Energy & Fuels 2009, 23, 2075–2085
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Coke Gasification: The Influence and Behavior of Inherent Catalytic Mineral Matter Mihaela Grigore,* Richard Sakurovs,† David French, and Veena Sahajwalla‡ The Commonwealth Scientific and Industrial Research Organisation (CSIRO) - Energy Technology, Lucas Heights, Bangor 2234, Australia ReceiVed August 18, 2008. ReVised Manuscript ReceiVed NoVember 26, 2008
Gasification of coke contributes to its degradation in the blast furnace. In this study, the effect of gasification on the inherent catalytic minerals in cokes and their reciprocal influence on gasification are investigated. The catalytic mineral phases identified in the cokes used in this study were metallic iron, iron sulfides, and iron oxides. Metallic iron and pyrrhotite were rapidly oxidized during gasification to iron oxide. The catalysts had a strong influence on the apparent rates at the initial stages of reaction. As gasification proceeds, their effect on the reaction rate diminishes as a result of reducing the surface contact between catalyst and carbon matrix because of carbon consumption around the catalyst particles; with extended burnout the reactivity of the coke becomes increasingly dependent on surface area. The reaction rate in the initial stages was also influenced by the particle size of the catalytic minerals; for a given catalytic iron level, the cokes whose catalytic minerals were more finely dispersed had a higher apparent reaction rate than cokes containing larger catalytic particles. Iron, sodium, and potassium in the amorphous phase did not appear to affect the reaction rate.
1. Introduction The blast furnace is a major source of greenhouse gas emissions and also a large energy consumer. To make the blast furnace more sustainable, new technologies1-3 have been or are being implemented to improve furnace efficiency, reduce energy consumption, and reduce greenhouse gas emissions. These changes in iron production will make new demands on coke quality, and, to prepare coke of suitable quality, a better understanding of the factors that affect coke degradation in the furnace is required. Gasification has been identified as an important factor that contributes to coke degradation as it descends through the furnace. Previous studies4-12 have indi* To whom correspondence should be addressed. Telephone: +61.2.97106875. Fax: +61 0.2.97106800. E-mail:
[email protected]. † Present address: The Commonwealth Scientific and Industrial Research Organisation (CSIRO) - Energy Technology, Newcastle, NSW 2300, Australia. ‡ Present address: School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. (1) Yagi, J.; Nogami, M.; Chu, M. Japanese National Project: Study on Half Energy and Minimum EnVironmental Impact of Ironmaking Process 2003, 1–14. (2) Schmole, P.; Lungen, H. B. 2nd Int. Meet. Ironmaking; 1st Int. Symp. Iron Ore 2004, 2–17. (3) Nomura, S.; Kitaguchi, H.; Yamaguchi, K.; Naito, M. The characteristics of catalyst-coated highly reactive coke. ISIJ Int. 2007, 47, 245– 253. (4) Vogt, D.; Depoux, M. Coke reactivity prediction by texture analysis. Fuel Process. Technol. 1990, 24, 99–105. (5) Mitchell, G. D.; Benedict, L. G. Ironmaking Conf. Proc. 1983, 42, 347–356. (6) Graham, J. P.; Wilkinson, H. C. Ironmaking Conf. Proc. 1978, 37, 421–436. (7) Duval, B.; Guet, J. M.; Richard, J. R.; Rouzaud, J. N. Coke properties and their microtexture. Part III: First results about relationship between microtexture and reactivity of some cokes. Fuel Process. Technol. 1988, 20, 163–175. (8) Van der Velden, B.; Trouw, J.; Chaigneau, R.; Van den Berg, J. Ironmaking Conf. Proc. 1999, 58, 275–285. (9) Kerkkonen, O.; Mattila, E.; Heiniemi, R. Ironmaking Conf. Proc. 1996, 55, 275–281.
