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Synthesis protocol. Schematic illustration of the hydrothermal approach developed here to synthesize template-assisted, MgO-stabilized, CaO-based CO2 sorbents
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Calcium looping, a CO2 capture technique, may offer a mid-term if not near-term solution to mitigate climate change, triggered by the yet increasing anthropogenic CO2 emissions. A key requirement for the economic operation of calcium looping is the availability of highly effective CaO-based CO2 sorbents. Here we report a facile synthesis route that...
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Herein we report the development of synthetic CaO-based sorbents for enhanced CO2 capture in calcium looping via a template-assisted synthesis approach, where carbonaceous spheres (CSs) derived from hydrothermal reaction of starch are used as the templates. Cage-like CaO hollow microspheres are successfully synthesized only using urea as the precip...
The use of cement raw meals as sorbent precursors for CO2 capture can reinforce the synergies between the cement production process and calcium looping CO2 capture technology. In this work, we measure the CO2-carrying capacity of different calcined samples of a particular marl, which were obtained under very different calcination conditions and set...
In this work the integration of the Calcium-Looping (CaL) process, used as a post-combustion CO2 capture system, into a cement kiln was analyzed by means of process simulations. The results show that capture efficiencies of about 90% can be achieved with operating conditions of CaL reactors similar to those for power generation applications. The in...
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... Of diverse catalyst supports, CaO is widely utilized as a CO 2 sorbent in the sorption-enhanced steam reforming (SESR) process [24,25]. CaO is also found to improve catalytic cracking of tar components, which is favorable for H 2 production [26]. ...
... 7 However, the synthesis cost is relatively high, limiting its industrialized application. Using Ca(NO 3 ) 2 and Mg(NO 3 ) 2 as CaO and MgO precursors, Naeem et al. 21 yielded highly effective CaO-MgO sorbents via a hydrothermal approach. They also found that the presence of a carbonaceous template during synthesis allowed for the formation of multi-shelled microstructures. ...
The main limit for the calcium looping process is the sharp decrease of the capture capacity of the CO2 sorbents during multiple cycles. In this research, a solution combustion method was employed to synthesize MgO-stabilized CaO sorbents. Polyethylene glycol (PEG) was used as the fuel and dispersant, with the purpose to enhance the uniformity of the Ca and Mg distributions in the sorbent. The results show that highly reactive MgO-stabilized CaO sorbents can be obtained through a solution combustion method using PEG as the fuel and dispersant. The existence of MgO can effectively restrain the sintering of the sorbent, resulting in a more porous and stable micro-structure of the sorbent. The CO2 capture capacity of the MgO-stabilized CaO sorbent synthesized under the optimum conditions is 0.40 g(CO2)/g(sorbent) after 20 cycles, which is 75.3% higher than CaCO3.
... It was shown that the MWCNTassisted templating method raised the ultimate CO 2 sorption capacity of CaO, under severe calcination conditions, from 0.13 to 0.26 g CO 2 / g sorbent. Naeem et al. [72] evaluated the effect of the template-assistance synthesis method on the multicyclic activity of modified CaO sorbent for the CaL technique under severe calcination conditions. Employing the hydrothermally xylose-based templating preparation approach resulted in 1.42 times higher CO 2 uptake capacity at the 10th multiple CaL cycle and a more porous structure containing higher pores. ...
The CaO-based sorbents suffer from a high-intensity sintering rate and require costly promoters to improve their CO2 capture capacity through the Calcium Looping (CaL) technique. Herein, for the first time, the eco-friendly and efficient eggshell-derived CaO sorbents were promoted with tea waste for the CO2 capture process. 0, 2.5, and 5 wt.% of ball-milled tea waste nanoparticles (NPs) were merged with sieved eggshell-based NPs throughly. Their CO2 capture activity was evaluated under severe calcination conditions, including calcination under 100 vol.% CO2 for 5 min at 950 °C. According to the textural/morphological data, 92 % and 89.8 % enhancement were seen for the prepared sample's surface area and pore volume, including 5 wt.% tea waste NPs. Additionally, the average grain size of as-prepared specimens decreased from 39.3 nm to 25.2 nm. Furthermore, the sorbent having 5 wt.% tea waste presented the multicyclic durability of 48.1 % and the average CO2 uptake capacity of 0.133 g CO2/g sorbent during fifteen multiple carbonation/calcination cycles under severe calcination conditions. The incorporation of 5 wt.% tea waste as the structural promoter resulted in 100 % and 58 % improvements in the capture potential of CaO at kinetically- and diffusion-controlled carbonation stages of the 15th cycle.
