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

2.2 Theory of Exhaust Gas Formation
The formation of N 2 O as an intermediate species (Eq. (2.14)) and its subsequent
conversion to NO (Eq. (2.15)) is important in fuel-lean mixtures
(φ< 0.8) at low temperatures and elevated pressures, as experienced in lean
premixed gas turbine combustion (see Chap. 2.1.1) [88, 428]. Turns [443] summarizes
Equations (2.13) through (2.15) as the “N 2 O-intermediate route”.
O+N 2 + M ⇋ N 2 O+ M (2.14)
O+NO⇋N 2 O+CO (2.15)
NNH-Intermediate Route
In 1994, Bozzelli et al. [53] proposed that NO can also be formed via the intermediate
NNH, that itself is metastable with respect to N 2 and atomic hydrogen
(H) by about 2.510×10 4 J mol −1 (Eq. (2.16)) [171, 173]. The authors suggest the
following main route:
N 2 + H⇋NNH, (2.16)
NNH+O⇋NH+NO. (2.17)
Evidence from different experimental setups confirms this pathway and supports
its importance for NO x modeling [171, 173, 215]. The associated investigations
included lean, stoichiometric, and fuel-rich mixtures burned at low
and high pressures in premixed flames as well as in stirred reactors. Hughes
et al. [185] provide a detailed discussion of the reactions of NNH, while Rutar
et al. [369] evaluate the NO x formation pathways in lean premixed, prevaporized
combustion of seven different fuels. The authors of the latter study
conclude that the Fenimore and NNH pathways dominate the formation of
NO x in the flame, while the Zeldovich and N 2 O pathways are primary contributors
to the NO x formation in the post-flame zone [149, 369]. Thus, this
mechanism can pose additional limits on the lowest NO x emissions achievable.
Fuel Nitrogen Mechanism
If fuels, including coal and HFO, contain nitrogen in their molecular structure,
this organically bound nitrogen runs through the ordinary combustion pathways.
It is rapidly converted into hydrogen cyanide (HCN) or ammonia (NH 3 )
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