Effectiveness of CO2 capture by calcium looping with regenerated calcium sorbents - last step calcination

The Ca looping process offers many advantages over the other CO2 capture technologies, such as a relatively small decrease in power plant efficiency and a potential for is further reduction. However, calcium sorbents in the consecutive cycles of adsorption and regeneration by calcination undergo deactivation, which is a considerable drawback of this technology. The depletion of sorbent reactivity is due to sintering, which causes changes in the sorbent's porous and crystal structure, whereby its reacting surface area decreases. The approaches to reducing sorbent deactivation (sorption capacity loss) include thermal preactivation, reactivation by hydration and/or the production of synthetic sorbents and doping. This paper studies three methods of regenerating deactivated sorbents. The sorbents underwent deactivation after working in the calcium looping of 10 calcination/carbonation cycles at respectively 900 degrees C and 650 degrees C. The first regeneration method consisted in hydration with water in controlled time and temperature conditions. The second regeneration method consisted in hydration with steam in controlled time and temperature conditions. The third method - 2-stage regeneration - besides water or steam hydration, included a second stage consisting in the controlled exposition of the sorbent to a CO2 enriched atmosphere. In all the methods the regeneration of the spent sorbent was conducted after the final calcination process. After regeneration the sorbent was subjected to 24 calcination/carbonation cycles in similar conditions as before regeneration. This study compares the three methods of regeneration in terms of the CO2 capture efficiency of the regenerated sorbent. The sorbents collected after the looping tests and after each regeneration step were analysed by mercury porosimetry, X-ray diffraction and scanning electron microscopy to determine the interdependences between the sorbents' physicochemical properties and their CO2 capture capacity. The investigations showed that: 1) an improvement in CO2 capture capacity in the calcium looping process was achieved by applying both water/steam hydration and 2- stage regeneration after 10 calcination/carbonation cycles), 2) the size of CaO crystallites increased during looping cycles and decreased as a result of the 2- stage regeneration, 3) the sorbent's pore and particle structure showed significant changes after the reactivation, which may explain the increase in its CO2 capture capacity. The regeneration and CO2 sorption capacity of deactivated sorbent can be effected by radically changing the process temperature and simultaneously feeding saturated steam and then washing the sorbent with a gas with a high CO2 content. The novelty of this idea consists in the fact that the desired regeneration effects can be achieved by changing the regeneration parameters and only slight modifying the structure of the regenerators. (C) 2017 Published by Elsevier Ltd.