Environmental Engineering Reference
In-Depth Information
The variation of temperature and concentration of propane, oxygen, and other
main species with time for the case in which the initial temperature was set to
T
0
=
1000 K is shown in
Figure 2.64
.
In spite of the large value of φ, propane decomposes
completely into H
2
, CO, H
2
O, C
2
H
4
and C
2
H
2
.
The concentration of C
2
H
4
w
as found to increase early, from
t
= 0 s, while the
production of the other species became apparent much later. C
2
H
2
is seen to be
formed after ignition and its presence in the post-ignition period is very important
for the PAH formation reactions, as will be discussed below. The concentrations of
H
2
, CO, H
2
O and CO
2
, the main combustion products, remain almost constant after
1.8 s.
The temperature shows a peak at around
t
= 1.8 s, revealing that exothermic
reactions occur first and endothermic reactions occur subsequently. It also shows
that combustion reactions are not sufficiently intense to provoke a “real” ignition of
the mixture, as the term is usually understood. However, the temporal profiles of
species concentrations and reaction rates do support the use of the term
ignition
here. The time at which the temperature peaks is then referred to as the ignition
time, for convenience. The most important exothermic reactions at early stages are
rate coefficient parameters.
One property of the flame under investigation is the absence of H, OH, and O
from the scene. The most abundant of these three active species, H atoms, have a
maximum of just 62 ppm; OH and O are, at most, 3 and 1 ppm, respectively. This
confirms that combustion reactions do not occur intensely, as suggested by
Figure 2.64
.
The variation of concentration of PAH-related species with time is shown in
Figures 2.65
through
2.68
.
A detailed analysis of the reaction rates reveals that
propane is first decomposed intoC
3
H
7
mainly through reactions R837 and R838. A
fraction of the formed C
3
H
7
then breaks into C
2
H
5
through reaction R847, and from
C
2
H
5
, reactions belonging to the well-known C
2
route occur, forming C
2
H
4
, C
2
H
3
,
and C
2
H
2
. There are no reactions in the scheme linking C
3
H
7
to C
3
H
6
, so that the
latter species is formed not through H abstraction from the former, but through
reactions involving C
2
species, such as C
2
H
4
+ CH
2
(R513).
PAH-related reactions are numerous, each having its own characteristic temporal
spectra, so that it is difficult to group them into temporal stages. To analyze them
methodically, it is convenient to group the reactions according to the period in which
they show their largest rates. The chemical reactions are described here as if they
occurred in three different stages. From these, the time interval around the ignition
time is very well characterized and this stage can be easily isolated from the others.
The reaction rates in this first stage are large in comparison with those at later stages.
The first stage lasts for about 30 ms. The main starting species for the PAH formation
reactions are C
2
H
2
and C
2
H
3
. These species, through R457 and -R482(-: reverse
reaction), give rise to C
4
species such as CH
2
CHCCH
2
and H
2
CCCCH
3
.
CH
2
CHCCH
2
is converted into C
4
H
4
through R451 and R459, and a considerable
fraction of the latter species forms H
2
CCCCH through R466. H
2
CCCCH, however,
is oxidized by molecular oxygen into CH
2
CO + HCCO (R479). C
2
H
2
combines also
with CH
3
to give C
3
H
4
(allene) through M R533.
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