Char porosity characterisation by scanning electron microscopy and image analysis

No significant change in either the morphotype composition or the macroporosity (pores > 5 μm) 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 (C1) and a Permian Gondwana coal (C2) were therefore examined by scanning electron microscopy to provide information on the combustion process up to approx. 60 wt.% char burnout. This study documents a significant mesopore (approx. 0.1-5 μm) development on the fused chars in the burnout interval studied. A method to quantify the size and amount of the mesopores is described and both the parameters increased with increasing char burnout. Above a char burnout of approx. 30 wt.% an increase in macroporosity was detected and ascribed to coalescence of mesopores to form larger pores. Although the measurement of mesoporosity is restricted to fused chars, i.e. pores in fragments and the char morphotypes inertoid, fusinoid and solid could not be measured, the consideration of mesoporosity seems to be fundamental in understanding, evaluating and modelling combustion processes in the char burnout interval studied.