Comparison of reactor performance in the nonthermal plasma chemicalprocessing of hazardous air pollutants

The performance of three different types of plasma reactors such as ferroelectric packed-bed (FPR), pulsed corona (PCR), and silent discharge (SDR) were compared in the decomposition of trichloroethylene (Cl₂C=CHCl, TCE), bromomethane (CH₃Br), and tetrafluoromethane (CF₄). Irrespective of reactors, hazardous air pollutant (HAP) reactivity in dry N₂ decreased in the order: TCE>CH₃Br>CF₄. Similar byproducts were obtained with any of the above reactors, and similar trends were observed in the HAP decomposition rate-retarding effect by water. Only for SDR, TCE decomposition was accelerated by O₂ in the background gas. The most plausible active oxygen species is considered to be the triplet oxygen atom. In the reaction systems where chemically induced decomposition of HAPs can occur, as in the case of TCE, PCR is expected to exceed FPR and SDR in performance. In the cases of CH₃ Br and CF₄, residence time has been the most important factor governing their decomposition rates, and FPR and SDR have shown higher performance than PCR     

Decomposition of benzene using alumina-hybrid and catalyst-hybridplasma reactors

In order to investigate the effects of alumina and metal ions in plasma discharge, plasma reactors packed with a mixture of BaTiO₃ pellets and porous Al₂O₃ pellets (alumina-hybrid reactor), and with a-mixture of BaTiO₃ pellets and metal-supported Al₂O₃ pellets (catalyst-hybrid reactor) were examined for oxidation of dilute benzene in air. It was found that the oxidative decomposition of benzene was enhanced by concentrating benzene on the Al₂O₃ pellets and the catalyst pellets. Furthermore, the selectivities to CO ₂ in the alumina-hybrid reactor and the catalyst-hybrid reactors were higher than those in the plasma reactor packed with BaTiO ₃ pellets alone. In particular, the selectivities to CO₂ in the catalyst-hybrid reactors using Ag, Co, Cu and Ni/Al₂ O₃ were higher than those from the alumina-hybrid reactor. In addition, the presence of the alumina and catalysts suppressed the formation of N₂O     

Catalysis-assisted plasma technology for carbon tetrachloridedestruction

Single-stage, catalysis-assisted plasma technology is a new concept developed to decompose carbon tetrachloride (CCl₄), one of the greenhouse gases, and to reduce the reaction by-products at the same time. A laboratory-scale plasma reactor used a packed-bed reactor with 1-mm spherical BaTiO₃ pellets. The configuration employed a unique one-stage catalysis/plasma process in which the BaTiO ₃ pellets were coated or impregnated by active catalysts such as Co, Cu, Cr, Ni, and V. The power supply used for these experiments was either a 50-Hz neon transformer or an 18-kHz inverter neon transformer. Enhancement of the CCl₄ destruction and the conversion of by-product CO to CO₂ were demonstrated using Ni catalyst in the one-stage plasma reactor     

Breakdown Characteristics of N2O Gas Mixtures for Quasiuniform Electric Field under Lightning Impulse Voltage

From the viewpoint of mitigating global warming by SF₆ gas, this paper discusses breakdown (BD) characteristics of different electronegative gas mixtures with N₂O gas as SF₆ gas substitutes for quasi-uniform electric field under lightning impulse voltage applications. Experimental results revealed the positive synergism in breakdown strength of binary N₂O / CO₂ and ternary N₂O / CO₂ / O₂ gas mixtures, respectively. Furthermore, N₂ gas as a retardant gas was also mixed with the electronegative gas mixtures in order to reduce the electron energy into the effective levels of electron attachment ability by the electronegative gas mixtures. As the result, ternary N₂O / CO₂ / N₂ and quaternary N₂O / CO₂ / O₂ / N₂ gas mixtures could exhibit the significant synergistic effect in breakdown strength. The optimum mixture rate of quaternary N₂O / CO₂ / O₂ / N₂ gas mixtures was consistent with that estimated by assuming the independent contribution of component gases to the improvement of impulse BD characteristics.     

