Analysis of the causes of damage to the furnace front wall in coke ovens and measures to reduce the frequency of repairs

Thread Content

Analysis of the Causes of Damage to the Wall Surface at the Coke Oven Front and Measures to Reduce the Frequency of Repair Work Abstract: By investigating the reasons for damage to the wall surface at the coke oven front, scientific and reasonable measures were taken, which effectively reduced the occurrence of such damage as well as the frequency of repair work. Keywords: coke oven front; melting hole; patching; measures Chinese Library Classification Number: TQ522 Document Code: A Article ID: 1004-7050(2004)01-0043-02 Introduction The two 58—II type coke ovens in the first coking plant of our company were put into operation in 1979 and 1981 respectively, and they are now in an aged state. As the furnace age increases, the damage to the walls of the carbonization chamber becomes more severe, especially in the burner area; the number of melting holes is on the rise year by year, which seriously affects coking production. During the process of filling the molten holes, it is necessary to lower the surface temperature of the furnace walls, which can cause varying degrees of damage to the entire wall surface as well as the adjacent flues, directly affecting the lifespan of the furnace. Therefore, it is of great significance for coking production and the maintenance of coke oven structures to explore and analyze the causes of damage to the furnace wall surfaces, seek countermeasures, and adopt scientific and reasonable measures to effectively reduce the frequency of repairs to the furnace channels. 1 Analysis of the causes of damage to the furnace head wall. Generally, after a coke oven begins operation, as its service life increases, factors such as thermal stress, mechanical pressure, and chemical corrosion resulting from repeated operations like coal loading, door opening/closing, and coke pushing cause gradual changes in various parts of the oven. The surface of the furnace wall may develop cracks, erosion, pitting, corner loss, deformation, misalignment, melting holes, or even collapse to varying degrees. Usually, the first and fastest to be damaged are the flues at the end of the combustion chamber, that is, the furnace head flues. Based on the actual production conditions of our plant’s coke ovens, and taking into account the issue of melting holes that occur due to damage to the oven heads, after discussion and analysis, it was determined that the main reasons for the damage to the walls of the oven heads in our plant’s 58—Ⅱ type coke ovens, as well as the formation of melting holes, are as follows: 1.1 Thinning of the lining bricks at the oven doors and the impact of increased oven head temperatures on the oven walls. In order to improve the quality of coke and enhance the company’s profitability, in 1993 the lining bricks at the oven doors were changed from dense clay bricks to lightweight polymeric bricks. Additionally, the thickness of the lining bricks on both the machine side and the coke side was reduced by 10 cm, allowing each carbonization chamber to hold an additional 0.27 tons of dry coal. This resulted in an annual increase in coke production of 8,500 tons, as well as 353,000 cubic meters of coke oven gas. To ensure proper maturation of the coke at the furnace tip, the diameter of the gas orifice plates in the furnace tip’s combustion channel was increased by 1 mm, which heightened the amount of gas used for heating in that channel. As a result, the temperatures at the furnace tips on the machine side and the coke side rose to 1,300°C and 1,340°C respectively. This led to a maximum temperature in the flame core area of the furnace tip combustion channel, located 1 meter below the bottom of the carbonization chamber, of 1,450°C to 1,500°C. This accelerated the conversion of SiO2 in the silica bricks into gaseous silica. At 1,470°C, it caused the transformation of phosgene quartz in the silica bricks into cristobalite, thereby increasing the proportion of cristobalite in those bricks. Cristobalite has poor corrosion resistance and a high linear expansion coefficient; over time, this leads to the peeling of the wall surface in that area, causing melting cavities to form in the furnace wall from the inside outward. Coupled with the erosion of the walls of the carbonization chamber, this accelerates the formation of such melting cavities. It has been found that in the molten holes repaired in our plant, melting occurs from the combustion chamber toward the carbonization chamber, and these molten holes are all located at a distance of 1 meter from the bottom of the carbonization chamber along the flame channel at the furnace head. 1.2 Impact of filling molten holes on the furnace wall: During the process of filling molten holes at the furnace tip, it is necessary to remove coke, construct a sealing wall (or install a fake furnace door), and bring the temperature of the area to be filled below 200°C before hot repair workers can enter to clear debris, remove leftover bricks, measure dimensions, and process the bricks layer by layer before construction can proceed. The entire filling process takes around 8 hours. Such a long cooling time, which reduces the temperature of the exposed furnace walls from 900°C to below 200°C, inevitably causes phase transformations within the silica bricks, leading to wider cracks in the walls and exacerbating the damage to adjacent flues as well as the exposed furnace walls. The newly installed silica bricks, after the furnace door is placed on them, experience uneven heating due to rapid temperature increases, which causes significant changes in their volume and leads to cracking of those bricks. This affects the quality of the repair; as a result, the repaired wall surface can only last for 2 to 3 years before perforations occur again. As the number of repaired fire channels increases, the secondary repair rate also rises year by year, which is another major factor contributing to the increased frequency of repairs. 