Name two common NOx control methods used in fossil-fired boilers.

• A. Low-NOx burners with staged combustion and Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR).
• B. Increased boiler pressure and reduced fuel input.
• C. Water injection and increased air temperature.
• D. Thermocouples calibration.

Answer: (A)

Two common NOx control approaches in fossil-fired boilers involve shaping the combustion process and treating the exhaust gas. Low-NOx burners with staged combustion reduce NOx by carefully controlling how fuel and air mix and by keeping flame temperatures and oxygen concentrations lower in the burn zone. This directly limits the chemical pathways that form NOx during combustion.

For exhaust treatment, Selective Catalytic Reduction uses a catalyst and injected reducing agent (like ammonia or urea) to convert NOx into nitrogen and water in the flue gas. Selective Non-Catalytic Reduction achieves a similar conversion without a catalyst, but it relies on a precise high-temperature window and is typically less effective. Plants often use the primary combustion technique together with SCR or SNCR to meet stricter NOx limits.

The other options don’t target NOx formation or removal in a meaningful way: changing boiler pressure or fuel input mainly shifts energy balance, water injection or higher air temperature isn’t a reliable NOx control method, and calibrating thermocouples is about measurement rather than reducing NOx.