SHOCK TUBE STUDY OF NITROGEN-CONTAINING

FUELS

A DISSERTATION

SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING

AND THE COMMITTEE ON GRADUATE STUDIES

OF STANFORD UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

Sijie Li

June 2014

http://creativecommons.org/licenses/by-nc/3.0/us/

This dissertation is online at: http://purl.stanford.edu/nw843zs8610

© 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.

ii

http://creativecommons.org/licenses/by-nc/3.0/us/http://creativecommons.org/licenses/by-nc/3.0/us/http://purl.stanford.edu/nw843zs8610

I certify that I have read this dissertation and that, in my opinion, it is fully adequatein 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 adequatein 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 adequatein 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 adequatein 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 inUniversity Archives.

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iv

v

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 (C4H9NO, 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 (OH) with dimethylamine

(DMA: CH3NHCH3) and ethylamine (EA: CH3CH2NH2) were measured behind

reflected shock waves using UV laser absorption of OH radicals near 306.7 nm. The

overall rate constants were determined by fitting the measured OH time-histories with

the computed profiles using the detailed dimethylamine and ethylamine sub-

mechanisms contained in the morpholine mechanism. Variational transition state theory

was used to compute the H-abstraction rates by OH 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.

vi

Dimethylamine (DMA) ignition delay times and OH time-histories were

investigated behind reflected shock waves. The dimethylamine ignition delay time

measurements were carried out in 4% oxygen/argon. OH time-histories were measured

in stoichiometric mixtures of 500 ppm DMA/O2/argon. The morpholine mechanism was

then updated by adding the DMA unimolecular decomposition channel: DMA =

CH3NH + CH3. With this modification, the simulation results are in excellent agreement

with both the dimethylamine ignition delay times and OH time-history data.

Ethylamine (CH3CH2NH2) pyrolysis and oxidation were studied behind

reflected shock waves. For ethylamine pyrolysis, NH2 time-histories were measured in

2000 ppm ethylamine/argon mixtures. For ethylamine oxidation, ignition delay times,

NH2 and OH time-histories were measured in ethylamine/O2/argon mixtures. By fitting

the simulations to the early time-histories of NH2 and OH, 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 NH2 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 = CH3N.H + NH2 was determined based on early time of

vii

the NH2 time-histories. Pressure dependence of this reaction was observed at 0.3-5 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 NH2 time-histories, a new reaction

rate expression was recommended for the reaction: NHNH2 + H = NH2 + NH2.

ix

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.

xi

Table of Contents

Abstract ................................................................................................................................v

Acknowledgments.............................................................................................................. ix

Table of Contents ............................................................................................................... xi

List of Tables .................................................................................................................. xvii

List of Figures .................................................................................................................. xix

Chapter 1. Introduction ..................................................................................................1

1.1. Background and Motivation ........................................................................1

1.2. Overview of Thesis ......................................................................................3

Chapter 2. Experimental Method...................................................................................5

2.1. Shock Tube Facility .....................................................................................5

2.1.1. Stanford High Pressure Shock Tube (HPST)...................................6

2.1.2. Stanford Kinetic Shock Tube (KST)................................................7

2.1.3. Stanford NASA Shock Tube (NASA) .............................................7

2.2. Laser Diagnostic ..........................................................................................8

2.2.1. Overview ..........................................................................................8

xii

2.2.2. IR Diagnostic of Fuel ...................................................................... 9

2.2.3. NH2 Diagnostic ................................................................................ 9

2.2.4. OH Diagnostic ............................................................................... 10

Chapter 3. Shock Tube and Modeling Study of Morpholine ...................................... 13

3.1. Introduction ............................................................................................... 13

3.2. Experimental Details ................................................................................. 13

3.3. Results and Discussion .............................................................................. 15

3.3.1. Morpholine Ignition Delay Times ................................................. 15

3.4. Model Development and Simulations ....................................................... 19

3.5. Summary .................................................................................................... 24

Chapter 4. Reactions of OH with Dimethylamine and Ethylamine ............................ 27

4.1. Introduction ............................................................................................... 27

4.2. Experimental Setup ................................................................................... 29

4.3. Kinetic Measurements ............................................................................... 30

4.3.1. Dimethylamine (DMA) + OH ....................................................... 31

4.3.2. Ethylamine (EA) + OH .................................................................. 36

4.4. Theoretical Study ....................................................................................... 42

4.5. Summary .................................................................................................... 46

Chapter 5. Dimethylamine Oxidation ......................................................................... 49

