Detailed kinetic modeling of 1,3-butadiene oxidation at ...ignis.usc.edu/Mechanisms/C4H6/C4H6.pdf · Detailed Kinetic Modeling of 1,3-Butadiene Oxidation at High Temperatures

Detailed Kinetic Modelingof 1,3-Butadiene Oxidationat High TemperaturesALEXANDER LASKIN,1,* HAI WANG,1 CHUNG K. LAW2

1Department of Mechanical Engineering,University of Delaware, Newark, DE 19716-31402Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263

Received 31 October 1999; accepted 16 May 2000

ABSTRACT: The high-temperature kinetics of 1,3-butadiene oxidation was examined with de-tailed kinetic modeling. To facilitate model validation, flow reactor experiments were carriedout for 1,3-butadiene pyrolysis and oxidation over the temperature range 1035–1185 K and atatmospheric pressure, extending similar experiments found in the literature to a wider rangeof equivalence ratio and temperature. The kinetic model was compiled on the basis of anextensive review of literature data and thermochemical considerations. The model was criti-cally validated against a range of experimental data. It is shown that the kinetic model com-piled in this study is capable of closely predicting a wide range of high-temperature oxidationand combustion responses. Based on this model, three separate pathways were identified for1,3-butadiene oxidation, with the chemically activated reaction of H� and 1,3-butadiene toproduce ethylene and the vinyl radical being the most important channel over all experimentalconditions. The remaining uncertainty in the butadiene chemistry is also discussed. � 2000

John Wiley & Sons, Inc. Int J Chem Kinet 32: 589–614, 2000

INTRODUCTION

The oxidation kinetics of 1,3-butadiene (1,3-C4H6) isconsiderably important to the hierarchical develop-ment of the kinetic mechanisms of hydrocarbon com-bustion. In several publications [1–3], we reported acomprehensive kinetic model of allene, propyne, pro-pene, and propane combustion. This model uses GRI-Mech 1.2 [4] as the C1-C2 kinetic subset, with the re-actions relevant to ethylene and acetylene combustioncarefully reexamined recently [5,6]. In the presentwork, we report an experimental and kinetic modelingstudy of 1,3-butadiene oxidation, a recent step in thedirection towards a comprehensive and self-consistentkinetic model of liquid hydrocarbon fuel combustion.

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Brezinsky et al. [7] examined 1,3-butadiene oxi-dation in an atmospheric flow reactor at temperatureof �1100 K and equivalence ratios (�) of 0.55, 1.18,and 1.65. Their analysis revealed that the concentra-tion of crotonaldehyde (CH39CH"CH9CHO)peaked at an early reaction time. It was concludedthat crotonaldehyde was formed from 3-butenal(CH2"CH9CH29CHO) as a result of rapid isom-erization during passage through the GC injector sys-tem. Following this analysis, Brezinsky et al. [7] pro-posed that the oxidation of 1,3-butadiene starts mainlythrough O atom addition to the double bond to form3-butenal. This is followed by its decomposition toallyl and CO:

1,3-C H � O !:4 6

� �CH "CH9 CH9CH 9O !:2 2

CH "CH9CH 9CHO !:2 2

� �CH "CH9CH 9 CO � H2 2

� �CH "CH9CH 9 CO !: aC H � CO2 2 3 5

Correspondence to:H. Wang ([email protected])*Current address: William R. Wiley Environmental Molecular

Sciences Laboratory, Pacific Northwest National Lab., P.O. Box999, MSIN K8-88, Richland, WA 99352.� 2000 John Wiley & Sons, Inc.

It was also found that the H abstraction of 1,3-buta-diene by H� and �OH radicals to yield the CH2"CH9�C"CH2 (i-C4H5) and CH2"CH9CH"�CH (n-C4H5) isomers may also play a significant rolein butadiene oxidation.

Reaction intermediates and products in two 1,3-bu-tadiene flames were determined by Cole et al. [8] usingMolecular Beam Mass Spectrometry. These were lam-inar, burner-stabilized flames, burning mixtures of 1,3-butadiene, oxygen, and argon with equivalence ratiosequal to 1.0 and 2.4. The emphasis of that work, aswell as the later kinetic modeling of these flames [9–11], was the mechanism of aromatics formation. Theoxidation pathway, especially the mechanism of O-atom addition to 1,3-butadiene, was left beyond thefocus of these studies.

Recently, Dagaut and Cathonnet [12] published ex-perimental and modeling results of their study on theoxidation of 1,3-butadiene in a jet-stirred reactor. Vi-nyloxirane was observed in the postreaction mixtures.This finding prompted the authors to propose that theattack of the O atom on 1,3-butadiene leads primarilyto the formation of vinyloxirane, that is,

�1,3-C H � O !: vinyloxirane4 6

The disappearance of vinyloxirane was assumed toproceed via unimolecular channels,

Vinyloxirane!: C H � CO3 6

!: C H � CH CO2 4 2

or through H abstraction by the radical pool.The reaction pathway mentioned above is signifi-

cantly different from that of Brezinsky et al. [7], asthe Dagaut-Cathonnet pathway leads to effective rad-ical-chain termination, whereas the pathway of Bre-zinsky et al. leads to effective chain branching. Despitethis difference, both studies emphasized the dominantrole of the reaction between 1,3-butadiene and the Oatom. For this reason, we conducted a critical reviewof the studies on the thermal reaction of various C4H6Oisomers [13–20]. Our analyses led us to adopt andpropose a viewpoint quite different from the previouswork.

A large volume of the fundamental kinetic data of1,3-butadiene oxidation has been accumulated overthe last decade. Significant progress was made in thepyrolysis kinetics. Hidaka et al. [21] reviewed studieson the pyrolysis of 1,3-butadiene [22–27], 1,2-buta-diene [28,29], 1-butyne [30], and 2-butyne [31]. A ki-netic model was proposed [21] which was capable ofreconciling a wide spectrum of experimental data. In-vestigations were also made on the elementary kinetics

relevant to 1,3-butadiene oxidation. These studies in-clude rate-constant measurements for the reactions of

and with 1,3-butadiene [32–34] and ofO OH� �radicals with O2 [35]. The thermochemicalC H� 4 5

stability of the C4H5 radicals was also examined [36].Finally, additional combustion data of 1,3-butadienesuch as the ignition delay time [10] and the flamespeed [37] were reported recently.

In this article, we present a detailed kinetic modelof 1,3-butadiene oxidation. The kinetic model wascompiled by incorporating mechanistic and rate-con-stant information of C4HxOy species into the C3Hx

mechanism published previously [1–3]. A large num-ber of the rate constants were estimated based on ther-mochemical considerations and Rice-Ramsperger-Kassel-Marcus (RRKM) [38,39] calculations usingpotential energies and vibrational frequencies obtainedfrom molecular orbital calculations. For a critical val-idation of the model, we extended the flow reactorstudy of Brezinsky et al. [7] to a wider range of equiv-alence ratios and temperatures, using the same Prince-ton Turbulent Flow Reactor. The proposed model wasvalidated against the experimental data of 1,3-butadi-ene oxidation in the flow reactor, shock tube, andflames.

For all experimental conditions considered here, theoxidation of 1,3-butadiene was found to be controlledmostly by H� � 1,3-C4H6 : C2H4 � �C2H3, followedby the consumption of�C2H3 by O2. The O-atom at-tack on 1,3-C4H6 also influences the overall oxidationprocess, but this influence is not as significant as thepreceding reaction. The addition of the butadienechemistry did not affect the predictive capability ofthe C2-C3 model.

