Chemical and Physical Pathways of
PAH and Soot Formation in Laminar Flames
Thesis submitted by Warumporn PEJPICHESTAKUL
in fulfilment of the requirements of the PhD Degree in Mechanical
Engineering (ULB - “Docteur en Science de l’ingénieur et technologie”) and
in Industrial Chemistry and Chemical Engineering (POLIMI)
Academic year 2018-2019
Supervisors: Prof. Alessandro PARENTE (Université libre de Bruxelles)
Aero-Thermo-Mechanics Laboratory
and Prof. Alessio FRASSOLDATI (Politecnico di Milano)
CRECK Modeling Laboratory
Thesis jury:
Prof. Axel COUSSEMENT (Université libre de Bruxelles, Chair) Prof. Tiziano FARAVELLI (Politecnico di Milano)
Prof. Andrea D’ANNA (Università degli Studi di Napoli Federico II)
DIPARTIMENTO DI
CHIMICA, MATERIALI
E INGEGNERIA CHIMICA “Giulio Natta”
Dottorato di Ricerca in Chimica Industriale e Ingegneria Chimica (CII)
PhD in Industrial Chemistry and Chemical Engineering
XXXI ciclo 2015 - 2018
Chemical and Physical Pathways of PAH and Soot
Formation in Laminar Flames
Tesi di Dottorato di
WARUMPORN PEJPICHESTAKUL Matricola 861657
Acknowledgement
This work would not have been possible without the guidance and support of following individuals.
I would like to express my appreciation to Prof. Alessio Frassoldati and Prof. Tiziano Faravelli for their constant patience and guidance. I specifically thank Prof. Eliseo Ranzi for his invaluable assistance. I appreciate Prof. Alberto Cuoci for his support regarding “SMOKE” families and Prof. Alessandro Parente for his support during my time at ULB. I am grateful to Prof. Andrea D’Anna for his constructive comments and for being my external committee. I thank Prof. Axel Coussement for serving on my examination committee. I also thank Prof. Joaquin Camacho for being my external reviewer.
Profound gratitude goes to all present and former colleagues at the CRECK modeling group. Thanks to Isabella Branca for her valuable help. Gratefulness to all my colleagues at POLIMI, ULB and in the CLEAN-Gas project who have made these PhD years considerably more enjoyable.
Finally, I am deeply thankful to my family for the endless support and for their faith in me.
Publications
W. Pejpichestakul, E. Ranzi, M. Pelucchi, A. Frassoldati, A. Cuoci, A. Parente, T. Faravelli,
Examination of A Soot Model in Premixed Laminar Flames at Fuel-Rich Conditions,
Proceedings of Combustion Institute. 37 (2019) 1013–1021. doi: 10.1016/j.proci.2018.06.104.
W. Pejpichestakul, A. Frassoldati, A. Parente, T. Faravelli, Kinetic Modeling of Soot Formation
in Premixed Burner-Stabilized Stagnation Ethylene Flames at Heavily Sooting Condition, Fuel.
234 (2018) 199–206. doi: 10.1016/j.fuel.2018.07.022.
W. Pejpichestakul, A. Frassoldati, A. Parente, T. Faravelli, Soot Modeling of Ethylene
Counterflow Diffusion Flames, Combustion Science and Technology, 191.9 (2019), 1473–
1483.. doi: 10.1080/00102202.2018.1540472
W. Pejpichestakul, A. Cuoci, A. Frassoldati, M. Pelucchi, A. Parente, T. Faravelli, Buoyancy
Effect in Sooting Laminar Premixed Ethylene Flame, Combustion and Flame 205 (2019), 135–
Contributions to Meetings and Symposia
37th International Symposium on Combustion, Dublin, Ireland, 29th July – 3rd August 2018. Oral Presentation: “Soot Effect on Intermediate PAHs Concentration in Premixed Laminar
Flames” Poster: “A Wide Range Validation of A Soot Kinetic Model Based on the Discrete Sectional Method”
4th International Sooting Flame Workshop, Dublin, Ireland, 27th – 28th July 2018. Poster:
“Modeling of Soot Formation in Laminar Counterflow Diffusion Flames”
Joint Meeting the German and Italian Sections of the Combustion Institute, Sorrento, Italy,
23rd – 26th May 2018. Oral Presentation: “Soot Modeling of Ethylene Counterflow Diffusion
Flames”
25th "Journées d'étude" Belgian Section of the Combustion Institute, Mons, Belgium, 15th –
16th May 2018. Oral Presentation: “Modeling of Soot Formation in Laminar Counterflow
Diffusion Flames”
10th Mediterranean Combustion Symposium, Naples, Italy, 17th – 21th September 2017. Oral Presentation: “Kinetic Modeling of Soot Formation in Premixed Burner-Stabilized Stagnation
Ethylene Flames at Heavily Sooting Condition”
CLEAN-Gas Combustion Summer School, Brussels, Belgium, 26th – 29th June 2017. Poster:
“Detailed Kinetic Modeling of Soot Formation in Lightly Sooting Laminar Premixed Ethylene Flames”
International Bunsen Discussion Meeting: Chemistry and Diagnostics for Clean
Combustion, Bielefeld, Germany, 21th – 23th June 2017. Poster: “Detailed Kinetic Modeling
of Soot Formation in Lightly Sooting Laminar Premixed Ethylene Flames”
40th Meeting of the Italian Section of the Combustion Institute, Rome, Italy, 7th – 9th June
2017. Oral Presentation: “Detailed Kinetic Modeling of Soot Formation at Lightly Sooting Conditions”
39th Meeting of the Italian Section of the Combustion Institute, Naples, Italy, 4th – 6th July
2016. Poster: “Detailed Kinetics Modeling of Soot Formation”
International Combustion Institute Summer School on Near-Wall Reactive Flows,
Bensheim, Germany, 6th – 10th June 2016. Poster: “Detailed kinetics modeling of soot
Table of Contents
Acknowledgement ... I Publications ... III Contributions to Meetings and Symposia ... V Table of Contents ... VII List of Figures ... XI List of Tables ... XIX Abstract ... XXI Sommario ... XXIIIChapter 1 – Introduction ... 1
1.1 Combustion-Generated Pollutants ... 1
1.2 Soot Morphology ... 5
1.3 Temperature Effect on Soot ... 9
1.4 Goal and Thesis Organization ... 11
Chapter 2 – State of the Art ... 13
2.1 Aromatic Chemistry ... 13
2.1.1 First Aromatic Ring ... 13
2.1.2 Polycyclic Aromatic Hydrocarbon (PAH) ... 15
2.1.3 Extension to Heavy PAH and Soot ... 19
2.2 Soot Formation and Oxidation ... 20
2.2.1 Soot Inception ... 21
2.2.2 Soot Growth ... 23
2.2.3 Soot Oxidation ... 25
2.3 Modeling Approaches for Soot Formation ... 27
2.3.1 Stochastic Approach ... 28
2.3.3 Discrete Sectional Method ... 31
Chapter 3 – Kinetic Mechanism and Numerical Method ... 35
3.1 Gas-Phase and PAH Kinetic Model ... 35
3.1.1 Overview ... 35
3.1.2 Thermodynamic Properties of PAH ... 36
3.1.3 Kinetics ... 38
3.2 Soot Kinetic Model ... 45
3.2.1 Lumped Pseudo-Species BINs ... 46
3.2.2 Thermochemical Properties ... 50
3.2.3 Kinetics of soot mechanism ... 54
3.2.4 Transport Properties ... 62
3.2.5 Soot Sub-Mechanism Generator ... 62
3.3 Numerical Simulations ... 63
3.3.1 Governing Equations ... 65
3.3.2 Radiation Model ... 65
3.3.3 Multicomponent Transport Properties ... 66
3.3.4 1-D Simulation ... 67
3.3.5 2-D and Multi-Dimensional Simulation ... 70
Chapter 4 – Soot Effects on Intermediate PAHs ... 71
4.1 Introduction ... 71
4.2 Rich Laminar Premixed Flames Database and Assessment Approach ... 72
4.3 Results ... 75
4.3.1 Major Species Formation in Flames ... 76
4.3.2 Heavier Aromatics and PAHs Formation in Flames ... 78
4.3.3 Effect of Soot on PAH Predictions ... 80
4.3.4 Experimental Uncertainties and Temperature Effect on Soot Predictions. ... 83
Chapter 5 – Intermolecular and Particle Interactions ... 87
