Application of Raman spectroscopy in characterization of coal macromolecular structure

Coals are a special kind of organic rocks, which are mainly composed of macromolecule polymer. Accurate description and quantitative characterization on the structural characteristics of coal macromolecules can provide important information for understanding the evolution mechanism of coal structure. Raman spectroscopy is a fast and non-destruction approach to characterize carbonaceous materials, which has been widely used in the characterization of coal macromolecular structures. The research progress of Raman spectroscopy in the characterization of coal macromolecules both in China and abroad was systematically analyzed and summarized.(1) For the Raman spectrum test of coals, some sample processing methods including the epoxy resin molding and polishing, the selection of different laser light sources, laser energy and signal collection time will affect Raman results. In addition, the coal rank and the structural information contained in the peak of the Raman spectrum should be fully considered in peak fitting.(2) The effect of temperature on the structural evolution of organic in coals can be divided into two stages: the Raman spectrum changes irregularly before graphitization and the Raman disorder parameters gradually decrease during high-temperature graphitization. Under high-pressure and normal temperature, the Raman characteristic peaks linearly shift to higher frequencies, which is consistent with the change of Raman spectrum response to stress, and the structural defects caused by tectonic stress will reduce the structural order degree of coal-based graphite.(3) There is no strict corresponding relation between Raman disorder parameters and coalification, which are suggested to be closely related to many factors, including the asynchronous evolution between coal macromolecular structure and chemical structure, and the effect of stress on structure ordering by inducing structure defects.(4) Other applications include the calculation of aromatic layer size, the indication of special peaks and the use of second-order Raman spectroscopy in the characterization of coal structure. However, these parameters need to be used carefully in practice.