Thermal Decomposition of C3-C5 Ethyl Esters

Home , Ethyl formate, Ethyl propionate

8th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013

Thermal decomposition of C3-C5 ethyl esters:

CO, CO2 and H2O time-history measurements behind reflected shock waves

Wei Ren, R. Mitchell Spearrin, David F. Davidson, Ronald K. Hanson High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA

Abstract: The thermal decomposition of three simple ethyl esters: ethyl formate (EF), ethyl acetate (EA), and ethyl propionate (EP), were studied behind reflected shock waves using laser absorption of CO, CO2 and H2O. Experimental conditions covered temperatures of 1248–1634 K, pressures near 1.5 atm, and reactant concentrations of 2000 ppm in argon. At long times after decomposition (~1.5 ms), the three product species provided oxygen balances of more than 90% of the oxygen atoms in the reacting system. During the pyrolysis of the ethyl esters, the initial process has been generally accepted to be a unimolecular six-centered decomposition producing equi-molar ethylene and a corresponding acid. Under the current shock tube conditions, the acid continues to decompose via a dehydration reaction to form

H2O, and a decarboxylation reaction to form CO2.

During the pyrolysis of EF, formic acid (HCOOH) was the expected major intermediate formed; a large amount of CO and H2O was also produced (at a ratio of 1:1) consistent with the dominant dehydration channel of formic acid HCOOH = H2O+CO. CO2 was formed as a minor product, with a CO2/H2O ratio of 0.05-0.07 at 1370-1636 K. In contrast, during the pyrolysis of EA where acetic acid (CH3COOH) was expected to be the major intermediate formed, the CO2/H2O ratio was measured to be 0.6 at 1430-1634 K. Much slower CO formation was also observed during EA decomposition, as the CO formation rate is controlled by ketene dissociation, produced from acetic acid dehydration reaction CH3COOH = H2O+CH2CO. Among these three ethyl esters, EP generated the largest CO2/H2O ratio of 0.9 at 1351- 1580 K. The dehydration reaction of propanoic acid produces an important intermediate methyl ketene

(CH3CHCO), which subsequently decomposes to CO2 and CO. The experiments provide the first laser- based time-history measurements of CO, CO2 and H2O during pyrolysis of these bio-diesel surrogate fuels in a shock tube.

1. Introduction

Alternative fuels are intensively studied nowadays as full replacements or supplements for fossil fuels. Biodiesel, an alternative diesel fuel derived from vegetable oils, animal fat, or waste cooking oil for example, can be used directly in diesel engines without major modifications to the engine system or the fuel distribution infrastructure. Typical biodiesel fuel is composed of saturated and unsaturated methyl or ethyl esters containing carbon chains of 12 or more atoms in length [1]. Due to the difficulty of studying these long-chain esters with complex chemical structure, the small esters have been widely used as biodiesel surrogates in the combustion community. Numerous studies of small methyl esters, especially methyl butanoate, have been performed experimentally and theoretically in the past few years [2-9]. However, there are still very few kinetic studies concerning ethyl esters. Blades [10] studied the pyrolysis of ethyl formate and ethyl acetate in a flow reactor and found the major decomposition products were the corresponding acid and alkene. Saito et al. [11] investigated the thermal

decomposition of ethyl acetate in a shock tube in the temperature range of 1300-1800 K. The initial process was identified as the unimolecular decomposition of ethyl acetate to produce equimolar ethylene and acetic acid. Blades and Gilderson [12] studied the kinetics of ethyl propionate pyrolysis in a toluene gas carrier system, measuring the rate constant of the six-centered elimination reaction to produce ethylene and propanoic acid. In addition, Barnard et al. [13] measured the thermal decomposition of ethyl propionate behind reflected shock waves, reproducing the results of Blades and Gilderson [12]. The only known detailed chemical kinetic model for the ethyl ester was developed by Metcalfe et al. [14] based on the jet-stirred reactor experiments of ethyl propionate. This model contains 139 species and 790 reactions, and has been validated against experimental data in the jet-stirred reactor at 10 atm pressure, using 0.1% fuel at equivalence ratios of 0.3, 0.6, 1.0 and 2.0 and at temperatures over 750-1100 K.

