Direct Current Thermal Plasma Research Articles

Morphology-controlled atmospheric pressure plasma synthesis of zinc oxide nanoparticles for piezoelectric sensors

Zinc oxide nanoparticles, especially those with a high aspect ratio (i. e., nanorods and nanowires), are of great interest for many applications as they are piezoelectric, photocatalytic and antimicrobial. In the present study, a plasma flight-thru synthesis method was developed that allows controlling the particle size and shape of the zinc oxide nanoparticles. In a direct current thermal plasma reactor operated at atmospheric pressure, zinc powder injected into the plasma jet was molten, vaporized and oxidized, which allowed growing zinc oxide nanoparticles. The particle spectrum ranged from small nanospheres to nanorods, nanowires and multipodic nanoparticles such as tetrapods. The influence of the oxygen rate and the plasma power (correlated to the discharge current) on the particle morphology was studied, and the feasibility of the nanowire-like particles as piezoelectric sensor material was investigated. Piezoelectric test sensors, equipped with the plasma-synthesized zinc oxide nanowires, successfully responded to mechanical stimulation after poling.

DC Thermal Plasma Design and Utilization for the Low Density Polyethylene to Diesel Oil Pyrolysis Reaction

The exponential increase of plastic production produces 100 million tonnes of waste plastics annually which could be converted into hydrocarbon fuels in a thermal cracking process called pyrolysis. In this research work, a direct current (DC) thermal plasma circuit is designed and used for conversion of low density polyethylene (LDPE) into diesel oil in a laboratory scale pyrolysis reactor. The experimental setup uses a 270 W DC thermal plasma at operating temperatures in the range of 625 °C to 860 °C for a low density polyethylene (LDPE) pyrolysis reaction at pressure = −0.95, temperature = 550 °C with τ = 30 min at a constant heating rate of 7.8 °C/min. The experimental setup consists of a vacuum pump, closed system vessel, direct current (DC) plasma circuit, and a k-type thermocouple placed a few millimeters from the reactant sample. The hydrocarbon products are condensed to diesel oil and analyzed using flame ionization detector (FID) gas chromatography. The analysis shows 87.5% diesel oil, 1,4-dichlorobenzene (Surr), benzene, ethylbenzene and traces of toluene and xylene. The direct current (DC) thermal plasma achieves 56.9 wt. % of diesel range oil (DRO), 37.8 wt. % gaseous products and minimal tar production. The direct current (DC) thermal plasma shows reliability, better temperature control, and high thermal performance as well as the ability to work for long operation periods.

Rapid preparation of tritium breeder material Li2TiO3 pebbles by thermal plasma

A route to obtain tritium breeder material Li2TiO3 pebbles in one step was investigated using a direct-current (DC) thermal plasma system. For this purpose, irregular and agglomerate powders were used as raw powders which were injected into DC thermal plasma torch with a plasma power level of ∼18kW. The morphology, particles size distribution and phase change of the prepared Li2TiO3 pebbles were characterized by field-emission scanning electron microscope and X-ray diffraction (XRD). The results showed that prepared Li2TiO3 pebbles presented excellent spherical degree with average size about 100µm and the spheroidization rate was close to 100%. The XRD patterns demonstrated that higher degree of crystallinity of Li2TiO3 pebbles could be obtained by thermal plasma processed. Formation mechanism of Li2TiO3 pebbles in DC thermal plasma system was discussed.

Experimental and thermodynamic comparison between a novel CO2/CH4 and an oxygen submerged DC thermal plasma for treatment of sebacic acid in basic aqueous solution

