In situ synchrotron XRD investigation of the dehydration and high temperature carbonation of Ca(OH)₂

Abstract

The dehydration and high temperature carbonation reactions of CaIJOH) 2 are an essential part of a number of existing schemes aiming to apply the thermochemical CaO-CaCO 3 cycle ('Ca Looping') in emerging energy industries such as CO 2 capture, gasification and thermal energy storage. However, the underlying chemical mechanisms are poorly understood and the reaction kinetics exhibit a number of strange features such as the increased thermal stability of CaIJOH) 2 , dubbed "superheating", observed in the presence of CO 2 . In this work, in situ X-ray diffraction (XRD) measurements using a synchrotron source are coupled with full pattern fitting analysis to provide high-resolution and high-accuracy data on the evolution of CaIJOH) 2 crystalline structural parameters during the dehydration and carbonation reactions. This method has allowed new observations to be made that provide insight into the studied reaction mechanisms and disprove existing hypotheses as to the origin of superheating. A new hypothesis, involving the formation of an intermediate and permeable CaIJOH) 2 structure was formulated which can explain the growing body of observations in this area. 7306 | CrystEngComm, 2015, 17, 7306-7315 This journal is

Surface and bulk carbonate formation in calcium oxide during CO2 capture

“…The presence of molecular water (1630 cm -1 ) and hydroxyl functionality (3750 -3250 cm -1 ) during direct carbonation (Figure 7b) supports findings (Materic and Smedley, 2011) whereby the carbonation reaction proceeds via an adsorbed water film on the surface. Furthermore, the presence of molecular water on the surface can somewhat confirm previous findings, whereby water molecules departing the Ca(OH)2 lattice act as a liquid-like interface for H 2 O and CO 2 to be exchanged (Materic et al, 2015). The identification of bicarbonate species (Figure 7b & Figure 8) provides further support for this hypothesis.…”

“…In this work prominent issues surrounding calcium looping are investigated primarily through the use of in-situ FTIR, as has been previously called for to further elucidate the role of molecular water (Materic et al, 2015). It is proposed that the onset of the "slow" regime can be spectroscopically identified through an increase in the integrated intensity of spectral features due to vibrational modes of bulk carbonate, initiated at a lower temperature in the presence of water or hydroxyl groups on the surface of particles; 300°C as compared to 400°C in the absence of water.…”

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances

“…Lastly, it has been suggested that CaO and steam could form Ca­(OH) 2 as an intermediate species, which improves the formation of CaCO 3 . − Under typical carbonation conditions (>650 °C) employed for the calcium looping cycle, Ca­(OH) 2 cannot exist stably from a thermodynamic point of view, making this pathway an unlikely reason for the improved CO 2 uptake in the presence of steam . However, for carbonation performed at lower temperatures, e.g., for MgO to MgCO 3 , hydroxide species can be thermodynamically stable and affect the carbonation mechanism on the metal oxide; this is discussed in the following section.…”