Properties of high ash char particles derived from inertinite-rich coal: 1. Chemical, structural and petrographic characteristics

An investigation was undertaken to determine the properties of high ash coal–chars derived from South African discards rich in inertinites, for the development of suitable overall reaction rate models at low temperatures (<900 °C). Detailed characterisation results of the parent coal and chars prepared at 700 °C and 900 °C obtained from standard coal analytical methods, petrographic techniques, CCSEM image analysis and a surface adsorption method are presented. The parent coal consisted of 32% by volume of inertite (“pure” inertinite), 7% of vitrite (pure “vitrinite”), and 13% of bi- and tri-macerite, 30% of maceral/mineral mixtures (carbominerite) with 18% of mineral-rich material. Reflectances obtained from measurements taken on vitrinites and total maceral reflectance scans increased dramatically on charring at 900 °C and were accompanied by an extension of vitrinite reflectance class distributions indicating higher molecular ordering. Volatiles were liberated essentially from the original parent vitrinites, creating fine gas pores. Inertinites increased in reflectance but not in porosity and were characterised as dense char fractions in the final charred product, according to a coal form analysis. Structural change due to low temperature thermal stress fracturing (passive deflagration) occurred early on in the temperature regimes, creating increased surface areas and porosity. The chars consist of a high proportion (52%) of extraneous rock fragments together with minerals mainly as fine inclusions in carbon rich particles (13%). The chars had very low porosities and surface areas. These were created by the devolatisation of reactive maceral associations and deflagration. Such materials could introduce intra-particle diffusional effects during gasification and combustion of millimetre size particles at low temperatures.