EXPERIMENTAL DATA FOR MODELVALIDATION

Experimental data used for model verification aresummarized in Table I. The data consist a total of 15cases, which range from pyrolysis and oxidation of1,3-butadiene in a flow reactor, to shock-tube pyrol-ysis and ignition, and finally to laminar flames. Theburner-stabilized flame of Cole et al. [8] was not in-cluded because the temperature profile of the flame isnot known [9,11]. Below, we shall discuss the presentflow-reactor experiments. Other experiments given inTable I are also briefly described.

Flow Reactor Data

The Princeton Turbulent Flow Reactor (PTFR), itsmode of operation, and the sampling system are de-

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scribed elsewhere [40,41]. The reactor is a near-adia-batic, continuous-flow device with nitrogen as the car-rier gas. The reactor operates at the atmosphericpressure. The mixture was sampled at distinct loca-tions along the centerline of the reactor. The distancesbetween the locations were transformed into reactiontime by knowledge of the temperature and flow veloc-ity. Reaction products were quenched in a water-cooled sampling probe and analyzed using gas chro-matography. Temperature was measured by a type-Bthermocouple (Pt-Pt/13% Rh).

The reactor conditions are specified in Table I(Cases 1a–1c and 4–9). These conditions were de-signed to overlap with those of Brezinsky et al. [9](Cases 10 and 11) and to extend the flow-reactor datato wider ranges of temperature and equivalence ratio.1,3-Butadiene, listed as 99% pure, was supplied byMatheson. The impurity contains mainlyp-tert-butyl-catechol, which serves as an inhibitor against 1,3-bu-tadiene dimerization. Oxygen was supplied by Aircowith purity �99.993%. The nitrogen carrier gas wastaken from a liquid reserve, supplied by Liquid Car-bonic and was found to contain 25–30 ppm of oxygen.All gases were used without further purification.

GC analysis was performed on the PlotQ and DB-5 columns, each equipped with a flame-ionization de-tector. For detection of CO and CO2, the PlotQ columnwas also equipped with a nickel catalyst methanizer.Identification of reaction products was based on theirretention time and assisted by comparison with the

results of Brezinsky et al. [9]. The concentrations ofreaction products were calculated from their GC peakareas using calibration factors obtained from the anal-ysis of a certified standard gas mixture. Carbon bal-ance was checked and was found to be within�5%of the total carbon value.

Ignition Delay Times

Ignition delay times of 1,3-butadiene were reported byFournet et al. [10] (Nos. 12–14 in Table I). In thatwork, mixtures of 1,3-butadiene, oxygen, and argonwere heated behind reflected shock waves to temper-atures between 1200 and 1700 K and pressures be-tween 8.5–10 atm. Ignition delay times were deter-mined for two equivalence ratios (� � 0.69 and 1.38)by monitoring the intensity of light emission from OHradicals at 306 nm. The ignition delay time was de-fined as the elapsed time between the reflected shockarrival and the instant when the emission signalreached 10% of its maximum value.

Flame Speed

Laminar flame speeds of 1,3 butadiene/air mixtureswere reported recently by Davis and Law [37] overthe equivalence ratio range of 0.7 to 1.7 and at atmo-spheric pressure (No. 15 in Table I). The laminar flamespeed was determined using the counterflow twin-

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Table I List of Experimental Data of 1,3-Butadiene High-Temperature Oxidation Used in Model Validation

CaseNo. Experiment Type

Reactant Composition, %

� 1,3-C4H6 O2 Diluenta

Initial Conditions

T (K) p (atm)References/Comments

1a Flow reactor � 0.3 — 99.7 1100 1 This work1b 11501c 11852 Single-pulse shock tube � 0.175 — 99.825(Ar) 1200–1800 6.5 Colket [27]3 Single-pulse shock tube � 0.5 — 99.5(Ar) 1200–1800 1.4–2.2 Hidaka et al. [21]4 Flow reactor 1.63 0.142 0.48 99.378 1035 1 This work5 Flow reactor 0.55 0.14 1.4 98.46 1035 1 This work6 Flow reactor 4.7 0.14 0.12 99.74 1120 1 This work7 Flow reactor 1.62 0.144 0.488 99.368 1110 1 This work8 Flow reactor 1 0.14 0.78 99.368 1120 1 This work9 Flow reactor 0.55 0.14 1.4 98.46 1120 1 This work

10 Flow reactor 1.65 0.143 0.477 99.38 1125 1 Brezinsky et al. [7]11 Flow reactor 1.18 0.143 0.626 99.231 1125 1 Brezinsky et al. [7]12 Shock-tube ignition 0.69 1 8 91(Ar) 1300–1500 8.5–10 Fournet et al. [10]13 Shock-tube ignition 1.38 1 4 95(Ar) 1300–1700 8.5–10 Fournet et al. [10]14 Shock-tube ignition 1.38 3 12 85(Ar) 1200–1500 8.5–10 Fournet et al. [10]15 Flame speed 0.7–1.7 — — — — 1 Davis and

Law [37]

Unless otherwise indicated, the diluent in all experiments is nitrogen (N2).a

flame technique, employing linear and nonlinear ex-trapolations [42–44] to eliminate the flame-stretch ef-fect.

Additional Data

In addition to 1,3-butadiene pyrolysis in PTFR, theshock-tube data reported by Colket [27] and Hidakaet al. [21] were also included for model validation(Nos. 2, 3 in Table 1). Colket [27] studied the thermaldecomposition of 1,3-butadiene behind reflectedshock waves for a 0.175% 1,3-butadiene–argon mix-ture over the temperature rangeT5 � 1200–1900 Kand pressurep5 � 6.5 atm. A similar experimentaltechnique was also used in the work of Hidaka et al.[21], who conducted experiments atT5 � 1200–1700K andp5 � 1.4–2.2 atm with a 0.5% 1,3-butadiene–argon mixture.

COMPUTATIONAL DETAILS

The kinetic model contains 92 species and 613 ele-mentary reactions. The reactions, their associated rateconstants, and thermochemical data are not presentedhere because of excessive space requirement. Instead,they are provided at the World Wide-Web address:http://ignis.me.udel.edu/13-butadiene. The reactionscheme is presented there in the Sandia Chemkin [45]format. References and notes for the individual reac-tions are given at the end of the file. The thermody-namic properties of the species were taken mostlyfrom previous compilations [4,46–48] and in partfrom individual works [2,31,36,49]. The heats of for-mation of several species were estimated using theNIST Structures and Properties code [50].

Simulation of the flow-reactor and shock-tube ex-periments was carried out using the Sandia ChemkinII [45] and the Senkin codes [51]. Computations wereperformed with a constant-pressure model for flow re-actor and with a constant-density model for reflectedshock waves [52]. Computational ignition delay timeswere determined following the same fashion as that inthe experimental study.

Simulation of flow-reactor experiments was carriedout using an adiabatic, zero-dimension, constant-pres-sure, and homogeneous-mixture model. In practice,this model cannot adequately treat the mixing region(upstream part) of the reactor. Nonideality in the mix-ing region and its influence on the experimental resultswere studied and discussed in great detail in the lit-erature [53,54]. In general, this nonideality tends toaccelerate initial fuel consumption. Based on numer-

ical simulation, Held and Dryer [54] showed that thisnonideality just shifts the experimental concentrationand temperature profiles along the time axis, and itdoes not affect the shape of the profiles. Consequently,the experimentally observed reaction time is only rel-ative time, and a proper comparison between experi-mental data and computational results can be made bytime shifting the experimental data to longer times. Forthe same reason, however, the species profiles do notyield information on initiation reactions. In this work,the experimental profiles from the flow-reactor studieswere artificially time shifted. The magnitude of thetime shift ranges from 5 to 70 ms and is within 30%of the full observation time in each experiment. In allcases, the amount of the time shift is given in the re-spective figure captions.

