Efficient Management of the Charging of Blast Furnaces and the Application of Contemporary Means of Control Over the Variable Technological Conditions
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Under the conditions of unstable qualitative and quantitative compositions of charge materials, we propose an approach to the selection of programs of bell-less top (BLT) charging. It is based on the reduction of the number of working angular positions of the tray and on the shifts (from batch to batch) of the conditional geometric ridges along the radius of the furnace top. A positive experience of realization of this charging program is shown by an example of blast furnace No. 3 at the Enakievo Iron & Steel Works. We discuss the main specific features of mounting of stationary temperature probes with positive temperature coefficient (PTC) on the blast furnaces, as well as the specific features of temperature distributions along the radius of the furnace for various consumptions of the reduced fuel. The relationship between the temperature over the surface of charge bed and the content of pellets in the charge is established.
Keywordsblast furnace charging program Bell-less top (BLT) charging mass flow ore load bed height temperature probe quality of coke dusty coal fuel pellet content of the charge
The improvement of the modes of charging of burden materials in blast furnaces (BFs) and their rational distribution along the radius and circumference of the furnace top can be regarded as an important direction of elevation of the efficiency of operation of BFs. According to the modern ideas, the level of technical and economic parameters of the operation of BF is, to a large extent, determined by the efficient formation of the melting-stock column and gas flow. This is promoted by the application of bell-less top (BLT) charging systems in the BFs [1, 2]. In the recent years, the reconstructed or newly constructed BFs are equipped with BLT charging systems characterized, unlike the bell top charging systems (BTCS), by wide possibilities of control over the distribution of burden materials along the radius and over the circumference of BF .
The experience of development of the programs of charging for the furnaces equipped with BLT charging systems shows  that the batches of burden materials are traditionally distributed over almost the entire radius of the furnace top, whereas coke is charged with various cyclicities into the central zone of the furnace top with an aim of formation of axial vents (zones with high gas permeability). As a rule, it is customary to distribute burden materials over the working angular positions of the tray with a given nonuniformity, which enables one to rapidly change the ore loads and/or the volumes of materials in different zones without changes in the structure of the charging program [1, 3]. The indicated formation of charging programs guarantees the possibility of placing of the materials with the minimum redistributions of the charge mixture along the radius.
In the present work, we describe the results of practical mastering of the charging programs on the BF-3 at the Enakievo Iron & Steel Works (EISW). The furnace was put in operation in October 2011 and equipped, for the second time in Ukraine, with the BLT charging system produced by the Paul Würth firm . The indicated programs enable us to obtain new data on the influence of the modes of charging on the parameters of operation of the furnace under the conditions of unstable and low quality of burden materials and also to reconsider the available approaches to the development of rational charging programs and substantiate the possibility of application of the data of contemporary means of monitoring for the control over the processes of melting under the existing operating conditions.
Rational modes of charging of the BF under the conditions of operation with unstable low quality of burden materials and low supplied mass. The charging programs developed according to the concepts formulated at the Nekrasov Institute of Ferrous Metallurgy of the Ukrainian National Academy of Sciences (IFM) [1, 2] and realized on the BF-3 guaranteed stable operation of the furnace and acceptable technical and economic parameters of the blast-furnace smelting in the initial stage of the operation of the furnace because the quality of iron-ore raw materials and coke was satisfactory . However, starting from the second half of 2012, the quality of burden materials significantly worsen. The transition to the high-basicity agglomerate (the basicity of CaO/SiO2 increased from 1.2 to 2.2) and the unsatisfactory state of the equipment of the sintering plant, together with the low quality of agglocharge led to a significant increase both in the relative part of small fractions in the skip agglomerate and in the mean-square deviation of its basicity.
Moreover, as compared with the initial stage of operation of the furnace, the quality of coke also became worse. Together with the absence of possibility of using washing materials, this resulted in the increase in the number of burnt tuyeres. The existing conditions caused the operation of BF with low supplied masses and low bed levels (lower than 2.5 m). The mass of coke batches was 7.0–8.0 tons (while the maximum possible mass is as high as 10.5 tons), which corresponded to the thicknesses of the coke layer smaller than 0.40 m. At that time, the indicated parameters of charging made it possible to minimize the upper “suspensions” and to maintain the central distribution of gases. However, the intense burning of air tuyeres was preserved, the productivity of the furnace decreased, and the consumption of coke under these conditions of operation of the furnace was quite high. All these factors led to the reconsideration of the approaches to the creation of rational charging programs of the BLT systems and to the search of new (never applied) decisions.
Thus, as one of the specific features of the new charging program, we can mention the decrease in the number of working angular positions of the tray of the BLT system from 6–8 down to five in the course of discharge of each batch in a charging cycle. This enabled us to increase the thickness of layers of charging materials in the loaded sections along the radius of the furnace. In addition, in the proposed charging program, every subsequent batch in a charging cycle shifts the conditional “ridge” of charging materials relative to the conditional ridge formed by the discharge of the previous batch, which leads to changes in the direction of motion of the gas flow.
