Main Features of a Modern High Capacity Blast Furnace
Main Features of a Modern High Capacity Blast Furnace
The performance of an integrated steel plant is highly dependent on the performance of its ironmaking units. In integrated steel plants, production of hot metal (HM) in a blast furnace (BF) is a preferred route globally for ironmaking. The specific cost of steel production is highly dependent on the specific cost of HM in the BF. Hence, BF is a very important unit of a steel plant.
Modern BF is a high capacity BF having large useful volume. It has many advanced technological features. Because of the advanced technological features, it has higher campaign life and better production indexes. It has larger indirect reduction zone and smaller specific surface area, which is beneficial to improve gas utilization efficiency, decrease heat loss, and reduce fuel rate. The coke rate, coal rate and fuel rate of the modern high capacity BF is usually low since it has an integrated technology for low carbon operation.
Modern high capacity BF has a higher productivity determined as tons of hot metal (tHM) production per cubic meter of useful volume per day and lower specific fuel consumption. Hence it has lower specific cost of HM production and lower investment per ton of HM capacity. This is since modern high capacity BF uses several modern state of the art technologies and has features for ensuring quality of raw materials which is charged in the BF, smoother operation of the BF, and better utilization of the gas thermal and chemical energy to reduce fuel rate.
Modern high capacity BF incorporates many state of the art technologies. These technologies include raw material preparation technologies and the technologies related to the improvement of the utilization efficiency of BF gas. In addition, the modern high capacity furnaces have many advanced features. The major advanced technologies and the features of a modern high capacity BF are shown in Fig 1 and described subsequently.
Fig 1 Major advanced technologies and the features of a modern high capacity blast furnace
Raw materials preparation technologies
Major raw materials preparation technologies include (i) use of sinter with low silica content and high reducibility, (ii) optimizing of coal blending for coking of coals, (iii) blending technology for raw materials, (iv) control of load of harmful elements, (v) screening technology of raw materials, (vi) utilization of smaller size of sinter, (vii) utilization of nut coke, and (viii) injection of dust of coke dry quenching.
Use of sinter with low silica content and high reducibility Sinter having low silica content has high content of iron (Fe) and hence it reduces the slag volume in the BF. It has improved metallurgical properties especially its softening characteristics and its use in the BF shifts the position of cohesive zone downward and thus reduces the thickness of the cohesive zone, promotes the indirect reduction, and improves the permeability of burden column. Statistically silica content of the sinter is to be around 5 % to 5.5 %. The specific technologies for the production of sinter of low silica content and high reducibility include the following.
Increase of the basicity of the sinter is needed to the desired level. Since the amount of bonding phase is decreased due to the reduction of the silica content, the binary basicity of sinter is to be raised appropriately to increase the CaO (calcium oxide) content, thereby increasing the content of calcium ferrite, which is beneficial to maintain necessary amount of bonding phase and to improve the reducibility of the sinter.
Improvement in the ratio of the fines and micro-fines of the materials in the sinter mix has desirable effect on the bonding phase. The bonding phase originates in small size fines, which can promote the solid phase reaction and the generation of sintering liquid phase.
The type and characteristics of iron ores have important impact on the formation of calcium ferrite and the compactness of the sintering mix. Based on the values of the sintering characteristics of iron ores, the suitable sintering phase can be formed by appropriate selections of the types of iron ores, which can satisfy the requirement of bonding phase amount for producing low silica content sinter while meeting the requirement of high reducibility sinter.
Increasing the height of the sintering bed in the sintering machine has many advantages. It has self-regenerative effect of the sintering bed and hence reduces the solid fuel consumption and overall heat consumption. Thus, the sintering at high temperature drops, oxidizing atmosphere is strengthened, the FeO content of sinter is decreased, the calcium ferrite content is increased, and the reducibility of sinter is improved. This is beneficial in improving the size fraction of the sinter with low silica content.
