Effect of Addition of Methyl Hexanoate and Ethyl Pentanoate on the

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Effect of Addition of Methyl Hexanoate and Ethyl Pentanoate on the Structure of Premixed n‑Heptane/Toluene/O2/Ar Flame Ksenia N. Osipova,†,‡ Tatyana A. Bolshova,† Oleg P. Korobeinichev,† Leonid V. Kuibida,†,‡ and Andrey G. Shmakov*,†,‡,§ †

Voevodsky Institute of Chemical Kinetics and Combustion, Novosibirsk 630090, Russia Novosibirsk State University, Novosibirsk 630090, Russia § Siberian State University of Geosystems and Technologies, Novosibirsk 630108, Russia ‡

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S Supporting Information *

ABSTRACT: The effect of adding isomeric esters ethyl pentanoate (EPe) and methyl hexanoate (MHe) to a model diesel fuel on the chemical structure of its flame has been investigated. A 7/3 (vol) n-heptane/toluene mixture was used as a model diesel fuel. The concentration of EPe or MHe additives to the n-heptane/toluene mixture was 50 vol %. The studied flames were stabilized over a flat burner at a pressure of 1 atm. The structures of three rich (φ = 1.6) flames of n-heptane/toluene/O2/Ar, MHe/n-heptane/toluene/O2/Ar, and EPe/n-heptane/toluene/O2/Ar mixtures were studied. The flame structure data were obtained using the method of molecular beam mass spectrometry (MBMS) with soft electron-impact ionization, gas chromatography−mass spectrometry (GCMS), and microthermocouples. In the flames studied, concentration profiles of more than 30 species were identified and measured, and, in particular, peak concentrations of heavy polycyclic aromatic compounds, the main soot precursors, were measured at a pressure of 1 atm for the first time by MBMS and GCMS techniques. The structures of the n-heptane/toluene and MHe/n-heptane/toluene flames were modeled using published chemical-kinetic mechanisms. For the EPe/n-heptane/toluene mixture, such a mechanism was first developed in this work. Comparison of the experimental and calculated species concentration profiles shows that they are mostly in satisfactory agreement. The key reactions involved in the formation of soot precursors were determined by analyzing the chemical-kinetic mechanisms of oxidation of the investigated fuel blends. The results of the study demonstrate that the applied mechanisms need to be considerably modified to provide an adequate description of the formation and consumption of heavy polycyclic aromatic hydrocarbons.

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INTRODUCTION Petroleum-derived hydrocarbons are the main fuel for conventional internal combustion engines. However, their use, without proper emission control technologies, could have an impact on the environment because of emissions of soot, NOx, and other criteria emissions. Therefore, the study of alternative fuels and their combustion characteristics is of importance. Such fuels include oxygenated hydrocarbons such as alcohols and biodiesel. Biodiesel contains fatty acid methyl and ethyl esters derived from vegetable or animal fats and corresponding alcohols. Because the physical properties of biodiesel, unlike those of alcohols, are close to the physical characteristics of diesel fuel, biodiesel can be used without significant modification of the existing engines, at lower blend levels. The effect of adding biodiesel to petroleum fuel on the performance of diesel engines has been widely investigated. The effect of biodiesel addition on the performance of diesel engine and emissions has been the subject of works.1−5 The low heat of combustion of biodiesel and the presence of oxygen atoms in its molecules increase the break specific fuel consumption. The low volatility and high viscosity of biodiesel slow down its injection, resulting in an increase in the concentration of heavy organic compounds in emissions. The presence of oxygen atoms in biodiesel molecules makes the combustion process more complete, which results in reduction of CO emissions; however, © XXXX American Chemical Society

it causes a rise in the combustion temperature and hence the NOx concentration. Because biodiesel contains no aromatic compounds, its addition reduces soot emissions. However, formation of CO and NOx depends strongly on the operating mode:5 at a low engine speed, CO concentration is lower for blends with biodiesel additives, and at high speed, it is lower for pure diesel. At low speeds, NOx concentration decreases with the increasing fraction of biodiesel in the blend, and at high speeds, the opposite happens. Similar results were obtained by Zhu et al.6 Lapuerta et al.7 showed that at a sufficiently high temperature in the chamber, the addition of biodiesel reduces emission of CO and unburned hydrocarbons. At low temperatures, however, concentration of CO and unburned hydrocarbons is higher for blends with biodiesel. Menkiel et al.8 revealed that with good dispersion, even more viscous biodiesel has a “lower” ignition delay time than diesel fuel. According to available data, the effect of biodiesel addition to a fuel blend depends on many different factors, including the engine design, and therefore requires detailed examination. Nevertheless, the physical and chemical effects of adding biodiesel to fuels can be considered separately. Received: January 17, 2019 Revised: March 28, 2019

A

DOI: 10.1021/acs.energyfuels.9b00166 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

but the contribution of this pathway to its consumption is not more than 10% even in rich flames. Significant progress has been made in the development of detailed combustion mechanisms for hydrocarbons and their blends. The n-heptane/toluene system is most widely used as a model diesel fuel.18−23 First, the oxidation of each of the components of this system was studied. Curran et al.24 developed a detailed mechanism for n-heptane, which included the high-temperature oxidation of the fuel in unimolecular decomposition reactions, with subsequent decomposition and isomerization of the formed alkyl radicals and the lowtemperature reactions of hydrogen abstraction and oxygen addition to the resulting alkyl radicals. Subsequent studies were focused on autoignition of n-heptane and toluene mixtures.21 The main point is the difference in reactivity of the components of this fuel blend. Hellier et al.25 showed that in n-heptane/ toluene mixture at low temperature, toluene reacts with radicals involved in the oxidation of n-heptane, but it is not completely oxidized to the final combustion products. A detailed mechanism for the oxidation of an iso-octane/n-heptane/ toluene mixture was proposed by Li et al.26 and validated against measured ignition delays at different pressures, temperatures, and equivalence ratios. Xu et al.27 compared primary pathways for the n-heptane and toluene conversion. For nheptane, the main conversion pathways are the molecule decomposition to shorter alkyl radicals, and for toluene− hydrogen abstraction, these are reactions involving small radicals. The main way of polycyclic aromatic hydrocarbons (PAH) formation is toluene conversion. The authors showed that naphthalene is formed by recombination of benzyl and propargyl radicals, C6H5CH2 + C3H3C10H8 + 2H, and benzyl radicals, in turn, are formed from toluene, according to the reaction C6H5CH3 + HC6H5CH2 + H2. Andrae et al.19 proposed a mechanism for oxidation of n-heptane, with addition of iso-octane and toluene. This mechanism describes the interaction of each fuel component with the radicals formed in oxidation of the other components. The developed mechanism correctly predicts that toluene addition leads to increase of the ignition delay time. Cross-reactions of n-heptane, iso-octane, and toluene oxidation were also considered in ref 28. The authors suggested that if the molecules have similar structures, crossoxidation reactions should be necessarily included in the mechanism. If the molecules have significantly different structures, these reactions are not very important. This suggestion was experimentally confirmed for n-heptane/toluene mixtures. A detailed mechanism for the oxidation of n-heptane/ toluene/iso-octane fuel blends was also developed in ref 29 and validated against flame speeds of iso-octane, n-heptane, and toluene and their mixtures. The proposed mechanism satisfactorily described experimental data, except for fuel blends with a high equivalence ratio. Many studies are focused on esters oxidation chemistry. Westbrook et al.30 developed a mechanism for oxidation of methyl acetate, ethyl formate, methyl formate, and ethyl acetate. The rate constants of primary hydrogen abstraction reactions and reactions of unimolecular decomposition were evaluated using the principle functional groups similarity. Dayma et al.31 proposed a mechanism for ethyl pentanoate oxidation-based experiments in a jet-stirred reactor and flame propagation speed. Knyazkov et al.32 studied the chemical structure of a lowpressure ethyl pentanoate flame and compared the experimental data with the results of simulation using the mechanism from ref 31. It was found that decomposition kinetics of the fuel primary

