Investigation on the transformation behaviours of Fe-bearing minerals of coal in O2/CO2 combustion atmosphere containing H2O

The transformation behaviors of Fe-bearing minerals in coals of Xinjiang (XJC) and Shenhua (SHC) were investigated in an O₂/CO₂ atmosphere containing H₂O in a drop-tube-furnace (DTF). The solid products were characterized using XRD, Mössbauer spectroscopy, particle size analyzer and SEM-EDX techniques. The results show that the change in the combustion atmosphere does not significantly alter the main phases of Fe-bearing minerals in the coal ashes, but does affect their relative contents. The ratio of Fe²⁺-glass to Fe³⁺-glass in the ashes produced from the O₂/CO₂ combustion atmosphere was significantly increased. During the XJC combustion and under different combustion conditions examined, the content of Fe-glass phases remained almost unaltered. However, in SHC samples, combustion under O₂/CO₂ atmosphere resulted in a higher amount of iron melting into Fe-glass phases and less amount of iron oxide formation. This could be attributed mainly to the presence of Fe-bearing minerals mostly included in nature in SHC samples, which more easily interacted with clays or other silicates inside coal-formed Fe-glass phases. Increasing the O₂ level of the O₂/CO₂ atmosphere during SHC combustion could promote the formation of iron oxides. In O₂/CO₂ atmosphere, with the same oxygen level, the replacement of 10% of CO₂ with H₂O promoted the formation of iron oxides, regardless of the occurrence form (included or excluded) of iron minerals in coal. Furthermore, the addition of steam resulted in an increase in the size of the particles in ash, resulting probably in a decrease in the deposition and slagging propensity of coal ash.

The ratio of Fe²⁺-glass to Fe³⁺-glass in ashes from O₂/CO₂ atmosphere is significantly increased. The iron oxides (hematite or magnetite) formation of included iron minerals may be delayed in O₂/CO₂. H₂O promotes iron oxides formation.