Incipient Soot Research Articles

The effect of sodium chloride on the charge state of soot particles in a laminar diffusion flame

This study investigates the effect of sodium chloride on the charge state of soot nanoparticles formed in a laminar diffusion flame by measuring the soot particle size distribution, average charge per particle, and charge fraction at various heights within the flame. The well-studied Santoro burner with methane as the fuel (at 0.35 L/min) and co-flow air (at 70 L/min) was used, which produced a stable laminar diffusion flame with flame height of 61 mm. Samples of soot nanoparticles were extracted via a 0.3 mm orifice in a 3-mm stainless steel tubular probe at various heights above the burner and were immediately diluted by a factor of a few thousand for aerosol measurement. The effect of sodium chloride on a methane flame was investigated by comparing the experimentally measured data for methane-only and methane-NaCl flames. The addition of NaCl particles to the laminar diffusion flame did not have a significant effect on the particles in the nucleation region of the flame. The majority of the incipient soot particles in both flames are uncharged, and their sizes are nearly the same, with diameters of approximately 5 nm or less. However, the size of soot particles differs by approximately 10 % to 25 % between methane-only and methane-NaCl flames in the coagulation-dominated region of the flame. The net charge on soot particles within the coagulation region of the methane-only flames is negative, while it is positive with NaCl addition. The fraction of charged particles and ion concentration decreases with NaCl addition within the coagulation region. The study indicates that the smaller particle size observed in methane-NaCl flames may be attributed to reduced coagulation via altered particle charge states. These factors could be the major contributor to the variations in soot formation between methane-only and methane-NaCl flames. Novelty and Significance StatementThis research addresses a notable knowledge gap concerning the influence of NaCl, a prevalent component of hydraulic fracturing fluids, on soot particle behaviour in flames. By explaining the impact of NaCl on soot formation and charge states, the findings contribute to a deeper understanding of combustion processes in environments influenced by hydraulic fracturing operations, thereby informing strategies for reducing emissions and improving environmental sustainability in energy production. Overall, this article represents a significant advancement in the field of combustion science, with implications for environmental stewardship in energy production.This study provides valuable insights into how the introduction of NaCl alters the electrostatic properties of soot particles. This investigation expands upon existing knowledge by shedding light on the intricate mechanisms underlying soot formation and evolution in the presence of NaCl, elucidating its potential implications for combustion processes involving fossil fuels and industrial flaring.

Physical, chemical and morphological evolution of incipient soot obtained from molecular dynamics simulation of acetylene pyrolysis

Incipient soot particles obtained from a series of reactive molecular dynamics simulations were studied to understand the evolution of physical, chemical, and morphological properties of incipient soot. Reactive molecular dynamics simulations of acetylene pyrolysis were performed using ReaxFF potential at 1350, 1500, 1650, and 1800 K. A total of 3324 incipient soot particles were extracted from the simulations at various stages of development. Features such as the number of carbon and hydrogen atoms, number of ring structures, mass, C/H ratio, radius of gyration, surface area, volume, atomic fractal dimension, and density were calculated for each particle. The calculated values of density and C/H ratio matched well with experimental values reported in the literature. Based on the calculated features, the particles were classified in two types: type 1 and type 2 particles. It was found that type 1 particles show significant morphological evolution while type 2 particles undergo chemical restructuring without any significant morphological change. The particle volume was found to be well-correlated with the number of carbon atoms in both type 1 and type 2 particles, whereas surface area was found to be correlated with the number of carbon atoms only for type 1 particles. A correlation matrix comparing the level of correlation between any two features for both type 1 and type 2 particles was created. Finally, based on the calculated statistics, a set of correlations among various physical and morphological parameters of incipient soot was proposed.

Internal Structure of Incipient Soot from Acetylene Pyrolysis Obtained via Molecular Dynamics Simulations.

