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Properties and CO2 reactivity of the inert and reactive maceral-derived components in cokes
International Journal of Coal Geology 98 (2012) 1–9
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Properties and CO2 reactivity of the inert and reactive maceral-derived components in cokes Mihaela Grigore a,⁎, Richard Sakurovs a, David French a, Veena Sahajwalla b a b
The Commonwealth Scientific and Industrial Research Organisation (CSIRO), Energy Technology, 11 Julius Avenue, North Ryde 2113, Australia School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e
i n f o
Article history: Received 15 February 2012 Received in revised form 4 April 2012 Accepted 5 April 2012 Available online 12 April 2012 Keywords: Gasification Catalytic mineral phases X-ray diffraction SIROQUANT
a b s t r a c t The response of coke to gasification affects its degradation in the blast furnace. Coke gasification with carbon dioxide at high temperature is selective, with the inert maceral-derived component reacting more readily than the reactive maceral-derived component. Previous reactivity studies on carbonised vitrinite- and inertinite-rich fractions indicated that the amount of catalytic mineral phases control their reactivity. However, total Fe, Ca, K and Na from the ash chemistry were used as indicators of the abundance of catalytic material rather than the concentration of the catalytic mineral phases. Additionally, there is disagreement regarding the influence of micropore surface area and average carbon crystallite size on reactivity of the inert maceral-derived component and reactive maceral-derived component. Here we examine the influence of the abundance of the catalytic mineral phases on the reactivity of the inert maceral-derived component and reactive maceral-derived component, and also the influences of micropore surface area and average carbon crystallite size. Cokes from inertinite- and vitrinite-rich fractions prepared from four Australian bituminous coals were reacted with carbon dioxide at temperatures between 855 °C and 934 °C. The major factors that make inert maceral-derived component more reactive than reactive maceral-derived component at the initial stages were found to be the concentration of catalytic mineral phases and micropore surface area. The catalytic mineral phases identified in the coked inertinite- and vitrinite-rich fractions were metallic iron, pyrrhotite, troilite, wustite, magnetite and hematite. No Ca, K and Na catalytic mineral phases were identified in any of the studied cokes. The average crystallite height was not found to be a major factor controlling coke reactivity at the initial stages. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Any improvement of the blast furnace efficiency operated under current technologies and any implementation of new technologies will make new demands on coke quality. In order to prepare coke of suitable quality and to address these requirements, a better understanding of the factors that affect coke degradation in the furnace is required, in particular, coke gasification. Coke gasification with carbon dioxide at high temperature (over 800 °C) shows selective attack, with the isotropic microtexture reacting more readily than the anisotropic microtexture (Duval et al., 1988; Kerkkonen et al., 1996; Koba and Ida, 1980; van der Velden et al., 1999; Vander et al., 1996; Vandezande, 1982; Vogt et al., 1988). The anisotropic microtexture forms from the coal macerals that fuse during coking, such as vitrinite, liptinite, and a part of the inertinite. Inertinite
⁎ Corresponding author. Tel.: + 61 2 9490 5321; fax: + 60 2 9490 8530. E-mail address: [email protected] (M. Grigore). 0166-5162/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2012.04.004
that does not fuse yields the isotropic microtexture. The isotropic and anisotropic microtextures in the coke are also referred to as the inert maceral-derived component and the reactive maceral-derived component, respectively. One way to investigate the contribution of each component to the reactivity of coke is to separate the coal macerals before coking them. Studies on cokes prepared from whole coals found that their reactivity is influenced by surface area (Vogt et al., 1991; Zamalloa et al., 1995), carbon crystallite size (Duval et al., 1988), and catalytic material (Turkdogan and Vinters, 1972; Walker et al., 1968). The influence of these properties on the reactivity of the carbonised vitrinite- and inertinite-rich fractions prepared from a range of bituminous coals with carbon dioxide has been investigated in several studies (Czechowski and Kidawa, 1991; Huang et al., 1991; Sakawa et al., 1982; Sun et al., 2004). Huang et al. (1991), Sun et al. (2004) and Czechowski and Kidawa (1991) used the concentration of calcium, iron, potassium and sodium in the ash to determine the relationship between reactivity and the abundance of catalytic material, whereas Sakawa et al. (1982) used the alkali index as indicator of the relative proportion of the catalytic material in the ash. Huang et al. (1991)
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M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
also investigate the influence of micropore surface area and carbon crystallinity on reactivity. In these studies, the carbonised inertinite-rich fractions were not always more reactive than the carbonised vitrinite-rich fractions. Sakawa et al. (1982) observed that the reactivity of carbonised densityseparated maceral fractions, prepared from coals of different rank, increased with increasing inertinite content of the fraction. The carbonised inertinite-rich fractions of two out of three coals prepared by Huang et al. (1991) were more reactive than their corresponding carbonised vitrinite-rich fractions, whereas the carbonised inertinite-rich fraction of the other coal was less reactive than its corresponding carbonised vitrinite-rich fraction. Additionally, the carbonised vitrinite-rich fractions of the coals used by Sun et al. (2004) and Czechowski and Kidawa (1991) were found to be more reactive than their corresponding carbonised inertinite-rich fractions. The differences in reactivity between the carbonised maceral-rich fractions were believed to be mainly due the catalytic material present in the samples. In these studies, surface area and carbon crystallinity were not considered to play a major role in controlling the reactivity of the carbonised inertinite- and vitrinite-rich fractions. Huang et al. (1991) and Sun et al. (2004) attempted to establish the influence of the catalytic material on the reactivity of the carbonised vitrinite- and inertinite-rich fractions by demineralising the chars or the maceral-rich fractions prior to carbonisation. Demineralisation reduced the reactivity of some of the investigated carbonised maceral-rich fractions. The reactivities of the carbonised vitrinite- and inertinite-rich fractions prepared by Sun et al. (2004) were reduced by demineralisation, but the carbonised vitrinite-rich fraction was still more reactive than the carbonised inertinite-rich fraction. Huang et al. (1991) found in their study that after demineralisation only one carbonised vitrinite-rich fraction has become less reactive. The reactivity of the other carbonised vitrinite- and inertinite-rich fractions did not change significantly after demineralisation. Grigore et al. (2009) have identified and quantified the mineral phases in a series of cokes and shown that not all iron, calcium and potassium crystalline mineral phases present in coke catalyse the gasification reaction. The sodium, potassium, calcium and iron associated with the amorphous phase, which formed during carbonisation from the decomposition of clays, are not known to catalyse gasification, although Kerkkonen et al. (1996) believe that small amounts of these elements may be released during coking to form catalytic materials. Walker et al. (1968) believe that even traces amounts of catalyst (less than 1 ppm) are able to affect the reaction rate. Grigore et al. (2009) found that the initial apparent rate is strongly related to the catalyst abundance. The aim of this study is to determine the relative importance of the amounts of catalytic minerals, micropore surface area and carbon crystallinity in explaining the differences in reactivity between the inert maceral-derived component and the reactive maceral-derived component in cokes. This study is undertaken on coked inertiniteand vitrinite-rich fractions prepared from four bituminous coals. 2. Experimental procedure For this study, four Australian bituminous coals (B, C, F and G) from New South Wales and Queensland were selected based on their differences in rank, mineral matter content and ash chemistry. Coals B and C were lower rank than coals F and G (Table 1). The ash yield of the coals ranged between 5.6 and 9.8 wt.%, and the ash composition varied to a great extent between coals. 2.1. Preparation of maceral-rich fractions The coal macerals were separated using the sink-float method, which is based on the different densities of the macerals: vitrinite density is less than that of inertinite (van Krevelen, 1993). Mixtures
Table 1 Coal rank, proximate and ash analyses of the coals. Coal
B
C
F
Coal rank (%, mean maximum vitrinite reflectance in oil) R0 max. 1.00 1.05
G 1.27
1.29
Proximate analysis (wt.%, air-dried basis) Fixed carbon 65.5 Ash 5.6 Volatiles 28.9 Moisture 2.4
66.1 7.7 26.2 2.5
71.7 7.0 21.3 1.4
70.0 9.8 20.2 1.1
Ash analysis (wt.%) SiO2 Al2O3 Fe2O3 CaO Na2O K2O MgO TiO2 P2O5 Mn3O4 SO3 V2O5 BaO SrO Total
53.6 28.4 7.6 3.0 0.57 1.0 0.95 1.4 1.7 0.05 0.76 0.05 0.15 0.08 99.4
56.9 18.3 12.8 3.7 0.45 0.92 1.6 1.1 1.3 0.06 2.0 0.04 0.09 0.05 99.4
48.3 37.9 5.3 2.5 0.65 0.54 0.58 1.4 1.9 0.03 0.32 0.02 0.22 0.13 99.9
61.4 28.3 4.3 1.3 0.30 0.48 0.34 1.5 0.79 b 0.02 0.26 0.05 0.03 0.04 99.2
of hexane and perchlorethylene in different ratios were used to separate the coal macerals. Prior to density separation, the coals were crushed to less than 1 mm size and screened at 0.45 mm. The fraction 0.45–1.00 mm was used to prepare the maceral-enriched fractions. In this study, the densities of the fractions richest in vitrinite were less than 1.30 g/cm 3 (Table 2). The percentage of vitrinite in these fractions varied from 84.5 to 96.6%. The fractions with densities between 1.35 and 1.45 g/cm 3 had the highest concentration of inertinite, ranging from 75.6 to 79.5% inertinite. The ash yields of the inertinite-rich fractions were greater or similar to those of the original coals, but the vitrinite-rich fractions had very low ash yield. 2.2. Coking of the maceral-enriched fractions and the original coals Sub-samples of the maceral-enriched fractions and original coals were coked in two stages. The 70 g retort used in the first stage was made of aluminium, which melts at 660 °C. Therefore, the maximum Table 2 Petrographic analyses on a dry, mineral-matter-free basis, and the ash yields from the proximate analyses of the maceral-rich fractions and the original coals. Vitrinite
