Modeling the Rich Combustion of Aliphatic Hydrocarbons
A. D’ANNA*, A. VIOLI, and A. D’ALESSIO
Dipartimento di Ingegneria Chimica, Università di Napoli “Federico II”, 80125 Naples, Italy
A new kinetic mechanism has been developed for the formation of benzene and high-molecular-mass aromatic
compounds in rich flames of aliphatic hydrocarbons. The kinetic scheme emphasizes both the role of resonantly
stabilized radicals in the growth of aromatics and the standard acetylene addition mechanism. The model has
been used to simulate premixed flames of acetylene and ethylene where the concentrations of radicals and
high-molecular-mass compounds are known. The kinetic scheme accurately reproduces the concentrations and
trends of radicals and stable species including benzene and total aromatic compounds. Formation of benzene
is controlled by propargyl radical combination. The model reproduces well the profiles of benzene for the
different hydrocarbons and in the different operating conditions. Key reactions in the formation of highmolecular-mass aromatics are the combinations of resonantly stabilized radicals, including cyclopentadienyl
self-combination, propargyl addition to benzyl radicals, and the sequential addition of propargyl radicals to
aromatic rings. The predicted amounts of total aromatic compounds increase at the flame front and remain
constant in the postoxidation zone of the flames, attaining the final concentrations of soot, in slightly-sooting
conditions. As a consequence, the carbonaceous species which contribute to soot formation are already present
at the flame front as high-molecular-mass structures. Soot is formed through dehydrogenation and aromatization of the high-molecular-mass compounds, rather than by surface growth. © 2000 by The Combustion
Institute
INTRODUCTION
Despite successful efforts to control the emissions of combustion-generated pollutants, epidemiological studies continue to implicate combustion sources as the probable cause of
increased mortality. These studies suggest an
association between exposure to low levels of
ambient particles and both acute and long-term
adverse health effects. In particular, many studies link higher levels of particulate matter to
increased risks of respiratory problems [1– 4].
The problem of particulate formation during
incomplete combustion of hydrocarbons is
hence an issue of practical importance and
demands complete knowledge of the processes
leading from hydrocarbon molecules to highmolecular-mass structures, in order to better
control such emissions in the atmosphere.
In fuel-rich combustion, the formation of
different classes of carbonaceous compounds,
including aromatic hydrocarbons and soot, is
often observed. The relative abundance of these
classes of compounds is strongly related to the
temperature and C/O distribution inside the
flame. The higher the C/O ratio, the higher is
the formation of soot, if the temperature in the
Corresponding author. E-mail: [email protected]
0010-2180/00/$–see front matter
PII S0010-2180(99)00163-7
pyrolysis region is in the domain for the formation of soot. On the other hand, lower, but still
rich C/O ratios, as well as lower temperatures
favor the formation of aromatic hydrocarbons.
These species comprise low-molecular-weight,
gas-phase polycyclic aromatic hydrocarbons
(PAHs), well characterized in many combustion
conditions, and high-molecular-mass tar-like
carbonaceous material of difficult chemical
characterization. The latter species, which represent a large fraction (70 – 80%) of the aromatic compounds, remain unresolved in the
analyses of combustion exhausts, as well as in
the atmosphere [5– 8]. Spectroscopic studies
have shown that tar-like material is transparent
to visible radiation, but it absorbs, scatters and
fluoresces in the ultraviolet region, suggesting it
consists of high-molecular-mass structures with
both aliphatic and aromatic chemical functionalities. Furthermore, these species are formed
in the main oxidation zone of the flame more
rapidly than PAHs and may have a role both as
pollutants and as soot precursors [9, 10].
Many kinetic schemes in the literature predict
with a reasonable level of accuracy the concentration of soot and PAHs in rich laminar premixed flames of aliphatic fuels [11–14], but
neglect the presence of high-molecular-mass
aromatic compounds. These compounds repreCOMBUSTION AND FLAME 121:418 – 429 (2000)
© 2000 by The Combustion Institute
Published by Elsevier Science Inc.
RICH COMBUSTION OF ALIPHATIC HYDROCARBONS
sent a large fraction of the total particulates
(their concentration is four or five times higher
than that of PAHs) both in flames below the
point of soot formation and in rich flames across
the soot threshold limit, which are operating
conditions typical of soot formation in diffusion
flames.
