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

2 Combustion Theory
before it follows the prompt NO mechanism [443]. The formation of fuel NO is
relevant for temperatures above 1100 K. For coal and medium fuel oil (MFO),
for instance, 60 to 80 % of the fuel-bound N 2 are converted into NO [19, 451].
Formation of NO 2
Equation (2.18) illustrates the exothermic reaction that is responsible to some
extent for NO 2 present in the combustion emissions but only of minor importance
in most ambient situations due to its slow progress under the associated
ambient conditions [214]:
2NO+O 2 → 2NO 2 . (2.18)
According to Sano [373, 374] and Miller and Bowman [289], the essential step
of NO 2 formation in flames is represented by Equation (2.19). Significant
amounts of the hydroperoxy radical (HO 2 ) are produced in flame regions of
lower temperatures and transported by diffusion into the high-temperature
regime, where NO is formed and available for oxidation. The subsequent HO 2
attack on NO (Eq. (2.19)) is two orders of magnitude faster than the reaction
of Equation (2.18) [149]:
NO+HO 2 → NO 2 + OH. (2.19)
As NO 2 is an efficient absorber of light over a broad range of ultraviolet and
visible wavelengths, it participates in photochemical smog reactions by photolysis
(i.e. photodissociation). Nitrogen dioxide absorbs sunlight and subsequently
decomposes to NO and O, thus triggering a complex series of reactions
involving organic compounds that lead to photochemical smog:
NO 2 + hν→NO+O, (2.20)
O+O 2 + M → O 3 + M. (2.21)
Here, M represents a third body molecule that absorbs the excess vibrational
energy and thereby stabilizes the O 3 molecule formed [149, 391, 451].
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