Structural rearrangement of strained coals

Native coals are strained and glassy. When coals are swollen, this strain is relieved as the coal structure rearranges to a lower free energy and more highly noncovalently associated state. Four coals ranging in carbon content from 77% C to 84% C were warmed in the weak swelling solvent chlorobenzene at 132 °C for 2 weeks and samples were withdrawn at intervals. After evaporation of the chlorobenzene, the pyridine extractability of the treated coals had decreased, sometimes by a factor of 2. The pyridine swelling of Pittsburgh No. 8 coal was sharply reduced. The extractability and swelling decreases with time demonstrate that changes in coal structure occurred with the rearranged coal being more associated. This increased association is not due to hydrogen bond formation because pyridine is known to break most if not all of the hydrogen bonds which occur in coals. The rearranged Pittsburgh No. 8 coal was studied by differential scanning calorimetry. Over the 2 week chlorobenzene reflux period, the heat ca...