Pelletized Goonyella coal was heated under N_2 gas atmosphere of 1.0MPa at a rate of 10K/min to a holding temperature around 700K, where change in apparent viscosity of the coal was evaluated by means of a needle penetrometry. During the holding period at 690K the viscosity decreased and then increased, while the mass fraction of pyridine solubles (PS), φ_p, decreased monotonously. This result can not be explained by existing models assuming softening coal to be a slurry in which PS behaves as liquid. The validity of the assumption was thus examined by measuring and analyzing the needle penetration into a model pellet of inert semicoke and PS in a temperature range from 500 to 650K. Softening characteristics of mixtures with φ_p of 0.4-1.0 were quantitatively described by Mooney's equation employing a critical value of solid mass fraction of 0.72. Time-dependent change of liquid fraction was calculated for the coal at 690K using the above equation and observed needle penetration data. The fraction was much larger (35-38%) than observed φ_p which was evaluated less than 20% from pyridine extraction.
