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nitrates salt by time of flight mass spectrometry
Solar Energy 183 (2019) 823–828
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Probing thermal decomposition mechanism of molten nitrite/nitrates salt by time of flight mass spectrometry
T
⁎
Zejie Feia, Yu Zhangb, Min Gea, Yongtian Wanga,c, Yanli Lia,c, Jinhui Chenga, , Baoren Weib, ⁎ Huiqi Houa, Hongtao Liua, a
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China Shanghai EBIT Laboratory, Modern Physics Institute, Fudan University, Shanghai 200433, China c University of Chinese Academy of Sciences, Beijing 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal energy storage Thermal stability Molten salts Time of flight mass spectrometer Thermal decomposition
A time of flight mass spectrometer (TOF-MS) combined with a specially designed in-situ heating furnace was used for analyzing gaseous products evolved during salt decomposition. The thermal decomposition behavior of solar salt (60 wt% NaNO3–40 wt% KNO3) and sodium nitrite at temperature above melting point was investigated. According to mass spectra recorded at different temperature, both solar salt and sodium nitrite thermally decomposed above their melting point and the main products are NO and N2 respectively. An unusual N2O was also observed in gaseous products by TOF-MS, while no O2 or NO2 was found at temperature bellow 300 °C. The result shows a very different reaction process from the common sense that O2 or NO2 should exist in the decomposition process. Based on these new evidences, the thermal decomposition mechanism of molten nitrite/nitrates salt is proposed, which suggests that there should be intermediate state such as superoxide and peroxide ions in the molten salt in the initial thermal-chemical reaction process.
1. Introduction Molten salt are most promising heat transfer fluid (HTF) and thermal energy storage (TES) media applied in Concentrating Solar Power (CSP) plants (Gil et al., 2010; Medrano et al., 2010; Dunn et al., 2012; Kelly et al., 2007; Eck and Hennecke, 2008). As HTF, alkali nitrate/nitrite salt are widespread used since decades due to its low cost and definite thermal-physical advantages such as low viscosity and high heat capacity. However, the thermal decomposition and non-negligible vapor pressure prevent the usage of nitrate/nitrite molten salt at temperature higher than 550 °C (Pacheco et al., 2002). Lots of work have been focus on the thermal stability of nitrate/nitrite molten salt, such as adding kinds of additive to the solar salt in Wu et al. (2017) and Peng’s et al. (2008) experiments, and designing new kind of multi-component systems by Wu et al. (2018), Mantha et al. (2013) and Peng et al. (2013), all of this researches received very good results. Whereas to improve the thermal stability, the decomposition behavior of molten nitrate/nitrite salt and corresponding chemical reaction also need to be thoroughly investigated. Although the thermal stability of nitrate/nitrite based molten salt has been studied since 1960s (Gordon and Campbell, 1955; Freeman, 1956), there are still confusions in regard to decomposition ⁎
temperature and thermal chemical behavior. For example, the decomposition temperature of the heat transfer salt (HTS) mixture (53 wt% KNO3–7 wt% NaNO3–40 wt% NaNO2) ranged from 450 °C (Alexander and Hindin, 2002; Silverman and Engel, 1977) to 535 °C (Kearney et al., 2003). From the thermal-gravimetric and mass spectrometric (TG-MS) experiment, Olivares et al. found that the thermal decomposition of HTS occurred at 470 °C with the evolution of NO in the inert atmosphere of argon, and the oxidation of nitrite to nitrate took place over the temperature range 218–510 °C in air environment (Olivares, 2012). The thermal stability of NaNO2 plays a key role in the HTS for determination of the decomposition temperature. In the work of Gordon et al., the alkali nitrates were observed to undergo a thermal reaction above their melting points as indicated by gas emitting from the molten salt and the evolution of nitrous fumes occurred at temperatures ranging between 200 and 350 °C, which showed the result of the decomposition of the nitrate and nitrite to oxide (Gordon and Campbell, 1955). For the ternary eutectic mixture LiNO3-NaNO3-KNO3 (30 wt% LiNO3–18 wt% NaNO3–52 wt% KNO3) molten salt, trace amount of weight loss was detected using the Thermogravimetric-Differential Thermal Analyzer (TG/DTA) at 435 °C, indicating onset of the thermal instability (Wang et al., 2012). However, the detection of NO from the
Corresponding authors. E-mail addresses: [email protected] (J. Cheng), [email protected] (H. Liu).
https://doi.org/10.1016/j.solener.2019.03.067 Received 15 November 2018; Received in revised form 5 March 2019; Accepted 17 March 2019 Available online 27 March 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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TG-MS experiment was first noticed at 325 °C and 425 °C in the inert atmosphere of argon and nitrogen, respectively. Those results suggest the decomposition temperature of the alkali nitrate mixture salt was effected by the different atmospheres (Olivares and Edwards, 2013). According to the literature, the nitrate based binary molten salt could be thermally stable up to 500 °C (Cordaro et al., 2011; Nissen and Meeker, 1983); and the proposed dissociation reaction was given in Eq. (1), which is a general understanding of thermal decomposition in nitrate based molten salts (Freeman, 1956; Kramer et al., 1982; Bond et al., 1966; Benaissa and Carson, 2011), although further decomposition would take place with the evolution of the nitrogen oxides at a higher temperature (Bartholomew, 1966).
NO3− ↑ NO2− + 1/2O2 ↑
vacuum and the decomposition gases from the molten salt were in situ detected by the time of flight mass spectrometer (TOF-MS). The sample salts were controlled in the low temperature range from the melting point to 300 °C (Tm < T < 300 °C). Surprisingly, the gas products were immediately detected above the melting point for both the mixture solar salts and NaNO2, which indicate the nitrate/nitrite based molten was not stable and thermal decomposition reaction takes place just above the melting point. Further analysis shown there were no NO2 and O2 in the gases evolved from the molten salt below 300 °C, however, the nitrous oxide (N2O) was found in gas products of both samples. The detail information about the intrinsic reaction of the molten salt will be shown and discussed in this paper.
(1)
These investigations indicated that the O2 should certainly exist as the products of the decomposition, while the gas analysis of Freeman (1956), Bond et al. (1966), Freeman (1956) had shown that only about 1/8 of the available oxygen evolved from NaNO2 under argon after heating 130 min at 750 °C, which strongly suggested the superoxide ion as an intermediate in the complete reaction Eqs. (2)–(4) (Bond et al., 1966), the peroxide and superoxide had also been postulated, the pertinent literature had been reviewed by Zambonin and Cavaggioni (1971) and Burke and Kerridge (1974).
2NO2− → N2 ↑ + 2O2− −
2O2 → O2
2−
+ O2 ↑
4O2− → 2O 2 − + 3O2 ↑
2. Experiment 2.1. Materials All of the materials used in this work, sodium nitrate (> 99% pure), sodium nitrite (> 99% pure) and potassium nitrate (> 99% pure) were bought from Sinopharm as analytical reagent (AR) without further purification.
(2) 2.2. Preparation of solar salt (3) The nitrate based binary mixture molten salt (60 wt% NaNO3–40 wt % KNO3) known as solar salt, is wildly used as thermal energy storage medium in the CSP plant for its better economic profit (Bauer et al., 2013) and usually thought to be stable even at the temperature higher than 500 °C. In this work, the sodium nitrate and potassium nitrate are mixed in certain proportion in a glove box, and kept at 300 °C in a high temperature oven for about 2 h to ensure completely melting and mixing. The melting point of this mixture salt was measured to ensure the accurate composition with the differential scanning calorimetry (DSC) in argon atmosphere. The DSC was calibrated with certified reference materials including In, Bi, Sn, Zn, Al and Au. The measurement of salt mixture was repeated thrice from 50 °C to 300 °Cat a rate of 5 °C/min and the mean values of the last two measurements were adopted. The melting temperature was determined from the extrapolated onset temperatures. According to DSC scanning shown in Fig. 1, the melting point of the solar salt used in this work is about 220 °C, which is well correspond to the recognized melting temperature range of 215–245 °C of solar salt (Rogers et al., 1982).
