Calcium-looping for post-combustion CO2 capture. On the adverse effect of sorbent regeneration under CO2

• Published in: Applied energy Vol. 126; pp. 161 - 171
• Main Authors: Valverde, J.M., Sanchez-Jimenez, P.E., Perez-Maqueda, L.A.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01-08-2014; Elsevier
• Subjects: Applied sciences; Calcium looping; Carbonation/calcination cycles; CO2 capture; Energy; Exact sciences and technology; Solid sorbents; Calcium looping; Carbonation/calcination cycles; CO2 capture; Solid sorbents; Regeneration; Calcium; Carbon dioxide; CO2 sequestration; capture; Combustion; Sorbent; CO
• ISSN: 0306-2619; 1872-9118
• DOI: 10.1016/j.apenergy.2014.03.081

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•Carbonation/calcination tests are reported in which the sorbent is regenerated under high CO2 partial pressure.•A drastic drop of CaO conversion occurs in just a few carbonation/calcination cycles.•Recarbonation has an adverse effect.•The growth of the CaO crystal structure along reactive oriented surfaces is precluded.•The presence of H2O would counteract the adverse effect of regeneration under CO2.
The multicyclic carbonation/calcination (c/c) of CaO solid particles at high temperature is at the basis of the recently emerged Calcium-looping (CaL) technology, which has been shown to be potentially suitable for achieving high and sustainable post-combustion CO2 capture efficiency. Despite the success of pilot plant projects at the MWth scale, a matter of concern for scaling-up the CaL technology to a commercial level (to the GWth scale) is that the CaO carbonation reactivity can be recovered only partially when the sorbent is regenerated by calcination at high temperatures (around 950°C) as required by the CO2 high concentration in the calciner. In order to reactivate the sorbent, a novel CaL concept has been proposed wherein a recarbonator reactor operated at high temperature/high CO2 concentration leads to further carbonation of the solids before entering into the calciner for regeneration. Multicyclic thermogravimetric analysis (TGA) tests demonstrate the feasibility of recarbonation to reactivate the sorbent regenerated at high calcination temperatures yet at unrealistically low CO2 partial pressure mainly because of technical limitations concerning low heating/cooling rates. We report results from multicyclic c/c and carbonation/recarbonation/calcination (c/r/c) TGA tests at high heating/coling rates and in which the sorbent is regenerated in a dry atmosphere at high CO2 partial pressure. It is shown that at these conditions there is a drastic drop of CaO conversion to a very small residual value in just a few cycles. Moreover, the introduction of a recarbonation stage has actually an adverse effect. Arguably, CaCO3 decomposition in a CO2 rich atmosphere is ruled by CO2 dynamic adsorption/desorption in reactive CaO (111) surfaces as suggested by theoretical studies, which would preclude the growth of the regenerated CaO crystal structure along these reactive surfaces, and this effect would be intensified by recarbonation. Nevertheless, the presence of H2O in the calciner, which is also adsorbed/desorbed dynamically in CaO reactive planes, would shield CO2 adsorption/desorption thus mitigating the deeply detrimental effect of CO2 on the carbonation reactivity of the regenerated CaO structure. Oxy-combustion, which produces a significant amount of H2O, is currently used in pilot-scale plants to raise the temperature in the calciner. Auxiliary techniques are being explored to help heating the partially carbonated solids since oxyxombustion represents an important penalty to the CaL technology. Our study suggests that steam injection would be necessary in a dry calciner environment to avoid a sharp loss of CaO conversion if the sorbent is regenerated at high CO2 partial pressure.