In the present shock tube study, we have measured CO, CO2 and H2O time-histories using laser- absorption techniques during the pyrolysis of three ethyl esters: ethyl formate (EF), HCOOC2H5; ethyl acetate (EA), CH3COOC2H5; and ethyl propionate (EP), C2H5COOC2H5. Figure 1 shows their corresponding molecular structures. These measurements are performed at relatively low concentrations (2000 ppm), so the influence on laser-absorption coefficient due to temperature change is negligible. It is the first shock tube study of these small ethyl esters using species time-history measurements.

(a) (b) (c) Fig. 1 The molecular structure of (a) ethyl formate (b) ethyl acetate (c) ethyl propionate.

2. Experimental setup

The high-temperature chemical kinetics studies were performed in a stainless-steel high-purity shock tube with a 15.24 cm inner diameter. The driven section has a length of 10 m, 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 high-quality test time of 1-2 ms. Between experiments, the shock tube driven section and mixing manifold were turbo-pumped for 20-30 minutes down to ~6 µtorr to remove residual impurities. Research grade high-purity argon (99.999% pure, Praxair Inc.) was used without further purification. All fuels (>99% pure, Sigma-Aldrich) were frozen and degassed three times to remove dissolved volatiles before making the mixtures. 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. In this study, three laser absorption diagnostics were utilized for the accurate and time-resolved measurements of CO2, CO and H2O concentration time-histories.

2.1 ECQCL laser absorption of CO2 near 4.2 µm

The recent commercial availability of external cavity quantum cascade lasers (ECQCLs) operating in continuous wave (cw) mode at room temperature has opened up a new diagnostic window to combustion products. In this study, we used a mid-IR laser absorption sensor recently developed in our laboratory, incorporating an ECQCL from Daylight Solutions Co., to provide sensitive and quantitative measurements of carbon monoxide. The CO2 sensor uses the R(76) transition line in the CO2 fundamental

band near 4.2 µm. Compared to previous CO2 sensors detecting the overtone and combinational bands near 2.7 µm [15, 16], the new diagnostic scheme provides orders-of-magnitude greater sensitivity. A fixed-wavelength direct-absorption strategy was used to detect the peak intensity of the R(76) -1 absorption line at 2390.52 cm . CO2 concentration was measured using the Beer-Lambert relation, It/I0 = exp(-kvL), where It and I0 are the transmitted and incident laser intensity, respectively; kv is the spectral absorption coefficient, L(cm) is the path length, and the product kvL is known the absorbance. The spectral absorption coefficient can be expressed as kv=S(T)ΦvxiP, where S(T) is the temperature-dependent line- -2 -1 strength (cm atm ) of the transition at T(K), Φ(cm) is the line-shape function, xi is the absorbing species mole fraction, and P(atm) is the total pressure. In the present study, the spectroscopic parameters for R(76) transition including line-strength and self- broadening coefficient were taken from HITRAN 2004 database. The collisional-broadening coefficient for argon (not available in HITRAN) was measured in the shock tube over a temperature range of 1000- 1800 K. A minimum CO2 detection sensitivity of 10 ppm can be achieved at 1400 K and 1.5 atm.

2.2 DFB-QCL laser absorption of CO near 4.6 µm

Absorption measurements of CO were made using a distributed feedback (DFB) quantum cascade laser (QCL) operating in continuous wave (cw) mode at room temperature. A fixed-wavelength direct- absorption strategy was employed in the present study to monitor the peak intensity of the R(13) absorption line at 2193.36 cm-1. The spectroscopic parameters for the R(13) transition have been measured in shock tube experiments over the temperature range of 1000–1800 K; results can be found in the literature [17]. Due to the strong absorption strength in the fundamental band, this new diagnostic scheme achieves ppm-level detectivity in shock tube experiments.

