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Ethyl Tertiary Butyl Ether Ignition and Combustion Using a Shock Tube and Spherical Bomb M. Yahyaoui, N. Djebaili-Chaumeix,* P. Dagaut, and C.-E. Paillard CNRS-ICARE, 1C, aVenue de la recherche scientifique 45071 Orle´ans Cedex 2, France ReceiVed May 13, 2008. ReVised Manuscript ReceiVed August 31, 2008
Ignition delay times and laminar flame speeds of Ethyl tertiary butyl ether (ETBE) were measured in both a shock tube and a spherical bomb, respectively. Ignition delay times of ETBE/O2/Ar mixtures were derived from OH radicals emission. Mixtures containing 0.1-0.4% of fuel were oxidized over the temperature range 1280-1750 K and for two different pressures, 0.2 and 1 MPa. The equivalence ratio was varied from 0.25 to 1.5. Furthermore, laminar flame velocities of ETBE/Air (79% N2 + 21% O2) mixtures were measured at room temperature and atmospheric pressure over an extensive range of equivalence ratios (0.5-1.5). The laminar flame speed measurements were performed in a spherical bomb using shadowgraph imaging system coupled to a high speed camera. Experimental results from both shock tube and spherical bomb were compared to those computed using a detailed chemical kinetic reaction mechanism. The mechanism used, containing 145 species and 998 reversible reactions, was validated earlier by simulating jet-stirred reactor (JSR) and on shock tube experiments on the oxidation of gasoline surrogate mixtures. The detailed chemical kinetic mechanism satisfactorily reproduces the experiments in shock tube at low pressure, but some improvements are required for relatively low temperature at high pressure. For the laminar flame velocities, the mechanism slightly overpredicts the experimental results around stoichiometric conditions. The main pathways and sensitivity analyses for ETBE oxidation under freely propagating flame and shock tube conditions were examined.
Introduction Alternative fuels use is expanding, due to the increase of oil prices driven by the oil resources limitation, also due to the climate change caused by the green house. Among the serious candidates of alternative fuels, one can find ETBE (ethyl tertiary butyl ether), considered as biofuel at 45% by mass, since it can be produced from 45% of bioethanol (biomass) and 55% of iso-butene (petrol). ETBE is also used as gasoline octane enhancer (RON ) 118 1). By improving the engine performances, it helps to significantly reduce monoxide carbon and unburned and soot emissions.2,3 The modeling of the combustion of ETBE is of great importance for a better understanding of autoignition in engines. Dunphy and Simmie4 measured ignition delay times of ETBE in a shock tube, at high temperature (1160-1830 K) and under 350 kPa. They compared ignition behaviours of ETBE and MTBE (methyl tertiary butyl ether) and observed that their rates of oxidation are very similar, and they concluded that iso-butene, produced in large amount during the oxidation of both hydrocarbons, controls their ignition. El Kadi and Baronnet5 studied the oxidation of ETBE in a static reactor, at low temperature (580 K), and explained its antiknock effect by the formation of * To whom correspondence should be sent. Phone: +(33) 2 38 25 54 23; fax: +(33) 2 38 69 60 04; e-mail:
[email protected]. (1) Guibet , J. C., Carburants et moteurs, e´ditions Technip, Paris, 1997, p. 259. (2) Kaiser, E. W.; Andino, J. M.; Siegel, W. O.; Hammerie, R. H.; Butler, J. W. J. Air and Waste Management Association 1991, 41 (2), 195–197. (3) Reuter, R. M.; Gorse, R. A.; Painter, L. J.; Benson, J. D.; Hocchauser, A. M.; Rippon, B.; Burns, V. R.; Koehl, W. J.; Rutherford, J. A. SAE Technical Paper Series 920326; 1992. (4) Dunphy, M. P.; Simmie, J. M. Int. J. Chem. Kinet 1991, 23, 553– 558. (5) El Kadi, B.; Baronnet, F. J Chim Phys 1995, 92, 706–725.
