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Generation of Hydrogen-Rich Gas Using Non Equilibrium Plasma Discharges
A Thesis
Submitted to the Faculty
Of
Drexel University
By
Olufela O. Odeyemi
In partial fulfilment of the requirement for the degree
Of
Doctor of Philosophy
Mechanical Engineering
June 2012
ii
© Copyright 2012
Olufela O. Odeyemi. All rights reserved
iii
DEDICATION
This work is dedicated to my parents, Olu and Lara Odeyemi, and my brother, Tunde
Odeyemi - who thought me the value of hard work, honesty, decency, perseverance and
humility. They exemplify the belief that knowledge is most powerful when it benefits the
most vulnerable and less privileged. They have provided me with constant support,
guidance and love that have given me an undeserved advantage in life. I am grateful.
iv
ACKNOWLEDGEMENT
I would like to express my profound appreciation to my research advisor, Dr
Alexander Fridman, who gave me a chance and ensured that I have a successful
dissertation. He provided the necessary guidance, encouragement as well as a conducive
research environment while never running out of ideas on how to solve problems. I am
also grateful to Dr Alexander Rabinovich, who patiently and painstakingly provided the
daily guidance on my research and dissertation. I have benefitted immensely from his
wealth of knowledge and experience.
I want to thank my committee members, Dr Young Cho, Dr Gary Friedman, Dr
Nicholas Cernansky and Dr Greg Fridman all who have positively impacted me either in
the classroom, in the lab or during interpersonal interactions.
I am grateful entire staff and students at the A.J Drexel plasma institute for
sustaining a friendly and cordial research environment. I especially thank Dr Mikhail
Pekker for his help with thermodynamic and kinetic simulations and Dr. Anatoliy
Polevich for his help with the design and simulation of plasma reactors with Pro/E. I will
like to single out Gary Nuremberg and Ivan Chernets for their constant support and
assistance with regards to machining, fabrications and power supply issues. Kirill Gutsol
and Robert Geiger were particularly always willing and available to offer their assistance.
I also benefitted from the advice and experiences of Mike Gallagher and Thomas
Nunnally. My appreciation goes to Yelena Alekseyeva for her motherly role, jovial
attitude and her kind words of encouragement.
Lastly, I want to thank my brothers Tunde, Kunle, Lanre and Yemi Odeyemi from
their encouragement and support; and my wonderful friends – Opeyemi Akanbi, Dr.
Steve Cox, Jackie Yim, Gabe Carryon, Ciira Maina, Manuel & Danielle Figueroa, Pat
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Kirby, Nick Vacirca, Yohan Seepersad, Jonathan Campos, Marko Janko, Kamau Wright,
Dave Delaine, Lamide Lawal, Cecilia Pena, Ashley Freeman, Shariel Fonville, Dr. Fang
Zhi, Sanghee Lee, Liang Yu, Ivi Kusta, Kamau Wright and too many to mention.
vi
TABLE OF CONTENTS DEDICATION ....................................................................................................................................................................... iii
ACKNOWLEDGEMENT ....................................................................................................................................................... iv
LIST OF FIGURES ................................................................................................................................................................. ix
LIST OF TABLES ................................................................................................................................................................. xii
1.0 INTRODUCTION TO PLASMA STIMULATED HYDROCARBON REFORMING AND
HYDROGEN PRODUCTION ................................................................................................................................................. 1
Concept of Plasma ............................................................................................................ 1 1.1
1.1.1 Natural Plasma .................................................................................................................. 3
1.1.2 Artificial Plasma ............................................................................................................... 3
Plasma Classifications ...................................................................................................... 5 1.2
1.2.1 Thermal Plasma ................................................................................................................ 5
1.2.2 Cold Plasma ...................................................................................................................... 6
1.2.3 Warm Plasma .................................................................................................................... 7
Types of Plasma Gas Discharges ...................................................................................... 8 1.3
1.3.1 Thermal Arc Plasma Discharge ........................................................................................ 8
1.3.2 Gliding Arc Plasma ........................................................................................................ 10
1.3.3 Dielectric Barrier Discharge ........................................................................................... 12
1.3.4 Corona Discharge ........................................................................................................... 14
1.3.5 Glow Discharge .............................................................................................................. 14
Fuel Reforming Chemistry ............................................................................................. 15 1.4
1.4.1 Fuel Combustion............................................................................................................. 15
1.4.2 Partial Oxidation ............................................................................................................. 16
1.4.3 Dry CO2 reforming ......................................................................................................... 16
1.4.4 Steam reforming ............................................................................................................. 17
1.4.5 Auto-Thermal Reforming ............................................................................................... 18
1.4.6 Pyrolysis ......................................................................................................................... 18
1.4.7 Gasification ..................................................................................................................... 18
Non - Equilibrium Plasma Fuel Reforming and Hydrogen Production .......................... 19 1.5
1.5.1 Plasma Reforming of Light, Gaseous Hydrocarbons (Methane) into Synthesis Gas ..... 20
vii
1.5.2 Plasma reforming of liquid hydrocarbons into synthesis gas.......................................... 22
Advantages of Plasma catalysis for hydrocarbon reforming .......................................... 24 1.6
1.6.1 Minimal soot formation .................................................................................................. 24
1.6.2 Preheating Not Necessary (Quick start) .......................................................................... 25
1.6.3 Insensitivity to Contaminants ......................................................................................... 25
1.6.4 Size and Compactness of Plasma Reactors ..................................................................... 26
Comparison of Plasma Systems and Other Engineering Solutions for Fuel Reforming . 26 1.7
1.7.1 Thermal Cracking ........................................................................................................... 26
1.7.2 Catalytic Cracking .......................................................................................................... 27
1.7.3 Other Plasma Systems .................................................................................................... 28
Conclusion ...................................................................................................................... 29 1.8
Introduction to Gliding Arc Reformer Technology ........................................................ 30 2.1
Low Current Gliding Arc in Vortex Flow Reactor Designs ........................................... 33 2.2
2.2.1 Introduction .................................................................................................................... 33
2.2.2 Gliding Arc Plasmatron .................................................................................................. 37
Characterization of gliding arc discharge power supplies .............................................. 44 2.3
2.3.1 Introduction .................................................................................................................... 44
2.3.2 Direct current power supply ........................................................................................... 45
2.3.3 Alternating current power supply ................................................................................... 45
Introduction .................................................................................................................... 46 3.1
Pyrogas ........................................................................................................................... 46 3.2
Thermodynamic analysis of Pyrogas reforming reactions .............................................. 50 3.3
Thermodynamic analysis of Plasma assisted Tar removal.............................................. 53 3.4
Experimental Investigation of Pyrogas Reforming With Non Equilibrium Plasma ....... 61 3.5
3.5.1 Experimental Set up ........................................................................................................ 61
3.5.2 Gas detection and Gas chromatography ......................................................................... 65
3.5.3 Voltage - Current Characteristics ................................................................................... 65
Evaluation of the experimental investigation of plasma assisted pyrogas reforming ..... 68 3.6
3.6.1 Plasma - Steam reforming .............................................................................................. 68
viii
3.6.2 Plasma - Dry CO2 reforming .......................................................................................... 69
3.6.3 Hydrocarbon conversion ................................................................................................. 75
3.6.4 Effect of size of the electrodes on hydrocarbon conversion ........................................... 76
3.6.5 Effect of preheating pyrogas on hydrocarbon conversion .............................................. 78
Conclusion ...................................................................................................................... 80 3.7
Introduction .................................................................................................................... 81 4.1
Carbon suboxide ............................................................................................................. 83 4.2
4.2.1 Background ..................................................................................................................... 83
4.2.2 Properties of carbon suboxide ........................................................................................ 84
4.2.3 Thermodynamic analysis of carbon suboxide ................................................................. 85
Thermodynamic calculation of the efficiency of hydrogen and carbon suboxide 4.3
production from various hydrocarbon feedstocks ..................................................................................... 88
Evaluation of Experimental Investigation of Plasma Assisted Hydrocarbon Conversion 4.4
Into Carbon Suboxide Via Partial Oxidation Reaction ............................................................................. 93
4.4.1 Experimental set up ........................................................................................................ 93
4.4.2 Role of Dielectric Barrier Discharge .............................................................................. 95
4.4.3 Voltage – Current Characteristics of DBD discharge ..................................................... 96
4.4.4 Reaction mechanism of n-Butane conversion to carbon suboxide ................................. 98
Evaluation of non-equilibrium plasma reforming of hydrocarbons for carbon suboxide 4.5
production 99
4.5.1 Experimental results and discussion ............................................................................... 99
4.5.2 Product Characterization .............................................................................................. 101
4.5.3 Characterization Procedure ........................................................................................... 101
Conclusion .................................................................................................................... 105 4.6
5.0 Conclusions and Summary .......................................................................................................................................... 107
REFERENCES ..................................................................................................................................................................... 109
APPENDIX .......................................................................................................................................................................... 118
VITAE .................................................................................................................................................................................. 123
ix
LIST OF FIGURES
Figure 1-1: Temperature of different plasmas with their equivalent electron density range [1]. .................... 2
Figure 1-2: Process of ionization. Electrons e- collides with neutral atoms 'Θ', knocking off electrons e- and
forming ions ‘+’ in a cascading effect. ............................................................................................................ 4
Figure 1-3: Typical example of the erosion of the stainless steel electrodes of a thermal arc ......................... 9
Figure 1-4: Thermal arc discharge used for welding purposes (courtesy Drexel university machine shop) . 10
Figure 1-5: The different stages of gliding arc progression from a simple spark to the non- equilibrium
phase. A – Gas breakdown stage; B – Quasi equilibrium phase; C – Non equilibrium phase [1] ................ 11
Figure 1-6: The planar configuration of a Dielectric Barrier discharge set up .............................................. 13
Figure 1-7: The cylindrical configuration of dielectric barrier discharge set up ........................................... 13
Figure 1-8: Methane conversion to syngas as a function of O: C ratio. Solid lines with circles represent the
kinetic simulation results; solid bars represent the experimental results; dotted line and bar represent kinetic
simulation and experimental results without plasma[7]. ............................................................................... 21
Figure 1-9: Electric energy cost of syngas production as a function of O:C ratio[1]. ................................... 22
Figure 1-10: Thermodynamic equilibrium and experimental energy conversion efficiency of H2 + CO, and
H2 + CO + light hydrocarbons (HCs) as a function of O/C ratio. Solid line: Equilibrium H2 + CO + light
HCs; dotted line: Equilibrium H2 + CO; ▴, H2 + CO + light HCs for RVF-GA reactor; ●, H2 + CO for RVF-
GA reactor; , H2 + CO + light HCs for GA-Plasmatron; , H2 + CO for GA-Plasmatron[49]. .................. 23
Figure 1-11: Thermodynamic equilibrium and experimental points showing product yields of H2 and CO as
a function of O/C ratio. Solid line: Equilibrium CO yield; dashed line: Equilibrium H2 Yield; ■, H2 yield
for RVF-GA reactor; ▴, CO yield for RVF-GA reactor; ♦, H2 yield for GA-Plasmatron reactor; ×, CO yield
for GA-Plasmatron reactor [49]. .................................................................................................................... 24
Figure 2-1: Typical schematic for the electric circuit of a gliding arc discharge ......................................... 32
Figure 2-2: Transition from quasi-thermal to non-thermal regime during the gliding arc discharge
evolution. ....................................................................................................................................................... 32
Figure 2-3: Schematic of the reverse vortex flow system which the Gliding Arc Tornado (GAT) reactor
principle is based on. On the left is the streamline of the rotating flow (circumferential velocity) and on the
right is the streamline of the axial flow[7]. ................................................................................................... 35
Figure 2-4: Two configurations for the reverser vortex reactor design for GAT stabilization. Movable ring
electrode (left) and Spiral configuration (right)[1, 2]. ................................................................................... 36
Figure 2-5: The high temperature plasmatron made from stainless steel. Electrodes are separated by a
macor or teflon dielectric material ................................................................................................................ 39
Figure 2-6: The Gliding Arc Plasma reformer consists of 2 stainless steel electrodes separated by Macor
which serves as a dielectric material. The ground electrode consists of multiple tangential gas jets
incorporated into the stainless steel flange which provide a swirl effect. ..................................................... 40
Figure 2-7: 3-D drawing of the high temperature plasmatron depicting a swirl forming tangential jet. ....... 41
Figure 2-8: A cross sectional 3D drawing of the high temperature gliding arc plasmatron. ......................... 41
x
Figure 2-9: The 3D model of the reverse vortex configuration low temperature plasmatron ....................... 43
Figure 2-10: Schematic of the low temperature plasmatron. The forward vortex configuration is on the left
and the reverse vortex configuration is on the right. ..................................................................................... 43
Figure 2-11: The vortex chamber containing swirl forming tangential jets .................................................. 44
Figure 3-2: Equilibrium mole factions of Pyrogas components as a function of temperature. ...................... 52
Figure 3-3: Graph showing the thermodynamic simulation of pyrogas without steam and oxygen. Initial gas
composition taken from table 3-2. ................................................................................................................. 54
Figure 3-4: Graph showing the thermodynamic simulation of pyrogas with 20% steam and no oxygen.
Initial gas composition and concentration were taken from table 3-3. .......................................................... 56
Figure 3-5: Graph showing the thermodynamic simulation of pyrogas with steam and oxygen in the initial
gas composition and concentrations. ............................................................................................................. 57
Figure 3-6: Graph showing Pyrogas mixture with naphthalene without the addition of water to the initial
concentration. ................................................................................................................................................ 59
Figure 3-7: Graph showing Pyrogas mixture with naphthalene with the addition of water to the initial
concentration. ................................................................................................................................................ 60
Figure 3-8: Experiment schematic for the pyrolysis gas conversion to syngas. ............................................ 62
Figure 3-9: Experimental set-up showing a well- insulated plasma reactor. ................................................. 64
Figure 3-10: Voltage - current characteristics at low gas flow rates. ............................................................ 66
Figure 3-11: Voltage current characteristic test at high flow gas rate. .......................................................... 66
Figure 3-12: Power vs Current plot at high gas flow rate. ............................................................................. 67
Figure 3-13: Graph shows results from plasma-steam reforming reaction of pyrogas. Results show the
concentrations of constituent gases with varying H2O/C ratios. Increases in hydrogen and decreases in CO
can be attributed to water gas shift phenomenon........................................................................................... 69
Figure 3-14: Graph shows plasma - dry CO2 reforming results for pyrogas at different enthalpies. Changes
in the concentration of individual gases with increase in enthalpy (kWh/m3) can be observed .................... 71
Figure 3-15: The hydrogen yield quantifies the hydrogen produced from the hydrocarbons in the pyrogas
mix after the plasma – dry CO2 reforming reaction. ..................................................................................... 72
Figure 3-16: Graph shows increases in the CO yield of the dry CO2 reforming reaction of pyrogas. .......... 73
Figure 3-17: A close up view of the gliding arc plasma discharge. ............................................................... 74
Figure 3-18: A new generation of high temperature plasmatron developed at the A.J Drexel Plasma
Institute. ......................................................................................................................................................... 74
Figure 3-19: Steam reforming versus dry CO2 reforming of methane gas in pyrogas. ................................. 75
Figure 3-20: The low temperature plasma reformer with the different size configuration of high voltage
electrodes and ground electrode inserts. ........................................................................................................ 77
Figure 3-21: Methane conversion with large electrode plasmatron configuration ........................................ 77
Figure 3-22: The effect of pyrogas-preheating in comparison to cold pyrogas mixture on methane
conversion. .................................................................................................................................................... 79
xi
Figure 3-23: The effect of pyrogas-preheating in comparison to cold pyrogas mixture on ethane conversion.
