The Ca-looping process shows a considerable potential for reducing postcombustion CO2 emissions from power plants in the short-term as demonstrated by the recent success of a 1.7 MWt pilot plant. This process involves the carbonation reaction of CaO to capture CO2 and the subsequent calcination of limestone (CaCO3) to regenerate the sorbent. Yet the capture capacity of natural limestones decreases with the increasing number of calcination/carbonation cycles, which is mainly attributed to a decrease of the reactive surface area with the number of cycles as a result of material sintering during calcination. A number of techniques have been developed in the last few years to improve the durability of Ca-based sorbents and minimize their loss in adsorption capacity. The goal is to increase the active surface area and the stability of the pore structure of the sorbent, which would enhance their efficiency for CO2 capture. Material chemistry methods oriented to this objective are generally focused on the use of rigid porous materials as carriers of the Ca-based sorbents, use of additives to improve the sorbent thermal stability, reduction of the sorbent particle size down to the nanometer scale, and use of synthetic precursors to produce novel sorbents with a rich micropore structure. Besides enhancing the thermal stability in newly developed synthetic sorbents, an issue of concern is to promote their mechanical stability. Attrition of natural limestone particles is a main problem affecting the sorbent performance in the Ca-looping process. This paper is devoted to a critical review on the novel Ca-based sorbents developed in the last few years with improved thermal and mechanical stability.
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