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WHEC 16 / 13-16 June 2006 – Lyon France

Economical Aspects of Sodium Borohydride for Hydrogen Storage

İ. Engin TÜRE, F. Öznur TABAKOĞLU, Gülbahar KURTULUŞ

UNIDO-ICHET, Sabri Ülker Sk. 38/4, Cevizlibag, Zeytinburnu, 34015 İstanbul

[email protected]; [email protected]; [email protected] ABSTRACT Hydrogen is the best fuel among others, which can minimize the cause to global warming. Turkey has an important location with respect to hydrogen energy applications. Moreover, Turkey has 72.2% of the world’s total boron reserves. Sodium borohydride (NaBH4) which can be produced from borax has high hydrogen storage capacity. Hence, it is important for Turkey to lead studies about sodium borohydride to make it one of the most feasible hydrogen storage methods. In this paper an approximate process cost analysis of a NaBH4-H2 system is given, starting with NaBH4 production till recycling of it. It is found that, the usage of NaBH4 as hydrogen storage material is relatively an expensive method but after improving reactions and by-product removal in the system and reducing the energy and reactant costs, sodium borohydride is one of the best candidates among hydrogen storage technologies. KEYWORDS: hydrogen storage, sodium borohydride, economic analysis 1. INTRODUCTION The increasing energy demand since the industrial revolution has been mainly met by fossil fuels up to now. However, utilization of fossil fuels has had many serious consequences such as global warming, climate change, damage on ozone layer, acid rains and environmental pollution. Besides the negative impact of environmental damage on people caused by fossil fuels, the finite reserves of oil and natural gas made scientists search for alternative fuels and new energy resources. The main properties which an ideal fuel should have are; environment friendly, renewable, light, having high calorific value, capable of being produced, stored and used in a safe, clean and economic way. As a fuel, hydrogen meets almost all these criteria and with further research and development can be expected to fulfill the remainder. In addition, taking into account that the fossil fuel will be depleted completely in 40-50 years, there is not another alternative. Hydrogen energy is also referred as “independence fuel” since it eliminates the countries’ dependence on the importation of oil and natural gas. Hydrogen can be used in transportation, spacecrafts, electricity generation, industry, houses and any other application where fossil fuels are currently used. 2. HYDROGEN STORAGE METHODS For widespread use of hydrogen energy it is necessary to develop hydrogen storage systems which are safe, capable of storing large amounts in small volumes. Today, hydrogen can be stored in gaseous, liquid or solid forms such as metal hydrides, chemical hydrides and carbon nanotubes. However, these methods are still in development phase and it will take some time before they are economically feasible. Although compressed hydrogen can be used in vehicle applications such as buses and cars, its use is not appropriate in small portable device applications with power outputs less than 100 W. In such cases, chemical hydrides, having high volumetric and gravimetric storage densities are being developed. Direct methanol fuel cells have been developed as a solution to the portable liquid fuel usage (DMFC), but their poor performance and the other problems caused by methanol have initiated scientists search for better devices. Particularly, the liquid solutions of sodium borohydride (NaBH4) having both high energy capacity and reliability, have been the subject of considerable interest in recent years [1]. However, the


high cost of chemical hydrides inherent to all chemical storage methods is also a disadvantage for sodium borohydride. The selling prices of sodium borohydride produced via commercial production methods such as Bayer, Rohm and Haas are US$ 55/kg for powder form and US$ 47/kg in a 12% NaOH solution [2]. 3. BORAX AND ITS IMPORTANCE FOR TURKEY Borax is boron mineral and is a compound used in sodium borohydride synthesis. Boron minerals contain boron oxide (B2O3) in different ratios and the common ones found in Turkey are tincal (Na2B4O7.10H2O), colemanite (Ca2B6O11.5H2O) and ulexite (NaCaB5O9.8H2O). The total boron reserves of Turkey are 851 million tons and constitute 72.2% of the world’s total estimated boron reserves. The boron reserves in the world and in Turkey are shown in Table 1 and Figure-1.

