Wr Wijnand Rutgers

Pulsed positive corona streamer propagation and branching

The propagation and branching of pulsed positive corona streamers in a short gap is observed with high resolution in space and time. The appearance of the pre-breakdown phenomena can be controlled by the electrode configuration, the gas composition and the impedance of the pulsed power circuit. In a point-wire gap the positive corona shows much more branching than in the parallel plane gap with a protrusion. In air, the branching is more pronounced than in argon. The pulsed power circuit appears to operate in two modes, either as an inductive circuit creating a lower number of thick streamers or as a resistive circuit giving a higher number of thin streamers. A possible cause for branching is electrostatic repulsion of two parts of the streamer head. The electric field at the streamer head is limited, the maximum values found are ∼170 kV cm −1 in air and ∼100 kV cm −1 in argon. At these maximum field strengths, the electrons have 5–10 eV energy, so the ionization is dominated by two-step processes. Differences between argon and ambient air in the field strength at which streamers propagate are ascribed to the difference in de-excitation processes in noble and molecular gases. The fact that the pulsed power circuit can control the streamer structure is important for applications, but this effect must also be taken into account in fundamental studies of streamer propagation and branching.

Gas phase corona discharges for oxidation of phenol in an aqueous solution

A new method for the removal of harmful organic molecules from water is described. A low power corona discharge is created over the aqueous solution. Chemically active species diffuse into the water and then oxidize the target compound, which in this case is the model compound phenol. The energy consumption per removed phenol molecule is one order of magnitude lower compared to the discharge techniques that create a plasma in the water. The reaction mechanism of the conversion is shown by measuring the ozone concentration over the water, the intermediate/final oxidation products and the release of CO2 from the water. Indications are found that the discharge is more than merely an ozone generator.

The degradation of aqueous phenol solutions by pulsed positive corona discharges

The Advanced Oxidation Process pulsed corona discharges have been utilized for the degradation of phenol in aqueous solution. The pulsed positive corona discharges are struck in the ambient gas phase over the solution. Experiments have been performed using both an air and argon atmosphere. Phenol conversion and the production of major oxidation products in the solution have been determined, using ion-exclusion chromatography with UV absorbance and conductivity detectors in series. The corona pulse energy has been measured from voltage and current waveforms using capacitive current correction. Oxidation products are polyhydroxybenzenes and carboxylic acids. Even though phenol conversion efficiencies by pulsed positive corona discharges in air and argon are similar, the degradation pathways are different.

Energy efficiency of NO removal by pulsed corona discharges

Pulsed positive corona discharges are used to remove NO from the flue gas of a methane burner. At low power input this leads to an increase in NO2, which shows that the process is oxidative. Removal efficiency is greatest when discharges are produced with high-voltage pulses, which are shorter in duration than the time required by the primary streamers to cross the discharge gap, in combination with a dc bias. Other important parameters are input power density and residence time. The best result obtained so far is an energy consumption of 20 eV per NO molecule removed, at 50% deNOx i.e., a removal of 150 ppm NOx, using a residence time of 15 s and an input power density, of 3.5 Wh/Nm3. [Wh/Nm3 stands for watt-hour per normal cubic meter, i.e., at normal conditions (273 K and 1 bar). This implies that 1 Nm3 contains 2.505 1025 molecules.] There appears to be room for improvement by the addition of gaseous and particulate chemicals or the use of multiple corona treatment. It is argued front comparison between results from models and experiments that the direct production of OH by the discharge is only the initiation of the cleaning process.

Breakdown of methylene blue and methyl orange by pulsed corona discharge

The recently developed corona above water technique is applied to water containing 10 mg l−1 methylene blue (MB) or methyl orange (MO). The corona discharge pulses are created with a spark gap switched capacitor followed by a transmission line transformer. The pulse amplitude is 40 kV; its duration is 50 ns. At a pulse repetition rate of 10 Hz this leads to an average power of 0.6 W into the discharge. MB and MO are completely decolourized in ~20 min. This corresponds to a yield of ~4.5 gr kW−1h−1, which is much higher than obtained with other discharge techniques or sonoluminescence. The high yield is reflected in the observed temperature increase of only ~1 K. Tests with additional chemicals show that the initial speed of the conversion can be influenced but the total time required for total decolourization is constant. Further, it follows that the main oxidation path of the dyes is by direct ozone attack and the conversion products are strong acids.

