UNDERSTANDING THE ROLE OF MULTIFUNCTIONAL …

The Pennsylvania State University

The Graduate School

Department of Mechanical and Nuclear Engineering

UNDERSTANDING THE ROLE OF MULTIFUNCTIONAL NANOENGINEERED

PARTICULATE ADDITIVES ON SUPERCRITICAL PYROLYSIS AND COMBUSTION OF

HYDROCARBON FUELS/PROPELLANTS

A Dissertation in

Mechanical Engineering

by

Hyung Sub Sim

2016 Hyung Sub Sim

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Doctor of Philosophy

May 2016

ii

The dissertation of Hyung Sub Sim was reviewed and approved* by the following:

Richard A. Yetter

Professor of Mechanical Engineering

Dissertation Advisor

Chair of Committee

Domenic A. Santavicca


Adri C.T. van Duin

Professor of Mechanical Engineering and Chemical Engineering

Robert M. Rioux

Friedrich G. Helfferich Associate Professor of Chemical Engineering

Karen A. Thole


Department Head of Mechanical and Nuclear Engineering

*Signatures are on file in the Graduate School

iii

ABSTRACT

This dissertation aims to understand the fundamental effects of colloidal nanostructured

materials on the supercritical pyrolysis, injection, ignition, and combustion of hydrocarbon

fuels/propellants. As a fuel additive, functionalized graphene sheets (FGS) without or with the

decoration of metal catalysts, such as platinum (Pt) or polyoxometalates (POM) nanoparticles, were

examined against conventional materials including nanometer sized fumed silica and aluminum

particles.

Supercritical pyrolysis experiments were performed as a function of temperature, residence

time, and particle type, using a high pressure and temperature flow reactor designed to provide

isothermal and isobaric flow conditions. Supercritical pyrolysis results showed that the addition of

FGS-based particles at a loading concentration of 50 ppmw increased the conversion rates and

reduced apparent activation energies for methylcyclohexane (MCH) and n-dodecane (n-C12H26) fuels.

For example, conversion rates, and formations of C1-C5 n-alkanes and C2-C6 1-alkenes were

significantly increased by 43.5 %, 59.1 %, and 50.0 % for MCH decomposition using FGS19 (50

ppmw) at a temperature of 820 K and reduced pressure of 1.36. In addition, FGS decorated with 20

wt % Pt (20wt%Pt@FGS) at a loading concentration of 50 ppmw exhibited additional enhancement

in the conversion rate of n-C12H26 by up to 24.0 % compared to FGS. Especially, FGS-based particles

seem to alter initiation mechanisms, which could result in higher hydrogen formation. Hydrogen

selectivities for both MCH and n-C12H26 decompositions were observed to increase by nearly a factor

of 2 and 10, respectively.

Supercritical injection and combustion experiments were conducted using a high pressure and

temperature windowed combustion chamber coupled to the flow reactor through a feed system.

Supercritical injection/combustion experiments indicated that the presence of a small amount of

iv

particles (100 ppmw) in the fuel affected the injection, ignition, and subsequent combustion for

hydrocarbon fuels. For instance, the addition of Pt@FGS reduced the core lengths of the supercritical

jets by up to 56.7 % (MCH) and 68.8 % (n-C12H26) depending on flow rate. Supercritical combustion

studies demonstrated that the addition of 100 ppmw 20wt%Pt@FGS to n-C12H26 fuel reduced ignition

delay times by nearly a factor of 3 (12.4 to 4.1 ms), increased spreading angles by approximately

32.0 % (15.4 to 20.3o), reduced the flame lift-off length by 54.0 % (1.74 to 0.8 mm), and

demonstrated an increase in conversion by 35.0 % relative to the pure fuel baseline at a volumetric

flow rate of 5.0 mL/min.

The enhancing mechanisms of FGS-based materials on pyrolysis and combustion were

studied using ReaxFF molecular dynamics (MD) simulation. The simulation results were in good

agreement with the experimental observations, showing enhanced conversion rates and lowered

activation energies in the presence of the particles. A combination of Pt and FGS facilitated catalytic

dehydrogenation forming a n-C12H25 radical. Another catalytic initiation step is hydrogenation of the

fuel molecule resulting in production of an alkanium ion, as a free hydrogen atom is donated to the

fuel molecule. Furthermore, hydroxyl radical (OH), which is dissociated from FGS, was found to

participate in dehydrogenation of the fuel forming water molecules. The recovery of the catalyst was

observed, resulting from dissociation of H2 molecule from the surface of the Pt. Using the ReaxFF

embedded with nudged elastic band (NEB) method, it was found that the bicomposite structure of

Pt@FGS served to lower the reaction energy and energy barrier for dehydrogenation, which could

result in the enhanced conversion rates and increased product yields.

These results demonstrate that a low mass loading of a high surface area material employed

either as an additive or a means of distributing another additive can significantly enhance, and be used

to tailor, the conversion of liquid hydrocarbon fuels/propellants under supercritical conditions,

resulting in reduced ignition delay time and improved combustion. Such enhancements benefit

practical propulsion systems which require high conversion efficiency in a short residence time.

v

TABLE OF CONTENTS

LIST OF FIGURES ................................................................................................................. vii

LIST OF TABLES .................................................................................................................. xiv

ACKNOWLEDGEMENTS .................................................................................................... xvi

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

1.1 Motivation ................................................................................................................. 1 1.1.1 Thermal Management for Advanced Liquid-Fueled Propulsion .................... 2 1.1.2 Combustion Issues of Liquid Hydrocarbon Fuels/Propellants ....................... 8

1.2 Colloidal Nanostructured Materials for Replacement of Structural Catalysts ........... 11 1.3 Multifunctional Nanoenergetic Materials .................................................................. 13 1.4 Research Objectives and Outline ............................................................................... 15 1.5 Dissertation Structure ................................................................................................ 17

Chapter 2 Literature Review ................................................................................................... 19

2.1 Combustion of Liquid Fuels with Reactive and Catalytic Materials ......................... 19 2.1.1 Metallic Nanoparticles .................................................................................... 19 2.1.2 Graphene-based Materials .............................................................................. 24

2.2 Thermal and Catalytic Decomposition of Hydrocarbons/Propellants

under Supercritical Conditions ................................................................................................ 27 2.2.1 Hydrocarbons/Propellants ............................................................................... 28 2.2.2 Catalytic Decomposition of Hydrocarbons/Propellants .................................. 33

2.2.2.1.Structural Catalysts .............................................................................. 33 2.2.2.2.Chemical Initiator ................................................................................ 36 2.2.2.3.Dispersible and/or Soluble Nanoparticles ............................................ 38

2.3 Supercritical Injection and Combustion Studies of Liquid Fuels .............................. 40 2.3.1 Supercritical Droplet Vaporization and Combustion ...................................... 40 2.3.2 Supercritical Injection and Combustion of Cryogenic Fuels .......................... 41 2.3.3 Supercritical Injection and Combustion of Hydrocarbons/Propellants ........... 43

2.4 Theoretical and Numerical Studies of Enhancement Mechanisms of

Hydrocarbons with Nanoparticles ........................................................................................... 45 2.5 Summary of Literature Review and Contribution of This Study ............................... 47

Chapter 3 Experimental Methods ............................................................................................ 50

3.1 Liquid Fuels ............................................................................................................... 50 3.2 Particle Synthesis ....................................................................................................... 51 3.3 Particle Dispersion ..................................................................................................... 53 3.4 High Pressure and High Temperature Flow Reactor ................................................. 54 3.5 Sub-to-Supercritical Injection (non-reacting) Experiment ........................................ 59 3.6 Supercritical Combustion Experiment Facility.......................................................... 66 3.7 Gas-Liquid Separation, Dilution, and Vaporization System ..................................... 71 3.8 Product Analysis ........................................................................................................ 73

vi

3.9 Surface Characterization Analysis of the Particles .................................................... 75

Chapter 4 Supercritical Pyrolysis Experiments ....................................................................... 77

4.1 Pure MCH and MCH containing Particles ................................................................ 77 4.2 Decomposition of n-C12H26/FGS-Based Particles ..................................................... 99 4.3 Summary of Decomposition Studies ......................................................................... 115

Chapter 5 Supercritical Injection and Combustion Experiments ............................................ 116

5.1 Injection Experiments ................................................................................................ 116 5.2 Combustion Experiments .......................................................................................... 131 5.3 Summary of Injection and Combustion Studies ........................................................ 154

Chapter 6 Molecular Dynamics Simulations ........................................................................... 157

6.1 Computational Details ............................................................................................... 157 6.1.1 ReaxFF Reactive Force Field....………………………………………….......157 6.1.2 ReaxFF with Nudged Elastic Band (NEB) Method ........................................ 158 6.1.3 Molecular Dynamics Simulations ................................................................... 158

6.2 Results and Discussion .............................................................................................. 161 6.3 Summary of ReaxFF MD Simulations ...................................................................... 192

Chapter 7 Summary of Research and Recommendations for Future Work ............................ 193

7.1 Summary .................................................................................................................... 193 7.2 Recommendations for Future Work .......................................................................... 196

References ............................................................................................................................... 199

vii

LIST OF FIGURES

Figure 1-1. Comparison of heat sink capacities available from LH2 and hydrocarbon

fuels as a function of temperature: Redrawn with additional lines and descriptions (Source: Ref. [9]).

................................................................................................................................................. 3

Figure 1-2. (a) n-dodecane phase diagram and (b) specific heat capacity (calculated using the NIST

code SUPERTRAPP [31]). ...................................................................................................... 5

Figure 1-3. Impact of coke deposition on heat exchanger catalyst coating

(Source: Ref. [34]). ................................................................................................................. 7

Figure 1-4. Comparison of ignition delay times with different hydrocarbon fuels and hydrogen

(Source: Ref. [2]). .................................................................................................................... 8

Figure 1-5. Limitation of the current structural catalysts to be applied into the advanced propulsion

systems. ................................................................................................................................... 11

Figure 1-6. Effect of adding energetic materials into propellants on the combustion energetics

(calculated using NASA Chemical Equilibrium with Applications (CEA) [72], [73]). .......... 13

Figure 1-7. Motivation of the current research (Image of rocket chamber taken from Ref. [75]).

................................................................................................................................................. 15

Figure 1-8. Overall program of the current research. .............................................................. 16

Figure 2-1. Comparison of decomposition mechanisms between supercritical and gas-phase reactions

(Source: Refs. [13], [125])....................................................................................................... 29

Figure 2-2. Effect of pressures (A:0.79 MPa, B:3.55 MPa, C:7.58 MPa) on wet-bubble temperature of

the droplet (Source: Ref. [159])............................................................................................... 41

Figure 3-1. Schematic of the high pressure flow reactor. ........................................................ 55

Figure 3-2. Photograph of the high pressure flow reactor. ...................................................... 55

Figure 3-3. Representative temperature and pressure profiles in the flow reactor over run for MCH

pyrolysis at the flow rate of 1.0 mL/min. ................................................................................ 57

Figure 3-4. Assembly of 3-way diverting valve, fuel injector, and windowed chamber......... 61

Figure 3-5. Schematic diagram of the supercritical reactor experiment and optically accessible

chamber used for sub-to-supercritical injection experiments. ................................................. 62

Figure 3-6. Representative firing sequence of supercritical non-reacting injection experiments.

................................................................................................................................................. 64

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Figure 3-7. Schematic diagram of the supercritical combustion experimental setup. ............. 67

Figure 3-8. Representative firing sequence of supercritical combustion experiments. ........... 69

Figure 3-9. High-speed shadowgraph. ..................................................................................... 70

Figure 3-10. High-speed schlieren setup. ................................................................................ 70

Figure 3-11. Specialized separation, vaporization, and dilution apparatus. ............................ 72

Figure 4-1. Effects of FGS19 on conversion of MCH under supercritical-phase decomposition at

temperatures ranging from 745 to 840 K at a fixed reduced pressure of 1.36, and a fixed flow rate of 1

mL/min. ................................................................................................................................... 78

Figure 4-2. Major pyrolysis products of MCH/FGS19 50 ppmw at temperatures ranging from 745 to

840 K, a fixed reduced pressure of 1.36, and a fixed volumetric flow rate of 1.0 mL/min. (a) methane,

(b) ethane, (c) propene, (d) propane, (e) cyclohexene, (f) DMCPs, and (g) toluene. .............. 82

Figure 4-3. Comparison of major product selectivities in the presence of FGS19 50 ppmw with those

for pure MCH pyrolysis at different conversions with temperatures varying from 745 to 840 K. (a)

methane, (b) ethane, (c) propene, (d) propane, (e) cyclohexene, (f) DMCPs, and (g) toluene.

................................................................................................................................................. 86

Figure 4-4. Arrhenius plots for thermal decomposition of MCH with FGS19 additive under

supercritical conditions at temperatures ranging from 745 to 840 K, a fixed reduced pressure of 1.36,

and a fixed volumetric flow rate of 1.0 mL/min. ..................................................................... 89

Figure 4-5. Effects of FGS19 on the measured mole fractions of the major products with respect to

residence time (800 K and 4.72 MPa). (a) methane, (b) ethane, (c) propene, (d) propane, (e) toluene,

and (f) DMCPs. ....................................................................................................................... 91

Figure 4-6. Effects of FGS19 on the conversion of MCH with respect to residence time (800 K and

4.72 MPa). ............................................................................................................................... 92

Figure 4-7. Conversion of MCH for two different loading concentrations of FGS at 800 K, 4.72 MPa

and flow rate of 1.0 mL/min. ................................................................................................... 93

Figure 4-8. Comparison of conversion of MCH containing various additives at 800 K and 4.72 MPa.

................................................................................................................................................. 94

Figure 4-9. Comparison of selectivities of the major products with the different particles at 800 K,

4.72 MPa, and flow rate of 1mL/min. ..................................................................................... 98

Figure 4-10. Major reaction mechanisms for pure MCH decomposition under supercritical conditions

[13], [38]. ................................................................................................................................. 98

Figure 4-11. Effect of FGS and Pt-FGS on the n-C12H26 conversion rate at three different

temperatures and at a fixed pressure of 4.75 MPa. .................................................................. 100

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Figure 4-12. Possible initiation mechanisms of n-C12H26 decomposition with FGS or Pt-FGS.

................................................................................................................................................. 101

Figure 4-13. Arrhenius plots and global kinetic parameters for pure n-C12H26 and n-C12H26 mixtures

containing FGS and Pt-FGS. ................................................................................................... 103

Figure 4-14. Product distributions measured for decomposition of pure n-C12H26 and n-C12H26

mixtures containing FGS and Pt-FGS at (a) 480 oC, (b) 500

oC, and (c) 530

oC at the fixed pressure of

4.72 MPa. ................................................................................................................................ 106

Figure 4-15. Comparison of mole fractions of (a) C1-C4 alkanes and (b) C2-C4 alkenes for different

temperatures at a fixed pressure of 4.72 MPa. ........................................................................ 107

Figure 4-16. Selectivities for (a) hydrogen and (b) C1-C4 alkanes and C2-C4 alkenes from n-C12H26

pyrolysis with or without Pt-FGS. ........................................................................................... 108

Figure 4-17. Effect of Pt-FGS on n-C12H26 conversion as a function of time at 530 oC and 4.75 MPa.

................................................................................................................................................. 110

Figure 4-18. Effect of Pt-FGS on the selectivities for (a) hydrogen, C2-C4 alkenes, and C5-C12 alkenes,

(b) C1-C4 alkanes and C5-C11 alkanes, and (c) branched and cyclic structures as a function of time at

530 oC and 4.75 MPa. .............................................................................................................. 112

Figure 4-19. TEM images of colloidal Pt-FGS nanoparticles (a) before and (b) after supercritical

pyrolysis. ................................................................................................................................. 113

Figure 5-1. Comparison of cold flow (ambient temperature) injections of pure MCH and MCH

containing 100 ppmw Pt@FGS at 20 mL/min and 4.09 MPa (Re=~1540). ........................... 117


containing 100 ppmw Pt@FGS at 50 mL/min, 4.09 MPa, and t=0.4 sec (Re=~3850). .......... 118

Figure 5-3. Supercritical jets of pure MCH and MCH containing Pt@FGS at a chamber temperature

of 673 K, reactor temperature of 853 K, and chamber pressure of 4.24 MPa. ........................ 119

Figure 5-4. Supercritical jets of pure n-C12H26 and n-C12H26 containing Pt@FGS at a chamber

temperature of 723 K, reactor temperature of 823 K, and chamber pressure of 4.24 MPa. .... 120

Figure 5-5. Core regions enhanced by averaging 25 frames for pure MCH and MCH containing

Pt@FGS at a chamber temperature of 673 K, reactor temperature of 853 K, and chamber pressure of

4.24 MPa. ................................................................................................................................ 121

Figure 5-6. Core regions enhanced by averaging 25 frames for pure n-C12H26 and n-C12H26 containing


4.24 MPa. ................................................................................................................................ 122

Figure 5-7. Relative gray values of pure MCH and MCH containing Pt@FGS at various flow rates.

................................................................................................................................................. 124

x

Figure 5-8. Relative gray values of pure n-C12H26 and n-C12H26 containing Pt@FGS at various flow

rates. ........................................................................................................................................ 125

Figure 5-9. Dark core lengths for pure MCH and MCH containing Pt@FGS at a chamber temperature


Figure 5-10. Dark core lengths for pure n-C12H26 and n-C12H26 containing Pt@FGS at a chamber


Figure 5-11. Snapshots of jet boundaries for pure MCH and MCH containing Pt@FGS at a chamber


Figure 5-12. Snapshots of jet boundaries for pure n-C12H26 and n-C12H26 containing Pt@FGS at a

chamber temperature of 723 K, reactor temperature of 823 K, and chamber pressure of 4.24 MPa.

................................................................................................................................................. 129

Figure 5-13. Spreading angles of pure MCH and MCH containing Pt@FGS at a chamber temperature


Figure 5-14. Spreading angles of pure n-C12H26 and n-C12H26 containing Pt@FGS at a chamber


Figure 5-15. Fuel pressures for n-C12H26 and n-C12H26 containing various particles at a flow rate of

5.0 mL/min (equivalence ratio of 0.38), Pch,ini=3.46 MPa, Tfuel=832 K, and Tair=777 K. ........ 134

Figure 5-16. Fuel pressure for n-C12H26 and n-C12H26 containing various particles at a flow rate of 2.5

mL/min (equivalence ratio of 0.19), Pch,ini=3.44 MPa, Tfuel=831 K, and Tair=772 K. .............. 134

Figure 5-17. Chamber pressures for n-C12H26 and n-C12H26 containing various particles at a flow rate

of 5.0 mL/min (equivalence ratio of 0.38), Pch,ini=3.49 MPa, Tfuel=832 K, and Tair=777 K. .... 136


of 2.5 mL/min (equivalence ratio of 0.19), Pch,ini=3.44 MPa, Tfuel=831 K, and Tair=772 K. .... 136

Figure 5-19. Comparison of percentage increase in conversion for n-C12H26 containing various

particles at a flow rate of 5.0 mL/min (equivalence ratio of 0.38), Pch,ini=3.49 MPa, Tfuel=832 K, and

Tair=777 K. The results are shown relative to the pure fuel baseline. ...................................... 138



Tair=772 K. The results are shown relative to the pure fuel baseline. ...................................... 138

Figure 5-21. Time sequence of schlieren images of the start of injection, penetration, autoignition, and

developed-turbulent diffusion flames for pure n-C12H26 at a flow rate of 5.0 mL/min (equivalence

ratio of 0.38), Pch,ini=3.49 MPa, Tfuel=832 K, and Tair=777 K. ................................................. 141

Figure 5-22. Time sequence of schlieren images, including pre-injection, start of injection,

penetration, autoignition, and developed-turbulent diffusion flames for n-C12H26 containing FGS19

xi

100ppmw at a flow rate of 5.0 mL/min (equivalence ratio of 0.38), Pch,ini=3.49 MPa, Tfuel=832 K, and

Tair=777 K. Time zero corresponds to the start of injection. ................................................... 142


penetration, autoignition, and developed-turbulent diffusion flames for n-C12H26 containing Pt@FGS

100 ppmw at a flow rate of 5.0 mL/min (equivalence ratio of 0.38), Pch,ini=3.49 MPa, Tfuel=832 K, and



penetration, autoignition, and developed-turbulent diffusion flames for pure n-C12H26 at a flow rate of

2.5 mL/min (equivalence ratio of 0.19), Pch,ini=3.44 MPa, Tfuel=831 K, and Tair=772 K. Time zero

corresponds to the start of injection. ........................................................................................ 144









Figure 5-27. Effect of particle additives on measured ignition delay times for n-C12H26 fuel, at a flow

rate of 5.0 mL/min (equivalence ratio of 0.38), Pch,ini=3.49 MPa, Tfuel=832 K, and Tair=777 K.

................................................................................................................................................. 148



................................................................................................................................................. 148

Figure 5-29. Spreading angles for n-C12H26 as well as n-C12H26 containing FGS and Pt@FGS for two

different flow rates (a) 2.5 mL/min and (b) 5.0 mL/min. ........................................................ 150

Figure 5-30. Effect of FGS and Pt@FGS on the flame growth rate for two different flow rates (a) 2.5

mL/min and (b) 5.0 mL/min. ................................................................................................... 150

Figure 5-31. Effect of FGS and Pt@FGS on the measured lift-off distance for two different flow rates

(a) 2.5 mL/min and (b) 5.0 mL/min. ....................................................................................... 152

Figure 5-32. Comparison of measured fuel and chamber pressures for pure n-C12H26 and n-C12H26

containing Pt@FGS at an initial chamber pressure of 1.41 MPa (Pr=0.78, subcritical), a flow rate of

5.0 mL/min, Tfuel=832 K, and Tair=777 K. ............................................................................... 153

Figure 5-33. Comparison of percentage increase in conversion for n-C12H26 containing Pt@FGS at

two different pressures of 1.4 MPa (subcritical) and 3.46 MPa (supercritical). ...................... 153

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Figure 5-34. Luminous regions for n-C12H26 containing 100 ppmw Pt@FGS at an initial chamber

pressure of 1.41 MPa (Pr=0.78, subcritical), a flow rate of 5.0 mL/min, Tfuel=832 K, and Tair=777 K.

................................................................................................................................................. 154

Figure 6-1. Snapshots of the initial configurations of FGS, Pt-cluster, and Pt@FGS. Hydrogen atoms

are white, oxygen atoms are red, carbon atoms are orange, and platinum atoms are yellow. . 160

Figure 6-2. Snapshots of the initial configurations of n-C12H26 with FGS, Pt-cluster, and Pt@FGS at

0.31 g/cm3. ............................................................................................................................... 161

Figure 6-3. Time evolution of conversion rates for n-C12H26 and n-C12H26 containing FGS, Pt-cluster,

and Pt@FGS at temperatures of 1500 K (a), 1550 K (b), 1650 K (c), 1700 K (d), 1800 K (e), and

1900 K (f) from the NVT ReaxFF simulations. ...................................................................... 166

Figure 6-4. (a) major species and (b) minor species for decomposition of n-C12H26 and n-C12H26

containing FGS, Pt-cluster, and Pt@FGS at a temperature of 1500 K. ................................... 169











Figure 6-10. Comparison of kinetics for pyrolysis of n-C12H26 and n-C12H26 containing various

additives at a fixed density of 0.31 g/cm3. (low concentration: 1 Pt@FGS molecule in 48 fuel

molecules, high concentration: 1 Pt@FGS molecule of in 24 fuel molecules) ....................... 176

Figure 6-11. Comparison of selectivities for n-C12H26 pyrolysis for the two different loading

concentration of Pt@FGS. (low concentration: 1 Pt@FGS molecule in 48 fuel molecules, high

concentration: 1 Pt@FGS molecule of in 24 fuel molecules) ................................................. 177

Figure 6-12. Plausible initiation mechanisms for n-C12H26 decomposition catalyzed by Pt atoms on

FGS. ......................................................................................................................................... 179

Figure 6-13. Possible initiation mechanisms for n-C12H26 decomposition catalyzed by oxygen-

functional groups on Pt@FGS. ................................................................................................ 180

Figure 6-14. Comparison of initiation mechanisms for pyrolysis of n-C12H26 without or with various

additives for temperatures ranging from 1500 to 1900 K at a fixed density of 0.31 g/cm3. .... 184

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Figure 6-15. An example of catalytic dehydrogenation by Pt@FGS observed during simulations.

................................................................................................................................................. 186

Figure 6-16. An example of catalytic dehydrogenation by oxygen-functional groups on Pt@FGS.

................................................................................................................................................. 186

Figure 6-17. An example of catalytic hydrogenation by a free hydrogen atom and subsequent C-C

bond scission to form n-heptane and n-C5H11. ........................................................................ 186

Figure 6-18. Interactions between a hydroxyl radical (dissociated from Pt@FGS) and a fuel molecule

to form water molecule. ........................................................................................................... 187

Figure 6-19. An example of H2 formation from the surface of Pt@FGS and the recovery of Pt cluster

on the FGS. .............................................................................................................................. 188

Figure 6-20. Final configurations of Pt@FGS for three different temperatures of (a) 1500 K and 2 ns,

(b) 1700 K and 1 ns, and (c) 1900 K and 1 ns. ........................................................................ 189

Figure 6-21. ReaxFF-NEB simulation of reaction and barrier energies for catalytic dehydrogenation

for Pt-cluster and Pt@FGS cases. ............................................................................................ 191

Figure 6-22. ReaxFF-NEB simulation of reaction and barrier energy for catalytic dehydrogenation by

an oxygen-functional group on the FGS. ................................................................................ 191

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LIST OF TABLES

Table 1-1. Comparison of chemical heat sink values for various endothermic fuels (Source: Refs. [2],

[9], [14]). ................................................................................................................................. 4

Table 1-2. Critical properties of hydrocarbon fuels (Source: Refs. [24], [26]–[30]). ............. 5

Table 1-3. Comparison of Isp for major operational liquid propellants families (Source: Ref. [9]).

................................................................................................................................................. 9

Table 2-1. Comparison of the two reaction mechanisms of HAN decomposition (Source: Ref. [100]).

................................................................................................................................................. 32

Table 3-1. Properties of MCH and n-C12H26 used in this study............................................... 51

Table 3-2. Experimental conditions for thermal decomposition of MCH and n-C12H26 containing

different particles. .................................................................................................................... 57

Table 3-3. Characteristics times for thermal decompositions of pure MCH and n-C12H26. .... 58

Table 3-4. Dimensionless parameters for thermal decompositions of pure MCH and n-C12H26.

................................................................................................................................................. 59

Table 3-5. Experimental conditions for injections of MCH and n-C12H26 containing different particles.

................................................................................................................................................. 65

Table 3-6. Experimental conditions for combustion experiments of n-C12H26 containing different

particles. .................................................................................................................................. 71

Table 3-7. GC methodology for product analysis. .................................................................. 75

Table 4-1. Molar product yields from thermal decomposition of MCH with and without FGS19 50

ppmw at a fixed reduced pressure of 1.36 and a fixed volumetric flow rate of 1.0 mL/min.

................................................................................................................................................. 83

Table 4-2. Kinetic parameters for thermal decomposition of MCH with and without FGS19 additive.

................................................................................................................................................. 89

Table 4-3. Molar product yields from the decomposition of MCH with different particles at 50 ppmw

loading concentration, 800 K, 4.72 MPa, and flow rate of 1.0 mL/min. ................................. 96

Table 4-4. Kinetic parameters for decomposition of n-C12H26 with various particles. ............ 103

Table 6-1. Reaction energies of initiation steps for thermal decomposition n-C12H26. ........... 163

xv

Table 6-2. Kinetic parameters for decomposition of n-C12H26 and n-C12H26 containing various

particles. .................................................................................................................................. 177

xvi

ACKNOWLEDGEMENTS

I would like to thank my advisor, Prof. Richard A. Yetter for his invaluable advice, financial

support, patience, and guidance throughout my Ph.D. years. I would like to thank Prof. Adri van Duin

for being a part of my doctoral committee, and also for his guidance and discussion with regards to

the ReaxFF molecular dynamics (MD) simulations. I am also grateful to my doctoral committee

members, Dr. Domenic Santavicca and Dr. Robert Rioux for their guidance and assistance. I would

like to thank Dr. Jong-guen Lee for recommendation to join the research group of Dr. Yetter and his

guidance in the initial stage of my Ph.D. study.

I would like to express my gratitude to Mr. Terrence Connell, Jr. for his advice, assistance,

and support with many of the experiments in supercritical combustion and injection as well as

discussion and proofreading. I am thankful to Mr. Sungwook Hong, Mr. Chowdhury Ashraf, and Dr.

Yun Kyung Shin in Dr. van Duin’s research group for their assistance to use the ReaxFF and

discussion. I would like to thank Dr. Eric Boyer and Dr. Andrew Cortopassi for their assistance and

expertise in high-speed schlieren imaging as well as for conducting experiments at the Cryogenic

Combustion Lab. I would also like to thank all of the members of Prof. Yetters lab group, both the

former and current members, Drs. Michael Weismiller, Pulkit Saksena, Sharat Parimi, Steven Dean,

Nicholas Tsolas, Kuninori Togai, Mr. Matthew Loomis, Mr. Reed Johansson, Ms. Bo-Han Kuo and

Ms. Paige Nardozzo. I am also grateful to Dr. Dongxiang Wang for her technical support on Gas

Chromatography. I would like to acknowledge Dr. Daniel Dabbs and Dr. Ilhan Aksay at Princeton

University for supplying the materials for my experiments. I would like to thank our machinist Mr.

Larry Horner and administrative support assistant Mrs. Mary Newby.

I would also like to thank the financial resources provided by the two MURI programs of

“Multifunctional Colloidal Nanocatalysts for Liquid Fuel Combustion” and “Smart Functional

xvii

Nanoenergetic Materials” sponsored by the Air Force Office of Scientific Research. I would like to

thank the teaching assistantship from the Mechanical and Nuclear engineering department at Penn

State. I would like to acknowledge the scholarship for the first two years of my Ph.D. study supported

by Korea Institute of Energy Technology Evaluation and Planning.

I would like to thank all of my friends that I have met here at the Penn State. Especially, I am

thankful to Drs. Sung Yup Kim, Hyun Jae Kim, Kyu Tae Kim for their guidance with regards to

research and future career. I also would like to thank my former advisor for my master study, Dr.

Seong Hyuk Lee and my former Lab. members at the Chung-Ang University for their encouragement.

I would like to thank Dr. Chang Kyoung Choi at Michigan Technological University for his guidance

to my future career.

Finally, I would like to thank my parents (Sang-Do Sim and Suk-hui Shin) and parents-in-law

(Young-Heung Yoon and Jae-Bok Lee) for all their love, patience, encouragement, and support

throughout my time in Ph.D. study. I also thank my sister (Yeon-Ju), brother-in-law (Seo-Ryong),

Soo-hyun, and Ji-U for their love and encouragement. I would like to thank my grandma in heaven

for her love and I believe that she would be very proud of me. This long journey would not have been

possible without the support, patience, and love of my wife, KyungJin, my son, Minjoon, and my

daughter, Yunha.

Chapter 1

Introduction

1.1 Motivation

A key challenge for advanced liquid-fueled propulsion systems is to develop the ability to use

liquid hydrocarbons as both the fuel and coolant because of their high energy density, heat sink

capacity, environmentally-friendly characteristics, costs, and availability [1]–[3]. It is well known that

for advanced propulsion systems, including ramjets, scramjets, pulse detonation engines, and high

speed turbines, the severe limitations of hydrocarbon-air mixtures are their long ignition delays and

slow combustion rates. Furthermore, as flight speed increases to supersonic and hypersonic regimes, a

high fuel heat sink capacity is desirable for efficiently managing heat loads generated by the system.

Recently, colloidal nanostructured materials suspended in liquid hydrocarbon fuels have

demonstrated significantly reduced ignition delay times and enhanced combustion rates [4], [5].

Dispersible nanostructured materials, more recently, have also proven to increase the cooling capacity

and conversion rates of the liquid fuels [6]–[8]. The benefit of a colloidal nanostructured material

which can be readily dispersed in a hydrocarbon fuel, is that it can increase the cooling capacity and

conversion efficiency as well as improve combustion and ignition characteristics. Dispersible

nanoadditives may eliminate not only the complexity of current systems but the deactivation

associated with conventional structural catalysts. For liquid-fueled rocket applications, suspension of

nanoenergetic and/or catalytic materials in the hydrocarbon fuel may serve to increase the energy

density of the fuel benefiting performance with respect to density-specific impulse (ρ* Isp), while

taking advantage of regenerative cooling processes to pyrolyze the fuel, producing a more reactive

mixture which could potentially be consumed in a shorter residence time. This thesis aims to study

2

the effects of colloidal nanostructured additives on the decomposition, injection, ignition, and

combustion of liquid hydrocarbon fuels.

1.1.1 Thermal Management for Advanced Liquid-Fueled Propulsion

This doctoral study is firstly motivated towards investigating the effects of dispersible

nanostructured particles on fuel decomposition under high temperatures and pressures which simulate

regenerative cooling for hypersonic vehicles or rocket engines. For advanced aircraft, rocket engines,

and missiles, thermal management becomes an important challenge. For instance, a scramjet traveling

at Mach 8 requires minimum a 3500 kJ/kg heat sink capacity for an uncooled scramjet combustor that

exceeds 2700 oC [9]–[11]. In order to deal with this high heat load, the fuel can be employed as the

primary cooling media and, once injected into the combustor, as the energy source for combustion.

Generally, cryogenic fuels such as liquid hydrogen (LH2) are attractive because their sensible heat

sink capacity is high enough to remove heat around airframe and combustor unit as well as the

external surface recovery temperature over the full range of flight speeds, as depicted in Fig 1-1. In

addition, hydrogen is considered as the highest performance fuel which offers the highest Isp, and

combustion and ignition properties. Hydrogen, however, has several drawbacks. The on-board

systems for LH2 handling, storage, control, and delivery are very expensive. LH2 theoretically

produces a heat flux up to 30 MW/m2, which is beyond the capability of the current materials,

attributing to the use of high strength materials or compromising the reusability of the rocket or

scramjet engine [11]. LH2 also has high explosion hazards, even at room temperature, and has a low

liquid density of approximately 0.07 g/cm3. For these reasons, alternative fuels are of considerable

interest for advanced propulsion systems.

Such limitations of LH2 have led to significant efforts to develop liquid hydrocarbon fuels

which can provide heat sink capacity and combustion performance close to liquid hydrogen. Liquid

3

hydrocarbons such as jet fuels have several advantages over cryogenic fuels that make them

potentially useful. First, liquid fuels have 11 times greater density than LH2, are ambient-storable, and

relatively inexpensive without changing any existing structure and material. However, additional heat

sink capacity is required in order to use a liquid hydrocarbon as a coolant for hypersonic vehicles or

rocket engines, as the sensible heat sink capacity available from a liquid hydrocarbon is much lower

than the heat load in the hypersonic regime. Figure 1-1 shows that as the fuel temperature exceeds

480 oC, the fuel undergoes thermal and/or catalytic decomposition, which is typically an endothermic

process, and therefore offers additional heat sink capacity up to 4500 kJ/kg [1], [2], [9], [10], [12],

[13]. This endothermic cracking of liquid hydrocarbons has the potential to provide the required

cooling capacity for high-speed flight. Table 1-1 compares endothermic capacities and ideal final

products for several liquid hydrocarbon candidates for hypersonic vehicles. As depicted in Fig. 1-1 by

the red dotted line (---), a key challenge for endothermic hydrocarbons is to lower the initiation

temperature of endothermic reactions and maximize the endothermicity for these reactions.

Figure 1-1. Comparison of heat sink capacities available from LH2 and hydrocarbon fuels as a

function of temperature: Redrawn with additional lines and descriptions (Source: Ref. [9]).

4

Table 1-1. Comparison of chemical heat sink values for various endothermic fuels (Source: Refs. [2],

[9], [13]).

