Oxygen Flame Studied with Tunable

Energy & Fuels 2006, 20, 1505-1513

1505

Lean Premixed Gasoline/Oxygen Flame Studied with Tunable Synchrotron Vacuum UV Photoionization Chaoqun Huang, Lixia Wei, Bin Yang, Jing Wang, Yuyang Li, Liusi Sheng, Yunwu Zhang, and Fei Qi* National Synchrotron Radiation Laboratory, UniVersity of Science and Technology of China, Hefei, Anhui 230029, P. R. China ReceiVed April 4, 2006. ReVised Manuscript ReceiVed May 18, 2006

A gasoline-fueled engine is a major source emitting toxic compounds. It is important and necessary to detect and identify these exhaust emissions from engines. The gasoline combustion process in engines approximates to a premixed flame. In this experiment, a lean premixed gasoline/oxygen/argon flame at 2.00 kPa with an approximate fuel equivalence ratio (φ) of 0.75 has been studied with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry. About 80 species produced in the flame have been unambiguously identified by measurements of the photoionization mass spectrum and photoionization efficiency (PIE) spectra. In addition, mole fraction profiles of these species are derived at the selected photon energies near ionization thresholds, and the temperature profile was measured with a Pt/Pt13%Rh thermocouple. Combined with the mole fraction profiles, the formation mechanism of some important radicals, oxygenated compounds, and stable intermediates are analyzed in detail.

1. Introduction Fossil fuels will remain the major energy source in the next several decades. Combustion of fossil fuels at present provides about 90% of our worldwide energy support, and gasoline plays a key role among fossil fuels. Gasoline-fueled engines are very important in vehicles and machines and prove to be powerful, versatile, and ubiquitous in our lives. However, gasoline-exhaust emissions such as saturated and unsaturated hydrocarbons and oxygenated organic compounds are known to be hazardous to humans. Numerous hydrocarbons, including 1,3-butadiene, benzene, toluene, ethylbenzene, xylene, and polycyclic aromatic hydrocarbons (PAHs) are mutagenic or carcinogenic.1,2 And oxygenated compounds such as carbonyl compounds, enols, alcohols, and organic acids are important intermediate products in hydrocarbon oxidation.3,4 Formaldehyde and many of other aldehydes are associated with the formation of photochemical smog, and they are also known to cause irritation of the skin, eyes, and nasopharyngeal membranes.5 Therefore, it is very important and necessary to acquire knowledge about gasolineexhaust emissions and the gasoline combustion mechanism.6-8 * Corresponding author. E-mail: [email protected]. Fax: +86-5515141078. Tel.: +86-551-3602125. (1) Marr, L. C.; Kirchstetter, T. W.; Harley, R. A. EnViron. Sci. Technol. 1999, 33, 3091-3099. (2) Al-Farayedhi, A. A. Int. J. Energy Res. 2002, 26, 279-289. (3) Kean, A. J.; Grosjean, E.; Grosjean, D.; Herley, R. A. EnViron. Sci. Technol. 2001, 35, 4198-4204. (4) Zervas, E.; Poulopoulos, S.; Philippopoulos, C. Fuel 2006, 85, 333339. (5) Mitchell, C. E.; Olsen, D. B. J. Eng. Gas Turbines Power 2000, 122, 603-610. (6) Cathonnet, M. Proceedings of the European Combustion Meeting, October, 2003, Orle´ans, France. (7) Schuetzle, D.; Siegl, W. O.; Jensen, T. E.; Dearth, M. A.; Kaiser, E. W.; GORSE, R.; Kreucher, W.; Kulik, E. EnViron. Health Perspect. 1994, Suppl. 102, 3-12. (8) Hakansson, A.; Stromberg, K.; Pedersen, J.; Olsson, J. O. Chemosphere 2001, 44, 1243-1252.

The emissions of organic compounds formed in gasoline flame are very complicated because of gasoline containing hundreds of hydrocarbons that have 5-12 carbons.9 Hence, the variety and complication of the exhaust species make the study of exhaust emissions very difficult. A partial reason for this is the lack of established on-line sensitive and selective gas monitoring techniques for these volatile and semivolatile organic compounds.10 Mass spectrometry serves as an important tool for a wide range of applications in chemistry, physics, and biology, and it has played an important role in aiding our understanding of environmental pollution and processes.10,11 It was used to analyze complicated systems (including the combustion process, exhaust emissions, and so on) combining gas chromatography (GC)1,8,12 with chemical ionization (CI),13,14 selected ion flow tube (SIFT) technology,15 atmosphere pressure ionization (API),16 and photoionization (PI) technology.17-22 The GC extractive method always eludes the detection of transient (9) Caprino, L.; Togna, I. G. EnViron. Health Perspect. 1998, 1998, 115125. (10) Butcher, D. J. Mirochem. J. 2000, 66, 55-72. (11) Richardson, S. D. Chem. ReV. 2001, 101, 211-254. (12) Riservato, M.; Rolla, A.; Davoli, E. Rapid Commun. Mass Spectrom. 2004, 18, 399-404. (13) Heeb, N. V.; Forss, A. M.; Bach, C.; Reimann, S.; Herzog, A. H.; Jackle, H. W. Atomos. EnViron. 2000, 34, 3103-3116. (14) Heed, N. V.; Forss, A. M.; Saxer, C. J.; Wilhelm, P. Atomos. EnViron. 2003, 37, 5185-5195. (15) Simth, D.; Cheng, P.; Spanel, P. Rapid Commun. Mass Spectrom. 2002, 16, 1124-1134. (16) Dearth, M. A.; Glerczak, C. A.; Siegl, W. O. EnViron. Sci. Technol. 1992, 26, 1573-1580. (17) Welckhardt, C.; Bosel, U.; Schlag, E. W. Anal. Chem. 1994, 66, 1062-1069. (18) Butcher, D. J. Mirochem. J. 1999, 62, 354-362. (19) Butcher, D. J.; Goeringer, D. E.; Hurst, G. B. Anal. Chem. 1999, 71, 489-496. (20) Muhlberger, F.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2002, 74, 3790-3801. (21) Oudejans, L.; Touati, A.; Gullett, B. K. Anal. Chem. 2004, 76, 2517-2524. (22) Zimmermann, R. Anal. Bioanal. Chem. 2005, 381, 57-60.

