Molten Alkali Carbonates Research Articles

Alkali-carbonate melts in the cratonic mantle evidenced by a wehrlite xenolith from the Majuagaa kimberlite, West Greenland

Carbonate melts are critically important for the deep carbon cycle, mantle melting, redox reactions, and transport of highly incompatible elements. The presence of carbonate melts in the cratonic mantle has been inferred from experimental studies, metasomatic transformations, and melt/fluid inclusions in xenoliths, kimberlites, and diamonds. However, the exact composition of such melts is difficult to determine due to their ephemeral nature and highly reactive properties. Once formed, they migrate away from the source and react with silicate mantle minerals, especially orthopyroxene, causing mantle metasomatism. Wehrlite is one of the products of interaction between the carbonate melt and peridotitic mantle and hence is an excellent candidate for locating in situ carbonate melts. Here, we report petrological, geochemical, and melt inclusion data for a garnet wehrlite xenolith in the Majuagaa kimberlite dike, West Greenland. The xenolith, which last equilibrated with the mantle at 4.5 GPa and 1000 °C, contains abundant melt pools composed of dolomite, calcite, serpentine, spinel, apatite, and phlogopite. Although the original magmatic mineralogy was largely destroyed by low-temperature alteration, remnants of the crystallized carbonatitic melt are preserved as primary melt inclusions in the liquidus Ti-Mg-Fe spinel. These melt inclusions, composed of carbonates, alkali carbonates, periclase/brucite, and minor halides, K-sulfide, apatite, and phlogopite, are the first direct evidence for in situ alkali-carbonate melt in the deep cratonic mantle. Compositionally, they are very similar to primary Na-dolomite melt found in experiments and in fluid inclusions within diamonds.

Sodium Manganese Ferrite Water Splitting Cycle: Unravelling the Effect of Solid-Liquid Interfaces in Molten Alkali Carbonates.

In this work, the Na2CO3 of the sodium manganese ferrite thermochemical cycle was substituted by different eutectic or eutectoid alkali carbonate mixtures. Substituting Na2CO3 with the eutectoid (Li0.07Na0.93)2CO3 mixture resulted in faster hydrogen production after the first cycle, shifting the hydrogen production maximum toward shorter reaction times. Thermodynamic calculations and in situ optical microscopy attributed this fact to the partial melting of the eutectoid carbonate, which helps the diffusion of the ions. Unfortunately, all the mixtures exhibit a significant loss of reversibility in terms of hydrogen production upon cycling. Among them, the nonsubstituted Na mixture exhibits the highest reversibility in terms of hydrogen production followed by the 7%Li-Na mixture, while the 50%Li-Na and Li-K-Na mixtures do not produce any hydrogen after the first cycle. The loss of reversibility is attributed to both the formation of undesired phases and sintering, the latter being more pronounced in the eutectic and eutectoid alkali carbonate mixtures, where the melting of the carbonate is predicted by thermodynamics.

Study of Speciation and Transport Properties for Different Compositions of Carbonates in Li2CO3–Na2CO3 and Li2CO3–K2CO3 Binary Systems at High Temperature in Molten State

Molten alkali carbonates play a central role in carbon-capture technologies. To characterize the structural and transport properties, we used NMR, pulse field gradient (PFG) NMR, electrical impedancemetry, and molecular dynamics simulation. Different compositions in Li2CO3–Na2CO3 and Li2CO3–K2CO3 binary systems at 1003 K have been studied. The NMR measurements confirmed only the presence of anionic species CO32– whatever the composition. Some structural evolution of K+ and Li+ cations around the oxygen was detected in the composition Li2CO3–K2CO3. With the addition of K2CO3 in the carbonate mixture, the interaction between the charge carriers present in the liquid increases and results in a decrease of electrical conductivity.

