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

2.1 Classification of Combustion Processes
too, for lean mixtures on account of excess air that does not participate in the
combustion process but acts as thermal ballast. However, if the mixture is
too lean, it may not ignite properly, leading to misfiring and large amounts
of fuel passing through the combustion zone unburned. As a consequence,
lean premixed combustion is employed in many technical applications with
a fuel-air ratio sufficiently above the lower flammability limit. Increasing the
initial/inlet temperature of a premixed fuel-air mixture increases the burned
gas temperature by about the same amount. Dissociation and temperaturedependent
effects usually cause a small deviation of the results from this direct
dependence. However, there will be a significantly greater increase in
flame speed S L with that increase in initial/inlet temperature. The thermal
NO mechanism is also strongly dependent on even moderate temperature
changes, which have to be well-controlled by avoiding positive peaks, when
aiming for overall low NO x emissions [149, 391, 443].
The luminescence of the flame is due to visible radiation, and its color changes
with the equivalence ratio φ. For hydrocarbon flames that are operated with
an excess of air (φ < 1), a blue to deeply violet radiation appears. It results
from excited CH ∗ radicals. Green radiation can be found when the mixture is
fuel-rich, which is due to excited C 2
∗ molecules. When the fuel-air mixture is
adjusted to be very rich, an intense yellow radiation appears, which can be
attributed to soot formation. Furthermore, OH ∗ radicals contribute to the visible
radiation at any fuel-air ratio with the maximum peak in the ultraviolet
range at 308 nm. The burned gases at high temperatures typically show a reddish
glow, which arises from CO 2
∗ and water vapor radiation [146, 149, 443].
Laminar Diffusion Flames
In contrast to premixed flames, nonpremixed combustion does not intrinsically
propagate but occurs in a flame into which fuel and oxidizer are transported
from opposite sides. As molecular diffusion of species is decisive to this
type of combustion, nonpremixed flames are often called diffusion flames.
Unlike premixed flames, they do not have a burning velocity and tend to be
nonexplosive because their heat release rates are limited by diffusion rates.
Since the fuel-air mixture ratio varies in space, an additional variable such as
the mixture fraction Z is needed to describe and model diffusion flames [253].
Tsuji [442], for instance, categorizes laminar counterflow diffusion flames into
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