Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors

This article presents a chemical mechanism for the high temperature combustion of a wide range of hydrocarbon fuels ranging from methane to iso-octane. The emphasis is placed on developing an accurate model for the formation of soot precursors for realistic fuel surrogates for premixed and diffusion flames. Species like acetylene (C₂H₂), propyne (C₃H₄), propene (C₃H₆), and butadiene (C₄H₆) play a major role in the formation of soot as their decomposition leads to the production of radicals involved in the formation of Polycyclic Aromatic Hydrocarbons (PAH) and the further growth of soot particles. A chemical kinetic mechanism is developed to represent the combustion of these molecules and is validated against a series of experimental data sets including laminar burning velocities and ignition delay times. To correctly predict the formation of soot precursors from the combustion of engine relevant fuels, additional species should be considered. One normal alkane (n-heptane), one ramified alkane (iso-octane), and two aromatics (benzene and toluene) were chosen as chemical species representative of the components typically found in these fuels. A sub-mechanism for the combustion of these four species has been added, and the full mechanism has been further validated. Finally, the mechanism is supplemented with a sub-mechanism for the formation of larger PAH molecules up to cyclo[cd]pyrene. Laminar premixed and counterflow diffusion flames are simulated to assess the ability of the mechanism to predict the formation of soot precursors in flames. The final mechanism contains 149 species and 1651 reactions (forward and backward reactions counted separately). The mechanism is available with thermodynamic and transport properties as supplemental material.     

Soot formation with C1 and C2 fuels using an improved chemical mechanism for PAH growth

Recently, an improved chemical mechanism of PAH growth was developed and tested in soot computations for a laminar co-flow non-premixed ethylene–air diffusion flame. In the present work, the chemical mechanism was enhanced further to accommodate the PAH gas phase growth in methane, ethylene and ethane co-flow flames. The changes in the mechanism were tested on a methane/oxygen and two ethane/oxygen premixed flames to ensure no degradation in its application to C₂ fuels. The major soot precursors were predicted in a satisfactory matter. The robustness of the soot solution methodology was tested for different fuels by solving methane/air, ethane/air and ethylene/air co-flow laminar diffusion flames using a single solution algorithm for all three cases. The peak soot volume fractions, which varied by two orders of magnitude between fuels, were predicted within a factor of two for all flames. The computations were also able to reproduce the spatial distributions of soot and to explain the variation in soot formation pathways among the fuels. Despite a similarity in bulk properties of the flame, the soot particles in different flames exhibit significantly different growth modes. Ethylene/air flames tend to form soot earlier than methane/air flames and inception plays a bigger role in the latter.     

Soot precursor measurements in benzene and hexane diffusion flames

To clarify the mechanism of soot formation in diffusion flames of liquid fuels, measurements of soot and its precursors were carried out. Sooting diffusion flames formed by a small pool combustion equipment system were used for this purpose. Benzene and hexane were used as typical aromatic and paraffin fuels. A laser-induced fluorescence (LIF) method was used to obtain spatial distributions of polycyclic aromatic hydrocarbons (PAHs), which are considered as soot particles. Spatial distributions of soot in test flames were measured by a laser-induced incandescence (LII) method. Soot diameter was estimated from the temporal change of LII intensity. A region of transition from PAHs to soot was defined from the results of LIF and LII. Flame temperatures, PAH species, and soot diameters in this transition region were investigated for both benzene and hexane flames. The results show that though the flame structures of benzene and hexane were different, the temperature in the PAHs-soot transition region of the benzene flame was similar to that of the hexane flame. Furthermore, the relationship between the PAH concentrations measured by gas chromatography in both flames and the PAH distributions obtained from LIF are discussed. It was found that PAHs with smaller molecular mass, such as benzene and toluene, remained in both the PAHs-soot transition and sooting regions, and it is thought that molecules heavier than pyrene are the leading candidates for soot precursor formation.     

