The erosion of the lining in hot blast stove is the quality problem of refractory bricks?

July 27, 2018

The purpose of the hot blast stove is to use the heat of combustion of the furnace gas,and provide high temperature hot air for the blast furnace with the help of the heat exchange of the checker brick (about 1200~1350℃,the vault temperature is 150~250℃). In the modern modernization technology of blast furnace is to adopt high temperature technology. Increasing the hot blast temperature to the blast furnace can not only reduce the coke ratio, but also increase the output of molten iron.

The masonry of each part of the hot blast stove is subjected to the high temperature effect of gas combustion, the gas is eroded by dust, the scouring of combustion gas and the thermal stress of the temperature change during the heat exchange, as well as mechanical load. Because the hot blast stove is a kind of high structure, the brick of the regenerator and the combustion chamber, under the effect of mechanical load and high temperature, produces the shrinkage and deformation of the masonry and crack, which affects the service life of the hot blast stove.

1. Internal combustion hot blast stove

A large number of inspection results show that the corrosion of the combustion chamber is mainly caused by the erosion of the combustion chamber. The erosion condition of the partition wall is : the partition wall burns through, it is outburst inward, there are many cracks( the width, shape and length of the crack are different), the wall brick at the vertex hole is burned seriously , even broken and powdered. At the same time,the combustion chamber is changed from the original circle to the ellipse.The destruction of the ceramic burner is mainly at the outlet of the burner. It’s a great fluctuates in the operating temperature, and the high temperature (> 1000℃) radiation is subjected to the air supply, and the low temperature (<150℃) airflow is passed through the combustion, which makes the burner is subjected to a large alternating thermal stress. Secondly, the alkaline substance in the gas and water vapor, causing the burners to peel off and loosen. In addition, the combustion gas passage shrinks here, the air flow speed increase,and the mechanical vibration generated also causes the burner to loosen. Therefore, the front distribution cap of ceramic burner is usually selected as cordierite or andalusite brick with low thermal expansion rate and good thermal shock resistance, and high alumina brick or dense clay brick can be used for the burner gas and air passage section.In the regenerator section, since the entire regenerator checker brick are constructed by checker bricks from bottom to top, the independent checker brick of the entire regenerator are generally used for a long time, and the “S” deformation of the independent checker brick of the whole regenerator has a different degree of deformation, and twisted and protruded in the direction of the perforated hole in the combustion chamber wall. There is a wedge-shaped hole between the large wall of the regenerator and the checker brick, which runs through entire hot blast stove in the height direction. The checker brick and large wall bricks are fragmented and pulverized. A large number of investigation results show that slag, crack and deformation are common in the checker bricks of regenerator. Due to slag and cracks, cracked and powdered checker bricks have been found in the pores of the regenerator compartment checker brick and in the cracks between the independent checker brick.The problem of brick wall is concentrated on cracks and surface pulverization. The beam portion of the furnace raft pillar is deformed, and the nearby pillars are sunk. The furnace itself also has problems with deformation, displacement and bending. The large wall masonry of regenerator has been working under high temperature and high pressure for a long time, and the conditions are very bad. The main reasons for the erosion of the large wall masonry are as follows,

1.1Thermal stress. The hot blast stove masonry works under high temperature conditions for a long time, and the temperature changes periodically within a certain range. As a result, on the one hand, the volume of the masonry changes, and on the other hand, it works under the effect of the thermal stress for a long time, causing the masonry to crack, peel off, the masonry is loose.

1.2.Mechanical load. Due to the mechanical load generated by the masonry itself, the masonry works under pressure for a long time, and the compressive stress and thermal stress are combined, cause creep deformation of the masonry and the brick joints.

1.3Chemical erosion. Because the dust in the gas contains certain alkaline oxides, these substances adhere to the surface of the masonry and penetrate into the interior, gradually chemically reacting with the refractory material to form low-melting materials, which leads to the high temperature properties of the masonry, such as the degree of refractoriness and the decrease of the thermal strength, and the formation of deformation or mutually fused to form a slag phenomenon. In recent years, due to the improvement of gas purification technology, the dust content of gas has been reduced, and the damage of dust to masonry has been greatly reduced.

1.4 Pressure effect. The hot blast stove periodically carries on the combustion and air supply, and is in a low pressure state during the combustion period, and is in a high pressure state during the air supply period. In the traditional large wall and vault structure, there is a large scale between the top of the furnace and the furnace shell. The filler layer between the large wall and the furnace shell also has some space after shrinking and natural compaction under long term high temperature. Due to the existence of these spaces and the pressure of high pressure gas, the masonry bears a lot of outward thrust, which easily causes the masonry to tilt,crack and loosen,then the outer space is periodically pressurized and relieved by the brick joint, thus aggravating the damage to the masonry.The “plate”type structure furnace wall is used in modern times, and the expansion joint is arranged according to the single block masonry. The overall expansion space is not provided between the large wall and the furnace shell, thus the effect of pressure on the masonry is eliminated.

2. External combustion type hot blast stove

The history of using external combustion type hot blast stoves in China is not long. Since the beginning of Baosteel construction, more steel companies in China have used external combustion hot blast stove.

According to the results of the use, the external combustion type hot blast stove is different from the internal combustion type hot blast stove due to its structural characteristics. The damage parts are in the combustion chamber, regenerator, the top of furnace, the contact pipe, the hot blast pipe, checker brick and the grate, but the most serious problems are concentrated on the contact pipe and the hot blast pipe on the top of the furnace.From the state of erosion, it shows the metamorphism of the working layer on the upper surface of the burner, the cracks, sinking and cracking of the large wall, the brick body in the end of the hot blast pipe,the fracture and fall off of the inner lining of the hot blast pipe, the hot blast pipe. The checker brick are broken and so on.

3. Crack in the top of the furnace and a large wall

Generally, after the hot blast stove is cooled, all kinds of cracks appear on the top of the furnace and on the large wall of the cylinder. In particular, the problem of silica brick masonry is more prominent.

Cracks can be divided into permanent cracks and non-permanent cracks:

a.Through the long-term research and practice of the hot blast stove technology, the main reason for the permanent crack is due to the unreasonable design of the hot blast furnace masonry structure and the defects in the key technical performance of the lining material. The basic feature of this type of crack is that the surface of the brick inside the crack has obvious burning marks. This crack reaches a certain level and will jeopardize the overall use of the hot blast stove. For such problems, it is necessary to study and improve the selection of the lining, the design of the structure, and the supporting aspects of the structure and materials.

b. Non-permanent cracks, which are mainly formed during the cooling process. Due to the mineral and physical properties of the silica bricks, if the speed of the furnace is too fast in the middle and low temperature stages, the temperature stress inside the silicon brick masonry exceeds its structural strength. Cracks are formed in both the brick body and the structure, so there is also a literature called "structural crack". The basic characteristics of this kind of crack are :there is no burning mark on the surface of the brick inside the crack,and the surface state is similar to the fracture surface after the brick body is stressed. The direction of such cracks is substantially vertical, from bottom to top, and varies in size. Such cracks can generally be automatically closed after heating in the reheating oven, but if the crack is too large, or the penetration depth of the crack is too deep, reaching the multilayer refractory bricks on the back of the working layer, or even the furnace shell, the crack may become a permanent crack. This situation can be alleviated or even eliminated by controlling the furnace technology. Special attention is required for this in hot blast stove when cooled.