Water-stabilized Plasma Torch Research Articles

THERMAL PLASMA GASIFICATION OF BIOMASS FOR FUEL GAS PRODUCTION

Thermal plasma offers possibility of decomposition of biomass by pure pyrolysis in the absence of oxygen, or with steichiometric amount of oxygen (gasification) to produce mixture of hydrogen and carbon monoxide. The paper analyses energy balance of biomass plasma gasification and presents results of experiments in plasma reactor with water stabilized plasma torch. Heat needed for production of syngas was determined from heats of combustion and thermo-dynamic analysis of all processes in the reactor. Oxygen and carbon dioxide were added in experiments as oxidizing mediums to reduce production of solid carbon from the surplus of carbon in treated biomass. Production of higher hydrocarbons was suppressed due to higher temperatures and high level of UV radiation in the reactor volume. Syngas with high content of hydrogen and carbon monoxide and very low content of carbon dioxide was produced. Very low content of complex hydrocarbons and tar was detected.

EFFECT OF FLUCTUATIONS OF DC CURRENT ON PROPERTIES OF PLASMA JET GENERATED IN PLASMA SPRAYING TORCH WITH GERDIEN ARC

Two different power supplies used for dc arc plasma spraying unit with water stabilized plasma torch were compared classical thyristor controlled unit with the frequency of the current ripple 300 Hz and the rectifier with the high frequency converter and frequency of the current modulation 30 kHz. For the both types of power supply the arc characteristics were measured, time averaged plasma jet properties were determined from spectroscopic measurements and plasma jet fluctuations were analysed on the basis of diagnostics with an array of high frequency photodiodes. The properties of coatings sprayed with the two power supplies were compared. While time averaged profiles of plasma jet were the same for the both power supplies, the fluctuations of plasma jet differed substantially. Also the dynamic arc voltage characteristics of the arc were different and could be described by modified Cassie equation. The differences of properties of TiO2 coatings deposited with the use of the two power supplies were not significant.

Fabrication of Bulk Nanocrystalline Ceramic Materials

An alternative method to produce bulk nanocrystalline materials is to produce amorphous material by rapid solidification followed by controlled solid state crystallization. Four different materials studied in this work are based on near-eutectic mixtures of alumina, zirconia, and silica. Powder feedstock materials have been plasma sprayed using a water stabilized plasma torch (WSP®) and subsequently heat-treated to prepare materials with nanocrystallites of 12 nm diameter on average. The as-sprayed materials have very low porosity and are mostly amorphous. The as-sprayed amorphous materials crystallize at temperatures around 950 °C. The resulting structure is best described as nanocomposite with the very small crystallites embedded in an amorphous matrix. The role of silica on phase composition, microstructure, and mechanical properties of the as-sprayed and annealed materials is discussed. The as-sprayed amorphous materials exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat-treated nanocrystalline samples.

Pyrolysis/gasification of biomass for synthetic fuel production using a hybrid gas–water stabilized plasma torch

An experimental plasma-chemical reactor equipped with a novel hybrid gas–water stabilized torch is available at IPP Prague for the innovative and environmentally friendly plasma treatment of waste streams with a view to their sustainable energetic and chemical valorization and to a reduction of the emission of greenhouse gases. Gasification/pyrolysis of biomass was experimentally studied using crushed wood as a model substance. The experimental results demonstrate homogeneous heating of the reactor volume and proper mixing of plasma with treated material in spite of the low plasma mass flow rate and constricted plasma jet. The conditions within the reactor ensure complete destruction of the tested substance. The economical viability, environmental performance and safety of biofuels/hydrogen produced from syngas resulting from the plasma-thermochemical gasification of a very broad range of second generation biomass feedstock will be investigated. The performance of several types of plasma torches and of possible combinations of torches will be compared. The final biofuels will be tested in the existing Internal Combustion Engine (ICE) test stands.

In-flight oxidation of iron particles sprayed using gas and water stabilized plasma torch

This paper is devoted to the study of the oxidation of iron (low carbon) particles during their flight in d.c. plasma jets flowing in air as well as the oxidation after the impact and solidification of particles onto the substrate. Two types of torches have been studied: a conventional one: gas stabilized plasma (GPS) with an effective power P eff=18 kW and a more exotic one: water stabilized plasma (WSP) with P eff=139 kW. Besides their power levels, their main difference lies in the plasma gas specific mass downstream of the anode attachment almost 10 times lower in GSP than in WSP with correspondingly the plasma temperature 10,000 K higher for WSP. The GSP oxidation is promoted by the convective movement induced within the liquid metal droplets by the plasma flow in the jet core. In-fight oxidation shows strong similarities for both techniques with mainly the formation of non-stoichiometric Fe x O. In contrast in coatings depending on the temperature during spraying, Fe 2O 3 or Fe 3O 4 are observed.

