Nascent Soot Research Articles

Numerical investigation of pressure effects on soot formation in laminar coflow ethylene/air diffusion flames

This study aims to provide fundamental understandings of the pressure effects on the soot formation and compare the performances of different soot aerosol models. Numerical simulations are performed in laminar coflow diffusion flames at pressures ranging from 1 to 16 bar. Two soot aerosol models are considered: the acetylene-based semi-empirical (SE) model and polycyclic aromatic hydrocarbons (PAH) based hybrid method of moment (HMOM). To study the effect of large-sized PAH species, a detailed reaction mechanism is used with PAH pathways up to coronene. Results show that the SE model provides good predictions of pressure scaling of peak soot mass with a deviation of 7%, while HMOM obtains better soot predictions on the flame centerline. Due to the shifting of PAH position towards the burner with increasing pressure, the nascent soot is formed earlier. The increase in the particle residence time is found to be an additional factor that further promotes the increased soot formation with pressure, apart from the increase in density, temperature, and PAH concentration. The residence time at 8 bar case is 2.5 times and 3.0 times longer than those at 1 bar case on the flame centerline and flame wings, respectively. Moreover, the pressure effects on the PAH contribution to the nucleation process are studied. Although small-sized PAH species (A2 and A2R5) dominate the nucleation process, the contribution of large-sized PAH species (larger than A4) increases from 5% to 20% of the total on the flame wings, when the pressure increases from 1 bar to 8 bar.

Formation of nascent soot during very fuel-rich oxidation of ethylene at low temperatures

Soot formation during very fuel-rich oxidation of ethylene (with equivalence ratio, φ, ranging from 3 to infinity) at relatively low temperatures (1473–1673 K) was studied in a flow reactor, in which temperature was well controlled through an external heater, and φ of the reactants was adjusted by adding different amounts of O2 (0–6000 ppm) at the inlet of the reactor. Detailed sooting properties including soot volume fraction, number density, particle size distribution (PSD), and morphology were examined. The results show that at a temperature of 1673 K, the total soot yield increases monotonously with φ. However, at 1473 and 1573 K, the soot volume fraction increases first and then decreases as φ increases from 3 to infinity, which means a small amount of oxygen can promote soot formation. The PSD and number density results indicate that this promotion effect is more prominent at the nucleation stage and weakens as soot grows at longer residence times. In addition, soot morphology is affected mostly by temperature rather than φ, and the decrease of temperature leads to larger primary particles and less fractal aggregates. Kinetic modeling results indicate that the concentrations of PAHs and soot increase because a small amount of O2 addition can enhance the C3 + C3 pathways through the generation of reactive oxygenated species.

On the use of PIV, LII, PAH-PLIF and OH-PLIF for the study of soot formation and flame structure in a swirl stratified premixed ethylene/air flame

Soot formation and oxidation are investigated in swirl stratified premixed ethylene/air flames at atmospheric pressure. The effects of both swirl and stratification are studied to understand the relationship between the flame structure, soot precursors and soot. The topology of the flame is obtained with particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) on hydroxyl radical (OH). The production of polycyclic aromatic hydrocarbons (PAHs) is investigated using PLIF by mainly probing the aromatic compounds with two benzene rings (i.e. naphthalene) that are known to actively participate in soot nucleation and growth. Soot production is investigated using laser-induced incandescence (LII), giving quantitative data on the soot volume fraction. Extensive information on the flame structure and the mechanisms of formation/consumption of soot is gathered based on the coupling of these laser diagnostics. An image analysis of velocity, OH, PAHs and soot distributions enables us to propose a scenario that describes the link between the inception, growth, aggregation and oxidation processes. In particular, the data reveal the presence of distinct zones for these processes: a thermal decomposition region in which PAHs contributes to nascent soot formation, organised as filaments along the interface between the PAH region and the inner recirculation zone (IRZ); a mixing region controlled by large moving structures that favour the growth and aggregation of nascent soot into mature soot; and an oxidation region leading to the fast consumption of soot particles. These processes are impacted to varying extents by the intensities of swirl and stratification.

