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In this paper, the enhancement of thermochemical energy storage by alkali metal chloride salts-doped Ca-based sorbents is revealed by experiments and DFT calculations. The results indicate that NaCl and KCl doping increases the reaction rate and cycle stability. Compared to CaO, the conversion of NaCl-CaO and KCl-CaO after one cycle is increased by 59.1% and 61.9%, respectively. This enhancement originates from the oxygen vacancies generated by Na2O and K2O and the significantly increased surface area by CaCl2 as well as the sintering delay. The synergistic effect between Na2O, K2O, and CaCl2 increases the reaction rate of calcium-based materials. Meanwhile, the penetration of low-viscosity molten NaCl and KCl into the calcium-based materials successfully segregates the CaO grains and allows the calcium-based material to maintain the porous structure after 80 cycles, thus exhibiting a high effective conversion rate. In addition, the KCl-CaO composites show the best combined performance in terms of effective conversion and averaged thermal energy density. This work paves the way for the application of chloride salts-doped calcium-based materials. [ABSTRACT FROM AUTHOR]

The impregnated layer solution combustion synthesis (SCS) technique was employed to synthesize CaO-based sorbents supported by inert stabilizers so as to improve the cyclic CO2 uptake stability. Four different inert stabilizers were evaluated for their impact on the cyclic CO2 uptake stability of sorbents. It found that the Mg-based inert stabilizer significantly outperforms the other three in improving cyclic CO2 sorption capabilities of sorbents synthesized by the impregnated layer SCS method. Moreover, MgO-supported sorbent pellets were prepared via graphite hydrophobic layer-assisted casting, with the magnesia load within the pellets being regulated between 15 and 45 wt %. It is the addition of MgO stabilizer that significantly improves the performance of CaO-based sorbents, with the improvements being notably more pronounced at higher MgO loads. The calcium-based sorbent pellets containing 45 wt % MgO show extremely stable CO2 sorption performance; after 17 cycles, the carbonation conversion rate is 73.6%, about three times the rate of pellets without MgO loading. The uniform dispersion of MgO inert stabilizer inside pellets improves high-temperature sintering resistance and, concurrently, enhances the mechanical property. Furthermore, the use of waste cigarette butts as impregnated layer templates, rather than other organic additives, will significantly reduce the cost of synthesizing MgO-supported CaO-based sorbents. [ABSTRACT FROM AUTHOR]

Abstract The impregnated layer solution combustion synthesis (SCS) technique was employed to synthesize CaO-based sorbents supported by inert stabilizers so as to improve the cyclic CO2 uptake stability. Four different inert stabilizers were evaluated for their impact on the cyclic CO2 uptake stability of sorbents. It found that the Mg-based inert stabilizer significantly outperforms the other three in improving cyclic CO2 sorption capabilities of sorbents synthesized by the impregnated layer SCS method. Moreover, MgO-supported sorbent pellets were prepared via graphite hydrophobic layer-assisted casting, with the magnesia load within the pellets being regulated between 15 and 45 wt %. It is the addition of MgO stabilizer that significantly improves the performance of CaO-based sorbents, with the improvements being notably more pronounced at higher MgO loads. The calcium-based sorbent pellets containing 45 wt % MgO show extremely stable CO2 sorption performance; after 17 cycles, the carbonation conversion rate is 73.6%, about three times the rate of pellets without MgO loading. The uniform dispersion of MgO inert stabilizer inside pellets improves high-temperature sintering resistance and, concurrently, enhances the mechanical property. Furthermore, the use of waste cigarette butts as impregnated layer templates, rather than other organic additives, will significantly reduce the cost of synthesizing MgO-supported CaO-based sorbents. Graphical Abstract