2.3 DFB diode laser absorption of H2O near 2.5 µm

−1 H2O concentration was measured using a DFB diode laser at 2550.96 nm (3920.09 cm ) within the ν3 fundamental vibrational band. This absorption feature has been well-characterized previously in our laboratory [18]. During experiments, the beam path (outside the shock tube) was continuously purged with pure N2 to minimize the laser attenuation due to ambient water. A minimum H2O detection sensitivity of 25 ppm can be achieved at 1400 K and 1.5 atm.

3. Experimental results

Pyrolysis experiments of ethyl esters in argon were conducted at temperatures from 1248 to 1634 K and at pressures from 1.4 to 1.7 atm. All the experiments were performed with 2000 ppm mixtures. The measured time-histories of CO, H2O and CO2 concentration are plotted in Figs. 2-4 for ethyl formate, ethyl acetate, and ethyl propionate, respectively. At long times after decomposition (~1.5 ms), the three product species provided oxygen balances of more than 90% of the oxygen atoms (95% for EF, 91% for EA and 93% for EP) in the reacting system.

The product fractional yield (xprod/xreact) for each ethyl ester at 1 ms is plotted as a function of temperature in Fig. 5. CO yields vary significantly for these three fuels. At temperatures higher than 1500 K, EF produces a CO yield of almost 100%, but is only 40% for EA. There does not appear to be a relationship between the CO yield and the length of the alkyl chain for these three ethyl esters, since the CO yield

(60%) for EP falls between those of EF and EA. Similarly, EF also produces the highest H2O yield over the temperature range between 1314 K and 1629 K, reaching 90% at temperatures higher than 1500 K.

The H2O yields for EA and EP overlap at the temperature of 1450 K, which is 45% lower than that observed for EF at the same temperature. However, CO2 seems to be the minor product during the pyrolysis of EF with a yield value of only 0.04-0.05 over the temperature range between 1370 K and 1636

K. In comparison, EA and EP produce the CO2 yield of 0.3 and 0.4 at temperatures higher than 1500 K, respectively.

To our knowledge, the Metcalfe et al. [14] mechanism is the only chemical kinetic model available for modeling EP combustion (no detailed kinetic models were found for EF and EA). The simulated CO, H2O and CO2 concentration time-histories are also plotted in Fig. 4 for comparison. None of these species concentration profiles are accurately captured by the Metcalfe et al. [14] mechanism; the CO2 yield is underpredicted with nearly an order of magnitude. Further discussion can be found in the next Section.

2500 200

2500 2000ppm E F/Ar 2000ppm E F/Ar 2000ppm E F/Ar 2000 1629K, 1.48atm 1467K, 1.68atm 2000 1629K, 1.48atm 150

1500 1636K, 1.49atm 1500 1467K, 1.68atm

100

1402K, 1.56atm 1000 1402K, 1.56atm 1000 1449K, 1.63atm Mole F raction [ppm] 2 O Mole Fraction [ppm]

2 50

C O Mole Fraction [ppm] 1314K, 1.69atm 1370K, 1.65atm 500 H 500 1314K, 1.69atm CO

0 0 0.0 0.3 0.6 0.9 1.2 1.5 0 0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.3 0.6 0.9 1.2 1.5 T ime [ms ] T ime [ms ] T ime [ms ] (a) (b) (c) Fig. 2 Measured (a) CO (b) H2O and (c) CO2 concentration time-histories behind reflected shock waves for 2000 ppm EF in Ar.

1800 1800 1000

2000ppm E A/Ar 2000ppm E A/Ar 2000ppm E A/Ar 1500 1500 800 1566K, 1.52atm 1200 1200 1634K, 1.49atm 600 1566K, 1.52atm 1578K, 1.54atm 1472K, 1.48atm 1492K, 1.61atm

900 900

400 1430K, 1.61atm 600 600 Mole F raction [ppm] 2

1472K, 1.48atm O Mole Fraction [ppm] 2 CO

C O Mole Fraction [ppm] 200 H 300 300 1393K, 1.54atm 1393K, 1.54atm 0 0 0 0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.3 0.6 0.9 1.2 1.5 T ime [ms ] T ime [ms ] T ime [ms ] (a) (b) (c) Fig. 3 Measured (a) CO (b) H2O and (c) CO2 concentration time-histories behind reflected shock waves for 2000 ppm EA in Ar.