10.1021/ef0601465 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/21/2006

1506 Energy & Fuels, Vol. 20, No. 4, 2006

emission species and cannot readily be used to analyze exhaust gases in real time. Moreover, GC/MS is time-consuming in the chromatographic separation step and lacks isomer selectivity, and it also prevents full speciation of some isomers. Recently, laser ionization, i.e., resonance-enhanced multiphoton ionization (REMPI) and single photon ionization (SPI), has been developed in the analysis of complex vehicle exhaust emissions incorporating time-of-flight mass spectrometry (TOFMS).17,19-21 The gasoline combustion process in engines can approximate to a premixed flame started by spark ignition. Hakansson et al.8 studied a premixed laminar gasoline flame with GC/MS, and about 40 compounds were detected in their study. However, few combustion intermediates were detected owing to the disadvantages of GC/MS as mentioned above. In this paper, we studied the lean premixed laminar gasoline/oxygen flame by using the synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS) techniques. The high resolution of photoionization mass spectrometry (PIMS) and the tunable photon energy from synchrotron allow isomer-specific detections of the intermediates. About 80 compounds formed in the gasoline flame were detected by the photoionization mass spectrum and photoionization efficiency (PIE) spectra measurements. Also, the mole fractions for these species were deduced in this work. 2. Experimental Section The experiment was performed at the combustion endstation of the National Synchrotron Radiation Laboratory (NSRL), University of Science and Technology of China. Detailed description of the instrument will be published elsewhere,23 which is identical to the previous report by Cool et al.24 Briefly, the apparatus consists of a flat burner situated in the flame chamber, a differentially pumped flame sampling system, and a photoionization chamber with a reflectron TOF mass spectrometer (RTOF-MS). A low-pressure laminar premixed flame on a 6.0 cm diameter flat burner (McKenna, USA) is sampled through a quartz nozzle with an orifice diameter of ∼500 µm. Here, the sampling flow is same as the forming flow of the combustion products; therefore, the sampling condition is isokinetic. The sampled gas forms a molecular beam, which is skimmed and then passes into a differentially pumped ionization region where it is crossed by the tunable synchrotron light. Synchrotron radiation from a bend magnet of the 800 MeV electron storage ring is monochromized by using a 1 m Seya-Namioka monochromator equipped with two gratings (2400 and 1200 lines/ mm) covering the wavelength range from 40 to 200 nm. The wavelength is calibrated with the known ionization energies (IEs) of the inert gases. The energy resolution (E/∆E) is about 5001000 depending on the width of the slits. A 150 µm width is normally used for this study with an energy resolution of ∼500. The photon flux is monitored by a silicon photodiode (SXUV-100, International Radiation Detectors, Inc., USA). The average photon flux was measured to be 5 × 1010 photons/s in the photon energy range of 7-11 eV. A LiF window (1.0 mm thickness) is used to eliminate higher-order radiation of the monochromatic light in the wavelength region longer than 105 nm. The photoions are collected and analyzed by a RTOFMS with the mass resolving power (m/ ∆m) of ∼1400. The mass spectrometer was described in detail previously.25 Movement of the burner toward or away from the quartz nozzle allows the mass spectrum to be taken at different positions in the flame. The ion signal is recorded using a multiscaler (FAST Comtec P7888, Germany) with a bin width of 2 ns. A DG535 is used to trigger a pulsed power supply and the multiscaler as well. (23) Qi, F.; Yang, R.; Yang, B.; Huang, C. Q.; Wei, L. X.; Wang, J.; Sheng, L. S.; Zhang, Y. W. ReV. Sci. Instrum. 2006, in press. (24) Cool, T. A.; McIlroy, A.; Qi, F.; Westmoreland, P. R.; Poisson, L.; Peterka, D. S.; Ahmed, A. ReV. Sci. Instrum. 2005, 76, 94102. (25) Huang, C. Q.; Yang, B.; Yang, R.; Wang, J.; Wei, L. X.; Shan, X. B.; Sheng, L. S.; Zhang, Y. W.; Qi, F. ReV. Sci. Instrum. 2005, 76, 126108.

Huang et al. Table 1. Gasoline Specificationsa density (kg‚L-1) 10 vol % evaporated boiling point (°C) 50 vol % evaporated boiling point (°C) 90 vol % evaporated boiling point (°C) final boiling point (°C)

0.737 59 105 159 184

Compositions (vol %) alkanes alkenes aromatics

47 29 24

a The 90# standard unblended gasoline for this experiment was provided by Fangyuan Inc, Liaoning, China. Data of the specifications is provided by the vendor.

