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Effect of CO2 Gasification on the Transformations of Coke Minerals at High Temperatures Sushil Gupta,* Maria Dubikova, David French,† and Veena Sahajwalla School of Materials Science and Engineering, The UniVersity of New South Wales, Sydney, NSW 2052, Australia ReceiVed September 12, 2006. ReVised Manuscript ReceiVed NoVember 6, 2006
Mineral phases of three metallurgical cokes were characterized after reactions with CO2 at 1373 and 1773 K in a horizontal furnace for 2 h using X-ray diffraction (XRD) and scanning electron microscope/energy dispersive spectroscopy (SEM/EDS). After reaction at 1373 K, all cokes displayed an increased degree of mineral decomposition, dissemination within the carbon matrix, and inclusions in pores such that the intensity of transformation varied with coke types. As for the original cokes, XRD analysis of the reacted cokes indicated the presence of quartz, mullite, and high melting point aluminosilicate as the major refractory phases while pyrrhotite, triolite, iron oxides, and metallic iron phases were the main reactive phases. Framboidal sulfide appeared to diminish after CO2 reactions. Iron-spinel was the common reaction product while olivine was observed in only one coke. All minerals occurred in a wide variety of sizes and distributions such that fine minerals occurred mainly within or close to pores and were comprised of iron sulfide and Al2O3. At 1773 K, aggregates of Al-silicates decomposed further to form mullite and cristobalite while low melting point Alsilicates transformed to running slag in coke. Iron was observed to occur in many forms including metallic iron, silicates, and oxides but less frequently as sulfides and appeared to occur within or in close proximity to pores. At 1773 K, pore inclusions were the prominent features of the reacted cokes, particularly a high proportion of Al2O3 (corundum). Calcium was observed as silicates as well as thermally stable phosphate and sulfides. This study demonstrated that characteristics of coke minerals after reactions at standard coke strength after reaction (CSR) test temperatures of 1373 and 1773 K were significantly different in terms of phase distribution which would influence coke strength and reactivity accordingly. This study implies that coke properties measured at relatively lower temperatures in the conventional tests may not provide a true assessment of coke performance in an operating blast furnace.
Introduction Coke plays multiple roles in a blast furnace, as it provides a heat energy source, performs the role of reducing agent and acts as a spacer for maintaining the bed permeability. Recently, there is an increasing trend to reduce the coke rates such that less amount of coke is available to perform the chemical and physical roles. At low coke rate operations, coke quality issues become more critical, as less coke is available to supply reducing gases and provide sufficient physical support. Under these conditions, the coke to ore volume ratio decreases resulting in a thinner coke layer in the stack and cohesive zones. Consequently, coke may take a longer time in descending from top to bottom, particularly at high injection rates.1-2 Thus, coke will experience longer periods of mechanical, thermal, and chemical stresses and consequently degrade more depending on coke quality. Coke fines influence the liquid and gas permeability as well as metal/slag drainage conditions as a consequence of their accumulation at lower regions in a blast furnace. Coke reactions including gasification, graphitization, * Corresponding author. Tel.: + 61-2-93854433. Fax: 61-2-93855956. E-mail:
[email protected]. † Present address: CSIRO Energy Technology, Lucas Heights Science and Technology Centre, Bangor, 2234, Australia. (1) Ishii, K. AdVanced PulVerised Coal Injection Technology and Blast Furnace Operation; Pergamon Publishers: Elmsford, NY, 2000. (2) Negro, P.; Steiler, J. M.; Beppler, E.; Jahusen, U.; Bennington; Willmers, R. R. Assessment of coke degradation in the blast furnace from tuyeres probing investigations. Proceedings of 3rd European Ironmaking Congress, CRM-VDEh, Gent, Belgium, 1996; Vol. 20, p 13.
