METHANE INJECTION THROUGH BORED GRAPHITE ELECTRODES INTO THERMAL ARCS OF EATS FOR HIGHER POWER AND CLEANER STEELS

Abstract

Methane injection into the arcs of electric arc furnaces has been shown on pilot scale to lead to a remarkable arc voltage increase at constant arc current and arc length. The concept is to inject natural gas through bored graphite electrodes of DC EAFs in order to raise their productivity or to operate at constant power with shorter arcs and/or lower currents. Recent investigations have been concerned with heat transfer and metallurgical effects in a gas-tight 150-kg arc furnace operated with two AC plasma torches. A first test with bored graphite electrodes in this furnace confirmed the power increase observed during methane injection. The carburization slowly occurring when CH4 was injected could be avoided by adding minor amounts of CO2, decarburization by CO2 being faster than carburization by CH4. A slag layer decreased mass transfer rates without noticeably affecting heat transfer. Hydrogen was quickly absorbed by the steel melt but also rapidly desorbed after CH4 injection was stopped. Bottom stirring improved heat dissipation in the melt and hydrogen removal. Manganese loss by evaporation was measured to investigate the influence of power increase and slag layers. From the results, an increase of 200 K was concluded for the melt Surface temperature with Ar + 6 % CH4 as compared to pure argon. Low nitrogen steelmaking in EAFs being a challenge to metallurgists, methane injection into the arcs proved to accelerate nitrogen removal considerably down to values below 20 ppm.