Endothermic Reactions Reaction Types

Theoretical chemical heat sink

capacity, kJ/kg

C7H14(MCH)C7H8(toluene)+3H2 Dehydrogenation 2190

C12H24(kerosene)6C2H4(ideal) Cracking 3560

2NH3(ammonia)N2+3H2 Dehydrogenation 2720

CH3OH (methanol)CO+2H2 Dehydrogenation 4000

C10H18(decalin)C10H8(naphthalene)+5H2

Dehydrogenation 2210

CH4(cryogenic methane) Sensible 3400

H2(cryogenic hydrogen) Sensible 15119

The fuel conditions that prevail during pre-combustion for current air-breathing vehicles can

exceed the critical pressures and temperatures for liquid hydrocarbon fuels as shown in Table 1-2. In

thermodynamics, a critical point, also known as a critical state, occurs under conditions at which no

phase boundaries exist. A fluid above its critical temperature and pressure, referred to as a

supercritical fluid, exhibits unique characteristics with liquid-like density and solvation properties,

gas-like transport properties, zero surface tension, and a high isothermal compressibility [12]. As an

example, n-dodecane (n-C12H26) exhibits liquid-like density under supercritical conditions and near

the critical point, the specific heat capacity approaches infinity (Fig. 1-2). Because of these unique

properties, supercritical fluids are used in many applications including advanced propulsion [1], [14],

chemical reactions [15], nanoparticle formation [16], hydrolysis [17]–[20], and power generation [21],

[22]. Under supercritical conditions, the fuel may reside for several seconds in a heat-exchanger and

undergo thermal decomposition, such as during scramjet fuel-passage cooling or during injection to

5

the combustor. This operational environment of high temperatures and pressures above the critical

point causes several major concerns [9], [23], [24].

Table 1-2. Critical properties of hydrocarbon fuels (Source: Refs. [23], [25]–[29]).

Fuel Tc, oC Pc, MPa

n-decane (n-C10H22) 345 2.10

n-dodecane (n-C12H26) 385 1.81

JP-8 393 2.45

Jet-A 400 2.38

JP-7 405 2.10

decalin 429 3.24

RP-1 410 2.18

MCH (C7H14) 298.9 3.47

(a) n-dodecane phase diagram (b) specific heat capacity

Figure 1-2. (a) n-dodecane phase diagram and (b) specific heat capacity (calculated using the NIST

code SUPERTRAPP [30]).

Pressure, MPa

De

ns

ity

,k

g/m

3

1 2 3 4 5 6 7 8 9100

100

200

300

400

500

600

700

625 K - 700 K

500 K

550 K

600 K

Critical Points at658.1 K,1.817 MPa,227 kg/m

3,

Pressure, MPa

Cp

(sp

ec

ific

he

at

ca

pa

cit

y),

J/(

mo

l*K

)

1 1.5 2 2.5 30

2000

4000

6000

8000

Above the critical temperature

Approachingthe critical temperature

6

Solid carbon deposition, termed coking, is one of the major concerns, and must not occur in

fuel lines, as illustrated in Fig. 1-3. Coking, if it occurs, creates several severe problems, such as a

decrease in heat transfer and clogging of fuel line passages and filters, eventually leading to engine

failure [1], [9], [14]. Understanding mechanisms which promote coke formation is one of the most

important topics concerning the potential use of liquid hydrocarbons as both a fuel and coolant for the

system. Much work on thermal stability of commercial and military fuels has been extensively carried

out to examine decomposition mechanisms and carbon deposition under near-critical or supercritical

conditions. Excellent accomplishments have been made to understand coke formation mechanism and

thus suppress coke and/or carbon deposition [1], [31]–[34]. Three different mechanisms on coke

formation were developed, suggesting the adsorption of cracked products can form filamentous

carbon on the metal surface of the heat exchanger, the condensation of high-molecular-weight liquids

(tars) can form amorphous carbon, and amorphous carbon can accumulate on the surface of the

exchanger by reactions between small molecules and radicals [1], [9], [14]. These coke formations

strongly depend on temperature and residence time. Generally, the rate of carbon deposition seems to

increase above 400 oC and exhibit an exponential correlation with temperature [1]. Interestingly, a

benefit of the supercritical condition is shown to reduce coking under catalytic cracking conditions

due to the enhanced solubilities of coke precursor at elevated pressure [9], [35]. In addition, the use of

catalytic additives could lower the temperature required to initiate decomposition of the fuel and

reduce the residence time necessary to achieve conversion with the desirable endothermicity.

7

Figure 1-3. Impact of coke deposition on heat exchanger catalyst coating (Source: Ref. [33]).

Another challenge is that high temperatures, above at least 480 oC, are necessary to obtain the

desirable endothermicity [9]. Unfortunately, this high fuel temperature, as shown in Fig. 1-1, may

reduce allowable stress of the heat exchanger materials and require increased wall thickness as well as

total flight weight [3]. Strategies to lower this high cracking temperature, therefore, are being

investigated together with high heat sink capacity and desirable products. The use of conventional

structural catalysts such as wall-coated cooling channels has extensively been studied to determine

whether the initiation temperature for endothermic reactions can be lowered by altering reaction

pathways and whether the conversion rates can be accelerated [23], [24], [29]. Deactivation of

catalysts by deposition of coke on the catalytic wall-coated heat exchanger, however, becomes a

serious issue, resulting in operational problems and decreased performance. This deactivation of

structural catalysts includes solid deposition, sulfur poisoning, and sintering. Carbon deposition or

metal-oxide formation on the surface of catalysts can occur. These layers of carbon deposits or metal

oxides can be grown on catalysts supported with silica or alumina having low thermal conductivities

[36]. These layers, illustrated in Fig. 1-3, can hinder heat flow from the combustor to the coolant due

to their low thermal conductivities. Thus, this increase of heat resistance due to the insulating layer

formed on the wall of the cooling system reduces reaction rates and heat sink capacity. While

structural catalysts would enhance cooling capacity and lower initiation temperature, it is not practical

8

to retrofit catalysts in existing systems [4]. Therefore, new strategies of integrating nanostructured

material technology directly and easily into existing systems are of interest.

1.1.2 Combustion Issues of Liquid Hydrocarbon Fuels/Propellants

Although the use of liquid hydrocarbons is appealing as a dual-function fuel, these fuels

exhibit shortcomings which need to be addressed for advanced liquid-fueled propulsion. Relative to

LH2, gaseous fuels or hypergolic propellants, liquid hydrocarbon fuels exhibit lower combustion

performances such as lower Isp, longer ignition delay times, and lower flame speeds. In particular,

ignition delay times for liquid hydrocarbons such as MCH and toluene are observed to be much

longer than LH2, as illustrated in Fig. 1-4 [2]. Table 1-3 presents the specific impulse, Isp, of

hydrocarbon fuels and propellants for rocket applications, indicating the desire to seek improvements.

Figure 1-4. Comparison of ignition delay times with different hydrocarbon fuels and hydrogen

(Source: Ref. [2]).

9

Table 1-3. Comparison of Isp for major operational liquid propellants families (Source: Ref. [9]).

Family Example Stoichiometry Isp, sec

Cryogenic Liquid Hydrogen/Liquid Oxygen 2H2+O2 2H2O 391

Kerosene RP-1/Liquid Oxygen aCH2+1.5O2CO2+H2O 300

Storable nontoxic Kerosene/peroxide aCH2+3H2O2CO2+4H2O 273

a:RP-1 and kerosene are thought to be approximate CH2 based on C11.7H22.8

In addition, there is little information on the behaviors of thermally or catalytically

decomposed fuel from regenerative cooling processes, which may differ from the pure fuel when the

fuel is injected into the combustion chamber and combusts. While the cracked fuel under high

temperatures and pressures may contain a variety of products, such as low-carbon-number and high-

carbon-number species, or polycyclic aromatic hydrocarbon (PAH), this fuel composition may

deteriorate combustion rates and ignition characteristics in the combustor. For example, under

supercritical conditions, higher yields of alkane species are typically observed in the products of

cracked fuels due to the promoted bimolecular reactions such as H-abstraction [12], [25], [26], [37]

rather than β-scission which yields higher alkene products under subcritical condition. These alkanes

in the cracked fuel have shown to cause longer ignition delays.

A simple way to enhance combustion rates and reduce ignition delay times of liquid fuels is

to increase the pressure in the combustor. This motivates the trend toward developing high pressure

and high temperature operation for next-generation liquid-fueled propulsion systems. For instance,

the combustion chamber pressure was about 9.7 MPa for the Merlin 1D (2011-2012), which used RP-

1 and liquid oxygen as fuel and oxidizer as well as a regeneratively cooled nozzle and combustion

chamber [38]. A similar direction is also evident for advanced gas turbines and diesel engines. The

Joint Strike Fighter (high performance combat aircraft) using the next-generation gas turbine engine

operates at pressures up to 6.1 MPa [39]. Such combustor environments approach and/or exceed the

10

thermodynamic critical point of liquid hydrocarbons. Supercritical combustion and injection of the

fuels themselves would be beneficial towards higher combustion efficiency and shorter ignition delay

[40]–[46]. More recently, supercritical diesel engine operation has demonstrated both an enhancement

in efficiency and a reduction in emissions [47].

Injection processes of cryogenic liquids, such as nitrogen and oxygen, under supercritical

conditions have widely been investigated since understanding the injection process for these liquids is

becoming increasingly important in research related to rocket propulsion [40]–[42], [44], [48].

Moreover, supercritical injection of cryogenic fuels behaves differently from that of transcritical or

subcritical injection [42], [49]–[51]. While previous studies involving supercritical fluid injection

largely consider studies of droplets and sprays [45], [46], [52], fundamental understanding of

supercritical fluid jets is still limited, and thus more research involving injection of supercritical

hydrocarbons and propellants is needed. Furthermore, the injection and combustion of cracked fuel

from endothermic reactions in the heat exchanger has not been well investigated at high temperatures

and pressures. Hence, qualitative as well as quantitative information on the combustion and injection

processes of a decomposed fuel for these applications is necessary to the design of combustion

chambers for optimum performance.

Supercritical-phase cracked fuel mixtures with dispersible nanostructured additives or

energetic materials also may present combustor development challenges concerning combustion

instabilities and emissions. Combustion instabilities occur due to the non-homogeneity of the fuel

mixture and can result in a combination of high pressure oscillations in the combustion chamber and

fuel delivery system [24]. The existence of nanostructured additives in fuel flow under high

temperatures and pressures could lead to either intensifying or diminishing combustion instability by

increasing or decreasing mixing and heat release prior to ignition. In addition, it is an important

challenge to control emissions, such as carbon monoxide (CO), sulfur oxides (SOx), or mono-

nitrogen oxides (NOx) which could be related to the addition of particles into the fuel. Particle

11

emissions themselves also require analysis and understanding. This doctoral study attempts to couple

the supercritical/endothermic fuel, which consists of a decomposed mixture produced by the

regenerative cooling process, with the combustion/injection of that fuel under high pressures and

temperatures.

1.2 Colloidal Nanostructured Materials for Replacement of Structural Catalysts

As a means to address the issues of liquid-fueled propulsion applications discussed in the

previous section, catalytic combustion with the conventional structural catalysts is being employed

[53]–[58]. Much effort on applying structural catalysts into combustion systems is underway in order

to accelerate combustion rates and shorten ignition delays of liquid hydrocarbons at lower

temperature. As previously discussed, structural catalysts seriously suffer from deactivation both

during cooling as fuel is flowed through the heat exchanger and during combustion, as illustrated in

Fig. 1-5. The limitation of the conventional structural catalysts leads to significant research interests

in investigating dispersible or soluble nanostructured materials as fuel additives because of their

benefits, such as readiness of integration into the current and future systems, free from deactivation,

and multifunctionality with different doping or decoration of the materials.

Figure 1-5. Limitation of the current structural catalysts to be applied into the advanced propulsion

systems.

Advantages

Shorter Ignition Delays

Higher Reaction Rates

Lower Breakdown or Ignition Temperatures

Enhanced Cooling Performance

Disadvantages

Impractical to Retrofit the current approaches into Existing System

Deactivation of Catalyst

Formation and/or Deposition of Solid Oxide Layer

Excessive Pressure Drop

12

To date, there exist several types of additives that researchers have considered, which can be

divided roughly into chemical initiators and dispersible and/or soluble nanoparticles. A chemical

initiator is known to accelerate thermal cracking reactions by forming highly reactive radicals from

the bond scission of the initiator, which is followed by H-abstraction to form hydrocarbon radicals [3],

[36], [59]–[61]. Instead of C-C bond cleavage which requires a higher bond breaking energy for

hydrocarbons, the H-abstraction reactions initiated by radicals can accelerate conversion rates [59].

The addition of small amounts of initiators can lower initiation temperature, likely due to the lower

energy barriers of radical branching reactions. These chemical initiators were found to enhance

cracking rates of the fuels and inhibit the formation of pyrolytic deposits [60], [62]. In diesel engine

applications, chemical additives to the liquid fuels such as dimethyl ether (DME) and 2-ethylhexyl

nitrate (2-EHN) could result in shorter ignition delays than the fuels without the initiators [10].

Accompanied by the investigation of chemical initiators, the suspension of nanostructured

materials in liquid fuels and propellants is of great interest [4], [5], [63]–[69] because particles can

offer additional benefits such as high specific areas and high active sites compared to structural

catalysts and chemical initiators. Recently, the role of dispersible particles has been studied on

conversion rates and heat sink capacity for regenerative cooling and exhibited positive effects. Most

dispersible or soluble particles are metals and/or metal oxides or metal oxyhydroxides supported by

high surface area materials such as silica or alumina [3]. While colloidal metal particles have

exhibited better performance, there exist accompanying drawbacks. Essentially, these metal

nanoparticles can form a nonenergetic oxide passivation layer on the particle surface and solid oxide

products during the combustion process [5], [70]. These drawbacks show that current approaches of

metal nanoparticles and endothermic fuel technologies have reached their limits for advanced high

speed aircrafts involving jet engines and scramjet engines where complete combustion must occur

within a shorter residence time.

13

1.3 Multifunctional Nanoenergetic Materials

The current study seeks novel nanoenergetic materials which are dispersible in liquid fuels

and propellants and provide multifunctional roles as catalysts in the chemical processes and energetic

materials in the combustion phases. Most additive materials are designed for targeting specific

performance (i.e., burning rate, conversion rate, or ignition delay, etc). For example, the addition of

conventional energetic materials, such as nanosized aluminum or boron, can provide the possibilities

for increased stored potential energy and increased thermodynamic performance. Figure 1-6 presents

a few illustrative examples of enhanced combustion energetics. Relative to the baseline bipropellant

mixture consisting of n-C12H26 and liquid oxygen, the addition of aluminum or boron to the fuel

increases the adiabatic flame temperature, rocket chamber temperature, and density specific impulse.

(a) adiabatic temperature calculation (b) performance calculation of rocket propellants

Figure 1-6. Effect of adding energetic materials into propellants on the combustion energetics

(calculated using NASA Chemical Equilibrium with Applications (CEA) [71], [72]).

Pressure, MPa

Ad

iab

ati

cT

em

pe

ratu

re,K

0 2 4 6 8 103000

3100

3200

3300

3400

3500

3600

3700

3800

3900

no particle

/w Boron

/w Al

Equivalence Ratio

I sp

*d

en

sit

y,k

g*s

ec

/m3

Ch

am

be

rT

em

pe

ratu

re,K

0 0.4 0.8 1.2 1.6 2 2.40.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

3.5E+05

4.0E+05

1000

1500

2000

2500

3000

3500

4000

4500

5000

Isp*density_no_additives

TC_no_additives

Isp*density_w/ Boron

TC_/w Boron

Isp*density_w/ Al

TC_/w Al

Isp*density_w/ Carbon (graphite)

TC_/w C (graphite)

Pc=6.7 MPa

Pc/P

e=69

14

Nanosized aluminum and born particles, however, have a passivating oxide layer, which

reduces the energy content of the particles. In addition, the large surface area of these nanosized

materials leads inherently to formation of aggregates or agglomerates, which result in the loss of

nanoscale features of these materials. To address these challenges, coating techniques with transition

metals and long-chain hydrocarbons has been developed. Coating by transition metal on the

aluminum has been proposed, but seemed to be limited to the low loading concentration of the metals

and exhibit still lowered active content of the particle [73]. In this respect, the use of graphene-based

particles as a substrate for energetic materials can be a potential alternative to transition metal coating.

Nanosized aluminum coated on FGS having high surface area and energy density may be beneficial

to prevent agglomeration due to the wrinkle structures of FGS and reduce the sintering by keeping the

particles separated. Tetrazines (high density and high nitrogen compounds) are also candidate,

attached to FGS via simple nucleophilic aromatic substitution.

Development of multifunctional additives is of importance in the drive towards high activity,

low loading concentration, and high dispersibility. In this respect, novel graphene-based

nanoengineered structures with a combination of catalytic and energetic materials are promising

alternatives to the metal-based or zeolite additives which can serve separately to extend regenerative

cooling or to enhance combustion characteristics. The current work is the first attempt to examine the

feasibility of graphene-based materials to enhance endothermic reactions and combustion processes

of liquid hydrocarbons and to provide their multifunctionality on both processes. As demonstrated in

Fig. 1-7, the current research is motivated to seek multifunctional materials for combustion

applications with regenerative cooling. During cooling processes, additives consisting of catalysts and

energetic materials in liquid fuels and propellants could exhibit catalytic activity without suffering

from sintering, aggregation, or deactivation. Once cracked fuel containing these highly energetic

materials is injected into the combustor, enhanced ignition and combustion properties could be

provided.

15

Figure 1-7. Motivation of the current research (Image of the rocket chamber taken from Ref. [74]).

While graphene-based particles have been shown experimentally to enhance combustion rates

[5], [75], little is understood about the mechanism by which such materials function. Further, there is

little evidence for either efficacy or mechanism of graphene-based catalysts in high temperatures and

high pressures of endothermic reactions and combustion. Thus, theoretical and numerical analysis is

needed to understand potential interactions with and on the catalyst substrate and deposited catalytic

metal, coupled with fundamental combustion and pyrolysis experiments.

1.4 Research Objectives and Outline

The overall objective of this study is to explore the impact of dispersing colloidal particles

into liquid fuels, while investigating the multifunctionality of these particles with respect to

enhancement of fuel decomposition, injection, and combustion. As is evident from the literature

review in the following chapter, studies involving colloidal particles have been limited to

16

endothermic reactions such as conversion rates and endothermicity or combustion characteristics such

as burning rates. Therefore, a study involving a combination of these endothermic reactions and

combustion/injection processes is necessary to develop the design of advanced liquid-fueled

propulsion systems for optimum performance. Basic mechanisms for the enhancement of fuel

conversion and combustion performance of liquid fuels and colloidal nanostructured materials will be

discussed, coupled with fundamental flow reactor studies, combustion studies, and numerical

simulations. An overall approach of the present dissertation research is illustrated in Fig. 1-8.

Figure 1-8. Overall program of the current research.

A fundamental understanding of decomposition of liquid fuels such as methylcyclohexane

(MCH) and n-dodecane (n-C12H26) with the addition of colloidal nanostructured materials is studied

under near-critical or supercritical conditions in a high pressure flow reactor. The impact of the

particle addition on the conversion rate, kinetics, and product distribution, is examined.

Phenomenological enhancing mechanisms based on the product yields will help to elucidate the key

role of the graphene-based particles on the decomposition of the hydrocarbon fuels.

17

The effects of the multifunctional nanostructured materials on injection, ignition, and

combustion of hydrocarbon fuels under supercritical conditions will be examined. Studies focus on

the global effect of these particles on conversion efficiency and ignition delay times under high

temperature and pressure operating conditions. The impact of these particles on the flame structures

and lift-off distances which are interesting parameters for understanding turbulent flames, is studied

using high speed cinematography.

The current study investigates the reaction mechanisms and kinetics of pyrolysis of both the

pure fuel and fuel containing various types of particles using theoretical and numerical simulations.

These MD simulations will help elucidate enhancing mechanisms and bridge a gap between

experimental findings and theoretical understandings. The role of the particles on the conversion rates,

product distributions and kinetics will be extensively investigated as a function of temperature,

reaction time, and particles type. Individual reaction energy barriers, especially for catalytic C-C bond

cleavage and dehydrogenation which could be expected as catalytic initiation mechanisms will be

examined.

1.5 Dissertation Structure

Chapter 2 presents a review of previous literature related to the current dissertation research,

and will cover a broad range of studies from fundamental chemical reactions and combustion to

molecular-scale studies with catalyzed and uncatalyzed systems. Chapter 3 gives a detailed

description of particle preparations, experimental setups, and analysis methodologies. Chapter 4

discusses the role of graphene-based particles on supercritical pyrolysis of hydrocarbon fuels.

Catalytic activity of the materials is determined as a function of temperature, residence time, and

particle type. Chapter 5 describes the influence of multifunctional graphene-based particles on

supercritical injection and combustion of the hydrocarbon fuel. The effect of particle addition on

18

conversion efficiency, ignition delay, and flame spreading angle is presented as a function of

temperature, pressure, residence time, and particle type. Chapter 6 follows with an investigation of

theoretical and numerical modeling for enhancing mechanisms using the ReaxFF MD [76]

simulations. The simulation results of initiation mechanisms and kinetics are evaluated to determine

the catalytic role of the materials on decomposition and combustion of the fuel. Finally, Chapter 7

concludes this research with a summary of this thesis and recommendations for future work.

19

Chapter 2

Literature Review

In this chapter, the literature relevant to the current study will be reviewed. This literature

review will cover four different fields, including thermal and catalytic decomposition of hydrocarbons

under supercritical conditions, combustion studies of hydrocarbons dispersed with nanoparticles,

supercritical combustion and injection of liquid fuels, and computational works of catalyzed

hydrocarbon systems.

2.1 Combustion of Liquid Fuels with Reactive and Catalytic Materials

2.1.1 Metallic Nanoparticles

A great deal of research has been performed on the use of nanosized metal particles (such as

metals, metal oxides, or metal oxyhydroxides) in solid fuels and propellants. This research has shown

that there are many advantages of incorporating nanosized particles into solid fuels and propellants,

such as increased energy densities, shortened ignition delays, enhanced heat transfer rates, and high

burning rates [57], [64], [77]–[86]. In recent years, researchers have shown considerable interest in

the addition of nanoparticles to fluids, referred to as nanofluids, as a way to enhance the fluids

physical properties, such as thermal conductivity [87]–[89], mass diffusivity [90], and radiative heat

transfer [65], [66], [91], [92]. Such studies have demonstrated that by addition of nanoparticles into

liquid fuels and propellants, it is possible to tailor fuels to exhibit enhanced thermal properties.

However, little work has been conducted on combustion and decomposition of liquid fuels and

propellants involving reactive and catalytic nanoparticles.

20

Colloidal nanostructured materials have many advantages for use in liquid fuels and

propellants [4], [5], [63]–[69]. One of the advantages is that colloidal particles can potentially be used

in existing combustion engines and cooling systems without modification. Another advantage is that

compared to conventional methods, such as catalyst coating or using a packed-bed catalyst, the

colloidal particles dispersed in liquid fuels and propellants are nearly free from deactivation because

of their consumability during operation. Wickham et al. [4] have shown that colloidal nanocatalysts

have the potential to accelerate combustion in relevant kinetic regimes and to reduce ignition delay.

In their research, carboxylato-alumoxanes were evaluated as combustion catalysts. The boehmite

particles in these nanocatalysts are very small, between 20 and 200 nm, and can be easily suspended

in liquid, by adjusting the functional group on the carboxylic acid to make the particles dispersible in

a wide variety of liquids. For example, if the functional group is a non-polar paraffin, the particles

will be readily dispersed in organic materials like jet fuels. Although alumoxanes themselves could be

catalytic materials for combustion, their catalytic properties can be significantly enhanced by

incorporating other metals into the boehmite core. As part of their research, nanocatalysts

functionalized with long chain paraffins were dispersed in toluene to study the suspension of the

nanocatalysts. It was noted that the metal substituted into the catalyst played an important role in their

dispersion and that the catalysts had good stabilities up to 13,000 ppm concentration. Their catalyst

consisted of a boehmite (aluminum oxyhydroxide) support attached with dispersed metal.

Experiments showed that the activity of the metal was most important to the acceleration of the

reaction of hydrocarbon liquid fuels, including JP-10 and a surrogate JP-5, at low reaction

temperatures. The characteristics of catalytic enhancement on CO2 production were studied using

boehmite supports with dispersed metal. The addition of only 5 ppm catalyst to the surrogate JP-5

reduced the temperature required to initiate combustion by about 300 oC. The enhancement was

attributed to the reaction on the metal catalyst and resulting exothermicity that increased the gas

21

temperature to the hot ignition point of the fuel more rapidly than gas-phase reactions that propagate

through the negative temperature coefficient regime.

The dispersion of nanocatalysts in diesel fuels for automotive applications has also received

attention as a means to improve cetane numbers and emission control [93], [94]. Tyagi et al. [69]

have studied the addition of aluminum and aluminum oxide nanoparticles in diesel fuels. Droplet

ignition tests were carried out over the temperature range of 688 – 768 oC by adding particle sizes of

15 and 50 nm at volume fractions of 0.1 and 0.5 %. Their results showed that the addition of

nanoparticles increased the hot plate ignition probability of the diesel fuel and was attributed to the

improvement in the radiative and heat/mass transfer properties between the gas and liquid. The

ignition probability, however, did not show a correlation with the change in nanoparticle material or

nanoparticle size. This finding may relate to the fact that these nanosized particles are known to

readily agglomerate and that with decreasing particle size, the active energy content is significantly

reduced by the passivating oxidized layer.

Gan and Qiao [67] also investigated the burning characteristics of fuel droplets in the

presence of nano and micron-sized aluminum particles. They have shown the differences in burning

mechanisms between nano-sized and micron-sized particles in n-decane (n-C10H22). Long chain

surfactants, such as Sorbitan Oleate (C24H44O6), were attached onto the nano-sized aluminum (nAl) to

reduce particle agglomeration. When nAl was present in n-C10H22, five distinctive stages were

observed, including preheating and ignition, classical combustion, microexplosion, surfactant flame,

and nAl droplet flame. On the other hand, in the case of micron-sized particles only the first three

stages were identified. The nAl droplet flame, followed by the microexplosion sequence, further

increases the temperature compared to micron-sized aluminum burning sequences. They hypothesized

that the different burning behaviors of the micron-sized and nano-sized aluminum particle

suspensions are mainly due to the different structure of particle agglomerates formed during droplet

evaporation and combustion. For micron-sized aluminum, a densely packed and impermeable shell

22

was formed, whereas a porous and more uniformly distributed spherical aggregate was generated for

nAl. Thus, the porous spherical aggregate formed by nAl offers higher surface areas to react more

readily with the oxidizer than the micron-sized aluminum. This finding can support the enhancement

mechanism of nAl particles in liquid fuels.

Allen et al. [83] studied auto-ignition characteristics of liquid fuels containing nAl using an

aerosol rapid compression machine. In their experiments, nAl with an average particle size of 50 nm

were treated with surfactants of 0.03%-wt alizarin and 0.004%-volume triethanolamine prior to

dispersing in liquid fuels. The ignition results showed that the addition of 2wt% nAl in hydrocarbons

such as ethanol and JP-8 reduced ignition delays by 32 % and 50 %, respectively. Enhancing

mechanisms of ignition delay were discussed, based on the general aspect of nanofluids, but were not

evaluated to determine whether the faster ignition resulted from particle ignition.

Gan et al. [68] examined the influence of nanosized boron and iron particles as fuel additives

on the droplet ignition of n-C10H22 and ethanol at different loading concentrations. Boron is an

attractive fuel and/or a fuel additive to increase the energy density of liquid fuels and propellants at

high concentrations. At a dense concentration, particle agglomerates were easily formed and were

likely to burn as a large particle after all the liquid fuel had been consumed. On the contrary, dilute

suspensions showed both the liquid fuel and the particles burned at the same time. As a result, they

argued the effect of loading concentration also caused different burning characteristics. The impact of

the surfactant (Sorbitan Oleate used in their experiments) on the droplet burning characteristics was

also evaluated. Due to surfactant pyrolysis, multiple disruptions with strong intensity were observed

during combustion, whereas for the fuels having no surfactant, continuous disruptions with mild

intensity were found. From a series of works by Gan et al., it can be concluded that the loading

concentration and the surfactant play an important role in burning processes of liquid fuels containing

particulate additives [67], [68].

23

Devener and Anderson [84] explored the effects of soluble cerium oxide (CeO2) and iron

oxide (Fe2O3) nanoparticles on thermal decomposition and combustion of JP-10 fuel. Decomposition

and oxidation experiments of JP-10 in the presence of CeO2 and Fe2O3 nanoparticles were carried out

in a small alumina flow-tube reactor over the residence time range of 3.0 – 14.0 ms, and

decomposition products were analyzed by an in-situ mass spectrometer. Their results showed that the

addition of CeO2 lowered the ignition temperature by about 240 K due to the change in initiation step.

Multifunctionality of CeO2 as an oxidizer or catalyst was examined in this study. When the oxygen

(O2) was absent in the experiment, CeO2 apparently acted as an oxidizer, but in the presence of O2,

the combustion reaction was clearly catalytic. The results showed that CeO2 not only initiated JP-10

decomposition but also oxidized the initial decomposition products, generating final products such as

carbon monoxide (CO), carbon dioxide (CO2), water (H2O), and formaldehyde (H2CO).

As mentioned above, nanoparticles can be used to reduce soot formation during combustion.

Rotavera et al. [95] conducted shock tube experiments with a hydrocarbon fuel containing ceria

nanoparticles. In their research, the influence of adding the ceria nanoparticles on the soot-reducing

capacity and formation delay times during toluene pyrolysis and oxidation was evaluated. They also

examined the effect of surfactant (sodium dioctyl sulfosuccinate) on soot formation. When the fuel

was highly diluted with the surfactant, the ceria particles had no effect on soot formation. On the

other hand, under conditions of low surfactant concentration, the particles were found to reduce soot

accumulation on the tube walls. Regardless of whether the surfactant is added to the fuel or attached

to the particles, the surfactant effect should be considered.

Catalysts are also important to the ignition of liquid monopropellants, and dispersion of

nanocatalysts in liquid monopropellants may also provide benefits. For example, hydroxylammonium

nitrate (HAN), NH3OHNO3, is a promising substitute for hydrazine because of its low toxicity, high

density, high specific impulse, and low freezing point [96]–[100]. In order to find proper catalysts for

HAN-based propellants in thruster applications, Meinhardt et al. [97] conducted rocket engine testing

24

using a HAN, glycine, and water solution. In the presence of conventional metal catalysts such as

iridium (Ir), platinum (Pt), and rhodium (Rh), deactivation by loss of the catalyst surface as well as

active metal was observed during combustion. Wucherer et al. [101] developed iridium-based Shell

405 catalysts having a maximum working temperature of 1423 K. Zube et al. [102] evaluated HAN-

based blends with high temperature ceramics as catalyst carriers. It was observed that an Aerojet in-

house catalyst, LCH233 with active iridium, exhibited reliable ignition and long operation time.

Oommen et al. [103] investigated catalytic decomposition of HAN/water solution containing iridium

metal catalyst supported on alumina using thermo-gravimetric analysis (TGA). They found that the

addition of the catalyst decreased the decomposition temperature by 30 oC.

There exist different types of HAN-based propellants which are formulated by a tri-

component mixture of a 13m HAN/water and fuel. The selection of the fuel and the mixture

composition allows for manufacture of liquid propellants with different energy densities. To date,

catalysts that can survive at the highest energy density formulations and high combustion

temperatures have not been developed.

2.1.2 Graphene-based Materials

Nanostructured graphene materials are known to have unique electrical, optical,

electrochemical, and mechanical properties [104]–[108]. Recently, graphene-based materials have

been considered as energetic materials or catalyst supports for use in combustion engines and energy

conversion devices [5], [65], [75], [109], [110]. Functionalized graphene sheets (FGS) exhibit a high

surface area to volume ratio with surface areas as large as 1,850 m2/g measured by Schniepp et al.

[107] and McAllister et al. [108] using the methylene blue (MB) technique. Decoration of this surface

with functional catalysts or introduction of a large number of defect sites makes them useful for

25

reaction enhancement. While previous results [5], [65], [75], [111] with graphene sheets are quite

limited, they have been employed as both a catalyst and a support structure.

FGS is fundamentally a 2D colloidal polymer produced in bulk quantities either through

ultrasonic or thermal expansion of graphite oxide [112]–[114]. FGS has a tunable carbon-to-oxygen

(C/O) ratio ranging from 2 to higher values, produced by varying the number density of epoxide,

hydroxyl, and carboxyl sites [107], [108]. FGS presents a variety of active sites that can affect

combustion and ignition properties. These include defects (coordination vacancies) that can act as

surface radical centers, and oxidized functions such as epoxide, carboxyl, and hydroxyl structures that

can participate in exothermic surface reaction, or whose removal by thermal processes can generate

new surface radical centers. Such functions on high-surface-area carbonaceous materials have been

shown to catalyze a surprising variety of reactions, including acid-catalyzed additions and

rearrangements, and hydrocarbon and alcohol oxidations [115]–[118]. The ability to carry out

oxidations requires that these materials also be able to activate oxygen for reaction with organic

molecules. The performance of these carbon-based catalysts for oxidation can be further enhanced by

addition of metal oxide components that are active for redox chemistry, such as molybdenum trioxide

(MoO3) [119]. Typical reaction temperatures for oxidation catalysis by carbons are 523-623K, and

these can be decreased by metal oxide addition [119].

FGS, in combination with dispersed metal nanoparticles or metal oxide clusters, has strong

potential to perform multifunctional catalytic tasks [75], [120], [121]. Nano-sized platinum particles

can be supported with FGS. As is evident in the following section, the combinations of

polyoxometalates (POM) and FGS can improve performance of a model fuel. Molecular metal oxides,

such as POM, are also known to increase the reaction rate of FGS. POM are early transition metal

oxygen-anion heteropolyanion clusters that exhibit a wide range of molecular architectures, surface

charge densities, and chemical and electronic properties [75], [121], [122]. For instance, POM

containing redox-active metals such as Mo and V in their frameworks are active oxidation catalysts.

26

Unlike nano-sized metal particles, POM exhibit greater molecular stability in the presence of water

than do alumoxanes [75], [121], [122]. They are also thermally stable and resistant to sintering.

Sabourin et al. [5] have studied the use of FGS as nanocatalysts in monopropellants such as

liquid nitromethane (NM, CH3NO2). The addition of FGS in NM was shown to increase the linear

burning rate of the monopropellant up to 175 % over neat NM, outperforming more conventional

additives such as aluminum monohydroxide and silica nanoparticles. It was apparent that the colloidal

FGS reduced reaction initiation temperatures and accelerated transition to rapid reaction. Since FGS

is a material that can be fully consumed by oxidation during combustion, the material also contributes

to the fuel energy density. Thus, surface oxidation can provide additional exothermicity to potentially

drive the reacting mixtures into the high temperature regime of explosive kinetic branching.

Furthermore, no exhaust plume “signature” beyond that of the fuel combustion products themselves

would result. In their research, possible mechanisms for the enhancement of nitromethane burning

rates with the inclusion of FGS were proposed as follows: (1) radiation heat transfer enhancement via

increased reaction zone emission and liquid phase absorption, (2) enhanced heat sink capacity, (3)

catalytic decomposition of nitromethane, thus altering the thermal reaction pathway, and (4) enhanced

heat generation by reactions between nitrogen oxides and carbon thereby accelerating the reduction

rate of nitric oxide.

Loomis [65] has investigated the material characteristics and combustion behavior of

nanostructured carbon materials, involving single walled carbon nanotubes (SWCNT) and FGS with

three C/O ratios. From the surface characterization on the particles, a high degree of variability and

impurities were found. Droplet burning rates of NM and Jet-A with SWCNT and FGS were measured.

FGS exhibited burning rate enhancement in NM on the order of 10 % - 30 %, and little change was

observed in Jet-A. Platinum-coated FGS (Pt@FGS) enhanced burning rates of both fuels, with the

degree of enhancement increasing with platinum content.

