Development of Isopentanol Reaction Mechanism ... - ACS Publications

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Development of Isopentanol Reaction Mechanism Reproducing Autoignition Character at High and Low Temperatures Taku Tsujimura,*,† William J. Pitz,‡ Fiona Gillespie,§ Henry J. Curran,§ Bryan W. Weber,∥ Yu Zhang,∥ and Chih-Jen Sung∥ †

National Institute of Advanced Industrial Science and Technology, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94551, United States § National University of Ireland, Galway, University Road, Galway, Ireland ∥ The University of Connecticut, 191 Auditorium Road, Unit 3139, Storrs, Connecticut 06269−3139, United States ‡

ABSTRACT: Isopentanol is one of a range of next-generation biofuels that can be produced by advanced biochemical production routes (i.e., genetically engineered metabolic pathways). Isopentanol is a C5 branched alcohol and is also called 3-methyl-1-butanol. In comparison with the most frequently studied ethanol, the molecular structure of isopentanol has a longer carbon chain and includes a methyl branch. The volumetric energy density of isopentanol is over 30% higher than ethanol. Therefore, isopentanol has the capability to be a better alternative than ethanol to gasoline. In this study, a detailed chemical kinetic model for isopentanol has been developed focusing on autoignition characteristics over a wide range of temperatures. The isopentanol model developed in this study includes high- and low-temperature chemistry. In the isopentanol model, hightemperature chemistry is based on a reaction model for butanol isomers whose reaction paths are quite similar to isopentanol. The low-temperature chemistry is based on a reaction model for isooctane which is a branched molecular structure similar to isopentanol. The model includes a new reaction mechanism for a concerted HO2 elimination, a process recently examined by da Silva et al. for ethanol (J. Phys. Chem. A 2009, 113, 8923). In addition, important reaction mechanisms relevant to lowtemperature chemistry were considered in this model. The authors conducted experiments with a shock-tube and a rapid compression machine to evaluate and improve accuracies of this model. The experiments were carried out over a wide range of temperatures, pressures, and equivalence ratios (652−1457 K, 0.7−2.3 MPa, and 0.5−2.0, respectively). Excellent agreement between model calculations and experimental data was achieved under most conditions. Therefore, it is believed that the isopentanol model developed in this study is useful for prediction and analysis of combustion performance involving autoignition processes such as a homogeneous charge compression ignition.

1. INTRODUCTION Bioderived alcohols are promising gasoline substitutes for conventional spark ignition engines and for advanced homogeneous charge compression ignition (HCCI) engines. Ethanol is the most popular bioderived alcohol which has been used in many countries as a gasoline substitute in blends and standalone. If a large amount of ethanol is blended in gasoline, some modifications in materials of fuel pipes, tank, hoses, etc., and in compatibility of engine controls, are required due to the low volumetric energy density (35% less than gasoline), high O/C ratio, and high hygroscopicity of ethanol. In addition, severe problems can occur due to ethanol’s significantly higher solubility into water which makes it difficult to use existing pipelines and infrastructure. On the other hand, a great increase in vapor pressure can occur when a small amount of ethanol is blended into gasoline (∼E10). Compared to ethanol, higher alcohols have several advantages because of their higher energy content, lower hygroscopicity, and lower corrosivity. The higher alcohols can be produced from several feed stocks either directly or via gasification.1−3 A genetically engineered metabolic pathway has been established to produce C4−C5 alcohols without the traditional fermentation route. Such advanced biochemical production routes for next-generation biofuels promise production of a great amount © 2012 American Chemical Society

of bioderived fuels which would be more compatible with existing fuel distribution and combustion infrastructure.4 Isopentanol, also known as 3-methyl-1-butanol, is a branched alcohol with 5 carbon atoms, and is one of the targets at the Department Of Energy (DOE) Joint BioEnergy Institute in the U.S.2,3 Isopentanol has some advantages over ethanol as a gasoline substitute since isopentanol has a greater similarity to gasoline in physiochemical properties. For example, isopentanol has much lower miscibility in water, and has a higher volumetric energy density (over 30% higher than ethanol, and also 2% higher than n-butanol). Despite the status of isopentanol as a prospective bioderived fuel, to the authors’ knowledge, the combustion fundamentals of isopentanol have not been significantly studied. Yang et al.5 have investigated combustion fundamentals of isopentanol in their HCCI engine. They found that, similarly to ethanol, isopentanol lacks two-stage ignition for typical HCCI operating conditions despite the fact that isopentanol has higher HCCI reactivity than gasoline or ethanol. Isopentanol did not show two-stage ignition even if the engine ran at very low engine speed (350 rpm) with considerable Received: May 21, 2012 Revised: July 15, 2012 Published: July 17, 2012 4871

dx.doi.org/10.1021/ef300879k | Energy Fuels 2012, 26, 4871−4886

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intake pressure boost (200 kPa abs).7 In addition, they mentioned that intermediate temperature heat release (termed ITHR) can be seen between low- and high-temperature heat release for gasoline, ethanol, and isopentanol in HCCI engines. ITHR is critical for HCCI operation at high loads using intakepressure boosting. ITHR is needed to retard combustion phasing and avoid unacceptably high pressure rise rates.5−7 As mentioned above, isopentanol has some advantages as a gasoline substitute; however, there are still unclear issues about how equivalence ratios, temperatures, pressures, and diluents would influence combustion because of insufficient data on isopentanol combustion fundamentals. Therefore, the objective of this study is to develop a detailed chemical kinetic model for isopentanol which could reproduce reactions under engine-like temperature and pressure conditions. Autoignition characteristics of isopentanol for various conditions should be examined by means of both experimental and modeling works. To develop an accurate model, the authors conducted validations of the model with shock-tube (ST) and rapid compression machine (RCM) experiments for a wide range of temperatures, pressures, and equivalence ratios. Recent advances in modeling of the butanol isomers are a great help when considering an isopentanol model,8−19 since although isopentanol has five carbons, it has methyl and hydroxyl groups similar to the butanol isomers (see Figure 1).

Figure 2. Measured cetane numbers for n- alkanes and alcohols.

Figure 1. Structure of isopentanol with bond dissociation energies in kcal/mol and butanol isomers.

numbers of n-alkanes and alcohols shows distinct differences, despite many structural similarities. The main difference between n-alkanes and alcohols is the presence of a hydroxyl group. Therefore, the high-temperature chemistry based on butanols and low-temperature chemistry based on isooctane or isopentane cannot be simply combined to predict autoignition characteristics of isopentanol, especially at low to intermediate temperatures. An alcohol model should include reaction steps relevant to the hydroxyl group. As described in detail in this study, the authors focus on a concerted elimination of HO2 from isopentanol, which is a process recently examined by da Silva et al. for ethanol.25 The concerted HO2 elimination progresses much faster than another HO2 path, which produces HO2 via isomerization of α-hydroxy-pentylperoxy radical. This elimination step is significantly related to the fact that isopentanol does not show two-stage ignition in the HCCI5 and RCM experiments. Details of these processes, as well as others relevant to the development of the isopentanol model, will be discussed in due course. In the following sections of this paper, a mechanism formulation is described in detail, and comparisons with ST and RCM experiments are presented to certify the accuracy of the isopentanol model.

It is easy to see that isopentanol and isobutanol have a similar methyl branch, and that isopentanol has four carbons in a row like n-butanol. Therefore, reactions and their rate constants recommended in chemical kinetic models for butanols could provide guidance for constructing a new isopentanol model. However, previous studies on the butanols have emphasized high-temperature chemistry rather than low-temperature chemistry9−14,19 because, in general, the butanols are expected to be a jet fuel.20 Therefore, high-temperature chemistry for the butanols is used to assist the development of the hightemperature portion of the isopentanol model. The lowtemperature chemistry portion of the current isopentanol model is taken by analogy from the low-temperature chemistry of the isooctane reaction mechanism. Isooctane has a branched molecular structure similar to isopentanol, and its model includes the reaction mechanism for isopentane which also resembles isopentanol in its molecular structure, except for lack of a hydroxyl group.21−23 Cetane number is described as an index of fuel reactivity, and should be attributed to low-temperature chemistry of a fuel. The National Renewable Energy Laboratory (NREL) has measured the cetane number for various kinds of fuel,24 and the cetane numbers as a function of carbon number for n-alkanes and n-alcohols are plotted in Figure 2. Comparing the cetane

2. DETAILED CHEMICAL KINETIC MODEL 2.1. Mechanism Formulation. As mentioned in the previous section, the isopentanol model should include highand low-temperature chemistries, which are based on models for butanol isomers and isooctane, respectively. Recent experimental work implemented by Weber et al. shows low-temperature autoignition characteristics of butanol isomers at elevated pressures, and comparisons of the experiments with model predictions have also been conducted.18 One interesting feature is that autoignition delay times for all butanol isomers decrease monotonically as temperature increases, indicating single-stage characteristics for the given experimental conditions (stoichiometric fuel/air mixtures for 725−855 K at 1.5 MPa),18 while the autoignition delays are much faster than ethanol.26 In addition, it is not surprising that the butanol reaction models overpredict ignition delays for n-butanol, since the n-butanol models do not include low-temperature chemistry.18 Figure 3 is a conceptual schematic on developing a higher alcohol model. Similar to the experimental results for butanol isomers,9,12,14,18,19,27 it was assumed that autoignition delays of isopentanol would monotonically vary along with variation of temperature, and that a negative temperature coefficient (NTC) behavior would hardly appear. Therefore, it seemed appropriate to use the existing C1−C4 alcohol models as a basis 4872


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Figure 3. Conceptual schematic on developing a higher alcohol model. Figure 4. Schematic diagram for HO2 elimination mechanism.

