O ratios

Combustion and Flame 159 (2012) 44–54

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Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e

Experimental and kinetic modeling study of methyl butanoate and methyl butanoate/methanol flames at different equivalence ratios and C/O ratios Wu Yu a, Gen Chen a, Zuohua Huang a,⇑, Zhaoyang Chen a, Jing Gong a, Jiuzhong Yang b, Zhandong Wang b, Fei Qi b a b

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, PR China

a r t i c l e

i n f o

Article history: Received 11 March 2011 Received in revised form 20 May 2011 Accepted 23 May 2011 Available online 16 June 2011 Keywords: Methyl butanoate Methanol Molecular-beam mass spectrometry Premixed laminar flame CO2 Soot precursors

a b s t r a c t Premixed laminar methyl butanoate/oxygen/argon and methyl butanoate/methanol/oxygen/argon flames were studied with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry at 30 torr (4.0 kPa). Three flames were investigated in the experiment: MB (methyl butanoate) flame F1.54 (/ = 1.54, C/O = 0.479), MB flame F1.67 (/ = 1.67, C/O = 0.511) and MB/methanol flame F1.67M (/ = 1.67, C/O = 0.479). By measuring the signal intensities at different distances from the burner surface, the mole fraction profiles of intermediates are derived. Experimental results show that the flame front shifts downstream and peak mole fractions of intermediates increase remarkably with the increase of equivalence ratio for pure MB fuel. When methanol is added, the peak mole fractions of most intermediates including those of soot precursors decrease remarkably at the same equivalence ratio, while peaks of soot precursors vary little (only slightly decreasing) at same C/O ratio. It is concluded that the formation of soot precursors is more sensitive to C/O ratio than to equivalence ratio. Besides, more CO2 is produced near the burner surface in MB flame than that in MB/methanol flame, and this validates an early production of CO2 in methyl ester oxidation. In addition, a modified MB detailed mechanism is used to model flame structure, and improved agreements between the experimental and predicted results are realized. Based on the simulation results, reaction flux and sensitivity are analyzed for CO2 and C3H3, respectively. Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction An increase in oil price and strengthening in emission regulations have motivated a growing interest in alternative and hybrid fuel of internal combustion engines. Biodiesel, the complex mixtures of long carbon chain methyl esters (C14AC22) of animal fats or plant oils, has been tested as an alternative fuel for diesel. Previous studies demonstrated that using biodiesel in diesel engines could decrease HC, CO and PM emissions but may increase the NOx emission [1–5]. Methanol is a promising biomass fuel, which has high oxygen content, high latent heat of evaporation and low viscosity. Based on these physical characteristics of methanol, previous studies realized the reduction of NOx emission and further decreased soot emission in diesel engines by using biodiesel–methanol fuel [6,7]. However, it was reported that the O atom in methyl esters oxygenate had a small influence on the inhibition of soot formation, and easier to form CO2 early than ⇑ Corresponding author. Fax: +86 29 82668789. E-mail address: [email protected] (Z. Huang).

those in other oxygenates [8–12]. Therefore, the effect of the chemistry structure of methanol on inhibiting soot formation of biodiesel–methanol needs to be investigated. Normally, it is difficult to directly study a biodiesel flame in laboratory conditions and propose a detailed kinetic mechanism due to its high boiling point and complex chemical structure. A common approach to solve this issue is to choose a surrogate which has a simple and well characterized composition to represent the real fuel. Methyl butanoate (MB) which has the methyl ester termination and similar alkyl chain to biodiesel is a surrogate candidate and employed in this study. For MB, Fisher et al. [13] firstly proposed a chemical kinetic model in 2000 to describe its combustion. However, this model was validated only against the limited combustion data [14]. Westbrook et al. [12] used Fisher’s model in computation, and they found that MB has a small effect on soot reduction compared with other oxygenated fuels. Later, the oxidation of MB was investigate in a flow reactor [15], jet stirred reactor [16–18], opposed-flow diffusion flame [19], shock tube [18,20–24] and rapid compression machine [20,25,26]. Meanwhile, detailed chemical kinetic models for the combustion of MB were

0010-2180/$ - see front matter Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2011.05.018

