Soot Formation In Combustion Research Articles

Soot formation in combustion of spherically symmetric isolated fuel droplets with different initial diameters

Spherically-symmetric, isolated droplets are ideal systems to investigate the physics and the chemistry of combustion of liquid fuels. Despite their simplicity, most phenomena involved in spray combustion are still accounted for: evaporation and diffusion-induced transport, complex liquid thermodynamics, radiation, aerosol chemistry. In this work we analyzed the formation and evolution of soot particles and aggregates from the combustion of n-heptane isolated droplets. A 1D mathematical model including detailed description of thermodynamics and transport properties of liquid and gaseous phases, radiative heat transfer, and detailed homogeneous chemistry is adopted. Soot formation and evolution is described via a discrete sectional method, accounting for nucleation, surface growth, coagulation, aggregation, and oxidation.The mathematical model was utilized in this study to investigate how the initial diameter of droplets affects their tendency to form soot. The simulations were specifically designed to examine the experimental findings of Choi et al. (2001), which indicated that droplets with intermediate diameters (∼ 1.9 mm) result in the highest soot volume fraction. The numerical results reveal that this maximum is due to a competition between two phenomena: (i) increasing droplet diameter leads to a longer lifetime, thereby promoting soot formation, but (ii) larger diameters cause more radiative losses, lower flame temperatures, and subsequently, a decrease in soot formation.

Accurate thermochemistry prediction of extensive Polycyclic aromatic hydrocarbons (PAHs) and relevant radicals

Polycyclic aromatic hydrocarbons (PAHs) are important intermediates to soot formation in combustion. A reliable database of their thermochemistry is required for the development of chemical kinetic models describing the gas-phase chemistry of hydrocarbon fuels. In this study, temperature-dependent thermodynamic properties are consistently determined using high-accuracy quantum chemistry calculations for an extensive set of PAH compounds. The developed database comprehensively consists of 125 C6-C18 PAH molecules and radicals, which are important and commonly included in chemical mechanisms studies. At the M06-2X/6-311++G(d,p) level of theory, geometry optimizations, vibrational frequency calculations, and dihedral angle scans are performed for all PAH species. The G3 method, together with the atomization reaction approach, is selected to derive the average atomization formation enthalpy. This method produces the most accurate thermochemistry quantities for PAHs, as demonstrated in a previous study. The entropy and heat capacity values are calculated using statistical thermodynamics in MultiWell. These results exhibit good agreement with the databases in literature. To examine the application of the group additivity (GA) method for PAHs, the Bland−Altman plot, a statistical analysis approach, is employed to visualize the agreement between the results from the quantum chemical calculations and GA methods. Two GA methods are examined and significant differences are found, which indicates that GA values of relevant groups need to be further updated. The database of thermodynamic quantities developed in this study are of particular value in modeling studies and important for exploring mechanisms of the PAH growth.

Effects of the electric field on soot formation in combustion: A coupled charged particle PBE-CFD framework

In this article, a coupled PBE-CFD framework has been proposed to study counterflow non-premixed flames and soot formation under an external electric field. This framework integrates the population balance equation (PBE) for nanoparticle dynamics into an in-house CFD solver for the multicomponent reactive flows. Different electric properties have been considered in this model. An ion mechanism used in both fuel-rich and fuel-lean combustion is combined with a detailed chemistry for neutral gaseous species and small-size aromatics to retain the full chemistry. In order to model soot particles carrying charges and the movement of the reacting fluid medium in the electric field, a second PBE for the production and transport of charges on soot particles is introduced for the first time and incorporated into the original PBE for the number density of particles. Also, the electric force for the gas mixture is included in the momentum equations. The electric drift velocities for ions and soot particles are also considered in the transport equations of ions and the PBE of soot particles, respectively. The simulations have shown that the presence of the electric field modifies the stagnation plane of the counterflow flames and reduces the soot formation in both rich-fuel and lean-fuel conditions in agreement with experimental observations. The application of the soot particle charging model, accompanied by a proper electric correction factor on the nanoparticle processes of nucleation and surface growth, significantly improves the stability of the flame structure. The introduction of the electric correction factor reveals that the suppression of soot formation in an electric field is mainly caused by the inhibited chemical reactions of the PAH nucleation and particle surface growth, which is more important than the electric drift of the charged particles. Reducing the critical size of the particle charging process enhances the electric drift of nascent soot, thus lessening its subsequent evolution.

