SHOCK TUBE STUDY OF NITROGEN-CONTAINING FUELS

Transcription

1 SHCK TUBE STUDY F NITRGEN-CNTAINING FUELS A DISSERTATIN SUBMITTED T THE DEPARTMENT F MECHANICAL ENGINEERING AND THE CMMITTEE N GRADUATE STUDIES F STANFRD UNIVERSITY IN PARTIAL FULFILLMENT F THE REQUIREMENTS FR THE DEGREE F DCTR F PHILSPHY Sijie Li June 2014

2 2014 by Sijie Li. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. This dissertation is online at: ii

3 I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Ronald Hanson, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Craig Bowman I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. David Davidson I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Hai Wang Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost for Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii

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5 Abstract The combustion chemistry of nitrogen-containing fuels is important in the study of bio-derived fuels and nitrogen-based propellants. However, little high-quality shock tube kinetics data exists for these systems. The primary objective of the research presented in this dissertation is to augment the experimental database and to improve understanding of the chemical kinetics for four nitrogen-containing fuels: morpholine, dimethylamine, ethylamine and monomethylhydrazine. Morpholine (C 4 H 9 N, 1-oxa-4-aza-cyclohexane) is a good representative candidate of a nitrogen-containing fuel because of its cyclic structure and wide industrial applications. Morpholine ignition delay times were measured behind reflected shock waves. A morpholine mechanism was developed based on this shock tube study and previous works in the literature. The simulations from this morpholine mechanism were in good agreement with the current morpholine experiments as well as previous morpholine flame data. Refinement of this morpholine mechanism required improvements in the sub-mechanisms of two major intermediate species dimethylamine and ethylamine, as discussed in a progressive manner in this dissertation. The overall rate constants of hydroxyl radicals (H) with dimethylamine (DMA: CH 3 NHCH 3 ) and ethylamine (EA: CH 3 CH 2 NH 2 ) were measured behind reflected shock waves using UV laser absorption of H radicals near nm. The overall rate constants were determined by fitting the measured H time-histories with the computed profiles using the detailed dimethylamine and ethylamine submechanisms contained in the morpholine mechanism. Variational transition state theory was used to compute the H-abstraction rates by H for dimethylamine and ethylamine. The calculated reaction rate constants are in good agreement with the experiment. The calculated reaction rate constants were used to update the morpholine mechanism for simulations in the following sections. v

6 Dimethylamine (DMA) ignition delay times and H time-histories were investigated behind reflected shock waves. The dimethylamine ignition delay time measurements were carried out in 4% oxygen/argon. H time-histories were measured in stoichiometric mixtures of 500 ppm DMA/ 2 /argon. The morpholine mechanism was then updated by adding the DMA unimolecular decomposition channel: DMA = CH 3 NH + CH 3. With this modification, the simulation results are in excellent agreement with both the dimethylamine ignition delay times and H time-history data. Ethylamine (CH 3 CH 2 NH 2 ) pyrolysis and oxidation were studied behind reflected shock waves. For ethylamine pyrolysis, NH 2 time-histories were measured in 2000 ppm ethylamine/argon mixtures. For ethylamine oxidation, ignition delay times, NH 2 and H time-histories were measured in ethylamine/ 2 /argon mixtures. By fitting the simulations to the early time-histories of NH 2 and H, the rate constants for the two major ethylamine decomposition pathways in the morpholine mechanism were updated for better agreement with the experiment. In addition, recommendations from recent theoretical studies of ethylamine radical reactions were implemented. With these modifications, the final updated morpholine mechanism provides significantly improved agreement with the species time-history measurements and the ignition delay times of ethylamine. The morpholine mechanism, after implementing the aformentioned updates based on the dimethylamine and ethylamine data, was compared with the morpholine ignition delay time data again. It was shown that those modifications improve the agreements of the mechanism with the morpholine data. Amine groups are common structural features for rocket propellants as well, and using the same approach as above, the pyrolysis of an important propellant monomethylhydrazine (MMH) was studied using NH 2 time-histories in MMH/argon mixtures. The MMH pyrolysis mechanism developed by Sun et al. (2009), with the updates by Cook et al. (2011), was used to compare with the experiment. The rate constant of the reaction: MMH = CH 3 N.H + NH 2 was determined based on early time of vi

7 the NH 2 time-histories. Pressure dependence of this reaction was observed at atm. The measured reaction rate constants follow a pressure dependence trend close to the theoretical results by Zhang et al. (2011) based on transition state theory master equation analysis. Using the high and low-pressure limit expressions by Zhang et al., a new Troe s expression in the fall-off region was proposed based on the current experimental data. Utilizing the later times of the NH 2 time-histories, a new reaction rate expression was recommended for the reaction: NHNH 2 + H = NH 2 + NH 2. vii

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9 Acknowledgments First, I would like to thank my advisor, Prof. Ronald K. Hanson, for his generous support during my PhD process. Without his constant encouragement and suggestions, I doubt I would ever reach this point. I also want to thank Dr. David F. Davidson for offering generous advice and guidance throughout my time in Stanford, not only about research but also about life in general. I am also thankful to Prof. Bowman and Prof. Wang for serving on my reading committee. Every time when things are not going well in my life as a PhD candidate, I often look into the thesis database of our group. I was not looking for solutions, but went directly to the thesis acknowledgements. I want to see what previous students had in mind, did they experience similar disappointment, fake hope, frustration, excitement, happiness...? Whom did they want to say thanks to? How did it feel like when they finally started to write their thesis? And more hauntingly, I always asked myself whether I will ever be able to write my own PhD acknowledgement? What I will say? I forgot in which thesis acknowledgement I read this line "I don t like the idea of listing people s name in the acknowledgement, because every list starts somewhere and has an end". I m grateful to all the previous and current members I met in the Hanson group. I still remember all the homework we finished together, the data we collected as a team and the free time we spent together. I m also grateful to all my friends; you made my life in Stanford more colorful. Lastly, I want to say thank you to my family and Ting. Thank you for having faith in me, even when I am lacking belief myself. Thank you for reminding me that people with steady, instead of fast pace, finish the Marathon. Thank you for encouraging me to try and never be afraid of losing. ix

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11 Table of Contents Abstract...v Acknowledgments... ix Table of Contents... xi List of Tables... xvii List of Figures... xix Chapter 1. Introduction Background and Motivation verview of Thesis...3 Chapter 2. Experimental Method Shock Tube Facility Stanford High Pressure Shock Tube (HPST) Stanford Kinetic Shock Tube (KST) Stanford NASA Shock Tube (NASA) Laser Diagnostic verview...8 xi

12 IR Diagnostic of Fuel NH 2 Diagnostic H Diagnostic Chapter 3. Shock Tube and Modeling Study of Morpholine Introduction Experimental Details Results and Discussion Morpholine Ignition Delay Times Model Development and Simulations Summary Chapter 4. Reactions of H with Dimethylamine and Ethylamine Introduction Experimental Setup Kinetic Measurements Dimethylamine (DMA) + H Ethylamine (EA) + H Theoretical Study Summary Chapter 5. Dimethylamine xidation Introduction xii

13 5.2. Experimental Setup Results and Discussion Dimethylamine Ignition Delay Times H Time-Histories Update to the Morpholine Mechanism Summary...63 Chapter 6. Ethylamine Pyrolysis and xidation Introduction Experimental Methods Experimental Results Ethylamine Pyrolysis Ethylamine xidation Update to the Morpholine Mechanism Summary...80 Chapter 7. Revisiting the Morpholine Data Introduction Morpholine Ignition Delay Times Sensitivity Analysis Summary...86 Chapter 8. MMH Pyrolysis...89 xiii

14 8.1. Introduction Experimental Method Results and Discussion Summary Chapter 9. Summary and Future Work Summary of Results Morpholine xidation Dimethylamine and Ethylamine Combustion MMH Pyrolysis Recommendations for Future Work Dimethylamine and Ethylamine Pyrolysis Dimethylamine and Ethylamine xidation Morpholine Pyrolysis and xidation Conclusion Appendix A. Morpholine xidation Set Appendix B. Morpholine Pyrolysis Set Appendix C. Thermochemistry for Morpholine Species Appendix D. Computational Methods D.1. CHEMKIN Simulation D.2. Multiwell Calculation xiv

15 D.3. Gaussian Calculation Bibliography xv

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17 List of Tables Table 3.1. Shock tube ignition delay times. Test gas mixture: morpholine/ 2 /argon...15 Table 4.1. Reactions describing DMA and EA + H experiments...31 Table 4.2. Measured rate constants for DMA + H = Products...35 Table 4.3. Measured rate constants for EA + H = Products...40 Table 4.4. Summary of the zero Kelvin electronic energies and rotational data used for dimethylamine and ethylamine + H VTST calculations...43 Table 4.5. The vibrational frequencies computed at the BH&HLYP/ G(2d,2p) level of theory...44 Table 4.6. VTST reaction rate constants of individual channels for DMA and EA + H Table 5.1. Ignition delay time data for dimethylamine...53 Table 5.2. Summary of rate recommendations to the dimethylamine submechanism of the morpholine mechanism Table 6.1. Updated reaction rate constants to ethylamine sub-mechanism...72 xvii

18 Table 7.1. Modifications to the morpholine mechanism [54], recommended in Chapters Table 8.1. Measured reaction rate constants for the N-N bond scission of MMH xviii

19 List of Figures Figure 2.1. Shock tube schematic....6 Figure 2.2. He-Ne laser diagnostic of fuel....9 Figure 2.3. Ring dye laser diagnostic of NH 2, using the fundamental output of a Spectra Physics 380 ring dye laser...10 Figure 2.4. Ring dye laser diagnostic of H, using the frequency doubled output of a Spectra Physics 380 ring dye laser Figure 3.1. Sample pressure trace for morpholine ignition delay time measurements Figure 3.2. Ignition delay time measurements in morpholine/air mixtures near 15 atm and with equivalence ratios of 0.5, 1 and 2. Dashed lines: linear fit to data Figure 3.3. Ignition delay time measurements in morpholine/air Φ = 1 mixtures near 15 and 25 atm. Dashed lines: linear fit to data Figure 3.4. Ignition delay time measurements in stoichiometric morpholine/air mixtures and morpholine/4% 2 /argon mixtures around 15 atm. Dashed lines: linear fit to data Figure 3.5. Comparison of model predictions to morpholine/air ignition delay time measurements around 15 atm and under different equivalence xix

20 ratios. Dashed lines: Simulation results using the mechanism in [8]. Solid lines: Simulation results using the morpholine mechanism [54] Figure 3.6. Comparison of model predictions to morpholine/air ignition delay time measurements for stoichiometric mixtures around 15 and 25 atm respectively. Dashed lines: Simulation results using the previous mechanism [8]. Solid lines: Simulation results using the morpholine mechanism [54] Figure 3.7. Comparison of model predictions to morpholine ignition delay time measurements around 15 atm for morpholine/4% 2 /argon and morpholine/air mixtures. Dashed lines: Simulation results using the previous mechanism [8]. Solid lines: Simulation results using the morpholine mechanism [54] Figure 4.1. Sensitivity analysis of H using the dimethylamine sub-mechanism [25] with TBHP chemistry set, in the mixture of 320 ppm DMA/Ar with 22 ppm TBHP and 140 ppm H 2, at 1176 K and 0.9 atm Figure 4.2. Sample H trace in 320 ppm DMA/Ar with 22 ppm TBHP and 134 ppm H 2, at 1176 K and 0.9 atm Figure 4.3. Error analysis for measured k 4.1 in 320 ppm DMA/Ar with 22 ppm TBHP and 134 ppm H 2, at 1176 K and 0.9 atm Figure 4.4. Measured overall reaction rate for k 4.1 : DMA+H = Products, in comparison with the estimation by Lucassen et al. [25] Figure 4.5. Sensitivity analysis of H using the ethylamine sub-mechanism [25] with inclusion of TBHP chemistry, in a mixture of 470 ppm EA,50 ppm TBHP, 190 ppm H 2, in Ar at 1067 K and 0.83 atm xx

21 Figure 4.6. Sample H trace in in a mixture of 470 ppm EA, 50 ppm TBHP, 190 ppm H 2, in Ar at 1067 K and 0.83 atm Figure 4.7. Error analysis for measured k 4.2 in 470 ppm EA/Ar with 50 ppm TBHP and 190 ppm H 2, at 1067 K and 0.83 atm Figure 4.8. Measured overall reaction rates for EA + H = Products, in comparison with the estimation by Lucassen et al. [25]...42 Figure 4.9. Comparison of the measured reaction rates and theoretical study results for DMA + H and EA + H Figure 5.1. Sample pressure traces for dimethylamine ignition delay times Figure 5.2. Ignition delay times in stoichiometric DMA/4% 2 /argon mixtures, with P ~ 0.9, 1.5 and 2.8 atm, measurements and simulation results using the morpholine mechanism described in Chapter 3 [54]...52 Figure 5.3. Ignition delay times in stoichiometric DMA/4% 2 /argon mixtures at P ~ 1.5 atm, with Φ = 0.5, 1, and 2, measurements and simulation results using the morpholine mechanism described in Chapter 3 [54] Figure 5.4. H time-histories in stoichiometric mixture of 500 ppm DMA/2/argon at 1417 K and 2.2 atm, with simulation results using the morpholine mechanism described in Chapter 3 [54] Figure 5.5. H time-histories in stoichiometric mixture of 500 ppm DMA/ 2 /argon at 1504 K and 2.1 atm, with simulation results using the morpholine mechanism described in Chapter 3 [54] Figure 5.6. Sensitivity analysis of temperature, at initial condition of 1300 K and 1.5 atm in a stoichiometric mixture of DMA/4% 2 /argon, using the morpholine mechanism described in Chapter 3 [54] xxi

22 Figure 5.7. Ignition delay times in stoichiometric mixtures of DMA/4% 2 /argon, at P ~ 0.9, 1.5 and 2.8 atm, measurements and simulations using the morpholine mechanism [54] with the modifications in Table Figure 5.8. Ignition delay times in stoichiometric mixtures of DMA/4% 2 /argon at P ~ 1.5 atm, with Φ = 0.5, 1, and 2, measurements and simulation results using the morpholine mechanism [54] with the modifications in Table Figure 5.9. Measured dimethylamine ignition delay times in 4% 2 /argon, scaled to Φ = 1 and P = 1.5 atm, in comparison with the simulations using the morpholine mechanism [54] with and without the changes recommended in Table Figure Comparison of the simulated H time-histories, using the morpholine mechanism [54] with and without the modifications in Table 5.2, to the experiment in stoichiometric mixture of 500 ppm DMA/ 2 /argon at 1417 K and 2.2 atm Figure Comparison of the simulated H time-histories, using the morpholine mechanism [54] with and without the modifications in Table 5.2, to the experiment in stoichiometric mixture of 500 ppm DMA/ 2 /argon at 1504 K and 2.1 atm Figure 6.1. NH 2 time-histories in 2000 ppm ethylamine/ar mixtures, measurements and simulation results using the morpholine mechanism [54] reported in Chapter Figure 6.2. Ethylamine ignition delay time measurements near 0.85, 1.35 and 2 atm in stoichiometric mixture of ethylamine/4% 2 /Ar, and simulations based on the morpholine mechanism [54] in Chapter xxii

23 Figure 6.3. Ethylamine ignition delay time measurements near 1.35 atm, with Φ = 0.75, 1 and 1.25, and simulation results using the morpholine mechanism [54] presented in Chapter Figure 6.4. NH 2 time-histories in 2000 ppm ethylamine/0.8% 2 /Ar mixtures; simulations are based on the morpholine mechanism presented in Chapter Figure 6.5. H time-histories in 500 ppm ethylamine/0.2% 2 /Ar mixtures, measurements and simulations using the morpholine mechanism [54] presented in Chapter Figure 6.6. NH 2 sensitivity analysis using the morpholine mechanism [54], with updates shown in Table 6.1, at 1428 K, 1.2 atm in 2000 ppm ethylamine/ar mixtures Figure 6.7. Measured NH 2 time-histories in 2000 ppm ethylamine/ar mixtures; simulation results are based on the morpholine mechanism [54] with updates shown in Table Figure 6.8. Measurements of ethylamine ignition delay times near 0.85, 1.35 and 2 atm in stoichiometric mixture of ethylamine/4% 2 /Ar; simulation results utilize the morpholine mechanism [54] with updates in Table Figure 6.9. Measured ethylamine ignition delay times near 1.35 atm, with Φ = 0.75, 1 and 1.25; simulation results utilize the morpholine mechanism [54] with updates in Table Figure NH 2 sensitivity analysis using the morpholine mechanism [54] with updates in Table 6.1, at 1441 K, 2.1 atm in 2000 ppm ethylamine/0.8% 2 /Ar mixtures xxiii

24 Figure NH 2 time-histories in 2000 ppm ethylamine/0.8% 2 /Ar mixtures: measurements (solid lines) and simulation results based on the morpholine mechanism [54] with updates in Table 6.1 (dash-dotted lines) Figure H sensitivity analysis at 1399 K, 1.9 atm in 500 ppm ethylamine/0.2% 2 /Ar mixtures, using the morpholine mechanism [54] with updates in Table Figure H time-histories in 500 ppm ethylamine/0.2% 2 /Ar mixtures: measurements (solid lines) and simulation results using the morpholine mechanism [54] with updates in Table 6.1 (dash-dotted lines) Figure 7.1. Comparisons of model predictions with the ignition delay time data in morpholine/air mixtures near 15 atm and under different equivalence ratios. Solid lines: simulation results using the morpholine mechanism in Chapter 3 [54]. Dash-dotted lines: simulation results using the morpholine mechanism with modifications in Chapter Figure 7.2. Comparisons of model predictions to ignition delay time data in stoichiometric morpholine/air mixtures near 15 and 25 atm respectively. Solid lines: simulation results using the morpholine mechanism in Chapter 3 [54]. Dash-dotted lines: simulation results using the morpholine mechanism with modifications in Chapter Figure 7.3. Comparisons of model predictions to ignition delay time data near 15 atm in morpholine/4% 2 /argon and morpholine/air mixtures. Solid lines: simulation results using the morpholine mechanism in Chapter 3 [54]. Dash-dotted lines: simulation results using the morpholine mechanism with modifications in Chapter xxiv

