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The evolution of mobility particle size distribution function (PSDF) for premixed propene flame soot was investigated in the burner stabilized stagnation (BSS) flame configuration. The measurements were carried out for two series of flames with the equivalence ratio equal to 1.8 and 2.07, as a function of the burner-to-stagnation surface separation distance. The flame temperature was varied from 1820 to 1940 K in each series to examine flame temperature effects on the measured PSDF in a systematic manner. Measurements were compared to those determined previously for ethylene flames. Qualitatively, bimodal PSDFs were observed in all propene flames. Like the ethylene flames studied previously, the mobility diameter at the trough of the bimodal distribution and the median mobility diameter of the large size lognormal mode both shift towards smaller particle size as the flame temperature increases. Quantitatively, soot nucleates and grows in propene flames earlier and faster than in ethylene flames under comparable conditions. In particular, the propene flames produce a substantially higher number of soot nuclei than in the ethylene flames. The observations made for the propene flames and the comparison with ethylene flames strongly indicate that detailed soot formation processes, such as the PSDF, can be extremely sensitive to parent fuel structure in premixed flames. [ABSTRACT FROM AUTHOR]

The evolution of the nascent soot particle size distribution function (PSDF) was determined by mobility sizing for two series of atmospheric pressure premixed ethylene flames in the burner stabilized stagnation flame configuration. The first series of flames has an equivalence ratio of 1.8, corresponding to conditions just above the sooting limit. The second series has an equivalence ratio of 2.5 and is quite heavy in soot production. In each series, six flames were tested in which the cold gas velocity is varied to obtain flame temperatures ranging from 1559 to 1941 K. The temperature profiles were carefully determined and the comparison to pseudo-one dimensional simulations was satisfactory. It was found that the evolution of the PSDFs with respect to flame stoichiometry, temperature and growth time is consistent with the understanding of kinetic competition during soot formation. Finite rate kinetic limitations are observed at lower temperatures and thermodynamic reversibility occurs at higher temperatures. The observed PSDF features are highly sensitive to competition among the various processes of soot formation, from nucleation to coagulation and gas–surface reactions. The PSDFs are mostly bimodal with both nucleation and coagulation mode particles present. The evolution of the PSDF indicates a strong contribution to the mass of coagulation-mode by the nucleation-mode particles. The measured PSDFs offer comprehensive, canonical data sets useful for testing models of soot formation. [ABSTRACT FROM AUTHOR]

Abstract: This study addresses soot oxidation and NO x reduction over a BaAl2O4 catalyst. By XRD analysis, the catalyst was shown to be of spinel structure. Temperature Programmed Oxidation (TPO) and Constant Temperature Oxidation (CTO) at 673K show that the presence of O2 decreases the ignition temperature of soot, and it enhances the conversion of NO x to N2 and N2O. The kinetic features of soot oxidation in the TPO test are similar to that in the TG–DTA analysis. Analysis by Diffuse Reflectance Fourier Infrared Transform Spectroscopy (DRIFTS) indicates that the nitrates formed from NO x adsorption and the C(O) intermediates from soot oxidation are the key precursors of the redox process between soot and NO x over surfaces of the BaAl2O4 catalyst. Moreover, DRIFTS tests suggest that nitrates act as the principal oxidants for C(O) oxidation, through which nitrates are reduced to N2 and N2O. The O2 in the gas mixture presents a positive effect on the conversion of NO x to N2 and N2O by promoting the oxidation of nitrites into nitrates species. [Copyright &y& Elsevier]

Laser-induced fluorescence (LIF) technique has been widely applied in PAH measurement in sooting flames, but it is still a big challenge to quantitatively identify the specific PAH species due to the complexities of spectral characteristics and calibration method. In this study, we aimed at developing a spectral-resolved LIF analytic model that can obtain PAH mole fractions in flames. A comprehensive LIF spectrum database in the wide temperature and concentration ranges was firstly established by the LIF experiments of gas-phase PAHs in an optical cell. Then, a spectral-resolved LIF analytic model was presented based on linear superposition regulation and an optimization fitting algorithm, and the LIF spectra of PAH components can be decomposed from the LIF spectra of the flame using this model. Finally, the integral fluorescence intensities of each PAH component were calibrated to their mole fractions by the calibration factors at corresponding temperatures. In this way, we can measure the mole fractions of several PAHs. Subsequently, the presented LIF method was applied to investigate the PAH formation characteristics at different flame temperatures. The experimental results showed that the mole fractions of each PAH except naphthalene decreased around 60% to 70% when the temperature increased from 1186 K to 1483 K, while the mole fraction of naphthalene kept nearly constant. The mole fractions simulated by a chemical kinetic model well reproduce the experimental results. According to the chemical kinetic analysis, C 2 H 2 oxidation reaction was promoted and the C 2 H 2 -addition reactions were suppressed by increasing temperatures, which suppressed the PAH formation. [ABSTRACT FROM AUTHOR]

