Formation Of Soot Particles Research Articles

Experimental and numerical study on rich methane/hydrogen/air laminar premixed flames at atmospheric pressure: Effect of hydrogen addition to fuel on soot gaseous precursors

Experimental and numerical atmospheric chemical structure of methane flames with and without hydrogen operating under sooting conditions has been investigated. Mole fraction profiles of reactants, final products and intermediate C2C7 hydrocarbon species (from ethane to toluene) have been obtained using gas chromatography and gas chromatography -mass spectrometry. The most unsaturated species, which are believed to play a key-role in the formation of PAHs and soot, such as acetylene, propyne, allene, 1-butyne and small aromatic compounds, such as benzene and toluene, seemed to be particularly sensitive to the presence of hydrogen in the post-flame zone. A new kinetic scheme proposed predict well the present results (sooting premixed flames) and was also applied successfully to literature data on the oxidation of methane/hydrogen mixtures under shock-tube and jet-stirred reactor (JSR) conditions. Our results suggest that hydrogen can be able to affect the reaction steps that bring to the formation of PAH and soot particles, by inhibiting or promoting their formation in a function of the particular working conditions and inlet flow composition of the flames. We discuss here the main reactions which can explain the ambiguous effects of hydrogen.

Lanthanide 5-sulfosalycilates and 3-amino-5-sulfosalycilates: Synthesis and estimation of thermal stability

Trivalent lanthanum, europium, terbium, erbium and lutetium 5-sulfosalycilates and terbium, europium and holmium 3-amino-5-sulfosalycilates have been synthesized with their composition studied using thermogravimetric, IR and quantum chemical analysis. Quantum chemical calculations were performed for anhydrous complexes. According to calculated data, coordination of La and Lu with 5-sulfosalycilic acid anion via carboxylic oxygen is more favorable compared to coordination via sulfonate oxygen, while the reverse situation is observed for Eu, Tb and Er 5-sulfosalycilates. In the case of Ho 3-amino-5-sulfosalycilate metal atom is coordinated by both groups while coordination via sulfonate group appears to be more preferable for Eu and Tb 3-amino-5-sulfosalycilates.A good correlation (R2=0.96) was found between calculated Ln–Oligand bond energy and temperature of complex decomposition, thus showing that semi-empirical methods can be used for preemptive estimation of thermal stability of lanthanide complexes. Synthesized compounds have potential as anti-exhaust additives to diesel fuel as their decomposition correspond with formation of soot particles, thus, inhibiting their growth.

Importance of detailed chemical kinetics on combustion and soot modelling of ventilated and under-ventilated fires in compartment

A novel in-house computation code based on large eddy simulations (LES) incorporating fully coupled subgrid-scale (SGS) turbulence, combustion, soot and radiation models for turbulent reacting flows in compartment fires has been developed. It uniquely embraces the detailed reaction mechanisms for the chemical processes involved during combustion, which provides a comprehensive description of the fuel oxidation processes. Furthermore, it gives a more complete representation of the generation of intermediate chemical by-products including toxic gases such as carbon monoxide and irritant gases such as soot. Since the modelling of hydrocarbons by-products are enabled when considering the full chemical profile, the formation of soot particles is correlated to the concentration of main soot incipient acetylene, which provides an appropriate representation of nucleation, surface growth processes. The LES code has been validated with experimental fire tests results for both ventilated and under-ventilated fires in compartment for confirmation of its robustness. The importance of incorporating the detailed reaction mechanisms in compartment fire simulations has been confirmed by comparing with experiments. For under-ventilated fires in compartment, the chemical kinetics become increasingly important since the combustion efficiency drops significantly involving generation of intermediate chemical species. It is discovered that species concentrations especially CO2 and CO are more accurately predicted by the detailed scheme comparing to the multi-step scheme, since the formation of hydrocarbons and nitrogen oxides are considered. In general, the simulation incorporating detailed kinetics showed an averaged improvement of 9.2% and 81.7% in the prediction of CO2/CO ratio and volume fraction of soot respectively. This also improves the replication of the flame structure as the fire is chemically-driven within the combustion zone.

