18th, Laser Aided Plasma Diagnostics

Polydiagnostic studies on CO2 plasmas at elevated pressures

Sep 28, 2017, 10:30 AM

25m

INVITED TALK Low-temperature plasmas Topical

Speaker

Stefan Welzel (DIFFER)

Description

The conversion of ${CO}_2$ into value-added chemicals or ${CO}_2$-neutral fuels are widely regarded as partial solution to integrate renewable electricity into process chains and the existing energy infrastructure. Non-equilibrium plasmas have been shown to enhance the efficiency of the ${CO}_2$-to-$CO$ dissociation process under certain conditions, though fundamental understanding of plasma-chemical processes is still lacking. Dielectric barrier discharges in different configurations have been characterised by (i) infrared spectroscopy as well as phase- and time-resolved laser spectroscopy, (ii) spatio-temporally resolved optical emission spectroscopy, and (iii) Raman scattering in order to quantify e.g. the conversion degree, byproducts, gas and surface temperatures. The conversion efficiency remained low (< 5%). Apart from unfavourably high reduced electrical fields, the recycling of (atomic) oxygen seemed to be hindered, as e.g. indicated by the ozone concentrations. The CO production was found to be mainly driven by electron-impact dissociation and could be directly linked with the total number of transferred charges during the residence time of ${CO}_2$ in the active plasma zone. Recent modelling efforts confirmed electron-impact dissociation as the dominant production channel next to dissociative electron attachment and recombination. Vibrationally stimulated dissociation seemed to play a negligible role. By contrast, microwave plasmas have received widespread attention given high conversion efficiencies that have been reported. More recently, two operating regimes -- a diffuse and constricted mode -- have been identified as function of pressure. In constricted mode gas temperatures in excess of 3000 K are determined suggesting a significant contribution of thermal dissociation. Moreover, these temperatures may hamper the desired vibrationally stimulated dissociation. To further disentangle individual dissociation processes a microwave phase shift technique has been developed to retrieve electron densities. Values between ${10}^{18}$ and ${10}^{19}$ m${}^{-3}$ were measured in the diffuse and constricted mode.