a large amount of iso-butene, since it leads, by hydrogen abstraction, to a resonantly stabilized radical. Bo¨hm et al.6 confirmed that the booster octane effect of ETBE as well as other ethers is due to the large production of some alkenes such as iso-butene, 2-methyl 1-butene, and 2-methyl 2-butene, which can form resonantly stabilized radicals. ETBE oxidation was studied in a jet-stirred reactor (JSR) between 800 and 1150 K, at 1 MPa, and for a large range of equivalence ratio (0.5-2) by Goldaniga et al.;7 the experimental results were used to propose a detailed chemical kinetics scheme. The inhibiting effect of ETBE on n-heptane oxidation was studied in a JSR by Dagaut et al. 8 at high pressure (1 MPa), and from low to high temperature (570-1500 K). According to Dagaut et al.,8 the presence of ETBE during n-heptane oxidation favors formaldehyde and acetaldehyde formation. Glaude et al.9 modeled the experimental results of Dagaut et al.,8 using a detailed chemical kinetics mechanism, generated automatically. Recently, ETBE was used up to 10% (molar percentage) to represent oxygenated molecules, in surrogate gasoline mixtures, both in JSR and in shock tube.10 More recently, Ogura et al.11 derived an ETBE submechanism from 2,2-dimethyl-pentane mechanism by substituting a CH2 group with an O atom. The (6) Bo¨hm, H.; Baronnet, F.; El Kadi, B. Phys. Chem. Chem. Phys. 1929, 2 (2000)), 1933. (7) Goldaniga, A.; Faravelli, T.; Ranzi, E.; Dagaut, P.; Cathonnet, M. Proc. Combust. Inst. 1998, 27, 353–360. (8) Dagaut, P.; Koch, R.; Cathonnet, M. Combust. Sci. Technol. 1997, 122, 345–361. (9) Glaude, P. A.; Warth, V.; Fournet, R.; Battin-Leclerc, F.; Coˆme, G. M.; Scacchi, G.; Dagaut, P.; Cathonnet, M. Combustion 2000, 1 (2), 123–139. (10) Yahyaoui, M.; Djebaili-Chaumeix, N.; Dagaut, P.; Paillard, C.-E.; Gail, S. Proc. Combust. Inst. 2007, 31, 385–391. (11) Ogura, T.; Sakai, Y.; Miyoshi, A.; Koshi, M.; Dagaut, P. Energy Fuels 2007, 21, 3233–3239.
10.1021/ef8003448 CCC: $40.75 2008 American Chemical Society Published on Web 10/21/2008
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Figure 2. Scheme of the spherical bomb used for ETBE/air laminar flame measurements. Figure 1. Example of the pressure and emission signals evolution recorded by the oscilloscope as a function of the time.
2,2-dimethyl-pentane mechanism was generated using KUCRS.12 They simulated experimental data of Dagaut et al.8 Several studies were available in the literature on ETBE oxidation using JSR and shock tubes, but no laminar flame speed measurements are available. Laminar flame velocity is an important property of hydrocarbons mixtures because it provides fundamental information on reactivity and heat release rate of hydrocarbons mixtures. Laminar flame speed information is very useful for engine design and turbulent combustion modeling. The present work presents experimental data on ETBE ignition delay times and ETBE/air laminar flame velocities. The experimental results are compared to computation using a detailed chemical kinetics elaborated in our laboratory and validated earlier for the oxidation of gasoline surrogate mixtures in JSR and shock tube.10
The visualization of the flame was obtained by shadowgraphy. The setup consists of two spherical mirrors (diameter 100 mm, focal length 0.5 m). The source light was an argon ion laser (488-514 nm), STABILITY 2017 (10 W max) supplied by Spectra Physics; it was made a point source via two lenses (diameter 75 and 20 mm with 150 and 22 mm focal lengths, respectively). A numerical high speed camera (Kodak EKTAPRO HS 4540) with a frequency ranging between 30 and 40 500 images per second was used to register the shadowgraph images of the growing flame. The images initially stored in the camera memory are transferred to a PC before being processed (Visilog 5.2 image processing). The image processor allows the measurements of the flame radius versus time. The bomb is evacuated with two primary pumps (250 L/ mn), and the mixture prepared in a stainless steel tank is introduced in the spherical bomb at a pressure up to 100 kPa. After 5 min, the camera and the scopes are readied, and a spark is produced at the electrodes to ignite the mixture and to trigger the scope and the camera twice. The flame speed for each condition was measured at least two times.