A series of reactive molecular dynamics simulations is used to study the internal structure of incipient soot particles obtained from acetylene pyrolysis. The simulations were performed using the ReaxFF potential at four different temperatures. The resulting soot particles are cataloged and analyzed to obtain statistics of their mass, volume, density, C/H ratio, number of cyclic structures, and other features. A total of 3324 incipient soot particles were analyzed in this study. Based on their structural characteristics, the incipient soot particles are classified into two classes, termed type 1 and type 2 incipient soot particles in this work. The radial distribution of density, cyclic (5-, 6-, or 7-member rings) structures, and C/H ratio inside the particles revealed a clear difference in the internal structure between type 1 and type 2 particles. These classes were further found to be well represented by the size of the particles, with smaller particles in type 1 and larger particles in type 2. The radial distributions of ring structures, density, and the C/H ratio indicated the presence of a dense core region in type 2 particles. In contrast, no clear evidence of the presence of a core was found in type 1 particles. In type 2 incipient soot particles, the boundary between the core and shell was found to be around 50-60% of the particle's radius of gyration.

Importance of resonance-stabilized radicals in soot formation mechanism of diphenyl ether pyrolysis

Soot formation during fuel pyrolysis can pose environmental and human health hazards. As a typical coal-related model compound, diphenyl ether (DPE) represents the specific structures in coal. In this study, soot formation mechanism of DPE pyrolysis was systematically investigated from theoretical principle. The molecular dynamics (MD) simulations show that the variety and number of DPE pyrolysis products increase with the raising temperature. In addition, the higher temperature also promotes the soot formation. Quantum mechanics (QM) calculations indicate that the energy barrier of CO removal from the phenoxy radical is lower than that of the phenyl isomerization reaction, leading to the easier depletion of the phenoxy radical. It is consistent with the result observed from the MD simulation that the phenoxy is consumed more rapidly than phenyl. The main pathways for initial ring formation are the isomerization of polyyne-like chains and the decomposition of DPE molecules which form the ring molecules of C6H5 and C5H5. C2H2 molecule and resonance-stabilized radical (RSR) species like C3H3, C6H5 and C5H5 are precursors for the growth of polycyclic aromatic hydrocarbons (PAHs), which have an important role in the growth process of soot. Subsequently, larger PAHs aggregate with each other to form incipient soot particles which will undergo surface growth and graphitization processes. We hope this work can provide worthy insights into the mechanism of soot formation in DPE and the suppression of carbon soot.

Effects of ferrocene addition on soot formation characteristics in laminar premixed burner-stabilized stagnation ethylene flames

The influence of ferrocene addition (∼60 ppm on a molar basis) on soot formation in laminar premixed burner-stabilized stagnation ethylene flames (with an equivalence ratio of 2.0) is investigated both experimentally and computationally. The results show that ferrocene addition has a very small effect on both flame temperature and concentrations of small soot precursors from kinetic modeling, but apparently promotes soot inception and particle size growth from particle size distribution measurements, and decreases soot maturity from elemental analysis. Transmission electron microscope (TEM) images and energy dispersive spectrometry (EDS) results show that distinct iron cores are present in large soot aggregates, consistent with the kinetic modeling results that iron predominates over iron oxide in ferrocene-doped flames. Iron in the incipient soot may be too dispersed or form too small cores to be observed by TEM; and as the soot particles coagulate, iron in the soot coalesces into large cores. Since iron is not exposed on the soot surface, and the concentrations of soot precursors are also not significantly increased by ferrocene, ferrocene promotes soot size growth mainly through enhanced coagulation rather than surface growth.

Portraits of Soot Molecules Reveal Pathways to Large Aromatics, Five-/Seven-Membered Rings, and Inception through π-Radical Localization.

