Molten Salt Technology Research Articles

Eutectic molten salt assisted fabrication of microporous biochar for greenhouse gases adsorption

The utilization of eutectic molten salt technology has emerged as a prominent area of investigation aimed at fabricating a novel carbon material sorbent based on biomass wastes. The adsorption mechanism on CO2 adsorption remains inadequately elucidated as well. Herein, the biochar fabrication process involving eutectic molten salt method at intermediate-low temperature of 550 °C was successfully achieved and investigated in detail. The impacts of molten salt composition, monomer salt’s anion and cation, and pyrolysis parameters were firstly studied on the resulting microporous biochar of 1.8 nm pore diameter and 975 m2/g surface area. The optimized conditions are 550 °C, 15 °C min−1 ramp rate, 120 min holding time, Na+ cation, NO3– anion, and ternary salt KCl/NaNO3/Na2SO4 of 32.5/30.4/37.1. The efficient adsorbent exhibited abundant micropores and a substantial specific surface area, leading to enhanced CO2 adsorption quantity of 3.75 mmol/g (the calculated capacity is up to 4.54 mmol/g) at pressure state of 1.6 MPa. Investigation into key factors, adsorption kinetics, and adsorption isotherm revealed that CO2 capture by optimal adsorbent predominantly stemmed from micropore filling, van der Waals forces, hydrogen bonds, and Lewis acid-base interactions. This research contributes novel insights into the utilization of biochar adsorbents for CO2 capture in the realm of industry.

A reactive force field approach to modeling corrosion of NiCr alloys in molten FLiNaK salts

The interface between NiCr alloys and FLiNaK molten salt exhibits complex corrosion behavior, mainly driven by intricate chemical interactions involving Cr and F−ions. Understanding these dynamic reactions is crucial for developing effective corrosion mitigation strategies to ensure the long-term durability of Ni-based alloy components in molten salt technologies. However, obtaining molecular-level understanding through experiments is challenging. To address this, we utilize reactive molecular dynamics simulations enabled by a reactive force field, ReaxFF, to investigate detailed reaction dynamics at the atomic level. We first present the development of the ReaxFF parameter set for Ni/Cr/F/Li/Na/K based on extensive first-principles calculations. With this force field, we achieve a strong agreement for the structure of FLiNaK molten salt by comparing the pair distribution functions with experimental and simulation results. Furthermore, our simulations successfully reproduce the experimental phenomenon of Cr dissolution in fluoride salt, with the corrosion rate depending on the alloy and salt compositions. Particularly, our simulations reveal that increasing the concentration of Li can enhance the formation of a compact double layer, mitigating Cr dissolution. This work enables a fundamental understanding of the interfacial behavior between fluoride salt and NiCr alloys.

A simulation study of tritium distribution in a 10WM(e) thorium-based molten salt reactor

The TMSR, with its thorium fuel cycle and molten salt technology, is an increasingly popular alternative to traditional nuclear reactors. In this study, an enhanced computational model was used to assess the distribution of tritium in SINAP's proposed 10 MW(e) TMSR system. The findings reveal that, in the absence of tritium removal measures, approximately one-third of the produced tritium could contaminate the environment. Moreover, the saturation of graphite adsorption can result in even greater tritium permeation over time. However, by implementing a method akin to the MSRE spray gas removal, approximately two-thirds of the tritium can be eliminated, reducing permeation to around 10 % and facilitating control. Furthermore, reducing the tritium yield from the reactor can further diminish the overall tritium amount and the proportion of permeation. For instance, substituting FNaBe salt for FLiBe salt as the fuel carrier yields lower tritium, thereby rendering control unnecessary or significantly reduced.

Preface: SolarPACES 2022, 28th International Conference on Concentrating Solar Power and Chemical Energy Systems

