In situ/operando plug-flow fixed-bed cell for synchrotron PXRD and XAFS investigations at high temperature, pressure, controlled gas atmosphere and ultra-fast heating

A plug-flow fixed-bed cell for synchrotron powder X-ray diffraction (PXRD) and X-ray absorption fine structure (XAFS) idoneous for the study of heterogeneous catalysts at high temperature, pressure and under gas flow is designed, constructed and demonstrated. The operating conditions up to 1000°C and 50 bar are ensured by a set of mass flow controllers, pressure regulators and two infra-red lamps that constitute a robust and ultra-fast heating and cooling method. The performance of the system and cell for carbon dioxide hydrogenation reactions under specified temperatures, gas flows and pressures is demonstrated both for PXRD and XAFS at the P02.1 (PXRD) and the P64 (XAFS) beamlines of the Deutsches Elektronen-Synchrotron (DESY).

Understanding the Role of Mono and Ternary Alkali Metal Salts on CO2 Uptake of MgO Sorbents

CO2 uptake by MgO-based sorbents at intermediate temperatures is attractive for pre- and post-combustion CO2 capture applications. However, besides the high CO2 uptake potential of these materials (1.1 g CO2 g-1 sorbent), in practice, the realistic CO2 capture is far from that of the theorical values. In this work, the sol-gel method was used to synthetize unsupported and supported MgO sorbents (10% Ca- or 10% Ce- support, mol) that were impregnated with different fractions (15, 25, and 35; % mol) of a NaNO3 single salt or a ternary alkali salt (NaNO3, LiNO3 and KNO3 (18/30/52; % mol)). To understand the role of alkali metal salts (AMSs) in the MgO sorbents' performance, the working and decomposition temperature ranges of AMS under different atmospheres (CO2 and air) were evaluated. The findings show that the CO2 uptake temperature range and maximum uptake (20-500 °C, CO2 atmosphere) of sorbents are correlated. The cyclic CO2 uptake of the most promising sorbents was tested along five carbonation-calcination cycles. For the first and fifth cycles, respectively, the 15 (Na, K, Li)-MgO sorbents showed the highest carrying capacity, i.e., 460-330 mg CO2 g-1 sorbent, while for the 15 (Na, K, Li)-MgO-Ca sorbents, it was 375-275 mg CO2 g-1. However, after the first cycle, the carbonation occurred faster for the 15 (Na, K, Li)-MgO-Ca sorbents, meaning that it can be a path to overpassing carbonation kinetics limitations of the MgO sorbent, making it viable for industrial applications.

CO2 Mineralization by MgO Nanocubes in Nanometric Water Films

Water films formed by the adhesion and condensation of air moisture on minerals can trigger the formation of secondary minerals of great importance to nature and technology. Magnesium carbonate growth on Mg-bearing minerals is not only of great interest for CO2 capture under enhanced weathering scenarios but is also a prime system for advancing key ideas on mineral formation under nanoconfinement. To help advance ideas on water film-mediated CO2 capture, we tracked the growth of amorphous magnesium carbonate (AMC) on MgO nanocubes exposed to moist CO2 gas. AMC was identified by its characteristic vibrational spectral signature and by its lack of long-range structure by X-ray diffraction. We find that AMC (MgCO3·2.3-2.5H2O) grew in sub-monolayer (ML) to 4 ML thick water films, with formation rates and yields scaling with humidity. AMC growth was however slowed down as AMC nanocoatings blocked water films access to the reactive MgO core. Films could however be partially dissolved by exposure to thicker water films, driving AMC growth for several more hours until nanocoatings blocked the reactions again. These findings shed new light on a potentially important bottleneck for the efficient mineralization of CO2 using MgO-bearing products. Notably, this study shows how variations in the air humidity affect CO2 capture by controlling water film coverages on reactive minerals. This process is also of great interest in the study of mineral growth in nanometrically thick water films.

