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Energy & Fuels 2003, 17, 744-754
Variation of the Crystalline Structure of Coal Char during Gasification Bo Feng,† Suresh K. Bhatia,*,† and John C. Barry‡ Department of Chemical Engineering, Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, QLD 4072, Australia Received October 27, 2002
The variation of the crystallite structure of several coal chars during gasification in air and carbon dioxide was studied by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques. The XRD analysis of the partially gasified coal chars, based on two approaches, Scherrer’s equation and Alexander and Sommer’s method, shows a contradictory trend of the variation of the crystallite height with carbon conversion, despite giving a similar trend for the crystallite width change. The HRTEM fringe images of the partially gasified coal chars indicate that large and highly ordered crystallites exist at conversion levels as high as 86%. It is also demonstrated that the crystalline structure of chars can be very different although their pore structures are similar, suggesting a combination of crystalline structure analysis with pore structure analysis in studies of carbon gasification.
Introduction It is well-known that carbons and chars consist of crystalline phases which are normally imperfect and randomly arranged. These crystallites are consumed gradually during gasification and thus the crystalline structure as well as the pore structure is expected to change with conversion. Compared with the pore structure change, the crystalline structure change shows directly the carbon consumption process, thus providing more reliable information about the gasification process. On the basis of the experimental observations of the edge recession of graphite using microscopy,1-3 several structural models of carbon gasification have been proposed.4-9 These models can address some features in char gasification, such as gasification-induced particle shrinkage,10,11 surface chemistry,12,13 constitution of * Corresponding author. Phone: 61 7 3365 4263. Fax: 61 7 3365 4199. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Centre for Microscopy and Microanalysis. (1) Hennig, G. R. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Dekker: New York, 1951; pp 1-49. (2) Yang, R. T.; Wong, C. J. Chem. Phys. 1981, 75, 4471. (3) Thomas, J. M. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Dekker: New York, 1965; pp 121-202. (4) Bhatia, S. K. AIChE J. 1998, 44, 2478-2493. (5) Chen, W. Y.; Kulkarni, A.; Milum, J. L. AIChE J. 1999, 45, 2557-2570. (6) Kyotani, T.; Ito, K.; Tomita, A.; Radovic, L. R. AIChE J. 1996, 42, 2303-2307. (7) Kyotani, T.; Leon, C.; Radovic, L. R. AIChE J. 1993, 39, 11781185. (8) Miura, K.; Hashimoto, K. Ind. Eng. Chem. Proc. Des. Dev. 1984, 23, 138-145. (9) Wolff, W. F. J. Phys. Chem. 1959, 63, 653-660. (10) Hurt, R. H.; Dudek, D. R.; Longwell, J. P.; Sarofim, A. F. Carbon 1988, 26, 433-450. (11) Wong, B. A.; Gavalas, G. R.; Flagan, R. C. Energy Fuels 1995, 9, 493-499. (12) Kasaoka, S.; Sakata ,Y.; Tong, C. Int. Chem. Eng. 1985, 25, 160-175. (13) Menendez, J. A.; Phillips, J.; Xia, B.; Radovic, L. R. Langmuir 1996, 12, 4404.
