The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part I. The role and kinetics of volatile
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I. INTRODUCTION
WITH the world’s iron and steel demand on the rise and the growing restriction of coke as the reducing agent due to environmental and economic factors, there has been an increased interest for the implementation of new ironmaking technologies with less capital investment. Natural gas–based processes such as Midrex and HYL are relatively proven technologies with a combined market share of 90 pct for direct reduced iron (DRI)/hot briquetted iron (HBI). Since these processes can only be used where natural gas is relatively inexpensive, a family of coal-based processes using a rotary hearth furnace (RHF) such as FASTMET, INMETCO, Sidcomet, and ITmk3 have been developed. These RHF technologies combine low material costs with simplicity and flexibility of operation. The unprocessed coals may contain large amounts of volatiles, and the evolution of volatiles consumes the energy needed for the reduction of the iron oxides. The amount and type of volatiles that evolve varies with the rank and heating rate of the coal.[1,2] At low temperatures of up to 350 °C, superheated water vapor evolves, and from 350 °C to 600 °C, light gases of CO, CO2, H2, CH4, and C2H6 occur. Above 600 °C, small amounts of complex hydrocarbons are observed. Thus, in the devolatilization of high- and medium-volatile–containing coals, the majority of gases evolved have been found to be H2, CO, H2O, CO2, and CH4, where the possible reducing species are CO, H2, and CH4. Previous studies[3,4] done with volatiles for a single-layer RHF have shown that the short retention time of the volatiles in the pellets and the lower temperatures at which these volatiles are released results in negligible amounts of reduction by the volatiles. However, with multiple layers of three or more, the bottom layers are at a lower temperature and retain their volatiles longer compared to the upper layers. This allows the delayed release of volatiles to react with the I. SOHN, Graduate Student, and R.J. FRUEHAN, Professor, are with the Materials Science and Engineering Department, Carnegie Mellon University, Pittsburgh PA, 15213. Contact e-mail: [email protected] Manuscript submitted February 8, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
top layers, which are at higher temperatures when the kinetics are favored. Thus, by simulating the individual reducing gaseous species, it may be possible to gain some information on the effects of volatile reduction. In addition, although the RHF process uses pellets, the intrinsic rates of the reduction with the simulated reducing gases must be measured, and in this study, the reduction of iron oxide powders has initially been investigated. The emphasis will be on the first 50 pct of reduction by H2, where reduction by volatiles will occur, and the effect of H2S, CO, and carbon is compared to the role of H2 in the temperature range of interest. For higher reduction degrees, devolatilization will have been completed, and the reduction by carbon will be dominant. II. EXPERIMENTAL The rates were measured using standar
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