ReaxFF molecular dynamics pyrolysis simulations of a large-scale model of Illinois no. 6 coal including the role of organic sulfur

Reactive molecular dynamics simulations are a useful computational approach for investigating chemical reactions and structural transformations in complex processes. Pyrolysis of coal is one such area were additional insight would be beneficial for utilization improvements and pollution control. Here, ReaxFF was utilized to perform pyrolysis simulations on a large-scale molecular model for Illinois no. 6 coal to examine coal pyrolysis chemistry. A previously constructed largescale molecular model of Illinois no. 6 coal composed of 51,001 atoms within 728 molecules was used in ReaxFF pyrolysis simulations at 2000 K for 250 ps. The ReaxFF simulation was performed until 50% of the cross-links had been cleaved primarily through thermolysis. During pyrolysis the molecular weight distributions shifted to lower molar mass values as a result of thermal decomposition of coal molecules to form smaller fragments. Analysis of sulfur forms distribution showed that aliphatic-bonded sulfurs decomposed more rapidly while thiophenicbonded sulfurs were more thermally stable in agreement with expected chemistry. The thermal degradation of sulfur-containing cross-links was more substantial than that of alkyl linkages, in accordance with their higher reactivity (aliphatic > aromatic > thiophenic). To further analyze the role of organic sulfur forms on coal pyrolysis chemistry, a non-sulfur containing Illinois coal model was created by substituting sulfurs for carbons, that were hydrogen adjusted, and the Reaxff simulation was repeated. Analysis of trajectories showed that the rate of light gases and tar generation was higher for the Illinois coal model (sulfur containing) compared to the non-sulfur containing coal structure, indicating that sulfur atoms enhanced reaction kinetics during coal pyrolysis as expected. This work further demonstrates that ReaxFF integration with representative coal molecular models can be a useful tool for probing chemical processes associated with coal pyrolysis.