Plasma waste treatment has over the past decade become a more prominent technology because of the increasing problems with waste disposal and because of the realization of opportunities to generate valuable co-products. Plasma vitrification of hazardous slags has been a commercial technology for several years, and volume reduction of hazardous wastes using plasma processes is increasingly being used. Plasma gasification of wastes with low negative values has attracted interest as a source of energy and spawned process developments for treatment of even municipal solid wastes. Numerous technologies and approaches exist for plasma treatment of wastes. This review summarizes the approaches that have been developed, presents some of the basic physical principles, provides details of some specific processes and considers the advantages and disadvantages of thermal plasmas in waste treatment applications.

http://iopscience.iop.org/article/10.1088/0022-3727/41/5/053001/pdf

Thermal plasma waste treatment

The physics of gas discharge lamps is reviewed, with particular emphasis on new developments in the field. In addition to the most important light sources (fluorescent and high-pressure discharge lamps), a number of less common sources are also discussed. The review covers not only the physics of the discharges themselves, but also their interactions with lamp materials and external driving circuits. Important historical steps in lamp product development are described and special sections are devoted to the properties of light, diagnostics, and modeling of the discharges. A detailed, but not exhaustive, review of the published literature is provided and some possible future directions for research in the subject are discussed.

The physics of discharge lamps

A two-dimensional numerical model of the interaction between an electric arc and a solid anode of different types is presented in this study. The CFD commercial code FLUENT is used to model the plasma flow and the solid anode domain. Quantities such as the velocities or the temperature are presented, and the energy transfer components between the plasma and the anode are quantified. Comparisons of the calculated results with the available experimental data in the literature show that the model predictions are in good agreement. In the case of argon gas and a copper anode, with the distance between the two electrodes 10 mm, the maximum temperature near the cathode tip is 21 000 K for a current of I = 200 A. For the same configuration, the maximum of the current density in the copper electrode is found to be −2.5 × 106 A m−2. The electrical flux is the main component of the transferred flux on the anode. Once validated, our model is applied to other theoretical and experimental configurations and allows us to study several parameters when attention is focused on the influence of metal vapour from the vaporization of the anode or the current-carrying path in the electrode on the arc behaviour. According to the current-carrying path in the anode, the current density distribution is affected in the material and its surface.

A numerical modelling of an electric arc and its interaction with the anode: Part I. The two-dimensional model

In order to clarify the formative mechanism of weld penetration in an arc welding process, the development of a numerical model of the process is quite useful for understanding quantitative values of the balances of mass, energy, and force in the welding phenomena because there is still lack of experimentally understanding of the quantitative values of them because of the existence of complicated interactive phenomena between the arc plasma and the weld pool The present article is focused on a stationary tungsten-inert-gas (TIG) welding process for simplification, but the whole region of TIG arc welding, namely, tungsten cathode, arc plasma, workpiece, and weld pool is treated in a unified numerical model, taking into account the close interaction between the arc plasma and the weld pool Calculations in a steady state are made for stationary TIG welding in an argon atmosphere at a current of 150 A The anode is assumed to be a stainless steel, SUS304, with its negative temperature coefficient of surface tension The two-dimensional distributions of temperature and velocity in the whole region of TIG welding process are predicted The weld-penetration geometry is also predicted Furthermore, quantitative values of the energy balance for the various plasma and electrode regions are given The predicted temperatures of the arc plasma and the tungsten-cathode surface are in good agreement with the experiments There is also approximate agreement of the weld shape with experiment, although there is a difference between the calculated and experimental volumes of the weld The calculated convective flow in the weld pool is mainly dominated by the drag force of the cathode jet and the Marangoni force as compared with the other two driving forces, namely, the buoyancy force and the electromagnetic force

A unified numerical modeling of stationary tungsten-inert-gas welding process

A method is described to predict the two-dimensional distributions of temperature, velocity and potential of free burning arcs and their electrodes for cathodes of tungsten and thoriated tungsten. The effects of non-equilibrium due to the ambipolar diffusion of charged particles are included for the calculation of the plasma electrical conductivity. The electron diffusion current is explicitly included in the solution of the current continuity equation. The plasma for the arc and the electrode sheath regions is treated as a continuum, so that the thickness of the non-equilibrium regions near the electrodes is determined within the model, depending upon the arc current and the arc and electrode configuration. This new treatment allows the calculation of the negative anode fall that may occur across the anode sheath when the electron diffusion current near the anode surface becomes larger than the total arc current. For a thoriated tungsten cathode we take the work function for cooling by electron emission to be that of tungsten, as, for small percentages of thoria in tungsten, cooling effects from electrons passing through the interfaces for tungsten-thoria and then thoria-plasma will add up to be that of a tungsten-plasma interface. Calculations have been made for arcs in argon at currents between 2.5 A and 200 A. For currents above 120 A, we calculate the anode fall voltage to be negative, being -2 V at 200 A. For currents less than 50 A, non-equilibrium effects in the plasma extend across the whole arc and electron number densities can be several orders of magnitude below the values for local thermodynamic equilibrium. Calculated arc voltages, arc temperatures and electrode temperatures are in agreement with experimental measurements to within 20%.

https://iopscience.iop.org/article/10.1088/0022-3727/33/2/309/pdf

Prediction of properties of free burning arcs including effects of ambipolar diffusion

Electric probe measurements have been performed at and close to a plane, water‐cooled anode surface in atmospheric pressure, high intensity argon arcs for different arc configurations which in turn, result in two distinctly different anode arc roots (diffuse and constricted) An adjustable fine wire probe penetrating through a small hole in the center of a flat anode allows for potential and electron temperature measurements in the anode boundary layer For the parameter range covered by this investigation (100 A≤I≤250 A) the results reveal negative anode falls ranging from approximately −21 to −14 V Electron temperatures at the anode surface range from 8800 to 9800 K Calorimetric energy balances from which the contributions of the various modes of heat transfer to the anode may be derived, indicate the importance of convective heat transfer to the anode The results of the calorimetric studies as well as of the anode fall measurements are consistent with a revised anode heat transfer model which take

N. A. Sanders

Papers

Measurement of anode falls and anode heat transfer in atmospheric pressure high intensity arcs