After pre-dedusting, a bag filter is used because pre-dedusting alone only removes coarse particles from the exhaust gas stream, while fine dusts and pollutant particles pass through largely unimpeded. The bag filter, as the second cleaning stage, handles the separation of these fine particles and thus ensures that statutory emission limit values are reliably met. The following sections explain how both stages work together and why neither stage can be dispensed with, particularly in thermal soil treatment.
How does a bag filter work in exhaust gas cleaning?
A bag filter is a textile filter system that directs exhaust gases through tubular filter media made of woven fibers. The finest particles deposit on the filter surface and form a filter cake layer that actually improves separation performance over time. The cleaned gas stream exits the filter on the clean side, while the separated dust is periodically cleaned off and collected.
Technically, the principle operates through several mechanisms simultaneously: inertial separation, interception, and diffusion separation act in parallel, so that even ultrafine particles in the sub-micrometer range are reliably retained. Cleaning of the filter cake is typically carried out by compressed air pulses, known as jet-pulse systems, or by mechanical shaking. The dust then falls into a collection container and is disposed of separately as filter dust.
In the exhaust gas cleaning of industrial plants, the bag filter is regarded as one of the most efficient dust removal technologies. Separation efficiencies of over 99 percent are achievable with correct design. This makes it particularly suitable for applications where residual dust must comply with strict limit values.
What does pre-dedusting achieve on its own – and where are its limits?
Pre-dedusting, typically implemented as a cyclone or multi-channel separator, separates coarse particles with a diameter of approximately ten micrometers and above from the exhaust gas stream. It operates without filter media, relying solely on centrifugal force, and is therefore low-maintenance and robust. It is well suited for the pre-separation of large quantities of dust, but clearly reaches its limits with fine dusts.
Particles below five micrometers are only inadequately captured by cyclones. Yet these fine particles are particularly critical for two reasons: firstly, they are respirable and therefore relevant to health. Secondly, they frequently carry a disproportionately high share of pollutants, as volatile organic compounds and heavy metal compounds preferentially accumulate on small particle surfaces.
Pre-dedusting protects the downstream bag filter from overloading and significantly extends its service life. However, when operated alone, it falls far short of meeting the legally prescribed emission limit values for total dust.
Why is single-stage dust removal insufficient for thermal desorption?
During thermal desorption in a rotary kiln, exhaust gases with a particularly heterogeneous particle spectrum are generated. In addition to coarse mineral particles from the treated soil, the exhaust gas contains fine dusts, condensation products, and pollutant particles that only form as the gas stream cools. Single-stage dust removal cannot fully capture this broad spectrum.
Furthermore, during the thermal treatment of contaminated soils, pollutants such as PAHs, PCBs, or heavy metal compounds are transferred into the gaseous state and condense back onto particle surfaces as the exhaust gas cools. These newly formed pollutant-laden fine particles only arise after the rotary kiln and cannot by their nature be captured by upstream pre-dedusting. The bag filter as a downstream stage is therefore indispensable.
Only the interaction of both stages ensures that both coarse mineral particles and pollutant-laden fine dusts are reliably removed from the exhaust gas stream before it enters the further cleaning chain.
Which pollutants does the bag filter retain after pre-dedusting?
After pre-dedusting, the bag filter primarily retains particle-bound pollutants. These include heavy metal compounds such as mercury compounds, lead, and cadmium, which accumulate on fine particles, as well as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) in particulate form. Dioxins and furans (PCDD/PCDF) are also partially separated in particle-bound form.
In addition, sorbents such as activated coke or hydrated lime are frequently mixed into the exhaust gas stream upstream of the bag filter. These react with gaseous pollutants and bind them to particles, which are then separated together with the filter dust. In this way, the spectrum of retained substances is considerably extended:
- Acidic pollutant gases such as hydrochloric acid (HCl) and sulfur dioxide (SO2) through the addition of hydrated lime
- Organic trace substances and residual quantities of volatile compounds through activated coke
- Mercury vapors through specific sorbents
- The finest dusts of all fractions through the filter fabric itself
The bag filter is therefore not merely a pure dust removal unit, but when operated accordingly, a combined separation and sorption system.
How is the bag filter operated during thermal soil treatment?
During thermal soil treatment, the bag filter is operated within a defined temperature window that on the one hand prevents condensation of moisture in the filter fabric and on the other hand optimizes the sorption effect of the mixed-in additives. Typical operating temperatures range between 120 and 200 degrees Celsius, depending on the dew point of the exhaust gas and the sorbents used.
Operation requires continuous monitoring of several parameters:
- Differential pressure across the filter as an indicator of the loading level of the filter cake
- Exhaust gas temperature to avoid falling below the dew point
- Volume flow and dust loading of the raw gas
- Residual dust on the clean side as a measure of separation performance
Cleaning of the filter bags is carried out cyclically and with compressed air support, so that the filter cake is removed in a controlled manner without completely interrupting the filtration process. The filter dust generated is classified as hazardous waste and is sent for proper disposal separately from the treated material. Material flow management for these residual materials is an integral part of the overall process.
Which statutory requirements make the bag filter mandatory?
In Germany, the Ordinance on the Incineration and Co-incineration of Waste (17th BImSchV) governs the emission requirements for plants that thermally treat waste. It prescribes a daily mean value of 10 mg/m³ and a half-hourly mean value of 30 mg/m³ for total dust. These limit values are technically unachievable with pre-dedusting alone.
In addition, the 17th BImSchV sets strict limit values for a range of individual pollutants, including:
- Heavy metals such as cadmium, thallium, mercury, and lead
- Organic compounds, expressed as total carbon
- Dioxins and furans (PCDD/PCDF) in nanogram ranges
- Acidic pollutant gases such as HCl, HF, and SO2
Operating a plant under the Federal Immission Control Act (BImSchG) requires that emission limit values are permanently maintained and demonstrated through continuous measurement. In practice, approval of a thermal treatment plant for contaminated soils cannot be obtained without multi-stage exhaust gas cleaning in which the bag filter plays a central role. The quality management of such a plant must systematically document and ensure compliance with these requirements.
How ARE Deutzen GmbH implements safe exhaust gas cleaning in thermal soil treatment
ARE Deutzen GmbH has been operating a thermal soil treatment plant with multi-stage exhaust gas cleaning at its Deutzen site in western Saxony for more than 30 years, in which pre-dedusting and bag filter work together as coordinated cleaning stages. The plant is approved under the Federal Immission Control Act and certified to ISO 9001, 14001, and 50001.
For customers from civil and structural engineering as well as industrial companies remediating contaminated sites, this means in concrete terms:
- Thermal treatment of soils with complex contaminations such as PAHs, PCBs, PFAS, dioxins, and heavy metals in a rotary kiln at up to approximately 600 degrees Celsius, depending on the material and contaminant load
- Multi-stage exhaust gas cleaning with pre-dedusting, bag filter, thermal post-combustion, flue gas scrubbing, and activated carbon filter
- Proper disposal of all resulting residual materials including filter dust
- Comprehensive documentation and emission monitoring to meet regulatory requirements
- Over 2.5 million tonnes of treated mineral materials as evidence of many years of operational experience
If you are planning a remediation project or wish to safely dispose of contaminated materials, contact the experts at ARE Deutzen directly. All information on thermal treatment as well as contact details can be found on the website. The team is available for expert initial consultation and will accompany you from waste characterization through to completed disposal. Get in touch now and develop the right solution together.