Experimental Setup
Results
The shock tubes used here for ignition delay times measurements were described in detail earlier.10,13 Therefore, only the spherical bomb used for the ETBE laminar flame speed measurements will be presented. The ignition delay time is defined as the time interval between the pressure risesindicated by the last pressure transducersdue to the passage of the reflected shock wave, and 50% of the maximum OH emission signal at 306 nm. In case of less diluted mixtures, the OH emission corresponds to the run-away of the explosive reaction detected by a small pressure signal increase. The error on the temperature is estimated to be less than 1%, whereas the one on the pressure is 1.3%. Concerning the error on the autoignition delay time, the estimated error depends on the temperature range and varies between 2 and 14%. Figure 1 shows an example of recorded signals during experiments, representing pressure evolution and OH emission at 306 nm. As one can see on this figure, ignition delay time was measured as the time between pressure jump and 50% of OH emission signal. The bomb is a stainless steel sphere (I.D. 250 mm) equipped with two opposite quartz windows (70 mm diameter, 40 mm thick) and has a black, polished surface. Two tungsten electrodes (2 mm diameter), located along a diameter of the sphere, are linked to a high voltage source (about 5 kV). The gap between the electrodes is adjustable and is usually fixed around 1 mm. Ignition was produced at the center of the sphere. A schematic of the experimental set up is shown in Figure 2. The reactive gas mixtures were prepared in a 10 L stainless steel tank using ETBE (Aldrich 99+ %) and a synthetic air (air liquid: air 1) containing 21% of oxygen and 79% of nitrogen. ETBE was degassed several times by pumping before the mixtures were prepared. The mixtures were allowed to mix for 1-2 h to ensure a homogeneous composition.
The experimental results are summarized in Table 1 for ignition delay times measurements and in the Table 2 for laminar flame speed measurements. Ignition Delay Times. The ignition of several ETBE/O2/Ar mixtures was studied in this work under different conditions to characterize the effects of each parameter on ignition delay times such as temperature, reactants composition, and pressure. For all the mixtures, the ignition delay time shows a strong dependence on the temperature. The effect of oxygen content at fixed fuel concentration is presented in Figure 3 at 0.2 MPa. The effect of ETBE concentration on ignition delay times, at fixed oxygen concentration, was examined at 0.2 MPa (Figure 4). At constant oxygen and diluent mole fraction, ETBE has an inhibiting effect on its own oxidation. This inhibiting effect is probably due to the reaction ETBE + H f R + H2 that consumes H-atoms, at the expense of the chain branching process H + O2 f OH + O. This H-atoms removal consequently increases ignition delay times. On the other hand, the promoting effect of pressure on ignition delay times was also demonstrated, as one can see in Figure 5. Ignition delay times decrease by increasing pressure at a fixed reactants mole percent; ETBE is more reactive at 1 MPa than at 0.2 MPa. The experiments performed by Dunphy and Simmie4 were compared to those carried out under the same conditions in this work, that is, at 350 kPa. An excellent agreement was observed between our experimental results and those of Dunphy and Simmie (Figure 6). Our experiments at higher pressure could not be compared to others because of the lack of data in the literature.
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Table 1. Experimental Data for Ignition Delay Times Measurements in the Shock Tubea P5/MPa ETBE O2 Ar T5/K τign/µs P/MPa ETBE O2 Ar T5/K τign/µs 0.2
0.2
0.2
0.2
0.2
a
0.1 0.9 99
1501 1498 1450 1492 1571 1525 1625 1673 1666 1717 0.1 1.8 98.1 1420 1570 1574 1504 1510 1464 1408 1395 1424 1552 1590 1619 1656 1677 0.1 3.6 96.3 1514 1440 1559 1438 1414 1393 1454 1607 1696 0.1 0.6 99.3 1544 1535 1496 1479 1437 1643 1667 1706 1731 1597 0.4 3.6 96 1470 1424 1401 1479 1477 1633 1678 1724 1479 1553
614 608 1061 650 209 414 128 68 81 49 614 608 1061 650 209 414 128 68 81 49 87 73 48 37 122 272 82 402 537 852 262 45 22 464 458 674 861 1334 136 112 73 43 226 501 737 1045 556 576 82 46 30 591 214
0.35
1
0.3 4.5 95.2 1463 1458 1337 1377 1493 1524 1525 1551 1564 1464 1507 1326 1403 1424 1326 1504 0.1 0.9 99 1449 1383 1455 1491 1436 1447 1690 1642 1509 1545 1398 1460 1481 1694 1476 1476 1500
187 226 609 363 123 77 83 74 56 188 145 656 383 277 657 101 319 683 333 226 331 332 32 53 229 174 464 288 237 27 283 226 205
Figure 4. Comparison between experimental (symbols) and computed (lines) ignition delay times of ETBE/O2/Ar mixtures for two different fuel mole fractions: 0.1 and 0.4%; P5 ) (0.2 ( 0.02) MPa. [ and solid lines: 0.4% ETBE + 3.6% O2 + 96% Ar; φ ) 1. g and dashed line: 0.1% ETBE + 3.6% O2 + 96.3% Ar; φ ) 0.25.