Predicting the soot emission tendency of real fuels – A relative assessment based on an empirical formula

Significant progress is going on in the development of non-fossil based alternative fuels which are designed to be sustainable, have potential to substitute (fully or partially) the fossil-based fuels and importantly are targeted to reduce emissions. Though soot emission characteristics has not been focus of fuel certification process yet, due to growing environmental concerns it is important to consider this aspect in the fuel design and understand its emission patterns.It is known that soot formation in combustion is linked to the degree of branching, degree of unsaturation or the degree of cyclization of a fuel molecule. This information is used to extract soot emission properties of complex real fuel mixtures. To do so, we use the definition of hydrogen deficiency (HD) which gives information on the number of double bonds or cyclic structures present in a fuel. With the help of one measured emission property, the same property of different fuels is predicted using HD. This way, we obtain a relative assessment of fuels’ sooting tendency compared to other known fuels. In this work, soot emission pattern of six jet fuels (reference fuels and synthetic fuels) is predicted based on HD and compared them with the recent ground-test measurements of V2527-A5 engine of an Airbus A320. In addition, two more test cases are provided: aviation fuels and diesel fuels, to predict sooting tendency using HD to support our concept. The prediction of the sooting propensity based on HD was found in excellent agreement with the measurements.

Structures and binding energies for complexations of different spin states of Ni+ and Ni2+ to aromatic molecules

Density functional theory calculations, using the B3LYP parameterisation, were performed to determine structures, vibrational frequencies, and binding energies for complexation of Ni+ and Ni2+ cations with benzene and naphthalene molecules and clusters. The calculations employed the Stuttgart basis set with ECP pseudo potentials for the Ni cations and basis sets of at least triple ζ plus polarisation, and diffuse quality for C and H. The effect of electron correlation on non-bonded interactions was accounted for by the Grimme GD3 dispersion correction. Counterpoise computations were made for BSSE. Comparison between experiment and theory provide fascinating new insight into the bonding for these prototypical organometallic (OM) complexes. These structures have a sandwich topology, indicating major structural reorganisations occuring when benzene or naphthalene interact with Ni cations. Adiabatic electron affinities and ionisation potentials agree well with experiment when available. Binding energies were also determined, providing insight into the stability of the complexes. The results presented here provide important information for future studies to address additional investigations of both problems of the electronic structure properties of these complexes, as well as the role of the polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM) and soot formation in combustion. The Ni+/Ni2+ + aromatic organometallic bonding is of the same order of stability as an aromatic C–H bond. Such bonding modifies the IR spectrum of the complexed aromatic molecules by enhancing the 3.3 μm feature and decreasing the C–H bands in the 11–12 μm range (γ C–H). Organometallic complexation reactions may contribute significantly to metal depletion in the ISM.

Exploring Soot Particle Concentration and Emissivity by Transient Thermocouples Measurements in Laminar Partially Premixed Coflow Flames

Soot formation in combustion represents a complex phenomenon that strongly depends on several factors such as pressure, temperature, fuel chemical composition, and the extent of premixing. The effect of partial premixing on soot formation is of relevance also for real combustion devices and still needs to be fully understood. An improved version of the thermophoretic particle densitometry (TPD) method has been used in this work with the aim to obtain both quantitative and qualitative information of soot particles generated in a set of laminar partially-premixed coflow flames characterized by different equivalence ratios. To this aim, the transient thermocouple temperature response has been analyzed to infer particle concentration and emissivity. A variety of thermal emissivity values have been measured for flame-formed carbonaceous particles, ranging from 0.4 to 0.5 for the early nucleated soot particles up to the value of 0.95, representing the typical value commonly attributed to mature soot particles, indicating that the correct determination of the thermal emissivity is necessary to accurately evaluate the particle volume fraction. This is particularly true at the early stage of the soot formation, when particle concentration measurement is indeed particularly challenging as in the central region of the diffusion flames. With increasing premixing, an initial increase of particles is detected both in the maximum radial soot volume fraction region and in the central region of the flame, while the further addition of primary air determines the particle volume fraction drop. Finally, a modeling analysis based on a sectional approach has been performed to corroborate the experimental findings.

