Evaluation of Effect of Predrying on the Porous Structure of Water-Swollen Coal Based on the Freezing Property of Pore Condensed Water Koyo Norinaga,* ,† Jun-ichiro Hayashi, Norihide Kudo, and Tadatoshi Chiba Center for Advanced Research of Energy Technology (CARET), Hokkaido University N13, W8, Kita-ku, Sapporo 060-8628, Japan Received February 16, 1999. Revised Manuscript Received May 17, 1999 The effect of the extent of predrying on the porous structure of water-swollen coal was examined. As-received Yallourn (YL), Beulah Zap (BZ), and Illinois #6 (IL) coals were used as the samples. They were predried at 303 K to different extents. Upon predrying, the coal samples released water in the following order: free water identical to bulk water, bound water that froze at around 226 K, and finally, nonfreezable water that never froze even at 123 K. Predried samples were swollen in water at 303 K and subjected to 1 H NMR measurements to characterize the freezing property of water retained in pores at a temperature range from 170 to 294 K. The total volume of the pores filled with water (V p ) was defined as the amount of water that was not frozen at 260 K. The removal of the nonfreezable water from YL coal by the predrying decreased the V p of the water-swollen coal, while removal of the other types of water had little effect on V p . Complete predrying of the other coals also reduced V p , but to a smaller extent than for YL coal. The freezing point distribution (FPD) for pore condensed water that froze at 213-260 K was determined experimentally by NMR and also simulated numerically using a Gaussian function. A modified Gibbs-Thompson equation, which relates the freezing point depression to the pore dimensions employing a cylindrical-shaped pore model, was applied to convert FPD into pore size distribution (PSD). The PSD, expressed as pore radius, ranged from 1 to 3 nm, suggesting that the reduction of V p for the YL coal was mainly due to the shrinkage or collapse of pores with radii around 2 nm, which are abundant in water-swollen coal before predrying. Introduction When partially or completely dried brown coals or lignites are exposed to water, they swell, but often do not regain their original volumes. 1,2 This irreversible change induced by drying has been partly attributed to collapse of the colloidal gel 1,3,4 accompanied by the formation of stronger and shorter hydrogen-bond bridges between coal macromolecules. The gel collapse could limit the accessibility of organic solvents 5 and mass transfer in aqueous media. 4 The water sorption isotherm on bed-moist brown coal 6 shows strong hysteresis between the desorption and readsorption curves, and the hysteresis persists at very low relative vapor pres- sures. At relative vapor pressures above 0.5, the normal capillary condensation mechanism explains the hyster- esis with lowering of vapor pressure according to the Kelvin equation. 7 Although there is no generally ac- cepted mechanism to explain the persistence of the hysteresis loop in the multilayer and monolayer water regions of the isotherms, it is attributed to difference in the adsorption and desorption mechanisms, which are associated with swelling and shrinkage effects such as the irreversible shrinkage or collapse of capillaries with drying. This study was undertaken to examine the irreversible nature of the colloidal gel structure of coal in the cycle of water removal and swelling, focusing on its porous structure. Conventional techniques, such as gas adsorption/ desorption and mercury porosimetry, are only utilized to characterize dry materials, and can hardly be applied to the pore structure analysis of water-containing materials. Drying induces irreversible pore collapse and a considerable reduction in the internal porosity. Hence, water itself is the only suitable probe molecule for investigating the porous structure of coal-sorbing water. In general, water sorbed in or on solid materials, such as coal, has properties that differ from those of bulk water in its normal thermodynamic states. 8-14 Norinaga * Author to whom correspondence should be addressed. † Present address: Institute for Chemical Reaction Science, Tohoku University Katahira, Aoba-ku, Sendai, 980-8577, Japan. Fax: +81- 22-217-5655. E-mail: norinaga@hisui.icrs.tohoku.ac.jp. (1) Deevi, S. C.; Suuberg, E. M. Fuel 1987, 66, 454. (2) Woskoboenko, F.; Stacy, W. O.; Raisbeck, D. The Science of Victorian Brown Coal; Durie, R. A., Ed.; Butterworth-Heinemann Ltd.: Oxford, 1991; p 152. (3) Evans, D. G. Fuel 1973, 52, 186. (4) Gorbaty, M. L. Fuel 1978, 57, 796. (5) Suuberg, E. M.; Otake, Y.; Yun, Y.; Deevi, S. C. Energy Fuels 1993, 7, 384. (6) Allardice, D. J.; Evans, D. G. Fuel 1971, 50, 236. (7) Thompson, W. L. K. Philos. Mag. 1871, 42, 448. (8) Mraw, S. C.; Naas-O’Rourke, D. F. Science 1979, 205, 901. (9) Mraw, S. C.; Naas-O’Rourke, D. F. J. Colloid Interface Sci. 1982, 89, 268. (10) Lynch, L. J.; Webster, D. S. Fuel 1979, 58, 429. (11) Lynch, L. J.; Barton, W. A.; Webster, D. S. Proceedings of the 16th Biennial Low-Rank Fuels Symposium; Groenewold, G. H., Ed.; Energy and Environmental Research Center: Montana, 1991; p 187. 1058 Energy & Fuels 1999, 13, 1058-1066 10.1021/ef990024v CCC: $18.00 © 1999 American Chemical Society Published on Web 07/10/1999