Flue gas recirculation

Flue gas recirculation is one of the most effective techniques to directly limit the formation of nitrogen oxides (NO_x) during the combustion process. By recirculating part of the flue gases back into the combustion zone, flame temperature and oxygen partial pressure are reduced, resulting in lower thermal NO_x formation.

The Callens Group designs and integrates both internal and external flue gas recirculation systems for industrial heat installations. Based on an analysis of your combustion process, our engineers determine which configuration will achieve the required emission reduction without compromising on the efficiency or operational reliability of your installation.

Discuss your flue gas recirculation project

When is flue gas recirculation worth considering?

Flue gas recirculation is especially worth considering when you want to limit NO_x formation at source. Because the technology acts on the combustion process directly, significant emission reductions are achieved without additional flue gas aftertreatment being required.

Our engineers assess how flue gas recirculation can best be integrated into your installation. To do so, we consider the required emission values, boiler type, fuel, flue gas temperatures, and available space.

Why choose flue gas recirculation?

To limit thermal NO_x formation at the source.
To meet emission standards without complex flue gas aftertreatment.
To optimize existing boiler installations.
To prepare new heat installations for stricter emission requirements.
To combine NO_x reduction with high boiler efficiency.

Which type of flue gas recirculation is best?

Internal and external flue gas recirculation serve the same purpose: limiting thermal NO_x formation at source by lowering the flame temperature. Even so, the two techniques differ in their technical design and field of application. The optimal choice will, among other factors, depend on your burner type, boiler configuration, required emission reduction, available space, and the options to modify the existing installation.

That is why the Callens Group always starts with a thorough analysis of your entire combustion process. Our engineers not only examine flue gas flows, but also map out fuel properties, air ratios, temperature profiles, heat balance, and your installation’s operating conditions. Based on that assessment, we determine whether internal or external flue gas recirculation offers the greatest added value, or whether a combination with other NO_x reduction techniques is recommended.

Our analysis is based on questions such as:

How does NO_x formation occur within your current combustion process?
Which emission values must the installation meet today and in the future?
Is the existing burner or boiler suitable for internal or external flue gas recirculation?
How does flue gas recirculation affect efficiency, capacity, and combustion stability?
Which modifications are needed to achieve the required emission reduction?
Is flue gas recirculation sufficient, or is a combination with SCR, SNCR, or an adapted boiler design recommended?

Only once all these parameters have been evaluated do we develop a solution that combines the required NO_x reduction with stable combustion, high installation efficiency, and maximum operational reliability.

Your challenge	Most suitable solution
New boiler with integrated NO_X reduction	Internal flue gas recirculation
Retrofitting an exisiting installation	External flue gas recirculation
Further reducing NO_X without SCR/SNCR	External flue gas recirculation
Compact integration	Internal flue gas recirculation
Large industrial capacity	External flue gas recirculation
Wondering about the best configuration	Analysis by the Callens Group

Internal or external flue gas recirculation?

Internal flue gas recirculation	External flue gas recirculation
Recirculation in the furnace	Separate recirculation loop
Mainly used in new installations	Suitable for retrofits and new-build projects
Compact solution	Greater flexibility
Limited installation impact	Greater NO_x reduction possible
Fewer components	Suitable for higher-capacity installations

BENEFITS

Benefits of
flue gas recirculation

Flue gas recirculation limits the formation of nitrogen oxides without requiring aftertreatment of the entire flue gas stream. As a result, this technology is often a cost-effective first step toward lower emissions.

Less NOx
at source

Thermal NO_x formation is limited during combustion itself.

Maintains high boiler efficiency

Emission reduction is achieved without losing sight of the installation’s energy efficiency.

Useful for retrofit projects

Existing installations can often be modified without full replacement.

Lower
investment cost

In many applications, flue gas recirculation offers an economically attractive alternative to SCR or SNCR.

Fully
customized

Every configuration is tailored to your installation, fuel, and emission targets.

Ready for stricter emission standards

Your installation is prepared for future environmental permits and emission requirements.

“Maximizing energy efficiency does not start with choosing a technology. It starts with understanding your specific environment and goals.”

FAQ

Questions?
We have the answers.

Flue gas recirculation returns part of the flue gases to the combustion zone. This lowers flame temperature and oxygen partial pressure, reducing thermal NO_x formation during the combustion process.

With internal flue gas recirculation, the system is integrated into the burner or boiler. External flue gas recirculation uses a separate recirculation line and fan. The most suitable solution will depend on the installation and the required emission reduction.

Yes. The Callens Group supports projects from analysis and engineering through delivery, installation, start-up, and maintenance. This gives you one partner throughout the entire process and ensures a solution that aligns perfectly with your production process.

You can find more information about the different steps in our workflow here.

External flue gas recirculation in particular is highly suitable for adapting existing boiler installations to stricter emission standards without a full replacement being required.

Yes. Flue gas recirculation is often the first step in a broader NO_x reduction strategy. Depending on the desired emission values, the technique can be combined with an adapted boiler design or flue gas aftertreatment using SCR or SNCR. The Callens Group analyzes which combination achieves the required emission reduction with the lowest investment and operating costs.

Curious which combination best suits your installation? Our experts are happy to think along with you.

Do you have a project inquiry?

Tell us about your challenges. We turn them into
practical advice.

Select your language