carbons with different reactivity,14 and heat annealing,15 which cannot or are difficult to be adopted in the pore structure model. However, there are only a few investigations on the variation of the crystalline structure during gasification.16-22 Therefore the validation of a structural model is still required. Palotas et al.16 found that the ordering of the carbon structure increases with increase of oxidation, using high-resolution transmission electron microscopy. The increased order is measured in increases in the fractional coverage of a cross section of the particles with a layered structure, a decrease in the mean interlayer spacing, and a decrease in the spread of the interlayer spacing. However, the crystallite size was not determined in their study. Davis et al.17 oxidized pulverized coal in a laboratory entrained flow reactor and observed that the early stages of heterogeneous oxidation proceed in parallel with the latter stages of carbonization, leading to preferential loss of hydrogen, a reduction in surface area, and the development of crystalline order. The (14) Franklin, R. E. Acta Crystallogr. 1951, 4, 253-261. (15) Suuberg E. M. Thermally induced changes in reactivity of carbons. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publisher: Norwell, MA, 1991. (16) Palotas, A. P.; Rainey, L. C.; Sarofim, A. F.; Vander Sande, J. B.; Ciambelli, P. Energy Fuels 1996, 10, 254-259. (17) Davis, K. A.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 31-40. (18) Sharma, A.; Kadooka, H.; Kyotani, T.; Tomita, A. Energy Fuels 2002, 16, 54-61. (19) Sharma, A.; Kyotani, T.; Tomita, A. Fuel 1999, 78, 1203-1212. (20) Rouzaud, J. N.; Duval, B.; Leroy, J. Coke microtexture: one key for coke reactivity. In Fundamental issues in control of carbon gasification reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Norwell, MA, 1991; 257-267. (21) Furuta, T.; Yamashita, Y.; Shiraishi, M. Tanso 1989, 140, 241247. (22) Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 131-143.
10.1021/ef0202541 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/24/2003
Crystalline Structure of Coal Char during Gasification
development of turbostratic order occurred gradually over a time scale comparable to the combustion process itself, on the order of 100 ms at particle temperatures of 1800 K and oxygen concentration of 12%. The crystallite size increased with the increase of conversion, determined by both XRD and HRTEM image analysis, which was attributed to heat annealing effects. Sharma et al.18,19 studied the structure of coal char during low-temperature gasification using HRTEM. Different coal chars show different behavior in the change of ordering with conversion. Chars from a lowvolatile bituminous coal (Pocahontas No. 3) contained both ordered and disordered parts in the initial stage, and the ordered part was dominant after gasification. Lignite (Beulah-zap) chars mainly consisted of lessordered carbon, and the crystallite ordering increased upon gasification. On the other hand, chars from a highvolatile bituminous coal (Illinois No. 6), and all the demineralized chars from these coals, did not show a large change before and after gasification. Rouzaud et al.20 studied the microtexture evolution of cokes during gasification in steam and in carbon dioxide using TEM. Cokes with medium-size molecular orientation domains (MOD) were observed to be consumed preferentially, which is explained as the best compromise between active site density and accessibility of these MOD compared with the small and large MOD. Wornat et al.22 studied the structural change of biomass char during gasification using HRTEM. After devolatilization, very little additional ordering of the carbon structure occurred in either of the two chars studied. Even at the highest levels of conversion, the carbon-rich portions of the biomass chars did not approach the graphitic structures as coal char did. This was attributed to the lower mobility of carbon crystallites in the biomass chars to align and coalesce.22 The present work studies the variation of the crystalline structure of several coal chars during gasification and attempts to combine this with the pore structure variation to obtain a more complete picture of the structural changes during gasification. The raw coals were heat treated at various temperatures to study the effect of heat treatment on structural evolution. One coal was also demineralized to study the effect of mineral matter in coal, and one coal char was gasified in both air and CO2 to study the effect of a gasifying agent. The crystalline structure was studied using X-ray diffraction and high-resolution transmission electron microscopy. Further, since there are competing methods for interpreting XRD data, several methods were considered, and the results were compared in an effort to determine meaningful trends. Experimental Section Sample Preparation. An Australian semi-anthracite, Yarrabee, was used in this study, and the properties of this coal can be found elsewhere.23 The raw coal was dried and sieved to a particle size range of 90-180 µm and then subjected to heat treatment at various temperatures. The raw coal was also demineralized, and the ash-free coal was heat-treated. The raw coal char and the demineralized coal char were gasified in a tube furnace in air at 653 K and in CO2 at 1073 K. Details (23) Feng, B.; Bhatia, S. K.; Barry, J. C. Carbon 2002, 40, 481496.
