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Calcium looping (CaL) has being considered a CO 2 capture technology with great promise because of its wide suitability and low-cost. In this work, the Ca(OH) 2 core was coated with porous alumina coverings via a dip-coating approach to produce CaO-based sorbent pellet with core-shell structure. The coating shell was prepared by evaporation-induced self-assembly sol-gel strategy using P123 as the pore-forming agent with different acidic conditions (i.e., acid-alcohol ratio varied from 0.05 to 3). It reveals that the cyclic CO 2 capture performance of core-shell structured CaO-based sorbent pellets decreases with the increase of acidity of sol-gel using for covering spherical cores. When coating gel with excessive acidity, the formation of dense alumina shell and the irreversible chlorination inactivation, which together lead to the low cyclic CO 2 capture performance. Particularly, the CO 2 capture capability of core-shell structured CaO-based sorbent pellets whose shell gel used for coating in low acid-alcohol ratio of 0.05 is as high as 1.4 times that of non-shell CaO-based pellets after 17 cycles. Meanwhile, the core-shell structured CaO-based sorbent holds good potentials to inhibit the adverse impact of in-situ coal combustion compared with conventional non-shell pellets in practical CaL system. [Display omitted] • Core-shell structured CaO-based pellets were prepared by wet coating method. • Acid-alcohol ratio closely affects the morphology of the outer alumina shell. • Excessive acidity causes dense outer shell and irreversible chlorination inactivation. • Core-shell structured pellets inhibit adverse impact of in-situ coal combustion. [ABSTRACT FROM AUTHOR]

• Co-firing fly ash was impregnated with TEPA to fabricate CO 2 sorbent. • TFA-25 exhibited good CO 2 sorption and sorbent regeneration performance. • Operating parameters for CO 2 sorption and sorbent regeneration were optimized. • The amine-tethered sorbent retained stable CO 2 uptake in repeated cycles. • Recycling co-firing fly ash presented environmental and economic implications. Co-firing of coal and biomass blends provides a promising route for clean energy utilization with high efficiency and mitigated pollutants emission. However, the technology meets an important challenge in effective utilization of the co-firing fly ashes. Efforts have been made to valorize the co-firing fly ash residues as potential CO 2 sorbents. To achieve enhanced CO 2 sorption performance, the samples were modified with tetraethylenepentamine (TEPA). Effects of amine loading, CO 2 sorption temperature, initial CO 2 concentration and gas flow rate on CO 2 sorption behaviors of the amine-tethered sorbents were investigated. The desired sorbent with 25% TEPA loading presented the highest CO 2 sorption capacity of 1.19 mmol CO 2 /g when tested under 60 °C and 15%CO 2 with a total gas flow rate of 1200 ml min−1. The effects of regeneration temperature and ramping rate on sorbent regeneration performance were demonstrated. The maximum sorbent regeneration efficiency of 97.2% was achieved under the regeneration condition of 110 °C and 5 °C min−1. CO 2 sorption capacity of the amine-tethered sorbent kept stable within 5 repeated cycles. These findings indicate that the amine-tethered sorbent could be a good candidate for on-site CO 2 capture from flue gas in co-firing power plants. [ABSTRACT FROM AUTHOR]

Highlights • Coal/biomass co-firing fly ash was impregnated with TEPA to prepare CO 2 sorbent. • CO 2 adsorption behaviors were described by different kinetic equations. • Breakthrough modeling and the mass transfer kinetic characteristics were studied. • Effects of operation parameters on CO 2 adsorption and mass transfer kinetic were investigated. Abstract Coal and biomass fly ash was impregnated with tetraethylenepentamine (TEPA) to prepare solid CO 2 sorbent. Experimental CO 2 adsorption capacities of the sorbent in fixed-bed column were fitted to several kinetic equations. The modified Avrami fractional kinetic model could accurately predict the CO 2 adsorption kinetic behaviors of the sorbent under different conditions. CO 2 adsorption rate increases with the increase in temperature, initial CO 2 concentration and gas flow rate, as a result of the decreased CO 2 diffusion resistance. A mathematical model was proposed and demonstrated to well predict the CO 2 adsorption breakthrough curves in the fixed-bed column. Effects of temperature, initial CO 2 concentration and gas flow rate on the mass transfer kinetic behaviors of the desired sorbent were investigated. The positive effect of these factors on promoting the overall mass transfer coefficient and the internal mass transfer coefficient is not significant. The external mass transfer coefficient will be remarkably reinforced with the increase in these operating parameters, due to the change in viscosity and density of the gas phase and the reduced external mass transfer resistance. Internal mass transfer process is determined as the rate-limiting step for CO 2 adsorption. The results will lay the groundwork for future scale-up application of the sorbent in post-combustion CO 2 capture. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

