Silicon as energy carrier—Facts and perspectives

Energy 31 (2006) 1395–1402 www.elsevier.com/locate/energy

Silicon as energy carrier—Facts and perspectives Norbert Auner *, Sven Holl Department of Inorganic Chemistry, John Wolfgang Goethe-University of Frankfurt, Marie-Curie-Str. 11, D-60439 Frankfurt am Main, Germany

Abstract Due to the diminishing reserves of carbon based primary energy carriers and the need to reduce carbon dioxide (CO2) emissions worldwide, an alternative energy concept was developed using elemental silicon as secondary energy carrier. Starting from sand, silicon can be accessible on a carbon/carbon dioxide free route in a process cycle using cost-effective—at best renewable—energy anywhere in the world. The reduction process sand/silicon, just as the generation of every synthetic secondary energy carrier, requires a significant amount of energy, which then is partially stored in the metal. Using existing technology, silicon can be transported and stored without any risk. Reactions of silicon with oxygen or nitrogen are exothermic and result in the release of thermal energy as well as formation of economically valuable products—instead of CO2. From silicon nitride, ammonia is obtained as a feed stock for the fertilizer industry as well as for hydrogen production. Alternatively, hydrogen is produced from silicon directly by simple reactions with water or alcohols, giving sand or silicon-based compounds as byproducts. These are available for a variety of different technical applications and, if required, can be recycled easily. q 2006 Elsevier Ltd. All rights reserved. Keywords: Silicon production; Silicon as energy carrier; Ammonia generation; Hydrogen generation

1. Introduction Our present concept of energy generation is based essentially on carbon (crude oil, natural gas) as raw material [1]. The resulting energy-supplying process consequently produces the greenhouse effect caused by the gas carbon dioxide (CO2). Not only have past oil crises shown that this raw material is not available in unlimited abundance, but scientific studies show that natural carbon-based resources are being dramatically reduced. The reason is twofold: the expected increase in the world’s population, and the transfer of technology to developing countries, especially in Asia. One possible approach to this problem is the use of biomass to generate renewable energy. Wind energy, hydroelectric power, and solar energy will certainly be used for energy generation purposes, where the geographical and climatic prerequisites are favourable. Unfortunately, such regions seldom coincide with the areas of high energy consumption, namely industrial regions with a high population density. Therefore, the problem is to find an efficient secondary energy carrier for permanent energy storage and safe energy transportation. This is essential because the direct transport of primary electrical energy via long-distance high-voltage power lines suffers from losses and requires a suitable economic and technical infrastructure often missing in those geographic regions that are suitable for large-scale generation of regenerative energy. The losses are estimated to amount to 2–4% per 1000 km for high-voltage DC transmission. According to numerous experts, this * Corresponding author. Tel.: C49 69 798 29591; fax: C49 69 798 29188. E-mail address: [email protected] (N. Auner).

