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Maintenance of coke oven body – coke oven plant

Coke ovens are industrial kilns that perform continuous production at high temperatures for a long time – coke oven plant. Compared with other industrial furnaces, it uses a large amount of bricks, has a complex structure, has high infrastructure costs, and has a long furnace life. It generally does not stop after being put into operation, otherwise the furnace body will be seriously damaged. The main parts of the coke oven are made of silica brick refractory materials. In daily production, due to the impact of temperature stress caused by cold and hot changes during coal loading and coking, as well as the extrusion and friction of the mechanical force of coke pushing, coupled with the erosion of harmful substances in the gas, as time goes by Defects such as peeling, chipping, dents, cracks, misalignment, etc. are inevitable on some masonry parts (especially the furnace head of the carbonization chamber). Under special circumstances, large-area deformation of the furnace wall may occur. , even bricks fell and collapsed. These are all specific manifestations of the aging of coke ovens. Severe operation and aging of coke ovens will cause adverse consequences such as reduced thermal efficiency, difficulty in pushing coke, deterioration of operating conditions, and even production reduction or suspension. The aging damage of coke ovens can be divided into two types: normal natural aging and abnormal aging damage. Normal aging is inevitable, while abnormal aging is accidental and avoidable. A coke oven that is normally damaged and properly maintained can generally have a service life of more than 30 years. Coke ovens that have suffered various abnormal damages or have not been properly maintained often fail to reach the designed service life, or even only have a service life of more than ten or several years. Therefore, strengthening the production technology management of coke ovens and timely thermal repair and maintenance are important ways to extend the life of coke ovens.

coke oven plant

Causes of coke oven damage

1.1 Causes of abnormal damage

⑴The refractory materials used to build the furnace do not meet the standards, especially when the physical and chemical properties are too poor, it is easy to cause premature aging of the coke oven.

⑵ The masonry quality of the coke oven is too poor, the geometric dimensions exceed the tolerance requirements, the mortar joints of the brickwork are not full, and after the operation is put into operation, the raw gas or the clean gas for heating leaks seriously, causing local high temperatures and causing damage to the furnace body.

⑶The quality of the oven is poor. Due to poor management of oven temperature rise, step cracks opened up on the walls of the carbonization chamber, chute, and regenerator, and roof bricks and furnace bottom bricks broke.

⑷ Improper management of the iron parts protecting the furnace or the coke oven door smokes and catches fire and burns out the iron parts protecting the furnace, causing the furnace body to lose its due protection, causing the furnace head masonry to loosen, deform, or even collapse.

⑸ Long-term use of coking coal with excessive expansion pressure or too small contraction makes it difficult to push coke.

⑹ Coking or overcoking often occurs, coking time is too long, too short, or coking is not pushed as planned, and production management is chaotic, causing difficulty in pushing coke and deforming the furnace wall.

⑺Lack of timely hot repair and maintenance accelerated the damage of the furnace body. The above-mentioned abnormal damage phenomenon of coke ovens is purely a matter of production management and technical management. As long as management in all aspects is strengthened, the above situation can be avoided.

1.2 Causes of normal damage

⑴The influence of temperature changes. During the production process, when the coke oven door is repeatedly opened and closed, coal is loaded, and coke is discharged, the thermal stress produced by temperature changes affects the furnace wall. This impact causes the burner head to begin to suffer erosion or cracks 3-5 years after it is put into production. As production time went on, the damage increased and extended into the furnace. The coal loading port is greatly affected by the external cold air flow, and soon causes damage such as erosion and cracks. The roof bricks on the stove head are often broken due to sudden temperature changes.

 

⑵The role of mechanical force. After cracks or deformation appear on the wall of the carbonization chamber, the mechanical stress generated by removing and installing the coke oven door and pushing coke will promote the expansion of cracks in the furnace wall and the intensification of wall deformation. Especially when it is difficult to push the focus, the impact is more serious. In order to reduce the mechanical force endured by the furnace wall, all factors that cause difficulty in pushing coke must be eliminated as much as possible.

 

⑶Physical and chemical effects. The main component of silica bricks, silica, is an acidic oxide. It corrodes strongly at room temperature, but does not cause slag corrosion with alkaline substances at high temperatures. It can interact with the metal oxides Na2O and FeO in the coal.

