Experimental and Modeling Study of the Thermal Decomposition of C3

Article pubs.acs.org/JPCA

Experimental and Modeling Study of the Thermal Decomposition of C3−C5 Ethyl Esters Behind Reflected Shock Waves Wei Ren,*,† R. Mitchell Spearrin, David F. Davidson,* and Ronald K. Hanson High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: The thermal decomposition of three ethyl esters, ethyl formate (C3H6O2), ethyl acetate (C4H8O2), and ethyl propanoate (C5H10O2), was studied behind reflected shock waves using laser absorption to measure concentration time-histories of H2O, CO2, and CO. Experimental conditions covered temperatures of 1301−1636 K, pressures of 1.48−1.72 atm, and reactant concentrations of 2000 ppm in argon. Recently developed mid-infrared laser diagnostics for H2O (2.5 μm), CO2 (4.3 μm), and CO (4.6 μm) provide orders-of-magnitude greater detectivity compared to previous near-infrared absorption sensors. The experimental results have highlighted significant differences among these three ethyl esters: negligible CO2 production during ethyl formate pyrolysis, quite slow CO formation rate during ethyl acetate pyrolysis, and nearly equal formation rate of H2O, CO2, and CO during ethyl propanoate pyrolysis. Detailed kinetic modeling was performed to understand the destruction pathways of these three ethyl esters with different alkyl chain lengths. Rate of production and sensitivity analyses were also carried out to interpret the experimental results and to identify the key reactions affecting experimental results.

1. INTRODUCTION Alternative fuels are intensively studied nowadays as full replacements or supplements for fossil fuels.1 Biodiesel is typically produced through the transesterification of vegetable oils or animal fats with methanol yielding fatty acid methyl esters (FAMEs).2,3 These methyl esters can be blended with petroleum diesel and used in diesel engines without major modifications. Methanol instead of ethanol has been preferred in industrial application to produce biodiesel mostly due to its lower cost. However, ethanol is less toxic, less volatile, and less corrosive than methanol and therefore provides a safer work environment during the transesterification process. Additionally, some countries like Brazil have started to produce ethanol in large quantities.4 Therefore, biodiesel in the form of fatty acid ethyl esters (FAEEs) produced through the conversion of biolipids with ethanol would further enhance the sustainability and commercialization of biofuels.5 Previous chemical kinetic studies on ethyl esters have aimed to compare the small methyl and ethyl esters with the same chemical formula (isomers) while varying the length of the alkyl chains to investigate the effect of the molecular structure on the combustion chemistry. In 2009, Westbrook et al.6 developed a detailed chemical kinetic mechanism describing the laminar premixed flames of four small alkyl ester fuels: methyl formate (HCOOCH3), methyl acetate (CH3COOCH3), ethyl formate (HCOOC2H5), and ethyl acetate (CH3COOC2H5). The model development employs a principle of similarity of functional groups in constraining the H-atom abstraction and unimolecular decomposition reactions for each of these esters. AkihKumgeh and Bergthorson7 investigated the ignition behavior of © 2014 American Chemical Society

three pairs of methyl/ethyl esters, including methyl/ethyl formate, methyl/ethyl acetate, and methyl/ethyl propanoate, by measuring the ignition delay times behind reflected shock waves. Ethyl esters are generally characterized by shorter ignition delay times than those of methyl esters of the corresponding alkanoic acid. El-Nahas et al. studied the bond dissociation energies and enthalpies of formation for ethyl propanoate and methyl butanoate at the CBS-QB3 level of theory.8 Investigation of the various decomposition channels available to both molecules reveals that energy barriers and rate constants follow a similar trend favoring the channel to produce ethylene via a sixmembered transition state or from the O-alkyl moiety of the molecule. The calculations by El-Nahas et al.8 were then adopted in the detailed kinetic mechanism developed by Metcalfe et al.9 describing methyl butanoate and ethyl propanoate oxidation. This model was validated against the ignition delay times for a series of mixtures of varying fuel/ oxygen equivalence ratios (ϕ = 0.25−1.5) behind reflected shock waves (1100−1670 K; 1.0 and 4.0 atm). The work was complemented later by jet-stirred reactor (JSR) experiments and modeling by Metcalfe and other coauthors.10 The rate constants of ethyl propanoate unimolecular decomposition to produce propanoic acid and ethylene were modified to reproduce the JSR data at 10 atm over a range of stoichiometries (ϕ = 0.3−2.0). Received: November 30, 2013 Revised: January 18, 2014 Published: January 22, 2014 1785

dx.doi.org/10.1021/jp411766b | J. Phys. Chem. A 2014, 118, 1785−1798

The Journal of Physical Chemistry A

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The study by Walton et al.11 for methyl butanoate and ethyl propanoate combustion refined the mechanism of Metcalfe et al.,9 so that it could reproduce new experimental results from a rapid compression machine. In 2011, Yang et al. performed low-pressure flame studies of three C5H10O2 ester isomers12 and four saturated/unsaturated C4 methyl/ethyl esters.13 Detailed kinetic mechanisms were constructed to describe differences in the concentrations of key reaction intermediates between the flames of these ester isomers. Very recently, Dayma et al.14 investigated the laminar burning velocities of C4−C7 ethyl esters at 1, 3, 5, and 10 bar in a spherical combustion chamber. The main purpose of this work is to provide new experimental results of the thermal decomposition of ethyl esters behind reflected shock waves and their kinetic interpretation. We have measured CO, CO2, and H2O concentration time-histories using laser-absorption techniques during the pyrolysis of three ethyl esters: ethyl formate (EF, C3H6O2), ethyl acetate (EA, C4H8O2), and ethyl propanoate (EP, C5H10O2). Figure 1 shows their corresponding molecular

species mole fraction, P(atm) is the gas pressure, and L(cm) is the path length. The spectroscopic parameters of line-strength and broadening coefficients were taken from the HITRAN database16 or experimentally determined. Mid-infrared laser absorption diagnostics were utilized for the time-resolved measurements of CO, H2O, and CO2 concentrations, due to their higher sensitivity in this wavelength range compared to near-infrared diagnostic schemes.17−19 First, CO concentration was measured using a distributed feedback quantum cascade laser (DFB-QCL) operating in continuous wave (cw) mode at room temperature. A fixedwavelength direct-absorption strategy was employed to monitor the peak intensity of the R(13) absorption line centered at 2193.36 cm−1, enabling ppm-level detectivity over the temperature range of 1000−1800 K in shock tube experiments.17 Second, absorption measurements of H2O were performed using a distributed feedback (DFB) diode laser at 2550.96 nm within the ν3-fundamental vibrational band. This absorption sensor achieves a minimum H2O detection sensitivity of 25 ppm at 1400 K and 1.5 atm for a path length of 15 cm.19 Third, a new mid-infrared CO2 diagnostic was developed in this work by incorporating an external cavity quantum cascade laser (ECQCL), to provide sensitive and quantitative measurements of carbon dioxide. The R(76) transition line in the CO2 fundamental band near 4.3 μm was selected due to its high absorption strength and negligible interference from other combustion products. The Ar-broadening coefficient (2γCO2‑Ar) for this transition was measured behind reflected shock waves using the same method as described in ref 19. The Arbroadening coefficient over the temperature range of 1200− 1900 K was measured to be 0.0762 ± 0.0012 cm−1/atm with a temperature exponent of n = 0.57 ± 0.02. Compared to previous CO2 sensors detecting the overtone and combinational bands near 2.7 μm,20,21 the new diagnostic scheme provides more than an order-of-magnitude greater sensitivity for shock tube experiments. 2.2. Experimental Results. The exact experimental conditions behind reflected shock waves are summarized in Table 1. The measurements covered the temperature range of 1301−1636 K and pressure range of 1.48−1.72 atm with fuel concentration of 2000 ppm. Such dilute mixtures result in negligible temperature variation during fuel pyrolysis and hence no variation in the absorption coefficients.

