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Characteristics of Chars Prepared from Various Pulverized Coals at Different Temperatures Using Drop-Tube Furnace Liming Lu,† Veena Sahajwalla,† and David Harris*,‡ School of Materials Science and Engineering, The University of New South Wales, Sydney NSW 2052, Australia, and CSIRO Division of Energy Technology, Queensland Centre for Advanced Technologies, 2643 Moggill Road, Pinjarra Hills, QLD 4069, Australia Received November 10, 1999. Revised Manuscript Received March 10, 2000
Char samples were prepared from five Australian black coals at different temperatures using a well-characterized drop-tube furnace. The characteristics of resultant chars and their parent coals were determined by such techniques as chemical analysis, XRD (X-ray diffraction), FESEM (field emission scanning electron microscopy), and the newly developed FIB (focused ion beam) miller. Some conventional particle analysis techniques were also used. The atomic structure, physical structure, and chemistry of chars were investigated as a function of pyrolysis conditions including pyrolysis temperature and coal type. Pyrolysis temperature is one of the key parameters influencing the char atomic structure. In general, the chars become more ordered and condensed with increasing pyrolysis temperature. This could be seen in their crystallite size (L11), aromaticity (fa), and interlayer spacing (d002/dγ). After being treated at 1200 °C, the L11 value increased from 5.9-6.5 Å for raw coals to 7.8-10 Å for the chars, and fa increased from 58-72% for raw coals to 75-82% for chars. Meanwhile d002 decreased from 3.5-3.59 Å for raw coals to 3.46-3.53 Å for chars, and dγ decreased upon charring from 4.72-5.01 to 4.33-4.74 Å. The char atomic structure is also dependent on coal type. However, the strong dependence on volatile matter observed for raw coals is diminished for the chars. Although all chars, except chars from coal AC-5, have similar spherical morphologies and surface areas, chars from different coals demonstrate extremely different pore structure. Some chars are very porous and light, while the others are solid and dense. The chemical analysis of chars shows that the atomic ratios H/C and O/C significantly decrease with increasing pyrolysis temperature. At pyrolysis temperatures of 1200 °C or greater, all the obtained chars have similar H/C and O/C ratios regardless of their origins. The decrease in H/C ratio is in agreement with the disappearance of γ band and increasing aromaticity, which were observed in char XRD spectra with increasing pyrolysis temperature.
Introduction In coal utilization processes, heterogeneous reactions of chars with oxidizing gases play an important role in determining the level of coal conversion, that can be achieved. It is therefore important that such reaction kinetics be understood when assessing coals for use in technologies, such as pulverized coal injection (PCI) in blast furnaces and those being developed for advanced power generation technologies. Heterogeneous char reaction can be limited by gas-phase diffusion, a combination of pore diffusion and chemical reaction, or purely chemical reaction depending on the reaction temperature, particle size, reactor type, and other reaction conditions. Therefore, the chemical and physical structures of the char produced during pyrolysis could be expected to have a significant influence on the subsequent reactions involved in coal combustion and gasification. * Corresponding author. † The University of New South Wales. ‡ CSIRO Division of Energy Technology, Queensland Centre for Advanced Technologies.
Often the char structure and kinetics of char/gas reactions are investigated separately with their relationship usually not studied in great detail. Many studies have been conducted to characterize the ultrafine structure of coal and its resultant char. An extensive range of analysis techniques,1-12 including TEM, (1) Oberlin, A. Carbon 1979, 17, 7. (2) Davis, K. A.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 31. (3) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (4) Orendt, A. M.; Solum, M. S.; Sethi, N. K.; et al. In Advance in Coal Spectroscopy; Meuzelaar, H. L. C., Ed.; Plenum: New York, 1992; pp 215-254. (5) Schoening, F. R. L. Fuel 1983, 62, 1315. (6) Lin, Q.; Guet, J. M. Fuel 1990, 69, 821. (7) Bladen, H. E.; Gibson, J.; Riley, H. L. Conference of Ultrafine Structures of Coals and Cokes [Proceedings]; BCURA: London, British, 1944; p 176. (8) Farrell, K.; Lu, L.; Sahajwalla, V.; Harris, D. 8th Australian Coal Science Conference [Proceedings]; The Institute of Energy Australia: Sydney, Australia, 1998; p 1. (9) Radovic, L. R.; Walker, P. L.; Jenkins, R. G. Fuel 1983, 62, 849. (10) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; et al. Energy Fuels 1990, 4, 319. (11) Kajitani, S.; Matsuda, H. 8th Australian Coal Science Conference [Proceedings]; The Institute of Energy Australia: Sydney, Australia, 1998; p 295.
