Nascent Soot Particles Research Articles

Effect of co-combustion of coal with biomass on the morphology of soot

This study performed an experimental investigation to explore the effect of co-combustion of coal with biomass on the morphological parameters and nanostructure of soot. Three types of soot, namely Shenghua coal (SH) soot, Pinewood biomass (PW) soot, and Co-combustion of SH and PW (CC) soot, were collected from the combustion flames of Shenhua coal, Pinewood biomass, and a mixture of coal and biomass. Thereafter, the soot samples were observed and analyzed by means of transmission electron microscope (TEM) and scanning electron microscope (SEM) to obtain their morphological parameters and nanostructure characteristics. The TEM results showed that the size of nascent soot particles generated in co-combustion flame is the largest, rather than between the sizes of PW and SH soot. The nanostructure of the three types of soot primary particles is mainly composed of straight parallel crystal stripes. The separation distance of crystal stripes in SH soot was measured to be larger than that in PW soot, indicating that the crystal structure of soot generated in Pine biomass combustion flame is more compact. It was also observed that both coal-derived primary soot particles and biomass-derived primary soot particles were formed in the co-combustion flame. SEM results indicated that PW soot agglomerates have stronger aggregation performance and more compact structure then SH soot and CC soot. The primary particle size of SH and CC soot agglomerates was mainly concentrated between 50 and 70 nm, while that of PW soot agglomerates was relatively uniform. Furthermore, the number of primary particles in SH soot and CC soot was distributed over a wide range compared to PW soot.

Experimental and simulation studies on flame characteristics and soot formation of C2H2 jet flames

This work aims to identify the relationship between the temperature and soot formation in the C2H2 laminar flames. The effects of flow rates and equivalence ratios on the flame temperatures and structures were investigated. The flame length increases with the increase of the equivalence ratio. Meanwhile, the flame color changes from pale blue to bright green from the fuel-lean to stoichiometric condition, and further to bright yellow under the ultra-rich conditions. By sampling and observing the morphology of soot at different heights of the flame, we reveal the factors affecting the size of soot. As the flame height increases, the growth rate of soot agglomerates becomes faster. The contribution of surface growth to the soot particle size decreases gradually. The collision and aggregation predominate the soot growth. To uncover the soot formation mechanism, the average particle size and particle distribution as a function of height above burner were correlated with flame temperatures. Furthermore, the underlying mechanisms behind the nucleation and growth of nascent soot particles from polycyclic aromatic hydrocarbons (PAHs) were reveal via ReaxFF molecular dynamics simulations. The results show that the aggregation ability of PAHs decreases as the temperature increases. The aggregation ability of PAHs is also affected by size, and the larger the size, the stronger the aggregation ability of PAHs. Both the experimental and simulation results provide novel insights into the development of combustion kinetic model for soot formation in sooting fuels.

Infrared spectral soot emission for robust and high-fidelity flame thermometry.

Spectral soot emission (SSE) in the visible spectrum is a popular technique for non-intrusive thermometry in sooting flames. However, its accuracy is restricted by uncertainties in the wavelength dependence of soot optical properties. We propose a novel infrared spectral soot emission method that successfully addresses this issue. Comprehensive light extinction experiments were firstly conducted to explore the spectral variation of soot optical property. The results indicated a wavelength independence of the soot absorption function provided the wavelength of the incident light is larger than 1000 nm, thereby indicating through the Kirchhoff law the potential of a robust thermometry using infrared (>1000 nm) spectral soot emissions. Proof-of-concept experiments were performed for sooting premixed flames of ethylene with different equivalence ratios. The results demonstrated that the new method provided more accurate temperature results compared with its visible-NIR counterpart, particularly at flame positions where nascent soot particles are present. The proposed method is, to our knowledge, the first infrared spectral soot emission-based thermometry, and is believed to offer a solution to improving the fidelity of SSE with a cost-effective optical setup.