5.1. Introduction ............................................................................................... 49

xiii

5.2. Experimental Setup ....................................................................................50

5.3. Results and Discussion ..............................................................................50

5.3.1. Dimethylamine Ignition Delay Times ...........................................50

5.3.2. OH Time-Histories.........................................................................54

5.3.3. Update to the Morpholine Mechanism ..........................................56

5.4. Summary ....................................................................................................63

Chapter 6. Ethylamine Pyrolysis and Oxidation .........................................................65

6.1. Introduction ................................................................................................65

6.2. Experimental Methods ...............................................................................65

6.3. Experimental Results .................................................................................66

6.3.1. Ethylamine Pyrolysis .....................................................................66

6.3.2. Ethylamine Oxidation ....................................................................67

6.4. Update to the Morpholine Mechanism ......................................................71

6.5. Summary ....................................................................................................80

Chapter 7. Revisiting the Morpholine Data .................................................................81

7.1. Introduction ................................................................................................81

7.2. Morpholine Ignition Delay Times .............................................................82

7.3. Sensitivity Analysis ...................................................................................85

7.4. Summary ....................................................................................................86

Chapter 8. MMH Pyrolysis ..........................................................................................89

xiv

8.1. Introduction ............................................................................................... 89

8.2. Experimental Method ................................................................................ 91

8.3. Results and Discussion .............................................................................. 92

8.4. Summary .................................................................................................. 106

Chapter 9. Summary and Future Work ..................................................................... 107

9.1. Summary of Results ................................................................................ 107

9.1.1. Morpholine Oxidation ................................................................. 107

9.1.2. Dimethylamine and Ethylamine Combustion ............................. 108

9.1.3. MMH Pyrolysis ........................................................................... 110

9.2. Recommendations for Future Work ........................................................ 111

9.2.1. Dimethylamine and Ethylamine Pyrolysis .................................. 111

9.2.2. Dimethylamine and Ethylamine Oxidation ................................. 112

9.2.3. Morpholine Pyrolysis and Oxidation ........................................... 113

9.3. Conclusion ............................................................................................... 113

Appendix A. Morpholine Oxidation Set ......................................................................... 115

Appendix B. Morpholine Pyrolysis Set ........................................................................... 119

Appendix C. Thermochemistry for Morpholine Species ................................................ 121

Appendix D. Computational Methods ............................................................................. 127

D.1. CHEMKIN Simulation ................................................................................ 127

D.2. Multiwell Calculation .................................................................................. 128

xv

D.3. Gaussian Calculation....................................................................................130

Bibliography ....................................................................................................................133

xvi

xvii

List of Tables

Table 3.1. Shock tube ignition delay times. Test gas mixture:

morpholine/O2/argon........................................................................................15

Table 4.1. Reactions describing DMA and EA + OH experiments ...................................31

Table 4.2. Measured rate constants for DMA + OH = Products .......................................35

Table 4.3. Measured rate constants for EA + OH = Products ...........................................40

Table 4.4. Summary of the zero Kelvin electronic energies and rotational data

used for dimethylamine and ethylamine + OH VTST calculations .................43

Table 4.5. The vibrational frequencies computed at the BH&HLYP/6-

311++G(2d,2p) level of theory ........................................................................44

Table 4.6. VTST reaction rate constants of individual channels for DMA and EA

+ OH. ...............................................................................................................45

Table 5.1. Ignition delay time data for dimethylamine ......................................................53

Table 5.2. Summary of rate recommendations to the dimethylamine sub-

mechanism of the morpholine mechanism. .....................................................59

Table 6.1. Updated reaction rate constants to ethylamine sub-mechanism .......................72

xviii

Table 7.1. Modifications to the morpholine mechanism [54], recommended in

Chapters 4-6 ..................................................................................................... 81

Table 8.1. Measured reaction rate constants for the N-N bond scission of MMH ............ 98

xix

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 NH2, using the fundamental output of a

Spectra Physics 380 ring dye laser...................................................................10

Figure 2.4. Ring dye laser diagnostic of OH, using the frequency doubled output

of a Spectra Physics 380 ring dye laser. ..........................................................11

Figure 3.1. Sample pressure trace for morpholine ignition delay time

measurements. ..................................................................................................15

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. .........................................................................................................17

Figure 3.3. Ignition delay time measurements in morpholine/air Φ = 1 mixtures

near 15 and 25 atm. Dashed lines: linear fit to data. ........................................18

Figure 3.4. Ignition delay time measurements in stoichiometric morpholine/air

mixtures and morpholine/4% O2/argon mixtures around 15 atm.

