Which combination is commonly used as a NOx control approach in fossil-fired boilers?

• A. Low-NOx burners with staged combustion and SCR.
• B. No NOx controls; rely on dilution.
• C. Only low-NOx burners without staged combustion.
• D. Only SNCR without burners.

Answer: (A)

Controlling NOx in fossil-fired boilers relies on reducing NOx formation during combustion and removing what remains in the flue gas. Low-NOx burners are designed to limit peak flame temperatures and improve fuel–air mixing, which lowers NOx formation at the source. Staged combustion adds fuel and/or air in steps, further reducing peak temperatures and the available oxygen in the most reactive zones, producing even less NOx. After combustion, Selective Catalytic Reduction uses a catalyst and a reducing agent (like ammonia or urea) to convert NOx into nitrogen and water, providing a strong post-combustion reduction across a wide range of conditions.

The combination of these approaches—primary measures (low-NOx burners with staged combustion) plus a secondary, post-combustion reduction (SCR)—offers the most effective and reliable NOx control for fossil-fired boilers. Relying on dilution by itself won’t reduce NOx formation. Using only low-NOx burners without staging misses significant reductions achievable from temperature and mixing management. Relying on SNCR alone (without burners) is limited by a narrow and difficult-to-control temperature window and is generally less effective than SCR, especially for stringent NOx targets.