Radical–radical reactions, pyrene nucleation, and incipient soot formation in combustion

We present a combined experimental and probabilistic simulation study of soot-precursor. The experiments were conducted using aerosol mass spectrometry coupled with tunable vacuum ultraviolet radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. Mass spectra and photoionization efficiency (PIE) curves of soot precursor species were measured at different heights in a premixed flat flame and in a counter-flow diffusion flame fueled by ethylene and oxygen. The PIE curves at the pyrene mass from these flames were compared with reference PIE scans recorded for pyrene. The results demonstrate that other C16H10 isomers than pyrene are major components among species condensed onto incipient soot in this study, which is in agreement with the simulations. Species with mass 202u only have a high prevalence in incipient soot particles drawn from the premixed flame, but hydrocarbon species with sizes in the range 200–400u are important to incipient-soot formation in both flames. The simulations predict that some species form through combination reactions involving relatively large radicals and bypass traditional molecular-growth pathways through addition of small hydrocarbon species. The experimental results support this prediction; they demonstrate that these species have higher relative abundances in particles formed close to the fuel outlet than smaller, lighter molecular species and indicate that these species are important to early formation of incipient-soot precursors. The results also imply that a leading role in incipient-soot precursor formation is played by species with lower thermal stability than the even-carbon numbered, unsubstituted polycyclic aromatic hydrocarbons known as “stabilomers”.

Effect of soot formed in result of coal combustion with two types of solid fuel additives on soot – soil bacteria interaction – biodegradation of soot components by soil bacteria

During coal combustion soot formation is unwanted. There are many fuel additives used to reduce soot production. In this study toxicity and biodegradation of three types of soot by two soil bacteria strains i.e. Pseudomonas aeruginosa and Bacillus subtilis were compared. Two additives were selected to produce soot samples: (i) commonly used and containing significant quantities of NaCl (40–44%) and copper (Cu-additive), (ii) newly developed without NaCl and with triiron tetraoxide instead of copper (Feadditive). Average number of bacterial colonies was similar when soot produced during combustion without additive and with Feadditive were used. It was about 50% higher than number of colonies when soot formed with Cu-additive was applied. The soot biodegradation was tested by HS-GC-MS analysis. The highest biodegradation level (about 80%) was obtained for samples with P. aeruginosa cultures and Fe-additive soot. It can be concluded that newly developed additive does not increase toxicity of the soot to soil bacteria and will be removed more quickly from the environment.

Time- and Isomer-Resolved Measurements of Sequential Addition of Acetylene to the Propargyl Radical.

Soot formation in combustion is a complex process in which polycyclic aromatic hydrocarbons (PAHs) are believed to play a critical role. Recent works concluded that three consecutive additions of acetylene (C2H2) to propargyl (C3H3) create a facile route to the PAH indene (C9H8). However, the isomeric forms of C5H5 and C7H7 intermediates in this reaction sequence are not known. We directly investigate these intermediates using time- and isomer-resolved experiments. Both the resonance stabilized vinylpropargyl (vp-C5H5) and 2,4-cyclopentadienyl (c-C5H5) radical isomers of C5H5 are produced, with substantially different intensities at 800 K vs 1000 K. In agreement with literature master equation calculations, we find that c-C5H5 + C2H2 produces only the tropyl isomer of C7H7 (tp-C7H7) below 1000 K, and that tp-C7H7 + C2H2 terminates the reaction sequence yielding C9H8 (indene) + H. This work demonstrates a pathway for PAH formation that does not proceed through benzene.

Exploring the Role of PAHs in the Formation of Soot: Pyrene Dimerization

A critical step in currently accepted models for soot formation in combustion is the dimerization of polycyclic aromatic hydrocarbons as small as pyrene, which is necessary within these models to reproduce correctly the soot particle size distribution. We present experimental measurements on the kinetics of pyrene dimerization performed in low-temperature supersonic flows with photoionization mass spectrometric detection, coupled with theoretical results based on careful consideration of the intermolecular interaction energies, binding energy, equilibrium constant, and intermolecular dynamics. These results demonstrate that the equilibrium of the reaction strongly favors the dissociation of the pyrene dimer at high temperature and that physical dimerization (involving van der Waals forces) of pyrene cannot be a key step in carbon particle formation in hot environments such as flames and circumstellar shells.