The chemical effects of biodiesel addition are related to the structure of biodiesel molecules, that is, the lengths of molecules and the number of double bonds. Zhu et al.9 found that the longer alkyl chain leads to an increase in the amount of unburned hydrocarbons and CO concentration and reduces emissions of NOx. Because the relative oxygen content in the biodiesel molecule decreases with the increase of the chain length, soot concentration becomes higher. The higher number of double bonds leads to an increase in the ignition delay time and adiabatic flame temperature and, hence, to higher NOx emissions. In addition, the presence of double bonds enhances unsaturated C2hydrocarbon formation and, thus, formation of C3H3 which plays the key role in the soot mechanism. Longer alcohol moiety (ethyl/methyl) causes increase of the CO and reduction of the NOx emissions. The same trend was observed in refs.10,11 The study of autoignition of binary blends (n-heptane and C7 methyl esters) showed that at low temperatures, more CO forms in the presence of saturated esters.12 Saturated esters have higher reactivity because of a longer alkyl chain, which is available for transfer of the hydrogen atom. However, the ignition delay time for blends with saturated esters is longer than that for pure n-heptane. The alcohol moiety in the biodiesel molecule is supposed to slow down formation of the transition state, which makes the fuel consumption process slower. Biodiesel contains both methyl and ethyl esters of fatty acids. Ethanol used for production of ethyl esters is derived from plant biomass, whereas production of methyl esters requires toxic methanol. This makes the use of ethyl esters more environmentally friendly.13 However, comparative studies of the oxidation chemistry of isomeric methyl and ethyl esters are very limited. Most works were focused on esters with a short alkyl chain, while real biodiesel molecules have a much higher molecular weight.14−16 Osswald et al.14 studied combustion of methyl acetate and ethyl formate. The concentration of C2 and C4 hydrocarbons is higher in ethyl formate flames than in methyl acetate flames, so the concentration of benzene is also higher in ethyl formate flames. There are also other differences between the flames of these esters: in methyl acetate flames, the abstraction of the hydrogen atom from the methoxy group results in formation of formaldehyde, and in the flame ethyl formate, the abstraction of the hydrogen atom from the ethoxy group leads to acetaldehyde formation. Yang et al.15 studied esters such as methyl butanoate, methyl isobutanoate, and ethyl propanoate. They showed that the decomposition of the fuel molecules occurs mainly by two pathways: radical reactions and unimolecular decomposition reactions. Methyl esters form a four-center transition state which decomposes into methanol and ketene, whereas ethyl esters form a six-center transition state which subsequently decomposes into acid and ethylene. Metcalfe et al.16 compared ignition delay times for methyl butanoate and ethyl propanoate. Ethyl esters ignite faster because of the existence of a pathway for its unimolecular decomposition, with an acid and ethylene formed. Ethyl propanoate has a shorter ignition delay time because of high reactivity of propanoic acid and ethylene. Dmitriev et al.17 performed a comparative analysis of the chemical structure of ethyl butanoate and methyl pentanoate flames. In stoichiometric flames, radical reactions are predominant in fuel consumption. Ethyl butanoate also undergoes unimolecular decomposition. Contribution of this reaction increases with the increase of the equivalence ratio. In fuel-rich conditions, reaction kinetics of butyric acid determines the further oxidation of ethyl butanoate. Methyl butanoate also undergoes unimolecular decomposition, B


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Energy & Fuels Table 1. Studied Flames mole fraction (% by volume) fuel

φ

n-C7H16/C7H8 EPe/n-C7H16/C7H8 MHe/n-C7H16/C7H8

1.6 1.6 1.6

[ester]

[n-C7H16]

[C7H8]

[O2]

[Ar]

