KCl ZnCl2 Research Articles

Microstructure and thermal properties of ternary chloride eutectic salts for high temperature thermal energy storage

Chloride molten salts are attractive candidate materials used for effective thermal energy transfer and storage in renewable energy system as concentrating solar power, but associated thermal properties at high temperature are still insufficient in research. In this work, eutectic point verification, microstructure and thermal properties of NaCl–KCl–ZnCl2 eutectic salts with four different contents are investigated by molecular dynamics simulation based on Born-Mayer-Huggins potential and experimental measurement. The density, specific heat capacity, viscosity, self-diffusion coefficient and thermal conductivity of four ternary eutectic salts are predicted with experimental validation and their correlations with different temperature and composition are proposed, and the mechanism of desirable thermal performance variation is revealed from the microscopic point of view. As ZnCl2 content increases, the thermal conductivity and specific heat capacity increase significantly, and eutectic salt #4 (18.6 %NaCl: 21.9 %KCl: 59.5 %ZnCl2) has maxima of density, specific heat capacity and thermal conductivity. The above phenomena can be attributed to the increase in potential energy and Columbic energy for more charge of Zn2+, which strengthens the interaction between atoms in the system and intensifies the collision. This work is expected to provide effective guidance on the design and application of molten salts for high temperature thermal energy storage.

Corrosion behavior of Ni–25Cr–10Fe–3Si–xMo alloys beneath KCl–ZnCl2 deposits at 650°C: Role of long‐term aging

AbstractThe corrosion behavior of unaged and thermally aged Ni–25Cr–10Fe–3Si–xMo (x = 0, 3 wt%) alloys was studied beneath KCl–ZnCl2 deposits at 650°C. The results indicated that the addition of Mo promoted the formation of a continuous Si‐rich protective layer and improved the corrosion performance of the unaged Ni–25Cr–10Fe–3Si–xMo alloys. The precipitates formed after long‐term aging greatly affect the corrosion resistance of the thermally aged alloys. The M23C6 along the grain boundaries is beneficial to improve corrosion resistance, however, the dispersed Laves phase and Sigma deteriorate corrosion resistance.

Three-Layer Electrorefining of Molten Zinc Held on Porous Carbon or Silica Sheets in Molten LiCl–KCl–ZnCl2

We investigated a three-layer electrolysis process utilizing porous holding materials with poor wettability to molten metal, which enables to establish a three-layer configuration of metal(l)/electrolyte(l)/metal(l) by holding metals above the electrolyte regardless of their densities. In this study, porous carbon or silica sheets were used as the holding materials for electrorefining experiments of Zn metal in molten LiCl–KCl–ZnCl2 at 723 K. Three types of experiments were carried out focusing on anodic polarization, electrorefining with cathodic carbon, and three-layer electrolysis using liquid Zn as cathode. The concentration of impurity elements in the Zn was reduced by a factor of 8 to 200 in this experimental condition, highlighting the purification effectiveness of the method. From the results obtained, we have identified certain challenges associated with the three-layer electrolysis process. Furthermore, we discuss potential prospects for industrialization.

Hot corrosion behaviors of 921A alloy and HVAF-sprayed Fe-based amorphous coating covered with KCl–ZnCl2 salts

Hot corrosion behaviors of 921A alloy and HVAF-sprayed Fe-based amorphous coating covered with KCl–ZnCl2 salts

Effect of KCl–ZnCl2 Ball-Milling Fluxing on Microstructure and Corrosion of Iron Ingot Hot-Dip Zinc Coating

Effect of KCl–ZnCl2 Ball-Milling Fluxing on Microstructure and Corrosion of Iron Ingot Hot-Dip Zinc Coating

Effect of enamel coating on the hot corrosion of 304 stainless steel beneath KCl–ZnCl2 deposits at 450 °C

