Study of transient processes in a blast furnace based on the heat exchange scheme analysis

A blast furnace is a complicated metallurgical facility, which is characterized by considerable delay and inertia in the flow of heat and mass exchange. Therefore, the analysis of transient processes based on modern ideas about heat transfer is an important issue in solving technological problems of blast furnace smelting managing. A two-stage heat transfer scheme along the height of a blast furnace of modern technology presented. When studying the thermal state of a blast furnace as a control object, it is advisable to divide it into two thermal zones - the upper zone and the lower zone. The border between the zones is located in the upper part of the mixed reduction region, between the start level of coke carbon gasification and the horizon below which iron oxides are directly reduced. It was shown, that the upper and lower thermal zones have fundamental differences in heat exchange conditions and are interconnected through the index of iron direct reduction degree. The transient processes of silicon variation in the hot metal studied at variation of iron ore load, natural gas flow rate, temperature and humidity of the hot blast, oxygen content in the hot blast and slag basicity. It was shown that the oscillatory transition process is observed in case, after applying the perturbation, it will have the opposite effect on the thermal conditions of the lower and the upper stages of heat exchange in the blast furnace. The iron ore load, hot blast humidity and slag basicity were found to be the most predictable input parameters affecting the concentration of silicon in hot metal. Change in oxygen concentration in hot blast and natural gas consumption have an alternating character of influence on thermal conditions of the blast-furnace hearth. At that, the characteristics of the transient processes of blast furnaces through various channels of action vary and depend significantly on the properties of the smelted raw materials, design and operational parameters of the furnaces

Effect of the natural gas hydrogen on variation of the heat and reducing processes along the blast furnace radius

The expenses for the blast furnace coke are one of most significant part of the hot metal cost. To save the coke, various technologies are used, capable to replace the coke by cheaper additional fuel (AF), in particular by natural gas (NG). The injection of considerable volumes of NG results in an increase of hydrogen share in the blast furnace gases and in a significant variation in the blast furnace technology. Study of peculiarities of such variations is necessary to use the NG more effectively. Based on the mathematical model of the blast furnace process, estimation of the effect of natural gas hydrogen on changes in the heat and reducing processes along the blast furnace radius was accomplished. A formula was elaborated, confirmed by practice, for calculating the degree of hydrogen usage ηН2 along the radius of the furnace. It was determined, that the reducing action of hydrogen along the furnace radius takes place unevenly –decreasing from the periphery to the axial zone of the blast furnace. To estimate the quantitative relations of the reducing action of hydrogen, parameters of the PAO “MMK” and PAO “ArcelorMittal Krivoy Rog” blast furnaces for a long period of operation were analyzed. It was determined, that in the axial and intermediate zones of a blast furnace, values of criterion RН2, designating the oxygen share in the burden removed by hydrogen, are in the range of 0.11–0.16 and weakly depend on the NG consumption. In the peripheral zone near the walls, the value of R Н 2 sharply increases to 0.22–0.27. In this zone of the blast furnace the quantity the burden oxygen, removed by hydrogen, accounts for 80–85%. Therefore, hydrogen accomplished the heat and reducing processes mainly in the peripheral zone of the furnace. At the NG consumption increase, the ore load should be increased for the peripheral zone, near the walls individually accounting hydrogen action along the furnace radius. This will make possible to increase the degree of hydrogen usage and decrease the coke consumption.

Benefits and risks of blast furnaces with high intensity

Increase in the intensity of blast furnace smelting at NLMK JSC is achieved by improving quality of coke and iron ore materials, by increasing pressure under the blast furnace mouth and by oxygen enrichment. It is accompanied by an increase in the rate of wustite indirect reduction and decrease in specific heat losses with cooling water. However, the risks of burden yield problems are significantly lower with high intensity. It has been established that with the change in quality of charge materials, reason for burden yield problems can be also the variation in ore load ratio along the furnace radius. A new method for diagnosing causes of burden yield problems was developed. Using method of mathematical modeling, effect of change in ore load and size of iron-ore materials on peripheral gases temperature change and the degree of CO use were analyzed. The method is based on analysis of joint change in gas temperature and degree of CO use in peripheral area. Analysis of peripheral gases temperature variation and degree of CO use makes it possible to identify reasons of pressure drops along the furnace height. As a result of analysis of pressure drops dynamics changes along the furnace height, there have been found signs that can be used to judge the probability of burden yield problems. The possibility of reducing pressure drops due to redistribution of ore load along the furnace radius and the amount of gases is considered. It is shown that various methods of reducing the amount of gases are accompanied by different changes in coke consumption and furnace productivity. Mathematical model has been developed to select the best combination of parameters changing of the fuel-enriched blast in specific conditions.