Binary Nitrate Research Articles

Effects of constant load on corrosion cracking of 304 ASS at 600°C in molten salts

AbstractThe effects of constant load (CL) on corrosion cracking of 304 austenitic stainless steel (ASS) in molten salts (binary nitrate salts, 60 wt% NaNO3, and 40 wt% KNO3) were investigated. A pair of novel clamping fixture was designed to fix the tensile sample, making the gauge section immersed in molten salts. The CL tensile tests were applied with the initial stress of 212 MPa and the temperature of 600°C. Optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were employed to characterize the deformation behavior and corrosion features. The results show that corrosion of 304 ASS in molten salts was aggravated by the continuous deformation due to the cracking of protective oxide scale. Grain boundaries were oxidized and weakened by the molten salt, leading to a crack propagating. Compared with the creep test result, a reduced rupture life of 304 ASS under constant loading in molten salts was observed.

Graphitization as efficient inhibitor of the carbon steel corrosion by molten binary nitrate salt for thermal energy storage at concentrated solar power

Hot corrosion by molten salts has detrimental effect on various industrial and infrastructural systems particularly on the Concentrated Solar Power (CSP) plants. To enhance durability of the CSP plants′ components, while decreasing the production cost of electricity, the graphitization anticorrosion method for carbon steel in contact with molten binary nitrate salt is proposed in this work. To shed some light on the corrosion mechanisms and quantify corrosion rates a combination of advanced material characterization techniques like Focused Ion Beam, SEM, EDX, XPS depth profiling, XRD was applied. It has been demonstrated that the surface spray-coating by graphite reduces the corrosion rate at least by factor of 6 and stabilizes the corrosion scale by iron carbide crystals at 390 °C.

Nanoparticles as a high-temperature anticorrosion additive to molten nitrate salts for concentrated solar power

Hot corrosion is profoundly detrimental to construction elements in Concentrated Solar Power (CSP) plants affecting their lifetime, running costs and safety. In this work we have studied the anticorrosion effect of TiO2 nanoparticles additives on carbon steel using XRD, XPS with depth profiling, EDX and FIB/SEM techniques. The results revealed that the addition of 1 wt% of TiO2 nanoparticles to molten binary nitrate salt reduces the corrosion rate of carbon steel more than twice and stabilizes the corrosion scale at 390 °C. The anticorrosion effect of TiO2 nanoadditive was attributed to the formation of iron-titanium mixed oxide on the carbon steel surface. It was confirmed by XRD and TGA techniques that addition of TiO2 nanoparticles does not alter the stability of the salt. In view of presented results, the feasibility of molten salts based nanofluids in the CSPs can be reconsidered in terms of improved compatibility with construction materials.

The evolution of the mushy zone during the melting process of a binary nitrate salt

In this study, an experimental setup was built to investigate the evolution of the mushy zone during the melting process of a binary nitrate salt (40% mol. LiNO3-60% mol. KNO3). The thermophysical properties of the salt, including the solidus and liquidus temperatures and latent heat, were firstly determined. The melting process of the salt was then conducted in a thin container with one-sided heating. The temperature variations of the salt were measured and the change of the melting front were recorded by a digital camera for the melting processes under different heating temperatures. Based on the recorded transient locations, the moving rate of the melting front was calculated. The resulting moving rate, together with the measured temperatures, was further used to calculate the mushy zone width. Moreover, the effect of the heating temperature on the mushy zone was studied. The results show that the moving rate of the melting front decreases during the melting process while the mushy zone width increases. It is also found that the moving rate of the melting front increases while the mushy zone width decreases with the increasing heating temperature. Finally, both the dimensionless location of the melting front and the dimensionless mushy zone width during the melting process were developed as functions of Stefan and Fourier numbers.

Research on thermal properties of novel silica nanoparticle/binary nitrate/expanded graphite composite heat storage blocks

