Selective Non-Catalytic Reduction Techniques for Nitrogen Oxide Control

Selective non-catalytic reduction (SNCR) is a post-combustion process for reducing nitrogen oxides (NOx) emissions by injecting a chemical reductant—commonly ammonia, urea or hydrocarbon fuels—directly into the flue gas. At high temperatures (typically 850–1100 °C), the reductant reacts with NOx to produce molecular nitrogen and water, offering a catalyst-free alternative to selective catalytic reduction. Its principal advantages are lower capital cost and simplified operation, yet its effectiveness is constrained by a narrow temperature window, potential ammonia slip and the formation of by-products such as nitrous oxide. Recent advances address these limitations through optimised reagent blends, additive-enhanced mechanisms and adapted injection strategies. SNCR is applied across power generation, industrial boilers, marine engines and the ceramics sector, where it contributes to compliance with increasingly stringent air-quality regulations and supports global efforts to mitigate acid rain, ozone formation and climate forcing from NOx pollutants.

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Selective Non-Catalytic Reduction Techniques for Nitrogen Oxide Control publication trend

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Technical terms

NOx: Collective term for nitrogen monoxide (NO) and nitrogen dioxide (NO2), key contributors to smog and acid deposition.

Selective non-catalytic reduction (SNCR): A flue-gas treatment process that injects reductants into high-temperature gases to convert NOx to N2 and H2O without a catalyst.

Reductant: A chemical agent (e.g. ammonia, urea, hydrocarbons) that reacts with NOx to achieve denitrification.

Temperature window: The narrow range of gas temperatures in which SNCR achieves effective NOx removal without generating excessive by-products.

Ammonia slip: The unintended emission of unreacted ammonia following reductant injection, which can lead to secondary pollution.

References

1. Reaction Characteristics of NOx and N2O in Selective Non-Catalytic Reduction Using Various Reducing Agents and Additives. Atmosphere (2021).
2. Experiment and simulation study of the effect of ethanol and compound additives on the urea-based selective non-catalytic reduction process under moderate temperature conditions. Royal Society Open Science (2018).
3. Application of SNCR denitration technology in circulating fluidized bed boiler. IOP Conference Series Earth and Environmental Science (2021).