cated that coke properties such as coke microtexture, porosity, and the inherent mineral matter are the main factors that control coke gasification rates. Ash composition has been considered to be an important parameter in determining coke reactivity.4,11,13 Although iron, calcium, potassium, and sodium have been identified as catalytic elements, subsequent studies have indicated that knowledge of the elemental composition of the ash of the coke is insufficient to predict coke reactivity, and determination of coke mineralogy is also required as only some forms of these elements in the coke are catalytic.9,14-16 However, to predict coke reactivity, not only must the minerals be identified but they must also be quantified. Walker et al.17 believed that even trace amounts of catalyst (less than 1 ppm) are able to affect the reaction rate. Grigore et al.18 found that the initial apparent gasification rate increased linearly with the concentration of the catalytic minerals in the cokes. (10) Aderibigbe, D. A.; Szekely, J. Studies in coke reactivity: Part 1 reaction of conventionally produced coke with CO-CO2 mixtures over temperature range 850-1000 °C. Ironmaking Steelmaking 1981, 11–19. (11) Gill, W. W.; Brown, N. A.; Coin, C. D. A.; Mahoney, M. R. Ironmaking Conf. Proc. 1985, 44, 233–238. (12) Beesting, M.; Hartwell, R. R.; Wilkinson, H. C. Coal rank and coke reactivity. Fuel 1977, 56, 319–324. (13) Vander, T.; Alvarez, R.; Ferraro, M.; Fohl, J.; Hofherr, K.; Huart, J. M.; Mattila, E.; Propson, R.; Willmers, R.; Van der Velden, B. 3rd Eur. Ironmaking Congr. Proc. 1996, 28–37. (14) Samaras, P.; Diamadopoulos, E.; Sakellaropoulos, G. P. The effect of mineral matter and pyrolysis conditions on the gasification of Greek lignite by carbon dioxide. Fuel 1996, 75, 1108–1114. (15) Grigore, M.; Sakurovs, R.; French, D.; Sahajwalla, V. Influence of mineral matter on coke reactivity with carbon dioxide. ISIJ Int. 2006, 46, 503. (16) Sakurovs, R.; French, D.; Grigore, M. Quantification of mineral matter in commercial cokes and their parent coals. Int. J. Coal Geol. 2007, 72, 81–88. (17) Walker, P. L., Jr.; Shelef, M.; Anderson, R. A. Catalysis of carbon gasification. In Catalysis of Carbon Gasification; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1968; Vol. 4, pp 287-383. (18) Grigore, M.; Sakurovs, R.; French, D.; Sahajwalla, V. International Conference on Coal Science and Technology [CD-ROM], 2005.
10.1021/ef8006728 CCC: $40.75 2009 American Chemical Society Published on Web 03/17/2009
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Table 1. Rank of the Parent Coals and Both Proximate and Ash Analyses of the Cokes Prepared in the 9-kg Oven A
R0 max
Coke moisture 0.7 ash 9.1 volatiles 0.7 fixed 90.2 carbon SiO2 Al2O3 Fe2O3 CaO MgO TiO2 Na2O K2 O P 2 O5 Mn3O4 SO3 BaO SrO
B
C
D
E
F
G
H
Rank of the Parent Coal (%, Mean Maximum Vitrinite Reflectance in Oil) 0.95 1.00 1.05 1.18 1.19 1.27 1.29 - Proximate Analysis (wt %, 0.7 0.8 0.7 1.1 7.9 10.3 9.3 11.9 0.8 0.3 0.7 0.5 91.3 89.4 90.0 87.6
affect gasification rates with increasing carbon burnout (conversion).
I
2. Experimental Section 1.40
Air-Dried Basis) 1.9 0.8 0.8 9.0 11.8 12.2 0.6 0.6 0.4 90.4 87.6 87.4
1.61 0.6 11.9 0.6 87.5
Coke - Ash Analysis (wt %) 51.5 62.0 54.5 48.1 54.3 58.0 48.0 61.3 56.9 38.3 29.0 29.3 36.8 33.8 19.1 37.5 28.3 27.0 4.2 3.9 7.3 4.7 4.3 12.7 5.6 3.1 8.5 1.1 1.2 2.7 2.6 2.1 2.9 2.4 1.3 1.6 0.21 0.20 0.90 0.59 0.43 1.6 0.52 0.38 0.78 1.8 1.5 1.4 1.8 1.4 0.99 1.4 1.4 1.4 0.42 0.11 0.20 0.38 0.27 0.32 0.58 1.5 0.35 0.76 0.55 1.1 1.1 0.75 0.92 0.52 1.1 1.0 0.72 0.74 1.6 1.8 1.4 1.2 1.8 0.84 0.71 0.00 0.00 0.05 0.05 0.04 0.05 0.04 0.03 0.14 0.03 0.16 0.41 0.48 0.21 1.10 0.25 0.17 0.43 0.06 0.00 0.08 0.15 0.08 0.00 0.24 0.06 0.04 0.08 0.04 0.07 0.28 0.09 0.05 0.14 0.05 0.04
Table 2. Measured Activation Energies of the Reactions of the Cokes with Carbon Dioxide and the Temperature Ranges over Which They Were Measured coke
activation energy (Ea) (kJ mol-1)
burnoff (%)
temp range (K)
A B C D E F G H I
247 266 245 252 256 222 258 239 229
16.3 16.8 17.5 18.1 15.3 17.5 18.4 17.6 17.3
1203-1128 1193-1119 1166-1079 1189-1128 1191-1118 1143-1043 1191-1116 1204-1137 1203-1128
The degree of dispersion of the catalyst is another important factor that controls reactivity.19,20 Lindert and Timmer21 and Tanaka et al.22 observed an increase of coke reactivity to carbon dioxide as the dispersion of metallic iron increased. Coke mineralogy undergoes changes during gasification,23 and the dispersion of the mineral matter changes.19 This means that the influence of mineral matter in cokes on gasification rates can change during gasification. Here, we extend the previous study that investigated the influence of the original mineral matter on initial gasification rates in cokes made from nine Australian bituminous coals to monitor the changes of the nature and distribution of mineral matter in these cokes during gasification and how these changes
Figure 1. Apparent reaction rate at 900 °C versus carbon conversion (100% CO2).
