Effect of temperature and CO₂ concentration on gasification behavior of carbon fiber containing fine iron particles

In direct reduced iron (DRI) process, CO–H₂ gas mixture is used as a reducing agent, which may make the operation unstable owing to a carbon deposition reaction and metal dusting reaction through Fe₃C. The Fe₃C decomposition reaction forms iron particles which acts as a catalyst for a carbon fiber deposition reaction at around 600°C. Such a carbon fiber deposition not only causes the loss of the carbon but also decrease the reducibility of the gas. On the other hand, the carbon fiber is likely gasified by CO–CO₂ gas mixture at around 1 000°C. In the present study, the carbon fiber gasification was quantitative analyzed using thermobalance to clarify the mechanism of the carbon fiber gasification reaction. To prepare a carbon fiber sample, the carbon was deposited with a reduced iron catalyst at 600°C in 50vol%CO-50vol%H₂. Carbon fiber containing fine iron particles was gasified with various compositions of CO–CO₂ gas mixture at 1 000°C. Further, the effect of temperature of the gasified reaction was also investigated at 800°C, 900°C and 1 000°C in 100vol%CO₂. According to XRD analysis of the sample after gasification, Fe₃C in the sample before gasification decomposed to iron and a portion of iron was oxidized to Fe₃O₄ and FeO when gasification ratio was high. The transition of carbon fiber shape was confirmed by SEM observation. Fine iron particles located on the tip of carbon fiber were sintered during gasification of carbon fiber. The mechanism of carbon fiber gasification was evaluated considering crystalline size, such as L_a and L_c that show (002) and (110) determined by XRD analysis, respectively. It was found that the L_a decreased with gasification ratio at all temperatures. In addition, L_c decreased after gasification at 1 000°C.