Early-stage operation experience of German wide-chamber, large-volume coke ovens

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This post was last edited by Xinglai on 2011-4-23 08:24. Summary: For coke ovens with wide carbonization chambers and large capacity, when the coke output per hole remains unchanged, the number of times coke is taken out can be significantly reduced, which is beneficial for environmental protection as well as for protecting the oven walls. It helps to extend the lifespan of the furnace. As the packing density of the coal fed into the furnace increases, both the coke yield and quality improve as well. However, there are also issues such as an excessively high proportion of coking coal and fat coal, limited proportions of bituminous coal, and the requirement that the volatiles content of the coal blend not be too high. In the 1970s, thanks to advances in coking technology, a number of large-capacity coke ovens were developed; for example, the carbonization chambers of Cokes ovens No. 1 and No. 2 at the Keihin Coke Plant of Nippon Steel Corporation had dimensions of 7.55 meters in height, 17 meters in length, and 450 mm in width. Even when these coke ovens use standard blended coal for coking, operational difficulties can sometimes occur. The main problem is that the coke cake cannot separate properly from the furnace wall; due to the difficulty in pushing out the coke, high forces have to be used, which causes excessive pressure on the furnace wall and bottom. This leads to premature damage of the furnace wall, increased maintenance costs, abnormal operation, and greater environmental pollution. As is well known, even large-volume coke ovens with excellent thermal control are still quite sensitive to changes in the composition of the coal used for charging; therefore, it is necessary to choose the composition of the blending coal carefully, which limits the options available for selecting coal. According to experts, the main reason is that in coke ovens with narrow carbonization chambers and large volumes, the shrinkage of coal during the coking process is too small; moreover, the improper ratios of the width to length and height of the carbonization chamber exacerbate the instability of the coke cakes during the pushing process. German experts, through years of laboratory and industrial testing, have shown that as the width of the carbonization chamber increases, the coking time does not increase in proportion to the square of this increase; rather, the rate of increase is slightly higher than the rate of increase in width. The results of this experiment have made it possible to construct large-volume coke ovens with industrialized wide carbonization chambers. With the coke production per carbonization chamber remaining unchanged, widening the carbonization chamber significantly reduces the number of times coke needs to be removed, which is beneficial for environmental protection and helps to protect the furnace walls; the force required to push out the coke can also be reduced accordingly. In the mid-1980s, large-volume coke ovens with wide carbonization chambers were put into operation at the Hugenhain and Prospero coke plants in Germany. In the 1990s, extra-large large-volume coke ovens with wide carbonization chambers were also brought online at the Kaiserslautern and Schwelgen coke plants. A total of 9 such large-volume coke ovens with wide carbonization chambers were installed in Germany, and the following experiences were drawn from this. (1) The sources of coking coal have been expanded; low-volatility, lean coals that cannot be used for coking on their own (with a volatility of Vdaf=16.5% and an ultimate shrinkage rate of only 2%) can account for up to 42% of the total amount used. (2) The volatiles content of the blending coal was reduced to Vdaf 22.5% (Vad 21.5%), thereby increasing the coke yield. (3) When coking with the above-mentioned blended coal, the coke cake exhibits good lateral contraction, smooth coke pushing, and a low coke pushing current. (4) The single-chamber carbonization chamber has a high output; when the coking plant has the same scale, it allows for a reduction in the number of times coke is produced per unit time, which is beneficial for environmental protection. Furthermore, as the number of times coke is discharged per unit time decreases, the frequency of compression and friction on the furnace