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Experimental researches on hydrogen generation by aluminum with adding lithium at high temperature
Energy 93 (2015) 451e457
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Energy journal homepage: www.elsevier.com/locate/energy
Experimental researches on hydrogen generation by aluminum with adding lithium at high temperature Weijuan Yang*, Tianyou Zhang, Jianzhong Liu, Zhihua Wang, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2015 Received in revised form 9 September 2015 Accepted 11 September 2015 Available online xxx
In order to recover the released heat of AleH2O reaction and promote the reaction itself, the hydrogen production processes of aluminum with lithium addition in molten state are investigated. Experiments are conducted by both a thermogravimetric analyzer and a special experimental facility at high temperature. The results on both apparatuses show that the addition of Li can promote the reactivity of aluminum with water. Compared with pure aluminum, only 5% of Li content can achieve a great improvement: the H2 yield increases from 8.7% to 53% and the average H2 generation rate from 15 to 112 mL min1 g1. With the increase of Li content, H2 yield is improved distinctly and the period with a high H2 generation rate is prolonged. In the Ale20%Li case, the H2 yield of 88% is obtained, and it appears a stable period in which the H2 generation rate keeps high. When adding lithium, LiAlO2 appears in the products and the products are made of columnar crystals. The pores with an average size of 17e33 nm in the LiAlO2 products are manyfold bigger than the pores of alumina, which takes an important role in improving the reactivity of aluminum and water. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen generation Aluminum Lithium Water High temperature
1. Introduction Hydrogen possesses a high mass energy density and does not emit any pollution gas when used as a fuel. As a regenerative and environmentally friendly fuel, it has attracted much attention due to fossil fuel depletion and air pollution from its combustion [1]. At present, steam/partial oxidation reforming of natural gas and coal gasification produce approximately 95% hydrogen [2]. In order to develop new promising technologies, the generation of H2 from suitable starting materials, such as water [3,4], NaBH4 [5,6], metallic hydrides [7] and Al based materials [8e11], has been investigated in the last years. Aluminum is a good choice for hydrolyzing metal and aluminum-based hydrogen production is potentially low-cost and high H2 capacity. Some research results have been published on AleH2O reaction in neutral condition, with assistance of alkalis or oxides or salt, at elevated temperature and with metal additives [11e14]. When producing H2, an inert oxide film easily formed on the Al particle surface, preventing water contacting Al and hindering the continued H2 production [12]. The size of Al particles performed obvious impacts on H2 generation and 10e50 mm was the normal * Corresponding author. Tel.: þ86 571 87951040; fax: þ86 571 87951616. E-mail address: [email protected] (W. Yang). http://dx.doi.org/10.1016/j.energy.2015.09.048 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
size in experimental researches [14e18]. Most of the published researches were carried out by using Al particles with a size of 10e200 mm. The H2 generation rate in neutral condition, even with some additives (C, TiO, Li, In, Zn, Bi, NaCl, KCl et al.), was not beyond 170 mL min1 g1 generally [19]. Although the reactions in NaAlO2, Na2SnO3or NaOH solution achieved the H2 generation rate of over 200 mL min1 g1 [20], the reacted solution had to be disposed specially because of its corrosivity. NiCl2, CoCl2, NaCl or Al2O3 was used to activate Al, but the great amount of the addition reduced the H2 produced per unit mass [11,21e24]. Aluminum based metals presented better performance than aluminum alone [14,25]. AleLi alloy with Li contents of 10%e30% by ball milling [26] and melting method [27] had been researched and the hydrogen yield was able to reach 100%. The addition of NaCl could enhance the reactivity of AleLi alloys with low Li content, and the hydrogen generation rate and hydrogen yield were improved [28]. Temperature was one of the key factors influencing hydrogen production [12,27]. The water initial temperature rising from 50 to 70 C could make the average hydrogen generation rate increase from 101 to 210 mL min1 g1 [21]. Increasing the temperature from 20 to 30, 40, 50 C, the AleLieIneZn alloy presented hydrogen generation efficiencies of 53%, 75%, 100% and 44% [16]. In the case of low melting point metal additives (Ga, In, Sn), liquid phases were observed even at room temperature [29,30].
