The Effect of Methyl Pentanoate Addition on the Structure of a Non

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The effect of methyl pentanoate addition on the structure of a non-premixed counterflow n-heptane/O2 flame Denis A. Knyazkov, Tatyana A. Bolshova, Artem M. Dmitriev, Andrey G. Shmakov, and Oleg P. Korobeinichev Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03185 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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The Effect of Methyl Pentanoate Addition on the Structure of a Non-Premixed Counterflow n-Heptane/O2 Flame Denis A. Knyazkov1,2 *, Tatyana A. Bolshova1, Artem M. Dmitriev1,3, Andrey G. Shmakov1,2, Oleg P. Korobeinichev1 1

Voevodsky Institute of Chemical Kinetics and Combustion, Novosibirsk 630090, Russia 2

3

Tomsk State University, Tomsk 634050, Russia

Novosibirsk State University, Novosibirsk 630090, Russia

*Corresponding author: E-mail: [email protected]

ABSTRACT

The influence of methyl pentanoate (MP) addition to n-heptane on the species pool in a non-premixed counterflow flame fueled with n-heptane at atmospheric pressure has been investigated experimentally and numerically. Two non-premixed flames in counterflow configuration have been examined: (1) nheptane/Ar (5.3%/94.7%) vs. O2/Ar (24.1%/75.9%) and (2) n-heptane/MP/Ar (2.5%/2.5%/95%) vs. O2/Ar (19.6%/80.4%). Both flames had similar strain rates and stoichiometric mixture fractions to allow

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an adequate comparison of their structures. The mole fraction profiles of the reactants, major products and intermediates in both flames were measured using flame sampling molecular beam mass spectrometry. These experimental data were used for validation of a detailed chemical kinetic mechanism, which was proposed earlier for prediction of combustion characteristics of n-heptane/isooctane/toluene/MP mixtures. Addition of MP to n-heptane reduced the flame temperature and the peak mole fractions of many flame intermediates, responsible for formation of polycyclic aromatic hydrocarbons, specifically, of benzene, cyclopentadienyl, acetylene, propargyl, and vinylacetylene. Significant discrepancies between the calculated and measured mole fractions of cyclopentadienyl and benzene were found. A kinetic analysis of the reaction pathways resulting in formation of these intermediates in both flames and a sensitivity analysis of cyclopentadienyl and benzene were carried out to understand the origins of the observed discrepancies. The peak mole fractions of the major flame radicals (H, O, OH, CH3) were found to decrease with MP addition. The influence of MP addition on the relative contributions of the primary stages of n-heptane consumption is discussed.

Keywords: n-heptane, methyl pentanoate, flame-sampling molecular beam mass spectrometry, precursors of PAH, counterflow flame, first aromatic ring formation

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1. Introduction Blending traditional diesel fuels with biodiesel is considered as one of the most reasonable ways to cut down the consumption of petroleum-based diesel fuels and to reduce emissions of hazardous combustion products, such as soot and polycyclic aromatic hydrocarbons (PAH) from diesel combustion.1-9 Over the recent years, biodiesel based on methyl esters of fatty acids (FAME) derived from the transesterification of fats and oils with methanol is becoming a more attractive option to utilize it in blends with diesel fuels.10 This is basically due to the fact that it is not required to introduce any significant modifications in diesel engines to use the FAME-based biodiesel fuel, because its inherent characteristics are similar to those of petroleum-based diesel. Therefore, interest in fundamental research for FAME-based biodiesel and its surrogates has been increasing. Building robust chemical reaction mechanisms for combustion and oxidation of biodiesel surrogates and their mixtures with diesel surrogates provides a basis for understanding the combustion processes in engines fueled by biodiesel/diesel blends, as well as for development of advanced engines to ensure better control of the combustion processes. Significant efforts have been taken by different research groups to develop detailed chemical kinetic mechanisms for co-combustion of diesel surrogates and FAME-biodiesel surrogates, as well as of individual components relevant to biodiesel fuels.11-16 These mechanisms serve as starting points for development of skeletal kinetic models, which can be utilized in computational fluid dynamics software to simulate processes in the combustion chambers of engines.17-20 The capability of a detailed chemical kinetic mechanism to reproduce adequately various parameters of combustion largely determines the predictive capability of skeletal models built on the basis of the original mechanism. Therefore, the detailed kinetic models need to be validated thoroughly against numerous experimental data, to verify their robustness in various conditions.


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Kinetic reaction mechanisms for combustion of heavy methyl esters (the main components of FAMEbased biodiesel), however, are extremely large, and simulations using them are computationally quite expensive. Methyl pentanoate (MP, methyl valerate) can be considered as an optimal model of FAMEbased biodiesel molecules. The length of its alkyl chain is not so large (5 carbon atoms) as compared to that of biodiesel molecules (16 and more carbon atoms). Nevertheless, MP is commonly used for simulating the reactions of ester groups in large saturated fatty acid methyl esters.21 Smaller methyl esters like methyl acetate and methyl propionate are less suited for this purpose, because the carboxyl group significantly influences the decomposition chemistry of alkyl chain, which is too short in these esters.22 The oxidation and combustion chemistry of methyl pentanoate has been thoroughly studied previously by Hayes and Burgess Jr.,21 Dayma et al.23 and Korobeinichev et al.24 In our recent work, the effect of MP addition on the species pool in a fuel-rich premixed n-heptane/toluene flame stabilized on a flat flame burner was investigated using numerical simulation and flame sampling molecular beam mass spectrometry.25 It was found that addition of MP reduces mole fractions of many flame intermediates, which are important in PAH formation processes, specifically, of propargyl, cyclopentadienyl, benzene, vinylacetylene, and acetylene. It was also found that production of naphthalene (a typical representative of small PAH) is basically determined by the amount of toluene, which is the main precursor of naphthalene, in fresh mixture. Premixed burner-stabilized flames are commonly used for studying the chemical kinetics of combustion, because the flame propagation process is dominated by chemistry when the fuel and the oxidizer are premixed.26 However, in many combustion processes taking place in real combustion devices, the fuel and the oxidizer enter the reaction zone separately, i.e. the burning is controlled not only by chemistry but also by the diffusion rates of the components. The lack of information on the effect of addition of methyl esters on the species pool in the non-premixed flames of hydrocarbons 4 ACS Paragon Plus Environment