The composition is given in molar percent.
Table 2. Experimental Data Obtained for Laminar Flame Velocity Measurements in the Spherical Bomba ETBE
O2
N2
φ
T/°C
SL/(cm/s)
1.83 2.17 2.17 2.28 2.28 2.5 2.61 2.61 2.94 3.38 3.38
20.6 20.54 20.54 20.52 20.52 20.47 20.45 20.45 20.38 20.29 20.29
77.50 77.27 77.27 77.19 77.19 77.01 76.93 76.93 76.67 76.33 76.33
0.8 0.95 0.95 1 1 1.1 1.15 1.15 1.3 1.5 1.5
19 21 23 24 25 21 24 19 16 18 17
25.64 30.08 29.79 29.95 31.48 32.60 31.76 31.16 26.67 14.73 12.28
a
Figure 3. Comparison between experimental (symbols) and computed (lines) ignition delay times of ETBE/O2/Ar mixtures for two different equivalence ratio: 0.25 and 1.5; P5 ) (0.2 ( 0.02) MPa. 2 and solid lines: 0.1% ETBE + 0.6% O2 + 99.3% Ar; φ ) 1.5. O and dashed line: 0.1% ETBE + 3.6% O2 + 96.3% Ar; φ ) 0.25.
pressure, and compositions was derived. The different exponents over concentrations are determined using multiple regression analysis, whereas pre-exponential factor and activation energy were derived from Figure 7:
( 24 T204 )
τign ) 6.44 × 10-15[ETBE]0.41[O2]-0.94[Ar]-0.14 exp
(1) cm-3,
where [X] represents the concentration in mol T is the temperature in K, and τign is the autoignition delay time in s. This equation is validated for the present experimental conditions and gives the ignition delay times with a relative error of ( 20%. This expression reports the inhibiting effect of ETBE on its own oxidationsshown by the positive coefficient obtained for ETBE concentrationsas well as the promoting effect of oxygen
The composition is given is molar percent.
From all these experimental results, an expression of ignition delay times as a function of the initial conditions of temperature,
(12) Miyoshi A., Jpn. Soc. Autom. Eng. 2005, 36, 35-40 (in Japanese). (13) Yahyaoui, M.; Djebaili-Chaumeix, N.; Dagaut, P.; Paillard, C.-E.; Gail, S. Combust. Flame 2006, 146, 67–78.
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coefficients of the three elements, since: [Xi] ) RiP/RT (where P is the pressure in Pa, and Ri is the mole fraction of the reactants). Thus, the coefficient upon pressure is equal to -0.67. Laminar Flame Speed. The spark obtained by high tension discharge is produced between the two electrodes in the center of the spherical bomb, yielding ignition of the mixture and flame growth from the center to the bomb walls as a growing sphere. An example of the images obtained is shown on Figure 8. The flame radius varies quasi-linearly as a function of time, and the flame front propagates at a constant speed, as we can see in Figure 9. Therefore, a global spatial velocity can be derived from the slope of a linear fit. Vs)drf ⁄ dt
Figure 5. Comparison between experimental (symbols) and computed (lines) ignition delay times of ETBE/O2/Ar mixtures for two pressures; φ ) 1. b and solid lines: 0.1% ETBE + 0.9% O2 + 99% Ar; P5 ) (0.2 ( 0.02) MPa. ] and dashed line: 0.1% ETBE + 0.9% O2 + 99% Ar; P5 ) (1 ( 0.1) MPa.
