Abstract
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.
Original language | English |
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Publication status | Published - 2015 |
Event | Aerosol Technology 2015, AT2015 - Tampere, Finland Duration: 15 Jun 2015 → 17 Jun 2015 |
Conference
Conference | Aerosol Technology 2015, AT2015 |
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Abbreviated title | AT2015 |
Country/Territory | Finland |
City | Tampere |
Period | 15/06/15 → 17/06/15 |
Keywords
- particle filtration
- electrostatic precipitation
- iodine oxide particles