Dashed lines: linear fit to data. ........................................................................19

Figure 3.5. Comparison of model predictions to morpholine/air ignition delay

time measurements around 15 atm and under different equivalence

xx

ratios. Dashed lines: Simulation results using the mechanism in [8].

Solid lines: Simulation results using the morpholine mechanism [54]. .......... 22

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

Figure 3.7. Comparison of model predictions to morpholine ignition delay time

measurements around 15 atm for morpholine/4% O2/argon and

morpholine/air mixtures. Dashed lines: Simulation results using the

previous mechanism [8]. Solid lines: Simulation results using the

morpholine mechanism [54]. ........................................................................... 24

Figure 4.1. Sensitivity analysis of OH 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 H2O, at 1176 K and 0.9 atm. ..................... 33

Figure 4.2. Sample OH trace in 320 ppm DMA/Ar with 22 ppm TBHP and 134

ppm H2O, at 1176 K and 0.9 atm. ................................................................... 33

Figure 4.3. Error analysis for measured k4.1 in 320 ppm DMA/Ar with 22 ppm

TBHP and 134 ppm H2O, at 1176 K and 0.9 atm. .......................................... 34

Figure 4.4. Measured overall reaction rate for k4.1: DMA+OH = Products, in

comparison with the estimation by Lucassen et al. [25]. ................................ 36

Figure 4.5. Sensitivity analysis of OH using the ethylamine sub-mechanism [25]

with inclusion of TBHP chemistry, in a mixture of 470 ppm EA,50

ppm TBHP, 190 ppm H2O, in Ar at 1067 K and 0.83 atm. ............................. 38

xxi

Figure 4.6. Sample OH trace in in a mixture of 470 ppm EA, 50 ppm TBHP, 190

ppm H2O, in Ar at 1067 K and 0.83 atm. ........................................................39

Figure 4.7. Error analysis for measured k4.2 in 470 ppm EA/Ar with 50 ppm

TBHP and 190 ppm H2O, at 1067 K and 0.83 atm. .........................................40

Figure 4.8. Measured overall reaction rates for EA + OH = 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 + OH and EA + OH. .............................................................46

Figure 5.1. Sample pressure traces for dimethylamine ignition delay times. ....................51

Figure 5.2. Ignition delay times in stoichiometric DMA/4% O2/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% O2/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]. ...........53

Figure 5.4. OH time-histories in stoichiometric mixture of 500 ppm

DMA/O2/argon at 1417 K and 2.2 atm, with simulation results using

the morpholine mechanism described in Chapter 3 [54]. ................................55

Figure 5.5. OH time-histories in stoichiometric mixture of 500 ppm

DMA/O2/argon at 1504 K and 2.1 atm, with simulation results using

the morpholine mechanism described in Chapter 3 [54]. ................................56

Figure 5.6. Sensitivity analysis of temperature, at initial condition of 1300 K and

1.5 atm in a stoichiometric mixture of DMA/4% O2/argon, using the

morpholine mechanism described in Chapter 3 [54]. ......................................57

xxii

Figure 5.7. Ignition delay times in stoichiometric mixtures of DMA/4%

O2/argon, at P ~ 0.9, 1.5 and 2.8 atm, measurements and simulations

using the morpholine mechanism [54] with the modifications in

Table 5.2. ......................................................................................................... 60

Figure 5.8. Ignition delay times in stoichiometric mixtures of DMA/4%

O2/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 5.2. .............................................................................. 60

Figure 5.9. Measured dimethylamine ignition delay times in 4% O2/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. ............................................................................. 61

Figure 5.10. Comparison of the simulated OH 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/O2/argon at 1417 K and 2.2 atm. ........................................................... 62

Figure 5.11. Comparison of the simulated OH 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/O2/argon at 1504 K and 2.1 atm. ........................................................... 63

Figure 6.1. NH2 time-histories in 2000 ppm ethylamine/Ar mixtures,

measurements and simulation results using the morpholine

mechanism [54] reported in Chapter 3. ........................................................... 67

