Transformation of indigenous iron-bearing phases during coal gasification

Abstract

The reactivity of coal toward CO2 and steam is enhanced by indigenous mineral impurities or by added catalysts. The prominence of iron compounds among the natural impurities in coal and the known catalytic effect of metallic iron for carbon gasification suggests that such ‘‘iron-specific’’ techniques as Mössbauer spectroscopy and magnetization may provide unique insight into these effects. The early stages of coal gasification have been examined using the cited reactant gases on an Illinois no. 6 coal char at 900–950 °C, with no added catalysts. The iron-bearing minerals in the coal transformed in different ways depending on the gaseous environment. The dominant impurity pyrite FeS2 in the raw coal converted during the preliminary inert-gas pyrolysis at 700 °C to a pyrrhotite Fe1−xS. During CO2 gasification a major portion of the pyrrhotite was converted into magnetite Fe3O4. By contrast, during steam gasification, the iron remained in less oxidized states with important fractions found as wüstite, Fe1−xO, ferrous glass/silicates, and metallic iron. These differences may influence the catalytic activity of the iron in the two gaseous environments.