Incipient soot early in the flame was studied by high-resolution atomic force microscopy and scanning tunneling microscopy to resolve the atomic structure and orbital densities of single soot molecules prepared on bilayer NaCl on Cu(111). We resolved extended catacondensed and pentagonal-ring linked (pentalinked) species indicating how small aromatics cross-link and cyclodehydrogenate to form moderately sized aromatics. In addition, we resolved embedded pentagonal and heptagonal rings in flame aromatics. These nonhexagonal rings suggest simultaneous growth through aromatic cross-linking/cyclodehydrogenation and hydrogen abstraction acetylene addition. Moreover, we observed three classes of open-shell π-radical species. First, radicals with an unpaired π-electron delocalized along the molecule's perimeter. Second, molecules with partially localized π-electrons at zigzag edges of a π-radical. Third, molecules with strong localization of a π-electron at pentagonal- and methylene-type sites. The third class consists of π-radicals localized enough to enable thermally stable bonds, as well as multiradical species such as diradicals in the open-shell triplet state. These π-diradicals can rapidly cluster through barrierless chain reactions enhanced by van der Waals interactions. These results improve our understanding of soot formation and the products formed by combustion and could provide insights for cleaner combustion and the production of hydrogen without CO2 emissions.

Effect of NH3 addition on soot morphology and nanostructure evolution in laminar ethylene diffusion flame

Ammonia, as a carbon neutral alternative fuel, has attracted wide attention. However, due to the defects in the combustion performance of pure ammonia, it is still necessary to further solve this problem in practical utilization. The purpose of this paper is to study the influence of ammonia on the morphology of soot and the evolution of nanoparticle structure in laminar flame of ethylene. In this work, the soot collected by thermophoresis probe was observed by the high resolution transmission electron microscope (HRTEM), and the image information in HRTEM images was statistically processed, and the graphitization degree of soot was measured by X-ray diffraction (XRD) spectrum of soot samples. The experimental results show that the addition of ammonia can inhibit the growth of soot nuclei and delay the formation of incipient soot, and has different effects on the evolution of soot morphology and structure in ethylene flame. It is noteworthy that the soot samples collected in the upstream part of the ammonia-doped flame show more mature core–shell structure in HRTEM images and more graphitized XRD diffraction data than those collected in pure ethylene flame. The main reason may be the chemical effect of ammonia and the thermal effect caused by the reduction of flame radiation loss, which have different effects on the nanostructure and morphology of soot at different stages of soot formation. Fringe structure model is used to describe and explain the influence of soot in flame with height before and after ammonia doped.

A molecular investigation on the effects of OMEX addition on soot inception of diesel pyrolysis

This study aimed at revealing the atomic-level chemical pathways involved in the soot inception inhibition of diesel pyrolysis with oxymethylene dimethyl ether (OMEX) addition. Using the reactive force field parameters in molecular dynamics simulation, the results effectively identified the specific pathways of OME3 pyrolysis and soot inception, via the analysis of the conversion of three main species: CH2O, CH3O, and CH3, generated from the initial decomposition of OME3. It has been found that CH2O molecule would be converted to CO through continuous dehydrogenation, and the carbon atoms convertered into CO would not participate in forming soot precursors, thus reducing soot formation. The oxidizing groups (mainly OH) are primarily produced from the CH3O radicals. These oxidizing species would react with small gaseous soot precursors to form stable oxides, thus inhibiting soot formation. However, this influence is relatively small under conditions where no oxygen is involved. The CH3 radicals would participate in the formation of soot precursors. In this study, it was observed that the addition of OME3 did not reduce the number of main gaseous precursors of polycyclic aromatic hydrocarbons such as C2H2, C2H3, and C3H3 during diesel-OME3 co-pyrolysis, because there were reactions between CH3 radicals and C1 and C2 species to form C2 and C3 hydrocarbon products. Based on the simulation results, the process from diesel decomposition to incipient soot production under the effects of OME3 is panoramically demonstrated. It is also found that the chain length increase of OMEX with a fixed molar fraction does not influence soot inhibition noticeably. Increasing the proportion of OMEX added to diesel helps reduce soot as carbon atoms involved in the formation of soot precursors are reduced, and more oxidizing species would be produced to slow the formation of gaseous soot precursors.

