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Propylene Oxide Addition Effect on the Chemical Speciation of a Fuel-Rich Premixed n -Heptane/Toluene Flame

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1,2-Propylene oxide (PO, C3H6O) is considered as a promising agent for improving fuel. In this work, the effect of PO additives on the species pool in a premixed burner-stabilized fuel-rich (ϕ = 1.6) flame fueled by n-heptane/toluene mixture (7/3 by volume of liquids) at atmospheric pressure is studied by the flame-sampling molecular beam mass spectrometry and numerical modeling in order to get insight into the chemical aspects of the influence of oxygenates with an epoxy group on the formation of abundant intermediates (including PAH precursors) during combustion of fossil fuels. The flames with various loadings of PO in the fuel blend (from 0 to 16.3% in mole basis) are examined, and detailed kinetic mechanisms available in the literature are validated against the measurements of mole fraction profiles of reactants, major products, and many intermediate species. A higher reactivity of the fresh mixture and a reduction in the peak mole fractions of intermediates playing an important role in PAH formation (benzene, styrene, ethylbenzene, phenol, acetylene, diacetylene, etc.) are observed when PO is added. This was found to be due to simultaneously two factors: the partial replacement of "sooting" fuel (toluene, which is the main precursor of these species) with oxygenated additive, and the changes in the flame radical pool caused by PO addition. Propylene oxide additive was found to change the ratio between H, OH, O, and CH3 toward an increase in the proportion of O and CH3. The detailed kinetic mechanisms considered in the work are found to overpredict the peak mole fraction of acetylene, a key species playing a crucial role in PAH growth. Its chemistry is revisited in order to provide a better prediction of C2H2 and, as a result, PAHs.
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Propylene Oxide Addition Eect on the Chemical Speciation of a
Fuel-Rich Premixed nHeptane/Toluene Flame
Artëm M. Dmitriev, Ksenia N. Osipova, Denis A. Knyazkov,*and Andrey G. Shmakov
Cite This: ACS Omega 2022, 7, 46900−46914
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Supporting Information
ABSTRACT: 1,2-Propylene oxide (PO, C3H6O) is considered as a promising
agent for improving fuel. In this work, the eect of PO additives on the species
pool in a premixed burner-stabilized fuel-rich (ϕ= 1.6) flame fueled by n-heptane/
toluene mixture (7/3 by volume of liquids) at atmospheric pressure is studied by
the flame-sampling molecular beam mass spectrometry and numerical modeling in
order to get insight into the chemical aspects of the influence of oxygenates with an
epoxy group on the formation of abundant intermediates (including PAH
precursors) during combustion of fossil fuels. The flames with various loadings of
PO in the fuel blend (from 0 to 16.3% in mole basis) are examined, and detailed
kinetic mechanisms available in the literature are validated against the
measurements of mole fraction profiles of reactants, major products, and many
intermediate species. A higher reactivity of the fresh mixture and a reduction in the
peak mole fractions of intermediates playing an important role in PAH formation
(benzene, styrene, ethylbenzene, phenol, acetylene, diacetylene, etc.) are observed when PO is added. This was found to be due to
simultaneously two factors: the partial replacement of “sooting” fuel (toluene, which is the main precursor of these species) with
oxygenated additive, and the changes in the flame radical pool caused by PO addition. Propylene oxide additive was found to change
the ratio between H, OH, O, and CH3toward an increase in the proportion of O and CH3. The detailed kinetic mechanisms
considered in the work are found to overpredict the peak mole fraction of acetylene, a key species playing a crucial role in PAH
growth. Its chemistry is revisited in order to provide a better prediction of C2H2and, as a result, PAHs.
1. INTRODUCTION
Propylene oxide (PO, C3H6O, 1,2-propylene oxide) is an
important industrial chemical that is used primarily in the
synthesis of other compounds. Extensive studies are focused on
the direct PO synthesis mostly from propylene.
14
However,
the direct epoxidation of propylene appeared to be challenging
due to the low selectivity of catalysts. Although considerable
eorts are being made to understand the kinetics of this
conversion, the detailed mechanism of the epoxidation process
is still unclear.
5,6
Large-scale production of PO is often
considered dangerous due to a high volatility, high
flammability, and wide explosion limits of PO.
7
In this regard,
it is also considered as a model and practical fuel-air
explosive.
810
Thus, extensive experimental and numerical
studies of PO/air explosion mechanisms have been carried out
using simplified
11
and detailed
9
kinetic schemes.
Particular attention was paid to PO thermal decomposition,
oxidation, and combustion kinetics, because propylene oxide is
an important intermediate formed during the oxidation of
other hydrocarbons.
12,13
Lifshitz and Tamburu
14
studied PO
decomposition and ring opening by measuring the products
formed behind the reflected shock waves in the PO/Ar mixture
in a single-pulse shock tube. They proposed a reaction scheme
for PO isomerization and thermal decomposition. Later
Burluka et al.
15
measured the laminar burning velocities of
C3H6O isomers (PO, propanal, and acetone) in air at
atmospheric pressure. Propylene oxide was found to burn
faster than its isomers. A new kinetic scheme was proposed to
simulate the data obtained, but quantitative agreement
between measurements and predictions of PO/air flame
speed was not reached.
The experimental and numerical characterization of the
chemical structure of one-dimensional premixed burner-
stabilized laminar PO/O2/Ar flames was reported by Knyazkov
et al.
16
The authors have measured spatial distributions of
mole fractions of reactants, major products, and various
intermediates in the flames with dierent equivalence ratios
using flame sampling molecular beam mass spectrometry
(MBMS). Three kinetic models with dierent detail levels
were validated against the measurement data. It was shown
that the branching ratio between the isomerization pathways of
PO should be revisited.
Received: September 16, 2022
Accepted: November 24, 2022
Published: December 9, 2022
Article
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© 2022 The Authors. Published by
American Chemical Society 46900
https://doi.org/10.1021/acsomega.2c05999
ACS Omega 2022, 7, 4690046914
Ramalingam et al.
17
have investigated PO reactivity at low
and intermediate temperatures in a series of shock tube and
rapid compression machine experiments. Ignition delay times
were measured at dierent stoichiometries and pressures.
Kinetic analysis revealed vital sections for the improvement of
the model like the ketene subchemistry and pathways leading
to the formation of benzene.
Since PO is produced in large industrial scales and could be
easily transported, it was proposed as an oxygenated additive to
transportation fuels
18,19
to reduce soot content in the exhaust
of diesel engines. Recent study has shown that there is no
evidence that typical environmental or occupational exposures
to PO constitute a health risk for humans.
20
Therefore, the use
of PO as a fuel-improving agent may be promising.
It is well-known that PAHs and soot are formed in flames of
oxygen-containing fuels in smaller amounts than in flames of
hydrocarbons of comparable molecular weight.
