Structural analysis of Greek and Bulgarian coals by solid-state 13C nuclear magnetic resonance spectroscopy

In this study, 32 peat, xylite-rich and matrix lignites, and sub-bituminous coal samples from representative lignite and sub-bituminous coal basins of Greece and Bulgaria were used to investigate how their NMR differences reflect the organic matter origin, genesis conditions and coalification process in each one of these basins. The evolution of the major organic structural units in the samples was identified by using high-resolution, solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, using a spectrometer with cross-polarisation and magic angle spinning. Solid-state 13C CP/MAS NMR is considered as a very powerful method for determining the chemical structure of complex organic substances such as coals of various ranks. To investigate a possible influence of paramagnetic species on the CPMAS-NMR spectra of the samples, electron paramagnetic resonance (EPR) spectroscopy was also applied. Similarities were observed between the spectra of coals of similar rank or of similar forming conditions. The peat-lignite and lignite-sub-bituminous coal transition is marked by pronounced differences in the NMR spectra, as do the different lithotypes of lignite, i.e., matrix and xylite-rich. Determination of the coal organic constituents offers valuable knowledge about the chemical and thermal behaviour of coals in order to investigate major issues associated with coal mining, combustion, and post-combustion processes.