On the Formation of Nitrogen Oxides During the Combustion of ...

2 Combustion Theory
nor generally considered to be an air pollutant. Still, it is a greenhouse gas
and participates in atmospheric chemistry involving the ozone layer by forming
NO and destroying the ozone [428, 451]. Currently, N 2 O emission is the
single most important ozone-depleting emission and expected to remain so
throughout the 21 st century. In the troposphere, N 2 O has a lifetime of 100 to
200 years, compared to NO x in the range of minutes to days. Thus, the contribution
of combustion processes to the overall N 2 O formation is meaningful,
despite the fact that combustion is only a minor direct source of N 2 O formation
and NO x formation is predominant with regard to the sum of all oxides of
nitrogen [19, 51, 135, 347].
The remaining representatives of oxides of nitrogen, as introduced above,
(NO 3 , N 2 O 3 , N 2 O 4 , and N 2 O 5 ) are present in the lower atmosphere only in very
low concentrations, even in polluted environments. Nonetheless, they play a
role in atmospheric chemistry leading to the transformation, transport, and
ultimate removal of nitrogen compounds from ambient air [451]. Discussing
the mechanisms on photochemical smog formation, Demerjian et al. [98] for
instance provide estimates of concentrations of the various oxides and acids
of nitrogen that would be present in the equilibrium state, initially assuming
only molecules of nitrogen and oxygen at atmospheric pressure, 25 ◦ C, and a
relative humidity of 50 %.
Extended Zeldovich Mechanism
The two most important parts of NO x chemistry are the thermal NO and
prompt NO reactions. These two pathways comprise the major initial steps
of breaking up air nitrogen molecules. Thermal NO can be described by
the chain reactions of Equations (2.7) and (2.8) that were first postulated by
Zel’dovich in 1946 [471, 472]. The strong temperature dependence from which
this particular type of NO derives its name is due to the fact that the N 2
molecule has an extremely strong triple bond. This bond needs to be broken
in order to form NO [19]:
O+N 2 ⇋ NO+N, (2.7)
N+O 2 ⇋ NO+O. (2.8)
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