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Coke quality and blast furnace performance
Abstract
The blast furnace is a counter-current reactor. The reduction gases must flow from the raceway to the blast furnace top against the descending burden column and the liquid products hot metal and slag have to vertically drain off against the up-streaming gases to the hearth and on a horizontal way to the taphole. Solid and big-sized coke with sufficient free voidage guarantees the gas permeability in the total burden column and the drainage for the liquid products in the lower furnace.
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The charge of HBI (Hot Briquetted Iron) to the blast furnace is an option to reduce reductant consumption and increase productivity of the blast furnace. A blast furnace model is used to calculate the metallurgical, ecological and economic aspects of this blast furnace operation mode with the charge of 100 kg HBI/t HM. The reductants decreasing effect of HBI charge is considered in two cases, decrease of coke rate under constant coal injection rate and constant coke consumption with decreased coal rates. As a general result it can be said, that from a metallurgical point of view it makes no sense to use pre-reduced material in the blast furnace and that the ecological potentials of such a proceeding are marginal. Possible advantages exist for those works with clear bottlenecks in hot metal production.
The injection of different auxiliary reducing agents into the tuyeres of a blast furnace has fundamental effects on the metallurgical reactions and on the total energy and material balance of the process. For a comparison of the different tuxiliary reducing agents it is of utmost importance to determine the changes of all balance values and to take all effects on the hot metal costs into consideration with an overall approach. Considering the prices for all input materials and the estimated prices for the products of the blast furnace different solutions are possible to realize, in dependence on the circumstances, an optimization in the process and plant technique, a minimization of the hot metal costs and therefore integrated approaches.
There are principally four routes for steelmaking. Three are based on iron ore reduction via blast furnace, smelting reduction and direct reduction; one is based on melting steel scrap via the electric arc furnace. Blast furnaces and smelting reduction plants produce liquid hot metal and separate the main amount of the charge materials gangue components via a slag. Direct reduction plants reduce iron ores in the solid state to sponge iron (DRI, HBI) whilst the gangue components remain in the product. The report describes the process developments and current status of iron ore reduction routes as well as some future ideas.
The growing production of hot metal - in due relation to the demand - forms the basis of the world's increasing demand for steel. In this context the blast furnace process will remain the number one ore reduction process. In Germany hot metal and crude steel are produced on a competitive basis at several integrated steel works using blast furnaces and basic oxygen converters which are world leading in terms of plant technology and operation. Within the framework of research programs, the development of an oxygen blast furnace process is underway, which - in combination with a CO2 capture and storage (CCS) system - is expected to massively reduce CO2 emissions from hot metal making. Direct reduction processes produce solid direct reduced iron (DRI) from iron ore without the use of coke. DRI is mostly used as input material in electric steelmaking. The majority of the DRI is produced in gas-based processes, especially at locations where low-cost natural gas is available. The intention of coal-based smelting reduction processes is to produce liquid hot metal without or only with very small amounts of coke. Of the various technologies, the Corex and the Finex processes have so far been applied to industrial-scale production. Whereas the Corex process relies on the use of lumpy ore charge, Finex can be operated with fine ores. Both processes will require the CCS technology to achieve a substantial reduction in CO 2 emissions.