(2)
where Vs is the apparent velocity, rf is the flame radius, and t is the time. In our case we must take into account the stretch due to the curvature and strain rate, because the flame is not planar but spherical. The velocity defined in eq 2 is stretched and differs from the unstretched one that corresponds to the zero-stretch extrapolation. The unstretched spatial velocity VsO differs from the spatial velocity by a coefficient known as Markstein length (L) as long as the stretch rate (k) is weak: V sO ) Vs + Lk
(3)
The Markstein length is on the order of a flame thickness and depends on the gas mixture composition; it can be positive or negative, leading, respectively, to an increase or decrease of the spatial flame velocity. The stretch rate k is well-defined in the case of a spherical expanding flame and is given by eq 4; k ) 2Vs ⁄ rf
(4)
The following expression was proposed by Eschenbach and Agnew14 to derive the laminar flame velocity from the spatial one;
(
SL ) Vs + Figure 6. Comparison between experimental (symbols) and computed (line) ignition delay times of ETBE/O2/Ar of this work and that of Dunphy and Simmie:4 0.1% ETBE + 0.9% O2 + 99% Ar; P5 ) (0.35 ( 0.03) MPa. ]: this work. b: Dunphy and Simmie.4
)( )( )( )
rb dPb Mb Tu Pb 3Pb dt Mu Tb Pu
(5)
With M, the molar mass; P and T, pressure and temperature; u, the relative to unburnt gas; b, the relative to the burnt gas; r, the flame radius; and t, the time. When the observation is limited to the first steps of the flame expansion, without pressure increase, a simple relationship links the unstretched special velocity to the fundamental (laminar) one, SOL ) V sO ⁄ σ
(6)
Figure 7. Normalized ignition delay times versus 104/T, according to eq 1.
where σ ) Fu/Fb is the expansion factor, and Fu and Fb are the unburned and burned density, respectively. The expansion factor is evaluated using the adiabatic flame calculation via the CHEMKIN II code package.15 It is well-known that under weak stretch the flame velocity presents a linear variation with stretch. In the present work, the stretch was below 2000 s-1. From eq 3, VsO ) Vs + Lk we deduce that VsOt ) r + 2L ln r + constant, so the unstretched spatial velocity can be obtained mathematically via multiple regression. Laminar flame speed of the ETBE/air mixtures versus equivalence ratio is reported on Figure 10, with uncertainty of
on ETBE oxidationsshowed by the negative coefficient obtained for oxygen concentration. This expression also shows the promoting effect of pressure on ETBE ignition (Figure 5). The coefficient of pressure can be calculated by adding the
(14) Eschenbach, R. C.; Agnew, J. T. Combust. Flame 1958, 2, 273– 285. (15) Kee, R. J., Rupley, F. M., Miller, J. A., CHEMKIN-II: a Fortran chemical kinetics package for the analysis of gas phase chemical kinetics, Report No. SAND89-8009B; Sandia International Laboratories: 1993.
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Figure 8. Shadowgraph images of ETBE flame at different times in the spherical bomb. The mixture contains 2.5% ETBE and 97.5% Air. Tini ) 292 K; Pini ) 100 kPa, framing rate is 4500 images/s, every 10th image is shown.
Figure 9. An example of the ETBE flame radius evolution versus time (φ ) 1.3).
about 5%.16 The main sources of inaccuracy in our experimental setup are nonspherical flame distortion produced by ignition, observed specially during the first steps of the flame propagation; heat loss to the electrodes used for ignition; and flame stretch effects. As for all hydrocarbons, laminar flame speeds of ETBE/ air mixtures presents a bell shape as a function of equivalence ratio. The maximum occurs for φ ) 1.1, with a laminar flame speed of about 33 cm/s. For both lean and rich mixtures, the laminar flame speed decreases drastically approaching the higher and lower flammability limits. Modeling Ignition Delay Times. The mechanism used here was designed to simulate high-temperature experiments. The C0-C4 subscheme was elaborated by Dagaut.17 To this base-set a submechanism of ETBE oxidation as described in a previous paper10 was added; only its main features are detailed here. The unimolecular reactions of ETBE are considered, including C-C, C-H, and C-O scissions and four-center unimolecular decomposition, which yields ethanol and iso-butene: TC4H9OC2H5 f i-C4H8 + C2H5OH. The i-C4H8 submechanism was available in the original mechanism;17 Marinov’s submechanism of ethanol18 was added. The H-abstractions from ETBE by small radicals were also added following the recommendations of Glaude et al.9 In the temperature range of this work, only the isomerisations and R-scissions reactions for radicals resulting from the ETBE oxidation are considered. We followed the (16) Daly, C. A.; Simmie, J. M.; Wu¨rmel, J.; Djebaı¨li, N.; Paillard, C. Combust. Flame 2001, 125, 1329–1340. (17) Dagaut, P. Phys. Chem. Chem. Phys. 1846, 4 (2002)), 1854. (18) Marinov, N. M. Int. J. Chem. Kinet 1999, 31, 183–220.
