CO2 Capture Rate Research Articles

Assessing Best Practices in Natural Gas Production and Emerging CO2 Capture Techniques to Minimize the Carbon Footprint of Electricity Generation.

Natural gas (NG) is expected to provide a substantial portion of electricity generation in many jurisdictions for the foreseeable future. Postcombustion carbon capture and storage (CCS) effectively abates direct CO2 emissions; however, indirect NG supply chain emissions in most jurisdictions are incompatible with climate change mitigation goals. This life cycle assessment evaluates specific opportunities to reduce the carbon footprint of combined cycle gas turbine (CCGT) generation with CCS using existing low-emission NG production practices, technologies, and processes combined with emerging CCS techniques to achieve high CO2 capture rates and mitigate startup emissions. We find baseload life cycle greenhouse gas (GHG) emission intensity ranges from 22 to 62 kgCO2e/MWh for 95-98.5% CO2 capture, within the range of published estimates for wind and photovoltaic power and considerably below prior estimates of CCGT with CCS. Low-emission NG production practices reduce other environmental impacts, which are dominated by combustion-related air pollution. We also show how interim solvent storage can effectively mitigate emissions from CCGT start/stop cycles. This work highlights the importance of mitigating both CO2 and methane emissions from NG supply chains and proposes a more nuanced discussion regarding the potential contribution of NG to the future energy supply. A surrogate model is provided to estimate life cycle GHG emissions for CCGT with CCS and user-input parameters.

Transforming carbon-intensive coal-fired power plants into negative emission technologies via biomass-fired calcium looping retrofit

Abstract Calcium looping is a promising CO2 capture technology due to reduced energy and economic penalties compared to mature solvent scrubbing technologies. It also can enable negative CO2 emissions when biomass is used to drive the sorbent regeneration process in the calciner. However, the trade-off between the energy, economic and environmental performance under different biomass co-firing fractions in the calciner and process operating scenarios has not yet been well understood. This study examined the potential for transforming a 580 MWel coal-fired power plant into a negative CO2 emitter via retrofit of calcium looping with biomass co-firing in the calciner. High-fidelity process models were developed in Aspen Plus and used to analyse the effect of biomass co-firing fraction, CO2 capture rate in the carbonator, and the fraction of flue gas fed to the carbonator on the techno-economic performance indicators. The results revealed that co-firing 30% biomass with coal in the calciner was sufficient for the retrofitted process to achieve negative CO2 emissions (−3.9 gCO2/kWelh). In this scenario, the levelized cost of electricity was 5% lower (81.1 €/MWelh) than that in the reference retrofit scenario without biomass co-firing (85.4 €/MWelh) at a carbon tax of 100 €/tCO2. Further improvement to the techno-economic performance was achieved by reducing the amount of CO2 captured in the carbonator by reducing either the CO2 capture rate (81.1 €/tCO2) or the amount of flue gas processed (80.4 €/tCO2). Although this was achieved at the expense of the increased specific CO2 emissions to 65.2 gCO2/kWelh and 109.0 gCO2/kWelh, respectively, the net specific emissions were still about 90% lower than those of the unabated host plant (792.3 gCO2/kWelh). This study demonstrated that depending on design priorities, biomass co-firing in the calciner can transform the existing coal-fired power plants into negative CO2 emission technologies or improve the process techno-economic viability.

