PLASMA ASSISTED DECOMPOSITION OF METHANE AND PROPANE AND CRACKING OF LIQUID HEXADECANE A thesis submitted for the degree of Doctor of Philosophy by Irma Aleknaviciute School of Engineering and Design Brunel University, Uxbridge April, 2013 Abstract Non-thermal plasmas are considered to be very promising for the initiation of chemical reactions and a vast amount of experimental work has been dedicated to plasma assisted hydrocarbon conversion processes, which are reviewed in the fourth chapter of the thesis. However, current knowledge and experimental data available in the literature on plasma assisted liquid hydrocarbon cracking and gaseous hydrocarbon decomposition is very limited. The experimental methodology is introduced in the chapter that follows the literature review. It includes the scope and objectives section reflecting the information presented in the literature review and the rationale of this work. This is followed by a thorough description of the design and construction of the experimental plasma reformer and the precise experimental procedures, the set-up of hydrocarbon characterization equipment and the development of analytical methods. The methodology of uncertainty analysis is also described. In this work we performed experiments in attempt the cracking of liquid hexadecane into smaller liquid hydrocarbons, which was not successful. The conditions tested and the problems encountered are described in detail. In this project we performed a parametric study for methane and propane decomposition under a corona discharge for CO x free hydrogen generation. For methane and propane a series of experiments were performed for a positive corona discharge at a fixed inter-electrode distance (15 mm) to study the effects of discharge power (range of 14 - 20 W and 19 – 35 W respectively) and residence time (60 - 240 s and 60 – 303 s respectively). A second series of experiments studied the effect of inter-electrode distance on hydrogen production, with distances of 15, 20, 25, 30 and 35 mm tested. The analysis of the results shows that both discharge power and residence time, have a positive influence on gaseous hydrocarbon conversion, hydrogen selectivity and energy conversion efficiency for methane and propane decomposition. Longer discharge gaps favour hydrogen production for methane and propane decomposition. A final series of experiments on corona polarity showed that a positive discharge was preferable for methane decomposition. i Declaration The research presented in this thesis is the original work of the author except where otherwise specified, or where acknowledgements are made by references. This project was carried out at the School of Engineering and Design, Brunel University, under the supervision of Prof. T. G. Karayiannis and Prof. M. W. Collins. The work has not been submitted for another degree or award to any other institution. ii Acknowledgements It is with immense gratitude that I acknowledge the support and encouragement of Professor T. G. Karayiannis. I attribute the success of this project to his close guidance and invaluable advice throughout the project. He also sacrificed his own personal time to study chemistry. I would also like to acknowledge my second supervisor Professor M. W. Collins. His constructive suggestions and comments have greatly facilitated the writing of this thesis and the successful publications. I am grateful to the technician Mr. C. Xanthos for his support in the installation and construction of the facility and test reactor. The success of the experimental work was facilitated by his constant help and advice. To the School of Engineering and Design, Brunel University, for providing me the scholarship and the opportunity to complete this study. Special thanks to Professor L. Wrobel for enabling the project to continue. My heartfelt thanks go to my partner Mr. T. Doyle for his patience, understanding, encouragement and love throughout my PhD research. In addition, his constructive suggestions and advice have greatly facilitated the writing of this thesis. Finally I would like to express my great thanks to my dearest friend Miss. I. Desnica. It is with her encouragement and belief that I started this project, and continuous support that I completed it. iii Contents Abstract i Declaration ii Acknowledgements iii List of Figures xi List of Tables xxii Nomenclature xxvii Chapter 1: Introduction 1 1.1. Research Background 1 1.2. Project objectives 3 1.3. Overview of this thesis 4 Chapter 2: Importance and Processing of Fuels 5 2.1. Introduction: From ‘Oil Era’ to ‘Hydrogen Economy’ 5 2.2. Hydrocarbons 7 2.2.1. Composition, structure and bonding 7 2.2.2. Bond dissociation energies 8 