1.6 1.6

2.1 1.1 1.1

1.3 0.7 0.7

21.6 21.6 21.6

75 75 75

radicals forming via β-bond scission requires particular attention. The same observation was made in ref 33. The authors developed the oxidation mechanism of methyl hexanoate and methyl butanoate. The results confirmed that the energy of C−H bonds at the β-position in the alkyl chain mainly determines the oxidation process. Oxidation of methyl pentanoate and methyl hexanoate was studied experimentally and numerically by Korobeinichev et al.34 The basis of the developed mechanism was the oxidation mechanism for C0−C4 hydrocarbons. Most high-temperature oxidation reactions were generated by adding one or two CH2 groups to methyl butanoate molecules and its radicals. This mechanism was validated against the data on the stoichiometric and rich flame structure. The main drawback of this mechanism is the absence of low-temperature oxidation reactions. In work of An et al.,35 a skeletal mechanism for the oxidation of biodiesel blend surrogates containing methyl decanoate, methyl-9-decenoate, and n-heptane was proposed and tested against ignition delay data for this blend. There is only one paper36 focused on the effect of adding methyl pentanoate to diesel surrogate fuel (nheptane/toluene). The structure of rich flame was studied by molecular beam mass spectrometry (MBMS) at atmospheric pressure. Using the published mechanisms, the authors developed a model for oxidation of a methyl pentanoate/nheptane/toluene mixture and validated it against the experimental data. Pathways analysis of PAH formation showed naphthalene to be mainly formed in reaction 2C5H5 ↔ C10H8 + H2. The main reaction of cyclopentadienyl radicals’ production is C6H5O ↔ C5H5 + CO. Phenoxy radicals are formed in reactions of phenyl radicals with oxygen: C6H5 + O2 ↔ C6H5O + O. The main source of phenyl radicals is benzyl radicals formed from toluene: C6H5CH3 + H ↔ C6H5CH2 + H2. Thus, the sequence of reactions resulting in naphthalene formation starts from toluene. Replacing part of toluene with methyl pentanoate leads to reduction of its initial fraction and thus to decrease of naphthalene concentration. It is also important to note that replacing part of the fuel with methyl pentanoate does not lead to changes in the concentration of most radicals in the flame. Thus, there are no experimental and theoretical comparative studies of the effect of adding isomeric esters on combustion of hydrocarbon fuels and their oxidation chemistry. The purpose of this work was to study the effect of adding isomeric [methyl hexanoate (C7H14O2, MHe) and ethyl pentanoate (C7H14O2, EPe)] esters to model diesel fuel by an experimental and numerical study of premixed fuel-rich flames stabilized over a flat burner at atmospheric pressure.

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and the purity of the gas componentsO2 and Arwas 99.99%. Table 1 shows the composition of the flames. The flames have the equivalence ratio φ = 1.6 because combustion in a diesel engine occurs mainly in a diffusion mode, in which the flame has fuel-rich zones where intense formation of PAH and soot occurs. Flames were stabilized over a Botha−Spalding flat burner37 which was a perforated brass disk 16 mm in diameter and 3 mm thick. The holes had a diameter of 0.5 mm and were spaced 0.7 mm apart; the centers of the holes were located at the nodes of a hexagonal grid. The disk was mounted in a brass casing fitted with a cooling jacket connected to a thermostat maintaining a constant burner temperature of 95 °C. A premixed mixture of liquid fuel vapor, O2, and Ar was supplied to the burner from a vaporizer. The vaporizer was a pyrex vessel equipped with an electric heater and a thermocouple for temperature measurement. The vaporizer was filled with metal beads 3 mm in diameter to provide uniform vaporization of the liquid fuel supplied to it. During the experiments, the vaporizer temperature was kept constant at 90 °C with an accuracy of ±2 °C using temperature controller REX-C100 (Berme Instruments Inc., China) with feedback from the thermocouple mounted in the vaporizer. Liquid fuel was supplied to the vaporizer through a metal capillary using a syringe pump. The liquid fuel flow rate was controlled by a stepper motor. Argon forced fuel vapor from an evaporator. The fuel and argon vapor flow coming from vaporizer were mixed with oxygen and argon and entered the burner. The flow rates of oxygen and argon were set by mass flow controllers (MKS Instruments, USA). The burner was mounted in and moved vertically with a micrometric screw mechanism relative to the probe or thermocouple with an accuracy of ±0.01 mm. The accuracy of the burner movement was monitored with a cathetometer. The chemical structure of studied flames was measured by MBMS with soft electron ionization. A detailed description of the setup and the sampling system is elsewhere.38−41 To reduce the spread of ionizing electron energy, the ion source was equipped with a system for compensating the cathode voltage drop. This provides an electron energy spread of ±0.25 eV.42 In the present study, the ionizing electron energy values were within 10.9−16.65 eV. The sensitivity of the mass spectrometer is not high enough to measure heavy aromatic hydrocarbons, such as naphthalene, indene, styrene, and ethyl benzene. Therefore, we used gas chromatography− mass spectrometry (GCMS) to measure the mole fractions of these aromatic hydrocarbons and to refine the mass spectrometry data for styrene. Gas samples from the flame were extracted in a 1.8 L glass flask using a probe similar to that used for the mass spectrometric analysis. It makes sampling conditions to be close to the conditions in the experiments with mass spectrometry. The distance between the probe orifice and the burner was chosen according to the position of the maximum concentration of intermediates in the flame. According to the results of calculations and mass spectrometric measurements, this distance is equal to 1.2 and 1.4 mm. Before sampling, the flask was vacuumed to a pressure of about 100 Pa with a pump. Then, the pumping line was closed, and the flask was filled through the probe with sampled gas until the pressure of 48 kPa was reached within 14−15 min. After that, the flask was filled with argon to atmospheric pressure. Then, combustion products were washed from the flask walls by using 4 mL of liquid hexane. Hexane was used as solvent because it dissolves all products formed in the flame and is not present in the expected combustion products. An Agilent HP 6890/5973N gas chromatography−mass spectrometer was used for species identification. These species were

EXPERIMENTAL DETAILS

An n-heptane/toluene (n-C7H16/C7H8) mixture was used as a model diesel fuel. The liquid components were mixed in a ratio of 7/3 by volume. Combustion of model diesel fuels with a similar composition was studied in refs.21,27 In experiments with esters, half the volume of the n-heptane/toluene mixture was replaced with the corresponding estermethyl hexanoate (C7H14O2, MHe) or ethyl pentanoate (C7H14O2, EPe). The purity of the liquid components was 99.0%, C


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Energy & Fuels

Table 2. Identified Flame Species, Their Ionization Potentials, the Ionizing Electron Energy in the Measurement, and the Calibration Methoda m/z

species

species name

ionization energy (eV)

energy of ionizing electrons (eV)

calibration method

2 15 16 18 26 28 28 30 30 32 39 40 42 44 44 44 50 52 54 56 65 66 78 92 94 100 102 104 106 116 128 130 130

H2 CH3 CH4 H2O C2H2 C2H4 CO CH2O C2H6 O2 C3H3 Ar C3H6 + CH2CO C2H4O C3H8 CO2 C4H2 C4H4 C4H6 C4H8 C5H5 C5H6 C6H6 C7H8 C6H5OH n-C7H16 C4H9COOH C6H5C2H3 C8H10 C9H8 C10H8 MHe EPe

hydrogen methyl radical methane water acetylene ethylene carbon monoxide formaldehyde ethane oxygen propargyl radical argon propene + ketene acetaldehyde propane carbon dioxide diacetylene vinylacetylene 1,2-butadiene + 1,3-butadiene 1-butene + 2-butene cyclopentadienyl cyclopentadiene benzene toluene phenol n-heptane pentanoic acid styrene ethyl benzene indene naphthalene methyl hexanoate ethyl pentanoate

