Plasma Chemistry and Plasma Processing (v.28, #4)

Synthesis of Ammonia Using CH4/N2 Plasmas Based on Micro-Gap Discharge under Environmentally Friendly Condition by Mindong Bai; Zhitao Zhang; Mindi Bai; Xiyao Bai; Honghui Gao (405-414).
The synthesis of ammonia has been studied in methane-nitrogen plasmas using a micro-gap discharge under an environmentally friendly condition. The effects of some parameters such as the specific input energy, the discharge gap, the volume ratio of CH4/N2, the residence time, and the gas temperature on the yield of NH3 and conversion rate of CH4 are discussed in the paper. The results show that the highest yield of NH3 is 8000 ppm for a residence time of 1.6 s. In addition, the yield and generating rate of H2 are 9.1% (v/v) and 1879.8 μmol/min, respectively. Therefore, the micro-gap discharge is an efficient method for NH3 synthesis and H2 generation from CH4.
Keywords: Concentration of synthesized NH3 ; Conversion rate of CH4 ; Micro-gap discharge; Environmentally friendly condition

We examined the influence of the gas flow-rate, microwave power and trichloroethylene concentration on the destruction of trichloroethylene with a system based on a microwave helium plasma operating at atmospheric pressure. Based on the experimental results obtained, the proposed system allows input concentrations of C2HCl3 in the ppmv range to be reduced to output concentrations in the ppbv range (i.e. virtually quantitative destruction) by using a microwave plasma power below 1000 W. High helium flow-rates and C2HCl3 concentrations allow energy efficiency values above 600 g/kW h to be obtained. Analyses of the output gases by gas chromatography and species present in the plasma by optical emission spectroscopy confirmed the negligible presence of halogen compounds resulting from the destruction of C2HCl3, and that of CCl4 and C2Cl4 as the sole chlorine species exceeding levels of 30 ppbv. Gaseous by-products consisted mainly of CO2, NO and N2O in addition to Cl2 traces.
Keywords: Microwave plasma; VOC removal; Atmospheric pressure; Plasma

Plasma Reactivity and Plasma-Surface Interactions During Treatment of Toluene by a Dielectric Barrier Discharge by N. Blin-Simiand; F. Jorand; L. Magne; S. Pasquiers; C. Postel; J.-R. Vacher (429-466).
Toluene removal is investigated in filamentary plasmas produced in N2 and in N2/O2 mixtures by a pulse high voltage energised DBD. Influence of the oxygen percentage (lower than 10%) and of the temperature (lower than 350°C) is examined. Toluene is removed in N2 through collisions with electrons and nitrogen excited states. The removal efficiency is a few higher in N2/O2. It increases when the temperature increases for N2 and N2/O2. Both H- and O-atoms play an important role in toluene removal because H can readily recombine with O to form OH, which is much more reactive with toluene than O. H follows from dissociation of toluene and of hydrogenated by-products by electron collisions. Detection of cyanhidric acid, acetylene, formaldehyde, and methyl nitrate strengthens that dissociation processes, to produce H and CH3, must be taken into account in kinetic analysis. Formation and treatment of deposits are also analysed.
Keywords: Dielectric barrier discharge; Toluene; Plasma kinetic; Ozone; Oxygen atom; Hydroxyl radical

Degradation of Methylene Blue by RF Plasma in Water by T. Maehara; I. Miyamoto; K. Kurokawa; Y. Hashimoto; A. Iwamae; M. Kuramoto; H. Yamashita; S. Mukasa; H. Toyota; S. Nomura; A. Kawashima (467-482).
Radio frequency (RF) plasma in water was used for the degradation of methylene blue. The fraction of decomposition of methylene blue and the intensity of the spectral line from OH radical increased with RF power. RF plasma in water also produced hydrogen peroxide. The density of hydrogen peroxide increased with RF power and exposure time. When pure water (300 mL) is exposed to plasma at 310 W for 15 min, density of hydrogen peroxide reaches to 120 mg/L. Methylene blue after exposed to plasma degraded gradually for three weeks. This degradation may be due to chemical processes via hydrogen peroxide and tungsten. The comparison between the experimental and calculated spectral lines of OH radical (A–X) shows that the temperature of the radical is around 3,500 K. Electron density is evaluated to be ≃3.5 × 1020 m−3 from the stark broadening of the Hβ line.
Keywords: Radio frequency plasma; In water; Degradation; AOPs

Naphthalene and Acenaphthene Decomposition by Electron Beam Generated Plasma Application by Anna Ostapczuk; Teruyuki Hakoda; Akihiko Shimada; Takuji Kojima (483-494).
The application of non-thermal plasma generated by electron beam (EB) was investigated in laboratory scale to study decomposition of polycyclic aromatic hydrocarbons like naphthalene and acenaphthene in flue gas. PAH compounds were treated by EB with the dose up to 8 kGy in dry and humid base gas mixtures. Experimentally established G-values gained 1.66 and 3.72 mol/100 eV for NL and AC at the dose of 1 kGy. NL and AC removal was observed in dry base gas mixtures showing that the reaction with OH radical is not exclusive pathway to initialize PAH decomposition, however in the presence of water remarkably higher decomposition efficiency was observed. As by-products of NL decomposition were identified compounds containing one aromatic ring and oxygen atoms besides CO and CO2. It led to the conclusion that PAH decomposition process in humid flue gas can be regarded as multi-step oxidative de-aromatization analogical to its atmospheric chemistry.
Keywords: Non-thermal plasma; Polycyclic aromatic hydrocarbons; Electron beam; Flue gas treatment

