Causes of Damage to Refractory Materials in Coke Oven Regenerator and Small Flue

The main body of the coke oven is composed of refractory masonry and furnace protection iron parts. Understanding the basic requirements and damage mechanisms of coke oven management is crucial to avoid abnormal damage to the coke oven and extend the service life of the coke oven. In the following, RS Refractory Brick Manufacturer takes the coke oven regenerator and small flue as examples to elaborate on the causes and characteristics of refractory damage in this area.

Cracks in the Single Wall, Main Wall, and Sealing Wall of the Regenerator

The cracks in the single wall and main wall of each coke oven (for the sake of simplicity, the single wall and main wall in the regenerator are referred to as single wall and main wall) are not the same. Some have 1 to 2 vertical cracks with a width of 0 to 40 mm from top to bottom in almost all the heads of the single wall and main wall. Some have narrower and shorter vertical cracks in only a few heads of the single wall.

For cracked single walls, the end cracks are generally wider and longer than the internal cracks. For the same coke oven, the main wall has fewer cracks than the single wall, and they are narrower and shorter.

The distribution and number of cracks in the single wall and main wall are not easy to observe clearly when they are hot, and can only be observed after the whole furnace is cooled. The single wall of the regenerator with a furnace age of 20 years has 2 to 4 vertical cracks after cooling. They are mostly concentrated in the middle of the furnace head machine side and the middle of the coke side. Each crack is basically connected to the inclined crack (or mortar joint), with a crack width of 0 to 15 mm, the widest in the middle, followed by the upper part, and the narrower as it goes down. Generally, the vertical crack is from the inclined to the middle of the heat storage chamber, and appears in a stepped gap from the middle to the bottom. That is, the bricks are broken above the middle, and the mortar joints are stretched below. It must be pointed out that after the furnace is cooled, many new cracks are generated due to the overall shrinkage of the masonry. Therefore, the above situation cannot fully represent the distribution of cracks inside the single and main walls under production conditions.

The sealing wall is in contact with the atmosphere and is greatly affected by temperature changes, so many cracks will occur in the masonry and mortar joints. Generally speaking, whether it is a sealing wall of a new furnace or an old furnace. If it is not maintained frequently, cracks can occur everywhere, especially around the contact between bricks and ironware (such as pressure measuring holes, etc.).

There are many reasons for the cracks in the single and main walls, and the main ones are as follows.

① Since the growth rate of the combustion chamber masonry in the direction of the furnace length is much greater than that of the regenerator, it is subjected to outward tensile stress (the same is true for the ramp), resulting in cracks in the single and main walls, or the original cracks continue to expand.

② The ends of the single and main walls are closer to the outside world, dissipate more heat, and the sealing wall is not tight, which is affected by the cold air that leaks in. In addition, the temperature in the regenerator changes with the periodic change of the airflow, and the ends of the single and main walls often fluctuate near the silicon dioxide crystal transformation point and crack.

③ When cleaning, replacing checker bricks or repairing single and main walls, a large amount of cold air invades, causing the temperature of the single and main walls to drop sharply and crack.

④ The gaps in the single and main walls are related to the furnace structure, the type of combustion chamber protection plate, and the material of the bricks.

The Otto furnace of a certain factory uses double gas collectors and small protection plates, and there are no small furnace columns and other protective facilities on the front of the single and main walls. When the production furnace age is 30 years, the single and main walls of its heat storage chamber have much fewer cracks than the single and main walls of the ΠBP furnace, which uses a large protective plate plus a staggered furnace head and a small furnace column on the front. The reason is that the Otto furnace uses a small protective plate structure, which makes the furnace length extension rate of the carbonization chamber slow, and the tensile stress on the single and main walls is small. It is also because its single and main walls are all semi-silicon bricks with good resistance to rapid cooling and heating. The head of its main wall is a straight seam structure, while the ΠBP furnace uses silica bricks to build the main wall head.

