Molten Media Research Articles

Developing methane pyrolysis as a viable alternative for hydrogen production

Methane pyrolysis using molten media can produce hydrogen with no greenhouse gas emissions.

Solar methane pyrolysis in a liquid metal bubble column reactor: Effect of medium type and gas injection configuration

Solar methane pyrolysis in different molten media and reactor configurations was experimented to improve hydrogen production. Pure Sn, Ni0.18Sn0.82, Cu0.45Bi0.55, and KCl melts were compared at three different temperatures (1030–1130–1230 °C), and no significant difference was observed except for KCl and only at 1230 °C (XCH4 = 72 % vs. 57 % for pure Sn). This enhanced performance was attributed to possible carbon dispersion in the salt, which probably modified the physical properties and enhanced hydrodynamics. A porous quartz sparger (downward injection) did not significantly enhance the bubbles hydrodynamics, mainly because bubbles were trapped and coalesced below the sintered disc. A custom-made sparger (lateral bubbling) did not either improve conversion due to non-uniform pores. A sparger with an upward injection should be preferred to generate small bubbles with longer residence time. When a solid bed of tungsten carbide particles was placed around the injector, overlaid by molten tin, the conversion was improved even at a relatively low temperature (XCH4 = 17 % at 1030 °C). The immersed bed likely behaved as a porous device and increased the gas-solid surface contact. Combining particle bed and liquid bubbling system is very promising for further optimization of methane pyrolysis in molten media. The carbon collected above molten metals showed mostly a sheet-like structure with significant metal contamination. In case of KCl, most carbon was entrained with the gas, while the remaining was mixed with KCl in the reactor. The density of KCl is close to that of carbon, which prevented a good separation.

Radiochemical and chemical characterization of fuel, salt, and deposit from the electrorefining of irradiated U-6 wt% Zr in hot cells

Abstract Metal fuels are considered as the promising candidates for future fast breeder reactors. Pyro-chemical reprocessing is the ideal method for reprocessing spent metallic fuels due to the inherent process advantages. Electrorefining run was demonstrated in a hot cell facility with irradiated U-6 wt% Zr alloy at 500 °C using LiCl–KCl eutectic melt. In order to understand the behavior of the actinides and various fission products during high-temperature electrolysis, various process streams, viz., irradiated metal alloy fuel, the eutectic salt, and the cathode deposit were analyzed for the uranium, plutonium, and other fission product contents. Various methods employed for characterizing the process streams and the behaviors of some of the fission products during the electrolysis process are highlighted. The major gamma emitting radionuclides present in the irradiated fuel were 106Ru, 125Sb, 134Cs, 137Cs, 144Ce, and 154Eu. During electrorefining, cesium, cerium and europium were oxidized and dissolved in the molten media, whereas ruthenium and antimony remained in the anode basket. A minor contamination of zirconium was found in the cathode deposit.

Molten Metals and Molten Carbonates in Solid Oxide Direct Carbon Fuel Cell Anode Chamber: Liquid Metal Anode and Hybrid Direct Carbon Fuel Cells

To enhance the contact between the electrolyte (source of O2−) and the carbon fuel in solid oxide–direct carbon fuel cells (SO-DCFCs), molten metals and molten salts were used in the anode chamber. Oxygen ions can dissolve and be transported in the molten medium to the anode three-phase boundary to reach and oxidize the carbon particles. To improve the sluggish kinetics of the electrochemical oxidation of carbon, the same molten media can act as redox mediators. Moreover, using a liquid metal/salt anode, tolerant to fuel impurities, the negative effect of carbon contaminants on cell performance is mitigated. In this work, an overview of SO-DCFCs with liquid metals, liquid carbonates, and mixed liquid metals/liquid carbonates in the anode chamber is presented and their performance was compared to that of conventional SO-DCFCs.

