Kinetic Modeling Of Soot Formation Research Articles

Two-route formation of soot nuclei: experimental and modeling evidence

The results of shock-tube measurements of acetylene concentration and characteristics of the ensemble of soot particles during the pyrolysis of benzene, ethylene, ethylene–methane and ethylene–propane diluted with argon are compared with the predictions of the unified kinetic model of soot formation implemented in the MACRON program based on the Galerkin method. The simulation results demonstrate good agreement with experimental data for the pyrolysis of ethylene alone and with the specified additives, thereby confirming the two route mechanism of soot nuclei formation. The agreement for benzene pyrolysis is less satisfactory.

Experimental and kinetic modeling of soot formation in counterflow non-premixed flames of surrogate fuel components: n-dodecane and iso-dodecane

Normal dodecane (n-C12) and iso-dodecane (i-C12) are often used as components for surrogate mixtures of aviation and diesel fuels. Although studies have been performed to understand the combustion and spray behavior of dodecane isomers, the soot formation from the combustion of n- and i-C12 in non-premixed flames is not well studied. In this work, soot volume fraction (SVF) profiles of n- and i-C12 were measured across a wide range of strain rates and mixture conditions in a counterflow burner facility at the University of Connecticut. Neat and binary mixtures of n- and i-C12 were investigated to study the influence of alkane branching on soot formation. A soot model was developed and validated in this study by extending the detailed kinetic model developed at the Lawrence Livermore National Laboratory (LLNL) for the formation of polycyclic aromatic hydrocarbons (PAHs) to simulate soot formation and growth based on the discrete sectional method. Additional gas phase reactions forming pyrene and ring enlargement reactions were added to the existing PAH model. The LLNL kinetic model was validated with SVF data obtained in this work for n-C12 and i-C12 flames along with data available in literature for ethylene, iso-butene, n-heptane, and iso-octane. The soot precursor reactions added in this work were found to play a critical role in simulating the experimentally observed non-linear trend of the peak SVF with increased branched alkanes. Reaction path analysis was conducted to illustrate the fuel structure effects on soot formation pathways in n- and i-C12 mixtures. In view of the satisfactory agreement between modeling results and experimental data, as well as capturing the non-linear variation in peak SVF with the alkane branching for the first time, further investigation within the framework of the soot model is discussed and critical insights are provided into the reaction pathways which require further attention.

An experimental and simulation study of the effect of acetone and propane additives on soot formation in acetylene pyrolysis behind reflected shock waves

Experimental investigations and detailed kinetic simulations of the formation of soot particles during pyrolysis of mixtures of acetylene with acetone and propane behind reflected shock waves are performed. Acetone and propane additives are found to substantially promote the process of soot formation as compared with that in acetylene–argon mixtures. Detailed kinetic simulations closely reproduce our own experimental results and published data. The kinetic model of soot formation is comprised of 4782 direct and reverse reactions involving 372 species. The predictive possibility of the kinetic model of soot formation is tested by describing the effect of acetone and propane additives to acetylene–argon mixtures on soot formation. All the kinetic parameters of the unified kinetic model are kept constant. The indicated additives enhance the soot yield because polyyne-dominated pathway of soot nucleation, characteristic of unseeded acetylene–argon mixtures, is augmented by the aromatic pathway, typical of most hydrocarbons.

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.

Experimental and Numerical Investigations of Soot Formation in Laminar Coflow Ethylene Flames Burning in O2/N2 and O2/CO2 Atmospheres at Different O2 Mole Fractions

This paper presents an experimental and numerical study of the distributions of the temperature and soot volume fraction in laminar coflow ethylene diffusion flames burning in O2/N2 and O2/CO2 atmospheres with the O2 mole fraction varying from 21 to 50% in both atmospheres. The fuel flow rate was maintained constant in all of the experiments and simulations. The two-color flame emission method based on the response spectrum of R and G bands of a three-color charge-coupled device camera was applied to measure the temperature and soot volume fraction. Numerical calculations were conducted using the C2 chemistry model [Appel–Bockhorn–Frenklach (ABF), Appel, J.; Bockhorn, H.; Frenklach, M. Kinetic modeling of soot formation with detailed chemistry and physics: Laminar premixed flames of C2 hydrocarbons. Combust. Flame 2000, 121 (1–2), 122–136, DOI: 10.1016/S0010-2180(99)00135-2] with formation of polycyclic aromatic hydrocarbons (PAHs) up to pyrene and a soot model incorporating the dimerization of two pyrene...