Liptinite
Inertinite
Ash, db
(vol.%)
(vol.%)
(vol.%)
(wt.%)
Coal B Vitrinite-rich fraction, b1.25 g/cm3 Inertinite-rich fraction, 1.35–1.45 g/cm3 Original coal
95.0 21.0 60.3
1.8 3.4 3.3
3.2 75.6 36.5
0.79 9.7 5.6
Coal C Vitrinite-rich fraction, b1.30 g/cm3 Inertinite-rich fraction, 1.35–1.45 g/cm3 Original coal
86.1 16.5 52.9
1.5 3.9 2.8
12.4 79.5 44.3
2.1 9.7 7.7
Coal F Vitrinite-rich fraction, b1.27 g/cm3 Inertinite-rich fraction, 1.35–1.45 g/cm3 Original coal
96.6 22.9 58.9
0 0 0
3.4 77.1 41.1
1.2 10.1 7.0
Coal G Vitrinite-rich fraction, b1.30 g/cm3 Inertinite-rich fraction, 1.35–1.40 g/cm3 Original coal
84.5 21.3 34.1
0 0 0
15.5 78.7 65.9
2.5 9.7 9.8
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
2.3. Reactivity test The cokes were crushed to less than 1 mm and screened at 0.6 mm. The 0.6–1.0 mm fraction was used to carry out the reactivity test. The reactivity test procedure has been described elsewhere (Grigore et al., 2006). Sub-samples weighing approximately 1.4 g were reacted with 100% CO2 in a fixed-bed reactor to a carbon conversion level of 15%. The reactivity measurements were made under conditions of chemical rate control (Regime I), free of any physical limitations due to gas pore diffusion and mass transfer. The verification of the ‘Regime I’ conditions has been described elsewhere (Grigore et al., 2009). The flow rate of the reactant gas was 750 ml/ min. The reaction temperature was selected on the basis of the reactivity of the samples, so that the percentage of carbon monoxide in the outlet gas did not exceed 1%. Such low concentrations of carbon monoxide had negligible inhibition effect on the reaction rate (Harris and Smith, 1989; Turkdogan and Vinters, 1970). For these samples, the reaction temperatures were between 855 °C and 934 °C for the most reactive and the least reactive samples, respectively. The activation energies were measured to provide information about reaction conditions and to normalise the reaction rates during the experiment to 900 °C. The apparent reaction rate is expressed in grams of carbon reacted per gram of carbon remaining per second. In the calculation, the sample mass is ash-free. 2.4. Sample characterisation The inert maceral-derived component in the cokes prepared from the inertinite- and vitrinite-rich fractions and original coals in this work was unfused inertinite. Petrographic analysis was used to determine the abundance of the inert maceral-derived component in the cokes (Table 3). The component that had the original cell structure preserved, with no indications of being fused or softened and
Table 3 Abundance of the inert maceral-derived component in the coked vitrinite- and inertinite-rich fractions and cokes prepared from original coals. Inert maceral-derived component (vol.%)
B
C
F
G
Coked vitrinite-rich fraction Coke Coked inertinite-rich fraction
3.0 36.3 68.0
12.0 43.5 71.0
2.7 35.2 56.4
12.3 45.2 56.0
did not display anisotropy, was classified as the inert maceralderived component. Micropore surface area was measured using carbon dioxide at 0 °C. The Dubinin–Radushkevich equation was used to determine the micropore surface area of the cokes from carbon dioxide isotherms. The X-ray diffraction (XRD) patterns were used to measure carbon crystallite size (Lc – average crystallite height and La – average crystallite width). The American Standard Test Method D 5187–91 was the procedure used to determine Lc. A similar procedure was used to determine La. This technique allows measuring the carbon crystallite height – Lc (002 band; 2θ ~ 30°) and width – La (100 band; 2θ ~ 51°). The crystallite size was calculated using the Scherrer equation (Cullity, 1978). The mineral matter characterisation was carried out on samples that underwent low-temperature ashing (approximately 120 °C) using radio-frequency oxygen plasma ashing (low-temperature ashing, LTA) (Gluskoter, 1965). This procedure enables carbon removal with minimal alteration of the mineral species. The samples subjected to low temperature ashing were then analysed using XRD. The XRD patterns were used for identification and quantification of the mineral phases using Bruker Eva search/match software and SIROQUANT™ (Taylor, 1991), a personal computer quantitative XRD analysis software package. 3. Results and discussion The reactivity data show that the coked inertinite-rich fractions were consistently more reactive than the coked vitrinite-rich fractions (Fig. 1). The initial apparent rates of the coked inertinite-rich fractions (3.1–19.5 * 10 − 6 gg − 1 s − 1) varied to a greater extent than those of the coked vitrinite-rich fractions (0.63–3.4 * 10 − 6 gg − 1 s − 1). Although the coked inertinite-rich fractions were more reactive than the coked vitrinite-rich fractions, the relationship between the initial apparent rate and the concentration of the inert maceralderived component is poor when all of the cokes were considered (Fig. 2). This indicates that there is significant variation between the properties of the different maceral-derived components among the cokes prepared from different coal sources. However, the reactivity of the cokes increased approximately linearly with increasing amount
Initial apparent rate (*10-6gg-1s-1)
temperature of carbonisation was set at 470 °C. The semi-coke prepared in this first stage was subsequently calcined at 1050 °C in a horizontal tube furnace. Sub-samples of 70 g were packed into a retort at a bulk density in the range of 0.68 to 0.80 g/cm 3. The 70 g retort was introduced in an oven preheated to 300 °C. The oven was continually purged with nitrogen to avoid the oxidation of the sample. After the temperature in the centre of the charge reached 300 °C, the coal sample was heated to 470 °C at a constant rate and held at this temperature for 2 h. Then the semi-coke produced was cooled under flowing nitrogen. The heating rate selection was based on the plastic properties of the macerals. The vitrinite-rich fractions swelled more than the coals and inertinite-rich fractions, leading to high porosity in the carbonization product. Therefore, to control the porosity in the coke, the heating rate of the vitrinite-rich fractions had to be lower than the heating rate of the coals and the inertinite-rich fractions. The heating rate from 300 to 470 °C was 1 °C per minute for the original coals and the inertinite-rich fractions. The vitrinite-rich fractions of coals B, C, and F were carbonized at 0.1 °C per minute. The vitrinite-rich fraction of coal G required an even lower heating rate of 0.05 °C per minute to prevent swelling. The semi-coke samples produced in the 70 g retort were loaded in alumina boats and inserted into a horizontal tube furnace at ambient temperature. The furnace was purged with high-purity nitrogen. The furnace was heated to 500 °C at a heating rate of 1 °C per minute, then at 10 °C per minute from 500 °C to 1050 °C. The purge gas was changed from high-purity nitrogen to ultra high-purity argon after the temperature reached 500 °C. At the completion of the calcination stage, the samples were cooled down inside the furnace under ultra high-purity argon. Below 500 °C, the purge gas was changed back to high-purity nitrogen.
3
25 20 coked vitrinite-rich fraction
15 coke
10 coked inertinite-rich fraction
5 0
B
C
F
G
Fig. 1. The initial apparent reaction rates of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals.
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
14
25 20
Mineral matter (wt,%)
Initial apparent rate (*10-6gg-1s-1)
4
B
15
C
10
F
5
G
12 10
B
8
C
6
F
4
G
2 0
0 0
20
40
60
80
0
20
40
60
80
Inert maceral-derived component (vol,%)
Inert maceral-derived component (vol,%)
Fig. 3. Concentration of mineral matter in the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals against concentration of the inert maceral-derived component.
Fig. 2. Initial apparent rate of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus concentration of the inert maceral-derived component.
The XRD analyses of the low temperature ashes show that the mineralogical composition of the coked inertinite-rich fractions is generally similar to that of their corresponding cokes (Table 4). But the coked vitrinite-rich fractions contain a larger variety of mineral phases than the coked inertinite-rich fractions, particularly coked vitrinite-rich fractions B and G (Table 4). The formation of the new mineral phases in the coked vitriniterich fractions is thought to be due to the slower heating rate used to prepare them (the final coking temperature was similar for all cokes). This may have led to the formation of strongly reduced phases such as aluminium nitride and silicon carbide. Additionally, fayalite, hercynite, gehlenite and tridymite were found only
of inert maceral-derived component when the cokes from each coal source were considered separately. This suggests that their reactivity is mainly controlled by the abundance of inert derived-maceral component. 3.1. Mineral matter The coked inertinite-rich fractions had a greater ash yield than the coked vitrinite-rich fractions (Table 4). The concentration of mineral matter increases as the amount of non-fused inertinite increases (Fig. 3).
Table 4 Mineral phases identified in the low temperature ash of the cokes and their relative concentrations (wt.%). Mineral phase
Chemical formula
Coal source B Coke-V
LTA (%) Akermanite Diopside Gehlenite Fluorapatite Oldhamite Bassanite Iron Hematite Magnetite Wustite Pyrrhotite Troilite Jarosite Fayalite Hercynite Iron phosphate Ferrosilicon Mullite Spinel Kalsilite Leucite γ-Alumina Aluminium nitride Quartz Cristobalite Tridymite Silicon carbide Brookite Anatase Rutile Tungsten carbide Titanium carbide Amorphous
Ca2MgSi2O7 CaMgSi2O6 Ca2Al2SiO7 Ca5(PO4)3F CaS CaSO4 × 0.5H2O Fe Fe2O3 Fe3O4 FeO Fe1-xS FeS (K,H3O)Fe3(SO4)2(OH)6 (Fe,Mg)2SiO4 FeAl2O4 FePO4 Fe3Si Al6Si2O13 MgAl2O4 KAlSiO4 KAlSi2O6 Al2O3 AlN SiO2 SiO2 SiO2 SiC TiO2 TiO2 TiO2 WC TiC
Coke-I
Coal source C Coke
1.9 0.1 0.1 1.0 3.2 0.3 0.7 0.1
12.3 0.1 0.5
8.6 0.2 0.4
0.5 0.1 0.2
1.6 0.1 0.3 0.1
0.1
0.2
0.2 0.1
Coke-V 4.0 0.4 0.8 0.6 5.3 0.3 0.5
Coal source F
Coke-I
Coke
13.9
11.0
Coke-V 1.7
2.1 0.1 0.6 0.1
2.5 0.1 0.6 0.3
0.3 5.3 0.3 0.4 0.1 0.2
0.2
0.2
0.3
0.2 0.2 2.5 0.6
0.3
0.1
8.0 0.9
6.6 1.0
0.3
0.3 3.2
0.9 12.2 0.1 0.2 0.5 0.6 0.1 15.0 6.0 54.0
9.4
0.4
0.4
1.6 0.1 0.3 0.1
2.8 0.3 1.3 0.7
0.2 0.2 9.7 1.3
0.2 0.9 9.4 1.4
0.2 0.2 7.0 0.4
0.3 1.8
0.2
0.6
0.8
24.5
27.9
15.7 0.1
16.8
16.0
17.7 0.1
31.3 0.2
33.1 0.1
0.4 0.1
0.8 0.1
0.7
0.7
0.7 0.4
0.4
0.3
57.5
63.3
67.7
4.0 0.3 0.7 2.8 0.1 0.2 0.1
1.8 0.2
6.3
0.2 0.2 5.4 1.3
0.2 2.1 6.4 1.8
0.7 0.1 5.7
3.4
8.9 1.0 52.4
3.4
Coke-V
Coke-I
Coke
13.6 0.2
12.7 0.2
2.3 0.2 0.4 0.2 0.2
2.6 0.2 0.2 0.4
0.1
1.7 0.6
60.3
1.1
Coke
13.2
0.1 0.1
0.2 0.5 0.6
Coke-I
Coal source G
0.2 0.5 5.5 2.0 0.6
6.4 0.9 58.3
Coke-V – coked vitrinite-rich fraction; Coke-I – coked inertinite-rich fraction; Coke – coke made from the original coal.