Composition Optimization and Phase Transformation of Si-Nanocrystal-Doped $hbox{SiO}_{x}$ for Enhancing Luminescence From MOSLED

Near-infrared (NIR) photo- and electroluminescence (PL and EL) of Si nanocrystals buried in Si-rich SiO_x, film, and their correlation with the structural phase transformation and the varied oxygen composition of SiO_x, are investigated. By detuning the N₂O flowing ratio (Y_{N 2 O} = [N₂O/(N₂O + SiH₄)] times 100%) from 93% to 80% during plasma-enhanced chemical vapor deposition growth, the oxygen composition ratio of the Si-rich SiO_x, can be adjusted from 1.64 to 0.88. The grazing incident X-ray diffraction and X-ray photoelectron spectroscopy spectra indicate that the SiO_x, transforms its structural phase from Si + SiO₂ isomer to Si + SiO + SiO₂ isomer. With O/Si ratio >1.24, the SiO_x, matrix becomes SiO₂ isomer, whereas the SiO_x, structure approaches SiO phase at O/Si ratio that is nearly 1.0. The formation of SiO matrix in SiO_x, grown at Y_{N 2 O} below 85% reduces the precipitated Si nanocrystal density from 2.8 times 10¹⁸ to 7 times 10¹⁶ cm^-3, and monotonically attenuates the NIR PL by one order of magnitude. Such a structural phase transformation from SiO₂ to SiO in SiO_x with lower O/Si ratio causes the degradation in EL power conversion efficiency and external quantum efficiency (EQE). Maximum EL power of 0.5 muW and EQE of 0.06% are obtained from MOSLED made on SiO_x, with optimized O/Si ratio of 1.24.     

Aerosol generation and decomposition of CFC-113 by theferroelectric plasma reactor

Plasma chemical decomposition, aerosol generation and product distribution of trichlorotrifluoroethylene (CFC-113 [CCl₂F-CClF₂]) were investigated using a ferroelectric plasma reactor. CFC-113 decomposition was the highest with dry hydrogen, but was not significantly affected by background gases (N ₂ and air) and humidity. Most of the aerosols produced under plasma were of a particle size less than 0.1 μm. The aerosol generation rate increased with applied voltage and concentration, but was not affected by humidity and background gases, indicating that aerosol generation is associated with plasma energy per volume (plasma power density). Aerosolization of 1000-ppm CFC-113 was approximately 10 aerosols/cm³ for air, significantly greater than N₂. The greatest quantity of reaction gas phase byproducts was for dry H₂, followed by dry N₂, wet N₂ , wet air and dry air. Reaction gas phase byproducts were minimal with aerated condition. The plasma reaction starts out by breaking the C-C bond to form radicals, which react with background gas radicals. The formation of CHClF-CClF₂ during CFC-113 decomposition indicates that the C-F bond is much stronger than the C-Cl bond and the C-Cl bond with more P atoms is stronger than that with fewer F atoms     

A novel plasma reactor for NOx control usingphotocatalyst and hydrogen peroxide injection

The authors have developed a new type of plasma reactor combining discharge plasma with a photocatalyst (TiO₂) which improves the performance of NO_x removal. This reactor is designated as a plasma-driven catalyst (PDC) reactor. The authors found that hydrogen peroxide (H₂O₂) is a very effective additive in this PDC reactor and the formation of undesirable by-product (such as O ₃, N₂O) was reduced significantly. Comparative test results showed that the combination of discharge plasma with TiO ₂ catalyst is a very effective method in NO_x, removal over a conventional wire-cylinder reactor. NO_x was effectively oxidized to HNO₃ on the TiO₂ catalyst and trapped on the catalyst surface. Specific energy consumption of this de-NO_x process is significantly reduced, in particular, with the injection of H₂O₂     

Factors and intermediates governing byproduct distribution fordecomposition of butane in nonthermal plasma

