影响硝酸熔盐高温稳定性的因素
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摘要
熔盐是太阳能热利用领域重要的储热材料之一,熔盐使用温度上限越高,储热量就越大,能量传递效率越高。为了提高熔盐使用温度上限,以Solar Salt与HITEC两种常用硝酸熔盐为对象,将实验和量子化学计算相结合研究了影响高温稳定性的因素。首先通过静态熔融法制备了Solar Salt与HITEC熔盐,并通过DSC-TG曲线分析了二者的高温稳定性,通过XPS分析了分解产物,通过计算软件模拟了硝酸盐分解为亚硝酸盐的过程,采用的是B3LYP泛函和6-31+G~*基组。最后从反应机理的角度对影响二者高温稳定性的因素进行了讨论。研究结果表明,当熔盐升温至600℃时,Solar Salt和HITEC的失重分别为2%和1%,并且亚硝酸盐没有进一步分解为金属氧化物或反应进度较小,K~+和Na~+的半径不一致,二者比例的不同是造成这两种熔盐稳定性差异的因素之一。同时,由反应物浓度不同导致化学平衡的移动,这也使HITEC较难分解。
Molten salts play a key role in thermal energy storage of solar thermal utilization. The higher the upper limit of the usable temperature of molten salt, the greater the heat storage density and the higher the energy transfer efficiency. This work uses both experiments and quantum chemical calculations to study factors affecting the stability of two molten salts of Solar Salt and HITEC at a high temperature. Solar Salt and HITEC were firstly prepared, which involved drying of NaNO_3 and KNO_3 powder in a vacuum oven at 120 ℃ for 24 hours; mixing the dried NaNO_3 and KNO_3 in a mortar with 60%:40%(mass ratio) followed by grinding of the mixture; heating the ground powder mixture in a resistance furnace at 400 ℃ for 12 hours; cooling the molten salt down and grounding the sample to a powder form. The resulting material gave the Solar Salt. The HITEC were prepared by using the same method with mass ratio of KNO_3, NaNO_3 and NaNO_2 being 53%:7%:40%(weight fraction). High-temperature stabilities of samples were studied by DSC-TG analyses. The decomposition of the products was analyzed by XPS. The nitrate decomposition into nitrite was simulated by a software based on B3LYP functional with a base group of 6-31+G~*. Finally factors affecting the stability of the nitrate salts were discussed from the reaction mechanisms. The results showed that, when heated to 600 ℃, Solar Salt and HITEC mass losses were 2% and 1%, respectively. No metal oxides were produced or the production rate is too low to be observed. The difference in the metal ion proportion and the difference in the acid radical proportion were regarded as possible factors for the difference in the stabilities of the molten salts. The radii of the metal ions were found to be different, leading to different energies of intermediates and transition states. The movement of chemical equilibrium, caused by reactant, was considered as an additional reason for the different thermal stabilities of the Solar Salt and HITEC.
Molten salts play a key role in thermal energy storage of solar thermal utilization. The higher the upper limit of the usable temperature of molten salt, the greater the heat storage density and the higher the energy transfer efficiency. This work uses both experiments and quantum chemical calculations to study factors affecting the stability of two molten salts of Solar Salt and HITEC at a high temperature. Solar Salt and HITEC were firstly prepared, which involved drying of NaNO_3 and KNO_3 powder in a vacuum oven at 120 ℃ for 24 hours; mixing the dried NaNO_3 and KNO_3 in a mortar with 60%:40%(mass ratio) followed by grinding of the mixture; heating the ground powder mixture in a resistance furnace at 400 ℃ for 12 hours; cooling the molten salt down and grounding the sample to a powder form. The resulting material gave the Solar Salt. The HITEC were prepared by using the same method with mass ratio of KNO_3, NaNO_3 and NaNO_2 being 53%:7%:40%(weight fraction). High-temperature stabilities of samples were studied by DSC-TG analyses. The decomposition of the products was analyzed by XPS. The nitrate decomposition into nitrite was simulated by a software based on B3LYP functional with a base group of 6-31+G~*. Finally factors affecting the stability of the nitrate salts were discussed from the reaction mechanisms. The results showed that, when heated to 600 ℃, Solar Salt and HITEC mass losses were 2% and 1%, respectively. No metal oxides were produced or the production rate is too low to be observed. The difference in the metal ion proportion and the difference in the acid radical proportion were regarded as possible factors for the difference in the stabilities of the molten salts. The radii of the metal ions were found to be different, leading to different energies of intermediates and transition states. The movement of chemical equilibrium, caused by reactant, was considered as an additional reason for the different thermal stabilities of the Solar Salt and HITEC.
引文
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[2]王鹏,罗尘丁,巨星.光热电站熔盐传热储热技术应用[J].电力勘测设计,2017(2):67-71.WANG Peng,LUO Chending,JU Xing.Application of molten salts for heat transfer and storage technique for molten salts in concentrating solar power plant[J].Electric Power Survey&Design,2017(2):67-71.
[3]FERNANDEZ A G,USHAK S,GALLEGUILLOS H,et al.Development of new molten salts with LiNO3 and Ca(NO3)2 for energy storage in CSP plants[J].Applied Energy,2014,119:131-140.
[4]魏小兰,尹月,丁静,等.镁基三元氯化物熔盐储热过程性能变化机理分析[J].太阳能学报,2018,39(1):37-43.WEI Xiaolan,YIN Yue,DING Jing,et al.Mechanismanalysis of performance change of ternary chloride molten salt storage process[J].Acta Energiae Solaris Sinica,2018,39(1):37-43.
