While calcium oxide has been identified to be the best candidate for capturing CO 2 at high temperature, it suffers a well-known problem of loss-in-capacity; that is, its capacity for carbon capture decays dramatically during cyclic carbonation/ calcination processes. Recently, extensive research work has been conducted on the enhancement of the cyclic performance of calcium oxide through either improving the performance of natural minerals, such as water/steam hydration and pretreatment or modification of calcium oxide sorbents by some techniques such as doping and synthesis. This work summarizes the state-of-theart research in the literature aiming to identify potential solutions to the loss-in-capacity problem. It is found that hydration during or after calcination is effective in recovering the capacity of natural minerals and mixing can produce highly effective synthetic sorbents. Periodic hydration of synthetic sorbents could be a good strategy to overcome the technical issues associated with loss-in-capacity while meeting the requirements of the physical properties of sorbents in many potential applications.

The reversible reaction of CaO with CO2 can be used for CO2 capture. However, two challenging problems, i.e., loss-in-capacity and high attrition rate for CaO-based sorbents, must be solved before it can be practically applied. In this paper, sorbents with various CaO contents were prepared from calcium hydroxide and cement using a screw extruder and the physical and chemical properties of the sorbents were obtained. The mechanical properties of sorbent particles were tested by friability and compression testers, and the sorption capacity and regenerability were measured in a thermogravimetric analyzer. It appears that the sorbents occupied acceptable attrition resistance and good mechanical strength. In the meantime, the sorbent particles showed higher CO2-capture capacities compared to pure CaO after 18 cycles, which was attributed to the stable phase of Ca12Al14O33 in the sorbents. The sorbent particles would be suitable for the calcium looping processes.

Three types of sorbent were fabricated using various calcium and support precursors via a simple mixing method, in order to develop highly effective, durable, and cheap CaO-based sorbents suitable for CO(2) capture. The sorption performance and morphology of the sorbents were measured in a thermogravimetric analyzer and a scanning electron microscopy, respectively. The experimental results indicate that cement is a promising low-cost support precursor for contributing to the enhancement of cyclic CO(2) sorption capacity, especially when organometallic calcium precursors were used. A sorbent (with 75% CaO content) made from calcium l-lactate hydrate and cement showed the highest CO(2) sorption capacity of 0.36 g of CO(2)/g of sorbent and its capacity decreased only slightly after 70 cycles of carbonation and calcination.

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