Mathematical simulation of direct reduction

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tion and desulfurization of relatively abundant, higher sulfur coals would seem to assure substantial supplies of coal gas being available in the not distant future. Coal gas, as initially produced, is rich in hydrogen and carbon monoxide which are natural reducing agents for iron oxides. In this respect coal gas has a distinct advantage as a reducing agent over natural gas (methane) which must be processed through a reformer to produce hydrogen and carbon monoxide. Thus, the factors of availability and suitability make it seem highly likely that the long-range future of steelmaking will be strongly based upon direct reduction of iron oxides using coal gas. In general, direct reduction utilizes either solid or gaseous fuels, depending on local conditions. Most solid-fuel processes are carried out in rotary kilns. In these, the ore is heated rapidly in an oxidizing atmosphere to eliminate sulfur pick-up, minimize sticking, and ensure easy temperature control of the reduction. Some direct reduction processes using gaseous fuels employ shaft-type reduction furnaces and reformed natural gas or coal gas. On the other hand, some processes are based on the fluidized-bed technique, in which, during reduction, reformed natural gas or waste gases from oil refineries are used to keep the particles of finely divided iron ore in suspension and to make them behave like fluids. Among these processes, the countercurrent shaft furnace has many advantages. The furnace is a vertical shaft. A charge of pelletized or lump ore is loaded into the top of the furnace and descends by gravity through the reducing gas. The gas is moved upward past the ore. Reduction takes place in the central and upper parts of the furnace at a temperature near 800 ~ The reduced sponge iron is then discharged from the lower part of the furnace and cooled to about 40 ~ prior to release to the atmosphere. The efficient gas-solid contact and heat exchange that can be obtained from this type of furnace leads to maximum productivity per unit furnace volume, and minimum consumption of fuel per ton of reduced iron. The objective of the present work is to develop a mathematical model of the direct reduction of iron ore in a countercurrent shaft furnace. The work is divided

ISSN 0360-2141/8110311-0111500.75f0 METALLURGICAL TRANSACTIONS B 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME

VOLUME 12B, MARCH 1981--111

into two parts: the reduction kinetics of single ore particles; the incorporation of these kinetics and energy balance equations into a model of a countercurrent shaft furnace and numerical calculation of illustrative results using this model. SINGLE PARTICLE KINETICS Iron has three solid oxides that are involved in the reduction process: hematite (Fe203), magnetite ( F e 3 0 4 ) and wustite (F%O). The atomic ratio, w, of iron to oxygen in wustite is shown by LP to vary from 0.95 along the wustite-iron boundary to 0.85 along the wustite-magnetite boundary at atmospheric pressure. It is generally agreed that the reduction of hematit

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