Ahmed Khacef

NOx remediation in oxygen-rich exhaust gas using atmospheric pressure non-thermal plasma generated by a pulsed nanosecond dielectric barrier discharge

It is clearly seen that the application of non-thermal plasmas (NTP) to remove NOx from gas mixture containing a large amount of oxygen (O2) is dominated by NO to NO2 oxidation. Experiments have been conducted using a NTP generated by a nanosecond pulsed dielectric barrier discharge in synthetic exhaust gas, prepared from N2, O2, NO, H2O, and C3H6, over a large range of gas temperature (20-300r{}C). Results show that the NOx removal rate significantly increased with increasing specific energy deposition. For example, at a temperature of 100r{}C and an energy deposition of 27 J l-1, 92% of the NO molecules have been removed. The W values for NO is dramatically reduced to values scaling from ≈15 eV at 27 J l-1 down to ≈4 eV at 7 J l-1. NOx removal efficiency around 43% was obtained at a temperature of 260r{}C and a space velocity of 60 000 h-1 for a specific input energy of 27 J l-1. W values for NOx were less than ≈30 eV. Such treatments in exhaust gas with and without the presence of water vapour induced reactions leading to the production of a large variety of by-products such as acetaldehyde, propylene oxide, formic acid, methyl nitrate, and nitromethane.

Enhanced seed germination and plant growth by atmospheric pressure cold air plasma: combined effect of seed and water treatment

The combined effect of non-thermal plasma treatment of water and seeds on the rate of germination and plants growth of radish (Raphanus sativus), tomato (Solanum lycopersicum), and sweet pepper (Capsicum annum) have been investigated using dielectric barrier discharges in air under atmospheric pressure and room temperature. A cylindrical double dielectric barrier discharge reactor is used for water activation and a plate-to-plate double DBD reactor is employed for seed treatment. The activation of water, for 15 and 30 min, lead to acidic solutions (pH ≈ 3) with moderate concentrations of nitrate (NO3−) and hydrogen peroxide (H2O2). Plasma activated water (PAW) has shown a significant impact on germination as well as plant growth for the three types of seeds used. Interestingly, the positive effect, in seed germination and seedling growth, has been observed when the PAW and plasma-treated seeds (10 and 20 min) were combined. In one hand, when the seeds were (tomato and pepper) exposed to 10 min plasma and watered with PAW-15 for first 9 days followed by tap water for 51 days, the stem length is increased about 60% as compared to control sample. On the second hand, for longer exposures of seeds and water to plasma discharges, a negative effect is observed. For instance, plasma-treated seeds watered with PAW-30, the plant growth and vitality were decreased as compared to control sample. These results revealed that the developed cold plasma reactors could be used to significantly improve the seed germination as well as plant growth, nevertheless, the plasma treatment time has to be optimized for each seeds.

High repetition rate compact source of nanosecond pulses of 5–100 keV x-ray photons

A powerful, compact, and repetitive flash x-ray system based on a cable transformer technology powered by ceramic capacitors in a Blumlein-like configuration has been developed. Open circuit voltages in excess of 100 kV can be achieved while commutation occurs at low voltage (<20u2009kV). The x-ray emission from a low impedance x-ray diode with a hollow cathode configuration was observed under a wide range of experimental conditions. The critical parameters limiting the flash x-ray performances are mainly the pressure in the x-ray diode and the anode–cathode space. This true table top device is able to produce doses up to 1 R per shot, measured at the output window, of x-rays between 5 and 100 keV. The pulse widths were about 20 ns and the maximum repetition rate was about 60 Hz. Operation is possible in air or in other gases (He, Ne, Ar, Kr, Xe, H2, N2) at pressures varying from 10−3u2009mbar for xenon to about 1 mbar for helium.

