Mechanism Of Soot Formation Research Articles

Soot formation during biomass pyrolysis: Effects of temperature, water-leaching, and gas-phase residence time

Soot particles from biomass incomplete combustion, as a major source of particulate matter, arouse significant concerns due to its harmful impacts on environment and human health. In this paper, the effects of pyrolysis temperature, volatile residence time and water-leaching of biomass on soot formation during biomass pyrolysis were studied in a lab-scale furnace. The physicochemical characteristics of soot particles were comprehensively analyzed. Results show that soot is formed by the secondary reactions of volatiles from biomass thermal-cracking requiring a certain high temperature and long volatile residence time. With a short volatile residence time of 0.2 s, no soot is formed at 900–1200 °C. When the residence time increases to 2 s, soot is significantly formed at >1000 °C, when onion-like shell/core structures is observed. With pyrolysis temperature increasing, the soot particle size decreases and becomes uniform, the carbon content in soot increases, and the ordered structure of carbonization is enhanced, which leads to the degradation of soot oxidation reactivity. The water-leaching pretreatment for biomass leads to the increases of soot yield, particle size, and carbon content, as well as the KCl absence in soot and significantly lowered oxidation reactivity of soot. This indicates strong catalysis of potassium on soot production and consumption. Both gas and tar analyses support soot formation mechanism: (1) H2 yield increases due to formation of soot, but C2H2 yield reaches peak at 1000–1100 °C; (2) tar is dominated by PAHs, which molecules grow with the increase of temperature and residence time.

Kinetic modeling of soot formation in premixed burner-stabilized stagnation ethylene flames at heavily sooting condition

A detailed kinetic mechanism of soot formation and oxidation is revised and extended to include temperature-dependent collision efficiencies. The collision efficiency for various particle size is studied and compared with experimental data and molecular dynamics simulations for the PAH dimerization where the experimental data are not available. This revised kinetic model is validated in comparison with the premixed burner-stabilized stagnation ethylene flames at heavily sooting conditions. The results showed that quasi-one-dimensional numerical simulations can capture the flame structure and predict soot formation quite satisfactorily. The predicted particle size distribution function (PSDF) is in reasonable agreement with experimental results, but the model only partially reproduces the distinct separation between nucleation and coagulation modes observed experimentally. This leads to some discrepancies in the prediction of soot number density, while the predicted soot volume fraction, which is dominated by the large particles of the PSDF, is in generally good agreement with the experimental data. There is an overestimation of the initial soot volume fraction in the flame region close to the burner, which is a consequence of the over-prediction of the amount of young particles. Therefore, the prediction of PAHs formation and their condensation on soot, which controls the nucleation rate, will require further attention. The comparison between the temperature-dependent model and the model neglecting the temperature dependency showed that the temperature-dependent model could improve the prediction of soot number density, which is controlled by small particles.

Time-resolved study of transient soot formation in an aero-engine model combustor at elevated pressure

The mechanisms of transient formation and oxidation of soot in an aero-engine model combustor at elevated pressure are studied for the first time using a combination of high-speed simultaneous stereo-PIV and OH-PLIF and results from a recent detailed LES. A combined analysis of experiment and LES shows that the highly transient and intermittent evolution of soot in this combustor is governed by an unsteady interplay of distinct pockets of burned gas in the inner recirculation zone (IRZ) with either relatively rich or relatively lean composition. The former originate from reaction of fuel-rich unburned gas, whereas the latter result from additional admixture of secondary air further downstream. The analysis further enables distinction and localization of premixed and diffusion-type flame fronts within the flame zone. The time-resolved complementary measurements of velocity field and flame structure allow accurate tracking of both the burned gas pockets and soot filaments. It is seen that soot generally forms in the rich pockets if their residence time in the IRZ is sufficient, whereas oxidation occurs in the lean zones carrying OH. Correlating the dynamics of flow field and soot indicates that the intermittency of soot is driven by an intermittent flow of lean burned gas into the IRZ that affects the residence time of rich pockets. The results suggest that the formation of soot might be further reduced by a proper adjustment of secondary air injection aiming at a sufficient and more constant recirculation of lean burned gas. A remarkably good agreement of measured and simulated instantaneous flame structures is observed, indicating that flow field and gas-phase reactions are well predicted by the LES. The experimental insights into the transient mechanisms of soot formation and oxidation, on the other hand, may provide useful input for LES soot models where deviations from measurements are generally larger.

