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Journal ArticleDOI

A Comprehensive Modeling Study of iso-Octane Oxidation

01 May 2002-Combustion and Flame (Elsevier)-Vol. 129, Iss: 3, pp 253-280
TL;DR: In this paper, a detailed chemical kinetic mechanism has been developed and used to study the oxidation of iso-octane in a jet-stirred reactor, flow reactors, shock tubes and in a motored engine.
About: This article is published in Combustion and Flame.The article was published on 2002-05-01 and is currently open access. It has received 1279 citations till now. The article focuses on the topics: Combustion & Ignition system.

Summary (7 min read)

INTRODUCTION

  • This interest is motivated by the need to improve the efficiency and performance of currently operating combustors and reduce the production of pollutant species emissions generated in the combustion process.
  • All of these systems exhibit phenomena including self ignition, cool flame, and negative temperature coefficient (NTC) behav-ior.
  • This mechanism was further used to perform a wide range simulation of iso-octane oxidation [26] .
  • More recently, Curran et al. [28] used a detailed chemical kinetic mechanism to simulate the oxidation of PRF mixtures.
  • In addition, a sensitivity analysis was carried out on each set of experimental results by changing the rate constants for different classes of reaction in the kinetic mechanism.

MODEL FORMULATION

  • Computer modeling of iso-octane oxidation was performed using the HCT (Hydrodynamics, Chemistry and Transport) program [30] , which solves the coupled chemical kinetic and energy equations, and permits the use of a variety of boundary and initial conditions for reactive systems depending on the needs of the particular system being examined.
  • The present detailed reaction mechanism was constructed based on the hierarchical nature of hydrocarbon-oxygen systems.
  • The mechanism was built in a stepwise fashion, starting with small hydrocarbons and progressing to larger ones.
  • The current detailed mechanism incorporates both low-and high-temperature kinetic schemes.
  • The authors have updated the H/C/O and bond dissociation group values based on the recent studies of Lay and Bozzelli [45] and have modified these slightly so that the thermodynamic functions of R ˙ϩ O 2 º RO ˙2 reactions agree with the experimental and calculated values of Knyazev and Slagle [46] for ethyl radical.

Classes of Reactions

  • The oxidation mechanism developed for isooctane is formulated in the same way as that published previously for n-heptane [41] .
  • The mechanism is developed in a systematic way, with each type of reaction in the oxidation process treated as a class of reaction with the appropriately assigned rate constant expression.
  • The authors outline these changes in the discussion below.
  • Thus, the authors assume that the abstraction of a tertiary H atom by reaction with O ˙H radicals has exactly the same rate in 2-methyl butane, 2-methyl pentane, 3-methyl pentane, and in iso-octane.
  • The authors treatment of these reaction classes in described in the following sections.

HIGH TEMPERATURE MECHANISM

  • Reactions in classes 1 through 9 are sufficient to simulate many high temperature applications of iso-octane oxidation.
  • The authors have made a number of ad hoc assumptions and approximations that may not be suitable for some problems involving alkene and alkyne fuels and further analysis is needed to refine details for these fuels.
  • Under the conditions of this study, iso-octane oxidation is relatively insensitive to these assumptions.

Reaction Type 1: Unimolecular Fuel Decomposition

  • These reactions produce two alkyl radicals or one alkyl radical and one hydrogen atom.
  • Similar to their work on n-heptane, the authors calculate the rate constant expressions in the reverse direction, the recombination of two radical species to form the stable parent fuel with the decomposition reaction being calculated by microscopic reversibility.
  • There is very little information available on the rate of recombination reactions for C 2 alkyl radicals and larger.
  • All of these rate constant expressions were used as input in HCT and the high-pressure decomposition rate expressions calculated.
  • These were then treated using a chemical activation formulation based on Quantum Rice-Ramsperger-Kassel (QRRK) theory, as described by Dean [52, 53] and rate constant expressions were produced at various pressures.

Reaction Type 3: Alkyl Radical Decomposition

  • The authors estimate the decomposition of alkyl radicals in the way described in their n-heptane paper [41] .
  • Allowing the forward, ␤-scission rate constant to be calculated from thermochemistry.the authors.
  • The authors rate constants for these addition reactions are based on the recommendations of Allara and Shaw [49] .
  • The rate constant for the addition of an alkyl radical has a lower A-factor and higher activation energy than for the addition of a H atom.
  • The authors assume these reactions are in their high pressure limit for the conditions considered in this study.