The cokes were prepared in a 9-kg cylindrical retort at a temperature of 1050 °C. The carbonization procedure was presented elsewhere.15 The cokes were crushed to -1 mm, and the fraction 0.6-1.0 mm was used for the reactivity test. Table 1 shows the rank of the parent coals and both proximate and ash analyses of the 0.6-1.0 mm cokes. A fixed-bed reactor system was used to carry out the reactivity experiments. The fixed-bed reactor system was described elsewhere.15 Two sets of subsamples of 1.4 g of each coke were reacted with 100% CO2 up to approximately 15 and 75% carbon burnoff, respectively. For these samples, the temperature chosen ranged between 873 and 930 °C for the most reactive and the least reactive cokes, respectively. The apparent reaction rates were measured only for the subsamples reacted up to approximately 15% burnoff. The apparent reaction rate (Fa) was calculated according to eq 1, where w is the mass of sample remaining at time t. The apparent reaction rate was reported as grams of carbon reacted per gram of carbon remaining per second. In the calculation, the sample mass was calculated as ash-free.
Fa )
1 dw w dt
( )
(1)
The reactivity was measured under conditions of chemical rate control, free of any physical limitations due to gas pore diffusion and mass transfer. Under chemical controlled conditions, particle size and gas flow rate should not affect the reaction rate. Harris and Smith24,25 showed that the reaction rate of coke with carbon dioxide at 800 and 890 °C, respectively, was not affected by coke particle size between 0.2 and 2.0 mm and the flow rate of the reactant gas (100% CO2) between 500 and 1000 mL min-1 at atmospheric pressure. In this work, varying the size of the particles from 0.212 to 1.0 mm did not change the reaction rate significantly. Also, changing the gas flow rate through the sample bed from 750 to 850 mL min-1 had negligible effect on reaction rate. The activation energies were measured to provide information about reaction conditions and to normalize the reaction rates during the experiment to a selected temperature, which enable comparison between cokes. The activation energy was derived from an Arrhenius plot of data collected during the sample cooling at the end of the reactivity test (the calculation procedure was presented elsewhere15). The corresponding activation energies are presented in Table 2. All the activation energies are in the range between 222 and 266 kJ mol-1. The magnitude of these numbers was consistent with the data available in the literature for chemically controlled reaction rates.25-28 Surface area measurements, using carbon dioxide at 0 °C, were performed on both raw cokes and reacted cokes at 15% burnoff. Carbon dioxide measured the surface area of the micropores in the range 0.4-1.6 nm. The Dubinin-Radushkevich equation was used to fit the carbon dioxide isotherm and determine the surface area of the sample. The mineral phases present in the nine raw cokes and four of the reacted cokes (B, C, F, and G) at both 15 and 75% burnoff were identified using X-ray diffraction analysis. Quantitative analysis was also carried out on the mineral matter of the cokes. The carbon was removed from the mineral matter with minimal alteration of the mineral species using radio frequency oxygen plasma ashing at low temperature (120 °C).29 Some artifacts of low(19) Tomita, A. Catalysis of carbon-gas reactions. Catal. SurV. Jpn. 2001, 5, 17–24. (20) Gopalakrishnan, R.; Bartholomew, C. H. Effects of CaO, hightemperature treatment, carbon structure, and coal rank on intrinsic char oxidation rates. Energy Fuels 1996, 10, 689–695. (21) Lindert, M.; Timmer, R. M. C. Ironmaking Conf. Proc. 1991, 50, 233–237.
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Figure 2. Dependence of both initial and final (15% burnoff) apparent rates on total surface area measured by CO2 at initial stages of reaction and after reaction (15% burnoff), respectively. Table 3. Surface Area of the Raw Cokes and Approximately 15% Burnoff Cokes (after Reaction), Measured by Carbon Dioxide coke surface area (m2 g-1)
A
B
C
D
E
Table 4. Mineral Phases Identified in the LTA of the Cokes and Their Relative Concentrations (%, Mass) mineral phase LTA (%)
F
G
H
A 10.0
B
I
raw cokes 3.4 2.3 13.4 6.0 8.8 64.5 8.5 12.8 4.5 15% burnoff 21.7 54.5 96.6 64.2 45.5 131.9 56.4 31.3 23.9 cokes
temperature ashing (LTA) have been identified, such as bassanite (formed from oldhamite), jarosite, and coquimbite (both formed from pyrrhotite).15 Consequently, the mineral phase compositions were recalculated with bassanite, coquimbite, and jarosite being proportioned to oldhamite and pyrrhotite. SIROQUANT,30 a personal computer quantitative X-ray diffraction (XRD) analysis software package, was used to quantify the minerals in the low-temperature ash. The software was developed by CSIRO and uses the full-profile Rietveld method of refining the shape of a calculated XRD pattern against the profile of a measured pattern. The distribution and association of the minerals in the raw and reacted cokes was observed using a field emission scanning electron microscopy (FESEM) technique on polished blocks. An energydispersive X-ray analyzer (EDS) was used for the semiquantitative microchemical analysis of materials in the samples.
