Modeling and Simulation of Chemical System Vaporization at High Temperature: Application to the Vitrification of Fly Ashes and Radioactive Wastes by Thermal Plasma

Abstract

The treatment, at high temperatures, of such chemical systems as the fly ashes and radioactive wastes requires the control of its element volatility. Precisely, it requires following the evolution of the system during treatment and determining the composition of the system in all phases. For a closed chemical system, the calculation of its composition is carried out by the method of free enthalpy minimization developed by Eriksson [1]. However, for open systems, the problem is not definitively solved yet. A computer code simulating the volatility of the elements present in an oxide system was developed by Pichelin [2] and Badie [3]. This computer code was modified by Ghiloufi to control the vaporization of the elements present in a chloride and oxide system during the fly ashes vitrification by plasma and to study the radioelement volatility during the treatment of radioactive wastes by thermal plasma [4-9]. In this chapter we present a method used in our computer code, which is developed to simulate and to modulate a chemical system vaporization at high temperature. This method is based on the calculation of composition by free energy minimization of the system, coupled with the mass transfer equation at the reactional interface. This coupling is ensured by fixing the equivalent partial pressure of oxygen in the mass transfer equation and those characterizing the complex balances. This chapter contains five parts: In the first part we will present the method used to the calculation of composition by free energy minimization of a closed system, precisely we will develop the equations characterizing the complex balances at the reactional interface. In the second part we will give the mass transfer equation of oxygen. In the third part we will present the method used in our study to determine the diffusion coefficients of gas species essentially for complex molecules like vapor metals. In the fourth part we will apply the computer code to simulate the radioelement volatility during the vitrification of radioactive wastes by thermal plasma. In the last part we will present the results obtained by the computer code during the study of radioaelement volatility.