Study on CO2 based thermal plasma torch and its effective utilization for material processing in atmospheric pressure

Abstract

Reuse of greenhouse gases is one of the promising approaches towards controlling global warming to some extent. As a part of that, an attempt has been made to utilize CO2 gas as one of the primary gases to form a thermal plasma jet for material processing. For that purpose, a unique direct current (DC) non-transferred arc thermal plasma torch was designed and its current-voltage characteristics and electro-thermal efficiency with different discharge currents and gas compositions were studied. The torch is made of rod type cathode and a nozzle type anode, which can produce a stable thermal plasma jet under different gas compositions. CH4 was used as a secondary mixing gas in an Ar-CO2 gas composition to produce a carbon-rich thermal plasma with high electro-thermal efficiency. The maximum electro-thermal efficiency of 73 % were reached while the torch operated at 100 A current with 6 slpm of CH4 flow along with 15 and 20 slpm of Ar and CO2 gas flow, respectively. Torch efficiency decreased to 12 % with increase in discharge current from 100 A to 200 A. Optical emission spectrum of the plasma jet was recorded at the exit of the torch nozzle to understand the emission characteristics of the plasma and estimate temperature and electron density. Temperature of the plasma increased from 13,400 to 17,800 K with respect to the discharge current. Thermodynamical studies were carried out for the given experimental conditions to understand the thermal decomposition of CO2-CH4 gas mixture and select the suitable condition for the material processing. Under the selective conditions, plasma torch was utilized for the spheroidization of tungsten metal powder and carburization of the AISI 316 L stainless-steel coupons in atmospheric pressure without using any shielding gases. Results showed that around 85 % of the metal powders were spheroidized perfectly at 15 kW input power. Plasma carburization results showed that the microhardness of the steel surface reached 605 HV with a short processing time of 3 min which was 4 times higher than the untreated steel surface. This work provides useful insight for the utilization of CO2 as primary plasma forming gas to generate thermal plasma for material processing.