Identification of Nano-Metal Oxides That Can Be Synthesized by Precipitation-Calcination Method Reacting Their Chloride Solutions with NaOH Solution and Their Application for Carbon Dioxide Capture from Air-A Thermodynamic Analysis

Several metal oxide nanoparticles (NPs) were already obtained by mixing NaOH solution with chloride solution of the corresponding metal to form metal hydroxide or oxide precipitates and wash-dry-calcine the latter. However, the complete list of metal oxide NPs is missing with which this technology works well. The aim of this study was to fill this knowledge gap and to provide a full list of possible metals for which this technology probably works well. Our methodology was chemical thermodynamics, analyzing solubilities of metal chlorides, metal oxides and metal hydroxides in water and also standard molar Gibbs energy changes accompanying the following: (i) the reaction between metal chlorides and NaOH; (ii) the dissociation reaction of metal hydroxides into metal oxide and water vapor and (iii) the reaction between metal oxides and gaseous carbon dioxide to form metal carbonates. The major result of this paper is that the following metal-oxide NPs can be produced by the above technology from the corresponding metal chlorides: Al₂O₃, BeO, CaO, CdO, CoO, CuO, FeO, Fe₂O₃, In₂O₃, La₂O₃, MgO, MnO, Nd₂O₃, NiO, Pr₂O₃, Sb₂O₃, Sm₂O₃, SnO, Y₂O₃ and ZnO. From the analysis of the literature, the following nine nano-oxides have been already obtained experimentally with this technology: CaO, CdO, Co₃O₄, CuO, Fe₂O₃, NiO, MgO, SnO₂ and ZnO (note: Co₃O₄ and SnO₂ were obtained under oxidizing conditions during calcination in air). Thus, it is predicted here that the following nano-oxides can be potentially synthesized with this technology in the future: Al₂O₃, BeO, In₂O₃, La₂O₃, MnO, Nd₂O₃, Pr₂O₃, Sb₂O₃, Sm₂O₃ and Y₂O₃. The secondary result is that among the above 20 nano-oxides, the following five nano-oxides are able to capture carbon dioxide from air at least down to 42 ppm residual CO₂-content, i.e., decreasing the current level of 420 ppm of CO₂ in the Earth's atmosphere at least tenfold: CaO, MnO, MgO, CdO, CoO. The tertiary result is that by mixing the AuCl₃ solution with NaOH solution, Au nano-particles will precipitate without forming Au-oxide NPs. The results are significant for the synthesis of metal nano-oxide particles and for capturing carbon dioxide from air.

Structural modification of salt-promoted MgO sorbents for intermediate temperature CO2 capture

MgO-based sorbents are a promising option for CO2 capture at intermediate temperatures. MgO-based sorbents are often hybridized with alkali metal salts to promote the CO2 capture performance. However, MgO-based sorbents often suffer a rapid decrement of CO2 capture performance during multicycle carbonation-calcination reactions due to the reduction of active sites. In this study, we attempted to enhance the durability of MgO-based sorbents by modifying their morphology. A tubular-shaped MgO-based sorbent was synthesized using a carbon nanotube template. Various characterization experiments and evaluations were performed with the synthesized MgO-based materials. The MgO sample with modified structure exhibited a specific morphology consisting of elongated plate-like structures separated by empty spaces. This separation is expected to prevent MgO agglomeration and preserve the modified morphology during iterative CO2 capture reactions. The MgO with modified structure achieved higher cycling stability with four times slower performance decay than the control MgO, even though identical chemical compositions were applied.

Hydrogen Production with In Situ CO2 Capture at High and Medium Temperatures Using Solid Sorbents

Hydrogen is a versatile vector for heat and power, mobility, and stationary applications. Steam methane reforming and coal gasification have been, until now, the main technologies for H2 production, and in the shorter term may remain due to the current costs of green H2. To minimize the carbon footprint of these technologies, the capture of CO2 emitted is a priority. The in situ capture of CO2 during the reforming and gasification processes, or even during the syngas upgrade by water–gas shift (WGS) reaction, is especially profitable since it contributes to an additional production of H2. This includes biomass gasification processes, where CO2 capture can also contribute to negative emissions. In the sorption-enhanced processes, the WGS reaction and the CO2 capture occur simultaneously, the selection of suitable CO2 sorbents, i.e., with high activity and stability, being a crucial aspect for their success. This review identifies and describes the solid sorbents with more potential for in situ CO2 capture at high and medium temperatures, i.e., Ca- or alkali-based sorbents, and Mg-based sorbents, respectively. The effects of temperature, steam and pressure on sorbents’ performance and H2 production during the sorption-enhanced processes are discussed, as well as the influence of catalyst–sorbent arrangement, i.e., hybrid/mixed or sequential configuration.