27

Tessonnier et al. [75] have developed an innovative technique for POM (H3PMo12O40 and

H4PMo11VO40) supported on grafted functionalized graphene sheets to improve the suspension of the

catalyst in the liquid fuels without the need of surfactants. Spray flame experiments have been

performed with the addition of a variety of particles in MCH at a concentration of 50 ppmw. Using

the grafting technique, the dispersibility of the catalysts dramatically improved in a wide range of

organic liquid fuels. They demonstrated that POM supported on the grafted FGS can be used as

nanocatalysts, showing the burning rate enhancements of MCH.

As reviewed above, studies involving FGS-based particles are limited to

combustion/oxidation experiments. These catalysts suspended in the liquid fuels were shown to

enhance combustion characteristics, such as burning rates. However, the basic mechanisms for

catalytic decomposition of liquid fuels and propellants in the presence of such particles under near-

critical or supercritical conditions have not been studied. An understanding of the thermal

decomposition of pure liquid fuels and propellants under near-critical or supercritical conditions is of

great importance to a fundamental study on the effects of particles on thermal decomposition of liquid

fuels and propellants. In the following section, the previous studies on thermal decomposition of

liquid fuels and propellants are reviewed.

2.2 Thermal and Catalytic Decomposition of Hydrocarbons/Propellants under

Supercritical Conditions

Thermal decomposition, also called thermal cracking, thermolysis, or pyrolysis, of

hydrocarbons has been studied due to the importance of these processes to the petrochemical industry.

Therefore, a voluminous amount of literature is available concerning experimental and theoretical

studies of hydrocarbon cracking. However, a majority of the previous works have focused on thermal

decomposition of hydrocarbons at relatively high temperatures (above 500 oC) and near atmospheric

28

pressure, which is typically a gas phase reaction. At this condition, a free radical chain mechanism

predominates thermal cracking of n-alkanes, as formulated by Rice and his coworkers [123]. Heavy

n-alkane is initiated by carbon-carbon (C-C) bond cleavage to form the primary radicals, and then

these radicals decompose to smaller alkene products via β-scission. This initiation mechanism can be

confirmed by the facts that at high temperatures and low pressures the major products from thermal

cracking of n-alkanes are a series of 1-alkenes and that the abundant product is ethylene. These gas

phase reactions can be explained reasonably well by the Rice-Kossiakoff (R-K) mechanism [123].

Even though initiation mechanisms at high pressures are analogous to those at low pressures,

branching and termination mechanisms clearly appear to be different between low and high pressures.

2.2.1 Hydrocarbons/Propellants

Only limited work has been done on the supercritical pyrolysis of MCH and n-C12H26

considered in the current study. Stewart [12] and Davis [124] have studied the thermal decomposition

of MCH in a high pressure flow reactor under supercritical conditions. Stewart conducted detailed

product analysis to determine the effects of temperature, pressure, and residence time on thermal

decomposition of MCH under supercritical conditions. From the comprehensive review of

supercritical pyrolysis mechanisms including the previous works of Davis [124], Lai and Song [37],

Brown and King [125], and Zeppieri, Brezinsky, and Glassman [126], detailed reaction pathways

were proposed. Two major differences in reaction mechanisms between sub- and super-critical phase

pyrolysis are discussed in Stewarts’ dissertation [12]. One mechanism considers that cage effects such

as solute-solute interactions surrounding a high pressure medium play an important role in the

supercritical phase and thus ring closing processes to form alkylcyclopentanes are dominant over ring

opening to form light alkenes via β-scission, as illustrated in Fig. 2-1. Supporting cage effects in

supercritical environments to form favorable alkylcyclopentane isomers unlike in gas phase

29

decomposition, Lai and Song [37] also reported the abundance of alkylcyclopentane isomers at a

temperature of 723 K and variable pressures above the critical point. Another difference is that

compared to the gas phase pyrolysis, the observation of relatively higher yields of alkanes such as

ethane, propane, or butane is also due to the pressure effect on the reaction. This is likely because the

high pressure environment increased collision frequencies of multi reactant reactions, while β-

scission reactions became suppressed and slower than the bimolecular reactions [12], [127].

(a) Gas-phase and supercritical-phase

(b) Supercritical-phase

Figure 2-1. Comparison of decomposition mechanisms between supercritical and gas-phase reactions

(Source: Refs. [12], [124]).

Zhou and Crynes [128] studied n-C12H26 thermolysis in a batch autoclave reactor under

moderate temperatures and high pressures of N2 or H2. Their thermal stability results showed n-

C12H26 is stable below 600 K, and with increasing temperature beyond that point, the fuel began to

decompose to a series of paraffins and olefins up to C22 but with C13 missing. They discussed the

- H

+ H

- H

- H + H

CH•

ring-contraction

30

dominant initiation mechanism in n-C12H26 thermolysis was C-C bond cleavage forming the primary

radicals. Bimolecular chain reactions such as H-abstraction with the parent fuel was involved forming

the n-C12H25 radical. Once these radicals formed, they easily decomposed to smaller products forming

a series of olefins and secondary radicals via β-scission reactions. The observation of significant

amounts of n-alkanes and long-chain products having higher carbon number than the parent fuel in

the products, however, cannot be explained by the R-K mechanism and the Fabuss, Smith, and

Satterfield (F-S-S mechanism) [129], [130].

Yu and Eser [25], [26], [131] studied thermal decomposition of heavy normal alkanes from

C10 to C14 under near-critical and supercritical conditions. This paper discussed in detail the reaction

mechanisms using a combination of the R-K mechanism and the F-S-S mechanism to explain thermal

decomposition of heavy alkanes under supercritical conditions. Yu and Eser [25], [26], [131],

however, argued that the R-K and F-S-S mechanisms are limited for high conversions because those

mechanisms did not take into account the secondary reactions of the primary radicals and products.

Their initiation reaction is C-C bond cleavage with H-abstraction following to initiate the formation

of n-C12H25. According to their product distributions, Yu and Eser [25], [26], [131] expanded the

previous models with secondary reactions to explain the formation of secondary products such as

branched alkanes and long-chain alkanes. They pointed out that the addition of a primary parent

radical or a secondary parent radical to a 1-alkene could produce a heavy n-alkyl radical or a heavy

branched alkyl radical and that the double-bond isomerization of 1-alkene to 2-alkene could occur.

They also mentioned that the isomerization of the low-carbon-number primary radicals only occurs as

an intermediate step during their decomposition. In addition, they hypothesized that the formation of

lighter branched alkanes in addition to heavy ones is due to reactions between low-carbon-number

secondary radicals and 1-alkenes.

It is evident that much work has focused on solid carbon deposition during supercritical

pyrolysis. Unfortunately, solid deposition is beyond the scope of this study. However, a brief review

31

on this topic helps to understand the decomposition mechanism and the reactivity of liquid fuels and

propellants. Edwards [1], [9] provided an excellent review on thermal decomposition and carbon

deposition behavior of liquid hydrocarbon aviation fuels under supercritical conditions. In the review

article by Edwards [1], [9], a brief history of aviation fuels is summarized and possible barriers to the

use of high temperature fuels is described. Dissolved oxygen at a concentration of about 100 ppm in

fuel reacts with the radicals to form peroxides and thus oxidative deposits. As the temperature

increases further, cracking products such as alkanes and alkenes interact/condense to form additional

solid pyrolytic deposits. Moreover, a hydrocarbon fuel exposed to a high temperature environment

leads to formation of polycyclic aromatic hydrocarbons which can readily convert to pyrolytic

deposits. The paper also reviewed progress in understanding and mitigating the thermal instability

and deposition problem.

Thermal decomposition kinetics of kerosene-based rocket propellants such as RP-1 and RP-2

have been studied by Widegren and Bruno [132]–[136], and Anderson and Bruno [137]. For RP-1,

supercritical pyrolysis was performed between 375 and 500 oC in stainless steel ampule reactors.

Measured rate constants ranged from 6.92 x 10-5

s-1

at 375 oC to 1.07 x 10

-3 s

-1 at 500

oC. For RP-2,

thermal decomposition under supercritical conditions was conducted over the temperature range of

375 – 425 oC. Decomposition reaction rate constants were measured ranging from 1.33 x 10

-5 s

-1 at

375 oC to 5.47 x 10

-3 s

-1 at 425

oC. Widegren and Bruno [134] also compared RP-2 decomposition

with RP-1, and found that there was no noticeable differences between the two propellants over the

temperature range studied. Since these projects mainly focused on kinetics of the fuels, there were no

interpretations of reaction mechanisms of thermal decompositions. From the literature survey on jet

fuels and rocket propellants such as Jet-A and RP-1, it should be noted that the thermal

decomposition of these fuels is very complex and is not completely understood.

As nanoparticle dispersions could also be advantageous for improving ignition and burning

characteristics as well as overall energy densities of liquid monopropellants, previous research on

32

HAN-based propellants, as an example, is also presented. First, thermal decomposition studies of

HAN are briefly reviewed to understand the basic decomposition mechanism of a pure HAN solution.

Lee and Litzinger [99] summarized and compared two condensed-phase reaction mechanisms which

were proposed by Klien [138] and Oxley and Brown [139], as described in Table 2-1. Lee and

Litzinger [140] also have investigated thermal decomposition of HAN-water solution using FTIR

spectroscopy to identify the decomposed products.

Table 2-1. Comparison of the two reaction mechanisms of HAN decomposition (Source: Ref. [99]).

Klein [31] Oxley and Brower [30]

Initiate Reaction HAN → NH2OH + HNO3

NH2OH + HNO3 → HONO + HNO +

H2O

HAN →NH2OH + HNO3

NH2OH + HNO3 →HONO + HNO +

H2O

N2O Formation NH3OH+ + HONO

→H3O+ + O=N-NH-OH

→HO-N=N-OH

NH2OH + HONO →N2O + 2H2O

→N2O + H2O

N2 Formation NH3OH+ + HNO →N2 + H2O + H3O+ NH2OH + HNO →N2 + 2H2O

Major NO

Formation

None None

NO2 Formation HONO + HNO3 →2NO2 + H2O

2HONO →NO + NO2 + H2O

None

Other Reaction HNO + NO3-→HONO + NO

- HNO + HNO3 →2HONO

Researchers [19], [20] have investigated reactions and kinetics of aqueous HAN using a flow

reactor with FT Raman spectroscopy at a temperature range of 420 – 470 K and at a fixed pressure of

27.5 MPa. Due to a rapid reaction at about 460 K, it was difficult to measure the decomposition rate

of HAN. From analyzing FT Raman spectra, the stoichiometry was determined as follows:

[NH3OH+][NO3

−] → 0.8N2O + 0.4HNO3 + 1.8H2O +0.1O2.

This reaction was somewhat different from the overall reaction obtained by Pembridge and Stedman

[141]. In their research, apparent activation energies of 129±29 kJ/mol for 0.87-1.52 M HAN and

33

66±8 kJ/mol for 1.58-1.74 M HAN were obtained by measuring the induction times-to-exotherm in a

flow cell.

2.2.2 Catalytic Decomposition of Hydrocarbons/Propellants

Catalytic decomposition of liquid hydrocarbons and propellants is of interest for advanced

cooling technology. Considerable research has been conducted to develop efficient catalyst materials

which can provide enhanced decomposition rates and cooling capacities for endothermic fuels. There

are several different types of catalytic materials, which can be roughly categorized as conventional

catalysts, chemical initiators, and dispersible nanoparticles. This section briefly reviews previous

researches of those types of catalytic materials.

2.2.2.1. Structural Catalysts

Structural catalysts, such as packed bed or wall-coated catalysts, are mainly under

investigation to improve cooling capacity and conversion rate for endothermic fuels and to understand

enhancing mechanisms. A few works have studied the catalytic cracking of supercritical

hydrocarbons over conventional catalysts. Most of the hydrocarbon catalytic cracking has been

studied using zeolite-based catalysts consisting of a porous and crystalline alumniosillicate, which

provides active sites with a strong Bronsted acidity for hydrocarbon adsorption.

Cooper and Shepherd [142] explored the effects of industrial zeolite catalysts on cracking of

JP-10 for pulse detonation engine (PDE) applications. Their comparison between thermal and

catalytic cracking showed that the zeolite catalysts increase conversion rates from 3.15 % to 34 % and

yield more small-carbon-number components, particularly the C5 components. Three zeolites such as

34

HY, USY, and Beta were compared to determine their effects on the conversion rates, but none of

these materials were observed to be resistant against rapid deactivation.

Sicard et al. [143] investigated catalytic cracking of n-C12H26 using common zeolite catalysts

under supercritical conditions in a stirred batch reactor. At different temperatures, they observed

higher final pressures in the presence of catalysts than with the pure fuel alone, meaning that fuel

cracking reactions were catalyzed to form more gaseous products. They confirmed the enhancing

effects by measuring the conversion rates of the fuel with the catalysts, as well. In their study, rapid

deactivation seemed to occur faster at the highest temperature because the zeolite catalysts lost their

activity at that temperature. This deactivation was possibly related to coke deposit formation on the

catalyst because their high temperature conditions may have facilitated the formation of ring-

structured products such as polyaromatic hydrocarbons (PAH). In addition, they argued that the

catalytic reaction mechanisms totally differ from those of thermal cracking by comparison of their

product distributions. Khennache et al. [144] examined the influence of the ion exchange on

endothermic fuel cracking using the same experimental setup as Sicard et al. [143]. Their zeolite

catalysts exhibited no enhancing effect on the decomposition rates of n-C12H26 at 400 oC due to the

rapid deactivation. The ion exchange between the fuel and zeolites, however, may affect the reaction

mechanisms because the product distribution from the catalytic cracking was observed to differ from

that produced by thermal cracking.

Another type of structural catalyst, a wall-coated catalyst, has been extensively investigated

as a candidate to lower the initiation temperature of endothermic fuel cracking and enhance cooling

capacity for advanced high-speed aircraft. Meng et al. [61], [145] developed HZSM-5 and Pd-

Pt/HZSM-5 coating catalysts for endothermic cracking under supercritical conditions. They explored

the effect of HZSM-5 coating thickness varying from 6.13 to 18.27 µm on supercritical catalytic

cracking of n-C12H26 by washcoating on the bare stainless steel tube. Conversion rates were found to

increase from 12.5 to 19.2 % with increasing coating thickness, but fatal deactivation was also

35

reported due to coke formation onto the coating. The addition of transition metals such as Pd and Pt

on HZSM-5 catalysts was investigated to determine whether a bifunctional enhancement of catalytic

cracking can be achieved using the wall-coated tube reactor. The activity of different catalysts was

compared in terms of heat sink capacity, conversion rate, and hydrogen selectivity. The results

indicated that Pt-Pd/HZSM-5 catalysts provide better catalytic activity with higher conversion rates

and enhanced dehydrogenation of n-C12H26. Pt and Pd coating on the catalyst can yield additional heat

sink capacity, which is probably due to the enhanced dehydrogenation. If their catalysts produce more

hydrogen by catalyzed dehydrogenation, combustion and ignition characteristics may be improved

once these fuel mixtures are injected into the combustion chamber. Similar research on catalytic

cracking of n-C12H26 over HSZM-5 zeolite under supercritical condition was performed by Xian et al.

[146]. They developed a first-order Langmuir kinetic model with a novel decay function to estimate

supercritical catalytic cracking of the fuel associated with supercritical extraction effect on catalyst

stability. From the estimated reaction rate and adsorption constant of n-C12H26 on HZSM-5 at

different temperatures, the activation energy of the catalytic reaction was calculated to be 125.4

kJ/mol, which is remarkably lower than approximately 260 kJ/mol for thermal cracking of n-C12H26.

Qu et al. [147] postulated that the acidity of HZSM-5 with different Si/Al ratios is the

controlling factor for enhanced decomposition of n-C12H26. While most of the previous works mainly

focused on the enhancement effects of the zeolite catalysts on the conversion rate and heat sink

capacity, they attempted to explain the possible mechanisms of coke formation with the varying

acidity of the catalyst. As a result, the higher catalytic performance may be associated with increasing

Lewis and decreasing Bronsted acid amounts. In addition, the deactivation rate was found to be

correlated with the acidity, showing that the coke amount deposited on the catalyst coating increased

with the Si/Al ratio. Much effort is underway to develop catalysts which are resistant to deactivation

under high temperatures and high pressures and to understand the reaction mechanisms.

36

2.2.2.2. Chemical Initiator

As discussed in the introduction, the use of structural catalysts may complicate the design and

construction of the regenerative cooling system and present rapid deactivation of the catalysts which

can result in fatal failure. The use of chemical initiators has been attractive as a fuel additive to reduce

the complexity of the system and the rapid deactivation of the catalysts due to their dispersibility and

consumability. As an example, chemical initiators such as dimethyl ether (DME) and 2-ethylhexyl

nitrate (2-EHN) in diesel fuels for automotive applications have received considerable attention as a

means to improve combustion and ignition characteristics, stabilize fuel mixtures, protect the motor

from abrasion and wax deposition, and reduce pollutant emissions [148]. These fuel additives are

known to consist of the oxygenated compounds (R-O-O-R) or azo compounds (R-N=N-R·) due to

their high reactivity [59], [149].

Wichkam et al. [59] provided a brief introduction to chemical initiators and emphasized their

advantages over structural catalysts. When the initiator is present in the fuel under supercritical

conditions, the initiation occurs easily to form highly reactive radicals due their low activation energy.

Homolytic bond cleavage initiates the initiator molecules to form radicals:

A-A 2A·

Once these radicals have formed, H-abstraction with the parent fuel (for example, n-heptane) follows:

C7H16 + A· C7H15· + AH

The addition of chemical initiators put the initiation mechanisms quickly from C-C bond cleavage

reaction to H-abstraction. This alteration in initiation mechanism can enhance fuel conversions under

supercritical conditions. Activation energies for C-C bond scission and C-H bond scission are

estimated at 82 ~ 87 kcal/mol and above 95 kcal/mol, respectively. These high energy barriers require

high temperatures and pressures for endothermic cracking. On the contrary, the chemical initiators

can facilitate the reactions towards energetically favorable H-abstraction (under 10 kcal/mol).

37

Wickham et al. [59] first evaluated the effects of initiators and showed significant

enhancements in conversion rates and heat sink capacity for endothermic fuel cracking. The addition

of their initiators at a concentration of 2.0 wt% increased the conversion rate of n-heptane by up to a

factor of six over the temperature range of 450 – 550 oC. The initiators were observed not to affect the

product distribution so that the propagation and termination reactions did not seem to change.

Another work by Wickham et al. [3] provided results on conversion rates and endothermic capacities

of JP-7 and n-C10H22 with and without the initiator. Consistently, their initiators gave higher

conversion rates and endothermicity for hydrocarbon fuels. They found significant reduction in the

activation energy from 65 to 39 kcal/mol by addition of the initiator and also the initiator had a larger

effect at lower temperature than at higher temperature. They did not provide information on the

composition of their initiator tested.

Liu et al. [60], [62] also studied the effects of several initiator additives such as 1-

nitropropane (NP), triethylamine (TEA), and 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane

(TEMPO) in terms of heat sink capacity and resistance to coke formation in the products from n-

C12H26 cracking. The additives increased decomposition rates of n-C12H26 from 20 to 150 %. In

contrast to Wickham et al.’s [59] finding, the product distributions from the thermal cracking of n-

C12H26, with and without initiators, indicated that the initiator type had a slight effect on the gas

product selectivity, but a non-negligible effect on the liquid product distribution. Using global first-

order kinetics, the apparent cracking activation energies were found to be lower by the addition of the

initiators. Attempts were made to explain the experimental results with the reaction mechanisms

proposed by Yu and Eser [25] including the possible initiation and propagation steps for the cases in

the presence of the additives. In addition, they argued that the initiators can affect coke formation in

the products and found a significant effect on the deposit morphologies.

38

2.2.2.3. Dispersible and/or Soluble Nanoparticles

Relative to the previous studies with the conventional catalysts and chemical initiators,

information on the introduction of dispersible nanoengineered materials to hydrocarbon cracking

under supercritical conditions is rare. The concept of these dispersible nanoparticles is very similar to

the chemical initiator. Highly dispersible nanoparticles blended in liquid fuels or propellants can offer

additional benefits over the chemical initiators, such as high surface areas with active sites, high

energy density with energetic materials, and multifunctionality with different decorating materials or

dopants.

Bao et al. [6], [150] first explored the use of dispersible nano-HZSM-5 catalysts for

hydrocarbon decomposition under supercritical conditions, which they referred to as the

pseudohomogenous method. Using surface modifications via organic silanization of the zeolite

surface, they made suspensions of nano-HZSM-5 catalysts with hydrocarbon fuels. This grafting

HZSM-5 nanocatalyst exhibited highly stable dispersions in the liquid hydrocarbon. The catalytic

decomposition of these nanocatalysts was performed in a stainless batch reactor. Bao et al. [6]

showed that the conversion rate of n-C12H26 increased by a factor of 4 during the first 34 min by the

addition of 1000 ppmw HZSM-5. The zeolite grafted with a hexyl group was found to exhibit the best

catalytic activity and butyl-functionalized HZSM-5 gave the highest H2 yield (7.5 % in the gaseous

product for the residence time of 40 min). Using a similar technique proposed by Bao et al. [6], [150],

Sun et al. [7] developed new pseudohomogenous additives called beta nanozeolites, which can be

highly dispersed in liquid hydrocarbons such as JP-10 and n-C10H22. The addition of these particles at

a loading concentration of 100 ppm was found to lower the initiation temperature by 50 oC and

increase conversion rates by a factor of three at 680 oC. This result seems quite remarkable, however,

at a temperature below 600 oC, their materials presented a negligible enhancement on the conversion

and formation of the gaseous products.

39

Guo et al. [151] studied the endothermic reaction of JP-10 catalyzed with well-dispersed

nickel (Ni) nanoparticles. The major hurdles on the use of metallic nanosized materials into liquid

hydrocarbons are related to their dispersibility and agglomeration. Resorcinarene-encapsulated Ni

nanoparticles with an average diameter of 35 nm were synthesized. These nanoparticles showed good

dispersibility when resorcinarene was attached to the surface of the Ni nanoparticles in JP-10. Guo et

al. [152] also developed Ni-B nano-amorphous alloys for catalytic cracking of JP-10. Their Ni–B

nanoparticles were suspended in the JP-10 and increased conversion rates of JP-10 from about 4.0 to

6.1 % at 570 oC. The hydrogen yield was observed to be lower in the presence of the catalysts than

with the pure fuel alone.

Courtheoux et al. [153]–[155] studied thermal and catalytic decomposition of HAN and water

solution. A thermal analysis (TA) to determine the decomposition temperature was conducted to

evaluate the activity of different catalysts on the decomposition of aqueous HAN. From the results, it

was shown that platinum supported on silica-doped alumina catalysts (Pt/Al2O3-Si) can lower the

decomposition temperature by 75 oC. All these experiments exhibited good activity and catalyst

stability. Amariei et al. [156] also showed the catalytic and thermal decomposition of HAN using a

dynamic reactor. Mass spectroscopic product analyses were performed to evaluate the activity of

different catalysts on the decomposition of HAN and water solutions. The catalyst geometry, e.g.,

powder or spheres, was found to affect the decomposition of HAN. For example, the powder type

shows the best activity due to a good contact between catalyst-bed and HAN.

40

2.3 Supercritical Injection and Combustion Studies of Liquid Fuels

2.3.1 Supercritical Droplet Vaporization and Combustion

Droplet vaporization and combustion under supercritical conditions has been studied

previously [157]–[161]. The review on key findings of liquid droplet vaporization and combustion at

elevated pressure helps to provide fundamental understanding of combustion and injection processes

for hydrocarbon fuels considered in the current study.

Based on the quasi-steady (QS) approximation, neglecting transient terms in mass,

momentum, and energy conservations, the droplet “D2 Evaporation Law” provides the vaporization

characteristic time of a droplet in the following [157]–[160]:

2-1

where d0 is the initial droplet diameter, t is the time, and βv is the vaporization rate constant as follows:

2-2

where kg is the thermal conductivity of the gas, Cp,g is the specific heat of the gas, ρl is the liquid

density, and B is the Spalding constant:

∞

2-3

where hfg is the effective latent heat of vaporization, T∞ is the gas temperature, and Tboil is the boiling

temperature of the liquid droplet. As the pressure and temperature conditions approach the critical

points of the pure substance, surface tension and hfg decreases to zero and thus the B-Spalding

constant becomes infinite or very large. Upon approaching the critical points, the vaporization rate

increases dramatically and the vaporization time shortens.

41

Figure 2-2. Effect of pressures (A:0.79 MPa, B:3.55 MPa, C:7.58 MPa) on wet-bubble temperature of

the droplet (Source: Ref. [158]).

For combustion, the droplet gasification time decreases with increasing pressure and

approaches a minimum with a further increase in the relative pressure of (Pr=P/Pc) up to about 5. The

combustion lifetime of the droplet showed a minimum near the critical pressure and then increased

above the critical pressure. This is relevant to the onset of supercritical burning, defined by Faeth et al.

[158], with no wet-bubble plateau. As shown in Fig. 2-2, with increasing pressure, the wet-bubble

plateau disappears for an n-C10H22 droplet burning in air. In summary, due to the characteristics of a

supercritical droplet, such as no surface tension or heat of vaporization, and the increased solubility,

liquid fuel atomization and combustion can be enhanced under supercritical conditions.

2.3.2 Supercritical Injection and Combustion of Cryogenic Fuels

There are a few studies on the injection of cryogenic fuels into sub-to-supercritical

environments. A series of works by Mayer et al. [51], [162]–[164] studied non-reacting jets and

combustion of cryogenic fuels, such as liquid nitrogen (LN2) and liquid oxygen (LOx), into different

42

gaseous environments with varying chamber pressures from sub to supercritical conditions. A

significant change (with no droplet formations) in jet appearance of LN2 was observed near and above

the critical pressures. They also revealed a remarkable difference between subcritical spray formation

and evaporation and the supercritical injection and mixing processes for combustion conditions with

LOx/gaseous hydrogen (GH2) as rocket propellants. As the chamber pressure increases near the

critical point, there no longer exists spray droplet formation but rather a fluid/fluid mixing in both

injection and combustion phases. Mayer et al. [50] reported that the transition of atomization and

breakup of cryogenic propellants is strongly influenced by the environment composition and initial

conditions. From the LOx droplet tests, droplets were observed to deform with increasing pressure

due to the absence of surface tension so that the clumps from deformation of the droplets are likely to

enhance the mixing process.

Chehroudi et al. [40], [42], [48] performed extensive research on supercritical injection and

combustion of cryogenic fuels into various gas environments. Similar observations of injection and

combustion processes of cryogenic fuels under supercritical conditions were reported, which are

consistent with Mayer et al.’s findings. Chehroudi et al. [42] attempted to correlate initial growth rate

of a round jet into a sub-to-supercritical environment with their visual characteristics. From this

approach, theoretical as well as quantitative information can be provided with extensive comparison

with the conventional jet theories. The results of the jet spreading angle or jet growth rate confirmed

that supercritical jets near or above the supercritical points behaves like incompressible but variable-

density gas jets. This analysis, taken from both liquid and gaseous jet theories, gave the profound

understanding of supercritical injection and combustion of the cryogenic fuels.

43

2.3.3 Supercritical Injection and Combustion of Hydrocarbons/Propellants

The use of the endothermic hydrocarbon as a regenerative coolant for advanced propulsion

systems must involve the fuel and its products that result from thermal or catalytic decomposition.

Understanding behaviors of the cracked fuel mixture during injection and combustion as well as the

effects on the ignition characteristics, therefore, is important. Very limited information is available in

the literature on the cracked fuel. To the author’s knowledge, the current study is the first attempt to

inject and combust fuel which is cracked under supercritical conditions into the non-reacting and

reacting supercritical environment.

Wu et al. [165], [166] investigated the injection and expansion processes of supercritical

ethylene near and above the thermodynamic critical point. By varying the injection temperature and

pressure ratio between the injector and chamber, jet structures were studied. The jet image indicated

gas-like expansion with a visible shock structure as the temperature rises. In addition, the supercritical

hydrocarbon jet can be assumed to behave like an ideal gas jet by the comparison of Mach disk

location of the supercritical jet with that of an ideal-gas jet. It was also noted that condensation during

injection was observed approaching the critical temperature, but detailed mechanisms relevant to

condensation were not discussed in these studies.

Lin et al. [167], [168] studied in more detail, experimentally and theoretically, the structure

and phase transition of supercritical methane/ethylene jets injected into a quiescent environment of

nitrogen. Their observations of jet structures supported the notion that supercritical jets behave as an

ideal-gas-like expansion jet found by Wu et al. [165], [166]. They also noticed condensation over the

injection and expansion processes, and attempted to explain the condensation process with

homogeneous droplet nucleation at the injection plane or inside the injector to create the observed

opaque jet appearance. Upon approaching the critical points, the degree of supersaturation increased,

and the surface tension decreased, possibly resulting an increase in droplet nucleation rate and a

reduction in the nucleus size. An effort was made to measure incident nucleation inside the injector

44

using a small angle X-ray scattering technique. Their preliminary results showed that at elevated

temperature, near or above the critical temperature, nucleation upstream of the injector accelerates

and correspondingly larger drops form inside and downstream of the injector.

Doungthip et al. [52] first performed supercritical injection of a hydrocarbon fuel, such as

Jet-A, into a supercritical N2 environment. Using schlieren image analysis, jet penetration depth,

spreading angle, and phase changes were studied as functions of temperature, pressure, and fuel flow

rate. As the fuel temperature increases under supercritical conditions, penetration depths of

supercritical jets were observed to decrease due to the enhanced mixing. Their numerical simulations

accurately predicted penetration depths and spreading angles of the supercritical jets from the

experiments. Rachedi et al. [45], [46] and Crook et al. [43] also conducted injection experiments with

hydrocarbons under supercritical conditions. Spreading angles, penetration depths, and spatial

concentrations were investigated on supercritical jets of JP-10. While the previous studies provide

useful information on pure fuel jets under supercritical conditions, the injection and mixing

mechanisms are still unknown.

Puri et al. [169] numerically examined ignition characteristics of cracked JP-7 fuel. With

different fuel compositions, ignition delays were estimated over the temperature range of 1200 - 2000

K, the pressure range of 1 - 20 atm, and the equivalence ratio range of 0.5 - 1.5. Their calculations

indicated that ignition delays decrease by 8.9 % with the fuel mixture consisting of the C3 compounds.

Helfrich et al. [170] investigated the effect of supercritical fuel injection on cycle performance of a

pulsed detonation engine (PDE). Supercritical fuel injection with increasing temperatures reduced

ignition delay, deflagration to detonation transition time, detonation distance, and increased the

percent of ignitions resulting in a detonation.

45

2.4 Theoretical and Numerical Studies of Enhancement Mechanisms of Hydrocarbons with

Nanoparticles

Enhancement mechanisms for fuel decomposition and combustion under high pressure and

temperature conditions for hydrocarbons containing colloidal nanostructured materials are not well

understood. Catalyzed reactions are being studied by several methodologies, for example, kinetics

based on product distributions from experiments, molecular dynamics (MD) simulations, and

quantum mechanics (QM) such as density functional theory (DFT) calculations [171]–[181].

Experimental approaches are still valuable and available for individual reactions, but at high pressure

and temperature are very expensive and difficult. Simulations based on MD or QM can provide

accurate transition states and reaction rate constants for individual elementary reactions. Moreover,

these calculations give intuitive understanding of catalytic reactions and interactions between fuel

molecules and catalytic materials. There exists a vast literature available for reaction mechanisms of

hydrocarbons over catalysts. Some examples of these catalytic mechanisms on hydrocarbons with

catalytic materials of interest to this study will be reviewed in this section. Reaction barriers for the

specific reactions can be estimated using DFT calculations and individual and global interactions of

catalytic materials with fuel components can be observed in MD simulations.

Tang and Cao [182] studied the oxidative hydrogenation (ODH) of propane catalyzed by

graphene oxides (GOs) using DFT simulations. The results showed high catalytic activity of GOs

with modified oxygen-containing groups for the ODH of propane to propene. This is attributed to the

fact that hydroxyl groups around the active sites provided by epoxides can enhance the C–H bond

activation of propane. It was also found that the activity enhancement strongly depends on the site

location of the functional groups.

Zhang et al. [183] proposed possible mechanisms for CNT-catalyzed butane oxidation using

the DFT calculations. They found that butane is oxidized energetically by H-abstraction with an

46

endothermic heat of reaction of 21.2 kcal/mol. The oxygen-functional sites are likely to participate in

the conversion of butane to 1-butene or 2-butene. They also showed the regeneration mechanisms of

active sites and provided the feasibility of the CNTs with oxygen functional groups as metal-free

carbo-catalysis.

Vajda et al. [181] performed DFT calculations to investigate the catalytic oxidative

dehydrogenation of propane with subnanometer Pt clusters on a θ-alumina surface. Their Pt cluster

catalyzed the OHD of propane to propene with a much lower energy barrier of 9.7 kcal/mol. In their

reaction pathway analysis, the C-H bond cleavage on the CH2 group of propane occurs first, followed

by C-H bond cleavage on the CH3 group.

To suggest possible enhancement mechanisms of FGS on NM burning rates, ab initio MD

simulations [111] have been recently performed. This simulation work has supported the

enhancement of NM burning rates in the presence of FGS and revealed that the catalytic activity of

FGS on the decomposition of NM and its radicals can originate from carbon vacancy defects within

the plane of the FGS, functionalized with oxygen-containing groups. FGS accelerated the NM

decomposition through the exchange of protons or oxygens between the oxygen-containing functional

groups and NM and its derivatives [111]. One of the important findings which explain the

enhancement of burning rates is the formation of aci-ions, CH2NO2−. This formation from the

interaction between NM and functionalities is likely to play an important role in the detonation

kinetics of dense NM. Moreover, the initiation reactions happened at the early stage of decomposition,

leading to a rapid reduction of the initial NM population. The final products observed in the

simulation were mainly H2O, CO2, and N2. When suppressing the catalytic activity of FGS, no

reactions took place in a time span of ∼5 ps.

Wang et al. [178], [184] studied thermal cracking of hydrocarbons such as n-C10H22 and n-

C12H26 and catalytic cracking of n-C10H22 in the presence of several fuel additives. Using reactive

molecular dynamics (MD) simulations employing the ReaxFF reactive force field, the presence of

47

additives were shown to enhance conversion rates of the fuel. ReaxFF MD simulation predicted the

initiation mechanisms which are in good agreement with existing chemical kinetic models of thermal

decomposition of n-C10H22 and n-C12H26. Catalytic decompositions of n-C10H22 with several additives

such as diethyl ether (DEE), methyl tert-butyl ether (MTBE), NP, TEMPO, triethylamine (TEA), and

diacetonediperodixe (DADP) were investigated. Using first-order kinetic analysis, the fuel additives

were found to lower the apparent activation energy and pre-exponential factors. The additives

accelerated conversion rates compared to that of pure n-C10H22.

Castro-Marcano and van Duin [58] extensively performed ReaxFF MD simulations to

understand fundamental initiation mechanisms and branching reactions for 1-heptene (C7H14) with

different particles such as amorphous silica, hydrated amorphous silica, and amorphous

aluminosilicate nanoparticles. The addition of particles in the fuel showed catalytic activity to alter

initiation mechanisms from C-C bond scission to a combination of C-C bond scission, protonation,

and dehydrogenation. This alteration was found to enhance fuel conversion rates and lower global

activation.

2.5 Summary of Literature Review and Contribution of This Study

The preceding literature review was divided into four major categories, including thermal and

catalytic decomposition of hydrocarbons under supercritical conditions, combustion studies of

hydrocarbons dispersed with nanoparticles, supercritical combustion and injection of liquid fuels, and

computational works of catalyzed hydrocarbon systems. First, as seen in the literature review on

combustion, several studies were reported on liquid fuels and propellants containing metal

nanoparticles. However, the published literature is very qualitative, which essentially reports several

observations such as enhanced burning rates and lowered ignition temperatures. These research

studies showed that the particles have the potential to accelerate combustion and reduce ignition delay

48

time for use in future advanced propulsion systems and energy conversion devices. Since the

combustion of fuels with or without particles is very complex, it is difficult to reveal the enhancement

mechanism when the particles are present in the fuel. Second, studies regarding pyrolysis experiments

were reviewed to understand the effects of catalytic materials on altering the reaction mechanisms.

