DC Arc Plasma Torch Research Articles

Experimental study on the life and performance of an improved DC arc plasma torch

A novel DC arc plasma torch is designed in this paper in order to reduce the electrode erosion, and a series of experiments are carried out to investigate how to improve the lifetime and performance of the nitrogen DC arc plasma torch. The analysis of voltage characteristics of the plasma torch indicates that the interelectrode insert can increase the average arc voltage and the sudden expansion structure can reduce the voltage fluctuation, which is helpful to improve the working stability to some extent. The spectrum characteristics at the plasma torch outlet and the cold flow simulations show that the dual shielding gas mainly act near the anode and can effectively cover the entire anode wall. Combining the shielding gas distribution with anode heat transfer processes in argon and nitrogen plasma torch, it is inferred that argon shielding gas plays an important role on reducing the anode heat transfer processes in nitrogen plasma torch, which can effectively suppress the anode erosion. The life testing experimental results find that there is no significant erosion of the cathode, anode, and interelectrode insert after cumulative working time exceeding 20 h. The maximum nitrogen plasma jet length can reach ∼35 mm with the outlet jet temperature of about 20 000 K at the current of 100 A and nitrogen gas flow rate of 10 slm. The maximum average specific enthalpy and thermal efficiency are respectively about 14 MJ kg−1 and 75% in the nitrogen plasma torch. Therefore, this newly designed DC arc plasma torch not only can suppress the electrode erosion but also has good working performance, which is expected to have excellent application prospects.

Effect of H2 addition on the preparation of ZrO2 powder from zircon (ZrSiO4) using a plasma torch

In this study, the plasma torch is utilized for the preparation of highly pure ZrO2 from zircon (ZrSiO4). The incorporation of H2 into the discharge gas Ar improves the plasma physical parameters and serves as a reducing agent to promote the irreversibility of the pyrolysis reaction of ZrSiO4. ZrSiO4 powder is fed internally into a DC arc plasma torch through a powder feeder device. The DC torch generates thermal plasma that can fully heat all the ZrSiO4 within a short period for pyrolysis, extraction, and separation, eventually producing high-purity ZrO2. According to the results, the addition of H2 to the working gas increases the material temperature in the flame by increasing the plasma temperature, thereby promoting the pyrolysis of ZrSiO4. The temperature of the material is 2500–2600 °C, which is higher than the boiling point of the pyrolysis product SiO2 while maintaining the high melting points of ZrSiO4 and ZrO2. Furthermore, H2 plasma acts as a reducing agent in the pyrolysis reaction, where it combines with the O of the product to prevent the reversibility of the pyrolysis reaction of ZrSiO4 and retains only ZrO2.

Experimental and numerical investigation of medical waste disposal via plasma gasification

The COVID-19 pandemic has increased global awareness of proper medical waste disposal. Conventional methods like incineration and gasification are insufficient due to low energy efficiency and harmful emissions. Plasma gasification, however, offers significant advantages, including economical benefits, higher efficiency, lower emissions, and greater hydrogen production potential. Hence, plasma gasification can be an influential alternative to conventional methods with further developments. In this study, gasification characteristics of medical waste are investigated in a novel laboratory-scale updraft plasma gasifier. The gasifier utilizes a DC arc plasma torch with a maximum capacity of 5 kWh. Medical waste samples are prepared by mixing various consumables like face masks and cotton bandages, whereas waste capacity of the gasifier is estimated to be 7.0 kg/h for an equivalence ratio (ER) of 0.2. Simulations are conducted for ER values of 0.1, 0.2, 0.3, and 0.4 using ANSYS Fluent software, and the plasma jet is modeled as hot gas. Results demonstrate that integrating the throat configuration to the updraft plasma gasifier and increasing the ER value enhance H2 production and reduce CO content in the syngas. The minimum H2 content is found as 48.5% for ER = 0.1 with 24.1% CO, while the maximum H2 content is 54.7% for ER = 0.4 with 17.1% CO. The gasifier configuration achieves a carbon conversion rate (CCR) of 29.1% and a cold gas efficiency (CGE) of 28.5%.

