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Technologies for control of sulfur and nitrogen compounds and particulates 147

• The reagent storage unit, including storing, cooling, and evaporation of the reagent.
• The SNCR unit itself, where the injection of the reagent and the reaction of NO to N and
water take place.
Using ammonia as a reagent, the following chemical reactions take place. At the
lower temperature, both reactions are too slow. At the higher temperature, the un-
wanted by-reaction dominates with an increase in NO x emissions.
The main reaction is

4NO þ 4NH 3 þ O 2 /4N 2 þ 6H 2 O ðreductionÞ

The unwanted by-reaction is

4NH 3 þ 5O 2 /4NO þ 6H 2 O ðoxidationÞ

This technique does not require high capital investments and shows lower operating
cost than other methods. Also, it is applicable in existing plants and to all types of
stationary-fired equipment, and there is no need for the use of catalysts. In most
full-scale applications, the reagents of choice are either NH 3 or urea. In certain labo-
ratory and pilot-scale experiments, cyanuric acid has also been tested as a potential re-
agent (Tayyeb et al., 2007).
Selective catalytic reduction (SCR). SCR technology needs to select an appropriate
catalyst to make a reaction between reagents with NO x on its surface (Fig. 6.3). The
main advantage of this technique is that the temperature required is lower than in
SNCR technology reactions. The most typical SCR catalyst used is a blend of titanium
oxide and vanadium oxide. Noble metal blends of platinum and palladium have been
developed during recent years, but they are not so widely used due to their high cost.
Typical reagents are urea and ammonia (Bell and Buckingham, 2010).
The reducing agent is injected into the flue-gas upstream of the catalyst. NO x con-
version takes place on the catalyst surface at a temperature usually between 170 and

510 C by one of the following main reactions:
With ammonia as a reduction agent:

4NO þ 4NH 3 þ O 2 /4N 2 þ 6H 2 O

6NO 2 þ 8NH 3 /7N 2 þ 12H 2 O

With urea as a reduction agent:

4NO þ 2ðNH 2 Þ CO þ 2H 2 O þ O 2 /4N 2 þ 6H 2 O þ 2CO 2
2
6NO 2 þ 4ðNH 2 Þ CO þ 4H 2 O/7N 2 þ 12H 2 O þ 4CO 2
2
Base metal oxide SCR catalysts operating in the above mentioned temperature
range are available on the market and used in numerous applications. The fraction