Characterization of Partially Carbonized Coals by Solid-State 13C NMR and Optical Microscopy

Abstract

Partially carbonized samples from three coals were prepared using a Juranek furnace, and the resultant fractions corresponding to temperatures over the range 290−630 °C were characterized by quantitative solid-state 13C NMR at low field using single-pulse excitation (SPE) and optical microscopy to determine and correlate the bulk chemical and microscopic transformations that occur during the thermoplastic stage of coking. As found previously for coals, chars, and pitch fractions, the carbon skeletal parameters derived from cross polarization experiments for the partially carbonized coals are significantly lower than those found by SPE. In the temperature range from ambient to 350 °C, there were no appreciable changes in the carbon structures. The softening and vesiculation processes (370−450 °C) observed by optical microscopy correspond to the loss of volatile aliphatic material and lead to a small increase in aromatic bridgehead carbon. During resolidification and formation of mosaic textures at temperatures above 460 °C, the aromaticity continues to rise with a vast increase in bridgehead carbon, due to dehydrogenation and condensation of aliphatic and aromatic structures. The number of rings in the aromatic clusters determined by NMR correlates with the size of the mosaic optical texture, with the two higher volatile coals having both the same mosaic size of 0.7 μm and 19 rings per aromatic cluster at 600 °C, while the medium volatile coal has a mosaic size of 2.0 μm with 26 rings per cluster.