21,22
This
suggests using oxygenates as PAH and soot-reducing additives
to traditional fuels. It was shown that the ability of oxygenates
to influence the pathways of PAHs and soot formation
depends not only on the number of oxygen atoms in a
molecule but also on the chemical structure of the molecule
itself.
2326
In this regard, many studies have been devoted to
the search and testing of oxygenated additives among alcohols,
ethers, esters, and ketones.
27
Various representatives of these
classes have been studied, either as individual fuels or as
additives to the conventional hydrocarbon fuels. Thus, the
search for an optimal oxygen-containing fuel/additive has
become a current research hotspot. At present, it is mainly
based on an understanding of the combustion chemistry of
these compounds. From this point of view PO looks attractive
since in flames it mostly isomerizes to acetone and
propionaldehyde and therefore can act as a blend of two
classes of oxygenates: ketone and aldehyde. However, to our
knowledge, there are no studies of the eect of PO additives on
soot and PAH formation during combustion of hydrocarbon
fuels.
Therefore, this work is focused on experimental and
numerical investigation of the eect of PO addition on the
species pool in a premixed burner-stabilized fuel-rich flame
fueled by n-heptane/toluene mixture. Examining this model
system in laboratory conditions, we aim to get insight into the
chemical aspects of influence of oxygenates with an epoxy
group on formation of abundant intermediates (including PAH
precursors) during combustion of fossil fuels. Similar to the
previous works,
2831
a mixture of n-heptane and toluene with
fixed ratio of these constituents (7/3 by volume of liquids) is
considered here as a surrogate of diesel fuel. Spatial
distributions of various species in the flames fuelled by the
n-heptane/toluene mixture with addition of dierent amounts
of PO are measured using the flame-sampling molecular beam
mass spectrometry and simulated with the detailed chemical
kinetic mechanisms available in the literature. Since the
uncertainty in MBMS measurements of the absolute mole
fractions of many intermediates in flames is quite high (up to
∼±50%), the ratio of the peak mole fraction of a certain
intermediate in the flame with and without PO addition
(which is determined with decreased uncertainty) is used in
this work as a target for validation of the kinetic models.
Performances and deficiencies of the kinetic models in
prediction of the observed tendencies are discussed. Kinetic
analysis is also conducted in order to elucidate if the eect of
PO on PAH precursors is due to partial replacement of
“sooting” fuel with oxygenated additive or due to chemical
interactions between the primary oxidation products of
dierent fuels.
2. EXPERIMENTAL DETAILS
The flames of n-heptane/toluene/O2/Ar mixtures with and
without propylene oxide addition stabilized on a Botha
Spalding type burner
32
at atmospheric pressure have been
investigated in this work. The burner and the mixture
preparation system have been thoroughly described earlier;
33
thus, only essential details are given below.
The matrix of the burner represented a perforated disc (0.5
mm orifices with 0.7 mm center-to-center spacing) 16 mm in
diameter and 3 mm thick, which was screwed into the brass
body of the burner. The burner temperature was kept at 95 °C
by thermostated water circulating through the water jacket in
the burner body. A fresh gaseous mixture consisting of gaseous
oxygen, argon, and vapors of n-heptane and toluene was
supplied into the burner from the electrically heated vaporizer
via a heated line. n-Heptane and toluene were blended in a
ratio of 7:3 (by volume of liquids) and supplied into the
vaporizer through a steel capillary using a syringe pump driven
by a stepper motor. The temperature of the vaporizer was kept
at 90 °C. The flow rates of gaseous oxygen and argon were
regulated by calibrated mass-flow controllers (Bronkhorst).
Propylene oxide has a significantly lower boiling temperature
(34 °C at 1 bar) as compared to that of toluene and n-heptane;
thus, it was supplied from a separate syringe pump via a
hypodermic needle directly into the heated line connecting the
vaporizer and the burner. This made it possible to ensure a
stable supply of highly volatile propylene oxide to the unburnt
mixture.
Fuel-rich, but nonsooting, laminar premixed flames with the
following parameters kept unchanged have been investigated in
this work:
n-heptane/toluene ratio: 7:3 by volume of liquids, which
is equivalent to their ratio of 1.68 in mole basis;
equivalence ratio: ϕ= 1.6;
argon content in the mixture: 0.75 in mole basis;
total mass flow rate of unburnt mixture: 0.0133 g/cm3s.
The flame conditions diered in the content of propylene
oxide in the fuel mixture. The ratio of liquid volumes of n-
Table 1. Molar Composition of the Reactant Mixtures
mole fraction, %
flame n-heptane/toluene/PO ratio (by vol. of liquids) PO content in fuel blend, % (mole basis) n-heptane toluene PO O2Ar
A (base flame) 7:3:0 0 2.12 1.26 0 21.63 75
B 7:3:0.1 1.9 2.10 1.25 0.07 21.59 75
C 7:3:0.2 3.8 2.08 1.24 0.13 21.56 75
D 7:3:0.5 8.9 2.02 1.20 0.32 21.46 75
E 7:3:1 16.3 1.94 1.15 0.60 21.31 75
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heptane/toluene/PO was varied from 7:3:0 to 7:3:1. Oxygen
content was also adjusted correspondingly to maintain the
same equivalence ratio ϕ= 1.6 ±0.02. The procedure on how
the uncertainty in the equivalence ratio determination was
estimated is given in the Supporting Information. In total, 5
flames have been examined with PO variation from 0% to
16.3% in mole basis in fuel mixture; their molar composition is
given in Table 1.
Mole fractions of various species in the flames were
measured as a function of the height above the burner
(HAB) using flame-sampling molecular beam mass spectrom-
etry (MBMS) with soft ionization by electron impact. The
experimental apparatus was thoroughly described earlier and
used in studies of chemical speciation of flames of liquid
fuels.
3336
Therefore, only essential details are provided in the
following.
Gas sampling was performed using a quartz conical probe
with an inner opening angle 40°and an orifice diameter of 0.08
mm. The wall at the nozzle tip had a thickness of 0.08 mm.
Behind the sampling probe, sampled gases formed a molecular
beam, which passed through a skimmer and a collimator and
entered the area of soft ionization by electrons emitted from a
tungsten filament. Compensation of voltage drop on the
filament allowed the spread in the electron energies to be
defined only by thermal distribution (fwhm 0.5 eV). The
energy of ionizing electrons (which could be varied in the
range of 820 eV) was selected individually for each species so
that, on the one hand, it was higher than the ionization energy
and provided a reasonable signal-to-noise ratio, and, on the
other hand, not high enough to cause the contribution of
fragmented ions from other species. The ions formed were
analyzed by a quadrupole mass spectrometer MS 7302. The
burner was mounted on a translation mechanism to manually
vary its vertical position (with the accuracy of ∼±10 μm) and
to scan over the distance between the burner surface and the
sampling probe tip. This allowed sampling the flames and
recording the signal intensities of the mass peaks of interest at
dierent heights above the burner (HAB).
