Nitric Oxide Conversion and Ozone Synthesis in a Shielded Sliding Discharge Reactor with Positive and Negative Streamers

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.     

Coupled Sliding Discharges: A Scalable Nonthermal Plasma System Utilizing Positive and Negative Streamers on Opposite Sides of a Dielectric Layer

A nonthermal plasma system based on simultaneously formed positive and negative streamers on either side of a dielectric layer is described. The coupled sliding discharge (CSD) reactor based on this concept was found to be scalable by stacking and operating multiple electrode assemblies in parallel, similarly to the shielded sliding discharge (SSD) reactor reported earlier. A comparison of the two systems showed that although the energy density in the CSD reactor was lower, the efficiency for NO conversion and ozone synthesis from dry air were significantly higher. The energy cost for 50 % NO removal was ~30 eV/molecule compared to ~60 eV/molecule in the case of the SSD under the same conditions of 330 ppm initial NO concentration in air. The energy cost decreased to ~12 eV/molecule in both cases when NO was mixed with plasma-activated air at the outlet of the reactor to utilize ozone for NO conversion i.e., indirect plasma treatment. The energy yield for ozone generation from dry air was at ~70 g/kWh, comparable in both systems. The results show that the concept of a CSD, as that of SSDs, allows the construction of compact, efficient plasma reactors.     

Characteristics of the Discharge and Ozone Generation in Oxygen-Fed Coaxial DBD Using an Amplitude-Modulated AC Power Supply

In this study, a traditional tubular reactor and an amplitude-modulated AC power supply were employed to develop a unique practical ozone generator with a widely adjustable ozone concentration and simultaneously a constant ozone yield. The characteristics regarding discharge and ozone generation in oxygen were experimentally investigated in detail. The amplitude-modulated AC waveform consisted of T_ON (burst of four consecutive AC cycles) and T_OFF with a duty cycle of 0.4. The experimental results show that a unique ozone generator can be developed through changing the applied voltage amplitude when an amplitude-modulated AC power supply producing periodic bursts of several consecutive AC cycles during the T_ON period is used. A quite high and stable ozone yield of 165?±?16 g/kWh was achieved and a wide range of ozone concentrations could be obtained. Moreover, we observed a very interesting phenomenon that the discharge energy and voltage peak for every AC cycle showed some difference, resulting from the accumulation and release of charge on the dielectric. The first AC cycle had the highest discharge energy and positive voltage peak as well as the lowest negative voltage peak, which was particularly obvious at a high energy density. Additionally, water cooling of the grounded electrode seemed to have a small influence on the basic electrical characteristics of the discharge and had a positive effect on the concentration and yield of ozone due to a reduction in gas temperature in the discharge gap.     

Influence of anodizing voltage mode on the nanostructure of TiO2 nanotubes

The nanostructure of self-ordered porous anodic TiO₂ nanotubes (PATNTs) has extraordinary influence on their physical and chemical properties. For this reason, extensive attention has been paid on pulse anodization to regulate the nanostructure of PATNT. However, the relationships between the nanostructures and current curves still remain unclear. Based on the traditional potentiostatic and pulse anodizations, five different modes (i.e., potentiostatic, pulse, triangle wave, decrease, and increase step by step) of applied voltage and their influences on the nanostructures of PATNT have been investigated in detail. The growing rates of the nanotubes anodized under five different modes were compared for the first time. The results show that the growing rate of pulse voltage anodization is the fastest, reaching 116.4 nm min^?1. The slowest is triangle wave voltage anodization, only 59.3 nm min^?1. When the applied voltage decreases step-by-step, branched nanotubes can be formed in the bottom of PATNT. Yet, when the applied voltage increases step-by-step, triple-layer nanotubes with different diameters are formed, and the forming mechanism of this special nanostructure is discussed. The present results may be helpful to understand the mechanism of PATNT and facilitate the assembling diverse nanostructures for extensive applications in photocatalysis, dye-sensitized solar cells, and biomedical devices.     