....................................................................................................................................................................... 79
Figure 4-1: Schematic showing the methods of extracting energy from Biomass and the resulting products
....................................................................................................................................................................... 83
Figure 4-2: Basic structure of carbon suboxide (C3O2) n [101]. ..................................................................... 85
Figure 4-3: Energy efficiency of production of carbon suboxide and hydrogen with respect to heating value
of production CO gas and hydrogen. x=0.4 corresponds to C3O2. HW lies in the limits 0.038 - 0.060. ........ 92
Figure 4-4: Schematic of the experimental setup for the formation of carbon suboxide from hydrocarbon
oxidation reaction. ......................................................................................................................................... 95
Figure 4-5: Voltage (left) and current (right) characteristics of the plasma discharge, data points taken from
a 500 MHz Tektronix digital phosphor oscilloscope ..................................................................................... 97
Figure 4-6: Oscilloscope (500 MHz Tektronix digital phosphor) screenshots of the discharge voltage (top in
yellow) and current (bottom in green) signals at two different resolutions and sampling periods. ............... 98
Figure 4-7: Initial stage of carbon suboxide formation in the DBD reactor (left) and the later stage of
gradual carbon suboxide formation along the walls of the DBD reactor (right). ........................................ 101
Figure 4-8: SEM image (left) and EDX spectrum (right) of deposit formed from plasma assisted butane
oxidation. The pink square in the SEM image represents the region on the sample where EDX signals were
collected. The EDX spectrum shows the peaks of carbon, oxygen. ............................................................ 102
Figure 4-9: SEM image (left) and EDX elemental analysis (right) of sample B ......................................... 102
Figure 4-10: SEM image (left) and EDX elemental analysis (right) of sample C ....................................... 103
Figure 4-11: SEM image (left) and EDX elemental analysis (right) of sample C ....................................... 103
Figure 4-12: SEM image (left) and EDX elemental analysis (right) of sample E ....................................... 104
xii
LIST OF TABLES
Table 1 1: Standard enthalpies of Methane and Isooctane at 298 K, 1 atm. ................................................. 17
Table 2 1: Typical Gliding arc plasma discharge characteristics [1] ............................................................. 33
Table 3 1: The molar concentration of the respective gases that constitute pyrogas [61] ............................. 48
Table 3 2: Model 1- Gas composition and respective concentration (No steam and Oxygen) ...................... 53
Table 3 3: Model 2- Gas composition and respective concentration (With added steam only) .................... 55
Table 3 4: Model 3- Gas composition and respective concentration (With Added steam and Oxygen) ....... 57
Table 3 5: Pyrogas mixture and respective concentration with tar surrogate (No water) .............................. 58
Table 3 6: Pyrogas mixture and respective concentration with tar surrogate (with added water) ................. 60
Table 3 7: Operating parameters of the plasma reforming experiment ......................................................... 64
Table 4 1: Energy efficiency of producing carbon suboxide with respect to syngas production (100%) from
cellulose and two hydrocarbons .................................................................................................................... 89
Table 4 2: Energy efficiency of producing hydrogen and solid carbon suboxide from wood, sunflower oil,
and castor oil. HVW represents heating value in MJ/kg ............................................................................... 90
Table 4 3: Energy efficiency of producing hydrogen and solid carbon suboxide from peat ......................... 91
Table 4 4: Energy efficiency of producing hydrogen and solid carbon suboxide from coal ......................... 91
Table 4 5: Parameters and conditions of partial oxidation of n-butane experiments ..................................... 94
Table 4 6: Gas phase reaction mechanism for low temperature n-butane oxidation ..................................... 99
Table 4 7: Possible surface reaction mechanism ........................................................................................... 99
Table 4 8: Atomic composition of an analyzed sample, each column shows the atomic percent obtained for
each of the six (A-F) areas of the sample analyzed with EDX .................................................................... 104
xiii
Abstract Generation of Hydrogen-Rich Gas Using Non-Equilibrium Plasma Discharges
Fela Odeyemi Alexander Fridman Supervisor, Ph.D
This dissertation investigates Non equilibrium plasma discharges, particularly gliding arc
plasma discharge and dielectric barrier discharge (DBD) as alternative techniques to
thermal or catalytic conversion of hydrocarbons to hydrogen rich gas mixtures. This
dissertation comprehensively addresses two important problems associated with the large
scale use of biomass, municipal wastes, coal and other hydrocarbons as energy sources.
One of the problems is the fouling effect of the by-product of pyrolysis and gasification
on process equipment. The by-products are collectively known as pyrogas. Pyrogas
comprises of light hydrocarbons, heavy hydrocarbons, tar as well as water, CO2, CO and
hydrogen. Some of the negative impacts of tar in fuel reforming include engine wear,
disruption of process equipment and high maintenance costs. The other important fuel
reforming drawback is the emission of carbon dioxide (a major greenhouse gas
responsible for climate change) into the atmosphere. Many scientific studies have
elaborated on the impact of anthropogenic CO2 on global climate. Non equilibrium
plasma discharge is demonstrated to effectively reform pyrogas into hydrogen rich
synthesis gas. A syngas (hydrogen and CO) concentration from 47% to 80%, hydrogen
yield of 55% and CO yield of 60% were achieved after plasma assisted pyrogas
reforming. Non equilibrium plasma discharge is also shown to be effective in the
reforming of hydrocarbon fuels into hydrogen and carbon suboxide without the release of
carbon dioxide into the atmosphere. Detailed analyses, evaluation and results are
presented.
xiv
1
1.0 INTRODUCTION TO PLASMA STIMULATED HYDROCARBON REFORMING
AND HYDROGEN PRODUCTION
Concept of Plasma 1.1
Plasma is the universal form of matter. It is often referred to as the fourth state of
matter and simply put, plasma is an ionized gas. Plasma consists of a collection of free
moving electrons and ions - atoms that have lost electrons. Ionization refers to
phenomenon whereby at least one electron is not bound to an atom or a molecule,
converting the atoms or molecules into positively charged ions. Plasma is a word that was
first coined by Irving Langmuir, an American chemist and physicist in 1928 because the
strongly interacting ionized gas had a semblance to blood plasma coagulation
phenomenon. Free electrons and ions make plasma responsive to electromagnetic fields
and electrically conductive.
Plasmas occur naturally and are also artificially created in the laboratory or in the
industry. Laboratory development of plasma has led to the development of ubiquitous
applications such as electronics, fluorescent lamps, thermonuclear synthesis,
semiconductors, computers, color coating, welding and many others.
Plasma has 3 main characteristics that make it ideal for numerous applications,
namely:
Ability to produce a volume of chemical active and energetic species such as radicals,
ions, electrons, atoms and photons.
Ability to attain very high temperatures and high densities in comparison to traditional
chemical processes.
2
Ability to be non-equilibrium and simultaneously providing high concentrations of active
species at very low temperatures (room temperature)[1].
These plasma characteristics greatly promote conventional chemical processes by
increasing their efficiency and stimulating many challenging chemical reactions. The
physics and chemistry of plasma has led to rapid development of advanced technological
applications such as electronic micro-fabrication, fiber and polymer treatments, protective
coatings in land, air and sea vehicles, ozone production, blood coagulation etc.
Figure 1-1: Temperature of different plasmas with their equivalent electron density range [1].
In plasma, only a portion of the particles in a gas need to be ionized. The ratio of
the density of charged particles to the density of the neutral atoms is known as ionization
degree. Conventional plasma processes have an ionization degree of 10-7 – 10-4[1].
Plasmas such as tokomaks and stellarators have ionization degrees closer to 1. These
plasmas can be described as being completely ionized[2].
3
1.1.1 Natural Plasma
A majority of the entire universe is composed of plasmas. Some of these plasmas
are solar plasma, nebula, solar wind and the earth’s ionosphere. A very common and
familiar natural plasma is lightning which is usually accompanied by thunder. Lightning
occurs when there is a strong electric field to cause an electric discharge in the clouds or
between the clouds and the ground. This atmospheric electrical discharge can reach
temperatures of about 30,000 °C and it is usually accompanied by the ionization of the air
around it. The most important naturally occurring plasma from a human perspective is the
sun. The sun is singled-handedly the most significant source of light and the energy from
the sun is responsible for photosynthesis – a chemical process that is the source of energy
for nearly all forms of life on earth. Aurora borealis is another naturally occurring plasma
at high altitudes. Aurora is caused by the interaction of the charged energetic particles
with atoms in the thermosphere. The earth’s magnetic field collides with charged solar
streaming particles and these charged particles become embedded in the earth’s magnetic
field forming a spectacular light display more profound and visible in the polar regions of
the earth.
1.1.2 Artificial Plasma
Due to the unique characteristics of the plasma discharges, many useful plasma
applications have been developed artificially in an effort to solve many industrial,
scientific and engineering problems. Artificial plasmas have become very mundane and
ubiquitous such that it is easy to be unaware that a simple fluorescent light tube is a
plasma device. There are many different types of artificially made plasma as a result of the
ability to vary or manipulate properties that affect the characteristics of plasma. In its most
basic form, a plasma discharge is formed when a critical breakdown voltage Vb is achieved
4
between two electrodes. A simple plasma device consists of a ground electrode and a high
voltage electrode separated by a dielectric material at a gap distance d. When a potential
difference applied across the electrodes reaches a breakdown voltage and the electric field
increases, a discharge (comprising of ionized gases) jumps from one electrode to another.
The voltage breakdown is usually marked by an electrical spark. At this stage, the
successive collisions of electrons and neutral gas atoms create an avalanche of more
electrons and ions over a small mean free path (distance an electron travels before
colliding with a neutral gas atom). With a sustained current density, a luminous spark or
arc is observed between the electrodes.
Figure 1-2: Process of ionization. Electrons e- collides with neutral atoms 'Θ', knocking off electrons e- and forming ions ‘+’ in a cascading effect.
Artificial plasma temperature varies from ambient temperature to the temperature
of the center of some galactic stars. Plasmas with practical applications usually have
5
electron densities of about 106 to 1018 cm-3 and electron temperatures of 1-20 electron
volts (eV)[2]. Some examples of common artificial plasma include fluorescent light tubes,
plasma televisions and spark plugs.
Plasma Classifications 1.2
Since there are a significant number of different types of plasma, it is important to
classify them based on their unique characteristics in order to better understand them.
Plasma can be classified based on an important parameter such as temperature. With
plasma, temperature is evaluated based on the average energies of the neutral and charged
particles as well as their degrees of freedom especially in relation to vibrational, rotational,
translational and electronic excitation. In this respect, it is safe to say plasma is
characterized by multiple temperatures. Excited electrons acquire energy from the electric
field while moving between impacts with heavy neutrals (joule heating). This energy is
later transferred to the heavy neutral particles depending on the rate (or frequency) of
collision with electrons. Due to the lighter weight of electrons in comparison to heavy
particles, a small portion of the acquired energy is lost during the electron-heavy particle
collision. Therefore, the electron temperature in plasma discharge is initially greater than
the temperature of the heavier particles they come in contact with. The electron-heavy
particle collision maintains an equilibrium temperature unless there is insufficient energy
to sustain the equilibrium state or there is a significant cooling process that inhibits the
heating of the entire gas involved [1].
1.2.1 Thermal Plasma
Thermal plasma is generally any plasma in thermodynamic equilibrium mainly
because the electron temperature Te is relatively similar to the gas temperature To. The
6
temperature difference between heavy atoms and electrons is proportional to the square of
the ratio of the energy received by an electron from the electric field E to the pressure P.
When the value of the ratio of the electric field to the pressure is small, the gas
temperature approaches the electron temperature. When this occurs the plasma is usually
called Thermal plasma. The relationship between the different temperatures in thermal
plasma can be expressed by Te ≈ Tv ≈ Tr ≈ Ti ≈ To. Where Te represents electron
temperature, Tv represents vibrational temperature, Tr represents rotational temperature, Ti
represents ion temperature and To represents gas temperature. Here, the values of the
different temperatures are relatively close to the each other. Thermal plasma is
characterized by higher gas temperature, higher power and higher density as well as low
selectivity of chemical processes. Most thermal plasmas usually have gas temperatures
greater than 11600 Kelvin or 1 eV. Typical examples of thermal plasma is the sun, plasma
torch and arc discharge.
1.2.2 Cold Plasma
Cold plasma is a type of non-thermal plasma that is not in thermodynamic
equilibrium mainly as a result of the wide disparity in the electron temperature and the gas
temperature (or ion temperature). Cold plasma has found numerous applications due to
their low gas temperatures, high electron temperature and abundant chemically active
species and radicals such as atomic oxygen O, hydroxyl OH. These radicals and active
species have the potential to drive chemical reactions processes while keeping the overall
gas temperature low. In cold plasma, the chemical and ionization processes are dependent
on the electron temperature and are therefore unaffected by either the gas temperature or
thermal processes. Here, the temperature characteristics vary significantly such that the
7
electron temperature Te is by far greater than the gas temperature To (Te >> To). As a
result, the temperature component of this kind of non-equilibrium plasma can be
expressed as Te > Tv > Tr ≈ Ti ≈ To. Again, Te represents electron temperature, Tv
represents vibrational temperature, Tr represents rotational temperature, Ti represents ion
temperature and To represents gas temperature. The electron temperature of cold plasmas
is typically about 11600 Kelvin (1 eV) but the gas temperature is usually about room
temperature. The word cold plasma stems from the fact that the gas temperature is about
the same as ambient temperature and can be touched by the human skin without any major
harmful effect. Cold plasma is generally formed as a result of direct electron impact
ionization. Direct ionization refers to the ionization of neutral and unexcited atoms,
radicals or molecules by an electron with significant energy to provide the ionization act
in a single collision. Cold plasmas are usually generated at low pressures or at relatively
low powers. They are also characterized by high chemical selectivity. Examples of cold
plasmas include corona discharge, dielectric barrier discharge and glow discharge. A
fluorescent light tube is a good example of an application of cold plasma.
1.2.3 Warm Plasma
Like the name suggests, the properties of warm plasma lie between that of thermal
plasma and cold plasma. The term ‘warm plasma’ was first used by Alexander Gutsol to
describe plasma discharges with both equilibrium and non-equilibrium characteristics. The
gas temperature of Warm plasmas is generally not too high (as is the case of thermal
plasma) and not too low (as is the case of cold plasma). Warm plasma is a direct result of
step wise electron impact ionization. Stepwise ionization refers to a phenomenon which
occurs when the plasma density and excited neutrals concentration are high enough such
8
that the high energy of these electronically excited neutrals can be converted in the
ionization act. The stepwise ionization is achieved in 2 stages. First, electron–neutral
collisions prepare highly excited species and secondly, a final collision with the low
energy electron provides the actual ionization effect [1]. Warm plasmas are characterized
by high chemical selectivity, moderately low temperature and high power density [1, 2].
Due to its high power density and high selectivity, warm plasma has found applications in
fuel conversion for syngas production, hydrogen sulfide dissociation, carbon dioxide
dissociation etc. Examples of warm plasma discharges include gliding arc plasma
discharge.
Types of Plasma Gas Discharges 1.3
There are many different types of plasma gas discharges and their applications
range from fuel conversion, medical treatment, and water treatment to environmental
pollution control and lighting & aesthetics. The most common of the gas discharges is the
glow discharge which comes in the form of fluorescent lamps. Fluorescent lights are a
form of non-thermal plasma which has become a very important illuminating device.
1.3.1 Thermal Arc Plasma Discharge
Thermal arc discharge can be formed from 2 electrodes (cathode and anode)
connected to an external power source or circuit at atmospheric pressure. Thermal arc
plasma discharges usually draw high currents (above 1 A) at high voltages. Thermal arc
plasma discharges have the capacity to release large amount of heat energy at temperature
greater than 11,600 Kelvin. The high power, high temperature plasma stream in an arc
discharge is usually propelled by a gas flow. Arc cathodes are known to emit electrons by
thermionic and field emission. One of the main problems of thermal arc is the erosion of
9
the electrodes due to high amount of joule heating initiated by the discharge current.
Thermal plasma arcs are used for various welding and metallurgy applications. They are
also used for incineration and management of municipal solid wastes [3-6].
Figure 1-3: Typical example of the erosion of the stainless steel electrodes of a thermal arc
Visible electrode erosion
10
Figure 1-4: Thermal arc discharge used for welding purposes (courtesy Drexel university machine shop)
1.3.2 Gliding Arc Plasma
Gliding arc has been defined as an auto-oscillating periodic discharge between at
least 2 diverging electrodes propelled by a gaseous flow[1]. The first gliding arc device
was developed about 100 years ago in the form of Jacobs ladder and was initially used to
produce nitrogen based fertilizer in 1904. Gliding arc plasma is formed by an arc
discharge which develops at the shortest gap distance between 2 electrodes; it is extended
and convectively cooled by a stream of gas. At this point, the cooling of the discharge is
balanced by a rise in the electric field strength; hence the ionization mechanism transitions
from stepwise ionization to a direct impact ionization mechanism. The transition between
ionization mechanisms gives rise to non-equilibrium conditions which ensure increases in
the formation of radicals and other active species. The extension of the plasma discharge
11
is sustained until a maximum length is attained which corresponds to the maximum power
available to maintain the elongation of the arc discharge. The arc finally cools and
extinguishes but the next arc cycle initiates instantly once the breakdown voltage has been
achieved, reigniting the arc discharge at appropriate spark gap distance and the process is
repeated [2, 7, 8]. Some of the main advantages of gliding arc plasma discharge includes
its ability to achieve high power for high reactor productivity as wells as maintaining a
high degree of non-equilibrium to sustain a selective chemical process. It can operate at
atmospheric pressure and power of up to 40 kW between two electrodes.
Figure 1-5: The different stages of gliding arc progression from a simple spark to the non- equilibrium phase. A – Gas breakdown stage; B – Quasi equilibrium phase; C – Non equilibrium phase [1]
12
Many scientists have done tremendous work toward the development of the gliding
arc discharge. Notably, Czernichowski, Lesueur, & Chapelle (1990) made significant
progress in the development and detailed study of gliding arc[9]. Fridman et al (1994)
published details of the equilibrium nature of the transitional regime of gliding arc[10].
Examples of gliding arc plasma reactors include the gliding arc tornado (GAT) reactor[7],
the gliding arc plasmatron (GAP)[11], three discharge glid arc reactor (RotArc)[12] and
magnetic blow out discharge reactor[13, 14].
1.3.3 Dielectric Barrier Discharge
Dielectric barrier discharge is a very colorful discharge and it is comprised of
multiple micro discharges. Dielectric barrier discharge is strongly non-equilibrium at
atmospheric pressure conditions for various gases at high voltages. Dielectric barrier
discharge is a quiet discharge due to the absence of sparks (sparks are generally known for
their local heating and high noise levels). Dielectric barrier discharges operate at
frequencies in the range 0.05 – 500 kHz. In its basic form, a dielectric barrier discharge
reactor consists of one or multiple dielectic materials between the current paths of two
metal electrodes. Planar and cylindrical are the two main types of DBD configuration. The
more common dielectric materials are usually quartz, glass or ceramic. DBD has found
applications in polymer treatment and wound treatment. Also, dielectric barrier discharge
has been used to investigate the production of syngas or hydrogen by fuel reforming of
hydrocarbons and alcohols [15-17]. Figures 1-6 and 1-7 show the 2 common
configurations dielectric barrier discharge.
