Hot Blast Stove Refractories
Hot Blast Stove Refractories
Hot blast stove is used to preheat air used in the blast furnace. It works as a counter-current regenerative heat exchanger. It consists of tall, cylindrical steel structures lined with different kinds of refractories and almost completely filled with checker bricks where heat is stored and then transferred to the fresh air to heat it to a specified temperature.
The hot blast stove technology has progressed from conventional stove with internal combustion chamber to high temperature stove with internal combustion chamber to high temperature stove with external combustion chamber. Fig 1 shows the three types of stoves with the refractories used in them. Hot blast stoves having no combustion chamber in the conventional sense have been developed. These stoves are named as dome combustion stove, or shaft-less stove, or Kalugin stove.
Fig 1 Three types of stoves
Main feature of conventional hot blast stove is a tall combustion chamber. It is located at the hot blast stove with internal combustion chamber along with the checker chamber within the same shell. The internal combustion chamber design with the division wall has a weak spot on the corners of the combustion chambers.
Hot blast stove technology especially the design and qualities of its refractories has undergone vast improvements in recent years and now a level of maturity has been achieved which ensures the campaign life of a stove of over 30 years.
A variety of designs have been developed for the hot blast stoves. Each design has distinct characteristics, but also resembles many similarities. For example, each hot blast stove includes a ceramic burner, checker support, and checker bricks.
Hot blast stove consists of several parts which are referred to as (i) the combustion chamber, (ii) ceramic burner, (iii) dome, (iv) regenerator which is a chamber containing a thermal storage and which is also being called checker-work or the brick zone, (v) crossover in case of stove with an external chamber, and (vi) large walls. All these parts of the hot blast stove use refractory materials.
Conventional hot blast stoves used metallic burner which is located outside the stove. In 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. This burner has, of course, a long service life. The ceramic burner, with its mixing chamber, is 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.
The combustion chamber is equipped at the base with a burner for mixing the fuel gas and air, burning the mixture, and producing high-temperature combustion gas. The dome is the highest temperature region of a stove. The checker chamber is constructed of a very large number of checker bricks (around 300,000 bricks) stacked together to store the generated heat from the combustion chamber. Normally three or more hot-blast stoves are needed per blast furnace. Hot-blast stoves for large blast furnaces are of 40 m or more in height.
In hot-blast stove operation, high-temperature combustion gas is generated in the combustion chamber which is sent to the checker chamber to heat the checker bricks for a certain period of time. After that, cold air is sent by a blower in the direction opposite to that of combustion gas flow as per the timing of blowing the hot blast into the blast furnace. The cold air is heated to around 1,200 deg C to 1,300 deg C by the heat of the checker bricks, and is sent to the blast furnace through hot blast main for blowing through the tuyeres into the blast furnace.
Fig 2 shows typical refractory lining of a stove and examples of refractory arrangement in three different types of stoves with external combustion chamber. Each type uses silica bricks in the combustion chamber heated by the hot blast, in the pipe to connect the combustion chamber and checker chamber, and in the upper part of the checker chamber.
Fig 2 Typical refractory linings of blast furnace stove
The parts of the hot blast stove are divided into high temperature areas, medium temperature areas, and low temperature areas as per the operating temperature existing in the areas. The highest temperature region of a stove is the dome where the temperature goes to the level of 1,550 deg C in modern stoves. The high temperature areas are those areas where the temperature remains in the range of 1,100 deg C to 1,400 deg for a long time.
Thermal shock resistance and strength, accurate shape, and no residual expansion are also important properties of the refractories. In order to meet the needs of high blast temperature and long life of hot blast stove, refractory materials are to be carefully selected. For refractory materials used in the high-temperature area of hot blast stoves, in addition to the need for a low creep rate, they are also to have a high load softening point, a low reheat line change rate (permanent linear change), and strong resistance to chemical erosion and airflow. The selection of refractory materials in these areas is based on the need to have large heat capacity, high temperature strength, good creep resistance, high load softening temperature, good thermal shock resistance, high temperature volume stability, and good erosion resistance. For medium and low temperature areas, refractory materials need large heat capacity, good hot air conductivity, low permeability, high density, high strength, and good thermal shock stability.
Zoning of refractories is used to minimize the refractory costs. Since zoning combinations of products in a hot-blast stove are numerous, it is important to specify the design and operating parameters for a given stove to ensure that the most economical refractory product can be installed in each zone.
Due to the different working temperatures, operating conditions, and structure, different qualities and grades the refractory materials are used in different areas of the stove. Refractory bricks for hot blast stove include checker bricks, straight bricks, tapered bricks, and special-shaped bricks. Refractory castables are also used in certain areas of stoves. Straight bricks are the normal refractory bricks, which are used to build the lining and partition wall.
