Environmental Engineering Reference
In-Depth Information
ther directly within a plasma or with its help, one can conduct chemical processes
of practical importance. Another advantage of a plasma follows from the possibility
of introducing large specific energies in a simple fashion.
The oldest applications of a plasma as a heat-transfer agent [9, 13] are in the weld-
ing or cutting of metals. Since the maximum temperature of chemical torches is
about 3000 K, they cannot be used for some materials. The arc discharge (electric
arc) makes it possible to increase this temperature by a factor of three, so melting
or evaporation of any material is possible. Therefore, an electric discharge is used,
starting a century ago, for welding and cutting of metals. Presently, plasma torches
with a power of up to 10MW are used for iron melting in cupolas, for scarp melt-
ing, for production of steel alloys, and for steel reheating in tundishes and landlies.
Plasma processing is used for extraction of metals from ores. In some cases, plas-
ma methods compete with traditional ones based on chemical heating. Comparing
the plasma methods with chemical methods in cases when either can be used, one
can conclude that plasma methods provide a higher specific output, a higher qual-
ity of product, a smaller amount of waste, but require greater energy expenditure
and more expensive equipment.
Another application of a plasma as a heat-transfer agent relates to fuel energet-
ics. The introduction of a plasma into the burning zone of low-grade coals leads to
improvements in the efficiency of the burning, with an accompanying reduction
in particulate emissions, in spite of a relatively low energy input from the plas-
ma. Plasmas are also used also for pyrolysis and other methods of processing and
cleaning of fuel. As a powerful heat-transfer agent, a plasma is used for treatment
and destruction of waste [14].
Plasmas have been employed extensively for the processing and treatment of sur-
faces or for the preparation of new materials in the form of films [15-20]. The good
heat-transfer capabilities of a plasma are useful for treatment of surfaces. During
plasma processing of surfaces, the chemical composition of the surface does not
change, but its physical parameters may be improved. Another aspect of the pro-
cessing of a surface by a plasma refers to the case when active particles of the plas-
ma react chemically with the surface. The upper layer of the surface can acquire
a chemical composition different from that of the substrate. For example, plasma
hardening of a metallic surface occurs when metal nitrides or carbides are formed
in the surface layer. These compounds are generated when ions or active atoms in
the plasma penetrate the surface layer. A third mechanism for the plasma action
on a surface is realized when the surface material does not itself participate in the
chemical process, but material from the plasma is deposited on the surface in the
form of a thin film. This film can have special mechanical, thermal, electric, op-
tical, and chemical properties depending on specific problems and requirements.
It is convenient to use for this purpose plasma beams that flow from jets. Beam
methods for deposition of micrometer-thickness films are widespread in the man-
ufacture of microelectronics, mirrors, and special surfaces.
An important area of plasma applications is plasma chemistry [11, 15, 17, 21, 22],
which relates to the production of chemical compounds. The first industrial plas-
machemical process was employed for ammonia production at the beginning of