Figure 10. Comparison between experimental (symbols) and computed (lines) laminar flame speed of ETBE/Air mixtures versus equivalence ratio.
recommendation of Heyberger19 for the isomerization reactions rate, whereas for the decomposition reactions, the reactions rate proposed by Glaude et al.9 were adopted. Simulations were performed using the SENKIN20 code of the CHEMKIN II15 software library. The assumption of constant volume behind the reflected shock wave was made. The simulated ignition delay times were defined as the time at which 50% of the maximum concentration of OH occurs. As one can see on Figures 3-6, the agreement obtained between data and modeling is rather satisfactory. Nevertheless, some discrepancies exist at relatively low temperature, especially at high pressure (Figure 5) and for Dunphy and Simmie experiments (Figure 6). Figure 11 shows a scheme of the significant reactions at 1300 K, for stoichiometric composition under 2 atm and for 50% fuel conversion. Under these conditions, ETBE is mainly consumed by the four-center unimolecular decomposition producing i-C4H8 and C2H5OH. Ethanol is consumed by H-abstraction (mainly by OH radical) to give the ethoxy radical CH3CH2O, which easily decomposes to produce formaldehyde and acetaldehyde (CH2O and CH3HCO). At higher temperature (1700 K) the four-center unimolecular decomposition remains the main route of ETBE consumption and represents only 70% of ETBE consumption instead of 90% under the conditions shown in Figure 11. In this temperature range, the unimolecular reactions corresponding to C-C bond scission from both tertiobutyl and ethyl group become important. The reaction R875: TC4H9OC2H5 ) (CH3)2COC2H5 + CH3 represents 10.72% instead of 1.2% under the conditions of Figure 11. The reaction (19) Heyberger, B. Me´canismes de combustion d’alcanes, d’alce`nes et de cyclanes, Ph.D. Thesis; Institut National Polytechnique de Lorraine, Nancy, France, 2002. (20) Lutz, A. E.; Kee, R. J.; Miller, J. A. SENKIN: A Fortran Program For Predicting Homogeneous Gas Phase Chemical Kinetics with SensitiVity Analysis, Report No. SAND87-8248UC-401; SANDIA National Laboratories: 1992.
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Figure 11. Reaction paths for ETBE oxidation in a shock tube at 1300 K, 0.2 MPa, and 50% of fuel conversion (0.1% ETBE/0.9% O2/99% Ar).
R876: TC4H9OC2H5 ) TC4H9OCH2 + CH3 consumes the same amount of ETBE instead of 1.2% under the conditions of Figure 11. The formed radicals decompose immediately by β-scission to from acetone and the ethyl radical (from (CH3)2COC2H5), and formaldehyde and tertio-butyl radical (from TC4H9OCH2). A detailed sensitivity analysis was carried out for ETBE ignition with respect to the OH radical. A sensitivity analysis spectrum is represented in Figure 12. The branching reaction (R6) H + O2 ) OH + O is obviously the most important favoring ignition of ETBE. Among the reactions favoring ignition, one finds reactions producing H atoms, and so promoting the branching reaction (R6), which accelerates the ETBE ignition: R25: HCO + M ) H + CO + M R9: H2 + OH ) H2O + H
(7) (8)
Among the last reactions accelerating ETBE ignition, one finds reaction R875: TC4H9OC2H5 ) (CH3)2COC2H5 + CH3. This reaction competes with the four-center unimolecular decomposition and tends to reduce ignition delay times. The last one is the reaction R44: CH3 + HO2 ) CH3O + OH, which converts reactive radicals, CH3 and HO2, into more reactive radicals, OH and CH3O. On the other hand, reactions producing relatively stable species and less reactive radicals inhibit the rate of ignition. The most inhibiting reaction is that consuming ETBE in large amounts: (R877) TC4H9OC2H5 ) i-C4H8 + C2H5OH. Iso-butene is mainly consumed by H-abstractions (from allylic position), implying radical species: i-C4H8 + X ) i-C4H7 + HX. This reaction converts reactive radicals such as H, OH, and O into resonantly stabilized radical, with very low reactivity like i-C4H7. When this radical reacts, it decomposes via the reaction i-C4H7 f C3H4 + CH3, leading to even less reactive species (allene and CH3). In conclusion, iso-butene removes radical species and produces relatively unreactive species. These interpretations agree with earlier studies by Brezinsky and Dryer.21 Other reactions inhibit ETBE ignition. This is the case for R932, which produces water and ethylene, and reaction R966: CH3 + H(+M) f CH4(+M) acting as a chain termination, by removing H and methyl radicals and slowing down the ignition rate. (21) Brezinsky, K.; Dryer, F. L. Combust. Sci. Technol. 1986, 45, 225.
