Selective non-catalytic reduction (SNCR) for NOx control
In SNCR systems, a reagent is injected into the flue gas in the furnace within an appropriate temperature window. Emissions of NOx can be reduced by 30% to 50%. The NOx and reagent (ammonia or urea) react to form nitrogen and water. A typical SNCR system consists of reagent storage, multi-level reagent-injection equipment, and associated control instrumentation. The SNCR reagent storage and handling systems are similar to those for SCR systems. However, because of higher stoichiometric ratios, both ammonia and urea SNCR processes require three or four times as much reagent as SCR systems to achieve similar NOx reductions.
The temperature window for efficient SNCR operation typically occurs between 900°C and 1,100°C depending on the reagent and condition of SNCR operation. When the reaction temperature increases over 1000°C, the NOx removal rate decreases due to thermal decomposition of ammonia. On the other hand, the NOx reduction rate decreases below 1000°C and ammonia slip may increase. The optimum temperature window generally occurs somewhere in the steam generator and convective heat transfer areas. The longer the reagent is in the optimum temperature window, the better the NOx reduction. Residence times in excess of 1 second yield optimum NOx reductions. However, a minimum residence time of 0.3 second is desirable to achieve moderate SNCR effectiveness.
Ammonia slip from SNCR systems occurs either from injection at temperatures too low for effective reaction with NOx or from over-injection of reagent leading to uneven distribution. Controlling ammonia slip in SNCR systems is difficult since there is no opportunity for effective feedback to control reagent injection. The reagent injection system must be able to place the reagent where it is most effective within the boiler because NOx distribution varies within the cross section. An injection system that has too few injection control points or injects a uniform amount of ammonia across the entire section of the boiler will almost certainly lead to a poor distribution ratio and high ammonia slip. Distribution of the reagent can be especially difficult in larger coal-fired boilers because of the long injection distance required to cover the relatively large cross-section of the boiler. Multiple layers of reagent injection as well as individual injection zones in cross-section of each injection level are commonly used to follow the temperature changes caused by boiler load changes. However, it is difficult to make fine adjustments due to the complexity of these injection levels and zones.
A potentially troublesome reaction is unreacted ammonia combining with SO3 to form ammonium bisulphate. Ammonium bisulphate will precipitate at air heater operating temperatures and can ultimately lead to air heater fouling and plugging. Although no SO2 is oxidised by the SNCR system, naturally occurring SO3 concentrations are sometimes high enough (especially from higher sulphur coals) to be a concern with potentially high ammonia slip rates.
An SNCR process can produce nitrous oxide (N2O), which contributes to the greenhouse effect. N2O formation resulting from SNCR depends upon the reagent used, the amount of reagent injected and the injection temperature.
SNCR technologies came into commercial use on oil- or gas-fired power plants in Japan in the middle of the 1970s. In Western Europe, SNCR systems have been used commercially on coal-fired power plants since the end of the 1980s. In the USA, SNCR systems have been used commercially on coal-fired power plants since the early 1990s.
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