In order to study the effects of EG (expanded graphite) content and SiO2 nanoparticles on the specific heat and thermal conductivity of nano-SiO2/NaNO3–KNO3/EG composite heat storage materials, a series of nano-SiO2/NaNO3–KNO3/EG composites are prepared by the aqueous solution-ultrasound oscillation method. The nano-SiO2/NaNO3–KNO3 and EG with different mass fractions (5%, 10%, 15% and 20%) are used as heat storage material and matrix material, respectively. The specific heat and thermal conductivity of the composite are measured by STA (Simultaneous Thermal Analysis) and LFA (Laser Flash Apparatus), respectively. The results show that nano-SiO2/NaNO3–KNO3/EG composite exhibits obvious better performance than pure binary nitrate NaNO3–KNO3. With 15 wt% EG, the average specific heat (from 250 °C to 430 °C) and thermal conductivity of the composite are 2.574 J/(g·K) and 6.05 W/(m·K), respectively, which are 1.69 times and 9 times higher than that of pure binary nitrate. After adding 1 wt% nano-SiO2 to NaNO3–KNO3/EG (15 wt%) composite, the average specific heat and thermal conductivity increase by 44% and 60.9%, respectively. From the above results, the composite heat storage materials prepared by adding EG and nano-SiO2 into binary nitrate are considered as promising candidate materials for high heat transfer performance.

Microkinetics of the reaction [formula omitted] in molten sodium nitrate and potassium nitrate salt

Latest development in the concentrating solar power (CSP) tower technology for energy production and storage intends to find a heat transfer and storage material that provides stability at elevated temperatures. The binary nitrate salt mixture Solar Salt (sodium nitrate, 60 wt.% and potassium nitrate, 40 wt.%) is utilized up to its state-of-the-art temperature limit of 560 °C. Knowledge of the ongoing reactions is demanded to assess the potential of Solar Salt for higher temperature applications. The reversible reaction NO3-⇌NO2-+0.5 O2 is fundamental in a sequence of decomposition reactions. It is investigated by thermogravimetric experiments and ion chromatography post-analysis with regard to the chemical equilibrium, and to reaction kinetics. The reaction enthalpy and entropy of 97 +/- 9 kJ mol−1 and 85 +/- 12 J·mol−1 K−1, respectively, coincide with literature values. A detailed reaction mechanism based on literature findings is drafted and discussed. The corresponding kinetic rate law is simplified by means of rate law approximations and by focusing on the reduction reaction path. The kinetic rate constants of this study exceed previous findings, probably due to successful exclusion of mass transport limitations in the <1 mm thin salt samples. The activation energies and pre-exponential factors amount to 212 +/- 11 kJ·mol−1 and 694546727 +/- 6 s−1, and 115 +/- 11 kJ·mol−1 and 25770 +/- 6 s−1 for the reduction and oxidation reaction, respectively.

Experimental investigations on the thermal properties of binary molten salt based AlN nanofluids for high temperature applications

Typical binary nitrate (NaNO3-KNO3 as 60:40 rat io by weight) based AlN nanofluid was prepared by a two-step solutionmethod. The specific heat capacities of nanofluids with AlN amount ranging from 0.5 to 4 wt. % were investigated. The results suggest a good compatibility between AlN and molten binary nit rates. Meanwhile, AlN nanoparticles are proved to be an effective additive in improving the thermal properties of binary nitrates. For binary nitrate-based materials, samples with AlN amount as 1 wt. % shows the best thermal enhancement of specific heat capacity.

Experimental study on the effect of SiO2 nanoparticle dispersion on the thermophysical properties of binary nitrate molten salt

Molten salts, good heat transfer and thermal storage media, show potential for various applications, particularly solar thermal power. Nanofluid technology has been applied to molten salts to improve specific heat. In this study, molten salt nanofluids with a base solution of binary molten salt and SiO2 nanoparticles were prepared through high-temperature melting. The specific heat of the molten salt nanofluids was examined with different diameters and mass fractions of nanoparticles, and their optimal diameter and mass fraction were determined. The thermophysical properties of the optimized molten salt nanofluids were experimentally tested and compared with those of pure molten salt. Results reveal that the specific heat of the molten salt nanofluids is higher than that of the pure molten salt. The optimal mass fraction and size of SiO2 nanoparticles at the improved specific heat of the molten salt nanofluids are 1% and 20 nm, respectively. The optimal specific heat of the molten salt nanofluids is 17.8% more than that of the pure molten salt. The density of the optimized molten salt nanofluids is similar to that of the pure molten salt, and the average thermal conductivity is 20.2% more than that of the pure molten salt.

