19 research outputs found
Computational Study of the Combustion and Atmospheric Decomposition of 2āMethylfuran
There
is a growing interest in alkylfurans as potential biofuels. Recent
work has highlighted the need for further study of the atmospheric
oxidation mechanism of 2-methylfuran (2MF). This study utilizes the
high level composite computational methods, G4 and CBS-QB3, to determine
the bond dissociation energies for the CāH bond in 2MF and
the reaction enthalpies and barrier heights of several of the known
possible initiation reaction pathways. This study also investigates
the possible subsequent low temperature reaction pathways following
the addition of OH and then O<sub>2</sub> onto the 2MF ring. The placement
of the OH and O<sub>2</sub> on the ring, either cis or trans to each
other, dictates the viability of subsequent reactions. Of particular
interest is the observation that 1,4 H-migrations that abstract the
hydrogen bound to the same carbon as the OH have abnormally low barrier
height. This dramatic decrease puts the reaction barrier lower than
concerted eliminations and 6-membered ring Waddington-type reactions.
In addition, a novel reaction type, described as a Waddington concerted
elimination, is reported herein. This reaction, when viable, is generally
more favorable than other reactions. The results presented here are
of interest to combustion modelers and atmospheric chemists, particularly
those working on aromatic hydrocarbons and systems with conjugated
double bonds
Numerical modelling of ion transport in flames
<p>This paper presents a modelling framework to compute the diffusivity and mobility of ions in flames. The (<i>n</i>, 6, 4) interaction potential is adopted to model collisions between neutral and charged species. All required parameters in the potential are related to the polarizability of the species pair via semi-empirical formulas, which are derived using the most recently published data or best estimates. The resulting framework permits computation of the transport coefficients of any ion found in a hydrocarbon flame. The accuracy of the proposed method is evaluated by comparing its predictions with experimental data on the mobility of selected ions in single-component neutral gases. Based on this analysis, the value of a model constant available in the literature is modified in order to improve the model's predictions. The newly determined ion transport coefficients are used as part of a previously developed numerical approach to compute the distribution of charged species in a freely propagating premixed lean CH<sub>4</sub>/O<sub>2</sub> flame. Since a significant scatter of polarizability data exists in the literature, the effects of changes in polarizability on ion transport properties and the spatial distribution of ions in flames are explored. Our analysis shows that changes in polarizability propagate with decreasing effect from binary transport coefficients to species number densities. We conclude that the chosen polarizability value has a limited effect on the ion distribution in freely propagating flames. We expect that the modelling framework proposed here will benefit future efforts in modelling the effect of external voltages on flames. Supplemental data for this article can be accessed at <a href="http://dx.doi.org/10.1080/13647830.2015.1090018" target="_blank">http://dx.doi.org/10.1080/13647830.2015.1090018</a>.</p
Combustion Characteristics of C<sub>5</sub> Alcohols and a Skeletal Mechanism for Homogeneous Charge Compression Ignition Combustion Simulation
C<sub>5</sub> alcohols are considered
alternative fuels because they emit
less greenhouse gases and fewer harmful pollutants. In this study,
the combustion characteristics of 2-methylbutanol (2-methyl-1-butanol)
and isopentanol (3-methyl-1-butanol) and their mixtures with primary
reference fuels (PRFs) were studied using a detailed chemical kinetic
model obtained from merging previously published mechanisms. Ignition
delay times of the C<sub>5</sub> alcohol/air mixtures were compared
to PRFs at 20 and 40 atm. Reaction path analyses were conducted at
intermediate and high temperatures to identify the most influential
reactions controlling ignition of C<sub>5</sub> alcohols. The direct
relation graph with expert knowledge methodology was used to eliminate
unimportant species and reactions in the detailed mechanism, and the
resulting skeletal mechanism was tested at various homogeneous charge
compression ignition (HCCI) engine combustion conditions. These simulations
were used to investigate the heat release characteristics of the methyl-substituted
C<sub>5</sub> alcohols, and the results show relatively strong reactions
at intermediate temperatures prior to hot ignition. C<sub>5</sub> alcohol
blending in PRF75 in HCCI combustion leads to a significant decrease
of low-temperature heat release (LTHR) and a delay of the main combustion.
