Technologies for Improving Blast Furnace Operating Performance
Technologies for Improving Blast Furnace Operating Performance
A blast furnace (BF) is an investment in the future. Hence it is necessary that there is the proper dimensioning of all equipment, systems and components as well as incorporation of technologies which assure the desired production and quality so that improved performance of the blast furnace can be achieved. This is particularly true when blast furnace goes for capital repairs. During capital repairs incorporation of technologies for the improvement of blast furnace operating performance also meet the new demands placed on the performance of the blast furnace, personnel safety, lower maintenance requirements and environmental compliance.
A key challenge for blast furnace operators has always been to assure a continuous and reliable supply of hot metal for the steel melting shop at uniform quality and at the lowest possible costs. Any interruption in the production of hot metal can lead to potential standstills in the downstream production and processing facilities. Downtime must be kept to a minimum while the blast furnace campaign life must be extended for as long as possible. Fluctuations in blast furnace operating parameters must be avoided for uniform hot metal quality, which is only possible through the application of proper technologies as well as sophisticated automation and process control solutions.
There are several technologies (Fig 1), which when adopted greatly improve blast furnace operating performance and increase its efficiency both with respect to productivity and fuel consumption. This results into improved hot metal production rate per unit of volume of blast furnace and reduced consumption of BF coke. Some of the major technologies are described below.
Fig 1 Technologies for improving BF operating performance
Increase in furnace internal volume
By using advanced technologies for furnace refractory lining and furnace cooling, it is possible to reduce the lining thickness in the furnace during the furnace capital repairs while simultaneously increasing the furnace campaign life. Reduction in the lining thickness results into increase in the internal volume of the blast furnace resulting into increase in the furnace production capacity. This also gives consistent furnace temperature profile throughout the furnace campaign. Improved refractories used for refractory lining of the furnace include erosion resistant alumina refractories in upper stack, silicon carbide refractories in bosh and belly, and erosion resistant carbon hearth walls with ceramic pad. For furnace cooling, copper staves are used in high heat flux zones while cast iron staves are used in other zones. Cast iron staves are normally with independent cooling.
Quality of ferrous burden
To ensure a permeable blast furnace, essential for stable operation, it is important that the ferrous burden is strong, closely sized and efficiently screened to remove fines. It must not disintegrate excessively in the stack, which generates additional fines. It must be sufficiently porous, reducible and of an appropriate size to allow the material to be adequately reduced by the time it reaches the softening zone. In this way the cohesive zone is less restrictive, with less FeO rich slag, and the thermal load in the lower regions of the furnace is lower, encouraging smooth operation. Requirements on the physical and metallurgical properties of sinter, calibrated lump ore, and/or pellets for efficient operation are to be met. The softening and melting properties of the ferrous components have an important effect on blast furnace operation. Restrictions in the cohesive zone and poor melting characteristics can result in erratic burden descent, unstable operation and thermal fluctuations.
An important aspect to consider when selecting individual burden components is their softening and melting characteristics. The major part of the pressure drop across a blast furnace is in the region where the ferrous burden is softening, melting and dripping down the coke bed through which the gases are ascending. A wide melting and softening range results in an increased pressure drop and a large cohesive zone root impinging on the lower shaft brickwork.
Quality of coke
For stable blast furnace operation at reasonable productivities, good quality coke is essential. It is one of the most often cited reasons for a poor period of operation. Coke must be strong and stabilized, to support the weight of the burden with minimal mechanical breakdown. It must be sufficiently large and closely sized, with minimal fines, to create a permeable bed through which liquids can drip down into the hearth without restricting the ascending gases. A consistent size is required to avoid undesired variations in permeability and to support the concept of varying coke layer thickness across the furnace radius to control radial gas flow. The coke must be sufficiently unreactive to solution loss (Coke reactivity index, CRI, normal value 20 % to 23 %), retain its strength under such conditions (Coke strength after reaction, CSR, normal value 65 % to 68 %), and be low in alkalis to minimize alkali gasification in the raceway, which has a deleterious effect on coke breakdown. Low sulphur content is also needed to minimize hot metal sulphur. Coke moisture and carbon content variations must be controlled to minimize their effect on the thermal state of the process.
Coke in the furnace centre gradually replaces the dead-man and the coke in the hearth, which must remain permeable to allow the liquids to drain across the centre of the hearth. This avoids excessive peripheral flow of hot metal in the hearth. An increase in the hearth pad centre temperatures is usually observed with an increase in deadman coke size, which indicates increased hearth centre activity. The aperture size of the coke screens is an important parameter to maintain hearth permeability. It is usually beneficial to increase the screen size and charge the additional small coke arising, mixed in with the ferrous burden, away from the furnace centre line.
The aim of specifying high quality coke is to ensure that large coke reaches the lower regions of the furnace. To monitor this objective in the long term, it is advisable occasionally to sample coke from tuyere level to assess the coke breakdown through the furnace. This is usually carried out during planned maintenance, often in conjunction with tuyere changes. A large sample of coke is raked from a tuyere aperture and its properties compared with a sample of the corresponding feed coke.
