Session PR21: Environmental and Energy Applications for Plasma Processes

Assessment of dielectric barrier discharge non-thermal plasma for the removal of siloxanes from landfill gas

Biogases including landfill gas (LFG) continue to be a vital renewable energy source, with a methane potential of approximately eight million tons/yr which can displace about 5% of current natural gas consumption in the electric power sector and 52% in the transportation sector. However, trace contaminant volatile methyl siloxanes (VMS) present in LFG causes deterioration of combustion engines. Conventional technologies for siloxane require periodic media replacement, is expensive and re-enters the waste cycle.  In this study, dielectric barrier discharge was applied on pure streams of D4 (octamethylcyclotetrasiloxane), L3 (octamethyltrisiloxane) and on mixtures of D4 and L3 in a tubular reactor using helium as carrier gas. The goal is to determine the experimental conditions under which the removal of siloxane as, polydimetylsiloxane (PDMS), could be optimized; PDMS is commonly used in biomedical research, medical equipment as well as in electronic sealants.  The discharge influence was explored over varying durations and flow rates, with most of the removal occurring in the first 20 minutes.  Maximum removal of ~80% for D4 and ~50 % for L3 was achieved at the highest gas flow rate of 500 sccm.  Further analysis is focused on determining kinetic rate constants for removal and PDMS generation. Preliminary operations with a pilot scale modular demonstration unit with planar electrodes has been completed that can utilize atmospheric air with other LFG components instead of helium as a discharge medium.   

Plasma assisted surface engineered natural fibers and its derivatives for adsorptive removal of heavy metals

Plasma surface modification is a greener technique that has been used for surface modification of materials for wide variety of applications. Heavy metal pollution is one of the serious issues in the current scenario. The ability of plasma to make surface functionalisation on the substrate surface made this a potential candidate for heavy metal remediation. Surface modification of substrate is currently handling through wet chemical techniques which is tedious as well as require more chemicals and hence causes pollution. Plasma assisted surface modification makes the scenario greener because of the less requirement of chemicals. In the current study, we have fabricated heavy metal binding functionalities on the surface of natural carbon and soy fibers and their derivates through low temperature plasma technique. Since large amounts of soy and carbon fibers are available from nature, the possibility of developing products from them is of interest. Plasma exposure time can be varied to make controlled and uniform surface modification. A solution of lead ion (Pb²⁺), manganese ion (Mn²⁺) or cadmium ion (Cd²⁺) was used as wastewater for studying adsorption efficiencies.   

Understanding the Optimization and Scaling of a Plasma Reactor for the Decomposition of Per- and Polyfluoroalkyl Substances

A plasma-water reactor featuring a packed-bed geometry of water streams exposed to non-equilibrium plasma has shown promise for the decomposition of per- and polyfluoroalkyl substances (PFAS) in water.  Several questions remain as to how various reactor operating conditions affect the decomposition efficiency.  This research explores the sensitivity of this efficiency to the reactor pulse frequency, power input, and the resulting plasma properties.  Waters contaminated with PFAS are treated using the plasma-water reactor at varying input powers, and efficiency performance metrics are evaluated using the calculated discharge power and the measured PFAS concentrations over the treatment time.  Optical emission spectroscopy is used to investigate how plasma parameters, such as gas temperature and plasma density, are related to the discharge power and the resulting PFAS decomposition efficiency.    

Not Participating

A combination of microreactor technology and dielectric barrier discharge (DBD) plasmas enable investigation of alternate chemical pathways for generation of plasma activated liquids [1]. This work focuses on an efficient method for generating methyl radicals from methane using low temperature microplasmas for in-liquid synthesis of methylated organometallic complexes.  This technique addresses the need for conversion of methane to higher value products.  The microreactor consists of a 500 μm channel etched on a Si substrate and covered with borosilicate glass as the dielectric. Nanosecond high voltage pulses of 10 kV operating at frequencies of 1-10 kHz were used to generate atmospheric pressure plasmas in an Ar/CH₄ feed gas. nonPDPSIM, a 2D plasma hydrodynamics model, was used to simulate the generation of methyl radicals and their transport to a bounding liquid containing organometallic catalysts.  nonPDPSIM solves Poisson’s equation and transport equations for charged and neutral particles in the plasma. The spatial and temporal evolution of CH₃ radicals, and their interaction with organic solvents are discussed for geometries having different plasma-liquid interfaces, including flowing films and droplets.

[1] Y. Liu et al., Journal Phys. D: Appl. Phys. 54, 194003 (2021).   

