Plasma Chemistry and Plasma Processing (v.34, #1)

Reduction of Oxide Minerals by Hydrogen Plasma: An Overview by K. C. Sabat; P. Rajput; R. K. Paramguru; B. Bhoi; B. K. Mishra (1-23).
Carbothermic reduction of oxide minerals is one of the major routes to obtain the corresponding metals. This process produces a lot of CO2, which is responsible for greenhouse effect. Alternatively, hydrogen plasma containing hydrogen in atomic, ionic, and excited states can reduce almost every metal oxide even at lower temperatures. Besides this advantage, plasma processing also offers kinetic advantages. Further, hydrogen-water cycle does not pose any environmental problems. However, reduction of metal oxides in hydrogen plasma is not so straightforward—there are issues relating to introduction of material into the plasma zone, residence time, reverse reaction, and scale-up that must be resolved—yet, it holds the key to future environmental challenges particularly with respect to CO2 emission. This paper provides an overview of reduction of oxide minerals by hydrogen plasma. The influences of various reaction conditions particularly with respect to reduction of oxides are discussed and some aspects of both thermal and non-thermal cold plasma linking oxidative as well as dissociative reduction are presented.
Keywords: Hydrogen plasma; Thermal plasma; Non-thermal plasma; Oxide minerals

Miniaturized atmospheric pressure glow discharges (APGDs) were generated in contact with small sized flowing liquid cathode systems. As anodes a solid pin electrode or a miniature flow Ar microjet were applied. Both discharge systems were operated in the open to air atmosphere. Hydrogen peroxide (H2O2) as well as ammonium (NH4 +), nitrate (NO3 ), and nitrite (NO2 ) ions were quantified in solutions treated by studied discharge systems. Additionally, an increase in the acidification of these solutions was noted in each case. Emission spectra of the near cathode zone of both systems were measured in order to elucidate mechanisms that lead to the formation of active species in gas and liquid phases of the discharge. Additionally, the concentration of active species in the liquid phase (H2O2, NH4 +, NO3 and NO2 ) was monitored as a function of the solution uptake rate and the flow rate of Ar. The suitability of investigated discharge systems in the water treatment was tested on artificial wastewaters containing an organic dye (methyl red), hardly removable by classical methods non-ionic surfactants (light Triton x-45 and heavy Triton x-405) and very toxic Cr(VI) ions. Preliminary results presented here indicate that both investigated flow-through APGD systems may successfully be applied for the efficient and fast on-line continuous flow chemical degradation of toxic and hazardous organic and inorganic species in wastewater solutions.
Keywords: Atmospheric pressure glow discharge; Degradation of hazardous chemicals; Plasma water treatment processes; Plasma–liquid interaction

Scaling of Shielded Sliding Discharges for Environmental Applications by Karl H. Schoenbach; Muhammad Arif Malik (39-54).
Shielded sliding discharges are nanosecond streamer discharges which develop along a dielectric between metal foil electrodes, with one of the foils extended over the entire rear of the dielectric layer. The electrode configuration not only allowed rearranging discharges in parallel due to the decoupling effect of the metal layer, but also to modify the electric field distribution in such a way that components normal to the surface are enhanced, leading to an increased energy density in the discharge plasma. By varying the electrode gap, the applied voltage, and the repetition rate, it is shown that by keeping the average electric field constant, the discharge voltage can be reduced from tens of kV to values on the order of a few kV, but only at the expense of a reduced energy density of the plasma. Varying the repetition rate from 20 to 500 Hz resulted in a slightly reduced energy per pulse, likely caused by residual charges on the dielectric surface. Measurements of the NO conversion to NO2 and ozone synthesis in dry air showed that the conversion is only dependent on the energy density of the discharge plasma. Although reducing the pulse voltage from the tens of kV range to that of few kV, and possibly even lower, causes a reduction in energy density, this loss can be compensated for by increasing the electrode gap area. This and the possibility to form discharge arrays allows generating large volume discharge reactors for environmental applications, at modest pulsed voltages.
Keywords: Shielded sliding discharges; Streamer discharges; Nanosecond discharges; Nonthermal plasma; Nitric oxide; Ozone; Air pollution