Such pyrolysis studies were limited to gas phase pyrolysis for the fuel containing solid particles.

Moreover, in the research field of catalysts, much work has been devoted to the development of

nanocatalysts for use in packed-bed or coating applications. As mentioned previously, it is impractical

to apply these types of catalysts into existing high speed airbreathing propulsion systems. Colloidal

nanostructured materials, therefore, have been considered for endothermic fuels in order to increase

heat sink capacity and conversion of the fuel. Very few research studies on the dispersion of

nanocatalysts in liquid fuels and propellants, however, have been performed. An understanding of the

effects of colloidal nanocatalysts on reaction mechanisms and decomposition rates of liquid fuels and

propellants under near-critical or supercritical conditions is also very limited. Third, supercritical

combustion and injection studies mainly focused on cryogenic fuels or gaseous hydrocarbons such as

methane and ethylene. While fundamental experiments and simulations were made on supercritical

droplet vaporization and combustion for heavy hydrocarbons, only limited research is available for

supercritical injection and combustion of heavy hydrocarbons without any cracking. Experimental

and numerical investigations studying the injection of fuel and particle mixtures pyrolyzed from

reactors at supercritical conditions into supercritical environments and subsequent ignition and

combustion of the fuel mixture is needed. This research is devoted to elucidating the role of colloidal

nanostructured materials on reaction mechanisms of liquid fuels. In particular, using MD simulations,

the decomposition mechanisms of liquid hydrocarbon fuels containing colloidal particles will be

analyzed. This research will help to better understand the supercritical-phase thermal/catalytic

decomposition of the fuel or fuel/particle mixtures. Comparing existing kinetic modeling based on

49

combustion and pyrolysis of pure fuels or propellants, much needed insight into the catalytic reactions

and better kinetic modeling for combustion and pyrolysis will be provided.

50

Chapter 3

Experimental Methods

Supercritical pyrolysis and combustion of hydrocarbon fuels/propellants with and without

particulate matter were studied experimentally using a high pressure/temperature flow reactor to

simulate reactions in fuel lines, which was coupled to an optically accessible pressurized combustion

chamber for observation of the subsequent injection, ignition, and combustion processes. Pyrolysis

products were analyzed using gas chromatography to characterize the extent of pyrolysis and effect of

various particle additives. High speed shadowgraph and schlieren techniques were employed to study

the injection, ignition, and subsequent combustion processes while combustion chamber pressure and

exhaust species measurements were recorded to elucidate any enhancement to conversion efficiency.

3.1 Liquid Fuels

MCH and n-C12H26, having a reported 99+% purity, were obtained from Sigma-Aldrich and

Alfa Aesar. MCH and n-C12H26, which are considered endothermic fuels and in the case of n-C12H26 a

major component of jet fuel, were tested as model fuels without further purification. Table 3-1

provides the properties of MCH and n-C12H26. Prior to mixing with particle additives, liquid fuels

were sparged under 0.17 MPa N2 for 2 hours to remove any dissolved oxygen which may be in the

fuel [12], [14], [127]. For injection and combustion experiments, the sparging procedure was not

applied because the comparison of product molar yields and distributions from the fuel pyrolysis

showed no observable differences with or without sparging.

51

Table 3-1. Properties of MCH and n-C12H26 used in this study.

Compound Structure TBP (K) Tc (K) Pc (MPa)

methylcyclohexane, C7H14

374 572 3.475

n-dodecane, C12H26

489.3 658 1.824

3.2 Particle Synthesis

In this study, a wide variety of colloidal particles, including functionalized graphene sheets

(FGS), Pt-decorated FGS (Pt@FGS or Pt-FGS), polyoxometalates-decorated FGS (POM@FGS),

coated nanosized aluminum (nAl) with attached ligands to prevent formation of agglomerates and

maintain suspension in liquid fuels, and inert silica were studied. FGS, Pt@FGS, and POM@FGS

were obtained from co-workers at Princeton University and the University of Delaware. Nanometer

aluminum with a nominal size of 80 nm were obtained from Novacentrix Inc.. These particles were

coated with Lauric acid (C12H24O2) using a process reported by Malchi [185].

The graphene was prepared through a thermal exfoliation process recently pioneered at

Princeton University [107], [108]. The C/O ratio of the sample represents the degree of

functionalization, and is described by a subscript; for example, FGS having a carbon to oxygen ratio

of 19 is denoted as FGS19. According to the Staudenmaier method [186], graphite oxide (GO) was

produced via oxidation of graphite flakes in a concentrated solution of nitric acid, sulfuric acid, and

potassium chlorate. After cleansing and drying, GO formed in the concentrated solution, and rapid

heating in a high temperature furnace was used to produce hydroxide and epoxide functional groups

within the GO. High rates of CO2 gas production during rapid heating also lead to sample break up

into single, functionalized, graphene sheets.

52

FGS decorated with platinum nanoparticles were prepared using a modification of the

procedure described by Kou et al. [187]. FGS having a C/O ratio of 22 or 100 were used as substrates

for the deposition and stabilization of the metal nanoparticles. The FGS were first mixed with 10 mL

of pure ethanol and sonicated in an ice bath for 45 minutes to break up agglomerates. The ethanol was

then evaporated from the mixture overnight. An appropriate amount of chloroplatinic acid hydrate,

H2PtCl6·xH2O (~38% Pt basis), enough to yield 20% by weight platinum in the final Pt@FGS powder,

was dissolved in 10 mL of acetone. The solution was added dropwise to dry FGS powder with gentle

stirring over a period of 10 min, and then dried overnight at 100 °C. The powder was heated to

300 °C at 10 °C/min in a sealed furnace, under a continuously purged 5% H2/95% N2 gas mixture.

The particles were held at temperature for 3 hours under the same atmospheric composition to reduce

the salt to metal ratio. This procedure resulted in formation of platinum particles as small as 2 nm in

diameter which were pinned to the FGS surface [187]. Decorated FGS were stored in desiccator until

used.

To improve the dispersibility of the particles in non-polar solvents (such as toluene), the

grafting technique, described in detail in reference [121], was used to attach decyl (C10) and eicosyl

(C20) chains onto the surface of FGS as a support for POM. FGS with a carbon to oxygen ratio of 16 –

19 were dispersed in toluene using a sonicating horn for 15 mins. Then, n-butyllithium and 1-

bromodecane were added to the solution to activate the FGS and graft decyl groups onto the particle

surface. The mixture was kept initially at room temperature for 1 h and the heated to 70 oC for 4 h.

Upon completion of the heating process, the solution temperature was reduced to room temperature

and neutralized with methanol. Using distilled water, successive liquid phase extractions were

performed to remove the LiBr formed during the reaction. Additional treatment with toluene and

methanol was conducted to restore and clean the extracted powder. Finally, the FGS was dried at 60

oC overnight.

53

POM@FGS was synthesized by an incipient wetness impregnation method, discussed in

detail in references [75], [120]. Polyoxometalates (H3PMo12O40 and H4PMo11VO40) were chosen as

catalysts for use in the FGS substrate. Grafted FGS samples were dispersed in a vial of methanol to

produce a paste. An aqueous solution containing the dissolved POM was then added to the paste. To

obtain homogeneous POM@FGS, the mixture was stirred and sonicated. The walls of the vial were

rinsed with methanol, and the vial was heated on a hot plate (50 oC) overnight for drying. Grinding

the dried sample in an agate mortar produced a fine powder of POM@FGS.

Nanometer sized aluminum particles were coated with lauric acid using the method described

by Malchi [185] with modifications. nAl and lauric acid were obtained from Novacentrix Inc. having

a nominal diameter of 80 nm and from Fluka Inc., respectively. Luaric acid (380 mg) was added to

125 mL of toluene and stirred for approximately 15 minutes, while one half gram of nAl was

sonicated in 375 mL of toluene to break up all aggregates. The dissolved ligands were added to the

nAl/toluene mixture and left sonicating for an additional 15 minutes. The coated nAl were produced

using a molar ratio of 0.1 mol ligand/mol particle. Once sonication was completed, the solution was

divided into four vials, with each having the same mass to permit balancing the centrifuge while

separating the particles from the solution. The vials containing the mixture were centrifuged at 3000

rpm for 10 minutes followed by decanting. The remaining solvent/lauric acid was washed from the

particles using hexanes. The coated particles were then placed in an oven at approximately 60 oC

overnight to evaporate any remaining solvent.

3.3 Particle Dispersion

FGS, Pt@FGS, and POM@FGS particles were dispersed in liquid hydrocarbons using a

sonicating horn (Branson Sonifier). Five sonication cycles having a 30 second duration were used.

After approximately one hour following sonication, particle setting was observed to initiate at a FGS

54

concentration of 100 ppmw. Grafted FGS with decyl (C10) and eicosyl (C20) chains showed better

dispersibility than FGS. Most pyrolysis and combustion experiments using FGS-based particles were

finished within an hour. Note that POM@FGS particles were considered only for pyrolysis

experiments.

In order to disperse the coated nAl in n-C12H26, 200 mg of nAl was added to 5 g of n-C12H26

and then the mixture was sonicated using an ultrasonic bath for approximately 10 minutes. This

concentrated mixture was then added to 194.8 g of n-C12H26 and sonicated again for an additional 10

minutes. Prior to pyrolysis and combustion experiments, all mixtures were dispersed immediately to

minimize any effects of sedimentation and formation of particle aggregates.

3.4 High Pressure and High Temperature Flow Reactor

The high pressure and temperature flow reactor was designed to provide isothermal, isobaric,

flow conditions, similar to Stewart [12] and Davis [124]. The reactor system (illustrated in Figs. 3-1

and 3-2) consists of a siltek/sulfinert deactivated stainless steel tube (2.159 mm i.d., 3.175 mm o.d.,

Restek) coiled and immersed in a temperature-controlled fluidized bath (Techne IFB51). This tubing

has similar characteristics to the silca-lined stainless steel tube used in Stewart [12] and Davis [124]

which was shown to prevent wall-catalyzed deposit formation that occurs with unlined stainless steel

[12], [124], [188]–[190].

55

Figure 3-1. Schematic of the high pressure flow reactor.

Figure 3-2. Photograph of the high pressure flow reactor.

The reactor pressure was controlled using a high-pressure piston sensing back-pressure

regulator (Swagelok) downstream of the reactor coil. The inlet and exit temperatures of the reactor

coil were controlled using a water-cooled heat exchanger and ensured a well-defined residence time.

syringe

pump

vent

pump

fill

line

sparge

vessel

N2

air supply

regulator

fluidized bath

reactor

coil

TC

PT

burst disk

filter filterPG

10-position sample

trapping valve

back pressure

regulator

vent

DAQ

PG

PT: pressure transducer

PG: pressure gauge

TC: thermocouple

water-cooled

heat exchanger

High

Pressure

Syringe

Pump

Fluidized

Heating

Bath

Water-

cooled

Heat

Exchanger

Multiposition

Sampling Valve

Back Pressure

Regulator

56

Frits having a 0.5 micron porosity were located both upstream and downstream of the reactor and

were used to filter large solids formed during pure methylcylohexane (MCH) and n-dodecane (n-

C12H26) pyrolysis. In contrast, for experiments conducted with FGS, the frit upstream of the reactor

was removed to avoid filtering the particles from the liquid fuel. Quenched intermediates were

automatically collected using a high–pressure 10-position sample trapping valve. The multi-position

valve (MPV), comprised of a servo-controlled actuator and multiple sealed loops with well-defined

volumes to trap samples, can store multi-phase mixtures under high pressures up to approximately 30

MPa. To prevent potential reactor contamination, separate coils were used for pure liquid fuels and

similar composition containing the particles. After the completion of an experiment, the cold reactor

was flushed several times with solvent, such as methanol or hexane, followed by a nitrogen gas purge

(0.55 MPa) to remove any remaining solvent. Pressure transducers (OMEGA high performance

pressure transducer PX309-7.5KG5V with a reported ±0.25 % error) were used to measure and record

the reactor pressure during each experiment. The pressure transducers were located at the reactor inlet

and exit. Measured pressure drops across the reactor were typically less than 0.05 MPa. Three K-type

thermocouples were affixed to the reactor coil on its outer surface to measure the temperature within

the fluidized bath. For data acquisition of temperatures and pressures, a custom National Instrument

DAQ system was used. Figure 3-3 provides representative temperature and pressure profiles (1.0

mL/min) which demonstrate the isothermal and isobaric conditions over the time period of a

supercritical pyrolysis experiment. The maximum uncertainties of temperature and pressure

measurements were ± 1.25 % and ± 2.20 %, respectively. The experimental conditions considered in

the current study are summarized in Table 3-2.

57

Figure 3-3. Representative temperature and pressure profiles in the flow reactor over run for MCH

pyrolysis at the flow rate of 1.0 mL/min.

Table 3-2. Experimental conditions for thermal decomposition of MCH and n-C12H26 containing

different particles.

Parameters MCH n-C12H26

temperature (K) 745 – 840 K 750 – 823 K

reduced pressure (Pr) 1.36 2.61

volume flow rate (mL/min) 0.9 – 2.0 2.5 – 8.0

additive concentration (ppmw) 50 50

To understand the fluid dynamics of the laminar flow reactor, characteristic times for

chemical reactions, fluid properties, and dimensionless parameters were determined prior to

conducting supercritical pyrolysis experiments. Tables 3-3 and 3-4 provide characteristic times and

Time, min

Pre

ss

ure

,M

Pa

0 5 10 15 200

1

2

3

4

5

Time, min

Te

mp

era

ture

,K

0 5 10 15 20700

750

800

850

900

TC_1

TC_2

TC_3

58

dimensionless parameters for pure MCH and n-C12H26 at representative experimental conditions. The

results of this characterization indicate that the reactor flow conditions meet the laminar plug flow

criteria proposed by Cutler et al. [191].

Table 3-3. Characteristics times for thermal decompositions of pure MCH and n-C12H26.

Characteristics Times Definition MCH (1.0 mL/min, 820 K,

4.48 MPa)

n-C12H26 (5.0 mL/min,

803 K, 4.7 MPa)

Global chemical kinetic,

t(ck) , sec 84.44 14.94

Forced convection,

radius, t(fc,r) r/V, sec 0.02 0.01

Forced convection,

length, t(fc,L) L/V, sec 54.37 30.14

Species diffusion,

radius,

t(sd,r)

r2/D, sec 26.35 11.65

Species diffusion,

length, t(sd,L) r

2/G, sec 5384.04 3742.8

Thermal diffusion,

t(td) r

2/α, sec 3.97 13.51

Momentum diffusion,

t(md) r

2/ν, sec 4.48 9.36

k:reaction rate measured from experiments, 1/sec

r: reactor radius, m

V: maximum velocity, m/s

D: diffusivity, m2/s, for n-C12H26 from Ref. [192]

G: Taylor diffusivity, m2/s

α: thermal diffusivity, m2/s, from SUPERTRAPP

ν: kinematic viscosity, m2/s, from SUPERTRAPP

59

Table 3-4. Dimensionless parameters for thermal decompositions of pure MCH and n-C12H26.

Dimensionless

Parameters Definition

MCH (1.0 mL/min, 820 K,

4.48 MPa)

n-C12H26 (5.0 mL/min,

803 K, 4.7 MPa)

Reynolds # (Re) t(md)/t(fc,r) 198.63 776.55

Prandtl # (Pr) t(td)/t(md) 0.89 1.44

Schmidt # (Sc) t(sd,r)/t(md) 5.88 1.25

Peclet # (Pesd) t(sd,r)/t(fc,r) 1167.33 966.9

Peclet # (Petd) t(td)/t(fc,r) 175.82 1121.26

Damkohler # (Da) t(sd,r)/t(ck) 0.31 0.78

Assuming that the fuel density varies as a linear function of reactor length [12], the residence

time can be calculated using the following equation:

3-1

In Eq. 3-1, ρavg is the average density, calculated using the densities of the parent fuel and the final

product, A is the cross-sectional area of the reactor, ṁ is the mass flow rate, and L is the length of the

reactor.

3.5 Sub-to-Supercritical Injection (non-reacting) Experiment

Non-reacting fuel jet experiments were conducted by coupling the previously discussed flow

reactor experiment with an optically accessible pressure vessel and three-way diverting valve (as

shown in Fig. 3-4). This valve was designed for high temperature operation, using Inconel stem tips

and electro-pneumatic solenoid valves via relays. This setup permits the injection of fuels under

60

controlled temperature and pressure environments. A schematic of the experimental setup is

illustrated in Fig. 3.5. The windowed chamber, bypass chamber, and three-way diverting valve were

placed in a boom-box, which was designed to contain fragmentation in the event of a pressure-related

failure. The pressure vessel had a constant volume (66.6 cm3) with layered quartz windows (6.35 mm

thick sacrificial window and 31.75 mm thick pressure window) on two sides for back-lighting to

visualize the jets under high pressure and temperature operational conditions. A capillary tube

injector (inner diameter, de, of 254 m) was chosen with a length to diameter ratio (L/de) of

approximately 500. These experiments were conducted under an inert (nitrogen, N2) environment to

prevent ignition, and thus the effect of non-reacting fuel jets containing additives, injected into high

temperature and pressure environments were examined.

61

(a) Overview

(b) Section-view

Figure 3-4. Assembly of 3-way diverting valve, fuel injector, and windowed chamber.

Windowed Chamber

Fuel Injector

Normally-Closed Pneumatic Valve

Normally-Open Pneumatic Valve

ToBypass Line

From Reactor

Travelling

62

Figure 3-5. Schematic diagram of the supercritical reactor experiment and optically accessible

chamber used for sub-to-supercritical injection experiments.

Both the fuel and nitrogen pressurant were heated using the fluidized bath, and multiple high

temperature heating cords regulated using variable AC voltage controller and insulated with ultra-

high temperature foil-faced strips maintained the line temperature from the bath to the valve and

chamber. The fuel line pressure upstream of the injector valve, as well as the chamber pressure, were

monitored and recorded at 1,000 Hz, and temperatures were recorded from multiple locations within

the system, as indicated by the system schematic, presented in Fig. 3-5. Following system pre-heating

(during which a continuous flow of inert gas was purged through both the reactor and pressurant

lines), fuel flow was initiated via the high-pressure syringe pump. Fuel was continuously pumped

through the reactor into a high-pressure bypass chamber until a steady-state flow condition was

63

achieved at the desired temperature and pressure. Following flow equilibrium through the bypass

system, the fuel diverting valve was actuated, closing the bypass portion of the flow system and

initiating fuel flow to the injector. Both the high-pressure bypass portion of the system and the

chamber utilized the same diameter injectors with the same L/de ratio to maintain a constant pressure

drop (and thus a constant upstream fuel pressure), and both were pressurized by the same pressurant

system to maintain a constant gas temperature and pressure between the high-pressure bypass portion

of the system and the optical pressure vessel. The fuel injection process was controlled using a

custom LabVIEW operating program which actuated the pneumatic valves via relays. Valve initiation

was timed based on empirically determined offsets which accounted for any lagging during the

actuation process (i.e., timing is such that the bypass portion of the valve finished closing just before

the injection valve began opening, minimizing any pressure fluctuation in the feed system, and thus

minimizing the injection transient).

In a typical series of experiments, the particles were dispersed in the liquid fuel using a

Branson digital sonifier, which utilizes high-frequency horn tip oscillations to induce cavitation in the

liquid fuel, break up agglomerates, and disperse the particles. Following several sonication mixing

cycles, the liquid fuel containing dispersed particles was drawn into the high-pressure syringe pump

reservoir. Air was purged from the system by pumping through a 4-way switching valve (shown in

Fig. 3-5) until liquid was observed to flow through the low-pressure bypass line. The injection

process always began by pump initiation to the low-pressure bypass line, while nitrogen purge gas

was vented through the high-pressure bypass chamber. Once the purge gas has vented (the fuel

reactor has depressurized), the 4-way valve was rotated, switching the flow paths for both the high-

pressure pump and the nitrogen purge line. The blow-down valve (not shown) at the base of the high-

pressure bypass system remained open (permitting remaining nitrogen and the initial fuel flow

reaching the bypass system injector to continue to purge from the system) until the fuel line pressure

increased, indicating nitrogen had been purged from the reactor and fuel had reached the high-

64

pressure bypass system injector. While the blow-down valve on the high-pressure bypass system was

closed, the fuel was being continuously injected into and stored within the high-pressure bypass

system reservoir. The system pressure was increased to the target value and after a period of

equilibration the previously described LabVIEW control sequencing program was initiated. This

control program also triggers a Phantom V7.3 high-speed camera through a Stanford Research

Systems delay/pulse generator which is used to observe and capture the injection process. High-speed

images are recorded at 10,000 frames per second with the exposure time of 1.0 µs. Following

completion of the experiment, the system pressure was reduced to ambient (using some of the stored

inert pressurant to blow-down the high-pressure bypass system reservoir into a nitrogen purged water

trap). Purge gas flow was then initiated through the low-pressure bypass line. The 4-way switching

valve was rotated back to the initial position, sending high-pressure inert gas into the reactor, purging

any remaining fuel through the high-pressure bypass system, while the high-pressure pump (now

diverted to the low-pressure bypass line) was shut off. Figure 3-6 shows a representative injection

sequence over the entire experiment. The run time was offset and zeroed to the trigger timing of the

Phantom v7.3 camera.

Figure 3-6. Representative firing sequence of supercritical non-reacting injection experiments.

Offset Time, sec

Pre

ss

ure

,M

Pa

Sig

na

l,V

dc

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.84

4.2

4.4

4.6

4.8

5

5.2

0

1

2

3

4

Fuel Line

Vessel

Trigger

Bypass valve

close

Trigger the

camera

Steady-state

Camera recording

Pressure drop:

0.09 MPa

Fuel valve close

& Bypass valve

Open

Fuel valve

open

at -0.95 sec Fuel valve open fully

65

High-speed video post-processing (which includes subtracting a pre-recorded background

taken prior to initiation of the experiment from each video frame) permits measurement of the dark

core length and spreading angle of the fuel jet. ImageJ software was used to analyze the high speed

videos [193]. This software aids in rendering jet images with enhanced contrast and also permits

time-averaging of the images. Each frame can be easily analyzed, pixel by pixel, according to gray

scale value and subtracted from the background noise. Jet boundary and spreading angle

measurements of supercritical fuels injected into the pressurized chamber were calculated using an

image processing program developed by Engine Combustion Network [194], which was modified for

output data and applied to all shadowgraphs with identical parameters. The analysis process included

calculating the temporal and spatial standard deviations of the individual pixel grey scale across the

entire region of interest, after which a threshold was applied which permitted determination of the jet

region, jet boundary, and ultimately the jet spreading angle. These values were compared against

spreading angles calculated manually using ImageJ software. The experimental conditions for

injections are summarized in Table 3-5.

Table 3-5. Experimental conditions for injections of MCH and n-C12H26 containing different particles.

Parameters MCH n-C12H26

reactor temperature (K) 853 K 823 K

chamber temperature (K) 673 K 723 K

chamber pressure (MPa) 4.24 4.24

fuel volume flow rate

(mL/min) 2.5 - 20.0 2.5 – 20.0

additive concentration (ppmw) 100 100

66

3.6 Supercritical Combustion Experiment Facility

Supercritical combustion experiments were conducted in a test cell (rather than using a boom-

box). The previously developed non-reacting experimental setup was modified to permit remote

operation for combustion experiments, as shown in Fig. 3-7. The same-diameter injector was used

with a corresponding length to diameter ratio (L/de) of ~ 400. The entire experiment was remotely

controlled using a custom LabVIEW operating program. Oxidizer (air) and inert gas (N2) mass flow

rates were controlled using choked flow orifices. Prior to conducting each supercritical combustion

experiment, the pressure upstream of each valve was set, using dome-loaded pressure regulators,

based on the calculated pressures required to achieve desired mass flow rate, (for a given orifice size

and discharge coefficient), gas type, and nozzle exit area [195]:

3-2

where CD is the discharge coefficient, Pup,ori is the pressure upstream of the orifice, Tup,ori is the

temperature upstream of the office, Ru is the universal gas constant, MW is the air molecular weight, γ

is the ratio of specific heats of the gas, and At is the orifice throat area. Each orifice was calibrated

prior to use. The fuel mass flow rate was determined by the following equation:

3-3

where ρfuel is the density at the measured pump pressure and room temperature and Qpump is the

volumetric flow rate (m3/sec), defined by the syringe pump. The global equivalence ratio therefore

was calculated as:

3-4

where O/Fstoichiometric is the stoichiometric air-fuel mass ratio for n-C12H26. In the current study,

industrial air was used as the oxidizer. Ambient-temperature N2 gas flowed through the bypass

chamber to mitigate the possibility of autoignition within the bypass chamber. Calibrated Setra

67

pressure transducers were employed to monitor and record the pressure upstream of each orifice, the

fuel line pressure upstream of the injector valve, as well as the bypass and chamber pressures.

Pneumatically-actuated valves were employed to initiate and shut off fuel, nitrogen, and air flow into

the bypass and combustion chamber using a custom LabVIEW control sequencing program and

external relay board.

Figure 3-7. Schematic diagram of the supercritical combustion experimental setup.

For combustion experiments, the bypass pressures were approximately 10 to 15 % higher

than the combustion chamber pressure to achieve the residence time and to prevent the combustible

gas mixture from back-flowing into the bypass chamber. The fuel pressure was set by the N2

pressures in the bypass chamber and the pressure drop (ΔP) across the bypass injector:

3-5

where f is the friction factor. The pressure drop across the fuel injector was observed to be slightly

higher when Pt@FGS was added to the liquid fuel component. With the rough approximation of

Syringe

Pump

Sand Bath

N2

RD (2500 psi)

Vent

4-Way Valve

PT

Fuel

Fill

TCsRD (1500 psi)

Low_pressure

bypass N2 purgeInjector

BypassChamber

Combustion Chamber

PT

AIROrifice (0.35 mm)

Variable

Orifice

PT

CV

CV Orifice (0.2 mm)

N2

PT PT

Fuel Heater

Reactor

3-Way Diverting Valve

Nozzle (0.762 mm)

Vent

Vent

Pump

Filter

MPV

Water-cooled

probeAir Heater #2

Air Heater

#1RD: Rupture disk

PT: Pressure transducer

TC: Thermocouples

CV: Check valve

Quartz tube

68

f=0.03 for high Reynolds number (approximately 14,000), the pressure drop across the bypass

injector were estimated to be approximately 66.3 kPa. The measured pressure drop was 69.0 ± 5.7 kPa

for the pure fuel at 831 K and 5.0 mL/min. Higher fuel pressures were measured when the catalytic

particles were introduced into fuel, which is likely due to the higher conversion rate and more moles

of product formed.

Stagnant hot air, which initially filled inside the fuel injector during chamber pressurization,

was purged using a high-pressure nitrogen pulse, prior to fuel injection. The injector purge prevented

fuel from being injected into a high temperature and pressure oxidizing environment upstream of the

injector. Figure 3-8 presents a typical motor firing sequence over the entire run. The time is offset,

time zero corresponding to the time of start of injection (based on the trigger timing of the Phantom

v3.10 camera). The flows of air and N2 were initiated by the LabVIEW operating program via an

external relay board, following which both chambers quickly achieved the desired operating pressures.

Fuel injection to the bypass chamber initiated after adequate N2 purging of the flow system. The fuel

pressure (reactor pressure shown in Fig. 3-8) and bypass chamber pressure reach steady-state

conditions nearly 30 seconds before fuel reaches the bypass injector. Prior to the injection of fuel into

the combustor, the high-pressure N2 purge was initiated, observed as a slight pressure increase and

subsequent decrease in the chamber pressure profile. Then, the injection process begins by pneumatic

valve actuation. Combustion products such as unburnt hydrocarbons, CO, or CO2 were collected

downstream of the nozzle using a water-cooled sample probe, a vacuum pump and a 16-position-

sample trapping valve. Once the firing duration has been reached, the flows of air and N2 were shut

off and the 4-way switching valve was rotated, permitting N2 purging of the reactor and bypass

injector. The fuel injector was also purged at this time to prevent coking, and the system was reduced

to ambient pressure. The pressure transducers and camera trigger signal were recorded at a sampling

rate of 1,000 Hz. High-speed images were recorded at 10,000 frames per second with the exposure

time of 4.0 µs. In addition to high-speed shadowgraph, schlieren imaging was used to visualize

69

injection, autoignition, and turbulent flames. Figures 3-9 and 3-10 show the setups for both high-

speed shadowgraph and schlieren. High-speed video post-processing permitted measurements of the

spreading angle and ignition delay using ImageJ software. The experimental conditions for

combustion tests are summarized in Table 3-6.

Figure 3-8. Representative firing sequence of supercritical combustion experiments.

70

Figure 3-9. High-speed shadowgraph.

Figure 3-10. High-speed schlieren setup.

Boom Box Bellows and 100-300

mm Nikon Zoom Lens

Delay

Generator

Xenon

Lamp

Phantom V310Mirror

Diffuser

Test Section

DelayGenerator

Phantom V310

Filters

Test Section

LEDLight

Source

FoldingMirror

Parabolic

Mirror

KnifeEdge

Pinhole

Parabolic

Mirror

Zoom Lens

FoldingMirror

PulseGenerator

71

Table 3-6. Experimental conditions for combustion experiments of n-C12H26 containing different

particles.

Parameters n-C12H26

reactor temperature (K) 831 ± 3.16

air temperature (K) 777 ± 3.34

combustion chamber pressure (MPa) 1.41 (only for 5 mL/min)

3.46 ± 0.03

bypass chamber pressure (MPa) 3.76 ± 0.02

volume flow rate (mL/min) 2.5, 5.0

air mass flow (g/sec) 2.5

equivalence ratio (ϕ) 0.186, 0.372

3.7 Gas-Liquid Separation, Dilution, and Vaporization System

During supercritical pyrolysis experiments, samples of decomposed fuel were collected using

the previously discussed multi-position sample trapping valve (MPV) under operating pressure

conditions. The samples were released from the MPV one loop at a time into a specially constructed

separation and dilution system, a schematic of which is shown in Fig. 3-11. Using this apparatus,

gaseous and liquid products were separated and diluted to concentrations which could be accurately

analyzed using gas chromatography. The apparatus was connected to a high-accuracy pressure

transducer (MKS model 398HD) to measure pressures within the system. To perform this separation,

a glass sample bulb, glass flask, and connecting lines were evacuated below 1.5 mmHg. After closing

the line to the vacuum pump, the sample stored in the MPV loop was released into the flask by

actuating the valve. The gas trapped in the sample was drawn into the glass bulb and equilibrated to a

certain pressure, while the liquid component remained stored in the flask. After equilibration, the

72

pressure increase was recorded to permit calculation of the total moles of gaseous products. The

liquid product, stored in the flask, was quantified on a mass basis with a high-accuracy (±0.01 mg)

analytical balance (Mettler Toledo model AB265-S). The gaseous sample was diluted 3 times with

argon such that the final sample was stored at approximately 600 mmHg for later analysis using gas

chromatography. Then, by closing the glass sample bulb valve, the diluted gaseous product was

trapped, ready for GC analysis. The MPV sample loop was purged with UHP Argon to force the

remaining liquid stored in the multi-position valve loop into the flask, and to clean the sample

reservoir.

Figure 3-11. Specialized separation, vaporization, and dilution apparatus.

Liquid sample processing was similar to gaseous sample manufacture. The liquid sample was

injected through an injection port near the flask using a syringe, the system having been evacuated

73

and stored under 1.5 mmHg. The dilution procedure was the same as the gaseous sample preparation.

After completion of the sample preparation for GC analysis, the entire system was cleaned to

minimize any carry-over effect of the previous samples and to permit accurate concentration

measurement of the products. The flasks were cleaned with acetone and distilled water followed by

evacuation which removed any remaining solvent and then purged 5 times with inert gas using the

low-pressure/backfill system. The diluted liquid MCH products were compared against a direct

injection of liquid sample into GC/Flame Ionization Detector (FID) and GC/Mass Spectrometry (MS)

without any dilution. Major species were the same for both liquid sample preparations. The direct

injection of liquid samples provided more accurate information concerning the minor species which

were not observed in the diluted samples due to the significantly reduced concentrations. For liquid

product analysis from n-C12H26 pyrolysis, direct injections to GC/FID and GC/MS were only used

because the major compounds in the liquid samples were not readily vaporized due to their low vapor

pressures.

In addition to the product analysis from the pyrolysis, combustion products such as unburnt

hydrocarbon, CO, CO2 were collected and diluted with this apparatus. Due to the sensitivity of the

GC/FID, dilution of the captured samples was applied with the same procedure described above.

Calibration mixtures, used to identify and quantify species and concentrations from the resulting GC

analysis were also manufactured using this system.

3.8 Product Analysis

Gaseous and liquid products of MCH and n-C12H26 pyrolysis were analyzed using GC/FID,

GC/Thermal Conductivity Detector (TCD), and GC/MS, as shown in Table 3-7. The samples were

injected on the GC column using either a low-pressure sample injection (GC/FID) or pressurized flow

(GC/TCD). The system operated using low-pressure sample injection. The samples were stored under

74

near atmospheric pressure condition in the glass bulb, while a stainless steel bulb was used when

samples were stored above 760 mmHg.

GC/FID calibrations were performed on 21 gaseous and 40 liquid species. For this case,

calibration factors within 5 % were obtained for all samples, supporting the dilution accuracy of the

system. The products were identified by matching retention times with those of reference standards,

and quantification for the gaseous products was achieved by injecting reference standards with known

concentrations on the GC column and then correlating retention times and chromatogram areas with

the known species from the calibration gas. A separate micro GC/TCD was used to measure hydrogen.

Even at the highest conversion, about 5 % of the total molar yield was not identified. The number of

moles of the liquid products can be calculated:

3-6

3-7

where Ci,liquid is the molar yield measured using GC analysis, mliquid is the total mass of the liquid

product, MWfuel is the molecular weight of the fuel using the assumption that the average molar

density of the liquid product was nearly the same as the molar weight of the parent fuel [12]. The

moles of gaseous product can then be obtained:

3-8

3-9

where Ci,gas is the molar yield measured using GC analysis, Pgas is the pressure increase measured by

the separation apparatus, V is the known volume of the separation apparatus, Ru is the ideal gas

constant, and T is the room temperature. Since the separation described above was performed under

vacuum (less than approximately 600 mmHg), the ideal gas law can be applied, thus the mole fraction

of each species identified was determined from the following equation:

75

3-10

where χi is the mole fraction, and i denotes the species in the products, N is the number of moles, and

C is the concentration obtained from GC analysis.

Table 3-7. GC methodology for product analysis.

Varian GC-

FID CP-3800

Shimadzu QP-

5000 MS Agilent 7890A

Agilent

3000A

MicroGC

Injection Split Split Splitless

Detector FID MS FID TCDs

Column

Type Capillary Capillary Capillary

PLOT

Q/MolSieve

Name Rxi R- 5Sil

ms Rti R-5ms H-Plot/Q

Specifications 30m,

0.25mmID 30m, 0.25mmID 30m, 0.53mmOD

Sample Type

Liquid

products:

MCH, n-

C12H26

Liquid products:

MCH, n-C12H26

Gaseous and/or

products: MCH, n-

C12H26

Hydrogen

3.9 Surface Characterization Analysis of the Particles

Parallel to decomposition and combustion studies of liquid fuels/propellants with the particles,

surface characterization analysis using transmission electron microscopy (TEM) was employed to

investigate the particles before and after reaction. The TEM images provide additional information on

76

structural characteristics of the particles to better understand the fundamental features of the particles

before and after reaction. Especially, this analysis determines whether Pt nanoparticles attached to the

FGS remained stable through the course of the reaction.

