Potassium and calcium-modified ilmenites for improved reactivity and hydrogen yield in chemical looping

Ilmenite is a promising oxygen carrier for scaling up chemical looping hydrogen production processes but shows limited redox kinetics and thus low H2 yield. On-site modification of ilmenite with alkali and alkaline earth metals (AAEM) offers a cost-effective way to reduce costs and streamline procedures since these metals could be naturally derived from feedstock such as biomass. In this study, calcium and potassium were used to modify ilmenite, the Ca- and/or K-modified ilmenites were prepared, characterized, and comprehensively studied in terms of reactivity and H₂ yield with a fluidized bed reactor. The Kunii-Levenspiel model was applied to analyze the reaction insights and predict upscaled reaction design. The formation of new phases of K-Ti-Fe and Ca-Ti-Fe oxides and their distribution led to elevated performances for the modified ilmenites. Ilmenite co-modified with Ca and K (5wt.%K2O30wt.%CaO-IL, 5K30Ca-IL) showed the best kinetics and highest H₂ yield, followed by K-only and then Ca-only modified ilmenite. At 900 °C, 5K30Ca-IL demonstrated a maximum improvement of 60 % in the steam oxidation rate constant, a reduction in CO consumption of up to 57 %, and a maximum enhancement in H₂ yield of ∼440 %, and up to ∼5.5 times higher energy efficiency. The reactor size for 5K30Ca-IL was only one-third of that for neat ilmenite to produce the same amount of H₂ per cycle in the upscale desgin.