Flue Gas Recirculation

The use of a varied percentage of the flue gases to reduce the burner flame temperature, injecting them into the secondary air, is a “dilution” technology whose scope is to obtain a “softer” and “colder” flame by decreasing the excess air.
By reducing the concentration of oxygen, the flame temperature is reduced and so is the formation of NOx originating from heat.
Another effect of flue gas recirculation is to minimize the CO levels becouse of its re-introduction into the combustion cycle.
Naturally, precautions have to be taken to ensure flame stability as well as for the presence of particles inside the burner that need to be filtered

The technique is typically used in U-shaped radiant tubes, as  in the figure, which shows the  ESA patent for the configuration with :
Radiant Tube burner, SIC PLUG IN recuperator,  FGR recirculation valve and valve for eliminating CO at minimum.