POLLUTION AND ITS CONTROL 423N 2 + O → NO + N
N + O 2 → NO + O
N + OH → NO + H
When this process occurs in the post flame zone, the resulting NO is called thermal NO, because
the amount of NO formed is strongly temperature sensitive. In order to limit the amount of NO formed,
it is necessary to reduce the combustion temperature (which is generally very effective because of the
strong temperature dependency of the NO formation), reduce the residence time (though this will in
general increase the CO emission), or limit the availability of oxygen.
NO also forms in the flame zone. In this case, the O-atom and OH concentrations affecting the
formation of NO have much higher concentrations than in the post-flame zone. Other mechanisms also
contribute, such as the prompt mechanism:
N 2 + CH → HCN + N
The hydrogen cyanide (HCN) and N-atom oxidize to NO. Because of the high free radical con-
centrations in the flame zone, the rate of production of NO is faster in the flame zone than in the post
flame zone. However, the time available in the flame zone is generally short compared to the time
available in the post-flame zone.
Some of the NO may oxidize to NO 2 in the combustor. Thus, the emission is expressed as NOx =
NO + NO 2.
Many methods are used to control the emission of NOx, and a great deal of research has been
done on this. A great deal of money is spent on NOx control throughout the world. As indicated above,
automotive NOx is controlled though the use of the three-way catalyst. Another name for this is non-
selective catalytic reduction (NSCR). Some industrial and utility combustors use a different type of
exhaust catalyst. This is a selective catalytic reduction or SCR. In this case, ammonia (NH 3 ) is injected
into the exhaust stream ahead of the catalyst. Across the catalyst, the NOx and NH 3 react to form N 2. The
conversion efficiency is about 80 to 90%.
NOx + NH 3 → N 2 + H 2 O + O 2 (not balanced)
Another method with ammonia injection is selective non-catalytic reduction (SNCR). If ammo-
nia is injected into the exhaust at higher temperatures (i.e., at about 1200K) than used in the SCR
process, the ammonia reduces the NOx without the need for the catalyst. However, if the temperature is
too high, the ammonia oxidizes into NO.
Combustion modification is also widely practiced to control NOx and a whole class of low-NOx
engines and combustors has grown up. These have NOx emissions anywhere from about 10% to 70% of
the ‘dirty’ pre-NOx-control combustors. The concepts used to effect the NOx control are no mystery.
Reduction of combustion temperature is very effective, thus, injection of water, or a diluents such as
steam or recycled exhaust (or flue) gas, is widely practiced. Another diluents is air, and thus, lean
premixed combustion is very effect in controlling NOx. Another method widely used is staged combus-
tion. That is, the first stage of combustion is conducted fuel rich. This creates a lot of CO and UHCs, but
it doesn’t create much NOx because of the lack of O 2. Some heat is transferred from the rich gases (for
example, to the working fluid of a steam-electric power-plant burner), and then the remaining air is
added into the flame to burn off the CO and UHCs. Now, NOx formation is limited, because of the
reduced combustion temperature.