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XRD, NMR, FT-IR, ROMAN, MS, and others, have been applied. However, most of these studies mainly focus on the coal chemistry to establish fundamental understanding of coal chemical structure. The combustion/ gasification kinetics of carbonaceous materials has also been extensively investigated.9,13-20 In an extensive review, Smith 13 examined the results obtained from different laboratories and reactors. His results support the following two facts: (1) The combustion/gasification rate of different carbonaceous materials can be significantly different; (2) As the combustion proceeds, the carbonaceous materials become less reactive to oxidizing gases. However, the underlying reasons for the observed deactivation during reaction are unclear and could be associated with surface area change, microstructure evolution such as thermal annealing, or perhaps loss of active sites. Smith et al.21 attempted to correlate coal/char structure to its reaction behavior. However, because of a lack of reliable data on both sides, they reported only semiquantitative conclusions; no quantitative relationship between the structural parameters and reaction kinetics has been derived. The fact that the char structure changes significantly during reaction further complicates the situation. In their recent papers, Hurt and co-workers2,22 observed the structure evolution during combustion and explained the deactivation of carbonaceous materials during combustion as a consequence of thermal annealing. Some other authors9,11,23 have attempted to correlate XRD measurements to reaction kinetics and hence make prediction of reaction performance based on structure analysis possible. An objective of this research is to correlate coal/char structure to char reaction kinetics. This paper focuses on structural characterization of chars generated under various pyrolysis conditions and from different coals. These well-defined chars will subsequently be used in studies of char combustion and gasification where the reaction kinetics will be measured and correlated with char structure. Experimental Section Coal Samples. Five Australian black coals, ranging from semi-anthracite to high volatile bituminous, were selected for this investigation. The lump coals were first crushed to millimeter size in a jaw crusher, then ground to micrometer (12) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42. (13) Smith, I. W. 19th International Symposium on Combustion [Proceedings]; The Combustion Institute: Pittsburgh, 1982; p 1045. (14) Harris, D. J.; Smith, I. W. 23rd International Symposium on Combustion [Proceedings]; The Combustion Institute: Pittsburgh, 1990; p 1185. (15) Valix, M.; Harris, D. J.; Smith, I. W.; et al. 24th International Symposium on Combustion [Proceedings]; The Combustion Institute: Pittsburgh, 1992; p 1217. (16) Wells, W. F.; Smoot, L. D. Fuel 1991, 70, 454. (17) Walker, P. L.; Tayler, R. L.; Ranish, J. M. Carbon 1991, 29, 411. (18) Cai, H. Y.; Gu¨eu, A. J.; Chatzakis, I. N.; et al. Fuel 1996, 75, 15. (19) Gale, T. K.; Bartholomew, C. H.; Fletcher, T. H. Energy Fuels 1996, 10, 766. (20) McDonald, K. M.; Hyde, W. D.; Hecker, W. C. Fuel 1992, 71, 319. (21) Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; et al. The Structure and Reaction Processes of Coal; Plenum: New York, 1994; p 1. (22) Hurt, R. H.; Davis, K. A.; Yang, N. Y. C.; et al. Fuel 1995, 74, 1297. (23) Kashiwaya, Y.; Ishii, K. ISIJ Int. 1991, 31, 440.
