FRACTAL PORE SURFACES IN BROWN-COAL, THEIR CHANGES ON PROCESSING AND THEIR EFFECT ON COMBUSTION

Using small angle X-ray scattering, we show that the scattering patter n caused by the pore surfaces of dried coal may often be interpreted a s arising from fractally rough surfaces. In practice a wide range of v alues of the surface fractal dimension, D(s), was found covering virtu ally the whole allowable range. In the case of brown coal we find that as it is dried from the bed-moist state, the scattering curve changes from that characteristic of a mass fractal to that appropriate to a s urface fractal. Furthermore, during upgrading of brown coal by deoxyge nation at various processing temperatures, T, changes of the parameter s characterizing the fractal surfaces may occur. Under some conditions D(s) and the surface measure, N0, change very slightly with increasin g T. However, heating in hydrogen-rich atmospheres causes dramatic inc reases in D(s) and dramatic decreases in N0 for T above about 320-degr ees-C. The results seem to indicate that after a very slight initial s moothing of the pore surfaces, they become very rough and of very much smaller extent indicating pore collapse. This correlates well with ot her properties of the coal, and in fact we are able to relate the oxid ation (combustion) behavior of these coals with the measured values of N0.