Study on the nano-morphology and chemical characteristics of soot produced by the combustion of kerosene mixed with ethanol

In this paper, we independently designed and built an experimental platform, including a combustion system and a soot particles collection system. Soot generated inside the flame and soot emitted outside the flame were collected during the combustion of kerosene and kerosene mixed with ethanol fuel. Transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and thermogravimetry were used to characterize both soot samples. The results show that the addition of ethanol to kerosene, i.e., the formation of mixed fuel, causes the lag of soot emission. With the increase in ethanol addition, the incipient soot particles were generated inside the flame, and small, unoxidized, and heat-aging soot particles were emitted outside the flame. The oxidation activity of soot particles produced by adding ethanol decreased, which was related to their particle size.

Effects of Iron Addition on the Collision of Polycyclic Aromatic Hydrocarbon Clusters: A Molecular Dynamics Study

In this work, soot particle size distributions in iron-doped premixed ethylene flames are examined using scanning mobility particle sizer measurements. It is found that iron addition promotes the growth in soot particle size, and the enhanced particle coagulation is inferred to be an important reason. To support that, the influence of iron addition on the coagulation of polycyclic aromatic hydrocarbon (PAH) clusters, the analogue of incipient soot particles, is further investigated using molecular dynamics simulations. Based on the results of hundreds of binary head-on collision simulations, the collision between two coronene-Fe-coronene dimers is found to have a significantly higher coagulation efficiency than that between two coronene dimers. However, this enhancement effect weakens as the size of the PAH monomer increases. Although the coagulation efficiency can be increased by iron addition, the collision frequency is almost unaffected, as revealed from the binary off-central collision simulations. Moreover, the simulation results of coronene cluster growth via coagulation show that iron addition promotes coronene cluster growth, leading to larger cluster size, which may explain the larger soot particle size observed in iron-doped flames.

Optical properties of incipient soot

The exact knowledge of the optical properties of soot nanoparticles is fundamental for several aspects including the correct determination of the soot concentration in combustion environments using optical diagnostics and the correct estimation of the environmental impact of the emitted particles. Although extensive researches over the years have led to a substantial agreement on the optical properties of mature soot particles, the optical properties of the incipient soot nanoparticles are still uncertain. From the particle inception point to the formation of large and more mature soot particles, the evolution of the optical properties must account for variations due to the size and the physicochemical transformation of the investigated particles. This work aims to determine the refractive index and optical properties of inception particles formed in lightly sooting flames. A previous determination based on in-situ light absorption and scattering measurements is revisited taking advantage of particle size measurements by differential mobility analysis. The spectral dependencies of the optical properties are derived by the Kramers-Krӧnig analysis of the ex-situ VUV-NIR light absorption measurements. Results confirm the strong decrease in the absorptivity in the vis-NIR region of inception particles with unimodal size distribution and d63∼3 nm, and confirm a strong size dependency of soot optical properties. The thermal-optical analysis of the sampled particles shows that the particle mass absorption coefficient also correlates with organic carbon content.

Incipient sooting tendency of oxygenated fuels doped in ethylene counterflow diffusion flames

The incipient sooting tendencies of oxygenated fuels in counterflow diffusion flames were systematically assessed by doping selected oxygenate fuels into the baseline fuel of ethylene with various mixing ratios. Laser light scattering and planar laser-induced fluorescence (PLIF) techniques were adopted. The critical oxygen mole fractions at different mixing ratios were identified with the use of laser scattering. It is found that the critical oxygen mole fraction varies with oxygenates, suggesting that the effect of mixing ethylene with oxygenates is highly sensitive to the types of oxygenate. While the doping of acetone moderately promoted the incipient sooting tendency, the doping of methanol, acetic acid, formic acid, and diethyl carbonate suppressed the formation of incipient soot. On the other hand, the doping of ethanol, dimethyl carbonate, diethyl ether, and dimethoxymethane had weak influences on the incipient sooting tendency. The production of polycyclic aromatic hydrocarbons (PAHs) of different sizes were measured by PLIF. The results show there was a strong correlation between the incipient sooting tendency and the production of large PAHs. One important finding is that, at their sooting limits, the total oxygen-to-carbon ratio of a stoichiometric fuel/oxygen mixture is linearly correlated with the mixing ratio of the ethylene-oxygenate mixture. This slope of these two quantities can be defined as an incipient sooting index (ISI) of oxygenated fuels for ranking their tendency of incipient sooting formation. Having the slopes correlated well with the oxygenates’ hydrogen/oxygen ratio, a reasonably accurate prediction of critical oxygen mole fractions from the molecule information of oxygenated fuels can be achieved.

