CaO-based CO2 Research Articles

Exploring the role of surface and porosity in CO2 capture by CaO-based adsorbents through response surface methodology (RSM) and artificial neural networks (ANN)

In this study, synthetic CaCO3 materials were utilized as precursors for CaO-based CO2 sorbents. The investigation examined how various operating parameters—such as synthesis temperature (ST), stirring rate (SR), and surfactant percentage (SP)—impact the properties of the adsorbents. Samples were firstly characterized by X-ray diffraction and Scanning Electron Microscopy (SEM), which revealed that the prevalence of calcite or aragonite crystal phases in the synthetic CaCO3 precursors can be tuned by adequately choosing the dose of surfactant (Triton-X100®), so that it can be used as a crystal habit and growth modifier. The calcination process applied to the CaCO3 precursors leads to the formation of partially sinterized cubic crystals of CaO, accompanied by minor quantities (< 5 %) of additional compounds like Ca(OH)2 or CaSO4. Specific surface area (SBET) and porosity were determined by measuring the N2 adsorption isotherms. A CaCO3 sample with an unprecedented value of SBET as large as 116 m2/g was prepared operating under optimal conditions. SBET and pore volumes were successfully correlated with the CO2 uptake capacity of the samples. SBET is more influential for experiments carried out under diluted CO2 atmosphere. When pure CO2 is used, the influence of meso- and micropore volumes (Vme and Vmi) is clearly predominant, which suggests that in this latter case diffusion through the porous texture of the samples plays a more remarkable role. A double-way approach through Response Surface Methodology (RSM) and the use of Artificial Neural Networks (ANNs) has been used to analyze the CO2 uptake capacity of the samples. Within the operational interval, excellent results were obtained for pure and diluted CO2 flow, and RSM and ANNs have demonstrated to be a very efficient tool to correlate the behavior of the CaO-based materials as CO2 sorbents with the surface area and pore volumes of the samples. Valuable information on (i) the importance of the different factors under study; (ii) their influence on the surface and porosity of the CaO-derived sorbents; and (iii) the subsequent CO2 capture performance of the sorbents has been obtained. The results suggest that four parameters have a statistically significant influence on CO2 uptake. These parameters are SR, the square of SR, its interaction with SP, and the square of SP. Additionally, the study assessed the stability of the CaO-based sorbents over 11 consecutive calcination-carbonation cycles. By adequately choosing the synthesis strategy and conditions, an almost negligible shrinkage effect can be achieved, resulting in a more sustained uptake capacity throughout the cycles.

Of Glasses and Crystals: Mitigating the Deactivation of CaO-Based CO2 Sorbents through Calcium Aluminosilicates.

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.

CaO-based high temperature CO2 sorbents – Literature review

CaO-based high temperature CO2 sorbents – Literature review

Research on the preparation of CO2 renewable sorbent from calcium-based waste: Towards enhanced biomass gasification for H2 production

The potential of industrial waste—carbide slag (CS) as a high-temperature CaO-based CO2 sorbent and its anti-sintering modification strategies are explored by examining the changes in maximum carbonation conversion (Xn,max) during multiple carbonation/calcination cycles. Although the pristine CS performs satisfactorily during initial carbonation, its Xn,max steadily declines with increasing cycle numbers due to sintering. To develop CO2 sorbent with robust, stable structures, various refractory dopants, including Al2O3, MgO, SiO2, Y2O3, and two industrial by-products rich in Al2O3 and SiO2—coal gangue (CG) and fly ash (FA)—are utilised to modify the CS. The SiO2-doped CS sorbent exhibits superior CO2 cycling adsorption capacity under identical testing conditions, attributed to the formation of Ca2SiO4 with a high Tammann temperature (TT) during high-temperature calcination. Notably, CSCG5 (CS/CG mass ratio of 95:5) presents a CO2 capture capacity comparable to CSSi2 (CS/SiO2 mass ratio of 98:2), providing a potential solution for low-cost and anti-sintering modification of CS sorbents. Relative to pure CaO or CS, the CSSi2 and CSCG5 samples exhibit superior stability in terms of H2 concentration and yield throughout the biomass cyclic hydrogen production tests. Adsorption kinetic analysis results suggest that the carbonation of CS sorbent during rapid reaction predominantly adheres to the Pseudo-first-order, Elovich, and Intra-particle diffusion models. The carbonation conversion (Xt) is excellently fitted (R2 > 0.99) by a Logistic equation considering CO2 concentration (xCO2) and carbonation temperature (Tcar). Moreover, comprehensive data analysis yields an innovative semi-empirical equation suitable for predicting the Xn,max during long-cycle carbonation/calcination processes. We also discussed the sintering and modification mechanisms of CS sorbent in depth.

