Flue Gas Recirculation for NOx Reduction
General Remarks
How does it work? FGR lowers NOx in two ways:
How is it done?
Figure 1: FGR by Air Vitiation
Flue gas recirculation (FGR) can be a highly effective technique for lowering NOx emissions from burners, and it’s relatively inexpensive to apply. Most of the early FGR work was done on boilers, and investigators found that recirculating up to 25% of the flue gases through the burner could lower NOx emissions to as little as 25% of their normal levels. Word of this success has spread, and now operators of industrial processes are interested in learning if FGR can do the same for them.
1.) The cooled, relatively inert, recirculated flue gases act as a heat sink, from the flame and lowering peak flame temperatures.
2.) When mixed with the combustion air, recirculated flue gases lower the average oxygen content of the air, starving the NOx-forming reaction for one of the ingredients they need.
The simplest way is to use the combustion air blower to suck the flue gases out of the stack and blend them with fresh air before they go into the burner (Figure 1). This technique is also known as air vitiation, and it has been applied to many types of burners without any other modifications. The second way is to use a separate blower to pull the flue gases from the stack and push them through some sort of manifold or bustle ring into the flame. This normally requires factory-modified burners. Both methods seem to be equally effective, although certain burner designs may respond better to one method than the other. Air vitiation has a number of practical advantages, so the rest of this article will deal with that method.