Interaction of Iron Oxygen Carriers and Alkaline Salts Present in Biomass-Derived Ash

Abstract

Chemical-looping combustion (CLC) is a unique method that is developed for carbon capture and storage (CCS) to mitigate the climate change. In CLC, an oxygen carrier is used to convert the fuel and the produced CO2 is inherently separated from air components, which makes it suitable for CCS. The CLC of biomass is a way to generate negative CO2 emissions. However, interactions between ash and oxygen carriers are a tough challenge as biomass-derived ashes consist of large amounts of reactive ash-forming matter such as alkaline and alkali earth metals. As iron-based oxygen carriers are one of the most commonly used ones, the interaction of the pure iron oxide and biomass-derived ash-forming matter needs to be further understood to overcome the deactivating effects of ash components on the oxygen carriers. Even though ash components may exist in different forms, the effects of K- and Na-based carbonates, chlorides, nitrates, phosphates, and sulfates on the pure iron oxide were mainly investigated in this study. The effect of synthetic biomass-derived ash on the iron oxygen carriers was also investigated to reveal the phases causing agglomeration or deactivation of the oxygen carriers. Experiments were performed at 950 °C for 5 h under both oxidizing and reducing atmospheres. After experiments, the obtained phases were analyzed by X-ray diffraction, and elemental mapping was performed by using scanning electron microscopy–energy-dispersive X-ray spectroscopy. Results showed that the Fe oxygen carrier was worst affected by KCl, KH2PO4, and NaNO3 in terms of agglomeration among the used salts. The presence of K and Si together in the ash caused a “bridge” formation between the oxygen carrier and the ash constituent, which increased the agglomeration. A strong Ca deposit on the outer layer of the Fe oxygen carrier was also observed when a mixture of salt was used to mimic ash. Even though some discrepancies were observed, generally thermodynamic calculations were successful in estimating the experimentally observed phases.