Characteristics of several typical dry quenching processes and the current status of dry quenching technology in China!

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Part 1: A brief introduction to the characteristics of several typical dry quenching processes for coke! I. Features of the dry quenching technology of German company ISOA The features of the dry quenching technology designed by the German company TSOA are as follows: (1) The dry quenching furnace is designed to be square in shape ； (2) No primary dust collector is installed between the dry quenching furnace and the boiler ； (3) The circulating fan uses variable frequency speed control ； (4) Grid-type swinging coking discharge is adopted to ensure uniform coking discharge temperature. The square dry quenching furnace designed by TSOA Company tapers to 25% of its original cross-section between the pre-storage section and the cooling section; the pre-storage section is supported by a steel structure, which is a significant difference from circular dry quenching furnaces. Due to the narrowing between the pre-storage section and the cooling section, the coke in the cooling section takes on a conical shape, creating large amounts of empty space; as a result, the velocity of the circulating gas decreases at this location, and the larger coke particles settle inside the quenching vessel. According to TSOA, the largest coke particles carried away by the circulating gas from the dry quenching furnace are 0.5–1 mm, and this is also an important reason why TSOA’s design for dry quenching of coke eliminates the need for a dust collector between the dry quenching furnace and the boiler. The lower part of the square dry quenching furnace is divided into 12 sections, each equipped with 12 hydraulically driven swinging coke discharge mechanisms for discharging the cooled coke in a section-by-section manner. Thermocouples are used to monitor the temperature of the coke; once the temperature in a particular section drops to the level allowing coke discharge, the shutter of the swinging discharge mechanism in that section opens, thereby ensuring uniformity in the coke discharge temperature. TSOA Company installs cooling walls in the upper section of the dry quenching furnace’s cooling zone. In the dry quenching furnace, red coke is cooled not only by the circulating cooling gas, but also 30% of its heat is absorbed by the cold water in the cooling walls; in fact, the cooling walls can be regarded as part of the boiler. The advantage of using cooling walls is that it allows for a reduction in the amount of circulating cooling gas, by about 20%, which in turn reduces the power consumption of the circulation fans and thus lowers operating costs ； The downside is that it increases the amount of maintenance required; if the water inside the cooling wall tubes is not controlled properly, it can leak into the dry quenching furnace, causing significant damage to the dry quenching equipment and even leading to accidents. Of course, the dry quenching furnaces designed by TSOA can also be constructed without cooling walls. II. Characteristics of Nippon Steel’s dry quenching technology The characteristics of the dry quenching technology developed by Nippon Steel are as follows: (1) The dry quenching furnace is designed to be circular ； (2) The loading device is equipped with a hopper ； (3) Continuous coking discharge using a rotary seal valve ； (4) CO in the fully burned cycle gas ； (5) The circulating fan has no speed control ； (6) Rotating coke drums are used for coke charging. The charging device equipped with a material hopper facilitates the even distribution of red coke within the dry quenching furnace, promotes uniform cooling of the coke, and can also reduce the volume of circulating air as well as the electrical consumption of the circulation fans ； The coking discharge device uses a rotary seal valve in place of the traditional intermittent type, which can reduce the height of the dry quenching system by about 4 meters, thereby lowering construction costs; it also enables true continuous coking discharge. The dry quenching process designed by Nippon Steel uses a method to completely burn H2 and CO in the circulating gas, with any excess circulating gas being able to be discharged directly. According to Nippon Steel, the burn loss rate for incompletely burned coke is 6–7 kg/t, while that for completely burned coke is 9.9 kg/t. However, theoretically, among the coke that suffers burn loss, 20% has a particle size of 20 mm or more, and 80% has a particle size of less than 20 mm; therefore, in reality, most of it is fine coke. Measurements conducted by Nippon Steel on the dry quenching of coke in furnaces No. 3 and No. 4 at Guangyuan showed that 82.1% of the coke particles with losses due to burning had a diameter of 25 mm or less ； When the burned coke accounts for 1% of the total coke loaded, the depth of surface burn on the large pieces of coke is only 22 mm. Between the inlet of the circulating gas pipeline to the dry quenching furnace and the outlet of the annular flue, there is a bypass that primarily serves to reduce the temperature of the circulating gas entering the boiler, thereby preventing any adverse effects on the boiler caused by excessively high temperatures of the circulating gas. However, this bypass is not always open. Nippon Steel believes that it is not necessary to adjust the speed of the recirculation fan; as long as the time during which coke supply is stopped does not exceed the time specified in the design for that section, there is no need to change the fan’s speed. Therefore, only flap valves are used in the design of the recirculation fan to make rough adjustments to its air volume. Nippon Steel believes that a baffle wall is not necessary for the primary dust collector, which