... 20,29,31−36 For example, a MgO-stabilized (11 wt %) multishell-type architecture of CaO yielded a CO 2 uptake of approximately 0.64 g CO2 g sorbent −1 after 10 carbonationregeneration cycles, which corresponds to a notable 400% increase in CO 2 uptake compared to the benchmark limestone. 31 Using atomic layer deposition (ALD) to allow for both the nanostructuring of the sorbents and the use of a stabilizer, it has been demonstrated that nanometer-thin metal oxide layers (e.g., Al 2 O 3 ) can appreciably stabilize the cyclic CO 2 uptake of CaO-based sorbents. 20,37 Furthermore, such fabrication of well-defined, model, nanostructured CaO-based sorbents has contributed to the elucidation of deactivation mechanisms in stabilized CaO-based sorbents. ...
... 20,37 Furthermore, such fabrication of well-defined, model, nanostructured CaO-based sorbents has contributed to the elucidation of deactivation mechanisms in stabilized CaO-based sorbents. 29,31,33 That being said, efficient measures to mitigate the segregation and (surface) agglomeration of the stabilizer and/or mixed phases are currently lacking, not least because we still have an incomplete fundamental understanding of the underlying interactions between CaO and the stabilizer phases and their dynamics under CO 2 capture-regeneration conditions. ...
... For example, multishelled, MgOstabilized, CaO exhibited a CO 2 uptake of ca. 0.65 g CO2 g Sorbent −1 after 10 carbonation-regeneration cycles (conditions comparable to the present work), 31 and a CO 2 uptake of 0.44 g CO2 g Sorbent −1 after 50 cycles was reported for CaO stabilized with MgO via mechanical mixing. 45 In summary, the mixed (Al,Si)O x -stabilized sorbent Ca@(Al,Si)O x notably outperformed the most promising single metal oxide-stabilized sorbents, i.e., Ca@Al (15) and Ca@Si(60), over consecutive carbonation-regeneration cycles. ...
CaO-based sorbents are cost-efficient materials for high-temperature CO2 capture, yet they rapidly deactivate over carbonation-regeneration cycles due to sintering, hindering their utilization at the industrial scale. Morphological stabilizers such as Al2O3 or SiO2 (e.g., introduced via impregnation) can improve sintering resistance, but the sorbents still deactivate through the formation of mixed oxide phases and phase segregation, rendering the stabilization inefficient. Here, we introduce a strategy to mitigate these deactivation mechanisms by applying (Al,Si)Ox overcoats via atomic layer deposition onto CaCO3 nanoparticles and benchmark the CO2 uptake of the resulting sorbent after 10 carbonation-regeneration cycles against sorbents with optimized overcoats of only alumina/silica (+25%) and unstabilized CaCO3 nanoparticles (+55%). ²⁷Al and ²⁹Si NMR studies reveal that the improved CO2 uptake and structural stability of sorbents with (Al,Si)Ox overcoats is linked to the formation of glassy calcium aluminosilicate phases (Ca,Al,Si)Ox that prevent sintering and phase segregation, probably due to a slower self-diffusion of cations in the glassy phases, reducing in turn the formation of CO2 capture-inactive Ca-containing mixed oxides. This strategy provides a roadmap for the design of more efficient CaO-based sorbents using glassy stabilizers.
... Typically, the surface functionalities of the surfactants used provided a physical barrier between the adjacent CaO-NPs and resulted in the highly stabilized structure of the nanocomposites, as shown in Figure 2c,d. This concept was discussed in the previously reported work [54]. By considering the stability of the designed materials in terms of hygroscopicity, it is clearly shown by the SEM images of the samples (CaO-NPs, Ag-CaO-NPs, PVP@Ag-CaO, and SDS@Ag-CaO) how well their original appearance and quality were maintained despite their exposure to moisture or humidity. ...
The demand for lithium is constantly increasing due to its wide range of uses in an excessive number of industrial applications. Typically, expensive lithium-based chemicals (LiOH, LiCl, LiNO3, etc.) have been used to fabricate adsorbents (i.e., lithium manganese oxide) for lithium ion (Li+) adsorption from aqueous sources. This type of lithium-based adsorbent does not seem to be very effective in recovering Li+ from water from an economic point of view. In this study, an innovative nanocomposite for Li+ adsorption was investigated for the first time, which eliminates the use of lithium-based chemicals for preparation. Here, calcium oxide nanoparticles (CaO-NPs), silver-doped CaO nanoparticles (Ag-CaO-NPs), and surfactant (polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS))-modified Ag-CaO (PVP@Ag-CaO and SDS@Ag-CaO) nanocomposites were designed by the chemical co-precipitation method. The PVP and SDS surfactants acted as stabilizing and capping agents to enhance the Li+ adsorption and recovery performance. The physicochemical properties of the designed samples (morphology, size, surface functionality, and crystallinity) were also investigated. Under optimized pH (10), contact time (8 h), and initial Li+ concentration (2 mg L−1), the highest Li+ adsorption efficiencies recorded by SDS@Ag-CaO and PVP@Ag-CaO were 3.28 mg/g and 2.99 mg/g, respectively. The nature of the Li+ adsorption process was examined by non-linear kinetic and isothermal studies, which revealed that the experimental data were best fit by the pseudo-first-order and Langmuir models. Furthermore, it was observed that the SDS@Ag-CaO nanocomposite exhibited the highest Li+ recovery potential (91%) compared to PVP@Ag-CaO (85%), Ag-CaO NPs (61%), and CaO NPs (43%), which demonstrates their regeneration potential. Therefore, this type of innovative adsorbents can provide new insights for the development of surfactant-capped nanocomposites for enhanced Li+ metal recovery from wastewater.