Variation of diesel soot characteristics by different types and blends of biodiesel in a laboratory combustion chamber

Very little information is available on the physical and chemical properties of soot particles produced in the combustion of different types and blends of biodiesel fuels. A variety of feedstock can be used to produce biodiesel, and it is necessary to better understand the effects of feedstock-specific characteristics on soot particle emissions. Characteristics of soot particles, collected from a laboratory combustion chamber, are investigated from the blends of ultra-low sulfur diesel (ULSD) and biodiesel with various proportions. Biodiesel samples were derived from three different feedstocks, soybean methyl ester (SME), tallow oil (TO), and waste cooking oil (WCO). Experimental results showed a significant reduction in soot particle emissions when using biodiesel compared with ULSD. For the pure biodiesel, no soot particles were observed from the combustion regardless of their feedstock origins. The overall morphology of soot particles showed that the average diameter of ULSD soot particles is greater than the average soot particles from the biodiesel blends. Transmission electron microscopy (TEM) images of oxidized soot particles are presented to investigate how the addition of biodiesel fuels may affect structures of soot particles. In addition, inductively coupled plasma mass spectrometry (ICP-MS), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) were conducted for characterization of soot particles. Unsaturated methyl esters and high oxygen content of biodiesel are thought to be the major factors that help reduce the formation of soot particles in a laboratory combustion chamber.

Diesel–hydrogen dual-fuel combustion and its impact on unregulated gaseous emissions and particulate emissions under different engine loads and engine speeds

Experimental investigations were conducted on a diesel engine to evaluate the effects of hydrogen addition on engine combustion, unregulated gaseous emissions and particulate emissions based on the Japanese 13-mode testing cycle. A diesel–hydrogen co-operated combustion strategy is proposed in which hydrogen is naturally aspirated into the diesel engine to substitute 10, 20 and 30% of the total fuel energy at 10 selected modes and diesel is used as the sole fuel for the other 3 modes. Gradual increase of peak in-cylinder pressure and heat release rate is observed at medium and high engine loads. Pre-ignition might occur at high engine load and speed. Cyclic variation of the indicated mean effective pressure and maximum pressure derivative increased with hydrogen addition. The brake specific emissions of acetaldehyde (CH3CHO), alkenes (C2H4, C3H6 and C4H6), arenes (C6H6, C7H8 and C8H10) and particulate mass can be evidently reduced. However, with H30 operation, the weighted total HC and formaldehyde (HCHO) increased by around 27% and 8%, respectively, while NOx increased by around 17%. The simultaneous decrease of particle size and number are believed to be associated with the inhibition effect of hydrogen on soot particle formation based on the HACA (H2 abstraction and C2H2 addition) mechanism.

Numerical Investigation of Soot Formation in Turbulent Diffusion Flame With Strong Turbulence–Chemistry Interaction

The present work is aimed at examining the ability of different models in predicting soot formation in “Delft flame III,” which is a nonpremixed pilot stabilized natural gas flame. The turbulence–chemistry interactions are modeled using a steady laminar flamelet model (SLFM). One-step and two-step models are used to describe the formation, growth, and oxidation of soot particles. One-step is an empirical model which solves the soot mass fraction equation. The two-step models are semi-empirical models, where the soot formation is modeled by solving the governing transport equations for the soot mass fraction and normalized radical nuclei concentration. The effect of radiative heat transfer due to gas and soot particulates is included using P1 approximation. The absorption coefficient of the mixture is modeled using the weighted sum of gray gases model (WSGGM). The turbulence–chemistry interaction effects on soot formation are studied using a single-variable probability density function (PDF) in terms of a normalized temperature or mixture fraction. The results shown in this work clearly elucidate the effect of radiation and turbulence–chemistry interaction on soot formation. The soot volume fraction decreases with the introduction of radiation interactions, which is consistence with the theoretical predictions. It has also been observed in the current work that the soot volume fraction is sensitive to the variable used in the PDF to incorporate the turbulence interactions.

