Reactivation of CaO-Based Sorbents for CO₂ Capture: Mechanism for the Carbonation of Ca(OH)₂

Abstract

Calcium looping is an emerging technology for CO 2 capture that uses a regenerable CaO-based sorbent. Here, a novel hydration-based reactivation strategy for spent sorbent, proposed by Industrial Research Limited of New Zealand, is investigated. They have called the process Ca(OH) 2 "superheating" and suggested that Ca(OH) 2 becomes more chemically stable under CO 2 , allowing release of steam at an elevated temperature ("superheated dehydration"). To investigate this, Ca(OH) 2 powder and pellets and hydrated calcined limestone and dolomite have been heated in various different atmospheres in a thermogravimetric analyzer with a mass spectrometer performing online gas analysis of the off-gas. The "superheated dehydration" effect was observed for Ca(OH) 2 pellets and hydrated calcined limestone, but not for Ca(OH) 2 powder or hydrated calcined dolomite. These findings are consistent with a mechanism involving formation of an impermeable carbonate layer, which prevents H 2 O diffusion until rupture. The carbonate layer has a critical thickness that is not reached in the case of the Ca(OH) 2 powder, but is in the case of the Ca(OH) 2 pellets and hydrated calcined limestone. The network of MgO in the dolomitic particles results in CaO grains that are not large enough to accommodate the impermeable carbonate layer.

In situ X-ray diffraction of CaO based CO2 sorbents

“…Particle size has been shown to influence the mechanism of CO 2 capture and calcination in CaO based materials like for example effects regarding heat and mass transfer. 32,[69][70][71] TEM analysis showed the presence of Ca(OH) 2 crystallites nominally 83×16 nm in size, which compares well with the values calculated from synchrotron diffraction of 75×16 nm. The good agreement between these two very different methods provides strong evidence that the analysis of the anisotropic broadening is robust, and that the calculations of strain and stress presented here are also reliable.…”

CO2 capture by calcium aluminate pellets in a small fluidized bed

“…These were particles to subsequent carbonation [39,40]. A similar process will have this occurred here upon aging; though the carbonation mechanism would be different, likely going through a Ca(OH)2 intermediate, which has also been shown to increase sorbent reactivity [33,34]. However, once steam is introduced a marked improvement in reactivity was observed for aged pellets over the original pellets.…”

“…In addition, some carbonation occurred as a result of reaction with CO2 in the atmosphere during the pelletization process. Further hydration and carbonation can be expected from further exposure to the air, and these are in fact a potential reactivation method for spent natural sorbent [2,33,34] , although it is much less clear what they might do to pellets produced by a partial hydration method.…”

Thermodynamics of hydrogen production from urea by steam reforming with and without in situ carbon dioxide sorption

“…limestone, dolomite), it makes sense to use the material over several cycles and to perform multiple regenerations before disposing of the used carbonate, so as to achieve genuine energy and CO 2 emissions reductions. Research efforts are taking place worldwide to understand the reasons for the loss of CO 2 capacity with repeated cycling and increase the durability of Ca-based CO 2 sorbent with this very aim [35][36][37][38][39][40][41][42][43]. Table 2 allows comparison of the minima of enthalpy changes of the urea-water system at 1 atm (and the temperatures at which they occur) with those of the urea-water-CaO system (with Ca:C=1), in kJ/mol of H 2 produced, alongside the H ratio, maxima of H 2 yield and of H 2 purity, also listed with their corresponding temperatures.…”