is related to its design for dry quenching of coke based on complete combustion ； Furthermore, Nippon Steel believes that in a single-stage dust collector, particles larger than 1 mm can settle due to their own gravity, and it is only particles larger than 1 mm that can have an adverse effect on the boiler. For a dust collector without retaining walls, the silo can be much smaller, which reduces construction costs; moreover, maintenance is also easier. However, the vast majority of primary dust collectors for dry quenching of coke designed by Nippon Steel are equipped with baffle walls. III. Technical features of dry quenching for Cokes Ovens No. 7 and No. 8 at Wuhan Iron and Steel Group The quenching process for Cokes Ovens No. 7 and No. 8 at Wuhan Iron and Steel Group utilizes technology from Nippon Steel of Japan, with some improvements made; it represents a relatively advanced level at present. Its technical features are as follows: (1) The dry quenching furnace is designed to be circular in shape ； (2) The use of a circular rotating coke holder facilitates uniform coke charging ； (3) The loading device is equipped with an advanced cross-shaped hopper, which helps to ensure even coke loading and protects the air cap of the dry quenching furnace ； (4) A continuous coking discharge device equipped with a rotary seal valve facilitates uniform coking discharge ； (5) The circulating fan uses variable frequency speed control, which facilitates more precise adjustment of the circulating gas flow rate ； (6) The secondary dust collector uses a multi-tube cyclone dust collector, which helps to improve the dust removal efficiency of the circulating gas ； (7) A feedwater preheater is designed to further cool the circulating gas entering the dry quenching furnace ； (8) Completely burn H and CO in the cycle gas. The circular rotating coke drum makes full use of its volume, which facilitates a uniform distribution of coke within it; as a result, the weight of the coke drum can be reduced, thereby lowering the power required by the hoist. Additionally, the lifespan of the lining material in the coke drum can also be extended. The loading device equipped with an advanced cross-shaped material hopper facilitates a more even distribution of red coke inside the dry quenching furnace. Both this loading device and the circular rotating coke tank help to reduce the volume of circulating air, thereby lowering the power consumption of the circulation fans. A continuous coking discharge device equipped with a hermetically sealed rotary valve can **reduce the leakage of circulating gas ； Depending on the level of material in the pre-storage section of the dry quenching furnace, the amount of coke discharged can be controlled with great precision using a vibrating feeder, thereby allowing the cooled coke to be discharged in a quantitative and continuous manner ； With a variable-frequency speed control circulator fan, the flow rate of the circulating gas can also be adjusted with high precision. A continuously precise coke discharge amount, combined with an accurate circulating gas flow rate, is more conducive to stabilizing the temperature at the boiler inlet and ensuring stable operation of the dry quenching boiler. A primary dust collector equipped with a retaining wall can remove coarse coke particles from the circulating gas, ensuring that the concentration of coke particles in the gas entering the boiler remains between 10 and 12 g/m3, with the particle diameter of these coke particles being less than 1 mm; this reduces wear on the heat transfer tubes in the boiler. At the bottom of the dust collector, there are 4 water-cooled sleeves used for cooling and removing coke powder. The circulating gas exiting the boiler passes through an advanced multi-tube cyclone dust collector, which removes most of the fine particulate coke dust; as a result, the mass concentration of coke dust in the circulating gas can be reduced to less than 1 g/m³, thereby eliminating any harm to the circulation fan. Compared to multi-stage cyclone dust collectors, multi-tube cyclone dust collectors offer better dust removal performance, require less space, use less tubing and steel structures, thereby reducing construction costs. The feedwater preheater consists, from top to bottom, of an upper shell, an upper section, a lower section, and a lower shell, among which the upper section and the lower section are shell-and-tube heat exchangers. The circulating gas is pressurized by a circulation fan and enters the feedwater preheater horizontally from the lower shell. It undergoes heat exchange in the lower and upper shell-and-tube exchangers, and then exits from the upper shell of the feedwater preheater. The feedwater preheater serves two purposes: first, it further reduces the temperature of the circulating gas entering the dry quenching furnace, lowering it from around 180°C at the outlet of the circulation fan to about 130°C, thereby improving the cooling effect on the coke and reducing the flow rate of the circulating gas ； Secondly, the heat from the recycled gas is further utilized to reduce the amount of steam required by the deaerator. The advantage of using the method that burns off H2 and CO in the cycle gas is that the excess cycle gas can be discharged directly ； Secondly, it can increase the inlet temperature of the circulating gas to the boiler, thereby increasing the steam output of the boiler ； Third, it reduces the corrosion of the boiler caused by circulating gases. Part Two: The current status of dry quenching technology in China! Since the early 1980s, when Baosteel’s first phase introduced dry quenching technology from Japan, six plants in China have now put dry quenching systems into operation. There are also certain differences in the way these systems are used across various