carbothermal reduction of alkali hydroxides using concentrated solar energy

Energy 26 (2001) 441–455www.elsevier.com/locate/energy

Carbothermal reduction of alkali hydroxides usingconcentrated solar energy

Michael Epstein a,*, Amnon Yogev a, Chengcai Yao b, Alexander Berman a

a Solar Research Facilities Unit, Weizmann Institute of Science, Rehovot 76100, Israelb Climate Control Systems, Visteon Corporation, Plymouth, MI, USA

Received 7 September 2000

Abstract

The reduction of hydroxides of various alkali metals (i.e., Na, K and Li with carbon) using concentratedsolar radiation at high temperatures in the range of 900–1600°C results in the production of CO, H2 andthe alkali metal. These reactions are highly endothermic; for instance, C+LiOH→Li+0.5H2+CO requires523 kJ/mol (at 298 K). The reaction is performed in two basic stages. In the first stage, at a temperaturerange of 900–1300°C, the carbonate of the alkali metal is formed as an intermediate compound. In thesecond stage, at slightly higher temperatures in the range of 1200–1600°C, the carbonates are decomposedand reduced to the metal element and additional CO. The metal element can be reoxidized with water andthen produce additional hydrogen. The hydroxide is recovered and recycled. The metal can also be usedas a chemical, fuel or as an intermediate material for production of other energy-intensive metals, such asmagnesium. Thermodynamic calculations and experimental results, which verify this hypothesis, arepresented. Potential applications and advantages of the process are discussed. 2001 Elsevier ScienceLtd. All rights reserved.

1. Introduction

Concentrated solar energy can be converted to chemical energy via high-temperature endo-thermic reactions for storage, transportation and conversion to fuels [1]. Various examples havebeen demonstrated, e.g., reforming [2,3], thermochemical reduction of metal oxides such as ZnO[4,5], and others. The present paper describes the work that has been done on a new family ofreactions [6], where alkali hydroxides are reduced to the elemental metals by carbon accordingto the following general equation:

* Corresponding author. Tel.: +972-8-934-3804; fax: +972-8-934-4117.E-mail address: [email protected] (M. Epstein).

0360-5442/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.PII: S0 360- 544 2(01 )000 10- X

442 M. Epstein et al. / Energy 26 (2001) 441–455

C�MOH→M�CO�0.5H2, (1)

where M can be Na, K, Li or their combinations. The reaction is carried out at the temperaturerange of 1000–1600°C, depending on the choice of M. The gaseous products, which includeprimarily the alkali metals and CO/H2 mixture, can be cooled to condense the metal and separateit from the synthesis gas mixture. CO/H2 can be combusted directly in a gas turbine to generateelectricity or can be converted by a shift reactor to CO2 and an additional mole of H2, as shownin Fig. 1.

The alkali metal from Eq. (1) can find a wide range of applications in industry. For example,a potential application lies in the displacement of valuable metals from their inorganic compounds,such as magnesium from its chloride:

2Na�MgCl2→Mg�2NaCl, �Ho298 K��395.2 kJ/mol. (2)

This reaction is exothermic and can be performed at relatively low temperatures in the rangeof 150–250°C. Note that, in industry, magnesium is produced by electrolysis of molten salt athigh temperatures (MgCl2→Mg+Cl2). This process is energy-intensive due to poor efficiency and,moreover, the chlorine gas is harmful to the environment. In cases where MgCl2 is not available,carnallite (KCl·MgCl2·6H2O) can be used as well.

The alkali metal can also be reoxidized as follows:

M�H2O→MOH�0.5H2. (3)

The heat of this exothermic reaction can be recovered for generating steam. The hydrogen can

Fig. 1. Carbothermal reduction of alkali hydroxides as an energy-storage process.

443M. Epstein et al. / Energy 26 (2001) 441–455

Fig. 2. A two-step steam gasification of carbon.

be utilized as a chemical or as a fuel, for instance, in fuel cells. Particularly, by applying theabove reaction, the hydroxide can be recovered and recycled. As can easily be seen, Eqs. (1) and(3) form a closed cycle (Fig. 2), with water and carbon as the input and the CO/H2 mixture asthe product.

Eq. (1) is highly endothermic, with the enthalpy of reaction being 523 kJ/mol for the reductionof LiOH, 414 kJ/mol for NaOH, and 391 kJ/mol for KOH. Therefore, the benefit of using solarthermal radiation as the heat supply to the reaction is obvious: solar energy is transformed intoa storable and transportable form. The effectiveness of a solar-driven thermochemical processmay be best described by the renewable energy content of the products, which is proportional tothe enthalpy of reaction.