K 2 CO 3 -modified silica aerogels (K 2 CO 3 /SG) were synthesized by the two-step sol-gel and wet impregnation method using K 2 CO 3 and tetraethoxysilane (TEOS) as starting materials and ethanol as solvent. CO 2 sorption behaviors of the sorbent were tested under ultra-dilute flue gas conditions of 20–60 °C, 1%CO 2 , 2%H 2 O and balanced N 2 with gas hourly space velocity (GHSV) of 4000 h −1 using a modified fixed-bed reactor. Experimental CO 2 uptakes of the sorbent under different temperatures were correlated to several kinetics models to study the kinetics performances. The effects of temperature, K 2 CO 3 loading, GHSV, H 2 O:CO 2 molar ratio and H 2 O pretreatment time on CO 2 sorption capacity and kinetics performances of the sorbent were further demonstrated. It is found that the two-step sol-gel and wet impregnation method could benefit the sorbent improved physical properties, superior surface characteristics and enhanced CO 2 sorption performances. Amongst various kinetics equations, the modified Avrami fractional kinetics model could well describe the CO 2 sorption process over K 2 CO 3 /SG. CO 2 sorption capacity decreases with the increase of temperature. With the increasing K 2 CO 3 loading, GHSV, H 2 O:CO 2 molar ratio and H 2 O pretreatment time, CO 2 sorption capacity increases first and then decreases. CO 2 sorption kinetics increases with the increasing temperature and H 2 O pretreatment time, while it decreases with the increasing K 2 CO 3 loading. CO 2 sorption kinetics keeps rather stable and then increases significantly with the increasing GHSV and H 2 O:CO 2 molar ratio. [ABSTRACT FROM AUTHOR]

[Display omitted] • Graphite-casted K 2 CO 3 -based adsorbent pellets with different supports were prepared. • ZrO 2 -supported K 2 CO 3 pellets exhibit outstanding performance for CO 2 adsorption. • 500 °C is the optimal calcination temperature for SiO 2 -supported K 2 CO 3 pellets. • SiO 2 -supported K 2 CO 3 adsorbent pellets possess excellent practicality for its low cost. K 2 CO 3 -based adsorbent pellets produced by graphite-casting method are promising alkali metal-based solid CO 2 adsorbents. The incorporation of porous metal oxide supports is an effective to enhance CO 2 adsorption capacity of K 2 CO 3 -based adsorbent pellets. In this work, graphite-casted K 2 CO 3 -based adsorbent pellets incorporated with different porous supports (i.e., TiO 2 , ZrO 2 , and SiO 2) were prepared for CO 2 capture. Comparatively, the ZrO 2 -supported, K 2 CO 3 pellets loaded with 30 wt% of K 2 CO 3 exhibit the highest CO 2 adsorption capacity, approximately 0.93 mmol/g. For the SiO 2 -supported K 2 CO 3 adsorbent pellets, the high calcination temperature (>700 °C) required to remove the graphite layer covering the pellet surface will cause the formation of new eutectics (i.e., K 2 Si 2 O 5 and K 2 Si 4 O 9), which leads to the collapse of the adsorbent pellets. Low calcination temperature of 500 °C can avoid the collapse of the SiO 2 -supported K 2 CO 3 adsorbent pellets and remove most of the graphite on the surface. Moreover, the CO 2 adsorption capacity can be further increased to 1.30 mmol/g by adding 20 wt% of microcrystalline cellulose to the SiO 2 -supported K 2 CO 3 adsorbent pellet. Although the adsorption capacity of structurally improved, SiO 2 -supported K 2 CO 3 pellets is still inferior to that of structurally improved, ZrO 2 -supported K 2 CO 3 pellets, they exhibit great cost advantage. Therefore, SiO 2 -supported K 2 CO 3 pellets with 20 wt% of microcrystalline cellulose have industrial application prospects. [ABSTRACT FROM AUTHOR]

