Premixed Benzene Flames Research Articles

Molecular-growth pathways in premixed flames of benzene and toluene doped with propyne

In a combined experimental and modeling effort, we investigated the molecular-growth pathways in propyne-doped low-pressure premixed flames of benzene and toluene. We determined the chemical structures of these two flames with flame-sampling molecular-beam mass spectrometry. The mole fraction profiles of the aromatic intermediates served as validation targets for two chemically detailed mechanisms that were independently developed at the German Aerospace Center (DLR) and at the Lawrence Livermore National Laboratory (LLNL). Reaction path analyses reveal the important pathways for indene, naphthalene, and phenanthrene. There is no appreciable fuel-structure effect and molecular growth was observed to be driven by radical-radical recombination reactions, and ring-closure and ring-enlargement reactions with little contribution from the classical HACA mechanism. Indene is formed in both flames through the reactions of the phenyl and propargyl radicals. The benzyl radical plays only a very minor role in the formation of indene through the reaction with acetylene. Reactions of the propargyl radical with fulvenallenyl, a C7H5 isomer, contribute significantly to naphthalene formation in both flames investigated here. Benzyl radicals contribute to naphthalene formation via reactions with propargyl radical through formation of phenyl-substituted butadienyl and vinylacetylene isomers. Ring-enlargement reactions converting indene's five-membered ring into naphthalene's six-membered ring also contribute in small amounts to naphthalene. Fulvenallenyl radicals also contribute substantially to phenanthrene formation.

Soot size distribution in lightly sooting premixed flames of benzene and toluene

The evolution of particle size distribution function (PSDF) of soot in premixed flames of benzene and toluene was studied on a burner stabilized stagnation (BSS) flame platform. The cold gas velocities were changed to hold the maximum flame temperatures of different flames approximately constant. The PSDFs of all the test flames exhibited a bimodal distribution, i.e., a small-size nucleation mode and a large-size accumulation mode. It was observed that soot nucleation and particle growth in the benzene flame were stronger than those in the toluene flame at short residence times. At longer residence times, the PSDFs of the two flames were similar, and the toluene flame showed a larger particle size distribution range and a higher particle volume fraction than the benzene flame.

Experimental and kinetic modeling investigation on laminar premixed benzene flames with various equivalence ratios

Chemical structures of six laminar premixed benzene/oxygen/argon flames with equivalence ratios from 0.75 to 2.0 were investigated at 30Torr. Dozens of flame species, especially radicals and large aromatics, were qualitatively identified and quantitatively measured using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A kinetic model was developed from our previous models of aromatic fuels to simulate the decomposition of benzene and formation of large aromatics, and was validated against the measured mole fraction profiles of flame species. Based on the rate of production analysis in the leanest and richest flames, the decomposition processes of benzene and the fate of C6 and smaller products were discussed in detail. Benzene, phenyl radical and cyclopentadienyl radical are concluded as key precursors of large aromatics, and the low formation of PAHs in rich benzene flames compared with rich alkylbenzene flames mainly derives from the low production of benzyl radical, which is consistent with the observed trends of the sooting tendencies of benzene and alkylbenzenes in previous studies.

The effect of temperature on the condensed phases formed in fuel-rich premixed benzene flames

The sooting structure of premixed fuel-rich atmospheric pressure benzene flames burning at the same C/O molar ratio=0.8 was studied in different temperature conditions (Tmax=1720K and 1810K) by changing the cold gas velocity. Compositional profiles of gaseous and condensed phases, measured by probe sampling and chemical analysis, indicated that pyrolytic routes leading to higher soot formation are more favoured in the lower temperature conditions.The structural analysis of condensed phases, including condensed species and soot, has been carried out by using FT-IR and UV–Visible spectroscopy sensitive to the hydrogen bonding and carbon network, respectively.The very low hydrogenated character, as evaluated by FT-IR and elemental analysis, and the high aromatic/graphitic nature of the benzene soot, as shown by a detailed examination of UV–Visible spectral parameters, confirmed the effect of benzene fuel on the internal structure of soot particles already in the early stages of particle inception.

Analyzing the effects of temperature on soot formation with a joint volume-surface-hydrogen model