Reaction Type 4: Alkyl Radical ؉ O 2 ‫؍‬ Olefin ؉ HO ˙2

  • This reaction type was discussed in some detail in their n-heptane paper.
  • For reasons, described in that paper, the authors do not included reaction type 4 in their mechanisms for alkyl radical containing more than four carbon atoms.
  • Further discussion is given below for reaction type 20.

Reaction Type 5: Alkyl Radical Isomerization

  • The authors treatment of this reaction type has been described in their n-heptane paper.
  • In summary, the rate constant depends on the type of C-H bond (1°, 2°, or 3°) being broken, and the ring strain energy barrier involved.
  • As olefins get larger in carbon number, the double bond affects only a small portion of the molecule, the rest of which remains paraffinic in character.
  • The authors are aware that the most important H atom abstraction reactions from olefins are those from allylic sites.
  • The authors are currently generating the appropriate thermodynamics for the radical species formed with the associated reactions and rate constant expressions.

Reaction Type 9: Olefin Decomposition

  • The authors have found that in model computations, the large olefin species decompose at appreciable rates.
  • This is another area in which further work would improve the mechanism.

LOW TEMPERATURE MECHANISM

  • This sequence, as it produces three radicals from one iso-octyl radical, is responsible for the low-temperature chainbranching process.
  • In many ways, the first addition of an alkyl radical to O 2 is the most important reaction for low temperature oxidation, even though it does not immediately determine the overall rate of chain branching.
  • Pollard [56] carried out an extensive kinetic analysis of hydrocarbon oxidation under low temperature conditions and identified the most important features of the low temperature submechanism.
  • Many recent modeling studies have incorporated these features in their mechanisms.
  • Their recent studies in detailed kinetic modeling of hydrocarbon oxidation [33, 37, 39, 41] have made it possible to address a wide variety of issues related to ignition, and have led to improved descriptions of ignition in internal combustion engines and engine knock.

Reaction Type 10: R ˙؉ O 2 Addition

  • The reverse decomposition rate constants are calculated from microscopic reversibility.
  • In addition, collisional stabilization is fast under the high pressure (13.5-40 bar) conditions of this study.
  • The activation energy for the addition reaction is zero but is quite large (Ϸ30 kcal/mol) in the reverse, dissociation direction.
  • Therefore, the equilibrium constant for this reaction is very strongly temperature dependent.
  • The authors do not include this dependence in their current iso-octane mechanism, but intend to add this feature to later mechanisms.

Reaction Type 12: RO ˙2 Isomerization

  • This is consistent with the recommendations of Baldwin et al. [67] and Hughes and Pilling [68, 69] in their analysis of peroxy radical isomerizations.
  • The authors modeling studies suggest that, in order to reproduce experimentally observed low to intermediate temperature alkane fuel reactivity profiles, it is necessary to use these lower activation energy values.
  • The authors rate choice is about a factor of two slower and has a stronger temperature dependence than the recommendations mentioned above.

Reaction Type 14: RO

  • The authors treat this reaction type in exactly the same way as described in their n-heptane paper.
  • There is little information available for this reaction rate constant except for the case where R ˙is C ˙H3 .
  • The rate constants of these reactions are not particularly well known, but have been estimated as follows.
  • The rate expression for type (a) above is the one the authors currently use for all type 15 reactions.

Reaction Type 18: RO ˙Decomposition

  • Large alkoxy radicals undergo ␤-scission to generate smaller stable oxygenated species, primarily aldehydes or ketones, and a hydrogen atom or an alkyl radical species.
  • Because alkoxy radical ␤-scission is endothermic the authors calculate the rate constant in the reverse, exothermic direction, that is, the addition of an alkyl radical (or H ˙atom) across the double bond of an aldehyde or ketone.
  • Allowing the forward, ␤-scission rate constant to be calculated from thermochemistry.the authors.

Reaction Type 19: Q ˙OOH ‫؍‬ Cyclic Ether ؉ O ˙H

  • This reaction type is treated in a similar way to that published in their n-heptane paper, but has been slightly altered to be consistent with their approach of estimating A-factors for R ˙O2 radical isomerization.
  • The rate parameters the authors use for these reactions are reported in Table 4 .

Reaction Type 20: Q ˙OOH ‫؍‬ Olefin ؉ HO ˙2

  • Q ˙OOH species that have a radical site beta to the hydroperoxy group can decompose to yield a conjugate olefin and HO ˙2 radical.
  • The rate constant for this reaction was considered in the reverse direction that is, the addition of an HO ˙2 radical at an olefinic site, in the same way as alkyl radical decomposition (type 3 above).
  • All values are in the high-pressure limit.
  • It is worth nothing that the activation energy for this class of reaction decreases with the type (1°, 2°, or 3°) of olefinic carbon atom to which the HO ˙2 radical is bonding.
  • The authors have not, as yet, included this new information in their reaction mechanism.

