Solid-state 13C NMR characterization of matched tars and chars from rapid coal devolatilization

Matched tar/char sets were prepared by pyrolysis of a lignite and a bituminous coal in two entrained flow reactors at temperatures between 900 K and 1650 K and heating rates of 10(4)-10(5) K/s. Detailed chemical structural characterization of these tars and chars was performed using elemental analysis and solid-state C-13 NMR. This is the first set of detailed solid-state C-13 nuclear magnetic resonance (NMR) data on coal tar samples. The average aromatic cluster sizes of the primary tars from these experiments are quite similar to those of their parent coals, confirming an assumption often made in network devolatilization models. Carbon aromaticities increase in char and tar samples until about 1250 K, after which line broadening in the NMR signal is observed. This line broadening is interpreted as formation of large aromatic radicals, Increases in bridges and loops per cluster are evidence for increased crosslinking above 1250 K. The measured molecular weights per cluster of the primary tars are lower than expected, indicating that some multiple cluster molecules (i.e., dimers) may exist in the tar. Tar and char nitrogen chemical structure is shown to correlate with changes in the carbon aromaticity, which may have implications for nitrogen release models that treat secondary reactions in the tar.