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developed based on those experiments [15–22]. Furthermore, some computational studies on bond dissociation energies and the reaction pathways of MB destruction [27–29] were conducted using quantum chemistry and dynamics calculations. Previous studies have provided important information for the understanding of MB combustion, but identification of combustion intermediates, especially for radicals and isomeric species remains incomplete, and detailed information is required to further improve the existing models. Meanwhile, the effect of methanol addition on the inhibition of soot formation for biodiesel oxidation needs further studies and the early production of CO2 caused by the methyl ester structure needs further validation under different experimental conditions. The objectives of this study are to detect more MB flame species and clarify the effects of methanol addition on the concentrations of major species and intermediates, especially soot precursors, at the same equivalence ratio and C/O ratio using molecular-beam mass spectrometry (MBMS) and tunable vacuum ultraviolet (VUV) synchrotron photoionization. The effects of equivalence ratio and C/O ratio on the formation of soot precursors are compared. Early production of CO2 from methyl ester structure is expected to be verified via observing the variation of CO2 mole fraction profiles caused by methanol addition. Moreover, a modified model is proposed based on the experimental results, and then used to simulate the flame structure. The chemical kinetic process of CO2 formation and the reactions that affect the consumption and formation of C3H3 in methyl ester oxidation are analyzed. The study will deepen the understanding on combustion chemistry of MB and MB/methanol. 2. Experimental apparatus and procedures The experiments were conducted at the Combustion and Flame Endstation of the National Synchrotron Radiation Laboratory in Hefei, China. The instruments have been reported in previous publications [30,31]. In brief, the apparatus consists of a low-pressure flame chamber which contains a movable McKenna burner with a diameter of 6.0 cm, a differentially pumped chamber with a molecular beam sampling system, and a photoionization chamber with a reflectron time-of-flight mass spectrometer (RTOF-MS). Flow rates of the gas reagents are controlled separately by mass flow controllers, and flow rates of the liquid fuels into the vaporizer are controlled separately by syringe pumps. Flame species are sampled by a quartz cone-like nozzle. The sampled gas forms a molecular beam which passes through a nickel skimmer into the differentially pumped ionization region where it is crossed by the tunable synchrotron VUV light. The photoions are then collected and analyzed by the RTOF-MS. The McKenna burner is driven by a step motor. Movement of the burner toward or away from the quartz nozzle allows mass spectra to be taken at different distances from the quartz cone to the burner surface along the central line of the flame (sampling positions). Tunability of the synchrotron radiation allows mass spectra to be taken at different wavelengths. Signal profiles for different flame species relative to the photon wavelength, i.e., photon energy, can be obtained from the spectra. These signal profiles are termed as photoionization efficiency (PIE) curves. And the ionization energy (IE) can be measured from the onset in the PIE. The scanning range of photon energy is from 7.9 eV to 11.7 eV. Error of the measured IE is 0.05 eV for the species with strong signals and 0.1 eV for those with weak signals. Species mole fractions are derived from the signal profiles using the methodology in literatures [32,33]. Photoionization cross sections (PICS) are taken from literatures [34–40], as are methods [33,41] used to estimate the cross section values for the intermediates with unknown PICS. The uncertainties of the mole fractions are within 10% for the major species, 25% for intermediates with known PICS, and a factor of 2 for those with esti-

Table 1 Flame conditions of the investigated flames. Flame

F1.54

F1.67

F1.67M

X (C5H10O2) X (O2) X (Ar) X (CH3OH) / C/O DM (g/s/cm2)