Influence of Duty Cycle on Ozone Generation and Discharge Using Volume Dielectric Barrier Discharge

The influence of duty cycle on ozone generation and discharge characteristics was investigated experimentally using volume dielectric barrier discharge in both synthetic air and pure oxygen at atmospheric pressure. The discharge was driven by an amplitude-modulated AC high voltage–power supply producing T_ON (a single AC cycle) and T_OFF periods with a widely variable duty cycle. The experimental results show that the energy delivered to the discharge during each AC cycle remains roughly constant and is independent of feed gas, duty cycle and T_OFF. Both average discharge power and ozone concentration show an initial linear increase with duty cycle, and deviate gradually from linearity owing to an increase in gas temperature at higher duty cycles. Nevertheless, ozone yield remains nearly constant (45.7 ± 3.5 g/kWh in synthetic air and 94.7 ± 3.1 g/kWh in pure oxygen) over a wide range of applied duty cycles (0.02–1). This property can be conveniently employed to develop a unique ozone generator with a widely adjustable ozone concentration and simultaneously a constant ozone yield. Additionally, the discharges in synthetic air and pure oxygen have similar electrical characteristics; however, there are observable differences in apparent luminosity, which is weak and white-toned for synthetic air discharge, and bright and blue-toned for pure oxygen discharge.     

Performance Evaluation of Hybrid Gas–Liquid Pulse Discharge Plasma-Induced Degradation of Polyvinyl Alcohol-Containing Wastewater

A multi-needle-to-plate pulsed discharge plasma reactor was designed to investigate its potential for polyvinyl alcohol-containing wastewater (PVA) treatment. The effects of some operational parameters such as PVA initial concentration, pulse peak discharge voltage, air flow rate, solution pH value, and iron additives on PVA degradation were examined. The results indicated that PVA could be effectively degraded from aqueous solutions. PVA degradation efficiency was 76.0 % within 60 min’s discharge plasma treatment with 1.5 mmol L^?1 Fe²⁺ addition. Decreasing PVA initial concentration and increasing pulse peak discharge voltage were both beneficial for PVA degradation. There existed appropriate air flow rate for obtaining great PVA degradation efficiency in the present study. A little acid environment was conducive to PVA degradation. The presence of Fe²⁺ and Cu²⁺ could both benefit PVA degradation, and the increment of Fe²⁺ and Cu²⁺ concentrations to a certain extent could enhance its degradation efficiency, as well as energy yield. PVA possible degradation mechanisms were discussed, and the degradation processes were mainly triggered by the reactions of PVA with \(^{ \cdot } {\text{OH}}\) radicals.     

Low Cost Compact Nanosecond Pulsed Plasma System for Environmental and Biomedical Applications

Nanosecond pulsed non-thermal atmospheric-pressure plasmas are promising for numerous applications including air and water purification, ozone synthesis, surface sterilization, material processing, and biomedical care. However, the high cost of the nanosecond pulsed power sources has hindered adaptation of the plasma-based technologies for clinical and industrial use. This paper presents a low cost (<100US$) nanosecond pulsed plasma system that consists of a Cockcroft–Walton high voltage charging circuit, a compact nanosecond pulse generator using a spark gap as switch, and a plasma reactor. The nanosecond pulse power source requires only a 12 V DC input, hence is battery operable. Through the optimization of the experimental parameters, pulses with a peak voltage >10 kV, a 3 ns rise time (10 to 90 %), and a 10 ns pulse duration (full width at half maximum) at a pulse repetition rate of up to 500 Hz were achieved in the present study. It has been successfully tested to power three different plasma reactors to form pulsed corona discharges, dielectric barrier discharges, and sliding discharges. The energy efficiency of such a nanosecond pulsed sliding discharge system was assessed in the context of ozone synthesis using air or oxygen as the feed gas, and was found comparable to a previously reported non-thermal plasma system that used commercial high voltage pulsed power sources. This study demonstrated that this low-cost nanosecond pulsed power source can prove to be an energy efficient and simple supply to drive various non-thermal atmospheric-pressure plasma reactors for environmental, medical and other applications.     