Energy & Fuels, Vol. 17, No. 3, 2003 745 on the gasification process are described elsewhere.24 Three coal chars, namely, rawy1050 (Yarrabee coal heat-treated at 1050 °C for 2 h), rawy1150 (Yarrabee coal heat-treated at 1150 °C for 2 h), and HFy1050 (demineralized Yarrabee coal heattreated at 1050 °C for 2 h), were gasified in a tube furnace to various conversions. The partially gasified char samples were XRD analyzed and observed using HRTEM. HRTEM. The samples were prepared for electron microscopy by crushing under ethanol in a mortar. A suspension of the crushed particles was then deposited onto a holey carbon film. The high-resolution transmission electron microscope (HRTEM) images were obtained using a JEOL 2010 microscope (Cs ) 1.4 mm; structure resolution limit ) 0.25 nm, information limit ) 0.19 nm), which was operated at 200 kV, with an EDS detector for determining elemental composition. Images were then digitized. XRD Analysis. The powder diffractometer (Philips PW 1710), fitted with a copper radiation source (λ1 ) 1.54060 Å, λ2 ) 1.54439 Å), was configured in the Bragg-Brentano pseudo-focusing geometry. Measurements were recorded from a start angle 2θ ) 10° to an end angle of 100° with a scanning speed of 0.5°/min. The XRD patterns were analyzed for the structural parameters using two methods. One is the classical Scherrer equation, and the other is the method used by Lu et al.,25 originally developed by Alexander and Sommer.26 The following structural parameters were obtained. The lateral size of the crystallite, La, the stacking height of the crystallite, Lc, the interlayer spacing, d002, the parallelism indicator, R, fraction of disordered carbon in char, xa, and fraction of carbon contained in a group of n parallel layers, pn. The average number of aromatic layers, na, can be obtained from the value of Lc.
Quantitative Analysis of XRD Patterns Scherrer’s Equation. The values of La and Lc can be calculated using the conventional Scherrer equations
La )
1.84λ 0.9λ , Lc ) Ba cos(θa) Bc cos(θc)
(1)
where λ is the wavelength of the radiation used, Ba and Bc are the widths of the (100) and (002) peaks, respectively, at 50% height, and θa and θc are the corresponding scattering angles. This was done by using an XRD pattern analysis software, accounting for instrumental broadening. The parallelism indicator, R, defined to be the peak height (above zero) divided by the background height at the position of the (002) peak, is obtained directly from the XRD patterns. This parameter has been used by Dahn et al.27 to estimate the fraction of graphene sheets which have no parallel neighbors. The (002) peak comes from constructive interference between X-rays scattered from parallel stacked graphene sheets. As the proportion of graphene layers with parallel neighbors increases, so will R. Therefore the larger the value of R, the greater the proportion of layers in crystallites having more than one layer. Method of Alexander and Sommer. This method26 was first developed in studies of the structure of carbon black. More recently it has also been employed by Lu (24) Feng, B.; Bhatia, S. K. Carbon 2003, 41, 506-523. (25) Lu, L.; Sahajwalla, V.; Kong, C.; Harris, D. Carbon 2001, 39, 1821-1833. (26) Alexander, L. E.; Sommer, E. C. J. Phys. Chem. 1956, 60, 16461649. (27) Dahn, J. R.; Xing, W.; Gao, Y. Carbon 1997, 35, 825-830.