5.1 Introduction ... 87
5.2 Model Description and Validation Cases ... 88
5.3 Comparison of Collision Efficiency ... 89
5.4 BSSF Flames Results and Discussions ... 92
5.4.1 Model with Temperature- and Size- Dependent Collision Efficiency ... 92
5.4.2 Effect of Temperature Dependency ... 95
5.4.3 Comparison with the Previous Model ... 96
5.4.4 Sensitivity Analysis ... 97
5.5 Summary ... 101
Chapter 6 – Validation of Soot Model in Complex Flames ... 103
6.1 Ethylene Counterflow Diffusion Flames ... 103
6.1.1 Introduction ... 103
6.1.2 Results and Discussions ... 104
6.1.3 Conclusion ... 110
6.2 Buoyancy-induced Premixed Ethylene Flame ... 111
6.2.1 Introduction ... 111
6.2.2 Validation Data Set ... 113
6.2.3 Numerical Simulation Methods ... 113
6.2.4 Results and Discussions ... 116
6.2.5 Physical Analysis ... 133
6.2.6 Conclusion ... 143
Chapter 7 – Conclusions and Future Perspectives ... 145
References ... 149
List of Figures
Figure 1.1: Primary energy consumption by fuels (left panel) and shares of primary energy (right panel) [2].1Figure 1.2: PAH emissions globally in 2007 [5]. ... 2
Figure 1.3: Relative source contribution to PM2.5 in different urban sites [9]. ... 3
Figure 1.4: Images of soot aggregate from TEM [21] (left panel) and HRTEM [19] (right panel). ... 5
Figure 1.5: TEM images of soot aggregate particles at 2.3 atm (A), 4 atm (B), 5.4 atm (C), 7.1 atm (D), and 10 atm (E) [23]. ... 6
Figure 1.6: TEM (left panel) and HRTEM (right panel) images methane soot (nascent (a), young (b), intermediate (c) and mature (d)) [25]. ... 7
Figure 1.7: HRTEM Images of soot derived from pyrolysis different fuels at 1650 °C and 100 sccm [26]. ... 8
Figure 1.8: Left panel: Amount of soot versus temperature at different residence times in shock tube studies of heptane/O2/Ar (φ = 5.0) at 20 bar [36]. Right panel: Sooting limit region regarding equivalence ratio versus maximum flame temperature [37]. ... 9
Figure 1.9: TEM images of pre-nascent (left panel) and mature (right panel) ethylene soot from two flames with different maximum flame temperatures. Top panel: LT-E flame (v0 = 3 cm/s, Tmax = 1620 K). Bottom panel: HT-E flame (v0 = 4 cm/s, Tmax = 1690 K) [39]. ... 10
Figure 2.1: HACA mechanism [71]. ... 16
Figure 2.2: PES of cyclopentadienyl radicals recombination [71]. ... 17
Figure 2.3: Schematic representation of oligomers of benzene (high H/C values) and maximally PCAH (low H/C values) [98]. ... 20
Figure 2.4: Schematic representation of soot formation, growth and oxidation. Adapted from [99]. ... 21
Figure 2.5: Scheme of the coagulation process [98]. ... 25
Figure 3.1: Scatter plots showing the distribution of the aromatic species in a C–H plot. ... 36
Figure 3.2: Standard state molar Gibbs free energy of C14H10 and C14H9 isomers. ... 37
Figure 3.3: Standard molar heat capacity, entropy and enthalpy of formation at 298 K. ... 38
Figure 3.4: Rate constants of H-abstraction reactions at different H-atoms. ... 39
Figure 3.5: Rate constants of acetylene addition on phenyl radical at different pressures. Red color: C2H2 + C6H5• + → C6H5C2H2. Black color: C2H2 + C6H5• + → C6H5C2H2 + H•. ... 40
Figure 3.6: Rate constants of resonantly-stabilized radical recombination at high pressure limit. ... 42
Figure 3.8: Rate constants of ipso substitution reactions. Dashed line:
C₁₀H₆CH₃+C₇H₈=>0.57143C₁₆H₁₀+0.3928625C₁₆H₁₆+H₂+CH₃. Solid line:
C₁₀H₇CH₂+C₇H₈=>0.57143C₁₆H₁₀+0.3928625C₁₆H₁₆+H₂+CH₃. ... 43
Figure 3.9: Rate constants of AAC (solid line) and ARC (dashed line) mechanisms. ... 44
Figure 3.10: Rate constants of key reactions in benzene oxidation. ... 45
Figure 3.11: Rate constants of phenoxy and phenol decomposition. Black lines represent rate constants of phenoxy decomposition (C₆H₅O=CO+C₅H₅) at a different pressure. Red line represents rate constant of phenol decomposition (C₆H₅OH=C₅H₆+CO). ... 45
Figure 3.12: Left panel: Comparison between H/C ratio respect to the molecular weight of each BINs between the AE assumption (red dotted lines) and present work (black solid lines) in comparison with the experimental observations [100] (symbol). Right panel: predicted PSDF from different assumptions. Dotted line: AE assumption. Solid line: present work. Dashed line: assumption by Saggese et al. [197]. ... 49
Figure 3.13: Comparison between H/C ratio assigned to different BIN as a function of particle mass. Symbol represents experimental data from [100]. Solid lines: present work. Dashed lines: assumption adopted by Saggese et al. [197]. Different colors represent hydrogenation classes. ... 49
Figure 3.14: Comparison of predicted (line) and experimental (symbols) H/C ratio of soot formed in a rich atmospheric premixed flames. Red color: methane flames (=2.4; C/O=0.6; v0=5 cm/s) [226]. Black color: ethylene flames (=2.4; C/O=0.8; v0=4 cm/s) [20,225,226]. ... 50
Figure 3.15: Example of functional groups. ... 51
Figure 3.16: Soot mechanism generation using SootSMOKE. ... 63
Figure 3.17: Solvers employed for numerical simulations. ... 64
Figure 4.1: Example of Interpolating splines. Left: Data vs smoothing function. Right: Centered finite difference of the data and first derivatives of the smoothing function. ... 74
Figure 4.2: Curve Matching and model performance evaluation. ... 75
Figure 4.3: Laminar flame speed of acetylene[254–256], propyne[257] , cyclopentadiene[258], benzene[257,259,260] and toluene[261,262]. ... 76
Figure 4.4: Scatter plots of the maximum calculated and measured concentration of major species. Each marker represents main fuels, ∆:CH4, □: C2H2, ◇: C2H4, ▽: C2H6, and ○: C3+. Void symbol: low-pressure flames. Filled symbol: atmospheric flames. ... 77
Figure 4.5: Scatter plots of the maximum calculated and measured concentration of C4H2 using CRECK, KAUST [263] and Blanquart models [264]. Symbols as in Figure 4.4. ... 78
Figure 4.7: Scatter plots of the maximum calculated and measured concentration of phenylacetylene and naphthalene using CRECK, KAUST [263] and Blanquart models [264]. Symbols as in Figure 4.4. ... 79 Figure 4.8: Error index of different species. Filled symbol: aromatic species. Void symbol: gaseous species. ... 80 Figure 4.9: Comparison between experimental (symbols) and predicted C6H6 profiles. Model predictions with
(dashed lines) and without soot kinetic model (solid lines). Left panel: CH4 flames (Flame3-5) [265]. Middle
panel: C2H6 flames (Flame31-34) [267]. Right panel: C2H4 flames (Flame24) – ϕ=2 [266]. ... 81
Figure 4.10: Reduction of error index of intermediate PAHs at different equivalence ratio. ... 82 Figure 4.11: Comparison between predicted (lines) and measured (symbols) soot volume fraction in different flames. ... 82 Figure 4.12: Relative contributions of HACA mechanism and PAH condensation reactions to the growth of soot particles at low (dashed lines) and atmospheric pressure (solid lines). The thickness of the arrows represents the importance of the different paths. ... 83 Figure 4.13: Temperature effect on soot formation of C2H4 flame (Flame 27) [268,269]. Comparison of