Modelling of Electric Processes in Circuits of Arc Plasma Torches Part 1. Approximations of Static Characteristics of Arc Plasma Torches

The article discusses selected structural and operational properties of DC and AC arc plasma torches. References cited in the article contain generalised analytical expressions identifying experimental current-voltage characteristics of typical arc plasma torches. Values of coefficients in formulas approximating static current-voltage characteristics depend not only on the type and the polarity of the plasma torch but also on values of criterial factors. The study discusses separately DC plasma torches and single-phase AC plasma torches.

Non-equilibrium modeling on the plasma–electrode interaction in an argon DC plasma torch

A 2D two-temperature chemical non-equilibrium model considering both cathode and anode space-charge sheath is applied to investigate the plasma–electrode interaction in a laminar argon DC plasma torch working at atmospheric pressure. In order to validate the model, the predicted arc voltage is compared with experimental values under different working conditions, and a reasonable agreement is obtained. The distributions of arc characteristics in the plasma column and electrode regions of a DC arc plasma torch are analyzed in detail. Moreover, the effects of thoriated tungsten, pure tungsten and non-uniform thoriated tungsten cathodes on the plasma characteristics inside the DC plasma torch are investigated. It is found that a constricted arc attachment with the highest current density is obtained at the non-uniform cathode, leading to the largest velocity and electric potential drop in the plasma column. The pure tungsten cathode produces the highest temperatures and heat flux along the cathode surface, which is caused by the largest arc voltage and intense space-charge sheath heating. The temperature and heat flux density on the thoriated tungsten cathode are the lowest, leading to a diffuse distribution of current density. The results indicate that the different cathodes can exert an important influence upon the plasma behavior near the cathode. Furthermore, the torch exit parameters are also slightly different due to their different arc voltages and overall input power.

Numerical Simulation of Temperature Field and Temperature Control of DC Arc Plasma Torch

In order to study and analyze the internal heat transfer process of DC arc plasma torch, a two-dimensional axisymmetric model of DC arc plasma torch was established based on magnetohydrodynamics and heat transfer theory. The finite element analysis software COMSOL was used to simulate the temperature field under different conditions. The results show that when the terminal current is constant, the DC plasma arc voltage and the average temperature of the torch outlet increase with the increase of nitrogen standard flow rate, and the maximum temperature in the plasma torch decreases with the increase of nitrogen standard flow rate. When the standard flow rate of nitrogen is constant, the arc voltage decreases with the increase of arc current, and the average temperature of plasma torch exit and the maximum temperature in the torch increase with the increase of arc current. The analysis of simulation data provides theoretical guidance for the temperature control of experimental DC plasma torch.

Experimental investigations on plasma-assisted wood pellet ignition for the start-up of biomass-fired power stations

The share of power production from biomass-fired plants in the fuel mix energy system is increasing. These plants are responsible to compensate the residual load and therefore need to provide a high level of flexibility. Plasma-assisted ignition for biomass power plants is an alternative start-up technology that offers a carbon-free start-up with high flexibility. The application of an electrical ignition system, using a plasma torch, is investigated in a 400 kW pilot-scale facility at the Institute of Combustion and Power Plant Technology at the University of Stuttgart. A DC arc plasma torch is integrated centrally in a movable block swirl burner which is to be used as a start-up burner. Milled wood pellets are considered for this study to investigate the applicability of the electrical ignition system for cold start-ups of biomass-fired power plants. An evaluation method is developed to characterize the ignition and flame status under various operational conditions. Additionally, the flame stability is evaluated by post-processing the flame images recorded by a high-speed camera. Several operational parameters, such as burner load, air ratio, and primary air flow rate are used to optimize the burner configurations for a successful ignition and a stable flame. The results showed that biomass particles can be ignited at the burner load of 25%, but a rise of 13% in the load is required to produce a stable flame cloud. The flame is formed at the burner mouth with the support of the plasma torch. Nevertheless, when the plasma system is turned off, a flame lift-off is observed in most cases. The investigation of the effect of air ratio on combustion performance highlights the importance of the available heat for the combustion reactions, which is directly related to the burner load.