15.43 9.84 12.71 12.62 11.41 10.53 14.01 10.88 11.52 12.07 8.68 15.76 9.74/9.6 10.23 10.94 13.8 10.18 9.63 9.23 9.86/9.38 8.4 8.57 9.37 8.8 8.5 9.93 10.53 8.46

16.65 13.2 14.35 15.4 12.3 12.3 14.35 11.5 12.3 14.35 12.3 16.2 12.3 10.9 12.3 15.4 12.3 12.3 12.3 12.3 12.3 14.35 12.3 15.4 14.35 15.4 12.3 16.65

11.0

12.3 14.35

H-element balance RICS(CH4) direct O-element balance RICS(C2H4) direct direct RICS(CO) RICS(C2H4) direct RICS(CO2) direct RICS(CO2) RICS(CO2) RICS(CO2) C-element balance RICS(C4H6) RICS(C4H6) direct RICS(C4H6) RICS(C7H8) RICS(C7H8) RICS(C7H8) direct RICS(C7H8) direct direct RICS(C7H8) GCMSb GCMSb GCMSb direct direct

a

RICS-relative ionization cross section. bGCMS.

identified using standard databases of mass spectra at ionizing electron energies of 70 eV, and quantitative measurement of the mole fraction of the compounds was performed using direct calibrations. The flame temperature profiles were measured with microthermocouples. In the experiments, a thermocouple was placed at a distance of 0.2 mm from the probe orifice, and the burner was moved relative to the fixed probe and thermocouple. Thus, the flame temperature distribution was measured as a function of the distance between the burner and the probe. The temperature profile is a necessary boundary condition for the flame structure simulation, so that the temperature profiles were measured in the presence of the probe in order to consider the thermal and aerodynamic perturbations of the flame by the probe. This approach of taking probe perturbations into account proved to be effective and was described in previous papers.36,43,44 In this work, we used thin S-type thermocouples (Pt−Pt 10% Rh) made of a 0.02 mm diameter wire. The thermocouple surface was coated with silicon oxide (SiO2) to avoid catalytic chemical reactions on its surface. The total diameter of the thermocouple with the coating was 0.04 mm. The spatial resolution was about three thermocouple diameters, that is, about 0.1 mm. To consider the radiative heat loss by the thermocouple, we used the approach described in refs.45,46 The error of the temperature measurement was estimated at 40± K. For most species, MBMS with soft electron ionization enables researchers to consider the contribution to their mass spectra from the signals of fragment ions formed from high molecular weight compounds; for some species, however, this is difficult to do. Therefore,

the contributions from the initial fuel components with the highest molecular weight to the mass peaks of the lighter compounds are considered via calibration for vapors of fuel mixtures in an inert diluent (under conditions without decomposition and oxidation of the fuel components), in which the peak intensities in the fuel mass spectrum are measured as a function of ionizing electron energy. To separate the mass peak intensity profile for certain species, we need to subtract the contribution of heavier compounds. The calculation of the mole fraction of species i from the corresponding mass peak intensity was performed using the formula

Ii = Si(T )Xi Here, I is the intensity of the signal corresponding to the mass peak of species i, S is the calibration coefficient, and X is the mole fraction of the species. The following methods were applied to determine the calibration coefficients Si: (1) direct calibration; (2) mass balance calibration (for H2, CO, CO2, and H2O); and (3) the relative ionization cross section (RICS) method47 in the case where the first two methods were not applicable. In the third method, the values of the ionization cross sections for most species were taken from the database.48 If the value of the cross section for particular species is not present in the database, it was estimated with the methods described in refs.49,50 As mentioned above, the mole fractions of certain intermediate species with a high molecular weight cannot be determined by MBMS because of the insufficient sensitivity of this method. In this case, samples were collected in a vacuumed flask and then analyzed by D


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Energy & Fuels GCMS as described above. The mole fractions of such species were calculated from the area of corresponding chromatographic peaks using the results of direct calibration. The identified flame species, their ionization potentials, the ionizing electron energy in the measurement, and the calibration method are given in Table 2. The detailed experimental and simulation data for the profiles of temperature and species concentration in the studied flames are given in the Supporting Information in the table format. The mole fractions for the reactants (MHe/EPe/n-C7H16/C7H8/ O2/Ar) were determined within ±10%; for CO, CO2, H2, and H2O within ±15%, and for intermediate species, up to a factor of 2 (1.5 for GCMS). The gas-dynamic perturbations of the flame by the probe were considered by shifting the experimentally measured concentration profiles by the amount calculated with the formula39

involving 345 species. All three mechanisms are available on the web page.53 Simulation data were also used to analyze the reaction pathways of the major intermediate products of fuel conversion. The integrated rate of reaction i was calculated from the formula ωi =

1/2

where d is the diameter of the sampling probe orifice, Q is the volume flow rate of the gas through the probe orifice, S is the area of the probe orifice, and V is the linear velocity of the gas coming on the probe. In this work, the maximum shift was 0.2 mm.

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COMPUTER SIMULATION Numerical simulation of the structure of premixed burnerstabilized was performed using the PREMIX code from the CHEMKIN-II package.51 The mechanisms used for the numerical simulation and the number of species and reactions in the flames are given in Table 3. Table 3. Mechanisms Used for Modeling of the Flame Structure of the Studied Flames fuel mixture

number of species

number of reactions

303 362 345

2213 2495 2445

∞

ωi′ dt =

∫0

∞

ωi′ dx v

where ω′i is the local rate of reaction i (mol/cm3s), v is the local gas velocity (cm/s), and x is the distance from the burner surface (integration was over the entire flame zone). The integrated reaction rates were used because in a multicomponent fuel mixture, individual components have consumption zones with different widths, so that it is impossible to choose a certain characteristic distance from the burner surface. It should be noted that many mechanism reduction methods have been successfully developed and demonstrated in the literature. The manual removal approach such as peak concentration analysis,54 systematic reduction models such as directed relations graph (DRG),55−57 DRG with error propagation (DRGEP),58 DRG-aided sensitivity analysis,59 and DRGEP and sensitivity analysis60 are often used to identify and eliminate unimportant species. Genetic algorithm methodologies for mechanism reduction and optimization against kinetic targets have also been described and successfully applied.61−70 In the future, we plan to use the genetic algorithm to optimize the developed oxidation mechanism for EPe/nC7H16/C7H8 ternary blend and refine the rate constants of the most important reactions.