Figure 5.7: Comparison of soot number density and volume fraction at different burner-to-stagnation surface separation distance between the temperature-dependent model (solid lines) and the models neglecting the temperature dependency: the model considering collision efficiency at 500 K (dashed lines) and the model considering collision efficiency at 1500 K (dotted lines). ... 96 Figure 5.8: Comparison between computed (lines) and measured (symbols) PSDFs at different burner-to-stagnation surface separation distance of L1 (left panel) and L4 (right panel). Solid lines represent temperature- and size-dependent model. Dashed lines represent the previous CRECK soot model. Different colors represent PSDFs at each separation distance (cm). ... 97 Figure 5.9: Comparison of collision efficiency of different PCAHs between collision efficiency obtaining from different separation distances of heavy PAHs and Hamaker constants at H/C = 0.5 (lines) and the MD results (symbols) as a function of temperature. Left panel: comparison of collision efficiency with Totton et al. [43]. Right panel: comparison of collision efficiency with Chung et al. [42]. Solid lines: collision efficiency used in this work with DHPAH = 0.21 nm, A0.5 = 5E-20 J. Dashed lines: collision efficiency calculated with DHPAH = 0.35
nm, A0.5 = 5E-20 J. Dotted lines: collision efficiency calculated with DHPAH = 0.35 nm, A0.5 = 3E-19 J. ... 98
Figure 5.10: Comparison of collision efficiency as a function of particle diameter between the observation from D’Alessio et al. [41] (symbol) and the calculated with a separation distance of DHPAH = 0.21 nm (solid
lines) and DHPAH = 0.35 nm (dashed lines) at different Hamaker’s constants (A). A = 3E-20 J (blue lines), A =
1E-19 J (green lines) and A = 5E-1E-19 J (red lines). ... 98 Figure 5.11: Comparison between computed (lines) and measured (symbols) PSDFs at a different burner-to-stagnation surface separation distance of L1 (left panel) and L4 (right panel) derives from different separation distances of BIN1-4 and Hamaker constant at H/C = 0.5. Different colors represent PSDFs at each separation
Figure 6.3: Comparison of soot volume fraction profiles between experimental data (symbol) and model (lines) of SF flames (left panel) and SFO flames (right panel). Dashed lines: model neglecting soot oxidation. Solid lines: model including soot oxidation. ... 107 Figure 6.4: Mass loss rate of particle oxidation by different oxidizers. ... 107 Figure 6.5: Comparison of soot volume fraction profiles between experimental data (symbol) and model (lines) of SF flames (left panel) and SFO flames (right panel). Dashed lines: model neglecting thermophoretic effect. Solid lines: model including thermophoretic effect. ... 108 Figure 6.6: Diffusion coefficient of lumped pseudo species respect to molecular weight. ... 109 Figure 6.7: Comparison of soot volume fraction profiles between experimental data (symbol) and model (lines) of SF flames (left panel) and SFO flames (right panel). Dashed lines: model accounting particle diffusivity from gas kinetic theory. Solid lines: model accounting particle diffusivity from Stoke-Cunningham correlation. ... 110 Figure 6.8: Schematic of the burner and computational mesh. ... 115 Figure 6.9: Comparison of temperature field and stream lines calculated from the “complete” model (right) and the model neglecting buoyancy effect (left). ... 117 Figure 6.10: Comparison of measured (symbol) temperature profiles and results from simulation using CRECK mechanisms (black lines) and 3-D simulation from Carbone et al. [266] with frozen chemistry. Solid line: effect of neglecting the soot sub-mechanism. Dashed line: effect of including the soot sub-mechanism. Dotted lines: 3-D simulation from [266]. ... 119 Figure 6.11: Mole fraction profiles of small gaseous species between the measurement (symbols) and the model predictions using CRECK soot mechanism (lines). ... 121 Figure 6.12: Global rate of production analysis of small gaseous species. ... 122 Figure 6.13: Comparison between measured (symbols) and predicted (lines) mole fraction profiles of aromatic species. Solid lines: CRECK PAH mechanism. Dashed lines: CRECK PAH + soot mechanism. ... 123 Figure 6.14: Comparison between measured (symbols) and calculated (lines) mole fraction profiles of phenylacetylene. ... 124 Figure 6.15: Plateau analysis of phenylacetylene. Black solid line: formation rate. Black dashed line: rate of phenylacetylene condensation on soot particle. Red line: mole fraction profile. ... 124 Figure 6.16: Reaction pathway analysis of PAH formation and flux of acetylene addition (kg/m3/s) at HAB = 1
Figure 6.18: Comparison between measured (symbols) and calculated (lines) temperature profiles. Solid line: CRECK PAH mechanism. Dashed line: KAUST mechanism [263]. Dotted line: Blanquart mechanism [264]. Dash-dotted line: ABF mechanism [68]. ... 128 Figure 6.19: Comparison between measured (symbols) and predicted (lines) mole fraction profiles of gaseous species (C0-C5). Solid lines: CRECK PAH mechanism. Dashed line: KAUST mechanism [263]. Dotted line:
Blanquart mechanism [264]. Dash-dotted line: ABF mechanism [68]. ... 129 Figure 6.20: Comparison between measured (symbols) and predicted (lines) mole fraction profiles of aromatic. Solid lines: CRECK PAH mechanism. Dashed line: KAUST mechanism [263]. Dotted line: Blanquart mechanism [264]. Dash-dotted line: ABF mechanism [68]. ... 130 Figure 6.21: Benzene formation rate and pathways from different mechanisms at flame location. ... 132 Figure 6.22: Rate of production analysis of C6H5C2H from different kinetic mechanisms. ... 133
Figure 6.23: Figure 16: Comparison between measured (symbols) and calculated temperature profiles from 2-D simulations using CRECK PAH mechanism (black lines) and 3-D simulation with frozen chemistry from Carbone et al. [266]. Solid line: 2-D simulations using the complete model. Dashed line: 2-D simulations using the model neglecting buoyancy. Dotted lines: 3-D simulation from Carbone et al. [266]. ... 134 Figure 6.24: Profiles of temperatures as a function of residence time at the centerline from 2-D models. 134 Figure 6.25: Radiative heat losses effect on axial temperature profile. ... 135 Figure 6.26: Radiative heat losses profile at the centerline and the normalized heat losses contribution by different gaseous species. Normalized heat losses profiles are color coded in black. Radiative heat losses profiles are color coded in red color. Solid lines: no soot sub-mechanism. Dashed lines: soot sub-mechanism. ... 136 Figure 6.27: Comparison between experimental (symbols) and predicted mole fraction profiles of hydrogen (black) and aromatic species (red) along the centerline. Solid lines: 2-D simulation using the complete model. Dashed lines: 2-D simulation neglecting Soret effect. ... 136 Figure 6.28: Mole fraction of H2 fields predicting from the 2-D simulation neglecting Soret effect (left panel)
Abstract
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.
