3694
Energy & Fuels 2009, 23, 3694–3702
Effect of Thermal Treatment on Coke Reactivity and Catalytic Iron Mineralogy Byong-chul Kim,*,† Sushil Gupta,† David French,‡ Richard Sakurovs,§ and Veena Sahajwalla† Centre for Sustainable Materials Research and Technology, UniVersity of New South Wales, Sydney, NSW 2052, Australia and CSIRO Energy Technology, Lucas Heights, NSW, 2234, and CSIRO Energy Technology, Newcastle, NSW, 2300, Australia ReceiVed March 16, 2009. ReVised Manuscript ReceiVed May 20, 2009
Iron minerals in coke can catalyze its gasification and may affect coke behavior in the blast furnace. The catalytic behavior of iron depends largely upon the nature of the iron-bearing minerals. To determine the mineralogical changes that iron could undergo in the blast furnace, cokes made from three coals containing iron present in different mineral forms (clays, carbonates, and pyrite) were examined. All coke samples were heat-treated in a horizontal furnace at 1373, 1573, and 1773 K and then gasified with CO2 at 1173 K in a fixed bed reactor (FBR). Coke mineralogy was characterized using quantitative X-ray diffraction (XRD) analysis of coke mineral matter prepared by low-temperature ashing (LTA) and field emission scanning electron microscopy combined with energy dispersive X-ray analysis (FESEM/EDS). The mineralogy of the three cokes was most notably distinguished by differing proportions of iron-bearing phases. During heat treatment and subsequent gasification, iron-containing minerals transformed to a range of minerals but predominantly iron-silicides and iron oxides, the relative amounts of which varied with heat treatment temperature and gasification conditions. The relationship between initial apparent reaction rate and the amount of catalytic iron mineralsspyrrhotite, metallic iron, and iron oxidesswas linear and independent of heat treatment temperature at total catalyst levels below 1 wt %. The study showed that the coke reactivity decreased with increasing temperature of heat treatment due to decreased levels of catalytic iron minerals (largely due to formation of iron silicides) as well as increased ordering of the carbon structure. The study also showed that the importance of catalytic mineral matter in determining reactivity declines as gasification proceeds.
Introduction One of the properties of coke that determines its suitability for blast furnace applications is its reactivity to carbon dioxide in the blast furnace. Coke reactivity is well-known to depend on parent coal properties, coking conditions, and in particular, the mineral matter in coke.1-3 Iron, calcium, and potassium oxides are known to catalyze the gasification reactions.2,3 Recent attempts have been made to modify coke reactivity by employing iron species present in coal blends or by introducing iron compounds during coking.4 These iron species occur in coke in a variety of mineral forms such as metallic iron and pyrrhotite as well as oxides such as magnetite.5 Gasification reactions are * Corresponding author. Phone: 61 2 9385 4433; fax: 61 2 9385 4292; e-mail:
[email protected] (B.-c.K.);
[email protected]. (S.G.). † University of New South Wales. ‡ CSIRO Energy Technology, Lucas Heights. § CSIRO Energy Technology, Newcastle. (1) Diez, M. A.; Alvarez, R.; Barriocanal, C. Int. J. Coal Geol. 2002, 20, 389–412. (2) Lindert, M.; Timmer, R. M. C. An analysis of the Japanese reactivity and CSR of plant coke and the corresponding pilot oven coke. Proceedings of the 50th Ironmaking Conference, 1991: Iron and Steel Society, Warrendale, PA, 1991; Vol 50, pp 233-237. (3) van der Velden, B.; Trouw, J.; Chaigneau, R.; Van den Berg, J. Coke reactivity under simulated blast furnace conditions. Proceedings of the 58th Ironmaking Conference, 1999; Iron and Steel Society: Warrendale, PA, 1999; Vol 58, pp 275-285. (4) Nomura, S.; Terashima, H.; Sato, E.; Naito, M. ISIJ Int. 2007, 47 (6), 823–830. (5) Grigore, M.; Sakurovs, R.; French, D.; Sahajwalla, V. ISIJ Int. 2006, 46 (4), 503–512.
catalyzed by metallic iron,6,7 pyrrhotite,8 hematite,2 and magnetite.9 However, other iron compounds such as iron phosphate and iron silicates are believed not to catalyze gasification.5 Coal minerals transform into other minerals to various degrees according to the carbonization conditions,10 with consequent effects upon coke reactivity.5 Thus, the amount of catalytic iron would be expected to be more closely related to coke reactivity than the amount of total iron. Quantitative determination of coke mineralogy and understanding its implications for coke gasification is still far from complete. In this paper, mineral matter of the original cokes was characterized using SIROQUANT and field emission scanning electron microscopy combined with energy dispersive X-ray analysis (FESEM/EDS) to identify the nature and amount of the different minerals formed in coke and their effect on the coke reactivity. Additionally, the transformation of minerals in coke after thermal treatment and gasification is also characterized (6) Walker, P. L., Jr.; Shelef, M.; Anderson, R. A. Catalysis of Carbon Gasification. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker Inc.: New York, 1968; pp 287-383. (7) Turkdogan, E. T.; Vinters, J. V. Carbon 1972, 10 (1), 97–106. (8) Vandezande, J. A. The structure and properties of metallurgical coke. Proceedings of the 44th Ironmaking Conference, 1985; Iron and Steel Society: Warrendale, PA, 1991; Vol 44, pp 189-206. (9) Price, J. T.; Iliffe, M. J.; Khan, M. A.; Gransden, J. F. Minerals in coal and high temperature properties of coke. Proceedings the of 53rd Ironmaking Conference, 1994; Iron and Steel Society: Warrendale, PA, 1984; Vol 53, pp 79-87. (10) Grigore, M.; Sakurovs, R.; French, D.; Sahajwalla, V. ISIJ Int. 2007, 47 (1), 62–66.