for an isopentanol model. However, as indicated by a dotted line in Figure 3, a model based on only existing alcohol models greatly overpredicts autoignition delays at low temperatures, despite correctly predicting autoignition delays at high temperatures. There are several useful studies of the low-temperature chemistry of several kinds of alkanes up to C16 available in the literature.21−23,28,29 A wide variety of experiments including shock-tube, RCM, jet stirred reactor, counter-flow flame, and others have played important roles on validating these models to achieve good agreement with the experimental results.30−42 Therefore, it would be a reasonable idea to combine hightemperature chemistry based on alcohol models and lowtemperature chemistry based on alkane models in order to develop a higher alcohol model like isopentanol. However, such a combined model would still only reproduce autoignition delays for high temperatures, and improperly predict the behavior of higher alcohols at low temperature. Consequently, it must be necessary to take the special chemical features of alcohols that are different from those of alkanes into consideration. da Silva et al.25 focused on the reason why autoignition delays for ethanol at low temperatures (600−1000 K) are longer than those for ethane, despite the fact that the dissociation energy for a C−C bond in ethanol is considerably (2.9 kcal/mol) smaller than one in ethane. Their findings are that α-hydroxyethyl + O2 reaction proceeds almost completely to acetaldehyde + HO2 via a concerted HO2 elimination reaction due to quite a low energy barrier. Specifically, they found that the elimination of HO2 proceeds through a weakly bonded complex of HO2 and acetaldehyde that is followed by dissociation to HO2 and acetaldehyde with a low barrier. As compared to this concerted mechanism, an intermolecular hydrogen shift from the hydroxyl group to a peroxy radical site is less important because of its significantly higher barrier than the concerted HO2 elimination. Figure 4 indicates the reaction pathways following α-hydroxypentyl radical + O2 reaction.43 This figure roughly indicates an energy diagram for α-hydroxypentyl + O2 reaction which eventually forms HO2 and aldehyde on the right side in this figure. As described in a later section, alcohols have a weak C−H bond on the carbon which has a hydroxyl group, and the C−H bond dissociation energy at the α site is a few kilocalories smaller than those at other sites. Therefore, it can be easy to abstract a hydrogen atom from the α site of isopentanol, and plenty of α-hydroxypentyl radicals can be formed via H abstraction reactions followed by O2 addition reaction to α-hydroxypentyl radicals. Similarly to ethanol, there are two possible pathways (termed I and II in Figure 4) to make HO2 and aldehyde. Pathway I occurs via a concerted elimination reaction. This pathway is

similar to the dominant addition elimination pathway in the hydroxymethyl + O2 reaction.25,44 As mentioned above, pathway I requires a much lower energy barrier than pathway II. Pathway I suppresses low-temperature chemistry because the RO2 radical formed goes to HO2 and an aldehyde, rather than abstracting a H-atom in the hydrocarbon portion of the molecule that leads to OH radicals and chain branching. As described above, the isopentanol model developed in this study mainly consists of high-temperature chemistry, lowtemperature chemistry, and the HO2 elimination step. The base C0−C4 chemistry is the same as one included in the reaction model for butanol isomers.13 Additionally, rate constants for some other important reactions need to be updated or reconsidered on the basis of recent considerations in the literature. THERM software47 was used to estimate thermochemical data for the relevant radicals and stable parents in the present model. Also, CHEMKIN-PRO software46 was used for calculations of autoignition character in a constant volume zone. CHEMKIN-PRO can solve the coupled chemical kinetic and energy equations. To simulate ST experiments, an assumption of the constant volume adiabatic zone is used. For simulating RCM experiments, autoignition delay times are calculated taking heat transfer to the chamber walls into consideration. The heat transfer model used in this study is the same as in previous work.47 The heat transfer model accounts for heat losses after the end of compression by introducing a volume expansion. A detailed hypothesis and discussion are given in ref 47. In the following sections, naming rules of species used in this study and detailed explanations on the model formulation are presented. The details are described only for important reactions. Brief descriptions of ST and RCM experimental procedures follow, and finally measured autoignition delay times are compared with those computed for a wide range of temperatures, pressures, and equivalence ratios (652−1457 K, 0.7−2.3 MPa, and 0.5−2.0, respectively). 2.2. Classes of Reactions. Similar reaction classes were used for the present isopentanol model that Curran et al.22 and Westbrook et al.48 constructed for the kinetic mechanisms of n- and iso-alkanes. In the present model, there are 25 total reaction classes, of which 9 deal with high-temperature chemistry and the remaining 16 describe the low-temperature chemistry. The following is the list of high-temperature mechanism reaction classes: reaction class 1, unimolecular fuel decomposition; reaction class 2, H atom abstractions from fuel; reaction class 3, alkyl radical decomposition; reaction class 4, alkyl radical + O2 = alkene + HO2; reaction class 5, alkyl radical isomerization; 4873


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reaction class 6, H atom abstraction from alkenes; reaction class 7, addition of radical species to alkenes; reaction class 8, alkenyl radical decomposition; reaction class 9, alkene decomposition. The following is the list of low-temperature mechanism reaction classes: reaction class 10, alkyl radical addition to O2 (R + O2); reaction class 11, R + R′O2 = RO + R′O; reaction class 12, alkylperoxy radical isomerization; reaction class 13, RO2 + HO2 = ROOH + O2; reaction class 14, RO2 + H2O2 = ROOH + HO2; reaction class 15, RO2 + CH3O2 = RO + CH3O +O2; reaction class 16, RO2 + R′O2 = RO + R′O + O2; reaction class 17, ROOH = RO + OH; reaction class 18, RO decomposition; reaction class 19, QOOH = cyclic ether + OH; reaction class 20, QOOH = alkene + HO2; reaction class 21, QOOH = alkene + aldehyde (or carbonyl) + OH; reaction class 22, addition of QOOH to molecular oxygen O2; reaction 23, O2QOOH isomerization to carbonylhydroperoxide + OH; reaction class 24, carbonylhydroperoxide decomposition; reaction class 25, reactions of cyclic ethers with OH and HO2. As mentioned above, new reaction mechanisms are necessary for the present model; therefore, they are regarded in the additional classes described below. Furthermore, since isopentyl radical is formed through a unimolecular fuel decomposition step, the isopentane mechanism which has low- and hightemperature chemistry23 is included in the present model. The following is a list of additional mechanism reaction classes: reaction class A1, intramolecular dehydration (fuel = H2O + alkene); reaction class A2, concerted HO2 elimination; reaction class A3, Waddington mechanism to produce aldehyde + OH; reaction class A4, direct elimination of HO2 from RO2. Reaction Class 1: Unimolecular Fuel Decomposition. Unimolecular fuel decomposition occurs when a fuel is thermally activated by collisions with surrounding molecules. The reactions in this class produce one alkyl radical and one hydrogen atom or two alkyl radicals. Since the reactions in this class are the first step for all the following reactions, they are significantly important for the whole kinetic mechanism. For the reactions which form one alkyl radical and one H atom in the present model, the rate constants are quoted from the n-butanol model.13 However, for the reactions which form more reactive or inactive radicals, the rate constants are updated. Recently, Sivaramakrishnan et al.49 examined the thermal decomposition of ethanol and its reactions with OH. They proposed rate constants of the decomposition of ethanol in modified Arrhenius expressions, with pressure dependencies up to 10 MPa.49 For the present model, the rate constants shown in Table 1 are used which are based by analogy to ethanol + OH. For example, the rate constants for iC5H11OH → iC4H9 +CH2OH are the same as C2H5OH → CH3 + CH2OH from ref 49. For iC5H11OH → CH3 +bC4H9OH, the pre-exponential factor was doubled because there are two possible ways to form methyl compared to one in C2H5OH decomposition. Note that in the table, dC5H11 stands for isopentyl radical which is formed by decomposition occurring at the C−OH bond of isopentanol. Reaction Class 2: H Atom Abstractions from Fuel. As shown in Figure 1, it is interesting that isopentanol has five different sites where H atom can be abstracted by several radicals. This means that H atom abstraction from C−H sites takes place at primary, secondary, tertiary, α, and O−H sites. In the present model, rate constants for the forward reaction (i.e., fuel + radical → alkyl radical + product) are given in Table 2, and the reverse rates are calculated during the simulation on the basis of the principle of microscopic reversibility. Note that, in Table 2, frequency factors are described on the basis of per H atom.