Low-melting silicates Na2SiO3 and Fe2SiO4 are formed on the surface of the silica bricks. These low-melting silicates and the silica in the silica bricks have different linear expansion rates and wear resistance, so they gradually fall off from the silica brick body under the action of temperature stress and mechanical forces such as coal loading and coking. . Such repeated action will continue to corrode the brick surface. In the long-term production process, the silica in the wall bricks of the carbonization chamber is “enriched” toward the side close to the combustion chamber, which is called the hot side. Other oxides are “enriched” toward the side close to the carbonization chamber, called the cold side, and are carburized in the pores of the silica bricks. Therefore, the hot side of the wall bricks contains more tridymite, which not only has different petrographic and physical properties, but also has different temperature changes between the cold side and the hot side after pushing coke and loading coal. The thermal stress caused by periodic and drastic changes in temperature difference causes the carbonized surface of the wall bricks to peel off. During the coking process, the carbonization and decomposition of coal produces a large amount of hydrogen, carbon monoxide and other gases. The silica in the silica bricks in this reducing gas will be reduced to silicon monoxide at a temperature of 1300°C and escape in the gaseous state. The higher the temperature, the faster this reaction will occur. In the presence of metallic iron, this reaction also occurs at a lower temperature of 1050°C. This reaction will reduce the silica content on the surface of the wall tiles, causing the structure to become porous and loose, forming pits.

 

⑷ Furnace length growth and graphite deposition. During the heating process of the oven, the volume of silica brick masonry expands due to the crystal transformation of silica. This transformation continued after the coke ovens were put into operation. Under the conditions of the flux (Fe2O3, CaO, Na2O, etc.) in the brick and the external reducing atmosphere, part of the quartz transforms into tridymite. When the flux is insufficient, quartz transforms into cristobalite. All these crystalline transformations are accompanied by volume changes [expansion]. This change will weaken year by year after the coke oven is put into operation. It will still be large in the first year, significantly reduce in the second year, and finally disappear. This real expansion of the expanded masonry itself is an inevitable phenomenon.

coke oven plant

A few years after the coke oven was put into operation, the expansion caused by the crystalline transformation of the silica bricks came to an end. However, the length of the furnace body has continued to increase year by year since then. This is because the cracks on the wall of the carbonization chamber are gradually generated due to mechanical stress and thermal stress due to temperature changes, and become larger due to the cooling shrinkage of the masonry when loading coal. A few hours after coal is loaded, the temperature of the masonry gradually increases again, and corresponding expansion occurs at the same time. However, since the original cracks (cracks) have been filled with graphite decomposed by gas, the cracks cannot be completely closed and can only expand outward, causing the furnace body to elongate. Repeating this cycle, the width of the cracks becomes larger and larger (from cracks to cracks, from small cracks to large cracks), and the number of rows of cracks becomes more and more, so that the furnace length continues to extend every year.

It can be seen that the elongation of the furnace body is mainly caused by the crystal transformation of the silica brick masonry in the oven and the initial years of operation, because cracks have not yet generally occurred in the furnace wall at this time. From a few years after it was put into operation until the coke oven was shut down for overhaul, this long period of time was mainly caused by the continuous increase in the number of masonry cracks and the continuous increase in crack width. For large coke ovens with an oven length of more than 14m, practical experience has proven that. When the total elongation of the furnace body, including the elongation of the oven and production period, reaches 450-500mm, it is difficult for the coke oven to maintain production. Cracks on the wall of the carbonization chamber should be filled with graphite. If there is no graphite, they can also be filled with refractory powder. , and the large cracks in the chute area, the inside of the regenerator and the small flue, [the leakage caused cannot be repaired], need to be dismantled and rebuilt [overhaul].

The expansion of the furnace body caused by amorphous transformation is closely related to the production management and technical management of coke ovens. The annual elongation of a well-managed large coke oven is about 3-4 mm. The annual elongation of a poorly managed coke oven will increase greatly, thereby accelerating the aging and damage of the furnace body and shortening the service life of the coke oven. Therefore, the annual elongation during the coke oven production process is a comprehensive indicator to measure the quality of daily management of coke ovens.

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