Speciation and Transport Properties in Molten Alkali Carbonates at High Temperature

Speciation and Transport Properties in Molten Alkali Carbonates at High Temperature

Raman study of quench products of alkaline carbonate melt at 3 and 6 GPa: Link to the pressure of origin

AbstractInclusions of former alkaline carbonate melts occur in igneous minerals, mantle xenoliths, and diamonds from kimberlites and lamproites. However, the identification of most daughter phases remains undetermined owing to the lack of their Raman spectra. Yet, the phase compositions of the alkali carbonate melt at different pressure are interesting to know because Na‐Ca, Na‐Mg, K‐Ca, and K‐Mg double carbonates have a number of pressure‐induced phase transitions. Here, we studied the quench products of K‐Ca, K‐Mg, and Na‐Ca carbonate melts obtained at 3 and 6 GPa. Scanning electron microscopy coupled with energy‐dispersive spectroscopy with silicon drift detector (EDS‐SDD) and Raman spectroscopy were employed for phase characterization. The following carbonates were detected among the quench products: γ‐Na2CO3, Na4Ca(CO3)3 (Ia3d), Na2Ca3(CO3)3 (P1n1), K2Ca(CO3)2 ( ) bütschliite, disordered K2Ca3(CO3)4 (Pnam), K2Mg(CO3)2 ( ), aragonite, magnesite at 6 GPa; and CaCO3‐bearing K2CO3, K2Ca(CO3)2 ( ) bütschliite, K2Ca2(CO3)3 (R3), and ordered K2Ca3(CO3)4 (P212121) at 3 GPa. The results indicate that the quench products of carbonate melt consist of carbonates thermodynamically stable at the pressure of quenching. Thus, the daughter phases in carbonatitic inclusions could be identified by obtained Raman spectra and reflect the pressure conditions of their entrapment.

Evaporated Alkali Carbonate Effect on an Aluminum Diffusion Coated 253MA Vessel after 4000 h Discontinuous Operation—Lessons Learned

A vessel of a laboratory setup for hosting molten alkali carbonate immersion experiments was recently decommissioned after about 4000 h discontinuous operation at 800 °C. In this article, we want to describe the long-term damage of the vessel caused by evaporated alkali carbonate species in a carbon dioxide gas environment. The vessel is made of alloy 253MA and coated by an aluminum powder pack diffusion process. The degree of material loss and internal degradation did not correlate with the temperature profile across the vessel but rather with the vicinity to the gas and salt evaporation source. One millimeter of the vessel’s initial 5 mm wall thickness was completely consumed at the strongest attacked location plus another 800 µm of internal attack beneath the metal–oxide interface.

Non-Arrhenian Temperature-Dependent Viscosity of Alkali(ne) Carbonate Melts at Mantle Pressures

The viscosity of carbonate melts is a fundamental parameter to constrain their migration and ascent rates through the mantle and ultimately, their role as carbon conveyors within the deep carbon cycle. Yet, data on the viscosity of carbonate melts have remained scarce due to experimental limitations and the lack of appropriate theoretical descriptions for molten carbonates. Here, we report the viscosity of K2Mg(CO3)2 and K2Ca(CO3)2 melts up to 13 GPa and 2,000 K by means of classical molecular dynamics (MD) simulations using optimized force fields and provide first evidence for non-Arrhenian temperature-dependent viscosity of molten carbonates at mantle pressures. The viscosity of K2Mg(CO3)2 and K2Ca(CO3)2 melts ranges respectively between 0.0056–0.0875 Pa s and 0.0046–0.0650 Pa s in the investigated pressure-temperature interval. Alkali(ne) carbonate melts, i.e. mixed alkali and alkaline earth carbonate melts -K2Mg(CO3)2 and K2Ca(CO3)2− display higher viscosity than alkaline earth carbonate melts -CaCO3 and MgCO3− at similar conditions, possibly reflecting the change in charge distribution upon addition of potassium. The non-Arrhenian temperature-dependence of the viscosity is accurately described by the Vogel-Fulcher-Tammann model with activation energies Ea for viscous flow that decrease with temperature at all investigated pressures, e.g. from ∼100 kJ/mol to ∼30 kJ/mol between 1,300 and 2,000 K at 3 GPa. Pressure is found to have a much more moderate effect on the viscosity of alkali(ne) carbonate melts, with activation volumes Va that decrease from 4.5 to 1.9 cm3/mol between 1,300 and 2,000 K. The non-Arrhenian temperature-viscosity relationship reported here could be exhibited by other carbonate melt compositions as observed for a broad range of silicate melt compositions and it should be thus considered when modeling the mobility of carbonate melts in the upper mantle.

