Synthesis gas production using non-thermal plasma reactors

textToday we face the formidable challenge of meeting the fuel needs of a growing population while minimizing the adverse impacts on our environment. Thus, we search for technologies that can provide us with renewable fuels while mitigating the emission of global pollutants. To this end, use of non-thermal plasma processes can offer novel methods for efficiently and effectively converting carbon dioxide and water vapor into synthesis gas for the production of renewable fuels. Particularly, non-thermal plasma technologies offer distinct advantages over conventional methods including lower operating temperatures, reduced need for catalysts and potentially lower manufacturing and operation costs. The non-thermal plasma reactors have been studied for ozone generation, material synthesis, decontamination, thruster for microsatellites, and biomedical applications. This dissertation focuses on producing synthesis gas using a non-thermal, microhollow cathode discharge (MHCD) plasma reactor. The prototype MHCD reactor consisted of a mica plate as a dielectric layer that was in between two aluminum electrodes with a through hole. First, electrical characterization of the reactor was performed in the self-pulsing regime, and the reactor was modeled with an equivalent circuit which consisted of a constant capacitance and a variable, negative differential resistance. The values of the resistor and capacitors were recovered from experimental data, and the introduced circuit model was validated with independent experiments. Experimental data showed that increasing the applied voltage increased the current, self-pulsing frequency and average power consumption of the reactor, while it decreased the peak voltage. Subsequently, carbon dioxide and water vapor balanced with argon as the carrier gas were fed through the hole, and parametric experiments were conducted to investigate the effects of applied voltage (from 2.5 to 4.5 kV), flow rate (from 10 to 800 mL/min), CO₂ mole fraction in influent (from 9.95% to 99.5%), dielectric thickness (from 150 to 450 [mu]m) and discharge hole diameter (from 200 to 515 [mu]m) on the composition of the products, electrical-to-chemical energy conversion efficiency, and CO₂-to-CO conversion yield. Within the investigated parameter ranges, the maximum H2/CO ratio was about 0.14 when H2O and CO₂ were dissociated in different reactors. Additionally, at an applied voltage of 4.5 kV, the maximum yields were about 28.4% for H2 at a residence time of 128 [mu]s and 17.3% for CO at a residence time of 354 [mu]s. Increasing residence time increased the conversion yield, but decreased the energy conversion efficiency. The maximum energy conversion efficiency of about 18.5% was achieved for 99.5% pure CO₂ at a residence time of 6 [mu]s and an applied voltage of 4.5 kV. At the same applied voltage, the maximum efficiency was about 14.8% for saturated CO₂ at a residence time of 12.8 [mu]s. The future work should focus on optimizing the conversion yield and efficiency as well as analyzing the temporal and spatial changes in the gas composition in the plasma reactor.Mechanical Engineerin

Instability in Nonequilibrium and Nonthermal Plasma Discharges

Microplasma, or plasma in micron scale interelectrode separation, is an effective way to attain nonthermal plasma operation at atmospheric and higher pressure. However, the small size causes the effect of other operating parameters to be crucial in stable operation and make the microplasma discharge system to be susceptible to instabilities. The two major instabilities that are commonly observed are the instability in the negative differential resistance (NDR) region and the Striations or the ionization waves. The physics and reaction kinetics of NDR instability for high pressure system is not well understood. This study pursues both experimental characterization and development of mathematical models to understand the physicochemical processes of direct current driven self pulsing non-thermal plasma discharge. The second category of instability that is observed in microplasma discharges is striations, which was previously found in the low pressure discharges. Striations in the positive column is a major efficiency barrier for many potential applications in microplasma discharge system. However, even though there are several investigations, a consistent theoretical framework for describing this phenomenon, especially for diatomic gases, is absent. For this purpose, we propose a detailed mathematical model that considers elaborate vibrational kinetics that is associated with diatomic gases and has shown that for diatomic gases the energy cascades from electrons to vibrational excited states, contributing to the striation formation. We studied a low pressure discharge system for the Striaions modeling due to the availability of experimental data in such conditions. One of the important applications of plasma discharge is the application in plasma enhanced chemical vapor deposition and micro patterning where the smallest feature size is dictated by smallest discharge current at which stable discharge can be attained. There are no existing method that is aimed at suppressing the instability of atmospheric and high pressure. In this study, a mathematical model was developed to suppress the NDR instability through an external circuit. The mathematical model was validated experimentally and found that the instability in the NDR region can be modulated and suppressed using an external circuit element. Although microplasma offers significant technical benefits, the small size of the system makes it extremely difficult to perform diagnostics and ion detection. In addition traditional OES, Laser or other spectroscopic measurements are challenging due to the high collisionality of microplasma discharge at elevated pressure. In this study a mathematical model is proposed to predict the ion number density in the microplasma discharges based on the readily available relaxation frequency in the NDR region. The model was validated experimentally and was found to be in a good quantitative agreement with the multiphysics numerical model

Simulation of atmospheric pressure dielectric barrier discharges at low driving frequencies