Figure 6.2. Ethylamine ignition delay time measurements near 0.85, 1.35 and 2

atm in stoichiometric mixture of ethylamine/4% O2/Ar, and

simulations based on the morpholine mechanism [54] in Chapter 3. .............. 68

xxiii

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 3. ..........................................................69

Figure 6.4. NH2 time-histories in 2000 ppm ethylamine/0.8% O2/Ar mixtures;

simulations are based on the morpholine mechanism presented in

Chapter 3. .........................................................................................................70

Figure 6.5. OH time-histories in 500 ppm ethylamine/0.2% O2/Ar mixtures,

measurements and simulations using the morpholine mechanism [54]

presented in Chapter 3. ....................................................................................71

Figure 6.6. NH2 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. ..................................................................................74

Figure 6.7. Measured NH2 time-histories in 2000 ppm ethylamine/Ar mixtures;

simulation results are based on the morpholine mechanism [54] with

updates shown in Table 6.1. .............................................................................75

Figure 6.8. Measurements of ethylamine ignition delay times near 0.85, 1.35 and

2 atm in stoichiometric mixture of ethylamine/4% O2/Ar; simulation

results utilize the morpholine mechanism [54] with updates in Table

6.1.....................................................................................................................76

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 6.1. .....................................................76

Figure 6.10. NH2 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% O2/Ar mixtures. ....................................................................77

xxiv

Figure 6.11. NH2 time-histories in 2000 ppm ethylamine/0.8% O2/Ar mixtures:

measurements (solid lines) and simulation results based on the

morpholine mechanism [54] with updates in Table 6.1 (dash-dotted

lines). ............................................................................................................... 78

Figure 6.12. OH sensitivity analysis at 1399 K, 1.9 atm in 500 ppm

ethylamine/0.2% O2/Ar mixtures, using the morpholine mechanism

[54] with updates in Table 6.1. ........................................................................ 79

Figure 6.13. OH time-histories in 500 ppm ethylamine/0.2% O2/Ar mixtures:

measurements (solid lines) and simulation results using the

morpholine mechanism [54] with updates in Table 6.1 (dash-dotted

lines). ............................................................................................................... 80

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 4-6. .......................................................................... 83

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 4-6. ............ 84

Figure 7.3. Comparisons of model predictions to ignition delay time data near 15

atm in morpholine/4% O2/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 4-6. ............................................... 85

xxv

Figure 7.4. . 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. ..........................................................................................86

Figure 8.1. Representative NH2 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. NH2 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. .....................................................................94

Figure 8.3. NH2 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

Figure 8.4. Representative NH2 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 NH2 cross section, in comparison with the Cook et al.

mechanism with the best-fit k8.1a (dotted line), and the Cook et al.

mechanism with the best-fit k8.1a and with k8.2 increased by a factor of

3 (dash-dotted line). .........................................................................................96

Figure 8.5. NH2 time-history measurement near 0.3 atm, in comparison with the

simulation results using the modified Cook et al. mechanism [34]

with the updated k8.1a and k8.2 (dash-dotted line). ............................................97

Figure 8.6. NH2 time-history measurement near 1 atm, in comparison with the

simulation results using the modified Cook et al. mechanism [34]

with the updated k8.1a and k8.2 (dash-dotted line). ............................................97

xxvi

Figure 8.7. NH2 time-history measurement near 5 atm, in comparison with the

simulation results using the modified Cook et al. mechanism [34]

with the updated k8.1a and k8.2 (dash-dotted line). ............................................ 98

Figure 8.8. Measured MMH N-N bond scission reaction rate constants k8.1a, in

first-order reaction form. Data points: current study. Dashed line:

reaction rate constant expression for k8.1a near 2.5 atm by Cook et al.

[34]. ............................................................................................................... 100

Figure 8.9. Uncertainty analysis for k8.1a at 1217 K and 0.34 atm in 350 ppm

MMH/argon. .................................................................................................. 102

Figure 8.10. Representative MMH N-N bond scission reaction rate constants

k8.1a, in comparison with the previous studies. .............................................. 104

Figure 8.11. Pressure dependence of the measured k8.1a at representative

temperatures, in comparison with the theoretical study by Zhang et al

[94]. ............................................................................................................... 105

Figure 9.1. Representative species time-histories for ethylamine oxidation in

stoichiometric mixture with 4% O2/argon, at 1500 K and 1 atm:

simulations using the modified morpholine mechanism. .............................. 112

1

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, in-

depth studies of simpler model biofuels are needed first to gain insight into fuel-

nitrogen chemistry. Morpholine (C4H9NO, 1-oxa-4-aza-cyclohexane), which is a six-

membered 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 ring-

down spectroscopy (CRDS) was also employed to detect the profiles of intermediate

2

species including CH2, CH and NH2 [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 sub-

mechanisms 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.