Besides different shapes, different qualities of the refractories are used in different areas of the stove. These qualities include fire clay refractories, alumina refractories with different content of alumina, low creep high alumina and fire clay refractories, andalusite refractories, silica refractories, and insulation refractories including lightweight heat insulation bricks, heat insulation refractory material, spray coating, and refractory mortar.
The insulating refractory material refers to a refractory material having a high porosity, a low bulk density, and a low thermal conductivity. The normal thermal insulation materials used in hot blast stoves are insulation refractory bricks and refractory ceramic fiber blanket and felt etc.
The refractory castables used in hot blast stoves are fire-resistant and acid-resistant. Besides, heat-insulating refractory castable is used mainly in stove shell and hot blast main in high-temperature areas. This prevents burning of the steel shell. Acid-resistant castable is mainly used for dome, combustion chamber, and regenerator chamber upper parts. It prevents the corrosion of steel shell by acidic combustion products generated at high temperatures. Insulating refractory castable is used to reduce the heat loss. The shell of the hot blast stove in the high temperature zone is required to be covered with thermal-insulation guniting refractories. Fig 3 shows different types of refractories used in stoves.
Fig3 Refractories used in stove
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.
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. This is shown in Fig 4.
Fig 4 Refractory lining of dome and hot blast outlet and ceramic burner
Refractories for ceramic burners
In several modern hot blast stoves, ceramic burners are used. Fig 4 shows a ceramic burner. 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 (Fig 4) 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. 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. 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.
The refractory materials used in the ceramic burner of the hot blast stove are made of cordierite, mullite, low-creep fire clay refractory, or an andalusite based high alumina refractory with high thermal shock resistance. The material used for the burner is to have special sustainability against carbon monoxide, high load softness, good thermal shock resistance, high compressive strength, and stable volume.
Dome area refractories
The dome area of the hot blast stove is divided into a working layer, a filling layer and a heat insulating layer. Maintenance of the structural stability of the dome at the high temperatures is important. The dome evenly distributes high-temperature flue gas into the regenerator during combustion. The temperature of the dome area is very high. It is around 1,400 deg C in conventional stoves and around 1,550 deg C in high temperature modern stoves. The refractories used in the working layer are mostly silica brick, mullite brick, andalusite brick, low creep high alumina brick. In the outside of the working layer insulation refractory bricks are used.
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. One of the approaches is to have the refractory design of the spherical dome design so that a long service life can be achieved with simple geometry. The dome is normally supported by the ring wall. The design of the ring wall aims for a most uniform vertical thermal expansion. This needs detailed calculations which 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 (Fig 4). 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.
Design where the dome refractory is not resting on the ring walls is also possible and has been successfully used. However in this case, it is to be noted that the critical areas like the expansion joint between wall and dome lining, the ‘overlapping areas’ of the refractories of the wall and dome lining and the shape of the pressure carrying steel shell need special attention and effort.
Division wall refractories
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. Division wall refractories normally consist of layers of bricks and a layer of insulation. The bricks layer are made of different refractory bricks as per the different temperatures, and the thickness is between 300 mm and 500 mm. Due to the high temperature of the upper part, the use of silica brick, mullite brick, high aluminum bricks are used while andalusite bricks and fire clay bricks are used in the middle part and the lower part.
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 wall due to different vertical thermal expansions. The optimum division wall construction needs numerous walls, which expand independently of each other. Multiple wall construction can 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 used.
Typically division wall is made of 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.
Fig 5 shows the principle of the division wall insulation. It also shows the leaning of the division wall (widely known as the Banana effect) towards the checker chamber as a result of the temperature difference, particularly in the lower part of the stove. 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.
Fig 5 Division wall refractories and checker support system
The internal combustion chamber design with the division wall has a weak spot on the corners of the combustion chambers. In the past frequent exceptional disturbances of operation, like explosions etc. led to damages in this area. Hence, these corners are to be supported properly. Normally the refractory material is reinforced in this area to solve this problem, while still allowing the required sliding gaps.
Regenerator is a chamber which contains a thermal storage. The regenerator chamber is constructed of refractory materials and is filled with a large number of checker bricks. It is also being called checker-work or the checker brick zone. Checker brick is used as the carrier of high temperature heat. The checker-work consists of a large volume of alumina and silica hexagonal bricks with several thousand of passage ways (normally called channels). A hot blast stove has around 300,000 chequer bricks in it. 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.
Checker bricks are assembled in the regenerator in such a way that the holes of two adjoining bricks are matched to allow the gas to pass. Checker bricks are heat storage and transfer medium. They are to have high thermal conductivity, good heat accumulation characteristics, strong heat exchange capacity, large heat storage area, smooth flow of gases, and small resistance to facilitate heat exchange and heat storage.
The checker bricks are supported on checker grid (Fig 5). 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. 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.
