Formation Of Propargyl Research Articles

In-situ laser diagnostic and numerical investigations of soot formation characteristics in ethylene and acetylene counterflow diffusion flames blended with dimethyl carbonate and methyl formate

Oxygenated fuels have emerged as promising energy carriers for storing renewable energy given their compatibility with current energy systems and their potential of reducing pollutant emissions when burning in blends with traditional fossil fuels. In particular, dimethyl carbonate (DMC) and methyl formate (MeFo) have raised increasing interest for their use as gasoline substitutes in recent years. However, the effect of fuel blending on soot formation remains unclear, which prevents the utilization of the full potential of these fuels. In this work, soot formation of ethylene-based and acetylene-based fuel blended with DMC and MeFo in laminar counterflow diffusion flames was investigated by laser-induced incandescence (LII) in combination with laser-induced breakdown spectroscopy (LIBS) and spontaneous Raman scattering (RS). When substituting the base fuels with DMC or MeFo, the flame temperature profiles remain unchanged, allowing to exclusively focus on the chemical kinetic effect of fuel blending on soot formation. The soot formation in the ethylene-based flames can be enhanced by blending a small amount of DMC, while the soot volume fraction begins to decline at high substitution rates of DMC. Blending DMC with acetylene shows no soot enhancement effect. In comparison, the blending of ethylene with MeFo leads to a direct reduction of soot production. This soot formation behavior was further examined using a reaction pathway analysis based on chemical kinetic modeling. The propargyl recombination pathway plays a more important role than the HACA pathway in the formation of benzene and naphthalene, especially for ethylene-based flames. Different from MeFo, DMC can decompose to generate CH3 radicals and subsequently enhance propargyl formation. Therefore, blending DMC means a reduction of C2H2 and an increase of CH3, which results in the soot formation characteristics of DMC blends. Our kinetic analysis also implies the need for improving the decomposition kinetics of C2H4 to C2H2.

An experimental and modeling study on sooting characteristics of laminar counterflow diffusion flames with partial premixing

The effects of partial premixing on soot formation were investigated both experimentally and numerically in counterflow diffusion flames (CDFs) of ethylene, propane and ethylene/propane binary mixtures. The partial premixing ratio was maintained at relatively low levels so as to minimize the hydrodynamic effects; while the chemical effects of partial premixing on soot formation can be highlighted. The experimental results and simulations consistently showed that partial premixing notably increased soot formation in ethylene flames; while it slightly reduced the soot formation in propane flames, suggesting that partial premixing played different chemical roles in the soot formation process of ethylene and propane flames. Kinetic analyses revealed that partial premixing showed opposing effects on propargyl formation of ethylene and propane flames; this in turn led to the opposing effects on the formation of benzene and polycyclic aromatic hydrocarbons (PAH), and finally on the soot inception process. For partial premixing in propane/ethylene mixtures, the observed synergistic effects with propane added to ethylene CDFs disappeared, indicating a chemical cross-linking effect between partial premixing and ethylene/propane mixture. In addition, the present soot model showed satisfactory performance in reproducing the mismatch between PAH and soot formation tendencies in ethylene and propane CDFs. This result implies that great care is needed when using PAH concentration as an indicator of sooting tendency of different fuels. The present modeling results provided new insights into the chemical role of partial premixing in the soot evolution process of ethylene, propane and their blends. Further investigations towards the partial premixing effects on soot formation of larger liquid hydrocarbons such as diesel surrogates are required.

Experimental and kinetic modeling study of ammonia addition on PAH characteristics in premixed n-heptane flames

Due to concerns over pollutant emissions and ever-increasing energy crisis, ammonia is being considered as a promising alternative fuel to control soot particles and their precursors (polycyclic aromatic hydrocarbons, PAHs) emissions in combustion for diesel engines. This paper aims to investigate the chemical effects of NH3 addition on PAH formation in n-heptane laminar premixed flames. The laser-induced fluorescence (LIF) technique was used to measure the relative PAH fluorescence intensities qualitatively. A kinetic modeling study was performed to predict PAH concentrations by comparing with the experimental results, and then clarify the underlying effects of NH3 on PAH formation. Both experimental and numerical results revealed that PAH concentrations decreased monotonically as ammonia fraction increased, suggesting a PAHs-inhibiting role for NH3 in n-heptane baseline flames. Reaction pathway analysis revealed that the suppression of NH3 on PAH formation could be attributed to the competition for H, OH and O radicals in fuels' decomposition, and chemical interactions of amine radicals with the C1-C2 species. For this reason, the NH3 addition produced a relatively decreased C2H2, thereby inhibiting the formation of propargyl. As a result, benzene formation was inhibited through C3H3 self-recombination reaction, resulting sequentially in the suppression of PAH formation and growth. Besides, the decreased C2H2 yield in the post-flame region of NH3 doping flames also inhibited the formation of PAHs through the hydrogen-abstraction-acetylene-addition (HACA) mechanism.

