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NOx emissions and NO2- formation in thermal energy storage process of binary molten nitrate salts
Energy 74 (2014) 215e221
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NOx emissions and NO2- formation in thermal energy storage process of binary molten nitrate salts Xiaolan Wei a, b, Yan Wang a, Qiang Peng a, Jianping Yang a, Xiaoxi Yang c, Jing Ding d, * a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China Key Laboratory of Fuel Cell Technology of Guangdong Province, Guangzhou 510640, PR China c Dongguan University of Technology, Dongguan 523000, PR China d Center for Energy Conservation Technology, School of Engineering, Sun Yat-sen University, Guangzhou Higher Education Mega Center, No. 132 Waihuan Dong Road, Guangzhou 510006, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2013 Received in revised form 10 March 2014 Accepted 17 May 2014 Available online 17 June 2014
Referring to the national environmental protection standard of PR China, HJ479-2009, NOx emissions and the effect of 45# carbon steel (1045, ASTM) on it in thermal energy storage (TES) process of binary molten nitrate (BMN) salts (a eutectic salt mixture named as solar salt with the component of 60% NaNO3 and 40% KNO3) are discussed in this paper. The accumulative absorption concentrations of NOx emissions in tail gases of molten salts heated at different temperatures were measured to study the thermal decomposition situation of molten salts in TES process. Furthermore, the concentrations of nitrite ion in molten salts samples were determined and chemical thermodynamic calculations of related reactions were done to explain the results. The research shows that BMN salts contained in silica or carbon steel container in the air under certain conditions can release NOx in its use of temperature range. The concentrations of NOx emissions and nitrite ion in molten salts both increase with the rise of the temperature. After contacting with 45# carbon steel, the total concentrations of NOx emissions and nitrite ion in molten salts increased, which may be caused by Fe component in the carbon steel. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Binary molten nitrate salts NOx emissions Carbon steel
1. Introduction In recent years, alkali metal nitrate salts are used as heat transfer and thermal energy storage (HT and TES) medium in concentrating solar power (CSP) system due to low melting point and vapor pressure, cheap price and wide operating temperature range. In 2011, the world's first commercial solar power station, Spain Gemasolar power plant, was built and solar salt was fully applied in the HT and TES system. However, due to the thermal decomposition of nitrate salts, there are some problems such as the changes in the component of molten salts as well as potential risks of NOx released in HT and TES process [1]. Change in the composition of molten salts will cause the variation of its thermal properties, which may result in the decrease of TES efficiency and affect the working life of molten salts. Meanwhile, large amounts of NOx emissions will have a major effect on the container surface by impacting on corrosion. Therefore, monitoring changes in the composition and NOx emissions of BMN (binary molten nitrate) salts in HT and TES process has
* Corresponding author. Tel.: þ86 20 39332320; fax: þ86 20 39332319. E-mail addresses: [email protected], [email protected] (J. Ding). http://dx.doi.org/10.1016/j.energy.2014.05.064 0360-5442/© 2014 Elsevier Ltd. All rights reserved.
important significance on the TES efficiency of molten salts and the utilization of renewable energy. The pyrolysis reaction of nitrate salts is very complex [24]. By conducting thermodynamic calculations of thermal decomposition reactions of ternary molten nitrate salts at high temperatures, Long [5] showed that the DrGqm ðTÞ values of reactions 2NaNO2(l) þ O2(g) ¼ 2NaNO3(l) and 5NaNO2(l) ¼ 3Na NO3(l) þ Na2O(s) þ N2(g) were both negative between 600 K and 1000 K, and the phenomenon that the concentration of nitrite ion decreased constantly was the result of thermodynamic calculation. Gordon [6] found that the bubbling phenomenon was observed at the surface of molten nitrate salts when the temperature was slightly higher than the melting point and the release of gaseous products was shown by this phenomenon. Erik Pihl [7] pointed out that when using nitrate salts there would be some level of decomposition to nitrite and other secondary products, mainly NOx. Freeman [8,9], Bartholomew [10] and Stern [11] studied the thermal decomposition of single NaNO3 or KNO3 respectively, and found that each of them decomposed to nitrite salt and oxygen, that was NO 3 ¼ NO2 þ 1/2O2. Freeman [8] detected N2 and a small amount of NO2 except for the monitoring of O2 in the experiment with a stainless steel as container under the cover gas of argon. Bartholomew [10] revealed that some
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side reactions could involve the evolution of NOx in addition to the release of O2 when molten salts were heated in a stainless steel or fine silver crucible. NaNO2 generated from decomposition of NaNO3 was unstable above 330 C in the air. Stern [11] found the further decomposition of NaNO2 would produce Na2O, NO, and NO2 and it also could decompose to Na2O, N2 and O2 in an inert atmosphere; Stern [11] also noted that nitrate salts decomposed at high temperatures might release N2, O2, NO, NO2, N2O3, N2O4, N2O5, etc., which of these gases were produced depended on the experimental conditions such as the salt itself and temperature; NO and NO2 were the main gases at temperature above 500 K, there was no need to consider the presence of other gases. Olivares [4] suggested that ternary molten nitrate salts (7%NaNO3e53%KNO3e40%NaNO2) melt in master alloy would release NO firstly and NO2 secondly at four different atmospheres (Ar, N2, O2, and air) in the heating process from room temperature to 1000 C. Kust [12] researched the content of thermal decomposition product of solar salt at 300 C in oxygen and demonstrated that the temperature of nitrate ion (NO 3 ) decomposed to nitrite ion (NO 2 ) to a measurable extent was as low as 295 C. Nissen and Meeker [13] measured the content of NO in decompo2 sition products of solar salt at 550 and 600 C in different oxygen partial pressures (21e100, vol%) and found that the percentage of nitrite ion increased with the rising of temperature while decreased with the climbing of the oxygen partial pressure; In other words, temperature and atmosphere affected the decomposition of molten nitrate salts. By carefully analyzing gaseous decomposition products of molten nitrate salts in above literature, it was found that container material used in the experiment may have some effects on thermal decomposition of molten nitrate salts. Although Freeman [8] indicated that NOx emissions in pyrolysis of potassium nitrate was less than 1% when using the stainless steel container, however, no previous studies in literature about the impact of container material on NOx emissions and composition changes of molten salts have been reported systematically. Thus, it seems necessary to design an experimental device to investigate the effect of container material on secondary gaseous products (mainly NOx) of thermal decomposition of molten nitrate salts except for the evolution of O2. It's obvious that thermal decomposition happened in molten salts if NOx emissions can be detected in the gas phase. As a result, the composition of molten salts will change, followed with the variation of TES stability. This is also the reason that molten salts needed to be replaced after running for a long time. In actual large scale applications, molten salts had a certain degree of contact with carbon steel or stainless steel tube or tank. Therefore, investigating the reason of NOx emissions of molten nitrate salts and the corresponding effect of various steels have important significance on the practical application of molten nitrate salts. Hence, the effect of 45# carbon steel on changing the component and NOx emissions of BMN salts in its TES process was reported here. Quartz boat (60 200 mm) was used as the container in the experiment given that no suitable container. The concentration of NOx in ambient air was measured by N-(1naphthyl) ethylene diamine dihydrochloride spectrophotometric method with reference to the national environmental protection standard of PR China HJ479-2009 [14], according to the same standard, NO and NO2 emissions and changes in component of BMN salts were determined before and after contacting with 45# carbon steel at high temperatures for 1 h continuous absorption. 2. Experiment 2.1. Materials The binary eutectic salt mixture (99%) of KNO3 and NaNO3 was prepared by mixing the two salt components according to the
composition of solar salt. All the ingredients obtained from Guangzhou Chemical Regent Factory were at least 99% pure and used without any further purification. Amounts of salts were held in two separate glass pans and placed in a drying oven overnight at 120 C before cooling in a desiccator for being weighed. The 45# carbon steel (1045, ASTM) with the size of 25 mm 15 mm 3 mm was bought from Guangdong Hongda Metal Materials Co., LTD, and its surface area was 9.90 104 m2. It's a kind of Fe-based alloy which contains C 0.42e0.50%, Si 0.17e0.37%, Mn 0.50e0.90%, P 0.04%, S 0.05% and Ni/Cr/ Cu 0.25%. 2.2. Apparatus and procedure With reference to the standard [14], sodium nitrite (NaNO2) standard solution was prepared in advance and the absorbance of the standard solution was measured by the spectrophotometer at the wavelength of 540 nm. Then, the absorbance A was plotted with the concentration of NO 2 in the standard solution and the relationship between them (standard curve) was obtained by linear fitting: A ¼ 0.009 þ 0.84762 C (mg/mL). The linear correlation coefficient R was 0.9998. According to the standard HJ479-2009 [14], the research method of tail gases of BMN salts was designed. It's a static open system with air flowed over the surface of molten salts. Schematic diagram of experiment device is as follows (see Fig. 1): The absorption solution is the mixture of color developing solution and water at a volume ratio of 4:1, color developing solution which consists of 4-aminobenzenesulfonic, N-(1-naphthyl) ethylene diamine dihydrochloride and the glacial acetic acid was prepared beforehand according to the standard [14]. The principle of color developing is that NO2 first is absorbed into the solution and reacts with water to generate nitrous acid, which diazotizes with sulfanilic acid first and couples with N-(1-naphthyl) ethylene diamine dihydrochloride second to generate azo dyes. KMnO4 solution and absorption solution were both prepared according to the standard [14]. The monitoring of NOx emissions in TES process of BMN salts was carried out as follows. At first, 40 g KNO3 and 60 g NaNO3 were mixed in the boat to form 100 g BMN salts. This procedure was repeated at least three times to ensure a homogeneous mixture. Next, the boat was placed quickly in the tubular furnace with flanges been tightened in the following. The heating and cooling sequence were programmed after using soap film flow meter for flow calibration to ensure good air tightness, then the salt mixture was statically heated to the melting temperature 220 C, 1# and 2# three-way valves were adjusted to make the air go though branch A; The pump in the end was opened and the flow rate was regulated to 80 mL/min, pumping for 1 h until the mixture melted completely. Afterwards, 2# three-way valve was adjusted again when the set temperature such as 300 C was reached and showed identical and stable reading after opening the second pump; The flow meter was adjusted finely to stay at 80 mL/min after rotating 2# three-way valve; the air was pumped to flow through branch B at constant temperature for 1 h continuous absorption. Airflow that passed over the surface of molten mixtures would carry liberated NOx into the gas washing bottle 10#, 11# and 12# in sequence, wherein NO2 was absorbed by absorption solution in bottle 10# to form a kind of red azo dye; While unabsorbable NO was then oxidized to NO2 by KMnO4 solution in bottle 11#, subsequently absorbed in bottle 12# to form red azo dyes. Absorbance of the absorption solution in bottle 10# and 12# was measured by ultraviolet and visible spectrophotometer (Shimadzu UV2450, þ0.00001) 1 h later respectively. Finally, the absorbance was taken into the standard curve acquired from NaNO2 standard
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Fig. 1. Schematic diagram of experimental device. 1 e rotameter; 2 e Corundum tube; 3 e Quartz boat; 4 e thermocouple; 5 e Program control panel; 6 e Filtration tube; 7, 11 e Monteggia Gas-washing bottle, equipped with deionized water and KMnO4 solution, respectively; 8, 13 e Safety bottle; 9, 14 e Vacuum pump; 10, 12 e Porous Gas-washing bottle, filled with absorption solution, used as NO2 and NO absorption bottle, respectively.
