Influence of Long-Term CaO Storage Conditions on the Calcium Looping Thermochemical Reactivity

Long-term storage capability is often claimed as one of the distinct advantages of the calcium looping process as a potential thermochemical energy storage system for integration into solar power plants. However, the influence of storage conditions on the looping performance has seldom been evaluated experimentally. The storage conditions must be carefully considered as any potential carbonation at the CaO storage tank would reduce the energy released during the subsequent carbonation, thereby penalizing the round-trip efficiency. From lab-scale to conceptual process engineering, this work considers the effects of storing solids at low temperatures (50-200 °C) in a CO₂ atmosphere or at high temperatures (800 °C) in N₂. Experimental results show that carbonation at temperatures below 200 °C is limited; thus, the solids could be stored during long times even in CO₂. It is also demonstrated at the lab scale that the multicycle performance is not substantially altered by storing the solids at low temperatures (under CO₂) or high temperatures (N₂ atmosphere). From an overall process perspective, keeping solids at high temperatures leads to easier heat integration, a better plant efficiency (+2-4%), and a significantly higher energy density (+40-62%) than considering low-temperature storage. The smooth difference in the overall plant efficiency with the temperature suggests a proper long-term energy storage performance if adequate energy integration is carried out.