The exhaust gas purification system of a thermal soil treatment plant operates in multiple stages: raw gases exiting the rotary kiln pass sequentially through thermal post-combustion, dust removal, flue gas scrubbing, and an activated carbon filter stage before the cleaned gas is released into the atmosphere. Each stage is designed for specific groups of pollutants and complements the preceding purification steps. The following sections explain the individual process steps and the legal requirements that must be met.
What pollutants are produced during the thermal desorption process?
During the thermal desorption process, organic pollutants are transferred from the soil into the gaseous state. The raw gas then contains volatile organic compounds such as BTEX, TPH, PAH, and phenols, as well as heavy metal compounds, dusts, hydrogen chloride, sulfur oxides, and in the case of particularly complex contaminations, also dioxins, furans, and PCBs.
The exact composition of the raw gas depends on the type of materials used. Soils from decommissioned gasworks typically contain high concentrations of PAH and cyanide, while soils from chemical plants or military sites may be contaminated with chlorinated hydrocarbons, nitroaromatics, or PFAS. Mercury (Hg) presents a particular challenge because it is volatile even at low temperatures and therefore requires special separation units. The exhaust gas purification system of a thermal soil treatment plant must therefore be flexibly designed to handle these varying pollutant profiles.
How is a multi-stage exhaust gas purification system structured?
A multi-stage exhaust gas purification system for thermal desorption typically consists of four successive main stages: thermal post-combustion, dust removal, flue gas scrubbing, and the activated carbon filter. Each stage performs a specific purification task while simultaneously protecting the subsequent stage from overload.
The raw gas exiting the rotary kiln is first directed into the afterburner chamber. This is followed by heat exchangers for cooling, fabric filters or electrostatic precipitators for dust separation, a multi-stage wet scrubber or flue gas scrubber for acidic gas components and heavy metals, and finally an activated carbon filter for the adsorption of residual pollutants such as mercury, dioxins, and furans. Thermal soil treatment at this technical level requires precise coordination of all stages to ensure that statutory emission limit values are reliably met.
What happens to the pollutants in the afterburner chamber?
In the afterburner chamber, organic pollutants originating from the rotary kiln are destroyed by high-temperature combustion. The raw gases are heated to temperatures of at least 850 °C and maintained at this temperature level for a minimum residence time of two seconds. Under these conditions, organic compounds are completely oxidized.
These conditions are legally prescribed and ensure that even persistent compounds such as PCDD/PCDF (dioxins and furans) are thermally destroyed. The afterburner chamber is therefore the heart of the exhaust gas treatment system: it converts the pollutant-rich raw gas into a flue gas that, while still containing dust, heavy metals, and acidic gas components, is free of organic pollutants. Only after this stage do the mechanical and chemical purification steps begin.
How are dust and heavy metals removed from the exhaust gas?
Dust and heavy metals are removed from the exhaust gas after post-combustion through a combination of fabric filters and wet scrubbers. Fabric filters physically separate fine dust and dust-bound heavy metal compounds, while the subsequent flue gas scrubbing washes out soluble heavy metals as well as acidic gas components such as hydrogen chloride and sulfur dioxide.
Dust removal using fabric filters
Fabric filters are particularly effective at separating fine dusts and particle-bound pollutants. The exhaust gas flows through a fine-mesh filter fabric to which the particles adhere. The separated dust is periodically cleaned off and disposed of separately as a residue.
Heavy metal separation in the flue gas scrubber
In the flue gas scrubber, the exhaust gas is brought into contact with washing liquid. Soluble heavy metal compounds and acidic gases are transferred into the aqueous phase and can be selectively removed from the process. Mercury, which poses special requirements due to its volatility, is subsequently adsorbed in the activated carbon filter. The material flow management of the residues generated in these stages is an essential component of the overall process.
What statutory limit values apply to exhaust gas emissions?
In Germany, exhaust gas emissions from thermal soil treatment plants are governed by the limit values of the 17th Federal Immission Control Ordinance (17. BImSchV), which applies to plants for the incineration and co-incineration of waste. It sets limit values for, among other things, total dust, heavy metals, carbon monoxide, nitrogen oxides, sulfur dioxide, hydrogen chloride, organic compounds, and dioxins and furans.
The limit values for dioxins and furans are particularly stringent: they are in the range of 0.1 nanograms of toxicity equivalent per cubic meter of exhaust gas. Very low limit values also apply to mercury. Compliance with these requirements is monitored through continuous measurement of certain parameters as well as regular individual measurements by accredited measurement institutions. Operators of thermal plants are required to document the measurement results and make them available to the authorities. The quality management system encompasses not only technical monitoring but also complete documentation of all emission-relevant process parameters.
How is it ensured that the purified exhaust gases are safe?
The safety of the purified exhaust gases is ensured through a multi-level control system: continuous online measurement of relevant emission parameters, regular external individual measurements by accredited testing institutes, and a documentation obligation monitored by the authorities. If a parameter exceeds the limit value, plant operation is automatically adjusted or interrupted.
The continuously measured parameters typically include carbon monoxide, total dust, sulfur dioxide, hydrogen chloride, nitrogen oxides, and the exhaust gas temperature in the afterburner chamber. These measured values are recorded and stored in real time. In addition, individual measurements for heavy metals, dioxins, furans, and organic compounds are carried out at defined intervals by independent measurement institutes. The combination of technical plant reliability, seamless measurement technology, and external controls forms the basis for safe and legally compliant plant operation.
How ARE Deutzen GmbH implements safe exhaust gas purification in practice
ARE Deutzen GmbH has been operating a thermal soil treatment plant with a rotary kiln and a fully integrated, multi-stage exhaust gas purification system at its Deutzen site in western Saxony since 1994. The plant is approved under the Federal Immission Control Act and certified to ISO 9001, 14001, 50001, and 18001. Over more than 30 years of operation, more than 2.5 million tonnes of mineral materials have been treated.
Specifically, ARE Deutzen offers the following services related to thermal soil treatment and exhaust gas purification:
- Treatment of soils with complex contaminations, including TPH, BTEX, PAH, PCB, PFAS/PFOS, PCDD/PCDF, cyanides, nitroaromatics, and chlorinated hydrocarbons
- Thermal desorption in a rotary kiln at temperatures of up to approximately 600 °C, depending on the material and contamination profile
- Multi-stage exhaust gas treatment with thermal post-combustion, dust removal, flue gas scrubbing, and activated carbon filter
- Continuous emissions monitoring and external individual measurements by accredited institutes
- Acceptance and disposal of process residues such as slags, ashes, and dusts
- Certified waste management company (EfB) pursuant to § 52 of the Circular Economy and Waste Act
If you are planning a remediation project involving contaminated soils or mineral waste and require a legally compliant, technically sound solution, contact ARE Deutzen directly. Get in touch now and develop the right disposal solution together.