Optimizing of coal blending for coking of coals Modern high capacity BF needs coke with higher quality. There are several requirements for the quality of coke needed for the modern high capacity BF. The requirements of the coke for the maintenance of the burden column in the lumpy zone and the maintenance of the permeability in the hearth area are materially different since the volume of the BF is high. With the increase the BF volume, hearth diameter is higher, ore batch is increased, load on the coke is increased, and hence coke of higher cold strength is needed. The activity of modern high capacity BF hearth has higher impact on yield, smooth operation, pulverized coal injection and tapping, thereby the higher requirements for improving the degradation of the coke in the BF and for ensuring the size of the coke which is needed before the tuyeres and in the deadman area.
The increase of the injection rates of the pulverized coal has drastically lengthened the residence time of coke in the BF. Hence, it has also increased the chemical and physical stresses on coke. With pulverized coal injection (PCI) rates of 200 kg/tHM to 250 kg/tHM, the residence time of coke is twice as long compared to a case without the injection of the pulverized coal. An increase of coke porosity and a decrease of strength in the lower parts have been observed when increasing the PCI rates. Hence, the requirements for coke quality are significantly higher in modern high capacity BF with high injection rates of pulverized coal.
These days for controlling the cost of metallurgical coke, several types of coals with different proximate analysis and with different coking properties are used in a blended form for the production of the BF coke. In some plants, coal blends even contain a small quantity of non-coking coal. For the production of BF coke having the required strength and thermal properties which is needed for the modern high capacity BF, there are requirements to be met for the selection of coals for the coal blend and there is a need for the uniform blending of coals before the coking of coal blend.
Blending technology for raw materials – The fluctuations in the Fe content of the ferrous burden and in the slag basicity cause unsteady operation of the BF and it increases the coke rate and decreases the output from the BF. These days, with the modern large capacity BF there is built a raw material yard to carry out the storage, blending and bulk handling functions for reducing the fluctuations in the composition of the ore, sinter or pellet. The moisture content and the size fraction of the raw materials being charged in the BF are also to be controlled for meeting the requirement of the BF. Also, the varieties and proportion of raw material are adjusted as per the production requirement of the BF. Also, the entire process of the raw material preparation is to adapt to the changes in supply of raw materials. The entire process is automated by utilizing the ore intelligent model. This model programs the general plan the mixing of ore, dynamic allocation of raw materials into dosing tanks, and intelligent control of the cutting velocity of the constant feed weighers for ensuring uniform composition of the ore burden to be charged in the BF. The standard deviations are controlled in percentage of silica at a level of plus/minus 0.125 % and in the percentage of total iron at a level of plus/minus 0.375 %.
Control of load of harmful elements – The load of harmful elements is required to be controlled since the accumulation of harmful elements in the BF damages refractory lining. This, in turn, leads to frequent fluctuations in the thermal load and results into an unstable furnace. Mainly, the harmful elements cause degradation of coke, destroy the load bearing function of coke, and result into higher coke rate. Investigations carried out in one of the high capacity BF has shown that under alkali load of around 4 kg/tHM and zinc load of around 280 g/tHM, the degradation rate of coke size located at 0.5 m to 2.5 m above tuyere is around 70 %, which means that the coke is highly degraded. Hence, special attention is needed in high capacity BF for the control of the alkali load to a level which is below 2 kg/tHM, and the zinc load to a level which is below 150 g/tHM.
Screening technology of raw materials BF has a vertical moving column of raw materials which needs good permeability for the smooth operation of the furnace. Hence, the charging of fines is required to be controlled by the continuous screening of the burden and this means that a close control on the screening operation of the burden materials of the BF. The proportion of materials less than 5 mm in the BF burden is not to exceed 5 %. The reduction in charging of burden materials with a size of less than 5 mm also reduces the dust content of the top BF gas. Every 1 % decrease of charging of the fines in the BF reduces the coke rate by around 0.5 %.
Utilization of smaller size of sinter – Utilization of smaller size of sinter increases the yield of sinter and reduces the fuel rate at the sinter plant. In addition, the utilization of smaller size sinter also reduces the circulating load in the sinter plant. Normally sinter is charged in the modern high capacity BF in two size ranges consisting of (i) greater than 10 mm, and (ii) in the range of 4 mm to 10 mm. The 4 mm to 10 mm fraction is charged at the periphery for better utilization efficiency of the sinter and improvement in the gas generation as well as reduction in the fuel rate.