Molecular content of nascent soot: Family characterization using two-step laser desorption laser ionization mass spectrometry

Molecules constituting nascent soot particles have been analyzed by two-step laser desorption laser ionization mass spectrometry. Three samples have been collected from a slightly sooting ethylene/air premixed flame with the aim to investigate soot composition in the transition from nucleated to just-grown soot particles. Sampling locations have been selected based on the evolution of the particle size distribution along the flame axis. The mass spectrometric results point to a strong evolution of the molecular composition. Just-nucleated soot is rich in polycyclic aromatic hydrocarbons (PAHs) dominated by medium sizes from 18 to 40 carbon atoms but containing sizes as large as 90 carbon atoms. Most abundant PAHs are in the form of peri-condensed structures. The presence of a large fraction of odd numbered carbon species shows that pentagonal cycles are a common feature of the detected population. Increasing the distance from the burner outlet, i.e., the particle residence time in flame, leads to an evolution of the chemical composition of this population with a major contribution of carbon clusters including also fullerenes up to about 160 carbon atoms. Our data support a scenario in which large PAHs containing pentagonal rings evolve very efficiently upon thermal processing by a series of dehydrogenation and isomerization processes to form fullerenes. This chemistry happens in the early steps of soot growth showing that carbonization is already active at this stage.

Radicals in nascent soot from laminar premixed ethylene and ehtylene-benzene flames by electron paramagnetic resonance spectroscopy

In this work, we present particle size distribution (PSD), electron paramagnetic resonance (EPR) and Raman spectroscopy measurements of soot particles, collected across the inception region, in two ethylene-air premixed laminar flames and two flames obtained by substituting 30% of the ethylene carbon with benzene. The experimental results are used to investigate the effect of aromatic fuel on the nature and role of radicals in particle nucleation and growth. For all the particles sampled, both in pure ethylene flames and in the ethylene/benzene ones, an EPR signal typical of persistent carbon-centered aromatic radicals is measured. The results point out an involvement of π-radicals during particle nucleation and initial growth process. Under the conditions investigated, our results show that the electron spin density of particles produced by different fuels is mainly related to the amount of H-atoms available in the particles. Moreover, for all the investigated flames, the spin density presents an inverse dependence with the particle size suggesting that particle surface plays an important role in the early steps of the soot formation process.

Determination of the volume fraction of soot accounting for its composition and morphology

The soot volume fraction, fv, is essential for combustion engineering, air quality and climate modeling. It is commonly obtained from mobility or optical data assuming soot spheres of constant composition that tend to overestimate fv by up to an order of magnitude. Here, a method is presented for estimation of the fv from soot mobility or optical diagnostics data accounting for the realistic soot morphology and composition. With mobility data, the fv is determined by a relation for soot morphology (fractal-like structure) and the average primary particle diameter from microscopy, gas adsorption or power laws. Such fv and the corresponding soot volume/mass size distributions are in excellent agreement with accurate but tedious soot mass-mobility measurements in premixed flames along various heights above the burner (HAB) as well as from the exhaust of diffusion flames and diesel engines. With light extinction data, the fv is obtained by explicitly accounting for soot composition (through its optical band gap and refractive index, RI) and the light absorption enhancement (due to multiple light scattering within soot agglomerates) in the absorption function, E(RI). So as nascent soot matures by surface growth and agglomeration, its optical band gap decreases while its mobility diameter, carbon to hydrogen (C/H) ratio and fv increase, increasing the soot E(RI) from 0.22 to 0.40, in excellent agreement with laser induced incandescence data from flat premixed ethylene flames. Using this variable E(RI), the soot fv from light extinction data in these flames is up to 50% lower than that using standard constant E(RI) for soot spheres.