Relics of Deep Alkali–Carbonate Melt in the Mantle Xenolith from the Komsomolskaya–Magnitnaya Kimberlite Pipe (Upper Muna Field, Yakutia)

Relics of Deep Alkali–Carbonate Melt in the Mantle Xenolith from the Komsomolskaya–Magnitnaya Kimberlite Pipe (Upper Muna Field, Yakutia)

Differentiation in corrosion performance of alumina forming alloys in alkali carbonate melts

Alkali carbonate melts are promising high temperature thermal storage media. In this work five alumina forming alloys have been exposed to a ternary LiNaK carbonate melt and CO2 at 800 °C. The corrosion propagation was found to depend on the formation of a slow-growing LiAlO2 scale. Furthermore, the two polymorphs contributing to the LiAlO2 phase were monitored for up to 1000 h: a dense α-LiAlO2 scale and γ-LiAlO2 crystallites. We suggest a growth stress assisted formation of α-LiAlO2 relaxing into the outwards growing γ-LiAlO2 phase. This implies a deceleration of the α-LiAlO2 scale growth towards a steady state-thickness.

Mineralogy and Geochemistry of Ocelli in the Damtjernite Dykes and Sills, Chadobets Uplift, Siberian Craton: Evidence of the Fluid–Lamprophyric Magma Interaction

The study reports petrography, mineralogy and carbonate geochemistry and stable isotopy of various types of ocelli (silicate-carbonate globules) observed in the lamprophyres from the Chadobets Uplift, southwestern Siberian craton. The Chadobets lamprophyres are related to the REE-bearing Chuktukon carbonatites. On the basis of their morphology, mineralogy and relation with the surrounding groundmass, we distinguish three types of ocelli: carbonate-silicate, containing carbonate, scapolite, sodalite, potassium feldspar, albite, apatite and minor quartz ocelli (K-Na-CSO); carbonate–silicate ocelli, containing natrolite and sodalite (Na-CSO); and silicate-carbonate, containing potassium feldspar and phlogopite (K-SCO). The K-Na-CSO present in the most evolved damtjernite with irregular and polygonal patches was distributed within the groundmass; the patches consist of minerals identical to minerals in ocelli. Carbonate in the K-Na-CSO are calcite, Fe-dolomite and ankerite with high Sr concentration and igneous-type REE patterns. The Na-CSO present in Na-rich damtjernite with geochemical signature indicates the loss of the carbonate component. Carbonate phases are calcite and Fe-dolomite, and they depleted in LREE. The K-SCO was present in the K-rich least-evolved damtjernite. Calcite in the K-SCO has the highest Ba and the lowest Sr concentration and U-shaped REE pattern. The textural, mineralogical and geochemical features of the ocelli and their host rock can be interpreted as follows: (i) the K-Na-CSO are droplets of an alkali–carbonate melt that separated from residual alkali and carbonate-rich melt in highly evolved damtjernite; (ii) the Na-CSO are droplets of late magmatic fluid that once exsolved from a melt and then began to dissolve; (iii) the K-SCO are bubbles of K-P-CO2 fluid liberated from an almost-crystallised magma during the magmatic–hydrothermal stage. The geochemical signature of the K-SCO carbonate shows that the late fluid could leach REE from the host lamprophyre and provide for REE mobility.