13
Figure 1-6: The planar configuration of a Dielectric Barrier discharge set up
Figure 1-7: The cylindrical configuration of dielectric barrier discharge set up
14
1.3.4 Corona Discharge
Corona plasma discharge is a faint glowing discharge that can be observed at
atmospheric pressure near curved regions such as projecting points, edges of wires,
corners or thin wires where there is a significant electric field. Corona discharge forms
when the electric field around the electrode is sufficient to form a conductive region but
insufficient to cause an electrical breakdown to surrounding objects. Coronas can be easily
observed on high voltage electrical transmission lines and lightning rods. Corona can be
either positive or negative, depending on the polarity of the electrode where the electric
field is located. Negative corona forms when the electric field is significant around the
cathode electrode while positive corona forms when the electric field is significant around
the anode. Applications of corona include ozone production and photocopying. Corona
has been used to remove NOx and SOx from flue gas [18, 19]. Also, corona has been used
in fuel reforming to investigate the dry reforming, steam reforming and partial oxidation
of methane and the auto thermal reforming of isooctane [20-22].
1.3.5 Glow Discharge
Of all the plasma gas discharge types, glow discharge is by far the most well-
known. Glow discharge is the basis for neon lamps and plasma TV screens. As the name
suggests, glow discharge is an incandescent discharge. Glow discharge is described as a
self-sustained continuous DC discharge with a cold cathode that emanates electrons due to
secondary emission induced by positive ions. In its simplest form, glow discharge consists
of two electrodes in an evacuated container at very low pressures (0.1 – 10 torr). The
container usually contains neon or a noble gas. Potential difference of 100 volts to 500
volts is applied between the electrodes. A few of the heavy neutrals are ionized via
thermal ionization. The resulting positively charged ions gravitate toward the cathode and
15
the electrons move toward the anode. The resulting ions further strike other heavy
neutrals. The heavy neutrals get excited and quickly lose their newly acquired energy by
emitting photons. A normal glow discharge operates in a current regime of 10-4 Amp – 0.1
Amp. A current increase above 0.1 Amp will result in a transition to an abnormal glow
discharge. Further current increase (up to 1 Amp) will result in a transition to an arc
discharge. Apart from illumination purposes, glow discharge has been explored for its
potential in hydrocarbon reforming, especially in the conversion of methane in to syngas
[23-26].
Fuel Reforming Chemistry 1.4
Fuel reforming is a technique of producing hydrogen, syngas or other useful
products from hydrocarbon fuels. The fuels can be gaseous, liquid or solid hydrocarbons.
Fuel reforming is achieved in multiple ways which will be discussed in subsequent
sections.
1.4.1 Fuel Combustion
Most of the chemical energy derived from hydrocarbons today is as a result of a
common phenomenon called combustion. Fuel combustion is an exothermic reaction
whereby a fuel is oxidized leading to the release of energy and conversion of the chemical
species. The Fuels involved in combustion can be gaseous, liquid or solid hydrocarbons.
Quite often, the release of energy during combustion is accompanied by a glow or a flame.
Complete combustion of a hydrocarbon with oxygen as the oxidizer usually results in the
production of water and carbon dioxide. The stoichiometric equation for the oxidation of a
hydrocarbon in air is given by:
CxHy + (x + ) (O2 + 3.76N2 ) x CO2 + ( ) H2O +
16
(x + ) (3.76N2) + energy (1-1)
Here, CxHy represents a hydrocarbon. Simply put, combustion in air is given by:
Fuel + Air Heat energy + Water + Carbon dioxide + Nitrogen
Electricity generation and combustion engines are is still largely dependent on the
combustion of coal, natural gas and other hydrocarbons.
1.4.2 Partial Oxidation
Partial oxidation may be described as a chemical reaction whereby a sub-
stoichiometric fuel and air mixture leads to a partial combustion in a reformer producing a
hydrogen rich gas mixture. The main products of partial oxidation are carbon monoxide
and hydrogen (syngas). In comparison to combustion, partial oxidation is less exothermic
i.e., the heat energy given off during combustion is less than the energy released during
partial oxidation. A lot of work has been done on the partial oxidation of methane [7, 27-
29], ethane [30, 31], propane [32, 33], diesel [11] and biofuel [11] for syngas production.
Partial oxidation of hydrocarbons can be described further by the equation stated below:
CxHy + (O2 + 3.76N2) x CO + H2 + (3.76N2) + energy (1-2)
1.4.3 Dry CO2 reforming
Dry CO2 reforming can be described as a reforming technique whereby the
reaction between carbon dioxide and a hydrocarbon yields a combination of hydrogen and
carbon monoxide. Dry CO2 reforming is an endothermic reaction that requires a
significant amount of energy in order for the reforming reaction to progress. Dry CO2
reforming has also been used for pyrogas conversion[8], methane conversion[34-37],
propane conversion[38] and biogas conversion[39]. Dry CO2 reforming of hydrocarbons
into synthesis gas can be further described by the reaction below:
17
CxHy + xCO2 2xCO + H2 (1-3)
1.4.4 Steam reforming
Steam reforming is a reforming technique whereby the reaction between steam at a
high temperature and a hydrocarbon yields carbon monoxide and hydrogen (syngas).
Steam reforming is a reforming technique that provides the best hydrogen yields[20].
Partial oxidation is exothermic unlike dry CO2 reforming and steam reforming which are
endothermic. The equation below shows the steam reforming reaction of a hydrocarbon.
CxHy + xH2O xCO + ( H2 (1-4)
Sometimes, steam can react with the resulting CO and this affects the
concentration of hydrogen and CO. By using water gas shift reaction (WSG), the resulting
carbon monoxide when reacted with steam at about 400 °C can be converted to hydrogen
and CO2[40]. This reaction is effective in an effort to produce more hydrogen. The water
gas shift reaction is exothermic and is reversible at temperatures greater than 1000 °C
[41]. The water gas shift reaction is shown in the equation below:
H2O + CO H2 + CO2, ∆H = -41 kJ/mol (1-5)
Steam reforming has been used to convert ethanol [42-44] and hydrocarbons such
as methane [45, 46], propane [46] into hydrogen rich syngas. Table 1-1 shows the
standard enthalpies of different reforming reaction techniques of methane and isooctane.
Table 1-1.4: Standard enthalpies of Methane and Isooctane at 298 K, 1 atm.
Methane (kJ/Mol) Isooctane (kJ/Mol)
Partial oxidation -36.1 -675.8
Steam reforming 205.7 1258.8
Dry CO2 reforming 246.9 1596.3
18
1.4.5 Auto-Thermal Reforming
Auto-thermal reforming can be described as a reforming reaction which combines
partial oxidation, dry CO2 reforming and steam reforming to produce syngas from a
hydrocarbon fuel. Here, oxygen, steam and carbon dioxide are combined together to attain
an auto-thermal chemical reaction where the overall enthalpy is zero[20]. The heat energy
released from the partial oxidation is balanced by heat energy required by the endothermic
reaction of steam reforming and dry CO2 reforming. This effectively leads to the
reduction of energy cost of the reforming mechanism.
1.4.6 Pyrolysis
Pyrolysis is a term used for a fuel reforming reaction whereby an irreversible
chemical change is brought about by heat energy is the absence of oxygen. Pyrolysis is
sometimes referred to as thermal decomposition or destructive distillation. Pyrolysis of
biomass often yields Pyrogas, soot, organic liquids and water [47]. Pyrogas contains
hydrogen, carbon monoxide, carbon dioxide, methane and trace quantities of heavier
hydrocarbons such as ethylene, ethane and propane [8]. Pyrolysis reactions are
endothermic, requiring an external source of heat. As an example, thermal decomposition
of methanol is expressed below:
CH3OH 2H2 + CO (1-6)
1.4.7 Gasification
Gasification is fuel reforming method that converts fossil fuels such as biomass or
coal into carbon monoxide and hydrogen. Gasification is achieved by the reaction of the
fuel with a limited amount of oxygen and/or steam at high temperatures (>700 °C).
19
Gasification usually involves the thermal processing of biomass where sub-stoichiometric
oxygen required by the fuel is admitted into the fuel zone to release the heat required for
the endothermic gasification reaction [47]. The resulting gas mixture of this process is
syngas. Syngas is gas with a high mass heating value which can be combusted or used to
run gas turbines [8].
Non - Equilibrium Plasma Fuel Reforming and Hydrogen Production 1.5
Environmental and pollution problems has led to an effort in the scientific and
engineering fields into finding and developing alternative and renewable methods for
energy and electricity production. As a result, interest in fuel cells as a better alternative to
fuel based combustion engines with respect to efficiency and environmental impact has
grown. Fuel cells generate electricity from oxygen and hydrogen, creating water without
combustion. Hydrogen as a fuel has significant drawbacks - especially storage. Despite
hydrogen’s high mass heating value: 120 kJ/g compared to gasoline (42.8kJ/g), methanol
(20kJ/g) and methane (50kJ/g); hydrogen has a low density which results in a very low
volumetric heating value (11kJ/l) compared to 16000 kJ/l for methanol [20]. A possible
solution to hydrogen’s storage problem is the reforming of hydrocarbons fuels.
Hydrocarbons already naturally store hydrogen (i.e. hydrogen is already contained in
hydrocarbon fuels). The hydrogen is extracted from the hydrocarbon fuel by reforming.
Non equilibrium plasma technology is a better technique for fuel reforming for hydrogen
production. Non equilibrium plasma eliminates problems associated with catalytic
converters such as cost, sulfur poisoning and short start up time [8, 48].
20
1.5.1 Plasma Reforming of Light, Gaseous Hydrocarbons (Methane) into Synthesis Gas
Hydrogen plays an important factor in the development of fuel cell technology.
Conversion of natural gas into syngas is very important for natural gas liquefaction and
some processes of organic synthesis. Conventional thermo-catalytic technology for
hydrogen production is limited by low specific productivity, high metal capacity and large
equipment size. Non equilibrium plasma assisted production of hydrogen rich gases result
in higher specific productivity, lower capital, operating and maintenance costs [1]. Gliding
arc is particularly suitable for methane conversion to syngas due to its proven non
equilibrium effect. Studies have also shown that gliding arc provides some of the highest
conversion efficiencies and syngas yields [7, 49]. Methane to syngas conversion rate
reached 80-85% using non equilibrium plasma as a reaction stimulant but thermal
processes on the other hand were able to produce 60% conversion. The non-equilibrium
plasma reforming experiments were conducted in a Gliding arc tornado (GAT) plasma
reactor made from quartz with a 0.2L volume [7]. Methane and air are injected into the
GAT reactor through the inlet tubes connected to a well-insulated counter flow heat
exchanger. Electric power input in the non-equilibrium GAT plasma is relatively low
(about 200 W), indicating low electric energy cost of the partial-oxidation process and its
plasma-catalytic nature[7]. The minimum energy cost was measured to be 0.09 kWh/m3 of
syngas (only 3% of the energy is consumed in the form of electricity; everything else is
chemical or thermal energy of the initial products) [1]. Figures 1-8 and 1-9 show the result
for methane conversion and energy cost respectively.
21
Figure 1-8: Methane conversion to syngas as a function of O: C ratio. Solid lines with circles represent the kinetic simulation results; solid bars represent the experimental results; dotted line and bar represent kinetic simulation and experimental results without plasma[7].
22
Figure 1-9: Electric energy cost of syngas production as a function of O:C ratio[1].
1.5.2 Plasma reforming of liquid hydrocarbons into synthesis gas
Liquid fuels such as gasoline, diesel, JP 8, ethanol, biodiesel etc. can be converted
in to syngas by non-equilibrium plasma with partial oxidation or steam air reforming
reactions. This technique is particular beneficial for on board hydrogen production for fuel
cell units. More hydrogen can be derived from reformed liquid fuels which are already a
natural store house for hydrogen. Syngas can be directly used in internal combustion
engines as an admixture to the conventional fuel–air mixture to reduce toxic exhausts and
improve the major engine characteristics[50, 51]. Bromberg et al used a low current
gliding arc reformer called plasmatron for the syngas conversion process. Plasma-catalytic
conversion of kerosene (total composition C11H22; with major components undecane
(C11H24), 21%; decane (C10H22), 14%; and dodecane (C12H26), 14%) into syngas has been
achieved with non-equilibrium atmospheric pressure pulsed microwave discharge[52].
23
The process has been investigated in regimes of partial oxidation and steam reformation
with different fractions of fuel, air and water vapor. Also, Gallagher et al demonstrated the
plasma assisted reforming of n-tetradecane (C14H30) into syngas using two different
plasma reformers[49]. The results show that gliding arc systems are capable of reforming
heavy hydrocarbon fuels with high conversion efficiency. Gallagher et al compared the
conversion efficiency and product yield of the gliding arc plasmatron and the reverse
vortex flow gliding arc reactor. Figures 1-10 and 1-11 show the results of the conversion
efficiency and product yield respectively.
Figure 1-10: Thermodynamic equilibrium and experimental energy conversion efficiency of H2 + CO, and H2 + CO + light hydrocarbons (HCs) as a function of O/C ratio. Solid line: Equilibrium H2 + CO + light HCs; dotted line: Equilibrium H2 + CO; ▴, H2 + CO + light HCs for RVF-GA reactor; ●, H2 + CO for RVF-GA reactor; , H2 + CO + light HCs for GA-Plasmatron; , H2 + CO for GA-Plasmatron[49].
24
Figure 1-11: Thermodynamic equilibrium and experimental points showing product yields of H2 and CO as a function of O/C ratio. Solid line: Equilibrium CO yield; dashed line: Equilibrium H2 Yield; ■, H2 yield for RVF-GA reactor; ▴, CO yield for RVF-GA reactor; ♦, H2 yield for GA-Plasmatron reactor; ×, CO yield for GA-Plasmatron reactor [49].
Advantages of Plasma catalysis for hydrocarbon reforming 1.6
Plasma catalysis is particularly effective for hydrocarbon reforming due to the low
electrical power in non-equilibrium plasma (couple hundred watts). This is coupled with
the low temperature of the heavy neutrals species, high temperature of the electrons (up to
11, 600 K). The role of plasma is simply to provide radicals and excited species which
stimulates and enhancing chemical reactions [1, 20, 48]. It should be noted that non-
equilibrium plasma does not provide energy to the system[1, 2]. Other advantages are
discussed in the following sections.
1.6.1 Minimal soot formation
Compared to combustion and other reforming techniques, non-equilibrium plasma
has shown to produce minimal soot during hydrocarbon reforming. This may be due to the
25
strong oxidative property of plasma. The strong oxidative nature of plasma can be
attributed to acidic nature of air propelled plasmas. Air comprises of about 21% oxygen
and 79% Nitrogen. The ionization of air leads to the production of N+ ions which easily
reacts with the small quantity of water in air. This reaction leads to the production of
hydronium H3O+ and hydroxyl radicals OH. The efficiency of production of the species
determines the acidity of air propelled plasma and in turn, determines its oxidative
property[41].
1.6.2 Preheating Not Necessary (Quick start)
Unlike catalytic conversion which require long preheating times, non - equilibrium
plasma conversion of hydrocarbons do not need to be preheated. Catalysts need to be
heated in order to reach their activation temperatures. Depending on the density of the
catalysts, the preheating time may increase with increasing quantities of catalysts. This
drawback has been eliminated by plasma which is characterized by instantaneous
activation of the chemical reforming process. This is beneficial for on board production of
hydrogen from gasoline, diesel or JP 8 fuels for fuel cell applications.
1.6.3 Insensitivity to Contaminants
Catalysts easily suffer from fouling, plugging or poisoning by particulates during
catalytic reforming and this leads to high maintenance costs due to the increased need for
the replacement of catalysts which are expensive to begin with. Common catalysts poisons
include fuel sulfur, lead and manganese which are found in liquid fuels such as gasoline
and diesel. Non-equilibrium plasma fuel conversion systems are devoid of these
limitations because they are gaseous-phase, volumetric catalysts with no surfaces with
which these contaminants can stick on.
26
1.6.4 Size and Compactness of Plasma Reactors
Most non equilibrium plasma reformer units are small and compact. A typical
example is the different generation of the plasma reformers called gliding arc plasmatron
and developed at the A.J Drexel plasma institute, Drexel University. The plasmatron unit
is capable of handling flow rates of up to 200 LPM and capable of producing over 20kW
of syngas from methane and diesel. The compact design is advantageous because it ensure
easy maintenance.
Comparison of Plasma Systems and Other Engineering Solutions for Fuel Reforming 1.7
There are multiple techniques of achieving effective fuel reforming for the production
of syngas. Each method has its own advantages and drawbacks. The by-products of
biomass gasification contain very useful fuels as well as contaminants which need to be
cleaned up. Aside from non-equilibrium plasma assisted removal of contaminants such as
tar, other methods of contaminant removal include thermal cracking [53-57] and catalytic
cracking with catalyst such as Nickel based catalysts [28, 32, 35, 58-60], alkali metal
catalyst[31, 32, 61, 62] and dolomite catalysts[63-67].
1.7.1 Thermal Cracking
Thermal cracking for fuel reforming involves heating up pyrogas or hydrocarbons
to very high temperatures to convert them to lighter hydrocarbons. Although unreacted
light hydrocarbons and tar in pyrogas can be converted via thermal cracking in a gasifier
at 1250 °C and a residence time of 0.5 seconds[53], however this method alone is
ineffective for the decomposition of some biomass based hydrocarbons[68]. This problem
may be mitigated by increasing the residence time and ensuring a direct contact between
27
the pyrogas and a very hot surface. Unfortunately, this approach leads to a significant
energy cost and reduction in overall conversion efficiency[55].
1.7.2 Catalytic Cracking
The first known commercial application of catalytic cracking happened in 1915
when Almer McAfee of the gulf refining company demonstrated a batch process using
Aluminum chloride to convert hydrocarbon oils. But due to the high costs of catalysts at
that time, the technique struggled to gain widespread use[69]. Catalytic cracking gained
widespread attention in the mid-1980s due to the advantages of converting hydrocarbons
to syngas and modification of pyrogas composition. Catalytic cracking is preferred to
thermal cracking due to the potential for higher conversion yields and efficiency. The
three common groups of catalysts for catalytic cracking are dolomite based, alkali and
metal based catalysts and nickel based catalysts.
