Experimental examination of existing slurry models for coal softening and resolidification

Pelletized Goonyella coal was heated at a rate of 10 K min^-1 up to holding temperatures from 690 to 710 K, while the apparent viscosity of the coal was measured by needle penetrometry and was found to change with time through a minimum viscosity of ca 10⁶ Pa s. Existing slurry models, any of which predict the minimum viscosity at the maximum volume fraction of liquid without variation of its chemical composition, were adopted to analyze the change in the coal viscosity at a constant temperature of 690 K. The assumption that softening coal behaves as a slurry was experimentally examined by analyzing the viscosity of pelletized mixtures of semicoke as an inert solid and pyridine extract from the coal at temperatures over 500 K where the extract behaved as a liquid. The results revealed that a model based on Mooney's equation can quantitatively describe the viscosity of the mixtures that varied with temperature and the fraction of the extract with an Einstein coefficient and critical mass fraction of solid of 5.9 and 0.77, respectively. Using these parameters and the viscosity of liquid given as that of the extract, Mooney's equation predicted the time-dependent change in the mass fraction of metaplast assumed to represent the liquid in the softening coal at 690 K. The mass fractions of pyridine, quinoline and pyridine/CS₂ extracts, which were obtained from the heat-treated and quenched coal pellets, were found to decrease monotonously with time and to be appreciably smaller than the predicted liquid fraction throughout the holding time. No significant changes with time of the chemical composition of the pyridine extracts were detected either. The solvent extracts were thus suggested to represent only a portion of the metaplast. Then the fraction of liquid in the coal, upon heating at 690 K, was evaluated by means of an in-situ proton magnetic resonance (¹H NMR) that can detect mobile hydrogen existing in the liquid phase distinguishing it from rigid hydrogen in the solid phase on the basis of molecular mobility. The observed fraction of mobile hydrogen was found to change in a manner similar to that for the predicted liquid fraction, although the former was slightly larger than the latter.