Thermal Decomposition of C3-C5 Ethyl Esters
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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 1 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 2 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.