Session GW4: Green Plasma Technologies II

Production of Oxygenated Hydrocarbons in Ar Containing Microplasmas

Atmospheric pressure low-temperature plasmas are being investigated to convert CH₄ into value-added compounds.  In the presence of O₂, oxygenated hydrocarbons, including alcohols and aldehydes, can be produced.  Although the desire for high throughput to produce large quantities of product typically motivates use of large volume reactors, arrays of micro-plasmas may also provide large throughput while enabling more precise control of reaction pathways.  For example,  mixtures of methane with Ar can enhance up-conversion due to Penning reactions where Ar^* dissociates or ionizes molecular gases.  In this work, the production of oxygenated hydrocarbons, including CH₃OH, C₂H₅OH, and CH₂O, is examined in a plasma formed in a microfluidic channel using results from the 0D plasma chemistry model GlobalKin.  Mixtures Ar/CH₄/O₂ can be used to regulate production of pure hydrocarbon species such as CH₃, C₂H₆, and C₃H₈ as well as selectively controlling production of oxygenated hydrocarbon densities.  The underlying reactions responsible for the changes in production of oxygenated hydrocarbons will be discussed.   

Synthesis of Carbon Nanoparticles from Methane Using a Non-Thermal Plasma

Our study proposes a novel solution for two concurrent environmental challenges: methane flaring during oil and gas extraction, and the increasing need for carbon nanoparticles in Lithium-ion batteries. Non-thermal plasma is employed to convert waste methane into valuable battery material, incentivizing carbon footprint reduction and creating additional income streams. Previous research by Woodard et al. (Plasma Chemistry Plasma Processing 2018) emphasized the efficacy of acetylene for the nucleation of small carbon nanoparticles. Conversely, methane exhibits limited nucleation abilities but rapid film growth on the reactor walls. To address these constraints, we devised a non-thermal plasma system comprising two consecutive reactors. The first plasma reactor nucleates seed carbon nanoparticles from acetylene onto which more carbon from methane is grown in-flight within the second reactor. Optimization of the various experimental variables led to a yield ~220 mg/hr of nanoparticles with a methane consumption rate of ~78%. Raman spectroscopy showed improved graphitization with increased RF power and methane flow rate, suggesting that the addition of the C:H bond in methane enhances graphitization. Carbon NP produced in this study showed similar capacity and Coulombic efficiency to commercially manufactured carbon black when tested in Li-ion batteries. Preliminary results will be presented on the use of metalorganic precursors in place of acetylene for the nucleation of seed particles.   

Abatement of Volatile Organic Compounds via a DC Arc Plasma Discharge

Due to its enclosed nature, spacecraft habitat air quality is a topic of utmost importance in which NASA places great emphasis for crew safety.  Numerous potential sources of air contaminants exist for current and proposed crewed space vessels including cleaning solvents, hygiene products, lubricants, coatings, and coolants, to name a few.  NASA maintains a list of 56 air contaminants that are ubiquitous to spacecraft operations codified in the Spacecraft Maximum Allowable Concentrations (SMAC) list.  Most items on the SMAC list are volatile organic compounds (VOCs).  For the work reported here, the authors developed a DC arc discharge plasma system wherein a test gas containing a VOC was recirculated through the plasma discharge.  During the recirculation, small aliquots of the VOC were collected and analyzed via gas chromatography/mass spectrometry to quantify the reduction in VOC concentration over time.  Five VOCs were analyzed (benzene, pentane, acetone, ethanol, and Freon-113) in two different background gases (CO₂ and N₂).  The reactions obeyed first-order rate kinetics in all cases, and VOC concentrations were reduced to below detection limits (<~1 ppm) within 5 minutes of plasma treatment.  Rate constants and half-lives were also determined for each analyte.   

Mass Separation by Electromagnetic Centrifugation in the Continuum Regime<!--EndFragment -->

In this talk, we demonstrate mass separation within gaseous and solid feedstocks in an inductively coupled plasma electromagnetic centrifuge (ICP-EMC) and derive experimentally relevant scaling relations for potential use in meeting the demand for critical materials, which are necessary for the development of green technologies. The device leverages ExB rotation acting on the broken-down feedstock passing through the ICP discharge. Through collisions, the momentum of the charged components transfers to their neutral neighbors to establish a bulk azimuthal fluid velocity, resulting in a radial concentration gradient based on mass differences. Experimental work focuses on developing and characterizing the ICP-EMC using gaseous mixtures of argon and nitrogen and solid particulates in a low-vacuum (0.1-1 Torr) regime. In a single-component argon gas, the relationship between DC current and rotational velocity is explored for measured azimuthal velocities exceeding 100 m/s and extrapolated to greater than 500 m/s. In a gaseous mixture of argon and nitrogen, we prove the ICP-EMC as a means of separating species by mass through centrifugation. Furthermore, using a time of flight secondary ion mass spectrometer, the separation of a solid feedstock is shown and is compared against our extrapolation model. Finally, a techno-economic analysis of the ICP-EMC technology is used to compare its viability against current chemical methods and similar devices such as rotary centrifuges and mass spectrometers.   

Mass-separation by waves near the cyclotron frequency in the presence of electrostatic self-fields

The ponderomotive force by standing waves near the ion cyclotron frequency pushes ions along the axial steady magnetic field. The direction of this axial force depends on the ion mass, what can be used for ion mass separation, a much-needed process for various applications. We calculate single-particle trajectories under forces by standing waves, where there is an axial ponderomotive force, and compare them to the trajectories under traveling waves, where the axial ponderomotive force is absent. The equations of motion are solved for the case that the particle displacement is much smaller than a quarter wavelength. In that case we present analytical solutions that show the particle pushing along the magnetic field with the mass-dependent direction as well the growth of particle perpendicular velocity. We find numerically the particle trajectories when the particle displacement is close to a quarter wavelength and show when the trajectory deviates from the analytical solution. We then examine theoretically the possible role of electrostatic self- fields in a plasma. We show that, under certain conditions, the self-fields can be used to enhance the process of ion mass separation.   

Investigating the Thermal Effect of Nanosecond Repetitively Pulsed Glow Discharges on a Methane-Air Flame by Coherent Anti-Stokes Raman Scattering and Optical Emission Spectroscopy

Nanosecond Repetitively Pulsed (NRP) glow discharges have been shown to alter combustion characteristics such as ignition, flame speed, lean stability and thermoacoustic instability, with a minimal electrical power. However, the pathways in which the discharges affect the flames are not fully understood. In this work, the thermal effect of NRP glow discharges applied to a methane-air flame is investigated. Hybrid fs/ps rotational Coherent Anti-Stokes Raman Scattering (CARS) measurements are used to determine the temperature of ground state nitrogen and the oxygen-to-nitrogen concentration ratio in the discharge region. The temperature of nitrogen in the discharge region was also measured using Optical Emission Spectroscopy (OES) of the second positive system of nitrogen. The spatial profile of temperature and the oxygen-to-nitrogen concentration ratio in the discharge region as well as the special and temporal profiles of the vibrational temperature of electronically excited nitrogen N2(C) are reported. The temperature and oxygen-to-nitrogen concentration ratio profiles in the discharge region were found to be in steady state during and in between discharges within the uncertainty of the measurements. The results suggest that NRP glow discharges have a negligible thermal effect on flames, indicating that the discharges influence flame characteristics through non-thermal pathways.