Detailed numerical modeling of PAH formation and growth in non-premixed ethylene and ethane flames

A chemical kinetic mechanism for C₁ and C₂ fuel combustion and PAH growth, previously validated for laminar premixed combustion, has now been modified and applied to opposed flow diffusion flames. Some modifications and extensions have been made to the reaction scheme to take into account recent kinetic investigations, and to reduce the stiffness of the reaction model. Updates have been made to the cyclopentadienyl reactions, indene formation reactions, and aromatic oxidation and decomposition reactions. Reverse reaction rate parameters have been revised to account for numerical stiffness. Opposed flow diffusion flame simulation data for ethylene and ethane flames with the present mechanism are compared to data computed using two other mechanisms from the literature and to experimental data. Whereas the fuel oxidation chemistry in all three mechanisms are essentially the same, the PAH growth pathways vary considerably. The current mechanism considers a detailed set of PAH growth routes, and includes hydrogen atom migration, possible free radical addition schemes, methyl substitution/acetylene addition pathways, cyclopentadienyl moiety in aromatic ring formation, and numerous reactions between aromatic radicals and molecules. It is shown that while bulk flame properties and major species profiles are the same for the three mechanisms, the enhanced PAH growth routes in the present mechanism are necessary to numerically predict the correct order of magnitude of PAHs that were measured in the experimental studies. In particular, predicting concentrations of naphthalene, phenanthrene, and pyrene, to within the correct order of magnitude with the present mechanism show a significant improvement over predictions obtained using mechanisms in the literature. Sensitivity and production rate analyses show that this improvement is attributable to the enhanced PAH growth pathways and updated reaction rates in the present mechanism. The overreaching goal of this research is to generate and fully validate a detailed chemical kinetic mechanism, with as few fitted rates as possible, that can be applied to premixed or diffusion systems, and used with any type of soot model. To that end, in recently published works, the present mechanism has been used to simulate premixed flames, while coupled to a method of moments to determine soot formation, and to simulate diffusion flames, while coupled to a sectional representation for soot formation. The present work extends the validation of the mechanism by applying it to counterflow diffusion flames, for which measurements of large PAH molecules are uniquely available. The validation of PAH growth predictions are of key interest to soot modeling studies as soot inception from PAH combination and PAH condensation are often major constituents of soot production.     

Synergistic effect on soot formation in counterflow diffusion flames of ethylene-propane mixtures with benzene addition

The characteristics of the formation of polycyclic aromatic hydrocarbon (PAH) and soot in counterflow diffusion flames of ethylene/propane mixtures have been investigated experimentally to identify the effect of fuel structure. The synergistic effect, that is, the enhancement of PAH and soot formation by the fuel mixing of ethylene and propane has been further analyzed to examine the suggested mechanisms based on the competition between PAH and soot growths through the H-abstraction-C₂H₂-addition (HACA) mechanism and the incipient ring formation through the propargyl recombination reaction. To mitigate the effect of incipient ring formation on the synergistic effect, a small amount of benzene was added to the fuel stream. Planar laser-induced incandescence and laser-induced fluorescence techniques were employed to measure relative soot volume fractions and PAH concentrations, respectively. Results showed that the synergistic effect on soot formation remained, even though the synergistic effects for relatively small aromatics mitigated with the benzene addition. Larger size PAHs have shown enhanced synergistic effects compared to smaller size PAHs regardless of benzene addition. These results implied that the role of propane mixing on the synergistic effect cannot be explained solely by the incipient ring formation via a propargyl recombination reaction; thus, it is suggested that the C₃ pathways could also contribute to the growth of PAH species.     

汽油与异辛烷/正庚烷的燃烧特性分析

利用定容燃烧弹系统,研究了相同辛烷值的两种燃料-标准汽油和异辛烷/正庚烷在化学当量比条件下的预混燃烧过程.通过高速摄像系统拍摄了标准汽油和异辛烷/正庚烷的层流预混阶段火焰传播过程,着重分析、比较了两种燃料的滞燃期,层流火焰传播速度和马克斯坦长度.结果表明,虽然两者辛烷值相同,但是异辛烷/正庚烷拉伸和无拉伸层流火焰传播速度均高于标准汽油的,而且标准汽油对应负的马克斯坦长度,其火焰稳定性较差,异辛烷/正庚烷对应正的马克斯坦长度,其火焰的稳定性较好.从火焰外观上,两种燃料的前期火焰基本一致,为淡蓝色预混火焰;但后期火焰的差异很大,汽油呈碳粒型燃烧,而异辛烷/正庚烷则为充分预混型.     