Recent Progress in Automotive Gasoline Direct Injection Engine Technology

Gasoline direct injection (GDI) engines are currently the dominant powertrains for passenger cars. With the implementation of increasingly stringent fuel consumption and emission regulations worldwide, GDI engines are facing challenges owing to high particulate matter emissions and a tendency to knock, leading to a change in the research and design (R&D) issues compared with those in the twentieth century. This paper reviews the progress in research regarding GDI engine technologies over the past 20 years, focusing on combustion system configurations, and also highlights common issues in GDI R&D, including pre-ignition and deto-knock, soot formation and PM emissions, injector deposits and gasoline compression ignition (GCI). First, an overview of recent developments in the field as driven by regulations is provided, following which progress in injection and combustion systems is examined. Third, the review addresses the occurrence and mechanism of deto-knock and considers means of suppressing this phenomenon. The fourth section discusses soot formation mechanisms and particulate matter emission characteristics of GDI engines and describes the application of gasoline particulate filter (GPF) after-treatment. The subsequent section summarizes studies regarding injector deposit formation, as well as pioneering research into GCI combustion modes. Finally, a summary and future prospects for GDI engine technologies are provided.

Soot formation during polyurethane (PU) plastic pyrolysis: The effects of temperature and volatile residence time

Soot is an undesired co-product during thermal-chemical disposal (incineration, pyrolysis, etc.) of plastic wastes at high temperatures, while also regarded as a valuable nanoscale carbon-based material if with proper production and post-treatment methods. In this paper, the pyrolysis of polyurethane (PU) plastics, a major composition of plastic waste, was conducted in a fixed-bed reactor to investigate the soot formation mechanism in depth. The effects of pyrolysis temperature (1000–1300 °C) and volatile residence time (0.2–2 s) on the yield, micro-morphology, composition and reactivity of soot were studied. Results show that the initialization and growth of soot particles during PU pyrolysis requires a certain high temperature and long volatile residence time. With a volatile residence time of 2 s, the soot yield is 11.0 wt.% at 1000 °C and increases to 24.5 wt.% with the pyrolysis temperature increasing to 1300 °C; while when the volatile residence time decreases to 0.2 s, the soot cannot be observed until 1200 °C. When the pyrolysis temperature increases 1000–1100 °C, the X-ray diffraction (XRD) patterns indicates an enhanced graphitization thereby a lower reactivity of soot oxidation, which is approved by onion-like layered structures initially observed by the high-resolution transmission electron microscope (HR-TEM) at 1100 °C and 2 s. When the pyrolysis temperature increases from 1100 °C to 1200–1300 °C, NaCl is observed in soot particles, which catalysis promotes the reactivity of soot oxidation. Gas chromatography and mass spectrometer (GC–MS) was used to measure the gas and tar compositions, especially the polycyclic aromatic hydrocarbons (PAHs), are proved essential for the formation of soot precursor. A large number of PAHs with ring number ≥4 start to form at 1100–1200 °C. Based on the soot characterization and gas-tar compositions, a pathway from light fragments and mono-ring aromatics to heavier aromatic compounds is proposed aiming to explain the chemical evolution of soot precursor particles.

Morphology and nano-structure analysis of soot particles sampled from high pressure diesel jet flames under diesel-like conditions

