Small angle X-ray scattering study on change of fractal property of Witbank coal with heat treatment

Change in the surface fractal dimension (Ds) of Witbank coal with heat treatment was studied using small angle X-ray scattering. It was found that Ds changes systematically depending on the temperature and the heating rate. Especially in the pre-softening stage, Ds decreased with increasing temperature. From thermogravimetric and differential scanning calorimetric analyses, we concluded that this was derived from a physical structural change in coal. It was also found that there are lower limits (Fmin) of fractal distribution of pore surface. Further, these limits change greatly at the threshold of the coal softening temperature. It is concluded that the physical structural change of coal takes place at lower temperatures than that thought so far, and that Ds and Fmin are valuable parameters for representing the structural changes with heat treatment.