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motivated us to perform experimental and numerical investigations on this topic. In particular, the effect of methyl pentanoate addition to n-heptane flame on formation of flame intermediates, specifically those playing an important role in formation of PAH and soot, has been investigated in non-premixed counterflow flame configuration at atmospheric pressure using flame-sampling molecular-beam mass spectrometry for detection and quantification of the flame species. A detailed reaction mechanism for combustion of n-heptane/methyl pentanoate fuel mixture has been used to predict the mole fraction profiles of the flame species. This work aimed to validate this mechanism against new experimental data and to ascertain how MP addition influences the mole fractions of intermediates in n-heptane flame stabilized in conditions when fuel and oxidizer are not premixed. The detailed kinetic analysis of the model was performed to explain the observed tendencies.

2. Experimental details Figure 1 shows a schematic of the experimental setup used to study counterflow flames of prevaporized fuels. The counterflow burner is similar to that used in our previous work.27 The reactant nozzles of the burner were straight quartz tubes with internal diameter d = 7 mm, and the separation distance between them L = 7 mm. Argon was supplied into annular ducts surrounding fuel and oxidizer jets in order to guard the flame from ambient air. A mixture of prevaporized fuel and argon was introduced from the left nozzle. A mixture of argon and oxygen was used as oxidizer and introduced from the right nozzle. Oxygen and argon flows were introduced and controlled using mass flow controllers (MKS Instruments). The oxidizer was at room temperature (25 °C), while the fuel stream was heated, and its temperature was maintained at 125 °C to prevent condensation of liquid fuel vapors inside the burner. The fuel vapors were formed in a vaporizer. A syringe pump was used to feed the liquid fuel into the vaporizer. The fuel supply system is described in details in our recent work.

25

The vaporizer temperature was 5


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maintained at 90 °C, which was high enough to evaporate completely the liquid fuel in the vaporizer. The line between the burner and the vaporizer was kept at 125 °C by an electrical heater. T-type thermocouples were used to control the temperatures of the fuel stream inside the burner, the line and the vaporizer. Two non-premixed counterflow flames fueled by n-heptane and a mixture of n-heptane and methyl pentanoate (1:1 in mole basis), respectively, were studied in this work. The fuel mixture for the nheptane/MP flame was prepared by premixing liquids of n-heptane and methyl pentanoate in the ratio of 10 to 9 (by volume of liquids). In order to adequately assess how the replacement of 50% of n-heptane (base fuel) with MP changes the flame structure, both flames were stabilized in similar conditions. Two parameters were kept the same in both flames. They are the global strain rate and the stoichiometric mixture fraction. In both flames, the global strain rate was 165 c-1, as defined by the following formula28: a=2

VO  V 1 + F  L  VO

ρF ρO

 ,  

where ρ is density, V is gas velocity, and the subscripts O and F refer to oxidizer and fuel nozzles, respectively. As is known, the mole fraction of fuel in the fuel stream and oxygen in the oxidizer stream essentially determine the flame location and therefore the flame structure.29-33 In particular, if the region of large fuel concentration is shifted towards the oxidizer side of the flame, then the processes of formation of soot and its precursors are stimulated. Conversely, if the region of high oxygen concentration is located closer to the fuel side of the flame, these processes are inhibited. In combustion theory, the flame location is usually depicted by the stoichiometric mixture fraction Zst defined as  Y F W O 2 ν O2 Z st = 1 +  YO 2 W Fν F 

   

−1


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where YO2 is the mass fraction of O2 at the oxidizer nozzle, YF is the mass fraction of fuel at the fuel nozzle, WO2 and WF are the molecular weights of O2 and fuel, respectively, νO2 and νF are the stoichiometric coefficients of O2 and fuel, respectively.34 When Zst < 0.5, the flame is situated on the oxidizer side and hence there are favorable conditions for formation of soot precursors. The flames studied in this work had the same Zst equal to 0.318. Therefore, in the present study, when comparing the flame structures, the effect of Zst on the mole fractions of soot precursors was not considered. The value of Zst=0.318 was chosen from considerations that the flames are non-sooting (to keep the sampling probe from clogging with the combustion products) but have fairly high concentrations of small soot precursors to measure them as accurately as possible. The molar composition of the fuel and the oxidizer streams for the base n-heptane flame was as following: n-heptane/Ar = 0.053/0.947 (fuel), O2/Ar = 0.241/0.759 (oxidizer); and for the flame with methyl pentanoate addition: n-heptane/MP/Ar = 0.025/0.025/0.950 (fuel), and O2/Ar=0.196/0.804 (oxidizer). The velocities of the fuel and the oxidizer streams were chosen to satisfy the momentum balance, ρΟV2O=ρFV2F. In both flames, the total oxidizer flow (at 25 °C) at the oxidizer nozzle exit was 11.2 ml/s, and the total fuel flow at the fuel nozzle exit (at 125 °C) was 10.7 ml/s. A flame sampling molecular beam mass-spectrometric setup (MBMS) was used to measure species mole fractions in the flames (Fig. 1). Detailed description of the setup has been presented previously.3536