Figure 1. Molecular structures of (a) ethyl formate, (b) ethyl acetate, and (c) ethyl propanoate.

structures. Considering the different groups (−H, −CH3, and −CH2CH3) adjacent to the CO function, our experimental results should reveal the effects of varying the alkyl chain length on the decomposition characteristics of small ethyl esters. It is the first shock tube study of the thermal decomposition of these small ethyl esters using species time-history measurements.

2. EXPERIMENTAL SECTION 2.1. Shock Tube and Laser Diagnostics. All the experiments were performed in a stainless-steel high-purity shock tube with a 15.24 cm inner diameter. The driven section is 10 m long and is separated from the helium-filled driver section (3.7 m) by a polycarbonate diaphragm. Gas temperatures and pressures behind the reflected shock waves were calculated using standard normal-shock relations and the measured incident shock speed, with an uncertainty in temperature of ±1% over the test time of 1−2 ms.15 Between experiments, the shock tube driven section and mixing manifold were turbo-pumped for ∼30 min down to ∼8 μTorr to remove residual impurities. All fuels (>99% pure, Sigma-Aldrich) were frozen and degassed three times to remove dissolved volatiles before making the mixtures with argon as the bath gas. All the test mixtures were manometrically prepared in a stainless-steel mixing tank (40 L) heated uniformly to 50 °C with an internal magnetically driven stirrer. Laser absorption and side-wall pressure measurements (Kistler 601B1 PZT) were located 2 cm from the shock tube end wall. Laser-absorption diagnostics are widely used for shock tube chemical kinetics studies, due to their species-specific and nonintrusive properties with fast time response (microsecond). The absorbing species concentration is measured using the Beer−Lambert relation, It/I0 = exp(−SΦvxiPL), where It and I0 are the transmitted and incident laser intensity, respectively; S (cm−2 atm−1) is the temperature-dependent line-strength of the transition, Φv (cm) is the line-shape function, xi is the target

Table 1. Summary of Reflected Shock Conditions for Ethyl Ester Pyrolysis mixture

2000 ppm EF/Ar

2000 ppm EA/Ar

2000 ppm EP/Ar

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CO

H2O

CO2

T5 (K)

P5 (atm)

T5 (K)

P5 (atm)

T5 (K)

P5 (atm)

1314 1402 1467 1629 1472 1566 1634 1310 1351 1440 1567

1.69 1.56 1.68 1.48 1.48 1.52 1.49 1.58 1.56 1.56 1.51

1314 1402 1467 1629 1393 1472 1566 1310 1351 1440 1567

1.69 1.56 1.68 1.48 1.54 1.48 1.52 1.58 1.56 1.56 1.51

1370 1449 1636

1.65 1.63 1.49

1492 1578 1634 1301 1366 1454 1580

1.61 1.54 1.49 1.72 1.69 1.57 1.52



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Figure 2. Representative (a) CO, (b) H2O, and (c) CO2 concentration time-histories measured during the pyrolysis of 2000 ppm EP/Ar at pressures near 1.5 atm.

Figure 3. Measured product fractional yields for (a) EF, (b) EA, and (c) EP at 1 ms.

Figure 4. Major decomposition pathways of (a) EF, (b) EA, and (c) EP.

EA pyrolysis exhibits the slowest formation rate among all these esters. Additionally, CO, H2O, and CO2 are observed to have almost the same formation rate during the pyrolysis of EP. Although the C3−C5 esters demonstrate completely distinct product yield behaviors, the final O-atom carrying products are mainly found to be CO, H2O, and CO2. At the highest temperature (∼1600 K) when the product time-histories reached the plateau level within the test time of 2 ms, the Oatom balance in the measured CO, H2O, and CO2 profiles were counted to be 98%, 93%, and 95% for EF, EA, and EP, respectively.

Representative CO, H2O, and CO2 concentration timehistories during EP pyrolysis at varied temperatures are illustrated in Figure 2 at pressures near 1.5 atm. Good signalto-noise ratios were achieved with the current laser absorption diagnostics. Note that the species time-history data for all three ethyl esters will be presented later in the Discussion section. The product fractional yields (defined by xproduct/xfuel; here xfuel = 2000 ppm) at t = 1 ms for the C3−C5 ethyl esters are plotted as a function of temperature as shown in Figure 3. In general, the product yields for these ethyl esters differ significantly from each other. EF shows the highest yields of CO and H2O but produces a negligible amount of CO2. The CO profile during 1787



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3. KINETIC MODELING Kinetic modeling of shock tube species concentration timehistories using the CHEMKIN code22 is carried out to gain further insight into the pyrolysis of EF, EA, and EP. A detailed chemical kinetic mechanism (139 species and 786 reactions) for EP oxidation has been developed by Metcalfe et al.10 This EP oxidation model was able to simulate previous shock tube ignition delay times (1100−1670 K, 1 and 4 atm9) and jetstirred reactor data (750−1100 K, 10 atm10). Under pyrolysis conditions, the major EP destruction pathway analysis using the Metcalfe et al.10 mechanism is summarized in Figure 4c. Nearly all EP decomposes to ethylene and propanoic acid through the concerted dissociation pathway (EP = C2H4 + C2H5COOH). The produced propanoic acid is then consumed mainly via the H-atom abstraction reactions yielding ·CH2CH2COOH and CH3·CHCOOH radicals. Subsequent β-scissions of these two radicals produce stable intermediates (ethylene and methyl ketene) or smaller radicals (hydroxyl radical and HOCO radical). Yang et al.12 recently studied the low-pressure premixed flat flames of ethyl propanoate, reporting that the rate constants of EP six-center unimolecular elimination and H-atom abstraction reactions needed to be modified for better agreement with the measured compositions of reaction intermediates in the lowpressure flames. In the current study, the Metcalfe et al.10 mechanism combined with the recommended modifications by Yang et al.12 is used for analyzing the shock tube data of EP pyrolysis. There are no mechanisms optimized for EF and EA pyrolysis, so the kinetic modeling in this work includes the construction of EF and EA pyrolysis models in a hierarchical manner. The core C1−C4 kinetic submechanisms from Metcalfe et al.10 are used as the starting point for the current EF and EA mechanisms. Here we only discuss the modeling efforts in terms of the added reactions or modified reaction rate constants in the Metcalfe et al.10 mechanism. The thermodynamic data for radicals related to EF and EA were taken from the literature and compared with the values estimated using THERM23 and THERGAS24 codes. All the data for the three ethyl esters were simulated with one combined mechanism, which is provided in the Supporting Information. Figure 4a presents the major EF destruction pathways. The initial EF decomposition is generally accepted to be the unimolecular elimination (EF = C2H4 + HCOOH) through a six-center transition state,25 followed by formic acid decomposition taking two competing pathways of dehydration (HCOOH = H2O + CO) and decarboxylation (HCOOH = CO2 + H2). The rate constants of EF decomposition are taken from Westbrook et al.,6 including the unimolecular decomposition to stable molecules or radicals through bond cleavage, and the H-atom abstractions by H, OH, and CH3 radicals. Recent theoretical and experimental studies on methyl esters reveal that the alcohol elimination reaction is the dominant pathway during the pyrolysis of methyl formate.26−28 Considering the similar molecular structure between methyl and ethyl formate, another concerted unimolecular reaction (EF = CO + C2H5OH) is added into the EF pyrolysis submechanism with rate constants estimated by analogy with methyl formate.28 The branching ratio of this ethanol elimination reaction is evaluated to be 0.03 and 0.1 at 1200 and 1800 K, respectively, proving to be a minor channel during EF pyrolysis especially at lower temperatures.