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et al.25,28 for the structure of coal chars. All the parameters mentioned above could be obtained using this method, and the procedures can be found in Alexander and Sommer26 as well as Lu et al.25 The method is based on the established XRD theory of Franklin14 which assumes that disordered carbon will distort the peaks, leading to their asymmetry. The fraction of disordered carbon is determined from the degree of asymmetry, and the symmetric bands obtained after removing the effect of the disordered carbon are then interpreted to determine the crystallite size distribution and the mean crystallite size. The procedure first involves normalization of the X-ray intensity curve. The observed X-ray intensities (in arbitrary units) were first normalized to electron units, and then to atomic units. The reduced intensity curves were used in the subsequent analysis to determine the values of d002, La, xa, pn, and Lc. Shi et al.’s Method. Shi et al.29,30 developed a model that takes into account the inherent disorder present in graphitic carbons to simulate XRD patterns, based on the work of Franklin14 and Ruland.31 Their model has been proved to be able to simulate well the XRD patterns of numerous carbons generated in widely varying conditions.23,29 In their model, some parameters representing orderness of carbon such as the probability for finding a random displacement between adjacent layers, fraction of low-strain material in carbon, and inplane strain as well as the parameters for crystallite size are considered and determined by fitting the X-ray diffraction intensity curve. Further details are provided elsewhere.23,30 Results and Discussion Crystallite Size Based on the Scherrer Equations. The XRD patterns are normalized to atomic units before quantitative analysis of the crystalline structure. The normalized XRD intensity curves for the ash-free coal char HFy1050 are shown in Figure 1. Typically, three peaks, (002), (10), and (11), are visible for all the chars, although the intensity of (10) and (11) peaks decreases with an increase of conversion level. The curves for all the coal chars are analyzed using the methods discussed above. The crystallite width, La, and stacking height, Lc, in the partially gasified coal chars, determined using Scherrer’s equation, are shown in Figure 2. The value of La decreases with an increase of carbon conversion for the Yarrabee coal chars gasified in both air and carbon dioxide. The value of Lc also decreases with an increase of carbon conversion for rawy1050 gasified in air and rawy1150 in CO2. However, Lc increases slightly with an increase of carbon conversion for the ash-free coal char, HFy1050, when gasified in air. The figure also shows that the rawy1150 coal char when gasified in CO2 decreases faster in the values of Lc than the rawy1050 coal char when gasified in air. (28) Lu, L. M.; Sahajwalla, V.; Harris, D. Energy Fuels 2000, 14, 869-876. (29) Shi, H. Disordered carbons and battery applications. In Department of Physics; Simon Fraser University: Burnaby, BC, Canada, 1993; p 155. (30) Shi, H.; Reimers, J. N.; Dahn, J. R. J. Appl. Crystallogr. 1993, 26, 827-836. (31) Ruland, W. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1968; pp 1-84.
Figure 1. Reduced intensity curve of HFy1050 coal chars gasified to various conversions in air at 653 K.
Figure 3 shows the variation of the parameter R with carbon conversion, which is similar to the variation of Lc with carbon conversion by Scherrer’s equation. The value of R decreases with an increase of carbon conversion for rawy1050 gasified in air and for rawy1150 gasified in CO2, while it remains essentially unchanged for HFy1050 gasified in air. Since R reflects the proportion of layers in crystallites having more than one layer, the number of crystallites with multiple parallel layers decreases with an increase of carbon conversion for rawy1050 gasified in air and for rawy1150 gasified in CO2. The above results indicate that carbon is gasified in different ways for the raw coal char and the ash-free coal char, whereas it appears that the raw coal char is gasified in the same way in CO2 and in air. For the raw coal char gasified in CO2 and in air, at the early stages of gasification (60% conversion), entire graphene layers are also removed so that the stacking height starts decreasing with an increase of conversion. On the other hand, for the ash-free coal char, graphene layers are apparently not gasified completely even at high conver-
Crystalline Structure of Coal Char during Gasification
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Figure 3. Variation of parameter R of Yarrabee coal chars with carbon conversion in air at 653 K, and in carbon dioxide at 1073 K. The value of R was obtained directly from the XRD pattern.