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Intergranular diffusion of these additives led to embrittlement of Al, which was regarded as the reason for Al becoming chemically active in water [31]. The reaction of AleGa and AleGaeIneSn alloys with water was enabled by liquefaction of the eutectic and a liquid phase through which Al could diffuse [32]. The United states patent proposed a method of producing H2 by Ga-rich liquid AleGa alloys [33]. The AleGa alloys showed limited water-reactivity at low temperature and they needed heating to initiate reaction by intensifying liquefaction. AleGaeIneSn alloys displayed considerable reactivity at lower temperatures [34]. The U.S. patents mentioned molten aluminum-lithium alloys were sprayed with liquid water through a nozzle in a reaction chamber in order to produce H2 [35,36], but no detail information on reaction rates was reported. In this paper, it was focused on the reaction of molten aluminum-based metal and water vapor at high temperature to prove the feasibility of H2 generation by Al blocks and heat recovery. A steel reactor was proposed and designed, in which aluminum was preheated to be molten and reacted with water vapor over 700 C. This paper concentrated on the effects of Li addition on the hydrogen generation of molten aluminum with water vapor. Different approaches were adopted to analyze the products, so as to discover the promoting mechanism of Li. The results would be valuable and referable to underwater power and distributed energy supply based AleH2O reaction. 2. Experimental methods Aluminum powder (99.99%, 10 mm) and aluminumelithium alloy (99.99%, 45 mm) were used as initial samples. The AleLi alloy was composed of 80% of aluminum and 20% of lithium. In order to reduce lithium content of samples, aluminum powder was added into the alloy particles. The samples with lithium content of 5%, 10% and 20% were adopted as well as the pure aluminum powder. Thermogravimetric measurements were conducted with THERMO CAHN's Thermax500 pressurized thermogravimetric analyzer. The temperature program rose up straightly from the room temperature to a certain temperature below 1100 C at a heating rate of 25 C/min. The argon flow as a carrier was set as 500 ml/min and the steam vapor flow was 100 ml/min. 10 mg of the metal sample was used in each test. A special experimental facility was designed and built as Fig. 1 showed. The reactor made of stainless steel had a cylindrical shell with an inner diameter of 5 mm and a height of 10 mm. Before the test, argon was fed into the reactor to drain air completely. 5 g of the metal sample in the reactor was heated to 700 C in order to ensure the sample melting fully. The sample temperature was metered by a K-type thermocouple which was mounted in the reactor and inserted in the middle of the sample. Only argon was fed into the
reactor when heating the sample. The purpose was avoiding the oxidization and hydrolysis of the sample. As soon as the temperature reached 700 C, water was pumped by an injection pump and the heater of the reactor was turned off. After water and argon mixed, they were heated to 200 C by an electric heater, and then flowed into the reactor. Vapor and argon were just exposed to the surface of the sample, and did not bubble through the molten metal. The gas out of the reactor was cooled, dried, and measured by a hydrogen analyzer with a precision of 1%. The precision of the injection pump was 0.5% and the flowmeter measured the argon flow was 1%. In the whole test, the flow of water and argon were 5 ml/min and 500 ml/min. The test would end until no hydrogen was detected by the hydrogen analyzer. The products were collected after the test ended and then measured using XRD (X'Pert PRO, Netherlands) and TEM-EDS (Tecnai G2 F20 S-TWIN, America). 3. Results and discussion Fig. 2 gives the thermogravimetric results of the four samples: pure aluminum (0% Li), aluminum samples with 5%, 10% and 20% of Li contents. The sample with 20% of Li (Ale20%Li) presented a better reactivity, and it had a greater final weight and a higher peak of DTG (differential thermogravimetry). With the reduction of Li content, both the final weight and the DTG peak decreased. The WGR (weight gaining ratio) was defined as the ratio of the final weight gaining to the theoretical weight gaining after the reaction ended. The WGR of the Ale20%Li sample reached 84%, which indicated 86% of Al had reacted with the addition of Li. The data was a little higher than the result by Fan using AleLi alloy with 20% of Li content [26]. The weight gain completed 90 percent when the temperature reached 500 C. However, only 19% of Al hydrolyzed in the pure aluminum case. Comparing the DTG curves in Fig. 2b, the sample with greater Li content displayed a earlier and higher DTG peak. The pure aluminum did not present a weight increasing until the temperature was over 900 C. The addition of Li brought forward and accelerated the AleH2O reactivity. The greater addition of Li performed a greater effect. The DTG peaks appeared at 195 and 365 C in the Ale20%Li case. The first was primarily due to LieH2O reaction and the second to AleH2O reaction. The hydrolysis with Al/Li in molten state was investigated using the special experimental facility and the results in the four cases were presented in Table 1. The average H2 generation rate (ra) was the average of the rates which were greater than 10% of the maximum H2 generation rate (rm), and the tiny values were ignored. The time of H2 generation (t) was also calculated as the same way. There were two values of H2 yield: one was defined as the rate of the volume of generated H2 to the theoretical H2 generation, the other was calculated according to the WGR data, the metal components and the theoretical reactions of
Fig. 1. Schematic of designed experiment system.
W. Yang et al. / Energy 93 (2015) 451e457
190 180
(a)
20% Li
Weight %
170 160
10% Li
150 140 130
5% Li
120 110 100 90
0% Li 0
200
400
o
600
T/ C
800
1000
Weight difference rate (%/min)
(a) TG 18 16 14 12 10 8 6 4 2 0 -2
(b) 20% Li
10% Li 5% Li
0
200
400
600
0% Li
800
1000
o
T/ C (b) DTG Fig. 2. Thermogravimetric results of different samples: (a) TG curves; (b) DTG curves.
