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 n m by small angle X-ray scattering (SAXS). The scattering profile from the u nprocessed lignite specimen exhibits two distinct power laws indicating dif ferent fractal morphologies over different length scales: a pore fractal (d imension D-p similar to 2.7) for the pore-coal interface below 17 nm and a surface fractal (dimension D-s, similar to 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 scat tering profiles. The fractal roughness of the pore-coal interface becomes s mooth (average D-s similar to 2) for a large window of the length scale, wh ereas a part of the pore fractal transforms to a surface (D-s similar to 2. 8). The specimen treated at relatively higher temperature exhibits further Variation from the fractal geometry with a tendency to form a new nonfracta l micropore structure for the length scale less than or equal to 8 nm. (C) 2000 Elsevier Science Ltd. All rights reserved.