Generation of Hot Air Blast and Hot Blast Stoves

Generation of Hot Air Blast and Hot Blast Stoves           

A hot blast stove is a facility to supply continuously the hot air blast to a blast furnace. Before the blast air is delivered to the blast furnace tuyeres, it is preheated by passing it through regenerative hot blast stoves that are heated primarily by combustion of the blast furnace top gas (BF gas). In this way, some of the energy of the top gas is returned to the blast furnace in the form of sensible heat. This additional thermal energy returned to the blast furnace as heat reduces the requirement of blast furnace coke substantially and facilitates the injection of auxiliary fuels such as pulverized coal as a replacement for expensive metallurgical coke. This improves the efficiency of the process. An additional benefit resulting from the lower fuel requirement is an increase in the hot metal production rate. All of these have a significant effect in terms of reducing the hot metal cost.

History of hot blast stoves

The use of blast furnaces dates back as far as early as fifth century B.C. in China. However, it was not until 1828 that the efficiency of blast furnaces was revolutionized by preheating them using hot stoves in conjunction with the process, an innovation created by James Beaumont Nielson, previously foreman at Glasgow gas works. He 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. In 1860, the cooperative use of hot stoves with blast furnaces was further transformed by Edward Alfred Cowper by recycling the top gas of the blast furnace rather than receiving solid fuel as did the earlier designs.

Early designs of hot stoves used with blast furnaces were originally placed on top of the furnace rather than next to it, the current layout used today. They used waste heat from the blast furnace delivered via cast iron pipes to the hot 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 furthered the design of the layout of the hot stove with the blast furnace to the use of two to four hot stoves placed in series beside the blast furnace. This allowed for the heating of one blast stove by blast furnace top gas as the other one was being drained of its heat to preheat the air blast into the blast furnace. As the air blast entered the stove, it was preheated by hot bricks and exited the stove as a hot blast. Cambria Iron Works was the first company in the U.S. to use regenerative stoves in 1854. These stoves were constructed of iron shells lined with refractory and contained multiple passageways of refractory for the blast throughout. A typical stove of this design had about 186-232 sq m of heating surface. In 1870, Whitwell Stoves designed and produced larger stoves with heating surfaces of about 8546 sq m, which could deliver 454-566 deg C hot blast to the blast furnace. These were also the first stoves to use hexagonal refractory checkers, cast iron checker supports, and semi-elliptical combustion chambers to enhance the distribution of gas throughout the checkers.

Characteristics of a modern hot blast stove

The calorific value of blast furnace top gas is not high enough in value to achieve the high flame temperature required for the higher hot blast temperatures of 1000 deg C to 1200 deg C.  Hence the blast furnace gas for the stoves is normally enriched by the addition of a fuel of much higher calorific value, such as coke oven gas for obtaining the high flame temperature. However many of the modern blast furnaces are having hot blast stoves, which have burners designed for the use of only the blast furnace gas.

Hot blast stoves of a modern blast furnace have the following characteristics.

  • Achievement of high efficiency combustion – Achievement of high efficiency combustion even in the operation with only blast furnace gas.
  • Smaller heat radiation from the stove body.
  • Low construction costs.
  • High stove service life -Expected service life of a modern stove is around 40 years
  • Complete elimination of stress corrosion cracking.
  • Low concentration of uncombusted CO above the upper surface of checker bricks.

Most blast furnaces are equipped with three hot blast stoves, although in a few instances there are four. The stoves are 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. Each stove is about as large in diameter as the blast furnace, and the height of the column of checkers is about 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. 1 shows 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.

Cross section of a BF stove

Fig 1 Typical cross sectional view of a conventional hot blast stove

There is an insulating lining just inside the steel shell. This is normally very thick on the side near the combustion chamber. The combustion chamber is completely surrounded by a brick well wall, which is lined with super duty refractory bricks containing 50 % to 60 % of alumina. For very high hot blast temperatures in excess of 1200 deg C, the entire combustion chamber and the dome are lined with this type of brick. Also, the top 8 m to 10 m of checkers are normally of super duty bricks.