The gasoline flame in the present experiment was fueled by 90# standard unblended gasoline, provided by Fangyuan Inc, Liaoning, China, and specifications for the gasoline are given in Table 1. The major components of the gasoline were analyzed to be C5H10 (4.96%), C5H12 (4.80%), C6H12 (4.81%), C6H14 (11.10%), C7H8 (3.91%), C7H14 (3.79%), C7H16 (10.40%), C8H10 (6.97%), C8H18 (8.70%), C9H12 (5.57%), C10H8 (6.87%), C10H16 (4.92%), C10H22 (3.95%), etc. with tunable vacuum ultraviolet single-photon ionization technology. The flow rates of oxygen and diluent argon are 1.67 and 0.90 standard L‚min-1 (SLM), respectively. The flow rate of gasoline was controlled to be 0.699 mL‚min-1 by a syringe pump (ISCO 1000D, USA) and fed to a vaporizer with the temperature kept at 200 °C. Therefore, the mass flow rate is calculated to be 2.7 × 10-3 g‚cm-2‚s-1. The gasoline is a complex mixture of many hydrocarbons; therefore, an accurate equivalence ratio cannot be calculated by the flows of the gasoline and oxygen. In this work, the gasoline approximates to isooctane; thus, the equivalence ratio (φ) is derived to be 0.75 for this study. The pressure in the flame chamber was 2.00 kPa (15 Torr). The temperature profile was measured by using the Pt/Pt-13%Rh thermocouple with a diameter of 0.076 mm, coated with Y2O3BeO anticatalytic ceramic. The flux-normalized ion signals, measured as a function of the photon energy, yield the PIE spectra. To avoid fragmentation and keep near-threshold photoionization, we scanned the burner at some selected photon energies: 16.50, 11.80, 10.87, 10.00, 9.54, 9.00, and 8.49 eV. The mole fractions of these species were derived according to the method described by Cool et al.26 However, the mole fractions are difficult to calculate by the method described in the reference because the gasoline is a mixture of hydrocarbons. To calculate the mole fractions of species formed in the gasoline flame, we added a small quantity of acetylene to the flame for calibration. The calculation method will be discussed in detail below. The calculation of mole fraction needs photoionization cross section data. However, the photoionization cross sections of some intermediates are not available. Thus, we derive these data empirically based on the method described by Koizumi.27 The mole fractions have an uncertainty of (25% for the stable intermediates and a factor of 2 for radicals.

3. Results and Discussion 3.1. Isomeric Identification of Flame Intermediates and Products. A lot of stable intermediates and unstable radicals can be formed in the premixed gasoline/oxygen flame. Figure 1 shows a photoionization mass spectrum of the gasoline flame at the photon energy of 11.87 eV, where the quartz nozzle was located at a distance of 5.0 mm from the burner surface. A series of peaks are detected and shown in the photoionization mass spectrum. These peaks correspond to hydrocarbons and oxygenated compounds with carbon numbers from C1 to C12. The relative intensity of the peaks changes with the different (26) Cool, T. A.; Nakajima, K.; Taatjes, C. A.; McIlroy, A.; Westmoreland, P. R.; Law, M. E.; Morel, A. Proc. Combust. Inst. 2004, 30, 16811688. (27) Koizumi, H. J. Chem. Phys. 1991, 95 (8), 5846-5892.

Lean Premixed Gasoline Flame Study

Figure 1. Photoionization mass spectrum of the lean premixed gasoline/oxygen/argon flame at the pressure of 2.0 kPa (15.0 Torr) and the photon energy of 11.87 eV, taken at the nozzle sampling position of 5.0 mm from the burner surface.

sampling positions and photon energies. The intensity of ion signals of m/e larger than 140 is amplified with a factor of 10, as indicated in Figure 1. There are many possible chemical compounds corresponding to each mass, and the total number of possible isomers increases rapidly with the increase of molecular weight. Species identification by ionization threshold becomes important in the combustion process. Therefore, identifying isomers responsible for an observed mass peak is a critical step to the prediction of the mechanism of gasoline combustion. The tunability and resolution of the present VUV light source allow measurement of PIE spectra for each observed mass; identification of the species is realized by mass and ionization threshold. Moreover, selective ionization of products can minimize fragmentation. To obtain PIE spectra, we recorded a series of mass spectrum at a fixed burner position by scanning photon energy. Each mass peak was integrated with the background subtraction and plotted versus photon energy to obtain the PIE spectra. The ionization energy (IE) of species can be directly obtained from the PIE spectra. In this paper, we do not display all measured PIE spectra and choose some of these just for illustration as follows. All detected species are listed in Table 2 with the measured IE and the literature values. a. Free Radicals. A lot of radicals were detected and identified in gasoline flame for the first time. The following figures are chosen to show PIE spectra for some radicals. Figure 2a shows the PIE spectra of m/e ) 15. A sharp onset is observed at 9.83 ( 0.05 eV, and this value is in good agreement with the IE of the methyl radical measured by photoelectron spectroscopy.28 Thus, m/e ) 15 can be attributed to the methyl radical according to the PIE measurement. A PIE curve of m/e ) 39 is presented in Figure 2b, and a clear onset is observed at 8.67 ( 0.05 eV. This value is also in excellent agreement with the IE of the propargyl radical, which was determined by photoelectron spectroscopy and photoionization mass spectrometry.28 The C3H3 radical is believed to play an important role in hydrocarbon flame, chemical-vapor deposition, and interstellar media, and it is also considered as a precursor in the formation of benzene, PAH, and soot in flame.29 As shown in Figure 2b, a series of autoionization Rydberg states can be resolved at (28) Linstrom, P. J.; Mallard, W. G. NIST Chemistry Webbook, NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg, MD, 2003 (http://webbook.nist.gov/ chemistry). (29) Miller, J. A.; Melius, C. F. Combust. Flame 1992, 91, 21-39.