Formic acid is the major intermediate during EF pyrolysis, and its decomposition has been the subject of several research groups.29−31,34 Our current shock tube measurements result in CO2/H2O ratios between 0.05 and 0.07, which is in excellent agreement with the experimental observation reported by Saito et al.31 Hence, the branching ratio of those two formic acid unimolecular reactions recommended by Saito et al.31 were used in the current EF submechanism. The H-atom abstraction reactions are also possible consumption pathways for formic acid, with rate constants taken from dimethyl ether (DME) reaction kinetics by Fischer et al.32 The added EF pyrolysis reactions or modified rate constants are summarized in Table 2. Table 2. EF Pyrolysis Submechanism: Added Reactions or Modified Rate Constants in the Metcalfe et al.10 Mechanism; Ea in Units of cal/mol and A in Units of mol/cm3/s or 1/s A

n

Ea

ref

0.00 0.00 2.80 1.00 0.00 0.00 0.00 −0.40 2.40 1.60 0.00 0.00 0.00 −0.90 2.40 1.60 3.50 1.50 2.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

5.00 × 104 5.87 × 104 6.28 × 103 1.58 × 103 1.16 × 104 1.04 × 104 1.04 × 104 2.46 × 104 4.47 × 103 −3.50 × 101 9.50 × 103 1.04 × 104 1.04 × 104 1.40 × 104 4.47 × 103 −3.50 × 101 5.48 × 103 3.74 × 104 5.73 × 103 0.00 0.00 0.00 0.00 0.00 0.00 5.40 × 104

6 28 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 31a

4.90 × 1015

0.00

5.70 × 104

31a

1.00 × 1014 2.62 × 106

0.00 2.06

0.00 9.16 × 102

32 32

1.85 × 107

1.51

−9.62 × 102

32

4.24 × 106

2.10

4.86 × 103

32

6.03 × 1013

−0.35

2.98 × 103

32

reaction EF = HCOOH + C2H4 EF = CO + C2H5OH EF + H = EFp + H2 EF + OH = EFp + H2O EF + CH3 = EFp + CH4 EF + C2H3 = EFp + C2H4 EF + C2H5 = EFp + C2H6 EFP = C2H4 + OCHO EF + H = EFs + H2 EF + OH = EFs + H2O EF + CH3 = EFs + CH4 EF + C2H3 = EFS + C2H4 EF + C2H5 = EFS + C2H6 EFs = CH3CHO + HCO EF + H = EFf + H2 EF + OH = EFf + H2O EF + CH3 = EFf + CH4 C2H5 + CO2 = EFf C2H5O + CO = EFf EFp + H = EF EFs + H = EF EFf + H = EF OCHO + C2H5 = EF HCO + C2H5O = EF CH3 + CH2OCHO = EF HCOOH + M = CO + H2O + M HCOOH + M = CO2 + H2 + M HCO + OH = HCOOH HCOOH + OH = CO2 + H + H2O HCOOH + OH = CO + OH + H2O HCOOH + H = CO2 + H + H2 HCOOH + H = CO + OH + H2 a

1.00 1.15 1.88 1.05 1.29 1.00 1.00 1.34 3.25 1.16 3.98 1.00 1.00 4.17 6.50 2.33 1.51 4.76 1.55 1.00 1.00 1.00 1.00 1.00 1.00 3.50

× × × × × × × × × × × × × × × × × × × × × × × × × ×

1013 1013 105 1010 1012 1011 1011 1013 105 107 1011 1011 1011 1015 105 107 100 107 106 1014 1014 1014 1012 1012 1012 1016

The total rates are increased by a factor of 3; see text.

In the case of EA pyrolysis, the unimolecular decomposition of EA produces one ethylene molecule and one acetic acid molecule via a six-center transition state, followed by the subsequent decomposition of acetic acid to the final products of H2O, CO, CO2, and CH4. The major fuel destruction pathways for EA are described in Figure 4b. Similarly, the rate constants 1788



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of EA decomposition are also taken from Westbrook et al.,6 including EA unimolecular decomposition reactions and the Hatom abstractions by H, OH, and CH3 radicals. Acetic acid is the major intermediate species during EA pyrolysis. Leplat and Vandooren33 recently performed a numerical and experimental study of the combustion of acetic acid in three CH3COOH/ O2/Ar flat premixed flames burning at low pressure (50 mbar) and with equivalence ratios equal to 0.77, 0.9, and 1.05, respectively. Therefore, in this work, the CH3 COOH submechanism from Leplat and Vandooren33 has been added to the current EA pyrolysis model; see Table 3 for all the added reactions.