25 Figure Sensitivity analysis of temperature using the morpholine mechanism [54] with modifications in Chapter 4-6, in a stoichiometric mixture of morpholine/air under initial condition of 1000 K and 15 atm Figure 8.1. Representative NH 2 time-history measurement at 1217 K and 0.34 atm in 350 ppm MMH/argon, in comparison with simulation results using the Cook et al. mechanism [34]...93 Figure 8.2. NH 2 sensitivity analysis at 1217 K and 0.34 atm in 350 ppm MMH/argon, using the Cook et al. mechanism [34] with the constant energy and volume assumptions Figure 8.3. NH 2 sensitivity analysis at 1163 K and 5.2 atm in 170 ppm MMH/argon, using the Cook et al. mechanism [34] with the constant energy and volume assumptions Figure 8.4. Representative NH 2 time-history measurement at 1217 K and 0.34 atm in 350 ppm MMH/argon, with an error bar ±15% due to the uncertainty of NH 2 cross section, in comparison with the Cook et al. mechanism with the best-fit k 8.1a (dotted line), and the Cook et al. mechanism with the best-fit k 8.1a and with k 8.2 increased by a factor of 3 (dash-dotted line) Figure 8.5. NH 2 time-history measurement near 0.3 atm, in comparison with the simulation results using the modified Cook et al. mechanism [34] with the updated k 8.1a and k 8.2 (dash-dotted line) Figure 8.6. NH 2 time-history measurement near 1 atm, in comparison with the simulation results using the modified Cook et al. mechanism [34] with the updated k 8.1a and k 8.2 (dash-dotted line) xxv

26 Figure 8.7. NH 2 time-history measurement near 5 atm, in comparison with the simulation results using the modified Cook et al. mechanism [34] with the updated k 8.1a and k 8.2 (dash-dotted line) Figure 8.8. Measured MMH N-N bond scission reaction rate constants k 8.1a, in first-order reaction form. Data points: current study. Dashed line: reaction rate constant expression for k 8.1a near 2.5 atm by Cook et al. [34] Figure 8.9. Uncertainty analysis for k 8.1a at 1217 K and 0.34 atm in 350 ppm MMH/argon Figure Representative MMH N-N bond scission reaction rate constants k 8.1a, in comparison with the previous studies Figure Pressure dependence of the measured k 8.1a at representative temperatures, in comparison with the theoretical study by Zhang et al [94] Figure 9.1. Representative species time-histories for ethylamine oxidation in stoichiometric mixture with 4% 2 /argon, at 1500 K and 1 atm: simulations using the modified morpholine mechanism xxvi

27 Chapter 1. Introduction 1.1. Background and Motivation Biofuels, as additives and alternatives to petroleum-based transportation fuels, are of increasing interest in national strategic fuel planning. These fuels may have more nitrogen-containing compounds than petroleum-based fuels, especially in those biofuels derived from biomass [1 4]. This is because nitrogen atoms bound in biomass, in the form of proteins and free amino acids for example, may stay as nitrogen-containing compounds in the derived fuels. Thus, understanding the combustion chemistry of nitrogen-containing fuels is becoming of great importance; however, little shock tube kinetics data exists for these systems.. Significant new insights into fuel-nitrogen chemistry can be gained by applying shock tube/laser absorption methods to study the kinetics of nitrogen-containing fuels. Previous works have shown that the structure of nitrogen-containing compounds and the pyrolysis conditions affect the decomposition pathways of biomass and biofuels, with mechanisms involving complicated oxygenated and nitrogenated ring structures [1,4]. Because of the complicated structure of real biomass and biofuels, indepth studies of simpler model biofuels are needed first to gain insight into fuelnitrogen chemistry. Morpholine (C 4 H 9 N, 1-oxa-4-aza-cyclohexane), which is a sixmembered ring both oxygenated and nitrogenated, is an excellent representative candidate for these studies [5 8]. In previous studies, molecular-beam mass spectrometry (MBMS) has been used to identify intermediates and products for morpholine flames stabilized on a flat low-pressure burner at 40 mbar [6]. Cavity ringdown spectroscopy (CRDS) was also employed to detect the profiles of intermediate 1

28 species including CH 2, CH and NH 2 [7] in morpholine flames under the same condition as in [6]. Combining photoionization (PI) and electron ionization (EI) MBMS, the mole-fraction profiles of major and intermediate species in a morpholine flame were further determined in [8]. In combination with the MBMS experimental work, a morpholine combustion mechanism was developed using analogies to cyclohexane combustion [8]. That mechanism for morpholine captures relevant features of the morpholine flame quite well. However, the morpholine combustion database needs to be augmented with shock tube data, and the morpholine mechanism developed in [8] needs to be validated and updated. The combustion of morpholine as a 6-membered cyclic amine starts with ring opening and pyrolysis process to form smaller aliphatic amine compounds, in particular, dimethylamine and ethylamine radicals. Accurate dimethylamine and ethylamine submechanisms are thus needed if the morpholine mechanism is to be refined. Dimethylamine and ethylamine are among the most abundant amines found in the atmosphere, with sources found in agricultural and industrial processes such as fish and meat production [1 3,9,10]. In industry applications, dimethylamine and ethylamine are the base structures for various substances used for crop and wood protection, paints, and finishes [11], as well as in amine-based fuel additives. nly a few early works on the reactions of aliphatic amines related to atmospheric chemistry are available in the literature [12 16]. Atkinson et al. [12] and Slagle et al. [17] examined the kinetics of the reactions of oxygen atoms with amines, using a photoionization technique. Atkinson and coworkers also investigated the reactions of methylamine with H over the temperature range of K [13]. A similar study was then carried out to measure the rate constants of dimethylamine, ethylamine and trimethylamine with H over the temperature range of K [14]. Carl et al. studied the reaction rate constants of aliphatic amines with H at 295 K, including those for methylamine, dimethylamine, ethylamine and trimethylamine [15]. Galano and Alvarez-Idaboy analyzed the different reaction channels of methylamine, dimethylamine and ethylamine with H, using variational transition-state theory [16]. 2

29 Even fewer studies of aliphatic amines have been carried out at combustion temperatures, and most of these were early studies of the effects of aliphatic amines on hydrocarbon ignition as combustion inhibitors [18 23]. Votsmeier et al. [24] studied methylamine thermal decomposition in a shock tube, employing NH 2 concentration time-history measurements. Recently, Lucassen et al. studied the laminar premixed flames of dimethylamine and ethylamine under one-dimensional low-pressure conditions [25]. In that work, a detailed combustion model was developed to analyze the major pathways in the two flames, which successfully captured many trends observed in the flame experiments [25]. More detailed investigations of dimethylamine and ethylamine are required for both their own research values and better understanding of morpholine combustion. Amine groups are common structural features for rocket propellants as well. For example, the important rocket propellant monomethylhydrazine (MMH) contains both primary and secondary amino groups (in the form of a hydrazine group). The combinations of MMH with certain oxidizers such as nitrogen tetroxide (N 2 4, NT) are hypergolic and can ignite spontaneously [26 28]. Hypergolic propellants play vital roles in orbital maneuvering and reaction control systems in aerospace industries. While MMH satisfies the flight performance requirements, it presents ground safety hazards because of its toxic, corrosive and carcinogenic properties, which make it challenging to study MMH experimentally. Safety precaution is very important for MMH experiments. As researchers have worked to develop detailed MMH pyrolysis and oxidation mechanisms for MMH with a variety of oxidizers, accurate reaction rate constants for the MMH thermal decomposition reactions have become increasingly important verview of Thesis The dissertations is divided into nine chapters: 3

30 Chapter 2 describes the shock tube facilities used for the study of the thesis, and the laser diagnostic techniques utilized in this work for species time-history measurements. Chapter 3 presents the ignition delay time study during morpholine oxidation in a high-pressure shock tube, and a theoretical study based on the experiment to develop a morpholine mechanism. Chapter 4 describes the overall rate constant measurements of hydroxyl radicals (H) with dimethylamine (DMA: CH 3 NHCH 3 ) and ethylamine (EA: CH 3 CH 3 NH 2 ). Accompanied with the experimental study, a variational transition state theory study is also included in this chapter with the potential energy surface, geometries, frequencies and electronic energies at CCSD(T)/ G(2d,2p) level of theory in the literature. Chapter 5 discusses the oxidation study of dimethylamine, including ignition delay time and H time-history measurements. Based on the experimental data, the mechanism discussed in Chapter 3 was further updated. Chapter 6 presents the ethylamine pyrolysis and oxidation study behind reflected shock waves. With the current experimental data and the recommendations from recent studies of ethylamine reactions, final modifications to the morpholine mechanism were recommended. Chapter 7 revisits the morpholine ignition delay time data to show the effects of the modifications, recommended in Chapter 4-6, on predicting morpholine ignition delay times. Chapter 8 describes the pressure dependence study of the important MMH decomposition reaction CH 3 NHNH 2 = CH 3 N.H + NH 2, making use of NH 2 time-history measurement during MMH pyrolysis process. Chapter 9 provides a summary of the thesis and proposes several future works. 4

31 Chapter 2. Experimental Method Three different shock tubes were used for this thesis work. This chapter provides an overview of a shock tube facility in general and presents some details for the three shock tubes used for the thesis work, respectively. Three different laser absorption diagnotic methods were used to monitor different species during the combustion processes of the fuels covered in this thesis. This chapter first discusses the fundamental theory for laser absorption measurement in general and then provides a more detailed description of each diagnotic method Shock Tube Facility A shock tube is a test facility close to an ideal zero-dimensional reactor with uniform temperature and pressure that can be readily generated by shock heating. A diaphragm is used to separate the tube into the driver and driven sections. When the diaphragm breaks a shock wave will form, travel down the tube, reach the end wall of the shock tube and then reflect back. ptical diagnostics can be implemented in the heated test gas behind the incident or the reflected shock waves. With accurate measurement of the incident shock speed, the test conditions behind the shock waves can be determined accurately using the ideal shock jump relations [29]. A schematic for shock tube operation is shown in Figure 2.1. Three different shock tubes in Stanford were used for this work, and are described in the following sections. 5

32 Figure 2.1. Shock tube schematic Stanford High Pressure Shock Tube (HPST) Morpholine ignition delay times in morpholine/air and morpholine/oxygen/argon mixtures were measured behind reflected shock waves using the Stanford high-pressure shock tube (HPST). This shock tube has a stainless steel driven section of 5 m length with a 5 cm inner diameter and a driver section that is 3 m long with an inner diameter of 7.5 cm. Shock tube driver inserts were used to achieve uniform test conditions at lower temperatures where facility effects at long test times (dp/dt and dt/dt) are most significant [30]. In the current study, a test time about 2.5 ms was achieved with uniform test conditions using helium driver gas. The incident shock speed, which is critical to the accurate determination of reflected shock pressure and temperature, was determined using five piezoelectric pressure transducers that were spaced at approximately 30 cm intervals over the last 2 m of the shock tube. The driven section was heated to 86 o C to mitigate condensation of fuel on the wall. Temperatures and pressures in the post-shock region were determined from the incident shock speed at the end wall using standard normal shock relations. Ignition pressure was monitored 6

33 using a piezoelectric pressure transducer (Kistler Model 603B1) located 1 cm from the end wall. ther details concerning this shock tube can be found in [31] Stanford Kinetic Shock Tube (KST) Dimethylamine and ethylamine + H, and dimethylamine oxidation were studied behind reflected shock waves in a shock tube that has a 3.35 m driver section and an 8.54 m driven section, both with an inner diameter of cm. Shock tube driver inserts were used to achieve uniform test conditions. In the current study, a test time about 2 ms was achieved with uniform test conditions using helium driver gas. The incident shock speeds were measured using five piezoelectric pressure transducers near the driven section endwall. Between experiments, the shock tube was routinely evacuated to ~5 µtorr to ensure purity of the test mixtures. More details concerning this shock tube are included in [32,33] Stanford NASA Shock Tube (NASA) Ethylamine pyrolysis and oxidation, and MMH pyrolysis were studied behind reflected shock waves in a shock tube with a 3.7 m driver section and a 10 m driven section, both with an inner diameter of cm. The incident shock speeds were measured using five piezoelectric pressure transducers over the last 1.5 meters of the shock tube and linearly extrapolated to the endwall. The ignition pressures were monitored using a piezoelectric pressure transducer (Kistler Model 603B1) located 2 cm from the end wall; laser absorption measurements were conducted at the same axial location. More details concerning this shock tube can be found in [34]. 7

34 2.2. Laser Diagnostic verview In this thesis, the primary laser diagnostic method utilized is the fixedwavelength direct absorption technique, which is a powerful tool for chemical kinetics study. ne advantage of a laser absorption diagnostic is that it enables rapid real-time measurement at khz-mhz rates, which can be used to determine the time evolution of important species in combustion processes. Besides, laser absorption diagnostics are non-intrusive and do not perturb the chemical kinetics processes. In this work, species time-history measurements for important combustion compounds were used to provide valuable kinetic information for mechanism validation and modification. Species concentration can be inferred from laser absorption measurement via the Beer-Lambert law shown in equation 2.1: ( ) ( ) (Eq 2.1), where α is the absorbance, T is the transmission, I is the transmitted laser intensity with absorption through the test region, I 0 is the laser intensity without absorption, P is the pressure, x is the mole fraction of the absorbing species, k is the absorption coefficient of the target species, and L is the laser pathlength in the test region. With the measured absorbance and pressure, and the known absorption coefficient and laser pathlength, the mole fraction of the absorbing species can be derived as: x = α/pkl (Eq 2.2) More details on the specific laser absorption diagnostic methods used for the current work can be found in the following sections. 8

35 IR Diagnostic of Fuel For morpholine ignition delay time measurements, initial fuel concentrations were monitored using the 3.39 m emission of a Spectral Physics model 124B He-Ne laser. This fuel diagnostic relies on the strong absorption band near 3.39 m due to the C-H stretch vibration. Common mode rejection was used to reduce laser intensity noise. The experimental setup for this fuel diagnostic is shown in Figure 2.2. In support of this work, the absorption coefficient of morpholine at 3.39 m and 86 o C was also measured using an FTIR instrument. Details on the FTIR measurement technique can be found in [35,36]. Figure 2.2. He-Ne laser diagnostic of fuel NH 2 Diagnostic NH 2 was measured using the output of a narrow-linewidth ring dye laser near nm. This NH 2 laser absorption diagnostic employed the overlapping ÃA 1 X 2 B 1 ( ) P Q 1,N 7 doublet lines, which was previously characterized in our laboratory [24,34,37]. Visible light near nm was generated by pumping 9

36 Rhodamine 6G dye in a Spectra Physics 380 laser cavity with the 5 W, continuous wave, output of a Coherent Verdi laser at 532 nm. Using a common-mode rejection detection setup, a minimum NH 2 detection sensitivity of 5 ppm could be achieved for most conditions studied in this work. A schematic of the NH 2 diagnostic is shown in Figure 2.3, and more details of the NH 2 laser diagnostic setup are described in [34]. Figure 2.3. Ring dye laser diagnostic of NH 2, using the fundamental output of a Spectra Physics 380 ring dye laser H Diagnostic H was measured near nm using the frequency-doubled output of the same ring dye laser system as for the NH 2 diagnostic. The chosen wavelength was the peak of the well-characterized R 1 (5) absorption line in the H A-X(0,0) band [32]. Visible light near nm generated in the Spectra Physics 380 laser cavity was intracavity frequency-doubled using a temperature-tuned AD*A nonlinear crystal to generate ~1 mw of UV light near nm. Using a common-mode rejection detection setup, a minimum H detection sensitivity of 0.5 ppm could be easily achieved. A schematic of the H diagnostic is shown in Figure 2.4, and more details of the laser diagnostic setup are can be found in [32]. 10

37 Figure 2.4. Ring dye laser diagnostic of H, using the frequency doubled output of a Spectra Physics 380 ring dye laser. 11

38

39 Chapter 3. Shock Tube and Modeling Study of Morpholine 3.1. Introduction As is introduced in Chapter 1, morpholine (C 4 H 9 N, 1-oxa-4-aza-cyclohexane) is an excellent representative candidate to study oxygenated and nitrogen-containing biofuel because of its unique structure and wide industrial applications [5 8]. The morpholine combustion experimental database needs to be augmented, and we are aware of no shock tube data that have been published for morpholine ignition delay times before this study. In this chapter, morpholine ignition delay times measured behind reflected shock waves were provided. A morpholine combustion mechanism, developed based on a previous morpholine flame study [8] and the current shock tube data, was used for comparison with the experiment Experimental Details Morpholine ignition delay times in morpholine/air and morpholine/oxygen/argon mixtures were measured behind reflected shock waves using the Stanford high-pressure shock tube (HPST). More details about this shock tube have been introduced in section Prior to each experiment, morpholine mixtures were manometrically prepared in a 12.8 L, magnetically stirred stainless steel mixing tank. To avoid condensation of the fuel, the mixing tank and mixing assembly were heated to approximately 86 C. Liquid morpholine was added into the mixing tank using a gas-tight syringe. A sufficient time 13

40 Pressure [A. U.] period (about 15 min) was provided to ensure the full evaporation of fuel liquid inside the tank; then the oxidizer and bath gas were added. The test mixtures were stirred using a magnetically driven vane assembly for at least 1 hour before actual shock tube experiments. At 86 C the vapor pressure of morpholine is around 25 kpa [38], while the partial pressures of morpholine inside the tank, mixing assembly, and the shock tube driven section never went above 3 kpa during the experiments, so that morpholine remained in the vapor phase throughout the experimental process. The infrared diagnostic of fuel at 3.39 mm, together with FTIR measurements of morpholine absorption cross section as described in section 2.2.2, was used to confirm the initial morpholine concentration in the shock tube test section. Ignition delay times were determined by extrapolating, back to the baseline pressure, the steep increase in pressure concurrent with ignition. A sample pressure trace for ignition delay time determination can be found in Figure 3.1. Shock tube driver inserts were used for all the experiments to reduce the non-ideal pressure variation caused by viscous effects, and to achieve test time with small pressure variation behind the reflected shock waves [30] Morpholine/Air 925 K, 15.8 atm = ign = 1070 s Time [ s] 14