Only 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 OH over the temperature range of 299-426 K [13]. A

similar study was then carried out to measure the rate constants of dimethylamine,

ethylamine and trimethylamine with OH over the temperature range of 298-426 K [14].

Carl et al. studied the reaction rate constants of aliphatic amines with OH 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 OH, using variational transition-state theory [16].

3

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 NH2 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 (N2O4, NTO)

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.

1.2. Overview of Thesis

The dissertations is divided into nine chapters:

4

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

(OH) with dimethylamine (DMA: CH3NHCH3) and ethylamine (EA: CH3CH3NH2).

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)/6-311++G(2d,2p) level of theory in the literature.

Chapter 5 discusses the oxidation study of dimethylamine, including ignition

delay time and OH 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 CH3NHNH2 = CH3N.H + NH2, making use of NH2 time-history

measurement during MMH pyrolysis process.

Chapter 9 provides a summary of the thesis and proposes several future works.

5

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.

2.1. 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. Optical 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.

6

Figure 2.1. Shock tube schematic.

2.1.1. 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 oC 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

7

using a piezoelectric pressure transducer (Kistler Model 603B1) located 1 cm from the

end wall. Other details concerning this shock tube can be found in [31].

2.1.2. Stanford Kinetic Shock Tube (KST)

Dimethylamine and ethylamine + OH, 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 14.13 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].

2.1.3. 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 15.24 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].

8

2.2. Laser Diagnostic

2.2.1. Overview

In this thesis, the primary laser diagnostic method utilized is the fixed-

wavelength direct absorption technique, which is a powerful tool for chemical kinetics

study. One 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, I0 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.

9

2.2.2. 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 oC 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.

2.2.3. NH2 Diagnostic

NH2 was measured using the output of a narrow-linewidth ring dye laser near

597.4 nm. This NH2 laser absorption diagnostic employed the overlapping ÃA1 ←

X2B1(090 ← 000)∑PQ1,N7 doublet lines, which was previously characterized in our

laboratory [24,34,37]. Visible light near 597.4 nm was generated by pumping

10

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 NH2 detection sensitivity of 5 ppm could be achieved for

most conditions studied in this work. A schematic of the NH2 diagnostic is shown in

Figure 2.3, and more details of the NH2 laser diagnostic setup are described in [34].

Figure 2.3. Ring dye laser diagnostic of NH2, using the fundamental output of a Spectra

Physics 380 ring dye laser.

2.2.4. OH Diagnostic

OH was measured near 306.7 nm using the frequency-doubled output of the

same ring dye laser system as for the NH2 diagnostic. The chosen wavelength was the

peak of the well-characterized R1(5) absorption line in the OH A-X(0,0) band [32].

Visible light near 613.4 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 306.7 nm. Using a common-mode rejection detection

setup, a minimum OH detection sensitivity of 0.5 ppm could be easily achieved. A

schematic of the OH diagnostic is shown in Figure 2.4, and more details of the laser

diagnostic setup are can be found in [32].

11

Figure 2.4. Ring dye laser diagnostic of OH, using the frequency doubled output of a

Spectra Physics 380 ring dye laser.

13

Chapter 3. Shock Tube and Modeling Study of

Morpholine

3.1. Introduction

As is introduced in Chapter 1, morpholine (C4H9NO, 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.

3.2. 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 2.1.1.

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

14

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].

0 500 1000 1500 2000

0.0

0.5

1.0

1.5

Pre

ssure

[A

. U

.]

Time [s]

ign

= 1070 s

Morpholine/Air

925 K, 15.8 atm

= 1

15

Figure 3.1. Sample pressure trace for morpholine ignition delay time

measurements.

3.3. Results and Discussion

3.3.1. 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 3.2-3.4 and listed in Table 3.1.

Table 3.1. Shock tube ignition delay times. Test gas mixture: morpholine/O2/argon.