Utilization of nut coke The size range of nut coke is generally 10 mm to 25 mm. Nut coke is normally charged with the ore burden in the BF. Charging of nut coke in the BF improves the utilization of energy in the ironmaking. The practice of charging nut coke in high capacity BF has shown that it is beneficial to the smooth operation of the BF and has distinct effect on reducing the fuel rate.
Injection of dust of coke dry-quenching A substantial amount of coke dust is generated during the dry quenching of coke. This coke dust has significantly higher calorific value (CV) than the CV of coal used for PCI. Around 8 % of coke dry quenching dust can be added in the raw coal used for PCI. This improves the fixed carbon content of the coal used for injection and results into reduction in the fuel rate. Around 4-5 kg/tHM consumption of coke dust can be achieved with suitable adjustments in pulverizing and injection processes.
Improvement of the utilization efficiency of BF gas
BF is a continuous reactor where the burden materials are charged in alternate layers of ore and coke intermittently. This layered structure is retained as the burden materials descend through the furnace. Burden distribution refers to the achievement of proper arrangement of the layers of different materials inside the furnace and mainly to the radial distribution (as axial symmetry is usually desired). The various burden materials charged into the furnace are very different from each other. Ore is around four times heavier than coke and the particle size is 2 to 4 times smaller, which affects the gas permeability and heating of the charged layers. As the reducing gas rises from below, it encounters the burden layers which re having very different level of permeability. Hence, the radial distribution of ore and coke is an important factor governing the gas flow distribution in the furnace.
Generally, the fraction of ore of the total volume or mass is used to quantify the material distribution. The (radial) region with higher fraction of ore results in a lower gas flow. In some operating prctices, higher gas flow at the centre of the furnace is preferred, because it is effective in decreasing discontinuous motion of the solid burden, resulting in smoother BF operation. Hence, batches of large-sized coke, known as centre-coke, or larger sinter and lump ore are charged near the centre of the furnace to improve the gas permeability in the region. BF with bell less charging is equipped to charge coke directly into the furnace centre. However, higher gas flow also results in higher gas temperatures as the gas does not have enough time for heat exchange and the thermal flow ratio (defined as the heat capacity ratio between burden and gas) is low.
The regions with higher gas temperature usually correspond to a higher cohesive zone level. Hence, the temperature readings of the above burden probe are important indicators of the burden distribution inside the furnace. As the burden descends into the furnace, the ore is reduced and at around 1200 deg C (depending on the quality of ore), it starts to soften and eventually melts at around 1350 deg C. Coke, on the other hand, maintains its form (except the quantity consumed by the solution-loss reaction) until it reaches the tuyere level. The semi-molten portion of the burden is extremely impermeable to the gas flow, so the gas has to flow through more permeable regions, coke slits, in the cohesive zone where it changes to more horizontal direction, until it reaches the lumpy zone. If the coke slits are blocked or not pervious enough, furnace irregularities such as hanging or erratic burden descent can occur. The burden distribution has a major role in affecting the size of coke slits in the cohesive zone. It also influences the formation deadman zone in the furnace as well as the wear rate of the lining of the furnace by controlling the gas flow and thus the heat losses. Most of the high capacity BF operation practices focus on the growing lack of high-quality burden materials and in the improvement of the furnace efficiency. These new practices require very precise control of the burden distribution which is carried out by accurate modeling and fast calculations. Thus, simulation of the burden distribution is becoming an increasingly important necessity for the smooth operation of the furnace. In addition, high coal injection rates through the tuyeres in BFs reduce the coke rates in the furnace, so the thickness of the coke layers is becoming further less. All this requires precise control of distribution of the burden materials for allowing sufficient permeability in the furnace and suitabe locating of the coke slits in the cohesive zone.