Table 1Energy inputs

Reaction �Ho298 K (kJ/mol) Solar energy contribution

C+H2O→CO+H2 131 0.33C+LiOH→Li+0.5H2+CO 523 1.33C+NaOH→Na+0.5H2+CO 414 1.05C+KOH→K+0.5H2+CO 391 0.99

The solar energy contribution to Eq. (1) is more than three times that for the solar gasificationprocess, as shown in Table 1. The solar contribution is defined here as the ratio between theenthalpy of the reactions listed in Table 1, added by solar energy, to the enthalpy of combustionof carbon with oxygen, which is �Ho

298 K=�393.5 kJ/mol. The source of carbon for Eq. (1) canbe coal or biomass. The coal or biomass can be pyrolyzed first using solar energy to remove thevolatile materials.

This paper presents the thermodynamic analyses of the process, followed by a detailed descrip-tion of the laboratory and solar reactor designs, testing procedures and results. NaOH has beenchosen for demonstration purposes.


2. Thermodynamic considerations and solar contribution

The thermodynamic equilibrium as a function of temperature of Eqs. (4)–(6):

LiOH�C→Li�0.5H2�CO, �Ho298 K�523 kJ/mol, (4)

NaOH�C→Na�0.5H2�CO, �Ho298 K�414 kJ/mol, (5)

KOH�C→K�0.5H2�CO, �Ho298 K�391 kJ/mol, (6)

is shown in Fig. 3(a)–(c), respectively [7]. In the case of LiOH [Eq. (4), Fig. 3(a)], Li2CO3 andLi2O are formed as intermediate species, with a maximum concentration at 500°C and 1250°C,respectively. However, above about 1600°C, the major products are CO, Li(g) and hydrogen. Inthe case of NaOH [Eq. (5), Fig. 3(b)], Na2O and Na2CO3 are formed as intermediate compoundswith a maximum concentration at 350°C and 950°C, respectively. Above 1130°C, they disappearcompletely and the products are CO, H2 and Na(g). Finally, the KOH system, Eq. (6), is shownin Fig. 3(c). Again, K2CO3 is formed with a maximum concentration at 900°C and decomposedcompletely at 1050°C. Above this temperature, the main products are CO, H2 and K(g).

In summary, each of the above reactions [Eqs. (4)–(6)] is executed in two steps. The first step,at a temperature range of 900–1300°C, involves mainly the conversion of the hydroxide to oxideor carbonate as follows:

0.5C�LiOH(l)→0.5Li2O(l)�0.5H2�0.5CO, �Ho298 K�137 kJ/mol, (7)

0.33C�NaOH(l)→0.33Na(g)�0.33Na2CO3(l)�0.5H2, �Ho298 K�77 kJ/mol, (8)

0.33C�KOH(l)→0.33K(g)�0.33K2CO3(l)�0.5H2, �Ho298 K�54 kJ/mol. (9)

The second step is carried out at slightly higher temperatures, in the range of 1100–1600°C,and involves mainly the following reactions:

0.5C�0.5Li2O(l)→Li(g)�0.5CO(g), �Ho298 K�386 kJ/mol, (10)

0.67C�0.33Na2CO3(l)→0.67Na(g)�CO(g), �Ho298 K�337 kJ/mol, (11)

0.67C�0.33K2CO3(l)→0.67K(g)�CO(g), �Ho298 K�337 kJ/mol. (12)

When the products of the above carbothermal process [for example, Eq. (5)] are cooled tocondense and separate the metal, the back reaction may occur. In addition, depending on the


Fig. 3. Thermodynamic equilibrium of (a) LiOH and carbon; (b) NaOH and carbon; (c) KOH and carbon.


source of carbon there always will be some CO2 in the product gas. The thermodynamics of thereverse reaction, with an initial CO to CO2 molar ratio of 9 (Na:CO:CO2=1:0.9:0.1), is shown inFig. 4. At a temperature near to the condensation point of sodium, the carbonate Na2CO3 is formedand as the liquid sodium is cooled further, the oxide Na2O is also formed. These undesirablesolids reduce the yield of the process and can cause practical problems to the equipment. Thekinetics is expected to play an important role here. Fast cooling and synthesis gas removal canminimize the problem, as well as high temperatures and surplus of carbon.

3. Experimental results and discussion

3.1. Laboratory tests

The reaction of carbothermal reduction of NaOH was first performed in a batch reactor usingan electrical furnace. A stoichiometric mixture of NaOH and graphite was introduced into a cru-cible inside the reactor and heated slowly until a temperature of 1250°C was reached. The reactorwas purged with argon prior to heating. The reactor was heated up at a rate of about 20°C/min.At this rate and due to the small mass of the reactants, there was only a small lag between thetemperature of the furnace and the reactants.