A quartz cone nozzle with 40° inner angle and 0.08 mm orifice diameter was used as a sampling

probe. The wall at the nozzle tip had a thickness of 0.08 mm. The setup was equipped with a system of soft ionization of sampled gas by electron impact. Spread in energies of ionizing electrons was about ±0.25 eV. A quadrupole mass-spectrometer with mass resolution m/∆m~100 was used for analysis of ions.


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Figure 1. A schematic view of the experimental setup for studying counterflow flames of pre-vaporized fuels. The burner was mounted on a three-axis table, which allowed moving the burner and adjusting its position relatively to the sampling probe with an accuracy of ±0.01 mm. The burner position was controlled using a cathetometer. Flame gases were sampled at flame periphery (at a distance of ~3 mm from the burner axis) as in our previous work27 in order to introduce minimal perturbations into the flame. In that study,27 we have provided experimental evidences of the fact that a counterflow flame has one-dimensional structure. The temperature profile were measured in this work using a Pt-Pt/Rh(10%) Π-shaped thermocouple made of wire 0.05 mm in diameter and coated with SiO2. The design of the thermocouple unit is similar to that described in ref 27. The total diameter of the thermocouple with the SiO2 layer was 0.08 mm. The thermocouple readings were corrected for radiative heat losses using the formulas available in the literature.37-38 The uncertainties in temperature measurements were about ± 40 К. To take into account 8 ACS Paragon Plus Environment

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the flame cooling effect of the probe, the temperature measurements were carried out with a thermocouple located ~ 0.2 mm down from the tip of the sampling probe.

3. Modeling The counterflow flame structure was simulated using the OPPDIF code.39 The calculations were carried out using the measured temperature profiles as input data. In this work, we used the chemical kinetic mechanism, which was employed in our recent study25 for simulating premixed burner-stabilized flame of n-heptane/toluene/MP mixture. This mechanism is available online in the CHEMKIN format in the supplemental material (Reaction_Mechanism.zip), so it is described only briefly below. In this mechanism, the combustion chemistry of n-heptane and smaller hydrocarbons including PAH precursors is represented similarly as in the detailed chemical kinetic mechanism proposed by Dirrenberger et al.40. The sub-mechanism for methyl pentanoate oxidation is similar to that proposed by Dayma et al.23. The resultant mechanism contains 350 species and 2472 reactions. Similarly to our previous work,27 the flame calculations were performed for both potential flow (nonzero radial velocity gradients at the nozzles’ exits, assigned by AOXI and AFUE keywords) and plug flow (AOXI=AFUE=0) boundary conditions. Chelliah et al.

41

have experimentally shown that the

actual velocity field is somewhere between the two approximations. Our calculations showed that the species mole fraction profiles in n-heptane and n-heptane/MP counterflow flames depend on the flow’s boundary conditions only slightly. Thus, in this work, we provide only the numerical results for boundary conditions corresponding to the plug flow. A multicomponent diffusion option was considered in the calculations.

4. Results and Discussion Species mole fractions in both flames were measured as functions of the distance from the fuel nozzle. To convert mass peak intensities to mole fractions, we used a procedure similar to that described in our 9 ACS Paragon Plus Environment

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recent work.25 For most of the species, the energies of ionizing electrons were the same as those used earlier.25 All the species, which were unambiguously identified and quantified in the flame, are listed in Table 1. In this table, their ionization energies, the corresponding energies of ionizing electrons and the calibration method are also given. The species corresponding to the mass peaks with m/z = 40 (allene+propyne), m/z=42 (propene+ketene), m/z = 56 (1-butene+2-butene), and m/z=70 (1-pentene+2pentene) were not separated because of a small difference between their ionization energies. Calibration coefficients (relative to argon) derived from direct calibration experiments with gas mixtures of known composition were determined for the reactants (n-C7H16, MP, O2), the major products (H2O, H2, CO, CO2) and some stable intermediates (methane, acetylene, ethylene, ethane, 1,3butadiene, benzene), similarly as was done earlier.25 Calibration coefficients for flame radicals (methyl, propargyl, cyclopentadienyl) and some stable intermediates (allene+propyne, propene+ketene, diacetylene, vinylacetylene, 1-butene+2-butene, 1pentene+2-pentene), to which the direct calibration could not be applied, were determined using the relative ionization cross-section (RICS) method described by Cool et al.42 and used in our previous work.25 The electron ionization cross sections at a given electron energy were calculated using the NIST Electron Impact Cross Section Database.43 For species for which data were not available in the NIST database (ketene, cyclopentadienyl, 1-pentene and 2-pentene), ionization cross sections were estimated by the method described in our recent work.44 We have analyzed the experimental errors in determining the absolute mole fractions of the flame species similarly as was done in our previous works,25, 44 so here we provide only the results of this analysis. The uncertainty of determining the absolute mole fractions of the flame products calibrated directly was estimated to be ±20% of the maximum mole fraction values. The uncertainty in the calibration coefficients determined using the RICS method was estimated to be at least about 50%. The resulting uncertainty of determining the mass peak signal intensity was different for various mass peaks, 10 ACS Paragon Plus Environment

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but for most species measured, this uncertainty did not exceed 15%. Although the overall uncertainty of determining the absolute mole fractions was fairly high, their relative change with changing the fuel from n-C7H16 to n-C7H16/MP mixture was measured more accurately. This uncertainty was estimated to

Temperature, K

2000 1800 1600 1400 1200 1000 800 600 400

Axial velocity, cm/s

be basically determined by the measurement error of the corresponding mass peaks.

