Behaviour of selected major elements during fixed-bed gasification of South African bituminous coal

The Sasol–Lurgi Fixed Bed Dry Bottom gasifier (S–L FBDB) treats a feed coal containing c.a. 30% ash forming minerals, which means that there are complex mineralogical processes occurring simultaneously with each other, and in the gasification reactions, within the reactor. To obtain an understanding of the mineral transformational behaviour, a quenched commercial-scale S–L FBDB gasifier was sampled and characterized mineralogically. Crystalline phases measured by XRD analyses show the ash exiting the reactor contains anorthite, quartz, mullite, cristobalite, diopsite, mayenite, anhydrite, muscovite, hematite, and magnetite, with the non-crystalline proportion in the ash bed c.a. 53%. While many of the reactions in gasification are kinetically controlled, the equilibrium analysis using the Fact-Sage 5.3 modelling program provided a computational approach useful in supporting some, though not all, of the experimentally observed chemical and physical behaviour of elements in gasification. Interpretation of hypothetical model output results was of limited utility, since interstitial/matrix aluminosilicate glasses, which are known to form in practise, cannot be modelled at present because no thermodynamic data is available for these species in the Fact-Sage databases.