This is a study on the properties of a dual-mode plasma torch system, carried out to provide technical information and to develop a practical plasma melting system for non-combustible waste such as metal scraps, concrete, soil, and so forth. A development of practical technology for waste treatment is in high demand based on several problematic issues such as environmental protection, the difficulties in disposal site selection and disposal cost increases. One of the new notable applications is a high temperature melting technology using a plasma torch, as it is able to effectively convert into more stable waste forms with maximum volume reduction at a reasonable cost.
This paper presents an idea and an experiential development of a 500 kW dual mode plasma torch system for demonstration. A preliminary test was carried out in order to evaluate the fundamental characteristics of the dual mode plasma torch in advance. An experimental apparatus for simulation was provided with drawings. Furthermore operational characteristics with power ratio change between the transferred mode and non-transferred mode were investigated.
The result shows that dual mode operation is possible with no harmonics. It is well understood that the transferred mode operation is much more effective in transferring heat to the pool object than the non-transferred operation due to joule heating. Also, it is common in the melting process for the transferred mode operation to be somewhat unstable due to the instability of the arc column in the furnace environment.
Based on the experimental results of preliminary testing, a dual-mode plasma torch system has been developed for the purpose of the treatment of non-combustible waste using surrogate radioactive wastes. This system consists of a 500 kW hollow type plasma torch, a melter with a refractory structure capable of treating more than 45 kg per hour, a power supply built with high power insulated gate bipolar transistor (IGBT) modules, a feeder for waste, a tapping device, an associated gas supply and cooling system, and a control and monitoring system with various functions and displays. The dual mode plasma torch system is able to operate in transferred, non-transferred or both, and is quite adequate for melting mixed wastes including non-conductive materials such as concrete, asbestos, etc., since it exploits both the high efficiency of heat transfer to the melt in transferred mode and stable operation in non-transferred mode. The dual mode torch is a hollow type and operates in reverse biased mode, that is to say, the rear electrode is at positive potential, and the nozzle and melter is at negative potential. After ignition, the dual mode plasma torch will operate only in the non-transferred mode because the slag layer in the pool will slow very high resistivity, and will melt slowly from the surface to the bulk. As the solid layer gets thinner, the slag will start conducting and the appreciable current will flow at the limiting voltage of the power supply. Once the slag is conducting, the melting process will be accelerated, because the direct joule heating will melt slag much more efficiently. The time from the start of conduction to fully conducting will be very short, while the complete melting of the slag layer will take more time because heat still must be transferred through conduction, convection and radiation. When the pool level increases as a result of waste melting, the distance is shortened between the nozzle of the dual mode torch and the surface of the molten pool. Therefore, it is necessary to keep the distance constant. This function can be performed by the measurement of bias voltage of the transferred mode operation. Average voltage of the dual mode torch is proportional to the distance, and so the distance is to be controlled by keeping the voltage constant. To ensure safety, the power supply should be constructed so that the front electrode is connected to the ground when it is operated in the non-transferred mode. Also, system operation, including restarting, is reliable and very easy. A stationary melter with a refractory structure was designed and manufactured taking into account the melting behavior of slags to minimize the refractory erosion.
The power supply for the dual mode plasma torch system, built with high power insulated gate bipolar transistor (IGBT) modules, has functions for both current control and voltage control and is sufficient to suppress the harmonics during the operation of the plasma torch.
The power supply provides two different voltages for transferred operation and non-transferred operation. It is confirmed that the transferred operation voltage is always higher than in non-transferred operation.
A stationary furnace with a slant tapping port on the side of the furnace is, for the first time, equipped with an electrical heating method by electrodes for tapping out. At the end of each melting cycle, the tapping hole needs to be opened with a tapping device. When the non-combustible wastes such as concrete debris, glass, sand, etc., are melted, they become acid slags with very low electrical conductivity and basicity because the chemical composition has much more acid oxides than basic oxides. Also, a molten slag does not have good characteristics for discharge and is mainly responsible for refractory erosion due to its low liquidity. The electrical tapping device is adequate for the application of discharging melts, except that the consumption of the guide electrode is somewhat faster than expected.
The dual mode plasma torch system was successfully developed and is under operation for a melting experiment. The results of this study will be useful and informative. Through a series of melting experiments using various types of surrogates, valuable fundamental data will be produced for commercialization in the future.