Porous carbons produced using poplar sawdust are promising CO 2 adsorbent applied under ambient conditions due to the low-cost preparation approach. CO 2 uptake capability of the porous carbons produced from poplar sawdust can be effectively enhanced via doping an approximate amount of microalgae. The N-doped porous carbon produced from poplar sawdust using microalgae as N-doping source (microalgae/poplar sawdust=0.5:1) possesses an excellent capacity of 4.14 mmol·g−1 for CO 2 uptake, nearly over 1.7 times that of the carbon without N-doping. The improved performance for CO 2 uptake of porous carbons is mainly ascribed to the two-fold effect of microalgae doping: (i) generating extensive narrow pores and channels within the porous carbons owe to co-pyrolysis; (ii) introduction of N-containing moieties derived from the intrinsic N of microalgae increasing the surface polarity and alkalinity. The N-doped porous carbon granules derived from poplar sawdust are inferior to the powdered porous carbons for CO 2 adsorption due to the destroyed structure. They still maintain the relatively desirable capability for CO 2 adsorption (~3.0 mmol·g−1 during 5 cycles) and good mechanical property (10.96 ± 1.43 Mpa). Comprehensively considering the advantaged properties (low-pressure drop, high bulk density) required for practical application, the N-doped porous carbon granules derived from poplar sawdust with low-doping of renewable microalgae possess strong practicability. [Display omitted] • Microalgae replaces the chemical N-containing reagents as the N-doping source. • Microalgae doping plays a two-fold role in enhancing CO 2 adsorption for porous carbon. • N-doped, sawdust-derived porous carbons exhibit desirable CO 2 adsorption capacity. • N-doped, sawdust-derived porous carbon extrudates possess superior practicability. [ABSTRACT FROM AUTHOR]

Reducing or removing CO 2 is critical to the confined spaces such as submarines, space-crafts or aircrafts while using solid sorbents has been regarded as a promising method. In this work, K 2 CO 3 loaded on activated carbon (K 2 CO 3 /AC) was developed as a new and regenerable sorbent for CO 2 removing in confined spaces. CO 2 sorption performances of K 2 CO 3 /AC were investigated under different conditions by varying the K 2 CO 3 loadings, CO 2 concentrations, H 2 O concentrations, CO 2 sorption temperatures and water pretreatment durations as well as the purge gas flow rates. The CO 2 sorption capacity and carbonation conversion of K 2 CO 3 /AC decrease with increasing temperature and increase with increasing mole ratio of H 2 O concentration over CO 2 concentration. Sufficient water vapor pretreatment is found to be beneficial to the sorption-enhanced performance. Increasing flow rate will weaken the CO 2 sorption performance. The carbonation kinetics was also investigated with the correlation between the shrinking core model and experimental data. Additionally, the sorbent is proved to be regenerable and stable during 20-cycle CO 2 sorption–desorption experiments. K 2 CO 3 /AC presents high carbonation conversion efficiency, high thermal stability, and low dependency on CO 2 partial pressure. Therefore, it can be considered as a new option for CO 2 removal in confined spaces. [ABSTRACT FROM AUTHOR]

[Display omitted] • Four materials are successfully used to produce CaO-based pellets. • Nano-TiO 2 is the most suitable for producing CaO-based pellets as hydrophobic layer. • Al(NO 3) 3 surpasses alumina powder and sol on improving stability of CaO-based pellets. • Higher load of Al-based stabilizer contributes to the superior cyclic stability. Granulation serves as an effective strategy for mitigating the elutriation of CaO-based sorbents in the fluidized bed-based Calcium Looping system designed for CO 2 capture. Here, hydrophobic interface-assisted casted CaO-based sorbent pellets were prepared via a range of hydrophobic materials, i.e., graphite, Nano-titanium dioxide, Nano-silica, and polyethylene powders. The contact Angle values of the composite slurry droplets for the four hydrophobic materials range from 126 to 143°, which can provide enough surface tension to form CaO-based sorbent pellets. Moreover, the Al-based stabilizer precursors in the form of oxide, nitrate and sol, respectively, were incorporated into CaO-based pellets aiming to enhance their cyclic CO 2 uptake stability. Soluble aluminum nitrate exhibits the superior performance on enhancing CO 2 uptake stability compared to Nano-alumina and alumina sol, whose superiority lies in its ability to disperse more evenly with the active CaO component. Therefore, the Al-supported CaO-based pellet (∼30 wt% of Ca 12 Al 14 O 33) with aluminum nitrate as the precursor of Al-based stabilizer remains excellent CaO carbonation conversion of 50.3 % after 20 cycles, approximately 2 times that of pure CaO pellet. Even under harsh calcination condition (950 ℃, 100 vol% CO 2), the Al-supported CaO-based pellet is still markedly superior to CaO pellet for cyclic CO 2 capture. The freshly calcined Al-supported CaO-based pellet exhibits highly desirable compressive strength (3.4 ± 0.83 MPa), compared to 0.35 ± 0.07 MPa for pure CaO pellet. The generated inert Ca 12 Al 14 O 33 skeleton provides structural stability and strength to the Al-supported, CaO-based pellets, which simultaneously contributes to the enhanced cyclic CO 2 uptake capacity and mechanical property. [ABSTRACT FROM AUTHOR]