Lu et al. Table 1. Summary of Chemical Analysis of Five Selected Coals item proximate: moisture (ad), % ash volatile matter fixed carbon ultimate: carbon (daf), % hydrogen nitrogen sulfur oxygen
AC-1
AC-2
3.3 11.5 34.5 50.7 81.9 5.49 1.81 1.28 9.6
2.7 1.6 18.6 8.4 26.9 15.1 51.8 74.9 82.9 90.0 5.02 4.36 1.66 1.82 0.43 0.82 10.0 3.0
AC-3
AC-4
AC-5
0.8 2.5 11.8 6.5 24.6 9.6 62.8 81.4 88.3 91.3 5.27 3.70 1.85 1.88 0.68 0.56 3.9 2.6
Figure 1. Temperature profile along centerline with different distances between injector and sampling probe (×: 160 mm; O: 200 mm; 4: 280 mm). size in a vibrating grinder, and finally wet sized at 75 and 45 µm. Chemical analysis of the graded samples (-75 + 45 µm) is summarized in Table 1. Char Preparation. The drop-tube furnace (DTF) used in this investigation consists of a coal feeding system, sampling probe, gas distribution system, and a high-temperature furnace. The composition of gaseous reactants, the position of the sampling probe, as well as the feeding rate of coal particles are adjustable depending on the devised experimental conditions. The furnace can be operated up to a maximum temperature of 1600 °C. The gas temperature profile along the center line of the working tube has been measured as a function of axial distance from the injector. As shown in Figure 1, hightemperature gradients near both the injector and sampling probe and a long isothermal zone were attained. It can also be seen from the measurement that the temperature profile is independent of the position of the sampling probe, which is inserted from the bottom of the reactor. All chars were prepared in the DTF mentioned above at three different gas temperatures (900, 1200, and 1500 °C), with a residence time of around 1 s. A particle heating-rate of approximately 104 °C/s can be expected for the particle size used here.24 The pyrolysis process was completed at 1 atm and under a slightly oxidizing atmosphere (1 v/v % O2). This was considered necessary to avoid contamination of the char samples with soot and condensed tars. Char Characterization. Char atomic structure was characterized by X-ray diffraction. All the XRD samples were chemically treated to remove mineral matter and hence the disturbance of mineral matter on the quantitative analysis of XRD spectra. The demineralization process was conducted as follows: A weighed sample of char (5 g) was dispersed in 30 mL of concentrated HCl solution (36.5 wt %). The mixture was stirred for 3 h at 50 °C. The char was filtered off and washed (24) Field, M. A.; Gill, D. W.; Morgan, B. B.; Hawksley, P. G. W. Combustion of Pulverised Coal; The British Coal Utilisation Research Association: Leatherhead, 1967; p 25.
Chars Prepared from Various Pulverized Coals
Figure 2. Corrected X-ray intensity curve of char prepared from AC-1 coal at 1200 °C. with distilled water. The HCl-treated sample was then mixed with 30 mL of concentrated HF solution (48 wt %). The mixture was stirred for another 3 h at the same temperature and filtered. Finally, the treated char was washed with distilled water and air-dried at ambient temperature. A Philips 112 X-ray diffractometer was used to record X-ray diffraction spectra from the chemically treated chars and coals. Copper KR radiation (30 KV, 30 mA) was used as the X-ray source. Samples were packed into an aluminum holder and scanned in a step-scan mode (0.1°/step) over the angular 2θ range of 5-115°. Intensities were collected for 8 s at each step. The obtained spectrum was corrected for polarization and then converted to reduced intensity using techniques described in the literature.25-27 Figure 2 is a typical corrected intensity of AC-1 coal char, in which the background has been subtracted. The position of the peak in low angle region (2θ ) 5-35°) in Figure 2 corresponds to the (002) peak of graphite, which is generally accepted as the stacking of the graphitic basal plans of char crystallite. Theoretically the (002) peak should be symmetric, the apparent asymmetry of this peak is concluded the existence of γ band on its left-hand side. The γ band and (002) band can be separated using techniques developed by Yen and co-workers.25 It is assumed that the γ band will not contribute to the high angle side of this peak due to its low intensity. Therefore, the high angle side of this peak can be used as a guide to delineate the (002) band, as shown in Figure 2. After the (002) peak is resolved, the remaining intensity in the low angle side of this peak is hence attributed to the γ band. Figure 2 presents the separated (002) and γ band. The spectra for other coals/chars have also been resolved into (002) and γ using the same techniques. The γ band, which usually occurs in the angular range of 16-23°(2θ) according to Ergun,28 has also been observed by many other authors,5,25,26,28 although its interpretation is not clear. Ergun28 suggested it was associated with packing of the saturated structure such as aliphatic side chains or condensed saturated rings. The other two peaks in the higher angle region of Figure 2, denoted as (10) and (11), can be attributed to hexagonal ring structures in chars. The chars were also characterized using FESEM, FIB, and other techniques. The details of these techniques are described elsewhere8 and are not discussed here.