0360-5442/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.12.001

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energy carrier will definitely be hydrogen, which not only produces water as the result of the energy-delivering combustion process, but also does so with high efficiency [2]. In spite of huge advantages, however, the concept of hydrogen-based energy is still controversial, and rightly so, because: (i) today’s hydrogen generation requires crude oil or natural gas as raw material (catalytic cracking of hydrocarbons, steam-reforming). (ii) water is used as an alternative hydrogen source. This requires considerable amounts of energy, because water must be split into hydrogen and oxygen in order to generate water and energy again. The problem here is that large areas of our globe lack water, and other hydrogen sources, such as metal hydrides, are less efficient. (iii) the thermodynamically most efficient source for hydrogen generation is ammonia. Decomposing ammonia into hydrogen and nitrogen, takes only approximately 10% of the energy necessary for water decomposition. However this also is not without problems: Using the Haber–Bosch process for the synthesis of ammonia, hydrogen has to be obtained from hydrocarbons leading to the problems discussed previously. Such a process route makes ammonia dependent on carbon. (iv) the generation, storage, and transport of liquid or gaseous hydrogen involve an enormous expenditure of energy and the diffusion behaviour is difficult to control, which increases the hazard. Hydrogen is only favourable, if it can be produced close to the consumer and is used immediately to generate energy. Many experts therefore consider an age of energy based on hydrogen unrealistic in view of the enormous cost expenditure and the hazard involved. 2. Silicon as energy carrier: definition of requirements Taking into account the cautionary global discussions about the urgent need to reduce CO2 emissions, it becomes clear that the search for, and research into, alternative energy sources is absolutely necessary. The above discussion places specific demands on a future energy carrier. Furthermore, mother nature no longer offers such a material, which would need to: (i) be easy to produce by synthesis, if possible via renewable energy such as that from solar, wind, or hydroelectric sources, (ii) be available to an unlimited degree or at least suitable for recycling, (iii) be transportable without hazard and be capable of permanent energy storage, (iv) demonstrate a high energy density, (v) release no carbon dioxide in high-tech population centres, and (vi) supply as many valuable combustion products as possible. These prerequisites are met by combining three raw materials: silicon, water, and air. This paper proposes that silicon is the desired energy carrier. 3. Silicon as energy carrier: availability, production, and reactivity In the context introduced above, it is interesting that none of the currently discussed global concepts involve an alternative energy form and energy generation based on sand—a non-toxic natural material that is available in unlimited supply. About 75% of the earth’s crust accessible to us, including the various types of biomass in the form of plants (rice, horsetail, inter alia), rocks, and diatoms (in salt water), is comprised of silicon dioxide, SiO2 (Si: 26.3%, O2: 48.9%; Silicon is as common as all of the other elements combined) [3]. Nowadays, quartz sand (SiO2) is converted into crystalline silicon on a megaton scale by reduction with carbon using an electric arc process (Tw1800–2000 8C), whereby large amounts of electric energy (11,000 kWh/t Si; technically realised energy efficiency: 84%) [4] are required and considerable amounts of CO2 are released. A reduction of SiO2 with H2 instead of carbon is not feasible, and it only leads to silicon monoxide SiO [4]. Silicon dioxide, SiO2, is chemically inert and dissolves in alkaline bases only maintaining the silicon oxygen bonds. It does not react with protic acids, except with hydrogen fluoride (HF), giving silicon tetrafluoride (SiF4)

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quantitatively and under moderate conditions. Technically, HF is mainly produced by reaction of calcium fluoride, CaF2, with sulfuric acid, H2SO4. Due to the carbon/carbon dioxide problem stated earlier, a completely new C/CO2 free chemical cycle was developed by the authors [5]. Starting from conventional desert or sea sand, which contain about 80–90% of a-quartz, silicon is produced according to Scheme 1 on the basis of technologically well-established large scale processes without formation of any byproducts and with high purity in powder form. More detailed, this process comprises some key steps: (a) Addition of H2SO4 to a mixture of conventional sand and an alkaline fluoride such as the sodium salt in a temperature range between 20 and 80 8C starts the in-situ formation of HF; this directly transfers SiO2 into gaseous SiF4. When SiF4 is purified by condensation, all impurities of the SiO2 source (mainly metal oxides) remain as a solid residue (metal fluorides and/or sulfates) and might be used for different applications. (b) SiF4 is reduced by alkaline metals, especially sodium, to give elemental silicon and alkaline fluoride, preferably sodium fluoride, NaF. (c) NaF is separated and reacted with H2SO4 to give HF in a recycling step and sodium sulfate, Na2SO4. (d) After closing the hydrogen fluoride cycle, the alkaline metal used for reduction of SiF4 has to be regenerated. Thus, Na2SO4 is reacted with calcium chloride (CaCl2) in aqueous solution to yield calcium sulfate, CaSO4$2 H2O. Water from the remaining sodium chloride solution is evaporated and solid NaCl is split into sodium and chlorine by the Downs-Process (‘molten salt electrolysis’). (e) The calcium sulfate that forms is a suitable starting material to recycle the sulfuric acid used in step (c). At 1200 8C, CaSO4 decomposes to calcium oxide (CaO), sulphur dioxide (SO2), and oxygen (O2). Using technically well-established processes, H2SO4 is produced from the gas mixture. (f) The CaCl2 needed in step (d) is obtained by reacting the CaO formed in step (e) with hydrogen chloride, HCl. The latter is isolated from the combustion of gaseous chlorine produced in step (d) (electrolysis) with hydrogen, which might originate from water electrolysis. A second, less extensive process according to Scheme 2 has been verified on laboratory scale. Silicon is deposited from a SiF4/H2 mixture by a microwave-induced plasma discharge [6]. The resulting byproduct HF can be recycled for the generation of SiF4. This greatly simplifies the overall process presented in Scheme 1 making it an analogue of the Siemens process, which is already used to produce electronic grade silicon from chlorinated silicon precursors. SiO2 4 HF O2 2 H 2O

2 H2

SiF4 4 Na 4 HF 4 NaF Si

2 Na2SO4 2 H2SO4

4 NaCl

2 H2O 2 CaCl2 2 H2SO4

Cat.