The SolarPACES 2022 International Conference was held from September 27 to September 30, 2022 in Albuquerque, New Mexico. As the premier international conference and network for advancing commercial deployment and research and development of concentrating solar-thermal power (CSP) and related technologies, the community was excited to convene in person after two years of virtual collaboration. The conference brought together more than 400 participants from 29 countries. The technical presentations and plenary sessions highlighted exciting advances in state-of-the-art commercial deployment and next-generation research and development. There was much focus on emerging commercial markets, including the value of thermal energy storage for cost-effective long duration energy storage – charged either with electricity or solar heat – and the use of solar thermal energy for industrial and chemical processes, especially to decarbonize the large portion of energy that’s currently used for industrial process heating. By holding the conference in the U.S., we were able to highlight particular technical and strategic objectives being advanced by the U.S. Department of Energy (DOE) and the US Government, including the progress in advancing the Generation 3 (Gen3) CSP test facility [1] at Sandia National Laboratories. This megawatt-scale system will demonstrate a fully integrated thermal transport and storage system that can collect, store, and deliver heat above 700° Celsius, using solid particle heat transfer media. Attendees to the conference were able to tour the globally renowned CSP research center at Sandia National Laboratories and the National Solar Thermal Test Facility (NSTTF). The NSTTF opened in 1978 as the world’s first tower research facility for CSP and has been operating for more than 40 years. The site has been a center of excellence in the development of molten salt technologies for CSP and has played a key role in the demonstration of technologies used in some of the largest solar power plants in the world. We also highlighted the recent announcements from the U.S. Department of Energy of the Energy Earthshot Initiatives to accelerate breakthroughs of abundant, affordable, and reliable clean energy solutions by the end of this decade. Specifically, the Long Duration Storage Shot [2] established a target to reduce the cost of grid-scale energy storage by 90% for systems that deliver 10 or more hours of duration, to $0.05 per kilowatt-hour of electricity delivered from storage. The Industrial Heat Shot [3] aims to develop cost-competitive industrial heat decarbonization technologies with at least 85% lower greenhouse gas emissions by 2035. Hydrogen Shot [4] seeks to reduce the cost of clean hydrogen by 80%, to $1 per 1 kilogram. CSP technologies have promising potential to contribute to all of these important goals. The 2022 conference covered important contributions in all of these areas, and more, across 15 technical tracks, coordinated and chaired by international experts in each field. We had high quality contributions across the board, including 211 oral presentations and 111 posters. Additionally, the plenary sessions brought together leading experts on energy storage, US and global markets for CSP technologies, and a specific focus on industrial decarbonization. On behalf of SolarPACES, I thank all of the conference participants, authors, presenters, track chairs, and the conference organizing committee for an extremely successful meeting! Avi Shultz Chairman of the 28th SolarPACES conference

Investment-based optimisation of energy storage design parameters in a grid-connected hybrid renewable energy system

Grid-connected hybrid renewable power systems with energy storage can reduce the intermittency of renewable power supply. However, emerging energy storage technologies need improvement to compete with lithium-ion batteries and reduce the cost of energy. Identifying and optimising the most valuable improvement path of these technologies is challenging due to the non-linearity of the energy system model when considering parameters as independent variables. To overcome this challenge, a novel investment-based optimisation method is proposed. The method involves linear optimisation of the hybrid renewable energy system and subsequent investment optimisation, accounting for diminishing improvements per investment. The results from applying the investment-based optimisation to thermal energy, pumped thermal energy, molten salt, and adiabatic compressed air energy storage technologies, show that improving the discharge efficiency is the most valuable for all technologies. The second most important parameters are the costs of discharge capacity and energy storage capacity, and the least important parameters are the charge capacity cost and charge efficiency. The study provides detailed improvement pathways for each technology under various operational conditions, assisting developers in resource allocation. Overall, the investment-based optimisation method and findings contribute to enhancing the competitiveness of emerging energy storage technologies and reducing reliance on batteries in renewable energy systems.

Thermodynamics of corrosion products in molten nitrate salts

Structural material corrosion is one of the major concerns regarding the implementations of molten salt technologies and thermodynamic calculation is crucial to understand the corrosion mechanism occurring in molten salts. The present study unlocked the limits of thermodynamic calculations in molten nitrate salt systems by using a new reference state. Based on which, the Ellingham and E-pO2− diagrams for the corrosion products including Cr, Fe, Mn, and Ni were developed, unveiling the stability domains of these metals and the various overall equilibria which may participate in the corrosion and passivation process in molten nitrate salts.