(4)
All of these works showed that the nitrate/nitrite based molten salt was unstable at temperature much higher than 500 °C (Peng et al., 2008; Olivares, 2012; Olivares and Edwards, 2013; Bartholomew, 1966; Kramer et al., 1982; Wei et al., 2013; Wright et al., 2012), though the reported decomposition temperature and decomposition products were not consistent. Scarcely studies about the intrinsic dissociation reaction in the nitrate/nitrite based molten salt at lower temperature (below 300 °C) had been reported before. Kust and Burke (1970) conducted the experiments with a pure equimolar sodium-potassium nitrate melt over the temperature range 295–340 °C, where the emf (electromotive force) measurement showed that high oxide dianion (O2−) concentration in the molten salt and the nitrate ion was postulated to be a Lux-Flood base which dissociated to produce the conjugated acid and oxide ion as followed Eq. (5) (Kust and Duke, 1963).
NO3− ↑ NO2+ + O 2 −
(5)
Further evidences for the existence of dissociation products are still desirable, such as directly detecting NO2+ cation. Usually the nitrate/ nitrite based molten salt is considered to be stable at 300 °C due to the difficulty monitoring the dissociation reaction in the melt, especially when the reaction rate is too slow to get enough valid data. The dissociation reaction also could be affected by environmental atmosphere because the component of oxide and carbon dioxide in the atmosphere might be directly involved into the reactions (Bradshaw et al., 2008). For these reasons, the investigation of the intrinsic reaction in the molten salt was difficult and complicated. The atmosphere can make an important effect on the molten nitrate/nitrite salts decomposition, for example, the N2 or/and O2 can participate in the decomposition reaction. Therefore, to avoid the effect from the atmosphere, the vacuum condition was adopted. Molten nitrate/nitrite salts were selected to be studied mainly because it was ambiguous of decomposition reaction. It was deemed that nitrate can decompose into nitrite and oxygen gas which is a reversible reaction and not discerned. If the decomposition took place in the vacuum condition, then two reaction process can be distinguished, it means that a transition state may be existed between these processes. In the present work the solar salt and sodium nitrite were heated in
cycle-1 up cycle-2 up cycle-3 up
0.4
DSC (mW/mg)
0.2
0.0
-0.2 Onset (melting temperature): 220.7 (
)
-0.4
-0.6 140
160
180
200
220
240
260
280
300
320
Sample Temperature (°C)
Fig. 1. DSC analysis for the melting point of solar salt, the second (cycle-1), third (cycle-2) and fourth (cycle-3) cycle from 150 °C to 300 °C are selected. 824
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Accelerated plate Ionization region
Deflection plate
Ion lens
MCP detector
Furnace
Cooling circuits Power Fig. 2. Schematic diagram of the in-situ high temperature time of flight mass spectrometer (TOF-MS).
0
2.3. Measurement of the gas components from molten salt decomposition by TOF-MS
10
20
30
40
50
60
70
80
50
60
70
80
solar salt 260 +
To investigate the gas components from molten salt decomposition, a time of flight mass spectrometer (TOF-MS) combined with an in-situ heating furnace was designed as Fig. 2. The sample was heated by the furnace placed in the main vacuum chamber, and the gaseous products were transfused into the ionization region to be ionized into different ions by the electron beam. These ions were accelerated by the electric field and flied to the microchannel plate (MCP) detector, from which we could get the mass information according to the time of flight of the different ions. In order to eliminate the background interference, a blanket reference mass spectrum should be taken at first. An empty crucible was putted into the heating furnace in the ion source chamber of the TOF-MS system, which has equipped with an electron impact ion source. The temperature of the furnace can be tuned from room temperature up to 400 °C, the mass spectra of the background were taken at several temperature points in the range of 30–400 °C, The background experiment showed that the vacuum changed slightly from 1 × 10−5 Pa to 6 × 10−5 Pa as the temperature raising up to 200 °C due to the emitting of moisture from the surface of furnace. After the vacuum background being measured and calibrated, 5 g self-prepared solar salt was placed into the molybdenum crucible and heated in the vacuum environment (about 1.0 × 10−5 Pa). Considered the melting point of the sample and the change of the vacuum environment, the temperature points for measurement were chosen at 30 °C, 50 °C, 100 °C, 150 °C, 200 °C, 240 °C, 260 °C for solar salt and 30 °C, 150 °C, 220 °C, 280 °C for NaNO2. The TOF-MS data were obtained at these temperature points, and each point was kept stable for at least 10 min before further measurement. The vacuum was affected by the emission of gases from the thermal decomposition reaction (in molten salt from 1.0 × 10−5 Pa to 5.0 × 10−3 Pa as temperature increased. The evolution gases were ionized by the low energy electron beam and detected by the microchannel plate (MCP). The data were recorded for about 1 h at each temperature point. The sodium nitrite was also investigated in the same way in present work. Due to the melting point of sodium nitrite is about 270 °C, the temperature of this sample was raised up to 280 °C. The detailed mass spectra of the gas components are discussed in the following part.
+
H2O
NO N2
+
+
N2O
intensity
solar salt 240
solar salt 200
solar salt 30
0
10
20
30
40
m/z Fig. 3. Mass spectra of the evolutions of gas from the molten solar salt (60 wt% NaNO3–40 wt% KNO3) change at different temperature points, the H2O+ is from the background and the new materials come out at the temperature above 220 °C.
molten solar salt (60 wt% NaNO3–40 wt% KNO3). As it reached the melting point (220 °C according to the DSC analysis in this work), some gases were emitting from molten salt, which would be shown from both
3. Results and discussion Fig. 3 shows the mass spectra of the evolutions of gas from the 825
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the vacuum gauge and the detector of the TOF-MS. According to mass spectrum the ion intensity of these gases were increasing with the rising temperature. Based on comparison with the mass spectra of the gas products from the molten solar salt decomposition, there were few changes at different temperature point in the range of 30–200 °C. In Fig. 3 four temperature points at 30 °C, 200 °C, 240 °C and 260 °C were selected to display the reaction process. Compared with the 200 °C mass spectrum, a dramatically change is observed in the spectrum at 240 °C (above the melting point 220 °C), in the 200 °C mass spectrum there were only four peaks obtained from the ionization of the moisture and hydrogen from the background, while some new peaks appeared clearly in the 240 °C mass spectrum. However these peaks emerge in the spectra at very low counting rate as the temperature raised higher than 230 °C, which means that there would be some gases coming out from the molten solar salt when it was melt at the temperature above its melting point 220 °C. The sodium nitrite was also conducted in the similar way, while there was no change in the spectrum before the temperature up to 260 °C in the heating process, which means that no evolutions of gases from the solid state salt as the temperature below 270 °C. When the temperature of the sodium nitrite was up to 280 °C, even only a few degrees higher than the melting point of the NaNO2, there were four new peaks appeared in the mass spectrum as shown in Fig. 4. This indicated there should be some new gas species emerged from the liquid phase.
0
10
20
30
40
50
60
70
intensity
+
N /N2
0
+
H2O
10
20
N2
+
+
N2O
30
40
50
60
70
80
Fig. 5. Calibrated mass spectrum of the evolution of gases from the molten solar salt at 260 °C.
In order to identify these ions in the spectrum, All the spectra of the solar salt and sodium nitrite were calibrated by the lines of H+, H2+, OH+ and H2O+, which were ionized from the background gases in the vacuum chamber at 280 °C. As shown in Fig. 5, A mass spectrum of molten solar salt recorded at 260 °C, considering the compound of the solar salt and the calibration information, except the background ions, the other peaks of the m/z at 14, 28, 30 and 44 were identified as N+, N2+, NO+ and N2O+ ions respectively, while the CO+ (28) and CO2+ (44) ions could be ruled out completely because there was no C+ ion (12) observed in the spectrum. As shown in the calibrated spectrum of the evolution of gases from the sodium nitrite (at 280 °C) Fig. 6, except the H2O+ peak from the background, the other four peaks were identified as N+, N2+, NO+ and N2O+ respectively. Compared with Fig. 5, the relative intensity of the evolution gases from molten solar salt and sodium nitrite were different. Usually these peaks in the mass spectrum were ionized from the related substance, while not each of them corresponds to a molecular in the gas phase, as for the N+, which might be a fragment ion from N2, NO or some other N element contained molecules. According to the Caroline’s work (O'Connor et al., 1997) the high temperature hydrolysis reaction to form HNO3 could be excluded in current work because there were no HNO3+ and its fragment NO2+ in the mass spectrum, which should appeared as peaks corresponded to their mass number at the horizontal m/z axis. In Caroline’s ionization
80
+
+
N2O
+
+ + N /N2 H2O
2+
m/z
NaNO2 280 N2
solar salt 260
+
NO
+
NO
intensity
NaNO2 260
N2
NaNO2 280
+
intensity
NaNO2 30
+
N /N2
+
2+
N2O +
NO +
H2O
0
10
20
30
40
50
60
70
80
m/z 0
Fig. 4. Mass spectra of the evolution of gas from the molten sodium nitrite salt change at different temperature points: 30 °C, 260 °C and 280 °C are selected, the H2O+ is from the background and the new materials come out at 280 °C, about 10 °C higher than the melting point of NaNO2.