Experimental investigation of specific heat capacity improvement of a binary nitrate salt by addition of nanoparticles/microparticles

Abstract Several recent studies reported anomalous enhancement of specific heat capacity by doping minute concentration (e.g. 1 wt.%) of nanoparticles into the eutectic salt. In this work, silica particles with three different sizes and nano-sized silicon nitride and silicon carbide particles were dispersed in solar salt with particle concentrations of 0.5–7 wt.%. To prepare the composite material, a conventional two-step aqueous solution method was used and the drying approach was modified to suit large-scale production. The specific heat capacities of the composites, pure solar salt and particles were measured using differential scanning calorimetry in a temperature range of 300–400 ᵒC and the results were compared. Silicon nitride and silicon carbide nanoparticles have marginal effect on the specific heat capacity of solar salt over the examined concentration of 0.5–3 wt.%. The specific heat capacity of silica composites with 3 wt.% of silica were enhanced by 4.3–9.7% over that of the pure solar salt. However, when 5 wt.% and 7 wt.% of silica (30 μm) were dispersed, the specific heat capacity was reduced by 10.6% and 9%, respectively. Material characterization of the composite material proved the formation of sodium silicate due to reaction of silica and sodium nitrate, which is probably correlated to the reduction of specific heat capacity.

The effect of added magnesium nitrate on the thermophysical property of sodium nitrate

Abstract In the centralized solar power generation system, the molten salt is a key component for storing and transferring heat energy. By adding Mg(NO3)2 to NaNO3 at various mass ratios, nine binary nitrate specimens were prepared.The melting point, decomposition temperature, specific heat and latent heat of the mixed nitrates were experimentally studied with differential scanning calorimetry and thermogravimetric analysis. The results show that within the nine mixed molten salts at different mass ratios, No.05, i.e. Mg(NO3)2 and NaNO3 with the identical mass proportion, can reach the state of eutectic crystal, with the initial melting temperature at 355.4℃, the melting temperature at 364.3℃ and the decomposition temperature at 440℃. The remaining binary nitrate samples which could reach eutectic showed the similar melting temperature at around 340℃, the decomposition temperature in the range of 430-500℃,the specific heat maintained at around 1 J·g-1·k-1 and the melting latent heat at around 154.2 J·g-1.

Enhanced heat capacity of binary nitrate eutectic salt-silica nanofluid for solar energy storage

In concentrating solar power plants, the heat capacity of thermal storage media is a key factor that affects the cost of electricity generation. This work investigated the effective specific heat capacity of binary nitrate eutectic salts seeded with silica nanoparticles, using both experimental measurements and molecular dynamics simulations. The effects of the mass concentration (0–2.0 wt%) and average size (10, 20, and 30 nm) of the nanoparticles on the specific heat capacity value of nanofluids were analyzed. The results show that specific heat capacity increases when adding 10 nm silica nanoparticles up to 1.0 wt%, and then it decreases at higher concentrations. At this optimal mass concentration, the 20 nm nanoparticles displayed a maximum enhancement in the average specific heat capacity (by ~26.7%). The simulation results provided information about the different energy components in the system. The rate of potential energy change versus nanoparticle mass concentration was found to be maximized at 1.0 wt% concentration, which agrees with the experimental measurements. The potential energy components in the simulation system indicate that the change of Coulombic energy contributes the most to the variation of specific heat capacity.

The Binary Alkali Nitrate and Chloride Phase Diagrams: NaNO3-KNO3, LiNO3-NaNO3, LiNO3-KNO3, and NaCl-KCl

LiNO3-NaNO3, LiNO3-KNO3, NaNO3-KNO3, and NaCl-KCl phase relations were formulated using the regular solution model to obtain desired compositions for their potential application as a heat transfer fluid. The simultaneous nonlinear equations formed were solved numerically using a contour plot technique in Matlab. An algorithm was developed to determine the real roots of the coupled nonlinear equations and subsequent phase diagrams were constructed. The phase diagrams were compared with those predicted using FactSage thermochemical package and literature data from experimental studies. The calculated NaNO3-KNO3 phase diagram shows that this system has a eutectic point at 0.5 mole fraction of KNO3 with a eutectic temperature of 220.85 °C, while the LiNO3-KNO3 system was found to have a eutectic point at 0.44 mole fraction LiNO3 at 137 °C. The LiNO3-NaNO3 system shows a eutectic at 0.524 mole fraction LiNO3 at 192.8 °C, and finally the NaCl-KCl system has a eutectic point at 0.48 mole fraction NaCl at 634 °C. The liquidus and solidus curves obtained fitted close against values obtained from the quasi-chemical model used in FactSage and experimental values from the literature. This study showed that the regular solution model combined with the contour plots approach can sufficiently describe the liquidus–solidus curves of the alkali binary nitrate and chloride systems and could be a useful method for predicting the phase diagrams for higher order alkali nitrates and chlorides.