Optimum Planning of Carbon Capture and Storage Network Using Goal Programming

Carbon capture and storage (CCS) is a critical technology used for mitigating climate change by capturing carbon dioxide emissions from industrial sources and storing them underground to prevent their release into the atmosphere. Despite its potential, optimizing CCS systems for cost-effectiveness and efficiency improvement remains a significant challenge. In this paper, the optimization of CCS systems through the development and application of two mathematical optimization techniques is introduced. The first technique is based on using a superstructure optimization model, while the second technique relies on applying a goal programming optimization model. These models were solved using LINGO software version API 14.0.5099.166 to enhance the efficiency and cost-effectiveness of CCS systems. The first model, seeking to maximize the exchange of CO2 flowrate from sources to sinks, achieved a CO2 capture rate of 93.36% with an annual total cost of USD 1.175 billion. The second model introduced a novel mixed-integer non-linear programming (MINLP) approach for multi-objective optimization, targeting the minimization of total system cost, alternative storage, and unutilized storage while maximizing CO2 load exchange. The application of the second model, when prioritized to maximize CO2 flowrate exchange using the goal programming technique, resulted in a cost reduction of 36.46% and a CO2 capture rate of 75.87%. In contrast, when the second model prioritized minimizing the total annual cost, a 48% cost reduction was achieved, and the CO2 capture rate was decreased by 68.37%. A comparison of the two models’ results is presented. The results showed that the second model, with the priority of maximizing CO2 capture, provides the best economic–environmental objective balance, which offers notable cost reductions while keeping an efficient CO2 capture rate. This study highlights the potential of advanced mathematical modeling in increasing the feasibility of CCS as one of the very important strategies of mitigating climate change and reducing global warming.

Dynamic modeling of post-combustion carbon capture process based on multi-gate mixture-of-experts incorporating dual-stage attention-based encoder-decoder network

Solvent-based post-combustion carbon capture (PCC) technology is a promising, near-term solution for decarbonizing power generation and industrial facilities. Model-based process simulation is crucial for the optimal design and operation of the PCC process. Recently, data-driven models have gained attention due to their adaptability, efficient computation and high accuracy. However, the nonlinearity, strong couplings and multi-time scale features of the PCC process pose significant challenges for model identification. To this end, this paper proposes a multi-gate mixture-of-experts incorporating dual-stage attention-based encoder-decoder (MMoE-DAED) network for dynamic modeling of the PCC process under wide operating conditions. An encoder-decoder composed of long short-term memory (LSTM) network is employed to extract features from the time-dependent input data and learn the complex dynamic interactions caused by the inertial and delay properties of the process. Dual-stage attention mechanism is incorporated into the encoder and decoder respectively to select the most relevant input features and their correlations within the time series data. To enhance multi-output prediction accuracy, multi-gate mixture-of-experts (MMoE) framework that considers correlations of multitask learning is implemented. Simulation results using operating data from a PCC experimental setup indicate that the proposed modeling approach accurately predicts the steady-state values and dynamic trends of the CO2 capture rate and stripper bottom temperature over a wide operating range. The RMSE, MAPE and R2 indices for the CO2 capture rate are 2.1592, 0.0295, 0.9641, respectively, and for the stripper bottom temperature are 0.1491, 0.0003, 0.9833, respectively. Validations on a PCC simulator further verify the accuracy and efficiency of the MMoE-DAED model, which enables an 80.87% reduction in computation time compared to the simulator. This paper points to a new direction for the data-driven dynamic modeling of complex energy conversion processes.

Synthetic natural gas (SNG) production by biomass gasification with CO2 capture: Techno-economic and life cycle analysis (LCA)

The integration of renewables with CCUS technologies has the promising potential to deliver energy applications with overall negative CO2 emissions which would significantly contribute to the climate neutrality. Moreover, the renewable-based synthetic fuels are predicted to replace the conventional fossil fuels. The current work evaluates key techno-economic and environmental indicators for the SNG production from various renewable fuels (e.g., sawdust, residual wood, organic wastes, agricultural and urban wastes etc.) through gasification process fitted with decarbonization capability. The investigated plant capacity was set to 500 MWth SNG with 60 % CO2 capture rate of primary fuel (biomass). The overall concept was evaluated using various process engineering approaches e.g., reactor & process modelling and simulation, model validation, heat and power analysis, techno-economic assessment combined with detailed environmental impact by Life Cycle Analysis (LCA). The investigated design shows promising performance indicators such as high cumulative energy efficiency (69 %) and carbon conversion to SNG, almost zero plant specific CO2 emissions (3 kg/MWh) and overall negative carbon emissions (−457 kg/MWh) from the whole biomass chain. The SNG production cost is not yet fully competitive versus the EU natural gas prices (53 vs. 30–35 €/MWh) but it shows promising potential considering the trends of increasing fossil prices and CO2 emission tax.