10
20
30
40
50
60
70
80
m/z Fig. 6. Calibrated mass spectrum of the evolution of gases from the molten NaNO2 salt at 280 °C. 826
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2012; Olivares and Edwards, 2013; Bartholomew, 1966; Kramer et al., 1982; Wei et al., 2013). And if the O2 and NO2 existed in the gas phase, it could be ionized into O2+ and NO2+ ions by the electron impacted beam and detected by the TOF-MS (Pal et al., 2003; Kim et al., 1997), while based on the TOF-MS analysis in current work, there were no O2+ and NO2+ ions observed in Figs. 5 and 6, which suggested that there were no O2 or NO2 gases released from the thermal decomposition reactions in the nitrate/nitrite based molten salt when the temperature was below 300 °C. Previous report shown an intermediate state existed in the reaction process and only about 1/8th of the available oxygen at 750 °C in 130 min (Bond et al., 1966; Freeman, 1956). According to the electron paramagnetic resonance and ultravioletvisible absorption spectra absorption spectroscopy (Shu-ting Liu and Zhang, 2017), the superoxide anion (O2−) was identified in the molten nitrate salt. When considering the existence of superoxide anion (O2−) in the nitrate/nitrite based molten salt, the possible thermal decomposition reaction mechanism in the nitrate based molten solar salt could be expressed as Eq. (6)
Table 1 Relative intensity of the ions N2+, NO+ and N2O+ in the spectrum of solar salt at 260 °C and NaNO2 molten salt at 280 °C, relative to the background ions, H2O+. Relative intensity
Solar salt
NaNO2
N2+/H2O+ NO+/H2O+ N2O+/H2O+
0.85 12.09 1.76
56.33 24.76 26.43
cross sections research for HNO3 by TOF-MS, as the incident electron energies rising from 40 to 450 eV, the parent ions HNO3+ and the fragmentation productions H+, N+, NO+ and NO2+ were detected, while the most abundant ion in the mass spectrum was NO2+ all the time. For the similar reason, these observed ions should not be fragments from nitrate/nitrite salt vapor molecule for there were no Na+ or K+ detected in the spectrum. All of these evidence indicated that the evolution of gases should be produced by the thermal decomposition reaction in the nitrate/nitrite based molten salt, which means that the molten nitrate/nitrite salt was undergoing a slow dissociation reaction when it was heated above its melting point. It was interesting to compare the spectrum of molten solar salt (Fig. 5) with NaNO2 salt (Fig. 6), though the N2+, NO+ and N2O+ ions appeared clearly in both of them, the relative intensity of these ions were great different. For convenient comparison, the background H2O+ ions was taken as reference and the relative intensity of the N2+, NO+ and N2O+ ions were normalized with H2O+ in Table 1. As listed in Table 1 the intensity of N2O+ ion, which was ionized from N2O molecular by the electron impact beam, the N2O+ ion from the NaNO2 experiment was much higher than that of solar salt experiment. Since there was no background signal of the N2O+ ion, the N2O molecular in the evolution of gases should be formed from the salt decomposition; Based on the partial ionization cross sections of N2O and NO at the energy of 200 eV by electron impact experiment as shown in Table 2 (Iga et al., 1996), the enhanced N2+ and NO+ ions intensities were assigned to be the ionization fragments of N2O. As shown in Table 2, the relative intensity of NO+/N2O+ and N2+/ N2O+ from ionization fragments of N2O should be about 0.46 and 0.16, while the measured ratios were about 6.87 and 0.48 in Fig. 5, about 0.94 and 2.13 in Fig. 6, respectively. This indicate only part of the N2+ and NO+ ions were produced fromN2O fragmentation, while the others should be direct electron impact from the N2 and NO molecule. Based on the mass spectrum, the products of the evolved gases in the molten sodium nitrite and solar salt were N2 and NO respectively, which was agree with Freeman’s work that N2 was the major gaseous product of NaNO2 at the beginning (Freeman, 1956). There was few evidence of the production of N2O from the thermal decomposition reaction in nitrated based molten salt except the detection by Olivares using the TG-MS equipment at high temperature (above 800 °C) under the Argon, Nitrogen and Oxygen atmospheres (Olivares and Edwards, 2013), however, the mass spectrum in present work shows the existence of N2O evolved from the molten salt at much lower temperature. The O2 and NO2 were considered to be the dominant productions from the thermal decomposition reaction in nitrate/nitrate based molten salt especially at high temperature (Peng et al., 2008; Olivares,
NO3−→NO ↑ +O2−
(6)
And the Eq. (7) expressed one of the most possible dissociation reaction of sodium nitrite molten salt
2NO2− → N2 ↑ + 2O2−
(7)
Based on the basis of Lux-Flood acid concept, the nitrate ions was postulated to dissociate into nitronium ions and oxide ions as Eq. (5), and then the oxide ions would continue react with the nitrate ions in the molten salt as follow:
O 2 − + NO3− ↑ O22 − + NO2−
O2
2−
+
2NO3−
−
↑ 2O2 + 2NO2
(8) −
(9)
For the detection of N2 and NO from the molten salt in different abundance, there might exist NO2− anion in the molten nitrate salt and NO3− anion in the molten sodium nitrite salt as shown in the Eqs. (8) and (9). According to Figs. 5 and 6, there were lots of N2O gas evolution from both of the molten solar salt and molten sodium nitrite salt detected by the TOF-MS. These N2O gas should be produced as the reactions of Eqs. (10) and (11)
NO2− + NO3− → 2O2− + N2 O↑ −
4NO2 →
2NO3−
+
O2−
+ N2 O↑
(10) (11)
Molecular
N2O+
NO+
N2+
N+
O+
Since there were no O2 and NO2 emerged from the nitrate/nitrite based molten salt when the temperature was below 300 °C in present work, while as shown in Eqs. (3) and (4) when the temperature rising up to more than 500 °C, peroxide and superoxide could be further decomposition into O2, which agreed with the investigation in the thermal stability of molten nitrite/nitrates salt (Peng et al., 2008; Olivares, 2012). The thermal stability of molten salt is sensitive to the environmental atmosphere, if salt exposures to the air, the oxygen can participate the decomposition reactions, either involved the surface reaction or oxidize the NO to NO2. While in present work, the vacuum environment could supposedly avoid the influence by the surrounding atmosphere. According to the TOF-MS experiment, the molten nitrate/nitrite salt produced gases by the thermal decomposition, while the reaction rate appeared very slow at low temperature for the changes of the composition in the salt were too weak to be detected by the Ion Chromatography System, nevertheless, they would grow quickly with increasing temperature, which could be monitored by the vacuum ion gauge and the decreasing vacuum constantly.
N2O NO
178.1 –
82.3 264.1
28.2 –
31.9 44.5
15 18.3
4. Conclusions
Table 2 Partial ionization cross sections of N2O and NO by electron impact at 200 eV (Iga et al., 1996).
Unit of measure is 10−18 cm2.
The thermo-chemical behavior and thermal stability of molten solar 827
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salt and sodium nitrite are studied by in situ salt heating oven system combined with TOF-MS. Evolutions of gases from molten salt have been detected above the melting temperature of salt in vacuum environment. The mass spectra of the solar salt (from room-temperature to 260 °C) and sodium nitrite (from room-temperature to 280 °C) are obtained.
