Interaction Between Alkali Chlorides and Ilmenite in Chemical Looping Combustion

Journal article, 2024

Chemical looping combustion (CLC) is an emerging combustion technology recognized for its high efficiency and minimal environmental impact in terms of CO2 emissions. When biomass is used in CLC, the process referred to as Bio-CLC, it can result in negative CO2 emissions. However, the presence of alkalis (K and Na) in biomass can pose operational challenges. To better understand how alkalis interact with oxygen carriers, this study utilized a method involving the impregnation of charcoal particles with alkali salts (KCl or NaCl or both). The influence of alkali chlorides on fluidization/agglomeration, reactivity, and interaction with ilmenite was investigated in a batch fluidized bed reactor operating at 950 °C. During the reduction step of the simulated CLC cycles with solid fuels, charcoal impregnated with KCl (K-charcoal), NaCl (Na-charcoal), and a combination of both (Na-K charcoal) was used. A total of 33 alkali cycles were performed with K-charcoal, Na-charcoal, and Na-K charcoal with calcined ilmenite, as well as Na-K charcoal with synthetic ilmenite, including additional cycles using nonimpregnated charcoal as a reference. Na-K charcoal with calcined ilmenite led to an earlier onset of agglomeration and defluidization than single-impregnated charcoal. In contrast, no defluidization was observed in the case of synthetic ilmenite with Na-K charcoal. EDX analyses revealed minimal Na and no K accumulation in the particle bridges of the final Na-ilmenite and K-ilmenite samples. Conversely, Na and K were found together with Si and Al in the particle bridges of the final Na-K ilmenite, while no accumulation of Na and K was observed in the final Na-K synthetic ilmenite. ICP-OES analyses showed that approximately 28% of K and 49% of Na were retained in the final K-ilmenite and Na-ilmenite, respectively. Negative K retention and 40% Na were observed in the final Na-K ilmenite, whereas 35% K and 40% Na retention were found in the final Na-K synthetic ilmenite. This research significantly enhances our understanding of the interactions between alkali chlorides and ilmenite.