Advancing greener LNG-fueled vessels: Compact simultaneous reduction system for CH4, NOX and CO2 emissions

Stringent ship regulations necessitate simultaneous emission reductions, challenged by cargo loss and energy constraints. This study proposes a compact reduction system that simultaneously mitigate CH4, NOX, and CO2 emissions onboard LNG-fueled vessels. The proposed system integrates two pivotal technologies: a metal support-based compact methane oxidation catalyst (MOC)-selective catalytic reduction (SCR) system to capture CH4 and NOX, and a cryogenic CO2 capture (CCC) system to capture and store CO2 in its solid phase utilizing LNG's cold energy. This innovative approach addresses two challenges. First, the compact MOC-SCR system employs metal support with an enhanced surface area-to-volume ratio surpassing the capabilities of ceramic support materials, achieving a volume reduction of 80.02% and 79.82% respectively. Secondly, the CCC system significantly lowers specific energy consumption by 61.54% by harnessing LNG's cold energy compared to the absorption-based CO2 capture system. As a result, the proposed system not only reduces CH4, NOX, and CO2 emissions simultaneously with a high capture rate but also reduces volume and weight. Furthermore, it achieves a high CO2 capture rate of 92.12%, while diminishing cargo losses by 25.30%. These findings provide valuable guidance for developing environmentally sustainable vessels, aligning with the continuously increasing stringency of regulations.

Industrial-scale 12-layered-bed vacuum pressure swing adsorption for fuel cell-grade H2 production from carbon-captured steam methane reforming syngas

Vacuum pressure swing adsorption (VPSA) is primarily used to produce high-purity H2 from CO2-rich syngas in steam methane reforming (SMR) plants. Since carbon capture is essential, the performance of current H2 VPSA processes is affected by feeding carbon-captured SMR syngas (CO2-lean SMR syngas). This study conducted the performance evaluation and optimization for a fuel cell-grade H2 production by an industrial-scale 12-layered-bed VPSA process with a 24-step cycle from CO2-lean SMR syngas. The multi-scale dynamic model for the VPSA process was validated with petrochemical industry data from a current SMR-VPSA plant. A sensitivity analysis using CO2-lean SMR syngas indicated the high loss of H2 during desorption steps, suggesting the need for optimization of the VPSA process. Owing to the complex interconnectivity of 12 beds via 24 steps, the framework of a dynamic model-based deep neural network (DNN) and DNN-based optimization was developed to design and optimize the VPSA process. Even without changing equipment or adsorbent configuration, the VPSA process achieved fuel cell-grade H2 production (>99.9999 % H2 and > 88 % recovery with < 0.2 ppm CO). The results indicate that CO2 capture rates, ranging from 90 to 99 %, can be independently determined under techno-economic and environmental conditions, as this capture range of CO2-lean SMR syngas gives a minor impact on the performance of VPSA process. The proposed three-step framework, which encompasses dynamic modeling, surrogate modeling, and multi-objective optimization, provides a systematic and feasible approach for designing and optimizing the VPSA process in an SMR plant integrated with a carbon capture process, to achieve a low-carbon economy.

Process simulation and techno-economic analysis of CO2 capture by coupling calcium looping with concentrated solar power in coal-fired power plant

