Toshikazu Ohkubo

NO removal characteristics of a corona radical shower system under DC and AC/DC superimposed operations

In this paper, the effects of operating applied voltage modes on the corona discharge morphology and NO removal characteristics from air stream are experimentally investigated. By applying a DC superimposed high frequency AC voltage, a uniform streamer corona can be generated, which is also less sensitive to electrodes mis-arrangements. Hermstein glow can be transferred to streamer by applying a DC superimposed AC power supply if the peak to peak voltage is larger than 1.0 kV at the voltage change rate of 0.2 kV//spl mu/s. A significant amount of NO removal is observed under streamer corona. While for Hermstein glow the removal is negligible.

Two-dimensional imaging of NO density profiles by LIF technique in a pipe with nozzles electrode during NO treatment

Two-dimensional NO concentration distribution was studied by a planar laser-induced fluorescence (PLIF) technique in nonthermal plasma during NO treatment. A pipe with a nozzles-to-plate electrode system, having an electrode gap of 50 mm, was used. A stable DC streamer corona discharge was generated in an NO/air mixture at atmospheric pressure. Laser pulses in the form of a sheet were shot between the electrodes during the discharge. LIF signal emitted at 90/spl deg/ to the laser sheet was imaged onto a gated-ICCD camera and two-dimensional distributions of NO concentration in the reactor were measured as a function of time during NO treatment. NO concentration was also monitored at the reactor outlet. The images of NO concentration covering almost the whole length of the reactor show that the density of NO molecule decreased not only in the plasma region formed by corona streamers but also in the upstream region of the reactor. This information is important for modeling and optimizing the plasma processes and designing the nonthermal plasma reactors.

Two-dimensional distribution of ground-state NO density by LIF technique in DC needle-to-plate positive streamer coronas during NO removal processing

Two-dimensional distribution of the ground-state NO molecules density was investigated using a laser-induced fluorescence (LIF) technique in a DC positive streamer corona reactor (needle-to-plate electrode geometry) during NO removal from a flue gas simulator (NO/air). NO density in the corona discharge reactor was monitored under the steady-state DC corona discharge condition. It was found that NO molecules density decreased due to the corona discharge processing not only in the discharging region but also in the upstream vicinity of the discharge.

Simultaneous measurements of wire electrode surface contamination and corona discharge characteristics in an air-cleaning electrostatic precipitator

Contamination of the corona wire in a wire-to-plate type air-cleaning electrostatic precipitator is studied experimentally. In order to enhance the contamination of wire, air containing dust is directly supplied to a part of the wire electrode. Spores of Lycopodium and cigarette smoke particles are used as test dusts. Simultaneous measurements of wire electrode optical images and corona discharge modes are carried out during contamination processes. Results show that corona discharge modes and optical emission from the wire electrode change with time due to the surface contamination, in the case of cigarette smoke, after a time elapsed, streamer coronas appear due to the buildup of smoke particles on the wire surface. After the first streamer generation, the corona current fluctuates with time because the formation and diminution of the projections occur alternately at the different parts on the wire electrode surface.

Electrohydrodynamic flow and its effect on ozone transport in corona radical shower reactor

New arguments supporting the supposition that the ozone is transported along a corona discharge radical shower (CDRS) reactor by the electrohydrodynamic (EHD) flow are presented. The arguments are based on the analysis of the corona discharge, which is a precursor of the EHD flow in the CDRS reactor, and on the measurements of velocity field of the EHD flow in the CDRS reactor by the particle image velocimetry (PIV). The obtained velocity flow structures and the possible causes of the ozone transport in the CDRS, i.e., diffusion, additional gas flow, EHD flow, and convection by the main flow, were discussed basing on the conservation equations for the EHD flow. The discussion showed that the EHD flow plays a dominant role in the ozone transport. This is also supported by the results of a simple phenomenological model for one-dimensional description of EHD-induced ozone transport in the CDRS reactor. The results of the computer simulation based on this model explained the main features of the measured ozone distribution in the CDRS reactor, establishing the EHD flow as the main cause of the ozone transport from the discharge region upstream, i.e., against the main flow.