40 30 20 10 0 -10 -20 -30 -40 -50 -60

A

n-heptane + MP

n-heptane

0

1

2

3

4

5

6

7

Distance from fueln-heptane+ nozzle, mm MP flame

B n-heptane flame

stagnation plane 2.8 mm

0

1 2 3 4 5 6 7 Distance from fuel nozzle, mm

Figure 2. A: Temperature profiles measured by a thermocouple in the flames fueled by the pure nheptane and by n-heptane/methyl pentanoate (1:1) blend. B: Calculated axial velocity profiles in the flames.

Mole fraction

0.06

0.30

0.030

n-Heptane

0.05

Methyl Pentanoate

0.025

0.25

0.020

0.03

0.015

0.15

0.02

0.010

0.10

0.01

0.005

0.05

0.00

0.000 1

2

3

4

5

6

7

0.00 0

Distance from fuel side, mm 0.10

1

2

3

4

5

6

0

7

1

2

3

0.05

H2O

0.08 0.06

0.03

0.04

0.04

0.02

0.02

0.02

0.01

0.00

0.00

0.00

5

6

7

0.012 0.010

CO

0.04

0.06

4

Distance from fuel side, mm


0.10

CO2

0.08

O2

0.20

0.04

0

Mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H2

0.008 0.006

0

1

2

3

4

5

6


7

0

1

2

3

4

5

6


7

0.004 0.002 0.000 0

1

2

3

4

5

6


7

0

1

2

3

4

5

6

7



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Figure 3. Mole fraction profiles of reactants and major stable products in the flames of pure n-heptane and of n-heptane/methyl pentanoate mixture. Symbols: experimental data; curves: simulation. Filled symbols and solid curves: flame fuelled by n-heptane. Open symbols and dashed curves: flame of nheptane/MP fuel mixture. Temperature profiles measured in the flames of pure n-heptane and n-heptane/MP fuel blend are shown in Fig. 2a. In Figure 2b, calculated axial velocity profiles in both flames are also plotted. As expected, both flames are located at the oxidizer side: maximum flame temperatures are to the right of the stagnation plane (a plane where axial velocity changes sign), which is at ~2.8 mm from the fuel side. In both flames the maximum flame temperature is reached at 3.3 mm from the fuel nozzle. The replacement of 50% of the base fuel (n-heptane) with MP leads to a decrease in maximum temperature by about 200 K (from ~1780 K in the flame of n-C7H16 to ~1580 K in n-C7H16/MP flame). The flame conditions can be considered as nearly adiabatic, so replacing a part of n-heptane with MP resulted in the flame temperature decrease. This fact is in qualitative agreement with that for ester-based biodiesel, the heating value is lower than that of standard diesel, see e.g. ref 45 and references therein. Therefore, the flame temperature decrease with changing fuel from n-C7H16 to n-C7H16/MP is expected to influence the chemical transformations in the flame. Figure 3 shows the measured and calculated mole fraction profiles of the reactants and major stable products in the flames with and without MP addition. As seen, the mole fraction profiles of these species are reproduced well by the mechanism. Only some discrepancies between the measurements and predictions are observed for H2O and CO peak mole fraction: however, these are probably due to experimental errors in determining the calibration coefficients for these species. It should be also noted that the experimental CO2 and H2O profiles are slightly broader than those predicted by the model. It is interesting to note that in our previous study of a counterflow non-premixed diffusion flame CH4/N2 O2/N2, ref 27, the experimental profiles of these species were also broader than those calculated using a 12 ACS Paragon Plus Environment

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detailed kinetic mechanism. Nevertheless, despite the indicated discrepancies, both the numerical and experimental data show that addition of MP leads to a decrease in the peak mole fraction of CO2, H2O,

Mole fraction (x 10-3)

CO and H2. 3.5

0.6 0.5 0.4

CH3 methyl

Mole fraction (x10-3)

2.5 2.0

C2H2 acetylene

8 6

1.5

0.2

25

10

CH4 methane

3.0

0.3

0.5

0.0

0.0 2

3

15

4

10

2

5

1

1.5 1.0 0.5 0.0 1

2

3

2

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

C2H6 ethane

2.0

3

4

1

0.1

3

0.03 0.02

0.01

0.01

0.00

0.00 1

2

3

4

1


Distance from fuel nozzle, mm

2

3

0.03

C5H5 cyclopentadienyl

0.02

x30 0.01

2

1

2

0.8

3

4


3

4

1

2

3

4

1.0

C4H8

0.8 0.6 0.4 0.2 0.0

1

2

3

4

1


0.8

2

3

4


0.08

C5H10

0.6

C6H6 benzene

0.06

0.4

0.04

0.2

0.02

0.0

0.00

propene+ ketene

1.2

C4H6 1,3-butadiene

4


4

0.0 1

C4H4 vinylacetylene

3

0.4

4 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

2

1.6

0.0 2

0.04

0.02

1

4

C3H4 propyne + allene

0.2

1

C4H2 diacetylene

3

2.0

0.3

0.05

0.03

2

0.4

C3H3 propargyl

4

0.04

0

0

4

C2H4 ethylene

20

1.0

0.1 1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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x10