77

Chapter 4

Supercritical Pyrolysis Experiments

4.1 Pure MCH and MCH containing Particles

Figure 4-1 illustrates the effect of FGS19 additive on MCH conversion under supercritical-

phase pyrolysis at temperatures ranging from 745 to 840 K, a fixed reduced pressure of 1.36, and a

fixed volumetric flow rate of 1.0 mL/min. In order to provide a baseline, supercritical-phase thermal

decomposition experiments of pure MCH were performed at the same conditions. Due to the change

in the average density of the final products during supercritical pyrolysis, the calculated residence

times were not exactly same for both non-catalytic and catalytic experiments. At temperatures below

800 K, conversion rates of MCH increase slowly with increasing temperature. In contrast, conversion

rates become much faster at temperature above 800 K. It is found that conversion enhancement in the

presence of 50 ppmw FGS19 is particularly more noticeable at higher temperatures than at low

temperatures. For example, the conversion of MCH with FGS19 at 768 K is about 2.62 %, whereas in

absence of FGS19 the conversion is about 2.32 %. However, when the temperature rises to 820 K, the

conversion in the presence of FGS19 is measured about 64.2 %, whereas the conversion with pure

MCH is observed about 44.8 %. It has been suggested that FGS can act as a catalyst. While the exact

mechanism of enhancement is not known, an accelerating effect on the thermal decomposition of

MCH at higher temperatures is clearly observed. When the FGS additive is present in MCH,

conversion rates were increased, hence the temperature required to achieve 50 percentage conversion

is approximately 10.0 K lower with the particle additive. While this difference in temperature is not

large, the trends are repeatable and consistent. When the temperature increases from 820 K to 840 K,

conversion improvement decreases from about 30.3 % to 5.3 %. A reasonable explanation for this

78

decrease might be that thermal decomposition above 820 K depresses the catalytic effect of FGS on

the conversion of MCH and that the reaction is approaching completion in the residence times of the

experiments. Note that conversions of pure MCH in this study are in relatively good agreement with

the previous study [12], indicating self-consistency between the two experiments.

Figure 4-1. Effects of FGS19 on conversion of MCH under supercritical-phase decomposition at

temperatures ranging from 745 to 840 K at a fixed reduced pressure of 1.36, and a fixed flow rate of 1

mL/min.

For the supercritical pyrolysis of both pure MCH and MCH/FGS19 50 ppmw, the gaseous and

liquid products identified and quantified in this study are C1-C5 n-alkanes, C2-C6 1-alkenes,

dimethylcyclopentane isomers, C5-C6 cycloalkanes and cycloalkenes, methylcyclopentane,

Temperature, K

Co

nv

ers

ion

of

MC

H,%

720 740 760 780 800 820 840 860-20

0

20

40

60

80

100

w/ 50 ppmw FGS19

no additive

no additive (J.F. Stewart, 1999)

Temperature, K

Co

nv

ers

ion

of

MC

H,%

740 750 760 770 780 790-1

0

1

2

3

4

5

79

methylcyclopentene, 2-butene, benzene, and toluene. The product distribution found in this study

compares well with the previous research studies [12], [32], [37]. Figure 4-2 shows major product

yields from the decomposition of MCH with and without FGS19 with reactor temperatures varying

from 745 to 840 K at a fixed reduced pressure of 1.36 and a fixed volumetric flow rate of 1.0 mL/min.

The major gaseous products from thermal decomposition of MCH with and without FGS19 were the

same, such as methane, ethane, propene, propane, ethene and gas-phase MCH. Only a small quantity

of n-butane, 1-butene, iso-butane, iso-butene, 2-butene, pentane, pentene, cyclohexene and

cyclohexane, methylcyclopentane and methylcyclopentane, 1-hexene, benzene, and toluene were

observed in the gas phase. The major liquid products were methylcyclohexane, dimethlycyclopentane

(DMCP) isomers, cyclohexene, benzene, and toluene. Mole fractions of 4 major gaseous products,

methane, ethane, propene, and propane, in the presence of FGS19 are higher than those in pure MCH

at higher temperatures. At lower temperatures, only small differences in gaseous product yields were

observed between experiments with and without FGS because of the low conversion rate of MCH.

Moreover, unlike the gas-phase pyrolysis of MCH, methane and ethane are the most abundant in the

products due to slow β-scission and high collision frequencies with H-donor molecules. As discussed

in the literature review section, alkane species formation due to bimolecular stabilization via

abstraction are more favored in supercritical-phase pyrolysis than in gas-phase pyrolysis of MCH.

From the results of higher methane and ethane yields, FGS are likely to help methyl radicals to

collide with H-donor molecules to form methane and ethane. Over the range of temperatures from

780 K to 820 K, major liquid product yields such as DMCP isomers and cyclohexene from thermal

decomposition of MCH with FGS are slightly lower than those in the absence of FGS. For

cyclohexene and DMCP isomers, standard deviation errors on the liquid product yield at these

temperatures are fairly large. Nevertheless, we would stress that FGS might retard decomposition or

isomerization reactions of MCH to cyclohexene and dimethylcyclopentanes. Less formation of

DMCPs in the presence of FGS is likely due to the fact that FGS facilitates ring breaking of DMCPs

80

to produce gaseous products instead of further DMCP formation. Moreover, the toluene yield is much

higher in the presence of FGS than in the absence of FGS over the range of temperature investigated

in the present study, as illustrated in Fig. 4-2 (g). Possible pathways to form toluene and hydrogen are

successive abstractions from the resulting cyclical olefinic structure [12] or aromatization of

methylcyclohexene isomers [196] as shown below:

- 2H- 2H - 2H

It is apparent that the dehydrogenation of MCH to toluene and hydrogen in the presence of

FGS19 is much more favored than without FGS19. Table 4-1 depicts the product distributions from

thermal decomposition of MCH with and without 50 ppmw FGS19 under supercritical conditions at

different temperatures and a constant reduced pressure of 1.36. At temperatures of 800, 820, and 840

K, conversion enhancement rates of MCH are 19.3, 43.5, and 5.6 %, respectively. Enhancement in

mole fractions of methane, ethene, ethane, propene, and propane at 820.0 K and 4.68 MPa, are 38.0,

35.5, 39.2, 35.7, and 38.6 %, respectively. As shown in Table 4-1, most product yields with FGS19 are

higher than those with pure MCH, except for cyclopentene and cyclopentane at 820 K, cyclohexene at

820 K, 1,3-dimethylcyclopentane at 820 K, cyclopentane at 840 K, and 1,1 dimethylcyclopentane at

840 K. In particular, a decrease in mole fraction yields of cyclohexene and 1,3 dimethylcyclopentane

at 820 K at 11.1 and 6.8 %, respectively, is observed. A comparison of the product distributions on

the thermal decomposition of MCH between experiments with or without FGS19 particle showed that

there was a significant effect of FGS19 on the reaction pathway for toluene, cyclohexene, and DMCP

formation mechanisms, while conversion and product yields increased in the presence of FGS19.

81

(a) methane (b) ethane

(c) propene (d) propane

(e) cyclohexene (f) DMCPs

Temperature, K

Mo

leF

rac

tio

n,p

pm

720 740 760 780 800 820 840 860-5.0E+04

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

3.5E+05

4.0E+05

MCH + FGS19

50 ppmw

pure MCH

Temperature, K

Mo

leF

rac

tio

n,p

pm

720 740 760 780 800 820 840 860-5.0E+04

0.0E+00

5.0E+04

1.0E+05

1.5E+05

MCH + FGS19

50 ppmw

pure MCH

Temperature, K

Mo

leF

rac

tio

n,p

pm

720 740 760 780 800 820 840 860

0.0E+00

1.0E+04

2.0E+04

3.0E+04

4.0E+04

5.0E+04

6.0E+04

MCH + FGS19

50 ppmw

pure MCH

Temperature, K

Mo

leF

rac

tio

n,p

pm

720 740 760 780 800 820 840 860

0.0E+00

1.0E+04

2.0E+04

3.0E+04

4.0E+04

5.0E+04

6.0E+04

MCH + FGS19

50 ppmw

pure MCH

Temperature, K

Mo

leF

rac

tio

n,p

pm

720 740 760 780 800 820 840 860-1.0E+04

0.0E+00

1.0E+04

2.0E+04

3.0E+04

4.0E+04

5.0E+04

6.0E+04

7.0E+04

MCH + FGS19

50 ppmw

pure MCH

Temperature, K

Mo

leF

rac

tio

n,p

pm

720 740 760 780 800 820 840 8600.0E+00

2.0E+04

4.0E+04

6.0E+04

8.0E+04

1.0E+05

1.2E+05

MCH + FGS19

50 ppmw

pure MCH

82

(g) toluene

Figure 4-2. Major pyrolysis products of MCH/FGS19 50 ppmw at temperatures ranging from 745 to

840 K, a fixed reduced pressure of 1.36, and a fixed volumetric flow rate of 1.0 mL/min. (a) methane,

(b) ethane, (c) propene, (d) propane, (e) cyclohexene, (f) DMCPs, and (g) toluene.

Temperature, K

Mo

leF

rac

tio

n,p

pm

720 740 760 780 800 820 840 860

0.0E+00

1.0E+04

2.0E+04

3.0E+04

4.0E+04

5.0E+04

6.0E+04

7.0E+04

MCH + FGS19

0.005 wt.%

pure MCH

83

Table 4-1. Molar product yields from thermal decomposition of MCH with and without FGS19 50

ppmw at a fixed reduced pressure of 1.36 and a fixed volumetric flow rate of 1.0 mL/min.

Temperature 800 K 820 K 840 K

Reactant

MCH +

FGS19

0.005

wt.%

Pure

MCH

MCH +

FGS19

0.005

wt.%

Pure

MCH

MCH +

FGS19

0.005

wt.%

Pure

MCH

Conversion (%) 20.00 16.77 64.25 44.78 78.46 74.28

Product yield (mol %)

methane 5.95 5.16 23.37 14.49 28.14 27.85

ethylene 0.48 0.45 2.12 1.37 2.24 2.20

ethane 1.31 1.02 6.15 3.74 8.64 8.68

propylene 0.99 0.83 3.62 2.33 4.01 3.90

propane 0.71 0.56 3.14 1.93 4.55 4.55

iso-butane 0.01 0.00 0.19 0.12 0.53 0.57

iso-butylene 0.35 0.31 1.15 0.74 1.33 0.70

1-butene 0.25 0.21 0.78 0.51 0.90 0.85

1,3-butadiene 0.07 0.07 0.15 0.10 0.13 0.10

n-butane 0.18 0.15 0.79 0.49 1.25 1.24

2-butene 0.08 0.07 0.31 0.20 0.43 0.43

1-pentene 0.05 0.05 0.28 0.10 0.70 0.23

n-pentane 0.04 0.04 0.23 0.13 0.63 0.14

cyclopentene 0.01 0.02 1.35 1.50 2.35 2.08

cyclopentane 0.03 0.03 0.00 0.02 0.00 0.11

1-hexene 0.00 0.00 0.81 0.13 1.04 0.75

methylcyclopentene 0.01 0.00 0.23 0.22 0.30 0.28

benzene 0.00 0.00 1.72 0.17 3.28 3.16

cyclohexene 2.79 3.34 5.01 5.35 3.37 3.23

1,1-DMCP 0.00 0.00 0.00 0.00 0.00 1.71

1,3-DMCP 4.15 4.39 7.43 8.25 8.21 6.70

toluene 2.54 0.08 5.41 2.88 6.43 4.81

84

The effect of FGS19 on major product selectivities was investigated. Figure 4-3 indicates the

variation of major product selectivities with conversion for thermal decomposition of pure MCH and

MCH/FGS19 mixture. For the gaseous products including methane, ethane, propylene, and propane at

low conversion, selectivities are slightly higher in the presence of FGS19 than in the absence of the

particles. At high conversion, there is no significant difference in the selectivities of the major

gaseous products between experiments with or without the addition of FGS19. For DMCP isomers and

cyclohexene, selectivities at low conversion are slightly lower in the presence of FGS than without

the particles. At higher conversion, the selectivities of DMCP and cyclohexene in the presence of

FGS are comparable to those without the particles. On the other hand, selectivity to form toluene and

hydrogen over the temperature range considered in this study is much higher in the presence of FGS19

than without FGS19, as shown in Figure 4-3 (g). Again, it is confirmed that the presence of FGS19 is

likely to facilitate dehydrogenation of MCH to form toluene and hydrogen.

85



(e) DMCPs (f) cyclohexene

Conversion, %

Se

lec

tiv

ity

[mo

les

/MC

Hm

ole

sre

ac

ted

]

0 10 20 30 40 50 60 70 80 900

0.1

0.2

0.3

0.4

0.5

MCH+FGS19

50 ppmw

pure MCH

Conversion, %

Se

lec

tiv

ity

[mo

les

/MC

Hm

ole

sre

ac

ted

]

0 10 20 30 40 50 60 70 80 900

0.03

0.06

0.09

0.12

0.15

MCH+FGS19

50 ppmw

pure MCH

Conversion, %

Se

lec

tiv

ity

[mo

les

/MC

Hm

ole

sre

ac

ted

]

0 10 20 30 40 50 60 70 80 900

0.02

0.04

0.06

0.08

0.1

MCH+FGS19

50 ppmw

pure MCH

Conversion, %

Se

lec

tiv

ity

[mo

les

/MC

Hm

ole

sre

ac

ted

]

0 10 20 30 40 50 60 70 80 900

0.02

0.04

0.06

0.08

0.1

MCH+FGS19

50 ppmw

pure MCH

Conversion, %

Se

lec

tiv

ity

[mo

les

/MC

Hm

ole

sre

ac

ted

]

0 10 20 30 40 50 60 70 80 900

0.1

0.2

0.3

0.4

0.5

0.6

MCH+FGS19

50 ppmw

pure MCH

Conversion, %

Se

lec

tiv

ity

[mo

les

/MC

Hm

ole

sre

ac

ted

]

0 10 20 30 40 50 60 70 80 900

0.05

0.1

0.15

0.2

0.25

0.3

MCH+FGS19

50 ppmw

pure MCH

86

(g) toluene

Figure 4-3. Comparison of major product selectivities in the presence of FGS19 50 ppmw with those

for pure MCH pyrolysis at different conversions with temperatures varying from 745 to 840 K. (a)

methane, (b) ethane, (c) propene, (d) propane, (e) cyclohexene, (f) DMCPs, and (g) toluene.

MCH pyrolysis may be approximated by a first-order relationship as follows [12], [37]:

4-1

4-2

4-3

4-4

where C is the reactant concentration (ppm), t is the reaction time (sec), and k is the first-order rate

constant (sec-1

). The conversion rate of X (the conversion of MCH) can be defined:

4-5

Conversion, %

Se

lec

tiv

ity

[mo

les

/MC

Hm

ole

sre

ac

ted

]

0 10 20 30 40 50 60 70 80 90-0.05

0

0.05

0.1

0.15

0.2

toluene_MCH+FGS19

50 ppmw

toluene_pure MCH

87

Using simple first-order kinetics for thermal decomposition of the fuels, the overall rates can

be determined in the following:

4-6

where X is the conversion of MCH and t is the residence time (sec) based on the density of the

unreacted pure MCH. The apparent activation energies (Ea, kJ/mol) and pre-exponential factors (A,

sec-1

) can be determined using the following Arrhenius expression:

4-7

Figure 4-4 demonstrates the Arrhenius plots for MCH decomposition with and without 50

ppmw FGS19. Table 4-2 shows the apparent activation energies and pre-exponential factors

determined from Arrhenius plots. Kralikova et al. [197] have reported the apparent activation

energies ranging from 201.25 to 339.32 kJ/mole and pre-exponential factors ranging from 1012

to 1018

sec-1

. Based on this investigation, the apparent activation energy of supercritical pyrolysis of pure

MCH was determined to be 302.44 kJ/mol with the pre-exponential of 1017.26

sec-1

. The activation

energy for thermal decomposition of pure MCH obtained in the current study falls within the range of

values reported by Kralikova et al. [197]. Stewart [12] also found an activation energy of 278.26

kJ/mole with a pre-exponential factor of 1015.4

. The 8 % difference in the activation energies between

this study and Stewart’s can be attributed to slight differences in experimental conditions. The

reduced pressure in the current research was about 1.36, which is slightly higher than Stewarts’

pressure of 1.31. For the residence time, Stewarts’ values [12] ranged from 48 to 72 sec over the

temperature of 720 K – 820 K, whereas the values in this study changed from 48.6 to 58.6 sec over

88

the temperature range of 745 to 840 K. Nevertheless, it should be emphasized that the addition of 50

ppmw FGS19 decreased the activation energy to 273.33 kJ/mol with the pre-exponential of 1015.56

sec-1

.

This decrease in the activation energy and frequency factor can be due to the surface activity of the

FGS19. The defects on both surfaces of FGS may act as anchoring sites for radicals and hence lower

the activation energy. Functional groups on the surface may provide active sites to accelerate

intermediate reactions during the supercritical thermal decomposition [107]. Furthermore, FGS

additives may behave like nanoparticles in nanofluids that can enhance heat transfer properties [5],

although this is not expected to be important in the present experiments. It should be noted that,

although the overall activation energy has been accounted for in this study, there still seems to be two

different regimes on the activation energy, as discussed above. Even in Stewarts’ result on global

activation energy, two different activation energies over 720 – 820 K appear to exist. By comparing

activation energies between low and high temperature regimes for this study, a higher activity of FGS

was observed at the high temperature regime than at the low temperature regime.

89

Figure 4-4. Arrhenius plots for thermal decomposition of MCH with FGS19 additive under

supercritical conditions at temperatures ranging from 745 to 840 K, a fixed reduced pressure of 1.36,

and a fixed volumetric flow rate of 1.0 mL/min.

Table 4-2. Kinetic parameters for thermal decomposition of MCH with and without FGS19 additive.

Reactant Ea

(kJ/mol)

log A

(sec-1

) R

2

MCH + FGS19 50 ppmw 273.33 15.56 0.97

MCH 302.44 17.26 0.97

The effect of residence time variation at constant temperature and pressure was examined by

changing volume flow rate from 0.9 to 2.0 ml/min. Figures 4-5 and 4-6 represent mole fractions of

major species and MCH conversion with respect to residence time ranging from 26.0 sec to 59.0 sec

at a fixed temperature of 800 K and reduced pressure of 1.36. As shown in Figs. 4-5 (a) through 4-5

(d), mole fractions of major gaseous products and MCH conversion change linearly with residence

1000/T, K-1

ln(k

),s

ec

-1

1.2 1.25 1.3 1.35-10

-8

-6

-4

-2

MCH + FGS19

50 ppmw

pure MCH

90

time, at least within the uncertainties of the data. Stewart [12] found that mole fractions of major

gaseous products from supercritical pyrolysis of pure MCH linearly increase with increasing

residence time, supporting this finding of the current work. In Fig. 4-5 (e), when FGS are present in

MCH, toluene yields increase gradually with residence time similar to the gaseous products. However,

for pure MCH pyrolysis, the mole fraction of toluene is nearly negligible for residence times below

55 sec. It is apparent that FGS facilitate toluene formation under supercritical-phase pyrolysis of

MCH. As shown in Fig. 4-5 (f), however, FGS are likely to retard 1,3 DMCP formation. Compared to

pure MCH pyrolysis, the ratio of MCH conversion is larger (Fig. 4-6), supporting the finding that

FGS accelerate the MCH decomposition. For example, at a given conversion of 25.0 %, residence

times could be estimated at 51.5 sec by the addition of 50 ppmw FGS, whereas a residence of 64.6

sec was estimated in the absence of FGS, based on the linear regressions for both cases.


Residence Time, sec

Mo

leF

rac

tio

n,p

pm

20 30 40 50 60 700.0E+00

2.5E+04

5.0E+04

7.5E+04

1.0E+05

1.3E+05

1.5E+05

MCH + FGS19

50 ppmw

pure MCH

Residence Time, sec

Mo

leF

rac

tio

n,p

pm

20 30 40 50 60 700.0E+00

5.0E+03

1.0E+04

1.5E+04

2.0E+04

2.5E+04

3.0E+04

3.5E+04

MCH + FGS19

50 ppmw

pure MCH

91


(e) toluene (f) DMCPs

Figure 4-5. Effects of FGS19 on the measured mole fractions of the major products with respect to

residence time (800 K and 4.72 MPa). (a) methane, (b) ethane, (c) propene, (d) propane, (e) toluene,

and (f) DMCPs.

Residence Time, sec

Mo

leF

rac

tio

n,p

pm

20 30 40 50 60 700.0E+00

5.0E+03

1.0E+04

1.5E+04

2.0E+04

2.5E+04

MCH + FGS19

50 ppmw

pure MCH

Residence Time, sec

Mo

leF

rac

tio

n,p

pm

20 30 40 50 60 700.0E+00

5.0E+03

1.0E+04

1.5E+04

2.0E+04

MCH + FGS19

50 ppmw

pure MCH

Residence Time, sec

Mo

leF

rac

tio

n,p

pm

20 30 40 50 60 70

0.0E+00

1.0E+04

2.0E+04

3.0E+04

4.0E+04

5.0E+04

MCH + FGS19

50 ppmw

pure MCH

Residence Time, sec

Mo

leF

rac

tio

n,p

pm

20 30 40 50 60 700.0E+00

2.0E+04

4.0E+04

6.0E+04

8.0E+04

1.0E+05

MCH + FGS19

50 ppmw

pure MCH

92

Figure 4-6. Effects of FGS19 on the conversion of MCH with respect to residence time (800 K and

4.72 MPa).

The present study carried out supercritical pyrolysis experiments to examine the influence of

FGS19 concentration on the conversion of MCH. Figure 4-7 illustrates the conversion of MCH for two

different concentrations of FGS19 (50 and 100.0 ppmw). For supercritical conditions in these

experiments, temperature and reduced pressure were set to 800 K and 1.36, respectively. It was found

that the conversion of MCH increases from 16.77 % to 22.51 % with the addition of 100 ppmw FGS19.

This result shows that higher surface areas may result in the enhanced conversion rates of MCH.

Residence Time, sec

Co

nv

ers

ion

of

MC

H,%

20 30 40 50 60 700.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

MCH + FGS19

50 ppmw

pure MCH

93

Figure 4-7. Conversion of MCH for two different loading concentrations of FGS at 800 K, 4.72 MPa

and flow rate of 1.0 mL/min.

This study also examined the possibilities of metal catalyst candidates such as platinum and

polyoxometalates supported by FGS as additives to liquid fuels and compared the effects of these

particles on the thermal decomposition of MCH. Pt@FGS and POM@FGS in the following figures

stand for Pt-decorated FGS and polyoxometalates-decorated FGS, respectively. In order to improve

the dispersibility of FGS in non-polar organic solvents such as toluene, a grafting technique to attach

eicosyl (C20) chains onto the surfaces of FGS was used in this study. FGS with a carbon to oxygen

ratio of 16 to 19 were grafted with the chains. FGS-C20 represents grafted FGS with eicosyl chains,

respectively. For example, POM@FGS-C20 designates decyl-grafted FGS decorated with

polyoxometalates.

Compared to the baseline, metal-decorated FGS shows enhanced MCH decomposition and

gaseous product formations, like MCH/FGS pyrolysis. Figure 4-8 shows MCH conversion with the

addition of various particles at a constant temperature of 800 K, pressure of 4.72 MPa, and 50 ppmw

additive. Adding 10 ppmw Pt to the MCH using a Pt@FGS powder containing 20% by weight

94

platinum further increased the conversion rate of MCH from 20.6 % to 24.1%, suggesting that even at

the low loadings of this study, the well-dispersed Pt nanoparticles can assist in catalyzing supercritical

MCH pyrolysis. Interestingly, POM@FGS gives higher enhancement of MCH decomposition

(25.6 %) compared to Pt-FGS. In the presence of the particles, the conversion rates increased

compared to those of the pure MCH alone, with the POM@FGS-C20 particles having the greatest

enhancements by approximately 33 % relative to the pure fuel baseline.

Figure 4-8. Comparison of conversion of MCH containing various additives at 800 K and 4.72 MPa.

For the supercritical pyrolysis of MCH in the presence of different particles, the products are

C1-C5 n-alkanes, C2-C6 1-alkenes, dimethylcyclopentane (DMCP) isomers, C5-C6 cycloalkanes and

cycloalkenes, methylcyclopentane, methylcyclopentene, 2-butene, benzene, and toluene. Table 4-3

shows mole fractions of the product species from decomposition of MCH with different particles at

the loading concentration of 50 ppmw under supercritical conditions at 800 K, 4.72 MPa, and flow

rate of 1.0 mL/min. It is noted that hydrogen measurements were obtained for the gaseous products as

well. Compared to pure MCH and MCH/FGS pyrolysis, mole fractions of gaseous products such as

95

hydrogen, C1-C4 alkanes, and C2-C4 alkenes are observed to be higher in the presence of Pt-FGS and

POM@FGS-C20. In addition, the presence of both Pt@FGS and POM@FGS have a significant

impact on reaction mechanism for toluene formation.

96

Table 4-3. Molar product yields from the decomposition of MCH with different particles at 50 ppmw

loading concentration, 800 K, 4.72 MPa, and flow rate of 1.0 mL/min.

Particles No Particle FGS19 Pt@FGS POM@FGS-

C20

Conversion (%) 16.9 20.6 24.5 25.6

Product yield (mol %)

hydrogen 1.22 2.77 3.67 3.12

methane 4.03 4.21 4.98 4.94

ethylene 0.33 0.34 0.46 0.46

ethane 0.83 0.94 1.09 1.15

propylene 0.68 0.69 0.95 0.95

propane 0.48 0.52 0.67 0.73

iso-butane 0.00 0.01 0.00 0.00

iso-butylene 0.28 0.26 0.43 0.40

1-butene 0.19 0.18 0.31 0.29

1,3-butadiene 0.06 0.05 0.09 0.07

n-butane 0.14 0.14 0.24 0.25

2-butene 0.07 0.06 0.12 0.11

1-pentene 0.05 0.04 0.09 0.07

n-pentane 0.04 0.03 0.07 0.06

cyclopentene 0.04 0.02 0.14 0.46

cyclopentane 0.02 0.03 0.05 0.04

1-hexene 0.01 0.01 0.00 0.00

methylcyclopentene 0.00 0.00 0.29 0.05

benzene 0.01 0.01 0.00 0.24

cyclohexene 2.97 2.98 2.06 2.69

1,1-DMCP 0.02 0.00 1.03 0.96

1,3-DMCP 5.40 4.76 4.33 4.77

toluene 0.08 2.60 3.38 3.75

97

Figure 4-9 shows the selectivities of methane, cyclohexene, 1,3-dimethylcycopentane and

toluene formation with different particles. When the particles are present in MCH, the selectivities of

methane, cyclohexene, and 1,3-dimethylcycopentane decreased compared to those of pure MCH

decomposition. On the other hand, toluene selectivity with the addition of the particles was much

higher than that of pure MCH decomposition. In Fig. 4-10, Scheme 1 and 2 depict the major reaction

mechanisms proposed by Stewart [12] and Lai and Song [37] for pure MCH decomposition under

supercritical conditions. It was suggested that the isomerization of MCH to dimethylcyclopentane

(Scheme 1) is favored under supercritical conditions. In addition, cyclohexene can be formed from β-

scission of the methylcyclohexyl radical (Scheme 2). Scheme 3 shows the dehydrogenation of MCH

to form toluene. For pure MCH decomposition, toluene formation is nearly negligible. Whereas

toluene was the most abundant aromatic formed in the present study. The selectivity of toluene was

negligible in the previous works [12], [37]. However, the particles significantly increase toluene

formation, as shown in Fig. 4-9. It is clear that the dehydrogenation of MCH to toluene (scheme 3) in

the presence of the particles is more favored than the pathways in Schemes 1 and 2. This result can

also show that metal catalysts supported by FGS can be used for the dehydrogenation of MCH to

form hydrogen and toluene, resulting in enhanced endothermic heat sink capacity. For the hydrogen

measurements, it can be confirmed that the addition of the particles used in this study produce more

hydrogen, which is a highly reactive fuel for combustion. Furthermore, the drawback of MCH

dehydrogenation with the solid catalysts mentioned above can be solved by the use of the particles

supported by FGS because FGS has a higher thermal conductivity than that of silica or alumina [3].

98

Figure 4-9. Comparison of selectivities of the major products with the different particles at 800 K,

4.72 MPa, and flow rate of 1mL/min.

+ CH3 +

Scheme 1- H

3H2+

- H

+ H

+ H

CH4 Scheme 2

Scheme 3

Figure 4-10. Major reaction mechanisms for pure MCH decomposition under supercritical conditions

[12], [37].

99

4.2 Decomposition of n-C12H26/FGS-Based Particles

Figure 4-11 illustrates the effects of FGS and Pt-FGS additives on n-C12H26 conversion under

supercritical pyrolysis at three different temperatures (480, 500, and 530 oC), under a constant

pressure of 4.75 MPa, and using a constant flow rate of 5.0 mL/min. In the mixtures, the loading

concentrations of the two particles were set to 50 ppmw, regardless of the platinum content, numbers

of defect sites or functional groups. The C/O ratio of the FGS was fixed at 100 for all tests. In order to

provide a baseline comparison, supercritical-phase thermal decomposition experiments of pure n-

C12H26 were performed at a similar temperature and pressure with the same flow rate. It should be

mentioned that due to the composition of products, the residence times were slightly shorter for the n-

C12H26/particle mixture than for the pure n-C12H26. Compared to the baseline conversion, FGS-

containing n-C12H26 at 480 and 500 oC showed minimal, if any, conversion enhancement. On the other

hand, the inclusion of 10 ppmw Pt in the n-C12H26 (requiring the addition of 50 ppmw Pt-FGS to the

n-C12H26) clearly showed conversion enhancement over the entire temperature range, with different

degrees of enhancement. For example, as temperature increased from 480 to 530 oC, the accelerating

factor (AF), which is defined as the ratio of fuel conversion with Pt-FGS to that without Pt-FGS,

changed from about 1.20 to 1.33.

100

Figure 4-11. Effect of FGS and Pt-FGS on the n-C12H26 conversion rate at three different

temperatures and at a fixed pressure of 4.75 MPa.

Although not as significant when compared to the combustion of NM containing high

concentrations of FGS (750 – 5,000 ppmw as reported by Sabourin et al. [5]), the addition of 50

ppmw Pt-FGS increased reaction rates of n-C12H26 thermal cracking in the absence of oxygen under

supercritical conditions. The mechanism for enhancing n-C12H26 pyrolysis using Pt-FGS additives is

unknown. However, the previously described model [111] for the combustion of NM in the presence

of FGS provides clues to the catalytic activity of Pt-FGS on the decomposition of n-C12H26,

suggesting that the exchange of protons or oxygens between the oxygen-containing functional groups

on FGS and n-C12H26 and its derivatives catalyzes decomposition. In particular, catalytic activity of

the functionalized surfaces on FGS or Pt-FGS can accelerate the initiation of fuel decomposition.

While thermal decomposition of n-alkanes is mainly initiated by homolytic carbon-carbon bond

cleavage [25], [123], [128], [130], [198], the inclusion of the particles may alter the initiation

mechanism via catalytic dehydrogenation, protonation, and deprotonation processes that accompany

0

10

20

30

40

50

60

70

80

480 °C 500 °C 530 °C

n-C

12H

26 C

onver

sion, %

no additive FGS Pt-FGS

101

the thermal homolysis reactions as shown in Fig. 4-12. These catalytic initiation reactions can

produce highly reactive radicals such as dehydrogenated radicals, alkanium ions, or carbenium ions.

Similar effects on the initiation mechanism were found to occur during the decomposition of n-

alkanes on solid acids such as zeolite HZSM-5 catalysts [199]. Moreover, Castro-Marcano and van

Duin [58] recently studied the catalytic enhancement mechanisms of fuel decomposition over

amorphous silica, hydrated amorphous silica, and amorphous aluminosilicate nanoparticles using MD

simulations. They found that the addition of the nanoparticles to 1-heptene accelerated the initiation

of fuel decomposition via catalytic protonation and dehydrogenation processes combined with

homolysis reactions.

Enhancing thermal properties such as heat capacity and thermal conductivity by introducing

nanoparticles into liquid fuels is unlikely, as such small mass loading levels are unlikely to

appreciably change the thermal properties [200]. This is also because as the reaction time in this study

is relatively short, on the order of several seconds, so the conversion process is more likely to be

governed by kinetics and not thermo-chemical properties [144].

Figure 4-12. Possible initiation mechanisms of n-C12H26 decomposition with FGS or Pt-FGS.

n-C12H26

C12H27 C12H25 C12H25 + H1 1

i jR R

Dep

roto

natio

n

Deh

yd

rog

enatio

n

Pro

ton

ation

C-C

bon

d cleav

age

(therm

al)

102

Arrhenius plots for pure n-C12H26 and n-C12H26 mixtures containing FGS and Pt-FGS are

reported in Fig. 4-13, calculated using first-order global kinetics. From a linear-least squares fit of the

data illustrated in Fig. 4-13, the activation energies (Ea) and pre-exponential factors (A) were

determined. Table 4-4 shows the kinetic parameters for decomposition of n-C12H26 with different

particles. Relative to pure n-C12H26 pyrolysis, the apparent activation energy decreased from 278.6 to

266.0 kJ/mol, implying that the small amount (50 ppmw) of FGS itself accelerates n-C12H26 pyrolysis.

Adding 10 ppmw Pt to the n-C12H26 using a Pt-FGS powder containing 20% by weight platinum

further decreases the activation energy to 239.8 kJ/mol, suggesting that even at the low loadings of

this study, the well-dispersed Pt nanoparticles can assist in catalyzing supercritical n-C12H26 pyrolysis.

The decrease in global activation energies is thought to result from formation of radicals during the

initiation phase of n-C12H26 pyrolysis altered by the addition of Pt-FGS, which would increase

participation of low activation energy radical abstraction or decomposition reactions (below ~170

kJ/mol) and decrease participation of high activation energy homolytic cleavages (typically ~350

kJ/mol). The combination of enhancement mechanisms in the initial stage would tend to reduce the

apparent activation energies.

103

Figure 4-13. Arrhenius plots and global kinetic parameters for pure n-C12H26 and n-C12H26 mixtures

containing FGS and Pt-FGS.

Table 4-4. Kinetic parameters for decomposition of n-C12H26 with various particles.

Additives Ea

(kJ/mol)

log A

(sec-1

) R

2

Pt-FGS 50 ppmw 239.8 15.1 0.99

FGS 50 ppmw 266.0 16.2 0.99

No particle 278.6 17.0 0.99

1000/T, K-1

lnk

,s

ec

-1

1.23 1.26 1.29 1.32 1.35-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

Pt-FGS

FGS

no additive

104

For the supercritical pyrolysis of both pure n-C12H26, n-C12H26/FGS 50 ppmw mixtures, and

n-C12H26/Pt-FGS 50 ppmw mixtures, the gaseous and liquid products identified and quantified in this

study are mainly hydrogen, C1 to C11 n-alkanes, C2 to C12 1-alkenes, and C4 to C12 2, 3, 4-alkenes; this

is in good agreement with previous studies of pure n-C12H26 [27], [60], [144], [201], [202]. Figure 4-

14 shows the major product distributions of n-C12H26 decomposition with different particles at three

different temperatures and at a fixed pressure of 4.72 MPa. At all three temperatures considered, mole

fractions of hydrogen and C1-C7 hydrocarbons are found to be higher in the presence of the particles

than the pure fuel baseline. The trends at 480 oC and 500

oC are very similar, showing that the

particles accelerate decomposition of n-C12H26 forming all of the products. At 530 oC, C8-C12

hydrocarbons seem to be lower in the presence of the particles with higher mole fractions of lower

carbon-number compounds. This result suggests that the particles are likely to affect not only the

initiation reactions but also the secondary reactions such as bimolecular reactions and β-scission. The

product speciation observed from the thermal decomposition of n-C12H26 with or without the additives

was nearly the same, although significant changes were found in the relative concentrations,

particularly hydrogen and low-carbon-number products. In addition, increasing temperature and

conversion yielded more C4 to C12 branched alkanes, cycloalkanes, and cycloalkenes in n-C12H26/Pt-

FGS mixtures than in pure n-C12H26. It should also be mentioned that measurements for oxygenates,

and in particular CO and CO2, were made using GC/TCD and GC/FID coupled with a methanation

catalyst having a minimum detectable limit of approximately 1 ppm for CO and CO2. Oxygenated

species were not detected in any of the experiments. Furthermore, the maximum oxygen content that

would be added from the 50 ppmw FGS100 additives was estimated to be approximately 0.5 ppmw.

