Predicting the thermal state of the blast-furnace hearth

Abstract

402 The thermal state of the blast furnace determines the fuel consumption in hot-metal production, the productivity, and the hot-metal composition. The change in thermal state of the furnace is estimated from the silicon content in the hot metal at discharge. Accordingly, predicting the silicon content in the hot metal during smelting is an important stage in regulating the furnace’s thermal state. Conventional methods of monitoring the thermal state of the hearth are based on the material and thermal balances of smelting in the lower part of the furnace; the time delay between direct and indirect reduction in the batch volume is taken into account [1]. The balance equations for predicting the silicon content in the hot metal include those items of the thermal balance that may be calculated from continuously monitored parameters. This entails continuous measurement of a large set of parameters with high metrological requirements and subsequent analysis of the data. However, such methods of monitoring the furnace state describe a process with constant quantity and characteristics of the batch, constant blast parameters, and constant thermal and physicochemical processes in the furnace. However, blast-furnace smelting is a nonsteady process, and correspondingly the familiar methods may only be used if the input and output items are averaged over certain time periods (as a rule, a few days). For more inertial processes, this period is extended. In controlling the ore load and the Si content in the hot metal, the inertia of the process is 4‐10 h (or as much as 14 h, according to Italsider data) [2]. One method of regulating the thermal state of the furnace is to predict the silicon content in the hot metal some time (6‐8 h) before discharge, by measuring the depth of the axial funnel within the batch surface in the charge hole and determining its mean depth for each hour and the variation in mean depth from hour to hour [3]. This method is only applicable for furnaces equipped with conical charging devices characterized by a batch surface that contains a pronounced axial funnel after each charging episode. At the same time, for furnaces with nonconical charging units, the axial gas flow is intensified by means of special charging programs, with the discharge of coke portions in the axial zone. When using such programs, the surface profile that forms is often either horizontal or a negative axial funnel (a pillar), and a relation cannot be established between the change in the axial funnel and the silicon content in the hot metal. Besides calculating the thermal balance at the bottom of the furnace, other approaches are possible, according to [4]. For example, we might look for direct characteristics of the thermal imbalance in the hearth. One possibility is the heat transfer at the tuyeres, determined by the heat in the gases within the tuyere zone, the temperature in the combustion source and the tuyere channel, and the state of the wall lining, which is also associated with the thermal state of the hearth. Monitoring of the thermal state of the hearth from the heat transfer at the tuyeres, with subsequent prediction of the silicon content in the next hot-metal discharge, has been introduced at blast furnace 8 at OAO ArcelorMittal Krivoi Rog. The radar system for measuring the charge profile in blast furnace 9 permits the calculation of the rate of descent of the batch surface over the charge-hole cross section under the action of the batch and gas-flow distributions, which, in turn, are related to the thermal state of the hearth. Statistical analysis of experimental data shows that the variation in silicon content in the hot metal from discharge to discharge is best related to the change in mean (over the cycle) charge, consisting of ten portions of batch materials, and the rate of batch descent in the axial zone of the furnace. Therefore, the calculated rate of descent of the batch surface in the axial zone may be used to predict the silicon content in the hot metal.