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It should be admitted that the installation of small furnace column protection equipment on the single and main walls is effective in preventing the formation and expansion of cracks, but it must be achieved when the furnace column is not deformed. Because the small spring that pressurizes the single and main walls is fixed on the cross iron connected to the furnace column, when the curvature of the furnace column increases, the cross iron moves outward, causing the load of the small spring to decrease. The greater the load of the small spring, the greater the curvature of the furnace column, and the more the cross iron moves outward. When the distance between the furnace column and the front of the heat storage chamber reaches a certain value, the small spring loses its function of protecting the single and main walls. Therefore, in general, the protective effect of the small furnace column device is greater in new coke ovens. After the furnace body ages, its function gradually decreases or disappears completely. Therefore, it is very important to protect the furnace column.

⑤ After the furnace is cooled, the lower part of the single and main walls of the heat storage chamber is pulled apart from the mortar joints to form a stepped gap. This is because the temperature of this part is low and the mortar and bricks are not sintered.

⑥ For the inclined area with silica bricks and the single and main walls with clay bricks, the cracks they form are because although a sliding layer is laid between the silica bricks and the clay bricks, it is still impossible to achieve ideal sliding, so the masonry is pulled apart.

Melting and Deformation of Single and Main Walls

Melting and deformation of single and main walls mostly occur at the top of the heat storage chamber. In addition to the melting of bricks into a whole, the molten material will also flow down the wall. It not only blocks the checker bricks, but also causes the masonry above it to sink or deform.

The main reason for the melting of single and main walls is that a large amount of raw gas or heated gas rushes into the heat storage chamber and burns violently in the presence of a large amount of air.

The deformation of single, main walls and sealing walls in the direction of furnace length is larger in the upper part and gradually becomes smaller as it goes down to the small flue. The deformation also increases with the year, but the growth rate is lower. For the entire heat storage chamber and small flue, the annual growth rate is larger at the top and smaller as it goes down. The reason for the annual growth of single and main walls is the same as that of the inclined part.

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Checker Brick Blockage and Melting

After years of use, the checker bricks of coke ovens are blocked to varying degrees by blast furnace gas ash, mud used for gunning, and bricks and debris dropped from the furnace top and fire channel. The checker bricks on the top, especially the coke side area, are mostly blocked by baking. This is because the fire channel temperature is high, the furnace body is not tight, and the raw gas in the carbonization chamber leaks into the heat storage chamber and burns at the top.

After years of use, the chemical composition and physical properties of clay checker bricks have also changed to varying degrees. For example, their refractoriness, true density, apparent porosity, aluminum oxide content and iron oxide content have all been significantly improved. The content of silicon dioxide has been significantly reduced. Among them, the influence of the chemical composition of the checker bricks plays a leading role, especially the high content of Al2O3, potassium and sodium in the checker bricks will cause the checker bricks to break. Because under the condition of more than 1200℃, diaspore (Al2O3·H2O) and kaolinite (Al2O3·SiO2·2H2O) in the checker brick will undergo chemical changes, release Al2O3 and SiO2, and Al2O3 and SiO2 will generate mullite for the second time, accompanied by volume expansion (about 10%). In addition, too much potassium and sodium low melting point substances are extremely harmful to refractory products. The eutectic temperature of K2O is 769℃, and the eutectic temperature of Na2O is 782℃. Each 1% content of both will produce nearly 4% liquid phase. The temperature at the top of the regenerator is about 1100℃, which is much higher than its eutectic temperature. Since the heating airflow is a reducing gas, chemical reaction and erosion are also a reason for accelerating the damage of the checker brick.

After the semi-siliceous checker brick is melted, there is no volume expansion except for surface magnetization. Under the action of high temperature, the clay checker brick has a cracked, loose and porous appearance, similar to blooming steamed buns. The width of the airflow channel of this checker brick becomes narrower, and it is often completely blocked by swelling.

The use of low-aluminum checker bricks or semi-silicon bricks with an Al2O3 content of 30% to 35% in the top three layers of the regenerator has better solved this problem.

