Étude théorique et expérimentale de décharges électriques à haute pression et faible courant en milieu non-réactif et réactif appliqué à la synthèse d'hydrocarbures

Eduardo Izquierdo 1
1 CEP/Sophia
CEP - Centre Énergétique et Procédés
Abstract : In the field of non-thermal plasmas, the term "High-Pressure" generally refers to pressures ranging between 0.01 MPa and 0.1 MPa. For pressures higher than 1 MPa, the behaviour of electrical discharges at low current remains a subject hardly explored. The study and the control of this kind of discharge can, however, open new prospects in the field of reacting chemistry and in particular in the field of hydrocarbons synthesis. This work aims to study a "tip-tip" electrical discharge under very high-pressure and low-current conditions through a parametric analysis (pressure, electrode gap and geometry, current, reactive and non-reactive gas) thanks to both theoretical and experimental investigation. The theoretical study concerns three complementary aspects. The first deals with the "magneto-hydrodynamic" modelling inside the reacting chamber under non-reactive conditions. The plasma is supposed to be on local thermodynamic equilibrium and the arc is assumed to be a conducting channel whose radius follows a parabolic function of the distance from the cathode. The model takes into account the gravity force. The parameters of the parabolic function are derived from the minimization of the energy losses of discharge (or energy minimum principle of Steenbeck). The second theoretical study concerns the modelling of the physical phenomena in the near-electrode regions. The objective of the model is to analyse the influence of the parameters (P, Te, Th, Tw) on the anode and cathode voltage drops. Finally, the last contribution aims at the evaluation of the non-thermal characteristic of discharge inside the positive arc column. An Elenbaas-Heller approach has been used for a two temperature-plasma coupled with the Ohm's Law. For our numerical models, the composition and properties of plasma have been calculated from a kinetic model specially developed for high pressure and two temperatures plasma. The experimental study is based on the development of an experimental set up specifically elaborated for this work. The study is performed, first, under non-reactive conditions (Ar, N2, H2) in order to determine the current-voltage characteristics of the discharge. The adjustment of an Ayrton model modified for high-pressure conditions is considered. The study is then extended toward reactive conditions (CO-H2). These exploratory results demonstrate the feasibility of hydrocarbon synthesis under very high-pressure conditions. They confirm that the pressure tends to support the selectivity towards the heavy hydrocarbon species. This behaviour validates the theoretical study developed in parallel.
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Eduardo Izquierdo. Étude théorique et expérimentale de décharges électriques à haute pression et faible courant en milieu non-réactif et réactif appliqué à la synthèse d'hydrocarbures. Autre. École Nationale Supérieure des Mines de Paris, 2008. Français. ⟨NNT : 2008ENMP1592⟩. ⟨pastel-00685101⟩

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