solution to calculate the concentrations of the absorption solutions, by which concentrations of NO and NO2 absorbed were calculated for the reason that the absorbance at wavelength of 540 nm of azo dye generated is proportional to the concentration of NO2 absorbed; All of these were also needed to calculate the result such as volume of the absorption solution, the dilution ratio, oxidation and Saltzman experiment coefficient et al. [14]. To study the effect of carbon steel on NOx emissions, 45# carbon steel was immersed in molten salts to repeat the experiment at 300 C. The experiments were conducted at an interval of 50 C in the temperature range from 300 C to 600 C with air flow of 80 mL/ min. Different temperatures were changed to repeat the experiment before and after contacting with 45# carbon steel. Each experiment was repeated for at least two times to ensure reproducibility of the results. To exclude the impact of NO and NO2 in the air itself on results, monitoring was carried out first without molten salts in the tubular furnace under the above described operation and deducting them in subsequent experiments. In the end, the salt mixture was removed and then cooled to ambient temperature until it solidified to a white mass. The solidified salt mixture was then ground into powder using mechanical rolling, sealed in polyethylene bag and kept in desiccators for measuring the concentration of nitrite ion in the sample. 2.3. Experimental error The experimental temperature has been calibrated for an error of ±5 C. At the flow rate of 80 mL/min, the relative error is less than 5%. Relative deviation of repeatability is less than 10%. Absorbance of each sample was analyzed at least twice in order to ensure the reproducibility of the measurements, the average value being recorded, and the relative deviation of average is less than 0.5%. Besides, details in the process of experiment were paid great attention to; for instance, absorption bottles were covered with black mask to avoid light in the process of sampling. The change in color of absorption solution in the process of sampling was observed per 15 min to avoid penetration as a result of high concentration of nitrogen oxide. After sampling, the sample was analyzed as soon as possible.
mixtures to repeat the experiment. The same size steel was used in each subsequent experiment. The concentrations of NO, NO2 and NOx emissions (1 h average) of BMN salts at different temperatures before and after contacting with 45# carbon steel with the flow rate of 80 mL/min are shown in Fig. 2. The item of 1 h average concentration provided by GB30952012 [15] means arithmetical mean of concentration of pollutant for any 1 h. And that’s the reason why each experiment was repeated at least two times. BMN salts contained in silica boat will release NOx (NO and NO2) at temperature range from 300 C to 600 C, and its concentration of NOx emissions increases after contacting with 45# carbon steel (line 5 and 6 in Fig. 2). And the effect of 45# carbon steel on NOx emissions of BMN salts become visible above 500 C. There is no exact reference standard on NOx emissions limit of molten salts used in open system because it's an innovative research, only related emission standards can be found to make a comparison. Beneath 450 C, the concentration of NOx emissions after contacting with 45# carbon steel is less than 160 mg/m3, which is in accordance with the latest emission standard 200 mg/ m3 [16] of thermal power plant in China and the second directive of EU in coal-fired power plant emission limit 200 mg/m3 [17], but still exceeds the emission limit 135 mg/m3 of US [18]. At 500 C, the concentration of NOx emissions of molten salts before and after contacting with 45# carbon steel is 259 mg/m3 and 288 mg/m3 respectively. When the temperature is higher than 550 C, concentrations of NO, NO2 and NOx emissions increase significantly with the rising temperature. At 550 C, the 1 h
3. Results and discussions 3.1. NOx emissions Absorbance measured was used respectively to calculate the concentrations of NO, NO2 and NOx (NOx ¼ NO þ NO2, calculated as NO2) in accordance with the calculation method in the standard [14]. The average concentrations of NO and NO2 in the air were respectively 0.119 mg/m3 and 0.159 mg/m3 and their total was 0.278 mg/m3. A piece of 45# carbon steel was immersed in molten
Fig. 2. The concentrations of NO, NO2 and NOx before and after contacting with 45# carbon steel as a function of temperature (1 h average).
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average concentration of NOx emissions is larger than 500 mg/m3 after contacting with carbon steel, but that is still below the allowable emissions limit 650 mg/m3 of key air pollutants from coal-fired power plant in EU [19] and 1400 mg/m3 of nitric acid, nitrogen fertilizer and explosive production industry in China [20]. At 600 C, the concentration of NOx emissions exceeds the limit. It's suggested that BMN salts should be used below 600 C. Below 450 C, the concentration of NO2 emissions is extremely small regardless of contacting with carbon steel or not, NO is the main gaseous product; The NO line almost overlap with the total line (see line1, 2, 5, 6 in Fig. 2). Between 450 C and 550 C, the increment of concentration of NO2 emissions is relatively larger than that of NO because that the NO2 line is much steeper than the NO line. The concentration of NO emissions after contacting with carbon steel is larger than that without contacting with carbon steel in the whole temperature range. Whereas, it's not the case for NO2; the concentration of NO2 emissions after contacting with carbon steel is slightly smaller than that without contacting with carbon steel below 500 C, however, above this temperature, the situation is completely opposite. At 600 C, NOx emissions increase sharply and solar salt decomposes seriously. It’s visible that the concentration of NOx emissions will be increased when molten nitrate salts circulate in carbon steel pipe. In order to clarify the relationship of NO and NO2, the molar ratios of concentrations of NO and NO2 emissions vary with temperature are shown in Table 1. Molar ratio of NO and NO2 emissions decreases gradually with rise of the temperature no matter that 45# carbon steel exposed to the molten salts or not (see Table 1). Below 450 C, the concentration of NO is much larger than that of NO2. Between 500 C and 600 C, the molar ratio of NO and NO2 continues to fall and tends to a certain value 2:1 within the error range. The experimental results described above have a certain degree of similarity with others at some points, for instance, NO was observed first at the initial period of sampling, which is consistent with the experiment of Rene I. Olivares [4]. A small amount of NO2 was produced and this is similar to Freeman's research [8]. The phenomenon of NOx emissions was also found in Bartholomew's [10] experiment. Major environmental concerns resulting from NOx emissions are ground level ozone and acid rain. Only a small amount of NOx is released if molten salt is used in open system, and cannot emit a lot of SO2 and CO2 as that from coal-fired power plant, thus the pollution caused by solar salt is much smaller than that caused by coal-fired plant. As a result, BMN salts have a mild effect on the atmospheric environment when its use temperature is below 600 C. For molten nitrate salts used in closed system, the increased NOx in closed cycle loop will increase the pressure of pipeline if it cannot be exhausted in time, and if it reacts with water vapor, nitric and nitrous acid formed will cause corrosion to container material, which may result in potential safety hazard. It's suggested that an absorptive device for NOx from molten salts would be added in the solar power system.
