Experiment study of oxygenates impact on n-heptane flames ...

[Pages:8]Fuel 88 (2009) 2297?2302

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Experiment study of oxygenates impact on n-heptane flames with tunable synchrotron vacuum UV photoionization

Jinou Song a,*, Chunde Yao a, Shiyu Liu a, Zhenyu Tian b, Jing Wang b

a State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China b National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China

article info

Article history: Received 4 May 2008 Received in revised form 24 April 2009 Accepted 27 April 2009 Available online 21 May 2009

Keywords: Oxygenate Premixed flame Heptane Synchrotron radiation photoionization

abstract

In order to determine the effects of oxygenates on the fuel combustion, the experiments reported here investigated the premixed n-heptane flame chemistry. Heptane typifies the large alkanes that comprise the bulk of most hydrocarbon fuels. The specific flames were low-pressure (25 Torr), laminar, premixed flames of n-heptane/oxygen/argon and n-heptane/oxygenate/oxygen/argon at an equivalence ratio of 1.0. Two different fuel oxygenates (i.e. MTBE and ethanol) were tested, these are the main oxygenates used to improve motor vehicle fuel properties. The experiment was performed with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry. Major species on the centerline of each flame were identified by measurements of the photoionization mass spectrum and photoionization efficiency (PIE) spectra. Mole fraction profiles of these species were derived at the selected photon energies near the ionization thresholds. A large amount of oxygenated intermediates was detected in the oxygenate containing flames. The species measurements indicated that MTBE and ethanol enhanced the heptane oxidation via different routes, and reduced the mole fractions of aromatics and cycloalkenes in varying degrees. The results are a useful databases for testing detailed chemical kinetic mechanism of fuel decomposition.

? 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Combustion of fossil fuels is one of the major sources of world commercial energy. Heptane is a component of commercial gasoline and one of the primary reference fuels for the determination of the gasoline octane number. Consequently, the combustion research community has endeavored to develop detailed chemical kinetic mechanisms for the combustion of heptane. The oxidation of n-heptane has been investigated in shock tubes [1], jet-stirred reactors [2], premixed [3,4], and diffusion [5] flames. The ignition of n-heptane/air mixtures has been studied by Zhukov et al. [6]. Temperature and species (stable and free radicals) mole fraction profiles in laminar, premixed n-heptane/oxygen/argon flames have been obtained by Dout? et al. [3]. Recently, Mc Enally and co-workers [7] have studied fuel decomposition and hydrocarbon growth processes in non-premixed flame. Temperature and species measurements were reported, and PAHs (polycyclic aromatic hydrocarbons) growth mechanisms were analyzed in this study.

For the sake of the improvement in motor vehicle fuel properties, fuel oxygenates were first used as an octane replacement for lead since lead inactivates the exhaust catalyst. They are also known for their ability to reduce exhaust CO emissions by leaning

* Corresponding author. Tel.: +86 022 2740 6842; fax: +86 022 2738 3362. E-mail address: songjinou@tju. (J. Song).

0016-2361/$ - see front matter ? 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2009.04.034

the fuel?air mixture. In the USA [8], the addition of ethers to gasoline helps to attain the minimum amount (up to 2.7 wt% of oxygen in some metropolitan areas) of oxygenated compounds required to get ``cleaner" emissions; commercial gasoline can include more than 10 wt% of ethers. Several oxygen-containing compounds (i.e. oxygenates) such as alcohols (e.g. methanol, ethanol, and tertiary butyl alcohol) and ethers (e.g. methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether (TAME)) were considered as the most common fuel oxygenates. Ethers' use is in decline due to their effect on groundwater.

The gas-phase oxidation of pure MTBE and ETBE has been investigated using several techniques, such as static reactors [9], a flow reactor [10], research engines [11], and a metallic jet-stirred reactor [12]. A recent paper [13] has already partly presented the experimental results for the oxidation of MTBE and ETBE, together with computer modeling.

Inal and Senkan [14] have studied the effects of three fuel oxygenates (methanol, ethanol, and MTBE) on the formation of PAHs and soot, and reported the species mole fraction and temperature profiles in laminar, premixed, atmospheric pressure, fuel-rich flames of n-heptane. Several studies of the effect of oxygenate on the combustion of fuels in engines [15,16] have also been published. The effects of blending unleaded gasoline with different proportions of oxygenate on exhaust carbon monoxide, carbon

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J. Song et al. / Fuel 88 (2009) 2297?2302

Table 1 Experimental conditions (equivalence ratios: 1.0; pressure: 3.33 kPa; reactants temperature: 200 ?C).