41 Figure 3.1. Sample pressure trace for morpholine ignition delay time measurements Results and Discussion Morpholine Ignition Delay Times Morpholine ignition delay times were measured during morpholine oxidation experiments under different conditions behind the reflected shock waves and are shown in Figure and listed in Table 3.1. Table 3.1. Shock tube ignition delay times. Test gas mixture: morpholine/ 2 /argon. T /T 5 P 5 Φ X 2 IDT [K] [1/K] [atm] [µs]

42 A shock tube can reproduce close, but not identical, pressures from shock experiment to shock experiment. For a uniform graphic presentation of the results, a pressure scaling of all the data in a similar pressure regime is needed. Many previous studies have observed that ignition delay times have pressure dependence close to P -1. Since the actual pressures are close to the reported pressure for one set of data on an ignition delay time plot, this simple power law dependence is used for Figure A more accurate pressure dependence will be established based on regression analysis of the ignition delay time data over the entire pressure range of the current study. In Figure 3.2, ignition delay times in morpholine/air mixtures at pressures near 15 atm are shown for different equivalence ratios. Synthetic air with 21% 2 and 79% N 2 was used. The stoichiometric case was defined with the following reaction: C 4 H 9 N ( N 2 ) = 4C H N 2 (Eq 3.1) As can be seen from Figure 3.2, in morpholine/air mixtures, when other conditions are held the same, auto-ignition occurs faster with increasing equivalence ratio. The data are characterized by small scatter, and within the current temperature range, ignition delay times vary monotonically with temperature. In Figure 3.2, the slopes of the ignition delay time data are similar at different equivalence ratios, thus the 16

43 Ignition Delay Time [ s] activation energy of morpholine/air ignition is not sensitive to equivalence ratio. At the current relatively low-temperature and high-pressure conditions, fuel-rich mixtures are fastest to ignite due to the major chain-branching reactions emanating from the fuel. 1111K 1000K 909K Morpholine/Air Scaled to 15 atm with P = 0.5 = 1 = /T [K -1 ] Figure 3.2. Ignition delay time measurements in morpholine/air mixtures near 15 atm and with equivalence ratios of 0.5, 1 and 2. Dashed lines: linear fit to data. The ignition delay times in morpholine/air mixtures are shown in Figure 3.3 for an equivalence ratio of 1, and pressures around 15 and 25 atm. The ignition delay times near 25 atm are shorter than those near 15 atm, as expected. 17

44 Ignition Delay Time [ s] K 1000K 909K Morpholine/Air = 1 Scaled with P P = 15 atm P = 25 atm /T [K -1 ] Figure 3.3. Ignition delay time measurements in morpholine/air Φ = 1 mixtures near 15 and 25 atm. Dashed lines: linear fit to data. To study the effects of oxidizer concentration, ignition delay times were measured in stoichiometric morpholine/4% 2 /argon mixtures as well, and compared with morpholine/air mixtures in Figure 3.4. The ignition delay time clearly decreases with an increasing oxygen concentration, due in part to the decreasing dilution. 18

45 Ignition Delay Time [ s] 1250K 1111K 1000K 909K Morpholine/4% 2 /Ar Morpholine/Air = 1 Scaled to 15 atm with P /T [K -1 ] Figure 3.4. Ignition delay time measurements in stoichiometric morpholine/air mixtures and morpholine/4% 2 /argon mixtures around 15 atm. Dashed lines: linear fit to data. A regression analysis was carried out based on all the experimental data reported in this section, and the following scaling relation was found for morpholine ignition delay time: τ = Φ -0.8 P -0.9 X exp(13400/t) [ s] (Eq 3.2) over the temperature range of K, pressures atm and equivalence ratios Model Development and Simulations A mechanism for morpholine flame chemistry has been previously presented, constructed using simple analogies with cyclohexane combustion [8]; however, this mechanism was tested only against low-pressure flat-flame data, and the conditions of shock tube oxidation were not considered. A new mechanism was thus proposed for morpholine oxidation (see Appendix A), based on a previous cyclohexane oxidation 19

46 study [39] (see Appendix A) and the current data. Additionally, the H/C/ chemistry was updated to reflect recent works on acetylene [40] and tetrahydropyran (THP) [41]. Improvements were also made to the base nitrogen-chemistry set, with rate constants drawn from several sources [25,28,42 47]. Rate coefficients for morpholine will be different from those for cyclohexane or other 6-membered ring species, but the transition-state structures have useful similarities. There are six saturated heavy atoms (C, N, ) in the morpholine ring, so it is proposed that, similar to cyclohexane, morpholine oxidation in the shock tube occurs by 2 addition to a radical site. The key difference is that while each hydrogen on a cyclohexane ring is symmetrically equivalent, morpholine has three different sites from which an H-atom may be abstracted: the carbon ortho to the ether oxygen, the meta carbon, and the para amine nitrogen. Thus, three distinct hydrogen-abstraction routes exist. nce an R 2 morpholine species is formed, as in cyclohexane, the 2 group can internally abstract a hydrogen atom from the ring and either form H 2 + an unsaturated morpholine-ene cyclic species or one of several morpholine QH species. The resulting QH radicals can then undergo β-scission by ring-breaking, forming linear and branched, unsaturated radicals. The linear and branched radicals can undergo further β-scission reactions until small products with 2 to 3 heavy atoms are formed. Reaction rate coefficients for the oxidation reactions for morpholine were derived based on analogous reactions from the cyclohexane [39] model for oxidation. Accounting for the thermal decomposition of the ring requires assumptions about the product channels and rate constants. There are no previous morpholine pyrolysis data in the literature, and the products and rate constants are not settled in the literature for morpholine or similar ring species that might provide analogies. However, for cyclohexane, Tsang [48] detected only 1-hexene as an experimental product. In this work, the ring decomposition was assumed to proceed analogously to the mechanisms proposed for 1,4-dioxane [49] and cyclohexane [50], modifying Arrhenius pre-exponential factors for the proper reaction-path degeneracy (RPD). Three 20

47 decomposition mechanisms were drawn from theoretical studies: 1) homolytic cleavage of the ring into three 2-heavy-atom species, 2) opening of the ring into a diradical species which can then internally abstract a hydrogen in a 6-centered transition state, then decomposing into two 3-heavy-atom radicals, and 3) 1,4-hydrogen shifts, transforming the ring into 6-heavy-atom linear species with a single π bond at the end, similar to the 1-hexene from cyclohexane observed by Tsang. Rate constants from Altarawneh and Dlugogorski [51] were also tested where possible. They had applied G3MP2B3 and RRKM calculations to investigate these pathways, concluding that the fastest channel in the present temperature range was to CH 2 =CH--CH 2 -CH 2 -NH. Their rate constants could not all be used, as their second fastest channel was homolytic scission of the C-N bond to an unphysical product, CH 2 --CH 2 -CH 2 -CH 2 -N. Thermochemistry for the 28 species introduced in [8], as well as five further species from the new pyrolysis mechanism and 41 additional species from the oxidation mechanism, were calculated theoretically using the complete-basis-set method CBS- QB3 [52] (see Appendix C). Geometry and frequency calculations were completed with Gaussian09 software [53] using the tight convergence criteria and rigid-rotor/harmonicoscillator approximations. Thermochemistry was estimated for an additional nine radical species for the oxidation set based on their non-radical analogues. The resulting mechanism is 290 species and 2130 reactions. This mechanism was also published in [54], with the morpholine oxidation reaction set, pyrolysis reaction set and thermochemistry included in Appendix A-C. This mechanism will be referred to as the morpholine mechanism in the following sections, with reference to [54]. Shock tube simulations were performed using a closed homogenous reactor model using CHEMKIN Pro [55] assuming constant volume and constant internal energy conditions. The predictions, using the current morpholine mechanism developed based on ignition delay time data, for the morpholine oxidation system (dashed lines) are compared to the shock tube data in Figure As can be seen in Figure 3.5, for the equivalence ratio dependence of the ignition delay time, the newly proposed morpholine mechanism captures the same trend as the experiment. Also shown in the figure are the 21

48 Ignition Delay Time [ s] simulation results using the mechanism of [8], represented by dash lines. It is evident that the current model matches much better with the experimental data than the model from [8] because of the addition of the 2 addition chemistry. The current mechanism is quite good at the equivalence ratio 0.5, but overpredicts the ignition delay times as richness increases. 1250K 1111K 1000K 909K 833K Morpholine/Air Scaled to 15 atm with P = 0.5 = 1 = /T [K -1 ] Figure 3.5. Comparison of model predictions to morpholine/air ignition delay time measurements around 15 atm and under different equivalence ratios. Dashed lines: Simulation results using the mechanism in [8]. Solid lines: Simulation results using the morpholine mechanism [54]. A sensitivity analysis for morpholine concentration was performed for the stoichiometric case at P=15 atm. At lower temperatures (~800K), the model has a heightened sensitivity to morpholine unimolecular decomposition to three-heavy-atom products. However, as temperature increases (1000K and higher), the unimolecular decomposition reactions become less important. Instead, the model predictions for morpholine become more sensitive to radical chemistry, especially to the orthomorphyl CH 2 CH 2 NHCH 2 CH and the metamorphyl CH 2 CH 2 CH 2 CHNH beta-scission reactions. 22

49 Ignition Delay Time [ s] Pressure effects are shown in Figure 3.6 at 15 and 25 atm, using both the current mechanism (solid lines) and the mechanism published in [8] (dashed lines). Significant improvement is evident in the modeling results using the morpholine mechanism [54]. The influence of oxidizer concentration on ignition delay time can be seen in Figure 3.7. The mechanism used in [8] overpredicts ignition delay times by a factor of 5, whereas the new model performs well. It overpredicts the ignition delay times only by about 50% in stoichiometric morpholine mixtures, using air (21% 2 in N 2 ) or 4% 2 in Ar K 1111K 1000K 909K 833K Morpholine/Air = 1 Scaled with P P = 15 atm P = 25 atm /T [K -1 ] Figure 3.6. Comparison of model predictions to morpholine/air ignition delay time measurements for stoichiometric mixtures around 15 and 25 atm respectively. Dashed lines: Simulation results using the previous mechanism [8]. Solid lines: Simulation results using the morpholine mechanism [54]. 23

50 Ignition Delay Time [ s] 1250K 1111K 1000K 909K 833K Morpholine/4% 2 /Ar Morpholine/Air = 1 Scaled to 15 atm with P /T [K -1 ] Figure 3.7. Comparison of model predictions to morpholine ignition delay time measurements around 15 atm for morpholine/4% 2 /argon and morpholine/air mixtures. Dashed lines: Simulation results using the previous mechanism [8]. Solid lines: Simulation results using the morpholine mechanism [54] Summary Morpholine ignition delay times were measured in the Stanford high-pressure shock tube, covering temperatures from 866 to 1197 K, equivalence ratios of 0.5, 1 and 2, two pressure groups near 15 and 25 atm, and two oxygen concentration values of 4% 2 in Ar and synthetic air with 21% 2. The current shock tube work extends the morpholine combustion experimental database and a new morpholine mechanism was generated using the current data. The morpholine mechanism developed for low-pressure flames in [8] significantly over-predicts the ignition delay times under all conditions. The simulations, using the morpholine mechanism proposed in this chapter, are much closer matches with the morpholine ignition delay times than those from the previous 24

51 mechanism developed in [8], and can successfully capture the equivalence ratio dependence near 15 atm. Combustion of morpholine as a 6-membered cyclic amine may start with ring opening and pyrolysis process to form smaller aliphatic amine compounds, in particular, dimethylamine and ethylamine radicals. Further refinement of the morpholine mechanism requires improvements in the sub-mechanisms of dimethylamine and ethylamine. In the following chapters, shock tube studies of dimethylamine and ethylamine combustion are presented, to improve understanding of the reaction kinetics of those two important aliphatic amines, and also to improve the morpholine mechanism. 25

52

53 Chapter 4. Ethylamine Reactions of H with Dimethylamine and 4.1. Introduction The efforts to update the dimethylamine and ethylamine sub-mechanisms of the morpholine mechanism begin with the direct reaction rate measurements of H with dimethylamine and ethylamine. The reactions of aliphatic amines are relevant to both atmospheric chemistry and biofuel combustion processes. In the context of atmospheric chemistry, aliphatic amines are potential precursors of HCN and stratospheric N x [56 58]. Additionally, the degradation of dimethylamine within the environment can lead to carcinogenic nitrosamines [59]. In the context of combustion, the amine group is a common functional group in bio-derived fuels [5 7,25,54]. The hydrogen abstraction reactions by H radical are important steps in the combustion of amines, thus dimethylamine and ethylamine + H reactions are of great research value. Experimental and computational studies of the reactions between aliphatic amines and H are scarce. Atkinson et al. investigated the reactions of methylamine (MA: CH 3 NH 2, CAS: ) with H over the temperature range of K using a flash photolysis-resonance fluorescence technique [13]. The same method was used to measure the rate constants for the reactions of dimethylamine, ethylamine and trimethylamine with H over the temperature range of K [14]. Carl et al. studied the reaction rate constants of aliphatic amines with H at 295 K, including those for methylamine, dimethylamine, ethylamine and trimethylamine, using the sequential two-photon dissociation of N 2 in the presence of H 2 as a source of H [15]. Galano and Alvarez-Idaboy analyzed the different reaction channels of methylamine, 27

54 dimethylamine and ethylamine with H, using variational transition-state theory [16]. Geometry optimization and frequency calculations were performed at the BH&HLYP/ G(2d,2p) level of theory, with electronic energy values improved by single-point calculations at the CCSD(T) level of theory and using the same basis set. The overall reaction rate constants and the branching ratios for reactions of amines with H were reported within the temperature range K [16]. Recently, Lucassen et al. studied the laminar premixed flames of dimethylamine and ethylamine under one-dimensional low-pressure conditions [25]. A detailed combustion model was developed to analyze the major pathways in those two flames, which successfully reproduced many trends observed in the flame experiments. Lucassen et al. estimated the reaction rates for amine + H reactions based on previous work in the literature. To the best of the author s knowledge, there is no direct experimental or theoretical study of the reactions of aliphatic amines with H under combustion conditions. The present work determines the reaction rate constants for the overall reactions of dimethylamine (DMA: CH 3 NHCH 3, CAS: ) and ethylamine (EA: CH 3 CH 2 NH 2, CAS: ) with H. DMA + H = Products EA + H = Products (R4.1) (R4.2) The H radical was generated by near-instantaneous pyrolysis of tert-butyl hydroperoxide (TBHP, CAS: ). The pseudo-first order decay of H behind reflected shock waves was monitored using laser absorption at nm, and the reaction rate constants of amine with H were inferred from the measured H timehistories. Variational transition state theory was used to compute the H-abstraction rates by H for dimethylamine and ethylamine, with potential energy surface geometries, frequencies and electronic energies calculated by Galano and Alvarez-Idaboy at CCSD(T)/ G(2d,2p) level of theory [16]. 28

55 4.2. Experimental Setup The Stanford Kinetic Shock Tube as described in section was used for the dimethylamine and ethylamine + H experiments, with the H decay time-histories monitored using the H diagnostic presented in section The chemicals used in the experiments include 97% ethylamine, anhydrous 99% dimethylamine, and a solution of 70%, by weight, tert-butyl hydroperoxide (TBHP) in water, all supplied by Sigma- Aldrich with no further purification. Research grade argon (99.99%) supplied by Praxair was employed as the bath gas. All the mixtures were prepared manometrically using a double-dilution method in a 12 liter electro-polished stainless steel tank, and mixed with a magnetically driven stirring vane for at least one hour prior to the experiments. Before each experiment, the shock tube was passivated to avoid loss of amine to the shock tube wall. Since each TBHP decomposes near-instantaneously to form one H, the TBHP concentrations were determined based on the peak H value for each experiment. Before the experiment, controlled mixtures of fuel diluted in argon were made; the amine concentrations were then confirmed by sampling a portion of the mixture, after filling into the shock tube, from near the endwall to an external multipass cell with 29.9 m pathlength. The fuel concentration in the multipass cell was measured using a Jodon helium-neon laser at 3.39 μm, and Beer s law was used to convert the measured absorption data to the fuel mole fraction. Further details about this multipass cell laser diagnostic of fuel are reported in [33,60]. The absorption cross sections of dimethylamine and ethylamine from the PNNL database [61] were used in the Beer s law concentration calculation, and the measured fuel concentrations were consistent with the manometric values to within ±5%, which gives confidence in the manometric values. The manometric fuel concentrations were used for comparisons with simulations. 29

56 4.3. Kinetic Measurements Experiments were performed behind reflected shock waves over the temperature range of K and pressures near 1.2 atm. At temperatures greater than 1000 K, TBHP dissociates near-instantaneously to form an H radical and a tert-butoxy radical. The tert-butoxy radical, (CH 3 ) 3 C, further dissociates into acetone and a methyl radical. TBHP also reacts with H radical to form other products. The TBHP chemistry set can be described as follows: TBHP = (CH 3 ) 3 C + H (CH 3 ) 3 C = CH 3 CCH 3 + CH 3 TBHP + H = H tert-c 4 H 9 TBHP + H = H 2 + H 2 + iso-c 4 H 8 (R4.3) (R4.4) (R4.5) (R4.6). Further details about the TBHP chemistry set can be found in the literature [33,60,62 65]. The rate constants for Reaction 4.3, 4.5, and 4.6 were adopted from Pang et al. [33], and the reaction rate constant for Reaction 4.4 was obtained from Choo and Benson [66]. The thermodynamic parameters for TBHP and tert-butoxy radical were taken from the thermodynamic database by Goos et al. [67]. Methyl radical is formed in Reaction 4.4 and previous works [33,62] have shown that the accuracy of the CH 3 + H rate constant around 1.5 atm is important for accurate determination of the fuel + H reaction rate constant. There are two major channels for CH 3 + H, CH 3 +H + M = CH 3 H + M CH 3 + H = CH 2 (s) + H 2 (R4.7) (R4.8). Reaction 4.7 was updated using the results from Srinivasan et al. [68] at ~ atm, and their values agree well with the theoretical study by Jasper et al. [69] and the 30