Laminar flame speeds were calculated using theSandia Chemkin II [45] and Premix [55] codes, em-ploying windward differencing for the convectiveterms, multicomponent transport formula, and includ-ing the thermal diffusion of H and H2. A large numberof mesh points were used in the simulation to ensureproper convergence. Further reduction in gradient andcurvature results in less than�1 cm/s difference in thecomputed flame speed.

REACTION MECHANISM

In this section, we shall discuss the key elements ofthe kinetic model. Although emphasis will be given tothe reactions of 1,3-butadiene, the combustion chem-istry of C1-C3 hydrocarbons will be discussed briefly.

C1-C2 Chemistry

The C1-C2 subset of the reaction mechanism is basedon GRI-Mech 1.2 [4]. This model was expanded todescribe acetylene and ethylene oxidation in burner-stabilized fuel-rich flames and in counterflow diffusionflames [49,56], and subsequently in the prediction[5,6] of acetylene and ethylene flame speeds and ig-nition delay times. The more recent work on propyne,propene, and propane combustion [1–3] also used thesame C1-C2 subset. Several changes were made hereto include more recent literature results crucial to theoxidation of 1,3-butadiene and its major reaction in-termediates, including acetylene, vinyl, and ethylene.Specifically, the rate expressions for HO2� � H� :�OH � �OH and HO2� � H� : H2 � O2 were up-dated by those reported by Mueller et al. [57]. Thereaction of triplet methylene with O2 produces bothHCO� and CO2,

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3 � �CH � O !: HCO � OH2 2

3 � �CH � O !: CO � H � H2 2 2

The total rate constant was set equal to that used inGRI-Mech, but the branching ratiok /CH �O :HCO�OH2 2

was assigned with a value of 0.8, based on thektotal

CO/CO2 ratio reported in [58] and [59]. This changewas necessary to properly predict the ignition delaytimes of acetylene [5]. The reactions

� � �C H � O !: HCCO � H2 2

3� �C H � O !: CH � CO2 2 2

and

� � �C H � O !: CH CO� H2 3 2

� � �C H � O !: CH � CO2 3 3

were assigned, respectively, with the branching ratiosreported by Michael et al. [60] and Donaldson et al.[61]. The expressions of the total rate constants weretaken from GRI-Mech [4] and Tsang’s compilation[62], respectively.

Preferred initiation pathway for acetylene oxidationwas found to proceed via the formation of vinylideneas the first step, followed by the reaction of vinylidene(H2CCC) with O2 [5],

C H � M !: H CCC � M2 2 2

H CCC � O !: products2 2

Based on energy considerations, the addition of vi-nylidene to O2 leads to the production of a variety ofradical products. In the present work the formation oftwo HCO� radicals was assumed.

The reaction between�C2H3 and O2 influencesmarkedly the combustion characteristics of ethyleneand 1,3-butadiene. The reaction products and branch-ing ratios have been examined by theoretical methodsin recent years [63–67]. We adopted the rate coeffi-cients of Mebel et al. [67] and assigned three relevantreaction channels,

� �C H � O !: C H � HO2 3 2 2 2 2

� � �C H � O !: CH CHO � O2 3 2 2

� �C H � O !: CH O � HCO2 3 2 2

With this assignment the channel to HCO� and CH2Ohas the largest rate constant atT � 900 K andp � 1atm, while at temperatures most relevant to combus-tion (T � 900 K) CH2CHO� � O� becomes the majorchannel.

The rates of vinoxy (CH2CHO�) decompositionwere taken from our recent quantum mechanical andRRKM calculations [68]. It was found that the disso-ciation of vinoxy is mainly

� �CH CHO !: CH � CO2 3

via 1,2-H shift followed by the rupture of the C9Cbond in CH3CO. The C9H bond breaking,

� �CH CHO !: CH CO� H2 2

is competitive only at temperatures�1600 K.At relatively low temperatures, the oxidation of al-

kenes is strongly influenced by the addition of HO2�radicals to the�-bond, which produces the�OH rad-ical and the highly reactive alkene oxides (oxiranes)[69]. The formation of ethylene oxide from the reac-tion of HO2� with ethylene was included in the modelwith the rate expression taken from the compilation ofBaulch et al. [70]. The reactions of ethylene oxide andtheir rate expressions were assigned on the basis of theexperimental studies of Wu¨rmel et al. [71] and Lifshitzand Ben-Hamou [72].

C3 Chemistry

The details of model development and verificationagainst experimental data of C3 hydrocarbons arefound elsewhere [1–3]. Briefly, the rates of the mutualisomerization of propyne and allene and of the rele-vant reactions on the C3H5 potential energy surfacewere obtained from molecular orbital and RRKM cal-culations [2]. The reaction kinetics of allyl and pro-pene was compiled largely based on the review ofTsang [73]. The rate expressions for the reaction ofallyl with O2 and with HO2� were taken respectivelyfrom Bozzelli and Dean [64] and Baulch et al. [70].Combined with the C1-C2 subset, the C3 model wasshown to predict a wide range of combustion data. Thedata included product distribution in the pyrolysis andoxidation of propyne and propene in a flow reactorunder fuel-lean, stoichiometric, and fuel-rich condi-tions; the shock-tube ignition delay times of propyne,allene, and propene; and the laminar flame speeds ofpropyne, propene, and propane.



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C4 Chemistry

The reaction subset was constructed based on a criticalreview of the literature. The emphasis was placed onthe C4H6 species. The C4H2 and C4H4 reaction mech-anisms were established as a logical part of the C4H6

mechanism. The present model also includes the 1-butene chemistry.

C4H6 Pyrolysis.A detailed description of the pyrol-ysis kinetics of 1,3-butadiene and its isomers is beyondthe primary scope of the present study. However, toensure that artifacts in the pyrolytic part of the modeldo not influence the oxidative kinetics of 1,3-butadi-ene, we examined closely the literature on the thermalreactions of C4H6 and incorporated the relevant kineticfeatures into the model.

The mechanisms of the thermal decomposition of1,3-butadiene have been discussed extensively in theliterature [21–27]. The initial step was thought to be:(a) C9C bond rupture to form two vinyl radicals[23,25,26]

� �1,3-C H !: C H � C H4 6 2 3 2 3

and/or (b) the formation of ethylene and acetylene[22,24,25]. The second pathway, originally proposed[22,24] to occur as a concerted unimolecular process,was lately considered [25] to proceed in two steps viathe formation of vinylidene.

1,3-C H !: C H � H CCC4 6 2 4 2

H CCC � M !: C H � M2 2 2

Studies on 1,2-butadiene [28,29] and 2-butyne [31]demonstrated relatively low-energy barriers for theirmutual isomerization, and particularly for their isom-erization to 1,3-butadiene. Based on these results Hi-daka et al. [21] proposed a reaction mechanism thatwas shown to reconcile a large amount of experimentaldata for 1,3-butadiene [22–27] and its isomers [28–31]. Hidaka and coworkers showed that the isomeri-zation of 1,3-butadiene to 1,2-butadiene and 2-butyneis much faster than its fragmentation. The decompo-sition of the isomers plays an important role in theoverall pyrolysis mechanism of 1,3-butadiene. Fur-thermore, the formation of CH4, allyl (a-C3H5), pro-pyne (p-C3H4), C2H6, and propene (C3H6) couldnot be systematically predicted without consideringC9C bond rupture in 1,2-butadiene.