The calcium looping (CaL) cycle is an effective high-temperature CO2 capture technology, its primary energy consumption arises from the calcination process, which limits the development of CaL technology. Utilizing concentrated solar power (CSP) instead of combustion for the calcination process in the CaL cycle can significantly reduce the energy penalty associated with CO2 capture of coal-fired power plants. Currently, the application of coupling CaL with concentrated solar power (CaL-CSP) systems in coal-fired power plants is still in its infancy, and there is limited research on its thermal performance and techno-economic analysis. In this study, a process simulation model of a 660 MW supercritical coal-fired power plant is developed as a reference. Subsequently, four different heat transfer routes are integrated into the design of CaL-CSP systems. The results indicate that the coupled power plant achieves a CO2 capture rate of 90 % compared to the reference coal-fired power plant, and reduces annual CO2 emissions by 93 % to only 58.2 g CO2/kWh. The CO2 capture energy penalty is 0.24 MJ/kg CO2. A sensitivity analysis of power generation cost reveals that if the cost of solar concentrator components is halved, the power generation cost will decrease to just 25 % higher than the reference power plant. Given the rapid advancements in CSP technology, this reduction is likely to be realized soon. Moreover, with the increase in carbon taxes and carbon prices, the power generation cost is expected to further decrease.

Developing slurry based on immobilized and aqueous [MEACl][EDA] for CO2 capture

The urgent need to mitigate climate change and reduce greenhouse gas emissions has underscored the importance of developing effective CO2 capture technologies. In this study, a novel strategy was proposed and developed, where adding cosolvent and immobilizing deep eutectic solvent (DES) were combined to enhance the performance from capacity to rate. To this end, [MEACl][EDA] (1:5) was chosen as the DES, H2O was used as the cosolvent, and silica was selected as the substrate to immobilize the DES; the effects of the DES immobilization amount and the concentration of the immobilized DES particles in the aqueous system as well as temperature on the viscosity and CO2 capture performance were investigated systematically. Compared to the aqueous DES, the slurry exhibited higher CO2 uptake and improved kinetics, comparable viscosity before CO2 capture, and slightly high viscosity after CO2 capture, where the best sample (3 wt% of (5 wt% DES@silica) in 40 wt% aqueous DES) showed a capacity of 24.93 wt% at the room temperature (22 ℃) and viscosities of 7.32 and 21.82 mPa·s before and after CO2 capture. Also, the CO2 capture rate of the slurry reached 1.4 mol CO2/(kg sorbent·min) after 2 min at 22 ℃, which was higher than the aqueous DES without immobilized DES (1.24 mol CO2/(kg sorbent·min) under the same condition). The prepared slurries showed high stability (over one month) and high regeneration (∼91 % CO2 capture recovery after 5 cycles).

Effect of backbone linker (O vs NH) on the ability of pincer-supported nickel hydrides to reduce CO2

The reactivity of nickel hydrides supported by a pincer ligand often can be tuned by modifying the ligand at the three donor sites. In this study, ligand modification is made at the backbone, more specifically the linkers X and Y in {2,6-C6H3(XPtBu2)(YPtBu2)}NiH. Of the three nickel hydrides investigated herein, the PNCNP-pincer complex (X = Y = NH) is the most reactive one towards CO2, whether it is for the rate of CO2 capture from air or for the thermodynamic favorability of CO2 insertion into the Ni–H bond. The POCOP-pincer complex (X = Y = O) is the least reactive hydride whereas the POCNP-pincer complex (X = O, Y = NH) is ranked in the middle. To have access to the hybrid pincer complex, an improved synthetic method for the proligand, m-C6H4(OPtBu2)(NHPtBu2), is also developed.

Carbon management technology pathways for reaching a U.S. Economy-Wide net-Zero emissions goal

The Carbon Management Study Group of the 37th Energy Modeling Forum (EMF 37) designed seven scenarios to explore the role of three potentially key technology suites – point source carbon dioxide capture and storage (PSCCS), direct air capture of carbon dioxide (DACCS), and hydrogen systems (H2) – in shaping the broader technology pathways to reaching net-zero carbon dioxide (CO2) emissions in United States by 2050. Each scenario was run by up to 13 models participating in the EMF 37 study. Results show that carbon dioxide removal technologies were consistently a major part of successful pathways to net-zero U.S. CO2 emissions in 2050. Achieving this net-zero CO2 goal without any form of carbon dioxide capture and storage was found to be impossible for most models; some models also found it impossible to reach net-zero without DACCS. The marginal cost of achieving net-zero CO2 emissions in 2050 was between two and 10 times higher without PSCCS and/or DACCS available. The carbon price at which DACCS was deployed as a backstop technology depended upon the assumed cost at which DACCS was available at scale. Carbon prices were between $250 and $500 per ton CO2 when DACCS deployed as a backstop. The average CO2 capture rate across all models in 2050 in the central net-zero scenario was 1.3 GtCO2/year, which implies a substantial upscaling of capacity to move and store CO2. Hydrogen sensitivity scenarios showed that H2 typically constituted a relatively small share of the overall U.S. energy system; however, H2 deployed in applications that are considered hard to decarbonize, facilitating transition towards net-zero emissions.