Table 1. Arrhenius Parameters for Unimolecular Fuel Decomposition Reactions rate constants in modified Arrhemus expressions P (MPa)

A (cm3 mol−1 s−1)

0.0001 0.001 0.01 0.1 1 10

3.41 × 1059 2.62 × 1057 1.65 × 1052 5.23 × 1043 4.59 × 1032 3.84 × 1020

0.0001 0.001 0.01 0.1 1 10

2.40 × 1054 1.03 × 1060 3.24 × 1066 1.11 × 1065 3.10 × 1058 3.56 × 1047

0.0001 0.001 0.01 0.1 1 10

1.20 5.18 1.62 5.55 1.55 1.78

× × × × × ×

1054 1059 1065 1064 1058 1047

0.0001 0.001 0.01 0.1 1 10

8.10 1.82 4.65 1.46 2.79 6.17

× × × × × ×

1046 1056 1063 1065 1061 1051

K∞

1.60 × 1015

K∞

1.67 × 1014

n

EA (cal)

iC5H11OH → H2O + cC5H10 −14.22 83 673 −13.29 85 262 −11.52 84 746 −8.90 81 507 −5.60 76 062 −2.06 69 466 iC5H11OH →CH3 + bC4H8OH −12.94 100 006 −13.98 99 906 −15.30 105 390 −14.47 107 099 −12.29 105 768 −8.96 101 059 iC5H11OH → iC4H9 + CH2OH −12.94 100 006 −13.98 99 906 −15.30 105 390 −14.47 107 099 −12.29 105 768 −8.96 101 059 iC5H11OH → dC5H11 + OH −11.33 111 053 −13.49 107 238 −14.99 109 623 −14.89 112 345 −13.40 113 080 −10.34 109 941 dC5H11 + OH → iC5H11OH −0.609 −76.30 iC4H9 + CH2OH → iC5H11OH −0.189 45.50

These constants are dependent on the radical species and the type of H atom being abstracted. The rate constants for abstractions by H, O, CH3, CH3O, CH3O2, and O2 are referred to the butanol model.13 For the abstractions by C2H5 and C2H3, the rate constants are referred to the isooctane mechanism in the gasoline surrogate model.23 Relating to abstractions of H atoms from an α carbon site, some of the butanol models consider that the dissociation energy of the C−H bond at an α carbon site is weaker than energies at the other sites [e.g., 9, 12, 16]. According to the list of C−H bond dissociation energy,45 101.1, 98.5, 96.5, 95.2, and 104.1 kcal/mol are required to abstract an H atom from primary, secondary, tertiary, α, and alkoxy sites, respectively. In addition, C−C or C−O bond dissociation energies can be seen in Figure 1. Therefore, H atom bond strength at the α site is the weakest in comparison with others. For the present model, rate constants for H atom abstraction from the α site were estimated by means of the estimation procedure proposed by Dean and Bozzelli.50 Equation 1 was used to estimate a new activation energy E. Eref is the activation energy for a reference reaction, and f is a proportionality constant which is known as the Evans−Polanyi factor. Equation 1 is based on the theory 4874


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Table 2. Arrhenius Parameters for H Atom Abstraction from Fuel Reactions C−H type H primary secondary tertiary α OH O primary secondary tertiary α OH OH primary secondary tertiary α OH HO2 primary secondary tertiary α OH CH3 primary secondary tertiary α OH

A (cm3 mol−1s−1)

n

EA (cal)

4.90 6.50 6.02 6.50 1.50

× × × × ×

105 105 105 105 107

2.43 2.40 2.40 2.40 1.60

5160 4471 2583 2333 3038

8.55 1.41 1.97 1.41 1.58

× × × × ×

1010 1013 105 1013 107

0.21 0.00 2.40 0.00 2.00

4890 5200 1150 2733 4448

9.00 1.30 1.10 1.20 8.90

× × × × ×

105 106 106 106 105

2.00 2.00 2.00 2.00 2.00

450 −765 −1865 −2200 450

2.04 9.48 6.50 9.48 3.16

× × × × ×

101 101 102 101 101

3.59 3.37 3.01 3.37 3.37

17 160 13 720 12 090 11 746 17 086

6.60 1.99 9.04 1.99 1.34

× × × × ×

1010 1011 10−1 1011 102

0.00 0.00 3.46 0.00 2.90

9784 9500 4598 7362 7452

C−H type CH3O primary secondary tertiary α OH CH3O2 primary secondary tertiary α OH O2 primary secondary tertiary α OH C2H5 primary secondary tertiary α OH C2H3 primary secondary tertiary α OH

n

7.23 1.50 1.90 1.50 3.00

× × × × ×

1010 1011 1010 1011 1011

0.00 0.00 0.00 0.00 0.00

6460 7000 2800 4862 7000

2.83 1.00 1.37 1.00 2.00

× × × × ×

1012 1012 102 1011 1012

0.00 0.00 3.12 0.00 0.00

20 460 17 000 13 190 14 862 17 000

1.01 2.00 1.00 2.00 4.00

× × × × ×

1013 1013 1013 1013 1013

0.00 0.00 0.00 0.00 0.00

48 500 47 500 48 200 45 362 47 500

1.67 2.50 1.00 2.50 2.50

× × × × ×

1010 1010 1011 1010 1010

0.00 0.00 0.00 0.00 0.00

13 400 10 400 7 900 8 262 14 050

1.67 1.99 2.00 1.99 1.99

× × × × ×

1011 1011 1011 1011 1011

0.00 0.00 0.00 0.00 0.00

18 000 16 800 14 300 14 660 20 450

EA (cal)

H atom abstraction from n-butanol by HO2 to form the α-hydroxybutyl radical is much more important than the rest at low temperatures. Aguilera-Iparraguirre et al. conducted accurate benchmarks for H atom abstraction by HO2 for hydrocarbons from methane through butane and isobutane.52 They comprehensively compared the relative rate measurements with either calculated or estimated rate constants in the literature for the hydrocarbons they studied. In general, their study is in good agreement with previous calculations or estimates except for those recommended by Carstensen et al.53 The rate constants calculated by Aguilera-Iparraguirre et al.52 are somewhat slower than the relative rate measurements for abstraction from the primary sites of n-propane and isobutane. However, the calculations agree quite well for the secondary and tertiary sites of n-propane and isobutane, respectively. By contrast, the rate constants calculated by Carstensen et al.53 are in good agreement with those for ethane, and the primary sites of n-propane and isobutane. For most of the cases, the rate constants calculated by Carstensen et al.53 are 3−6 times faster than those calculated by Aguilera-Iparraguirre et al.52 Through these examinations, it can be roughly supposed that the calculated or estimated rate constants for H atom abstraction by HO2 probably have uncertainties approximately a factor of 3−6. In this study, the isopentanol model uses rate constants that are 3 times faster at primary, secondary, and α abstraction sites than those suggested by Aguilera-Iparraguirre et al.52 Reaction Class 3: Alkyl Radical Decomposition. Except for decomposition of alkoxy radical, the rest of the alkyl radical decomposition reactions are based on the n-butanol model.13

that the activation energy for a set of reactions is proportional to the enthalpy change of reaction. Since the set of reactions should be similar, in this study, reactions where H atom abstractions occur at secondary sites were chosen as a reference reaction for estimating H atom abstraction at α site. ° ° ) E = Eref − f (Δr Href,298 − Δr H298

A (cm3 mol−1s−1)

(1)

H atom abstraction by an OH radical is one of the most important reactions among these of fuel + radical. There are already many kinetic models for butanol isomers including H atom abstraction by OH with different rate constants.9,10,12−14,19 Vasu et al.17 conducted a direct measurement of the rate constant for n-butanol + OH, and compared overall rate constants k1 with those computed by several existing models. The k1 values for n-butanol + OH computed by the models of Zhou et al.,16 Veloo et al.,13 and Moss et al.9 are in excellent agreement with the measured k1 for a given condition (973−1428 K, 0.23 MPa). In the present model, rate constants for H atom abstraction by OH are referred to those in the butanol isomers model proposed by Moss et al.9 With regard to H atom abstraction from a fuel by HO2, this reaction is supposed to be very important for not only hightemperature chemistry but also low-temperature chemistry, since the abstraction reactions with HO2 form both hydroxypentyl radicals and H2O2 which would initiate chain branching reactions for high- and low-temperature chemistry through the reaction H2O2 = OH + OH. In addition, Weber et al.51 argued with their brute force sensitivity analysis that 4875


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Since alkyl radical decomposition, i.e., alkyl radical β-scission, is endothermic, the rate constants are given in the reverse, exothermic, direction in the present model. The reverse reaction represents the addition of an alkyl radical or H atom across the double bond of an alkene. Relating to alkyl radical decomposition at a C−OH site, this reaction in the reverse direction indicates the addition of OH to an alkene. As mentioned above and in detail in a later section, the Waddington reaction mechanism54 is applied to the present model for OH addition to isopropylethylene (2-methyl-3-butene). This mechanism subsequently makes two aldehydes and OH. Relating to the alkoxy decomposition, the rate constants recommended by Grana et al. are used for the present model.14 Reaction Class 4: Alkyl Radical + O2 = Alkene + HO2. This reaction type was discussed when Curran et al. made n-heptane mechanism. The reaction of alkyl radical with O2 has several reaction channels. In the present model, alkyl radical + O2 = alkene + HO2 is not included while O2 addition to alkyl radical (class 10) and direct elimination of HO2 from RO2 are considered in the present model. Reaction Class 5: Alkyl Radical Isomerization. Isopentanol has five carbons in its structure; therefore, it is easier for isopentanol to form a five-membered than lower alcohols. Theoretically, alkyl radicals can isomerize by transferring an H atom from any carbon site to the radical site. The energy barrier for isomerization depends on the type of C−H bond being broken and on the ring strain energy barrier involved.22 In the case of isopentanol, isomerizations for the 1−4 and 1−5 shifts are more important than those for 1−2 and 1−3 shift isomerizations. Hence, the present model has only 1−4 and 1−5 shift isomerizations. The H atom transfer at a primary site with a radical on the α site is the most likely isomerization, because the C−H bond dissociation energy at the α site is the weakest and because this isomerization involves a 1−4 shift making a five-membered ring (six-membered isomerizations have even lower activation energies but none are possible in isopentanol). The activation energy for this isomerization is quoted from the work of Tsang et al. on n-hexyl radical.55 Since n-hexyl radical has no C−H bond at the α site, it is necessary to recalculate the activation energy by means of the estimation procedure with eq 1 mentioned above. Regarding an isomerization involving a six-membered ring, this is an H atom transferring from an O−H site to a primary site. The rate constants for this 1−5 shift isomerization are from the work of Atkinson.56 Reaction Class 6: H Atom Abstraction from Alkenes. In the isopentanol reaction mechanism, there are possibilities to form alkenes via reaction classes 1, 3, 20, 21, 25, A1 (intramolecular dehydration), and A4 (direct elimination of HO2 from RO2). Larger alkenes with four or five carbons in their structure can be generated. Similarly to the models for n-heptane and isooctane,22,57 it is assumed that each alkene can undergo H atom abstraction by radicals. Most of the rate constants for the reactions in this class are based on butanol isomers13 and on the examination by Mehl et al. on hexene isomers58 whose model includes more detailed chemistry on unsaturated hydrocarbons. Reaction Class 7: Addition of Radical Species to Alkenes. The reactions in this class are analogous to the reverse of reactions in class 3 where alkenes are generated. The addition of H and CH3 are similar to the decomposition of alkyl radicals already described in class 3, and HO2 addition to alkenes will be mentioned in class 20. Only relevant reactions not considered in class 3 are included here. All the reactions in this class are