The species detected in the flames are listed in Table S1,
which shows also the electron energies used, the calibration
method applied in this work, and the uncertainty in
determination the mole fractions for each species. The
calibration factors used to convert measured mass peak
intensities to the species mole fractions have been determined
according to the procedures thoroughly described in our earlier
works.
34,36,37
Briefly, the following two calibration methods
were used: direct calibration method (using gas mixtures of
known composition) and relative ionization cross-section
(RICS) method proposed by Cool et al.
38
(for radicals and
species for which gas mixtures were not feasible).
To eliminate the uncertainties induced by instrumental
inconstancies,
39
the measurements in all flames have been
carried out in the same experimental run. The following three
main sources of uncertainties in the mole fraction evaluation
can be distinguished: (1) the statistical error of the
measurement of the mass peak signals in the flame, (2) the
error in the determination of calibration coecients, which
depends on the calibration method used, and (3) the degree of
reliability in the separation of the contributions from parent or
fragmentary ions of other species. Typical measurement error
(relative standard deviation) of signal intensities of most mass
peaks relevant to this work was as a rule below 10%. Therefore,
a relative comparison of mole fraction of each species from
dierent flames oers a precision within 10%.
Flame disturbances are inevitable when using conical nozzles
for gas sampling. However, extensive studies carried out
earlier
4045
have shown that hydrodynamic perturbations
caused by the probe mainly due to gas suction may be
corrected by shifting the measured species mole fraction
profiles upstream to the burner. Similarly to as was done in
previous works focused on flame structure measure-
ments,
16,30,33,46
in this study all experimental mole fraction
profiles in each flame were shifted toward the burner by the
same distance (0.2 mm) to superpose the peak values of the
water mole fraction and temperature profiles.
The “disturbed” flame temperature profiles were measured
by an S-type thermocouple made of wires 0.03 mm in diameter
and coated with SiO2. During the measurements, the
thermocouple junction was located 0.2 mm upstream of the
probe tip. Corrections for radiation heat losses by the
thermocouple were made according to the procedure
described earlier.
47,48
The maximum uncertainty in the
temperature measurements did not exceed ±60 K. All
experimental data (temperature and species mole fraction
profiles) reported in this work are tabulated in the Supporting
Information.
3. MODELING
Chemical structures of the laminar one-dimensional premixed
flames studied experimentally in this work were simulated
using the PREMIX code from the CHEMKIN II package. The
calculations were carried out using the “burner-stabilized
flame” option in the PREMIX code simulations. The
temperature profiles measured in a manner described above
were used as input to account for the flame cooling eects by
the sampling probe.
Propylene oxide is usually ignored in detailed chemical
kinetic models for surrogate fuels, since it is a much less
abundant C3H6O intermediate in contrast with its isomers,
acetone and propanal. There are only few detailed chemical
kinetic mechanisms available in the literature, which involve
simulataneously all reactants presented in the fuel blend
studied in this work (n-heptane, toluene, propylene oxide). To
the best of our knowledge, they are (1) the kinetic mechanism
provided by CRECK Modeling at Politecnico di Milano
49
and
(2) C3MechV3.3 (3761 species and 16 522 reactions), a
detailed kinetic model for surrogate fuels developed by the
Computational Chemistry Consortium research team.
50
Both
mechanisms are hierarchically organized and self-consistent.
They contain detailed chemistry for small species (including
many oxygenates, and PO is among them) and important
surrogate fuel components for jet fuel, diesel, gasoline,
including PAHs. These mechanisms were used for kinetic
modeling of the flames in this work.
Recently, three dierent mechanisms for PO combustion
kinetics
5153
with dierent level of detail were validated against
the MBMS data for the chemical structure of premixed burner-
stabilized PO/O2/Ar flames.
16
Among all the mechanisms, the
kinetic mechanism of Capriolo et al.,
51
incorporating the most
of PO decomposition pathways including isomerization to
propanal and acetone, demonstrated a generally good
predictive capability.
This motivated us to also use it in the current work for
further validation against the new experimental data. The
mechanism of Dirrenberger et al.
29
was chosen as a base
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kinetic mechanism describing combustion kinetics of the n-
heptane/toluene mixture, similarly as was done previ-
ously.
30,31,36
Therefore, the specific intermediates and
reactions from the mechanism of Capriolo et al.
51
were
incorporated to the mechanism of Dirrenberger et al.
29
to
model the flames with propylene oxide additive. The resultant
mechanism (hereinafter denoted as D+K) involves 313 species
and 2258 reactions.
4. RESULTS AND DISCUSSION
This Section is organized as follows. First, the eect of PO
additive on temperature and mole fraction profiles of major
flame species (reactants and final products) is discussed. Then,
the influence of partial replacement of fuel with PO on the
mole fraction profiles of primary aromatic intermediates, major
flame radicals, and small hydrocarbon intermediates is
considered, and performances and deficiencies of the kinetic
mechanisms in predicting the observed tendencies are
discussed. Finally, some improvements to the chemistry of
acetylene, the major intermediate playing a crucial role in PAH
growth, are suggested, and a modified kinetic mechanism is
validated against the experimental data.
4.1. Temperature and Major Flame Species. Figure 1
shows the temperature profiles measured in the base flame
(Flame A) and the flames with dierent amount of PO in fuel
mixture (Flames BE). As is seen, the width of the flame zone
determined as the HAB where maximum temperature is
attained becomes narrower with the PO loading in the fuel
mixture; i.e., the flame with higher PO content in the mixture
tends to stabilize closer to the burner. This indicates the fact
that the flame speed rises as the PO content in the unburnt
mixture increases, which is quite expected because of higher
laminar burning velocity of PO
15
as compared to those of n-
heptane and toluene.
29
Thus, the addition of PO enhances
combustion of the base fuel. Nevertheless, as seen from Figure
1a, the postflame temperature gradually decreases with the rise
of PO loading (up to 110 K when 16.3% of the base fuel is
replaced with PO), because as the flame gets closer to the
burner, heat losses to the burner increase.
In Figure 2, mole fraction profiles of the major species (n-
heptane, toluene, PO, oxygen, water, CO, CO2, and H2)
measured in the base flame (Flame A) and the flame with
addition of 8.9% of PO in the fuel blend (Flame D) are
compared with the predictions by dierent detailed kinetic
mechanisms. As seen, all three mechanisms adequately
reproduce the experimental data and provide very similar
predictions of the profiles. Both experiment and modeling
show that addition of PO does not result in any notable change
in the postflame mole fractions of CO, CO2, H2O, H2. It can
also be seen from Figure 2 (as well as from Figure1) that the
flame zone becomes narrower with addition of PO in fuel.
