Numerical Investigation of Hybrid-Stabilized Argon-Water Electric Arc Used for Biomass Gasification

Plasma generators with arc discharge stabilization by water vortex exhibit special performance characteristics; such as high outlet plasma velocities (up to 7 000 m ⋅ s-1), temperatures (~ 30 000 K), plasma enthalpy and, namely, high powder throughput, compared to commonly used gas-stabilized (Ar, He) torches (Hrabovský et al., 1997). In a water-stabilized arc, the stabilizing wall is formed by the inner surface of water vortex which is created by tangential water injection under high pressure (~ 10 atm.) into the arc chamber. Evaporation of water is induced by the absorption of a fraction of Joule power dissipated within the conducting arc core. Further heating and ionization of the steam are the principal processes which produce water plasma. The continuous inflow and heating lead to an overpressure and plasma is accelerated towards the nozzle exit. The arc properties are thus controlled by the radial energy transport from the arc core to the walls and by the processes influencing evaporation of the liquid wall. A combination of gas and vortex stabilization has been utilized in the so-called hybridstabilized electric arc, its principle is shown in Fig.1. In the hybrid H2O-Ar plasma arc the discharge chamber is divided into the short cathode part where the arc is stabilized by tangential argon flow in the axial direction, and the longer part which is water-vortexstabilized. This arrangement not only provides additional stabilization of the cathode region and protection of the cathode tip, but also offers the possibility of controlling plasma jet characteristics in wider range than that of pure gas or liquid-stabilized torches (Březina et al., 2001; Hrabovský et al., 2003). The arc is attached to the external water-cooled rotating disc anode a few mm downstream of the torch orifice. The characteristics of the hybridstabilized electric arc were measured and the effect of gas properties and flow rate on plasma properties and gas-dynamic flow characteristics of the plasma jet were studied. Experiments (Březina et al., 2001; Hrabovský et al., 2006) proved that plasma mass flow rate,