Plasma chemical decomposition of butane was investigated with a ferroelectric parked-bed plasma reactor to obtain the information on the fundamental chemical processes occurring in nonthermal plasma. It has been shown that butane decomposition efficiencies are higher in nitrogen rather than in air. This fact suggests that energy transfer from hot electrons to butane is mainly responsible for the initial decomposition of butane. Nitrogen incorporation was observed for acetonitrile only in dry nitrogen and for nitromethane in air. Barium titanate and water have been shown to act as monooxygen transfer agents in nitrogen. Lattice oxygen atoms in barium titanate can be consumed in the formation of N ₂O and CO, depending on reaction conditions. Water is much more reactive than barium titanate as an oxidant in nonthermal plasma, and it can oxygenate butane to butanols, epoxidize 1- and 2-butenes, and oxidize CO to CO₂. Water, which has a dichotomic nature regarding oxygenation/hydrogenation in plasma, can act as a hydrogen source toward alkyl radicals formed in the initial decomposition of butane. In air, triplet oxygen molecules are the most reactive oxygen source in the presence or absence of water and carbon balance can be improved with suppression of byproducts due to promoted autoxidation processes     

Recovery of SF6 from N2/SF6 gasmixtures by using a polymer membrane

The investigation of gas recovery from N₂/SF₆ gas mixtures using a polymer membrane has been reported. It has been shown that the purity of recovered gas, the recovery loss, and the gas handling speed depend on various parameters such as original gas mixing ratio, gas feeding pressure, gas flow rate, and temperature of the membrane. A gas recovering system with two membrane separators connected in cascade has proved to be very efficient in attaining high purity in a recovered gas, with negligibly small recovery loss and high handling speed for practical use. Furthermore, this system could be applied for CO₂/SF₆, He/SF ₆ and even for ternary mixtures such as N₂/CO₂/SF₆ without any modification in the system, where we expect much better performance in comparison with N ₂/SF₆     

Comparison of SF6/N2 and SF6/CO2 gas mixtures as alternatives to SF6 gas

The interest in SF₆ gas mixtures has been re-ignited in recent years by the issue of the greenhouse effect of the SF₆ gas, and most research work is now focused on a SF₆/N₂ gas mixture, which is suitable for application in electrical apparatus with slightly non-uniform fields. This paper presents a comparison of SF₆/N₂ and SF₆/CO₂ gas mixtures with a viewpoint of their possible applications to gas-insulated transformers, where both highly non-uniform field problems and partial discharges in gas/film insulation are inevitable. It is shown that in this case the dielectric strength of SF₆/CO₂ is superior to that of SF₆/N ₂ with a minor disadvantage related to the gas decomposition in SF₆/CO₂. However, this may not be a problem for the SF₆/CO₂ gas mixture to be used in gas-insulated transformers, where internal breakdown is not allowed     

$hbox{CO}_{bf 2}$ Reforming of Aliphatic Hydrocarbons With Silent Discharge Plasma

CO₂ reforming of methane ( CH₄) and propane (C₃H₈) was performed with a silent discharge reactor (SDR). The reactor performance was evaluated in terms of energy efficiencies for the conversion of the substrates and formation of H₂ and CO. The reactivity of C₃H₈ was 2- to 3-fold higher than that of CH₄, and both of CH₄ and C₃H₈ were reformed in the order of 10¹⁶ molecules/J at 298 K. The energy efficiencies for the conversion of these substrates increased with their initial concentrations, but decreased with an increase in reactor energy density. On the other hand, the energy efficiencies for the conversion of CO₂, which were not affected by the hydrocarbon types, were lower than those for the hydrocarbon substrates. A positive temperature effect was observed in the conversion of the hydrocarbon substrates only at low reactor energy densities from 298 to 433 K.     

非运行状态电容式电压互感器绝缘体系生物降解分析

探究非运行状态电容式电压互感器（CVT）油-纸绝缘体系中CH₄、H₂和CO₂等气体含量增高的原因,对提高CVT运行安全很有必要。采取自然污染、人工接种及高温杀菌方式对CVT油-纸绝缘体系进行微生物生长试验,用气相色谱仪、微水仪等仪器测定生物降解绝缘油-纸的产物及含量,并采用生物学鉴定微生物种属,分析油-纸绝缘体系生物降解特性。试验结果表明：CVT油-纸绝缘体系在微生物降解作用下产生了H₂O、酸、CH₄、H₂和CO₂等,其与非运行状态下CVT内绝缘油中气体组分相同;各组分气体含量在增加过程中出现一减小的拐点;显微镜观察和生物学鉴定出4种球状厌氧微生物。结合生物降解产物及变化趋势和微生物种属分析,CVT油-纸绝缘体系生物降解分为2个阶段,即多种微生物降解和厌氧微生物沼气发酵阶段,CH₄、H₂和CO₂等气体是生物降解的特征气体。     