T5 1000/T5 P5 Φ XO2 IDT

[K] [1/K] [atm] [µs]

910 1.099 14.71 1 0.21 1350

915 1.093 15.76 1 0.21 1130

925 1.081 15.83 1 0.21 1070

938 1.066 14.66 1 0.21 1000

966 1.035 14.79 1 0.21 636

1009 0.991 14.04 1 0.21 374

1062 0.942 13.77 1 0.21 197

1097 0.912 13.02 1 0.21 115

921 1.086 13.79 0.5 0.21 1970

983 1.017 13.94 0.5 0.21 1030

1023 0.978 13.37 0.5 0.21 547

1110 0.901 13.19 0.5 0.21 192

1168 0.856 12.53 0.5 0.21 97

875 1.143 16.45 2 0.21 969

901 1.110 16.60 2 0.21 775

935 1.070 16.29 2 0.21 562

965 1.036 15.25 2 0.21 402

994 1.006 16.13 2 0.21 223

16

1041 0.961 15.08 2 0.21 136

1027 0.974 16.50 1 0.04 1150

1042 0.960 16.64 1 0.04 845

1091 0.917 16.30 1 0.04 430

1139 0.878 15.94 1 0.04 244

1197 0.835 15.56 1 0.04 145

932 1.073 25.21 1 0.21 625

955 1.047 29.18 1 0.21 389

986 1.014 26.70 1 0.21 279

994 1.006 27.02 1 0.21 241

1046 0.956 27.05 1 0.21 117

1074 0.931 26.36 1 0.21 82

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 3.2-3.4. 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% O2 and 79% N2 was used. The

stoichiometric case was defined with the following reaction:

C4H9NO + 5.75(O2 + 3.76N2) = 4CO2 + 4.5H2O + 22.12N2 (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

17

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.

0.9 1.0 1.1

100

1000

= 0.5

= 1

= 2

Ignitio

n D

ela

y T

ime [s]

1000/T [K-1]

Morpholine/Air

Scaled to 15 atm with P-1

1111K 1000K 909K

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.

18

0.9 1.0 1.1

100

1000

1111K 1000K 909K

P = 15 atm

P = 25 atm

Ignitio

n D

ela

y T

ime [s]

1000/T [K-1]

Morpholine/Air = 1

Scaled with P-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% O2/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.

19

0.8 0.9 1.0 1.1

100

1000

1250K 1111K 1000K 909K

Morpholine/4% O2/Ar

Morpholine/Air

Ignitio

n D

ela

y T

ime [s]

1000/T [K-1]

= 1

Scaled to 15 atm with P-1

Figure 3.4. Ignition delay time measurements in stoichiometric morpholine/air mixtures

and morpholine/4% O2/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 = 1.7×10-3

Φ-0.8

P-0.9

XO2-0.84

exp(13400/T) [s] (Eq 3.2)

over the temperature range of 866-1197 K, pressures 15-25 atm and equivalence ratios

0.5-2.

3.4. 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

20

study [39] (see Appendix A) and the current data. Additionally, the H/C/O 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, O) in the morpholine ring, so it

is proposed that, similar to cyclohexane, morpholine oxidation in the shock tube occurs

by O2 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. Once an RO2 morpholine species is formed, as in cyclohexane, the O2 group can

internally abstract a hydrogen atom from the ring and either form HO2 + an unsaturated

morpholine-ene cyclic species or one of several morpholine QOOH species. The

resulting QOOH 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

21

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 CH2=CH-O-CH2-CH2-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, •CH2-O-CH2-CH2-CH2-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/harmonic-

oscillator 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 3.5-3.7. 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

22

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 O2 addition chemistry. The current mechanism

is quite good at the equivalence ratio 0.5, but overpredicts the ignition delay times as

richness increases.

0.8 0.9 1.0 1.1 1.2

100

1000

10000

= 0.5

= 1

= 2

Ignitio

n D

ela

y T

ime [s]

1000/T [K-1]

Morpholine/Air

Scaled to 15 atm with P-1

1250K 1111K 1000K 909K 833K

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

→ CH2CH2NHCH2CHO and the metamorphyl → CH2CH2OCH2CHNH beta-scission

reactions.

23

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% O2 in N2) or 4%

O2 in Ar.

0.8 0.9 1.0 1.1 1.2

100

1000

10000

1250K 1111K 1000K 909K 833K

P = 15 atm

P = 25 atm

Ignitio

n D

ela

y T

ime [s]

1000/T [K-1]

Morpholine/Air = 1

Scaled with P-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].

