What reagents are used in SNCR to reduce NOx?

• A. Sodium hydroxide.
• B. Sulfuric acid.
• C. Water.
• D. Ammonia or Urea.

Answer: (D)

SNCR relies on a reducing agent to convert NOx into harmless nitrogen and water in the hot flue gas. The practical reducing agents are ammonia or urea. When ammonia is injected into the furnace gas within the right temperature window, it reacts with NOx to form N2 and H2O (a commonly cited simplified reaction is 4NO + 4NH3 + O2 → 4N2 + 6H2O). Urea works similarly, as it decomposes to release ammonia, providing the same reducing species. The other substances listed don’t serve as NOx-reducing agents in this process: sodium hydroxide is used for scrubbing acids, sulfuric acid is itself an acid and not a reducing agent for NOx, and water is not a reducing agent and won’t drive NOx reduction on its own. So the correct reagents are ammonia or urea.