Coking Pressure Control by Selective Crushing of High Coking Pressure Coal

Transcription

1 , pp Coking Pressure Control by Selective Crushing of High Coking Pressure Coal Seiji NOMURA, 1) Takashi ARIMA, 1) Atsushi DOBASHI 2) and Kazuhide DOI 3) 1) Nippon Steel Corporation, Environment & Process Technology Center, 20-1, Shintomi, Futtsu, Chiba, Japan. 2) Nippon Steel Corporation, Kimitsu Works, Kimitsu 1, Kimitsu, Chiba, Japan. 3) Nippon Steel Corporation, Oita Works, Nishinosu 1, Oita, Oita, Japan. (Received on March 11, 2011; accepted on May 30, 2011) Controlling coking pressure is one of the most important aspects of the cokemaking process, since excessive coking pressure increases the force needed for coke cake pushing and in some cases leads to operational problems such as hard pushes or stickers, causing wall damage. Against this backdrop, we investigated the selective fine crushing of high coking pressure coal as a way to reduce coking pressure. It was shown on a laboratory scale that the fine crushing of high coking pressure coal increases the permeability of the plastic coal layer, which decreases coking pressure (internal gas pressure). Based on the basic investigation, we tried the fine crushing of high coking pressure coal at commercial cokemaking plants, and it was confirmed during a long-term commercial-scale experiment that the fine crushing of high coking pressure coal decreases coking pressure and decreases the maximum power current of coke pushing. Thus, the selective fine crushing of high coking pressure coal is a promising way to reduce coking pressure and prolong coke oven life. KEY WORDS: coking pressure; crushing; plastic layer; permeability; coke pushing. 1. Introduction During coal carbonization in a coke oven chamber, the swelling of molten coal causes a load called coking pressure on oven walls. Since excessive coking pressure increases the force needed for coke cake pushing and in some cases leads to operational problems such as hard pushes or stickers, causing wall damage, one of the most important aspects of the cokemaking process is to control and reduce the coking pressure. 1 11) With this in mind, Nippon Steel has developed dry coal-charging processes for cokemaking, such as CMC (Coal Moisture Control) 12) and DAPS (Dry-cleaned and Agglomerated Precompaction System). 13,14) With the coal moisture being reduced to 5 6 mass% using CMC and 2 4 mass% using DAPS, the advantages of less heat consumption for carbonization, higher productivity, and better coke quality were gained. Since a decrease in coal moisture leads to an increase in coal bulk density in the coke oven chamber, which increases coking pressure to a great extent, it is quite important to control the coking pressure during the dry coal charging processes. We have reported on the coal blending theory to control coking pressure and produce high-quality coke under high coal charge bulk density in the dry coal charging process. 15) However, as coke ovens have been deteriorating recently, an additional method to reduce coking pressure is needed. Furthermore, since high coking pressure coal consists of low-volatile-matter coal (high-rank coal), a method to use a larger amount of high coking pressure coal as well as to decrease coking pressure is required in order to reduce coke cake pushing force and increase coke yield. The addition of inert material, such as coke breeze, and the fine crushing of coal are known as one method to decrease coking pressure, 1) however, the former is not favorable since inert addition leads to the deterioration of coke strength, and high strength coke is demanded for the stable operation of large blast furnaces. On the other hand, the latter may improve coke strength, 16,17) but the mechanism of coking pressure decrease caused by fine crushing is unclear. While, a report 18) suggests that a decrease in coking pressure with the fine crushing of coal observed in a top-charge movable-wall pilot coke oven is caused by a decrease in bulk density and that the fine crushing of coal increases coking pressure under the same bulk density. Therefore, the fine crushing of coal in order to reduce coking pressure has not been positively used on a commercial scale. In the current work, firstly we investigated the effect of the fine crushing of coal on coking pressure under the same coal charge bulk density through use of a test coke oven. Then, we developed a method to measure plastic coal layer permeability, which is considered one of the dominant factors of coking pressure, and investigated the effect of the fine crushing of coal on plastic coal layer permeability. Finally, based on the basic investigations, we tried the fine crushing of coal in order to reduce coking pressure on a commercial scale ISIJ