Table 1 Molar ratios of NO and NO2 emissions of BMN salts at different temperatures. Temperature/ C
NO/NO2 (molar ratio)
NO/NO2 (45# carbon steel)
300 350 400 450 500 550 600
41.6 22.1 14.7 7.1 2.4 2.2 2.0
58.8 41.2 17.1 12.1 1.9 2.0 2.4
3.2. Thermodynamic calculations To find the source of NOx emissions of BMN salts, thermodynamic calculations of some reactions about nitrite salts have been carried out. It was shown in these literature [25,11,13,2126] that more than a dozen thermal decomposition reactions of molten nitrate salts might occur above 500 C. In order to calculate the likelihood of occurrence of these reactions, the thermodynamic equation Dr Gm ðTÞ ¼ Dr Gqm ðTÞ þ R T ln Q (kJ/mol) was used to compute Dr Gm ðTÞ of a series of reactions; In this formula, P vB $Df GqB ðTÞ, vB is the stoichiometric number, Df GqB ðTÞ DrGqm ðTÞ ¼ B
is the standard Gibbs free energy of formation per molar substance B, the DrGqm ðTÞ values can be found in the literature [5] or calculated by Df GqB ðTÞ data referred in the literature [27]; R is the molar gas constant, R ¼ 8.314 J/(mol K); T (K) is the thermodynamic temperature when the reaction occurs; Q is the reaction quotient, Q Q ¼ fðPB =P q ÞðCB =C q ÞgvB , the concentrations of NO and NO2 in the B
air measured and nitrate ions in initial mixtures, the nitrogen and oxygen pressure in the air (79 and 21, vol%) and the density of BMN salts r(kg/m3) ¼ 2263.628e0.636 T (K) [28] can be used to calculate Q. Owing to the shortage of thermodynamic data of potassium nitrate/nitrite (KNO3/KNO2), the relationship between standard molar Gibbs formation function of KNO3/KNO2 and temperature acquired by experimental measurement and relevant calculation was shown in the dissertation [29], the formulas are Df Gqm ðTÞ ¼ 489:36771 þ 0:32077$TðKÞ andDf Gqm ðTÞ ¼ 371:4917 þ0:2114$TðKÞ; therefore the Df Gqm ðTÞ values of KNO3/KNO2 were calculated by the two formulas respectively. In this experiment, the time for TES process of BMN salts is short; therefore, the effect of CO2 and H2O on reactions can be ignored; only the thermal decomposition reactions of Na/KNO3 were calculated. These decomposition reactions of which Dr Gm ðTÞ values vary with temperature are shown in Table 2. As we know, for a reaction at certain conditions, Dr Gm ðTÞ ＜ 0 indicates that the reaction is thermodynamically favored, the more negative theDr Gm ðTÞ value, the greater the spontaneous degree of the reaction. Therefore, the Dr Gm ðTÞ values of Eqs. (1) and (5) at the temperature range 300e600 C listed in Table 2 are less than zero and this indicates that these reactions are thermodynamically favored. However, Eq. (2) and Eq. (4) are only favored thermodynamically at 600 C, Eq. (3) and Eqs. (6)e(8) are not thermodynamically feasible. Meanwhile, the Dr Gm ðTÞ values of Eq. (1) change more negative with the rise of temperature; and it’s the same case for Eq. (5). The fact that the Dr Gm ðTÞ values of Eq. (1) listed in Table 2 are less negative than those in Eq. (5) in the temperature range investigated, would suggest that, Eq. (1) is less thermodynamically favored. Eq. (1) and Eq. (5) manifest the existence of nitrite ion in BMN salts. NO and NO2 can be produced gradually at 600 C by Eq. (2) and Eq. (4). However, NOx emissions can be observed at 300 C, and thermodynamic calculations also show that the gaseous product of thermal decomposition of BMN salts used in its temperature range is O2, not NOx. Hence, the reason for the source of NOx needs to be explored step further. Nissen [13] revealed that if molten salts which spread spontaneously contact with materials such as quartz or stainless steel at temperatures＞500 C, there would be extensive corrosion with the 2 liberation of NOx, those are SiO2 þ 2NO 3 ¼ SiO4 þ 2NO2 and 2 Cr þ 2NO ¼ CrO þ 2NO. Although the inner surface of the quartz 4 3 boat was still smooth as ever at low temperatures from 300 C to 450 C after containing BMN salts for 1 h, in order to find the source of NOx, the thermodynamic calculations of Na/KNO3 and SiO2 have to be done carefully. However, the Df Gqm ðTÞ data of K/Na2SiO4
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Table 2 Dr Gm ðTÞ values of different reactions at different temperatures (kJ mol1). Temperature/ C
Equations
2NaNO3(l) ¼ 2NaNO2(l) þ O2(g) 4NaNO3(l) ¼ 2Na2O(s) þ 4NO2(g) þ O2(g) 2NaNO3(l) ¼ Na2O(s) þ 5/2O2(g) þ N2(g) 2NaNO3(l) ¼ Na2O(s) þ NO(g) þ NO2(g) þ O2(g) 2KNO3(l) ¼ 2KNO2(l) þ O2(g) 4KNO3(l) ¼ 2K2O(s) þ 4NO2(g) þ O2(g) 2KNO3(l) ¼ K2O(s) þ 5/2O2(g) þ N2(g) 2KNO3(l) ¼ K2O(s) þ NO(g) þ NO2(g) þ O2(g)
(1) (2) (3) (4) (5) (6) (7) (8)
300
350
400
450
500
550
600
26.35 332.72 162.40 178.02 58.55 585.29 288.68 304.30
45.62 276.16 138.48 145.66 73.72 519.89 260.35 267.53
64.89 219.79 114.60 113.40 88.89 454.70 232.05 230.85
84.15 163.62 90.75 81.23 104.06 389.69 203.78 194.26
103.42 107.62 66.93 49.15 119.23 324.86 175.55 157.77
122.69 51.80 43.15 17.16 134.40 260.21 147.35 121.36
141.96 3.85 19.40 14.74 149.57 195.73 119.19 85.04
to the limited contact area of nitrate with silica rock or sand and SiO2 percentage in the rock or sand. The phenomenon that the concentrations of NOx emissions increase after contacting with 45# carbon steel and that NO is the major increment can also be clarified. As we know, carbon steel belongs to Fe-based alloy with trance amount of Cr, Ni and Cu. Nitrate salts can oxidize most alloys until a protective layer is established. C. M. KRAMER [31], S. H. Goods [32], A. G. Ferna'ndez [33] and Manohar S. Sohal [34] showed that Fe2O3 and Fe3O4 were the principal corrosion products formed on the carbon steel materials. Hence, the thermodynamic calculation of the reaction Fe reacts with Na/KNO3 has been done. The reactions of which Dr Gqm ðTÞ values vary with temperature are shown in Table 4. It can be seen from Table 4 that the Dr Gqm ðTÞ values of Eqs. (1)e(4) are both negative between 600 K (327 C) and 1000 K (727 C). Allowing for the small concentration of NO and large content of K/NaNO3, Q ＜ 1, lnQ ＜ 0, hence, the Dr Gm ðTÞ values of Eqs. (1)e(4) are negative. That’s to say, Eqs. (1)e(4) are thermodynamically favored. Therefore, the reason for the increase of NOx emissions after contacting with 45# carbon steel can be explained. NO is the major increment as a result of that Fe with limited surface reacts with K/NaNO3. When the concentration of NO emissions is larger than that of NO2, NO can react with O2 to form NO2. The formed thin but dense and adherent Fe2O3/Fe3O4 protective layers can passivate the interface of metal and molten salts in its oxidation process, as a result, prevent these reactions from going ahead. The reaction in Table 4 shows that oxide ions exist in the decomposition product, but no obvious alkaline characteristic turned up when trying to measure the basicity of aqueous solution of molten salts samples by PH strips. Nissen and Meeker [13] also implied that the method of measuring PH could not prove oxide 2 2 ions such as O 2 , O2 , O , etc. exist in molten samples, even if there is some, the concentration is also lower than the lowest detection value of 105 mol/kg. The thermal decomposition of potassium nitrate is similar with that of sodium nitrate when compared with Eqs. (1)e(4) and Eqs. (5)e(8) in Table 2; And Na/KNO3 decomposes to Na/KNO2 is the main thermal decomposition reaction no matter what the container is, which is in accordance with the research of Freeman [8,9], Bartholomew [10] and Kurt H. Stern [11]. Nitrite ion was yield by the two equations (Eq. (1) and Eq. (5) in Table 2).
cannot be found in the literature [27] which contains most of the compounds, therefore it's unable to judge whether the reaction can happen or not. Meanwhile, the six valence state of silicon in nature is not stable, it’s doubtful for its existence. Thus, K/Na2SiO3 was assumed to be the resultant. The reactions of which Dr Gm ðTÞ values vary with temperature are shown in Table 3. It can be seen from Table 3 that Eq. (1) and Eq. (2) are thermodynamically favored between 300 and 600 C; Eq. (3) and Eq. (4) is not thermodynamically feasible at 300 C, but feasible above this temperature. As to the reaction 2NO (g) þ O2 (g) ¼ NO2 (g), its Dr Gqm ðTÞ values are negative below 450 C, but positive above 500 C. If the concentration of NO2 (g) is relatively large, NO (g) and O2 (g) can coexist at certain temperatures. The Dr Gm ðTÞ values of Eq. (1) and Eq. (3) become negative faster with the rise of temperature than that of Eq. (2) and Eq. (4); Below 450 C, Eq. (1) is less negative than Eq. (2), but more negative than Eq. (2) above this temperature. It's the same case for Eq. (3) and Eq. (4). Therefore, Eq. (1) and Eq. (3) are somewhat less stable than Eq. (2) and Eq. (4) below 450 C, but more stable above this temperature. From Table 3, the reason for NOx emissions in TES process of BMN salts is that Na/KNO3 reacts with SiO2, the main component of the container, which is consistent with the standpoint of Nissen [13], but the reaction principle is not the same. The inner face of the quartz boat was observed nontransparent and blur in sight after use for a long time or at a relative higher temperature. Due to the short time for TES process and limited contact area between molten salts and silica boat, relevant oxides or molten silicate salts in BMN salts have not been found by XRD, the main substance is still Na/KNO3; Even if there is some, the concentration of the oxides produced is also lower than the lowest detection limit of XRD. And silicon oxide can form thin but dense and adherent protective oxide layers that passivate the surface of the quartz boat in the oxidation process; therefore, although the whole quartz boat is made of SiO2, the concentration of NOx emissions measured is relatively small. The results above showed that molten nitrate salts will generate NOx in its use of temperature range after a period of time contact with quartz material, although the US Sandia laboratory [30] showed that BMN salts have a good compatibility with filler materials such as silica rock or sand after contact for a long time. The possibility of NOx emissions cannot be ruled out and ignored. It’s also possible that no corrosive characteristics of filler materials can be found due
Table 3 Dr Gm ðTÞ values of different reactions at different temperatures (kJ mol1). Equations
(1) (2) (3) (4)
4NaNO3(l) þ 2SiO2(s) ¼ 2Na2SiO3(s) þ 4NO(g) þ 3O2(g) 4NaNO3(l) þ 2SiO2(s) ¼ 2Na2SiO3(s) þ 4NO2(g) þ O2(g) 4KNO3(l) þ 2SiO2(s) ¼ 2K2SiO3(s) þ 4NO(g) þ 3O2(g) 4KNO3(l) þ 2SiO2(s) ¼ 2K2SiO3(s) þ 4NO2(g) þ O2(g)
Temperature/ C 300
350
400
450
500
550
600
82.63 129.27 77.29 30.65
154.98 185.30 5.21 35.52
227.14 241.14 87.51 101.51
299.10 296.79 169.63 167.30
370.89 352.25 251.56 232.91
442.50 407.55 333.31 298.35
513.94 462.67 414.90 363.62
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Table 4 Dr Gqm ðTÞ values of different reactions at different temperatures (kJ mol1). Equations
(1) (2) (3) (4)
Temperature/K
9Fe(s) 2Fe(s) 9Fe(s) 2Fe(s)
þ þ þ þ
8NaNO3(l) ¼ 3Fe3O4(s) þ 8NO(g) þ 4Na2O(s) 2NaNO3(l) ¼ Fe2O3(s) þ 2NO(g) þ Na2O(s) 8KNO3(l) ¼ 3Fe3O4(s) þ 8NO(g) þ 4K2O(s) 2KNO3(l) ¼ Fe2O3(s) þ 2NO(g) þ K2O(s)
600
700
800
900
1000
1291.77 298.59 826.27 182.22
1376.73 317.86 941.20 208.98
1458.32 336.09 1095.05 236.27
1537.43 353.36 1179.67 263.92
1613.10 379.66 1301.30 301.72
# Fig. 3. The mass fraction of NO 2 before and after contacting with 45 carbon steel in the samples as a function of temperature.
