Allied Environmental Technologies, Inc.

- Plasma Technology

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The need for the ultimate Waste Destruction offered by plasma discharge technology is evident from population growth in all nations, deterioration of water supplies, increased production of toxic wastes and infectious hospital wastes, and the diminishing amount of land that can be used for municipal solid waste dumps. Because of these increasing volumes of all types of wastes and the increased risk to human health associated with such growth, it is prudent to use a technology that offers the destruction of wastes and does not generate new toxic wastes.

The major advantages of plasma treatment systems are cost effectiveness and technical efficiency. Upon acceptance of the waste stream, the plasma system destroys organics creating harmless elemental gases and particulate and at the same time reduces inorganic oxides to a glass-like byproduct. As it destroys the wastes, the plasma system can create salable by-products, which offset operating costs and have value to society, e.g., hot water for district heating, steam for industrial and hospital use, fuel gas for heating, and aggregate for a variety of uses.

While the application of plasma discharge technology to environmental purposes is a relatively new process, this technology has been used for decades in the metals industry. It is a proven technology with an extensive track record.

Because of the shift in the US to plasma discharge technology in the final destruction of radioactive and hazardous wastes, other nations can learn from the US experience and avoid intermediate technologies. They can move directly to the proven plasma discharge technology approach for destroying industrial and municipal wastes. 

Allied can offer assistance in determining alternatives, designing the most optimum system for a client, insuring the most effective technology transfer, and operation.

What is Plasma?

On an astronomical scale, plasma is very common. Our sun is composed of plasma, fire is plasma, fluorescent and neon lights contain plasma. According to plasma physicists, 99.9% of the Universe is made up of plasma. The loosest definition of a plasma is that it is an electrically conducting gas. At normal temperatures and pressures gases are usually very good electrical insulators. This is because the electrons in the gas are tightly bound inside gas atoms and are not free to move in response to externally applied electric or magnetic fields.

Under certain conditions, however, some or all of the electrons can be removed from their parent atoms, a process called ionization. The gas then consists of a mixture of negatively charged electrons, positively charged atoms, called ions, and un-ionized neutrally charged atoms. Now the electrons and ions are free to move under the action of applied electromagnetic fields and the gas can conduct electricity. Due to their much smaller mass the electrons respond to the applied fields much more readily than the ions and, consequently, carry most of the current. Since electrons and ions are produced in pairs and have opposite charges most of the plasma remains electrically neutral.

There are three principal methods for ionizing a gas:

  1. The first, called filed ionization, involves applying an extremely high electrical field that acts on the electrons ina neutral atom and essentially disrupts the atom.

  2. The second method involves bombarding the gas with high-energy radiation or other sub-atomic particles.

  3. The third, called thermal ionization, involves raising the temperature of the gas until collisions knock electrons out of the atoms, Thus, a plasma does not have to be 'hot', although some are extremely so.

At normal temperatures and pressures gases are usually good electrical insulators. The reason for this is the fact that the electrons in the gas are tightly bound inside the gas atoms and are not free to move in response to externally applied electric or magnetic fields. Under certain conditions, however, some or all of the electrons can be removed from their parent atoms, a process called ionization. The gas then consists of a mixture of negatively charged electrons, positively charged atoms, called ions, and un-ionized neutrally charged atoms. Now the electrons and ions are free to move under the action of applied electromagnetic fields and the gas can conduct electricity. Due to their much smaller mass the electrons respond to the applied fields much more readily than the ions and, consequently, carry most of the current. Sine electrons and ions are produced in pairs and have opposite charges most of the plasma remains electrically neutral.  Because the properties of plasma are so very different from those of a neutral gas, the plasma state is sometimes refereed to as 'the fourth state of mater.'

In practice, the plasma state covers an extremely large range of temperature and pressure, from the gas in the fluorescent lamps in your house to the fusion reactions in the center of the sun. Although one may have to search for plasma in ones daily life, most of the visible matter in the universe is in the plasma state.

Technological applications of plasma include: fluorescent lights, welding arcs, steel making furnaces, experimental fusion reactors, semiconductors processing, flat panel displays, photo-voltaic devices, solar coatings, architectural coatings, and hazardous waste processing.

  • Thermal plasma discharge technology has been long ago identified as a potential tool for the destruction/immobilization of hazardous wastes.

  • The technology involves subjecting hazardous waste streams to high plasma temperatures such that the resulting by-product satisfies U.S. Environmental Protection Agency leach resistance requirements.

  • Plasma discharge technology can be applied 'selectively' for wastes in which no disposal alternative exists or as the final process in a treatment train.

  • The potential exists to recycle metals.

  • A plasma is an electrically conductive gas -- essentially sustained lightning.

