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

Soot Modeling of Ethylene Counterflow Diffusion Flames

Combustion-generated nanoparticles cause detrimental effects to not only health and environment but also combustion efficiency. A detailed kinetic mechanism employing a discrete sectional model is validated using experimental data obtained in laminar counterflow diffusion flames of ethylene/oxygen/nitrogen. Two configurations, named Soot formation (SF) and soot formation/oxidation (SFO) flames, are modeled using one-dimensional simulations. Radiative heat losses reduce the maximum flame temperature in the range of 20–60 K and therefore reduce soot volume fraction by ~ 10%. The model predictions accounting for the radiation effects are quite satisfactory. The model can reproduce the qualitative trends of soot volume fraction peaks that are slightly shifted toward the oxidizer zone with the increased oxygen content. In SF flames, the model predicts the maximum soot volume fraction quite well with the largest discrepancy of two folds. The particle stagnation locations can be reproduced by the model, although they are slightly shifted toward the oxidizer nozzle by ~ 0.4 mm. In SFO flames, the most considerable discrepancy is observed at the least sooting flame (xF,o = 0.23) in which the model over-predicts the maximum soot volume fraction by a factor of two. The effect of soot oxidation is important. The model shows that neglecting oxidation of soot significantly increases soot volume fraction in SFO flames by two folds while SF flames are only marginally affected. Also, ignoring soot oxidation leads to the presence of soot particles in the oxidizer zone where they are not observed experimentally. OH is the most effective oxidizer because the sooting zone is located inside the flame region. The effect of thermophoresis is also investigated. It strongly influences SFO flames due to the high temperature gradient. The model accounting particle diffusivities from Stokes–Cunningham correlation can better characterize the distinct particle stagnation plane of SF flames due to their low diffusion coefficients

Evaluation of Polycyclic Aromatic Hydrocarbon Formation in Counterflow Diffusion Flames

Polycyclic aromatic hydrocarbons (PAHs) have been heralded as mutagenic and carcinogenic substances and currently, their emissions are subject to regulatory control due to recently imposed stricter environmental regulations. Hence, it has become necessary to have a detailed understanding of their chemistry. In this work, a short review of the available PAH relevant counterflow diffusion flame datasets is presented. Following that, the reliability of four widely used PAH mechanisms and the revised PAH mechanism, within the scope of this work, is assessed by validating them against these collected experimental datasets. The formation of the first aromatic ring is investigated based on the performed reaction path analyses. The results show that the dominant reaction pathways for the formation of benzene are “even carbon atom” pathways (H-abstraction acetylene addition) and “odd carbon atom” pathways (recombination of propargyl radicals). The dominance of one pathway over the other was found to be strongly dependent on the fuel structure and its doping with other components

Mechanism Comparison for PAH Formation in Pyrolysis and Laminar Premixed Flames

Polycyclic aromatic hydrocarbons (PAHs) are known precursors of harmful carbonaceous particles. Accurate predictions of soot formations strongly rely on accurate predictions of PAHs chemistry. This work addresses the detailed kinetic modeling of PAH formation using two models: CRECK [8] and ITV [12], aiming to compare the model predictions with experimental data in olefin pyrolysis and laminar premixed flames. The two kinetic mechanisms are validated and compared highlighting similarities and differences in PAHs formation pathways. The validation highlights the critical role of resonance-stabilized radicals leading to the PAH formation

Voies chimiques et physiques de la formation de HAP et de suie dans les flammes luminaires