2Alþ3H2O¼Al2O3þ3H2 and 2Li þ H2O¼Li2O þ H2. The H2 yields by WGR data was greater 2.3e3.7% absolutely than those by the volume of H2. The H2 yields by the two ways appeared less than 5% of relative error except the pure Al case, and the pure Al case had an absolute error of 2.86%. The error indicated that the H2 yields by the two ways coincided well, and hereinafter the H2 yields by H2volume were adopted because the H2 generation processes were concentrated on. The Ale20%Li case achieved 86.62% of the WGR, which is similar to the result by the thermogravimetric analysis. 5% of Li content improved the H2 yield from 8.72% to 53.28% and ra from 7.6 to 112.3 mL min1 g1 which was the highest in the tests. The H2 yield of 53.28% is much higher than 33% reported by Chen [28]. Furthermore the peak rate of 234 mL min1 g1 in the Ale5%Li case was maximal in all tests. More Li addition could prolong the time of H2 generation and promoted the H2 yield, but reduced H2 generation rate a little. 88.51% of the H2 yield could be obtained in the Ale20%Li case. The crossing-section area of the reactor was the reacting area since vapor just exposed to the molten metal surface. The average reacting rate of the sample changed little with the Li
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content as well as ra did. The average reacting rate of the sample per surface area was increased to 26.7 from 1.9 mL min1 mm2 by adding Li. Fig. 3 shows the sample temperature and hydrogen generation rate in the four cased of Table 1. In the pure Al case in Fig. 3a, the sample temperature did not rise up obviously and the maximum was just 743 C. The curve of H2 generation rate showed two small peaks and the maximum was 15.02 mL g1 min1. Because the reaction rate was not high enough, the heat released by AleH2O reaction was too small to prevent the sample temperature falling down. Li addition promoted the H2 generation rate and the sample temperature dramatically. The temperature peak could be over 1000 C and the rate peak over 230 mL min1 g1. The H2 generation rate climbed up to the peak straightly and then dropped down. Greater Li content of the sample speeded up the climbing and slowed down the dropping. 20% of Li content made the rate peak appear on the 130th second after feeding the vapor, while 5% of Li on the 255th second. Moreover the dropping process in the Ale20%Li case obviously performed a three-phase feature, and there was a gently-dropping phase between two sharply-dropping phases. This three-phase feature could be found but not so apparent in the other cases. Although the maximum of H2 generation rate in the Ale20%Li case was a little lower than that in Ale10%Li and Ale5%Li cases, the Ale20%Li case gained the best H2 yield because its three-phase feature in the rate dropping process prolonged the time of H2 generation significantly as Table 1 showed. The peak of the sample temperature appeared later than the peak of the H2 generation rate, especially in the Ale5%Li case. In Ale10%Li and Ale5%Li cases, the temperature performed a multiphase climbing feature, and there were several even phases and dropping phases between the climbing phases. However the performance of the Ale20%Li case showed different. The curve of the temperature straightly climbed as fast as the curve of H2 generation rate. When the rate reached the peak, the temperature arrived at 966 C, which was close to the peak value of 1011 C. The period from 966 to 1011 C presented a tiny oscillation and lasted about 400 s. Moreover the period was companied with the gentlydropping phase of the H2 generation rate. It was the most important phase during the H2 generation process because the stable temperature and H2 generation rate were favorable and expected by the practical application. As to the other cases, the oscillation was enlarged greatly and the multi-phase climbing feature appeared. In Fig. 3b the temperature even dropped down in the process of H2 generation, which implied that the AleH2O reaction was not stable and sometimes hindered by the products which isolated the vapor from unreacted aluminum. When the hindrance was destroyed by the shock of argon, vapor and H2, the reaction went on and the temperature rose up quickly. The results of the products by XRD (X-ray diffraction) were given in Fig. 4. X-ray diffraction results showed that the products of pure Al were Al2.667O4 and Al2O3 and that LiAlO2 appeared with Li addition. Al peaks were also detected since some of Al was
Table 1 Thermogravimetric results and hydrogen generation of samples with different Li contents. Item
WGR
H2 generation
rm
ra
t
Unit
%
ml g-1
ml g-1 min-1
ml g-1 min-1
s
%
0% Li 5% Li 10% Li 20% Li
10.29 50.13 67.16 86.62
108.46 672.27 945.16 1164.1
15.02 234 232.98 191.24
7.6 112.3 103.96 99.82
852 338 526 686
8.72a/11.58b 53.28a/55.61b 73.86a/73.45b 88.51a/92.19b
a b
It was defined as the ratio of the volume of generated H2 to the theoretical H2 generation. It was calculated according to the WGR data and the reactions of AleH2O and LieH2O.
H2 Yield
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Fig. 4. XRD patterns of the condensed products: (1-Al; 2-Al2.667O4; 3-Al2O3; 4-LiAlO2; 5-LiAl2(OH)7$xH2O; 6-LiAl5O8).
Li2O þ 5Al2O3 / 2LiAl5O8
Fig. 3. Hydrogen generation processes of the samples with different Li contents: (a) 0%, (b) 5%, (c) 10%, (d) 20%.
unreacted. With more Li content, the LiAlO2 peak was higher, but LiAl2(OH)7$xH2O was lower. LiAl5O8 was detected for Ale10%Li and Ale5%Li. The reactions could be described as R1eR5, and R2 was promoted by both the heat of R1 and the products of R3eR5. Li þ H2O / Li2O þ H2
R1
Al þ H2O / Al2O3 þ H2
R2
Li2O þ Al2O3 / LiAlO2
R3
Li2O þ 2Al2O3 þ (7 þ x) H2O / 2LiAl2(OH)7$xH2O
R4
R5
The graphs of the products by TEM (transmission electron microscope) were demonstrated in Fig. 5. EDS (energy dispersive spectrometry) test was carried out through the irradiation onto the products surface (as red circles (in the web version) showed in Fig. 5). The products of pure aluminum and Ale20%Li demonstrated a great difference. The product of pure aluminum had an almost complete spherical shell and only few shells cracked. The surface appeared compact and no visible pores could be found. The compact surface provided some resistance to AleH2O contact and was disadvantageous to Al hydrolysis, as many researchers studied [37]. However, the product of Ale20%Li was built up by columnar crystals and the size of columnar crystals was 100e500 nm. For Ale10%Li and Ale5%Li, no visible crystal was formed. There were some needles or flakes distributed in the surface of samples. The EDS results for Al, Ale5%Li, Ale10%Li and Ale20%Li were listed in Table 2. Because the atomic Al/O ratios for Al, Al2.667O4, Al2O3, LiAlO2, LiAl2(OH)7$xH2O and LiAl5O8 were constant, the products can be estimated based on the EDS results. Take Ale10%Li for example, the atomic ratio was approximate to 0.64 which was close to 0.63 (LiAl5O8). Thus the products should be LiAl5O8. The results in Table 2 and Fig. 5 agreed well. Considering the high activity of Li, the R1 should occur firstly and quickly. The heat released by R1 could provide R2 some hot micro-spots where the temperature was enough to make R2 occur. After oxides of Li and Al generated, R3eR5 reacted subsequently. The generation of LiAlO2, LiAl5O8 and LiAl2(OH)7$xH2O reduced the concentration of Al2O3, which was helpful to R2. Moreover, LiAlO2, LiAl5O8 and LiAl2(OH)7$xH2O changed the surface morphologies and broke compactness, as Fig. 5 presented. This benefited to the contact of Al and H2O, which was important to AleH2O reaction. The products of pure Al and AleLi had much higher melting point than that of themselves, and the products covered the surface and
W. Yang et al. / Energy 93 (2015) 451e457
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Fig. 5. TEM graphs of the products: (a) Al; (b) Ale5%Li; (c) Ale10%Li; (d) Ale20%Li.