carburization, and slag reactions could influence coke fines generation in a blast furnace. Particularly, coke gasification has been most extensively studied due to the well-known impact on blast furnace operations. For example, coke strength is believed to be influenced by gasification such that high reactivity coke is believed to generate greater coke fines.3 In a blast furnace, coke undergoes a reduction in size caused by mechanical and thermal stresses and gasification by CO2 and H2O. In order to assess the effect of the thermal transformations of coke during its passage through the furnace, the Nippon Steel Corporation (NSC) developed a test, in which coke is reacted with pure carbon dioxide at 1373 K for 2 h. In this test, coke strength, known as coke strength after reaction (CSR), is measured in terms of the percentage of coke retained on a 6 mm sieve after tumbling the reacted coke for a fixed number of revolutions. Accordingly, the CSR index should be greater than 57 for trouble-free operation particularly for a large blast furnace.4 Generally, high CSR cokes have lower reactivity to CO2 gasification and are therefore desirable. Recently, this led to an increased demand for high CSR cokes with an index of the order of 65.5-7 Despite the wide-scale popularity of the test, the mechanistic understanding of the relationship between high CSR values and blast furnace performance is not clear.5 In the CSR test, the (3) Iwanaga, Y. ISIJ Int. 1991, 31 (1), 32-39. (4) Ishikawa, Y. Relationship between coke behavior in the lower part of a blast furnace and blast furnace performance. 18th Iron & Steel Smelting Research Symposium, Tohoku University, Japan, 1982.
10.1021/ef060462j CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007
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Table 1. Chemical Composition of Total Inorganic Matter in Coke (wt%) cokes
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
P2O5
TiO2
ash yield
A B C
5.24 4.48 4.41
3.15 2.36 2.54
0.86 0.82 1.47
0.24 0.30 0.14
0.08 0.10 0.08
0.07 0.10 0.17
0.08 0.08 0.07
0.04 0.10 0.02
0.08 0.02 0.04
0.16 0.14 0.15
10.00 8.50 9.10
inerts-derived carbons are believed to show greater reactivity compared to the reactive-derived carbons. Unexpectedly, in a recent study, the inerts-derived carbons (often poorly ordered) appeared to be less reactive in an operating blast furnace as indicated by their higher residuals in the tuyere coke.8 No clear mechanisms for this apparent contrasting gasification behavior of coke in a bench-scale and full-scale blast furnace have been established so far. However, this could be attributed to the preferential response of reactives-derived carbon toward alkali attack and abrasion as well as due to mineral-carbon reactions at much higher temperatures in the lower zone of a blast furnace. In a blast furnace, most of the coke gasification with CO2 occurs at relatively higher temperatures such that gases react mainly on the coke surface and coke is not expected to degrade. Coke strength is related to the carbon structure of coke9 while changes in strength after gasification are often related its modification in pore volume and coke texture after gasification.10 Previous studies suggested that mineral-carbon reactions could weaken the coke particularly at its center.11-12 In addition to inherent coke minerals, an external reaction environment such as high temperatures and recirculating alkalis could also affect high-temperature gasification and graphitization and subsequently fines behavior.13-14 Recently, coke minerals were also found to influence coke gasification even at much lower temperature such as those experienced above the thermal reserve zone.15 Therefore, in order to obtain a better understanding of the influence of mineral matter on the coke behavior in an operating blast furnace and associated bench-scale tests, the characterization of coke minerals and their transformations at high temperatures is essential. With this aim, three industrial grade metallurgical cokes were reacted with CO2 at a typical CSR test temperature of 1373 K and a much higher temperature of 1773 K. Mineral phases of original cokes were already characterized and reported in a separate study16-17 and hence were not included in this paper. The main aim of this study is to highlight the effect of temperature on the mineral characteristics after CO2 reactions particularly on the heterogeneous nature of their distribution (5) Best, M. H.; Burgo, J. A.; Valia H. S. 61st Ironmaking Conference Proceedings, Nashville, TN, March 10-13; AIST: Warrendale, PA, 2002; pp 213-240. (6) Diez, M. A.; Alvarez, R.; Barriocanal, C. Int. J. Coal Geol. 2002, 50, 389. (7) Todoschuk, T. W.; Price, J. P.; Gransden, J. F. Iron Steel Technol. 2004, 3, 73-84. (8) Horrocks, K. R. S.; Cunningham, R. B.; Ellison, J. F.; Nightingale, R. J. Coke quality at BHP steel Port Kembla. 4th European Coke and Ironmaking Congress, Paris, 2000; p 167. (9) Sato, H.; Patrick J. W.; Walker, A. Fuel 1998, 77, 1203-1208. (10) Yamaoka, H.; Suyama, S. ISIJ Int. 2003, 43 (3), 338-347. (11) Gill, W. W.; Brown, N. A.; Coin, C. D. A.; Mahoney, M. A. The influence of ash on the weakening of coke. 44th Ironmaking Conference Proceedings, ISS: Warrendale, PA, 1985; p 233. (12) Gill, W. W.; Brown, N. A.; Coin, C. D. A.; Dubrawski, J. V.; Rogers, H.; Scaife, P. H. Comparitive study of world blast furnace cokes-behavior of ash components. Report No. 0133; NERDDC Project C0333, ACARP: Australia, 1983. (13) Hilding, T.; Gupta, S.; Sahajwalla, V.; Bjorkman, B.; Wikstrom, J. O. ISIJ Int. 2005, 45 (7), 1041-1050. (14) Gupta, S.; Sahajwalla V.; Burgo, J.; Chaubal, P.; Youmans, T. Metall. Mater. Trans. B 2005, 36, 386-394. (15) Grigore, M.; Sakurovs, R.; French, D.; Sahajwalla, V. ISIJ Int. 2006, 46 (4), 503.
