Plasma Arc Technology
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.
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
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