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Devolatilization and Cracking Characteristics of Australian Lumpy Coals Byong-chul Kim,* Sushil Gupta, Si-hyung Lee,† Sung-man Kim,† and Veena Sahajwalla School of Materials Science and Engineering, The UniVersity of New South Wales, Sydney, NSW 2052, Australia ReceiVed July 11, 2007. ReVised Manuscript ReceiVed September 25, 2007
An experimental study was conducted to investigate the devolatilization characteristics of five Australian coals in a thermogravimetric analysis (TGA) reactor by varying the coal lump size and the temperature. The swelling ratio was measured after thermal treatment of coal lumps in a horizontal tube furnace at 1273 K while the cracks generated in the lumpy char samples were examined using scanning electron microscopy (SEM). Physical and chemical properties of coal and char samples were measured using CO2 gas adsorption, Hg porosimetry, X-ray fluorescence (XRF) and X-ray diffraction (XRD). Under all the tested conditions, the total volatile yield of lumpy coals was found to be not influenced by either the temperature or particle size and was similar to that indicated in proximate coal analysis. However, as expected, the devolatilization rates were found to increase with increasing temperature as well as the increasing amount of volatiles present in the coal. The study further demonstrated that the effect of coal properties on the devolatilization rates of lumpy coals may not be significant as the rates decrease with increasing lump size, such that coal lumps with sizes more than 10 mm indicated similar orders of reaction rates. The apparent activation energy of coal lumps indicated a linear correlation with the stack height of the carbon crystallite of coals. The study demonstrated that the cracking and swelling behavior of coals was influenced by physical as well as chemical properties, particularly their modification during devolatilization conditions. The study showed that coals with low volatiles indicated high cracking which would increase further with increasing lump size in accordance with the size effect. The cracking tendency of coals appeared to have a reciprocal association with swelling tendency such that less swelling coals are more vulnerable to cracking.
Introduction Coal is a complex heterogeneous substance which undergoes a variety of physical and chemical changes during pyrolysis. Understanding the high temperature behavior of coal, particularly lumpy coals, such as the devolatilization kinetics, swelling, and cracking is important in order to improve the process efficiency of ironmaking in current and emerging smelting processes such as Corex. The thermoplastic behavior of coal would indirectly affect current blast furnace efficiency due to the implications on coke quality during carbonization and would have direct impact in emerging smelting processes due to impact on coal decrepitation, attrition, and breakup behavior which are gaining popularity due to strong economical and environmental benefits including low SOX, NOX, and net CO2 emissions.1–9 In * Corresponding author. Tel.: 61 2 9385 6597. Fax: 61 2 9385 5956. E-mail:
[email protected]. † Ironmaking Research Group, POSCO Technical Research Laboratories POSCO, P.O. Box 36, Pohang, Korea 780 795. (1) Wright, J. K.; Taylor, I. F.; Philp, D. K. Miner. Eng. 1991, 4 (7– 11), 983–1001. (2) Zervas, T.; McMullan, J. T.; Williams, B. C. Int. J. Energy Res. 1996, 20 (1), 69–91. (3) Joo, S.; Kim, H. G.; Lee, I. O.; Schenk, J. L.; Gennari, U. R.; Hauzenberger, F. Scand. J. Metal. 1999, 28 (4), 178–183. (4) Shin, S. K.; Sahajwalla, V.; Kang, T. I. Coal Char Gasification in the COREX Process. 59th Ironmaking Conference Proceedings, Pittsburgh, PA, March 26–29, 2000; ISS: Warrendale, PA, 2000; Vol. 59, pp 351– 356. (5) Emi, T.; Seetharaman, S. Scand. J. of Metal. 2000, 29 (5), 185– 193. (6) Usachev, A. B.; Romenets, V. A.; Lekherzak, V. E.; Balasanov, A. V. Metallurgist 2002, 46 (3–4), 117–130.