For the analysis of charging programs and the estimation of the distribution of materials along the radius of the furnace obtained by using these programs, it is customary to use the commonly accepted parameters of the distributions, such as the volumes and OL in a charging cycle of the furnace in ten zones of the furnace top with identical areas [1, 2]. The given distribution of these parameters along the radius in a charging cycle can be guaranteed by numerous versions of charging programs. In the development of a new charging program for the operating conditions of the BF-3 at the EIMW, we performed the analysis of both mentioned programs in order to find their distinctive signs and specific features. In this case, we used the parameter  of computed OL in ten zones of the furnace top with identical areas in a charging cycle after the discharge of each pair of batches of coke and iron-containing materials (in what follows, we write: after the discharge of each supply). By using this parameter, we can estimate the changes in the OL for each zone in the structure of the melting-stock column formed by the discharge in a charging cycle.
time of the stay of gases in the furnace.
After changing the charging program, the operation of the BF-3 was first characterized by the unstable chemical heating of cast iron with changes in the content of silicon from 0.21 to 1.01% ([Si]av = 0.62%) and in the content of sulfur (0.026–0.269%, [S]av = 0.072%), which can be explained by the renewal of the coke head caused by the reconstruction of the melting-stock column in the furnace. After this, the descent of the mixture became more stable without “bridgings” and “breaks” and the temperature of peripheral gases (after stabilization of the working level of the bed (1.3 m)) decreased by 65°C (since the amount of hot agglomerate in the charge increased, the service life of the tray of BLT system became longer). The temperature of the top gas insignificantly increased from 295 to 310°C.
After changing the charging program, the intense burning of tuyeres was terminated and the operation without upper “suspensions” but with a bed height of 1.3 m accompanied by relatively stable upper pressure drops became possible. In order to reduce the operation of the BF-3 to the identical working conditions prior to and after the replacement of the charging program, we determined the influence of technological factors on the specific consumption of coke  and found that the reduced consumption of coke decreased by 2% for the new charging program .
Thus, the developed charging program was successfully used as the basic program in the BF-3 since February 2013 with some corrections aimed at guaranteeing the developed central distribution of gases in the case of changes in the raw-materials conditions. On the basis of this program, we also developed and used some corrected versions of the charging program: a version for the blowing-up of the BF after scheduled shutdowns, which differs from the basic program by a more “disclosed periphery” (lowered ore loads in this zone) and a version of charging program for the elevated (more than 50%) contents of pellets in the charging mixture, which ensures their minimum concentrations in the peripheral and central zones of the furnace top.
By using the proposed approach, we developed and realized charging programs with the use of the technology of injection of the dusty coal fuel (DCF), which was realized in the BF-3 of the EIMW in 2016 . The main modification of the charging program in the case of application of the DCF is the decrease in the number of working angular positions of the tray of the BLT system for the discharge of coke from five to four and their increase from five to six for the discharge of the batches of iron-containing materials. These changes are explained by the operation of BF with elevated mean ore loads in the case of injection of the DCF and are directed toward guaranteeing both a sufficiently high gas permeability of the layers of charging materials along the height of the column and the intensification of the mutual overflow of gases between the central and peripheral zones of the furnace .
Application of the data on temperature over the burden bed surface for the control over blast-furnace smelting. For the efficient choice and analysis of the charging modes, we need reliable data about the distributions of gas flows along the radius and circumference of the BF. As a rule, it is estimated according to the readings of thermocouples of the stationary temperature probes and/or by the chemical composition of gases obtained with the help of gas-sampling machines [1, 8, 9, 10]. In recent years, the distributions of gas flows are more and more extensively analyzed with the help of temperature probes mounted over the burden bed surface in the furnace along one or several diameters of the furnace top. Unlike gas-sampling machines, their important advantage is the continuity of measurements, which allows one to use the information obtained with the help of temperature probes for the rapid evaluation and correction of the charging modes of BFs.
As the main disadvantage of temperature probes, we can mention the incorrect information obtained for lowered working bed levels. In these cases, the incorrectness of the accumulated information on the distribution of temperature of the gases along the radii of the furnace is explained by mixing of the gas flows with different chemical compositions and temperatures in the created extended zone of the furnace from the burden bed surface to the temperature probes. This reveals higher efficiency of the application of temperature probes, especially for the control over the radial distribution of charging materials in BFs equipped with BLT systems, where the variations of the bed level are not used as a tool for controlling the distribution of the charging mixture.
In order to get reliable and stable readings, the temperature probes should be placed at a distance of ~0.50 m from the bed surface . As a rule, to get almost identical distances from the surface of the charging mixture to the thermocouples along the radius of the furnace, the temperature probes are placed with a certain slope to the horizon with an aim to form a funnel-like profile of the burden bed surface in the BF. However, this requirement is actual only in the case of formation of this surface with the help of BTCS and is not necessary for the equipment of the BFs with the BLT charging systems for which charging programs include the discharge of the batches of coke into the axial zone of the BF.
