Atmospheric dielectric barrier discharge (DBD) was found to be promising in the context of plasma chemistry, plasma medicine, and plasma-assisted combustion. In this paper, we present a detailed fluid modeling study of abundant radical species produced by a positive streamer in atmospheric dense methane-air DBD. A two-dimensional axisymmetric fluid model is constructed, in which 82 plasma chemical reactions and 30 different species are considered. Spatial and temporal density distributions of dominant radicals and ions are presented. We lay our emphasis on the effect of varying relative permittivity (εr = 2, 4.5, and 9) on the streamer dynamics in the plasma column, such as electric field behavior, production, and destruction pathways of dominant radical species. We find that higher relative permittivity promotes propagation of electric field and formation of conduction channel in the plasma column. The streamer discharge is sustained by the direct electron-impact ionization of methane molecule. Furthermore, the electron-impact dissociation of methane (e + CH4 = >e + H+CH3) is found to be the dominant reaction pathway to produce CH3 and H radicals. Similarly, the electron-impact dissociations of oxygen (e + O2 = >e + O+O(1D), e + O2 = >e + O+O) are the major routes for O production.

In this paper, plasma kinetics of CH 4 /O 2 at atmospheric pressure are of interest for applications in plasma chemistry where partial oxidation of methane to oxygenates and syngas are investigated by zero-dimensional global model. Effect of methane mole fraction ranging from 5% to 99% on plasma kinetics is studied in detail. It is found that electron density and electron temperature change periodically with periodic variation of power density pulse. The number densities of free radicals, ions and stable molecules are discussed as a function of simulation time and CH 4 /O 2 molar ratio. Conversions of inlet gases, selectivities and yields of important products, and optimum methane content for methanol and formaldehyde formation are calculated and presented. In addition, CH 3 radical is found to be the key intermediate for oxygenates production by analyzing the underlying dominant reaction pathways. Finally, a schematic overview of the transformation relationship between dominant plasma species is also summarized and shown.

In this paper, a two-dimensional axisymmetric fluid model is applied to investigate the streamer discharge characteristics in an atmospheric pressure methane-air plasma jet as a function of methane flow velocities (2.5 m/s and 20 m/s, respectively). Although the streamer ignition and propagation in the dielectric tube are not sensitive to the methane gas flow velocity, the concentration field of methane and air in the mixing layer established by a balance between convective methane flow and back-diffusion of air ambient is crucial for streamer propagation in the gap. As the methane flow velocity is 2.5 m/s, the structure of the streamer head transits from ring-shape into solid disk-shape, while the streamer head always maintains a donut-shaped pattern at high flow velocity of 20 m/s until it impinges on the substrate. At lower gas velocity, the back-diffusion of ambient air into the methane jet is even more pronounced, which causes a larger space charge density at the streamer head, and thus the local electric field near streamer head is greater. Therefore, the overall trend in streamer propagation speed versus methane flow velocity is that the larger the flow velocity, the lower plasma bullet speed. Besides, as the gas flow velocity increases from 2.5 to 20 m/s, less oxygen/nitrogen radical species and charged ions are produced in the streamer discharge, while the produced methane-related particles increase slightly. As for different methane flow velocities, the streamer advances within the methane core.

It was proved that atmospheric non-equilibrium plasma can be deemed as “reaction carrier”, and is an effective method of partial oxidation of methane to methanol and other higher hydrocarbons. In this paper, hydrogen peroxide vapor is selected as oxygen-containing oxidizer and used to activate and convert methane into methanol in an atmospheric dielectric barrier discharge. A detailed axisymmetric 2D fluid model in CH4/H2O/H2O2 gas mixture is developed, with an emphasis on gas-phase plasma chemistry for partial oxidation of methane and methanol formation. Especially, the effect of hydrogen peroxide concentration on the conversion of methane to methanol is studied. The spatial and temporal distributions of various plasma species are shown as a function of hydrogen peroxide concentration. In addition, the main plasma species and reaction pathways governing the production and loss of CH3OH and OH are determined. It is shown that the increasing hydrogen peroxide concentration results in increase of OH and CH3OH production. Hydroxyl appears to play a significant role during the process of methanol synthesis, which is primarily produced by electron-impact dissociation of H2O2 and H2O molecules.

In this paper, a detailed fluid model is developed for the conversion of methane to methanol with hydrogen peroxide vapor in an atmospheric dielectric barrier discharge. The two-dimensional axisymmetric fluid model is constructed, in which 107 plasma chemical reactions and 28 different species are considered. Our attention is focused on physicochemical mechanism of methanol formation during the complicated chemical reaction processes of reactant molecules dissociation by electron impacting and neutral radical recombination. First, spatial and temporal characteristics of main radicals and ions, such as H, CH3, OH, CH3OH, CH3O, CH2OH, CH4+, CH3+, H2O2+, and H2O+, are presented. It is found that the streamer discharge is sustained by the direct electron-impact ionization of methane molecules. The dominant positive ion flux on to the dielectric surface is methane ion, and its peak value is located at axis. Then, the dominant chemical routes governing the production and loss of CH3OH and OH are discussed in detail. Finally, a schematic overview of dominant plasma reaction pathways for partial oxidation of methane to methanol with hydrogen peroxide vapor in atmospheric dielectric barrier discharge is summarized.