Results and Discussion Atomic Structure of Chars by XRD. The atomic structure of carbonaceous materials can be examined (25) Yen, T. F.; Erdman, J. G.; Pollack, S. S. Anal. Chem. 1961, 33, 1587. (26) Schwager, I.; Farmanlan, P. A.; Kwan, J. T.; et al. Anal. Chem. 1983, 55, 42. (27) Franklin, R. E. Acta Crystallogr. 1950, 3, 107. (28) Ergun, S.; Tiensuu, V. H. Fuel 1959, 38, 64.
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directly or indirectly by TEM.1,2, NMR.3,4, XRD.5-8, FTIR.10, ROMAN,11 MS,12 and other advanced techniques, depending on the objectives and magnification required. The X-ray diffraction technique is nondestructive and well developed with good reproducibility. The XRD quantitative analysis technique was first used by Warren29,30 for carbonaceous materials and further developed by Franklin27 and Diamond.31,32 So far, various carbonaceous materials, such as graphite,6 carbon black,29,30 petroleum/coal asphaltenes,25,26 raw coals,33-36 and coal chars,37 have been examined using XRD. Different structural parameters, such as crystallite size, aromaticity, interlayer spacing, and other properties of carbonaceous materials, have been derived from the XRD spectrum. In the present investigation, the first three of the above parameters (L002/L11, fa, d002/dγ) and their relationship with coal type and pyrolysis temperature are examined in detail. Crystallite Size. Each band in XRD spectrum can be characterized by its peak position (φ), peak width at half-maximum intensity (B), as well as area under the peak (A). For crystallites suffering no lattice strain or distortion, the average stacking height, L002, can be expressed by the Scherrer’s equation,25,26,30,39
L002 )
0.89λ B002 cos φ002
(1)
where λ is the wavelength of the incident X-rays (Å). For copper KR radiation, the λ value is 1.5409 Å. B002 is the width of (002) peak at half-maximum intensity, and φ002 is the peak position. According to eq 1, L002 increases with decreasing B002. In other words, the sharper the (002) peak, the larger the crystallite. Figure 3 presents the X-ray intensity curves of raw coals and their chars produced at different temperatures. Based on the intensity curves, the average stacking height of crystallites in both coals and chars can be calculated from eq 1, as shown in Figure 4a. It is worthwhile to mention that different coals behave differently under heat treatment in terms of L002. As shown in Figures 3 and 4a, for low rank coals such as AC-1 and AC-4, the (002) band of char becomes sharper with increasing pyrolysis temperature. It is likely that increasing pyrolysis temperature will promote the growth of crystallites in chars, consequently increase the crystallite size. This is not the case for all the coals examined. The crystallite size in char generated from high rank coals such as AC-3 and AC-5 (Figures 3b and 4a) at moderate temperature (900-1200 °C) is slightly smaller than its parent coal. The larger crystallites observed for high rank coals may be attributed to coplanar coalescence of smaller nuclei as a result of long(29) Warren, B. E. Phys. Rev. 1941, 59, 693. (30) Biscoe, J.; Warren, B. E. J. Appl. Phys. 1942, 13, 364. (31) Diamond, R. Acta Crystallogr. 1957, 10, 359. (32) Diamond, R. Acta Crystallogr. 1958, 11, 129. (33) Ergun, S. Fuel 1958, 37, 365. (34) Wertz, D. L.; Bissell, M. Energy Fuels 1994, 8, 613. (35) Wertz, D. L.; Quin, J. L. Energy Fuels 1998, 12, 697. (36) Wertz, D. L. Fuel 1998, 77, 43. (37) Lu, L.; Sahajwalla, V.; Harris, D. 58th Ironmaking Conference [Proceedings]; The Iron and Steel Society: Chicago, 1999; p 599. (38) Ju¨ngten, H. In Fundamentals of the Physical-chemistry of Pulverised Coal Combustion; Lahaye, J., Prado, G.; Eds.; Martinus Nijhoff: Dordrecht, The Netherlands, 1987; pp 4-59. (39) Cullity, B. D. Elements of X-ray Diffraction; Addison-Wesley: Reading, MA, 1978; p 102.
