Hot Blast Stoves
Hot Blast Stoves
High temperature of the air blast blown in the blast furnace has been and remains one of the most important parameters which determine the economic efficiency of the blast-furnace operation. Increase of the air blast temperature reduces the consumption of expensive coke and normally improves blast furnace performance. The hot blast air is produced by passing cold blast air through preheated chambers or ‘stoves’, which results in the heating of the air to the required temperature.
Hot blast stove is used to preheat air used in the blast furnace. It works as a counter-current regenerative heat exchanger. It is a facility for the continuous supply of hot air to a blast furnace. It is a tall, cylindrical, thermal regenerative heat exchanger. It serves the function of providing hot air, called blast (combustion air), to the blast furnace at high temperature and constant flow. Before the blast air is delivered to the blast furnace tuyeres, it is preheated by passing it through hot blast stove which is heated primarily by combustion of the blast furnace top gas or enriched blast furnace gas. Normally there are three to four hot blast stoves operating in a system for providing hot blast for one blast furnace. Typical layout with three hot blast stoves is shown in Fig 1.
Fig 1 Typical layout with three hot blast stoves
Operation of the hot stove is cyclic, divided into two pass or periods namely (i) on-gas (or firing) period, and (ii) on-blast period as shown in Fig 2. During the on-gas period, the fuel gas is combusted to heat up a thermal storage and during the on-blast period air is heated by flowing it through the thermal storage. Normally one hot stove is operating in on-blast mode and supplies hot blast to the blast furnace for a certain time period, while the other two stoves in the system are being heated. The on-blast period is normally set such that the hot stove in the on-blast mode can complete the entire on-blast period whilst sustaining the outgoing blast temperature to be higher than the desired level. Once the hot stove in the on-blast mode completes its on-blast period, it is switched into on-gas phase, and another stove in the system takes the turn to supply the hot blast air to the blast furnace.
The operation of the hot-stove system is highly dynamic as it involves two alternating main phases during the operation of each stove, namely a heating phase and a cooling phase. Furthermore, because of the highly interconnected characteristics of the blast preheating system, both operation parameters and performance of each stove influence the other stoves in the set.
Fig 2 Typical operation sequence of one full cycle in a system of three hot blast stoves
The use of blast furnaces dates back as far as early as fifth century BCE in China. However, it was not until 1828 that the efficiency of blast furnaces was revolutionized by preheating the air blast using hot stoves in conjunction with the process. The development of hot blast has been described as ‘the most important single innovation in the iron age’. Certainly it ranks alongside the development of the blast furnace, and the introduction of mineral fuel in the blast furnace, in enabling a significant step-change in the production of the liquid iron.
The invention of hot blast is conventionally associated with James Beaumont Nielson, previously foreman at Glasgow gas works. He took out his first patent in October 1828. Nielson’s story is one of an outsider to the industry developing a new technology, initially in the face of opposition from a suspicious and conservative establishment, but subsequently universally accepted. Nielson invented the system of preheating the blast for a furnace. He found that by increasing the temperature to 300 deg F (149 deg C), he could reduce the fuel consumption from 8.06 tons to 5.16 tons with further reductions with higher temperatures.
The regenerative heat exchangers have been first used in the 19th century when Eduard Alfred Cowper submitted a patent application in 1857 for a regenerative brick type heater for heating of the air blast, a prototype of the hot blast stove. Cowper was designed for the recycling of the top gas of the blast furnace rather than receiving solid fuel as did the earlier designs. Since then, the hot blast stove system has been subject to many improvements and has evolved to a hot blast system characterized by high efficiency and long campaign life.