Natural convection heat transfer for eutectic binary nitrate salt based Al2O3 nanocomposites in solar power systems

Molten salts are widely used as thermal energy storage materials in a concentrated solar plant. In these thermal storage systems, natural convection heat transfer for molten salts is important for both energy storage and transfer, however the research on nature convection heat transfer is quite limited. In present work, the natural convection heat transfer for eutectic binary nitrate salt based Al2O3 nanocomposites was numerically simulated using a two-phase lattice Boltzmann model. Various interactions affecting the distribution of nanoparticles were considered in the simulation. The effects of the nanoparticle mass fraction (0–2.0%) and Rayleigh number (104–108) on natural convection heat transfer were also analyzed. Results indicated that natural convection heat transfer increased with increasing the Ra. With the addition of nanoparticles, natural convection heat transfer decreased at low Ra numbers but increased at high Ra numbers. Among all interaction forces considered in this study, the temperature difference driving force FS was much larger than other forces, which means the nanoparticle distribution was mainly influenced by FS.

Optical properties of high temperature molten salt mixtures for volumetrically absorbing solar thermal receiver applications

Molten salts are promising candidates for liquid volumetric absorbers in concentrated solar power systems. To characterize absorption and heat transfer performance in high temperature applications, their optical properties are required. Thus a method for experimentally determining the absorption coefficient of non-scattering high temperature semi-transparent liquids for large (∼1m-deep) direct absorption solar receiver applications was developed. It was used to measure the absorption coefficient in liquids over a broad spectral range and temperatures up to 800°C in a 40wt.% KNO3:60wt.% NaNO3 binary nitrate molten salt mixture (solar salt) and a 50wt.% KCl:50wt.% NaCl binary chloride molten salt mixture. The binary nitrate and binary chloride both demonstrated well distributed solar absorption (>95% absorption through 1m and 2m, respectively). At 400°C, the binary nitrate is optically thick in its re-emission spectrum and behaves as a blackbody radiator. The effects of thermal decomposition were also shown to have significant consequences on the overall performance of the binary nitrate mixture, transforming it into an opaque surface absorber following thermal degradation (>95% in <0.25m). The implications of these results for solar receiver design are discussed in terms of volumetric absorption, total effective emissivity, and capture efficiency. The measurement technique developed and results are relevant in a variety of high temperature applications including heat transfer systems, materials processing, pharmaceuticals, and food processing facilities.

Inelastic intermolecular exchange of vibrational quanta and relaxation of vibrationally excited states in binary solid systems

The processes of molecular relaxation in the binary nitrate–perchlorate solid systems LiNO3–LiClO4, NaNO3–NaClO4, and KNO3–KClO4 have been investigated using Raman spectroscopy. It has been found that the relaxation time of the ν1(A) vibration of the NO3- anion in the binary solid system is shorter than that in the pure metal nitrates. It has been shown that an increase in the relaxation rate is caused by the existence of an additional mechanism of relaxation of vibrationally excited states of the nitrate ion in the system. This mechanism is associated with the excitation of a vibration of another anion (ClO4-), as well as with the “creation” of a lattice phonon. It has been established that the condition for the realization of the relaxation mechanism is that the difference between the frequencies of the aforementioned vibrations should correspond to the range of sufficiently high densities of states of the phonon spectrum.

Effect of Al2O3 nanoparticle dispersion on the specific heat capacity of a eutectic binary nitrate salt for solar power applications

Molten salts can be used as heat transfer fluids or thermal storage materials in a concentrated solar power plant. Improving the thermal properties can influence the utilization efficiency of solar energy. In this study, the effect of doping eutectic binary salt solvent with Al2O3 nanoparticles on its specific heat capacity (cp) was investigated. The effects of the mass fraction of nanoparticles on the cp of the composite nanofluid were analyzed, using both differential scanning calorimetry measurements and molecular dynamics simulations. The specific heat capacity of the nanocomposites was enhanced by increasing the nanoparticle concentration. The maximum enhancement was found to be 8.3%, at a nanoparticle concentration of 2.0%. A scanning electron microscope was used to analyze the material morphology. It was observed that special nanostructures were formed and the specific heat capacity of the nanocomposites was enhanced by increasing the quantity of nanostructures. Simulation results of cp agreed well with the experimental data, and the potential energy and interaction energy in the system were analyzed. The change in Coulombic energy contributed to most of the large change in cp, which explains the discrepancy in values between conventional nanofluids and molten salt-based nanofluids.

