Plasma Chemistry Applications Research Articles

Electrical and Optical Characterization of Gliding Arc Discharge (GAD) Operated at Line Frequency (50 Hz) Power Supply

In this study, an atmospheric alternating-current gliding arc device using line frequency (50 Hz) has been designed for the generation of various reactive species in different working gases. Electrical characteristics of the generated discharge are investigated by oscilloscope while the optical characteristics are analyzed using optical emission spectroscopy. The role of different working gases (oxygen, argon and air) on discharge voltages and power consumption per cycle in the discharge are calculated and compared. Electron density and electron temperature of the discharge are estimated by electrical and optical method respectively. The production of reactive species in the discharge is affirmed by optical emission spectroscopy. The outcomes of the results confirm that the GAD can generate non-equilibrium plasma having reactive nitrogen and oxygen species (RONS) which are essential for plasma chemistry applications.

Selective generation of excited species in ns pulse/RF hybrid plasmas for plasma chemistry applications

Hybrid plasmas, sustained by a repetitive ns pulse discharge and a sub-breakdown RF waveform in N2 and its mixtures with H2, CO, and CO2, are studied using laser diagnostics and kinetic modeling. Plasma emission images show that adding the RF waveform to the ns pulse train does not result in a discharge instability development, since the RF field does not produce additional ionization. Unlike a ns pulse/DC discharge, the ns pulse/RF plasma is sustained using a single pair of electrodes external to the discharge cell. Measurements of electronically excited molecules, N2(A3Σu +), and vibrationally excited molecules in the ground electronic state, N2(X1Σg +, v), demonstrate that these species are generated selectively. N2(A3Σu +) molecules are produced predominantly by the ns pulse discharge waveform, while vibrational excitation of the ground electronic state N2 is mainly due to the RF waveform. Strong vibrational nonequilibrium is maintained at a low translational–rotational temperature. The ns pulse/RF discharge data demonstrate that the quenching of N2(A3Σu +) is not affected by N2 vibrational excitation. Kinetic modeling shows that the rate of N2(A3Σu +) quenching in a ns pulse discharge in nitrogen is underpredicted, and the modeling predictions agree with the data only if the rate of N atom generation by electron impact dissociation of N2 is increased by approximately an order of magnitude. This suggests a significant effect of excited electronic states on the net dissociation rate. Infrared emission spectra of ns pulse/RF hybrid plasmas in CO–N2 and CO2–N2 mixtures show that the present approach also generates strong vibrational excitation of CO and CO2, with the CO yield in the CO2–N2 mixture approximately a factor of two higher compared to that in a ns pulse discharge alone. This indicates a significant contribution of the vibrationally enhanced CO2 dissociation in the hybrid plasma. The present results demonstrate that sustaining the hybrid plasma in reacting molecular gas mixtures may isolate the plasma chemical reaction pathways dominated by vibrationally excited molecules from those of electronically excited molecules and atomic species.

Steam Plasma Flows Generated in Gerdien Arc: Environment for Energy Gas Production from Organics and for Surface Coatings

Abstract Electric arc stabilized by a vortex of water (Gerdien arc) is used in a special type of plasma torch for generation of thermal plasma jet with extreme plasma parameters. Extremely high plasma enthalpy and plasma temperature and very low plasma density result in very high efficiency of utilization of plasma enthalpy for heating of material injected into plasma. Plasma torches with Gerdien arc are used in industrial scale for plasma spraying, other plasma chemistry applications like waste treatment or gasification of biomass for syngas production are being investigated. In these applications, special properties of steam plasma produced in this type of arc are beneficial. Experiments with steam plasma gasification of carbon containing materials were made in plasma reactor with Gerdien arc plasma torch. Gasification of polyethylene, plastic waste, wooden pellets and saw dust were studied for arc power 100 - 140 kW. Syngas with high content of hydrogen and carbon monoxide and very low content of carbon dioxide was produced. Very low content of complex hydrocarbons and tar was detected. In optimal conditions the calorific value of produced syngas was substantially higher than electric energy spent for plasma generation. Possibility of usage of the proc ess is discussed for energy storage during excess of electricity available from power plants with fluctuating output.