Kinetic modeling of counterflow diffusion flames of butadiene

A comprehensive, semi-detailed kinetic scheme was used to simulate the chemical structures of counterflow diffusion and fuel-rich premixed 1,3-butadiene flames, to better understand the formation of polycyclic aromatic hydrocarbons (PAH). The results showed that model predictions were in good agreement with the experiments for most of the species in both the flames. In the counterflow flames higher-molecular weight products are slightly over predicted. The pathways characterizing the pollutant formation are very different in the premixed and in the counterflow flames confirming or suggesting the need to verify and refine the detailed mechanisms tuned for premixed conditions when they are extrapolated and used in diffusion flames. Reaction paths analysis for PAH formation in the counterflow flame shows that both the HACA mechanism and the resonantly stabilized radicals are important for the growth of PAH. The kinetic model was unsuccessful in predicting the increased reactivity in O₂-doped diffusion flames, indicating the need for improved models and also the opportunity of new experiments of butadiene oxidation in the intermediate temperature region.     

PAH formation in one-dimensional premixed fuel-rich atmospheric pressure ethylbenzene and ethyl alcohol flames

This study addresses health-hazardous emissions from combustion of aromatic and oxygenated components of engine fuel blends. An investigation was conducted on the evolution of polycyclic aromatic hydrocarbons (PAH) and other pollutants (soot, CO, unburned light hydrocarbons) emitted from one-dimensional ethylbenzene and ethyl alcohol flames. The study of ethylbenzene combustion is also pertinent to that of waste polystyrene, as past work has indicated that ethylbenzene may be a surrogate for initial products of polystyrene pyrolysis. Both liquid fuels were prevaporized in nitrogen, mixed with oxygen and additional nitrogen, and upon ignition, premixed flat flames were stabilized over a burner. Temperature measurements and product sampling were conducted at various heights above the burner. Results showed that ethyl alcohol combustion generated small yields of PAH, even under the adverse fuel-rich conditions of this study (?=2.5). PAH mole fractions in the ethyl alcohol flame were 2-5 orders of magnitude lower than those in the ethylbenzene flame at the same location. PAH mole fractions often peaked in the postflame region and remained at relatively high levels thereafter. PAH mole fractions in premixed fuel-rich benzene, ethane, ethylene, and methane flames, published in the literature, were typically in between those found in the two flames of this study. Computations were conducted using a currently developed detailed kinetic model, allowing for the prediction of formation and depletion of major PAH and soot particles of different sizes. The computed chemical flame structures were compared to experimental data obtained in the present work. Predicted mole fractions were often close to the experimentally obtained values or, at worse, within the same order of magnitude for both fuels. Reaction pathways are discussed.     

An experimental and numerical investigation of n-heptane/air counterflow partially premixed flames and emission of NOx and PAH species

An experimental and numerical investigation of counterflow prevaporized partially premixed n-heptane flames is reported. The major objective is to provide well-resolved experimental data regarding the detailed structure and emission characteristics of these flames, including profiles of C₁-C₆, and aromatic species (benzene and toluene) that play an important role in soot formation. n-Heptane is considered a surrogate for liquid hydrocarbon fuels used in many propulsion and power generation systems. A counterflow geometry is employed, since it provides a nearly one-dimensional flat flame that facilitates both detailed measurements and simulations using comprehensive chemistry and transport models. The measurements are compared with predictions using a detailed n-heptane oxidation mechanism that includes the chemistry of NO_x and PAH formation. The reaction mechanism was synergistically improved using pathway analysis and measured benzene profiles and then used to characterize the effects of partial premixing and strain rate on the flame structure and the production of NO_x and soot precursors. Measurements and predictions exhibit excellent agreement for temperature and major species profiles (N₂, O₂, n-C₇H₁₆, CO₂, CO, H₂), and reasonably good agreement for intermediate (CH₄, C₂H₄, C₂H₂, C₃H_x) and higher hydrocarbon species (C₄H₈, C₄H₆, C₄H₄, C₄H₂, C₅H₁₀, C₆H₁₂) and aromatic species (toluene and benzene). Both the measurements and predictions also indicate the existence of two partially premixed regimes; a double flame regime for ?<5.0, characterized by spatially separated rich premixed and nonpremixed reaction zones, and a merged flame regime for ?>5.0. The NO_x and soot precursor emissions exhibit strong dependence on partial premixing and strain rate in the first regime and relatively weak dependence in the second regime. At higher levels of partial premixing, NO_x emission is increased due to increased residence time and higher peak temperature. In contrast, the emissions of acetylene and PAH species are reduced by partial premixing because their peak locations move away from the stagnation plane, resulting in lower residence time, and the increased amount of oxygen in the system drives the reactions to the oxidation pathways. The effects of partial premixing and strain rate on the production of PAH species become progressively stronger as the number of aromatic rings increases.     

