Re-oxidation Kinetics of Flash-Reduced Iron Particles in H 2 -H 2 O(g) Atmosphere Relevant to a Novel Flash Ironmaking P
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A novel flash ironmaking process is currently under development at the University of Utah as a promising alternative ironmaking process that would greatly reduce CO2 emissions and energy requirement compared with ironmaking in a blast furnace.[1–3] In this process, pure H2 would be the most attractive reducing agent, although other H2-containing gases such as natural gas or syngas could also be used.[3] In the temperature range of 1500 K to 1900 K (1227 C to 1627 C) expected in the flash reduction process, H2 is the main reductant even if CO may be present when gases from hydrocarbon fuel resources are used. The last stage of iron oxide reduction by hydrogen, i.e., the reduction of wustite, is substantially limited by equilibrium, which means the equilibrium gas mixture contains a significant amount of unreacted hydrogen, and the reduction by CO has an even smaller equilibrium constant at these temperatures. For example, at 1673 K (1400 C), the equilibrium gas mixture contains about equal amounts of H2 and H2O, as shown below: FeO þ H2 ðgÞ ¼ Fe þ H2 OðgÞ K ¼ 0:97
½1
½pH2 O =ðpH2 O þ pH2 Þeq ¼ 0:49:
½2
ZHIXUE YUAN, Graduate Student, HONG YONG SOHN, Professor, and MIGUEL OLIVAS-MARTINEZ, Graduate Student, are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] Manuscript submitted: February 24, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
This requires the reaction to be carried out in an excess amount of hydrogen to drive the reaction forward. The equilibrium gas composition over a wide temperature range was calculated with the Outokumpu’s HSC software in Figure 1.[4] It shows the equilibrium gas composition over a wide temperature range and also indicates the possibility of re-oxidation as the reaction mixture cools down. Thus, the re-oxidation kinetics of reduced iron particles is of concern. This is a simplified equilibrium diagram without considering the nonstoichiometric nature of wustite. This does not present a critical problem as far as the process is concerned because the equilibrium constant of the reduction reaction is quite close whether FeO or Fe0.947O, representing a wide range of nonstoichiometry, is used as the reactant. Kaushik and Fruehan[5] reported on the re-oxidation of pellets of direct-reduced iron (DRI) and hot-briquetted iron in CO2. The fraction oxidized of DRI pellets can reach 50 pct at their highest temperature 1173 K (900 C) within 25 minutes in a 100 pct CO2 atmosphere. In the novel ironmaking process, however, CO2 accounts for less than 10 pct in the gas mixture produced by partially burning natural gas, especially in the presence of a higher CO partial pressure, and the temperatures at which re-oxidation would be of concern are much lower. Therefore, Fe re-oxidation by CO2 in flash ironmaking is expected to be much less significant. Nevertheless, this effect is planned to be investigated in further detail in this laboratory. Previous studies have shown that DRI (sponge iron) of large specifi
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