48.6
45.6
0.5 0.4 0.3 0.1 8.3 1.0 0.3 0.1 1.2 1.0 1.9 0.1 0.2 0.2 0.6 0.2 0.3 6.3 0.7 71.9
0.2
1.2
0.4
0.3 0.1 16.2 0.4
0.5 16.4 1.5
0.3 3.7
0.4 2.1
3.2 0.1
3.3 0.1
0.6 0.3 3.4 70.9
67.5
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
5
Table 5 The concentration of the mineral phases present in cokes, expressed on a coke basis (wt.%). Mineral phase
Akermanite Diopside Gehlenite Fluorapatite Oldhamite Iron Hematite Magnetite Wustite Pyrrhotite Troilite Fayalite Hercynite Iron phosphate Ferrosilicon Mullite Spinel Kalsilite Leucite γ-Alumina Aluminium nitride Quartz Cristobalite Tridymite Silicon carbide Brookite Anatase Rutile Amorphous
Chemical formula
Coal source B Coke-V
Coke-I
Coke
Coke-V
Ca2MgSi2O7 CaMgSi2O6 Ca2Al2SiO7 Ca5(PO4)3F CaS Fe Fe2O3 Fe3O4 FeO Fe1-xS FeS (Fe,Mg)2SiO4 FeAl2O4 FePO4 Fe3Si Al6Si2O13 MgAl2O4 KAlSiO4 KAlSi2O6 Al2O3 AlN SiO2 SiO2 SiO2 SiC TiO2 TiO2 TiO2
0.002 0.002 0.024 0.078 0.016 0.002
0.012 0.062
0.017 0.035
0.062 0.025
0.139 0.022 0.009
0.018 0.036 0.027 0.240 0.025
0.002
0.025
0.017 0.009 0.017
0.002
0.007
Coal source C
0.015 0.037
0.009
0.988 0.111
0.573 0.087
0.007
0.037 0.395
1.31
Coke
0.301 0.057 0.014
0.284 0.045 0.034
0.029
0.023
0.114
0.068
0.027
0.005 0.005 0.061 0.015
0.022 0.296 0.002 0.005 0.012 0.015 0.002
0.014
Coal source F
Coke-I
Coke-V
0.006 0.099 0.009 0.002 0.004
Coal source G
Coke-I
Coke
0.056
0.040
0.223 0.035 0.014
0.278 0.094 0.070
0.006
0.029 0.129 1.348 0.201
0.023 0.023 0.794 0.045
0.026 0.156
0.009
0.086
0.091
3.027
2.420
0.711 0.005
2.410
1.815
0.330 0.002
4.365 0.028
3.291 0.010
0.049 0.012
0.069 0.009
0.100
0.079
0.013 0.007
0.056
0.030
7.45
4.99
1.09
6.78
7.68
Coke
0.013
0.027
0.026
0.316 0.055 0.027 0.027 0.014
0.343 0.040 0.053
0.029
0.009
0.041 0.014 2.225 0.055
0.066
0.041 0.508
0.053 0.277
0.440 0.014
0.436 0.013
0.030 0.121 0.009 0.004
0.169 0.010
0.004
9.08
Coke-I
0.004 0.014 0.168
0.009 0.009 0.439 0.059
2.37
Coke-V
0.004 0.004 0.101 0.024
0.028 0.293 0.893 0.251
0.020 0.050 0.547 0.199 0.060
4.53
0.022 0.017 0.013 0.004 0.358 0.043 0.013 0.004 0.052 0.043 0.082 0.004 0.009 0.009 0.026 0.009 0.013 3.10
2.166 0.198
0.079 0.041 9.74
8.91
Coke-V – coked vitrinite-rich fraction; Coke-I – coked inertinite-rich fraction; Coke – coke made from the original coal.
(fayalite, hercynite, ferrosilicon and iron phosphate) along with the calcium and potassium bearing minerals (akermanite, diopside, gehlenite, fluorapatite, oldhamite, kalsilite and leucite) are either not catalysts or not known to catalyse gasification. Additionally, any iron, calcium, potassium and sodium in the amorphous phase formed due to clay decomposition, is not known to catalyse gasification (Kerkkonen et al., 1996; Lang and Neavel, 1982). Hence, using total iron, calcium, potassium and sodium from ash chemistry to assess the influence of the catalytic mineral phases on gasification is not appropriate. The catalytic mineral phases must be identified and quantified in order to assess their influence on gasification. The total iron present in the catalytic mineral phases was calculated as mmol fraction of element per 1 kg coke (Table 6). The coked vitrinite-rich fractions had the lowest concentrations of catalytic iron. The abundance of catalytic iron in the coked inertinite-rich fractions was greater than that in their corresponding cokes made from the original coal, with the exception of coked inertinite-rich fraction B, which had less catalytic iron than coke B. Although a strong relationship was observed between the mineral matter yield and the concentration of non-fused inertinite (Fig. 3), the abundance of catalytic mineral matter is not related to the concentration
in the coked vitrinite-rich fractions. Although aluminium nitride generally occurs at higher temperatures than 1050 °C, it may have been formed by reaction of γ-alumina with nitrogen in the presence of carbon (nitrogen was the purging gas of the horizontal tube furnace used to calcine the semi-coke samples). Silica reduction by carbon in cokes can occur at temperatures as low as 1100 °C (Van der Velden et al., 2002). These reduced phases have not been previously found in cokes prepared at such low temperatures. Some samples were contaminated during sample preparation. The small amounts of tungsten carbide (WC) and titanium carbide (TiC) found in some coked vitrinite-rich fractions, are contaminants that arose during cutting of the cokes using abrasive disc blades. Additionally, oxidation artefacts of the low temperature ashing process were found in all cokes, namely bassanite and jarosite (Grigore et al., 2006; Ward, 2002). Hence, the mineralogical composition of the cokes was recalculated by proportioning the sulfates to oldhamite and pyrrhotite, and eliminating tungsten and titanium carbides from the calculation. Table 5 shows the recalculated mineralogical composition on a coke basis. The catalytic mineral phases identified in the cokes are metallic iron, pyrrhotite and iron oxides. The other iron-bearing minerals
Table 6 Deportment of total iron present in the mineral phases known to catalyse gasification (Fe, Fe1-xS, Fe2O3, Fe3O4 and FeO), expressed as mmol fraction of iron per 1 kg coke. Mineral phase
Chemical formula
Coal source B
Iron Hematite Magnetite Wustite Pyrrhotite and troilite Total
Fe Fe2O3 Fe3O4 FeO Fe1-xS
0.44
Coke-V
0.32 0.72 1.5
Coke-I
3.2 2.2 5.4
Coal source C Coke
Coke-V
Coal source F
Coke-I
Coke
1.5
2.5
5.9
2.2 1.2 5.7 10.6
3.6
2.9
4.5 4.5
31.5 37.6
22.4 31.2
Coke-V 0.33 0.47
Coal source G
Coke-I
Coke
Coke-V
2.4
11.8
0.79
Coke-I
Coke
4.9 3.4
9.3
0.57 2.0 2.8
Coke-V – coked vitrinite-rich fraction; Coke-I – coked inertinite-rich fraction; Coke – coke made from the original coal.
1.8 51.3 55.5
40.7 52.5
1.4
3.5 9.8 18.1
3.1 15.9
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M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
Catalytic iron (mmol/1 kg coke)
60
Table 7 Micropore surface areas of the raw coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals.
50 B 40 C 30 F 20
Coal source B C F G
Surface area (m2g− 1) Coked inertinite-rich fraction
Coke
Coked vitrinite-rich fraction
19.0 17.0 30.7 11.8
10.4 11.0 14.1 11.1
4.1 8.0 6.5 7.3
G 10 0 0
20
40
60
80
Inert maceral-derived component (vol,%) Fig. 4. Concentration of catalytic iron in cokes versus concentration of the inert maceral-derived component.
of non-fused inertinite (Fig. 4). This indicates that the concentration of catalytic mineral phases can vary significantly between cokes containing similar amounts of non-fused inertinite. Fig. 5 shows good agreement between the initial apparent rate and the concentration of catalytic iron. The scatter could be due to differences in particle size of catalytic iron or the degree of dispersion of the catalyst (Gopalakrishnan and Bartholomew, 1996; Tomita, 2001). Lindert and Timmer (1991) and Tanaka et al. (1995) observed an increase in coke reactivity to carbon dioxide as the dispersion of metallic iron increased. Additionally, Grigore et al. (2009) observed that the reaction rate is sensitive to the size of catalyst particle; finer catalyst particles enhance coke gasification more than coarser particles. The relationship between reactivity and the abundance of the catalytic phases that we found would not be significantly affected by the different heating rates used in preparation of the cokes. The amounts of catalytic iron phases in the coked vitrinite-rich fractions were low compared to those in the cokes prepared from the inertinite-rich fractions and original coals, and the inclusion of the iron present in fayalite and hercynite, mineral phases that may have formed at the expense of the catalytic iron phases, would not significantly increase the total amount of catalytic iron. These results establish that the abundance of catalysts is one dominant factor that makes the inert maceral-derived component in cokes more reactive than the reactive maceral-derived component, and accounts for the variation in reactivity between the inert maceralderived components in cokes made from different coals. Since micropore surface area and carbon crystallite size have also been reported as factors affecting the reactivity of cokes prepared
from whole coals (Duval et al., 1988; Vogt et al., 1991; Zamalloa et al., 1995), we investigated their importance on the initial reaction rates of the cokes prepared from the vitrinite- and inertinite-rich fractions and original coals, as well. 3.2. Micropore surface area The micropore surface area of the coked vitrinite-rich fractions was significantly lower than that of their corresponding coked inertiniterich fractions (Table 7). This observation is consistent with the findings of Huang et al. (1991). Micropore surface area generally increased with increasing the amount of inert maceral-derived component in cokes (Fig. 6). This suggests that the micropore network in the inert maceral-derived component is more extensively developed than that in the reactive maceral-derived component. The good relationship observed between the micropore surface area and the amount of inert maceralderived component in cokes indicates that the different heating rates used for preparation of the cokes did not affect significantly microporosity of the cokes. Surface area plays an important role when the reaction rate is chemically controlled (Zamalloa et al., 1995). The reactivity of the cokes increases as their surface area increases (Vogt et al., 1991). Fig. 7 shows a good correlation between the initial apparent rate and the micropore surface area. This confirms that the more extensive micropore network of the inert maceral-derived component increases its reactivity compared to that of the reactive maceral-derived component as a greater surface area is exposed to gasification. The initial apparent rate was divided by micropore surface area to obtain the initial intrinsic rate (Harris and Smith, 1989). If micropore surface area is the only factor that controls the reaction rate, then all cokes would have similar intrinsic rates. The intrinsic rates of the coked inertinite-rich fractions were greater than those of the coked vitrinite-rich fractions (Fig. 8). This confirms that micropore surface area is not the only factor that makes the inert maceral-derived component more reactive than the reactive maceral-derived component.