2 2. Laboratory-scale Investigation 2.1. Experiment Measurement of the Internal Gas Pressure in a Test Coke Oven The properties of the coal used in the laboratory-scale test and commercial-scale test are shown in Table 1. Coal A, Coal B, and Coal C are high coking pressure coals, and the maximum internal gas pressure measured in the test coke oven 6) was 170 kpa, 370 kpa, and 50 kpa, respectively (where each single coal was crushed 3 mm 85% and charged at a bulk density of 850 kg/m 3 ). The blend design for the internal gas pressure measurement tests are shown in Table 2. In tests 1 4, each single coal was crushed into a desired grain size and then mixed (crushing before blending). In tests 5 and 6, coal was mixed according to a blending ratio, and then the blended coal was Coal Ash (db) Proximate Analysis VM (db) Table 1. Coal characteristics. Gieseler Plastometry Log 10 (Max. Fluidity/ ddpm) Ruhr Dilatometry Total Dilatation Petrographic Analysis Mean Vit. Ref. Total Inerts (vol.%) A B C D E F G H I J K L M Table 2. Blend design for internal gas pressure measurement tests. Test 1 Test 2 Test 3 Test 4 Test 5 Test 6 B C 50 D E F G H I J 25 K 25 L 25 M 25 Total crushed (crushing after blending). The crushing conditions for internal gas pressure measurement tests are shown in Table 3. The coal-crushing grain size was controlled by adjusting the rotation speed of the test crushing machine. A sample of 1.5 mm 100% was prepared by sieving crushed coal with a 1.5 mm-size screen and by crushing +1.5 mm fractions repeatedly. Moreover, in tests 3 and 4, a +1 mm fraction of crushed Coal B was used. The prepared coal was then charged at a bulk density of 850 dry kg/m 3 in an electrically heated pilot coke oven (420 mm wide, 600 mm long, and 400 mm high 15) ) and was carbonized for 18.5 hours under heating conditions equivalent to the flue temperature of C in an actual coke oven. The internal gas pressure was measured at the oven width center (210 mm from the oven wall), the oven length center, and 120 mm from the oven sole, using stainless steel tube probes (inner diameter 1 mm and outer diameter 2mm). 6) Previous experiments in our laboratory showed that there was a good correlation between the internal gas pressures measured in this way and the coking pressure measured with a movable wall pilot oven Measurement of Plastic Coal Layer Permeability In order to investigate the effect of coal particle size on the plastic coal layer permeability, the permeability was measured using the apparatus shown in Fig ,20) A given amount of a coal sample was charged in a constant volume cell and carbonized with the expansion of the coal being restricted. The permeability was then evaluated by measuring the pressure drop caused by forced inert gas flow through the plastic coal layer. The reactor tube is made of stainless steel and the dimension of the space where the coal sample is charged is 10 mm in diameter and 10 mm in length. In order to keep the coal Table 3. Crushing conditions for internal gas pressure measurement tests. Test 1 Test 2 Test 3 Test 4 Test 5 Test 6 Crushing-blending Blending-crushing Coal B Coal C Coal B Blended coal 3 mm 85% 3 mm 85% +1 mm 100% 3 mm 70% 3 mm 95% 3 mm 95% 3 mm 75% 3 mm 80% 3 mm 95% 3 mm 90% 1.5 mm 100% 1.5 mm 100% Other coals ( 3 mm 85%) Fig. 1. Plastic coal permeability test apparatus ISIJ 1426