[5]程晓敏,徐凯,朱闯,等.高温处理对LiNO3-NaNO3-KNO3熔盐固相线温度的影响[J].储能科学与技术,2017,6(5):1094-1098.CHENG Xiaomin,XU Kai,ZHU Chuang,et al.Influence of heat treatment on solidus temperature of LiNO3-NaNO3-KNO3 molten salt[J].Energy Storage Science and Technology,2017,6(5):1094-1098.
[6]朱教群,陈维,周卫兵,等.三元硫酸熔盐的制备及其热稳定性能[J].储能科学与技术,2016,5(4):498-502.ZHU Jiaoqun,CHEN Wei,ZHOU Weibing,et al.Preparation and thermal stability of a ternary sulfate molten salt[J].Energy Storage Science and Technology,2016,5(4):498-502.
[7]李彦,余杨敏,李鹏,等.LiNO3-KNO3二元混合硝酸盐热稳定性分析[J].上海电力学院学报,2018,34(1):37-40.LI Yan,YU Yangmin,LI Peng,et al.Thermal stability analysis of LiNO3 binary mixed nitrates[J].Journal of Shanghai University of Electric Power,2018,34(1):37-40.
[8]彭强,杨晓西,丁静,等.三元硝酸熔盐高温热稳定性实验研究与机理分析[J].化工学报,2013(5):1507-1512.PENG Qiang,YANG Xiaoxi,DING Jing,et al.Experimental study and mechanism analysis for high-temperature thermal stability of ternary nitrate salt[J].Journal of Chemical Industry and Engineering(China),2013(5):1507-1512.
[9]聂挺,单艳,胡川,等.量子化学计算对4种抗氧化肽清除自由基活性机理判别分析[J].南昌大学学报(理科版),2015,39(1):70-75.NIE Ting,SHAN Yan,HU Chuang,et al.Study on the activity mechanism of free redical scavenging of antioxidant peptides by quantum chemical calculation[J].Journal of Nanchang Universiy(Natural Science),2015,39(1):70-75.
[10]黄晶晶,齐永锋,王妹婷,等.不同气氛下煤焦吸附NO的量子化学计算[J].化学通报,2015,78(7):655-658.HUANG Jingjing,QI Yongfeng,WANG Meiting,et al.Quantum chemistry calculation of nitric oxide adsorption on char under different atmospheres[J].Chemistry Bulletin,2015,78(7):655-658.
[11]王瑜,刘建,曾勇,等.量子化学计算在硫化铅锌矿浮选机理中的研究进展[J].矿产保护与利用,2018(3):37-42+48.WANG Yu,LIU Jian,ZENG Yong,et al.Quantum chemistry calculation in lead-zinc sulfide ore flotation:A review[J].Conservation and Utilization of Mineral Resources,2018(3):37-42+48.
[12]IVERSON B D,BROOME S T,KRUIZENG A M,et al.Thermal and mechanical properties of nitrate thermal storage salts in the solid-phase[J].Solar Energy,2012,86(10):2897-2911.
[13]NISSEN D A,MEEKER D E.Nitrate/nitrite chemistry in sodium nitrate-potassium nitrate melts[J].Inorganic Chemistry,1983,22(5):716-721.
[14]NANAYAKKARA C E,JAYAWEERA P M,RUBASINGHEGE G,et al.Surface photochemistry of adsorbed nitrate:The role of adsorbed water in the formation of reduced nitrogen species on alpha-Fe2O3particle surfaces[J].Journal of Physical Chemistry A,2014,118(1):158-166.
[15]AL-REFAIE A A,WALTON J,COTTIS R A,et al.Photoelectron spectroscopy study of the inhibition of mild steel corrosion by molybdate and nitrite anions[J].Corrosion Science,2010,52(2):422-428.
[16]NESBITT H W,BANCROFT G M,HENDERSON G S,et al.Bridging,non-bridging and free(O2-)oxygen in Na2O-SiO2 glasses:An X-ray photoelectron spectroscopic(XPS)and nuclear magnetic resonance(NMR)study[J].Journal of Non-Crystalline Solids,2011,357(1):170-180.
[17]HYUNJIN Kim,DONG Young Kim,YONGSU Kim,et al.Na insertion mechanisms in vanadium oxide nanotubes for na-ion batteries[J].ACS Applied Materials and Interfaces,2015,7(6):1477-1485.
[18]LEE Jaeryeong,KIM Youngjin.Chemical dissolution of iridium powder using alkali fusion followed by high-temperature leaching[J].Materials Transactions,2011,52(11):2067-2070.
[19]OLIVARES R I.The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres[J].Solar Energy,2012,86(9):2576-2583.
[20]STERN K H.High temperature properties and thermal decomposition of inorganic salts with oxyanions[M].Boca Raton:CRC Press,2001.
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[22]FLORI M,GRUZZA B,BIDEUX L,et al.A study of the 42CrMo4steel surface by quantitative XPS electron spectroscopy[J].Applied Surface Science,2008,254(15):4738-4743.
[23]CHAUVAUT V,ALBIN V,SCHNEIDER H,et al.Study of cerium species in molten Li2CO3-Na2CO3 in the conditions used in molten carbonate fuel cells.Part I:Thermodynamic,chemical and surface properties[J].Journal of Applied Electrochemistry,2000,30(12):1405-1413.