Reaction Type 22: Addition of Q ˙OOH to O 2

  • The rate expressions for this class of reaction are identical to those for alkyl addition to O 2 type 10.
  • The reverse dissociation rate was then calculated from microscopic reversibility.

Reaction Type 23: O ˙2 QOOH Isomerization to Carbonylhydroperoxide ؉ O ˙H

  • The rate constant for this and other isomerizations via an internal hydrogen atom transfer are analogous to those for RO ˙2 º Q ˙OOH isomerization.
  • Similar to their n-heptane work, the activation energy has been reduced by 3 kcal mol Ϫ1 as the hydrogen atom being abstracted is bound to a carbon atom which is bound to a hydroperoxy group and should be more easily removed.
  • In addition, the A-factors have been reduced by a factor of three consistent with their treatment of alkyl-peroxy radical isomerization, reaction type 12.

Reaction Type 24: Carbonylhydroperoxide Decomposition

  • The decomposition of carbonylhydroperoxide molecules leads to the formation of two radicals, a carbonyl radical and a second O ˙H radical, which results in chain branching as two radical species are produced from one stable reactant.
  • A rate expression of 1.5 ϫ 10 16 exp(Ϫ42,000 cal/RT) s Ϫ1 was chosen for the decomposition of each carbonylhydroperoxide molecule even though each species has slightly different thermodynamic properties.
  • The authors rate constant expression is similar to that used for alkylhydroperoxide decomposition based largely on the recommendations of Sahetchian et al. [76] for 1-heptyl and 2-heptyl hydroperoxide decompo-sition.
  • Carbonylhydroperoxide decomposition is especially important at low temperatures as the high activation energy ensures an induction period during which carbonylhydroperoxide concentration builds up.
  • The subsequent decomposition products help accelerate the overall rate of fuel oxidation, raising the temperature and allowing the remaining carbonylhydroperoxide species to decompose more easily.

Reaction Type 25: Cyclic Ether Reactions with O ˙H and HO ˙2

  • Cyclic ethers are produced under low temperature conditions during hydrocarbon oxidation and, in the present study are large C 8 species with an oxygen atom embedded in the molecule.
  • As the two most prevalent radicals are O ˙H and HO ˙2 at low and intermediate temperatures, where the cyclic ethers are produced, these are the only two radical attack reactions included.
  • The rate expressions used are provided in Table 5 [41] .
  • The authors have not included these reactions in the current study.

REACTION MECHANISM

  • The overall reaction scheme included in this study for iso-octane oxidation is depicted in Fig. 2 .
  • At low temperatures, chain branching is mainly because of the reaction pathway leading through the carbonylhydroperoxide species.
  • As the temperature increases, the chain propagation reactions of Q ˙OOH species increase, because the energy barrier to their formation is more easily overcome, leading to the formation of cyclic ether species, conjugate olefins, and ␤-decomposition products, at the expense of the reaction pathways through the carbonylhydroperoxide species.
  • The increasing importance of these propagation channels leads to a lower reactivity of the system which is observed as the NTC region.
  • The difference in the chain branching mechanisms at low and high temperatures leads to varying reactivity depending on the fuel to air equivalence ratio.

MECHANISM VALIDATION

  • An explaination of the chemical kinetic mechanism formulation has been given in in the preceeding section.
  • Below, the authors describe how this mechanism was used to simulate experimental results obtained in an atmospheric pressure flow reactor [20] , a variable pressure flow reactor [15, 22] at 12.5 atm, and a complementarily different flow reactor [23] .
  • Overall, good agreement is obtained between model and experiment.
  • The authors indicate the major pathways responsible for fuel oxidation under the specific conditions of each study and also point out where they believe improvements may be made.

Atmospheric Pressure Flow Reactor

  • Experiments, carried out in an adiabatic flow reactor, provide a well-characterized environment that is designed to minimize mixing and diffusion effects.
  • Constant pressure and adiabatic walls were also assumed.
  • 2-methyl-1-butene is produced by the reaction of methyl radicals with 2-methyl-allyl radicals.
  • It appears from the experimental result that there is a direct path forming CO while the model predicts that CO is mainly formed from formaldehyde oxidation via formyl radical consumption, that is, secondary processes.
  • The authors believe that there may be a mechanism that they are not fully reproducing correctly in the model.