30
25 20 coked inertinite-rich fraction
15 coke
10 coked vitrinite-rich fraction
5
Surface area (m2g-1)
Initial appatent rate (*10-6 gg-1s-1)
35 B
25 C 20 15
F
10
G
5 0 0
20
40
60
80
Inert maceral-derived component (vol,%)
0 0
20
40
60
Catalytic iron (mmol/1 kg coke) Fig. 5. The initial apparent rate versus concentration of catalytic iron.
Fig. 6. Surface area of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus concentration of the inert maceralderived component.
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
7
Table 8 The average crystallite height and width of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals.
25 20 coked inertinite-rich fraction
Coal source
B
C
F
G
coke
Lc (nm) Coked inertinite-rich fraction Coke Coked vitrinite-rich fraction
1.22 1.36 1.39
1.21 1.37 1.37
1.35 1.54 1.72
1.35 1.40 1.63
La (nm) Coked inertinite-rich fraction Coke Coked vitrinite-rich fraction
3.41 3.85 3.41
3.37 3.96 3.57
4.09 3.76 3.40
3.33 3.53 3.80
15 10 coked vitrinite-rich fraction
5 0 0
10
20
30
40
Fig. 7. Micropore surface areas of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus initial apparent rate.
3.3. Carbon crystallinity The average crystallite height and width of the coked inertiniteand vitrinite-rich fractions were investigated in this study, since increasing the degree of ordering of carbon structure is reported to lead to a lower reactivity (Duval et al., 1988). It has also been reported that the gasification reaction occurs mainly at the edges of the crystallites rather than on the basal planes (Rouzaud et al., 1988; Walker et al., 1959). Since the density of the free edges increases as the La decreases, cokes with a greater La should have a reduced reactivity. The coked inertinite-rich fractions have lower Lc than their corresponding coked vitrinite-rich fractions (Table 8), indicating less ordering of the carbon structure. This observation is consistent with Huang et al. (1991) findings. It needs to be noted that the vitriniterich fractions were coked at lower heating rate than the inertiniterich fractions. Heating rate is one of the factors that affect Lc. The Lc increases as the heating rate increases (Rouzaud et al., 1988). If the vitrinite-rich fractions would have been coked at a similar heating rate to that used for inertinite-rich fractions then their Lc would be even greater than those reported in this study. The average crystallite heights of the coked vitrinite-rich fractions vary over a wider range than those of the coked inertinite-rich fractions. The coked vitrinite-rich fractions with smaller Lc (B and C) were prepared from coals of lower rank than those with larger Lc (F and G). The greater Lc values of coked inertinite-rich fractions F and G compared to those of B and C is probably due to the greater amount of reactive maceral-derived component in these cokes, rather than any variation in the Lc of the inert maceral-derived component. Fig. 9a shows a good agreement between Lc and the concentration of inert maceral-derived component for the cokes prepared from coals F and G. Since rank of the parent coal had stronger influence on the Lc of the reactive maceral-derived component but not on that of the inert maceral-derived component, the relationship between
Lc and the concentration of inert maceral-derived component becomes more obvious when the cokes are prepared from coals of higher rank. Unlike Lc, there were only small variations in La of the cokes (Table 8). No consistent trend was observed between La and the concentration of the inert maceral-derived component or rank of the parent coal (Fig. 9b). There is no obvious relationship between Lc of the cokes and the reaction rate (Fig. 10). The average crystallite heights of the coked inertinite-rich fractions were not expected to have a strong influence on their reaction rates, since Lc values did not vary greatly between the fractions. Although the Lc values of the coked vitrinite-rich fractions might have been expected to demonstrate this correlation since they exhibited greater variability, as shown in Table 8, no such relationship was found. Our observations are not consistent with the conclusions drawn by Huang et al. (1991) who attributed the lower reactivity of the carbonised vitrinite-rich fractions compared to that of the carbonised
(a) 1.8 1.7
B
1.6 C
1.5 1.4
F
1.3 1.2
G
1.1 0
20
40
60
80
Inert maceral-derived component (vol,%)
(b) 4.4 4.2
1.0
B
4.0
0.8
C
3.8 coked vitrinite-rich fractions
0.6
3.6
F
3.4 coke
0.4
G
3.2 coked inertinite-rich fractions
0.2
3.0 0
20
40
60
80
Inert maceral-derived component (vol,%)
0.0 B
C
F
G
Fig. 8. Initial intrinsic rates of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals.
Fig. 9. (a) Average crystallite height and (b) average crystallite width of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals against concentration of the inert maceral-derived component.
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M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
25 20 coked inertinite-rich fractions
15 coke
10 coked vitrinite-rich fractions
5 0 1.0
1.2
1.4
1.6
inertinite-rich fractions to their greater degree of ordering of the carbon structure. Although we also found the degree of ordering of the carbon structure of the coked inertinite-rich fractions to be lower than that of the coked vitrinite-rich fractions, the degree of ordering of the carbon structure does not appear to influence the initial apparent rate of the carbonised vitrinite- and inertinite-rich fractions from our study (Fig. 10). The initial apparent rate of the coked vitrinite- and inertinite-rich fractions was not related to La. This is not unexpected since the La of the inert maceral-derived component and reactive maceral-derived component was not determined by the type of macerals and their abundance in the parent coal. We concluded that the reaction rate is not controlled by either Lc or La at the initial stages. 3.4. Statistical analysis The reactivity of the coke was affected by both amount of catalytic matter and micropore surface area. Since both factors increased with the amount of inert maceral-derived component they correlated to each other as well. In order to separate out the contribution of each to reactivity, a multiple regression was performed on the data. The regression equation found was Initial apparent rate ¼ 0:143ð0:044ÞFe þ 0:46ð0:12ÞSurface area 2:0ð1:2Þ
ð1Þ
The r 2 correlation coefficient is 0.91. Fig. 11 shows the relationship between the predicted initial apparent rate from Eq. (1) and the
25
20
15
10
5
0 5
10
15
4. Conclusions From a study of the reactivity of cokes made from inertinite- and vitrinite-rich fractions and original samples of four Australian bituminous coals, it was found that:
1.8
Fig. 10. Initial apparent rates of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus the average crystallite height.
0
measured initial apparent rate. It can be seen that the contribution of both amount of catalytic matter and micropore surface area to reactivity is positive and statistically significant: both contribute independently to the initial reactivity of the coke. Introducing Lc and La as independent parameters did not significantly improve this relationship suggesting that their influence on reactivity is small. This confirms the findings above.
20
25
Fig. 11. Initial apparent rates of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus that predicted from Eq. (1).
• The reactivities of the coked inertinite-rich fractions were consistently greater than those of the coked vitrinite-rich fractions. The major factors that make the inert maceral-derived component more reactive than the reactive maceral-derived component in cokes are the concentration of catalytic iron phases and micropore surface area. • The catalytic mineral phases identified in the coked inertinite- and vitrinite-rich fractions are metallic iron, pyrrhotite, troilite, wustite, magnetite and hematite. Iron also occurred in mineral phases that do not catalyse gasification, such as fayalite, hercynite, ferrosilicon and iron phosphate. No catalytic calcium-, potassium- and sodiumbearing mineral phases have been identified in any coke. • The Lc of coke was related to maceral composition and coal rank of the parent coal, whereas La was not related to any of these properties. Although the degree of ordering of carbon structure (Lc) of the reactive maceral-derived component is greater than that of the inert maceral-derived component, it was not found to be a major factor controlling coke reactivity at the initial stages.
Acknowledgments 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 Centres Program of the Commonwealth Government of Australia. References Cullity, B.D., 1978. Elements of X-Ray Diffraction. Addison-Wesley Publishing Company INC. Czechowski, F., Kidawa, H., 1991. Reactivity and susceptibility to porosity development of coal maceral chars on steam and carbon dioxide gasification. Fuel Processing Technology 29, 57–73. Duval, B., Guet, J.M., Richard, J.R., Rouzaud, J.N., 1988. Coke properties and their microtexture. Part III: First results about relationship between microtexture and reactivity of some cokes. Fuel Processing Technology 20, 163–175. Gluskoter, H.J., 1965. Electronic low temperature ashing of bituminous coal. Fuel 44, 285–291. Gopalakrishnan, R., Bartholomew, C.H., 1996. Effects of CaO, high-temperature treatment, carbon structure, and coal rank on intrinsic char oxidation rates. Energy & Fuels 10, 689–695. Grigore, M., Sakurovs, R., French, D., Sahajwalla, V., 2006. Influence of mineral matter on coke reactivity with carbon dioxide. ISIJ International 46, 503. Grigore, M., Sakurovs, R., French, D., Sahajwalla, V., 2009. Coke gasification: the influence and behaviour of inherent catalytic mineral matter. Energy & Fuels 23, 2075–2085. Harris, D.J., Smith, I.W., 1989. Intrinsic reactivity of coke and char to carbon dioxide. 197th ACS National Meeting (preprints of papers), 34, pp. 94–101. Huang, Y.H., Yamashita, H., Tomita, A., 1991. Gasification reactivities of coal macerals. Fuel Precessing Technology 29, 75–84. Kerkkonen, O., Mattila, E., Heiniemi, R., 1996. The correlation between reactivity and ash mineralogy of coke. Ironmaking Conference Proceedings 55, 275–281. Koba, K., Ida, S., 1980. Gasification reactivities of metallurgical cokes with carbon dioxide, steam and their mixtures. Fuel 59, 59–63. Lang, R.J., Neavel, R.C., 1982. Behaviour of calcium as a steam gasification catalyst. Fuel 61, 620–626. Lindert, M., Timmer, R.M.C., 1991. An analysis of the Japanese reactivity and CSR of plant coke and the corresponding pilot oven coke. Ironmaking Conference Proceedings 50, 233–237.
