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

2.2 Theory of Exhaust Gas Formation
Review on NO x Formation with Regard to Technical Applications
In 1975, Ludwig et al. [258] reported on experimental measurements 12 of the
major stable species, including NO, within diffusion flames around simulated
ethanol (C 2 H 5 OH) droplets burning in air under atmospheric conditions. Despite
the fact that the measured flame temperatures were considerably lower
than the estimated ones, the NO concentrations were greater than predicted
theoretically. In another investigation and a further step towards technical applications,
Nizami et al. [315, 316] discuss experimental results of NO and NO x
emissions from the atmospheric, monodisperse fuel spray combustion of different
hydrocarbon fuels. The authors observed a significant effect of droplet
size, with NO and NO x reaching minima around a droplet size of 48 to 55µm.
These minima shift towards a smaller droplet size for single component fuels
of lower vapor pressure. Pre-vaporization in the spray and the transition from
diffusive to pre-vaporized, premixed combustion are considered important
factors in determining the minimum NO x point.
A comprehensive case study on the contribution of the different NO x formation
mechanisms was conducted by Nishioka et al. [314] for methane-air double
flames. 13 The numerical study specifies production of NO via the thermal
mechanism, Fenimore pathway, N 2 O-intermediate route, and NO 2 formation.
In this regard, the authors highlight the importance of the velocity gradient
within the flame. Rutar and Malte [367] discuss the contribution of the different
pathways of NO x formation for high pressure jet-stirred reactors, being
significant for lean premixed gas turbines. Aggarwal et al., on the one hand, investigated
the behavior of single and multi-component fuels by applying different
vaporization models [8–10], and on the other hand, studied the impact
of various NO formation mechanisms, including thermal and prompt NO, in
partially premixed flames in a counterflow configuration [303, 304, 468, 469].
Chen and Driscoll [68, 69] examined the contribution of the different NO x
species in experiments, studying a jet diffusion flame. An increased level of
partial premixing, as for instance realized by forcing coaxial air into the flame,
could drastically (up to sixfold) decrease NO x emissions. Here, the authors attribute
the large NO x reduction to the diminishing flame length and the flame
12 This work is in a series with the theoretical studies of Bracco [54, 55] stated above.
13 Double flames have a structure that is characterized by an initial rich premixed flame that produces CO and
H 2 as the two main intermediate products. A final diffusion flame consumes these intermediate products
together with the surrounding air.
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