Energy & Fuels, Vol. 20, No. 4, 2006 1507

higher photon energies above the ionization threshold. These autoionization peaks were assigned to two vibrational progressions with the excitation of the C-C-C stretching and the CH2 scissors vibrational modes, previously.30 The allyl radical (C3H5) is also important in the formation of PAH and soot in combustion processes. It exhibits several characteristic features that make it an ideal model system in understanding chemical dynamics in general.31 In the gasoline flame, C3H5 was detected and identified according to the measurement of the PIE spectra, as shown in Figure 2c. Two onsets are observed from the PIE spectra of m/e ) 65 shown in Figure 2d, which correspond to the IEs of the cyclopentadienyl radical and the pent-1-en-4-yn3-yl radical. Besides these radicals discussed above, C2H3, C2H5, C3H7, C4H5, C4H7, C5H7, C7H7, and C8H9 were also detected in the study, as listed in Table 2. Here, we should note that the C4H5 radical, contribution from the CH2CHCCH2 (i-C4H5), and some combination of the CH3CCCH2 and CH3CHCCH isomers are evident according to the measurement of the PIE spectra. But the role in soot formation of the CH3CCCH2 and CH3CHCCH isomers is currently absent from flame models and requires further investigation. In the high-temperature reaction zone of flame, n-C4H5 can be easily converted to the more stable i-isomer by H-atom-assisted isomerization and no clear evidence for n-C4H5 was found in the flame. The C4H5 radical was also identified in allene/oxygene flame, propyne/oxygene flame, cyclopentene/oxygene flame, and benzene/oxygene flame.32 Schuetzle et al. suggested that partial combustion of the isoparaffins and alkyl aromatics in the gasoline would result in an increase of methyl and ethyl radicals. Moreover, the alkyl radical can react with excess oxygen, resulting in an increased production of oxygenated intermediates, such as formaldehyde, acetaldehyde, acetone, and so on, under the fuel-lean condition.7 Therefore, the amount of radicals produced in the lean flame is less than those in the rich flame owing to the existence of excessive oxygen. b. Oxygenated Compounds. Oxygenated compounds are important intermediates in the oxidation of hydrocarbons and are formed easily in the lean flame. The most common oxygenated compounds are carbonyl (formaldehyde, acetaldehyde, etc.), alcohols (methanol, ethanol, etc.), and organic acids. These compounds were found in the exhaust gas of internal combustion engines.4 In this experiment, a lot of oxygenated compounds, including HCHO, CH3OH, C2H2O, C2H4O, C3H4O, C3H6O, C4H8O, C7H14O, C8H8O, C10H8O, and C12H10O, were identified. Some of them were observed for the first time in gasoline flame. Figure 3 shows some PIE spectra of the oxygenated compounds. Two onsets are observed at 9.34 and 10.23 ( 0.05 eV for the PIE spectra of m/e ) 44, as shown in Figure 3a. They correspond to the IEs of ethenol (IE ) 9.33 eV) and acetaldehyde (IE ) 10.229 eV), respectively.28 Similarly, m/e ) 58 contains the contributions of 2-propenol and acetone according to its PIE spectra, as presented in Figure 3b. Three onsets are observed for m/e ) 72 in Figure 3c. This implies that the mass 72 contains the contributions of butenol, isopentane, and 2-butanone. Enols are common intermediates in hydrocarbon oxidation and have a structure that includes (30) Zhang, T.; Tang, X. N.; Lau, K.-C.; Ng, C. Y.; Nicolas, C.; Peterka, D. S.; Ahmed, M.; Morton, M. L.; Ruscic, B.; Yang, R.; Wei, L. X.; Huang, C. Q.; Yang, B.; Wang, J.; Sheng, L. S.; Zhang, Y. W.; Qi, F. J. Chem. Phys. 2006, 124, 074302. (31) Leung, K. M.; Lindstedt, R. P. Combust. Flame 1995, 102, 129160. (32) Hansen, N.; Klippenstein, S. J.; Taatjes, C. A.; Miller, J. A.; Wang, J.; Cool, T. A.; Yang, B.; Yang, R.; Wei, L. X.; Huang, C. Q.; Wang, J.; Qi, F.; Law, M. E.; Westmoreland, P. R. J. Phys. Chem. A 2006, 110, 36703678.

methyl radical acetylene vinyl radical ethylene ethyl radical formaldehyde methyl alcohol propargyl radical allen propyne allyl radical ketene propylene isopropyl radical ethenol acetaldehyde 1,3-butadiyne 1,2,3-butatriene vinylacetylene but-2-yn-1-yl radical or 1-butyn-3-yl radical i-C4H5 1,3-butadiene 1-butyne 1-buten-3-yl radical methylketene 2-butene propenol acetone cyclopentadienyl radical pent-1-en-4-yn-3-yl radical 1,3-cyclopentadiene cyclopropylacetylene 3-cyclopentenyl radical 1,3-pentadiene 2-methyl-1,3-butadiene 2-methyl-2-butene 2-pentene butenol 2-butanone isopentane benzene fulvene 1,3-cyclohexadiene

species

7.60 9.05 10.16 7.50 8.95 9.15 8.73 9.72 8.43 7.80 8.57 8.70 7.56 8.61 8.87 8.67 9.03 8.39 9.46 10.33 9.25 8.43 8.27

9.83 11.38 8.57 10.51 8.28 10.87 10.82 8.67 9.70 10.35 8.12 9.63 9.76 7.51 9.34 10.23 10.16 9.25 9.57 7.99

9.84 11.4 8.59 10.5138 8.26 10.88 10.84 8.67 9.692 10.36 8.138 9.617 9.73 7.55 9.33 10.229 10.17 9.25 9.58 7.95 d 7.97d 7.55 9.07 10.18 7.49 8.95 9.11 8.70 9.703 8.41 7.88 8.57 8.70 7.54 8.62 8.86 8.69 9.01 8.42 9.52 10.32 9.24 8.36 8.25

literatb

IE (eV) this worka

5.0 4.5 4.5 4.0 5.5 5.5 5.0

3.7× 10-4 7.5× 10-4 6.5× 10-5 5.7× 10-4 7.5× 10-5 2.9× 10-4 10-5

4.0

5.0 4.5

5.7× 10-5 7.4× 10-4

6.0× 10-4

5.0

2.0× 10-3

5.5

5.5

6.4× 10-5

1.7× 10-3

3.0

2.1× 10-4

3.0

4.5 4.0

7.8× 10-5 2.5× 10-3

1.0× 10-5

5.0

1.2× 10-3

3.6×

1.1× 3.1× 10-3 1.2× 10-4 4.5× 10-3 3.2× 10-4 2.9× 10-3 5.0× 10-4 2.7× 10-4 4.4× 10-4

5.0 5.5 5.5 5.0 4.5 4.5 4.5 5.5 5.0

position (mm)