With this level of addition, the oxygenated species would be below the detection limit for GC analysis,

would likely form stable products in the reducing environment without continuing the chain, and

therefore cannot account for the amount of conversion observed.

105

(a) 480 oC

(b) 500 oC

106

(c) 530 oC

Figure 4-14. Product distributions measured for decomposition of pure n-C12H26 and n-C12H26

mixtures containing FGS and Pt-FGS at (a) 480 oC, (b) 500

oC, and (c) 530

oC at the fixed pressure of

4.72 MPa.

With the addition of the particles, the product yields were observed to be higher at different

temperatures, particularly for the low molecular weight species. In regards to mole fractions of C1-C4

alkanes and C2-C4 alkenes with the addition of 50 ppmw Pt-FGS, we found percent increases in the

C1-C4 alkanes/C2-C4 alkenes of 47.1/42.3 % at 480 oC, 23.8/24.8 % at 500

oC, and 30.7/35.2 % at 530

oC, as shown in Fig. 4-15. Results presented in Fig. 4-16 compare the selectivities (i.e., the percent

yields of products) for hydrogen, C1-C4 alkanes, C2-C4 alkenes, C5-C11 alkanes and C5-C11 alkenes

from supercritical pyrolysis of n-C12H26 with or without Pt-FGS. The inclusion of Pt-FGS gave higher

selectivities for the low molecular weight species such as hydrogen, C1-C4 alkanes, and C2-C4 alkenes,

which could lead to higher endothermicity and burning rates during combustion [29], [169].

Moreover, lower selectivities were found for the high molecular weight species, such as C5-C11

alkanes and C5-C12 alkenes, corresponding to a low heat sink capacity [6], [29]. This result indicates

that Pt-FGS can serve as multifunctional additives in endothermic fuels, providing higher

endothermicity, reaction rates, and energy density over the pure fuels. Interestingly, the hydrogen

107

yield was observed to increase from 200 ppm to 2516 ppm (a factor of ten) at 480 oC. Even at 500

and 530 oC, the selectivity for hydrogen increases by a factor of approximately 2.5.

(a) C1-C4 alkanes

(b) C2-C4 alkenes

Figure 4-15. Comparison of mole fractions of (a) C1-C4 alkanes and (b) C2-C4 alkenes for different

temperatures at a fixed pressure of 4.72 MPa.

Temperature,oC

Mo

leF

rac

tio

n,%

480 500 520 540

5

10

15

20

25

30C

1-C

4alkanes_w/ Pt-FGS

C1-C

4alkanes_w/ FGS

C1-C

4alkanes_no additive

Temperature,oC

Mo

leF

rac

tio

n,%

480 500 520 540

3

6

9

12

15

18C

2-C

4alkenes_w/ Pt-FGS

C2-C

4alkenes_w/ FGS

C2-C

4alkenes_no additive

108

(a) hydrogen

(b) C1-C4 alkanes and C2-C4 alkenes

Figure 4-16. Selectivities for (a) hydrogen and (b) C1-C4 alkanes and C2-C4 alkenes from n-C12H26

pyrolysis with or without Pt-FGS.

Temperature,oC

Se

lec

tiv

ity

,%

480 500 520 5400

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Pt-FGS

FGS

no additive

Temperature,oC

Se

lec

tiv

ity

,%

470 480 490 500 510 520 530 54010

15

20

25

30

35

40

45

50

C1-C

4alkanes_w/ Pt-FGS

C1-C

4alkanes_w/ FGS

C1-C


C2-C

4alkenes_w/ Pt-FGS

C2-C

4alkenes_w/ FGS

C2-C


109

A comparison with the previous observations of Bao et al. [6] helps to illuminate the effect of

Pt-FGS on the supercritical pyrolysis of n-C12H26. Bao et al. [6] showed that the conversion rate of n-

C12H26 increased by a factor of 4 during the first 34 min pyrolysis by the addition of 1000 ppmw

HZSM-5 which is much higher than the 50 ppmw for Pt-FGS and longer than the reaction times of 9

to 20 sec used in this study. This result suggests that the influence of Pt-FGS on the pyrolysis rate

may be increased by adding more Pt-FGS particles into n-C12H26. In the present work, the

concentration of Pt-FGS was limited to less than 100 ppmw due to higher sedimentation and

agglomeration rates of Pt-FGS at higher concentrations in the n-C12H26. It is important to mention that

the dispersibility of graphene nanocomposite in non-polar hydrocarbon fuels can be significantly

improved using a grafting technique to attach alkyl chains onto the surface of FGS [75], [120], [121].

The Pt-FGS used in this study was not modified by grafting or the use of surfactant, but the particles

at the concentration of 50 ppmw level were qualitatively observed to remain suspended in n-C12H26

for a few hours following sonication.

Similar results of high hydrogen yields were reported by Zhao et al. [201] using Pd/HZSM-5

coating catalysts and by Bao et al. [6] using dispersed nano-HZSM-5 catalysts. Notably, Bao et al. [6]

showed increased production of hydrogen and low molecular weight species, whereas the production

of high molecular weight species using suspended nano-catalysts were found to be lower, compared

to those produced during the decomposition of pure n-C12H26. Zhao et al. [201] found Pd/HZSM-5

coating gave higher conversion and hydrogen yield compared to the HZSM-5 coating or pure n-

C12H26, showing that the incorporation of Pd facilitates dehydrogenation reactions of n-C12H26. Since

Pd and Pt are platinum group metals and they have similar physical and chemical properties, the

addition of Pt on FGS, which is a counterpart of HZSM-5 as support, is also likely to enhance

dehydrogenation reactions, resulting in higher conversion and hydrogen. It is clear from our similar

studies with MCH that the presence of Pt-FGS enhanced dehydrogenation, and increased the yields of

110

hydrogen and toluene during MCH pyrolysis, consistent with other studies using Pt catalysts to

catalyze dehydrogenation of MCH and form increased yields of toluene and hydrogen [13].

In this investigation, experiments were performed at flow rates varying from 2.5 to 8.0

mL/min, corresponding to residence times ranging from 9.1 to 19.6 sec. Figure 4-17 represents the

comparison of n-C12H26 conversion and AF at 530 oC, 4.75 MPa, and 50 ppmw of Pt-FGS. Over the

entire residence time range tested, the inclusion of Pt-FGS gave higher conversions than the pure n-

C12H26 baseline, and thus showed the accelerating effect on reaction rates even at different residence

times. For simplicity, the AF in Fig. 4-17 was calculated at the same flow rate. As the residence time

increases, AF decreases from about 1.26 to 1.05 with the maximum AF of 1.26.

Figure 4-17. Effect of Pt-FGS on n-C12H26 conversion as a function of time at 530 oC and 4.75 MPa.

Residence Time, sec

n-C

12H

26

Co

nv

ers

ion

,%

AF

8 10 12 14 16 18 200

20

40

60

80

100

0.8

1

1.2

1.4

1.6

1.8

2

Pt-FGS

no additive

AF

111

In Fig. 4-18, the comparisons of selectivities for hydrogen, C1 to C11 alkanes, C2 to C12

alkenes, and branched and cyclic structures at 530 oC, 4.75 MPa, and 50 ppmw of Pt-FGS as a

function of residence time are described. Compared to the results with varying temperature at the

given flow rate, Fig. 4-18 shows remarkable differences in selectivities with and without Pt-FGS.

Interestingly, the selectivities for hydrogen and C2-C4 alkenes are higher with Pt-FGS at shorter

residence times than without Pt-FGS. With increasing residence time, the selectivities for hydrogen

and C2-C4 alkenes in the presence of Pt-FGS tended to decrease after about 12 sec. This result implies

that polymerization to form branched or cyclic structures was favored in the presence of Pt-FGS, and

thus gave higher selectivities for those structures, as illustrated in Fig. 4-18 (c). On the other hand, the

selectivities for C1-C4 alkanes with and without Pt-FGS increased in a nearly parallel manner with

residence time.

(a) H2 and C2-C4 alkenes, and C5-C12 alkenes

Residence Time, sec

Se

lec

tiv

ity

,%

8 10 12 14 16 18 2015

20

25

30

35

H2

and C2-C

4alkenes_w/ Pt-FGS

H2

and C2-C


C5-C

12alkenes_w/ Pt-FGS

C5-C


112

(b) C1-C4 alkanes and C5-C11 alkanes

(c) branched and cyclic structures

Figure 4-18. Effect of Pt-FGS on the selectivities for (a) hydrogen, C2-C4 alkenes, and C5-C12 alkenes,

(b) C1-C4 alkanes and C5-C11 alkanes, and (c) branched and cyclic structures as a function of time at

530 oC and 4.75 MPa.

Residence Time, sec

Se

lec

tiv

ity

.%

8 10 12 14 16 18 200

10

20

30

40

50

C1-C

4alkanes_w/ Pt-FGS

C1-C


C5-C

11alkanes_w/ Pt-FGS

C5-C


Residence Time, sec

Se

lec

tiv

ity

,%

8 10 12 14 16 18 200.4

0.8

1.2

1.6

2

2.4

branched and cyclic structures_w/ Pt-FGS

branched and cyclic structures_no additive

113

Figure 4-19 shows a comparison of transmission electron micrograph (TEM) images of the

Pt-FGS before and after supercritical pyrolysis at 530 oC, 4.75 MPa and 5.0 mL/min. The TEM

images indicate that the Pt-FGS structure did not change, and in particular, the Pt nanoparticles

attached to the FGS remained stable through the course of the reaction. Their survival also indicates

the potential to further use the Pt-FGS in the gas-phase following injection of the fuel into a

combustor.

Figure 4-19. TEM images of colloidal Pt-FGS nanoparticles (a) before and (b) after supercritical

pyrolysis.

114

The reaction mechanism under supercritical conditions differs significantly from the gas

phase reaction at high temperature and low pressure. For the gas phase reaction, the unimolecular

reactions are dominant and successive decomposition by β-scission produce abundant low molecular

weight alkenes such as ethene and propene [12], [27], [144]. This gas phase reaction is, however, in

striking contrast to the supercritical phase reaction, which is dominated by the bimolecular reactions

between alkyl radicals via hydrogen abstraction [12], [27], [144]. The concentration of alkyl radicals

is so high under supercritical conditions at extremely high densities that forming alkanes such as

ethane and propane is favored unlike in the gas phase reaction. In this investigation, the inclusion of

Pt-FGS was found to promote both unimolecular and bimolecular reactions, supported by the

formation of higher amounts of C1-C4 alkanes and C2-C4 alkenes, compared to amounts formed by the

pyrolysis of pure n-C12H26. As discussed previously, Pt-FGS appears to accelerate the overall

decomposition during initiation via production of a larger radical pool resulting in reactions with

lower activation energy. Bimolecular reactions involving secondary products are also likely to be

promoted due to higher concentrations of radicals, combined with the high collision frequency under

supercritical conditions. In addition, lower C5-C11 alkanes and C5-C12 alkenes were found to form in

the presence of Pt-FGS, indicating that the particles are likely to catalyze the decomposition of high-

molecular weight products via secondary reactions [27], [60]. This change in selectivity indicates that

the particles may affect the secondary reactions more than the initial decomposition reaction of n-

C12H26. Based on this analysis, it can be concluded that the effects of Pt-FGS result in the formation

of hydrogen, C1-C4 alkanes and C2-C4 alkenes, which are more likely to enhance endothermicity and

combustion characteristics, such as shorter ignition delays and higher reaction rates.

115

4.3 Summary of Decomposition Studies

This work investigated the effects of colloidal FGS-based additives on the decomposition of

MCH and n-C12H26 under supercritical conditions in a high pressure and temperature flow reactor.

The conversion rates and major product yields from MCH decomposition in the presence of the

particles were compared with those of pure MCH. The addition of FGS to MCH enhanced conversion

rates and increased major gaseous products yields at higher temperatures. It was also found that FGS

significantly changes the potential reaction mechanisms for toluene, cyclohexene, DMCPs, and

hydrogen formations, while the effectiveness of mechanism was altered. For example, selectivity to

form toluene and hydrogen was increased to 13.0 % in the presence of FGS over the temperature

range tested from 0 % in the absence of the particles. According to first-order reaction kinetics, the

apparent activation energy of pure MCH was 302 kJ/mol, which decreased to 273 kJ/mol by adding

FGS19 50 ppmw. This study also tested the possibilities of FGS as a support for metal particles,

showing the addition of Pt and POM gives further enhancement of the conversion rates of MCH.

For n-C12H26 in the presence of FGS and Pt-FGS, the conversion rates, product yields and

selectivities from the supercritical pyrolysis of n-C12H26 in the presence of Pt-FGS were compared

with those of pure n-C12H26. The inclusion of only 50 ppmw Pt-FGS (equivalent to adding 10 ppmw

Pt) in the n-C12H26 enhanced conversion rates and increased specific product yields, preferentially

selecting for the production of low molecular weight species while diminishing the production of high

molecular weight species. In particular, hydrogen production yield and selectivity were observed to

increase by nearly a factor of 13 (200 ppm to 2516 ppm) at 480 oC and 4.75 MPa when 50 ppmw Pt-

FGS was present in n-C12H26. Selectivities in product yields with and without the graphene additive

also varied as a function of temperature and time, showing that the activity of the particles on the

product distribution could be controlled by the operational conditions. The results of this investigation

support using colloidal hierarchical nanocomposites to enhance for future advanced propulsion and

energy conversion devices.

116

Chapter 5

Supercritical Injection and Combustion Experiments

5.1 Injection Experiments

Cold flow injections of methylcyclohexane (MCH) into a nitrogen (N2) environment at room

temperature and elevated pressure were performed to examine the effects of particle addition on the

liquid fuel jets, using flow rates of 20 and 50 mL/min. The fuel was injected into the windowed

chamber at ambient temperature, using high-speed cinematography to observe and record the

injection process. Typically, 100 ppmw of Pt-decorated functionalized graphene sheets containing 20

weight % Pt (20wt%Pt@FGS) was suspended in the fuel using the process described in Chapter 3.

The diameter and L/De ratio of the injectors were chosen as 305 µm and 500, respectively. Figure 5-1

illustrates the effect of Pt@FGS addition to the laminar jet of MCH. Pure MCH (baseline fuel) is

shown in the figure as well. In both cases, the chamber pressure was set to 4.09 MPa (Pr=1.18). At a

flow rate of 20 mL/min, straight laminar jets (Fig. 5-1 (a)) were observed for pure MCH. A Reynolds

number of approximately 1540 was estimated at the injector exit. When 100 ppmw Pt@FGS was

suspended in MCH, the fuel jet was observed to break up downstream of the injector (Fig. 5-1 (b)).

The generation of stream instabilities indicated that the presence of the FGS additives affected the

injection of MCH under laminar flow conditions (as no pyrolytic reactions occur during cold flow

injection). For pure MCH and MCH/Pt-FGS, the measured pressure drops across the injector were the

same (90 kPa). Figure 5-2 shows both pure MCH and MCH/Pt@FGS injected at a volumetric flow

rate of 50 mL/min under ambient temperature and 4.09 MPa. Both cases at the higher flow rate

showed surface instabilities and ligaments and droplets shed from the surface of the jets. The

Reynolds number at the flow rate of 50 mL/min was approximately 3850. Qualitatively, the pure

117

MCH jet appeared to break up close to the injector and form larger fluid segments and fine drops

from the stream surface, while the fuel spray containing the particles may shed smaller droplets and

ligaments. The low particle loading in the fuel may affect the liquid spray due to the influence of the

particles on the fluid properties (i.e., viscosity or surface tension) of the liquid fuel.

(a) Pure MCH

(b) MCH/Pt@FGS


containing 100 ppmw Pt@FGS at 20 mL/min and 4.09 MPa (Re=~1540).

Normalized Time, sec

Pre

ss

ure

,M

Pa

Sig

na

l,V

dc

-0.8

-0.8

-0.6

-0.6

-0.4

-0.4

-0.2

-0.2

0

0

0.2

0.2

0.4

0.4

0.6

0.6

0.8

0.8

4

4.2

4.4

4.6

4.8

5

5.2

0

1

2

3

4

Fuel Line

Vessel

Trigger

Bypass valve

close

Trigger the

camera

Steady-state

Camera recording

Pressure drop:

0.09 MPa

Fuel valve close

& Bypass valve

Open

Fuel valve

open

at -0.95 sec

1.59 mm

at 0.0 sec at 0.2 sec at 0.4 sec

Normalized Time, sec

Pre

ssu

re,M

Pa

Sig

na

l,V

dc

-0.8

-0.8

-0.6

-0.6

-0.4

-0.4

-0.2

-0.2

0

0

0.2

0.2

0.4

0.4

0.6

0.6

0.8

0.8

4

4.2

4.4

4.6

4.8

5

5.2

0

1

2

3

4

Fuel Line

Vessel

Trigger

Bypass valve

close

Trigger the

camera

Steady-state

Camera recording

Pressure drop:

0.09 MPa

Fuel valve close

& Bypass valve

Open

Fuel valve

open

at -0.95 sec

1.59 mm

at 0.0 sec at 0.2 sec at 0.4 sec

118

(a) MCH (b) MCH/Pt@FGS


containing 100 ppmw Pt@FGS at 50 mL/min, 4.09 MPa, and t=0.4 sec (Re=~3850).

Following the cold flow studies, hydrocarbons pyrolyzed under supercritical conditions both

with and without Pt@FGS suspensions were injected into an optically accessible, pressurized (using

inert N2), chamber to provide a qualitative comparison of jet appearance and quantitative analysis of

jet spreading angle or growth rate. MCH and n-C12H26 were employed as the liquid fuels, and flow

rates were varied from 2.5 to 20 mL/min. The diameter of the injector and L/De were 254 µm and 600,

respectively. Measurements of jet boundary and spreading angle during supercritical fuel injection

were determined using an image processing program developed by Engine Combustion Network

[194], which was modified and applied to all shadowgraphs using the identical parameters. The

process included calculating the temporal and spatial standard deviations of the individual pixel grey

scale across the entire region of interest, after which a threshold was applied which permitted

determination of the jet region, jet boundary, and ultimately the jet spreading angle. Spreading angles

calculated using the processing program were also compared against values calculated manually

using ImageJ [193] software. High-speed images were recorded at 10,000 frames per second with the

exposure time of 1.0 µs. From these injection experiments, the effects of particle additives on the fuel

jets under sub-to-supercritical conditions could be evaluated.

119

Unlike cold flow sprays, supercritical fuel jets are difficult to distinguish because the density

gradients between the supercritical fuel and the chamber pressurant are relatively small, consistent

with previous researchers observations [43], [45], [46], [52]. Figures 5-3 and 5-4 show snapshots of

supercritical MCH and n-C12H26 jets both with and without the particle additives, for flow rates

ranging from 2.5 to 20 mL/min. Upon injection, supercritical jets of both hydrocarbons rapidly

expanded beyond the injector producing an opaque region which diffused quickly from the core

without any observable jet breakups. This observation clearly differs from the liquid sprays under

cold flow conditions. As the flow rate was reduced, the opaque region of supercritical jets became

less dark. In addition, the visible length of the supercritical jet decreased with flow rates.

Figure 5-3. Supercritical jets of pure MCH and MCH containing Pt@FGS at a chamber temperature

of 673 K, reactor temperature of 853 K, and chamber pressure of 4.24 MPa.

20 mL/min

2.5 mL/min

10 mL/min

5.0 mL/min

Pure MCH MCH + 100 ppmw Pt@FGS

120

Figure 5-4. Supercritical jets of pure n-C12H26 and n-C12H26 containing Pt@FGS at a chamber

temperature of 723 K, reactor temperature of 823 K, and chamber pressure of 4.24 MPa.

The opaque regions of the supercritical jets, near the injector, were analyzed to examine the

effects of the particles on the injection processes. In automotive applications, the dark (intact) liquid

core length is an important parameter to evaluate the atomization process, as high velocity liquid is

injected into a highly dense, gaseous environment [203]. Supercritical fuel jets above their critical

points are known to have the liquid-like densities [40], [42], [158], [159]. In the current study, the

fuels, which exhibit much higher density under supercritical phase than gas phase, were injected with

Reynolds numbers ranging from 6,000 to 60,000 into a high temperature and pressure gaseous

environment. Given the similarity, a supercritical fuel may be considered as a dense fluid, so the

concept of core region typically used for the liquid spray can be applied to the current system [40],

[42]. Time-averaged images were obtained for each condition at three different times during injection

using ImageJ [193]software. Figures 5-5 and 5-6 show the 25-frame-averaged images in which the

fuel jet core length for both pure fuel as well as fuel mixtures containing dispersible Pt@FGS

20 mL/min

2.5 mL/min

10 mL/min

5.0 mL/min

Pure n-C12H26 n-C12H26 + 100 ppmw Pt@FGS

121

particles at a chamber pressure of 4.24 MPa are observed. Injection experiments were performed with

fuel volumetric flow rates varying from 2.5 to 20.0 mL/min, corresponding to a mass flow rate

ranging from 0.032 to 0.257 g/s. At higher flow rates, core regions of the supercritical jet were

opaque and easily observable. With increasing residence time, the core regions were considerably

reduced and became difficult to distinguish from the ambient gas. The same trends were observed for

both MCH and n-C12H26.

Figure 5-5. Core regions enhanced by averaging 25 frames for pure MCH and MCH containing


4.24 MPa.

20 mL/min

2.5 mL/min

10 mL/min

5.0 mL/min

Pure MCH MCH + 100 ppmw Pt@FGS

122

Figure 5-6. Core regions enhanced by averaging 25 frames for pure n-C12H26 and n-C12H26 containing


4.24 MPa.

Dark core lengths of injected fuel streams were calculated using the gray pixel value along

the jet centerline from the images of Figs 5-5 and 5-6, which were obtained using ImageJ [193]

software. The gray value for the lowest flow rate and a fuel mixture containing Pt@FGS was used as

a reference point to define the dark core lengths of the supercritical jets. This value was applied to all

high speed injection videos, and used to calculate the respective dark core length of each jet.

Chehroudi et al. [40]–[42] proposed that the intact core length of the jet is governed by:

20 mL/min

2.5 mL/min

10 mL/min

5.0 mL/min

Pure n-C12H26 n-C12H26 + 100 ppmw Pt@FGS

123

5-1

where C is an empirical constant ranging from 3.3 to 11 [40]–[42] and ρ is the density either of the

fuel or pressurant (as denoted by the subscript). The length of the core region, at 20 mL/min, was

estimated to be about 8 to 10 injector diameters, which is in good agreement with the previous results

[40], [42], [204] based on the ratio of fuel and N2 densities. Under this flow condition, the

supercritical fuels may undergo slight conversion due to the very short residence time of

approximately 3 sec. With the assumption of isothermal uniform-density axisymmetric and two-

dimensional jets, the dark core length of the supercritical jet at the condition shown was calculated

between 6 to 10 injector diameters. This core length range agrees well with values estimated by

Abramovich [204]. While the supercritical jets studied here were expected to exhibit a non-uniform

density and the injection process was not perfectly isothermal, this approach may still be valid based

on the comparison of core length between the theory and the current experimental results.

According to the criterion explained above, dark core lengths were estimated for both fuels

with and without Pt@FGS for a range of flow rates under supercritical conditions, at chamber

temperatures of 673 K (Tr=1.18) for MCH and 723 K (Tr=1.10) for n-C12H26 and a fixed pressure of

4.25 MPa (Pr=1.25 for MCH and Pr=2.35 for n-C12H26). As shown in Figs. 5-7 and 5-8, the core

critical value of unity is defined empirically, which approximately matches the analytical length

defined by the above equation. Results presented in Figs. 5-7 and 5-8 show the relative gray values

along the jet centerline (axis length scale) for pure fuels and fuels containing Pt@FGS. As shown, the

relative gray values increased with axial distance, as the jets diffused rapidly outward, eventually

approaching background pixel values. These relative gray value results can provide important

information concerning particle effects on the lengths of dark core region. When the particles were

present in the fuel, the relative gray values at longer residence times were increased relative to the

124

pure fuel. For the shortest residence time of 3.0 sec (20 mL/min), the dark core lengths for both cases

appeared to be virtually the same.

(a) 2.5 mL/min (b) 5.0 mL/min

(c) 10 mL/min (d) 20 mL/min

Figure 5-7. Relative gray values of pure MCH and MCH containing Pt@FGS at various flow rates.

125


(c) 10 mL/min (d) 20 mL/min

Figure 5-8. Relative gray values of pure n-C12H26 and n-C12H26 containing Pt@FGS at various flow

rates.

Comparison of the core lengths in Figs. 5-9 and 5-10 indicates that at the lower flow rates

(2.5 and 5.0 mL/mini), the core length and fuel jet thickness are significantly reduced for fuels

containing particle additives relative to the pure fuels. At the higher flow rates (10 and 20 mL/min),

the particle effects on core length were significantly reduced due to the short residence times which

lead to minimal conversion rates and thus a small change in the composition of the injected fuel. The

substantial decrease in the core length under low flow (longer residence time) conditions can be

explained by the experimental pyrolysis results. At longer residence times, the presence of Pt@FGS

in the liquid fuel exhibited higher conversion relative to the pure fuel baselines, hence more low-

carbon-number gaseous products were formed during the pyrolysis process (in the presence of

126

Pt@FGS) prior to injection. This change in the product composition reduces the average density of

the injected mixtures. According to Chehroudi’s findings, the reduction in fuel density results in a

decreasing jet core length at a given chamber pressure and temperature. Moreover, the decrease in the

jet length with addition of Pt@FGS implies that fuel and nitrogen gas would mix over a shorter

injection distance than the pure fuel. These results suggest the addition of multifunctional Pt@FGS as

a fuel additive not only enhance the pyrolysis processes in a given residence time, but may permit

better mixing in a shorter residence time, beneficial to the design and performance of practical

systems. These values were obtained using a relative gray scale comparison, therefore further analysis

should be considered to define more accurately the dark core region.

Figure 5-9. Dark core lengths for pure MCH and MCH containing Pt@FGS at a chamber temperature


Mass Flow Rate, g/s

Da

rkC

ore

Le

ng

th,m

m

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.5

1.0

1.5

2.0

2.5

3.0

MCH/Pt-FGS

MCH-baseline

127

Figure 5-10. Dark core lengths for pure n-C12H26 and n-C12H26 containing Pt@FGS at a chamber


Measurements of supercritical jet spreading angle were also obtained to aid in understanding

the effects of particle additives on the injection processes. Jet spreading angles were calculated at a

downstream length of 10 injector diameters (10de) using the image processing program developed by

Engine Combustion Network [194] which was modified for the current study. Figures 5-11 and 5-12

show the comparison of jet boundaries for MCH and n-C12H26 with and without Pt@FGS injected

into an inert (N2) environment, pressurized under supercritical conditions. The jet spreading angle

was measured automatically from the center line (dotted line), as shown in Figs. 5-11 and 5-12, to a

tangent line drawn along the outer portion of jet mixing layer. As a means of comparison, these

angles were also calculated manually at multiple points, and showed good agreement with values

calculated using the program. As the flow rate decreased, the visibility of the jets reduced, however

the image processing program can distinguish very small differences in the pixel gray level. Based on

the difference in gray value across the domain, the jet boundary can be defined, and thus the angles

Mass Flow Rate, g/s

Da

rkC

ore

Le

ng

th,m

m

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.5

1.0

1.5

2.0

2.5

3.0

n-dodecane/Pt-FGS

n-dodecane-baseline

128

can be calculated relative to the jet centerline. Note that at lower flow rates it is very difficult to

define boundaries without image processing. As shown in Figs. 5-11 and 5-12, the jet spreading

angles appear to reduce with addition of Pt@FGS relative to the pure fuel baseline. While the high-

speed shadowgraph used in the current study can provide both qualitative and quantitative

information about the injection process, when coupled with image processing programs, further

enhancement in the image resolution and contrast would be beneficial, and simplify the analysis

process.

Figure 5-11. Snapshots of jet boundaries for pure MCH and MCH containing Pt@FGS at a chamber


Pure MCH MCH+100ppmw Pt@FGS

20 mL/min

2.5 mL/min

10 mL/min

5.0 mL/min

129

Figure 5-12. Snapshots of jet boundaries for pure n-C12H26 and n-C12H26 containing Pt@FGS at a

chamber temperature of 723 K, reactor temperature of 823 K, and chamber pressure of 4.24 MPa.

Figures 5-13 and 5-14 show the effect of Pt@FGS on the jet spreading angle for both MCH

and n-C12H26 at a fixed chamber pressure of 4.24 MPa. The spreading angle was observed to decrease

with decreasing mass flow rate, which is in good agreement with results reported by Doungthip et al.

[52]. Unlike the dark core length results, Pt@FGS nanoparticles were observed to have little effect on

the spreading angle due to the large scattering in the measured angles. Consistently, however, average

values for the particle-doped fuels were slightly reduced from the pure fuel baseline case. Chehroudi

et al. [40]–[42] found that increasing the chamber-to-injectant density ratio led to greater spreading

Pure n-C12H26 n-C12H26 +100ppmw Pt@FGS

20 mL/min

2.5 mL/min

10 mL/min

5.0 mL/min

130

angle, which means that the decreasing spreading angle due to the reduced fuel density at a given

nitrogen density (fixed chamber pressure) disagrees with the spreading angle correlation proposed by

Chehroudi et al. [40]–[42]. Similar to the current observation, Rachedi et al. [45], [46] found that the

spreading angle increases as a function of the density ratio between the injected fuel (JP-10) and the

surrounding environment. They explained that this trend is due to a loss in angular momentum as the

fluid density decreases. This inconsistency between Chehroudi et al. [40]–[42], and Rachedi et al.

[45], [46] and the current findings should be investigated in more detail. In addition, since the

experimental errors are relatively large for all data, further analysis is clearly warranted to illuminate

the particle effects on spreading angle.

Figure 5-13. Spreading angles of pure MCH and MCH containing Pt@FGS at a chamber temperature


Mass Flow Rate, g/s

Sp

rea

din

gA

ng

le,d

eg

ree

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

MCH/Pt-FGS

MCH-baseline

131

Figure 5-14. Spreading angles of pure n-C12H26 and n-C12H26 containing Pt@FGS at a chamber


5.2 Combustion Experiments

Supercritical combustion experiments were performed using five types of particles, including

fumed silica, lauric acid coated nAl, FGS19, 5wt%Pt@FGS20, and 20wt%Pt@FGS20 to investigate the

macroscopic effect of these additives on the ignition and combustion processes. As described

previously, 5wt%Pt@FGS indicates the surface of the FGS was decorated with platinum

nanoparticles which comprise 5.0 weight percent of Pt@FGS. Pressure measurements of the fuel,

bypass chamber, and combustion chamber were acquired through the experiments at a sample rate of

1 kHz. In particular, the pressure achieved in the chamber during the experiment was directly related

to the amount of conversion of fuel and oxidizer to products. The effects of the different particle

additives can therefore be evaluated by comparing the pressure rise in the combustion chamber.

Based on the ideal gas law and the first law of thermodynamics in the closed system, the equivalent

Mass Flow Rate, g/s

Sp

rea

din

gA

ng

le,d

eg

ree

0.00 0.05 0.10 0.15 0.20 0.25 0.3010.0

15.0

20.0

25.0

30.0

n-dodecane/Pt-FGS

n-dodecane-baseline

132

chemical release Q of the fuel and oxidizer mixture during the combustion process can have a linear

relationship with the chamber pressure as shown in the following equation [205]:

5-2

where P is the combustion chamber pressure, γ is the ratio of specific heats, and V is the chamber

volume. In addition to the chemical energy of the mixture, the characteristic combustion efficiency

can be defined as follows [195], [205], [206]:

5-3

where and are the initial chamber temperature and adiabatic flame temperature. The

combustion efficiency, , provides a means for evaluating the conversion of reactants to products

in a given system by relating the theoretical product gas velocity at the nozzle throat to the

experimental value [71]. For combustion experiments, n-C12H26 was chosen as the fuel. The particle

loading concentrations were set to 100 ppmw, with the exception of silica which was loaded to 104

ppmw, and the coated nAl loaded at 103 ppmw. The nanosized silica particles were considered to

investigate whether the addition of inert particles into the hydrocarbon fuel exhibited any effect

during the pyrolysis process and subsequent combustion event. In the case of the nAl suspension, the

effect of reactive particles on the ignition and combustion was compared against graphene-based

particles. Two equivalence ratios, 0.19 (fuel flow rate = 2.5 mL/min) and 0.38 (fuel flow rate = 5.0

mL/min), were considered. Figure 5-15 shows the measured fuel pressures with the various particles

at a volumetric flow rate of 5.0 mL/min (equivalence ratio of 0.38), Pch,ini=3.46 MPa, Tfuel=832 K, and

Tair=777 K. The presence of 20wt%Pt@FGS exhibited the highest pressure (approximately 4.4 MPa)

133

compared to other particles. The higher fuel pressure was likely due to the catalytic activity of the

particles during supercritical pyrolysis in the reactor, forming a higher number of moles of gaseous

products, which is consistent with the higher conversion rate observed during the pyrolysis

experiments. As described in Chapter 3, the fuel pressure strongly depends on the fuel composition,

bypass chamber pressure, and the bypass injector. The variation of bypass pressure between

experiments was approximately 1.6 %, indicating all tests were conducted at virtually the same

conditions. As shown in Fig. 5-15, the Pt-decorated FGS at two different Pt loadings produced higher

fuel pressures compared to FGS. With increasing Pt loading, further fuel conversion can be expected.

The addition of 104 ppmw silica showed a slightly higher fuel pressure due to the high silica particle

loading compared to the pure fuel, FGS, and nAl loaded fuel. Fuel pressures for 38 and 80

nanometer-sized of aluminum particles exhibited no effects on the conversion in the phase of

supercritical reaction. This observation is quite reasonable, as these nAl particles are expected to be

non-catalytic materials. Figure 5-16 shows fuel pressures for pure n-C12H26 and n-C12H26 containing

various particles at a flow rate of 2.5 mL/min (equivalence ratio of 0.19), Pch,ini=3.44 MPa, Tfuel=831

K, and Tair=772 K. For both Pt@FGS, higher fuel pressures were measured, whereas for FGS19 lower

fuel pressures were found than the pure fuel. The measured pressure drops observed in the profile

(approximately 200 ms after the offset time at 0.0 sec) occurred during valve switching, and were

0.10 (no particle), 0.09 (silica), 0.09 (FGS19), 0.08 (5wt%Pt@FGS), and 0.15 (20wt%Pt@FGS) MPa,

respectively.

134

Figure 5-15. Fuel pressures for n-C12H26 and n-C12H26 containing various particles at a flow rate of

5.0 mL/min (equivalence ratio of 0.38), Pch,ini=3.46 MPa, Tfuel=832 K, and Tair=777 K.

Figure 5-16. Fuel pressure for n-C12H26 and n-C12H26 containing various particles at a flow rate of 2.5

mL/min (equivalence ratio of 0.19), Pch,ini=3.44 MPa, Tfuel=831 K, and Tair=772 K.