A novel submerged direct current (DC) thermal plasma torch operating with a mixture of carbon dioxide and methane has been compared with an oxygen DC submerged thermal plasma torch for treatment of a basic solution containing organic acid. Sebacic acid, a long-chain organic contaminant in Bayer liquor, was selected as a representative of organic acid in wastewater and contaminated industrial liquids. The effects of different operating conditions including treatment time, reactor pressure as well as the role of oxidizing agents, such as H2O2, were investigated on the decomposition rate of sebacic acid and the resulting degradation by-products. Intermediate products were quantified by IC/MS (ion chromatography/mass spectrometry) and a TOC (total organic carbon) analyzer. It was shown that both plasmas would decompose the sebacic acid however, oxygen plasma showed a higher conversion rate. The sebacic acid conversion rate was increased five times by adding H2O2 with the CO2/CH4 plasma. It was increased up to the same conversion rate with the oxygen plasma. However, adding H2O2 had no positive effect on the conversion rate with the oxygen plasma. Increasing the pressure also enhanced the conversion of sebacic acid in both plasmas. The decomposition mechanism of the CO2/CH4 plasma was mainly due to the UV radiation of the plasma while in the case of the oxygen plasma, it was more attributed to the plasma oxidizing species. This work, therefore, shows the possibility of using the CO2/CH4 plasma and oxygen plasma torches for treatment of the contaminated species in basic solution.

Application of a Novel CO2 DC Thermal Plasma Torch Submerged in Aqueous Solution for Treatment of Dissolved Carboxylic Acid

A novel direct current (DC) plasma torch, operating with a gas mixture consisting of carbon dioxide and hydrocarbon (methane), has been adapted and used for the first time for treatment of a solution containing dissolved carboxylic acid. Compared with conventional DC thermal plasma torches, this novel torch has high plasma enthalpy, high thermal conductivity, low cost and often readily available operating gas (CO2) and long electrode life-time. Performance of this CO2/CH4 DC thermal plasma torch submerged in aqueous solution was investigated for decomposition of dissolved organic contaminants. Sebacic acid was selected as a representative of organic pollutants in wastewater of chemical process industries. Effect of different operational conditions including treatment time, initial solution pH, and the reactor pressure as well as the role of oxidizing agents such as (H2O2) were investigated on decomposition rate of sebacic acid and its intermediate products. It was shown that the submerged CO2/CH4 DC thermal plasma could decompose carboxylic acid. The highest decomposition rate was obtained at neutral solution pH. Adding H2O2 could significantly enhance the decomposition rate due to synergetic effect of ultraviolet radiation (UV) absorption and hydrogen peroxide oxidation. Photo-oxidation was also proposed as the decomposition mechanism with the novel CO2/CH4 plasma torch based on its high level of UV. This work shows the ability of this novel plasma torch for treatment of organic acids in liquids and solutions and therefore offers a new application for the CO2/CH4 thermal plasma in wastewater treatment.

Detoxifying PCDD/Fs and heavy metals in fly ash from medical waste incinerators with a DC double arc plasma torch

Medical waste incinerator (MWI) fly ash is regarded as a highly toxic waste because it contains high concentrations of heavy metals and dioxins, including polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). Therefore fly ash from MWI must be appropriately treated before being discharged into the environment. A melting process based on a direct current thermal plasma torch has been developed to convert MWI fly ash into harmless slag. The leaching characteristics of heavy metals in fly ash and vitrified slag were investigated using the toxicity characteristic leaching procedure, while the content of PCDD/Fs in the fly ashes and slags was measured using method 1613 of the US EPA. The experimental results show that the decomposition rate of PCDD/Fs is over 99% in toxic equivalent quantity value and the leaching of heavy metals in the slag significantly decreases after the plasma melting process. The produced slag has a compact and homogeneous microstructure with density of up to 2.8 g/cm3.

Synthesis of Ultrafine Carbon Black by Pyrolysis of Polymers Using a Direct Current Thermal Plasma Process

A direct current thermal plasma system was developed and applied to synthesize ultrafine carbon black by using PS (polystyrene) and HDPE (high density polyethylene) as carbon sources. The precursors were pyrolyzed at different temperatures and the pyrolysis products were employed to investigate the actual synthesis of carbon black through the plasma jet. Spherical carbon nanoparticles with a high degree of turbostratic structure were obtained, showing a fingerprint graphite structure with a large surface area and ultrafine particle size. The carbon black was characterized by transmission and scanning electron microscopy, powder X-ray diffraction, Raman spectroscopy, and nitrogen physisorption. Particle size distribution and dispersion stability in solvents were also investigated.