The heat release features demonstrated by C<sub>5</sub> alcohols can
be used to improve the design and operation of advanced engine technologies
PAH Growth Initiated by Propargyl Addition: Mechanism Development and Computational Kinetics
Polycyclic aromatic hydrocarbon (PAH)
growth is known to be the
principal pathway to soot formation during fuel combustion, as such,
a physical understanding of the PAH growth mechanism is needed to
effectively assess, predict, and control soot formation in flames.
Although the hydrogen abstraction C<sub>2</sub>H<sub>2</sub> addition
(HACA) mechanism is believed to be the main contributor to PAH growth,
it has been shown to under-predict some of the experimental data on
PAHs and soot concentrations in flames. This article presents a submechanism
of PAH growth that is initiated by propargyl (C<sub>3</sub>H<sub>3</sub>) addition onto naphthalene (A2) and the naphthyl radical. C<sub>3</sub>H<sub>3</sub> has been chosen since it is known to be a precursor
of benzene in combustion and has appreciable concentrations in flames.
This mechanism has been developed up to the formation of pyrene (A4),
and the temperature-dependent kinetics of each elementary reaction
has been determined using density functional theory (DFT) computations
at the B3LYP/6-311++GĀ(d,p) level of theory and transition state theory
(TST). H-abstraction, H-addition, H-migration, Ī²-scission, and
intramolecular addition reactions have been taken into account. The
energy barriers of the two main pathways (H-abstraction and H-addition)
were found to be relatively small if not negative, whereas the energy
barriers of the other pathways were in the range of (6ā89 kcalĀ·mol<sup>ā1</sup>). The rates reported in this study may be extrapolated
to larger PAH molecules that have a zigzag site similar to that in
naphthalene, and the mechanism presented herein may be used as a complement
to the HACA mechanism to improve prediction of PAH and soot formation
Quantities of Interest in Jet Stirred Reactor Oxidation of a High-Octane Gasoline
This work examines
the oxidation of a well-characterized, high-octane-number
FACE (fuel for advanced combustion engines) F gasoline. Oxidation
experiments were performed in a jet-stirred reactor (JSR) for FACE
F gasoline under the following conditions: pressure, 10 bar; temperature,
530ā1250 K; residence time, 0.7s; equivalence ratios, 0.5,
1.0, and 2.0. Detailed species profiles were achieved by identification
and quantification from gas chromatography with mass spectrometry
(GC-MS) and Fourier transform infrared spectrometry (FTIR). Four surrogates,
with physical and chemical properties that mimic the real fuel properties,
were used for simulations, with a detailed gasoline surrogate kinetic
model. Fuel and species profiles were well-captured and -predicted
by comparisons between experimental results and surrogate simulations.
Further analysis was performed using a quantities of interest (QoI)
approach to show the differences between experimental and simulation
results and to evaluate the gasoline surrogate kinetic model. Analysis
of the multicomponent surrogate kinetic model indicated that iso-octane
and alkyl aromatic oxidation reactions had impact on species profiles
in the high-temperature region; however, the main production and consumption
channels were related to smaller molecule reactions. The results presented
here offer new insights into the oxidation chemistry of complex gasoline
fuels and provide suggestions for the future development of surrogate
kinetic models
High-Pressure Limit Rate Rules for Ī±āH Isomerization of Hydroperoxyalkylperoxy Radicals
Hydroperoxyalkylperoxy
(OOQOOH) radical isomerization is an important
low-temperature chain branching reaction within the mechanism of hydrocarbon
oxidation. This isomerization may proceed via the migration of the
Ī±-hydrogen to the hydroperoxide group. In this work, a combination
of high level composite methodsīøCBS-QB3, G3, and G4īøis
used to determine the high-pressure-limit rate parameters for the
title reaction. Rate rules for H-migration reactions proceeding through
5-, 6-, 7-, and 8-membered ring transitions states are determined.