using the X-ray diffraction (XRD) and the scanning electron microscope (SEM). Potential implications of mineral distribution on associated coke performance in a blast furnace are also discussed. Experimental Three metallurgical cokes which had high CSR (>60) values were obtained from three US operating blast furnaces. Ash contents of the three cokes were less than the typical ash content of metallurgical coke (Table 1). Approximately 5-10 g of bulk coke pieces, 5-10 mm in size, was reacted in a horizontal furnace (internal diameter 50 mm) with 100% CO2 flowing at the rate of 2 L/min (Figure 1). Coke specimens were located in an alumina
Figure 1. Schematic of horizontal furnace used for coke reactions with CO2.
sample holder connected to a steel rod. After 2 h, the coke sample was extracted from the reaction zone of the furnace and left in the cold zone of the furnace for 30 min before removing it to avoid oxidation. Two sets of coke samples were prepared after reactions at 1373 and 1773 K. The former temperature was selected to simulate the typical CSR test temperature and the thermal reserve zone, while the higher temperature simulated the blast furnace temperature below the cohesive zone. In real operations, the reacting gas composition varies in different parts of a blast furnace. As the main aim of this study was to distinguish the effect of gasification temperature on minerals, the gas composition in all tests was fixed to 100% CO2. Crystalline phases of the reacted cokes were identified using powdered coke specimens as discussed elsewhere.16 Reacted coke pieces were analyzed using the SEM as discussed below. A field emission scanning electron microscope (Hitachi S4500) was used to investigate the distribution and chemical composition of inorganic matter of the original and the reacted cokes. Coke pieces were embedded into epoxy resin and polished. Qualitative and semiquantitative analyses were obtained using an energy dispersive X-ray (EDX) microanalyzer equipped with a LINK ISIS 200 microanalytical system. Analyses were performed on particles larger than 5 µm using an accelerating voltage of 20 kV. At least 20-40 spots were analyzed for each coke sample.
Results and Discussion Mineral phases of the original cokes were characterized and reported elsewhere.16 For the sake of continuity, some of the key features of the original coke minerals, particularly their (16) Gupta, S.; Dubikova, M.; Sahajwalla, V.; Best, M. H.; Chaubal, P.; Youmans, T. Mineral phase transformations of metallurgical cokes and their implications. AISTech06 Proceedings, Vol-1; May 1-4, Cleveland; AIST: Warrendale, PA, 2006; pp 19-30. (17) Gupta, S.; Dubikova, M.; French, D.; Sahajwalla, V. Energy Fuels 2006, 21, 303-313.
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Figure 2. SEM images illustrating the heterogeneity of size and distribution of coke minerals; (a and b) coke C; (c and d) coke B; and (e and f) coke A. Bright spots, gray, and darker phase show minerals, the carbon matrix, and pores, respectively. Typical aggregate (b), disseminated (c and d), and pore minerals (f).
phase distributions, are briefly discussed here. Figure 2 illustrates the mineral distribution within the original cokes. In all cokes, quartz, mullite, and a range of high melting point Al-silicates were observed as refractory and semirefractory phases while pyrrhotite, triolite, iron oxides, and metallic iron phases, as well as framboidal sulfide crystals, were the main the reactive phases. It may be noted that tridymite was not observed in any cokes most likely due to slow kinetics of quartz transformation in the tested conditions. There was no significant differences in the identified minerals in all three cokes except in abundance and the phase distribution patterns such that Coke A, B, and C indicated the presence of high abundance of fine iron bearing
phase as pore inclusions, fine disseminated high melting point Al-silicates, and discrete aggregates of Al-silicates, respectively.16-17 Minerals of the cokes after CO2 reactions at 1373 and 1773 K were identified using the XRD analysis.16 On the basis of peak intensity, quartz was found to decrease progressively with increasing temperatures and disappeared at 1773 K.16 Mullite and cristobalite content of all cokes increased with increasing reaction temperatures. Corundum started to increase with increasing reaction temperature such that it was clearly identified only in Coke A at 1773 K.16 In general, mullite, cristobalite, and corundum were identified as the major crystalline phases
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Figure 3. Mineral distribution of coke A (a and b), coke B (c and d), and coke C (e and f) after reactions at 1373 K. Framboids of Fe-sulfide (bright phase) in an Al-silicate aggregate and typical Al-silicate slag are shown in parts e and f, respectively.