emerging technologies, coal quality requirements are expected to be different compared to those required for pulverized coal injection (PCI) and coking applications for blast furnace processes,9–11 particularly high temperature phenomena such as pyrolysis, swelling, and degradation mechanisms.4 For example, high volatile coals initiate gasification at low temperatures due to less cracking of volatiles leading to excess coal consumption12 while high moisture increases adverse endothermic effects.10,12 High ash content of coal is invariably discouraged in all of the processes in order to avoid the negative effects of high slag volume and flux requirements.13 During pyrolysis, surface area (7) Lee, I. O.; Shin, M. K.; Cho, M.; Lee, H. G. ISIJ Int. Suppl. 2002, 42, S33–S37. (8) Kim, B. C.; Gupta, S.; Lee, S. H.; Kim, S. M.; Sahajwalla, V. Devolatization and Cracking Behavior of Australian Lumpy Coals at High Temperatures. AISTech 2007 Proceedings, Indianapolis, IN, May 7–10; AIST: Warrendale, PA, 2007; Vol. 1. (9) Kumar, P. P.; Gupta, D.; Naha, T. K.; Gupta, S. S. Ironmaking Steelmaking 2006, 33 (4), 293–298. (10) Heckmann, H. Coals and Coal Requirements for the COREX Process. Proceedings of the 13th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 3–7, 1996; Center for Energy Research, University of Pittsburgh: Pittsburgh, PA, 1996; Vol. 2, pp 1200– 1205. (11) Chatterjee, A. Technical Evaluation of the Romelt Process for Possible Application under Indian Conditions. Proceedings of the 1st International Conference on Process DeVelopment in Iron and Steelmaking, Scanmet I, Luleå, June 7–8, 1999, MEFOS: Sweden, 1999; pp 325–335. (12) Gupta, S. K. J. Mines Met. Fuels 2002, 50 (7–8), 300–305. (13) Wibberley, L. J.; Olivares, R. I. New Iron and Steelmaking Processes—Impact on Utilization of Australian Coal. NERDDC/ACARP Report C1414, Brisbane, Australia, 1993.
10.1021/ef700397t CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2007
Characteristics of Australian Lumpy Coals
and porosity are often modified depending on coal rank,14–17 e.g., low-rank coals contain a higher proportion of macropores and a lower proportion of micropores compared to high-rank coals.18 In addition to the effect of coal properties, swelling and cracking can also be influenced by the process conditions such as the residence time, temperature, and heating rate, e.g., slow heating rates can significantly suppress coal swelling.19,20 Coal can also breakup under severe temperatures and rapid heating rates depending on the size. Fragmentation of small coal particles has been shown to depend on the temperature and the volatile yield, which is believed to increase with increasing plasticity.21 Swelling can weaken coal strength due to local pressure and viscosity gradients occurring within the particle during devolatilization.20,22 There is limited understanding of the high temperature behavior of lumpy coals other than the few exceptions where swelling was shown to increase with increasing particle size and volatile matter.23 A high heating rate is shown to increase coal swelling.4 The devolatilization rate is known to be independent of particle size for particles less than 0.5 mm,24 as smaller particles would offer less resistance to volatile release due to smaller escape lengths20,25 while the yield decreases as particle size is further increased up to 1.0 mm.24 Despite extensive research on coal pyrolysis, there is limited information available about the high temperature behavior of coal lumps which are of particular interest to a variety of smelting processes in which optimization of the feed coal size is critical to avoid the adverse effect of fines generation due to coal breakup. Improved understanding of the thermal behavior of lumpy coals is also required for developing a bench-scale methodology to assess coal performance in emerging smelting reactors, as currently no specific test is available. Therefore, an experimental study was carried out to simultaneously investigate the devolatilization, swelling, and cracking behavior of Australian coals with special focus on the lump size of coals and the devolatilization temperature. Experimental Sample Preparation. Five Australian coals were selected on the basis of volatile matter (VM) varying from 10-35%. Table 1 provides the properties of coals, which can be divided into three groups: namely, the low VM (LVM; A, B), medium VM (MVM; C), and high VM (HVM; D, E) groups. All the coals were from Bowen Basin, Queensland, except coal E which was from Hunter Valley, New South Wales. Coal samples were dried by heating in (14) Kinney, C. R.; Nunn, R. C.; Walker, P. L., Jr. Ind. Eng. Chem. 1957, 49, 880–884. (15) Nandi, S. P.; Ramadass, V.; Walker, P. L., Jr. Carbon 1964, 2, 199–210. (16) Nandi, S. P.; Walker, P. L., Jr. Fuel 1971, 50 (4), 345–366. (17) Walker, P. L., Jr. Fuel 1980, 59 (11), 809–810. (18) Grey, V. R. Fuel 1988, 67 (9), 1298–1304. (19) Saxena, S. C. Prog. Energy Combust. Sci. 1990, 16, 55–94. (20) Howard, J. B. Fundamentals of Coal Pyrolysis and Hydropyrolysis in Chemistry of Coal Utilization; Elliot, M. A., Ed.; John Wiley and Sons: New York, 1981; second supplementary volume, pp 665–784 (21) Cho, M. Y.; Lee, S. D.; Lee, J. H.; Shin, M. K.; Joo, S.; Lee, I. O.; Jung, B. J. Quality of Coal for the New Ironmaking Process. 13th Coal Conference Proceedings, Pittsburgh, PA, September 3–7, 1996; Center for Energy Research, University of Pittsburgh: Pittsburgh, PA, 1996; Vol. 2, pp 1213–1218. (22) Melia, P. F.; Bowman, C. T. Combust. Sci. Technol. 1983, 31 (3– 4), 195–201. (23) Fu, Z.; Guo, Z.; Yuan, Z.; Wang, Z. Fuel 2007, 86 (3), 418–425. (24) Anthony, D. B.; Howard, J. B.; Hottel, H. C. Fuel 1976, 55 (4), 121–128. (25) Griffin, T. P.; Howard, J. B.; Peters, W. A. Energy Fuels 1993, 7 (2), 297–305.