3. Results and Discussion 3.1. Introduction. The apparent and intrinsic reaction rates of the cokes will be discussed initially, followed by a description of the changes occurring in coke mineralogy during gasification. The relationship between the coke mineralogy and the apparent reaction rate will then be presented. The apparent reaction rate
Figure 3. Intrinsic reaction rates at initial stages and 15% burnoff of the nine cokes (calculated using CO2 surface area).
iron hematite magnetite wustite pyrrhotite troilite oldhamite jarosite coquimbite bassanite
C
D
E
8.9 12.1 10.1 12.3 Catalysts 0.5 0.5 0.4 0.3
0.2
0.3
0.3
0.4 0.2
0.8
0.3
0.1 0.3 0.3 0.1
0.1 0.3 0.2 0.3
0.1 1.0 0.2 0.7
0.1 0.5 0.3 0.7
akermanite diopside 0.4 fluorapatite 0.6 2.3 iron phosphate 0.1 0.1 alumina 6.9 0.2 mullite 9.0 12.3 spinel 0.9 leucite 0.3 0.3 quartz 3.0 26.8 cristobalite 0.1 0.1 brookite 0.3 anatase 0.3 0.2 rutile 0.1 0.3 amorphous 76.9 55.8
0.9
F
G
H
I
9.8 12.4 12.7 13.0 1.6
0.1
0.1 0.2
2.9 1.1 0.6 4.3
0.3
1.6
0.6 0.1 0.2
0.4
0.1
0.3
0.1 0.6 0.1 0.3
1.4
0.2
0.2 0.5
0.1 0.1 0.1 0.4 0.2 0.6
Others 0.3 0.2 0.3 0.1 0.2 0.3 0.3 0.5 3.6 3.3 1.9 3.5 3.4 1.5 1.5 0.3 0.5 0.1 0.3 0.2 0.1 0.2 2.5 1.7 0.4 12.8 15.4 10.5 5.7 14.1 9.8 9.7 1.3 0.6 0.3 0.3 0.3 0.4 0.1 0.1 0.6 14.4 6.8 12.8 32.7 4.1 24.1 21.3 0.1 0.1 0.1 0.1 0.5 0.5 0.3 0.1 0.2 0.6 0.2 0.4 0.4 0.1 0.5 0.3 0.3 0.3 0.5 0.2 62.0 67.2 70.3 44.0 74.9 61.1 62.9
is a measure of the rate of carbon loss as a function of the degree of carbon conversion and has been normalized here to 900 °C (22) Tanaka, S.; U-emura, T.; Ishizaki, K.; Nagayoshi, K.; Ikenaga, N.; Ohme, H.; Suzuki, T.; Yamashita, H.; Ampo, M. CO2 gasification of ironloaded carbons: Activation of the iron catalyst with CO. Energy Fuels 1995, 9, 45–52. (23) Grigore, M.; Sakurovs, R.; French, D.; Sahajwalla, V. Mineral reactions during coke gasification with carbon dioxide. Int. J. Coal Geol. 2008, 75, 213–224. (24) Harris, D. J.; Smith, I. W. Prepr. Pap. Am. Chem. Soc. 1989, 34, 94–101. (25) Harris, D. J.; Smith, I. W. 23rd Symp. (Int.) Combust., Proc. 1990, 1185–1190. (26) Pang, B. Y.; Harris, D. J.; Tyler, R. J.; Sakurovs, R. 13th Annu. Int. Pittsburgh Coal Conf. Proc. 1996, 1, 506–511. (27) Laurendeau, N. M. Heterogeneous kinetics of coal char gasification and combustion. Prog. Energy Combust. Sci. 1979, A, 221–270. (28) Kawakami, M.; Mizutani, Y.; Ohyabu, T.; Murayama, K.; Takenaka, T.; Yokoyama, S. Reaction kinetics of coke and some carbonaceous materials with CO2 and coke strength after reaction. Steel Res. Int. 2004, 75, 93–98. (29) Gluskoter, H. J. Electronic low temperature ashing of bituminous coal. Fuel 1965, 44, 285–291. (30) Taylor, J. C. Computer programs for standardless quantitative analysis of minerals using the full powder diffraction profile. Powder Diffr. 1991, 6, 2–9.