Figure 12. Sensitivity analysis for computed ignition delay times for the mixture 0.1% ETBE/0.9% O2/99% Ar with respect to OH radical in a shock tube; φ ) 1; P ) 0.2 MPa; T ) 1300 K.
Under shock tube conditions, the unimolecular initiation via C-C scission represent 21.4% to form the radicals (CH3)2COC2H5 and TC4H9OCH2, whereas those corresponding to the C-O scission are responsible for 6.5% of ETBE decomposition. Laminar Flame Speed. The laminar flame speed calculations were performed using the PREMIX22 code of the CHEMKIN II15 software library, using the thermo diffusion assumption. Computed laminar flames are compared to the experiments in (22) Kee, R. J.; Grcar, J. F.; Smooke, M. D., Miller, J. A., A Fortran Program For Modeling Steady Laminar One-Dimensional Premixed Flames, Report No. SAND85-8240; SANDIA National Laboratories: 1993.
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Figure 13. Reaction paths for ETBE oxidation in the freely propagating flame at 100 kPa, at an equivalence ratio of 1. Table 3. Comparison of the Main Consumption Pathways under Shock Tube and Spherical Bomb Conditions reactions R877: R887: R886: R888: R893: R890: R891: R889: R892: R894: R875: R876: R879: R878:
ETBE ETBE ETBE ETBE ETBE ETBE ETBE ETBE ETBE ETBE ETBE ETBE ETBE ETBE
f i-C4H8 + C2H5OH + H f TC4H8OCHCH3 + H2 + H f C4H8OC2H5 + H2 + H f TC4H8OCH2CH2 + H2 + OH f TC4H9OCHCH3 + H2O + O f TC4H9OCHCH3 + OH + O f TC4H9OCH2CH2 + OH + O f C4H8OC2H5 + OH + OH f C4H8OC2H5 + H2O + OH f TC4H9OCH2CH2 + H2O f (CH3)2COC2H5 + CH3 f TC4H9OCH2 + CH3 f TC4H9 + CH3CH2O f TC4H9O + C2H5
laminar flame
shock tube
19.64% 14.52% 11.57% 11.57% 9.36% 6.49% 6.49% 6.49% 6.19% 6.19% 0.37% 0.37% 0.16% 0.12%
70% 0.32 0.28 0.28 0.06% 0.03% 0.11% 0.03% 0.04% 0.04% 10.72 10.72 4% 2.5%
Figure 10. The agreement is satisfactory, although the model slightly overpredicts experimental results, especially around stoichiometric conditions. Reaction paths in laminar flame for stoichiometric conditions were analyzed using the program PATH.EXE elaborated in our laboratory. As shown in Figure 13, the ETBE is mainly consumed by H-atom abstraction (78.9%) involving small reactive radicals as H (26.09%), OH (21.74%), and O (19.47%). The formation of the radical TC4H9OCHCH3 by abstraction of a secondary hydrogen is favored (30.4%); the C-H bond in this position is also weaker due to the presence of an O atom. This, it is confirmed by bond strength calculation, showing that the secondary hydrogen has the weaker C-H bond strength: 87.82 kcal/mol instead 100.02 kcal/mol for the primary hydrogen of the tertio butyl group and 100.22 kcal/mol for the primary hydrogen of the ethoxy group. Thermodynamic calculations also show that the C-C bonds are 82.71 kcal/mol in the tertio butyl group and 82.95 kcal/mol in the ethoxy group and that they are weaker than the C-O bond (84.31 kcal/mol) between O and the tertio butyl group, and that (80.37 kcal/mol) between O and the ethyl group. All the ETBE consumption pathways (H-atom abstraction, unimolecular decomposition, etc.) lead to the formation of oxygenated intermediates: ethanol, formaldehyde, acetaldehyde, and acetone. This is the major disadvantage of using ETBE as a fuel, because formaldehyde and acetone, as well as acetaldehyde, present a direct hazard on human health and participate in the photochemical mechanisms producing ozone.23 (23) Folkins, I.; Chatfield, R. J. Geo. Resea 2000, 105, 585–599.