kept in solid state during the whole hydrolysis process. Before contacting and reacting with molten metals, water vapor had to cross the layer of products firstly. Therefore, the surface feature of the products could impact the reacting process. The data of the specific surface in Table 3 indicated that there were some differences between the products of pure Al and AleLi. The product of pure aluminum had greater specific surface area and pore volume, both of them were several times greater than those of the products of AleLi. However, the pore size of Al product was only 3.98 nm while those of AleLi were 17e33 nm. The tiny pores increased the flow resistance manyfold and hindered the vapor contacting the unreacted aluminum, thus pure Al showed a very low hydrogen yield. The big pores between LiAlO2 crystals provided a good channel with a small resistance to allow the vapor to diffuse, which guaranteed the continuous production of hydrogen. That was the reason why the addition of Li could promote the hydrogen yield of AleLi sample. 4. Evaluation of the proposed concept at industrial scale Fig. 6 gave a schematic of the concept application at industrial scale. The system include the reactor, condenser, dryer and fuel cell Table 2 EDS result for Al, Ale5%Li, Ale10%Li and Ale20%Li. Samples
Al Ale5%Li Ale10%Li Ale20%Li
Atomic ratio (%) Al
O
57 44 39 37
43 56 61 63
Al/O
Species
1.33 0.79 0.64 0.59
Al2O3or Al2.667O4, Al LiAl2(OH)7$xH2O LiAl5O8 LiAlO2, LiAl5O8
and pump. The spiral pipe coiled round the reactor. Water fed by a pump flowed into the condenser and spiral pipe, and was heated to hot water/steam. One of hot water/steam was utilized by users and the other was flowed into as the reactant water. Water reacted with aluminum in the reactor and H2 generated, and then H2 flowed out of the reactor with the unreacted water. The mixture gas flowed into condenser and dryer, pure H2 was achieved. Fuel cell changed H2 to H2O and provided electricity energy for users. Generally, both heat of AleH2O and H2 energy were utilized at the same time. The starting energy was necessary for melting aluminum and vapor generation, which could be provided by igniters or electric heating. 992 KJ starting energy was needed for 1 kg of Al, which occupied 3.42% of the energy of Al. The starting energy would be reduced by lowering starting temperature, optimizing reactor design and recycling heat. The performance of industrial scale depended on the reactivity of Al-based metal fuel, the optimal design of the reactor, the feeding strategy of both reactant water and coolant water. For the application of the proposed concept, the cost was not one of its advantages. It should be higher than conventional source of energy and hydrogen energy by reforming and electrolysis. However, it would possibly be cheaper than batteries with the same high energy density. The cost was determined by the aluminum price and the equipment price. If the equipment was reusable, the aluminum price would be the main factor. The proposed concept at industrial scale would be suitable for underwater power and distributed energy supply outdoors where water source was easy to get. The process of AleH2O reaction releases abundant heat which occupies 51% of the chemical energy of Al, but this portion is abandoned in some H2 generation system concept [11,14,25]. Thus the energy efficiency cannot exceed 50% with reaction heat utilization. An energy conversion system based
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Acknowledgment
Table 3 Specific surface area by BET nitrogen adsorption apparatus. The product
0% Li 5% Li 10% Li 20% Li
Specific surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
BJH Adsorption
BJH Desorption
BJH Adsorption
BJH Desorption
21.3163 2.7817 0.5362 1.8867
0.021041 0.015747 0.007748 0.009203
0.019397 0.015665 0.007747 0.009213
3.9777 22.3610 33.5059 20.9361
3.9866 17.3702 23.7333 16.7161
on AleH2O reaction was conceived and its efficiency was in the range of 0.62e0.85 with both H2 and heat utilization [38]. Our system would present a higher efficiency because the reaction heat could be utilized. Compared to other H2 production processes (gas/ coal reforming, SeI or CueCl chemical looping, biomass fermentation, water electrolysis and photoelectrolysis with solar energy) [2,12,39,40], there existed these advantages:(i) equipment of the proposed concept would be simple, portable and safe; (ii) the products and equipment would be recycled, the process would be completely environment-friendly; (iii) energy density, power density and efficiency would be high.
5. Conclusions The hydrogen production processes of aluminum with lithium addition were investigated by both a thermogravimetric analyzer and a special experimental facility. The reaction of molten aluminum samples and water vapor was carried out in a reactor with over 700 C. Four samples were tested: pure aluminum (0% Li), aluminum with 5%, 10% and 20% of Li contents. The results on both apparatuses showed the addition of Li could promote the reactivity of AleH2O and improve both the reaction rate and the reaction efficiency. Only 5% of Li content can achieve a great promotion: 53% of H2 yield and 112 mL min1 g1 of the average H2 generation rate. More addition of Li improved H2 yield distinctly, and 88% of H2 yield was obtained in the Ale20%Li case. Moreover, in the Ale20%Li case, the sample temperature was stable during the main part of H2 generation process. With the addition of Li, LiAlO2 appeared in the products and the products were made of columnar crystals. The differences in morphologies and products explained the activation mechanism of Li. The pores with an average size of 17e33 nm in the LiAlO2 products took an important role in promoting the reactivity of aluminum and water.
