03.08.2015   Pretreatment Techniques for Biofuels and Biorefineries

Direct Current Plasma Torches

Until now, most gasification and pyrolysis systems use DC plasma torches. Fig­ure 12.2a shows the typical schematic diagram of power-supply circuit. The ballast resistor is used to stabilize burning of DC arc that causes considerable real power losses.

The most widespread DC plasma torches are Westinghouse Plasma Corp plasma torches with cylindrical electrodes. Sketch of this type of plasma torch is presented in Fig. 12.2b.

Power ranges for plasma torches MARC-3A and MARC-11L are 130-300 kW and 300-800 kW [87, 88], respectively, which are most attractive for industry. These are the most advanced designs. Their efficiency of energy transfer from the arc to plasma (thermal efficiency) is 70-85 %. The total efficiency of the system including

Fig. 12.2 DC plasma torches: a Typical power-supply schematic diagram of DC plasma torch (P plasma torch; D rectifier; L-C filter; R ballast resistor, SF automatic circuit breaker); b After [87] schematic representation of Westinghouse Plasma Corporation plasma torch (1 plasma column; 2 electrode; 3 entering process gas; 4 heated process gas); c Photo of 300 kW Europlasma’s plasma torch. (Reprinted from [89], with permission from the International Plasma Chemistry Society)

ohmic resistance losses in power-supply system is significantly lower. The lifetimes of electrodes for these models are 600 and 1,000 h, respectively.

EUROPLASMA designed a series of plasma torches of the same type which could be fed by CO2, CO, CH4, H2, N2, and their mixtures. Figure 12.2c shows a 300 kW plasma torch [89].

Series of “Linde” plasma torches are described in [83-85]. These devices are designed for short-time operation at high pressures and power up to 20 MW. These plasma torches use cylindrical copper electrodes. Some models have the working gas enthalpy up to 10 MJ/kg. Stabilized rectifiers power these devices. Voltage of power sources is up to 10 kV. Typically, the flow rates of cooling agents for ballast resistor and plasma torch are virtually equal. Pressure in the cooling system is about 4 MPa. Plasma torches MDC-200 and MDC-300 [86] use air as a working gas and operate in relatively short-time mode, they are intended for special experiments. Maximum working pressure of gas in the plasma torch chamber is 25 MPa, a range of operating currents 100-1,200 A, voltage 1.2-14 kV, thus power changes from 0.7 to 10.5 MW.