Unified kinetic model of soot formation in the pyrolysis and oxidation of aliphatic and aromatic hydrocarbons in shock waves

The formation of soot particles in the pyrolysis and oxidation of various aromatic and aliphatic hydrocarbons in argon behind reflected shock waves has been investigated by computational and theoretical methods. The hydrocarbons examined include methane, ethane, propane (aliphatic hydrocarbons with ordinary bonds), acetylene, ethylene, propylene (aliphatic hydrocarbons with multiple bonds), benzene, toluene, and ethylbenzene (simplest aromatic hydrocarbons). Soot formation in the pyrolysis and oxidation of both aromatic and aliphatic hydrocarbons can be simulated in detail within a unified kinetic model. The predictive power of the unified kinetic model has been verified by directly comparing the calculated kinetic data for the formation of products and reactive radicals in the pyrolysis and oxidation of various hydrocarbons to the corresponding experimental data. In all calculations, all the kinetic parameters of the unified kinetic model were strictly fixed. A good quantitative fit between the data calculated via the unified kinetic model and experimental data has been attained.

Formation of soot particles in pyrolysis and oxidation of aliphatic and aromatic hydrocarbons: Experiments and detailed kinetic modeling

Experiments on pyrolysis and oxidation of rich mixtures of various aliphatic and simple aromatic hydrocarbons in reflected shock waves have been performed. The mixtures C2H2/Ar, C2H6/Ar, C2H4/Ar, C2H4/O2/Ar, CH4/Ar, CH4/O2/Ar, C3H8/Ar, C3H6/Ar, toluene/Ar, and benzene/Ar were studied. The yield of soot and the temperature of soot particles were determined experimentally by the double-beam absorption emission method. The kinetic model of soot formation during the pyrolysis and oxidation of rich mixtures of aliphatic and aromatic hydrocarbons complemented with a set of nucleations of soot particles from both polyaromatic fragments and unsaturated aliphatic hydrocarbons was suggested. This kinetic model of soot formation was successfully tested. It describes the experimental literature data on the yield of the products of pyrolysis and oxidation of acetylene and diacetylene in a shock tube. The results of our experiments and kinetic calculations of the time, temperature, and concentration dependences are in good agreement for all hydrocarbons under study. All the kinetic parameters of the model remained strictly constant.

Soot formation during the pyrolysis and oxidation of acetylene and ethylene in shock waves

Experimental and computational studies of soot formation during the pyrolysis and oxidative pyrolysis of hydrocarbons with multiple bonds have been performed on ethylene and acetylene as examples. The experiments have been carried out behind reflected shock waves over the temperature range from 1700 to 2800 K at a pressure of 3 to 5 bar. The process of soot formation was recorded using the absorption-emission method, which enables one to simultaneously measure the mass of soot particles per unit volume and their temperature. The experimental data are interpreted within a new kinetic mechanism that takes into account the nucleation of soot particles from both polyaromatic hydrocarbon fragments and unsaturated aliphatic hydrocarbons. The proposed kinetic model of soot formation closely reproduces the time dependence of the soot yield and the temperature of soot particles measured in the present work, as well as the concentrations of several key components formed in the early stages of pyrolysis and oxidation of acetylene and ethylene, reported in a number of works by other authors.

