Numerical modeling of pulsed volume discharges for the generation of ozone

Sammanfattning: The pulsed electric volume discharge is a new alternativefor the efficient generation of ozone in compact systems. Thisthesis presents a parameter study of the reactions in this kindof homogeneous discharge, using a here developed numericalmodel DISCHEM which solves plasma chemical kinetic rate andenergy equations. The thesis also presents necessary conditionsfor initiating and sustaining a pulsed volume discharge forparameter values of interest. Matching of the discharge to apulsed power driving circuit is also demonstrated. Single pulse simulations are performed for 10-9- 10-5discharges with oxygen gas density in the range1≤n≤10 amagat, initial gas temperature 100≤T0≤300K and normalized electric field of100≤E/n≤200 Td. Results are presented of ozonegeneration efficiency versus ozone concentration for differentparameter combinations. Two parameter regimes are identified and analyzed. In theplasma phase ozone formation regime ozone formation occursmainly In the discharge plasma. For this regime, not accessiblein conventional dielectric barrier discharge ozone generators,it is found that higher ozone concentrations can be obtainedthan in the neutral phase ozone formation regime, where ozoneis formed after the discharge pulse. In both regimes the ozonegeneration efficiency increases asnis incraesed orT0decreased. The maximum concentration, at 10amagat and 100K, is 3%. For the efficiency of the two-step ozone formation process,the rate of conversion of atomic oxygen plays a key role. Threelimiting reactions are identified, with rates mainly determinedby the concentration of vibrationaIly excited ozone and atomicoxygen and by the gas temperature. Results on ozone accumulation in multiple pulse dischargesare presented. In contrast to the single pulse case, higherefficiency is achieved at lower gas density. This scaling canbe explained by losses due toion currents. A trade-off can bemade between ozone generation efficiency and the number ofpulses requierd to reach a certain concentration. Thus, forexample en ozone concentration of 10% can be achieved In 15pulses at 200 gO3/kWh, whereas 30 pulses with lower energy resultsin 300 gO3/kWh. Conditions necessary for creating and sustaining a stablepulsed volume discharge are identified. A scaling law isderived for the dependence of critical preionization on gasdensity and electric field rise time. Conditions leading tothermal instabilities are examined. Good matching of the discharge to an inductive storagepulsed power system can be obtained, according to simulationsusing the discharge numerical model self-consistently coupledto a driving circuit model. In summary, results of the present work show that thepreviously demonstrated high efficiency of ozone generation atlow concentration in single pulse volume discharges can bescaled to compact, efficient systerms with production rates andconcentrations of interest to industrial applications by properselection of parameters in multipulse discharges.