Samenvatting

With sufficient electrical energy CO2 from the atmosphere, combined with water, can be converted to hydrocarbons. The first step in converting CO2 into a carbon based fuel is the dissociation of CO2 into CO and O. DIFFER explores the possibilities of this process by using a microwave plasma. This project focusses on this step of the process, studying the plasma that converts CO2 into CO and O.
The dissociation process in the plasma is strongly influenced by the conditions in the plasma. One of these conditions is the electron temperature. Finding a method to measure the electron temperature in a CO2 plasma could improve the understanding of CO2 plasma and the conversion processes within. One method of analyzing plasmas is by studying its natural light emission using spectroscopy. This report is centered around the research question: How can visible and near UV emission spectroscopy be used to determine the electron temperature in a microwave CO2 plasma? The goal of this research was to find a method to obtain an absolute emission spectrum of CO2 plasma from which the electron temperature can be determined by analyzing the continuum emission.
In order to reach this goal, an optical collection system was built. The spectrometer was found to have a dark-offset in the transmitted signal. This offset is both pixel- and temperature dependent. To eliminate this offset, for every pixel the relation between the offset of the pixel and the temperature was determined. Using two calibrated light sources, the optical setup was calibrated. The resulting calibration curve corrects the signal for losses in all parts of the setup, such as the quartz tube, lenses, fiber and spectrometer. Using the dark correction and the calibration curve a quantitative emission spectrum could be calculated. By measuring spectra at different locations in the plasma, the location-dependent light emission could be reconstructed with a spatial resolution of 200 μm. This method was used to analyze two CO2 plasmas under different pressures, 100 mbar and 250 mbar. Both plasmas had a power input of 1.4 kW and a flow of 6 slm.
A theoretical model for bremsstrahlung and an empirical model for CO-O recombination emission was subsequently fitted onto the continuum emission spectrum. Values for the electron temperature, gas temperature and the [CO][O] density product were obtained by this fit. The electron temperature was 2.2 eV (or 25000 K) for both plasmas. The electron temperature does not vary for different radial positions in the plasma, but the uncertainty increases with radius. The method is also shown to be effective in regions where CO-O recombination radiation is dominant over bremsstrahlung, as is the case downstream from the plasma. Here, the model can be used to determine the local gas temperature and give information about the CO and O concentrations.