Synergistic effect of mixing dimethyl ether with methane, ethane, propane, and ethylene fuels on polycyclic aromatic hydrocarbon and soot formation

Characteristics of polycyclic aromatic hydrocarbon (PAH) and soot formation in counterflow diffusion flames of methane, ethane, propane, and ethylene fuels mixed with dimethyl ether (DME) have been investigated. Planar laser-induced incandescence and fluorescence techniques were employed to measure relative soot volume fractions and PAH concentrations, respectively. Results showed that even though DME is known to be a clean fuel in terms of soot formation, DME mixture with ethylene fuel increases PAH and soot formation significantly as compared to the pure ethylene case, while the mixture of DME with methane, ethane, and propane decreases PAH and soot formation. Numerical calculations adopting a detailed kinetics showed that DME can be decomposed to produce a relatively large number of methyl radicals in the low-temperature region where PAH forms and grows; thus the mixture of DME with ethylene increases CH₃ radicals significantly in the PAH formation region. Considering that the increase in the concentration of O radicals is minimal in the PAH formation region with DME mixture, the enhancement of PAH and soot formation in the mixture flames of DME and ethylene can be explained based on the role of methyl radicals in PAH and soot formation. Methyl radicals can increase the concentration of propargyls, which could enhance incipient benzene ring formation through the propargyl recombination reaction and subsequent PAH growth. Thus, the result substantiates the importance of methyl radicals in PAH and soot formation, especially in the PAH formation region of diffusion flames.     

Structure of laminar sooting inverse diffusion flames

The flame structure of laminar inverse diffusion flames (IDFs) was studied to gain insight into soot formation and growth in underventilated combustion. Both ethylene-air and methane-air IDFs were examined, fuel flow rates were kept constant for all flames of each fuel type, and airflow rates were varied to observe the effect on flame structure and soot formation. Planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF), planar laser-induced incandescence of soot (soot PLII), and thermocouple-determined gas temperatures were used to draw conclusions about flame structure and soot formation. Flickering, caused by buoyancy-induced vortices, was evident above and outside the flames. The distances between the OH, PAH, and soot zones were similar in IDFs and normal diffusion flames (NDFs), but the locations of those zones were inverted in IDFs relative to NDFs. Peak OH PLIF coincided with peak temperature and marked the flame front. Soot appeared outside the flame front, corresponding to temperatures around the minimum soot formation temperature of 1300 K. PAHs appeared outside the soot layer, with characteristic temperature depending on the wavelength detection band. PAHs and soot began to appear at a constant axial position for each fuel, independent of the rate of air flow. PAH formation either preceded or coincided with soot formation, indicating that PAHs are important components in soot formation. Soot growth continued for some time downstream of the flame, at temperatures below the inception temperature, probably through reaction with PAHs.     