The hot corrosion behavior of 304 stainless steel (304SS) and the enamel coating in KCl–ZnCl2 salts is studied. The corrosion kinetics of the samples shows that the mass loss of enamel coating is far lower than that of 304SS, indicating that the enamel coating can effectively block the diffusion of Cl element to the enamel coating/alloy interface due to its high chemical stability in chloride salts and low grain boundary density. Fe2O3 and (Fe,Cr)2O3 are mainly formed on the surface of 304SS covered by KCl–10%ZnCl2, whereas no corrosion products are found on the surface of the enamel coating. When the content of ZnCl2 is increased to 55%, Fe2O3 and Cr2O3 are mainly formed on the surface of 304SS, and Zn2SiO4 is formed on the surface of the enamel coating. In the two salts, the corrosion layer of 304SS is composed of Fe-rich oxides (external) and Cr rich oxides (internal). The enrichment of high amounts of Cl element near the interface between the corrosion products and alloy matrix indicate that metal chlorides are formed at the interface. These results are in line with the characteristics of “active oxidation” mechanism.

Impact of the additive manufacturing process on the high-temperature corrosion of 316L steel in the presence of NaCl–KCl–ZnCl2 molten solar salt

A comparative study of the high-temperature oxidation and corrosion resistance of wrought and additive manufacturing selective laser melting (SLM) 316L stainless steel was performed in presence of molten NaCl–KCl–ZnCl2 salt used for thermal energy storage in the new generation of concentrated solar plants. Isothermal tests at 650 and 700 °C in a dry air atmosphere were conducted to evaluate (i) the effect of the manufacturing process and (ii) the effect of molten salts on the corrosion behavior of the steel by dimensional metrology analysis, scanning electron microscopy and X-ray diffraction characterization. The manufacturing process had a significant influence on the oxidation resistance. The wrought samples had a mass gain after 48 h that was four times greater than that of SLM samples at 650 °C, and 50% greater at 700 °C. The presence of the salt primarily increased the corrosion rate of all the samples and, again, the wrought pieces suffered 13% and 6% more corrosion in terms of metal loss than the SLM ones at 650 and 700 °C, respectively. The presence of more pathways for Cr diffusion associated with a higher number of defects in the microstructure of the SLM sample, as dislocations and grain boundaries, facilitated the formation of Cr2O3 scales in the SLM sample, avoiding the intergranular attack shown in the wrought one.

Design and development of novel LiCl–NaCl–KCl–ZnCl2 eutectic chlorides for thermal storage fluids in concentrating solar power (CSP) applications

Molten chloride mixtures are of great scientific and technical interest for sensible heat storage in solar power generation. In this work, the condensed phases in a LiCl–NaCl–KCl–ZnCl 2 multicomponent system were critically evaluated and predicted via literature review and simulations of LiCl–KCl and KCl–ZnCl 2 systems. Two innovative LiCl–NaCl–KCl–ZnCl 2 quaternary chloride systems with low melting points were proposed and developed on the basis of a thermodynamic design. The thermal–physical properties of these two eutectic compositions, including melting point, mixing enthalpy, specific heat capacity, density, thermal conductivity, and mass loss, were then experimentally determined. The experimental results were consistent with the theoretical predictions. The novel quaternary eutectic chlorides with excellent thermophysical properties developed herein are potential candidates for thermal energy storage and transfer materials in concentrating solar power plants. • Thermodynamic prediction of LiCl–NaCl–KCl–ZnCl 2 system was performed via CALPHAD. • Two novel quaternary eutectic chlorides were successfully designed for thermal energy storage and transfer. • Thermophysical properties of the system were determined by experiment and classical prediction method. • Thermodynamic performance of the novel chlorides was substantially improved compared with other potential materials. • The novel quaternary eutectic chlorides were proposed as potential candidate materials in CSP applications.

2D and 3D cellular automata simulation on the corrosion behaviour of Ni-based alloy in ternary molten salt of NaCl–KCl–ZnCl2

When molten salt is used as a high-temperature heat carrier or electrolyte solution, the corrosion of molten salt is necessary to be studied regarding the device's safety. In this work, a cellular automata method is used to simulate the corrosion behaviour of alloys in ternary molten salt of NaCl–KCl–ZnCl 2 . The CA method described in this paper can be used to reproduce the distribution of corrosion products and components and the change in morphology and thickness of the corrosion layer in the phase of high-temperature corrosion, which provides a unique insight into the corrosion behaviour of chloride molten salt over time. In addition, we also consider the pitting corrosion that may occur during the corrosion process. Furthermore, the applicability of the model can predict the corrosion behaviour of different metals in molten chloride salt. • Cellular automaton (CA) method is proposed to study molten chloride salt corrosion. • The simulation of cross-sectional corrosion morphology of alloy is proposed. • The growth process of a single corrosion pit is simulated innovatively. • A novel 3D CA model realizes the calculation of the corroded volume.