To minimize thermal and chemical variations, a homogeneous burden is desirable. The burden components should be as intimately mixed as possible. This depends on the number of burden components and the individual charging system, but it can usually be achieved to a reasonable degree by selection of storage bunkers and the sequence of material discharge.
Charging of nut coke
A flexible charging system allows the use of nut coke. The size of nut coke available for charging depends on the size and efficiency of the blast furnace coke screens at coke sorting unit of coke oven batteries, but is typically in the range 10 mm to 25 mm. The charging of nut coke mixed in the ferrous material and positioned along the mid-radius, improves operation by improving reduction efficiency and permeability of the ore layer in the cohesive zone. Nut coke charging also reduces belly temperatures. Nut coke is also charged at the wall, sandwiched between the two ore charges to prevent an inactive wall region when fine ore is being charged at the wall.
Burden distribution is one of the main factors which not only affect the stability of operation but, by determining the radial gas flow in the furnace, it is one of the major factors controlling the rate of wear of the furnace walls. As a means of obtaining better control of burden distribution in the blast furnace stack and thereby improving the gas–solid contact and the fuel efficiency, several new developments have been used in recent years. The two types of distribution system that enable sufficient control for high productivity are the bell-less top using a tiltable rotating chute, and a bell charging system with movable throat armour.
Primarily, radial gas flow is controlled by the proportion of ferrous burden to coke, since coke is generally much larger in size. This is most easily achieved by charging the material in discrete layers and varying the layer thickness across the furnace radius. Protection of the furnace walls is therefore achieved by increasing the proportion of the ore layer at the wall, which results in a reduced quantity of heat removed by the wall cooling system. However, there is a limit to the proportion of ferrous material close to the furnace wall, otherwise an inactive layer forms, which may encourage the formation of wall accretions and allow unprepared burden into the lower regions of the furnace and increasing tuyere failures. The proportion of coke at the centre of the furnace must be sufficient to allow stable furnace operation at the desired level of production. A large proportion of coke creates a relatively permeable region with fewer descending liquids, allowing the use of maximum blast volume without large fluctuations in blast pressure and erratic burden descent.
The coke at the centre of the furnace replaces the coke in the hearth and a coke rich permeable centre encourages a permeable hearth, which relates the liquid flow across the hearth. The central coke chimney should not be unnecessarily wide, however, or inefficiency results and damage may be incurred to certain parts of the furnace top due to excessively high heat capacity of the ascending gas.
Split size charging of materials
More sophisticated distribution systems permit additional control of burden distribution by utilizing more than one size range of a given material. One of the most commonly used practices is the charging of fine ferrous materials, often from screenings of the main ferrous burden. Fines are charged separately in small quantities close to the furnace wall, to give a localized reduction in permeability and thereby protect the walls. Charging a separate small batch of finer material normally reduces the charging capacity of the furnace. Charging of small batches with a bell and movable throat armour system causes fewer delays than with a bell-less top due to the reduced discharge time. It may be possible to charge small quantities of finer materials to the furnace wall by charging them first into the top hopper or large bell hopper and using the corresponding initial chute angle or movable throat armour setting. However, the quantity is limited by the hopper discharge characteristics to that which will pass through the hopper without mixing with the remainder of the charge. There is also a financial benefit in using such ferrous fines directly as opposed to returning them to be re-sintered. In a similar manner, the ferrous burden may be split into large and small sizes which are then charged over different parts of the furnace radius to control the radial permeability.
High pressure operation
One of the limiting factors in attempting to increase the blast volume rate in the blast furnace is the lifting effect that is caused by the large volumes of gases blowing upward through the burden. This lifting effect (the mass flow rate) prevents the burden from descending normally and causes a loss rather than an increase in production. To increase production rates above normal, the blast furnace is equipped with septum valve in the top gas system to increase the exit gas pressure. This increase in pressure compresses the gases throughout the entire system and permits a larger amount of air blast to be blown. With this increase in the quantity of air blown per minute, there is a corresponding increase in production rate. In addition, this also suppresses the formation of SiO resulting in lowering of the hot metal silicon content.
When the pressure of the top gas is increased, the pressure of the inlet air blast is also to be increased proportionately. Further, if the top pressure is increased then it is necessary to use a larger blower, capable of delivering the increased blast volume at the higher pressure. The furnace shell, stove shells, dust catcher, primary washer, and gas mains also need to have the structural integrity to withstand the increased pressure. The throttling valve that is used to increase top pressure is located beyond the primary gas washer where the sandblasting effect of the gas has been reduced by removal of a large portion of the dust carried by the gas from the furnace. The exit water line from the primary washer needs to be equipped with a regulator so that the gas pressure within the washer does not destroy the water seal. Clean gas or nitrogen is used for the equalization of the pressure at the furnace charging equipment. Furnaces with top pressures of 2 -2.5 kg/sq cm are operating successfully. At some of these furnaces, top pressure recovery turbines are used to recover some of the energy of compression and to produce power.
