Electrical Arcs and Plasma Jets

When the current density of a luminous discharge exceeds a certain limit, the potential difference between the electrodes diminishes and the discharge becomes an arc. The arc is characterized by the low potential difference of tens of volts between the electrodes and the high current density which may reach several thousand amperes per square centimeter on the electrodes. When an arc is established between two electrodes in a stream of rapidly flowing gas, the plasma is pushed in the direction of flow. If the arc is formed in a chamber with a proper exit nozzle, a plasma jet is produced outside the chamber with electrons and ions present, in the complete absence of an external electric field. 

Following the invention of the high intensity electric arc several attempts have been made to convert methane and natural gas to acetylene. Unlike solids, gaseous reactants induce instability in the arc column by appreciable forced convection56.  

It is known that higher hydrocarbons such as propane and butane give good yields of acetylene with less expenditure of energy74-77. In a coaxial arc reactor at a specific energy consumption of 3.3 kWhr m-3 natural gas conversion into acetylene reached a maximum of 70%. 

In a series of papers and books published over a period of nearly two decades Polak has reported the work of his group at the Petrochemical Research Institute of the U.S.S.R. Academy of Sciences, Moscow80-88. An electrode plasmatron was used which operated on a direct current with a maximum voltage output of 15 kV (Fig. 4). Argon containing 10—30 vol % of methane was decomposed at temperatures of 10,000 K and contact times of 10-4—10-5 s. Most of the methane was decomposed and 67—87% conversions to acetylene were obtained. The acetylene yield could be increased by using gas mixtures richer in methane89-91.  

With natural gas83-86 containing 91% CH4 by vol introduced into a hydrogen plasmatron at 4000-4500 K and with water chilling injected perpendicular to the gas flow the total decomposition was 94% and the conversion to C2 H2 was as high as 76%. The composition by vol % of a typical gas mixture after the conversion was 

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Both the electric arc and the plasma jet have been used for the pyrolysis of coal. Acetylene is the principal hydrocarbon product, its yield being three times more in a hydrogen atmosphere than in an argon atmosphere. Since the thermodynamic stability of acetylene decreases rapidly below about 1600 K, the product gases must be quenched rapidly in order to prevent the decomposition of acetylene (Sect. 2). 

Arcs can be considered as gaseous resistance heaters and offer temperatures up to 50,000°K. The sustained temperatures realizable from electric arcs can be divided into three general regions according to the current density of the conducting path. The lower temperatures (up to 4000°K. and a current density of 60 amp./cm.2) make the anode material incandescent, but as the current density is increased beyond a critical level the voltage drop shifts suddenly from a uniform drop between cathode and anode to a drop concentrated at the anode surface (for a dc arc—at both electrodes for an ac arc). The transition from the conventional to a high intensity arc is marked by pronounced increases in brilliance and temperature the arc path becomes distorted, and a jet of plasma, called the tail flame, issues from the rapidly... 

With low-pressure cascade arc, plasma formation (ionization/excitation of Ar) occurs in the cascade arc generator, and the luminous gas is blown into an expansion chamber in vacuum. The majority of electrons and ions are captured by the anode and the cathode, respectively, of the cascade arc generator, and there is no external electrical field in the expanding plasma jet. Consequently, the photon-emitting excited neutrals of Ar cause the majority of chemical reactions that occur in the plasma jet. The luminous gas coming out of the nozzle interacts with gases existing in the space into which it is injected or the surface that is placed to intercept the jet. 

No electric arc is created, which would damage parts as the ions generated by the plasma are filtered off at the plasma jet nozzle. Thus, the potential-free plasma jet can be used on or around metals. Corona treatment generates arcs on metals. As the velocity and distance of the plasma generator in relation to the substrate must be constant, manual application is not recommended. 

Insulators for plasma jet furnaces and electric arc impulse generators V V ... 

FIGURE 2 Photograph of a vacuum arc showing anode spot and cathode spots with plasma jets. (Photograph courtesy of Dr. Gerhard Frind, Corporate Research and Development, General Electric Company.)... 

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