10-3

Xmaxc

134 142 144 154 156 170

130 132

122 126 128

120

118

116

110 112 114

105 106

100 102 104

94 96 98

86 90 91 92

82 84

mass

C10H12 C10H14 C11H10 C10H8O C11H22 C12H12 C12H10O

C6H10 C6H12 C6H12 C6H14 C7H6 C7H7 C7H8 C7H8 C7H10 C7H12 C7H14 C7H14 C7H14 C7H16 C8H6 C8H8 C8H8 C8H9 C8H10 C8H10 C8H14 C8H16 C7H14O C8H18 C9H8 C9H8 C9H10 C9H10 C8H8O C9H12 C9H14 C10H6 C10H8 C9H20 C10H10 C10H12

formula 4-methyl-1,3-pentadiene 2-methyl-2-pentene 2-hexene hexane 5-ethenylidene-1,3-cyclopentadiene benzyl radical 5-methylene-1,3-cyclohexadiene toluene 3-methyl-3-isopropenyl-cyclopropene 2,4-heptadiene 2-methyl-2-hexene 3-heptene cycloheptane heptane phenylacetylene 3,6-bis(methylene)-1,4-cyclohexadiene styrene methylbenzyl radical 1,3,5,7-octatetraene xylene 2,4-octadiene 3-ethyl-3-hexene isopropyl ketone octane indene phenylallene allylbenzene R-methylstyrene dihydrobenzofuran 1,2,4-trimethylbenzene 1,2,3,4-tetramethylcyclopentadiene 1,4-diethynylbenzene naphthalene nonane 3-methylindene 1,4-dimethyl-3,6-bis(methylene)1,4-cyclohexadiene 1,3-dimethyl-2-vinylbenzene durene methylnaphthalene 1-naphthalenol 2,3-diethyl-3-heptene 2,3-dimethyl-naphthalene biphenylol

species

8.10 8.09 8.00 7.80 8.07 7.84 7.76

8.22 8.6 8.92 10.16 8.30 7.27 8.04 8.84 7.80 8.16 8.57 8.76 9.92 10.04 8.87 7.76 8.49 7.09 7.71 8.46 8.09 8.42 8.92 9.80 8.16 8.28 7.77 8.34 7.62 8.27 7.48 8.52 8.18 9.74 7.94 7.58 8.10 8.06 7.96 7.76 8.041 7.89 7.80

8.26 8.58 8.88 10.13 8.29 7.242 7.90 8.828 7.84 8.17 8.62 8.77 9.9 10.08 8.82 7.87 8.464 7.07 ∼7.8 8.44 8.13 8.480 8.96 8.80 8.14 8.29 7.80 8.35 7.65 8.27 7.52 8.58 8.144 9.71 7.97 7.58

literatb

IE (eV) this worka

1.4× 10-4

2.1× 10-4

4.5

5.0

4.5

4.5

8.4× 10-4

4.7× 10-6

5.0

5.0

3.9 × 10-4

6.9 × 10-5

5.5 5.0

5.0

7.6× 10-4

4.3× 10-4 9.6× 10-4

5.0

position (mm)

8.9× 10-5

Xmaxc

a

b

The experimental error for IEs is (0.05 eV. Refers to ref 28 except for those which have the specific label. c The values are the total mole fraction of a specific mass, for example, the mole fraction of mass 40 includes contributions from both allene and propyne. d Refers to ref 32.

80

78

72

70

67 68

66

65

58

55 56

54

53

50 52

43 44

C4H5 C4H6 C4H6 C4H7 C3H4O C4H8 C3H6O C3H6O C5H5 C5H5 C5H6 C5H6 C5H7 C5H8 C5H8 C5H10 C5H10 C4H8O C4H8O C5H12 C6H6 C6H6 C6H8

CH3 C2H2 C2H3 C2H4 C2H5 HCHO CH3OH C3H3 C3H4 C3H4 C3H5 C2H2O C3H6 C3H7 C2H4O C2H4O C4H2 C4H4 C4H4 C4H5

15 26 27 28 29 30 32 39 40

41 42

formula

mass

Table 2. Lists of Intermediates Measured in the Gasoline Flame along with Their IEs, Maximum Mole Fractions (XMAX), and Positions (from the Burner Face)

1508 Energy & Fuels, Vol. 20, No. 4, 2006 Huang et al.


Figure 2. PIE spectra of (a) m/e ) 15 (CH3), (b) m/e ) 39 (C3H3), (c) m/e ) 41 (C3H5), and (d) m/e ) 65 (C5H5) measured at the nozzle sampling position of 5.0 mm from the lean premixed gasoline/oxygen/ argon flame.

Figure 3. PIE spectra of (a) m/e ) 44 (C2H4O), (b) m/e ) 58 (C3H6O), (c) m/e ) 72 (C4H8O/C5H12), and (d) m/e ) 144 (C10H8O) measured at the nozzle sampling position of 5.0 mm from the lean premixed gasoline/oxygen/argon flame.

properties of both alkenes and alcohols. But they have never been detected in hydrocarbon flames until 2005. Taatjes et al. first detected enols in 24 different flames of 14 prototypical single fuels by using the tunable synchrotron photoionization technique.33 The detailed consequences of enols chemistry for practical combustion remain unclear; nevertheless, the identification of enols in flames will revise the current model of hydrocarbon oxidation. One clear onset at 7.80 eV is observed in Figure 3d, which is attributed to the IE of 1-naphthalenol based on the IE (7.76 ( 0.03 eV) from the literature.28 This oxygenated compound was detected for the first time in gasoline flame. The other oxygenated compounds, such as biphenylol (C12H10O), dihydrobenfuran (C8H8O), and isopropyl ketone (C7H14O) were also identified for the first time in the gasoline flame. The measured IEs are listed in Table 2, along with the literatures values as well. The measurement and identification of these oxygenated compounds will be helpful for study of combustion kinetics. c. Hydrocarbon and Aromatic Hydrocarbon Intermediates. The main products in the gasoline flame are hydrocarbons and (33) Taatjes, C. A.; Hansen, N.; McIlroy, A.; Miller, J. A.; Senosiain, J. P.; Klippenstein, S. J.; Qi, F.; Sheng, L. S.; Zhang, Y. W.; Cool, T. A.; Wang, J.; Westmoreland, P. R.; Law, M. E.; Kasper, T.; Kohse-Hoinghaus, K. Science 2005, 308, 1887-1889.


