THE MOLECULAR-BASIS OF COAL THERMOPLASTICITY

The molecular mobility of a range of Australian bituminous coals during heating and pyrolysis to 875 K has been studied by proton nuclear magnetic resonance (**1H n. m. r. ) thermal analysis. Mobility acquired at lower temperatures is correlated with both exinite content and H/C ratio. Mobility occurring at higher temperatures is assessed to involve aromatic structures and is identified with the onset of coal thermoplasticity. The transient nature of the thermoplastic event is explained in terms of three partly overlapping processes: 1. a physical mobilization of aromatic-rich structures; 2. a rate-limiting thermochemical decomposition of the molecular structure to form reactive highly-aromatic molecular units in the residue; and 3. after a critical level of decomposition has been reached, there is a rapid condensation of the residual units to produce a rigid semi-coke.