We introduced the following set of the reactions inthe present kinetic model:

1. The mutual isomerization of 1,3-butadiene, 1,2-butadiene, 1-butyne, and 2-butyne are describedby the reactions,

1,3-C H !: 1,2-C H4 6 4 6

1,3-C H !: 2-C H4 6 4 6

2-C H !: 1,2-C H4 6 4 6

1,2-C H !: 1-C H ,4 6 4 6

where 1,2-C4H6, 1-C4H6, and 2-C4H6 are, re-spectively, 1,2-butadiene, 1-butyne, and 2-bu-tyne. These reactions are not elementary steps[21,29–31,74]. Figure 1 shows a schematic en-ergy diagram for C4H6 mutual isomerization,where the energy barriers represent the effectiveactivation energies obtained from experiments[21,29–30]. It is seen that the energy barrier of1,3-C4H6 isomerization to 1,2-C4H6 and 2-C4H6

are about 85 kcal/mol or about 17 kcal/mollower than that of C9H fission in 1,3-C4H6 andabout 33 kcal/mol lower than the C9C bondstrength in 1,3-C4H6. On the other hand, theisomerization of 1,3-C4H6 does not yield the iso-mers in significant concentrations. Computersimulations showed that the equilibrium concen-tration of 1-butyne never exceeds 0.5% (mol) of1,3-butadiene at 1000 K. For this reason, we in-cluded 1,2-C4H6 and 2-C4H6 in the model, butexcluded 1-C4H6. The combined uncertainty inthe chemistry of 1-C4H6 and 2-C4H6 decompo-sition renders an inclusion of the 1-C4H6 chem-istry not worthwhile at this time.

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Figure 1 Energy diagram of the C4H6 mutual isomeriza-tions. The energy levels are in kcal/mol.

2. The formation of the�C4H5 radicals—namely,HC�"CH9CH"CH2 (n-�C4H5),H2C"C�9CH"CH2 (i-�C4H5), andCH2"C"C�9CH3, are described by the re-actions of H ejection and abstraction from C4H6:

� �1,3-C H !: n- C H � H4 6 4 5

� �!: i- C H � H4 5

� �1,2-C H !: i- C H � H4 6 4 5

� �2-C H !: CH "C"C 9CH � H4 6 2 3

� �1,3-C H � R !: n- C H � HR4 6 4 5

�!: i- C H � HR4 5

� �1,2-C H � R !: i- C H � HR4 6 4 5

� �2-C H � R !: CH "C"C 9CH � HR4 6 2 3

where R� � H�, �CH3, �C2H3, and�C3H3. Bothi-�C4H5 and CH2"C"C�9CH3 are reso-nantly stabilized and are thus more stable thann-�C4H5. In the present model, the mutual isom-erization of�C4H5 is described by 1,2-H shift:

� � �n- C H ;: i- C H ;: CH "C"C 9CH4 5 4 5 2 3

3. Additional fragmentation reactions of 1,3-buta-diene and 1,2-butadiene were also considered.They are

1,3-C H !: C H � H4 6 4 4 2

!: H CCC � C H2 2 4

� �!: C H � C H2 3 2 3

� �1,2-C H !: C H � CH4 6 3 3 3

4. The H-atom attack on the�-bonds in 1,3-buta-diene, 1,2-butadiene, and 2-butyne leads to theformation of five rovibrationally excited�C4H7

adducts, which may mutually isomerize. The�C4H7 potential energy surface is quite complex,as demonstrated in Figure 2, where only 1,2-Hshift was considered. The energy barrier of thisprocess was estimated to be 40 kcal/mol on thebasis of previous studies of�C3H5 [2], �C4H3

[75], and�C4H5 [76]. It can be inferred fromFigure 2 that a variety of C2 � C2 and C3 � C1

products can form as a result of chemicallyactivated reactions. These products includeC2H4 � �C2H3, p-C3H4 � �CH3, and C2H2 ��C2H5. To obtain the pressure-dependent ratecoefficients for all reaction channels is beyondthe scope of the present study. Here we included



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Figure 2 Energy diagram of the reactions occurring on the C4H7 potential energy surface. The relative energy levels of theradical adducts were obtained from quantum chemical calculation at the G2(B3LYP) level of theory [2]. The energy barriersof all isomerization steps were estimated. All other energy values (kcal/mol) were determined from thermochemical data.

the following six chemically activated reactions:

� �1,3-C H � H !: C H � C H4 6 2 4 2 3

�!: p-C H � CH3 4 3

�!: a-C H � CH3 4 3

� �1,2-C H � H !: p-C H � CH4 6 3 4 3

�!: a-C H � CH3 4 3

� �2-C H � H !: p-C H � CH4 6 3 4 3

Among these reactions, the channel leading tothe formation of vinyl and ethylene is the mostcritical to the oxidation kinetics of 1,3-butadi-ene. Our initial simulation tests showed that theoverall oxidation rate of 1,3-butadiene is highlysensitive to the rate parameter of 1,3-C4H6 �H� : C2H4 � �C2H3. For this reason, we justifybelow our rate-constant choice.

RRKM Analysis of Reaction 1,3-C4H6 � H :C2H4 � C2H3. To simplify the analysis, only the partof the potential energy surface involving the formationof the CH2"CH9CH29�CH2 adduct followed byits dissociation to C2H4 � �C2H3 was included in theanalysis (see Fig. 2). Based on a previous study [76]of C2H2 � �C2H3 : C4H4 � �H, we do not expectthat this simplification yields significant errors in thepredicted rate coefficients. The most favorable out-come of the hot CH2"CH9�CH9CH3 isomer isfor it to dissociate back to 1,3-C4H6 � H�.

RRKM parameters are given in [49]. The energybarrier of H-atom addition to 1,3-C4H6 was loweredby 0.6 kcal/mol from that used in [49] to fit the roomtemperature data [77–83], as seen in Figure 3a. Thesedata were collected under a wide range of pressures(0.38 to 760 Torr), but our analysis shows that all ofthem are at the high-pressure limit. Figure 3a showsalso the RRKM rates for

� �1,3-C H � H !: C H � C H4 6 2 4 2 3

� �1,3-C H � H !: CH "CH9CH 9 CH4 6 2 2 2

at 100 Torr, the sum of the rate coefficients previouslyestimated [2] for

� �1,3-C H � H !: p-C H � CH4 6 3 4 3

� �1,3-C H � H !: a-C H � CH ,4 6 3 4 3

the rate coefficients previously estimated [49] for re-actions

� �1,3-C H � H !: n- C H � H4 6 4 5 2

� �1,3-C H � H !: i- C H � H ,4 6 4 5 2

and the total rate constant,ktot, for 1,3-C4H6 � H� :products. It is seen thatktot is in close agreement withthe available data at�1100 K [84,85].

Figure 3b presents a comparison between experi-mental and computed rate coefficients for C2H4 ��C2H3 : 1,3-C4H6 � H� at 1 mTorr of pressure. It isseen that the computed rate constant is slightly largerthan the experimental data [86]. This is reasonableconsidering that the experimental rates were obtainedrelative to that of vinyl recombination with an as-sumed rate constant of 2� 1013 cm3 mol�1 s�1. Recentstudies [87,88] showed that the vinyl-recombination

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Figure 3 Comparison of the experimental (symbols) andcomputed (lines) rate coefficients of 1,3-C4H6 � H : prod-ucts (top panel) and C2H3 � C2H4 :1,3-C4H6 � H (bottompanel). In the top panel, the values for 1,3-C4H6 � H :n-C4H7 and C2H4 � C2H3 were obtained at 100 Torr, whichis equal to the pressure under which the rate coefficients arereported [84,85]. The rate coefficients for the production ofp-C3H4 anda-C3H4 � CH3 and the two H-abstraction reac-tions were estimated from previous studies [2,49].ktot is thetotal rate of the 1,3-C4H6 � H reactions considered in thepresent model (see text).

rate constant is a few factors larger than 2� 1013 cm3

mol�1 s�1.

