2.2.1. Low-NOx burners (LNB) Low-NOx burner is a technology allowing a controlled mixing of fuel and air, resulting in a
cooler flame and consequently less thermal NOx formation. It is designed as a stage
combustion with either staged air or staged fuel. It is applicable to tangential and wall-fired
boilers of various sizes and heaters. It reduces emissions from 40 to 60%.
The basic principle of Low-NOx burner is the separated injection of air and fuel
in the furnace
resulting in the destruction of NOx in the flame (fuel-rich combustion zones) and the peak
flame temperature suppression. Moreover the better air flow distribution allows fuel ignition
and flame stability.
2.2.2. Staged combustion air (SCA) Staged combustion air allows the reduction of fuel NOx by suppressing the amount of air
below that required for complete combustion. It is achieved by injecting a portion of the total
combustion air downstream of the fuel-rich primary combustion zone.
The SCA can be accomplished by several means such as burners out of service (BOOS),
biased firing or overfire air (OFA), depending on the type of boiler. The SCA technique is
highly effective on high nitrogen fuels such as residual oil. It reduces NOx emissions by 20 to
50%.
2.2.3. Flue gas recirculation (FGR) Flue gas recirculation consists in the rerouting of a portion of flue gases from the stack back
to the furnace. Thus, furnace temperature and oxygen concentration are reduced and so is
thermal NOx formation. Large modifications to the burner and windbox in old boilers are
expensive so this technique is better for new boilers.
2.2.4. Water or steam injection (WI/SI) Water or steam injection in the flame reduces thermal NOx formation by lowering the peak
temperature of the flame. This technique has a relatively low initial cost so it is considered as
quite efficient for smaller boilers. However this technique can lead to thermal losses and
increase in CO emissions.
