Study On Thermal Stability Of Chloride Salt And Enhanced Thermal Properties Of Molten Salt

Facing the ever-increasing energy crisis and escalating environmental issues,energy transition cannot be delayed.The efficient heat transfer and heat storage technology provides a good solution for the large-scale industrialization of renewable energy.The nature of the heat transfer material will directly affect the conversion and utilization of the entire energy exchange system.Because of its low price,wide operating temperature range,low vapor pressure,and good fluidity,molten salt is widely used in heat transfer and heat storage systems as an ideal transfer heat storage material.Common molten salt materials include carbonate salt,nitrate salt and chloride salt.Different types of molten salt can meet the heat transfer requirements of different temperature ranges.China is a country with rich resources of saline lake.The design and development of chloride salts has always been a research hotspot.However,when chloride salts are used in pipelines,there are problems such as high temperature decomposition and evaporation,which resulting in the short service life of chloride salts and serious damage to equipment.Therefore,it is of great significance to study the thermal stability of chloride salts at high temperatures and its instability mechanism.At the same time,when molten salts is used as heat transfer material in the system,there exist common problems such as low thermal conductivity and low specific heat capacity,especially in the liquid phase of molten salts.Therefore,on the basis of studying the stability of chloride salt,this article investigate the thermal physical enhancement of different kinds of molten salt which aim to provide molten salt composite with more practical application value.In this paper,the high-temperature thermal stability of ternary chloride salt system?NaCl-CaCl₂-MgCl₂?in open and closed systems were both studied.The components before and after heat storage were analyzed by titration and atomic absorption spectrometry.Scanning calorimeter?DSC?and laser thermal conductivity tester?LFA457?were employed to monitor the thermal properties change of chloride salts before and after thermal storage.X-ray diffraction?XRD?and scanning electron microscopy?SEM?were used to analyze the phase and morphology of the degraded hydrolyzed products.The instability mechanism of the chloride salts stored in the open and closed system was given.The results demonstrate that the mass loss of chloride salts after heating in the open system is very serious.It is mainly caused by the hydrolysis of MgCl₂ and evaporation of NaCl.The mass loss of chloride salts leads to the performance deterioration of molten salt,which mainly manifested as a substantial increase in melting point and large decrease of latent heat.Instead,the thermal stability of molten salt after heating in a closed system is significantly improved.The results comparison shown that reduce the probability of contact air with chloride salts can prevent the performance deterioration of the molten salt and improve high-temperature thermal stability of molten salt.Meanwhile,the hydrolysis product of MgCl₂ results in a significant increase in the thermal diffusivity of chloride salts.In the study of the high temperature thermal stability of chloride salts,it was found that the formation of MgO can increase its thermal diffusivity.On the basis of this,the mass fraction of 10wt%MgO nanoparticles were doped in carbonate salts liquid,nitrate salts liquid and chloride salts liquid to prepare different kinds of MgO-molten salts composites?MgO-carbonated salts,MgO-nitrate salts,MgO-chloride salts?.Different experimental instruments were used to determine the specific heat capacity,density,and thermal diffusivity of three kinds of molten salts composites.SEM was used to observe the dispersion of MgO nanoparticles in different molten salts to preliminary explain the thermal physical enhancement mechanism of molten salt composite materials.The results shown that the addition of MgO nanoparticles significantly improved the thermal conductivity of carbonate salts,nitrate salts and chloride salts,and MgO nanoparticles can uniformly dispersed in molten salt liquid.The maximum increase rates compared to the original molten salt were122.9%?MgO-carbonate salts?,60.5%?MgO-chloride salts?,and 114.0%?MgO-nitrate salts?,respectively.The MgO-nitrate salts composites with different mass fraction of MgO nanoparticles were prepared by adding different concentration of MgO nanoparticles to the nitrate salts liquid.DSC thermal analyzer,LFA457 laser thermal conductivity tester and MT-ZRIG-12melt physical properties were used to measure the specific heat,thermal diffusiviy,density,melting point and latent heat of the composites,respectively.The surface morphology of MgO-nitrate salts composites and MgO nanoparticles were observed by SEM.Meanwhile,the crystal structure of MgO nanoparticles was observed by TEM,which can be used to explain the thermal physical enhancement mechanism of MgO-nitrate salts composites.The study found that the addition of MgO nanoparticles resulted in a significant increase in the specific heat capacity and thermal conductivity of nitrate salts.The specific heat and thermal conductivity of nitrate salts composites almost increased linearly with the increase mass faction of MgO nanopaticles.The abnormal increase in the specific heat capacity of MgO-nitrate salts is due to the semi-solid boundary layer structure formed between the nitrate salts and MgO nanoparticles,and the reason for the thermal conductivity enhancement is the interaction of the semi-solid boundary layer and the Brownian motion of nanoparticles in the molten salt liquid.Since MgO nanoparticles exist in the form of solid particle in the nitrate salts liquid,the latent heat of the MgO-nitrate composites decreases with the increase of MgO nanoparticles.