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9 Rouzaud, J.N., Vogt, D., Oberlin, A., 1988. Coke properties and their microtexture; Part I: Microtextural analysis: a guide for cokemaking. Fuel Precessing Technology 20, 143–154. Sakawa, M., Sakurai, Y., Hara, Y., 1982. Influence of coal characteristics on CO2 gasification. Fuel 61, 717–720. Sun, Q., Li, W., Chen, H., Li, B., 2004. The CO2-gasification and kinetics of Shenmu maceral chars with and without catalyst. Fuel 83, 1787–1793. Tanaka, S., U-emura, T., Ishizaki, K., Nagayoshi, K., Ikenaga, N., Ohme, H., Suzuki, T., Yamashita, H., Ampo, M., 1995. CO2 gasification of iron-loaded carbons: activation of the iron catalyst with CO. Energy & Fuels 9, 45–52. Taylor, J.C., 1991. Computer programs for standardless quantitative analysis of minerals using the full powder diffraction profile. Powder Diffraction 6, 2–9. Tomita, A., 2001. Catalysis of carbon-gas reactions. Catalysis Surveys from Japan 5, 17–24. Turkdogan, E.T., Vinters, J.V., 1970. Effect of carbon monoxide on the rate of oxidation of charcoal, graphite and coke in carbon dioxide. Carbon 8, 39–53. Turkdogan, E.T., Vinters, J.V., 1972. Catalytic oxidation of carbon. Carbon 10, 97–111. van der Velden, B., Trouw, J., Chaigneau, R., Van den Berg, J., 1999. Coke reactivity under simulated blast furnace conditions. Ironmaking Conference Proceedings 58, 275–285. Van der Velden, B., Trouw, J., Atkinson, C., Bol, L.C.G.M., 2002. High temperature properties of coke, critical for the lower part of blast furnace. Ironmaking Conference Proceedings, CD.
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van Krevelen, D.W., 1993. Coal: typology, physics, chemistry, constitution. Elsevier Scientific Publishing Company. Vander, T., Alvarez, R., Ferraro, M., Fohl, J., Hofherr, K., Huart, J.M., Mattila, E., Propson, R., Willmers, R., der Velden, V., 1996. Coke quality improvement. Possibilities and limitations: 3rd European Ironmaking Congress Proceedings, pp. 28–37. Vandezande, J.A., 1982. Correlation of coke microstructure and properties. Ironmaking Conference Proceedings 41, 12–23. Vogt, D., Weber, J.V., Rouzaud, J.N., Schneider, M., 1988. Coke proprieties and their microstructure Part II: coke carboxyreactivity: relations to their texture. Fuel Processing Technology 20, 155–162. Vogt, D., Duchene, J.M., Rouzaud, J.N., Isler, D., 1991. Coke reactivity: influence of texture importance of weight loss on mechanical strength. Ironmaking Conference Proceedings 50, 225–231. Walker Jr., P.L., Rusinko, F., Austin, L.G., 1959. Gas reactions of carbon. Advances in Catalysis 11, 133–221. Walker Jr., P.L., Shelef, M., Anderson, R.A., 1968. Catalysis of carbon gasification. In: Walker Jr., P.L. (Ed.), Chemistry and Physics of Carbon. Marcel Dekker Inc., New York, pp. 287–383. Ward, C.R., 2002. Analysis and significance of mineral matter in coal seams. International Journal of Coal Geology 50, 135–168. Zamalloa, M., Ma, D., Utigard, T.A., 1995. Oxidation rates of industrial cokes with CO2 and air. ISIJ International 35, 458–463.
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International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo
Properties and CO2 reactivity of the inert and reactive maceral-derived components in cokes Mihaela Grigore a,⁎, Richard Sakurovs a, David French a, Veena Sahajwalla b a b
The Commonwealth Scientific and Industrial Research Organisation (CSIRO), Energy Technology, 11 Julius Avenue, North Ryde 2113, Australia School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e
i n f o
Article history: Received 15 February 2012 Received in revised form 4 April 2012 Accepted 5 April 2012 Available online 12 April 2012 Keywords: Gasification Catalytic mineral phases X-ray diffraction SIROQUANT
a b s t r a c t The response of coke to gasification affects its degradation in the blast furnace. Coke gasification with carbon dioxide at high temperature is selective, with the inert maceral-derived component reacting more readily than the reactive maceral-derived component. Previous reactivity studies on carbonised vitrinite- and inertinite-rich fractions indicated that the amount of catalytic mineral phases control their reactivity. However, total Fe, Ca, K and Na from the ash chemistry were used as indicators of the abundance of catalytic material rather than the concentration of the catalytic mineral phases. Additionally, there is disagreement regarding the influence of micropore surface area and average carbon crystallite size on reactivity of the inert maceral-derived component and reactive maceral-derived component. Here we examine the influence of the abundance of the catalytic mineral phases on the reactivity of the inert maceral-derived component and reactive maceral-derived component, and also the influences of micropore surface area and average carbon crystallite size. Cokes from inertinite- and vitrinite-rich fractions prepared from four Australian bituminous coals were reacted with carbon dioxide at temperatures between 855 °C and 934 °C. The major factors that make inert maceral-derived component more reactive than reactive maceral-derived component at the initial stages were found to be the concentration of catalytic mineral phases and micropore surface area. The catalytic mineral phases identified in the coked inertinite- and vitrinite-rich fractions were metallic iron, pyrrhotite, troilite, wustite, magnetite and hematite. No Ca, K and Na catalytic mineral phases were identified in any of the studied cokes. The average crystallite height was not found to be a major factor controlling coke reactivity at the initial stages. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Any improvement of the blast furnace efficiency operated under current technologies and any implementation of new technologies will make new demands on coke quality. In order to prepare coke of suitable quality and to address these requirements, a better understanding of the factors that affect coke degradation in the furnace is required, in particular, coke gasification. Coke gasification with carbon dioxide at high temperature (over 800 °C) shows selective attack, with the isotropic microtexture reacting more readily than the anisotropic microtexture (Duval et al., 1988; Kerkkonen et al., 1996; Koba and Ida, 1980; van der Velden et al., 1999; Vander et al., 1996; Vandezande, 1982; Vogt et al., 1988). The anisotropic microtexture forms from the coal macerals that fuse during coking, such as vitrinite, liptinite, and a part of the inertinite. Inertinite
⁎ Corresponding author. Tel.: + 61 2 9490 5321; fax: + 60 2 9490 8530. E-mail address: [email protected] (M. Grigore). 0166-5162/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2012.04.004
that does not fuse yields the isotropic microtexture. The isotropic and anisotropic microtextures in the coke are also referred to as the inert maceral-derived component and the reactive maceral-derived component, respectively. One way to investigate the contribution of each component to the reactivity of coke is to separate the coal macerals before coking them. Studies on cokes prepared from whole coals found that their reactivity is influenced by surface area (Vogt et al., 1991; Zamalloa et al., 1995), carbon crystallite size (Duval et al., 1988), and catalytic material (Turkdogan and Vinters, 1972; Walker et al., 1968). The influence of these properties on the reactivity of the carbonised vitrinite- and inertinite-rich fractions prepared from a range of bituminous coals with carbon dioxide has been investigated in several studies (Czechowski and Kidawa, 1991; Huang et al., 1991; Sakawa et al., 1982; Sun et al., 2004). Huang et al. (1991), Sun et al. (2004) and Czechowski and Kidawa (1991) used the concentration of calcium, iron, potassium and sodium in the ash to determine the relationship between reactivity and the abundance of catalytic material, whereas Sakawa et al. (1982) used the alkali index as indicator of the relative proportion of the catalytic material in the ash. Huang et al. (1991)
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M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
also investigate the influence of micropore surface area and carbon crystallinity on reactivity. In these studies, the carbonised inertinite-rich fractions were not always more reactive than the carbonised vitrinite-rich fractions. Sakawa et al. (1982) observed that the reactivity of carbonised densityseparated maceral fractions, prepared from coals of different rank, increased with increasing inertinite content of the fraction. The carbonised inertinite-rich fractions of two out of three coals prepared by Huang et al. (1991) were more reactive than their corresponding carbonised vitrinite-rich fractions, whereas the carbonised inertinite-rich fraction of the other coal was less reactive than its corresponding carbonised vitrinite-rich fraction. Additionally, the carbonised vitrinite-rich fractions of the coals used by Sun et al. (2004) and Czechowski and Kidawa (1991) were found to be more reactive than their corresponding carbonised inertinite-rich fractions. The differences in reactivity between the carbonised maceral-rich fractions were believed to be mainly due the catalytic material present in the samples. In these studies, surface area and carbon crystallinity were not considered to play a major role in controlling the reactivity of the carbonised inertinite- and vitrinite-rich fractions. Huang et al. (1991) and Sun et al. (2004) attempted to establish the influence of the catalytic material on the reactivity of the carbonised vitrinite- and inertinite-rich fractions by demineralising the chars or the maceral-rich fractions prior to carbonisation. Demineralisation reduced the reactivity of some of the investigated carbonised maceral-rich fractions. The reactivities of the carbonised vitrinite- and inertinite-rich fractions prepared by Sun et al. (2004) were reduced by demineralisation, but the carbonised vitrinite-rich fraction was still more reactive than the carbonised inertinite-rich fraction. Huang et al. (1991) found in their study that after demineralisation only one carbonised vitrinite-rich fraction has become less reactive. The reactivity of the other carbonised vitrinite- and inertinite-rich fractions did not change significantly after demineralisation. Grigore et al. (2009) have identified and quantified the mineral phases in a series of cokes and shown that not all iron, calcium and potassium crystalline mineral phases present in coke catalyse the gasification reaction. The sodium, potassium, calcium and iron associated with the amorphous phase, which formed during carbonisation from the decomposition of clays, are not known to catalyse gasification, although Kerkkonen et al. (1996) believe that small amounts of these elements may be released during coking to form catalytic materials. Walker et al. (1968) believe that even traces amounts of catalyst (less than 1 ppm) are able to affect the reaction rate. Grigore et al. (2009) found that the initial apparent rate is strongly related to the catalyst abundance. The aim of this study is to determine the relative importance of the amounts of catalytic minerals, micropore surface area and carbon crystallinity in explaining the differences in reactivity between the inert maceral-derived component and the reactive maceral-derived component in cokes. This study is undertaken on coked inertiniteand vitrinite-rich fractions prepared from four bituminous coals. 2. Experimental procedure For this study, four Australian bituminous coals (B, C, F and G) from New South Wales and Queensland were selected based on their differences in rank, mineral matter content and ash chemistry. Coals B and C were lower rank than coals F and G (Table 1). The ash yield of the coals ranged between 5.6 and 9.8 wt.%, and the ash composition varied to a great extent between coals. 2.1. Preparation of maceral-rich fractions The coal macerals were separated using the sink-float method, which is based on the different densities of the macerals: vitrinite density is less than that of inertinite (van Krevelen, 1993). Mixtures