0.115 0.485 0.400 – 1.540 0.479 2.845E3

0.123 0.477 0.400 – 1.670 0.511 2.881E3

0.090 0.426 0.400 0.084 1.670 0.479 2.731E3

mated PICS. Since the experimental apparatus and data reduction methods are the same for the tested flames, the relative comparisons on mole fraction among the flames should have smaller uncertainties. The flame temperature is measured by using a 0.1-mm-diameter Pt–6%Rh/Pt–30%Rh thermocouple coated with Y2O3–BeO anti-catalytic ceramic, and is corrected for the radiation heat loss and cooling effects of sampling nozzle [42,43]. The experimental error of the maximum flame temperature is estimated to be ±100 K. Table 1 lists the flame conditions in this work. Here X (C5H10O2), X (O2), X (Ar) and X (CH3OH) are the inlet mole fractions of MB, oxygen, argon and methanol, respectively. / is equivalence ratio, C/O is the carbon-to-oxygen ratio of the mixture, and DM is the mass flow rate of the mixture. The choosing of flame inlet composition refers to literature [44]. However, for better comparison on chemical issues, the inlet percentage of Ar is kept identical in all flames. The equivalence ratio of the flame with methanol addition, F1.67M, is the same as that of pure MB flame F1.67, with the value of 1.67. Since the O content in methanol is higher than that in MB, the C/O ratio of the flame decreases with methanol addition. By changing the concentration of MB, the C/O ratio of the pure MB flame can be adjusted to the same as that of flame F1.67M and the equivalence ratio of the flame is 1.54. The pressure in the combustion chamber is kept at 30 torr (4.0 kPa). 3. Kinetic modeling The detailed kinetic mechanism used to simulate the flame structure is based on Dooley’s model [20]. Dooley’s model estimates the pressure-dependence of MB decomposition by quantum Rice–Ramsper–Kassel (QRRK) theory [45]. In this study, some intermediates that are not taken into account in Dooley’s model are detected, such as 1,3-butadiyne, 1-buten-3-yne and 3-cyclopentadiene. And this model cannot predict the observed mole fractions of other unsaturated C3AC4 hydrocarbons measured in our experiment as described in the next section. To make the model cover the new detected species and obtain a better prediction of our experimental unsaturated C3AC4 hydrocarbons data, some reactions and species related to the chemistry of these species taken from the C3H4 mechanism of Hansen et al. [46] were added to this model. Hansen’s model was developed according to the low pressure (25 torr) premixed flame data of allene and propyne. It includes the latest theoretical rate coefficients for most reactions, and some of the rate coefficients are for a pressure of 30 torr. The modified model contains 290 species and 1703 reactions. Since the MB mechanism includes the detailed methanol oxidation submechanism, no modification was made when it was used to model the MB/methanol flame. Chemkin Premix codes were used to carry out model computation. Before the temperature profiles were input in the calculations, the perturbation induced by the quartz probe and the thermocouple had been considered [42,43] (the reactions taken from the mechanism of Hansen et al. are listed and stated in the Supplemental material).

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4. Results and discussions 4.1. Identification of the intermediate species Figure 1a and b show the photoionization mass spectra of the MB/methanol flame at the sampling positions of 4.0 mm and 5.0 mm, and photon energy of 11.80 eV. As increasing the sampling position, signal of MB (M/Z = 102) decreases while those of intermediates increase. This reflects the decomposition process of large molecular fuel. Figure 1b–d give the mass spectra of three flames at 5.0 mm and 11.80 eV. Direct comparisons on the signals of intermediates among three flames are meaningless due to the difference in photon fluxes. However, comparison of the relative signal intensity variation can provide valuable information. At 5.0 mm, the relative signal intensity of MB in flame F1.54 is smaller than those in other flames, which indicates faster fuel consumption in the leaner flame. It also shows that the relative signal intensities of M/Z = 30 and 32 increase remarkably with methanol addition. The species can be identified by comparing the measured IEs with literature IEs [47]. For candidate species without literature IEs, their IEs are calculated by using GAUSSIAN03 program. According to the methodology reported in literature [48], a B3LYP method with the 6-31++G(d, p) basis set is used for geometry optimization and frequency calculation of these molecules. Single point energies are calculated using the following equation, and the error bars of the calculated IEs are ±0.1 eV.

E½B3-PMP2 ¼ ðE½B3LYP=6-311 þ ð3df;2pÞ þ E½MP2=6-311 þ ð3df;2pÞÞ  0:5 þ Zero Point EnergyðZPEÞ Table 2 lists the identified intermediates, along with their measured IEs, peak mole fractions and peak positions. For the same species, the measured IEs are almost the same in different flames.

Here only IEs measured in flame F1.67 are listed. For M/Z = 29, two isomers have the same literature IEs: one is formyl (IE = 8.12 eV [47]) and the other is ethyl (IE = 8.12 eV [47]). The simulation results show that the mole fraction of ethyl is one order of magnitude smaller than that of formyl in the tested flames, indicating that formyl is the dominant one in the tested conditions. For M/ Z = 100, the measured IE (9.57 eV) is close to the calculated IE of MB1D (9.50 eV) and MB2D (9.54 eV). Besides, the dehydrogenation of MB is easy to form MB1D and MB2D. Thus, it can predict the coexisting of these two species in the flame. Comparing the flame intermediates in three flames, it is found that the chemical compositions are the same, and no new species are detected even when methanol is added. This can be attributed to the similarity of the investigated flames. These three flames contain a large proportion of MB and the equivalence ratios are close, and moreover, MB oxidation also forms methanol. Although methanol addition has little effect on the intermediate pools, it influences the concentrations of the related species. For example, at an equivalence ratio of 1.67, the peak mole fraction of formaldehyde increases and its corresponding position shifts upstream with methanol addition. Table 2 shows that the peak position of the intermediates in flame F1.54 is closer to the burner surface than those in other flames. This may be due to the fast burning of fuel in leaner flame. The consumption of the saturated C/H/O fuel is generally linked to unimolecular decomposition and bimolecular hydrogen abstraction. The reaction barrier studies showed that H-abstraction reactions from MB by reactive radicals have rather lower barriers than MB unimolecular decomposition reactions [28]. Hence, the initial destruction steps of MB are mainly through H-abstractions. Then the radicals undergo b-scission. The species formed by these reaction pathways are illustrated in Fig. 2. An H atom is abstracted from C(a), C(b), C(c) or methoxy radical of MB. Then CH2CH2CH2OCOCH3, CH3CHCH2OCOCH3, CH3CH2CHOCOCH3 and CH3CH2CH2OCOCH2 are formed. The b-scission of CH3CHCH2O-