Freeman, E.S., 1956. The kinetics of the thermal decomposition of potassium nitrate and of the reaction between potassium nitrite and oxygen.pdf. J. Phys. Chem. 79, 838–841. Alexander, J., Hindin, S.G., 2002. Phase relations in heat transfer salt systems. Ind. Eng. Chem. 39, 1044–1049. Silverman, M.D., Engel, J.R., 1977. Survey of Technology for Storage of Thermal Energy in Heat Transfer Salt. Off-Peak Energy Storage. Kearney, D., Herrmann, U., Nava, P., Kelly, B., Mahoney, R., Pacheco, J., Cable, R., Potrovitza, N., Blake, D., Price, H., 2003. Assessment of a molten salt heat transfer fluid in a parabolic trough solar field. J. Sol. Energy Eng. 125, 170–176. Olivares, R.I., 2012. The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres. Sol. Energy 86, 2576–2583. Wang, T., Mantha, D., Reddy, R.G., 2012. Thermal stability of the eutectic composition in LiNO3–NaNO3–KNO3 ternary system used for thermal energy storage. Sol. Energy Mater. Sol. Cells 100, 162–168. Olivares, R.I., Edwards, W., 2013. LiNO3–NaNO3–KNO3 salt for thermal energy storage: thermal stability evaluation in different atmospheres. Thermochim. Acta 560, 34–42. Cordaro, J.G., Rubin, N.C., Bradshaw, R.W., 2011. Multicomponent molten salt mixtures based on nitrate/nitrite anions. J. Sol. Energy Eng. 133, 169–175. Nissen, D.A., Meeker, D.E., 1983. Nitrate/nitrite chemistry in NaNO/sub 3/-KNO/sub 3/ melts. Inorg. Chem. (United States) 22, 5. Kramer, C.M., Munir, Z.A., Volponi, J.V., Kramer, C.M., Munir, Z.A., Volponi, J.V., 1982. Differential scanning calorimetry of sodium and potassium nitrates and nitrites. Thermochim. Acta 55, 11–17. Bond, B.D., Jacobs, P.W.M., Bond, B.D., Jacobs, P.W.M., 1966. The thermal decomposition of sodium nitrate. J. Chem. Soc. A 1265–1268. Benaissa, W., Carson, D., 2011. Oxidation properties of “Solar Salt”. Aiche Spring Meeting Global Congress on Process Safety, 2011. Bartholomew, R.F., 1966. A study of the equilibrium KNO3(l) ⇄ KNO2(l) + 1/2O2(g) over the temperature range 550–750°. J. Phys. Chem. 70, 3442–3446. Freeman, E.S., 1956. The kinetics of the thermal decomposition of sodium nitrate and of the reaction between sodium nitrite and oxygen. J. Phys. Chem. 60, 1487–1493. Zambonin, P.G., Cavaggioni, A., 1971. Redox mechanisms in an ionic matrix. Autocatalysis in the process superoxide + nitrite.dbr. peroxide + nitrate in fused salts. J. Am. Chem. Soc. 93, 2854–2858. Burke, J.D., Kerridge, D.H., 1974. Concerning oxygen anions in a nitrate melt. Electrochim. Acta 19, 251–256. Kramer, C.M., Munir, Z.A., Volponi, J.V., 1982. Screening tests of sodium nitrate and potassium nitrate decomposition ☆. Sol. Energy 29, 437–439. Wei, X., Peng, Q., Ding, J., Yang, X., Yang, J., Long, B., 2013. Theoretical study on thermal stability of molten salt for solar thermal power. Appl. Therm. Eng. 54, 140–144. Wright, S., Tran, T., Chen, C., 2012. Thermal stability of potassium and sodium nitrate molten salt mixtures above 500°C. Kust, R.N., Burke, J.D., 1970. Thermal decomposition in alkali metal nitrate melts. Inorg. Nucl. Chem. Lett. 6, 333–335. Kust, R.N., Duke, F.R., 1963. A study of the nitrate ion dissociation in fused nitrates. J. Am. Chem. Soc 85, 170–172. Bradshaw, R.W., Cordaro, J.G., Siegel, N.P., 2008. Molten nitrate salt development for thermal energy storage in parabolic trough solar power systems. In: ASME 2009 International Conference on Energy Sustainability Collocated with the Heat Transfer and Interpack09 Conferences, pp. 631–637. Bauer, T., Pfleger, N., Breidenbach, N., Eck, M., Laing, D., Kaesche, S., 2013. Material aspects of solar salt for sensible heat storage. In: Innostock 2012 - the International Conference on Energy Storage, pp. 1114–1119. Rogers, D.J., Janz, G.J., Rogers, D.J., Janz, G.J., 1982. Melting-crystallization and premelting properties of NaNOâ-KNOâ. J. Chem. Eng. Data 27. O'Connor, C.S.S., Jones, N.C., Price, S.D., 1997. Electron-impact ionization of nitric acid. Int. J. Mass Spectrom. Ion Process. 163, 131–139. Iga, I., Rao, M.V.V.S., Srivastava, S.K., 1996. Absolute electron impact ionization cross sections for N2O and NO from threshold up to 1000 eV. J. Geophys. Res. Planets 101, 9261–9266. Pal, S., Kumar, J., Bhatt, P., 2003. Electron impact ionization cross-sections for the N2 and O2 molecules. J. Electr. Spectrosc. Relat. Phenom. 129, 35–41. Kim, Y.K., Hwang, W., Weinberger, N.M., Ali, M.A., Rudd, M.E., 1997. Electron-impact ionization cross sections of atmospheric molecules. J. Chem. Phys. 106, 1026–1033. Shu-ting Liu, T.S., Zhang, Peng, Fei, Ze-jie, Liu, Hong-tao, 2017. Identification of superoxide O2-during thermal decomposition of molten KNO3-NaNO2-NaNO3 salt by electron paramagnetic resonance and UV-Vis absorption spectroscopy. Chin. J. Chem. Phys. 30, 7.
(1) The gas products are identified to be N2, NO and N2O, and the main dissociation product of the molten solar salt and molten sodium nitrite salt is NO and N2, respectively. O2 and NO2 are not detected in the intrinsic reaction below 300 °C, which shows a very different reaction process from the common sense that O2 or NO2 should exist in the decomposition process. (2) Based on these new evidences, the thermal decomposition mechanism of molten nitrite/nitrates salt was proposed to be 4NO2−→ NO3−→NO ↑ +O2−, 2NO2− → N2 ↑ + 2O2− and 2NO3− + O 2 − + N2 O↑ at the temperature below 300 °C, which suggests the superoxide ions and peroxide ions should exist in the molten salt as intermediate products in the initial thermal-chemical reaction process. Acknowledgements This work was supported by the National Natural Science Foundation of China [Grant No. 21703281; 51604275; 21573273; 11504397], and “Strategic Priority Research Program” of the Chinese Academy of Sciences [Grant No. XDA02002400]. H.T. Liu would like to acknowledge support of Hundred Talents Program (CAS). References Gil, A., Medrano, M., Martorell, I., Lazaro, A., Dolado, P., Zalba, B., Cabeza, L.F., 2010. State of the art on high temperature thermal energy storage for power generation. Part 1-concepts, materials and modellization. Renew. Sust. Energ. Rev. 14, 31–55. Medrano, M., Gil, A., Martorell, I., Potau, X., Cabeza, L.F., 2010. State of the art on hightemperature thermal energy storage for power generation. Part 2-case studies. Renew. Sust. Energ. Rev. 14, 56–72. Dunn, R.I., Hearps, P.J., Wright, M.N., 2012. Molten-salt power towers: newly commercial concentrating solar storage. Proc. IEEE 100, 504–515. Kelly, B., Price, H., Brosseau, D., Kearney, D., Kelly, B., Price, H., Brosseau, D., Kearney, D., 2007. Adopting nitrate/nitrite salt mixtures as the heat transport fluid in parabolic trough power plants. In: ASME 2007 Energy Sustainability Conference, pp. 1033–1040. Eck, M., Hennecke, K., 2008. Heat Transfer Fluids for Future Parabolic Trough Solar Thermal Power Plants. Springer, Berlin, Heidelberg. Pacheco, J.E., Showalter, S.K., Kolb, W.J., 2002. Development of a molten-salt thermocline thermal storage system for parabolic trough plants. J. Sol. Energy Eng. 124, 153. Wu, Y.T., Li, Y., Ren, N., Ma, C.F., 2017. Improving the thermal properties of NaNO3KNO3 for concentrating solar power by adding additives. Sol. Energy Mater. Sol. Cells 160, 263–268. Peng, Q., Wei, X., Ding, J., Yang, J., Yang, X., 2008. High-temperature thermal stability of molten salt materials. Int. J. Energy Res. 32, 1164–1174. Wu, Y.T., Li, Y., Ren, N., Zhi, R.P., Ma, C.F., 2018. Experimental study on the thermal stability of a new molten salt with low melting point for thermal energy storage applications. Sol. Energy Mater. Sol. Cells 176, 181–189. Mantha, D., Wang, T., Reddy, R.G., 2013. Thermodynamic modeling of eutectic point in the LiNO3–NaNO3–KNO3–NaNO2 quaternary system. Sol. Energy Mater. Sol. Cells 118, 18–21. Peng, Q., Yang, X., Ding, J., Wei, X., Yang, J., 2013. Design of new molten salt thermal energy storage material for solar thermal power plant. Appl. Energy 112, 682–689. Gordon, S., Campbell, C., 1955. Differential thermal analysis of inorganic compounds. Anal. Chem. 27, 1–4.
