding pollutant destruction in the afterburner chamber.
At what temperature are pollutants destroyed in the afterburner chamber?
In the afterburner chamber, organic pollutants are destroyed at temperatures of at least 850 °C. At this temperature and a minimum residence time of two seconds, even persistent organic compounds such as dioxins and furans are reliably destroyed. Higher temperatures may be required for particularly difficult-to-degrade pollutants such as chlorinated hydrocarbons.
Temperature requirements are regulated by law and are based on the provisions of the 17th Federal Immission Control Ordinance and the European Industrial Emissions Directive. What matters is not only the peak temperature, but the interplay of temperature, residence time, and turbulence in the exhaust gas stream. Only when all three parameters are correct is complete thermal destruction of pollutants guaranteed.
In thermal soil treatment, the rotary kiln is operated at temperatures of up to approximately 600 °C under exclusion of oxygen. The pollutants released in this process enter the afterburner chamber as raw gas, where they are completely oxidized at significantly higher temperatures.
What chemical reactions take place in the afterburner chamber?
Predominantly oxidation reactions take place in the afterburner chamber. Organic molecules react with oxygen to form carbon dioxide and water. Carbon-containing compounds are completely combusted, while sulfur and nitrogen compounds are converted into their inorganic oxides.
The basic reaction mechanism can be described in simplified terms as follows: hydrocarbon compounds react with the supplied oxygen at sufficiently high temperatures to form CO₂ and H₂O. This process is referred to as complete combustion or complete oxidation. Incomplete combustion processes, in which carbon monoxide or unburned hydrocarbons are produced, are largely prevented by the high temperatures and sufficient residence time.
In the case of halogenated compounds such as PCB or chlorinated hydrocarbons, combustion produces hydrogen halides, for example hydrochloric acid. These are removed from the exhaust gas stream in the downstream flue gas scrubbing purification stages. Thermal afterburning is therefore only one link in a multi-stage exhaust gas purification chain.
Which pollutants can an afterburner chamber render harmless?
An afterburner chamber is effective against all volatile organic pollutants that were converted into a gaseous state in the upstream process. These include mineral oil hydrocarbons (MKW), BTEX compounds, phenols, polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), PFAS, as well as dioxins and furans (PCDD/PCDF).
Particularly significant is the effect on persistent organic pollutants, which are barely broken down in the environment. Dioxins and furans, for example, are known for their high thermal stability. Nevertheless, they are reliably destroyed at temperatures of 850 °C and above with sufficient residence time. The same applies to many fluorinated compounds from the PFAS group, which place special demands on treatment temperature due to their chemical stability.
Non-volatile inorganic pollutants such as heavy metals, on the other hand, remain in the solid material and are not eliminated by the afterburner chamber. They must be removed from the gas stream by other process steps, such as dust separation. Mercury occupies a special position: it evaporates at low temperatures and is captured in the exhaust gas purification system by activated carbon filters.
How does the afterburner chamber prevent the formation of new pollutants?
The afterburner chamber prevents the formation of new pollutants primarily by avoiding the so-called de novo synthesis temperature range. Dioxins and furans can re-form in a temperature range of approximately 250 °C to 450 °C when organic compounds and chlorine come into contact with catalytically active surfaces. Rapid cooling of the exhaust gas below this critical temperature range prevents this re-formation.
Technically, this is achieved through targeted exhaust gas routing after the afterburner chamber. The hot exhaust gas is rapidly cooled after thermal treatment so that the critical temperature range is passed through as quickly as possible. This principle is known as „quench“ and is an essential component of modern exhaust gas purification systems.
In addition, complete combustion in the afterburner chamber ensures that no unburned organic compounds enter the downstream purification stages, where they could act as precursor substances for new pollutants. Low CO values in the exhaust gas are a reliable indicator that combustion has been completed fully.
How is the success of pollutant destruction in the afterburner chamber demonstrated?
The success of pollutant destruction in the afterburner chamber is demonstrated through continuous and discontinuous emissions measurements. Parameters such as temperature, oxygen content, carbon monoxide, and total carbon are monitored continuously. Discontinuous measurements by accredited measurement institutes capture pollutants such as dioxins, furans, and heavy metals at regular intervals.
The statutory requirements for emissions monitoring are regulated in the 17th Federal Immission Control Ordinance. Operators of thermal treatment plants are required to document compliance with emission limit values without gaps and to report to the competent authorities. The measurement results demonstrate whether the plant is operating within the prescribed operating range and whether pollutant destruction is actually being carried out completely.
In addition to exhaust gas measurements, the treated solids are also examined for residual contamination. A comparison of pollutant levels in the untreated input material with those in the treated output material demonstrates the success of the treatment. An effective quality management system ensures the traceability of all relevant process parameters and measurement results throughout the entire treatment period.
How ARE Deutzen GmbH supports safe pollutant destruction
ARE Deutzen GmbH has been operating a thermal soil treatment plant with a rotary kiln and multi-stage exhaust gas purification at its Deutzen site in western Saxony since 1994. The plant is licensed for the treatment of a wide range of organic pollutants and meets all statutory requirements under the Federal Immission Control Act. Customers from civil and structural engineering as well as industry benefit from:
- Over 30 years of experience in the thermal desorption of contaminated mineral waste
- Licensed treatment of complex contaminations, including PAH, PCB, PFAS, PCDD/PCDF, MKW, BTEX, and chlorinated hydrocarbons
- Multi-stage exhaust gas purification with thermal afterburning, flue gas scrubbing, and activated carbon filters
- Certified operation in accordance with ISO 9001, 14001, 50001, and 18001
- Comprehensive material flow management for soils, construction rubble, and mineral process residues
- Complete documentation and emissions monitoring as the basis for regulatory evidence
Are you planning a remediation project involving complex contaminations, or do you need a reliable disposal solution for contaminated mineral waste? Get in touch and discuss your project directly with the experts at ARE Deutzen.