1.3 The impact of spraying on the walls of the carbonization chamber and the removal of graphite on the walls of the furnace head: After cracks, pitting, and erosion appeared on the walls of the furnace head, spraying treatment was applied to some of the areas with severe erosion between 1994 and 1995. Due to the thin consistency of the slurry and its room temperature, while the wall temperature is higher, the application of the slurry causes the wall temperature to drop sharply, resulting in a sudden change in the local volume of the furnace wall and exacerbating the erosion of the wall. During production operations, the areas with erosion and pitting are at higher temperatures, making it easy for graphite to accumulate there. Once this accumulation reaches a certain level, it can affect the process of coke extraction. During the manual removal of graphite, it is inevitable that damage will be caused to the furnace walls, making their surface even more uneven. The greater the unevenness, the easier it is for graphite to accumulate, and this vicious cycle is also a factor that leads to the formation of molten holes in the furnace head area. 2. Take measures to effectively reduce the frequency of repairs to the burner flues. In response to the causes of damage to the burner walls, we took the following measures, which helped to reduce the frequency of such repairs. 2.1 Restoring the original design thickness of the lining bricks for the machine side and coke side furnace doors, thereby reducing the temperature at the furnace tip. From September 1998 to August 1999, we gradually restored the lining bricks for these furnace doors to their original design thickness, and also restored the diameter of the gas orifice plates in the furnace tip channels to their original values; this reduced the area of the furnace wall surface that was subject to corrosion due to coal exposure. At the same time, the temperature at the furnace tip was reduced to 1,260°C on the machine side and 1,290°C on the coke side, keeping the temperature in the flame center area of the furnace tip combustion chamber below 1,450°C. This effectively prevented damage to the furnace tip walls and reduced the formation of new melting holes. 2.2 Improve the patching method to enhance the quality of repairs and reduce the rate of subsequent repairs. By improving and refining the patching process, it is possible to make such repairs more rational, scientific, and standardized, thereby improving the quality of the repairs, extending the lifespan of the patched walls, and reducing the need for further repairs. 2.2.1 Apply insulation measures to the exposed wall surfaces after constructing a sealing wall (or installing a fake furnace door). Using 20 mm thick aluminum silicate fiber mats bonded with water glass to cover the wall surfaces helps keep the temperature on the furnace wall above 600°C, while the surface temperature of the fiber mats remains below 200°C. This reduces damage to the exposed wall surfaces and adjacent flues, saves time required for cooling, improves the working conditions for maintenance personnel, and shortens the repair time by more than 2 hours. 2.2.2 Extend the preheating time for newly fired silica bricks. Before carrying out the patching, preheat the silica bricks to ensure they are dry; after the patching is completed, extend the preheating time for the newly built wall surface – that is, do not supply gas to the first three burners after construction. Remove 3 to 5 layers of bricks from the upper part of the sealing wall, install a furnace door, and use the heat from the adjacent burners, the carbonization chamber, and the excess gas in the upper part of the sealing wall to preheat the patched wall surface. After heating the repaired carbonization chamber for 12 hours, 2/3 of the gas valves in the basement are closed to create a sealed environment within the furnace; after two cycle times, the walls are sealed again, gas is supplied, and the coking process is completed. For adjacent carbonization chambers, the coking time can be appropriately extended to use them as buffer furnaces, keeping them in a semi-hermetically sealed state. Preheating prevents the new silicon bricks from cracking due to rapid temperature rise, ensuring the quality of the repairs. Coupled with proper maintenance, the repaired walls can remain free from deformation and perforations for 5 years or even longer, effectively reducing the need for further repairs. 2.2.3 No expansion joints shall be reserved during patching, and no loose bricks shall be left. Since only one flue is repaired at a time, it is sufficient to widen the vertical and horizontal mortar joints appropriately, leaving a mortar joint of 10 mm at the joint area; there is no need to leave a \"loose brick\" in the first layer. This not only simplifies the repair process but also ensures the quality of the repair work. Practice has shown that this bricklaying method does not cause cracking or deformation of the silicon bricks in the newly constructed areas. 2.3 Strengthening the daily maintenance of the furnace: First, it is necessary to improve the control of furnace temperature, ensuring stability in terms of coking time, heating procedures, and the quality standards of coal fed into the furnace, thereby guaranteeing a consistent supply of heat to the coke oven and preventing any damage to the furnace walls due to fluctuations in temperature. Second, the maintenance methods used by the technicians responsible for thermal repairs should be improved; individual responsibility should be assigned for each section of the furnace walls, and the frequency of inspections should be reduced. The goal is to carry out minor repairs instead of major ones, and thermal repairs rather than cold repairs. For small melting holes in the furnace nozzle, high-temperature adhesives mixed with aggregates are used for patching; for large cracks and eroded surfaces, semi-dry flame welding is employed; for furnace numbers that have been patched, regular inspections should be carried out to ensure timely re-patching. In addition, the organization replaced the longitudinal and transverse struts on the top of the furnace, as well as some steel columns with excessive curvature, to ensure that the furnace components could protect the furnace body and effectively curb the further deterioration of the furnace wall. 3 Conclusions By implementing the above series of measures, on the one hand, the occurrence of new melting holes in the burners was reduced, and on the other hand, the rate of secondary repairs was decreased. As a result, the frequency of repairing melting holes in the burners was effectively lowered, yielding significant benefits that contributed to the stable operation and longer service life of the coke ovens.