Influence of Magnesium Oxide on Carbonation of Cement Paste Containing Limestone and Metakaolin

One of the major durability concerns for limestone calcined clay cement (LC3) concrete is its high susceptibility to atmospheric carbonation that could lead to an early onset of electrochemical corrosion of reinforcing steel in concrete structures. Aimed at designing innovative LC3 formulations with potentially enhanced carbonation resistance, this preliminary study investigates the influence of reactive magnesia (MgO) on the early-age strength development, hydrates assemblage, and atmospheric carbonation resistance of ternary ordinary Portland cement-metakaolin-limestone blends with a constant 45% ordinary Portland cement (OPC) replacement level. The results show that the MgO addition impedes the formation of AFm phases (hemicarbonate and monocarbonate), likely through interfering reactions between metakaolin and portlandite. The formed brucite due to MgO hydration can uptake atmospheric CO2 to some extent, but at a considerably slower rate, in comparison with other hydrates in LC3 including AFm, AFt, and portlandite. The enhancement of carbonation resistance of LC3 pastes is insignificant by MgO addition of less than 5%.

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO₂ emissions. Solid materials capable of reversibly absorbing CO₂ have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO₂ sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO₂ absorption both at surfaces and within solid materials.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

Molten Salt-Promoted MgO Adsorbents for CO2 Capture: Transient Kinetic Studies

Optimization of MgO adsorbents is predominantly focused on the regulation of appropriate adsorption sites for CO₂ associated with Mg²⁺-O^2- sites of low coordination. Here, for the first time, we conducted transient kinetic experiments to identify and characterize changes of the CO₂ molecular path in MgO-based CO₂ adsorbents upon the addition of molten salt modifiers. Among the optimized samples, addition of 10 mol % NaNO₂ on the surface of MgO exhibited the highest CO₂ uptake (15.7 mmol g^-1) at 350 °C compared to less than 0.1 mmol g^-1 for the unpromoted MgO. Kinetic modeling showed that the interaction of molten salt-promoted MgO with CO₂ at 300 °C involves three different processes, namely, fast surface adsorption associated with surface-active basic sites, chemical reaction associated with MgCO₃ formation, and a slow diffusion step being the rate-limiting step of the carbonation process. Furthermore, transient kinetic studies coupled with mass spectrometry under low CO₂ partial pressure agreed well with the kinetic simulation results based on TGA measurements, demonstrating an in-depth understanding of the CO₂-capturing performance gained and its considerable significance for future practical designs of precombustion CO₂ capture.

A review on ex situ mineral carbonation

The increased CO₂ quantities in the environment have led to many harmful effects. Therefore, it is very important to decrease the CO₂ levels in the environment. CO₂ capture along with safe and permanent storage using mineral CO₂ sequestration method can play an important role to reduce carbon emissions into the environment. Mineral sequestration is a stable storage method that provides long-term storage and an appropriate substitute for the more popular geological storage method. The process is most suited for places where there is a lack of underground cavities for underground geological storage. Minerals rich in Ca and Mg are used predominantly in carbonation reactions. In addition, those alkaline wastes that are rich in Mg and Ca such as cement waste, steel slag and many process ashes can also be employed in CO₂ sequestration. Mineral carbonation could be used for the sequestration of billions of tonnes of CO₂ every year. However, various drawbacks related to mineral carbonation still need to be addressed, such as resolving the slow rate of reactions, necessity of large amounts of feedstock, decreasing the high overall cost of CO₂ sequestration and reducing the huge energy requirements to accelerate the carbonation reaction. This study explores a number of carbonation methods, parameters that control the process and future potential applications of carbonated products.

An Experimental Study of the Decomposition and Carbonation of Magnesium Carbonate for Medium Temperature Thermochemical Energy Storage