57 measured values from Vasudevan et al. at 1.3 atm [62]. The rate constant for Reaction 4.8 was updated with the value measured by Pang et al. [33], which agrees well with the values by Srinivasan et al. [68] and Vasudevan et al. [62]. Sangwan et al. recently measured the reaction rate for Reaction 4.8 over the temperature range of K [70]. The rate constant for Reaction 4.8 by Pang et al. also agrees with extrapolation of the Sangwan et al. results, within the uncertainty limit used for error analysis in the following sections. The rate constants for reactions are provided in Table 4.1. The same set of reaction rate constants for TBHP chemistry has been used before by Lam et al. [65,71] Table 4.1. Reactions describing DMA and EA + H experiments # Reaction Rate Constant [cm 3 mol -1 s -1 ] A n E Ref. 4.1 DMA + H = Products See text This work 4.2 EA + H = Products See text This work 4.3 TBHP = (CH 3 ) 3 C + H [33] (CH 3 ) 3 C = CH 3 CCH 3 + CH [66] 4.5 TBHP + H = H tert-c 4 H [33] 4.6 TBHP + H = H 2 + H 2 + iso-c 4 H [33] 4.7 CH 3 + H + M = CH 3 H + M [68] 4.8 CH 3 + H = CH 2 (s) + H [33] 1 Rate coefficient units for 1 st order reactions: s -1 The above TBHP chemistry set was implemented into the base dimethylamine and ethylamine sub-mechanisms of the morpholine mechanism. The dimethylamine and ethylamine sub-mechanisms were originally developed by Lucassen et al. [25]. The CHEMKIN PR [55] package was used to simulate the H time-histories, with the standard constant energy and volume assumptions Dimethylamine (DMA) + H The reaction of dimethylamine with H consists of two different channels: DMA + H = CH 3 NHCH 2 + H 2 31 (R4.1a)

58 DMA + H = CH 3 NCH 3 + H 2 (R4.1b) In the dimethylamine sub-mechanism by Lucassen et al. [25], the total rate constant of DMA with H at 295 K measured by Carl et al. [15], and the branching ratio by Galano and Alvarez-Idaboy at 295 K [16] was used for all temperatures, with k 4.1a = cm 3 mol -1 s -1 and k 4.1b = cm 3 mol -1 s -1. A H sensitivity analysis was carried out for the overall rate constant determination of Reaction 4.1 (k 4.1 = k 4.1a + k 4.1b ) in the mixture of 320 ppm DMA with 22 ppm TBHP (and 140 ppm water) diluted in argon, at 1176 K and 0.9 atm. The H sensitivity is defined as S H = ( X H / k i ) (k i /X H ), where X H is the local H mole fraction and k i is the rate constant for reaction i. As illustrated in Figure 4.1, the sensitivity analysis shows that Reaction 4.1 is the dominant reaction with minor interferences from secondary reactions. The measured H time-history under the same conditions is shown in Figure 4.2. The mechanism with the TBHP chemistry set in Table 4.1 was used to simulate the experimental data, and a best-fit overall rate constant of k 4.1 = cm 3 mol -1 s -1 was obtained between the experiment and the simulation. Also shown in Figure 4.2 are the simulations for the perturbations of ±50% in the bestfit overall rate constant. Note in this figure that non-kinetic effects, i.e. laser beam steering by the shock passage, contribute to the measured absorption profiles at times before t=0. The branching ratios for Reaction 4.1 in the mechanism were kept the same in the simulations. It is worth noting that the presence of H 2 in the test mixture has no noticeable influence on the simulated H profiles. 32

59 H [ppm] H Sensitivity ppm DMA / Ar 22 ppm TBHP / 140 ppm H K, 0.9 atm DMA + H = Products TBHP = tert-butoxy + H CH 3 +H=CH 2 (S)+H 2 CH 3 +CH 2 =C 2 H 4 +H CH 3 NHCH 2 =CH 3 +CH 2 NH Time [ s] Figure 4.1. Sensitivity analysis of H using the dimethylamine sub-mechanism [25] with TBHP chemistry set, in the mixture of 320 ppm DMA/Ar with 22 ppm TBHP and 140 ppm H 2, at 1176 K and 0.9 atm ppm DMA/ Ar 22 ppm TBHP/ 134 ppm H K, 0.9 atm Experiment k 4.1 = 3.2E13 cm 3 mol -1 s -1 (best-fit) 2 k k Time [ s] Figure 4.2. Sample H trace in 320 ppm DMA/Ar with 22 ppm TBHP and 134 ppm H 2, at 1176 K and 0.9 atm. Under the current pseudo first-order conditions, H decays exponentially and close to be a straight line in the semi-log plot of Figure

60 Figure 4.3. Error analysis for measured k 4.1 in 320 ppm DMA/Ar with 22 ppm TBHP and 134 ppm H 2, at 1176 K and 0.9 atm. A detailed error analysis was conducted to evaluate the overall uncertainty of the measured rate constant for Reaction 4.1 in the mixture of 320 ppm dimethylamine with 22 ppm TBHP and 140 ppm water in argon at 1176 K and 0.9 atm. The primary sources of uncertainty for the rate constant determination include 2 uncertainties in (a) pressure (±1%), (b) temperature (±1%), (c) mixture composition (±5%), (d) H cross section(±3%), (e) fitting data (±7%), (f) the rate constant for TBHP = tert-butoxy + H (Reaction 4.3, ±30%), (g) the rate constant for CH 3 + H = CH 2 (s) + H 2 (Reaction 4.8, uncertainty factor used: 2), (h) the rate constant for CH 3 + CH 2 = C 2 H 4 + H (uncertainty factor used: 2), (i) the rate constant for CH 3 NHCH 2 = CH 3 + CH 2 NH (uncertainty factor used: 2). Figure 4.3 presents the contributions from each source of uncertainty, which were obtained by perturbing each uncertainty source to its error limits and refitting an overall rate constant for Reaction 4.1. The uncertainty in Reaction 4.8 is the major contributor to the measured rate constant k 4.1, and no significant influence on k 4.1 determination was observed due to the uncertainties in other secondary 34

61 reactions. All the uncertainties were assumed to be uncorrelated and combined in a rootsum-squared method to yield a total uncertainty of ±26% in the rate constant k 4.1 at 1176 K. Similar tests were carried out for the reaction of DMA with H, over the temperature range of K, and pressures of atm, with the measured overall rate constants summarized in Table 4.2. Different initial dimethylamine concentrations were implemented to confirm the pseudo-first order kinetics in H. Table 4.2. Measured rate constants for DMA + H = Products T P k 4.1 x [K] [atm] [cm 3 mol -1 s -1 ] 60 ppm TBHP, 440 ppm DMA, Ar ppm TBHP, 320 ppm DMA, Ar ppm TBHP, 570 ppm DMA, Ar

62 k 4.1 [cm 3 mol -1 s -1 ] Figure 4.4 presents the Arrhenius plot for the overall rate constant k 4.1 over the temperature range of K, and pressures of atm, together with the estimation by Lucassen et al. [25]. Lucassen et al. assumed a constant reaction rate, and this value overpredicts the measured reaction rate constant. Similar error analyses as in Figure 4.3 were carried out for 925 K and 1307 K, and the uncertainties were estimated to be ±29% and ±21% respectively. ver the temperature range studied, the measured values can be fitted with k DMA+H = T 1.93 exp(1450/t) cm 3 mol -1 s -1. 5x x K 1250 K 830 K DMA + H = Products 3x x10 13 Experiment Fit to Data Lucassen et al. (2012) /T [K -1 ] Figure 4.4. Measured overall reaction rate for k 4.1 : DMA+H = Products, in comparison with the estimation by Lucassen et al. [25] Ethylamine (EA) + H The reaction of ethylamine with H consists of three different channels: EA + H = CH 3 CH 2 NH + H 2 (R4.2a) 36

63 EA + H = CH 3 CHNH 2 + H 2 EA + H = CH 2 CH 2 NH 2 + H 2 (R4.2b) (R4.2c) In the ethylamine sub-mechanism by Lucassen et al. [25], the reaction rate constant for Reaction 4.2a was estimated by analogy to methylamine + H, with k 4.2a = T 2 exp(-447[cal/mol]/rt) cm 3 mol -1 s -1 from Dean and Bozzelli [72]. The theoretical study by Galano and Alvarez-Idaboy shows that channel 4.2b contributes ~98% of the total ethylamine + H rate within the temperature range K [16], and was assumed to be the dominant channel by Lucassen et al. [25]. Thus, the overall reaction rate constant measured by Carl et al. at 295 K [15] was used by Lucassen et al. for this channel with k 4.2b = cm 3 mol -1 s -1. For Reaction 4.2c, Lucassen et al. estimated the reaction rate constant to be k 4.2c = exp(-1300[cal/mol]/rt) cm 3 mol -1 s -1, by analogy to CH 3 CH 2 H + H [73]. A sensitivity analysis for H, using the ethylamine sub-mechanism with the TBHP chemistry set as in Table 4.1, in the mixture of 470 ppm ethylamine with 50 ppm TBHP and 140 ppm water in argon at 1067 K and 0.83 atm, is shown in Figure 4.5. As can be seen in Figure 4.5, the fast formation of H is controlled by TBHP decomposition, Reaction 4.3, and the ethylamine + H reaction dominants the H removal process. Reaction 4.8 is the most important secondary reaction that affects H time-histories at the later times, with smaller contributions from H + H 2 = H + H 2 CH 3 H + H = CH 2 H + H 2 (R4.9) (R4.10). The effects of the secondary reactions are included in the error analysis, and no modification to Reaction 4.9 and 4.10 in the ethylamine sub-mechanism was made. 37

64 H Sensitivity ppm EA / Ar 50 ppm TBHP / 190 ppm H K, 0.83 atm EA + H = Products TBHP = tert-butoxy + H CH 3 + H = CH 2 (S)+H2 H + H 2 = H + H 2 CH 3 H + H = CH 2 H + H Time [ s] Figure 4.5. Sensitivity analysis of H using the ethylamine sub-mechanism [25] with inclusion of TBHP chemistry, in a mixture of 470 ppm EA,50 ppm TBHP, 190 ppm H 2, in Ar at 1067 K and 0.83 atm. Illustrated in Figure 4.6 is a sample measured H time-history in the mixture of 470 ppm ethylamine with 50 ppm TBHP and 140 ppm water in argon at 1067 K and 0.83 atm. Under the current pseudo first-order conditions, the H removal process is close to an exponential decay and nearly linear on a semi-log plot. The ethylamine submechanism with the TBHP chemistry set was used to simulate the experimental data, and a best-fit overall rate constant for Reaction 4.2 of cm 3 mol -1 s -1 was obtained between the experiment and the simulation, as is presented in Figure 4.6. Also shown in Figure 4.6 are the simulations for the perturbations of ±50% in the best-fit overall rate constant. No discernible effect, due to the branching ratio of Reaction 4.2, on the overall reaction rate determination was observed, and the branching ratios proposed by Lucassen et al. were kept in the simulations. 38

65 H [ppm] ppm EA / Ar 50ppm TBHP/ 190ppm H K, 0.83 atm Experiment k 4.2 = 1.8E13 cm 3 mol -1 s -1 (best-fit) k k Time [ s] Figure 4.6. Sample H trace in in a mixture of 470 ppm EA, 50 ppm TBHP, 190 ppm H 2, in Ar at 1067 K and 0.83 atm. A detailed error analysis was conducted to evaluate the overall uncertainty of the measured rate constant for Reaction 4.2 in the mixture of 470 ppm ethylamine with 50 ppm TBHP and 140 ppm water in argon at 1067 K and 0.83 atm. The primary sources of uncertainty for the rate constant determination include 2 uncertainties in (a) pressure (±1%), (b) temperature (±1%), (c) mixture composition (±5%), (d) H absorption cross section (±3%), (e) fitting data (±5%), (f) the rate constant for TBHP = tert-butoxy + H (Reaction 4.3, ±30%), (g) the rate constant for CH 3 + H = CH 2 (s) + H 2 (Reaction 4.8, uncertainty factor used: 2), (h) the rate constant for H + H 2 = H + H 2 (Reaction 4.9, ±25%), (i) the rate constant for CH 3 H + H = CH 2 H + H 2 (Reaction 4.10, ±50%). Figure 4.7 shows the contributions from each source of uncertainty, which were obtained by perturbing each uncertainty source to its error limits and refitting an overall rate constant for Reaction 4.2. The uncertainty in Reaction 4.8 is again the major contributor to the measured rate constant k 4.2, and no noticeable influence on k 4.2 determination due to the known uncertainties in Reaction 4.3, 4.9 and 4.10 was observed. All the uncertainties were assumed to be uncorrelated and combined 39

66 in a root-sum-squared method to yield a total uncertainty of ±26% for the rate constant k 4.2 at 1067 K. Figure 4.7. Error analysis for measured k 4.2 in 470 ppm EA/Ar with 50 ppm TBHP and 190 ppm H 2, at 1067 K and 0.83 atm. Similar experiments were carried out over the temperature range of K, and pressures of atm, with the measured overall rate constant summarized in Table 4.3. Different initial ethylamine concentrations were implemented to confirm the pseudo-first order kinetics in H. Table 4.3. Measured rate constants for EA + H = Products T P k 4.2 x [K] [atm] [cm 3 mol -1 s -1 ] 50 ppm TBHP, 470 ppm EA, Ar

67 ppm TBHP, 670 ppm EA, Ar Figure 4.8 presents an Arrhenius plot for the overall rate constant k 4.2 over the temperature range of K, and pressures of atm. Similar error analyses as in Figure 4.7 were carried out for 901 K and 1368 K, and the uncertainties were estimated to be ±31% and ±22% respectively. ver the temperature range studied, the measured values can be fitted with k EA+H = T 2.69 exp(1800/t) cm 3 mol -1 s -1. Also shown in Figure 4.8 is the estimated overall rate constant by Lucassen et al., which underpredicts the observed activation energy. Based on extrapolation of the current study to higher temperatures, the reaction rates for EA + H = products used in Lucassen et al. might be lower than the actual values at flame temperatures. 41

68 k 4.2 [cm 3 mol -1 s -1 ] 4x x x K 1000 K EA + H = Products 830 K 2.5x x x Experiment Fit to Data Lucassen et al. (2012) /T [K -1 ] Figure 4.8. Measured overall reaction rates for EA + H = Products, in comparison with the estimation by Lucassen et al. [25] Theoretical Study To the author's best knowledge, there are no theoretical studies for the reaction rates of dimethylamine and ethylamine + H at high temperature. Galano and Alvarez- Idaboy performed CCSD(T)/ G(2d,2p) single-point calculations along the internal reaction coordinates corresponding to various H-abstraction pathways for DMA and EA + H [16]. They initially explored the potential energy surface through DFT calculations at the BH&HLYP/ G(2d,2p) level of theory and then used refined CCSD(T) energies for both conventional and variational transition state theory (VTST) to compute H-abstraction rates by H for various channels in DMA and EA for the temperature range of K. In this work, analogous calculations were performed for the temperature range of K, with all geometries, frequencies, and electronic energies adopted from the work of Galano and Alvarez-Idaboy [16]. Eckart tunneling corrections were included, but are expected to be negligible at the temperatures of this work. Multiwell 2013 [74 76] was used for all calculations. For 42

69 DMA + H, internal degrees of freedom corresponding to CH 3 and H rotations were treated as internal hindered rotors. For EA + H, internal degrees of freedom corresponding to NH 2, CH 3, and H rotations were treated as internal hindered rotors. Because not all possible pathways were explored at the CCSD(T) level of theory, stereochemical and symmetry information required for calculation of reaction path degeneracy was not available for all reaction pathways. Thus, reaction path degeneracy was determined by multiplying the rate for a specific channel by the number of equivalent H-atoms. The electronic energies at 0 K, relative to corresponding reactants, and rotational data used in the VTST calculations can be found in Table 4.4. The transition states for DMA + H (R4.1) are referred to as TS_1a and TS_1b for two channels respectively, and the transition states for EA + H (R4.2) are referred to as TS_2a, TS_2b, and TS_2c for three channels. Table 4.4. Summary of the zero Kelvin electronic energies and rotational data used for dimethylamine and ethylamine + H VTST calculations external rotors b internal rotors e species E a (kcal/mol) inactive c active d V 0,1 sym V 0,2 sym V 0,3 sym DMA TS_1a TS_1b H EA TS_2a TS_2b TS_2c a Relative to reactants; all geometries, frequencies, and electronic energies were adopted from the work of Galano and Alvarez-Idaboy[16]. b Units of all rotational data are in cm -1. c two-dimensional (all symmetry number σ =1); d one-dimensional (all symmetry number σ =1). e Rotor barriers for CH 3 and NH 2 are estimated from Benson[77]; internal rotational barriers for H estimated as B = 420 cm -1 and the 43

70 potential calculated as V = v 2 /B [74], where v is the vibrational frequency associated with the H rotation B is its estimated rotational constant. The vibrational frequencies at the BH&HLYP/ G(2d,2p) level of theory, previously computed by Galano and Alvarez-Idaboy [16], are provided in Table 4.5. Table 4.5. The vibrational frequencies computed at the BH&HLYP/ G(2d,2p) level of theory Species vibrational frequencies [cm -1 ] computed at the B3LYP/ G(2d,2p) * DMA TS_1a 314i TS_1b 406i EA TS_2a 2041i TS_2b 425i TS_2c 1713i H 3891 * frequencies in italics are replaced as hindered internal rotors. Taken from Galano and Alvarez-Idaboy For the two abstraction channels in DMA + H treated by VTST, each corresponding transition state is geometrically fixed over the temperature range of K, corresponding to H H distances of Å (R4.1a) and Å (R4.1b). The same observation was made for the three abstraction channels of EA + H that were treated with VTST, each corresponding transition state is geometrically fixed over the temperature range of K, corresponding to H H distances of Å (R4.2a), Å (R4.2b) and Å (R4.2c). The resulting reaction rate constants using VTST are fitted using modified Arrhenius expressions and presented in Table