Table 3. EA Pyrolysis Submechanism: Added Reactions or Modified Rate Constants in the Metcalfe et al.10 Mechanism; Ea in Units of cal/mol and A in Units of mol/cm3/s or 1/s reaction

A

n

Ea

ref

2.00 × 10 0.00 5.00 × 10 6 EA = CH3COOH + C2H4 13 0.00 0.00 6 C2H5O + CH3CO = EA 3.00 × 10 C2H5 + CH3CO2 = EA 3.00 × 1013 0.00 0.00 6 C2H5OCO + CH3 = EA 6.00 × 1013 0.00 0.00 6a EA + H = EAp + H2 1.88 × 105 2.80 6.28 × 103 6 EA + OH = EAp + H2O 1.05 × 1010 1.00 1.58 × 103 6 EA + CH3 = EAp + CH4 1.29 × 1012 0.00 1.16 × 104 6 EA + C2H3 = EAp + 1.00 × 1011 0.00 1.04 × 104 6 C2H4 EA + C2H5 = EAp + 1.00 × 1011 0.00 1.04 × 104 6 C2H6 −0.40 2.46 × 104 6 EAp = C2H4 + CH3CO2 1.34 × 1013 5 EA + H = EAs + H2 3.25 × 10 2.40 4.47 × 103 6 EA + OH = EAs + H2O 1.16 × 107 1.60 −3.50 × 101 6 EA + CH3 = EAs + CH4 3.98 × 1011 0.00 9.50 × 103 6 EA + C2H3 = EAs + 1.00 × 1011 0.00 1.04 × 104 6 C2H4 1.00 × 1011 0.00 1.04 × 104 6 EA + C2H5 = EAs + C2H6 −0.90 1.40 × 104 6 4.17 × 1015 EAs = CH3CHO + CH3CO 5 3 EA + H = EAm + H2 6.50 × 10 2.40 2.58 × 10 6 EA + OH = EAm + H2O 1.40 × 1010 0.50 6.30 × 101 6 EA + CH3 = EAm + CH4 1.51 × 10−10 6.40 8.93 × 102 6 EA + C2H3 = EAm + 1.00 × 1011 0.00 1.04 × 104 6 C2H4 EA + C2H5 = EAm + 1.00 × 1011 0.00 1.04 × 104 6 C2H6 1.00 × 1013 0.00 0.00 6 CH2CO + C2H5O = EAm EAp + H = EA 1.00 × 1013 0.00 0.00 6 EAs + H = EA 1.00 × 1013 0.00 0.00 6 EAm + H = EA 1.00 × 1013 0.00 0.00 6 CH3COOH = CH4 + 7.08 × 1013 0.00 7.46 × 104 35 CO2 CH3COOH = CH2CO + 4.47 × 1014 0.00 7.98 × 104 35 H2O HOCO + CH3 = 0.00 0.00 b 1.20 × 1012 CH3COOH CH3COOH + H = 8.40 × 107 2.00 7.70 × 103 33 CH2COOH + H2 5.55 × 10−23 10.6 −4.46 × 103 33 CH3COOH + H = CH3CO2 + H2 CH3COOH + OH = 1.10 1.81 × 103 33 1.29 × 1010 CH2COOH + H2O CH3COOH + OH = 0.00 −4.00 × 102 33 2.40 × 1011 CH3CO2 + H2O 6.60 × 1011 0.00 2.78 × 103 33 CH3COOH + CH3 = CH2COOH + CH4 6.11 × 100 3.57 7.72 × 103 33 CH3COOH + CH3 = CH3CO2 + CH4 CH2 + CO(+M) = 8.10 × 1011 0.50 4.51 × 103 36 CH2CO(+M) k∞ k0 2.69 × 1033 −5.11 7.09 × 103 2 6 α = 0.5907, T*** = 2.75 × 10 , T* = 1.226 × 10 , T** = 5.185 × 103 CH2CO + H = HCCO + 5.00 × 1013 0.00 8.00 × 103 10c H2 CH2CO + CH3 = C2H5 0.00 0.00 × 100 33 5.00 × 1012 + CO 13

4. DISCUSSION 4.1. Ethyl Formate Pyrolysis. Simultaneous measurements of H2O and CO concentration time-histories are plotted together in Figure 5 at temperatures between 1314 and 1629 K, pressures between 1.48 and 1.69 atm. Nearly equal formation rate (CO is slightly faster) is observed for H2O and CO during EF pyrolysis over the entire temperature range, except for the case at the highest temperature (1629 K). As EF decomposition is dominated by the concerted unimolecular elimination to produce ethylene and formic acid, the measured 1:1 of H2O/ CO ratio is good evidence for the subsequent dehydration reaction of formic acid (HCOOH = H2O + CO). Note that the H2O/CO ratio is less than 1 (actually 0.87) at the highest temperature (1629 K) when these species time-histories reached their plateau levels at long times. Such overproduction of CO relative to H2O can be attributed to the added EF competing decomposition channel (EF = CO + C2H5OH), which is more pronounced at higher temperatures. Additionally, the product fractional yield shown in Figure 3a reveals that CO2 is a minor product during EF pyrolysis with the CO2/H2O ratio between 0.05 and 0.07. Saito et al.31 measured the branching ratio of formic acid decomposition in a shock tube by monitoring the time-resolved infrared radiation from CO (4.63 μm) and CO2 (4.23 μm). The branching ratio recommended by Saito et al.31 was adopted in the current model, but the authors of the present work found it necessary to increase the total decomposition rates by up to a factor of 3 for the best fit to the experimental data. Although some independently corroborated experimental data exist for some conditions, there is an overall large spread in the total rate (differing by a factor of 2−6 at temperatures between 1200 and 1600 K) across all previous studies.29,31,37 The rate constants for H-atom abstractions of formic acid are taken directly from the dimethyl ether (DME) reaction kinetics by Fischer et al.,32 as formic acid is also an intermediate species in DME oxidation. Figure 6 compares the measured H2O, CO, and CO2 time-histories with the model predictions; see Table 2 for the added reactions. Excellent agreement can be seen between the measured H2O and CO time-histories and simulations over the entire temperature range. CO2 is also predicted to be a minor product as shown in Figure 6c, in good agreement with the measurements. Rate of production (ROP) and sensitivity analyses of CO are plotted in Figure 7. The ROP analysis in Figure 7a indicates that CO is largely produced by the dehydration reaction of formic acid (HCOOH = CO + H2O), which originates from EF decomposition via a six-center transition state. Hence, CO exhibits strong sensitivity to EF and formic acid decomposition reactions as illustrated in Figure 7b. However, EF is completely consumed so quickly (within 15 μs at 1500 K) that the CO

4

a A-factor increased by a factor of 2; see text. bEstimated with the best fit to the experimental data, see text. cA-factor divided by a factor of 4; see text.

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Figure 5. Measured H2O and CO concentration time-histories during the pyrolysis of 2000 ppm EF/Ar.

