(388ad) Fundamental Aspects of the Carbonation and Calcination Reactions and the Synthesis of Stable, Calcium-Based CO2 Sorbents

The increasing concentration of greenhouse gases in the atmosphere is most likely linked to climate change. One of the most promising options to mitigate climate change is CO₂ capture and storage (CCS). Here, CO₂ is captured from power plants, compressed, transported and stored in geological formations rather than emitted into the atmosphere.However, the costs of CO₂ capture using amine based scrubbing technology are estimated to be around 39-96$ per ton of CO₂ captured.¹ Thus, there is an urgent need to develop more efficient and less costly CO₂ capture technologies. One of the emerging CO₂ capture technologies is based on the carbonation and calcination of CaO and CaCO₃, respectively, i.e. CaO + CO₂ ↔ CaCO₃. This CO₂ capture technology is commonly referred to as calcium looping. Economic analyses showed that such a process is economically attractive, in particular due to the relatively inexpensive, naturally occurring sorbents, i.e. limestone or dolomite.¹ However, most natural sorbents show a rapid decrease in CO₂ uptake with cycle number. It has been argued that the drop in CO₂ capture capacity is mainly caused by thermal sintering resulting in a reduction of surface area and the volume of pores with diameter < 100 nm.² It has been hypothesized that the carbonation reaction can be divided into two regimes.³ The first, fast reaction stage finishes when the volume available in small pores is filled by the newly-formed CaCO₃. Once the internal pores in the grain are filled, a layer of product (CaCO₃) is deposited on the surface of a grain. Owing to the typically short contacting times in industrial fluidized bed reactors, only the CO₂ uptake achieved during the fast reaction stage will be of practical relevance. However, it is likely, that this simple carbonation model does not describe the dynamics of the carbonation reaction in full detail. In this work focused ion beam – high resolution scanning electron microscopy (FIB-HRSEM) was used to probe the structural changes occurring on a nanometer scale during the carbonation and calcination reactions. Using FIB-HRSEM we are able to determine whether CaCO₃ is formed on the CaO skeleton homogeneously layer by layer or via the merging of heterogeneous CaCO₃ islands. However, to derive robust conclusions a Ca-based material with regular pore structure and a high stability over repeated cycles is required. So far, mostly "simple" techniques, such as wet-impregnation, mechanical mixing or co-precipitation^4,5 were applied to develop synthetic Ca-based sorbents. However, these methods do not allow to tailor key structural properties, such as pore size distribution, that have shown to influence strongly the overall CO₂ uptake. In this work we report the development of a Ca-based sorbent using a sol-gel technique. An important facet was to determine relationships between (i) the preparation conditions, (ii) the structure and (iii) the performance of the material. The calcium precursor and the ratio of Ca²⁺ to Al³⁺ were identified as the sol-gel parameters which influenced the CO₂ uptake characteristics most. Calcium precursors containing a weak organic acid, i.e. Ca(C₅H₇O₂)₂ or Ca(CH₃COO)₂ yielded sorbents which possessed a homogeneous, nano-structured morphology as well as a high surface area and pore volume. In turn, these sorbents also showed a high and very stable CO₂ uptake over many cycles. After 30 cycles of calcination and carbonation the CO₂ uptake was 0.51 g CO₂ /g sorbent which is about 250 % higher than for natural Havelock limestone.

References

(1) A. MacKenzie, D. L. Granatstein, E. J. Anthony, J. C. Abanades, Energy Fuels 2007, 21, 920-926.

(2) J.C. Abanades, E.J. Anthony, D.Y. Lu, C. Salvador, D. Alvarez, AIChE J. 2004, 50, 1614-1622

(3) J. S. Dennis, R. Pacciani, Chem. Eng. Sci. 2009, 64, 2147-2157.

(4) M. Broda, A. M. Kierzkowska, C. R. Müller,  ChemSusChem 2012, 5, 411-418.

(5) A. M. Kierzkowska, C. R. Müller, Energy Environ. Sci. 2012, 5, 6061-6065.