Standard gas kinetic mechanisms consider
soot formation to occur through three different
steps. The first comprises elementary reactions
for benzene formation through the oxidation
and early pyrolysis of the fuel; the second step
regards the modeling of planar PAH growth,
starting from smaller hydrocarbons, by a sequence of H-atom abstraction and acetylene
addition reactions (H-Abstraction aCetylene
Addition or HACA mechanism). The third step
concerns the formation of solid soot particles by
coagulation of planar PAHs to form soot nuclei
and soot loading from surface growth [11].
In the last few years, a new concept has
emerged from both experimental and theoretical studies of the formation of aromatics in
combustion: the potential importance of resonantly stabilized radicals. These radicals are
relatively unreactive, as a result of the delocalization of the unpaired electron, and can therefore attain high concentrations in combustion,
becoming attractive as building blocks for
higher hydrocarbons [15–19]. Starting from this
evidence, we developed a new detailed kinetic
mechanism for aromatic growth, which emphasizes the role of resonantly stabilized radicals.
The modeling results were compared with the
total amount of particulate carbon collected in
different rich premixed flames of ethylene/oxygen across the soot threshold limit. The results
showed that the amount and net formation rate
of two- and three-ring aromatics agree well with
experimentally determined concentrations and
formation rates of soot and high-molecularmass structures [20, 21]. Two major outcomes
have been reached: the formation of small
aromatics is the rate-determining step in the
formation of total organic carbon [20], and
prediction of the concentration of these compounds gives the possibility of determining the
final concentration of soot and high-molecularmass aromatics [21].
Since this kinetic model stresses the role of
resonantly stabilized radicals in the growth of
419
aromatics, it is important to verify its capability
to correctly simulate the formation of these
intermediate species for different fuels and operating conditions. To this end, in this paper we
use the developed kinetic mechanism to simulate flames of acetylene and ethylene, where the
concentrations of radicals and high-molecularmass compounds are known. Unfortunately,
due to experimental reasons, well-characterized
flames in terms of radicals lack information on
high-molecular-mass species. We therefore use
two sets of data for the validation of the kinetic
scheme: (a) low-pressure flames, studied by
molecular beam mass spectrometry, to check
the capability of the model to predict the concentrations of small radicals and benzene, and
(b) atmospheric pressure experiments, performed by sampling, weighing, and chemical
analysis of the collected material, for the simulation of particulates in different flame conditions.
MODEL
The compiled chemical kinetic mechanism used
in this study includes reaction pathways recently
proposed in the literature. Below, the key points
concerning the reaction mechanism are discussed. A modified version of Miller and Melius’s light hydrocarbon/fixed gas kinetic model
[15] was used as the basis upon which to build
detailed molecular growth chemistry of hydrocarbons. Minor modifications were made to
describe ethylene flames better. For the reactions: CH4 ⫹ H ⫽ CH3 ⫹ H2 and HCO ⫹ M ⫽
CO ⫹ H ⫹ M, the kinetic data of Mauss and
coworkers were adopted [22]. According to
Miller [19], the reactions i-C4H5 ⫹ H ⫽ C3H3 ⫹
CH3, i-C4H5 ⫹ H ⫽ CH3CCCH2 ⫹ H,
CH3CCCH2 ⫹ H ⫽ C3H3 ⫹ CH3 were included
in the kinetic scheme to account for the production of C3H3 and 1-methylallenyl radicals.
Formation of the First Aromatic Ring
The pathways considered in this study for the
formation of phenyl radicals and benzene comprise three routes. The first is the addition of
n-C4H3 (HC⬅C-CH⫽CH) to C2H2 leading to
phenyl and similarly, the addition of n-C4H5
420
(H2C⫽CH-CH⫽CH) to C2H2 leading to benzene ⫹ H [23, 24]:
n-C4H3 ⫹ C2H2 3 phenyl
n-C4H5 ⫹ C2H2 3 benzene ⫹ H
The rate coefficients of these reactions were
adapted from Wang and Frenklach’s RRKM
calculations [11].
The second route is the self-combination of
propargyl radicals by:
A. D’ANNA ET AL.
such as naphthalene, phenanthrene, and higher
order rings. By-products of the process are
ethynyl substituted PAHs and five-membered
aromatics such as acenaphthylene [11, 24].