On the opposing effects of methanol and ethanol addition on PAH and soot formation in ethylene counterflow diffusion flames

An experimental and numerical analysis of the effects of methanol and ethanol addition on polycyclic aromatic hydrocarbon (PAH) and soot formation in non-premixed ethylene flames is reported here. Laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were used to measure soot volume fractions and relative PAH concentrations in counterflow diffusion flames, respectively. A comprehensive chemical kinetic analysis was performed by modeling soot with detailed gas-phase chemistry and a sectional method. The results showed that although both methanol and ethanol are typically regarded as clean fuels, their presence in ethylene diffusion flames had the opposite effects on PAH and soot formation. The LIF and LII signals decreased significantly as methanol fraction increased, suggesting a soot-inhibiting role for methanol. Apart from the fact that methanol addition reduced the carbon supply for soot thus having a fuel-dilution effect (methanol converted primarily to CO), the increased H2 concentration from methanol decomposition was seen to chemically suppress incipient benzene ring formation and subsequent PAH and soot growth processes. In contrast, a small amount of ethanol addition enhanced soot formation, which was well captured by the numerical model. Reaction pathway analysis showed that ethanol decomposition produced a relatively large amount of methyl radicals, enhancing the chemical interaction between CH3 and C2 species and, thereby promoting the formation of propargyl and C4 species. As a result, benzene formation was promoted through reactions between C2H2 and C4 species and via C3H3 recombination reaction, leading sequentially to the enhancement of PAH growth and soot formation.

The Role of Methyl Radical in Soot Formation

ABSTRACT It was noted a significant increase in soot formation with the addition of methane in the pyrolysis of acetylene in shock tubes. Qualitative kinetic analysis revealed an increase of propargyl formation due to methyl/methylene and acetylene recombination. Here, we study the role of methyl radicals in the soot formation process more carefully. Mixtures of acetylene with various sources of methyl radicals – methane, diacetyl and dimethyl ether (DME) – were studied under shock tube pyrolysis conditions over a temperature range of 1650–2250 K and a pressure range of 3.5–4.5 bar. Laser light extinction was used for soot volume fraction and induction time of the particle inception measurements. A more than threefold increase in soot volume fraction with the addition of 1% of methane and 2% of acetylene diluted in argon was observed. Additions of diacetyl and DME resulted in less significant increases in soot volume fraction. All additions of methane, diacetyl and DME reduced the induction time of particle inception. The numerical modeling of the initial gas phase part of the process confirmed our hypothesis that in all cases the increase in the soot yield was proportional to the appearance of an additional amount of CH3 radicals, which promoted the formation of propargyl, benzene and pyrene.

Experimental and kinetic modeling study of laminar coflow diffusion methane flames doped with iso-butanol

In this work, laminar coflow diffusion flames fueled with methane/iso-butanol were investigated with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Mole fractions of C2–C16 flame species, including small hydrocarbons, oxygenates and aromatics were measured along the flame centerline. The enhancement in the formation of aromatics was observed through the increment of the iso-butanol doping ratio (1.95% and 3.90% in two flames) in the inlet fuel gas mixture. A detailed kinetic model was developed from previous models to help understand the experimental findings. According to rate of production analysis in laminar coflow diffusion flames, unimolecular decomposition and H abstraction reactions provide equivalent contributions to the consumption of iso-butanol. Compared to the coflow flames doped with other butanol isomers with the identical condition, the water elimination reaction of iso-butanol has the lowest contribution to fuel consumption. Two reaction pathways provide major contribution to the formation of propargyl radical, i.e. iC4H9OH→iC3H7→C3H6→aC3H5→aC3H4→C3H3 and iC4H9OH→βC4H8OH→iC4H8→iC4H7→aC3H4→C3H3. Propargyl is concluded as the key precursor for aromatics, while phenyl and benzyl radicals are the main PAH precursors. Finally, based on the studies of laminar coflow flames fueled with butanol isomers, the sooting tendency of butanol isomers relates strictly to the efficiency of the propargyl formation pathways in fuel decomposition.

Experimental and numerical investigation of fuel mixing effects on soot structures in counterflow diffusion flames

Experimental and numerical analyses of laminar diffusion flames were performed to identify the effect of fuel mixing on soot formation in a counterflow burner. In this experiment, the volume fraction, number density, and particle size of soot were investigated using light extinction/scattering systems. The experimental results showed that the synergistic effect of an ethylene-propane flame is appreciable. Numerical simulations showed that the benzene (C6H6) concentration in mixture flames was higher than in ethylene-base flames because of the increase in the concentration of propargyl radicals. Methyl radicals were found to play an important role in the formation of propargyl, and the recombination of propargyl with benzene was found to lead to an increase in the number density for cases exhibiting synergistic effects. These results imply that methyl radicals play an important role in soot formation, particularly with regard to the number density.