3 Fig. 2. Particle size distribution of Coals B and C (tests 1 4). volume constant during carbonization, the expansion of the coal sample was restricted by sandwiching it between two wire mesh plates with a 0.15 mm sieve, which were fixed using inner tubes. Five reactor tubes could be set in an electric furnace at the same time, and a thermocouple was placed in the center reactor tube to control the temperature. After placing the four reactor tubes with the coal sample into the furnace, the reactors were connected to the gas supply lines at the bottom and N 2 was forced to flow at a constant flow rate of 10 cm 3 /min (equivalent to a linear velocity of 2 cm/s), controlled by a mass flow controller. The upper end of the reactor was open to the atmosphere and connected to a vent. The sample was heated from room temperature to 300 C at 10 C/min and from C at 5 C/min. Changes in pressure with time were measured using a pressure gauge placed between the reactor tube and the mass flow controller. High coking pressure Coal A and Coal B (coal particle size 2.8 mm, 1.5 mm, 1.0 mm, 0.6 mm, 0.3 mm, and 0.15 mm) were used, and the bulk density was 800 dry kg/m Results and Discussion Effect of the Fine Crushing of Coal on Coking Pressure The particle size distribution of Coal B and Coal C are shown in Figs. 2 and 3, respectively. The coal was crushed to a desired level. The relationship between the weight-average particle size and internal gas pressure is shown in Figs. 4, 5 and 6, with the particle sizes of Coal B or Coal C on the x-axis in tests 1 4 and that of the blended coal on the x- axis in tests 5 and 6. It is clear that the fine crushing of high coking pressure coal decreases internal gas pressure. In addition, not only the selective fine crushing of high coking pressure Coal B and Coal C (crushing before blending, in tests 1 4) but also the fine crushing of blended coal including Coal B and Coal C (crushing after blending, in tests 5 and 6) reduces internal gas pressure Effect of the Fine Crushing of Coal on Plastic Coal Layer Permeability The relationship between superficial velocity and permeability through a packed bed is described as follows according to Darcy s law: Fig. 3. Particle size distribution of blended coal (test 5). Fig. 4. Effect of mean particle size of Coals B and C (in tests 1 4) and blended coal (in tests 5 and 6) on internal gas pressure. u = (K/μ) (ΔP/L)... (1) where u [m/s] is the superficial velocity, K [m 2 ] is the permeability coefficient, μ [Pa s] is the gas viscosity, L [m] is the length of the layer, and ΔP [Pa] is the pressure drop. The permeability, K, can be obtained by assuming μ as has been reported; 19,20) however, here we used ΔP to represent permeability, since the lower ΔP is, the higher the permeability of the layer. Figure 7 shows the changes in the pressure drop of Coal B during the permeability test (hereinafter referred to as ISIJ

4 Fig. 5. Effect of 3 mm % of Coals B and C (in tests 1 4) and blended coal (in tests 5 and 6) on internal gas pressure. Fig. 8. Effect of coal particle size on maximum pressure drop (ΔP max) measured in permeability test rig. Fig. 6. Effect of 1 mm % of Coals B and C (in tests 1 4) and blended coal (in tests 5 and 6) on internal gas pressure. Fig. 9. Coal crushing plant. the fine crushing of high coking pressure coal could be ascribed to the increase in the permeability of the plastic coal layer. Fig. 7. Changes in pressure drop in permeability test (ΔP) with temperature. ΔP ), with temperature. As temperature increases, ΔP starts to increase and shows a peak (ΔP max). Then, it decreases as the temperature approaches the resolidification temperature. The temperature range over which ΔP appears is nearly equivalent to the plastic temperature range of Coal B (softening temperature 454 C to resolidification temperature 507 C in a Gieseler fluidity test), and ΔP max decreases as the particle size becomes finer. As shown in Fig. 8, the fine crushing of high coking pressure Coal A and Coal B increases the permeability of the plastic coal layer. This shows that the decrease in internal gas pressure caused by 3. Commercial-scale Investigation 3.1. Using Larger Amounts of High Coking Pressure Coal by Fine Crushing It Based on the basic investigations, we trialed the fine crushing of high coking pressure coal at the Kimitsu cokemaking plant (No. 4 and No. 5 coke oven batteries, 184 chambers in total, oven width 430 mm, length mm, and height mm). This plant is equipped with four crushers, and each crusher has eight coal hoppers as shown in Fig. 9. One crusher was used only for crushing high coking pressure Coal B, and Coal B was crushed finely by increasing the rotation speed of the crusher. Before starting a long-term experiment, a batch test was carried out where blended coal, including finely crushed Coal B, was charged in several coke oven chambers. In this test, the internal gas pressure was measured at the oven width center below a charging hole by inserting stainless steel tube probes (inner diameter 1 mm and outer diameter 2 mm) with a guide bar through a hole in the oven door. Firstly, 3 mm of Coal B was increased from 85% to 93%, and the blending ratio of Coal B was kept constant at 6%. As shown in Fig. 10, the fine crushing of high coking pressure coal decreased internal gas pressure and decreased the maximum power current of pushing. Secondly, 3 mm of 2011 ISIJ 1428