Evaluation of Energy Utilization Efficiencies for SO2 and NO Removal by Pulsed Corona Discharge Process

Pulsed corona discharge process was applied to the removal of sulfur dioxide and nitrogen oxides from simulated flue gas. The energy transfer efficiency of the pulse generation circuit and the energy utilization efficiencies for SO ₂ and NO removal are evaluated and discussed. When the pulse-forming capacitance was five times larger than the geometric capacitance of the reactor, the energy utilization efficiency was maximized, and the energy requirements for NO and SO ₂ removal could be lowered. With regard to radical utilization efficiency, producing small amounts of radicals frequently was found to be more advantageous than producing large amounts of radicals less frequently. Removal efficiency of SO ₂ increased with the applied peak voltage, but the energy utilization efficiency was nearly independent of the peak voltage when the peak field intensity was high enough to induce corona discharge (above 10 kV cm ^–1 in this system).     

Plasmas in Saline Solution Sustained Using Bipolar Pulsed Power Source: Tailoring the Discharge Behavior Using the Negative Pulses

Plasmas in saline solution driven by a repetitive bipolar pulsed power source are studied. We use a negative pulse to generate electrolytic gas with a controllable amount, followed by a positive pulse to ignite the plasma. With an increase in the negative voltage pulse amplitude from 0 to ?80 V, we observed an increase in the amount of electrolytic gas (hydrogen) formation, resulting in a reduced time delay, from 65 to 6 μs, required to ignite the plasma upon the onset of the positive pulse. A decrease, from 1.75 to 1.0 A, in the peak currents within the positive voltage pulse is also observed. Optical emission spectroscopy shows that the intensity ratio of the H_α (656 nm) to Na (588 nm) emission lines increases from zero to 0.0035. These observations can be well explained by the increase in the gas coverage on the electrode surface and the change in the gas composition within which the plasma is ignited with the application of the negative pulse.     

A non-aqueous Li/organosulfur semi-solid flow battery

A non-aqueous Li/organosulfur semi-solid flow battery is constructed. The battery with a high cell voltage of 3.36 V achieves coulombic efficiency of 99%, voltage efficiency of 73% and energy efficiency of 72% at the current density of 5 mA/cm².     

A pathway of free radical generation via copper corrosion and its application to oxygen and ozone activation for the oxidative destruction of organic pollutants

This study investigated the commercially available zero-valent copper powder and copper foil to activate molecular oxygen (O₂) and ozone for the degradation of organic pollutants. Under aerobic atmospheric conditions, copper powder effectively removed 50 mg/L of acetaminophen (ACT) within 2 h, though the degradation rate using the foil was less than 20% of the powder. However, copper foil activated ozone to effectively degrade ACT. The total organic carbon (TOC) removal reached a high of 58.3% at a catalyst concentration of 40 g/L, but only 26.8% with ozone alone. The initial solution pH and dosage of copper foil were key operational parameters affecting the ozone activation process. H₂O₂ and Cu(I) were important intermediates in the process as hydroxyl radicals (^·OH) were identified via EPR (electron paramagnetic resonance) experiments and free radical scavengers. The generation of ^·OH was attributed to a Fenton-like reaction between Cu(I) and H₂O₂; this free-radical generation mechanism differs from typical transition metal oxide catalysts. This study outlines a promising approach to significantly increase the generation of ^·OH and effectively remove refractory organic compounds. Furthermore, these copper products are applied in structural components of practical water treatment. Thus, the study of corrosion resistance to oxygen and ozone in aqueous solution have both a practical and theoretical significance. It was determined that copper products were resistant to oxygen corrosion in aqueous solution, but not resistant to ozone corrosion.     

A platinum-like behavior electrocatalyst and solid polymer electrolyte technique used on high concentration of electrochemical ozone water generation

This study presents an advanced ozone production process using the solid polymer electrolyte (SPE) technique, similar to the fabrication of proton exchange membrane fuel cell (PEMFC) membrane electrode assembly (MEA). Tungsten carbide and platinum on carbon black are coated on anode and cathode sides of a polymer membrane (Du Pont), respectively, to produce high concentration of ozone water. The water electrolysis of ozone generation requires a higher voltage than that of hydrogen production. On one hand, tungsten carbide, which is a platinum-like behavior electrocatalyst, plays a key role in preventing the MEA from corroding or oxidizing under high voltage. On the other hand, the carbon paper is replaced by a titanium porous disc to bear higher voltage. Moreover, an outstanding electronic control system can produce 1.37 ppm ozone water at atmosphere by adjusting the voltage range (6–10 V) with a current set to the maximum of 3 A for a household demand of ozone water generation.     

