(269e) On the Mechanism of Morphological Enhancement of CaO by Hydration

As an effective and efficient approach for CO₂ separation from coal-combustion flue gas, CaO-based chemical looping processes attracted world-wide research attention in the past decades. Similar to many other chemical looping particles, the reactivity and recyclability of pure CaO steadily deteriorate as the cyclic reaction proceeds. One of the most effective sorbent reactivation strategies to date is to use hydration onto the deactivated CaO, which restores the surface area and pore volume. The hydration mechanism has been studied for decades, but most previous studies followed the “black-box” approach and tried to explain this phenomenon by varying single reaction parameter, such as temperature, pressure, water-to-CaO ratio, etc. However, a hydration process, especially a water hydration, is a multi-phase reaction, which involves several physical and chemical sub-steps. Hence, many previous theories on this mechanism are inconsistent and sometimes conflicting. This work explores the increases of surface area and pore volume of CaO by hydration. Firstly, a widely-believed mechanism: “physical attrition theory” is experimentally examined, which shows its limitation in explaining this phenomenon. To explain this increase of morphological properties by hydration, a typical water hydration process is examined by dividing it into four independent chemical and physical sub-steps. The morphological changes of Ca(OH)₂ and derived CaO by each sub-step are measured by BET. During the first step – the intrinsic chemical conversion from CaO to Ca(OH)₂, the formed Ca(OH)₂ disintegrates because of its low tensile strength and weak crack resistance, which explains the increases of surface area and pore volume by steam/moisture hydration as well as the rapid heat release during hydration. Physical interaction with water slightly decreases the surface area and pore volume, possibly by mobilizing micro-particles into the porous structure of bigger particles and inducing stronger particle agglomeration. The Ca(OH)₂ solid can further chemically bond water molecules, which significantly enlarges the solid volume during water-bonding and generates a more porous structure during dehydration. The final precipitation of dissolved Ca(OH)₂ decreases the solid’s surface area and pore volume. This decrease is attributed to the formation of micro-particles from solution, which plug some of the surface pores on the bigger particles during the drying process.