Soot particles emitted from diesel engines have a significant impact on the atmospheric environment. Detailed understanding of soot formation and oxidation processes is helpful for reducing the pollution of soot particles, which requires information such as the size and nano-structure parameters of the soot primary particles sampled in a high-temperature and high-pressure diesel jet flame. Based on the thermophoretic principle, a novel sampling probe minimally disturbing the diesel jet flame in a constant volume combustion vessel is developed for analysing soot particles. The injected quantity of diesel fuel is less than 10 mg, and the soot particles sampled by carriers with a transmission electron microscope (TEM) grid and lacey TEM grid can be used to analyse the morphologies of soot aggregates and the nano-structure of the soot primary particles, respectively. When the quantity of diesel fuel is more than 10 mg, in order to avoid burning-off of the carriers in higher temperature and pressure conditions, single-crystal silicon chips are employed. Ultrasonic oscillations and alcohol extraction are then implemented to obtain high quality soot samples for observation using a high-resolution transmission electron microscope. An in-house Matlab-based code is developed to extract the nano-structure parameters of the soot particles. A complete sampling and analysis procedure of the soot particles is provided to study the formation and oxidation mechanism of soot.

Soot formation characteristics of ethylene premixed burner-stabilized stagnation flame with dimethyl ether addition

Effect of dimethyl ether (DME) addition on PAH and soot formation mechanisms as well as particle-size distribution function (PSDF) behavior of C2H4 premixed burner-stabilized stagnation (BSS) flame was studied. A wide range of DME addition from pure C2H4 to pure DME was considered. The sectional aerosol dynamics model with detailed gas- and particle-phase kinetic mechanism was employed for the simulations. The results indicate that soot growth rate by nucleation, and especially the H-abstractionC2H2addition (HACA) mechanism and polycyclic aromatic hydrocarbons (PAH) condensation increased significantly inside the stagnation boundary layer, due to strong flame-wall interaction. The PSDF curve in the post-flame region was unimodal, while that in the stagnation boundary layer was bimodal. The first peak of the PSDF curve in the stagnation boundary was resulted from the enhanced nucleation rate, and the second peak was due to the intensified soot surface growth by HACA reaction and PAH condensation. The rich premixed C2H4 BSS flame did not exhibit the synergistic effect of DME addition on PAH and soot formations. Soot formation rate decreased monotonously with DME addition for the C2H4 premixed BSS flame. Topology of soot PSDF behavior of the C2H4 premixed BSS flame changed significantly when DME addition exceeded above 20%.

Impact of oil aging and composition on the morphology and structure of diesel soot

HypothesisSoot is a black powder-like substance consisting of carbonaceous amorphous particles, formed by incomplete combustion of hydrocarbons. In the last decades, industrial research on lubricant oils has grown to develop more efficient formulations, mainly to reduce the formation of soot in oil so to increase engine lifetime. The comprehension of the mechanism of soot formation and particles growth/aggregation in times during real applications is of fundamental importance for the design of better performing lubricants. ExperimentsIn this work, we report on a multi-technique investigation of soot-in-oil samples, drawn from the oil sump of a direct-injected heavy-duty diesel engine at increasing working times in a standard engine test. Scanning Electron Microscopy, Small Angle X-ray Scattering and Dynamic Light Scattering are employed to study the evolution in term of soot size, morphology and fractal dimensions. FindingsOur results evidence that, in a complete lubricant formulation, exhaust oil viscosity evolution correlates well with the increase of soot amount inside the oil. Moreover, the growth of both primary soot particles and aggregates during the engine test is limited to about 24 and 200 nm, respectively, because of the active role of the dispersant present in the investigated oil.

The soot handling ability requirements and how to solve soot related viscosity increases of heavy duty diesel engine oil

PurposeThe purpose of this paper is to identify the feature of soot in diesel engine oil and provide a method to stably disperse these soots using effect additives which is benefical for lubricants to pass related engine tests.Design/methodology/approachThis paper designed experiments to investigate the dispersant type, treat level and different dispersant interactions which influence on lubricant soot-related viscosity increase. The research work developed an effective dispersant package which can well solve the soot-related viscosity increase, allowing pass Mack T-11 and Mack T-8 engine tests and demonstrated the helpfulness of using a quickly screening method developed by a steel piston diesel engine CA 6DL2-35.FindingsThe effect of dispersant treat level on the viscosity increase of the oil samples was negligible. Dispersant booster can effectively improve the soot handling ability of heavy-duty diesel engine oils (HDDEO), and the appropriate treat level of dispersant booster can help HDDEO pass Mack T-8 and Mack T-11 engine tests.Practical implicationsThe test results are useful for formulators to select the appropriate dispersants or dispersant booster to develop the HDDEO packages which can meet the modern diesel engine lubrication requirements.Originality/valueMost previous studies in this field were carried out on soot formation mechanism and soot-related wear rather than how to solve the soot-related viscosity increasing of HDDEO. This paper describes the soot dispersing requirements of different HDDEO specifications and developed an effective dispersant package which can well deal with Mack T-11 and Mack T-8E standard engine tests soot handling ability requirements.