A High-Efficiency Double Surface Discharge and Its Application to Ozone Synthesis

Surface discharge with the flat plate configuration tends to generate a uniform and high-density plasma during ozone synthesis, but suffers from relatively low energy yield at high ozone concentration. Here we report that a double surface discharge reactor can produce, at the same input power, two uniform plasma zones that locate two sides of the thin dielectric layer simultaneously, which results in a high ozone energy yield at high ozone concentration. Discharge characteristics confirm that reducing dielectric thickness and discharge gap favors the achievement of high plasma-density and energy efficiency. The optical emission spectroscopy diagnosis suggests that the double surface discharge with thinner dielectric thickness and narrower discharge gap possesses much higher electron density, as well as higher excitation temperature and low rotational temperature, which is responsible for the excellent performance in ozone synthesis. The optimal parameters of 0.25 mm dielectric thickness and 2 mm discharge gap enable ozone synthesis to proceed with an energy yield of 295.2–108.7 g/kWh at ozone concentration of 11.1–48.3 g/Nm³ and exhibit a good stability during a 4-h test. This performance surpasses the performance of many other typical discharge processes for ozone synthesis.     

Abatement of Gaseous Xylene Using Double Dielectric Barrier Discharge Plasma with In Situ UV Light: Operating Parameters and Byproduct Analysis

Ultraviolet (UV) light with a wavelength of 254 nm was applied to a double dielectric barrier discharge (DDBD) system to decompose of gaseous xylene. The results show that a significantly synergistic effect can be achieved with the introduction of UV light into the DDBD system. When UV light is applied, the system show a 21.8 % increase in its removal efficiency for xylene at 35 kV with an ozone concentration close to 971 ppmv. The CO _x (x = CO₂ and CO) selectivity of outlet gas rises from 6.54 to 76.2 %. The optimal synergetic effect between UV light and DDBD can be obtained at a peak voltage of 30 kV. The system is robust for humidity, which only slightly reduces the xylene removal efficiency at a high peak voltage (30–35 kV). With the increase of gas flow rate, the removal efficiency for xylene decreases due to a reduced residence time. In addition, the products of xylene degradation were also analyzed. The major products of the degradation were found to be CO₂ and H₂O while byproducts such as O₃ and HCOOH were observed as well.     

Influence of Power Modulation on Ozone Production Using an AC Surface Dielectric Barrier Discharge in Oxygen

The measurements of electro-optical discharge characteristics and concentration of produced ozone were performed to evaluate the efficiency of ozone production in an AC surface dielectric barrier discharge (SDBD) in pure oxygen at atmospheric pressure. The discharge was driven in an amplitude-modulated regime with a driving AC frequency of 1 kHz, variable discharge duty cycle of 0.01–0.8 and oxygen flow rate of 2.5–10 slm. We observed asymmetric SDBD behaviour as evidenced by the variation in the ratio of the O^I/O₂ ⁺ emission intensities registered during the positive/negative half-periods and complemented by the transferred charge measurements through the Lissajous figures. We also found a strong dependence of O₃ concentration on the discharge duty cycle. The highest calculated ozone production yield reached 170 g/kWh with a corresponding energy cost of about 10 eV/molecule when combining the lowest inspected duty cycle with the lowest AC high voltage amplitude.     

Hydrogen Production from Methane Through Pulsed DC Plasma

A non-equilibrium warm plasma reactor has been constructed for methane reforming and hydrogen production. The discharge reactor was derived with 20 kV pulsed DC power supply with pulse duration of 4 µs, pulse frequency of 33 kHz. Electrical and optical characterizations of the reactor have been investigated. The electrical characteristics of the discharge revealed that the discharge was ignited by streamer to glow transition. The optical characteristics of the discharge revealed that the discharge was found to be strongly non-equilibrium with rotational temperature (T_rot) of 2873 K and vibrational temperature (T_vib) of 12,130 K. The Stark broadening of the emitted H_α line profile was used to deduce the electron density, which was found to be in the order of 10¹⁶ cm^?3. Methane conversion was strongly dependent upon the applied voltage and the methane flow rate. In general, under the specified operating condition, a methane conversion percentage of about 92% and a maximum hydrogen selectivity of 44.6% have been achieved. Specific energy consumption of methane conversion (SEC) and specific energy requirements for hydrogen formation (SER) of 5 eV/molecule has been achieved simultaneously with a maximum hydrogen production energy cost of about 3.8 µg/J. Finally, the decomposition of methane gas resulted in the deposition of an important byproduct namely graphene oxide.     