Experimental comparison of solar methane pyrolysis in gas-phase and molten-tin bubbling tubular reactors

This study experimentally compares solar methane pyrolysis in gas phase and molten tin for hydrogen and solid carbon production. Molten media are expected to facilitate carbon separation and to enhance solar-to-gas heat transfer. Effects of temperature (1000–1400 °C), gas feed flow rate (0.5–1.0 NL/min), and methane molar fraction in the feed (0.1–0.5) on methane decomposition were investigated in a tubular solar reactor. Methane conversion in molten tin was significantly lower than in gas phase at all temperatures (e.g., 64% vs. 92% at 1300 °C). It was justified by the negligible catalytic activity of tin, the small gas-liquid surface contact (bubble diameter ∼5 mm) and the reduced bubble-tin contact time, in comparison with gas-phase pyrolysis (e.g., 0.83 s vs. 0.5 s for 0.5 NL/min gas feed at 1300 °C). For both routes, a temperature increase (1000–1400 °C) improved methane conversion (gas phase: 2–98%, molten tin: 0–91%). Increasing gas feed flow rate (0.5–1.0 NL/min) decreased conversion (gas phase: 93-85%, molten tin: 64-48% at 1300 °C) by reducing space-time in gas phase (0.83–0.42 s) or by inducing bubbles coalescence in molten tin. Increasing methane molar fraction in the feed generally decreased methane conversion but energy efficiencies were enhanced. The carbon produced in gas-phase pyrolysis was carbon black powder (50–100 nm particle size) and carbon sheets in molten tin.

Methane Cracking for Hydrogen Production: A Review of Catalytic and Molten Media Pyrolysis

Currently, hydrogen is mainly generated by steam methane reforming, with significant CO2 emissions, thus exacerbating the greenhouse effect. This environmental concern promotes methane cracking, which represents one of the most promising alternatives for hydrogen production with theoretical zero CO/CO2 emissions. Methane cracking has been intensively investigated using metallic and carbonaceous catalysts. Recently, research has focused on methane pyrolysis in molten metals/salts to prevent both reactor coking and rapid catalyst deactivation frequently encountered in conventional pyrolysis. Another expected advantage is the heat transfer improvement due to the high heat capacity of molten media. Apart from the reaction itself that produces hydrogen and solid carbon, the energy source used in this endothermic process can also contribute to reducing environmental impacts. While most researchers used nonrenewable sources based on fossil fuel combustion or electrical heating, concentrated solar energy has not been thoroughly investigated, to date, for pyrolysis in molten media. However, it could be a promising innovative pathway to further improve hydrogen production sustainability from methane cracking. After recalling the basics of conventional catalytic methane cracking and the developed solar cracking reactors, this review delves into the most significant results of the state-of-the-art methane pyrolysis in melts (molten metals and salts) to show the advantages and the perspectives of this new path, as well as the carbon products’ characteristics and the main factors governing methane conversion.

Corrosion Resistance of Hastelloy G-35 Nickel-Based Superalloy in Various Aggressive Media

Nickel-based superalloys are widely used in various fields of engineering and technology in contact with aggressive liquid media. High-temperature molten salts are one of the types of such corrosive liquids. Molten electrolytes are widely used in technologies for the electrowinning and refining non-ferrous and rare metals, in nuclear industry and solar storage systems. In the present study the corrosion resistance of Hastelloy G-35 Аlloy (UNS N06035) nickel-based alloy was investigated. Corrosion properties of the material were examined in NaCl–KCl melt at 750 °C under inert (Ar) and oxidizing (air) atmosphere, and in acidic KCl–AlCl3 melts under an inert atmosphere (Ar). KCl–AlCl3 melts were chosen in addition to NaCl–KCl as having higher oxidation ability. Samples were kept in contact with the melts for 30 h. Additional corrosion tests were performed in a boiling sulfuric acid solution in the presence of iron (III) sulfate (according to the ASTM G28-02A procedure) and these lasted for 120 h. The corrosion rates of the alloy samples in various molten media were calculated and are given in Table 1. The slowest corrosion was observed in NaCl–KCl melt under an inert atmosphere. The corrosion rate in NaCl–KCl melt increased noticeably in the presence of oxygen in the atmosphere, and the corrosion resistance of the alloy under these conditions was low. The rate of the alloy corrosion in KCl–AlCl3 melt at 750 °C was extremely high, and this alloy cannot, therefore, be recommended for application as a construction material in this melt under the conditions studied. The data obtained were compared with results of corrosion tests performed according to the ASTM G-28-02A standard. Samples of Hastelloy G-35 alloy were examined in the initial state and after preliminary aging in KCl–AlCl3 at 650 °C for 200 hours (Table 2). The corrosion rate of the alloy in boiling sulfuric acid solution in the presence of iron (III) sulfate was comparable to that in the NaCl–KCl melt under an inert atmosphere. Preliminary aging in the KCl–AlCl3 melt led to intensification of the corrosion processes in aqueous solutions. Figure 1