Offset Time, sec

Pre

ss

ure

,M

Pa

-0.5 0 0.5 1 1.5 2 2.5 3 3.52

2.5

3

3.5

4

4.5

5

no particle

1wt%_silica

100 ppmw_FGS19

100 ppmw_5wt%Pt@FGS20

100 ppwm_20wt%Pt@FGS20

1000 ppmw_38nm_nAl

1000 ppmw_80nm_nAl

Offset Time, sec

Pre

ss

ure

,M

Pa

-0.5 0 0.5 1 1.5 2 2.5 3 3.53

3.2

3.4

3.6

3.8

4

4.2

no_particle

100 ppmw_FGS19



135

The effect of the particles on fuel pyrolysis with respect to pressure rise in the combustion

chamber was studied and related to conversion efficiency of that specific fuel mixture at the

experimental operating conditions. Figure 5-17 shows a series of measured chamber pressures using

n-C12H26 with the particle additives at a flow rate of 5.0 mL/min (global equivalence ratio of 0.38),

Pch,ini=3.46 MPa, Tfuel=832 K, and Tair=777 K. In Fig. 5-17, when 20wt%Pt@FGS at a concentration

of 100 ppmw was present in the fuel, the chamber pressure rise was observed 0.8 MPa at 3.5 sec,

indicating the highest enhancement among the particles which is consistent with the highest

conversion of fuel to pyrolysis products. Addition of 1.0 wt% silica exhibited no influence on the

pressure rise. Relative to the baseline, the FGS19 doped fuel showed a slight enhancement (9.0 %) in

the measured pressure rise. By decorating Pt nanoparticles onto FGS, higher chamber pressures for

5wt% and 20wt% Pt-decorated FGS were observed, with measured pressures increasing by 27.3 and

45.5 % with Pt loading. Addition of 38 nm aluminum particles (1000 ppmw) to n-C12H26 produced no

measurable increase in chamber pressure while the 80 nm nAl particles (74.5% active aluminum

[207]) which have a considerably higher active aluminum content than the 38 nm particle (49 %

active aluminum [185]) showed a slight enhancement in the pressure rise. However, the benefit of the

nAl additives in the liquid fuel was still limited, because the loading concentration was too low to see

any significant enhancement with respect to the adiabatic flame temperature. The comparison

between nAl and FGS-based particles supports the hypothesis that FGS-based materials can be used

as fuel additives to enhance combustion efficiency. Figure 5-18 illustrates the chamber pressures with

the various particles at a flow rate of 2.5 mL/min (global equivalence ratio of 0.19), Pch,ini=3.44 MPa,

Tfuel=831 K, and Tair=772 K. At this low flow rate, chamber pressure trends are similar to those shown

in Fig. 5-17. There seems to be a time delay in the chamber pressure rise at the lower flow rate in the

following order: no particle > FGS19 ≈ 20wt%Pt@FGS > 5wt%Pt@FGS. Consequently, the

enhancements in the pressure rise for both fuel flow rates were found when Pt@FGS in the n-C12H26

at a low loading concentration at 100 ppmw was suspended.

136


of 5.0 mL/min (equivalence ratio of 0.38), Pch,ini=3.49 MPa, Tfuel=832 K, and Tair=777 K.


of 2.5 mL/min (equivalence ratio of 0.19), Pch,ini=3.44 MPa, Tfuel=831 K, and Tair=772 K.

Offset Time, sec

P-P

ini,

MP

a

-0.5 0 0.5 1 1.5 2 2.5 3 3.5-0.2

0

0.2

0.4

0.6

0.8

1

1.2no_particle

1wt%_silica

100 ppmw_FGS19



1000 ppmw_38nm_nAl

1000 ppmw_80nm_nAl

Offset Time, sec

P-P

ini,

MP

a

-0.5 0 0.5 1 1.5 2 2.5 3 3.5-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

no_particle

100 ppmw_FGS19



137

Complimentary to the pressure rise data presented in Figs. 5-17 and 5-18, calculated increases

in conversion for the various fuel compositions containing particle additives are illustrated in Figs. 5-

19 and 5-20. The percentage increase was calculated based on the averaged pressure rise at the offset

time of 3.5 sec for all combustion experiments. Fumed silica, having a 104 ppmw loading case

showed a negative effect even though a higher fuel pressure was observed relative to the baseline.

The Pt@FGS showed a significant percentage increase relative to the baseline, silica, and nAl fuels

with increases up to 25.0 and 34.9 % for 5wt%Pt@FGS and 20wt%Pt@FGS, respectively.

Conversion enhancements (at 5 mL/min) were observed in the following order: 38 nm nAl < FGS19 ≈

80 nm nAl < 5wt%Pt@FGS < 20wt%Pt@FGS. As shown in Fig. 5-20, the highest percent increase in

conversion (33.5 %) is obtained in the presence of 20wt%Pt@FGS at 2.5 mL/min. FGS-based

particles including FGS and Pt@FGS showed an enhancing effect on the percentage increase in

conversion at both flow rates with similar degrees of enhancement. From these results, higher loading

concentration of Pt onto FGS should be studied to determine whether higher conversion efficiency

can yet be achieved.

138



Tair=777 K. The results are shown relative to the pure fuel baseline.



Tair=772 K. The results are shown relative to the pure fuel baseline.

139

Autoignition of supercritical fuel containing particle suspensions was investigated using high-

speed schlieren shadowgraph techniques to determine the influence of the particles on the ignition

delay time. Figures 5-21 – 26 show typical schlieren images captured using the schlieren setup

illustrated in Chapter 3. Images were collected at 10,000 frames per second with a 4-μs exposure time

for injection of n-C12H26 containing various particle additives including FGS and Pt@FGS into

elevated temperature and pressure environment. Sequences of still frame images depicting

autoignition of various fuels at select timings after the start of injection and at a flow rate of 5.0

mL/min, Pch,ini=3.49 MPa, Tfuel=832 K, and Tair=777 K are shown in Figs. 5-21 – 23. The time shown

in the upper left corner of each image was calculated with time zero being that frame in which

injection initiation was observed. The appearance of the fuel jets for the baseline and FGS19

containing fuels was quite similar at 3.0 ms, whereas the Pt@FGS containing fuel jet appeared fully

developed. This difference is likely due to the change in fuel composition, catalyzed by Pt@FGS.

From the pyrolysis experiments, increased low-carbon-number products in the fuel may facilitate jet

development in a shorter time. In the presence of Pt@FGS, the ignition kernel was observed to form

around 3.5 ms, leading to the flame that propagated along the jet. The flame propagation was quite

similar in all cases. Relative to Pt@FGS, the fuel jet development time was increased to about 6.0 and

9.0 ms for no particle and FGS19 cases, respectively. It was clear that for the three cases shown in Figs.

5-21 to 5-23 autoignition occurs by an explosive ignition kernel which forms downstream of the

supercritical jet. Once the diffusion flame is fully developed, there exist fast-moving eddies along the

flow stream which can enhance the local turbulence and affect mixing and combustion [208]. The

location of ignition kernel formed downstream of the injector exit was observed in the following

order: 32.8 de for Pt@FGS < 50.3 de for FGS19 < 61.6 de (no particle). Figures 5-24 - 26 show a time

sequence of schlieren images prior to injection and up to full combustion at a flow rate of 2.5 mL/min.

The supercritical fuel jets developed slower at 2.5 mL/min than 5.0 mL/min. The location of the first

ignition kernel appeared to be independent on particle type. Images of the turbulent flame showed

140

fine flame structures and wrinkles. These structures may be related to pressure effects on the flame

propagating through the fuel jet. In the flame images, when ignition occurs, many small-scale

wrinkles were observed, whereas when the turbulent flame becomes stabilized, large-scale wrinkles

were observed. The flame front for n-C12H26 dispersed with Pt@FGS appeared to be smooth and

layered, as shown in Figs. 5-23 and 5-26. In addition, divisions between unburned fuel and burned

fuel were observed near the injector. At the low flow rate (2.5 mL/min), the flame showed

instabilities with eddy shedding formed throughout the injection processes. For example, there seems

to form an instant vacancy where the flame was not present, and the flame was found to fluctuate with

a wave-like motion downstream of the turbulent flame at 45.0 ms (Fig. 5-26). This phenomenon may

cause combustion instabilities throughout the injection processes. Further investigation of the

combustion instabilities should be considered as a function of flow rate, temperature, pressure,

particle type, and particle loading concentration.

141

Figure 5-21. Time sequence of schlieren images of the start of injection, penetration, autoignition, and

developed-turbulent diffusion flames for pure n-C12H26 at a flow rate of 5.0 mL/min (equivalence

ratio of 0.38), Pch,ini=3.49 MPa, Tfuel=832 K, and Tair=777 K.

-0.1 ms 0.0 ms 3.0 ms 6.0 ms

9.0 ms 10.7 ms

12.0 ms 15.0 ms

20.0 ms 25.0 ms

142




Tair=777 K. Time zero corresponds to the start of injection.

-0.1 ms 0.0 ms 3.0 ms 6.0 ms

8.1 ms 9.0 ms

15.0 ms 20.0 ms

25.0 ms 30.0 ms

143





-0.1 ms 0.0 ms 3.0 ms 3.5 ms

6.0 ms 9.0 ms

15.0 ms 20.0 ms

25.0 ms 30.0 ms

144


penetration, autoignition, and developed-turbulent diffusion flames for pure n-C12H26 at a flow rate of

2.5 mL/min (equivalence ratio of 0.19), Pch,ini=3.44 MPa, Tfuel=831 K, and Tair=772 K. Time zero

corresponds to the start of injection.

-0.1 ms 0.0 ms 15.0 ms 30.0 ms

49.3 ms

65.0 ms60.0 ms

55.0 ms

145





-0.1 ms 0.0 ms 15.0 ms 30.0 ms

43.4 ms

50.0 ms

45.0 ms

60.0 ms

146





Measurement of ignition delay was determined using the known frame rate of the camera and

the number of frames captured between the start of injection and ignition kernel formation. Figure 5-

27 shows a comparison of measured ignition delay times for four different particle types including

fumed silica, nAl, FGS19, and Pt@FGS20 at a flow rate of 5.0 mL/min (global equivalence ratio of

0.38), Pch,ini=3.49 MPa, Tfuel=832 K, and Tair=777 K. For pure fuel, the ignition delay was found to be

about 12.4 ms. Karwat et al. [209] studied autoignition of n-C12H26 using the rapid compression

experiments, measuring ignition delay times for the fuel ranging from 44.0 to 56.0 ms over a

temperature and pressure range of 710 – 790 K and 0.29 – 0.36 MPa, for fixed equivalence ratio of

0.97. Compared to the lower pressure results of Karwat et al. [209], a considerably shorter ignition

-0.1 ms 0.0 ms 15.0 ms 30.0 ms

34.7 ms

45.0 ms

35.0 ms

60.0 ms

147

delay for cracked n-C12H26 under supercritical conditions was measured (12.4 ms, 5.0 mL/min,

ϕ=0.38). This finding suggests a potential benefit of using a supercritical fuel in an advanced

propulsion system. The addition of particles to baseline fuel resulted in reduced ignition delay, which

agrees well with the pressure rise and conversion efficiency results. Pt@FGS particle addition

reduced the ignition delay by nearly a factor of 3 compared to the baseline fuel. Specifically,

20wt%Pt@FGS reduced ignition delay time from 12.4 ms to nearly 4.0 ms. These results suggest that

graphene-based particles can be successfully used as an ignition enhancer for combustion applications.

The shorter ignition delay likely results from the combination effects of Pt@FGS on both the

pyrolysis and combustion processes. As shown in results of Sabourin et al. [5], FGS having defect

sites can serve as catalysts for enhancing burning rates of the nitromethane monopropellant. In

addition, pyrolyzing the fuel prior to injection produces more reactive gaseous products, (for example,

hydrogen or ethylene), which may accelerate the ignition process. Addition of nAl particles into the

fuel resulted in approximately 22.8 % reduction in ignition delay time. The preheated nAl particles

which are heated near the core melting temperature prior to injection, may facilitate ignition by

reacting with available oxidizer. This high temperature and pressure injection condition may allow

the fuel and nanosized aluminum mixture to ignite faster than the baseline case. Results presented in

Fig. 5-28 show a similar positive effect of the particles on the ignition delay time at 2.5 mL/min. The

ignition delay time was increased when the flow rate was reduced to 2.5 mL/min. Consistent with the

5.0 mL/min results, fuels containing particle additives exhibited shorter ignition delays relative to the

pure fuel. The highest reduction in the ignition delay was approximately 22.7 % when

20wt%Pt@FGS was dispersed in the fuel.

148





149

The impact of particle additives on the flame structure, such as flame spreading angle, was

investigated using images obtained from high speed videos captured during steady-state combustion.

Flame spreading angles based on the flame boundary for different particle types were analyzed.

Figure 5-29 provides a comparison of 25-frame-averaged images (high speed shadowgraphs) at two

different flow rates (2.5 and 5.0 mL/min) and three different particle types including FGS19,

5wt%Pt@FGS20, and 20wt%Pt@FGS20. From these averaged images, flame spreading angles were

defined at a downstream length of 26 nozzle diameters (26de) using the ImageJ [193] software. The

flame spreading angle was measured from the center line as shown in Fig. 5-29, to a tangent line

drawn along the outer portion of flame layer at pre-defined number of jet diameters downstream of

the injector. Spreading flame angles at the flow rate of 5.0 mL/min were measured in the following

order: 15.4 ° (no particle) < 17.2 ° (FGS) < 19.1 ° (5wt%Pt@FGS) < 20.3 ° (20wt%Pt@FGS). At 2.5

mL/min, similar trends and angles were obtained (18.3 ° (no particle) < 18.5 ° (FGS) < 19.2 °

(5wt%Pt@FGS) < 20.9 ° (20wt%Pt@FGS)).

Measured spreading angles for volumetric flow rates of 2.5 and 5.0 mL/min for the various

particles considered are provided in Fig. 5-30. For both flow rates, a positive impact of particle

addition to the fuel was observed. When 20wt%Pt@FGS was present in the fuel, the calculated

tangent of the spreading angle was increased by 34.6 %, at a flow rate of 5.0 mL/min. Based on the

axisymmetric assumption, increasing the flame spreading angle corresponds with a larger flame area

which could result in greater chemical energy release within the combustor volume and thus result in

an increased chamber pressure.

150


Figure 5-29. Spreading angles for n-C12H26 as well as n-C12H26 containing FGS and Pt@FGS for two

different flow rates (a) 2.5 mL/min and (b) 5.0 mL/min.


Figure 5-30. Effect of FGS and Pt@FGS on the flame growth rate for two different flow rates (a) 2.5

mL/min and (b) 5.0 mL/min.

No Particle

100 ppmw FGS19

100 ppmw

5wt%Pt@FGS20

100 ppmw

20wt%Pt@FGS20

No Particle

100 ppmw FGS19

100 ppmw

5wt%Pt@FGS20

100 ppmw

20wt%Pt@FGS20

151

The effects of the particles on the flame lift-off length were studied during steady-state

combustion. Time-averaged images of light emitting from the diffusion flame were measured using

the high-speed shadowgraphs. The lift-off distance can be visualized and calculated using the ImageJ

software [193]. The lift-off distance was defined using steady-state flame structures obtained

approximately 1.5 sec after the camera triggered. From the averaged images, the flame front can be

determined based on the brightness of the images. The lift-off distance was determined by defining

the distance between the injector exit and the flame front, using the known outer diameter of the

injector to obtain an appropriate measurement scale. High speed images of the flames indicate that at

5.0 mL/min the addition of the particles significantly reduces the lift-off length by up to 53.8 %

(Pt@FGS), while at 2.5 mL/min the effects of the particles are nearly indistinguishable. The

comparison of measured lift-off distance at 5.0 mL/min showed that the reaction zone stabilized much

faster when 20wt%Pt@FGS was added to the fuel. This finding of the reduced lift-off distance for

different particle additives, especially under the flow condition at 5 mL/min, correlates well with the

change in pyrolyzed fuel composition prior to injection. As discussed previously, the extent of fuel

pyrolysis could produce a reduced bulk density, prior to injection, which in turn may affect the

injection velocities of the fuel into the pressurized chamber.

152


Figure 5-31. Effect of FGS and Pt@FGS on the measured lift-off distance for two different flow rates

(a) 2.5 mL/min and (b) 5.0 mL/min.

Reduction of the initial chamber pressure to achieve subcritical fuel injection (Pr=0.78, 1.41

MPa) was investigated at a flow rate of 5.0 mL/min, Tfuel=832 K, and Tair=777 K, with the addition of

100 ppmw 5wt%Pt@FGS. Figure 5-32 shows measured fuel and combustion chamber pressures.

Similar to the high pressure (supercritical) combustion experiments, the presence of Pt@FGS resulted

in a higher fuel pressure and greater chamber pressure rise. The fuel pressure for the particle case was

roughly 10 % higher than the baseline, while the chamber pressure rise was increased by 18 %. A

comparison of subcritical and supercritical chamber conditions is presented in Fig. 5-33. Greater

conversion (24.5 %) was achieved under a supercritical condition than a subcritical condition (13.8 %)

due to the fact that the fuel pyrolysis and combustion under supercritical conditions could be

accelerated by enhanced interactions between the particles and fuel molecules during the entire

process based on the collisional theory. Figure 5-34 shows high speed shadowgraphs captured during

the injection and ignition for n-C12H26 containing Pt@FGS under subcritical conditions at 5.0 mL/min.

153

In a series of the images, high temperature luminous regions (Fig. 5-34) were observed when

Pt@FGS was added to the fuel. These luminous regions may come from hot spots resulting from FGS

combustion, which could lead to shorter ignition delays. No luminous regions were observed in the

pure fuel baseline experiment.

Figure 5-32. Comparison of measured fuel and chamber pressures for pure n-C12H26 and n-C12H26

containing Pt@FGS at an initial chamber pressure of 1.41 MPa (Pr=0.78, subcritical), a flow rate of

5.0 mL/min, Tfuel=832 K, and Tair=777 K.

Figure 5-33. Comparison of percentage increase in conversion for n-C12H26 containing Pt@FGS at

two different pressures of 1.4 MPa (subcritical) and 3.46 MPa (supercritical).

Offset Time, sec

Ch

am

be

rP

res

su

re(P

-Pin

i),M

Pa

Fu

elP

res

su

re,M

Pa

-0.5 0 0.5 1 1.5 2 2.5 3 3.5-0.2

0

0.2

0.4

0.6

0.8

0

0.5

1

1.5

2

2.5

Pch

_no_particle

Pfuel

_no_particle

Pch

_100 ppmw 5wt%Pt@FGS20

Pfuel

_100 ppmw 5wt%Pt@FGS20

154

Figure 5-34. Luminous regions for n-C12H26 containing 100 ppmw Pt@FGS at an initial chamber

pressure of 1.41 MPa (Pr=0.78, subcritical), a flow rate of 5.0 mL/min, Tfuel=832 K, and Tair=777 K.

5.3 Summary of Injection and Combustion Studies

The effects of particle addition on injection and combustion of hydrocarbon fuels under

supercritical conditions were experimentally studied using a high pressure and temperature windowed

combustor coupled to a flow reactor and feed system. Images of fuel jets and turbulent flames,

captured using high-speed cinematography, were analyzed to provide comparisons between pure

(baseline) fuels and the same fuel containing inert, catalytic, or reactive particle additives. In this

155

study, fumed silica, aluminum, FGS, and FGS decorated with varying amounts of nanometer sized

platinum particles (5.0 and 20.0 percent by weight) were considered. Spreading angle, ignition delay,

and conversion efficiency were characterized as a function of flow rate, chamber pressure, and

particle type. Quantitative as well as qualitative effects of the particle addition on injection, ignition,

and combustion process were compared against the pure fuel baseline.

Cold flow injections were performed to investigate the effect of Pt@FGS on the liquid jet.

The presence of a small amount of particles (100 ppmw) in the fuel was observed to lead to

instabilities on the jet stream. Supercritical injection studies using MCH and n-C12H26 containing

Pt@FGS were conducted under a high temperature and pressure inert (N2) environment. The addition

of the particles was observed to affect the supercritical injection process of the hydrocarbon fuels. It

was found that when the particles were present in the fuels, the core lengths of the supercritical jets

were reduced by up to approximately 56.7 % (MCH) and 68.8 % (n-C12H26). Pyrolysis experiments

show that the additives enhance the conversion rates of the fuels and change the speciation of the fuel

mixture prior to injection. The change in the fuel mixture density may cause the reduction in core

lengths of the jets.

Combustion under supercritical conditions for hydrocarbon fuel containing various particle

additives was investigated as well. The injection, ignition, and combustion process was recorded

using high speed cinematography, and pressure rise, ignition delay, flame spreading angle, lift-off

length, and flame structure were examined as a function of pressure, particle type and flow rate.

Addition of 100 ppmw 20wt%Pt@FGS reduced ignition delay of n-C12H26 by nearly a factor of 3,

increased spreading angles by approximately 35.0 %, reduced the flame lift-off length by 50.0 %, and

demonstrated an increase in conversion by 35.0 % relative to the pure fuel baseline at a volumetric

flow rate of 5.0 mL/min. Such enhancements benefit practical propulsion systems which require high

conversion efficiency in a short residence time.

156


either as an additive or a means of distributing another additive (i.e., platinum) can significantly

enhance, and be used to tailor, the conversion of liquid hydrocarbon fuels/propellants under

supercritical conditions, resulting in reduced ignition delay and improved combustion.

157

Chapter 6

Molecular Dynamics Simulations

6.1 Computational Details

6.1.1 ReaxFF Reactive Force Field

The current study investigated the enhancing mechanisms on the catalytic initiation of

hydrocarbon and nanoparticle mixtures using the molecular dynamics program ReaxFF, which makes

use of empirical force fields derived from quantum mechanics-based parameterization [76] or

experiments. Basically, the ReaxFF reactive forces are divided into various energy contributions as

follows:

5-1

where the total energy term, , consists of the bond energy ( ), over-coordination energy

( ), under-coordination energy ( ), lone-pair energy ( ), valence angle energy ( ),

torsion angle energy ( , and bond order-independent terms, van der Waals ( ) and

Coulomb ( ) energies, respectively [76]. Different from traditional force fields which are not

capable of simulating chemical reactions, the ReaxFF reactive force can provide bond-breaking and

bond-forming reactions using bond orders based on the interatomic distance between atoms.

Individual reaction pathways, therefore, can be obtained during the molecular dynamics (MD)

simulations. van Duin et al. [76] first developed the ReaxFF force field for hydrocarbon systems to

investigate pyrolysis and combustion with or without catalysis [58], [174], [176], [179], [210]. In the

158

current study, the ReaxFF MD simulations were performed to shed light on the study of the catalytic

or reactive mechanisms of hydrocarbon and particle mixtures.

6.1.2 ReaxFF with Nudged Elastic Band (NEB) Method

Estimation of reaction energies and barriers for elementary chemical reactions such as

dehydrogenation, hydrogenation, and H-abstraction, is important to understand the effects of

additives on the chemical reactions. The ReaxFF embedded with the nudged elastic band (ReaxFF-

NEB) method can be used to calculate reaction energies and barriers for specific initiation steps of

thermal and catalytic decomposition [211]. Especially, ReaxFF-NEB can predict the transition state

between a given initial and final state [211]. With a series of intermediate images (transition states), a

linear interpolation using a spring force between neighboring images allows one to find the minimum

energy path, which represents the transition state of the elementary reaction [212], [213]. All

geometrical structures are optimized using the conjugate gradient method. Typically, the number of

images varies from 5 to 8 with the end point criterion of 1.0 for energy minimization.

6.1.3 Molecular Dynamics Simulations

For thermal and catalytic decomposition simulations, four different sizes of cubic periodic

boxes were built at constant densities of 0.12 and 0.31 g/cm3 containing 24 n-dodecane (n-C12H26)

molecules without a particle, as well as containing a functionalized graphene sheet (FGS), a Pt cluster

(Pt-cluster), and a Pt cluster-decorated graphene sheet (Pt@FGS). By changing the size of the cubic

periodic boxes in the presence of the particles, the pressure effect, which may be induced by adding

the particles into the system, can be minimized to precisely measure the decomposition kinetics of the

hydrocarbon fuel. The FGS, Pt-cluster, and Pt@FGS were prepared using Material Studio 6.1 of

159

Accelrys Inc., as illustrated in Fig. 6-1. Snapshots of initial configurations for pure n-C12H26 (a), n-

C12H26/FGS (b), n-C12H26/Pt-cluster (c), and n-C12H26/Pt@FGS (d) systems at 1500 K are showed in

Fig. 6-2. These molecular simulations of thermal and catalytic cracking of n-C12H26 were repeated at

least 10 times to generate statistical information on the effect of particle type on kinetics and reaction

mechanisms. The systems for each case were energy-optimized and equilibrated via low-temperature

(100 K) ReaxFF simulations (time step of 0.25 femtosecond and 200,000 iterations). These

equilibrated systems were ramped up to 3000 K at a rate of 10 K/picosecond (ps) to obtain the initial

configurations with different particles at different temperatures. Using constant number of atoms (N)

in a constant volume (V) at a constant temperature (T), referred to as NVT, pyrolysis simulations

were performed over the temperature range from 1500 to 1900 K for 1.0 or 2.0 nanoseconds (ns) with

a time step of 0.2 femtosecond (fs). The results from these simulations were used to explore the effect

of different particles on reaction mechanisms, overall product distributions, and kinetics as a function

of temperature. In order to perform parallel MD simulations, the ReaxFF version integrated into ADF

software was used for all simulations. ReaxFF force field parameters for C, H, O, and Pt atoms

derived from previous work based on quantum mechanics and experimental data were used [211].

Each simulation was carried out using from 1 to 4 processors on an Intel Xenon X5560 Quad-Core

2.8 GHz computer at the Institute for CyberScience’s Lion-X clusters of the Pennsylvania State

University.

160

(a) oxidized graphene with

divacancy (C51H20O2)

at C/O=22.5

(b) Pt6 cluster (c) Pt-decorated graphene

sheets (Pt6@ C51H20O2)

at C/O=22.5

Figure 6-1. Snapshots of the initial configurations of FGS, Pt-cluster, and Pt@FGS. Hydrogen atoms

are white, oxygen atoms are red, carbon atoms are orange, and platinum atoms are yellow. (a) a FGS,

(b) a Pt-cluster, and (c) a Pt@FGS.

(a) n-C12H26 (b) n-C12H26/FGS

161

(c) n-C12H26/Pt6-cluster (d) n-C12H26/Pt@FGS

Figure 6-2. Snapshots of the initial configurations of n-C12H26 with FGS, Pt-cluster, and Pt@FGS at

0.31 g/cm3. (a) n-C12H26, (b) n-C12H26/FGS, (c) n-C12H26/Pt6-cluster, and (d) n-C12H26/Pt@FGS.

6.2 Results and Discussion

The reaction mechanisms and kinetics of pyrolysis of pure fuels and mixtures with catalytic

particles were investigated using ReaxFF simulations. Thermal decomposition simulations of a pure

hydrocarbon were performed with reaction pathway analysis to find the initiation mechanisms of n-

C12H26 decomposition. The simulations confirmed that homolytic carbon-carbon (C-C) bond cleavage

is one of the major initiations of supercritical pyrolysis of n-C12H26, which is in good agreement with

the previous findings discussed in the previous section. Then, smaller hydrocarbon radicals formed by

C-C bond scission, which subsequently underwent β-scission generating a series of alkenes, where the

most abundant product is ethylene (C2H4). Another initiation mechanism observed during thermal

decomposition simulations was carbon-hydrogen (C-H) bond scission forming H atom and n-C12H25,

where this reaction occurred at high temperature (above 1800 K) rather than at low temperature. This

162

finding is quite reasonable because the C-H bond scission of the long-chain alkanes has a high

activation and bond energies greater than 90 kcal/mol [27], [184]. Once small hydrocarbon radicals

including H, CH3, C2H5, and C3H7 were generated in the system, secondary steps such as H-

abstraction, rapidly participated in the fuel consumption. Interaction between the parent fuel molecule

and these radicals can produce H2 or a series of alkanes (methane, ethane, propane, etc.) with the n-

C12H25 radical. Table 6-1 shows initiation steps for thermal cracking of n-C12H26 and their reaction

energies estimated using the ReaxFF-NEB. Comparison of reaction enthalpies with the literature in

Table 6-1 indicates that ReaxFF-NEB calculations exhibit reasonable reaction energies for initiation

reactions. The C-C and C-H bond scission require higher reaction energies than the secondary

initiation steps such as H-abstraction.

163

Table 6-1. Reaction energies of initiation steps for thermal decomposition n-C12H26.

Reactions This Study (kcal/mol) Literature (kcal/mol)

n-C12H26 CH3 + n-C11H23 87.19 85.8a/88.93

b

n-C12H26 n-C2H5 + n-C10H21 81.39 81.5a/85.97

b

n-C12H26 n-C3H7 + n-C9H19 80.05 81.5a/85.59

b

n-C12H26 n-C4H9 + n-C8H17 79.66 81.5a/85.44

b

n-C12H26 n-C5H11+ n-C7H15 80.12 81.5a/84.56

b

n-C12H26 n-C6H13+ n-C6H13 79.84 81.5a/84.34

b

n-C12H26 1n-C12H25+ H 99.70 101.55b

n-C12H26 2n-C12H25+ H

93.65 96.74b

n-C12H26 3n-C12H25+ H 94.28 97.38b

H+n-C12H26 n-C12H25+ H2 -8.73 -6.26b

H+n-C12H26 n-C12H25+ H2 -9.74

CH3+n-C12H26 n-C12H25+CH4 -5.82 -4.65b

CH3+n-C12H26 n-C12H25+CH4 -5.40

CH3+n-C12H26 n-C12H25+CH4 -5.10

n-C2H5+n-C12H26 n-C12H25+n-C2H6 0.17 0.138b

n-C3H7+n-C12H26 n-C12H25+n-C3H8 1.45

n-C4H9+n-C12H26 n-C12H25+n-C4H10 -1.02

n-C5H11+n-C12H26 n-C12H25+n-C5H12 -0.37

n-C6H13+n-C12H26 n-C12H25+n-C6H14 2.90

References: a[202],

b[184]

1: CH3(CH2)10CH2,

2: CH3(CH2)7CH(CH2)2CH3,

3: CH3(CH2)9CHCH3

164

In this study, temperature-dependent NVT ReaxFF MD simulations were performed for

temperatures ranging from 1500 to 1900 K at a time step of 0.2 fs for a total simulation time of 1.0 ns.

Figure 6-3 shows the time evolution of the conversion rates of n-C12H26 with a FGS, a Pt-cluster, and

a Pt@FGS. During the simulation time, conversion rates of the pure fuel were increased from 0.83 to

45 % with increasing temperature. When the FGS was present in the fuel, conversion rates was raised

from 2.1 to 39.28 %. When the Pt-cluster and Pt@FGS were present in the fuel, the decomposition

rates accelerated even more over the temperature range studied. At 1500 K, the conversion rates in

the presence of Pt@FGS (22.9 %) were increased by nearly a factor of 2.5 compared to the Pt-cluster

(9.2 %). A similar trend in conversion rates during simulations was observed for Pt-cluster and

Pt@FGS. With increasing temperature, the cracking rates seem to be accelerated, and the reaction

approaches complete conversion during the simulation time. As the percent conversion becomes high,

the conversion rate slows due to the decrease in fuel molecules. Addition of the Pt@FGS and Pt-

cluster exhibited the promoting effect on the conversion rate of n-C12H26 over the temperature ranges

of 1500 to 1900 K, whereas the presence of FGS in the fuel showed a slight enhancement (except for

1800 K). For instance, the inclusion of FGS at 1800 K increased the conversion rate by 53 %

compared to the pure fuel. Especially, Pt@FGS was found to exhibit higher catalytic activity at lower

temperatures compare to Pt-cluster case. As illustrated in Fig. 6-3, the enhancing effect of additives

on the decomposition of the fuel is in the following order: Pt@FGS > Pt-cluster > FGS. From these

results, the decoration of Pt onto FGS can exhibit further promoting effect on the decomposition of n-

C12H26. This observation agreed well with the pyrolysis experimental results discussed in Chapter 4.

165

(a) 1500 K (b) 1550 K

(c) 1650 K (d) 1700 K

Time, ns

Co

nv

ers

ion

of

n-C

12H

26,%

0 0.2 0.4 0.6 0.8 10

20

40

60

80

100

no particle

w/ FGS

w/ Pt6-cluster

w/ Pt6@FGS

Time, ns

Co

nv

ers

ion

of

n-C

12H

26,%

0 0.2 0.4 0.6 0.8 10

20

40

60

80

100

no particle

w/ FGS

w/ Pt6-cluster

w/ Pt6@FGS

Time, ns

Co

nv

ers

ion

of

n-C

12H

26,%

0 0.2 0.4 0.6 0.8 10

20

40

60

80

100

no particle

w/ FGS

w/ Pt6-cluster

w/ Pt6@FGS

Time, ns

Co

nv

ers

ion

of

n-C

12H

26,%

0 0.2 0.4 0.6 0.8 10

20

40

60

80

100

no particle

w/ FGS

w/ Pt6-cluster

w/ Pt6@FGS

166

(e) 1800 K (f) 1900 K

Figure 6-3. Time evolution of conversion rates for n-C12H26 and n-C12H26 containing FGS, Pt-cluster,

and Pt@FGS at temperatures of 1500 K (a), 1550 K (b), 1650 K (c), 1700 K (d), 1800 K (e), and

1900 K (f) from the NVT ReaxFF simulations.

Pyrolysis simulations of the pure n-C12H26 and the n-C12H26 containing three different

additives were performed for 1.0 and 2.0 ns to investigate the effect of the additives on the product

distribution. Figure 6-4 demonstrates the product distribution observed from the ReaxFF simulations

for temperatures varying from 1500 to 1900 K. Note that radicals observed at the final simulation

time were not included in the product distribution. The first two distributions at 1500 K and 1550 K

were obtained for the simulation time of 2.0 ns. For temperatures from 1650 to 1900 K, the final

product data were collected at the simulation time of 1.0 ns. From the product distribution analysis,

the most abundant product was ethylene for all simulations, due to the predominant β-scission of the

initial fuel radicals. The species found in the simulations were hydrogen, water, C1 to C15 alkanes (C12

= fuel), C2 to C12 alkenes, C4H6, C5H6, C5H8, C6H8, C6H10, C7H12, C8H14, and some oxygenated

compounds such as CH4O, C2H4O, and C5H12O. Major species observed in the MD simulations were

Time, ns

Co

nv

ers

ion

of

n-C

12H

26,%

0 0.2 0.4 0.6 0.8 10

20

40

60

80

100

no particle

w/ FGS

w/ Pt6-cluster

w/ Pt6@FGS

Time, ns

Co

nv

ers

ion

of

n-C

12H

26,%

0 0.2 0.4 0.6 0.8 10

20

40

60

80

100no particle

w/ FGS

w/ Pt6-cluster

w/ Pt6@FGS

167

in good agreement with those from the pyrolysis experiments. For the temperature range investigated,

when the Pt-cluster and Pt@FGS were dispersed in the fuel, higher hydrogen formation was found

compared to the fuel with or without FGS. For instance, hydrogen yield at 1500 K was increased from

0 (pure fuel) to approximately 5 % by adding Pt@FGS in the fuel. In particular, the case with

Pt@FGS exhibited a higher hydrogen yield from n-C12H26 decomposition than the case with only Pt

cluster. It was believed that FGS can serve as a substrate for well-distributed catalytic materials like

Pt nanoparticles. A combination role of Pt and FGS may offer enhanced activity on the fuel

consumption relative to the case with only the Pt cluster. Consistent with the experimental results,

formation of low-carbon-number products, such as the major species shown in Fig. 6-4, was increased

by adding Pt@FGS in the fuel compared to the cases with FGS and the pure fuel. In addition to the

major products, generation of C2-C15 alkanes (missing C12) and C5-C12 alkenes was observed to

increase in the presence of Pt@FGS in the fuel. As a minor species, the water molecule was observed

to form in the presence of FGS and Pt@FGS due to the participation of the hydroxyl radical (OH) in

the fuel decomposition. Some oxygenated compounds such as acetaldehyde (C2H4O) were also found

for both cases, indicating that the oxygen-functional groups that decorate FGS reacted with the fuel

molecules or their derivatives. The observation of minor species including C4H6, C5H6, C5H8, C6H8,

C6H10, C7H12, and C8H14, can be related to the measurements of branched and cyclic species from the

pyrolysis experiments. These minor species can also be involved in the secondary reactions to form

branched and cyclic species. Unlike the experimental observation under supercritical conditions, a

series of 1-alkenes was a major group of the products in the simulations. This clear difference in the

product distribution between the experiments and simulations is due to the much higher temperatures

of the simulations, which lead to β-scission rather than recombination of the small hydrocarbon

radicals to form alkanes. Furthermore, the simulation time may be too short for these radicals to

recombine. While the density of the fuel molecules (0.31 g/cm3) was close to the experimental

condition, gas-phase reactions seemed to be favored during the simulations. However, the initiation

168

mechanisms found in the simulations agreed well with the mechanisms suggested by the theory in the

literature [25], [123], [128], [130] for the supercritical pyrolysis of n-alkanes, which is where the

ReaxFF MD simulations are most valuable in the present analysis.