Experiments were also performed with separate stoichiometric mixtures of NaOH/C below1000°C and Na2CO3/C above 1000°C. Fig. 5 shows the experimental set-up. Argon was utilizedas a carrier gas to remove the gaseous products from the reactor. The sodium vapors were collectedin two different ways: through the bottom of the reactor and through its top. Nickel and platinumcrucibles were used. The reactor was made of a 304 stainless steel tube. The NaOH feed was inthe shape of small spheres of 0.5 mm diameter. It was mixed with either graphite powder or

Fig. 4. Thermodynamic equilibrium of the back reaction of CO2 and Na. The initial ratio of Na:CO:CO2=1:0.9:0.1.


Fig. 5. Schematics of the laboratory experimental set-up of the reaction between NaOH and carbon.

graphite particles of 1 mm size (the graphite is manufactured by BDH; it contains 99.5% particleswith maximum limits of impurities: residue — 1% weight, loss on drying at 120°C — 0.5%,soluble matter in ethanol — 0.2%). Charcoal particles of 0.2–0.5 mm size were also employed(the charcoal is manufactured by Fluka: loss on drying at 110°C — �5%, soluble matter inHCl — �2%, soluble matter in water — �0.5%, minimum carbon content — 92% weight).

Typical results are shown in Fig. 6. Below 800°C, the reaction was slow. The gas yield reachesa peak at 910–930°C and as soon as the NaOH was consumed, the gas flow rate was sharplyreduced. A second peak of gas yield was obtained at 1120°C. Gas chromatograph analysis shows

Fig. 6. Gas yield as a function of temperature of the NaOH/graphite stoichiometric mixture. Gas flow rate is relatedonly to the products, excluding the inert gas.


that the gas at about 930°C contains mostly hydrogen (H2/CO=99:1) and at �1150°C a typicalcomposition was H2/CO/CO2=7:88:5.

Temperatures were measured using K-type thermocouples sheathed with Inconel tubes. Thegases were analyzed by a Tracer-540 gas chromatograph with a 10 ft column consisting of astainless steel tube of 1/8 in. diameter filled with Supelco Carbosieve. The flow rate of the gaseswas measured by a conical float rotameter with 2% accuracy. The total volume of the gases (afterthe separation of the sodium) was measured by a gasometer with a capacity of 4 l and an atmos-pheric pressure and volume accuracy of ±0.1 cm3.

3.1.1. Study of the first stepThe first step of the reduction process, which proceeds at higher temperatures, was analyzed

according to the following equation:

C�3NaOH→Na2CO3�Na�1.5H2. (13)

The heating of the reactor was stopped immediately after the first peak of gas yield was reached.The solid residues in the crucible were analyzed by powder X-ray diffraction (Rigaku, RotaflexRU 200B with graphite monochromator and rotating anode, Cu Kα radiation). A typical resultcan be seen in Fig. 7. It can be seen that the residue was almost pure Na2CO3.

An interesting result was obtained when the kinetics of reduction by graphite particles wascompared with that by charcoal particles. The results are shown in Fig. 8. The reaction with thegraphite reached its peak at 930°C (Run II). However, the reaction with charcoal reached thepeak at a much lower temperature (Run I). At 650°C, it reaches the peak and at around 750°Cthe reaction is almost completed. The charcoal is much more reactive than the graphite. In bothcases the product gas contained primarily hydrogen (�99%) and the balance is CO and CO2. The

Fig. 7. X-ray diffraction of the solid residue at the outlet of the reactor.


Fig. 8. Gas yield as a function of temperature of the NaOH/charcoal mixture. Run I: C=charcoal; Run II: C=graphite,no argon flow.

results revealed in Fig. 8 were obtained with the upstream reactor. However, experiments donewith the downstream reactor show a similar tendency.

3.1.2. Study of the second stepThe second step of the process is developed according to:

C�0.5Na2CO3→1.5CO�Na. (14)

This reaction was studied at a temperature range of 1000–1250°C. The maximum yield wasobtained at 1130–1140°C. The entire carbonate was consumed. However, the Na yield was onlyabout 30% because of the back reaction [Eq. (18)]. To understand the mechanism of Eq. (14),the decomposition of sodium carbonate was studied.