Concentrated solar power (CSP) integrated with calcium looping (CaL) technology has garnered significant interest as a solution to mitigate the issue of intermittency in solar power production. However, the deactivation of CaO‐based materials during cycling limits the application of CaL technology on a large scale. Herein, the dynamic characteristics of the CSP‐CaL system indirectly integrated with the s–CO2 Brayton cycle using CaO–ZnO–Na2SO4 composites are studied. Energy analysis shows that the energy storage density varies from 1328.13 to 1232.03 J g−1, and its average value increases by 32.0%. Round‐trip efficiency of the system varies from 40.0% to 39.5%, and its average value increases by 22.6%. Exergy analysis shows that the co‐doping of ZnO and Na2SO4 into CaO enhances the exergy efficiency of the calciner and the heat exchange network, but reduces the exergy efficiency of the carbonator. Exergy round‐trip efficiency of the CSP‐CaL system has slightly decreased from 44.0% to 43.4%, with an average increase of 22.8%. Techno‐economic analysis shows that the levelized cost of electricity for different sizes of CSP‐CaL systems using CaO–ZnO–Na2SO4 ranges from 176.44 to 153.70 $ MWh−1. Therefore, the CaO–ZnO–Na2SO4 composite shows promising prospects for application in solar thermal power generation. [ABSTRACT FROM AUTHOR]

The University of Newcastle (UON) and Jord International Pty Ltd. (Jord) have jointly developed a novel concept for the storage of energy from renewable and fossil fuel sources. The process, referred to as the pressurized regenerative calcium cycle (PRC2), relies on cyclic carbonation and calcination of CaO/CaCO3, in which low-cost electrical energy (i.e., off-peak, or excess generation from renewables) drives the calcination reaction and electricity is generated as required through the carbonation reaction. Initial proof-of-concept testing of the process was previously conducted within an existing fluid bed reactor at UON. The PRC2 concept was successfully demonstrated by maintaining the fluid bed reactor at a constant temperature by using the heat released during the reaction of calcium oxide and carbon dioxide. Following proof-of-concept testing, further refinement of the PRC2 process, which is the subject of this paper, was conducted to address its shortcomings and, importantly, facilitate the detailed design, construction, and operation of a large-scale demonstration plant. Nine different configurations were examined for the PRC2 process, for each of which a combined experimental, process modelling, and techno-economic assessment was completed. Experimental investigations were conducted to determine the suitability of carbonate materials for the PRC2 process. Process modelling and levelized cost of storage (LCOS) calculations were concurrently conducted and revealed that the molten salt approach (Option 9) was the most promising, having superior round-trip efficiency and lowest LCOS. For practical reasons (e.g., technical difficulties of working with molten salts), Option 3 (indirect power generation using a fluid bed reactor) was deemed the most feasible option for a demonstration scale plant. The LCOS for Option 3 (assuming a 100 MWe capacity) was calculated to be AUD 245 per MWh, which is on par with the cost of batteries for peak power replacement applications (the cost associated with lithium-ion batteries is AUD 370 per MWh). [ABSTRACT FROM AUTHOR]

The calcium looping integrated with the chemical looping combustion (CaL-CLC) process is an efficient and cost-effective CO2 capture technology that avoids the energy-intensive air separation unit in the calcium looping (CaL) process. However, in these CaL-CLC and CaL system integration schemes, the carbonation heat is utilized for steam generation, resulting a significant temperature difference and considerable irreversible loss. To prevent temperature mismatch, this paper proposes a novel Ca-Cu looping post-combustion CO2 capture method with thermochemical recuperation based on steam methane reforming. Additionally, a novel system integrated with turbine exhaust heat recovery is introduced to effectively reduce carbon emissions from flue gas. Results show that the proposed system has superior performance compared to the reference system based on the Ca-Cu looping method. The specific primary energy consumption for CO2 avoidance decreased from 2.40 MJLHV/kg CO2 in the reference system to 2.02 MJLHV/kg CO2. Exergy analysis indicates that a total of 3.0 % reduction in exergy destruction can be achieved in chemical reaction processes and heat recovery processes, contributing to the superior performance of the proposed system. Furthermore, the effects of key operating parameters indicate that cascaded turbine exhaust recovery is essential for improving the thermodynamic efficiency of the proposed system. Overall, recovering the mid-temperature carbonation heat via thermochemical regeneration and integrating with exhaust heat recovery contribute to reducing SPECCA, thus providing a promising low-energy-consumption alternative for CO2 capture.