Mixture Flow rates/slma n-Heptane (mol%) Ethanol (mol%) MTBE (mol%)

Flame A

C7H16/O2/Ar 0.092/1.0/0.8 4.832 0 0

Flame B

C7H16/C2H5OH/O2/Ar 0.065/0.12/1.0/0.8 3.242 6.983 0

Flame C

C7H16/MTBE/O2/Ar 0.050/0.06/1.0/0.8 2.618 0 3.321

a slm is the abbreviation of liters per minute at standard ambient condition.

dioxide, and hydrocarbon emissions from a fixed compression ratio SI engine were reported.

This paper studied the species mole fraction in premixed, laminar, low-pressure (25 Torr) flames of n-heptane/oxygen/argon and n-heptane/oxygenate/oxygen/argon with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry. The goals of the research were to develop experimental information on heptane combustion and to gain additional understanding of the effects of oxygenate on heptane oxidation kinetics. Oxygenates tested were MTBE and ethanol.

2. Experimental

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 has been published elsewhere [17], which is identical to the previous report by Cool et al. [18], therefore, only a brief description will be given here. 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 time-of-flight mass spectrometer (RTOFMS). 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 lm. Here, the sampling flow is the 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 passed into a differentially pumped ionization region where it is crossed by the tunable synchrotron light.

The photoions are collected and analyzed by a RTOFMS (reflection time-of-flight mass spectrometer) with the mass resolving power (m/Dm) of $1400. 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.

The flux-normalized ion signals, measured as a function of the photon energy, yield the PIE (Photoionization efficiency) spectra.

Temperature/K

2000

1600

1200 800 400 0

flame A flame B flame C

1234 Distance Z/mm

Fig. 2. Temperature profiles in three flames.

To avoid fragmentation and keep near-threshold photoionization, the burner was scanned at 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. [18]. The mole fractions have an uncertainty of ?25% for the stable intermediates and a factor of 2 for radicals. A Pt/ Pt?13%Rh thermocouple coated with Y2O3?BeO antioxidant on the surface and with a diameter of 0.076 mm was used to measure the flame temperature. Since it was placed 15 mm in front of the quartz probe, the thermocouple had no effect on flame structure. It is estimated that the flame temperatures have an uncertainty of about 100 K.

The experimental conditions are listed in Table 1. Argon was also used as inert gas. Oxygen (99.99%) and argon (99%) were obtained from Nanjing Special Gas, China. Liquid fuel and oxygenates were commercially supplied: n-heptane (97%) and ethanol (HPLC grade, 99.7%) from China Pharmaceutical Group, and MTBE (HPLC grade) from Alfa Aesar. The flow rates of oxygen and argon were controlled by calibrated mass flow controllers (Multi Gas Controller 647C, MKS). Two high precision syringe pumps (Isco 1000D USA) were used to introduce the fuel and oxygenate into a vaporizer with the temperature kept at 200 ?C. The syringe pumps had a flow rate resolution of 1 ml/min and flow rate accuracy of ?0.5%. The argon?oxygen stream was heated to the temperature of fuel to prevent condensation.

3. Results and discussion

Since a significant amount of pollution is formed in the combustion, some important large intermediates, components of fuel in the flames were analyzed in detail. In these figures, distance Z indicates the sampling position above the burner surface. To avoid the possible sampling probe?burner surface interactions, the sampling position 0 mm is at 1.2 mm above the burner surface. When distance Z exceeds 4 mm, the data begins to be unstable, and is considered questionable, possibly due to the surrounding air interactions.

Normalized mole fraction Normalized mole fraction

0.5 0.4 0.3 0.2 0.1 0.0

0

flame A flame B flame C

1

2

3

4

Distance Z/mm

a) Overall fuel

0.5 0.4 0.3 0.2 0.1 0.0

0

flame A flame B flame C

1

2

3

4

Distance Z/mm

b) Heptane

Fig. 1. Normalized mole fraction profiles.