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Figure 4. Relationship between pyrolysis temperature/coal type and crystallite size (L002/L11).
Figure 3. X-ray intensity curves of raw coals and chars prepared at different temperatures: (a) AC-1, (b) AC-3, and (c) AC-4.
term pressure-induced orientation and packing.33 When exposed to high temperature, such coalescence will break up, causing the observed reduction in char crystallite size at low and moderate temperatures. The average stacking diameter of char crystallites, Lhk, can be calculated from the width of either (10) or (11) band.25,26,28-30 According to Warren’s mathematical derivation,29 the expression for Lhk has exactly the same form as eq 1 except for a numerical prefactor about twice as large, i.e.,
Lhk )
1.84λ Bhk cos φhk
(2)
Due to its partial overlap with the (002) band on its left and with the (004) band on its right, the results from the (10) band are thought less reliable when compared to the (11) band.25 In this investigation, the (11) band was used to calculate the average stacking diameter of char crystallites, denoted as L11. Figure 4b presents the influence of pyrolysis temperature on L11 values. Unlike L002, L11 increases with pyrolysis temperature for all examined coals. It might be associated with strong bonds between carbon atoms on the graphitic basal plan. Figure 5 shows the relationship between coal/char crystallite size and the volatile matter (VM) of raw coal. According to Figure 5a,b, it appears that the coal crystallite size is very sensitive to its VM, and linearly decreases with increasing VM. This strong dependence of crystallite size on VM observed for raw coals is reduced for the chars. The crystallite size (L002/L11) for chars shows a less significant correlation with VM. This may imply that the parameters such as VM, which are used as a rank indicator to predict the coal properties, cannot be solely used to predict the properties of chars produced from them. Aromaticity. As shown in Figure 2, the peak in the low angle region of X-ray intensity curve, i.e., 2θ ) 5-35°, can be resolved into a (002) band and a γ band, which reflect the aromatic and saturated structures, respectively. Theoretically, the area under the (002) peak (A002) should be proportional to the number of atoms with graphitic order (Car). Similarly, the integrated area under γ band, Aγ, is proportional to the number of saturated carbon atoms (Csa). Therefore, the aromaticity of char can be defined:25
fa )
Car A002 ) Car + Csa A002 + Aγ
(3)
where A is the area under the corresponding peak, Csa
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Figure 5. Dependence of crystallite size of coal and char on the VM of raw coals.
Figure 7. Influence of pyrolysis temperature and coal type on the interlayer spacing of chars.
ticity corresponds to the disappearance of the γ band. Therefore the same trend can be observed in the X-ray intensity curves in Figure 3. For all coals, the (002) band becomes more symmetrical with increasing pyrolysis temperature since the γ band disappears gradually. According to Figure 6b, the aromaticity value increases from 58-72% for the selected coals to 75-82% after being treated at 1200 °C. The aromaticity values obtained in this research agree very well with the data reported in the literature.3 Again, the fa of chars is not a strong function of VM compared to that for raw coals, as shown in Figure 6b. Interlayer Spacing. Given the (002) peak position, the spacing between graphitic sheets, d002, can be calculated according to Bragg’s law:
d002 ) λ/(2 sin φ002)
Figure 6. Relationship between pyrolysis temperature/coal type and aromaticity, fa.
and Car, the number of saturated and aromatic carbon atoms per structure unit, respectively. Figure 6a presents the measured aromaticity values as a function of pyrolysis temperature. With increasing pyrolysis temperature more aliphatic side chains, which are not strongly bonded to the coal matrix, will detach from the coal matrix. They will then be transported out of the coal particles as volatiles, leading to an increasing aromaticity (fa), as shown in Figure 6a. According to the definition of aromaticity in eq 3, an increasing aroma-
(4)
Figure 7a illustrates the influence of pyrolysis temperature on the interlayer spacing of resultant chars. As shown in Figure 7a, d002 is strongly affected by pyrolysis temperature when the pyrolysis temperature is over 900 °C. With increasing pyrolysis temperature, the d002 value decreases, causing a more condensed char structure. According to the coal pyrolysis model suggested by Ju¨ngten,38 large amounts of radicals are generated in the early stage of pyrolysis due to the cleavage of linkages in the macromolecular structure of the coal. If insufficient hydrogen is present, these radicals can be recombined to form molecules which are too large to be transported out of the particle. These aromatic molecules condense further at higher temperatures to form higher aromatic coke structures. The
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Figure 8. FESEM morphologies of AC-1 (a) and its chars prepared at 900 (b) and 1500 °C (c).