Figure 14. Sensitivity analysis of ETBE flame velocity, at an equivalence ratio of 1.
Figure 15. Comparison of experimental data of laminar flame speed of ETBE and DME at room temperature and 0.1 MPa. O: DME, Daly et al.16 b: ETBE, this work.
The four-center unimolecular decomposition reaction of ETBE, forming i-C4H8 and C2H5OH represents only 19% instead of 70-90% under the shock tube conditions. The ETBE consumption through monomolecular scission via C-C as well as C-O bond scission are of minor importance under laminar flame conditions and represents only, respectively, 0.74 and 0.28% of the total fuel consumption. The comparison between ETBE consumption pathways under shock tube and laminar flame conditions is summarized in Table 3. ETBE is mainly consumed by H-atom abstraction under laminar flame conditions, whereas the major ETBE consumption pathways under shock tube conditions are unimolecular initiations. Sensitivity analyses were carried out for laminar flame speed of ETBE and are shown in Figure 14. Under laminar flame conditions, it appears that the reactions involving small reactive radicals are very important, because these species diffuse easily and participate mainly in the oxidation of ETBE. As in the case of ETBE ignition, the most important reaction increasing laminar
3708 Energy & Fuels, Vol. 22, No. 6, 2008
Yahyaoui et al.
flame speed is the chain branching reaction: (R6) H + O2 f OH + O. Therefore, all the reactions that increase the flame speed produce H-atoms, as shown on sensitivity analysis diagram: R22: CO + OH ) CO2 + H
(9)
R25: HCO + M ) H + CO + M
(10)
R168: C2H2 + O ) HCCO + H
(11)
R9: H2 + OH ) H2O + H
(12)
Conversely, the reactions decreasing the laminar flame speed are those acting as a chain termination by removing H-atoms and produce stable species or other radicals less reactive in these conditions, such as HO2 and CH3. By comparing the two sensitivity analyses in Figures 12 and 14, there are some common reactions: R6, R25, and R9. Unlike ignition delay times sensitivity results, no reaction from the ETBE submechanism appear as sensitive for the computation of laminar flame speeds. The laminar flame velocity measurements obtained in this work could not be compared to others, because of the lack of data in the literature. However, we compared our experiments to those of DME (dimethyl ether) measured earlier by Daly et al.16 (Figure 15). As shown in Figure 15, the laminar flame speed of DME is higher than that of ETBE over all the equivalence ratio range. The maximum value of DME laminar flame speed is higher than that of ETBE by about 13 cm/s. The DME is, therefore, more reactive than ETBE. This difference is due to difference in their structures: aliphatic versus branched ethers. DME oxidation directly forms a significant concentration of methoxymethyl and methoxy radicals, which can easily increase the radical pool, thereby increasing the overall oxidation rate of DME and its laminar flame speed, whereas ETBE oxidation,
as presented above, forms less reactive species and more species with great scavenging effect, which limits its laminar flame speed. Conclusion In the present work, ETBE oxidation at high temperature was studied by measuring ignition delay times and laminar flame speeds. A new and unique laminar flame speed data set was obtained over a wide range of equivalence ratios. Our measurements in both shock tube and spherical bomb were compared to computations using a detailed chemical kinetic mechanism. The agreement between experiments and calculations is rather satisfactory. Oxidation pathway of ETBE oxidation under shock tube and flame conditions were performed and compared. Although ETBE is mainly consumed by unimolecular reactions under shock tube conditions, it is essentially consumed by H-atom abstraction involving small reactive radicals. The sensitivity analyses with respect to laminar flame velocity and ignition delay times were compared. For laminar flame speeds, the more-sensitive reactions are those implying small radicals and no reaction from ETBE submechanism strongly influence the computations. Laminar flame speeds of ETBE are compared with those of DME measured under similar conditions. The DME laminar flame speeds are higher than those of ETBE, due to the different oxidation mechanisms (and products) occurring (and formed) during aliphatic and branched ethers oxidation. Acknowledgment. The authors would like to thank Dr. Christian Vovelle for his valuable help and advise concerning the pathflow analysis. This work has been financially supported by CNRS, TOTAL, PSA, La Région Centre in the framework of a national program. EF8003448