CaO-based materials have attracted considerable interest because of their potential roles in thermochemical CO2 capture via the calcium looping (CaL) process. The steel manufacturing sector inevitably generates substantial amounts of slag as a by-product, posing environmental issues such as soil contamination if left untreated. Despite its abundance and low cost, steel slag, which contains 20–60 % CaO, has not been extensively researched for its potential in the CaL process. This study introduces ladle furnace slag (LFS) as an optimal CaO-rich material for developing mesoporous composites to improve CO2 sequestration in the CaL process. Using an autoclave reactor/muffle furnace setup, we conducted a parametric investigation on operational variables including reaction time, temperature, pressure, and liquid-solid ratio and determined the kinetics of carbonation/calcination reaction of CaL process. Our findings reveal that the CO2 capture performance of modified LFS surpassed that of the bare LFS, achieving a CO2 uptake of 7.55 ± 0.01 g per g of sorbent over 20 cycles. Additionally, the modified LFS exhibited the capability to undergo a minimum of 20 regeneration cycles, reaching steady state after the 15th cycle with minimal variation of 0.01 g per g of sorbent. The enhanced stability was linked mainly due to the presence of ceramics such as Al2O3 and Fe2O3 in ratios of 1:5 and 1:6.5 respectively, with respect to CaO, achieved through acid etching. Such mineralogical transformation of the modified LFS improved its resistance towards sintering while ensuring 100 % recycling of metals in the LFS. Therefore, this study highlights the sustainable utilization of LFS as a valuable and efficient sorbent for CO2 capture, showcasing its potential for repurposing in environmental applications.

The cement industry is exceptionally energy-intensive and a major global carbon emitter, with CO2 primarily arising from the calcination of carbonate raw meal and the combustion of fossil fuels. This study proposes a novel process integrating calcium looping and dry reforming of methane (CaL-DRM) based on an “in-situ carbon capture and conversion” strategy to enhance the energy efficiency and decarbonization in the cement production process. Models for both conventional cement production process model and the CaL-DRM processes were developed using Aspen Plus to compare the mass flow and process energy balances of conventional cement production with the CaL-DRM process. The modelling results were validated by the cement plant operating data and published results. Sensitivity analyses were performed to optimize key production parameters, including CH4/O2 = 1.37 and CaCO3/CH4 = 0.5, which resulted in the highest conversion efficiencies of CO₂ and CH₄. Subsequently, the optimization of the tertiary air volume and the proportion of hot raw meal entering the carbonator was carried out. The optimal tertiary air volume was found to be less than 28529 Nm³/h, and 13% of the hot raw meal was directed to the carbonator. With these conditions, the process thermal efficiency can be increased from 58 % to 86 %. CO2 emissions were analyzed at key stages of cement production process, focusing on fuel combustion and carbonate decomposition at the calciner and rotary kiln, with a comparison of the conventional method and the CaL-DRM process to quantify emissions at each stage. The results indicate that 852.3 kg CO₂ per ton of cement clinker can be converted to produce 1680 kg of syngas per ton of cement clinker along with cement clinker. Additionally, up to 62.5 kg CO₂ per ton of cement clinker can be captured by the carbonator, reducing the CO₂ volume fraction in flue gas from 23.29 % to 0.24 %, thus eliminating the need for subsequent CO₂ purification and transport. These findings demonstrate the significant potential of this novel method for sustainable development in the cement industry.