Enhancing electrochemical carbon dioxide capture with supercapacitors

Supercapacitors are emerging as energy-efficient and robust devices for electrochemical CO2 capture. However, the impacts of electrode structure and charging protocols on CO2 capture performance remain unclear. Therefore, this study develops structure-property-performance correlations for supercapacitor electrodes at different charging conditions. We find that electrodes with large surface areas and low oxygen functionalization generally perform best, while a combination of micro- and mesopores is important to achieve fast CO2 capture rates. With these structural features and tunable charging protocols, YP80F activated carbon electrodes show the best CO2 capture performance with a capture rate of 350 mmolCO2 kg–1 h–1 and a low electrical energy consumption of 18 kJ molCO2–1 at 300 mA g–1 under CO2, together with a long lifetime over 12000 cycles at 150 mA g–1 under CO2 and excellent CO2 selectivity over N2 and O2. Operated in a “positive charging mode”, the system achieves excellent electrochemical reversibility with Coulombic efficiencies over 99.8% in the presence of approximately 15% O2, alongside stable cycling performance over 1000 cycles. This study paves the way for improved supercapacitor electrodes and charging protocols for electrochemical CO2 capture.

Near-zero emission trigeneration system coupling molten salt methane pyrolysis with solid oxide fuel cell based on carbon capture and utilization: Thermodynamic and exergoenvironmental evaluation

Natural gas-based energy systems have garnered widespread attention for their potential to facilitate the transition from traditional to renewable energy sources. Among these, methane pyrolysis technology stands out as a promising pathway. Consequently, a novel methanol-electricity-carbon trigeneration system featuring methane pyrolysis coupled with chemical looping combustion, methanol synthesis, and solid oxide fuel cell unit was designed. To validate this innovative concept, thermodynamic model of the process was developed. Compared to conventional single-generation system, the proposed system demonstrated superior thermodynamic performance, with energy efficiency of 69.51 %, representing improvement of 28.78 %. Sensitivity analysis indicated that optimal carbon utilization-to-storage ratio of 0.68 effectively balanced environmental benefits with energy conversion efficiency. Additionally, the CO2 capture rate and emission of the proposed system reached impressive 99.24 % and 2.15 kg/MWh, respectively, confirming its carbon mitigation potential. Based on this, exergoenvironmental analysis demonstrated that rb,k of the novel system was 21.14 %, indicating room for further environmental impact reduction. Moreover, due to the high rb,k of 16.61 % for the preheater-2 identified as a key component for optimization. Finally, the economic analysis verified the system’s feasibility, showing the annual net income of 6774.06 k$. This novel concept highlights the potential of natural gas-based systems in optimizing carbon utilization and energy efficiency, contributing to a more environmentally sustainable energy infrastructure.

Dynamic modeling and comprehensive analysis of an ultra-supercritical coal-fired power plant integrated with post-combustion carbon capture system and molten salt heat storage

