Abstract:

During coalification, the chemical structure of coal is dramatically altered.In order to establish a geochemical scheme for coalification pathways and to reconstruct the key structural transformations of coaly materials, a series of Late Palaeozoic coals from North China and some synthetically coalified samples were analyzed by means of C-13 cross-polarization, magic angle spinning (CP/MAS) NMR spectra. Besides observable changes in carbon aromaticity encountered in coals of different rank, the data revealed also significant changes in aromatic as well as aliphatic structural components. At the early metamorphic stage (lower than 1.28% R_max^o), the skeletal carbon structure shows relatively low aromatization and contuiris a relatively high amount of substituents. Methylenic and methinic groups are the dominant aliphatic components. With the increase of coal rank and, especially, in the range of major oxygen decrease, these components exhibit relatively high mobility and their amount is gradually reduced. At the medium metamorphic stage (from 1.28 to 2.69% R_max^o), there is a rapidincrease in aromaticity. This is accompanied by a significant reduction in number and size of aliphatic components. At the late metamorphic stage (higher than 2.65% R_max^o), aliphatic carbon groupsal most have disappeared, except for a small number of methyl and its adjacent methylene groups. The quantitative variations in the abundancy of the aromatic and aliphatic components tend to be quite moderate. We suggest that the predominant geochemical mechanism in the late metamorphic stuge has changed from aromatization into ring condensation. The changes in the geochemical structure of coal obviously result in differences of yield and composition of hydrocarbons with increasing coalification. The three strutural evolutionary stages correspond with the generation stages of oil, condensate+wet gas and late catagenetic methane respectively.