CO2 sorption capacity and cyclic performance of quicklime (CaO) under gasification conditions

Calcium looping (CaL), based on the cyclic CaCO3/CaO calcination/carbonation process, is one of the crucial features of sorption enhanced gasification (SEG). CaO sorbent has been validated for in-situ CO2 capture in the gasification unit where gasifying agent steam (H2O) and syngas products CO and H2 co-exist. Even though steam is hypothesized to enhance sorbent reactivity, its behavior in practical conditions is debatable. Additionally, carbonation performance of CaO-based sorbents in the presence of CO and H2 is not well understood. This contribution investigates CO2 sorption performance of CaO under relevant gasification conditions (in the presence of steam and syngas) using a laboratory-scale fluidized bed reactor over a range of representative carbonation temperatures. CO2 sorption increases with increasing carbonation temperature, but is limited by the reaction equilibrium of the CaCO3/CaO system. CO2 sorption performance of CaO sorbent increases notably when steam is introduced into the carbonation atmosphere or followed by ex-situ hydration of CaO sorbent. Steam promotes formation of a microporous structure, thereby facilitating CO2 diffusion through the CaCO3 layer. On the contrary, CO and H2 in the carbonation atmosphere hinder CO2 sorption performance, whereas the combined influence of steam and syngas is insignificant. We hypothesize that CO and H2 limit CO2 sorption performance of CaO through competitive sorption at CaO active sites and by carbon formation that blocks the pores. This work demonstrates that practical gasification conditions for in-situ CO2 sorption change the sorption behavior of CaO significantly, which provides insights for the use of CaO-based sorbents to capture CO2.