The Breakdown Phenomena in Micrometer Scale Direct-Current Gas Discharges by M. Radmilović-Radjenović; B. Radjenović; Š. Matejčik; M. Klas (55-64).
This paper reports on experimental and theoretical studies of the direct current breakdown voltage characteristics for several gases (argon, nitrogen, helium, hydrogen, dry air, oxygen and carbon dioxide) in discharges with parallel-plane electrodes separated from 1 to 100 μm. The breakdown phenomena have been discussed in terms of field emission of electrons from the cathode. Based on the measured breakdown voltage curves, realistic values of the effective yields and the field emission thresholds for considered gases have been estimated. It was found that the secondary electron emission due to high electric field generated in microgaps depends primarily on the electric field E leading directly to the violation of the Paschen’s law. The effective yields due to the field emission for all gases are determined for the first time. Experimental data are supported by the theoretical predicitions that suggest departure from the scaling law and a flattening of the Paschen curves at higher pressures. The obtained results may provide better understending of the breakdown phenomena in microgaps.
Keywords: Field emission; Secondary electron; Breakdown voltage; Microgaps

Pilot-Scale Aftertreatment Using Nonthermal Plasma Reduction of Adsorbed NOx in Marine Diesel-Engine Exhaust Gas by Takuya Kuwahara; Keiichiro Yoshida; Tomoyuki Kuroki; Kenichi Hanamoto; Kazutoshi Sato; Masaaki Okubo (65-81).
Regulations governing marine diesel engine NOx emissions have recently become more stringent. As it is difficult to fulfill these requirements by combustion improvements alone, effective aftertreatment technologies are needed to achieve efficient NOx reductions. In this study, we develop an effective NOx-reduction aftertreatment system for a marine diesel engine that employs combined nonthermal plasma (NTP) and adsorption. Compared with selective catalytic reduction, the proposed technology offers the advantages of not requiring a urea solution or harmful heavy-metal catalysts and low operating temperatures of less than 150 °C. The NOx reduction comprises repeated adsorption and desorption flow processes using NTP combined with NOx adsorbents made of MnOx–CuO. High concentrations of NOx are treated by NTP after NOx adsorption and desorption, and this aftertreatment system demonstrates excellent energy efficiencies of 161 g(NO2)/kWh, which fulfills the most recent International Maritime Organization emission NOx standards in the Tier II–III regulations for 2016 and requires only 4.3 % of the engine output power.
Keywords: Marine diesel engine; Nonthermal plasma; NOx ; Aftertreatment; Emission; Adsorption

Production of Reactive Oxygen Species and Application for NOx Control by Mindi Bai; Baiyu Leng; Shaolei Mao (83-92).
Current gas ionization discharge techniques used in the removal of NOx from waste gases require large plasma sources, have high energy consumption, and may feature low NOx removal rates. We develop a system to generate reactive oxygen species through a strong ionization discharge, which is injected into a flow of simulated waste gas. The relative proportions and temperatures of input gases were controlled and the rate of consumption by reactive species was monitored. HNO3 oxidization products of NOx were also collected and measured. The molar ratio of reactive oxygen species to NO was optimized to improve the rate of NOx removal. A input gas temperature of 58–60 °C was also found to be optimal. The O2 volume fraction has almost no influence on NOx removal, while H2O volume fractions above 6 %, gave rise to NOx removal rates of 97.2 %. The present study addresses disadvantages of current gas ionization discharge and requires no catalyst, reducing agent or oxidant.
Keywords: NOx removal rate; NO3 recovery rate; Strong ionization discharge; Reactive oxygen species; ·OH radicals

