Ethylene Flames Research Articles

Skeletal kinetic mechanism for predicting formation of non-fuel hydrocarbons and soot in ethylene flames – A CFD approach

Since existing reduced mechanisms of ethylene oxidation are either too large or lack of sufficient chemical reactions to describe aromatics formation, the study aims at developing a skeletal mechanism with extended predictive capability and lowered computational cost in computational fluid dynamics modeling of ethylene combustion. The newly assembled and refined kinetic mechanism composed of 664 species and 3582 reactions is minimized to 79 species and 538 reactions without empirically adjusting kinetic parameters. In the 1-D simulation for a premixed flame of ethylene, the ethylene-PAH mechanism generally well predicts experimental profiles of five hydrocarbons and nine aromatics. Integrated into a 2-D axisymmetric laminar finite-rate model, the ethylene-PAH mechanism reproduces experimental data of seven C2–C5 hydrocarbons, five aromatic hydrocarbons and soot in a diffusion flame of ethylene. In comparison with the 158-species (Narayanaswamy) mechanism, the newly proposed ethylene mechanism leads to a speedup of approximately 5.5 times in the 2-D laminar diffusion flame simulation and improved accuracy for predicting species formation in both premixed and nonpremixed flames. Moreover, we unveil the correlation between reaction pathways and intermediate formation from ethylene oxidation at the low temperature regime.

Effect of NH3 addition on soot morphology and nanostructure evolution in laminar ethylene diffusion flame

Ammonia, as a carbon neutral alternative fuel, has attracted wide attention. However, due to the defects in the combustion performance of pure ammonia, it is still necessary to further solve this problem in practical utilization. The purpose of this paper is to study the influence of ammonia on the morphology of soot and the evolution of nanoparticle structure in laminar flame of ethylene. In this work, the soot collected by thermophoresis probe was observed by the high resolution transmission electron microscope (HRTEM), and the image information in HRTEM images was statistically processed, and the graphitization degree of soot was measured by X-ray diffraction (XRD) spectrum of soot samples. The experimental results show that the addition of ammonia can inhibit the growth of soot nuclei and delay the formation of incipient soot, and has different effects on the evolution of soot morphology and structure in ethylene flame. It is noteworthy that the soot samples collected in the upstream part of the ammonia-doped flame show more mature core–shell structure in HRTEM images and more graphitized XRD diffraction data than those collected in pure ethylene flame. The main reason may be the chemical effect of ammonia and the thermal effect caused by the reduction of flame radiation loss, which have different effects on the nanostructure and morphology of soot at different stages of soot formation. Fringe structure model is used to describe and explain the influence of soot in flame with height before and after ammonia doped.

Experimental study on soot formation and primary particle size in oxy-combustion ethylene diffusion flames under CO2 substitution for N2

This paper reports an experimental study on the effects of O2 concentration and CO2 concentration on soot volume fraction and primary particle size of flame centre in ethylene laminar co-diffusion flame. The incandescent light was induced by a two-color band method and laser-excited by wavelength 1064 nm. Previous studies have shown that the oxygen concentration affects the soot volume fraction in the flame. However, the effects of O2 concentration on the primary particle size under CO2 atmosphere are unclear and need to be further discussed. Seven flames were optically detected, in which the oxygen concentration of the accompanying gas varied from 21% to 50% by volume. One primary flame was burned in co-flowing ethylene/air. Six ethylene flames were burned in an oxygen-rich O2/N2 or O2/CO2 atmosphere with an oxygen mole fraction from 30% to 50%. The C2H4 flow rate remained constant in all operating conditions. The results show that the SVF observed by the LII method and spectroscopy in the oxygen-enriched flame are in good agreement. Furthermore, the soot's primary particle diameter measured by the time-resolved LII method is in good agreement with the transmission electron microscope image analysis.With the increase of oxygen concentration, the flame height becomes shorter, the luminosity becomes brighter, and the soot yield increases. Compared with an N2 atmosphere with the same oxygen concentration, the soot yield in a CO2 atmosphere was significantly inhibited. In all the flames studied, the SVF along the flame axis showed a similar distribution: it first increased to the maximum value and then decreased rapidly until the flame tip. The maximum SVF on the axis appears between 0.64 and 0.78 based on the normalized height of the visible flame. In all ethylene/(O2/N2) flames, the maximum primary particle diameter and average primary particle diameter increase with oxygen content. For ethylene/(O2/CO2) flame, the maximum diameter of primary particles decreases with the increased oxygen content. The uncertainty of primary diameter is mainly caused by the thermal adaptation coefficient, the lower LII signal of smaller particles and the uncertainty of local flame temperature.