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Table 5. Mineral Phases Identified in the LTA of the Reacted Cokes B, C, F, and G (15 and 75% Burnoff) and Their Relative Concentration (%, Mass) B mineral phase iron hematite magnetite magnesioferrite wustite oldhamite calcite jarosite bassanite anorthite akermanite clinopyroxene rankinite calcium ferrite fluorapatite hercynite iron phosphate fayalite leucite mullite spinel cristobalite quartz tridymite anatase rutile amorphous
15% 0.1 0.2 0.3 0.6 0.6 0.5 0.3 0.2 0.1
1.0
C 75%
F 75%
Catalysts 0.2 0.3 0.6 0.1 0.1
1.3 1.0
15%
1.4 0.6
3.8 0.8
0.3
0.3 0.3 0.9 0.6
1.2 0.8 0.4
Others 0.3 1.4 0.4 0.6 0.8
0.6 0.8
0.7
0.3 11.1
0.2 17.6
0.1 30.4
0.8 26.2 0.6 0.1 0.9 49.2
G 75%
0.1
0.1 0.7 1.5 0.4
0.2
0.2 1.0 52.8
15%
0.3 2.6 0.9 0.2 0.5 0.1 11.9 0.2 0.1 16.9
0.5 1.4 4.2 0.5 0.6 0.4 18.5 0.3 2.0 15.1 0.3
0.5 59.3
0.7 51.3
15%
75%
0.1 0.3 1.7 0.4 0.2 0.5 2.4 0.9 1.4 0.6 0.8 6.2
0.8 2.1 0.9 0.1 0.6 0.2 4.2 0.6 0.1 35.8
1.4 0.3 9.8 4.3 0.9 28.3
0.2 0.7 45.6
0.3 39.3
2.8 0.2 0.1 0.1 0.1 0.1 0.2
0.3 0.3 0.4 2.4 0.1 0.2 11.7 0.2 4.3 0.4 1.1 74.9
0.6 1.1 0.6
0.3 1.5 0.6 2.1 0.7 0.7 1.2 0.1 0.4 28.6 0.6 3.0 2.6 0.2 0.1 0.9 54.1
using the activation energy measured at the end of each experiment. The intrinsic rate is determined by dividing the apparent reaction rate by the surface area. The initial rate is the reaction rate measured at the start of the experiment, and the final rate is that measured at the conclusion of the experiment. 3.2. Apparent Rate. Figure 1 shows the apparent reaction rates with CO2 (100%) of the nine cokes as a function of carbon conversion. The apparent reaction rate tends to increase with increasing carbon conversion. The apparent reaction rates for the different cokes varied by more than an order of magnitude at 15% carbon conversion. 3.3. Coke Surface Area and the Intrinsic Rate. The surface areas of the raw and approximately 15% burnoff cokes are shown in Table 3 (no surface area measurements are available for the 75% burnoff cokes). The surface areas of the raw cokes varied to a great degree from 2.3 to 64.5 m2 g-1. The surface area of micropores increased after reaction, which is consistent with previous work and is attributed to
Figure 5. Relationship between initial apparent rate against (a) catalytic iron and (b) catalytic calcium.
Figure 6. Catalytic calcium against catalytic iron in the raw cokes.
(a) An increase of pore size at early stages of reaction31,32 (b) Formation of new microporosity and opening of closed pores, which was probably developed at the early stages33or (c) Improved access of the coke to carbon dioxide caused by changes in the mineral matter23 If surface area was the major factor responsible for the differences in the apparent reactivity of the cokes, the
Figure 4. Deportment of total iron present in the mineral phases known as catalysts of gasification (Fe, Fe1-xS, Fe2O3, Fe3O4, and FeO), expressed as mole fraction of iron per 100 g of coke (original coke).
(31) Szekely, J.; Aderibigbe, D. A. Coke reactivity and its effect on blast furnace performance. Iron Steel Soc. 1990, 3–12. (32) Kawakami, M.; Taga, H.; Takenaka, T.; Yokoyama, S. Micropore structure and reaction rate of coke, wood charcoal and graphite with CO2. ISIJ Int. 2004, 44, 2018–2022. (33) Turkdogan, E. T.; Olsson, R. G.; Vinters, J. V. Pore characteristics of carbons. Carbon 1970, 8, 545–561.
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Figure 7. Initial apparent rate against (a) iron content present in the catalytic iron phases and (b) total iron.
Figure 8. FESEM images of a pyrrhotite particle (A) and a metallic iron particle (B) in coke C. Right: EDS spectra of the particles.
apparent reaction rates should increase with increasing surface area. Figure 2 shows that the initial apparent reaction rate was poorly related to carbon dioxide surface area, demonstrating that the surface area of accessible microporosity is not the dominant factor in determining reaction rate at initial stages for the cokes used in this study. At 15% burnoff, however, the relationship between reaction rate and carbon dioxide surface area was much improved (Figure 2), suggesting that the reaction rate at 15% burnoff is controlled mainly by the surface area of micropores. The apparent rates were divided by carbon dioxide surface areas to determine the intrinsic rates. Figure 3 shows that the initial intrinsic rate varied by a factor of 5 between cokes, but the final intrinsic rate varied by only a factor of 2 at 15% burnoff. These observations indicate that the relative importance of factors that affect the intrinsic rate decreases during the course of the reaction.
