F. Jorand

Removal of formaldehyde in nitrogen and in dry air by a DBD: importance of temperature and role of nitrogen metastable states

The removal of traces of formaldehyde (150?ppm) in nitrogen and in dry air, at atmospheric pressure, by the filamentary plasma of a dielectric barrier discharge (in a cylindrical geometry) energized by a HV pulse is experimentally studied, at ambient temperature (20??C) and at 300??C. It is found that the pollutant molecule is more efficiently removed in nitrogen than in air at 20??C, whereas it is the opposite at 300??C. In air, the removal of CH2O strongly increases when the temperature increases. This effect also occurs in nitrogen, but it is less important. A qualitative explanation for these results can be found in the competitive influence of quenching collisions of the nitrogen metastable states by formaldehyde and oxidation reactions of this molecule.

OH kinetic in high-pressure plasmas of atmospheric gases containing C2H6 studied by absolute measurement of the radical density in a pulsed homogeneous discharge

The absolute value of the hydroxyl radical was measured in the afterglow of an homogeneous photo-triggered discharge generated in N2/O2/H2O/C2H6 mixtures, using a UV absorption diagnostic synchronized with the discharge current pulse. Measurements show that OH is efficiently produced even in the absence of water vapour in the mixture, and that the radical production is closely linked to the degradation kinetic of the hydrocarbon. Experimental results for dry mixtures, both for OH and for the removal of ethane in the discharge volume, are compared with predictions of a self-consistent 0D discharge and the kinetic model. It appears that the oxidation reaction of the ethane molecule by O(3P) atoms plays a minor role. Dissociation of the hydrocarbon through quenching collisions of the nitrogen metastable states are of great importance for a low oxygen concentration value. Also, the oxidation of ethane by O(1D) cannot be neglected at high oxygen concentration. The most probable exit channel for N2 states quenching collisions by ethane is the production of ethene and hydrogen molecules. Afterwards C2H4 should be dissociated to produce H and H2. As previously suggested from the study of the OH density time evolution in relative value, the recombination of H and O atoms appears as a main process for the production of OH in transient low temperature plasmas generated in atmospheric gases at high pressure. Another important reaction is the reduction of the HO2 radical by O, this radical coming from the addition of H on the oxygen molecule. H atoms come from numerous kinetic processes, amongst which is the dissociation of ethene.

Experimental Study and Kinetic Modeling for Ethanol Treatment by Air Dielectric Barrier Discharges

This paper deals with the ethanol (EtOH) removal in both dry and humid air fed dielectric barrier discharges. The experimental results were compared to the predictions of a zero dimension kinetic model to elucidate the main chemical routes occurring in the plasma phase. This comparison shows that both the dissociative quenching of the nitrogen metastables and the oxidation reactions by the oxygen atom or the hydroxyl radical should be taken into account to explain the EtOH abatement in these kinds of discharges. The CH3CHOH radical seems to be the main product of the nitrogen dissociative collisions, whereas radicals issued from the α- and β-H atom cleavage are the dominant ethanol oxidation by-products. These radicals account for the production of acetaldehyde, the main by-product of the ethanol/air fed discharges investigated here. Apart the complete oxidation products, i.e. carbon oxides and water, aldehydes containing up to six carbon atoms, ketones, carboxylic acids, ozone, nitrogen oxides, nitric acid and organic nitrates were found in the exhaust gas. A kinetic pathway is proposed to explain the formation of the detected by-products. Water vapour addition to the feeding gas slightly improves the EtOH removal and promotes further oxidation of the main by-products, thus enhancing the CO2 selectivity. This behaviour could be ascribed to the higher amount of hydroxyl radicals, which could boost the production of the direct precursors of CO2.

Nanosecond Scale Discharge Dynamics in High Pressure Air

The use of pulsed nanosecond scale discharges is promising for automotive engine ignition because air-hydrocarbon mixtures can be chemically activated with a nonthermal plasma. Very short high voltage pulses are a good way to control the energy which is transferred into the gas at pressures above atmospheric. The development of such a discharge, in a point-to-plane configuration, under a short and high overvoltage, is investigated in air for different pressures through fast imaging and electrical records.

Towards a kinetic understanding of the ignition of air-propane mixture by a non-equilibrium discharge: the decomposition mechanisms of propane

The decomposition of propane in non-thermal plasmas of N2/C3H8 and N2/O2/C3H8 mixtures (oxygen percentage up to 20%) at low temperature is studied in a photo-triggered discharge. Quenching of nitrogen metastable states dissociate C3H8 to produce propene and hydrogen. Oxidation reactions are growing in importance when the O2 concentration increases, but the dissociation quenching reactions still occurs for the air-based mixture. Even for a low concentration of oxygen, OH is an important specie involved in the conversion of the hydrocarbon. A kinetic analysis emphasises that OH comes in great part from the production of H, in which the methyl radical plays a role, strengthening the role of the dissociation processes of propane and propene in the medium reactivity. Results of PLIF measurements performed on OH during the diffuse afterglow of a nanosecond corona discharge correlate with results obtained on the photo-triggered discharge.

Filamentation of a Nanosecond Pulse Corona Discharge in Air–Propane Mixtures at Atmospheric Pressure

The space and time development of a single nanosecond pulsed corona discharge is studied in atmospheric air-propane mixtures with different concentrations of propane up to 8%. Time-resolved imaging of the discharge shows a complete diffuse pattern in pure dry air, which becomes more and more filamentary when propane is added. Filaments are thinner with higher concentrations of propane, and the whole discharge energy increases and gets saturated.

Optical and electrical characterization of pulsed dielectric barrier discharges in heterogeneous structures

In the context of expanding plasma-catalytic cleaning techniques where discharges develop inside heterogeneous catalytic supports like packed beds or honeycomb monoliths, the understanding of streamers propagation physics in such nonhomogeneous media is of great interest. For that purpose, have been compared pulsed discharges inside two identical plane-to-plane dielectric barrier devices, one of them being coupled with a honeycomb monolith of cordierite. Electrical records and corresponding images of plasma properties in both these reactors are presented.