Role of the Temperature on the Interaction Mechanisms Between Argon–Oxygen Post-Discharge and Hexatriacontane by M. Mafra; T. Belmonte; F. Poncin-Epaillard; A. S. da Silva Sobrinho; A. Maliska (495-509).
Interactions between a late Ar–O2 post-discharge and the hexatriacontane (HTC), a long-chain alkane, are shown to depend on the thermal flux released by surface reactions that makes the temperatures of the sample and the gas phase drift in an uncontrolled manner as a function of time. Since the transformations of the hexatriacontane depend on these temperatures, the initial value of the temperature and that of the oxygen concentration are key parameters that control the whole transformation process. A thorough description of the different steps of the transformation undergone by the hexatriacontane is given, explaining the origins of the limitation of the material etching. Pulsing the plasma shows that optimizing the etching process requires to work at low temperature, a too strong heating of the sample leading to functionalization and reticulation that limit the etching of the HTC.
Keywords: Hexatriacontane; Plasma cleaning; Post-discharge

The influence of polarity and rise time of the pulse voltage on the removal of particulate matter (PM) emitted from a diesel engine was investigated using a dielectric barrier discharge reactor. Four kinds of pulse voltage waveforms (positive, negative, positive–negative and negative–positive) were used. It was found that the energy efficiency for PM removal is just a function of energy injection and that there are no obvious influences on PM removal and energy efficiency within the voltage waveforms except the negative pulse voltage of a peak voltage below 8 kV. A comparison of PM removals using various kinds of pulse voltage waveforms and different types of plasma reactors is given.
Keywords: Pulse voltage waveform; PM removal; Corona discharge; Uneven DBD reactor

The effects of gas composition on hybrid gas–water gliding arc discharge plasma reactor have been studied. The voltage cycles are characterized by a moderate increase in the tension which is represented by a peak followed by an abrupt decrease and a current peak in the half period (10 ms). Emission spectrum measurements revealed that OH hydroxyl radicals are present in the discharge with feeding any gas. The H2O2 concentrations reach 38.0, 15.0, 10.0, and 8.0 mg/l after 25 min plasma treatment with oxygen, argon, air, and nitrogen, respectively. O3 was produced when oxygen and air are used, but not when nitrogen and argon. The O3 concentration reached the highest value 1.0 mg/l after 25 min plasma treatment with oxygen feeding gas, but gradually decreased to 0.2 mg/l after that. With feeding nitrogenous gas, NO2 and NO3 byproducts were formed by the plasma chemical process.
Keywords: Plasma; Gliding arc discharge; Gas–water; Active species; Byproducts

Influence of Plasma Treatments on the Hemocompatibility of PET and PET + TiO2 Films by Ionut Topala; Nicoleta Dumitrascu; Valentin Pohoata (535-551).
A dielectric barrier discharge (DBD) in helium was used to ameliorate the interface between the blood and the surface of polymeric implants: polyethylene terephthalate (PET) and PET with titanium oxide (PET + TiO2). A higher crystallinity degree was found for the DBD treated samples. The wettability of polymers was improved after the treatment. The chemical composition, analyzed by infrared spectroscopy was preserved during the DBD treatment. The surface modifications have been correlated with polymers hemocompatibility. Concerning the polymer surface–blood interaction, the treatment induced a decrease of the interfacial tension between the blood components and the treated surfaces. The in vitro tests of hemocompatibility showed no perturbation in the blood composition when the polymer samples are present in the blood volume. An interesting result is related to the whole blood clotting time that shows a dramatic increase on the treated surfaces. Moreover, the coagulation kinetics on the treated surfaces is modified.
Keywords: Blood–material interface; Dielectric barrier discharge; Hemocompatibility; Polymers; Surface modification

Thermodynamic and Transport Properties of Two-Temperature Nitrogen-Oxygen Plasma by S. Ghorui; J. V. R. Heberlein; E. Pfender (553-582).
Thermodynamic and transport properties are computed for a 17 species model of nitrogen-oxygen plasma under different degrees of thermal non-equilibrium, pressures and volume ratios of component gases. In the computation electron temperatures range from 300 to 45,000 K, mole fractions range from 0.8 to 0.2, pressures range from 0.1 atmosphere to 5 atmospheres, and thermal nonequilibrium parameters (Te/Th) range from 1 to 20. It is assumed that all the electrons follow a temperature Te and the rest of the species in the plasma follow a temperature Th. Compositions are calculated using the two temperature Saha equation derived by van de Sanden et al. Updated energy level data from National Institute of Standards and Technology (NIST) and recently compiled collision integrals by Capitelli et al., have been used to obtain thermodynamic and transport properties. In the local thermodynamic equilibrium (LTE) regime, the results are compared with published data and an overall good agreement is observed.
Keywords: Thermodynamic property; Transport property; Two temperature plasma; Nitrogen oxygen plasma; Air plasma