J. Song et al. / Fuel 88 (2009) 2297?2302

2299

The mole fractions of heptane and overall fuel at the burner inlet were different for three flames, although the equivalence ratios were equal to 1.0. For the sake of the comparison of oxidation rates of heptane and overall fuel in flames, the normalized mole fractions were used. The normalized mole fraction is defined as overall fuel mole fraction/overall fuel mole fraction at the burner inlet (see

Fig. 1a) heptane mole fraction/heptane mole fraction at the burner inlet (see Fig. 1b).

The normalized mole fraction profiles of overall fuel and n-heptane are shown in Fig. 1 for the neat heptane and oxygenate containing flames. The data points represent the experimental results and the solid lines represent trends in all the figures pre-

Mole fraction/10-3

1-butene

2.0

1-pentene

1.6

1-hexene

1,3-butandiene

1.2

2-butene

0.8

2-pentene

0.4

1,3-pentadiene

0.0 01234

Distance Z/mm

a) Major alkenes in flame A

Mole fraction/10-3

1-pentene

2.0

1-butene

1-hexene

1.6

2-pentene

1,3-pentadiene 1.2

0.8

0.4

0.0

0

1

2

3

4

Distance Z/mm

b) Major alkenes in flame B

Mole fraction/10-3

1.4

1-butene 3-methyl

1,3-butadiene

1.2

1-butene 2-methyl

1.0

1-butene 2,3-methyl

0.8

0.6

0.4

0.2

0.0

0

1

2

3

4

Distance Z/mm

c) Major alkenes in flame C

Fig. 3. Comparison of noted alkenes in various flames.

Mole fraction/10-4

12

1-butyne propyne

1-buten-3yne

8

4

0

0

1

2

3

4

Distance Z/mm

a) Major alkynes in flame A

Mole fraction/10-4

8

propyne

6

1-butyne

1,3-butadiyne

4

1-buten-3-yne

2

0

0

1

2

3

4

Distance Z/mm

b) Major alkynes in flame B

8

propyne

1-butyne

6

Mole fraction/10-4

4

2

0

0

1

2

3

4

Distance Z/mm

c) Major alkynes in flame C

Fig. 4. Comparison of noted alkynes in various flames.

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J. Song et al. / Fuel 88 (2009) 2297?2302

sented here. As expected, the normalized mole fractions of overall fuel and heptane decreased steadily with the increase of distance Z. The addition of oxygenate decreased the normalized mole fractions of overall fuel and heptane. As can be seen from these figures, the

lowest normalized mole fraction of heptane was obtained for flame B, and the lowest normalized mole fraction of overall fuel was obtained for flame C. This result indicated that during the early stage of combustion heptane oxidized faster in flame B than in flame C.

Mole fraction/10-5 Mole fraction/10-5

2.4 2.0 1.6 1.2 0.8 0.4 0.0

0

benzene/flame A benzene/flame B toluene/flame B benzene/flame C

1

2

3

4

Distance Z/mm

a) major aromatic

1,3cyclopentadiene/flame A

1,3cyclopentadiene/flame B

8

1,3cyclohexadiene/flame B

1,3cyclopentadiene/flame C

6

4

2

0

1

2

3

4

Distance Z/mm

b) major cycloalkenes

Fig. 5. Mole fraction profiles of benzene, toluene, and cyclopentadiene in various flames.

Mole fraction of acetaldehy/10-4

Mole fraction/10-4

60

50

40

flame A

30

flame B flame C

20

10

0

0

1

2

3

4

Distance Z/mm

a) Acetaldehyde

4

propanone/flame B 3-pentanone/flame B

3-heptanone/flame B

3

2-butanone/flame B

2

1

0 01234 Distance Z/mm c) major ketones in flame B

Mole fraction/10-4

Mole fraction/10-4

propanone/flame A

5

2-butanone/flame A

3-heptanone/flame A

4

3

2

1

0 01234 Distance Z/mm b) major ketones in flame A

7

propanone/flame C

6

2-butanone/flame C

5

3-heptanone/flame C

4

3

2

1

0 01234 Distance Z/mm

d) major ketones in flame C

Mole fraction of ketone/10-4

12

10

8

6

flame A

flame B

4

flame C

0.0 0.5 1.0 1.5 2.0 2.5 Distance Z/mm

e) summing the major ketones in each flame

Fig. 6. Major oxygenated intermediates in various flames.