Fig. 1. Photoionization mass spectra of flame F1.67M taken at (a) 4.0 mm, (b) 5.0 mm, (c) F1.54 and (d) F1.67 at 5.0 mm (photo energy = 11.80 eV).

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W. Yu et al. / Combustion and Flame 159 (2012) 44–54 Table 2 List of species measured in flames. M/Z

IE (eV) measured literature

Formula

Species

Peak mole fraction (Xmax) and peak position (Pmax, mm) F1.67

15 26 28 29

9.83 11.37 10.51 8.28

30 32 39 40

60 66 68 70 86 88 100

10.88 10.83 8.73 9.76 10.38 8.16 9.61 9.76 9.29 10.25 10.20 10.42 10.20 9.61 9.08 8.92 9.72 9.92 10.60 8.58 8.43 8.82 10.04 10.16 9.57

102

10.00

41 42 44 46 50 52 54 56 58

a b c d

9.84 11.40 10.51 8.12 8.12 10.88 10.84 8.67 9.69 10.36 8.13 9.62 9.73 9.33 10.23 10.20 10.48 10.17 9.58 9.07 8.95 9.70 9.96 10.65 8.57 8.45 8.80 9.90 10.17 9.54d 9.50d 10.07

CH3 C2H2 C2H4 HCO C2H5 CH2O CH4O C3H3 C3H4 C3H4 C3H5 C2H2O C3H6 C2H4O C2H4O C2H6O C2H6O C4H2 C4H4 C4H6 C3H4O C3H6O C3H6O C2H4O2 C5H6 C4H4O C4H6O C4H6O2 C4H8O2 C5H8O2 C5H8O2 C5H10O2

Methyl Acetylene Ethylene Formyla Ethyl Formaldehyde Methanol Propargyl Allene Propyne Allyl Ketenea Propylene Ethenol Acetaldehyde Dimethyl ether Ethanol 1,3-butadiyne 1-buten-3-yne 1,3-butadiene Methylketene Acetone Propanal Acetic acida 1,3-cyclopentadienea 1,3-butadienala Ethylketenea MP2Dc Propanoic acid, 2-methyl-a MB2Dc MB1Dc MB