Time-resolved spectroscopy of high pressure rare gases excited by an energetic flash X-ray source

Abstract Fluorescence from high pressure (0.1–30 bar) rare gas plasmas has been excited using an intense flash X-ray source specially developed and optimized for this experiment. Spectral analysis of the so-called “third continuum” of rare gases is presented. Time-resolved spectroscopy, absorption measurements and pressure dependence studies have provided an extended database on this longer wavelength continuum of rare gases. These data allow us to understand some of the previously contradictory results reported in the literature. They strongly support a multi-component spectral aspect of the previously called “third continuum”. In fact, the observed fluorescence result from the superposition of several continua whose relative intensities are strongly dependent upon pressure. This suggests that different species are at its origin. The name of “third continuum” does not seem anymore appropriate when speaking of the whole fluorescence emitted in the longer wavelength continuum by high pressure rare gas plasmas. Generally speaking, the flash X-ray device was proven to be a very convenient and powerful tool for the study of high pressure plasmas resulting in UV-VUV fluorescence.

Compact flash X-ray sources and their applications

Abstract Compact flash X-ray machines are opening up extended fields of applications. X-ray diodes driven by repetitive small size pulsers have been shown able to deliver high dose rates of X-rays in pulses of nanosecond to microsecond duration. Improvements have been carried out on the lifetime of the diodes and reliability of the systems to allow operation at constant emitted dose over long periods of time. After a brief review of recent developments, described here, in more details, is the progress obtained at the GREMI laboratory in the development of true table-top flash X-ray sources producing strong X-ray doses in nanosecond pulses at a high repetition rate (50 Hz). Doses up to 2 R (5.2 × 10−4 C/kg), measured at the output window, of X-rays between 5 and 200 keV can be generated from either a linear source (up to 10 cm long) or from a focal spot of less than 300 μm in diameter depending on the electrode configuration. Of the numerous applications of compact flash X-ray sources, an example is given of the use of these devices for the excitation of high pressure gas samples, realized at GREMI. Such an energetic excitation leads to important populations of highly excited ionic species.

Plasma-Assisted Diesel Oxidation Catalyst on Bench Scale: Focus on Light-off Temperature and NOx Behavior

The effect of a non-thermal plasma reactor over a commercial Diesel oxidation catalyst (DOC) was investigated. Studies have been focused on the gas treatment efficiency together with lowering light-off temperature when a DOC catalyst was connected downstream to plasma reactor in test bench scale. Experiments have been conducted using multi-DBDs (dielectric barrier discharge) reactor in planar configuration driven by a HV AC generator (11xa0kV–15xa0kHz). The specific input energy was set to 57 and 85xa0J/L. Experiments were performed in gas composition simulating Diesel exhaust. Commercial DOC, monolith-supported Pt–Pd/Al2O3, was used at gas hourly space velocities of about 55,000 and 82,000xa0h−1. CO and hydrocarbons light-off curves were determined for DOC, plasma, and plasma-DOC systems by temperature programmed surface reaction from 80 to 400xa0°C. Particular attention has been paid to the gas temperature between the plasma reactor and the DOC. Results show that the plasma-catalyst system provides the lowest light-off temperatures for CO and HCs. Under conditions of this study, light-off temperature improvement by about 57xa0°C was obtained and the plasma reactor totally oxidized NO to NO2 at low temperature.

Syngas production by plasma treatments of alcohols, bio-oils and wood

Exploitation of forest resources for energy production includes various methods of biomass processing. Gasification is one of the ways to recover energy from biomass. The Syngas produced from biomass can be used to power internal combustion engines, or, after purification, to supply fuel cells. The paper is summarizing results obtained through a non thermal arc plasma reactor at laboratory scale. A stationary discharge (I = 150mA) is used to perform physical diagnostics and also chemical analysis. The arc is formed between two electrodes made of graphite. We first present results on plasma-steam reforming of alcohols and bio-oils mixed in water. The outlet gas compositions are given from various alcohols and-bio-oils obtained at different experimental conditions. The second part of the paper is dedicated to a direct plasma treatment of wood (beech) at laboratory scale. One of the electrodes is surrounded by wood. The final part of the paper is a general discussion about efficiencies and comparisons of plasma treatments presented. The results obtained are discussed by considering the steam reforming reactions and the water gas shift reaction.

