Mineralogy of Fixed-Bed Gasification Ash Derived from North Dakota Lignite

Abstract

ABSTRACTThe mineralogy of nine ash samples derived from North Dakota lignite and gasified in three different fixed-bed producers was determined by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The gasifiers had maximum temperatures in the 1100–1250°C range, but included several differences in ash residence time, oxidizing gas and pressure, and ash handling conditions. The lignite gasified came from the same geologic formation, but the mines were separated by 20 km. Despite these differences, all of the ash specimens had the same basic mineralogy: glass at the 15–30 wt.% level; a crystalline phase assemblage of ortho- and pyro-silicates (merwinite, dicalcium silicate (C2S), C2S-like phases, and melilite), network aluminosilicates (nepheline, carnegieite, and a sodalite-structure phase), oxides (ferrite spinel, hematite, and periclase), and calcite along with residual lignite minerals (quartz and felspar). SEM morphologies and XRD comparisons of the amounts of crystalline phases in size fractions both indicated that some of the phases (particularly merwinite and carnegieite) crystallized during cooling of portions of the ash that had melted during gasification.