i Q yz zz Z = d· jjj k S·V {

n-C7H16/C7H8 MHe/n-C7H16/C7H8 EPe/n-C7H16/C7H8

∫0

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refs

RESULTS AND DISCUSSIONS Figure 1 shows the concentration profiles of the reactants (EPe, MHe, n-C7H16, C7H8, and O2), the major combustion products (H2O, CO, and CO2), the temperature profiles, and the results of numerical simulation of the mole fraction profiles. The temperature in the zone of combustion products is almost the same for all flames. In addition, the width of the combustion zone for all flames is approximately the same and is 1.7 mm. For all species listed above, the results of the simulation and experiment are consistent with each other within the experimental error. The mole fraction profiles of the intermediate species identified in the flames are given in Figure 2. For most species, the applied mechanisms53 correctly predict the maximum position and the shape of the mole fraction profile, so that there is generally satisfactory agreement between the experimental and numerical results. However, for some species, there are significant discrepancies. In addition, ester additives have different effects on the formation of some intermediates. In flames with added esters, the measured hydrogen concentration in the reaction zone is 2.5−3 times higher than in the n-heptane/toluene flame. However, the simulation results do not describe this trend and predict similar mole fraction profiles of hydrogen for all the three flames in both the reaction zone and the zone of combustion products. The observed differences between the calculated and measured H2 concentration profiles far exceed the measurement error. The measured maximum mole fractions of the methyl radical in all the three flames are approximately the same. Although the simulation predicts significantly higher values than those observed experimentally, the simulated peak concentrations

29 29,33 29,52

The mechanism for the n-C7H16/C7H8 blend oxidation was developed previously.29 The mechanism for the MHe/n-C7H16/ C7H8 blend oxidation consists of two sub-mechanisms, the first one describing oxidation of the n-C7H16/C7H8 mixture29 and the second one dealing with oxidation of methyl hexanoate and lighter methyl esters.33 As mentioned above, a mechanism for the oxidation of the EPe/n-C7H16/C7H8 fuel mixture is not available in the literature. Therefore, the mechanism for this mixture was composed of two sub-mechanisms describing the oxidation of individual components. The oxidation reactions for the n-C7H16/C7H8 blend were also taken from ref 29, and the oxidation reactions of ethyl pentanoate and lighter esters were taken from ref 52. The procedure for composing the mechanism is described below. The oxidation mechanism for the EPe/n-C7H16/C7H8 ternary blend was combined from sub-mechanisms for the n-heptane/ toluene mixture and ethyl pentanoate. The resulting mechanism contained more than 2700 reactions. It was reduced using the SENKIN module51 with a 0-dimensional closed homogeneous reactor model. Reactions with a negligible contribution to the consumption of the EPe/n-C7H16/C7H8 fuel blend were eliminated from the mechanism. The procedure of reaction elimination was based on the results of calculating the reaction rate profiles over time in the temperature range 1000−1700 K. The initial composition of the blend was the same as in the experiments. The final mechanism contained 2445 reactions E


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Energy & Fuels

Figure 1. Mole fraction profiles of reactants, major combustion products, and temperature in the flame of pure n-heptane/toluene fuel mixture (at the top), in the flame of n-heptane/toluene/EPe blend (in the middle) and in the flame of n-heptane/toluene/MHe blend (at the bottom). Symbols: experimental data; curves: modeling.

are also almost identical. Because the three flames have similar temperature profiles, their propagation rates are also almost the same. As is known from the literature, the flame speed is determined mainly by radical formation processes.71 This may explain the fact that the maximum mole fraction of methyl radicals is almost the same for all the three flames. According to the experimental data, the maximum mole fraction of methane in the flame with added ethyl pentanoate is slightly lower than in the other two flames. The simulated methane concentration profiles in all three flames almost coincide. However, the simulation for all three mechanisms predicts the presence of methane in the zone of combustion products, which is inconsistent with the experimental data. For C2H2, there is a slight increase in the maximum mole fraction in the flame with methyl hexanoate. The simulation predicts that the acetylene mole fraction in the zone of combustion products is only 1.5−2 times lower than the peak concentration in all three flames, which disagrees with the experimental data. In other words, the models incorrectly describe conversion of acetylene to the final products. As for C2H4, in the experiment, the addition of ethyl pentanoate increases its maximum concentration only slightly,

and in the modeling, this effect is more pronounced. The reason for this will be analyzed below. The addition of methyl hexanoate has little effect on the measured and calculated C2H4 profiles. In the experiment, addition of esters does not lead to any significant changes in the mole fraction of CH2O within the experimental error. According to the simulation results, the mole fraction of formaldehyde is ∼1.5 times higher in the flame with added methyl hexanoate than in the other flames. This is due to the existence of an additional pathway for the formation of formaldehyde as a result of hydrogen abstraction from the methoxy group of methyl hexanoate. Similar results were previously obtained for rich flames of methyl acetate.72 According to the experimental data, addition of methyl hexanoate leads to an increase in the mole fraction of C2H6 in the flame, and the simulation also correctly reproduces this trend. According to the simulation data, addition of ethyl pentanoate also leads to a slight increase in the mole fraction of C2H6; however, in the experiment, this effect is within the experimental error. In the case of propargyl radicals, the experimental results show that the addition of esters does not lead to any significant F


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Energy & Fuels

Figure 2. Mole fraction profiles of intermediates in the flame of the pure n-heptane/toluene fuel mixture (opened circles and solid lines), in the flame of n-heptane/toluene/EPe blend (gray triangles and dashed lines) and in the flame of n-heptane/toluene/MHe blend (dark squares and dotted lines). Symbols: experimental data (measured by MBMS technique); curves: modeling.

for the C2H2 concentration profiles) and indicates a drawback of the mechanisms used. In flames with added esters, the maximum concentration of C3H4 and C3H6 + CH2CO is significantly reduced according to both the simulation and experimental results; however, these changes are within the experimental error.

changes in its maximum mole fraction. At the same time, the simulated maximum concentration of C3H3 is lower in flames with added esters than in the n-heptane/toluene flame; however, in the experiment, this difference is within the experimental error. It can also be seen that according to the simulation, the concentration of propargyl radicals remains significant in the final flame zone, which is inconsistent with the experiment (as G