3.4. Mineral Matter in Cokes and Its Changes on Gasification. An amorphous aluminosilicate is the dominant phase in most of the cokes (Table 4), occurring in association with a highly variable mineralogy. The amorphous mineral matter is the result of decomposition during carbonization of aluminosilicates (kaolinite, illite, montmorillonite, chamosite) present in the parent coals.15 Of all the mineral phases identified in the cokes, metallic iron, iron oxides, and pyrrhotite were the only minerals considered to be gasification catalysts.17,21,34-36 Oldhamite is
(34) Turkdogan, E. T.; Vinters, J. V. Catalytic oxidation of carbon. Carbon 1972, 10, 97–111. (35) Vandezande, J. A. Ironmaking Conf. Proc. 1985, 44, 189–206. (36) Price, J. T.; Iliffe, M. J.; Khan, M. A.; Gransden, J. F. Ironmaking Conf. Proc. 1994, 53, 79–87.
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Figure 9. FESEM images of a pyrrhotite particle (A) and metallic iron particles (B and C) in coke I. The enlarged image of an area of a metallic iron particle is shown in D. The EDS spectra of the particles are included.
potentially a catalyst because carbon dioxide can oxidize it to calcium oxide, which is a known gasification catalyst.19 In this study, oldhamite was considered as a potential source of calcium oxide.
Four of the nine cokes (B, C, F, and G) were selected to investigate the effect of gasification on the catalytic mineral matter in coke. Table 5 shows the mineralogy and the relative concentration of the mineral phases in these reacted cokes at
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Figure 10. SEM images and EDS spectra of metallic iron and pyrrhotite particles in the 15% burnoff coke F.
Figure 11. SEM/EDS micrographs of iron oxide particles in the 15% burnoff coke F.
15 and 75% burnoff. The mineralogical composition of the reacted cokes was recalculated on a coke basis with jarosite
and coquimbite being proportioned to pyrrhotite and bassanite being proportioned to oldhamite as noted above.
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Grigore et al. Table 6. Elemental Composition (Mol Fraction * 10-4/100 g of Coke) of Crystalline and Amorphous Mineral Forms in Cokes coke A B C D E F G H I
Figure 12. Apparent rates of cokes B, C, F, and G at 15% burnoff versus (a) catalytic iron and (b) catalytic calcium.
total crystalline form amorphous form total crystalline form amorphous form total crystalline form amorphous form total crystalline form amorphous form total crystalline form amorphous form total crystalline form amorphous form total crystalline form amorphous form total crystalline form amorphous form total crystalline form amorphous form
Fe
Ca
K
Na
51.8 10.5 41.2 43.9 14.6 29.3 82.1 38.4 43.7 52.9 29.0 23.8 50.6 25.8 24.8 142.9 99.9 43.0 63.0 30.3 32.7 36.0 23.4 12.6 95.6 41.6 54.0
17.7 9.9 7.8 19.3 23.4 -4.1 43.4 53.7 -10.3 41.8 39.3 2.5 37.0 29.2 7.7 46.6 56.5 -9.9 38.6 46.1 -7.5 20.9 24.2 -3.3 25.7 31.4 -5.7
14.6 2.0 12.6 10.5 1.8 8.8 21.1 4.1 17.0 21.1 2.9 18.2 13.2 1.1 12.2 17.6 8.4 9.2 10.0 2.1 7.9 21.1 4.0 17.1 19.1 1.0 18.1
16.0 16.0 3.2 3.2 5.8 5.8 11.0 11.0 11.9 11.9 9.3 9.3 16.8 16.8 20.9 20.9 10.2 10.2
The amounts of catalytic minerals in the reacted cokes were different from those in the raw cokes, and also two new, potentially catalytic mineral phases were identified, namely magnesioferrite and calcite. Magnesioferrite is not known to catalyze gasification, but it could be a potential catalyst because it has a structure very similar to that of magnetite. Calcite is not actually formed during gasification but during cooling when the activation energy of reaction was measured.15 Calcite is
Figure 13. SEM/EDS micrographs of iron oxide particles in the 75% burnoff coke F.
Coke Gasification
Figure 14. Initial apparent rate of the nine cokes versus (a) iron content in the amorphous mineral phase, (b) potassium content in the amorphous mineral phase, and (c) sodium content in the amorphous mineral phase.
believed to be the result of carbonation of calcium oxide, which is formed during gasification. To establish the influence of the catalytic iron and calcium on coke reactivity, the total iron and calcium present in the catalytic mineral phases were calculated as mole fraction of element per 100 g of coke. Figure 4 shows the deportment of iron between the catalytic mineral phases in the raw cokes, expressed as moles per 100 g of coke. The iron present in metallic iron and pyrrhotite accounted for most of the catalytic iron. Coke F had the greatest content of catalytic iron followed by cokes I and C.