Reduction of uranyl nitrate ions in a continuous flow electrochemical reactor

In a contemporary salt-free flowsheet of nuclear fuel reprocessing, uranous ions are used as a reducing agent to accomplish the separation of uranium and plutonium from each other which is normally produced electrochemically. In the present study, continuous mode electrochemical reduction of uranyl ions in nitric acid medium and in the presence of hydrazine nitrate was carried out in a divided cell at three different feed flow rates and cathodic current densities for the purpose of optimizing the process conditions for better conversion efficiency. A correlation function is described as a mathematical model in which, mass transfer parameter is expressed as a function of current density and flow rate. From the multivariable nonlinear regression of the experimental data, rate parameter was determined. The electro reduction of uranyl ions was not influenced by the initial acidity. The maximum conversion rate of 87 % was obtained for the electro-reduction of 100 g/l U(VI) in 3 M nitric acid with feed flow rate as 0.3 l/h and current density as 15 mA/cm². The results for the reduction of 100 g/l uranyl ions indicate that an optimum flow rate of about 0.5 l/h and 15 mA/cm² as cathodic current density may suffice for the conversion with efficiency better than 60 %. The calculated steady state concentrations for U(IV) were found to be in good agreement with the experimental results.     

Softening Hard Water Using High Frequency Spark Plasma Discharge

Effects of high frequency spark plasma discharge as a time efficiency method in order to softening the natural hard water has been investigated experimentally. A very hard water sample with 331 ± 19 mg/l of CaCO₃ hardness was used. The current and voltage of each spike was about 9.6 A and 3.5 kV respectively at 16 kHz frequency with 35 μs pulse width. Hard water was treated for 2, 4, 6, and 8 min. The concentration of CaCO₃, Ca²⁺ ions, Mg²⁺ ions and pH as well as water conductivity was controlled before and after treatment. The concentration of CaCO₃ dropped by 70%, after 8 min treatment. During the treatment, the pH had a fluctuation about 1.5 and finally remained in neutral state. Also the elemental composition, crystalline structure and morphology of the precipitates were identified. Molecular dynamics simulation revealed that the ozone and hydroxyl play important roles in the softening of the hard water.     

Experimental Study of Pulse Polarity and Magnetic Field on Ozone Production of the Dielectric Barrier Discharge in Air

We studied the combined effect of the pulse or sinusoidal driving voltage and magnetic field on electrical characteristics and ozone production of a dielectric barrier discharge in air. These effects were studied for both polarities of the pulse. We found that the pulse polarity, as well as the magnetic field, affects discharge ozone production only for higher powers. The discharge driven by sinusoidal voltage produces higher ozone concentration than the discharge driven by voltage pulses of negative polarity. The discharge driven by the voltage pulses of negative polarity produces higher ozone concentrations than the discharge driven by positive voltage pulses. Ozone production yield for the voltage pulses of negative polarity is similar to that of sinusoidal driving voltage. For the positive polarity of voltage pulses, the yield is lower. Magnetic field slightly increases the concentration of ozone produced by the discharge and ozone production yield only for the discharge driven by positive voltage pulses. For the discharge driven by the sinusoidal voltage or by the pulses of the negative polarity magnetic field has only small effect on the concentration of ozone produced by the discharge.     

Measurement of Ozone Production in Non-thermal Plasma Actuator Using Surface Dielectric Barrier Discharge

Plasma actuators for flow control are intensively studied, but the production of ozone by the surface dielectric barrier discharge used in the actuators has never been quantified. Since ozone is harmful to human health, it is important to quantify its production for an application of this type of actuator on a land vehicle. This paper describes an experimental study to measure the concentration of ozone produced by an actuator with different parameters: amplitude and frequency of the applied high voltage, and the electrode configuration (shape, spacing and length). The results show that, under our experimental conditions, the production of ozone is directly proportional to the power dissipation. The production rate was measured at 21 g/kWh. Although the rate is much lower than that of an industrial ozonizer, it is still far from being negligible and should be taken into account for the future application of these actuators.