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Energy & Fuels

combustible mixture with added ester; toluene, in turn, is the main precursor of benzene. A similar situation is observed for phenol: the addition of esters leads to a decrease in its maximum concentration in the flames. Simulation data qualitatively reproduce the observed trend. Pentanoic acid is observed only in the flame with added ethyl pentanoate. The simulation satisfactorily describes the experimentally measured concentration profile of this acid. Hexanoic acid in the flame with added methyl hexanoate is formed in such small quantities that its concentration is below the detection limits of the methods used. The addition of both methyl hexanoate and ethyl pentanoate leads to a decrease in the maximum mole fraction of styrene in the experiment. GCMS measurements also indicate a decrease in the maximum concentration of styrene in flames with added esters. The simulation qualitatively reproduces this trend. Because, as mentioned above, the concentration profiles of heavy aromatics, such as ethyl benzene, naphthalene, and indene are difficult to measure with a mass spectrometer, their mole fractions were determined by GCMS. In the experiment, the addition of esters leads to a decrease in the maximum concentration of ethyl benzene and this trend is satisfactorily described by the detailed kinetic mechanisms used (Figure 3). In the case of indene and naphthalene, addition of esters also leads to a decrease in the maximum concentration of these compounds in both the experiment and simulation. However, it should be noted that the calculated maximum concentration of indene and naphthalene decreases approximately four times, whereas according to the experimental data, their concentration decreases only about two-to threefold. This difference requires a detailed analysis because naphthalene and indene are key compounds in the soot formation process. Formation of Acetaldehyde. One of the most significant differences in the structure between the flames studied is a significant increase in the mole fraction of acetaldehyde during combustion of the mixture with added ethyl pentanoate. The main pathways for the formation of acetaldehyde in the flames determined by analyzing the rate of formation and consumption of this species are given in Figure 4. In all flames, acetaldehyde is formed from ethenol, which, in turn, is obtained from ethylene. The source of ethylene is all the initial components of the fuel mixture. The addition of EPe gives rise to additional pathways for acetaldehyde to be formed from the intermediate products of conversion of this ester. The presence of such additional pathways explains the increase in the peak concentration of acetaldehyde with the addition of EPe to the n-heptane/toluene mixture. Formation of the Pentanoic Acid. The next significant difference between the flames is the presence of the pentanoic acid as an intermediate product of ethyl pentanoate conversion. As mentioned above, the molecular structure of ethyl esters makes the unimolecular decomposition reaction possible.73 This reaction explains the presence of pentanoic acid as an important intermediate product of ethyl pentanoate conversion and also explains the slight increase in the maximum concentration of C2H4 which is clearly observed in the experiment and is particularly pronounced in the calculations. The products of this reaction can lead to an increase in the burning rate (or reduction in the ignition delay) of fuel mixtures with added ethyl pentanoate, especially in fuel-rich flames at high temperature. This is consistent with available papers.73 The structure of the

According to the experiment, addition of ethyl pentanoate leads to an increase in the maximum concentration of acetaldehyde (C2H4O) in the flame, which is in qualitative agreement with the simulation results. However, if we refer to quantitative characteristics, the experimental results show a double increase in the mole fraction of acetaldehyde in the flame with added ethyl pentanoate, whereas the simulation predicts a tenfold increase. This suggests that the part of reactions describing formation and consumption of acetaldehyde in the flame with added ethyl pentanoate requires substantial refinement. According to the experiment, the addition of ethyl pentanoate also leads to an increase in the mole fraction of C3H8; however, in the calculations, a noticeable increase in the mole fraction of propane occurs in the flame with added methyl hexanoate. The effect observed in the experiment is within the measurement accuracy. As for the model, the reactions involving propane also should be refined. According to the experimental results, addition of ethyl pentanoate leads to reduction in the maximum concentration of C4H2 in the flame, but it is within the experimental error. The simulation significantly underestimates the peak concentration of this species. For C4H4, the experimental results show slight reduction in its mole fraction with addition of esters. The simulation results are only in qualitative agreement with the experiment. As in the case of C4H2, the models predict a significantly underestimated maximum mole fraction of C4H4. As for the mole fraction profiles of C4H6, the experimental data show that the addition of ethyl pentanoate leads to onethird reduction in the peak concentration of this species, compared to the n-heptane/toluene flame. This trend is correctly described by the model. In the flame with added methyl hexanoate, the experimental mole fraction of C4H6 has almost no change. However, the simulation predicts an increase in the maximum concentration of C4H6 in the flame with added methyl hexanoate and the presence of C4H6 in the zone of combustion products. Because this effect is observed only for this flame, the reactions of C4H6 in the block of oxidation reactions of methyl esters require further refinement. According to the experiment, addition of ethyl pentanoate reduces the maximum concentration of the cyclopentadienyl radical, and addition of methyl hexanoate increases it. However, according to the simulation, the addition of both esters leads to reduction in the C5H5 mole fraction, compared to the flame without the additive. For cyclopentadiene, the experimental data show a slight increase in its mole fraction in the flames with added esters. The simulation predicts the opposite trend for the maximum concentration of this species with addition of esters. This disagreement can also be due to the large experimental error in measuring the concentration of C5H6. The measured peak concentration of benzene in the flame with added methyl hexanoate is found to be slightly higher than that in the other two flames, but this difference is within the experimental error. In addition, methyl hexanoate is not an aromatic compound, and the proportion of the only aromatic compound in the mixturetoluenedecreases in the flame with the addition of ester, so that this fact is not indicative. According to the simulation, addition of esters leads to twofold reduction in the mole fraction of C6H6, which is just associated with twofold reduction in the proportion of toluene in the H


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Energy & Fuels C7H8 + HC6H6 + CH3

(2) Phenol C6H6 + HC6H5 + H 2

C6H5 + O2 C6H5O + O C6H5O + H( +M)C6H5OH( +M)