Background and Objective: One of the major challenges in the calcium looping process is the loss of CO2 capture capacity of calcium-based sorbents during consecutive cycles due to increased sintering mechanism. This article aims to improve the performance of carbonation conversion in different cycles by employing hydration and acidification methods. Materials and Methods: Following the preparation of both CaO-based and modified sorbents, scanning electron microscopy, X-ray diffraction, and Brunauer-Emmett-Teller analyses were employed to verify the proper preparation of the sorbents and examine their structures. Subsequently, the adsorption performance of different samples was assessed using a thermogravimetric analyzer device . Results: The results of the carbonation reaction showed that the effective conversion rates for the raw and modified sorbents, employing acidification and hydration methods, at the end of the first cycle were 75%, 86%, and 73%, respectively. By the twentieth cycle, these rates decreased to 24%, 38%, and 26%, respectively. This decline indicates an improvement in the sorbent's capture capacity, attributed to the formation of calcium acetate, resulting in a more stable and porous structure. The findings from utilizing raw limestone sorbents and their modified versions through acidification and hydration techniques demonstrated a decrease in sorbent capture capacity by 69, 58, and 67% across twenty cycles. This highlights the enhanced sorbent stability achieved with the acidification method. Conclusion: Based on obtained results, acidification method can be considered as one of the most efficient approaches for improving the performance of calcium oxide sorbents. [ABSTRACT FROM AUTHOR]

Calcium looping carbon capture was simulated for a steel plant and three fuel cases were studied: coal, coke, and half coal/coke. The steelmaking production was about 1.2 t of liquid steel per hour at >99.85% purity by weight. This study evaluated the effect of carbon capture efficiency and raw material cost on the lime flow rate in the calcium looping section. Increasing the lime feeding flow rate achieved higher efficiency but at a higher cost. The overall captured efficiency was higher than the flue gas captured efficiency at a low lime feeding flow rate but lower at a high lime feeding flow rate. Then, the overall captured efficiency tended to decrease at high lime feeding flow rate because raw materials were used too much for carbon capture and caused overall carbon dioxide emission more than the content of captured carbon dioxide in the product stream. Coal as fuel plays an important role in carbon emission reduction as one of the raw materials, along with the utility cost and the raw material cost in calcium looping section. [ABSTRACT FROM AUTHOR]

Aluminium industry stands out as a significant source of CO2 emissions, due partially to the high energy demand of the alumina extraction stage. Accordingly, this study explores the implementation of direct resistive heating of mid-temperature processes in alumina production as an alternative to decrease CO2 emissions. Additionally, two different strategies are evaluated to decarbonize alumina industry: the generation of renewable electricity through solar photovoltaic panels and the integration of a CO2 capture plant based on calcium looping technology. This work comprehends the modelling and sizing of these plants and the assessment of their economic performance through the calculation of their Net Present Value and their respective payback periods. The integration of both strategies into an alumina refinery model reveals that electrification of low and mid-temperature processes yields a 15 % reduction in CO2 direct emissions, whereas calcium looping demonstrates the potential to capture 97 % of emissions with a 7 % energy penalty. Also, economic assessments indicate substantial potential for improvement through in-site electricity generation via solar photovoltaic panels, exhibiting a payback time of 4.5 years. Conversely, the feasibility of a calcium-looping plant is hindered by high capital expenses, necessitating a longer payback period of 19–24 years. Sensitivity analyses underscore the suitability of in-site renewable electricity generation, whereas carbon emission taxes emerge as crucial in incentivizing carbon-neutral processes, with thresholds around 95–125 €/tonne of CO2. Despite potential deviations from real industrial settings, this study provides evidence for environmentally friendly strategies in alumina production that demonstrate limited adverse effects on economic performance.