Early designs of hot blast stoves used with blast furnaces were consisting of placing them on top of the furnace rather than next to it which constitutes the present layout being used for the hot blast stoves. The earlier hot blast stoves used waste heat from the blast furnace delivered via cast iron pipes to the stove to preheat the cold air blast. One major problem with using cast iron pipes was the generation of cracks throughout them. This was remedied by eliminating the pipes and using refractory instead. This also led to further changes in the design of the layout of the hot blast stoves with the blast furnace due to the use of two to four hot stoves which are to be placed in series beside the blast furnace. This allowed for the heating of one hot blast stove by blast furnace top gas while the other one was being drained of its heat to preheat the hot air blast into the blast furnace. As the cold air blast entered the stove, it was preheated by hot bricks and exited the stove as a hot air blast. Cambria Iron Works was the first company in the USA to use regenerative stoves in 1854. These stoves were constructed of iron shells lined with refractory and contained multiple passageways of refractory for the air blast throughout. A typical stove of this design had around 186 sqm to 232 sqm of heating surface. In 1870, Whitwell Stoves designed and produced larger stoves with heating surfaces of about 8546 sqm, which could deliver hot air blast at the temperatures of 454 deg C to 566 deg C to the blast furnace. These were also the first hot blast stoves to use hexagonal refractory checkers, cast iron checker supports, and semi-elliptical combustion chambers to enhance the distribution of gas throughout the checkers.
Hot blast stove is around as large in diameter as the blast furnace, and the height of the column of checkers is around 1.5 times as tall as the working height of the blast furnace. At the modern blast furnaces, the relation of the stove size to the furnace size is even larger. As an example, one typical new blast furnace has a hearth diameter of 9.75 m and a working height of 25.9 m, and it is equipped with three stoves with each stove having an inside diameter of 10.36 m and a checker height of 40 m.
Fig 3shows the typical cross sectional views of a conventional two pass hot blast stove. As seen in the figure, the oval shaped combustion chamber occupies around 10 % of the total cross sectional area of the stove. It extends from the bottom of the stove to within around 4 m of the top of the stove dome. A sturdy brick breast wall separates the combustion chamber from the balance of the stove, which is filled with checker bricks resting on a steel grid supported by steel columns.
Fig 3 Typical cross sectional views of a conventional two pass hot blast stove
Hot blast stove consists of tall, cylindrical steel structures lined with insulation and almost completely filled with checker bricks where heat is stored and then transferred to the blast air. It consists of three main parts referred to as (i) the combustion chamber with metallic or refractory burner, (ii) the dome, and (iii) a chamber containing a thermal storage, called checker-work or the brick zone. It is constructed of refractory materials. The checker-work consists of a large volume of alumina and silica hexagonal bricks (Fig 4) with several thousand channels. The checker-work is capable of withstanding elevated temperature as well as storing thermal energy. It provides large heat storage volume as well as a large surface area for heat transfer.
Fig 4 Checker brickwork of a blast furnace stove
High performance checkers feature optimum heat transmission characteristics (heating surface) and heat storage capacity (refractory weight) over a very long service life. While the refractory weight of the bricks is resulting from other parameters and the material of the bricks, the heating surface can be greatly influenced by the design of the bricks. The size and shape of the holes is one typical parameter to optimize the heating surface. Small holes and complicated shaped flue gas channels increase the specific heating surface of the bricks and allow the reduction of stove size. However they also influence the pressure loss of the checker work and greatly increase the risk of blockages and therefore serious reduction of the performance.
The waste flue gasses enter the upper zones of checker bricks with very high temperature allowing high heat transfer rates due to heat transfer by radiation. Hence, the checkers used in the upper zones of the checker chamber can have smaller flue channels and thicker walls than the checkers installed in the lower zones of the stove. This leads to lower heating surface in favour of higher heat storage capacity in the upper part of the stove checker column. Such optimization allows the reduction of stove size without negative influence of critical diameters of the hole or complicated hole shapes. The checker bricks used in the lower temperature range have larger flue channels and thinner walls in order to increase the heating surface. Furthermore, such design results in reduced pressure loss but decreased heat storage capacity at the lower parts of the checker column. Fig 4 shows the checker brickwork of a blast furnace stove.
The main function of the checker grid is to support the checker bricks during stove operation, when high temperatures occur in the area of the grid. A high design temperature of the grid increases therefore the safety for stove operation. Since high waste flue gas temperatures also increase the efficiency of heat transfer, they allow the reduction of stove size, while waste flue gas heat recovery system (described later) allow feeding back the higher energy in the waste flue gas to the stove. Normally, the grid is suitable for short term maximum waste flue gas temperatures of 450 deg C, and of 400 deg C for the continuous operations. For special applications, a high temperature grid, suitable for temperatures upto 550 deg C has been developed.