walls also decreases, which helps to extend the lifespan of the furnace. (5) The bulk density of the coal loaded in the carbonization chamber increases, but it is appropriate to keep it at around 830 kg/m3. German experts believe that the quality of coke produced in coke ovens with wide carbonization chambers will not decline. Results from experiments conducted by Japanese experts using a 250 kg test furnace to simulate industrial coke ovens show that, at the same furnace temperature, increasing the furnace width leads to a decrease in the coke production rate per oven cavity (output per unit time) ； Under the current conditions with blended coal, even increasing the furnace width will have little impact on the quality of coke ； However, when coal blends with lower adhesiveness are used, the decrease in the final carbonization temperature leads to a reduction in microstrength ； The decrease in the heating rate of the gel layer leads to an increase in porosity, thereby reducing the strength of the coke. It is worth noting that in coke ovens with wide carbonization chambers and large volumes, the coke cakes experience significant lateral and longitudinal contraction within these chambers; as a result, the amount of weakly bonded bituminous coal (Pro coal) used must be limited to 15% only, as using too much of it will lead to the formation of large amounts of graphite in the furnace walls and ceiling areas. Therefore, the volatile matter content of the coal blend at Germany’s Prospehl coking plant has always been relatively low, as shown in Table 1. Table 1: Coal blending composition at Germany’s Prospehl coking plant from 1985 to 1990
Coal type | Volatility % | Blending percentage %
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1–6 | 7–12 | 1–6 | 7–12 | 1–6 | 7–12 | 1–6 | 7–12 | 1–6 | 7–12 | 1–6
Hugo coal | 25 | 30 | 28 | 25 | 24 | 19 | 13 | 10 | 8 | 12 | 10
Heinrich low-volatility coal | 16 | 5 | 15 | 25 | 31 | 38 | 39 | 33 | 39 | 36 | 40
Heinrich bituminous coal | 23 | 11 | 7 | 4
Osselfeld coal | 27 | 8 | 5 | 5 | 4
Westphalian coal | 23 | 6
Monopol coal | 16 | 3 | 8
Haus Ardennen coal | 29 | 6 | 13 | 2 | 5
Niedberg coal | 11 | 1 | 5
Prospehl No. 2 coal | 36 | 19 | 16 | 15 | 18 | 16 | 14 | 12 | 10 | 10 | 5
Rhineland coal | 29 | 51 | 51 | 45 | 33 | 26 | 28 | 32 | 30 | 15 | 25 | 33
Volatility of blended coal (DAF) % | 29.13 | 27.55 | 27.1 | 26.05 | 25.17 | 24.56 | 24.41 | 24.73 | 23.49 | 23.27 | 21.97
As can be seen from Table 1, the volatility of the coal blend used by this plant decreased over the years, from 27.55% in 1985 to 21.97% in 1990. The document explains: \"The improved shrinkage of coke and the structure of the heating flue across the holes provide more free space in the carbonization chamber, resulting in an increase in the temperature of the furnace roof area and the accumulation of graphite in that region; therefore, it is necessary to change the composition of the coal used as feedstock.\" ” The structure of double-layer crossing holes in the wide carbonization chamber and vertical flue results in a high temperature in the furnace roof area; it also causes large lateral contraction of the coke cake within the wide carbonization chamber, leading to a larger gap between the coke cake and the furnace walls. Therefore, when coking with highly volatile coals, graphite tends to accumulate in the roof area and furnace walls. The coal blending ratio used over the long term in the German-designed Prospero coking plant, featuring wide carbonization chambers and large-volume coke ovens, is as follows: low-volatility, lean coals that cannot be used for coking on their own (Vdaf 16.5%) account for about 40%, anthracite makes up 5%, coke dust accounts for 2%, weakly bindable, high-volatility bituminous coals account for 5%, and the remainder is medium to high-quality rich coals (such as Rhineland coal with Vdaf 29%). The main technical parameters of blended coal around 1990 are as follows: Moisture, % – 9%; Ash (on a dry basis), % – 7; Volatiles (Vdaf), % – 22.5, 21.5; G-value (German index for coking capacity of blended coal) ≮7; Softening temperature, °C – 395; Solidifying temperature, °C – 491; Shrinkage value – 26; Expansion value – 5. Studies indicate that very lean blends of coal are highly sensitive to changes and fluctuations in particle size with regard to their expansion rate. For blended coals with a volatile matter content of 20%–24%, since