Understanding the role of oxygen ion (O2−) activity in 1-D crystal growth of rutile TiO2 in molten salts

The role of oxygen ion (O2−) activity in molten media in the phase and morphology of the final nanocrystal.

ChemInform Abstract: Lithium‐Silicon Alloys: Phase Diagram, Electrochemical Studies, Thermodynamic Properties, Application in Chemical Power Cells

AbstractReview: 170 refs.

Generalized method for determining fluoroacidity by electrochemical diffusion coefficient measurement (application to HfF4)

A universal method for fluoroacidity evaluation was developed and based on a mass transport approach: it simply consists in measuring the diffusion coefficient of an electroactive species in various molten media. The reduction behaviour of Hf(IV) ions was investigated in molten fluorides and diffusion coefficients of Hf(IV) ions were measured. Results showed that diffusion coefficients decrease with fluoroacidity, due to the effect of solvent viscosity (which is linked to bridged fluorines). A global approach of mass transport in solution was then proposed, taking into account both solute and solvent. The Schmidt number (Sc) defined as the ratio between solvent viscosity and solute diffusivity was calculated in order to take into account these two parameters. Results showed that Sc increases with fluoroacidity, in a much more sensitive way than D. This universal method can extended to all electroactive species and to all bath fluoroacidity.

Comparison of Different Synthesis Methods for LiNi0.5Mn1.5O4 Regarding the Electrochemical Performance and Different Aging Mechanisms