Migrations from primary, secondary and tertiary carbon sites to the
peroxy group are considered. Chirality is also investigated by considering
two diastereomers for reactants and transition states with two chiral
centers. This is important since chirality may influence the energy
barrier of the reaction as well as the rotational energy barriers
of hindered rotors in chemical species and transition states. The
effect of chirality and hydrogen bonding interactions in the investigated
energies and rate constants is studied. The results show that while
the energy difference between two diastereomers ranges from 0.1ā3.2
kcal/mol, chirality hardly affects the kinetics, except at low temperatures
(atmospheric conditions) or when two chiral centers are present in
the reactant. Regarding the effect of the H-migration ring size,
it is found that in most cases, the 1,5 and 1,6 H-migration reactions
have similar rates at low temperatures (below ā¼830 K) since
the 1,6 H-migration proceeds via a cyclohexane-like transition state
similar to that of the 1,5 H-migration
Computational Kinetics of Hydroperoxybutylperoxy Isomerizations and Decompositions: A Study of the Effect of Hydrogen Bonding
Hydroperoxyalkylperoxy
(OOQOOH) radicals are important intermediates
in combustion chemistry. The conventional isomerization of OOQOOH
radicals to form ketohydroperoxides has been long believed to be the
most important chain branching reaction under the low-temperature
combustion conditions. In this work, the kinetics of competing pathways
(alternative isomerization, concerted elimination, and H-exchange
pathways) to the conventional isomerization of different Ī²-,
Ī³- and Ī-OOQOOH butane isomers are investigated. Six-
and seven-membered ring conventional isomerizations are found to be
the dominant pathways, whereas alternative isomerizations are more
important than conventional isomerization, when the latter proceeded
via a more strained transition state ring. The oxygen atoms in OOQOOH
radicals introduce intramolecular hydrogen bonding (HB) that significantly
affects the energies of reacting species and transition states, ultimately
influencing chemical kinetics. Conceptually, HB has a dual effect
on the stability of chemical species, the first being the stabilizing
effect of the actual intramolecular HB force, and the second being
the destabilizing effect of ring strain imposed by the HB conformer.
The overall effect can be quantified by determining the difference
between the minimum energy conformers of a chemical species or transition
state that have HB and that do not have HB (non-hydrogen bonding (NHB)).
The stabilization effect of HB on the species and transition sates
is assessed, and its effect on the calculated rate constants is also
considered. Our results show that, for most species and transition
states, HB stabilizes their energies by as much as 2.5 kcal/mol. However,
NHB conformers are found to be more stable by up to 2.7 kcal/mol for
a few of the considered species. To study the effect of HB on rate
constants, reactions are categorized into two groups (<i>groups
one</i> and <i>two</i>) based on the structural similarity
of the minimum energy conformers of the reactant and transition state,
for a particular reaction. For cases where the reactant and transition
state conformers are similar (i.e., both HB or NHB structures), <i>group one</i>, the effect of HB on reaction kinetics is major
only if the magnitudes of the stabilization energy of the reactant <i>and</i> transition state are quite different. Meanwhile, <i>for group two</i>, where the reactant and transition state prefer
different conformers (one HB and the other NHB), HB affects the kinetics
when the stabilization energy of the reactant <i>or</i> transition
state is significant or the entropy effect is important. This information
is useful in determining corrections accounting for HB effects when
assigning rate parameters for chemical reactions using estimation
and/or analogy, where analogies usually result in inaccuracies when
modeling atmospheric and combustion processes
Effect of the Methyl Substitution on the Combustion of Two Methylheptane Isomers: Flame Chemistry Using Vacuum-Ultraviolet (VUV) Photoionization Mass Spectrometry
Alkanes with one or more methyl substitutions
are commonly found
in liquid transportation fuels, so a fundamental investigation of
their combustion chemistry is warranted. In the present work, stoichiometric
low-pressure (20 Torr) burner-stabilized flat flames of 2-methylheptane
and 3-methylheptane were investigated. Flame species were measured
via time-of-flight molecular-beam mass spectrometry, with vacuum-ultraviolet
(VUV) synchrotron radiation as the ionization source. Mole fractions
of major end-products and intermediate species (e.g., alkanes, alkenes,
alkynes, aldehydes, and dienes) were quantified axially above the
burner surface. Mole fractions of several free radicals were also
measured (e.g., CH<sub>3</sub>, HCO, C<sub>2</sub>H<sub>3</sub>, C<sub>3</sub>H<sub>3</sub>, and C<sub>3</sub>H<sub>5</sub>). Isomers of
different species were identified within the reaction pool by an energy
scan between 8 and 12 eV at a distance of 2.5 mm away from the burner
surface. The role of methyl substitution location on the alkane chain
was determined via comparisons of similar species trends obtained
from both flames. The results revealed that the change in CH<sub>3</sub> position imposed major differences on the combustion of both fuels.