4.2. Primary Aromatic Intermediates. Mole faction
profiles of some aromatic intermediates (benzene, phenol,
styrene, and ethylbenzene) measured in the flames with and
without PO addition are compared in Figure 3 with those
simulated using dierent detailed chemical kinetic mecha-
nisms. As seen, in general, all three mechanisms give
reasonable description of these experimental profiles in all
flames: the HAB where the peak mole fractions are attained
and also the shape of the profiles are predicted fairly well.
There are, however, some insignificant discrepancies between
the predictions and measurements of the peak mole fractions
of these species. Particularly, benzene peak mole fraction is
only slightly overpredicted by all mechanisms, phenol peak
mole fraction is overestimated by the D+K mechanism,
whereas styrene peak mole fraction is predicted precisely
only by D+K and CRECK mechanisms. As seen from the
experimental data presented in Figure 3, addition of up to
16.3% of PO to the fuel blend results in some changes in the
peak mole fractions of these aromatic intermediates, which are
however within the uncertainty in the determination of the
Figure 1. Temperature profiles measured in the flames with dierent
loading of PO in the fuel blend.
Figure 2. Measured (symbols) and simulated (lines) mole fraction profiles of the reactants and major products in the flames. Base flame (Flame
A): open symbols and solid lines. Flame with addition of 8.9% PO in fuel (Flame D): filled symbols and dashed lines.
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absolute mole fractions. Nevertheless, these experimental data
can be used to assess the eect of PO addition on the relative
change in the species maximum mole fractions, which was
measured with higher precision (see Experimental Details
section).
Both experiment and modeling showed that the higher
content of PO in the fuel blend, the lower is the peak mole
fractions of these intermediates in the flame. To clearly
demonstrate this fact, in Figure 4, we plotted normalized peak
mole fractions of these four species (calculated as a ratio of the
peak mole fraction of a particular intermediate in the flame
with PO to that in the base flame) versus the percentage of PO
in the fuel blend. As seen from this figure, all three mechanisms
provide quite reasonable reproduction of the eect of PO
additive in terms of relative reduction of the maximum mole
fractions of the aromatic intermediates in the flame of n-
heptane/toluene mixture. Nevertheless, CRECK mechanism
and C3Mech provide a better agreement with the experimental
data.
Both experiment and simulations show that the eect of PO
additive on these species is linear with PO loading, at least up
to 16% of PO in fuel. If this eect was due exclusively to the
partial replacement of “sooting” fuel (toluene) with oxygenated
additive, then (all other conditions being equal) one would
expect that the PO influence to the peak mole fractions of all
the considered aromatic intermediates would be the same.
However, according to measurements, benzene and phenol
peak mole fractions are reduced by 25 ±3%, whereas styrene
and ethylbenzene are reduced by 36 ±4% and 33 ±4%,
respectively, when 16.3% PO is added in fuel. As seen from
Figure 4, all mechanisms also predict the same tendency. In
order to reveal the reasons for such behavior and, therefore, to
understand the influence of PO on the kinetics of formation of
these species, we analyzed (1) the temperature at which the
peak mole fractions of these species in the flames are reached
(Tpeak), and (2) the reaction pathways of formation of these
species in the flame with and without PO additive.
Figure 3. Measured (symbols) and simulated (lines) mole fraction profiles of some aromatic intermediates (benzene, phenol, styrene,
ethylbenzene) in the base flame (Flame A) and in the flames with dierent loading of PO (1.9%, 3.8%, 8.9%, 16.3% in fuel: Flames BE,
respectively).
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Figure 5 shows the Tpeak plotted versus the PO content in
the fuel mixture (exemplarily, according to CRECK mecha-
nism). As seen, in all flames this temperature is nearly the same
for a particular species, and it varies only within ±15 K. The
dependencies shown in Figure 5 clearly demonstrate the order
(from the burner) in which these aromatic intermediates form
in the flames without and with PO: ethylbenzene styrene
phenol and benzene. The fact that the amount of PO loading
does not influence this order may indicate that PO and its
primary decomposition products have no eect on the
chemistry of primary pathways of toluene oxidation, since all
these intermediates are expected to be mainly the products of
toluene decomposition.
Figure 6 schematically summarizes the major reaction
pathways of formation of these intermediates from toluene,
as derived from the rate-of-production analysis in the flame
without PO additive according to C3Mech at the temperatures
corresponding to particular Tpeak values shown in Figure 5
(e.g., 1420 K for benzene). This schematic does not indicate
the percentage contribution of each pathway to the total rate of
production of corresponding intermediate, because it slightly
varies when switching between flames with dierent amount of
PO additive. Analysis of other kinetic mechanisms considered
in this work provides very similar structure of reaction
pathways.
As seen from Figure 5, among all these intermediates,
ethylbenzene peak mole fraction is reached at the lowest
temperature in the flames (1285 K), and it is obvious,
because, according to all chemical kinetic mechanisms
considered in this work, this intermediate is generally formed
virtually directly from toluene via the formation of benzyl
radical (C7H7):
+ +C H H C H H
7 8 7 7 2
(1)
+C H CH C H C H
7 7 3 6 5 2 5
(2)
Styrene mole fraction peaks at a higher temperature (1385
K), because its main precursor is 1-phenylethyl radical
(C6H5C2H4), which is formed during H-abstraction from
ethylbenzene:
+ +C H C H H C H C H H
6 5 2 5 6 5 2 4 2
(3)
+C H C H C H C H H
6 5 2 4 6 5 2 3
(4)
Noteworthy that benzene and phenol peak mole fractions are
attained at even higher temperature (1420 K) although it
would seem that there are all the prerequisites for their
formation practically directly from toluene, particularly for
benzene, via the reaction of CH3abstraction:
+ +C H H C H CH
7 8 6 6 3
(5)
Nevertheless, according to all three models, a decomposition
of 2-methyl-phenoxy radical contributes to a greatest extent to
benzene formation:
+ +OC H CH C H H CO
6 4 3 6 6
(6)
This radical in all the mechanisms is formed via the following
chain of transformations:
Figure 4. Normalized peak mole fraction of benzene (C6H6), phenol (C6H5OH), styrene (C6H5C2H3), and ethylbenzene (C6H5C2H5) vs the
content of PO in the fuel blend.
Figure 5. Temperature, at which the maximum peak mole fractions of
some aromatic intermediates (see the legend) is reached in the flames,
plotted vs PO content in the fuel mixture (according to CRECK
mechanism).
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+ +
toluene C H CH OC H CH
OH,OH / H , H
6 4 3
O , O
6 4 3
2 2 2
(7)
where C6H4CH3is 2-methylphenyl radical.
