What Refractory Bricks Are Used For Each Part Of The Hot Blast Stove?

A hot blast stove is equipment for supplying hot air to the blast furnace. It is a regenerative heat exchanger used to preheat hot blast for the blast furnace and provide stable high-temperature hot air for efficient blast furnace operation. The air volume and blast temperature of the hot blast stove shall meet the requirements of blast furnace ironmaking. Blast temperature is an inexpensive energy source for blast furnace ironmaking. Increasing blast temperature can significantly raise the coal injection rate of the blast furnace, reduce coke ratio and cut the production cost of pig iron. High blast temperature is an important means to achieve high efficiency and low energy consumption in blast furnace ironmaking. To continuously supply hot air to the blast furnace and facilitate equipment maintenance without affecting blast temperature during overhaul, one blast furnace is generally equipped with 3 to 4 hot blast stoves, whose service life should be twice that of the blast furnace.

The combustion chamber of a hot blast stove forms an empty tower structure enclosed by furnace walls. The inner side of the furnace wall is lined with one layer of refractory bricks, and the outer side has 1 to 2 layers of insulating bricks and buffer packing materials. The burner at the bottom fully mixes the introduced gas and combustion-supporting air, which burn inside the combustion chamber. The generated high-temperature gas flows to the dome of the regenerator and is then distributed to all checker holes to heat the checker bricks. A hot blast outlet is arranged on the upper part of the combustion chamber, connected with a hot blast valve and hot blast duct to deliver hot air.

The furnace wall of the regenerator is fully built with heat storage checker bricks. The checker holes of the checker bricks run through from top to bottom. The hole shapes include round and rectangular ones. Rectangular holes are laid with alternating long and short sides to enhance convective heat exchange. All checker bricks inside the regenerator are supported on heat-resistant cast iron posts and a grate. The holes of the grate shall align with those of the checker bricks. Waste gas from the hot blast stove converges between the posts and is discharged through the flue. Cold blast is introduced from the cold air inlet, distributed to all checker holes between the posts and heats the holes.

According to different layout structures of combustion chamber and regenerator, hot blast stoves are generally divided into three types: internal combustion type, external combustion type and top combustion type. At present, the three types coexist in China. Internal combustion hot blast stoves include ordinary internal combustion type and the improved Hoogovens (DanieliCorus) internal combustion type; external combustion hot blast stoves include Didier type, Cowper type and Martin type; top combustion hot blast stoves include Kalugin top combustion type and the improved top combustion type developed by Shandong Metallurgical Design Institute.

Compared with internal combustion hot blast stoves (including improved versions) and external combustion hot blast stoves, top combustion hot blast stoves feature a symmetrical structure, small engineering quantity, less floor area and low maintenance cost, making them simple and reasonable. The inner shell area is utilized efficiently, and 100% of the cross-section inside the main wall is used for heat storage. High blast temperature and long service life can be achieved at a relatively low dome temperature, and the working conditions of refractory materials are obviously improved, realizing both high blast temperature and long service life. In recent years, a considerable proportion of newly built blast furnaces in China have adopted top combustion hot blast stoves. To raise temperature, many internal combustion hot blast stoves have been reconstructed into top combustion type. Top combustion hot blast stoves have become the main development direction of hot blast stove technology.

The improved internal combustion hot blast stove adopts a design with the combustion chamber and regenerator arranged longitudinally and parallelly inside one shell. Only about 67% of the area inside the main wall is used for heat storage; refractory materials work under poor conditions and have high quality requirements.

For external combustion hot blast stoves, the combustion chamber is arranged outside, leading to large floor area, high consumption of refractory materials and steel, heavy capital construction work and high investment. It is connected to the regenerator via connecting ducts, with a complex structure. Manufacturing of refractory components, especially the dome connecting ducts, duct ports and ceramic burners, is difficult and complicated with high cost. The furnace body bears uneven stress, so it is hard to achieve long service life.

Due to different structural forms of hot blast stoves, the damage conditions of furnace linings vary: partition walls are vulnerable parts for internal combustion hot blast stoves, while for external combustion hot blast stoves, damage easily occurs at high-temperature zones such as domes of combustion chambers and regenerators, and the connecting bridge between the two chambers. The main comprehensive damage causes are as follows:

(1) Thermal stress. The furnace lining and checker bricks of hot blast stoves are always under alternating cold and hot conditions. Thermal stress tends to cause cracks, spalling and loosening of masonry. Therefore, refractory materials shall have good thermal shock resistance.

(2) Chemical erosion. The gas for combustion and combustion-supporting air contain certain alkaline oxides (iron oxide, zinc oxide, etc.). These substances adhere to the masonry surface and penetrate inward, gradually reacting chemically with refractory materials to form low-melting substances and degrade the high-temperature performance of refractories. Hence, refractory materials shall have high purity and low impurity content.

