Thermal soil treatment requires a multi-stage exhaust gas cleaning system because heating contaminated soils in a rotary kiln transfers a complex mixture of various pollutants into the gas phase. No single cleaning process can simultaneously and reliably reduce all relevant pollutant classes to the legally prescribed limit values. The following sections explain which pollutants are produced, how the individual cleaning stages work together, and why each stage is indispensable.
Which pollutants are produced during thermal desorption in a rotary kiln?
During thermal desorption in a rotary kiln, organic pollutants are released from the soil and converted into a gaseous state. Depending on the contamination of the source material, the resulting raw gas contains hydrocarbons such as TPH and BTEX, polycyclic aromatic hydrocarbons (PAHs), phenols, chlorinated compounds such as PCBs and PCDD/PCDF, as well as mercury in gaseous form.
The exact composition of the raw gas depends heavily on the source substrate. Soils from decommissioned gasworks sites typically contain high concentrations of PAHs and cyanides. Contaminated sites from military installations may contain nitroaromatics or chlorinated solvents. Soils contaminated with PFAS or PFOS place special demands on the post-combustion stage due to the chemical stability of these compounds. In addition, inorganic dusts and particles are mechanically released as material passes through the kiln.
This range of pollutants in a single exhaust gas stream explains why thermal soil treatment requires a multi-stage cleaning chain. Different pollutants respond to different physical and chemical cleaning mechanisms, and no single stage covers the entire spectrum.
How does a multi-stage exhaust gas cleaning system work?
A multi-stage exhaust gas cleaning system consists of coordinated process stages that treat the raw gas from the rotary kiln in sequence. The typical sequence includes dust removal, thermal post-combustion, flue gas scrubbing, and activated carbon filtration. Each stage performs a specific function within the overall process.
Dust removal
In the first step, coarse and fine particles are separated from the raw gas. Cyclones or fabric filters remove dusts and solid particles before the gas enters the post-combustion stage. This step protects downstream components from contamination and increases their efficiency.
Thermal post-combustion
The de-dusted gas is completely oxidized in a post-combustion chamber at temperatures above 850 °C and a defined residence time. Organic pollutants that were only desorbed in the rotary kiln but not completely destroyed are thermally broken down in this stage. This stage is particularly critical for persistent organic compounds such as PCDD/PCDF.
Flue gas scrubbing and activated carbon filter
After post-combustion, the exhaust gas passes through wet or dry flue gas scrubbing, which neutralizes acidic gas components such as hydrogen chloride, sulfur dioxide, and hydrogen fluoride. In the final activated carbon filter, remaining organic trace substances, mercury, and other volatile heavy metal compounds are adsorbed. Only after this final step is the cleaned gas discharged into the atmosphere via the stack.
Why is single-stage cleaning insufficient for contaminated soils?
Single-stage cleaning is insufficient because the pollutants contained in the exhaust gas are too chemically and physically diverse to be fully captured by a single process. Particles can be mechanically separated, organic compounds must be treated thermally or by adsorption, and acidic gases require chemical neutralization.
A further problem is the formation of secondary pollutants. During the thermal treatment of chlorine-containing compounds, new chlorinated hydrocarbons — including dioxins and furans — can form within certain temperature ranges. A single cleaning stage could not reliably control this re-formation. The combination of high-temperature post-combustion and subsequent rapid cooling is specifically designed to suppress the re-formation of such compounds.
Contaminated soils from remediation projects also frequently exhibit mixtures of pollutants whose composition is not always fully known. A multi-stage exhaust gas cleaning system provides the necessary safety margin to reliably retain even unexpected pollutant components.
Which limit values apply to exhaust gases from thermal soil treatment plants?
Thermal soil treatment plants in Germany are subject to the 17th Federal Immission Control Ordinance (17. BImSchV), which sets strict emission limit values for a wide range of pollutants. These include limit values for dust, carbon monoxide, total organic carbon compounds, nitrogen oxides, sulfur dioxide, hydrogen chloride, heavy metals, and PCDD/PCDF.
A particularly strict limit value of 0.1 nanograms of toxicity equivalent per cubic meter of exhaust gas applies to dioxins and furans. Mercury is governed by its own limit value, as it is present in many contaminated soils and occurs in gaseous form in the exhaust gas. Compliance with these limit values must be demonstrated through continuous and discontinuous measurements.
The permit issued under the Federal Immission Control Act (BImSchG) forms the legal basis for plant operation. In addition to emission limit values, it also specifies the operating conditions under which the plant may be operated, including minimum temperatures and residence times in the post-combustion stage.
What happens to the residues from exhaust gas cleaning?
The residues from exhaust gas cleaning — including filter ashes, scrubber sludges, and loaded activated carbon — are classified as hazardous waste and must be properly disposed of. They concentrate the pollutants separated from the exhaust gas and may not simply be landfilled or otherwise recovered.
Filter ashes from dust removal and residues from flue gas scrubbing are either sent to a secure landfill for hazardous waste or subjected to specialized treatment, depending on their composition. Loaded activated carbon is generally thermally regenerated or disposed of as special waste. The material flow management of these residues is an essential component of the overall concept of a thermal treatment plant.
The quantities of these residues are small relative to the volume of soil treated, but their safe disposal is required by law and demands careful documentation and record-keeping.
How is exhaust gas cleaning continuously monitored?
Exhaust gas cleaning at thermal soil treatment plants is monitored by a combined system of continuous online measurement technology and regular discontinuous measurements by accredited monitoring bodies. Parameters measured continuously include carbon monoxide, total carbon, nitrogen oxides, sulfur dioxide, hydrogen chloride, dust, and exhaust gas temperature.
Measured values are recorded in real time and compared against the approved limit values. Exceedances automatically trigger alarms and can lead to a controlled shutdown of the plant. These automated protective functions are a central element of safe plant operation.
Discontinuous measurements by external measurement institutes recognized by the regulatory authorities supplement the online monitoring. They capture pollutants that cannot be determined continuously by measurement technology, including dioxins, furans, and heavy metals. The results are reported to the competent authorities and documented. Consistent quality management ensures that all monitoring obligations are fulfilled without gaps.
How ARE Deutzen GmbH helps with thermal soil treatment using multi-stage exhaust gas cleaning
ARE Deutzen GmbH has been operating a thermal soil treatment plant at its Deutzen site in western Saxony since 1994 and has more than 30 years of experience in treating complexly contaminated mineral waste. The plant is equipped with a complete multi-stage exhaust gas cleaning system and is permitted under the Federal Immission Control Act.
For civil engineering companies, project developers, and industrial customers wishing to remediate or revitalize contaminated sites, ARE Deutzen GmbH offers concrete solutions:
- Thermal desorption at temperatures up to approximately 600 °C, depending on the material and pollutant load, for a broad spectrum of pollutants, including TPH, BTEX, PAHs, PCBs, PFAS/PFOS, PCDD/PCDF, and mercury
- Multi-stage exhaust gas cleaning with dust removal, thermal post-combustion, flue gas scrubbing, and activated carbon filter to ensure reliable compliance with all statutory emission limit values
- Acceptance and disposal of soils, dredged material, construction rubble, and process residues within the framework of comprehensive material flow management
- Certified waste management company (EfB) with certifications to ISO 9001, 14001, 50001, and 18001
- Complete documentation and record-keeping for all treated materials
If you are planning a remediation project or have questions about the treatment of contaminated soils, speak directly with the experts at ARE Deutzen GmbH. Contact us for expert advice on your project.