5 Fig. 10. Effect of particle size of high coking pressure coal B on internal gas pressure and maximum power current of pushing in an actual coke oven chamber. Fig. 11. An increase in the blending ratio of high coking pressure coal B by the fine crushing of the coal. Coal B was increased from 85% to 93%, and the blending ratio of Coal B was increased from 6% to 10%. As shown in Fig. 11, the fine crushing of high coking pressure coal allowed for the blending of larger amounts of high coking pressure coal without any influence on coke pushing performance. Based on the batch test result, a long-term test for the fine crushing of high coking pressure coal was carried out in two coke oven batteries with 184 chambers. The target of the grain size of high coking pressure Coal B was 3 mm >90% (the result was 3 mm 93 96%). The working rate of the coke oven was 125%, the flue temperature at the top was 1130 C, and the moisture of the charged coal was 4.0%. The blending ratio of Coal B was increased from 6% to 10% during the test period, as shown in Fig. 12. In order to maintain coke strength (DI ), as the blending ratio of Coal B increases, the blending ratio of the coal other than Coal B was slightly adjusted in such a way that the weightedaverage of the volatile matter (VM), maximum fluidity (log MF), and total dilatation (TD) was 26.1%, 2.1, and 66%, respectively. As a result, coke strength (DI ) was kept nearly constant, as shown in Fig. 12. As shown in Fig. 13, 3 mm% of the blended coal increased by increasing the blending ratio of the finely crushed Coal B, while 0.3 mm% of the blended coal increased by increasing 3 mm% of the blended coal, as shown in Fig. 14. In addition, there had been a concern that fine crushing might lead to a decrease in charged coal bulk density; however, the amount of charged coal showed little change against 3 mm% of the Fig. 12. Fig. 13. Changes in the blending ratio of high coking pressure coal B, 3 mm % of blend coal, 0.3 mm % of blend coal, the mass of charged coal in a coke oven chamber, maximum power current of pushing and DI Relationship between the blending ratio of high coking pressure coal B and 3 mm % of blend coal. blended coal, as shown in Fig. 15. Moreover, there had been another concern that fine crushing might lead to an increase in carbon deposits on the coke oven chamber walls 21) and thus would worsen coke pushing performance; however, the ISIJ

6 Fig. 14. Relationship between 3 mm % and 0.3 mm % of blend coal. Fig. 15. Relationship between 3 mm % of blend coal and the mass of charged coal in a coke oven chamber. Fig. 17. Changes in 3 mm % of coal A, 0.3 mm % of coal A, the mass of charged coal in a coke oven chamber, maximum power current of pushing and DI. Fig. 16. Relationship between the blending ratio of high coking pressure coal B and maximum power current of pushing. maximum power current of the pushing showed little change against the blending ratio of Coal B, as shown in Fig. 16. As described above, it was confirmed during the long-term commercial-scale experiment that the fine crushing of high coking pressure coal allows for the blending of larger amounts of high coking pressure coal without any influence on coke pushing performance Decreasing the Maximum Power Current of Coke Pushing by Fine Crushing High Coking Pressure Coal Furthermore, we tried the fine crushing of high coking pressure coal at the Oita cokemaking plant (No. 1 and No. 2 coke oven batteries, 156 chambers in total, oven width 440 mm, length mm, and height mm). This plant is equipped with three crushers, and each crusher has six coal hoppers. One crusher was used only for crushing high coking pressure Coal A, and Coal A was crushed finely by increasing the rotation speed of the crusher. The working rate of the coke oven was 121%, the flue temperature at the top was C, and the moisture of the charged coal was 4.5%. The blending ratio of Coal A was kept constant at 4% during the test period. The grain