Strengthening performance of Al-stabilized, CaO-based CO2 sorbent pellets by the combination of impregnated layer solution combustion and graphite-moulding

Calcium looping (CaL) process is recognized as one of the most promising CO2 capture technologies. However, the CaO-based sorbents commonly encounter serious sintering and elutriation problem during CO2 capture cycles. In this work, Al-stabilized, CaO-based sorbents were synthesized by the impregnated layer solution combustion method, and cigarette butt was selected as the porous reactive skeleton. The Al-stabilized, CaO-based sorbent containing 15 wt% of Al2O3 exhibits the highly stable CO2 capture performance, whose CO2 capture capacity after 20 cycles is almost the same as its initial capacity, ∼0.354 g/g. It is mainly attributed to the synergistic effect of thermal pretreatment (cigarette butt-assisted combustion) and incorporation of Al-based stabilizer (i.e., Ca12Al14O33) provide a stable inert framework for the synthetic sorbents, ensuring their highly stable capability for CO2 sorption during cycles. Further, graphite-moulding is a facile and energy-saving approach to make Al-stabilized, CaO-based sorbent powders into pellets to enhance its resistance to elutriation. The produced sorbent pellets (85 wt% CaO/15 wt% Al2O3) exhibit excellent long-term CO2 capture capacity (∼0.248 g/g after 49 cycles) and high compression strength of 18.47 ± 3.47 MPa. Therefore, the combined method involved of impregnated layer solution combustion and graphite-moulding is promising to produce highly efficient CaO-based CO2 sorbent pellets used in CaL.

Fabrication of robust CaO-based sorbent via entire utilization of MSW incineration bottom ash for CO2 capture

CO2 capture using CaO-based sorbent is a promising and effective CO2 mitigation strategy due to its high theoretical capacity of CO2 adsorption and potential use in large scale. The preparation of waste-derived CaO-based CO2 sorbents has received extensive interests due to its economic feasibility via waste recycling. In this work, the development of a novel robust CaO-based sorbent via the entire utilization of municipal solid waste incineration (MSWI) bottom ash (BA) was reported. Specifically, residues from the extraction of CaO from BA were added back to the extracted CaO after thermal passivation. The results showed that the addition of BA residue-derived stabilizer (BAS) promoted the homogeneous dispersion of CaO and increased the specific surface area (17.66 m2/g) of the BAS/CaO sorbents and pores in the range of 20–100 nm. Meanwhile, the small amounts of Al2O3 and SiO2 in the Ca-Si-Al-O species of BAS reacted with CaO to form Ca12Al14O33 and Ca2SiO4 during cyclic carbonation/calcination, which was beneficial to alleviate the aggregation and sintering between CaO particles and maintain porous structure of sorbents. In addition, the CO2 uptake exhibited a volcano-type variation with the increase of BAS additive amount and carbonation temperature, where the optimum carbonation temperature for BAS(20 wt%)/CaO sorbents was 700 °C. Among the various sorbents, the BAS(20 wt%)/CaO sorbent (BAS/CaO = 2/8) exhibited the most superior CO2 uptake of 0.14 gCO2/gsorbent after 20 cycles. This study provides a promising method to achieve a cost-effective CO2 capture candidate with enhanced capacity by completely reusing solid wastes, which eliminates the disposal of secondary residues that was rarely considered in previous studies.

