Environmental Engineering Reference
In-Depth Information
nitrogen emitted from combustion systems are nitric oxide, NO, and nitric dioxide,
NO
2
. NO
2
is usually lower in concentration than NO in burned products, but it is
one of the most toxic gases and can be a serious health hazard. Although relatively
large concentrations of NO
2
can be formed in the flame zone, most of them are
converted to NO in the subsequent postflame region. Therefore NO
2
is considered
to be a transient species existing at flame conditions. It has been reported that rapid
mixing of hot burned gas and cold air surrounding the turbulent flame can result in
a rapid quenching of the NO
2
, resulting in relatively large NO
2
concentrations in the
cooled downstream regions of the flame. With HiTAC in furnaces, however, local
and temporal high temperatures, often seen in open flames, do not appear. A relatively
uniform temperature occurs throughout the flow in the furnace, and the rapid quench-
ing in the mixing layer with cold air does not occur. Accordingly, NO can be
considered as the major species of NO
x
in the exhaust gas produced by HiTAC
systems.
As the major species of oxides of nitrogen, NO is considered to be formed in
one of the following three ways:
1.
N
2
reacts with an O-atom to form thermal NO in high temperature cir-
cumstances, such as flames.
2.
Prompt NO is formed considerably fast in flame fronts through a mech-
anism other than the thermal NO.
3.
Fuel NO is formed at comparatively low temperatures from the nitrogen
released from the nitrogen-containing compounds in the fuel.
Thermal NO is the principal source of NO emission from combustion systems.
The principal reactions governing the formation of thermal NO from molecular
nitrogen during the combustion of lean and near stoichiometric fuel air mixtures are
given by the Zel'dovich or extended Zel'dovich mechanism.
N
2
+ O ⇔ NO + N
(R1)
O
2
+ N ⇔ NO + O
(R2)
N + OH ⇔ NO + H
(R3)
Reaction R3 becomes important only in near stoichiometric and rich flames
where high temperature is held long enough to produce significant amounts of NO.
The thermal NO formation rate is generally slower than that of combustion reactions
and most of the NO is formed in the postflame region. Therefore, the NO formation
process is often decoupled from the combustion processes and the NO formation
rate is conveniently calculated using the Zel'dovich mechanism assuming equilib-
rium of the combustion reactions. Because the Zel'dovich mechanism shows strong
nonlinear dependency on temperature, calculation of NO based on the time-averaged
temperature is not appropriate to predict the emission level of turbulent combustion
where instantaneous temperature usually fluctuates depending on the intensity of
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