Structural variations in lignite coal: a small angle X-ray scattering investigation

The structural morphology of raw and processed lignite coal specimens from Neyveli (Tamil Nadu, India) is characterized over a length scale of 5–100 nm by small angle X-ray scattering (SAXS). The scattering profile from the unprocessed lignite specimen exhibits two distinct power laws indicating different fractal morphologies over different length scales: a pore fractal (dimension Dp∼2.7) for the pore–coal interface below 17 nm and a surface fractal (dimension Ds∼2.3) above 17 nm. As a result of industrial processing at different temperature, pressure and carbonization, significant changes in the structure are noticed as indicated by the scattering profiles. The fractal roughness of the pore–coal interface becomes smooth (average Ds∼2) for a large window of the length scale, whereas a part of the pore fractal transforms to a surface (Ds∼2.8). The specimen treated at relatively higher temperature exhibits further variation from the fractal geometry with a tendency to form a new non-fractal micropore structure for the length scale ≤8 nm.