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.

Morphology and electronic properties of incipient soot by scanning tunneling microscopy and spectroscopy

Soot nucleation is one of the most complex and debated steps of the soot formation process in combustion. In this work, we used scanning tunneling microscopy (STM) and spectroscopy (STS) to probe morphological and electronic properties of incipient soot particles formed right behind the flame front of a lightly sooting laminar premixed flame of ethylene and air. Particles were thermophoretically sampled on an atomically flat gold film on a mica substrate. High-resolution STM images of incipient soot particles were obtained for the first time showing the morphology of sub-5 nm incipient soot particles. High-resolution single-particle spectroscopic properties were measured confirming the semiconductor behavior of incipient soot particles with an electronic band gap ranging from 1.5 to 2 eV, consistent with earlier optical and spectroscopic observations.

On the reactive coagulation of incipient soot nanoparticles

In this work, we studied the coagulation process of two PAH clusters with diameter ∼2nm using reactive molecular dynamics (MD) simulations. To describe the coagulation process quantitatively, the distance between the center of mass (COM) of the two PAH clusters, as well as the inter-cluster potential energy and kinetic energy of the COM of the clusters were calculated. Head-on coagulation efficiencies (η) of two PAH clusters at typical flame temperatures where soot inception is most likely to occur, i.e., 1500K—2000K, were determined based on hundreds of MD simulated trajectories. Our simulation results showed that η decreases with increasing temperature, which is mainly due to the increased kinetic energy of atoms within the PAH clusters at higher temperature. In addition, introduction of surface σ-radical site fraction in the range of 0.01 to 0.1 can only moderately improve η by ∼10% by forming carbon–carbon bonds between the two coagulating clusters, which suggests η of incipient soot nanoparticles with surface σ-radicals in high temperature flame regions is very low even if with reactive coagulation taken into consideration.

The coalescence of incipient soot clusters

Reactive molecular dynamics (MD) simulations are employed to investigate the coalescence of incipient soot clusters. Initially, one thousand acetylene molecules collide and react with each other, allowing bond breakage and new bond formation upon collision, leading to various species (e.g., linear hydrocarbons, branched polyaromatic hydrocarbons) up to the formation of nascent soot clusters with diameter of up to 3.5 nm. The structure and composition of the formed soot clusters are quantified by the packing density and carbon-to-hydrogen (C/H) ratio, respectively, during nucleation and up to the formation of large nascent soot nanoparticles. Then, the nucleated incipient soot clusters are isolated from the surrounding reactive species and are allowed to coalesce with each other isothermally to investigate soot coalescence. The coalescence between incipient soot clusters of different sizes is elucidated at various process temperatures, ranging from 800 to 1800 K. The characteristic coalescence time of nascent soot is quantified by tracking the evolution of the particle surface area, for the first time. Soot clusters consisting of up to 760 atoms coalesce instantly (within 0.1 ns), especially at relatively low temperatures (i.e., 800–1000 K). At higher temperatures (1200–1600 K), incipient soot clusters are less prone to coalescence due to the larger fraction of constituent aromatic rings leading to more rigid particles. Large clusters consisting of more than 1300 atoms do not coalesce within the time scales investigated here (i.e., up to 5 ns). The employed reactive MD approach gives significant insight into fundamental soot formation and growth mechanisms, which are typically treated semi-empirically, facilitating a better understanding and more efficient control of soot in combustion processes.