2000 1200 2000 Measurement Measurement Measurement Metcalfe et al. 2008 Metcalfe et al. 2008 Metcalfe et al. 2008 1000 2000ppm E P/Ar 1600 2000ppm E P/Ar 2000ppm E P/Ar 1580K, 1.52atm 1500 1567K, 1.51atm 800 1200 1440K, 1.56atm 1454K, 1.57atm 1440K, 1.56atm

1567K, 1.51atm 600

1000 800 1351K, 1.56atm 1366K, 1.69atm 1351K, 1.56atm 400 Mole F raction [ppm]

500 2 1301K, 1.72atm C O Mole Fraction [ppm] O Mole Fraction [ppm] 2 400

H 1310K, 1.58atm CO 200 1248K, 1.73atm 1310K, 1.58atm 0 0 0 0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.3 0.6 0.9 1.2 1.5 T ime [ms ] T ime [ms ] T ime [ms ] (a) (b) (c) Fig. 4 Calculated (dashed line, Metcalfe et al. [14]) and measured (a) CO (b) H2O and (c) CO2 concentration time-histories behind reflected shock waves for 2000 ppm EP in Ar.

1.2 1.2 0.5

1.0 EF 1.0 0.4 EP EF 0.8 0.8 0.3 EA EP

0.6 0.6 EA

0.2 EP Fractional Y ield 0.4 0.4 2 O Fractional Yield 2 EA CO CO Fractional Yield 0.1 H EF 0.2 0.2

0.0 0.0 0.0 0.60 0.65 0.70 0.75 0.80 0.60 0.65 0.70 0.75 0.80 0.60 0.65 0.70 0.75 0.80 -1 1000/T [K ] -1 -1 1000/T [K ] 1000/T [K ] (a) (b) (c) Fig. 5 Measured (a) CO (b) H2O and (c) CO2 fractional yields at 1 ms. Pressure: 1.4-1.7 atm.

4. Discussion

In general, for those ethyl esters with a β-hydrogen in the ethyl group, the products are ethylene and the corresponding acid through a six-centered unimolecular elimination reaction [10, 11]. Under the current shock tube conditions, the produced acid continues to decompose via a dehydration reaction to form H2O, and a decarboxylation reaction to form CO2. Quantitative measurements of these important oxygen- carrying products enable the investigation of the decomposition pathways of EF, EA and EP.

4.1 Ethyl formate pyrolysis

Previous experimental study of EF indicates that the initial reaction of ethyl formate does not involve radical decomposition reactions, which rapidly produce hydrogen atom through a consecutive decomposition [10]. Rather, the initial step of EF decomposition is generally accepted to be the following molecular process:

HCOOC2H5 = C2H4 + HCOOH (EF1)

At high temperatures, the produced formic acid continues to decompose taking the following two pathways:

HCOOH = H2O + CO (EF2a)

= CO2 + H2 (EF2b)

The dehydration Rxn. EF2a implies equimolar H2O and CO formed during the formic acid decomposition. The H2O and CO concentration time-histories measured under the same test conditions are plotted together in Fig. 6 in the range of temperatures between 1314 and 1629 K, pressure between 1.48 and 1.69 atm. The ratio of H2O and CO mole fractions is measured to be 1:1 over most of the temperature range, except for the case at the highest temperature (1629 K). The measured 1:1 of H2O/CO ratio is good evidence for the dehydration reaction of formic acid. However, at much higher temperatures, the non-negligible free radicals enhance the hydrogen abstraction reactions of both ethyl formate and formic acid, resulting in more CO production.