Until recently, hot blast stove checkers were literally the staggered arrangement of bricks with communicating passage ways of open spaces in between. The improved design which is now popular consists of fluted and parallel-bored hexagonal bricks, stacked in the regenerator so as to provide a dense triangular arrangement of parallel tubular channels, each one the full height of the column. Each open tube is around 35 mm face to face dimension. The main parameters of the checker brick include the regenerative area per unit volume, the quality of the unit volume, the quality of the brick, the diameter or the size of the channel, the equivalent thickness and the effective channel area.
The checker bricks are constantly subjected to compressive stress under the weight of the brickwork. The important features of the checker bricks are (i) good volume stability, (ii) good high-temperature loading creep property, (iii) high bulk density, and (iv) low apparent porosity. It has a plurality of open lattice holes parallel to the side surface, as well as the positioning bulge and positioning grooves on the two parallel surfaces. Also, the checker bricks need to have properties to withstand high temperatures, corrosion resistance, heat storage, heat transfer, quick cooling, good heat resistance, and long service life.
The size of the checker brick hole depends not only on the dust content of the combustion gas, but also on the air volume in the production. When the hot air blast temperature is high, then the hot blast stove need to have higher heat storage, which means a need to increase the number of checker holes, increase the heat storage area, and increase the heat storage capacity. However, this can cause the blockage of checker resulting into the increase of resistance and lowering the heat transfer.
At different height of the checker brick chamber, checker bricks made from different types of materials are used. In the high temperature zone at the top of the checker chamber, silica checker bricks are used. In the lower temperature zones, lower of the checker chamber, alumina checker bricks with different composition of alumina are used. Still lower in the chamber, checker bricks of andalusite are used. Finally in the lowest portion of the checker chamber, checker bricks made from fire clay material are used.
Since the temperature of the hot-blast stoves where silica bricks are used exceeds 1,400 deg C, the silica checker bricks are to have better creep resistance. For this reason, the silica bricks used in the hot blast stoves have a higher SiO2 content and a smaller amount of the liquid phase to cause creep. The phase transition of SiO2 due to the dissolution and precipitation reactions through the liquid phase is less likely to occur in the silica bricks of the hot-blast stoves.
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 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.
The thermal characteristics of checker bricks play a decisive role in the heat storage capacity, heat exchange capacity, and thermal efficiency of the hot blast stove. The normal requirement is to have a large heat transfer surface for heat exchange. There are to be a sufficient number checker bricks so as to ensure that there is no excessive hot blast temperature drop in the air supply cycle, and the flow of air in the hole is in turbulent state in order to improve the thermal efficiency.
The checker bricks which are normally used have hexagonal shape. The normal checker bricks used in the hot blast stoves are of three types namely (i) 7 holes bricks, (ii) 19 holes bricks, and (iii) 32 holes bricks. 7 holes brick is the traditional checker brick which is used in hot blast stoves. Fig 6 shows different types of checker bricks used in hot blast stoves.
Fig 6 Stove checker bricks
The holes in the checker bricks can be either round or hexagonal. The higher is the number of holes lower is the dimensions of the hole. For example, as per the GOST standard, the hole diameter is 40 mm for 7 hole brick, 30 mm for 19 hole brick, and 20 mm for 32 hole brick. However, the lower the dimension of the hole, the higher is the probability of its clogging, especially if there is high dust content of the fuel gas used in the stove. Also, there is higher loss of pressure across the checker work in case of lower dimension of the brick hole.
On the other hand, higher number of holes in the checker bricks provides higher area for heat storage and heat transfer. Hence the selection of the type of checker brick depends upon the quality of the fuel gas available for heating, and the quantity and the parameters of the hot blast air needed for the blast furnace operation.
Damage to the hot blast stoves refractories
There are different types of damages which can take place in the hot blast stove refractories. These damages are described below.
In the dome area, the type of refractory damage can be subsidence of dome, vertical cracking, and drop out of the bricks. In the dome base the refractory damage can be cracking and rupture of the bricks with load from the dome.
In the combustion chamber, the type of refractory damage can be lightening shaped cracking on the side and the partition wall. The damage in the burner can be spalling and cracking with rapid temperature change and load from wall bricks.
The deformation and failure of checker bricks can take place because of formation of cracks, and chemical corrosion of the brick material under the action of low-melting dust at high temperatures.
At the hot blast outlet mouth, the type of refractory damage can be drop out of the mouth bricks because of the complicated refractory structure. At the hot blast outlet pipe, the type of refractory damage can be cracking because of the steel shell movement around the expansion. At the junction of the hot blast outlet pipe and hot blast main, the type of refractory damage can be loosening and dropout of bricks with thermal expansion of the hot blast outlet pipe and hot blast main refractories.