Optical properties of organic carbon and soot produced in an inverse diffusion flame

The carbonaceous particulate matter (soot plus organic carbon) sampled downstream of an ethylene inverse diffusion flame (IDF) was chemically and spectroscopically analyzed in detail. In particular, the H/C ratio, the UV–Visible absorption coefficient and Raman parameters were measured and found to be representative of a highly disordered sp2-rich carbon as the early soot sampled in a premixed flame. In contrast, the optical band gap was found to be relatively low (0.7eV), closer to the optical band gap of graphite than to that of medium-sized polycyclic aromatic hydrocarbons (>2eV) which are widely considered to be soot precursors and are mostly contained in the organic carbon. The significance of the optical band gap as signature of different structural levels (nano-, micro- and macro-structure) of sp2-rich aromatic disordered carbons was critically analyzed in reference to their molecular weight/size distribution. The relevance of the optical band analysis to the study of the soot formation mechanism was also highlighted.

Extracting of fullerene-like nanoparticles from environmental soots

ABSTRACTIt is well known that burning graphite electrodes in electric arc is an efficient method for obtaining of fullerenes. However, fullerenes form in any sooting flames. Therefore, detection fullerene in natural burning fuel is of a great interest for understanding of mechanism of soot formation. This research extracted samples of environmental soots in toluene. Soots and products of extraction were characterized by UV-vis spectroscopy, sedimentation analysis, atomic force microscopy. Analysis of extracts found fullerene-like clusters. The concentration of particles decreased in the following sequence: charcoal – carbon black – gas soot.

Study on inversion of morphological parameters of soot aggregates in hydrocarbon flames

This study proposes an effective inversion method to detect the morphological characteristics of soot aggregates in hydrocarbon flames. Firstly, the imaging method is used to measure the distribution of soot aggregates absorption coefficients in flames. Then, the primary particle diameter, primary particle number and volume fraction of soot aggregates are retrieved from the detected absorption coefficients using the particle swarm optimization algorithm. Meanwhile, the thermophoretic sampling particle diagnostic-transmission electron microscopy (TSPD-TEM) technique and the generalized multi-particle Mie-solution theory is employed to verify the absorption coefficients detected by the imaging method and the retrieved morphological parameters of soot aggregates by the proposed inversion technique. The comparison results reveal a deviation below 30% in absorption coefficients between obtained by the imaging method and the TSPD-TEM analysis combined with the generalized multi-particle Mie-solution theory. And the morphological parameters of soot aggregates retrieved using the inversion method are in good agreement with that obtained by the TSPD-TEM technique, which suggests that the inversion method is robust and can accurately retrieve the morphological parameters. Finally the distributions of primary particle diameter, primary particle number and volume fraction in flames for two different cases are presented, and effect of combustion condition and soot formation mechanism on the distribution rules of these morphological parameters is discussed. The proposed inversion method can provide detailed morphological information about soot aggregates for a deeper understanding and control of soot formation.

Formation of incipient soot particles from polycyclic aromatic hydrocarbons: A ReaxFF molecular dynamics study