2.3. From crude oil to gasoline 9 2.3.1. Crude oil composition 9 2.3.2. Gasoline – importance and production 10 2.3.2.1. Thermal cracking 10 2.3.2.2. Catalytic cracking 12 2.3.2.3. Problems with conventional cracking technologies 13 2.4. Fuelling the future: from hydrocarbon to hydrogen 14 2.4.1. Hydrogen Economy 14 2.4.2. Applications of hydrogen 15 iv 2.4.3. Hydrogen sources and production 16 2.5. Pyrolysis of methane: the Hydrogen-Carbon economy 20 2.5.1. Thermo-catalytic methane decomposition 20 2.5.2. The use of solid carbon 27 2.6. Propane decomposition for hydrogen production 28 2.6.1. Advantages of propane pyrolysis 28 2.6.2. Catalytic propane reforming 29 2.6.3. Pyrolysis and thermo-catalytic decomposition 30 2.7. Summary 34 Chapter 3: The Fourth State of Matter 36 3.1. Introduction 36 3.2. Plasma sources and processes 37 3.3. Thermal and non-thermal plasmas 37 3.4. Non-thermal atmospheric pressure discharges 38 3.5. Corona discharge 40 3.5.1. What is corona? 40 3.5.2. Positive pin-to-plate corona processes 41 3.5.3. Negative pin-to-plate corona processes 43 3.5.4. Multipoint and point-to-liquid electrode configurations 44 3.5.5. Important parameters reported in the literature 46 3.5.6. Corona discharge applications 48 Chapter 4: Plasma Processing of Hydrocarbons 50 4.1. Introduction 50 4.2. Conversion rate, selectivity and energy consumption 51 4.3. Plasma-assisted generation of hydrogen from methane 52 4.3.1. General theory 52 4.3.2. Syngas production by partial oxidation of methane 54 4.3.2.1. Atmospheric pressure glow discharge and plasma jet 54 v 4.3.2.2. Microwave and corona discharges 57 4.3.2.3. Arc discharges 59 4.3.2.4. Thermal plasma 62 4.3.2.5. Comparing plasma assisted partial oxidation systems 64 4.3.3. Hydrogen production by methane decomposition 66 4.3.4. Steam and autothermal reforming of methane 69 4.3.5. Comparing PO, pyrolysis and SR plasma systems 71 4.3.6. Important parameters reported in literature 73 4.3.6.1. Flow rate 73 4.3.6.2. Power input 76 4.3.6.3. Gas composition 78 4.3.6.4. Other parameters investigated 86 4.3.6.5. Summary 92 4.4. Hydrogen generation from propane 98 4.5. Plasma assisted cracking of liquid hydrocarbons 102 4.6. Plasma-catalytic systems for hydrocarbon reformation 105 4.6.1. Two stage plasma-catalyst system 106 4.6.2. Single stage plasma-catalytic system 109 4.6.3. Plasma-catalytic systems conclusions 113 4.7. Summary 113 Chapter 5: Experimental Rig 116 5.1. Introduction: rationale of this study 116 5.2. Scope and objectives 117 5.3. Plasma reactor ` 121 5.3.1. Overview 121 5.3.2. Top plate 122 5.3.3. Bottom plates 126 5.3.4. Temperature and pressure measure and control 127 vi 5.4. Experimental methodology 130 5.4.1. Liquid samples 130 5.4.2. Gaseous samples 130 5.4.3. Residence time and flow 131 5.5. Sample characterization 132 5.5.1. Overview 132 5.5.2. GC-MS instrument 132 5.5.2.1. GC-MS configuration for liquid analysis 134 5.5.2.2. Liquid hydrocarbon characterization 134 5.5.2.3. GC-MS configuration for gas analysis 137 5.5.2.4. Gaseous hydrocarbon characterization 138 5.5.3. Chromopack CP9001 GC 141 5.5.3.1. GC configuration for hydrogen measurement 141 5.5.3.2. Hydrogen quantification 142 5.5.4. Energy conversion efficiency calculations 143 5.6. Calibration and uncertainty analysis 145 5.6.1. General theory 145 5.6.2. Calibration process and error analysis 146 5.7. Summary 160 Chapter 6: Experimental results for hexadecane cracking under corona discharge 161 6.1. Introduction 161 6.2. Underlying theory 161 6.3. Experimental conditions 165 6.4. Discussion 165 6.5. Conclusions 166 vii Chapter 7: Experimental results for methane decomposition under corona discharge 168 7.1. Introduction 168 7.2. Methane decomposition under a positive corona discharge 168 7.2.1. 15 mm inter electrode distance: Effects of discharge power and residence time 170 7.2.1.1. Methane conversion 172 7.2.1.2. Hydrogen production and selectivity 174 7.2.1.3. Energy conversion efficiency 177 7.2.1.4. Other compounds generated 181 7.2.1.5. Summary 184 7.2.2. The effects of inter-electrode distance on hydrogen production 184 7.2.3. Experimental repeatability 188 7.3. Methane decomposition under a negative corona discharge 188 7.4. Summary 190 Chapter 8: Experimental results for propane decomposition under corona discharge 192 8.1. Introduction 192 8.2. 15 mm inter electrode distance: Effects of discharge power and residence time 192 8.2.1. Plasma cut-off phenomenon 195 8.2.2. Propane conversion 196 8.2.3. Hydrogen production and selectivity 198 8.2.4. Energy conversion efficiency 200 8.2.5. Other compounds generated 202 8.3. The effects of inter-electrode distance on hydrogen production 210 viii 8.3.1.