Thermal Plasma Technology: Where Do We Stand and Where Are We Going?

In this overview, an attempt is made to assess the present and future research and development in thermal plasma processing of materials restricted to (1) thermal plasma coating technologies, (2) thermal plasma synthesis of fine powders, (3) thermal plasma waste destruction, and (4) thermal plasma spheroidization and densification. Since thermal plasma processing is, in general, governed by a large number of parameters, implementation of controls becomes mandatory. The lack of sufficient controls combined with economic drawbacks in some cases has been the main obstacle for the growth of thermal plasma technology. Present R&D efforts, however, address these problems.  

Study of SO2 Removal Using Non-thermal Plasma Induced by Dielectric Barrier Discharge (DBD)

Dielectric Barrier Discharge (DBD) non-thermal plasma reactors built with three different dielectric materials for SO₂ removal were studied. The discharge characteristics of the three dielectrics, namely glass, Teflon, and glass fiber-based epoxy resin, were analyzed using Lissajous figures. From the Lissajous figures, the transition charge and energy deposition for each dielectric material were determined. When both the discharge characteristics and mechanical processability were considered, glass fiber-based epoxy resin was regarded as the best dielectric barrier among the three for DBD plasma reactors. A multi-cell DBD reactor built with glass fiber-based epoxy resin was used for treating air stream containing SO₂. SO₂ % removal decreased with increasing initial SO₂ concentration in a biphasic fashion. SO₂ removal was greatly improved by adding NH₃ into the air stream. Raising the relative humidity of the air stream also helped SO₂ removal. A SEM (scanning electron microscope) test illustrated some changes in surface morphology of Teflon and glass fiber-based epoxy resin.  

Novel AC and DC Non-Thermal Plasma Sources for Cold Surface Treatment of Polymer Films and Fabrics at Atmospheric Pressure

Novel types of non-thermal plasma sources at atmospheric pressure based on multi-pin DC (direct current) diffusive glow discharge and AC (alternative current) streamer barrier corona have been elaborated and tested successfully for cold surface treatment of polymer films [polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET),] and polyester fabric. Results on physical properties ofdischarges mentioned and output energy characteristics of new plasma sources as well as data on after-treatment changes in wettability of films and fabrics are presented. The main goal of this study was to find out the experimental conditions for gas discharge and surface processing to achieve a remarkable wettability change for a short treatment time.  

Concentration Dependence of VOC Decomposition by Dielectric Barrier Discharges

A comparative study of the decomposition of Volatile Organic Compounds (VOCs: Vinyl Chloride (VC), Ethyl Acetate (EA), Toluene (T), Acetone (A)) in dielectric barrier discharges (DBD) in zero and humidified air at atmospheric pressure was performed. Small scale tubular-flow with pulse excitation and large scale planar-flow with sinusoidal excitation were used to determine the removal efficiency in dependence on inlet concentration S₀ and air humidity. According to the destruction law S=S₀ exp(–E/), where E is the plasma energy density, linear functions were found for the -parameters with respect to S₀ containing an absolute term ₀. By modeling the reaction kinetics it was possible to discriminate active species responsible for the decomposition. Ozone was confirmed to be involved in VC decomposition in zero air whereas OH radicals were best suited to explain the absolute efficiency of EA and toluene destruction in humid air. Their decomposition in zero air however, as well as the degradation of acetone cannot be explained in a similar way.  

Depolymerization of Polyethylene Using Induction-Coupled Plasma Technology

A significant, valuable percentage of today's municipal solid wastestream consists of polymeric materials, for which almost no economicrecycling technology currently exists. This polymeric waste is incinerated,landfilled, or recycled via downgraded usage. Thermal plasma treatment is apotentially viable means of recycling these materials by converting themback into monomers or into other useful compounds. The technical, laboratoryscale, feasibility of using an induction-coupled RF plasma (ICP) heatedreactor for this purpose has been demonstrated in the presentstudy. Polyethylene powder was injected axially through the center of anICP torch. Results from the initial set of experiments, analyzed using astatistical design of experiment technique, showed that plasma plate power,central gas flow rate, probe gas flow rate, powder feed rate, and theinteraction between the quench gas flow rate and power input were the keyprocess parameters affecting the yield of ethylene in the product gasstream. The gaseous products obtained were mainly mixtures of ethylene andpropylene. The amount of propylene obtained was significantly higher thananticipated and was believed to be due to -scission reactionsoccurring at the higher plasma temperatures.  