Evidence on the formation of dimers of polycyclic aromatic hydrocarbons in a laminar diffusion flame

The role of polycyclic aromatic hydrocarbons (PAHs) in the formation of nascent soot particles in flames is well established and yet the detailed mechanisms are still not fully understood. Here we provide experimental evidence of the occurrence of dimerization of PAHs in the gas phase before soot formation in a laminar diffusion methane flame, supporting the hypothesis of stabilization of dimers through the formation of covalent bonds. The main findings of this work derive from the comparative chemical analysis of samples extracted from the gas to soot transition region of a laminar diffusion methane flame, and highlight two different groups of hydrocarbons that coexist in the same mass range, but show distinctly different behavior when processed with statistical analysis. In particular, the identified hydrocarbons are small-to-moderate size PAHs (first group) and their homo- and heterodimers stabilized by the formation of covalent bonds (second group).

Comparative study on soot characteristics of non-swirling and swirling inverse diffusion iso-octane flames with biofuel 2,5-dimethylfuran addition

Due to the heavy consumption of the fossil fuels in the transportation sector and the furnace factory, a large amount of soot is discharged and seriously threatened the environment and the human health. The present work focuses on examining the characteristics of soot produced from iso-octane/2,5-dimethylfuran (DMF) non-swirling and swirling inverse diffusion flames (IDFs). Morphology, nanostructure, graphitization and oxidation reactivity of nascent soot particles from 100% iso-octane, 50% iso-octane/50% DMF and 100% DMF IDFs were investigated by using high resolution transmission electron microscopy, X-ray diffraction, and thermogravimetric analysis, respectively. The effect of swirling on the soot characteristics was discussed. Results demonstrate that the nascent soot from pure iso-octane flames shows the morphology with liquid-like deposition and turbostratic nanostructure, while the soot from pure DMF displayed more primary particle with the nanostructure contained by many well-organized lattice fringes. Because the mixability of air and fuel under the action of swirling is higher than that of non-swirling condition, the soot particles from swirling conditions reveal more ordered nanostructure with longer fringe length and lower fringe tortuosity, as well as higher soot graphitization and lower oxidation reactivity.

Effect of phosphorus on soot formation and flame retardancy in fires

The mechanism by which phosphorus imparts flame retardancy to a burning polymeric solid remains controversial because of the complex coupling of gas and condensed phase reactions. To address that, an organophosphorous compound was mixed into epoxy at different concentrations and burned under diffusion flame conditions. The yield of carbon monoxide and soot was found to increase in the presence of phosphorus, significantly reducing the combustion efficiency. The reduction in heat release was attributed to a heterogeneous mechanism that promotes soot formation. Phosphorus does not exhibit a noticeable effect on soot morphology. Element analysis reveals that 75% of phosphorus in the virgin sample is incorporated into soot particles. More aromatic structures were identified under the influence of phosphorus. Soot surface reactivity was found to decrease as phosphorus loading increases. The main flame retardancy mechanism of phosphorus is through promoting soot formation in the gas phase. It is speculated that it increases direct condensation of aromatic structures in the soot formation path and decreases the number of the active sites on the particle surface for soot oxidation. The increased soot yield cools the flame by radiative heat loss, further hindering the dehydrogenation process that converts nascent soot to mature soot.

Role of dimethyl ether in incipient soot formation in premixed ethylene flames

Emissions of soot particles exert crucial impact on human health and the environment. Previous researches show that a partial replacement of fossil fuels with oxygenated fuels is supposed to reduce soot emission. However, the crucial information of soot size distributions is unclear, although small particles are more dangerous than large particles. In this paper, dimethyl ether (DME) is selected among the oxygenated fuels for the investigation of the doping effect on nascent soot formation. DME was doped in the burner-stabilized rich premixed ethylene/oxygen/argon flames at various mixing ratios. Flame temperature profiles were measured using R-type thermocouples with radiation correction. A scanning mobility particle sizer (SMPS) with a sampling system was used to determine particle size distributions (PSDs) of soot-containing combustion products with immediate dilution. Thermo-gravimetric analysis (TGA) complemented with elemental analysis (EA) was conducted to detect the chemical properties of formed soot particles. Additionally, species profiles of the experimental flame conditions were provided from simulations. The experimental PSDs showed that DME addition could slow down soot evolution process, while lead to a slight increase in incipient soot with size below 4 nm. When DME added, the production of benzene was suppressed due to the reduced concentrations of acetylene and propargyl, and thus soot nucleation. Meanwhile, soot oxidiation was enhanced because the OH radical and the oxidizability of soot particles are both increased. Considering the slight increase of sub-4 nm soot, the effect of oxygenated-PAHs (OPAH) was emphasized. The production of OPAH suppressed soot surface growth, which leading to the relatively lower consumption of sub-4 nm soot in DME added flames.As a result, DME cannot be simply regarded as a clean fuel due to the enhanced formation of sub-4 nm soot particles. When applying oxygenated fuels into practice, it is necessary to pay more attention to the size distributions of the emitted particles, and conduct appropriate post-processing techniques to reduce the emissions of small particles.