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.
Sommario
La combustione di combustibili idrocarburici è una delle principali fonti di inquinanti, con effetti negativi per l'ambiente e la salute umana. Gli idrocarburi policiclici aromatici (IPA) e il particolato carbonioso (soot) sono tra gli inquinanti più abbondanti e nocivi generati dalla combustione di carburanti idrocarburici. La riduzione delle emissioni inquinanti non solo è benefica per l'ambiente e la salute umana, ma anche per aumentare l'efficienza dei processi di combustione. Questo lavoro si inserisce nel contesto del progetto Combustion for Low Emission Application of Natural Gas (CLEAN-Gas), il programma di ricerca e innovazione Horizon 2020 dell'Unione Europea nell'ambito della rete di formazione innovativa Marie Sklodowska-Curie (ITN), che mira a proporre un approccio innovativo per migliorare la combustione del gas naturale nei processi industriali, compresa la chimica dettagliata e la fluidodinamica computazionale. A tal fine, lo scopo di questo lavoro è quello di caratterizzare e comprendere i fenomeni chimici e fisici alla base della formazione di inquinanti attraverso lo sviluppo di un meccanismo cinetico dettagliato e completo con capacità predittive in una vasta gamma di condizioni operative di interesse per i sistemi reali. I sotto-meccanismi cinetici che descrivono gli IPA e la formazione del soot sono accoppiati al meccanismo centrale che descrive la combustione in fase gassosa e la cinetica di pirolisi di specie più piccole. Questo lavoro si concentra sullo sviluppo dei sotto-meccanismi degli IPA e del soot e li convalida in un'ampia gamma di condizioni operative mediante confronti approfonditi e critici con un gran numero di dati sperimentali. La validazione rispetto ai dati sperimentali presentati in questa tesi coinvolge principalmente fiamme laminari utilizzando simulazioni 1-D e 2-D. Considerando le difficoltà nelle misurazioni quantitative degli IPA, è stata effettuata un'ampia raccolta di dati sulle fiamme premiscelate ricche di premiscelati. Questo ampio database è utile per migliorare l'affidabilità dei modelli cinetici in un'ampia gamma di condizioni. Anche l'effetto della formazione di soot è stato studiato quantitativamente utilizzando il modello cinetico sviluppato, evidenziando l'importanza di descrivere l'interazione con il soot per prevedere le concentrazioni di IPA pesanti.Un modello che tiene conto della temperatura e della dipendenza dalle dimensioni delle particelle fornisce anche una validità più generale, in particolare sulla densità numerica di particelle di soot. Viene effettuata un'analisi di sensitività dei diversi parametri chiave che controllano i tassi di coagulazione per evidenziare gli impatti di ciascun parametro sulla PSDF. La caratterizzazione della modalità di coagulazione della PSDF si basa fortemente sui processi di coagulazione delle particelle. La validazione nelle fiamme a flusso laminare contrapposto ha evidenziato che le proprietà fisiche influenzano il comportamento delle particelle nelle fiamme in modo anche importante. La diffusione termica delle specie gassose e delle particelle di soot gioca un ruolo vitale nelle fiamme diffusive, in particolare per caratterizzare il piano di ristagno delle particelle, come osservato sperimentalmente.
Il modello cinetico dettagliato della formazione di IPA e soot sviluppato in questo lavoro di tesi è stato ulteriormente convalidato utilizzando le misurazioni sperimentali ottenute in uno studio completo della fiamma premiscelata laminare che segue la transizione dalla fase gassosa alle particelle di soot. Tuttavia, questa fiamma è caratterizzata dalla presenza di una significativa galleggiabilità, che influenza il campo di flusso convettivo. Per studiare questa fiamma è quindi necessaria una simulazione 2-D. Questa indagine ha evidenziato che non solo la descrizione accurata delle proprietà chimiche e fisiche è importante, ma anche l'approccio di simulazione appropriato è critico. Una simulazione numerica impropria può portare a un'interpretazione errata del modello cinetico. Inoltre, il modello è in grado di caratterizzare il comportamento del plateau, che è stato osservato sperimentalmente per alcuni aromatici nella regione post-fiamma a causa di un effetto di compensazione tra la loro formazione, da specie gassose, e il loro consumo dovuto alla crescita di soot. Anche in questo caso, ciò ha confermato che la validazione degli IPA senza sotto-meccanismo soot è fuorviante nelle fiamme ricche.
Chapter 1 – Introduction
This chapter provides information about combustion-generated pollutants, mainly polycyclic aromatic hydrocarbon (PAH) and soot particles, with a focus on its morphology. Temperature effect is also described relatively to the different stages of soot formation process because it affects the interactions of the particles with the environment.1.1
Combustion-Generated Pollutants
Over 75% of world energy requirements are met through combustion from hydrocarbon fuels including, natural gas, petroleum, and coal [1]. It is expected that the global energy demand will continue to grow with particularly large increases in the demands from fast-growing emerging economies [2]. Particularly, natural gas demand grows strongly, driven by China, the Middle East and Europe [1]. Figure 1.1 shows the primary energy consumption, presented as tons of oil equivalent (toe), by different fuels and their shares.
(a) (b)
Figure 1.1: Primary energy consumption by fuels (left panel) and shares of primary energy (right panel) [2]. Combustion of hydrocarbon fuels is major sources of the current energy generation process, but it also results in the main source of pollutants. Besides products of combustion like CO, CO2, NOx, SOx, polycyclic
PAHs are an important class of organic pollutants, because of their toxic effects on ecosystem and harmful for human health. As of great concern, the list of 16 PAHs was issued by the U.S. Environmental Protection Agency (EPA) to use chemical analysis for assessing risks to human health and environment [4]. Figure 1.2 shows the global emission of listed PAHs. Among the total PAH emissions, roughly 6% of the emissions were in the form of high molecular weight carcinogenic compounds [5]. In addition to these negative effects, PAHs are also soot precursors by means of radical addition reactions on double bonds, cyclization, and formation of resonantly stabilized radicals, finally resulting in incipient soot [6].
Figure 1.2: PAH emissions globally in 2007 [5].
Soot or carbonaceous nanoparticles are airborne particulate matter, harmful to human health and environment. Commonly, particulate matter is divided into two main classes of ambient air pollution particles depending on the diameter [7]. The first class is coarse particles with diameter below 10 µm (PM10), defined
as inhalable particles, which can be breathed through mouth and nose. The second class is fine particles with smaller than 2.5 µm (PM2.5), defined as respirable particles, which can penetrate deeply into the respiratory
and cardiovascular systems, causing extremely serious health diseases such as lung cancer [8]. However, the other size class is defined for ultrafine nanoscale particles smaller than 0.1 µm or 100 nm (PM0.1). Therefore,
PM is considered to be one of the best indicators for health effects of ambient air pollution [9]. As a result, legislation is becoming more restrictive, particularly, for particulate emission, i.e., the EU air quality directive defines the PM2.5 concentration of 25 μg/m3 on an annual basis and will lower a threshold to 20 μg/m3 by
2020 [3].