higher the temperature to which the coal is exposed, the more energy it gets for its structural rearrangement and final condensation, causing more condensed structure in the resultant char. Similarly, the interchain spacing between two neighboring aliphatic side chains, dγ, can be calculated based on the γ band by eq 4. As shown in Figure 7b, the same trend was found. The char prepared at higher temperature is much more condensed than chars produced at low temperature. It is also seen that both d002 and dγ of char from a high rank coal are generally less than those for the low rank coal char. It is understood that the existing crystallites present in the parent coal can act as the nuclei for the growth of char crystallites during coal pyrolysis. Therefore coal type could be expected to influence the interlayer spacings of crystallite in char. The reason for why the changes in d002 are not significant at temperatures lower than 1200 °C for AC-3 coal is not clear. Although the peak position for both (10) and (11) can also be defined, these peaks represent twodimensional diffuse reflections and have no physical significance. In conclusion, pyrolysis temperature is a key parameter influencing the char atomic structure. In general, char becomes more ordered and condensed with increasing pyrolysis temperature. Although the char crystallite structure becomes more graphitic with increasing pyrolysis temperature, both (002) and (10) as well as (11) reflections of char are much broader than the spectrum of high purity graphite. This is due to its relatively limited crystallite size and low degree of graphitisation. The char atomic structure is also dependent on coal type. However, it does not show a strong correlation with coal VM as the coal did. It appears that the atomic structure of chars, especially those obtained at high pyrolysis temperature, become similar regardless of their origins. This is in agreement with results of Fletcher and co-workers.40 Physical Structure. Thus far, we have focused mainly on the atomic structure of coal and its derived chars. These properties are thought to perhaps play an important role when the char combustion or gasification is chemically controlled. At elevated temperatures, where either external or internal diffusion becomes rate limiting, the physical structure of char, such as pore structure, particle size, and other physical properties, becomes increasingly significant. Although some work has been done to attempt to model the pore structural evolution during heterogeneous char reaction, experimental measurements are still necessary as input and (40) Fletcher, T. H.; Solum, M. S.; Grant, D. M.; Pugmire, R. J. Energy Fuels 1992, 6, 643.
Figure 9. Physical structure of chars (a) surface area, (b) particle swelling.
verification for these models. In the present study, the field emission SEM (FESEM), focused ion beam miller (FIB), and some conventional techniques were used to characterize char physical structure. Figure 8 illustrates the external structures of AC-1 and its chars. It is obvious that char particles prepared at high temperature are more porous at the macropore level. The morphology observations show that all chars generated at 1200 °C and above have a similar spherical shape except AC-5 char, which being from a semianthracite coal maintains its angular shape without apparent softening during pyrolysis. No meaningful features, such as micropores and mesopores that are usually observed for partly burnt chars, were found for any of the chars even at extremely high magnification. The FESEM observation agrees with the surface area measurements very well. According to N2 adsorption surface area measurements, all examined chars regardless of the coal type and pyrolysis temperature have a very low and comparable surface area (around 0.5-3 m2/g). This is consistent with the apparent absence of micropores and mesopores in chars. As shown in Figure 9a, the surface area slightly increases with pyrolysis temperature. The relatively high surface area of raw
Chars Prepared from Various Pulverized Coals
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Figure 10. FIB images of chars prepared from AC-1, AC-2, and AC-5 at 1200 °C.