Hot blast temperature
Hot blast temperature improves the fuel efficiency of the blast furnace and allows higher furnace temperatures, which increases the capacity of furnaces. High hot blast temperatures are essential for efficient blast furnace operation since they reduce the furnace coke requirement substantially and facilitate the injection of auxiliary fuels such as pulverized coal as a replacement of blast furnace coke. The total energy savings possible by a combination of techniques is of the order of 0.12 million kcal /ton of hot metal. It results into lower operating costs because coke ratio reduces by 2.8 % per 100 deg C rise in blast temperature when it is maintained between 1000 deg C to 1200 deg C. Many modern furnaces operate at a hot blast temperature which is higher than 1300 deg C.
Oxygen enrichment of the hot air blast
The purpose of oxygen enrichment into the blast is to control raceway adiabatic flame temperature (RAFT), hearth gas generation, and intensity of melting. When the blast air is enriched with oxygen, there is an increase in the RAFT. High flame temperatures are normally incompatible with the relatively low quality burden materials and need burden materials of right quality. Further high flame temperatures due to oxygen enrichment need to be controlled with blast moisture and fuel injection. There are furnace operations using in excess of 12 % oxygen enrichment. For every percent of oxygen above that for normal air blast (approximately 21 % of oxygen), the production rate increases by about 2 % to 4 %. In instances where the burden materials have good reducibility, that is, they will reduce rapidly, the flame temperature can be increased significantly and the fuel efficiency can be improved. The judicious use of oxygen provides a means of controlling the bosh gas mass flow rate so that furnace throughput can be maximized while controlling hot metal quality.
Auxiliary fuel injection
With the development of techniques for increasing hot-blast temperatures to the range of 1000 deg C to 1300 deg C and the need for controlling the RAFT because of the type of burden materials in use, it has become possible to inject hydrocarbon fuels into the blast furnace through the tuyeres to control the flame temperature, increase the reducing power of the bosh gas and at the same time replace some of the blast furnace coke. In the presence of large quantities of coke, the hydrocarbon fuels can burn only to carbon monoxide and hydrogen; consequently, they produce less heat than the coke they replace resulting in control of the flame temperature, but the reducing gas they produce is more effective than that produced by combustion of coke.
Many different fuels have been tried—natural gas, coke oven gas, oil, tar and pulverized coal, even slurries of coal in oil. Pulverized coal is the most used injectant in the blast furnace in the present day because of its relative abundance and low cost. When coal is used, it is also introduced into the air blast by a lance entering the air stream through the sides of the blowpipes. It is most desirable to have the injected coal completely gasified and combusted before it leaves the raceway just inside the furnace. When injecting fuel special precautions are required to avoid the buildup of fuel in the bustle pipe or blowpipe and its subsequent combustion. Pulverized coal injection process is described below.
Coal feedstock is conveyed to a coal preparation plant where tramp material is removed by screening and an over-head magnet. The coal is then crushed and simultaneously dried in a stream of hot gas or in a combined grinding unit/drier, followed by extraction through the system by means of an induced draught fan. Coal with the correct product size distribution is drawn up through a velocity separator and captured in a bag filter unit. The final product is screened prior to transfer to a storage silo. A proportion of the exhaust gases are recycled back to the hot-gas generator at the grinding unit/drier. This control feature ensures that the total oxygen content of the hot gas, in contact with the coal, is kept below 12 % to eliminate any chance of ignition of the ground coal. The coal-injection system is comprised of lock hoppers and injector units. The coal flow rate to each tuyere can be independently controlled by a mechanical feeder. Alternatively a simpler system with less accuracy of flow control to each tuyere can be provided, using a splitter based system. The equipment for PCI is quite sturdy these days with availability in excess of 98 % and accurate coal injection rates to within 2 %.
Automation and control
Automation and control system these days provides the ideal solution for all aspects of furnace operation. These include namely (i) furnace top control on skip or belt charged tops with complex charging patterns and burden distribution, (ii) unique spiral charging system for bell less top to increase the portion of fines that can be charged, (iii) stock house control of sequentially batched materials with ‘in-flight’ weighing and material layering, (iv) gas cleaning control, (v) stoves control for cyclic, parallel, lapped parallel and staggered parallel four stove operation, (vi) coal injection system controls, (vii) cast house operation and control, and (viii) slag granulation plant controls. Besides the automation and control also have features for plant safety and shutdown sequences.
For ensuring high performance blast furnace operation at low cost, blast furnaces these days are provided with a closed loop optimization system. This system functions on the basis of advanced process models, artificial intelligence, enhanced software applications, graphical user interfaces and operational know-how. Excellent process performance and significantly lower production costs are being achieved in the furnaces with closed loop optimization system. In closed-loop expert system the main parameters of the blast furnace to be controlled are carried out without the need for operator interaction. For example, control of the coke rate, basicity, the steam injection rate and even the burden distribution can be simultaneously and automatically executed in a closed loop mode to ensure stable and consistent process operations at low production costs. Precise control of the blast furnace is achieved on the basis of advanced process models.
The process information management system provided in the present day blast furnace collects, prepares and stores all relevant data for subsequent use.