10.1021/ef900229p CCC: $40.75 2009 American Chemical Society Published on Web 06/15/2009
Catalysis of Coke by Iron-bearing Minerals
Energy & Fuels, Vol. 23, 2009 3695
Table 1. Proximate and Ash Analysis of Coal and Cokes coals A moisture, ada volatile ash fixed carbon SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 Mn3O4 SO3 P2O5 SiO2/Al2O3 BIb
B
cokes C
A
B
C
Proximate Analysis (wt %, dry basis) 2.0 2.0 12.7 0.40 0.10 20.8 38.8 39.2 1.50 1.30 7.1 9.5 14.5 9.1 15.8 72.1 51.7 46.3 89.4 82.9 Oxide 4.13 1.55 0.72 0.21 0.14 0.04 0.07 0.09 0.07 0.11 2.67 0.21
Analysis (wt %, in coal) 4.64 5.92 5.10 2.76 2.15 1.94 0.60 4.26 0.97 0.52 0.75 0.31 0.19 0.11 0.2 0.17 0.16 0.05 0.09 0.23 0.09 0.14 0.10 0.12 0.01 0.01 0.38 0.64 0.09 0.04 0.04 0.14 1.68 2.76 2.63 0.21 0.68 0.23
0.50 2.30 23.0 74.7
7.60 4.50 1.17 0.96 0.35 0.25 0.16 0.24 0.01 0.38 0.06 1.69 0.24
9.32 3.54 6.30 1.31 0.23 0.30 0.39 0.18 0.03 1.10 0.07 2.63 0.66
a ad: air-dried basis. b BI, basicity index ) (Fe O + CaO + MgO + 2 3 Na2O3)/(SiO2 + Al2O3).
Table 2. Maceral Composition and Maximum Vitrinite Reflectance of Coals maceral
A
B
C
vitrinite inertinite liptinite Ro max
66.5 33.4 0.2 1.32
86.7 9.6 3.7 0.8
89.5 8.5 2.0 0.47
with particular focus on iron-bearing minerals in order to simulate typical blast furnace temperature conditions corresponding to the temperatures from the thermal reserve zone to the cohesive zone. The influence of other coke properties, such as carbon crystallite height and micropore surface area, on thermal treatment and their effect on coke reactivity is also examined. Experimental Section Sample Preparation. Three coals were selected on the basis of differing abundance and nature of the iron-bearing minerals. The coal samples also differed in rank, maceral and ash content, and composition as given in Tables 1 and 2. Vitrinite levels of coals A, B, and C were 66.5, 86.7, and 89.5, and mean maximum reflectance of vitrinite was 1.32, 0.8, and 0.47, respectively. On the basis of rank and vitrinite values, coal C is not a typical coking coal, but can be used in blending; however, it was specifically selected here due to its high pyrite content. Coke samples were prepared using a 9 kg electrically heated coke oven at CSIRO with a maximum exposure temperature of 1273 K and were quenched in nitrogen as detailed elsewhere.10 Table 1 also provides the properties of the coals after carbonization that indicate an increase in ash yields in all cokes due to volatile and moisture removal during carbonization. All coke samples prepared were crushed to ∼1 mm and the 0.6-1.0 mm fraction was used for all characterization including coke reactivity, CO2 surface area, XRD and SEM analysis. Coke A contains about 9% ash with the least iron content, whereas coke C contains 23% ash. The cokes were heat-treated in a horizontal tube furnace as shown in Figure 1. Coke samples were crushed to pass ∼1.0 mm and the 0.6-1.0 mm fraction was used for all characterization. Samples were put into a graphite sample holder and pushed in the cold zone of the furnace until the desired atmosphere was attained, then inserted into the hot zone. The inert gaseous atmosphere was created by flowing nitrogen (2.5 L/min). Heat treatment of cokes was examined at three different temperatures (1373, 1573, and 1773
Figure 1. Schematic of horizontal tube furnace used for heat treatment of cokes.