Energy & Fuels, Vol. 22, No. 1, 2008 515 Table 1. Chemical and Thermal Properties of Five Coals A
B
C
D
E
mositure volatiles fixed carbon ash FSI
Proximate (%, air-dried basis) 1.7 2.1 2.6 8.7 20.6 27.5 79.8 70.6 62.8 9.8 6.7 7.1 8.0 7.5
2.5 32.5 58.3 6.7 9.0
4.7 35.1 52.1 8.1 4.5
carbon hydrogen nitrogen sulfur oxygen
Ultimate (%, dry, ash-free basis) 91.4 89.9 87.7 86.2 3.8 4.9 5.1 5.5 1.8 2.0 2.1 2.2 0.8 0.5 0.6 0.7 2.2 2.7 4.5 5.4
83.7 4.8 2.2 0.6 8.7
Ash Component (%, expressed as oxides) 53.2 60.6 53.2 51.1 25.4 21.4 28.7 37.2 8.6 8.4 6.9 3.5 4.9 2.7 3.8 1.9 1.5 1.8 1.1 0.5 0.7 1.1 1.3 0.9 1.1 0.6 0.5 0.6 0.6 0.5 0.5 0.2 3.2 1.2 2.4 1.3 0.7 1.2 1.5 2.0
68.6 24 2.7 0.7 0.6 1.1 0.5 0.3 0.3 1.1
SiO2 Al2O3 Fe2O3 CaO MgO K2 O Na2O SO3 P2O5 TiO2 vitrinite inertinite liptinite Rra
Maceral Analysis (%, mineral free) 67.1 59.5 66.0 87.4 32.4 40.8 33.2 10.6 0.5 0 0.7 2.0 2.4 1.24 0.96 0.89
Tsb Tf c Trd Tr - Ts
Gieseler Plastometer Test (K) 713 678 668 733 718 713 nae 753 753 743 40 75 75
68.1 18.2 13.7 0.66 678 703 713 35
a R , mean vitrinite random reflectance. b T , initial softening r s temperature. c Tf, maximum fluidity. d Tr, resolidification temperature. e na, not available.
an oven at 383 K for 24 h as per ASTM standards to remove the moisture. The coal samples had a wide range of sizes and irregular shapes. Coals were separated into different size groups using a 5 mm sieve. Coal particles below 5 mm were used as such, while large particles (>5 mm) were shaped into a regular hexahedron shape by manual grinding. A symmetric cubic shape was used in order to keep the identical physical dimensions of different coals and improve the accuracy of measuring changes in surface dimensions of each sample after devolatilization. The arithmetic mean of vertical and horizontal axes of the shaped specimens was considered as the original particle size of coal. Thermogravimetric Analysis (TGA). A custom-made TGA furnace was used to monitor the weight loss of coal particles during pyrolysis.4 The TGA consists of a mechanically movable vertical alumina tube furnace, Precisa (1212 MSCS) analytical balance having 1.2 kg of capacity (1 mg accuracy), data logging computer, and furnace. The furnace temperature was controlled by using a thermocouple external to the reaction tube, while the sample temperature was monitored through a thermocouple located under the sample holder inside the reaction tube. Weight loss was continuously recorded every 1 s by the data logger connected to a balance and computer. The coal specimen was placed in the sample holder and held in the sealed cold zone of the furnace. The furnace was purged for 30 min with N2 (99.99% flowing at the rate of 2.5 L/min). The furnace was heated to the specified temperature by heating at the rate of 2 °C/min. The sample assembly containing the coal specimen was pushed into the hot zone after the furnace acquired the specified temperature, and then, the pyrolysis of the lumpy coal was initiated under isothermal conditions. After completion of pyrolysis, the sample assembly was moved back to the cold zone before removing the sample. Pyrolyzed coal samples were subsequently used for other analyses including X-ray diffraction (XRD).