(3) Ethyl benzene C7H8 + HC7H 7 + H 2 C7H 7 + CH3C6H5C2H5

(4) Styrene C6H5C2H5 + HC6H5C2H4 + H 2

C6H5C2H4C6H5C2H3 + H

(5) Cyclopentadiene C6H5OC5H5 + CO C5H5 + HC5H6

As can be seen, the main source for formation of these species is toluene. Therefore, the decrease in the mole fraction of toluene in the fuel blend leads to a decrease in the mole fraction of the above species. Formation of Naphthalene and Indene. As already mentioned above, for indene and naphthalene, the reaction mechanism is not able to quantitatively predict the reduction in the maximum mole fraction (Figure 3). According to the simulation, their maximum concentration in the flame with added esters decreases approximately fourfold, whereas the experimentally obtained value of this decrease is only about twoto threefold. Analysis of the pathways of these species’ formation showed that the key reaction involved in the formation of naphthalene and indene is the recombination of two cyclopentadienyl radicals: 2C5H5C10H8 + H2. The main source of cyclopentadienyl radical is toluene. Therefore, a twofold decrease in the toluene fraction in the initial fuel mixture will cause a fourfold decrease in the mole fraction of naphthalene, according to the law of mass action. In this case, the mole fraction of indene will also decrease fourfold because indene is formed from naphthalene. Because, in the experiment, the addition of esters to the n-heptane/toluene mixture leads only to a two-to threefold decrease in the mole fraction of naphthalene and indene, the validity of the rate constant of the above reaction in the mechanism is questionable. Therefore, to modify the mechanism and to obtain better agreement with the experimental results, we decreased the rate constant pre-exponential of the reaction 2C5H5C10H8 + H2 by 3.5 orders of magnitude. Initially, the value of the rate constant of the reaction 2C5H5C10H8 + H2 was taken from ref 74, where there is no information about its experimental measurements, and this provides ground for varying it in the developed mechanism. The rate constants for the other important reactions involved in the formation of indene were also modified to improve the quantitative agreement between the modeling and experimental concentration profiles of the species. The list of modified rate constants is given in Table 4. In the first column are given the reactions which mainly contribute to naphthalene and indene formation. In the modified mechanism, only the pre-exponential

Figure 3. Mole fraction profiles of intermediates in the flame of the pure n-heptane/toluene fuel mixture (opened circles and solid lines), in the flame of n-heptane/toluene/EPe blend (grey triangles and dashed lines), and in the flame of n-heptane/toluene/MHe blend (dark squares and dotted lines). Symbols: experimental data (measured by GCMS); curves: modeling.

Figure 4. Main pathways for the formation of acetaldehyde in the flames.

MHe molecule is such that the formation of hexanoic acid as a result of its unimolecular decomposition is fairly unlikely; therefore, in the flame with added MHe, it is formed in such small quantities that it is not possible to detect it in the experiment. Formation of Benzene, Phenol, Styrene, Ethyl benzene, and Cyclopentadiene. Reaction pathways analysis shows that these aromatic hydrocarbons are formed in the following reactions (1) Benzene I


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Energy & Fuels Table 4. Modified Reactionsa reaction

A

b

E

Amod

2C5H5 ⇒ C10H8 + H2 C6H5CH2 + C3H3 ⇒ C10H8 + 2H C10H8 + H ⇒ C10H7 + H2 C10H8 + OH ⇒ C10H7 + H2O C10H8 + CH3 ⇒ C10H7 + CH4 C10H8 + O ⇒ C9H7 + CO + H C9H7 + H ⇒ C9H8

4.3 × 1036 6.0 × 1011 8.0 × 108 2.1 × 108 2.7 × 1012 2.7 × 1013 1.0 × 1014

−6.3 0 1.8 1.42 0 0 0

22 835 0 16 800 1450 15 000 3600 0

0.86 × 1033 2.1 × 1012 8.0 × 109 2.1 × 109 2.7 × 1013 2.7 × 1014 1.0 × 1015

k = A × Tb exp(E/RT), A units: mol cm s K, E units cal/mol.

a

Figure 5. Mole fraction profiles of naphthalene (at the top) and indene (at the bottom). On the leftmodeling by the original version of the mechanism and on the rightmodeling by the modified version of the mechanism. For the n-heptane/toluene fuel mixture (opened circles and solid lines), in the flame of n-heptane/toluene/EPe blend (gray triangles and dashed lines) and in the flame of n-heptane/toluene/MHe blend (dark squares and dotted lines). Symbols: experimental data (measured by GCMS); curves: modeling.

experimentally measured in the work, except for naphthalene, indene, and the cyclopentadienyl radical. Thus, the modification of the mechanisms provided better agreement between the measured concentration profiles of soot precursors and the numerical simulation results.

factors of these seven reactions were changed. In column 2 are pre-exponential factors of rate constants (A) from the initial mechanism. The pre-exponential factors of rate constants (Amod) for the modified mechanism are presented in the last column. The rate constant of the reaction C6H5CH2 + C3H3 ⇒ C10H8 + 2H in the original mechanism29 was taken from ref 75, where its value was not measured experimentally. The rate constants of the reactions C10H8 + HC10H7 + H2, C10H8 + OHC10H7 + H2O, C10H8 + CH3C10H7 + CH4, and C10H8 + O ⇒ C9H7 + CO + H in the original mechanism29 were also estimated similar to that for the reactions involving benzene. The rate constant of the reaction C9H7 + HC9H8 in the original mechanism29 was evaluated similar to that of the reaction of n-butyl benzene with atomic hydrogen. Figure 5 compares the experimental and simulation results for the mechanisms with unchanged reaction rate constants (left) and for the mechanism with the modified rate constants of the above-mentioned reactions (right). It should be noted that the above modification of the mechanisms did not lead to noticeable changes in the calculated concentration profiles for the species

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CONCLUSIONS New experimental data were obtained on the chemical structure of rich (φ = 1.6) premixed flames of n-heptane/toluene EPe/nheptane/toluene, and MHe/n-heptane/toluene stabilized over a flat burner at a pressure 1 atm. For studied flames, temperature profiles and concentration profiles of the reactants, final combustion products, and intermediate species, including heavy aromatic hydrocarbons (naphthalene and indene), were measured. A detailed chemical-kinetic reaction mechanism for the EPe/n-heptane/toluene flame was developed based on the published sub-mechanisms for individual components of this mixture. The flame structures were simulated numerically using the available mechanisms for n-heptane/toluene and MHe/nheptane/toluene flames and the mechanism developed in this study for EPe/n-heptane/toluene mixtures. The simulation J


