Encyclopedia of Environmental Science and Engineering, Volume I and II

764 NITROGEN OXIDES REDUCTION

One of the drawbacks of using anhydrous ammonia is

that the chemical is hazardous and dangerous to handle and

transport. Secondly, it requires a double lined steel tank due

to its high vapor pressure. The other type of ammonia used

for injection is aqueous ammonia, NO 3 H 2 O, which typi-

cally comes in an industrial grade of 27% NH 3 , 73% H 2 O by

weight. An injection system for aqueous ammonia contains

the following:^29

• Storage of the aqueous ammonia in an ordinary
  tank


• Vaporizing the ammonia through atomizing
  nozzles


• Mixing of the ammonia with a predetermined
  amount of heated ambient air in the vaporizer


• Injection of the ammonia-air mixture into the flue
  gas stream via a parallel network of pipes contain-
  ing several orifices.

The advantages of this type of system are that the aqueous

ammonia is less hazardous, can be transported on U.S. high-

ways, and can be kept in an ordinary storage tank.

The second part of the SCR system, the catalyst reactor,

is the most important. There are a variety of catalysts avail-

able on the market today, such as aluminum oxide (Al 2 O 3 ),

vanadium pentoxide (V 2 O 5 ), titanium dioxide (TiO 2 ), tung-

sten trioxide (WO 3 ), molybdenum trioxide (MoO 3 ), and iron

oxides. 29,30 In addition to metal catalysts, many SCR systems

use zeolites or activated carbon. The use of zeolites has been

found to increase catalyst activity, reduce the amount of

metals for disposal, and reduce the size and capital cost of

the SCR system.

As noted above, SCR’s can provide 70–90% reductions

in NO x emissions. The performance of a SCR, however,

depends on many parameters, including:^29

• sulfur content in the fuel


• type and age of catalyst


• residence time of flue gas in the reactor


• oxygen and water vapor concentration in the
  flue gas


• flue gas temperature


• effective mixing and distribution of ammonia and
  air in the flue gas stream

100

80

60

40

20

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Water Injection Rate, Ib/Ib fuel

NO

Reduction, %x

FIGURE 7 NOx reduction as a function of water injection rate.^22

C014_002_r03.indd 764C014_002_r03.indd 764 11/18/2005 1:26:55 PM11/18/2005 1:26:55 PM