considered in an approximate way to limit the number of additional species in the mechanism and are assumed to be one way reactions. The rate constants are from the models for isooctane,22 n-heptane,57 and hexanes.58 Reaction Class 8: Alkenyl Radical Decomposition. This class includes the reactions for decomposition of the alkenyl radicals formed in class 6 for H atom abstraction from alkenes. The reactions have been greatly simplified, and the rate constants in this class are all the same, 2.5 × 1013 exp(−45 000 cal/RT) s−1, as used in the isooctane model.22 Reaction Class 9: Alkene Decomposition. The reactions in this class are also simplified, and the decomposition is assumed to occur at the same rate, 2.5 × 1016 exp(−71 000 cal/RT) s−1 as used by Curran et al. for the isooctane model,22 originally recommended by Edelson and Allara.59 Reaction Class 10: Alkyl Radical Addition to O2 (R + O2). At temperatures below about 900 K, O2 addition to alkyl radical is the predominant first step followed by internal H atom isomerization, a second O2 addition, H atom isomerization, and subsequent decomposition to form two OH radicals and a carbonyl radical. In this study, the rate constants for this step were assumed to depend on the type of alkyl radical undergoing addition, and they were referred to the isooctane mechanism in the gasoline surrogate model.23 2.26 × 1012, 7.54 × 1012, 1.41 × 1013, and 3.00 × 1012 cm3 s−1 mol−1 are used for primary, secondary, tertiary, and α sites, respectively, and their reverse rates were calculated from microscopic reversibility. As described in the new reaction class A2, the concerted HO2 elimination though lc5h10oh + o2 → ic4h9cho + ho2 is taken into consideration. Therefore, R + O2 for α site is set at 3.00 × 1012 cm3 s−1 mol−1 so that the total rate of R + O2 reactions at this site should be within the collisional limit. Reaction Class 11: R + R′O2 = RO + R′O. Similarly to the previous n-heptane, isooctane, and gasoline surrogate models, the rate constants in this class are set to 7.0 × 1012 exp(+1000 cal/RT) cm3 s−1 mol−1.22,23,57 This value is originally based on the recommendation of Keiffer et al.60 for the reaction CH3 + CH3O2 = CH3O + CH3O. Reaction Class 12: Alkylperoxy Radical (RO2) Isomerization. Alkylperoxy and hydroperoxyalkyl radicals are key reactive intermediates not only in hydrocarbon oxidation, but also alcohol oxidation mechanisms since both radials are sources for low-temperature chemistry. The reactions in this class relate directly to consumption or formation of each radical. As shown in Figure 5a, isomerization of an alkylperoxy

Figure 5. Schematics for (a) alkylperoxy radical isomerization and (b) direct HO2 elimination.

radical involves a ring-like transition state. The RO2 isomerization rate constants depend on the size of the ring-like 4876


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QOOH + O2 = O2QOOH. For the present model, a similar way as in the literature23 is used to estimate the rate constants. Reaction Class 20: QOOH = Alkene + HO2. QOOH species in this class have a radical site β to the hydroperoxy group, and the reactions in this class form a conjugate alkene and HO2. Because of the inclusion of the new reaction class A2, this reaction is now a minor source of alkene and HO2 radicals in the model. The rate constants are assigned in the reverse direction, referring to the addition of HO2 at an alkene site. The rate constant expression for this reverse reaction is 1.0 × 1011 exp(−11 750 cal/RT) cm3 s−1 mol−1 for all reactions in this class which is similar to that found in Curran et al.22 The rate constants in the forward direction are calculated from thermochemistry. Reaction Class 21: QOOH = Alkene + Aldehyde (or Carbonyl) + OH. QOOH species in this class have a radical site γ to the hydroperoxy group which allows β scission to an alkene followed by a second β scission to a carbonyl + OH. To avoid adding too many minor species in the mechanism, this reaction is assumed to occur in one step and made irreversible. To obtain a rate constant for this reaction, the reverse rate constants were estimated assuming that the aldehyde (or carbonyl) adds to the alkene, and the forward rate constants were calculated from thermochemistry. For the present model, the rate expression is from similar molecular structures in the isooctane submodel of the gasoline surrogate model.22,23 Reaction Class 22: Addition of QOOH to Molecular Oxygen O2. The rate expressions for reactions in this class are assumed to be similar to those in reaction class 10. Also, similarly to class 10, the rate constants are dependent on the type of alkyl radical undergoing addition. Therefore, similar rate expressions are used for primary, secondary, tertiary, and α sites, respectively, and their reverse rates were calculated from microscopic reversibility. Reaction Class 23: O2QOOH Isomerization to Carbonylhydroperoxide + OH. For the present model, the analogy of the isomerization of O2QOOH is similar to reactions in class 12 (RO2 isomerization), which involve an internal hydrogen atom transfer. There is an assumption in this class that the hydrogen atom being abstracted is bound to the carbon atom attached to the hydroperoxy group, because the C−H bond dissociation energy at this carbon (93.6 kcal/mol) is weaker even than an α site (95.2 kcal/mol). For the present model, the rate expression is from the isooctane model in the gasoline surrogate model.22,23 Reaction Class 24: Carbonylhydroperoxide Decomposition. The reactions in this class form two important radicals, a carbonyl alkoxy radical and an OH radical, both of which result in chain branching. The rate expression is based on the recent mechanism including iso-alkanes,23 and the reactions in this class are written in the decomposition direction similar to those in the reaction class 17. These reactions are considered irreversible. The energy barrier for this decomposition is high. However, once this decomposition occurs, the overall rate of fuel oxidation is accelerated, raising the temperature and allowing the remaining carbonylhydroperoxide species to decompose more easily.22 Reaction Class 25: Reactions of Cyclic Ethers with OH and HO2. Cyclic ethers are produced via reactions in class 19 at low temperatures, with OH radicals being produced simultaneously. It is assumed that cyclic ethers in which an oxygen atom is embedded within the molecule are likely to react with OH and HO2 radicals, and that they would abstract an H atom from the

transition state and the type of H atom being abstracted (primary, secondary, tertiary). Reaction rate constants in this class are essentially related to the nature of the broken C−H bond and to the ring strain energy barrier. They are originally from the work on the isooctane model of Curran et al.,22 and recently revised by Mehl et al. on their n-heptane/iso-octane model.23 The activation energies are reduced by 400 cal/mol in accordance with the recommendations of Zhu et al.61 The rate constants are summarized by Sarathy et al. in ref 62. Special consideration was taken for abstraction from α C−H bond which is attached to the OH functional group. This bond is weaker than a normal secondary C−H so that that activation energy for abstraction was reduced by 3.3 kcal/mol. The abstraction of the H from the OH (hydroxyl) group is too slow to be considered because of the high O−H bond strength. However, as indicated in Figure 5 b, another type of RO2 reaction involving this site that leads to HO2 elimination is considered and discussed later (reaction class A2). Reaction Class 13 and 14: RO2 + HO2 = ROOH + O2 and RO2 + H2O2 = ROOH + HO2. In both of these reaction classes, an alkyperoxy radical takes an H atom from HO2 or H2O2 followed by forming the molecular product ROOH. For the present model, the rate constants for these classes, 13 and 14, are 1.75 × 1010 exp(+3275 cal/RT) and 2.4 × 1010 exp(−10 000 cal/RT) cm3 s−1 mol−1, respectively. These constants are from the models for isooctane and gasoline surrogate.22,23 Reaction Class 15: RO2 + CH3O2 = RO + CH3O +O2. The reactions in this class are not well-known; however, Lightfoot et al. measured the rate constant for CH3O2 + CH3O2 ⇄ products and the branching ratio to two possible product paths including the path for this reaction class.63 From these expressions, Curran et al.22 extracted the rate constant 1.4 × 1016T−1.61 exp(−1860 cal/RT) cm3 s−1 mol−1, which is used for all reactions in this reaction class. Reaction Class 16: RO2 + R′O2 = RO + R′O + O2. Basically the same analogy can be adopted for this reaction class as for the reaction class 15. Therefore, the same rate expression, 1.4 × 1016T−1.61 exp(−1860 cal/RT) cm3 s−1 mol−1, is used. Reaction Class 17: ROOH = RO + OH. The decomposition of hydroperoxides (i.e., ROOH) is a potentially important reaction because it involves a relatively weak O−O bond and forms two very reactive radical species. However, the main sources of ROOH are reaction classes 13 and 14 which do not produce a lot of ROOH under most conditions. The rate expression in this reaction class is similar to one in the models for iso-alkanes.62 The reactions in this class are written in the decomposition direction with a reaction rate constant of 1.0 × 1012 exp(−39 000 cal/RT) s−1. Reaction class 18: RO Decomposition. The reactions in this class progress via β-scission to produce an alkyl radical, aldehydes, or ketones. The rate constants for endothermic RO decomposition are specified in the reverse direction as 1.0 × 1011 exp(−11 900 cal/RT) cm3 s−1 mol−1,22 and the forward rate constants are calculated from thermochemistry during the simulation. Reaction Class 19: QOOH = Cyclic Ether + OH. The reactions in this class generate cyclic ethers and OH radicals via a cyclic transition state that involves the concerted breaking of an O−O bond and the forming of a new C−O bond. As previously provided,22,57 the rate constants for cyclic ether formation from QOOH radicals depend on the ring size of the cyclic ether, and a larger ring size needs less activation energy. Isopentanol could make a five-membered cyclic ether ring, and this would compete with the critical chain branching channel 4877


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ethers. Once the H atom is abstracted, the subsequent cyclic ether radicals undergo β scission to form smaller species. Rate expressions are similar to previous works,22,23 and 2.5 × 1012 and 5.0 × 1012 exp(−17 700 cal/RT) cm3 s−1 mol−1 are used for abstractions by OH and HO2, respectively. New Reaction Class A1: Intramolecular Dehydration (Fuel = H2O + Alkene). Figure 6 shows the intramolecular dehydration

Figure 7. Schematics for Waddington mechanism to produce aldehyde and OH. Figure 6. Schematics for intramolecular dehydration.

