Evidence for network formation during the carbonization of coal from the combination of rheometry and 1H NMR techniques

High-temperature rheometry and 1H NMR have been combined to assess the microstructural changes taking place during carbonization of a number of different coals. A linear relationship exists between the logarithm of the material's complex viscosity (η*) and the fraction of hydrogen present in rigid structures (ϕrh) for the resolidification region in which the material is liquid-like with small amounts of dispersed solid. The relationship is best characterized by the Arrhenius viscosity equation given by η*=η0*exp([η]ϕrh), where η0* is the complex viscosity of the liquid medium and [η] is the intrinsic viscosity of the resolidified material. Attempts to fit the Krieger–Dougherty suspension equation showed that the solid regions formed do not pack together like a normal suspension. Instead, it is more likely that cross-linking and cyclization reactions within the liquid medium give rise to a network structure of solid material and a characteristic gel point. The ratio of hydrogen present in rigid structures to that still present in liquid form at the gel point is approximately 2:3. The resolidified material was found to have a higher [η] than the components of the coal that remained unsoftened, which suggests that while the unsoftened components have a fairly equant shape, the resolidified components have a much higher hydrodynamic volume. The resolidification process bears similarity with thermosetting polymer networks and the η* measurements taken for a blend of two coals follow a common two-component polymer blending rule.