Mineralogical characterisation of Secunda gasifier feedstock and coarse ash

The formation of clinkers in a Sasol–Lurgi fixed bed dry bottom (FBDB) gasifier is a complex process controlled by coal mineralogy and operating conditions. This paper explores the link between the coal mineralogy and the mineralogy of the corresponding clinkers/coarse ash. CCSEM and XRD results indicate that kaolinite (Al2Si2O5(OH)4) and to a lesser extent quartz (SiO2) are the major coal minerals. Minor minerals are calcite (CaCO3), dolomite (CaMg(CO3)2) and pyrite (FeS2). Kaolinite and quartz are associated with rock fragments (“stone”) or as fine inclusions in organic rich coal particles. Calcite/dolomite and pyrite occur in coarse and fine cleats or fine inclusions in carbon-rich particles. The coarse ash and clinker samples contain the following phases: (1) Ca- and Fe-bearing aluminosilicate glass, (2) Crystallised phases in glass (anorthite and mullite), (3) Remnant rock fragments, (4) Large extraneous Fe-oxide/pyrrhotite/Fe–S–O particles (originally coarse pyrite cleat fragments), (5) Large extraneous Ca-oxide/Ca–Mg-oxide/carbonate particles (originally coarse carbonate cleat fragments) and (6) Uncombusted carbon-rich particles (char) with their associated minerals (kaolinite, quartz and carbonates). The equilibrium phases (as calculated with Factsage) for temperatures above 1220 °C correspond approximately to the phases which were identified in ash and clinker by CCSEM.