Numerical Modeling of Hybrid Stabilized Electric Arc With Uniform Mixing of Gases

The paper is concerned with numerical modeling of an electric arc in the plasma torch with combined stabilization of arc by gas and water vortex. The axisymmetric model includes the arc discharge area between the outlet nozzle for argon and the outlet nozzle of the hybrid plasma torch. It is proved that the addition of argon increases the plasma velocity but does not influence substantially the plasma temperature. The outlet velocities exhibit the maximum difference up to 3000 ms/sup -1/ for 500 A, regarding a water-stabilized plasma torch. Transition to supersonic flow in the axial discharge region near the outlet occurs as a result of higher velocity and lower temperature for 500 A. The calculated velocities, temperature, and electron concentration profiles exhibit good agreement with experiments.

Thermal and mechanical properties of cordierite, mullite and steatite produced by plasma spraying

Cordierite, mullite and steatite are silicate materials widely used in the ceramics industry. There is, however, only sparse information on their application in plasma spraying and properties of sprayed materials. Plasma sprayed deposits of cordierite, mullite and steatite contain amorphous phases as the result of rapid cooling of molten particles. The amorphous phase has a significant influence on physical properties, especially in connection with heat treatment. This article deals with phase changes of cordierite, mullite and steatite, resulting from plasma deposition. Special attention is paid to amorphous phase crystallization in subsequent heat treatment. Plasma sprayed deposits were produced using a water stabilized plasma torch WSP ®. The processes taking place in the deposits were examined primarily by differential thermal analysis (DTA), thermal dilatometry (TMA) and X-ray diffraction (XRD). These measurements were complemented by the determination of physical properties (namely, density, Young’s modulus and strength) and morphological observations. Important thermal characteristics like crystallization temperature and the coefficient of thermal expansion (CTE) of all materials were determined.

Tosoh’s benzene-to-phenol catalyst

The presented studies are focused on the microstructure characterization of zirconia-based coatings deposited by two types of high-energy plasma torches: (i) Axial III; and, (ii) hybrid version of Water-Stabilized Plasma (WSP) torch. The suspensions were formulated using solid dispersed phase of: (i) zirconia stabilized with 14 wt% of Y2O3 and (ii) zirconia stabilized with 24 wt% of CeO2 + 2.5 wt% of Y2O3 and continuous phase of water with ethanol. The spray process parameters were optimized for each plasma set-up individually. The in-flight observations (shadowgraphy) were performed to optimize the injection of the liquid feedstock into the plasma jet. Then the coating's morphology and coating/substrate interface were characterized using conventional light microscopy and scanning electron microscopy (SEM). The results showed that through the change of deposition parameters various coatings microstructures could be obtained, in particular columnar and two-zones structures. The EDS/EDX and XRD studies showed that there was no significant change in chemical/phase composition of zirconia material before and after spraying. Electron backscatter diffraction (EBSD) method allowed to analyze the grain size in the coating microstructure as well as crystallographic orientation of individual grains. The results showed that coatings were characterized by submicrometric microstructure what corresponded to the size of powder particles used to formulate suspension. No texture was observed in the coatings microstructure. The surface topography analysis which was performed by confocal scanning laser microscopy (CSLM) and Shape From Shading (SFS) technique proved the great influence of suspension concentration on the coating structure.

An effect of magnetic field on arc plasma flow

MM equations for electric arcs predict recirculation zones in front of electrodes of plasma torches that are consumed faster as a result. The same equations suggest that a weak applied uniform dc magnetic field parallel to the presumably axisymmetric plasma flow can remove the reverse flows, and thus protect the electrodes. As an example, results from a model for a water-stabilized plasma torch and its near-cathode region are presented. At an arc current of 400 A, an applied magnetic field of -0.06 T completely removes the recirculation zone.

Elastic and Inelastic Effects in Compression in Plasma-Sprayed Ceramic Coatings

Four-point bend tests of a plasma-sprayed zircon (ZrSiO4) coating are presented, the coating being prepared by atmospheric plasma spraying (APS) spraying onto a titanium alloy substrate, using a water-stabilized plasma (WSP) torch. The mechanical behavior in compression of the coating material is nonlinear, including hysteresis (9% of the maximum strain) and permanent deformation. The effective in-plane Young’s modulus is about 13 GPa for very small strains and doubles for a compressive strain of −0.18%. Possible micromechanisms to explain this behavior are proposed, and some general protocols concerning the testing of plasma-sprayed ceramic coatings are deduced.

Study of phase changes in plasma sprayed deposits

The formation of a plasma-sprayed coating that exhibits predictable properties requires the control of many process variables. The phase changes that take place during plasma spraying are significant material variables that should be controlled. Several different materials were deposited in air with a water-stabilized plasma torch (model PAL 160). Usually, air was used as a carrier gas for the powder; however, argon was also used for some coatings. The injected powders (NiAl, Ni, ZrSiO4-based, Al2O3-based, etc.) as well as the coatings were studied for, among other properties, their structure, particle size, microhardness, and chemical and phase composition. Phase changes induced by the different cooling rates of molten particles after their impact on a substrate are illustrated for ZrSiO4. It has also been found that the oxidizing power of the water-stabilized torch is less than previously believed. For example, coatings produced with nickel powder injected with argon as the carrier gas exhibited almost no oxides. Significant element redistribution during plasma spraying was demonstrated with a two-phase NiAl feedstock powder. The coating exhibited almost all the phases that are present in the binary NiAl alloy as well as envelopes of oxides and traces of amorphous phase.