Effect of heat pretreatment/recarbonation in the Ca-looping

7 Even though an increasing number of pilot-scale plants are demonstrating the potential ef8 ficiency of the Ca-looping technology to capture CO2 at a commercial level, a still standing 9 matter of concern is the loss of carbonation reactivity of the regenerated CaO by calcination, 10 which is expected to be particularly marked at realistic conditions necessarily implying a high 11 CO2 partial pressure in the calciner. In this work, we address the effect of previously reported 12 strategies for sorbent reactivation, namely heat pretreatment and the introduction of a recar13 bonation stage before regeneration. Both techniques, either combined or separately, are shown 14 to favor the carbonation reactivity albeit CaO regeneration is usually carried out at low CO2 15 partial pressure in lab-scale tests. Novel results reported in this paper show the opposite when 16 the sorbent is regenerated by calcination at high CO2 concentration, which is arguably due to 17 the diverse mechanisms that rule decarbonation depending on the CO2 concentration in the 18 calciner atmosphere. Dynamic and reversible adsorption/desorption of CO2 is thought to gov19 ern decarbonation during calcination at high CO2 partial pressure, which would be hindered 20 by the introduction of a recarbonation stage before carbonation. Moreover, carbonation in the 21 fast phase is severely hampered as a result of the marked loss of reactivity of the surface of 22 CaO regenerated under high CO2 partial pressure. On the other hand, heat pretreatment and 23 harsh calcination conditions lead to a notable enhancement of diffusion, which would favor 24 the process efficiency. In these conditions, diffusion controlled carbonation becomes a signifi25 cant contribution to CaO conversion, which is notably increased by prolonging the carbonation 26 stage. Heat pretreatment allows also reducing the calcination temperature at high CO2 partial 27 pressure while still achieving full decarbonation in short residence times. 28