3.3. NO-2 formation Freeman [8,9], Stern [11], Nissen and Meeker [13] showed that the thermal decomposition reaction of single sodium nitrate or potassium nitrate was NO 3 ¼ NO2 þ 1/2O2. Thermodynamic calculation also indicates the formation of nitrite ion (Eq. (1) and (5) in Table 2). Therefore, the mass fraction of nitrite ion in each sample was determined by using the naphthyl ethylene diamine hydrochloride spectrophotometric method according to the standard [14]. Each sample was analyzed at least twice to ensure the reproducibility of the measurements, the average value being recorded, and the relative deviation of average is less than 4%.The results were shown in Fig. 3: The mass fractions of NO 2 formed in BMN salts which had been kept at constant temperatures for 1 h are extremely small at temperatures below 450 C (see Fig. 3), indicating that NO 3 decomposes hardly to NO 2 and decomposition reaction rates are relatively slow at low temperatures below 450 C because of the multistep reactive path and high activation barrier from NO 3 to NO 2 [35]. Above 450 C, the mass fractions of NO2 increase rapidly with the increasing temperature and this reveals that temperatures have a significant impact on the formation of NO 2 . After contacting with 45# carbon steel, the mass fractions of NO 2 in the samples increase with the temperature compared with that without
contacting with carbon steel. At 550 C, the mass fraction of NO 2 in molten salts samples increase from less than 0.0007% (impurity) in the original reagent to 0.061% and that increases further to 0.11% after adding 45# carbon steel. It's clear that nitrite ion will appear after a long period of time in the TES process of BMN salts looped in metal pipes and tanks. The reason for the increase of the concentration of NOx after contacting with 45# carbon steel may be that the Fe component in the steel reacts with NO 3 to generate NO. Meanwhile, the increase of the mass fraction of NO 2 after contacting with carbon steel was also resulted from Fe component in the carbon steel. The relevant reactions of which Dr Gqm ðTÞ values vary with temperature are shown in Table 5. From Table 5, it can be seen that the Dr Gqm ðTÞ values of Eqs. (1)e(4) are both negative between 600 K (327 C) and 1000 K (727 C). And the concentration ratio of NaNO2/NaNO3 or KNO2/ KNO3 in BMN salts is small, consequently, Q ＜ 1, lnQ ＜ 0, hence, the Dr Gm ðTÞ values of Eqs. (1)e(4) are negative too. That’s to say, Eqs. (1)e(4) are thermodynamically favored between 600 K and 1000 K. Therefore, the reason for the increase of the mass fraction of NO-2 after contacting with 45# carbon steel can be explained. In practical applications, molten nitrate salts loop in carbon steel or stainless steel pipe. The Dr Gqm ðTÞ values are both negative by calculating reactions that molten nitrate salts react with metals (Fe, Cr, Ni, Mn) with the evolution of NOx thermodynamically. Hence, molten nitrate salts contained in different kinds of stainless steels can also release a small amount of NO and NO2 with the formation of dense and adherent protective layers and some spinel. The thermal decomposition of BMN salts which contact with silica or alloy container not only generates nitrite ion but also may produces NOx. Occurrence of these reactions can also change the environment of inert and pure atmosphere; if generated gas cannot be removed quickly, the atmosphere which molten salts in will change.
4. Conclusion BMN salts contained in silica or carbon steel container will release a small amount of NOx from some side reactions in its use of temperature range except for the formation of nitrite ion. The concentration of NOx emissions creeps with the rise of temperature, it's advisable that BMN salts should be used under 600 C in order to relief the burden to the environment and the container surface. The concentration of NOx emissions has an increase after contacting with 45# carbon steel, the increment mainly comes from NO
Table 5 Dr Gqm ðTÞ values of the reactions at different temperatures (kJ mol1). Equations
(1) (2) (3) (4)
Temperature/K
2Fe(s) þ 3NaNO3(l) ¼ Fe2O3(s) þ 3NaNO2(l) 3Fe(s) þ 4NaNO3(l) ¼ Fe3O4(s) þ 4NaNO2(l) 2Fe(s)þ3KNO3(l) ¼ Fe2O3(s) þ 3KNO2(l) 3Fe(s) þ 4KNO3(l) ¼ Fe3O4(s) þ 4KNO2(l)
600
700
800
900
1000
524.68 732.04 504.68 705.38
525.89 736.28 511.70 717.36
526.78 740.36 519.16 730.20
527.40 744.54 527.01 744.02
537.60 748.28 545.01 758.16
X. Wei et al. / Energy 74 (2014) 215e221
emissions, which may be result from Fe element in carbon steel. Fe element can also result in the increment of the concentration of nitrite ion in the molten salts samples. As a result, the concentration of nitrite ion in the sample increases with the temperature. This should be considered carefully when choosing steels as container materials. Except for the secondary product NOx, the nitrate ion decomposes to nitrite ion is still the main thermal decomposition reaction. Acknowledgment The authors acknowledge financial support from National Natural Science Foundation of China (51376067), the National Basic Research Program of China (No. 2010CB227103) and the Postdoctoral Science Foundation of China (2013M531847). References [1] Wang Yan, Wei Xiaolan, Ding Jing, et al. The cumulative monitoring of NOx emissions in heat storage process of molten nitrate salts. J Lanzhou Univ Technol 2013;39(5):54e7. [2] Peng Qiang, Wei Xiaolan, Ding Jing, et al. High-temperature thermal stability of molten salt materials. Int J Energy Res 2008;32:1164e74. [3] Bradshaw RW, Meeker DE. High-temperature stability of ternary nitrate molten salts for solar thermal energy systems. Sol Energy Mater 1990;21: 51e60. [4] Olivares Rene I. The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres. Sol Energy 2012;86: 2576e83. [5] Long Bin, Wei Xiaolan, Ding Jing, et al. Calculation of chemical thermodynamics deterioration of ternary nitrate salts at high temperatures. J Sol Energy 2011;32(2):252e6. [6] Gordon S, Campbell C. Differential thermal analysis of inorganic compounds nitrates and perchlorates of the alkali and alkaline earth groups and their subgroups. Anal Chem 1955;27(7):1102e9. n Bjo €rn, et al. Material constraints for [7] Pihl Erik, Kushnir Duncan, Sande concentrating solar thermal power. Energy 2012;44:944e54. [8] Freeman Eli S. The kinetics of the thermal decomposition of sodium nitrate and of the reaction between sodium nitrite and oxygen. J Phys Chem 1956;60(11):1487e93. [9] Freeman Eli S. The kinetics of the thermal decomposition of potassium nitrate and of the reaction between potassium nitrite and oxygen. J Am Chem Soc 1957;79(4):838e42. [10] Bartholomew RF. A study of the equilibrium KNO3 (l) ¼ KNO2 (l) þ1/2O2 (g) over the temperature range 550e750 C. J Phys Chem 1966;70(11):3442e6. [11] Stern Kurt H. Thermal decomposition of inorganic salts. Part 3. Nitrates and nitrites. J Phys Chem Ref. Data 1972;1(3):747e71. [12] Kust RN, Burke JD. Thermal decomposition in alkali metal nitrate melts. Inorg Nucl Chem Lett 1970;6:333e5. [13] Nissen DA, Meeker DE. Nitrate/nitrite chemistry in NaNO3eKNO3 melts. Inorg Chem 1983;22(5):716e21.
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[14] Ministry of Environmental Protection of PR China. HJ479-2009 ambient airdetermination of nitrogen oxides-N-(1-naphthyl) ethylene diamine dihydrochloride spectrophotometric method. Beijing: Chinese Environmental Science Press; 2009. [15] Ministry of Environmental Protection of PR China. GB3095-2012 ambient air quality standards. Beijing: Chinese Environmental Science Press; 2012. [16] Ministry of Environmental Protection of PR China. GB13223-2011 emission standard of air pollutants for thermal power plants. Beijing: Chinese Environmental Science Press; 2011. [17] EU. Integrated pollution prevention and control (IPPC) reference document on best available techniques in the large combustion plants. Brussels: European Union; 2006. [18] US EPA. Standards of performance for electric utility steam generating units. Washington DC: U.S EPA; 2006. [19] EU. On the limitations of certain pollutants into air from large combustion plants, Directive 2001/80/EC of the European parliament and of council. Brussels: European Union; 2001. [20] Ministry of Environmental Protection of PR China. GB16297-1996 integrated emission standard of air pollutants. Beijing: Chinese Environmental Science Press; 1996. [21] Halmann M, Zuckerman K. Stability of molten nitrate salts containing light absorbing additives as solar flux absorbers. Sol Energy Mater 1988;17(4): 311e8. [22] Kramer CM. Screening test of sodium nitrate and potassium nitrate decomposition. Sol energy 1982;29(5):437e9. [23] Kust RN, Burke JD. Thermal decomposition in alkali metal nitrate melts. Inorg Nucl Chem Lett 1970;6(3):333e5. [24] Bond BD, Jacobs PWM. The thermal decomposition of sodium nitrate. J Chem Soc Inorg, Phys, Theor 1966;9:1265e8. [25] Protsenko PI, Brodyushkova EA. Kinetics of the thermal decomposition of alkali metal nitrites. Zhurnal Neorganicheskoi Khimii 1965;10(5):1215e9. [26] Kramer CM, Munir ZA, Volponi JV. Screening test of sodium nitrite decomposition. Sol Energy Mater 1981;6(1):85e95. [27] Barin Ihscn. In: Sora Karin, Gardiner James, editors. Thermochemical data of pure substances. 3rd ed. Wenham: VCH Verlagsgesellschaft mbH; 1995. p.116e2003. Ger VCH Publ Inc N.Y (USA). pez R, Fradera J, Cuesta-Lo pez S. Molten salts database for energy [28] Serrano-Lo applications. Chem Eng Process Intensif 2013;73:82e102. [29] Peng Qiang, Ding Jing. Design and performance control for multiple molten salts materials with heat transfer and thermal storage. Guangdong: Sun Yatsen University; 2011. pp. 49e51. [30] Pacheco J, Showalter S, Kolb W. Development of a molten salt thermocline thermal storage system for parabolic trough plants. Sol Energy Eng Spec Issue: Sol Therm Power 2002;124(2):153e9. [31] Kramer CM, Smyrl WH, Estill WB. Corrosion of Fe alloys in NaNO3eKNO3eNaNO2 at 823K. Mater Energy Syst 1980;1:59e65. [32] Goods SH, Bradshaw RW. Corrosion of stainless steels and carbon steel by molten mixtures of commercial nitrate salts. J Mater Eng Perform 2004;13(1): 78e87. ndez AG, Lasanta MI, Pe rez FJ. Molten salt corrosion of stainless steels [33] Ferna and low-Cr steel in CSP plants. Oxid Met 2012;78:329e48. [34] Sohal Manohar S, Ebner Matthias A, Sabarwal Piyush, et al. Engineering database of liquid salt thermophysical and thermochemical properties. Idaho National Laboratory; 2010. http://www.inl.gov. [35] Wei Xiaolan, Peng Qiang, Ding Jing, et al. Theoretical study on thermal stability of molten salt for solar thermal power. Appl Therm Eng 2013;54(1): 140e4.