Effects of Temperature, Voltage Properties, and Initial Gas Composition on the Plasma Reforming of Aliphatic Hydrocarbons With $hbox{CO}_{{bf 2}}$

CO₂ reforming of methane, propane, and neopentane was investigated with a ferroelectric packed-bed and silent discharge plasma reactors in N₂ at temperature from 303 to 433 K. The conversions of the substrate hydrocarbons and CO₂, and the yields of H₂ and CO were expressed as functions of reactor energy density irrespective of voltage waveform. The positive temperature effect on the hydrocarbon conversions and the product yields can be ascribed to the promotion of secondary decomposition of the hydrocarbons induced by radicals formed in situ because reactor power consumption was not affected by reaction temperature at the same frequencies and peak-to-peak voltages. At 303 K, the reactivity of the hydrocarbon decreased in the following order: neopentane > propane > methane. At 433 K, propane and neopentane showed the same reactivities. The molar ratio of H₂ to CO was affected by hydrocarbon structure and the initial concentration ratio of CO₂ to the carbon atom in hydrocarbon, but not by reaction temperature. Better carbon balances were obtained for all the hydrocarbons at 433 K than at 303 K, suggesting higher reaction efficiencies at 433 K. Carbon balance was also affected by initial gas composition, and the carbon atoms in reacted neopentane were quantitatively recovered as CO, CO₂ and lighter hydrocarbons at 10 of [CO₂]/[neopentane].     

Reactivity of methane in nonthermal plasma in the presence ofoxygen and inert gases at atmospheric pressure

Partial oxidation of methane to methanol and formaldehyde as liquid fuels with oxygen diluted with inert gases was investigated experimentally using the pulsed discharge plasma method under conditions of room temperature and atmospheric pressure. The experimental results indicated that ethylene (C₂H₄), ethane (C₂ H₆), methanol (CH₃OH), formaldehyde (HCHO), hydrogen (H₂), water (H₂O), carbon monoxide (CO), and carbon dioxide (CO₂) were detected as products and, especially, methanol, formaldehyde, and carbon monoxide were the major products of the plasma chemical reactions. Particularly, it was found that the methanol and formaldehyde production has an optimum specific input energy (SIE: defined as the electrical applied energy per unit volume). The highest methanol and formaldehyde production ability and selectivity were achieved with a relatively low SEE of 360 J/L and concentration of Argon of 50 vol.%. Under this optimum condition, the maximum liquid fuel (methanol and formaldehyde) production ability of about 0.7 μmole/J, and selectivity of 64 mol%, were obtained     

Reduction of CO2 from combustion gases by DC coronatorches

An experimental investigation has been conducted to reduce CO₂ from combustion gases by using DC corona torches. This plasma device, the corona torch, consists of two small-diameter hollow electrodes. The gas flow enters the upstream cylindrical hollow electrode and exits at a downstream cylindrical hollow electrode. Therefore, all the reactive gas passes through the active corona-induced plasma zone. High-speed gas flow near the exit of the electrode cools the electrodes; hence, the chemical reactions and the stability of discharge are enhanced. Various amounts of argon gas were added to the model combustion gas (N₂:O₂:CO₂=0.745:0.15:0.105). The results show that the CO₂ gas reduction rate increases with increasing corona current when the model gas is seeded with argon. The CO₂ gas reduction rate increases when the percentage of argon gas in the mixture increases up to 20%, and decreases when the percentage of argon gas in the mixture increases above 20%     