Shock tube and modeling study of soot formation during the pyrolysis and oxidation of a number of aliphatic and aromatic hydrocarbons

An experimental and modeling study of soot formation during the shock-tube pyrolysis and oxidation of a number of aromatic and aliphatic hydrocarbons was performed. Hydrocarbons with different chemical structures, both aliphatic (methane, propane, and propylene) and aromatic (benzene, toluene, and ethylbenzene), were tested. The experiments showed that the soot formation during the pyrolysis, even in the presence of small amounts of oxygen, is accompanied by a temperature decrease. A new, essentially modified detailed kinetic model of soot formation is proposed, which was successfully tested by describing the published data on the temporal behavior of a number of key species during the initial stages of pyrolysis and oxidation of a number of hydrocarbons under various conditions. When applied to modeling our experimental data, it closely reproduced the time profiles of the soot yield and temperature for the shock-tube pyrolysis of methane, propane, and propylene, as well as benzene, toluene, and ethylbenzene, two sets of compounds of drastically different natures.

Kinetic Modeling of Soot Formation in Turbulent Nonpremixed Flames

A coupled radiation/flamelet combustion model was applied to the simulation of two jet diffusion flames fed with ethylene and methane. The scope of this work was to demonstrate the application of several strategies for the solution of a two-equation model for describing formation of soot in turbulent non-premixed flames using simple semi-empirical models of soot nucleation, growth, oxidation, and coagulation. Three methods for closing soot production terms in turbulent flames are presented and compared. It is shown that turbulent fluctuations must be carefully taken into account to obtain a reliable prediction of soot volume fraction. Moreover, the coupling between the soot production rate and the radiative heat loss cannot be neglected. The predicted soot amount in the two turbulent flames investigated was relatively insensitive to the semi-empirical model adopted for nucleation. On the contrary, growth and oxidation models significantly influenced soot formation.

Detailed kinetic modeling of soot formation in shock tube pyrolysis and oxidation of toluene and n-heptane

A new detailed kinetic model of soot formation in shock tube pyrolysis and oxidation of aliphatic and aromatic hydrocarbons is proposed. The model is based on the comprehensive kinetic model of PAH formation and growth [H. Richter, J.B. Howard, Phys. Chem. Chem. Phys. 4 (2002) 2038–2055; H. Richter, S. Granata, W.H. Green, J.B. Howard, Proc. Combust. Inst. 30 (2005) 1397–1405; J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122–136; M. Frenklach, D.W. Clary, T. Yuan, W.C. Gardiner, Jr., S.E. Stein, Combust. Sci. Tech. 50 (1986) 79–115; M. Frenklach, J. Warnatz, Combust. Sci. Tech. 51 (1987) 265–283; M.S. Skjøth-Rasmussen, P. Glarborg, M. Østberg, J.T. Johannessen, H. Livbjerg, A.D. Jensen, T.S. Christensen, Combust. Flame 136 (2004) 91–128], on the new concepts of soot particle nucleation [A. Violi, Combust. Flame 139 (2004) 279–287; A. Violi, A.F. Sarofim, G.A. Voth, Combust. Sci. Tech. 176 (2004) 991–1005; A. D’Alessio, A. D’Anna, P. Minutolo, L.A. Sgro, A. Violi, Proc. Combust. Inst. 28 (2000) 2547–2554; A. D’Anna, A. Violi, A.D’Alessio, A.F. Sarofim, Combust. Flame 127 (2001) 1995–2003] and the traditional H-abstraction/C 2H 2-addition (HACA) route of PAH and soot particles surface growth [H. Wang, M. Frenklach, Combust. Flame 110 (1997) 173–221; J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122–136]. The gas-phase kinetic scheme was validated against the experimentally measured concentration profiles of the main gas-phase species formed during toluene pyrolysis and H and OH radicals during benzene and phenol pyrolysis and toluene oxidation behind reflected shock waves. The model describes the main characteristics of soot formation in pyrolysis and oxidation of toluene and n-heptane oxidation under conditions typical of shock tube experiments. Both hydrocarbons have the same number of carbon atoms but different structures, which causes different behavior of the systems. The discrete Galerkin technique was applied for direct counting of the mean number of active sites formed on the surface of soot precursors and soot particles in reactions of activation, deactivation, and surface growth.