K), and the exposure time was 2 h. During the heat treatment, the off-gas was channeled into an infrared analyzer (IR) for continuous online monitoring and analysis of the off-gas composition. Reactivity. Coke reactivity was measured using a fixed bed reactor (FBR) as described elsewhere.5 Coke specimens were dried at 378 K overnight, and then 1.2 g of sample was supported on a sintered glass frit inside a quartz tube placed in an electrically heated furnace. A mass-flow controller was used to pass the gases through the sample bed from top to bottom at a flow rate of 0.750 L/min. A thermocouple was used to monitor the sample bed temperature. Carbon dioxide was passed through an oxygen and moisture trap prior to injection into the furnace. The CO concentration of the exhaust gas was continuously monitored by an infrared analyzer via computer to calculate the apparent reaction rates and their activation energies. The normalized reaction rates of cokes to 1173 K were obtained from both the apparent reaction rate and activation energy using procedures outlined elsewhere.5 Coke Mineral Analysis. Coke minerals were quantified using radio frequency low-temperature oxygen plasma ashing (LTA) to prepare a carbon-free mineral sample that was used for XRD analysis. The XRD analysis of the prepared ash was then carried out on a Philips PW1050 goniometer using Cu KR radiation at 40 kV and 35 mA, with step scans from 2-90° 2θ, a step interval of 0.04° 2θ, and a 5 s count time per step.5 SIROQUANT, PC based quantitative X-ray diffraction analysis software, was used to quantify the minerals in the ash.11 The error of mineral matter quantification is typically less than 0.3% in the LTA samples, which translate to less than 0.03% on a coke basis.5 Coke minerals were also examined using a high-resolution field emission scanning electron microscope (FESEM) (Hitachi 4500II) fitted with an Oxford Isis energy dispersive X-ray analyzer (EDS). Cokes were embedded in resin then polished and carbon coated prior to analysis. Physical and Chemical Properties of Cokes. A Micrometrics ASAP 2010 surface area analyzer at RMIT (Melbourne) was used to measure the micropore surface area of cokes using carbon dioxide at 273 K as the adsorbate. The Dubinin-Radushkevich (DR) surface area is used in this study. A Siemens D5000 X-ray diffractometer was used to determine the carbon structural parameters. Powder samples were scanned in a step-scan mode (0.05°/step) over the angular range of 5-105° (2θ) using Cu KR radiation (30 kV, 30 mA). Diffracted X-ray intensities were collected for 5 s at each step, and the carbon structural parameters were calculated from the resultant pattern as described elsewhere.12
Results and Discussion Coke Reactivity and Coke Properties. Figure 2 shows that the apparent CO2 reaction rate of the cokes increased uniformly with increasing carbon conversion except at the initial stages. The initial and final (15% carbon conversion) apparent reaction rates of the cokes varied from 9.0 to 14.8 g · g-1 · s-1 and from 21.4 to 32.1 g · g-1 · s-1, respectively. The coke reactivity and modification of the reactivity trend during progressive carbon conversion are influenced to some extent by the properties of (11) Taylor, J. C. Powder Diffr. 1991, 6 (1), 2–9. (12) Lu, L.; Sahajwalla, V.; Kong, C.; Harris, D. J. Carbon 2001, 39, 1821–1833.
3696
Energy & Fuels, Vol. 23, 2009
Kim et al. Table 3. SIROQUANT Analysis of LTA Samples of Coke (wt %)
Figure 2. Variation of CO2 apparent reaction rates of three cokes with carbon conversion.
the organic part of the coke.1 In this study, carbon crystallite height (Lc) of the three cokes varied from 0.82 (coke C) to 1.32 nm (coke A). The initial surface areas of the cokes were of similar range, varying from 31 to 36 m2/g. Neither surface area nor Lc correlated with the reactivities of the three cokes. Moreover, neither did rank or maceral composition of the parent coal (Table 2). There was no apparent correlation between any of these measurements of the organic part of the coke with reactivity. The ash components including basicity of coke have previously been related to coke reactivity,1 however the apparent reaction rates of the cokes analyzed in this study did not show any clear relationship with the concentrations of iron, calcium, sodium, potassium, and magnesium oxides in the cokes or the basicity index (Table 1). Thus, the reactivity of the tested cokes could not be explained on the basis of rank, inertinite content, or ash chemistry of the parent coals or on ash chemistry of the coke. This observation is consistent with our previous experience.5,13 Iron-bearing minerals such as metallic iron, sulfides, and oxides have been related to the apparent reaction rate of coke. A detailed description of association of the reaction rates with iron minerals will be described in the next section. Iron Minerals and Coke Reactivity. The amount and nature of minerals present in the cokes varies. The total amount of iron, calcium, etc. measured by oxide ash analysis (Table 1) provides some indication, but the amount of catalytic minerals is not necessarily proportional to the amount of iron, calcium, etc. Table 3 shows that most of the mineral matter in the cokes was present as an amorphous phase (≈50%), which largely consists of poorly ordered aluminosilicate materials. Clay minerals are well-known to produce amorphous material during carbonization.10 Quartz, mullite, spinel, pyrrhotite, metallic iron, fluorapatite, and oldhamite were invariably present in all three cokes. Other minerals such as albite, troilite, magnetite, hercynite, fayalite, gehlenite, and akermanite were present in some cokes. Small amounts of calcite were present in cokes B and C, which could be produced in the coke due to reaction of lime or oldhamite with a partially oxidizing atmosphere during cooling.14 Oldhamite itself can be formed by the reaction between lime derived from calcite and sulfur released from pyrite decomposition.10 Coke made from coal C had the highest oldhamite content, followed by cokes B and C. This is a reasonable trend when considering their higher calcium and sulfur contents in the respective ash analyses (Table 1). Anatase and rutile were found in cokes A and C. (13) Gupta, S.; French, D.; Sakurovs, R.; Grigore, M.; Sun, H.; Cham, T.; Hilding, T.; Hallin, M.; Lindblom, B.; Sahajwalla, V. Prog. Energy Combust. Sci. 2008, 34 (2), 155–197. (14) Sakurovs, R.; French, D.; Grigore, M. Coal Geol. 2007, 72, 81– 88.
