Evaluation of effect of predrying on the porous structure of water-swollencoal based on the freezing property of pore condensed water

The effect of the extent of predrying on the porous structure of water-swol len coal was examined. As-received Yallourn (YL), Beulah Zap (BZ), and Illi nois #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 H-1 NMR measurements to characterize the freezing property of water retained i n pores at a temperature range from 170 to 294 K. The total volume of the p ores filled with water (V-p) was defined as the amount of water that was no t frozen at 260 K. The removal of the nonfreezable water from YL coal by th e predrying decreased the V-p of the water-swollen coal, while removal of t he 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. Th e freezing point distribution (FPD) for pore condensed water that froze at 213-260 K was determined experimentally by NMR and also simulated numerical ly using a Gaussian function. A modified Gibbs-Thompson equation, which rel ates the freezing point depression to the pore dimensions employing a cylin drical-shaped pore model, was applied to convert FPD into pore size distrib ution (PSD). The PSD, expressed as pore radius, ranged from 1 to 3 nm, sugg esting that the reduction of V-p for the YL coal was mainly due to the shri nkage or collapse of pores with radii around 2 nm, which are abundant in wa ter-swollen coal before predrying.