Corrosion behaviors of 316 stainless steel and Inconel 625 alloy in chloride molten salts for solar energy storage

AbstractCorrosion behaviors of 316 stainless steel (316 ss) and Inconel 625 alloy in molten NaCl–KCl–ZnCl2 at 700°C and 900°C were investigated by immersion tests and electrochemical methods, including potentiodynamic polarization and electrochemical impedance spectroscopy. X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy were used to analyze the phases and microstructures of the corrosion products. Inconel 625 alloy and 316 ss exhibited high corrosion rates in molten chlorides, and the corrosion rates of these two alloys accelerated when the temperature increased from 700°C to 900°C. The results of the electrochemical tests showed that both alloys exhibited active corrosion in chloride molten salt, and the current density of 316 ss in chloride molten salt at 700°C was 2.756 mA/cm−2, which is about three times the value for Inconel 625 alloy; and the values of the charge transfer resistance (Rt) for Inconel 625 were larger than those for 316 ss. The corrosion of these two alloys is owing to the preferred oxidation of Cr in chloride molten salt, and the corrosion layer was mainly ZnCr2O4 which was loose and porous and showed poor adherence to metal.

Solid Liquid Phase Equilibria in the Ternary Systems KCl–ZnCl2–H2O and MgCl2–ZnCl2–H2O at 278 K

This paper presents the solid–liquid equilibria phase diagrams of two ternary systems KCl–ZnCl2–H2O and MgCl2–ZnCl2–H2O at 278 K using the method of isothermal dissolution equilibrium. Integrated with the determined equilibrated liquid phase and humid residue composition, the phase diagrams of the two systems were generated. At the same time, the invariant points, the univariant solubility curves and the solid crystalline phase regions are displayed and discussed. The results of the experiment show that the two ternary systems are classified as complex owing to the formation of double salt. The phase diagram of the ternary system KCl–ZnCl2–H2O at 278 K is comprised of two invariant points, three univariant solubility curves and three solid crystalline phase regions corresponding to KCl, ZnCl2·2H2O and 2KCl·ZnCl2·2H2O. The phase diagram of the ternary system MgCl2–ZnCl2–H2O at 278 K includes two invariant points, three univariant solubility curves and three solid crystalline phase regions corresponding to MgCl2·6H2O, MgCl2·ZnCl2·6H2O and ZnCl2·2H2O. The densities of ternary mixtures at 278 K were measured. The possibility of Laliberte model to predict densities of investigated ternary saturated solution was checked. The results show that this model is allowed to offer fundamental description for densities of ternary mixtures and calculated results are in good agreement with the experimental values.

Heat Transfer Performance Comparisons of Supercritical Carbon Dioxide and NaCl–KCl–ZnCl2 Eutectic Salts for Solar s-CO2 Brayton Cycle

Supercritical carbon dioxide (s-CO2) and the new generation molten salts can both be used as the heat transfer fluid in the solar supercritical carbon dioxide Brayton cycle. One of the main concerns in the design of solar s-CO2 Brayton cycle is that the working fluids need to meet the desired heat transfer coefficient to reach the high operating temperature, responding to the high concentrated heat flux. In this paper, to satisfy the SunShot Initiative’s goal of a 10 MWe solar s-CO2 Brayton cycle power plant, the receiver is designed to heat the s-CO2 and the NaCl–KCl–ZnCl2 eutectic salts from 500 ºC to 750 ºC to gain heat of 20 MWth. A tubular receiver made of pressureproof Haynes 230 is assumed. A one-dimensional computational heat transfer model is developed. The influences of receiver operating and structure parameters on the performance of the HTFs are evaluated. Finally, the outlet temperature, the heat gain, the solar field efficiency and the solar exergy efficiency for the s-CO2 and the new molten salts are compared. It is shown that both the fluids can reach the desired temperature and heat gain under special operating and structure parameters, but their relevant effectiveness to convert solar radiation into heat are different. The results obtained in this study are expected to provide references for the further development of the solar s-CO2 Brayton cycle technology.

