Abatement of fluorinated compounds in thermal plasma flow

Fluorinated compounds, widely used substances in semiconductor manufacturing, represent a potent source of global warming effect with direct global warming potential much higher than that of carbon dioxide, methane or nitrous oxide. These gases are extremely chemically stable and thus very high temperature as generated by thermal plasma torch is effective for their destruction. Compared to conventional methods, thermal plasma offers higher efficiency of decomposition as it enables reaching sufficiently high temperature and enthalpy. The aim of this work was to apply N2 stabilized direct current-plasma torch (with the input power up to 16 kW) to generate steam plasma for an efficient abatement of model fluorinated substances (CF4, C2F6, CHF3, NF3, and SF6). The effect of arc power of the plasma torch, gas flow rate and the concentration of fluorinated compounds on their destruction efficiency was tested. Determined destruction and removal efficiency can be ordered with respect to the treated chemical substance in the following way: NF3 > C2F6 > CHF3 > SF6 >> CF4. Removal greater than 99.99% level of the most persistent gas, i.e., CF4 was attained at 16 kW torch power and inlet concentration of 1% (vol.) for feed rate 50 standard L/min. For C2F6 abatement it was found that steam addition is essential to prevent CF4 by-product formation even though this addition reduces destruction and removal efficiency. The general trend observed at 10 kW torch power showed that destruction efficiency increases with increasing inlet gas concentration. The only exception is SF6 that exhibit opposite tendency for any applied torch power. To assess the energy efficiency of the abatement process the dependence of residual concentrations of the abated gases on the feed rate to torch power ratio was evaluated.