J. Song et al. / Fuel 88 (2009) 2297?2302

2301

Mole fraction/10-4 Mole fraction/10-4

ethane methoxy

ethenol

4

butanol

DME(in flame C)

3

2

1

0

0

1

2

3

4

Distance Z/mm

Fig. 7. Characteristic species in flame B.

The temperature profiles for the three flames are shown in Fig. 2, and correspond to direct thermocouple readings. The temperature profiles for the three flames were generally similar with maximum temperatures in the range 1990?2025 K. The measurement results showed that the temperature of flame C at position 0 mm was the lowest, but soon exceeded the temperatures of flame A and B. It is observed from Figs. 1 and 2 that ethanol enhanced the heptane oxidation more obviously than MTBE and the MTBE containing flame had the maximum combustion rate.

The previous studies have shown that at high temperatures, n-heptane consumption occurs by the thermal decomposition via C?C bond rupture and H-atom abstraction [19], and that ethanol

25

20

15

10

ethene methoxy

5

propane 2-methoxy

0.0 0.5 1.0 1.5 2.0 2.5 Distance Z/mm

Fig. 8. Characteristic species in flame C.

oxidations are initiated mostly by reactions with the OH and H radicals [20]. In the case of MTBE, the major oxidation pathway is unimolecular decomposition, producing branched butene and methanol [21]. In order to identify the chemical pathways that cause heptane decomposition and to develop detailed kinetic mechanisms that describe it, about 80 species produced in the flames have been unambiguously identified by measurements of the photoionization mass spectrum and photoionization efficiency (PIE) spectra. In addition, mole fraction profiles of about 40 species (listed in Appendix A) are derived at the selected photon energies near ionization thresholds. Low-molecular weight alkenes or alkynes have been studied extensively [4,14]. In this study, major

Table A1 List of species measured in flames.

m/e

Species

Formula

Ionization energies/eV

Experimenta

Literatureb

Maximum mole fraction

Flame A

Flame B

15

Methyl radical

CH3

28

Ethylene

C2H4

29

Ethyl radical

C2H5

40

Propyne

41c

Allyl radical

C3H4 C3H5

42

Ketene

C2H2O

42

Propene

C3H6

43

n-Propyl radical

C3H7

44

Acetaldehyde

C2H4O

44

Ethenol

C2H4O

46

Ethanol

C2H6O

46

DME

C2H6O

52

1-Buten-3-yne

C4H4

54

1,3-Butadiene

C4H6

54

1-Butyne

C4H6

55

Allyl 2-methyl-

C4H7

56

2-Butene, (E)-

C4H8

56

1-Butene

C4H8

58

Acetone

C3H6O

58

Ethene, methoxy-

C3H6O

60

Ethane, methoxy-

C3H8O

66

1,3-Cyclopentadiene

C5H6

68

1,3-Pentadiene, (Z)-

C5H8

70

1-Pentene

C5H10

70

2-Pentene, (E)-

C5H10

72

2-Butanone

C4H8O

74

Propane, 2-methoxy

C4H10O

74

Propanoic acid

C3H6O2

78

Benzene

C6H6

80

1,3-Cyclohexadiene

C6H8

82

Furan,2-methyl-

C5H6O

84

1-Hexene

C6H12

84

1-Butene, 2,3-dimethyl

C6H12

86

2-Pentanone

C5H10O

92

Toluene

C7H8

100

n-Heptane

C7H16

114

3-Heptanone

C7H14O

9.84 10.54

8.36 10.37

8.15 9.63 9.75 8.12 10.23 9.34 10.50 10.01 9.61 9.08 10.18 7.91 9.08 9.63 9.72 9.08 9.78 8.58 8.64 9.52 9.06 9.52 9.41 10.43 9.25 8.27 8.37 9.44 9.08 9.34 8.87 10.08 9.16

9.84 10.51

8.26 10.36

8.13 9.62 9.73 8.10 10.23 9.33 10.48 10.02 9.58 9.07 10.18 7.9 9.10 9.55 9.70 8.95 9.72 8.57 8.62 9.49 9.04 9.52 9.45 10.44 9.24 8.25 8.38 9.44 9.07 9.38 8.83 10.08 9.15

