Emissions of NOx can be either abated or controlled by primary measures or flue gas treatment technologies. Primary measures for NOx control may be divided into the following categories:
burner optimisation for NOx control (excess air control, burner fine tuning)
air staging (overfire air or two-stage combustion)
flue gas recirculation
fuel staging (burner out of service, fuel biasing, reburning or three stage combustion)
low NOx burners
Burner optimisation techniques and low NOx burners are used to minimise the formation of NOx during combustion. In air staging and flue gas recirculation technologies, the aim is to reduce the oxygen availability to help reduce NOx formation and complete the combustion. In fuel staging, previously formed NOx is reduced to nitrogen and oxygen within the furnace (Soud and Fukasawa, 1996).
Primary measures for NOx control (burner optimisation, low NOx burners and overfire air) are now considered integral parts of a newly built power plant and existing units retrofit them whenever they are required to reduce their NOx emissions. SCR is accepted throughout the world as the proven commercial option to achieve high NOx removal. However, new developments in primary measures, such as application of the reburning process at large scale, advanced reburning as well as post-combustion flue gas treatment such as SNCR and combined SO2/NOx control systems should offer alternatives to SCR in the future.