1.7.2.1 Nickel-based Catalysts
Most of the experimental investigations on catalytic cracking for fuel reforming
are based on Nickel catalysts. Nickel based catalysts are capable of reversing ammonia
reaction; therefore they are beneficial in the reduction of nitrogen oxides (NOx)
production during biomass gasification. Some of the common types of Nickel based
catalyst used for catalytic cracking include ICI46-1, UCG90-C, Z409 and RZ409[62]. For
pyrogas conversion experiments, hydrogen yields of 6-11% and conversion of light
hydrocarbons (such as methane and ethylene) were achieved with ICI46-1, Z409 and
RZ409 catalysts with increased temperature [70]. Azner et al tested 8 catalysts produced
by BASF, ICI-Katalco, UCI and Haldor Topsoe for Tar removal at 780 to 830 °C. Four of
the catalysts were tested for steam reforming of light hydrocarbons while the other four
were tested steam reforming of the heavier hydrocarbons such as naphthalene. Azner et al
were able to conclude that the catalysts for steam reforming of heavy hydrocarbons were
more active than the catalysts for steam reforming the light hydrocarbons. The efficiency
28
of removal of tar is mostly dependent on the catalysts bed conditions, temperature, space
time, particle size and the pyrogas composition [71, 72].
1.7.2.2 Alkali metal catalysts
Alkali metal catalysts are usually embedded directly in biomass by dry mixing or
wet impregnation. These methods make the recovery of the catalysts difficult which is not
cost effect from the point of view of biomass gasification. These techniques also result in
increases in the fly ash content after gasification and the disposal of the ash content will
also prove to be expensive and problematic[62].
1.7.2.3 Dolomite catalysts
Dolomite is a calcium magnesium ore - CaMg (CO3)2, and it is generally used as
raw material in pidgeon process for the production of magnesium via thermal reduction.
Dolomite is a cheap and easily disposable catalyst capable of converting pyrogas into
syngas in an industrial gasifier. It is applied as primary catalyst or dry mixed with
biomass[62]. The presence of calcined dolomite in the fluidized bed of a gasifier for
pyrogas conversion decreases tar content and increases syngas yields [73, 74].
Unfortunately, dolomite has no significant effect on the concentration of gaseous
hydrocarbons in pyrogas. 20-30% wt of dolomite in the gasifier reduced tar content to
about 1 g/m3 at an equivalence ratio of 0.3[63].
1.7.3 Other Plasma Systems
Plasma discharge has been demonstrated to remove tar, light and heavy
hydrocarbons in pyrogas [8, 75-77]. The removal fraction for 300 ppm methane in
atmospheric pressure air, with a residence time less than 0.5 ms, was measured at 80%
with an energy density of 4 KJ/l. At 400 °C, about 50% naphthalene can be thermally
29
decomposed in 20 minutes. With corona discharge, 50% removal can be achieved with an
energy density of 40 J/L at 400 °C in less than 3 minutes. This can be attributed to the
high-energy electrons generating reactive species and radicals by way of collisions [1, 2,
55].
Conclusion 1.8
Plasma has a wide variety of applications beyond fuel conversion. It is a unique
phenomenon that is still being studied by scientists and researchers all over the world.
Plasma is phenomenon with tremendous amount of applications in the different facets of
life. The plasma catalytic effect for fuel reforming is a promising technology for fuel
reforming. Further studies and optimizations are still needed to make non equilibrium
plasma such as gliding arc plasma more attractive for commercialization. Non thermal
plasma has the potential to overcome all the drawbacks associated with catalytic
reforming techniques. The following chapters will describe the design, experimental and
simulation efforts in the advancement of non-equilibrium plasma for solving the problems
associated with fuel reforming in particular tar and heavy hydrocarbon conversion as well
as avoidance of carbon dioxide emission.
30
Introduction to Gliding Arc Reformer Technology 2.1
As emphasized in earlier sections, non-equilibrium plasma is well suited for fuel
reforming due to the availability of active species and radicals which stimulates a
reforming reaction and eventually leads to higher efficiency and lower energy cost when
compared to catalytic conversion [1, 78]. For pyrogas reforming, gliding arc plasma
reformers are considered most effective due to its low energy cost as well as its non-
equilibrium characteristics such as its potential for high chemical selectivity in the
reforming reaction process. The main hurdle with generating non equilibrium plasma is
the ability to control the power of the discharge as well as the current density. These
parameters can be managed by focusing on the power supply, reactor design, gas flow
rates and gas pressure. Gliding arc plasma discharge is a dynamic discharge propelled by a
gas flow along a reactor [1]. Gliding arc reformer is considered to be an important system
for fuel reforming for hydrogen production and on board applications [20, 49]. Gliding arc
reformer is a simple and flexible device that is capable of operating over high flow rates
and handling a great number of chemical species. The gliding arc plasma reformer can
operate on both a direct current power supply and alternating power supply.
Czernichowski et al developed a simple gliding arc reforming which consisted of
two thin diverging electrodes and the gas is introduced at the base of the electrodes[9]. A
high voltage is applied to between this electrodes and an arc develops between the
electrodes. The arc is pushed by the rising gas upward and it extinguishes at the top of the
electrodes. Fridman et al provided details about the initial breakdown phenomenon at the
shortest distance between the two diverging electrodes. The shortest distance between the
electrodes is typically between 1-3mm. The cycle of the gliding arc evolution originates
during the initial breakdown. At a breakdown voltage Vb of about 3kV and 1µs, low
31
resistance plasma is formed and the voltage between the electrodes drops. The equilibrium
stage develops right after the formation of the plasma channel. The incoming gas flow
pushes the plasma column with a typical average velocity of about 10 m/s (velocity
depends largely on design). The length l of the arc increases with voltage and the power
goes up to the maximum value Pmax as dictated by the power supply [1, 2]. The electric
current also goes up to its maximum value of Im = ≈ 40A. During the quasi-equilibrium
stage, the gas temperature TO remains stable (remaining around 3000 K; Fridman et al.,
1993, 1997, 1999). The non-equilibrium stage begins when the length of the gliding arc
exceeds its critical value lcrit. Heat loss from the plasma column start to go beyond the
energy supplied by the source, and it becomes impossible to sustain the plasma in quasi
equilibrium. Then the plasma quickly cools down to To = 1000 – 2000 K, while
conductivity is maintained by high electron temperature, Te = 1 eV. After decay of the
non-equilibrium discharge, a new breakdown takes place at the shortest distance between
the electrodes and the cycle repeats [1, 2, 7]. Table 2-1 shows the typical characteristics of
the gliding arc plasma discharge. Figure 2-1 shows the electrical schematic of a simple
gliding arc discharge. Figure 2-2 shows the transition of the gliding arc discharge from an
equilibrium state to a quasi-equilibrium stage and finally to a non-equilibrium state.
32
Figure 2-12: Typical schematic for the electric circuit of a gliding arc discharge
Figure 13: Transition from quasi-thermal to non-thermal regime during the gliding arc discharge evolution.
33
Table 2.1-1: Typical Gliding arc plasma discharge characteristics
Electron Temperature ~ 1 - 1.5 eV
Maximum Electron Density
(1/cm3) 1011 – 1014
Maximum Gas Temperature 300 - 3000K
Average Power Density 10 - 300 W/cm3
Ionization Mechanism Stepwise / Direct
Discharge Cycle 2 - 10 mSec.
Current (A) 0.01 - 10
Power (kW) 0.1 - 100
Low Current Gliding Arc in Vortex Flow Reactor Designs 2.2
2.2.1 Introduction
The plasma vortex flow can either be reverse vortex flow or forward vortex flow.
The reverse vortex flow design is an efficient concept which consists of an outer swirling
rotation and a low pressure area in the middle of the discharge zone. The name ‘gliding
arc tornado’ derived its name from the natural tornado due to the similarity in their shape
and formation. The tornado flow in the reverse vortex flow systems allows for higher gas
velocities needed for gliding arc motion. Other advantages of this design includes the high
capability for heat and mass exchange at the middle of the plasma zone in the inner
cylindrical volume as a result of fast radial migration of chaotic micro-volumes
decelerated around the walls of the tube [7]. The vortex flow is designed to push and
34
elongate the plasma discharge as Fuel gas and air mix with the discharge. The interaction
ensures heat energy and the active species from plasma stimulate an oxidation reaction of
the fuel gas [1, 7]. The forward vortex flow design has many similarities with the reverse
vortex flow. The main differences in the design are in the location of the swirl jets and the
size of the exhaust diaphragm. In the reverse vortex flow, the diameter of the exhaust
diaphragm is much smaller than the internal diameter of the vortex chamber. This forms a
barrier which reverses the direction of the vortex flow back into the cylindrical vessel.
Also, the swirl jets in the reverse vortex flow design are located on the same plane as the
exhaust diaphragm [41].
2.2.1.1 Gliding Arc "Tornado" Discharge Reactor
The gliding arc tornado reactor was first designed and developed for the
investigation of the partial oxidation of methane to syngas at Drexel University [7]. The
walls of the reactor were made from quartz most especially for easy visual observation of
the plasma discharge as well as the fuel reforming process. The reactor consisted of
movable stainless steel electrodes. The design is based on the reverse vortex flow concept.
35
Figure 2-3: Schematic of the reverse vortex flow system which the Gliding Arc Tornado (GAT) reactor principle is based on. On the left is the streamline of the rotating flow (circumferential velocity) and on the right is the streamline of the axial flow[7].
Figure 2-3 shows the schematic of the gliding arc tornado principle. Two flows are
injected into the cylindrical volume. There are 2 distinct flow systems, which are – the
axial flow system and the tangential flow system. The products of the fuel reforming
reaction exits the cylindrical volume at the top central portion of the quartz reactor (flow
exit). The flow exit happens to be located on the same side as the tangential gas entrance.
The plasma zone is well stabilized by the near wall tangential flow. Typical dimensions
for the quartz tube reactor are 40mm – inner diameter and 50mm – length. Figure 2-4
shows the physical outcome of the two different reverse vortex configurations.
36
Figure 2-4: Two configurations for the reverser vortex reactor design for GAT stabilization. Movable ring electrode (left) and Spiral configuration (right)[1, 2].
The first configuration is essentially a movable ring electrode that moves toward or
away from the top fixed electrode. At the initial spark discharge breakdown, the cathode
ring is 3 mm away from the fixed ground electrode. Upon the ignition of the discharge, the
high voltage electrode (cathode) moves up and down, thereby elongating the discharge arc
until it reaches a non-equilibrium state. Elongating arc demands more power to sustain
itself. Thus the voltage on the arc increases and the current drops. As the arc stretches out,
the electric field and electron temperature increases but the gas temperature declines
making the resulting plasma discharge more non-equilibrium. The second configuration
consists of a spiral cathode placed coaxially with the quartz tube wall. The arc discharge
ignites at the top of the spiral electrode and glides along the spiral cathode until it
approaches a smaller diameter ring and eventually stabilizes [1, 7, 20, 79]. Reverse vortex
37
flow (RVF) provides excellent thermal insulation of the discharge zone from the reactor
walls, increased retention time of fuel - air oxidation reaction and convective cooling of
the plasma arc. Also, formation of a recirculation zone near the gas exit is a possibility and
the active species from the plasma zone are retained inside the reactor. This recirculation
is very important for plasma-catalytic reactions[1, 79]. The gliding arc tornado reactor has
been used to investigate the partial oxidation of methane into syngas. The results obtained
with the GAT has been cited as one of the best in plasma assisted partial oxidation of
methane [20].
2.2.2 Gliding Arc Plasmatron
The gliding arc plasmatron is regarded as an improvement over the inadequacies of
the gliding arc tornado plasma reformer. The Gliding arc plasmatron is an advanced fuel -
plasma reforming reactor that was initially developed at Massachusetts Institute of
Technology (MIT) [50] and has since been modified to its current state at Drexel
University. The gliding arc plasma reformer is designed with the capability to reform both
liquid and gaseous fuels. It has been successfully adopted in reforming fuels such as
gasoline, methane, diesel, propane, JP-8 fuel; biofuels such as ethanol, canola oil and
soybean [20, 49-51, 80]. Applications include hydrogen production for fuel cell feeds,
NOx absorber regeneration, spark ignition engine [11]. There are currently 2 main
generations of gliding arc plasmatron that is in operation at the Drexel plasma institute and
developed by Rabinovich et al [8, 11, 49, 50, 80].
38
2.2.2.1 High Temperature Plasmatron
One generation of the gliding arc plasmatron is the high temperature (forward-
vortex-only) plasma reformer that can convert both liquid and gaseous hydrocarbon fuels
into syngas. The high temperature plasmatron is small and compact and lacks a pre-
mixing compartment for fuel and air admixing. As an alternative to a premixing
compartment, the device has a fuel atomization nozzle which can supply hydrocarbon
fuels axially into the reaction compartment (see figures 2-5, 2-6). This reformer is
especially designed to function at temperatures of up to 900 °C which allows for gas
preheating as well as endothermic reactions. The dielectric material that separates the
electrodes is MACOR. MACOR is a machineable glass-ceramic with excellent thermal
characteristics. It is stable at temperatures as high as 1000 °C. MACOR has a density of
2.52 g/cm3, and a thermal conductivity of 1.46 W/ (m·K). MACOR was chosen mainly
because of its high melting point and low thermal conductivity. This gliding arc reformer
consists of a small top unit which comprises of swirl generating fluid jets from which the
vortex flow is generated. The top unit also includes the high voltage electrode (1.5 cm
outer diameter) separated from the ground electrode by MACOR. Gases are injected into
the tangential jets which further flows toward a 3mm gap between electrodes (spark gap)
where the plasma discharge is initially formed. There is also a post plasma region which
contains a cylindrical ceramic for thermal insulation. The high temperature plasmatron is
designed to handle gas flow rates of up to 100 LPM at atmospheric pressure.
39
Figure 2-5: The high temperature plasmatron made from stainless steel. Electrodes are separated by a macor or teflon dielectric material
40
Figure 2-6: The Gliding Arc Plasma reformer consists of 2 stainless steel electrodes separated by Macor which serves as a dielectric material. The ground electrode consists of multiple tangential gas jets incorporated into the stainless steel flange which provide a swirl effect.
41
Figure 2-7: 3-D drawing of the high temperature plasmatron depicting a swirl forming tangential jet.
Figure 2-8: A cross sectional 3D drawing of the high temperature gliding arc plasmatron.
42
2.2.2.2 Low Temperature Plasmatron
The other generation of plasmatron is known as the low temperature plasmatron. It
is best suited for low temperature hydrocarbon reforming processes. The low temperature
plasmatron (figures 2-9, 2-10) is designed mainly to reform gaseous or vaporized
hydrocarbon fuels. It is also designed to be adaptable to a reverse vortex flow
configuration as well as a forward vortex flow configuration unlike the high temperature
plasmatron. In the low temperature plasmatron, the inlet gas mixture arrives into the
reformer via a swirl generating tangential jet (figure 2-11) within the stainless steel flange.
The low temperature plasmatron has stainless steel inserts that help to easily modify the
vortex flow configuration. At the gas exit is also a stainless steel cylinder which helps to
elongate the plasma discharge thereby maintaining the contact between the gas mixture
and the plasma discharge, eventually increasing residence time. The dielectric material is
usually made out of Teflon. The low temperature plasmatron can operate at atmospheric
pressure, flow rates of up to 120 L/Min and at temperatures up to 300 °C.
43
Figure 2-9: The 3D model of the reverse vortex configuration low temperature plasmatron
Figure 2-140: Schematic of the low temperature plasmatron. The forward vortex configuration is on the left and the reverse vortex configuration is on the right.
44
Figure 2-11: The vortex chamber containing swirl forming tangential jets
Characterization of gliding arc discharge power supplies 2.3
2.3.1 Introduction
A power supply is a device that provides electrical energy to a plasma unit. The
electrical energy required to generate a plasma discharge is usually provided by a power
supply. In order to generate a plasma discharge, a potential difference needs to be applied
between two electrodes. This results in current being draw from the power supply. A
power supply obtains the energy it supplies a discharge load from an energy source,
typically an electrical energy transmission system. Two important attributes considered
when choosing a power supply unit are:
The amount of voltage and current that can be provided to the load and
The level of stability of the output voltage and current under load and sometimes line
conditions.
The two main power supply types are alternating current (AC) power supply and direct
current (DC) power supply.
45
2.3.2 Direct current power supply
A Direct current power supply converts alternating current from a transmission
line to a pulsating direct voltage. The pulsating direct voltage is filtered by removing most
of the pulsation. The filter typically consists of capacitors, resistors and inductors. The left
over unwanted alternating voltage component (ripple) is superimposed on the direct
voltage output. The direct current power used for the generation of the gliding arc plasma
discharge (to activate the plasmatron) is the BRC 10000 Universal voltronics high voltage
power supply unit. The power supply is connected to an 11 kilo-ohm ballast resistor unit
which helps to limit the plasma discharge current. The power is converted using IGBT
switching at frequencies above 20 kHz, and controlled using tuned pulse width modulation
techniques. The unit has an operational output voltage of 0- 10 kV, output current of 0 – 1
A DC and input frequency of 50/60 Hz. This high voltage power supply was chosen for
our plasma application because of its capability for low ripple, fast transient response,
endurance to repetitive arcing and stable output even in the face of line voltage and load
charges. Front panel mode indicator LEDs automatically show which regulating mode
(current or voltage) is controlling the supply. Ten-turn locking potentiometer controls for
voltage and current are located on the front panel to allow full-range adjustment of voltage
or current. The BRC 10000 universal power supply was used for the pyrogas reforming
experiments described in chapter 3.
