Analysis of the reasons for graphite formation in the roof space of 7.63m coke ovens and treatment methods

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Analysis of the Causes and Treatment of Graphite Formation in the Roof Space of 7.63m Coke Ovens Abstract: The No. 1 and No. 2 coke ovens in the new area of MaSteel are large-scale, modern coke ovens of 7.63m in size, introduced from Wood Company in Germany. Since their commissioning, severe graphite formation has occurred in the roof space of these ovens; it is therefore necessary to take effective measures as soon as possible to eliminate the impact of graphite on production and help the coke ovens reach full operational capacity at an early date. Keywords: graphite, cause analysis, treatment measures. Introduction: The No. 1 and No. 2 coke ovens in the new area of MaSteel are large-scale modern coke ovens with a diameter of 7.63 meters, introduced from Wood Company in Germany. They were put into operation on January 13 and April 24, 2007 respectively. After operation, graphite formation in the oven roof area and at the coal charging ports occurred very rapidly; without effective measures, this would have a serious impact on production within about a month ； Coal loading is difficult; the coal loading port and screw tend to get clogged, resulting in frequent smoke and fire emissions ； The coal loading amount **has decreased** and fails to meet the standards ； When leveling coal, the coal leveling rod gets stuck, etc. In response to this situation, a detailed analysis of the causes was conducted, appropriate measures were taken, and effective actions were implemented in a timely manner to ensure the smooth progress of production. 1 Analysis of the reasons for severe graphitization 1.1 Temperature-related reasons The fundamental cause of graphitization is the excessively high temperature in the roof space of the coke oven. Since their commissioning, the temperature in the roof space of ovens No. 1 and No. 2 has remained high, staying above 900 °C, and at times it even approaches 1,000 °C. The temperature in the furnace top space when heated with blast furnace gas is about 40°C higher than that when heated with coke oven gas. The formation of graphite is essentially caused by the pyrolysis of gas; when the temperature in the furnace top space exceeds 900°C, pneumatic pyrolysis increases significantly, leading to an increase in free carbon and a decrease in phenols. When the temperature in the furnace top space approaches 1,000°C, graphite is formed rapidly and becomes hard. Therefore, the key to solving the problem of graphite formation is to reduce the temperature in the coke oven roof space to below 850°C. The main reason for the excessively high temperature in the furnace top space is that the coal loading amount does not meet the required standards; this is one of the key factors. Moreover, the coal loading amount and the formation of graphite in the furnace top space influence each other, and if this issue is not resolved properly, it will lead to a vicious cycle. The coal loading amount does not meet the standards, and the average space above the furnace is too large. The ideal coal level according to the design is 400–450 mm, whereas the current coal level ranges between 550–650 mm. Additionally, the amounts of coal in the four hoppers of the coal loading vehicle are not set appropriately; as a result, the space above the furnace is uneven, leading to excessively high temperatures in that area. At the same time, the standard temperature set at the beginning of operation was on the high side. Additionally, the suction and pressure systems have not yet been adjusted to their optimal state. 1.2 Reasons related to coke oven design 1.2 Reasons related to coke oven design 1.2.1 Difficulty in adjusting the regulating bricks. The design of this 7.63m coke oven, imported from Germany, takes vertical heating into good account; the main measures include: 1) Air is supplied in three stages. Adjustment can be made using the air control plates at the upper part of the small flue in the heat storage chamber, as well as the upper heating outlets in the combustion chamber; the adjustment bricks W523 and W524 (located at the upper heating air inlets on the 18th/19th and 34th/35th floors) are used for this purpose, and these bricks should be placed at both ends of the air inlets during construction. 2) Exhaust gas recycling. The exhaust gas circulation amount between the twin flues can be adjusted using the circulating exhaust gas adjustment brick W16, which is located at the bottom of the combustion chamber; during construction, this adjustment brick is placed as far away as possible from the circulation port. 3) Furnace top space temperature fine-tuning brick W522. On the 53rd layer of the combustion chamber, above each flame channel, there is a regulating brick with a shape similar to that of the viewing hole. This brick is placed next to the viewing hole, and by moving it, it is possible to adjust the circulating exhaust gas entering the space above the furnace, thereby fine-tuning the temperature in that space. According to the design concept, the adjusting bricks are an important means for regulating vertical heating during the production process; however, in practice, it is very difficult to adjust the W16, W523, and W524 bricks, with the W16 being practically impossible to adjust ； The W522 brick can only provide fine adjustments; its purpose is to reduce the temperature