Carbon NMR of coals: the effects of coal type and rank

Abstract

Carbon-13 nuclear magnetic resonance (NMR) spectra, which provide direct measurements of the chemical structure of organic matter, have been obtained for a large number of New Zealand coals ranging from peat to semi-anthracite. Converted into numerical parameters, the spectra show differences related to coal type and rank. Four NMR parameters— f a, S ox, f CO2H and f COH—are examined, whereas only f a is commonly treated in previous publications. Previously, the effects of coal type have not been considered in interpreting whole-coal samples, although f a had been shown to vary inversely with hydrogen content in the lower rank coals. A wide variety of analytical determinations were used as rank indicators. In the present study, Rank( S r), which is type-compensated, is used as the principal rank index; T max and reflectance are also considered. Coals within the middle one-third of the New Zealand Coal Band, representing a restricted type range of moderately high hydrogen coals, exemplify the changes in NMR parameters as rank increases. These are all particularly sensitive to the earliest changes in kerogen, when the distinguishing oxygenated functional groups are abundant, both within the peat and as peat changes to the lowest-rank lignite. As rank increases, both f CO2H and f COH progressively decline, but neither f a nor S ox shows such uniform change. In particular, inflections in the f a/Rank( S r) and S ox/Rank( S r) curves closely correspond, for New Zealand coals, to the onset of oil expulsion and the culmination (before rapid decline) of coal coking properties.