Raman characterization of structural evolution in the low-middle rank coals

In this study,the structural parameters of 28 vitrain samples with the maximum vitrinite reflectance (Ro,max ) ranging between 0. 30% -2. 05% were obtained from Raman spectra. The results show that the regular changing trend is found for the structural parameters of Raman spectra with the increasing Ro,max ,which further indicates the complexi- ty of coal structural evolution. Specifically,well correlation exists between the position of D1 and G,the difference of peak position and peak width at half height (PWHM) of G and Ro,max . The PWHM ratio of D1 to G designated as ID1 /IG also changes sectionally with Ro,max . Accordingly,the coal structural evolution process can be divided into three sta- ges,i. e. ,0. 3% -0. 8% ,0. 8% -1. 6% and 1. 6% -2. 0% . For the stage of Ro,max range of 0. 3% -0. 8% ,D1 shifts to- ward lower wavenumber,while G shifts to higher wavenumber. The decreasing trend is found in the PWHM of G with Ro,max . The aforementioned analyses confirm that the coal structure becomes ordered. There is no clear change found for the PWHM of D1 and ID1 / IG,indicating the slow formation process of the aromatic structure. For the vitrain samples of Ro,max range of 0. 8% -1. 6% ,the extreme point appears in the position of D1 and G,the difference of peak position and PWHM of G,which can be considered as the index of the mutation of the second coalification process. The related pa- rameters change slightly for the Ro,max of 1. 6% -2. 0% ,which shows that the microcrystal structure of coal become stable,and the decreasing PWHM of G indicates the growing stage of the aromatic structure.