minerals
chemical formula
A
B
C
quartz mullite albite spinel pyrrhotite troilite iron magnetite hercynite fayalite calcite oldhamite gehlenite fluorapatite akermanite anatase rutile amorphous
SiO2 Al6Si2O13 NaAlSi3O8 MgAl2O4 Fe1-xS FeS Fe Fe3O4 FeAl2O4 Fe2SiO4 CaCO3 CaS Ca2Al2SiO70.10 Ca5(PO4)3F Ca2MgSi2O7 TiO2 TiO2 5.34
2.21 0.62 0.26 0.27 0.24 0.05 0.15 0.08
0.92 1.61
3.47 1.04
0.61 0.18 0.41 0.16 0.11 0.11
0.07 4.97
0.19 0.23
0.11 0.68
0.19 0.18 0.06 0.02 11.48
0.02
0.20
0.03 0.04 0.07 0.28 0.05 0.03 11.19
0.02
The pyrrhotite content of coke C was about an order of magnitude greater than those in the cokes A and B. Minor magnetite was found in cokes A and B. Metallic iron, known for its catalytic effect, was present in all three coke samples. Even though all three cokes have a similar iron mineralogy, the catalytic effects may differ depending upon the size and mode of distribution. Figure 3 illustrates differences in ironbearing minerals present in each of the three cokes. Figure 3a shows iron oxide in coke A pores, whereas iron-rich aluminosilicates and iron sulfides were mainly present in cokes B (Figure 3b) and C (Figure 3c), respectively. Further SEM examination of these cokes indicated that the average size range of iron minerals in coke A is smaller compared to the size of iron bearing minerals in the other two cokes. Therefore, it is possible that the relatively smaller size of iron oxide grains will also be contributing to the higher final apparent reaction rates of coke A due to the higher contact surface area. The data from a previous study, which lists the amount of total catalytic iron minerals (Fe, Fe1-xS, FeO, Fe2O3 and Fe3O4) and initial apparent reaction rates of cokes, is shown in Figure 4 as open circles.5,15 The data from this study are superimposed and shown as filled circles. Figure 4 shows a strong, almost proportional, relationship between the initial apparent reaction rates of the cokes and the sum of catalytic iron minerals (Fe, FeS, Fe1-xS FeO, Fe2O3, and Fe3O4). For cokes containing less than 1.0 wt % of catalytic iron minerals, the initial apparent rate shows an almost linear relationship with catalytic iron minerals as expected (Figure 4b).5 Coke C, although it is the most reactive coke, is clearly less reactive than would be expected on the basis of the linear trend seen for other cokes. Figure 5c shows that some of the pyrite derived grains in coke C are larger than 200 µm, which therefore have a lower contact surface area available for reaction, consequently resulting in a less than expected initial apparent reaction rate. In contrast to the strong relationship seen between the amount of catalytic material and initial apparent rate, the final apparent rate did not show a strong relationship with total catalytic iron minerals (Figure 6), indicating that the catalytic effect of minerals is significantly modified at a later stage of gasification. Effect of Heat Treatment on Coke Mineral Transformations. After heating at 1373 K, the mineral matter in all three cokes (Table 4) was modified (Table 3). After annealing at 1373 K, mullite levels increased in all cokes, most likely due to further (15) Grigore, M.; Sakurovs, R.; French, D.; Sahajwalla, V. Int. J. Coal Geol. 2008, 75, 301–308.
Catalysis of Coke by Iron-bearing Minerals
Energy & Fuels, Vol. 23, 2009 3697
Figure 3. Typical iron minerals in three cokes: (a) iron oxide in pore in coke A; (b) Fe-rich aluminosilicate in coke B; (c) cluster of framboidal iron sulfide in coke C. Bright areas are mineral phases, and gray and dark gray areas are the carbon matrix and pores, respectively.
Figure 4. Correlation between initial apparent reaction rates and total catalytic iron minerals. Open circles show previous data5,15 and solid circles indicate current data.
Figure 5. Comparison of minerals of coke A and coke B (a, b) with large size of pyrrhotite in coke C (c).
iron minerals in all cokes including pyrrhotite, troilite, and metallic iron increased marginally except in coke A. Gupeite was found in coke A, which is consistent with a previous study,18 even though it is reported to form only at much higher temperatures.19 In all cokes, after annealing at 1773 K, the percentage of quartz, mullite, pyrrhotite, troilite, and metallic iron decreased significantly (Table 4). A number of new phases comprising nitrides (Si nitride, Al nitride, Al oxide nitride) and carbides (moissanite) were formed in all cokes. Moissanite can be formed due to in situ quartz reduction followed by carbon reaction.20-22 Figure 6. Correlation between final apparent reaction rates and total catalytic iron minerals. Open circles show previous data5,15 and solid circles indicate current data.
decomposition of metakaolinite.16 In coke B, calcite disappearance was accompanied by the formation of small amounts of anorthite, which can be attributed to reactions between calcite and metakaolinite.17 After heating at 1373 K, the amount of (16) Wang, M. J.; Wada, H. J. Mater. Sci. 1990, 25, 1690–1698. (17) Mitchell, R. S.; Gluskoter, H. J. Fuel 1976, 55, 90–96.