In a BF, several reactions are taking place due to the counter-current movement of the burden materials and the gases. Hence, the BF operates efficiently when there is smooth downward movement of the burden materials and balanced distribution of the gas flow. This results into improvement in the gas utilization efficiency and reduction in the fuel rate. Improvement of gas utilization efficiency can be achieved due to the full utilization of the thermal and chemical energies of the gases. The gas utilization efficiency of the modern high capacity BF is generally high and in many of the BFs it is above 50 %. The primary means for the improvement of the gas utilization efficiency and smelting of the ferrous burden materials in the BF is achieved through the appropriate distribution of the burden in the furnace which in turn adjusts the gas flow distribution.
Adjustment in the upper part of the BF It is achieved by the control of the charging of the burden materials. The charging of the burden materials can be through (i) central charging, (ii) peripheral charging, or (iii) controlled radial charging in order to achieve rational gas flow distribution. The control of the charging of the burden materials constitutes (i) mode of charging, (ii) weight of the batch, and (iii) the maintenance of the level of the stock line.
Mode of charging needs regulations for improvement of gas utilization efficiency. The batch of the material and its size and charging sequence determine the depth of material in the furnace. Generally, when the rate of the PCI increases, there is an increase in the ore to coke ratio due to the decrease in the weight the coke batch thus narrowing down of the coke window in the furnace. Due to it, the ore/coke ratio at the boundary of lumpy zone increases, thus the gas flow distribution is affected, leading to deterioration of the permeability and increase of pressure difference. Hence, for adjusting the central gas flow, the central charging of coke need to be increased and the depth of the coke window need to be increased. Also, at the periphery, the charge need to be properly adjusted either by increasing the coke amount or decreasing the ore amount. Overall, the central working of the furnace is to be improved and the restricting of the amount of the ore rolling to the centre is to be ensured, to avoid the obstruction of central gas flow, deterioration of smooth operation, and increase in the fuel rate. The central charging of the coke and the depth of the coke window is to be determined as per the condition of the BF.
In the large capacity BF, the smelting period extends and the degradation of coke is more serious and because of it, the gas flow increases. As the cross sectional area increases, the volume of deadman zone increases and the uniformity of gas flow distribution deteriorates. Hence, there is a requirement to strengthen the central gas flow and to increase the height of the inverted V shaped cohesive zone for ensuring enough area of coke window and smooth gas flow. The mode of central coke charging can enhance and stabilize the central gas flow, reduce the solution loss of central coke, prevent the degradation of coke, suitably increase the height of the cohesive zone, and ensure the rational gas flow distribution and smooth furnace operation. Hence, the high capacity BF can adopt central coke charging mode when necessary, but at the same time is to pay attention that it is not done excessively.
The biggest advantages of the regulated mode of charging are the higher gas utilization efficiency and lower fuel rate, but it requires higher and uniform quality of the burden materials. While the biggest advantages of the central coke charging mode is that it adapts the fluctuation in the quality of the burden materials, but with a big disadvantage of lowering the gas utilization efficiency which in turn results into higher fuel rate.
The weight of ore batch has significant impact on the burden distribution at the throat of the furnace, and has some influence on the gas flow distribution. The batch weight has different control range for different BFs. With the increase of BF volume, the throat area increases and hence the weight of ore batch need to be increased accordingly. Hence all the equipments at the charging side of the BF are required to be designed and sized to suit the higher ore batch weight. The larger ore batch is beneficial to furnace stability, improves gas utilization efficiency and reduces the fuel rate. Thus, from the point of view of stabilizing coke layer in the cohesive zone and reducing the change of gas flow distribution, the upper adjustment in the BF is to stabilize the coke batch weight for a suitable coke depth and accordingly change the ore batch weight.
The maintenance of the stock line level is carried out by adjusting the falling height of the burden to change the position of depositing of the peak, and in combination with the initial angle of the chute. With the different stock line level, the burden distribution at the surface is different and this has a high influence on the gas flow distribution. The stock line level has relation with the profile of throat, profile of upper shaft and the properties of the burden materials. In practice, the control of stock line level is to be combined with the bell-less chute charging angle, and the initial falling point is to be located within 300 mm from wall.
