(739g) Calcium Looping Process for CO2 Capture - Investigation of Direct Carbonation of Ca(OH)2

CO₂ capture using calcium-based sorbents is rapidly developing into a viable technology. However, the decrease in CO₂ capture capacity of calcium sorbents when used over many cycles, presents a significant challenge. Among the various techniques that have been used to overcome this challenge, intermediate hydration seems most promising. Intermediate hydration results in the formation of Ca(OH)₂ making the traditional 2 step (CaO-CaCO₃) process a 3-step (CaO-Ca(OH)₂-CaCO₃) process. In such a scenario, the CO2 contacts the Ca(OH)₂ and not CaO. It has been believed that the Ca(OH)₂ decomposes to yield CaO which then reacts with CO₂. However, direct carbonation of Ca(OH)₂cannot be ruled out at the operating conditions.

The direct carbonation of Ca(OH)₂ was investigated in this work and the effects of CO₂ concentration and heating rate were studied. It was found that direct carbonation resulted in very high conversion to CaCO₃, as compared to indirect carbonation. It is believed that the conversion is enhanced due to equimolar counterdiffusion of H₂O which helps the CO₂ reach the unreacted CaO core through the CaCO₃ product layer. Direct carbonation also resulted in lower sintering due to the instant occupation of the pores formed because of H₂O evolution, by CO₂ (in the form of CaCO₃). This results in better carbonation conversion in the succeeding cycles. While CO₂ concentration did not affect direct carbonation conversion, different heating rates had an effect. Final conversions and reaction kinetics were poor when lower heating rates were used. Surface area and pore volume analyses were able to corroborate the theory of mitigation of sintering during direct carbonation. Using the shrinking core model (SCM), it was found that the effective diffusion coefficients during direct carbonation are higher than during indirect carbonation. The non-isothermal direct carbonation data was also fitted using the SCM and the experimental and model results were found to be in good agreement. These results will be presented during this talk and the implications on the process will be briefly highlighted.