Specific Features of the Structure of Various Coal Ranks at the Nano Level

Abstract

Structural factors and functions of radial distribution of atoms of different coal ranks have been calculated by X-ray diffraction analysis. This made it possible to establish that the main structural component is the clusters with graphite-like packing of atoms. The predominant size of these clusters (calculated from the peaks of small-angle X-ray scattering at 5° in CoKα-radiation) for coals with a carbon content of 83--95 % is within 2--3 nm. Fossil coal is an amorphous carbonaceous substance consisting of polymorphic modifications in the sp2- and sp3-states. According to the results obtained using Raman scattering spectroscopy, the physical properties of amorphous natural coals are strongly dependent on the ratio of sp2- and sp3-hybridization of atomic orbitals. Moreover, studies have shown that the carbon atoms of the coal matrix in the sp2-state are represented by both aromatic and aliphatic conjugated chain fragments. It was found that the degree of ordering in rank LF coals is higher than in coals of a higher stage of metamorphism. This is possible if the hydrocarbon matrix in rank LF coals can be represented as a polymer consisting of conjugated chains with periodicity. With an increase in the stage of coal metamorphism, starting with coal rank G, the transformation of the coal structure occurs with a violation of the periodicity of the conjugated fragments of the polymer matrix connecting graphite-like clusters, a decrease in their length, and an increase in randomness in their arrangement