In this study, we present the results from a series of ReaxFF molecular dynamics (MD) simulations to uncover the underlying mechanisms behind the nucleation and growth of incipient soot particles from polycyclic aromatic hydrocarbons (PAHs). PAHs, namely, naphthalene, anthracene, pyrene, coronene, ovalene and circumcoronene, are selected for ReaxFF MD simulations over a range of temperatures from 400 to 2500 K. Distinctive mechanisms of incipient soot formation are identified with respect to PAH mass and temperature. At low temperatures (e.g., 400 K), all types of the above PAHs can nucleate into incipient soot particles in stacked structures due to physical interactions. With the increase of temperature, the possibility of physical nucleation decreases for each PAH. At moderate temperatures (e.g., 1600 K), it becomes difficult for these PAH monomers, except circumcoronene grows into incipient soot particles. When the temperature increases to 2500 K, all the PAHs become chemically active, which not only leads to the formation of incipient soot particles but also takes the graphitization with the increase of the carbon-to-hydrogen (C/H) ratios in the particles. In addition to the formation of fullerene-like soot particles, stacked particles connected by ‘carbon bridges’ are also observed for large PAHs like coronene, ovalene and circumcoronene.

Swirl ratio and post injection strategies to improve late cycle diffusion combustion in a light-duty diesel engine

Nitrogen oxides (NOx) and soot emissions are the most important pollutants from direct-injection diesel engines. In particular, soot formation and oxidation determine the net engine-out soot emissions. These phenomena are complex and competing processes during diesel combustion. Despite many researches implicate the mechanisms of soot formation with soot emissions, the enhancement of the late cycle soot oxidation is the dominant mechanism for a reduction of engine-out soot emissions. The mixing process and the in-cylinder bulk temperature are two important parameters in the development of soot oxidation process. The current research compares different engine strategies to enhance the late cycle mixing controlled combustion process and therefore enhance soot oxidation while maintaining similar gross indicated efficiency in a light-duty engine. For this purpose, a simplified methodology has been used, which analyzes the effect of mixing process and in-cylinder bulk gas temperature on soot oxidation during the late cycle combustion. For carrying out this research, theoretical and experimental tools were used. In particular, the experimental measurements were made in a single-cylinder direct-injection light-duty diesel engine varying the swirl ratio and the injection pattern as injection pressure, Start of Energizing (SoE), Energizing Time (ET) and number of injections events. To analyze soot emissions, the combustion luminosity was measured by an optoelectronic probe and the optical thickness parameter (KL) was evaluated by the two-color pyrometry method. The apparent combustion time (ACT-1) was used as mixing time tracer. Results show that an increase in swirl ratio implies an improvement on the mixing process and higher values of average bulk temperature during the late-cycle diffusion combustion. Both phenomena produce an enhancement in the soot oxidation process. In the lowest swirl ratio case, a suitable injection strategy based on multiple injections, provides similar results of soot oxidation process (and therefore, the emissions) as high swirl ratio case.

Computational Investigation on Soot Mechanism of Diesel and Diesel/n-Butanol Blend in Constant Volume Chamber with Various Ambient Temperatures

Computational investigation was carried out in an optical constant volume chamber to explore the combined effects of initial ambient temperatures (800, 900, and 1000 K) and n-butanol additive (20% volume fraction of n-butanol) on diesel combustion and soot characteristics. An improved phenomenological soot model integrated with a reduced n-heptane/n-butanol/polycyclic aromatic hydrocarbon (PAH) mechanism was developed and implemented into the KIVA-3V R2 code to study the soot formation and oxidation mechanism. The predicted chamber pressure and heat release rate as well as soot mass trace and distribution showed good agreements with the experimental data. Results indicated that the ignition delay was retarded and total soot mass was reduced with the decrease of initial ambient temperature and with the n-butanol additive. The heat release rate of pure diesel demonstrated a transition from diffusion-dominated combustion at 1000 K to premix-dominated combustion at 800 K. Diesel/n-butanol blend showed no obvi...

Implementation of two color method to investigate late cycle soot oxidation process in a CI engine under low load conditions