K 2 CO 3 supported on activated carbon (K 2 CO 3 /AC) shows promise as a potential candidate for removing low-concentration CO 2 from environmental control and life support systems (ECLSS). However, K 2 CO 3 /AC would suffer a decreased CO 2 sorption capacity due to sorbent deactivation in acid impurities such as SO 2 and NO 2 . To solve this problem, water vapor pretreatment, KOH addition and polyethyleneimine (PEI) modification were proposed as efficacious means for improving its CO 2 sorption capacity and mitigating the sorbent deactivation in these acid impurities. CO 2 sorption performances of the pretreated and modified sorbents were evaluated in a fixed-bed reactor under simulated ECLSS conditions of 20 °C, 1.0% CO 2 , 2.0% H 2 O, 30 ppm SO 2 , 20 ppm NO 2 and balanced N 2 . The mechanisms for sorbent deactivation inhibition by water vapor pretreatment, KOH addition and PEI modification were revealed by N 2 adsorption-desorption, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and Fourier transform infrared spectroscopy (FT-IR). Effects of water vapor pretreatment duration, KOH addition and PEI loading on sorbent deactivation inhibition performances were demonstrated. Furthermore, the long-term working stabilities of the pretreated and modified sorbents in impurities were evaluated. The results will provide fundamental support on practical application of K 2 CO 3 /AC for low-concentration CO 2 removal in ECLSS. [ABSTRACT FROM AUTHOR]

• Alkali metal salts promoted MgO pellets were prepared for CO 2 capture. • Sacrificial pore-forming templates were employed to improve CO 2 uptakes. • Template modification improved pore structure and facilitated CO 2 bulk diffusion. • Structure-performance relationships of AMS-MgO-P were unraveled. • AMS-MgO-P-MC exhibited high CO 2 uptake and desirable mechanical property. Alkali metal salts (AMS) promoted MgO with high CO 2 uptakes represents a promising adsorbent candidate for CO 2 capture. The AMS-MgO composite adsorbents are facing the challenges of increased pressure drop or elutriation when employed in dual fixed-bed reactor or twin circulating fluidized-bed configurations. Granulation by the extrusion-spheronization technique represents an effective problem-solving strategy. However, textural properties and CO 2 uptakes of MgO-based adsorbent pellets might be adversely affected by the granulation process. In this work, AMS-MgO composite adsorbent pellets were prepared by the extrusion-spheronization method. Citric acid (CA), ammonium bicarbonate (AB), urea (UA) and microcrystalline cellulose (MC) were employed as pore-forming templates to improve the porous structures and CO 2 uptakes of the AMS-MgO pellets. CO 2 uptakes and adsorption kinetics of the adsorbent pellets were studied using a fixed-bed reactor. Effects of granulation and pore templating on the structure-performance relationships of the AMS-MgO pellets were unraveled. Granulation had resulted in the remarkable decrease in CO 2 uptake from 10.87 to 4.24 mmol CO 2 /g due to the impaired textural parameters. The employed sacrificial templates endowed the AMS-MgO pellets with enhanced CO 2 uptakes and improved carbonation kinetics. This was associated with extended surface area and expanded pore structures due to gas liberation in the pyrolysis of templates, which had facilitated the kinetics of CO 2 bulk diffusion. The desired AMS-MgO-MC pellet modified with MC template showed a high CO 2 uptake of 7.54 mmol CO 2 /g in 50%CO 2 at 340 °C, and its CO 2 uptake was stabilized around 5.0 mmol CO 2 /g within 20 cycles. Besides, the AMS-MgO-MC pellet also exhibited good mechanical properties. The results will guide the rational design of highly efficient and robust MgO-based adsorbent pellets for CO 2 capture applications. [ABSTRACT FROM AUTHOR]