Phenoxy radical (C6H5O) is the main precursor of phenol
due to the reaction of H addition: C6H5O+HC6H5OH,
while C6H5O radical is formed mainly via the following chain
of reactions:
(8)
+ +C H O C H O O
6 5 2 6 5
(9)
Therefore, the fact that the predictions of the eect of PO
addition on the peak mole fraction of such intermediates like
ethylbenzene, styrene, phenol, and benzene are in good
agreement with our measurements (Figure 5) indicates the
validity of the reaction pathways mentioned above in the
flames of n-heptane/toluene mixture with and without the
addition of PO.
4.3. Major Flame Radicals. The “chemical” eect of PO
additive may be due to its influence on the pool of major flame
radicals (H, O, OH, CH3), playing a crucial role in fuel
conversion in flame conditions (as also seen from the above
analysis). Since addition of PO to the fuel blend enhances
combustion of the mixture, this may have a notable eect on
the ratio of the abundances of these radicals, because their
concentrations are determined by the balance in the reactions
of chain branching (e.g., H+O2OH+O), chain propagation,
chain termination, and heat release (CO+OH CO2+H). In
order to validate the kinetic mechanisms in their ability to
predict the eect of partial replacement of parent fuel with PO
on the major flame radicals, we compared the predictions with
the measurements of H, OH, and CH3mole fraction profiles
(Figure 7). As seen, all three models adequately capture the
shape of the profiles, and taking into account the diculties
and high uncertainties in measurements of the radicals, one can
observe a fairly good agreement between predictions and
observations of the peak mole fractions of H, OH, and CH3in
the flame without PO. All mechanisms predict a reduction in H
and OH peak mole fractions, whereas CH3maximum
abundance remains practically unchanged when PO is added.
As seen, the experimental observations demonstrate the similar
tendency. Although PO addition enhances the combustion of
the fuel studied, the H and OH abundances decrease due to
increased heat losses into the burner.
Unfortunately, we could not measure O mole fraction
profiles in the flames, since this is a challenging task due to the
diculties in separating O from methane (a major
intermediate), having the similar m/zratio and close ionization
energies (13.6 and 14.25 eV, respectively). Thus, in Figure 8
we present the calculated O mole fraction profiles only. As is
seen, all three mechanisms predict quite low (2.52.7 x105)
and virtually unchanged peak mole fraction of this radical when
switching from the base flame to the flame with 8.9% PO.
Therefore, summarizing the obtained results on the eect of
PO on the flames radicals, one can conclude that propylene
oxide additive changes the ratio between H, OH, O, and CH3
Figure 6. Major reaction pathways of formation of ethylbenzene, styrene, phenol, benzene in the flames (according to C3Mech).
Figure 7. Measured (symbols) and simulated (lines) mole fraction profiles of the major flame radicals. Base flame (Flame A): open symbols and
solid lines. Flame with addition of 8.9% PO in fuel (Flame D): filled symbols and dashed lines.
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toward an increase in the proportion of O and CH3. This in
turn cannot but aect the concentrations of other compounds.
4.4. Small Hydrocarbon Intermediates. Addition of PO
may result in significant changes in the mole fractions of small
hydrocarbons, which are believed to be important species
involved in the reactions of formation of PAH (see recent
reviews, e.g.,
54
). We analyzed the main reaction pathways
responsible for formation of cyclopenta[c,d]pyrene radical
(C18H9), one of the heaviest PAH included in the detailed
kinetic mechanism C3MechV3.3 (the schematic of these
reaction pathways is shown in Figure S1). Figure S1 clearly
shows that the species with one aromatic ring serve as
precursors for formation of intermediates with 2, 3, and more
aromatic rings. Small hydrocarbons involved in the corre-
sponding reactions of PAH growth, like acetylene, propargyl
radical, allene, propyne, cyclopenadienyl (C5H5), are also
indicated in the diagram in Figure S1. Adequate prediction of
mole fractions of these hydrocarbons in the flames with PO
addition is of particular importance to reliably describe the
chemistry of PAH formation during combustion of fuel blends
with addition of propylene oxide.
This motivated us to validate the chemical kinetic
mechanisms against our experimental data for C1C4
intermediate hydrocarbons detected in the flames with and
without PO addition. A comparison of predicted and measured
mole fraction profiles of these intermediates in the base flame
and, exemplarily, in the flame with 8.9% PO in fuel blend
(Flame D) is shown in Figure 9. The species corresponding to
mass peaks 40 (allene+propyne) and 42 (ketene+propene), 56
(1-butene+2-butene+acrolein) were not separated due to very
low dierence between the ionization energies of correspond-
ing components (see Table S1), so their summarized mole
fraction profiles are represented in Figure 9.
In general, all three mechanisms give reasonable prediction
of these experimental profiles in the base flame: the HAB
where the peak mole fractions are attained and also the shape
of the profiles are predicted fairly well for most species.
However, a qualitative discrepancy between the predictions
and measurements can be seen for methane (CH4) and
acetylene (C2H2) mole fraction profiles: all three models
predict a nonzero concentration of these intermediates in the
postflame zone, while their complete consumption is observed
in the experiment. This disagreement for methane and
acetylene in the postflame zone in fuel-rich atmospheric-
pressure premixed flames is a well-known diculty encoun-
tered earlier,
30,31,34,50,5557
indicating that methane and
acetylene chemistry in fuel-rich flames still needs to be further
improved. Particular attention should be paid to acetylene
chemistry, since, as clearly seen from Figure 8, it pays a crucial
role in PAH growth reactions (basically according to the H-
abstraction-carbon (acetylene) addition (HACA) mecha-
nism
54,58,59
)
Figure 8. Simulated mole fraction profiles of O radical in Flame A
(solid lines) and Flame D (dashed lines).
Figure 9. Measured (symbols) and simulated (lines) mole fraction profiles of C1C4intermediate products in the flames. Base flame (Flame A):
open symbols and solid lines. Flame with addition of 8.9% PO in fuel (Flame D): filled symbols and dashed lines.
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As seen from Figure 9, the addition of 8.9% PO to the fuel
mixture does not result in any notable change in the peak mole
fractions of such intermediates, like propargyl, ketene
+propene, 1-butene+2-butene+acrolein. In this regard, the
predictions by all three models and the experimental
observations are in good agreement with each other.
Noteworthy that all the mechanisms also predict the peak
mole fractions of other intermediates (methane, acetylene,
ethylene, allene+propyne and diacetylene) to be practically
unchanged when 8.9% PO is added to the fuel mixture, as
compared to the base flame. However, the measured peak mole
fractions of methane, acetylene, ethylene, allene+propyne and
diacetylene significantly decrease (by nearly 2 times) with the
replacement of 8.9% of fuel with PO. Noteworthy that in the
fuel-rich burner-stabilized flames fuelled by pure PO,
16
the
kinetic mechanisms available in the literature for propylene
oxide
5153
also overpredicted methane, ethylene, and acetylene
peak mole fractions. Thus, we can conclude that the
discrepancies for small hydrocarbons observed in this and
earlier
16
works can be explained by (1) the deficiencies of
propylene oxide primary decomposition submechanisms used
in the kinetic models, and (2) the deficiencies of the core
hydrocarbon submechanisms used in the models.