Fig. 6. Schematic illustration of the application prospect via the proposed concept.
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Experimental researches on hydrogen generation by aluminum with adding lithium at high temperature Weijuan Yang*, Tianyou Zhang, Jianzhong Liu, Zhihua Wang, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2015 Received in revised form 9 September 2015 Accepted 11 September 2015 Available online xxx
In order to recover the released heat of AleH2O reaction and promote the reaction itself, the hydrogen production processes of aluminum with lithium addition in molten state are investigated. Experiments are conducted by both a thermogravimetric analyzer and a special experimental facility at high temperature. The results on both apparatuses show that the addition of Li can promote the reactivity of aluminum with water. Compared with pure aluminum, only 5% of Li content can achieve a great improvement: the H2 yield increases from 8.7% to 53% and the average H2 generation rate from 15 to 112 mL min1 g1. With the increase of Li content, H2 yield is improved distinctly and the period with a high H2 generation rate is prolonged. In the Ale20%Li case, the H2 yield of 88% is obtained, and it appears a stable period in which the H2 generation rate keeps high. When adding lithium, LiAlO2 appears in the products and the products are made of columnar crystals. The pores with an average size of 17e33 nm in the LiAlO2 products are manyfold bigger than the pores of alumina, which takes an important role in improving the reactivity of aluminum and water. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen generation Aluminum Lithium Water High temperature
1. Introduction Hydrogen possesses a high mass energy density and does not emit any pollution gas when used as a fuel. As a regenerative and environmentally friendly fuel, it has attracted much attention due to fossil fuel depletion and air pollution from its combustion [1]. At present, steam/partial oxidation reforming of natural gas and coal gasification produce approximately 95% hydrogen [2]. In order to develop new promising technologies, the generation of H2 from suitable starting materials, such as water [3,4], NaBH4 [5,6], metallic hydrides [7] and Al based materials [8e11], has been investigated in the last years. Aluminum is a good choice for hydrolyzing metal and aluminum-based hydrogen production is potentially low-cost and high H2 capacity. Some research results have been published on AleH2O reaction in neutral condition, with assistance of alkalis or oxides or salt, at elevated temperature and with metal additives [11e14]. When producing H2, an inert oxide film easily formed on the Al particle surface, preventing water contacting Al and hindering the continued H2 production [12]. The size of Al particles performed obvious impacts on H2 generation and 10e50 mm was the normal * Corresponding author. Tel.: þ86 571 87951040; fax: þ86 571 87951616. E-mail address: [email protected] (W. Yang). http://dx.doi.org/10.1016/j.energy.2015.09.048 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
size in experimental researches [14e18]. Most of the published researches were carried out by using Al particles with a size of 10e200 mm. The H2 generation rate in neutral condition, even with some additives (C, TiO, Li, In, Zn, Bi, NaCl, KCl et al.), was not beyond 170 mL min1 g1 generally [19]. Although the reactions in NaAlO2, Na2SnO3or NaOH solution achieved the H2 generation rate of over 200 mL min1 g1 [20], the reacted solution had to be disposed specially because of its corrosivity. NiCl2, CoCl2, NaCl or Al2O3 was used to activate Al, but the great amount of the addition reduced the H2 produced per unit mass [11,21e24]. Aluminum based metals presented better performance than aluminum alone [14,25]. AleLi alloy with Li contents of 10%e30% by ball milling [26] and melting method [27] had been researched and the hydrogen yield was able to reach 100%. The addition of NaCl could enhance the reactivity of AleLi alloys with low Li content, and the hydrogen generation rate and hydrogen yield were improved [28]. Temperature was one of the key factors influencing hydrogen production [12,27]. The water initial temperature rising from 50 to 70 C could make the average hydrogen generation rate increase from 101 to 210 mL min1 g1 [21]. Increasing the temperature from 20 to 30, 40, 50 C, the AleLieIneZn alloy presented hydrogen generation efficiencies of 53%, 75%, 100% and 44% [16]. In the case of low melting point metal additives (Ga, In, Sn), liquid phases were observed even at room temperature [29,30].
