Filtration of gaseous and particulate iodine with a wet electrostatic precipitator

The electrostatic precipitation (ESP) technique is widely used in the industry to filter out impurities in gases. It has been found to be a reliable and effective filtration method in a variety of applications (Riehle 1997). The filtration efficiency of a modern wet electrostatic precipitator (WESP) can be higher than 99.9 % for the particles (Chang 2011). That leads to a decontamination factor higher than 1000. Another advantage of the WESP is that the impurities are removed from the system with water. They can thus directly be transported to a water container, such as a sump in a nuclear power plant (NPP). Radiotoxic iodine has a significant contribution to a possible source term in a severe NPP accident. Therefore, the applicability of the WESP technique on the filtration of fission products, especially gaseous and particulate iodine, is investigated in this study. Inside an ESP a corona is formed on the tip of the centre electrode in a high electric field. Ions travelling from the centre electrode to the collection electrode charge particles during their flight. In the electric field charged particles drift to the collection electrode. For small particles, less than 1 µm in diameter, charging efficiency may be too low for effective filtration. These particles are filtered with an ion wind principle. Momentum of the ions travelling to the collection electrode drives small aerosol particles there as well. The filtration efficiency has been further enhanced in the studied WESP filter by spraying water droplets to the gas flow. Inside the filtration unit large droplets are charged and driven to the collection electrode. The droplets are very efficient in trapping small particles. The ESP technique can also be used to filter gaseous pollutants when the gas flow is pre-treated before the filtration unit. The water droplets fed into the system adsorb gaseous impurities. The effect can be enhanced by injecting additives with the water. The gaseous compounds can also be oxidized to form solid particles which are filtered with the ESP technique. In this study on iodine filtration with a WESP, both proposed methods are used for the decontamination of gaseous species in containment conditions. Gaseous iodine is oxidized with additional ozone and water droplets are fed to the gas flow just before the filtration unit of WESP. The oxidation of gaseous iodine by ozone has previously been studied at VTT (Kärkelä 2009). The preliminary experiments were carried out with TiO2 aerosol. The applied electric voltage between the electrodes, the residence time of particles inside the ESP chamber and the injection of water droplets before the ESP chamber were varied in the first experiments. As it is shown in Figure 1, the injection of water droplets significantly increased the trapping efficiency of TiO2 particles for applied electric voltage less than 15 kV (negative). In the iodine experiments, the gas flow containing elemental iodine (2.2 l/min, 16 ppm of I2) was mixed with ozone (3 l/min, > 1000 ppm) in order to oxidize all gaseous iodine to iodine oxide particles. The number size distribution and the number concentration of the formed iodine oxide particles were measured online with ELPI and SMPS devices. A result of the first filtration experiments with iodine oxides, more than 98 % of IOx particles was filtered with the WESP when the applied electric voltage was in a range from -10 kV to -25 kV. This study is partly funded by the European Atomic Energy Community's (Euratom) Seventh Framework Programme FP7/2007-2013 under grant agreement n° 323217.