F1.54

F1.67M

Pmax

Xmax

Pmax

Xmax

Pmax

Xmax

6.0 7.0 5.5 5.5

4.14E03 1.37E02 1.74E02 1.40E04b

5.0 6.0 5.0 4.5

2.82E03 8.02E02 1.44E02 1.04E04b

6.0 7.0 5.5 5.5

2.46E03 7.99E03 1.25E02 1.90E05b

5.0 1.5 6.5 6.0 3.0 6.0 4.0

5.54E03 2.47E03 1.36E04 8.49E05 2.04E04 1.04E04 3.82E03b

4.5 1.5 5.5 5.0 1.5 5.0 3.5

4.74E03 2.13E03 5.48E05 6.45E05 1.42E04 6.92E05 2.60E03b

4.5 Fuel 6.5 6.0 3.5 6.0 4.5

8.51E03

6.0 3.5 3.5

2.13E05 3.92E04 7.23E05b

4.5 1.5 1.5

1.94E05 2.90E04 7.01E05b

6.5 3.5 3.0

2.11E05 3.13E04 6.24E05b

7.0 6.0 5.5 5.5 2.5

8.15E05 6.71E05 3.56E05 6.52E05 5.25E05b

6.0 5.0 4.5 4.5 2.5

4.96E05 4.12E05 2.44E05 4.74E05 4.09E05b

7.0 6.5 5.5 5.0 2.5

4.58E05 3.58E05 2.20E05 4.51E05 3.36E05b

3.5 2.5 2.0 2.0 4.5 3.5 2.0

1.90E05 5.75E06 3.54E04 3.41E04 1.66E03 1.80E04 2.64E04b

3.5 5.0 1.5 2.0 4.0 1.5 2.0

1.22E05 4.26E06 1.66E04 2.27E04 1.35E03 1.41E04 1.88E04b

3.0 3.5 3.0 2.5 5.0 3.5 2.5

1.14E05 3.72E06 1.56E04 2.07E04 1.07E03 1.13E04 1.38E04b

4.69E05 5.15E05 1.29E04 5.86E05 3.41E04b

Fuel

PICS of which was estimated. Total maximum mole fraction of the corresponding M/Z. Using abbreviated name in this paper: MP2D (CH2CHOCOCH3), MB1D (CH2CHCH2OCOCH3), MB2D (CH3CHCHOCOCH3). Calculated value.

Fig. 2. Possible intermediates formed in the first destruction steps of MB.

COCH3 yields propylene and CH3OCO. And CO2 is directly formed by further decomposition of CH3OCO. Reaction flux analysis for CO2 in Section 4.4 supports that this reaction is the dominant pathway for CO2 early production in MB oxidation. The final products of reactions in Fig. 2 are all listed in Table 2. It justifies the possibility of these fuel consumption pathways.

4.2. Temperature profiles The temperature profiles of three flames have similar tendencies as demonstrated in Fig. 3. For rich flames of MB fuel, the temperature decreases with the increase of equivalence ratio. The temperature profile of flame F1.67M has the smallest peak

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W. Yu et al. / Combustion and Flame 159 (2012) 44–54

value compared with those of other flames, but its peak position is similar with that of flame F1.67. The similar peak positions of temperature profiles together with the similar maximum mole fraction positions of most intermediates between flame F1.67M and flame F1.67 indicate the reaction zone of these two flames are close. 4.3. Mole fraction profiles Figures 4–6 show the experimental and predicted (by modified model) mole fraction profiles of the major species, C2AC4 unsaturated hydrocarbons, and aldehydes and ketones of three flames respectively.

Fig. 3. Temperature profiles of three flames.

4.3.1. Effect of equivalence ratio (flames F1.67 and F1.54) As shown in Fig. 4, the mole fraction profiles of major species shift downstream with the increases of equivalence ratio. MB and O2 are completely consumed before 5.0 mm and 9.0 mm in F1.54 flame, and these positions shift to 6.0 mm and 10.0 mm in

Fig. 4. Mole fraction profiles of major species in flames F1.67, F1.54 and F1.67M. Mole fraction profiles of CH4O are plotted with those of MB for flame F1.67M. Symbols are the measured and lines are the predicted with modified model.

W. Yu et al. / Combustion and Flame 159 (2012) 44–54

49

Fig. 5. Mole fraction profiles of C2AC4 unsaturated hydrocarbons in flames F1.67, F1.54 and F1.67M. Symbols are the measured and lines are the predicted with modified model.

F1.67 flame. This indicates that the flame front shifts downstream with the increase of equivalence ratio. The maximum mole fraction of CO increases by 12%, and that of CO2 decreases by 17% as / increases from 1.54 in F1.54 flame to 1.67 in F1.67 flame. The reduction of O2 in flame inhibits the following reactions.

H þ O2 ! O þ OH 



CO þ OH ! CO2 þ H

H2 gives the peak shaped mole fraction profiles in all flames. As / increases, the maximum mole fraction of H2 increases by 20% and that of H2O decreases by 7%. The peak positions of both species shift downstream. Comparing the mole fraction profiles of the intermediate (Figs. 5 and 6) between flame F1.67 and flame F1.54, it’s noticeable that the peak positions of them shift away from the burner surface as / increases. The unsaturated C2AC4 hydrocarbons (Fig. 5) are likely to be the soot precursors [49]. Their peak mole fractions increase significantly with the increase of /. The peak value of propargyl (C3H3), known as a dominant species for benzene formation through recombination of two propargyl radicals [50–52], is increased by 150%. The peak values of acetylene (C2H2), ethylene (C2H4), allene (C3H4-a), propyne (C3H4-p), allyl (C3H5), 1,3-butadiyne (C4H2), 1-buten-3-yne (C4H4), and 1,3-butadiene (C4H6)