... In addition, the special shaped Ca-based absorbents such as microspheres, microtubules, cage structures, and nanosheets can be synthesized using template-assisted techniques (Table 5) [121][122][123]. For example, to ensure homogeneous mixing of the stable phases, Muhammad Awais et al. synthesized CaMg absorbents using a simple single-tank hydrothermal template-assisted method [124]. Firstly, the aqueous solutions of xylose, urea and glycine were hydrothermally processed with calcium nitrate and magnesium nitrate precursors for 24 h. ...
... (A) Mechanisms for porosity enhancement by the sacrificial template method; (B) Schematic diagram of the hydrothermal template assisted MgO stabilized Ca-based absorbent and SEM/TEM images of Ca-based absorbent; (C) Schematic diagram of the CaO/Ca 2 SiO 4 hollow nanoparticles prepared without template method and TEM images of CaO/Ca 2 SiO 4[120,124,126]. ...
... While in the sorptionenhanced steam reforming processes, as shown in Fig. 2 (b), the steam and bio-fuel mixtures are passed through the reforming reactor, where the bio-fuels are reformed into H 2 and other by-products like CO 2 , which are then in-situ captured by the sorbents; accordingly, the H 2 purity is significantly improved. Subsequently, the carbonated sorbents are transferred into the calcination reactor and regenerated under elevated temperatures ( Naeem et al., 2018 ). The calcination process is projected to generate the pure CO 2 stream for storage or utilization, and accordingly, closes the looping process. ...
... The common synthesis methods still failed to manufacture Ca-based sorbents that are resistant to sinter. Hence, some advanced synthesis methods, such as flame spray pyrolysis ( Peng et al., 2015 ), hydrothermal Naeem et al., 2018 ), and atomic layer deposition ( Armutlulu et al., 2017 ), have been proposed to produce Cabased sorbents with highly porous and hollow properties. For instance, Naeem et al. (2018) synthesized a kind of highly porous and hollow CaO sorbent by a hydrothermal method and found that the obtained sorbent achieved a CO 2 capture capacity of 0.66 g -CO2 /g -sorbent , which exceeded the natural occurring limestone-derived materials by 500%, as shown in Fig. 14 . ...
... Hence, some advanced synthesis methods, such as flame spray pyrolysis ( Peng et al., 2015 ), hydrothermal Naeem et al., 2018 ), and atomic layer deposition ( Armutlulu et al., 2017 ), have been proposed to produce Cabased sorbents with highly porous and hollow properties. For instance, Naeem et al. (2018) synthesized a kind of highly porous and hollow CaO sorbent by a hydrothermal method and found that the obtained sorbent achieved a CO 2 capture capacity of 0.66 g -CO2 /g -sorbent , which exceeded the natural occurring limestone-derived materials by 500%, as shown in Fig. 14 . ...
... Another approach is the modification of the sorbent morphology. Multishelled hollow microspheres of CaO have been synthesized with MgO stabilizer through a hydrothermal fabrication route [23]. Their CO 2 uptake was about 5 times higher than the limestonederived CaO after 30 cycles of carbonation at 650 • C and decarbonation at 900 • C. Coating the CaO sorbent layer on an existing backbone material was proposed as a simple and effective way to control the CaO microstructure. ...
The carbonation behavior of calcium-containing sorbents, CaO and Ca(OH)2, was investigated under pressurized CO2 at nominal room temperature. The carbonation reaction was mechanically driven via reactive ball milling. The carbonation rate was determined by monitoring the CO2 pressure inside the sealed milling jar. Two different versions of CaO were fabricated as starting materials. The addition of citric acid in CaO synthesis resulted in a significant increase in sorbent surface area, bringing up the conversion of CO2 from 18% to 41% after 3 h of reactive milling. The hydroxide formation from these two oxides closed the surface area gap. Nevertheless, we found that hydroxides had a higher initial carbonation rate and greater final CO2 uptake than their oxide counterparts. However, the formation of byproduct water limited the further carbonation of Ca(OH)2. When we added a controlled amount of water to the CaO-containing milling jar, the highest carbonation rate and most extensive CO2 uptake were attained due to the in situ formation of reactive Ca(OH)2 nanoparticles. We saw CaCO3 X-ray diffraction peaks only when Ca(OH)2 was involved in this low-temperature carbonation, indicating that the grain growth of CaCO3 is easier on the Ca(OH)2 surface than on the CaO surface. We used the Friedman isoconversional method to calculate the effective activation energy of decarbonation for the high surface area CaO sorbent milled with water. The average effective activation energy was found to be about 72 kJ mol−1, and its magnitude started to decrease significantly from 50% sorbent regeneration. The drastic change of the effective activation energy during decarbonation suggests that CaCO3, formed at nominal room temperature by reactive milling under pressurized CO2, should undergo a more drastic morphology change than the typical thermally carbonated CaCO3.
