This study has been undertaken aiming at establishment of a needle penetrometry combined with a volumetric dilatometry that can quantitatively evaluate characteristics of softening and resolidification of coals upon heating as changes with time and temperature of apparent viscosity. Coal pellets were heated in nitrogen gas atmosphere at a rate of 3 or 10K/min, and net needle penetration depth into the pellets was measured as a function of temperature for a range of loads applied on needle, diameters of needle and gross penetration rates. The results reveal the validity of the analysis of net penetration curves based on an equation of motion of needle assuming that softening coal behaves as a Newtonian fluid. Changes in the apparent viscosity of coals at temperatures ranging from 600 to 800K can be estimated by analysing the net penetration curves measured under constant gross penetration rates with varying loads on needle.