Positive and negative streamer discharges in atmospheric pressure air were generated in a shielded sliding discharge reactor at operating voltages as low as 5 kV for a gap length of 1.6 cm. In this reactor, electrodes are placed on top of a dielectric layer and one of the electrodes, generally the one on ground potential, is connected to a conductive layer on the opposite side of the dielectric. The energy per pulse, at the same applied voltage, was more than a factor of seven higher than that of pulsed corona discharges, and more than a factor of two higher than that of sliding discharges without a shield. It is explained on the basis of enhanced electric fields, particularly at the plasma emitting electrode. Specific input energy required for 50 % removal from ~1,000 ppm initial NO could be reduced to ~18 eV/molecule when ozone in the exhaust of negative streamers was utilized. For sliding discharges and pulsed corona discharges this value was ~25 eV/molecule and it was 35 eV/molecule for positive shielded sliding discharges. Also, the ozone energy yield from dry air was up to ~130 g/kW h and highest for negative streamer discharges in shielded sliding discharge reactors. The high energy density in negative streamer discharges in the shielded discharge reactor at the relatively low applied voltages might not only allow expansion of basic studies on negative streamers, but also open the path to industrial applications, which have so far been focused on positive streamer discharges.
Keywords: Non-thermal plasma; Pulsed corona discharges; Sliding discharges; Shielded sliding discharges; Positive streamer discharges; Negative streamer discharges; Nitric oxide conversion; Ozone generation

Optimization of NH3 Decomposition by Control of Discharge Mode in a Rotating Arc by Dae Hoon Lee; Kwan-Tae Kim; Hee Seok Kang; Sungkwon Jo; Young-Hoon Song (111-124).
In this study, the characteristic behavior of a rotating arc was investigated. Various modes, depending on the electric power supplied, can be observed in a rotating arc. Each mode produces different discharge characteristics and thermal environments and, accordingly, chemical processes hosted in the plasma reaction volume can be controlled differently in each mode. General thermal to non-thermal transitions observed in a gliding arc are based on the longitudinal expansion of the arc column. In a rotating arc, the reverse transition or non-thermal to thermal transition can be hosted by controlling the reactor geometry. The reverse transition can be achieved by self-adjustment of the arc column where longitudinal expansion of the arc column is confined. The reverse transition enhances the conversion efficiency of electric power to thermal energy. Then, optimization of thermal activation was obtained by controlling the mode of operation, and it was verified using the NH3 decomposition reaction.
Keywords: Rotating arc; Mode; Expansion; Non-thermal to thermal transition

Reduction Characteristics of Carbon Dioxide Using a Plasmatron by Seong Cheon Kim; Mun Sup Lim; Young Nam Chun (125-143).
To decompose carbon dioxide, which is a representative greenhouse gas, a 3-phase gliding arc plasmatron device was designed and manufactured to examine the decomposition of CO2, either alone or in the presence of methane with and without water vapour. The changes in the amount of carbon dioxide feed rate, the methane to carbon dioxide ratio, the steam to carbon dioxide ratio, and the methane to steam ratio were used as the parameters. The carbon dioxide conversion rate, energy decomposition efficiency (EDE), carbon monoxide and hydrogen selectivity, and produced gas concentration were also investigated. The maximum values of the carbon dioxide conversion rate, which is a key indicator of carbon dioxide decomposition, in different cases were compared. The maximum carbon dioxide conversion rate was 12.3 % when pure carbon dioxide was supplied; 34.5 % when methane was injected as a reforming additive; 7.8 % when steam was injected as a reforming additive; and 43 % when methane and steam were injected together. Therefore, this could be explained that the methane-and-steam injection showed the highest carbon dioxide decomposition, showing low EDE as 0.01 L/min W. Furthermore, the plasma produced carbon-black was compared with commercial carbon-black chemicals through Raman spectroscopy, surface area measurement and scanning electron microscopy. It was found that the carbon-black that was produced in this study has the high conductivity and large specific surface area. Our product makes it suitable for special electric materials and secondary battery materials applications.
Keywords: Gliding arc plasma; Plasmatron; Greenhouse gas; Global warming; Carbon dioxide