Figure 4. PIE spectra of (a) m/e ) 54 (C4H6), (b) m/e ) 78 (C6H6), (c) m/e ) 92 (C7H8), and (d) m/e ) 106 (C8H10) measured at the nozzle sampling position of 5.0 mm from the lean premixed gasoline/oxygen/ argon flame.

aromatic hydrocarbon intermediates. More than 50 intermediates were detected and identified in this flame. Here, we choose four PIE spectra as an illustration. Figure 4 shows the PIE spectra of m/e ) 54, 78, 92, and 106. Two onsets at the PIE spectra of m/e ) 54, shown in Figure 4a, correspond to the IEs of 1,3butandiene (IE ) 9.07 eV) and 1-butyne (IE ) 10.18 eV), respectively. 1,3-Butadiene is considered to be a hazardous air pollutant by the 1990 U.S. Clean Air Act, and it is classified as a probable human carcinogen; its cancer risk potency is over 25 times higher than that of benzene.12 1-Butyne was identified for the first time in gasoline flame. Aromatic hydrocarbons and PAHs play important roles in the formation of soot. These aromatic compounds are formed from incomplete combustion, and soot is thought to pose a particularly great risk to health.1 Some aromatic hydrocarbons and PAHs were detected and identified even if the flame is under the lean condition. Figure 4b-d shows the PIE spectra of some selected aromatic hydrocarbons. A PIE spectrum for m/e ) 78, shown in Figure 4b, reveals the presence of benzene and fulvene isomers in the flame. Their IEs are demonstrated clearly in the figure, and the fulvene isomer was detected for the first time in gasoline flame. Two onsets are clearly observed at 8.04 and 8.84 ( 0.05 eV for the PIE spectra of m/e ) 92, as shown in Figure 4c. This implies that mass 92 contains two isomers: methylbenzene and 5-methylene-1,3-cyclohexadiene. Similarly, the PIE spectra of m/e ) 106 with two onsets implys that mass 106 includes xylene and 1,3,5,7-octatetraene. Some observed hydrocarbons, i.e., 1,2,3-butariene (m/e ) 52), 1,3-cyclopentadiene and cyclopropylacetylene (m/e ) 66), 1,3-pentadiene and 2-methyl-1,3butadiene (m/e ) 68), 1,3-cyclohexadiene (m/e ) 80), 5-methylene-1,3-cyclohexadiene (m/e ) 92), phenylacetylene (m/e ) 102), and styrene (m/e ) 104) were identified for the first time in the gasoline flame. 3.2. Mole Fraction Profiles. The measurement of mole fractions is very important for the study of the combustion kinetic model. However, it is difficult to deduce the accurate mole fractions of species produced in the gasoline flame because gasoline is a complicated mixture of hundreds of hydrocarbons. Here, we introduce an empirical method to calculate the mole fractions. As mentioned above, a small mount of acetylene was used as a calibrated species for the calculation of mole fractions of the flame species because acetylene does not dissociate at the photon energy below 16.70 eV,28 which is higher than the photon energies used for this experiment. In addition, the gasoline is approximated as isooctane. Thus, the mole fractions


Figure 5. Mole fraction profiles of major species formed in the lean premixed gasoline/oxygen/argon flame.

Figure 6. Mole fraction profiles of C1 and C2 species formed in the lean premixed gasoline/oxygen/argon flame.

of flame intermediates were derived by the mole fraction of acetylene in light of the method described by Cool et al.26 The mole fractions of all observed species were calculated, and also listed in Table 2 along with the maximum mole fractions and their positions from the burner face. a. Mole Fraction Profiles of Major Species. Mole fraction profiles for the major species H2, H2O, CO, O2, CO2, and Ar are displayed in Figure 5 along with the temperature profile. It is noted that all mole fraction profiles and the temperature curve are functions of distance from the burner face. The temperature was measured, and a maximum temperature of 1900 ( 100 K is observed at 9.0 mm, as shown in Figure 5. It is obvious that the concentration of CO2 increases along with the decrease of that of CO under the fuel-lean condition. In addition, the mole fractions of CO2, CO, H2O, H2, and O2 are 0.41, 0.05, 0.17, 0.0046, and 0.19 when the sampling position is located at 30 mm from the burner face, as shown in Figure 5. Then, the C/H/O ratio of the gasoline flame is calculated to be 0.46/0.35/1.42 in the postflame zone. Therefore, we can deuced that the equivalence ratio of the gasoline flame is about 0.77. The value is in good agreement with the data (φ ) 0.75) which was derived by the approximates method. b. Mole Fraction Profiles of C1 and C2 Species. Figure 6 shows the mole fraction profiles of C1 and C2 species. The reaction zone is located at 2-7 mm away from the burner surface, as we can see from the gradient of the concentration profiles. As shown in Figure 6, the smallest hydrocarbon species detected in this flame is the methyl radical (CH3) with a maximum mole fraction of 1.1 × 10-3. Methyl is considered

Huang et al.