Thermogravimetric analyses of the decomposition of Na2CO3 (in an environment of argon anda heating rate of 10°C/min) show that below 950°C the decomposition is negligible (see Fig. 9).Appreciable decomposition starts at about 1120°C. At 1200°C, 20% of the carbonate is decom-posed. This happened with continuous removal of the CO2 from the reactor. In case of a closedreactor and no removal of the product gases, the composition was lower. At 1200°C, only 9% ofthe initial carbonate was decomposed. Therefore, it is assumed that Eq. (14) proceeds accordingto the following steps:

� Decomposition of Na2CO3

Na2CO3→Na2O�CO2. (15)

� Boudouard reaction

C�CO2→2CO. (16)


Fig. 9. Results of the thermogravimetric analysis of Na2CO3 decomposition.

� Reduction of sodium oxide

C�Na2O→2Na�CO. (17)

Consequently, since the thermal decomposition of the carbonate [Eq. (15)] is achieved only ata higher temperature relative to its reduction [Eq. (4)], it is reasonable to assume that once theoxide is formed, its reduction is fast. Thermodynamic equilibrium composition of Eq. (17) showsalso that at �1130°C the reaction is completed.

3.1.3. Back reactionThe thermodynamic equilibrium shows that sodium and CO react at high temperatures (above

1100°C typically; see Fig. 4). Although the product gases from the second step of the processwere cooled rapidly, solid sodium carbonate was found on the surface of the cooler. This carbonateis apparently the product of the reaction between CO2 and sodium at temperatures below 500°C,according to the following equation:


3CO2�4Na→C�2Na2CO3. (18)

The CO2 may be formed from decomposition of the carbonates [Eq. (15)] and the Boudouardreaction [Eq. (16)]. Eq. (18) must be suppressed in order to increase the efficiency of the processand avoid solid sediments downstream the reactor, resulting in plugging and encrustation of thecondensing apparatus. This reaction is the main difficulty in obtaining economic yields in thecarbothermal production of sodium [8].

Three actions were performed. The first action was degassing of the carbonaceous material ata temperature of 600–700°C during the preparation of this material. The second action was touse a surplus of carbon (20–30%), above the requirement for the stoichiometric composition, todecrease the Boudouard reaction. These two methods were performed by introducing first intothe reactor the carbon particles, heating and degassing them by pumping all the volatile materialsreleased during this preheating stage. Then, NaOH was added gradually and in a controllablemanner into the surplus of carbon. NaOH can be added as a liquid at 320–350°C or as a solid,which will melt immediately after the contact with the hot carbon. This way of feeding the NaOHenables also the control of the reaction rate, which becomes vigorous at 1200–1300°C. In thecase of feeding molten NaOH, a separate electrically heated container, needle valve and feedingtube were built and connected to the reactor. The third action was fast quenching, where separationand removal of the liquid sodium are accomplished. A water-cooled stainless steel trap was usedin the laboratory, downstream the reactor. In a larger-scale system, a direct contact condenser,where the vapors are passed through a molten pool of sodium, can be a preferable arrangement [9].

3.2. Solar tests

Three types of solar reactor have been tested at the solar furnace of the Weizmann Institute ofScience (WIS), Rehovot. The first reactor is shown in Fig. 10. It was made of a stainless steel

Fig. 10. A tubular solar reactor inside the cavity.


tube of 20 mm inside diameter, placed inside a well-insulated cavity. Concentrated solar radiationentered the cavity through a side aperture of 7 cm diameter. The reactor was loaded with a typicalmixture of 1.8 g of sodium hydroxide and 0.5 g of graphite particles (molar ratio of 1.08, slightlyabove the stoichiometric ratio of 1.0). The reactor was heated up to 1200°C at a rate of about30°C/min, under a flow of argon at an atmospheric pressure. The solar power input to the cavitywas 2 kW.

The product gas volumetric flow rate and its composition were monitored and recorded as afunction of the reaction time. The gases were cooled down using a water-cooled finger. Thesodium was trapped in a paraffin oil trap. The gases started to merge out at about 675°C. Amaximum rate is obtained at 800–925°C. At �925°C, sodium began to be trapped in the paraffinoil trap. After sodium removal, the typical gas composition was primarily hydrogen. A typicalcomposition ratio is H2:CO=90:5. By increasing the temperature to approximately 1125°C, theproduct gas contains primarily CO, and a typical composition is CO:H2:CO2=88:7:5.

Powder X-ray diffraction analysis of the solid gray material found on the cooling finger andthe reactor outlet shows that this material contains primarily Na2CO3, formed due to the reversereaction between Na and CO2. At higher temperatures the reaction was fast, large amounts ofgases were evolved, and it was difficult to control the reaction by adjusting the size of the cavityaperture using a small door. Therefore, the procedure of feeding the reactants was modified asdescribed hereinafter.

