Abstract
The effects of the arc current and the gas flow rate in a plasma torch on the thermal plasma decomposition process have been analyzed to improve its economic feasibility for the pyrolysis of perfluorocompounds (PFCs). A 200-slpm waste gas mixture of 1 % $CF_4$ and 99 % $N_2$ purging gas was decomposed by using a nitrogen thermal plasma generated by using a plasma torch with hollow electrodes. A thermal efficiency of over 50 % was achievable with a torch input power of 40 kW operated at 150 A and 70 slpm as a result of a stable attachment of the cathodic arc root near the plasma gas injection area. Measurements in the thermal plasma pyrolysis showed a more than 96 % destruction and removal efficiency (DRE) of $CF_4$, which turned out to be mainly affected by the net plasma thermal power, which depended on the thermal efficiency of the torch, regardless of the input electric power. The plasma torch operated with a higher gas flow rate showed an increased $CF_4$ DRE by enhancing the turbulent mixing effects of the thermal plasma jet having a relatively higher axial velocity with a steep gradient from the reactor entrance. However, a higher flow-rate gas disturbs the cathodic arc attachment around the gas injection area, which causes a considerably increased electric power consumption. Plasma torch operation at a low gas flow rate and high current is expected to be effective in increasing its thermal efficiency with a sufficiently high DRE for industrial application of thermal plasma pyrolysis to the treatment of PFCs due to its low running cost for electric power and working gas.