Solid state interaction studies on binary nitrate mixtures of uranyl nitrate hexahydrate and lanthanum nitrate hexahydrate at elevated temperatures

Interaction behavior of uranyl nitrate hexahydrate (UNH) and lanthanum nitrate hexahydrate (LaNH) have been investigated on the mixtures in different molar ratios of the two precursors and monitoring the reactions at elevated temperatures with thermoanalytical and X-ray diffraction measurement techniques. During the decomposition of equimolar mixture of LaNH and UNH, formation of lanthanum uranate (U0.5La0.5)O2, was seen by the temperature of 500 °C along with lanthanum oxide (La2O3) and uranium trioxide (UO3). By the temperature of 700 °C, the formation of uranium sesquioxide (U3O8) was observed along with (U0.5La0.5)O2 as end products in uranium rich mixtures. Lanthanum rich compositions decomposed by the temperature of 700 °C to give (U0.5La0.5)O2 and La2O3 as end products.

Experimental and numerical study of heat transfer performance of nitrate/expanded graphite composite PCM for solar energy storage

Eutectic molten salt can be used as the latent thermal energy storage medium in solar energy applications. Nitrates and their binary mixtures are suitable phase change material (PCM) for solar energy applications in middle-temperature-range of 200–300°C. In the present study, binary nitrate (50wt.% NaNO3, 50wt.% KNO3) with a melting temperature of about 220°C was employed as the PCM, and expanded graphite (EG) with the mass fraction of 5%, 10% or 20% was used to enhance the thermal conductivity. The thermal conductivities of pure nitrates and nitrate/EG shape-stabilized composites were measured with a steady-state test rig firstly. Results showed that the addition of EG significantly enhanced the thermal conductivities, e.g., the thermal conductivities of sodium nitrate/20wt.% EG composite PCM were measured to be 6.66–7.70W/(mK) in the temperature range of 20–120°C, indicating about seven times larger than those of pure sodium nitrate. Furthermore, pure binary nitrate and nitrate/EG composite PCM were encapsulated in a cylindrical storage unit with a diameter of 70.0mm and a length of 280.0mm. Heat storage and retrieval tests were conducted extensively at different heating temperatures of 250°C, 260°C and 270°C, and different cooling temperatures of 30°C, 70°C and 110°C. Time-durations from temperature evolutions showed that both the melting and solidification processes were accelerated by EG, and the heat transfer characteristics were interpreted by the numerical analysis based on enthalpy–porosity and volume-of-fluid models. The evolution of nitrate/air interface caused by volume expansion ascended gradually during melting, while that caused by volume shrinkage descended during freezing.

Mechanical Dispersion of Nanoparticles and Its Effect on the Specific Heat Capacity of Impure Binary Nitrate Salt Mixtures.

In this study, the effect of nanoparticle concentration was tested for both CuO and TiO2 in eutectic mixture of sodium and potassium nitrate. Results showed an enhancement in specific heat capacity (Cp) for both types of nanoparticles (+10.48% at 440 °C for 0.1 wt % CuO and +4.95% at 440 °C for 0.5 wt % TiO2) but the behavior toward a rise in concentration was different with CuO displaying its highest enhancement at the lowest concentration whilst TiO2 showed no concentration dependence for three of the four different concentrations tested. The production of cluster of nanoparticles was visible in CuO but not in TiO2. This formation of nanostructure in molten salt might promote the enhancement in Cp. However, the size and shape of these structures will most likely impact the energy density of the molten salt.

Preparation and Heat Transfer/Thermal Storage Properties of High-Temperature Molten Nitrate Salts

The thermal stability of molten salts, operating temperature range and latent heat of melting for the molten salts at high temperature have been studied in the present investigation. The multi-component molten salts composed of purified potassium nitrate, purified sodium nitrate were prepared by statical mixing method [1]. The stability experiments were carried out at 500 to 600°C, and the experimental result showed that the purified nitrate molten salts performed better high-temperature thermal stability and its optimum operating temperature was increased from 500°C to 550°C. DSC analysis indicated that the purified nitrate molten had a lower melting point and a higher phase change latent heat. The melting point of purified binary nitrate molten salts was sharp decreased to 225.2°C and latent heat of melting for molten salts was also reduced from 78.41J/g to 81.15J/g compared with unpurified nitrate salts. Besides, the change in the concentration of impurities by analyzing in the binary molten salts, and combination of XRD test results can be found that the degree of degradation reduce and improve the thermal efficiency of the storage of binary molten salts by purified sodium nitrate and potassium nitrate.