After combustion, hot gas rises along the combustion chamber, enters the regenerator to heat the checker bricks, and then discharges through the flue below. Taking the internal combustion hot blast stove as an example, fuel is injected from the lower part of the combustion chamber. During the heating process, high-pressure air is blown into the regenerator, heated by the heat stored in the checker bricks, and then extracted from the middle section of the combustion chamber to be blown into the blast furnace. Once the air supply temperature drops below the required level, the cycle switches back to the combustion heating phase, repeating continuously. Therefore, to ensure a continuous supply of high-temperature preheated air to the blast furnace, each blast furnace is generally equipped with 3-4 hot blast stoves.
For the normal operation of regenerative hot blast stoves, the valves in the equipment system must continuously switch between open and closed states, and the flow rates and pressures of various gases (such as gas and air) must be continuously adjusted. Therefore, the transmission and control systems of hot blast stove equipment are critical components. The simplest operation is manual, while electric operation is widely used. Hydraulic and pneumatic operation devices have also been developed.
Hot blast stoves are subjected to the following effects during operation:
- Temperature Gradients:
- During the heating phase, the temperature in the combustion chamber is extremely high, with the furnace top temperature reaching 1500–1560°C. The temperature gradually decreases from the vault along the furnace wall and checker bricks.
- During the air supply phase, high-velocity cold air enters from the bottom of the regenerator and is gradually heated.
- Alkali Metal Erosion:
- Gas and combustion air contain certain amounts of alkaline oxides. Combustion ash contains approximately 20% iron oxide, 20% zinc oxide, and 10% alkaline oxides (mainly potassium oxide). Most of these substances are discharged out of the stove, but a small portion adheres to the surface of the masonry and penetrates into the brickwork, accumulating over time to a depth of about 20 mm.
- These substances react chemically with refractory bricks to form alkaline aluminosilicates, causing volume changes, structural damage, cracking, and reducing strength and high-temperature performance. This chemical erosion is particularly prominent in the upper part of the regenerator.
- Under certain dust content and operating temperature conditions:
- Clay bricks react to form products with lower melting points, creating a glassy layer on the brick surface that diffuses inward through capillaries, leading to deeper erosion.
- High-alumina bricks produce products with higher melting points that adhere to the brick surface and grow gradually.
- Silica bricks have reaction products that are volatile, keeping the surface smooth and reducing erosion.
- Load Effects:
- Hot blast stoves are tall structures, typically 25–50 meters in height.
- The maximum static load on the lower part of the regenerator checker bricks can reach 0.8 MPa.
- The lower lining of the combustion chamber bears a static load of 0.4–0.5 MPa.
- The pressure at the arch foot bricks of the furnace top is 0.02–0.2 MPa.
Refractory Materials for Hot Blast Stoves
The selection of refractory materials for hot blast stoves is primarily determined by the hot air temperature:
- When the hot air temperature is below 900°C, clay bricks are generally used for masonry, with some having a service life of up to 20 years.
- When the air temperature exceeds 900°C, high-alumina bricks, mullite bricks, sillimanite bricks, and silica bricks are used for the linings and checker bricks in high-temperature zones. However, countries may differ in their refractory material choices.
- Large blast furnaces in Japan mainly use the Koppers type, with the Nippon Steel type being increasingly constructed in recent years.
- Germany uses Koppers or MacIntyre-type hot blast stoves.
- Other Western European countries mostly adopt the Didier type.
- The United States focuses on retrofitting internal combustion hot blast stoves in addition to building some external combustion types, aiming to achieve higher air temperatures and extended service life.
In recent years, newly built or retrofitted blast furnace hot blast stoves worldwide predominantly use the external combustion type, with air supply temperatures of 1200–1350°C and top temperatures generally 1500–1550°C, even approaching 1600°C. Therefore, silica bricks are widely used for masonry in high-temperature lining zones and the upper layers of checker bricks, yielding good performance results.