The presence of water vapour has been proven to stimulate carbonation for the calcium looping process, while its effect mechanism during the slow reaction stage still lacks insight understanding at the atomic level, where H2O molecules may interact with the product layer to affect the transportation of CO2. This study tried to reveal the role of water vapour on CO2 mobility on the calcite surface through density functional theoretical (DFT) calculation. H2O molecule has higher adherence to calcite surface than CO2, and it can react with CO3 to generate HCO3− and OH− ions. Then, the formed OH− ion above the surface layer can adsorb CO2. Moreover, H2O can also react with O2− defect to form OH− ions, and the OH ion at the deeper position can still adsorb CO2 through a two-step process with remarkable energy barriers. However, the formation of HCO3− can degrade the energy barriers to CO2 release from the calcite surface, owning to the weaker CO bond. Two directions of CO2 movement between anions were involved in this investigation, including crossing through the surface layer to the second layer and the movement inside the surface layer, where the higher mobility of CO2 from HCO3− to O2− ion occurs in both movement directions. For the case inside the surface layer, the movement from O2− ion to HCO3− has a higher energy barrier, indicating that the stimulation by H2O is a one-way effect, and the enhancement by H2O for CO2 mobility is caused by the reaction with CO32− other than O2− ion.

CO2 emissions have posed numerous global challenges, leading to an increasing consensus on the need for carbon neutrality in future development. CO2 capture and energy storage technologies represent a critical step in the carbon neutrality journey. Calcium looping (CaL), a promising technology for both CO2 capture and energy storage, holds significant potential in future carbon neutral technology strategies. In this paper, a comprehensive review of the application of CaL in CO2 capture and thermochemical heat storage (TCHS) is offered to inform further advancements in this field. Firstly, a brief overview and analysis of the fundamental technical routes and principles underlying of CaL for CO2 capture and TCHS are provided. Then, the research progress in the development of CaL-integrated systems for CO2 capture and TCHS is subsequently reviewed, with the existing limitations and outlining future prospects for further development highlighted. Additionally, a comprehensive summary of the proposed improvements in the performance of calcium-based materials is presented, focusing on enhancing carbonation reactivity in the multiple cycles and improving sunlight absorption performance of calcium-based materials. Finally, based on the current status of CaL development, insights and perspectives on potential avenues for further technological advancement are offered. Solar-driven CaL is a promising avenue for future CaL development, calling for greater research efforts on optimizing relevant equipment and enhancing calcium-based materials for sunlight-driven CaL systems. In addition, the CO2 in-situ conversion in the calcination stage of CaL is also a great potential direction for technological evolution.

In this paper, the enhancement of thermochemical energy storage by alkali metal chloride salts-doped Ca-based sorbents is revealed by experiments and DFT calculations. The results indicate that NaCl and KCl doping increases the reaction rate and cycle stability. Compared to CaO, the conversion of NaCl-CaO and KCl-CaO after one cycle is increased by 59.1% and 61.9%, respectively. This enhancement originates from the oxygen vacancies generated by Na2O and K2O and the significantly increased surface area by CaCl2 as well as the sintering delay. The synergistic effect between Na2O, K2O, and CaCl2 increases the reaction rate of calcium-based materials. Meanwhile, the penetration of low-viscosity molten NaCl and KCl into the calcium-based materials successfully segregates the CaO grains and allows the calcium-based material to maintain the porous structure after 80 cycles, thus exhibiting a high effective conversion rate. In addition, the KCl-CaO composites show the best combined performance in terms of effective conversion and averaged thermal energy density. This work paves the way for the application of chloride salts-doped calcium-based materials.

Concentrating solar power (CSP) technology with thermal energy storage (TES) could contribute to achieving a net zero emissions scenario by 2050. Calcium looping (CaL) is one of the potential TES processes for the future generation of CSP plants coupled with highly efficient power cycles. Research on CaL as a system for thermochemical energy storage (TCES) has focused on efficiency enhancement based on hybridization with other renewable technologies. This work proposes a novel solid management system to improve the efficiency of a CaL TCES system. The inclusion of a solid–solid separation unit after the carbonation step could lead to energy and size savings. The role of segregation between carbonated and calcined material on plant requirements is assessed, given the experimental evidence on the potential classification between more and less carbonated particles. The results show lower energy (up to 12%) and size (up to 76%) demands when the circulation of less carbonated material through the CaL TCES system diminishes. Moreover, under a classification effectiveness of 100%, the retrieval energy could increase by 32%, and the stored energy is enhanced by five times. The present work can be a proper tool to set the design and size of a CaL TCES system with a partial separation of the carbonated material. [ABSTRACT FROM AUTHOR]