(3) Mechanical load. A hot blast stove is a tall structure. The checker bricks in the lower part of the regenerator bear a load up to 0.8 MPa, and the lining in the lower part of the combustion chamber bears a load of about 0.5 MPa. Under long-term heating, the masonry tends to produce large shrinkage deformation, which seriously affects the service life of the hot blast stove. Therefore, refractory materials in high-temperature zones shall have good creep resistance, and those in medium-temperature zones shall have high strength.

A hot blast stove is a thermal equipment with cyclic repeated heating and heat release. Based on current application conditions, the main requirements for heat storage materials of hot blast stoves are listed below:

(1) Good volume stability. That is, no excessive expansion or shrinkage caused by internal crystal phase changes during high-temperature service, to ensure good volume stability of heat storage bricks and masonry.

(2) Excellent high-temperature load creep resistance. The service life of a hot blast stove can exceed 20 years. Refractory materials bear heavy load from their own weight, so good creep resistance at high temperature is required. Silica bricks have extremely outstanding high-temperature creep resistance, followed by high-alumina products with composition close to mullite.

(3) High heat storage performance. This requires materials with high bulk density and high specific heat capacity. Silica bricks have low bulk density and poor heat storage capacity. Moreover, crystal transformation easily occurs below 600℃, destroying the integrity of the material. Therefore, silica bricks are not suitable for areas below 600℃. In addition, slow heating or cooling shall be adopted during hot blast stove drying-out and shutdown to ensure slow transformation of quartz crystals without material damage.

(4) Relatively high thermal conductivity. High thermal conductivity enables the regenerator to absorb and release heat at high speed and efficiency in each heat exchange cycle. Thermal conductivity is one of the key physical properties of refractory materials. It is related to chemical composition, mineral phase composition, densification, microstructure and temperature of the material. For alumina-silica refractory materials, thermal conductivity increases with the rise of Al₂O₃ content. Therefore, if high-alumina bricks with good heat storage capacity are used as checker bricks, the blast temperature of the hot blast stove can be greatly improved.

(5) Low linear expansion coefficient. Linear expansion coefficient refers to the property that the volume and length of refractory materials increase with temperature rise. Before 600℃, silica bricks have a large linear expansion rate; above 600℃, the linear expansion rate remains almost unchanged. Therefore, the service temperature of silica bricks shall not be lower than 600℃.

The dome is the space connecting the combustion chamber and regenerator, consisting of working layer bricks, filler layer and thermal insulation layer. It shall maintain structural stability at high temperature during operation and distribute high-temperature flue gas evenly into the regenerator during combustion. For this reason, the dome is designed to be spherical to avoid unstable lateral thrust on the furnace shell. Since the dome area of a hot blast stove operates at ultra-high temperature exceeding 1400℃, low-creep high-alumina bricks are adopted for the working layer. Silica bricks, mullite bricks, sillimanite bricks, andalusite bricks or low-creep high-alumina bricks can also be selected for this zone. Outside the working layer, diatomite bricks or insulating fireclay bricks are laid, and the outermost layer is filled with granulated blast furnace slag or granulated slag-diatomite packing.

The main wall refers to the enclosure of the hot blast stove body, including working layer bricks, filler layer and thermal insulation layer. Different refractory bricks with thickness of 300~500 mm are selected for working layer bricks according to temperature variation from top to bottom. Due to the highest temperature at the upper part, silica bricks and mullite bricks are mostly used. High-alumina bricks, sillimanite bricks and fireclay bricks can be adopted for the middle and lower parts. For thermal insulation, one layer of lightweight fireclay bricks is often built outside the working layer bricks in high-temperature zones, and thermal insulation filler is filled between the two insulating layers.

The regenerator is a space filled with checker bricks. Its main function is to realize heat exchange between internal checker bricks, high-temperature flue gas and combustion-supporting air. Silica checker bricks are mostly used in the upper section of the regenerator; low-creep high-alumina bricks, mullite bricks, sillimanite bricks, andalusite bricks, etc. are used in the middle section. Fireclay bricks are generally used for the lower section.

The combustion chamber is the space for gas combustion. The layout of the combustion chamber is closely related to the type and structure of the hot blast stove. Silica bricks, mullite bricks or low-creep high-alumina bricks are used in high-temperature zones of the combustion chamber, while ordinary high-alumina bricks and fireclay bricks can be used in medium and low-temperature zones.

The partition wall is a furnace wall arranged inside internal combustion hot blast stoves to separate the combustion chamber and regenerator. The partition wall of a hot blast stove is heated on both sides, while the main wall is heated on one side only. Therefore, the internal furnace wall of internal combustion hot blast stoves has large expansion, which may cause the partition wall to collapse toward the checker brick side. Besides, the temperature difference on both sides of the lower partition wall is large, leading to a big difference in thermal expansion. The partition wall may bend and crack in service, resulting in burnout of the bottom grate and posts. Expansion joints shall be reserved and reinforcement measures adopted during masonry. The refractory materials for partition walls are the same as those for combustion chambers.

A hot blast stove burner is equipment that mixes gas and air and feeds the mixture for combustion. Burners used in hot blast stoves are divided into metal type and ceramic type. Among them, ceramic burners are mainly made of mullite or cordierite-mullite.