1,3-Butadiene Oxidation.It was concluded [7,12]that the initial step during the oxidation of 1,3-buta-diene involves mainly the addition of the O atom tothe double bond. Two different mechanisms were pro-posed, as discussed in the Introduction section. Herewe propose a pathway entirely different from the pre-vious suggestions and show that the formation of cro-tonaldehyde [7] and vinyloxirane [12] are accountedfor by the mechanism proposed here.

1. Reactions of 1,3-butadiene with O� and HO2�.As a result of O-atom addition to the� bond in1,3-butadiene, two possible biradical isomerscan be formed, that is,

O��H2C"CH9CH9CH2 (A) or

O��H2C"CH9CH9CH2 (B)

Because of resonant stabilization, isomerA ismore stable thanB and is thus favored overB.Based on energy considerations, we believe thatthe favorable reaction ofA is for it to isomerizeto 2,5-dihydrofuran or vinyloxirane,

H2C�9CH"CH9CH29O�O

H2C"CH9�CH9CH29O�O

2. Reactions of 2,5-dihydrofuran and vinyloxiranehave been studied in great detail [15–20,89–91] and are illustrated in Figures 4 and 5, re-spectively.

It is known [17,18] that 2,5-dihydrofuran un-dergoes H2 elimination to produce furan,

H2 �O O

Furan dissociates in nearly the same rate [89–91] to,

C2H2 � CH2CO

p-C3H4 � CO

O

It is seen in Figure 4 that the energy level of 1,3-C4H6 � O� is high enough to yield furan� H2,C2H2 � CH2CO � H2, andp-C3H4 � CO � H2

in one step through chemically activated pro-cesses.

The reaction path via vinyloxirane is morecomplex than that via 2,5-dihydrofuran. It isknown that vinyloxirane undergoes rapid ringexpansion to 2,3-dihydrofuran at relatively lowtemperatures [13,14]

O

O

The reactions of 2,3-dihydrofuran were reported



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Figure 4 Energy diagram of the reactions of 1,3-butadiene with O and HO2 radicals, via the formation 2,5-dihydrofuran. Theenergy values are in kcal/mol.

[15] to be unimolecular isomerizations to cro-tonaldehyde and cyclopropanecarboxaldelde-hyde.

CH39CH"CH9CHOO

O

In the work of Lifshitz et al. [15] on the thermaldecomposition of 2,3-dihydrofuran, these twoisomers were not fully separated by GC analy-sis. As a result, the reaction sequence was notclearly identified. However, the formation ofcrotonaldehyde can be inferred based on a laterwork [92] on the thermal reactions of 2-methyl-4,5-dihydrofuran, which showed that 2,3-dihy-drofuran can isomerize either directly to a non-cyclic aldehyde or via the cyclopropane to formthe noncyclic aldehyde. In addition, the uni-molecular fragmentation of 2,3-dihydrofuran,

O

OC2H2 �

C2H4 � CH2CO

was also reported, but both reactions are ex-pected to be slow [15].

The thermal decomposition of crotonalde-hyde produces propene and carbon monoxide[20] or radical products [19],

CH 9CH"CH9CHO3

!: C H � CO3 6

� �!: CH 9CH"CH9 CO � H3

� �!: CH 9CH"CH9CHO � H2

Figure 5 shows the energy of the entrance chan-nel is high enough to allow many products toform through chemically activated process. Insummary, the reaction channels of potential sig-nificance are

1,3-C H � O !: H � C H � CH CO4 6 2 2 2 2

!: H � p-C H � CO2 3 4

!: C H � CH CO� CO2 4 2

!: C H � oxirane2 2

� �!: CH 9CH"CH9 CO � H3

�(CH 9CH"CH � CO)3

� �!: CH 9CH"CH9CHO � H2

�(aC H � CO)3 5

In the present study, the total rate constant of1,3-C4H6 � O� reported [32,33] for the tem-perature range of 280–1016 K was used as is

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Figure 5 Energy diagram of the reactions of 1,3-butadiene with O and HO2 radicals, via the formation vinyloxirane. Theenergy values are in kcal/mol.

without modification. The largest uncertainty isthe branching ratio of various product channels.In particular, the choice of radical versus mo-lecular products has a significant influence onthe overall reaction rate of 1,3-butadiene oxi-dation. Our initial computer simulation showsthat the fuel-disappearance rates can be pre-dicted only if the reaction products are radicalspecies. Therefore, we assumed that the reactionof 1,3-C4H6 � O� produces CH39CH"CH9�CO � H� and �CH29CH"CH9CHO � H� with a natural branching ratio of1:3. Dissociation of the CH39CH"CH9�CO and �CH29CH"CH9CHO radicalsyields CO� CH39CH"CH� and a-�C3H5,respectively.

Our model differ principally from the previ-ous proposals [7,12], in that we assume that thereaction between 1,3-C4H6 � OD is chemicallyactivated, whereas previous studies assumedthat the major products of the reaction comefrom the collisional stabilization of the adduct.Based on the energy levels shown in Figures 4and 5, it is questionable whether the stabilizationof adducts can be of any significance.

tion reactions are included in the model,

� �1,3-C H � OH !: i- C H � H O4 6 4 5 2

�!: n- C H � H O4 5 2

The rate constant reported in [34] was adoptedand split into two parts. We assumed that theA-factor for H-abstraction from the"CH2 groupis twice that of the9CH" group. The tem-perature exponent was assumed to be equal to2. The activation energy toi-�C4H5 was as-sumed to be 3 kcal/mol lower than that ton-�C4H5 because of the difference in reactionenthalpy. The resulting rate expressions arek(cm3 mol�1 s�1) � 6.2 � 106 T2 exp(�1730/T)for n-C4H5 and 3.1� 106 T2 exp(�220/T) fori-C4H5.

4. Reactions of�C4H5 with O2. We took the rateexpression of Slagle et al. [35] and assigned thereaction products to be CH2CO � CH2CHO�,based on a similar channel for the�C2H3 � O2

reaction.The reaction of O2 with n-�C4H5 is expected

to be faster than withi-�C4H5. We assumed thatthe rate constant was equal to that of the anal-ogous�C2H3 � O2 reaction but found that then-�C4H5 � O2 reaction has little or no influenceon model predictions under all tested conditions.

RESULTS

Figure 6 presents selected experimental and computedspecies and temperature profiles during 0.3% 1,3-bu-tadiene pyrolysis in the flow reactor. The experimentaldata were collected at three initial temperatures�1100–1200 K. GC analyses identified ethylene andacetylene to be the dominant products at all tempera-tures. The concentrations of methane, allene, propyne,propene, 1,2-butadiene, 2-butyne, cyclopentadiene,benzene, toluene, styrene, and naphthalene were alsonotable. The concentration profiles of ethylene andacetylene are nearly identical for each temperature.The concentration of propyne is about a factor of 2larger than that of allene, because of rapid attainmentof partial equilibrium at these temperatures [2]. Theconcentration of methane reaches�240 ppm forT �1200 K, but it is still much smaller than those of eth-ylene and acetylene. The sharp rise of the 1,2-butadi-ene concentration to its equilibrium value at early re-action times is certainly indicative of rapidisomerization of 1,3-butadiene and of partial equilib-rium of the isomerization products thereafter. Themodel predicts quite well the profiles of 1,2-butadiene



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Also shown in Figures 4 and 5 are the reac-tions between 1,3-butadiene and the HO2�radical. Here we assume that in addition toH-abstraction of 1,3-C4H6 by HO2D, the reactionalso leads to the formation of vinyloxirane and2,5-dihydrofuran,

� �1,3-C H � HO !: vinyloxirane� OH4 6 2

�!: 2,5-dihydrofuran� OH

The trace amounts of crotonaldehyde [7] andvinyloxirane [12] can be well accounted for bya single mechanism. Simulation of the flow re-actor experiment [7] showed that while vinylox-irane and crotonaldehyde are formed in detect-able amounts (�10 ppm), the computedconcentrations of furan, 2,3- and 2,5-dihydro-furans are less than 1 ppm. The source of cro-tonaldehyde is vinyloxirane, which is producedfrom the reaction of 1,3-butadiene with HO2,and not with O atom, as originally proposed.