One-pot preparation of H2-mixed CH4 fuel and CaO-based CO2 sorbent by the hydrogenation of waste clamshell/eggshell at room temperature.

The preparation of clean fuel or CO2 adsorbents using industrial and domestic garbage is an alternative way of meeting global energy needs and alleviating environmental problems. Herein, H2-mixed CH4 fuel and CaO-based CO2 sorbent were first prepared in one pot by the mechanochemical reaction of pretreated clamshell or eggshell wastes (carbon and calcium source) with calcium hydride (hydrogen source) at room temperature. In the above reactions, CH4 was the sole hydrocarbon product, and its yield reached 78.23%. The H2/CH4 ratio of the produced H2-mixed CH4 fuel was tunable according to the need by changing the reaction conditions. It is inspiring that the simultaneously formed solid CaO/carbon products were efficient CaO-based sorbents, which possessed a higher CO2 adsorption capacity (49.81-58.74wt.%) at 650°C and could maintain good adsorption stability in 30 carbonation/calcination cycles (average activity loss per cycle of only 1.6%). The three achievements of the idea are that it can simultaneously eliminate clamshell or eggshell wastes, obtain valuable clean fuel, and acquire efficient CaO-based sorbents.

Structure and surface insight into a temperature-sensitive CaO-based CO2 sorbent

Calcium looping process is a prospective strategy to remove CO2 from flue gas. However, the CO2 capture capacity of the CaO-based sorbents would be spoiled severe over the repeated cycles. In this work, a kind of NaCl-modified CaO-based sorbents were prepared by a simultaneous hydration and impregnation process and explored for cyclic CO2 capture by a simultaneous thermal analyzer. The promotion mechanism was discussed by electron microscope, X-ray diffraction, N2 physisorption, X-ray photoelectron spectroscopy, and so on. Results showed that NaCl tailored the temperature sensitivity of CaO-based sorbents, and it was a good promoter to enhance the carbonation activity of CaO-based sorbents, especially under higher temperature modes. At higher temperatures, NaCl would cover the surface of CaO, and promote the CO2 transportability in the modified sorbent, and then greatly improve the carbonation ability of the sorbents over the cycles. After 50 repeated cycles, the CO2 capture capacity of “10 NaCl/CaO” still retained at 0.288 g/g and 0.237 g/g when carbonated at 650 °C and 750 °C, which were 2.05 and 2.63 times that of “CaCO3 AR” respectively. Hence, the temperature-sensitive CaO-based sorbent promoted by NaCl shows a promising prospect for CO2 capture from flue gas.

The effect of BeO on heat transfer and durability of nano-CaO-based CO2 adsorbents.

The solution of decreasing the decomposition temperature of CaCO3 and the development of the durability of the CaO-based CO2 adsorbent are the key issues in reducing the energy consumption and cost of CO2 capture in calcium looping technology. In this work, BeO with high thermal conductivity was chosen as a dopant of the adsorbent to increase the thermal conductivity properties and decomposition properties of CaCO3. The endothermic rate of the nano-CaO-BeO/Al2O3 adsorbent with 15.6 wt% BeO dopant increased by 12.3% compared with that of the nano-CaO/Al2O3 adsorbent at 720 °C, leading to an increase of 10.1% of CaCO3 decomposition rate. The enhancement of the decomposition rate of the nano-CaO-BeO/Al2O3 adsorbent was significant to lower the regeneration temperature by 50 °C compared with that of the nano-CaO/Al2O3 adsorbent under calcium looping conditions, which made the total average deactivation rate decrease by 21.0% and made the total residual stable carbonation conversion increase by 27.0% in infinite calcium looping cycles. Strengthening the heat transfer inside the adsorbent material can effectively decrease the regeneration temperature, so as to improve the sorption durability.