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Intergranular diffusion of these additives led to embrittlement of Al, which was regarded as the reason for Al becoming chemically active in water [31]. The reaction of AleGa and AleGaeIneSn alloys with water was enabled by liquefaction of the eutectic and a liquid phase through which Al could diffuse [32]. The United states patent proposed a method of producing H2 by Ga-rich liquid AleGa alloys [33]. The AleGa alloys showed limited water-reactivity at low temperature and they needed heating to initiate reaction by intensifying liquefaction. AleGaeIneSn alloys displayed considerable reactivity at lower temperatures [34]. The U.S. patents mentioned molten aluminum-lithium alloys were sprayed with liquid water through a nozzle in a reaction chamber in order to produce H2 [35,36], but no detail information on reaction rates was reported. In this paper, it was focused on the reaction of molten aluminum-based metal and water vapor at high temperature to prove the feasibility of H2 generation by Al blocks and heat recovery. A steel reactor was proposed and designed, in which aluminum was preheated to be molten and reacted with water vapor over 700 C. This paper concentrated on the effects of Li addition on the hydrogen generation of molten aluminum with water vapor. Different approaches were adopted to analyze the products, so as to discover the promoting mechanism of Li. The results would be valuable and referable to underwater power and distributed energy supply based AleH2O reaction. 2. Experimental methods Aluminum powder (99.99%, 10 mm) and aluminumelithium alloy (99.99%, 45 mm) were used as initial samples. The AleLi alloy was composed of 80% of aluminum and 20% of lithium. In order to reduce lithium content of samples, aluminum powder was added into the alloy particles. The samples with lithium content of 5%, 10% and 20% were adopted as well as the pure aluminum powder. Thermogravimetric measurements were conducted with THERMO CAHN's Thermax500 pressurized thermogravimetric analyzer. The temperature program rose up straightly from the room temperature to a certain temperature below 1100 C at a heating rate of 25 C/min. The argon flow as a carrier was set as 500 ml/min and the steam vapor flow was 100 ml/min. 10 mg of the metal sample was used in each test. A special experimental facility was designed and built as Fig. 1 showed. The reactor made of stainless steel had a cylindrical shell with an inner diameter of 5 mm and a height of 10 mm. Before the test, argon was fed into the reactor to drain air completely. 5 g of the metal sample in the reactor was heated to 700 C in order to ensure the sample melting fully. The sample temperature was metered by a K-type thermocouple which was mounted in the reactor and inserted in the middle of the sample. Only argon was fed into the
reactor when heating the sample. The purpose was avoiding the oxidization and hydrolysis of the sample. As soon as the temperature reached 700 C, water was pumped by an injection pump and the heater of the reactor was turned off. After water and argon mixed, they were heated to 200 C by an electric heater, and then flowed into the reactor. Vapor and argon were just exposed to the surface of the sample, and did not bubble through the molten metal. The gas out of the reactor was cooled, dried, and measured by a hydrogen analyzer with a precision of 1%. The precision of the injection pump was 0.5% and the flowmeter measured the argon flow was 1%. In the whole test, the flow of water and argon were 5 ml/min and 500 ml/min. The test would end until no hydrogen was detected by the hydrogen analyzer. The products were collected after the test ended and then measured using XRD (X'Pert PRO, Netherlands) and TEM-EDS (Tecnai G2 F20 S-TWIN, America). 3. Results and discussion Fig. 2 gives the thermogravimetric results of the four samples: pure aluminum (0% Li), aluminum samples with 5%, 10% and 20% of Li contents. The sample with 20% of Li (Ale20%Li) presented a better reactivity, and it had a greater final weight and a higher peak of DTG (differential thermogravimetry). With the reduction of Li content, both the final weight and the DTG peak decreased. The WGR (weight gaining ratio) was defined as the ratio of the final weight gaining to the theoretical weight gaining after the reaction ended. The WGR of the Ale20%Li sample reached 84%, which indicated 86% of Al had reacted with the addition of Li. The data was a little higher than the result by Fan using AleLi alloy with 20% of Li content [26]. The weight gain completed 90 percent when the temperature reached 500 C. However, only 19% of Al hydrolyzed in the pure aluminum case. Comparing the DTG curves in Fig. 2b, the sample with greater Li content displayed a earlier and higher DTG peak. The pure aluminum did not present a weight increasing until the temperature was over 900 C. The addition of Li brought forward and accelerated the AleH2O reactivity. The greater addition of Li performed a greater effect. The DTG peaks appeared at 195 and 365 C in the Ale20%Li case. The first was primarily due to LieH2O reaction and the second to AleH2O reaction. The hydrolysis with Al/Li in molten state was investigated using the special experimental facility and the results in the four cases were presented in Table 1. The average H2 generation rate (ra) was the average of the rates which were greater than 10% of the maximum H2 generation rate (rm), and the tiny values were ignored. The time of H2 generation (t) was also calculated as the same way. There were two values of H2 yield: one was defined as the rate of the volume of generated H2 to the theoretical H2 generation, the other was calculated according to the WGR data, the metal components and the theoretical reactions of
Fig. 1. Schematic of designed experiment system.
W. Yang et al. / Energy 93 (2015) 451e457
190 180
(a)
20% Li
Weight %
170 160
10% Li
150 140 130
5% Li
120 110 100 90
0% Li 0
200
400
o
600
T/ C
800
1000
Weight difference rate (%/min)
(a) TG 18 16 14 12 10 8 6 4 2 0 -2
(b) 20% Li
10% Li 5% Li
0
200
400
600
0% Li
800
1000
o
T/ C (b) DTG Fig. 2. Thermogravimetric results of different samples: (a) TG curves; (b) DTG curves.