Entrainment of cold gas into thermal plasma jets

There is increasing evidence that the entrainment of cold gas surrounding a turbulent plasma jet is more of an engulfment type process rather than simple diffusion. A variety of diagnostic techniques have been employed to determine the development of turbulence in a plasma jet and to measure concentration and temperatures of the cold gas entrained into atmospheric-pressure argon plasma jets in ambient argon or air. The results indicate that the transition to turbulence causes a rapid drop of the axial jet velocity due to entrainment of the cold gas surrounding the plasma jet. Dissipation of the cold engulfed gas bubbles by molecular diffusion is relatively slow if molecular gases (for example air) are entrained, as indicated by conditional sampling and CARS measurements. Temperature measurements using emission spectroscopy and enthalpy probes show strong discrepancies in the jet fringes.  

低温等离子体技术治理气态污染物

本文综述了低温等离子体技术治理气态污染物及其反应装置的研究状况与技术优势，并展望了其应用前景。  

废聚丙烯塑料等离子体热解研究——水蒸气的加入对改善气体品质的影响

以聚丙烯为试验物料，利用N2热等离子体在等离子体反应器内进行了一系列热解试验，重点考察了气体产物成分及含量。在反应过程中加入过热水蒸气以改善气体品质，在本实验条件下，气体产物中CO与H2之和可以达到40％，C2H2可达到5％。  

Plasma Thermal Conversion of Methane to Acetylene

This paper describes a re-examination of a known process for the direct plasma thermal conversion of methane to acetylene. Conversion efficiencies (% methane converted) approached 100% and acetylene yields in the 90–95% range with 2–4% solid carbon production were demonstrated. Specificity for acetylene was higher than in prior work. Improvements in conversion efficiency, yield, and specificity were due primarily to improved injector design and reactant mixing, and minimization of temperature gradients and cold boundary layers. At the 60-kilowatt scale cooling by wall heat transfer appears to be sufficient to quench the product stream and prevent further reaction of acetylene resulting in the formation of heavier hydrocarbon products or solid carbon. Significantly increasing the quenching rate by aerodynamic expansion of the products through a converging–diverging nozzle led to a reduction in the yield of ethylene but had little effect on the yield of other hydrocarbon products. While greater product selectivity for acetylene has been demonstrated, the specific energy consumption per unit mass of acetylene produced was not improved upon. A kinetic model that includes the reaction mechanisms resulting in the formation of acetylene and heavier hydrocarbons, through benzene, is described.  

Counter-flow liquid injection plasma synthesis of spinel powders

A novel counter-flow liquid injection plasma synthesis (CF-LIPS) reactor has been developed to produce ceramic powders. By using a counter-flow plasma configuration, entrainment of reactant particles into the plasma is improved compared to conventional injection methods. The counter-flow process also creates large recirculation zones which increase the residence time to more than 100 ms as predicted by modeling results [1]. The long residence time ensures complete evaporation and decomposition of precursor particles and complete reactions to the desirable products. Also, the process employs liquid precursors rather than solids, resulting in less contamination of products from unevaporated reactants. Results show that CF LIPS is an excellent method for producing single-phase and spherical spinel powders with a narrow particle size distribution. Particle size increases with increasing precursor concentration based on the synthesis of magnesium aluminate powders. Characterization techniques include X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), X-ray mapping, centrifugal sedimentation particle size distribution analysis, and vibrating sample magnetometer (VSM) measurements. In addition, crystallographic studies are conducted to determine the bond lengths, bond angles, and stoichiometries of the as-produced spinels.