4.5. Refinement of Acetylene Chemistry. Understand-
ing the chemistry of formation and consumption of acetylene is
of particular importance to correctly predict the formation of
PAH in the flames. In this relation, here we focused on
revealing the reaction pathways that should be revised to
improve predictive ability of the kinetic models. For this
purpose, first of all, we carried the sensitivity analysis to
elucidate the reactions to the rate constants of which the
acetylene mole fraction is sensitive in the flames with PO but
not sensitive in the base flame.
Figure 10 shows the C2H2sensitivity coecients calculated
for the height above burner of 2 and 1.5 mm corresponding to
peak mole fraction of acetylene in the flames without and with
8.9% PO in fuel (Flame A and Flame D, respectively). As seen,
the peak mole fraction of C2H2has a significant sensitivity to
the reactions involving oxygen atom: H2+OH+OH, H+O2
O+OH, and O+C2H2H+HCCO, since the reactions of
acetylene with O atoms play a key role in C2H2consumption
(in particular, the latter one) in the flames. The C2H2peak
mole fraction is also very sensitive to the reaction H
+C2H2(+M)C2H3(+M), which plays an important role in
acetylene formation. It is particularly remarkable that in the
flame with PO addition almost all reactions shown in Figure 10
have higher sensitivity coecients than in the flame without
PO, which is most likely related to the changes in the ratio
between the flame radicals noted in Section 4.3. Besides, C2H2
peak mole fraction is much more sensitive to the reactions
HCO+M = H+CO+M (with a negative sensitivity coecient)
and 2CH3(+M)C2H6(+M) (with a positive sensitivity
coecient) in the flame with PO than in the base flame, in
which their sensitivity coecients are negligible. This can also
be associated with the increased relative fraction of methyl
radicals in the flame with PO, which also was discussed in
Section 4.3. Therefore, an improvement of chemistry of
formation of methyl, formyl and ethane in the PO
submechanism may probably help to provide more accurate
prediction of acetylene in the flames fueled by the blends with
PO.
To check this idea, we carried out an analysis of the key
reaction pathways of PO transformation to these small
hydrocarbons in the flame of triple n-heptane/toluene/PO
fuel blend. For the sake of brevity, the primary PO
decomposition pathways constructed basing on the rate of
production (ROP) analysis in the flame with addition of 8.9%
PO in fuel (Flame D) are given and discussed in the
Supporting Information. Summarizing the results of this
analysis, we come to a conclusion that PO presence in the
flames favors to rapid formation of CH3, C2H5, and HCO
radicals. Since PO is dominantly consumed in the flame via the
isomerization to propanal and acetone, propanal largely serves
as a precursor of HCO and C2H5radicals, whereas acetone
mainly decomposes to form CH3radicals. This explains the
positive sensitivity coecient of C2H2to 2CH3(+M)
C2H6(+M) reaction and the negative one to the reaction
involving HCO: HCO+MH+CO+M (Figure 10). Thus, a
more accurate prediction of acetylene peak mole fraction in the
flames with PO may be reached by a refinement of the
branching ratio for the reactions of PO isomerization to
acetone and propanal in the flame. Particularly, according to
the ROP-analysis, the presence of PO in the flame contributes
to the formation of acetylene due to the chain of H-abstraction
reactions with flame radicals: C2H5C2H4C2H3C2H2.
Therefore, an increase in the kacetone/kpropanal ratio (isomer-
ization reaction rate constants of PO to acetone and propanal,
respectively) could probably decrease the predicted peak mole
fraction of acetylene in the flames with PO additive. This
hypothesis is supported by the fact that in the burner-stabilized
flames of acetone, a lower mole fraction of acetylene is formed
as compared to the flame fuelled by propanal in similar flame
conditions.
60
We varied the ratio of pre-exponential factors of
kacetone and kpropanal within 2 orders of magnitude in the
mechanism C3Mech; however, no notable change in the
predicted C2H2peak mole fraction as compared to unmodified
C3Mech was observed. This, most likely, is related to too low
C2H2sensitivity to the reactions R154 and R130 (Figure 10),
Figure 10. Sensitivity coecients of C2H2peak mole fraction
(C3Mech).
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and also related to the conversion of ethane to ethyl radicals
via H-abstraction reactions.
As was mentioned above and as seen from Figure 10,
another option suggesting an improvement of C2H2prediction
in the flames (particularly, in the postflame zone) is a revision
of key reactions responsible for acetylene consumption and
production in the core hydrocarbon mechanism. To reveal
such reactions in dierent zones of the flame, we analyzed the
C2H2sensitivities at various heights above the burner, and they
are depicted in Figure 11.
As seen, for most reactions, the sensitivity coecient of
C2H2strongly depends on the height above the burner;
therefore, reconsidering the rate constants of the reactions, the
sensitivity of which is dierent in the high and low temperature
zones of the flame, is needed to improve the model in its
prediction of C2H2across the flame zone. Among these
reactions, there are the reactions whose rate constants were
extensively studied in the literature and reliably evaluated, and
they are the following: the chain branching reactions (R3,
R10), the key heat release reaction (R42), the reaction of vinyl
radical with O2(R309), and the reaction of H abstraction from
ethylene (R283). Therefore, in this work, we paid a particular
attention to the rate constant parameters of only some of the
reactions shown in Figure 11 and also listed in Table 2.
For the reaction of C2H3consumption (R306), the rate
constant parameters suggested by Miller and Klippenstein
61
are used in C3Mech. However, Ma et al.
62
in their study of
premixed ethylene flames used the rate constant for this
reaction as suggested in a more recent investigation of Wang et
al.,
63
who proposed using its direct approximation via
Chebyshev expansion
64
to yield a more reliable and accurate
dependence on pressure and temperature. The rate constant
parameters for the reaction R372 used in the C3Mech are
taken from the work of Senosiain et al.
65
However, in a more
recent study, Liu et al.
57
suggested a new rate coecient
expression formulated by regressing the available experimental
data for this reaction rate constant in high temperature region.
Thus, we modified the C3Mech mechanism by adopting the
updated rate constant parameters for the reaction R306 and
R372 from the works of Wang et al.
63
and Liu et al.,
57
respectively. Moreover, in the modified mechanism, for the
reaction R58, we used the rate constant recently measured by
Wang et al.,
67
and for the reactions of C2H2with O (R375 and
R376), we adopted the rate constants from the LLNL detailed
mechanism for n-heptane combustion.
66
The C3Mech with the
modifications mentioned above is available in CHEMKIN
format in the Supporting Information. The comparisons of the
predictions of C2H2and selected PAHs mole fraction profiles
using the original C3Mech and its modified version
(C3Mech_mod) in the base flame (Flame A) and the flame
with 8.9% PO in fuel (Flame D) are presented in Figures
1213.