2Alþ3H2O¼Al2O3þ3H2 and 2Li þ H2O¼Li2O þ H2. The H2 yields by WGR data was greater 2.3e3.7% absolutely than those by the volume of H2. The H2 yields by the two ways appeared less than 5% of relative error except the pure Al case, and the pure Al case had an absolute error of 2.86%. The error indicated that the H2 yields by the two ways coincided well, and hereinafter the H2 yields by H2volume were adopted because the H2 generation processes were concentrated on. The Ale20%Li case achieved 86.62% of the WGR, which is similar to the result by the thermogravimetric analysis. 5% of Li content improved the H2 yield from 8.72% to 53.28% and ra from 7.6 to 112.3 mL min1 g1 which was the highest in the tests. The H2 yield of 53.28% is much higher than 33% reported by Chen [28]. Furthermore the peak rate of 234 mL min1 g1 in the Ale5%Li case was maximal in all tests. More Li addition could prolong the time of H2 generation and promoted the H2 yield, but reduced H2 generation rate a little. 88.51% of the H2 yield could be obtained in the Ale20%Li case. The crossing-section area of the reactor was the reacting area since vapor just exposed to the molten metal surface. The average reacting rate of the sample changed little with the Li
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content as well as ra did. The average reacting rate of the sample per surface area was increased to 26.7 from 1.9 mL min1 mm2 by adding Li. Fig. 3 shows the sample temperature and hydrogen generation rate in the four cased of Table 1. In the pure Al case in Fig. 3a, the sample temperature did not rise up obviously and the maximum was just 743 C. The curve of H2 generation rate showed two small peaks and the maximum was 15.02 mL g1 min1. Because the reaction rate was not high enough, the heat released by AleH2O reaction was too small to prevent the sample temperature falling down. Li addition promoted the H2 generation rate and the sample temperature dramatically. The temperature peak could be over 1000 C and the rate peak over 230 mL min1 g1. The H2 generation rate climbed up to the peak straightly and then dropped down. Greater Li content of the sample speeded up the climbing and slowed down the dropping. 20% of Li content made the rate peak appear on the 130th second after feeding the vapor, while 5% of Li on the 255th second. Moreover the dropping process in the Ale20%Li case obviously performed a three-phase feature, and there was a gently-dropping phase between two sharply-dropping phases. This three-phase feature could be found but not so apparent in the other cases. Although the maximum of H2 generation rate in the Ale20%Li case was a little lower than that in Ale10%Li and Ale5%Li cases, the Ale20%Li case gained the best H2 yield because its three-phase feature in the rate dropping process prolonged the time of H2 generation significantly as Table 1 showed. The peak of the sample temperature appeared later than the peak of the H2 generation rate, especially in the Ale5%Li case. In Ale10%Li and Ale5%Li cases, the temperature performed a multiphase climbing feature, and there were several even phases and dropping phases between the climbing phases. However the performance of the Ale20%Li case showed different. The curve of the temperature straightly climbed as fast as the curve of H2 generation rate. When the rate reached the peak, the temperature arrived at 966 C, which was close to the peak value of 1011 C. The period from 966 to 1011 C presented a tiny oscillation and lasted about 400 s. Moreover the period was companied with the gentlydropping phase of the H2 generation rate. It was the most important phase during the H2 generation process because the stable temperature and H2 generation rate were favorable and expected by the practical application. As to the other cases, the oscillation was enlarged greatly and the multi-phase climbing feature appeared. In Fig. 3b the temperature even dropped down in the process of H2 generation, which implied that the AleH2O reaction was not stable and sometimes hindered by the products which isolated the vapor from unreacted aluminum. When the hindrance was destroyed by the shock of argon, vapor and H2, the reaction went on and the temperature rose up quickly. The results of the products by XRD (X-ray diffraction) were given in Fig. 4. X-ray diffraction results showed that the products of pure Al were Al2.667O4 and Al2O3 and that LiAlO2 appeared with Li addition. Al peaks were also detected since some of Al was
Table 1 Thermogravimetric results and hydrogen generation of samples with different Li contents. Item
WGR
H2 generation
rm
ra
t
Unit
%
ml g-1
ml g-1 min-1
ml g-1 min-1
s
%
0% Li 5% Li 10% Li 20% Li
10.29 50.13 67.16 86.62
108.46 672.27 945.16 1164.1
15.02 234 232.98 191.24
7.6 112.3 103.96 99.82
852 338 526 686
8.72a/11.58b 53.28a/55.61b 73.86a/73.45b 88.51a/92.19b
a b
It was defined as the ratio of the volume of generated H2 to the theoretical H2 generation. It was calculated according to the WGR data and the reactions of AleH2O and LieH2O.
H2 Yield
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Fig. 4. XRD patterns of the condensed products: (1-Al; 2-Al2.667O4; 3-Al2O3; 4-LiAlO2; 5-LiAl2(OH)7$xH2O; 6-LiAl5O8).
Li2O þ 5Al2O3 / 2LiAl5O8
Fig. 3. Hydrogen generation processes of the samples with different Li contents: (a) 0%, (b) 5%, (c) 10%, (d) 20%.
unreacted. With more Li content, the LiAlO2 peak was higher, but LiAl2(OH)7$xH2O was lower. LiAl5O8 was detected for Ale10%Li and Ale5%Li. The reactions could be described as R1eR5, and R2 was promoted by both the heat of R1 and the products of R3eR5. Li þ H2O / Li2O þ H2
R1
Al þ H2O / Al2O3 þ H2
R2
Li2O þ Al2O3 / LiAlO2
R3
Li2O þ 2Al2O3 þ (7 þ x) H2O / 2LiAl2(OH)7$xH2O
R4
R5
The graphs of the products by TEM (transmission electron microscope) were demonstrated in Fig. 5. EDS (energy dispersive spectrometry) test was carried out through the irradiation onto the products surface (as red circles (in the web version) showed in Fig. 5). The products of pure aluminum and Ale20%Li demonstrated a great difference. The product of pure aluminum had an almost complete spherical shell and only few shells cracked. The surface appeared compact and no visible pores could be found. The compact surface provided some resistance to AleH2O contact and was disadvantageous to Al hydrolysis, as many researchers studied [37]. However, the product of Ale20%Li was built up by columnar crystals and the size of columnar crystals was 100e500 nm. For Ale10%Li and Ale5%Li, no visible crystal was formed. There were some needles or flakes distributed in the surface of samples. The EDS results for Al, Ale5%Li, Ale10%Li and Ale20%Li were listed in Table 2. Because the atomic Al/O ratios for Al, Al2.667O4, Al2O3, LiAlO2, LiAl2(OH)7$xH2O and LiAl5O8 were constant, the products can be estimated based on the EDS results. Take Ale10%Li for example, the atomic ratio was approximate to 0.64 which was close to 0.63 (LiAl5O8). Thus the products should be LiAl5O8. The results in Table 2 and Fig. 5 agreed well. Considering the high activity of Li, the R1 should occur firstly and quickly. The heat released by R1 could provide R2 some hot micro-spots where the temperature was enough to make R2 occur. After oxides of Li and Al generated, R3eR5 reacted subsequently. The generation of LiAlO2, LiAl5O8 and LiAl2(OH)7$xH2O reduced the concentration of Al2O3, which was helpful to R2. Moreover, LiAlO2, LiAl5O8 and LiAl2(OH)7$xH2O changed the surface morphologies and broke compactness, as Fig. 5 presented. This benefited to the contact of Al and H2O, which was important to AleH2O reaction. The products of pure Al and AleLi had much higher melting point than that of themselves, and the products covered the surface and
W. Yang et al. / Energy 93 (2015) 451e457
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Fig. 5. TEM graphs of the products: (a) Al; (b) Ale5%Li; (c) Ale10%Li; (d) Ale20%Li.