4 Na + 2 Cl2 2 H2

2 CaSO4

2 SO2 + O2 2 CaO

4 HCl 4 HCl 2 H2O

2 CaCl2

Scheme 1. HF-route for the production of highly pure amorphous silicon in the gas phase and in solution by reduction of SiF4 with sodium.

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SiO2 4 HF O2 2 H2 O

2 H2

SiF4 2 H2 4 HF 4 HF Si Scheme 2. HF-route for the production of highly pure silicon by gas-phase deposition from a SiF4/H2 mixture.

The overall processes shown in Schemes 1 and 2 are not meant to compete with today’s electric arc technology, but they might develop into an attractive alternative for a low-cost production of highly pure silicon (electronic-grade quality) or as a replacement for a future without carbon reserves. Of course, the discussed cycles contain some energyconsuming steps: on one hand, the molten salt electrolysis, the production of hydrogen by water electrolysis, the thermal decomposition of calcium sulfate, and the evaporation of water from aqueous salt solutions, on the other hand the production of hydrogen by water electrolysis and the plasma deposition of silicon. Fortunately, the closed processes remain completely carbon/CO2 free with no formation of byproducts. The only raw material needed is sand, which, in contrast to today’s technology, is suitable even in low quality. Based on the synthetic conditions chosen for silicon production, the reactivity of silicon can be adjusted for different applications. Low temperature reduction of silicon tetrahalides SiX4 (XZCl, F) with sodium in donor solvents yields an yellow-brown amorphous silicon powder (Siam,ye); the reduction in non-polar hydrocarbon solvents gives a black powder (Siam,bl) mixed with the corresponding sodium salts NaX (XZCl, F). After separation from these salts, no signal can be detected by X-ray powder diffractometry in either case, thus proving the non- or microcristalline nature of the amorphous silicon. The difference in color indicates a different silicon surface coverage and thus different reactivities. This is demonstrated by the low temperature formation of amorphous silica by simple oxidation of silicon powder in air, even at room temperature. Notably, this reaction does not occur with conventional silicon metal but gives amorphous silica (SiO2) with hitherto unknown properties. The oxidation process of silicon is temperature dependent, which is shown for four different silicon samples in Fig. 1. Metallurgical silicon proves a comparatively low reactivity against air, which allows the material to be transported and stored like coal. The energy required for the sand/silicon transformation is in part stored in the energy carrier, which fulfills all the requirements defined earlier; thus it stores energy permanently and can be conventionally and environmentally friendly transported anywhere without risk. Its energy is released by simple oxidation under controlable conditions. 4. Silicon as energy carrier: combustion processes The reduction process sand/silicon produces a durable secondary energy carrier with an energy density and content comparable to carbon; silicon is as good as carbon, and the resource is not limited (Table 1) [7]. It is well known that conventional metallic silicon reacts with oxygen only at high temperatures (TO1500 8C) exothermally to give silicon dioxide and heat (Eq. (1)). The thermal energy released, which is about 9 kWh/kg Si, can then be transformed into electric energy by conventional routes. Besides the possibility of burning silicon in fluidized bed systems, the oxidation of silicon powder in an open flame has been tested with a device optimized for burning powdered aluminum metal [8]. In this case, the silicon flame was not self-preserving but needed continuous ignition.

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At % O (c)

60

(e)

84.8 % conversion

50

(b)

(d)

(a)

40

74.4 % conversion

30 20

51.6 % conversion

10 0 0

200

400

600

800

1000

1200

Temperature [°C] Fig. 1. Oxidation of different silicon samples in air as f(T): (a) metallic silicon, particle size 0–70 mm; (b) amorphous silicon, produced by reduction of SiO2 with Mg at high temperature; (c) Siam,ye; (d) Siam,bl; (e) Siam,bl after 5 days at room temperature.