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Figure 1. Schematic of the horizontal furnace used for visualization of coal swelling during pyrolysis.
Figure 3. (a) Comparison of weight loss of five coals with time at 1173 K. (b) Variation of rate constants of five coals with volatile matter at four temperatures.
Scattered X-ray intensities were collected for 5 s at each step. Carbon structure parameters were calculated from the XRD patterns.26
Results and Discussion Figure 2. (a) Comparison of mass loss in TGA with time of coal A (VM, 8.7%) and coal C (VM, 27.5%) at four devolatilization temperatures. (b) Comparison of rate constant variation of five coals with temperature.
Coal Swelling. The swelling behavior of coals was examined in a custom-made high horizontal tube furnace (Figure 1). The hexahedron coal particles were kept in the cold zone followed by insertion into the hot zone at the specified test temperature. Nitrogen (99.99% purity, 2.5 L/min) was purged through the furnace during the tests. A high resolution charge-coupled device (CCD) camera was used to monitor the dynamic changes of the physical configuration of the samples. The swelling test for each coal was repeated five times. Each side of the original and devolatilized coal lumps was photographed using a digital camera. The swelling ratio was then calculated on the basis of changes in the total area of each external surface of the lump, by averaging the three results with minimum standard deviation. Coal and Char Properties. A Quantachrome micropore analyzer (NOVA 3200) at UNSW was used to analyze the surface area of the coal and char particles using CO2 adsorption at 273 K. The Dubinin–Radushkevich (DR) surface area was calculated using NovaWin 1.12 software. The bulk porosity of samples was measured using a high-pressure mercury intrusion porosimeter (Micromeritics AutoPore IV) from a commercial laboratory. A scanning electron microscope (JEOL-840) was used to examine the breakage or swelling features of devolatilized char samples using a voltage of 10 kV. The XRD spectra of powder specimens were obtained using Siemens D5000 X-ray diffractometer by recording scattered intensities using Cu KR radiation (30 kV, 30 mA). Powder samples were adhered to a regular square glass plate and scanned in step-scan mode (0.05°/step) over the angular range of 5 to 105° (2θ).
Effect of Temperature. Figure 2a illustrates the effect of temperature on the weight loss of lumpy coal particles (8–10 mm) in two coal types during devolatilization in a TGA reactor. Figure 2a shows that total weight loss of both coals occurred in less than 120 s; however, the rate of weight loss becomes faster as the temperature of devolatilization is increased from 1173 to 1473 K. Figure 2a further shows that the relative amount of weight loss of coal C is greater than that of coal A, which is again similar to that expected on the basis of the higher volatile yield of coal C (Table 1). Figure 2a further indicated that the maximum weight loss observed in the TGA test was of the same order as reported in the proximate analysis on the basis of Australian standards, being 27.5% and 8.7% for coals C and A, respectively (Table 1). A similar relationship between the maximum weight loss observed in the TGA and the reported volatile yield was noted for remaining coals. This implies that under the tested conditions maximum volatiles released are not influenced by the selection of the devolatilization temperature. Figure 2a further shows that the devolatilization temperature does influence the rates at which volatiles come out of coal lump, such that same amount of volatiles can be released in half of the time (60 s) when the devolatilization temperature of coal A is increased from 1173 to 1473 K. A similar effect of temperature in decreasing the duration of volatile release was observed for coal C as well as other coals. The intensity of volatile release can be quantitatively compared in terms of the devolatilization rates,27,28 which are (26) Lu, L.; Sahajwalla, V.; Kong, C.; Harris, D. Carbon 2001, 39 (12), 1821–1833. (27) Biagini, E.; Lippi, F.; Petarca, L.; Tognotti, L. Fuel 2002, 81 (8), 1041–1050.
