Browsing by Author "Leins, Martina"

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    Development and spectroscopic investigation of a microwave plasma source for the decomposition of waste gases
    (2010) Leins, Martina; Schumacher, Uwe (Prof. Dr.)
    In view of the world climate change the cleaning and purification of waste gases has become one of the most urgent tasks for humankind nowadays. Harmful gases are volatile organic compounds (VOC). However, even more hazardous gases are perfluorinated compounds (PFC) which have a 7400..23000 times higher green house potential compared to carbon dioxide and are widely used for etching processes in growing industry sectors like the production of semiconductors. Conventionally these waste gases are treated in oil or gas combustions which are complex to handle and produce additional carbon oxides. Hot plasma processes offer a promising alternative for this purpose, since the production of additional carbon oxides is prevented. Common RF and DC discharges have the disadvantage of electrodes which would erode when they come in contact with the waste gases. An excellent option is provided by an electrodeless microwave plasma torch at atmospheric pressure. This work deals with the development and spectroscopic study of a plasma source at atmospheric pressure powered by 2.45 GHz microwaves (APS) for the abatement of waste gases. The plasma source is based on an axially symmetric cavity. The plasma is confined in a quartz tube and a metallic nozzle is used for the gas inlet. For a successful application in industry simple ignition of the plasma as well as stable plasma operation are indispensable. To guarantee that the plasma can be ignited without any additional igniters detailed information about the electric field distribution is required. Therefore, finite element simulations of the electric field were conducted by using the simulation software COMSOL Multiphysics. The simulation results were verified by measurements with a network analyser. The simulations combined with the measurements led to a configuration which provides an ignition of the plasma without any additional igniters as well as stable plasma operation. The characterisation of the plasma was performed by means of optical emission spectroscopy for different microwave powers and air flows. The gas temperature was measured by using a transition of the free OH radical while the electron temperature was estimated from a Boltzmann plot of two atomic oxygen lines. The neutral particle and electron density were calculated from these temperatures. Furthermore, the decomposition of as exemplary VOC propane and toluene in air plasmas and as exemplary PFC tetrafluoromethane and sulphur hexafluoride in nitrogen plasmas was studied. The analyses of the raw and clean gases were performed using Fourier-Transform Infra-Red spectroscopy, a flame ionisation detector, a quadrupole mass spectrometer, and a gas phase chromatograph. The measurements revealed that the suitability for the abatement of VOC is questionable even though destruction and removal efficiencies of over 99 % are reached since critical by-products and large amounts of nitride oxides are produced. However, the measurements also showed that the PFC can be completely decomposed and that no critical by-products are formed and therefore the APS is well suited for the abatement of PFC.