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

2.3 Kinetic Modeling
2.3.2 Nitrogen Oxide Chemistry
Since neither the Princeton nor the Aachen mechanism contains nitrogen oxide
chemistry, it had to be added to the hydrocarbon mechanism. In terms
of applicable NO x kinetics, literature refers to Li and Williams [250], based on
the NO x chemistry of Hewson and Bollig [176], as well as to Hughes et al. [186],
known as the “Leeds mechanism”. Thus, the following discourse deals with the
approach towards a combined mechanism (Chap. 2.3.3).
Thermal
Since the extended Zeldovich mechanism is a strong function of temperature,
the concentration of NO is evaluated for different reference mechanisms and
representative combinations of hydrocarbon and NO x chemistry. Figures 2.7
and 2.8 show results obtained from an adiabatic, perfectly stirred reactor after
1ms residence time (reactor loading). This is a reasonable time because the
present process is only a function of the chemical time scale, which is well
below this time. No fuel (hydrocarbon) is applied to the initial conditions on
the reactant side within this NO x chemistry study. Thus, the air is initialized
with a composition identical to ISO standard reference conditions, with the
following mole fractions [190, 298]:
X N2 = 0.782028, (2.22)
X O2 = 0.207881, (2.23)
X H2 O= 0.010091. (2.24)
The exponential increase of NO formation with temperature is obvious and a
result of the extended Zeldovich mechanism. Since there is no carbon in the
gas phase, NO formation via the prompt NO path can be neglected. Although
the thermal NO chemistry is similar in the mechanisms shown in Figure 2.7,
the scatter in the results appears to be large. This is due to the fact that the
kinetics compared are lumped mechanisms including only a reduced set of
species. Moreover they are optimized to fit certain experimental data and validated
against a limited range of problems. The formation of O, H, and OH
radicals is critical to the production of NO x in air at high temperatures but
dominated by the respective radicals due to fuel oxidation in the presence of
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