0.00 1

2

3

4


1

2

3

4


Figure 4. Mole fraction profiles of intermediates in the flames of pure n-heptane and of n-heptane/MP mixture. Symbols: experimental data; curves: simulation. Filled symbols and solid curves: flame of nheptane. Open symbols and dashed curves: flame of n-heptane/MP fuel blend. In Fig. 4 the measured mole fraction profiles of intermediates detected in the flames are compared with those predicted by the model. In this work, we focused on the the species with a common formula CxHy, because they are expected to be closely related to formation of soot and its precursors. Therefore, the oxygenated intermediates including those formed in the reactions of MP decomposition are not 13 ACS Paragon Plus Environment

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discussed in this paper. As can be seen from Fig. 4, the mechanism, in general, reproduces qualitatively the mole fraction profiles of the majority of intermediates, which were measured in the flames. The calculated mole fraction profiles of methyl, methane, ethylene, ethane, propyne+allene, propene+ketene, 1,3-butadiene, 1-butene+2-butene are in good agreement with the experimental profiles in both flames. As seen, the model underpredicts the peak mole fractions of acetylene, propargyl, diacetylene, and vinylacelene, but overpredicts the peak mole fraction of C5H10 (1-pentene+2-pentene). The discrepancies between the measurements and simulations for these species do not exceed, however, a factor of 2-3 and could be explained by the experimental uncertainties. fuel

intermediates

C3H6 tC3H5 aC3H4 C3H3 l-C5H5 C6H6

C6H5

C5 H5

products

products

products

Figure 5. A schematic outline of the formation of cyclopentadienyl (C5H5) and benzene (C6H6) in nonpremixed counterflow flames of n-heptane with and without methyl pentanoate addition. Particular emphasis should be placed upon the mole fraction profiles of cyclopentadienyl (C5H5) and benzene (C6H6). As seen from Fig. 4, the model significantly (~ 30 and 10 times respectively for C5H5 and C6H6) underpredicts the peak mole fractions of these species in both flames. However, our measurment data on mole fractions of these intermediates, reported earlier in ref 25, have shown a good 14 ACS Paragon Plus Environment

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agreement with the simulations carried out in premixed burner-stabilized flames of n-heptane/toluene mixture with and without MP addition using the same kinetic model. We assume that the observed discrepancies between the model and measurement data in counterflow configuration can be assosiated with uncertainties not only of the kinetic parameters used in the model but also of transport data. In counterflow non-premixed flames, the transport parameters play a particularly important role, because the total combustion process depends on the mixing rate by diffusion. We are aware of the fact that the sensitivity to transport parameters can be of the same order of magnitude as the sensitivity to the reaction rates.46 However, quantifying the uncertainty of transport data when validating kinetic mechanisms represents a fairly complicated problem.47 Therefore, in this work we will provide possible explanations for the observed discrepancies in terms of uncertainties in kinetic parameters only. To determine the reactions, the rate constants of which need to be revised in the mechanism to better predict mole fractions of cyclopentadienyl and benzene in the flames, we carried out a kinetic analysis of the reaction routes leading to formation of these species in both flames (with and without MP) and calculated sensitivity coefficients of cyclopentadienyl and benzene. Figure 5 schematically shows formation of cyclopentadienyl and benzene in the counterflow flames at 2.5 mm from the fuel nozzle, i.e. where the mole fractions of these species are close to their peak values. In this schematic, primary intermediates formed from fuel molecules, like fuel radicals formed in Habstraction reactions and the products of successive decomposition of these radicals via β-scission reactions, are not shown, nor the minor reaction pathways. According to the mechanism used, no matter whether the methyl pentanoate is added to n-heptane or not, formation of 6- or 5- membered rings proceeds via a chain of transformations, typical of large alkanes.48 In both flames, the chain starts from propene (C3H6). Sequential H atom abstraction from propene, then from allyl radical (tC3H5, CH2=ĊCH3) and allene (aC3H4, CH2=C=CH2) produces propargyl radical (C3H3). The propargyl is the main precursor of the so-called “first aromatic ring”, benzene (C6H6) and phenyl radical (C6H5), due to the 15 ACS Paragon Plus Environment

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reactions of C3H3 recombination (C3H3+C3H3↔C6H6 and C3H3+C3H3↔C6H5). Hereinafter, the reactions are written in a form in which they are given in the mechanism used. The main pathway of cyclopentadienyl formation is the isomerisation reaction of the open chain form, 1-vinylpropargyl (lC5H5, Ċ≡C-CH=CH-CH3), which is formed in the reaction of addition of acetylene (C2H2) to the propargyl l-C5H5↔C2H2+C3H3. It is interesting to note that in the premixed flame fuelled by nheptane/toluene25 the contribution of the above mentioned reaction pathways to formation of benzene and cyclopentadienyl was negligible as compared to the reactions, in which they were formed virtually directly from the toluene (fuel component). Benzene, phenyl and cyclopentadienyl radicals are interconvertible to each other basically due to the following reactions: C6H6+H↔C6H5+H2, C6H5+O↔C5H5+CO. A sensitivity analysis was performed to make reasonable suggestions for updating the mechanisms in order to provide better agreement between the simulations and the experimental data for C5H5 and C6H6. The sensitivity coefficients of C5H5 and C6H6 are shown in Fig. 6. They were calculated in pure nheptane flame 2.5 mm from the fuel nozzle, where these species have nearly maximum mole fractions. C 6H 6