To improve the energy efficiency of an industrial process thermochemical energy storage (TCES) can be used to store excess or typically wasted thermal energy for utilisation later. Magnesium carbonate (MgCO3) has a turning temperature of 396 °C, a theoretical potential to store 1387 J/g and is low cost (~GBP 400/1000 kg). Research studies that assess MgCO3 for use as a medium temperature TCES material are lacking, and, given its theoretical potential, research to address this is required. Decomposition (charging) tests and carbonation (discharging) tests at a range of different temperatures and pressures, with selected different gases used during the decomposition tests, were conducted to gain a better understanding of the real potential of MgCO3 for medium temperature TCES. The thermal decomposition (charging) of MgCO3 has been investigated using thermal analysis techniques including simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), TGA with attached residual gas analyser (RGA) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (up to 650 °C). TGA, DSC and RGA data have been used to quantify the thermal decomposition enthalpy from each MgCO3.xH2O thermal decomposition step and separate the enthalpy from CO2 decomposition and H2O decomposition. Thermal analysis experiments were conducted at different temperatures and pressures (up to 40 bar) in a CO2 atmosphere to investigate the carbonation (discharging) and reversibility of the decarbonation–carbonation reactions for MgCO3. Experimental results have shown that MgCO3.xH2O has a three-step thermal decomposition, with a total decomposition enthalpy of ~1050 J/g under a nitrogen atmosphere. After normalisation the decomposition enthalpy due to CO2 loss equates to 1030–1054 J/g. A CO2 atmosphere is shown to change the thermal decomposition (charging) of MgCO3.xH2O, requiring a higher final temperature of ~630 °C to complete the decarbonation. The charging input power of MgCO3.xH2O was shown to vary from 4 to 8136 W/kg with different isothermal temperatures. The carbonation (discharging) of MgO was found to be problematic at pressures up to 40 bar in a pure CO2 atmosphere. The experimental results presented show MgCO3 has some characteristics that make it a candidate for thermochemical energy storage (high energy storage potential) and other characteristics that are problematic for its use (slow discharge) under the experimental test conditions. This study provides a comprehensive foundation for future research assessing the feasibility of using MgCO3 as a medium temperature TCES material. Future research to determine conditions that improve the carbonation (discharging) process of MgO is required.

Atmospheric Carbon Capture Performance of Legacy Iron and Steel Waste

Legacy iron (Fe) and steel wastes have been identified as a significant source of silicate minerals, which can undergo carbonation reactions and thus sequester carbon dioxide (CO₂). In reactor experiments, i.e., at elevated temperatures, pressures, or CO₂ concentrations, these wastes have high silicate to carbonate conversion rates. However, what is less understood is whether a more "passive" approach to carbonation can work, i.e., whether a traditional slag emplacement method (heaped and then buried) promotes or hinders CO₂ sequestration. In this paper, the results of characterization of material retrieved from a first of its kind drilling program on a historical blast furnace slag heap at Consett, U.K., are reported. The mineralogy of the slag material was near uniform, consisting mainly of melilite group minerals with only minor amounts of carbonate minerals detected. Further analysis established that total carbon levels were on average only 0.4% while average calcium (Ca) levels exceeded 30%. It was calculated that only ∼3% of the CO₂ sequestration potential of the >30 Mt slag heap has been utilized. It is suggested that limited water and gas interaction and the mineralogy and particle size of the slag are the main factors that have hindered carbonation reactions in the slag heap.

The Indirect Mineral Carbonation of Electric Arc Furnace Slag Under Microwave Irradiation

The indirect mineral carbonation of industrial residues is one of the potential technologies for CO₂ sequestration. In this paper, the leaching and carbonation of electric arc furnace (EAF) slag under microwave irradiation was investigated. The experimental results show that the main reactive calcium-containing phase in the EAF slag carbonation process is calcium silicate, and the final leaching ratio of larger particles is lower than that of smaller particles due to the silica layer produced on the surface of the calcium silicate. The Drozdov equation with a self-impeding coefficient can describe EAF slag leaching kinetics under microwave irradiation. The explosive homogeneous nucleation phenomenon under microwave irradiation contributes to the thinning and narrowing of crystals. Microwave irradiation can inhibit the crystaltype transformation of vaterite.

Factors Influencing CO2 and Energy Penalties of CO2 Mineralization Processes

Carbon mineralization is one of the carbon capture utilization, and storage (CCUS) technologies that can be used to capture large quantities of CO₂ and convert it into stable carbonate products that can be stored easily. Several CO₂ mineralization processes have been proposed; however, there are no commercial-scale projects because there are still significant issues that need to be improved before commercialization can take place. In this work, we evaluate the CO₂ and energy penalties related to the most well-known types of mineralization processes developed to date, in which the mineralization reaction takes place directly under aqueous conditions, high pressures and temperatures, and compared these with newer T-P swing processes and ball-mill reactor processes, which are under development. The data used in the evaluation are taken from published literature. By comparing the three processes, we identify important variables that contribute to high CO₂ and energy penalties so that future research can focus on optimization of these variables. It is observed that slurry concentration (heating) and particle size (grinding) are critical factors, with mineral calcination and operating pressure constituting other important factors.