(18) Wall, T. F.; Badat, Y. A.; Gupta, R. P.; Shrestha, P. L.; Mahoney, M. R. M.; Rogers, H. H. Formation of Silicon Carbide in Heat-treated Cokes. Proceedings of the 24th International Pittsburgh Coal Conference; 2007 (in CD-ROM). (19) Morishita, K.; Ichimura, N.; Takarada, T. High-temperature Interactions between Coal Char and Mixtures of Iron compounds, Quartz and Kaolinite. Proceedings of the 21st International Pittsburgh Coal Conference; 2004 (in CD-ROM). (20) Wang, J.; Ishida, R.; Takarada, T. Energy Fuels 2000, 14, 1108– 1114. (21) Wada, H.; Wang, L. J. Mater. Sci. 1992, 27, 1528–1536. (22) Biernacki, J. J.; Wotzak, G. P. J. Am. Ceram. Soc. 1989, 72 (1), 122–129.
3698
Energy & Fuels, Vol. 23, 2009
Kim et al.
Table 4. Minerals in Annealed Cokes Based on SIROQUANT Analysis (wt %) A
B
C
minerals
formula
1373 K
1773 K
1373 K
1773 K
1373 K
1773 K
quartz cristobalite moissanite silicon nitride mullite leucite corundum aluminum oxide nitride aluminum nitride spinel pyrrhotite troilite iron magnetite hematite iron silicon iron silicon gupeite hercynite calcite oldhamite gehlenite calcium iron oxide fluorapatite akermanite anorthite anatase osbornite amorphous
SiO2 SiO2 SiC Si3N4 Al6Si2O13 KAlSi2O6 Al2O3 Al5O6N AlN MgAl2O4 Fe1-xS FeS Fe Fe3O4 Fe2O3 FeSi Fe5Si3 Fe3Si FeAl2O4 CaCO3 CaS Ca2Al2SiO7 CaFe2O4 Ca5(PO4)3F Ca2MgSi2O7 (Ca,Na)(Si,Al)4O8 TiO2 TiN
2.35 nda 0.03 nd 1.19 nd nd nd nd 0.34 0.09 0.04 0.07 nd nd nd nd 0.32 nd nd 0.04 nd nd 0.21 nd nd 0.03 nd 5.28
0.16 0.02 1.05 0.39 0.31 0.01 0.01 0.25 0.38 nd 0.19 nd 0.09 0.06 0.06 0.17 0.23 1.24 nd nd 0.03 nd 0.14 nd nd nd 0.02 0.10 4.92
1.16 nd nd nd 2.76 nd nd nd nd 0.32 0.58 0.12 0.2 nd nd nd nd 0.16 0.12 nd 0.38 0.06 nd 0.06 0.04 0.22 nd nd 13.86
0.13 nd 0.96 0.64 0.93 nd 0.01 0.38 0.70 nd 0.09 nd 0.15 nd nd nd 0.20 1.33 nd 0.16 0.10 0.03 0.19 nd nd nd 0.01 nd 8.58
3.10 nd nd nd 2.55 nd nd nd nd nd 5.28 0.73 0.53 nd nd nd nd nd nd nd 0.43 0.08 nd 0.18 nd nd nd nd 12.17
0.35 0.05 1.13 0.53 0.9 nd 0.10 0.18 1.36 nd 1.53 0.03 0.23 nd nd nd 0.55 5.57 nd nd 0.53 nd nd nd nd nd 0.10 nd 11.96
a
nd: not determined.
In the presence of nitrogen, silicon nitride can form.20,21,23 Both aluminum oxide nitride and aluminum nitride can be formed due to reaction of alumina with nitrogen.24-26 Other new phases including cristobalite, corundum, leucite, calcium iron oxide, and FeSi alloys were seen in some cokes. Corundum is known to form due to mullite reduction, which may also increase moissanite contents.16 Ferrosilicon phases can be formed either due to quartz reaction with pyrite derivatives19 or the reaction of iron oxide with moissanite formed at 1674 K.27 Coke C had a higher content of catalytic iron minerals even after heat treatment at 1773 K compared to other cokes, possibly due to slower kinetics of transformation of pyrite derivatives in the coke. Kinetics of coal pyrite transformation during coking as well as those of pyrite derivatives in coke is dependent on grain size and reaction environment.28-31 The larger grains in coke C could also be responsible for the slow and incomplete transformation of its iron bearing minerals to iron silicide during heat treatment, most likely in the core of the lumps. In all annealed cokes at 1773 K, calcium iron oxide was another new mineral observed in cokes A and B along with simultaneous disappearance of most of the calcium minerals except oldhamite. This is most likely the reaction product of (23) Komeya, K.; Inoue, H. J. Mater. Sci. Lett. 1975, 10, 1243–1246. (24) Lefort, P.; Billy, M. J. Am. Ceram. Soc. 1993, 76 (9), 2295–2299. (25) Ish-Shalom, M. J. Mater. Sci. Lett. 1982, 1 (4), 147–149. (26) Yawei, L.; Nan, L.; Runzhang, Y. J. Mater. Sci. Lett. 1997, 16 (3), 185–186. (27) Kurunov, I. F.; Tuktayshev, I. I. Metallurgist 2003, 47 (7-8), 277– 283. (28) Hu, G.; Dam-Johansen, K.; Wedel, S.; Hansen, J. P. Prog. Energy Combust. Sci. 2006, 32, 295–314. (29) Boyabat, N.; Ozer, A. K.; Bayrakceken, S.; Gulaboglu, M. S. Fuel Process. Technol. 2003, 85, 179–188. (30) Fegley, B., Jr.; Lodders, K. ICARUS 1995, 115, 159–180. (31) Hong, Y.; Fegley, B., Jr. Plane. Space Sci. 1998, 46 (6-7), 683– 690.