Measurement and analysis of soot inception limits of oxygen-enriched coflow flames

This study explores the criteria for soot inception in oxygen-enriched laminar coflow flames. In these experiments we select an axial height in the coflow flame at which to identify the sooting limit. The sooting limit is obtained by varying the amount of inert until luminous soot first appears at this predefined height. The sooting limit flame temperature is found to increase linearly with stoichiometric mixture fraction, regardless of fuel type. To understand these results, the relationships between flame structure, temperature, and local C/O ratio is explored through the use of conserved scalar relationships. Comparison of these relationships with the experimental data indicates that the local C/O ratio is a controlling parameter for soot inception in diffusion flames (analogous to the global C/O ratio in premixed flames). Analysis of experimental results suggests that soot inception occurs when the local C/O ratio is above a critical value. The values for critical C/O ratios obtained from the analysis of experiments using several fuels are similar in magnitude to the corresponding C/O ratios for premixed flames. In addition, temperatures and PAH fluorescence were measured to identify regions in these flames most conducive to particle inception. Results indicate that the peak PAH concentration lies along a critical iso-C/O contour, which supports a theory that soot particles first appear along this critical contour, given sufficient temperature.     

Mapping surrogate gasoline compositions into RON/MON space

In this paper, new experimentally determined octane numbers (RON and MON) of blends of a tri-component surrogate consisting of toluene, n-heptane, i-octane (called toluene reference fuel TRF) arranged in an augmented simplex design are used to derive a simple response surface model for the octane number of any arbitrary TRF mixture. The model is second-order in its complexity and is shown to be more accurate to the standard “linear-by-volume” (LbV) model which is often used when no other information is available. Such observations are due to the existence of both synergistic and antagonistic blending of the octane numbers between the three components. In particular, antagonistic blending of toluene and iso-octane leads to a maximum in sensitivity that lies on the toluene/iso-octane line. The model equations are inverted so as to map from RON/MON space back into composition space. Enabling one to use two simple formulae to determine, for a given fuel with known RON and MON, the volume fractions of toluene, n-heptane and iso-octane to be blended in order to emulate that fuel. HCCI engine simulations using gasoline with a RON of 98.5 and a MON of 88 were simulated using a TRF fuel, blended according to the derived equations to match the RON and MON. The simulations matched the experimentally obtained pressure profiles well, especially when compared to simulations using only PRF fuels which matched the RON or MON. This suggested that the mapping is accurate and that to emulate a refinery gasoline, it is necessary to match not only the RON but also the MON of the fuel.     

Experimental investigation and detailed modeling of soot aggregate formation and size distribution in laminar coflow diffusion flames of Jet A-1, a synthetic kerosene,and n-decane

A fully-coupled soot formation model is developed to predict the concentration, size, and aggregate structure of soot particles in the atmospheric pressure laminar coflow diffusion flames of a three-component surrogate for Jet A-1, a three-component surrogate for a Fischer–Tropsch Synthetic Paraffinic Kerosene (SPK), and n-decane. To model the chemical structure of the flames and soot precursor formation, a detailed chemical kinetic mechanism for fuel oxidation, with 2185 species and 8217 reactions, is reduced and combined with a Polycyclic Aromatic Hydrocarbon (PAH) formation and growth scheme. The mechanism is coupled to a highly detailed sectional particle dynamics model that predicts the volume fraction, structure, and size of soot particles by considering PAH-based nucleation, surface growth, PAH surface condensation, aggregation, surface oxidation, fragmentation, thermophoresis, and radiation. The simulation results are validated by comparing against experimental data measured for the flames of pre-vaporized fuels. The objectives of the present effort are to more accurately simulate the physical soot formation processes and to improve the predictions of our previously published jet fuel soot formation models, particularly for the size and aggregate structure of soot particles. To this end, the following improvements are considered: (1) addition of particle coalescence submodels to account for the loss of surface area, reduction of the number of primary particles, and increase of primary particle diameters upon collision, (2) consideration of a larger PAH molecule (benzopyrene instead of pyrene) for nucleation and surface growth to enhance the agreement between the soot model and the measured chemical composition of soot particles, and (3) implementation of a dimerization efficiency in the soot inception submodel to account for the collisions between PAH molecules that do not lead to dimerization. The results of two different particle coalescence submodels show that this process is too slow to account for the growth of primary particles, mainly because of the limited rate of particle collisions. Soot volume fraction predictions on the wings and at lower flame heights are considerably improved by using benzopyrene, due to the different distribution of the soot forming PAH molecule in the flame. The computed number of primary particles per aggregate and the diameters of primary particles agree very well with the experimentally measured values after implementing the dimerization efficiency for PAH collisions, because of the reduced rate of soot inception compared to growth by PAH condensation. Concentrations of major gaseous species and flame temperatures are also well predicted by the model. The underprediction of soot concentration on the flame centerline, observed in previous studies, still exists despite minor improvements.     