Molecular dynamics simulation of soot formation during diesel combustion with oxygenated fuel addition.

This study investigates the soot formation process of diesel combustion using molecular dynamics simulations with reactive force fields by examining the effects of five oxygenated additives on diesel soot reduction. The newly improved ReaxFF CHO2016 parameters are employed due to their feasibility in modelling the kinetics of large hydrocarbon fuels and describing the chemistry of carbon condensed phases. The detailed pathways of soot formation examined include the thermal decomposition of fuels, the formation of aromatic rings, the mechanism of nucleation, and the mass growth of nascent soot. The morphological developments of the soot formation are obtained, together with quantities indicating the physical and chemical properties, such as mass, size, C : H ratio and aliphatic-to-aromatic C : H ratio. The role of aliphatic-substituted aromatics in nascent soot coalescence and the appearance of aliphatic side chains in soot structures are identified. The concentrations of C-atoms in nascent soot particles contributed from oxygenates blended with diesel are quantified and visualized for the first time. The effects of the molecular structure of the oxygenated additives, i.e. the existence of esters, alcohols, carbonyl groups and ethers, on soot precursor mitigation are elucidated by evaluating the early formation of CO and CO2 quantitatively during the thermal decomposition.

Reactive sites on the surface of polycyclic aromatic hydrocarbon clusters: A numerical study

The surface growth of soot is a key process in its mass growth, depending crucially on surface properties. In this work, we directly extract the detailed surface properties, such as surface area, number density of reactive sites on a particle and parameter α, from the microscopic configuration of polycyclic aromatic hydrocarbon (PAH) clusters. Five representative PAHs, including pyrene (C16H10), coronene (C24H12), ovalene (C32H14), hexabenzocoronene (C42H18) and circumcoronene (C54H18), are used to build the model configurations of nascent soot. We develop a numerical scheme to determine the detailed surface properties based on the approximation of solvent-excluded surface. The assumption of spherical particles is found to introduce a large uncertainty in the estimation of the surface area, and the error can be a factor of 2.5 in the worst case. The number density of atoms or sites on cluster surfaces does not depend on the chemical composition of a particle larger than 2 nm in diameter, and our study indicates that the number density of hydrogen atoms is overestimated by a factor of 3 in the literature. Finally, we propose a new equation for parameter α, which includes the effects of the size of gaseous species in surface reaction, local temperature, particle size and chemical composition.

The role of fuel chemistry in dictating nanostructure evolution of soot toward source identification

Laser derivatization is proposed as a diagnostic technique toward identifying the sources contributing to combustion produced soot. Fuel chemistry and the resultant oxygen content in nascent soot h...

Size Distribution of Nascent Soot in Premixed n-Hexane, Cyclohexane, and Methylcyclohexane Flames

The evolution of particle size distribution function (PSDF) of nascent soot in premixed flames of n-hexane, cyclohexane, and methylcyclohexane was studied on the burner-stabilized stagnation flame platform. The cold gas velocities were changed to keep the maximum flame temperatures of different flames approximately constant. The PSDFs of all fuels tested transited from the unimodal (nucleation mode only) to the bimodal (both nucleation and accumulation modes) distribution with increased particle residence time. Soot nucleation and growth in the n-hexane flame were found to appear later than those in the cyclohexane and methylcyclohexane flames. In addition, the methylcyclohexane flame exhibited the highest nucleation and accumulation rate at the early stage of soot formation. However, the particle nucleation strength and the soot mass growth rate of the cyclohexane flame exceeded those of the methylcyclohexane flame at long particle residence times.

