Castellanos Mata, Antonio

Fernández Rueda, A.

Moreno González, H.

Pontiga Romero, Francisco de Paula

idUS. Depósito de Investigación Universidad de Sevilla

 NEGATIVE CORONA DISCHARGE IN SYNTHETIC AND DRY AIR  F. Pontiga(1) , (*), A. Fernández-Rueda(1), H. Moreno (1), A. Castellanos(2)  (1) Dpt. Física Aplicada II, Universidad de Sevilla, Spain.  (2) Dpt. Electrónica y Electromagnetismo, Universidad de Sevilla, Spain.   pontiga@us.es  Electrical discharges in air generate many different species, some of them toxic and corrosive. Ozone is a predominant species, but also nitrogen oxides, such as N2O, NO, NO2, NO3 and N2O5, have been confirmed by means of spectroscopic techniques and mass spectrometry in air-fed ozonizers based on dielectric barrier discharge [1,2]. The production of chemical species by means of corona discharge in air has received less attention [3,4] and, although the fundamental physico-chemical processes in dielectric barrier and corona discharges are essentially identical, both discharges are very different from the electrical point of view. In this work, a numerical modelling of negative DC corona discharge in dry air (0.03% CO2) is performed for wire-to-cylinder coaxial geometry. The chemical kinetic model used here includes nine charged species (e, N2+, NO+, NO−, O2+, O2−, O−, O3− and CO3−), ten neutral species in their ground states (N2, O2, ,CO2, N, NO, N2O, NO2, O, O3 and CO), four excited species (N2(A3Σu+), N(2D), O2(1∆g) and O(1D)), with a total number of 150 reactions. The electrical discharge is assumed to be homogeneous along the wire and stationary in time. Therefore, the concentration of species will only depend on the radial distance to the wire (1D model). The equations governing the electrical discharged are continuity equations for each species (which include the chemical kinetics) and Poisson’s equation for the electric field.  The electric field dependence of reaction rate constants has also been taken into consideration, particularly in electron induced reactions. The numerical modelling is complemented with the experimental measurements of ozone and nitrogen dioxide concentrations generated by a negative corona discharge in synthetic air. The electrode arrangement used in the experiments (tungsten wire ∅ 0.1 mm, stainless steel Topic number: 6, 10 Fig. 1 Radial distribution of electrons and ions corresponding to an applied voltage of -6.5 kV (numerical simulation). Fig. 2 Radial distribution of neutral species corresponding to an applied voltage of -6.5 kV (numerical simulation). 1041051061071081091010 0.01  0.1  1N (cm-3 )r (cm)eN+2NO+NO−CO−3O+2O−2O−O−310610810101012101410161018 0.01  0.1  1N (cm-3 )r (cm)N2OO3NO2 , CO , CO2NOONO2(1∆g)N2(A)O(1D)cylinder ∅ 17 mm) is identical to the one considered in the numerical simulation. The gas inside the corona discharge reactor was prepared as a mixture of 20% oxygen and 80% nitrogen, and species concentrations were determined by means of UV absorption spectroscopy, in the range 190-330 nm. The high voltage was applied from 4.5 kV (slightly above the onset of corona) up to 7.5 kV. Each applied voltage was maintained for about 50 minutes so as to enable that a stationary current and, presumably, stationary species concentrations, were reached. Figures 1 and 2 show the radial distribution of charged and neutral species, respectively, according to the numerical simulation for an applied voltage of -6.5 kV and a current intensity of -20.3µA/cm. In the vicinity of the wire, ionization of nitrogen and oxygen molecules by electron impact gives rise to the formation of N2+ and O2+. However, O2+ ions are predominant due to the existence of a fast charge transfer reaction from N2+ to oxygen molecules to form O2+. In the drift region, the principal negative ion is CO3−, followed by O3−. This result is in agreement with mass spectrometric study of ions produced by negative corona in air [5], and it can be explained by the presence of a fast ion conversion reaction between O3− and CO3−. Concerning the radial distribution of neutral species (Fig. 2), the predominant species is N2O, which is formed at the expenses of NO2 and atomic nitrogen. Moreover, ozone density is almost two orders of magnitude smaller than in the case of corona in pure oxygen [6], even though the density of oxygen has only been reduced by a factor of five. This dramatic reduction of ozone is the consequence of catalytic reactions chains, which lead to the recombination of oxygen atoms. In figure 3, the averaged ozone and nitrous oxide densities measured in the experiments are compared with the results obtained from the numerical simulation at different current intensities. As predicted by the numerical simulation, the concentration of N2O was always higher than that of O3, but the numerical simulation tends to underestimate the actual concentrations.  References [1] U. Kogelschatz  and P. Baessler, 1987 Ozone Science Engineering 9 195-206. [2] B. Eliasson and U. Kogelschatz 1991 IEEE Transactions on Plasma Science 19 309-323 [3] K. G. Donohoe, F. H. Shah and O. R. Wulf 1977 Industrial and Engineering Chemistry Fundamentals 16 208-215. [4] P. Martinez and D. K. Brandvold 1996 Atmospheric Environment 30 4177-4182. [5] J. Skalny 1987 Acta Physica Universitatis Comenianae 28 161-176. [6] C. Soria, F. Pontiga and A. Castellanos, 2004 Plasma Sources Science and Technology 13 95-107. Fig. 3 Averaged ozone and nitrous oxide densities as a function of corona current intensity 10151016101710181019 0  5  10  15  20  25  30N (cm-3 )I (µA/cm)O3 experimentalO3 numericalN2O experimentalN2O numerical

Negative corona discharge in synthetic and dry air

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Negative corona discharge in synthetic and dry air

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