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Energy & Fuels

(3) Zhu, L.; Zhang, W.; Liu, W.; Huang, Z. Experimental study on particulate and NOx emissions of a diesel engine fueled with ultra low sulfur diesel, RME-diesel blends and PME-diesel blends. Sci. Total Environ. 2010, 408, 1050−1058. (4) Pourkhesalian, A. M.; Stevanovic, S.; Salimi, F.; Rahman, M. M.; Wang, H.; Pham, P. X.; Bottle, S. E.; Masri, A. R.; Brown, R. J.; Ristovski, Z. D. Influence of Fuel Molecular Structure on the Volatility and Oxidative Potential of Biodiesel Particulate Matter. Environ. Sci. Technol. 2014, 48, 12577−12585. (5) Tan, Y. H.; Abdullah, M. O.; Nolasco-Hipolito, C.; Zauzi, N. S. A.; Abdullah, G. W. Engine performance and emissions characteristics of a diesel engine fueled with diesel-biodiesel-bioethanol emulsions. Energy Convers. Manage. 2017, 132, 54−64. (6) Zhu, L.; Cheung, C. S.; Zhang, W. G.; Huang, Z. Combustion, performance and emission characteristics of a DI diesel engine fueled with ethanol-biodiesel blends. Fuel 2011, 90, 1743−1750. (7) Lapuerta, M.; Rodríguez-Fernández, J.; García-Contreras, R. Effect of a glycerol-derived advanced biofuel -FAGE (fatty acid formal glycerol ester)- on the emissions of a diesel engine tested under the New European Driving Cycle. Energy 2015, 93, 568−579. (8) Menkiel, B.; Donkerbroek, A.; Uitz, R.; Cracknell, R.; Ganippa, L. Combustion and soot processes of diesel and rapeseed methyl ester in an optical diesel engine. Fuel 2014, 118, 406−415. (9) Zhu, L.; Cheung, C. S.; Huang, Z. Impact of chemical structure of individual fatty acid esters on combustion and emission characteristics of diesel engine. Energy 2016, 107, 305−320. (10) Schoenborn, A.; Ladommatos, N.; Williams, J.; Allan, R.; Rogerson, J. The influence of molecular structure of fatty acid monoalkyl esters on diesel combustion. Combust. Flame 2009, 156, 1396−1412. (11) Salamanca, M.; Mondragón, F.; Agudelo, J. R.; Benjumea, P.; Santamaría, A. Variations in the chemical composition and morphology of soot induced by the unsaturation degree of biodiesel and a biodiesel blend. Combust. Flame 2012, 159, 1100−1108. (12) Zhang, Y.; Boehman, A. L. Autoignition of binary fuel blends of n-heptane and C7 esters in a motored engine. Combust. Flame 2012, 159, 1619−1630. (13) Coniglio, L.; Bennadji, H.; Glaude, P. A.; Herbinet, O.; Billaud, F. Combustion chemical kinetics of biodiesel and related compounds (methyl and ethyl esters): Experiments and modeling - Advances and future refinements. Prog. Energy Combust. Sci. 2013, 39, 340−382. (14) Osswald, P.; Struckmeier, U.; Kasper, T.; Kohse-Höinghaus, K.; Wang, J.; Cool, T. A.; Hansen, N.; Westmoreland, P. R. Isomer-Specific Fuel Destruction Pathways in Rich Flames of Methyl Acetate and Ethyl Formate and Consequences for the Combustion Chemistry of Esters. J. Phys. Chem. A 2007, 111, 4093−4101. (15) Yang, B.; Westbrook, C. K.; Cool, T. A.; Hansen, N.; KohseHöinghaus, K. Fuel-specific influences on the composition of reaction intermediates in premixed flames of three C5H10O2 ester isomers. Phys. Chem. Chem. Phys. 2011, 13, 6901−6913. (16) Metcalfe, W. K.; Dooley, S.; Curran, H. J.; Simmie, J. M.; ElNahas, A. M.; Navarro, M. V. Experimental and modeling study of C5H10O2 ethyl and methyl esters. J. Phys. Chem. A 2007, 111, 4001− 4014. (17) Dmitriev, A. M.; Osipova, K. N.; Knyazkov, D. A.; Gerasimov, I. E.; Shmakov, A. G.; Korobeinichev, O. P. Comparative Analysis of the Chemical Structure of Ethyl Butanoate and Methyl Pentanoate Flames. Combust., Explos. Shock Waves 2018, 54, 125−135. (18) Farrell, J. T.; Cernansky, N. P.; Dryer, F. L.; Friend, D. G.; Hergart, C. A.; Law, C. K.; McDavid, R. M.; Mueller, C. J.; Patel, A. K.; Pitsch, H. Development of an experimental database and kinetic models for surrogate diesel fuels. SAE Paper, 2007-01-0201, 2007. (19) Andrae, J.; Johansson, D.; Björnbom, P.; Risberg, P.; Kalghatgi, G. Co-oxidation in the auto-ignition of primary reference fuels and nheptane/toluene blends. Combust. Flame 2005, 140, 267−286. (20) Agafonov, G. L.; Naydenova, I.; Vlasov, P. A.; Warnatz, J. Detailed kinetic modeling of soot formation in shock tube pyrolysis and oxidation of toluene and n-heptane. Proc. Combust. Inst. 2007, 31, 575− 583.

results were validated against the obtained experimental data. The mechanisms used to calculate the flame structures were modified, and refined rate constants were proposed for a number of reactions of heavy aromatic hydrocarbons. The use of the modified mechanism provides a more accurate description of the concentration profiles of naphthalene and indene in the flames. The comparative analysis of the effect of ethyl pentanoate and methyl hexanoate addition to the n-heptane/toluene mixture leads to the following conclusions: 1) Addition of ethyl pentanoate to the n-heptane/toluene mixture causes an approximately twofold increase in the maximum concentration of acetaldehyde. Thus, under conditions of incomplete combustion of the fuel, addition of ethyl pentanoate can cause a significant increase in the acetaldehyde concentration in the engine exhaust gases. 2) Addition of ethyl pentanoate to the n-heptane/toluene mixture leads to formation of the pentanoic acid in the reaction zone and increases the concentration of ethylene, which, at a high equivalence ratio and high temperature, can result in higher reactivity of the EPe/n-heptane/ toluene fuel mixture compared to heptane/toluene and MHe/n-heptane/toluene. 3) Addition of ethyl pentanoate and methyl hexanoate to the n-heptane/toluene mixture in the experiment causes a twofold decrease in the maximum concentration of soot precursors in the flame, such as the PAH naphthalene and indene. 4) The mechanisms used for the simulation of the flame structure predict a greater (fourfold) decrease in the maximum concentrations of naphthalene and indene with added ethyl pentanoate and methyl hexanoate to the nheptane/toluene mixture, compared to that obtained in the experiment (twofold), indicating the need to refine the rate constants of the reactions involved in the formation and consumption of PAH.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b00166. Detailed experimental and simulation data for the profiles of temperature and species concentration in studied flames (XLS)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andrey G. Shmakov: 0000-0001-6810-7638 Notes

The authors declare no competing financial interest.

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REFERENCES

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