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Given the gradual nature of the energy transition, retrofitting coal-fired power plants with carbon capture technology is crucial. The calcium looping (CaL) process is a promising solution, with challenges like absorbent deactivation and reduced thermal efficiency mitigated by absorbent reactivation and heat recovery systems. This study evaluated the techno-economic feasibility of integrating a novel wet extraction and precipitation process for absorbent reactivation within a solar-assisted CaL system, alongside an existing coal power plant. The process incorporated a secondary steam cycle and an ammonia absorption chiller for enhanced heat recovery and district cooling. The integrated project could increase daily power generation by 50% and reduce CO2 emissions from 820.4 g/kWh to 54.5 g/kWh. Over its lifespan, the reactivation facility could reduce limestone extraction by 21 Mt with 90% capture efficiency. With a levelized cost of electricity (LCOE) of 116.1 €/MWh and breakeven electricity selling price (BESP) of 56.6 €/MWh, the system demonstrated promising commercial viability, with the reactor and concentrated solar heating (CSH) system making up over 60% of investment costs. CSH cost and solar abundance were identified as key factors, indicating potential feasibility even in higher latitude regions. At CO2 revenues of 150 €/t, a stand-alone capture project can break even based solely on CO2 sales, demonstrating its potential for expansion to other areas. A case study highlighted the benefits of integrating absorbent reactivation and an ammonia absorption chiller, improving both economics and carbon capture efficiency. The study also confirmed the viability of solar-assisted projects in high-latitude regions, with optimistic future CO2 revenues and advancements in carbon capture technology enhancing feasibility.

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.

Waste eggshells have been investigated as biomineralised CO2 sorbents for the calcium looping cycle due to their high Ca content, low cost, availability, and high CO2 uptake. This work investigates a novel heat-pre-treatment using eggshells as raw material. The eggshells were simultaneously calcined and thermally pre-treated via microwave heating using different powers and times whilst maintaining the same input energy in a single mode cavity reactor. The calcined eggshells were characterised using standard characterisation techniques such as TGA and isotherms of N2 adsorption at 77 K. The dielectric properties were measured using the cavity perturbation technique at 2.45 GHz. Numerical electromagnetic simulations were performed using COMSOL Multiphysics and results were used to optimise the experimental setup. The treated materials were subjected to carbonation/calcination cycles in order to assess their suitability as a calcium-looping sorbent. The sorbent that exhibited a more stable CO2 uptake was the one treated at the highest power (800 W) 4.5 mmol CO2/g sorbent after 20 cycles under mild calcination conditions. Adsorbents prepared at 400, 600 and 800 W displayed similar CO2 uptake after 20 cycles when the calcination conditions were under more realistic conditions.

