Evaluation of the SCOPE21 coking process in Japan

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Evaluation of the Japanese SCOPE21 coking process by Hu Desheng (Baosteel Corporation) 1 Research on the SCOPE21 coking process Since the 1950s, countries around the world have made extensive efforts to research and develop new coking processes. Japan planned to replace chamber coke ovens with shaped coke production processes in the early 2000s. After large-scale tests conducted from the late 1970s to the early 1980s, a semi-industrial testing facility capable of producing 200 tons of shaped coke per day was established. The 6,000 tons of shaped coke produced were used in ironmaking experiments using 4,000 cubic meter blast furnaces. Test results showed that modern large-volume blast furnaces can use at most 20%–30% shaped coke to replace coke produced by traditional methods, which led Japan to abandon the idea of replacing conventional coking processes with shaped coke processes, and thus the SCOPE21 plan was introduced. SCOPE21 is \"a super coke oven for efficient production and environmental protection in the 21st century\", which is the English abbreviation of Super Coke Oven for Productivity and Environment enhancement toward the 21st century. This project was carried out over a period of 10 years (1994–2003) with funding from Japan, through collaboration between the Japan Iron and Steel Federation (JISF) and the Japan Coal Utilization Center (CCUJ), involving an investment of approximately 10 billion yen in total. The key technical aspects of the SCOPE21 process are as follows: First, a rapid coal heating system is employed, consisting of a fluidized bed drying section and a pneumatic bed preheating section, to preheat the coal to 350–400°C before feeding it into the furnace ； Second, the finely ground coal powder is thermally pressed into shape, mixed with preheated coarse coal, and then loaded into the furnace ； Third, the preheated coal is rapidly dry-distilled at moderate temperature in the coke oven to 750–850℃ ； Fourth, the coke produced through medium-temperature carbonization is transported in a sealed manner to the pre-storage section of the dry quenching system, where it is heated to 1000°C to achieve high-temperature modification of the medium-temperature coke. Research on the SCOPE21 process began in 1994. During the period from 1994 to 2000, efforts were focused on the development and research of key technologies. The main tasks included testing coal drying, heating, and grading techniques; examining the effects of rapid preheating on coal modification; as well as studying the structure of coke ovens, the uniformity of heating within these ovens, and oven designs that enable high productivity and low emissions. Among them, the small-scale tests on coal pretreatment were completed between October 1998 and March 2000. On this basis, they developed a semi-industrial test facility and conducted semi-industrial tests from March 2002 to March 2003. The coal pretreatment capacity of the semi-industrial test facility is 1/20 of that of a full-scale plant; the coke oven has a single carbonization chamber, whose height and width are the same as those of a real oven, while its length is half that of a real oven. Figure 1 Schematic diagram of the SCOPE21 coking process. 2 Main features of the SCOPE21 coking process. According to publicly available Japanese literature, the SCOPE21 coking process has the following features: (1) Effective utilization of coal resources. Through rapid preheating of coal and hot pressing of pulverized coal, the bulk density of the coal fed into the carbonization chamber is increased, thereby improving the quality of coke; or, while maintaining the quality of coke unchanged, the proportion of weakly bindable coal in the coal mixture can be increased. The above measures can increase the proportion of weakly caking coal in the coal blend to 50%, whereas in traditional processes its usage is only 20%. In the semi-industrial tests, strong-cohesive and weak-cohesive coals accounted for 50% each in the blend, and the cold strength of the coke was 84.8%, which is 2.5 percentage points higher than the 82.3% achieved by the conventional process. (2) High productivity. By preheating the coal before loading it into the furnace, thinning the furnace walls of the coke oven, using furnace wall bricks with high thermal conductivity, and reducing the temperature at which coke is extracted, it is possible to shorten the coking time of the coke oven and improve its productivity. In the semi-industrial tests, when the temperature of the coal fed into the furnace was 330°C, the temperature of the coke oven was 1250°C, and the temperature at which the coke was extracted was below 900°C, the coking time was 7.4 hours – significantly lower than the 17.5 hours required by conventional coke ovens. Moreover, the production capacity was 2.4 times that of conventional coke ovens. (3) Energy saving. By preheating the coal fed into the furnace to a higher temperature, the temperature