Experimental investigation on soot formation and characteristics of ethylene flames: Scale effects

Understanding the mechanism of scale effect was of great significance for both fundamental theory and actual combustion regulation. Using five combustors of different diameters (4, 6, 7, 8, and 10 mm), this study experimentally investigated the impact of scale effect on soot formation and combustion characteristics using optical and sampling diagnostic methods. For the optical results of two-color pyrometry, the scale effect exerted a non-monotonic effect on flame temperature. The flame temperature increased firstly with the enlargement of the combustor diameter from 4 to 7 mm, but it decreased rapidly when the combustor diameter continued to increase from 7 to 10 mm. However, the soot concentration increased at first and then remained constant approximately with the enlargement of combustor diameter, which exhibited a different tendency from flame temperature. For the sampling results of TEM and TGA, fringe tortuosity and oxidation rate of soot decreased with the combustor diameter enlargement, indicating the higher soot carbonization degree from the larger scale. Furthermore, for the result of exhaust gas analysis, the C2H4 concentration decreased first and then increased with the enlargement of combustor diameter, which also proved that the scale effect exerted a non-monotonic effect on the combustion process.

Investigation on the Effect of Charge Injection from Non-Thermal Plasma on Soot Formation in Laminar Coflow Diffusion Flame

ABSTRACT A novel, modified coflow burner was developed to study the effect of charge injection from a non-thermal plasma into three helium-diluted laminar coflow diffusion ethylene flames. The frequency of the high voltage (HV) signal was varied to control the ion concentration (charge) injected into the flames. Optical emission spectroscopy was used to characterize the non-thermal plasma while a bias plate methodology was used to gauge the relative amount of charge generated. For different HV signal frequencies, the laser-induced fluorescence of OH, chemiluminescence of CH*, and laser-induced incandescence of soot in flames were measured. The OH and CH* measurements showed that the flames retained the classic flame shape with charge injection. Significant soot reduction was observed at low HV signal frequencies, corresponding to an increase in charge injection. Notably, at low HV signal frequency, soot reduction in highly concentrated (60%) ethylene flame is three times lower than the less concentrated (32%) ethylene flame. This can be attributed to the decrease in the injected charge to soot precursor concentration ratio when the concentration of ethylene in the flame is increased. These results demonstrate that the current system is a promising candidate for studying the charge effect from non-thermal plasma on soot formation in laminar coflow diffusion flames.

Insight of soot nanostructure and oxidation behavior in ammonia / ethylene coaxial diffusion flame

In this work, the effects of ammonia addition on the morphology evolution, nanostructure and oxidation reactivity of soot particles in ethylene laminar diffusion flame were investigated. Soot sampling was performed using a thermophoresis probe and the morphological evolution of the particles was observed by transmission electron microscopy. In addition, high resolution transmission electron microscopy, Raman spectroscopy and thermogravimetric analysis were adopted to study the nanostructure and oxidation reactivity of soot particles sampled by quartz plate samples. The experimental results show that ammonia addition increases the flame height and compresses the soot formation region, delaying the evolution of each generation stage. For the primary soot particles, the surface growth is inhibited by ammonia doping, and the average particle size d¯p is reduced by about 33.13%. In addition, ammonia doping greatly inhibits the condensation between the primary particles of soot, and the parameters of the agglomerates such as the average area of the agglomerates A¯a, the number of primary particles N¯p and the fractal dimension Df are greatly reduced. Compared to the 100% argon doping ratio, ammonia doping reduced the average area of the agglomerates by approximately 76.27%, and the number of primary particles contained in the aggregate decreased by about 50%. The fringe length of soot particles decreases by about 15.38%, the fringe spacing increases by about 1.67% and the fringe tortuosity increases by about 4.23%, indicating that the degree of graphitization of soot decreases, the nanostructure becomes more disordered and the possibility of soot oxidation increases. The addition of ammonia results in the increase of disordered carbon in the soot of ethylene flame, and the activation energy of soot decreases significantly, so the soot has stronger oxidation reactivity.