sensitivity to EF reactions is only pronounced at the very early times. The very similar performance of the H2O sensitivity can be seen in Figure 8, as H2O and CO are mainly produced from the formic acid dehydration reaction. However, one of the EF unimolecular decomposition reactions (EF = C2H5OH + CO, Rxn. C in Figure 7b) shows up in the early time CO sensitivity but is negligible in the H2O sensitivity plot. Therefore, this EF decomposition channel accounts for the slight difference between CO and H2O concentration time-histories measured behind reflected shock waves. CO2 is observed to be a minor product during the pyrolysis of EF, which has also been well captured by the current kinetic model. ROP analysis indicates that CO2 is largely produced by the decarboxylation reaction of formic acid (HCOOH = CO2 + H2). However, the formic acid decomposition favors the dehydration pathway with the branching ratio of more than 0.9 under the shock tube conditions. The CO2 sensitivity analysis shown in Figure 9 indicates that the CO2 mole fraction is dominantly sensitive to these two competing pathways of formic acid unimolecular decomposition. 4.2. Ethyl Acetate Pyrolysis. Current experimental results reveal that the CO production rate is much slower compared to H2O and CO2 during EA pyrolysis, as shown in Figure 2b. Additionally, the CO time-histories during the pyrolysis of EF, EA, and EP at a temperature near 1450 K are plotted in Figure 10 for comparison. The CO formation rate during EA pyrolysis was measured to be only 0.5 ppm/μs in the first 500 μs, which is 9 times slower than EF pyrolysis and more than 6 times (considering the 30 K difference in temperature) slower than EP pyrolysis, respectively. Therefore, it is of interest to investigate the kinetic interpretation of this experimental observation. The kinetic mechanism specific to EA pyrolysis (in Table 3) has been built by considering the current experimental results and a literature study on ethyl acetate and acetic acid. The EA submechanism is taken from Westbrook et al.6 without modification except for the rate constant of EA bond-fission reaction (EA = CH3 + C2H5O·CO), which is increased by a factor of 2. Rate constants for this reaction used in the core EA kinetic mechanism6 were estimated from the reverse radical− radical recombination reaction while ignoring the barrier height, and thus, the adjustments implemented here are not unreasobable. The submechanism of acetic acid is one of the core subsets for the EA pyrolysis model since acetic acid is the major intermediate produced from the initial EA unimolecular

Figure 6. Comparison of the measured (a) CO, (b) H2O, and (c) CO2 concentration time-histories with the model predictions during the pyrolysis of 2000 ppm EF in argon: solid line, measurement; dashed line, simulation.

elimination (EA = CH 3 COOH + C 2 H 4 ). Leplat and Vandooren33 recently performed numerical and experimental study of the combustion of acetic acid in three CH3COOH/ O2/Ar low-pressure premixed flames. Considering the different experimental conditions, their model is adopted here as a starting point for modeling our shock tube data. The unimolecular decomposition of acetic acid was previously investigated over the temperature range of 1300− 1950 K in a shock tube.35 On the basis of the experimental observation of the principal products including ketene, H2O, CO2, and methane, Mackie and Doolan35 proposed that acetic acid decomposes mainly via the dehydration (CH3COOH = CH2CO + H2O) and decarboxylation (CH3COOH = CH4 + CO2) pathways. The rate constants recommended by Mackie and Doolan35 were used in this study without modification. Another radical-related channel for acetic acid decomposition 1790



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Figure 9. CO2 sensitivity: 2000 ppm EF/Ar, 1500 K, and 1.5 atm.

Figure 7. (a) ROP and (b) sensitivity analyses (using the current EF pyrolysis mechanism) of CO: 2000 ppm EF/Ar, 1500 K, and 1.5 atm.

Figure 10. Comparison of the measured CO concentration timehistories during the pyrolysis of EF, EA, and EP; pressure near 1.5 atm and fuel concentration of 2000 ppm.

Figure 8. H2O sensitivity: 2000 ppm EF/Ar, 1500 K, and 1.5 atm.

decomposition (CH2CO = CH2 + CO, Rxn. D in Figure 12a), and through ketene bimolecular reactions with CH3, H, and OH radicals (CH2CO + CH3 = C2H5 + CO, Rxn. A in Figure 12a; CH2CO + H = CH3 + CO, Rxn. B in Figure 12a; and CH2CO + OH = CH2OH + CO, Rxn. C in Figure 12a). These reactions also appear in the CO sensitivity plot as illustrated in Figure 12b. Nearly all EA takes the unimolecular elimination reaction (EA = C2H4 + CH3COOH, Rxn. A in Figure 12b) to produce the intermediate species acetic acid, which quickly decomposes via two competing pathways to CO2 + CH4 (decarboxylation) and H2O + CH2CO (dehydration). It should be noted that the existence of methyl group in acetic acid leads to the formation of ketene during the dehydration process instead of the direct formation of CO during the dehydration of formic acid. The initial EA decomposition involves little production of radicals since the major EA initiation reaction (EA = C2H4 + C2H5COOH) and subsequent acetic acid decomposition (C2H5COOH = CO2 + CH4 and C2H5COOH = H2O + CH2CO) are all concerted molecular reactions. Hence, the CO formation is constrained by the ketene decomposition rate and the size of CH3, H, and OH radical pool in the system. Acetic acid decomposition is different from formic acid, which significantly favors the dehydration channel. Acetic acid proceeds through dehydration and decarboxylation decomposition channels with almost equal rate. This explains the formation of much larger amount of CO2 during the pyrolysis of EA compared to that of EF (which differs by a factor of 6 or

(CH3COOH = CH3 + HOCO) is also considered in the modeling to achieve a best fit to the experimental data. The ketene reactions and other submechanims are all based on the core C1−C4 models in the Metcalfe et al.10 mechanism. All the measured H2O, CO, and CO2 concentration time-histories are plotted in Figure 11, along with simulations using the current EA pyrolysis submechanism. Simulations are in relatively good agreement with measurements over all the experimental conditions, though differences remain that merit future adjustments to the mechanism. The ROP analysis shown in Figure 12a indicates that CO is mainly produced from ketene, through ketene unimolecular 1791



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Figure 12. (a) ROP and (b) sensitivity analyses (using the current EA pyrolysis mechanism) of CO: 2000 ppm EA/Ar, 1500 K, and 1.5 atm.

Figure 11. Comparison of the measured (a) CO, (b) H2O, and (c) CO2 concentration time-histories with the model predictions for 2000 ppm EA/Ar: solid line, measurement; dashed line, simulation. Figure 13. CO2 sensitivity: 2000 ppm EA/Ar, 1500 K, and 1.5 atm.

more). As expected, the CO2 sensitivity, plotted in Figure 13, indicates that the CO2 mole fraction is dominantly sensitive to the decarboxylation reaction (CH3COOH = CO2 + CH4) of acetic acid. Much weaker sensitivity can be seen to EA unimolecular decomposition (EA = CH3COOH + C2H4 and EA = C2H5 + CH3CO2) at very early times and the H-atom abstraction reaction of acetic acid (CH3COOH + OH = · CH2COOH + H2O) at long times. Considering the remaining underprediction of CO2 at the lowest temperature (1492 K), the rate constants assigned to these reactions may require further investigation. These reactions also appear in the H2O sensitivity as illustrated in Figure 14. The H2O concentration is sensitive to the branching ratio of acetic acid unimolecular

elimination and the H-atom abstraction of acetic acid by hydroxyl radical. 4.3. Ethyl Propanoate Pyrolysis. The detailed kinetic mechanism by Metcalfe et al.10 is used for the analysis of ethyl propanoate pyrolysis. In general, modifications are required to improve the agreement between model predictions and the measured H2O, CO, and CO2 concentration time-histories during the pyrolysis of EP behind reflected shock waves. Figure 15 presents the measured product fractional yields at 1 ms, along with the model predictions using the Metcalfe et al.10 mechanism. At the highest temperature near 1600 K, the model significantly underpredicts the CO2 yield by a factor of 5, but 1792



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Figure 14. H2O sensitivity: 2000 ppm EA/Ar, 1500 K, and 1.5 atm.