Corrosion of construction materials of separator in molten carbonates of alkali metals

The mitigation of steels and alloys corrosion is one of the great challenges for application of high-temperature devices in contact with molten salts. The current research estimates corrosion resistance of the following construction materials: Crofer APU 22, austenitic steel 12Cr18Ni10Ti and two types of austenitic nickel-chromium alloy: CrNi78Ti, CrNi78Ti + 5% wt. W. The measurements are performed by exposition of samples within 24 h in the molten mixture of carbonates of lithium and potassium at 650 °C. The rates and mechanism of materials corrosion under these conditions are determined. The process occurring on the surface while contact of the materials and steels with carbonate melt are investigated.Accurate quantitative data on materials interaction with alkali carbonate melt have been obtained by gravimetrical and physico-chemical methods. The high-temperature corrosion of Fe and Ni austenitic materials has turned dramatically different from both their low-temperature corrosion and each other. There are mixed passivating oxide films on the surface of the only iron-based materials.In addition, the resulting layers of corrosion products are well bonded to the substrate and have protective properties and can be used as protective coatings in molten salts of other compositions, including alkali metal fluorides.

A brief review of CO2 utilization for alkali carbonate gasification and biomass/coal co-gasification: Reactivity, products and process

Carbon gasification in molten alkali carbonate salts over CO2 is a process to produce gaseous fuels with mitigating the greenhouse effect. Alkali carbonate salts can facilitate Boudouard reaction, which is difficult to occur even at high reaction temperature if the catalyst is absent. Alkali carbonate allows easily carbon−CO2 contact by changing the reaction interface from gas-solid to gas-liquid. This review focused on CO2 utilization for alkali carbonate gasification and biomass/coal co-gasification. CO2 gasification of biomass and coal with the alkali carbonate salts enhances the carbon conversion rate and CO yield. Furthermore, alkali carbonate salts gasification of biomass and coal could reduce reaction temperature, activation energy, and tar formation in the product gas. The factors that influence the carbon conversion rate were discussed, such as reaction temperature, CO2 concentration, alkali carbonate addition, and char feeding mode. On the other hand, blending coal with biomass is considered as potential fuel base for CO2 co-gasification and displays an advantage for improving the coal char reactivity. The alkali and alkaline earth metals (AAEMs) present in biomass ash acted as a natural catalyst to increase the coal reactivity and CO production. The catalytic effect in the co-gasification enhances by raising the percentage of biomass in the blend. In contrast, the inhibiting effect in co-gasification could be attributed to silica and alumina reactions with AAEMs to form aluminosilicates complexes.

Transport and Interfacial Properties of Mixed Molten Carbonate/Hydroxide Electrolytes by Molecular Dynamics Simulations

We use molecular dynamics simulations based on a recently developed force field to obtain the viscosity, ionic conductivity, and liquid–vapor surface tension of molten alkali-metal carbonate–hydroxide mixtures over a range of cation and hydroxide compositions. Recent experimental and simulation studies have suggested that molten carbonates contain non-negligible amounts of hydroxide ions in the presence of water at low partial pressures of CO2. However, due to the high temperatures (≃600 °C or higher) required to melt pure alkali carbonates and their mixtures, there is a lack of experimental thermodynamic and transport data for these molten phases. Here, we deploy a recently parametrized force field for molten alkali carbonates and hydroxides [ J. Chem. Theory Comput. 2020, 16, 5736−5746, DOI: 10.1021/acs.jctc.0c00285] to simulate some physical properties using atomistic molecular dynamics. Our predictions show a consistent decrease in viscosity and an increase in ionic conductivity with increasing hydroxide fractions, whereas a higher Li+ mole fraction leads to an increase in both viscosity and ionic conductivity. The computed surface tension values exhibit an upward trend with higher asymmetry in cation and anion sizes. Structural analysis suggests that in the carbonate–hydroxide melts the smaller/harder ions, OH– and Li+, are more favored at the interface than the larger/softer ions, CO32– and K+/Na+. The current approach provides a systematic route to obtaining physicochemical properties of molten alkali carbonate and hydroxide mixtures over a large domain of chemical compositions and thus can steer future experimental research for molten carbonates and their applications.