Characteristics of non-premixed oxygen-enhanced combustion: II. Flame structure effects on soot precursor kinetics resulting in soot-free flames

A detailed computational study was performed to understand the effects of the flame structure on the formation and destruction of soot precursors during ethylene combustion. Using the USC Mech Version II mechanism the contributions of different pathways to the formation of benzene and phenyl were determined in a wide domain of Z st values via a reverse-pathway analysis. It was shown that for conventional ethylene–air flames two sequential reversible reactions play primary roles in the propargyl (C 3H 3) chemistry, namely (1) C 2 H 2 + CH 3 = p C 3 H 4 + H (2) pC 3 H 4 = C 3 H 3 + H with the corresponding overall endothermic reaction of propargyl formation (3) C 2 H 2 + CH 3 = C 3 H 3 + 2 H. The contributions of these reactions to propyne (pC 3H 4) and propargyl formation and propargyl self-combination leading to benzene and phenyl were studied as a function of physical position, temperature, Z st , and H concentration. In particular, the role of H radicals on soot precursor destruction was studied in detail. At low Z st , Reactions 1 and 2 contribute significantly to propyne and propargyl formation on the fuel side of the radical pool at temperatures greater than approx. 1600 K. At higher local temperatures near the radical pool where the concentration of H is significant, the reverse reactions begin to dominate resulting in soot precursor destruction. As Z st is increased, these regions merge and only net propargyl consumption is observed. Based on the equilibrium constant of Reaction 3, a Z st value was estimated above which the rate of propargyl formation as a soot precursor is greatly reduced ( Z st = 0.3). This condition compares well with the experimental results for permanently blue counterflow flames in the literature.

Mercury-Free Preparation and Selective Reactions of Propargyl (and Propargylic) Grignard Reagents

ZnBr2 was found to catalyze formation of propargyl and propargylic Grignard reagents, and thus put an end to the standard method using a mercury catalyst. The Grignard reagents were submitted to addition reaction with carbonyl compounds and allylation with the cyclic monoacetate to afford the propargyl-type products selectively. Furthermore, the product from the monoacetate was transformed to an acetylene analogue of 2-(5,6-epoxyisoprostane A2)phosphorylcholines.

Highly Selective Formation of Propargyl‐ and Allenyltrichlorosilanes and Their Regiospecific Addition to Various Types of Aldehydes: Preparation of Both Allenic and Homopropargylic Alcohols.

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Highly selective formation of propargyl- and allenyltrichlorosilanes and their regiospecific addition to various types of aldehydes: preparation of both allenic and homopropargylic alcohols

The highly selective preparation of propargyl- and allenyltrichlorosilanes via metal-catalyzed silylation of propargyl chloride has been developed. These trichlorosilyl nucleophiles were then shown to add to various types of aldehydes to afford the corresponding allenic and homopropargylic alcohols, respectively, in high yields with complete regiospecificity. Remarkably, these carbon–carbon bond-forming reactions simply proceeded in N, N-dimethylformamide (DMF) without using any metal catalysts.

Effect of mixing methane, ethane, propane, and propene on the synergistic effect of PAH and soot formation in ethylene-base counterflow diffusion flames

The synergistic effect of the enhancement of PAH and soot formation by the fuel mixing of ethylene and propane has been previously investigated, and the effect has been explained based on the competition between the H-abstraction–C 2H 2-addition reaction and the incipient ring formation through propargyl recombination, and subsequently polycyclic aromatic hydrocarbon (PAH) growth through odd-carbon chemistries. To further elucidate the effect of fuel mixing on the formation of PAHs and soot, four different types of fuels—methane, ethane, propane, and propene—have been mixed to counterflow ethylene-base diffusion flames. Planar laser-induced incandescence and laser-induced fluorescence techniques were employed to measure relative soot volume fractions and PAH concentrations, respectively. Results showed that a small amount of ethane mixing in ethylene diffusion flame also exhibits the synergistic effect on PAH and soot formation. The mixing of propane in ethylene diffusion flame has a more pronounced synergistic effect as compared to the propene mixing when the mixture ratio is small. Since the decomposition reactions of ethane or propane can be the source of methyl radicals, such behaviors emphasize the role of methyl radicals in the formation of PAHs and soot. Propargyl formation could be additionally enhanced by the reactions related to methyl radical, which can be the reason for the synergistic effect in the mixture flames of ethylene with propane or ethane. Numerical results on the concentrations of CH 3, C 2H 2, and C 3H 3 substantiated the importance of methyl radical in the formation of propargyl and thereby the synergistic effect on the formation of PAHs and subsequent soot growth.