The soot particle size distribution in premixed flames of the light and heavy distillate fraction of gasoline was measured. The results were also compared with particles yielding from the not-distilled gasoline to study the difference in soot formation tendency between the practical fuel and its distillates. It was observed that the particle size distribution from the light and heavy distillates was similar with that of gasoline in a qualitative level, showing typical unimodal and bimodal distribution. Also, the heavy distillate had much higher particle growth rate than the light distillate, and those of gasoline were between the two distillates. However, the particle nucleation strength in premixed gasoline flames was higher than that in flames of either distillate and this also caused a higher particle number density, demonstrating that a synergistic effect on particle nucleation existed between the light and heavy distillate fractions. [ABSTRACT FROM AUTHOR]

Abstract The growth of polycyclic aromatic hydrocarbons (PAH) can proceed via multiple chemical mechanisms. The mechanism of naphthyl radical and vinylacetylene (C 4 H 4) addition reaction has been systematically investigated in this computational study. A combination of DFT/B3LYP/6-311+G(d,p), CCSD/6-311+G(d,p) and CBS-QB3 methods were performed to calculate the potential energy surfaces. It revealed that the products, including phenanthrene, anthracene, a PAH with a five-membered ring structure, and PAH with a C 4 H 3 radical substitution, can be formed in A 2 -1 (1-naphthyl)+C 4 H 4 and A 2 -2 (2-naphthyl) +C 4 H 4 reaction networks. The reaction rate constants at 0.1-100 atm were evaluated by RRKM theory by solving the master equation in the temperature range of 800–2500 K, which showed that the rate constants of reactions A 2 -1 (A 2 -2)+C 4 H 4 →product+H are highly temperature-dependent but nearly pressure-independent. The distribution of products was investigated in a 0-D batch reactor, wherein the initial reactant concentrations were taken from experimental measurements. The results showed that adduct intermediates were the main products at low temperature (T < 1000 K), and the phenanthrene and PAH with C 4 H 3 radical substitution became the dominant products at temperatures where PAHs and soot form in flames (T > 1000 K). It was observed that a significant amount of phenanthrene is formed from PAH with a C 4 H 3 radical substitution with the assistance of H atom. Reaction pathway sensitivity analysis for the PAH radical+C 4 H 4 reaction system was performed and showed that the new benzene rings are more likely to be generated near the zig-zag edge surface site instead of the free edge. For the development of a PAH mechanism, the analogous treatment of rate constants for larger PAH radical + C 4 H 4 reaction system are discussed. The formation rate of naphthalene from the reaction of phenyl+C 4 H 4 was found to be very close to that of phenanthrene from the reaction of naphthyl+C 4 H 4 , suggesting that the analogous treatment of the rates is reasonable in PAH mechanisms. [ABSTRACT FROM AUTHOR]

Abstract Hydrogen-abstraction/acetylene-addition (HACA) pathway has long been postulated as the dominant pathway for the formation of polycyclic aromatic hydrocarbons (PAHs) and the surface growth of soot. In this study, the site effect on PAH formation following HACA pathway is systematically investigated using density functional theory, transition state theory and premixed flame kinetic modeling. The entire reaction network includes 186 elementary reactions, starting from benzene to pyrene. Analysis of the potential energy surface and kinetic parameters show that H abstraction and C 2 H 2 addition reactions are greatly sensitive to the site position (ortho-, meta- and para-position) relative to the existing C 2 H chain and surface site type (zig-zag, free-edge and armchair). Specifically, H abstraction and C 2 H 2 addition reactions on the ortho-position and armchair surface site are kinetically unsupported due to the relatively high energy barrier and orientation hindrance effect compared with other site options. Therefore, the formation of a new benzene ring by the addition of the second C 2 H 2 molecule on the ortho-position (e.g., 1-ethynylnaphthalene + C 2 H 2 →phenanthrene) or the first C 2 H 2 molecule on the armchair surface site (e.g., phenanthrene + C 2 H 2 →pyrene) is unlikely, as demonstrated by PAH simulations in a premixed C 2 H 4 /O 2 /N 2 sooting flame. The yield distribution of various reaction products has been investigated using a 0-D reactor, where the combustion conditions are taken from experimental data. The results show that the dominant products are di-substituted PAHs in benzene-naphthalene reaction system and PAHs with 5-membered ring structures in larger PAHs reaction systems. The existence of abundant PAHs with 5-membered rings contributes to clarifying the PAHs signal detected using laser induced fluorescence technology. Additionally, the observed sequence of mass peaks at intervals of mass number 26 in C 2 H 2 /C 2 H 4 pyrolysis is reasonably explained by the HACA pathway with considering site effect. [ABSTRACT FROM AUTHOR]