Investigation on the LIF spectrum superposition of gas-phase PAH mixtures at elevated temperatures: potential for the analysis of PAH LIF spectra in sooting flames

Laser-induced fluorescence (LIF) technique has been widely used to measure the polycyclic aromatic hydrocarbons (PAHs) in sooting flames, but the assignment of the LIF signals measured in flames to specific PAH species is hard to obtain. Investigation on the LIF spectra of gas-phase PAH mixtures could be helpful to analyze LIF spectra measured in flames for more detailed PAH information. In this study, the LIF spectra of gas-phase PAH mixtures containing naphthalene, phenanthrene, pyrene, fluoranthene were experimentally investigated in an optical cell at 673 K and 1073 K. The experimental results indicated that the integral fluorescence intensity of each PAH was in direct proportion to its mole fraction. Moreover, the LIF spectra of PAH mixtures were linearly superposed from the spectra of their components weighted using mole fractions. The component concentrations of the PAH mixtures were able to be fitted from the LIF spectra using multiple linear regression method, and most of the relative errors were less than 5%. In the light of these results from gas-phase PAHs, the LIF spectra of flame-formed PAHs measured in a laminar premixed ethylene flame were discussed. Considering the temperature effects on LIF spectra and simulated PAH concentrations in the flame, the lighter PAHs will contribute to the fluorescence signals shorter than 450 nm measured in the flame. Using the multiple linear regression method established in this study, the relative ratios of naphthalene, phenanthrene and pyrene can be fitted from the LIF spectrum of the flame. In this way, more selective information of these lighter PAHs can be obtained from the LIF spectrum measured in flames. On the other hand, fluorescence signals in the flame longer than 450 nm are possibly emitted from the PAHs with five-membered-ring structures or (and) nanoparticles and nascent soot particles, which are still ambiguous and needs to be further investigated in future works.

Structural effects on the growth of large polycyclic aromatic hydrocarbons by C2H2

Soot particles are composed of planar and curved polycyclic aromatic hydrocarbons with different types of reactive sites, where their growth and oxidation reactions occur. This study presents the effect of curvatures in PAHs present in soot on their growth in the flame environment. For this, six planar and curved model PAH molecules having five to nine aromatic rings with armchair and zigzag sites are selected. Density functional theory (B3LYP functional and 6-311G (d,p) basis set) is used to study the reaction energetics for their growth through hydrogen-abstraction-C2H2-addition mechanism that is primarily used in all soot models for their chemical growth. The rate constants evaluated using transition state theory for the reactions involved in the mechanism are provided. Through energetics and kinetics comparison, the differences in the reactivity of planar and curved PAHs at different site types are observed. The growth at armchair site was found to be sensitive to the PAH structure, with the curved one having higher growth rate at all temperatures between 1000 and 2500 K studied in this work. The PAH curvature had less impact on the growth at zigzag sites of the model PAHs. The fast conversion of planar PAH to a curved one through ring addition and the high growth rate of curved PAHs at low temperatures suggests that tortuous PAHs can also be present in nascent soot.

Laser-induced structural modifications of differently aged soot investigated by HRTEM

Laser heating was applied to soot probed in the first inception region (nascent soot) and downstream (mature soot) of premixed methane and ethylene flames, burning in similar temperature and equivalence ratio conditions. Different properties of nascent and mature soot were investigated by TEM/HRTEM and EELS along with elemental analysis, UV–visible and Raman spectroscopy. Structural modifications were followed by HRTEM imaging applied to the soot before and after laser heating. Laser heating produced varied modifications of the lamellae organization dependent upon the morphology of pristine soot, in turn dependent on soot aging and fuel type (ethylene and methane). Specifically, structures in form of void shells versus multifaceted rosette structures were observed in the case of young nascent and mature soot, respectively, showing a relationship with their different nanostructures, namely amorphous versus disordered turbostratic structures. Additionally, the differentiation between void shells or multifaceted rosette structures can be used as signature for the occurrence of laser heating effects, which have to be avoided when the laser is instead employed for diagnostic purposes.

