AN EMPIRICAL-MODEL FOR COAL FLUIDITY BASED ON A MACROMOLECULAR NETWORK PYROLYSIS MODEL

We have developed a phenomenological model for coal fluidity based on a macromolecular network model for the decomposition and condensation of the network under the influence of bond breaking and cross-linking reactions. The macromolecular network model is the previously published FG-DVC model of coal pyrolysis. It employs a network consisting of aromatic ring clusters linked by bridges. The bond scissions are described by a single first-order reaction with a distribution of activation energies. Cross-linking is related to CO2 and CH4 formation which are described in multiple first-order reactions with distributions of activation energies. The fluidity is described by an empirical equation which depends on the relative amounts of the liquid (molecules detached from the network) and solid (the remaining network) and on the fluidity of the liquid component. The FG-DVC model predicts the yield of liquids. The fluidity of the liquid component is described by a second phenomenological equation which depends only on the temperature. The advantage of this model is that it is based on a previously demonstrated methodology which allows the incorporation of rank-dependent kinetics, cross-linking, weathering, and extraction phenomena into the fluidity predictions. Excellent agreement has been obtained between the model predictions and low-temperature fluidity measurements of Oxley and Pitt, van Krevelen, and Gieseler plastometer measurements for the Argonne premium coal samples. The trends for changes in the fluidity with weathering or extraction are predicted as well. Good agreement has been obtained at high temperatures between the model predictions and measurements of Fong for the onset of the fluidity. The loss of fluidity, however, is predicted to occur sooner than is indicated by the data and the maximum value of fluidity is overpredicted. The data cover over 5 orders of magnitude in fluidity, and eight coals with carbon concentration between 80 and 90%. This agreement is obtained using coal-independent equations for the dependence of the fluidity on the liquid fraction and the liquid fluidity. The coal-dependent variables are the kinetic rates for bond breaking and cross-linking and the extent of cross-linking as determined from laboratory pyrolysis measurements using a TG-FTIR (thermogravimetric analyzer with Fourier transform infrared analysis of evolved products). There are only two adjustable parameters in the model.