However, for in newer furnaces for the stoves. silica refractories are the ,material of choice for improved stability owing to the elimination of expansion movements in the upper structure during operation. Silica refractories have an additional advantage over alumina refractories, since they are resistant to dust accumulation. For this reason, seven layers of silica checkers are normally installed at the top of the checker shaft, in alumina based stoves.

In erecting the dome lining, arch bricks are used and a space is provided between the brick and the dome to allow for expansion of the ring wall from which it is supported. In some stoves, there is an offset in the steelwork at the top of the ring wall so that the dome brick can be supported independently.

Traditional hemisphere domes, although simple in shape, have a natural instability with a tendency for the upper part of the dome to collapse first. Hence some blast furnaces have an inverted catenary shaped dome. This dome has a statically balanced shape and can be built with a minimum of special shaped bricks. Since the mushroom dome refractories also expand and contract, a hinged support construction allows for these movements, without exerting any force on the structure.

With better gas cleaning facilities available these days,  it is possible to use checkers with smaller flue openings without any danger of plugging of the flues with dirt. With smaller flues, heat transfer rates are better because the ratio of heating surface to checker weight is large and more checker weight are installed in the available space. However, with the smaller flue openings, it became very important to lay up the checkers properly so that the flues match perfectly. Misaligned flues increase the pressure drop through the stoves significantly and prevent effective use of all the heat storage capacity.

The burner for the blast furnace 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 that 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.

Combustion continues while the gas ascends up the combustion chamber. When a stove is to be heated from the cold condition, an igniter is normally used to start combustion but, during normal operation, the residual heat in the target wall is sufficient to cause ignition.

In several modern hot blast stoves, ceramic burners are used. These 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 port through which the hot blast air exits from the stove is located on the side of the combustion chamber usually  4 m to 7 m above the burner. Between the stove and the hot blast main there is located a water cooled hot blast valve that prevents the high pressure air in the main from entering the stove during the heating process. The hot blast valve is usually located a short distance away from the stove to reduce the amount of radiation it receives from the combustion gases. In several blast furnace shops, the cold mixing air that is used for controlling the temperature of the hot blast is mixed with the hot air from the stove on the stove side of the valve. This is to prevent the valve from being exposed to air at the maximum temperature obtained in the stove dome. Some blast furnaces have a central single cold blast mixer opening that is located in the hot blast main between the closest stove and the furnace itself.

The central system has the advantage of fewer thermal cyclings of the hot blast main with the higher temperature systems. Most of the hot blast valves are of the gate type or of the mushroom type and are 1.2 m to 2.0 m in diameter.

The reheating of the stove requires instrumentation in the dome area, the checker refractory and the waste gas exit area at a minimum. There is an opening in the dome of the hot blast stove through which a thermocouple or a radiation type temperature detector is usually inserted. This instrument is to control the amount of gas and air during the firing process. The temperature monitoring instruments in the dome, checkers and waste gas area are also used to protect the refractories from an overheating condition.

In the plenum chamber below the grid that supports the checkers, there are openings to the chimney and to the cold blast main. Generally, there are two chimney valves, ranging in size from 1.5 m to 2.0 m in diameter, which are opened when the stove is being heated so that the products of combustion are drawn out to the stove stack. When the stove is on blast (heating the blast air), the chimney valves are closed. The seats of the valve are arranged so that when the stove is on blast, the pressure in the stove holds the seats together to prevent leakage. When the stove is to be taken off blast and put on heating, there is a blow off valve that is opened to relieve the pressure. Because of the need to depressurize the stove rapidly, the air is to exit at a very high velocity. Consequently, the blow off valves are equipped with silencers to keep the noise level within tolerable limits.

The cold blast valve is the type that is held closed by the pressure in the cold blast main. Before this valve can be opened, the small ports in the valve disc are opened to pressurize the stove and equalize the pressure on each side of the valve.

At several modern blast furnaces, the stoves are equipped with combustion chambers completely external to the stove shell. These stoves are having external metallic burners situated near the top of the stove.