Activity gradient mass transport accelerated corrosion of stainless steel in molten salt

Advanced energy systems based on molten salt technologies have gained increasing interest, and their design and construction are likely going to involve the use of materials of dissimilar chemistries. With dissimilar materials being used, galvanic coupling induced corrosion acceleration is one of the mechanisms believed to be an issue for the deployment of molten salt systems. However, non-galvanic interaction mechanisms such as activity gradient mass transport have also been theorized to represent a potential challenge. Unfortunately, the two effects have often been compounded in the literature, such that it is still unclear which degradation mechanisms are at play in these systems. In the following study, electrochemical measurements are used to predict the effect that galvanic coupling would have on dissimilar materials submerged in the same molten salt medium. Dissimilar material salt exposure tests are then performed to investigate whether galvanic coupling or activity gradient driven processes account for the levels of materials degradation observed. These separate effect electrochemical and corrosion experiments uniquely elucidate the degradation mechanisms in dissimilar materials systems.

Thermal Energy Storage Options: Comparisons between Molten Salt, Liquid Air, and Liquid Nitrogen Technologies

Thermal energy storage is crucial for the transition to a low-carbon, renewable energy future. This literature review critically compares and contrasts three sustainable thermal energy storage technologies: molten salt, liquid air energy storage (LAES), and the liquid nitrogen engine (LNE). Molten salt systems have a high energy density and long thermal stability, but are restricted by their high cost and maintenance needs. LAES has a high energy density and low environmental impact, yet suffers from low efficiency. LNE is highly efficient but limited to low-power applications and needs a high-pressure storage tank. Each technology has its benefits and drawbacks, and the most appropriate technology will depend on the specific application. Molten salt may be suitable for large-scale projects, LAES for urban environments and LNE for low-power applications. Therefore, research and development in thermal energy storage should focus on improving efficiency, and cost-effectiveness and exploring new technologies to aid in the transition to a low-carbon, renewable energy future.

Establishing a Design Strategy for Corrosion Resistant Structural Materials in Molten Salt Technologies

Establishing a Design Strategy for Corrosion Resistant Structural Materials in Molten Salt Technologies

Thermodynamic measurements and assessments for LiCl-NaCl-KCl-UCl3 systems

Thermodynamic descriptions of higher-order molten salt systems are essential for the effective application of molten salt technologies. However, for pyroprocessing and molten salt reactor development, a complete description of the key Li-Na-K-U(III) chloride system has yet to be provided. To remedy this, we applied the CALculation of PHAse Diagrams (CALPHAD) approach to all available phase equilibria and related thermodynamic measurements that include new differential scanning calorimetry (DSC) observations for pseudo-binary and pseudo-ternary systems to obtain consistent and accurate sets of thermodynamic properties. An outcome of that effort was the apparent discovery of a previously unreported intermediate compound in the NaCl-UCl3 system. In this effort, we adopted a unique approach to the quantification of uncertainty in phase equilibria obtained from DSC measurements, which can provide distinct advantages in phase diagram analysis. The methodology indicates that reported values of uncertainty are often underestimated. The assessed systems of the current effort were incorporated in the publicly available Molten Salt Thermal Properties Database - Thermochemical.

Integrated High-Throughput and Machine Learning Methods to Accelerate Discovery of Molten Salt Corrosion-Resistant Alloys.

Insufficient availability of molten salt corrosion‐resistant alloys severely limits the fruition of a variety of promising molten salt technologies that could otherwise have significant societal impacts. To accelerate alloy development for molten salt applications and develop fundamental understanding of corrosion in these environments, here an integrated approach is presented using a set of high‐throughput (HTP) alloy synthesis, corrosion testing, and modeling coupled with automated characterization and machine learning. By using this approach, a broad range of Cr—Fe—Mn—Ni alloys are evaluated for their corrosion resistances in molten salt simultaneously demonstrating that corrosion‐resistant alloy development can be accelerated by 2 to 3 orders of magnitude. Based on the obtained results, a sacrificial protection mechanism is unveiled in the corrosion of Cr—Fe—Mn—Ni alloys in molten salts which can be applied to protect the less unstable elements in the alloy from being depleted, and provided new insights on the design of high‐temperature molten salt corrosion‐resistant alloys.

