Page 320 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

10.4 NO x Reduction Using SCR and SNCR 297

at lower temperature, the NH 3 decomposes slowly and results in significant waste of
NH 3 . This problem can be solved by additives, such as, hydrogen peroxide (H 2 O 2 )
and hydrocarbons (C x H y ), which can shift the optimum temperature by about
200 °C.
The optimal temperature also drops with the decrease of NO concentration and
the increase of residence time or the CO concentration. At firing of pulverized coal,
the NO reduction efficiency usually falls to 40–80 %. Another potential problem of
the SNCR is the increase of N 2 O and CO emissions. Instead of ammonia, other
compounds may be used as reduction chemicals, such as, urea or cyanuric acid.

10.4.3 Reagents

Both ammonia and urea have been successfully employed as reagents for SCR and
SNCR. Ammonia is generally less expensive than urea. However, the choice of
reagent is based on not only cost but physical properties and operational consid-
erations. A popular replacement of ammonia is urea, which can produce ammonia
by heating.

NH 2 CONH 2 þ 1 þ xð ÞH 2 O ! NH 4 COONH 2 þ xH 2 O ! 2NH 3 þ CO 2 þ xH 2 O
ð10:25Þ

The reaction occurs in two steps and overall it is endothermic. The first reaction
involves the production of ammonium carbamate from the combination of urea and
water. Ammonium carbamate then breaks down in the hot flue gas to produce
ammonia and carbon dioxide.
Urea is a nontoxic, less volatile liquid that can be stored and handled more safely
than ammonia. Urea solution droplets can penetrate farther into the flue gas when
injected into the boiler. This enhances mixing with the flue gas, which is chal-
lenging for large boilers. Because of these advantages, urea is more commonly used
than ammonia in large boiler applications of SNCR systems. Use of the urea,
however, usually results in higher emissions of N 2 O than when using ammonia.
Furthermore, SNCR does not remove NO 2 , while SCR does.

10.5 Simultaneous Removal of SO x and NO x

Several technologies have been developed to remove NO x and SO 2 simultaneously
for the sake of low cost and small footprint. They include electron beam flue gas
treatment [3] and activated carbon-based adsorption [36]. Dry adsorption on acti-
vated carbon can be completed at a temperature as high as 220 °C. Principles of