In situ X-ray diffraction of CaO based CO2 sorbents

Abstract

In situ X-ray diffraction coupled with Rietveld refinement has been used to study CO 2 capture by CaO, Ca(OH) 2 and partially hydrated CaO, as a function of temperature. Phase quantification by Rietveld refinement was performed to monitor the conversion to CaCO 3 and the results were compared to those derived using thermogravimetric analysis (TGA). It was found that Ca(OH) 2 converted directly to 100% CaCO 3 without the formation of a CaO intermediate, at ca. 600˚C. Both pure CaO and partially hydrated CaO (33.6 wt% Ca(OH) 2 ) reached the same capture capacity, containing approximately 65 wt% CaCO 3 at 800˚C. It was possible to provide direct evidence of the capture mechanism. The stresses in the Ca(OH) 2 phase of the partially hydrated CaO were found to be more than 20 times higher than its strength, leading to disintegration and the generation of nano-sized crystallites. The crystallite size determined using diffraction (75×16 nm) was in good agreement with the average crystallite size observed using TEM (of 83×16 nm). Electron diffraction images confirmed coexistence of CaO and Ca(OH) 2 . The analysis provides an explanation of the enhanced capture /disintegration observed in CaO in the presence of steam.Keywords: Rietveld analysis, in-situ X-ray diffraction, CaO, CO 2 capture, hydration, capture mechanism, attrition, regeneration, synchrotron. Graphical abstractIn situ XRD, in conjunction with phase quantification using Rietveld analysis, was used successfully to study CO 2 capture in CaO and Ca(OH) 2 as well as in partially hydrated CaO.The work shows how in situ XRD can be used as a supplemental technique to TGA, by which the mechanism of capture and hydration can be elucidated. IntroductionConcerns about global warming have prompted both national and international efforts to curb CO 2 emissions. CaO based materials are being considered as CO 2 sorbents for removal of CO 2 from flue gases at temperatures between 400˚C and 800˚C. Applications include pre-and post-combustion carbon capture technologies. [1][2][3][4][5][6][7][8][9][10] In addition, with the growing replacement of fossil fuels by biomass sources, and due to the high oxygen content of biomass, many of the processes of thermochemical fuel upgrading generate significant CO 2 at medium-high temperatures. [11][12] This provides the opportunity for in situ CO 2 capture in the 400-800˚C range and for improved heat transfer arising from the coupling of the endothermic gasification reactions with the exothermicity of the CO 2 chemisorption. This results in lower reaction temperatures and lower fuel consumption. [13][14] The advantages of CaO based materials as CO 2 sorbents for pre-and post-combustion CO 2 capture are their low cost, high abundance, large sorption capacity and fast reaction kinetics. 15-17 However, a major shortcoming of CaO based materials as CO 2 sorbents is the degradation in sorption capacity after multiple capture and regeneration cycles, due to loss of surface area through sintering. 18-21 For large scale CO 2 capture, th...

Investigation of CaO–CO2 reaction kinetics by in-situ XRD using synchrotron radiation

“…Instead, the effect of the calcium oxide/carbonate crystallite size on the sorbent performances are pieces of information not available in the literature when referring to the carbonation reaction with CaO-based solid sorbents. Molinder et al (2012) recently used x-ray diffraction to measure the crystallite size of partially hydrated CaO sorbents in a study on simultaneous hydration and carbonation of CaO, partially hydrated CaO and of Ca(OH) 2 . However, to the best of our knowledge, the dependence of the carbonation kinetics on the calcium oxide/carbonate crystallite size has never been investigated.…”

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.…”

Durability of CaO–CaZrO3 Sorbents for High-Temperature CO2 Capture Prepared by a Wet Chemical Method

“…For this purpose a Philips CM200 FEG-TEM was operated at 197 keV and fitted with a Gatan Imaging The as-prepared sorbent powders contained a mixture of CaO (cubic) and CaZrO 3 (orthorhombic), as identified by powder XRD 27 (Figure 1). Rietveld refinement of the XRD data for as-prepared powders and for powders after multiple TGA carbonation/decarbonation cycles gave residual (R P ) and weighted residual (R WP ) Rietveld parameters <10% ( Figure S1, Table S1), indicating that the model provided a reliable indication of phase proportions (accurate to ± 1 wt%) 22 . The estimated weight fraction (%) of CaZrO 3 in the three as-prepared sorbents were as follows: Sample 1, 10CaZrO 3 :90CaO; Sample 2, 18CaZrO 3 :82CaO; Sample 3, Carbonation (%) The net CO 2 uptakes, expressed as g-CO 2 /g-total sorbent mixture, using the three as-prepared sorbent compositions under 'mild' conditions are shown in Figure 2 (15% CO 2 at 650 o C for Table 2).…”