Division of Environmental Physics -
Students Faculty of Mathematics, Physics and Informatics, Comenius University Bratislava
Michal LEŠTINSKÝ alumni student
PhD. study 2008 - 2012
Supervisor: Karol Hensel Consultant: -- Title: Optical emission spectroscopy of microdischarges and plasma assisted catalysis for environmental applications Abstract: -
MSc. study Supervisor: Karol Hensel Consultant: -- Title: Physical properties microdischarges in porous ceramics and their use for exhaust gas cleaning Abstract: The aim of the work was the research on generation of microdischarges in porous ceramics at atmospheric pressure using a high voltage AC power supply. To understand the physical properties of microdischarges we conducted various electrical and optical measurements. We investigated the effect of various parameters on discharge ignition and propagation, particularly the influence of pore size, discharge power, gas flow rate and composition of gas mixture.
We found that the generation of microdischarges is only possible for ceramics with a certain pore size and a minimum value of voltage/power. At very low voltages only a surface discharge was observed, while with further increase of the voltage brekdown across the ceramics was observed. Breakdown voltage decreased with increasing pore size. Microdischarges were accompanied by a large voltage drop and a rapid increase of discharge current. Current pulses were very short (~ 100 ns ) and had amplitudes of several tens of amperes. The highest amplitudes were observed for ceramics with pore size of 50 and 80 microns. Simultaneously with the electrical measurements we performed also optical diagmostics. The ceramics with a pore size of 50 and 80 microns appeared best in terms of homogeneity and distribution of microdischarges. We followed also effect of the gas composition on the discharge mixture homogeneity. In nitrogen the emission was homogenous all over the ceramics, while with an increasing amount of oxygen and carbon dioxide mixture discharge channels gradually moved to the edge of the ceramics.
Analysis of emission spectra of the discharge indicated the presence of the first and second positive systems of molecular nitrogen and also atomic lines of O and N. Fitting the experimentally measured spectra with a spectra simulated by Specair software we found that the generated plasma is strictly non-equilibrium.
Plasma chemical activity of the discharges we challanged through the generation of ozone and NO removal. The amount of generated ozone in pure oxygen increased with the discharge power and was found independent of flow rate if energy density (J/L) was kept constant. In air-like mixtures ozone yield first increased with with power, but declined if the power was too high. The maxium yield increased with the gas flow rate. Analysis of the gas mixture by infrared spectroscopy showed that besides ozone, also other products were generated (N2O5 and HNO3). We also addressed the removal of NO. Using microdischarges showed that they can be quite effective to remove pollutants from gas mixture.
Future plan is to continue experiments with the ceramics with deposited catalysts, such as Pt , Pd or Rh, which may further improve the efficiency of removal of harmful substances. Furthermore, we will investigate the influence of temperature and humidity on the stability and quality of discharge and will use the discharge to other types of polllutants, including VOCs.