Effect of ambient pressure on the crystalline phase of nano TiO2 particles synthesized by a dc thermal plasma reactor

The synthesis of nanoparticles of titanium dioxide (TiO2) with varying percentages of anatase and rutile phases is reported. This was achieved by controlling the operating pressure in a transferred-arc, direct current thermal plasma reactor in which titanium vapors are evaporated, and then exposed to ambient oxygen. The average particle size remained around 15 nm in each case. The crystalline structure of the as-synthesized nanoparticles of TiO2 was studied with X-ray diffraction analysis; whereas the particle morphology was investigated with the help of transmission electron microscopy. The precursor species responsible for the growth of these nanoparticles was studied with the help of optical emission spectroscopy. As inferred from the X-ray diffraction analysis, the relative abundance of anatase TiO2 was found to be dominant when synthesized at 760 Torr, and the same showed a decreasing trend with decreasing chamber pressure. The study also reveals that anatase TiO2 is a more effective photocatalytic agent in degrading methylene blue by comparison to its rutile phase.

Particle Injection in Direct Current Air Plasma Spray: Salient Observations and Optimization Strategies

External injection of high-melting point low thermal conductivity ceramics orthogonal to a typical direct current thermal plasma jet plays a vital role in determining the in-flight state of the particles and the process downstream. The interactions between low density ceramic particles and high temperature plasma jet is quite complex, which influences the spray process and associated deposition. Detailed in-flight particle diagnostics as well as spray stream visualization have significantly enhanced our capability to diagnose and control the process. In this paper we present some salient observations on the role of key variables on particle injection. A number of experiments were conducted using a 7MB torch (Sulzer Metco, Westbury, NY) with both Ar–H2 and N2–H2 plasma gases, where the carrier gas flow to inject Yttria Stabilized Zirconia (YSZ) was varied systematically and the resulting in-flight particle state was captured using an array of particle and spray stream sensors arranged in a 3D set-up. A notable observation is the existence of a “sweet-spot” in the plasma jet where the particle temperatures and velocities achieved a maximum. This sweet-spot can be characterized by the plume position (location of centroid of the spray stream) rather than carrier gas flow rate and is independent of primary gas flows and other process/material conditions. This result suggests a possible approach to optimize particle injection independent of plasma-forming-torch-parameters. Controlling particle injection at this sweet-spot has shown to benefit the overall process efficiency (in terms of melting) and process reliability (both in-flight measurement and coating build-up) with concomitant application benefits.

Diagnostics of thermal spraying plasma jets

Direct current thermal plasma jets are strongly affected on the one hand by the arc root fluctuations at the anode, resulting in a type of pulsed flow and enhanced turbulence, and on the other hand by the entrainment of surrounding cold gas in the plasma jet. These phenomena and the resulting temperature distributions have been studied using a wide range of diagnostic techniques, including fast cameras, laser doppler anemometry (LDA), coherent anti-Stokes Raman spectroscopy (CARS), Rayleigh scattering, emission spectroscopy, Schlieren photography, enthalpy probes, and sampling probes. The information obtained by these techniques is evaluated and compared. The effect of the arc fluctuations on the spectroscopic measurements is emphasized, and the possibility of using these fluctuations to determine information on the arc behavior and the axial velocity of the jet is presented. Optimization of plasma processing of solid particles requires information about their size and surface temperature, as well as number flux, and velocity distributions at various locations in the flow field. The different statistical techniques of inflight measurements are discussed together with their limitations. A method to determine the temperature and species density of the vapor cloud or comet traveling with each particle in flight is then presented. However, such statistical measurements present ambiguities in their interpretation, which can be addressed only by additional measurements to determine the velocity, diameter, and surface temperature of a single particle in flight. Moreover, information on single particles is required to understand the coating properties, which depend strongly on the way the particles flatten and solidify upon impact. A method to obtain data related to a single particle in flight and to follow the temperature evolution of the corresponding splat upon cooling is presented. The article concludes with the description of the experimental techniques to follow the temperature evolution of the successive layers and passes. This is important because temperature distribution within the coating and substrate controls the adhesion and cohesion of coatings as well as their residual stress.