Chemical, Dilutive, and Radiative Effects of Simultaneous CO2 Addition to the Fuel and Oxidizer Streams on Soot Formation in a Co-Flow Diffusion Ethylene Flame

Chemical, Dilutive, and Radiative Effects of Simultaneous CO₂ Addition to the Fuel and Oxidizer Streams on Soot Formation in a Co-Flow Diffusion Ethylene Flame

The impact of ammonia addition on soot formation in ethylene flames

The present work investigates computationally the impact of blending ammonia with ethylene on soot formation in four laminar diffusion flames and the corresponding turbulent partially premixed flames. The conditions conform to experimental investigations (Bennett et al., Combust. Flame, 2020, and Boyette et al., Combust. Flame, 2021) with both sets of flames featuring identical fuel blends. A mass and number density preserving sectional method was applied to all cases with the soot surface growth and oxidation models updated to include reactions with oxides of nitrogen. The gas phase chemistry was extended to include a comprehensively validated ammonia sub-mechanism. For the laminar flames, the inception of soot particles is based on detailed chemistry with pyrene treated as representative of the PAH pool. The accuracy of fitting a global acetylene-based inception step is evaluated for the different fuel blends and subsequently applied to the turbulent flames using a fully coupled transported joint probability density function method featuring an 84-dimensional joint-scalar space. Results obtained for laminar flames show that detailed chemistry coupled with the sectional soot model provides excellent agreement with the measured suppression of the soot volume fraction with increased use of ammonia. Computed particle size distributions (PSDs) show an increase in smaller soot particles under such conditions, consistent with experimental observations. The experimentally observed reduced impact in turbulent flames is also reproduced computationally. The suppression of soot is principally caused by changes in the radical pool leading to reduced soot surface growth and, to a lesser extent, soot inception. The contribution of oxides of nitrogen to soot oxidation is modest. Computed PSDs in laminar and turbulent flames highlight the importance of differences in flame structures and flowfield timescales.

An experimental study of the sooting behavior of a partially premixed flame under moderately rich conditions

Soot and its gaseous precursors are quantified in detail (precursors up to 166 amu, volume fraction, particle size, number concentration, and light emissivity dispersion exponent) in a laminar partially premixed counterflow flame of ethylene. The investigated flame has an equivalence ratio Φ = 2.43 and a mixture fraction Zst = 0.4, resulting in a distinct double-flame structure consisting of a rich premixed flame component and a diffusion flame component, both stabilized on the fuel side of the stagnation plane. The value of the equivalence ratio makes the premixed flame the dominant contributor to soot production, with soot being oxidized completely by OH from the diffusion flame component. Particle size is measured to increase quasi-monotonically, but remains within a few nanometers throughout the soot forming region. Aromatic species are primarily formed in the post flame region of the premixed flame. Their mole fractions peak close to the premixed flame and decrease as the diffusion flame is approached. The experimentally measured gaseous species are captured well by kinetic models, with the exception of two critical species in soot chemistry: benzene and naphthalene.

Investigation of soot suppression by ammonia addition to laminar ethylene flames at varying pressure

Blending ammonia with hydrocarbon fuels is a promising pathway to expedite its use in different industries, as it might offer a good compromise between energy output and minimizing the emissions of both NOx and carbonaceous combustion products, including soot. This work describes the effects of pressure on soot formation in ethylene/ammonia laminar co-flow diffusion flames. A high-pressure chamber was used to study the flames at pressures between 1 to 7 atm, and ammonia addition mole fractions between 0 and 50% (38% in mass fraction). Soot volume fraction and soot temperature measurements using a Spectral Soot Emissions technique show that adding a 50% mole fraction of ammonia at atmospheric pressure reduces soot concentration below the detection limit of the technique and between 70% and 80% for all the remaining studied pressures; that is, the ammonia soot suppression characteristics are demonstrated when increasing pressure. Maximum soot yield and total volumetrically integrated soot volume fraction results are investigated to reveal the contrasting effects of pressure and ammonia addition on the flame structure and the soot formation mechanisms. The results showcase the potential of NH3 in high-pressure applications when mixed with hydrocarbon fuels to reduce carbon emissions.