plants. As for the development of dry quenching facilities in China, the status of such systems in different plants is as follows: 1. Baosteel’s dry quenching system – To support its 12×50-hole (6 m) coke ovens, Baosteel built 12 sets of dry quenching units with a capacity of 75 t/h each, enabling the processing of 5.1 million tons of coke per year; the construction was carried out in three phases. The first phase of the 4×75 t/h dry quenching unit was put into operation in May 1985, while the second and third phases were commissioned in June 1991 and December 1997 respectively. The first phase of the dry quenching unit was entirely imported from Japan ； The second-phase dry quenching unit was developed based on the lessons learned from the first phase, and was primarily designed and constructed in China. The proportion of domestically produced equipment accounts for 80% of the total weight of the equipment, with some key components being imported from Japan ； In Phase 3, with the exception of a very small number of key components imported from Japan, the vast majority of the equipment has been domestically produced, achieving a localization rate of over 90%. Baosteel uses only dry quenching for coke, without wet quenching as a backup, and adopts a production mode of \"three in operation and one in standby\". 2. Dry quenching of coke at Pudong Gas Plant: To facilitate the quenching of coke produced in quantities of 560,000 tons per year by its coke ovens, Pudong Gas Plant introduced a set of dry quenching equipment with a capacity of 2×70 t/h from the Soviet Union in 1984. The State Design Institute for Coking in the Soviet Union was responsible for the core design of this entire dry quenching system; Anshan Coke and Refractory Institute as well as the Shanghai Chemical Research Institute contributed to the design of the supporting components. Anshan Coke and Refractory Institute took on the overall responsibility for the design and construction of this dry quenching system. Construction began in December 1991, and the project was completed and put into operation in December 1994. The entire set of dry quenching equipment for coking is imported from Russia, with wet quenching retained as a backup option. 3. Dry quenching of coke at Jigang: To support the quenching process of coke ovens that produce 1.1 million tons of coke per year, Jigang introduced dry quenching equipment with a capacity of 2×70 t/h from Russia in 1994; this equipment was designed jointly by the Russian State Design Institute for Coking and Jigang’s own design institute. Regarding the equipment, a combination of partial importation and cooperative manufacturing was adopted; the project commenced in 1996 and was completed and put into operation in March 1999. After commissioning, it was found that Russian technology lacked reliability and that it took a long time to reach full production capacity. Like the dry quenching unit at the Pudong Gas Plant, this device does not have a high level of automation, and wet quenching is still used as a backup option. 4. Shougang Dry Quenching of Coke: Shougang’s first-phase dry quenching plant with a capacity of 1×65 t/h was built under Japan’s green aid program; the main equipment was supplied by Japan, while the auxiliary equipment was purchased by Shougang itself. The design of this facility was carried out jointly by Nippon Steel and Shougang Design Institute; construction began in 1999, and it came online in January 2001. Since its commissioning, Shougang’s dry quenching unit has operated reliably, with an ideal level of automated control and excellent environmental protection performance. Shougang also retains wet quenching as a backup option. 5. Dry quenching of coke at Wuhan Iron and Steel Group: Wuhan Iron and Steel Group’s No. 7 and No. 8 coke ovens are 2×55-chamber 6 m coke ovens, and the design capacity of their dry quenching system is 1×140 t/h. The core design of this system was carried out by Japan, while the adaptation design was done by Anshan Coke and Refractory Research Institute. This project is a digestion and absorption project funded by the National Development and Reform Commission, and it represents the largest dry quenching unit for coke processing in China at present. The key equipment for dry quenching of coke was imported from Japan; some of the equipment was designed and supervised by Japanese parties while manufactured by domestic manufacturers. The automatic control system for dry quenching of coke was designed by Wuhan Iron and Steel Group itself. Construction began in October 2002, the facility was completed and put into operation in December 2003, and it reached full capacity by June 2004. The dry quenching of cokes in Wuhan Iron and Steel Group’s No. 7 and No. 8 coke ovens still retains wet quenching as a backup option. 6. Dry quenching of coke at MaSteel: MaSteel’s No. 5 and No. 6 coke ovens are 2×50-chamber 6 m coke ovens, equipped with a dry quenching system with a capacity of 1×125 t/h. This system was constructed under the overall supervision of Anshan Coke and Refractory Research Institute; some of the key equipment was imported from Japan, Germany, and the United States, while the remaining equipment was manufactured domestically. It is the dry quenching system in China that makes use of the most domestically produced components. It came online in April 2004, with a wet quenching method also available as a backup option. With the continuous improvement of environmental protection regulations and the rising awareness of environmental protection among the general public, it has become imperative to develop dry quenching technology; all steel mills that construct coke ovens with a diameter of over 4.3 meters must install corresponding dry quenching systems. This post was last edited by ryn on 2009-2-9 16:23]