Two-Temperature Chemical Non-equilibrium Modeling of Argon DC Arc Plasma Torch

A 2D two-temperature chemical non-equilibrium model has been developed to investigate the plasma characteristics inside a DC arc plasma torch of argon. In this study the chemical kinetic model is carefully examined in order to properly capture the spatial variations of plasma characteristics, especially near the electrodes and arc fringe, where large gradients of plasma parameters exist. Two different sets of chemical kinetic processes are adopted in the calculation. The first chemical kinetic model [model (a)] includes the ground-state argon atoms, excited argon 4s state, atomic ions, molecular ions, and electrons, while the second model [model (b)] only considers the ground-state argon atoms, excited argon 4s state, atomic ions, and electrons. The predicted exit temperature and arc voltage by model (a) are in better agreement with experimental measurements. A delayed anode attachment with a higher arc voltage occurs in model (a) due to the low electron density in the upstream region of arc-anode attachment. Compared with the wide arc-anode attachment zone of model (b), model (a) exhibits a more constricted anode arc root with higher current density, which leads to a higher heat flux and temperature on the anode. This phenomenon may be explained by the rapid loss of electrons outside the arc-anode attachment zone due to the dissociative recombination of argon molecular ions. Moreover, model (a) produces a longer arc column with slightly higher maximum temperature and velocity on the axis, which is caused by its larger input energy.

Spheroidization of Fe-Based Powders in Plasma Jet of DC Arc Plasma Torch and Application of These Powders in Selective Laser Melting

Spheroidization of Fe and steel powders in a plasma stream of an DC arc plasma torch is investigated. The possibility of obtaining a product with a spherical particle content of up to 100% is established. The fluidity of the best samples of obtained powder is 13–20 s with a fluidity of the feedstock of 39 s. The bulk density of the initial powder (3.05 g/cm3) during the plasma treatment increases to 4.1–4.4 g/cm3. The nanofraction content in spheroidized powders varied from 0 to 75 wt % and it depends mainly on the enthalpy of the plasma stream, consumption of raw materials, and the organization of the flow of the plasma-forming gas in the discharge channel of the plasma torch. It is established that Fe powders spheroidized in plasma can be used in selective laser melting technology to obtain compact samples whose density reaches 7.86 g/cm3.

A 2-D Axisymmetric Magneto-Hydrodynamic Model of a DC Arc Plasma Torch and Its Solution Methodology

The behavior of dynamic plasma is governed by the magneto-hydrodynamic (MHD) equation, and the efficient solution is still an open challenging issue in computational electromagnetics. In this article, to efficiently predict the behavior of an engineering direct current (dc) arc plasma torch, a 2-D axisymmetric MHD model is introduced and its solution methodology is proposed. In the methodology, the initial temperature field of the MHD model is estimated based on a predicted trajectory of the plasma arc, determined by using the single-particle orbital theory; a simple yet efficient iterative procedure using a prediction-correction approach to determine the magnetic field and to solve the corresponding 2-D finite-element equations of the mass, momentum, and energy conservations is proposed; and an approach to solve the extremely small conductivity near the electrodes is introduced. Finally, a finite element method code is developed and used to compute the multi-physics fields of the plasmas with promising results.

Experimental investigation of anode arc attachment area in DC arc plasma torch at low pressures

The anode area and anode erosion of a DC arc plasma torch with an external anode are experimentally investigated using different ambient pressures, arc currents, and anode configurations. A high-speed camera, high-voltage probe, electric probes, and a Schlieren system are utilized. The electric field in the anode area is measured using electric probes at arc electric currents lower than 100 A. It is found that a mean value of the arc electric field above the anode can be satisfactorily estimated in a non-intrusive way when the anode attachment is constricted. Anode erosion is proportional to a variable that can be compared in different experimental conditions just by quickly processing high-speed video footage, even while the plasma torch is in operation. Anode erosion decreases along with ambient pressure and it is lower when a diffuse anode arc attachment is present as opposed to a constricted anode attachment. The sources of plasma fluctuations inside and around the plasma jet are mainly the movement of the anode arc attachment and turbulent vortices. The average speed of the anode arc attachment increases with decreases in ambient pressure. On the contrary, the average period of the restrike process, or the distance between the exit nozzle and the average attachment’s position, decreases with reductions in ambient pressure. We report ambient pressure values for the transition between a constricted and diffuse anode arc attachment and for the transition between subsonic and supersonic plasma flow.