In-Depth Experimental Study on Thermochemical Conversion of Furan in Molten Alkali Carbonates

Furan is a typical heterocyclic compound in pyrolytic tar and has a strong tendency of coke deposition in thermochemical conversion, leading to catalyst deactivation for tar upgrading. The present study proposed a promising method for furan cracking and reforming using molten alkali carbonates (Li2CO3–Na2CO3–K2CO3). The results showed that a little gas was generated in the conversion process and CO dominated the gas species. In comparison to the experiment under salt-free conditions, the decarbonylation of furan in molten salt conversion was strengthened and oxygen could be removed as H2O into the liquid products. Under the catalysis of alkali metal cations, the conversion of furan to aromatic compounds was promoted at each temperature. Moreover, excess CO2 enhanced the reverse water–gas reaction, resulting in the consumption of H2 and the increase of CO in gaseous products. In addition, coke produced in molten salts was of higher reactivity and could hardly affect the stability of the molten salt system. On the basis of the good thermal conversion behavior as well as anticoking performance of molten carbonates, these findings offer a new way for the proper reforming and upgrading of furan through molten salt thermal treatment.

Predicting chemical reaction equilibria in molten carbonate fuel cells via molecular simulations

AbstractIt has been recently suggested that hydroxide ions can be formed in the electrolyte of molten carbonate fuel cells when water vapor is present. The hydroxide can replace carbonate in transporting electrons across the electrolyte, thereby reducing the CO2 separation efficiency of the fuel cell although still producing electricity. In this work, we obtain the equilibrium concentration of hydroxide in five molten alkali carbonate salts from molecular simulations. The results reveal that there can be a substantial amount of hydroxide in the electrolyte at low partial pressures of CO2. In addition, we find that the equilibrium concentration of molecular water dissolved in the electrolyte is over two orders of magnitude higher than that of CO2. Increasing the size and polarizability (or in other words reducing the “hardness”) of the cations present in the electrolyte can reduce the hydroxide fraction, but at the cost of lowering ionic conductivity.

Assessment of bismuth oxide-based electrolytes for composite gas separation membranes

Oxide + salt composites can be used in CO2 and NOx separation membranes, where high oxide-ion conductivity is crucial to improve performance. Pursuing this goal, the stability of three different bismuth oxide-based electrolytes (Cu + V, Y and Yb-doped) against molten alkali carbonates (Li, Na, K) or nitrates (Na, K) was tested firing them in the 450–550 °C temperature range, and with endurance tests up to 100 h. A well-known ceria-based composite was used as reference (CGO - Ce0.9Gd0.1O1.95). Oxides and composites were studied by X-ray diffraction, scanning electron microscopy and impedance spectroscopy (in air, 140–650 °C temperature range). Bi2Cu0.10V0.90O5.35 easily reacts with molten salts. Bi0.75Y0.25O1.5 and Bi0.75Yb0.25O1.5 have higher stability against molten carbonates and complete stability against molten nitrates. The Y-doped oxide stability against the molten carbonates was enhanced changing the molten salt composition (Y2O3 additions) and using lower firing temperatures. Above all, composites based on Y or Yb-doped Bi2O3 with molten alkali nitrates showed impressive 6× or 3× higher electrical conductivity at 290 °C, in air (4.88 × 10−2 and 2.41 × 10−2 S cm−1, respectively) than CGO-based composites (7.72 × 10−3 S cm−1), qualifying as promising materials for NOx separation membranes.