Figure 1.3 shows the relative source contribution to PM2.5 in a different region. Globally 25% of urban ambient
air pollution from PM2.5 is contributed by traffic or mobile sources, which includes different kinds of emissions
from various vehicle types. 15% of PM2.5 is contributed by industrial activities that the main emissions come
in large quantities by vehicular combustion and their role for causing several adverse human health effects [10].
Figure 1.3: Relative source contribution to PM2.5 in different urban sites [9].
PAHs and soot formation and destruction are kinetically controlled. Soot formation in the combustion devices systems affects the efficiency, i.e., reducing the heat exchange and increasing pressure drops in tubular reactors. The surviving soot particles can accumulate in the system, which requires preventive maintenance [11]. Furthermore, soot formation in practical applications influences the radiative heat losses and lowers the efficiency of combustors [8].
All practical devices often contain the locally non-ideal burning conditions that lead to incomplete combustion and result in the production of carbonaceous compounds. This is mainly caused by local cold spots, inadequate mixing of fuel and oxidizer, insufficient pulverizing of solid fuels or atomizing of liquid fuels, too short residence time at high temperatures, and sudden cooling of the flame gases through combustion chamber walls.
formation, i.e., the presence of sulfur and aromatics in fuels. The use of synthetic diesel fuel, the product of Fischer-Tropsch synthesis from feedstock such as natural gas or coal, significantly reduces all pollutant emissions, including soot particles. Other measures, which may result in lower soot emissions, are the use of oxygenated additives, i.e., biodiesel, ethanol and dimethylether [13,14]. Furthermore, the post-combustion treatment is also necessary to control emitted soot particles from exhaust gas through various types of particulate traps and catalysts before they leave the exhaust pipe [12].
The total amount and characteristics of soot generated from industrial activities, or stationary sources, depend on the fuel and the type of combustion [15]. Emission control techniques from large combustion plants rely on secondary abatement technologies, which are very efficient with up to over 99.8% by weight removal from the raw gas input. The widely-used technologies for reducing soot emission in industries are electrostatic precipitators (ESPs), fabric filters, cyclone precipitation, and wet scrubbers. However, the efficiency is lower for some fuels, i.e., oil, due to the different particle composition and size. The efficiency in controlling small particles, such as for PM10 and below, decrease to between 95% and 98%. Consequently,
most of the particles emitted from large combustion plants to the environment still are lower than 10 μm of diameter [15].
1.2
Soot Morphology
Soot reactivity is crucial for determining its toxicity [16] and the efficiency of soot abatement from engine exhaust on particulate filters, which require regeneration cycles to oxidize soot accumulated during the filtration phase [17]. Structure and morphology at a nanoscale level of soot particles, which are dependent on fuels and synthesis conditions, affects their reactivities [18].
The soot nanostructure is usually diagnosed by means of transmission electron microscopy (TEM) providing access to the soot multi-scale organization. Figure 1.4a shows the TEM image of soot particles from diesel engine conditions. It shows that a soot particle is composed of primary particles with dimensions about 10 nm that afterward coagulate into fractal aggregates with a chain-like structure [19]. The primary particles are near-spherical in shape, and are crystalline near the outer edge. This crystallinity arises due to the stacking of planar polycyclic aromatic hydrocarbons (PAHs) indicating that PAHs are soot precursors [19] and their H/C ratio in mature aggregates is about 0.1 [20]. The high-resolution mode of TEM (HRTEM) images show hollow centers, signifying the presence of an amorphous structure, which results in more reactive carbon sheets in the center of the primary particles as depicted in Figure 1.4b. This amorphous structure arises from the random alignment of PAHs [19].
(a) (b)
pressures, which indicates that the elevated pressure increases the overlap between primary particles [23]. Furthermore, Vargas et al., also observed that the mean diameter and its variance of primary particles decrease with the increase of pressures [24].
Figure 1.5: TEM images of soot aggregate particles at 2.3 atm (A), 4 atm (B), 5.4 atm (C), 7.1 atm (D), and 10 atm (E) [23].
The nanostructure of soot particles evolves with residence time or their aging. Figure 1.6 shows the TEM and HRTEM images of soot particles derived from premixed methane flames at a different height above the burner (HAB) representing different residence times [25]. Nascent soot presents large coalesced disordered structures, however, the structure disappears with the increasing of HAB. The dimensions of the primary particle, around 30 nm for the young soot and decrease to around 20 nm for mature soot. Electron energy loss spectroscopy (EELS) was applied to follow the transformation of soot nanostructures by measuring the relative concentration of hybridized carbon (sp2/sp3), where graphite has 100% sp2 content. The nascent
methane soot presented large coalesced disordered structures with a high ratio (84%) of sp2 hybridized
carbon indicating the predominant aromatic character of soot precursors in comparison to young, intermediate and mature soot presenting spherule aggregates. The steep increase of sp2 content up to 94%
TEM HRTEM
Figure 1.6: TEM (left panel) and HRTEM (right panel) images methane soot (nascent (a), young (b), intermediate (c) and mature (d)) [25].
(a) Acetylene (b) Benzene
(c) Pyrene (d) Ethanol
Figure 1.7: HRTEM Images of soot derived from pyrolysis different fuels at 1650 °C and 100 sccm [26]. Many factors (e.g., maturity, fuels, pressure, temperature, etc.) characterize different structures, change the number of active sites on the surface of the particle [27]. The different structures infer the reactivity of soot particles and their interactions with the surrounding environment. Specifically, a correlation between soot nanostructure and soot reactivity toward oxidation has been proposed assuming that the reactivity toward oxidative attack depends on these factors [28]:
The degree of the organization of layers: The highest oxidative reactivity soot has an amorphous nanostructure, which composed of short individual layer planes with no orientation relative to each other.
A higher ratio of carbon in the edge sites versus basal plane sites: accessibility of carbon in edge sites makes them more reactive than the basal plane carbon atoms.
1.3
Temperature Effect on Soot
The formation of soot particles and their nanostructure depends on a wide range of parameters including temperature, pressure, and fuel type, as discussed earlier in Section 1.2. Temperature plays a vital role among these parameters, particularly high temperature environment that favors soot formation. Several experimental studies, both in shock tubes [30–32] and in laminar premixed flames [22,33–35], show that soot volume fractions from various fuels exhibit a “bell-shaped” curve as a function of temperature as depicted in Figure 1.8. At low temperatures, generally below 1500 K, soot volume fraction increases with increasing temperature. Above a certain threshold temperature, the soot volume fraction decreases as temperature increases. This threshold temperature has been found to be a function of residence times (Figure 1.8a) and fuel (i.e. equivalence ratio in Figure 1.8b). Mätzing and Wagner [22] observed the apparent activation of about 180 kJ/mol for the soot mass growth in ethylene flames.
Figure 1.8: Left panel: Amount of soot versus temperature at different residence times in shock tube studies of heptane/O2/Ar (φ = 5.0) at 20 bar [36]. Right panel: Sooting limit region regarding equivalence ratio
versus maximum flame temperature [37].
flames in two different flame temperatures. Figure 1.9 shows the TEM images of soot particles in both low temperature (LT-E) and high temperature (HT-E) flames. It was found that the final nanostructure, mature soot, are rather similar regardless flame temperature. However, the nascent particle favors dehydrogenation and aromatization at high temperature with sp2 content of ~90%, while the nascent soot formed at lower
temperature has very low percentage of sp2 around 80% in the unstructured, amorphous-like fraction.
