SNCR

SNCR, or selective non-catalytic reduction, is a NOx abatement technology used in combustion systems. It reduces nitrogen oxides in flue gas by injecting a reducing agent, typically ammonia or urea, at high temperature to promote gas-phase reactions that convert NOx to nitrogen and water without a catalyst.

Operation and chemistry: The reducing agent is introduced into hot combustion gases within a defined temperature window, generally around 850–1100 °C (1560–2010 °F). Within the optimal range, ammonia or its derivatives react with NOx to form N2 and H2O, while side reactions can produce ammonia slip if dosing exceeds the conversion capacity. Because SNCR does not use a catalyst, it relies on temperature, residence time, and mixing quality to achieve selective reduction.

Reagents and dosing: Common agents are aqueous or anhydrous ammonia or urea solutions. Dosing is adjusted based on NOx levels, local temperatures, and gas mixing to maximize NOx reduction while minimizing ammonia slip. Poor mixing, high oxygen content, or temperatures outside the optimal window reduce effectiveness.

Applications and performance: SNCR is used in retrofits for utility and industrial boilers, and in some waste-to-energy plants, where catalyst-based systems are impractical due to cost, space, or maintenance considerations. Typical NOx reductions from SNCR alone range from roughly 20% to 60%, depending on temperature control, residence time, and mixing efficiency. It is often considered as part of a multi-stage NOx control strategy.

Limitations and concerns: The method has a narrow and sensitive temperature window and can produce ammonia slip and minor secondary emissions. Its effectiveness is limited compared with catalytic methods like SCR, particularly for stringent NOx targets. Proper integration requires careful control of reagent dosing, gas temperature, and mixing to avoid fouling and corrosion from reaction products.