Figure 2. Variation of (a) La and (b) Lc of Yarrabee coal chars with carbon conversion in air at 653 K and in carbon dioxide at 1073 K. The values of La and Lc were obtained using the Scherrer equations.
sions (up to 80%). This suggests the importance of the effects of inorganic impurities in coal on the gasification process. Crystalline Structure Based on the Alexander and Sommer Method. The variation of La with carbon conversion, obtained using the method of Alexander and Sommer,26 is shown in Figure 4. The value of La decreases with the increase of carbon conversion, consistent with the results obtained by Scherrer’s equation, although the value of La obtained using the Alexander and Sommer method is larger than that obtained using Scherrer’s equation. The variation of Lc with carbon conversion, obtained using the method of Alexander and Sommer,26 is shown in Figure 5. The value of Lc increases with an increase of carbon conversion for rawy1050 gasified in air at 653 K and for rawy1150 gasified in CO2 at 1073 K, while it decreases with an increase of carbon conversion for HFy1050 gasified in air at 653 K. The results are contradictory with those obtained using Scherrer’s equation in Figure 2. Also, the absolute value obtained
Figure 4. Variation of La of Yarrabee coal chars with carbon conversion in air at 653 K and in carbon dioxide at 1073 K. The value of La was obtained using the method of Alexander and Sommer.26
using the Alexander and Sommer method is higher than that obtained using Scherrer’s equation. The value of Lc increases significantly from 11.5 Å to 15 Å for Yarrabee raw coal chars when gasified from 0% to 90% conversion. This is unlikely to be true, although it could be explained as an apparent increase in crystallite size due to the preferential removal of unorganized carbon. The increase in fraction of organized carbon gives the appearance of larger crystallite size in XRD. Furthermore, the variation of Lc with conversion by Scherrer’s equation is supported by the variation of R. Consequently, it is more likely that the calculation of Lc in the Alexander and Sommer method, which is based on the crystallite size distribution, is suspect. This is supported by inconsistent estimates for the fraction of disordered carbon, to be discussed.
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Figure 5. Variation of Lc of Yarrabee coal chars with carbon conversion in air at 653 K and in carbon dioxide at 1073 K. The value of Lc was obtained using the method of Alexander and Sommer.26
The distributions of the stacking number of the partially gasified coal chars are shown in Figures 6 to 8, for HFy1050 gasified in air at 653 K, rawy1050 gasified in air at 653 K, and rawy1150 gasified in CO2 at 1073 K, respectively. It is clear that the distribution of the stacking number of the ash-free coal char is different from that of the raw coal chars. During the gasification of the ash-free char, the number of crystallites with two layers increases, and the number of crystallites with three layers decreases with the increase of carbon conversion. However, during the gasification of rawy1050 in air and rawy1150 in CO2, the crystallites with two layers disappear and the number of crystallites with three layers increases with the progress of gasification. The fraction of disordered carbon in the partially gasified coal chars is also obtained and shown in Figure 9. Surprisingly, the fraction of disordered carbon increases with increase of carbon conversion and reaches nearly unity for rawy1050 gasified in air at 653 K and for rawy1150 gasified in CO2 at 1073 K, while it decreases from 1.0 to 0.6 with an increase of conversion for HFy1050 gasified in air at 653 K. Again it is seen that the behaviors of the raw chars, rawy1050 gasified in air, and rawy1150 gasified in CO2, are similar to each other but different from that of the ash-free coal, HFy1050 gasified in air. That the fraction of disordered carbon increases with an increase of conversion is inconsistent with the results reported previously.16,18 It also appears to be contradictory to the interpretation of the earlier result that the crystallite height increases with an increase of conversion, for if this increase of the crystallite size is caused by the remaining larger crystallites after preferential removal of more disordered carbon, a decrease in the fraction of disordered carbon is expected. Both of these features together appear to indicate an inconsistency in the method. Although the variation of crystallite size Lc with conversion obtained by the Alexander and Sommer method is in disagreement with that obtained using Scherrer’s equation, both methods lead to the same
Figure 6. Distribution of stacking number for HFy1050 coal char at various conversion levels after gasification in air at 653 K.