Damage to the Single and Main Walls or Lining Bricks of the Small Flue

The single and main walls of the heat storage chamber and the small flue of large coke ovens are mostly built with silica bricks. In order to prevent the silica brick masonry from cracking due to the sudden change in temperature of the rising and falling airflows, the single and main walls in the small flue are generally lined with clay bricks. Single-mouth main walls (small coke ovens) built with semi-silica bricks and clay bricks do not need to be lined with bricks in the small flue. Regardless of the material of the single and main walls, vertical cracks may occur within 1m of the end. These vertical cracks are mostly extensions of the vertical cracks in the single and main walls of the heat storage chamber, but the width is smaller.

In general, the chemical composition of the single and main walls of a siliceous or clay small flue will not change fundamentally. Due to the low temperature here, the quartz remaining in the silica bricks has not been transformed. Only in individual cases, the single and main walls of small flue or clay lining bricks of siliceous small flue are built with clay bricks. After 3 to 5 years of production, deformation and flaking will occur. The degree of damage is most serious at the end of the small flue, and the further in, the lighter it is. The lining bricks (or single and main walls) at the two ends of the small flue of the whole furnace are the least corroded.

The deformation of clay lining bricks (or clay single and main walls) is mostly 20~30mm protruding from the middle of the brick, and the thickness of the peeling is 15~25mm. Compared with the uncorroded parts, the sulfuric acid content in the corroded parts is significantly increased, and the aluminum oxide content is significantly decreased. The reason for the bulging is that the single and main walls of the small flue crack due to the sudden change of temperature. The coke oven gas used for heating enters the downflow small flue and burns violently, causing uneven heating of the masonry.

The reason for the corrosion is: the sulfur content of coking coal is high (some exceed 1%), and 70%~80% of the sulfur remains in the coke in the carbonization chamber. The remaining sulfur generates sulfuric acid alcohol (RSH), thiophene (C4H4S), carbonyl sulfide (COS) and carbon disulfide (CS2), and 95% of the sulfur generates hydrogen sulfide (H2S). The raw coal gas containing a large amount of sulfide is directly burned in the fire channel without treatment (only the coke oven is put into production but the recycling has not yet been put into production), producing a large amount of sulfur dioxide gas. In addition to the trace amount of nitrogen monoxide originally carried by the raw coal gas, the large amount of ammonia (not recovered) it contains reacts with carbon dioxide in the exhaust gas to generate a large amount of nitrogen monoxide.

2NH3+5CO2→2NO+3H2O+5CO

Under normal circumstances, sulfur dioxide is not easy to convert into sulfur trioxide, but this reaction is easy to achieve when nitrogen monoxide is present.

2NO+O2→2NO2

NO2+SO2→SO3+NO

SO3+H2O→H2SO4

In addition, since the exhaust gas generated by the combustion of coke oven gas contains a large amount of water vapor, it condenses into water droplets when the temperature of the small flue is lower than the dew point. Sulfur trioxide reacts with water to generate sulfuric acid in the presence of excess air. Sulfuric acid reacts with trioxide cement in clay bricks). Since its expansion coefficient is inconsistent with that of the uncorroded part, it gradually peels off under the influence of the sudden change in temperature of the rising and falling airflows.

The lower the temperature in the small flue, the higher the corrosion rate; the lower the temperature. Usually at the end of the rising airflow, the temperature of the small flue from the entrance to 1m inside is relatively low, about below 100℃. The closer to the entrance of the small flue, the lower the temperature, the more serious the corrosion. From 1m away from the entrance of the small flue to the inside, the surface temperature of the brick is roughly above 100℃, and the corrosion is relatively light.

There is more excess air in the small flue at the end, and the content of sulfur trioxide in the unit volume of the exhaust gas is much lower than that of the internal small flue. So although its temperature is lower than that of the internal small flue, the corrosion is not serious.

The fundamental way to prevent the corrosion of clay lining bricks or clay single and main wall of small flue is: when using recycled coke oven gas for heating, use coking coal with low sulfur content. The equipment for recovering ammonia must operate normally, coking and recovery must be put into production at the same time, or stop using coke oven gas without recovered ammonia for heating, and use blast furnace gas for heating instead. The production experience of some coking plants shows that as long as the recovery workshop produces normally, even if the sulfur content of the coal is high and the recycled gas is not desulfurized, the temperature of the small flue is lower than 100℃, and the corrosion of the lining bricks or clay single and main wall of the small flue is not fast.