Direct Current Plasma Arc Research Articles

Ash Waste Utilization via Direct Current Arc Plasma with Production of SiC and AlN

Ash accumulation is a relevant problem due to the widespread use of coal-fired power plants and a very limited number of ash utilization technologies. Plasma processing of ash is a promising method of ash valorization for production of various useful products, which is limited by high energy consumption. In the current study, a mixture of waste ash from power plant and black carbon was processed using an original direct current plasma arc experimental setup in ambient air which allows lower energy consumption due to the absence of the vacuuming system. Four types of samples at different plasma exposure times were produced and together with the initial ash were studied by thermal analysis, X-ray diffractometry and scanning electron microscopy. The X-ray diffraction analysis identified the resulting phases of graphite, silicon carbide, and aluminum nitride in the synthesis products; at the same time, at the arc discharge energy increased from 26 to 105 kJ, the intensities of the maxima that correspond to metal and nonmetal oxides in the composition of the initial ash decreased to nearly zero. To remove graphite, the processed samples were annealed at 750 °C in air and studied. It was found that the applied approach made it possible to obtain a mixture of silicon carbide and aluminum nitride powders in direct current arc plasma initiated in ambient air, which reduced the energy consumption for ash processing by electric arc.

Interaction of dust with the DC plasma arc - a computational modelling investigation

The presence of dust and fume suspended in the freeboard region is a common feature of the operation of direct current (DC) plasma smelting furnaces. This occurs primarily as a result of the use of fine feed materials together with the open-arc, open-bath operation of such smelters, and is exacerbated by the high velocities and turbulent mixing of the gas in the vicinity of the arc jet. Dust and fume losses into the furnace off-gas system can be significant in some cases and may have economic, operational, and environmental impacts on the process. A computational modelling study is presented in which the concentration of dust material was considered as a continuous field subject to a governing partial differential equation. Settling behaviour was calculated as a function of particle size, local gas/plasma temperature, and other physical properties. Development of the coupling between the concentration field and a magneto-hydrodynamic description of the arc is shown, and the resulting models were used to compute various aspects of the behaviour of the concentration field in the arc region for a variety of furnace conditions. Time-averaged as well as transient models of the arc were used to generate the results presented. Qualitative case studies produced several practical suggestions for furnace operation, including increased dust capture by the bath when feed ports are located closer to the electrode, and the possible effects of feed segregation in the furnace freeboard based on dust particle size and density.

Computational modelling of shear-layer instabilities and vortex formation in DC plasma arcs

The direct-current plasma arc is the principal heating and stirring element in plasma arc furnaces. The arc is a highly dynamic system operating at very short time scales (milliseconds or less). This dynamic behaviour can be understood as the combination of several modes of instability acting together. These modes can include fluid dynamic/shear layer instabilities, near-electrode instabilities resulting from steep local temperature gradients and other electrode surface effects, or helical/twisting instabilities resulting from the magnetic field generated in one part of the arc column exerting a force on another.Computational magneto-hydrodynamic models of the arc which are able to reproduce the highly transient behaviour of the system are developed to study certain dynamic modes – in particular, the formation and evolution of axisymmetric vortices and shear layer instabilities in the arc jet. Results from this work are then compared to qualitative data from high-speed photographic imaging of large plasma arcs (up to 3kA current).

Effect of Feed Forms on the Results of Melting of Fly Ash by a DC Plasma Arc Furnace

Fly ash from a municipal solid waste incinerator (MSWI) without preprocessing (original fly ash, OFA) was melted by a direct current (DC) plasma arc furnace to investigate how the feed forms governed the results. Dioxins in flue gas from stack and bag-filter ash (BFA) were detected. The distribution of heavy metals of Pb, Cd, As, and Cr along the flue gas process system was analyzed. Through a comparison of the results for dioxins and heavy metals in this study and previous work, carrying-over of fly ash particles with the flue gas stream can be deduced. Based on the magnetic induction equation and Navier–Stokes equations, a magnetohydrodynamic (MHD) model for the plasma arc was developed to describe the particle-carrying effect. The results indicate that, a. when melted, the feed forms of MSWI fly ash affect the results significantly; b. it is not preferable to melt MSWI fly ash directly, and efforts should be made to limit the mass transfer of OFA from the plasma furnace.

Melting Decontamination of Radioactive Scrap Metal by Graphite Arc Melter

The partitioning phenomena of radioisotope cobalt-60 (60Co) and cesium-137 (137Cs) from a molten ingot, slag, and dust have been investigated for the effect of the slag type, slag concentration and basicity in a plasma arc melting process. A direct current plasma arc furnace was used to melt contaminated stainless steel, and mild steel with an acid, neutral, and basic slag (SiO2, CaO, Al2O3, Fe2O3, MgO) containing radioactive 60Co and 137Cs, to measure the partitioning phenomena. Calcium oxide and ferric oxide were added to provide an increase in the slag fluidity and oxidative potential, respectively. Most of the 60Co remained in the ingot phase and was barely present in the slag. 60Co decontamination factor was not highly dependent on the slag composition. The distribution ratio of 60Co the ingot and the slag phase showed that about 90% to 95% was recovered in the ingots. 137Cs was completely eliminated from the melt of the stainless steel as well as the carbon steel, and was distributed to the slag and dust phase. The partition remaining in the slag depended on whether the slag was basic or acidic and had a high oxidative flux (Fe2O3). A maximum of 52% of the 137Cs remained in the slag phase with an acid slag.