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15Microwave plasma (MWP) technology is currently being used in application fields such as 16 semiconductor and material processing, diamond film deposition and waste remediation. 17Specific advantages of the technology include the enablement of a high energy density source 18 and a highly reactive medium, the operational flexibility, the fast response time to inlet 19 variations and the low maintenance costs. These aspects make MWP a promising alternative 20 technology to conventional thermal chemical reactors provided that certain technical and 21 operational challenges related to scalability are overcome. Herein, an overview of state-of-22 the-art applications of MWP in chemical processing is presented (e.g. stripping of photo 23 resist, UV-disinfection, waste gas treatment, plasma reforming, methane coupling to olefins, 24 coal/biomass/waste pyrolysis/gasification and CO 2 conversion). In addition, two potential 25 approaches to tackle scalability limitations are described, namely the development of a single 26 unit microwave generator with high output power (> 100 kW), and the coupling of multiple 27 microwave generators with a single reactor chamber.

Plasma reactor technologies have the potential to enable storage of green renewable electricity into fuels and chemicals. One of the major challenges for the implementation of these technologies is the energy efficiency. Empirical enhancement of plasma reactors performance has proven to be insufficient in this regard. Numerical models are becoming therefore essential to get insight into the process for optimization purposes. The chemistry in non-thermal plasmas is the most challenging and complex part of the model due to the large number of species and reactions involved. The most recent reaction kinetic model for carbon dioxide (CO 2 ) dissociation in non-thermal microwave plasma considers more than one hundred species and thousands of reactions. To enable the implementation of this model into multidimensional simulations, a new reduction methodology to simplify the state-tostate kinetic model is presented. It is based on four key elements; 1) all the asymmetric vibrational levels are lumped within a single group, or fictitious species, CO 2 *, 2) this group follows a nonequilibrium Treanor distribution, 3) an algebraic approximation is used to compute the vibrational temperature from the translational temperature based on the Landau-Teller formula and 4) weighted algebraic expressions are applied, instead of complex differential equations, to calculate the rates of the most influencing reactions; this decreases substantially the calculation time. Using this new approach, the dissociation and vibrational kinetics are captured in a reduced set of 44 reactions among 13 species. The predictions of the reduced kinetic model regarding the concentrations of the heavy species in the afterglow zone are in good agreement with those of the detailed model from which the former was derived. The methodology may also be applied to other state-to-state kinetic models in which interactions of vibrational levels have the largest share in the global set of reactions.

A novel surface-wave microwave discharge reactor configuration to generate syngas via gaseous CO 2 reduction with H 2 (non-catalytic Reverse Water-Gas Shift reaction) is studied in the context of power-to-chemicals concept. Improvement of CO 2 conversion to maximize CO production is explored by adding an external cylindrical waveguide downstream of the plasma generation system. A 2D self-consistent argon model shows that power absorption and plasma uniformity are improved in the presence of the waveguide. We show experimentally that CO 2 conversion is increased by 50% (from 40% to 60%) at the stoichiometric feed ratio H 2 :CO 2 equal to 1 when using the waveguide. At higher H 2 :CO 2 ratios, the effect of the waveguide on the reactor performance is nearly negligible. Optical emission spectroscopy reveals that the waveguide causes significant increase in the concentration of O atoms at a ratio H 2 :CO 2 =1. The effects of the operating pressure and cooling rate are also investigated. A minimum CO 2 conversion is found at 75 mbar and ratio H 2 :CO 2 = 1, which is in the transition zone where plasma evolves from diffusive to combined operation regime. The cooling rates have significant impact on CO 2 conversion, which points out the importance of carefully designing the cooling system, among other components of the process, to optimize the plasma effectiveness.

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