Preparation And Experimental Study On Thermal Properties Of Mixed Bromide Salts

Solar thermal power generation is one of the most promising power generation methods in renewable energy applications,and the heat transfer and storage technology is a key factor of solar thermal power generation system.As a new type of medium for heat transfer and storage,molten salt has the advantages of low viscosity,low saturated vapor pressure,wide operating temperature range and low cost,and it has been tested and applied in large scale successfully in many solar thermal power plants.At home and abroad,researches on molten salts are mainly concentrated in nitrate,fluoride,carbonate and chloride,and the application of molten salt in the solar thermal power generation system is mainly based on the mixed nitrate salts.At present,there are few studies on the thermal properties and application of bromide salts,especially for the mixed bromide salts.The research object of this paper is bromide salts.Six samples of quaternary bromide salts from KBr,LiBr,NaBr,and CaBr2 were made with different mass ratios.After improving the melt thermal physical property measurement system,the densities of the bromide samples were measured by Archimedes method,and fitted as functions of temperature.The density values of the six samples were in the range of 2.4~2.7g/cm3.Relative to other molten salts,the density of bromide samples is relatively large.The density values of each sample show a linear trend to decline with the increase of temperature.At the same temperature,the density of 2# is the highest,while the density of 6# is the lowest.Also,expansion coefficients of the samples were calculated based on density values.In the measuring temperature range,expansion coefficient of each sample rises slowly with increasing temperature.All expansion coefficient values ranges within 2.30×10-4~3.09×10-4 1/?.The surface tensions of the six bromide samples were measured by the maximum pull force method.The measured values were in the range of 0.11~0.13 N/m and were fitted as functions of temperature.And they diminish linearly with increasing temperature.At the same temperature,the surface tension of 2# is the highest,while the surface tension of 6# is the lowest.In order to explore the high temperature corrosion characteristics of bromide salts,316 L,304 stainless steel and Q235 carbon steel were selected as test materials.The 2# bromide sample was used as the corrosive liquid.Static corrosion tests for different time of metal samples were performed in bromide salts at 450? with immersion method.The weight loss of metal samples increase with the increasing of etching time.After 360 h in bromide salts,the average weight losses of 316 L,304,Q235 are 1.45mg/cm2,3.77mg/cm2 and 24.14mg/cm2 respectively.The corrosive effect of bromide salts on Q235 carbon steel is the strongest,while that on 316 L stainless steel is the slightest.Being corroded for 288 h,316 L sample is relatively flat,but small corrosion cracks has formed on the surface.Surface of the 304 sample is corroded more badly.In the process of corrosion,some corrosion layers are peeled off from the substrate of 304 sample.Surface of Q235 is corroded most seriously,and its surface has some wider grooves,finally loose corrosion product layer is formed.The compositions of 316 L sample corrosion surface mainly contain carbide with rich Cr and LiFeO2.And on 304 sample corrosion surface,LiFeO2?LiMnO2 have been formed,while Iron and manganese oxides appear on Q235 sample corrosion surface.Electrochemical corrosion has occurred on the surfaces of three kinds of metal samples.