their expansion and contraction values are roughly equal, high expansion pressures are generated, especially when the particle size range of ≤0.5 mm accounts for ≤30%. This phenomenon is more pronounced in such cases. During continuous production in the large-volume coke ovens of Germany’s Prosselcoke plant, 36%–38% of the coal is crushed to a size of <0.5 mm; the bulk density is approximately 840 kg/m3, and the carbonization time is 24.5–25.0 hours. Compared with narrow carbonization chambers, coke oven gas production increases by about 5%–7% in wide carbonization chambers; the CH4 content in the gas decreases, while the CO and H2 contents increase. The calorific value of the gas drops, its density decreases, and the production of crude tar falls by 20%, with crude benzene production decreasing by 10%. The particle size of the coal used for charging typically results in a bulk density of the coal material in large-volume coke ovens with wide carbonization chambers of 900 kg/m3 or more (on a wet basis). Such a high bulk density can cause the gas pressure inside the oven to reach the limit allowed by the oven walls’ load capacity; the test results from the Prospero Coke Plant are shown in Table 2. Table 2 Relationship between the particle size and bulk density of the coal used in the Prospero coking plant, as well as the gas pressure inside the carbonization chamber. Test: Particle size <0.5mm %; Additives in the coal used for charging, %; Bulk density, kg/m3; Gas pressure, Pa; Shrinkage rate, %. Oil: Coke powder A – 27, 886, 371, 5.5; B – 45, 819, 88, 8.5; C – 43, 0.15, 860, 88, 8.2; D – 41, 2, 821, 50, 9.5. When the bulk density reached 900 kg/m3, the expansion pressure exceeded the limit for the stability of the furnace walls, resulting in bulging of these walls and cracking of the horizontal joints. This is similar to the experience encountered at the Hu Jingen coking plant, which uses large-volume coke ovens with wide carbonization chambers (550 mm wide). Thanks to the early detection of this phenomenon, the bulk density of the coal was promptly reduced to the normal value of 830 kg/m3, thereby preventing permanent deformation of the furnace walls. These tests indicate that the hypothesis that a wider carbonization chamber reduces expansion pressure has not yet been fully confirmed. The carbonization chamber facilitates coke pushing operations, but it is not necessarily possible to avoid the occurrence of critical internal gas pressure (expansion pressure) ； It also proves that the furnace wall stability of these two types of large-volume coke ovens with wide carbonization chambers is insufficient; it is necessary to widen the combustion chamber in order to improve the stability of the furnace wall, which was achieved in the 7.63m coke ovens at the Kaiserslautern coking plant. Operational experience shows that, compared to a carbonization chamber with a width of 450 mm, the coking time extension index of 1.3 to 1.4, which has been determined through numerous tests for chambers with a width of 600 mm, is correct. Therefore, at a certain coking rate, a larger carbonization chamber volume can be used and the number of furnace holes can be reduced without affecting the economic efficiency of the coking process. As a result, the number of coal loading and coke pushing operations can be reduced, thereby decreasing dust emissions. This article is the author’s notes after reading the Chinese translation of the journal “Foreign Coking Chemistry,” and it essentially consists of excerpts based on the original meaning of the translation. What is made public here is intended solely as a reference for industry professionals interested in coke ovens with large carbonization chambers and large volumes. Since reading notes are inherently incomplete and one-sided, readers are encouraged to consult the original text if there are any points that are not clear. References [1] “Current Status of Coke Oven Technology with Wide Carbonization Chambers and Its Impact on Coke Quality”, Foreign Coking Chemistry, Issue 1, 1992. [2] “Design and Commissioning of the Third Wide Carbonization Chamber Coke Oven at the Siddeprosper Coke Plant”, Foreign Coke Chemistry. [3] “Operational experience with 600 mm wide coke ovens at the Prospero coking plant”, Foreign Coking Chemistry, Issue 6, 1992. [4] “Broad carbonization chamber and large-volume coke ovens”, Foreign Coke Chemistry, 1996