71 Table 4.6. VTST reaction rate constants of individual channels for DMA and EA + H. # Reaction Rate Constant [cm 3 mol -1 s -1 ] A n E 4.1a DMA + H = CH 3 NHCH 2 + H E b DMA + H = CH 3 NCH 3 + H E a EA + H = CH 3 CH 2 NH + H E b EA + H = CH 3 CHNH 2 + H E c EA + H = CH 2 CH 2 NH 2 + H E The author notes that it is more appropriate to compute the rates for these systems using modern comprehensive methods like multi-structural and multi-path VTST while accounting for torsional anharmonicity [78], or with direct trajectory simulations. The largest potential sources of errors, for the current theoretical study, lie in the fact that only one 1-D potential surface was employed per reaction channel, that internal rotor barrier heights are estimated or obtained from group additivity contributions, and that reaction path degeneracy cannot be rigorously defined, as has been outlined in previous work [79], without further ab initio calculations. Considering all this, the uncertainty in the computed rate constants is likely a factor of two or larger. However, there is evidently a fortuitous cancellation of errors for the VTST calculations performed here, as total rate constants k 4.1 and k 4.2 only need to be slightly changed in order to bring the predictions into good agreement with the experimental data. Compared in Figure 4.9 are the computed results with the experimental data, for the overall reaction rates of DMA and EA + H, with the calculated total rate constants k 4.1 and k 4.2 slightly changed (increase of 20% for k 4.1 and a decrease of 13% for k 4.2 ). Besides, the theoretical study can provide the branching ratios between different reaction channels, while the experimental values are the summations of all DMA+H and EA+H channels respectively. The calculated reaction rates for different channels, with the branching ratios unchanged and the total reaction rates scaled to the experimental values, are recommended for the dimethylamine and ethylamine + H reactions respectively. 45

72 k [cm 3 mol -1 s -1 ] 5x K 1250 K 1111 K 1000 K 909 K 4x x x10 13 EA + H = Products DMA + H = Products Lines: Current Theoretical Study /T [K] Figure 4.9. Comparison of the measured reaction rates and theoretical study results for DMA + H and EA + H Summary The overall rate constants of hydroxyl radicals (H) with dimethylamine and ethylamine were investigated using UV laser absorption of H near nm, behind reflected shock waves over the temperature range of 901 to 1368 K and at pressures near 1.2 atm. ver the temperature range studied, the measured values can be expressed as k DMA+H = T 1.93 exp(1450/t) cm 3 mol -1 s -1 and k EA+H = T 2.69 exp(1800/t) cm 3 mol -1 s -1. Detailed error analyses were performed to estimate the overall uncertainties of these reactions, and the estimated (2σ) uncertainties were found to be ±29% at 925 K and ±21% at 1307 K for k DMA+H, and ±31% at 901 K and ±22% at 1368 K for k EA+H. Variational transition state theory was used to compute the H-abstraction rates by H for dimethylamine and ethylamine, with potential energy surface geometries, frequencies and electronic energies calculated by Galano and Alvarez-Idaboy [16] at CCSD(T)/ G(2d,2p) level of theory. The calculated reaction rate constants are in good agreement with the experimental data. The research work described in this chapter was published in [80]. To the author s best knowledge, 46

73 the current work presents the first direct high-temperature measurements and theoretical study of the overall rate constants for dimethylamine and ethylamine with H. The resulting reaction rate constants in this chapter will be implemented into the morpholine mechanism [54] and will be used for further analysis in the following chapters. 47

74

75 Chapter 5. Dimethylamine xidation 5.1. Introduction The reaction mechanism developed for morpholine [54], as described in Chapter 3, contains the dimethylamine sub-mechanism by Lucassen et al. [8], with updated C/H//N mechanisms. The dimethylamine sub-mechanism of the morpholine mechanism was developed by Lucassen et al. for laminar premixed flames of dimethylamine under one-dimensional low-pressure conditions, and has not been validated with experiments using shock tubes. After evaluating the morpholine mechanism in Chapter 3 with overall combustion parameter morpholine ignition delay times, it is worthwhile to further refine the mechanism with dimethylamine oxidation data taken in a shock tube. In this chapter, dimethylamine (DMA: CH 3 NHCH 3 ) oxidation was investigated using shock tube/laser absorption methods. Ignition delay times and H time-histories were measured in dimethylamine/oxygen/argon mixtures. The morpholine mechanism developed in Chapter 3, was used to simulate the H time-histories and ignition delay times. The morpholine mechanism was then modified in this chapter by adding the branching channel of: DMA (+M) = CH 3 NH + CH 3 (+M) (R5.1), with rate constants estimated by analogy to the dimethyl ether decomposition [81], and the reaction rate constants for DMA + H were updated using the measured values in Chapter 4 [80]. With those modifications, the morpholine mechanism now agrees well with both the dimethylamine ignition delay times and H time-history data. 49

76 5.2. Experimental Setup The Stanford Kinetic Shock Tube as described in section was used for the dimethylamine oxidation study, and the H diagnostic as presented in section was implemented for H time-history measurements. Anhydrous dimethylamine ( 99%) supplied by Sigma-Aldrich was used with no further purification. Research grade argon (99.99%) supplied by Praxair was employed as the bath gas. All the mixtures were prepared manometrically using a double-dilution method in a 12 liter electro-polished stainless steel tank, and mixed with a magnetically driven stirring vane for at least one hour prior to the experiments. Before each experiment, the shock tube was passivated to avoid loss of amine to the shock tube wall. Following this procedure, it was found by direct laser absorption of the fuel at 3.39 µm that the fuel concentration inside the shock tube matches well with the manometric value [80], and hence the manometric mixture composition was used for comparison with the simulations Results and Discussion Dimethylamine Ignition Delay Times Dimethylamine ignition delay times were determined by extrapolating, back to the baseline pressure, the steep increase in pressure concurrent with ignition. A sample pressure trace for the ignition delay time determination can be found in Figure 5.1. Shock tube driver inserts were used to reduce pressure variations due to non-ideal effects. Close to constant pressure was achieved for non-reactive shock, as can be seen from Figure 5.1. Also shown in Figure 5.1 is a simulated pressure trace using CHEMKIN Pro and with the standard constant energy and volume assumptions. Preignition pressure rises were observed for both the experiment and the simulation for the 50

77 Pressure [atm] reactive case. The observed pressure variation for the experiment is caused by heat release during dimethylamine oxidation process, instead of gasdynamic effects. The current ignition delay time measurements covered the temperature range K, with pressures near 0.9, 1.5 and 2.8 atm, and equivalence ratios of 0.5, 1 and 2 in 4% oxygen/argon. The stoichiometric case was defined as: CH 3 NHCH = 2C H N 2 (Eq 5.1) 1.5 DMA/4% 2 /Ar, = K, 0.93 atm 1.0 Simulation Reactive Non-reactive 0.5 ign = 1314 s Time [ s] Figure 5.1. Sample pressure traces for dimethylamine ignition delay times. A shock tube can reproduce close, but not identical, pressures from shock experiment to shock experiment. For a uniform graphic presentation of the results, a pressure scaling of P -1 for the data in a close pressure regime is implemented. Since the actual pressures are close to the reported pressure for one set of data on an ignition delay time plot, this simple power law dependence was used for Figure and More accurate pressure dependence will be studied based on regression analysis of the ignition delay times over the entire pressure range of the current study and shown in later section. Figure 5.2 shows the measured dimethylamine ignition delay times at three nominal pressures, 0.9, 1.5 and 2.8 atm, along with the simulation results using the 51

78 Ignition Delay Time [ s] morpholine mechanism described in Chapter 3 [54]. The CHEMKIN PR package [55] was used to simulate the dimethylamine ignition delay times and H time-histories, under the standard constant energy and volume assumptions. As can be seen in Figure 5.1, nonideal effects inside the shock tube have been successfully eliminated [30,82], and the pressure trace for nonreactive experiment has no significant variation behind reflected shock; thus the standard constant energy and volume assumptions are wellgrounded. As expected, the ignition delay times are shorter at higher pressures. The morpholine mechanism [54] is seen to overpredict the dimethylamine ignition delay times at all pressures. Analysis and modification to the morpholine mechanism will be carried out in the following sections K 1250 K 1111 K DMA/4% 2 /Ar, = 1 Dashed Lines: Morpholine Mech. 0.9 atm 1.5 atm 2.8 atm Data points: Current data P ~ 0.9 atm P ~ 1.5 atm P ~ 2.8 atm /T [K -1 ] Figure 5.2. Ignition delay times in stoichiometric DMA/4% 2 /argon mixtures, with P ~ 0.9, 1.5 and 2.8 atm, measurements and simulation results using the morpholine mechanism described in Chapter 3 [54]. Similar dimethylamine ignition delay time measurements were carried out in mixtures with different equivalence ratios. Figure 5.3 shows the measured ignition delay times at equivalence ratios of 0.5, 1 and 2, along with the simulations from the morpholine mechanism [54]. Similar to Figure 5.2, the morpholine mechanism is seen 52

79 Ignition Delay Time [ s] to overpredict dimethylamine ignition delay times. Both the mechanism and the experimental data show that, in 4% oxygen, fuel lean mixtures of DMA are more reactive and have shorter ignition delay times K 1250 K DMA/ 4% 2 /Ar P ~ 1.5 atm 1111 K Dashed Lines: Simulation = 2 = 1 = 0.5 Current Study: = 2 = 1 = /T [K -1 ] Figure 5.3. Ignition delay times in stoichiometric DMA/4% 2 /argon mixtures at P ~ 1.5 atm, with Φ = 0.5, 1, and 2, measurements and simulation results using the morpholine mechanism described in Chapter 3 [54]. A summary of all the dimethylamine ignition delay time data can be found in Table 5.1. Table 5.1. Ignition delay time data for dimethylamine DMA/4% 2 /Ar T [K] P [atm] Φ IDT [ s]

80 H Time-Histories H concentration time-histories were measured in stoichiometric mixtures of 500 ppm DMA/ 2 /argon. H time-histories were measured in highly diluted mixtures, thus there was little temperature or pressure variation behind the reflected shock waves due to heat release, and a constant H absorption coefficient was used for each experiment. Figure 5.4 and 5.5 show the H time-histories at two different temperatures and at pressures near 2 atm. Log-log plots are implemented to show the 54

81 H [ppm] large dynamic range of the H behavior. Also shown in the figures are the simulated H time-histories using the morpholine mechanism [54] in Chapter 3. As can be seen in the plots, the morpholine mechanism captures the plateau value of H concentration relatively well, but greatly overpredicts the time for H to reach that peak value. This observation is consistent with the longer ignition delay times predicted by the mechanism. In addition, the mechanism predicts a long induction time for H during the dimethylamine oxidation process, with almost no H formed, followed by fast formation of H at ignition, clearly different from observations ppm DMA / 2 /Ar = 1, 1417 K, 2.2 atm 10 Experiment Morpholine Mech Time [ s] Figure 5.4. H time-histories in stoichiometric mixture of 500 ppm DMA/2/argon at 1417 K and 2.2 atm, with simulation results using the morpholine mechanism described in Chapter 3 [54]. 55

82 H [ppm] ppm DMA / 2 /argon = 1, 1504 K, 2.1 atm 10 1 Experiment Morpholine Mech Time [ s] Figure 5.5. H time-histories in stoichiometric mixture of 500 ppm DMA/ 2 /argon at 1504 K and 2.1 atm, with simulation results using the morpholine mechanism described in Chapter 3 [54] Update to the Morpholine Mechanism The morpholine mechanism in Chapter 3 was modified based on the current experimental data. Shown in Figure 5.6 are the sensitivity analysis results using the morpholine mechanism in Chapter 3 for an initial condition of 1300 K and 1.5 atm in a stoichiometric mixture of DMA/4% 2 /argon. The temperature is a good indicator of ignition behavior, and sensitivity analysis for temperature is readily performed using CHEMKIN PR [55]. The mixture temperature is most sensitive to the well-studied reaction: H + 2 = + H (R5.2), and the reactions of dimethylamine and fuel radicals: DMA + CH 3 = CH 3 NCH 3 + CH 4 CH 3 NCH = CH 3 N + CH 3 (R5.3) (R5.4) 56

83 Mixture T Sensitivity DMA + H = CH 3 NHCH 2 + H 2 CH 3 NCH 3 + CH 3 = CH 3 NCH 2 + CH 4 DMA + H = CH 3 NHCH 2 + H 2 (R5.5a) (R5.6) (R5.7) R2. H+ 2 =+H R3. DMA+CH 3 =CH 3 NCH 3 +CH 4 R4. CH 3 NCH =CH 3 N+CH 3 R5a. DMA+H=CH 3 NHCH 2 +H 2 R6. CH 3 NCH 3 +CH 3 =CH 3 NCH 2 +CH 4 R7. DMA+H=CH 3 NHCH 2 +H 2 R 3 R 2 R DMA/ 4% 2 / Ar = 1, 1300K, 1.5atm Morpholine Mech. R 5a R Time [ s] Figure 5.6. Sensitivity analysis of temperature, at an initial condition of 1300 K and 1.5 atm in a stoichiometric mixture of DMA/4% 2 /argon, using the morpholine mechanism described in Chapter 3 [54]. Since there is a dramatic temperature change near the time of ignition, reactions that exhibit positive temperature sensitivities accelerate ignition and those that exhibit negative sensitivities decelerate ignition. It can be seen from Figure 5.6 that several reactions related with CH 3 radical play important roles in the DMA oxidation process. In the morpholine mechanism, the only DMA unimolecular decomposition channels are: DMA = CH 3 NHCH 2 + H and DMA = CH 3 NCH 3 + H. The following channel of forming CH 3 by DMA unimolecular decomposition is not included: R 7 DMA (+M) = CH 3 NH + CH 3 (+M) (R5.1). Previous work on MMH (CH 3 NHNH 2 ) showed that, even with a highly reactive N-N bond, the C-N bond scission still needs to be included for good prediction of 57

84 experimental data at high temperatures in a shock tube [47]. Hence, for DMA oxidation at the current conditions, Reaction 5.1 cannot be neglected. To the best of the author s knowledge, there is no theoretical or experimental study of Reaction 5.1 in the literature. In the current study, the reaction rate constant k 5.1 was estimated by analogy to the decomposition reaction of dimethyl ether (CH 3 CH 3, DME): DME (+M) = CH 3 + CH 3 (+M). The reaction rate constants by Cook et al. [81], at the high-pressure limit, low-pressure limit, and the Troe parameters in the fall-off region, were used with: k 5.1, = T exp(-42220/t), s -1 (Eq 5.2) k 5.1,o = exp(-21537/t), cm 3 mol -1 s -1 (Eq 5.3) F cent = exp(-t/2510) (Eq 5.4) To check the validation of estimating Reaction 5.1 with the DME reaction, the potential energy surfaces for DMA and DME decompositions were explored through DFT calculations at the BH&HLYP/ G(2d,2p) level of theory. The two resulting potential energy surfaces were seen to be almost identical. VTST calculations for the reaction rates of Reaction 5.1 were carried out using the calculated DMA potential energy surface. Since there is no previous data for Reaction 5.1, the VTST rates were compared with the DME data by Cook et al. [81]. With a reasonable exponential down model of E down = 50 cm -1, good agreements with the DME data [81] were achieved. The good agreements confirm the similarity of the DMA and DME reaction. The purpose of the current theoretical study is to give us more confidence in the feasibility of estimating the rates of Reaction 5.1 using the DME rates. Experimental study of Reaction 5.1 making use of a CH 3 diagnostic and more accurate theoretical work of Reaction 5.1 can be carried out in the future. Chapter 4 presented the reaction rate constants for DMA + H, with experimental results in good agreement with variational transition-state theory calculation [80]. Hence the reaction rate constants for DMA + H in the morpholine mechanism were updated with the value in Chapter 4, with k 5.5a =

85 T 2.09 exp(1365/rt) cm 3 mol -1 s -1 for DMA + H = CH 3 NHCH 2 + H 2 (R 5.5a ), and k 5.5b = T 2.11 exp(3208/rt) cm 3 mol -1 s -1 for DMA + H = CH 3 NCH 3 + H 2 (R 5.5b ). All the DMA rate adjustments to the morpholine mechanism are summarized in Table 5.2. Table 5.2. Summary of rate recommendations to the dimethylamine sub-mechanism of the morpholine mechanism. # Reaction Rate Constant [cm 3 mol -1 s -1 ] Reference R5.1 DMA (+M) = CH 3 NH + CH 3 (+M) See text [81] R5.5a DMA + H = CH 3 NHCH 2 + H 2 k 5a = T 2.09 exp(1365/rt) [80] R5.5b DMA + H = CH 3 NCH 3 + H 2 k 5b = T 2.11 exp(3208/rt) [80] The simulation results using the morpholine mechanism with the modifications shown in Table 5.2 are compared with the measured dimethylamine ignition delay time data in Figure 5.7 and 5.8. The morpholine mechanism with modifications in Table 5.2 is seen to be in good agreement with the DMA experimental data. The updates to DMA + H reaction rates have little influence on dimethylamine ignition delay times in the current study, and the differences between the simulated dimethylamine ignition delay times using the mechanism with and without the modifications in Table 5.2 are mostly caused by the addition of Reaction 5.1. Reaction 5.1 introduces an extra channel of forming CH 3 radicals, which attack more DMA, form reactive radicals, and speeds up the oxidation process. 59

86 Ignition Delay Time [ s] Ignition Delay Time [ s] 1429 K 1250 K 1111 K % DMA/4% 2 /Ar, = 1 Lines: Morpholine Mech. w/ DMA Updates 0.9 atm 1.5 atm 2.8 atm 100 Data points: Current data P ~ 0.9 atm P ~ 1.5 atm P ~ 2.8 atm /T [K -1 ] Figure 5.7. Ignition delay times in stoichiometric mixtures of DMA/4% 2 /argon, at P ~ 0.9, 1.5 and 2.8 atm, measurements and simulations using the morpholine mechanism [54] with the modifications in Table DMA/ 4% 2 /Ar P ~ 1.5 atm 1429 K 1250 K 1111 K 1000 Lines: Morpholine Mech. w/ DMA Updates 100 = 2 = 1 = 0.5 Data Points: = 2 = 1 = /T [K -1 ] Figure 5.8. Ignition delay times in stoichiometric mixtures of DMA/4% 2 /argon at P ~ 1.5 atm, with Φ = 0.5, 1, and 2, measurements and simulation results using the morpholine mechanism [54] with the modifications in Table