24

0.8 0.9 1.0 1.1 1.2

100

1000

10000

1250K 1111K 1000K 909K 833K

Morpholine/4% O2/Ar

Morpholine/Air

Ignitio

n D

ela

y T

ime [s]

1000/T [K-1]

= 1

Scaled to 15 atm with P-1

Figure 3.7. Comparison of model predictions to morpholine ignition delay time

measurements around 15 atm for morpholine/4% O2/argon and morpholine/air mixtures.

Dashed lines: Simulation results using the previous mechanism [8]. Solid lines:

Simulation results using the morpholine mechanism [54].

3.5. 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%

O2 in Ar and synthetic air with 21% O2. 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

25

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.

27

Chapter 4. Reactions of OH with Dimethylamine and

Ethylamine

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 OH 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 NOx [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 OH radical are important steps in the combustion of amines, thus

dimethylamine and ethylamine + OH reactions are of great research value.

Experimental and computational studies of the reactions between aliphatic

amines and OH are scarce. Atkinson et al. investigated the reactions of methylamine

(MA: CH3NH2, CAS: 74-89-5) with OH over the temperature range of 299-426 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 OH over the temperature range of 298-426 K [14]. Carl et al.

studied the reaction rate constants of aliphatic amines with OH at 295 K, including

those for methylamine, dimethylamine, ethylamine and trimethylamine, using the

sequential two-photon dissociation of NO2 in the presence of H2 as a source of OH [15].

Galano and Alvarez-Idaboy analyzed the different reaction channels of methylamine,

28

dimethylamine and ethylamine with OH, using variational transition-state theory [16].

Geometry optimization and frequency calculations were performed at the

BH&HLYP/6-311++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 OH were reported within the temperature range 290-310 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 + OH 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 OH under combustion

conditions. The present work determines the reaction rate constants for the overall

reactions of dimethylamine (DMA: CH3NHCH3, CAS: 124-40-3) and ethylamine (EA:

CH3CH2NH2, CAS: 75-04-7) with OH.

DMA + OH = Products (R4.1)

EA + OH = Products (R4.2)

The OH radical was generated by near-instantaneous pyrolysis of tert-butyl

hydroperoxide (TBHP, CAS: 75-91-2). The pseudo-first order decay of OH behind

reflected shock waves was monitored using laser absorption at 306.7 nm, and the

reaction rate constants of amine with OH were inferred from the measured OH time-

histories. Variational transition state theory was used to compute the H-abstraction rates

by OH for dimethylamine and ethylamine, with potential energy surface geometries,

frequencies and electronic energies calculated by Galano and Alvarez-Idaboy at

CCSD(T)/6-311++G(2d,2p) level of theory [16].

29

4.2. Experimental Setup

The Stanford Kinetic Shock Tube as described in section 2.1.2 was used for the

dimethylamine and ethylamine + OH experiments, with the OH decay time-histories

monitored using the OH diagnostic presented in section 2.2.4. 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 OH, the TBHP concentrations were determined based on the peak OH 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.

30

4.3. Kinetic Measurements

Experiments were performed behind reflected shock waves over the temperature

range of 901-1368 K and pressures near 1.2 atm. At temperatures greater than 1000 K,

TBHP dissociates near-instantaneously to form an OH radical and a tert-butoxy radical.

The tert-butoxy radical, (CH3)3CO, further dissociates into acetone and a methyl radical.

TBHP also reacts with OH radical to form other products. The TBHP chemistry set can

be described as follows:

TBHP = (CH3)3CO + OH (R4.3)

(CH3)3CO = CH3COCH3 + CH3 (R4.4)

TBHP + OH = H2O + O2 + tert-C4H9 (R4.5)

TBHP + OH = H2O + HO2 + iso-C4H8 (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 CH3 + OH

rate constant around 1.5 atm is important for accurate determination of the fuel + OH

reaction rate constant. There are two major channels for CH3 + OH,

CH3+OH + M = CH3OH + M (R4.7)

CH3 + OH = CH2(s) + H2O (R4.8).