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Energy journal homepage: www.elsevier.com/locate/energy
NOx emissions and NO2- formation in thermal energy storage process of binary molten nitrate salts Xiaolan Wei a, b, Yan Wang a, Qiang Peng a, Jianping Yang a, Xiaoxi Yang c, Jing Ding d, * a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China Key Laboratory of Fuel Cell Technology of Guangdong Province, Guangzhou 510640, PR China c Dongguan University of Technology, Dongguan 523000, PR China d Center for Energy Conservation Technology, School of Engineering, Sun Yat-sen University, Guangzhou Higher Education Mega Center, No. 132 Waihuan Dong Road, Guangzhou 510006, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2013 Received in revised form 10 March 2014 Accepted 17 May 2014 Available online 17 June 2014
Referring to the national environmental protection standard of PR China, HJ479-2009, NOx emissions and the effect of 45# carbon steel (1045, ASTM) on it in thermal energy storage (TES) process of binary molten nitrate (BMN) salts (a eutectic salt mixture named as solar salt with the component of 60% NaNO3 and 40% KNO3) are discussed in this paper. The accumulative absorption concentrations of NOx emissions in tail gases of molten salts heated at different temperatures were measured to study the thermal decomposition situation of molten salts in TES process. Furthermore, the concentrations of nitrite ion in molten salts samples were determined and chemical thermodynamic calculations of related reactions were done to explain the results. The research shows that BMN salts contained in silica or carbon steel container in the air under certain conditions can release NOx in its use of temperature range. The concentrations of NOx emissions and nitrite ion in molten salts both increase with the rise of the temperature. After contacting with 45# carbon steel, the total concentrations of NOx emissions and nitrite ion in molten salts increased, which may be caused by Fe component in the carbon steel. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Binary molten nitrate salts NOx emissions Carbon steel
1. Introduction In recent years, alkali metal nitrate salts are used as heat transfer and thermal energy storage (HT and TES) medium in concentrating solar power (CSP) system due to low melting point and vapor pressure, cheap price and wide operating temperature range. In 2011, the world's first commercial solar power station, Spain Gemasolar power plant, was built and solar salt was fully applied in the HT and TES system. However, due to the thermal decomposition of nitrate salts, there are some problems such as the changes in the component of molten salts as well as potential risks of NOx released in HT and TES process [1]. Change in the composition of molten salts will cause the variation of its thermal properties, which may result in the decrease of TES efficiency and affect the working life of molten salts. Meanwhile, large amounts of NOx emissions will have a major effect on the container surface by impacting on corrosion. Therefore, monitoring changes in the composition and NOx emissions of BMN (binary molten nitrate) salts in HT and TES process has
* Corresponding author. Tel.: þ86 20 39332320; fax: þ86 20 39332319. E-mail addresses: [email protected], [email protected] (J. Ding). http://dx.doi.org/10.1016/j.energy.2014.05.064 0360-5442/© 2014 Elsevier Ltd. All rights reserved.
important significance on the TES efficiency of molten salts and the utilization of renewable energy. The pyrolysis reaction of nitrate salts is very complex [24]. By conducting thermodynamic calculations of thermal decomposition reactions of ternary molten nitrate salts at high temperatures, Long [5] showed that the DrGqm ðTÞ values of reactions 2NaNO2(l) þ O2(g) ¼ 2NaNO3(l) and 5NaNO2(l) ¼ 3Na NO3(l) þ Na2O(s) þ N2(g) were both negative between 600 K and 1000 K, and the phenomenon that the concentration of nitrite ion decreased constantly was the result of thermodynamic calculation. Gordon [6] found that the bubbling phenomenon was observed at the surface of molten nitrate salts when the temperature was slightly higher than the melting point and the release of gaseous products was shown by this phenomenon. Erik Pihl [7] pointed out that when using nitrate salts there would be some level of decomposition to nitrite and other secondary products, mainly NOx. Freeman [8,9], Bartholomew [10] and Stern [11] studied the thermal decomposition of single NaNO3 or KNO3 respectively, and found that each of them decomposed to nitrite salt and oxygen, that was NO 3 ¼ NO2 þ 1/2O2. Freeman [8] detected N2 and a small amount of NO2 except for the monitoring of O2 in the experiment with a stainless steel as container under the cover gas of argon. Bartholomew [10] revealed that some
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side reactions could involve the evolution of NOx in addition to the release of O2 when molten salts were heated in a stainless steel or fine silver crucible. NaNO2 generated from decomposition of NaNO3 was unstable above 330 C in the air. Stern [11] found the further decomposition of NaNO2 would produce Na2O, NO, and NO2 and it also could decompose to Na2O, N2 and O2 in an inert atmosphere; Stern [11] also noted that nitrate salts decomposed at high temperatures might release N2, O2, NO, NO2, N2O3, N2O4, N2O5, etc., which of these gases were produced depended on the experimental conditions such as the salt itself and temperature; NO and NO2 were the main gases at temperature above 500 K, there was no need to consider the presence of other gases. Olivares [4] suggested that ternary molten nitrate salts (7%NaNO3e53%KNO3e40%NaNO2) melt in master alloy would release NO firstly and NO2 secondly at four different atmospheres (Ar, N2, O2, and air) in the heating process from room temperature to 1000 C. Kust [12] researched the content of thermal decomposition product of solar salt at 300 C in oxygen and demonstrated that the temperature of nitrate ion (NO 3 ) decomposed to nitrite ion (NO 2 ) to a measurable extent was as low as 295 C. Nissen and Meeker [13] measured the content of NO in decompo2 sition products of solar salt at 550 and 600 C in different oxygen partial pressures (21e100, vol%) and found that the percentage of nitrite ion increased with the rising of temperature while decreased with the climbing of the oxygen partial pressure; In other words, temperature and atmosphere affected the decomposition of molten nitrate salts. By carefully analyzing gaseous decomposition products of molten nitrate salts in above literature, it was found that container material used in the experiment may have some effects on thermal decomposition of molten nitrate salts. Although Freeman [8] indicated that NOx emissions in pyrolysis of potassium nitrate was less than 1% when using the stainless steel container, however, no previous studies in literature about the impact of container material on NOx emissions and composition changes of molten salts have been reported systematically. Thus, it seems necessary to design an experimental device to investigate the effect of container material on secondary gaseous products (mainly NOx) of thermal decomposition of molten nitrate salts except for the evolution of O2. It's obvious that thermal decomposition happened in molten salts if NOx emissions can be detected in the gas phase. As a result, the composition of molten salts will change, followed with the variation of TES stability. This is also the reason that molten salts needed to be replaced after running for a long time. In actual large scale applications, molten salts had a certain degree of contact with carbon steel or stainless steel tube or tank. Therefore, investigating the reason of NOx emissions of molten nitrate salts and the corresponding effect of various steels have important significance on the practical application of molten nitrate salts. Hence, the effect of 45# carbon steel on changing the component and NOx emissions of BMN salts in its TES process was reported here. Quartz boat (60 200 mm) was used as the container in the experiment given that no suitable container. The concentration of NOx in ambient air was measured by N-(1naphthyl) ethylene diamine dihydrochloride spectrophotometric method with reference to the national environmental protection standard of PR China HJ479-2009 [14], according to the same standard, NO and NO2 emissions and changes in component of BMN salts were determined before and after contacting with 45# carbon steel at high temperatures for 1 h continuous absorption. 2. Experiment 2.1. Materials The binary eutectic salt mixture (99%) of KNO3 and NaNO3 was prepared by mixing the two salt components according to the
composition of solar salt. All the ingredients obtained from Guangzhou Chemical Regent Factory were at least 99% pure and used without any further purification. Amounts of salts were held in two separate glass pans and placed in a drying oven overnight at 120 C before cooling in a desiccator for being weighed. The 45# carbon steel (1045, ASTM) with the size of 25 mm 15 mm 3 mm was bought from Guangdong Hongda Metal Materials Co., LTD, and its surface area was 9.90 104 m2. It's a kind of Fe-based alloy which contains C 0.42e0.50%, Si 0.17e0.37%, Mn 0.50e0.90%, P 0.04%, S 0.05% and Ni/Cr/ Cu 0.25%. 