Environmental Engineering Reference
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equivalence ratios (
U
t). The prompt NO
x
formation mechanism is seen to be active
majorly in the inner premixed
flame, while the thermal NO
x
mechanism is more
prevalent in the outer non-premixed
fl
flame structure. It is
the redistribution of and changes in the contributions of these mechanisms, as well as
the NO reburn and reduction reactions occurring in these zones, which dictate the net
EINO
x
values and the EINO
x
minima. For a
fl
flame zone, for the double
fl
xed
U
t, the EINO
x
decreases with
decreasing
U
p, reaches a minimum and then increases again. For laminar
fl
ames, the
closely grouped EINO
x
values; for
U
p
\
2
2, show lesser drop in the peak
fl
ame
:
temperature, which agrees with the prompt NO
x
mechanism. For
2, the drop
in temperatures is much higher and the EINO
x
values are widely spaced, and hence it
corresponds to the Zeldovich mechanism. Similarly, for turbulent
U
p
[
2
:
fl
ames, the EINO
x
values are closely spaced and show lesser sensitivity to
U
t
for
U
p
\
1
5, and are
:
widely spaced and show greater sensitivity to
5.
Lyle et al. (
1999
) studied the emission indices for NO
x
, CO and HC in a
turbulent partially premixed
U
t variations, for
U
p
[
1
:
flame, formed by injecting rich methane-air mixtures
through a central burner tube, into a co-
fl
fl
ow of air. A double
fl
ame structure was
also observed around
U
p
¼
1
:
5
1
:
2. The NO
x
emission index with a
fixed fuel
fl
ow rate and a
xed
U
t, is independent of
U
p, the
fl
flame height and the global
U
p
1
U
p
residence time for low premixing levels (5
). For 1
:
5
5,
the
EINO
x
decreases with decreasing
flame height and decreasing residence time and
reaches a minimum at around
U
p
¼
1
:
5. With further premixing till
U
p
¼
1, the
EINO
x
shows a rapid increase, with decreasing
fl
flame height and global residence
time, to a maximum for the completely premixed case. This is due to the formation
of thermal NO
x
. Similar trends are observed for EINO. In the rich premixed
fl
fl
ame,
NO is generally formed by the prompt NO
x
mechanism, and in the diffusion
ame,
NO is formed generally by the thermal NO
x
mechanism. The temperature increases
monotonically, while the
fl
flame height and the residence time monotonically
decrease with increasing premixing. Below
fl
5, NO production rate is faster
than the reduction in the residence time, so the NO
x
formation can be attributed to
the wider high temperature zones (thermal NO
x
mechanism). The NO
x
U
p
\
1
:
increase
from
2 is also probably due to the slowly increasing temperature.
The decrease in EINO
x
and EINO from
U
p
¼1
to
U
p
¼
5, despite the increase in
the mean temperature, can be due to the changes in the contributions of the prompt
and re-burn mechanisms. Suf
U
p
¼
2to
U
p
¼
1
:
35 %
reduction in the experimentally observed EINO
x
. The EICO and EIHC are negli-
gibly small, and of the order of ambient air measurements, indicating complete
combustion. Thus, there is no trade-off between reduced NO
x
and increased CO and
HC concentrations.
Mishra et al. (
2006
) compared the structures of methane-air and propane-air
partially premixed
cient partial premixing also results in a 25
-
fl
flames and saw that both these alkanes exhibit a double
fl
ame
structure for a certain range of equivalence ratios. The inner
flame for propane is
smaller than that of methane. The acetylene concentration is higher in the propane
fl
fl
flame, and since acetylene is a precursor of soot, it explains why propane
fl
ames are
sootier than methane
fl
flames. Also, the concentration of CO is higher in the inner
fl
flame, while the CO
2
concentration is higher in the outer
fl
flame, for both the fuels.
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