increase by 20–60%. Aldehydes and ketones are the toxic species. Figure 6 shows that the peak values of them typically increase by 20–50% as the equivalence ratio increases. The peak value of 1,3-butadienal (CH2CHCHCO) is especially sensitive to equivalence ratio, increasing by 110%. 4.3.2. Effect of methanol addition at identical equivalence ratio (flames F1.67 and F1.67M) For methanol addition at specified equivalence ratio, methanol occupies part of composition in the fuel blend. However, there appears no variation in flame front position between flame F1.67M and flame F1.67 as shown in Fig. 4. When comparing CO2 mole fractions between the two flames, it is found that methanol addition lead to a decrease of CO2 near the burner surface (maximum discrepancy of 17% at 1.5 mm), larger than the initial C atom content difference (a discrepancy of 9%). And this discrepancy of CO2 mole fractions between the two flames disappeared after 9 mm. These indicate that the chemistry structure of methyl ester is more likely to produce CO2 early, and adding methanol in MB fuel will inhibit this behavior. In the burned gases, CO concentration decreases, H2O concentration increases and H2 concentration varies little with methanol addition. Figure 5 shows that the maximum mole fractions of C2AC4 unsaturated hydrocarbons decrease significantly (30–65%) with

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W. Yu et al. / Combustion and Flame 159 (2012) 44–54

Fig. 6. Mole fraction profiles of aldehydes and ketones in flames F1.67, F1.54 and F1.67M. Symbols are the measured and lines are the predicted with modified model.

methanol addition at the same equivalence ratio. For most oxygenates including methanol, the C atom is initially bound to an O atom and one O atom in the oxygenate removes one C atom from the pool of species which may form the soot. But for the ester structure ((O@C)O) oxygenate, the two O atoms are bonded to one C atom. Approximately half of the (O@C)O in MB are unbroken during the combustion, resulting in the direct formation of CO2 [12]. This means that two O atoms only remove one C atom from the pool of species which may form the soot. Thus, the CAO structure in methanol is more convenient for inhibiting the formation of soot precursors than (O@C)O in MB.

The hydrogen abstraction reactions from methanol by flame radicals H and OH play a significant role in the consumption of methanol, particularly under fuel rich conditions [53,54]:

R þ CH3 OH ! CH2 OH þ RH ðR ¼ H; OHÞ R þ CH3 OH ! CH3 O þ RH ðR ¼ H; OHÞ CH2 OH ! CH2 O þ H CH3 O ! CH2 O þ H

W. Yu et al. / Combustion and Flame 159 (2012) 44–54

These reaction pathways may explain the increase of peak mole fraction of formaldehyde (CH2O) (Fig. 6b) with methanol addition. Formyl is main product in the consumption of formaldehyde. The peak value of formyl (Fig. 6a) also increases with methanol addition. For other aldehydes and ketones in Fig. 6, methanol addition leads to the decrease in their maximum mole fraction at the same equivalence ratio. As methanol oxidation produces little high-carbon species, the decreasing of MB fraction and the decomposition of methanol to hydroxyl which favors aldehydes and ketones oxidation maybe the two reasons for the reduction of large aldehydes and ketones.

4.3.3. Effect of methanol addition at identical C/O ratio (flames F1.54 and F1.67M) The C/O ratio of flame F1.54 is the same as that of flame F1.67M (the value is 0.479), and the C/O ratio of flame F1.67 is 0.511. Comparison on the mole fraction profiles of products between flame F1.67M and flame F1.54 can show the influence of methanol addition on flame structure at the same C/O ratio. The mole fraction profiles of major products shift downstream with methanol addition (Fig. 4). It is found that the mole fraction of CO2 decreases dramatically in all detection positions (maximum discrepancy of 30% at 1.5 mm) with methanol addition at the same C/O ratio. This indicates that the production of CO2 is greatly inhibited at the same C/O ratio with methanol addition. In the burned gas, the mole fraction of CO varies little while those of H2 and H2O increase with methanol addition. No pronounced change in mole fraction profile of C2AC4 unsaturated hydrocarbons intermediates (Fig. 5) is found with methanol addition at same C/O ratio. The peak mole fraction of allene (C3H4-a) has the largest decrease (25%). Peak values of other soot precursors (C2H2, C2H4, C2H3, C3H4-p, C3H5, C4H2, C4H4, and C4H6) decrease by approximately 10–15%. Thus, it can be concluded that the C/O ratio plays a key role in the formation of soot precursors. Renard et al. [44] gave the similar conclusion by studying ethylene and ethylene/dimethoxymethane flames. For aldehydes and ketones intermediates (Fig. 6): the maximum mole fractions of formyl and formaldehyde increase with methanol addition, and the increments are larger compared with those at the same equivalence ratio; the peak value of ketene + propylene (CH2CO + C3H6) also increases but those of other aldehydes and ketones vary little. Figures 5 and 6 also show that the peak positions of intermediates in flame F1.67M move approximately 1.0 mm downstream compared with those in flame F1.54. This indicates that pure fuel burns rapidly than relatively rich blended fuel at the same C/O ratio. 4.3.4. Comparison between experimental results and kinetic modeling predictions In this section, the experimental mole fraction profiles are compared with simulation results, using the modified model discussed in Section 3. The predicted mole fraction profiles of major species (Fig. 4) in three flames have a good agreement with the experimental data. The concentration of CO2 is slightly underpredicted in calculation (maximum discrepancy of 12% in F1.67M). The peak positions of H2 shift 1.5 mm upstream. Actually, it is difficult to accurately simulate the mole fraction of H2, since all H-abstraction reactions by the H atom lead to the formation of this species. The modified model can also predict well the mole fraction profiles of C2AC4 unsaturated hydrocarbons (Fig. 5) in flames. In general, the peak position from calculations slightly shifts upstream (1–1.5 mm). But the peak value of most of these species shows a

