Novel Simplification Approach for Large-Scale Structural Models of Coal: Three-Dimensional Molecules to Two-Dimensional Lattices. Part 3: Reactive Lattice Simulations

Various studies have used lattice structures combined with simplistic kinetics to explore the devolatilization of coal with the goal of predicting yields of gas, tar, and char. In our previous work, we have demonstrated the ability to reduce a three-dimensional (3D) large-scale (>50 000 atoms) atomistic representation of Illinois no. 6 bituminous coal to a coal-specific two-dimensional (2D) lattice with cross-links being depicted as linkage lines and aromatic clusters being depicted as nodes. Because there is a direct link between the full complexity of the atomistic representation and the 2D lattice, the cross-links within the coal topography can be identified. Using this structural information, a statistical simulation of the kinetically controlled liquefaction reactions of an Illinois no. 6 coal lattice was performed. The Illinois no. 6 coal lattice was composed of a distribution of singular nodes, dimers, and multi-linkage molecules that ranged from 3 nodes (or clusters) to a 16-node entity. Probabilities for the cleavage of coal interunit links were calculated on the basis of the mixture of 10 model compounds comprised of two benzene rings connected by a cross-link (e.g., diphenyl sulfide, benzyl phenyl ether, diphenyl ether, etc.) equivalent to those found in the Illinois no. 6 model. The mixture of model compounds was based on the concentrations of cross-links representative of the model (and, thus, the coal analysis). Cross-link-specific kinetic parameters (Arrhenius pre-exponential factor and activation energy) for each pyrolysis reaction were obtained from literature sources or tuned from composition data using the Kinetic Modeling Editor, a software tool used to generate and solve the balance equations in a kinetic model. At a heating rate of 5 degrees C/min, thermolysis calculations from 360 to 490 degrees C showed that the 2D lattice structure broke down extensively, generating mainly monomer cluster molecules. At 400 degrees C with residence times of 600 and 1600 s, the number of cross-links between aromatic units was reduced by 40 and 60%, respectively, from the original population. At higher temperatures, the breakdown of the weaker and more kinetically favored cross-links (-CH2S- and -OCH2-) occurred rapidly at the earlier stages of heating. The absolute probability of cleavage (P-i) of certain cross-links (-CH2CH2-, -CH2OCH2-, etc.) increased at longer residence times. Cross-links such as phenyl and doubly aromatic bound (-S- and -O-) remained essentially unreactive at the conditions explored.