As seen from Figure 12, the revision of the above-mentioned
reaction rate constants results in a decreased peak and
Figure 11. Sensitivity coecients of C2H2in Flame D (according to
C3Mech).
Table 2. List of Reactions Modified in C3Mech to Refine Prediction of Acetylene Mole Fraction Profile in the Flames
rate constant in modified Arrhenius form
k=ATnexp(Ea/RT), the units are K, s1, cm3and cal/mol
no. reaction A n E ref
R372 OH+C2H2= H+CH2CO 1.33 ×1013 0.11 11059 57
R375 C2H2+O = CH2+CO 6.120 ×1006 2.00 1.900 ×1003 66
rev/1.152 ×1006 2.00 5.257 ×1004/
R376 C2H2+O = HCCO+H 1.430 ×1007 2.00 1.900 ×1003 66
rev/2.021 ×1005 2.00 1.331 ×1004 /
R58 CH4(+M) = CH3+H(+M) 2.10 ×1016 0.00 104913.6 67
LOW/3.91 ×1017 0.00 89812.4/
TROE/0.50 1350 1350 7834 /
rate constant in Chebyshev format
64
R306 H+C2H2(+M) = C2H3(+M) 1.00 0.0 0.0 62
TCHEB / 500.0 2000.0 /
PCHEB / 1.00 ×1003 1.00 ×1002 /
CHEB/7 4 1.0631 ×10012.2524 ×10001.5114 ×10016.3036 ×1002/
CHEB/ 3.3509 ×10012.4814 ×10011.4262 ×10014.8996 ×10023.5145 ×1001/
CHEB/ 5.9670 ×10033.3570 ×10038.6600 ×10039.3740 ×10029.5393 ×1003/
CHEB/ 5.4786 ×10039.9965 ×10042.0417 ×10022.9275 ×10032.2531 ×1003/
CHEB/ 1.2784 ×10032.2207 ×10033.0803 ×10043.5237 ×10043.7207 ×1004/
CHEB/8.8859 ×10042.4405 ×10041.2987 ×10041.4597 ×1005/
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postflame mole fraction of acetylene in the flames with and
without PO and therefore leads to a better agreement with
experimental data. Nevertheless, the modified mechanism, as
well as the original one, overpredicts C2H2peak mole fraction
in the flame with addition of PO. It is interesting that previous
studies of the eect of oxygenates on the chemical speciation
of atmospheric-pressure premixed hydrocarbon flames (partic-
ularly, ethanol to ethylene,
34
and methyl pentanoate to n-
heptane/toluene mixture
30
) also showed a strong inhibiting
influence of the oxygenates on the peak mole fraction of
acetylene in the flames, which was underpredicted by detailed
kinetic models. Summarizing these findings, one can conclude
that the observed discrepancy may be related to an inability of
kinetic mechanisms to accurately predict O atoms’ concen-
tration (especially in the flames of oxygenated fuels), which are
involved in the reactions of acetylene consumption. Low
concentrations of atomic oxygen in fuel-rich conditions and
diculties encountered by experimentalists in its measure-
ments (they were mentioned in Section 4.3) do not allow an
adequate validation of the core kinetic mechanisms against the
O measurements in flames, which are fairly scarce, see, e.g.
68,69
The revisions made to the C3Mech resulted in negligibly
slight changes in the mole fraction profiles of small
hydrocarbons and small PAHs (with 13 aromatic rings),
however, led to some improvements in prediction of methane
profiles (they are shown in Figure S5). Figure 13 exemplarily
demonstrates the predicted mole fraction profiles of phenan-
threne (3 aromatic rings, C14H10), which is formed in the
reaction involving (but not limited to) acetylene and biphenyl
radical (Figure S1). Both modified and unmodified mecha-
nisms predict very similar phenanthrene profiles in Flame A
and Flame D, and the eect of PO addition is very weak
according to the modeling. As also seen from Figure 13, the
eect of acetylene chemistry is more significant on pyrene (4
aromatic rings) mole fraction profiles: the modified mechanism
predicts a lower peak mole fraction of this species in both
flames. Noteworthy that the updated acetylene chemistry has a
substantial influence on the predicted mole fraction profiles of
C18H10 and C20H10, also shown in Figure 13: their mole
fractions predicted by the modified mechanism are up to
75% lower than those calculated using the original
mechanism. Since formation of a particular PAH proceeds
via several steps of C2H2addition, the production of heavier
PAH molecules becomes more sensitive to C2H2concen-
tration.
Therefore, Figures 12 and 13 clearly demonstrate that
accurate prediction of acetylene is vital for adequate
reproducing the mole fraction profiles of PAHs. The fact
that the kinetic mechanism significantly underestimates the
eect of PO additive on C2H2peak mole fraction indicates
indirectly that the mechanism is expected to significantly
overpredict the heavy PAHs (4 and more aromatic rings) in
the flames with PO. We were not able in this work to measure
these PAH species using our experimental setup to verify this
modeling result. However, it should be noted that these
compounds are usually very dicult to measure due to their
Figure 12. Acetylene mole fraction profiles measured and predicted
by the original and modified (see text) C3Mech. Open symbols and
solid lines: base flame (Flame A); filled symbols and dashed lines:
flame with 8.9%PO in fuel (Flame D).
Figure 13. Mole fraction profiles of selected PAHs predicted by the original and modified C3Mech.
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low concentrations and therefore their observations in the
flames are very scarce in literature, see e.g.
70,71
The
measurements of heavy PAHs in the flames fueled by the
mixtures of hydrocarbons with propylene oxide are thus
needed to further validate and improve the PAH formation
submechanism in the available kinetic models for cocombus-
tion of oxygenates and hydrocarbon fuels.
5. CONCLUSIONS
In this work, flame sampling molecular beam mass
spectrometry was used to obtain new experimental data on
the chemical speciation of laminar premixed fuel-rich (ϕ= 1.6)
burner-stabilized flames fueled by n-heptane/toluene mixture
(7:3 by volume of liquids) with and without addition of
propylene oxide. In total, the chemical structures of 5 flames
with various loading of PO in the unburnt gas (from 0% to
16.3% in fuel mixture in mole basis) were studied. Spatial
distributions of mole fractions of reactants, major products,
and some intermediates (small hydrocarbons, some major
flame radicals and aromatic species) above the burner were
measured. Three detailed kinetic mechanisms were validated
against the measurement data: CRECK, C3Mech (both are
available in the literature), and D+K mechanism (developed by
merging the mechanism of Dirrenberger et al.,
29
and the
propylene oxide chemistry adopted from the work of Capriolo
et al.
51
). All three mechanisms have been shown to perform
generally well in reproducing the measurements, however,
some quantitative discrepancies between predictions and
observations were found.
