Optical diagnostics to study microwave plasma reactors

The valorization of waste gases is a critical component in a circular future particularly for closing the carbon cycle through, amongst others, the recycling of plastics and biomass. Plasma-based chemical processes, driven by renewable energy sources, operate at high temperatures and reactant ionization levels, facilitating rapid and complete conversion of waste-stream gases from the chemical industry to valuable products with high efficiency. Microwave-induced plasmas offer more options for specifically linking electrical power into the process compared to arc systems, in addition to providing higher plasma homogeneity. This reduces high temperature gradients, avoids the use of electrodes and improves process efficiency because less quenching is required and offers higher selectivity. Nevertheless, the complexity of plasma reactors necessitates further investigation to optimize conversion pathways and reactor performance.

This contribution provides an overview of optical diagnostics applied to study the microwave plasma reactors within the plasma lab at Maastricht University, which includes 2D vibrational and rotational Raman spectroscopy on nitrogen, hydrogen and oxygen (only 1D), Fourier transform infrared spectroscopy (FTIR), and ICCD imaging. The microwave plasmas are operated with up to 10 kW continuous power at 2.45 GHz operating frequency. Firstly, results obtained with these diagnostics and the developments made for improvement are discussed. Secondly, the plans for future diagnostics, including coherent anti-Stokes Raman spectroscopy (CARS), and electric field induced second harmonic generation (E-FISH), are discussed.