FROM SOLAR ENERGY TO HYDROGEN VIA MAGNESIUM: A CHALLENGING APPROACH

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1 FROM SOLAR ENERGY TO HYDROGEN VIA MAGNESIUM: A CHALLENGING APPROACH H. K. Abdel-Aal National Research Center, Doki, Cairo, Egypt E mail: habdelaal@link.net.eg ABSTRACT: In the proposed scheme, solar energy is used first to vaporize a dynamic stream of sea water flowing along an inclined Preferential Salt Separator (P S S). Magnesium chloride salts soluble in seawater- will separate as end products [1].Once obtained, anhydrous magnesium chloride is to be electrolyzed to produce magnesium metal, a reliable source of stored energy. When shipped to remote locations, it is used as electrode to construct a galvanic- electrolytic cell, in which water is electrolyzed producing hydrogen as end product. Small scale experimental results are presented. Reference to the work reported by Pacheco [2] is made. KEYWORDS : Seawater, Hydrogen, Galvanic-Electrolytic cell, Magnesium chloride, Magnesium INTRODUCTION It is most common to talk about energy systems that would utilize solar energy to split water to produce hydrogen. This way we manage to store solar energy in the form of hydrogen being an ideal energy carrier.however, we are still left with a formidable problem : how to store and transport hydrogen for end users? The answer to this is offered by the proposed scheme. Solar energy is utilized to vaporize a dynamic stream of saline water (seawater, desalination brines) fllowing along across an inclined P S S. Preferntial salt separation takes place, while the fluid is flowing leaving magnesium chloride as the end product. The recovery of this salt from sea water- to be used for the manufascture of magnesium metal has been a challenging endeavor over the years [1,2} Once obtained, electrolysis of anhaydrous magnesium chloride is carried out to produce magnesium metal. Now, if shipped by air, sea or other means to remote areas, magnesium metal is used to construt a galvenic cell, made of magnesium / iron electrodes { Mg/Fe}, in which seawater is electrolysed under the influence of self-generated e.m.f. Thus while magnesium metal is consumed for power generation, hydrogen is produced simultaneously by the electrolysis of water. The availability of magnesium metal from seawater plays a signifant role as a key element in our proposed energy system. Magnesium is recognised as the world s lightest structural metal and is the eighth element in order of occurance in the world [3].One cubic kilometer of seaawter contains a minimum of one million ton magnesium, which makes the ocean a «storehouse» containing about 1.7 x (10 raised to the power 24) tons of magnesium, a staggering figure. Magnesium represnts only 0.13 % of the totalk content of all soluble salts found in seawater. SYSTEM DESCRIPTION The energy system described in this paper involves three distincive processes (as shown in Fig.1) These are : 1- Preferntial Salt Separation ( PSS), for the recovery of magnesium chloride. 2- Electrolysis of magnesium chloride to produce magmesium metal. 3- Hydrogen production using galvanic-electrolytic cell. 1/6

2 1. Preferntial Salt Separation(PSS) Magnesium chloride has been traditionally produced from sea water by precipitaing it as magnesium hydroxide, then converting it to the chloride by adding hydrochloric acid. The PSS method is different in concept, since it is totally a physical operation (1). It is enviseged to build an industrial plant based on this method, to be located near the sea shore. It consists of a number of shallow inclined basins (10 meter wide by 10 meter long, arranged in a step-wise patteren [Fig.2]. Each basin is covered with a transparent plastic cover. Preconcentrated saline water is continusally fed to the upper basin, allowed to overflow by gravity to the next channels all the way to the very end. Solar energy, absorbed by the water and by the black polyethylene film covering the bottom of the evaporator, would supply the heat flux necessary for the evaporation with the subsequent selective separation of salts along the evaporator. This takes place according to the solubility product of each salt. Water condensing on the inside of the plastic cover is to be collected in a trough installed on each basin. Flow conditions and the level of saline water in the basins are controlled to resume optimum performance of operation. Whatever the evaporative process might be, the necessary end product of the evaporative seqquence would be a denase magnesium solution. The concentrated solutin is then dried ( dehydrated to yield granualrs of anhydrous magnesium chloride of approximate composition MgCl2. 3/2H2O, which contains about 74% MgCl2. 2. Electrolysis of Magnesium Chloride : Production of Magnesium metal The granuals of anhydrous magnesium chloride are fed to electrolytic cells, which contan a fused salt mixture that consists of : MgCl2, CaCl2,NaCl in the ratio of 20%, 20%and 60% respectively.the cells are fuelled by a high-amperage direct current. They are made of steels, which form the cathode, while the anodes are made of artificial graphite. Electrolysis takes place at 6-7 volts and 100,000 amps. Molten magnesium rises to the top of the cell, collected then cased into ignots of 99.8 % purity, or alloyed with zinc or other metals. The estimated materials and energy requirements for the production of one ton of magnesium are as follows [3] :- Magnesium Chloride 4.2 ton Natural gas (fuel) 36,000 cu.ft Electrical Energy 18,500 Kwh Electrodes 0.10 ton Renewable energy sources such as solar and wind could be utilized as a source of energy for the electrolysis process. Off-peak electricity is also a promising input in this regard. It is worth-mentioning that the production of magnesium from the Dead Sea is taking place industrially by the electroytic decomposition of Carnalite salt ( KCl.MgCl2.6H2O) [4]. 3. Hydrogen Production Using Galvanic-Electrolytic Cell The dissociation of water is a non-sponentaneous reaction and needs energy to split water producing hydrogen and oxygen (4.8 kwh / one cubic meter hydrogen). This energy is supplied in-situ by another sponeteneous rection using galvanic cells as explained next. Galvanic cells, also called voltaic cells allow us to harness a flow of electrons in a redox reaction to perform useful work. In other words, they are defined as electrochemical cells with positive potential that allows chemical energy to be converted to electrical energy. In this process, it is anticipated to utilize a magnesium electrode (anode) along with an iron electrode (cathode) to construct a galvanic cell, in which seawater is electolyzed. Once the cell is operated by connecting the two electrodes by a conductor from outside, a flow of electrons will take place from the higher electron concenmtration (higher negative potential) to the lower electron concentration (lhigher positive potential). This setup establishes what we may call it galvenic-electrolytic cell. A schematic diagram is illustrated in Fig.3. This will supply the energy necessary to electrolyze the seawater with a voltage diffeence of about 1.93 volt producing the hydrogen. The reactions taking place in our system when electrolyzing seawater- are classified into two catogeries : 1- Reactions responsible for the e m f generation. 2- Reactions underlying the electrolysis process. 2/6