at which carbonization begins is increased, while the final temperature during coking is reduced, thereby lowering the temperature of the coke produced. At the same time, by recovering the heat contained in the gas and the waste gases emitted at the gas outlet, the efficiency of heat utilization can be improved, which in turn reduces the energy consumption required for coking. In semi-industrial tests, the SCOPE21 process can reduce energy consumption by 21% compared to conventional processes. (4) Environmental protection. In the SCOPE21 process, coal and coke are both transported in enclosed systems, and leak prevention measures are also taken for the coke oven itself and the coking process to prevent the leakage of coke oven gas, thereby improving environmental standards. Through the optimized design of the coke oven heating system, the NOx content in exhaust gases can be reduced. (5) Economy. An increase in coke oven productivity allows for a proportional reduction in the number of chambers in the coke ovens, thereby reducing equipment costs. Furthermore, increasing the use ratio of non-cohesive and weakly cohesive coals, combined with energy-saving effects, can reduce the production cost of coke. In semi-industrial trials, compared with conventional processes, the SCOPE21 process resulted in a 16% reduction in equipment costs and an 18% decrease in coke costs. 3 Effect Evaluation Through an analysis of relevant literature published in Japan, it can be seen that the SCOPE21 coking process generally offers advantages such as improving coke oven productivity, enhancing coke quality, reducing energy consumption, and minimizing environmental pollution. However, there are still several issues, including: (1) the safety and reliability of coal preheating. During the rapid heating, storage, and charging of coal, its safety is one of the key considerations; improper handling can lead to coal dust explosions. Countries around the world have attempted to develop coal preheating processes for coking, but these were not put into industrial use because the critical safety-related technologies involved in preheating, storage, and charging could not be resolved. Compared with the coal preheating loading process studied previously, the SCOPE21 process involves a higher temperature of the coal being loaded into the furnace (the temperature of the coal in the previous preheating process was 150–200°C, while it is 350–400°C in the SCOPE21 process), which raises concerns regarding its safety. Additionally, preheated coal is transported via pipelines, and the friction between the coal and the pipe walls poses issues regarding the reliability of the equipment. If the pipe walls are damaged, it not only leads to severe environmental pollution but also makes maintenance very difficult. (2) Thermal shaping of coal. Based on the experience gained from the development of shaped coals, when coal is preheated to 350–400°C before shaping, demolding during the binder-free hot pressing process is very difficult, and the strength of the shaped coal is relatively low, resulting in a reduced effect on increasing the density of the coal pile. Furthermore, when hot coal at 350–400°C is loaded into the carbonization chamber, gas is generated quickly, creating an upward buoyant force on the coal being loaded, which is also unfavorable for increasing the bulk density of that coal. (3) The lifespan of the coke oven is shortened. Due to the reduction in coking time, the productivity of the coke oven has increased, and the number of times coke can be produced from each carbonization chamber will rise year on year. Coupled with the decrease in the coking temperature, which results in the coke not being fully matured and poor contraction of the coke cake, the friction between the coke cake and the sides of the carbonization chamber increases during coke pushing, necessitating a greater force to push the coke. These two factors will result in the coke ovens used in the SCOPE21 process having a much shorter lifespan compared to those in traditional processes. (4) The increase in the proportion of weakly bonded coal is limited. According to the test results published in the literature, the cold strength of coke produced using the SCOPE21 process with the same coal blend increases by only 2.5 percentage points compared to that produced by conventional coking processes; however, the hot strength of the coke does not improve, and it is even inferior to the improvement achieved by large-volume coke ovens with high chambers. Since different **definitions of weakly caking coal vary, and the range of weakly caking coal is wide with significant differences, the proportion of weakly caking coal in the coal blend does not indicate the quality of that blend. Based on the data published to date, the quality of the blended coal in which 50% weakly caking coal is incorporated is as follows: the maximum flowability logMF ranges from 1.97 to 2.2, and the expansibility TD ranges from 42 to 51. The quality of the coal blend produced by Baosteel in