The intent of the current work is to present and further validate a new tri-variate model for the formation of soot particles, to apply this model in analyzing the effects of temperature on the formation and growth of soot, and to compare the findings with the present understanding derived from numerous experimental studies. In this novel model, a particle is represented as a fractal shaped aggregate and is described by three independent quantities: the volume, the surface area, and the number of hydrogenated sites (or active sites) on the surface. The introduction of this third variable allows for a better description of the surface reactivity at high temperatures. This approach is extended by a model for the total carbon-to-hydrogen (C/H) ratio of the particle. The model is validated first in high temperature premixed ethylene flames, premixed benzene flames, an acetylene counterflow diffusion flame, and toluene pyrolysis in shock-tubes. Then, the soot volume fraction is computed for a series of atmospheric laminar ethylene premixed flames with varying flame temperatures. The soot model is shown to reproduce the well known bell-shaped temperature dependence of soot volume fraction, which was found in many experiments. It is observed that nucleation is the largest contributor to soot volume fraction at low temperatures while growth by surface reactions is more important at higher temperatures. The surface reactivity and the volumetric carbon-to-hydrogen ratio (C/H) are also computed as a function of temperature. The surface reactivity is found to depend not only on the temperature but also on the particle size and the residence time in the flame. Finally, as observed experimentally, the C/H ratio is found to be essentially constant and close to unity for low temperature flames and increases with residence time in high temperature flames.

Large molecules, ions, radicals, and small soot particles in fuel-rich hydrocarbon flames part V: Positive ions of polycyclic aromatic hydrocarbons (PAH) in low-pressure premixed flames of benzene and oxygen

Low-pressure benzene-oxygen flames (C/O = 0.30–0.80, p = 2.66 kPa) have been analyzed for large positive PAH ions using molecular beam sampling, combined with reflectron time-of-flight mass spectrometry. In the mass range studied (10–90 C atoms per ion) PAH + ions occur with every number of C atoms. Growth of PAH + is very similar to that of the neutral pendents in following a broad band in the C-H diagram, in which H-rich PAHs play an important role. PAHs with the same number of C atoms always occur with different numbers of H atoms indicating different structures of the carbon skeleton. They are initially very H-rich, but with increasing temperature their formulae correspond more to maximum pericondensed structures (standard structures) and to H-poor PAHs containing condensed C pentagons. The classification of PAHs according to the oddity of their numbers of C and H atoms reveals very regular intensities of C distributions for PAHs with a given number (even or odd) of H atoms. These regular C distributions have previously also been observed with neutral PAHs in the same flames. A novelty is the detection of regular C-distributions of very H-rich PAH + ions with fewer C atoms than that of the standard PAH structures with the same number of H atoms. They are interpreted as phenyl-substituted pericondensed PAHs, the C-distributions of which can be correlated with those of the unsubstituted pericondensed species. In cooler flame zones very near to the burner, extremely H-rich, oxygen-containing hydrocarbons ions are found in the mass range of about 150 to 400 u; they disappear rapidly with increasing temperature. Their formation is attributed to reactions between oxygen and the fuel, other than the usual combustion reactions, but also initiated by back diffusion of H atoms which attack benzene.

Formation mechanism of polycyclic aromatic hydrocarbons and fullerenes in premixed benzene flames

A better understanding of the formation of polycyclic aromatic hydrocarbons (PAH) and fullerenes is of practical interest due to the apparent environmental health effects of many PAH and potential industrial applications of fullerenes. In the present work, a kinetic model describing the growth of PAH up to coronene (C24H12) and of C60 and C70 fullerenes is developed. Comparison of the model predictions with concentration profiles in a nearly sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (equivalence ratio φ = 1.8, pressure = 2.67 kPa) measured by Bittner using a molecular beam system coupled to mass spectrometry shows reasonably good predictive capability for stable and radical intermediates and growth species up to C16H10 isomers. Cyclopentadienyl is found to be a key species for naphthalene formation. The further growth process is based on H abstraction and acetylene addition but also the contribution of small PAH is considered. Good to fair agreement between model predictions and experimental data for larger PAH including the different C16H10 isomers obtained by gas chromatography coupled to mass spectrometry and high performance liquid chromatography could be achieved for PAH in a sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (φ = 2.4, 5.33 kPa). C60 and C70 fullerenes are underpredicted, and possible reasons such as uncertainties in rate coefficients or the existence of other formation pathways are discussed. PAH depletion in the burnt gas is not reproduced by the model and is believed to involve supplementary sinks such as reactions involving PAH and growing soot particles.

Detailed kinetic modeling of premixed benzene flames

A detailed kinetic mechanism capable of modeling aromatic ring breakdown in benzene flames has been developed through reaction path and systematic sensitivity analyses. The model incorporates benzene oxidation and pyrolysis steps, a C 5 chemistry submechanism and some novel features such as linearization reactions of cyclic and aromatic species. Model predictions are compared with the MBMS data of Bittner and Howard obtained in a rich, near-sooting, laminar premixed C 6H 6/O 2/Ar flame. Excellent agreement is observed for major species and key intermediates. However, the C 5 chemistry is still tentative, though the influence of the uncertainties on the main reaction paths is small to moderate. It is shown that aromatic ring destruction occurs via linearization to C 6 species and subsequent breakdown to the C 3 and C 4 chains. It has been shown that a direct decomposition route for phenyl, or benzene, to propargyl leads to poor agreement with measurements. The present work provides a basis for further study of the formation of PAH in flames.