Effect of H₂ concentration on carbon deposition reaction by CO–H₂ gas mixture at 773 K to 973 K

CO–H₂ gas mixture is often used for gas-based DRI process where carbon deposition reaction and Fe₃C metal dusting play negative roles for a stable reduction operation. Fe₃C decomposition leads to the formation of iron particles which is a catalyst for carbon fiber deposition. Because of the parallel occurrence of these reactions, kinetic analysis of them would be complicated. In the present study, to simplify the kinetic analysis, quantitative analysis of carbon fiber deposition was conducted by using thermobalance. A powdery iron sample was prepared by reduction of Fe₂O₃ with 100vol%H₂ at 673 K. Carbon deposition on the iron sample was investigated under flowing 100%CO, 90vol%CO-10vol%H₂, 75vol%CO-25vol%H₂, 50vol%CO-50vol%H₂, 25vol%CO-75vol%H₂ and 10vol%CO-90vol%H₂ gas mixtures at 773 K, 873 K and 973 K. Results showed that amount of the deposited carbon in the CO–H₂ gas mixture are larger than that in the pure CO gas. The largest amount of deposited carbon was obtained in 75vol%CO-25vol%H₂ gas mixture at 873 K. According to SEM observations and weight change measurements, carbon was deposited in fiber shape on the iron surface and amount of it was increased linearly with an increase in sample’s weight change. The rate constant of carbon fiber deposition was calculated considering Rideal mechanism with focusing on elementary reaction steps. It was found that the rate constant of the hydrogen-oxygen reaction step was the largest indicating a significant effect of hydrogen on promoting carbon deposition 773 K and 873 K. This would be due to the removing oxygen from CO by hydrogen on the iron catalyst.