size of high coking pressure Coal A was changed from 3 mm 82% to 3 mm 96% stepby-step, as shown in Fig. 17. The blend composition was kept nearly the same during the test period, and the weighted-average of volatile matter (VM), maximum fluidity (log MF), and total dilatation (TD) were 27.2%, 1.8, and 41%, respectively. As shown in Fig. 18, the fine crushing of Coal A increased 0.3 mm% of Coal A. Further, the amount of charged coal showed little change against 3 mm% of Coal A, as shown in Fig. 19. As shown in Fig. 20, it was confirmed during the long-term commercial-scale experiment that the fine crushing of high coking pressure coal decreased the maximum power current of coke pushing. Despite these results, fine crushing may result in a decrease in charged coal bulk density, which can lead to a decrease in productivity and the deterioration of coke 2011 ISIJ 1430

7 a method to treat an increase in fine particles caused by fine crushing, the DAPS process 13,14) is suitable since fine coal particles are dry-classified and agglomerated during this process. Fig. 18. Relationship between 3 mm % and 0.3 mm % of high coking pressure coal A. 4. Conclusions In order to control and reduce coking pressure, we investigated the selective fine crushing of high coking pressure coal. (1) The fine crushing of high coking pressure coal increases the permeability of the plastic coal layer, which decreases coking pressure (internal gas pressure). (2) It was confirmed during a long-term commercialscale experiment that the fine crushing of high coking pressure coal decreases coking pressure and decreases the maximum power current of coke pushing. It was proved both on a laboratory scale and on a commercial scale that the selective fine crushing of high coking pressure coal is a promising way to reduce coking pressure and prolong coke oven life. REFERENCES Fig. 19. Fig. 20. Relationship between 3 mm % of high coking pressure coal A and the mass of charged coal in a coke oven chamber. Relationship between 3 mm % of high coking pressure coal A and maximum power current of pushing. strength. Further, an increase in fine coal particles caused by fine crushing may result in an increase in carbon deposition on coke oven walls, leading to an increase in the maximum power current of coke pushing, hard pushes, and stickers. It is thus necessary to pay attention to these issues in order to carry out coking pressure control by the selective crushing of high coking pressure coal on a commercial scale. As 1) R. Loison, P. Foch and A. Boyer: Coke Quality and Production, Butterworth Publications Ltd., London, (1989), ) J. Tucker and G. Everitt: 2nd Int. Cokemeking Cong., Vol. 2, IOM, London, (1992), 40. 3) G. C. Soth and C. C. Russell: Trans. AIME, 157 (1944), ) C. C. Russell, M. Perch and H. B. Smith: AIME Blast Furnace, Coke Oven and Raw Materials Proc. 12, the American Institute of Mining, Metallurgical, and Petroleum Engineers, New York, (1953), ) W. Rohde, D. Habermehl and V. Kolitz: Ironmaking Conf. Proc., 47 (1988), ) S. Nomura and K. M. Thomas: Fuel, 75 (1996), ) R. Alvarez, J. J. Pis, M. A. Diez, A. Marzec and S. Czajkowska: Energy Fuels, 11 (1997), ) S. Nomura and T. Arima: Fuel, 80 (2001), ) J. F. Gransden, M. A. Khan and J. T. Price: Ironmaking Conf. Proc., 44 (1985), ) J. F. Gransden, M. A. Khan and J. T. Price: Ironmaking Conf. Proc., 47 (1988), ) F. Huhn, F. Strelow and W. Eisenhut: Cokemaking Int., 4 (Special) (1992), S38. 12) S. Wakuri, M. Ohno, K. Hosokawa, K. Nakagawa, Y. Takanohashi, T. Ohnishi, K. Kushioka and Y. Konno: Ironmaking Conf. Proc., 45 (1986), ) Y. Nakashima, S. Mochizuki, S. Ito, K. Nakagawa, K. Nishimoto and K. Kobayashi: 2nd Int. Cokemeking Cong., Vol. 2, IOM, London, (1992), ) S. Tanaka, K. Okanishi, A. Kikuchi and Y. Yamamura: Ironmaking Conf. Proc., 56 (1997), ) S. Nomura, T. Arima and K. Kato: Fuel, 83 (2004), ) Y. Miura, T. Yamaguchi, T. Nishi and Y. Yone: J. Fuel Soc. Jpn., 60 (1981), ) Y. Kubota, S. Nomura, T. Arima and K. Kato: ISIJ Int., 48 (2008), ) O. P. Brysh and W. E. Ball: Research Bulletin 11, Institute of Gas Technology, Chicago, (1951). 19) K. Miura and K. Nishioka: Cokemaking Int., 4 (1992), No. 1, ) M. D. Casal, E. Diaz-Faes, R. Alvarez, M. A. Diez and C. Barriocanal: Fuel, 85 (2006), ) T. Nakagawa, T. Suzuki and I. Komaki: Int. Cong. Sci. Technol. Ironmaking and Ironmaking Conf. Proc., 57 (1998), ISIJ