The Identity and Chemistry of C7H7 Radicals Observed during Soot Formation

We used aerosol mass spectrometry coupled with tunable synchrotron photoionization to measure radical and closed-shell species associated with particle formation in premixed flames and during pyrolysis of butane, ethylene, and methane. We analyzed photoionization (PI) spectra for the C7H7 radical to identify the isomers present during particle formation. For the combustion and pyrolysis of all three fuels, the PI spectra can be fit reasonably well with contributions from four radical isomers: benzyl, tropyl, vinylcyclopentadienyl, and o-tolyl. Although there are significant experimental uncertainties in the isomeric speciation of C7H7, the results clearly demonstrate that the isomeric composition of C7H7 strongly depends on the combustion or pyrolysis conditions and the fuel or precursors. Fits to the PI spectra using reference curves for these isomers suggest that all of these isomers may contribute to m/z 91 in butane and methane flames, but only benzyl and vinylcyclopentadienyl contribute to the C7H7 isomer signal in the ethylene flame. Only tropyl and benzyl appear to play a role during pyrolytic particle formation from ethylene, and only tropyl, vinylcyclopentadienyl, and o-tolyl appear to participate during particle formation from butane pyrolysis. There also seems to be a contribution from an isomer with an ionization energy below 7.5 eV for the flames but not for the pyrolysis conditions. Kinetic models with updated and new reactions and rate coefficients for the C7H7 reaction network predict benzyl, tropyl, vinylcyclopentadienyl, and o-tolyl to be the primary C7H7 isomers and predict negligible contributions from other C7H7 isomers. These updated models provide better agreement with the measurements than the original versions of the models but, nonetheless, underpredict the relative concentrations of tropyl, vinylcyclopentadienyl, and o-tolyl in both flames and pyrolysis and overpredict benzyl in pyrolysis. Our results suggest that there are additional important formation pathways for the vinylcyclopentadienyl, tropyl, and o-tolyl radicals and/or loss pathways for the benzyl radical that are currently unaccounted for in the present models.

Magnetic field effects on characteristics of counterflow diffusion ethylene flame: An experimental study

The present study was aiming to obtain the effects of magnetic gradients with varying direction on counterflow diffusion ethylene flames, including the flame shape, chemical structure and soot concentration distribution. Experiments were performed using flame imaging with a digital camera, two-color pyrometry, and flame speciation with gas chromatography (GC)/mass spectrometer (MS) system. When upward and horizontal outward magnetic gradients were introduced to flames, it was discovered that the mean flame temperature rose and the mean soot concentration dropped in the sooting region, while an opposite trend was observed with the addition of downward magnetic gradient. Meanwhile, results revealed that the presence of the upward or horizontal outward magnetic gradients could accelerate the consumption of ethylene and encourage the conversion of intermediate hydrocarbon species to CO and CO2 in the flame area, while the oxidation of fuel and intermediate hydrocarbon species were inhibited and the formation of the aromatics was promoted with the application of the downward magnetic gradient. Furthermore, it was noted that the changing trend of aromatics was consistent with that of soot. The reason might be understood from the variation of magnetic force imposed on oxygen, which in turn affected the fuel consumption and soot distribution.

Effects of intramolecular oxygen and partial premixing on soot formation in ethane/ethanol flames

Detailed soot concentration and temperature fields were measured in coflow laminar diffusion flames of neat ethane and ethane/ethanol mixtures, and partially premixed flames of ethane/ethanol mixtures. Two-dimensional diffuse-light line-of-sight attenuation and spectral soot emission techniques were employed for soot volume fraction and temperature measurements, respectively. The choice of flow rates prioritized tractability and comparability between flames. To this end, the total carbon mass flow rate and the C/H ratio were kept constant for all ethane/ethanol mixture flames, i.e., oxygen was introduced into the fuel stream either as intramolecular oxygen in ethanol or both as intermolecular and molecular oxygen. Soot concentration was observed to decrease monotonically with an increasing ethanol content in ethane/ethanol mixtures without synergistic effects. Soot concentration was also measured in neat ethane flames with flow rates matching those of ethane in ethane/ethanol mixtures. A comparison of these flames with ethane/ethanol flames revealed a striking soot suppression effect by ethanol addition. Although the carbon flow rate was less in neat ethane flames than the total carbon flow rate of their corresponding ethane/ethanol flames, the peak soot volume fractions were significantly higher in neat ethane flames. This points to a chemical soot suppression effect beyond that can be explained by the lower sooting propensity of ethanol fuel. These results were discussed in light of the synergistic effect studies in the literature. Lastly, measurements from partially-premixed ethane/ethanol mixtures are reported. The increased oxygen content significantly increased the peak soot volume fraction in these flames. These results support that intermolecular oxygen in ethanol and molecular oxygen in the fuel tube behave drastically differently in an ethane base flame. Ethanol addition does not exhibit synergistic effects on soot formation, whereas molecular oxygen addition increases maximum soot concentration, indicating the activation of unique soot production pathways.