finding that the behavior of the raw char in air gasification is very similar to that in CO2 gasification, but is different from that of the ash-free char gasified in air. The results strongly suggest that the mineral matter (e.g., iron) plays an important role in gasification, as is well-known, and the effect of burnoff on the crystalline structure is independent of the gasifying agent, as shown by Mahajan et al. Crystallite Structure by Shi et al.’s Method. Attempts using the method of Shi et al. to obtain the structural change during gasification were not successful. Although the X-ray diffraction patterns could be fitted well, the values of the crystallite size obtained were unreasonable. For some chars at high conversion levels, it was difficult to achieve convergence of solutions. Figure 10 shows the fittings of the XRD patterns for rawy1050 at 0% conversion (Figure 10a) and at 35% conversion (Figure 10b). The fits for both chars are very good and the main structural parameters obtained for unreacted rawy1050 are as follows: d002 3.37 Å, La 18.71 Å, Lc 55.42 Å, and fraction of organized carbon 0.35. These numbers are reasonable and consistent with the
Crystalline Structure of Coal Char during Gasification
Figure 7. Distribution of stacking number for rawy1050 coal char at various conversion levels after gasification in air at 653 K.
results reported earlier.23 However, the structural parameters for rawy1050 at 35% conversion are larger than those for rawy1050 at 0% conversion, which is unlikely to be correct. The results for rawy1050 at higher conversions show a very high value of the crystallite size, and often the value of the fraction of organized carbon is negative. No clear trends could be found for the variation of the structural parameters. The fact that the model works well for the unreacted chars but not for the partially gasified chars might be a result of the following reasons. The XRD pattern of the chars at a high conversion level is not as good as that of the unreacted char. Particularly, the (11) peak of the partially gasified char is much lower than that of the fresh char, and this peak is very important in obtaining the structural parameters, such as the inplane strain and fraction of organized carbon. The (10) peak is also becoming lower with an increase of conversion, and this peak is particularly important in the calculation of the crystallite size. In other words, this method is more sensitive to the height and shape of the peaks compared with that of the other two methods. Another possibility is that the assumption of the strain
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Figure 8. Distribution of stacking number for rawy1150 coal char at various conversion levels after gasification in carbon dioxide at 1073 K.
probability distribution does not hold for the partially gasified chars any longer. In the model of Shi et al., a combination of a Dirac δ-function to represent the lowstrain regions and a Gaussian distribution to represent the highly strained layers is used. This distribution works well for the unreacted chars after heat treatment at various conditions. However, it might be inappropriate for the partially gasified chars because of preferential gasification of disorganized carbon in char. Gasification occurs preferentially at certain sites and probably at high energy sites at low conversion and later at lower energy sites. This affects the strain distribution, which is no longer random. HRTEM Images. The HRTEM fringe images of some partially gasified chars are shown in Figure 11. The graphene layers can be identified easily, although the crystallite size is difficult to estimate visually from the micrographs. It is clear from the images that the spaces between the layers do not change significantly, supporting the experimental findings that the locations of the peaks on the pore-size distribution curves obtained from physical gas adsorption do not change with carbon
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Figure 9. Variation of fraction of disordered carbon of Yarrabee coal chars with carbon conversion in air at 653 K and in carbon dioxide at 1073 K.