51

reasonable agreement with that of experiment when experimental uncertainties in calibrating mole fraction and the model uncertainties in prediction are taken into account. The model slightly overpredicts that of allene (C3H4-a) by a factor of approximately 1.5 and underpredicts that of allyl (C3H5), 1,3-butadiyne (C4H2), 1-buten-3-yne (C4H4), and 1,3-butadiene (C4H6) by approximately 40%. The tendencies of mole fraction profile of these species among different flames are also in general correctly predicted by the model. In particular, it is noteworthy that the most important soot precursor, propargyl (C3H3), is well predicted. The measured mole fraction profiles of C3AC4 unsaturated hydrocarbons are also compared with simulation results obtained by using Dooley’s model, as shown in Fig. 7. 1,3-butadiyne (C4H2) and 1-buten-3-yne (C4H4) are not displayed in Fig. 7, since they are not included in Dooley’s model. Dooley’s model underpredicts the peak values of propargyl (C3H3) and propyne (C3H4-p) by approximately 60% and overpredicts those of allene (C3H4-a) and 1,3-butadiene (C4H6) by approximately a factor of 2.0. At the same C/O ratio, the predicted peak values of propargyl (C3H3), allene (C3H4-a) and propyne (C3H4-p) increase with methanol addition at variance with the experimental results. Besides, as / increases in MB flames, the predicted peak values of allyl (C3H5) and 1,3butadiene (C4H6) vary little while differences are clearly observed in the experimental results. Thus, Dooley’s model cannot well predict unsaturated C3AC4 hydrocarbons profiles. Comparison on predicted mole fraction profiles of other intermediate species between the modified and Dooley’s model are conducted and only little variation is presented. For aldehydes and ketones (Fig. 6), the modified model accurately predicts the mole fraction profiles of formyl (HCO), ketene + propylene (CH2CO + C3H6) and methylketene (CH3CHCO). Maximum mole fractions of formaldehyde (CH2O) and acetaldehyde (CH3CHO) are slightly overpredicted by a factor of approximately 1.5 and that of propanal + acetone (C3H6O) by a factor of approximately 2.5. Peak value of ethylketene (C2H5CHCO) gives the largest discrepancy with a factor of approximately 3.0. Peak value of 1,3-butadienal (CH2CHCHCO) can relatively well be predicted, but the predicted peak position shifts approximately 3 mm downstream. In conclusion, the modified model can predict the mole fractions of concerned species reasonably. 4.4. Reaction flux analysis for CO2 and sensitivity analysis for C3H3 The production of CO2 in methyl ester oxidation is different to that in other oxygenates. Reaction flux analysis for CO2 in flames can be helpful to provide insight into the chemical kinetic process of its formation during methyl ester oxidation. Analyses are made at the distances of 3.8, 4.2 and 4.6 mm from the burner surface for the flames F1.54, F1.67 and F1.67M, respectively. These distances correspond to the maximum rate of CO2 production. As shown in Fig. 8, CO2 is produced mainly via the decomposition of CH3OCO. This is true for the three flames due to the presence of the ester (O@C)O structure. When methanol is added to MB, the contribution of this reaction for CO2 production is reduced, while the contribution of CO that reacts with OH to produce CO2 is enhanced. This can be attributed to the linear reaction path (CH3OH ? CH2OH ? CH2O ? CHO ? CO ? CO2) in methanol oxidation [12]. The reaction of HCO with HO2 is also enhanced to a considerable extent for CO2 production with methanol addition. Combination of two propargyl (C3H3) radicals is regarded as the dominant path for benzene formation [50–52]. Thus, it is important to identify the reactions that affect the consumption and formation of C3H3. Local sensitivity analysis for C3H3 in the investigated flames is carried out using the modified model, as shown in Fig. 9. Sensitivity analysis is performed at the position