3 The basic equations that represent these reactions, in general,are summarized as follows : ++ At the anode, Mg metal Mg + 2 e (1) These electrons generated by the formation of metallic ions at the anode, have passed through the conducting wire to the surface of the cathode. Here, they restore the electrical balance of the system by reacting with and neutralizing the positive ions of H ( remember H2O molecule is made up of H positive ions and OH negative group). Hydrogen ions are reduced to hydrogen atoms which combine to give hydrogen gas. Accordingly, the electrochemical reaction at the cathode is hydrogen generation : 2 H2O + 2 e 2OH + H2 (2) On the other hand, the electrochemical reaction at the anode is oxygen evolution : 2 OH H2O + ½ O2 + 2 e (3) The overall rection describing water electrolysis in the galvenic-electrolytic cell is given by : H2O H2 + ½ O2 (4) It should be mentioned that in the electrolysis of saline water, additional reactions would take place. Most important is the formation of Mg Cl2 and Cl2 gas (because of the presence of the chlorides in seawater)[5]. In addition, Mg(OH)2 is produced. SMALL-SCALE EXPERIMENTAL RESULTS To demonstrate the feasibility of hydrogen production using the galvenic-electrolytic cell, a small experimental set-up was constructed using Mg /Fe electrodes.the following is a sample of some typical results (6) :- Time (minutes) Hydrogen (cubic cm) Volt It could be observed from the above results, that polarization takes place at the anodes, which causes decrease in the current. DISCUSSION and CONCLUSIONS 1. In the proposed system, magnesium chloride was first recoverd from saline sources using solar energy. 2. Magnesium metal was next produced by the electrolysis of the anhydrous salt. 3. By building a galvanic-electrolytic cell utilizing magnesium metal, electrolysis of sea water was accomplished generating hydrogen as the end pruduct for energy carrier. 4. It has been reported in the literarture that the current output of magnesium is about 335 amp-hrs per one pound of magnesium consumed. 5. The proposed system could be compared with the Pacheco hydrogen generator in which Mg/Al electrodes were used [7]. 6. The system could be tailored to the needs of power plants to utilze the off-peak ouput as a mean of storing this surplus energy. REFERENCS 1. M.A. Kettani and H.K. Abdel-Aal, «Production of Magnesium Chloride from Brines using Solar Energy», Proceedings of the 4th International Sympsium on Fresh Water from Sea, Vol. 2, p.509, Heidelberg (1973). 2. H.K. Abdel-Aal, «Projected Economics of a New Magnesium Production Process», Int. J. Hydrogen Energy, 7(5),429 (1982). 3. Metal Statistics, American Metal Market, FairChild Publications, New York, N.Y., (!988). 4. Internet : 3/6

4 5. H.K. Abdel-Aal and I.A. Hussein, «Parametric Study for Saline Water Electrolysis», Int. J. Hydrogen Energy, 6(18),pp (1993). 6. M. Al-Sahli et al, Co-op Senior Project, Dept of Chem. Eng., KFUPM, Dhahran, Saudi Arabia (1995). 7. Pacheco Hydrogen Co-Generator, World Hydrogen Energy Conference Exihibt, Hawaii, (1992). P. S. S. PROCESS No 1 Electrolysis of Magnesium Chloride PROCESS No 2 Hydrogen Production by Galvanic- Electrolytic Cell PROCESS No 3 Fig.1 The Three Processes Involved in the Proposed Energy System 4/6

5 Fig. 2 Preferntial Salt Separator Inlet of Saline Water Deposited Mineral Salts 5/6

6 OXYGEN HYDROGEN Mg Anode Saline Iron Water Cathode Fig. 3 Shematic Illustration of Galvenic-Electroltic Cell 6/6

Rusting is an example of corrosion, which is a spontaneous redox reaction of materials with substances in their environment.

CORROSION WHAT IS CORROSION? Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Rusting is an example of corrosion, which is a spontaneous