Analysis and optimization of calcium looping processes aimed at the CO2 capture in dual interconnected fluidized bed systems

This work summarizes the research activities carried out by this group and concerning the development of calcium looping (CaL) processes (see Figure 1), based on the use of calcium-based sorbents, aimed at the CO2 capture from flue gases in dual interconnected fluidized bed systems (Coppola et al., 2012a; Coppola et al., 2012b; Montagnaro et al., 2012). Three main research topics were investigated. A) Effect of SO2 on the attrition propensity and on the CO2 capture capacity for sorbents upon CaL cycles in a lab-scale fluidized bed reactor. B) Development of a model of the two CaL stages, based on population balances on sorbent particles. C) Water-induced reactivation of the CO2 capture capacity for exhausted sorbents ('double looping' process shown in Figure 2). Pertinent works concerning the broader context of this research subject can be found in the literature (Shimizu et al., 1999; Stanmore and Gilot, 2005; Sun et al., 2007; Grasa et al., 2008; Zeman, 2008; Blamey et al., 2010; Charitos et al., 2010; Wang et al., 2012). Along the research pathway A), it has been highlighted that the contextual calcium sulphation reaction, determining the building-up of a non-removable CaSO4-based shell in the cortical region of the sorbent particles, is responsible for the observed changes in the sorbent performances with respect to an idealized case in which sulphur is absent upon both calcination and carbonation stages. Concerning the topic B), it has been underlined the relevant influence of process parameters, such as sorbent inventory and flow-rates, particle size distributions and fluidization velocity, on the CO2 capture efficiency in the carbonator, on the fines elutriation rate in both stages, and on the energy requirements at the calciner fuelled by an auxiliary fuel. Topic C) is relevant to environmental strategies as it is closely related to the issue of landfilling of CaO-rich spent sorbents. In this context it has been observed that the changes in the spent sorbent physico-chemical properties induced by hydration (e.g., CaO conversion to Ca(OH)2, increase in specific active porosity) were very important in determining the renewal of the CO2 capture capacity of the materials once they were re-submitted to calcium looping tests after reactivation. The influence that hydration-reactivation has on the attrition and fragmentation propensities of the sorbent particles has been also critically discussed.