2.2. Apparatus and procedure With reference to the standard [14], sodium nitrite (NaNO2) standard solution was prepared in advance and the absorbance of the standard solution was measured by the spectrophotometer at the wavelength of 540 nm. Then, the absorbance A was plotted with the concentration of NO 2 in the standard solution and the relationship between them (standard curve) was obtained by linear fitting: A ¼ 0.009 þ 0.84762 C (mg/mL). The linear correlation coefficient R was 0.9998. According to the standard HJ479-2009 [14], the research method of tail gases of BMN salts was designed. It's a static open system with air flowed over the surface of molten salts. Schematic diagram of experiment device is as follows (see Fig. 1): The absorption solution is the mixture of color developing solution and water at a volume ratio of 4:1, color developing solution which consists of 4-aminobenzenesulfonic, N-(1-naphthyl) ethylene diamine dihydrochloride and the glacial acetic acid was prepared beforehand according to the standard [14]. The principle of color developing is that NO2 first is absorbed into the solution and reacts with water to generate nitrous acid, which diazotizes with sulfanilic acid first and couples with N-(1-naphthyl) ethylene diamine dihydrochloride second to generate azo dyes. KMnO4 solution and absorption solution were both prepared according to the standard [14]. The monitoring of NOx emissions in TES process of BMN salts was carried out as follows. At first, 40 g KNO3 and 60 g NaNO3 were mixed in the boat to form 100 g BMN salts. This procedure was repeated at least three times to ensure a homogeneous mixture. Next, the boat was placed quickly in the tubular furnace with flanges been tightened in the following. The heating and cooling sequence were programmed after using soap film flow meter for flow calibration to ensure good air tightness, then the salt mixture was statically heated to the melting temperature 220 C, 1# and 2# three-way valves were adjusted to make the air go though branch A; The pump in the end was opened and the flow rate was regulated to 80 mL/min, pumping for 1 h until the mixture melted completely. Afterwards, 2# three-way valve was adjusted again when the set temperature such as 300 C was reached and showed identical and stable reading after opening the second pump; The flow meter was adjusted finely to stay at 80 mL/min after rotating 2# three-way valve; the air was pumped to flow through branch B at constant temperature for 1 h continuous absorption. Airflow that passed over the surface of molten mixtures would carry liberated NOx into the gas washing bottle 10#, 11# and 12# in sequence, wherein NO2 was absorbed by absorption solution in bottle 10# to form a kind of red azo dye; While unabsorbable NO was then oxidized to NO2 by KMnO4 solution in bottle 11#, subsequently absorbed in bottle 12# to form red azo dyes. Absorbance of the absorption solution in bottle 10# and 12# was measured by ultraviolet and visible spectrophotometer (Shimadzu UV2450, þ0.00001) 1 h later respectively. Finally, the absorbance was taken into the standard curve acquired from NaNO2 standard
X. Wei et al. / Energy 74 (2014) 215e221
217
Fig. 1. Schematic diagram of experimental device. 1 e rotameter; 2 e Corundum tube; 3 e Quartz boat; 4 e thermocouple; 5 e Program control panel; 6 e Filtration tube; 7, 11 e Monteggia Gas-washing bottle, equipped with deionized water and KMnO4 solution, respectively; 8, 13 e Safety bottle; 9, 14 e Vacuum pump; 10, 12 e Porous Gas-washing bottle, filled with absorption solution, used as NO2 and NO absorption bottle, respectively.
solution to calculate the concentrations of the absorption solutions, by which concentrations of NO and NO2 absorbed were calculated for the reason that the absorbance at wavelength of 540 nm of azo dye generated is proportional to the concentration of NO2 absorbed; All of these were also needed to calculate the result such as volume of the absorption solution, the dilution ratio, oxidation and Saltzman experiment coefficient et al. [14]. To study the effect of carbon steel on NOx emissions, 45# carbon steel was immersed in molten salts to repeat the experiment at 300 C. The experiments were conducted at an interval of 50 C in the temperature range from 300 C to 600 C with air flow of 80 mL/ min. Different temperatures were changed to repeat the experiment before and after contacting with 45# carbon steel. Each experiment was repeated for at least two times to ensure reproducibility of the results. To exclude the impact of NO and NO2 in the air itself on results, monitoring was carried out first without molten salts in the tubular furnace under the above described operation and deducting them in subsequent experiments. In the end, the salt mixture was removed and then cooled to ambient temperature until it solidified to a white mass. The solidified salt mixture was then ground into powder using mechanical rolling, sealed in polyethylene bag and kept in desiccators for measuring the concentration of nitrite ion in the sample. 2.3. Experimental error The experimental temperature has been calibrated for an error of ±5 C. At the flow rate of 80 mL/min, the relative error is less than 5%. Relative deviation of repeatability is less than 10%. Absorbance of each sample was analyzed at least twice in order to ensure the reproducibility of the measurements, the average value being recorded, and the relative deviation of average is less than 0.5%. Besides, details in the process of experiment were paid great attention to; for instance, absorption bottles were covered with black mask to avoid light in the process of sampling. The change in color of absorption solution in the process of sampling was observed per 15 min to avoid penetration as a result of high concentration of nitrogen oxide. After sampling, the sample was analyzed as soon as possible.
mixtures to repeat the experiment. The same size steel was used in each subsequent experiment. The concentrations of NO, NO2 and NOx emissions (1 h average) of BMN salts at different temperatures before and after contacting with 45# carbon steel with the flow rate of 80 mL/min are shown in Fig. 2. The item of 1 h average concentration provided by GB30952012 [15] means arithmetical mean of concentration of pollutant for any 1 h. And that’s the reason why each experiment was repeated at least two times. BMN salts contained in silica boat will release NOx (NO and NO2) at temperature range from 300 C to 600 C, and its concentration of NOx emissions increases after contacting with 45# carbon steel (line 5 and 6 in Fig. 2). And the effect of 45# carbon steel on NOx emissions of BMN salts become visible above 500 C. There is no exact reference standard on NOx emissions limit of molten salts used in open system because it's an innovative research, only related emission standards can be found to make a comparison. Beneath 450 C, the concentration of NOx emissions after contacting with 45# carbon steel is less than 160 mg/m3, which is in accordance with the latest emission standard 200 mg/ m3 [16] of thermal power plant in China and the second directive of EU in coal-fired power plant emission limit 200 mg/m3 [17], but still exceeds the emission limit 135 mg/m3 of US [18]. At 500 C, the concentration of NOx emissions of molten salts before and after contacting with 45# carbon steel is 259 mg/m3 and 288 mg/m3 respectively. When the temperature is higher than 550 C, concentrations of NO, NO2 and NOx emissions increase significantly with the rising temperature. At 550 C, the 1 h
3. Results and discussions 3.1. NOx emissions Absorbance measured was used respectively to calculate the concentrations of NO, NO2 and NOx (NOx ¼ NO þ NO2, calculated as NO2) in accordance with the calculation method in the standard [14]. The average concentrations of NO and NO2 in the air were respectively 0.119 mg/m3 and 0.159 mg/m3 and their total was 0.278 mg/m3. A piece of 45# carbon steel was immersed in molten
Fig. 2. The concentrations of NO, NO2 and NOx before and after contacting with 45# carbon steel as a function of temperature (1 h average).
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average concentration of NOx emissions is larger than 500 mg/m3 after contacting with carbon steel, but that is still below the allowable emissions limit 650 mg/m3 of key air pollutants from coal-fired power plant in EU [19] and 1400 mg/m3 of nitric acid, nitrogen fertilizer and explosive production industry in China [20]. At 600 C, the concentration of NOx emissions exceeds the limit. It's suggested that BMN salts should be used below 600 C. Below 450 C, the concentration of NO2 emissions is extremely small regardless of contacting with carbon steel or not, NO is the main gaseous product; The NO line almost overlap with the total line (see line1, 2, 5, 6 in Fig. 2). Between 450 C and 550 C, the increment of concentration of NO2 emissions is relatively larger than that of NO because that the NO2 line is much steeper than the NO line. The concentration of NO emissions after contacting with carbon steel is larger than that without contacting with carbon steel in the whole temperature range. Whereas, it's not the case for NO2; the concentration of NO2 emissions after contacting with carbon steel is slightly smaller than that without contacting with carbon steel below 500 C, however, above this temperature, the situation is completely opposite. At 600 C, NOx emissions increase sharply and solar salt decomposes seriously. It’s visible that the concentration of NOx emissions will be increased when molten nitrate salts circulate in carbon steel pipe. In order to clarify the relationship of NO and NO2, the molar ratios of concentrations of NO and NO2 emissions vary with temperature are shown in Table 1. Molar ratio of NO and NO2 emissions decreases gradually with rise of the temperature no matter that 45# carbon steel exposed to the molten salts or not (see Table 1). Below 450 C, the concentration of NO is much larger than that of NO2. Between 500 C and 600 C, the molar ratio of NO and NO2 continues to fall and tends to a certain value 2:1 within the error range. The experimental results described above have a certain degree of similarity with others at some points, for instance, NO was observed first at the initial period of sampling, which is consistent with the experiment of Rene I. Olivares [4]. A small amount of NO2 was produced and this is similar to Freeman's research [8]. The phenomenon of NOx emissions was also found in Bartholomew's [10] experiment. Major environmental concerns resulting from NOx emissions are ground level ozone and acid rain. Only a small amount of NOx is released if molten salt is used in open system, and cannot emit a lot of SO2 and CO2 as that from coal-fired power plant, thus the pollution caused by solar salt is much smaller than that caused by coal-fired plant. As a result, BMN salts have a mild effect on the atmospheric environment when its use temperature is below 600 C. For molten nitrate salts used in closed system, the increased NOx in closed cycle loop will increase the pressure of pipeline if it cannot be exhausted in time, and if it reacts with water vapor, nitric and nitrous acid formed will cause corrosion to container material, which may result in potential safety hazard. It's suggested that an absorptive device for NOx from molten salts would be added in the solar power system.
