3-D Modelling and Experimental Applications of Thermal Plasmas (RF and Transferred Arc) for Waste Treatment Process

3-D Modelling and Experimental Applications of Thermal Plasmas (RF and Transferred Arc) for Waste Treatment Processes V. Colombo, E. Ghedini Dipartimento di Ingegneria delle Costruzioni Meccaniche, Nucleari, Aeronautiche e di Metallurgia (DIEM) and Centro Interdipartimentale di Ricerca per le Applicazioni della Matematica (CIRAM), Università di Bologna, Via Saragozza 8, 40123 Bologna, Italy. A three-dimensional model for the simulation of inductively coupled (RF) plasma torches (ICPTs) and for DC transferred arc ones working at atmospheric pressure has been developed at the University of Bologna, using customized CFD commercial code FLUENT©. For what concerns ICPTs, the helicoidal coil is taken into account in its actual 3-D shape, showing its effects on the plasma discharge for various geometric, electric and operating conditions without axisymmetric hypotheses of simplification. The electromagnetic equations are solved in their vector potential form, while the steady flow and energy equations are solved for optically thin plasmas under the assumptions of LTE and laminar flow. An industrial TEKNA PL-35 plasma torch with a downstream reaction chamber fit for waste treatment, as part of a source operated in the laboratories of the University of Bologna at 3MHz with maximum generator power of 25kW, is modelled without geometry simplifications, refining the mesh at the injection points, in order to perform a more realistic simulation of the inlet region of the discharge, also taking into account turbulence effects using the Reynolds Stress Model. The effects of changing inlet gas flow rates, direction of the swirl velocity component, axial length and number of turns of the coil and the net amount of power dissipated in the discharge are evidenced, in order to give useful hints for avoiding the formation of a hot temperature spot in the confinement tube wall due to the axial displacement of the plasma fireball. Metallic and ceramic particle axial injection in the discharge through a carrier gas by means of a probe is simulated as well, taking into account the energy and momentum transfer between the continuous and the discrete phase and the effect of particle turbulent dispersion. Comparisons are made with experimental measurements carried out by means of an enthalpy probe technique. The impact of both the computational and experimental procedures on the feasibility of RF thermal plasma treatment of waste materials is outlined. On the other side, the behaviour DC transferred arc thermal plasma sources operating at atmospheric pressure for the treatment of a substrate material (for waste treatment purposes and for metallic substrate cutting or hardening) is also considered. A fully investigation of plasma velocity and temperature fields in high power twin torch transferred arc systems (electric arc discharge generated between two suspended metallic electrodes) designed for waste treatment purposes inside a furnace is considered for a plasma source designed and operated by Centro Sviluppo Materiali (CSM S.p.A.) in Castel Romano, Rome, with special interest for time-dependent effects. Turbulence effects are taken into account into the model using a RANS approach, as well as the effect on the discharge characteristics of using different types of plasma gas (Ar, air and Ar/H2 mixtures), for various geometric and operating conditions. Simulations can also give important information on non-axisymmetric anode attachment under particular operating conditions. All simulations are performed over a network cluster of double processor calculators in order to use the full capabilities of 3-D modelling, both in steady state and in a time-dependent framework.