Exploring the potential of petrographics in understanding coal pyrolysis

The effects of heating manner and charring temperature on both the physical and chemical properties of coal were investigated by petrographic techniques. Six coal samples of different rank were subjected to chemical treatment plus heat treatment. The pyrolysis experiments were carried out on acid washed coal in order to avoid ash formation. The chars used in the study were prepared in a tube furnace and the morphology was characterized. The morphology analysis reveals the differences in reactivity among the coals. It was found that a link existed between total reflectance and aromaticity, total reflectance and fuel ratio, total reflectance and surface area, and total reflectance and percentage of carbon dry ash-free basis (daf), which all gave a definite relationship confirming, that reflectance can be used as a measuring index in coal analysis. The mean total reflectance of the lignite coal for the temperature regime 450°C–700 °C is 1.26–4.36 %; sub-bituminous coal is 1.49–4.64 %; bituminous coal is 1.39–4.86 %; semi-anthracite is 2.15–4.86 %; and anthracite is 2.56–4.79 %. Thus, the large difference in the initial total reflectance between the different coal ranks became very narrow by 700 °C. •Total reflectance of coals converges to the same value at 700 °C irrespective of maceral content.•Char morphology analysis gives a good char formation tracking and reactivity.•Total reflectance can be used as a coal measuring index for coals subjected to high temperatures.•Links between total reflectance and other coal measuring indices were established.