Evolution in size and structural order for incipient soot formed at flame temperatures greater than 2100 K

Soot formed in flames hotter than conventional combustion applications is expected to undergo unique formation processes and develop a carbon structure distinct from typical soot. A complementary experimental and modeling approach is reported here to assess flame temperature and equivalence ratio effects for soot formed in the higher-temperature regime (1950 K < Tf < 2250 K). Computations using three separate combustion chemistry models show that predictions of polycyclic aromatic hydrocarbon (PAH) concentration profiles for the higher-temperature flames are more sensitive to the choice in mechanism rather than specific flame conditions. As for material properties, the Raman signatures transition from a typical soot spectrum to features observed in disordered sp2 carbon materials. The defect distance extracted from the Raman bands nearly doubles from values typically reported for soot as the flame temperature exceeds 2200 K. Higher concentrations of gas-phase precursors may facilitate development of an ordered carbon structure as indicated by the relatively high defect distance observed for the highest equivalence ratio series. Particle size distributions measured by mobility sizing show size and yield of soot decreases with increasing flame temperature and the bimodal distribution falls within the ultra-fine range for all flame conditions. This is especially promising if the significant transformation in carbon structure inferred from the evolution in Raman spectra enables development of functional high-surface area sp2 carbon materials. Namely, the current observations indicate that the flame-formed carbon structure evolves towards high-defect sp2 carbon with size and carbon structure that can be tuned to some extent.

Mechanistic insights into effect of feeding rate on soot formation during rapid pyrolysis of biomass model components in a drop-tube furnace at high temperature

This paper reports the significant effect of feeding rate on soot formation during rapid pyrolysis of water-washed cellulose and acid-washed lignin (denoted as W-cellulose and A-lignin, respectively) in a drop-tube furnace at 1300 °C and a residence time of ∼0.75 s in argon. Soot produced during W-cellulose pyrolysis has a unimodal distribution, with only a fine mode that its modal diameter increases from 0.043 to 0.246 µm as the feeding rate increases from 40 to 280 mg/min. However, at feeding rates of 12–200 mg/min, soot produced from A-lignin pyrolysis has a bimodal distribution with two fine modes located at 0.077 µm and 0.246 µm, respectively. As the A-lignin feeding rate further increases from 200 to 280 mg/min, the fine mode at 0.077 µm disappears and the particle size distribution of soot becomes unimodal with only a fine mode with diameter of 0.246 µm. Increasing feeding rate increases the yield and particle size of total soot but decreases the yield of non-mature soot for W-cellulose at feeding rates < 280 mg/min and A-lignin at feeding rates < 40 mg/min. The results suggest that a high feeding rate produces high concentration of soot precursors and enhances the collisions for increased formation of incipient soot to large mature soot. As the feeding rate further increases to be more than 40 mg/min, the soot yield from A-lignin pyrolysis levels off, the particle sizes of overall soot decease and the yields of non-mature soot increase. Under such conditions, further results show that soot growth is limited by the availability of soot-forming carbonaceous materials (including small gas molecules e.g. C2H2 and polycyclic aromatic hydrocarbons).

Effects of Magnetic Fields on Morphology and Nanostructure Evolution of Incipient Soot Particles from n-heptane/2,5-dimethylfuran Inverse Diffusion Flames

Differences of the morphology and nanostructure evolution of incipient soot particles generated in n-heptane/2,5-dimethylfuran (DMF) inverse diffusion flames (IDFs) with/without magnetic fields were investigated. Utilizing a high resolution transmission electron spectroscopy, the morphology and nanostructures of soot sampled from spatial locations at different heights in IDFs were analyzed. The graphitization and the oxidation reactivity of soot were tested by an X-ray diffraction and a thermogravimetric analyzer, respectively. Results demonstrated that the magnetic force on paramagnetic species, such as oxygen molecules, can modify the soot formation and oxidation. More incipient soot particles with larger diameters appeared in chains or branches or tufted forms on the flame wing region and the higher position than that on the flame centerline region and the lower position. With magnetic fields, greater amounts of clustered soot particles displayed more crowded distribution and larger diameters. Soot particles with typical structures of the core-shell were promoted to own more orderly bordered lamellae with longer fringe length and smaller fringe tortuosity by the magnetic force acting on oxygen at the same sample position. These modifications resulted in relatively larger diffraction angle of the peak, higher graphitization degree and slightly lower oxidation reactivity of 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.