Table 1 Molar ratios of NO and NO2 emissions of BMN salts at different temperatures. Temperature/ C
NO/NO2 (molar ratio)
NO/NO2 (45# carbon steel)
300 350 400 450 500 550 600
41.6 22.1 14.7 7.1 2.4 2.2 2.0
58.8 41.2 17.1 12.1 1.9 2.0 2.4
3.2. Thermodynamic calculations To find the source of NOx emissions of BMN salts, thermodynamic calculations of some reactions about nitrite salts have been carried out. It was shown in these literature [25,11,13,2126] that more than a dozen thermal decomposition reactions of molten nitrate salts might occur above 500 C. In order to calculate the likelihood of occurrence of these reactions, the thermodynamic equation Dr Gm ðTÞ ¼ Dr Gqm ðTÞ þ R T ln Q (kJ/mol) was used to compute Dr Gm ðTÞ of a series of reactions; In this formula, P vB $Df GqB ðTÞ, vB is the stoichiometric number, Df GqB ðTÞ DrGqm ðTÞ ¼ B
is the standard Gibbs free energy of formation per molar substance B, the DrGqm ðTÞ values can be found in the literature [5] or calculated by Df GqB ðTÞ data referred in the literature [27]; R is the molar gas constant, R ¼ 8.314 J/(mol K); T (K) is the thermodynamic temperature when the reaction occurs; Q is the reaction quotient, Q Q ¼ fðPB =P q ÞðCB =C q ÞgvB , the concentrations of NO and NO2 in the B
air measured and nitrate ions in initial mixtures, the nitrogen and oxygen pressure in the air (79 and 21, vol%) and the density of BMN salts r(kg/m3) ¼ 2263.628e0.636 T (K) [28] can be used to calculate Q. Owing to the shortage of thermodynamic data of potassium nitrate/nitrite (KNO3/KNO2), the relationship between standard molar Gibbs formation function of KNO3/KNO2 and temperature acquired by experimental measurement and relevant calculation was shown in the dissertation [29], the formulas are Df Gqm ðTÞ ¼ 489:36771 þ 0:32077$TðKÞ andDf Gqm ðTÞ ¼ 371:4917 þ0:2114$TðKÞ; therefore the Df Gqm ðTÞ values of KNO3/KNO2 were calculated by the two formulas respectively. In this experiment, the time for TES process of BMN salts is short; therefore, the effect of CO2 and H2O on reactions can be ignored; only the thermal decomposition reactions of Na/KNO3 were calculated. These decomposition reactions of which Dr Gm ðTÞ values vary with temperature are shown in Table 2. As we know, for a reaction at certain conditions, Dr Gm ðTÞ ＜ 0 indicates that the reaction is thermodynamically favored, the more negative theDr Gm ðTÞ value, the greater the spontaneous degree of the reaction. Therefore, the Dr Gm ðTÞ values of Eqs. (1) and (5) at the temperature range 300e600 C listed in Table 2 are less than zero and this indicates that these reactions are thermodynamically favored. However, Eq. (2) and Eq. (4) are only favored thermodynamically at 600 C, Eq. (3) and Eqs. (6)e(8) are not thermodynamically feasible. Meanwhile, the Dr Gm ðTÞ values of Eq. (1) change more negative with the rise of temperature; and it’s the same case for Eq. (5). The fact that the Dr Gm ðTÞ values of Eq. (1) listed in Table 2 are less negative than those in Eq. (5) in the temperature range investigated, would suggest that, Eq. (1) is less thermodynamically favored. Eq. (1) and Eq. (5) manifest the existence of nitrite ion in BMN salts. NO and NO2 can be produced gradually at 600 C by Eq. (2) and Eq. (4). However, NOx emissions can be observed at 300 C, and thermodynamic calculations also show that the gaseous product of thermal decomposition of BMN salts used in its temperature range is O2, not NOx. Hence, the reason for the source of NOx needs to be explored step further. Nissen [13] revealed that if molten salts which spread spontaneously contact with materials such as quartz or stainless steel at temperatures＞500 C, there would be extensive corrosion with the 2 liberation of NOx, those are SiO2 þ 2NO 3 ¼ SiO4 þ 2NO2 and 2 Cr þ 2NO ¼ CrO þ 2NO. Although the inner surface of the quartz 4 3 boat was still smooth as ever at low temperatures from 300 C to 450 C after containing BMN salts for 1 h, in order to find the source of NOx, the thermodynamic calculations of Na/KNO3 and SiO2 have to be done carefully. However, the Df Gqm ðTÞ data of K/Na2SiO4
X. Wei et al. / Energy 74 (2014) 215e221
219
Table 2 Dr Gm ðTÞ values of different reactions at different temperatures (kJ mol1). Temperature/ C
Equations
2NaNO3(l) ¼ 2NaNO2(l) þ O2(g) 4NaNO3(l) ¼ 2Na2O(s) þ 4NO2(g) þ O2(g) 2NaNO3(l) ¼ Na2O(s) þ 5/2O2(g) þ N2(g) 2NaNO3(l) ¼ Na2O(s) þ NO(g) þ NO2(g) þ O2(g) 2KNO3(l) ¼ 2KNO2(l) þ O2(g) 4KNO3(l) ¼ 2K2O(s) þ 4NO2(g) þ O2(g) 2KNO3(l) ¼ K2O(s) þ 5/2O2(g) þ N2(g) 2KNO3(l) ¼ K2O(s) þ NO(g) þ NO2(g) þ O2(g)
(1) (2) (3) (4) (5) (6) (7) (8)
300
350
400
450
500
550
600
26.35 332.72 162.40 178.02 58.55 585.29 288.68 304.30
45.62 276.16 138.48 145.66 73.72 519.89 260.35 267.53
64.89 219.79 114.60 113.40 88.89 454.70 232.05 230.85
84.15 163.62 90.75 81.23 104.06 389.69 203.78 194.26
103.42 107.62 66.93 49.15 119.23 324.86 175.55 157.77
122.69 51.80 43.15 17.16 134.40 260.21 147.35 121.36
141.96 3.85 19.40 14.74 149.57 195.73 119.19 85.04
to the limited contact area of nitrate with silica rock or sand and SiO2 percentage in the rock or sand. The phenomenon that the concentrations of NOx emissions increase after contacting with 45# carbon steel and that NO is the major increment can also be clarified. As we know, carbon steel belongs to Fe-based alloy with trance amount of Cr, Ni and Cu. Nitrate salts can oxidize most alloys until a protective layer is established. C. M. KRAMER [31], S. H. Goods [32], A. G. Ferna'ndez [33] and Manohar S. Sohal [34] showed that Fe2O3 and Fe3O4 were the principal corrosion products formed on the carbon steel materials. Hence, the thermodynamic calculation of the reaction Fe reacts with Na/KNO3 has been done. The reactions of which Dr Gqm ðTÞ values vary with temperature are shown in Table 4. It can be seen from Table 4 that the Dr Gqm ðTÞ values of Eqs. (1)e(4) are both negative between 600 K (327 C) and 1000 K (727 C). Allowing for the small concentration of NO and large content of K/NaNO3, Q ＜ 1, lnQ ＜ 0, hence, the Dr Gm ðTÞ values of Eqs. (1)e(4) are negative. That’s to say, Eqs. (1)e(4) are thermodynamically favored. Therefore, the reason for the increase of NOx emissions after contacting with 45# carbon steel can be explained. NO is the major increment as a result of that Fe with limited surface reacts with K/NaNO3. When the concentration of NO emissions is larger than that of NO2, NO can react with O2 to form NO2. The formed thin but dense and adherent Fe2O3/Fe3O4 protective layers can passivate the interface of metal and molten salts in its oxidation process, as a result, prevent these reactions from going ahead. The reaction in Table 4 shows that oxide ions exist in the decomposition product, but no obvious alkaline characteristic turned up when trying to measure the basicity of aqueous solution of molten salts samples by PH strips. Nissen and Meeker [13] also implied that the method of measuring PH could not prove oxide 2 2 ions such as O 2 , O2 , O , etc. exist in molten samples, even if there is some, the concentration is also lower than the lowest detection value of 105 mol/kg. The thermal decomposition of potassium nitrate is similar with that of sodium nitrate when compared with Eqs. (1)e(4) and Eqs. (5)e(8) in Table 2; And Na/KNO3 decomposes to Na/KNO2 is the main thermal decomposition reaction no matter what the container is, which is in accordance with the research of Freeman [8,9], Bartholomew [10] and Kurt H. Stern [11]. Nitrite ion was yield by the two equations (Eq. (1) and Eq. (5) in Table 2).