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3.5. Relationship between Catalytic Mineral Phases and Apparent Reactivity. 3.5.1. Coke Characterization Initial Stages. A good relationship was observed between the catalytic iron levels and the initial apparent rate, which indicates that initially the reaction rate was strongly influenced by these catalytic mineral phases (Figure 5a). Although catalytic calcium content also showed a good relationship to the initial apparent rate (Figure 5b), it was not possible to distinguish any independent effect of catalytic calcium in these samples as the correlation between catalytic calcium and catalytic iron was high and potentially catalytic calcium was less than a quarter of the amount of catalytic iron (on a mole basis) (Figure 6). Figure 7 shows that the relationship between initial apparent rate and iron levels in the catalytic mineral phases was closer than that between initial apparent rate and total iron, as determined from the ash chemistry. Since total iron includes phases such as iron phosphate and amorphous material (see section 3.6), which are probably noncatalytic, total iron in the coke would be expected to be a poorer predictor of reaction rate than catalytic iron. A similar observation can be made regarding calcium, potassium, and sodium. The calcium forms present in the cokes were akermanite, diopside, fluorapatite, and oldhamite. Leucite was the sole potassium mineral found in the cokes, and no sodium-bearing minerals were identified. Of these phases, fluorapatite36 is known to be a noncatalyst of the gasification reaction. Oldhamite can be a potential source of catalytic CaO, but the other calcium phases such as akermanite and diopside are not known to catalyze gasification. None of the naturally occurring potassium or sodium-bearing phases in the cokes are known to be catalytically active. Therefore, it is more appropriate to use the mineral phase composition as an indicator of coke reactivity rather than the ash chemistry. Although cokes I and C have similar levels of catalytic iron, they had different initial apparent reactivity (Figure 5a). Since the particle size and distribution of catalytic materials may affect reactivity,19,20,37 SEM/EDS analyses were performed on these cokes, and representative SEM images and EDS spectra of metallic iron and pyrrhotite particles present in these two cokes are presented in Figures 8 and 9, respectively. Pyrrhotite particles in coke I (Figure 9) were in general much larger than those observed in coke C (Figure 8), resulting in a reduced contact surface area between the catalyst and the carbon matrix. The metallic iron particles in coke I (Figure 9) were approximately 15-20 times larger than those in coke C (Figure 8). Moreover, because of their large size (300-600 µm) some of the metallic iron particles in coke I were not completely embedded in the coke matrix as shown in Figure 9B,C. Furthermore, metallic iron in coke I had a distribution different from that in coke C. Metallic iron in coke I also occurs as fine inclusions in a phase, most likely amorphous, consisting mainly of iron and oxygen, and small amounts of calcium, manganese, magnesium, and sulfur (about 15%) as shown in the EDS spectrum (Figure 9D). QEMSCAN analysis of coal I shows that pyrite (FeS2) and siderite (FeCO3), which are the principal precursors of pyrrhotite and metallic iron in coke,9,38,39 are often associated with calcite and ankerite. The decomposition of these minerals during the carbonization process could produce metallic iron and an amorphous phase rich in iron. If the iron present in this amorphous phase does not catalyze the gasification reaction, the contact surface between metallic iron and carbon in coke I would be even less. 3.5.2. Coke Characterization - 15% Burnoff. The XRD data of the 15% burnoff coke (Table 5) reveal that catalytic iron is mostly present as iron oxides (magnetite, magnesioferrite, and
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Figure 15. SEM images and EDS spectra of a mineral phase with a composition similar to that of chamosite, in the raw coke F.
Figure 16. SEM images and EDS spectra of a mineral phase with a composition similar to that of chamosite, in the 15% burnoff coke F.