Figure 1.9: TEM images of pre-nascent (left panel) and mature (right panel) ethylene soot from two flames with different maximum flame temperatures. Top panel: LT-E flame (v0 = 3 cm/s, Tmax = 1620 K). Bottom
panel: HT-E flame (v0 = 4 cm/s, Tmax = 1690 K) [39].
ethylene-air flames on a mica plate by atomic force microscopy technique (AFM). They observed very low collision efficiencies for the small particles, which could be ascribed to the high energy of these particles due to their Brownian motion, which causes thermal rebound effects prevailing over adhesion mechanisms due to van der Waals forces. Molecular dynamics simulations (MD) of the collision of monomers revealed that there is an increase of the collision efficiencies with the colliding monomer mass [14,15], while the collision efficiencies are inversely related to temperature. However, these theoretical studies were performed up to a maximum temperature of 1500 K, which is approximately at the lower limit of soot threshold temperature of ethylene flames [36] as shown in Figure 1.8b.
As highlighted earlier, the temperature dependence of the soot formation is crucial, particularly soot inception. To accurately predict soot formation, the analysis of temperature effect will be performed with the detailed kinetic mechanism of PAH and soot model developed in this work. This model accounts for different H/C ratio of soot particles. The final goal is to validate the model with a wide range of experimental data in both laminar premixed and diffusion flames considering PAH and soot particles and explain some of the phenomena observed experimentally through mechanistic analysis.
1.4
Goal and Thesis Organization
The objective of this work is to study the physical and kinetic mechanisms leading to the formation of pollutant species, which are polycyclic aromatic hydrocarbons (PAH) and carbonaceous particles (soot) in laminar flames fed with natural gas in operating conditions close to those of interest. This knowledge can then be used for improving the design of equipment for combustion of natural gas, characterized by low emissions of pollutants. The study focused on simplified geometries, like canonical reactors, laminar counterflow diffusion flames and premixed flat flames. The activities carried out during this study can be divided into two main parts according to the pollutants which are PAH and soot.
This chapter, Chapter 1, provides a brief overview of the combustion generated pollutants and some concerns regarding their negative effects. Key factors in soot formation and the nanostructure are also discussed together with the temperature effects that will often be recalled in the following sections. Chapter 2 provides a brief overview of the different possible kinetic pathways of PAH and soot formation, as well as, the approaches to modeling that available in the literature. Chapter 3 describes the model formulation of PAH and soot modules inside CRECK detailed kinetic mechanism, including thermodynamic, kinetic parameters and transport properties. A modeling approach to simulate various flames are also provided in this section. Chapter 4 is devoted to an extensive validation of rich laminar premixed flame of various hydrocarbon fuels from C1 to C10, which further supports the general reliability of the kinetic model, and
Chapter 2 – State of the Art
This chapter provides an overview of aromatic, PAH, soot formation pathways that have been discussed in the literature. The soot modeling approaches are also reviewed, highlighting several aspects, which are discussed in detail in the successive chapters.2.1
Aromatic Chemistry
2.1.1 First Aromatic Ring
The first aromatic compound, benzene, is a key precursor since it represents the starting point of PAH and thus soot formation. It is formed mainly through the recombination of C2, C3 and C4 molecules and radicals
[44]. Firstly, the “even-carbon-atom” pathways contributed by acetylene (known as HACA - H-Abstraction, acetylene addition) in forming benzene formation were proposed by Frenklach and co-workers [45]. Cole et al. [46] also proposed acetylene addition on n-C4H5• forming the cyclization of aromatic with instantaneous
release of hydrogen. High fidelity ab-initio calculations have been performed by Senosiain and Miller [47] on the acetylene addition onto the C4H5 isomers. The reaction with n−C4H5• was found to produce mainly
benzene and fulvene (C5H4CH2), while the reaction with i−C4H5• was found to produce mainly fulvene.
n-C4H5• + C2H2 → C5H4CH2 + H• (R1)
n-C4H5• + C2H2 → C6H6 + H• (R2)
i-C4H5• + C2H2 → C5H4CH2 + H• (R3)
Similarly, cyclic compound formation through acetylene addition on n-C4H3•was suggested by Cole and
coworkers [48].
n-C4H3• + C2H2 → C5H4CH• (R4)
n-C4H3• + C2H2 → C6H5• (R5)
i-C4H3• + C2H2 → C5H4CH• (R6)
Westmoreland et al. [49] confirmed the notable contribution to benzene formation from acetylene addition on C4 radicals in acetylene and 1,3-butadiene flames. Together with the addition by acetylene, Westmoreland
et al. [49] highlighted the contribution by vinyl radical (C2H3) in 1,3-butadiene flame.
However, Miller and Melius [50] proved the importance of the route forming the first aromatic ring by odd-carbon-atom pathways which are self-recombination of two propargyl radicals (C3H3•). Propargyl radical is a
resonantly stabilized species, which is relatively stable and can be present in significant amount in high temperature environment. Miller and Klippenstein [51] studied the potential energy surface (PES) of C6H6
and found these three product channels that are important in ring formation:
C3H3• + C3H3• → C6H6 (R8)
C3H3• + C3H3• → C6H5• + H• (R9)
C3H3• + C3H3• → C5H4CH2 (R10)
C5H4CH2 → C6H6 (R11)
C5H4CH2 → C6H5• + H• (R12)
Under fuel-rich flame conditions, the fulvene (C5H4CH2) formed in reaction (R10) rapidly rearranges to
benzene by reaction with hydrogen atoms [52,53]
H• + C5H4CH2 → H• + C6H6 (R13)
The latter odd-carbon-atom pathways via propargyl and allyl (a-C3H5•) radicals were discussed by Miller et
al. [53]. It forms linear C6H8 isomers, which isomerize fast into fulvene.
C3H3• + a-C3H5• → C5H4CH2 + 2H• (R14)
Another pathway involving cyclopentadienyl and methyl radicals has been proposed by Melius et al. [54] and also confirmed by methylcyclopentadiene pyrolysis of Ikeda et al. [55]. Sharma and Green [56] calculated the PES of this pathway proving that fulvene and C6H7 products could further isomerize and lose a hydrogen
atom to form benzene.
is followed by the oxidation of phenyl radical to form phenoxy radical (C6H5O•). The reaction rate constant
for (R15) is taken from the experimental study by Frank et al. [61].
O2 + C6H5• → C6H5O• + O• (R15)
Phenoxy radical plays a vital role leading to decomposition in the overall oxidation mechanism. In the high or intermediate temperatures, phenoxy radical mainly decomposes to form cyclopentadienyl and CO (R16) [62], while the recombination with H• to form phenol (R17) [63] is less important. In fact, phenol can also directly decompose to cyclopentadiene (R18) [64].
C6H5O• → C5H5• + CO (R16)
C6H5O• + H• → C6H5OH (R17)
C6H5OH → C5H6 + CO (R18)
It was found that benzene, phenol and cyclopentadiene associated with their relevant radicals, i.e., phenyl, phenoxy and cyclopentadienyl radicals have significant interactions [60]. Similarly, the interactions are also expected in larger aromatic species, like naphthalene, naphthol and indene.
2.1.2 Polycyclic Aromatic Hydrocarbon (PAH)
PAH formation mainly derives from benzene, but the understanding of PAH formation is still very uncertain in both experimental and theoretical studies. The low concentration and lifetime of PAHs, as well as interferences from soot particles in some conditions, cause the experimental measurements to become very complicated. The recent advances in optical diagnostics provide possibilities and useful information to modelers [40], but they were mostly performed in limited conditions, i.e., low pressure [65]. Theoretical calculations have been proved as an effective method to understand kinetic behaviors without limitation of temperature and pressure conditions. However, the high-level theoretical calculation can be computationally demanding in underlying the Potential Energy Surface (PES) of the complex process of multi-ring aromatics due to numerous numbers of products, wells, and transition states.