Development and diagnosis of an atmospheric pressure plasma torch for investigating magnetohydrodynamic instabilities

The development and diagnosis of an atmospheric pressure, non-transferred, DC arc plasma torch for investigating magnetohydrodynamic experiments is presented. Utilizing a custom AC starter circuit coupled with a high power DC power supply a long, laminar plasma jet was successfully generated by adjusting voltage and current characteristics, gas flow rates, and torch geometry (nozzle diameter and length). A complete and detailed description of the design and operation conditions of our torch is presented such that our results may be reproduced. Utilizing copper, aluminum, and tungsten alloy electrode components in conjunction with an electrically isolated water cooling system allows for continuous torch operation. The 50 kV AC starter circuit is coupled with a 15 kW DC power supply via a series injection transformer as a part of the custom arc starter circuit components. Axial and tangential gas flow, with nitrogen as the working gas, is regulated to generate a stable, laminar, atmospheric plasma torch with a jet length of over 80 cm under ideal operating conditions. To visualize the plasma interface in magnetohydrodynamics experiments, a technique was developed to image the cylindrical plasma jet in both the R-Z plane (axial) and R-θ plane (cross sectional) using planar-laser Mie scattering from particles added to the surrounding gas. Plasma emission wavelengths are filtered out allowing scattered light from the particle laden-flow field illuminated at 532 nm by an Nd:YAG laser, to be isolated. Plasma electron temperature and degree of ionization were estimated using a technique based on the well-known Fowler–Milne spectroscopic method. Electron temperatures of up to 14 000 K and degree of ionization up to 27% at the torch exit were obtained.

Role of return currents in the dynamics of a magnetically rastered plasma torch

The role of return currents in the plasma dynamics of a dc non-transferred arc plasma torch has been investigated. The anode arc root connects the main plasma column and return current in the anode and its motion governs the return current formation in the anode. Thus by investigating the return currents, we can explore complex spatio-temporal evolution of the plasma in detail. In this work, we have used electrical and magnetic diagnostics. The electrical diagnostic comprises of a high voltage probe, and the magnetic diagnostic consists of a garland of several B-dot probes incorporated inside the anode water cooling channels. The magnetic (B-dot) probes are designed to pick up the dominant component of the time-varying magnetic fields. Results show that, beyond a threshold, volume return currents transform to constricted line currents in the anode return path. This is attributed to eddy currents arising due to space varying rather than time varying return current components in the anode. The probes capture fast rotation of the arc root that is not captured by fast imaging. Also, arc root shunting (restriking) phenomena and its location on the anode periphery are clearly captured by the garland of magnetic probes. A physical model explaining the processes is also presented.

Prediction of operational characteristics of a dc non-transferred arc plasma torch using similarity criteria

The operational characteristics of a dc non-transferred arc plasma torch have been predicted using the techniques of dynamic similarity. The torch used has a distinct design incorporating both axial plasma generating gas and a shroud gas that initially forms a cold boundary layer but eventually takes part in the plasma formation. The current–voltage characteristics are correlated with electrode heat losses and plasma power for different gas flow conditions using the techniques of dynamic similarity. It is shown that this torch can operate in various power levels at high electro-thermal efficiencies. Unique dimensionless numbers are created for its wide operating regimes. The voltage and operational characteristics are predicted using these numbers for a number of experimental conditions, which are in good agreement with actual experimental results. Predictions are made on the limits of operational characteristics of this torch and class of torches at higher powers.

Direct probing of anode arc root dynamics and voltage instability in a dc non-transferred arc plasma jet

The transient dynamics of the anode arc root in a dc non-transferred arc plasma torch is captured through fast photography and directly correlated with the associated voltage instability for the first time. The coexistence of multiple arc roots, the transition to a single arc root, root formation and extinction are investigated for the steady, takeover and re-strike modes of the arc. Contrary to the usual concept, the emerging plasma jet of a dc non-transferred arc plasma torch is found to carry current. An unusually long self-propelled arc plasma jet, a consequence of the phenomenon, is demonstrated.