169

(a) major species

(b) minor species


containing FGS, Pt-cluster, and Pt@FGS at a temperature of 1500 K.

0

2

4

6

8

10

12

14

16

18

20

H2 CH4 C2H4 C3H6 C4H8

Mo

le F

ract

ion

, %

No particle

FGS

Pt-cluster

Pt@FGS

0

0.5

1

1.5

2

2.5

Mole

Fra

ctio

n, %

No particle FGS

Pt-cluster Pt@FGS

170

(a) major species

(b) minor species


containing a FGS, a Pt-cluster, and a Pt@FGS at a temperature of 1550 K.

0

5

10

15

20

25


Mo

le F

ract

ion

s, %

No Paritlce

FGS

Pt-cluster

Pt@FGS

0

0.5

1

1.5

2

2.5

3

Mole

Fra

ctio

ns,

%

No Paritlce FGS

Pt-cluster Pt@FGS

171

(a) major species

(b) minor species



0

5

10

15

20

25

30


Mo

le F

ract

ion

s, %

No particle FGS

Pt-cluster Pt@FGS

0

0.5

1

1.5

2

2.5

Mole

Fra

ctio

ns,

%

No particle FGS

Pt-cluster Pt@FGS

172

(c) major species

(b) minor species



0

5

10

15

20

25

30


Mo

le F

ract

ion

s, %

No particle FGS

Pt-cluster Pt@FGS

0

0.5

1

1.5

2

2.5

C2

H6

C3

H4

C3

H8

C4

H6

C4

H1

0

C5

H1

0

C5

H1

2

C6

H1

0

C6

H1

2

C6

H1

4

C7

H1

4

C7

H1

6

C8

H1

6

C8

H1

8

C9

H1

8

C9

H2

0

C1

0…

C1

0…

C1

1…

C1

1…

C1

1…

C1

2…

C1

2…

C1

5…

H2

O

C5

H…

Mole

Fra

ctio

ns,

%

No particle FGS Pt-cluster Pt@FGS

173

(a) major species

(b) minor species



0

5

10

15

20

25

30

35

40


Mo

le F

ract

ion

s, %

no particle FGS

Pt_cluster Pt@graphene

0

0.5

1

1.5

2

2.5

Mole

Fra

ctio

ns,

%

no particle FGS


174

(a) major species

(b) minor species



0

5

10

15

20

25

30

35

40

45


Mo

le F

ract

ion

s, %

no particle FGS


0

0.5

1

1.5

2

2.5

3

3.5

4

Mole

Fra

ctio

ns,

%

no particle FGS


175

The kinetics of pyrolysis for a pure fuel and fuel/particle suspension with a FGS, a Pt-cluster,

and a Pt@FGS over the temperature range from 1500 K to 1900 K was investigated using NVT

simulations. Figure 6-10 shows the Arrhenius plots of reaction rates for the pure fuel and fuel

mixtures with the FGS, Pt-cluster, and Pt@FGS. For particles suspended in the hydrocarbon fuel

suspended, decomposition rates were observed to be faster than the pure fuel, which is in agreement

with the experimental results. Pt@FGS resulted in the largest reduction in activation energy by

approximately 52.0 % because the Pt cluster was assisted in catalyzing the fuel pyrolysis by the FGS

as a substrate. The global activation energies were observed in the following order: pure fuel (281

kJ/mol) > FGS (226.24 kJ/mol) > Pt-cluster (173.9 kJ/mol) > Pt@FGS (135.4 kJ/mol). The effect of

the additive concentration was also investigated using two different concentration ratios of the

particle and fuel mixtures at a fixed density of 0.31 g/cm3 (one Pt@FGS molecule in 24 or 48 n-

C12H26 molecules). As shown in Fig. 6-10, the catalytic activity of Pt@FGS on the global activation

energy at the low concentration was reduced by 21.9 % relative to the high concentration, indicating

that there exists a concentration effect on the fuel decomposition. Although the concentration was

reduced by almost a factor of two, the promoting effect of Pt@FGS was observed to be higher than

the case with the Pt-cluster. Figure 6-11 presents the product selectivities for the two concentrations.

No observable difference in initiation mechanisms and product speciation (shown in Fig. 6-11) was

found for the two loading concentrations. A comparison of the kinetic parameters between the

experiments and simulations are summarized in Table 6-2. For the pure fuel, the pre-exponential

factor and activation energies determined from the simulations agreed well with the experimental

values found in the current study. In the simulations, reaction rates in the presence of FGS and

Pt@FGS were overestimated due to the higher particle concentration than the experimental value.

Consequently, the addition of Pt@FGS was found to lower the activation energy for decomposition of

the fuel, which is in good agreement with the experimental results.

176

Figure 6-10. Comparison of kinetics for pyrolysis of n-C12H26 and n-C12H26 containing various

additives at a fixed density of 0.31 g/cm3. (low concentration: one Pt@FGS molecule in 48 fuel

molecules, high concentration: one Pt@FGS molecule in 24 fuel molecules)

1000/T, 1/K

lnk

,1

/se

c

0.5 0.55 0.6 0.65 0.714

16

18

20

22

no particle

w/ FGS

w/ Pt6-cluster

w/ Pt6@FGS (high concentration)

w/ Pt6@FGS (low concentration)

177

Figure 6-11. Comparison of selectivities for n-C12H26 pyrolysis for the two different loading

concentration of Pt@FGS. (low concentration: one Pt@FGS molecule in 48 fuel molecules, high

concentration: one Pt@FGS molecule in 24 fuel molecules)

Table 6-2. Kinetic parameters for decomposition of n-C12H26 and n-C12H26 containing various

particles.

Additives Experiment MD Simulations

Ea log A Ea log A

Pt@FGS 239.8 15.1 135.4

a 13.0

a

165.0b 13.8

b

Pt-cluster 173.9 14.0

FGS 266.0 16.2 226.2 15.0

No particle 278.6 17.0 281.0 16.5

Units: Ea (kJ/mol), log A (1/sec) a: 1 molecule of particle in 24 molecules of the fuel

b: 1 molecule of particle in 48 molecules of the fuel

0

5

10

15

20

25

30

35

40

45

Sel

etiv

itie

s, %

low concentration high concentration

178

Figures 6-12 and 6-13 illustrate the plausible initiation mechanisms for the fuel

decomposition catalyzed by the combination of Pt and FGS surfaces. As presented in the previous

works [111], [214], functional groups of hydroxides and epoxides on the graphene surface can act as

active sites for catalyzing reactions, for example, decomposition or oxidative reactions of the

hydrocarbon fuels and propellants. In addition to the catalytic activity of FGS, Pt is known as a

common catalyst for many applications. Generally, Pt cluster or Pt-supported catalysts are known to

accelerate dehydrogenation of hydrocarbons to from hydrogen [215]–[217]. Based on these

characteristics, possible initiation mechanisms for the catalytic decomposition of the n-C12H26 studied

here are proposed in Figs. 6-12 and 6-13. Figure 6-12 illustrates the participation of Pt as a catalyst in

the initiation mechanism through dehydrogenation of n-C12H26 forming n-C12H25 and an H atom

absorbed onto the Pt surface (Scheme 1). In Scheme 2 of Fig. 6-12, the hydrogen atom, absorbed onto

the Pt surface from the hydrogenation process of Scheme 1, can interact with the fuel molecules and

then abstract an H-atom from the fuel forming a hydrogen molecule and n-C12H25. In Scheme 3 of Fig.

6-12, the hydrogen atom on the Pt surface may also participate in hydrogenation of the fuel forming

n-C12H27, which is an alkanium ion [199]. The alkanium ion is highly energetic species, so it rapidly

decomposes to a smaller hydrocarbon radial and n-alkane product via C-C bond cleavage [199]. In

addition, C-H bond scission of the alkanium ion can result in formation of a hydrogen molecule and

secondary radical [199]. In Fig. 6-13, the plausible initiation mechanisms involving the oxygen-

functional groups on the FGS that includes hydroxides and epoxides are illustrated. In Scheme 5, the

fuel molecule can undergo protonation or hydrogenation so that the alkanium ion is generated like the

mechanism described in Scheme 3. Similarly, hydroxide on FGS donates its hydrogen atom to the

fuel molecule, enabling hydrogenation to form the alkanium ion. This energetic ion subsequently

experiences H2 elimination with formation of n-C12H25, as illustrated in Scheme 6. Scheme 7 also

illustrates dehydrogenation of the fuel molecule by an epoxide, which is one of the oxygen-functional

groups on the FGS. In this study, the edge of the FGS was terminated with hydrogen atoms and an

179

oxygen atom. This oxygen atom can also dehydrogenate the fuel molecule to form n-C12H25. These

functional groups on the surface or around the edges can participate in the dehydrogenation of the

fuel.

Figure 6-12. Plausible initiation mechanisms for n-C12H26 decomposition catalyzed by Pt atoms on

FGS.

Pt O H

n-C12H26 n-C12H25 n-C12H26 n-C12H27

n-C12H26 n-C12H25 + H2n-C12H26 n-CxH2x+1 + n-CyH2y+1

H adsorption

Graphene

Scheme 1

Scheme 2

Scheme 3

H desorption

n-C12H26 adsorption

Scheme 4

180

Figure 6-13. Possible initiation mechanisms for n-C12H26 decomposition catalyzed by oxygen-

functional groups on Pt@FGS.

The initiation reactions observed during the ReaxFF MD simulations can be classified into

four categories including C-C bond cleavage, dehydrogenation, hydrogenation, and H-abstraction. In

this study, H-abstraction can be considered as the secondary initiation mechanism because this

reaction requires radicals that need to be generated from the first three initiations steps. Figure 6-14

shows a comparison of initiation mechanisms for each particle at different temperatures. Note that the

number of reaction steps counted were averaged from at least ten simulations for each condition, so

the height of the bars in Fig. 6-14 also represent the conversion rates of the fuel. The portioning of the

bars in Fig. 6-14 corresponds to the contribution of each individual initiation step to the total

conversion of the fuel. At the lower temperatures of 1500 K, 1550 K, and 1650 K in Fig. 6-14 (a)

through (c), dehydrogenation was observed as one of the major initiation mechanisms resulting from

the catalytic interactions between the fuel molecules and Pt@FGS. Followed by the dehydrogenation,

Pt O H

n-C12H26 n-C12H27

n-C12H26 n-C12H25 + H2

Graphene

Scheme 6

Scheme 5

n-C12H26 n-C12H25

H adsorption

Scheme 7

Scheme 8

n-C12H26

n-C12H25

181

H-abstraction became another important step as radical pools, begin to form from the

dehydrogenation step. For the pure fuel at 1500 and 1550 K, C-C bond cleavage was difficult to occur

due to its high activation barrier, greater than 80 kcal/mol. At the lowest temperature, catalytic

dehydrogenation by Pt@FGS, therefore, became dominant, and once the radical pools formed, bi-

molecular reactions such as H-abstraction having much lower activation barrier (shown in Table 6-1)

quickly introduce secondary steps. As demonstrated in Fig. 6-14, FGS at the lower temperatures may

serve as a catalyst to promote C-C bond cleavage or dehydrogenation. Frequent interactions between

the fuel molecules and FGS, observed during the simulations, may cause the weak C-C bond of the

fuel molecule to break. Moreover, in a few cases, dehydrogenation of the fuel molecules was found to

occur by losing their hydrogen atoms to the oxygen-functional groups on the FGS. Since the number

of these functional groups was limited to only two in this simulation, this process was rarely observed.

Even at the lowest temperature, hydrogenation can accelerate fuel decomposition in the presence of

Pt-cluster and Pt@FGS. Smaller hydrocarbon radicals, for example C2H5 or C3H7, underwent C-H

bond cleavage generating free hydrogen atoms. The free hydrogen atoms hydrogenated the fuel

molecule to the alkanium ions. In some cases, free hydrogen atoms can be donated from Pt@FGS to

the fuel molecules as proposed in Schemes 3 and 5 of Figs 6-12 and 6-14. With an increase in

temperature, the dehydrogenation step competed with C-C bond cleavage and H-abstraction as the

initiation reactions. This observation was quite reasonable because C-C bond scission as the initiation

mechanism becomes noticeable for the pure fuel at higher temperatures. In addition, more energetic

radicals in the reaction pool are generated by the C-C bond cleavage that enables H-abstraction

reactions to occur. In the simulations, the fuel molecules were observed to absorb onto the surface of

the particles and to subsequently desorb from the particles. During this reaction, some of the fuel

molecules underwent C-C bond cleavage on the weak bond of the fuel molecule due to the adsorption

onto Pt@FGS and left their small fractures behind on the particles.

182

(a) T=1500 K and t=2.0 ns

(b) T=1550 K and t=2.0 ns

0

1

2

3

4

5

6

7

8

9

No Particle FGS Pt-Cluster Pt@FGS

Aver

aged

Num

ber

of

Init

iati

on

Rea

ctio

ns

Hydrogenation

H-abstraction

C-C scission

Dehydrogenation

0

2

4

6

8

10

12


Aver

aged

Num

ber

of

Init

iati

on

Rea

ctio

ns

Hydrogenation

H-abstraction

C-C scission

Dehydrogenation

183

(c) T=1650 K and t=1.0 ns

(d) T=1700 K and t=1.0 ns

0

1

2

3

4

5

6

7

8

9

10


Aver

aged

Num

ber

of

Init

iati

on

Rea

ctio

ns

Hydrogenation

H-abstraction

C-C scission

Dehydrogenation

0

2

4

6

8

10

12


Aver

aged

Num

ber

of

Init

iati

on

Rea

ctio

ns

Hydrogenation

H-abstraction

C-C scission

Dehydrogenation

184

(e) T=1800 K and t=1.0 ns

(f) T=1900 K and t=1.0 ns

Figure 6-14. Comparison of initiation mechanisms for pyrolysis of n-C12H26 without or with various

additives for temperatures ranging from 1500 to 1900 K at a fixed density of 0.31 g/cm3. (a) 1500 K

and t=2.0 ns, (b) 1550 K and t=2.0 ns, (c) 1650 K and t=1.0 ns, (d) 1700 K and t=1.0 ns, (e) 1800 K

and t=1.0 ns, and (f) 1900 K and t=1.0 ns.

0

2

4

6

8

10

12

14

16

18


Aver

aged

Num

ber

of

Init

iati

on

Rea

ctio

ns

Hydrogenation

H-abstraction

C-C scission

Dehydrogenation

0

5

10

15

20

25


Aver

aged

Num

ber

of

Init

iati

on

Rea

ctio

ns

Hydrogenation

H-abstraction

C-C scission

Dehydrogenation

185

Figures 6-15 to 18 show some examples of the catalytic initiation steps observed from the

ReaxFF simulations. First, as one of the most important reaction steps at lower temperatures, catalytic

dehydrogenation is depicted in Fig. 6-15. Here, a fuel molecule was dehydrogenated by losing its

hydrogen atom to the Pt pinned to the FGS to form n-C12H25, which was short-lived and subsequently

underwent β-scission, where the products were 1-alkanes and small hydrocarbon radicals. It was also

found that when dehydrogenation occurred, the fuel molecule instantly absorbed onto the Pt atom.

This adsorption of the fuel molecule onto the Pt@FGS may lower the energy barrier for C-H bond

cleavage of the fuel. In Fig. 6-16, catalytic dehydrogenation by oxygen-functional groups on the FGS

is illustrated. These functional groups around the edge of FGS can dehydrogenate the fuel molecule

as does Pt catalysts. This oxidative dehydrogenation by the oxygen-functional functional groups on

the carbon-based catalysts is in good agreement with the previous simulations [176], [182], [183].

Similar to the dehydrogenation by Pt catalysts, the n-C12H25 radical was formed, which rapidly

decomposed to small hydrocarbon radicals and an alkene via β-scission. Another important initiation

step found in the simulations was hydrogenation of the fuel molecule resulting in the formation of

alkanium ions as a free hydrogen atom is donated to the fuel molecule. The free hydrogen atom

results mainly from the surface of Pt@FGS or C-H bond scission of hydrocarbon radicals, for

example, C2H5. As illustrated in Fig. 6-17, the alkanium ion, n-C12H27, quickly undergoes C-C bond

scission generating n-C7H16 and n-C5H11. As discussed earlier regarding the product distribution, the

water molecule was formed by interaction between the fuel and hydroxyl radical, which dissociated

from the surface of the FGS (Fig. 6-18). This hydroxyl radical, OH, can also participate in catalytic

dehydrogenation of the fuel molecules such that oxygen-functional groups dissociated from the

surface of the FGS can accelerate decomposition of the fuel.

186

Figure 6-15. An example of catalytic dehydrogenation by Pt@FGS observed during simulations.

Figure 6-16. An example of catalytic dehydrogenation by oxygen-functional groups on Pt@FGS.

Figure 6-17. An example of catalytic hydrogenation by a free hydrogen atom and subsequent C-C

bond scission to form n-heptane and n-C5H11.

187

Figure 6-18. Interactions between a hydroxyl radical (dissociated from Pt@FGS) and a fuel molecule

to form water molecule.

Figure 6-19 illustrates an example of sequences for the recovery of the Pt surface by

generating hydrogen molecules from interactions between hydrogen atoms absorbed on Pt. Absorbed

H atoms were easily observed to migrate on the surface of Pt. During migration, the interaction

between H atoms may form molecular hydrogen, which can easily desorb from the Pt surface. This

process may also help to recover a clean Pt catalyst surface. In the aforementioned results, it was

found that the Pt@FGS resulted in a higher conversion rate and higher hydrogen yield than did the Pt-

cluster. This simulation result may be explained by the fact that hydrogen atoms may roll over to the

FGS surface and then form the hydrogen molecule, which can separate from the FGS (referred to as

hydrogen spillover). Recently, Psofogiannakis and Froudakis [171], [172] found that H2 dissociates

quickly from a Pt-cluster, but migration of H atoms dissociated from the Pt surface to the graphite

surface must overcome a large energy barrier greater than 60 kcal/mol. As a result of this high energy

barrier, they argued that hydrogen spillover would be difficult to occur. While hydrogen spillover

could still be a plausible mechanism explaining why the Pt@FGS could result in a higher hydrogen

yield than Pt-cluster case, it was not observed in the ReaxFF MD simulations at the current conditions.

188

Figure 6-19. An example of H2 formation from the surface of Pt@FGS and the recovery of Pt cluster

on the FGS.

Figure 6-20 presents the final configurations of the Pt@FGS at different temperatures for a

1.0 ns simulation time. As shown, cracked hydrocarbons are bound to the surface, which can be

considered the initiation of coke deposition onto the Pt@FGS. This deposition of coke forming

hydrocarbons can reduce in the catalytic activity of Pt@FGS. Moreover, the breakdown of Pt

catalysts was found in the final configuration, which was typically observed during the interaction

with the hydrocarbons. While the bonding energies between Pt and FGS are relatively weak, Pt@FGS

tested here exhibited thermal stability at high temperatures. Some of the final configurations indicated

structural distortions and strain in the carbon network of Pt@FGS due to thermal stress. In addition,

an oxygen atom from the edge of FGS migrated to the surface of Pt.

189

(a) 1500 K and 2 ns

(b) 1700 K and 1 ns

(c) 1900 K and 1 ns

Figure 6-20. Final configurations of Pt@FGS for three different temperatures of (a) 1500 K and 2 ns,

(b) 1700 K and 1 ns, and (c) 1900 K and 1 ns.

190

Reaction energies and barriers for the major initiation mechanisms were studied using the

ReaxFF-NEB method. This ReaxFF-NEB simulation allows one to answer why the composite of Pt

and FGS provided higher conversion rates of n-C12H26 pyrolysis and combustion in the experiments

and simulations than either the Pt-cluster or FGS alone. Figure 6-21 presents a comparison of reaction

energies and barriers for the dehydrogenations between the Pt-cluster and Pt@FGS. For the case of

the Pt-cluster, the activation energy barrier and reaction energy for dehydrogenation were determined

to be 38.5 and 32.6 kcal/mol, respectively. On the other hand, when Pt@FGS participated in the

dehydrogenation process, the activation energy barrier and reaction energy significantly decreased to

9.4 and 11.3 kcal/mol, respectively. The reaction energies for dehydrogenation catalyzed by both

particles are much lower than C-H and C-C bond scission of >90 kcal/mol. This reduction in the

activation barrier is likely to alter the initiation mechanism from C-C or C-H bond cleavages to

dehydrogenation. In Fig. 6-22, the reaction energy and barrier for dehydrogenation catalyzed by FGS

are analyzed. In the simulations, the fuel molecules underwent the dehydrogenation process forming

the n-C12H25 radical. As seen, this process by FGS is energetically more favorable than that by Pt-

cluster or Pt@FGS. From the analysis of the initiation mechanisms by FGS in Fig. 6-14, this reaction

did not seem to be favored due to the limitation of the number of oxygen-functional groups in the

current study. If the number of these functional groups increases with increasing dimensions of FGS,

dehydrogenation onto FGS could be facilitated.

191

Figure 6-21. ReaxFF-NEB simulation of reaction and barrier energies for catalytic dehydrogenation

for the Pt-cluster and Pt@FGS cases.

Figure 6-22. ReaxFF-NEB simulation of reaction and barrier energy for catalytic dehydrogenation by

an oxygen-functional group on the FGS.

Number of Images

Re

lati

ve

En

erg

y,k

ca

l/m

ol

0 1 2 3 4 5-60

-40

-20

0

20

40

60

80

100Pt + n-C

12H

26--> H-Pt + n-C

12H

25

Pt@FGS + n-C12

H26

--> H-Pt@FGS + n-C12

H25

IS TS FS

IS TS FS

Ea=11.3 kcal/mol

Er=9.4 kcal/mol

Ea=38.5 kcal/mol

Er=32.6 kcal/mol

Number of Images

Re

lati

ve

En

erg

y,k

ca

l/m

ol

0 1 2 3 4 5 6 7 8-20

-10

0

10

20

30

40FGS + n-C

12H

26--> H-FGS + n-C

12H

25

Ea=9.99 kcal/mol

Er=-11.9 kcal/mol

TS

IS

FS

192

6.3 Summary of ReaxFF MD Simulations

ReaxFF MD simulations were performed to investigate the effects of the particulate additives

to hydrocarbon fuel on the initial reaction mechanisms, kinetics, and product distribution at different

temperatures. In addition, reaction energies and barriers for the specific initiation mechanisms were

estimated using the ReaxFF-NEB. The ReaxFF simulations indicated that the addition of Pt@FGS

can accelerate the cracking of the fuel. Based on the analysis of the kinetics, Pt@FGS exhibited a

much lower activation energy of decomposition of the fuel than that of the pure fuel, FGS, and Pt-

cluster. Consistent with the experimental results, hydrogen yield in the presence of Pt@FGS was

increased with rising temperatures. In addition, low-carbon-number products such as C2H4, C3H6, and

C4H8 were likely to form more often by addition of Pt@FGS than the other additives. Analysis of the

initial reaction mechanisms confirmed that Pt catalysts supported with or without FGS enhanced

dehydrogenation, which is deduced to be one of the major catalytic mechanisms. Followed by

dehydrogenation, H-abstraction became another important step as radical pools developed. Somewhat

less important, hydrogenation of the fuel molecule resulting in alkanium ion was observed. The free

hydrogen atom is produced from the surface of the Pt@FGS or from C-H bond scission of

hydrocarbon radicals (for example, C2H5). Hydroxyl radical dissociation from the FGS can participate

in dehydrogenation of the fuel forming water molecules. The recovery of the Pt cluster on the FGS

was observed, where the mechanism was directly related to the dissociation of the H2 molecule from

the surface of the catalyst. In the final configuration of Pt@FGS, precursors to coke formation were

observed to deposit on the surface of the particle as well as to contribute to the breakdown of the

particles. The ReaxFF-NEB simulations demonstrated that Pt@FGS lowered the reaction energy and

barrier for dehydrogenation considerably more than a Pt cluster alone. This reduction in the activation

energy may provide further enhancement on the conversion rate of the fuel in the presence of

Pt@FGS. In summary, the ReaxFF simulations have shed significant light on the enhancing

mechanisms provided by the presence of Pt@FGS on the fuel conversion process.

193

Chapter 7

Summary of Research and Recommendations for Future Work

7.1 Summary

This study explored experimentally and theoretically the role of dispersing colloidal

nanostructured materials, while investigating the multifunctionality of these particles with respect to

enhancement of fuel decomposition, injection, ignition, and combustion of hydrocarbon

fuels/propellants. The research presented here covers a range of fundamental and applied

investigations, from molecular dynamics simulations which aid in understanding potential reaction

mechanisms, flow reactor experiments which elucidate particle effects on the pyrolysis, and small

scale combustion experiments, during which ignition delay, flame spreading angle, and conversion

efficiency were studied.

The effects of colloidal FGS-based additives on the decomposition of methylcyclohexane

(MCH) and n-dodecane (n-C12H26) were investigated under near-critical or supercritical conditions in

a high pressure flow reactor designed to provide isothermal and isobaric flow conditions. Conversion

rates and major product yields generated during the fuel cracking were analyzed as a function of

temperature, residence time, and particle type. Supercritical pyrolysis results showed that the addition

of FGS-based particles, at a concentration of 50 ppmw, enhanced conversion rates and increased

major gaseous product yields during fuel decomposition. For example, conversion rates as well as

generation of C1-C5 n-alkanes and C2-C6 1-alkenes were significantly increased by 43.5 %, 59.1 %,

and 50.0 % for MCH decomposition using FGS19 (50 ppmw) at a temperature of 820 K and reduced

pressure of 1.36. According to first-order reaction kinetics, the apparent activation energy for MCH

decomposition was lowered by 10 % in the presence of FGS. It was also found that the introduction

194

of FGS in the fuel significantly changed the potential reaction mechanisms for toluene, cyclohexene,

dimethylcyclopentane, and hydrogen formation. Toluene selectivities were observed to increase from

0 to 13 % at low conversions and by 15 % at high conversions when the FGS was present in the fuel.

Suspension of 50 ppmw Pt@FGS (equating 10 ppmw Pt in the total mixture) further increased the

conversion rate of MCH from 20.6 to 24.5 % at 800 K, suggesting that even at the low loading, the

well-dispersed Pt nanoparticles can assist in catalyzing supercritical MCH pyrolysis.

Polyoxometalates-decorated FGS (POM@FGS) also exhibited an enhancement of MCH

decomposition by 21.6 % relative to FGS. The addition of FGS-based particles seems to alter the

initiation mechanisms, as hydrogen formation was observed to increase by a factor of nearly 2 at 800

K when Pt@FGS (50 ppmw) was dispersed in the MCH. This result implies that Pt@FGS can

facilitate dehydrogenation of MCH to form hydrogen and toluene, which could result in enhanced

endothermic heat sink capacity.

Similar to MCH pyrolysis, the inclusion of 50 ppmw Pt@FGS in the n-C12H26 enhanced

conversion rates and increased specific product yields, preferentially selecting the production of low

molecular weight species while diminishing the production of high molecular weight species.

Hydrogen production yield and selectivity were observed to increase by nearly a factor of 13 (200

ppm to 2516 ppm) at 480 oC and 4.75 MPa when 50 ppmw Pt@FGS was present in n-C12H26.

Transmission electron micrograph (TEM) analysis of Pt@FGS before and after supercritical pyrolysis

at 530 oC, 4.75 MPa and 5.0 mL/min indicated that the Pt@FGS structure did not change, and in

particular, the Pt nanoparticles attached to the FGS remained stable through the course of the reaction.

Their survival also indicates the potential to further use the Pt@FGS in the gas-phase following

injection of the fuel into a combustor. Furthermore, selectivities in product yields with and without

the graphene additives imply that the activity of the particles on the product distribution could be

controlled by the operational conditions. The results of this investigation support using colloidal

hierarchical nanocomposites for future advanced propulsion and energy conversion devices.

195

The multifunctional effects of these particle additives on the injection, ignition, and

combustion processes under supercritical conditions were also experimentally examined.

Supercritical injection and combustion experiments were performed using a high pressure and

temperature windowed combustor coupled to the flow reactor and feed system. Cold flow injection

experiments indicated the presence of a small amount of Pt@FGS particles (100 ppmw) in the fuel

led to instabilities on the jet stream, while supercritical injection studies demonstrated that the

addition of the particles influenced injection of the hydrocarbon fuels. It was found that when the

particles were dispersed in the fuels, the core lengths of the supercritical jets were reduced by up to

56.7 % (MCH) and 68.8 % (n-C12H26), depending on the flow rate. The reduction in the core lengths

of the jets may be attributed to the decrease in fuel mixture density, which resulted from increased

formation of low-carbon-number products during pyrolysis.

Combustion under supercritical conditions for hydrocarbon fuel containing various particle

additives was investigated as well. Supercritical combustion studies showed that the addition of 100

ppmw 20wt%Pt@FGS to n-C12H26 fuel reduced ignition delay times by nearly a factor of 3 (12.4 to

4.1 ms), increased spreading angles by approximately 32.0 % (15.4 to 20.3o), reduced the flame lift-

off length by 50.0 % (1.74 to 0.8 mm), and demonstrated an increase in conversion by 35.0 % relative

to the pure fuel baseline at a volumetric flow rate of 5.0 mL/min. Such enhancements benefit practical

propulsion systems which require high conversion efficiency in a short residence time.

The enhancing mechanisms introduced by addition of these colloidal nanostructured

materials on pyrolysis and combustion were investigated using theoretical and numerical simulation.

The current study performed ReaxFF molecular dynamics (MD) simulations to analyze the catalytic

initiation mechanisms, conversion rates, kinetics, and product distribution for n-C12H26 containing the

various particles. The simulation results were in good agreement with the experimental observations,

showing enhanced conversion rates and lowered activation energies in the presence of the particles.

Based on the analysis of the initial reaction mechanisms, Pt cluster supported with or without FGS

196

facilitated catalytic dehydrogenation rather than the carbon-carbon bond scission. Followed by the

initial dehydrogenation, H-abstraction participates in the decomposition as a secondary process

because radical pools were promptly generated by the dehydrogenation step. Another important

initiation step found in the simulations is hydrogenation of the fuel molecule resulting in formation of

an alkanium ion, as a free hydrogen atom is donated to the fuel molecule. Furthermore, the hydroxyl

radical, which is dissociated from FGS, can participate in dehydrogenation of a fuel molecule forming

a water molecule. The recovery of the Pt cluster on FGS was observed, resulting from dissociation of

H2 molecules from the surface of the Pt cluster. The ReaxFF program, embedded with nudged elastic

band (NEB) method simulations, showed that the bicomposite structure of Pt@FGS served to lower

the reaction barrier for dehydrogenation, which could result in the enhanced conversion rates and

increased product yields. Consequently, the ReaxFF simulations support the enhancing mechanisms

of the fuel conversion and combustion for the hydrocarbon fuels containing the particles.


either as an additive or a means of distributing another additive can significantly enhance, and be used

to tailor, the conversion of liquid hydrocarbon fuels/propellants under supercritical conditions,

resulting in reduced ignition delay and improved combustion. Such enhancements benefit practical

propulsion systems which require high conversion efficiency in a short residence time.

7.2 Recommendations for Future Work

While this research answered the multifunctional role of nanoengineered materials on the

supercritical pyrolysis, injection, ignition, and combustion processes for hydrocarbon

fuels/propellants, there are several theoretical and experimental studies that should be considered for

future research. First, additional combustion experiments should be performed where the particle

effects are isolated, to permit investigation of the sole role of the particles on the combustion. By

197

isolating the role of the particles during combustion, more confidence in the enhancing effect of the

particles can be achieved. Furthermore, ignition delays, flame spreading angles, and lift-off lengths of

the supercritical fuel containing the particles, should be analyzed using another imaging technique,

such as OH chemiluminescence. More detailed exhaust gas measurements from the combustion

chamber should be considered for future research to measure the product composition and hence

conversion efficiency.

Another recommendation is to build a detailed kinetic model which can account for the

catalytic mechanisms observed from the ReaxFF simulations. By incorporating these kinetic

mechanisms into the existing model, more confidence in the proposed initiation mechanisms can be

achieved. In addition, the comparison of the conversion rates and product distributions between the

kinetic modeling and the experimental results should be examined. Combined with the developed

kinetic model, computational fluid dynamics (CFD) simulation of the injection and combustion

should be considered for the future work. This CFD work can provide a direct comparison against the

experimental observation.

The present study demonstrated the positive effects of FGS-based particles on the pyrolysis

and combustion of single component fuels, so the effectiveness of these materials on pyrolysis and

combustion of Jet-A or RP-1, which are currently being used as jet fuel or fuel for rocket

bipropellants, should be considered as well. Future study on the particle effect on monopropellants,

such as hydroxylammonium nitrate (HAN), is also recommended. Investigation of the injection,

ignition, and combustion of cracked fuels containing the particles under supersonic combustor

conditions, should be considered. These experiments could provide insight on the use of

multifunctional energetic or reactive materials for hypersonic propulsion applications.

For future particle designs, composites with both catalysts and energetic particles (e.g.,

shell/core particles) should be considered, as well as the decoration of catalytic and energetic particles

onto FGS. Higher loadings of energetic materials (if necessary through gels) should be examined.

198

Furthermore, synthesis of mesoscopic particles with gasifying agents to further distribute the particles

in gas-phase fuel/air mixtures during combustion should be considered.

199

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VITA

Hyung Sub Sim

Combustion Research Lab., Center for Combustion, Power, and Propulsion

Mechanical Engineering, The Pennsylvania State University, University Park, PA 16802, USA

Cell Phone:+1 (814)777-8009 / E-mail: [email protected]

EDUCATION

Ph.D., Mechanical Engineering, The Pennsylvania State University (GPA: 3.97/4.0) May 2016

Advisor: Prof. Richard A. Yetter

Dissertation: Understanding the Role of Multifunctional Nanoengineered Particulate Additives on Supercritical Pyrolysis

and Combustion of Hydrocarbon Fuels/Propellants

M.S., Mechanical Engineering, Chung-Ang University, South Korea (GPA: 3.9/4.0) Feb 2008

Advisor: Prof. Seong Hyuk Lee

Thesis: Microscale Thermo-Optical Characteristics in Thin Films Irradiated by Ultrashort Pulse Lasers

B.S., Mechanical Engineering, Chung-Ang University, South Korea (GPA: 3.96/4.5) Feb 2006

ACADEMIC & WORK EXPERIENCES

Research Assistant, The Pennsylvania State University, University Park, PA, USA Jan 2011 ~ Present

Teaching Assistant, The Pennsylvania State University, University Park, PA, USA Aug 2010 ~ Dec 2010

Plant Engineer, STX Heavy Industries, Co., Ltd., Seoul, South Korea Jul 2008 ~ Jul 2010

Research Assistant, Chung-Ang University, Seoul, South Korea Mar 2006 ~ Feb 2008

Teaching Assistant, Chung-Ang University, Seoul, South Korea Mar 2006 ~ Feb 2008

HONORS AND AWARDS

Honor Scholarship, Advanced Energy Expert Development Program, Aug 2010 ~ Jul 2012

Korea Institute of Energy Technology Evaluation and Planning (KETEP)

Sigma Xi, Full Membership, The Scientific Research Society, Jan 2009 ~ Present

Outstanding Research Award, Graduate School of Chung-Ang University, Feb 2006/ Feb 2007

National Scholarship, Brain Korea 21 (BK21), Korea Research Foundation Mar 2006 ~ Feb 2008

Mechanical Promotion Society Award, 2004 Capstone-Design Fair, South Korea Dec 2008

mailto:[email protected]

UNDERSTANDING THE ROLE OF MULTIFUNCTIONAL …

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