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INTRODUCTION Until recently, heat processing was typically limited to conventional fuel combustion, but advancements in heating technology now offer more efficient options. One of the more exciting is the Plasma Arc Heating System. The Plasma System offers greater temperature control, faster reaction time, better processing control, lower capital costs, greater throughput, and more efficient use of energy as compared to the more conventional methods. Applications range from industrial and research environments to municipal waste management. They also include ladle and tundish heating, melting (ferrous and non-ferrous metals), vacuum melting, recovery processes, and hazardous waste treatments. Plasma energy is a common, naturally occurring resource. Simply stated, plasma energy is any gas that conducts, or can be made to conduc,t electricity. The lightning in a thunderstorm is an example of plasma energy. The plasma process is very efficient. Very quickly, and with very little combustion, temperatures up to 20,000 degrees Fahrenheit can be achieved. Since very little combustion takes place, stack gases are virtually eliminated, minimizing environmental concerns. Plasma heating has been around since the late 1800’s. Nearly 30 years ago, NASA used plasma heating to simulate the extremely high temperatures of re-entry into the earth’s atmosphere. Although the applications are widespread, plasma is just becoming recognized as one of the technologies of tomorrow, especially in waste treatment. PLASMA SYSTEM A complete plasma system is made up of many components. 1. Cooling Water System EXAMPLES OF PLASMA SYSTEM EQIIPMENT
(Photo courtesy of Callidus Technologies) Cooling Water System The Water System supplies the cooling water for the DC Power Supply, the Plasma Torch and often times the furnace. Not all Power Supplies or Furnaces require water-cooling but the Plasma Torch always does. The Water System is crucial to the reliability of the system. Loss of water always means trouble if proper interlocks are not included. Gas Supply System The Gas Supply System supplies the various process gasses in the amounts and for the time periods required by the Plasma Process. The torch manufacturer usually supplies this equipment because coordination of the gasses and flow rates is essential to proper torch operation. DC Power Supply This is the heart of the overall power control system. Specially designed electronic circuitry provides extremely stable arc current during changing load resistance conditions. The more stable the arc the more finely tuned the process. Power Supplies can be sized from a few kilowatts to many megawatts of power depending on process requirements.
AMETEK HDR Power Systems 1500kW Plasma Power Supply Includes PWM switching technology, High Voltage Switchgear, Arc Starter and Closed Loop Water Cooling System. Control System The Control System provides the inputs to the Power Supply and the outputs to other process equipment. It monitors process variables via a Programmable Logic Controller (PLC) or other misc. equipment. It usually includes a CRT and other “bells & whistles” as needed. Plasma Torch The Torch delivers the power (heat) to the load regardless of whether it is Toxic Waste or the Air Chamber in a Combustion Furnace. Two different Torch designs exist with specific applications in mind. The two basic types of Plasma Torches are Transferred and Non-Transferred. Each refers to the method in which the arc is utilized and each has its own outstanding features. TRANSFERRED and NON-TRANSFERRED TORCHES
(Drawing courtesy of Callidus Technologies) The Plasma Torch is loosely based on jet engine technology and is fairly complex. PLASMA TORCH CUTAWA
(Photo courtesy of Westinghouse Plasma Co.) APPLICATIONS Applications are many and wide-ranging. Although the plasma equipment is similar in most cases, the process designer must still specify the plasma operating parameters thereby making the equipment custom. Some of the applications include: 1. Municipal, Hazardous, and Medical Waste Municipal, Hazardous, and Medical Waste New challenges for Plasma incineration include low-level radioactive waste; soil and ground water contaminated by pesticides; and hazardous medical and biological waste. The high temperatures of the plasma process significantly reduce the volume of waste. All that remains is an inert, non-leachable slag. In some cases it provides the opportunity to recover and recycle metals from the process. Destruction removal efficiencies in excess of 99.99999% have been achieved. Westinghouse Electric has designed and built several mobile plasma systems on 48-foot semi-trailers that include everything except electricity and water. During recent tests performed at New York’s Love Canal using one of these mobile units it was found to be very efficient. Although they were still generating 200 to 300 gal/month of waste, it is conceivable that all of the available waste could be destroyed in a matter of a few months. WESTINGHOUSE PLASMA SYSTEMS - Mobile Unit
(Photo courtesy of Westinghouse Plasma Co.) Boiler Ignition As we all know, there are many electric utilities that use coal fired boilers to create steam and in turn, generate electricity. The boilers are shut down periodically and fired back up, as demand requires. In most electric utilities, gas is used to create a flame and in turn ignite the coal. By replacing the gas igniter with a plasma torch, the coal ignites much quicker and the utility utilizes their own electricity rather than a fossil fuel they must purchase. In addition, the plasma torch is much more energy efficient. Foundry Melters Years ago, plasma heating systems were uncommon sights in foundries. Low cost fossil fuels and relaxed environmental regulations limited the use of electro-technology. Years later, foundries facing stricter regulatory enforcement and increasing environmental control costs, are taking a fresh look at plasma heating systems as a source of high temperature process heat. Two applications are gaining widespread acceptance and recognition in the United States, Europe, and the Pacific Rim nations. The first is using plasma to heat the Tundish; the second is using plasma to heat the foundry Cupola during melting operations. TUNDISH
(Drawing courtesy of Callidus Technologies) Plasma heat contains more available energy for work than fossil fuel at high temperatures. Although the electricity used for making plasma may cost two to three times as much as fossil fuels, four times as much available energy is contained in plasma heat than is possible with fossil fuels. Fossil fuel flames have a practical limit of 3600° F (2000° C). At 2900° F (1600° C), only 20% of the energy in a fossil fuel flame is available for melting metal, but more than 80% of the energy in the plasma heated air is available.
CUPOLA FURNACE WORKING TEMPERATURE
(Reference graph courtesy of EPRI Journal) R & D Applications These applications are almost unlimited. From NASA using them for simulating re-entry temperatures to very exotic projects. Many of the national labs use these for various experiments. Diamond Coating Besides being the hardest material known, diamonds have many characteristics of importance. For electronic applications the superior heat and electrical conductivity make diamonds very advantageous to the Integrated Circuit industry. The Plasma Torch allows a manufacturer to deposit diamond at a very rapid rate. Another application for diamond coating is the coating of the cutting surfaces on tools. These can be cutting tools such as used on a machine mill or the abrasive surface on a grinding tool or cutting disc. Glass Coating In glass coating applications, rather than gas, a material such as copper or aluminum is sprayed through the nozzle of the plasma torch. The high temperature of the torch melts the material and sprays it on the glass. The more stable the arc is, the more uniform the coating. There are many applications for plated glass. The one that comes to mind most often is architectural glass. The colored windows on a modern building such as a skyscraper is a good example of plasma coated glass. Other applications include: 1. Metals Recovery CONCLUSION Although the Plasma Torch has been around for many years, the applications are just becoming known. Now with the advancements in power electronics and control methods the applications are more wide spread than ever due to the increased controllability of the arc. The small size, minimal installation costs and the ability to use different gasses are all benefits of the Plasma Torch. However, the greatest advantage is the ability to provide and maintain accurate temperature control. NOTE: © 2005 |
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