... To date, extensive investigations have been conducted on Ca-based materials (CaM) for thermochemical energy storage [18]. However, a common issue of CaM is the loss-in-capacity during cycles due to the loss of chemical reactivity associated with surface area reduction after sintering [25,26]. Over the past 20 years, extensive studies for maintaining cyclic stability using different approaches have been performed, such as (1) the addition of steam to favor hydration, (2) using ball milling or acidification to modify CaM, or (3) doping CaM with insert material [27]. ...
Calcium looping (CaL) is one of the most promising thermochemical energy storage technologies for high-temperature applications such as next-generation concentrated solar power (CSP) systems. However, most previous investigations have mainly focused on optimizing Calcium-based materials to maintain their reactivity during cycling, while their behavior in reactors under direct solar irradiation has rarely been reported. In this paper, highly efficient and stable direct solar-driven thermochemical energy storage in fluidized reactors is demonstrated. (AlMgFeMn)OxCaCO3 pellets demonstrated excellent long-term stability with an energy storage density of more than 85% of the initial value after 100 cycles. The underlying mechanism can be attributed to the presence of poly-oxide (AlMgFeMn)Ox crystals, which prevent crystallite growth and sintering, as confirmed by in-situ X-ray diffraction analysis. Moreover, the solar-thermal conversion efficiency of (AlMgFeMn)OxCaCO3 pellets in fluidized bed reactors is significantly improved from 9% to 19% thanks to the considerably increased average solar absorptance and fast reaction kinetics over white (AlMg)OxCaCO3 pellets. The experimental analysis using an operando fluidized thermogravimetric analyzer (F-TGA) further revealed that the interparticle diffusion control limitation in traditional TGA or fixed bed and localized overheating due to Gaussian distribution of solar irradiation are successfully relieved in a fluidized bed. We further suggest that steam has a positive effect on enhancing reaction kinetics and stability by performing 10 energy storage/release cycles of (AlMgFeMn)OxCaCO3 pellets under direct irradiation of concentrated light due to the increase of surface area after rehydration and the higher OH reactivity toward CaO. This work paves the way for the application of solar-driven fluidized bed reactors for scalable thermochemical energy storage.
... Although significant progress has been made, few current carbon capture and sequestration methodologies can meet the overall fossil energy performance goals set by the U.S. Department of Energy of a 90% CO 2 capture rate with 95% CO 2 purity at a cost of electricity 30% less than baseline capture approaches [8]. Current technologies for capturing CO 2 , including solvent-based (amines) and CaO-based materials, are still too energy intensive [9][10][11][12]. Hence, development of new materials that can capture and release CO 2 reversibly with acceptable energy costs is critical. ...
The electronic properties and thermal stabilities of MAlO2 and M5AlO4 (M = Li, Na, K) are investigated by density functional theory and lattice phonon dynamics. Based on the calculated electronic and lattice thermodynamic properties, their abilities to capture CO2 as solid sorbents are analyzed. The calculated electronic structural properties of MAlO2 and M5AlO4 indicate that all these alkali aluminates are semiconductors with a bandgap range of 2.4 ~ 6.4 eV. The 1st valence bands of these alkali aluminates are located 0 ~ − 6 eV under Fermi levels and are mainly contributed by p orbitals of O, s and p orbitals of Al and M. The phonon vibrational frequencies of M5AlO4 spread at a lower frequency range compared to their MAlO2 phases. With increasing temperature, the calculated phonon free energies of M5AlO4 decrease faster than their corresponding MAlO2 while their entropies have opposite trends. The reaction 2MAlO2 + CO2 = M2CO3 + Al2O3 has higher reaction heat and Gibbs free energy change than those of corresponding reaction ²/5M5AlO4 + CO2 = M2CO3 + ¹/5Al2O3, which shows the former reaction possesses lower turnover temperature. Among the alkali aluminates studied, the β-NaAlO2, lt-KAlO2, and γ-LiAlO2 are better candidates that could be applied for CO2 capture technologies.
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