Lean Oxidation in a Variable Pressure Flow Reactor

  • Complementary to the experiments performed above under stoichiometric conditions, Chen et al. [23] investigated iso-octane in a high pressure flow reactor at a temperature of approximately 925 K, at 3, 6, and 9 atm pressure and with a fuel/air equivalence ratio of approximately 0.05, in which the mole fractions of iso-octane, oxygen, and nitrogen were 0.08%, 20.99%, and 78.92%, respectively.
  • Many hydrocarbon and oxygenated hydrocarbon intermediates were identified and quantified as a function of time.
  • Overall, there is good agreement between model and experiment, and so it is reasonable to assume that the mechanism is predicting the major production routes correctly.
  • (Note that in the atmospheric-pressure experiments of Dryer and Brezinsky [20] 2-methyl-1butene is slightly over-predicted.).
  • As these experiments were performed under very lean conditions some methallyl-oxyl radicals also react with molecular oxygen to produce methacrolein and hydroperoxyl radicals.

Jet Stirred Reactor

  • Dagaut et al. [10] investigated iso-octane oxidation in a jet-stirred reactor at 10 atm, in the temperature range 550 to 1150 K, with equivalence ratios of 0.3, 0.5, 1.0, and 1.5, and 99% dilution by nitrogen with a residence time of 1 s. Simulations were performed under isothermal, constant pressure conditions, and assumed perfect mixing of the reactants.
  • Figure 9b depicts profiles for methane, isobutene, ethylene, and propene.
  • At 820 K, under stoichiometric conditions where 60% of the fuel has been oxidized, iso-octane undergoes hydrogen atom abstraction, primarily by hydroxyl radicals but also by hydrogen atoms and hydroperoxyl radicals, producing the four iso-octyl radicals.
  • Under fuel-lean conditions, Fig. 10 , the fuel is oxidised at lower temperature relative to stoichiometric conditions, while, under fuel-rich conditions, oxidation occurs at higher temperatures relative to the stoichiometric experiments, Fig. 11 .
  • This behaviour is well reproduced by the simulation.

Shock Tube

  • The authors detailed chemical kinetic model has been used, assuming constant-volume, homogeneous, adiabatic conditions behind the reflected shock wave, to simulate the autoignition of iso-octane, at high temperatures studied by Vermeer et al. [5] and at low temperatures as studied by Fieweger et al. [6, 7] .
  • Vermeer et al. studied the autoignition of iso-octane/oxygen/argon mixtures behind reflected shock waves in the pressure range of 1 to 4 atm and the temperature range of 1200 to 1700 K. Stoichiometric fueloxygen mixtures were diluted with 70% argon to reduce the influence of the boundary layer.
  • The authors speculated that this may have been because of their estimation of the unimolecular decomposition of the fuel being in the high pressure limit and that they should include a pressure-dependent treatment of this type of reaction to improve their predictions.
  • At temperatures below about 800 K the model is consistently slower than the experimental measurement.
  • Figure 14 shows comparisons of model predicted and experimentally measured ignition delay times versus inverse temperature at a reflected shock pressure of 40 bar for lean, stoichiometric and rich mixtures.

Co-Operative Fuels Research Engine

  • Leppard [17] performed autoignition experiments in a CFR engine motored at 500 RPM using n-heptane, iso-octane and PRF mixtures, and speciated the exhaust gases.
  • The compression ratio was increased in steps of 0.5, with continuous sampling, until iso-octane autoignited.
  • Moreover, even though the CCR for iso-octane is well reproduced by the model, it is apparent that, because the concentration of carbon monoxide formed is a direct measure of reactivity, at all compression ratios the model predicts more reactivity than is observed experimentally.
  • The only alkanes identified were methane (in trace amounts) and iso-octane.
  • Both formaldehyde and acetone are overpredicted by more than a factor of two.

SENSITIVITY ANALYSIS

  • A detailed analysis was carried out to investigate the sensitivity of each class of reaction denoted earlier, to the oxidation of iso-octane in each experiment.
  • In addition, for the low temperature shock tube experiments of Fieweger et al. their analysis has mainly focused on the classes of reaction which are represented in Fig. 2 .
  • Reactions which promote the route leading to Q ˙OOH formation and its subsequent route to chain branching via its addition to molecular oxygen result in a negative sensitivity coefficient (increased reactivity), while reactions which lead to the decomposition of the Q ˙OOH radical show a positive sensitivity coefficient (decreased reactivity).
  • Indeed, either increasing the rate expression by a factor of two, with or without a change in equilibrium constant, leads to almost identical negative sensitivity coefficients at all temperatures.
  • The authors define the sensitivity coefficient as the percentage change in time taken for 20% and 60% of the fuel to be consumed.