2.3.3 Alternating current power supply
A variable alternating current power supply was used to generate the DBD plasma
in experiments in chapter 5. The Quinta power supply has an operational frequency of
50Hz – 1.66 kHz and a maximum peak to peak voltage range of 20 kV– 34 kV.
46
Introduction 3.1
This chapter discusses plasma assisted conversion of pyrolysis gas (pyrogas) fuel
to synthesis gas (syngas, combination of hydrogen and carbon monoxide). Pyrogas is a
product of biomass, municipal wastes or coal - gasification process that usually contains
hydrogen, carbon monoxide, carbon dioxide, water, unreacted light and heavy
hydrocarbons and, tar. These hydrocarbons diminish the fuel value of Pyrogas thereby
necessitating the need for the conversion of the hydrocarbons. Various conditions and
reforming reactions were considered for the conversion of Pyrogas into syngas. Non-
equilibrium plasma reforming is an effective homogenous process which makes use of
catalysts unnecessary for fuel reforming. The effectiveness of gliding arc plasma as a non-
equilibrium plasma discharge is demonstrated in the fuel reforming reaction processes
with the aid of a specially designed low current device also known as gliding arc plasma
reformer. Experimental results obtained focuses on yield, molar concentration, carbon
balance, enthalpy kWhr/m3 at different conditions.
Pyrogas 3.2
Biomass, municipal wastes, hydrocarbon fuels or coal can be reformed via one or a
combination of pyrolysis, combustion and gasification processes [49, 64, 74, 81]. A series
of chemical reactions during the course of these processes usually result in the formation
of a complex mixture of combustible gases such as CH4, CO, H2, unreacted light and
heavy hydrocarbons; tar, moisture and a noncombustible gas – (CO2). A combination of
all these gases constitutes what is known as pyrolysis gas or pyrogas. Pyrogas is a hot gas
produced in industrial gasifiers at high temperatures.
CnHm + O2 → H2 + CO + CO2 + H2O + CxHy (3-1)
47
Where CnHm represents coal, municipal solid waste or biomass and CxHy represents
methane, ethane, and propane etc. As an example, figure 3-1 shows the different stages of
biomass processing.
Figure 3-1: Schematic of the different steps in biomass processing.[82]
The presence of heavy hydrocarbons and tar diminishes the quality of pyrogas
from the perspective of its use for power generation or as an intermediate for synthetic
fuel production. These hydrocarbons can condense to form tar aerosols and polymerize
into complex structures which are problematic to turbines or internal combustion
engines[83]. This drawback therefore necessitates the removal of the unreacted
hydrocarbons. The composition of pyrogas presented in table 3-1 is obtained from the US
department of energy’s world gasification database [84]. The molar concentrations of the
gases making up the pyrogas composition stated are within the range of concentrations
48
obtained from coal gasification processes. The temperature of pyrogas produced in an
industrial gasifier can be as high as 850oC. The mean value of the molar concentrations of
the respective gases was used for the steam reforming and dry CO2 reforming experiments
of pyrogas. Water vapor was not included in the pyrogas composition used in the
reforming experiments due to the limitation of the gas chromatography equipment in
detecting and measuring water vapor molar concentrations.
Table 3.2-1: The molar concentration of the respective gases that constitute pyrogas [84]
Gas Mole %
H2 32-37
CO 10-14
CH4 20-25
CO2 19-24
C2H6 6-10
C3H8 1-2
C3H6 < 1
C4H10 < 1
A good approach for accomplishing pyrogas reforming is the use of non-
equilibrium gliding arc plasma for the chemical reformation of pyrogas into synthesis gas
(or syngas). Non-equilibrium gliding arc plasma reforming is a fuel reforming process,
which eliminates the need for catalysts [49]. There are existing methods of removal of
heavy hydrocarbons from pyrogas using catalysts such as Nickel, activated alumina, alkali
metals and calcined dolomite [55, 64, 85, 86]. Unfortunately, catalytic conversion
49
techniques have been known to have extensive drawbacks such as high cost, large size and
significant carbon footprint[87]. As an alternative, an effective conversion of heavy
hydrocarbons with non-equilibrium gliding arc plasma reactor was demonstrated in [49].
This reactor is characterized by smaller size, fast start-up time, higher efficiency and low
electrical energy cost to produce plasma; about 2% – 5% of total chemical energy
produced within the system [20, 88].
Steam reforming:
Pyrogas H O → H CO Syngas (3-2)
Dry CO2 reforming:
Pyrogas CO → H CO Syngas (3-3)
The two reforming reactions shown in (2) and (3) are endothermic [20]. These two
main reforming reactions produce hydrogen rich synthesis gas which can be used for
power generation, utilized for fuel cells to produce electricity [89] and as a building block
for production of synthetic fuels via the Fischer Tropsch process [90].
Syngas is a gas comprising of a varying quantity of hydrogen (H2) and carbon
monoxide (CO). Gliding arc plasma serves as a resource for active species and radicals
such as O and OH which are necessary to stimulate the desired chemical reactions and
reduce the initial temperature required to jump start fuel reforming chemical reactions[78,
91]. Many researchers have done extensive work on reforming hydrocarbons (such as
methane [7, 23, 51, 92], ethane [93], propane [88], and diesel [49]) individually taking
different approaches in the reforming methods adopted. Gliding arc discharge has
successfully been used in fuel reforming of hydrocarbons such as methane, ethane, diesel,
gasoline, biofuels [20] etc. Gliding arc plasma has also found applications in hydrogen
50
sulfide (H2S) dissociation [94], volatile organic compounds (VOCs) decomposition [95]
and carbon dioxide (CO2) dissociation [96]. A main objective of these plasma pyrogas
reforming experiments is the detailed description of the plasma catalytic reforming of a
complex mixture of hydrocarbons (pyrolysis gas) in the presence of non-equilibrium
gliding arc plasma discharge with the aid of a gliding arc reactor which is essentially a
plasma reforming device. Conversion rates also quantified based on hydrogen yield and
carbon monoxide yield. The energy costs of the dry CO2 reforming and steam reforming
reactions were also discussed.
Thermodynamic analysis of Pyrogas reforming reactions 3.3
The pyrolysis gas plasma steam reforming process was simulated with all the
constituent gases that make up pyrogas. Chemical reactions calculations were conducted
based on the supposition that the all the participating reactants completely mix in the
reactor. The thermodynamic simulation was conducted by using the chemical reaction and
equilibrium software - HSC Chemistry (H-enthalpy, S-entropy, C-heat capacity). The
equilibrium composition is calculated using the GIBBS or SOLGASMIX solvers, which
use the Gibbs energy minimization method. Each equilibrium calculation was performed
at constant pressure and temperature. The calculations were performed for the temperature
range 25 – 1700 °C and constant pressure of 1 atm. Initial concentrations of the species
corresponded to those shown in table 3-1. The resulting distribution of molar fractions
with temperature is shown in figure 3-2. Steam reforming of pyrolysis gas was simulated
in an empty reactor with H2, O2, CO2, N2, CO, H2O, CH4, C2H6 and C3H8 as the reactant
gases.
51
Oxidizers such as CO2 and H2O can convert or reform the hydrocarbons to
hydrogen and CO from a thermodynamic point of view but this is unrealistic due to kinetic
limitations. It will take an infinite amount of time for the conversion to occur. Therefore
the thermodynamic simulations presented are intended to show the temperatures at which
the hydrocarbons are converted into hydrogen and syngas. The simulations were run over
an infinite amount of time at different temperatures.
52
Figure 3-15: Equilibrium mole factions of Pyrogas components as a function of temperature.
From the thermodynamic model (figure 3-2), increases in the molar concentrations
of hydrogen and carbon monoxide can be observed, while the molar concentrations of
steam (H2O), carbon dioxide (CO2), and hydrocarbons such as methane (CH4), ethane
(C2H6) and propane (C3H8) diminish. Changes in the molar concentrations with
temperature flatten out at about 800 C. The model shows the concentrations of the
different gases that constitutes pyrogas over time at different temperatures.
53
Thermodynamic analysis of Plasma assisted Tar removal 3.4
Tar is a complex mixture of poly-aromatic hydrocarbons. Tar in the product gases
will condense at low temperature, and lead to clog or blockage in fuel lines, filters and
engines [55]. The results of the thermodynamic simulations provide useful predictions for
the chemical reactions and the accompanying conditions and parameters necessary for
effective tar reforming experiments. The syngas and tar concentrations for the
thermodynamic simulation were based on the results obtained from an industrial
municipal solid waste thermal incinerator. The tar containing syngas gas concentrations
are provided below:
Simulated Pyrogas composition (dry basis):
CO (15 vol %); CO2 (15 vol %); H2 (15 vol %); CH4 (2 vol %); N2 (53 vol %).
Moisture content: 20 vol%
Tar was simulated using Naphthalene (C10H8) dissolved in Toluene (C7H8). The
resulting solution can serve as a tar surrogate due to the close similarities of the chemical
properties with tar [66, 75]. The concentration of the tar surrogate is provided below:
Tar surrogate:
- Composition: naphthalene dissolved in toluene at 1:3.2 mass ratios.
- Naphthalene concentration in inlet gas = 20 mg/L on a dry basis for the inlet gas.
The reaction temperature was varied from 25 °C to 1500 °C at atmospheric pressure.
Table 3.4: Model 1- Gas composition and respective concentration (No steam and Oxygen)
Gas Mole %
N2 53
CO2 15
54
CO 15
H2 15
CH4 2
Total 100
Figure 3-3: Graph showing the thermodynamic simulation of pyrogas without steam and oxygen. Initial gas composition taken from table 3-2.
The figure above shows carbon dioxide and the only hydrocarbon (methane) in the
mixture diminish significantly at about 650 °C and carbon monoxide and hydrogen
(syngas) concentrations reaching a significant peak at about 650 °C. The reaction here can
be described as dry CO2 reforming; where CO2 plays a major role in the hydrocarbon
conversion to produce more CO and hydrogen (see section 1.4.3). This explains the reason
why there is an increase in hydrogen and CO concentrations and a decrease in methane
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
0 200 400 600 800 1000 1200 1400 1600
Molar concentration (%)
Temperature (°C)
Thermodynamic case 1
H2
H2O
CH4
CO
CO2
N2
55
and CO2. Final hydrogen concentration varied between 23% and 27%. Here most of the
reaction occurs between the temperature 300 and 670 °C.
Table 3.4: Model 2- Gas composition and respective concentration (With added steam only)
Gas Mole %
N2 42.4
CO2 12
CO 12
H2 12
CH4 1.6
Steam (H2O) 20
Total 100
56
Figure 3-4: Graph showing the thermodynamic simulation of pyrogas with 20% steam and no oxygen. Initial gas composition and concentration were taken from table 3-3.
The figure above expresses what happens when some water in form of steam is
added to the pyrogas mixture. The graph shows the methane, steam and CO2 diminish
significantly at 600 °C. Hydrogen significant peaks at 600 °C while CO experiences its
most significant increase at 600 °C but continues to increase gradually up to 1500 °C. This
reaction can be described as steam reforming reaction (see section 1.4.4) and it is strongly
endothermic. Most of the reaction occurs between the temperatures 400 and 700 °C.
Hydrogen concentration peaked at 19% and CO concentration peaked at 18% resulting in
a syngas concentration of 37%.
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
0 200 400 600 800 1000 1200 1400 1600
Molar Concentration (%)
Temperature (°C)
Thermodynamic ‐ case 2
H2
H2O
CH4
CO
CO2
N2
57
Table 3.4: Model 3- Gas composition and respective concentration (With Added steam and Oxygen)
Gas Mole %
N2 40.4
CO2 11.4
CO 11.4
H2 11.4
CH4 1.5
Steam (H2O) 19
Oxygen 4.8
Total 100
Figure 3-5: Graph showing the thermodynamic simulation of pyrogas with steam and oxygen in the initial gas composition and concentrations.
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
0 200 400 600 800 1000 1200 1400 1600
Molar Concentration (%)
Temperature (°C)
Thermodynamic ‐ Case 3
H2
O2
H2O
CH4
CO
CO2
N2
58
The next thermodynamic simulation involves the addition of steam and oxygen
into the initial mixture. This is an auto-thermal reaction (see section 1.4.5) and therefore
the overall enthalpy should approach zero reducing the energy involved in the reaction.
Figure 3-5 shows methane and oxygen completely removed (at 575 °C for methane).
Hydrogen concentration peaks at 15% at 600 °C and begins to fall at the same time as
when steam begins to increase. This can be attributed to hydrogen reacting with oxygen to
form more water at temperatures greater than 600 °C. Adding oxygen only helps to
increase the temperature of the reaction, thereby minimizing the need for heat from the
plasma. Plasma will only be providing active species and radicals which stimulate the
reaction thereby reducing the energy cost of the system. Unfortunately, oxygen also
oxidizes hydrogen into water, reducing the concentration of hydrogen rich syngas.
Further thermodynamic simulation was carried out to investigate tar removal in
pyrogas via thermal decomposition and extrapolate the effects of radicals and heat energy
produced by non-equilibrium plasma in pyrogas reforming experiments (see appendix
section). Tar surrogates (Naphthalene and Toluene) are introduced into the pyrogas
mixture based on the concentration described at the beginning of section 3.4.
Table 3.4: Pyrogas mixture and respective concentration with tar surrogate (No water)
Gases Concentration (%)
CO 14.7
CO2 14.7
H2 14.7
CH4 2.0
H2O 0.0
59
N2 52.0
C10H8 0.4
C7H8 1.6
Total 100.0
Figure 3-6: Graph showing Pyrogas mixture with naphthalene without the addition of water to the initial concentration.
In figure 3-6, Methane and carbon dioxide completely vanish at about 800 °C.
Carbon monoxide and hydrogen concentrations peaked at 36% and 22% respectively at
800 °C. Naphthalene start to diminish at 750 °C and vanishes as the temperature
approaches 1600 °C. Since non equilibrium plasma is expected to provide a temperature
of only about 300 °C, external heat energy is needed to make up for the 1500 °C required
to remove the tar surrogate (naphthalene dissolved in toluene) from the mixture. The
lower the external heat energy involved the lower the overall energy cost of tar removal.
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
0 500 1000 1500 2000
Mol. Concentration (%)
Temperature (Celsius)
Nitrogen (g)
CO(g)
H2O(g)
Hydrogen(g)
CO2(g)
Nathphtalene(g)
Methane(g)
Oxygen (g)
60
Table 3.4: Pyrogas mixture and respective concentration with tar surrogate (with added water)
Gases Concentration (%)
CO 11.8
CO2 11.8
H2 11.8
CH4 1.6
H2O 19.6
N2 41.6
C10H8 0.4
C7H8 1.6
Total 100.0
Figure 3-7: Graph showing Pyrogas mixture with naphthalene with the addition of water to the initial concentration.
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0 500 1000 1500 2000
Mol. Concentration (%)
Temperature (Celsius)
Nitrogen (g)
H2O(g)
CO(g)
Hydrogen (g)
CO2(g)
Methane (g)
Naphthalene (g)
C7H8(TLUg)
61
In figure 3-7, with the addition of water in the form of steam to the Pyrogas
mixture, it can be observed that naphthalene is completely destroyed at 800 °C. If non
equilibrium plasma provides active species and radicals and a temperature of just 300 °C,
this is enough to completely destroy tar and other hydrocarbons in an already preheated
pyrogas from an industrial gasifier.
Experimental Investigation of Pyrogas Reforming With Non Equilibrium Plasma 3.5
3.5.1 Experimental Set up
The setup for the pyrogas reforming experiments, shown in figure 3-8 includes a
gliding arc plasma reformer attached to an insulated cylindrical stainless steel reactor
which forms the post plasma region. The high voltage electrode and ground electrode of
the plasma reformer were connected to a D.C power supply (universal voltronics BRC).
The high voltage power supply for the gliding arc plasma device is a BRC 10000
Universal voltronics device. The power is converted using IGBT switching at frequencies
above 20 kHz, and controlled using tuned pulse width modulation techniques. The unit has
an operational output voltage of 0- 10 kV, output current of 0 – 1 A DC and input
frequency of 50/60 Hz. The power supply is connected to an 11 kilo-ohm ballast resistor
unit which helps to limit the plasma discharge current. Omega FMA mass flow controllers
control the mass flow rates of the gases being directed to the plasma reformer. The plasma
reformer is placed inside a furnace (carbolite STF16/610) equipped with a temperature
controller (for experiments with the high temperature plasmatron). Reforming experiments
with the low temperature plasmatron were conducted in a kaowool insulated J-shaped
reactor (figure 3-9). A water tank is connected to a steam generator; the steam generator is
a spiral stainless tube wrapped in heat strips to generate steam at higher temperatures (400
62
Celsius). Thermocouple probes (Omega K type - TJ36-CAXL-116U-18 series) were used
to monitor the temperature within the post plasma region and the steam generator. The
thermocouples can measure temperatures up to 1300 Celsius. Exhaust gases from the
Pyrogas reforming reactions were collected with a 20 mL syringe and analyzed with a gas
chromatograph (Agilent 3000 micro GC) equipped with a heated injector, sample column,
reference column, thermal conductivity detector (TCD), electronic pressure control (EPC)
hardware, gas flow solenoids, and a control board. Information from the data acquisition
unit was monitored via a LabView computer program.
Figure 3-8: Experiment schematic for the pyrolysis gas conversion to syngas.