Waddington mechanism through this hydroxyalkyl radical. The Waddington mechanism continues with the addition of O2 to the hydroxyalkyl radical (step1). Subsequently, in step 2, the hydroxyperoxy radical undergoes an RO2 isomerization to form an alkoxyhydroperoxide which β scissions to formaldehyde and a QOOH radical. The QOOH radical is unstable and exothermically decomposes to C4 aldehyde and an OH radical. This sequence has been studied by Sun et al.67 for the case of isobutene. In LLNL models, step 2 is approximated as a single step with a rate constant of 2.5 × 1010 exp(−18 860 cal/RT) cm6 s−1 mol−2 from ref 23. Reaction Class A4: Direct Elimination of HO2 from RO2. As mentioned in reaction class 12, there is another pathway to form alkene and HO2 in addition to the isomerization of RO2 (reaction class 12). The pathway is the direct elimination of HO2 from RO2 whose steps are drawn in Figure 5b.68,69 Since the 1−4 hydrogen shift isomerization needs the additional 3.5 kcal/mol for its energy barrier compared to the direct elimination path, the direct elimination of HO2 becomes relatively important.29,70,71

mechanism which eliminates water from an alcohol to form an alkene. This path has been included in the LLNL submechanism for ethanol29 and is now included for higher alcohols. This reaction forms two stable species and releases significant amounts of energy. The rate expressions for this present model are from the work of Moss et al.9 New Reaction Class A2: Concerted HO2 Elimination. As described in Section 2.1, da Silva et al.25 used electronic structure calculations and master equation analysis to quantify the reaction of an α ethanol radical with O2 to explain why ignition delays for ethanol are much longer than those for alkanes such as ethane. Their findings are very important for the description of the low-temperature chemistry of alcohols. Inclusion of this reaction path gave reasonable autoignition delays for alcohols at low temperatures.43,64 To explain this reaction pathway, they found that the reaction proceeded through a weakly bound, five-membered ring complex that leads to formation of an aldehyde and HO2. They compute that the overall reaction of R + O2 → HO2 + aldehyde is chemically activated and barrierless. For the present work, the authors assume that an analogous reaction occurs for iso-pentanol, lc5h10oh + O2 → ic4h9cho + HO2. The rate constants for this reaction are from da Silva et al. with the pressure dependence from their RRKM/ master equation analysis. In addition, compared to the HO2 elimination, da Silva et al.25 give 21.64 kcal/mol as the activation energy for the first barrier of pathway II in Figure 4 and 15.86 kcal/mol for the second barrier, and describe that this pathway to acetaldehyde + HO2 is not expected to be of major significance. Zádor et al. examined the similar pathway for hydroxyethylperoxy radical, and gave an even higher energy barrier.65 However, since the activation energy given by da Silva et al.25 for the first step of pathway II is lower than those used for the five-membered and even some of the six-membered RO2 isomerization reactions in n-heptane and isooctane mechanisms,22,57 RO2 radicals are likely to be consumed through this pathway II, and the low-temperature chemistry could be slightly suppressed. Reaction Class A3: Waddington Mechanism to Produce Aldehyde + OH. The Waddington mechanism is well-known as a reaction pathway involving radical addition to an alkene which would form a hydroxyalkyl radical.54,58,66 This mechanism is already included in LLNL mechanisms and overlaps with alcohol chemistry through the hydroxyalkyl radical. Figure 7 indicates a schematic of the Waddington mechanism after terminal addition of OH to a C5 alkene with the same carbon skeletal structure as isopentanol. The hydroxyalkyl radical formed is one of the isopentyl radicals (β-C5H10OH) formed in isopentanol oxidation. The isopentanol mechanism accesses the

3. EXPERIMENTAL PROCEDURE AND SEVERAL DEFINITIONS 3.1. Procedure of Shock-Tube Experiments. The high-pressure shock-tube facility at NUI Galway is a stainless steel tube with an internal diameter of 6.3 cm and a length of 8.76 m, comprising a 3 m long driver section and a 5.73 m long driven section. The two sections are separated by a double-diaphragm. Two Mylar diaphragms, with thicknesses ranging from 75 to 500 μm, were used depending on the reflected shock pressure required. The driver gas used was helium (99.99% pure; BOC Ltd.). The diaphragms were burst by a pressure differential between the pressurized driver section and the nearevacuated driven section. Four pressure transducers (PCB, 113A24), located along the driven section of the shock-tube, were used to calculate the incident shock velocity and account for possible attenuation of the shock wave. The equilibrium program Gaseq72 was used to calculate the temperature and pressure behind the reflected shock wave using the initial temperature and pressure and the incident shock velocity. A Kistler 603B pressure tansducer, embedded in the end wall of the shock tube, was used to obtain all pressure profiles. The ignition delay time was defined as the interval between the pressure rise caused by the arrival of the shock wave at the end wall and the maximum rate of rise of the pressure signal, see Figure 8. Estimated uncertainty limits of the measurements are ±15 K in reflected shock temperature, T5, ± 15% in ignition delay time, and ±10% in mixture composition. Isopentanol (>98.0% purity, GC grade; Tokyo Chemical Industries, Europe) has a very low vapor pressure of 4.86 mmHg at 31.32 °C.73 For this reason, a heating system was used on the shock tube to ensure that no fuel condensed in the mixing tanks and that the fuel remained in the gas phase during experiments. The manifold, driven section, and part of the driver section of the shock tube were heated to 385 K (±3.5 K) using rope heaters (Omega, FGR) and heating tape 4878


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Table 4. Shock-Tube Experimental Conditions

% O2

% N2

1.38 2.72 5.30

20.71 20.43 19.89

77.90 76.85 74.81

7.9 7.6 7.0 7.0 6.9

982 1008 1053 1070 1077

Table 3. Composition of Mixtures Studied % iC5H11OH

1081 1110 1150 1191 1239

1075 1083 1127 1155 1203

(Flexelec, 1250 W) and insulated using layers of insulation tape (Zetec, 1000). This heating system was previously described in detail by Darcy et al.74 Mixtures were prepared by direct injection of the fuel through an injection port at the top of the heated mixing tanks using a gastight syringe. Oxygen (≥99.5% purity; BOC Ltd.) and nitrogen (≥99.998% purity; BOC Ltd.) were introduced to the mixing tanks through the heated manifold from gas cylinders. Three different mixtures were prepared using the method of partial pressures (lean, stoichiometric, and rich, see Table 3), and experiments were performed in the

ϕ

P (bar)

1054 1097 1109 1160 1171

Figure 8. Pressure trace obtained for oxidation of 2.72% isopentanol and 20.43% O2 in N2, ϕ = 1.0. T5 = 936 K, p5 = 20.2 bar, and τ = 1.107 ms.

0.5 1.0 2.0

T (K)

954 1010 1022 1066 1109 976 990 999 1015 1056

temperature range 954−1457 K and at pressures of 7.2 and 23 bar (0.72 and 2.3 MPa), the details of which are shown in Table 4. 3.2. Procedure of RCM Experiments. The RCM at the University of Connecticut used in the present study has been described extensively elsewhere.47 A short description is provided here for reference. The current RCM is a single piston, pneumatically driven, hydraulically stopped arrangement. The temperature and pressure at piston top dead center (TDC) are independently variable by adjusting the stroke of the piston as well as TDC clearance and initial pressure in the reaction chamber. The reactor conditions at TDC are referred to as the “compressed” conditions (TC and PC for temperature and pressure, respectively). In addition, the compression ratio can be further increased by increasing the length of the end-cap of the reaction chamber to reduce TDC reaction chamber volume. Changing this endcap also has some other effects on the heat transfer from the hot reactants to the cold chamber walls after the end of compression; these effects will be discussed below in conjunction with the experimental data. Fuel and air premixtures are prepared in a 17 L mixing tank prior to the start of experiments. The mixing tank and reaction chamber, as well as all the tubes that connect them, are equipped with heaters. This allows the study of fuels with relatively low vapor pressures. To ensure complete vaporization of the fuel, the preheat temperature is set above the saturation temperature of the fuel in the mixing tank. The mixing tank is also equipped with a magnetic stirring vane for mixing enhancement. At the start of mixture preparation, the mixing tank is vacuumed to less than 1 torr, whereupon liquid fuel (isopentanol, 99.6% purity) is injected through a septum by a syringe. The syringe is massed gravimetrically before and after injection to determine the mass of fuel in the mixing tank. This mass of fuel is used to compute the required

ignition delay (ms)

φ = 0.5, O2:N2 = 1.072 0.842 0.530 0.404 0.302 φ = 1.0, O2:N2 = 8.0 1.097 7.8 0.777 7.9 0.637 8.1 0.402 8.1 0.37 φ = 2.0, O2:N2 = 7.7 1.042 7.8 0.823 8.2 0.576 8.0 0.348 8.0 0.268 φ = 0.5, O2:N2 = 21.1 1.617 21.0 1.033 21.3 0.708 21.9 0.61 17.3 0.557 φ = 1.0, O2:N2 = 20.0 1.094 20.9 0.712 22.7 0.647 21.9 0.438 22.8 0.246 φ = 2.0, O2:N2 = 20.7 0.903 21.3 0.674 20.4 0.644 20.4 0.542 22.8 0.279