The decarboxylation of formic acid (Rxn. EF2b) to produce CO2 and H2 occurs as a minor channel in the current shock tube experiments. As shown in Fig. 7 (1629 K and 1.5 atm), the measured CO2 concentration is more than an order of magnitude lower than H2O and CO. The current measurement is in good agreement with the previous shock tube study by Saito et al. [19], who reported the CO2 production to be less than 5% of the initial reactant during the pyrolysis of formic acid. In addition, the time-

dependent concentrations of H2O and CO2 can be directly expressed by kEF2a and kEF2b as xCO2/xH2O = kEF2b/kEF2a. In the current study, we obtained values for the kEF2b/kEF2a ratio of 0.05 to 0.07 at temperatures between 1314 K and 1629 K, showing good agreement with the reported value of 0.03-0.07 by Saito et al. [19].

Fig. 6 Measured H2O and CO concentration time-histories during the pyrolysis of 2000 ppm EF/Ar.

H O 1000 2

CO 2 100

Mole F raction [ppm] 2000ppm E F/Ar 1629 K, 1.5 atm 10 0.10 O 2

/H 0.05 2

CO 0.00 0 100 200 300 400 500

Time [µs]

Fig. 7 CO, H2O and CO concentration time-histories at 1629 K and 1.5 atm during the pyrolysis of EF; the ratio of CO2/H2O is plotted in the bottom panel.

4.2 Ethyl acetate pyrolysis

CO, H2O and CO2 are all major products during the pyrolysis of EA, as illustrated in Fig. 3. Previous experimental study of EA indicates that in the temperature region lower than 1400 K, the initial reaction of EA pyrolysis does not involve the radical decomposition [11]. Therefore, the initial step of EA decomposition is likely the unimolecular elimination:

CH3COOC2H5 = C2H4 + CH3COOH (EA1)

to produce ethylene and acetic acid. According to Mackie and Doolan [20], the thermal decomposition of acetic acid takes the following competing paths at almost equal reaction rate:

CH3COOH = H2O + CH2CO (EA2a)

= CO2 + CH4 (EA2b)

However, as shown in Fig. 3 (b) and (c), H2O is produced at a much faster formation rate than CO2 and the final H2O yield is almost twice that of CO2. The current experimental results reveal the fact that those decomposition channels producing H2O are favored during the pyrolysis of EA. Hence, both the branching ratio of acetic acid unimolecular decomposition and other secondary reactions involved in the EA pyrolysis system need to be revised in the Mackie and Doolan [20] mechanism.

The slower CO formation in EA, among these three ethyl esters, is another significant observation during these studies. As illustrated in Fig. 5 (a), at 1450 K, the CO production of EA at 1 ms is four times and three times lower compared to that of EF and EP, respectively. Such slow CO production rate is controlled by the thermal stability of ketene, as CO is mainly formed through the subsequent decomposition of ketene (CH2CO = CO + CH2).

4.3 Ethyl propionate pyrolysis

The detailed chemical kinetic mechanism by Metcalfe et al. [14] is used for the analysis for ethyl propionate pyrolysis. According to the authors, the reaction rates for ethyl propionate unimolecular fuel decomposition and H-abstraction are taken directly by analogy with methyl butanoate. The propanoic acid submechanism is based on n-heptane and iso-octane kinetic mechanisms.

1667 K 1538 1429 1333 1250 1800

2000 ppm E P/Ar 1500

CO 1200

H O 900 2 CO 2 600 Mole F raction [ppm] 300

0 0.60 0.65 0.70 0.75 0.80 1000/T [K ]

Fig. 8 Measured (symbol) and simulated (dashed line, Metcalfe et al. [14]) CO, H2O and CO2 yields for 2000 ppm EP/Ar mixture at 1 ms. Temperature: 1248-1580 K; pressure: 1.4-1.7 atm.

The measured CO, H2O and CO2 concentration time-histories and the model simulations for EP pyrolysis are plotted in Fig. 4 for comparison. None of these species time-histories are captured by the Metcalfe et al. [14] mechanism in terms of the early-time formation rate and the final product yield. The product yields at 1 ms are also plotted in Fig. 8, for both experiments (symbol) and simulations (dashed line). The measurements show that these three species start to reach plateau values at temperatures higher than 1500

K. However, the simulations predict that CO and H2O yields still grow with the increase of temperature over between 1248 K and 1580 K. A large amount of CO2 was observed during EP pyrolysis and reached the final plateau of about 800 ppm. The Metcalfe et al. [14] mechanism failed to capture such CO2 yields.

