A Study on Direct Current Arc Plasma Torch Design with Preserve Nozzle for Perfluorinated Compounds (PFCs) Decomposition in Cement Kiln

Abstract

Perfluorinated compounds (PFCs), including sulfur hexafluoride (SF 6 ), are used as insulating gases for heavy electric equipment, and are green- house gases with a very high global warming potential. For the thermal decomposition of greenhouse gases, a high temperature of 3,000 K or more and a technology to prevent recombination of the decomposed gases are required. A direct current plasma arc with a flame temperature of 6,000 K or higher provides an effective ultra-high temperature for the decomposition of chemically stable PFCs. A large amount of CaO, which is a raw material for clinker, is in the cement kiln, hence the S and F radicals decomposed from SF 6 react with CaO to quickly convert it to CaF 2 and CaSO 4 and prevent their recombination. Therefore, the convergence technology of plasma and cement kiln is known as an effective method for treating PFCs. However, the interior of the cement kiln causes erosion of the plasma torch due to the turbulent flow of a large amount of combustion air at a high temperature of 1,500 ∘ C or more, and the scattering of the raw material particles of the clinker, which greatly affects the operation stability. In this study, a plasma torch was developed that can maintain stable plasma in a vulnerable environment and effectively decompose PFCs. Moreover, the design and results of the experiment on the torch were presented to enable stable plasma operation for a long time in vulnerable environmental conditions through durability, plasma efficiency, and analysis of the discharge characteristics. thermal efficiency, and durability of the plasma torch were analyzed. Continuous operation of the plasma torch for more than 1,000 h while maintaining the plasma characteristics even in en- vironments with a high temperature, vibration, and strong turbulent flow was verified using a molybdenum preserve nozzle. At this time, the plasma efficiency was measured as 79.1%, confirming the supe-rior durability and efficiency of the developed plasma torch to those of the conventional PFC treatment torch. These results show that the high-temperature plasma torch can be considered advanced technol- ogy with high utility in various waste gas treatment processes and plasma incineration fields by overcoming the short lifespan due to electrode erosion, which is a known disadvantage of high-temperature plasma torches.