The fuel mixing effect on soot formation was investigated based on a premixed propene flame, in which the C/O mole ratio was 0.6 and the maximum flame temperature was 1829 K. Different proportions of methane, ethane, propane and ethylene were mixed into the propene flame, with the constant C/O mole ratio and similar maximum flame temperatures and temperature–time histories. The particle size distribution function (PSDF) was measured in the burner stabilized stagnation (BSS) flame configuration, using the micro-orifice probe sampling technique and the scanning mobility particle sizer (SMPS). It was observed that the PSDF of the propene flame was not sensitive to methane, ethane or propane addition, while the number density of small nucleated particles was strengthened with ethylene addition. The particle growth in flame with a small amount of ethylene addition was stronger than that of either the pure propene or ethylene flame, indicating the synergistic effect between propene and ethylene on soot formation. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the chemical effects of CO 2 and H 2 O addition on PAH formation in laminar premixed C 2 H 4 /O 2 /Ar flames using laser-induced fluorescence (LIF) technique. For this purpose, the equivalence ratio, dilution ratio and maximum flame temperature were kept constant when the blending ratios of CO 2 and H 2 O varied. The experiment results indicated that the fluorescence signal intensities of different PAHs monotonically decreased with higher blending ratios of both CO 2 and H 2 O, and the suppression effect on PAH formation caused by H 2 O addition was stronger than that caused by CO 2 addition. The kinetic analysis was carried out to clarify the experimental phenomenon using a chemical kinetic model in Chemkin-Pro Software coupled with a PAH formation mechanism proposed by Appel et al. (ABF). The simulation results reproduced the tendencies observed in LIF experiments. The suppression on PAHs was likely resulted from the decreased concentrations of propargyl and phenylacetylene, which were influenced by the variation of H and OH radical concentrations. The CO 2 addition increased the reverse reaction rate of CO + OH ⇌ CO 2  + H reaction, resulting in the decrease of H radical concentration and the increase of OH radical concentration. On the other hand, the H 2 O addition significantly increased the reverse reaction rates of OH + OH ⇌ O + H 2 O and CH 3  + OH ⇌ CH 2 * + H 2 O reactions, promoting the formation of OH radical. In further steps, PAH growth pathway from benzene to large PAHs still need to be completed in more accurate PAH mechanism, which needs more useful data from LIF technique with better selectivity. [ABSTRACT FROM AUTHOR]

The burner stabilized stagnation flame technique coupled with micro-orifice probe sampling and mobility sizing has evolved into a useful tool for examining the evolution of the particle size distribution of nascent soot in laminar premixed flames. Several key aspects of this technique are examined through a multi-university collaborative study that involves both experimental measurement and computational modeling. Key issues examined include (a) data reproducibility and facility effects using four burners of different sizes and makers over three different facilities, (b) the mobility diameter and particle mass relationship, and (c) the degree to which the finite orifice flow rate affects the validity of the boundary condition in a pseudo one dimensional stagnation flow flame formulation. The results indicate that different burners across facilities yield nearly identical results after special attention is paid to a range of experimental details, including a proper selection of the sample dilution ratio and quantification of the experimental flame boundary conditions. The mobility size and mass relationship probed by tandem mass and mobility measurement shows that nascent soot with mobility diameter as small as 15 nm can deviate drastically from the spherical shape. Various non-spherical morphology models using a mass density value of 1.5 g/cm 3 can reconcile this discrepancy in nascent soot mass. Lastly, two-dimensional axisymmetric simulations of the experimental flame with and without the sample orifice flow reveal several problems of the pseudo one-dimensional stagnation flow flame approximation. The impact of the orifice flow on the flame and soot sampled, although small, is not negligible. Specific suggestions are provided as to how to treat the non-ideality of the experimental setup in experiment and model comparisons. [ABSTRACT FROM AUTHOR]