The grid material is in direct contact with refractory material, hence it is to have a thermal expansion coefficient as close to the refractory as possible to reduce constant differential movements to a minimum. Since metal has typically much larger thermal expansions than refractory material, the thermal expansion coefficient is to be as low as possible for the grid material. Hence, the checker grid is composed of a special cast iron with low thermal expansion. The grid is free standing without contact to the refractory wall lining to avoid unnecessary differential movement there. The contact surface between grid and checker work is protected with separated metal plates for every checker to avoid the constant ‘grinding’ which occurs if checker grid material and checker work is in direct contact.
While other elements have a direct influence on the performance of the hot blast stove, the dome construction is solely based on the avoidance of hot spots on the steel shell during a maximum service life. Different approaches exist to achieve this requirement. A refractory design has been for the spherical dome so that a long service life can be achieved with simple geometry.
The dome is usually supported by the ring wall. The design of the ring wall aims for a most uniform vertical thermal expansion. This needs detailed calculations that take into account the effects of local temperature variations and different thermal refractory expansion characteristics. The support bricks and transition bricks of the dome have tongues and grooves at their circumference in order to absorb the radial forces. In addition, the radial forces are absorbed by friction due to the vertical load of the dome refractory. This design avoids the reinforcement of the dome refractory through skew back bands.
Depending on the shape and the dimension of the dome in addition to the thermal expansion of the refractories, adequate expansion joints are to be provided, even for the support and transition bricks. This is to absorb the horizontal thermal expansion of the ring wall bricks and of the lower part of the dome. While the spherical design is technically and commercially convincing, the spherical design also is the perfect shape for the stove steel shell as a pressure vessel. Especially designs with sharp edges in the steel shell are problematic and result in local tension peaks in these areas.
A design where the dome refractory is not resting on the ring walls is also possible and has been successfully applied. However it is to be noted that in this case the critical areas like the expansion joint between wall and dome lining, the ‘overlapping areas’ of the refractory of wall and dome lining and the shape of the pressure carrying steel shell need special attention and effort.
The division wall is a critical item in internal combustion chamber stoves. It separates the combustion chamber with high flame temperature from the checker chamber which is designed for lower temperatures. The division wall also prevents the fuel gas from leaking to the checker grid area. Traces of diffused, unburned fuel gas can lead to high emission values of carbon mono oxide in the waste gas which can exceed the environmental requirements.
The division wall between the combustion chamber and the checker chamber is exposed to the most critical thermal stresses during operation of a hot blast stove. The face on the combustion chamber side is directly exposed to the high heat impact from the burner flame, whereas the lower part of the division wall which faces the checker chamber is located in the coldest part of the stove. This can cause leaning of the division (banana effect) wall due to different vertical thermal expansions. The optimum division wall construction consists of numerous walls, which expand independently of each other. Multiple walls also reduce the temperature gradient across each individual wall. Since this is not possible for construction reasons, other methods for solving the differential expansion need to be found.
Normally the division wall is typically constructed with three layers of refractory bricks, each utilizing a tongue and groove design on the radial face. Vertical sliding joints between each course allow free vertical expansion of each individual layer. If a metallic burner outside the combustion chamber is used, the lower part of the division wall has an additional refractory wall to protect against flame impingement. In the lower part, where the highest differential expansion occurs an additional wall of insulation is used, which is relatively easy to install. The installation of insulation on the cold face of the division wall reduces the temperature gradient in the division wall bricks which reduces the leaning to a minimum.
The internal combustion chamber design with the division wall has a weak spot on the corners of the combustion chambers. Exceptional disturbances of operation, like explosions etc. lead to the damages in this area. Hence these corners have to be supported properly. The normal practice is to reinforce the refractory material in this area to solve this problem.
In the hot blast outlet, the inner shaped ring consists of special bricks which are keyed into the combustion chamber brickwork, so that they cannot move towards the combustion chamber. The outer diameter profile of the shaped ring normally allows the installation of the combustion chamber bricks without cutting. The combustion chamber brickwork and the openings in the shell brickwork are separated from the ring wall bricks by a sliding joint in order to allow them to expand independently of each other. The opening in the combustion chamber brickwork is similar to the opening in the ring wall brickwork, and is again separated from the stove insulation layers by a sliding joint. It is normal to adopt a relief arch which helps unloading the hot blast outlet shapes from the brick dead load above.
