Fine particulate matter and volatile metals are removed from the flue gas of contaminated soils through a multi-stage exhaust gas purification system that combines fabric filters, thermal post-combustion, flue gas scrubbing, and activated carbon adsorption. Each stage performs a specific function: mechanical dedusting separates particles, while downstream processes capture gaseous metal compounds and residual organic pollutants. The following sections answer the most important individual questions about flue gas purification in thermal desorption.
What technologies are used to separate fine particulate matter?
Fabric filters (bag filters) and electrostatic precipitators are primarily used to separate fine particulate matter from flue gas. Fabric filters are considered particularly effective because they reliably capture even ultrafine particles in the micrometer range. In thermal soil treatment, the fabric filter is the central element of the dedusting stage.
In a fabric filter, the raw gas flows through a fine-pored filter medium made of specialty fibers. Particles deposit on the filter surface and gradually form a filter cake that actually improves separation performance over time. Regular cleaning via pressure pulses dislodges the filter cake, which is then collected in a hopper.
Electrostatic precipitators operate on a different principle: particles are electrically charged and then deposited on grounded collector plates. They are particularly suitable for high gas volumes, but do not always achieve the same separation rate as fabric filters for very fine particles. In practice, both technologies are sometimes combined or supplemented by upstream cyclones that pre-separate coarse dust fractions.
How do volatile metals behave in the rotary kiln?
Volatile metals such as mercury, zinc, lead, and cadmium evaporate in the rotary kiln at treatment temperatures of up to approximately 600 °C, depending on the material and pollutant load, and transition into the gas phase. They therefore leave the kiln not as solids, but as gaseous metal compounds or elemental metal within the flue gas stream.
Their behavior depends strongly on the respective boiling point and chemical bonding form. Mercury is particularly volatile and evaporates completely even at low temperatures. Zinc and lead form oxides or chlorides that are likewise transferred into the gas stream. Cadmium behaves similarly and preferentially accumulates in the fine dust fractions of the exhaust gas.
This transition into the gas phase is intentional: it separates the metals from the treated soil material and enables their targeted capture in the downstream flue gas purification system. Without this step, heavy metals would remain in the cleaned soil material and limit its recyclability.
How is mercury removed from flue gas?
Mercury is removed from flue gas by adsorption onto activated carbon. Activated carbon filters bind elemental mercury as well as mercury compounds with high efficiency, because the large internal surface area of the activated carbon provides an intensive contact surface. This stage is indispensable in flue gas purification for contaminated soils with mercury contamination.
In practice, activated carbon is either used as a fixed-bed filter or injected directly into the flue gas stream as a powder. In the injection method, the activated carbon reacts with the mercury in the gas stream and is subsequently separated together with the dust in the fabric filter. Fixed-bed filters offer a longer residence time and therefore higher adsorption performance.
In addition, wet condensers or flue gas scrubbers can be used to wash soluble mercury compounds out of the gas stream. Since mercury can exist in various chemical forms, combining multiple methods is often necessary to reliably comply with statutory emission limit values.
What happens to the separated dusts and metal compounds?
The dusts and metal compounds separated in the fabric filter and in further purification stages are classified as hazardous waste and must be disposed of through regulated channels. They are generally stored in approved underground repositories or special waste landfills, because their pollutant content rules out above-ground disposal.
Depending on their composition, individual fractions may be sent for recovery. Zinc-containing dusts from metallurgical processes, for example, are used as secondary raw materials in the zinc industry. In the case of dusts from thermal soil treatment, however, the pollutant concentration is often so high that recovery is not technically or legally feasible.
As part of a structured material flow management process, the various residual material fractions are analyzed, classified, and assigned to the appropriate disposal route. Careful documentation is mandatory in order to fulfill the verification obligations under the Circular Economy Act.
What limit values apply to fine particulate matter and heavy metals in exhaust gas?
In Germany, facilities for the thermal treatment of waste are subject to the limit values of the 17th Federal Immission Control Ordinance (17. BImSchV). A daily average value of 10 mg/m³ is prescribed for total dust. Separate limit values apply to heavy metals: mercury is limited to 0.05 mg/m³ as a daily average, and cadmium and thallium combined are subject to the same value.
For additional heavy metals such as lead, chromium, copper, manganese, nickel, and arsenic, a combined limit value of 0.5 mg/m³ applies. Particularly stringent requirements exist for dioxins and furans (PCDD/PCDF), which are limited to 0.1 ng/m³ toxicity equivalent.
These limit values are to be understood as minimum standards. The emission values actually approved for a facility may be stricter in individual cases if the permitting authority imposes special requirements. Continuous monitoring equipment tracks compliance with the limit values in real time and triggers automatic alarms in the event of exceedances.
Why is single-stage flue gas purification often insufficient?
Single-stage flue gas purification is usually insufficient for the treatment of contaminated soils because the raw gas simultaneously contains particles, organic pollutants, acidic gases, and volatile metals in very different concentrations. No single process can effectively separate all of these components at the same time.
Fabric filters reliably remove fine particulate matter, but are not suitable for gaseous pollutants such as mercury or sulfur dioxide. Flue gas scrubbers neutralize hydrochloric acid and sulfur dioxide, but do not separate fine particles. Thermal post-combustion destroys organic compounds, but in doing so generates acidic combustion products that must subsequently be scrubbed out again.
Added to this is the fact that contaminated soils exhibit very different pollutant profiles. A soil with a high mercury load places different demands on exhaust gas purification than a soil with a high PAH content. Only a multi-stage rotary kiln exhaust gas purification system that can be flexibly tailored to the specific waste characteristics guarantees reliable compliance with all statutory limit values. This is also the reason why modern thermal desorption facilities standardly combine four or more purification stages.
How ARE Deutzen GmbH helps with flue gas purification of contaminated soils
ARE Deutzen GmbH has been operating a thermal soil treatment facility since 1994, featuring a rotary kiln and a multi-stage exhaust gas purification system specifically designed for the requirements of complexly contaminated soils and mineral waste. Decades of experience with more than 2.5 million tonnes of treated materials form the basis for reliable, legally compliant disposal.
The range of services specifically includes:
- Thermal desorption at temperatures of up to approximately 600 °C, depending on the material and pollutant load, under oxygen exclusion for the treatment of soils contaminated with Hg, TPH, BTEX, PAH, PCB, PFAS/PFOS, PCDD/PCDF, and other pollutants
- Multi-stage flue gas purification with dedusting via fabric filter, thermal post-combustion, flue gas scrubbing, and activated carbon adsorption
- Continuous emissions monitoring to ensure compliance with limit values under the 17. BImSchV
- Complete material flow management for all resulting residual materials, including filter ashes and scrubber sludges
- Certified waste management company under § 52 KrWG, as well as certifications to ISO 9001, 14001, 50001, and 18001
If you need to safely and verifiably dispose of contaminated soils or mineral waste with complex pollutant profiles, contact ARE Deutzen directly. All information on acceptance conditions and contact persons can be found on the contact page.