Global CO2 concentration rapidly increases over the past few centuries, particularly in recent decades, and the greenhouse effect is sharply accelerated by the increase of CO2 emissions. CO2 emissions are mainly produced from fossil fuel combustion, in particular coal-fired power plants. Calcium looping, i.e. the carbonation/calcination cycles of CaO, is one of the most promising CO2 capture technologies for coal-fired power plant. At the same time, coal-fired power plants are also one of the main sources of NOx emissions. CO can be used as a reducing agent for NO due to its low cost and low toxicity. CaO is a suitable catalyst for NO reduction by CO. The combustion of biomass is used to provide the required energy for the decomposition of CaCO3 in the calciner. Few unburned char and CaO flow into the carbonator, where the unburned char reacts with O2 in the flue gas to generate CO. Thus, the simultaneous NO removal and CO2 capture in the carbonator of the calcium looping process can be realized. However, the reaction mechanism of the effect of CaO on NO removal by CO in the carbonation stage is unclear. The mechanism of NO reduction with CO catalyzed by CaO in the carbonation stage is investigated by the density functional theory (DFT) calculations. The structural, adsorption, and reaction path parameters, including atomic layout, bond length, adsorption energy, and energy barrier of CO and NO molecules on the CaO (100) surface, are determined. The optimal adsorption sites of CO and NO molecules on the CaO (100) surface are O-top sites, and the adsorption energies are −0.35 and −0.79 eV, respectively. The CO and NO adsorption energies on the CaO (100) surface in the presence of CO2 are −0.36 and −0.20 eV, respectively. The co-adsorption of CO and NO molecules appears feasible on the CaO (100) surface, while CO2 exhibits obvious inhibition for the co-adsorption. The reaction path of NO reduction by CO on the CaO (100) surface undergoes two elementary reaction stages: CO2 formation and adsorption, and N2 generation. Finally, one N2 molecule and three \begin{document}${\rm{CO}}_3^{2-} $\end{document} ions are formed, with a total energy barrier of 11.08 eV. CO2 formation and adsorption (R→IM1→IM2) are the rate-determining steps of the whole reaction process.

Poor CO2 capture stability and serious elutriation are the issues commonly occur when limestone circulating in the fluidized bed-based Calcium Looping (CaL) system. Hence, three types of coal fly ash-bound (CFA-bound) limestone-derived sorbents were synthesized via mixing the limestone, calcined lime and hydrated lime with coal fly ash, respectively. The generated ternary Ca-Al-O and Ca-Si-O phases offer structural stability for CFA-bound limestone-derived sorbents and improve their cyclic CO2 capture performance. Moreover, extrusion-spheronization method was adopted to produce CFA-bound limestone and hydrated lime sorbent pellets to synergistically enhance cyclic CO2 capture capability and mechanical properties. It is found that hydrated lime is superior to limestone as the calcium precursor to prepare CaO-based sorbent pellets due to its desirable pore structure and high binding force. Therefore, the CFA-bound hydrated lime pellets containing 15 wt % of fly ash (HL/CFA15-P) exhibits a high carbonation conversion of 40 % after 17 cycles, which is nearly 1.4 times that of limestone pellets. The discrepancy on carbonation conversion between the two sorbent pellets is enlarged to ∼1.6 times after 100 cycles, which is calculated using a semi-empirical equation. Moreover, HL/CFA15-P exhibits outstanding mechanical property, whose compressive strength reaches 6±1.7 MPa, which is comparable to the cement-bound CaO-based sorbent pellets.

This study presents process design and thermodynamic analysis of hydrogen production through a novel two-stage sorption-enhanced gasification (SEG) process. The SEG process integrates decoupled gasification with calcium looping to convert solid fuels such as biomass. The proposed two-stage SEG process employs a fluidized bed gasifier and a countercurrent moving bed reformer. The first stage involves high-temperature sorption-enhanced gasification in a fluidized bed. Then the product gas from the first stage undergoes sorption-enhanced reforming at relatively lower temperatures to further improve the hydrogen purity. This study employs an autothermal multistage model based on Gibbs free energy minimization to analyze and optimize the calcium looping ratio, steam-to-carbon ratio and feed temperatures of the moving bed reformer. The product gas distribution and temperature profile along the moving bed are discussed. Under optimal conditions, the two-stage SEG process can directly produce 99% H 2 (dry basis), and the overall energy efficiency of the two-stage SEG is 58.7%, which is nearly 10% higher than the conventional process. Therefore, two-stage sorption-enhanced biomass gasification shows the potential to significantly improve green hydrogen production. [Display omitted] • Novel two-stage sorption-enhanced biomass gasification that uses an autothermal moving bed reformer is proposed. • A non-dimensional, non-isothermal, multi-stage equilibrium moving bed model is developed. • Analysis and optimization of the moving bed reformer operating parameters are carried out. • Under optimal conditions, the proposed process produces hydrogen-enriched gas with up to 99% hydrogen. • Process simulation showed that the efficiency of two-stage gasification is 10% higher than the conventional process. [ABSTRACT FROM AUTHOR]

