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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.

This review aims to provide a comprehensive overview of sorption-enhanced gasification (SEG) with CaO, highlighting its potential as an efficient and sustainable energy conversion technology. SEG integrates dual bed steam gasification with in situ CO2 removal using CaO, efficiently converting solid carbonaceous fuels such as biomass and low-rank coal into hydrogen-rich (up to 80 vol% H2), low-carbon and tar, medium calorific value and nitrogen-free syngas that can be adapted for various downstream applications. The review details the working principle, operating conditions and reaction mechanisms of SEG, emphasizing how these factors influence product distribution. Calcium sorbents are central to the SEG process, so their reactions, catalytic activity and limitations are discussed in this review. Pilot-scale tests are examined to underscore process engineering advancements as well as to highlight scale-up challenges that currently limit the technology to TRL 5–6. Unsustainable long-term sorbent performance and energy penalties from sorbent regeneration and CO2 capture still need to be addressed through scalable and cost-effective material development and process engineering. This review discusses various process intensification concepts as potential solutions to the inherent shortcomings of SEG. Process systems analyses examined indicate the potential of SEG in hydrogen, synthetic fuel and electricity production, positioning it as a promising technology for decentralized sustainable energy conversion. Furthermore, the review explores the sustainable repurposing and disposal of spent solids to foster circular economies. Overall, this comprehensive review provides crucial insights to further leverage and advance the SEG process, offering a platform for future research and development.

The calcium looping (CaL) process stands out as a promising technology for carbon dioxide (CO2) capture, which exhibits two essential phases: carbonation and calcination. CaL process has several advantages over conventional systems such as availability of abundant and low cost CaO sorbents, reduced environmental impact, lower greenhouse emissions and energy requirements. CaL offers easy and innovative schemes to integrate renewable energy such as concentrated solar power, oxy-fuel and chemical looping process and steam dilution to further enhance the overall efficiency of the system. The review first focuses on summarizing the characteristics and operational parameters of these process integrated CaL facilities while highlighting key experimental findings. The examination of innovative sorbent materials utilized within integrated CaL processes has been addressed, emphasizing pathways directed towards enhancing reaction efficacy, energy conservation, and holistic sustainability attained via process integration and intensification. Meanwhile, strategies to overcome the limitation of CaL process in terms of rapid sintering of sorbent particles over time have also been discussed. Further, the approaches for integrating CaL into industrial plants such as power, cement and steel plants have been identified and compared to realize significant reduction of energy penalty compared to conventional system. The impact of multivariate latent variable (LV) modeling on the integrated CaL process has been examined. Based on the review, CaL showed equivalent or better performance in reducing CO2 emissions (global warming potential or climate change impact indicator) in comparison to alternative scenarios.

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.

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.

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.

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.

CaO-based materials are potential candidates for thermochemical energy storage in calcium looping (CaL) due to their low-cost and large theoretical heat storage capacity. The harsh energy storage mode can circumvent adopting the membrane separation techniques with uncertain availability, contributing to improve the practicability of CaL. Nevertheless, the high calcination temperature and CO2 partial pressure involved in harsh energy storage mode adversely affect the cyclic stability of CaO-based materials. Therefore, comparison investigation on cyclic stability of CaO-based composites with inert supports for energy storage under harsh mode (950 °C, pure CO2) was conducted. The single Zr-supported, CaO-based composite possesses the highest energy release density of 1.42 MJ/kg after 19 cycles, which is markedly higher than that of the single Al- or Zr/Al-supported CaO-based composites. The Fe/Mn-doped, Zr-supported CaO-based composites were further prepared aiming to achieve direct solar absorption in the CaL. Although the binary Fe/Mn ions doping causes the decreased energy release density due to consumption of active CaO, the optical adsorption capability is significantly enhanced. The dark composite with a molar ratio of Ca/Zr:Fe:Mn = 100:4:8 exhibits the highest spectral absorbance of 74.2%, which is approximately 5.8 times that of white Zr-supported CaO-based composite. It also possesses excellent energy storage/release stability with a low average energy release density loss of 0.005 MJ/kg per cycle during 19 cycles.

The calcium looping process has been considered as a promising technology for capturing CO2 from the flue gas, but the decay of CaO-based sorbent, which was caused by the sintering process and the poisoning of SO2, was the major obstacle for its application in industry. In this work, NaBr modification was proposed to enhance the cyclic CO2 capture capacity of the CaO-based sorbents, and the effect of SO2 on the cyclic CO2 capture capacity of the NaBr modified CaO-based sorbents was well studied. The enhancing mechanism of cyclic CO2 capture capacity of the modified CaO-based sorbents was analyzed by X-ray diffraction, field emission scanning electron microscopy, N2 physisorption, and fractal dimensions. The result showed that the SO2 in the flue gas inactivated the CO2 capture activity of the sorbents over the cycles, but the NaBr modification not only enhanced the SO2 capture capacity but also improved the cyclic CO2 capture capacity of the sorbents. The specific surface area and BJH cumulative pore volumes of the sorbents decayed rapidly over the repeated cycles, but the fractal dimensions (D) of the NaBr modified sorbents were still larger than that of unmodified reference sorbent. The “NaBr/CaO” captured 15 grams of CO2 per gram of sorbent over the 50 cycles, which was 2.2 times that of “CaO, AR”. Hence, NaBr modification would be a promising strategy to boost the CO2 capture capacity of the CaO-based sorbent in the calcium looping process that contributes to the carbon neutralization scenarios.

Carbon dioxide-abated hydrogen can be synthesised via various processes, one of which is sorption enhanced steam methane reforming (SE-SMR), which produces separated streams of high purity H2 and CO2. Properties of hydrogen and the sorbent material hinder the ability to rapidly upscale SE-SMR, therefore the use of artificial intelligence models is useful in order to assist scale up. Advantages of a data driven soft-sensor model over thermodynamic simulations, is the ability to obtain real time information dependent on actual process conditions. In this study, two soft sensor models have been developed and used to predict and estimate variables that would otherwise be difficult direct measured. Both artificial neural networks and the random forest models were developed as soft sensor prediction models. They were shown to provide good predictions for gas concentrations in the reformer and regenerator reactors of the SE-SMR process using temperature, pressure, steam to carbon ratio and sorbent to carbon ratio as input process features. Both models were very accurate with high R2 values, all above 98%. However, the random forest model was more precise in the predictions, with consistently higher R2 values and lower mean absolute error (0.002-0.014) compared to the neural network model (0.005-0.024).

In this study, a precipitation-based synthesis method has been employed to prepare magnesium calcites with the general formula Ca 1-x Mg x CO 3 , with the objective of use them in the calcium looping (CaL) process for CO 2 capture (CaL-CCS) and thermochemical energy storage (CaL-CSP). The structure and microstructure of the samples have been characterized. It has been found by X-ray diffraction that the samples with a Ca:Mg molar ratio of 0.5:0.5 and 0.55:0.45 are phase pure, while the samples with molar ratios of 0.7:0.3 and 0.8:0.2 are composed by two phases with different stoichiometry. In addition, the sample prepared with calcium alone shows the aragonite phase. The microstructure of the magnesium-containing samples is composed of nanocrystals, which are aggregated in spherical particles whereas the aragonite sample presents a typical rod-like morphology. The multicycle tests carried out under CaL-CCS conditions show that an increase on the MgO content in the calcined samples results in a reduced value of effective conversion when compared to aragonite. On the other hand, under CaL-CSP conditions, the samples with the higher MgO content exhibit nearly stable effective conversion values around 0.5 after 20 cycles, which improve the results obtained for aragonite and those reported for natural dolomite tested under the same conditions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)