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Figure 6. Correlation between the activation energy and stack height of carbon crystallites (Lc) of original coals.
Figure 4. (a) Arrhenius plots of five coals. (b) Association of activation energy (Ea) with the volatile matter of coals. Figure 7. Comparison of the rate constant variation of three coals with particle size.
Figure 5. XRD spectra of coal B illustrating modification of the carbon structure after devolatilization at 1273 and 1473 K. Table 2. Variation of Crystallite Height (Lc, nm) of Original Coals and Coal Chars after Devolatilization temperature coal 1173 1273 1373 1473
K K K K
A
B
C
D
E
1.34 0.80 0.79 0.78 0.93
0.89 0.89 0.91 0.92 0.92
0.75 0.76 0.87 0.96 0.97
0.71 0.84 0.90 0.94 0.95
0.67 0.71 0.73 0.87 0.89
commonly calculated from the weight loss data by considering the first-order kinetics using eq 1: -dW/dt ) k(W - W∞)
(1)
where W, W∞, and dW/dt are the initial mass of sample, the final mass of solid residue, and the rate of mass change at a given time, respectively. The initial linear section of the weight loss curve was used to calculate the rate constants (k) on the basis of the Arrhenius relation shown in eq 2: k ) A exp(-E/RT)
(2)
(28) Vuthaluru, H. B. Fuel Process. Technol. 2004, 85 (2–3), 141–155.
where A (1/min), E (J/mol), R (J/(mol K)), and T (K) are the pre-exponential factor, the activation energy, the universal gas constant, and the temperature, respectively. Figure 2b compares the rate constants of coals showing that the rate constant of coal A increases 3-fold (20 units) as the devolatilization temperature increases from 1173 to 1473 K. The strong effect of temperature on the rate constants of lumpy coals is similar to that reported in the case of smaller coal particles.20,29 Coal Volatile Matter and Rate Constant. Figure 3a plots the weight loss data of five coals (8–10 mm) against time during devolatilization at 1173 K and shows that the final volatile yield was similar to the proximate volatile matter as discussed previously. Figure 3a further demonstrates that high volatile coals will display rapid weight loss in a given time interval such that low volatile matter (LVM) coal A would release less volatile matter compared to high volatile matter (HVM) coal E. Figure 3b clearly shows that the devolatilization rate increases linearly with the increasing volatile matter of coals. A similar linear relationship between the volatile matter of coals and the rates can be seen at higher devolatilization temperatures (Figure 3b). Activation energies of coals were also calculated from the Arrhenius plots (Figure 4a) using a linear curve fitting as used in past8,27,28 and plotted with volatile matter of coals in Figure 4b. The activation energies of coals decrease with increasing volatile matter, are found to vary from 26 to 53 KJ/mol, and are consistent with previously reported data.27,28 Figure 4b further shows that the activation energy decreases exponentially with coal volatile matter such that this effect decreases appreciably as the volatile matter exceeds 25%. Differences in the activation energy of different coals can be related to the chemical structure of coals, and therefore, this (29) Badzioch, S.; Hawksley, P. G. W. Ind. Eng. Chem. Process DeV. 1970, 9 (4), 521–530.
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Figure 8. Transformations of original coal samples after devolatilization at 1173 K in TGA; coal B (a) 0.8–1.0, (b) 2.8–5, (c) 10, and (d) 15.5 mm; coal C (e) 0.8–1.0, (f) 2.8–5, (g) 10.5, and (h) 17.5 mm.