C 5H 5

C5H6C2H6(+M)

C3H6+H=>C3H7

-0.8

C3H6+H=>C3H7

-0.4

0

0.4

0.8

-1

-0.5

0

0.5

1

Figure 6. Sensitivity coefficients of cyclopentadienyl (left) and benzene (right) in non-premixed counterflow flames of n-heptane at 2.5 mm from the fuel nozzle. 16 ACS Paragon Plus Environment

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As seen, the formation (consumption) of cyclopentadienyl is most sensitive to the rate constants of the reaction of 1-vinylpropargyl formation (l-C5H5↔C2H2+C3H3), the reaction of H abstraction from propene

(C3H6+H↔tC3H5+H2),

the

reaction

of

cyclopentadiene

(C5H6)

formation

from

cyclopentadienyl (C5H5+H→C5H6) and the reaction of H abstraction from propargyl to form C3H2 (HĊ=C=ĊH) biradical (C3H3+H↔C3H2+H2). The latter reaction, as well as the abovementioned reaction of allyl formation, also exhibits high sensitivity coefficients for benzene. Besides, as expected, benzene formation is also very sensitive to the rate constant of the reaction of two propargyl radicals recombination (C3H3+C3H3↔C6H6). Therefore, the rate constants of the abovementioned reactions with high sensitivity coefficients have to be subjected to revision for refining the mechanism. Although we did not set a goal in this work to update the chemical kinetic mechanism, we compared the rate constants of some of these reactions used in the mechanism with some literature data and concluded that the mechanism update is indeed needed. As an example, in Fig. 7 the rate constant of the self-combination reaction of C3H3 is compared with those taken from the recently developed AramcoMech 2.0 mechanism49, and the mechanism proposed by Slavinskaya and Frank.50 As seen, in the mechanism used in this work the rate constant is estimated as temperature independent. In the flame zone with the temperature of 1300 K, where propargyl attains peak concentration, the rate constant used in the mechanisms from the refs 49 and 50 is 2.5 and 1.5 times, respectively, higher than the rate constant used in the mechanism from our work. Therefore, the use of the increased rate coefficient of C3H3 recombitation could probably improve agreement between the simulation and the experimental data on benzene mole fraction. It is most likely that using the rate constant proposed recently by da Silva51 for 1-vinylpropargyl formation from acetylene addition to the propargyl radical could also improve the performance of the mechanism.


Energy & Fuels

14 T=1300K

log(k, cm3mol -1s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Slavinskaya-Frank mech.

13 this work

12

Aramco mech. 2.0

11

C3H3+C3H3−>C6H6

10 9 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

1000K/T Figure 7. The rate constants for C3H3 recombination used in this work, in the Aramco 2.0 mechanism,50 and in the mechanism proposed by Slavinskaya and Frank.51 Another possible reason of the observed discrepancy betweeen predicted and measured peak mole fraction of benzene may be related to the fact that the kinetic mechanism used in this work does not involve fulvene. Fulvene (C5H4CH2) is an isomer of benzene, which can be formed also in recombination of two propargyl radicals.52, 53 The isomerization of fulvene to benzene can potentially provide an additional pathway of benzene formation, and probably improve the performance of the mechanism. Thus, we complemented the mechanism with the following 7 reactions involving fulvene, which were suggested in the AramcoMech 2.049: C3H3+C3H3↔C5H4CH2, C5H4CH2↔C6H5+H, C6H6+H↔C5H4CH2+H,

C3H3+tC3H5↔C5H4CH2+H+H,

iC4H5+C2H2↔C5H4CH2+H,

nC4H5+C2H2↔C5H4CH2+H, and C5H4CH2↔C6H6. The rate constants for these reactions were taken as those proposed in AramcoMech 2.0 for atmospheric-pressure conditions. The calculations of the flame structure with the modified mechanism showed, however, a negligibly low peak mole fraction of fulvene (10-20) and no notable effect on the mole fractions of benzene and other intermediates as compared to the results predicted with the original mechanism. 18 ACS Paragon Plus Environment

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In order to quantify the effect of MP addition to the fresh mixture on the peak mole fraction of the flame intermediates, similarly as it was done in our recent work,25 we calculated the parameter R=[(X0Xmp)/X0]·100%, where X is the maximum mole fraction of the species in the flame. Subscripts 0 and MP refer to the flame without and with MP, respectively. This parameter was calculated using both the experimental data (Rexp) and calculation data (Rcalc). It reflects the percentage decrease in the peak mole fraction of an intermediate species with MP addition. Table 2 lists the Rcalc and Rexp values for CxHy intermediates, as determined from the mole fraction profiles shown in Fig. 4. The errors indicated in the table were determined on the basis of the measurement accuracy of the intensity of the corresponding mass peak in two flames. As it can be seen from Table 2, the Rcalc values for all intermediates listed are positive, indicating that the addition of MP decreases their peak mole fractions. The experimental data demonstrate the same tendency for all species with the exception of methyl and ethane, but it should be noted that the accuracy of Rexp for these two intermediates is too low to make any valuable conclusions from this observation. The Rexp values agree well (within the experimental uncertainties) with corresponding Rcalc values for many species (see the last column in Table 2), and only for acetylene and ethylene Rcalc are slightly higher than Rexp, whereas for C4H8 the calculated effect of MP addition is weaker than that observed experimentally. As can be seen from Table 2, for most species the R-values are higher than 50%, i.e. the effect of replacement of half n-heptane with MP is not additive, as it was observed in our previous study of the effect of MP on premixed burner-stabilized n-heptane/toluene flame.25 In ref 25, we could not observe any influence of the MP addition (45% in mole basis of total fuel content) on the postflame temperature, because the flame conditions were non-adiabatic, and the R-values of the species considered as soot precursors, were very close to 45%, which was associated basically with replacing sooting “nheptane/toluene” with “non-sooting” methyl pentanoate. In the counterflow configuration, the peak 19 ACS Paragon Plus Environment