Adjustment in the lower part of the BF – The adjustment in the lower part of the BF is carried out by the adjustment of the blast parameters to control the combustion zone of the tuyere which has effect on the initial distribution of gas flow in the furnace. It is crucial for controlling the smooth furnace operation, rational gas flow distribution and improvement of gas utilization efficiency. A rational initial gas flow distribution can be achieved through the adjustment in the lower part of the BF. It is done by the control of the blast volume and control of the kinetic energy of the hot air blast.
The control of the blast volume is done to influence the bosh gas volume. When the bosh gas volume is small, with the increase of blast volume and the movement in the BF improves thus there is decrease in the fuel rate and coke rate. After the movement in the BF is achieved to a certain level then the bosh gas volume is to be restricted. Any further increase in the blast volume results into increase in the fuel rate and coke rate. It is because the gas flow is affected by the permeability of burden column. Any development of the peripheral gas flow or excessive central gas flow leads to decrease the gas utilization efficiency and increase in the fuel rate. In the modern high capacity BF the blast volume is controlled at the reasonable level to ensure rational gas distribution, improve gas utilization efficiency and reduce the coke and the fuel rate.
The control of the kinetic energy of the hot air blast is done to achieve optimal or near optimal depth of the raceway, and rational initial gas flow distribution. The tuyere combustion zone is adjusted suitably to maintain appropriate wind velocity and blast kinetic energy and to avoid the influence of excessively high wind velocity and blast kinetic energy on the coke in the raceway. The objective in the modern high capacity BF is to achieve a blast kinetic energy at a level of around 14,000 kilogram meter / second (kg.m/s) to 15,000 kg.m/s. For achieving this level of kinetic energy, the first measure is to maintain a reasonable wind velocity (normally in the range of 250 meters per second to 270 meters per second). If the coke quantity in the burden is high then the wind velocity can be higher, and vice versa. However, the wind velocity is subject to a ceiling by suitable matching of the blast volume, tuyere combustion zone, blast temperature, and top pressure within the reasonable limits for achieving good production indexes.
Important features of a modern high capacity BF
Some of the important features of a modern high capacity BF are described below.
High blast temperature – The heat needed to carry out the smelting process in the BF mainly comes from combustion of the fuel (coke + coal) and physical heat of hot air blast. Generally the physical heat of the hot air blast accounts for around 30 % of the total heat requirement in the BF. The higher the amount of the physical heat which is brought by the hot air blast, the lower is the required heat needed from the combustion of the fuel. Improvement in the blast temperature reduces the fuel rate and has a saving in the cost of the production of the HM. This is because the physical heat brought by hot blast can replace some of the fuel. Also, with the increase of blast temperature, the PCI rate can be improved. Increase in the PCI rate replaces some coke, and thus reduces the coke rate.
In recent years, hot blast stove technology has significant developments. The development of hot blast stove technology is from internal-combustion to external combustion, and then to top-combustion. The fuel gas which has been normally a mix gas of low calorific value (CV) usually consisting of BF gas enriched with a part of the high CV gas (usually coke oven gas) to only BF gas with gas-air double preheating technology to suit the high blast temperature requirement which is also increasing year by year. The blast temperature of some of the high capacity BFs is in the range of 1,250 deg C to 1300 deg C.
Dehumidified blast – The dehumidified blast eliminates the loss of heat needed for the decomposition of the water in the blast furnace. This in turn raises the flame temperature, promotes higher PCI rate and thus reduces the coke rate. Normally, for every 1 g/Ncum of moisture removed in the hot air blast, there is a decrease in the coke rate in the range of 0.8 kg/tHM to 1 kg/tHM. In order to maintain the thermal conditions of the hearth, for every 1 g/N cum moisture removal, the rate of PCI increases by 1.5 kg/tHM to 2.0 kg/tHM.
Modern high capacity BFs operate with high blast temperature and low blast humidity, by adjusting the amount of PCI to control the furnace temperature. In many furnaces, the blast temperature is stabilized at 1,230 deg C to 1,250 deg C, and the moisture in blast is stabilized at 10 g/N cum to 15 g/N cum.