Figure 16. Main reaction pathways for EP pyrolysis using the Metcalfe et al.10 mechanism: 2000 ppm EP/Ar, 1400 K, and 1.5 atm, at t = 100 μs.

small amount of propanoic acid (3%) takes the third pathway to produce the ·CH2COOH radical through bond cleavage, followed by the β-scission to produce ketene. Therefore, the CO2 yield is determined mainly by the branching ratios of HOCO thermal decomposition, H-abstractions of propanoic acid, and methyl ketene decomposition. As discussed before, the Metcalfe et al.10 mechanism adopted a CO2 branching ratio less than 0.1 for HOCO decomposition over the temperature range of 1000−2000 K. HOCO radical is treated as an important intermediate in the CO + OH reaction system.38−40 The rate constants for the reaction HOCO = CO + OH can be obtained from the reverse reaction with the wellknown equilibrium constant. However, no study can be found in terms of the branching ratios of those two HOCO thermal decomposition channels: HOCO = CO2 + H and HOCO = CO + OH. Interestingly, another radical CH3OCO, showing similar potential energy surface to that of HOCO,41 has been thoroughly studied recently as it is an important radical formed during methyl butanoate decomposition. Huynh et al.42 recently calculated the rate constants of CH3OCO decomposition (CH3OCO = CO2+ CH3 and CH3OCO = CO + CH3O) using the RRKM theory with corrections from tunneling, hindered rotation and variational treatments. The branching ratio for the CO2 channel was calculated to be 0.7− 0.9 over the temperature range of 1000−2000 K.42 Additionally, some preliminary results from the recent calculations by John Barker39,43 reveal that the branching ratio of HOCO thermal decomposition is strongly pressure sensitive. In this study, the rate expressions for HOCO decomposition are listed in Table 4 with the branching ratio of CO2 channel evaluated to be 0.4−0.7 over the temperature range of 1300−1800 K and near 1 atm. Without any experimental evidence, however, an uncertainty factor of 2 or more is still assigned to these reaction rate constants.

Figure 15. Comparison of measured (symbol-solid line) and simulated (dashed line, Metcalfe et al.10) CO, H2O, and CO2 yields for 2000 ppm EP/Ar mixtures at 1 ms: temperature, 1301−1580 K; pressure, 1.4−1.7 atm.

overpredicts the H2O and CO yields by 50% and 30%, respectively. These discrepancies indicate that the EP decomposition pathways need to be revised in the Metcalfe et al.10 mechanism. Figure 16 presents the main pathways for CO and CO2 production during the pyrolysis of EP using the Metcalfe et al.10 mechanism at 1400 K and 1.5 atm; in this figure, the thickness of the arrow is proportional to the importance of the reaction pathway. At t = 100 μs, more than 97% of EP decomposes to ethylene and propanoic acid (EP = C2H5COOH + C2H4) through a six-center transition state. Subsequent reactions of propanoic acid include three decomposition pathways. The first pathway (32%) involves H-atom abstraction to produce a · CH2CH2COOH radical, followed by the β-scission to yield one ethylene and one HOCO radical. The unstable HOCO radical continues to decompose to CO2 and CO with the CO2/CO product ratio being approximately 0.08 in the Metcalfe et al.10 mechanism; this indicates that nearly all of the HOCO is converted to CO. The second pathway (65%) involves the Hatom abstraction of propanoic acid to produce a CH3· CHCOOH radical, followed by a β-scission to yield methyl ketene and hydroxyl radical, or a second H-atom abstraction to produce a propenoic acid molecule. In the Metcalfe et al.10 model, methyl ketene is relatively stable under current experimental conditions; only 2% of methyl ketene reacts with OH radical to yield CO and CO2 at t = 100 μs. Finally, a 1793



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Table 4. Reaction Rate Constants Modified in the Metcalfe et al.10 Mechanism; Ea in Units of cal/mol and A in Units of mol/cm3/s or 1/s reaction

A

EP+ H = EP3J + H2 EP + H = EP2J + H2 EP + H = EPEJ + H2 EP + H = EPMJ + H2 EP + OH = EP3J + H2O EP + OH = EP2J + H2O EP + OH = EPEJ + H2O EP + OH = EPMJ + H2O C2H5COOH + H = CH2CH2COOH + H2 C2H5COOH + H = CH3CHCOOH + H2 C2H5COOH = CH3 + CH2COOH CH3CHCO + OH = sC2H4OH + CO CH3CHCO + H = C2H5 + CO HOCO = CO + OH HOCO = CO2 + H

× × × × ×

n

Ea

10 1013 105 105 1010

× × × × ×

ref

2.54 0 2.4 2.54 0.97

6.76 7.30 2.58 6.76 1.59

3

10 103 103 103 103

12 12 12 12 12

2.30 × 1010

0.51

6.30 × 101

12

2.29 × 1010

0.51

6.30 × 101

12

1.05 × 1010

0.97

1.59 × 103

12

2.60 × 106

2.54

6.76 × 103

10a

6.35 × 1013

0.00

7.30 × 103

10b

3.98 × 1015

0.00

8.84 × 104

10

2.00 × 1012

0.00

−1.01 × 103

10

4.40 × 1012

0.00

1.46 × 103

10

2.80 × 1026 2.00 × 1036

−5.12 −8.11

2.76 × 104 2.90 × 104

39,43c 39,43c

1.33 5.04 3.25 1.88 1.06

6

a

A-factor increased by a factor of 4; see text. bA-factor reduced by a factor of 4; see text. cPrivate communication: Prof. John Barker, University of Michigan (2013).

The branching ratio for H-atom abstractions of propanoic acid also needed to be adjusted to improve the agreement with experimental data. Here, the rate constant for abstraction of primary H-atom in propanoic acid (C2H5COOH + H = · CH2CH2COOH + H2) is increased by a factor of 4, while that for abstraction of secondary H-atom (C2H5COOH + H = CH3· CHCOOH + H2) is reduced by a factor of 4. The proposed changes to the rate constants of H-abstraction of propanoic acid are again not unreasonable, as the propanoic acid submechanism in the Metcalfe et al. model was constructed using estimated rates based on n-heptane and iso-octane rates.10 According to the recent flat flame study by Yang et al.,12 the rate constants for H-atom abstraction of EP were modified for better agreement with the measured composition of reaction intermediates in the low-pressure flames. The rate constants for these reactions were changed in general by a factor of 2−5 by Yang et al.12 Therefore, the Metcalfe et al.10 mechanism is also modified with the new rate values from Yang et al.12 All the reactions with rate constants modified in the Metcalfe et al.10 mechanism in the current study are summarized in Table 4. Figure 17 presents the comparison of the measured H2O, CO2, and CO time-histories and the model predictions using the original and modified Metcalfe et al.10 mechanism. The modifications proposed in this study significantly improve the agreement between measurements and simulations. The modified EP model captures the product yield of H2O (the plateau within the test time) well, and the product yields of CO and CO2 somewhat less well, especially at low temperatures. Notably, the early time formation rate of CO and CO2 is underpredicted. The ROP and sensitivity analyses for CO are illustrated in Figure 18, conducted at 1500 K and 1.5 atm for 2000 ppm EP