Colket and Seery [28] proposed reactions involving the resonantly stabilized benzyl radicals
leading to PAH formation. Specifically, the key
step forming PAHs is benzyl and propargyl
combination leading to naphthalene production
CH3CCCH2 ⫹ H2CCCH 3 C6H5CH2 ⫹ H
H2CCCH ⫹ H2CCCH 3 C6H5 ⫹ H
C6H5CH2 ⫹ H2CCCH 3 C10H8 ⫹ H ⫹ H
The rate constant used for this reaction is 3 ⫻
1012 cm3/(mol s), as derived by Stein et al. [18]
and Miller and Melius [15] through the modeling of the benzene levels measured in lowpressure, rich, premixed acetylene flames.
The last sequence for benzene formation
involves the 1-methylallenyl and propargyl combination reaction with the formation of benzyl
radicals and their decomposition to benzene
[13, 25]:
Following Marinov and coworkers [13] the third
reaction sequence is:
C6H5 ⫹ O2 3 C6H5O (phenoxy) ⫹ O
C6H5O 3 c-C5H5 (cyclopentadienyl) ⫹ CO
c-C5H5 ⫹ c-C5H5 3 C10H8 ⫹ H ⫹ H
C6H5CH2 ⫹ H 3 C6H5CH3
It considers that the reaction between two cyclopentadienyl radicals to form naphthalene takes
place in spite of a large intrinsic energy barrier. A
rate expression of 3.8 ⫻ 1012 cm3/(mol s) was
assigned to this reaction. Phenanthrene formation was predicted using a reaction step similar
to that used for naphthalene formation:
C6H5CH3 ⫹ H 3 C6H6 ⫹ CH3
indenyl ⫹ c-C5H5 3 phenanthrene ⫹ H ⫹ H
The rate expressions used for these reactions
were adjusted downward from Miller and Melius’s kinetic scheme [15]. Mechanisms for the
oxidation of toluene and benzene were also
included [26, 27].
A rate expression of 3.8 ⫻ 1012 cm3/(mol s) was
assigned to the combination of the resonantly
stabilized indenyl radical with cyclopentadienyl.
By analogy with the chemistry of benzene
formation through propargyl radical combination, a new reaction path for the formation of
PAHs is proposed. It considers the sequential
addition of propargyl radicals to phenyl leading
to bi-phenyl and subsequent cyclization to
phenanthrene by acetylene addition. The reaction rates for this reaction path are evaluated by
analogy with similar reactions [20]. A schematic
representation of the different pathways is reported in Fig. 1; the rate coefficients of the key
reactions used in the aromatic formation submechanisms are reported in Table 1.
CH3CCCH2 ⫹ H2CCCH 3 C6H5CH2(benzyl)
⫹H
Formation of Multi-Ring Aromatics
The kinetic submechanism for the formation of
larger aromatic structures includes either replicating the hydrogen abstraction acetylene addition (HACA) mechanism or kinetic pathways
involving resonantly stabilized free radicals.
Four different reaction sequences of resonantly
stabilized radicals were analyzed as potentially
important sources for PAH growth. The HACA
mechanism occurs by way of a two-step process
involving hydrogen abstraction to activate aromatics followed by subsequent acetylene addition. This process continues, leading to the
sequential formation of multi-ring structures,
Computational Procedure
The computations were carried out using the
Sandia laminar one-dimensional premixed
RICH COMBUSTION OF ALIPHATIC HYDROCARBONS
421
Fig. 1. A schematic diagram illustrating the kinetic pathways for aromatic growth.
TABLE 1
Key Reactions in the Aromatic Formation Submechanisma
K ⫽ A Tn exp(-E/RT)
Reactions
Benzene formatioin
C3H3 ⫹ C3H3 ⫽ C6H5 ⫹ H
nC4H5 ⫹ C2H2 ⫽ C6H2 ⫹ H
nC4H3 ⫹ C2H2 ⫽ C6H5
C3H3 ⫹ H3CCHCCH ⫽ C6H5CH2 ⫹ H
Larger aromatic formation
C6H5CH2 ⫹ C3H3 ⫽ C10H8 ⫹ H ⫹ H
C6H5 ⫹ C3H3 ⫽ C6H5C3H2 ⫹ H
C6H5C3H2 ⫹ C3H3 ⫽ C6H5C6H4 ⫹ H
C6H5C6H4 ⫹ C2H2 ⫽ C14H10 ⫹ H
c-C5H5 ⫹ c-C5H5 ⫽ C10H8 ⫹ H ⫹ H
indenyl ⫹ c-C5H5 ⫽ C14H10 ⫹ H ⫹ H
C6H5 ⫹ C2H2 ⫽ C6H5C2H ⫹ H
C6H5C2H ⫹ H ⫽ C6H4C2H ⫹ H2
C6H4C2H ⫹ C2H2 ⫽ C10H7
a
3
Units: cm , mol, s, kJ.
