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

2.1 Classification of Combustion Processes
overall NO x emissions, is formed in regions with mixture conditions close to
stoichiometry (φ≈1) and a correspondingly high temperature level [443].
2.1.2 Inhomogeneous and Partially Premixed Combustion
The design of many combustion devices, including liquid-fueled gas turbines,
is affected by factors such as combustion efficiency, combustion stability,
auto-ignition, flashback, relight-at-altitude capability, and exhaust gas formation
[241, 245, 246, 443]. Essential to these factors is the state of mixedness
(homogeneity) of the oxidant air with the gaseous and/or liquid fuel at
the moment of combustion [125, 194, 305, 422]. In gas turbine combustion,
lean premixed (LP) or lean premixed pre-vaporized (LPP) combustion can
be seen as opposing conventional diffusion flame combustion (Chap. 2.1.1)
[86, 87, 99, 263, 264]. However, premixers for gaseous fuels as well as liquid
fuel pre-vaporizing premixers rarely achieve homogeneity to such an extent
as to truly satisfy the conditions of a “perfectly premixed” flame. In light of
this, Barnes and Mellor [34, 35] present a method of quantifying unmixedness
in lean premixed gas turbine combustors and discuss the results, as determined
in their experiments. They follow the assumption of Mikus and Heywood
[287] and Fletcher and Heywood [137] assuming that the local equivalence
ratio has a Gaussian distribution around its true global value if the main
fuel-air mixture of the gas turbine is inhomogeneous. The study of Schlegel
et al. [388] highlights the influence of fuel-air unmixedness on NO x emissions
in lean premixed combustion for both non-catalytic and catalytically stabilized
combustion. 5 Gravity effects on partially premixed methane-air flames
are compared on an experimental and numerical basis by Lock et al. [255].
As summarized by Anderson [18], Lefebvre (ed.) [246], and Mularz [301], spray
atomization of fuel results in a wide range of the local equivalence ratio with
a spectrum from lean to rich values. Since the formation of thermal NO is
exponentially dependent on the local temperature level, this distribution of
stoichiometry leads to large quantities of NO being produced in some regions
and virtually none in others, even for global mixture ratios far from stoichiometry.
Consequently, it becomes a major challenge to predict NO and/or NO x
emissions from the average equivalence ratio of the reaction zone in a con-
5 Apart from yielding lower NO x emissions, a catalyst is capable of reducing temporal fluctuations in the fuel
concentration as well as in combustion temperature.
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