Energy & Fuels

mole fractions of the CxHy intermediates are reached in the region between 2 and 3 mm from the fuel nozzle (Fig. 4), where the temperature of the flames differs insignificantly (Fig. 2). Nevertheless, as mentioned above, the maximum temperature of the counterflow flames differs by about 200 K, which can have significant impact on peak mole fractions of the soot precursors and, therefore, can be reflected in higher R-values than 50%. This can be attributed to changes in the mole fractions of the main flame radicals (H, O, CH3, OH) with addition of MP, because the most of elementary processes responsible for formation of the CxHy species proceed with participation of these radicals. 0.005 CH3 with MP CH3 w/o MP

0.004

Mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H, with MP H, w/o MP OH, with MP

0.003

OH, w/o MP O, with MP O, w/o MP

0.002

0.001

0.000 2

3

4

5

6


Figure 8. Calculated mole fraction profiles of H, OH and O radicals in counterflow flames of n-heptane with and without MP addition. Indeed, in the premixed flame of n-heptane/toluene with and without MP addition25, the pool of the main flame radicals was virtually unchanged, whereas in the counterflow flame studied in this work the mole fractions of H, O, OH, CH3 radicals change essentially. In Figure 8, the calculated mole fraction profiles of these radicals in counterflow flames with and without MP addition are represented. As expected, the peak mole fractions of these radicals are reduced (by a factor equal to slightly less than 2) when switching from pure n-heptane to n-heptane/MP flame.


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The decrease in the mole fraction of the main flame radicals with MP addition influences not only the formation of the abovementioned intermediates but is expected to have an impact on the primary stages of n-heptane oxidation. To ascertain this, we analyzed the relative contributions of these reactions to the total rate of consumption of the n-heptane in both flames. The integrated contribution αi of ith reaction to the total rate of n-heptane consumption was calculated as follows: L

∫ ω ( x)dx ' i

αi =

× 100% ,

0 L

∫ω

' total

( x ) dx

0

where ω'i (x) is the local rate of the i-th reaction of n-heptane consumption, ω'total(x) is the local total rate of n-heptane consumption, x is the distance from the fuel nozzle (integration was carried out over the entire flame zone). A similar approach was used in our previous works.25, 44 The idea to use the ratios of integrated rates to the total rate of consumption or formation of a particular flame species was originally proposed by Held and Dryer.54

C7H16+CH3=>CH4+C7H15 with MPE without MPE

C7H16+OH=>H2O+C7H15

C7H16+H=>H2+C7H15

C7H16=>CH3+C6H13

C7H16=>C2H5+C5H11

C7H16=>C3H7+C4H9

0

10

20

30

40

50

60

α (%)

Figure 9. Relative integrated rate contributions (α) to the consumption of n-heptane in the flame with and without methyl pentanoate addition.


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In Figure 9, a diagram with αi values for the most valuable reactions of n-heptane consumption in both flames is shown. These reactions are unimolecular reactions of thermal decomposition forming two different radicals, and the reactions of H abstraction from n-heptane molecule by H, OH and CH3 attack. In this diagram, the contributions of the reactions of H abstraction from the fuel by H, OH and CH3 attack forming different C7H15 radicals are combined, respectively. It is noteworthy that the integrated contribution of H abstraction reactions by interactions with H radicals is significantly higher in both flames than that by OH and CH3 attack. This can be due to two reasons. Firstly, the position of the peak mole fraction of H radicals is closer to the fuel inlet, as compared to that of OH (Fig. 8) and, therefore, the H concentration is higher than that of OH in the area of the maximum consumption rate of n-heptane (~2.5 mm from the fuel inlet). Secondly, H reactivity is significantly higher than that of CH3 and, therefore, despite the fact that the CH3 mole fraction at 2.5 mm from the fuel inlet is significantly higher that of H (Fig.7), the H-abstraction from the fuel molecule initiated by H attack predominate. As seen, the unimolecular reactions of n-heptane thermal decomposition forming two radicals become less important in the flame with MP due to the flame temperature decrease. The relative contribution of the reactions of n-heptane with H radicals is nearly the same in both flames. It is interesting that similarly to premixed flames,25 the contribution of the reactions with OH radicals increases when MP is added. However, in the counterflow flame, as mentioned, these reactions are of minor importance for nheptane consumption.