Multicycle Performance of CaTiO3 Decorated CaO-Based CO2 Adsorbent Prepared by a Versatile Aerosol Assisted Self-Assembly Method.

Calcium oxide (CaO) is a promising adsorbent to separate CO2 from flue gas. However, with cycling of carbonation/decarbonation at high temperature, the serious sintering problem causes its capture capacity to decrease dramatically. A CaTiO3-decorated CaO-based CO2 adsorbent was prepared by a continuous and simple aerosol-assisted self-assembly process in this work. Results indicated that CaTiO3 and CaO formed in the adsorbent, whereas CaO gradually showed a good crystalline structure with increased calcium loading. Owing to the high thermal stability of CaTiO3, it played a role in suppressing the sintering effect and maintaining repeated high-temperature carbonation and decarbonation processes. When the calcium and titanium ratio was 3, the CO2 capture capacity was as large as 7 mmol/g with fast kinetics. After 20 cycles under mild regeneration conditions (700 °C, N2), the performance of CO2 capture of CaTiO3-decorated CaO-based adsorbent nearly unchanged. Even after 10 cycles under severe regeneration conditions (920 °C, CO2), the performance of CO2 capture still remained nearly 70% compared to the first cycle. The addition of CaTiO3 induced good and firm CaO dispersion on its surface. Excellent kinetics and stability were evident.

Single-step production of hydrogen-rich syngas from toluene using multifunctional Ni-dolomite catalysts

Tar compounds such as toluene can be oxidatively converted by reforming, followed by water–gas-shift and CO2 removal to produce H2-rich syngas. We report a type of low-cost multifunctional catalysts that are capable of producing hydrogen-rich syngas from toluene in a single reactor. The multifunctional catalysts, derived from Ni-loaded dolomite, also act as oxygen carriers and CO2 sorbents. When operating under a chemical looping-type scheme at 700 °C, the catalyst containing 15 wt% NiO could produce syngas containing >70% hydrogen, with a cold-gas-efficiency of 90.3%. No sign of deactivation, coking or structural change was observed over 10 consecutive cycles of reforming and regeneration. The remarkable performance is attributed to the promotional effects arising from the formation of the MgxNi1-xO solid solution. Additionally, MgO acts as a support and provides sintering resistance to both the Ni catalyst and the CaO-based CO2 sorbent, making the multifunctional structure highly regenerable over cyclic operation.

Mass transfer enhanced CaO pellets for CO2 sorption: Utilization of CO2 emitted from CaCO3 pellets during calcination

• Mass transfer enhanced CaO pellets were prepared and applied to capture CO 2 . • CO 2 evacuation from CaCO 3 pellets created mass transfer channels. • Developed channels significantly enhanced CO 2 sorption kinetics. • Enhanced mass transfer caused fast regeneration of CaO pellets after CO 2 sorption. In the preparation of CaO-based CO 2 sorbents, particle densification during pelletization significantly limits the mass transfer of CO 2 , thereby decreasing the CO 2 sorption performance. In this study, mass transfer enhanced CaO pellets (CaO–PC) were prepared through the formation of channels using CO 2 evacuation from the inside of the pellets. Calcination of CaCO 3 pellets induced CO 2 evacuation and the remaining evacuation pathways provided excellent mass transfer channels for CaO–PC. Conventional CaO pellets (CaO–CP) were also prepared for comparison. Unlike the severely agglomerated (or blocked) morphology of CaO–CP, well-developed channels were observed in CaO–PC. It was experimentally confirmed that these channels directly contributed to the initial stage of CO 2 sorption in CaO–PC, which significantly accelerated the CO 2 sorption kinetics. CaO–PC had increased CO 2 sorption uptakes of 58.9, 65.1, and 67.0 wt% at 500, 600, and 700 °C, respectively, whereas those for CaO–CP were 47.6, 55.4, and 56.8 wt%. In addition to CO 2 sorption, enhanced mass transfer had a positive effect on CO 2 release after capture. Under CO 2 flow, the regeneration of CaO–PC was faster than that of CaO–CP, even at lower temperatures. Both the fast CO 2 sorption and regeneration kinetics of CaO–PC significantly enhance the energy efficiency of continuous CO 2 capture processes. These improvements were accomplished easily without the need of any additional energy-consuming treatments other than the conventional preparation methods.