Plasma Treatments and Biomass Gasification

Exploitation of forest resources for energy production includes various methods of biomass processing. Gasification is one of the ways to recover energy from biomass. Syngas produced from biomass can be used to power internal combustion engines or, after purification, to supply fuel cells. Recent studies have shown the potential to improve conventional biomass processing by coupling a plasma reactor to a pyrolysis cyclone reactor. The role of the plasma is twofold: it acts as a purification stage by reducing production of tars and aerosols, and simultaneously produces a rich hydrogen syngas. In a first part of the paper we present results obtained from plasma treatment of pyrolysis oils. The outlet gas composition is given for various types of oils obtained at different experimental conditions with a pyrolysis reactor. Given the complexity of the mixtures from processing of biomass, we present a study with methanol considered as a model molecule. This experimental method allows a first modeling approach based on a combustion kinetic model suitable to validate the coupling of plasma with conventional biomass process. The second part of the paper is summarizing results obtained through a plasma-pyrolysis reactor arrangement. The goal is to show the feasibility of this plasma-pyrolysis coupling and emphasize more fundamental studies to understand the role of the plasma in the biomass treatment processes.

Non Thermal Plasma NOx Remediation: From Binary Gas Mixture to Lean-Burn Gasoline and Diesel Engine Exhaust

Abstract Experiments are presented on the plasma removal of NOx (sum of NO and NO2 concentrations) and hydrocarbons in atmospheric pressure gas streams by sub-microsecond pulsed dielectric barrier discharge processing. This investigation presents the effects of electrical input energy, hydrocarbon addition, and water addition on the NOx chemistry and by-products formation. Exhaust gas mixtures with composition containing up to seven gases (CO, CO2, NO, O2, H2O, C3H6, N2) were synthesized. The objective is to use synthetic gas exhaust simulating diesel and Lean Burn gasoline engine exhaust with propene as a reductant agent. It was established that the observed chemistry in the plasma includes conversion of NO to NO2 as well as the partial oxidation of hydrocarbon. In a given reactor under identical gas composition and equivalent energy density deposition, experimental results show that the main parameter which controls the efficiency of the plasma process is the energy deposition mode. The best results on NOx and hydrocarbon removal efficiencies have been obtained at low input energy per pulse and high discharge frequency. NOx removal improves with increasing input energy deposition and the presence of water in the gas mixture appears to essentially enhance the chemistry process efficiency, reducing by this way the energy cost of the processes. For example, for an input energy density of 27 J/L, the fraction of NOx removed was about 60% with an energy cost less than 30 eV/molecule in the case of simulated diesel engine exhaust. The data obtained suggest that aldehydes (CH2O and CH3CHO) are formed in concert with NO oxidation to NO2 in the plasma phase. Methyl nitrate (CH3ONO2) and nitromethane (CH3NO2) are the main R-NOx compounds produced and small amounts of nitrous acid (HNO2) and formic acid (CH2O2) were also detected.

Modeling of high-pressure rare gas plasmas excited by an energetic flash X-ray source

In this paper, a comprehensive description of the kinetics of argon plasmas has been formulated for atmospheric pressures. It has been used to attribute to molecular species the ultraviolet-vacuum ultraviolet (UV-VUV) continua, observed between 150 and 300 nm in pure argon. The participation of both ionic dimers and trimers has been found to be necessary to explain the whole fluorescence emitted by the rare gases (Rg). A simple kinetic model of high-pressure (1-30 bar) argon plasmas created by flash X-ray excitation is reported. The photoexcitation of Rg at high pressure by X-ray photons with energy of about 10 keV is shown to produce plasmas with significant density gradients of ionic and excited species. Spatial analysis of the energy position along the X-ray penetration path is performed in order to account for these concentration gradients. The calculation of electron density in different Rg (Ar, Kr, Xe) excited by X-ray photons of various energies (0.2-10 keV) is also reported. From these results, it appears that hash X-ray excitation of Rg can be a very interesting means to produce high-electron-density plasmas designed to selectively emit intense fluorescence in the UV-VUV spectral domain.