The other two solar reactors were directly irradiated. One reactor was made of a hollow solidgraphite cylinder (see Fig. 11), which served both as a reactor container and a reducing agent.The cylinder dimensions were: outside diameter — 40 mm, inside diameter — 30 mm, length —

Fig. 11. A solar reactor with graphite crucible and molten NaOH feeding.


60 mm. This graphite crucible was housed inside a quartz tube of 65 mm diameter, which wasdirectly irradiated by concentrated solar radiation. The graphite was sealed against a stainlesssteel tube that contained a cooled trap for product sodium. Molten NaOH was stored at 350°Coutside the reactor and dropped into the graphite crucible. The reactor was first purged by astream of argon and heated gradually at a rate of 30°C/min. At 400°C, the flow of argon wasstopped and the heating continued until the temperature of 1200°C was reached. The heating wasdone in a solar furnace at an average solar flux of 550 kW/m2. Once the temperature of 1200°Cwas reached, 1–2 g/min of molten NaOH at 350°C was added as drops into the graphite crucible.Gases and sodium were immediately produced. The lifetime of the crucible was short; it crackeddue to a large temperature gradient between the front and the back side. The sealing with thecooler was also problematic and some sodium vapor escaped and attacked the quartz housing.During the short-time operation (a few moments), the NaOH was fully consumed. However, theNa yield collected at the cooling trap was only around 10%.

A modified reactor was designed and tested. A stainless steel tubular reactor was housed insidea quartz tube to avoid sodium vapors from reaching the quartz (see Fig. 12). The quartz tube wasevacuated to reduce convection losses and oxidation of the metal tube. Graphite particles wereloaded into the reactor and molten NaOH at 350°C was stored in a separate electrically heatedcontainer, which was connected to the center of the reactor with an electrically heated tube. Thegraphite particles were first heated to about 1200°C by exposing the lower part of the reactortube to the concentrated solar energy. Molten NaOH feed was regulated with a valve accordingto the volume flow rate of the product gas, as monitored at the outlet of the reactor. Sodium wascondensed in a water-cooled sodium trap located above the graphite particles.

This reactor arrangement provided the possibility to work at high temperatures in a controlled

Fig. 12. A modified solar tubular reactor with molten NaOH feeding.


way, abating the problem of corrosion of the construction materials of the reactor container.Nevertheless, it is recommended to use high nickel alloys for future work. The NaOH reactedimmediately with the carbon and was consumed completely at the mentioned temperatures. About30% of the theoretical sodium was trapped on the surface of the cooler. The remaining sodiumwas converted to sodium carbonate.

4. Conclusions

1. A new family of reactions, i.e., carbon reduction of hydroxides to produce alkali metal andsynthesis gas using concentrated solar radiation, has been proposed as a candidate process forconversion of solar energy to fuels, chemicals and storage of solar energy. The feasibility ofthis process is demonstrated by carbon reduction of NaOH performed in laboratory, as well ason the WIS solar furnace.

2. Both thermodynamic analyses and experiments show that the reaction between carbon andNaOH falls in two major steps. In the first step, Na2CO3 is formed along with sodium andhydrogen at around 950°C. At higher temperatures, Na2CO3 reacts further with carbon to pro-duce more sodium and CO. The reaction proceeds vigorously at a temperature of approximately1050°C. It releases a large volume of product gases, which create a sudden increase of pressurein the reactor and causes difficulties in controlling the reaction. Separating the reactants, heatingthe carbon first to the operation temperature and adding molten NaOH in a controlled wayfrom a separate container can solve this problem. In addition, the corrosion issue of this reactorcontainer is alleviated.

3. Some Na2CO3 was found on the cooling finger and the reactor outlet. It was originated by theback reaction between Na and CO2. This reaction is kinetically controlled, since the carbonatewas found even when the carbothermal reaction was carried out at high temperatures whereonly Na2O and CO exist. Therefore, the CO2 level should be reduced if an efficient sodiumproduction is desired. High-temperature preheating of the carbon and preventing the use ofmetals in the system, which can catalyze CO2 formation caused by the Boudouard reaction(2CO→C+CO2), are means in this direction.

4. The possibility to perform the reduction reaction in two steps is advantageous because separ-ation of H2 (produced only in the first step) and CO is achieved. This is applicable for NaOHand KOH, but not for LiOH. In addition, the second step, the reduction of the carbonate, canbe treated as an independent process for obtaining the metal, if the carbonate is a more con-venient raw material (availability, cost, etc.).

Acknowledgements

We wish to thank D. Koebi Ehrensberger, of AAC Computers AG, Winterthur, Switzerland,for his helpful advice and assistance on behalf of this study.


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carbothermal reduction of alkali hydroxides using concentrated solar energy

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