Hydrogen Production from Ethanol Decomposition by Two Microwave Atmospheric Pressure Plasma Sources: Surfatron and TIAGO Torch by Rocío Rincón; Margarita Jiménez; José Muñoz; Manuel Sáez; María Dolores Calzada (145-157).
Molecular hydrogen production from ethanol decomposition by two microwave atmospheric pressure plasma sources (surfatron and Torche a Injection Axiale sur Guide d’Onde (TIAGO) torch) was studied by optical emission spectroscopy and mass spectrometry. In both cases ethanol was almost completely decomposed, thus giving place to molecular hydrogen. However, the atmosphere surrounding the discharge significantly influences the overall decomposition process. When the surfatron is used, C2H2 and CO are obtained as exhaust gases. Likewise, H2O and HCN are also detected at plasma exit when sustained with the TIAGO torch.
Keywords: Plasma; Torch; Atmospheric pressure; Hydrogen; Ethanol; Emission spectroscopy; Mass spectrometry

Plasma Burner for Active Regeneration of Diesel Particulate Filter by Dae Hoon Lee; Hongsuk Kim; Young-Hong Song; Kwan-Tae Kim (159-173).
This study proposes a diesel particulate filter system to remove particulate matter (PM) from diesel engines. Filters should be periodically burned (regenerated) to remove PM. In the proposed system, a plasma burner is utilized for filter regeneration. The plasma burner avoids the drawbacks of the electric heater and full-flow-type diesel burner by successively discharging and partially oxidizing the fuel, which produces hydrogen for flame stabilization under all revolution per minute and load conditions. The proposed system was verified by field testing three different vehicles for over 20,000 km. The field test results confirmed successful PM removal and the simultaneous removal of other emissions such as CO, NOx, and hydrocarbons. The plasma burner is applicable to the thermal management of emission control systems for both retrofitted and original equipment manufacturer cars.
Keywords: Plasma burner; DPF; Regeneration; Diesel engine; PM

Effect of the Electric Conductivity of a Catalyst on Methane Activation in a Dielectric Barrier Discharge Reactor by Sungkwon Jo; Taegyu Kim; Dae Hoon Lee; Woo Seok Kang; Young-Hoon Song (175-186).
The influence of catalyst electric conductivity on methane activation in a planar-type dielectric barrier discharge reactor is investigated by empirically comparing the degree of methane conversion of bare Al2O3 with that of Pt/Al2O3; from this, it is determined that the latter catalyst converts less methane owing to the presence of Pt. Calculations and comparisons of electric fields with and without Pt show that the presence of a Pt catalyst results in a lower electric field than does bare Al2O3. An analysis of product gases based on the correlation between the fragmentation of radicals and the electric field also indicates that the electric field is decreased by using Pt. From these results, it can be concluded that the synergies between the plasma and the conductive catalysts need to be reassessed for different electric field conditions, and that further studies of non-conductive catalysts that can enhance methane activation and synergistic effects are needed.
Keywords: Plasma; Catalyst; Methane; Dielectric barrier discharge; Reforming

Ethylene Epoxidation in an AC Dielectric Barrier Discharge Jet System by Thitiporn Suttikul; Satoshi Kodama; Hidetoshi Sekiguchi; Sumaeth Chavadej (187-205).
In this work, ethylene epoxidation was investigated in a dielectric barrier discharge jet (DBDJ) with a separate ethylene/oxygen feed under oxygen lean conditions. The ethylene (C2H4) stream was directly injected behind the plasma zone in order to reduce all undesired reactions, including C2H4 cracking and further reactions, while the oxygen (O2) balanced with argon was fed through the plasma zone totally to maximize the formation of active oxygen species. The effects of various operating parameters, such as total feed flow rate, O2/C2H4 feed molar ratio, applied voltage, input frequency, and C2H4 feed position on the ethylene epoxidation activity, were investigated to determine the optimum operating conditions for this new DBDJ system. The highest ethylene oxide (EO) selectivity (55.2 %) and yield (27.6 %), as well as the lowest power consumption (3.3 × 10−21 and 6.0 × 10−21 Ws/molecule C2H4 converted and EO produced, respectively) were obtained at a total feed flow rate of 1,625 cm3/min (corresponding to a residence time of 0.022 s), an O2/C2H4 feed molar ratio of 0.25:1, an applied voltage of 9 kV, an input frequency of 300 Hz, and a C2H4 feed position of 3 mm behind the plasma zone. The superior activity of the ethylene epoxidation in the DBDJ system resulted from a small reaction volume as well as a separate ethylene/oxygen feed.
Keywords: Epoxidation; Ethylene oxide; Dielectric barrier discharge; Atmospheric plasma jet