Figure 2.1: HACA mechanism [71].
The other pathway proposed by Zhang et al. [72] was introduced to explain the soot formation in conditions where the presence of H atoms is insufficient to sustain the HACA mechanism. It involves the direct addition of acetylene on to the edge of PAH, as some PAHs have significant di-radical or multi-radical characteristics, and finally, H atom migration reactions, so-called carbon addition and hydrogen migration (CAHM). The recent theoretical study by Liu et al. [73] shows that HACA mechanism is dominant in the main flame region due to the abundance of H atom, but the direct addition of acetylene forming PAH-C2H2 is preferred in the
post-flame region. It was also found that the direct addition on armchair site and bridge surface of PAH are more likely in comparison with zig-zag and 5-membered ring sites. On the contrary, Frenklach et al. [74] recently recalculated the reaction rate constant of HACA mechanism and compared with CAHM. The comparison concluded that both mechanisms could explain the formation of aliphatic groups chemisorbed at edges of aromatics, however, experimental observations are still required in order to quantify the relative contribution.
Similar to benzene formation, the role of resonantly stabilized radicals cannot be neglected in PAH formation. Dente et al. [75] firstly proposed the route of naphthalene (C10H8) formation via five-membered ring. Melius
et al. [54] performed the theoretical studies of naphthalene formation via self-recombination of cyclopentadienyl radicals (C5H5•) and the addition of cyclopentadienyl and cyclopentadiene. Several
experimental studies of cyclopentadiene (C5H6) pyrolysis [76–78] confirmed the importance of this pathway
not only in the formation of naphthalene but also the considerable amount of indene (C9H8). Theoretical
calculations of recombination of this pathway by Cavallotti et al. [79] re-examined the PES (in Figure 2.2) with complete set of C10H10 isomers and highlighted the recombination of cyclopentadienyl leading to the
(above 1020 K at 1 bar and 1230 K at 100 bar). The addition of cyclopentadienyl and cyclopentadiene could lead to indene as investigated theoretically by [79].
C5H5• + C5H5• → C10H8 + 2H• (R19)
Figure 2.2: PES of cyclopentadienyl radicals recombination [71].
The experimental study of toluene pyrolysis performed by Colket and Seery [81] observed the important role of benzyl radical (C7H7•) due to its decomposition leading to cyclopentadiene and the recombination of
radical products. This work also suggests that the addition of heavy unsaturated and aromatic species, such as phenyl addition to naphthalene, naphthyl addition to benzene and similar, may need to be considered. Marinov et al. [82] proposed the reaction of benzyl and propargyls to form naphthalene in flames. (R20). This reaction is a relevant pathway in the naphthalene formation at low temperatures, similarly to cyclopentadienyl recombination [71]. Matsugi and Miyoshi [83] performed theoretical calculation of the addition of propargyl radicals on benzyl radicals. They observed that the addition on alpha position of benzyl radical prevails and subsequently forms 1-methylindene or naphthalene.
C3H3•+ C7H7• → C10H8 + 2H• (R20)
Another pathway of naphthalene formation is the addition of phenyl radical to the triple bond of vinylacetylene (C4H4) to form naphthalene (R21). PES investigation by Moriarty and Frenklach [84] concluded
that this phenyl-vinylacetylene adduction to form naphthalene by rotation about the double bond is too slow, leading to the non-PAH product of C6H5C4H3. This channel was re-calculated by Aguilera-Iparraguirre and
Klopper [85], who observed that a cis-trans isomerization of side chain via a radical four-member ring has a lower energy barrier than the rotation. The reaction rate constants were also calculated and concluded that this pathway is feasible.
The five-membered ring aromatics like indene (C9H8) can form through several pathways, which can lead to
different C9Hx (x = 8-11), like C6H5•+C3Hy, C7H7•+C2H2, as well as, C5H5•+C5H6,which isdiscussed earlier. It can
convert to naphthalene via the methylation. This pathway is related to the reactions of C7H7•+C3H3•, where
the products from initial adduct can isomerize to form methylene-indanyl. The methylation of indene produces 1-methylindenyl and its radical as the intermediate products, which can decompose easily to naphthalene under flame conditions [86].
Several experimental findings and detailed kinetic studies [87,88] proposed an efficient pathway in larger aromatic formation, which is reactive coagulation [89] between aromatic radical and aromatic molecules assisted by H-abstraction and cyclization. Shukla and Koshi [90,91] also proposed this idea, so-called phenyl addition/cyclization (PAC) mechanism in order to explain the mass spectra observed in pyrolysis of benzene. PAC is the subsequent addition of phenyl radical on an aromatic molecule, following by the cyclization to form larger aromatic intermediates.
Constantinidis et al. [92] studied the recombination of phenyl radicals in the microreactor by IR/UV ion dip spectroscopy, using free electron laser radiation as a mid-infrared source. The reaction is the vital pathway leading to biphenyl (C12H10) and terphenyl (C18H14) formation, which are the main products. Interestingly,
indene and naphthalene formed in the experiments without the addition of acetylene. Similar experimental studied has been recently carried out by Hirsch [93], who studied dimerization of benzyl radical at high temperature. The resonantly-stabilized radical can accumulate in the system and form PAH. The main products are bibenzyl as expected, however, the formation of phenanthrene is observed, which indicates the cyclization and following aromatization of bibenzyl.
Keller et al. [94] analyzed large PAH molecules and radicals formed in low pressure premixed benzene flames using molecular beam sampling. The study suggested that observed PAH growth does not follow the narrow C-H diagram, but instead, it provides a broad range of higher hydrogen content PAHs. Large even-carbon number PAH are mostly closed-shells, while PAHs with odd-carbon atoms mainly present π-radicals. They also suggested that π-radicals of PAH does not require hydrogen abstraction to activate σ-radicals like HACA mechanism.
A recent analysis using large datasets of low pressure flames by Hansen et al. [96] allows a deeper understanding and reveals some correlations based on measured mole fraction of chemical species. This correlation is useful in order to identify the branching ratio when the theoretical calculations are not available. Additionally, the large experimental dataset allows the verification of experimental reliability. Despite all the recent advances, fundamental chemical kinetics of aromatics needs further research. Challenges remain especially in the context of soot precursor formation from real fuel combustion. Due to the challenges in predicting PAH and soot precursors formation, the validation of the kinetic model in a vast range of experimental data is necessary in order to improve model reliability. The activities regarding the extensive collection of experimental data in rich laminar premixed flame and the validation against those flames are provided in Chapter 4.
2.1.3 Extension to Heavy PAH and Soot
The number of chemical compounds isomers, particular PAHs, rises with their molecular mass, which increases the complexitiy of heavy PAHs and successive carbonaneous particle study. Two broad classes of the aromatic compound can be considered in order to follow each compound [97], as presented in Figure 2.3.
First, peri-condensed aromatic hydrocarbons (PCAH) consist of only π-bonds among C atoms, which lead to the lowest hydrogen content. They are maximally condensed six-membered ring structures. Their H/C ratio decreases with the increase of the molecular size. The largest of these compounds is a graphene sheet. The structure of the compound is planar if it is comprised of only six-membered ring structures. Instead, the presence of five-membered rings leads to a molecular distortion and induces a curvature in the molecule as discussed in Section 1.2.