87 Ignition Delay Time [ s] A regression analysis of the dimethylamine ignition delay times was carried out using all the experimental data, and the following correlation was inferred with R 2 = 0.99: τ ign = P Φ 0.45 exp(18265/t) (Eq 5.5), where τ ign is in μs, P is in atm, and T is in K. Shown in Figure 5.9 are all the dimethylamine ignition delay time data, scaled to Φ = 1 and P = 1.5 atm using Equation 5.5, together with the simulated ignition delay times using the morpholine mechanism [54] with and without the modifications in Table 5.2. The modified mechanism is in much better agreement with the experimental data DMA /4% 2 /Ar 1429 K 1250 K = 1, P = 1.5 atm 1111 K Current Study Morpholine Mech. Morpholine Mech. w/ DMA updates /T [K -1 ] Figure 5.9. Measured dimethylamine ignition delay times in 4% 2 /argon, scaled to Φ = 1 and P = 1.5 atm, in comparison with the simulations using the morpholine mechanism [54] with and without the changes recommended in Table 5.2. Shown in Figure 5.10 and 5.11 are the simulated H time-histories using the morpholine mechanism [54] with modifications in Table 5.1 (dash dotted lines), in comparison with the experimental data (solid lines) and the simulation results using the mechanism [54] without the modifications in Table 5.1 (dashed lines). The modified 61

88 H [ppm] mechanism is seen to capture the experimentally observed features in the H timehistories much better than the original mechanism. Similarly, as for ignition delay times, the differences between the simulated H time-histories using the morpholine mechanism and the modified mechanism are mostly caused by the addition of Reaction 5.1. Updating the DMA+H reaction rates has only a small effect on the H timehistories. With Reaction 5.1, forming CH 3 NH and CH 3 radical, becoming the major channel for DMA decomposition. The reactions for CH 3 NH radical have been investigated in a methylamine (CH 3 NH 2 ) study [24]. The morpholine mechanism [54] contains the methylamine subset, and recent H/C//N chemistry sets, thus with a reasonable rate constant for Reaction 5.1, the modified morpholine mechanism successfully captures the DMA oxidation process ppm DMA / 2 /Ar = 1, 1417 K, 2.2 atm Experiment Morpholine Mech. Morpholine Mech. w/ DMA Updates Time [ s] Figure Comparison of the simulated H time-histories, using the morpholine mechanism [54] with and without the modifications in Table 5.2, to the experiment in stoichiometric mixture of 500 ppm DMA/ 2 /argon at 1417 K and 2.2 atm. 62

89 H [ppm] ppm DMA / 2 /Ar = 1, 1504 K, 2.1 atm 10 1 Experiment Morpholine Mech. Morpholine Mech. w/ DMA Updates Time [ s] Figure Comparison of the simulated H time-histories, using the morpholine mechanism [54] with and without the modifications in Table 5.2, to the experiment in stoichiometric mixture of 500 ppm DMA/ 2 /argon at 1504 K and 2.1 atm Summary The combustion database of nitrogen-containing fuels was augmented with dimethylamine ignition delay times and H time-histories, studied behind reflected shock waves. The measured dimethylamine ignition delay times covered the temperature range of K, with pressures near 0.9, 1.5 and 2.8 atm, and equivalence ratios of 0.5, 1 and 2, in 4% oxygen/argon. The current dimethylamine ignition delay time data feature low scatter and can be correlated into a single expression (R 2 ~0.99): τ ign = P Φ 0.45 exp(18265/t), with τ ign in μs, P in atm, and T in K. H time-histories were measured in stoichiometric mixtures of 500 ppm DMA/ 2 /argon, using laser absorption of H near nm. The morpholine mechanism described in Chapter 3 [54] was used to simulate the measured dimethylamine ignition delay times and H time-histories. The mechanism, which was developed for morpholine combustion [54], overpredicts the measured dimethylamine ignition delay times, and cannot capture the measured H time-histories. Modification 63

90 to the mechanism was recommended by adding the DMA unimolecular decomposition channel: DMA = CH 3 NH + CH 3, with the reaction rates estimated by analogy to dimethyl ether decomposition previously investigated by Cook et al. [81] The reactions of DMA + H were also updated with the values presented in Chapter 4. The morpholine mechanism [54] with the aforementioned modifications is in good agreement with both the dimethylamine ignition delay times and H time-histories. 64

91 Chapter 6. Ethylamine Pyrolysis and xidation 6.1. Introduction In this chapter, we continue to evaluate and improve the morpholine mechanism [54] described in Chapter 3, by comparing the mechanism predictions with shock tube data of ethylamine. Ethylamine (EA: CH 3 CH 2 NH 2 ) pyrolysis and oxidation were investigated using shock tube/laser absorption methods. For ethylamine pyrolysis, NH 2 concentration time-histories in ethylamine/argon mixtures were measured. For ethylamine oxidation, ignition delay times and time-histories of H and NH 2 were measured in ethylamine/oxygen/argon mixtures. The CHEMKIN PR package [55] was used to simulate all the experimental data, under the standard constant energy and volume assumptions, using the morpholine mechanism in Chapter 3 [54]. Modifications to the mechanism were recommended based on the ethylamine data in this chapter Experimental Methods The Stanford NASA Shock Tube as described in section was used for the ethylamine study. The NH 2 diagnostic as described in section and the H diagnostic as presented in section were implemented for NH 2 and H time-history measurements respectively. Ethylamine (97%) supplied by Sigma-Aldrich was used in the current study without further purification. Research grade argon (99.99%) supplied by Praxair was employed as the bath gas. All the mixtures were prepared manometrically using a double-dilution method in a 40 liter Teflon-coated stainless steel tank, and mixed with a 65

92 magnetically driven stirring vane for at least one hour prior to the experiments. Before each experiment, the shock tube was passivated to avoid loss of amine to the shock tube wall Experimental Results Ethylamine Pyrolysis For the ethylamine pyrolysis study, NH 2 time-histories in 2000 ppm ethylamine/argon mixtures were measured. Figure 6.1 shows the representative NH 2 time-histories at different temperatures and at pressures near 1.3 atm. During the ethylamine pyrolysis process, NH 2 radicals form quickly though the ethylamine decomposition reaction: CH 3 CH 2 NH 2 = C 2 H 5 + NH 2, then the NH 2 radicals are consumed to form more stable species including NH 3. The resulting NH 2 time-histories are positive-skewed bell shapes as shown in Figure 6.1. Also included in Figure 6.1 are the simulation results in dashed lines, using the mechanism described in Chapter 3 [54]. The morpholine mechanism clearly overpredicts the NH 2 formation rates and the peak NH 2 concentrations, and hence fails to adequately capture the measured NH 2 timehistories. Sensitivity analysis using the mechanism and recommended modifications to the morpholine mechanism [54] based on the current ethylamine data will be presented in the following sections. 66

93 NH 2 [ppm] K, 1.4 atm 2000 ppm EA/ Ar Solid Lines: Experiment Dashed Lines: Morpholine Mech K, 1.2 atm K, 1.2 atm K, 1.3 atm Time [ s] Figure 6.1. NH 2 time-histories in 2000 ppm ethylamine/ar mixtures, measurements and simulation results using the morpholine mechanism [54] reported in Chapter Ethylamine xidation For ethylamine oxidation, ignition delay times and time-histories of NH 2 and H were measured in ethylamine/oxygen/argon mixtures Ethylamine Ignition Delay Times Ethylamine ignition delay times were determined by extrapolating, back to the baseline pressure, the steep increase in pressure concurrent with ignition. The current ignition delay time data cover the temperature range of K, with pressures near 0.85, 1.35 and 2 atm, and equivalence ratios of 0.75, 1 and 1.25 in 4% oxygen/argon. The stoichiometric case was defined as: CH 3 CH 2 NH = 2C H N 2 (Eq 6.1) For a uniform graphic presentation of the results, a pressure scaling of P -1 is implemented for the ignition delay time data in a narrow pressure regime for reach set of data. Since the actual pressures are close to the reported pressure (within 15%) for 67

94 Ignition Delay Time [ s] one set of data on an ignition delay time plot, this simple power law dependence was used for Figure and Figure 6.2 shows the measured ignition delay times at three nominal pressures, 0.85, 1.35 and 2 atm, along with the simulations using the morpholine mechanism in Chapter 3. As expected, the ignition delay times are shorter at higher pressures. The mechanism in Chapter 3 predicts shorter ignition delay times than the experimental data K 1333 K 1250 K 1176 K 1000 EA/ 4% 2 / Ar = atm 1.35 atm 2 atm 100 Solid Points: Experiment Dashed Lines: Morpholine Mech /T [K -1 ] Figure 6.2. Ethylamine ignition delay time measurements near 0.85, 1.35 and 2 atm in stoichiometric mixture of ethylamine/4% 2 /Ar, and simulations based on the morpholine mechanism [54] in Chapter 3. Figure 6.3 presents the measured ethylamine ignition delay times at three different equivalence ratios, i.e. 0.75, 1 and 1.25, along with the simulation results from the morpholine mechanism. The experimental data show that the ethylamine ignition delay times are not sensitive to the equivalence ratio, while the simulations results present small dependence of ignition delay times on equivalence ratio. Similar as is shown in Figure 6.2, the morpholine mechanism [54] clearly underpredicts the ethylamine ignition delay times. 68

95 Ignition Delay Time [ s] 1429 K 1333 K 1250 K 1176 K EA/ 4% 2 / Ar P ~ 1.35 atm 1000 = 1.25 = 1 = Solid Points: Experiment Dash Lines: Morpholine Mech /T [K -1 ] Figure 6.3. Ethylamine ignition delay time measurements near 1.35 atm, with Φ = 0.75, 1 and 1.25, and simulation results using the morpholine mechanism [54] presented in Chapter NH 2 Time-History in Ethylamine xidation For ethylamine oxidation studies, NH 2 time-histories were measured in 2000 ppm ethylamine/0.8% 2 /argon mixtures near 2 atm. A log-log plot is implemented in Figure 6.4 to show the NH 2 behavior at multiple time and concentration scales. As is presented in Figure 6.4, for oxidation cases, NH 2 radicals form rapidly at early times, rising to a peak value and then decaying gradually until ignition when NH 2 is rapidly and fully depleted. Also shown in Figure 6.4 are the simulated NH 2 time-histories using the morpholine mechanism [54]. The morpholine mechanism (dashed line) captures the general shape of the NH 2 time-history, but overpredicts the initial rate of NH 2 formation and the peak NH 2 concentration, while underpredicting the time for NH 2 depletion. 69

96 NH 2 [ppm] 2000 ppm EA/ 0.8% 2 / Ar 1558 K 2.05 atm K 2.07 atm 1441 K 2.09 atm 10 Solid Lines: Experiment Dashed Lines: Morpholine Mech Time [ s] Figure 6.4. NH 2 time-histories in 2000 ppm ethylamine/0.8% 2 /Ar mixtures; simulations are based on the morpholine mechanism presented in Chapter H Time-History in Ethylamine xidation H time-histories were measured in 500 ppm ethylamine/0.2% 2 /argon mixtures, using the well-characterized R 1 (5) transition in the A-X(0,0) system of H near nm. The H time-histories throughout the entire ignition process are of interest here. The low ethylamine concentration of 500 ppm was employed to reduce the temperature rise when ignition happens, thereby reducing uncertainty in the H absorption coefficient due to the temperature changes during ignition. With 500 ppm ethylamine, the temperature rise at ignition is less than 1% for the current study, enabling use of a constant H absorption coefficient throughout the test time for each shock experiment. Figure 6.5 shows the measured H time-histories at three different temperatures and at pressures near 2 atm. The measured H time-histories at low temperatures feature an initial H formation and first H plateau, after which the H concentration increases rapidly to its peak value at ignition, and finally the H concentration slowly decreases after ignition. At the highest temperature of 1593 K, the initial H plateau spans a very short time and is not obvious. Comparing the H time- 70

97 H [ppm] histories at different temperatures, it can be seen that both the initial plateau H concentration and the peak H concentration increase with temperature. Also included in Figure 6.5 are the simulated H time-histories using the morpholine mechanism [54] in Chapter 3. The mechanism predicts peak H concentrations slightly higher than the measured values and underpredicts the time for H concentration to reach the peak value K 2.1 atm 500ppm EA/ 2 / Ar = 1 Solid Lines: Experiment Dashed Lines: Morpholine Mech K 2.0 atm 1399 K 1.9 atm Time [ s] Figure 6.5. H time-histories in 500 ppm ethylamine/0.2% 2 /Ar mixtures, measurements and simulations using the morpholine mechanism [54] presented in Chapter Update to the Morpholine Mechanism The mechanism in Chapter 3 was developed for morpholine combustion, and contains the ethylamine sub-mechanism developed by Lucassen et al. [25], together with updated C/H/ chemistry and an improved nitrogen-chemistry base set [54]. The ethylamine sub-mechanism is not the dominant factor for morpholine ignition delay times and was not updated in Chapter 3. Furthermore, the Lucassen et al. ethylamine sub-mechanism has only been validated in a low-pressure flame study [25]. Hence, the discrepancy between the mechanism predictions and the current ethylamine pyrolysis 71

98 and oxidation data is most likely caused by weaknesses in the ethylamine submechanism. The ethylamine sub-mechanism in the morpholine mechanism contains four unimolecular decomposition channels for ethylamine, including decomposition to C 2 H 5 +NH 2, C 2 H 4 +NH 3, CH 3 CHNH 2 +H and CH 2 CH 2 NH 2 +H [25,54]. While this ethylamine decomposition set might be sufficient to reproduce many trends observed in flame experiments [25], a more complete description of the ethylamine decomposition process is needed for the current shock tube conditions. We also note that Lucassen et al. estimated the ethylamine + H, H, CH 3 and NH 2 reactions by analogy to ethanol and methylamine + H, H, CH 3 and NH 2 reactions [25]. In the current study, the ethylamine sub-mechanism was updated based on a recent theoretical study of ethylamine decomposition [83]. The reaction rate constants for ethylamine + H, CH 3 and NH 2 reactions are also updated, making using of the recent quantum chemistry calculation [84]. In addition, we have presented the measured reaction rate constants for ethylamine + H in Chapter 4, with experimental results in good agreement with variational transition-state theory calculations [80], and will use these values in the revised mechanism. Several reactions in the C/H/ subset of the morpholine mechanism were also updated using the USC Mech II [85]. A summary of all the modifications to the morpholine mechanism in this chapter can be found in Table 6.1. Table 6.1. Updated reaction rate constants to ethylamine sub-mechanism # Reaction A N Ea [cal/mol] Ref. EA Decomposition CH 3 CH 2 NH 2 = CH 2 NH 2 + CH E See text 6.2 CH 3 CH 2 NH 2 = C 2 H 5 + NH E See text 6.3 EA = C 2 H 4 + NH E [83] 6.4 EA = CH 4 + HCNH E [83] 6.5 EA = CH 3 CNH 2 + H E [83] 6.6 EA = CH 3 CH + NH E [83] 6.7 EA = CH 3 CHNH 2 + H 2.40E [83] 6.8 EA = CH 3 CH 2 NH + H 1.10E [83] 6.9 EA = CH 2 CH 2 NH 2 + H 2.00E [83] 72

99 6.10 CH 3 CNH = CH 3 CNH E [83] 6.11 HCNH = H 2 NCH E [83] 6.12 CH 3 CH = C 2 H E [83] 6.13 CH 3 CH + 2 = CH 3 CH E [83] EA + Radicals 4.2a EA + H = CH 3 CH 2 NH + H E [54] 4.2b EA + H = CH 3 CHNH 2 + H E [54] 4.2c EA + H = CH 2 CH 2 NH 2 + H E [54] 6.14 EA + H = CH 2 CH 2 NH 2 + H E [84] 6.15 EA + H = CH 3 CH 2 NH + H E [84] 6.16 EA + H = CH 3 CHNH 2 + H E [84] 6.17 EA + CH 3 = CH 3 CH 2 NH + CH E [84] 6.18 EA + CH 3 = CH 2 CH 2 NH 2 + CH E [84] 6.19 EA + CH 3 = CH 3 CHNH 2 + CH E [84] 6.20 EA + NH 2 = CH 2 CH 2 NH 2 + NH E [84] 6.21 EA + NH 2 = CH 3 CH 2 NH + NH E [84] 6.22 EA + NH 2 = CH 3 CHNH 2 + NH E [84] C/H/ Reactions 6.23 H + 2 = + H 2.64E [85] 6.24 H + H 2 = H + H E [85] 6.25 H 2 + CH 3 = H + CH E [85] 1 Rate coefficient units for 1 st order reactions: s -1 2 Rate coefficient units for 2 nd order reactions: cm 3 mol -1 s -1 A NH 2 sensitivity analysis using the morpholine mechanism with aforementioned modifications is shown in Figure 6.6, in a 2000 ppm ethylamine/ar mixture at 1428 K, 1.2 atm. As can be seen in Figure 6.6, NH 2 is strongly sensitive to the unimolecular decomposition reactions of ethylamine: CH 3 CH 2 NH 2 = CH 2 NH 2 + CH 3 (R 6.1) CH 3 CH 2 NH 2 = C 2 H 5 + NH 2 (R 6.2). The pre-exponential terms of the reaction rate constants of these two reactions were tuned based on the experimental data, with the resulted reaction rate constants shown in Table 6.1. As presented in Figure 6.6, Reaction 6.2 dominates the initial formation of NH 2, while its influence decreases at later times. Reaction 6.1 is the major reaction with negative NH 2 sensitivity, and becomes more important after initial NH 2 formation. The NH 2 formation at early times in pyrolysis mixtures were used to anchor the reaction 73