Defining the Challenges—Identifying the Key Poisoning Elements to Be Separated in a Future Integrated Molten Salt Fast Reactor Clean-Up System for iMAGINE

Nuclear fission technologies have the potential to play a significant role in the energy mix of a net-zero and sustainable society. However, to achieve the sustainability goal two significant challenges remain: efficient and sustainable fuel usage and the minimization of long-term nuclear waste. Civil nuclear molten salt systems and technologies offer the opportunity to address both, delivering future reactors at scale for efficient and effective power production and nuclear waste burnup. Potentially, both objectives could be fulfilled in one reactor system, which could significantly improve sustainability indices. The key to this innovation is demand driven development of a significantly reduced fuel cycle with enhanced proliferation resistance which offers further potential for improvement. To achieve these goals, a transformative approach for salt clean-up during molten salt reactor operation is proposed, by concentrating on the detection and removal of key neutron poisoning elements which prevent the reactor from long-term operation. To enable this highly innovative development work, a novel analysis of the evolving elementary fuel composition, their concentrations, and their criticality influence is now provided in this work. This, combined with consideration of the oxidation states of each of these elements then provides the basis for the selection of these key poisons and the development of advanced separation processes and process monitoring. This work also discusses the importance of the effective integration of physics and chemistry when systems modelling in achieving these system development goals.

Electrochemical production of hydrogen in molten salt

Clean and economical production of hydrogen is a key step for the decarbonization of energy systems. Current water electrolysis technologies developed for the green hydrogen production require high voltage values of around 2.0 V, making them uneconomical for large-scale applications. The voltage required for the production of hydrogen can be substantially reduced to around 1.0 V by performing the electrolysis at high temperatures. This article concerns the low-cost and green electrolytic production of hydrogen operating at such a low voltage performed by the dissolution of steam in high temperature molten salt electrolytes, and the subsequent electrochemical reduction of the dissolved hydrogen ions. The electro-generation of hydrogen is confirmed by performing cyclic voltammetry using molten lithium chloride as the electrolyte at 660 °C under a flow of moisturized argon. Furthermore, hydrogen is generated by the electrolysis of molten salt containing hydrogen ions using a nickel foam cathode at the constant cell voltage of 1.5 V with a high Faradaic efficiency of 83.2%. The results obtained provide crucial insights for further development of molten salt technologies for clean and efficient hydrogen production.

Molten salt modelling in SPECTRA applied to MSRE

In the Netherlands, NRG has an extensive experimental irradiation program on molten salt technology. In support of this program, also system thermal hydraulic modeling capabilities are being developed in the SPECTRA code. This paper describes the (fueled) molten salt capabilities developed and presently available in the code. These include flexible input of properties and (heat) transport correlations, delayed neutron precursor drift, fission product behavior, noble gas behavior, noble metal behavior, noble metal extraction, and chromium leaching and deposition. The paper introduces the models developed and where possible their application to the Molten Salt Reactor Experiment (MSRE) data. In general, satisfactory agreements were found between experimental data and SPECTRA results, especially taking into account all the uncertainties involved in the data retrieved from MSRE which is a large integrated system test. In the next development stages, the models will be validated using separate effect tests, where clear test definition and relatively few phenomena involved will allow to quantify the accuracy of models.

Engineering molten MgCl2–KCl–NaCl salt for high-temperature thermal energy storage: Review on salt properties and corrosion control strategies

Conventional thermal energy storage (TES) media and heat transfer fluids (HTFs) currently used in commercial concentrated solar power (CSP) plants are nitrate-based molten salts with working temperature up to about 565 °C. Current interest in raising the working temperature in next-generation CSP for higher energy conversion efficiency leads to chloride-based molten salt technology (>700 °C). MgCl2–KCl–NaCl salt is selected to be such a TES/HTF material due to its excellent salt properties, abundance and low costs. In this review, survey and evaluation of its exact eutectic composition by evaluation of the phase diagram as well as most relevant salt properties for design, construction and operation of a test loop and projection of a real TES/HTF system are presented. They include minimum melting temperature, vapor pressure, heat capacity, density, thermal conductivity and dynamic viscosity. The working temperature range 420–800 °C is suggested for the TES/HTF system using this salt. Moreover, the recommended values of the heat capacity, density, thermal conductivity and dynamic viscosity in the suggested working temperature range are given. In addition, corrosion control strategies for this salt including corrosion monitoring and mitigation techniques are briefly reviewed, since controlling the salt corrosivity is one main concern of the engineering aspects using the molten chlorides at high temperatures. Overall, this review is expected to supply reliable values of the most relevant salt properties and recommend the most promising corrosion control strategies for further material studies, as well as modeling, simulation, detailed design and construction of molten MgCl2–KCl–NaCl salt TES/HTF systems.