T (K)

P (bar)

ignition delay (ms)

1:3.76 (by mol), 1280 1287 1295 1407 1457 1:3.76 (by mol), 1203 1283 1327 1409

PC = 0.7 MPa 6.2 0.204 7.8 0.199 6.9 0.173 7.0 0.068 7.4 0.052 PC = 0.7 MPa 7.0 0.352 6.7 0.174 6.7 0.128 6.8 0.064

1:3.76 (by mol), 1243 1293 1349 1378

PC = 0.7 MPa 7.8 0.176 7.0 0.138 7.4 0.08 7.0 0.056

1:3.76 (by mol), 1099 1172 1205 1266 1370 1:3.76 (by mol), 1111 1125 1143 1246 1306 1:3.76 (by mol), 1068 1104 1181 1241

PC = 2.0 MPa 20.5 0.417 20.3 0.258 18.7 0.172 18.4 0.105 18.9 0.064 PC = 2.0 MPa 20.4 0.269 22.8 0.277 19.9 0.211 21.5 0.106 22.9 0.063 PC = 2.0 MPa 22.5 0.267 22.6 0.187 20.9 0.096 21.2 0.068

partial pressures of oxidizer components (O2, 99.994% purity, N2, 99.999% purity) to create the specified equivalence ratio. The oxidizer gases are added to the mixing tank at room temperature. Finally, the heaters and stirring vane are turned on for at least 1.5 h prior to the start of experiments to ensure complete homogeneity. Several checks of this procedure for handling liquid fuels have been performed. In particular, two analyses of the mixture composition by GC-MS and GC-FID have shown that the fuel mole fraction was within 5% of the expected value,51 and that little to no decomposition of the fuel occurs during the preheat phase as well as during experiments.75 Although these checks were performed, respectively, with n-butanol and n-decane, the procedure outlined above is deemed to be adequate for handling isopentanol as well. The experimental conditions studied in this work are detailed in Table 5, along with the ignition delay of each condition. The ignition delay is defined as the point of maximum rate of pressure rise after the piston reached TDC. Figure 9 shows a representative pressure trace for these experiments, including the definition of ignition delay. P(t) is the experimental pressure trace, P′(t) is the time derivative of the pressure, and P(t), nonreactive, is a nonreactive pressure trace, as described below. Each condition was repeated at least six times to ensure repeatability and the pressure trace closest to the mean of the scatter is chosen as the indicative pressure trace. The scatter in ignition delay is less than 10% of the reported value, which represents the current experimental uncertainty. Furthermore, each new mixture preparation 4879


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was checked against a previous condition to ensure reproducibility. It should be noted that there are no results listed for the equivalence ratio φ = 2.0, 0.7 MPa case. This is because no ignition could be found for this case under the operating conditions investigated here. The temperature in the reaction chamber at TDC (TC) is computed by the method described in Mittal et al.47 A detailed uncertainty analysis in the work by Weber et al. found that the typical uncertainty in the compressed temperature is 0.7−1.7%.51 Although that study considered n-butanol, the uncertainty for a similar fuel such as isopentanol should not be substantially different. The simulated pressure traces are matched to the experimental pressure traces up to TDC by varying four parameters: (1) a volume term that increases the geometric volume, in order to model heat loss during compression; (2) the total compression time; (3) the time the piston was accelerating; and (4) the time the piston was decelerating. Using the assumption of an “adiabatic core” of gases in the reaction chamber, the temperature at TDC is reported as the compressed temperature, TC. Heat loss after the end of compression is determined from the corresponding nonreactive experiment, by modeling the heat loss as a volume expansion of the reaction chamber. This nonreactive experiment is set up by creating a mixture where the oxygen has been replaced by nitrogen, to eliminate oxidation reactions but maintain a similar heat capacity ratio. Finally, the pressure traces prior to and after the end of compression are converted to volume traces by the adiabatic relation pVγ = constant using the temperature dependent heat capacity ratio, γ. This procedure allows an accurate simulation of the compression phase and an accounting of heat loss after compression. These volume traces are provided to CHEMKIN-Pro through the “Volume Profile” feature for use in RCM simulations. This approach has been validated in previous studies (e.g.,51,47,75).

Table 5. RCM Experimental Conditions initial T (K) φ= 413 413 413 413 φ= 413 413 413 413 φ= 360 360 360 360 360 360 370 380 φ= 360 360 360 360 360 360 360 370 413 φ= 390 390 390 390 413

initial P (bar)

compressed T (K)

0.5, O2:N2 = 1:3.76 (by mol), PC = 0.7 MPa 0.3082 897 0.2853 914 0.2495 941 0.2347 956 1.0, O2:N2 = 1:3.76 (by mol), PC = 0.7 MPa 0.2904 862 0.2668 877 0.248 891 0.2367 902 0.5, O2:N2 = 1: 3.76 (by mol), PC = 2.0 MPa 1.3397 721 1.1214 753 0.9077 792 0.7937 817 0.7057 843 0.6256 864 0.6287 882 0.6342 902 1.0, O2:N2 = 1:3.76 (by mol), PC = 2.0 MPa 1.3431 689 1.1855 707 1.0248 731 0.925 750 0.8012 773 0.7057 795 0.6356 815 0.6454 831 0.6962 896 2.0, O2:N2 = 1:3.76 (by mol), PC = 2.0 MPa 1.7946 652 1.6624 662 1.4475 682 1.154 711 1.1541 746

ignition delay (ms) 62.41 24.17 11.09 5.03 58.43 25.62 18.27 11.62 76.53 48.17 39.45 27.30 20.15 14.05 8.94 6.12 63.87 44.37 34.20 24.98 20.63 15.41 9.44 7.55 4.52

4. MODEL VALIDATIONS 4.1. Autoignition at High Temperatures (Comparisons with ST Experiment). The NUIG (National University of Ireland, Galway) group measured autoignition delay times of isopentanol in their shock tube (ST). Figure 10 indicates

78.40 41.44 20.28 10.60 8.40

Figure 10. Comparisons of measured and calculated ignition delay times for equivalence ratio ranging from 0.5 to 2.0 and pressures at 0.7−0.8 MPa. The ST ignition data from NUIG group.

autoignition delay times of isopentanol in air for equivalence ratios of 0.5, 1.0, and 2.0, pressures at 0.7−0.8 MPa, and temperatures ranging from 1054 to 1457 K. For convenience in the CHEMKIN simulation, the definition of autoignition timing for calculations is the time at which temperature increases 500 K compared to the given initial temperature. For all given conditions, temperature increases steeply so the definition of autoignition time in the simulations corresponds closely to the one described earlier for the experiments. The results from the ST experiments (plots in Figure 10) indicate

Figure 9. Representative experimental pressure trace of the RCM experiments. P(t) is the pressure trace, and P′(t) is the time derivative of the pressure. P(t), nonreactive, is the pressure trace from a nonreactive experiment, as described in the text. 4880


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that autoignition delay times slightly decrease with an increase in equivalence ratio. The calculated results (lines in Figure 10) show a similar relationship between equivalence ratio and autoignition delays. In addition, the calculated results indicate that the differences in autoignition delays for different equivalence ratios are smaller as the temperature is increased. On the basis of shock tube results for hydrocarbons, the ignition delay curves at different equivalence ratios are expected to cross at even higher temperatures. Autoignition delay times for equivalence ratios of 0.5, 1.0, and 2.0 at higher pressures are shown in Figure 11. The ST

Figure 12. Comparisons of measured and calculated ignition delay times for equivalence ratio ranging from 0.5 to 1.0 and pressures at 0.7 MPa. The RCM ignition data from UConn group.

remarkable pressure rise. On the other hand, experiments at P = 2.0 MPa could be accomplished even for an equivalence ratio of 2.0. Figure 13 shows measured autoignition delays for equivalence ratios of 0.5, 1.0, and 2.0 at pressure of 2.0 MPa, and a range of temperatures from 652 to 902 K.

Figure 11. Comparisons of measured and calculated ignition delay times for equivalence ratio ranging from 0.5 to 2.0 and pressures at 2.0 to 2.3 MPa. The ST ignition data from NUIG group.

experiments were conducted at pressures that are around three times higher in comparison with those in Figure 10, and at temperatures ranging from 954 to 1370 K. The measured and calculated autoignition delay times indicate that shorter ignition delays are obtained at higher equivalence ratio, which is similar to ignition delays at lower pressures indicated in Figure 10. However, the sensitivity of ignition delays to equivalence ratio at higher pressures is more significant than at lower pressures. This is probably due to the sequence (1) fuel + HO2 → R + H2O2, (2) H2O2 = OH + OH. This reactive sequence is favored at high pressures because more HO2 is formed at high pressure through H + O2 → HO2. Additionally, as the equivalence ratio is increased in a fuel/air mixture, the fuel concentration and the rate of reaction 1 increase. Therefore, reactions occurring at higher pressures can become more sensitive to fuel concentration (i.e., equivalence ratio). The isopentanol model developed in this study has an excellent agreement with the measured ignition delays. 4.2. Autoignition at Low Temperatures (Comparisons RCM Experiment.). In this section, the isopentanol model is validated by comparing computed autoignition delay times from the model to those measured in the rapid compression machine of the UConn (University of Connecticut) group. In general, by means of using an RCM, autoignition delays at relatively lower temperatures can be acquired compared to ST experiments. Figure 12 shows measured autoignition delay times for equivalence ratios of 0.5 and 1.0 at pressure of 0.7 MPa, and a range of temperatures from 862 to 956 K. No data could be collected for the experiments at an equivalence ratio of 2.0 and compressed pressure of 0.7 MPa. Even though the RCM was configured with the maximum possible compression ratio and initial temperature, the compressed temperature was too low for the mixtures to ignite and show a

Figure 13. Comparisons of measured and calculated ignition delay times for equivalence ratio ranging from 0.5 to 2.0 and pressures at 2.0 MPa. The RCM ignition data from UConn group.