Non-equilibrium Dynamic Control of Plasma-assisted Methane Dry Reforming in Nanosecond-Pulsed Dielectric Barrier Discharge (DBD) Reactors

The increasing greenhouse gas production, majorly by the consumption of fossil fuels, has led to consequences of climate change since the last century. Methane and carbon dioxide as the major contributors to the greenhouse effect is desired to be restrained or re-captured in industry. Noble metals, such as Rh, Ru, and Pd have shown the highest catalytic activity and stability in the dry reforming process. However, noble metals are too expensive for mass production. Nickel (Ni) catalysts are promising candidates with high conversion demonstrated in research studies, but their high coke formation led to the deactivation that constitutes a major operational drawback. The dry reforming process has the potential to be optimized with non-equilibrium plasma. Due to its low temperature, (150–450 °C) and atmospheric pressure operation conditions, plasma-assisted catalytic dry reforming exhibits a higher reaction rate and stronger resistance to coking by the synergistic effect. In this work, dry reforming is studied with a uniquely designed Plasma-Assisted Nanosecond-Pulsed DBD Reactor with nickel (Ni) catalysts. Instead of a typical dielectric catalyst, the metal catalyst sheet set between two electrodes. The catalyst plays as an additional electrode layer in the DBD reactor. Such a setup generates two independent plasma sections on each side of the catalyst. Electric field-induced second harmonic generation (E-FISH) used to characterize the two electric fields independently. Dynamic control strategies may be added to further improve the reforming outcome and catalyst deactivation performance.   

MVDC circuit breaker hollow cathode thermal and magnetic field modeling

We will present recent results of coupled thermal/magnetic field model of hollow thermionic cathode for MVDC circuit breaker. The effects of cathode geometry and material work function on cathode thermals and therefore device performance and life will be discussed. The model results indicate that shorter cathodes are required to limit the axial thermal gradient and support an increase in cathode life. The uncertainty in the cathode work function results in ~25 C variation in the predicted temperature, but is still within the temperature measurement error.  We will also show the importance of accounting for Schottky correction in the emission current calculations. According to the model, the Schottky correction results in an additional 30% of electron emission from the cathode. We will also describe the magnetic field modeling of various options to uniformly distribute the plasma across the control grid. We will compare the magnetic field lines and strength obtained for designs with either a permanent magnet or solenoid.   

Performance and life measurements of LaB6 thermionic hollow cathodes

We are developing a fast inline medium-voltage direct current (MVDC) circuit breaker based on a gas discharge tube (GDT). The GDT breaker is an enabling technology for development of multi-terminal MVDC and ultimately a meshed MVDC grid. A long-life, low forward-voltage drop thermionic hollow cathode is an essential part of the GDT. We will give an overview of the hollow cathode design considerations. We will present results of performance and life measurements of LaB₆ hollow cathodes of select geometries in various gases and operating conditions.  Plasma voltages below 25V were measured for cathode current densities in the 1-5 A/cm² range (discharge currents of 5A to 100A). At similar conditions (discharge current and gas pressure), the plasma voltage was found to be lower for gases with lower ionization potential. Measured material loss rates were consistent with the literature values of thermionic emission coefficients and LaB₆ vaporization rates.   

Mechanical, thermal, and electrical properties of plasma functionalized sustainable carbon-reinforced biocomposite.

Carbon-based nanomaterials are considered one of the most useful materials primarily due to their unique electrical, electronic and mechanical properties. However, Carbon obtained through pyrolysis of bio-based precursors is generally limited in its use due to their hydrophobic and inert surfaces. The nature of functional groups introduced across the surface of carbon materials is critical to the development of advanced materials for applications where surface-specific interactions or reactions determine performance.  Semicrysllaine carbon was synthesized from sustainable starch-based waste packaging material using a simple high-pressure/temperature pyrolysis reaction. Thus, obtained carbon was further modified using ultrasonication method. The surface energies of the biochar carbon were altered using low-temperature plasma treatment processes in presence of Sulphur Hexafluoride (SF6) gas. The carbon was treated at a chamber pressure of 0.2 Torr, constant flow of 5 ccm and Radio Frequency (RF) generated power of 150W. The carbon was treated for various durations of 5,10,15,20 and 30 minutes. Plasma-treated carbon was characterized via X-ray photoelectron spectroscopy (XPS) for surface binding energy changes and Fourier transform infrared spectroscopy (FTIR) for surface functionality changes. It was found that plasma treatment was effective in incorporating fluorine-related functionalities on the carbon surfaces. The surface-modified carbon will be effective in forming a good interface between reinforcement fillers and the host polymer matrix for composite application.    

An experimental and computational study of hydrogen production during hybrid plasma-catalytic conversion of ethanol using air plasma of rotating gliding discharge with vortex flows

Hybrid plasma-catalytic conversion has shown its potential for efficient hydrogen production from organic raw materials. However, the precise role of plasma and the main reaction mechanisms during the hybrid plasma-catalytic conversion are not yet firmly understood. This work aims to compare the production of hydrogen, which was calculated using chemical kinetics modeling of the hybrid plasma-catalytic conversion of ethanol into syngas, with the data obtained during the experimental reforming conducted at similar conditions. The experimental conversion of ethanol (810 ml/h) into syngas was done using a system consisting of a chemical reactor and a 100 W plasma source based on a rotating gliding discharge and using air as plasma gas. The chemical reactions during the ethanol conversion were modeled using ZDPlasKin and Bolsig+ software combined with the custom reaction database containing over 1400 reactions. Modeling has shown that hydrogen production during the hybrid plasma-catalytic reforming of ethanol has several stages. Data obtained in the studied temperature range has shown that the presence of plasma significantly influenced the first stage of the hydrogen production, while subsequent stages were mostly similar to the partial oxidation of ethanol without plasma.