高湿度気流中における液滴火炎内のすす生成特性(熱工学,内燃機関,動力など)

高湿度気流中における液滴火炎内のすす生成特性(熱工学,内燃機関,動力など)

Kinetic modeling of hydrogen production by thermal decomposition of methane

A kinetic model for the thermal decomposition of methane was developed by modifying a model for soot formation in combustion. The modifications consisted of adding five reactions representing surface chemistry on carbon particles. One kinetic parameter for the new surface reactions was fit to experimental data and the other parameters were determined by analogy to known gas-phase reactions. The modified model reproduces well the hydrogen concentrations at the beginning and end of recent experiments on the thermal decomposition of methane in the presence of carbon catalysts. The model predicts that increasing the pressure from 0.1 to 3.0 MPa will decrease hydrogen production with carbon catalysts by 48–60%. This prediction is robust to variations in the rate constants for the surface reactions and the rate constant for a key gas-phase reaction.

Soot Formation in Combustion Processes (Review)

A review is given of recent papers on the phenomenology, kinetics, and mechanism of soot formation in hydrocarbon combustion; the effects of various factors on the formation of polycyclic aromatic hydrocarbons, fullerenes, and soot, low-temperature soot formation in cool flames, combustion in electric field, and the paramagnetism of soot particles from an ecological viewpoint are considered.

Study of soot growth in a plug flow reactor using a moving sectional model

Combustion in a plug flow reactor (PFR) is similar to that occurring in the postflame region of laminar flames, and hence provides a well-defined environment to study those soot formation processes at a relatively larger residence time, which are identified as simultaneously occurring surface growth (typically by HACA mechanism) and PAH surface condensation together with the particle coagulation. The roles of HACA mechanism and PAH condensation are of great interest in recent research. A moving sectional aerosol dynamics model is implemented in this study for prediction of soot formation in a PFR. Coupled with the detailed gas phase chemistry, the soot model provides reasonable predictions of the molar concentrations of major species, PAHs and radicals, soot mass concentration, soot mean particle diameter, and the particle size distribution. The predicted size distribution shows the evolution of its shape from the power law to a bi-modal distribution. The study of the soot growth rate shows that both HACA surface growth and PAH surface condensation are important in the modeling of soot formation in the PFR. All available PAH radical species are included in surface condensation, which results in good prediction of mass growth rate without excessive depletion of the PAH species. However, the use of a single incipient species, pyrene, results in its concentration being underpredicted. This new soot model provides a solid approach to investigate further the soot formation in combustion with simultaneous particle nucleation, surface growth, PAH condensation, and coagulation.

Condensation flame of acetylene decomposition

Acetylene decomposition flame propagation was numerically analyzed and was found to be the result of the condensation reaction. Condensation processes provide reaction heat and act as a driving force for C2H2 flame propagation. The kinetic model reasonably predicts the level of burning velocity of the acetylene decomposition flame. The model does not demonstrate the relatively strong positive pressure dependence of burning velocity as was observed experimentally in the work of Cummings et al. [Proc. Combust. Inst. 8 (1962) 503–510]. Heat-release kinetics demonstrates a two-stage process. The first stage corresponds to heat release due to benzene formation, and the second stage of heat release corresponds to soot inception and carbonization processes. It was demonstrated that the burning velocity is sensitive to the surface growing rate constant. The use of a simplified form of presentation of the surface growing process [P.R. Lindstedt, in: Soot Formation in Combustion: Mechanisms and Models, Springer-Verlag, Berlin/New York, 1994, pp. 417–441] represents positive thermal feedback in the heat generation in a flame reaction zone.

Fouling phenomena in pyrolysis and combustion processes

Several large scale processes, e.g. liquid or vapour phase pyrolysis or combustion, involve hydrocarbon feedstocks. Their common mechanistic feature is characterised by complex radical reaction systems, involving unsaturated molecules and radicals, universally recognized as precursors in fouling and soot formation. This is an important and actual research field, both for the possible improvements in the processes and for the reduction of the environmental impact of combustion. This paper analyses the strong analogies existing on the mechanism of fouling and soot formation in liquid phase pyrolysis (such as visbreaking and delayed coking), gas phase pyrolysis to produce ethylene and finally combustion process of fuel hydrocarbons. The aim of this work is to move towards the ambitious objective of obtaining a global model for the description of fouling phenomena into the equipments and soot formation in combustion.