[a] Present work.
A
n
E
3.00 ⫻ 1012
1.00 ⫻ 1016
2.80 ⫻ 103
3.00 ⫻ 1012
⫺1.33
⫹2.90
22.57
5.85
3.00 ⫻ 1012
3.00 ⫻ 1012
3.00 ⫻ 1012
6.64 ⫻ 1033
3.80 ⫻ 1012
3.80 ⫻ 1012
5.13 ⫻ 1038
2.50 ⫻ 1014
1.43 ⫻ 1051
⫺5.92
94.47
⫺7.09
123.73
66.90
108.68
⫺11.58
Reference
[15,
[23,
[23,
[13,
18]
24]
24]
25]
[28]
[a]
[a]
[11, 24]
[a]
[a]
[11, 24]
[11, 24]
[11, 24]
422
flame code (PREMIX) [29] of the Chemkin
package [30]. The PREMIX code computes
concentration profiles for a burner-stabilized
premixed laminar flame using the cold mass
flow rate through the burner, feed-gas composition, pressure, and an estimated solution profile as input. The program can also compute the
temperature profile. However, heat losses to the
burner and the external environment are unknown, and therefore an experimentally determined temperature profile is used as input.
Thermochemical information was primarily
obtained from the Chemkin thermodynamic
database [31], from Stein et al. [18] and Marinov et al. [13]. Unavailable thermodynamic
properties for some species were estimated by
using Benson’s group additivity method [32].
The transport parameters were obtained from
the Chemkin database [33] and from Wang and
Frenklach [34]. The detailed reaction mechanism consists of 340 reactions and 90 chemical
species. Unless specifically mentioned, each elementary reaction in the mechanism is reversible and the rate coefficients of the forward
reactions were either taken from the literature
or estimated on the basis of analogous reactions. The reverse reaction rates were calculated
using equilibrium constants. For most of the
recombination and decomposition reactions,
the pressure dependence in the Troe format
and third-body efficiencies were taken into account [35].
MODEL RESULTS AND DISCUSSION
The detailed kinetic model was used to simulate
rich premixed flames of different aliphatic hydrocarbons where the concentrations of stable
species, radicals, soot, and high-molecular-mass
aromatic hydrocarbons are available. Figure 2
reports the experimental and computed mole
fraction profiles of the major stable species in an
acetylene– oxygen–argon flame with equivalence ratio of 2.4 and pressure of 2.67 kPa. The
flame was experimentally studied by Westmoreland et al. [36, 37] by sampling through a sonic
quartz nozzle and mass spectrometric analysis.
In order to account for probe perturbation from
the true profiles, the data have been translated
by ⬃ 1.1 mm, which corresponds to approxi-
A. D’ANNA ET AL.
Fig. 2. Comparison between experimental (symbols) [36,
37] and computed mole fraction profiles of reactants and
major products along the axis of a rich acetylene– oxygen–
argon flame with ␸ ⫽ 2.4 and pressure of 2.67 kPa.
mately two nozzle diameters, toward the burner
from the probe tip position, except for the data
very near the burner surface.
The concentrations of the fuel (C2H2) (Fig.
2a) and oxidant (O2) (Fig. 2b) rapidly decrease
in the flame zone; unreacted C2H2 persists well
into the postflame region in steady concentrations, as expected in fuel-rich conditions. The
formation of CO (Fig. 2a), a major product in
rich combustion, also occurs in the flame zone,
corresponding to the consumption of fuel and
oxidant. CO subsequently oxidizes to CO2 and
both CO and CO2 (Fig. 2a) have concentrations
which rise and remain quite constant in the
postflame zone. The model is seen to predict
reasonably well the mole fraction profiles of
reactants and products. The flame region is
characterized by peak concentrations of radicals
deriving from acetylene oxidation. The predicted concentration profiles of OH, H, CH3,
and CH2 radicals are compared with the experimental data in Fig. 3. The maximum values and
concentration profiles are well reproduced by
the model, probably to within the experimental
uncertainty for all the reported species. However, the mechanism tested displays a general
shift of the predictions 1 mm away from the
RICH COMBUSTION OF ALIPHATIC HYDROCARBONS
423
Fig. 4. Comparison between experimental (symbols) [36,
37] and predicted mole fraction profiles (lines) of propargyl
radical and benzene along the axis of a rich acetylene–
oxygen–argon flame with ␸ ⫽ 2.4 and pressure of 2.67 kPa.
