Flue gas scrubbing removes gaseous pollutants such as hydrochloric acid, sulfur dioxide, and heavy metal compounds from the exhaust gas produced during thermal soil treatment. It is an indispensable component of the multi-stage exhaust gas purification process and ensures that the treated gases comply with statutory emission limit values before being released into the atmosphere. The following sections explain how flue gas scrubbing works, which pollutants it captures, and why it is essential for safe soil remediation.
How is flue gas generated during thermal soil treatment?
During thermal soil treatment, contaminated soil is heated in a rotary kiln at temperatures of up to approximately 600 °C, depending on the material and pollutant load. Organic pollutants are vaporized or thermally destroyed in the process. The resulting gases are extracted from the kiln and then pass through an exhaust gas purification line before leaving the facility.
The raw gas leaving the rotary kiln contains not only water vapor and carbon dioxide but also a wide range of gaseous pollutants. These include volatile organic compounds, hydrogen halides such as hydrogen chloride and hydrogen fluoride, sulfur compounds, and heavy metal compounds. Depending on the type of contamination in the treated soil, the composition of the raw gas varies considerably. When treating soils contaminated with chlorinated hydrocarbons, PAHs, or PCBs, particularly complex gas mixtures are produced that require high-performance exhaust gas purification.
Before flue gas scrubbing, the exhaust gas first passes through a dust removal stage in which fine dusts and particles are separated out. This is followed by thermal post-combustion, in which any remaining organic compounds are completely oxidized at high temperatures. Only then does the exhaust gas enter the flue gas scrubbing stage.
What exactly does flue gas scrubbing do and how does it work?
Flue gas scrubbing is a wet exhaust gas purification stage in which the hot exhaust gas is brought into intensive contact with a scrubbing liquid. Gaseous pollutants are transferred into the liquid and chemically bound or neutralized. The cleaned gas leaves the scrubber with a significantly reduced pollutant concentration.
Technically, flue gas scrubbing operates via a countercurrent scrubber or venturi scrubber, in which the exhaust gas flows from bottom to top while the scrubbing liquid is sprayed in from above. Through the intensive mixing of gas and liquid, acidic gas components dissolve into the scrubbing liquid. By adding sodium hydroxide solution or lime water, these acids are neutralized and bound as salts.
In modern facilities, several scrubber stages are often connected in series to specifically separate different groups of pollutants. After flue gas scrubbing, an activated carbon filter typically follows as the final purification stage, adsorbing any remaining organic trace substances and dioxins. Only after this multi-stage process is the cleaned exhaust gas released into the ambient air via the chimney. More information on this overall process is available on the page for thermal soil treatment.
Which pollutants does flue gas scrubbing remove from the exhaust gas?
Flue gas scrubbing primarily removes acidic gas components such as hydrochloric acid (HCl), sulfur dioxide (SO₂), hydrogen fluoride (HF), and nitrogen oxides from the exhaust gas. In addition, it separates out soluble heavy metal compounds still present in the gas phase, as well as water-soluble organic compounds.
Particularly relevant is the separation of hydrogen chloride, which is produced during the thermal treatment of chlorinated compounds such as PCBs, chlorinated solvents, or PVC-containing materials. When treating PFAS-contaminated soils — an increasingly important topic in the remediation of contaminated soils — halogen-containing compounds also occur in the exhaust gas stream and are captured by the scrubbing process.
Mercury, which passes into the gas phase during the treatment of Hg-contaminated soils, is precipitated in dedicated scrubber stages through targeted chemical reactions. For dioxins and furans (PCDD/PCDF) as well as persistent organic pollutants, the downstream activated carbon filter handles the fine purification, as these compounds have only limited solubility in the scrubbing liquid.
What statutory limit values must exhaust gas purification comply with?
In Germany, thermal treatment facilities for contaminated soils are subject to the 17th Federal Immission Control Ordinance (17. BImSchV), which sets strict emission limit values for a wide range of pollutants. These limit values apply to dust, heavy metals, dioxins, furans, hydrochloric acid, sulfur dioxide, nitrogen oxides, carbon monoxide, and total organic carbon compounds.
For particularly hazardous substances such as dioxins and furans, a limit value of 0.1 nanograms of toxicity equivalent per cubic meter of exhaust gas applies. Mercury must generally not exceed 0.05 milligrams per cubic meter. Compliance with these limit values is monitored through continuous emission measurements, which are regularly verified by accredited measurement bodies.
The competent licensing authorities grant operating permits under the Federal Immission Control Act (BImSchG) only when the facility has demonstrably proven its ability to permanently comply with these limit values. A functioning quality management system is a prerequisite, as is the operation of a high-performance multi-stage exhaust gas purification system.
What happens to the wash water after flue gas scrubbing?
The wash water that becomes laden with pollutants during flue gas scrubbing must be professionally treated and disposed of as contaminated wastewater. It contains dissolved salts, heavy metal compounds, and, depending on the source material, also organic compounds that cannot simply be discharged into the sewage system.
As a rule, the wash water is treated in an on-site or external wastewater treatment facility. Heavy metals are separated out by precipitation and filtration and deposited as filter cake or subjected to further treatment. The cleaned water can, depending on its quality, be recirculated in a closed loop or discharged in an orderly manner once compliance with limit values has been demonstrated.
The disposal of the residual materials generated by wash water treatment — i.e., sludges, salts, and filter cake — is part of the material flow management process, which ensures complete and legally compliant disposal of all process residues. Information on this can be found under material flow management.
Why is flue gas scrubbing indispensable for safe soil remediation?
Flue gas scrubbing is indispensable because, while thermal desorption releases pollutants from the soil, it initially transfers them into the gas phase. Without effective exhaust gas purification, these pollutants would enter the ambient air in an uncontrolled manner, and thermal soil treatment would not be an environmentally sound solution but merely a relocation of the pollution problem.
Flue gas scrubbing must not be viewed in isolation; it is part of a multi-stage exhaust gas purification chain. Dust removal, thermal post-combustion, flue gas scrubbing, and activated carbon filtration work together to ensure that the cleaned exhaust gas remains below statutory emission limit values. If one stage fails or does not operate optimally, the overall purification performance can decline significantly.
For clients commissioning remediation projects, a functioning flue gas scrubbing system means specifically: the treatment of contaminated soils takes place without impermissible emissions, the facility operates in compliance with the Federal Immission Control Act, and the pollutant balance is fully traceable. This is not only a legal requirement but also a matter of responsibility toward local residents and the natural environment.
How ARE Deutzen GmbH ensures safe flue gas purification during soil treatment
ARE Deutzen has been operating a thermal soil treatment facility with a fully integrated, multi-stage exhaust gas purification system since 1994. Flue gas scrubbing is a permanent component of this facility and is continuously monitored to ensure compliance with all statutory emission limit values. Customers benefit from:
- Over 30 years of experience in the thermal desorption of complex pollutant mixtures
- A permitted facility under BImSchG with continuous emission monitoring
- Certifications to ISO 9001, 14001, 50001, and 18001
- Treatment of a broad spectrum of pollutants: TPH, BTEX, PAHs, PCBs, PFAS, PCDD/PCDF, Hg, cyanides, and more
- Complete material flow management including disposal of all process residues
- Over 2.5 million tonnes of treated mineral materials as a reference for facility reliability
Are you planning a remediation project and require reliable thermal soil treatment with proven exhaust gas purification? Get in touch with ARE Deutzen and discuss your project directly with the on-site specialists.