The metallic burner for the hot blast stove is located near the bottom of the combustion chamber. On the majority of hot blast stoves, the burners are external to the combustion chamber. There is a burner shutoff valve between the burner and the stove which is closed to isolate the burner when the stove is on blast, but open when the stove is being fired. The gas and combustion air are partially mixed in the metallic burner but, because of their high velocity through the burner, actual ignition probably does not occur until inside the stove. The mixture of gas and air impinges on the target wall directly opposite the burner port and then makes a 90 degree turn.
In several modern hot blast stoves, ceramic burners are used. The ceramic burner has the task to achieve complete combustion of air and gas in a large range of operation characteristics and of course a long service life. Since external metallic burners need regular maintenance and always impose larger stress by the direct impingement of the flame and hot gasses on the division wall, the maintenance free parallel stream ceramic burner has been developed. The parallel stream design allows thorough mixing of air and gas over a large range of flow rates and ensures an even load of the combustion chamber beside low emission values.
The ceramic burners, with their mixing chamber, are installed inside the combustion chamber and the firing is upward in a vertical direction instead of a horizontal direction as with the conventional metallic burner. With this type of burner, shutoff valves are required in both the gas main and the combustion air duct. These valves are capable of withstanding the force of the blast pressure. The ceramic burners have certain benefits because of their special design features.
The brickwork of the burner is self-supporting and is preferably not incorporated into the combustion chamber walls. The burner can easily absorb high temperature fluctuations. The material used for the burner is an andalusite based high alumina refractory with high thermal shock resistance and special sustainability against carbon monoxide. Combustion air and gas enter two separate chambers underneath the burner through two separate openings. Gas and air are then directed to a set of parallel slots, thus providing the initial distribution of air and gas across the burner. On top of these slots are refractory courses which provide a very intense mixing of air and gas, a prerequisite for complete combustion.
Main feature of conventional hot blast stove is a tall combustion chamber. It is located at the hot stove with internal combustion chamber along with the checker chamber within the same shell. Operation of this type of stove has shown a number of drawbacks as described below.
- Short circuit or direct leakage of fuel gases takes place through the separation wall between the combustion chamber and the checker chamber through cracks and joints between bricks. During the period of on gas, this leads to a substantial increase of carbon mono oxide content in the waste flue gases which can reach a level of 5000 milligrams/cum against a norm of 100 milligrams/cum. This results into a significant increase in carbon mono oxide emissions into the environment. Due to the same short circuiting reason during the period of on blast, part of cold blast leaks from checker-work chamber into combustion chamber and gets mixed with hot blast leading to reduction of hot blast temperature (sometimes by 100 deg C),
- Leaning of the combustion chamber towards the checker chamber (banana effect) takes place. This leads to (i) damage of both combustion chamber and checker brickwork, (ii) displacement of checkers resulting into partial loss of open channels, and (iii) a significant increase in hydraulic resistance of hot blast stove.
- High-temperature creep of refractory bricks takes place under the influence of high temperatures and brickwork load in the bottom part of the combustion chamber which causes deformation and collapse of brickwork, especially of hot blast branch.
- Pulsating combustion occurs which originates from the acoustic excitation of tall combustion chamber similar to that of organ pipe and leads to strong vibration of structures and to a collapse of its brickwork. This in turn deteriorates the performance of hot blast stoves.
- Uneven distribution of combustion products along the checker-work surface takes place. This can go upto plus / minus15 % which reduces the efficiency of the checker-work performance and results in temperature fluctuation across the checker-work. This also causes in development of cracks in its bricks and in the checker supporting grid.
- Cracking of refractories in the bottom part of combustion chamber (in the area where the burner is installed) takes place due to the thermal shock caused to refractory bricks during the change-over from gas period to blast period and vice versa.
These design deficiencies lead to frequent failures of the combustion chamber and, hence, the combustion chamber is considered as the weakest element of the conventional hot blast stove with internal combustion chamber. It limits hot blast temperature during long-term operation at the level of 1,200 deg С. Further, the higher the operating hot blast temperature, the more frequent are shutdowns and relines of stoves which affect the blast furnace techno-economics.