The plasma reforming of Pyrogas experiments involve the reaction of Pyrogas with
steam or CO2 in the presence of non-equilibrium gliding arc plasma discharge. The
63
pyrogas composition from table 3-1 was replicated by directing the individual constituent
gases into an air-tight stainless steel cylindrical chamber with volume capacity of 1900
cm3. For plasma steam reforming, the gases constituting Pyrogas mixes properly in the
cylindrical chamber (gas mixer) before they are further mixed with steam. The
concentration of the individual gases in the pyrogas mixture coming out of the cylindrical
chamber was compared with the concentrations before entering the cylindrical chamber
(gas mixer) to ensure proper gas mixing. The resulting Pyrogas and steam mixture is
finally directed toward the gliding arc plasma reactor where Pyrogas reforming takes place
as shown in figure 3-8. The steam generator comprises of a 183cm long, 0.32 cm diameter
stainless steel coil wrapped in multiple heat strips. The steam generator unit is further
insulated by ceramic fiber blankets. The ceramic fiber insulation helps prevent the
condensation of the steam formed in the heat strip wrapped coil. Water at flow rates
ranging from 5 ccm to 35 ccm was allowed to flow through the heated stainless steel coil
with the aid of a rotameter. The steam produced from this system, participated in the
plasma - steam reforming process of pyrogas. The experimental setup for dry carbon
dioxide (CO2) reforming of pyrogas is identical to that described in the plasma – steam
reforming set up described above but without the steam and the steam generator (figure 3-
8). The total flow rate of the pyrolysis gas fuel mixture used for the experiments is 30
L/min. It should be noted that the parameters and conditions under which the reforming
experiments were conducted were constant during experiments. A summary of the
operating conditions of the plasma fuel reformer for pyrogas reforming is provided in
table 3-7.
64
Table 3.5: Operating parameters of the plasma reforming experiment
Fuel Pyrolysis Gas
Maximum pressure 1 atm
Furnace Temperature 850 oC
Oxidants Water or CO2
Pyrogas Flow 30 L/min
Steam to Carbon Ratio 0 - 1.5
Plasma input power 3 kW
Spark Gap 3 mm
Figure 3-9: Experimental set-up showing a well- insulated plasma reactor.
65
3.5.2 Gas detection and Gas chromatography
Inlet gases and exhaust gases were detected and characterized by an Agilent 3000
Micro Gas Chromatograph. The gas chromatograph is a device used for testing the purity
of a particular substance, or separating the different components of a mixture and the
relative amounts of such components can also be determined. The instrument uses self-
contained GC module which consists of a heated injector, sample column, reference
column, thermal conductivity detector (TCD) which uses a Wheatstone bridge design,
electronic pressure control (EPC) hardware, gas flow solenoids, and control board. Gas
samples are introduced through a 1/16-inch Swagelok connection to the inlet on the front
panel. The inlet pressure can be nearly atmospheric since an internal vacuum pump
connected to the column exit eliminates column back pressure. The GC is equipped with 2
sample columns which are molecular sieve (molsieve) and plot U columns. The molsieve
column is calibrated to detect hydrogen, oxygen, methane, nitrogen, carbon dioxide and
carbon monoxide. The plot U column is calibrated to detect ethane, propane, acetylene and
ethylene. The carrier gases are helium and argon, the sample input pressure is 2 – 800
PSIG and flow operating temperature is 60 – 120 °C. The drawback of the Agilent micro
gas chromatography is its inability to detect water vapor properly.
3.5.3 Voltage - Current Characteristics
In order to realize the best power regimes for the operation of the gliding arc
plasma reformer for Pyrogas reforming, voltage- current characteristic tests were
conducted. The aim of the voltage- current characteristic test is to determine the critical
regime where the plasma discharge is most stable, effective and at the same time require
minimal electrical energy.
66
Figure 3-10: Voltage - current characteristics at low gas flow rates.
Figure 3-11: Voltage current characteristic test at high flow gas rate.
1.50
2.00
2.50
3.00
3.50
4.00
4.50
180 280 380 480 580 680
Voltage V (kV
)
Current I (mA)
Plasmatron ‐ 1
Flow Rate15LPM
Flow Rate22LPM
Flow Rate30LPM
Flow Rate38LPM
Flow Rate48LPM
2.8
3
3.2
3.4
3.6
3.8
4
200 250 300 350 400 450 500 550
Voltage (kV
)
Current (mA)
Plasmatron ‐2
50 LPM
60 LPM
70 LPM
80 LPM
90 LPM
100 LPM
67
In Figures 3-11 and 3-12, the plasma discharge is most stable at current of 375 mA
and voltage of 3.6kV. The plasma discharge is least stable as you approach the currents
lower than 300mA and it is characterized by intermittent burst of discharge in an effort to
sustain the breakdown voltage across the spark gap. As the current increases beyond 450
mA, the plasma discharge becomes less non-equilibrium.
Figure 3-12: Power vs Current plot at high gas flow rate.
The voltage – current tests were conducted with air as the plasma gas. The
tests were conducted at different gas flow rates at atmospheric pressure. The voltage and
current measurements were taken with a amp meter embedded in the power supply and a
volt-meter. The measurements were further tested and compared to results obtained from
an oscilloscope to confirm the accuracy of the electrical readings.
0.60
0.80
1.00
1.20
1.40
1.60
1.80
200 250 300 350 400 450 500 550
Power (kW)
Current (mA)
Plasmatron ‐2
70 LPM
60 LPM
80 LPM
90 LPM
100 LPM
50 LPM
68
Evaluation of the experimental investigation of plasma assisted pyrogas reforming 3.6
3.6.1 Plasma - Steam reforming
The gliding arc plasma reactor was externally heated up to 850 oC during the steam
reforming chemical process. The exhaust gas composition from the plasma catalysis -
steam reforming process, was directly sampled and analyzed by the gas chromatograph.
Analyzed results obtained from the exhaust gas show that hydrogen concentration
increased as H2O/C ratio increased. Concentration of methane and other heavier
hydrocarbons decreased slightly. At lower H2O/C ratio, CO2 concentration decreased
slightly before increasing upon further increase in H2O/C ratio. Also, CO concentration
increased at lower H2O/C ratios but began to decrease at higher H2O/C ratios (figure 3-
13). This phenomenon can be attributed to water-gas shift reaction (section 1.4.4). The
water gas shift reaction is responsible for the increase in the concentration of hydrogen
and CO2; and the decrease in the concentration of CO. For Syngas production, an increase
in the concentrations of hydrogen and CO; and a decrease in the concentration of CO2 and
hydrocarbons are desired.
69
Figure 3-116: Graph shows results from plasma-steam reforming reaction of pyrogas. Results show the concentrations of constituent gases with varying H2O/C ratios. Increases in hydrogen and decreases in CO can be attributed to water gas shift phenomenon.
3.6.2 Plasma - Dry CO2 reforming
Plasma – dry CO2 reforming is a reforming reaction which involves the reaction of
hydrocarbons with CO2 in the presence of plasma discharge to form syngas.
CH CO → 2CO 2H (3-4)
C H 2CO → 4CO 3H (3-5)
C H 3CO → 6CO 4H (3-6)
With the already available CO2 present in the pyrolysis gas composition, plasma -
dry CO2 reforming of pyrogas experiments were conducted at conditions stated in Table 3-
7. One of the advantages of this chemical reaction process is that the need for a
supplementary source for carbon dioxide (CO2) is unnecessary due to the existing CO2
70
already present in the original pyrogas concentration. This reduces the cost and increases
the efficiency of the chemical reaction process. The non-equilibrium plasma catalysis –
dry CO2 reforming process of pyrogas experiments were conducted at a temperature range
of 800 °C - 900 °C. Exhaust gases from the plasma reforming process were analyzed with
the gas chromatograph. Results (figure 3-14) show increases in the concentrations of
hydrogen and carbon monoxide (CO) after the plasma chemical catalysis process. Also,
the concentrations of methane, ethane, propane and carbon dioxide (CO2) reduced when
compared to their initial individual concentrations in the pyrogas mixture (figure 3-14).
The hydrogen and carbon monoxide yields also increased with increasing enthalpy
(figures 3-15, 3-16). The methodology for calculating the hydrogen and carbon monoxide
yields are expressed in (formula 3-7, 3-9). Figure 3-17 shows an up close image of the
gliding arc plasma discharge and 3-18 show the new generation of low temperature
plasma reformers for hydrocarbon reforming.
71
Figure 17: Graph shows plasma - dry CO2 reforming results for pyrogas at different enthalpies. Changes in the concentration of individual gases with increase in enthalpy (kWh/m3) can be observed
72
Figure 18: The hydrogen yield quantifies the hydrogen produced from the hydrocarbons in the pyrogas mix after the plasma – dry CO2 reforming reaction.
Hydrogen yield 100% (3-7)
Where α is mole fraction of produced hydrogen, αi is initial mole fraction of
hydrogen in pyrogas, and α0 is mole fraction of hydrogen in the hydrocarbons. Enthalpy is
defined as the plasma input power kW divided by the amount of input gas (m3) entering
the reactor per hour.
Enthalpy
(3-8)
73
Figure 19: Graph shows increases in the CO yield of the dry CO2 reforming reaction of pyrogas.
CO yield 100% (3-9)
Where βo is mole fraction of total carbon monoxide (CO), βi is initial mole
fraction of carbon monoxide (CO) in pyrogas mixture, and βx is mole fraction of the
hydrocarbons in pyrogas.
74
Figure 20: A close up view of the gliding arc plasma discharge.
Figure 21: A new generation of high temperature plasmatron developed at the A.J Drexel Plasma Institute.
75
3.6.3 Hydrocarbon conversion
Methane conversions were investigated with steam reforming and dry CO2
reforming reactions. Experimental conditions were the same as those described in section
3.6.1 and 3.6.2 Methane was investigated in this section because the concentration of
methane is the highest (of all hydrocarbons) in the Pyrogas composition. Steam reforming
experiments were conducted with a high temperature plasmatron while dry CO2 reforming
of the hydrocarbons were conducted with the low temperature plasmatron.
Figure 22: Steam reforming versus dry CO2 reforming of methane gas in pyrogas.
Figures 3-19 show the result of the comparison of the different reforming reaction
of methane and ethane respectively. The low conversion of methane in the steam
reforming reaction is due the inability of the experimental plasma system to maintain
steam at high temperatures for the reaction. The steam condensed to water before the
0.00
5.00
10.00
15.00
20.00
25.00
0 0.5 1 1.5 2
Mol. Concentration CH4(%
)
Enthalphy (KWhr/m3)
Dry CO2reforming(Low Temp.Plamsatron)
SteamReforming(High Temp.Plasmatron)
76
steam-Pyrogas-plasma reaction thereby negating the purpose of steam reforming effect.
The inability to maintain vapor temperature of steam eventually resulted in a short circuit
between the plasma electrodes making it impossible to vary input power and enthalpy.
3.6.4 Effect of size of the electrodes on hydrocarbon conversion
In an effort to optimize the gliding arc plasmatron for more effective hydrocarbon
conversion, the stainless steel high voltage and ground electrodes were modified to have
replaceable electrodes of 3 different sizes. The different electrodes are simply called large
electrodes, Medium electrodes and small electrodes based on their respective internal
diameters and lengths. Experiments were conducted to investigate the effect of the
electrode configurations on hydrocarbon conversion. Figure 3-20 shows the different
electrode sizes for the low temperature plasmatron. The internal diameter of the large
sized electrode is 2 cm, internal diameter of the medium sized electrode is 1.5 cm, and
internal diameter of the small sized electrode is 1cm.
77
Figure 23: The low temperature plasma reformer with the different size configuration of high voltage electrodes and ground electrode inserts.
Figure 241: Methane conversion with large electrode plasmatron configuration
0.00
5.00
10.00
15.00
20.00
25.00
0 0.5 1 1.5 2
Mol. Concentration CH4(%
)
Enthalphy (KWhr/m3)
Methane (CH4) ConversionLow Temp. Plasmatron
LargeElectrodeconfig.
MediumElectrodeConfig.
SmallElectrodeConfig.
78
For methane conversion, (figures 3-21), the large electrode configuration indicates
better conversion rates than the medium and small electrode configurations. This implies
that the larger the internal diameter of the electrodes and the longer the length of the
ground electrode inserts the greater the plasma vortex flow volume. This leads to a greater
residence time (reaction time) which results in greater hydrocarbon conversion and greater
syngas yields.
3.6.5 Effect of preheating pyrogas on hydrocarbon conversion
The effect of preheating the pyrogas mixture on methane and ethane conversion
via non equilibrium plasma was also investigated and compared to the methane and ethane
conversion results obtained from cold pyrogas mixture. Pyrogas was preheated to a
temperature of 400 oC before mixing and reacting with plasma in the expectation that heat
energy will increase the kinetic energy of the gas molecules will improve pyrogas
conversion into syngas. The tube carrying the cold pyrogas mixture was heated up to a
temperature of 400 oC by heat strips and insulated by kaowool to prevent heat losses to the
surrounding environment. Minimal differences can be observed from the methane and
ethane conversion results obtained from the preheated pyrogas mixture in comparison to
the cold pyrogas mixture (Figures 3-22 and 3-23). The inability to maintain higher
pyrogas preheating temperatures can be attributed to the unexpected methane and ethane
conversion results for the preheated pyrogas reforming experiments.
79
Figure 25-22: The effect of pyrogas-preheating in comparison to cold pyrogas mixture on methane conversion.
Figure 3-23: The effect of pyrogas-preheating in comparison to cold pyrogas mixture on ethane conversion.
0.00
5.00
10.00
15.00
20.00
25.00
0 0.5 1 1.5 2
Molar
Concentration CH4
(%)
Enthalpy (KwHr/m3)
Methane (CH4) ConversionLow temp. Plasmatron
Pre‐HeatedMixture
ColdMixture
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
0 0.5 1 1.5 2
Molar Concentration C
2H6(%
)
Enthalphy (KWhr/m3)
Ethane (C2H6) ConversionLow temp. Plasmatron
Pre‐HeatedMixture
ColdMixture
80
Conclusion 3.7
The main purpose of this work is to demonstrate the effectiveness of the gliding
arc plasma assisted reformer in removing light and heavy hydrocarbons contained in
pyrogas by converting the hydrocarbons and carbon dioxide to syngas using both steam
reforming reactions and dry CO2 reactions at atmospheric pressure conditions. The data
collected and results analyzed during the non-equilibrium plasma reforming experiments
indicate the plasma reforming of pyrogas into synthesis gas (syngas) using plasma - dry
CO2 reforming reaction. Note that pyrogas from an industrial gasifier is usually at a
relatively high temperature (800-900 oC). Hence plasma energy is mainly required to
stimulate chemical reactions and not as a source of heat energy in the reforming processes
discussed in this paper. This makes energy consumption for pyrogas reforming very low.
Conversion rates of the different gases were higher with plasma - dry CO2 reforming
reaction when compared to plasma - steam reforming reaction. Lower conversion rates
observed with plasma – steam reforming reaction may be attributed to the inability to
maintain steam temperature required for homogenous reaction. The ability to maintain
steam at a much higher temperature should eventually lead to better hydrocarbon
conversion results.
81
Introduction 4.1
An alternative process of extracting energy from fossil fuels (coal, biomass,
hydrocarbons etc.) without the emission of CO2 is possible with non-equilibrium plasma.
This chapter describes a novel process of extracting the energy from fossil fuels without
the emission of CO2 while producing hydrogen and carbon suboxide (a reddish, brown
polymer), an important constituent of organic fertilizers. This approach has the capability
of avoiding drawbacks associated with combustion of fossil fuels, such as CO2 emission.
A major problem associated with modern energy production from fossil fuels is
high CO2 emissions, which is linked to global climate change also known as global
warming. The transition from thermal power plants to nuclear energy would ensure a
significant decrease in CO2 emissions. In Europe, the total amount of electricity produced
by nuclear power plants reaches 80% in France, 60% in Belgium and 43% in Sweden
(14% of the world’s total electricity is produced by nuclear power plants) [97]. Nuclear
accidents at the Chernobyl nuclear power plant, Ukraine (April 1986), and Fukushima
nuclear power plant, Japan (March 2011), called into question not only further
developments in nuclear energy, but its viability as a primary source of safe and clean
energy. It turns out the current security measures cannot eliminate the possibility of a
nuclear accident, which can result in the spread of radioactive substances across national
borders. Taking all these facts into account, there is an ongoing debate over the direction
of the development of the power industry. One option is the transition to thermal power
stations running on fuels (such as peat, lignite, and human wastes) with low energy
content (10-25 MJ/kg). World reserves of lignite coal and sub-bituminous coal make up
more than half of all coal reserves in the world. Another energy source that can replace
nuclear power is natural gas; the world’s gas reserves are 1754 trillion cubic meters
82
(2006). It is estimated that there are also about 900 trillion cubic meters of
"unconventional" gas such as shale gas, of which 180 trillion cubic meters may be
recoverable [98]. The third source of energy is biomass. Biofuels are gaining increased
public and scientific attention, driven by factors such as oil price and the need for
increased energy security.
Three different methods of extracting chemical energy from a biomass feedstock
are shown in figure 4-1. The direct combustion of biomass approach results in release of
the most significant amount of energy as well as water and CO2 but this process has a low
thermal efficiency - about 26% [99]. Another method of energy extraction from biomass is
via reforming reactions (e.g. steam, dry CO2 or partial oxidation reactions), which results
in the production of syngas (combination of hydrogen and CO). Partial oxidation of a
hydrocarbon such as diesel results in an energy conversion efficiency of 70% [41]. Syngas
can be subsequently burned releasing a significant amount of energy but this process also
results in the eventual release of CO2. Direct combustion and reforming of biomass both
result in production of significant quantities of CO2, a greenhouse gas. This drawback
necessitates the adoption of expensive carbon sequestration methods (construction of
underground storages for CO2, chemical binding with amines, etc.) which can cost as
much as $1800 per ton of carbon for 2.3 x 106 tons of carbon [100]. A third possible
method of extracting energy from biomass is the partial oxidation reaction which results in
the production of hydrogen and solid (C3O2)n (poly(carbon suboxide)). This novel route,
which involves the production of hydrogen and solid carbon suboxide from a hydrocarbon
feedstock without CO2 emission is demonstrated in this chapter. This approach was first
suggested by A. Fridman et al [101] and was based mainly on thermodynamic
83
assumptions. Although this approach may result in less energy release in comparison to
direct combustion and reforming reactions, it avoids the release of CO2 in the atmosphere.