Fig. 3. Comparison between experimental (symbols) [36,
37] and computed mole fraction profiles of major radicals
OH, H, CH3, and CH2 along the axis of a rich acetylene–
oxygen–argon flame with ␸ ⫽ 2.4 and pressure of 2.67 kPa.
burner than the experimental concentration
profiles. This could be due to experimental
uncertainty (measured temperatures may be too
high in regions close to the burner) or to probe
effects.
Acetylene combination with the singlet methylene radical is the dominant route forming
propargyl radicals, whose concentration profile
is reported in Fig. 4a.
C2H2 ⫹ O ⫽ HCCO ⫹ H
H ⫹ HCCO ⫽ CH2 ⫹ CO
1
CH2 ⫹ C2H2 ⫽ C3H3 ⫹ H
Again, the model predicts well both the peak
value and profile for propargyl radicals. Due to
its high stability, C3H3 reaches high concentrations at the end of the flame zone, comparable
to those of stable species such as methane and
ethylene (order of 10⫺3). Methane and ethylene
are the major stable intermediates and are
mainly produced by the reactions of methyl
radicals with H atoms and CH2 (both singlet
and triplet methylene), respectively.
CH3 ⫹ H(⫹M) ⫽ CH4(⫹M)
CH3 ⫹ CH2 ⫽ C2H4 ⫹ H
Vinyl radicals in the main-flame zone are produced by H-abstraction from ethylene. The
combination of C2-radicals with acetylene leads
to the formation of C4 species, i.e. the selfcombination of vinyl radicals and its addition to
ethylene. The model predicts a low concentration of n-C4H3 radicals since they are easily
converted into the more stable isomers i-C4H3,
whose concentration in the flame is relatively
high. The same is true for the n-C4H5 radical,
whose concentration is significantly lower than
that of i-C4H5, in agreement with Miller and
Melius’s suggestions [15]. We note that there is
significant scatter in the C4H3 and C4H5 experimental data although the predicted values are
in the same range as the experimental ones,
indicating that the reaction model may predict
reasonably well all of the C3 and C4 radical
species relevant to aromatic formation.
424
Despite this good agreement, the concentration of benzene is not well reproduced by the
model although the model is able to correctly
reproduce the rise-decay profile and the reincrease of benzene in the postoxidation zone of
the flame as shown in Fig. 4b. The model
overpredicts peak benzene concentration by a
factor of 2 to 3 when a rate constant of 3 ⫻ 1012
cm3 mol⫺1 s⫺1 is assigned to the recombination
channels of propargyl radicals to form phenyl
and hence benzene. This latter pathway, together with the addition of propargyl to methylallenyl radicals, is the main source of benzene
in our kinetic model. The contribution of C2H2
addition to C4Hx leading to benzene and phenyl
is not insignificant. However, the rate coefficients used in the model for the n-C4H3 ⫹ C2H2
and n-C4H5 ⫹ C2H2 reactions, which were
taken from Wang and Frenklach calculations
[12], are almost certainly too large as suggested
by Walch [38].
The kinetic simulation indicates that the decrease in benzene concentration is due mainly
to phenyl radical decomposition, rather than to
its oxidation or molecular growth. As a consequence, the differences between the experimental and predicted benzene concentrations might
be related to the temperature profile used for
the simulation or to poor estimation of thermodynamic properties of the species involved in
benzene formation. However, a more accurate
description of the C3 ⫹ C3 reaction, which
cannot be considered a single-step reaction,
particularly in flame conditions, as suggested by
Wang and Frenklach [12], may resolve the
differences between experiments and simulations.
