Nitrogen is an essential element to plants, animals, human beings and all the other living things on earth. Nitrogen fixation, which converts inert atmospheric nitrogen into ammonia or other valuable substances, is a very important part of the nitrogen cycle. The Haber-Bosch process plays the dominant role in the chemical nitrogen fixation as it produces a large amount of ammonia to meet the demand from the agriculture and chemical industries. However, due to the high energy consumption and related environmental concerns, increasing attention is being given to alternative (greener) nitrogen fixation processes. Among different approaches, plasma-assisted nitrogen fixation is one of the most promising methods since it has many advantages over others. These include operating at mild operation conditions, a green environmental profile and suitability for decentralized production. This review covers the research progress in the field of plasma-assisted nitrogen fixation achieved in the past five years. Both the production of NOx and the synthesis of ammonia are included, and discussion on plasma reactors, operation parameters and plasma-catalysts are given. In addition, outlooks and suggestions for future research are also given.

The production of NO x from air and air 1 O 2 is investigated in a pulsed powered milli-scale gliding arc (GA) reactor, aiming at a containerized process for fertilizer production. Influence of feed mixture, flow rate, temperature, and Ar and O 2 content are investigated at varying specific energy input. The findings are correlated with high-speed imaging of the GA dynamics. An O 2 content of 40-48% was optimum, with an enhancement of 11% in NO x production. Addition of Ar and preheating of the feed resulted in lower NO x production. Lower flow rates produced higher NO x concentrations due to longer residence time in the GA. The volume covered by GA depends strongly on the gas flow rate, emphasizing that the gas flow rate has a major impact on the GA dynamics and the reaction kinetics. For 0.5 L/min, 1.4 vol % of NO x concentration was realized, which is promising for a containerized process plant to produce fertilizer in remote locations.

A Gliding Arc Plasmatron (GAP) is very promising for CO 2 conversion into value-added chemicals, but to further improve this important application, a better understanding of the arc behavior is indispensable. Therefore, we study here for the first time the dynamic arc behavior of the GAP by means of a high-speed camera, for different reactor configurations and in a wide range of operating conditions. This allows us to provide a complete image of the behavior of the gliding arc. More specifically, the arc body shape, diameter, movement and rotation speed are analyzed and discussed. Clearly, the arc movement and shape relies on a number of factors, such as gas turbulence, outlet diameter, electrode surface, gas contraction and buoyance force. Furthermore, we also compare the experimentally measured arc movement to a state-of-the-art 3D-plasma model, which predicts the plasma movement and rotation speed with very good accuracy, to gain further insight in the underlying mechanisms. Finally, we correlate the arc dynamics with the CO 2 conversion and energy efficiency, at exactly the same conditions, to explain the effect of these parameters on the CO 2 conversion process. This work is important for understanding and optimizing the GAP for CO 2 conversion.

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A gliding arc plasmatron (GAP), which is very promising for purification and gas conversion, is characterized in nitrogen using optical emission spectroscopy and high-speed photography, because the cross sections of electron impact excitation of N 2 are well known. The gas temperature (of about 5500 K), the electron density (up to 1.5×10 15 cm-3) and the reduced electric field (of about 37 Td) are determined using an absolutely calibrated intensified charge-coupled device (ICCD) camera, equipped with an in-house made optical arrangement for simultaneous two-wavelength diagnostics, adapted to the transient behavior of a gliding arc (GA) channel in turbulent gas flow. The intensities of nitrogen molecular emission bands, N 2 (C-B,0-0) as well as N 2 + (B-X,0-0), are measured simultaneously. The electron density and the reduced electric field are determined at a spatial resolution of 30 µm, using numerical simulation and measured emission intensities, applying the Abel inversion of the ICCD images. The temporal behavior of the GA plasma channel and the formation of plasma plumes are studied using a high-speed camera. Based on the determined plasma parameters, we suggest that the plasma plume formation is due to the magnetization of electrons in the plasma channel of the GAP by an axial magnetic field in the plasma vortex.

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