Study Of Soot Aggregate Formation And Oxidation In A Swirled Stratified Premixed Ethylene/Air Flame

Soot particles are one of the main causes of today's pollution because of their negative contribution to global warming and human health. Aviation is one of the domains dealing with soot reduction as the new engine concepts are developed to reduce fuel consumption and global emissions. Despite the fact that most combustion devices used for air transportation operate at high pressure (e.g., aircraft gas turbines up to 40 bar), our understanding of soot formation and oxidation in such conditions is not yet at an appropriate level, as there is still a fundamental lack of experimental data and corresponding predictive models in the literature. Thus, the objective of the current study is to evaluate soot formation and oxidation processes in stratified, swirled, premixed ethylene/air flames examined with a variety of laser diagnostics designed to simultaneously measure soot particle and soot precursor 2D-distributions, as well as the flame structure and the aerodynamic field. For that, the SIRIUS burner was selected because of its ability to produce flames with topologies similar to those encountered in aircraft combustors. Soot particle distributions are measured by Planar Laser-induced incandescence (PLII) diagnostic. The flame structure is obtained by detecting the hydroxyl radicals (OH) with Planar laser-induced fluorescence (PLIF). A second PLIF diagnostic is also used to investigate the production of polycyclic aromatic hydrocarbons (PAH) with the detection of two benzene rings molecules which are recognized as good precursors of soot nucleation and growth. Finally, the particle Image Velocimetry (PIV) diagnostic is used for measuring the velocity distributions. These laser diagnostics are coupled together in order to obtain cross-correlations between several scalar parameters playing a determining role in the soot formation/consumption processes. The experimental results collected at atmospheric pressure are reviewed and critically assessed. A scenario describing the link between the soot inception, growth, aggregation and oxidation processes is proposed by analyzing velocity, OH, PAHs and soot distributions. In particular, the data reveal the presence of distinct regions for these processes. Incipient soot production zone is strongly function of specific local conditions of velocity, PAH concentration, and strain rate encountered at the interface of the internal recirculation zone and the fuel/air jet. The central part of the inner recirculation zone in which large structures move at low velocities provides suitable conditions for the aggregation of nascent soot particles while an oxidation region located in the upper zone of the internal recirculation zone favors the consumption of soot.

Raman Spectroscopy of Nascent Soot Oxidation: Structural Analysis During Heating

The effect of oxidation on nascent soot particles is investigated in this work by means of Raman spectroscopy. Three different soot particle samples are produced in a lightly sooting ethylene/air laminar premixed flame and collected at different residence times. The samples are characterized by a different size of primary particles and a different degree of aging and hence graphitization, and they are representative of the early evolution of nascent soot in flames. Oxidation of particles is induced by exposing the samples to a high temperature oxidation treatment in air over a range of temperature 25–500°C. To this aim, a programmable heating microscope stage is used. Thermo-optical transmission (TOT) measurements are used to characterize the chemical composition of pristine particles in terms of organic and elemental carbon content, also providing the pyrolytic carbon fraction of the organic carbon. The TOT results show that the organic fraction is constant for the three conditions. On the other hand, the pyrolytic carbon fraction decreases and the elemental carbon increases when moving from the sample composed of just-nucleated particles to the sample mostly made of mature soot particles. Raman spectroscopy reveals that the thermal oxidation treatment performed on the sample of just-nucleated particles, with the highest organic carbon content, results in a reduction of the amorphous carbon component. Conversely, the sample of mature soot, with the highest elemental carbon content, shows an increase in the amorphous carbon phase after oxidation, which is attributed to fragmentation or the formation of point defects by O2 oxidation. Finally, the thermal oxidation procedure produces a strong reduction in the photoluminescence signal detected from Raman spectra.

The Effect of Flame Temperature on Nascent Soot Particles in a Series of Premixed Ethylene–Oxygen–Nitrogen Stagnation Flames

The Effect of Flame Temperature on Nascent Soot Particles in a Series of Premixed Ethylene–Oxygen–Nitrogen Stagnation Flames

Hydrogen as a fuel additive in laminar premixed methane flames: Impact on the nucleation and growth of soot particles

This work reports new experimental data regarding the impact of H2 on slightly sooting methane flames (Φ–1.91) characterized by the formation of soot particles undergoing growth processes. This study has been carried out in three atmospheric sooting premixed methane flames with added H2 (2% of the total flowrate) or substituted (10% molar comparing to CH4 fuel).The mole fraction profiles of selected aliphatic and aromatic compounds (C1-C16) have been measured by Gas Chromatography and Jet-Cooled Laser-Induced Fluorescence. Concerning the soot particles, we have implemented for the first time an analytical methodology based on in-situ measurements by Laser-induced Incandescence and Cavity Ring-Down Spectroscopy coupled with ex-situ measurements by Scanning Mobility Particle Sizing (SMPS). This original approach allows to determine the soot volume fraction profiles and corresponding evolution of the particle number density and particles size density function without requiring accurate knowledge of the dilution ratio during SMSP measurements.The reported data highlight that the introduction of H2 strongly influences the formation of aromatic species and soot depending on the operating conditions (H2 addition or substitution). The impact of H2 was evaluated on the nucleation step and the soot growth processes. In that context, we notably show that the H2 substitution inhibits while the H2 addition favors the formation of nascent soot particles, and furthermore the H2 addition generates substantial modifications in the dynamic of the soot growth processes. This work also supports the idea that the onset of the particle nucleation might be controlled by specific concentration thresholds of aromatic precursors.

