Environmental Engineering Reference
In-Depth Information
As has been discussed so far, most of the emissions from practical combustion
systems are due to either very high
flame temperature or incomplete combustion.
Therefore, reducing the combustion temperature can lead to a decrease in NO
x
emissions and soot but that will slow down the reaction leading to the formation CO
and UHC. PPC addresses both these issues and hence is a good candidate for
'
fl
c literature was explored and it was found
that various studies have been done on partially premixed
greener
'
energy production. The scienti
fl
flames, purely premixed
and purely diffusion
flames, which focus on either or both experimental measure-
ments and computational derivations of various pollutants as well as other
parameters like pressure and temperature variations. Some of the literature reviewed
has been enlisted here.
Xue and Aggarwal (
2003
) studied NO
x
fl
emissions in
n-heptane/air partially
premixed
fl
flames in the counter-
fl
ow con
guration using Held
'
s mechanism for
n-heptane and Li and William
s mechanism for NO
x
. They observed that the rate of
NO formation is generally higher in the non-premixed zone, and a NO
x
destruction
region is present in between the rich premixed and the non-premixed zone. This can
be explained by the transport of acetylene from the rich premixed to the non-
premixed zone, which forms CH and CH
2
radicals. These radicals enhance prompt
NO formation in the non-premixed zone, as well as act to consume NO in the
destruction region. The higher concentrations of O and OH radicals in the non-
premixed zone also enhance both thermal and prompt NO formation there. In the
rich premixed zone, the contribution of thermal NO is higher than prompt NO, and
vice versa in the non-premixed zone. They concluded that for prompt NO to form,
the presence of O and OH radicals is also necessary in addition to CH radicals. And
that there existed an optimum value of
'
, for which EINO
x
could be minimized for
U
moderate and high strain rate
ames.
Mitrovic and Lee (
1998
) used a laser induced incandescence (LII) setup to
measure the soot volume fraction in ethylene partially premixed
fl
fl
flames. The fuel
fl
flow rate was maintained constant and the equivalence ratios (
) were varied from a
U
pure diffusion
fl
ame (
) to about 2.5. The idea behind keeping the fuel constant
∞
was to
fix the carbon content, so that any changes in the soot concentration can only
be attributed to changes in the ef
ciency of combustion or the chemistry, rather than
due to any change in the fuel content. They observed that initially, on increasing the
premixing air, the soot emissions increase till an equivalence ratio of 20. This is
explained on the basis that the extra oxygen molecules serve to increase the local
radical pool in the soot pyrolysis region, and these radicals are in turn precursors to
soot formation. Partial premixing signi
cantly alters the chemistry of intermediate
hydrocarbons, which can account for the initial increase in soot production. But
eventually, at higher levels of premixing at an equivalence ratio of 10, the soot
concentration reduces as expected, because the oxidation of soot dominates over the
soot production. Also, the soot is
first formed in the annular region, while at
downstream locations; the soot is detected in the central region of the
fl
ames too.
guration
using LPG as the fuel. They observed that for zero and low levels of premixing, the
fl
Sreenivasan et al. (
2012
) studied partial premixing in co-
fl
ow con
flame height reduces and the soot inception point from the burner tip increases with
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