Fast, scalable master equation solution algorithms. IV. Lanczos iteration with diffusion approximation preconditioned iterative inversion

In this paper we propose a second linearly scalable method for solving large master equations arising in the context of gas-phase reactive systems. The new method is based on the well-known shift-invert Lanczos iteration using the GMRES iteration preconditioned using the diffusion approximation to the master equation to provide the inverse of the master equation matrix. In this way we avoid the cubic scaling of traditional master equation solution methods while maintaining the speed of a partial spectral decomposition. The method is tested using a master equation modeling the formation of propargyl from the reaction of singlet methylene with acetylene, proceeding through long-lived isomerizing intermediates.

Fast, scalable master equation solution algorithms. III. Direct time propagation accelerated by a diffusion approximation preconditioned iterative solver

In this paper we propose a novel fast and linearly scalable method for solving master equations arising in the context of gas-phase reactive systems, based on an existent stiff ordinary differential equation integrator. The required solution of a linear system involving the Jacobian matrix is achieved using the GMRES iteration preconditioned using the diffusion approximation to the master equation. In this way we avoid the cubic scaling of traditional master equation solution methods and maintain the low temperature robustness of numerical integration. The method is tested using a master equation modelling the formation of propargyl from the reaction of singlet methylene with acetylene, proceeding through long lived isomerizing intermediates.

Combined crossed-beam studies of C(3PJ)+C2H4→C3H3+H reaction dynamics between 0.49 and 30.8 kJ mol−1

The reaction C(3PJ)+C2H4(X 1A1)→C3H3+H(2S1/2) has been studied using complementary crossed molecular beam techniques. Integral cross sections have been obtained in the range of relative translational energies ET=0.49–24.9 kJ mol−1 in experiments conducted with pulsed supersonic beams coupled with laser-induced fluorescence detection of H(2S1/2) atoms. The major reaction pathway leading to HCCCH2 (propargyl)+H has been found without any barrier, with relative integral cross sections that are proportional to (ET)−0.60±0.03 below 8 kJ mol−1. Threshold for a minor pathway, leading also to H formation, occurs around 6 kJ mol−1; the relative importance of this second pathway increases with relative translational energy. Differential cross sections have been obtained at three relative translational energies: ET=9.1, 17.2, and 30.8 kJ mol−1 in experiments conducted with continuous supersonic molecular beams coupled with universal mass spectrometric detection and time-of-flight analysis. At the lowest ET of 9.1 kJ mol−1 formation of HCCCH2 (propargyl)+H is observed to be the dominant channel with a nearly forward–backward symmetric angular distribution in the center-of-mass (cm) frame; about 35% of the total available energy is channeled into translation indicating that the propargyl radical is highly internally excited; formation of less stable C3H3 isomer(s) is minor (2%). As ET increases, formation of appreciable, increasingly larger fractions of less stable propyn-l-yl and/or cyclopropenyl isomers is also observed. These findings are consistent with the integral cross-section measurements. While formation of propargyl is thought to proceed via an osculating complex mechanism following addition of C(3PJ) to the double bond of ethylene, the dynamics of formation of the less stable isomers is going through a long-lived complex, as witnessed by an isotropic cm angular distribution. The H2 elimination channel leading to C3H2 formation has not been found to occur, which suggests that inter-system-crossing to the ground singlet C3H4 potential energy surface manifold has low probability and/or the H2-elimination process on the triplet surface is characterized by a very large exit potential barrier.

Propargyl from the Reaction of Singlet Methylene with Acetylene

The technique of infrared kinetic spectroscopy has been used to study the production of propargyl radical from the reaction of singlet methylene with acetylene. The rate constant for this product channel was determined to be (3.5 ± 0.7) × 10-10 cm3 molecule-1 s-1 at 295 K, measured relative to the known rate for 1CH2 with H2 or CH4. Methylene was produced in the singlet state by excimer laser photolysis of ketene at 308 nm in the presence of acetylene and either H2 or CH4. Reaction of 1CH2 with acetylene produces propargyl, and reaction of 1CH2 with either H2 or CH4 produces CH3. The intensity of a propargyl infrared absorption line was compared with that of a methyl infrared absorption line, and the rate of formation of propargyl was determined from the ratio of these two intensities and the known rate of reaction of singlet methylene with H2 (or CH4) to produce CH3. The relative peak infrared absorption cross sections of methyl and propargyl were calibrated under the conditions of the experiment by photolyzing crotyl bromide at 193 nm to produce methyl and propargyl in equal concentrations.