5. Conclusions Two non-premixed counterflow flames (n-heptane/Ar vs. O2/Ar and n-heptane/MP/Ar vs. O2/Ar) at atmospheric pressure have been examined experimentally and numerically, in order to ascertain the effect of methyl pentanoate addition (1:1 with n-heptane in mole basis) to n-heptane on the flame 22 ACS Paragon Plus Environment

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species pool. This work was focused on the effect of MP on the mole fractions of intermediates, which are precursors of PAH, to get insight into the processes responsible for reduction of PAH when biodiesel is added to diesel in combustion devices. The mole fraction profiles of various stable and labile species including reactants, the major products and intermediate C1-C6 hydrocarbons were measured using flame sampling molecular beam mass spectrometry with soft electron ionization. The simulations were carried out using a detailed chemical kinetic mechanism, which was used earlier25. In general, satisfactory agreement between the new experimental data on the structure of the flames and the numerical modeling was observed with the exception that the kinetic model significantly underpredicted the measured peak mole fractions of cyclopentadienyl and benzene. The observed discrepancies were believed to be associated with inaccuracies in both the transport data and the rate coefficients of the reactions playing an important role in formation and consumption of these species. A kinetic analysis of the reaction routes leading to formation of these species in both flames and a sensitivity analysis of cyclopentadienyl and benzene has shown that refining the rate constants of some reactions, specifically, the reactions of benzene formation from C3H3 self-combination and 1vinylpropargyl formation from acetylene addition to the propargyl could improve the performance of the mechanism. Despite the abovementioned discrepancies, the mechanism used satisfactorily predicted the effect of MP addition on many CxHy intermediates in the counterflow flame: addition of MP was found to reduce the mole fractions of the intermediates, that play an important role in formation of PAH, in particular, acetylene, diacetylene, propargyl, benzene, cyclopentadienyl, and vinylacetylene. The peak mole fractions of these and some other hydrocarbon intermediates decreased by more than 50% as compared to the flame of pure n-heptane. This indicates that the effect of replacement of half n-heptane with MP was not additive, which was associated generally with the reduction in the flame temperature and the mole fractions of the major flame radicals (H, O, OH, CH3) with MP addition. 23 ACS Paragon Plus Environment

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The effect of MP addition on relative contributions of primary stages of n-heptane consumption was analyzed. n-Heptane decomposition occurred generally via reactions of H abstraction from the fuel molecule by H attack forming different fuel radicals (C7H15) and via unimolecular thermal decomposition forming two radicals. The MP addition was not found to have any significant influence on the contribution of the H abstraction reactions. However, the thermal decomposition of n-heptane became less important in the flame with MP due to the flame temperature decrease. The experimental data obtained in this work may be used for testing other kinetic mechanisms not mentioned in this paper that simulate the combustion of n-heptane and its mixtures with methyl pentanoate.

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Table 1. Species measured in non-premixed counterflow flames of n-heptane and n-heptane/MP mixture. IE: ionization energy; E: energy of ionizing electrons. m/z Formula

Species name

IE (eV)

E (eV)

Calibration method

2

H2

Hydrogen

15.43

16.65

Direct

15

CH3

Methyl radical

9.84

13.2

RICS vs. CH4

16

CH4

Methane

12.71

14.35

Direct

18

H2O

Water

12.62

15.4

Direct

26

C 2H2

Acetylene

11.41

12.3

Direct

28

CO

Carbon monoxide

14.01

15.4

Direct

28

C 2H4

Ethylene

10.53

12.3

Direct

30

C 2H6

Ethane

11.52

12.3

Direct

32

O2

Oxygen

12.07

14.35

Direct

39

C 3H3

Propargyl radical

8.68

12.3

RICS vs O2

40

C 3H4

Allene

10.22

12.3

RICS vs O2, not separated

40

C 3H4

Propyne

10.48

40

Ar

Argon

15.76

16.2

Direct

42

C 3H6

Propene

9.74

12.3

RICS vs CO2, not separated

42

CH2CO

Ketene

9.6

44

CO2

Carbon dioxide

13.80

15.4

Direct

50

C 4H2

Diacetylene

10.18

12.3

RICS vs CO2

52

C 4H4

Vinylacetylene

9.63

12.3

RICS vs CO2

54

C 4H6

1,3-Butadiene

9.23

12.3

Direct

56

C 4H8

1-butene

9.86

12.3

RICS vs C4H6, not separated

56

C 4H8

2-butene

9.38

65

C 5H5

Cyclopentadienyl

8.4

12.3

RICS vs C4H6

70

C5H10

1-pentene

9.49

12.3

RICS vs C4H6,

78

C 6H6

Benzene

9.37

12.3

Direct

100

n-C7H16

n-Heptane

9.93

12.3

Direct

116

C6H12O2

Methyl pentanoate

10.4

12.3

Direct 28


Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 2. R values for CxHy intermediates calculated as described in text. Index “exp” correspond to experimental data, and “calc” is for the results of calculations. Agreement between Species

Rcalc, % Rexp, % Rexp and Rcalc

Methyl

23

-33±34

-

Methane

36

34±13

+

Acetylene

67

49±12

-

Ethylene

57

38±6

-

Ethane

17

-23±17

-

Propargyl

64

67±11

+

Propyne+allene

53

41±12

+

Propene+ketene

42

50±17

+

Diacetylene

78

78±10

+

Vinylacetylene

74

62±12

+

1,3-butadiene

53

41±20

+

C 4H8

12

44±9

-

Cyclopentadienyl

68

58±13

+

C5H10

58

54±14

+

Benzene

82

67±15

+


The Effect of Methyl Pentanoate Addition on the Structure of a Non

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