of the furnace roof surface, but it cannot actually lower the temperature in the space above the furnace roof. 1.2.2 The designed heating level height for the coke ovens is too low. In the new area, the heating level height for coke ovens of 7.63 m is 1,200 mm (the difference in height between the combustion chamber and the top of the carbonization chamber). Practice has shown that this heating level height is insufficient; these coke ovens utilize three-stage heating, with the gas burning in the upper section providing sufficient heat. If the heating level height is too low, a large amount of excess heat will accumulate in the upper part, resulting in excessively high temperatures in the space above the oven, making it difficult to regulate these temperatures. The relatively low heating level height is a significant flaw in the design of this type of furnace. Wood Company is fully aware of this issue, and in the design of 7.63m coke ovens for other manufacturers in the country, it plans to increase the heating level height. 1.2.3 The use of a single-side gas collection pipe results in poor flow of raw gas. The carbonization chamber of these 7.63m coke ovens has a large volume, with a coal loading capacity of around 64.0 tons; as a result, a large amount of raw gas is generated after coal is loaded. The single-side gas collection pipe does not facilitate the proper guidance of this raw gas, causing it to remain in the space above the oven for an extended period, which increases the breakdown of the gas. Many 6.0m coke ovens in China use dual gas collection systems. 1.2.4 The impact of automatic lid closing on graphite formation in the furnace roof area has not been taken into account. With increasing environmental regulations, and in order to work in conjunction with coal loading and dust removal processes, many coke ovens in China now utilize automatic lid closing systems. The use of such systems reduces the risk of air infiltration into the furnace roof area, which is also one of the factors that can lead to graphite formation. In traditional manually operated coke ovens, the oven lid and the lid of the rising pipe were usually opened 20 minutes in advance; this allowed air to enter the space above the oven, which helped to burn away the graphite and facilitated its removal. 1.3 Operational factors: Improper coal loading and leveling procedures, as well as unreasonable settings for the amount of coal used, can also contribute to the formation of graphite in the space above the furnace. This is manifested in situations where the coal is not leveled properly, resulting in blockages; or when the amounts of coal in the four hoppers of the coal loader are not set appropriately. All these factors lead to unevenness in the space above the furnace after coal loading, making it difficult to discharge the waste gas. The gas pressure in this space increases, and the longer the gas stays there, the more likely it is to undergo decomposition, leading to increased graphite formation. 2 Methods and measures to address the problem of graphite formation 2.1 Reducing the temperature in the furnace roof area The fundamental solution to this problem is to reduce the temperature in the furnace roof area. According to the standards of the coking industry, this temperature should be maintained at 800±30°C, with a maximum of 850°C. Since their commissioning, the temperature in the roof space of Furnaces 1# and 2# has remained around 950°C. To reduce this temperature, the following measures have been taken. 2.1.1 Appropriate reduction of the standard temperature: The standard temperature provided by the foreign party was initially quite high; appropriate adjustments were made based on actual production conditions and observations of the coke cakes. The standard temperature currently in use is 15°C lower than that specified by the foreign party. The specific standard temperature in practice is shown in Table 1. Table 1: List of standard temperatures specified by the external party and those actually used. Coking time/h: 29, 28, 27, 26, 25.2. Standard temperature specified by external party/°C: 1,260, 1,275, 1,295, 1,300, 1,310. Actual temperature used/°C: 1,245, 1,260, 1,275, 1,285, 1,295. By appropriately reducing the standard temperature, the temperature at the center of the coke cake was reduced by about 10°C; the coke cake matured well, and the quality of the coke met the required standards. The standard temperature should be determined based on the temperature at the center of the coke cake. However, the temperature at the center of the coke cake in a 7.63m coke oven cannot be measured using traditional methods (such as inserting probes). It is planned to install an infrared portable device for automatic measurement of the temperature at the center of the coke cake, so as to help establish a more scientific and reasonable standard temperature. 