CHARACTERIZATION OF PARTIALLY CARBONIZED COALS BY SOLID-STATE C-13 NMR AND OPTICAL MICROSCOPY

Partially carbonized samples from three coals were prepared using a Ju ranek furnace, and the resultant fractions corresponding to temperatur es over the range 290-630 degrees C were characterized by quantitative solid-state C-13 NMR at low field using single-pulse excitation (SPE) and optical microscopy to determine and correlate the bulk chemical a nd microscopic transformations that occur during the thermoplastic sta ge of coking. As found previously for coals, chars, and pitch fraction s, the carbon skeletal parameters derived from cross polarization expe riments for the partially carbonized coals are significantly lower tha n those found by SPE. In the temperature range from ambient to 350 deg rees C, there were no appreciable changes in the carbon structures. Th e softening and vesiculation processes (370-450 degrees C) observed by optical microscopy correspond to the loss of volatile aliphatic mater ial and lead to a small increase in aromatic bridgehead carbon. During resolidification and formation of mosaic textures at temperatures abo ve 460 degrees C, the aromaticity continues to rise with a vast increa se in bridgehead carbon, due to dehydrogenation and condensation of al iphatic and aromatic structures. The number of rings in the aromatic c lusters determined by NMR correlates with the size of the mosaic optic al texture, with the two higher volatile coals having both the same mo saic size of 0.7 mu m and 19 rings per aromatic cluster at 600 degrees C, while the medium volatile coal has a mosaic size of 2.0 mu m with 26 rings per cluster.