Comparison of heating methods for 7.63M coke ovens in China and Germany

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Comparison of Heating Methods for 7.63m Coke Ovens in China and Germany Wang Xiaodong (MCC Coke & Refractory Engineering Co., Ltd., Anshan 114002) Nie Kefu (Engineering Management Department, Wuhan Iron and Steel Company, Wuhan 430083) In the past two years, with the trend toward larger coke ovens, China has successively introduced 7.63m coke ovens designed by the German company Wood. On June 28, 2006, the first batch of coke was successfully produced from Asia’s first 7.63m coke oven at Yankuang International Coking Company, with the furnace drying process carried out by MCC Jiaonai Company. By comparing and summarizing the heating methods for 7.63m coke ovens in China and Germany, this paper provides a reference for the subsequent heating of 7.63m coke ovens. 1. Kaiserslautern 3 coke oven: The baking method for German 7.63m coke ovens is illustrated using the Kaiserslautern 3 coke oven as an example. This coke oven is a regenerative coke oven with a carbonization chamber height of 7.63 m (in the hot state). Its basic parameters are as follows: carbonization chamber length – 18,560 mm; effective length of the carbonization chamber – 17,760 mm; total height of the carbonization chamber – 7,540 mm (in the cold state) and 7,630 mm (in the hot state); effective height of the carbonization chamber – 7,090 mm; thickness of the oven roof – 1,750 mm; heating level – 1,210 mm; average width of the carbonization chamber – 623 mm; width on the machine side – 598 mm; width on the coke side – 648 mm; center distance between carbonization chambers – 1,650 mm; number of vertical flues – 36; center distance between vertical flues – 498 mm; effective volume of the carbonization chamber – 78.4 m³. The main structural features of this coke oven include a segmented structure for the regenerator chambers, staged air supply to the combustion chambers. The oven roof is constructed using two types of materials: silica bricks and semi-silica bricks, with sliding joints installed at the boundary between these materials. The regenerator is also built using the aforementioned two types of materials, with sliding joints installed between them as well. 2 Kaiserslautern 3 Methods for controlling temperature during coke oven heating 2.1 Formulation of the heating plan The Kaiserslautern 3 coke ovens, like other coke ovens in Germany, determine the time required to raise the temperature from a cold state to 800°C to 55 days, based on the thermal expansion properties of silica bricks. During this period, the thermal expansion rate of masonry also varies significantly across different temperature ranges; for example, in the range of 75–200°C, with intervals of 25% each, the minimum daily heating expansion rate is 0.025%, while the maximum reaches 0.054%. 2.2 Selection of the heating control point: The heating control point for the Kaiserslautern 3 coke oven is located at the lowest point of all the furnace channels, namely those channels through which no air flow occurs during the heating process. The temperature control points are set at burners 18 and 19; this area is located in the center of the coke oven and is not exposed to the heating air currents, so the temperature there is the lowest. 2.3 Selection of temperature measurement points to control the uniformity of the entire furnace temperature: The degree of uniformity of the furnace heating temperature can only be determined based on burners 18 and 19. The uniformity of the temperature in the entire furnace is controlled using the uniformity of the temperatures in channels 18 and 19, ignoring the variations and uniformity of temperatures on the machine side and the coke side. Due to the lag in the increase in temperature of burners 18 and 19, once the temperature exceeds the specified limit, it directly affects the timeliness and sensitivity of regulation, leading to actual temperature unevenness. 2.4 Vertical temperature monitoring and horizontal temperature monitoring: Although there are measurement points for vertical temperature in the middle and lower parts of the coke oven, for control purposes, it is only required that the temperature at the exhaust gas valve be 1/3 of the temperature in the combustion chamber. It is also feasible to control the distribution of high-temperature areas using the ratio of the temperature at the exhaust gas shutter to the temperature in the combustion chamber. Since Germany maintained the pressure at the tuyeres at a level of +1 to +2 Pa during the heating process, this resulted in a limitation on the increase in temperature in the lower part of the coke oven. The monitoring points for lateral temperature are the same as those in Chinese-style drying ovens, that is, all the vertical flues in one combustion chamber are selected. 