Combustion of hydrocarbon fuels is a major source of pollutants, causing adverse effects to environment and human health. Combustion-generated polycyclic aromatic hydrocarbon (PAH) and soot particles are within the most abundant and harmful pollutants generated from burning of hydrocarbon fuels. Pollutant emission reduction not only is beneficial for the environment and human health but also to increase the efficiency of combustion processes. This work is in the context of Combustion for Low Emission Application of Natural Gas (CLEAN-Gas) project, European Union’s Horizon 2020 research and innovation programme under Marie Sklodowska-Curie Innovative Training Network (ITN), aiming to propose an innovative approach to improve natural gas combustion in industrial processes including detailed chemistry and computational fluid dynamics. Towards this goal, the aim of this work is to characterize and understand the chemical and physical phenomena behind pollutant formation through the development of a comprehensive detailed kinetic mechanism with predictive capabilities in a wide range of operating conditions of interest for real systems. The kinetic sub-mechanisms describing PAHs and soot formation are coupled to the core mechanism describing smaller species gas phase combustion and pyrolysis kinetics. This work focuses on the development of PAHs and soot sub-mechanisms and validate them in a wide range of operating conditions by means of extensive and critical comparisons with a large number of experimental data. The validation against the experimental data presented in this thesis mostly involves laminar flames using 1-D and 2-D simulations.Considering the difficulties in quantitative PAH measurements, an extensive data collection of rich premixed flames was carried out. This extensive database is beneficial for improving the reliability of kinetic models in a wide range of conditions. The effect of the soot formation was also quantitatively investigated using the developed kinetic model, highlighting the importance of describing the interaction with soot to predict heavy PAHs concentrations.The study of soot formation/oxidation pathways was performed using a discrete sectional model coupled with gas phase reactions and PAH sub-mechanism. The essential tool “SootSMOKE” was developed in order to generate the large soot sub-mechanism on the basis of rate rules and reaction classes. The effect of temperature-dependent collision efficiencies is also included in the model for soot formation due to their importance on particle size distribution. 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 kinetic model was validated in comparison with the premixed burner-stabilized stagnation ethylene flames at heavily sooting conditions. A model accounting for temperature and particle size dependence also provides a more general validity, especially on soot number density. Sensitivity analysis of different key parameters controlling coagulation rates is carried out to highlight impacts of each parameter on PSDFs. The characterization of the coagulation mode of PSDF strongly relies on the particle coagulation processes. The validation in laminar counterflow diffusion flames highlighted that physical properties affect the behavior of particles in flames and are also important. The thermal diffusion of gaseous species and soot particles play a vital role in diffusion flames, particularly, to characterize the particle stagnation plane, which was experimentally observed.The detailed kinetic model of PAH and soot formation developed in this thesis work has been further validated using the experimental measurements obtained in a comprehensive study of laminar premixed flame which follows the transition of gas-phase to soot particles. However, this flame is characterized by the presence of a significant buoyancy, which influences the convective flow field. Therefore, 2-D simulation is required to study this flame. This investigation highlighted that not only the accurate description of chemical and physical properties is important, but the appropriate simulation approach is also critical. An improper numerical simulation can lead to the misinterpretation of the kinetic model. Additionally, the model is able to characterize the plateau behavior, which was observed experimentally for some aromatics in the post-flame region because of a counterbalancing effect between their formation from gaseous species and their consumption due to soot growth. Again, this confirmed that the validation of PAH without soot sub-mechanism is misleading in rich flames. The overall validation clearly highlights the presence of critical gaps between the kinetic model and experimental studies of PAH and soot. The validation of PAH, soot precursors, is usually ignored during soot model development, while the inclusion soot model is also usually neglected during the PAH model development. To narrow the gap toward soot formation, the development of PAH and soot models should be carried out concurrently as the validity of the soot model cannot be assessed nor achieved without reasonable PAH prediction and vice versa. The concurrent study the formation of PAHs and soot formation requires more comprehensive experimental studies using different flame configurations or measurement techniques, especially those that can be simulated using quasi 1-D simulation. This will allow a deeper understanding of chemical and physical pathways in PAH and soot formation.Doctorat en Sciences de l'ingénieur et technologieinfo:eu-repo/semantics/nonPublishe

Detailed Kinetics Modeling of Soot Formation

The present work addresses the study of the detailed kinetic modeling of soot formation process, with the aim to compare different number of classes of lumped pseudo-species

Soot Modeling of Ethylene Counterflow Diffusion Flames

info:eu-repo/semantics/publishe

Examination of a soot model in premixed laminar flames at fuel-rich conditions

The primary objective of the present work is to collect and review the vast amount of experimental data on rich laminar premixed flames of hydrocarbon fuels reported in recent years, and to analyze them by using a detailed kinetic mechanism, identifying aspects of the mechanism of PAH and soot formation requiring further revisions. The kinetic assessment was hierarchically conducted, with the progressive extension of the core C 0 -C 2 NUIG mechanism up to the CRECK kinetic mechanism of PAH and soot formation. This mechanism is here adopted to evaluate and analyze the extensive amount of experimental data collected. Therefore, it provides a kinetic guideline, useful to critically compare and unify flames involving similar fuels and/or conditions from different sources. The relevant effect of soot particles formation on heavy PAHs concentration is also discussed, together with the kinetic analysis highlighting systematic deviations and critical issues still existing in the present model. The model performances were evaluated using the Curve Matching approach (Bernardi et al. 2016). Considering the challenges of quantitative PAH measurements and associated uncertainties, this extensive database is a further value of this paper and is beneficial for improving reliability of kinetic models in a wide range of conditions.SCOPUS: ar.jinfo:eu-repo/semantics/publishe