Interactions between oxygen carriers used for chemical looping combustion and ash from brown coals

Abstract

Iron-based oxides are considered as potential oxygen carriers in coal chemical looping combustion. However, their redox or oxidation behaviour can be affected by coal mineral matters. The interactions between oxygen carriers and coal ashes depend strongly on the type of ash, which varies significantly in Australian brown coals.We studied the interactions between two types of coal ash (silica-rich and iron and magnesium-rich) and two types of iron-based oxygen carriers (iron ore and industrial-grade ilmenite) via microstructural analysis of samples processed in reducing and oxidising atmospheres at 900 and 950°C, respectively.Both types of ash reacted locally with iron-based oxides, forming alumina silicates, spinels, magnesioferrites, and iron silicate phases as solid solutions or through liquid phase formation. These interactions can affect the oxidation and reduction kinetics of oxygen carriers. Our results show that iron-rich ash improves the oxidation and reduction kinetics of iron ore. It does not affect the reduction kinetics of ilmenite, but significantly increases ilmenite oxidation time. Silica-rich ash decreases oxidation rates of iron ore, but has less of an effect on the kinetics of ilmenite processing.