Definition
Selective Non-Catalytic Reduction
SNCR injects ammonia or urea into a furnace temperature window to reduce NOx without catalyst. It is simpler than SCR but usually less efficient.
- Subject
- SCR and SNCR
- Also known as
- SNCR, SNCR system
Selective Non-Catalytic Reduction (SNCR) is a NOx control method that injects ammonia or urea into hot furnace gas without using a catalyst. In the right temperature window, the reagent reacts selectively with NOx to form nitrogen and water.
SNCR is mechanically simpler than SCR because it avoids a catalyst reactor, but it has a narrower operating window and usually lower NOx reduction. It is used on boilers, cement kilns, waste-to-energy units, biomass plants, and other combustion systems where a moderate reduction is acceptable or where SCR is not practical.
Temperature window
The reaction needs gas hot enough for the chemistry to proceed but not so hot that reagent oxidises to more NOx. A common practical window is roughly 850 to 1100 deg C, although the exact range depends on reagent, residence time, mixing, oxygen, fuel chemistry, and furnace design.
If reagent is injected too cold, ammonia slip rises and ammonium salts may form downstream. If injected too hot, NOx reduction falls and reagent can increase NOx. Load swings move the temperature window, so fixed injection nozzles may perform well at one load and poorly at another.
Design and operating issues
An SNCR system includes reagent storage, dilution or preparation, pumps, lances or nozzles, injection zones, control logic, and monitoring. Good atomisation and mixing are essential because the reaction time is short. Multiple injection levels are often used to follow load and temperature changes.
Failure modes include plugged nozzles, poor atomisation, wall wetting, ammonia slip, ammonium bisulphate formation, corrosion, local reducing conditions, and unstable NOx control. Operators track inlet and outlet NOx, reagent flow, ammonia slip, furnace temperature, CO, load, and downstream deposits.
Relationship to acoustic cleaning
SNCR itself does not create a catalyst pluggage problem because there is no catalyst. Its relevance to acoustic cleaning is downstream. Excess ammonia slip can react with sulphur species to form sticky ammonium bisulphate or ammonium sulphate deposits in air heaters, ducts, ESPs, and baghouses. Sonic horns may help manage dry deposits, but sticky ammonium salts require temperature, reagent, and mixing control.
Operating variables
SNCR depends on a narrow temperature window, reagent droplet size, injection location, mixing, residence time, oxygen level, and furnace load. If the gas is too cool, ammonia slip rises. If it is too hot, reagent can oxidise back to NOx. Load changes move the useful window, so multi-level injection, lance retraction, or adaptive controls may be needed on boilers and kilns with wide operating ranges.
Common failure modes include plugged nozzles, poor atomising air, wall impingement, uneven distribution, high ammonia slip, ammonium-salt fouling downstream, and corrosion where deposits become sticky. Maintenance checks include reagent storage, pumps, strainers, injection lances, dilution water, air supply, flow meters, and the link between CEMS readings and injection control. Acoustic cleaning does not reduce NOx directly, but it can be relevant downstream if SNCR increases ammonium bisulphate or sticky particulate that fouls air heaters, ducts, catalysts, or filters.
Measurement and compliance context
SNCR performance is measured through stack NOx, ammonia slip where monitored, reagent consumption, carbon monoxide, oxygen, load, and furnace temperature indicators. A short test at one load is not enough for a unit that cycles. The useful operating map shows which injection levels work at low, normal, and high load, and where the reagent starts to create slip or downstream deposits.
Regulatory compliance can make SNCR tuning a tradeoff. More reagent may reduce NOx but increase ammonia emissions, visible plume risk, ammonium-salt fouling, or reagent cost. Maintenance and environmental teams should therefore review CEMS data together with air-heater pressure drop, baghouse differential pressure, and ash chemistry. If acoustic cleaning is added downstream, the target is deposit management caused by the side effects of reagent use, not the NOx reaction itself.
Seasonal fuel and load changes should be reviewed because they move the temperature window and can turn a stable SNCR setting into a source of slip or fouling.
Related terms
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Related terms
5 terms
- Selective Catalytic ReductionSCR reduces NOx by injecting ammonia upstream of a catalyst. Temperature, mixing, catalyst condition and ash control determine performance.
- DeNOxDeNOx is the collective term for post-combustion NOx-reduction technologies. SCR and SNCR are the dominant options; both rely on reaction of NOx with ammonia or urea.
- Urea SNCR / aqueous-ammonia SNCRUrea and aqueous ammonia are the main SNCR reagents for NOx reduction, with different handling, injection, by-product and ammonia-slip implications.
- Ammonia slipAmmonia slip is unreacted ammonia leaving the DeNOx system in the flue gas. It is regulated, expensive in lost reagent, and causes ammonium-bisulphate fouling downstream.
- Ammonium bisulphateAmmonium bisulphate is a sticky low-melting deposit formed when slipped ammonia reacts with SO3 in cooling flue gas. The dominant cold-end fouling species on SCR-equipped boilers.
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