Develop large-scale coke ovens suitable for China’s conditions

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Developing Large-Capacity Coke Ovens Suitable for China’s Conditions Abstract: By analyzing the production practices of 7.63m-wide chamber coke ovens introduced from Germany, it is shown that although these ovens have many advantages, they are not suitable for China’s coal resource situation. It is recommended that when constructing new large-scale coke ovens in the future, taking into account China’s limited supply of highly caking, low-volatility coking coals and fat coals, the width of the carbonization chambers should be kept around 500mm. I. Problems with 7.63m large coke ovens: Over the past 2–3 years, with the development of steel enterprises in China and the aging of these coke ovens, there is an urgent need to upgrade and renovate some of them. Some owners have decided to build coke ovens with a larger capacity, 6 meters higher than the current carbonization chambers. Since no suitable furnace type was available in the country at that time, a large coke oven with a carbonization chamber 7.63 meters high and about 600 mm wide was imported from Germany (hereinafter referred to as the 7.63m coke oven). Due to the large effective volume of the 7.63m coke oven, the number of coal charging and coke pushing operations can be reduced at the same coke production capacity. This not only contributes to environmental protection, improves labor productivity, and reduces land use, but also results in a higher level of automation. Currently, this type of coke oven has been put into operation at Yankuang, Taiyuan Iron and Steel, and Ma’anshan Iron and Steel. From the perspective of construction and production, in addition to high investment in infrastructure, the following issues have been identified that warrant careful consideration. ①The temperature at the top of the carbonization chamber is high; graphite forms at the base of the rising tube as well as inside the rising tube, and in some cases this condition is quite severe ； ②Smoke and dust are emitted during coal loading, and its smoke suppression effect is inferior to that of the dust removal stations on land in our country ； ③Tar is leaking from beneath the furnace door. In summary, these problems arise from a combination of factors, which are discussed below separately. 1. The high roof temperature leads to the formation of graphite at the base of the rising tube and within the rising tube. To discuss this issue in depth, it is first necessary to trace the development history of the 7.63m coke oven. The 7.63m coke oven was designed in Germany in the mid-to-late 1980s; it came online at the Kaiserstuhl plant in 1992 and was shut down at the end of 2000 after 8 years of operation. It can be said that, for a production facility, the production time is not long. The main type of coal used in such coke ovens is highly caking coal – coking coal with good caking properties and bituminous coal with high binding capacity – with a proportion of at least 70%; this is combined with high-quality lean coal. The volatile content of this highly caking, low-volatility coal ranges from 21% to 22%, which results in low shrinkage during coke formation. Therefore, it is necessary to widen the carbonization chamber to around 600 mm in order to increase the degree of shrinkage. In the coking industry, coke ovens with a carbonization chamber width of ≥550 mm are generally referred to as wide-carbonization-chamber coke ovens. Broadening the carbonization chamber has the following effects on the coking process of coal: Firstly, theories and practical experiences from experts in China and abroad have shown that coke produced in coke ovens with wide carbonization chambers using strongly bonded coal exhibits good strength properties both in the cold and hot states, whereas coke produced using weakly bonded coal shows poor performance in all these aspects. This is determined by the mechanisms underlying the coking process of coal. Secondly, coke ovens with wide carbonization chambers experience greater shrinkage during the coke formation process, with vertical shrinkage being the most significant factor; therefore, it is necessary to use coal blends with low volatile matter content. These two effects precisely confirm the reality that coke ovens with 7.63m wide carbonization chambers must use coal with strong cohesion and low volatile matter content. Additionally, there are the 7.63m coke ovens equipped with Schalke coal loading cars; when loading coal, these cars discharge coal from all four hoppers simultaneously. To accommodate the flow of raw gas, it is necessary to increase the space at the top of the carbonization chamber. In summary, due to the generally high volatile matter content in the coals used for coking in China (the volatile matter content of the coals used by Taiyuan Iron and Steel is around 24%, which is not considered high, but still not suitable for the requirements of 7.63m coke ovens), the large vertical contraction of the coke cakes in ovens with wide carbonization chambers, the need for ample ceiling space in Schalke-type coal loading cars, as well as the structure of the vertical flue openings in 7.63m coke ovens, these factors all contribute to high temperatures at the top of the carbonization chambers in China’s operational 7.63m coke ovens, as well as to the formation of graphite at the base of the rising pipes and within those pipes. In particular, the rise pipe of the 7.63m coke oven is quite tall, and coupled with the inadequate mechanical cleaning equipment, it is very difficult to clean the rise pipe manually. Given the limited availability of high-quality lean coal in our country, in order to reduce the volatility of the coal mixture used in 7.63m coke ovens, it is recommended to add an appropriate amount of powdered coke (with a particle size of <1 mm) to the coal mixture. 