CONCLUSIONS

  • A detailed chemical kinetic mechanism has been developed to simulate iso-octane oxidation over wide ranges of temperature, pressure and equivalence ratio.
  • The overall reactivity of iso-octane oxidation is well reproduced by the model, as indicated by the good agreement between modeling and experiment in the shock tube experiments.
  • In addition, experimentally quantified intermediate species profiles in flow reactors and jet-stirred reactors are well reporduced by the model, indicating that the chemical pathways leading to their formation are well understood.
  • The authors are unhappy that, in order to reproduce the very low reactivity observed experimentally at low temperatures (600 -770 K), they had to decrease the rates of alkyl-peroxyl radical isomerization (reaction type 12) and peroxy-alkylhydroperoxyl radical isomerization (reaction type 23) by a factor of three relative to their n-heptane work.
  • There must be some reason why the rates of these isomerization reactions are slower for iso-octane than for n-heptane or there may be some other pathway(s) occurring at lower temperatures.

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TL;DR: In this article, a detailed chemical kinetic mechanism has been developed and used to study the oxidation of n-heptane in flow reactors, shock tubes, and rapid compression machines, where the initial pressure ranged from 1-42 atm, the temperature from 550-1700 K, the equivalence ratio from 0.3-1.5, and nitrogen-argon dilution from 70-99%.

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Abstract: This document contains evaluated data on the kinetics and thermodynamic properties of species that are of importance in methanepyrolysis and combustion. Specifically, the substances considered include H, H2, O, O2, OH, HO2, H2O2, H2O, CH4, C2H6, HCHO, CO2, CO, HCO, CH3, C2H5, C2H4, C2H3, C2H2, C2H, CH3CO, CH3O2, CH3O, singlet CH2, and triplet CH2. All possible reactions are considered. In arriving at recommended values, first preference is given to experimental measurements. Where data do not exist, a best possible estimate is given. In making extrapolations, extensive use is made RRKM calculations for the pressure dependence of unimolecular processes and the BEBO method for hydrogen transfer reactions. In the total absence of data, recourse is made to the principle of detailed balancing, thermokinetic estimates, or comparisons with analogous reactions. The temperature range covered is 300–2500 K and the density range 1×1016–1×1021 molecules/cm3. This data base forms a subset of the chemical kinetic data base for all combustion chemistry processes. Additions and revisions will be issued periodically.

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Frequently Asked Questions (8)
Q1. What are the contributions in "A comprehensive modeling study of iso-octane oxidation" ?

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of iso-octane in a jet-stirred reactor, flow reactors, shock tubes and in a motored engine. This range of physical conditions, together with the measurements of ignition delay time and concentrations, provide a broad-ranging test of the chemical kinetic mechanism. This mechanism was based on their previous modeling of alkane combustion and, in particular, on their study of the oxidation of n-heptane. In addition, a sensitivity analysis was performed for each of the combustion environments in an attempt to identify the most important reactions under the relevant conditions of study. 

Because of recent changes in thermodynamic data, and in an attempt to improve their treatment of some of their estimated rate expressions, some of those expressions published in their n-heptane paper have been changed. 

The major classes of elementary reactions considered in the present mechanism include the following:1. Unimolecular fuel decomposition 2. H atom abstraction from the fuel 3. 

Another reaction type that increases the overall reactivity of the system is the addition of alkyl radicals to molecular oxygen, reaction type 10. 

The rate constant for the addition of an alkyl radical has a lower A-factor and higher activation energy than for the addition of a H atom. 

even though the CCR for iso-octane is well reproduced by the model, it is apparent that, because the concentration of carbon monoxide formed is a direct measure of reactivity, at all compression ratios the model predicts more reactivity than is observed experimentally. 

For 1° and 3° alkyl radical addition the authors use the Lenhardt et al. [61] measured rates of addition for n-butyl and tert-butyl radicals to O2 which are 4.52 10 12 and 1.41 1013 cm3 mol 1 s 1, respectively. 

the self reaction of hydroperoxyl radicals shows a positive sensitivity coefficient as it consumes hydroperoxyl radicals which could otherwise abstract a hydrogen atom from a stable species to ultimately produce two hydroxyl radicals from one hydroperoxyl radical, as depicted in the equation array above.