Reaction 4.7 was updated using the results from Srinivasan et al. [68] at ~0.3-1.1 atm,

and their values agree well with the theoretical study by Jasper et al. [69] and the

31

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 294-714 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 4.3-4.8 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 + OH experiments

# Reaction Rate Constant [cm

3mol

-1s

-1]

Ref. A n E

4.1 DMA + OH = Products See text This work

4.2 EA + OH = Products See text This work

4.3 TBHP = (CH3)3CO + OH1 3.57 × 10

13 0 3.575 × 10

4 [33]

4.4 (CH3)3CO = CH3COCH3 + CH31 1.26 × 10

14 0 1.530 × 10

4 [66]

4.5 TBHP + OH = H2O + O2 + tert-C4H9 2.30 × 1013

0 5.223 × 103 [33]

4.6 TBHP + OH = H2O + HO2 + iso-C4H8 2.49 × 1013

0 2.655 × 103 [33]

4.7 CH3 + OH + M = CH3OH + M 1.73 × 108 1.41 -3.32 × 10

4 [68]

4.8 CH3 + OH = CH2(s) + H2O 1.65 × 1013

0 0 [33] 1Rate 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 PRO [55] package was used to simulate the OH time-histories, with the

standard constant energy and volume assumptions.

4.3.1. Dimethylamine (DMA) + OH

The reaction of dimethylamine with OH consists of two different channels:

DMA + OH = CH3NHCH2 + H2O (R4.1a)

32

DMA + OH = CH3NCH3 + H2O (R4.1b)

In the dimethylamine sub-mechanism by Lucassen et al. [25], the total rate

constant of DMA with OH 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

k4.1a = 2×1013

cm3mol

-1s

-1 and k4.1b = 1.9 ×10

13 cm

3mol

-1s

-1.

A OH sensitivity analysis was carried out for the overall rate constant

determination of Reaction 4.1 (k4.1 = k4.1a + k4.1b) 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 OH

sensitivity is defined as SOH = (∂XOH/∂ki)×(ki/XOH), where XOH is the local OH mole

fraction and ki 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 OH 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 k4.1 = 3 × 1013

cm3mol

-1s

-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 best-

fit 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 H2O in the test mixture has no

noticeable influence on the simulated OH profiles.

33

0 20 40 60 80-7

-6

-5

-4

-3

-2

-1

0

1

320 ppm DMA / Ar

22 ppm TBHP / 140 ppm H2O

1176 K, 0.9 atm

OH

Se

nsitiv

ity

Time [s]

DMA + OH = Products

TBHP = tert-Butoxy + OH

CH3+OH=CH

2(S)+H

2O

CH3+CH

2=C

2H

4+H

CH3NHCH

2=CH

3+CH

2NH

Figure 4.1. Sensitivity analysis of OH 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 H2O, at 1176 K and 0.9 atm.

0 10 20 30 40

10

50

k4.1

1.5

OH

[pp

m]

Time [s]

Experiment

k4.1

= 3.2E13 cm3mol

-1s

-1

(best-fit)

320 ppm DMA/ Ar

22 ppm TBHP/ 134 ppm H2O

1176 K, 0.9 atm

k4.1

0.5

2

Figure 4.2. Sample OH trace in 320 ppm DMA/Ar with 22 ppm TBHP and 134 ppm

H2O, at 1176 K and 0.9 atm.

Under the current pseudo first-order conditions, OH decays exponentially and

close to be a straight line in the semi-log plot of Figure 4.2.

34

Figure 4.3. Error analysis for measured k4.1 in 320 ppm DMA/Ar with 22 ppm TBHP

and 134 ppm H2O, 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) OH cross

section(±3%), (e) fitting data (±7%), (f) the rate constant for TBHP = tert-Butoxy + OH

(Reaction 4.3, ±30%), (g) the rate constant for CH3 + OH = CH2(s) + H2O (Reaction

4.8, uncertainty factor used: 2), (h) the rate constant for CH3 + CH2 = C2H4 + H

(uncertainty factor used: 2), (i) the rate constant for CH3NHCH2 = CH3 + CH2NH

(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 k4.1, and no significant

influence on k4.1 determination was observed due to the uncertainties in other secondary

35

reactions. 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 rate constant k4.1 at

1176 K.

Similar tests were carried out for the reaction of DMA with OH, over the

temperature range of 925-1307 K, and pressures of 0.89-1.24 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 OH.

Table 4.2. Measured rate constants for DMA + OH = Products

T P k4.1 x 10-13

[K] [atm] [cm3mol

-1s

-1]

60 ppm TBHP, 440 ppm DMA, Ar

925 0.99 2.80

995 1.24 2.90

1016 0.93 3.00

1170 1.16 3.15

22 ppm TBHP, 320 ppm DMA, Ar

926 1.16 2.80

1141 1.16 3.15

1176 0.89 3.20

1278 1.17 3.40

1307 1.04 3.50

30 pp