  • Electricity is used to generate thermal energy thus enabling a plasma to generate high temperatures to dissociate waste molecules. Recombined molecules form an inert solid.

  • Controllable temperatures ranging from 1'500 to 7'000 degrees Celsius (oC) can be achieved.

Plasma discharge treatment is a non-incineration thermal process which uses extremely high temperatures in an oxygen-starved environment to  completely decompose input waste material into very simple molecules. The extreme heat and lack of oxygen results in pyrolysis of the input waste material. Pyrolysis is the decomposition of matter in the absence of oxygen. Incineration is merely the burning of waste material in the presence of oxygen, and incinerators have significant air emission control problems.

The byproducts of pyrolysis are: (1) combustible gas, which can be used to generate electricity, and (2) inert slag, which is a vitrified glassy rock, primarily composed of silicon. The heat source is a plasma discharge torch, a device which produces a very high temperature plasma gas. A plasma gas is the hottest, sustainable heat source available. The plasma discharge centerline temperature can be as high as 50,000 degrees Centigrade, and the resulting plasma gas has a temperature profile of between 3,000 and 8,000 degrees Centigrade.

A plasma discharge treatment system is designed specifically for the type, size and quantity of waste material which must be processed. The refractory-lined reactor vessel is preheated to a minimum of approximately 1,100 degrees Centigrade before any processing commences. The very high temperature profile of the plasma gas then provides an optimal processing zone with the reactor vessel through which all input waste material is forced to pass. The reactor vessel operates at atmospheric pressure.

Pyrolysis provides for virtually complete gasification of all volatiles in the source material, while non-combustible material, including glass and metal, is reduced to an inert slag. With municipal solid waste as the input waste material, the product gas and slag have very distinct characteristics. The product gas is high in hydrogen and carbon monoxide, with traces of methane, acetylene and ethylene; therefore, it can be combusted very efficiently resulting in carbon dioxide, nitrogen and water vapor as the only gaseous exhaust to the atmosphere. The carbon dioxide can be recovered through use of special membrane filters. The slag is a homogeneous, silico-metallic monolith with leachate toxicity several orders of magnitude lower than those specified in current landfill regulations.

The product gas and slag from the plasma discharge treatment of municipal solid waste both have commercial value. The product gas has a heating value approximately  1/4 to 1/3 the heating value of natural gas; therefore, it can be used as an efficient fuel source for industrial processes, including the generation of electricity, and the production of methanol and ethanol. The slag can be used in the construction industry or for road paving. All other byproducts, such as scrubber water and cyclone catch material, can be recycled into the process for reprocessing to alleviate disposal requirements. Plasma discharge treatment has no byproducts which must be disposed of as waste; therefore, it can be viewed as a totally closed treatment system and the ultimate recycling process.

Different types of waste will generate different product gas and slag characteristics. The chemical composition of different input waste materials will result in differing gas and slag characteristics. Input waste materials with a high carbon content and a high percentage of non-volatile material will produce results very similar to those with municipal solid waste. Other waste materials, such as biomass, liquid wastes and organic wastes, will not produce hardly any slag since virtually all of the waste is gasifiable

Below is an artist's rendition of a 'typical' plasma non-transferred torch operation:

It could be characterized:

  • High thermal efficiency.

  • Flexibility to choose inert, reducing, or oxidizing process gases.

  • No fossil fuel and lower demand for air -- less than 15% of conventional furnaces air demand.

  • Smaller reactors are available as high energy densities are produced.

  • Less pollution abatement equipment is required

This is not a 'brand new' technology, rather an application of an existing and commercially available technology that has been used for many years for metals processing. Mixed waste treatment in a plasma torch furnace has several potential benefits, including:

  • efficacy of organic destruction;

  • versatility of application;

  • a high-integrity vitrified final waste form that reduces leachability of both hazardous and radioactive contaminates; and

  • waste volume reduction.

The plasma furnace can process a wide variety of waste types including paper, cloth, plastics, metal, glass, concrete, soil, and sludge.

The technology has application to the treatment and vitrification of hazardous, radioactive (both low-level and TRU), mixed wastes, and contaminated soils. 

A high-temperature plasma technology is particularly effective in the treatment of mixed wastes. Advantages of plasma treatment include:

  • high efficiency destruction of organics;

  • separation of metal from slag in the molten state, with TRU components partitioning to the slag phase;

  • encapsulation of heavy metals and radionuclides in the final waste matrix;

  • high-integrity, vitrified final waste form;

  • improved criticality control;

  • maximum volume reduction;

  • smaller off-gas rates minimize particulate entrainment and carryover;

  • higher energy density and smaller gas rates allow smaller process equipment;

  • one-step treatment process (no pre- or post-treatment required); and

  • capability to process many waste types.

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