Figures 12 and 13 include calculated autoignition delay times for equivalence ratios of 0.5 to 2.0 at pressures of 0.7 and 2.0 MPa, respectively. Two approaches have been used to calculate the ignition delay. The first type is obtained assuming adiabatic and constant volume combustion. For the other type, calculations are conducted by considering heat loss of core gas to the walls of the RCM. As discussed previously, this heat loss is modeled as a volume expansion. Therefore, calculated ignition delays with the adiabatic assumption are shorter than those measured and calculated with heat loss of the RCM, since there is no NTC region present for this temperature range. In addition, ignition delays measured in the RCM have larger temperature dependence than those obtained in an adiabatic constant volume. The present isopentanol model can precisely reproduce ignition delays of RCM experiments by means of taking heat loss into consideration. It is obvious that an increase in equivalence ratio decreases ignition delays, which is similar to isooctane.22 Curran et al. described that chain branching at low temperatures depends on radical species which are formed 4881


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directly via the reactions involving the fuel.22 This may likely be due to fuel + HO2 → R + H2O2 followed by H2O2 → OH + OH. Therefore, as equivalence ratio increases, more chain branching would occur. Similarly to the results of model validation at high temperatures, the isopentanol model can precisely reproduce ignition delays at low temperatures. For lower pressure conditions (P = 0.7 MPa) shown in Figure 12, it appears that the calculated ignition delays without heat loss have large differences from those measured and those calculated with heat loss. These differences are related to how much heat loss the RCM experiments have. In fact, as a higher compression ratio was required to achieve a higher compressed temperature at a relatively lower compressed pressure (P = 0.7 MPa), the clearance of the piston at the top dead center (TDC) was reduced in comparison with the higher pressure cases (P = 2.0 MPa). Decreasing the clearance at TDC causes the ratio of the surface area and volume of the reaction chamber to increase. In addition, as higher temperatures and lower pressures at the bottom dead center (BDC) were required for the lower pressure cases, the charge masses of mixtures were reduced. Both the higher surface area to volume ratio and lower charge mass can result in an increase in heat loss during/after a compression stroke. However, it is clear from Figure 12 that by modeling the effects of the heat loss, the mechanism is able to precisely predict the ignition delay. In case of higher pressure conditions (P = 2.0 MPa), the calculated ignition delays with and without considering heat loss are closer compared to the lower pressure conditions even as ignition delays become longer at lower temperatures. In spite of pressure conditions, the effect of equivalence ratio is significant, which is probably due to R + O2 → RO2 and its effect on the limited lowtemperature chemistry of isopentanol, as well as to the sequence of fuel + HO2 → R + H2O2 followed by H2O2 → OH + OH. 4.3. Effect of Pressure on Autoignition for a Wide Range of Temperature (Comparisons with ST AND RCM Experiments). Figure 14 indicates effects of pressure and

and RCM experiments. As mentioned above, NTC behavior, where the slope of the ignition delay curve is negative, cannot be seen for the given experimental conditions. However, both results from the experiments and calculations seem to display minor curvature at relatively lower temperatures. As shown in Figure 13, ignition delays of the RCM experiments are close to those calculated without heat loss especially for the cases at higher pressures. This means the minor curvature mentioned above barely includes the effect of heat loss, and this curvature is likely the result of low-temperature chemistry. Similar curvature can be seen in Figures 15 and 16, which show measured

Figure 15. Comparisons of measured and calculated ignition delay times for equivalence ratio of 0.5 and pressures at 0.7 and 2.0 MPa. The ST ignition data from NUIG group, and the RCM ignition data from UConn group.

Figure 16. Comparisons of measured and calculated ignition delay times for equivalence ratio of 2.0 and pressures at 0.7 and 2.0 MPa. The ST ignition data from NUIG group, and the RCM ignition data from UConn group.

and calculated autoignition delay times for equivalence ratio of 0.5 and 2.0. The recent work for n-butanol experiments shows nonmonotonic autoignition trends at a pressure of 8.0 MPa.76 Although in the current isopentanol model developed in this study a pressure dependence on the concerted HO2 elimination is not considered, da Silva et al. proposed a pressure dependence of the HO2 elimination for ethanol. According to their expression, the reaction rate would decrease with an increase in pressure.25 Therefore, if the isopentanol model included the pressure dependence for the concerted HO2 elimination reaction, it is likely that more obvious NTC behavior could appear with an increase in pressure.

Figure 14. Comparisons of measured and calculated ignition delay times for equivalence ratio of 0.5 and pressures at 0.7 to 2.3 MPa. The ST ignition data from NUIG group, and the RCM ignition data from UConn group.

temperature on autoignition delay times measured or calculated for an equivalence ratio of 1.0, pressures of 0.7−2.3 MPa, and temperatures ranging from 689 to 1409 K. All plots and lines shown in Figure 14 correspond to those in Figures 10−13. It is clear that autoignition delay times predicted with the present model have excellent agreement with those measured in the ST 4882


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fuel turns into α-hydroxypentyl radicals (referred to lc5h10oh). Oxygen addition to a hydroxyalkyl radical progresses (reaction class 10); however, a concerted elimination reaction of HO2 also proceeds to form HO2 and an aldehyde (reaction class A2). The latter path competes with the conventional isomerization of α-hydroxy-pentylperoxy radical that leads to chain branching and retards the ignition process. Dayma et al. conducted a jetstirred reactor (JSR) experiment to better understand the combustion characteristics of isopentanol, and the oxidation of isopentanol was modeled using an extended detailed chemistry derived from their previous scheme for the oxidation of various fuels. However, because no low-temperature oxidation of isopentanol was observed (550−770 K at 10 atm) through their JSR experiments, the low-temperature chemistry of isopentanol was not included.77 By contrast, Welz et al. examined lowtemperature combustion chemistry of isopentanol by means of time-resolved tunable synchrotron photoionization mass spectrometry,78 and they observed isopentanal and 3-methyl1-buten-1-ol which are referred to ic4h9cho and ic5h9oh3-4 in Figure 18 and found that these species are formed associated with HO2 production. As shown in Figure 18, when an H-atom is abstracted from the β site followed by addition of O2, the secondary hydroxypentylperoxy radical (referred to as kc5h10oh-4o2-3) is likely to form a six membered ring involving the hydroxyl group. This route is called the Waddington mechanism and leads to chain propagation rather than chain branching that occurs in hydrocarbons with the same carbon skeletal structure. Thus, more hydroxyl-pentylperoxy radicals are consumed by alternative routes than the conventional low-temperature reaction routes found in hydrocarbons. As shown in Figure 3, the experimentally observed ignition behavior of alcohols is attributed to a partial lack of the low-temperature chemistry compared to hydrocarbons with similar structures. Therefore, reaction mechanisms for higher molecular weight alcohols should include reaction steps that suppress the low-temperature chemistry normally found in hydrocarbons. A sensitivity analysis at the same conditions of the reaction path analysis is shown in Figure 19. For this analysis, the firstorder sensitivity coefficients for OH concentration were calculated using CHEMKIN-PRO.46 The sensitivity of OH concentration to the reaction rate pre-exponential A-factors was used because low-temperature reactivity is controlled by OH radicals. If an increase in the A-factor for a reaction results in higher OH concentration, then this reaction promotes the overall system reactivity and is a positive sensitivity. A negative sensitivity indicates that a reaction inhibits the overall reactivity. The largest positive sensitivity is for abstraction by OH on one of the methyl groups in isopentanol (“p site”, Figure 19). This reaction is followed by O2 addition and an RO2 isomerization involving the α site that leads to low-temperature chain branching. The first-ranked positive sensitivity is for abstraction by OH on the α site which is followed by O2 addition to α-R. The second-ranked positive reaction of fuel with HO2 (labeled “H abstraction by HO2 (α site)” in Figure 19) exhibits high sensitivity because it is part of the chain branching sequence fuel + HO2 → R + H2O2 followed by H2O2 → OH + OH. The following reaction α-R + O2 has both positive and negative sensitivity. The positive reaction forms RO2 (α) followed by the RO2 isomerization. They are labeled “α-R+O2⇔RO2(α)” and “RO2⇔QOOH (1,5 shift)”. The RO2 isomerization from the α site to the tertiary site leads to chain branching. This RO2 isomerization rate is enhanced by the relatively weak tertiary

5. DISCUSSION AND CONCLUSION In this study, the authors focus on one of the next generation biofuels, isopentanol, which is being considered as a gasoline substitute for spark ignition engines or for advanced engines (e.g., HCCI engines). A new reaction mechanism for isopentanol was developed and validated with autoignition delay times measured for a wide range of temperatures, pressures, and equivalence ratios (652−1457 K, 0.7−2.3 MPa, and 0.5−2.0, respectively). The experiments were conducted in a shock-tube (ST) for ignition delays at relatively high temperatures and in a rapid compression machine (RCM) for relatively low temperatures. High-temperature chemistry for the present model is based on the recently developed reaction model for butanol isomers,13 and most of low-temperature chemistry is based on the isooctane model proposed by Curran et al.22 In addition, new reaction pathways particular to alcohols and recently investigated in the literature25,65,68 were included in the mechanism and are needed to reproduce autoignition delays for isopentanol over a wide range of temperature and pressure. The authors believe that the most important pathway is the concerted HO2 elimination mechanism which is theoretically examined for ethanol by da Silva et al.,25 and a similar study was conducted by Zádor et al.65,68 The oxidation of isopentanol progresses though pathways whose rates are dependent on temperature and pressure (Figure 17).

