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.