According to both modeling and experiment, addition of
propylene oxide to the base fuel results in a higher reactivity of
the mixture and also in a reduction of the peak mole fraction of
aromatic intermediates (benzene, styrene, ethyl benzene,
phenol). Analysis of the reaction pathways of formation of
these intermediates showed that their reduction in the flame
with PO is due to simultaneously two factors: (1) the partial
replacement of “sooting” fuel (toluene, which is the main
precursor of these species) with oxygenated additive, and (2)
the changes in the flame radical pool caused by PO addition.
Propylene oxide additive was found to change the ratio
between H, OH, O, and CH3toward an increase in the
proportion of O and CH3. This in turn aects the
concentrations of other hydrocarbon intermediates in the
flame.
All tested kinetic mechanisms were found to underestimate
the eect of PO additive on the peak mole fractions of
methane, acetylene, ethylene, allene+propyne and diacetylene.
Since acetylene is an intermediate playing a key role in PAH
growth, its chemistry was revisited in order to provide a better
prediction of PAHs. The sensitivity analysis for C2H2and the
ROP analysis of PO oxidation pathways have shown that PO
contributes to the formation of acetylene due to favoring the
rapid production of CH3, C2H5and HCO radicals. Since
primary products of PO transformation in the flame are
propanal and acetone, these two intermediates serve as
precursors of HCO and C2H5(propanal), and CH3(acetone).
This allowed concluding that a variation of the branching ratio
for the reactions of PO isomerization to acetone and propanal
may have an eect on the predicted C2H2mole fraction.
However, this hypothesis was not confirmed. The sensitivity
analysis allowed revealing the reactions playing a key role in
acetylene formation and consumption. Their rate constant
parameters were updated in the C3Mech, as the most detailed
mechanism including heavy PAHs, according to the recent
findings in literature. The modified C3Mech was shown to
have a better predictive ability of acetylene mole fraction
profiles in the flames with and without PO.
The revisions made to the mechanism substantially aected
on the predicted mole fraction profiles of PAHs with 4
aromatic rings and heavier ones (C18H10 and C20H10) clearly
demonstrating that a more accurate prediction of acetylene is
vital for adequate reproducing the mole fraction profiles of
PAHs. The measurement data reported in this work extend the
available experimental data in the literature for combustion of
propylene oxide and its cofiring with diesel fuel surrogate, and
can be used as a guidance for further refinement of the
submechanisms for PO combustion and PAH formation.
ASSOCIATED CONTENT
*
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsomega.2c05999.
Tabulated experimental data (profiles of temperature
and species mole fractions) (XLSX)
Files of the modified C3Mech mechanism in CHEM-
KIN format (ZIP)
Supplementary figures (Figures S1S5) and Table S1
(PDF)
AUTHOR INFORMATION
Corresponding Author
Denis A. Knyazkov Voevodsky Institute of Chemical
Kinetics and Combustion, Novosibirsk 630090, Russia;
orcid.org/0000-0002-6819-4935; Email: knyazkov@
kinetics.nsc.ru
Authors
Artëm M. Dmitriev Voevodsky Institute of Chemical
Kinetics and Combustion, Novosibirsk 630090, Russia;
orcid.org/0000-0002-0162-7636
Ksenia N. Osipova Voevodsky Institute of Chemical Kinetics
and Combustion, Novosibirsk 630090, Russia; orcid.org/
0000-0001-5025-033X
Andrey G. Shmakov Voevodsky Institute of Chemical
Kinetics and Combustion, Novosibirsk 630090, Russia;
orcid.org/0000-0001-6810-7638
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsomega.2c05999
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Ministry of Science and
Higher Education of the Russian Federation (Project No.: 075-
15-2020-806).
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... MCH vapors mixed with oxygen and argon, forming a premixed fuel mixture, and entered the burner through a heated line. The temperature and concentration of compounds in the flames were measured as a function of the distance from the burner surface using a microthermocouple method and the MBMS setup [27,28], respectively. The distance between the sampling probe and the burner was adjusted using a microscrew and measured with a cathetometer (measurement error no more than ±0.005 mm). ...
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Methylcyclohexane (MCH, C7H14) is a typical component in hydrocarbon fuels and is frequently utilized in surrogate fuel mixtures as a typical representative of alkylated cycloalkanes. However, comprehensive experimental studies on speciation during its combustion remain limited. This research investigates for the first time the chemical structure of laminar premixed flames of lean and stoichiometric mixtures (φ = 0.8 and 1.0) of MCH/O2/Ar under atmospheric pressure. Using probe-sampling molecular-beam mass spectrometry (MBMS), the spatial distribution of 18 compounds, including reactants, products, and intermediates, in the flame front was measured. The obtained results were compared with numerical simulations based on three established chemical–kinetic models of MCH combustion. The comparative analysis demonstrated that while the models effectively describe the profiles of reactants, primary products and key intermediates, significant discrepancies were observed for various C2–C6 compounds. To indicate the roots of the discrepancies, a rate of production (ROP) analysis was performed in each simulation. ROP analyses revealed that the primary cause for the discrepancies could be attributed to the overprediction of the rates of initial stages during MCH decomposition. Particularly, the role of non-elementary reactions was emphasized, indicating the need for refinement of the mechanisms based on new experimental data.
... In particular, according to AramcoMech 2.0, the major reaction pathway of acetylene production is H-abstraction from the vinyl radical by H attack: C 2 H 3 + H(+M) ↔ C 2 H 2 + H 2 (+M). The acetylene mole fraction is very sensitive to the rate parameter of this reaction [56]. Its value, used in AramcoMech 2.0, was suggested by Miller and Klippenstein [57]. ...
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Understanding and controlling the combustion of clean and efficient fuel blends, like methane + hydrogen, is essential for optimizing energy production processes and minimizing environmental impacts. To extend the available experimental database on CH4 + H2 flame speciation, this paper reports novel measurement data on the chemical structure of laminar premixed burner-stabilized CH4/H2/O2/Ar flames. The experiments cover various equivalence ratios (φ = 0.8 and φ = 1.2), hydrogen content amounts in the CH4/H2 blend (XH2 = 25%, 50% and 75%), and different pressures (1, 3 and 5 atm). The flame-sampling molecular-beam mass spectrometry (MBMS) technique was used to detect reactants, major products, and several combustion intermediates, including major flame radicals. Starting with the detailed model AramcoMech 2.0, two reduced kinetic mechanisms with different levels of detail for the combustion of CH4/H2 blends are reported: RMech1 (30 species and 70 reactions) and RMech2 (21 species and 31 reactions). Validated against the literature data for laminar burning velocity and ignition delays, these mechanisms were demonstrated to reasonably predict the effect of pressure and hydrogen content in the mixture on the peak mole fractions of intermediates and adequately describe the new data for the structure of fuel-lean flames, which are relevant to gas turbine conditions.