FT-Raman spectroscopic study of the evolution of char structure during the pyrolysis of a Victorian brown coal

FT-Raman spectroscopy with a 1064 nm laser was used to investigate chemical structural changes of char during the pyrolysis of Victorian Loy Yang brown coal samples. The chars were diluted with KBr in order to record Raman spectra with acceptable quality. The interpretation of the Raman spectral data for these highly disordered and heterogeneous chars differs distinctly from that for the highly condensed/graphitised carbon materials. The FT-Raman spectra of chars in this study over the range of 800-1800 cm^-1 were curve-fitted with 10 bands representing major structures in the chars. This has given information about the size of aromatic rings and the nature of substitutional groups and cross-links in char. The observed Raman intensity of a char is governed by its Raman scattering ability and its light absorptivity for both excitation laser and Raman scattering. The overall Raman intensity (peak area) as well as the ratios among the intensities of some major Raman bands has allowed some semi-quantitative evaluation of changes in char structure with increasing temperature during pyrolysis. The presence of ion-exchangeable Na and Ca in brown coal greatly affects the char-forming reactions during pyrolysis.