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 CO 2 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, particu- larly 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 SO X , NO X , and net CO 2 emissions. 1–9 In 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 consumption 12 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 * Corresponding author. Tel.: 61 2 9385 6597. Fax: 61 2 9385 5956. E-mail: byong-chul.kim@student.unsw.edu.au. † 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. (7) Lee, I. O.; Shin, M. K.; Cho, M.; Lee, H. G. ISIJ Int. Suppl. 2002, 42, S33–S37. (8) Kim, B. 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