coal in Figure 9a is probably due to the cracks generated during coal preparation processes, such as crushing and grinding. These cracks will coalesce during the softening stage of coal pyrolysis. The swelling property of coal during pyrolysis is strongly affected by coal type and pyrolysis temperature. As shown in Figure 9b, all the examined coals, except coal 5 (nonswelling), show the maximum swelling at 900 °C, with coal 4 doubling its diameter, while the others show a slight increase in size. After being treated at a higher temperature (>)1200 °C), the diameters of all coals show either a negligible increase or a decrease, due to the strong fragmentation occurring during hightemperature pyrolysis. In FIB milling an ultra-fine (5 nm) energetic gallium ion beam is used to interact with the specimen surface and remove the surface atoms of specimen, making an in-situ insight into internal pore structure possible. FIB is a newly developed technique and can be used to observe the internal pore structure up to a magnification of about 300 000 times. In this investigation, the authors have attempted to apply this technique to characterize coal chars. Figure 10 presents the typical FIB images of chars prepared at 1200 °C from AC-1, AC-2, and AC5. A window with a certain depth has first been cut out from the char particles; this allowed the internal structure to be observed. The internal pore structure of resultant chars varies from coal to coal, as illustrated in Figure 10, and might have important implications on char reactivity as well as ash formation. Char 1 has very thin pore walls with large holes in the middle, through which the reactant gas can access the solid carbon without any difficulties. Early fragmentation and high reactivity during its combustion or gasification may be expected. Char 5 is not porous at all and seems very dense, with some cracks around the particle surface. The pore structure of char 2 shows intermediate characteristics. Figure 11 presents the bulk density of the five chars. The bulk density is consistent with the FIB observations. Elemental Composition. The atomic ratios H/C and O/C are often used to characterize coals.41 The observed decrease in H/C with increasing coal rank is probably due to the loss of aliphatic materials, the occurrence of dehydrogenation reactions and preferential oxidation, as well as the coalescence of aromatic units and loss of reactive edge site concentration. These variables could be expected to have implications for char structure and (41) Van Krevelen, D. W. CoalsTypology-Physics-Chemistry-Constitution; Elsevier: The Netherlands, 1993; p 180.
Figure 11. Bulk density of chars prepared at 1200 °C from selected coals.
Figure 12. Elemental composition of chars generated at different temperatures from various coals.
reactivity. An attempt to correlate hydrogen and oxygen content to char reactivity has been made.21 The ratio of H/C for chars generated at different temperatures from selected coals is presented in Figure 12a. It is obvious that the H/C of char prepared at high temperature is much lower than that for raw coals and chars produced from low temperature pyrolysis. This implies more aliphatic groups lost and more condensation occurring during high temperature pyrolysis. This is in agreement with the aromaticity increasing with pyrolysis temperature as seen in Figure 6a. Similarly,
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Figure 13. Relationship between measured aromaticity and H/C ratio.
Figure 12b presents the analogous information on the O/C ratio of coals and their derived chars. The same trend as observed for H/C ratio is seen. Figure 13 presents the correlation between the measured aromaticity and chemical analysis of coals and chars. As shown in Figure 13, the measured aromaticity agrees with the chemical analysis, with aromaticity increasing as H/C ratio decreases. Although the difference in atomic ratios between coals is significantly distinct, they are approaching a similar level (Figure 12a,b), especially when the pyrolysis temperature is over 1200 °C. Conclusions (1) For chars from low rank coals, the L002 value increases with pyrolysis temperature. However, the L002 value of chars obtained at low or moderate temperature
Lu et al.
from high rank coals is slightly lower than those for their coals. (2) For the all examined chars, L11 and fa always increase with pyrolysis temperature, while the interlayer spacings both between graphitic sheets and aliphatic side chains drop significantly. (3) As pyrolysis temperature increases, chars tend to be more ordered and condensed in most cases as reflected by their crystallite size (L11), aromaticity (fa), and interlayer spacing (d002 and dγ). (4) Char atomic structure is also affected by coal type. However, the strong dependence on volatile matter observed for raw coals is diminished for the chars. (5) All generated chars except chars from AC-5 have a similar spherical morphology and surface area. Chars from different coals feature different pore structure. Some chars are very porous and light, while the others are solid and dense. (6) The atomic ratios of chars H/C and O/C significantly decrease with increasing pyrolysis temperature. When the pyrolysis temperature is 1200 °C or over, all the obtained chars show the similar H/C and O/C ratio regardless of their origins. The decrease in H/C ratio with temperature is in agreement with the disappearance of γ band and increasing aromaticity observed in char XRD spectrum. Acknowledgment. The authors acknowledge the support of CRC for Black Coal Utilization which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia. EF990236S