Measurement of Mineral Solubilities in the Ternary Systems CaCl2–ZnCl2–H2O and KCl–ZnCl2–H2O at 373 K

In this work, the solubilities of the salt minerals and the densities of solutions in two ternary systems, calcium chloride–zinc chloride–water and potassium chloride–zinc chloride–water, were measured at 373 K using an isothermal dissolution equilibrium method. Based on the determined equilibrium solubility data and the corresponding equilibrium solid phases, the phase diagrams and density diagrams of the two systems were plotted. The results show that the two ternary systems are complex. The invariant points, the univariant solubility curves and the solid crystalline phase regions are reported and discussed. The phase diagram of the ternary CaCl2–ZnCl2–H2O system at 373 K includes two invariant points, three univariant solubility curves and three solid crystalline phase regions corresponding to CaCl2·2H2O, CaCl2·ZnCl2·2H2O and ZnCl2. Also, the phase diagram of the ternary KCl–ZnCl2–H2O system at 373 K is constituted of two invariant points, three univariant solubility curves and three solid crystalline phase regions corresponding to KCl, 2KCl·ZnCl2 and ZnCl2. The influence of one of solute on the solubility of the ternary systems is also discussed in this work.

Thermal and Transport Properties of NaCl–KCl–ZnCl2 Eutectic Salts for New Generation High-Temperature Heat-Transfer Fluids

Three eutectic salts from a system of halide salts NaCl–KCl–ZnCl2 were chosen for detailed study of thermal and transport properties with the objective of developing a next generation high-temperature heat-transfer fluid (HTF) for concentrated solar thermal power (CSP) technology. The acceptable range of the working temperatures for the HTF is from below 250 °C to at least 800 °C. The tested properties are presented here for the three candidate eutectic salts, including melting point, heat of fusion, heat capacity, vapor pressure, viscosity, density, and thermal conductivity. Data-fitted equations are provided for all the measured properties for convenience in engineering application. It is concluded that the three eutectic salts can satisfy the needs for a high-temperature HTF and thus are recommended as a new generation high-temperature HTF.

Molecular dynamics simulations and thermodynamic modeling of NaCl–KCl–ZnCl2 ternary system

The NaCl–KCl–ZnCl2 ternary system is examined and modeled using the CALPHAD methodology in conjunction with molecular dynamics (MD) simulations. In particular, MD simulations are used for calculating liquid enthalpies of mixing as a function of composition for the ternary and its binary sub-systems. In addition, key structural features are obtained from MD that is then used for informing the employed two-sublattice ionic liquid model (Na+1, K+1: Cl−1, ZnCl2), which describes the ternary liquid phase. The structure of the simulated liquid systems show that Zn+2 cations primarily exhibit 4-fold coordination in addition to a smaller percentage of 5-fold followed by 3-fold coordination; in contrast, the coordination of both Na+ and K+ cations are distributed between 2- and 4-fold states. The optimized self-consistent thermodynamic model parameters show good agreement with MD data obtained in this work and available experimental literature data.

ZnCl2 and Liquid Zinc Assisted Electrochemical Extraction of Thulium from LiCl–KCl Melt

This work presents a study of the electrochemical behavior of Tm at liquid Zn electrode in LiCl–KCl–TmCl3 melt and the co-reduction of Zn, Li and Tm on W electrodes in LiCl–KCl–ZnCl2–TmCl3 melt at 753 K. The deposition potential of Tm in liquid Zn material was found to be 0.9 V more positive than that at a solid W electrode. In LiCl–KCl–ZnCl2–TmCl3 melt, five Zn–Tm intermetallic compounds were detected in Zn-coated W electrode by cyclic voltammetry and chronopotentiometry. Extraction of Tm was carried out on a W electrode in LiCl–KCl–ZnCl2–TmCl3 melt and at liquid Zn electrode in LiCl–KCl–TmCl3 melt. The obtained deposits showed Tm2Zn17 phase. The morphology and micro-zone chemical analysis of the deposits was characterized by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The composition of the deposits was analyzed by inductive coupled plasma atomic emission spectrometer (ICP-AES). The deposits obtained at W and liquid Zn electrodes were composed of 50.6 wt% and 22.3 wt% Tm, respectively.

