The rotary kiln and the afterburner chamber serve two fundamentally different purposes in the thermal soil treatment process: in the rotary kiln, contaminants are released from the solid material through heat and transferred into the gas phase, while the afterburner chamber subsequently destroys these gaseous contaminants completely at significantly higher temperatures. Both process stages are necessary to treat contaminated soils safely and in compliance with regulations. The following sections explain how each stage works, where the key difference lies, and which contaminants are treated in which way.
What happens to contaminants in the rotary kiln?
In the rotary kiln, contaminants are released from the solid material through thermal energy without being combusted. The soil or construction rubble is heated at temperatures of up to approximately 600 °C, depending on the material and contaminant load, under oxygen exclusion. The contaminants evaporate or are transferred into a gaseous state, while the solid material itself remains largely intact.
This process is referred to as thermal desorption or thermodesorption. The term describes precisely what occurs: contaminants bound to soil particles are released from the surface of the material through heat. The decisive factor here is the exclusion of oxygen. Without oxygen, no combustion takes place — only a physical phase change from the solid or liquid state into the gaseous state.
The cleaned solid material exits the rotary kiln at the end of the process as treated material that can subsequently be reused or properly disposed of. The resulting contaminant gas is further treated in the downstream purification stages.
What exactly happens in the afterburner chamber?
In the afterburner chamber, the contaminant gases generated in the rotary kiln are thermally destroyed at very high temperatures. The gases are heated in the chamber to above 850 °C and held at this temperature for a defined residence time. In the process, organic contaminants are broken down into their basic components — typically carbon dioxide and water.
The afterburner chamber is therefore not a purification stage in the strict sense, but a destruction stage. Contaminants that have left the rotary kiln as gas no longer exist in their original chemical form after afterburning. For persistent organic contaminants such as PCDD/PCDF (dioxins and furans) or PCB, a sufficiently high temperature and residence time in the afterburner chamber is absolutely essential to ensure complete destruction.
After the afterburner chamber, the exhaust gas passes through further purification stages, including dust removal, flue gas scrubbing, and activated carbon filters, before being released into the atmosphere. More information on the individual process steps of thermal soil treatment can be found on the corresponding page.
What is the core difference between desorption and destruction?
The core difference lies in what happens to the contaminants: desorption is a separation process, destruction is an elimination process. During desorption in the rotary kiln, contaminants are merely transferred from the solid material into the gas phase — they are still present afterwards. The actual destruction of contaminants only takes place in the afterburner chamber.
Both processes depend on each other and together form a complete treatment system:
- Rotary kiln (desorption): Physical separation of contaminant and solid material through heat under oxygen exclusion. Result: cleaned solid material and contaminant-laden gas.
- Afterburner chamber (destruction): Chemical decomposition of the contaminant gases through high-temperature combustion. Result: destroyed contaminants and treated exhaust gas, which is subsequently further purified.
Without the afterburner chamber, desorption would not be a complete treatment, but merely a transfer of the problem from the solid phase to the gas phase. Only the interaction of both stages makes thermal soil treatment a safe and legally compliant process.
Which contaminants are treated by desorption?
Thermal desorption in the rotary kiln is suitable for a wide range of organic and inorganic contaminants that can be released from the solid material through heat. Soils and mineral waste contaminated with the following substances can be treated:
- Mineral oil hydrocarbons (MKW)
- Benzene, toluene, ethylbenzene, and xylene (BTEX)
- Polycyclic aromatic hydrocarbons (PAH)
- Phenols
- Polychlorinated biphenyls (PCB)
- Per- and polyfluoroalkyl substances (PFAS/PFOS)
- Dioxins and furans (PCDD/PCDF)
- Mercury (Hg)
- Cyanides
- Nitroaromatics and other chlorinated hydrocarbon compounds
Many of these contaminants are typically found at decommissioned industrial sites, former gasworks, or military facilities. The ability to treat complex contaminant mixtures is a key characteristic of thermal desorption compared to other soil remediation processes.
When is desorption sufficient and when is destruction necessary?
Desorption alone is never sufficient, because the contaminant gases generated in the rotary kiln always require post-treatment. The real question is what temperatures and residence times in the afterburner chamber are required to completely destroy specific contaminants. This depends on the chemical stability of the respective compounds.
More volatile and less stable organic compounds such as MKW or BTEX can be reliably decomposed at moderate temperatures in the afterburner chamber. Persistent organic contaminants such as PCDD/PCDF or PCB, on the other hand, require higher temperatures and longer residence times, as their chemical bonds are significantly more stable.
The operating parameters of the plant are therefore set based on the contaminant characteristics of the respective material. Waste materials are analytically characterized beforehand to ensure that the process management is tailored to the specific contamination. Material flow management plays an important role here, as it forms the basis for a correct assessment and planning of the treatment.
How is it ensured that both processes run safely?
The safety of thermal soil treatment is ensured through a multi-stage system of technical measures, continuous monitoring, and regulatory requirements. Both process stages — the rotary kiln and the afterburner chamber — are permanently monitored for compliance with defined operating parameters.
The key safety elements include:
- Continuous emissions monitoring: Exhaust gas values are continuously recorded and compared with statutory limit values.
- Multi-stage exhaust gas purification: After the afterburner chamber, the exhaust gas passes through dust removal, flue gas scrubbing, and activated carbon filters before being discharged.
- Permits under BImSchG: The plant is operated on the basis of the required permits under the Federal Immission Control Act (BImSchG).
- Certifications: The technology is certified to ISO 9001, 14001, 50001, and 18001, covering quality, environmental, and energy management.
- Analytical incoming characterization: Every waste material is analyzed prior to treatment to correctly set the process parameters.
Quality management ensures that all process steps are documented, reviewed, and continuously improved. More than 30 years of operating experience and over 2.5 million tonnes of treated mineral materials demonstrate that this system reliably works in practice.
How ARE Deutzen GmbH helps with thermal soil treatment
ARE Deutzen GmbH has been operating one of the most powerful thermal desorption plants in Germany since 1994. For civil engineering companies, remediation planners, and industrial clients, this means a reliable partner for the treatment of even complex contaminations. The range of services specifically includes:
- Acceptance and treatment of contaminated soils, construction rubble, and mineral waste with a broad spectrum of contaminants
- Analytical incoming characterization and individual adjustment of process parameters to the respective contamination
- Thermal desorption in the rotary kiln at temperatures of up to approximately 600 °C, depending on the material and contaminant load, under oxygen exclusion with subsequent contaminant destruction in the afterburner chamber
- Multi-stage exhaust gas purification for reliable compliance with statutory emission limit values
- Complete material flow management including interim storage and mechanical-physical pre-treatment
- Certified waste management company with all required permits under BImSchG
Whether it involves a decommissioned industrial site, a former gasworks, or contamination with particularly persistent pollutants: ARE Deutzen has the experience, the technology, and the permits to reliably implement even demanding remediation projects. Get in touch to discuss your project and receive a concrete proposal.