Interaction between iron-based oxygen carrier and four coal ashes during chemical looping combustion

Chemical looping combustion (CLC) is a novel technology with inherent CO2 capture, especially for solid fuels. The existence of ash in solid fuels is one major challenge for CLC technology development. In this work, interaction between an iron-based oxygen carrier and four different types of coal ash was studied in a laboratory-scale fluidized reactor. Different factors - the ash component, the redox cycle number, and the ash size - were taken into account. Chemical composition of the ash had effect on the reduction time from Fe2O3 to Fe3O4 in the fluidized bed. The presence of reactive components (such as Fe2O3 and CaSO4) in the ash, functioning as oxygen carriers, extended the reduction time. However, the chemical combination between the ash contents and the carrier can shorten the reduction time. The effect of ash on the carrier's reactivity depended on the ash type. Most ashes decreased the reactivity of the carrier, except the ash mainly composed of CaSO4 which showed an increased reactivity due to the deposited reactive CaSO4. The effect of ash on decreasing the carrier's reactivity increased with the cycles. Meanwhile, the larger ash (900-1000 μm) corresponded to a higher CO conversion, and thus had less effect on the reactivity than the smaller ash (300-400 μm). This occurrence can be attributed to the non-uniform solid-solid contact between the larger ash and the carrier. Sintering and agglomeration of the carrier particles occurred in the existence of most ashes, except the lignite ash enriched in CaO. Ash deposition and the formation of new compounds were detected. One common compound formed in the presence of SiO2-rich ash was Fe2SiO4, which has a low melt point (1170 °C) and a low thermal conductivity with a greater adhesion. The physical ash deposition and the formation of Fe2SiO4 through chemical reactions were proposed to be the main reasons for the effect of ash on the carrier's reactivity and the occurrence of sintering and agglomeration. The existence of ash not only has impact on the carrier's reactivity, but also causes solid fluidization disturbances. More effort is deserved to put into the ash-related issue in solid fuel CLC.