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Probing thermal decomposition mechanism of molten nitrite/nitrates salt by time of flight mass spectrometry
T
⁎
Zejie Feia, Yu Zhangb, Min Gea, Yongtian Wanga,c, Yanli Lia,c, Jinhui Chenga, , Baoren Weib, ⁎ Huiqi Houa, Hongtao Liua, a
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China Shanghai EBIT Laboratory, Modern Physics Institute, Fudan University, Shanghai 200433, China c University of Chinese Academy of Sciences, Beijing 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal energy storage Thermal stability Molten salts Time of flight mass spectrometer Thermal decomposition
A time of flight mass spectrometer (TOF-MS) combined with a specially designed in-situ heating furnace was used for analyzing gaseous products evolved during salt decomposition. The thermal decomposition behavior of solar salt (60 wt% NaNO3–40 wt% KNO3) and sodium nitrite at temperature above melting point was investigated. According to mass spectra recorded at different temperature, both solar salt and sodium nitrite thermally decomposed above their melting point and the main products are NO and N2 respectively. An unusual N2O was also observed in gaseous products by TOF-MS, while no O2 or NO2 was found at temperature bellow 300 °C. The result shows a very different reaction process from the common sense that O2 or NO2 should exist in the decomposition process. Based on these new evidences, the thermal decomposition mechanism of molten nitrite/nitrates salt is proposed, which suggests that there should be intermediate state such as superoxide and peroxide ions in the molten salt in the initial thermal-chemical reaction process.
1. Introduction Molten salt are most promising heat transfer fluid (HTF) and thermal energy storage (TES) media applied in Concentrating Solar Power (CSP) plants (Gil et al., 2010; Medrano et al., 2010; Dunn et al., 2012; Kelly et al., 2007; Eck and Hennecke, 2008). As HTF, alkali nitrate/nitrite salt are widespread used since decades due to its low cost and definite thermal-physical advantages such as low viscosity and high heat capacity. However, the thermal decomposition and non-negligible vapor pressure prevent the usage of nitrate/nitrite molten salt at temperature higher than 550 °C (Pacheco et al., 2002). Lots of work have been focus on the thermal stability of nitrate/nitrite molten salt, such as adding kinds of additive to the solar salt in Wu et al. (2017) and Peng’s et al. (2008) experiments, and designing new kind of multi-component systems by Wu et al. (2018), Mantha et al. (2013) and Peng et al. (2013), all of this researches received very good results. Whereas to improve the thermal stability, the decomposition behavior of molten nitrate/nitrite salt and corresponding chemical reaction also need to be thoroughly investigated. Although the thermal stability of nitrate/nitrite based molten salt has been studied since 1960s (Gordon and Campbell, 1955; Freeman, 1956), there are still confusions in regard to decomposition ⁎
temperature and thermal chemical behavior. For example, the decomposition temperature of the heat transfer salt (HTS) mixture (53 wt% KNO3–7 wt% NaNO3–40 wt% NaNO2) ranged from 450 °C (Alexander and Hindin, 2002; Silverman and Engel, 1977) to 535 °C (Kearney et al., 2003). From the thermal-gravimetric and mass spectrometric (TG-MS) experiment, Olivares et al. found that the thermal decomposition of HTS occurred at 470 °C with the evolution of NO in the inert atmosphere of argon, and the oxidation of nitrite to nitrate took place over the temperature range 218–510 °C in air environment (Olivares, 2012). The thermal stability of NaNO2 plays a key role in the HTS for determination of the decomposition temperature. In the work of Gordon et al., the alkali nitrates were observed to undergo a thermal reaction above their melting points as indicated by gas emitting from the molten salt and the evolution of nitrous fumes occurred at temperatures ranging between 200 and 350 °C, which showed the result of the decomposition of the nitrate and nitrite to oxide (Gordon and Campbell, 1955). For the ternary eutectic mixture LiNO3-NaNO3-KNO3 (30 wt% LiNO3–18 wt% NaNO3–52 wt% KNO3) molten salt, trace amount of weight loss was detected using the Thermogravimetric-Differential Thermal Analyzer (TG/DTA) at 435 °C, indicating onset of the thermal instability (Wang et al., 2012). However, the detection of NO from the
Corresponding authors. E-mail addresses: [email protected] (J. Cheng), [email protected] (H. Liu).
https://doi.org/10.1016/j.solener.2019.03.067 Received 15 November 2018; Received in revised form 5 March 2019; Accepted 17 March 2019 Available online 27 March 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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TG-MS experiment was first noticed at 325 °C and 425 °C in the inert atmosphere of argon and nitrogen, respectively. Those results suggest the decomposition temperature of the alkali nitrate mixture salt was effected by the different atmospheres (Olivares and Edwards, 2013). According to the literature, the nitrate based binary molten salt could be thermally stable up to 500 °C (Cordaro et al., 2011; Nissen and Meeker, 1983); and the proposed dissociation reaction was given in Eq. (1), which is a general understanding of thermal decomposition in nitrate based molten salts (Freeman, 1956; Kramer et al., 1982; Bond et al., 1966; Benaissa and Carson, 2011), although further decomposition would take place with the evolution of the nitrogen oxides at a higher temperature (Bartholomew, 1966).
NO3− ↑ NO2− + 1/2O2 ↑
vacuum and the decomposition gases from the molten salt were in situ detected by the time of flight mass spectrometer (TOF-MS). The sample salts were controlled in the low temperature range from the melting point to 300 °C (Tm < T < 300 °C). Surprisingly, the gas products were immediately detected above the melting point for both the mixture solar salts and NaNO2, which indicate the nitrate/nitrite based molten was not stable and thermal decomposition reaction takes place just above the melting point. Further analysis shown there were no NO2 and O2 in the gases evolved from the molten salt below 300 °C, however, the nitrous oxide (N2O) was found in gas products of both samples. The detail information about the intrinsic reaction of the molten salt will be shown and discussed in this paper.
(1)
These investigations indicated that the O2 should certainly exist as the products of the decomposition, while the gas analysis of Freeman (1956), Bond et al. (1966), Freeman (1956) had shown that only about 1/8 of the available oxygen evolved from NaNO2 under argon after heating 130 min at 750 °C, which strongly suggested the superoxide ion as an intermediate in the complete reaction Eqs. (2)–(4) (Bond et al., 1966), the peroxide and superoxide had also been postulated, the pertinent literature had been reviewed by Zambonin and Cavaggioni (1971) and Burke and Kerridge (1974).
2NO2− → N2 ↑ + 2O2− −
2O2 → O2
2−
+ O2 ↑
4O2− → 2O 2 − + 3O2 ↑
2. Experiment 2.1. Materials All of the materials used in this work, sodium nitrate (> 99% pure), sodium nitrite (> 99% pure) and potassium nitrate (> 99% pure) were bought from Sinopharm as analytical reagent (AR) without further purification.
(2) 2.2. Preparation of solar salt (3) The nitrate based binary mixture molten salt (60 wt% NaNO3–40 wt % KNO3) known as solar salt, is wildly used as thermal energy storage medium in the CSP plant for its better economic profit (Bauer et al., 2013) and usually thought to be stable even at the temperature higher than 500 °C. In this work, the sodium nitrate and potassium nitrate are mixed in certain proportion in a glove box, and kept at 300 °C in a high temperature oven for about 2 h to ensure completely melting and mixing. The melting point of this mixture salt was measured to ensure the accurate composition with the differential scanning calorimetry (DSC) in argon atmosphere. The DSC was calibrated with certified reference materials including In, Bi, Sn, Zn, Al and Au. The measurement of salt mixture was repeated thrice from 50 °C to 300 °Cat a rate of 5 °C/min and the mean values of the last two measurements were adopted. The melting temperature was determined from the extrapolated onset temperatures. According to DSC scanning shown in Fig. 1, the melting point of the solar salt used in this work is about 220 °C, which is well correspond to the recognized melting temperature range of 215–245 °C of solar salt (Rogers et al., 1982).
