Plasma torch: excellence of plasma application

Author

Global User

UPDESH VERMA

Date :

The term ‘plasma’ was first applied by Langmuir in 1928 to ionized gas in electric discharge. It is the fourth state of matter composed of charge (electrons, ions and radicals) and neutral particles. Almost 99% of all the matter existing in the universe is found to be in this state and it is produced by ionizing gases to a certain degree of ionization. Because of the presence of charged particles it possesses electrical conductivity like that of metals. In our day to day life we come across different naturally occurring as well as manmade plasma. Ionosphere, solar corona, aurora borealis are examples of naturally occurring plasma where as among laboratory plasma, flames, glow discharge, fluorescent plasma, high pressure arcs, RF discharges and MHD generators are common. There are different ways to produce plasma and have several applications which are very much helpful to the human being and off course to our society. One of the prominent application of plasma is Plasma Torch.

A plasma torch is a device which generate direct flow of plasma also knows as plasma gun, plasma arc, plasmatron etc. One can achieve temperatures as high as 50,000°C by injecting a plasma gas tangentially into an electric arc formed between electrodes in a chamber. The resulting vortex of hot gases emerges at very high speed through a hole in the negative electrode, to form a jet for welding, spraying of molten metal, and cutting of hard rock or hard metals.

Plasma torch in operation

The plasma torch has several applications in industry, health sector as well as in day today life. Some of the most prominent applications of plasma Torch is like it can be used for plasma cutting, plasma spraying and plasma arc waste disposal, which reduces
waste to power generating gas and construction rubble. The details of these processes are as follows

Plasma cutting
Plasma cutting is a process used to cut steel and other metals (or sometimes other materials) using a plasma torch. In this process, an inert gas (in some units, compressed air) is blown at high speed out of a nozzle; at the same time an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas to plasma. This plasma is sufficiently hot to melt the metal and moving sufficiently fast to blow molten metal away from the cut. The result is very much like cutting butter with a hot jet of air.

The torch uses a two cycle approach to producing plasma. First, a high-voltage, low current circuit is used to initialize a very small high intensity spark within the torch body, thereby generating a small pocket of plasma gas. This is referred to as the pilot arc. The now conductive plasma contacts the workpiece, which is the anode. The plasma completes the circuit between the electrode and the workpiece, and the low voltage, high current now conducts. If the plasma cutter uses a high frequency/high voltage starting circuit, the circuit is usually turned off to avoid excessive consumable wear. The plasma, which is maintained between the workpiece and electrode, travels at over 15,000 km/h(over twelve times the speed of sound of the ambient air).

Block diagram of plasma cutting process

Plasma is an effective means of cutting thin and thick materials alike.Handheld torches can usually cut up to 1/2 in (13 mm) thick steel plate, and stronger computer-controlled torches can pierce and cut steel up to 12 inches (300 mm) thick. Formerly, plasma cutters
could only work on conductive materials, however new technologies allow the plasma ignition arc to be enclosed within the nozzle thus allowing the cutter to be used for non-conductive workpieces.

Plasma spraying
Plasma spraying, one of the thermal spraying family, is a materials processing technique for producing coatings and free-standing parts using a plasma jet. Deposits having thickness from micrometers to several millimeters can be produced from a variety of materials-metals, ceramics, polymers and composists. The material to be deposited (feedstock) – typically as a powder, rarely as a liquid, suspension or wire – is introduced into the plasma jet, emanating from a plasma torch. In the jet, where the temperature is in the order of 10 000 K, the material is melted and propelled towards a substrate. There, the molten droplets flatten, rapidly solidify and form a deposit. Commonly, the deposits remain adherent to the substrate as coatings; free-standing parts can also be produced by removing the substrate. These are generally mechanical properties, such as lower strength and modulus, higher strain tolerance, and lower thermal and electrical conductivity. Also, due to the rapid solidification, metastabl phases can be present in the deposits.


Block diagram of plasma spray process

This technique is mostly used to produce coatings on structural materials. Such coatings provide protection against high temperatures, corrosion, erosion, wear; they can also change the appearance, electrical or tribological properties of the surface, replace worn material, etc. When sprayed on substrates of various shapes and removed, free-standing parts in the form of plates, tubes, shells, etc. can be produced. It can be also used for powder processing – spheroidization, homogenization, modification of chemistry, etc. In that case, the substrate for deposition is absent and the particles solidify during flight or in a controlled environment (e.g. water).


Industrial application of plasma spray process

Plasma arc waste disposal
Plasma arc waste disposal is a method of waste management which uses the extreme high temperature created by a plasma torch (or arc), to break down waste into steam and gas used for power generation, and hard solid rock-like waste (slag) which can be used in construction. The process is intended to be a net generator of electricity and to allow waste to be used completely, avoiding the need for landfill. This technology has been used primarily to treat industrial, military, and medical wastes.

Electricity is passed between two electrodes, creating an arc. Inert gas is passed through the arc into a sealed reactor containing waste material, heating the gas to temperatures as high as 13,000 °C (25,000 °F). The temperature a meter from the torch can be as high as ~4000 °C (~8,000 °F). At these high temperatures the waste is completely destroyed and broken down into its basic elemental components. There are no tars or furans. At these high temperatures all metals become molten and flow out the bottom of the reactor. Inorganics such as silica, soil, concrete, glass, gravel, etc. are vitrified into glass and flow out the bottom of the reactor.

The reactor operates at a slightly negative pressure, meaning that the feed system is simplified because the process gas cannot escape. Any moisture in the waste consumes energy to vaporise and can impact the economy of the process; however, it will not affect the process. Gas from the plasma reactor can be burned to produce electricity or can be synthesised into liquid fuel to contribute to automotive fuel.
An introduction to plasma decomposition in Russia states that the process works at around 1300 °C and breaks down toxic compounds such as dioxins and other organochlorine waste to environmentally negligible levels. The first plasma based waste disposal system in the USA is scheduled to come into operation in St. Lucie County, Florida. The county states that they hope to not only avoid further landfill, but reprocess their historic landfills back to 1978 within 18 years as well. The plant is scheduled to come into operation in 2008-09, and to produce 600 short tons (550,000 kg) of solid rubble from around 3000 tons (2.7 million kg) of waste per day at around 5500 °C.
The authors have collected data from various sources like internet, books, industries, articles.

1Ms. ANCHAL GARG
Environmentalist and BRICS- YSF Alumni,
GGS Indraprastha University, Delhi

2Dr. UPDESH VERMA
PhD IIT Delhi
Asst. Professor Physics
Manyavar Kanshiram Govt Degree College Saddiq Nagar, Gaziabad, U.P.Reg. Representative, I-STEM, A Prime Minister office initiative, https://www.istem.gov.in/
BRICS- YSF Alumni,
Coordinator- NEEV, an IIT Alumni social Initiative
Past -SENATE member, IIT Delhi.
Ex-Treasurer and Joint Secy, IIT Delhi Alumni Association Raman Postdoc Fellow and Visiting Scientist,
at Princeton Plasma Physics laboratory(PPPL)
Princeton University, Princeton, New Jersey-08544.
Phone no.: 7599182718, updeshv@gmail.com