A variety of designs have been developed for the hot blast stoves. These designs include internal, external, and top (dome) combustion hot blast stoves (Fig 5). Each design has distinct characteristics, but also resembles many similarities. For example, each hot blast stove includes a ceramic burner, checker support, and checkers.
Fig 5 Schematic diagrams of different types of stoves
The most important modification in hot blast stove with internal combustion chamber was the separation of combustion chamber from the checker chamber where the combustion chamber is arranged as a separate shell. This eliminates the disadvantages of the short circuiting and the banana effect. This design of the hot blast stove has a more complicated dome design and a complex system of temperature expansion compensation of the checker chamber shell resulting into higher cost. Also more space is needed for such hot blast stove. Besides the shell of this type of stove has a tendency of stress corrosion cracks. This type of stove has a more reliable structure and such stoves are installed in large capacity blast furnaces. These stoves are able to maintain hot blast temperature of 1,250 deg C on a long term basis. Fig 6 shows different designs of hot blast stoves with external combustion chamber.
Fig 6 Different designs of hot blast stoves with external combustion chamber
For several decades, the maximum level of blast temperature remains unchanged at 1,200 deg C to 1,250 deg C. This shows the limited capacity of the existing hot stoves with internal and external combustion chambers. In the meantime, because of increased use of injectants through tuyeres such as pulverized coal, natural gas, etc. the necessity has arisen for higher hot blast temperature of 1,300 deg C to1,400 deg C. For achieving these temperatures, new hot stoves without drawbacks of conventional stoves and with better engineering, economic and environmental performance are needed.
The main disadvantages of conventional hot blast stove with internal and external combustion chamber described above can be resolved if the combustion chamber (known also as the shaft) is eliminated as such. Hence, hot blast stoves having no combustion chamber in the conventional sense have been named as dome combustion stoves or shaft-less stove or Kalugin stove (Fig 7). Hence, inter-reline service life of dome combustion stove is defined by the service life of its dome brickwork, life of the burner device located on top of it and that of the checker-work but not by the combustion chamber life.
Burner device is located on the top of the dome along the axis of the hot stove; it has a pre-chamber and a jet-vortex supply of gas and air (Fig 7). Jet-stream vortex of gas and air in the pre-chamber provides intensive mixing and combustion of gas which starts in the pre-chamber and is over in the middle part of the dome. Optimum degree of stream’s vortex provides full combustion of gas and uniform distribution of combustion products along the checker with a non-uniformity level of plus / minus 3 % to 5 %.
Fig 7 Dome combustion hot blast stove
In the dome combustion stoves, during the on-gas cycle, pure air, fuel gas is injected into the stove combustion chamber and chemical energy in the gas is converted into heat in the waste flue gas. While the waste flue gas enters the dome, descends down the brick chamber and exits the stove at the bottom, heat is transferred and stored into the refractory bricks. During the on-blast cycle, the flow direction is reversed. Fresh cold air is injected in the bottom from the other side of the stove. While it ascends through the brick chamber, heat is released from the brick and absorbed by the fresh air. After being mixed with some extra cold blast to achieve specified temperature and flow rate, the final product of the hot blast stove i.e. hot air blast is piped to the blast furnace.
In the past, stress corrosion cracking occurred when unprotected stoves were operated at dome temperatures in excess of 1,300 deg C and at high blast pressures. Studies have revealed that the formation of nitric oxide from the nitrogen of the air increases exponentially when the temperature is above 1,300 deg C. As a consequence, aggressive condensates are produced at the stove shell which attack especially in the area of tensile stress concentrations (welds, notches etc.). This leads over time to progressive corrosion damage, continuous crack formation, and ultimately to the failure of the stove as a pressure vessel.
Different approaches exist to address this major problem. All approaches consider eliminating one of the three influencing factors namely (i) tension forces, (ii) chemical attack and (iii) stress corrosion cracking sensitive material. The preventative measures start with the thermal layout of the stove. If possible, the heat storage level is to be designed in a way by which the needed stove performance is achieved with the lowest possible dome temperature. Normally, the temperature differential between the desired controlled hot blast temperature and the dome temperature is around 100 deg C to 150 deg C.