wustite), which are assumed to have formed by oxidation of metallic iron and pyrrhotite by carbon dioxide.23 However, some metallic iron and pyrrhotite were still present at low levels (Table 5). Figure 10 shows that the metallic iron and pyrrhotite particles remaining in the 15% burnoff cokes were invariably enclosed in the coke matrix and consequently inaccessible to carbon
dioxide. Closed porosity could explain the inaccessibility of carbon dioxide to these minerals. The SEM images of the iron oxide particles in the 15% burnoff coke show that the contact between the iron oxide particles and carbon matrix was less intimate than that between both pyrrhotite and metallic iron and the carbon matrix in the
Coke Gasification
raw coke (Figure 11), possibly due to a greater degree of gasification of carbon around the catalytic mineral phases. The concentration of catalytic iron and calcium mineral phases in the 15% burnoff cokes B, C, and G was greater than in the raw cokes (Tables 4 and 5), the most likely source being the amorphous phase as will be discussed further in section 3.6. The correlation between the final apparent rate at 15% burnoff and the levels of catalytic iron and calcium is poor (Figure 12), which is consistent with the above finding that surface area is the dominant factor in determining the final apparent rate at 15% burnoff. The SEM data show that the contact surface area between the catalyst and coke matrix diminishes, and thus it would be expected that the influence of the catalysts on the reaction rate is reduced. 3.5.3. Coke Characterization - 75% Burnoff. Iron oxides were the only catalytic iron forms present in the 75% burnoff cokes (Table 5); no metallic iron and pyrrhotite were present. The amount of catalytic iron and calcium mineral phases in the 75% burnoff cokes was less than in the 15% burnoff cokes (Table 5). Decrease of catalyst abundance in the 75% burnoff cokes can be explained by the occurrence of mineral reactions during gasification.23 In the 75% burnoff coke, the iron oxide particles showed a much poorer contact with the carbon matrix than those in the 15% burnoff coke (Figure 13), the strongly gasified coke maintaining only a weak bond with the catalytic minerals. Although the reaction rate was not measured at 75% burnoff, any influence of the catalysts on gasification is expected to be very low because of the small contact surface area between the catalyst and coke matrix. 3.6. Role of the Amorphous Phase in Gasification. Of the four elements (Fe, Ca, K, and Na) considered to have catalytic activity,11 sodium was present only in the amorphous phase as were much of the potassium and some of the iron (Table 6). Calcium was almost fully accounted for by the crystalline mineral phases. The negative estimates of calcium in the amorphous phase (Table 6) are attributed to the use of the stoichiometric composition of the minerals to calculate the element partitioning. This can overestimate the content of these elements in the minerals since substitution of some elements can occur in some minerals.40 Since the relationship between the levels of iron, potassium, and sodium in the amorphous phase and initial apparent rate was poor (Figure 14), these elements when present in the amorphous phase are considered to not catalyze the gasification reaction. The levels of the catalytic iron minerals in the 15% burnoff cokes B, C, and G were greater than in their corresponding raw (37) Radovic, L. R.; Walker, P. L.; Jenkins, R. G. Importance of catalyst dispersion in the gasification of lignite chars. J. Catal. 1983, 82, 382–394. (38) Earnest, C. M. Characterization of the low-temperature ash component of the Herrin 6 coal seam (southwestern Illinois) by thermal methods of analysis. Thermochim. Acta 1987, 121, 71–86. (39) Gotor, F. J.; Macias, M.; Ortega, A.; Criado, J. M. Comparative study of the kinetics of the thermal decomposition of synthetic and natural siderite samples. Phys. Chem. Miner. 2000, 27, 495–503. (40) Grigore, M.; Sakurovs, R.; French, D.; Sahajwalla, V. Mineral matter in coals and their reactions during coking. Int. J. Coal Geol. 2008, 76, 301–308.
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cokes (Tables 4 and 5), indicating that reaction of the amorphous phase had occurred resulting in the formation of catalytic minerals. As mentioned previously, the amorphous phase was formed from decomposition of clay minerals such as kaolinite, illite/smectite, illite, and chamosite during carbonization of the coals. Of these minerals, chamosite has the greatest content of iron, though some contribution by illite or illite/smectite is possible. Examination of raw coke F using SEM/EDS revealed the occurrence of small bright grains of metallic iron within a mineral phase of a composition similar to that of chamosite present in the parent coal (Figure 15). QEMSCAN analysis showed that chamosite was not associated with siderite or pyrite in the parent coal,40 both of which are known to be sources of metallic iron in coke. Therefore, the grains of metallic iron are most likely to have formed by the decomposition of chamosite during carbonization. In the 15% burnoff coke F, metallic iron was not observed to be present in the product of chamosite decomposition (Figure 16), most likely because, during gasification, the fine metallic iron grains reacted with the amorphous phase. The nature of the product phase is unknown but is likely to have been either an iron-rich spinel or an iron silicate such as fayalite. It cannot be ascertained to what degree metallic iron resulting from chamosite decomposition affects gasification rate, but it is likely to be small as the iron is encapsulated in the amorphous phase. Further investigation of the effect of the potentially catalytic minerals formed from the decomposition of clay minerals on gasification is required. 4. Conclusions This investigation of the effect of the catalytic mineral phases on coke reactivity showed the following: • Quantification of the catalytic mineral matter in coke results in a more reliable indicator of its initial reactivity than bulk ash chemistry. In this series of cokes, the most likely mineral phases responsible for the variation in coke reactivity were metallic iron, iron sulfides, and iron oxides. Oldhamite may also contribute. • None of the potassium or sodium in the cokes was in a form known to be catalytically active. • The particle size and association of the catalytic mineral matter are also important in determining the initial apparent reaction rate. • The reactivity of the coke is less affected by mineral matter composition as reaction proceeds; surface area becomes an increasingly important indicator of reactivity at higher conversion rates. This is probably because the contact surface between catalytic mineral phases and coke diminishes during gasification, due to consumption of carbon around the mineral grains. Acknowledgment. We 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 Centres Program of the Commonwealth Government of Australia. EF8006728