In the overall energy needed for silicon production (w12 kWh/kg Si), the storage efficiency factor is nearly 30% and thus comparable to the system ‘water/hydrogen (by electrolysis)/water’, using the same conversion factor (0.391) [9] used in industrial power plants for the transformation of thermal energy into electrical energy. Surprisingly, silicon burns even under a nitrogen atmosphere to give silicon nitride. Based on this reaction, air (w80% N2, 20% O2) would become the ideal reaction partner for silicon to produce thermal energy and would yield highly valuable ceramic materials as solid combustion products. The question is how silicon can be activated to react with air according to the Eqs. (1) and (2): Si C O2 / SiO2 C 912 kJ=mol

(1)

3 Si C 2 N2 / Si3 N4 C 750 kJ=mol

(2)

Fortunately, the answer to this question is already known [10]. With copper oxide as an activator, silicon reacts in a fluidized bed reactor under mild conditions (w600 8C) with nitrogen to give silicon nitride, a chemically and physically resistant ceramic material. 5. Silicon as energy carrier: a save source for hydrogen In chemically simple processes, silicon nitride reacts with bases, or at high temperatures even with water, to yield silicates, sand, and ammonia (Eq. (3)). Si3 N4 C 6 H2 O/ 3 SiO2 C 4 NH3

(3)

This basic compound for the fertilizer industry (production: w120 Mio t/year) is thermodynamically the most favourable source for the production of hydrogen (besides nitrogen; retro Haber–Bosch process) [11]. While 18.75 kJ is needed for the production of 1 g hydrogen from methane, the corresponding numbers for water and ammonia are 143 and 15.4 kJ, respectively [7]. Thus, the relative ratio of energy required for the formation of hydrogen from water and ammonia is 9.3/1. Table 1 Comparison between the energy carriers carbon and silicon

Energy content (kJ/g) Energy density (kJ/cm3)

Carbon

Silicon

32.8 74.2

32.6 75.9

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NH3[%]

1400

90 80 70 60 50 40 30 20 10 0

1. 2. Na2CO3 dec. 400

600

700

800

900

T[°C] Fig. 2. NH3 from Si3N4 1. low concentration of Si3N4$2. high concentration of Si3N4.

Fig. 2 shows the formation of ammonia from water and silicon nitride as f(T) with Na2CO3 as additive. Above 770 8C, Na2CO3 begins to decompose giving sodium oxide. With H2O, sodium hydroxide is formed. The latter is responsible for the quantitative formation of NH3 at high temperatures. In an alternative reaction path, silicon is reacted directly with water and sodium hydroxide to give silicate and hydrogen (Eq. (4)) [12]. Si C 2 NaOH C H2 O/ Na2 SiO3 C 2 H2[

(4)

In a very slow reaction, NaOH is recycled from the silicate (Eq. (5)). Na2 SiO3 C 3 H2 O/ 2 NaOH C SiðOHÞ4

(5)

To accelerate the reaction, Ca-hydroxide is added to finally give Ca-silicate [13]. The disadvantage of this process is the large scale consumption of base, water, and lime; moreover, the decomposition of calcium carbonate requires thermal energy and releases CO2. To overcome these problems, a system was optimized recently using the reaction of (Eq. (6)) to produce hydrogen in a reformer system at higher temperatures and pressures with water recycling [14]. Na2 SiO3 C H2 O/ 2 NaOH C SiO2

(6)

The crystallisation process has to be conducted at temperatures above 200 8C to obtain quartz instead of highly hydrated silicic acid. Correspondingly, high pressures are necessary to prevent vaporization of the solution. Needless to say, the reaction of silicon with a second partner without additives, which nevertheless give one product and hydrogen under ambient conditions, would be the most favourable storage system. And this can be done: when highly reactive amorphous silicon produced via the SiF4 route reacts directly with water at room temperature, sand and hydrogen are formed quantitatively. Even better, reactions of silicon with alcohols or carbon acids result in the formation of hydrogen and valuable silicon-based feed stocks for the chemical industry. The reaction of less reactive metallurgical silicon with alcohols, especially with methanol (MeOH), to give trimethoxysilane HSi(OMe)3 in an industrial process has been optimized for several years. Silicon powder is dispersed in a high-boiling inert solvent and transformed nearly quantitatively into the desired product by a copper-catalysed reaction with gaseous MeOH at about 250 8C [15]. Altered reaction conditions lead to Si(OMe)4 as the main product and thus to an optimized hydrogen yield [16]. The different synthetic methods for the formation of hydrogen from amorphous silicon are summarized in Scheme 3. If required, these silicon compounds could be recycled easily by reaction with water to release the alcohols or acids initially used and cause a simple formation into sand again. 6. Silicon as energy carrier: the overall process Summarizing, we developed a carbon and carbon-dioxide free route to ammonia. On the one hand, this creates an industrial feed stock for fertilizers and possibly a replacement for natural gas in combustion processes or as converter supply for fuel cells. On the other hand, the process leads to ammonia, which serves as a source for the hydrogen

N. Auner, S. Holl / Energy 31 (2006) 1395–1402 ROH

1401

Si(OR)4 + 2H2

(R=Me, Et)

-ROH H2O H2O

Siam,bl

SiO2 + 2H2

-HAc H2O HAc

Si(OAc)4 + 2H2

Scheme 3. Silicon as source for hydrogen.