To analyze the reliability of the kinetic
scheme in reproducing benzene, we have also
simulated a rich ethylene flame experimentally
studied recently by Bhargava and Westmoreland [39] using molecular beam mass spectrometry (C2H4/O2/50%Ar flame at 2.67 kPa with
equivalence ratio of 1.9). Comparison between
computational results and experimental data is
reported in Fig. 5 for the C3H3 radical (Fig. 5a)
and benzene (Fig. 5b). Again the model is able
to reproduce the amounts and trend for C3H3
radical, but unlike the acetylene flame, the
model underpredicts the peak benzene concentration by a factor of 2. The model predicts
A. D’ANNA ET AL.
Fig. 5. Comparison between experimental (symbols) [39]
and predicted mole fraction profiles (lines) of propargyl
radical (a) and benzene (b) along the axis of a rich ethylene–
oxygen–argon flame with ␸ ⫽ 1.9 and pressure of 2.67 kPa.
quite well the rise of C6H6, whereas the decline
of benzene is overestimated. As already discussed, the discrepancy between model and
experimental measurements may be attributed
to a number of factors, including the uncertainties in the reaction rate and thermodynamic
parameters in the present reaction mechanism,
but also in this case, an incorrect evaluation of
the temperature profile might affect the benzene profile.
To ascertain the role of propargyl self-combination in benzene formation, we also simulated two rich acetylene flames, experimentally
studied by Ancia et al. [40], in which both the
concentrations of propargyl and benzene were
measured. The flames, with equivalence ratios
of 2.0 and 2.25, were investigated by molecular
beam mass spectrometry in such a way that
fragmentation and isotopic contributions remained low. Stable species were calibrated by
comparing the signal intensity to those measured in a reference mixture, whereas radical
concentrations were measured by estimating
ionization cross sections for carbon-containing
radicals. The experimental and computed mole
fraction profiles of C3H3 and C6H6 are presented in Figs. 6 and 7 for flames with ␸ ⫽ 2 and
RICH COMBUSTION OF ALIPHATIC HYDROCARBONS
425
Fig. 6. Comparison between experimental (symbols) [40]
and predicted mole fraction profiles (lines) of propargyl
radical (a) and benzene (b) along the axis of a rich acetylene– oxygen–argon flame with ␸ ⫽ 2.0 and pressure of 2.63
kPa.
Fig. 7. Comparison between experimental (symbols) [40]
and predicted mole fraction profiles (lines) of propargyl
radical (a) and benzene (b) along the axis of a rich acetylene– oxygen–argon flame with ␸ ⫽ 2.25 and pressure of 3.42
kPa.
␸ ⫽ 2.25, respectively. The model accurately
reproduces the peak value and the profile of
C3H3 in both flames, whereas the benzene concentration is slightly overpredicted by the reaction model, within a factor of 1.5 or better,
which is probably to within the experimental
uncertainty. The model predicts a slower decomposition in the postoxidation zone of the
flame compared to the experimental data, which
however show a very sharp decrease in benzene
after the peak value. It is worth noting that the
modeling results were obtained with the same
value of the recombination rate constant for
propargyl, which failed to reproduce Westmoreland et al.’s [36, 37, 39] measurements.
From the previous comparisons, it is remarkable that, despite the uncertainty in the experimental measurements, in the values of the rate
constants, and also in the thermodynamic data
for radicals, the kinetic model reproduces the
experimental trends for C3H3 and C6H6 within a
factor of 3 (C6H6), or less (C3H3), in flames of
ethylene and acetylene in different operating
conditions.
To validate the capability of the kinetic
scheme to predict also the formation of hydro-
carbons with higher molecular masses in rich
flames, we simulated flames of ethylene/oxygen
in which a complete characterization of PAHs,
tar-like material, and soot, is available. It is
important to underline that the comparison
concerns the total aromatic hydrocarbons collected in flames, since the reaction model is
currently unable to describe in detail the distribution of the high-molecular-mass pyrolytic carbon in terms of PAHs, tar, and soot. The first set
of experimental data comprises three ethylene/
oxygen flames (at 1 atm.) with equivalence
ratios ranging from a non-sooting condition
(␸ ⫽ 1.5), a slightly-sooting (␸ ⫽ 2.0) and a
sooting one (␸ ⫽ 2.4) with maximum flame
temperature of ⬃ 1750 K. The flames were
stabilized on a water-cooled sintered-bronze
burner and experimentally characterized in
terms of stable gaseous species, soot, and condensed hydrocarbons, i.e. PAH and tar-like
material [41– 43]. The data are translated ⬃ 2
mm towards the burner from the tip position, to
account for probe perturbation from the true
profiles.