100 NH 2 Sensitivity rates for Reaction 6.2. The reaction rates for Reaction 6.1 need to be tuned at the same time to achieve good agreement with the measured NH 2 time-histories at the later times. The branching ratio between these two important ethylamine unimolecular decomposition reactions is defined as: BR=k 6.1 /k 6.2. With the reaction rates for Reaction 6.1 and 6.2 shown in Table 6.1, the recommended BR is 2.5 at 1300 K and 2.1 at 1800 K. Because of the strong monotonic influence of these two reaction rate constants on the initial NH 2 formation rate and the late time NH 2 removal behavior, this unique combination of reaction rate constants captures the initial NH 2 formation, peak NH 2 concentration, and NH 2 removal generally well. Also, the proposed reaction rates for Reaction 6.1, as shown in Table 6.1, are in good agreement with the similar C-C bond scission reactions of C 3 H 8 [86] and C 2 H 5 H [87]. Similarly for Reaction 6.2, the proposed reaction rates in Table 6.1 are close to the reaction rates for C-N bond scission of CH 3 NH 2 [88] and the C- bond scission of C 2 H 5 H [87] CH3CH2NH2=CH2NH2+CH3 CH3CH2NH2=C2H5+NH2 CH3CH2NH2+H=CH2CH2NH2+H2 CH3CH2NH2+H=CH3CHNH2+H2 NH+H2=NH2+H NH3+H=NH2+H2 CH3+NH2(+M)=CH3NH2(+M) CH3NH2+H=CH2NH2+H2 CH3+CHNH=CH3CHNH ppm EA/ Ar 1428 K, 1.2 atm Time [ s] Figure 6.6. NH 2 sensitivity analysis using the morpholine mechanism [54], with updates shown in Table 6.1, at 1428 K, 1.2 atm in 2000 ppm ethylamine/ar mixtures. The morpholine mechanism [54], with updates shown in Table 6.1, is compared with the NH 2 time-histories in 2000 ppm ethylamine/argon mixture in Figure 6.7. The modified mechanism is seen to provide good agreement with the experimental data. 74

101 NH 2 [ppm] Comparing Figure 6.7 and Figure 6.1, it is clear that the modifications in Table 6.1 greatly improve the model predictions of NH 2 time-histories for ethylamine pyrolysis ppm EA/ Ar 1790 K, 1.4 atm K, 1.2 atm Solid Lines: Experiment Dash Dotted Lines: Morpholine Mech. w/ EA Updates 1428 K, 1.2 atm K, 1.3 atm Time [ s] Figure 6.7. Measured NH 2 time-histories in 2000 ppm ethylamine/ar mixtures; simulation results are based on the morpholine mechanism [54] with updates shown in Table 6.1. Simulations employing the morpholine mechanism [54], with updates shown in Table 6.1, are compared with the ethylamine ignition delay time data in stoichiometric ethylamine/4% 2 /Ar mixtures at pressures of 0.85, 1.35 and 2 atm in Figure 6.8. These simulations are in quite good agreement with the ignition delay time data. Similarly, a comparison of the modified simulations with ignition delay time data at different equivalence ratios is presented in Figure 6.9. Analysis of the mechanism shows that besides the important H 2 / 2 reactions, ethylamine + radical reactions also affect ethylamine ignition delay times. In the morpholine mechanism [54], the reaction rates for ethylamine + radical were estimated by analogy with C 2 H 5 H and CH 3 NH 2 + radical reactions [25]. When the revised values for the ethylamine + radical reactions in Table 6.1 are employed in the simulations, there is significant improvement in the agreement between the simulations and the experiments. 75

102 Ignition Delay Time [ s] Ignition Delay Time [ s] K 1333 K 1250 K 1176 K EA/ 4% 2 / Ar = atm 1.35 atm atm 100 Solid Points: Current Study Dash Dotted Lines: Morpholine Mech. w/ EA Updates /T [K -1 ] Figure 6.8. Measurements of ethylamine ignition delay times near 0.85, 1.35 and 2 atm in stoichiometric mixture of ethylamine/4% 2 /Ar; simulation results utilize the morpholine mechanism [54] with updates in Table K 1333 K 1250 K 1176 K EA/ 4% 2 / Ar P ~ 1.35 atm 1000 = 1.25 = 1 = Solid Points: Experiment Dash Dotted Lines: Morpholine Mech. w/ EA Updates /T [K -1 ] Figure 6.9. Measured ethylamine ignition delay times near 1.35 atm, with Φ = 0.75, 1 and 1.25; simulation results utilize the morpholine mechanism [54] with updates in Table

103 NH 2 Sensitivity A NH 2 sensitivity analysis was carried out at 1441 K, 2.1 atm in the mixture of 2000 ppm ethylamine/0.8% 2 /Ar, with the results shown in Figure It can be seen that Reaction 6.1 and 6.2 are important for the initial NH 2 formation in ethylamine oxidation as well. Later on in the oxidation process, various C/H//N reactions become important at ignition ppm EA/ 0.8% 2 / Ar 1441 K, 2.1 atm CH3CH2NH2=CH2NH2+CH3 CH3CH2NH2=C2H5+NH2 CH3CH2NH2+H=CH3CHNH2+H2 H+2=+H H+H2=H+H2 +H2=H+H H+CH3(+M)=CH4(+M) N+H(+M)=HN(+M) CH2CHNH+H=CH3CHNH HCN+=NC+H Time [ s] Figure NH 2 sensitivity analysis using the morpholine mechanism [54] with updates in Table 6.1, at 1441 K, 2.1 atm in 2000 ppm ethylamine/0.8% 2 /Ar mixtures. The simulated NH 2 time-histories in oxidation mixtures of 2000 ppm ethylamine/0.8% 2 /Ar, using the morpholine mechanism [54] with updates shown in Table 6.1, are shown together with the experimental data in Figure Comparing with the simulations using the morpholine mechanism in Figure 6.4, the modified mechanism is in much better agreement with the experiment. The improved agreement with the experimental data is primarily because of the revised reaction rates for the ethylamine decomposition reactions and the ethylamine + radical reactions. 77

104 H Sensitivity NH 2 [ppm] 2000 ppm EA/ 0.8% 2 / Ar 1558 K, 2.05 atm K, 2.07 atm 1441 K, 2.09 atm Time [ s] Figure NH 2 time-histories in 2000 ppm ethylamine/0.8% 2 /Ar mixtures: measurements (solid lines) and simulation results based on the morpholine mechanism [54] with updates in Table 6.1 (dash-dotted lines). A sensitivity analysis for H time-history at 1399 K, 1.9 atm in stoichiometric mixture of 500 ppm ethylamine/0.2% 2 /Ar is shown in Figure Note firstly that the H + 2 = + H reaction is important throughout the whole H time-history. ther reactions, including the ethylamine unimolecular decomposition reactions 6.1 and 6.2, are also important for H time-history under the conditions in Figure CH3CH2NH2=CH2NH2+CH3 CH3CH2NH2=C2H5+NH2 CH3CH2NH2+H=CH3CHNH2+H2 +H2=H+H H+2=+H H+CH3(+M)=CH4(+M) CH3+2=H+CH2 H+C2H4(+M)=C2H5(+M) CH3CHNH+H=CH2CHNH+H2 CH3CH2NH2=CH3CHNH2+H ppm EA/ 2 / Ar = 1, 1399 K, 1.9 atm Time [ s] 78

105 H [ppm] Figure H sensitivity analysis at 1399 K, 1.9 atm in 500 ppm ethylamine/0.2% 2 /Ar mixtures, using the morpholine mechanism [54] with updates in Table 6.1. Shown in Figure 6.13 is a comparison of the modified morpholine mechanism simulations with the experimental data for ethylamine H time-histories. The modified morpholine mechanism predicts the peak H concentrations at different temperatures relatively well, and matches with the data within 40% for the time needed by H to reach half the peak concentration. Comparing with the morpholine mechanism [54] as shown in Figure 6.5, the modified mechanism is in better agreement with the experimental data. Similarly, as for NH 2 time-histories in ethylamine oxidation, the better agreement with the experimental data for the modified mechanism is primarily due to the revised reaction rate constants for the ethylamine decomposition reactions and the ethylamine + radical reactions. The discrepancies between model predictions and the experiment are likely to be caused by the uncertainties of the EA + H reaction K 2.1 atm 500ppm EA/ 2 / Ar = K 2.0 atm 1399 K 1.9 atm Time [ s] 79

106 Figure H time-histories in 500 ppm ethylamine/0.2% 2 /Ar mixtures: measurements (solid lines) and simulation results using the morpholine mechanism [54] with updates in Table 6.1 (dash-dotted lines) Summary Ethylamine (CH 3 CH 2 NH 2 ) pyrolysis and oxidation were studied behind reflected shock waves using shock tube/laser absorption methods. In addition to extending the shock tube database for ethylamine combustion, the morpholine mechanism [54] developed in Chapter 3 was improved with the experimental data of the current chapter. For ethylamine pyrolysis, NH 2 time-histories in 2000 ppm ethylamine/argon mixtures were measured and compared with the morpholine mechanism in Chapter 3. The mechanism overpredicts the NH 2 formation during ethylamine pyrolysis process and cannot capture the measured NH 2 time-histories. For ethylamine oxidation, ignition delay times, NH 2 and H time-histories were measured in ethylamine/oxygen/argon mixtures and were also analyzed using the morpholine mechanism described in Chapter 3 [54]. The mechanism significantly underpredicts the measured ethylamine ignition delay times. The mechanism also overpredicts the initial formation rate and the peak concentration of NH 2 for ethylamine oxidation in 2000 ppm ethylamine/0.8% 2 /argon mixtures. Finally, the mechanism cannot capture the measured H time-histories in 500 ppm ethylamine/0.2% 2 /argon mixtures. The unimolecular decomposition reaction rates, for EA = CH 2 NH 2 + CH 3 and EA = C 2 H 5 + NH 2, were determined based on the current experimental data. Modifications to the ethylamine decomposition reactions and the ethylamine + radical reactions in the morpholine mechanism were also recommended based on recent theoretical studies. With these modifications, simulations are in much better agreement with all the ethylamine species time-histories and the ignition delay time data. 80

107 Chapter 7. Revisiting the Morpholine Data 7.1. Introduction The mechanism described in Chapter 3 [54] was optimized for morpholine combustion, with the dimethylamine and ethylamine sub-mechanisms developed by Lucassen et al. together with updated C/H/ chemistry and an improved nitrogenchemistry base set. The dimethylamine and ethylamine sub-mechanisms are not the dominant factors for morpholine ignition delay times and were not updated in Chapter 3. Furthermore, the dimethylamine and ethylamine sub-mechanisms had only been validated in a low-pressure flame study [25] before this thesis work. In Chapter 4-6, we proposed several updates to the morpholine mechanism [54], especially the dimethylamine and ethylamine sub-mechanisms, based on the shock tube data and recent theoretical studies for dimethylamine and ethylamine. All the modifications are shown in Table 7.1. Table 7.1. Modifications to the morpholine mechanism [54], recommended in Chapters 4-6 # Reaction Rate Constant A n E Ref Modifications recommended in Chapter 4 4.1a DMA + H = CH 3 NHCH 2 + H E [80] 4.1b DMA + H = CH 3 NCH 3 + H E [80] 4.2a EA + H = CH 3 CH 2 NH + H E [80] 4.2b EA + H = CH 3 CHNH 2 + H E [80] 4.2c EA + H = CH 2 CH 2 NH 2 + H E [80] Modifications recommended in Chapter 5 81

108 5.1 2 DMA = CH 3 NH + CH 3 See Chapter 5 Modifications recommended in Chapter CH 3 CH 2 NH 2 = CH 2 NH 2 + CH 3 See Chapter CH 3 CH 2 NH 2 = C 2 H 5 + NH 2 See Chapter EA = C 2 H 4 + NH E [83] 6.4 EA = CH 4 + HCNHc 1.20E [83] 6.5 EA = CH 3 CNH 2 + H E [83] 6.6 EA = CH 3 CH + NH E [83] 6.7 EA = CH 3 CHNH 2 + H 2.40E [83] 6.8 EA = CH 3 CH 2 NH + H 1.10E [83] 6.9 EA = CH 2 CH 2 NH 2 + H 2.00E [83] 6.10 CH 3 CNH = CH 3 CNH E [83] 6.11 HCNH = H 2 NCH E [83] 6.12 CH 3 CH = C 2 H E [83] 6.13 CH 3 CH + 2 = CH 3 CH E [83] 6.14 EA + H = CH 2 CH 2 NH 2 + H E [84] 6.15 EA + H = CH 3 CH 2 NH + H E [84] 6.16 EA + H = CH 3 CHNH 2 + H E [84] 6.17 EA + CH 3 = CH 3 CH 2 NH + CH E [84] 6.18 EA + CH 3 = CH 2 CH 2 NH 2 + CH E [84] 6.19 EA + CH 3 = CH 3 CHNH 2 + CH E [84] 6.20 EA + NH 2 = CH 2 CH 2 NH 2 + NH E [84] 6.21 EA + NH 2 = CH 3 CH 2 NH + NH E [84] 6.22 EA + NH 2 = CH 3 CHNH 2 + NH E [84] 6.23 H + 2 = + H 2.64E [85] 6.24 H + H 2 = H + H E [85] 6.25 H 2 + CH 3 = H + CH E [85] 1 Rate coefficient units for 2 nd order reactions: cm 3 mol -1 s -1 2 Rate coefficient units for 1 st order reactions: s -1 To evaluate the effects of those modifications on the predictions of morpholine ignition delay times, in this chapter morpholine ignition delay times were simulated again using the morpholine mechanism [54] with all the modifications in Table Morpholine Ignition Delay Times Shown in Figure 7.1 are the ignition delay times of morpholine under different equivalence ratios, in comparison with the simulations using the morpholine mechanism with and without modifications in Table 7.1. The results using the mechanism without the modifications are shown in solid lines and those with the modifications are shown in dash dotted lines. The predictions of the modified mechanism are generally good, 82

109 Ignition Delay Time [ s] especially for ignition delay times at equivalence ratio of 0.5 and 1. The predictions of ignition delay times in fuel rich mixtures are too long for both the morpholine mechanism [54] and the modified mechanism, with slightly better predictions using the modified mechanism. 1250K 1111K 1000K 909K 833K Morpholine/Air Scaled to 15 atm with P = 0.5 = 1 = /T [K -1 ] Figure 7.1. Comparisons of model predictions with the ignition delay time data in morpholine/air mixtures near 15 atm and under different equivalence ratios. Solid lines: simulation results using the morpholine mechanism in Chapter 3 [54]. Dash-dotted lines: simulation results using the morpholine mechanism with modifications in Chapters 4-6. Similar comparisons were shown in Figure 7.2 for morpholine ignition delay times in stoichiometric mixtures near 15 and 25 atm. The modified mechanism predicts slightly longer ignition delay times and match somewhat better with the experimental data. 83

110 Ignition Delay Time [ s] 1250K 1111K 1000K 909K 833K Morpholine/Air = 1 Scaled with P P = 15 atm P = 25 atm /T [K -1 ] Figure 7.2. Comparisons of model predictions to ignition delay time data in stoichiometric morpholine/air mixtures near 15 and 25 atm respectively. Solid lines: simulation results using the morpholine mechanism in Chapter 3 [54]. Dash-dotted lines: simulation results using the morpholine mechanism with modifications in Chapters 4-6. Figure 7.3 presents the simulated morpholine ignition delay times in different oxidizers, using the morpholine mechanism and the morpholine mechanism with modifications in Table 7.1. The modified mechanism is also in somewhat better agreement with the experimental data. 84

111 Ignition Delay Time [ s] 1250K 1111K 1000K 909K 833K Morpholine/4% 2 /Ar Morpholine/Air = 1 Scaled to 15 atm with P /T [K -1 ] Figure 7.3. Comparisons of model predictions to ignition delay time data near 15 atm in morpholine/4% 2 /argon and morpholine/air mixtures. Solid lines: simulation results using the morpholine mechanism in Chapter 3 [54]. Dash-dotted lines: simulation results using the morpholine mechanism with modifications in Chapters Sensitivity Analysis Figure 7.4 presents a sensitivity analysis for mixture temperature in a stoichiometric morpholine/air mixture at 1000 K and 15 atm. The mixture temperature is a good indicator of ignition behavior, and sensitivity analysis for temperature can be readily performed using CHEMKIN PR [55]. Since there is a dramatic temperature change near the time of ignition, reactions that exhibit positive temperature sensitivities accelerate ignition and those exhibit negative sensitivities decelerate ignition. As is shown in Figure 7.4, the major reactions that affect the oxidation process are chain opening reactions of morpholine radicals and bond scission reactions of the radicals after chain opening. Besides, under the current test conditions for morpholine ignition, H 2 2 reactions are also important. More detailed studies of those reactions can be carried out in the future to further reduce the discrepancies between the morpholine 85

112 Ignition Temperature Sensitivity ignition delay time data and the modified mechanism. Even though the reactions of dimethylamine and ethylamine are not the determining factors for morpholine ignition delay times, they are of great research value on their own and efforts to validate and update the morpholine mechanism using shock tube data of dimethylamine and ethylamine are shown to be beneficial Morpholine/Air, = K, 15 atm cymetamorphyl=ch2ch2ch2chnh H22(+M)=2H(+M) cyrthomorphyl=ch2ch2nhch2ch cymorph+h2=cymetamorphyl+h22 CH2CH2CH2CHNH=C2H4+CH2CHNH -2-4 cyrthomorphyl=nhch2ch2chch2 CH2CH2NHCHCH2=CH2+CH2NHCHCH2 CH2CH2CH2CHNH+2=CH2CHCH2CHNH+H2 H+H22=H2+H2 cymetamorphyl=ch2ch2nhchch Time [ s] Figure Sensitivity analysis of temperature using the morpholine mechanism [54] with modifications in Chapters 4-6, in a stoichiometric mixture of morpholine/air under initial condition of 1000 K and 15 atm Summary ne of the goals of the current thesis work is to improve the understanding of the combustion chemistry of fuels with an amine group. We started with the model biofuel morpholine and developed a reaction mechanism for morpholine combustion based on previous research and the morpholine ignition delay time data in a shock tube. During the process of studying morpholine, we realized the need to update the submechanisms for smaller aliphatic amines: dimethylamine and ethylamine. High quality shock tube data were used to update the dimethylamine and ethylamine sub- 86

113 mechanisms in the morpholine mechanism. This chapter shows that those updates improve the mechanism predictions of the morpholine ignition delay times. More detailed studies of morpholine chemistry are needed to further improve the agreement between the mechanism predictions and the morpholine experimental data. 87