pyrite derivatives, together with iron in the amorphous phase, with other calcium species present in the cokes.32 The majority of new minerals formed in heated cokes can be attributed to carbothermal reactions.16,20-26 Figure 7 clearly shows the evolution of CO gas formed due to mineral reduction by carbon as well as possible participation of nitrogen during heat treatment. Some of the CO could also be attributed to liberation of chemisorbed oxygen on the carbon active sites of cokes.15 The amount of CO liberation increases with increasing amounts of coke minerals (Figure 7a) as well as with increasing temperature (Figure 7b), both of which supports the occurrence of carbothermal reduction. Comparison of Figure 8 with Figure 5 shows that the physical nature of the minerals including morphology and distribution does not change significantly at 1373 K but changes significantly as the annealing temperature increases from 1573 to 1773K. For example, at 1373 K, coke C minerals appear to be in closer contact with the carbon matrix, and melt becoming spherical in shape at 1573 K, resulting in less contact with the carbon matrix compared to the other cokes. Figure 9 further illustrates typical transformation of iron minerals in coke C with heat treatment temperature, such that most of the pyrrhotite remained unchanged at 1373 K (Figure 9b) followed by transformation to iron-silicide at 1573 K (Figure 9, panels c and d). With increasing annealing temperature, pyrrhotite transformation to iron silicides is significantly increased. Iron silicides, most likely gupeite in this case, are known to form due to quartz and iron sulfides reactions at high temperatures.19 Minerals Transformation of Heat Treated Coke during Gasification. The mineralogy of the heat-treated cokes was further modified by gasification, as shown in Table 5. Quartz and mullite proportions in the cokes that had been heat-treated at 1373 K increased consistently at 15% burnoff. After gasifica(32) Ward, C. R. Int. J. Coal Geol. 2002, 50, 135–168.
Catalysis of Coke by Iron-bearing Minerals
Energy & Fuels, Vol. 23, 2009 3699
Figure 7. Variation of CO liberation among three cokes with heat treatment time at 1373 K (a); similar plot for CO liberation at 1773 K (b).
Figure 8. Illustration of morphological changes of cokes during heat treatment.
Figure 9. Illustration of the effect of temperature on transformation of iron minerals in coke C.
tion of the annealed cokes A and B, the spinel content increased marginally while small amounts of cristobalite were formed. After gasification, pyrrhotite, troilite, and metallic iron contents of cokes decreased considerably, most likely to form
magnetite and hematite. In some cokes, pyrrhotite and troilite completely disappeared. In cokes A and B, hercynite was also observed, which may have formed due to reactions of iron minerals with decomposed amorphous aluminosilicates.
3700
Energy & Fuels, Vol. 23, 2009
Kim et al.
Table 5. SIROQUANT Analysis of the Heat-treated Cokes after 15% Carbon Conversion (wt %) A
B
C
minerals
formula
1373 K
1773 K
1373 K
1773 K
1373 K
1773 K
quartz cristobalite moissanite silicon nitride mullite leucite corundum aluminum oxide nitride aluminum nitride spinel pyrrhotite troilite iron magnetite hematite iron silicon gupeite hercynite calcite oldhamite gehlenite fluorapatite akermanite anorthite bredigite anatase rutile osbornite amorphous
SiO2 SiO2 SiC Si3N4 Al6Si2O13 KAlSi2O6 Al2O3 Al5O6N AlN MgAl2O4 Fe1-xS FeS Fe Fe3O4 Fe2O3 Fe5Si3 Fe3Si FeAl2O4 CaCO3 CaS Ca2Al2SiO7 Ca5(PO4)3F Ca2MgSi2O7 (Ca,Na)(Si,Al)4O8 Ca14Mg2(SiO4)8 TiO2 TiO2 TiN
2.62 0.01 nd nd 1.61 nd nd nd nd 0.67 0.03 nd 0.06 0.17 0.13 nd nd 0.13 0.01 0.05 nd 0.24 0.10 nd nd 0.01 0.06 nd 6.30
0.04 0.14 1.62 0.27 0.46 0.21 0.14 0.14 0.61 nd nd nd 0.11 0.24 0.38 0.04 0.98 nd nd 0.15 nd nd nd nd 0.12 0.05 nd 0.05 6.65
1.27 0.02 nd nd 3.33 nd nd nd nd 0.61 nd 0.02 0.10 0.32 0.34 nd nd 0.18 0.10 0.26 0.20 nd 0.18 0.42 nd nd nd nd 12.83
0.07 nda 2.17 0.55 0.66 nd 0.02 0.41 0.75 nd nd nd 0.16 nd 0.68 0.21 0.28 nd nd 0.07 nd nd 0.12 0.21 nd nd nd 0.20 11.25
3.59 nd nd nd 3.08 nd nd nd nd nd 1.39 0.06 0.28 2.46 2.91 nd nd nd 0.06 0.42 0.17 0.28 0.20 nd nd nd nd nd 13.39
0.14 0.06 1.38 0.39 0.72 nd 0.11 0.08 1.82 nd nd nd 0.28 1.66 0.86 0.69 3.64 nd nd 0.58 nd nd nd nd nd 0.06 nd nd 15.12
a
nd: not determined.