(4)
All of these works showed that the nitrate/nitrite based molten salt was unstable at temperature much higher than 500 °C (Peng et al., 2008; Olivares, 2012; Olivares and Edwards, 2013; Bartholomew, 1966; Kramer et al., 1982; Wei et al., 2013; Wright et al., 2012), though the reported decomposition temperature and decomposition products were not consistent. Scarcely studies about the intrinsic dissociation reaction in the nitrate/nitrite based molten salt at lower temperature (below 300 °C) had been reported before. Kust and Burke (1970) conducted the experiments with a pure equimolar sodium-potassium nitrate melt over the temperature range 295–340 °C, where the emf (electromotive force) measurement showed that high oxide dianion (O2−) concentration in the molten salt and the nitrate ion was postulated to be a Lux-Flood base which dissociated to produce the conjugated acid and oxide ion as followed Eq. (5) (Kust and Duke, 1963).
NO3− ↑ NO2+ + O 2 −
(5)
Further evidences for the existence of dissociation products are still desirable, such as directly detecting NO2+ cation. Usually the nitrate/ nitrite based molten salt is considered to be stable at 300 °C due to the difficulty monitoring the dissociation reaction in the melt, especially when the reaction rate is too slow to get enough valid data. The dissociation reaction also could be affected by environmental atmosphere because the component of oxide and carbon dioxide in the atmosphere might be directly involved into the reactions (Bradshaw et al., 2008). For these reasons, the investigation of the intrinsic reaction in the molten salt was difficult and complicated. The atmosphere can make an important effect on the molten nitrate/nitrite salts decomposition, for example, the N2 or/and O2 can participate in the decomposition reaction. Therefore, to avoid the effect from the atmosphere, the vacuum condition was adopted. Molten nitrate/nitrite salts were selected to be studied mainly because it was ambiguous of decomposition reaction. It was deemed that nitrate can decompose into nitrite and oxygen gas which is a reversible reaction and not discerned. If the decomposition took place in the vacuum condition, then two reaction process can be distinguished, it means that a transition state may be existed between these processes. In the present work the solar salt and sodium nitrite were heated in
cycle-1 up cycle-2 up cycle-3 up
0.4
DSC (mW/mg)
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-0.2 Onset (melting temperature): 220.7 (
)
-0.4
-0.6 140
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220
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Sample Temperature (°C)
Fig. 1. DSC analysis for the melting point of solar salt, the second (cycle-1), third (cycle-2) and fourth (cycle-3) cycle from 150 °C to 300 °C are selected. 824
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Accelerated plate Ionization region
Deflection plate
Ion lens
MCP detector
Furnace
Cooling circuits Power Fig. 2. Schematic diagram of the in-situ high temperature time of flight mass spectrometer (TOF-MS).
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2.3. Measurement of the gas components from molten salt decomposition by TOF-MS
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solar salt 260 +
To investigate the gas components from molten salt decomposition, a time of flight mass spectrometer (TOF-MS) combined with an in-situ heating furnace was designed as Fig. 2. The sample was heated by the furnace placed in the main vacuum chamber, and the gaseous products were transfused into the ionization region to be ionized into different ions by the electron beam. These ions were accelerated by the electric field and flied to the microchannel plate (MCP) detector, from which we could get the mass information according to the time of flight of the different ions. In order to eliminate the background interference, a blanket reference mass spectrum should be taken at first. An empty crucible was putted into the heating furnace in the ion source chamber of the TOF-MS system, which has equipped with an electron impact ion source. The temperature of the furnace can be tuned from room temperature up to 400 °C, the mass spectra of the background were taken at several temperature points in the range of 30–400 °C, The background experiment showed that the vacuum changed slightly from 1 × 10−5 Pa to 6 × 10−5 Pa as the temperature raising up to 200 °C due to the emitting of moisture from the surface of furnace. After the vacuum background being measured and calibrated, 5 g self-prepared solar salt was placed into the molybdenum crucible and heated in the vacuum environment (about 1.0 × 10−5 Pa). Considered the melting point of the sample and the change of the vacuum environment, the temperature points for measurement were chosen at 30 °C, 50 °C, 100 °C, 150 °C, 200 °C, 240 °C, 260 °C for solar salt and 30 °C, 150 °C, 220 °C, 280 °C for NaNO2. The TOF-MS data were obtained at these temperature points, and each point was kept stable for at least 10 min before further measurement. The vacuum was affected by the emission of gases from the thermal decomposition reaction (in molten salt from 1.0 × 10−5 Pa to 5.0 × 10−3 Pa as temperature increased. The evolution gases were ionized by the low energy electron beam and detected by the microchannel plate (MCP). The data were recorded for about 1 h at each temperature point. The sodium nitrite was also investigated in the same way in present work. Due to the melting point of sodium nitrite is about 270 °C, the temperature of this sample was raised up to 280 °C. The detailed mass spectra of the gas components are discussed in the following part.
+
H2O
NO N2
+
+
N2O
intensity
solar salt 240
solar salt 200
solar salt 30
0
10
20
30
40
m/z Fig. 3. Mass spectra of the evolutions of gas from the molten solar salt (60 wt% NaNO3–40 wt% KNO3) change at different temperature points, the H2O+ is from the background and the new materials come out at the temperature above 220 °C.
molten solar salt (60 wt% NaNO3–40 wt% KNO3). As it reached the melting point (220 °C according to the DSC analysis in this work), some gases were emitting from molten salt, which would be shown from both
3. Results and discussion Fig. 3 shows the mass spectra of the evolutions of gas from the 825
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the vacuum gauge and the detector of the TOF-MS. According to mass spectrum the ion intensity of these gases were increasing with the rising temperature. Based on comparison with the mass spectra of the gas products from the molten solar salt decomposition, there were few changes at different temperature point in the range of 30–200 °C. In Fig. 3 four temperature points at 30 °C, 200 °C, 240 °C and 260 °C were selected to display the reaction process. Compared with the 200 °C mass spectrum, a dramatically change is observed in the spectrum at 240 °C (above the melting point 220 °C), in the 200 °C mass spectrum there were only four peaks obtained from the ionization of the moisture and hydrogen from the background, while some new peaks appeared clearly in the 240 °C mass spectrum. However these peaks emerge in the spectra at very low counting rate as the temperature raised higher than 230 °C, which means that there would be some gases coming out from the molten solar salt when it was melt at the temperature above its melting point 220 °C. The sodium nitrite was also conducted in the similar way, while there was no change in the spectrum before the temperature up to 260 °C in the heating process, which means that no evolutions of gases from the solid state salt as the temperature below 270 °C. When the temperature of the sodium nitrite was up to 280 °C, even only a few degrees higher than the melting point of the NaNO2, there were four new peaks appeared in the mass spectrum as shown in Fig. 4. This indicated there should be some new gas species emerged from the liquid phase.
0
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intensity
+
N /N2
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Fig. 5. Calibrated mass spectrum of the evolution of gases from the molten solar salt at 260 °C.
In order to identify these ions in the spectrum, All the spectra of the solar salt and sodium nitrite were calibrated by the lines of H+, H2+, OH+ and H2O+, which were ionized from the background gases in the vacuum chamber at 280 °C. As shown in Fig. 5, A mass spectrum of molten solar salt recorded at 260 °C, considering the compound of the solar salt and the calibration information, except the background ions, the other peaks of the m/z at 14, 28, 30 and 44 were identified as N+, N2+, NO+ and N2O+ ions respectively, while the CO+ (28) and CO2+ (44) ions could be ruled out completely because there was no C+ ion (12) observed in the spectrum. As shown in the calibrated spectrum of the evolution of gases from the sodium nitrite (at 280 °C) Fig. 6, except the H2O+ peak from the background, the other four peaks were identified as N+, N2+, NO+ and N2O+ respectively. Compared with Fig. 5, the relative intensity of the evolution gases from molten solar salt and sodium nitrite were different. Usually these peaks in the mass spectrum were ionized from the related substance, while not each of them corresponds to a molecular in the gas phase, as for the N+, which might be a fragment ion from N2, NO or some other N element contained molecules. According to the Caroline’s work (O'Connor et al., 1997) the high temperature hydrolysis reaction to form HNO3 could be excluded in current work because there were no HNO3+ and its fragment NO2+ in the mass spectrum, which should appeared as peaks corresponded to their mass number at the horizontal m/z axis. In Caroline’s ionization
80
+
+
N2O
+
+ + N /N2 H2O
2+
m/z
NaNO2 280 N2
solar salt 260
+
NO
+
NO
intensity
NaNO2 260
N2
NaNO2 280
+
intensity
NaNO2 30
+
N /N2
+
2+
N2O +
NO +
H2O
0
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m/z 0
Fig. 4. Mass spectra of the evolution of gas from the molten sodium nitrite salt change at different temperature points: 30 °C, 260 °C and 280 °C are selected, the H2O+ is from the background and the new materials come out at 280 °C, about 10 °C higher than the melting point of NaNO2.