Among the high-voltage cathode materials, LiNi0.5Mn1.5O4 is one of the most promising candidates for the use in commercial lithium-ion batteries. Apart from the dominant potential plateau at 4.7 V, the spinel exhibits a high structural stability. Furthermore, the partial substitution of manganese by nickel can improve the capacity retention during cycling compared to LiMn2O4.[1]However, the high operating voltage of up to 5 V can lead to continuous electrolyte decomposition and capacity fading during cycling. Over the last years, a large variety of methods has been reported to synthesize LiNi0.5Mn1.5O4. The majority of these synthesis methods can be divided into wet chemistry based methods which implies the formation of a precursor in a solution (e.g. Co-Precipitation, Sol-Gel method, Modified Pechini method etc.) and solid-state methods where the reactants are heat-treated directly after mixing (Combinational Annealing method, One-step solid-state method etc.). While the wet chemistry based methods are able to produce powders of high chemical homogeneity with a narrow size distribution at low annealing temperatures, the synthesis procedures are rather complex and expensive due to long processing times and expensive precursor materials. The solid-state methods are mostly inexpensive and simple. However, the high annealing temperatures can lead to impurities and inhomogeneity. Alternatively, the Molten Salt method based on salts with a low melting point is a simple technique that yields high purity powders due to high diffusion rates in the molten media. [2] The different synthesis methods yield LiNi0.5Mn1.5O4 particles varying in morphology and purity that strongly influence the electrochemical performance and the aging mechanisms. At high potentials the oxidative decomposition of the electrolyte leads to the formation of a passivation layer on the surface of the LiNi0.5Mn1.5O4 particles. With regard to the different particle morphologies it was reported that smooth surfaces lead to moderate surface reactions. [3] Another aging mechanism that has been studied by several research groups is the manganese dissolution into the electrolyte from LiMn2O4 particles during cycling. [4] The manganese dissolution can be significantly reduced due to the enhanced structural stability in presence of Ni2+. However, the transition-metal dissolution can still occur on the surface of the LiNi0.5Mn1.5O4particles due to natural local inhomogeneity of the crystal structure. In this study LiNi0.5Mn1.5O4 powders were synthesized by three different methods and investigated regarding their electrochemical performance and aging behavior. The synthesis methods, namely Co-precipitation, Molten Salt method and Combinational annealing method [5] yield LiNi0.5Mn1.5O4 particles with varying purity and morphology as shown in the scanning electron microscopy (SEM) image in Fig. 1. The difference in the electrochemical performance of the electrodes based on these powders originates from several factors. X-ray diffraction (XRD) measurements were performed to investigate the crystal structure and impurities which are key factors to achieve high specific capacities and good performances. The post-mortem analysis of the cycled electrodes showed a passivation layer on the surface of the LiNi0.5Mn1.5O4particles due to the decomposition of the electrolyte at high potentials. The thickness of the passivation layer increased with the cycle number for the electrodes based on the powders synthesized by Co-Precipitation and Combinational Annealing method. In contrast, the Molten Salt electrodes showed only minor amounts of decomposition products on the surface of the particles. X-ray photoelectron spectroscopy (XPS) measurements confirmed these assumptions. The electrolyte analysis via gas chromatography mass spectrometry (GC-MS) revealed an increased concentration of decomposition products in the aged electrolytes. Furthermore the transition-metal dissolution was investigated by inductively coupled plasma optical emission spectrometry (ICP-OES). The results indicate increasing manganese and nickel dissolution and therefore a higher transition-metal concentration in the electrolyte of the aged cells with increasing cycle number. [1] J. Vetter, P Novák, M. R. Wagner, C. Veit, K.-C. Möller, J. O. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler, A. Hammouche J. Power Sources 2005, 147, 269. [2] J.-H. Kim, S.-T. Myung, Y.-K. Sun Electrochim. Acta 2004, 49, 219. [3] R. Marom, S. F. Amalraj, N. Leifer, D. Jacob, D. Aurbach J. Mater. Chem. 2011, 21, 9938. [4] R. J. Gummow, A. de Kock, M. M. Thackeray Solid State Ionics 1994, 69, 59. [5] H. Fang, Z. Wang, B. Zhang, X. Li, G. Li Electrochem. Commun. 2007, 9, 1077. Figure 1

Lithium-silicon alloys: Phase diagram, electrochemical studies, thermodynamic properties, application in chemical power cells

Review summarizes and analyzes data on the phase diagram, electrochemical behavior in molten media, and thermodynamic properties of alloys in the lithium-silicon system in the solid and liquid states. The use of Li-Si alloys in medium-temperature chemical power cells and prospects for application of silicon and silicon-containing components as the anode material in lithium-ion batteries are discussed.

Aerosol-Assisted Synthesis of Shape-Controlled CoFe2O4: Topotactic versus Direct Melt Crystallization

Compositionally complex materials can be prepared as nanocrystals with defined shapes by aerosol-assisted molten salt synthesis (AMSS), where the aerosol phase spatially and temporally confines crystal growth. Here, single-crystalline CoFe2O4 nanoplates and CoFe2O4 octahedra are achieved selectively by AMSS through the use of Na2CO3 and Na2MoO4 fluxes, respectively. The formation of CoFe2O4 nanoplates is generally unexpected given the cubic crystal symmetry of spinel ferrites, but this shape is accounted for by in situ formation of a layered double hydroxide (LDH) intermediate stabilized by the CO32– anions of the flux. Topotactic conversion of the LDH intermediate to CoFe2O4 preserves the plate morphology of the LDH crystals. In contrast, the formation of the LDH intermediate is not possible with the Na2MoO4 flux. Thus, product formation proceeds via crystallization from the molten media, with octahedral crystals consistent with the common crystal habit of the spinel phase produced. This demonstration of...