Evaluation of the chemical effect of carbon dioxide dilution to fuel on soot formation in co-flow laminar ethylene and propane diffusion flames

In this study, the effects of carbon dioxide (CO2) addition on soot evolution were isolated systematically by adding 50% N2, 31% Ar+19% CO2, and 50% CO2 to the fuel side, respectively. Two series of diffusion flames, ethylene and propane, were experimentally investigated. Optical diagnostics, including laser-induced fluorescence (LIF), ratio pyrometry, and laser-induced incandescence (LII), were utilized to measure the spatial distributions of polycyclic aromatic hydrocarbons (PAHs), flame temperature, soot volume fraction fv, primary particle size dp, and number density of soot np. It was found that the flame height decreased gradually for the ethylene flame with adding N2/Ar + CO2/CO2. In contrast, the flame height remained essentially unchanged for propane-based flames. The results showed that adding N2/Ar + CO2/CO2 increased the temperature in the upper part of the ethylene flame, while the temperature change in the upper part of the propane flame was subtle and almost negligible. The total PAHs-LIF loading and the spatial distributions of fv/dp/np indicated that the chemical effect of CO2 added to ethylene flames in inhibiting the formation of large-size PAHs and retarding the soot inception stage was insignificant. However, it was not negligible for propane flames. The quantitative results revealed that the inhibition of total soot loading ST in propane flames was dominated by the chemical effect of CO2, while in ethylene flames, it was dominated by the thermal and dilution effect of CO2. The same conclusion was also found in the results of the maximum carbon conversion factor ηmax and the average particle size of soot. The specific growth rate of soot mass was almost unchanged regardless of the mixture on the fuel side. Adding CO2 had a more substantial chemical effect in inhibiting soot formation in propane flames than that in ethylene flames, which could mainly be attributed to a more effective reduction in the time for the increase of soot mass τg during soot evolution.

High resolution mass spectrometric suspect screening, wide-scope target analysis of emerging contaminants and determination of legacy pollutants in adult black-tailed godwit Limosa limosa limosa in the Netherlands – A pilot study

The Dutch breeding population of the black-tailed godwit Limosa limosa limosa has declined substantially over recent decades; the role of contaminants is unknown. We analysed liver samples from 11 adult birds found dead on their breeding grounds in SW Friesland 2016–2020, six from extensive, herb-rich grasslands, five from intensive grasslands. We carried out LC and GC wide-scope target analysis of more than 2400 substances, LC suspect screening for more than 60,000 substances, target analysis for Cd, Hg, Ni and Pb, organo-phosphate flame retardants (OPFRs), dechlorane plus compounds and selected polybrominated diphenyl ether flame retardants (PBDEs), and bioassay for polybrominated dibenzo-p-dioxins and dibenzofurans (PBDDs/PDBFs) and dioxin-like polychlorinated biphenyls (dl-PCBs). Residues of 29 emerging contaminants (ECs) were determined through wide-scope target analysis. Another 20 were tentatively identified through suspect screening. These contaminants include industrial chemicals (personal care products, surfactants, PAHs and others), plant protection products (PPPs) and pharmaceuticals and their transformation products. Total contaminant load detected by wide-scope target analysis ranged from c. 155 to c. 1400 ng g−1 and was generally lower in birds from extensive grasslands. Heatmaps suggest that birds from intensive grasslands have a greater mix and higher residue concentrations of PPPs, while birds from extensive grasslands have a greater mix and higher residue concentrations of per- and polyfluoroalkyl substances (PFAS). All four metals and two OPFRs were detected. All tested PBDEs were below the respective LODs. Bioassay revealed presence of PBDDs, PBDFs and dl-PCBs. Further research is required to elucidate potential health risks to godwits and contaminant sources.

Infrared spectral soot emission for robust and high-fidelity flame thermometry.

Spectral soot emission (SSE) in the visible spectrum is a popular technique for non-intrusive thermometry in sooting flames. However, its accuracy is restricted by uncertainties in the wavelength dependence of soot optical properties. We propose a novel infrared spectral soot emission method that successfully addresses this issue. Comprehensive light extinction experiments were firstly conducted to explore the spectral variation of soot optical property. The results indicated a wavelength independence of the soot absorption function provided the wavelength of the incident light is larger than 1000 nm, thereby indicating through the Kirchhoff law the potential of a robust thermometry using infrared (>1000 nm) spectral soot emissions. Proof-of-concept experiments were performed for sooting premixed flames of ethylene with different equivalence ratios. The results demonstrated that the new method provided more accurate temperature results compared with its visible-NIR counterpart, particularly at flame positions where nascent soot particles are present. The proposed method is, to our knowledge, the first infrared spectral soot emission-based thermometry, and is believed to offer a solution to improving the fidelity of SSE with a cost-effective optical setup.