2.1.2 Reducing the pressure of the heating gas: By keeping the gas flow rate constant, increasing the orifice size, and reducing the pressure of the heating gas, it is possible to decrease the jet force of the gas, thereby appropriately lowering the temperature in the furnace roof area. In June, the gas pressure in Furnace 1 was reduced from 1320 Pa to 1280 Pa. At the end of August, tests were conducted on Furnace No. 1#, during which the pressure of the gas used to heat the blast furnace was lowered from 1280 Pa to 1180 Pa, while the size of the orifice plate was increased from 192 mm to 202 mm. With the gas flow rate remaining unchanged, the temperature in the space above the furnace was reduced by approximately 30°C. 2.1.3 Adjusting the upward heating adjustment bricks: Since it is extremely difficult to adjust the circulating exhaust gas bricks W16 at the bottom of the combustion chamber – as they are located at a distance of approximately 9.4 meters from the top of the furnace – special high-temperature resistant tools are required, and adjusting them can easily lead to the bricks blocking the access to the inclined passages. The vertical heating outlet control bricks W523 and W524 are located at the high-phase heating air inlets on the 18th and 19th floors, as well as on the 34th and 35th floors. It is difficult to see these bricks from the observation holes at the top of the furnace; moreover, they are prone to falling off during adjustment, and it is hard to make adjustments to them. At present, neither of these two types of bricks has undergone adjustment tests. In mid-August, adjustment tests were conducted on the W522 disc bricks located at layer 53 of the combustion chambers 31–35 in Furnace 1. All five rows of W522 bricks were shut off, and after three days, the temperature in the space above the furnace was measured to have decreased by about 20°C. However, once the W522 brick is turned off, the temperature of the transverse wall cannot be measured. 2.2 Increasing and stabilizing the coal loading rate – Increasing and stabilizing the coal loading rate and reducing the space above the furnace are fundamental to minimizing graphite formation and increasing coke production. An increased coal loading allows for the absorption of more heat, reducing the temperature in the furnace roof area and minimizing the formation of graphite. If the coal loading amount does not meet the requirements, it leads to excessive graphite formation on the furnace top; meanwhile, this graphite on the furnace top in turn makes it difficult to load coal, creating a vicious cycle. At the beginning of August, problems with graphite formation in furnaces No. 1 and No. 2 recurred on a frequent basis, and the situation was quite serious; the average coal loading volume during the first part of the month was only 61.5 tons. By taking measures such as intensified manual cleaning and emptying of the furnaces, the coal loading volume gradually increased. By the end of August and the beginning of September, the average coal loading volume for furnaces No. 1 and No. 2 was 63.54 tons and 63.92 tons respectively. 2.3 Purging Furnace The purging furnace is a fast and effective method for removing graphite. In May, one cycle of furnace blanketing was carried out on Furnace No. 1; five carbonization chambers were grouped together, and each cycle lasted 4 hours. This approach yielded some results, but it had significant negative effects: it caused chaos in the coke pushing schedule, made production organization difficult, and reduced coke output. 2.4 Strengthening mechanical and manual cleaning: Practice has shown that graphitization is particularly severe in 7.63m coke ovens, and this is the case for Taigang and Yankuang among the 7.63m coke ovens in operation in China. At present, our main method for cleaning graphite is still manual cleaning. We divide the work into four shifts, with each shift responsible for a specific area; the tools used for cleaning are mainly hexagonal steel picks and flat shovels. Additionally, a rock drilling machine was purchased to assist with cleaning. Through strengthened management, the implementation of systems, and the refinement of plans, good results have been achieved temporarily. Judging from the current situation, this effort must be pursued over the long term without any slack. 2.5 Reducing downtime and maintaining stable coke oven operation: Since its commissioning, the 7.63m coke ovens in MaSteel’s new plant have not operated stably. The reasons for this include design and manufacturing defects in some of the new equipment such as coke scrapers and coke powder scrapers, which result in high failure rates and long downtime each time a failure occurs ； The four locomotives require a long period of commissioning and running-in, exhibit unstable performance, and suffer from frequent failures ； Furthermore, the operating skills of the drivers of these four locomotives are not high, and there are many operational accidents; this leads to unstable and fluctuating coking times in the coke ovens. Prolonged pressure on the ovens occurs frequently, making temperature control difficult, which results in high temperatures in the oven roof area and accelerates the formation of graphite. 3 Conclusion In fact, completely resolving the problem of graphite formation in the furnace roof area is a systematic task. The various factors and related measures analyzed above are interconnected and influence one another. At present, through the gradual adjustment of the coal loading amount to meet the required standards, the step-by-step reduction of the temperature in the furnace top area, the manual cleaning of the graphite in the furnace top, as well as further standardization and regularization of operations, the 7.63m coke ovens in the new area are gradually reaching their designed production capacity. However, to reduce the temperature in the furnace roof space to 850°C and completely resolve the problem of graphite formation, further exploration and effort are still required. Last edited by Lawlessness on 2009-3-20 17:28]