Figure 17. Simplified kinetic scheme for isopentanol oxidation.

At high temperature, the fuel decomposes (reaction class 1) and hydrogen atom abstractions occur from the fuel to form a hydroxyalkyl radical (reaction class 2). In the case of alcohols, an intramolecular dehydration tends to occur to form stable species, water and alkene (reaction class A1). For low temperatures, a reaction path analysis was carried out under the conditions for equivalence ratio, temperature, and pressure of 0.5, 800 K, and 2 MPa, respectively. The reaction fluxes are given for 20% fuel consumption. As mentioned above, isopentanol has a weak C−H bond at the α site, and H atom abstraction from fuel is likely to occur at the α site. Therefore, as the reaction fluxes shows, almost 50% of 4883


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Figure 18. Reaction path analysis for low-temperature chemistry of isopentanol (ϕ: 0.5, 800 K, 2 MPa, 20% fuel consumed).

lean mixtures at low pressure (shown in Figure 10) where the model overestimates autoignition delays. These problems of the model will be addressed in future work. In addition, there is a considerable uncertainty in H atom abstraction from fuel by HO2 (in reaction class 2). For this reaction, the present model uses rate constants that are three times faster for all abstraction sites than those suggested by Aguilera-Iparraguirre et al.52 yet less than the rate constant recommended by Carlstensen et al.53 This change was made to obtain better agreement with the experimental data. Few experimental data exist for this class of H abstraction reactions, and it is estimated that the rate constants for H atom abstraction by HO2 have uncertainties of a factor of 3−6. This reaction has a great effect on the autoignition process at low and intermediate temperatures because H2O2 is formed via this reaction which decomposes into two reactive OH radicals. Therefore, more experimental data and more accurate computational chemistry calculations are needed for this reaction to reduce the uncertainties in its rate constant. Finally, note that the current isopentanol model has been validated only with the experimental data for autoignition delays obtained in the ST and RCM. In further future work on isopentanol, the model will be validated with experimental data obtained via laminar flames, stirred reactor, etc., providing an enhanced model for combustion analysis for additional applications.

Figure 19. Sensitivity analysis for low-temperature chemistry of isopentanol (ϕ: 0.5, 800 K, 2 MPa, 20% fuel consumed).

C−H bond. On the other hand, the negative one (the secondranked negative sensitivity) induces the concerted HO2 elimination which is faster than the RO2 isomerization as shown in Figure 19. Therefore, it is likely that the concerted HO2 elimination can strongly inhibit low-temperature chemistry by consuming α-R and O2 which would be source of lowtemperature chemistry. In addition, this path competes with a low-temperature chain branching path which leads to two reactive OH radicals (labeled “RO2⇔QOOH (1,5 shift)” on Figure 19). The third-ranked sensitivity is the reaction HO2 + HO2 which is chain terminating and competes with the fuel + HO2 chain branching path mentioned above. As shown in section 4, the present model has excellent agreement with the experiments for a wide range of conditions. However, the present model has not been tested at even higher pressures found in HCCI engines. In future work, the current isopentanol model will be validated with ST and RCM experiments at pressures that are higher than those examined in this paper. Moreover, the present model needs some improvements to obtain good agreement for experiments for

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+81) 29-861-7165. Fax: (+81) 29-861-7863. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Ministry of Economy, Industry, and Trade (METI) Japan as a part of Japan−U.S. cooperation project for research and standardization of Clean Energy 4884


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(23) Mehl, M.; Pitz, W. J.; Westbrook, C. K.; Curran, H. J. LLNL Report; 2010, LLNL-PROC-422312; https://e-reports-ext.llnl.gov/pdf/ 385061.pdf. (24) Murphy, M.; Taylor, J. D.; McCormick, R. L. Subcontractor Report; 2004, NREL/SR-540−36805; http://www.nrel.gov/ vehiclesandfuels/pdfs/sr368051.pdf. (25) da Silva, G.; Bozzelli, J. W.; Liang, L.; Farrell, J. T. J. Phys. Chem. A 2009, 113, 8923−8933. (26) Cancino, L. R.; Fikri, M.; Oliveira, A. A. M.; Schulz, C. Fuel 2010, 90, 1238−1244. (27) Van Geem, K. M.; Pyl, S. P.; Marin, G. B.; Harper, M. R.; Green, W. H. Ind. Eng. Chem. Res. 2010, 49, 10399−10420. (28) Westbrook, C. K.; Pitz, W. J.; Boercker, J. E.; Curran, H. J.; Griffiths, J. F.; Mohamed, C.; Ribaucour, M. Proc. Combust. Inst. 2002, 29, 1311−1318. (29) Lawrence Livermore National Laboratory, Combustion Chemistry, https://www-pls.llnl.gov/?url=science_and_technologychemistry-combustion. (30) Ciezki, H. K.; Adomeit, G. Combust. Flame 1993, 93, 421−433. (31) Fieweger, K.; Blumenthal, R.; Adomeit, G. Proc. 25th Int. Symp. Combust. 1994, 1579−1585. (32) Minetti, R.; Carlier, M.; Ribaucour, M.; Therssen, E.; Sochet, L. R. Combust. Flame 1995, 102, 298−309. (33) Minetti, R.; Carlier, M.; Ribaucour, M.; Therssen, E.; Sochet, L. R.; Proc, L. R. 26th Int. Symp. Combust. 1996, 747−753. (34) Mansurov, Z. A.; Mironenko, A. V.; Bodykov, D. U.; Rakhimetkaliev, K. N.; Westbrook, C. K. J. Therm. Sci. 2000, 9−4, 365−370. (35) Oehlschlaeger, M. A.; Davidson, D. F.; Herbon, J. T.; Hanson, R. K. Int. J. Chem. Kinet. 2003, 36, 67−78. (36) Smith, J. M.; Simmie, J. M.; Curran, H. J. Int. J. Chem. Kinet. 2005, 37−12, 728−736. (37) Shen, H.-P. S.; Vanderover, J.; Oehlschlaeger, M. A. Combust. Flame 2008, 155, 739−755. (38) Goldsborough, S. S. Combust. Flame 2009, 156, 1248−1262. (39) Vasu, S. S.; Davidson, D. F.; Hanson, R. K. Shock Waves, Part IV 2009, 293−298. (40) Oehlschlaeger, M. A.; Steinberg, J.; Westbrook, C. K.; Pitz, W. J. Combust. Flame 2009, 156, 2165−2172. (41) Oehlschlaeger, M. A.; Shen, H.-P. S.; Steinberg, J.; Vanderover, J. Proc. 6th U.S. Natl. Combust. Meet. 2010, 1−7. (42) Davidson, D. F.; Ranganath, S. C.; Lam, K. Y.; Liaw, M.; Hong, Z.; Hanson, R. K. J. Propul. Power 2010, 26-2, 280−287. (43) Tsujimura, T.; Pitz, W. J.; Yang, Y.; Dec, J. E. SAE Tech. Pap. Ser. 2011, 2011−24−0023. (44) Schocker, A.; Uetake, M.; Kanno, N.; Koshi, M.; Tonokura, K. J. Phys. Chem. A 2007, 111, 6622−6627. (45) Ritter, E. R.; Bozzelli, J. W. Int. J. Chem. Kinet. 1991, 23, 767− 778. (46) CHEMKIN-PRO Version 15101; Reaction Design Inc.: San Diego, CA, 2010; www.reactiondesign.com. (47) Mittal, G.; Sung, C.-J. Combust. Sci. Technol. 2007, 179, 497− 530. (48) Westbrook, C. K.; Pitz, W. J.; Herbinet, O.; Curran, H. J.; Silke, E. Combust. Flame 2009, 156, 181−199. (49) Sivaramakrishnan, R.; Su, M.-C.; Michael, J. V.; Klippenstein, S. J.; Harding, L. B.; Ruscic, B. J. Phys. Chem. A 2010, 114, 9425−9439. (50) Dean, A. M.; Bozzelli, J. W. Combustion Chemistry of Nitrogen. Gas-Phase Combustion Chemistry; Gardiner, W.C., Jr., Ed.; Springer: New York, 2000; Chapter 2, pp 138−141. (51) Weber, B. W.; Kumar, K.; Zhang, Y.; Sung, C.-J. Combust. Flame 2011, 158, 809−819. (52) Aguilera-Iparraguirre, J.; Curran, H. J.; Kloper, W.; Simmie, J. M. J. Phys. Chem. A 2008, 112, 7047−7054. (53) Carlstensen, H.-H.; Dean, A. M.; Deutschmann, O. Proc. Combust. Inst. 2007, 31, 149−157. (54) Stark, M. S.; Waddington, D. J. Int. J. Chem. Kinet. 1995, 27, 123−151.

Technologies, and also was supported in part by the U.S. Department of Energy, Office of Vehicle Technologies, Fuel Technologies Program. The authors thank program manager Kevin Stork for their support. The modeling work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. The RCM work was supported as part of the Combustion Energy Frontier Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (Award DE-SC0001198). The work at NUI Galway was supported by Science Foundation Ireland via their Principal Investigator Program under Grant [08/IN1./I2055]. The authors also thank Marco Mehl, Mani Sarathy, and Charles Westbrook (Lawrence Livermore National Laboratory, Chemical Combustion Group) for useful discussion on chemical kinetics and alcohols, and thank Kenji Yasunaga (NUI Galway at that time, National Defense Academy Japan at present) for his first trial on the shock-tube experiments with isopentanol fueling.

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