Gas cleaning with semi-wet type plasma reactor

An experimental study on the removal of NH₃, NO, NO_x in concentration of 10-40 ppm in air has been carried out using plasma chemical reactions in a streamer corona discharge. The results of the performance of dry type and semi-wet type reactors are compared. The effects of different type of applied voltages such as rectangular pulse, 60-Hz sinusoidal, and 18-kHz alternating voltages are investigated. During NO removal, O₃ and NO₂ are produced. NO₂ can, partially, be removed with higher power input into the discharge. Another undesirable pollutant, namely N₂ O, is also produced, especially, in case of dry reactors having long residence time (~2.4 s). N₂O production decreases, essentially, to zero at 0.6-s residence time while using a semi-wet reactor. In general, higher removal efficiency has been obtained with pulse voltage in a semi-wet reactor. NH₃ in air appears to produce ozone and ammonium nitrate in a discharge. The performance of semi-wet reactors an the removal of submicron dust particles has also been investigated and very high removal efficiency (~93% at 0.6-s residence time) has been obtained     

Interruption capability of CF/sub 3/I Gas as a substitution candidate for SF/sub 6/ gas

We studied the interruption performance of CF₃I gas because its environmental effect is smaller than that of SF₆ gas with a model arc-extinguishing chamber. First, we measured the arc time constant and arc power loss coefficient using Mayr's equation. Comparing CF₃I with other gases, the arc time constants are SF₆3I2< H₂2. The arc power loss coefficient is H₂>SF₆ >CO₂>Air>N₂ >CF₃I. Next, we evaluated the short line fault (SLF) interruption capability by measuring the di/dt-dv/dt characteristic. Consequently, the SLF interruption performance of pure CF₃I was about 0.9 times that of SF₆. However, CF₃I application to gas insulated switchgear (GIS) and gas circuit breakers (GCB) is difficult because the liquefying temperature of the gas is high. Therefore, we adopted a countermeasure to obtain a lower liquefying temperature mixing CF₃I with CO₂. The result showed that the SLF interruption performance of the mixture approximated that of pure CF₃I when the ratio of CF₃ I exceeds 20%     

Decomposition of benzene using a nonthermal plasma reactor packedwith ferroelectric pellets

The decomposition of benzene in air was carried out using a nonthermal plasma discharge reactor packed with ferroelectric materials. It was found that ferroelectric materials of 1-2 mm in diameter with a relative dielectric constant of ϵ_r>1100 decomposed benzene with greatest energy efficiency. Benzene at low concentrations (below 50 ppm) was completely decomposed to CO and CO₂ with no formation of other hydrocarbons. The ratio of CO to CO₂ produced from benzene was minimized under an atmosphere containing more than 5% O₂     

Electrical and Reliability Studies of “Wet N2O” Tunnel Oxides Grown on Silicon for Flash Memory Applications

In this paper, we report the electrical characteristics and reliability studies on tunnel oxides fabricated by "wet N₂O" oxidation of silicon in an ambient of water vapor and N₂O at a furnace temperature of 800 degC. Tunnel oxides that have an equivalent oxide thickness of 67 A are subjected to a constant-current stress (CCS) amount of -100 mA/cm² using a MOS capacitor to obtain information on stress-induced leakage current (SILC), interface, and bulk trap generation. The obtained results clearly demonstrate the superior performance features of the present tunnel oxides with reduced SILC, lower trap generation, minimum change in gate voltage, and higher charge-to-breakdown during CCS studies. X-ray photoelectron spectroscopy depth profile studies of the tunnel oxide interfaces have shown that the improved performance characteristics and reliability can be attributed to the incorporation of about 8.5% nitrogen at the oxide-silicon interface of the samples formed by the "wet N₂O" process that involves low-temperature oxidation and annealing at 800 degC.     

Polarization behavior of ferroelectricBi4Ti3O12 ceramics

The effect of oxidation and reduction atmospheres on the ferroelectric behavior of bismuth titanate was investigated. The effective surface density σ_eff of free charges in samples polarized by applying an electric field E_p=1 MV m^-1 at a temperature of 100°C for 90 min was determined by compensation voltage measurements. The thermally treated samples in oxidation atmospheres exhibit a significant decrease of σ_eff while those treated in a reducing atmosphere exhibit an increase accompanied by an improvement in the stability of the electret charge. Such observations indicate that the formation as well as the transformation of the electret state of Bi₄Ti₃O ₁₂ ceramics may be related to the enhancement and diminishing of oxygen vacancies. In all cases homocharge with a time independent sign was obtained