SOOT PARTICULATE FORMATION AND CHARACTERIZATION IN COMBUSTION

The subject of soot formation in combustion is being extensively studied to decipher the relevant mechanisms. Despite considerable progress made in understanding some to the features of soot formation, we still do not have sufficient details of the formation mechanisms to be able to calculate soot in the flame of a given hydrocarbon fuel from first principles. This paper first gives and overview of the soot particulate formation in combustion and its contribution to ambient particulate matter. The experimental burner types that are commonly used for soot research are then reviewed and basic features of the most commonly used laminar diffusion burners are summarized. Soot particulate formation and emissions in diesel and gasoline engines, and their relative contributions to ambient particulate matter, are discussed. Methods based on physical sampling and non-intrusive laxer-based soot diagnostic techniques are briefly reviewed and their capabilities and limitations are discussed.

Simultaneous Particle and Molecule Modeling (SPAMM): An Approach for Combining Sectional Aerosol Equations and Elementary Gas-Phase Reactions

ABSTRACT A method for writing the sectional aerosol equations in a form suitable for combination with a detailed kinetic model for gas-phase reactions has been developed. The sectional equations are given for arbitrary values of d, the ratio of molecular weights of adjacent bin boundaries, and are solved for d ≥ 2 for three different cases of the intra-bin distribution: (1) constant mass density w.r.t. ln(v), where v = molecular weight; (2) constant number density w.r.t. ln(v), or alternately, constant mass density w.r.t. v; (3) constant number density w.r.t. v. All the solutions given conserve mass; the extent of deviation from conservation of particle number is evaluated. An example of the approach is given for soot formation in combustion. The aerosol sections describe mass above 400 amu, with d = 2. Aerosol reactions are: bin-bin coagulation; addition of C2H2; addition of polycyclic aromatic hydrocarbons (PAH) with 2 to 10 benzenoid rings; oxidation via OH attack. The set of aerosol reactions is appended to a detailed elementary-step kinetic mechanism describing the gas-phase chemistry, including growth of PAH up to C32H14 (398 amu). The method also directly calculates the effect of the soot aerosol upon concentrations of gas-phase species such as PAH—a new capability for soot formation models. Predictions for a given set of conditions for the three cases of the intrabin distribution show striking changes not only in the particle size distribution but also in predictions of total aerosol mass and number concentrations. The extent of deviation from conservation of particle number for the three cases is shown to have a direct effect upon the predicted aerosol dynamics.

A New Mechanism for the Formation of Meteoritic Kerogen-Like Material

The carbon in ancient carbonaceous chondritic meteorites is mainly in a hydrocarbon composite similar to terrestrial kerogen, a cross-linked structure of aliphatic and aromatic hydrocarbons. Until recently, the composite has been commonly thought to have been produced in the early solar nebula by a Fischer-Tropsch-type process, involving the catalytic synthesis of hydrocarbons from carbon monoxide and hydrogen on grain surfaces. Instead, the aromatic hydrocarbons may form in gas-phase pyrolysis of simple aliphatics like acetylene and methane by a mechanism developed recently to explain formation of soot in combustion and of aromatic molecules in circumstellar envelopes. Nonequilibrium chemical kinetic calculations indicate that this mechanism can produce meteoritic aromatics if the initial concentration of simple hydrocarbons in the solar nebula was sufficiently but not unreasonably high.

Application of FT-IR emission/transmission (E/T) spectroscopy to study coal combustion phenomena

This paper describes the application of a recently described Fourier Transform Infrared (FT-IR) Emission and Transmission (E/T) technique to study coal flames produced in a transparent wall reactor. Comparisons are made for the ignition, soot formation, particle temperature and gas temperature for a number of coals varying in rank from lignite to low volatile bituminous. Samples of chars prepared at different temperatures and demineralized coals were also studied. Flame properties were compared with characteristics of the samples to determine the factors which control flame behavior. A comparison of the ignition of several samples suggests that the rate of ignition correlates with the initial rate of weight loss in air in a TGA experiment at lower temperatures. Ignition of chars is heterogeneous; ignition of high rank coals is homogeneous; but low rank coals exhibit both homogeneous and heterogeneous contributions to ignition. Soot formation in combustion correlates well with tar yield in pyrolysis suggesting that tar is the chief precursor of soot.