was further investigated. Figure 5 compares the XRD spectra of original coal B and the lumpy char samples after pyrolysis at two temperatures. A narrow and sharp (002) carbon peak reflects a high degree of ordered carbon in the specimen, which is often quantified in terms of carbon crystallite height (Lc) as shown in Table 2. Generally, the Lc values of coals decrease with increasing volatile contents (Table 2). Figure 5 shows that as the pyrolysis temperature is increased, the width of (002) carbon peak of coal B becomes narrow, consequently the ordering of the carbon structure increases; however, the changes are marginal as reflected by small changes in the respective Lc values (Table 2). A similar trend of carbon structure modification with temperature was noted for other coals except coal A. Unlike other coals, the Lc value of pyrolyzed char of coal A (0.8 nm) is lower as compared to that of parent coal A (1.34 nm), indicating a significant reduction in ordering of the carbon structure (Table 2). This observation is consistent with a previous study in which semianthracite coals are shown to indicate unexpected increased disordering of the carbon structure after thermal treatment and was attributed to possible cleavage of large carbon crystallites into smaller ones.14,15 The stack height of the carbon crystallites (Lc) of the original coals was found to
have a strong effect on the activation energy indicating a linear correlation between the two as shown in Figure 6. Effect of Particle Size. Figure 7 illustrates the effect of particle size by comparing the devolatilization rates of three coals at 1173 K by varying lump size up to 18 mm. Figure 7 shows that the rate constant of coal B decreases linearly with increasing particle size of the lump. Higher VM coals C and D also indicated a similar trend. Smaller coal lumps display larger differences in the rates such that rate constant of coal D is almost double than that of coal B for the same 1 mm size lump (Figure 7). Large variation of the rate constant of small particles can be attributed to differences in the coal properties including volatile matter and carbon structure. As the lump size exceeds 10 mm, the rates of all coals become comparable. Therefore, with smaller coal particles, chemical properties will exhibit a more pronounced effect on the devolatilization rate constant, while with larger particles, physical properties would dominantly dictate the rate constant, such that various coals will show similar rate constants. The effect of particle size on the rates can be attributed to the modification of evolution of tar and primary gases during devolatilization. As the particle size increases, total volatile yield
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Figure 9. Comparison of changes in physical dimensions of coals A and D lump (≈10 mm) specimens after 50 and 120 s of devolatilization at 1273 K.
is influenced by mass transfer and secondary reactions.30 During devolatilization, with increased particle size, the amount of tar decreases as a consequence of increased secondary reactions of primary volatiles.30 Therefore, the diminishing effect of coal properties on the devolatilization rates of the large lumps can be related to secondary reactions during devolatilization. Figure 8 shows the char samples made from different lump sizes of coals B and C after devolatilization at 1173 K. The smallest lumps of both coals clearly do not show any sign of fragmentation; however, they do appear to coalesce after swelling (Figure 8a and e). As the lump size is increased (2.8–5.0 mm), coal particles show some swelling but still remain separated without showing any appreciable sign of joining or coalescence (Figure 8b and f). When individual large coal lumps (>10 mm) are heated, they show a high degree of swelling accompanied by cracking (Figure 8c, d, g, and h). It is possible that when large lumps are heated together, they may show a lesser degree of coalescence compared to smaller particles due to decreased contact surface area; however, they will display similar fragmentation tendencies as shown by individual lumps. Comparison of the morphology of all char samples illustrates that, with increasing lump size, the swelling and cracking tendency of coal particles also increase. The size effect on the rate constants will be determined by the modification of the escape path of volatiles31 as well as the resistance of the pore structure, which will further be dictated by the combination of the coalescence and swelling tendency. The study further clearly shows that the intensity of the size variation on the rates is also dependent on coal properties such that coal D is more sensitive to the size variation when compared to coal B (Figure 7). This suggests that the effect of lump size on the rate variation of coals should be considered in order to optimize their performance in the operating smelters. Swelling and Cracking Characteristics. Figure 9 shows the physical changes in 10 mm lumps of coals A and D after 50 and 120 s during devolatilization at 1273 K. Coal A does not show any swelling which can be attributed to its semianthracite (30) Yu, J.; Lucas, J. A.; Wall, T. F. Prog. Energy Combust. Sci. 2007, 33, 135–170. (31) Craig, N. E.; Douglas, S. Fuel 1996, 75 (13), 1601–1605.
Figure 10. Variation of the coal swelling ratio with volatile matter.