3. Reactions of 1,3-butadiene with�OH. Liu et al.[34] reported the overall rate constant in thetemperature range of 305–1173 K. Based ontheir analysis, the major reaction channel is�OHaddition at temperatures�1000 K, while the H-abstraction byDOH is the major channel athigher temperatures. Here only the H-abstrac-

and other species. This includes both the absolute con-centration levels, as well as the temporal shapes of theconcentration profiles.

The effect of oxygen contamination was examinednumerically. It was found that the addition of 25–30ppm of oxygen in nitrogen affects only the initial pe-riod of reaction. It does not affect the concentrationprofiles after a small reaction time. Thus, the effect ofoxygen contamination is partly reflected by time-shift-ing the experimental profiles.

To examine the capability of the kinetic model forpredicting the pyrolysis of 1,3-butadiene at highertemperatures, we performed simulation for the shock-tube experiments of Colket [27] (Case 2) and Hidakaet al. [21] (Case 3). Figures 7 and 8 present the com-parison of model and experiment. It is seen that forboth cases the model predicts very well the

Figure 7 Experimental [27] (symbols, Case 2) and com-puted (lines) concentration profiles for species formed dur-ing the pyrolysis of 0.175% 1,3-butadiene in argon in a sin-gle pulse shock tube. The experimental and computationaldwell time is 0.7 ms.

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Figure 6 Experimental (symbols) and computed (lines)concentration and temperature profiles during the pyrolysisof 0.3% 1,3-butadiene–N2 in a flow reactor at initial tem-peratures of 1110, 1150, and 1185 K. The data were takenin the present work (Cases 1a, b, and c, see Table I) andwere time shifted by 55, 45, and 20 ms, respectively.

qualitative feature of the reactant, intermediate, andproduct concentrations as a function of temperature.In particular, the rise and fall in the concentrations ofintermediate species, including ethylene, vinylacety-lene, benzene, and propyne, are all well reproduced bythe model. The model also predicts quantitatively theconcentrations of all species over a wide temperaturerange.

We conducted seven experiments for 1,3-butadieneoxidation in the flow reactor. GC measurements werefocused largely on the species relevant to the majoroxidative pathway of 1,3-butadiene, as will be dis-cussed in further detail. We lowered the reaction tem-perature from that used in [7] to about 1035 K andperformed experiments at two equivalence ratios of1.63 and 0.55. These are Cases 4 and 5, respectively.In Cases 6–9, we conducted the experiments at thesimilar temperature as those in Cases 10 and 11 [7],but the range of the equivalence ratio was extended,

from the ultra fuel-rich condition of� � 4.7 (Case 6)to the fuel-lean condition of� � 0.55 (Case 9). Inaddition, Case 7 from the present study was designedto overlap with Case 10 from [7].

The experimental results of Brezinsky et al. [7] andthose of the present work showed that under all con-ditions only ethylene, acetylene, and carbon monoxidewere formed in large amounts. Other products, likemethane, ethane, allene, propyne, propene, vinylacet-ylene, benzene, other C4 and C5 species, and someoxygenated compounds, were also detected in thepost reaction mixtures, but they are formed in rela-tively small concentrations or even trace quantities(�10 ppm).

Figures 9 and 10 present data at the equivalenceratios of 1.63 and 0.55, respectively, at the identicalinitial temperature of 1035 K. The overall reaction inthe fuel-lean case is seen to be faster than that under

the fuel-rich condition. This is expected consideringthat the variation of the equivalence ratio is mainlycaused by a large initial oxygen concentration in thefuel-lean case.

It is seen in Figures 9 and 10 that the model cap-tures quite well the major species profiles. The con-centration profiles of propyne and allene are also rea-sonably well predicted. The temperature computed atlong reaction times is seen to be higher than the ex-perimental counterpart. This discrepancy is certainlycaused by the adiabatic assumption employed in thesimulation. The large rates computed for 1,3-butadienedisappearance and CO production may be a direct con-sequence of the temperature difference between sim-ulation and experiment.

Comparisons of the experimental and computedspecies profiles at the initial temperature of�1120 Kare presented in Figures 11–14, varying the equiva-lence ratio from 4.7 (Fig. 11), 1.6 (Fig. 12), 1.0 (Fig.13), to 0.55 (Fig. 14). Again, a comparison of the 1,3-butadiene and CO profiles in these figures show thatan increase in the initial oxygen concentration facili-tates faster reactions. It is seen that the model predictsvery well the major and minor species profiles for allfour mixtures. In particular, the sharp rise in the CO2

profiles and the rise– then fall of the CO concentra-tions for the stoichiometric (Fig. 13) and fuel-lean(Fig. 14) cases are well captured by the model, sug-gesting that the radical pool concentrations in theseexperiments are well reproduced.

Comparisons between experimental data and modelpredictions were also made for the original flow re-actor data of Brezinsky et al. [7]. Figures 15 and 16plot the experimental and computed concentrationprofiles for Cases 10 and 11, respectively. It is seenthat the concentrations of the C3 and C4 aldehydes arewell reproduced by the current model.

Having verified the model against the detailedstructure of 1,3-butadiene oxidation, we turned our at-tention to global combustion properties. Figure 17compares experimental [10] and computed ignitiondelay times for three 1,3-butadiene–oxygen–argonmixtures. In general, the predicted ignition-delay timesare smaller than the experimental data at low temper-atures, while they are large than the experimental val-ues at high temperatures. The predictions are in closeagreement in the mid-temperature range for each dataset. The discrepancy in the activation energies betweenmodel and experiment is certainly discomforting.Fournet et al. [10] also reported the experimental dataof acetylene, propyne, and allene. In each case, theactivation energy was larger, by a factor of 2 or more,than previously measured values. For example, theactivation energy of acetylene ignition obtained by



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Figure 8 Experimental [21] (symbols, Case 3) and com-puted (lines) concentration profiles for species formed dur-ing the pyrolysis of 0.5% 1,3-butadiene in argon in a single-pulse shock tube. The experimental and computational dwelltime is between 1.3 and 2.4 ms.

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Figure 9 Experimental (symbols) and computed (lines) concentration and temperature profiles during the oxidation of 0.142%1,3-butadiene–0.48% O2-N2 in a flow reactor at the initial temperature of 1035 K. The data were taken in the present work(Case 4, see Table I) and were time shifted by 18 ms.


Figure 12 Experimental (symbols) and computed (lines) concentration and temperature profiles during the oxidation of0.144% 1,3-butadiene–0.488% O2-N2 in a flow reactor at the initial temperature of 1110 K. The data were taken in the presentwork (Case 7, see Table I) and were time shifted by 12 ms.



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fitting previous data [93–99] was 20 kcal/mol. Incomparison, the data reported in [10] give an activa-tion energy of�60 kcal/mol. Similarly, the activationenergy of propyne ignition was reported by Curran etal. [100] to be 33 kcal/mol, whereas the data reportedin [10] yield an activation energy value of 64 kcal/mol. In each case the experimental activation energyfor the 1,3-butadiene data as seen in Figure 17 is alsolarge than that predicted by the current model by abouta factor of 2. Based on these comparisons, we con-clude that the discrepancy may well originate from theaccuracy of the data; and a rigorous validation of thecurrent model against ignition delay of 1,3-butadienecannot be made without having the experimental datavalidated first.