DETAILED KINETIC MODELING OF SOOT FORMATION DURING SHOCK-TUBE PYROLYSIS OF C6H6: DIRECT COMPARISON WITH THE RESULTS OF TIME-RESOLVED LASER-INDUCED INCANDESCENCE (LII) AND CW-LASER EXTINCTION MEASUREMENTS

The results of calculations of the main parameters of the soot formation process (τ, k f, SY, and r m) carried out with the use of the detailed kinetic model of soot formation are compared with the experimental measurements of these parameters by the continuous-wave (CW)-laser extinction technique and by the time-resolved laser-induced incandescence (LII) method during C6H6 pyrolysis behind reflected shock waves. The detailed kinetic model of soot formation that is developed incorporates the gas-phase mechanisms of acetylene pyrolysis and the mechanisms of formation of polyaromatic hydrocarbons, polyyne molecules, and pure carbon clusters. It combines the H abstraction/C2H2 addition and polyyne pathways of the soot formation process. The formation, growth, and coagulation of soot precursors and soot particles are described within the framework of the discrete Galerkin technique based on an error-controlled expansion of the size distribution function of heterogeneous species into the orthogonal polynomials of a discrete variable (in particular, the number of monomers in the heterogeneous particle) that makes it possible to preserve a discrete character of any elementary transformations of heterogeneous particles and to describe them as elementary chemical reactions for the heterogeneous particles of all sizes. The comparison of the calculations with the experimental measurements of the induction time τ, observable rate of soot particle growth, k f, and soot yield SY by the CW-laser extinction method in the pyrolysis of benzene/argon mixtures in shock-tube experiments clearly demonstrates that the coincidence is quantitatively good for all the main parameters of soot formation. A particular difference between the values of the mean soot particle radius r m experimentally measured by the time-resolved LII technique and calculated with the help of the detailed kinetic model is observed at the low and high temperatures. The results presented demonstrate the current level of the predictive capabilities of the detailed kinetic model of soot formation and the reliability of the time-resolved LII technique for the quantitative determination of the soot particle sizes.

Detailed kinetic modeling of soot formation in hydrocarbon pyrolysis behind shock waves

A new detailed kinetic model of soot formation in hydrocarbon pyrolysis under conditions typical for shock tube experiments was developed. It combines the mechanism of formation of polyaromatic hydrocarbons, polyyne, and H-abstraction-C 2 H 2 -addition (HACA) pathways of soot formation and the mechanisms of acetylene pyrolysis and pure carbon cluster formation. The kinetic model was tested for various aliphatic and aromatic hydrocarbons and their mixtures, and it showed the role of the polyyne and HACA pathways in soot formation under various conditions for different hydrocarbons. All the numerical results presented were calculated while keeping the reaction mechanism and the entire set of the rate coefficients fixed. The comparison of the calculations with the experimental measurements clearly shows that the coincidence is quantitatively good for all the main parameters of soot formation. It is worth remarking that the calculations have been performed for hydrocarbons with different chemical structures, both aliphatic and aromatic, and also for a wide range of temperature and pressure. Therefore, the model can be expected to describe soot formation for other molecules, including larger ones with more complex chemical structures and even for hydrocarbon mixtures.

00/02499 Kinetic modeling of soot formation with detailed chemistry and physics: laminar premixed flames of C 2 hydrocarbons

00/02499 Kinetic modeling of soot formation with detailed chemistry and physics: laminar premixed flames of C 2 hydrocarbons

Kinetic modeling of soot formation with detailed chemistry and physics: laminar premixed flames of C 2 hydrocarbons

The present study reports an updated detailed chemical kinetic model for soot formation. The model combines recent developments in gas-phase reactions, aromatic chemistry, soot particle coagulation, soot particle aggregation, and develops a new submodel for soot surface growth. The model was tested against experimental profiles of major and minor chemical species, aromatics, soot volume fractions, and soot particle diameters reported in the literature for nine laminar premixed flames of ethane, ethylene, and acetylene. The numerical agreement between the model predictions and experimental data is generally within a factor of 3. This level of agreement is very encouraging, considering the current uncertainties in the thermodynamics and kinetics of aromatics and soot chemistry. The principal accomplishment of the present study is that the demonstrated level of agreement all around—main flame environment, aromatics, and soot—can be attained with a single reaction model.