Nanostructure and reactivity of nascent carbon particles from 2,5-dimethylfuran/n-heptane swirling inverse diffusion flames

Carbon particles (Soot) have been the one of primary pollutants inevitably with the combustion of fossil fuels. A better understanding on incipient soot was useful to build the models of soot formation and even control soot emissions. The present work focuses on the nanostructure and oxidation reactivity of nascent carbon particles (soot) formed from 2,5-dimethylfuran (DMF)/n-heptane non-swirling and swirling inverse diffusion flames (IDFs). The nascent soot samples were derived from three different fuels: 100% n-heptane, 50% n-heptane/50% DMF and 100% DMF. In addition, the effects of swirling combustion and collection time on characteristics of nascent soot were investigated in detail using the high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Results demonstrated that nascent soot from pure n-heptane flames presented the film-like morphology and nanostructure of amorphous nature, while the nearly primary particles with more well-organized nanostructure were found in pure DMF soot. Swirling combustion increased the entire flame front areas and diameters due to the existence of tangential velocity at the outlet of burner. Moreover, swirling combustion could enhance the mixability of the oxidizer stream and fuel stream, leading to more young soot within the fuel stream being oxidized in the inception stage. However, the collection time on soot characteristics exhibited quite negligible impacts in comparison to the swirling effects.

Ion-Induced Soot Nucleation Using a New Potential for Curved Aromatics

ABSTRACTA potential able to capture the properties and interactions of curved polycyclic aromatic hydrocarbons (cPAHs) was developed and used to investigate the nucleation behavior and structure of nascent soot particles. The flexoelectric charge polarization of cPAHs caused by pentagon integration was included through the introduction of off-site virtual atoms, and enhanced dispersion interaction parameters were fitted. The electric polarization and intermolecular interactions of cPAHs were accurately reproduced compared to ab initio calculations. This potential was used within molecular dynamics simulations to examine the homogeneous and heterogeneous nucleation behavior of the cPAH corannulene and planar PAH coronene across a range of temperatures relevant to combustion. The enhanced interactions between cPAHs and potassium ions resulted in significant and rapid nucleation of stable clusters compared to all other systems, highlighting their importance in soot nucleation. In addition, the resulting cPAH clusters present morphologies distinct from the stacked planar PAH clusters.

Evaluation of the effect of nickel clusters on the formation of incipient soot particles from polycyclic aromatic hydrocarbons via ReaxFF molecular dynamics simulations.

In the present study, the ReaxFF reactive molecular dynamics simulation method was applied to investigate the effect of a small nickel cluster (Ni13) on the formation of nascent soot from polycyclic aromatic hydrocarbon (PAH) precursors. A series of NVT simulations was performed for systems of a Ni13 cluster and various PAH monomers, namely, naphthalene, anthracene, pyrene, coronene, ovalene, and circumcoronene, at temperatures from 400 to 2500 K. At low temperatures, the PAHs form soot particles via binding and stacking around nickel clusters. Larger soot particles are formed due to the early initiation of clustering provided by nickel compared to those observed in homogenous PAH systems. At 1200-1600 K, the PAH monomers show a chemisorption tendency onto the nickel surface, which results in incipient soot particles. Chemical nucleation was observed at 2000 K where nickel-assisted dehydrogenation and chemisorption of PAH led to the growth of stable soot particles, which did not occur in the absence of Ni-clusters. At a high temperature (2500 K), nickel significantly accelerates the ring-opening and graphitization of PAH molecules and increases the size of the fullerene-type soot as compared to that of homogenous systems.