One-step and sustainable preparations of inert additive-doped CaO-based CO2 adsorbents by hydrogenation reduction of CaCO3

• ▶ CaO-based adsorbent is sustainably synthesized by hydrogenation reduction of CaCO 3 . • ▶ The prepared CaO@C and CaO@MgO@C composites are high performance CO 2 adsorbent. • ▶ They exhibit higher CO 2 uptake capacity than that of the ball-milled CaO at 650 °C. • ▶ They also show faster adsorption rate and better cycling stability. • ▶ The work provides a novel and sustainable method to prepare CaO-based CO 2 adsorbent. The development of inert additive-doped CaO-based CO 2 adsorbents is a promising method to obtain efficient CO 2 adsorbents, while multiple programs are frequently involved, the organic solvent is often used, and CO 2 is sometimes released in the process of preparation. In this study, we first report that inert additive-doped CaO-based adsorbents can be synthesized in one pot by the mechanochemical hydrogenation reduction of CaCO 3 producing methane and avoiding CO 2 emission. The prepared CaO@C and CaO@MgO@C composites are high-performance CO 2 adsorbents with high CO 2 adsorption capacity and superior cycling stability. The results show that the CO 2 capture capacities of the CaO@C and CaO@MgO@C composites are 19.14 and 14.53 wt% higher than that of the BM-CaO (37.07 wt%), respectively, at 650 °C carbonation for 2 h. Notably, the activity losses of the CaO@C and CaO@MgO@C composites after 30 cycles are about 47.93 and 29.33%, which are much lower than that of the BM-CaO (60.39%). The performance of the CaO@C and CaO@MgO@C adsorbents is comparable to that of adsorbents synthesized by the conventional methods, but this work provides novel, simple, solution-free, and sustainable strategies to synthesize effective CaO-based CO 2 adsorbents in one pot.

Template-Free Synthesis of Hollow CaO/Ca2SiO4 Nanoparticle as a Cyclically Stable High-Capacity CO2 Sorbent

Cyclically stable and highly effective CaO-based CO2 sorbents are crucial to calcium looping, which is a CO2-capture technique that offers an effective and feasible way to achieve the goal of carbo...

Structure and Surface Insight into a Temperature-Sensitive Cao-Based Co2 Sorbent

Structure and Surface Insight into a Temperature-Sensitive Cao-Based Co2 Sorbent

Novel procedure of CO2 capture of the CaO sorbent activator on the reaction of one-part alkali-activated slag.

CaO derived naturally from limestone or dolomite is an inexpensive and widely available sorbent. Understanding the mechanisms of CaO carbonation at ambient temperature under the assistance of H2O is important for predicting the reaction of CaO in complex environments and designing novel CaO materials. In this study, we found that the reaction rate of alkali-activated slag is controlled by the CaCO3 layer on a partially carbonized CaO alkali activator. The size of the sorbent increased after the adsorption reaction and the physically adsorbed water in the pores accelerated the carbonation. The carbonation process was governed by CO2 diffusion. When the carbonation conversion rate reached 2–6%, the setting time increased rapidly with the increase in the carbonation rate. This is because the surface of the activator was modified by the thickened CaCO3 product layer, which increased the diffusional resistance and thus prolonged the setting time of the alkali-activated slag.