Oldhamite content showed minimal variation in all cokes, while some calcite was observed, which could have been formed by carbonation of calcium oxide, most likely during cooling of the sample. The content of minor calcium minerals such as gehlenite, akermanite, and anorthite also increased to some extent by the possible reaction of calcite formed from oldhamite and the amorphous phase. Fluorapatite content did not vary significantly in cokes A and C, whereas that in coke B completely disappeared after gasification. Minor rutile was formed only in the reacted coke A from anatase transformation and amorphous decomposition. Unlike the cokes annealed at lower temperature, the cokes annealed at 1773 K showed a completely different trend of mineral transformation during gasification. The quartz content decreased slightly in all the reacted cokes probably due to inversion to cristobalite and formation of other minerals such as moissanite, although moissanite can also be formed by reaction of alumina with silicon nitride and carbon. Silicon nitride and mullite contents decreased to some extent in the reacted cokes, probably due to their involvement in reactions, which result in the formation of cristobalite, corundum, and aluminum nitride. Leucite (the only crystalline form of potassium in annealed coke A) increased after gasification, most likely due to decomposition of amorphous aluminosilicates,33 whereas aluminum oxide nitride contents only slightly decreased in the reacted cokes A and C and showed a minimal decrease in coke B. Magnetite and hematite increased significantly in all cokes, probably from oxidation of pyrrhotite and troilite, both of which completely disappeared, in contrast metallic iron increased slightly. The amount of ferrosilicon decreased on gasification, probably contributing to the increased magnetite and hematite (33) Willmers, R. R.; Monson, J. R.; Wilkinson, H. C. The Degradation of Coke in the Blast Furnace, report EUR 8703; Commission of the European Communities, 1984.
contents. Decreased ferrosilicon after reaction at 1173 K is unexpected as it is known to have a high thermal stability in the tested temperature range. Calcite, gehlenite, and calcium iron oxide in the annealed cokes disappeared after gasification, whereas akermanite, anorthite, and bredigite were formed in some cokes. Oldhamite content was almost unchanged in all the reacted cokes. Figure 10, panels a and b, compares the majority of ironbearing minerals in cokes after annealing at 1373 and 1773 K, respectively. At 1373 K, most of the crystalline proportion of iron occurs as pyrrhotite and troilite and as iron silicides and is mainly gupeite and iron silicon at 1773 K, also seen in Figure 9. Comparison of Figure 10, panels a and c, as well as panels b and d suggest that during gasification, both iron sulfides and iron silicides oxidize to magnetite and hematite. However, this transformation appears to be slower in the case of 1773 K coke samples, particularly for gupeite, which was present in large amount even after 15% burnoff. The same figure further shows that after gasification, pyrrhotite and troilite are absent in the 1773 K coke samples (Figure 10d) but small amounts of pyrrhotite remained in the 1373 K samples (Figure 10c). Implications of Heat Treatment on Coke Reactivity. Figure 11 shows the apparent reaction rates of the three cokes annealed at different temperatures. The annealing temperature affected both the shape of the reaction curve and the order of reactivity of the three cokes. The apparent reaction rate of cokes annealed at 1373 K increased uniformly with increasing carbon conversion except coke C, whereas the cokes annealed at 1573 K had a reaction rate that was approximately constant with carbon conversion. The apparent reaction rates of cokes annealed at 1773 K showed the most significant modification, such that in the initial stage of the reaction, the reaction rates of all three cokes sharply increased and then decreased, and then after 10% carbon conversion, leveled off.
Catalysis of Coke by Iron-bearing Minerals
Energy & Fuels, Vol. 23, 2009 3701
Figure 10. Iron minerals in the cokes after annealing at 1373 K (a) and 1773 K (b). Minerals in both set of annealed cokes after CO2 reactions with 1373 K samples (c) and 1773 K samples (d).