Table 1: Boron Reserves in the World (thousand tonnes - B2O3) [3]

Country Proven Economical

Reserve

Possible Economical

Reserve

Total Reserve

Share In Total Reserve (%)

Turkey 227 624 851 72.2 USA 40 40 80 6.8 Russia 40 60 100 8.5 China 27 9 36 3.1 Argentina 2 7 9 0.8 Bolivia 4 15 19 1.6 Chili 8 33 41 3.5 Peru 4 18 22 1.9 Kazakhstan 14 1 15 1.3 Serbia 3 0 3 0.3 Total 369 807 1,176,000 100,000

Figure 1: Distribution of Boron Reserves in Turkey [3] Although Turkey has the highest share of proven and possible boron reserves in the world, USA has the highest production. The boron products produced in Turkey are borax decahydrate, anhydrous borax, borax pentahydrate, sodium perborate tetrahydrate, sodium perborate monohydrate and boric acid [4]. The boron reserves are in the state monopoly and are processed by the Eti Company. It is obvious that Turkey having a 230-240 million US dollars boron export income does not have enough share in the boron market based on the assumption that the boron trade value of the world is 1.25 billion US dollars [5]. The


boron compounds which constitute the raw material for more than 400 compounds in manufacturing industry can be used in various applications such as detergents, space technologies, toothpaste and engine oil. Therefore, boron mineral has a strategic importance similar to oil and natural gas [5]. Hydrogen storage technology is one of the application areas of borax and sodium borohydride can be produced from borax decahydrate and anhydrous borax. These compounds are produced by processing tincal mineral which is abundant in Turkey. As noted earlier the production cost of sodium borohydride today is quite expensive, constituting a major obstacle to commercialization of sodium borohydride as a hydrogen storage method. Nevertheless, it is expected that the cost will decrease to less than US$ 1/kg within the next five years [6]. 4. USAGE OF SODIUM BOROHYDRIDE AS HYDROGEN STORAGE MATERIAL 4.1. CHARACTERISTICS OF SODIUM BOROHYDRIDE Sodium borohydride, NaBH4, is classified as a flammable solid, is available either as a white powder or as a 12% solution in caustic soda. It is stable to 300°C in dry air and in vacuum to 400°C with only partial decomposition. In order to prolong the shelf life of sodium borohydride solutions, NaOH is added. Under normal storage conditions, the loss of NaBH4 in 12% solution by decomposition is less than 0.1% NaBH4 per year [2]. NaBH4 is hygroscopic and when it absorbs water rapidly from moist air, it decomposes slowly to form sodium metaborate and hydrogen. Hydrogen can be generated in a rapid and controlled manner by adding an acidic compound or an appropriate metal such as nickel, cobalt, ruthenium or platinum. Exothermic, catalytic hydrolysis reaction of sodium borohydride solution is shown in reaction (1). As a result of the complete hydrolysis of NaBH4, 2.37 l H2/g NaBH4 is released [2]. Half of the hydrogen comes from sodium borohydride and the half comes from water. Therefore, the hydrogen content of sodium borohydride is quite high. The theoretical maximum hydrogen capacity of sodium borohydride is 10.8%. NaBH4 + 2H2O → 4 H2 + NaBO2 + 300 kJ (1) ∆Gº (298) = -321 kJ/mol-NaBH4 NaBH4 has more hydrogen storage capacity than many complex hydrides, as shown in Figure 2. Furthermore, the studies have shown that NaBH4 has more hydrogen storage capacity than compressed hydrogen in a tank [7].

Figure-2. Comparison of chemical hydrides [8]


Briefly, the advantages of hydrogen storage using NaBH4 can be summarized as follows:

• Sodium borohydride solutions are stable for long period of time, • Sodium borohydride solutions are non-flammable, • The volumetric and gravimetric hydrogen storage capacity of NaBH4 is high, • The hydrogen release rate of NaBH4 can be controlled easily, • The by-product NaBO2 can be recycled back to NaBH4 .