Experimental Investigation and Simplistic Geochemical Modeling of CO₂ Mineral Carbonation Using the Mount Tawai Peridotite

In this work, the potential of CO₂ mineral carbonation of brucite (Mg(OH)₂) derived from the Mount Tawai peridotite (forsterite based (Mg)₂SiO₄) to produce thermodynamically stable magnesium carbonate (MgCO₃) was evaluated. The effect of three main factors (reaction temperature, particle size, and water vapor) were investigated in a sequence of experiments consisting of aqueous acid leaching, evaporation to dryness of the slurry mass, and then gas-solid carbonation under pressurized CO₂. The maximum amount of Mg converted to MgCO₃ is ~99%, which occurred at temperatures between 150 and 175 °C. It was also found that the reduction of particle size range from >200 to <75 µm enhanced the leaching rate significantly. In addition, the results showed the essential role of water vapor in promoting effective carbonation. By increasing water vapor concentration from 5 to 10 vol %, the mineral carbonation rate increased by 30%. This work has also numerically modeled the process by which CO₂ gas may be sequestered, by reaction with forsterite in the presence of moisture. In both experimental analysis and geochemical modeling, the results showed that the reaction is favored and of high yield; going almost to completion (within about one year) with the bulk of the carbon partitioning into magnesite and that very little remains in solution.

CO2 chemisorption in Li2CuO2 microstructurally modified by ball milling: study performed with different physicochemical CO2 capture conditions

Lithium cuprate (Li₂CuO₂) was obtained by a solid state reaction and a subsequent ball milling process. Then, the samples were characterized (structurally and microstructurally) and evaluated as CO₂ captors.

Structural and CO2capture analyses of the Li1+xFeO2(0 ≤ x ≤ 0.3) system: effect of different physicochemical conditions

α-Li_1+xFeO₂compounds have been synthesized by nitrate decomposition at low temperature. Their CO₂capture were evaluated in CO₂and CO₂+ steam atmospheres. The amount captured in CO₂+ steam atmosphere was 24 wt%, also magnetite was formed.

Low-temperature gas–solid carbonation of magnesia and magnesium hydroxide promoted by non-immersive contact with water

From high-pressure gas–solid thermogravimetry, the presence of water at high relative humidity (>25% RH) caused a drastic acceleration in the rate of CO₂ absorption into MgO and Mg(OH)₂ producing magnesite and hydrocarbonate precursors below 200 °C.

Direct gas-solid carbonation of serpentinite residues in the absence and presence of water vapor: a feasibility study for carbon dioxide sequestration

Mineral carbonation of serpentinite mining residue offers an environmentally secure and permanent storage of carbon dioxide. The strategy of using readily available mining residue for the direct treatment of flue gas could improve the energy demand and economics of CO2 sequestration by avoiding the mineral extraction and separate CO2 capture steps. The present is a laboratory scale study to assess the possibility of CO2 fixation in serpentinite mining residues via direct gas-solid reaction. The degree of carbonation is measured both in the absence and presence of water vapor in a batch reactor. The gas used is a simulated gas mixture reproducing an average cement flue gas CO2 composition of 18 vol.% CO2. The reaction parameters considered are temperature, total gas pressure, time, and concentration of water vapor. In the absence of water vapor, the gas-solid carbonation of serpentinite mining residues is negligible, but the residues removed CO2 from the feed gas possibly due to reversible adsorption. The presence of small amount of water vapor enhances the gas-solid carbonation, but the measured rates are too low for practical application. The maximum CO2 fixation obtained is 0.07 g CO2 when reacting 1 g of residue at 200 °C and 25 barg (pCO2 ≈ 4.7) in a gas mixture containing 18 vol.% CO2 and 10 vol.% water vapor in 1 h. The fixation is likely surface limited and restricted due to poor gas-solid interaction. It was identified that both the relative humidity and carbon dioxide-water vapor ratio have a role in CO2 fixation regardless of the percentage of water vapor.

Water steam effect during high CO2 chemisorption in lithium cuprate (Li2CuO2) at moderate temperatures: experimental and theoretical evidence

Li₂CuO₂ is able to chemisorb high quantities of CO₂ in the presence of water steam at low temperatures.

A review of mineral carbonation technologies to sequester CO2

Mineral carbonation is a promising and at the same time challenging option for the sequestration of anthropogenic CO₂.

Synthesis, electron microscopy and X-ray characterization of oxymagnesite, MgO·2MgCO3, formed from amorphous magnesium carbonate

The first electron microscopy images of the peculiar oxymagnesite crystal are presented along with the synthesis description and crystallographic data.