Figure 11. Effect of heat treatment temperature on variation of apparent reaction rates with carbon conversion at 1373 (a), 1573 (b), and 1773 K (c). Table 6. Initial and Final (15% Conversion) Apparent Reaction Rates of Three Cokes with CO2 after Heating at a Range of Temperatures apparent rate (×10-6, g · g-1 · s-1) coke
temperature
initial
final
A
original 1373 K 1573 K 1773 K original 1373 K 1573 K 1773 K original 1373 K 1573 K 1773 K
9.5 10.5 13 5.8 9 12 13.7 3.8 14.8 18.5 23 8.4
32.1 36.3 21.7 21.1 21.4 24.4 13.8 8.1 26.3 19.2 29 23.7
B
C
The initial and final apparent reaction rates of all the coke samples are provided in Table 6. The initial apparent reaction rates of all cokes increased with increasing treatment temperatures but sharply decreased on annealing at the highest temperature, 1773 K. The final reaction rates of cokes A and B decreased with increasing annealing temperature with the
Table 7. CO2 Surface Areas and Stack Height of Carbon Crystallite (Lc) of Coke Samples Surface area (m2/g)
Lc (nm) annealing temperature
A
B
C
A
B
C
untreated 1373 K 1573 K 1773 K
1.3 1.4 2.1 3.6
1.2 1.3 2.1 2.8
0.8 0.9 1.1 1.7
36 19 24 34
31 23 25 27
35 32 24 25
exception of cokes annealed at 1373 K. Coke C showed the highest initial apparent reaction rates even after heat treatment. Table 7 compares the micropore surface areas and carbon crystallite height (Lc) values of the cokes with annealing temperatures. As expected, the stack height of carbon crystallites of the three cokes increased continuously with increasing annealing temperatures. Coke A, made of higher rank coal, showed the highest Lc values (3.6 nm) at 1773 K, whereas Coke C, made of lower rank coal, only showed moderate improvement of Lc values (1.7 nm). On the basis of carbon structure, coke A annealed at 1773 K is expected to be the least reactive, however its reactivity was much higher than that of coke B and similar to that of coke C. Thus, the stack height of carbon crystallites
3702
Energy & Fuels, Vol. 23, 2009
Figure 12. Comparison of CO2 (DR) surface area variation of three cokes after heat treating at 1773 K with carbon conversion.
of annealed cokes could not explain the reactivity differences of heat treated coke. Surface area of cokes is well-known to influence reactivity. The micropore surface area of the cokes decreased on annealing as found previously34-37 due to sealing off of the micropore system35 and condensation of the turbostratic structure, such as hydrogen loss, edge coalescence, and defect elimination,37 but surface area and initial coke reactivity did not show any correlation. However, the significant modification of apparent reaction rates of annealed cokes at 1773 K during gasification (Figure 11c) could be due to a change in surface area. Figure 12 shows the evolution of the micropore surface area of annealed cokes at 1773 K during gasification. The reactivity of cokes annealed at 1773 K (Figure 11c) seemed to follow the evolution of surface area (Figure 12). If the relationship between the amount of catalytic iron minerals and the initial reactivity seen in Figure 4 was followed, the transformation of catalytic iron minerals to other noncatalytic iron minerals such as ferrosilicon upon thermal treatment would be expected to decrease initial rates. Figure 13 shows that this trend is followed for coke B; its reactivity at 1373 K, the amount of catalytic iron minerals is high compared to 1773 K, and the relationship is similar to that established for the other cokes. However, coke A has a low reactivity at 1773 K but high levels of catalytic iron, most likely due to a much significant improvement of the crystalline order of carbon. Coke C has high levels of catalytic iron, but not as high as expected from the linear relationship between catalytic iron levels and reactivity, which is presumably due to its coarser iron particles. Conclusions The influence of thermal treatment on mineral transformations and subsequent gasification with CO2 was investigated for cokes (34) Nandi, S. P.; Ramadass, V.; Walker, P. L. Carbon 1964, 2, 199– 210. (35) Razouk, R. I.; Saleeb, F. Z.; Youssef, A. M. Carbon 1968, 6, 325– 331. (36) Griffine, R. R.; Scaroni, A. W.; Walker, P. L. Jr. Carbon 1991, 29 (7), 991–998. (37) Arenillas, A.; Rubiera, F.; Pevida, C.; Ania, C. O.; Pis, J. J. J. Therm. Anal. Calorim. 2004, 76, 593–602.
Kim et al.
Figure 13. Correlation between initial apparent rates of cokes and total catalytic iron (Fe, FeS, Fe1-xS, Fe3O4, and Fe2O3) content present in the samples.
made from coals containing different contents of iron-bearing minerals. The following conclusions were made. (1) There was a linear relationship between initial apparent reaction rate and the amount of catalytic iron mineralsspyrrhotite, metallic iron, and iron oxidesswhich was independent of heat treatment temperature at total catalyst levels below 1 wt %. (2) The initial coke reactivity decreased with increasing temperature of heat treatment due to decreased levels of catalytic iron minerals (largely due to formation of iron silicides) as well as increased ordering of the carbon structure. (3) During heat treatment (g1373 K) and subsequent gasification, iron-containing minerals predominantly transformed to iron silicides and iron oxides, the relative amounts of which varied with heat treatment temperature and gasification conditions. (4) The catalytic impact of mineral matter in determining reactivity declines as gasification proceeds. The study implies that the assessment of coke mineral behavior using only ash analysis is not sufficient to explain the variation of coke reactivity. Improved understanding of coke mineralogy, in particular, iron mineral transformation will contribute to the optimization of coal selection for a suitable coke quality and also improve reliability of coke quality prediction models. The effect of mineral size and distribution should also be considered in order to obtain a reliable assessment of coke performance in an operating blast furnace. Acknowledgment. The authors wish to acknowledge the financial support provided by the Cooperative Research Centre for Coal in Sustainable Development (CCSD), which is funded in part by the Cooperative Research Centre Program of the Commonwealth Government of Australia. The authors are also greatly thankful to Dr. M. Grigore (CSIRO) and Mr. N. Saha-Chaudhury (UNSW) for their technical assistance in this research. EF900229P