• MgO adsorbents were prepared from magnesium oxalate precursor for CO 2 capture. • Effects of calcination conditions on structure-performance relationships were studied. • Calcination atmosphere and temperature significantly affected the CO 2 adsorption performance. • The desired MgO-C-500 adsorbent exhibited a high CO 2 uptake of 5.09 mmol CO 2 /g. Facile, green and solvent-free fabrication of efficient MgO-based adsorbents is highly necessary for their intermediate-temperature CO 2 capture applications. In this work, MgO adsorbents were prepared by calcining magnesium oxalate precursor under different calcination atmospheres and temperatures. N 2 adsorption-desorption, X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscopy and CO 2 temperature programmed desorption were employed to study their physicochemical properties. CO 2 adsorption performance was evaluated by testing the adsorbents in a dry stream containing 10%CO 2 at a mild temperature of 200 °C. The influence of calcination conditions on their structure-performance relationships was investigated. Calcination atmosphere and temperature will significantly affect the microstructure and CO 2 adsorption performance. Calcination in a flowing CO 2 stream will endow the adsorbent with minimized MgO crystal size, and this will offer more O2− active sites at the edges and corners for enhanced CO 2 adsorption capacity. The precursor can hardly be completely decomposed at a lower calcination temperature to form porous structure, and the basic active sites available for CO 2 adsorption are limited. A higher calcination temperature will result in sintering of MgO crystals, pore structure blockage and loss of the O2− basic active sites, and CO 2 adsorption capacity will therefore be reduced. The desired MgO-C-500 adsorbent calcined in a flowing CO 2 stream at 500 °C exhibits the highest CO 2 adsorption capacity of 5.09 mmol CO 2 /g. The results will pave the way for developing highly efficient MgO adsorbents for separating CO 2 from industrial exhaust gas. [ABSTRACT FROM AUTHOR]

• Three reactivation modes were adopted to recover CO 2 uptake of spent sorbent. • Hydration is effective to recover the initially high CO 2 uptake for spent sorbent. • Simultaneous hydration/impregnation contributes to improve CO 2 capture stability. • Filtration introduced in acidification can remove CaSO 4 within spent sorbent. Rapid loss-in-capacity of naturally occurring limestone for CO 2 capture is the most prominent issue in the calcium looping process. The synthesis of highly efficient sintering-resisting sorbents is a common solution to mitigate capacity decay. However, the long-term CO 2 looping cycles, particularly in the presence of SO 2 , still cause the generation of large amounts of spent CaO-based sorbents. Comparatively, the reactivation of spent sorbent is more economical than the replenishing of fresh sorbent. In this work, three types of reactivation modes (i.e., hydration, simultaneous hydration/ impregnation and acidification) were applied to recover the CO 2 capture capacities of the spent, synthetic CaO-based sorbents (CaO/MgO = 75 wt%/25 wt%). It is found that hydration is effective to recover the cyclic CO 2 capture performance of the spent sorbent due to the formation of more cracks and channels that extend to the interior of sorbent particles. Therefore, the sorbent reactivated via sole hydration exhibits a high capacity of 0.390 g/g after 40 cycles. Moreover, NaCl impregnation combined with hydration produce lager amounts of macropores within the reactivated sorbents after initial sever sintering that are responsible for their desirable CO 2 capture stability. The introduction of filtration during acidification reactivation process can effectively enhance the initial CO 2 capture capacity (an improvement percentage of ~14.7%) owing to the removal of the irreversible CaSO 4. [ABSTRACT FROM AUTHOR]

• Sugarcane bagasse-derived activated carbons were prepared for CO 2 capture. • Effect of activating agent on structure-performance relationships was studied. • CAC-S exhibited a high CO 2 uptake of 4.28 mmol CO 2 /g at 25 °C and 1 bar. • NaOH activation improved textural properties and surface basicity. Business cost concern of solid CO 2 adsorbents has motivated the production of porous activated carbons from waste biomass materials. In this work, porous activated carbons were prepared from sugarcane bagasse using different activating agents (air, CO 2 , H 3 PO 4 and NaOH). Chemically activated porous carbons exhibited better physicochemical properties as compared to the physically activated carbons. Particularly, NaOH-activated carbon (CAC-S) featured a high specific surface area (1149 m2/g) and a large pore volume (1.73 cm3/g). X-ray photoelectron spectroscopy (XPS) analysis confirmed that the CAC-S adsorbent exhibited enhanced total basicity. The desired CAC-S adsorbent showed superior isothermal CO 2 uptakes of 5.50 and 4.28 mmol CO 2 /g at 0 °C and 25 °C and 1 bar. The isosteric heat of adsorption was predicted as 43.96 kJ/mol from the Clausius-Clapeyron equation and Toth model, which indicated intensified interaction between CO 2 molecules and the basic carbon surface. The excellent CO 2 adsorption performance should be associated with the improved textural properties and enriched surface basicity benefited from NaOH activation. Overall, CAC-S is considered as a promising adsorbent for CO 2 adsorption. The recycling of sugarcane bagasse as solid CO 2 adsorbents presents environmental implications with respect to waste management and pollutants mitigation. [ABSTRACT FROM AUTHOR]