Figure 10. Experimental and fitted XRD patterns for (a) unreacted rawy1050, and (b) rawy1050 at 35% conversion. The XRD patterns are fitted using the model of Shi et al.30
conversion level.24 This implies structure shrinkage during gasification, which will be discussed in detail below. Interestingly, highly ordered structure is observed in the chars, as shown in Figure 12. The elemental composition was determined using an energy dispersive spectrometer. The carbon structure in the vicinity of clays is visually much more ordered. The large size of graphene layers can also be seen even in a char with a conversion level of as high as 86% in the vicinity of the iron particles. Ordered carbon is known to be less reactive than disordered carbon and remains even at the late stage of gasification. Also, the fact that it is associated with iron particles, which are good gasification catalysts, is intriguing and requires further study. Gasification of Yarrabee Char in Air. Complementary studies on the variation of crystalline structure and pore structure with carbon conversion for the Yarrabee coal chars gasified in air and CO2, reported
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recently,24 have led to some interesting observations, which are summarized as follows: ‚The pore volume and surface area of small micropores (ca. 6 Å) do not change with carbon conversion.24 ‚The positions of the peaks on the pore size distribution curves for different chars at various conversion levels are almost exactly the same.24 This suggests shrinkage processes occurring simultaneously with pore growth or enlargement.4,10 ‚The crystalline structure of carbon changes with conversion in the same way between gasification of the raw coal char in air and CO2, but in a different way between air gasification of the raw and the ash-free coal char. This suggests that the observed difference in pore structure development between gasification of the raw char in air and CO2 is due to a difference in the accessibility of the pores to adsorption between the two cases. ‚The mean lateral width of the crystallites in the Yarrabee coal chars decreases with an increase of carbon conversion, while the stacking height does not change until a high conversion (60% for the raw coal chars and >80% for the ash-free coal char, based on Scherrer’s method). ‚The difference in gasification behavior between the raw coal char and the ash-free coal char is the variation of Lc. This suggests that the inorganic impurities in coal influence gasification on the basal planes rather than on the edge sites. Combined with the structural variations reported here, the above observations permit the formulation of a conceptual model of the gasification process, schematically shown in Figure 13. This figure presents the gasification process of only a few crystallites in a single grain (drawn as a big circle). There are actually far more crystallites in a grain. Also the crystallites are not as flat as those shown in the figure and are aligned randomly and not so regularly as shown here. The gasification is considered to occur in the following manner. At the beginning of gasification (x ) 0%), some micropores are blocked by pyrolysis products of heavy molecules or closed due to edge fusing and are not accessible to gas adsorption.24 Therefore the measured surface area is very low (60%). The results of the variation of Lc with conversion obtained using the Alexander and Sommer method are considered to be doubtful, as discussed before. Therefore these results need to be validated by another independent method. Another XRD analysis method30 which was successfully used previously23 does not provide consistent and convincing results of the variation of Lc and the fraction of disordered carbon. Quantitative analysis of HRTEM images19,34 could serve as the benchmark at the end although it is laborious to obtain high-quality HRTEM images, and the validation of the results is still questionable. Implications for Gasification Studies. The present work demonstrates that the crystalline structure of two carbons might be very different although their pore (34) Shim, H.-S.; Hurt, R. H.; Yang, N. Y. C. Carbon 2000, 38, 2945.
structure appears to be similar. The reverse, namely, similar crystalline structure but different pore structure, is also possible, as seen here for oxygen and CO2 gasification. This suggests the importance of a simultaneous study of the porosity as well as the crystalline structure, while examination of only one of these provides incomplete information. Conclusions The variation of the crystalline structure of several coal chars with carbon conversion in air and in CO2 has been studied and the following conclusions can be made. ‚The variation of structure with conversion is almost the same for air and CO2 gasification. This suggests that the difference between air gasification and CO2 gasification in the development of porosity probably resulted from the slower opening of the closed micropores in CO2 gasification compared to air gasification. ‚There is an apparent difference between the raw coal char and the ash-free coal char in the variation of crystallite height during air gasification. This confirms the well-known fact that most mineral matter in coal catalyzes the gasification process. ‚Large and highly ordered crystallites are observed for a coal char gasified in air with a conversion level of
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86% in the vicinity of iron particles in char. This indicates that the organized carbon could remain ungasified, and contribute to the residue carbon in ash. These ordered graphene layers are probably formed during pyrolysis, as catalytic graphitization of the same coal by iron has been observed. ‚The variation of the crystalline structure of two carbons can be different, although the variation of their pore structure is similar. This suggests the importance
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of a combined study of the structure and texture for a full quantitative understanding of carbon structure. Acknowledgment. The financial support of the Australian Research Council (ARC) under the Large Research Grant Scheme is gratefully acknowledged EF0202541