Effects of Cold Plasma Treatment on the Performance of Polyurethane Laminated Glass by Xibao Li; Jinshan Lu; Junming Luo; Haizhong Zheng; Gangqin Shao (207-215).
The delamination of polyurethane (PU) laminated glass usually occurs at the interface between PU and inorganic glass. To prevent interlaminar delamination of PU and inorganic glass, PU was treated with cold plasma. The transparency of PU improved after cold plasma treatment. The cold plasma-treated PU showed 2 to 6 % higher transmittance than untreated PU. The adhesive strength of PU laminated glass increased by at least 96 % after cold plasma treatment. The adhesive strength of untreated PU samples rapidly decreased with increased hygrothermal aging time, whereas treated PU maintained almost 93 % of the original adhesive value even after hygrothermal aging for 30 days. FTIR analysis shows that –CO–O–C–, –C=O, and –CO–N groups appeared on the surface of cold plasma-treated PU, which resulted in the formation of hydrogen bonds at the interface between PU and silicate glass. After cold plasma treatment, the contact angle of PU changed from 105° to 79°, and the surface roughness of PU membranes changed from an average value of 19.74 to 57.16 nm.
Keywords: Cold plasma; Adhesion; Polyurethane; Interface

Oxidation of Elemental Mercury by Active Species Generated From a Surface Dielectric Barrier Discharge Plasma Reactor by Jiu Tao An; Ke Feng Shang; Na Lu; Yu Ze Jiang; Tie Cheng Wang; Jie Li; Yan Wu (217-228).
A surface dielectric barrier discharge plasma reactor was employed to study Hg0 oxidation in coal-fired flue gas. The experimental results showed that 98 % of Hg0 oxidation efficiency and 13.7 μg kJ−1 of energy yield were obtained under a specific energy density (SED) of 7.9 J L−1. Increasing SED was beneficial for Hg0 oxidation due to higher production of active species. Higher initial concentration resulted in lower Hg0 oxidation efficiency, but higher amount of Hg0 oxidation. Water vapor inhibited Hg0 oxidation because the generation of O3 was suppressed. The presence of NO remarkably restrained Hg0 oxidation, while SO2 showed little effect on Hg0 oxidation. Roles of active species in Hg0 oxidation were examined under different gas atmospheres (O2 and air), indicating that O3 played an important role in Hg0 oxidation. Deposits on the internal surface of the reactor were analyzed by energy dispersive spectroscopy and the product was identified as HgO.
Keywords: Surface discharge plasma reactor; Active species; Elemental mercury; Mercury oxidation

Deposition of a Continuous and Conformal Copper Seed Layer by a Large-Area Electron Cyclotron Resonance Plasma Source with Embedded Lisitano Antenna by Soo Ouk Jang; Hyun Jong You; Young-Woo Kim; Yong Ho Jung; In Uk Hwang; Jae Yang Park; Heon Lee (229-237).
To deposit copper seed layer on ultra large scale integration devices, a large-area (Ø 378 mm) electron cyclotron resonance plasma has been generated by using permanent magnets-embedded Lisitano antenna. The plasma source operates in the pressure range of 0.2–1.5 mTorr with microwave power range of 500–2,000 W. By using a Langmuir probe, the electron density and temperature have been measured near the DC sputter target position. Measurements indicate argon plasmas having electron densities of ~5 × 1010/cm and electron temperatures of 5 eV with 750 W microwave power at gas pressures of 0.5 mTorr. Using this plasma source and a DC sputter, we obtained excellent conformal copper seed layer with high aspect ratios of 12:1. This is in contrast with conventional methods using magnetron sputter, which has aspect ratios of 2–3:1. Also, improvements are observed in the smoothness (root mean square roughness of 1.345 nm), uniformity (2.5 % at 300 mm wafer), and sidewall symmetricity (more than 95 %) of the copper seed layer.
Keywords: Lisitano antenna; ECR plasma source; Copper seed layer; Physical vapor deposition