The heat storage level and the capital cost can be kept small if the design of the stove permits high waste flue gas temperatures. This adds to the importance of heat recovery systems, in addition to their advantages associated with the energy savings and elimination / reduction of enrichment gases.
The stress corrosion cracking protection system normally consists of the following measures.
- Tensile stress concentration in the shell is to be kept at a minimum or completely avoided. Design of the steel shell including the dome area, special welding procedures, and shot-peening of stress concentration areas are recommended practices.
- Chemicals responsible for the stress corrosion cracking are to be separated from the steel shell. Coating of the entire internal surface of the stove shell by a specially developed ‘Stellatar-D’ coating system is recommended practice.
- The shell is to be fabricated from stress corrosion resistant steel. Steel grade 16Mo3 (ASTM A 204), has both high resistance to corrosion and good welding characteristics. Another option to avoid the chemical attack is the external insulation of the steel shell in order to keep the shell temperature above 180 deg C, i.e. above the dew point of the aggressive chemicals. However this approach needs constant attention in case of changes of the shell temperatures and regular maintenance.
These days external waste flue gas heat recovery system is also frequently been used. The typical efficiency of a new hot blast stove is 75 % to 80 % and with the implementation of a waste gas heat recovery system it can increase the system efficiency upto 85 %.
In general, reduction of energy consumption of the hot blast stoves is one of the important criteria of the technical plant concept of the waste flue gas heat recovery system. The hot blast stoves heat recovery process allows the energy in the waste flue gas of the stoves to be recovered in order to save energy. The recovered energy is used for preheating the combustion air and fuel gas to the stoves. This leads to reduction in the overall gas consumption and replaces the expensive high calorific value fuel gas by the blast furnace top gas and thus reduces the operational expenditures for the production of hot metal in the blast furnace. The heat recuperation is made by heat exchangers based on heat pipe technology. Auxiliary burners increase the available energy for preheating to economize the heat utilization. Process diagram of heat recovery system of hot blast stoves is shown in Fig 8.
Fig 8 Process diagram of heat recovery system of hot blast stoves
The main advantage of the heat recovery system is that the total consumption of energy of the hot blast stoves plant can be reduced. With the increasing hot blast temperature needed for more efficient blast furnace operation, it has become necessary to add high calorific value fuel (coke oven gas or natural gas) to achieve the needed dome temperature. The use of preheated combustion fuel can considerably reduce, and even eliminate in some cases, the need for these expensive high calorific fuels.
Further, with the installation of a heat recovery system, the lifetime of refractory lining in the lower combustion chamber and ceramic burner can be increased because of the elimination of condensed water from preheated combustion gas and less temperature fluctuations. Also, hot blast stove performance can be increased by operating stoves on the upper temperature limits of the checker work. This reduces the capital cost of new hot blast stoves because of the construction of smaller hot stoves with higher performance which are calculated for higher waste gas temperatures.
The future of hot blast stoves is determined by historical and present developments and improvements, which are mainly driven by suppliers in order to secure a competitive edge and unique selling points. Industrial trends which are widely observed include the pursuit of lower emissions of carbon mono oxides and sulphur oxides, elimination of enrichment gas, implementation of waste gas heat recovery system and increased efficiency, which can be accomplished by changes to the checker and checker support system. These changes also reduce the hot blast stove dimensions and associated costs. At present, campaign life of over 30 years to 40 years is common practice and inter–crystalline stress corrosion is prevented by innovative coatings and external insulation or double-shell practice.
Until now, all designs converge towards most efficient systems, whereas construction schedule are frequently critical in selecting a specific technology. Significant changes are expected as a result of innovative ironmaking process technologies. These innovations target reduced carbon dioxide emissions and increased energy efficiencies as energy and environment is expected to change the nature of iron and steel industry comparable to changes as a result of health and safety. These innovative ironmaking technologies permit lower hot blast temperatures and several technologies eliminate the need for hot blast stoves. Further, in future hot blast stoves dimensions are expected to be reduced and can eventually disappear. The future of hot blast stoves is yet uncertain.