Analysis of Soot Formed during the Pyrolysis of Athabasca Oil Sand Asphaltenes

Asphaltenes are formed during the primary upgrading process of bitumen. Gasification is considered as a good option for conversion of the low-value asphaltenes into syngas. Syngas can later be used in the generation of steam for steam-assisted gravity drainage, and the hydrogen from the syngas can be used for upgrading. Understanding the decomposition of asphaltenes, the subsequent formation of soot, and the properties of soot enables optimization of the process of asphaltene gasification. In this work, the effects of feed particle size, temperature, and residence time on the formation of soot particles during the pyrolysis of Athabasca oil sand asphaltenes were investigated. Morphological, structural, and elemental properties of collected soot were also investigated. The experiments were carried out in an atmospheric entrained-flow reactor that was electrically heated to a range of set temperatures between 800 and 1400 °C, and the residence time was varied between 5 to 12 s by controlling the carrier gas nitrogen flow rate. The pyrolysis products were air-cooled in the collection probe after the reactor and passed through a cyclone to separate out particles larger than 10 μm. The particles smaller than 10 μm were subsequently passed through a cascade impactor for segregation of soot and ash particles in the size range from 0.03 to 3 μm. It was found that asphaltenes devolatilize to produce char, light gases, and tar. The different feed particle size ranges used in this study had a negligible effect on the analyzed properties of the soot. The yield of soot formed increased with the pyrolysis temperature. It was observed that the average size of primary soot particles decreases with an increase in temperature. The sulfur and hydrogen contents of soot decrease with temperature as a result of the liberation of sulfur as hydrogen sulfide during the pyrolysis reactions. With increasing residence time, the average size of the primary soot particles increases.

Physicochemical evolution of nascent soot particles in a laminar premixed flame: from nucleation to early growth

In this work, particle inception and early growth stages were investigated in an ethylene/air premixed flame by the evolution of the particle size and structure with flame residence time. Particle size distribution was measured by a scanning mobility particle seizer and chemical–physical investigation was carried out by Raman microspectroscopy, UV–visible light absorption and cyclic voltammetry.From early inception of particles, just downstream the flame front, to the formation of primary soot particles, in the post-oxidation flame zone, particles participate in a series of chemical and physical reactions that strongly modify their nanostructure and physicochemical properties, resulting in different optical and electronic characteristics. The results presented in this study show that the evolution from a mono-modal to a bi-modal size distribution is associated to a particle graphitization process consisting of a slight increase of the in plane average size of the polyaromatic units within the particles, La, and on the formation of stacks of polyaromatic planes. These outcomes suggest that in our flame conditions particle coagulation/coalescence has a major role in the initial soot formation, affecting both physical and chemical particle properties.

Combination of LII and extinction measurements for determination of soot volume fraction and estimation of soot maturity in non-premixed laminar flames

The measurement of soot and soot precursors is important for understanding the formation of soot particles in flames. In this paper, we use the difference between laser-induced incandescence (LII) and two-dimensional extinction measurements to assess the contribution of soot precursors to the extinction measurement. LII measurements are performed with a high spatial resolution of 100 µm to determine the soot volume fraction (f V) in a laminar ethylene/air non-premixed flame at the standard Gulder conditions. While LII is specific to mature soot only, the extinction data represent attenuation due to mature and young soot (absorption and elastic scattering) and also absorption by soot precursors. The difference between the two measurements indicates the contribution of soot precursors and allows a determination of the maturity of soot. This is important knowledge for those using extinction techniques to measure soot concentration, as the contribution from soot precursors may lead to an overestimation of the mature soot concentration. Further, regions with high soot-precursor concentrations, which lead to soot formation, can be identified.

