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

2 Combustion Theory
four types. Out of these, “Type I” may be the most common with a diffusion
flame established between two opposing jets [326, 343, 417]. “Type III” is very
close to droplet combustion with the diffusion flame established in the forward
stagnation region of a spherical porous burner [398, 416]. Based on the
fact that the characteristic chemical time τ c is much smaller than the characteristic
diffusion time τ d , Tsuji [442] draws the following conclusions for diffusion
flames [146, 249, 461]:
• A chemical reaction occurs in a narrow zone between fuel gas and oxidizer.
• Concentrations of the reactants (fuel and oxidizer) are very low in the reaction
zone.
• The combustion rate is controlled by the rate at which fuel and oxidizer
flow into the reaction zone.
A well-known example of a diffusion flame is the candle [253]. Turns [443]
also cites laminar jet flames as an example. Those are employed in many residential
appliances, particularly in the United States of America, where they
cause reoccurring concerns about the emission of toxic gases, such as NO 2
and CO. Since the early work of Burke and Schuhmann [61], published in
1928, the design parameters to control flame size, shape, and emission signature
have been of particular interest, and thus the subjects of various studies
[69, 176, 281, 328, 345, 354, 358–360, 442, 454].
Soot and nitrogen oxide (NO x ) formation are two prominent combustion phenomena
of diffusion flames. Particularly in hydrocarbon (diffusion) flames,
soot is present and often an issue in terms of air quality regulations, as well.
Soot is typically formed on the fuel side of the reaction zone and consumed
when it flows into the oxidizing region, in either case provided there is sufficient
time. The incandescent soot within the flame gives the diffusion flame
its typical orange to yellow appearance and contributes to its increased radiant
heat losses in contrast to premixed flames [62, 148, 174, 443, 455]. Temperature,
residence time, and composition of the reactants are decisive for NO x
formation, in a similar way to premixed flames. Here, however, the mixture
fraction Z varies from point to point within the flow, as does temperature distribution.
In a first estimate, thermal NO, representing the major portion of
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