An added advantage to this approach is it’s byproduct – solid poly (carbon suboxide)
(C3O2)n, major building block of organic soil fertilizers.
Figure 26: Schematic showing the methods of extracting energy from Biomass and the resulting products
Carbon suboxide 4.2
4.2.1 Background
Apart from CO and CO2, there exists carbon suboxide (C3O2 or O=C=C=C=O) - a
solid carbon oxide, which can be polymerized to form chemically and thermodynamically
stable substances. Carbon suboxide was first discovered by Benjamin Brodie in 1873
while subjecting carbon monoxide to an electric field [102]. Carbon suboxide is typically
84
synthesized artificially by the dehydration of malonic acid - CH2 (COOH)2 in phosphorus
pentoxide (P2O5)[101, 103, 104].
4.2.2 Properties of carbon suboxide
Carbon suboxide (C3O2) is a foul-smelling lachrymatory non-toxic gas. It has
linear symmetric structure that can be represented as O=C=C=C=O. The suboxide
monomer is stable at -78 oC; at 25 oC it polymerizes to form a highly colored solid
material with a polycyclic six-member lactone structure (figure 4-2). Carbon suboxide is
the anhydride of malonic acid, and it slowly reacts with water to produce malonic acid. It
can be stored at a pressure of a few Torr, but under standard conditions C3O2 forms a
yellow, red, or brown polymer (C3O2)n (ruby-red above 100oC, violet at 400oC, and it
decomposes into carbon at 500oC) [1, 101, 103, 105]. The enthalpy of formation for gas-
phase carbon suboxide C3O2 was reported to be between -97.6 kJ/mol [106] and -95.4
kJ/mol [107]. The enthalpy of formation for liquid polymerized carbon (C3O2)n is -100 kJ
per one mole of carbon, and the enthalpy of formation for solid polymerized carbon
suboxide (C3O2)n is above -112 kJ per one mole of carbon[108-110]. (C3O2)n is a building
block of humic substances, a main component of organic fertilizers [101, 111].
85
Figure 27: Basic structure of carbon suboxide (C3O2) n [101].
4.2.3 Thermodynamic analysis of carbon suboxide
An important aspect of transitioning to new energy sources or new technologies
for extracting energy from fuels is the energy cost involved and the advantages they have
over preceding technologies. The thermodynamic aspect of producing energy (hydrogen)
and suboxide from different types of hydrocarbon fuel is described in detail below. First,
the general formula for calculating energy efficiency is derived, followed by the results of
thermodynamic calculation for different types of fuel considered. The efficiency of
hydrocarbon conversion resulting in solid carbon suboxide and hydrogen is measured with
respect to the efficiency of producing syngas (particularly CO) from reforming reaction of
a hydrocarbon feedstock. In this regard, the efficiency (%) of producing solid carbon
suboxide is derived as a percentage of the efficiency of producing syngas (assuming the
efficiency of producing syngas is 100%). In solid state, carbon suboxide can be in form of
(C3O2)n and also in the form of monomers with carbon to oxygen ratio , 1 < < . The
general formula for polymerized carbon suboxide can be written as
(C1-xOx)n (4-1)
86
Here, coefficients x and 1 - x represent proportional amounts of atoms of oxygen
and atoms of carbon in mole monomers (C1-xOx)n respectively. Coefficient x lies in the
limits: 0.4 ≤ x < 0.5, where x=0.4 corresponds to C0.6O0.4≡C3O2 (tricarbon dioxide). Since
enthalpy of formation for solid carbon suboxide is known only for tricarbon dioxide,
enthalpy of formation for a mole of monomers (4-1) can be approximated by the linear
formula:
Ex = a*x + b (4-2)
Where Ex represents the enthalpy of formation and a & b are constants. Assuming that
enthalpy of formation for pure carbon (x=0) equals 0, then constant b=0. Also, since the
enthalpy of formation per one mole of monomers C1O2/3 is -112kJ, therefore the enthalpy
of formation for monomers C0.6O0.4 equals E0.4 = -112*(1-0.4) = -67.2 kJ. Substituting E0.4
in (4-2) allows calculating coefficient a
a = ∗ ,
.168
Ex = 168x (4-3)
Biofuels such as oil, wood, peat, lignite (type of coal) and natural gas consist mainly of
carbon, hydrogen and oxygen (95-98%). Therefore, the chemical formula for biofuel can
be expressed in the form:
CαHβOγ (4-4)
Where α, β and γ represent the ratios of molecules of carbon, oxygen and hydrogen in
biofuel and it can be assumed that α+β+ γ =1. Also, hydrogen, oxygen and carbon in fuels
can be expressed in weight percentage:
CwHwOw (4-5)
87
It can be assumed again that Cw + Hw + Ow =1. Formulas expressing values , , with
respect to CwHwOw can be further written as:
α =
, β =
, γ =
(4-6)
The formula expressing the heating value (HV) of hydrocarbon fuel in kJ per mole of
monomers (4-4) with respect to heating value in kJ per kilogram is shown below:
HVmol = 10-3 (12Cα + 16Oβ + Hγ) HVW (4-7)
Calculating the theoretical heating value of partial oxidation of biofuel (4-4) to suboxide
and hydrogen gives:
CαHβOγ +. β O2 *C1-xOx + H (4-8.1)
H + O H O (4-8.2)
Reaction (4-8.1) represents the conversion of the fuel into solid carbon suboxide
and hydrogen; while reaction (4-8.2) represents the combustion of hydrogen. The heating
value of carbon suboxide (HVsub) can be expressed as:
HVsub = Enthalpy of formation (CαHβOγ) – Enthalpy of formation ∗ C O H O (4-9)
Cα Oβ Hγ + (α - β) O2 = α CO + H2 (4-10.1)
H2 + O2 = H2O (4-10.2)
α CO + O2 = α CO2 (4-10.3)
Equation (4-10.1) describes the production of carbon monoxide and hydrogen from
biofuel (4-4). Equation (4-10.2) and (4-10.3) correspond to the subsequent combustion of
CO and H2 produced from conversion of biofuel. The energy released in reaction (4-10.1)
88
is small. For methane, energy released in (4-10.1) equals 35.6 kJ per mole of carbon,
which is only 4% of the full energy of combustion. For cellulose and lignite ( 45.01144.0 HOC
), reaction (4-10.1) is endothermic, it demands an additional 9.5 kJ per mole of carbon for
cellulose and 31 kJ per mole for lignite. This shows that the energy released in reaction (4-
10.1) is insignificant and can be neglected. Therefore, the heating value of CO (HVCO)
derived from (4-10.1 to 4-10.3) is stated below:
HVCO = Enthalpy of formation (α.CO) – Enthalpy of formation α ∗ CO H O (4-
11)
Energy efficiency of producing solid carbon suboxide and hydrogen from a hydrocarbon
is defined by the ratio of heating value of carbon suboxide (4-9) and that of carbon
monoxide (4-11):
EFsub = (4-12)
For a variety of fuels, the heating values are known but the energies of formation
are unknown. In such instances, the enthalpy of formation of a hydrocarbon fuel can be
calculated from the fuel’s composition and heating value:
Enthalpy of formation (CαHβOγ) = Enthalpy of formation αCO H O + HV (CαHβOγ) (4-13)
Thermodynamic calculation of the efficiency of hydrogen and carbon suboxide 4.3
production from various hydrocarbon feedstocks
The difficulty associated with the calculation of the energy efficiency of hydrogen
production from biofuels lies in the fact that the formation enthalpy is not known for many
biofuels. Therefore, the use of the formulas obtained from extrapolating experimental data
89
becomes essential for the calculation of energy efficiency. In Table 1, the energy
efficiency of hydrogen and solid carbon suboxide production with respect to the
production of hydrogen and CO gas from methane, n-butane, and cellulose is presented for
different contents of oxygen in the monomer C1-xOx. Energies of formation were obtained
from the National Institute of Standards and Technology (NIST), Chemical Kinetics
Database[112]. In table 4-1, x = 0.4 corresponds to C3O2, x = 0.5 corresponds to monomer
CO (x can be less than 0.5, but in this calculation, we have chosen x = 0.5).
Table 4.3: Energy efficiency of producing carbon suboxide with respect to syngas production (100%) from cellulose and two hydrocarbons
Efficiency (%)
x Methane (CH4)
n-Butane
(C4H10) Cellulose (C12O11H22)
0.40 68 49 43
0.45 71 54 48
0.50 75 59 54
In table 4-2, the energy efficiency of hydrogen and carbon suboxide production
with respect to the production of hydrogen and CO gas from wood, sunflower oil, and
castor oil are presented for different values of oxygen atoms in the monomer C1-xOx. For
these calculations, energies of formation were obtained from [113].
90
Table 4.3: Energy efficiency of producing hydrogen and solid carbon suboxide from wood, sunflower oil, and castor oil. HVW represents heating value in MJ/kg.
Efficiency (%)
x
Wood Fuel
(C6O5H10) Sun Flower Oil (C18O2H32)
Castor Oil
(C18O3H3)
HVW =21MJ/kg HVW =39.5MJ/kg HVW =39.5MJ/kg
0.40 59 66 67
0.45 65 71 72
0.50 71 77 78
Table 4-3 presents the energy efficiency of hydrogen and suboxide production with
respect to the production of hydrogen and CO gas from biofuels and peat. For these
energy efficiency calculations, formula from [114] was used:
HVW = 103 (-0.763 + 30.1CW + 52.5HW + 6.4OW) (4-14)
This is the best approximation of experimental data for different concentrations of
hydrogen HW, oxygen OW, and carbon CW for biofuel. In the calculations, HW = 0.06. It
should be noted that heating value of peat is variable; it depends on moisture, composition
and ash content. Biofuels and peat are categorized together due to the similarity in the
composition and properties of dry peat and biofuels.
91
Table 4.3: Energy efficiency of producing hydrogen and solid carbon suboxide from peat
Peat (Hw=0.06) Efficiency (%)
Cw x = 0.40 x = 0.45 x = 0.50
0.30 60 64 69
0.35 55 60 66
0.40 51 56 63
0.45 48 53 57
Figure 4-3 and table 4-4 show the energy efficiency of hydrogen and solid carbon
suboxide production with respect to the production of hydrogen and CO gas from coal. It
should be noted that heating value of lignite changes in wide limits from 10 MJ/kg to 20
MJ/kg and for peat, the heating value depends on humidity and coal composition. Heating
value calculations[115] were based on the formula:
HVW = 103 (34.01CW + 131.96HW – 11.83OW) (Dry, Without Ash) (4-15)
Table 4.3: Energy efficiency of producing hydrogen and solid carbon suboxide from coal
Lignite (Hw = 0.060-0.058) Efficiency (%)
Cw x = 0.40 x = 0.45 x = 0.50
0.60 48 54 61
0.65 50 57 64
0.70 53 59 66
0.75 54 61 69
92
Figure 28: Energy efficiency of production of carbon suboxide and hydrogen with respect to heating value of production CO gas and hydrogen. x=0.4 corresponds to C3O2. HW lies in the limits 0.038 - 0.060.
Results from thermodynamic calculations show that the energy efficiency of
producing hydrogen and solid carbon suboxide can be up to 78% of energy efficiency of
syngas production (depending on the type of hydrocarbon feedstock). The decrease in
energy efficiency is compensated for by the elimination of CO2 emissions and production
of two valuable products: hydrogen and carbon suboxide (a major component of organic
fertilizers). The costs of removal or transfer of carbon suboxide generated (soil
conditioner) in a partial oxidation reaction need to incorporated into future feasibility
studies.
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.6 0.65 0.7 0.75 0.8 0.85 0.9
Energy efficiency (x 100%) with respect to CO
Cw ‐ amount of carbon in coal (x 100%)
x=0.4
x=0.45
x=0.5
93
Evaluation of Experimental Investigation of Plasma Assisted Hydrocarbon 4.4
Conversion Into Carbon Suboxide Via Partial Oxidation Reaction
4.4.1 Experimental set up
Gliding arc plasma (GAP), arc discharge and dielectric barrier discharge (DBD)
are three atmospheric pressure plasma discharges considered for the carbon suboxide
(C3O2) formation from plasma assisted hydrocarbon oxidation reaction. Arc discharge and
gliding arc discharge proved to be ineffective in the formation of carbon suboxide due to
the high heat energy and power of these discharges. Other warm or hot plasmas will be
ineffective for the same reasons stated above. A far more non-equilibrium plasma
discharge (dielectric barrier discharge) is better suited for low temperature gaseous
hydrocarbon oxidation reactions due to its low temperature and low power requirement.
Low temperature plasma assisted oxidation of hydrocarbon was conducted in a dielectric
barrier discharge (DBD) reactor with n-butane (C4H10) as the hydrocarbon feedstock
mixed with air. The suitability of DBD for the plasma assisted partial oxidation reaction
for carbon suboxide production over other cold or warm plasma discharge types can be
attributed to a combination of the following properties: greater plasma volume (which
ensures greater residence time) due to the multiple micro-discharges it produces, almost
no local heating and its relatively low gas temperature (350 K – 500 K) properties. The
DBD reactor, figure 4-4, consists of a 1.22 m long quartz tube with 22 mm internal
diameter (ID). The quartz tube functions as a dielectric barrier. Inside the quartz tube is an
80 cm long stainless steel electrode that serves as the high voltage electrode. The high
voltage electrode was held in place inside the quartz tube with the aid of a silicone stopper
at the inlet of the quartz tube and a ceramic holder at the inner center of the reactor. The
uniform distance between the high voltage electrode and the quartz inner wall will ensure
94
the uniformity of the propagation of the plasma discharge from the high voltage electrode
to the inner wall of the quartz tube. A copper mesh is wrapped around the outer surface of
the quartz tube to align with the stainless steel electrode, spanning 40cm in length. The
copper mesh serves as the ground electrode. N-Butane gas and air were mixed before
entering the DBD reactor. The gas mixture was delivered to the reactor via ¼” tubes and
fed directly into the reactor through an aperture within the stainless steel electrodes.
Butane and air flow rates were each controlled with Omega FMA-2605A series mass flow
controllers according to the desired oxygen to carbon ratio.
The cylindrical DBD reactor was placed in a temperature controlled Carbolite STF
16/610 furnace. In order to accelerate the rate of deposition of solid products produced
from the plasma assisted oxidation of n-butane (C4H10), a water-cooled heat exchanger
made from 1/16” copper tubing wound into a spiral shape was inserted inside the post-
plasma region of the DBD reactor. Typical experiments involve the treatment of n-butane
and air mixture with DBD for 150 minutes after which n-butane and air flows are shut off
and the deposit dried up in the reactor via the heat from the furnace at 150°C.
Table 4.4: Parameters and conditions of partial oxidation of n-butane experiments
Gas n-Butane, Air
Total Flow rate 1.2 L/min
Temperature 150°C - 400°C
Oxygen : Carbon ratio 0.4-0.9
Discharge Power 80 Watts
95
Figure 29: Schematic of the experimental setup for the formation of carbon suboxide from hydrocarbon oxidation reaction.
4.4.2 Role of Dielectric Barrier Discharge
Dielectric barrier discharge is a non-thermal plasma discharge, which is strongly
non-equilibrium at atmospheric pressure. DBD generates atomic and molecular species,
free radicals and electrons (necessary for butane reforming into carbon suboxide and
hydrogen) with mean kinetic energy of 1-10eV [1]. DBD already has a wide range of
applications which include ozone generation, thin film deposition [116, 117], polymer
treatment [118, 119], pollution treatment [120], exhaust gas cleaning (from CO, NOx, SO2
and volatile organic compounds) [121], medical applications [1, 118, 122], in CO2 lasers
[123], plasma assisted combustion [1]. The dielectric barrier discharges have been
extensively discussed in [104, 124, 125] As a result of a dielectric barrier between the high
voltage and ground electrodes, the DBD requires alternating current for its operation.
96
The current passing through the dielectric is limited by factors such as the
dielectric thickness, dielectric constant and the time derivative of the applied voltage
dU/dt. High electric fields are required to propagate current through the discharge gap to
cause a break-down in the gas. The dielectric usually limits the current density in the gas
space thereby acting as ballast which does not consume energy. Multiple micro-discharges
can be observed when the electric field in the discharge gap is high enough to cause a
breakdown. At higher frequencies, the dielectric material’s current limitation diminishes
[123]. DBD is a non-uniform discharge which comprises of multiple, distributed micro-
discharges in the discharge gap. The micro-discharges are usually in constant motion and
interact frequently [1].
4.4.3 Voltage – Current Characteristics of DBD discharge
A variable alternating current (AC) power supply was used to generate the DBD
plasma. The power supply has an operational frequency of 50 Hz – 1.66 kHz and a
maximum peak-to-peak voltage range of 20 kV– 34 kV. Typical voltage and current
waveforms of the plasma discharge are shown in figure 4-5. A 500 MHz Tektronix digital
phosphor oscilloscope is connected to the electrodes to monitor and evaluate the electrical
characteristics of the plasma power. In order to obtain the total DBD power, integration of
the voltage and current was conducted using the formula stated below and the parameters
obtained from the oscilloscope. The average DBD power was calculated to be 80 Watts.
The DBD power is calculated with the expression
0
0
( )1
n m
T n nn
u iP u i dt
T m
(4-16)
97
P: DBD power, u: applied voltage, i: total current; m: the number of sample points, T:
sampling time.
