The Synthesis of Highly Efficient Ca-Based CO2 Sorbents Containing a Hierarchical Pore Structure

In 2010 the global CO₂ emissions increased by more than 5 % compared to 2009, reaching 33 billion tonnes.¹ One possible, mid-term solution to reduce the release of CO₂ into the atmosphere is the implementation of CO₂ capture and storage (CCS) technologies.² However, the currently available CO₂ capture technology, i.e. amine scrubbing, is associated with a large penalty on plant efficiency. A promising, new CO₂ capture technology is the use of alkaline earth metal based CO₂ sorbents. Due to its (i) high theoretical CO₂ capture capacity of 0.78 g CO₂/ g sorbent, (ii) low cost and (iii) relatively vast abundance in naturally occurring minerals, e.g. limestone or dolomite, CaO is the most promising candidate in this class of materials. The selective capture and release of CO₂ is achieved by the following, theoretically reversible, reaction:

CaO(s) + CO₂(g)  « CaCO₃ (s), DH⁰_298K = ± 178 kJ/ mol               (1)

However, CaO produced via the calcination of naturally occurring materials has a serious drawback, viz. the rapid decrease in the CO₂ capture capacity with repeated cycles of carbonation and calcination.³ The carbonation reaction can be divided into two regimes.⁴ It has been argued that the fast, kinetically controlled reaction stage, is associated with the filling of small pores. The importance of the available pore volume can be clearly seen by comparing the molar volume of the product, CaCO₃ (M_V = 36.9 cm³/mol), with that of CaO (M_V = 16.7 cm³/mol). Once the pores have been filled, the second, diffusion controlled reaction stage takes over, in which CaCO₃ is deposited on the partially carbonated CaO grains. Owing to the typically short contacting times in circulating fluidized beds, the reactor of choice for the commercial implementation of the calcium looping processes, only the CO₂ uptake achieved during the fast reaction stage will be of practical relevance. Since the Tammann temperature (temperature at which thermal sintering starts) of CaCO₃ (533 °C) is relatively low compared to the operating temperatures of 650-950 °C, the dramatic reduction in CO₂ uptake has been attributed to the reduction of surface area and the blockage of pores with diameter d_pore< 100 nm. One potential strategy to increase the CO₂ capture stability of Ca-based sorbents is to stabilize CaO with high Tammann temperature matrix. However, work in this field has largely focused on the use of “simple” synthesis techniques, which do not allow easily to adjust key structural properties, such as the pore volume. In this work we report the preparation of carbon gel templated, Ca-based CO₂ sorbents. Here, calcium nitrate and aluminium nitrate were used as the calcium and aluminium precursors, respectively, whereas resorcinol and formaldehyde were the precursors of the carbon gel. In addition, the diameter of the carbon sphere template was varied by changing the surfactant (CTAB) to resorcinol ratio. Upon removal of the carbon gel via calcination in air, hollow micro-spheres were obtained. The synthetic material was investigated using (i) scanning electron microscopy (SEM), (ii) surface area (BET) and pore volume (BJH) measurements, (iii) X-ray diffraction (XRD) measurements and (iv) mercury porosimetry measurements. The material synthesized possessed an excellent CO₂ capture capacity when compared to limestone or alternative CO₂ sorbents, such as hydrotalcites or metal organic frameworks. Furthermore, the current synthesis techniques allow the amount of Al₂O₃ required to stabilize the cyclic CO₂ capture capacity of the material to be reduced to only 5 wt.%.

References

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<>3. Grasa, G. S.; Abanades, J. C. CO₂ Capture Capacity of CaO in long series of carbonation/calcination cycles. Ind. Eng. Chem. Res. 2006, 45, 8846-8851.

<>4. Dennis, J. S.; Pacciani, R. The rate and extent of uptake of CO₂ by a synthetic, CaO-containing sorbent. Chem. Eng. Sci. 2009, 64, 2147-2157.