nature; coals with this nature are often shown to shrink rather than swell in the late stage of devolatilization.23 On the other hand, high volatile coal D indicates the highest degree of swelling (Figure 9 after 120 s) when compared to cubic dimensions of the original sample in Figure 9D. A high intensity of gas evolution is also evident from the high volume of gas accumulation around the coal D particle after 120 s. Visual observation of swelling of five coals was quantified on the basis of the changes in the total surface area of lumpy coals and chars formed after devolatilization in the horizontal furnace at 1273 K. Figure 10 plots the swelling ratio of coals with their volatile matter, showing that generally the swelling ratio increases exponentially with volatile matter with the exception of coal E. In contrast with other samples, coal A shows shrinkage rather than swelling, which can be related to discharge of moisture and volatiles causing micropore blockage leading to increase in density.15 It may be noted that the similar micropore blockage may not be significant in other coals with higher volatile matter which may dominate the swelling behavior. On the other hand, coal E indicated a lower swelling ratio than that expected on the basis of volatile matter alone. This can be explained on the basis of greater crack initiation during pyrolysis providing easier pathways for volatile escape without causing the sufficient gas buildup necessary for swelling. This study further shows that our swelling ratios are different than the crucible swelling number (also known as the free
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Figure 11. Comparison of morphological changes of five coals after devolatilization at 1273 K.
swelling index, FSI) which is obtained following Australian standards, AS 1038.12.1 (Table 1). Therefore, it can be inferred that swelling behavior of smaller particles as estimated by the crucible swelling number cannot be used to assess the similar behavior of lumpy coals. Figure 11a-e shows the cracking and swelling behaviors of coals A, B, C, D, and E after devolatilization in a TGA reactor at 1273 K of 8–10 mm lumps, respectively. An enlarged view of the dotted region of each coal particle is shown opposite to each figure. Each coal char lump shows different morphological features. Chars A and E show a smooth surface and wider cracks (Figure 11a and e). Figure 11f and j further illustrate the magnified view of chars A and E, respectively, which clearly shows coal splitting after devolatilization. Figure 11c and d show that the surface of chars C and D is rough and fluffy and has smaller cracks.
On the basis of visual comparison, the crack width of char B seems to be smaller than that of coals A and E and did not split as chars A and E did, even though the frequency of crack occurrence is high. Lower values of swelling of coal B compared to coals C and D can also be contributed by lower Gieseler plasticity. Generally, the Gieseler plasticity of coals increases with decreasing rank.32 The low swelling ratio of coal E (Figure 10) can be related to some extent to medium plasticity as expected on the basis of the rank. On the same basis, coal A is expected to have insignificant plasticity and shows the least swelling. Careful examination of the SEM images of char samples suggested that coal cracking initiates during the devolatilization (32) Diaz-Faes, E.; Barriocanal, C.; Diez, M. A.; Alvarez, R. J. Anal. Appl. Pyrolysis 2007, 79, 154–160.
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Figure 12. Comparison of surface area modification of three coals with devolatilization temperature.
process. These cracks could become intense by external impact or late stage combustion events. The cracking behavior of lumpy coal appears to be related to the swelling ratio, such that high swelling coals may show less cracking. It is also believed that coal degradation occurs if a coal particle has less resistance to swelling, as it will have a higher capability to reduce the pressure occurring within the coal during devolatilization.20,22 On the other hand, if the coal particle has higher resistance to swelling, increased pressure within the particle would lead to breakage of coal in order to secure the travel route of volatile matter. Thus, less cracking in coals C and D could be associated with their high swelling tendency. Similar to the swelling ratio, the cracking nature of coals can also be related to plasticity of coals such that coals B, C, and D with high plasticity display less cracks as compared to coal A with insignificant expected plasticity. It may be noted that coals A and E appeared to have higher inorganic matter compared to other coals. In this study, high ash coals appear to provide more cracks after devolatilization. The exact mechanism of the association of coal ash with cracking is not clear. It is possible that complex mineral transformations during devolatilization and particularly their association with the carbon matrix could have some implication on cracking. During devolatilization, both the physical and chemical properties of coal lumps change and would affect the devolatilization behavior of coals accordingly. However, due to known association of physical properties with swelling, the physical structures of coal and char lumps were analyzed in detail. Figure 12 shows that the surface areas of original coals vary from 80 to 201 m2/g and increase as the devolatilization temperature increases up to 1173 K. Figure 12 further shows that at subsequent higher temperature, the surface of char samples decreases such that the surface area modification is strongly influenced by coal types. After devolatilization at 1173 K, coal A char displays the highest improvement of the surface area (220 m2/g) while coal D char shows the least change in the surface area (80 m2/g) while other coals show intermediate changes in surface area. Figure 12 further shows that coal A char made at 1173 K indicates a much higher increase in surface area compared to other coal char samples made at the same temperature. Coal D which shows the least cracking also happens to show the least surface area growth after devolatilization. Therefore, the rate of evolution of the surface area can be related to the cracking behavior of coals such that a sharp change of micropore surface area over a narrow temperature range would promote severe cracks. In order to further clarify the nature of pore transformation after the devolatilization at 1273 K, Hg adsorption data was