High‐current variable‐voltage rectifiers: state of the art topologies

Applications such as electrolysis, electrowinning, DC arc furnaces and plasma torches require high-current (several kA) power supplies at low-voltage levels (few hundreds of volts). In this study, conventional rectifier topologies (diode- and thyristor-based multi-pulse rectifiers with on-load tap changing transformer and chopper-rectifier) used for high-current, variable-voltage applications are reviewed along with recent advancements in the field. The main applications themselves are also discussed. Issues with conventional topologies are highlighted. Power quality techniques like passive and active filters are discussed for power factor and current harmonics compensation. Advanced medium- to high-frequency transformer-based topologies (with current and voltage source rectifiers) are discussed along with simulation results.

Analysis of processes in DC arc plasma torches for spraying that use air as plasma forming gas

Developed in Saint Petersburg State Polytechnical University technological processes of air-plasma spraying of wear-resistant, regenerating, hardening and decorative coatings used in number of industrial areas are described. The article contains examples of applications of air plasma spraying of coatings as well as results of mathematical modelling of processes in air plasma torches for spraying.

Experimental observations of arc-anode attachment in steam-argon-air environment

DC arc plasma torches performance depends strongly on processes near anode. Arc-anode interaction not only defines lifetime of the electrode but also influences power distribution in the plasma and flow structure of the jet. This paper presents an experimental investigation of effects of gas properties and flow conditions on an arc-anode attachment. Several optical and electric diagnostic means were used to provide complex image of behaviour of the attachment and resulting state of anode surface. It is shown that even in one particular regime of the arc-anode attachment, which is called a restrike mode, dynamic behaviour and resulting erosion of anode can be significantly different. For example with argon in arc-anode gap the anode surface melt more intensively than with air environment. Despite increased melting weaker radiation of copper vapour was found in plasma. It was also shown that control of arc-anode attachment position in air environment had substantial effect on anode erosion pattern and plasma flow behaviour.

Comparisons of Two Models for the Simulation of a DC Arc Plasma Torch

The hypothesis of local thermal equilibrium (LTE) in thermal plasma has been widely accepted. Most of the simulation models for the arc plasma torch are based on the hypothesis of LTE and its results indicate good validity to mimic the pattern of plasma flow inside a plasma torch. However, according to the LTE hypothesis, electrical conductivity near electrodes is significantly lower because of the low gas temperature. Consequently, it is difficult for electrical current flows to pass between the anode and cathode. Therefore, the key subject for a model concentrating on the LTE assumption is to deal with the low electrical conductivity near the electrodes. In this study, two models determining the electrical conductivity at the vicinity of the electrodes with two different assumptions were used to calculate the flow patterns inside a non-transferred DC arc plasma torch. Gas temperature, velocity, voltage drop, and heat energy of the plasma arc were compared between the two models. The results indicated that the plasma arc inside the plasma torch fluctuates, as simulated by both models. It seems that the model can obtain comparable accuracy with the experimental results if the plasma gas electrical conductivity is determined by nominal electron temperature.

Spraying of Metallic Powders by Hybrid Gas/Water Torch and the Effects of Inert Gas Shrouding

A hybrid DC arc plasma torch, combining water and gas stabilization, offers a high flexibility in plasma characteristics. These can be controlled in a wide range by the torch operational parameters, such as arc current and secondary gas flow rate. In this study, their influence on plasma spraying of tungsten and copper was investigated. To suppress the in-flight oxidation of the metals, inert gas shrouding was applied. In-flight particle diagnostics and analysis of free-flight particles and coatings was performed for spraying experiments in the open atmosphere and with argon shrouding. Both in-flight particle behavior and coating properties were found to be sensitive to the torch parameters. The application of shrouding was found to affect particle in-flight parameters, reduce the oxide content in the coatings and generally improve their properties, such as thermal conductivity. However, a different degree of these effects was observed for copper and tungsten.