kept in solid state during the whole hydrolysis process. Before contacting and reacting with molten metals, water vapor had to cross the layer of products firstly. Therefore, the surface feature of the products could impact the reacting process. The data of the specific surface in Table 3 indicated that there were some differences between the products of pure Al and AleLi. The product of pure aluminum had greater specific surface area and pore volume, both of them were several times greater than those of the products of AleLi. However, the pore size of Al product was only 3.98 nm while those of AleLi were 17e33 nm. The tiny pores increased the flow resistance manyfold and hindered the vapor contacting the unreacted aluminum, thus pure Al showed a very low hydrogen yield. The big pores between LiAlO2 crystals provided a good channel with a small resistance to allow the vapor to diffuse, which guaranteed the continuous production of hydrogen. That was the reason why the addition of Li could promote the hydrogen yield of AleLi sample. 4. Evaluation of the proposed concept at industrial scale Fig. 6 gave a schematic of the concept application at industrial scale. The system include the reactor, condenser, dryer and fuel cell Table 2 EDS result for Al, Ale5%Li, Ale10%Li and Ale20%Li. Samples
Al Ale5%Li Ale10%Li Ale20%Li
Atomic ratio (%) Al
O
57 44 39 37
43 56 61 63
Al/O
Species
1.33 0.79 0.64 0.59
Al2O3or Al2.667O4, Al LiAl2(OH)7$xH2O LiAl5O8 LiAlO2, LiAl5O8
and pump. The spiral pipe coiled round the reactor. Water fed by a pump flowed into the condenser and spiral pipe, and was heated to hot water/steam. One of hot water/steam was utilized by users and the other was flowed into as the reactant water. Water reacted with aluminum in the reactor and H2 generated, and then H2 flowed out of the reactor with the unreacted water. The mixture gas flowed into condenser and dryer, pure H2 was achieved. Fuel cell changed H2 to H2O and provided electricity energy for users. Generally, both heat of AleH2O and H2 energy were utilized at the same time. The starting energy was necessary for melting aluminum and vapor generation, which could be provided by igniters or electric heating. 992 KJ starting energy was needed for 1 kg of Al, which occupied 3.42% of the energy of Al. The starting energy would be reduced by lowering starting temperature, optimizing reactor design and recycling heat. The performance of industrial scale depended on the reactivity of Al-based metal fuel, the optimal design of the reactor, the feeding strategy of both reactant water and coolant water. For the application of the proposed concept, the cost was not one of its advantages. It should be higher than conventional source of energy and hydrogen energy by reforming and electrolysis. However, it would possibly be cheaper than batteries with the same high energy density. The cost was determined by the aluminum price and the equipment price. If the equipment was reusable, the aluminum price would be the main factor. The proposed concept at industrial scale would be suitable for underwater power and distributed energy supply outdoors where water source was easy to get. The process of AleH2O reaction releases abundant heat which occupies 51% of the chemical energy of Al, but this portion is abandoned in some H2 generation system concept [11,14,25]. Thus the energy efficiency cannot exceed 50% with reaction heat utilization. An energy conversion system based
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Acknowledgment
Table 3 Specific surface area by BET nitrogen adsorption apparatus. The product
0% Li 5% Li 10% Li 20% Li
Specific surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
BJH Adsorption
BJH Desorption
BJH Adsorption
BJH Desorption
21.3163 2.7817 0.5362 1.8867
0.021041 0.015747 0.007748 0.009203
0.019397 0.015665 0.007747 0.009213
3.9777 22.3610 33.5059 20.9361
3.9866 17.3702 23.7333 16.7161
on AleH2O reaction was conceived and its efficiency was in the range of 0.62e0.85 with both H2 and heat utilization [38]. Our system would present a higher efficiency because the reaction heat could be utilized. Compared to other H2 production processes (gas/ coal reforming, SeI or CueCl chemical looping, biomass fermentation, water electrolysis and photoelectrolysis with solar energy) [2,12,39,40], there existed these advantages:(i) equipment of the proposed concept would be simple, portable and safe; (ii) the products and equipment would be recycled, the process would be completely environment-friendly; (iii) energy density, power density and efficiency would be high.
5. Conclusions The hydrogen production processes of aluminum with lithium addition were investigated by both a thermogravimetric analyzer and a special experimental facility. The reaction of molten aluminum samples and water vapor was carried out in a reactor with over 700 C. Four samples were tested: pure aluminum (0% Li), aluminum with 5%, 10% and 20% of Li contents. The results on both apparatuses showed the addition of Li could promote the reactivity of AleH2O and improve both the reaction rate and the reaction efficiency. Only 5% of Li content can achieve a great promotion: 53% of H2 yield and 112 mL min1 g1 of the average H2 generation rate. More addition of Li improved H2 yield distinctly, and 88% of H2 yield was obtained in the Ale20%Li case. Moreover, in the Ale20%Li case, the sample temperature was stable during the main part of H2 generation process. With the addition of Li, LiAlO2 appeared in the products and the products were made of columnar crystals. The differences in morphologies and products explained the activation mechanism of Li. The pores with an average size of 17e33 nm in the LiAlO2 products took an important role in promoting the reactivity of aluminum and water.
Fig. 6. Schematic illustration of the application prospect via the proposed concept.
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