Input on the Measurement and Comprehension of CO2 Solubility in Molten Alkali Carbonates in View of Its Valorization

As the capture and valorization of CO2 in molten carbonates is nowadays becoming a strategic energy topic, the need of accurate solubility measurements of such molecule is becoming compulsory. This study provides a new set of reliable CO2 solubility values, determined by a a carefully adapted manometric setup, in Li2CO3-K2CO3 (62:38 mol%), Li2CO3-Na2CO3 (52:48 mol%), Li2CO3-Na2CO3-K2CO3 (43.5:31.5:25.0 mol%) and Na2CO3-K2CO3 (56:44 mol%) carbonate eutectics in an overall range of temperatures from 450 to 850 °C. High solubility values are obtained for Li-K and Li-Na eutectics, respectively 1.7 × 10−1 and 1.1 × 10−1 mol atm−1 l−1 at 650 °C. It is shown that CO2 solubility increases with the temperature for most of the eutectic mixtures. Another important trend is that the Li content significantly increases the solubility.

Study on reaction mechanism of superior bamboo biochar catalyst production by molten alkali carbonates pyrolysis and its application for cellulose hydrolysis

Biochar has attracted wide interest due to its important role in the synthesis of functionalized carbon materials. Molten alkali carbonates (MC) pyrolysis as a novel mean was explored to obtain superior bamboo biochar compared with alkali impregnation pretreating bamboo powder associated with pyrolysis. The properties of biochar produced by MC pyrolysis were enhanced, suggesting an increase in BET surface area of 43.59 m2/g compared with non-pretreat biochar of 5.76 m2/g, and possessed preferable pore structure than the alkali impregnation one of 39.82 m2/g and 16.98 nm for catalyst support. Besides, pyrolysis temperature reinforced the porosity and the degree of carbonization as it rising, however, the optimized temperature for top-grade biochar materials was 450 °C. The MC weakened the hydrogen bonds network in bamboo powder acting a similar role just as alkaline pretreated, meanwhile neutralized the acid produced during pyrolysis process supported by the analysis of FTIR and TGA. The potential pathways of biomass pyrolysis with MC had also been proposed. In addition, the biochar obtained was utilized as a catalyst support of biochar-bearing sulfonic acid groups, which achieved the remarkable yields of TRS and glucose (52.8% and 43.5%) in hydrolysis of cellulose.

Structure and diffusion of molten alkali carbonate salts at the liquid-vacuum interface

The liquid-vacuum interface of molten alkali carbonate salts is studied with molecular dynamics simulations. Three salts comprised of LixNayKzCO3 near their respective eutectic concentrations are considered to understand the distribution of ions relative to a liquid-vacuum interface and their diffusivity. These simulations show that each of the cations accumulate at the interface preferentially compared to carbonate. The cation ordering is found to inversely correspond to cation radius, with K being the most likely occupant at the surface, followed by Na, Li, and then the anion. Similar to other studies, the carbonate is found to diffuse more slowly than the cations, but we do observe small differences in diffusion between compositions that present opportunities to optimize ion transport. These results hold consequences for our understanding of ion behavior in molten carbonate salts and the performance of devices employ these electrolytes.

Performance of calcium-added molten alkali carbonates for high-temperature desulfurization from pyrolysis gases

Co-pyrolysis of biomass with oxygen-lean polymer waste is a promising way to improve both quantity and quality of the derived oil. However, the release of sulfur contaminants causes serious adverse effects on the utilization of the derived oil. In order to promote the efficient and economic utilization of the derived products, the high-temperature desulfurization was conducted in this study using a ternary molten carbonates (Li2CO3–Na2CO3–K2CO3). And the effects of added Ca2+ or Cl−/SO42− on the desulfurization were estimated. The results demonstrated that the molten carbonates showed high efficiency in the removal of H2S/COS/SO2 at 500 °C. Alkali metals competitively reacted with H2S/COS to form various sulfides and sulfur species in the cooled products were probably determined by the crystallization processes. Besides, H2S removal was stimulated by adding CaCO3 and the formed CaS was precipitated from the molten salts. The addition of Cl−/SO42− changed the ion distribution characteristics of the molten salts and had slightly adverse effects on the removal of H2S. In contrast, SO2 removal was hardly affected while COS removal efficiency was decreased after adding Ca2+ or Cl−/SO42−. Anyway, the added Ca2+ favored the recovery of sulfur as well as the regeneration of spent molten carbonates.