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Chemical and Physical Pathways of PAH and Soot Formation in Laminar Flames

par Pejpichestakul, Warumporn
Président du jury Coussement, Axel
Promoteur Parente, Alessandro ;Frassoldati, Alessio
Publication Non publié, 2019-09-04

Thèse de doctorat

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Résumé :

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.