Figure 18 shows the variation of experimental andcomputed flame speeds as a function of equivalenceratio. The data for the fuel-lean mixtures are well pre-dicted by the model, whereas the computed flame

speeds are slightly lower than the experimental coun-terpart from the stoichiometric to fuel-rich condi-tions.

In addition to the results shown in Figures 6–18,we also repeated simulations for experiments previ-ously reported for C3 fuels [2,3]. In all cases, the ad-dition of the butadiene chemistry did not affect thepredictive capability for C3 fuels.

DISCUSSION

Based on the current model, the radical chain processin the early period of 1,3-butadiene pyrolysis (Cases1a–c) is initiated via the isomerization of 1,3-butadi-ene to 1,2-butadiene and 2-butyne. This is followedby the dissociation of these isomers and the furtherreactions of the dissociation products:

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Figure 13 Experimental (symbols) and computed (lines) concentration and temperature profiles during the oxidation of 0.14%1,3-butadiene–0.78% O2-N2 in a flow reactor at the initial temperature of 1120 K. The data were taken in the presentwork (Case 8, see Table I) and were time shifted by 20 ms. The dashed lines in the top panels are the model calculationsfor the 1,3-butadiene and CO concentrations assuming the reaction channel of 1,2-C4H6 � O : C2H2 � oxirane, instead of1,2-C4H6 � O : CH3 · CHCHCO� H and CH2CH · CHCHO� H (see text).

1,3-C H !: 2-C H4 6 4 6

1,3-C H !: 1,2-C H4 6 4 6

� �2-C H !: CH CCCH � H4 6 2 3

� �CH CCCH !: i- C H2 3 4 5

� �1,2-C H !: C H � CH4 6 3 3 3

� � �1,3-C H � CH !: i- C H andn- C H4 6 3 4 5 4 5

� CH4

� � �i- C H andn- C H !: C H � H4 5 4 5 4 4

We found that under the flow reactor condition the rateof unimolecular decomposition of 1,3-butadiene toform C2H4 � H2CCC was comparable with those ofthe isomerization reactions. Vinylidene isomerizesrapidly to acetylene, thus provides little to no contri-bution to the radical pool.

The radical-chain reaction process becomes domi-nant in just a few microseconds. This chain is con-

trolled by the following two reactions:

� �1,3-C H � H !: C H � C H4 6 2 3 2 4

� �C H !: C H � H2 3 2 2

The H abstraction of 1,3-butadiene by the H atom

� �1,3-C H � H !: i- C H � H4 6 4 5 2

proceeds at a rate that is an order of magnitude slowerthan the�C2H3 � C2H4 channel. Very little ethyleneis consumed via the H abstraction by the H atom. Thisexplains the nearly equal concentrations of acetyleneand ethylene for all cases shown in Figure 6.

Propyne and allene are also produced by the chem-ically activated reactions of 1,3-butadiene with the Hatom,

� �1,3-C H � H !: p-C H � CH4 6 3 4 3

� �1,3-C H � H !: a-C H � CH4 6 3 4 3



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Figure 14 Experimental (symbols) and computed (lines) concentration and temperature profiles during the oxidation of0.144% 1,3-butadiene–1.4% O2-N2 in a flow reactor at the initial temperature of 1120 K. The data were taken in the presentwork (Case 9, see Table I) and were time shifted by 18 ms.

Figure 16 Experimental [7] (symbols, Case 11, see Table I) and computed (lines) concentration and temperature profilesduring the oxidation of 0.143% 1,3-butadiene–0.626% O2-N2 in a flow reactor at the initial temperature of 1110 K. The datais time shifted by 10 ms.

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Figure 15 Experimental [7] (symbols, Case 10, see Table I) and computed (lines) concentration and temperature profilesduring the oxidation of 0.143% 1,3-butadiene–0.477% O2-N2 in a flow reactor at the initial temperature of 1110 K. The datais time shifted by 10 ms.

Figure 17 Experimental [10] (symbols) and computed(lines) ignition-delay times of 1,3-butadiene–oxygen–argonmixtures behind reflected shock waves. I: Case 12 (1% 1,3-C4H6–8% O2–91% Ar); II: Case 13 (1% 1,3-C4H6–4% O2–95% Ar); III: Case 14 (3% 1,3-C4H6–12% O2–85% Ar).The dashed lines are the calculated ignition-delay times as-suming the reaction channel 1,2-C4H6 � O : C2H2 � ox-irane, instead of 1,2-C4H6 � O : CH3CH � H andCH BCOCH2CH · CHCHO� H (see text).

Methane is produced mainly as a result of H abstrac-tion from 1,3-butadiene by the methyl radicals.

For 1,3-butadiene pyrolysis in shock tubes (Cases2 and 3), the overall reaction sequence is similar tothose found in the flow-reactor experiment. At tem-peratures higher than 1400 K, however, additional re-actions become important. In particular, the unimole-cular dissociation of 1,3-butadiene to C2H4 � H2CCCand C4H4 � H2 is increasingly important. The H ab-straction of 1,3-butadiene by the H atom begins tocompete effectively with the chemically activated re-actions. The resultingi-�C4H5 and n-�C4H5 isomersdissociate rapidly to form vinylacetylene, which isconsumed by

� �C H � H !: C H � C H4 4 2 2 2 3

In addition, intermediates such as ethylene, propyne,and allene begin to decompose via the following chan-nels:

� �C H � H !: C H � H2 4 2 3 2

� �p-C H � H !: C H � CH3 4 2 2 3

�p-C H !: C H � H3 4 3 3

�a-C H !: C H � H3 4 3 3



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Figure 18 Experimental [37] (symbols) and computed(line) laminar flame speeds of 1,3-butadiene–air mixtures atatmospheric pressures. The filled symbols represent data de-rived from linear extrapolation; open symbols are nonlinearextrapolation.

In the present model, we assume the rate constants ofthese reactions to be equal. This is not crucial to theprediction of the relative concentrations of propyneand allene, since they rapidly convert to each other viaunimolecular isomerization as well as H-atom cata-lyzed isomerization [2].

p-C H !: a-C H3 4 3 4

� �p-C H � H !: a-C H � H3 4 3 4

Diacetylene in these single-pulse shock-tube ex-periments is produced from vinylacetylene:

� � �C H � H !: n- C H or i- C H � H4 4 4 3 4 3 2

� � �n- C H or i- C H !: C H � H4 3 4 3 4 2

where n-�C4H3 and i-�C4H3 are the HC#C9CH"�CH and HC#C9�C"CH2 radicals, re-spectively. Under all conditions, the recombination ofthe propargyl radicals, along with the reaction pres-ently postulated,

� �CH CCCH � C H !: C H � H2 3 2 2 6 6

is the leading source of benzene in these shock tubeexperiments.

For 1,3-butadiene oxidation in the flow reactor(Cases 1 and 4–11), the radical-chain process is ini-tiated through the formation of vinylidene,

1,3-C H !: C H � H CCC4 6 2 4 2

Although vinylidene radicals isomerize rapidly toacetylene, some of them react with O2 to produce theinitial radical species, that is,

H CCC !: C H2 2 2

� �H CCC � O !: HCO � HCO2 2

� �HCO !: H � CO

The isomerization of 1,3-butadiene to 2-butyne fol-lowed by the C9H bond fission of 2-butyne also con-tributes to the initial radical pool.