2.5 Priority areas for heating: Since Germany uses positive pressure during furnace drying, it is obvious that the priority areas for heating are the upper and middle parts of the coke oven, which inevitably results in slower heating in the lower part. The upper-middle section, being the priority area for temperature increase, helps to prevent large amounts of cold air from entering the roof area, ensures normal temperature rise in this region, and thus prevents the two different types of masonry materials in the roof area from pulling apart from each other, maintaining the integrity of that area. However, this results in a smaller increase in temperature in the lower part of the coke oven, leading to the formation of condensation water there and causing the masonry in the middle and lower sections of the coke oven to crack apart. Later, the Germans adopted an external combustion method for heating the furnace: gas was burned outside the furnace and then introduced into the furnace chamber to raise its temperature. This approach not only improved the situation of lower temperature levels, prevented condensation water from forming in the lower part of the furnace, but also ensured that the upper part of the furnace heated up properly, thereby maintaining the integrity of all parts of the furnace. 3 Comparison of furnace heating methods between the two countries (1) Heating rate from cold state to 800°C. The Chinese approach involves first determining the maximum daily expansion rate for silicon bricks; generally, for large coke ovens, this maximum value is set at 0.035%. Within the specified temperature ranges, the number of days required for heating in that range is calculated based on the maximum expansion rate for that range. In Germany, the process is scheduled over a fixed period of 55 days, during which the daily expansion rates vary significantly across different temperature ranges; the highest daily expansion rate was 0.054% during the heating up of Coke Oven No. 3 in Kaiserslautern. From the perspective of silicon brick properties and furnace drying processes, the Chinese method is more rational. (2) Selection of the temperature rise control point. The Chinese method involves distributing the temperature control points evenly across all the burners in the furnace, which is more representative than the German method, which selects the lowest point among all the burners in the furnace, namely burners 18 and 19. (3) Control the temperature measurement points for uniformity of the entire furnace temperature. The Chinese method selects the standard flame channel in the middle of both the furnace and the coke sides, which better reflects the uniformity of temperatures on both sides of the furnace and coke as well as throughout the entire furnace. (4) In maintaining a proportional distribution of heat in the vertical direction, the Chinese method ensures that a higher proportion of the heat reaches the middle and lower parts of the coke oven, allowing the moisture in the refractory materials to be discharged properly without condensing in the lower part of the oven. At the same time, when the temperature in the middle and lower sections is relatively high, the furnace body should expand synchronously as much as possible, so that the masonry in those areas does not develop cracks due to separation. Ensure the tightness of the masonry. Germany adopts a top-priority approach to maintain integrity during the heating and expansion of masonry. Since the roof area of the 7.63 m coke oven is constructed using two different materials – silica bricks and semi-silica bricks – with sliding joints installed between them, and since there are also protective devices in this area that can apply elastic loads, during the heating process for oven drying, even though the expansion rates of these two different materials differ significantly, the sliding joints prevent cracks from forming in the masonry, thus maintaining its integrity. (5) Although German external heating ovens consume much more fuel than internal heating ovens, and require additional equipment and materials for transporting the hot gases from external combustion, they can achieve very good results. A reasonable temperature distribution is achieved along the height of the coke oven, eliminating the formation of condensation water at the lower part of the oven and preventing the various sliding joints in the masonry from cracking apart, thus ensuring the integrity of the masonry. The structural characteristics of China’s JN60 type coke ovens result in uniform, evenly spaced cracks forming in the roof area after baking. Subsequently, during the construction of the hot-state facilities, methods such as grouting are used to ensure the integrity of this area. In the early stages of operation, gas leakage from the carbonization chamber to the combustion chamber in the furnace top area, especially around the coal loading holes and the rising pipe holes, was quite common; in some coke ovens, such leakage continued to occur even during production. This post was last edited by ryn on 2009-3-19 14:05]