2. Smoking from the coal charging hole during coal loading: The 7.63m coke oven uses Schalke coal charging cars in combination with Proven technology (a technology equivalent to high-pressure ammonia water used for smoke suppression in China). However, such technical measures are not suitable for the characteristics of the coal used in our country’s furnaces. The volatile matter content of coal used in German furnaces is low (typically 21%–22%), and the coal moisture level is also low, remaining stable at 8%–9%. The coal used in furnaces in our country not only has a high volatility content, but also a high moisture content in the coal blend, generally ranging from 11% to 12%. At 11%, the moisture content is 3 percentage points higher than that of German coal blends. Assuming a coal loading of 60t per hole, an additional 30×60=1800 kg of water is added per hole. When this 1800 kg of water turns into steam, the density of the steam is 0.8 kg/m3. Then the volume of water vapor is: 1800/0.8=2250 m3. Of course, not all of this moisture can evaporate during coal loading; if 10% of the moisture evaporates, its volume is approximately over 200 m3. Therefore, it is impossible to load coal without a ground station for dust removal. To convert a 7.63m coke oven to a dust removal system at ground level, both the Schalke coal charging vehicle and the Proven system would need to be modified, which involves significant changes and poses considerable challenges. II. China is not suitable for building coke ovens with wide carbonization chambers. The carbonization chamber of the 7.63m coke oven at the Kaisersluhl plant in Germany has a width of 623 mm (610 mm in the hot state), and it has been purchased by China’s Yankuang Group as second-hand equipment. The 7.63m coke ovens introduced by Taiyuan Iron and Steel, Maanshan Iron and Steel, Wuhan Iron and Steel, and Shougang Caofeidian projects have a carbonization chamber width of 603mm (at cold state, 590mm at hot state). As mentioned earlier, this type of coke oven with a wide carbonization chamber has been used only by the Germans worldwide, and it requires coal with high caking strength and low volatility. At present, in Taiyuan Iron and Steel, the proportion of coking coal and fat coal in the coal mix exceeds 75%; in Maanshan Iron and Steel, it is 60%, while at Yankuang it is 65%. Nevertheless, high volatility levels are still a problem. It is entirely foreseeable that Maanshan Iron and Steel and Yankuang will need to increase the proportion of coking coal, fat coal, and lean coal in their coal mixes. In the coking coal mix used by large domestic steel companies, the proportion of coking coal to bituminous coal is around 55%; Baosteel uses about 50% of coking coal. Thanks to measures such as the use of blended coal and dry quenching of coke, the quality of the coke produced is not inferior to that of Taiyuan Iron and Steel or Maanshan Iron and Steel. As can be seen from the table below, although the quality of coke produced by Taiyuan Iron and Steel Group and Maanshan Iron and Steel Group’s 7.63m coke ovens is also very good, this is achieved by using considerably more coking coal and fat coal than Baosteel does, rather than being a result of the 7.63m wide carbonization chamber of those ovens. It is understood that the cost of coke produced by Taigang’s 7.63m coke ovens is about 80 yuan per ton higher than that of coke produced by Taigang’s other coke ovens. Comparison table of coke quality among Baosteel, Taiyuan Iron and Steel, and Maanshan Iron and Steel
Factory name: M40% M10% CRI% CSR%
Baosteel: 88–89% 5–6% 69–70%
Taiyuan Iron and Steel: 89–91% 5–7% 70–71%
Maanshan Iron and Steel: 89–90% 5–6% 69–70%

In summary, in coke ovens with a 7.63m wide carbonization chamber, the proportion of strongly caking coals and fat coals in the coal mixture used should be no less than 70%. However, in China, there is a shortage of such coals, and they are also difficult to process. As a result, the ratio of coals to fat coals in the coal mixture used in conventional coke ovens is generally only around 55%; such a mixture has a moderate degree of caking and is not suitable for use in coke ovens with wide carbonization chambers. If large amounts of coals and fat coals are imported from abroad, the cost of coke will inevitably increase further. Therefore, to suit the conditions in our country, even if the carbonization chamber of the coke oven is made taller, its width should be kept at around 500 mm. III. Developing large-scale coke ovens in China in a way that is both effective and cost-efficient: According to the estimates for Taiyuan Iron and Steel’s coking workshop, the capital costs for constructing 2×70-hole, 7.63m coke ovens, including costs related to coke screening, amount to 370 yuan per ton; among these costs, the control equipment for the Prover system alone costs 2.4 million euros. From the perspective of coking production, the appropriate size of coke ovens is determined by the scale of coke production. The principle followed is that two coke ovens form one production unit, with a group of workers operating the machinery for that set of ovens; this approach is the most economical and reasonable, as shown in the table below. For a coke production capacity of around 1.5 million tons per year, the best choice is: carbonization chamber height of 7 meters ； Charging chamber width: about 500mm ； Length of the carbonization chamber: 17–18 m. The primary advantage of this type of coke oven is its suitability for China’s coking coal resources, which helps to reduce production costs and takes advantage of China’s mature technologies (such as a well-structured oven body, dust removal at the coal loading station, and a properly designed coke oven door). At the same time, it incorporates the advantages of other large coke ovens, such as independently suspended control panels, quenchers that rest directly on the ground, and effective automation. This type of coke oven is now being used in the expansion projects of Ansteel, Benxi Iron and Steel, Handan Iron and Steel, Panzhihua Iron and Steel, etc. Ansteel plans to put it into operation by the end of 2007. The capital investment for a coking plant built with such coke ovens is ≯300 yuan per ton. With an annual coke production capacity of 2.5 to 3 million tons and a ratio of 7.63m coke ovens, savings of over 200 million yuan can be achieved.