The predicted and measured concentrations
of C3 and c-C5 species relevant to benzene and
426
A. D’ANNA ET AL.
Fig. 9. Comparison between experimental (symbols) [41–
43] and predicted mole fraction profiles (lines) of benzene
along the axis of three rich ethylene– oxygen flames with ␸ ⫽
1.5, 2, and 2.4 at atmospheric pressure.
Fig. 8. Comparison between experimental (symbols) [43]
and predicted mole fraction profiles (lines) of the sum of C3
(a), the sum of c-C5 (b) species and benzene (c) along the
axis of a rich ethylene– oxygen flame with ␸ ⫽ 2.4 at
atmospheric pressure.
higher aromatic formation are reported in Figs.
8a and 8b for the sooting flame (␸ ⫽ 2.4). The
computed data in this figure are reported as the
sum of C3 and c-C5 species, since radicals with
C3 and c-C5 structures may attain high concentrations in flames as a result of the delocalization of the unpaired electrons and hence may
substantially contribute to the stable C3 and
c-C5 species experimentally detected. The
agreement between experimental data and computed results is very good. The benzene concentration in this sooting flame is reported in Fig.
8c. It shows a rise-decay profile, and it is observed to increase again downstream of the
flame zone. This behavior, which is usually
found in sooting flames of aliphatic fuels, is well
reproduced by the model, together with the
peak concentration of benzene. The kinetic
simulation indicates that, also in these flame
conditions, the decrease in benzene concentration is mainly due to phenyl radicals decomposing, rather than their oxidation or molecular
growth leading to larger aromatics. Decomposition prevails in the formation process just after
the flame front. Benzene formation increases in
the main oxidation zone of the flame and
reaches a maximum just before the flame front.
The increase in radical species and the higher
temperature at the flame front favor benzene
decomposition, resulting in a decrease in its net
formation rate. Downstream from the flame
front the decreasing amount of radicals and
temperature inhibit the continuation of the
benzene decomposition process in such a way
that the benzene net formation rate increases
again. Simulations show that self-combination
of propargyl radicals is the main route leading
to benzene in the early flame region. In the
postoxidation zone of the flame, the acetylene
addition to C4 hydrocarbons contributes significantly, becoming the most important benzene
formation route.
Figure 9 shows the comparison between predictions and experimental data for benzene
concentration profiles in the three ethylene/
oxygen flames with different equivalence ratios.
The concentration profiles of benzene exhibit a
first maximum early on in all these flames, but
decrease in the non-sooting condition (␸ ⫽ 1.5)
without going through any further maximum. In
the slightly-sooting (␸ ⫽ 2.0) and in sooting
(␸ ⫽ 2.4) conditions, the benzene concentration
reincreases in the postoxidation zone. The
model is able to predict correctly the amounts
and trends of benzene for the three flames and
hence in a wide range of feed ratios. The peak
concentrations are also predicted reasonably
well within a factor of 1.5 or better.
RICH COMBUSTION OF ALIPHATIC HYDROCARBONS
Fig. 10. Comparison between predicted concentration profile of total aromatics (lines) and the experimental concentration of soot (square) and total aromatics (circles) along
the axis of two rich ethylene– oxygen flames with ␸ ⫽ 1.5
[41] (a) and 2.4 [42] (b) at atmospheric pressure.
The profile of the sum of species with molecular masses higher than benzene is reported in
Fig.10 for the slightly sooting (␸ ⫽ 2.0) and the
sooting (␸ ⫽ 2.4) ethylene flames at atmospheric
pressure. Modeled species comprise two- and
three-ring PAHs and are predicted considering
both the mechanisms involving H-abstraction
acetylene addition to aromatic radicals (HACA
mechanism) and the pathways involving resonantly stabilized free radicals. The modeling prediction is compared with the experimental concentration profile of soot and total particulate.