Materials performance in simulated waste combustion environments

Combustion of waste for power generation gives increased fouling and corrosion compared to fossil fuels, and leads to higher operating and maintenance costs. New materials solutions to increase lifetime include austenitic stainless steels, nickel base alloys, coatings and weld overlaying. In this work a simulated waste incineration environment with regularly renewed deposits of KCl–ZnCl2 has been used to evaluate the performance of candidate materials and to elucidate the operative degradation mechanisms.

Erosion corrosion properties of HVOF coatings for municipal solid waste incinerator protection

Municipal solid waste incinerators (MSWI) usually have erosion–corrosion (EC) problems. A possible solution for the damage caused by EC processes in the heat exchanger zone has been found in thermal spray coatings with Ni based and Fe based high velocity oxygen fuel (HVOF) coatings showing promising results. High velocity oxygen fuel coatings generally have a low porosity and good adherence. These properties help to prevent damage to exchanger tubes. In the present paper Ni and Fe alloys were compared under different laboratory tests and in service inside a MSWI heat exchanger. Coatings were also characterised by standardised tribological and microhardness tests. A eutectic mixture of KCl–ZnCl2 found on the boiler ashes was used as a corrosion agent in the laboratory tests. Scanning electron microscopy (SEM) images of sprayed coating structures before and after the tests were compared. Energy dispersive spectroscopy (EDS) analysis and X-ray diffraction (XRD) were used to characterise the coatings and corrosion products. Studies on bulk materials with similar composition help to understand some steps in the corrosion processes that take place during incineration. It is concluded that the low level of protection provided by Fe based alloy HVOF coating in these conditions may be attributed to the role of non-protective chromium oxides, and the good behaviour of Ni based alloy HVOF coating may be attributed to the formation of more protective Ni, Mo and Nb oxides.

Fundamental aspects of chlorine induced corrosion in power plants

Chlorine induced corrosion is the most important corrosion process in power plants, firing waste, coal and/or biomass. Depending on the process, chlorine is present as HCl gas, as solid KCl and NaCl in ashes or as eutectic melts i.e. KCl–ZnCl2 in deposits of waste fired boilers. In the presence of HCl gas and solid chlorides, ‘active oxidation’ is generally accepted as the major corrosion mechanism. Metal chlorides are formed by inward diffusion of chlorine to the metal/oxide interface, evaporating and subsequently oxidized to non-protective oxides and additional chlorine. In this mechanism, the role of alloying elements has not been fully understood until now. Hence, experiments were conducted on the influence of the alloying elements Mo, Ti, Si, Al on Fe–15Cr model alloys in an N2–5 vol.% O2 gas mixture with addition of 500 and 1500 vppm HCl with the use of thermogravimetric experiments at 600°C. The corrosive attack by ‘active oxidation’ strongly depends on the alloying elements. Fe–(15–25) wt% Cr alloys without any further addition of alloying elements show catastrophic ‘active oxidation’, characterized by the formation of metal chlorides at the metal–oxide interface. The addition of molybdenum, silicon and aluminium generally decreases the corrosive attack, whereas Ti has no beneficial effect and the corrosion is enhanced compared to Fe–15Cr.The presence of molten chlorides on heat exchanger tubes leads to catastrophic corrosion rates, even at relatively low temperatures of 250°C. Thick oxides scales of Fe2O3 are formed in contact with the gas phase. At the metal–oxide interface, a mixture of KCl–FeCl2 chloride phases are detected, probably molten at a reaction temperature of 400°C. Short term experiments on 2.25Cr–1Mo steel at 400°C in N2–5 vol.% O2–HCl gas mixtures covered with a eutectic KCl–ZnCl2 melt have shown that the metal is dissolved in the chloride melt at the melt/scale interface and precipitated as a thin layer of oxide at the melt/gas phase boundary. The kinetics of the corrosion strongly depends on gas phase composition i.e. p(HCl) and p(O2).