Table 1 Coal rank, proximate and ash analyses of the coals. Coal
B
C
F
Coal rank (%, mean maximum vitrinite reflectance in oil) R0 max. 1.00 1.05
G 1.27
1.29
Proximate analysis (wt.%, air-dried basis) Fixed carbon 65.5 Ash 5.6 Volatiles 28.9 Moisture 2.4
66.1 7.7 26.2 2.5
71.7 7.0 21.3 1.4
70.0 9.8 20.2 1.1
Ash analysis (wt.%) SiO2 Al2O3 Fe2O3 CaO Na2O K2O MgO TiO2 P2O5 Mn3O4 SO3 V2O5 BaO SrO Total
53.6 28.4 7.6 3.0 0.57 1.0 0.95 1.4 1.7 0.05 0.76 0.05 0.15 0.08 99.4
56.9 18.3 12.8 3.7 0.45 0.92 1.6 1.1 1.3 0.06 2.0 0.04 0.09 0.05 99.4
48.3 37.9 5.3 2.5 0.65 0.54 0.58 1.4 1.9 0.03 0.32 0.02 0.22 0.13 99.9
61.4 28.3 4.3 1.3 0.30 0.48 0.34 1.5 0.79 b 0.02 0.26 0.05 0.03 0.04 99.2
of hexane and perchlorethylene in different ratios were used to separate the coal macerals. Prior to density separation, the coals were crushed to less than 1 mm size and screened at 0.45 mm. The fraction 0.45–1.00 mm was used to prepare the maceral-enriched fractions. In this study, the densities of the fractions richest in vitrinite were less than 1.30 g/cm 3 (Table 2). The percentage of vitrinite in these fractions varied from 84.5 to 96.6%. The fractions with densities between 1.35 and 1.45 g/cm 3 had the highest concentration of inertinite, ranging from 75.6 to 79.5% inertinite. The ash yields of the inertinite-rich fractions were greater or similar to those of the original coals, but the vitrinite-rich fractions had very low ash yield. 2.2. Coking of the maceral-enriched fractions and the original coals Sub-samples of the maceral-enriched fractions and original coals were coked in two stages. The 70 g retort used in the first stage was made of aluminium, which melts at 660 °C. Therefore, the maximum Table 2 Petrographic analyses on a dry, mineral-matter-free basis, and the ash yields from the proximate analyses of the maceral-rich fractions and the original coals. Vitrinite
Liptinite
Inertinite
Ash, db
(vol.%)
(vol.%)
(vol.%)
(wt.%)
Coal B Vitrinite-rich fraction, b1.25 g/cm3 Inertinite-rich fraction, 1.35–1.45 g/cm3 Original coal
95.0 21.0 60.3
1.8 3.4 3.3
3.2 75.6 36.5
0.79 9.7 5.6
Coal C Vitrinite-rich fraction, b1.30 g/cm3 Inertinite-rich fraction, 1.35–1.45 g/cm3 Original coal
86.1 16.5 52.9
1.5 3.9 2.8
12.4 79.5 44.3
2.1 9.7 7.7
Coal F Vitrinite-rich fraction, b1.27 g/cm3 Inertinite-rich fraction, 1.35–1.45 g/cm3 Original coal
96.6 22.9 58.9
0 0 0
3.4 77.1 41.1
1.2 10.1 7.0
Coal G Vitrinite-rich fraction, b1.30 g/cm3 Inertinite-rich fraction, 1.35–1.40 g/cm3 Original coal
84.5 21.3 34.1
0 0 0
15.5 78.7 65.9
2.5 9.7 9.8
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
2.3. Reactivity test The cokes were crushed to less than 1 mm and screened at 0.6 mm. The 0.6–1.0 mm fraction was used to carry out the reactivity test. The reactivity test procedure has been described elsewhere (Grigore et al., 2006). Sub-samples weighing approximately 1.4 g were reacted with 100% CO2 in a fixed-bed reactor to a carbon conversion level of 15%. The reactivity measurements were made under conditions of chemical rate control (Regime I), free of any physical limitations due to gas pore diffusion and mass transfer. The verification of the ‘Regime I’ conditions has been described elsewhere (Grigore et al., 2009). The flow rate of the reactant gas was 750 ml/ min. The reaction temperature was selected on the basis of the reactivity of the samples, so that the percentage of carbon monoxide in the outlet gas did not exceed 1%. Such low concentrations of carbon monoxide had negligible inhibition effect on the reaction rate (Harris and Smith, 1989; Turkdogan and Vinters, 1970). For these samples, the reaction temperatures were between 855 °C and 934 °C for the most reactive and the least reactive samples, respectively. The activation energies were measured to provide information about reaction conditions and to normalise the reaction rates during the experiment to 900 °C. The apparent reaction rate is expressed in grams of carbon reacted per gram of carbon remaining per second. In the calculation, the sample mass is ash-free. 2.4. Sample characterisation The inert maceral-derived component in the cokes prepared from the inertinite- and vitrinite-rich fractions and original coals in this work was unfused inertinite. Petrographic analysis was used to determine the abundance of the inert maceral-derived component in the cokes (Table 3). The component that had the original cell structure preserved, with no indications of being fused or softened and
Table 3 Abundance of the inert maceral-derived component in the coked vitrinite- and inertinite-rich fractions and cokes prepared from original coals. Inert maceral-derived component (vol.%)
B
C
F
G
Coked vitrinite-rich fraction Coke Coked inertinite-rich fraction
3.0 36.3 68.0
12.0 43.5 71.0
2.7 35.2 56.4
12.3 45.2 56.0
did not display anisotropy, was classified as the inert maceralderived component. Micropore surface area was measured using carbon dioxide at 0 °C. The Dubinin–Radushkevich equation was used to determine the micropore surface area of the cokes from carbon dioxide isotherms. The X-ray diffraction (XRD) patterns were used to measure carbon crystallite size (Lc – average crystallite height and La – average crystallite width). The American Standard Test Method D 5187–91 was the procedure used to determine Lc. A similar procedure was used to determine La. This technique allows measuring the carbon crystallite height – Lc (002 band; 2θ ~ 30°) and width – La (100 band; 2θ ~ 51°). The crystallite size was calculated using the Scherrer equation (Cullity, 1978). The mineral matter characterisation was carried out on samples that underwent low-temperature ashing (approximately 120 °C) using radio-frequency oxygen plasma ashing (low-temperature ashing, LTA) (Gluskoter, 1965). This procedure enables carbon removal with minimal alteration of the mineral species. The samples subjected to low temperature ashing were then analysed using XRD. The XRD patterns were used for identification and quantification of the mineral phases using Bruker Eva search/match software and SIROQUANT™ (Taylor, 1991), a personal computer quantitative XRD analysis software package. 3. Results and discussion The reactivity data show that the coked inertinite-rich fractions were consistently more reactive than the coked vitrinite-rich fractions (Fig. 1). The initial apparent rates of the coked inertinite-rich fractions (3.1–19.5 * 10 − 6 gg − 1 s − 1) varied to a greater extent than those of the coked vitrinite-rich fractions (0.63–3.4 * 10 − 6 gg − 1 s − 1). Although the coked inertinite-rich fractions were more reactive than the coked vitrinite-rich fractions, the relationship between the initial apparent rate and the concentration of the inert maceralderived component is poor when all of the cokes were considered (Fig. 2). This indicates that there is significant variation between the properties of the different maceral-derived components among the cokes prepared from different coal sources. However, the reactivity of the cokes increased approximately linearly with increasing amount
Initial apparent rate (*10-6gg-1s-1)
temperature of carbonisation was set at 470 °C. The semi-coke prepared in this first stage was subsequently calcined at 1050 °C in a horizontal tube furnace. Sub-samples of 70 g were packed into a retort at a bulk density in the range of 0.68 to 0.80 g/cm 3. The 70 g retort was introduced in an oven preheated to 300 °C. The oven was continually purged with nitrogen to avoid the oxidation of the sample. After the temperature in the centre of the charge reached 300 °C, the coal sample was heated to 470 °C at a constant rate and held at this temperature for 2 h. Then the semi-coke produced was cooled under flowing nitrogen. The heating rate selection was based on the plastic properties of the macerals. The vitrinite-rich fractions swelled more than the coals and inertinite-rich fractions, leading to high porosity in the carbonization product. Therefore, to control the porosity in the coke, the heating rate of the vitrinite-rich fractions had to be lower than the heating rate of the coals and the inertinite-rich fractions. The heating rate from 300 to 470 °C was 1 °C per minute for the original coals and the inertinite-rich fractions. The vitrinite-rich fractions of coals B, C, and F were carbonized at 0.1 °C per minute. The vitrinite-rich fraction of coal G required an even lower heating rate of 0.05 °C per minute to prevent swelling. The semi-coke samples produced in the 70 g retort were loaded in alumina boats and inserted into a horizontal tube furnace at ambient temperature. The furnace was purged with high-purity nitrogen. The furnace was heated to 500 °C at a heating rate of 1 °C per minute, then at 10 °C per minute from 500 °C to 1050 °C. The purge gas was changed from high-purity nitrogen to ultra high-purity argon after the temperature reached 500 °C. At the completion of the calcination stage, the samples were cooled down inside the furnace under ultra high-purity argon. Below 500 °C, the purge gas was changed back to high-purity nitrogen.
3
25 20 coked vitrinite-rich fraction
15 coke
10 coked inertinite-rich fraction
5 0
B
C
F
G
Fig. 1. The initial apparent reaction rates of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals.
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
14
25 20
Mineral matter (wt,%)
Initial apparent rate (*10-6gg-1s-1)
4
B
15
C
10
F
5
G
12 10
B
8
C
6
F
4
G
2 0
0 0
20
40
60
80
0
20
40
60
80
Inert maceral-derived component (vol,%)
Inert maceral-derived component (vol,%)
Fig. 3. Concentration of mineral matter in the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals against concentration of the inert maceral-derived component.
Fig. 2. Initial apparent rate of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus concentration of the inert maceral-derived component.
The XRD analyses of the low temperature ashes show that the mineralogical composition of the coked inertinite-rich fractions is generally similar to that of their corresponding cokes (Table 4). But the coked vitrinite-rich fractions contain a larger variety of mineral phases than the coked inertinite-rich fractions, particularly coked vitrinite-rich fractions B and G (Table 4). The formation of the new mineral phases in the coked vitriniterich fractions is thought to be due to the slower heating rate used to prepare them (the final coking temperature was similar for all cokes). This may have led to the formation of strongly reduced phases such as aluminium nitride and silicon carbide. Additionally, fayalite, hercynite, gehlenite and tridymite were found only
of inert maceral-derived component when the cokes from each coal source were considered separately. This suggests that their reactivity is mainly controlled by the abundance of inert derived-maceral component. 3.1. Mineral matter The coked inertinite-rich fractions had a greater ash yield than the coked vitrinite-rich fractions (Table 4). The concentration of mineral matter increases as the amount of non-fused inertinite increases (Fig. 3).