Combustion Chemistry via Metadynamics: Benzyl Decomposition Revisited

Large polycyclic aromatic hydrocarbons (PAHs) are thought to be responsible for the formation of soot particles in combustion processes. However, there are still uncertainties on the course that leads small molecules to form PAHs. This is largely due to the high number of reactions and intermediates involved. Metadynamics combined with ab initio molecular dynamics can provide a very precious contribution because offers the possibility to explore new possible pathways and suggest new mechanisms. Here, we adopt this method to investigate the chemical evolution of the benzyl radical, whose role is very important in PAHs growth. This species has been intensely studied, and though most of its chemistry is known, there are still open questions regarding its decomposition. The simulation reproduces the most commonly accepted decomposition pathway and it suggests also a new one which can explain recent experimental data that are in contradiction with the old mechanism. In addition, quantitative free energy evaluation of some key reaction steps sheds light on the role of entropy.

Temperature and ozone anomalies as indicators of volcanic soot in the stratosphere

The role of volcanogenic aerosols in the formation of ozone and temperature anomalies in the tropical stratosphere recorded after the eruption of Mount Pinatubo in June, 1991, is analyzed in the work on the basis of 30-year balloon measurement data series at the Hilo station (Hawaii). Positive temperature and negative ozone deviations in vertical profiles ≥2σ from normal annual are considered as anomalies. The stratospheric anomalies observed in the second half of 1991 agree well with the presence of volcanic ash remaining in the stratosphere for about six months. However, temperature anomalies and stratospheric ozone depression, observed during next 2–3 years, cannot be explained by long-lived sulfuric acid aerosol. A mechanism is suggested for the formation of long-lived volcanic soot particles in the stratosphere due to thermal decomposition of methane in the eruption column, which actively absorb solar radiation and destroy ozone on their surfaces. The largest ozone anomaly observed in the lower stratosphere in the second half of 1992 is explained by the soot deposition rate calculated accounting for the high efficiency of ozone destruction on its surface.

10906 反射衝撃波背後におけるイソオクタン熱分解時の煤生成に及ぼす水素添加の影響

This paper discusses the effects of hydrogen addition on soot formation in iso-octane pyrolysis using a shock tube. In the experiments, 1% iso-octane + 99% argon is used as a base test gas in the temperature range of 1800-2600 K and in the pressure of 1.2±0.1 MPa behind a reflected shock wave. Soot formation process was characterized by induction time and soot volume fraction, which is obtained from laser light extinction measurement. Selected laser wavelength was 633 nm (He-Ne laser). In addition, tune history of soot particles temperature was calculated based on spectral dependence of monochromatic emissive power from thermal radiation from the soot particles. The experimental results showed that soot particle temperature T_p was higher than ambient temperature T_5 by about 200 - 400 K due to heat release during the soot particle formation process. The soot volume fraction curve showed the bell shape, and hydrogen addition to iso-octane reduced soot volume fraction for T_5=1800 K.

Soot Formation in Unstrained Diffusion Flames

The formation of soot particles has been investigated in CH4/O2 diffusion flames using a unique burner design, which allows the creation of a nearly unstrained planar reaction sheet. Spatially resolved soot volume fractions were obtained using laser-induced incandescence. These soot measurements and the sooting limits were obtained as a function of bulk flow across the flame and mixture strength. Samples were collected using thermophoretic sampling and analyzed using electron microscopy, revealing a broad range of microstructures including particles with unusually large primary diameters and carbon nanotubes. A theoretical model is presented, which confirms that under certain conditions the 1D nature of the flow field of the burner and the strong adverse temperature gradient on the fuel side of the flame result in the soot particles being held in place by thermophoretic forces and allowed to grow for very long time periods. Some of these so-called super aggregates reached sizes of tens of microns and became visible to the naked eye in the soot layer.

