Flue gas is rapidly cooled after post-combustion to prevent the reformation of dioxins and furans. These highly toxic compounds form within a specific temperature range of approximately 200 °C to 400 °C when the exhaust gas cools too slowly. The following sections explain the technical background, the legal requirements, and the further purification steps that follow cooling.
What happens in the flue gas after post-combustion?
After thermal post-combustion, the flue gas still contains residual pollutants in gaseous form as well as combustion products such as nitrogen oxides, sulfur dioxide, hydrochloric acid, and particulate matter. The organic pollutants are largely destroyed at the high temperatures of post-combustion; however, the gas in this state is not yet safe for emission. It must be cooled in a controlled manner and then cleaned in multiple stages.
During thermal soil treatment, pollutants are transferred from the contaminated material into a gaseous state. Post-combustion ensures that these gaseous pollutants are completely oxidized at temperatures above 850 °C. What remains afterward is a hot gas mixture that must not be released into the atmosphere without further treatment. The cooling step is not merely a technical necessity but a critical safety mechanism.
Why is the cooling phase so decisive for pollutant prevention?
The cooling phase is decisive because dioxins and furans (PCDD/PCDF) can preferentially reform in the temperature range between 200 °C and 400 °C. This process is known as de novo synthesis. If the flue gas cools too slowly within this range, chlorine, carbon, and oxygen in the exhaust gas can form new dioxin compounds in the presence of catalysts such as heavy metal dust.
The goal of rapid cooling is to pass through this temperature window as quickly as possible so that chemical reactions have no time to initiate. The residence time in the critical range must be reduced to a minimum. If this is achieved, the dioxin load in the exhaust gas remains at a very low level, well below the legally prescribed limit values.
How does rapid cooling work technically?
Rapid cooling, also known in technical jargon as quenching, is achieved by the targeted injection of water or air into the flue gas stream immediately after post-combustion. The goal is to reduce the gas temperature from above 850 °C to below 200 °C within a matter of seconds. The injected water evaporates, extracts heat from the gas, and cools it abruptly.
Wet quenching with water injection
In wet quenching, water is sprayed directly into the flue gas stream. The enthalpy of vaporization of the water extracts large amounts of heat from the gas in a short time. This method is particularly effective and is used in many industrial flue gas treatment plants. The resulting water vapor is condensed and separated in the subsequent purification process.
Dry and combined processes
Alternatively or in addition, cold air can be injected to increase the cooling rate. In some plants, heat exchangers are also used to make use of the waste heat from the flue gas before fine purification begins. In every case, what is decisive is that the cooling rate is high enough to safely fall below the critical temperature window.
What legal requirements govern flue gas cooling?
Flue gas cooling in combustion and desorption plants is regulated by the 17th Federal Immission Control Ordinance (17. BImSchV). This ordinance establishes, among other things, limit values for dioxins and furans in the exhaust gas and indirectly prescribes process management requirements to prevent the reformation of these compounds.
Specifically, the ordinance stipulates that exhaust gases from post-combustion must be cooled in a controlled manner and as quickly as technically possible after the last addition of heat. The limit value for PCDD/PCDF in the exhaust gas is 0.1 ng/m³ (toxicity equivalent). Plants that wish to comply with this value depend on a functioning rapid cooling system. Quality management in accordance with ISO 14001 and the permits issued under the Federal Immission Control Act ensure that these requirements are permanently met.
What are the consequences of cooling that is too slow?
Cooling the flue gas too slowly leads to the reformation of dioxins and furans in the exhaust gas. The result is an increased pollutant concentration in the exhaust gas that can exceed legal limit values. Associated with this are permitting risks, operational interruptions, and in the worst case a threat to the environment and local residents.
In addition, other chlorinated hydrocarbon compounds can also reform or accumulate if cooling is too slow. De novo synthesis is not a theoretical risk but a phenomenon documented in practice. Plants without a functioning rapid cooling system are therefore not eligible for approval. The consequences range from regulatory conditions imposed by authorities to the shutdown of the plant.
How is further purification carried out after cooling?
After rapid cooling, the flue gas passes through several further purification stages to remove all remaining pollutants. The exhaust gas purification system is built up in multiple stages and typically includes dust removal, flue gas scrubbing, and activated carbon filters. Only after these stages is the purified gas released into the atmosphere via the stack.
- Dust removal: Fabric filters or electrostatic precipitators separate fine dust and particles from the gas stream. These dusts can themselves be contaminated with pollutants and must be disposed of separately.
- Flue gas scrubbing: In a wet scrubber, acidic gas components such as hydrochloric acid, sulfur dioxide, and hydrogen fluoride are washed out of the gas stream by contact with an aqueous solution.
- Activated carbon filters: In the final step, activated carbon adsorbs remaining organic trace substances, mercury, and residual dioxin compounds. This step is particularly important for reliably complying with the strict limit values for heavy metals and PCDD/PCDF.
The interaction of these stages ensures that the purified exhaust gas reliably falls below the legal emission limit values. Material flow management also includes the residual materials generated during flue gas purification, which must themselves be subjected to proper disposal.
How ARE Deutzen GmbH implements safe flue gas treatment in practice
ARE Deutzen has been operating a thermal soil treatment plant in western Saxony since 1994, in which all the described process steps for flue gas treatment are consistently implemented. The plant features a multi-stage exhaust gas purification system encompassing rapid cooling, dust removal, flue gas scrubbing, and activated carbon filters. Over more than 30 years, more than 2.5 million tonnes of mineral materials have been treated, including soils contaminated with PCDD/PCDF, PCB, PFAS, mercury, and numerous other pollutants.
The strengths of ARE Deutzen in the field of flue gas treatment at a glance:
- Approved operation under the Federal Immission Control Act with continuous emissions monitoring
- Certified quality and environmental management system in accordance with ISO 9001 and ISO 14001
- Demonstrated compliance with the limit values of the 17. BImSchV, even with complex pollutant mixtures
- Treatment of soils with particularly demanding contaminations such as PCDD/PCDF and PCB
- Certified waste management company (EfB) pursuant to § 52 of the Circular Economy and Waste Act
If you wish to dispose of contaminated soils or mineral waste safely and in compliance with the law, contact ARE Deutzen directly. Via the contact page, you can reach the team for a no-obligation initial consultation on your project.