Coal plasticity at high heating rates and temperatures

The potential influence of gas bubbles on the viscosity of molten coal at elevated temperatures was estimated by calculations, using literature models for the viscosity of two and three phase suspensions. In one approach, gas bubbles in molten coal were approximated as the dispersed phase of deformable liquid globules in G.I. Taylor's model of emulsion viscosity. This model was incorporated into different models for the viscosity of solid-in-liquid suspensions, in which the dispersed solids are approximated as rigid spheres. Calculations show that in the presence of gas bubbles, the apparent viscosity of molten coal increases roughly in proportion to the volume fraction of gas with respect to the liquid continuum phase. It is experimentally and mathematically difficult to accurately determine gas volume fractions throughout the softening stage of coal pyrolysis. Our current plan is therefore to assume that the apparent viscosity of molten coal can be reasonably modeled in terms of a single liquid continuum phase and one dispersed phase consisting only of solids, using for example the two phase suspension viscosity model of Hatschek or of Frankel and Acrivos. In this approximation, gas bubble effects would force the model to underestimate metaplast concentrations and to overpredict the contribution of the dispersed solids to increase the viscosity of the liquid continuum