Effect of different organic compounds on the preparation of CaO-based CO2 sorbents derived from wet mixing combustion synthesis

CaO based sorbents have great potential for commercial use to capture CO 2 of power plants. In the demand of producing sorbents with better cyclic performance, CaO-based sorbents derived from different kinds of calcium precursors, containing calcium carbonate (CC-CaO), calcium gluconate monohydrate (CG-CaO), calcium citrate (CCi-CaO) and calcium acetate monohydrate (CA-CaO), were tested cyclically and compared using simultaneous thermal analyzer (STA). And further study was conducted on the sorbents modified with citric acid monohydrate and 50% gluconic acid solution by wet mixing combustion synthesis. The modified sorbents showed better performance and higher pore parameters as well as porous microstructure with more organic acid added. After 20 cycles of carbonation and calcination, the C2CCi8 (CaO: citric acid = 2:8 by mass ratio) and C2G8 (CaO: gluconic acid = 2:8 by mass ratio) sorbent possess CO 2 capture capacity of 0.45 g·g −1 (g CO 2 per g sorbents) and 0.52 g·g −1 respectively. The citric acid was more effective for modification than gluconic acid for extended 50 cycles. Furthermore, good linear relationship between CaO conversion and specific surface area as well as pore volume were determined, of which the specific surface area showed closer correlation with CaO conversion.

Facile reactivation of used CaO-based CO2 sorbent via physical treatment: Critical relationship between particle size and CO2 sorption performance

Herein we report a new and facile mechanical approach for maximizing the CO2 sorption performance of CaO sorbents, and also for recovering the sorption performance of the used (or deactivated) sorbent. We compared the physicochemical properties of CaO-based sorbents, which could change depending on their preparation methods, and determined that the CaO particle size was the most critical factor that affected their CO2 sorption performance. The CO2 sorption performance of CaO-based sorbents could be maximized via the simple physical reduction of their particle size, and this was experimentally demonstrated using ball-milling. The CO2 sorption uptake of CaO prepared using the conventional solid-state method was significantly increased to the highest ever reported level after ball-milling, and it reached 98.0% of the theoretical maximum CO2 sorption capacity. The most important application of the particle-size-dependency of CaO-based sorbents on their CO2 sorption uptake is the reactivation of used sorbents via reducing the size of the aggregated or sintered bulky particles that form after cyclic usage. The CO2 sorption uptake of the used CaO (~32.9 wt% after 10 alternative sorption–regeneration cycles) was successfully recovered (almost doubled) to ~64.7 wt% after ball-milling. This is the first time a facile mechanical-grinding-based reactivation method, which appeared to be highly efficient and directly applicable to the continuous calcium looping technology, has been developed and used.

Determination of Kinetic and Diffusion Parameters Needed to Predict the Behavior of CaO-Based CO2 Sorbent and Sorbent-Catalyst Materials

For about 10 years, this research group has developed and utilized a particle grain model (PGM), to simulate CO2-capture carried out by CaO-based porous particles. Chemical kinetics and diffusion parameters were either taken from literature studies or fixed by fitting experimental sorption data. As recently observed, this procedure was not fully satisfactory and revealed systematic, minor discrepancies between PGM numerical results and experimental data when predicting sorbents behavior during the initial chemically controlled regime of carbonation. This work deals with the experimental determination of kinetic and diffusion parameters, utilized in the PGM, by means of straightforward thermogravimetric analysis (TGA) tests on small samples of materials to be evaluated for CO2 sorption and sorption-enhanced processes. To validate this procedure, the carbonation of two Ni–CaO–mayenite combined sorbent-catalyst materials (CSCMs) was studied in TGA. The experimental data so obtained were used to infer carbona...

Experimental and Kinetic Study on CaO-based CO2 Sorbent Pellets with Different Binders

In this work, sorbent pellets were prepared based on six synthetic powdery sorbents. CO2 uptake tests and kinetic analysis were carried out to screen sorbent pellets. Experimental results showed th...