The enormous anthropogenic emission of the greenhouse gas CO2 is most likely the main reason for climate change. Considering the continuing and indeed growing utilisation of fossil fuels for electricity generation and transportation purposes, development and implementation of processes that avoid the associated emissions of CO2 are urgently needed. CO2 capture and storage, commonly termed CCS, would be a possible mid-term solution to reduce the emissions of CO2 into the atmosphere. However, the costs associated with the currently available CO2 capture technology, that is, amine scrubbing, are prohibitively high, thus making the development of new CO2 sorbents a highly important research challenge. Indeed, CaO, readily obtained through the calcination of naturally occurring limestone, has been proposed as an alternative CO2 sorbent that could substantially reduce the costs of CO2 capture. However, one of the major drawbacks of using CaO derived from natural sources is its rapidly decreasing CO2 uptake capacity with repeated carbonation-calcination reactions. Here, we review the current understanding of fundamental aspects of the cyclic carbonation-calcination reactions of CaO such as its reversibility and kinetics. Subsequently, recent attempts to develop synthetic, CaO-based sorbents that possess high and cyclically stable CO2 uptakes are presented.

The capacity of calcined limestone to react repeatedly with CO2, according to CaO(cr) + CO2(g) = CaCO3(cr) (eq I), and also its regeneration in the reverse reaction have been studied in a small, electrically heated fluidized bed of sand, for five different limestones. The forward step of eq I is a promising way of removing CO2 from the exhaust of, for example, a coal-fired power station, ready for sequestration or as part of a scheme to generate H2 using an enhanced water−gas shift reaction. The reverse step regenerates the sorbent. The uptake of CO2 by CaO, produced by calcining limestone, was measured using a bed of sand fluidized by N2 at ∼1023 K. For each experiment, a small quantity of limestone particles was added to the hot sand, whereupon the limestone calcined to produce CaO. Calcination was completed in ∼500 s for particles of a mean diameter of ∼600 μm. Next, CO2 was added to the fluidizing nitrogen to carbonate the CaO for ∼500 s. Measurements of [CO2] in the off-gases enabled the rates of calcination and the subsequent carbonation to be measured as functions of time. Many successive cycles of calcination and carbonation were studied. The forward step of reaction I is shown to exhibit an apparent final conversion, which decreases with the number of cycles of reaction; the final conversion fits well to a correlation from the literature. The reverse (calcination) reaction always proceeded to completion. Particles of limestone, removed from the reactor after several cycles, in either their partially carbonated or fully calcined state, were studied using X-ray diffraction, gas adsorption analysis, mercury porosimetry, and scanning electron microscopy. It was found that the carrying capacity of CaO for CO2 on the nth cycle of carbonation was roughly proportional to the voidage inside pores narrower than ∼150 nm in the calcined CaO before carbonation began. Thus, morphological changes, including reduction in the volume of pores narrower than 150 nm within a calcined limestone, were found to be responsible for much of the fall in conversion of reaction I with increasing numbers of cycles. The rate of attrition of the particles of limestone in a fluidized bed, while cycling between the calcined and carbonated states, was also studied. It was found that most limestones lost less than 10% of their mass due to attrition over the course of a typical experiment, lasting ∼8 h.

Coprecipitation and hydrolysis of CaO have been employed to produce Ca-based synthetic sorbents suitable for capturing CO 2 in a fluidized bed. Their composition, CO 2 uptake, volume in small pores (2-200 nm) and resistance to attrition were measured and compared to those of limestone and dolomite. Sorbents produced by hydrolysis showed the highest uptake and resistance to attrition. After 20 cycles of carbonation and calcination, two sorbents exceeded the uptake of both limestone and dolomite, when subjected to the same regimes of reaction. A sorbent's uptake of CO 2 was shown to be determined by the volume in pores narrower than ∼200 nm.On a eu recoursà la co-précicipation età l'hydrolyse de CaO pour produire des sorbants convenantà la capture de CO2 dans un lit fluidisé. Leur composition, le retrait de CO 2 , le volume dans les petits pores (2-200 nm) et la résistanceà l'attrition ontété mesurés et comparésà ceux du calcaire et de la dolomite. Les sorbants produits par hydrolyse montre l'absorption et la résistanceà l'attrition les plusélevées. Après 20 cycles de carbonisation et calcination, deux sorbants dépassent la capacité de retrait du calcaire et de la dolomite, lorsqu'on applique les mêmes régimes de réaction. On montre que le retrait de CO 2 par un sorbant est déterminé par le volume des pores plusétroit que 200 nm.

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A novel, high temperature solid absorbent based on lithium orthosilicate (Li(4)SiO(4)) has shown promise for postcombustion CO(2) capture. Previous studies utilizing a clean, synthetic flue gas have shown that the absorbent has a high CO(2) capacity, >25 wt %, along with high absorption rates, lower heat of absorption and lower regeneration temperature than other solids such as calcium oxide. The current effort was aimed at evaluating the Li(4)SiO(4) based absorbent in the presence of contaminants found in typical flue gas, specifically SO(2), by cyclic exposure to gas mixtures containing CO(2), H(2)O (up to 25 vol. %), and SO(2) (up to 0.95 vol. %). In the absence of SO(2), a stable CO(2) capacity of ∼ 25 wt % over 25 cycles at 550 °C was achieved. The presence of SO(2), even at concentrations as low as 0.002 vol. %, resulted in an irreversible reaction with the absorbent and a decrease in CO(2) capacity. Analysis of SO(2)-exposed samples revealed that the absorbent reacted chemically and irreversibly with SO(2) at 550 °C forming Li(2)SO(4). Thus, industrial application would require desulfurization of flue gas prior to contacting the absorbent. Reactivity with SO(2) is not unique to the lithium orthosilicate material, so similar steps would be required for other absorbents that chemically react with SO(2).

The water gas shift (WGS) reaction was conducted in the presence of two natural and two synthetic CaObased sorbents. It was shown that such sorbents can affect the WGS in two ways: (i) by catalysis of the reaction and (ii) by altering the equilibrium position by abstraction of CO 2 from the gas phase. It was shown that CaO can significantly enhance the production of H 2 during the WGS reaction; however, a trade-off between the production of H 2 and contamination of the product gas with CO 2 (the "CO 2 slip") has to be made. It was found that CaO catalyzes the WGS reaction. The carbonation reaction was very close to thermodynamic equilibrium, even at small contact times at 650 °C. However, the concentration of H 2 was significantly below that predicted from equilibrium considerations. In our experiments, once the sorbent had been fully carbonated, it was regenerated by heating to release the CO 2 so that it could be reused. In such a cyclic experiment, calcium magnesium acetate, a synthetic sorbent, was the best sorbent tested, albeit only over five cycles of reaction, with respect to the amount of H 2 produced. The other sorbents, especially limestone, revealed a decrease in the production of hydrogen with the number of cycles.

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