present in the reacted cokes while some phases containing Fe and Ti oxides (possibly rutile) and various silicates were also observed. Peaks of plagioclase and leucite phases were observed but not clearly identified. Most of the minerals observed in this study were also reported by others.18-19 Mineral distribution could have great influence on coke strength and reactivity at high temperatures, e.g., disseminated minerals could modify the internal porosity of the carbon matrix due to carbothermal reduction. Therefore, further discussion of this paper will be limited to highlight the effect of temperature on the mineral characteristics particularly on their phase distribution, being the main scope of this study. Coke Mineral Characteristics at 1373 K. Figure 3 shows the SEM images of the coke minerals and their distribution
patterns after reaction with 100% CO2 at 1373 K after 2 h. After reaction, carbon, mineral, and pore characteristics change with coke type depending on their nature in the original coke. The XRD could only identify a limited number of crystalline phases due to the small amounts present as well as to the poor crystalline nature since more than 60% of the inorganic matter of the original cokes comprised glassy or amorphous phases.17 Therefore, the distribution characteristics of the minerals in the (18) Kerkonnen, O.; Mattila, E.; Heiniemi, R. The correlation between reactivity and ash mineralogy of coke. 55th Ironmaking Conference Proceedings, Pittsburgh, PA, March 24-27; ISS: Warrendale, PA, 1996; p 275-282. (19) Mahoney, M.; Rogers, H.; Andriopoulos, N.; Gupta, R. Understanding mineral matter in Australian coking coals and PCI coals. Project Report C9059, ACARP: Australia, 2002.
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Figure 4. Crystallization of Fe-Al oxide in coke A after reactions at 1373 K. The EDS composition of the Al-silicate matrix and the crystalline phase are also shown.
Figure 5. Crystallization of Fe-Mg silicate in coke A after reaction at 1373 K. The EDS composition of the crystalline phase is also provided.
reacted cokes was only examined using SEM as discussed in the following sections. Phase Distribution. Figure 3 shows that the physical features of minerals in the reacted cokes were marginally modified compared to the original cokes as far as the occurrence of discrete aggregates, disseminated phases, and pore inclusions were concerned. However, disintegration of large aggregates was more pronounced after reaction at the mineral-carbon interface as seen in Figure 3b, d, and e. Minerals display a wide range of physical features varying from aggregate sintered phases (Figure 3f) to accumulation of fine Al-silicate grains in pores (Figure 3a). Few large mineral aggregates disintegrated resulting in the formation of clusters of fine grains (Figure 3d). In general, the reacted cokes (Figure 3) appeared to have a higher proportion of minerals present as the disseminated grains as well as the pore inclusions compared to the original cokes (Figure 2). After CO2 reactions, disintegration of mineral aggregates seemed to be more intense in coke B compared to in cokes A and C. This could be related to the variation in the nature of distribution as well as the chemical composition of the Alsilicate aggregates. Due to the well-known high swelling nature of illitic clay, the associated Al-silicate aggregates should display a high degree of disintegration; however, the presence of iron or potassium elements contributed in transforming the aggregates to slag. Visual observation as well as EDS analysis seems to suggest that high melting point Al-silicates (low iron/alkalis) are more vulnerable to disintegration into finer particles and, therefore, more likely to influence the carbon matrix of coke. In addition, the influence of mineral disintegration would also depend upon the particle size as finer minerals could improve the microporosity of the carbon matrix by vaporizing. Accordingly, coke B refractory minerals, which occur in relatively high proportion as dissemination phases, may cause greater stress in the carbon matrix.
Chemical Distribution. The chemical composition of the Alsilicates of cokes was modified after reaction particularly in terms of their alkalis and iron levels. Iron phases appeared to have further reacted with quartz/silicates phases resulting in the formation of Fe-rich crystalline and glassy material. Figure 4 shows features of typical Fe and Al rich crystalline phases observed in all reacted cokes. The individual crystals located within the carbon matrix were too small to obtain the chemical composition (