6.8E?3 9.5E?3 7.3E?4 5.8E?4 5.3E?4 2.0E?3 3.9E?3 ? 9.1E?4 ? ? ? 9.4E?5 1.4E?3 1.2E?3 ? 3.1E?4 1.9E?3 1.6E?4 ? ? 6.7E?5 1.4E?4 1.99E?3 2.8E?4 4.1E?4 ? ? 2.4E?5 ? ? 8.5E?4 ? ? ? 2.4E?2 2.8E?4

5.3E?3 1.4E?2 4.4E?4 2.0E?4 ? 1.2E?3 2.3E?3 ? 5.2E?3 3.3E?4 1.2E?2 ? 6.4E?5 8.6E?4 6.4E?4 ? ? 1.2E?3 1.8E?4 ? 5.3E?5 3.8E?5 3.3E?5 1.14E?3 1.6E?4 3.1E?4 ? ? 1.7E?5 2.8E?5 6.9E?5 5.4E?4 ? 1.1E?4 4.9E?6 1.5E?2 2.2E?4

a The experimental errors for measured IEs are ?0.05 eV for species with strong signals and ?0.10 eV for species with weak signals (See Ref. [22]). b Refers to Ref. [23]. C C2HO is very active and is hardly detected, so mass 41 is only listed as C3H5.

Flame C

4.6 E?3 8.6 E?3 2.4E?4 6.5E?4 ? ? 3.2E?3 2.4E?5 6.0E?4 ? ? 1.6E?4 ? 6.9E?4 8.0E?4 6.8E?5 1.25E?4 ? 4.5E?4 2.6E?3 ? 2.8E?5 3.4E?5 3.9E?4 4.2E?6 6.6E?4 2.1E?3 2.6 E?3 ? ? ? ? 1.1E?4 ? ? 1.2 E?2 1.2E?4

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J. Song et al. / Fuel 88 (2009) 2297?2302

larger species in flames are investigated. Concentration profiles of major alkenes are shown in Fig. 3. Major alkenes in flame A exhibited similar behavior as that in flame B (Fig. 3a and b). A large amount of branched alkenes were detected in flame C (Fig. 3c).

The mole fraction profiles of major alkynes are shown in Fig. 4. In contrast with Fig. 3, major alkynes in flame A exhibited similar behavior as observed in flame C (Fig. 4a?c). A larger amount of alkadienes was detected in flame B (Fig. 4b).

Benzene formation is a critical step to soot production and has been studied extensively [24]. In general, the dominant benzene/ phenyl formation pathways are reactions of propargyl (C3H3) [25,26], butenynyl (n-C4H3), and butadienyl radicals (n-C4H5) [27] for hydrocarbon flame. Although a large amount of butene, butyne and propyne existed in flame C, there was a small amount of benzene in flame C (Fig. 5a). Fig. 5b shows major cycloalkenes in the three flames. Less cycloalkenes were produced in oxygenate containing flames.

Fig. 6 shows the major oxygenated intermediates in the three flames. The peak mole fraction of acetaldehyde from flame B was higher than flame A or C by a factor of 5 and even higher than the peak mole fraction of alkene and alkyne from flame B. A larger amount of ketones was detected in flame C. Flame A and C contained about the same amount of acetaldehyde, and flame A and B contained about the same amount of ketones.

Figs. 7 and 8 show the characteristic species in flame B and C: a large amount of ethenol and butanol was detected in flame B only, and their mole fractions were at the same order of magnitude as that of alkynes; a large amount of ethane methoxy and propane 2-methoxy were detected in flame C, and their mole fractions were of the same order of magnitude as that of alkenes.

4. Conclusions

(1) Ethanol enhanced the heptane oxidation more noticeably than MTBE, and MTBE containing flame had the maximum combustion rate;

(2) more linear alkenes existed in ethanol containing flame, and more branched alkenes and alkadienes existed in MTBE containing flame;

(3) the ethanol addition increased the mole fraction of acetaldehyde, and the MTBE addition increased the mole fraction of ketones in the pre-mixed n-heptane flames;

(4) a large amount of ethane methoxy and propane 2-methoxy were formed in the n-heptane flame by the addition of MTBE, and a large amount of ethenol and butanol were formed in the n-heptane flame by addition ethanol.

Acknowledgments

The authors acknowledge the financial support of national natural science foundation of China (No. 50676065). Thanks to reviewers for constructive suggestions.

Appendix A

See Table A1.

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