Investigation of coal components of Late Permian different ranks bark coal using AFM and Micro-FTIR

•Structure characteristics of barkinite were coal rank dependent.•Barkinite ranges from fibrous to irregular network structure with rank increase.•Barkinite has similar AFM image feature to vitrinite at a coal rank of 1.12%.•Surface roughness degree of barkinite decrease with coal rank increase.

The structural characteristics of barkinite for different coal ranks were studied by both atomic force microscopy (AFM) and Fourier transform infrared spectroscopy with microscopy (Micro-FTIR). The surface morphological texture differences between barkinite and other macerals (sporinite and vitrinite) were characterized using AFM images. We found that both chemical structure and physical texture were rank-dependent parameters. With increase of vitrinite reflectance (Ro), the morphological structure of barkinite evolves from fibrous to irregular network structure, and the orientation arrangement seems enhanced. Based on our experimental data, the AFM image feature of barkinite is similar to that of vitrinite when Ro is with a minimum value of 1.12% or beyond. In chemical structure, the contents of aliphatic functional group of barkinite decrease whereas that of aromatic group increase. The values of CH2/CH3 ratios also decrease with increase in coal rank. The different morphological textures between barkinite and others macerals were characterized with AFM examination. The surface roughness degree of these three samples was found to be in sequence of vitrinite < barkinite < sporinite.