4.2. COSTS OF SYNTHESIS, HYDROLYSIS AND RECYCLING OF SODIUM BOROHYDRIDE The Bayer process is one of the best known commercial sodium borohydride production methods. In this process, a certain amount of anhydrous borax, sodium and quartz are heated together at 500°C under 3 atm hydrogen pressure [9]. After extraction of the products with ammonia and its evaporation, sodium borohydride is obtained with high efficiency. As a secondary product sodium metasilicate is produced. The reaction is given below:

1/4 Na2 B4O7 + 4Na + 2H2 + 7/4 SiO2 → NaBH4 + 7/4 Na2SiO3 (2)

∆Gº (298) = -411.3 kJ/mol-NaBH4 ; ∆Hº (298)=-541.348 kJ/mol NaBH4

The reaction enthalpy of the process is calculated and it is found that the energy cost of the process is US$ 2/kg NaBH4. This assumes ideal conditions and that the energy cost of by-product removal is excluded. On the other hand, in order to produce 1 kg NaBH4, approximately US$ 10 is spent for raw materials. If sodium borohydride produced as described in section 4.1 is used in hydrolysis reaction, 4.73 kg of NaBH4 is needed to generate 1 kg hydrogen and the cost will be approximately US$ 50. There is also a need for a catalyst in the hydrolysis reaction and depending on the catalyst type the cost required to produce 1 kg H2 out of NaBH4 is estimated to be around US$ 80 kg/H2. It should be noted that, if NaBH4 is not produced in the system and if it is purchased from industry at a price of around US$ 50/kg NaBH4, the cost of this reaction would be around US$ 230/kg H2 and including the cost of catalyst, the hydrolysis reaction cost would be approximately US$ 260/kg H2. It is crucial to recycle NaBO2 back to NaBH4, which is generated via a hydrolysis reaction. It has been shown that the recycling process can be realized by reacting NaBO2 with different chemicals such as MgH2 [10] or Mg2Si [11]. In this study, the cost of recycling process using Mg2Si is calculated as at least US$ 15/kg H2. Finally, under the assumption of ideal conditions on, the total cost starting with NaBH4 production until recycling amounts to around US$ 110/kg H2 and US$ 290/kg H2 if the sodium borohydride was purchased directly from industry. 5. CONCLUSION Fossil fuels are limited and it is foreseen that their amount will decrease considerably in the near future. Hence, the best alternative fuel for the 21st century is hydrogen. Hydrogen production and storage technologies are costly nowadays but in the future are expected to decrease. Turkey has 72.2% of world’s boron minerals and utilization of boron ores could make an important contribution to the Turkish economy drive. In particular, developing sodium borohydride for use as a hydrogen storage material and being one of the leaders in this field will contribute to the Turkish economy and its sustainable development. Although sodium borohydride is one of the best candidates to be used in hydrogen storage, work is needed to improve the efficiency of the NaBH4-H2 system reactions and energy and reactant costs must be decreased.


6. REFERENCES

1. Wee, J., A comparison of sodium borohydride as a fuel for proton exchange membrane fuel cells and for direct borohydride fuel cells, Journal of Power Sources, 2006

2. Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, 2001 3. http://www.etimaden.gov.tr 4. Boron ores and the status in the world, http://science.ankara.edu.tr/~kavusan/ 5 Alma,H., Acemioğlu, B., Boron Applications in Turkey, Science and Eng. J.,V.4, (2), 2001 6. http://merit.hydrogen.co.jp 7. Zeybek, O., Akın, S., “Hydrogen Storage Technology Using Sodium Borohyride”, Proceedings

International Hydrogen Energy Congress and Exhibition IHEC 2005, İstanbul. 8. Ying Wu, “Hydrogen Storage via Sodium Borohydride Current Status, Barriers, and R&D Roadmap”,

GCEP – Stanford University, April 14-15, 2003. 9. Schubert, F., Lang, K., Shabacher, W., Burger, A., US Patent Office, 3,077,376 (1963). 10. Z.P.Li,B.H.Liu, N. Morigasaki, S.Suda,Journal of Alloys Compounds 349 (1), 232-236, 2003., 11. Kojima,Y., Haga, T., “Recycling process of sodium metaborate to sodium borohydride” International

Journal of Hydrogen Energy, 2002, 989-9932

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