Achieving Better NOx Removal in Discharge Plasma Reactor by Field Enhancement

Application of plasma chemistry for gas cleaning is gaining prominence in recent years, mainly from an energy efficiency point of view. In this paper we conducted a comparative study of NO/NOx removal using two different types of dielectric barrier discharge electrodes, wire-cylinder reactor, pipe-cylinder reactor. Investigations were first carried out with synthetic gases to obtain the baseline information on the NO/NOx removal with respect to the two geometries studied. Further, experiments were carried out with raw diesel exhaust under loaded condition. A high NOx removal efficiency of 90% was observed for the pipe-cylinder reactor as compared to that of 53.4% for the wire-cylinder reactor. Furthermore, for the same energy consumed per NO molecule (about 73 eV/NO molecule), the removal efficiency increased from 67% for the wire-cylinder to about 98% for the pipe-cylinder which was quite appreciable.

Nonequilibrium effects in thermal plasmas

Abstract

14C dating of ancient rock art: A new application of plasma chemistry

We have developed a new statically operated oxygen plasma method that allows direct14C dates to be obtained from ancient rock paintings. The method is applicable even to paintings on limestone (CaCO3) walls. A sample of a pictograph which had naturally spalled offa shelter wall in the Lower Pecos region of Texas was subjected to a low temperature (∼ 150°C) oxygen plasma to selectively remove the organic carbon-containing material used in the paint as CO2, without contamination from the limestone substrate. The Zurich EHT accelerator mass spectrometer was then used to determine the radiocarbon age of this prehistoric rock painting. It was successfully dated at 3865 ± 100 years BP, in good accord with the archaeological context which has set the onset of this Pecos River style of pictograph between 4100 and 3200 years BP. The method appears feasible and is applicable to rock art in which organic materials were used in the paint.

Facility for non-LTE studies of a magnetically rotated arc

A facility is described for the spectroscopic diagnostics of an electric arc in a rotating magnetic field. The magnetic field is used to convert the arc into a ‘‘surface of revolution’’ which internally provides a more uniform and controllable plasma for particle treatment or plasma chemistry applications. The magnetic field also enhances the spectral lines indicating a non-LTE condition. A high-frequency, two phase amplifier and phase shifter are used to generate the rotating mag- netic field in the coil. Present operating conditions are 0–300 G at 500–2000 Hz and 100–1000 A.

Restoration and conservation of ancient artifacts: A new area of application of plasma chemistry

The application of low-pressure hydrogen plasma for the restoration and conservation of iron artifacts has been developed. This method of treatment at temperatures below 400°C is relatively fast and efficiently removes chlorides which, otherwise, would cause a fast postcorrosion of the excavated object. Furthermore, the application of this method to freshly excavated objects enables the restorer to easily uncover the original surface.

The industrial application of plasma technology for the production of fumed Silica

The industrial application of plasma chemistry is restricted for the time being to the use of a plasma torch as an efficient heat source for the production of existing chemicals. The techno-economic requirements for products suitable for plasma processing are outlined. There are only very few such processes at or near industrialization, such as the Tafa process for ZrO 2, the Noranda MoS 2 process and the Lonza process for fumed silica. Various types of plasma torches are evaluated according to such criteria as investment cost, efficiency, availability in industrial size, suitability for continuous operation and scale-up etc. The liquid-stabilized torch used in this work is described. The Lonza process for surface-active silica is based on the reduction of sand to silicon monoxide gas in a specially designed reactor, fired by a 200 kW alcohol-stabilized plasma torch. By direct oxidation of the SiO gas in a quench system it is possible to produce a surface-active product. At present the process has reached pilot plant scale and is being optimized. In comparison with the conventional process, based on the gas phase hydrolysis of silicon tetrachloride, the use of cheaper raw materials and the elimination of hydrogen chloride from the process lead to a cheaper process without ecological problems.