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W. Yu et al. / Combustion and Flame 159 (2012) 44–54

Fig. 7. Mole fraction profiles of some C3AC4 unsaturated hydrocarbons in flames F1.67, F1.54 and F1.67M. Symbols are the measured and lines are the predicted with Dooley’s model.

where maximum mole fraction of C3H3 is observed (4.8 mm for F1.54, 5.3 mm for F1.67 and 5.8 mm for F1.67M). With methanol added to MB, the effects of reactions H + O2@O + OH and C3H3 + O@CH2O + C2H on the consumption of C3H3 are reduced. This may be due to the factor that methanol oxidation increases

the concentrations of OH and CH2O, and then these reactions are inhibited. In contrast, the effects of reactions C3H3 + OH@C2H3 + HCO and C3H3 + OH@C2H4 + CO on the consumption of C3H3 are promoted due to the increase of OH radical. The obviously increasing sensitivity coefficient of reaction CH3 + HO2@CH3O + OH merits further analysis of the mechanism. For the reactions that effect the formation of C3H3 (CH2 + C2H2@C3H3 + H, C3H4-p + H@C3H3 + H2 and C3H6 + H@C3H5-a + H2), the positive sensitivity coefficients decrease with methanol addition.

5. Conclusions Flame species in premixed laminar methyl butanoate and methyl butanoate/methanol flames with different equivalence ratios and C/O ratios were investigated qualitatively and quantitatively using tunable synchrotron VUV photoionization and molecular beam mass spectrometry at 30 torr (4.0 kPa). The experimental data were compared with the predicted results using the modified mechanism. The main conclusions are summarized as follows:

Fig. 8. Reaction flux analyses for CO2 in flames F1.67, F1.54 and F1.67M.

1. With the increase of equivalence ratio in MB flames, peak mole fractions of soot precursors (C2AC4 unsaturated hydrocarbons), aldehydes and ketones increase significantly, and the flame front shifts downstream.

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Fig. 9. Sensitivity analyses for C3H3 in flames F1.67, F1.54 and F1.67M.

2. With methanol addition into the MB while maintaining the same equivalence ratio, peak mole fractions of soot precursors in flame decrease dramatically, peak mole fractions of formyl and formaldehyde increase, and peak mole fractions of other aldehydes and ketones species decrease. Methanol addition inhibits the formation of CO2 near the burner surface, which validates the early production of CO2 in MB oxidation. 3. For a specified C/O ratio, when methanol is added, production of CO2 is further inhibited compared with that at same equivalence ratio, and significant increases in peak mole fractions of formyl and formaldehyde are observed, and also positions of peak mole fraction for most intermediates move approximately 1.0 mm downstream. However, there are no pronounced variations (only slightly decreases) in peak mole fractions of soot precursors. Thus, C/O ratio plays a key role in the formation of soot precursors. 4. The modified MB model can reasonably predict the concentration profiles of major species and intermediates. Reaction flux analyses for CO2 indicate the contributions of reactions CO + OH@CO2 + H and HCO + HO2@CO2 + H + OH for CO2 production are enhanced with methanol addition. Sensitivity analyses for C3H3 demonstrate that the effects of reactions C3H3 + OH@C2H3 + HCO and C3H3 + OH@C2H4 + CO on C3H3 consumption are enhanced, while the sensitivity coefficients of the reactions related to C3H3 formation are decreased with methanol addition.

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Acknowledgments The authors thank National Synchrotron Radiation Laboratory of China for the experiment and Dr. James A. Miller of Sandia National Laboratory for sending C3H4 model. This work is supported by the Synchrotron Radiation Fund of Innovation Project of Ministry of Education (No. 20090604) and National Natural Science Foundation of China (50821064). Authors also express their thanks to the reviewers for their suggestions and comments in improving the paper.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2011.05.018.

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