Detailed kinetic modeling of soot formation in ethylene/air mixtures reacting in a perfectly stirred reactor

A series of perfectly stirred reactor (PSR) calculations has been performed that allow the comparison of numerical predictions for sooting trends in PSRs with those observed in experiments. A detailed chemical kinetics model for soot production is employed that has been extensively tested for laminar flames. The chemical kinetics model is used without any adjustments. The PSR code Aurora that is a CHEMKIN III application has been modified considerably to incorporate the method of moments. Gas-phase and surface reactions are rigorously coupled in the model. Predicted sooting characteristics are consistent with behavior observed experimentally for premixed laminar flames and well-stirred reactors. Specifically, soot volume fraction exhibited the same qualitative behavior with equivalence ratio, temperature, and pressure as observed for flames. Concentration profile shapes were consistent with profile shapes measured for flames as well. The model also predicts soot threshold values and the relationship of soot volume fraction with temperature and equivalence ratio that has been observed experimentally for well-stirred reactors. Under the conditions of the present investigation, soot formation dynamics was dominated by nucleation. The temperature dependence of surface reaction rates revealed the importance of the HACA mechanism (H-abstraction-C2H2 addition reaction sequence) in surface growth. Under PSR conditions, coupling the gasphase and surface chemistry is significant with the degree of coupling increasing with pressure.

Evolution of char chemistry, crystallinity, and ultrafine structure during pulverized-coal combustion

Detailed modeling of soot particle nucleation, growth and oxidation in laminar premixed ethylene-air flames at a pressure of 10 bar is presented. The employed kinetic model of soot formation, developed and tested earlier for subatmospheric and atmospheric flames, can quantitatively describe the experimental soot volume fraction profiles collected in a number of ehtylene flames at high pressures. The kinetic model consists of initial aromatic ring formation from nonaromatic species, planar polycyclic aromatic hydrocarbon (PAH) growth through the hydrogen-abstraction-acetylene-addition reaction mechanism, particle nucleation through coalescence of PAHs, particle growth by coagulation and surface reactions of the forming clusters and particles, and particle oxidation. Analysis of the computational results obtained at 10 bar and additional tests performed for a 1-bar flame suggest that the influence of pressure on soot production arises mainly from the larger concentration of acetylene, which affects both the soot surface growth and the nucleation of soot particles.

Detailed modeling of soot formation in laminar premixed ethylene flames at a pressure of 10 bar

Detailed modeling of soot particle nucleation, growth and oxidation in laminar premixed ethylene-air flames at a pressure of 10 bar is presented. The employed kinetic model of soot formation, developed and tested earlier for subatmospheric and atmospheric flames, can quantitatively describe the experimental soot volume fraction profiles collected in a number of ehtylene flames at high pressures. The kinetic model consists of initial aromatic ring formation from nonaromatic species, planar polycyclic aromatic hydrocarbon (PAH) growth through the hydrogen-abstraction-acetylene-addition reaction mechanism, particle nucleation through coalescence of PAHs, particle growth by coagulation and surface reactions of the forming clusters and particles, and particle oxidation. Analysis of the computational results obtained at 10 bar and additional tests performed for a 1-bar flame suggest that the influence of pressure on soot production arises mainly from the larger concentration of acetylene, which affects both the soot surface growth and the nucleation of soot particles.

Detailed kinetic modeling of soot formation in shock-tube pyrolysis of acetylene

The chemical reaction pathways to soot were investigated by experimenting with detailed kinetic models of soot formation under the conditions used in shock-tube pyrolysis experiments. The analyses of the computational results revealed a single dominant route for the main soot mass growth. Fused polycyclic aromatics play a particularly important role: their formation reactions are essentially irreversible and have the effect of “pulling” chains of reversible reactions. Hydrogen atoms reactivate aromatic molecules to radicals by abstraction reactions. The main bottleneck appears at the formation of the first aromatic ring. The model explains the time scale of soot formation and soot yields obtained in shock-tube pyrolysis of acetylene and also is in accord with product distributions observed in flames.