This dissertation investigates the self-pulsing of Dielectric Barrier Discharges (DBDs) at low driving frequencies, in particular, (a) the dependence of current on the product pd of gas pressure p and the gas gap length d, (b) the effects of lossy dielectrics (in resistive discharges) and large dielectric permittivity (in ferroelectrics) on current dynamics, (c) the transition from Townsend to a dynamic Capacitively Coupled Plasma (CCP) discharge with changing pd values, and (d) the transition from Townsend to a high-frequency CCP regime with increasing the driving frequency. A one-dimensional fluid model of Argon plasma is coupled to an equivalent RC circuit for lossy dielectrics. Our results show multiple current pulses per AC period in Townsend and CCP discharge modes which are explained by uncoupled electron-ion transport in the absence of quasineutrality and surface charge deposition at dielectric interfaces. The number of current pulses decreases with an increasing applied frequency when the Townsend discharge transforms into the CCP discharge. The resistive barrier discharge with lossy dielectrics exhibits Townsend and glow modes for the same pd value (7.6 Torr cm) for higher and lower resistances, respectively. Finally, we show that ferroelectric materials can amplify discharge current in DBDs. Similarities between current pulsing in DBD, Trichel pulses in corona discharges, and subnormal oscillations in DC discharges are discussed

Investigation and optimisation of a plasma cathode electron beam gun for material processing applications

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University London.This thesis describes design, development and testing work on a plasma cathode electron beam gun as well as plasma diagnosis experiments and Electron Beam (EB) current measurements carried out with the aim of maximising the power of the EB extracted and optimising the electron beam gun system for material processing applications. The elements which influence EB gun design are described and put into practice in a thermionic EB gun case study. The relevant principles of plasma EB gun systems, such low-temperature, low-pressure, RF excitation, are described along with the test rigs developed to investigate different plasma cathode configurations. The first experimental setup was for optical spectroscopy measurements of the light emitted from the plasma and the second included current measurements from EBs generated at –30 and –60 kV as well as the spectroscopic measurements. Comparison of EB current measurements with different plasma cathode configurations and correlation with spectroscopic measurements are presented. The maximum current extracted from the Radiofrequency (RF) gun was 38 mA at –60 kV using a hollow cathode geometry and permanent magnets for electron confinement. The RF gun was compared to a Direct Current (DC) gun which generated higher currents. This was reflected in the spectra which indicated a higher ionisation level than in the RF plasma. Simulation work carried out using Opera-2d to model beam trajectories indicated that the beam shape is largely influenced by the plasma boundary. Particle In Cell (PIC) simulations of a parallel plate RF plasma cathode demonstrated that higher excitation frequencies produced higher ionisation, however the RF sheaths were larger and thus the current extracted may be limited in practice due to fewer electrons being available near the aperture. The sheath thickness decreased in the simulations as the discharge gap was increased. RF plasma also produced larger currents from larger plasma chambers.TWI Ltd

Studies of dynamic processes related to active experiments in space plasmas

This is the final report for grant NAGw-2055, 'Studies of Dynamic Processes Related to Active Experiments in Space Plasmas', covering research performed at the University of Michigan. The grant was awarded to study: (1) theoretical and data analysis of data from the CHARGE-2 rocket experiment (1keV; 1-46 mA electron beam ejections) and the Spacelab-2 shuttle experiment (1keV; 100 mA); (2) studies of the interaction of an electron beam, emitted from an ionospheric platform, with the ambient neutral atmosphere and plasma by means of a newly developed computer simulation model, relating model predictions with CHARGE-2 observations of return currents observed during electron beam emissions; and (3) development of a self-consistent model for the charge distribution on a moving conducting tether in a magnetized plasma and for the potential structure in the plasma surrounding the tether. Our main results include: (1) the computer code developed for the interaction of electrons beams with the neutral atmosphere and plasma is able to model observed return fluxes to the CHARGE-2 sounding rocket payload; and (2) a 3-D electromagnetic and relativistic particle simulation code was developed

Magnetic enhancement of the electrical asymmetry effect in capacitively coupled plasmas

The authors gratefully acknowledge Prof. Mark J Kushner and Dr Andrew R Gibson for insightful discussions.Peer reviewe

Viking '75 spacecraft design and test summary. Volume 1: Lander design

The Viking Mars program is summarized. The design of the Viking lander spacecraft is described

Characteristics of pseudospark discharge in particle-in-cell simulations

In this article, the characteristics of the pseudospark (PS) discharge were studied through particle-in-cell simulations with the consideration of the external circuit. The different discharge stages, including the Townsend discharge, hollow cathode discharge, and super-dense glow, were characterized by the discharge voltage and current. The sources of electrons at different discharge stages were analyzed, with the electrons generated from the ionization of the background gas playing the dominant role. The electron densities at the Townsend discharge, hollow cathode discharge, and super-dense glow discharge stages were in the range of 1013, 1018, and 1020 m-3, respectively. The energy of the electron beam extracted at the anode aperture at different discharge phases was exported and postprocessed. The electron beams generated at the three stages have an energy spread of 10%, 20%, and 40%, respectively. The hollow cathode discharge phase has balanced parameters in beam energy, energy spread, and electron density in contrast to the Townsend discharge and super-dense glow discharge stages

Satellite auxiliary-propulsion selection techniques. Addendum: A survey of auxiliary electric propulsion systems

A review of electric thrusters for satellite auxiliary propulsion was conducted at JPL during the past year. Comparisons of the various thrusters for attitude propulsion and east-west and north-south stationkeeping were made based upon performance, mass, power, and demonstrated life. Reliability and cost are also discussed. The method of electrical acceleration of propellant served to divide the thruster systems into two groups: electrostatic and electromagnetic. Ion and colloid thrusters fall within the electrostatically accelerated group while MPD and pulsed plasma thrusters comprise the electromagnetically accelerated group. The survey was confined to research in the United States with accent on flight and flight prototype systems