Direct air capture (DAC) is widely investigated to capture carbon dioxide (CO 2) emissions from decentralised sources. As an alternative to geological storage, CO 2 from DAC can be reacted with hydrogen (H 2) from water electrolysis driven by renewable energy to produce synthetic methane (CH 4), increasing the penetration of renewable energies and leading to a circular carbon economy. This study presents a techno-economic assessment of an integrated system for DAC based on calcium looping and subsequent methanation of CO 2. Photovoltaics (PV) is considered as renewable energy source. For the DAC plant, heat exchangers represent the most expensive component, followed by blowers/compressors, reactors and gas heaters. The levelized cost of CO 2 removal for DAC ranges within 693–1587 € t CO2 −1 according to the parameters considered. For a methanation plant built in Abu Dhabi (United Arab Emirates), the electrolyzer represents the most expensive component, followed by PV field, H 2 storage tank, and reactors. Altogether, the levelized cost of methane (LCOM) ranges within 4.9–8.2 € kg CH4 −1 for integration with the DAC plant investigated. It reduces to 3.1–3.9 € kg CH4 −1 for integration with point source carbon capture. A comparison between Abu Dhabi and Benevento (Italy) reveals that cities with an uneven distribution of solar energy throughout the year may require a large investment cost for the H 2 storage tank, that makes the LCOM increase. In conclusion, LCOM values provided in this study exceeds the current price of methane, suggesting that further improvements or economic incentives are required to increase the competitiveness of this power-to-gas technology. • A techno economic analysis of CO 2 methanation by green hydrogen was performed. • Direct air capture (DAC) and point source carbon capture (PSC) were considered. • Levelized cost of DAC by calcium looping ranges within 693–1587 €/t CO2. • Levelized cost of methane (LCOM) ranges within 5–8 €/kg CH4 (DAC) and 3–4 €/kg CH4 (PSC). • H 2 storage cost boosts with a high seasonal variability of solar energy, increasing LCOM. [ABSTRACT FROM AUTHOR]

This research work deals with the application of the calcium looping concept for thermochemical energy storage. Experiments were carried out in a lab-scale fluidised bed reactor, which was electrically heated. An Italian limestone (98.5% CaCO3, 420–590 μm) was present in the bed alone, or in combination with silica sand/silicon carbide (this last material was chosen as per its high absorption capacity in the solar spectrum). Calcium looping tests (20 calcination/carbonation cycles) were carried out under operating conditions resembling the "closed-loop" scheme (calcination at 950 °C, carbonation at 850 °C, fluidising atmosphere composed of pure CO2 in both cases). Carbonation degree, particle size distribution, and particle bulk density were measured as cycles progressed, together with the application of a model equation to relate carbonation degree to the number of cycles. Mutual relationships between the nature of the bed material and possible interactions, the degree of CaO carbonation, the generation of fragments, and changes in particle density and porosity are critically discussed. An investigation of the segregation behaviour of the bed material has been carried out through tests in a devoted fluidisation column, equipped with a needle-type capacitive probe (to measure solid concentration). [ABSTRACT FROM AUTHOR]

The calcium looping process has become a promising technology for reducing carbon dioxide emissions because of its lower energy consumption compared with other methods. A three‐dimensional multifluid model based on the kinetic theory was employed to study the movement of granules in the calciner. The results demonstrated that an increase in the rotation speed and inclination angle of the calciner led to a decrease in the residence time of the particles. Simultaneously, the probability of the particles being heated was reduced. Insufficiently calcined particles reduce the carbon capture capacity in the next cycle; therefore, suitable operating conditions and mechanism designs are very important. [ABSTRACT FROM AUTHOR]