In addition, these three measured species time-histories enable investigation of the oxygen-atom balance in the pyrolysis system. The product yields summarized in Fig. 8 reveal that the measured CO, H2O and CO2 account for 92.7% of oxygen atoms in the system at 1440 K, compared to the value of 53.7% in the model prediction (dashed line in Fig. 8). Therefore, the decomposition rates of some intermediates must be underpredicted in the kinetic model. The major species time-histories during the pyrolysis of EP are simulated using the Metcalfe et al. [14] mechanism under a typical shock tube condition as shown in Fig. 9. The model shows that, at times after 0.5 ms, more than 30% of the oxygen atoms still exist in the decomposition intermediates methyl ketene (CH3CHCO) and C2H3COOH, which is suspicious as both are unstable species at such high temperature.

2000 C2H4

C2H5COOH 1500 H2O

CO 1000 C2H2

CH3CHCO

Mole F raction [ppm] 500 C2H3COOH CO2 0 0.0 0.5 1.0 1.5 2.0 T ime [ms ]

Fig. 9 Simulated species time-histories using the Metcalfe et al. [14] mechanism; 2000 ppm EP/Ar at 1440 K and 1.56 atm.

0.8 CH3CHCO+OH=C2H5+CO2 2000ppm E P/Ar C O+OH=C O2+H 1440K, 1.56atm 2 HOC O=C O2+H 0.4

2000ppm E P/Ar 0.0 mol/cc/s]

-6 1440K, 1.56atm

1 -0.4 S ensitivity E P =C 2H5C OOH+C 2H4 2 E P =C 2H5C O2+C 2H5 CO

R O P [10 C 2H5C OOH=C H3+C H2C OOH 2 -0.8 C2H5COOH+OH=CH2CH2COOH+H2O

CO C H3C HC O+OH=C 2H5+C O2 HOC O=C O+OH 0 -1.2 HOC O=C O2+H

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 T ime [ms ] T ime [ms ]

(a) ROP (b) Sensitivity

Fig. 10 (a) ROP and (b) sensitivity analysis (based on the Metcalfe et al. [14] mechanism) of EP pyrolysis: 1440 K, 1.56 atm, 2000 ppm EP/Ar. 8

The largest discrepancy between the EP experiments and model predictions is found to be in the CO2 concentrations, as the model underpredicted the CO2 yield by a factor of 8 over the entire temperature range. Based on the Metcalfe et al. [14] mechanism, rate-of-production (ROP) and sensitivity analyses for CO2 were performed. The results are presented in Fig. 10 (a) and (b), respectively, at 1440 K and 1.56 atm. The ROP analysis indicates that at 1440 K, there are two dominant pathways for the formation of CO2: (1) through one of the methyl ketene bimolecular decarboxylation pathways, Rxn. EP5; and (2) through HOCO unimolecular decomposition, Rxn. EP12 (competing with Rxn. EP11).

Sensitivity analysis supports the ROP interpretation. The early-time CO2 concentration is strongly sensitive (negative effect) to the initial unimolecular elimination reaction of EP through Rxn. EP1, which forms propionic acid and ethylene. The produced propionic acid can then decompose through Rxn. EP2 to methyl ketene and H2O, Rxn. EP3 to CH3CHCOOH radical (quickly decomposes to methyl ketene and hydroxyl radical), and Rxn. EP8 to CH2CH2COOH radical (quickly decomposes to HOCO and ethylene). As EP is consumed very rapidly (within 50 µs), at the later times, the CO2 concentration is predominantly sensitive to several secondary reactions of methyl ketene and HOCO radical: Rxn. EP5 to form CO2 and ethyl radical, and two other competing reactions Rxn. EP11 and EP12. Hence, those reactions favoring CO2 formation may need to be increased to achieve better agreement with the current measurement.