10
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m/z Fig. 6. Calibrated mass spectrum of the evolution of gases from the molten NaNO2 salt at 280 °C. 826
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2012; Olivares and Edwards, 2013; Bartholomew, 1966; Kramer et al., 1982; Wei et al., 2013). And if the O2 and NO2 existed in the gas phase, it could be ionized into O2+ and NO2+ ions by the electron impacted beam and detected by the TOF-MS (Pal et al., 2003; Kim et al., 1997), while based on the TOF-MS analysis in current work, there were no O2+ and NO2+ ions observed in Figs. 5 and 6, which suggested that there were no O2 or NO2 gases released from the thermal decomposition reactions in the nitrate/nitrite based molten salt when the temperature was below 300 °C. Previous report shown an intermediate state existed in the reaction process and only about 1/8th of the available oxygen at 750 °C in 130 min (Bond et al., 1966; Freeman, 1956). According to the electron paramagnetic resonance and ultravioletvisible absorption spectra absorption spectroscopy (Shu-ting Liu and Zhang, 2017), the superoxide anion (O2−) was identified in the molten nitrate salt. When considering the existence of superoxide anion (O2−) in the nitrate/nitrite based molten salt, the possible thermal decomposition reaction mechanism in the nitrate based molten solar salt could be expressed as Eq. (6)
Table 1 Relative intensity of the ions N2+, NO+ and N2O+ in the spectrum of solar salt at 260 °C and NaNO2 molten salt at 280 °C, relative to the background ions, H2O+. Relative intensity
Solar salt
NaNO2
N2+/H2O+ NO+/H2O+ N2O+/H2O+
0.85 12.09 1.76
56.33 24.76 26.43
cross sections research for HNO3 by TOF-MS, as the incident electron energies rising from 40 to 450 eV, the parent ions HNO3+ and the fragmentation productions H+, N+, NO+ and NO2+ were detected, while the most abundant ion in the mass spectrum was NO2+ all the time. For the similar reason, these observed ions should not be fragments from nitrate/nitrite salt vapor molecule for there were no Na+ or K+ detected in the spectrum. All of these evidence indicated that the evolution of gases should be produced by the thermal decomposition reaction in the nitrate/nitrite based molten salt, which means that the molten nitrate/nitrite salt was undergoing a slow dissociation reaction when it was heated above its melting point. It was interesting to compare the spectrum of molten solar salt (Fig. 5) with NaNO2 salt (Fig. 6), though the N2+, NO+ and N2O+ ions appeared clearly in both of them, the relative intensity of these ions were great different. For convenient comparison, the background H2O+ ions was taken as reference and the relative intensity of the N2+, NO+ and N2O+ ions were normalized with H2O+ in Table 1. As listed in Table 1 the intensity of N2O+ ion, which was ionized from N2O molecular by the electron impact beam, the N2O+ ion from the NaNO2 experiment was much higher than that of solar salt experiment. Since there was no background signal of the N2O+ ion, the N2O molecular in the evolution of gases should be formed from the salt decomposition; Based on the partial ionization cross sections of N2O and NO at the energy of 200 eV by electron impact experiment as shown in Table 2 (Iga et al., 1996), the enhanced N2+ and NO+ ions intensities were assigned to be the ionization fragments of N2O. As shown in Table 2, the relative intensity of NO+/N2O+ and N2+/ N2O+ from ionization fragments of N2O should be about 0.46 and 0.16, while the measured ratios were about 6.87 and 0.48 in Fig. 5, about 0.94 and 2.13 in Fig. 6, respectively. This indicate only part of the N2+ and NO+ ions were produced fromN2O fragmentation, while the others should be direct electron impact from the N2 and NO molecule. Based on the mass spectrum, the products of the evolved gases in the molten sodium nitrite and solar salt were N2 and NO respectively, which was agree with Freeman’s work that N2 was the major gaseous product of NaNO2 at the beginning (Freeman, 1956). There was few evidence of the production of N2O from the thermal decomposition reaction in nitrated based molten salt except the detection by Olivares using the TG-MS equipment at high temperature (above 800 °C) under the Argon, Nitrogen and Oxygen atmospheres (Olivares and Edwards, 2013), however, the mass spectrum in present work shows the existence of N2O evolved from the molten salt at much lower temperature. The O2 and NO2 were considered to be the dominant productions from the thermal decomposition reaction in nitrate/nitrate based molten salt especially at high temperature (Peng et al., 2008; Olivares,
NO3−→NO ↑ +O2−
(6)
And the Eq. (7) expressed one of the most possible dissociation reaction of sodium nitrite molten salt
2NO2− → N2 ↑ + 2O2−
(7)
Based on the basis of Lux-Flood acid concept, the nitrate ions was postulated to dissociate into nitronium ions and oxide ions as Eq. (5), and then the oxide ions would continue react with the nitrate ions in the molten salt as follow:
O 2 − + NO3− ↑ O22 − + NO2−
O2
2−
+
2NO3−
−
↑ 2O2 + 2NO2
(8) −
(9)
For the detection of N2 and NO from the molten salt in different abundance, there might exist NO2− anion in the molten nitrate salt and NO3− anion in the molten sodium nitrite salt as shown in the Eqs. (8) and (9). According to Figs. 5 and 6, there were lots of N2O gas evolution from both of the molten solar salt and molten sodium nitrite salt detected by the TOF-MS. These N2O gas should be produced as the reactions of Eqs. (10) and (11)
NO2− + NO3− → 2O2− + N2 O↑ −
4NO2 →
2NO3−
+
O2−
+ N2 O↑
(10) (11)
Molecular
N2O+
NO+
N2+
N+
O+
Since there were no O2 and NO2 emerged from the nitrate/nitrite based molten salt when the temperature was below 300 °C in present work, while as shown in Eqs. (3) and (4) when the temperature rising up to more than 500 °C, peroxide and superoxide could be further decomposition into O2, which agreed with the investigation in the thermal stability of molten nitrite/nitrates salt (Peng et al., 2008; Olivares, 2012). The thermal stability of molten salt is sensitive to the environmental atmosphere, if salt exposures to the air, the oxygen can participate the decomposition reactions, either involved the surface reaction or oxidize the NO to NO2. While in present work, the vacuum environment could supposedly avoid the influence by the surrounding atmosphere. According to the TOF-MS experiment, the molten nitrate/nitrite salt produced gases by the thermal decomposition, while the reaction rate appeared very slow at low temperature for the changes of the composition in the salt were too weak to be detected by the Ion Chromatography System, nevertheless, they would grow quickly with increasing temperature, which could be monitored by the vacuum ion gauge and the decreasing vacuum constantly.
N2O NO
178.1 –
82.3 264.1
28.2 –
31.9 44.5
15 18.3
4. Conclusions
Table 2 Partial ionization cross sections of N2O and NO by electron impact at 200 eV (Iga et al., 1996).
Unit of measure is 10−18 cm2.
The thermo-chemical behavior and thermal stability of molten solar 827
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salt and sodium nitrite are studied by in situ salt heating oven system combined with TOF-MS. Evolutions of gases from molten salt have been detected above the melting temperature of salt in vacuum environment. The mass spectra of the solar salt (from room-temperature to 260 °C) and sodium nitrite (from room-temperature to 280 °C) are obtained.
