Stability And Performance Enhancement Of Molten Nitrate Salt During Heat Transfer And Storage Process

Solar thermal power generation has shown great potential in mitigating environmental pollution and energy crisis.The heat transfer and storage system is an important part of solar thermal power generation,and the heat transfer and storage material is the core of the heat transfer and storage system.Molten nitrate salt has been successfully used as a heat transfer and storage material in commercial solar thermal power plants because of its low melting point,low vapor pressure,low viscosity,and wide temperature range.However,the progecess of molten nitrate salts flow through metal tubes,tanks and valves,or contact with storage solid materials in a single tank heat storage system may cause occcuration of reactions resulting in variation of performance of molten salt due to the oxidative and thermal decomposition of molten nitrate salt during the progecess of high temperature heat transfer and storage.In addition,the molten nitrate salt fluid has the same problems of low thermal conductivity and specific heat as other fluids.Therefore,it is of great practical significance to study the stability of the molten salt in the process of heat transfer and storage and to enhance its properties of heat transfer and storage.In this paper,the stability of molten nitrate salt during heat storage was studied by monitoring NOx release during heat storage.Refer to the HJ479-2009 national standard,the release of NOx from Binary nitrate salt?Solar salt?contacting with the metal and heat storage solid materials and ternary nitrate molten salt?Hitec?contacting with the metal was monitored at high temperature.The melting point,melting enthalpy and specific heat of Solar Salt and Hitec after long-term contact with metal were measured.The results showed that the NOx from Solar Salt contacting with the metal and storage solid materials and Hitec contacting with metal increases substantially with increasing temperature.During long-term heat storage at 500°C,the NOx emission from Solar Salt and Hitec contacting with the metal decreased due to the formation of oxide film on the metal surface.After 600 h of heat storage,its melting point,melting enthalpy and specific heat did not change much.Although heat storage process resulted in NOx release from molten nitrate salt,it had little effect on the thermal properties of the molten salt,and the heat storage stability of the molten nitrate salt is good.Thermodynamic analysis confirmed that production and differences of NOx are related to the reaction between molten nitrate and elements in the metal to produce NO/NO₂.The nano-fluid was formed by the composite nanoparticles and the molten salt to enhance the heat transfer and heat storage performance of the molten salt.Nano-MgO with low density and easy suspension and good thermal conductivity was selected to analyze its feasibility for strengthening the performance of molten nitrate salt.The MgO composite Hitec materials with different concentrations and particle sizes were prepared by mechanical mixing method at high temperature?two-step method?.The effects of nano-MgO addition amount and particle size on the specific heat,density,thermal diffusivity,thermal conductivity,viscosity and melting point,and melting enthalpy of MgO composite Hitec materials were investigated.The long-term stability of MgO composite Hitec materials was investigated.The surface morphology of MgO composite Hitec material and the crystal structure of nano-MgO were observed by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?,respectively,to explain the mechanism of enhancement of the thermal properties.The results showed that the optimum addition amount?4.5 wt.%?and the optimum particle size?60 nm?of MgO obtained the maximum specific heat increase,and the enhancement of thermal conductivity increases with the increase of the addition amount and the decrease of the particle size.The significant increase in the specific heat of the MgO composite Hitec material is related to the semi-solid layer formed on the MgO particles;the significant increase in the thermal conductivity is related to microperturbation of the molten salt caused by the Brownian motion of the nanoparticles and the preferential thermal conductivity of the liquid layer on the surface of the particles.The viscosity of MgO composite Hitec material did not change much compared with that of the base salt,the density of MgO composite Hitec material is slightly increased compared with that of the base salt,resulting in enhancement of the heat transfer coefficient.And the MgO composite Hitec had good long-term stability.The corrosion of the MgO composite Hitec material prepared by the two-step method on stainless steel 310S,nickel-based alloy Hastelloy C276?C276?and 45#carbon steel was investigated by immersion method.The effect of the addition of MgO on the corrosion of Hitec and the heating time on corrosion resistance of the metal in the MgO composite Hitec material was investigated.The kinetic curves and rate curves of corrosion of three metals in Hitec and MgO composite Hitec materials under the experimental conditions were given,respectively.Corrosion products and morphology of surface of the metals were analyzed by X-ray diffraction?XRD?and scanning electron microscopy?SEM?.The results showed that the addition of MgO does not increase the corrosiveness of Hitec.The surface of C276 will form a protective layer of MgFe₂O₄ with increasing heating time,which resulted in the best corrosion resistance.Based on above research,the low-cost preparation of MgO composite nitrate molten salt material was studied by in-situ method using the hydrous magnesia?MgCl₂·6H₂O?waste ore produced by potash fertilizer as raw material in the process of heating and storage heat of molten nitrate salt.The microstructure and specific heat of the MgO composite nitrate molten salt materials were characterized and measured by XRD,SEM and DSC.The results show that the MgO composite Solar salt material prepared by slow heating has a Tyndall effect but also has MgO coagulation,and the specific heat is hardly enhanced.The MgO composite Hitec material MgO prepared by rapid electromagnetic heating showed higher increase in the heat specific with the nanometer-sized and well dispersed MgO than that of the composite prepared by the two-step method...