Molten salt modelling capabilities in SPECTRA and application to MSRE and Mk1-PB-FHR

In support of extensive experimental irradiation program on molten salt technology at NRG in the Netherlands, system thermal hydraulic modeling capabilities are being developed in the SPECTRA code. This paper describes the (fueled) molten salt capabilities developed and presently available in the code. These include obviously flexible input of properties and (heat) transport correlations, delayed neutron precursor drift, fission product behavior, noble gas behavior, noble metal behavior, noble metal extraction, and chromium leaching and deposition. The paper will introduce the models developed and where possible their application to the Molten Salt Reactor Experiment (MSRE) data and simulation of the Mk1-PB-FHR.

Ab-initio investigation of the effect of adsorbed salts on segregation of Cr at the Ni (100) surface

The dissolution and depletion of chromium (Cr) in salt facing nickel (Ni) alloy surfaces is one of the predominant degradation mechanisms of structural components in molten salt technology. In this work, we use density functional theory to investigate the role of electronic level interactions that may underlie the depletion phenomenon of Cr in a Ni 100 surface exposed to various adsorbed salt species. Our results show that, under vacuum, Ni preferentially segregates to the surface layer. Conversely, in the presence of adsorbed anionic salt species (e.g., chlorine (Cl), fluorine (F) or the impurity oxygen (O)) Cr segregation becomes more favorable. In these cases, Cl has the weakest effect on segregation, while O has the strongest effect. Our analysis reveals the strong correlation between Cr segregation and the amount of valence charge transferred between the Cr atom and surface adsorbate: the greater the charge transfer, the lower the segregation energy. We also show that, when considered, secondary cations screen Cr-anion interactions, which in turn reduce the magnitude of the anions effect on segregation. These results shed light on the role of salt impurities likely play in the overall corrosion phenomena in molten salt environments. This work provides insights into the atomic level interactions fundamental to molten salt corrosion and on the importance of maintaining salt purity.

Molten‐Salt Technology Application for the Synthesis of Photocatalytic Materials

Most of the photocatalytic materials are prepared by wet chemical methods and calcination methods. These preparation methods usually have the problems of slow reaction speed and low crystallinity of products, which seriously restrict the development of photocatalytic technology. Molten salt method (MSM) has attracted much attention of researchers because of its unique advantages in optimizing the synthesis route of photocatalyst. Herein, the application of molten salt technology in the preparation of photocatalytic materials in recent years is summarized. The advantages of MSM in photocatalytic synthesis, such as promoting crystallization, adjusting morphology, accelerating the construction of heterostructure, stabilizing single atom, doping auxiliary elements and adjusting defect position, are also discussed in detail. It is indicated that MSM has important application potential in the preparation of photocatalytic materials. Meanwhile, the research achievements and existing problems of MSM are summarized, and suggestions for further application of MSM in photocatalyst's preparation are put forward.

Editorial for special issue on high-temperature molten salt chemistry and technology

Editorial for special issue on high-temperature molten salt chemistry and technology

Nanoporous Versus Nanoparticulate Carbon‐Based Materials for Capacitive Charge Storage

This article reviews recent progresses in the preparation and supercapacitor applications of selected nanoporous and nanoparticulate carbon‐based materials, namely activated carbons, graphenes and carbon nanotubes and their composites with redox materials. Simplified processing steps and graphitization are crucial for making activated carbons. Progressive liquid processing and furnace technology are mainly used in graphene preparation, including curlization, electrolyte incorporation, self‐propagating high‐temperature synthesis, molten salt technology, and flash Joule heating. Redox deposition and new dispersion methods open better routes to making carbon nanotube composites of high charge capacity. Charge storage mechanisms are compared with attention to the facts that activated carbons store ionic charges largely in desirable nanopores, while the ion‐accessible external surfaces of nanoparticulates affect the storage performance of graphenes and composites of carbon nanotubes. Especially, it is explained that pseudocapacitance can result from partial (or zone) delocalization of electrons in graphenes caused by hetero atoms, oxygen in particular. A highlight of this review is on novel molten salt densified holey graphene that has enabled a high specific energy of 618 Wh kg−1 in supercapattery.