After the radical pool is established, there are threeseparate pathways contributing to the overall reac-tions. These three pathways are described in Figure 19and are seen as a result of the different starting reac-tions of 1,3-butadiene, that is, with H, O, and OH. Therelative contribution of each pathway does not vary

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Figure 19 Oxidative reaction pathways of 1,3-butadiene.

drastically as a function of equivalence ratio. The ratesof these pathways range from being nearly equal infuel-lean cases to being different by just a few factorsunder the fuel-rich condition. Pathway I is the fastestunder all flow-reactor conditions. Similar to the py-rolysis case, this pathway starts with the chemicallyactivated reaction of 1,3-butadiene with the H atom toyield vinyl and ethylene. Ethylene is consumed eitherby its reaction with the O� atom to yield the methyland formyl radicals, or through H abstraction by H�and�OH radicals to produce the vinyl radical. Thus,Pathway I can be viewed as that of ethylene oxidationwith the addition of the initial, chemically activatedH-atom attack on 1,3-butadiene.

Pathway II starts from the reactions of 1,3-butadi-ene with the O atom, with the subsequent reactionsinvolving mostly the �C3H5 radicals. The CH39CH"�CH radical undergoes rapid�-scission, lead-ing to acetylene and the methyl radical. The allyl rad-ical, on the other hand, tends to combine with H� and

�CH3, forming propene and 1-butene, respectively.Propene and 1-butene are subsequently consumed bythe chemically activated reaction of H� (or �CH3) ad-dition : �CH3 (or H�) elimination to yield ethylene.The allyl pathway has a net effect of reducing the rad-ical pool concentrations, because each reaction stepfollowing allyl formation involves either radical– rad-ical combination or the exchange of the H atom for alesser reactive methyl radical.

In all flow-reactor experiments, the propene con-centrations in the oxidation butadiene are larger thanthose in pyrolysis. For a limited range of equivalenceratio, the propene concentration increases with a de-crease in the equivalence ratio (cf. Figs. 9 and 10).This trend is clearly caused by an increase in the con-tribution of Pathway II with a decrease in the equiv-alence ratio.

In general, Pathway III proceeds at a slower ratethan the first two pathways. This route starts with Habstraction of 1,3-butadiene by�OH and under fuel-



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Figure 20 First-order sensitivity coefficients computed at 25 ms of the reaction time for 1,3-butadiene oxidation in the flowreactor.

rich conditions, also by the H atom. Diacetylene isproduced from the decomposition of�C4H5 via C4H4

and C4H3 and is oxidized through its reactions withO� and�OH.

Sensitivity analyses reveal essentially the same fea-ture as the reaction pathway analysis. Figure 20 pre-sents the ranked first-order sensitivity coefficientscomputed for 1,3-butadiene concentration at a reactiontime of 25 ms in Cases 7 (� � 1.62), 8 (� � 1), and9 (� � 0.55). No major change in the ranking of sen-sitivity coefficients was observed. The reactions per-tinent to each pathway exhibit large influences on fuel-disappearance rates. The dominant influence comesfrom 1,3-C4H6 � H� � C2H4 � �C2H3.

For the oxidation of 1,3-butadiene in shock tubes(Cases 12–14), it was found that the initiation reactionsequence is the same as that in the flow-reactor case.The radical-chain reaction prior to ignition is largelyinitiated by 1,3-butadiene decomposition to vinyli-dene, followed by the reaction of vinylidene with O2.

This finding extends the critical role of vinylideneidentified [5] in the initiation reaction of acetylene ox-idation to the present case of 1,3-butadiene oxidation.

There is little difference in the major pathways of1,3-butadiene oxidation between flow reactor, shocktube, and flame. Prior to ignition, three separate path-ways proceed at the rates that are within a few factorsof each other. In shock tubes toward high tempera-tures, the isomerization of 1,3-butadiene to 2-butyne,followed by the C-H fission of 2-butyne, also proceedsvery rapidly. The ranked sensitivity coefficients areshown in Figures 21 and 22 for ignition delay times(Cases 12 and 13). Figure 23 presents the sensitivitycoefficients for flame speeds computed at three equiv-alence ratios. It is seen that the reactions between 1,3-butadiene and the H atom to yield ethylene and thevinyl radical, as well as the subsequent oxidation ofthe vinyl radical by O2 to form vinoxy radical, are oneof the major driving forces for the radical chain pro-cess. In addition, there is no specific kinetic reason to

610 LASKIN ET AL


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Figure 21 First-order sensitivity coefficients computed for the ignition-delay time of Case 12.

explain the difference in the experimental and com-puted activation energies of ignition delay shown inFigure 17.

Through the present analysis, we identified thegreatest uncertainty in the oxidation kinetics of 1,3-butadiene to be the branching ratio of the reaction

1,3-C H � O !: products4 6

From energy considerations (Figs. 4 and 5), a substan-tial amount of the products are molecular species. Inthe present model, however, we assumed that theabove reaction leads entirely to the production of rad-ical species. This assumption was critically needed inorder to predict the flow-reactor data. Having assumedthat the entire reaction of 1,3-C4H6 � O leads to mo-lecular species, we would have the rates of fuel dis-appearance and CO production seriously underpre-dicted. This is seen by the dashed lines in Figure 13,which were obtained by assuming the production of

acetylene� oxirane as the only reaction channel.While the product assignment has little effects onflame-speed prediction, it markedly affects the igni-tion-delay time. The dashed lines in Figure 17 wereobtained with acetylene� oxirane as the only reactionproducts and show that the ignition-delay times wereover-predicted. Lastly, we note that although our anal-ysis suggests that the formation of molecular speciesfrom the reaction cannot be very significant, this sug-gestion remains to be verified.

SUMMARY

The oxidation kinetics of 1,3-butadiene was examinedby detailed kinetic modeling. A comprehensive mech-anism is proposed and shown to predict well a varietyof combustion responses including the detailed speciesprofiles during the oxidation of 1,3-butadiene in a flowreactor, the shock-tube ignition-delay time, and lami-



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Figure 22 First-order sensitivity coefficients computed for the ignition-delay time of Case 13.

nar flame speed. The present study advances the un-derstanding of 1,3-butadiene oxidation by identifyingthe multiplicity of major oxidative pathways. Thesepathways are initiated by the reactions of 1,3-butadi-ene with the H�, O�, and�OH radicals, featuring, re-spectively, the C2, C3, and C4 intermediates, until theyconverge to small C1 and C2 intermediates. Unlike pre-vious findings, we found that the path following thechemically activated reaction of H and 1,3-butadieneto produce ethylene and the vinyl radical is the mostimportant in the overall oxidation mechanism. The in-itiation of the radical-chain process was identified toinvolve the production of vinylidene, followed by thereaction of vinylidene with molecular oxygen. The un-certainty in the oxidation kinetics of 1,3-butadiene ox-idation is discussed with specific emphasis on the re-action between 1,3-butadiene and the O atom. Inparticular, the branching ratio of the product channelswas found to be critical in order to achieve a betterdescription of 1,3-butadiene oxidation.

The authors thank Mr. Joe Sivo and Mr. Delin Zhu for theirgenerous help in the PTFR experiment, and Professors IrvinGlassman and Fredrick Dryer for making the PTFR facilityaccessible to us. This work was supported by the Air ForceOffice of Scientific Research under the technical monitoringof Dr. Julian M. Tishkoff. The work at the University ofDelaware was also partially supported by the NSF CAREERprogram (CTS-9874768) under the technical monitoring ofDr. Farley Fisher. A part of the computation was performedat the Facility for Computational Chemistry at the Universityof Delaware, which is funded by NSF (CTS-9724404).

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Detailed kinetic modeling of 1,3-butadiene oxidation at ...ignis.usc.edu/Mechanisms/C4H6/C4H6.pdf · Detailed Kinetic Modeling of 1,3-Butadiene Oxidation at High Temperatures