Streamer Corona Discharge Induced by Laser Pulses During LIF Measurements in a DC Non-thermal Plasma Reactor for NO Oxidation

Abstract A possible interference of the light emitted by the laser induced streamer coronas and the laser induced fluorescence (LIF) signal during the measurements of NO concentration in a DC positive corona discharge was experimentally investigated. For in-situ NO measurement a LIF system consisting of a XeF excimer laser, dye laser and BBO crystal, generating a tuned laser line at 226 nm was employed. From the measured occurrence timing between the regular streamer coronas, laser pulse, LIF signal and laser induced streamer it was found that the LIF signal appears almost immediately after the laser incidence and lasts over about 30 ns, while the induced streamer starts about 35 ns after the LIF signal and lasts about 350-500 ns. Due to the 5 ns interval between the LIF signal and the laser induced streamer the undisturbed detection of the LIF signal can be possible with a properly adjusted timing of the ICCD camera (the gate opening and exposure time). Two-dimensional distribution of NO molecules concentration in the discharge gap was measured using the LIF technique. The time-resolved evolution of the laser induced streamers was visualized using the ICCD camera with the proper timing adjustment. This resulted in determining the velocity of propagation of the streamer (about 2.5 × 105 m/s) and the averaged diameters of the leader channel and leader streamers (200 μm and 100 μm, respectively).

Decomposition of VOC in Air Using a Streamer Corona Discharge Reactor Combined with Catalyst

Abstract In this study, the improvement in the decomposition of volatile organic compounds (VOCs) was investigated by combining discharge plasma with catalyst. DC streamer corona plasma reactors combined with a catalyst were developed. The basic configuration of the reactors is a multineedle-to-plane mesh electrode system. By changing the position of catalyst against the plasma region, several reactors were developed and compared with each other. As a catalyst, a honeycomb catalyst (Mn type) was tested. A catalyst layer was located between the needles and plane mesh electrodes (type A reactor), or the catalyst layer was placed just behind the mesh electrode (type B reactor). In other reactors, the catalyst layer was followed by the needles-to- plane mesh electrode system (type C reactor) or the catalyst layer was removed (type D reactor). The needles- to-plane mesh electrode system produced the stable streamer corona discharge. The streamers covered the surface of the catalyst layer in the type A and type B reactors. However, the characteristics of the streamer were different for the type A and type B reactors. Toluene (C6H5CH3) was chosen as a test VOC. The type A reactor showed the most efficient toluene decomposition. It was considered that the gas-phase toluene and the toluene adsorbed on the catalyst were decomposed simultaneously.

Wide-range two-dimensional imaging of NO density profiles by LIF technique in a corona radical shower Reactor

Planar laser-induced fluorescence was employed during the process of NO/sub x/ removal in a corona radical shower system. Using the wide-range imaging (image size: 240 mm in width and 160 mm in height), two-dimensional distributions of ground-state NO could be observed not only in the discharge zone but also both in the downstream and the upstream regions of the reactor. The obtained results showed that the density of NO molecules decreased not only in the plasma region formed by the corona streamers and the downstream region of the reactor but also in the upstream region of the reactor. The effect of the gas injection through the nozzles electrode on the NO profile in the reactor was negligible. The NO removal rate was almost the same for both cases with and without the injection gas once the streamer discharge was produced. In the present reactor at low main gas flow rate, it was considered that electrohydrodynamic flow became to be dominant, and the flow toward the upstream affected the decrease of NO in the upstream region. This fact is important for optimizing the performance of the nonthermal plasma reactor.

Surface temperature profiles of the corona wire in a plate-wire type electrostatic precipitator

The corona wire surface temperature profiles in a plate-wire-type electrostatic precipitator have been measured by an IR thermal image technique and thermocouples under both DC positive and negative corona discharge conditions. The results show that: the corona wire surface temperature profiles under positive coronas are more uniform than those of the negative coronas, where the differences between maximum and minimum temperatures for positive and negative coronas are 0.5 degrees C and 6 degrees C, respectively; the theoretically calculated corona wire surface temperature always overestimates surface temperature, since a relatively significant electrohydrodynamic cooling can control corona wire surface temperature; and for a negative corona, the corona wire vibration frequency decreases slightly with increasing applied voltage.<<ETX>>

Progress in the Visualization of Filamentary Gas Discharges. Part 2: Visualization of DC Positive Corona Discharges

Abstract Recently the filamentary gas discharges at atmospheric pressure, such as dielectric-barrier and corona discharges regained their importance due to numerous new applications. These new applications impose a demand of better understanding of the fundamentals of the filamentary discharges. As a consequence, during the last ten years an essential progress in the diagnostics of the filamentary discharges has been made. The recent progress in the investigations of the dielectric-barrier discharges by cross-correlation spectroscopy was described in Part 1 of this paper. Part 2 is a survey of the recent progress in the visualization of DC positive corona discharges.