to be the most abundant hydrocarbon radical in the hydrocarbon flame, and it has been reported to contribute to PAH formation.34 The presence of large alkanes, such as isooctane, isopentane, heptane, etc. increases the amount of methyl radical by thermal cracking of the methyl group.35,36 Vinyl (C2H3) is the major acetylene precursor in the flame. Hydrogen abstraction from ethylene by H and OH should be its dominant formation route. It has a maximum mole fraction of 1.2 × 10-4 at 5.5 mm from the burner face. Radicals are key intermediates in flame chemistry, and the ability to predict their concentration at different stages of combustion is essential for accurate prediction of the flame properties and behavior of interest. Compared with the concentrations of stable species, the concentrations of radical are more difficult to measure.37,38 The mole fractions of all other radicals, including C3H3, C4H5, C5H7, etc. were calculated and will be discussed in detail below. Acetylene was detected in the gasoline flame and has a maximum mole fraction of 3.1 × 10-3. Its main formation path in benzene flame is C6H6 f C6H5 f C4H3 f C2H2.39 A similar pathway is possible in the gasoline flame. Ethylene is a dominant product with a maximum mole fraction of 4.5 × 10-3, as shown in Figure 6. Its main source is from the β-scission of octane.39 The PIE spectra of m/e ) 29 shows that there is an apparent onset at 8.28 ( 0.05 eV, which is close to the IE (8.26 eV)28 of C2H5. This implies that the C2H5 free radical is dominant for mass 29 and has a maximum mole fraction of 3.2 × 10-4. Formaldehyde is known to be a critical intermediate species, and it is the most important oxygenated compound in the exhaust of gasoline-fueled engines. As shown in Figure 6, formaldehyde is formed early in the flame and has a maximum mole fraction of 2.9 × 10-3. It was reported that exhaust formaldehyde was produced from methanol, ethanol, and hydrocarbons in the fuel.5 There are no methanol and ethanol in the gasoline in this study. Thus, we think the formaldehyde is mainly produced from the oxidation of octane, isooctane, and other hydrocarbons. As shown in Figure 6, the mole fraction of methanol has a maximum value of 5.0 × 10-4. Zervas et al. thought that methanol comes from hexane and isooctane or from a recombination of CH3 and the OH radical or CH3O and H.40 c. Mole Fraction Profiles of C2H4O and C3 Species. The mole fraction profiles of C2H4O and C3 species are displayed in Figure 7. The propargyl (m/e ) 39), allyl (m/e ) 41), and isopropyl radicals (m/e ) 43) were detected in the flame and have the maximum mole fractions of 2.7 × 10-4, 3.7 × 10-4, and 6.5 × 10-5, respectively. Hydrogen abstraction by H and OH from allene and propyne are the dominant propargyl formation pathways. Miller et al. suggested that reaction 1 is a major route to the formation of one-aromatic ring species in flame.29 Richter et al. thought that bimolecular decomposition giving the allyl, i.e., reaction 2, is one of the formation pathways of the allyl.34 C3H4 has two isomers, allene and propyne, according to the measurement of the PIE spectra of m/e ) 40. The total maximum mole fraction is 4.4 × 10-4 at 5.0 mm away from (34) Richter, H.; Howard, J. K. Phys. Chem. Chem. Phys. 2002, 4, 20382055. (35) Doute, C.; Delfau, J. L.; Akrich, R.; Vovelle, C. Combust. Sci. Technol. 1997, 124, 249-276. (36) Doute, C.; Delfau, J. L.; Vovelle, C. Combust. Sci. Technol. 1999, 147, 61-109. (37) Simmie, J. M. Prog. Energy Combust. Sci. 2003, 29, 599-634. (38) Wu, K. C.; Hochgreb, S. Combust. Flame 1996, 107, 383-400. (39) Zervas, E.; Montagne, X.; Lahaye, J. Fuel 2004, 83, 23132321. (40) Zervas, E.; Montagne, X.; Lahaye, J. EnViron. Sci. Technol. 2002, 36, 2414-2421.



Figure 7. Mole fraction profiles of C2H4O and C3 species formed in the lean premixed gasoline/oxygen/argon flame.


the burner face. According to the method described by Cool et al.,26 the isomeric composition of 40 ( 5% allene and 60 ( 5% propyne is yielded in this flame.

C3H3 + C3H3 h C6H5 + H

(1)

C5H6 + H h C3H5 + C2H2

(2)

Mass 42, containing the contribution of ketene and propylene, has a maximum mole fraction of 7.5 × 10-4. The ketene is observed for the first time in gasoline flame, and its formation pathway is unclear. It could be formed by the reaction(s) of C2H2 + O or/and C2H3 + OH. The propylene was formed by isooctane via more stages as discussed by Zervas et al.39 Acetaldehyde (m/e ) 44) was also identified in the flame. Zervas et al. thought that exhaust acetaldehyde is produced from ethanol and straight hydrocarbons.40 Besides the acetaldehyde, the ethenol also contributes to the mole fraction of m/e ) 44. Similarly, the m/e ) 58 contains the contributions of propenol and acetone and the m/e ) 72 contains the contributions of butenol, 2-butanone, and isopentane, as discussed above. Their mole fraction profiles are displayed in Figures 8 and 9. In a previous chemical model, enols were not involved and their formation routes were ambiguous. There is no ethanol in the gasoline. Thus, we suggest that enols are formed mainly from reactions of hydrocarbons with O or OH. d. Mole Fraction Profiles of C3H6O and C4 Species. Figure 8 shows some mole fraction profiles of C3H6O and C4 species. C4H5 (m/e ) 53) is an important radical in the formation of the first aromatic species and was identified in the gasoline flame


with the maximum mole fraction of 3.6 × 10-5. It contains the contribution of CH2CHCCH2 (i-C4H5) and some combinations of the CH3CCCH2 and CH3CHCCH isomers, as mentioned above. C4H7 is attributed to the 1-buten-3-yl radical, and it is formed by hydrogen abstraction from 1-butene. C4H2 (m/e ) 50) may be a potential species in the formation of aromatics, and it has a maximum mole fraction of 7.5 × 10-5. The PIE spectra measurement shows that the C4H6 (m/e ) 54) contains the contributions of 1,3-butadiene and 1-butyne. Ye et al. thought that more than 90% of the 1,3-butadiene in engine exhaust comes from the common alkane and aromatic constituents of the fuel.41 While Zervas et al. and Hakansson et al. thought that 1,3-butadiene was generated from 1-hexene and cyclohexene.8,42 Their maximum mole fraction is 1.2 × 10-3, as displayed in Figure 8. C4H4 contains the contributions of 1,2,3-butarien and vinylacetylene and has the maximum mole fraction of 2.9 × 10-4. Mass 56 has two isomers, methylketene and 2-butene, and its maximum mole fraction is 2.5 × 10-3. e. Mole Fraction Profiles of C4H8O and C5 Species. The mole fraction profiles of C4H8O and C5 species are displayed in Figure 9. C5H5 is attributed to the cyclopentadienyl radical and the pent-1-en-4-yn-3-yl radical according to the measurement of the PIE spectra and has the maximum mole fraction of 6.4 × 10-5, as shown in Figure 9. The cyclopentadienyl radical is formed by hydrogen abstraction from cyclopentadiene in this flame, i.e.,34