cannot be found in the literature [27] which contains most of the compounds, therefore it's unable to judge whether the reaction can happen or not. Meanwhile, the six valence state of silicon in nature is not stable, it’s doubtful for its existence. Thus, K/Na2SiO3 was assumed to be the resultant. The reactions of which Dr Gm ðTÞ values vary with temperature are shown in Table 3. It can be seen from Table 3 that Eq. (1) and Eq. (2) are thermodynamically favored between 300 and 600 C; Eq. (3) and Eq. (4) is not thermodynamically feasible at 300 C, but feasible above this temperature. As to the reaction 2NO (g) þ O2 (g) ¼ NO2 (g), its Dr Gqm ðTÞ values are negative below 450 C, but positive above 500 C. If the concentration of NO2 (g) is relatively large, NO (g) and O2 (g) can coexist at certain temperatures. The Dr Gm ðTÞ values of Eq. (1) and Eq. (3) become negative faster with the rise of temperature than that of Eq. (2) and Eq. (4); Below 450 C, Eq. (1) is less negative than Eq. (2), but more negative than Eq. (2) above this temperature. It's the same case for Eq. (3) and Eq. (4). Therefore, Eq. (1) and Eq. (3) are somewhat less stable than Eq. (2) and Eq. (4) below 450 C, but more stable above this temperature. From Table 3, the reason for NOx emissions in TES process of BMN salts is that Na/KNO3 reacts with SiO2, the main component of the container, which is consistent with the standpoint of Nissen [13], but the reaction principle is not the same. The inner face of the quartz boat was observed nontransparent and blur in sight after use for a long time or at a relative higher temperature. Due to the short time for TES process and limited contact area between molten salts and silica boat, relevant oxides or molten silicate salts in BMN salts have not been found by XRD, the main substance is still Na/KNO3; Even if there is some, the concentration of the oxides produced is also lower than the lowest detection limit of XRD. And silicon oxide can form thin but dense and adherent protective oxide layers that passivate the surface of the quartz boat in the oxidation process; therefore, although the whole quartz boat is made of SiO2, the concentration of NOx emissions measured is relatively small. The results above showed that molten nitrate salts will generate NOx in its use of temperature range after a period of time contact with quartz material, although the US Sandia laboratory [30] showed that BMN salts have a good compatibility with filler materials such as silica rock or sand after contact for a long time. The possibility of NOx emissions cannot be ruled out and ignored. It’s also possible that no corrosive characteristics of filler materials can be found due
Table 3 Dr Gm ðTÞ values of different reactions at different temperatures (kJ mol1). Equations
(1) (2) (3) (4)
4NaNO3(l) þ 2SiO2(s) ¼ 2Na2SiO3(s) þ 4NO(g) þ 3O2(g) 4NaNO3(l) þ 2SiO2(s) ¼ 2Na2SiO3(s) þ 4NO2(g) þ O2(g) 4KNO3(l) þ 2SiO2(s) ¼ 2K2SiO3(s) þ 4NO(g) þ 3O2(g) 4KNO3(l) þ 2SiO2(s) ¼ 2K2SiO3(s) þ 4NO2(g) þ O2(g)
Temperature/ C 300
350
400
450
500
550
600
82.63 129.27 77.29 30.65
154.98 185.30 5.21 35.52
227.14 241.14 87.51 101.51
299.10 296.79 169.63 167.30
370.89 352.25 251.56 232.91
442.50 407.55 333.31 298.35
513.94 462.67 414.90 363.62
220
X. Wei et al. / Energy 74 (2014) 215e221
Table 4 Dr Gqm ðTÞ values of different reactions at different temperatures (kJ mol1). Equations
(1) (2) (3) (4)
Temperature/K
9Fe(s) 2Fe(s) 9Fe(s) 2Fe(s)
þ þ þ þ
8NaNO3(l) ¼ 3Fe3O4(s) þ 8NO(g) þ 4Na2O(s) 2NaNO3(l) ¼ Fe2O3(s) þ 2NO(g) þ Na2O(s) 8KNO3(l) ¼ 3Fe3O4(s) þ 8NO(g) þ 4K2O(s) 2KNO3(l) ¼ Fe2O3(s) þ 2NO(g) þ K2O(s)
600
700
800
900
1000
1291.77 298.59 826.27 182.22
1376.73 317.86 941.20 208.98
1458.32 336.09 1095.05 236.27
1537.43 353.36 1179.67 263.92
1613.10 379.66 1301.30 301.72
# Fig. 3. The mass fraction of NO 2 before and after contacting with 45 carbon steel in the samples as a function of temperature.
3.3. NO-2 formation Freeman [8,9], Stern [11], Nissen and Meeker [13] showed that the thermal decomposition reaction of single sodium nitrate or potassium nitrate was NO 3 ¼ NO2 þ 1/2O2. Thermodynamic calculation also indicates the formation of nitrite ion (Eq. (1) and (5) in Table 2). Therefore, the mass fraction of nitrite ion in each sample was determined by using the naphthyl ethylene diamine hydrochloride spectrophotometric method according to the standard [14]. Each sample was analyzed at least twice to ensure the reproducibility of the measurements, the average value being recorded, and the relative deviation of average is less than 4%.The results were shown in Fig. 3: The mass fractions of NO 2 formed in BMN salts which had been kept at constant temperatures for 1 h are extremely small at temperatures below 450 C (see Fig. 3), indicating that NO 3 decomposes hardly to NO 2 and decomposition reaction rates are relatively slow at low temperatures below 450 C because of the multistep reactive path and high activation barrier from NO 3 to NO 2 [35]. Above 450 C, the mass fractions of NO2 increase rapidly with the increasing temperature and this reveals that temperatures have a significant impact on the formation of NO 2 . After contacting with 45# carbon steel, the mass fractions of NO 2 in the samples increase with the temperature compared with that without
contacting with carbon steel. At 550 C, the mass fraction of NO 2 in molten salts samples increase from less than 0.0007% (impurity) in the original reagent to 0.061% and that increases further to 0.11% after adding 45# carbon steel. It's clear that nitrite ion will appear after a long period of time in the TES process of BMN salts looped in metal pipes and tanks. The reason for the increase of the concentration of NOx after contacting with 45# carbon steel may be that the Fe component in the steel reacts with NO 3 to generate NO. Meanwhile, the increase of the mass fraction of NO 2 after contacting with carbon steel was also resulted from Fe component in the carbon steel. The relevant reactions of which Dr Gqm ðTÞ values vary with temperature are shown in Table 5. From Table 5, it can be seen that the Dr Gqm ðTÞ values of Eqs. (1)e(4) are both negative between 600 K (327 C) and 1000 K (727 C). And the concentration ratio of NaNO2/NaNO3 or KNO2/ KNO3 in BMN salts is small, consequently, Q ＜ 1, lnQ ＜ 0, hence, the Dr Gm ðTÞ values of Eqs. (1)e(4) are negative too. That’s to say, Eqs. (1)e(4) are thermodynamically favored between 600 K and 1000 K. Therefore, the reason for the increase of the mass fraction of NO-2 after contacting with 45# carbon steel can be explained. In practical applications, molten nitrate salts loop in carbon steel or stainless steel pipe. The Dr Gqm ðTÞ values are both negative by calculating reactions that molten nitrate salts react with metals (Fe, Cr, Ni, Mn) with the evolution of NOx thermodynamically. Hence, molten nitrate salts contained in different kinds of stainless steels can also release a small amount of NO and NO2 with the formation of dense and adherent protective layers and some spinel. The thermal decomposition of BMN salts which contact with silica or alloy container not only generates nitrite ion but also may produces NOx. Occurrence of these reactions can also change the environment of inert and pure atmosphere; if generated gas cannot be removed quickly, the atmosphere which molten salts in will change.
4. Conclusion BMN salts contained in silica or carbon steel container will release a small amount of NOx from some side reactions in its use of temperature range except for the formation of nitrite ion. The concentration of NOx emissions creeps with the rise of temperature, it's advisable that BMN salts should be used under 600 C in order to relief the burden to the environment and the container surface. The concentration of NOx emissions has an increase after contacting with 45# carbon steel, the increment mainly comes from NO
Table 5 Dr Gqm ðTÞ values of the reactions at different temperatures (kJ mol1). Equations
(1) (2) (3) (4)
Temperature/K
2Fe(s) þ 3NaNO3(l) ¼ Fe2O3(s) þ 3NaNO2(l) 3Fe(s) þ 4NaNO3(l) ¼ Fe3O4(s) þ 4NaNO2(l) 2Fe(s)þ3KNO3(l) ¼ Fe2O3(s) þ 3KNO2(l) 3Fe(s) þ 4KNO3(l) ¼ Fe3O4(s) þ 4KNO2(l)
600
700
800
900
1000
524.68 732.04 504.68 705.38
525.89 736.28 511.70 717.36
526.78 740.36 519.16 730.20
527.40 744.54 527.01 744.02
537.60 748.28 545.01 758.16
X. Wei et al. / Energy 74 (2014) 215e221
emissions, which may be result from Fe element in carbon steel. Fe element can also result in the increment of the concentration of nitrite ion in the molten salts samples. As a result, the concentration of nitrite ion in the sample increases with the temperature. This should be considered carefully when choosing steels as container materials. Except for the secondary product NOx, the nitrate ion decomposes to nitrite ion is still the main thermal decomposition reaction. Acknowledgment The authors acknowledge financial support from National Natural Science Foundation of China (51376067), the National Basic Research Program of China (No. 2010CB227103) and the Postdoctoral Science Foundation of China (2013M531847). References [1] Wang Yan, Wei Xiaolan, Ding Jing, et al. The cumulative monitoring of NOx emissions in heat storage process of molten nitrate salts. J Lanzhou Univ Technol 2013;39(5):54e7. [2] Peng Qiang, Wei Xiaolan, Ding Jing, et al. High-temperature thermal stability of molten salt materials. Int J Energy Res 2008;32:1164e74. [3] Bradshaw RW, Meeker DE. High-temperature stability of ternary nitrate molten salts for solar thermal energy systems. Sol Energy Mater 1990;21: 51e60. [4] Olivares Rene I. The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres. Sol Energy 2012;86: 2576e83. [5] Long Bin, Wei Xiaolan, Ding Jing, et al. Calculation of chemical thermodynamics deterioration of ternary nitrate salts at high temperatures. J Sol Energy 2011;32(2):252e6. [6] Gordon S, Campbell C. Differential thermal analysis of inorganic compounds nitrates and perchlorates of the alkali and alkaline earth groups and their subgroups. Anal Chem 1955;27(7):1102e9. n Bjo €rn, et al. Material constraints for [7] Pihl Erik, Kushnir Duncan, Sande concentrating solar thermal power. Energy 2012;44:944e54. [8] Freeman Eli S. The kinetics of the thermal decomposition of sodium nitrate and of the reaction between sodium nitrite and oxygen. J Phys Chem 1956;60(11):1487e93. [9] Freeman Eli S. The kinetics of the thermal decomposition of potassium nitrate and of the reaction between potassium nitrite and oxygen. J Am Chem Soc 1957;79(4):838e42. [10] Bartholomew RF. A study of the equilibrium KNO3 (l) ¼ KNO2 (l) þ1/2O2 (g) over the temperature range 550e750 C. J Phys Chem 1966;70(11):3442e6. [11] Stern Kurt H. Thermal decomposition of inorganic salts. Part 3. Nitrates and nitrites. J Phys Chem Ref. Data 1972;1(3):747e71. [12] Kust RN, Burke JD. Thermal decomposition in alkali metal nitrate melts. Inorg Nucl Chem Lett 1970;6:333e5. [13] Nissen DA, Meeker DE. Nitrate/nitrite chemistry in NaNO3eKNO3 melts. Inorg Chem 1983;22(5):716e21.
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