The other class is aromatic aliphatic linked hydrocarbons (AALH) or oligomers aromatic compounds (OAC) which contain both σ- and π-bonds providing incompletely-condensed oligomers of PCAH that lead to higher H/C ratio in comparison with PCAH. The active sites of PCAH, H atoms at the edge, react with another aromatic compound or its radical leading to non-completely condensed aromatic oligomers. Their H/C ratio of the oligomers remains almost constant as the molecular weight increases. The structure of AALH is usually assumed as non-planar.
reaction accounting for the growth of these compounds can be treated on the basis of gaseous reactions involving PAHs
Figure 2.3: Schematic representation of oligomers of benzene (high H/C values) and maximally PCAH (low H/C values) [98].
2.2
Soot Formation and Oxidation
Figure 2.4: Schematic representation of soot formation, growth and oxidation. Adapted from [99].
2.2.1 Soot Inception
The growth of precursor species eventually leads to soot nucleation to form nascent soot. Nascent soot has a core-shell structure composed of an aromatic core ranging from pyrene to ovalene and aliphatic shell [6]. Nucleation process is one of the least understood soot formation processes [95]. Several suggestions have been made regarding the general nature of soot particle nucleation, which involves polyyne species [100– 102], ionic species [103,104], and PAHs [89,105] as nucleating species. However, recent studies stated that the PAH are the most likely precursors [94,106–109].
The mechanism of soot inception, which characterizes the transition from gas-phase to condensed phase large aromatic and results in solid particles, remains elusive. As discussed by Wang [6], three conceptual pathways are hypothesized for soot nucleation.
The first pathway suggests the fullerenic mechanism, which is the evolution of two-dimensional PAH together with the presence of five-membered rings can bend the structure into curved fullerene-like structures [110]. These particles cannot close into a layer of carbon, but they develop in successive layers. This hypothesis was found to be too slow to explain the time scale of the soot inception phenomena. This large molecules of fullerene-like can be hardly found in the flame, perhaps only trace amount can be found in specific conditions. The second pathway considers the PAH dimerization driven by physical coagulation. To understand the non-equilibrium dynamics of PAH collision, Schuetz and Frenklach [111] carried out molecular dynamics simulation on the collision of pyrene using semi-empirical force fields to calculate dimer lifetimes for pyrene under flame conditions. They found that the internal rotations of PAH monomers greatly extend the dimer lifetimes and compatible with soot formation time scale. The calculation by Herdman and Miller [40] anticipated that the stacking of moderate size PAHs is possible at the flame temperature. The dimerization is controlled by equilibrium kinetics, in fact PAH dimers can survive only when PAHs reach the size of circumcoronene (C54H18), because the electrostatic and dispersive forces become sufficiently strong [6]. This
pathway seems to be able to explain soot formation in higher-temperatures, where the chemical growth, i.e., HACA, is limited by reaction reversibility [112].
this mechanism cannot explain the persistent particle nucleation into the post-flame region [35,117], where the presence of H atoms is too low to generate aryl radicals in sufficient amount.
The above conceptual pathways still cannot explain the mystery of the transition from gas to particles. Therefore, additional pathways are needed. Wang [6] suggested that the pathways of moderate-sized PAH-PAH interactions binding together with a bond as strong as a typical covalent bond. This suggestion drives the current studies on soot nucleation from physical dimerization toward chemical dimerization.
Several computational studies proposed the soot nucleation mechanism through covalent dimerization and oligomerization of PAHs. Koley et al. [118] suggested that the presence of small stretches of linear oligoacene would be adequate for enabling the nucleation of soot particles due to localized π-electron states. This localized π-electron allows PAH to have multiradical characteristics to form stable polymer molecules through covalent bonds. Zhang et al. [119] suggested that the number of covalent bonds is related to the aromaticity of individual six-membered ring structures. For instance, naphthalene and anthracene (C14H10)
are polyacenes species, PAHs containing linearly fused benzene rings, have higher aromaticity than non-linear PAHs (e.g., pyrene, coronene, etc.). They also observed that the binding energy of the dimerization increases proportionally to the diradical character in the range relevant to soot nucleation.
Elevati et al. [120] proposed the role of radical−radical combinations in nascent soot formation to form a molecule with a molecular mass of higher than 1200u. The interval time required by this recombination mechanism was observed to be around 2 ms, which corresponds to around 1 cm above the burner in typical laminar premixed flames.
The scanning probe microscopy technique recently used by Schulz et al. [121], which combines scanning tunneling microscopy (STM) and high-resolution atomic force microscopy (ATM), allows to image single building block molecules at atom level contributing to incipient soot particles. It also allows to identify the building block molecules and their electronic properties. Schulz et al. [121] investigated incipient soot particles generated in slightly sooting premixed ethylene flame. They observed a significant number of five-membered rings and several molecules composed of alkyl chains. They also observed the presence of unpaired π-electrons within the building block molecules.
closed-shell hydrocarbon, which is ready to generate large resonantly-stabilized radicals. Importantly, this mechanism does not require H-abstraction, which can have a high energy barrier, and it is independent of the presence of H atoms. However, this mechanism has not been implemented and tested, and thus this hypothesis leaves discussions open for future research. There is a need to investigate its potential regarding the detailed reaction rates of reactions involved in the mechanism, as well as, the formation of an initial cluster of soot particles and the growth [122].
2.2.2 Soot Growth
Nucleation is an entrance channel in soot formation. However, the total soot amount is mostly formed by soot growth[123]. Wagner [124] described the deposition rate of carbon in flames is a first order reaction proportional to surface growth kinetics with typical value range between 30 – 500 s-1. D’Alessio et al. [125]
numerically compared the detailed kinetic mechanism of particle formation with the measurements. They observed good agreement using the model neglecting soot growth reactions in flames with C/O <0.8. At the end of the flame with C/O=0.92, they also observed that surface growth contributes to the carbon mass in soot by 60%, while the remaining results from the transformation from chemical nature to particle nature. The growth can occur via several processes across a variety of flame conditions and fuels [126]. Main soot growth processes are chemical growth by acetylene, chemical growth by PAHs (biaryl recombination), condensation of PAHs, and coagulation of soot particles. The first three processes are surface reactions that increase the total soot mass, while the last process, coagulation, controls the number of particles and particle size.
It is important to note that the heterogeneous reaction involving gaseous species and solid soot particles are not well understood. This should consider adsorption and desorption of gaseous species on particles. Usually, the analogous surface growth reaction is usually assumed as PAH growth. However, the growth in heterogeneous reactions could be slower than the reaction of PAH in the gas phase due to steric hindrance [133].
The interaction of two PAHs can lead to soot growth. First, the chemical reaction of two aryl radicals so-called biaryl recombination can lead to internal rearrangement, transform into a more stable compound. Sarofim et al. [134] assessed the role of aryl-aryl reaction and suggested that this reaction class may be more important for fuels such as coal and wood that can readily generate aryl radicals.
PAH deposition on the surface of soot particles, so-called PAH condensation, is also considered a fast carbonization process for the growth of soot particles [135,136]. The growth of the particle, due to adsorption of gas-phase species to the surface of the soot particle, is similar to the nucleation based on the stacking of PAH. Condensation of PAH on soot particles may occur for a smaller monomer size aromatic compound [40,112]. Although some studies indicate that small PAH molecules like pyrene and coronene may be not stable at flame temperatures [137,138], some experiments suggest that PAH stacks are the building block of soot [139,140]. This growth process has been included in the many soot models [109,141,142] as well as, the model used in this study. The large contribution by PAH condensation to soot particles is also observed in Section 4.3.3, in which the condensed PAHs are formed via HACA mechanism. Still, a better understanding of this PAH condensation process is needed both theoretically and experimentally.
