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Low NOx burners are designed to control fuel and air mixing at each burner in order to create larger and more branched flames. Peak flame temperature is thereby reduced, and results in less NOx formation. The improved flame structure also reduces the amount of oxygen available in the hottest part of the flame thus improving burner efficiency. Combustion, reduction and burnout are achieved in three stages within a conventional low NOx burner. In the initial stage, combustion occurs in a fuel rich, oxygen deficient zone where the NOx are formed. A reducing atmosphere follows where hydrocarbons are formed which react with the already formed NOx. In the third stage internal air staging completes the combustion but may result in additional NOx formation. This however can be minimised by completing the combustion in an air lean environment.
Low NOx burners can be combined with other primary measures such as overfire air, reburning or flue gas recirculation. Plant experience shows that the combination of low NOx burners with other primary measures is achieving up to 74% NOx removal efficiency. A large number of low NOx burners have been developed and are currently used in over 370 coal-fired units (125 GWe). Nevertheless, developmental work continues to enhance the design, and improve the performance of existing burners and engineer and develop new and advanced low NOx burners.
Click on relevant text for details of other primary measures for NOx control:
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