Figure 17. Comparison of measured (a) H2O, (b) CO2, and (c) CO concentration time-histories with the model predictions during the pyrolysis of 2000 ppm EP/Ar. Solid line, current measurement; dashdot line, simulation using the Metcalfe et al.10 mechanism; dashed line, simulation using the modified (see Table 4) Metcalfe et al. mechanism.

in argon. The ROP analysis in Figure 18a reveals that CO is largely produced through the HOCO decomposition (HOCO = CO + OH) at the early times and H/OH assisted cleavage of methyl ketene (CH3CHCO + H = C2H5 + CO and CH3CHCO + OH = sC2H4OH + CO) at the long times. Therefore, the CO concentration must be sensitive to those reactions building radical pools. Sensitivity analysis shown in Figure 18b supports the ROP interpretation. The early time CO concentration is strongly sensitive to the initial EP 1794



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time-histories using the Metcalfe et al.10 mechanism, which is in suspicion. Similarly, while the modified Metcalfe et al.10 mechanism adequately captures the CO2 yield at long times for high temperatures (less well at low temperatures), it significantly underpredicts the early time CO2 concentrations; see Figure 17b. Figure 19 presents the CO2 sensitivity at 1500 K and 1.5

Figure 19. CO2 sensitivity analysis using the modified Metcalfe et al.10 mechanism: 2000 ppm EP/Ar, 1500 K, and 1.5 atm.

atm for 2000 ppm EP/Ar mixture. The branching ratio of the H-atom abstraction of propanoic acid to produce CH3· CHCOOH and ·CH2CH2COOH determines the final CO2 yield, as these two competing pathways show strong sensitivity over the entire CO2 time-histories but with opposite effects. Besides the EP unimolecular decomposition (EP = C2H5COOH + C2H4 and EP = C2H5CO2 + C2H5), the propanoic acid bond fission (C2H5COOH = CH3 + · CH2COOH) also plays a significant role in determining the early time CO2 formation rate. Considering the importance of propanoic acid and methyl ketene decomposition reactions, it is of interest to investigate the influence of adjusting these rate constants on predicting the early time formation of H2O, CO, and CO2. According to the previous discussion, hence, three more reactions were modified:

Figure 18. (a) ROP and (b) sensitivity analyses using the modified Metcalfe et al.10 mechanism: 2000 ppm EP/Ar, 1500 K, and 1.5 atm.

unimolecular elimination (EP = C2H5COOH + C2H4) with negative effect and bond fission reaction (EP = C2H5CO2 + C2H5) with positive effect. Subsequent decomposition reactions of C2H5CO2 and C2H5 radicals both produce the H atoms. CO also shows sensitivity to propanoic acid bond fission reaction (C2H5COOH = CH3 + ·CH2COOH, followed by ·CH2COOH = CH2CO + OH), as this is a radical branching reaction producing both CH3 and OH radicals. Large amounts of methyl ketene and HOCO radicals are produced by the β-scission of CH3·CHCOOH and ·CH2CH2COOH intermediates, both primarily originating from the H-atom abstractions of propanoic acid (C2H5COOH + H = CH3·CHCOOH + H2 and C2H5COOH + H = ·CH2CH2COOH + H2). These reactions exhibit high sensitivity at the longer times of CO time-histories as shown in Figure 18b. All of these reactions involving methyl ketene, propanoic acid, and EP may contribute to the discrepancies between the current measurements and simulations. It is not clear which of these reactions must be modified, and further study is necessary to improve the EP kinetic mechanism. One clue is that the decomposition rate of methyl ketene was likely underpredicted in the Metcalfe et al.10 mechanism. At the early times of EP pyrolysis, CO is directly produced via the thermal decomposition of HOCO radical (HOCO = CO + OH) or radical-assisted cleavage of methyl ketene (CH3CHCO + H = C2H5 + CO and CH3CHCO + OH = sC2H4OH + CO). However, a large amount of methyl ketene still exists in the predicted species

C2H5COOH ↔ CH3 + CH 2COOH

(R1), k × 6

CH3CHCO + OH ↔ sC2H4OH + CO CH3CHCO + H ↔ C2H5 + CO

(R2), k × 4

(R3), k × 4

with the rate constants increased by a factor of 4 to 6. The comparison of the current measurements and different model predictions are plotted in Figure 20. As expected, the proposed adjustments on R1−R3 further improved all three species timehistories at the early times, especially the CO formation rate as shown in Figure 20c. Because of the complex nature of EP decomposition, a final conclusion for all the key individual reaction rates in the EP pyrolysis mechanism cannot yet be made. The direct measurement of certain reaction rates such as methyl ketene decomposition, however, is feasible and may provide a worthwhile research path. High-level ab initio calculations are also recommended to reduce the uncertainties in the rate constants for EP and propanoic acid decomposition reactions. This assumption is supported by the very recent study of methyl propanoate (MP) pyrolysis that even for this small methyl ester the pyrolysis submechanism of MP including 1795



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Figure 21. Major reaction pathways for EP pyrolysis using the modified Metcalfe et al.10 mechanism: 2000 ppm EP/Ar, 1400 K, and 1.5 atm, at t = 100 μs. Modified reactions: Table 4 and R1−R3.

for CO is EP → C2H5COOH → CH3·CHCOOH → CH3CHCO → CO.

5. CONCLUSIONS Quantitative measurements of CO, CO2, and H2O concentration time-histories were carried out for ethyl formate, ethyl acetate, and ethyl propanoate pyrolysis using mid-infrared laser absorption techniques in a shock tube. More than 90% of oxygen balance was accounted for when the CO, CO2, and H2O concentrations reached their plateau values (within the test time of 2 ms) for all three ethyl esters. The C3−C5 ethyl esters produce ethylene and their corresponding acid (formic acid for EF, acetic acid for EA, and propanoic acid for EP) primarily through unimolecular elimination reactions. In general, for those ethyl esters with even longer alkyl substitution, the major intermediates are also expected to be the corresponding acid and ethylene. Hence, the final decomposition product yields are mainly determined by the submechanisms of the individual carboxylic acids. Detailed kinetic modeling was performed to understand the experimental results. The EF and EA pyrolysis models proposed in the current study can fairly well simulate the corresponding species time-histories, while the detailed EP mechanism10 needs to be modified to match our experimental data. Sensitivity and ROP analyses indicate that species time-histories are dependent on a complicated network of chemical reactions, many of which have not been well studied. Reaction pathway and sensitivity analyses identify the branching ratios of HOCO radical decomposition and H-abstraction of propanoic acid to be modified as the CO2 yield was significantly underpredicted in the Metcalfe et al.10 mechanism. Further experimental and theoretical studies of certain reaction rates such as HOCO,