Electrochemical Nucleation and Growth of Actinides on Tungsten Electrode from Molten Salts

Chronomamperometry (CA) was used to study the nucleation and growth mechanism of actinides (e.g. U, Pu) on a tungsten electrode. CA is a method widely used to study nucleation phenomena in aqueous or molten media. The objective of this work is to fundamentally characterize the initial stages of metal phase formation during electrodeposition of actinides from molten salt at inert electrodes in order to enhance the understanding of the interfacial electrochemistry at solid electrodes and to obtain kinetic parameters. Current-time (i-t ) curves measured at different fixed potentials were used to study the mechanism of the nucleation process and to determine quantitatively magnitude of many of its parameters (e.g. nucleation and growth rates, nucleation sites, nuclei density, diffusion coefficient, etc.). The shape of the current transients is associated with the nucleation and growth of a metal covering a foreign substrate. An example of current transients for various potential pulses in molten LiCl/KCl containing U3+ ions is shown in Figure 1. Each transient can be divided into three zones. The first zone (1) represents the double-layer charging current and the formation of the first nuclei on the electrode. This is followed by an increase in the current due to the total number of nuclei and growth of independent nuclei (2). This rise culminates in a maximum, the position of which depends on the magnitude of the potential step and is a characteristic of nucleation processes. Finally, in part 3, the slow decrease in current corresponds to limitation by semi-infinite linear diffusion of the metal ions to the electrode surface. From the experimental i-t curves recorded at different overpotentials, dimensionless i-t curves were constructed by plotting the current parameters (I/Im; Im≡ current maximum) vs. the time parameter (t/tm; tm≡ the time at which the maximum is reached).These curves were compared to theoretical models for the instantaneous and progressive limiting cases for three-dimensional nucleation and growth described by Scharifker and Hills.1,2 In the instantaneous case, all nuclei are created at the same moment at the beginning of the electrolysis. In the progressive case, the rate of nucleation is slow and continues to take place at the surface while other clusters are growing2. It was found that the nucleation mode of actinides depends on concentration of their ions in the electrolyte and changes from progressive to instantaneous as their concentration increases. At lower concentrations the nucleation is progressive, with nuclei continuing to form as U and Pu deposit on the cathode. At higher concentration, however, the nucleation is instantaneous, with an initial rapid formation of nuclei followed by continued deposition only at those initial nucleation sites.In addition to defining the nucleation mode, additional parameters were found from the current-time transients: (diffusion coefficient, number of nuclei created, nuclei density, nucleation rates, etc). These parameters were obtained at different salt compositions and different magnitudes of the overpotentials and the results were compared.

Current efficiency in electro-winning of lanthanum and cerium metals from molten chloride electrolytes

The objective of this study is to prepare lanthanum and cerium metals by fused salt electrolysis of their anhydrous chloride in molten media such as LiCl–KCl, NaCl–KCl, KCl, NaCl, and LiCl and to characterize the metal deposit by X-ray diffraction, energy dispersive X-ray fluorescence, and inductive coupled plasma-atomic emission spectroscopy. Deposit metal of purity more than 99 % was obtained in each of the experiments. The entire process starting from preparation of anhydrous lanthanum/cerium chloride to electrolysis yielding of metal deposits has been described. The effect of process parameters such as temperature, electrolyte composition, and current density on the current efficiency was studied. All these parameters were varied to get the highest current efficiency and metal yield. The major non-rare earth impurities with the deposit are found to be Fe, Cr, and Ni along with ~1 × 10−3 of total gaseous impurities.

A “Green Chemistry” Approach to the Synthesis of Rare‐Earth Aluminates: Perovskite‐Type LaAlO3 Nanoparticles in Molten Nitrates

Perovskite‐type LaAlO3 nanoparticles have been prepared by a facile, rapid, and environmentally friendly molten salts method using alkali metal nitrates as low‐temperature fluxes. Starting from hydrated lanthanum and aluminum nitrates and alkali metal hydroxides, the proposed methodology consists briefly of two steps: a mechanically induced metathesis reaction followed by short firing at temperatures above nitrates melting points. The purpose of the first is twofold: on the one hand to generate in situ the alkali metal nitrate flux and on the other hand, to obtain a La and Al‐containing precursor material suitable for the synthesis of bulk LaAlO3 nanoparticles in molten nitrates. Different alkali metal nitrates and eutectic mixtures were used to analyze the influence of melt basicity in the reaction outcome. Single phase LaAlO3 was obtained directly, without any purification step when using three molten media: LiNO3, NaNO3, and their mixture; using KNO3 as flux either alone or as part of eutectic compositions, prevents complete conversion, and the title material is obtained mixed with additional crystalline phases such as lanthanum hydroxinitrates and carbonates. As‐prepared LaAlO3 powders are composed of loosely agglomerated nanoparticles with very fine crystallite size (32–45 nm). The present method reduces considerably previously reported synthesis time/temperatures for this material.

