Efficient, Modular Microwave Plasma Torch for Thermal Treatment

A high-efficiency and high-power atmospheric pressure plasma torch has been operated in air and nitrogen by modularly combining two 2.54 GHz microwave plasma sources. Each modular plasma device uses a shorted waveguide within which the plasma is sustained. They are arranged in series so the second torch adds all its power to the output of the first torch to produce one high power flame. Total microwave power of 4 kW has been used in the present experiments by combining 2.5 and 1.5 kW sources. The microwave power coupling efficiency to the plasma has been determined to be approximately 95% by reflected power measurements in these initial unoptimized experiments. Plasma gas temperatures of approximately 5500 K have been obtained by spectroscopic measurements. A flame length of up to 25 cm beyond the second waveguide has been achieved for axial gas flows of up to 140 liters/minute through the 2.5 cm diameter discharge tube traversing the waveguides. The maximum power, number of modular stages, and gas flow volume is limited by the presently available hardware components. There is significant potential for scaling up. This novel approach can open new possibilities for thermal processing by using commercially available and low cost magnetrons at 2.45 GHz and 915 MHz. Capital costs could be less than a dollar per watt for high power systems (> 75 kW) and there would be no electrodes to wear out. These features could make this technology competitive with traditional plasma arc torches and RF induction plasmas. INTRODUCTION: The need for thermal processing in different areas of the industry such as municipal and solid waste destruction, radioactive waste vitrification, and clean material processing has triggered research and development of high power, contaminant-free, low-cost and lowmaintenance plasma torches. DC and AC plasma arc technologies have been around for almost a century and are used in many thermal processes including waste remediation and materials manufacturing. One of the main limitations of this technology is the limited electrode lifetime, which consequently requires frequent replacement, increasing costs and maintenance [1]. Much research has been expended on improving the technology of plasma arc electrodes. Some plasma arc systems use water-cooled metallic electrodes. This solution increases the lifetime of the electrodes only to a few hundreds of hours, but at the same time introduces a safety concern because a water leak into the plasma can produce an * Corresponding author: [email protected]