Effects of Iron Addition on the Collision of Polycyclic Aromatic Hydrocarbon Clusters: A Molecular Dynamics Study

In this work, soot particle size distributions in iron-doped premixed ethylene flames are examined using scanning mobility particle sizer measurements. It is found that iron addition promotes the growth in soot particle size, and the enhanced particle coagulation is inferred to be an important reason. To support that, the influence of iron addition on the coagulation of polycyclic aromatic hydrocarbon (PAH) clusters, the analogue of incipient soot particles, is further investigated using molecular dynamics simulations. Based on the results of hundreds of binary head-on collision simulations, the collision between two coronene-Fe-coronene dimers is found to have a significantly higher coagulation efficiency than that between two coronene dimers. However, this enhancement effect weakens as the size of the PAH monomer increases. Although the coagulation efficiency can be increased by iron addition, the collision frequency is almost unaffected, as revealed from the binary off-central collision simulations. Moreover, the simulation results of coronene cluster growth via coagulation show that iron addition promotes coronene cluster growth, leading to larger cluster size, which may explain the larger soot particle size observed in iron-doped flames.

Ion density-enhanced electrostatic precipitation using high voltage nanosecond pulses

This study evaluates the beneficial effects of discharging nanosecond pulse transient plasma (NPTP) in a coaxial electrostatic precipitator for capturing nanoscale soot particles (∼50 nm) produced by an ethylene flame.

On the combined effects of compositional inhomogeneity and ammonia addition to turbulent flames of ethylene

This paper is part of a broader program aimed at investigating the effects of co-firing clean fuels such as ammonia or hydrogen with hydrocarbons. The focus is on soot formation as well as flame stability in turbulent mixed-mode combustion, which is highly relevant in practical combustors. Ammonia substitution for nitrogen results in reduced flame stability, and this is correlated to differences in flame speed and extinction strain rate. While it is known that the addition of ammonia suppresses soot, visual inspection of compositionally inhomogeneous flames of ethylene-ammonia indicates a reduction in ammonia's ability to suppress soot formation. Measurements of soot volume fraction and laser-induced fluorescence in selected UV and visible bands are made along the centreline in selected flames to test this hypothesis. Experimental results are then compared to simulations in laminar diffusion flames, stratified counterflow flames, and partially premixed flames. All results confirm the soot-inhibiting ability of ammonia. Increasing inhomogeneity, leading to higher centreline mixture fractions, enhances soot formation, and the level of enhancement is greater for flames with ammonia than without. Moreover, it is found that partial premixing is ultimately responsible for determining the amount of soot formed as opposed to stratification of fuel mixtures near the pilot.

Detailed Study of the Formation of Soot Precursors and Soot in Highly Controlled Ethylene(/Toluene) Counterflow Diffusion Flames

We perform spatially resolved measurements of temperature, gaseous species up to three-ring Polycyclic Aromatic Hydrocarbons (PAHs), and soot in atmospheric pressure counterflow diffusion flames. First, we characterize fully a baseline ethylene flame and then a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of prevaporized toluene. The goal is twofold: to investigate the impact of a common reference fuel component of surrogates of transportation fuels and bypass the main bottleneck to soot formation from aliphatic fuels, that is, the formation of the first aromatic ring. The composition of the fuel and oxidizer streams are adjusted to maintain a constant stoichiometric mixture fraction and global strain rate, thereby ensuring invariance of the temperature-time history in the comparison between the two flames and decoupling the chemical effects of the fuel substitution from other factors. Major combustion products and critical radicals are fixed by the baseline flame, and profiles of critical C2-C5 species precursors to aromatic formation are invariant in both flames. On the other hand, doping with toluene boosts the aromatic content and soot volume fraction, increasing the mole fraction of benzenoid structures and soot volume fraction by a factor of 2 or 3, relative to the baseline ethylene flame. This finding is consistent with the expectation that the formation of the first aromatic ring is no longer a bottleneck to soot formation in the doped flame. In addition, toluene bypasses completely benzene formation, opening a radical recombination pathway to soot precursors through the production of C14H14 (via dimerization of benzyl radical) and pyrene (through dimerization of indenyl radical).