High solubility pathway for the carbon dioxide free production of iron

We report a fundamental change in the understanding of iron oxide thermochemistry, opening a facile, new CO(2)-free route to iron production. The resultant process can eliminate a major global source of greenhouse gas emission, producing the staple iron in molten media at high rate and low electrolysis energy.

Gasification and Desulfurization Characteristics of Carbonaceous Materials in Molten Alkali Carbonates

This study focuses on gasification technology of carbonaceous materials in molten carbonates since this gasifier can gasify those fuels catalytically and in-situ clean up the gasified gas at high temperature simultaneously. This gasification system has such advantages that alkali metal compounds show catalytic function for the gasification reactions, the molten alkali compounds can absorb sulfur compounds and the molten media can capture ash and unreacted particles produced. The molten carbonate employed in this study is an eutectic salt with the compositions of 43 mol% Na2CO3 and 57 mol% K2CO3. CO2, is selected as a gasification agent. Before the gasification experiments, the optimum gasification and desulfurization temperature is evaluated by the chemical equilibrium calculations. Fundamental gasification and desulfurization experiments were conducted, by using an electrically heated reactor. As a result, the main compositions of the gasified gas are CO, H2 and CH4. H2S and COS gases in the product can be absorbed almost completely by the molten alkali carbonates. These results suggest that this gasifier can gasify the carbonaceous materials and desulfurize even at high temperature inside the furnace simultaneously.

Lanthanide and actinide speciation in molten fluorides: A structural approach by NMR and EXAFS spectroscopies

The objective of this paper is to describe the local structure in fluoride melts of nuclear interest. Our experimental approach combines NMR and EXAFS spectroscopies both sensitive to the microstructure of solid and liquid materials. These techniques allow identifying the complex formation in molten media. Thanks to the development of specific cells adapted to molten fluorides up to 1300°C, we give some structural description of LiF–LnF3 systems (Ln=Y,Lu,Ce) and discuss the evolution of 19F spectra in terms of different fluorine configurations depending on the composition. This approach is further extended to the characterization of LiF–ThF4 system by 19F NMR in melts from 0 to 100mol% ThF4 at temperatures ranging from 600 to 1100°C. We report also very first EXAFS spectra obtained in situ at high temperature, at the Thorium LIII edge, in LiF–ThF4 molten mixtures.

Crosslinking of ethylene–octene copolymers under dynamic conditions: A new way to access polymeric hyperbranched structure

The crosslinking process of ethylene–octene copolymers by dicumyl peroxide (DCP) has been studied under static and dynamic conditions. Under static conditions, our results (gel time, insoluble fraction, and equilibrium modulus) qualitatively agree with the theories on the prediction of properties of random crosslink network. Furthermore, it can be concluded that the chain linking reaction is dominant and that the beta chain scission can be neglected for these copolymers. We report an original and new result: under dynamic conditions, inside the chamber of a batch mixer, the dynamically crosslinked samples are reprocessable, meaning that the final microstructure is totally different from that obtained under static conditions. Indeed, the samples crosslinked under shearing and mixing are soluble and the size exclusion chromatography analysis revealed a broad molecular weight distribution with a significant tail of high molecular weights situated around 106 g/mol. This molten media can be imagined as a continuum of clusters of different multiscale sizes from the size of a free precursor chain to larger clusters of high molecular weights. Actually, the balance between reaction and mixing efficiency is the controlling parameter for the development of this hyperbranched structure under dynamic conditions. POLYM. ENG. SCI., 46:1530–1540, 2006. © 2006 Society of Plastics Engineers