Performance of Ca-Based Sorbents for Calcium Looping Processes: Role of Steam

This study reports on a preliminary investigation concerning the role of water vapor in calcium looping cycles, and is based on results of an experimental campaign performed in a lab-scale fluidized bed reactor under operating conditions that are representative of a realistic calcium looping process. Tests have been designed so as to characterize the effect of steam in either the calcination or the carbonation stages, or in both. A reference limestone has been used as sorbent. Uptake of CO₂ by the Ca-based sorbent with and without exposure to steam during the calcination and carbonation stages has been correlated with results of porosimetric characterization of the samples. Results indicate that exposure to steam is beneficial as it improves in any case the ultimate CO₂ uptake. Exposure to steam during the calcination stage favors the development of accessible porosity, inducing incremental CO₂ uptake in the order of 10% with respect to a reference no-steam case. Exposure to steam during the carbonation stage is also favorable, due to the positive role of steam as a “catalyst” for CO₂ diffusion through the sorbent CaCO₃-based product layer. Synergistic effects were observed when steam was added during both the calcination and carbonation stages, resulting in a very pronounced increase of sorbent CO₂ capture capacity as compared with the no-steam case.