Impact of the maturation process on soot particle aggregation kinetics and morphology

Mature soot aggregates exhibit a morphology which is currently mimicked by diffusion-limited aggregation (DLCA) codes, i.e. with a sticking probability = 1 when two aggregates collide. Nevertheless, nascent soot particles may grow in the reaction-limited aggregation regime (RLCA), i.e. with a sticking probability ≪ 1. Yet, it remains to be seen how fast the transition from RLCA to DLCA occurs for soot particles and what is the impact on aggregation kinetics, particle size distribution and morphology. This work intends to fill this gap by exploring the aggregation of soot particles formed in a laminar premixed flame through numerical simulations. Results show that the transition from RLCA to DLCA is very fast and produces a moderate impact on soot formation dynamics and morphology. However, soot particles mass bulk density is found to play an important role and should be considered in future simulations of soot formation in flames.

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.

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.

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.

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.

Unveiling trends in soot nucleation and growth: When secondary ion mass spectrometry meets statistical analysis

At the present day, ultrafine soot particles have become the object of increasing attention due to their well-documented adverse effects on human health and climate. In particular, understanding soot nucleation is one of the most challenging problems toward a more controlled and cleaner combustion. Detailed information on the chemistry of nascent soot particles (NSPs) is expected to provide clues on the soot formation and growth reaction pathways. Herein, the early steps of soot formation in flames are addressed by investigating the chemical composition of NSPs and their molecular precursors by secondary ion mass spectrometry. The originality of this work lies on the combination of several factors that include: two different approaches to gain access to soot nucleation in premixed flames (analysis of a nucleation flame in which NSPs nucleate all along the reaction coordinate without growing in size) and in diffusion flames (separation of the precursors and soot regions); a sampling procedure that allows a rough separation of the condensable gas phase from soot particles; a large database of collected samples and high mass resolution that enable the use of an improved data reduction protocol based on hierarchical cluster analysis and principal component analysis for the data mining and interpretation.

Partitioning of polycyclic aromatic hydrocarbons in heterogeneous clusters

The morphologies of heterogeneous clusters of polycyclic aromatic hydrocarbons (PAHs) are investigated using molecular modelling. Clusters of up to 100 molecules containing combinations of the different sized PAHs circumcoronene, coronene, ovalene, or pyrene are evaluated. Replica exchange molecular dynamics simulations using an all-atom force field parameterised for PAHs sample many configurations at high and low temperatures to determine stable low energy structures. The resulting cluster structures are evaluated using molecular radial distances and coordination numbers, and are found to be independent of initial configuration and the cluster sizes studied. Stable clusters consist of stacked PAHs in a core-shell structure, where the larger PAHs are found closer to the cluster core and the smaller PAHs are located on the cluster surface. This work provides novel insight into the molecular partitioning of heterogeneous aromatic clusters, with particular relevance to the structure of nascent soot particles.

On imaging nascent soot by transmission electron microscopy

High-resolution Transmission Electron Microscopy (HRTEM) imaging of nascent soot was carried out with an emphasis in demonstrating the annealing of soot samples under continuous irradiation of the high-energy electron beam. Images were taken for several soot samples over the duration of 16 min in 2 min time intervals to reveal the crystallization process. Fringe properties, including fringe length, tortuosity, and spacing were analyzed over the duration of the imaging. The sensitivity of the fringe properties to the apparent changes in the nanostructures imaged was examined. The difficulties in quantifying soot composition and structures are further illustrated by analyzing simulated TEM images of molecular-dynamics generated particles. Together, the results highlight some of the challenges in using HRTEM to obtain unambiguous structural properties for nascent soot particles.

Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot

Abstract. Optical properties of flame-generated black carbon (BC) containing soot particles were quantified at multiple wavelengths for particles produced using two different flames: a methane diffusion flame and an ethylene premixed flame. Measurements were made for (i) nascent soot particles, (ii) thermally denuded nascent particles, and (iii) particles that were coated and then thermally denuded, leading to the collapse of the initially lacy, fractal-like morphology. The measured mass absorption coefficients (MACs) depended on soot maturity and generation but were similar between flames for similar conditions. For mature soot, here corresponding to particles with volume-equivalent diameters >∼160 nm, the MAC and absorption Ångström exponent (AAE) values were independent of particle collapse while the single-scatter albedo increased. The MAC values for these larger particles were also size-independent. The mean MAC value at 532 nm for larger particles was 9.1±1.1 m2 g−1, about 17 % higher than that recommended by Bond and Bergstrom (2006), and the AAE was close to unity. Effective, theory-specific complex refractive index (RI) values are derived from the observations with two widely used methods: Lorenz–Mie theory and the Rayleigh–Debye–Gans (RDG) approximation. Mie theory systematically underpredicts the observed absorption cross sections at all wavelengths for larger particles (with x>0.9) independent of the complex RI used, while RDG provides good agreement. (The dimensionless size parameter x=πdp/λ, where dp is particle diameter and λ is wavelength.) Importantly, this implies that the use of Mie theory within air quality and climate models, as is common, likely leads to underpredictions in the absorption by BC, with the extent of underprediction depending on the assumed BC size distribution and complex RI used. We suggest that it is more appropriate to assume a constant, size-independent (but wavelength-specific) MAC to represent absorption by uncoated BC particles within models.

Experimental and numerical study of the evolution of soot primary particles in a diffusion flame

The evolution of primary soot particles is studied experimentally and numerically along the centreline of a co-flow laminar diffusion flame. Soot samples from a flame fueled with C2H4 are taken thermophoretically at different heights above the burner (HAB), their size and nano-structure are analysed through TEM. The experimental results suggest that after inception, the nascent soot particles coagulate and coalesce to form larger primary particles ( ∼ 5 to 15 nm). As these primary particles travel along the centreline, they grow mainly due coagulation and condensation and a layer of amorphous hydrocarbons (revealed by HRTEM) forms on their surface. This amorphous layer appears to promote the aggregation of primary particles to form fractal structures. Fast carbonisation of the amorphous layer leads to a graphitic-like shell around the particles. Further graphitization compacts the primary particles, resulting in a decrease of their size. Towards the flame tip the primary particles decrease in size due to rapid oxidation. A detailed population balance model is used to investigate the mechanisms that are important for prediction of primary particle size distributions. Suggestions are made regarding future model development efforts. Simulation results indicate that the primary particle size distributions are very sensitive to the parameterization of the coalescence and particle rounding processes. In contrast, the average primary particle size is less sensitive to these parameters. This demonstrates that achieving good predictions for the average primary particle size does not necessarily mean that the distribution has been accurately predicted.

HOMO-LUMO energy splitting in polycyclic aromatic hydrocarbons and their derivatives

Abstract The HOMO-LUMO gap of polycyclic aromatic hydrocarbons (PAHs) with and without functional group substitutions was computed using the B3LYP methods with the 6-311 + + G(d,p) basis set. The functional groups considered are ethyl (C2H5), phenyl (C6H5), hydroxyl (OH), vinyl (C2H3), carboxyl (COOH) and formyl (CHO). The purpose was to explore the impact of substitution on the electronic structure of PAHs and provide a rational explanation for the optical band gap values measured for nascent soot particles formed in flames. Results suggest a high sensitivity of the HOMO-LUMO gap on the type and number of functional groups in a PAH molecule. It was found that single -OH, -COOH and -CHO groups on PAHs can reduce the HOMO-LUMO gap by 0.2, 0.4 and 0.5 eV, respectively. Further functionalization of PAH by several CHO groups could reduce the HOMO-LUMO gap by as much as 0.7 eV. It is concluded that the impact of functional groups in PAHs cannot be neglected while interpreting the optical band gap measured for nascent soot particles.

On the Response of Nascent Soot Nanostructure and Oxidative Reactivity to Photoflash Exposure

Soot particles are a kind of major pollutant from fuel combustion. To enrich the understanding of soot, this work focuses on investigating detailed influences of instantaneous external irradiation (conventional photoflash exposure) on nanostructure as well as oxidation reactivity of nascent soot particles. By detailed soot characterizations flash can reduce the mass of soot and soot nanostructure can be reconstructed substantially without burning. After flash, the degree of soot crystallization increases while the soot reactive rate decreases and the activation energy increases. In addition, nanostructure and oxidative reactivity of soot in air and Ar after flash are different due to their different thermal conductivities.