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

2.2 Theory of Exhaust Gas Formation
• NNH-intermediate route
• Fuel nitrogen mechanism
• Formation of NO 2
Nitrogen dioxide (NO 2 ) is a reddish-brown toxic gas with a characteristically
pungent odor. It is corrosive and highly oxidizing. Although the boiling point
of NO 2 is high at T = 294K [453], its low partial pressure in the atmosphere
prevents condensation. Nitrogen dioxide also has an uneven number of valence
electrons and forms the dimer N 2 O 4 at higher concentrations and lower
temperatures [391, 451]. For gas turbines of previous generations, characteristic
NO 2 /NO x ratios were determined to be in the range of 15 to 50 %, with
highest values prevailing under lean conditions or in idle mode. Yellow smoke
was frequently encountered and seen as a formidable challenge in combustion
engineering. The extensive formation of NO 2 was finally located in the
primary zone of the combustor [100, 229, 241, 248]. In typical modern combustion
processes, NO 2 is emitted in small quantities of 5 to 10% by volume
of the total NO x emissions along with NO, and it is formed in the atmosphere
by the oxidation of NO [391, 451].
In general, NO 2 is unlikely to be formed in a premixed flame under stoichiometric
conditions. However, nonpremixed flames and/or local areas of lower
temperature of 600 to 1200 K are conducive to NO 2 formation. Furthermore,
the temperature level succeeding the initial combustion stages is essential, as
NO 2 destruction is initiated at temperatures above 900 to 950 K due to equilibrium
with NO [214] and the presence of the radicals O, H, and OH that play
an important role in recombination reactions [229, 380]. Nevertheless, there
has been controversy surrounding the question to which extent the NO 2 measured
in some experiments is due to the actual combustion process or to sample
probe effects [15, 149, 241]. As summarized by Kramlich and Malte [217],
combustor-formed NO 2 is believed to be due to the rapid cooling of pockets
of combustion gases as they turbulently mix with cooling air. This could also
be demonstrated in measurements by a sharp increase in the NO 2 /NO x ratio
while varying the equivalence ratio as well as parameters responsible for flame
length and aerodynamic quenching [15, 69, 217].
Nitrous oxide (N 2 O) is ubiquitous, even in the absence of any anthropogenic
sources. It is neither known to be involved in photochemical smog reactions
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