Since the kinetic mechanism currently includes
only pathways of aromatic formation, but does not
describe in detail their transformation to tar and
soot, the comparison of the modeled concentrations has to be performed with the total concentration of aromatic compounds, i.e. the sum of
soot, PAH, and tar-like compounds. The predicted concentration of larger aromatics increases
in the main flame region with a high net rate,
thereafter increasing again with a lower rate in the
postflame region. The same behavior is also
shown by the experimentally determined concentration profile of total aromatic carbon in both
flames, which also shows a region of constant
427
concentration just after the flame front and in
correspondence to soot inception. It is worth
noting that in the ␸ ⫽ 2 flame the total aromatic
compounds are mainly PAH and tar species,
since the contribution of soot is very low, just
20% of the total particulates late in the flame.
In the richer flame, PAHs and tar are the
dominant species just downstream of the flame
front, whereas late in the postflame region soot
contributes to about 60% of the particulate in
this flame. The two profiles and concentration
levels are well reproduced by the model.
The most interesting result is that the model
is able to reproduce the formation of total
aromatic compounds in flames from the beginning of the postoxidation zone. The concentrations of these species remain quite constant
downstream of the postoxidation zone and only
in very fuel-rich conditions is soot formed.
Consequently, soot is formed through a slow
process of polymerization, dehydrogenation,
and internal rearrangement of these structures
without significant mass addition from the gas
phase. Numerical simulation shows that the
HACA mechanism contributes negligibly to the
formation of aromatics, whereas the cyclopentadienyl self-combination is the dominant route
in the main oxidation zone. This is due to the
considerable formation of c-C5H5 radicals from
phenyl oxidation in the main flame zone, as
previously suggested by Castaldi et al. [25].
Benzyl and propargyl combination forming naphthalene is less important than the cyclopentadienyl self-combination route, and it is prevalent in
the postflame region. The sequential addition of
propargyl to phenyl radicals forming biphenyl and
subsequently phenanthrene by acetylene addition
is the dominant route at the flame front. It is also
worth noting that the model is able to correctly
reproduce the concentration of total aromatic
carbon in flames below the soot threshold and
the increase in their concentration of about one
order of magnitude across the soot threshold
(experiments show that the soot threshold in
premixed ethylene flames is about ␸ ⫽ 2.0).
CONCLUSIONS
The modeling results show that the developed
kinetic scheme may predict reasonably well the
428
concentrations and trends of reactants, products, and stable intermediates, as well as all of
the radicals relevant for aromatic growth. The
introduction in the kinetic mechanism of reaction pathways for aromatic growth stresses the
role of resonantly stabilized radicals; in addition
to the acetylene mechanism (HACA mechanism), it gives the possibility of reproducing with
a good level of accuracy both the formation of
benzene and total aromatic species in rich
flames of acetylene and ethylene in different
operating conditions.
Benzene formation is controlled by propargyl
radical combination in the main flame zone,
whereas its decrease is due to oxidation of
phenyl radicals at the flame front, rather than to
growth toward larger aromatics. Good estimates
of the temperature profile and thermodynamic
properties of the species involved in benzene
formation are necessary to cover the gap between predicted and measured benzene concentrations in some operating conditions. However,
despite these uncertainties, the kinetic model
reproduces the experimental trends quite well
for the different fuels and in different operating
conditions. In particular, the model is able to
reproduce the rise-decay profile of benzene in
all the flames and its reincrease downstream of
the flame zone which is usually observed in rich,
atmospheric pressure flames.
Key reactions leading to the formation of
aromatics of high molecular mass are combinations of resonantly stabilized radicals, including
cyclopentadienyl radical combination, propargyl
addition to benzyl radicals, and the sequential
addition of propargyl radicals to aromatic rings.
The model is able to reproduce the formation of
total aromatic compounds formed in flames
from the beginning of the postoxidation zone
and with a high formation rate. The concentration of these compounds remains quite constant
downstream from the flame front and is comparable to the final soot concentration. As a
consequence, the amount of carbonaceous species that contribute to soot formation is already
present at the flame front as high-molecularmass structures. However, since the model simulates only the formation of two- and three-ring
aromatics, total organic material collected in
flames, i.e. a mass quantity much larger than the
PAHs, is the result of a fast reactive coagulation
A. D’ANNA ET AL.
of small aromatics, forming structures of high
molecular mass. Soot is formed through a slow
process of dehydrogenation and aromatization
of these high-molecular-mass compounds.
These latter processes are the controlling steps
of soot formation in slightly-sooting-conditions.
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Received 8 April 1999; revised 17 September 1999; accepted 27
October 1999