Figure 13. Comparison of pore volume distributions (a) before pyrolysis and (b) after pyrolysis. Table 3. Physical Changes of Lumpy Coal (8–10 mm) Particles after Pyrolysis at 1273 K (L ) large, M ) medium, S ) small, VS ) very small) swelling ratio, % macropore +5µm, % width of crack
A
E
B
C
D
-10 27 L
27 68 M
15 73 S
30 80 VS
59 87 VS
used. Figure 13a and b compares the pore volume of five coals and respective char samples. Coals B and C have similar macropore volumes of the order of 0.06 mL/g, which is also greater than other coals. Figure 13 further shows that the proportion of smaller pores of original coals, in the mesopore domain, is high (0.04 mL/g), while after pyrolysis the proportion of macropores increased by 1 order of magnitude ranging from 0.45 to 0.70 mL/g except for coal A. Coal A uniquely indicated the least modification of macropore volume while coal D indicated the most significant change, such that majority of mesopores changed to macropores (Figure 13). A small change in pore volume (Figure 13) and surface area (Figure 12) of coal A after devolatilization at 1273 K can be primarily related to less removal of volatiles. It seems that the mineral matter of coal A is also not making any significant contribution in modifying the pore characteristics even though coal A has the highest mineral content. Therefore, generally, the proportion of macropores larger than 5 µm increases after devolatilization and is often related to the coalescence of micro- and mesopores (except for coal A).33 On the basis of careful examination of the SEM images of cracks (Figure 11), coal lumps were divided into four groups. The presence of macropres in coal lump resists coal cracking or breakage as a consequence of decreased internal stresses. Particularly, the proportion of large macropores (>5µm) in char samples seems to have a good association with observed cracks in coal lumps (Table 3). A comparison of coal cracking and swelling data in Table 3 suggests that, (33) Arenillas, A.; Rubiera, F.; Pevida, C.; Ania, C. O.; Pis, J. J. J. Therm. Anal. Calorim. 2004, 76, 593–602.
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in general, high swelling coals are less likely to show cracks after devolatilization, although further studies with large number of coals and different lump sizes are necessary to validate this observation. In summary, the cracking behavior of coal is related to the evolution of physical and chemical properties of coals during devolatilization, which are closely interrelated, hence making it difficult to isolate the effect of individual coal properties. Mineral matter could influence the modification of pore growth and surface area in a complex manner as a function of coal rank as well as their nature of association, particularly for coal A which has the highest mineral content and the highest surface area. Mineral data is currently being interpreted and will be reported later in a separate paper. However, the volatile matter, size of lump, and pore characteristics of the original coals appear to have a strong effect on the cracking behavior of lumpy coals. In order to further understand the effect of coal properties, the heterogeneous nature of coal matter including mineral and maceral components needs to be considered, particularly their associations with each other. Conclusions The devolatilization and cracking behavior of five Australian coals were investigated as a function of temperature and particle size. Under all the tested conditions, the total volatile yield of lumpy coals was not influenced by either the temperature or particle size and was observed to be similar
Kim et al.
to that indicated in proximate coal analysis. The devolatilization rates were found to increase with increasing temperature as well as with the increasing amount of volatiles present in the coal. The study demonstrated that the effect of coal properties on the devolatilization rates of lumpy coals may not be significant as the rates decrease with increasing lump size, such that coal lumps more than 10 mm in size indicated a similar range of reaction rates. The apparent activation energy of coal lumps indicated a linear correlation with the stack height of carbon crystallites of coals. The cracking and swelling behavior of coals was influenced by physical as well as chemical properties, particularly their modification during devolatilization conditions. Coals with low volatiles indicated the most cracking which further increased in accordance with the size effect. The cracking tendency of coals appeared to have a reciprocal association with swelling tendency such that low swelling coals are more vulnerable to cracking. An improved understanding of the high temperature behavior of lumpy coals would assist in further improving coal performance in smelting operations by optimizing their physical and chemical properties. Acknowledgment. The authors thank POSCO for their financial support and permission for the publication of this research. We also appreciate the technical assistance provided by the staff from POSCO Technical Research Laboratories and Mr. N. Saha Chaudhury. EF700397T