The advantage of this design is that the entire stove shell can be filled with checkers. Furthermore, the thermal pattern in the stove is much more symmetrical and there are far fewer stresses that tend to distort and rupture the brickwork. However, there have been many stress induced problems that have caused rupturing in the steelwork of the junction section between the combustion chamber and the stove. As a result, frequent repairs to the steelwork are required in this location.

Air network

Between the hot blast stoves and the blast furnace blower is the cold blast main. It is unlined because the temperature of the cold blast is usually 150 deg C to 250 deg C, which is the temperature resulting from the heat of compression at the blower. At the stove end of the main are the cold blast valves for the stoves and the mixer line equipped with a butterfly valve. To maintain a constant hot blast temperature to the blast furnace, a thermocouple in the hot blast main controls this butterfly valve in the mixer line and proportions the amount of air delivered to the stove and the amount bypassing it.

When a heated stove first goes on blast, the temperature of the heated air is much higher than the desired hot blast temperature, so a significant portion of the air must bypass the stove. As heat is removed from the stove and the temperature decreases, the mixer line butterfly valve is gradually closed  and force more of the air through the stove. In some automatic stove changing systems, the position of the regulating valve is used as the signal that initiates a stove change.

The cold blast main is also equipped with a snort valve, usually located near the blast furnace, that is opened when it is necessary to decrease the blast pressure rapidly. This discharges the cold blast air to the atmosphere and keeps a positive pressure on the cold blast line so that gas from the furnace cannot travel back to the blower. Because of the rapid discharge of air when the snort valve is opened, it is also generally equipped with a silencer.

For generating the blast air, many blast furnaces are equipped with centrifugal turbo blowers provided with three or four stages. For very large blast furnaces, two blowers are generally provided  which operate in parallel. However, with very large blast furnaces an axial blower can be used more efficiently.

At plants, where the blast is enriched with oxygen, the oxygen is normally added at atmospheric pressure to the inlet of the turbo blower or it can be added under pressure in the cold blast main. Moisture is added in the cold blast main when it is required for blast moisture control.

The blowpipe, which connects the hot blast system to the tuyere, fits into a machined spherical seatat the base of the tuyere. The tuyere cooler and the tuyere are water cooled. In modern blast furnaces with hot blast temperatures of 1000 deg C to 1200 deg C, the tuyere body water passages are designed to keep the water velocity above 20 m/sec and the tuyere nose water passages are designed to keep the water velocity above 27.5 m/sec for improving the rate of heat transfer. Usually the nose of the blowpipe is also water cooled, although in the older blast furnaces it was not done. The auxiliary fuel injection lance enters through the wall of the blowpipe and usually discharges the fuel slightly off the centerline and about 50 mm back from the nose of the blowpipe. With the use of pulverized coal as a tuyere fuel, the injection lance placement is more critical to avoid impingement on the inside of the tuyere and for better combustion of the pulverized coal.

The blowpipe is held tightly against the tuyere by tension in the bridle rod, which connects the tuyere stock to the hearth jacket. The bridle spring on the end of the bridle rod allows limited motion as the blowpipe expands and contracts with changes in hot blast temperature. The blowpipe itself is an alloy steel tube lined with refractory material to prevent the metal from becoming too hot.

At the back of the tuyere stock on the centerline of the blowpipe and tuyere is a small opening through which a rod can be inserted for cleaning material out of the blowpipe. The opening is closed by a cap that can be opened when necessary but is gas tight when closed. In this cap, called a tuyere cap, there is a glass covered peep sight that permits the operator to inspect the interior of the furnace directly in front of the tuyere. The upper part of the stock is connected by a swivel joint to the refractory lined nozzle of the gooseneck to which it is clamped by lugs and keys that fit into seats of hanging bars. Each gooseneck in turn is connected by flanges and bolts to a neck extending radial from the inside diameter of the bustle pipe. The bustle pipe is a large, circular, refractory lined and insulated pipe that encircles the furnace at above mantle level and distributes the hot blast from the hot blast main to each tuyere connection.