Reducing carbon footprints and enhancing operational flexibility are crucial for the future development of coal-fired power plants (CFPPs). This necessitates the deployment of post-combustion carbon capture (PCC) and molten-salt heat storage (MSHS) systems. Given the various integration schemes and complex interactions, understanding the comprehensive performance of the integrated CFPP-PCC-MSHS system is important. This paper proposes an integration scheme for 1000MWe ultra-supercritical CFPP, solvent-based PCC and MSHS, achieving cascade energy utilization. A dynamic simulation model for the CFPP-PCC-MSHS system is developed to understand the dynamic couplings between subsystems. Comprehensive performance analyses are conducted to evaluate the thermodynamics, flexibility and safety of the integrated system under various operating conditions. Simulation results indicate that deploying MSHS reduces the thermal and exergy efficiencies of the CFPP-PCC system by 0.18 % and 0.19 %, respectively, but effectively expands the adjustable power load range by 6.08 % under the fixed 90 % CO2 capture rate mode. Meanwhile, the integration of MSHS offers alternative pathways to improve the power ramping rate to 13.33 MW/min. The heat charging/discharging process of MSHS induces fluctuations in temperature and pressure within the turbine, potentially affecting plant operating safety. This paper provides useful insights for the design, retrofit and operation of new generation of CFPPs.

Optimization and Tradeoff Analysis for Multiple Configurations of Bio-Energy with Carbon Capture and Storage Systems in Brazilian Sugarcane Ethanol Sector.

Bio-energy systems with carbon capture and storage (BECCS) will be essential if countries are to meet the gas emission reduction targets established in the 2015 Paris Agreement. This study seeks to carry out a thermodynamic optimization and analysis of a BECCS technology for a typical Brazilian cogeneration plant. To maximize generated net electrical energy (MWe) and carbon dioxide CO2 capture (Mt/year), this study evaluated six cogeneration systems integrated with a chemical absorption process using MEA. A key performance indicator (gCO2/kWh) was also evaluated. The set of optimal solutions shows that the single regenerator configuration (REG1) resulted in more CO2 capture (51.9% of all CO2 emissions generated by the plant), penalized by 14.9% in the electrical plant's efficiency. On the other hand, the reheated configuration with three regenerators (Reheat3) was less power-penalized (7.41%) but had a lower CO2 capture rate (36.3%). Results showed that if the CO2 capture rates would be higher than 51.9%, the cogeneration system would reach a higher specific emission (gCO2/kWh) than the cogeneration base plant without a carbon capture system, which implies that low capture rates (<51%) in the CCS system guarantee an overall net reduction in greenhouse gas emissions in sugarcane plants for power and ethanol production.

Flexibility analysis on absorption-based carbon capture integrated with energy storage

Post-combustion carbon capture in power plants is essential in reducing greenhouse gas emission. As the available heat in the plant is instable in time, it becomes a challenge to maintain the efficient operation of capture system. The lean/rich solvent storage proves to be a potential way for the capture system to keep high CO2 capture rate. However, the performance of the integrated system under different operating conditions is not fully evaluated, especially for the match criterion of lean/rich solvent process. The impact factors of single and coupled operating conditions of solvent storage still remain unclear. In this work, system performance during peak/off-peak period is first evaluated. Results indicates that heat duty, split ratio, solution concentration and absorption solvent flow rate are key factors for CO2 capture rate. Then, system performance under continuous operation is explored at the global CO2 capture rate from 0.918 to 0.978. It demonstrates that maintaining a relatively equal CO2 capture rate during peak/off-peak period efficiently improves the global CO2 capture rate, which can be achieved by adjusting flow rate and split ratio of solvent. Finally, volume and cost of integrated CO2 capture with solvent storage are compared with that of CO2 capture with heat storage. The volume of solvent storage decreases by over 69% and the cost of solvent storage decreases by over 84%, which indicates a promising application in terms of technical and economic aspects.

Performance of novel carbonate-based CO2 post-combustion capture process

A novel carbonate-based CO2 capture process was developed at VTT, and proof-of-concept was tested using a bench scale device. The process has closed-loop, carbonate liquid, and the captured CO2 is released at under-pressure. The temperature is around 65 °C, which enables, for example, utilisation of waste heat streams in process integrations. The process performance and its parameters were tested using 8 w-% sodium carbonate liquid with simulated gases, real flue gases and biogas. Also, the absorption mass transfer kinetics and suitable flow parameters of the invented microbubble generator were assessed. The device has successfully and continuously run for tens of hours, and the typical CO2 capture rate between 72 and 84 % were performed at the liquid pH value of 9.3, using a near-atmospheric pressure process. Mass transfer rates kLa for carbonate liquid and air were between 0.14 and 0.34 s−1, which indicates efficient gas absorption, enabling size reduction of the device size.