High-Power Pulsed Corona for Treatment of Pollutants in Heterogeneous Media

A technical overview of a European project on pulsed corona (PC) treatment of polluted streams is presented. Versatile high-power systems that are capable of cleaning both aqueous and gaseous streams in heterogeneous media, in either a corona above water reactor or an aerosol reactor, have been developed. Both reactors are capable of high-phenol removal yields from aqueous streams and increase the biochemical oxygen demand/chemical oxygen demand ratio for several nonbiodegradable wastewaters to such degree that further biodegradation becomes possible. The PC combined with a catalyst is capable of cleaning gaseous streams that are polluted by toluene, styrene, and malodorous constituents. Reduction rates of toluene that are higher than 99% have been achieved, and very high odor-removal efficiency has been demonstrated. The reliable operation of high-power all-solid-state compact nanosecond pulsers has also been demonstrated. For the second phase of the project, a high-power pulser was designed. One compression stage suffices for the formation of 60-kV 3-J pulses across a reactor that has a discharge impedance of approximately 100 Omega at a pulse repetition frequency of up to 500 Hz; the rise time is 15 ns, and the duration is 100 ns. The system scale-up is also analyzed. The estimated price of water treatment in systems that were scaled up to 50 m3 /h is 2 euro/m3. Incineration of aqueous organic waste streams can cost 500 euro/m3 or more, depending on the nature of the contamination, so the PC water-treatment technology may become highly competitive. It is planned in the future to investigate the cleaning efficiency of the developed processes with different industrial wastes, both at laboratory conditions and in the field

Influence of gaseous atmosphere on corona-induced degradation of aqueous phenol

The phenol degradation processes by pulsed corona discharges are investigated under three kinds of discharge atmosphere (air, argon and oxygen). The temporal variations of the concentrations of phenol and the intermediate products are monitored by LIF spectroscopy. The species of the intermediate products are identified by spectral analysis. It is clarified that the oxidative gaseous reagents produced from O2 and those from H2O degrade phenol to intermediate products with comparable degradation rates. The degradation via the reagents from H2O gives rise to the formation of molecules exhibiting fluorescence at 400-500 nm, in addition to dihydroxybenzene (DHB), while the degradation via the reagents from O2 produces only DHB. The reagents from O2 play an important role in the conversion of phenol to DHB.

Combined Effects of Pulsed Discharge Removal of NO, SO2, and NH3 from Flue Gas

Experiments have been performed using pulsed high-voltage discharges with the aim of removing NO and SO2from flue gas obtained from a methane burner. It is found that the NO conversion is strongly increased by the addition of SO2or NH3. When both gases are added simultaneously the increase almost disappears. The synergetic effect can be maintained, as is shown, when NH3is introduced much later than SO2. The SO2removal is already 70% upon stoichiometric addition of NH3, but the electric discharge improves this to >95% and reduces the NH3leak to a few ppm. This increase is probably related to aerosol production by the pulsed discharge which enhances the ammonium salt production. A so-called “history effect” is observed, i.e., the removal of NO and SO2depends on the time that is taken to reach the required energization. It appears that the discharge has to create favorable conditions for the cleaning process. Using the synergetic and history effects the best cleaning result, at initial concentrations of 300 ppm, is 80% NO removal and 95% SO2removal with 3 ppm NH3leak. In this case the energy cost is 13 eV/NO (or a yield of 90 g NO and 200 g SO2per kWh). Possibilities for further improvement are indicated.

Streamer branching in a short gap: the influence of the power supply

The formation of streamers in a 25-mm gap in air is studied with an intensified charge coupled device camera with high resolution in space and time. Strong branching is observed and streamers reach the cathode in an area that is much wider than the gap length. Switching the high voltage with either a spark gap or a semiconductor stack has a large influence on the branching: much more discharge channels appear when using the semiconductor switch. Pictures taken with 0.8-ns resolution show that the streamers propagate with 3 ms/mm near the electrodes and with 0.5 mm/ns in the middle of the gap.

Inception behaviour of pulsed positive corona in several gases

The inception probability and the streamer length of pulsed positive corona discharges is determined in argon, nitrogen, oxygen and air. This study is performed in a 25 mm point-plane gap at a pressure of 1 bar. The lowest voltage at which a discharge in argon starts is 3 kV but only with an inception probability of 1%. At 5 kV the corona discharge in argon transforms into a spark with a probability close to 100%. The inception probability of corona discharges in all molecular gases used here as a function of the voltage is identical, starting with 1% at 4 kV and going up to 100% at 9 kV. The streamer lengths are quite different for these gases, nitrogen requiring the lowest voltage for streamers to cross the gap and oxygen the highest. This is probably due to electron attachment in oxygen. A remarkable result is that in air streamers bridge the gap at 8 kV, but spark breakdown occurs only above 26 kV. This property makes it relatively easy to obtain powerful pulsed corona discharges in air.