Char porosity characterisation by scanning electron microscopy and image analysis

No significant change in either the morphotype composition or the macroporo sity (pores > 5 mu m) in the 0-30 wt.% char burnout interval were revealed by reflected light microscopy or image analysis. Two high temperature char series from a Tertiary South American coal (Cl) and a Permian Gondwana coal (C2) were therefore examined by scanning electron microscopy to provide in formation on the combustion process up to similar to 60 wt.% char burnout. This study documents a significant mesopore (similar to 0.1-5 mu m) develop ment on the fused chars in the burnout interval studied. A method to quanti fy the size and amount of the mesopores is described and both the parameter s increased with increasing char burnout. Above a char burnout of similar t o 30 wt.% an increase in macroporosity was detected and ascribed to coalesc ence of mesopores to form larger pores. Although the measurement of mesopor osity is restricted to fused chars, i.e. pores in fragments and the char mo rphotypes inertoid, fusinoid and solid could not be measured, the considera tion of mesoporosity seems to be fundamental in understanding, evaluating a nd modelling combustion processes in the char burnout interval studied. (C) 2000 Elsevier Science Ltd. All rights reserved.