Plasma Arc Technology
Description
A
plasma arc operates on principles similar to an arc-welding machine, where an
electrical arc is struck between two electrodes. The high-energy arc creates
high temperatures ranging from 3,000 degrees to 7,000 degrees Celsius. The
ÓplasmaÓ is highly ionized gas. The plasma arc is enclosed in a chamber. Waste
material is fed into the chamber and the intense heat of the plasma breaks down
organic molecules (such as oil, solvents, and
paint) into their elemental atoms. In a carefully controlled process, these
atoms recombine into harmless gases such as carbon dioxide. Solids such as
glass and metals are melted to form materials, similar to hardened lava, in which
toxic metals are encapsulated. With plasma arc technology there is no burning
or incineration and no formation of ash. There are two main types of plasma arc
processes: plasma arc (or DC) melter and plasma torch.
Plasma
arc melters have a very high destruction efficiency. They are very robust; they
can treat any waste with minimal or no pretreatment; and they produce a stable
waste form. The arc melter uses carbon electrodes to strike an arc in a bath of
molten slag. The consumable carbon electrodes are continuously inserted into
the chamber, eliminating the need to shut down for electrode replacement or
maintenance. The high temperatures produced by the arc convert the organic
waste into light organics and primary elements.
Combustible
gas is cleaned in the off-gas system and oxidized to CO2 and H2O in ceramic bed
oxidizers. The potential for air pollution is low due to the use of electrical
heating in the absence of free oxygen. The inorganic portion of the waste is
retained in a stable, leach-resistant slag.
In
plasma torch systems, an arc is struck between a copper electrode and either a
bath of molten slag or another electrode of opposite polarity. As with plasma
arc systems, plasma torch systems have very high destruction efficiency; they
are very robust; and they can treat any waste or medium with minimal or no
pre-treatment. The inorganic portion of the waste is retained in a stable,
leach-resistant slag. The air pollution control system is larger than for the
plasma arc system, due to the need to stabilize torch gas.
Limitations
and Concerns
A
chief concern about plasma arc technology is ensuring that gaseous emissions
are kept to a minimum and cleaned before being released to the atmosphere.
Concerns
have been raised regarding the reliability of plasma torch technology.
The
water-cooled copper torch must be replaced periodically to prevent burn-through
at the attachment point of the arc and a subsequent steam explosion due to
rapid heating of the released cooling water.
Applicability
The
plasma arc can be used for organic and inorganic wastes. It is being studied
for mixed radioactive waste treatment, because it separates the organic from
the inorganic portion of the waste. It is also being studied to reduce
explosive compounds and unexploded ordnance in place of traditional
technologies, such as open burning and open detonation that produced toxic
emissions and hazardous ash. It has also been used to thermally reduce
asbestos to a slag that can be disposed of in a municipal landfill.
Technology
Development Status
Both
plasma arc systems are developed and commercial applications exist for both
hazardous and radioactive waste
Web
Links
http://www.clu-in.org/download/remed/destruct_tech.pdf
(see p. 33)
http://owww.cecer.army.mil/facts/sheets/UL37.html
Other
Resources and Demonstrations
At
the Hawthorne Ammunition Depot, Nevada, site of the U.S. military's largest
munitions demilitarization stockpile, the Army is undertaking the large-scale
demilitarization of small caliber pyrotechnic ordnance using a new technology,
plasma arc thermal treatment. Montana-based MSE-Technology's Plasma Ordnance
Demilitarization System (PODS) uses electrically power plasma arc torches to
deliver heat up to 20,000 degrees Fahrenheit, destroying the ordnance by
maintaining the waste residue in a molten bath of at least 3,000 degrees.
See
Report of the Secretary of Energy Advisory BoardÕs Panel on Emerging
Technological Alternatives to Incineration, December 2000, Secretary of Energy
Advisory Board, U.S. Department of Energy