China in 2005 was as follows: the maximum flowability logMF ranged from 2.06 to 3.49, with an average of 2.9; the expansibility TD ranged from 13 to 70, with an average of 34.2. Comprehensive analysis shows that the expansibility (TD) of the coal blend used in the tests is better than that of the coal blend used in Baosteel’s production in 2005; the maximum fluidity (logMF) of the test coal blend is slightly lower than that of Baosteel’s production coal blend from 2005. The quality difference between the coal blend used in the tests and that used in Baosteel’s production is not very significant. Therefore, Baosteel is increasing the proportion of weakly caking coal significantly based on its current coal blending strategy; it is not possible to obtain coke of the same quality using this process. In other words, the quality of the coke produced by Baosteel using this process with its current coal blend will not see any significant improvement. Based on Baosteel’s many years of research and production experience, given the current quality of coke produced by Baosteel, this process can at most utilize 3% to 5% more weakly bonded coal, without enabling a significant improvement. If the proportion of weakly bonded coal is increased significantly, even if the cold strength of coke decreases only slightly, its hot strength will drop substantially. However, in large-volume blast furnaces where a large amount of coal powder is injected, the hot strength of coke is more important than its cold strength; this is why shaped coke can be used at most to 30% in such large blast furnaces. (5) There is no substantial breakthrough in the technology, and its economic viability is questionable. The SCOPE21 process essentially involves transferring part of the coking process, which was previously carried out inside the coke oven chambers, to outside the ovens. By combining various technical measures aimed at improving coke quality and reducing environmental impact, it is possible to increase the productivity of coke ovens, optimize coke quality, and eliminate environmental pollution. But in the end, the SCOPE21 process is merely an improvement and enhancement of the traditional coking process; it does not deviate from the essence of traditional chamber-type coke ovens and high-temperature dry distillation coking. The SCOPE21 process failed to simplify the coking process; instead, it increased the operational complexity. Coal preheating, hot forming technology, as well as the storage, transportation, and charging processes for coal at 350–400°C are all highly complex. Dry quenching of coke requires reheating coke at temperatures below 900°C to 1000°C, and the process is also more complex than that of current dry quenching equipment. Although the number of chambers in the carbonization section of the coke oven itself has decreased, the number of times coke is produced per day and night across the entire oven fleet remains unchanged; instead, the operating intensity of each individual carbonization chamber has increased significantly. This imposes quite high requirements on coke oven equipment and refractory materials, and the lifespan of coke oven equipment will decrease in proportion as productivity increases. Taking all the above factors into account, the economic benefits claimed in relevant literature are questionable. In summary, the problems associated with the SCOPE21 process can be summarized as follows: the increase in the use of non-coking coals and weakly caking coals does not have a significant effect; the cold strength of the coke produced from the same coal mixture increases by only 2.5 percentage points compared to the traditional process, while there is no change in the thermal properties of the coke ； Although production efficiency has increased, the lifespan of the coke ovens will be shortened; therefore, the total investment cost cannot decrease ； The significant increase in the investment costs for coal pretreatment raises the operating costs of coking, increases its complexity, and there are still many key technologies that have not been resolved. Although Japan’s SCOPE21 coking process has its shortcomings, it is undeniable that this process provides a reference for the development of future coking technologies. There are three points worth learning from: the first is the flexibility of resources. New coke ovens must be able to handle non-caking and weakly caking coals, which cannot be used extensively in traditional coke ovens. Second is the competitiveness of costs. High productivity and strong cost competitiveness. Third, it is environmentally friendly. New coke ovens must meet increasingly strict environmental regulations (NOx, SO2, CO2) and occupational health requirements (dust, fumes, odors). In the 21st century, the blast furnace process will remain an important iron-making technology. We believe that environmentally friendly and energy-efficient coking technologies will be further developed and brought into commercial use, contributing to the growth of the global steel industry.