Towards a predictive model for polycyclic aromatic hydrocarbon dimerization propensity

Soot particles are a significant pollutant formed as the result of incomplete combustion. Particle nucleation significantly impacts the formation and morphology of soot particles, yet remains a key knowledge gap. To elucidate the process of nucleation, we have investigated the thermodynamic stability of dimers of polycyclic aromatic hydrocarbons (PAHs), towards developing a more comprehensive model for PAH clustering behavior. Using a computational methodology based on molecular dynamics and well-tempered Metadynamics, we quantified the impact of morphological parameters on homo-molecular dimerization, as well as the relative size of monomers on the stability of hetero-molecular dimers. The results illustrated the substantial impact of PAH mass and geometry on the stability of homo-molecular and hetero-molecular dimers at flame temperatures. In particular, dimer stability was found to depend most strongly on monomer mass, followed by solvent-accessible surface area. Additionally, hetero-molecular dimer stability was found to be largely determined by the size of the smallest monomer. Identifying relationships between PAH morphology and thermodynamic stability is a significant step towards a more comprehensive understanding of the physical interactions between PAHs. Altogether, this work presents a framework for elucidating the clustering behavior of arbitrary PAHs and will greatly impact understanding and modeling of particle nucleation and growth.

A Modeling Study on the Nucleation, Growth and Coagulation of Soot Particles in Combustion Flame

Soot, a crucial component of PM2.5 in the ambient air, is a prime product of incomplete combustion of fule. The present paper uses a series of models, including nucleation of soot precursors, surface growth of soot particles as well as particles coagulation, to do numerical simulation of soot particles formation mechanism over time in combustion flame, with the aid of MATLAB platform by C language. Simulation results indicate that the concentration distribution of soot precursors like polycyclic aromatic hydrocarbons (PAHs) have direct effect on the concentration distribution of soot nuclei. Mass addition rate on soot particles surface rises over time since the enlargement of the surface area. In coagulation process, the number of small diameter particles decreases, while that of large diameter particles increases.

The role of dimethyl ether as substituent to ethylene on particulate formation in premixed and counter-flow diffusion flames

The role of dimethyl ether (DME) as substituent to ethylene on particulate formation has been evaluated in premixed and counter-flow diffusion flames. In the premixed flame, the equivalence ratio has been changed from 1.95 to 2.61 and dimethyl ether has been added from 2% to 30% of the total carbon fed. In the counter-flow diffusion flame, the addition of DME has been from 0% to 60% of total carbon fed. Laser induced fluorescence and incandescence have been used to follow the soot formation process: UV and visible fluorescence signals have been attributed to aromatic macromolecules and incipient nanoparticles, respectively, whereas incandescence has been attributed to soot particles and aggregates.In premixed flames results evidence that the formation of soot precursors is not so sensitive to DME addition. In very rich combustion environments, DME addition cannot completely avoid the formation of small precursors, although it can slow down the formation process. This behavior has been observed for all the equivalence ratios investigated.In the pyrolysis region of counter-flow diffusion flames, the formation of aromatic small precursors and soot particles is increased for small DME percentages, up to 20%. Then, the precursors are suppressed for larger amounts, going below the detection limit when 60% of DME is used. This suggests that DME can enhance the production of radicals and small reactive molecules in the pyrolytic side when it is added in small concentrations. For larger amounts of DME, oxidative pathways prevail and the conversion of carbon to precursors and then to soot is inhibited. In the oxidative region DME starts to effectively decrease the particle reduction also for amounts as small as 10–15%.

Kinetic Modeling Study of Polycyclic Aromatic Hydrocarbons and Soot Formation in Acetylene Pyrolysis

The aim of this kinetic work is a critical and detailed analysis of the acetylene pyrolysis in a wide range of conditions, especially from 900 to 2500 K, in order to further validate and refine the kinetic mechanism of C2–C4 acetylenic species. In particular, the successive reactions of the intermediate products, such as C4 species and polyynes, were especially investigated to better understand the primary C2H2 kinetics. The subsystem of acetylene pyrolysis reactions clearly constitutes an important portion in the overall kinetic scheme and also a crucial topic in the further extension of the model toward the formation of polycyclic aromatic hydrocarbons (PAHs) and soot particles. The analysis of acetylene experiments, often with the formation of large amount of soot, requires the coupling of the gas phase kinetic scheme with a soot kinetic model. The gas-phase kinetics involves the formation of the first polycyclic aromatic hydrocarbons up to the first components of the soot kinetic mechanism (C20H16 and...