Freeman, E.S., 1956. The kinetics of the thermal decomposition of potassium nitrate and of the reaction between potassium nitrite and oxygen.pdf. J. Phys. Chem. 79, 838–841. Alexander, J., Hindin, S.G., 2002. Phase relations in heat transfer salt systems. Ind. Eng. Chem. 39, 1044–1049. Silverman, M.D., Engel, J.R., 1977. Survey of Technology for Storage of Thermal Energy in Heat Transfer Salt. Off-Peak Energy Storage. Kearney, D., Herrmann, U., Nava, P., Kelly, B., Mahoney, R., Pacheco, J., Cable, R., Potrovitza, N., Blake, D., Price, H., 2003. Assessment of a molten salt heat transfer fluid in a parabolic trough solar field. J. Sol. Energy Eng. 125, 170–176. Olivares, R.I., 2012. The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres. Sol. Energy 86, 2576–2583. Wang, T., Mantha, D., Reddy, R.G., 2012. Thermal stability of the eutectic composition in LiNO3–NaNO3–KNO3 ternary system used for thermal energy storage. Sol. Energy Mater. Sol. Cells 100, 162–168. Olivares, R.I., Edwards, W., 2013. LiNO3–NaNO3–KNO3 salt for thermal energy storage: thermal stability evaluation in different atmospheres. Thermochim. Acta 560, 34–42. Cordaro, J.G., Rubin, N.C., Bradshaw, R.W., 2011. Multicomponent molten salt mixtures based on nitrate/nitrite anions. J. Sol. Energy Eng. 133, 169–175. Nissen, D.A., Meeker, D.E., 1983. Nitrate/nitrite chemistry in NaNO/sub 3/-KNO/sub 3/ melts. Inorg. Chem. (United States) 22, 5. Kramer, C.M., Munir, Z.A., Volponi, J.V., Kramer, C.M., Munir, Z.A., Volponi, J.V., 1982. Differential scanning calorimetry of sodium and potassium nitrates and nitrites. Thermochim. Acta 55, 11–17. Bond, B.D., Jacobs, P.W.M., Bond, B.D., Jacobs, P.W.M., 1966. The thermal decomposition of sodium nitrate. J. Chem. Soc. A 1265–1268. Benaissa, W., Carson, D., 2011. Oxidation properties of “Solar Salt”. Aiche Spring Meeting Global Congress on Process Safety, 2011. Bartholomew, R.F., 1966. A study of the equilibrium KNO3(l) ⇄ KNO2(l) + 1/2O2(g) over the temperature range 550–750°. J. Phys. Chem. 70, 3442–3446. Freeman, E.S., 1956. The kinetics of the thermal decomposition of sodium nitrate and of the reaction between sodium nitrite and oxygen. J. Phys. Chem. 60, 1487–1493. Zambonin, P.G., Cavaggioni, A., 1971. Redox mechanisms in an ionic matrix. Autocatalysis in the process superoxide + nitrite.dbr. peroxide + nitrate in fused salts. J. Am. Chem. Soc. 93, 2854–2858. Burke, J.D., Kerridge, D.H., 1974. Concerning oxygen anions in a nitrate melt. Electrochim. Acta 19, 251–256. Kramer, C.M., Munir, Z.A., Volponi, J.V., 1982. Screening tests of sodium nitrate and potassium nitrate decomposition ☆. Sol. Energy 29, 437–439. Wei, X., Peng, Q., Ding, J., Yang, X., Yang, J., Long, B., 2013. Theoretical study on thermal stability of molten salt for solar thermal power. Appl. Therm. Eng. 54, 140–144. Wright, S., Tran, T., Chen, C., 2012. Thermal stability of potassium and sodium nitrate molten salt mixtures above 500°C. Kust, R.N., Burke, J.D., 1970. Thermal decomposition in alkali metal nitrate melts. Inorg. Nucl. Chem. Lett. 6, 333–335. Kust, R.N., Duke, F.R., 1963. A study of the nitrate ion dissociation in fused nitrates. J. Am. Chem. Soc 85, 170–172. Bradshaw, R.W., Cordaro, J.G., Siegel, N.P., 2008. Molten nitrate salt development for thermal energy storage in parabolic trough solar power systems. In: ASME 2009 International Conference on Energy Sustainability Collocated with the Heat Transfer and Interpack09 Conferences, pp. 631–637. Bauer, T., Pfleger, N., Breidenbach, N., Eck, M., Laing, D., Kaesche, S., 2013. Material aspects of solar salt for sensible heat storage. In: Innostock 2012 - the International Conference on Energy Storage, pp. 1114–1119. Rogers, D.J., Janz, G.J., Rogers, D.J., Janz, G.J., 1982. Melting-crystallization and premelting properties of NaNOâ-KNOâ. J. Chem. Eng. Data 27. O'Connor, C.S.S., Jones, N.C., Price, S.D., 1997. Electron-impact ionization of nitric acid. Int. J. Mass Spectrom. Ion Process. 163, 131–139. Iga, I., Rao, M.V.V.S., Srivastava, S.K., 1996. Absolute electron impact ionization cross sections for N2O and NO from threshold up to 1000 eV. J. Geophys. Res. Planets 101, 9261–9266. Pal, S., Kumar, J., Bhatt, P., 2003. Electron impact ionization cross-sections for the N2 and O2 molecules. J. Electr. Spectrosc. Relat. Phenom. 129, 35–41. Kim, Y.K., Hwang, W., Weinberger, N.M., Ali, M.A., Rudd, M.E., 1997. Electron-impact ionization cross sections of atmospheric molecules. J. Chem. Phys. 106, 1026–1033. Shu-ting Liu, T.S., Zhang, Peng, Fei, Ze-jie, Liu, Hong-tao, 2017. Identification of superoxide O2-during thermal decomposition of molten KNO3-NaNO2-NaNO3 salt by electron paramagnetic resonance and UV-Vis absorption spectroscopy. Chin. J. Chem. Phys. 30, 7.
(1) The gas products are identified to be N2, NO and N2O, and the main dissociation product of the molten solar salt and molten sodium nitrite salt is NO and N2, respectively. O2 and NO2 are not detected in the intrinsic reaction below 300 °C, which shows a very different reaction process from the common sense that O2 or NO2 should exist in the decomposition process. (2) Based on these new evidences, the thermal decomposition mechanism of molten nitrite/nitrates salt was proposed to be 4NO2−→ NO3−→NO ↑ +O2−, 2NO2− → N2 ↑ + 2O2− and 2NO3− + O 2 − + N2 O↑ at the temperature below 300 °C, which suggests the superoxide ions and peroxide ions should exist in the molten salt as intermediate products in the initial thermal-chemical reaction process. Acknowledgements This work was supported by the National Natural Science Foundation of China [Grant No. 21703281; 51604275; 21573273; 11504397], and “Strategic Priority Research Program” of the Chinese Academy of Sciences [Grant No. XDA02002400]. H.T. Liu would like to acknowledge support of Hundred Talents Program (CAS). References Gil, A., Medrano, M., Martorell, I., Lazaro, A., Dolado, P., Zalba, B., Cabeza, L.F., 2010. State of the art on high temperature thermal energy storage for power generation. Part 1-concepts, materials and modellization. Renew. Sust. Energ. Rev. 14, 31–55. Medrano, M., Gil, A., Martorell, I., Potau, X., Cabeza, L.F., 2010. State of the art on hightemperature thermal energy storage for power generation. Part 2-case studies. Renew. Sust. Energ. Rev. 14, 56–72. Dunn, R.I., Hearps, P.J., Wright, M.N., 2012. Molten-salt power towers: newly commercial concentrating solar storage. Proc. IEEE 100, 504–515. Kelly, B., Price, H., Brosseau, D., Kearney, D., Kelly, B., Price, H., Brosseau, D., Kearney, D., 2007. Adopting nitrate/nitrite salt mixtures as the heat transport fluid in parabolic trough power plants. In: ASME 2007 Energy Sustainability Conference, pp. 1033–1040. Eck, M., Hennecke, K., 2008. Heat Transfer Fluids for Future Parabolic Trough Solar Thermal Power Plants. Springer, Berlin, Heidelberg. Pacheco, J.E., Showalter, S.K., Kolb, W.J., 2002. Development of a molten-salt thermocline thermal storage system for parabolic trough plants. J. Sol. Energy Eng. 124, 153. Wu, Y.T., Li, Y., Ren, N., Ma, C.F., 2017. Improving the thermal properties of NaNO3KNO3 for concentrating solar power by adding additives. Sol. Energy Mater. Sol. Cells 160, 263–268. Peng, Q., Wei, X., Ding, J., Yang, J., Yang, X., 2008. High-temperature thermal stability of molten salt materials. Int. J. Energy Res. 32, 1164–1174. Wu, Y.T., Li, Y., Ren, N., Zhi, R.P., Ma, C.F., 2018. Experimental study on the thermal stability of a new molten salt with low melting point for thermal energy storage applications. Sol. Energy Mater. Sol. Cells 176, 181–189. Mantha, D., Wang, T., Reddy, R.G., 2013. Thermodynamic modeling of eutectic point in the LiNO3–NaNO3–KNO3–NaNO2 quaternary system. Sol. Energy Mater. Sol. Cells 118, 18–21. Peng, Q., Yang, X., Ding, J., Wei, X., Yang, J., 2013. Design of new molten salt thermal energy storage material for solar thermal power plant. Appl. Energy 112, 682–689. Gordon, S., Campbell, C., 1955. Differential thermal analysis of inorganic compounds. Anal. Chem. 27, 1–4.
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