Page 235 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

7.7 NO x 211

7.7.1 Nitric Oxide

7.7.1.1 Thermal NO

For the analysis above using simple chemical stoichiometry (Chap. 3), it was
assumed that nitrogen (N 2 ) is inert and does not react with oxygen. This is no longer
true when the reaction temperature is high enough. As a result, NO is produced by
the oxidation of N 2 in the air through the following chemical reaction,
1 1
N 2 þ O 2 $ NO
2 2
ð7:53Þ
10;900
K P;NO ¼ 4:71 exp
T
As indicated by the formula for equilibrium constant (K P;NO ), oxidation of
nitrogen proceeds slowly at low temperatures, but very fast at high temperatures.
The formation of NO from molecular nitrogen was first proposed by Zeldovich
[50], and it is thus referred to as the Zeldovich mechanism. NO formed by this
mechanism is also called thermal NO.
Thermal NO formation was first introduced in the 1940s. The formation of
thermal NO is very sensitive to the combustion temperature, and its formation is
negligible when temperature is 1,000 °C or lower. On the other hand, the rate of NO
formation is significant and increases exponentially when the temperature is over
1,400 °C[20]. The reactions in Table 7.5 can be used to describe the step reactions
of thermal NO formation.
First of all, the concentration of oxygen atoms required for the initiation of
reaction is strongly dependent on temperature because more oxygen atoms are
available to the reaction at higher temperature. Secondly, this reaction produces
nitrogen atoms, which is the bottleneck that limits the rate of NO formation, which
is also sensitive to temperature.

Table 7.5 Thermal NO step reactions and rate constants
3

Reaction Rate constant m mol s Equation

8 38;370 (7.54)
k f1
N 2 þ O $ NO þ N k f1 ¼ 1:8 10 exp T
k b1 7 425
k b1 ¼ 3:8 10 exp
T
k f2 4 4;680 (7.55)
N þ O 2 $ NO þ O k f2 ¼ 1:8 10 T exp T
k b2 3 20;820
k b2 ¼ 3:8 10 T exp
T
7 450
k f3 k f3 ¼ 7:1 10 exp (7.56)
N þ OH $ NO þ H T
k b3 8 24;560
k b3 ¼ 1:7 10 exp
T