Controlling Plasma Reactivity Transfer to Gases, Solids and Liquids

Abstract

Plasma discharges at atmospheric pressure enable efficient conversion of the kinetic energy of electrons to chemical reactivity. In the process of breaking of bonds of molecular gases, plasmas produce high densities of reactive species. These species can then be utilized to treat flue gases and waste water. However, currently deployed systems suffer from poor energy efficiencies and throughputs, largely due to the lack of understanding of the underlying physics and chemistries. In this work, computational modeling was performed to investigate the transfer of reactivity (relative capacity to undergo or produce a chemical reaction) from plasmas to gases (via Packed Bed Reactors (PBRs)), solids (metallic catalysts, porous media) and liquid (micron-scale aerosols). The work was performed using the plasma hydrodynamics model – nonPDPSIM. Necessary changes and additions to the code included addition of source terms to the surface heating module, implementation of an updated mesh generator, and parallelization of radiation transport routines. The evolution and properties of plasmas in PBRs were characterized. Three plasma modalities were shown to exist, each leading to different rates of production of reactive species. Chemical selectivity could be achieved by choosing the packing fraction and materials that lead to preferential formation of one of the modalities over others – for example, formation of Surface Ionization Waves in air preferentially increases dissociation of nitrogen over oxygen. When metallic catalysts were added to the PBR, the discharge modalities changed, causing increased fluxes of charged species to the surfaces of the catalysts. This was, in part, due to realignment of charges within the metallic particles, which induced high local electric fields, and electric field emission of electrons. The high fluxes could lead to heating and self-cleaning of the catalysts, which would explain some of the plasma-catalytic synergies observed in experimental literature. Lastly, the interactions of liquid aerosols with Dielectric Barrier Discharges were investigated. The diameter of the droplets was shown to address diffusion transport limits of both ions and neutrals by maximizing the surface-to-volume ratio. Large surface areas enable rapid solvation from the gas-phase while small volume led to fast saturation of liquid-phase reactive species. Different species were shown to have different saturation time-scales, depending on the droplet size, pointing to an additional control mechanism of liquid-chemistry and selectivity. For example, for a 10 μm droplet, ozone (Henry’s law constant, h0 ≈ 0.3) saturates within one tenth of a millisecond. On the other hand, hydrogen peroxide (h0 ≈ 1.9×106) requires up to 10 seconds to saturate a droplet of the same size.