甲醇燃料火焰可见度的研究

甲醇火焰可见度低,因此就带来了使用中的安全性问题.作者采用一种实验室方法,对火焰的照度进行测量,从而提供了甲醇燃料火焰可见度的评估基础.对甲醇、柴油、汽油、正庚烷、异辛烷、甲苯及甲醇混合燃料共十二种燃料进行了研究,提供了不同燃料的火焰高度、燃烧速度及火焰照度的定量结果.纯甲醇火焰的照度约为汽油的千分之一,用汽油作添加剂可以提高甲醇火焰的照度,但在燃烧过程中随汽油的蒸发而照度下降.作者建议用着火一分钟后,测量期2秒内照度的平均值L与标准偏差σ的和L+σ作为评价可见度的参数.     

Physical and chemical comparison of soot in hydrocarbon and biodiesel fuel diffusion flames: A study of model and commercial fuels

Data are presented to compare soot formation in both surrogate and practical fatty acid methyl ester biodiesel and petroleum fuel diffusion flames. The approach here uses differential mobility analysis to follow the size distributions and electrical charge of soot particles as they evolve in the flame, and laser ablation particle mass spectrometry to elucidate their composition. Qualitatively, these soot properties exhibit a remarkably similar development along the flames. The size distributions begin as a single mode of precursor nanoparticles, evolve through a bimodal phase marking the onset of aggregate formation, and end in a self preserving mode of fractal-like particles. Both biodiesel and hydrocarbon fuels yield a common soot composition dominated by C_x ions, stabilomer PAHs, and fullerenes in the positive ion mass spectrum, and and C_2xH⁻ in the negative ion spectrum. These ion intensities initially grow with height in the diffusion flames, but then decline during later stages, consistent with soot carbonization. There are important quantitative differences between fuels. The surrogate biodiesel fuel methyl butanoate substantially reduces soot levels, but soot formation and evolution in this flame are delayed relative to both soy and petroleum fuels. In contrast, soots from soy and hexadecane flames exhibit nearly quantitative agreement in their size distribution and composition profiles with height, suggesting similar soot precursor chemistry.     

Experimental study of ethylene counterflow diffusion flames perturbed by trace amounts of jet fuel and jet fuel surrogates under incipiently sooting conditions

The structure of an ethylene counterflow diffusion flame doped with 2000 ppm on a molar basis of either jet fuel or two jet fuel surrogates is studied under incipient sooting conditions. The doped flames have identical stoichiometric mixture fractions (z_f = 0.18) and strain rates (a = 92 s⁻¹), resulting in a well-defined and fixed temperature/time history for all of the flames. Gas samples are extracted from the flame with quartz microprobes for subsequent GC/MS analysis. Profiles of critical fuel decomposition products and soot precursors, such as benzene and toluene, are compared.The data for C7-C12 alkanes are consistent with typical decomposition of large alkanes with both surrogates showing good qualitative agreement with jet fuel in their pyrolysis trends. Olefins are formed as the fuel alkanes decompose, with agreement between the surrogates and jet fuel that improves for small alkenes, probably because of an increase in kinetic pathways which makes the specifics of the alkane structure less important.Good agreement between jet fuel and the surrogates is found with respect to critical soot precursors such as benzene and toluene. Although the six-component Utah/Yale surrogate performs better than the Aachen surrogate, the latter performs adequately and retains the advantage of simplicity, since it consists of only two components.The acetylene profiles present a unique multimodal behavior that can be attributed to acetylene’s participation in early stages of formation of soot precursors, such as benzene and other large pyrolysis products, as well as in the surface growth of soot particles.