Figure 20. Performance of the modified Metcalfe et al.10 model on the predicted species time-histories of (a) H2O, (b) CO2, and (c) CO. Two versions of model modifications: modified model v1, only rate constants in Table 4 adopted in the Metcalfe et al.10 model; modified model v2, both reactions in Table 4 and R1−R3 modified in the Metcalfe et al.10 model.

unimolecular decomposition and H-abstraction reactions needs to be recalculated to enhance the accuracy of the kinetic modeling.44 Lastly, the major EP decomposition pathways to produce CO and CO2 are depicted again in Figure 21 using the current modified Metcalfe et al.10 mechanism. The simulation was performed for 2000 ppm EP/Ar mixture at the temperature of 1400 K and pressure of 1.5 atm. Note that the modified pathways are highlighted (red arrows) in the plot. The current kinetic interpretation indicates that during the pyrolysis of ethyl propanoate, the dominant CO2 formation pathway is EP → C2H5COOH → ·CH2CH2COOH → HOCO → CO2, and that 1796



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(10) Metcalfe, W. K.; Togbé, C.; Dagaut, P.; Curran, H. J.; Simmie, J. M. A Jet-Stirred Reactor and Kinetic Modeling Study of Ethyl Propanoate Oxidation. Combust. Flame 2009, 156, 250−260. (11) Walton, S. M.; Wooldridge, M. S.; Westbrook, C. K. An Experimental Investigation of Structural Effects on the Auto-Ignition Properties of Two C5 Esters. Proc. Combust. Inst. 2009, 32, 255−262. (12) Yang, B.; Westbrook, C. K.; Cool, T. A.; Hansen, N.; KohseHöinghaus, K. Fuel-Specific Influences on the Composition of Reaction Intermediates in Premixed Flames of Three C5H10O2 Ester Isomers. Phys. Chem. Chem. Phys. 2011, 13, 6901−6913. (13) Yang, B.; Westbrook, C. K.; Cool, T. A.; Hansen, N.; KohseHöinghaus, K. The Effect of Carbon−Carbon Double Bonds on the Combustion Chemistry of Small Fatty Acid Esters. Z. Phys. Chem. 2011, 225, 1293−1314. (14) Dayma, G.; Halter, F.; Foucher, F.; Mounaim-Rousselle, C.; Dagaut, P. Laminar Burning Velocities of C4−C7 Ethyl Esters in a Spherical Combustion Chamber: Experimental and Detailed Kinetic Modeling. Energy Fuels 2012, 26, 6669−6677. (15) Ren, W.; Davidson, D. F.; Hanson, R. K. IR Laser Absorption Diagnostic for C2H4 in Shock Tube Kinetics Studies. Int. J. Chem. Kinet. 2012, 44, 423−432. (16) Rothman, L. S.; Gordon, I. E.; Barbe, A.; Benner, D. C.; Bernath, P. F.; Birk, M.; Boudon, V.; Brown, L. R.; Campargue, A.; Champion, J.-P; et al. The HITRAN 2008 Molecular Spectroscopic Database. J. Quant. Spectrosc. Radiat. Transfer 2009, 110, 533−572. (17) Ren, W.; Farooq, A.; Davidson, D. F.; Hanson, R. K. CO Concentration and Temperature Sensor for Combustion Gases Using Quantum-Cascade Laser Absorption near 4.7 μm. Appl. Phys. B: Laser Opt. 2012, 107, 849−860. (18) Pyun, S. H.; Cho, J.; Davidson, D. F.; Hanson, R. K. Interference-Free Mid-IR Laser Absorption Detection of Methane. Meas. Sci. Technol. 2011, 22, 025303. (19) Hong, Z.; Farooq, A.; Barbour, E. A.; Davidson, D. F.; Hanson, R. K. Hydrogen Peroxide Decomposition Rate: A Shock Tube Study Using Tunable Laser Absorption of H2O near 2.5 μm. J. Phys. Chem. A 2009, 113, 12919−12925. (20) Farooq, A.; Jeffries, J. B.; Hanson, R. K. CO2 Concentration and Temperature Sensor for Combustion Gases Using Diode-Laser Absorption near 2.7 μm. Appl. Phys. B: Laser Opt. 2008, 90, 619−628. (21) Ren, W.; Jeffries, J. B.; Hanson, R. K. Temperature Sensing in Shock-Heated Evaporating Aerosol Using Wavelength-Modulation Absorption Spectroscopy of CO2 near 2.7 μm. Meas. Sci. Technol. 2010, 21, 105603. (22) Kee, R. J.; Ruply, F. M.; Miller, J. A. CHEMKIN-PRO | Reaction Design. http://www.reactiondesign.com/products/chemkin/chemkinpro/. (23) Ritter, E. R.; Bozzelli, J. W. THERM: Thermodynamic Property Estimation for Gas Phase Radicals and Molecules. Int. J. Chem. Kinet. 1991, 23, 767−778. (24) Muller, C.; Michel, V.; Scacchi, G.; Come, G. M. THERGAS: a Computer Program for the Evaluation of Thermochemical Data of Molecules and Free Radicals in the Gas Phase. J. Chim. Phys. 1995, 92, 1154−1178. (25) Blades, A. T. The Kinetics of the Pyrolysis of Ethyl and Isopropyl Formates and Acetates. Can. J. Chem. 1954, 32, 366−372. (26) Francisco, J. S. Mechanistic Study of the Gas-Phase Decomposition of Methyl Formate. J. Am. Chem. Soc. 2003, 125, 10475−10480. (27) Metcalfe, W. K.; Simmie, J. M.; Curran, H. J. Ab Initio Chemical Kinetics of Methyl Formate Decomposition: The Simplest Model Biodiesel. J. Phys. Chem. A 2010, 114, 5478−5484. (28) Ren, W.; Lam, K.-Y.; Pyun, S. H.; Farooq, A.; Davidson, D. F.; Hanson, R. K. Shock Tube/laser Absorption Studies of the Decomposition of Methyl Formate. Proc. Combust. Inst. 2013, 34, 453−461. (29) Chang, J.-G.; Chen, H.-T.; Xu, S.; Lin, M. C. Computational Study on the Kinetics and Mechanisms for the Unimolecular Decomposition of Formic and Oxalic Acids. J. Phys. Chem. A 2007, 111, 6789−6797.

ketene, and methyl ketene decomposition are required to improve performance of the EP kinetic mechanism.

■

ASSOCIATED CONTENT

S Supporting Information *

Reaction mechanism for the thermal decomposition of ethyl formate, ethyl acetate, and ethyl propanoate. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*(W.R.) E-mail: [email protected]. Phone: 650-7236850. Fax: 650-723-1748. Present Address

† (W.R.) Rice University, 6100 Main Street, Houston, Texas 77005, United States.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We would like to thank Dr. David Golden (Stanford University) and Dr. John Barker (University of Michigan) for useful discussion on the HOCO radical decomposition system. We would also like to thank Dr. Joseph Bozzelli (New Jersey Institute of Technology) for help in using THERM code. This work was supported by the Combustion Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001198.

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REFERENCES

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Experimental and Modeling Study of the Thermal Decomposition of C3

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