Comparison with numerical simulations was performed for kinetic model
testing. The results provide a comprehensive set of data about the
combustion of both flames, which can enhance the erudition of both
fuels combustion chemistry and also improve their chemical kinetic
reaction mechanisms
Effect of the Methyl Substitution on the Combustion of Two Methylheptane Isomers: Flame Chemistry Using Vacuum-Ultraviolet (VUV) Photoionization Mass Spectrometry
Alkanes with one or more methyl substitutions
are commonly found
in liquid transportation fuels, so a fundamental investigation of
their combustion chemistry is warranted. In the present work, stoichiometric
low-pressure (20 Torr) burner-stabilized flat flames of 2-methylheptane
and 3-methylheptane were investigated. Flame species were measured
via time-of-flight molecular-beam mass spectrometry, with vacuum-ultraviolet
(VUV) synchrotron radiation as the ionization source. Mole fractions
of major end-products and intermediate species (e.g., alkanes, alkenes,
alkynes, aldehydes, and dienes) were quantified axially above the
burner surface. Mole fractions of several free radicals were also
measured (e.g., CH<sub>3</sub>, HCO, C<sub>2</sub>H<sub>3</sub>, C<sub>3</sub>H<sub>3</sub>, and C<sub>3</sub>H<sub>5</sub>). Isomers of
different species were identified within the reaction pool by an energy
scan between 8 and 12 eV at a distance of 2.5 mm away from the burner
surface. The role of methyl substitution location on the alkane chain
was determined via comparisons of similar species trends obtained
from both flames. The results revealed that the change in CH<sub>3</sub> position imposed major differences on the combustion of both fuels.
Comparison with numerical simulations was performed for kinetic model
testing. The results provide a comprehensive set of data about the
combustion of both flames, which can enhance the erudition of both
fuels combustion chemistry and also improve their chemical kinetic
reaction mechanisms
High-Pressure Limit Rate Rules for Ī±āH Isomerization of Hydroperoxyalkylperoxy Radicals
Hydroperoxyalkylperoxy
(OOQOOH) radical isomerization is an important
low-temperature chain branching reaction within the mechanism of hydrocarbon
oxidation. This isomerization may proceed via the migration of the
Ī±-hydrogen to the hydroperoxide group. In this work, a combination
of high level composite methodsīøCBS-QB3, G3, and G4īøis
used to determine the high-pressure-limit rate parameters for the
title reaction. Rate rules for H-migration reactions proceeding through
5-, 6-, 7-, and 8-membered ring transitions states are determined.
Migrations from primary, secondary and tertiary carbon sites to the
peroxy group are considered. Chirality is also investigated by considering
two diastereomers for reactants and transition states with two chiral
centers. This is important since chirality may influence the energy
barrier of the reaction as well as the rotational energy barriers
of hindered rotors in chemical species and transition states. The
effect of chirality and hydrogen bonding interactions in the investigated
energies and rate constants is studied. The results show that while
the energy difference between two diastereomers ranges from 0.1ā3.2
kcal/mol, chirality hardly affects the kinetics, except at low temperatures
(atmospheric conditions) or when two chiral centers are present in
the reactant. Regarding the effect of the H-migration ring size,
it is found that in most cases, the 1,5 and 1,6 H-migration reactions
have similar rates at low temperatures (below ā¼830 K) since
the 1,6 H-migration proceeds via a cyclohexane-like transition state
similar to that of the 1,5 H-migration