What is a SNCR system and when is it used?

• A. Selective Catalytic Reduction uses a catalyst to reduce NOx in the presence of ammonia.
• B. A filtration system that removes NOx.
• C. A process that uses water spray to wash NOx from flue gas.
• D. Selective Non-Catalytic Reduction uses ammonia or urea injected into the furnace at specific temperatures to reduce NOx without catalysts.

Answer: (D)

Selective Non-Catalytic Reduction is a NOx control method that lowers nitrogen oxides in flue gas by injecting ammonia or urea directly into the furnace at a specific high-temperature range, without using a catalyst. In that temperature window, gas-phase reactions convert NOx into nitrogen and water, providing NOx reduction without the need for catalyst hardware. This approach is used when a simpler, lower-cost option is preferred or when catalysts are impractical or undesirable. The effectiveness depends on keeping the gas within the right temperature band and proper mixing; outside the window, the reactions slow and ammonia slip can increase, reducing efficiency and potentially causing other emissions issues. Compared with methods that use a catalyst, SNCR generally offers lower NOx removal but avoids catalyst-related maintenance and costs. The described option matches SNCR, whereas descriptions of catalytic reduction, filtration, or water washing do not fit SNCR.