Relationship between Organic Sulfur Occurrence and Coalification Stage: Insights from Thermodynamic Simulation and X-ray Photoelectron Spectroscopy

Organic sulfur in coal can pollute the atmosphere due to the emission of SOx gas during coal combustion and utilization. Although the relative distributions of the different organosulfur functional groups change with progressive coalification (maturity), how these groups are incorporated in the macromolecule structures of coals and the role they play in the coalification process remain ambiguous. The purpose of this study, then, is to elucidate these relationships by using thermodynamic simulation with a reactive force field program, calculation of the C–S bond dissociation energies of four types of organosulfur functional groups, X-ray photoelectron spectroscopy (XPS) analysis of seven coal samples, and published data to determine the changes in the types and proportions of organosulfur functional groups in three coalification stages (0–1, 1–2, and >2% Ro). The results show that the bond stabilities follow the order Cal–S < S–S < Car–S. Disulfides and thioethers occur in lower contributions in the coalification stage of 0–2% Ro, while mercaptans present in a relatively higher proportion. The contribution of sulfone increases and exceeds a half in organosulfur at 1–2% Ro; the major role of sulfur turns from edge bond (0–1% Ro) to bridge bond (1–2% Ro) gradually. Thiophene decreases in the 0–1% Ro coalification stage and increases in relatively high proportions in the 1–2% Ro coalification stage. When the coalification stage is above 2% Ro, the contributions of sulfides, sulfone, and mercaptan decrease, while that of thiophene increases, and sulfur gradually acts as a thiophenic ring bond. These results demonstrate that the role of organosulfur in the macromolecular skeleton of coal changes with progressive coalification.