Environmental Engineering Reference
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rate coefficient of R431 becomes very small in the post-ignition period, due to
the
T
-
7.3
factor. The inhibition of PAH formation at small values of equivalence
ratio is then attained not through oxidation reactions, but through a thermal effect
that seems artificial. This is a point of the scheme that can be improved in future
work. Experiments should be done to check if the predictions given by the scheme
are correct; if the PAH formation in a mixture of constant equivalence ratio were
actually much more intense at temperatures as low as 1500 K in comparison to
temperatures as high as 2000 K, the scheme, then, although in an artificial way,
could be considered as able to predict correctly the PAH formation in rich hydro-
carbon flames. In that case, the combustion of preheated mixtures would show
low soot emission, in addition to low NO emission.
S
UMMARY
The PAH formation mechanism in preheated mixtures of propane and air with
equivalence ratios of 5 and 2 was investigated through a well-stirred model. The
employed chemical reaction scheme comprised 902 reactions occurring among 251
species, and the results can be summarized as follows:
1. C
4
species were not seen to be precursors of PAH because, although
formed at large rates, they are oxidized through reaction with molecular
oxygen. C
3
species like allene (C
3
H
4
), propyne (C
3
H
4
P), and propargyl
radicals (H
2
CCCH) are the main precursors of the first aromatic ring.
2. Naphthalene and other larger species containing more than one aromatic
ring are seen to be formed through the HACA (H abstraction C
2
addition)
mechanism. An exception is fluoranthene, which forms mainly through a
rearrangement reaction from acephenanthrene.
3.
When the equivalence ratio was set to 2, the PAH species did not form.
At this condition, the temperature raised to a value higher than 2180 K.
Such a high temperature caused decomposition of C
3
H
4
and C
3
H
4
P and
a significant decrease in the rate coefficient of the step H
2
CCCH → C
5
H
5
L,
which is in the main route for the PAH formation. The expression for the
rate coefficient
k
of this step has a
T
-
7.3
factor that causes
k
to decrease
as temperature increases. This seems to be an artificial way to inhibit PAH
formation when combustion reactions proceed at large rates. The step
H
2
CCCH → C
5
H
5
L is a potential target for future work and a better
understanding of this reaction can lead to improvements in Richter's
scheme. At the same time, if this inhibition of PAH formation at high
temperatures were supported by experimental evidences, then the scheme
could be considered able to predict correctly the PAH formation in hydro-
carbon flames, with low soot emission — an additional property of the
combustion of preheated mixtures.
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