Table 4 Mineral phases identified in the low temperature ash of the cokes and their relative concentrations (wt.%). Mineral phase
Chemical formula
Coal source B Coke-V
LTA (%) Akermanite Diopside Gehlenite Fluorapatite Oldhamite Bassanite Iron Hematite Magnetite Wustite Pyrrhotite Troilite Jarosite Fayalite Hercynite Iron phosphate Ferrosilicon Mullite Spinel Kalsilite Leucite γ-Alumina Aluminium nitride Quartz Cristobalite Tridymite Silicon carbide Brookite Anatase Rutile Tungsten carbide Titanium carbide Amorphous
Ca2MgSi2O7 CaMgSi2O6 Ca2Al2SiO7 Ca5(PO4)3F CaS CaSO4 × 0.5H2O Fe Fe2O3 Fe3O4 FeO Fe1-xS FeS (K,H3O)Fe3(SO4)2(OH)6 (Fe,Mg)2SiO4 FeAl2O4 FePO4 Fe3Si Al6Si2O13 MgAl2O4 KAlSiO4 KAlSi2O6 Al2O3 AlN SiO2 SiO2 SiO2 SiC TiO2 TiO2 TiO2 WC TiC
Coke-I
Coal source C Coke
1.9 0.1 0.1 1.0 3.2 0.3 0.7 0.1
12.3 0.1 0.5
8.6 0.2 0.4
0.5 0.1 0.2
1.6 0.1 0.3 0.1
0.1
0.2
0.2 0.1
Coke-V 4.0 0.4 0.8 0.6 5.3 0.3 0.5
Coal source F
Coke-I
Coke
13.9
11.0
Coke-V 1.7
2.1 0.1 0.6 0.1
2.5 0.1 0.6 0.3
0.3 5.3 0.3 0.4 0.1 0.2
0.2
0.2
0.3
0.2 0.2 2.5 0.6
0.3
0.1
8.0 0.9
6.6 1.0
0.3
0.3 3.2
0.9 12.2 0.1 0.2 0.5 0.6 0.1 15.0 6.0 54.0
9.4
0.4
0.4
1.6 0.1 0.3 0.1
2.8 0.3 1.3 0.7
0.2 0.2 9.7 1.3
0.2 0.9 9.4 1.4
0.2 0.2 7.0 0.4
0.3 1.8
0.2
0.6
0.8
24.5
27.9
15.7 0.1
16.8
16.0
17.7 0.1
31.3 0.2
33.1 0.1
0.4 0.1
0.8 0.1
0.7
0.7
0.7 0.4
0.4
0.3
57.5
63.3
67.7
4.0 0.3 0.7 2.8 0.1 0.2 0.1
1.8 0.2
6.3
0.2 0.2 5.4 1.3
0.2 2.1 6.4 1.8
0.7 0.1 5.7
3.4
8.9 1.0 52.4
3.4
Coke-V
Coke-I
Coke
13.6 0.2
12.7 0.2
2.3 0.2 0.4 0.2 0.2
2.6 0.2 0.2 0.4
0.1
1.7 0.6
60.3
1.1
Coke
13.2
0.1 0.1
0.2 0.5 0.6
Coke-I
Coal source G
0.2 0.5 5.5 2.0 0.6
6.4 0.9 58.3
Coke-V – coked vitrinite-rich fraction; Coke-I – coked inertinite-rich fraction; Coke – coke made from the original coal.
48.6
45.6
0.5 0.4 0.3 0.1 8.3 1.0 0.3 0.1 1.2 1.0 1.9 0.1 0.2 0.2 0.6 0.2 0.3 6.3 0.7 71.9
0.2
1.2
0.4
0.3 0.1 16.2 0.4
0.5 16.4 1.5
0.3 3.7
0.4 2.1
3.2 0.1
3.3 0.1
0.6 0.3 3.4 70.9
67.5
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
5
Table 5 The concentration of the mineral phases present in cokes, expressed on a coke basis (wt.%). Mineral phase
Akermanite Diopside Gehlenite Fluorapatite Oldhamite Iron Hematite Magnetite Wustite Pyrrhotite Troilite Fayalite Hercynite Iron phosphate Ferrosilicon Mullite Spinel Kalsilite Leucite γ-Alumina Aluminium nitride Quartz Cristobalite Tridymite Silicon carbide Brookite Anatase Rutile Amorphous
Chemical formula
Coal source B Coke-V
Coke-I
Coke
Coke-V
Ca2MgSi2O7 CaMgSi2O6 Ca2Al2SiO7 Ca5(PO4)3F CaS Fe Fe2O3 Fe3O4 FeO Fe1-xS FeS (Fe,Mg)2SiO4 FeAl2O4 FePO4 Fe3Si Al6Si2O13 MgAl2O4 KAlSiO4 KAlSi2O6 Al2O3 AlN SiO2 SiO2 SiO2 SiC TiO2 TiO2 TiO2
0.002 0.002 0.024 0.078 0.016 0.002
0.012 0.062
0.017 0.035
0.062 0.025
0.139 0.022 0.009
0.018 0.036 0.027 0.240 0.025
0.002
0.025
0.017 0.009 0.017
0.002
0.007
Coal source C
0.015 0.037
0.009
0.988 0.111
0.573 0.087
0.007
0.037 0.395
1.31
Coke
0.301 0.057 0.014
0.284 0.045 0.034
0.029
0.023
0.114
0.068
0.027
0.005 0.005 0.061 0.015
0.022 0.296 0.002 0.005 0.012 0.015 0.002
0.014
Coal source F
Coke-I
Coke-V
0.006 0.099 0.009 0.002 0.004
Coal source G
Coke-I
Coke
0.056
0.040
0.223 0.035 0.014
0.278 0.094 0.070
0.006
0.029 0.129 1.348 0.201
0.023 0.023 0.794 0.045
0.026 0.156
0.009
0.086
0.091
3.027
2.420
0.711 0.005
2.410
1.815
0.330 0.002
4.365 0.028
3.291 0.010
0.049 0.012
0.069 0.009
0.100
0.079
0.013 0.007
0.056
0.030
7.45
4.99
1.09
6.78
7.68
Coke
0.013
0.027
0.026
0.316 0.055 0.027 0.027 0.014
0.343 0.040 0.053
0.029
0.009
0.041 0.014 2.225 0.055
0.066
0.041 0.508
0.053 0.277
0.440 0.014
0.436 0.013
0.030 0.121 0.009 0.004
0.169 0.010
0.004
9.08
Coke-I
0.004 0.014 0.168
0.009 0.009 0.439 0.059
2.37
Coke-V
0.004 0.004 0.101 0.024
0.028 0.293 0.893 0.251
0.020 0.050 0.547 0.199 0.060
4.53
0.022 0.017 0.013 0.004 0.358 0.043 0.013 0.004 0.052 0.043 0.082 0.004 0.009 0.009 0.026 0.009 0.013 3.10
2.166 0.198
0.079 0.041 9.74
8.91
Coke-V – coked vitrinite-rich fraction; Coke-I – coked inertinite-rich fraction; Coke – coke made from the original coal.
(fayalite, hercynite, ferrosilicon and iron phosphate) along with the calcium and potassium bearing minerals (akermanite, diopside, gehlenite, fluorapatite, oldhamite, kalsilite and leucite) are either not catalysts or not known to catalyse gasification. Additionally, any iron, calcium, potassium and sodium in the amorphous phase formed due to clay decomposition, is not known to catalyse gasification (Kerkkonen et al., 1996; Lang and Neavel, 1982). Hence, using total iron, calcium, potassium and sodium from ash chemistry to assess the influence of the catalytic mineral phases on gasification is not appropriate. The catalytic mineral phases must be identified and quantified in order to assess their influence on gasification. The total iron present in the catalytic mineral phases was calculated as mmol fraction of element per 1 kg coke (Table 6). The coked vitrinite-rich fractions had the lowest concentrations of catalytic iron. The abundance of catalytic iron in the coked inertinite-rich fractions was greater than that in their corresponding cokes made from the original coal, with the exception of coked inertinite-rich fraction B, which had less catalytic iron than coke B. Although a strong relationship was observed between the mineral matter yield and the concentration of non-fused inertinite (Fig. 3), the abundance of catalytic mineral matter is not related to the concentration
in the coked vitrinite-rich fractions. Although aluminium nitride generally occurs at higher temperatures than 1050 °C, it may have been formed by reaction of γ-alumina with nitrogen in the presence of carbon (nitrogen was the purging gas of the horizontal tube furnace used to calcine the semi-coke samples). Silica reduction by carbon in cokes can occur at temperatures as low as 1100 °C (Van der Velden et al., 2002). These reduced phases have not been previously found in cokes prepared at such low temperatures. Some samples were contaminated during sample preparation. The small amounts of tungsten carbide (WC) and titanium carbide (TiC) found in some coked vitrinite-rich fractions, are contaminants that arose during cutting of the cokes using abrasive disc blades. Additionally, oxidation artefacts of the low temperature ashing process were found in all cokes, namely bassanite and jarosite (Grigore et al., 2006; Ward, 2002). Hence, the mineralogical composition of the cokes was recalculated by proportioning the sulfates to oldhamite and pyrrhotite, and eliminating tungsten and titanium carbides from the calculation. Table 5 shows the recalculated mineralogical composition on a coke basis. The catalytic mineral phases identified in the cokes are metallic iron, pyrrhotite and iron oxides. The other iron-bearing minerals
Table 6 Deportment of total iron present in the mineral phases known to catalyse gasification (Fe, Fe1-xS, Fe2O3, Fe3O4 and FeO), expressed as mmol fraction of iron per 1 kg coke. Mineral phase
Chemical formula
Coal source B
Iron Hematite Magnetite Wustite Pyrrhotite and troilite Total
Fe Fe2O3 Fe3O4 FeO Fe1-xS
0.44
Coke-V
0.32 0.72 1.5
Coke-I
3.2 2.2 5.4
Coal source C Coke
Coke-V
Coal source F
Coke-I
Coke
1.5
2.5
5.9
2.2 1.2 5.7 10.6
3.6
2.9
4.5 4.5
31.5 37.6
22.4 31.2
Coke-V 0.33 0.47
Coal source G
Coke-I
Coke
Coke-V
2.4
11.8
0.79
Coke-I
Coke
4.9 3.4
9.3
0.57 2.0 2.8
Coke-V – coked vitrinite-rich fraction; Coke-I – coked inertinite-rich fraction; Coke – coke made from the original coal.
1.8 51.3 55.5
40.7 52.5
1.4
3.5 9.8 18.1
3.1 15.9
6
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
Catalytic iron (mmol/1 kg coke)
60
Table 7 Micropore surface areas of the raw coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals.