SiO2

air

silicon

SiO2 /Si3N4

H2O

NH3

H 2O

SiO2

+

glasses / fertilizers

H2

O2

ENERGY + N2 + H2O

NH3 energy

H2 + N 2

Scheme 4. Silicon as energy carrier—the overall process.

itself. Alternatively, hydrogen can be directly produced from silicon to feed conventional power plants as well as fuel cells in stationary and mobile systems (Scheme 4). Scheme 5 gives the overall thermodynamics of the process in which the energy values are calculated from the heats of formation [7].

Silicon

‘Combustion’ in N2

Siliconnitride 1.989 kJ

H2O 1.528 kJ

Hydrogen Ammonia

Combustion in O2

Combustion in O2

Sand

Coal

Sand

N2

Water

Scheme 5. Energy balance of the sand-silicon-water-cycle.

1.712 kJ

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7. Conclusion As carbon-based primary energy sources are becoming more and more exhausted and the still existing reserves should obviously be used and saved for the production of valuable products, this paper presents an innovative way to secure the future supply of ammonia and hydrogen on a ‘non carbon’ route for mobile and stationary energy production applications. Furthermore, the ammonia produced serves as feed stock for the fertilizer industry, and the silicic acid (SiO2) and silicon nitride (Si3N4) are ceramic materials of great economic value. Consequently, besides energy, this method would provide useful silicon-based combustion products instead of carbon dioxide. Hydrogen produced by the route sand/ silicon/ silicon nitride/ ammonia/ hydrogen; or directly formed from silicon and water or alcohols is truly ‘clean’. Transportation of silicon, in contrast to oil or even more to hydrogen, is without any risk using existing technical infrastructure and thus resembles coal. While coal eventually forms carbon dioxide, silicon is transformed back to sand and can be recycled again. References [1] German Federal Ministry of Economy and Work. Energy data 2003—national and international development (Energie Daten 2003— Nationale und Internationale Entwicklung); 2004. Available via: http://www.bmwa.bund.de [in German]. [2] Gretz J. Hydrogen as energy carrier in a clean energy supply system with direct and indirect solar energy, concept of a pilot project (Wasserstoff als Energietra¨ger eines sauberen Energiebereitstellungssystems mit direkter und indirekter Sonnenenergie, Konzept eines Pilotprojektes). In: Dechema-Monographien, vol. 106. Weinheim: VCH Verlagsgesellschaft; 1986 [in German].Hoffmann VU. Hydrogen— energy with future (Wasserstoff—Energie mit Zukunft). 2nd ed. Stuttgart: Teubner Verlag; 1994 [in German]. [3] Hollemann AF, Wiberg N. Textbook of inorganic chemistry (Lehrbuch der Anorganischen Chemie). Berlin: de Gruyter Verlag; 1985. p. 91– 100 [in German]. [4] Schei A, Tuset JK, Tveit H. Production of high silicon alloys. Trondheim: Tapir Trykkeri; 1998. [5] Auner N, Holl S. Silicon as energy carrier—facts and perspectives (Silicium als Energietra¨ger—Fakten und Perspektiven). Prax. Naturwissenschaften Chem. Schule 2003;52(8):2–7. Auner N. Method for producing silicon. WO 03059814; 2003. [6] Societe anonyme des manufactures des glaces et produits chimiques de Saint-Gobain, Chauney & Cirey. An improved process for the production of metals and other chemical elements of metallic character in a state of high purity. GB 838378; 1960. Sarma KR, Rice Jr MJ, Lesk IA (Motorola, Inc.). High pressure plasma deposition of silicon. US 4292342; 1981. Nagano M, Moriya T, Takoshima T, Mori N, Yamaguchi F (Tohoku Electric Power Company Incorporated/Nova Science Institute). Method and apparatus for production of high purity silicon. WO 0187772; 2001. [7] Lide DR, editor. 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