Direct air capture with amino acid solvent: Operational optimization using a crossflow air‐liquid contactor

AbstractDirect air capture (DAC) is a negative emission technology for removing CO2 from the atmosphere to maintain the CO2 level within a reasonable range so as to address greenhouse effects. In this study, the operational optimization of lab‐scale DAC has been investigated using a crossflow air‐liquid contactor loaded with a three dimensionally printed Gyroid packing structure and a potassium sarcosinate solvent. The effects of various parameters, including feed air flow rate, liquid solvent flow rate, contactor geometry, and ambient temperature, are examined. The results demonstrate that the Gyroid packing design achieves comparable CO2 capture performance to conventional packed beds but with a significantly lower pressure drop of up to 77.8%, suggesting its potential as an efficient and cost‐effective solution for gas–liquid contactors in DAC. Additionally, the study explores the climate impact on CO2 capture performance and finds that as the air temperature increases from 35 to 95°F at a fixed relative humidity of 80%, the CO2 capture rate increased from 23.2% to 46.8% with better stability. The research highlights the importance of optimizing contactor design and operational conditions to improve the CO2 capture rate and feasibility of DAC systems as a negative emission technology for addressing greenhouse effects.

Demonstration of CO2 capture with CaO and Ca(OH)2 in a countercurrent moving bed carbonator pilot

Decoupling the carbonation and calcination stages of Calcium Looping (CaL) facilitates the capture of CO2 from small point sources, or even the atmosphere, by transporting CO2 as CaCO3 to centralized calcination plants to obtain pure CO2 and regenerate the Ca-sorbent. In this work, we provide the experimental proof of a novel countercurrent moving bed carbonator particularly suited for such applications. We conducted experiments in a 3-meter-long moving bed reactor with an internal diameter of 0.15 m, using Ca-sorbent particles of varying sizes (in the mm and cm scale). Simulated flue gases with varied concentrations of CO2 (up to 8.5 %v), superficial gas velocities (0.5–2 m/s) and inlet gas temperatures (250–630 °C) have been tested. The temperature profiles we observed along the axial direction confirm the formation of a carbonation zone at optimum temperatures (i.e., 550–700 °C), together with a CO2 polishing region towards the gas exit that leads to CO2 capture rates over 99 % in some cases. Using Ca(OH)2 as a CO2 sorbent, we achieved a molar conversion to CaCO3 of approximately 0.7.

Continuous CO synthesis from ambient air by integrating direct air capture and direct carbonate reduction using an alkaline CO2-absorbing electrolyte operating at room temperature

We constructed an integrated system comprising direct air capture (DAC) and direct carbonate reduction (DCR) to facilitate the industrial implementation of negative CO2 emissions. The DAC-DCR system demonstrated continuous CO synthesis from ambient air at room temperature, in contrast to conventional methods, including high-temperature processes and previously reported a batch-type method that connect DAC and DCR. The CO2-absorbing electrolyte, a K2CO3-KHCO3 aqueous solution, is circulated between the DAC and DCR subsystems. The CO2 captured from the air with the CO2-absorbing electrolyte is converted to carbonate in the DAC; subsequently, the carbonate is electrochemically reduced to CO in the DCR. This system monitored the CO2 capture rate, carbonate reduction rate, and pH of the CO2-absorbing electrolyte in real time. Controlling the carbonate reduction rate by adjusting the DCR reactor current enabled the balancing of carbon capture and carbonate reduction rates. Consequently, stable and continuous operation of a DAC-DCR system was achieved for over one hour for the first time. The DAC-DCR is more suitable for combination with intermittent renewable electricity, including photovoltaic and wind electricity, than the previous methods because all the processes in the system work at room temperature. Thus, the present proof-of-concept study is an important step toward the widespread use of DAC.