Chemistry Reference
In-Depth Information
results and experimental data. It should be noted, however, that this model needs a
detailed information concerning the distribution of a surface charge over the dielectric
electrode, as well as the values of the secondary emission coefficient (for electrons
and ions) for a charged dielectric surface, which have never been measured.
There are also two essential gaps in the understanding of the MD behavior near
the cathode surface that should be filled, in order to develop an adequate numerical
model for it. In the case of a dielectric cathode, there is a need to simulate a three-
dimensional surface discharge (Lichtenberg figures) which has been proved to occur
simultaneously with a volume breakdown within the MD channel. In the case of a
metallic cathode, there is an evolution of the cathode sheath. Formation of the normal
cathode sheath under atmospheric pressure with the reduced field strength of some
thousand Td [15] seems hardly possible, and the mechanisms leading to a complete
or partial destruction of this sheath are not quite clear.
8.1.2 P
LASMA
T
REATMENT OF
F
LUE AND
E
XHAUST
G
ASES
Today the world is faced with a number of environmental problems like acid rain,
global warming, or ozone accumulation in the lower atmosphere. Common pollutants
emerge from the burning of fossil fuels and coal in power plants, chemical production
plants, diesel engines, and motor vehicles. Besides gaseous components like VOCs
harmful ultrafine particles, or aerosols, there are also substances with unpleasant
odors. Nonthermal plasma (NTP) techniques offer an innovative approach to solve
some of these environmental problems.
A strong electric field causes ionization of an airstream and, besides the formation
of charge carriers (electrons, ions), also causes a lot of radicals. These highly reactive
species can lead to partial or total oxidation of air pollutants. Since the typical
concentration range of the pollutants is mostly in the order of several hundred parts
per million (ppm), direct interactions between the electrons and pollutants are usually
negligible.
8.1.2.1 Overview of Methods for Pollution Control
Fields of NTP applications range from air-pollution control systems for electrostatic
precipitation to sterilization [24-26]. Table 8.2 shows an overview of different plasma
methods for pollution control and diverse examples. One of the most important appli-
cations of NTP is the corona discharge. This type of discharge is preferably applied
in electrostatic precipitators for aerosol removal. It was demonstrated originally by
Cottrell in a large-scale application for the collection of sulfuric acid mist [45].
Meanwhile large-scale facilities of pulsed corona processing for industrial incinera-
tors or collecting fly ash from a pulverized coal-fired power plant are state of the art.
Figure 8.6 shows typical electrostatic precipitator configurations. The discharge elec-
trodes contain parts with small radius of curvature or sharp edges, which cause locally
high electric fields to enable corona discharge. Typical electrode plate distances are
0.2-0.4 m; the applied voltage can reach 50-110 kV. The corona discharge can be
characterized by a weak glow in the high field region of the discharge electrode.
Depending on the discharge conditions, the breakdown is accompanied by streamer