Soot emissions from diesel engines are an important concern in meeting emissions regulations. Soot emissions are the result of two competing processes: soot formation and soot oxidation. Mechanisms of soot formation are discussed extensively in the literature. Equivalence ratio at lift-off length along with residence time and gas temperature play an important role for soot formation in a diffusion flame. Mixing capability and bulk gas temperature are the most important parameters that influence the in-cylinder soot oxidation process. Normally, research studies of soot formation-oxidation processes have been developed under controlled and not completely representative conditions of engine operation in the field. Therefore, the main objective of this work was to develop a simplified methodology to evaluate in-cylinder soot oxidation under ‘real’ engine conditions. In particular the impact of mixing process and bulk gas temperature on late cycle soot oxidation was evaluated. The experimental measurements were made in a production light-duty diesel engine varying those parameters that have been demonstrated in the literature as the most relevant in soot formation – oxidation processes; injection pressure, ambient density and intake air temperature. To measure soot, two color method was applied by means of an optoelectronic pyrometer. To evaluate the mixing capability a specific “tracer” Apparent Combustion Time (ACT−1) based on the experimental heat release and injection parameters was defined. The relationship between both parameters was used to explain the soot oxidation process.

Optical Investigations of Soot Formation Mechanisms and Possible Countermeasures on a Turbocharged Port Fuel Injection SI Engine

Optical Investigations of Soot Formation Mechanisms and Possible Countermeasures on a Turbocharged Port Fuel Injection SI Engine

Detailed particle nucleation modeling in a sooting ethylene flame using a Conditional Quadrature Method of Moments (CQMOM)

In this work, a detailed kinetic mechanism for soot formation and evolution (D'Anna et al. 2010) is implemented in a method of moments framework using the concept of ‘Conditional Quadrature Method of Moments’ (CQMOM) (Yuan and Fox, 2011). Particle nucleation and growth pathways are described in detail tracking moments of the number density function (NDF) for large PAHs, soot clusters and agglomerates in both stable and radical forms. Individual entities of these groups are characterized by both their size and their H/C ratio, the latter providing information on the chemical structure and the reactivity of the particles. This leads to a multivariate population balance statement. The CQMOM implementation of the soot model is then applied to predict particle formation in a lightly sooting burner-stabilized premixed ethylene flame with an equivalence ratio of ϕ=2.1. Several experimental data sets for this target flame are available in the literature and they are used in this study for comparison with the simulation results. The suitability of the soot model to predict particle matter under such conditions is shown, as both the onset of soot formation by nucleation and the subsequent growth are captured. Modeling results offer further insights into the relevance of agglomeration and dehydrogenation in this lightly sooting flame.

A skeletal mechanism modeling on soot emission characteristics for biodiesel surrogates with varying fatty acid methyl esters proportion

A new developed four-component biodiesel combustion skeletal mechanism comprising methyl decenoate, methyl-5-decenoate, n-decane and methyl linoleate is presented in this work, which could predict soot formation trends with varying fatty acid methyl esters proportion. It was established based on a detailed n-heptane mechanism, HACA-mechanism and our previously developed four-component biodiesel combustion skeletal mechanism without soot formation mechanism. This new developed mechanism consists of 134 species and 475 reactions. In order to validate the feasibility of this new skeletal mechanism, ignition delay testing, constant volume combustion chamber experiment testing and diesel engine experiment testing were performed. The result indicates that it is suitable to predict ignition behavior, flame lift-off length, soot shape and distribution, cylinder pressures, heat release rate and NOx emission trends of biodiesel with varying fatty acid methyl esters proportion.

Soot optical properties determined by analyzing extinction spectra in the visible near-UV: Toward an optical speciation according to constituents and structure

To improve the optical diagnostics of soot particles and gain an increased comprehension of the soot formation and aging mechanisms requires a better understanding of the soot refractive index. The present study evaluated the refractive index of soot generated by a miniCAST, an ethylene diffusion flame, and a PALAS GFG by interpreting specific extinction measurements in the near-UV spectrum with the help of an improved RDG-FA theory. Soot’s morphology, size distribution, and bulk densities were determined and taken into account for the inversion process. Findings indicate that absorption and scattering capacities of the considered soot were strongly dependent on the amount of organic or volatile carbons and the degree of graphitization of the carbon atoms in the primary sphere (amorphous to graphitic arrangement). The proposed speciation of particles according to these properties is based on the spectral dependence of the determined refractive index for three reference aerosols representative of organic, amorphous, and ordered soot particles. Results show that by combining the refractive indices of these three aerosols it is possible to explain the high variability of most of the optical properties of soot, paving the way for a simple optical technique that permits the speciation of soot particles.