Chemistry Reference
In-Depth Information
Dielectric
High voltage
Dielectric
FIGURE 8.9
Packed-bed reactor.
to an external electric field, the pellets are polarized and a high field is created at each
contact point of the pellets. This ends up in microdischarges at each pellet and in the
surroundings. Packed-bed reactors can easily be modified to incorporate a catalyst.
8.1.2.2 Selected Plasma Treatment Applications
8.1.2.2.1 Soot and VOC Decomposition
There are various technical processes that generate waste streams containing toxic
gases and aerosols mixed with air. Mainly submicron soot particles pose short-
and long-term health risks. The reasons for this are the particle-bound polycyclic
aromatic hydrocarbons (PAHs). The main sources of submicron particles are the
different combustion processes. The potential to use DBDs for the decomposition of
pollutants or toxic molecules was demonstrated in various contexts [24].
In many treatment applications of common DBD configurations, the flow rate,
frequency, and power consumption are fixed parameters. The geometrical properties
of the reactor (especially length and cross section) influence the results of the plasma
treatment to a large extent. The residence time is determined by the geometry of
the reactors, which should be optimized for a successful treatment. In the case of
soot-containing flue gas, especially, a high residence time of particulates should be
promoted.
The preferred system for the treatment of soot combines both plasma and filter
techniques. In this case, a special DBD configuration consists of two electrodes:
one of these is porous and gas-permeable: The porous electrode is made of high-
performance electrically conductive SiC-ceramics or sintered metal filter. Figure 8.10
shows a scheme of the reactor configuration (one unit). The reactor consists partly
of a wall flow filter with the channels at one end open and the outlet end plugged (in
the shape of a honeycomb). The dielectric barrier electrode is mounted in the axis of
the channels.
In this way, a plasma-processing station is formed by the two electrodes. In order
to decompose the toxic and soot-containing harmful substances, the flue gas is first
introduced into the processing station, in which it is subject to plasma processing, and
from which flue gas is let out through the porous electrode, which is gas-permeable
but filters and holds back the soot particles. Figure 8.11 shows a photograph of such
a scaled up reactor with 108 inlet channels (active length of each channel: 28.5 cm,
width of the honeycomb channels: 5.6 cm).