114

115 Chapter 8. MMH Pyrolysis 8.1. Introduction Amino groups are also common structural features of hypergolic fuels that are widely used as rocket propellants. Monomethylhydrazine (CH 3 NHNH 2, MMH) is an important example of such fuels, with both a primary and a secondary amino group (- NH and -NH 2 ). It is popular because MMH spontaneously ignites when combined with certain oxidizers like N 2 4 [28]. Due to the importance of MMH, comprehensive pyrolysis and oxidation MMH mechanisms have been developed, but the performance of those mechanisms needs to be evaluated over broader range of experimental conditions. In this chapter, we investigate MMH pyrolysis using NH 2 time-histories measured in a shock tube. Kerr et al. investigated the first-order rate constant for the homogenous dissociation of the N-N bond of MMH using the toluene-carrier technique, at pressures of atm [89]. Eberstein and Glassman studied the total thermal decomposition rates of MMH in an adiabatic flow reactor at atmospheric pressure and K [90]. The flow reactor experiment resulted in an overall reaction order close to 1. Golden and Solly carried out a pyrolysis study of MMH at very low pressures and measured the reaction rate constants for the MMH unimolecular decomposition induced by wall collisions [91]. Catoire et al. performed MMH pyrolysis study behind shock waves in a 38.4 mm shock tube, using mixtures of 1-3% MMH/argon, with a temperature range of K and a pressure range of atm [92]. MMH absorption at 220 nm was implemented as a diagnostic, and a chemical kinetic mechanism was proposed to model the measured MMH concentration time-histories. 89

116 Sun and Law carried out a theoretical study of MMH pyrolysis using quantum Rice- Ramsperger-Kassel (QRRK) theory and master equation analysis for the pressure falloff. A comprehensive reaction mechanism was developed based on the theoretical study [93], and the mechanism was further updated using shock tube data for MMH thermal decomposition at K and atm [47]. The MMH N-N bond scission reaction: CH 3 NHNH 2 = CH 3 N.H + NH 2 (R 8.1a) and the C-N bond scission reaction: CH 3 NHNH 2 = CH 3 + NHNH 2 (R 8.1b) were determined to be the dominant MMH decomposition channels. Zhang et al. provided more refined pressure-dependent reaction rates for these two important MMH decomposition channels using ab initio transition state theory-based master equation analyses [94]. More recently, Cook et al. measured NH 2, NH 3 and MMH time-histories in MMH pyrolysis behind reflected shock waves using laser absorption method. MMH concentrations of ~1% in argon were employed, and the shock tube experiments covered the temperature range of K, at pressures near 2.5 atm [34]. The reaction rate constant for Reaction 8.1a (2 atm, K) was uniquely determined as: k 8.1a = T exp (-39580/T) s -1 (Eq 8.1) and a value of k tot = k 8.1a + k 8.1b = T exp (-39580/T) s -1 was recommended for the overall unimolecular decomposition of MMH [34]. Even though the reaction rate constants were reported for 2 atm, the NH 2 measurements of Cook et al. were carried out at pressures of atm. The MMH decomposition process is exothermic, and choosing an appropriate gasdynamic model for the simulation is difficult under high energy release conditions [34,95]. As well, the temperature variation due to the energy release can lead to uncertainty in the absorption cross sections used for the laser absorption measurement. MMH concentrations of 1-3% MMH in argon were used in Catoire et al. [92], and an 90

117 MMH concentration of ~1% was implemented in Cook et al. [34]. In 1% MMH/argon mixtures, the simulated temperatures and pressures rise ~10% during the pyrolysis process, using the standard constant volume assumption [34]. To avoid the complications inherent with high fuel concentrations, the current study used low fuel concentrations of ppm MMH in argon. Low fuel concentration also extends the decomposition process and makes it possible to detect different stages of the pyrolysis sub-mechanism. Reaction 8.1a is clearly pressure dependent, and the pressure dependence of Reaction 8.1a has not previously been studied experimentally at combustion temperatures. Using the NH 2 laser absorption diagnostic at cm -1, the current study provides a more direct determination of k 8.1a over a wider pressure range than achieved by Cook et al. The measured reaction rate constants follow a pressure dependence trend which is close to the theoretical results by Zhang et al. [94] at atm. The pressure dependence of Reaction 8.1a in the falloff region, determined in the current study, provides important information for the thermal decomposition of MMH. Modification to the rate constant of a secondary NH 2 formation channel is also recommended based on the later times of the measured NH 2 time-histories. With these updates in the current study, the modified mechanism matches well with the measured NH 2 time-histories throughout the current conditions Experimental Method MMH pyrolysis was studied using the Stanford NASA shock tube as described in section 2.1.3, with the NH 2 diagnostic method presented in section MMH (98%) supplied by Sigma-Aldrich was used in the current study without further purification. Research grade argon (99.99%) supplied by Praxair was employed as the bath gas. All the MMH/argon mixtures were prepared manometrically using a doubledilution method in a 40 liter Teflon-coated stainless steel tank, and mixed with a magnetically driven stirring vane for at least one hour prior to the experiments. The MMH concentration was measured immediately before each shock by sampling a 91

118 portion of the MMH/argon mixture, after filling into the shock tube, from near the endwall to an external multipass cell with 29.9 m pathlength. The fuel concentration in the multipass cell was measured using a Jodon helium-neon laser at 3.39 μm, and Beer s law was used to convert the measured absorption data to the fuel mole fraction. The absorption cross section of MMH from the PNNL database [61] was confirmed in our laboratory (within 7%), and used in the Beer s law concentration calculation. The measured initial MMH concentrations were lower than the manometric values by 15%- 30%, depending on the filling pressures into the shock tube. This drop in MMH initial concentration was mostly caused by loss in the mixing assembly and to the shock tube wall, and no significant MMH decomposition was observed in the current diluted MMH/argon mixtures at room temperature. MMH absorbance was monitored during the filling process with no passivation. After filling ended, no noticeable drop in MMH absorbance was observed for periods of up to 5 min. The measured MMH concentration employing the multipass cell was used for comparisons with simulations Results and Discussion The MMH pyrolysis study was performed using NH 2 time-history measurements behind reflected shock waves. The CHEMKIN PR package [55] was used to simulate the NH 2 time-histories, under the standard constant energy and volume assumptions, for comparison with the experimental data. The base mechanism used for the data analysis in the current study was the MMH pyrolysis mechanism originally developed by Sun and coworkers [47,93], and updated by Cook et al. using the shock tube data obtained near 2.5 atm [34]. This mechanism will be referred to as the Cook et al. mechanism in the following sections. Figure 8.1 shows a measured NH 2 time-history example taken during the pyrolysis process of 350 ppm MMH in argon at 1217 K and 0.34 atm behind reflected shock wave, with error bars of ±15% due to the uncertainty in the NH 2 absorption cross section. Also shown in Figure 8.1 is the simulation result, at the same conditions, using the Cook et al. mechanism. It should be noted that the reaction rate constants 92

119 NH 2 [ppm] determined by Cook et al. were for pressures near 2.5 atm, and were not suitable for the lower pressures of the current study. The pressure dependencies of the MMH decomposition reactions have not been studied experimentally under combustion temperatures before, and will be the focus of the current study Experiment Cook et al. (2011) P ~ 2.5 atm ppm MMH / argon 1217 K, 0.34 atm Time [ s] Figure 8.1. Representative NH 2 time-history measurement at 1217 K and 0.34 atm in 350 ppm MMH/argon, in comparison with simulation results using the Cook et al. mechanism [34]. Shown in Figure 8.2 are the sensitivity analysis results using the Cook et al. mechanism at the same conditions of Figure 8.1. It can be seen that at early times the NH 2 formation is mostly determined by the MMH decomposition Reaction 8.1a. A similar sensitivity analysis for the pyrolysis of 170 ppm MMH in argon at 1163 K and 5.2 atm was shown in Figure 8.3 to confirm that Reaction 8.1a is dominant at the early times for all conditions. It is worth noting that for experiments at high pressure, reactions happen faster and the effective test time is shorter. 93

120 NH 2 Sensitivity NH 2 Sensitivity MMH = CH 3 N.H + NH 2 NH 2 + H = NH + H ppm MMH / argon 1217 K, 0.34 atm NHNH 2 + H = NH 2 + NH 2 MMH + NH 2 = CH 3 N.NH 2 + NH Time [ s] NH 2 + NH 2 = NH*NH + H 2 Figure 8.2. NH 2 sensitivity analysis at 1217 K and 0.34 atm in 350 ppm MMH/argon, using the Cook et al. mechanism [34] with the constant energy and volume assumptions MMH = CH 3 N.H + NH 2 NHNH 2 + H = NH 2 + NH ppm MMH / argon 1163 K, 5.2 atm NH 2 NH 2 = NH 2 + NH 2 NH 3 = NH 2 + H MMH + NH 2 = CH 3 N.NH 2 + NH Time [ s] Figure 8.3. NH 2 sensitivity analysis at 1163 K and 5.2 atm in 170 ppm MMH/argon, using the Cook et al. mechanism [34] with the constant energy and volume assumptions. 94

121 The measured NH 2 concentration time-histories were fit with simulated profiles at the early times, by varying k 8.1a in the Cook et al. mechanism. As can be seen in Figure 8.4, with the k 8.1a modified to fit the early times of the measured NH 2 timehistory, the simulated NH 2 time-history does not match the measured NH 2 at later times (dotted line in Figure 8.5). At the later times, secondary reactions other than Reaction 8.1a start to become more significant, as shown in Figure The reaction that forms NH 2 from the immediate decomposition product of MMH is: NHNH 2 + H = NH 2 + NH 2 (R 8.2). The reaction rate constant of this reaction in the Sun et al. mechanism was estimated as k 8.2 = exp (-755/T) cm 3 /mol/s; no modification to this reaction was made by Cook et al. It was found, under the current test conditions, that increasing k 8.2 in the Cook et al. mechanism by a factor of 3 greatly improves the match of the simulated NH 2 time-history with the experiment at the later times. wing to this modification to k 8.2, the optimum value of k 8.1a determined initially by fitting early time data (with initial k 8.2 ), needed to be reduced slightly (within 10% for all test conditions) to maintain a good match with the experiment at the early times. According to the sensitivity analysis results in Figure 8.2 and Figure 8.3, there are other reactions besides Reaction 8.2 that also affect the NH 2 concentrations at the later times. Modification to Reaction 8.2 was chosen because it leads to close match with the experimental data at all the current temperatures and pressures, which was not possible with any reasonable variations of the other reactions (NH 2 + H, NH 2 + NH 2, NH 2 + MMH) we explored. No update to the activation energy of Reaction 8.2 was made in the current study, and the newly proposed reaction rate constant for Reaction 8.2 is: k 8.2 = exp (-755/T) cm 3 /mol/s (Eq 8.2). The simulated NH 2 time-history using the Cook et al. mechanism with k 8.1a and k 8.2 modified are also included in Figure 8.4 (red dash-dotted line), along with perturbations of ±40% in the best-fit k 8.1a. The Cook et al. mechanism, with the 95

122 NH 2 [ppm] recommended modifications, matches closely with the experimental data, in both the NH 2 formation and removal processes. In the following sections, the Cook et al. mechanism with k 8.1a and k 8.2 updated will be referred to as the modified Cook et al. mechanism. 150 Best-fit k 8.1a 1.4, k Best-fit k 8.1a 0.6, k Experiment Best-fit k 8.1a Best-fit k 8.1a, k ppm MMH /argon 1217 K, 0.34 atm Time [ s] Figure 8.4. Representative NH 2 time-history measurement at 1217 K and 0.34 atm in 350 ppm MMH/argon, with an error bar ±15% due to the uncertainty of NH 2 cross section, in comparison with the Cook et al. mechanism with the best-fit k 8.1a (dotted line), and the Cook et al. mechanism with the best-fit k 8.1a and with k 8.2 increased by a factor of 3 (dash-dotted line). NH 2 concentration time-histories were measured behind reflected shock waves at different temperatures and pressures; representative data near 0.3, 1, and 5 atm together with the simulation results using the modified Cook et al. mechanism are shown in Figure The simulation results using the modified Cook et al. mechanism, with the experimentally inferred k 8.1a, and the k 8.2 as in Equation 8.2, match closely with the experiment over all the current temperatures and pressures. Fit to the later times of the time-histories can be further improved by changing the activation energy of k 8.2, but that was not attempted in the current study. The measured NH 2 timehistories were used to infer k 8.1a as described above and the results over K and atm are shown in Table

123 NH 2 [ppm] NH 2 [ppm] K, 0.32 atm 350 ppm MMH / argon Dash-Dotted Lines: Modified Cook et al K, 0.34 atm K, 0.34 atm K, 0.33 atm Time [ s] Figure 8.5. NH 2 time-history measurement near 0.3 atm, in comparison with the simulation results using the modified Cook et al. mechanism [34] with the updated k 8.1a and k 8.2 (dash-dotted line) K 0.95 atm 380 ppm MMH / argon Dash-Dotted Lines: Modified Cook et al K 0.99 atm 1206 K 0.98 atm 1119 K 1.01 atm Time [ s] Figure 8.6. NH 2 time-history measurement near 1 atm, in comparison with the simulation results using the modified Cook et al. mechanism [34] with the updated k 8.1a and k 8.2 (dash-dotted line). 97

124 NH 2 [ppm] ppm MMH / argon Dash-Dotted Lines: Modified Cook et al K, 4.5 atm K, 5.1 atm 1163 K, 5.2 atm Time [ s] Figure 8.7. NH 2 time-history measurement near 5 atm, in comparison with the simulation results using the modified Cook et al. mechanism [34] with the updated k 8.1a and k 8.2 (dash-dotted line). Table 8.1. Measured reaction rate constants for the N-N bond scission of MMH T P [atm] k 8.1a [s -1 ] 350 ppm MMH in argon ppm MMH in argon ppm MMH in argon

125 ppm MMH in argon % MMH in argon ppm MMH in argon The Arrhenius plot for the inferred k 8.1a is shown in Figure 8.8, over K and atm. Also shown in Figure 8.8 is the reaction rate expression of Reaction 8.1a recommended by Cook et al. at pressures near 2.5 atm [34] (dashed line). As can be seen in Table 8.1, several experiments were carried out in 1% MMH/argon near 2.5 atm, to confirm agreement with the Cook et al. data. 99

126 k 8.1a [S -1 ] 1333K 1250K 1176K 1111K 1053K P ~ 5 atm P ~ 2.4 atm P ~ 1.6 atm P ~ 1 atm P ~ 0.65 atm P ~ 0.33 atm Cook et al. (2011), P ~ 2.5 atm MMH = CH 3 N.H+NH /T [K -1 ] Figure 8.8. Measured MMH N-N bond scission reaction rate constants k 8.1a, in firstorder reaction form. Data points: current study. Dashed line: reaction rate constant expression for k 8.1a near 2.5 atm by Cook et al. [34]. A detailed error analysis was carried out to estimate the uncertainty of the experimental inferred k 8.1a at 1217 K and 0.34 atm in 350 ppm MMH/argon. The primary sources of uncertainty for the rate constant determination include uncertainties in (a) pressure (±1%), (b) temperature (±1%), (c) fitting data (±5%), (d) MMH concentration (±15%), (e) NH 2 absorption cross section (±15%), (f) the rate constant for NH 2 + H = NH + H 2 (±40%), (g) the rate constant for MMH = CH 3 + NHNH 2 (uncertainty factor used: 2), (h) the rate constant for MMH + NH 2 = CH 3 N.NH 2 + NH 3 (uncertainty factor used: 2), (i) the rate constant for NHNH 2 + H = NH 2 + NH 2 (uncertainty factor used: 5), (j) the rate constant for NH 2 + NH 2 = NH*NH + H 2 (uncertainty factor used: 5), (k) the rate constant for CH 3 N.H = CH 2 NH + H (uncertainty factor used: 2) and (l) the rate constant for MMH = CH 2 NH + NH 3 (uncertainty factor used: 2). Figure 8.9 shows the contributions from each source of uncertainty, which were obtained by perturbing each uncertainty source to its error limits and refitting an overall rate constant for Reaction 8.1a. 100

127 Besides the secondary reactions included in Figure 8.2, other MMH decomposition channels including MMH = CH 3 + NHNH 2 and MMH = CH 2 NH + NH 3, and the reaction of CH 3 N.H were analyzed using the same method. The roaming reaction MMH = CH 2 NH + NH 3 was not included in the Cook et al. mechanism (originally developed by Sun et al. [47]), and was added with the reaction rate constant by Golden and Solly. The influence on k 8.1a by adding this reaction is less than ±1%. The reaction of CH 3 N.H was perturbed by a factor of 2 and the experimental data were refitted for k 8.1a. The influence of CH 3 N.H reaction on k 8.1a result is also less than ±1%. The determination of k 8.1a in the current study is insensitive to secondary reactions mostly because current low MMH concentration makes the early times of NH 2 timehistory, which is dominated by Reaction 8.1a, sufficiently long for fitting determination of k 8.1a. The errors introduced by secondary reactions are small at the early times, even though the importance of secondary reactions increases with time. All the uncertainties were assumed to be uncorrelated and combined in a root-sum-squared method to yield a total uncertainty of ±26% in the reaction rate constant at 1217 K and 0.34 atm in 350 ppm MMH/argon. Similar error analysis was performed for k 8.1a at 1163 K and 5.2 atm in 170 ppm MMH/argon as well. The uncertainty in fitting data increases to ±10% at high pressure, and the uncertainty in initial MMH concentration increases to ±20% at low concentrations. The influences of the uncertainty in the rate constants of secondary reactions increase slightly as well, and the overall uncertainty was estimated to be ±37% at 1163 K and 5.2 atm in 170 ppm MMH/argon mixture. 101

128 Figure 8.9. Uncertainty analysis for k 8.1a at 1217 K and 0.34 atm in 350 ppm MMH/argon. The k 8.1a values determined in the current study, at pressures near 0.3, 1 and 5 atm, are compared with those from the previous MMH decomposition studies in Figure The data by Eberstein et al. [90], Golden et al. [91], and Sun et al. [93] are the overall MMH decomposition rate, and are shown in solid scatter points in Figure As is recommended by Cook et al., Reaction 8.1a accounts for ~90% of the overall unimolecular decomposition of MMH [34]. The data by Kerr et al. are the measured first-order rate constants for the homogenous dissociation of the N-N bond in MMH, which is exactly Reaction 8.1a, but at lower temperatures and pressures. The reaction rate constant for k 8.1a, recommended by Cook et al. [34] based on shock tube experiments in 1% MMH/argon mixtures near 2.5 atm, is shown as the black solid line in Figure Also shown in Figure 8.10 are the theoretical results by Zhang et al. [94], for the rate constants of Reaction 8.1a at different pressures, making use of ab initio transition state theory-based master equation analysis. In Zhang et al., the transition states for the N-N bond scission were studied using variable reaction coordinate transition state 102