EP = C2H5COOH + C2H4 (EP1) C2H5COOH = CH3CHCO + H2O (EP2) C2H5COOH+OH = CH3CHCOOH+H2O (EP3) CH3CHCOOH = CH3CHCO+OH (EP4) CH3CHCO+OH = C2H5 + CO2 (EP5) CH3CHCO+OH = sC2H4OH + CO (EP6) CH3CHCO+H = C2H5 + CO (EP7) C2H5COOH + H = CH2CH2COOH + H2 (EP8) C2H5COOH + OH = CH2CH2COOH + H2O (EP9) CH2CH2COOH = C2H4 +HOCO (EP10) HOCO = CO + OH (EP11)

HOCO = CO2 + H (EP12)

Sensitivity analyses for CO and H2O were also conducted at 1440 K and 1.56 atm for 2000 ppm EP in Ar, as demonstrated in Fig. 11 and Fig. 12, respectively. Although both species concentrations are complicated by secondary reactions, the unimolecular and H-abstraction reactions of EP and propanoic acid play significant roles in determining the CO and H2O time-histories. Figure 11 shows that CO is strongly sensitive to Rxns. EP8 and EP9, whose products both decompose to HOCO radicals. Then the competition between pathways Rxns. EP11 and EP12 determines how much CO is formed. As propanoic acid is the major intermediate, the H-abstraction reactions of propanoic acid with hydroxyl radical (Rxns. EP3 and EP9) are the major sources of H2O formation.

As evident from the discrepancies between model and data discussed above, several modifications to the Metcalfe et al. [14] mechanism are needed to achieve a better fit to the experimental data. Because of the large number of reaction pathways, a final recommendation for all the key individual reaction rates in the EP decomposition mechanism cannot yet be made. The direct measurement of certain reaction rates, however, is feasible and may provide a worthwhile research path. High-level ab initio calculations are also recommended to reduce the uncertainties in the methyl ketene decomposition pathways.

0.8 0.6 E P =C 2H5OC O+C 2H5 2000ppm E P/Ar E P =C 2H5C O2+C 2H5 1440K, 1.56atm C2H5COOH=CH3+CH2COOH 0.4 0.4 C2H5COOH+OH=CH2CH2COOH+H2O C2H5COOH+OH=CH3CHCOOH+H2O C2H5COOH+H=CH3CHCOOH+H2 CH3CHCOOH=C2H3COOH+H 0.0 0.2

2000ppm E P/Ar E P =C 2H5C OOH+C 2H4 1440K, 1.56atm E P =C 2H5C O2+C 2H5 O Sensitivity CO Sensitivity C2H5COOH=CH3+CH2COOH 2 -0.4 H C2H5COOH+OH=CH2CH2COOH+H2O 0.0 C2H5COOH+H=CH2CH2COOH+H2 C2H5COOH+H=CH3CHCOOH+H2 CH3CHCO+OH=SC2H4OH+CO -0.8 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 T ime [ms ] T ime [ms ]

Fig. 11 CO sensitivity analysis based on the Fig. 12 H2O sensitivity analysis based on the Metcalfe et al. [14] mechanism. Metcalfe et al. [14] mechanism.

5. Conclusions

Quantitative measurements of CO, CO2 and H2O time-histories were carried out for ethyl formate, ethyl acetate, and ethyl propionate pyrolysis using laser-absorption techniques in a shock tube. More than 90% of oxygen balance was achieved by measuring these three species concentrations. The decomposition pathways were then analyzed for these three ethyl esters. Compared with EA and EP with relatively high CO2 yield, EF shows a very minor production of CO2 yield (less than 0.05) during the pyrolysis. The CO formation during EA pyrolysis behaves the slowest, as the CO formation rate is mainly constrained by the ketene decomposition. Comparison of measured species concentration profiles and fractional yields from the EP experiments with the detailed kinetic model by Metcalfe et al. [14] reveals the need for significant model improvement. Sensitivity and ROP analyses were performed to identify the essential reactions controlling these species concentrations. Further experimental and theoretical studies of certain reaction rates such as methyl ketene decomposition are recommended to reduce the uncertainties in the current kinetic models.

Acknowledgements

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.

References

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