Modelling the relationship of coal structure to char porosity

Abstract

This paper investigates the ‘average’ coal molecular structure of Pittsburgh #8, and how that structure changes during pyrolysis to form char. Chars were prepared at different stages of pyrolysis in a heated wire mesh reactor, and analysed by FTIR spectroscopy of the C–H stretching regions. This yielded information on the aromaticities and the concentrations of aromatic and aliphatic hydrogen and carbon atoms within the coal and chars. From this, elemental analysis, and existing literature data on the structural properties of bituminous coals and chars, average molecular models were constructed which were geometry optimised by energy minimisation using a commercial molecular modelling package. The coal model structure has a molecular weight of 8189 amu, and an aromaticity of 0.79. Average char structures which are consistent with experimental data (aromaticities of 0.90 or higher) require large extended aromatic clusters and extensive Ar–Ar cross-linking The resulting molecular conformations exhibit microporosities and densities comparable to those expected from experimental measurement. The derived structures also have implications for predictions of char combustion behaviour.