50 B 40 C 30 F 20
Coal source B C F G
Surface area (m2g− 1) Coked inertinite-rich fraction
Coke
Coked vitrinite-rich fraction
19.0 17.0 30.7 11.8
10.4 11.0 14.1 11.1
4.1 8.0 6.5 7.3
G 10 0 0
20
40
60
80
Inert maceral-derived component (vol,%) Fig. 4. Concentration of catalytic iron in cokes versus concentration of the inert maceral-derived component.
of non-fused inertinite (Fig. 4). This indicates that the concentration of catalytic mineral phases can vary significantly between cokes containing similar amounts of non-fused inertinite. Fig. 5 shows good agreement between the initial apparent rate and the concentration of catalytic iron. The scatter could be due to differences in particle size of catalytic iron or the degree of dispersion of the catalyst (Gopalakrishnan and Bartholomew, 1996; Tomita, 2001). Lindert and Timmer (1991) and Tanaka et al. (1995) observed an increase in coke reactivity to carbon dioxide as the dispersion of metallic iron increased. Additionally, Grigore et al. (2009) observed that the reaction rate is sensitive to the size of catalyst particle; finer catalyst particles enhance coke gasification more than coarser particles. The relationship between reactivity and the abundance of the catalytic phases that we found would not be significantly affected by the different heating rates used in preparation of the cokes. The amounts of catalytic iron phases in the coked vitrinite-rich fractions were low compared to those in the cokes prepared from the inertinite-rich fractions and original coals, and the inclusion of the iron present in fayalite and hercynite, mineral phases that may have formed at the expense of the catalytic iron phases, would not significantly increase the total amount of catalytic iron. These results establish that the abundance of catalysts is one dominant factor that makes the inert maceral-derived component in cokes more reactive than the reactive maceral-derived component, and accounts for the variation in reactivity between the inert maceralderived components in cokes made from different coals. Since micropore surface area and carbon crystallite size have also been reported as factors affecting the reactivity of cokes prepared
from whole coals (Duval et al., 1988; Vogt et al., 1991; Zamalloa et al., 1995), we investigated their importance on the initial reaction rates of the cokes prepared from the vitrinite- and inertinite-rich fractions and original coals, as well. 3.2. Micropore surface area The micropore surface area of the coked vitrinite-rich fractions was significantly lower than that of their corresponding coked inertiniterich fractions (Table 7). This observation is consistent with the findings of Huang et al. (1991). Micropore surface area generally increased with increasing the amount of inert maceral-derived component in cokes (Fig. 6). This suggests that the micropore network in the inert maceral-derived component is more extensively developed than that in the reactive maceral-derived component. The good relationship observed between the micropore surface area and the amount of inert maceralderived component in cokes indicates that the different heating rates used for preparation of the cokes did not affect significantly microporosity of the cokes. Surface area plays an important role when the reaction rate is chemically controlled (Zamalloa et al., 1995). The reactivity of the cokes increases as their surface area increases (Vogt et al., 1991). Fig. 7 shows a good correlation between the initial apparent rate and the micropore surface area. This confirms that the more extensive micropore network of the inert maceral-derived component increases its reactivity compared to that of the reactive maceral-derived component as a greater surface area is exposed to gasification. The initial apparent rate was divided by micropore surface area to obtain the initial intrinsic rate (Harris and Smith, 1989). If micropore surface area is the only factor that controls the reaction rate, then all cokes would have similar intrinsic rates. The intrinsic rates of the coked inertinite-rich fractions were greater than those of the coked vitrinite-rich fractions (Fig. 8). This confirms that micropore surface area is not the only factor that makes the inert maceral-derived component more reactive than the reactive maceral-derived component.
30
25 20 coked inertinite-rich fraction
15 coke
10 coked vitrinite-rich fraction
5
Surface area (m2g-1)
Initial appatent rate (*10-6 gg-1s-1)
35 B
25 C 20 15
F
10
G
5 0 0
20
40
60
80
Inert maceral-derived component (vol,%)
0 0
20
40
60
Catalytic iron (mmol/1 kg coke) Fig. 5. The initial apparent rate versus concentration of catalytic iron.
Fig. 6. Surface area of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus concentration of the inert maceralderived component.
M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
7
Table 8 The average crystallite height and width of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals.
25 20 coked inertinite-rich fraction
Coal source
B
C
F
G
coke
Lc (nm) Coked inertinite-rich fraction Coke Coked vitrinite-rich fraction
1.22 1.36 1.39
1.21 1.37 1.37
1.35 1.54 1.72
1.35 1.40 1.63
La (nm) Coked inertinite-rich fraction Coke Coked vitrinite-rich fraction
3.41 3.85 3.41
3.37 3.96 3.57
4.09 3.76 3.40
3.33 3.53 3.80
15 10 coked vitrinite-rich fraction
5 0 0
10
20
30
40
Fig. 7. Micropore surface areas of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus initial apparent rate.
3.3. Carbon crystallinity The average crystallite height and width of the coked inertiniteand vitrinite-rich fractions were investigated in this study, since increasing the degree of ordering of carbon structure is reported to lead to a lower reactivity (Duval et al., 1988). It has also been reported that the gasification reaction occurs mainly at the edges of the crystallites rather than on the basal planes (Rouzaud et al., 1988; Walker et al., 1959). Since the density of the free edges increases as the La decreases, cokes with a greater La should have a reduced reactivity. The coked inertinite-rich fractions have lower Lc than their corresponding coked vitrinite-rich fractions (Table 8), indicating less ordering of the carbon structure. This observation is consistent with Huang et al. (1991) findings. It needs to be noted that the vitriniterich fractions were coked at lower heating rate than the inertiniterich fractions. Heating rate is one of the factors that affect Lc. The Lc increases as the heating rate increases (Rouzaud et al., 1988). If the vitrinite-rich fractions would have been coked at a similar heating rate to that used for inertinite-rich fractions then their Lc would be even greater than those reported in this study. The average crystallite heights of the coked vitrinite-rich fractions vary over a wider range than those of the coked inertinite-rich fractions. The coked vitrinite-rich fractions with smaller Lc (B and C) were prepared from coals of lower rank than those with larger Lc (F and G). The greater Lc values of coked inertinite-rich fractions F and G compared to those of B and C is probably due to the greater amount of reactive maceral-derived component in these cokes, rather than any variation in the Lc of the inert maceral-derived component. Fig. 9a shows a good agreement between Lc and the concentration of inert maceral-derived component for the cokes prepared from coals F and G. Since rank of the parent coal had stronger influence on the Lc of the reactive maceral-derived component but not on that of the inert maceral-derived component, the relationship between
Lc and the concentration of inert maceral-derived component becomes more obvious when the cokes are prepared from coals of higher rank. Unlike Lc, there were only small variations in La of the cokes (Table 8). No consistent trend was observed between La and the concentration of the inert maceral-derived component or rank of the parent coal (Fig. 9b). There is no obvious relationship between Lc of the cokes and the reaction rate (Fig. 10). The average crystallite heights of the coked inertinite-rich fractions were not expected to have a strong influence on their reaction rates, since Lc values did not vary greatly between the fractions. Although the Lc values of the coked vitrinite-rich fractions might have been expected to demonstrate this correlation since they exhibited greater variability, as shown in Table 8, no such relationship was found. Our observations are not consistent with the conclusions drawn by Huang et al. (1991) who attributed the lower reactivity of the carbonised vitrinite-rich fractions compared to that of the carbonised
(a) 1.8 1.7
B
1.6 C
1.5 1.4
F
1.3 1.2
G
1.1 0
20
40
60
80
Inert maceral-derived component (vol,%)
(b) 4.4 4.2
1.0
B
4.0
0.8
C
3.8 coked vitrinite-rich fractions
0.6
3.6
F
3.4 coke
0.4
G
3.2 coked inertinite-rich fractions
0.2
3.0 0
20
40
60
80
Inert maceral-derived component (vol,%)
0.0 B
C
F
G
Fig. 8. Initial intrinsic rates of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals.
Fig. 9. (a) Average crystallite height and (b) average crystallite width of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals against concentration of the inert maceral-derived component.
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M. Grigore et al. / International Journal of Coal Geology 98 (2012) 1–9
25 20 coked inertinite-rich fractions
15 coke
10 coked vitrinite-rich fractions
5 0 1.0
1.2
1.4
1.6
inertinite-rich fractions to their greater degree of ordering of the carbon structure. Although we also found the degree of ordering of the carbon structure of the coked inertinite-rich fractions to be lower than that of the coked vitrinite-rich fractions, the degree of ordering of the carbon structure does not appear to influence the initial apparent rate of the carbonised vitrinite- and inertinite-rich fractions from our study (Fig. 10). The initial apparent rate of the coked vitrinite- and inertinite-rich fractions was not related to La. This is not unexpected since the La of the inert maceral-derived component and reactive maceral-derived component was not determined by the type of macerals and their abundance in the parent coal. We concluded that the reaction rate is not controlled by either Lc or La at the initial stages. 3.4. Statistical analysis The reactivity of the coke was affected by both amount of catalytic matter and micropore surface area. Since both factors increased with the amount of inert maceral-derived component they correlated to each other as well. In order to separate out the contribution of each to reactivity, a multiple regression was performed on the data. The regression equation found was Initial apparent rate ¼ 0:143ð0:044ÞFe þ 0:46ð0:12ÞSurface area 2:0ð1:2Þ
ð1Þ
The r 2 correlation coefficient is 0.91. Fig. 11 shows the relationship between the predicted initial apparent rate from Eq. (1) and the
25
20
15
10
5
0 5
10
15
4. Conclusions From a study of the reactivity of cokes made from inertinite- and vitrinite-rich fractions and original samples of four Australian bituminous coals, it was found that:
1.8
Fig. 10. Initial apparent rates of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus the average crystallite height.
0
measured initial apparent rate. It can be seen that the contribution of both amount of catalytic matter and micropore surface area to reactivity is positive and statistically significant: both contribute independently to the initial reactivity of the coke. Introducing Lc and La as independent parameters did not significantly improve this relationship suggesting that their influence on reactivity is small. This confirms the findings above.
20
25
Fig. 11. Initial apparent rates of the coked vitrinite- and inertinite-rich fractions B, C, F and G and the cokes prepared from the original coals versus that predicted from Eq. (1).
• The reactivities of the coked inertinite-rich fractions were consistently greater than those of the coked vitrinite-rich fractions. The major factors that make the inert maceral-derived component more reactive than the reactive maceral-derived component in cokes are the concentration of catalytic iron phases and micropore surface area. • The catalytic mineral phases identified in the coked inertinite- and vitrinite-rich fractions are metallic iron, pyrrhotite, troilite, wustite, magnetite and hematite. Iron also occurred in mineral phases that do not catalyse gasification, such as fayalite, hercynite, ferrosilicon and iron phosphate. No catalytic calcium-, potassium- and sodiumbearing mineral phases have been identified in any coke. • The Lc of coke was related to maceral composition and coal rank of the parent coal, whereas La was not related to any of these properties. Although the degree of ordering of carbon structure (Lc) of the reactive maceral-derived component is greater than that of the inert maceral-derived component, it was not found to be a major factor controlling coke reactivity at the initial stages.
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