In selective noncatalytic reduction a NOx reduction agent such as ammonia or urea is injected into the boiler exhaust gases at a temperature of approximately 1400-1650°F. The ammonia or urea breaks down the NOx in the exhaust gases into water and atmospheric nitrogen, plus CO2 if urea is injected. This reaction takes place in a narrow range of temperatures; as shown in Fig. 4.4, ammonia is formed below a certain temperature, and above this temperature the NOx level increases. SNCR reduces NOx by about 70%. The SNCR method is used in large industrial and utility boilers, which have adequate residence times for the reduction reactions. In packaged boilers it is difficult to apply this method because the ammonia or urea must be injected into the flue gases at a specific flue gas temperature; however, the gas temperature profile varies with load, excess air, and fuel fired as shown in Fig. 4.5 And residence times in oil — and gas-fired packaged boilers are generally very small.

Typical reactions, that take place with ammonia injection are:

NO + NH3 + (1/4)O2—!N2 + (3/2)H2O

NH3 + (5/4)O2 ! NO + (3/2)H2O

Both oxidation and reduction take place. Ammonia oxidizes to form NO. Because reduction and oxidation reactions are temperature-sensitive, there is a narrow range of temperatures in which the conversions are efficient. An increase in ammonia increases the efficiency of conversion; however, excessive ammonia can slip through the reactions and cause plugging of components downstream. SNCR has a low cost of operation and may be used in conjunction with other methods such as a low-NOx burner to improve the efficiency of NOx reduction.

In large field-erected boilers, wall injectors are located at several locations to inject the ammonia or urea using specially designed lances. This method is not used in HRSGs because it is difficult to find such a temperature window and also have a suitable residence time.

Benefits of SNCR include

• Medium to high NOx reduction.

• No by-products for disposal—minimizes waste management concerns,

• Easy to retrofit—little downtime required.

• Minimum space required.

• Can be used along with other NOx reduction methods.

• Low energy consumption. Additional gas pressure drop of flue gases is zero, unlike in SCR method, where the catalyst could add about 3 to 4 in. WC to the gas pressure drop, adding to the operating cost.