High top pressure Improvement in the top pressure reduces the gas flow velocity, lowers the pressure loss in the burden, and promotes smooth operation of the BF. In addition, the increase of top pressure reduces the dust amount, increases the utilization efficiency of the coke and coal, and reduces the coke and fuel rate. At present, improvement of the top pressure has become an indispensable means for normal production in a BF. The top pressure of modern high capacity BFs is normally above 2.75 kg/sq cm.
Economical injection of pulverized coal – The purpose of injection of pulverized coal is to save coke and reduce the cost of the HM production. In spite of the price difference between coal and coke, if the coal-to coke replacement ratio decreases to a certain level, it increases the fuel rate, and the economic benefits of the injection of pulverized coal get offset. Hence, for the economical injection of pulverized coal, there is a necessity that a high replacement ratio is maintained while increasing the amount of injection of pulverized coal. If by increasing the PCI rate, there is the increase in the fuel rate due to the decrease of coal combustion rate and due to the decrease of the replacement ratio then it is not the economical injection of pulverized coal. Modern high capacity BFs aim for the economical injection of pulverized coal. The prerequisites for the economical injection of pulverized coal are as follows.
- Improvement in the quality of burden, including coke, sinter, pellet and lump ore, to reduce slag volume and improve permeability in upper and lower portions of the BF.
- Implementation of high level of oxygen enrichment, high temperature of the hot air blast, and dehumidification of the hot air blast is essential to control the flame temperature and accumulated amount of unburned pulverized coal and coke powder in the hearth, and increasing the combustion rate of the injected pulverized coal. Oxygen enrichment of the hot air blast reduces gas volume per ton of HM, increases the flame temperature, and changes the temperature distribution in the BF. Combining oxygen enrichment with injection of pulverized coal suitably reduces the change of heat flow ratio, maintains the flame temperature within reasonable limits, and makes the BF operation stable. Every increase of 1 % in the rate of the oxygen enrichment results into increase in the coal combustion rate by 1.51 %. High oxygen enrichment of the hot air blast is a characteristic of the modern high capacity BFs. There are high capacities BFs operating with the oxygen enrichment rate of more than 10 %. High enrichment of the hot air blast also compensates the deterioration in the quality of the BF coke.
- Rational gas flow distribution and stable operation of the BF is ensured when there is suitable central gas flow in the lower portion of the BF. It is ensured by the adjustment in the blast parameters which helps in the complete utilization of the unburned pulverized coal entering into hearth and thus the utilization efficiency of coal increases.
- Optimization of the blending of coals for injection improves the combustion property of mixed coal, improved the rate of PCI, expands the options of coal type, and reduces the cost of blended coal. Generally the anthracite coal with high fixed C (carbon) content and high CV is blended with bitumen coals with high content of volatile matter (VM) and good combustion property. The VM content of the blended coal is to be controlled in the range of 15 % t0 25 % and the ash content is to be lower than that of coke (usually less than 11 %).
Production of HM with low silicon Production of HM with low silicon results into reduction of fuel rate. Every decrease of 0.1 % of silicon content of HM, there is a reduction in fuel rate of around 4 kg/tHM to 6 kg/tHM. The hearth of modern high capacity BFs is normally active and has plenty of heat, so it is easier to produce HM with low silicon. The technological requirements for the production of HM with low silicon include the following.
- Reduction of silica load of the burden materials to be achieved by the decrease in the ash content of the coke, ash content of the coal for PCI, and the silica content of the sinter.
- With the requirements of ensuring sufficient heat in the BF hearth for the smooth operation, the flame temperature can be reduced suitably to contain the generation of SiO gas.
- Control of the reasonable shape and position of the cohesive zone is needed to reduce the contact of dripping liquid iron with SiO gas. This prevents the generation of the silicon.
- Optimization of the slag quality is required by reducing the activity of silica in the slag.
- Operation of the BF with high top pressure is needed for the suppression of the generation of SiO gas.