C5H6 + H h C5H5 + H2

(3)

C5H7 is identified to be the 3-cyclopentenyl radical and has a maximum mole fraction of 5.7 × 10-5. Up to now, the chemical reaction mechanism in gasoline flame of the pent-1en-4-yn-3-yl radical and the 3-cyclopentenyl radical has still been ambiguous. C5H6 contains the contributions of 1,3cyclopentadiene and cyclopropylacetylene. The recombination of the cyclopentadienyl radical with hydrogen may be one of the formation pathways of the 1,3-cyclopentadiene. C5H8 has a maximum mole fraction of 7.4 × 10-4, and it includes the contributions of 1,3-pentadiene and 2-methyl-1,3-butadiene. The recombination of the C4H5 radical with CH3 is one of the formation routes of C5H8.43 (41) Ye, Y.; Galbally, I. E.; Weeks, I. A. Atomos. EnViron. 1997, 31, 1157-1165. (42) Zervas, E.; Montagne, X. J. Air Waste Manage. Assoc. 1999, 49, 1304-1314. (43) Orme, J. P.; Curran, H. J.; Simmie, J. M. J. Phys. Chem. A 2006, 110, 114-131.


Figure 10. (a) Mole fraction profiles of selected C6 species observed in the lean premixed gasoline/oxygen/argon flame. (b) Mole fraction profiles of selected C7 species formed in the lean premixed gasoline/ oxygen/argon flame.

f. Mole Fraction Profiles of Selected C6-C11 Species. Mole fraction profiles of C6-C11 species are displayed in Figures 1012. Among these species are components of gasoline according to their mole fraction profiles. For C6H6, two isomers were identified, and it corresponds to benzene and fulvene, as discussed above. The maximum mole fraction of fulvene is estimated around 6 × 10-5. The mole fraction of both benzene and fulvene has a maximum of 1.7 × 10-3. Zervas et al. studied the qualitative and quantitative correlations between fuel composition and specific pollute emissions. They thought that benzene was emitted from aromatic fuels and cyclohexane.42 The reaction between benzene and H or O is one of the consumption pathways of benzene in flame. Lindstedt et al. described the formation pathways of benzene and fulvene in their kinetic model study.44 C7H7, corresponding to the benzyl radical, has a maximum mole fraction of 8.9 × 10-5, as shown in Figure 10, which had been detected in benzene flame by Yang et al.45 C7H8 contains the contributions of toluene and 5-methylene-1,3-cyclohexadiene, as mentioned above. Hakansson et al. found that toluene was enriched owing to its formation from o-xylene and ethyl-benzene.8 C8H10 with two onsets at 7.71 and 8.46 ( 0.05 eV could be a mixture of 1,3,5,7-octatetraene and xylene. Its mole fraction profile is displayed in Figure 11. (Here, we cannot distinguish the isomers of p- from o-xylene.) Zervas et al. thought that exhaust o-xylene is a product of unburned fuel, and other compounds of gasoline do not enhance the formation of o-xylene.39,42 C9H8 contains the contributions of (44) Lindstedt, R. P.; Skevis, G. Proc. Combust. Inst. 1996, 26, 703709. (45) Yang, B.; Li, Y. Y.; Wei, L. X.; Huang, C. Q.; Wang, J.; Tian, Z. Y.; Yang, R.; Sheng, L. S.; Zhang, Y. W.; Qi, F. Proc. Combust. Inst. 2006, in press.

Huang et al.

Figure 11. (a) Mole fraction profiles of C8 species observed in the lean premixed gasoline/oxygen/argon flame. (b) Mole fraction profiles of C9 species formed in the lean premixed gasoline/oxygen/argon flame.

Figure 12. Mole fraction profiles of C10 and C11 species observed in the lean premixed gasoline/oxygen/argon flame.

indene and phenylallene according to the measurement of the PIE spectra, and their maximum mole fraction is 3.9 × 10-4, as shown in Figure 11. Other PAHs, including naphthalene, dimethylnaphthalene, etc., were also identified. The mole fraction profiles of higher masses such as the C10 and C11 series show monotonic decrease, as shown in Figure 12. Thus, these are components of the gasoline, not the flame intermediates. We do not list mole fractions for gasoline components in Table 2. The mole fraction profiles of these species decrease with the increment of the flame temperature. This suggests that heavy hydrocarbons decompose into the small hydrocarbons (stable and unstable species). We did not calculate the mole fractions of C12 series species (i.e., C12H22 and C12H10O), C6H10, and C7H6 owing to their low concentrations in the gasoline flame.


4. Conclusion The low-pressure lean premixed gasoline/oxygen/argon flame was studied by using tunable synchrotron photoionization and molecular-beam mass spectrometry. More than eighty intermediates (including radicals, oxygenated compounds, and PAHs) produced in the flame were identified with the measurements of the photoionization mass spectrum and the PIE spectra by scanning photon energies. Some intermediates were observed for the first time in the lean gasoline/oxygen flame, and they will be helpful for understanding the gasoline combustion mechanism in gasoline-fueled engine. The mole fractions of


these flame species were measured by scanning the burner at selective photon energies near the ionization threshold. Detailed chemical reaction mechanisms of the main species were discussed. Acknowledgment. The authors thank the Chinese Academy of Sciences (CAS), Natural Science Foundation of China under Grant Nos 20473081 and 20533040, SRF for ROCS, and SEM for the funding support. EF0601465

Oxygen Flame Studied with Tunable

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