Figure 4-5 shows the voltage-current transient waveform obtained by plotting the
voltage-time, current-time data points with Matlab. Figure 4-6 shows the typical
waveforms obtained from the oscilloscope during the butane partial oxidation experiments
(producing carbon suboxide) at different resolutions and sampling times.
The overall butane oxidation chemical reaction to form hydrogen and carbon
suboxide is given below:
3C4H10 + 4O2 4 C3O2+ 15 H2 (4-17)
Figure 30: Voltage (left) and current (right) characteristics of the plasma discharge, data points taken from a 500 MHz Tektronix digital phosphor oscilloscope
98
Figure 31: Oscilloscope (500 MHz Tektronix digital phosphor) screenshots of the discharge voltage (top in yellow) and current (bottom in green) signals at two different resolutions and sampling periods.
4.4.4 Reaction mechanism of n-Butane conversion to carbon suboxide
The gas phase reaction and surface reaction mechanisms are presented in tables 4-6
and 4-7 where n-butane is represented by RH, R is C4H9 and R’ is C3H6 (propylene). At
low temperatures, initial stages of oxidation of saturated hydrocarbons (alkanes) start with
the dissociation of oxygen. Atomic oxygen reacts forming an alkyl radical and a hydroxyl
radical. Hydroxyl radical then eliminates a hydrogen atom from RH, forming water and an
alkyl radical. The alkyl radical will typically react quickly with oxygen forming a peroxy
radical [126-128].
99
Table 4.4:Gas phase reaction mechanism for low temperature n-butane oxidation
e + O2 O + O + e (4-18.1)
O + RH R˙ + OH˙ (4-18.2)
OH˙ + RH R˙ + H2O (4-18.3)
R˙ + O2 R˙O2 (4-18.4)
The surface reaction mechanism for low temperature n-butane oxidation has not
been fully investigated but a possible reaction mechanism is presented in table 4-7. At
appropriate temperatures, the peroxy radical decomposes into HCO˙ and propylene
releasing water. HCO˙ combines to form C2O and H2O[1]. C3O2 monomer is formed by
the reaction of C2O with CO [129].
Table 4.4: Possible surface reaction mechanism
R˙O2 HCO˙ + H2O + R’ (4-18.5)
HCO˙ + HCO˙ C2O + H2O (4-18.6)
C2O + CO C3O2 (4-18.7)
Evaluation of non-equilibrium plasma reforming of hydrocarbons for carbon 4.5
suboxide production
4.5.1 Experimental results and discussion
The low temperature oxidation experiments were conducted in the DBD plasma reactor
with n-butane (C4H10) and air as the reacting gases. The experiments were conducted at
100
atmospheric pressure conditions. The temperature around the external region of the DBD
reactor was constantly monitored with the aid of thermocouples connected to temperature
readers. Glass slides (19mm x 19mm x 0.5mm) were placed within the post plasma region
of the DBD reactor to collect solid deposits for analyses and characterization.
Solid deposits start to form within moments of n-butane-air reaction in the
presence of plasma as shown in figure 4-7. The deposits formed within the post plasma
region of the reactor usually appear reddish brown at temperatures between 150 ºC - 400
ºC and black at temperatures greater than 500 ºC. The reddish – brown deposits collected
after 150 minutes of plasma treatment at low temperatures (150 ºC - 400 ºC) appear wax-
like and sticky. This is attributed to some water produced during the butane - oxidation
reaction. The gas produced at the post plasma region of the DBD reactor as well as the
reddish brown deposits had a pungent smell and induced lacrimation. The reddish brown
deposits collected on the glass slides inside the reactor were dried (using the heat from a
furnace) and analyzed for its atomic composition via energy-dispersive x-ray spectroscopy
(EDX).
101
Figure 32: Initial stage of carbon suboxide formation in the DBD reactor (left) and the later stage of gradual carbon suboxide formation along the walls of the DBD reactor (right).
4.5.2 Product Characterization
The characterization of the deposits obtained from the DBD reactor was carried
out with Zeiss Supra 50VP Scanning Electron Microscope with Energy-dispersive X-ray
spectroscopy (EDX). This is an analytical technique used for the elemental analysis. EDX
works by detecting characteristic X-rays that are produced by a sample placed in an
electron beam [130]. In addition to visualization, SEM is capable of providing information
such as surface topography and composition of the sample
4.5.3 Characterization Procedure
To ensure the accuracy of the characterization results, a sample of caprolactam (C6H11NO)
was tested first. The Carbon:Oxygen atomic ratio for caprolactam came out to be
5.5~6.5:1, confirming the accuracy of the EDX analysis. Elemental analysis tests were
carried out on the solid deposit samples collected during the plasma assisted n-butane
oxidation experiments. The Carbon: Oxygen ratio of the sample analyzed was in the range
Deposits formed After 40 minutes
After 150 minutes
102
1.5~1.8: 1 (Table 4-8) and this ratio corresponds to carbon suboxide C3O2 (Carbon:
Oxygen = 1.5). The SEM images of the characterized samples are shown in figures 4-8, 4-
9, 4-10, 4-11, 4-12 (left). The raw spectrum results from the EDX spectroscopy are shown
in figures 4-8, 4-9, 4-10, 4-11, 4-12 (right) for multiple samples.
Figure 33: SEM image (left) and EDX spectrum (right) of deposit formed from plasma assisted butane oxidation. The pink square in the SEM image represents the region on the sample where EDX signals were collected. The EDX spectrum shows the peaks of carbon, oxygen.
Figure 34: SEM image (left) and EDX elemental analysis (right) of sample B
103
Figure 35: SEM image (left) and EDX elemental analysis (right) of sample C
Figure 36: SEM image (left) and EDX elemental analysis (right) of sample C
104
Figure 37: SEM image (left) and EDX elemental analysis (right) of sample E
Table 4.5: Atomic composition of an analyzed sample, each column shows the atomic percent obtained for each of the six (A-F) areas of the sample analyzed with EDX
Element
A
Atoms
(%)
B
Atoms
(%)
C
Atoms
(%)
D
Atoms
(%)
E
Atoms
(%)
F
Atoms
(%)
C 53.69 50.55 51.43 51.64 52.7 51.74
N 12.77 15.78 14.18 10.61 14.66 14.65
O 33.54 33.67 34.39 35.41 32.51 33.61
C:O ratio 1.60 1.50 1.50 1.46 1.62 1.54
Table 4-8 shows the analyses of a sample produced at Oxygen: Carbon = 0.42, 200
°C, 80W and 1.2 L/min. Six different spots of the sample have been analyzed by EDX
and based on the results the average C:O ratio at is 1.54. This result supports the
105
assumption that the polymerized deposit has the atomic ratio close to carbon suboxide
(C3O2)n. Carbon suboxide (C3O2)n has a carbon to oxygen ratio of 1.5 (exactly). From
further analyses, greater carbon: oxygen ratios were observed with increasing
temperature. With the low flow rates of gases in the experiments, hydrogen yield was
extremely low, making analysis of the gas phase difficult. Further work should be
focused on establishing the optimal regimes for carbon suboxide and hydrogen
production using different hydrocarbon feed stocks at different O/C ratios, temperature,
plasma power and reagents flow rates. The future work will also include more detailed
analysis of the gas phase products of the n-butane oxidation reaction.
Conclusion 4.6
Thermodynamic calculations show that it is possible to attain relatively high
energy efficiency of conversion of abundant, low quality energy feedstock such as lignite,
peat, and biofuels into hydrogen and carbon suboxide. The highlighted approach suggests
a major shift of the paradigm of hydrocarbon feedstock as an energy source. It allows a
substantial portion of energy to be released in the form of hydrogen, or eventually
electricity, together with simultaneous bonding of carbon into solid polymerized carbon
suboxide that can be used to produce organic fertilizers. Decrease in energy efficiency,
when compared to synthesis gas production is compensated by the elimination of CO2
emissions and production of two valuable products: hydrogen and carbon suboxide.
Experimental testing of non-thermal plasma reforming of n-butane in DBD reactor
confirms carbon suboxide production. Elemental analysis (carried out with EDX
spectroscopy) of the produced deposits suggests that C:O ratio in the analyzed sample is
close to 1.5 and this corresponds to the C:O ratio in carbon suboxide (C3O2)n. This is the
106
first time in history that carbon suboxide has been successfully produced from a
hydrocarbon feedstock. The thermodynamic calculations show the efficiency of
converting hydrocarbon feedstock such as low quality coal (lignite and peat) into carbon
suboxide and hydrogen. The thermodynamic conversion efficiency was calculated to be as
high as 78% with respect to the efficiency of converting the same hydrocarbon feedstock
into hydrogen and carbon monoxide. The experimental results have demonstrated the
possibility of producing carbon suboxide from a hydrocarbon feedstock – n-butane.
107
5 CONCLUSIONS AND SUMMARY
This thesis expresses in detail the effect of non-equilibrium plasma on fuel
conversion of hydrocarbons, municipal wastes, biomass, and coal into hydrogen and other
useful products. It specifically addresses two important fuel conversion issues, namely: the
clean up or conversion of pyrogas in to hydrogen rich gas mix (syngas) and the conversion
of hydrocarbons without the emission of carbon dioxide into the atmosphere. In both
cases, transitional plasma has been demonstrated to be effected in fuel conversion mainly
due to its non-equilibrium properties. Chapter one introduces the concept of plasma and
the applications of non-equilibrium plasmas for fuel conversion and hydrogen production,
Chapter two discussed further, gliding arc plasma as a viable non-equilibrium plasma
discharge in terms of its design, physical properties and characteristics. Chapter three
focuses on gliding arc plasma assisted conversion of pyrogas into syngas. The
effectiveness of using gliding arc plasma for the removal or clean-up of light and heavy
hydrocarbons present in pyrogas was successfully demonstrated through different
reforming reaction experiments. Plasma - Dry CO2 reforming was shown to be more
effective than steam reforming for pyrogas conversion into syngas mainly because of the
inability to maintain high steam temperatures necessary for steam reforming reactions.
Plasma - dry CO2 reforming reaction achieved up to 80% syngas concentration up from
47% syngas in the initial pyrogas mixture. Large electrode configuration showed better
hydrocarbon conversion when compared to small electrode configuration. Large
electrodes results in greater plasma discharge volume (in comparison to smaller
electrodes) resulting in better conversion results. Thermodynamic simulations were also
conducted in order to determine the respective temperatures of conversion for respective
hydrocarbons in pyrogas. Chapter four discusses the use of dielectric barrier discharge
108
(DBD) plasma for hydrocarbon conversion without CO2 emission. The conversion of
hydrocarbon feedstock – n-butane- into hydrogen and carbon suboxide via partial
oxidation reaction in the presence of dielectric barrier discharge was successfully
demonstrated. This happens to be the first time carbon suboxide has been produced from a
hydrocarbon feedstock. Also, thermodynamic calculations show the possibility of
attaining up to 78% efficiency for carbon suboxide production with different hydrocarbon
feedstock in comparison to Syngas production (100%). The resulting efficiency of
producing carbon suboxide from hydrocarbon feedstocks such as methane – 75%, n-
butane -59%, cellulose -54%, biofuels (castor oil) – 78%, peat – 69%, lignite – 69%. This
shows that abundant, low quality hydrocarbon feedstock can be converted into hydrogen
and carbon suboxide. Elemental analyses with energy dispersive X-ray and SEM results
show a carbon: Oxygen ratio of analyzed deposits to be 1.5 at flow rate of 1.2LPM, 80W,
200 Celsius. The carbon: oxygen ratio of 1.5 indicates that carbon suboxide is produced
from plasma assisted butane oxidation.
Finally, both experimental and thermodynamic analyses of the reforming reactions
were presented in detail. Further studies and optimization are necessary to demonstrate the
increasing viability of non-equilibrium plasma for fuel reforming. Better yields, energy
costs and efficiencies can still be achieved with increased emphasis on the research and
development of non-equilibrium plasma for fuel reforming.
109
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APPENDIX
Kinetics of Tar removal
In an effort to examine the effect of radicals and temperature on the decomposition
of hydrocarbons in pyrogas, kinetic models were done. Kinetic models try to show the
molecular interactions that occur when chemical bonds are broken, altered and converted
into new chemical compounds. The reaction mechanism for a kinetic model typically
consists of reaction pathways with their respective rate coefficient and reverses rate
coefficients. GRI Mechanism (version 3.0) was used for the kinetic modeling of the
hydrocarbon decomposition in pyrogas.
The chemical bonds of some hydrocarbons are difficult to breakdown, the kinetic
model is intended to investigate the effect of radicals (atomic oxygen) produced from
plasma on hydrocarbon conversion. Different concentrations of atomic oxygen were
considered for hydrocarbon decomposition and the results compared. The effect of heat
energy from plasma on the decomposition of hydrocarbons was also investigated.
Table A-1: Pyrogas mixture and concentrationGases Concentration (%)
(without H2O)
CO 14.69
CO2 14.69
H2 14.69
CH4 1.96
H2O 0
N2 51.91
C10H8 0
119
C7H8 2.05
Total 100
Figure A-1: Effect of radicals in the conversion of Toluene to benzene at 650 °C.
Figure A-2: The effect of no atomic oxygen and 0.1% atomic oxygen on the reaction temperature.
0
0.005
0.01
0.015
0.02
0.025
0 500 1000 1500 2000 2500 3000
MolFraction
Time (Sec)
C6H6(O=0,T=650)
C6H5CH3(O=0,T=650)
C6H6(O=0.001,T=650)
C6H5CH3(O=0.001,T=650)
645
650
655
660
665
670
675
680
685
690
0 1000 2000 3000 4000 5000
Temperature C
Time (Sec)
650 C
Temperature C[O=0]
Temperature C[O=0.001]
120
Toluene decomposition was investigated in place of Naphthalene decomposition
due to the unavailability of reliable reaction rates and reaction pathways for naphthalene.
The chemical properties of Toluene (C7H8) are similar to that of naphthalene (C10H8) to
some extent. Therefore, the kinetic modeling result of toluene can shed some light on
naphthalene decomposition. Figure A-1 shows the effect of radicals on the conversion of
toluene to benzene at an initial temperature of 650 °C over 3000 seconds at atmospheric
pressure. The graph shows a better conversion rate of Toluene when 0.1% atomic oxygen
is added to the pyrogas mixture as opposed to when no atomic oxygen is present.
Also, from Figure A-2, the addition of atomic oxygen (0.1%) seems to
instantaneously increase the temperature of the reaction when compared to the absence of
atomic oxygen.
Figure A-3: Effect of radicals in the conversion of Toluene to benzene at 700 °C
0
0.005
0.01
0.015
0.02
0.025
0 100 200 300 400 500 600
MolFraction
Time (Sec)
C6H6 (O=0,T=700)
C6H5CH3(O=0,T=700)
C6H6(O=0.001,T=700)
C6H5CH3(O=0.001,T=700)
121
Figure A-4: The effect of no atomic oxygen and 0.1% atomic oxygen on the reaction temperature
at 700 °C
Figure A-5: Effect of radicals in the conversion of Toluene to benzene at 800 °C
695
700
705
710
715
720
725
730
735
740
0 200 400 600 800 1000 1200
Temperature C
Time (Sec)
700 C
Temperature C[O=0]
Temperature C[O=0.001]
0
0.005
0.01
0.015
0.02
0.025
0 10 20 30 40 50
Mol Fraction
Time (Sec)
C6H6 (O=0,T=800)
C6H5CH3(O=0,T=800)
C6H6(O=0.001,T=800)
C6H5CH3(O=0.001,T=800)
122
Figure A-6: The effect of no atomic oxygen and 0.1% atomic oxygen on the reaction temperature
at 700 °C
A similar pattern of the effect of radicals and temperature on hydrocarbon
decomposition can be observed in figures A-3 to A-6. Further investigation will include
other radicals for the decomposition of hydrocarbons.
795
800
805
810
815
820
825
830
835
0 20 40 60 80
Temperature C
Time (Sec)
800 C
Temperature C[O=0]
Temperature C[O=0.001]
123
VITAE
Olufela Omotola Odeyemi
Birthplace: Ibadan, Nigeria
Education
Ph.D. Mechanical Engineering, Drexel University, Research Assistant, 2009 – 2012
M.S. Electrical/Telecommunication Engineering, Drexel University
2006-2008
B.S. Electrical Engineering, University of Ado-Ekiti, Nigeria
1999-2004
Publications
1. Odeyemi, F.; Rabinovich, A.; Fridman, A.; , "Gliding Arc Plasma-Stimulated Conversion
of Pyrogas into Synthesis Gas," Plasma Science, IEEE Transactions on , vol.40, no.4,
pp.1124-1130, April 2012; doi: 10.1109/TPS.2012.2185855
2. F. Odeyemi, M. Pekker, A. Rabinovich and A. Fridman, "Non Equilibrium Plasma
Assisted Conversion of Fossil Fuels without CO2 Release," Green and Sustainable
Chemistry, Vol. 2 No. 2, 2012, pp. 38-46; doi: 10.4236/gsc.2012.22007
3. Odeyemi, F.; Pekker, M.; Rabinovich, A.; Fridman, A. A.; Heon, M.; Mochalin, V. N.;
Gogotsi, Y.; , "Low Temperature Plasma Reforming of Hydrocarbon Fuels Into Hydrogen
and Carbon Suboxide for Energy Generation Without CO2 Emission," Plasma Science,
124
IEEE Transactions on , vol.40, no.5, pp.1362-1370, May 2012 doi:
10.1109/TPS.2012.2190106
4. Non Thermal Plasma assisted Conversion of Pyrolysis Gas into Syngas; Odeyemi, F.;
Gutsol K., Rabinovich, A.; Fridman, A. - Plasma Chemistry and Plasma Processing
Journal - 2012 (In Review).
125
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