However, the production of HM with low silicon has an impact on the campaign life of the BF since it has a detrimental effect on the erosion of the hearth lining. Hence, the silicon content of the HM is normally not dropped to a very low level and is normally maintained in the range of 0.4 % to 0.6 % in the high capacity BFs.
Control of thermal load of the BF – The heat loss of the BF is reduced mainly by controlling the thermal load on the lining. The thermal load reflects the cooling status of the lining and it is frequently used to know whether there is increase in the peripheral gas flow and erosion of the lining. Thermal load is to be controlled within a certain range, and as far as possible to reduce the heat loss, which is in turn helps in the reduction of the fuel rate. However, too low of thermal load can cause build-up (scaffolding) at the lining. This, in turn, affects the gas flow distribution in the BF. Further, when the build-up falls off, it affects the BF operation and even damages the tuyere, leading to reduction the blast or even shut down of the BF. Hence, control is needed for the balanced thermal load in the BF since it is beneficial for its stable operation and for the control of the fuel rate.
For ensuring the suitable thermal load during the BF operation, one of the measures is the maintenance of the proper gas flow distribution by adjusting the charging regulation. The other measure is the adjustment of the cooling water flow according to the erosion level and temperature of lining for the maintenance of the stable operation profile in the BF. Hence, the adjustment of gas flow distribution is the most important means for the control of the thermal load.
In the modern high capacity BFs, thermal load monitoring is carried out through the partition management along the vertical and horizontal direction. The management standard at different height of the BF is different, and the thermal load along the horizontal direction is to be kept uniform. The control of thermal load is done through the adjustments in the upper and lower parts of the BF in oder to gradually achieve the middle-part (between lower shaft and bosh) management of the BF.
The three-dimensional visual thermal load model (Fig 2), which shows the BF operator display of 3 dimension real-time thermal load, historical trend, video-interaction, and alarm function, is the latest trend in the modern high capacity BFs. Combining of the model with the profile management model determines the reasonable control range of thermal load and provides good guidance for the BF operator.
Fig 2 Three-dimensional visual thermal load model
BF operations through visuals
The technology of BF operation by observing the visuals of furnace inside is being followed in the modern high capacity furnaces. This technology of observing the visuals is being used to monitor the distribution of the burden and it helps the BF operator to know what is happening inside the BF. The visuals of the BF inside helps the operator to understand better what is taking place inside of the BF so that proper adjustments can be made in the operating parameters for improving the gas utilization efficiency and thus reducing the fuel rate, and ensuring smooth BF operation. Major implements for the use of this technology are given below.
Use of thermo-vision camera and image processing The video camera at BF top helps the BF operator to observe the movement of the chute, gas flow distribution and the stream of the falling burden materials throughout the whole burden surface. This helps the operator in monitoring of channeling and slipping tendency inside the furnace. Thermo-vision camera is used since during the normal working of the BF, the temperature at the furnace top is normally less than 120 deg C.
Use of laser technology to measure online the surface profile of the burden The laser technology is used to measure the burden surface profile online by installing the burden surface laser detector. These detectors provide the operator the visual burden surface profile image thus helping him in better monitoring of the burden surface profile.
Monitoring of tuyeres by video camera and image processing – With the help of the video camera, the BF operator can observe at the same time the brightness, coke movement and coal stream size at the each tuyere, and timely detect the falling of the scaffolds, skulls, and colder burden in front of the tuyere. By the subsequent image processing, operators can get the quantitative analysis of thermal state and coal stream status at the each tuyere. This helps him in better understanding of the working of the tuyeres and PCI system for taking timely action in case of abnormalities. A video camera with a spectroscope can help the operator in direct observation of the tuyere.
BF filling measurement during BF blow- in using laser technology The use of the laser technology to observe furnace inside during the blow-in of the furnace provides the operator, the filling measurement, the data of burden flow trajectory with different angle of the chute, and the data of burden surface profile after charging. This helps the operator to know the way the burden distribution is done by the charging equipment. This knowledge guides the operator in the charging operation of the BF during the normal operation.