As a parameter of evaluation of the distribution of burden materials over the radii of the BF required for the decision-making concerning the correction of charging modes, we take ore loads in each cross section of the furnace radius. The distribution of ore loads over the radius is closely connected with the distribution of CO2. This relationship has the form of a straight line . The procedure of estimation of the distribution of ore loads according to the information obtained by temperature probes is based on the experimentally established relationship between the distribution of CO2 and the temperature of gases over the burden bed surface along the radius of the furnace. This relationship is described by the inverse dependence . However, the indicated relationship is violated after the discharge of a coke batch into the axial zone of the furnace and in the case of lowering of the working bed level. The surface temperature of the charge also depends on the thickness of the layer and the heat capacity of materials in annular zones of the furnace . Hence, prior to undertaking managing actions, it is necessary to accumulate objective information about the variations of temperature of the burden bed surface with the use of special algorithms of data processing .
As a negative feature of temperature probes characterized by the contact surface with the flux of a charging mixture unloaded from the tray of the BLT charging system with a width greater than 200 mm, we can mention the formation of cavities under the temperature probes in the furnace whose depth can be as large as 0.5 m and even larger. This results in a nonuniform distribution of the charge and gas flows over the circumference of the furnace . Two temperature probes of this kind were mounted in the BF-3 EIMW in October 2011. The results of prestart investigations  revealed the presence of distortions of the distribution of burden materials over the circumference of the furnace introduced by the temperature probes. According to the recommendation of the authors of the cited paper, four new nitrogen-cooled temperature probes of domestic production with a width of the contact surface smaller than 200 mm were mounted in the BF-3 after the major overhaul of the 3rd class in June 2014. One more positive structural feature of the mounted temperature probes, unlike the structures of the major part of probes produced abroad, is connected with the possibility of objective control over the degree of development of the peripheral gas flow: the distance between the wall of the furnace top and the extreme peripheral thermocouple is 50 mm, while the distance to the second thermocouple in the near-wall zone is 310 mm.
Some works [9, 13, 14] give contradictory information about the influence of cooling of the temperature probes on their readings. Thus, the investigations of the influence of cooling on the readings of temperature probes carried out in  by means of periodic disconnections of the supply of nitrogen did not reveal any significant distortions of readings of the thermocouples, which contradicts the conclusions made in . According to , the procedure of cooling of temperature probes affects solely the readings of the extreme near-wall thermocouple due to the presence of released nitrogen. These assertions require additional experimental verification aimed at the analysis of the influence of short-time disconnections of the supply of cooling agent. For the objective estimation of the temperature of peripheral gas flows, it is necessary to use the mean values of two extreme thermocouples in the near-wall zone of the BF. At the same time, the use of cooling is especially urgently required in the case of operation of the BF on the hot agglomerate, when it is impossible to get the objective information in the case of operation of the temperature probes without cooling. In this case, it is worth noting that the practice of using hot agglomerates is extensively applied in the major part of BFs in Ukraine.
To estimate the influence of technical and economic parameters of smelting on the distribution of temperature of the burden bed surface for each day of the analyzed period of operation of the BF-3, we determined the consumption of fuel reduced to coke taking into account the amount of coke nuts used in the charge and the amount of injected natural gas. In Fig. 4, we present the distribution of the surface temperature of the charge over the furnace-top diameters for two periods of operation of the BF-3 with minimum (06.29–30.2014; see Fig. 4a) and maximum (07.03.2014, see Fig. 4b) consumptions of fuel. In the first period, the consumption of fuel was lower than in the second period by 3.5%. At the same time, the temperature on the furnace axis was higher as compared with the period with the maximum consumption of fuel by 128°C.
Thus, it is established that the increase (decrease) in the value of ωp in the PPh zone of the furnace by 1% corresponds to a decrease (increase) in the temperature over the burden bed surface by 3–8°C. This feature can be explained both by the influence of variable amounts of hot agglomerate (local production) and by the increase in the amount of FeO in the primary slag melt with the content of pellets, which is connected with high amounts of heat spent for the reduction of iron. This results in changes in the conditions of melting and reduction, which leads to a decrease in temperature over the burden surface. Thus, the information obtained from stationary temperature probes allows one to monitor the content of pellets in the peripheral zone of the furnace, which is of great importance for the formation of the protective scull in the furnace.
Conclusions. Under the conditions of unstable qualitative and quantitative compositions of the components of charging materials, we propose a new approach to the rational choice of charging programs for the BLT charging systems. This approach is based on decreasing the number of working angular positions of the tray and the shift of conditional geometric “ridges” along the radius of the furnace top from batch to batch. The positive experience of realization of this charging program is demonstrated by an example of the BF-3 at the Enakievo Iron & Steel Works, where this program was used, with some modification, for more than four years and guaranteed, together with the other measures, the required or higher technical and economic parameters. On the basis of the proposed approach, we also developed and realized special versions of charging programs for the blowing-up period of the furnace, for the modes of operation with high contents of pellets in the batch composition, and for the conditions of injection of the dusty coal fuel. We determined the specific features of the variations of temperature over the burden bed surface depending on the content of pellets in the near-wall zone, which gives us a possibility of subsequent application of the information obtained from temperature probes for the operative management of the distribution of charging materials in the process of formation of the protective scull in the furnace.
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