Stove operation

These days, with the use of well prepared burdens and good control of burden distribution, the operation of blast furnaces  is much more uniform. Hence blast furnaces are normally operated very near to the maximum hot blast temperature that the stoves can maintain or that the particular burden materials can accept without causing premature melting and poor burden movement.  With higher hot-blast temperature, the blast furnace operation is more efficient because a larger percentage of the heat consumed is furnished by the sensible heat of the hot blast and less fuel is needed in the blast furnace. In the operation of the hot blast system, the ceramic checker work of the stoves is heated by the combustion of blast furnace gas sometimes supplemented by coke oven gas, and then the air from the blowers is passed through the stoves and is heated by the hot checker work. In the heating cycle, the stoves are fired until the temperature of the exit gases at the stack valves has reached an established maximum temperature of around 400 deg C to 450 deg C, while simultaneously being careful not to overheat the stove domes. During the heating cycle the temperature at the dome of the stove is controlled so that it does not exceed a maximum, which is determined primarily by the type of refractory material used for the lining of the dome. If the dome temperature reaches this maximum before the stack temperature reaches its maximum, excess air is added through the burner to hold down the flame temperature and prevent the dome from being overheated while the firing is continued until the stack gas temperature reaches its limit. However, if the dome temperature does not increase rapidly enough to reach its maximum allowable temperature by the time the stack gas temperature reaches its maximum, the blast furnace gas is usually enriched with a fuel of higher calorific value to obtain a faster heating rate.

After the stove has been heated, it is ready to be put on blast. This is done by first shutting off the gas and the air supply to the burner and then closing the burner shutoff valve and the chimney valves. The cold blast valve is then opened in such a way that the air entering the stove brings it to a pressure equal to the blast pressure without reducing the blast pressure excessively. In some of the modern blast furnace installations, the blower controls are switched from constant volume control to constant pressure control during a stove change. In such a system, the blower speeds up so that the stove can be filled and pressurized rapidly without causing a detectable decrease in the blast pressure.

After the stove is filled, the mixer valve (which controls the amount of cold air which is bypassed around the stove to be mixed with the very hot air from the stove to produce the desired hot blast temperature) is set at approximately the correct opening. The hot blast valve is then opened to put the stove on blast and, once the stove is on blast, the hot blast temperature controller automatically adjusts the mixer valve opening to maintain the desired hot blast temperature.

The hot blast stove after its use, is then taken off blast by closing first the cold blast valve and then the hot blast valve. The blow off valve is then opened to depressurize the stove and, after depressurizing,  the chimney valves are opened and the blow off valve is closed. Next, the burner shutoff valve is opened, and the air supply to the burner is turned on. Finally, the gas shutoff valve is opened to obtain the desired gas flow rate.

At modern blast furnace installations, the stove valves are motorized and the valve changing is automated so that only about three minutes are needed for a stove change. With the shorter changing time, the heating time can be increased so that higher hot blast temperatures can be used and greater efficiency can be obtained. The automatic stove changing cycle can be initiated either by having the stove tender push a button when the change is required or by a completely automatic electronic signal. This signal can be based on the extent of the mixer valve opening (as, for example, when the mixer valve is 85 % closed), on the dome temperature, or strictly on a time cycle.

Typically, blast furnaces are equipped with three hot blast stoves, and each stove is kept on blast for around one hour. Thus, the amount of heat that is extracted from the stove while it is on blast must be put back into the stove in the heating period which is simply twice the on blast time minus twice the stove changing time. At some furnaces, there are four stoves. With the extra stove, the firing rate does not have to be as great because the heating cycle is three times the on blast cycle minus twice the stove changing time. Another advantage of the extra stove is that if there is a problem with the stove equipment, the stoves can be repaired one at a time without significantly affecting the operation of the furnace. Fig 2 give a typical layout with three hot blast stoves.

Typical layout with three BF stoves

Fig 2 Typical layout with three hot blast stoves