Analysis of Soot Particle Movement in Diesel Engine under the Influence of Drag Force

The formation of soot is influenced by the composition of air entrainment and structure of hydrocarbon in the fuel. Soot will then form during combustion in a diesel engine. Some of the soot particles will be released from the engine through the exhaust nozzle and some will stick to the cylinder walls. The soot that sticks to the cylinder wall can affect the lifetime of the lubricant oil. Subsequently this will decrease the durability of the diesel engine. By understanding the movement of the soot particles, the effect to the engine can be decreased. Therefore, the initial position and last position of soot particle was recognized through this study. The data for the formation of soot particles in the diesel engine was obtained from previous investigation. The study of soot movement at 8° crankshaft angle under the influence of drag force with different radial, axial and angular settings were carried out using a MATLAB routine. The results showed that the movement of soot particle will change with different parameter settings. Besides that, comparison of the results of soot particle movement influenced by drag force and without drag force has been carried out. It was observed that drag force caused shorter soot particle movement path and moves them away from the cylinder wall.

Detailed Kinetic Modeling of Soot-Particle and Key-Precursor Formation in Laminar Premixed and Counterflow Diffusion Flames of Fossil Fuel Surrogates

This study reports a chemical kinetics soot model for combustion of engine-relevant fuels. The scheme accounts for both low- and high-temperature oxidation, considering their crucial role in engine combustion process. The mechanism is validated against several ignition delay times and laminar burning velocities data sets for single and mixtures of hydrocarbons. To assess the mechanism ability to predict soot precursors, formation of aromatic and aliphatic species with critical effects on soot formation is investigated for several laminar premixed and diffusion flames. The model includes soot particle inception, surface growth, coagulation, and aggregation based on the method of moments. The performance of the model is evaluated by predicting the amount of produced soot during heavy alkanes and aromatic species mixtures pyrolysis. The results are encouraging, proving this methodology to be a suitable tool to simulate the all-round combustion features of engine fuel surrogates by a single reaction model.

Laser-induced incandescence technique to identify soot nucleation and very small particles in low-pressure methane flames

This paper presents the study we carried out on the formation of soot particles in low-pressure premixed CH4/O2/N2 flames by using Laser-Induced Incandescence (LII). Flames were stabilised at 26.6 kPa (200 torr). Four different equivalence ratios were tested (Φ = 1.95, 205, 2.15 and 2.32), Φ = 1.95 corresponding to the equivalence ratio for which LII signals begin to be measurable along the flame. The evolution of the LII signals with laser fluence (fluence curve), time (temporal decay) and emission wavelength is reported at different heights above the burner. We specifically took advantage of the low-pressure conditions to probe with a good spatial resolution the soot inception zone of the flames. Significant different behaviours of the fluence curves are observed according to the probed region of the flames and Φ. In addition, while the surface growth process is accompanied by an increase in the LII decay-times (indicator of the primary particle diameter) at higher Φ, decay-times become increasingly short at lower Φ reaching a constant value along the flame at Φ = 1.95. These behaviours are consistent with the detection of the smallest incandescent particles in the investigated flames, these particles having experienced very weak surface growth. Flame modelling including soot formation has been implemented in flames Φ = 2.05 and 2.32. Experimental quantitative soot volume fraction profiles were satisfactorily reproduced by adjusting the fraction of reactive soot surface available for reactions. The qualitative variation of the computed soot particle diameter and the relative weight of surface growth versus nucleation were consistent with the experimental observations.