Two-step water splitting thermochemical cycle based

ARTICLE IN PRESS

Energy 32 (2007) 1124–1133 www.elsevier.com/locate/energy

Two-step water splitting thermochemical cycle based on iron oxide redox pair for solar hydrogen production Patrice Charvina,, Ste´phane Abanadesa, Gilles Flamant, Florent Lemortb a

Processes, Materials and Solar Energy Laboratory (PROMES-CNRS,UPR 8521), 7 rue du four solaire, 66120 Odeillo—Font Romeu, France b CEA, BP 17171, 30 207 Bagnols-sur-Ce`ze Cedex, France Received 10 April 2006

Abstract This study deals with solar hydrogen production from the two-step iron oxide thermochemical cycle (Fe3O4/FeO). This cycle involves the endothermic solar-driven reduction of the metal oxide (magnetite) at high temperature followed by the exothermic steam hydrolysis of the reduced metal oxide (wustite) for hydrogen generation. Thermodynamic and experimental investigations have been performed to quantify the performances of this cycle for hydrogen production. High-temperature decomposition reaction (metal oxide reduction) was performed in a solar reactor set at the focus of a laboratory-scale solar furnace. The operating conditions for obtaining the complete reduction of magnetite into wustite were defined. An inert atmosphere is required to prevent re-oxidation of Fe(II) oxide during quenching. The water-splitting reaction with iron(II) oxide producing hydrogen was studied to determine the chemical kinetics, and the influence of temperature and particles size on the chemical conversion. A conversion of 83% was obtained for the hydrolysis reaction of non-stoichiometric solar wustite Fe(1y)O at 575 1C. r 2006 Elsevier Ltd. All rights reserved. Keywords: Iron oxides; Hydrogen; Water-splitting; Thermochemical cycle; Solar concentrated energy

1. Introduction Hydrogen is the most promising energy carrier for the next decades (in addition to electric power). Most of the specialists predict a substitution of fossil fuels by hydrogen used in fuel cells in the transportation sector [1,2]. This new energy carrier offers many advantages because it is a nonpolluting fuel and it is renewable if produced with water and solar energy. But new environmentally friendly processes must be developed to produce low-cost hydrogen at large scale. Solar-driven thermochemical water splitting cycles (TWSCs) constitute a promising high-efficiency pathway for hydrogen production from water and solar power (abundant and clean resources). When using directly high-temperature thermal energy, the expected efficiencies are potentially higher than the intermediate conversion of heat to electricity (currently 33% efficiency) combined with the use of electric power to electrolyse water (about 70% Corresponding author. Tel.: +33 4 68 30 77 31; fax: +33 4 68 30 29 40.

E-mail address: [email protected] (P. Charvin). 0360-5442/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2006.07.023

efficiency). The resulting overall efficiency of hydrogen production using electrolysis (from heat to hydrogen) yields to 20–25% (36% with improved technologies) [3]. The theoretical efficiencies reported for the cycles currently studied in the nuclear sector (I-S and UT-3) are in the range 35–50% [4]. Based on previous screening studies [5,6], two- and threestep cycles are the most suitable for a coupling with solar energy. Two-step cycles proceed with the endothermic reduction of a metal oxide at high temperature (above 1300 1C). Then, the reduced oxide or the metal reacts directly with water at lower temperature to generate hydrogen, which reforms the initial metal oxide. A twostep cycle based on metal oxide redox pair can be written as MOox ! MOred þ 12 O2

ðT41300 CÞ,

MOred þ H2 O ! MOox þ H2

ðTo1000 CÞ.

(1) (2)

Many studies [7,8] address the ZnO/Zn cycle proposed by Bilgen [9]. But other metal oxides are possible and Fe3O4/ FeO may be a good candidate according to Nakamura [10]

ARTICLE IN PRESS P. Charvin et al. / Energy 32 (2007) 1124–1133

and Steinfeld et al. [11]. Other metal oxides involving nickel [12,13], manganese [14], cobalt [15], or zinc [16–18] were added to iron oxide to form ferrites which can be reduced at lower temperatures than magnetite (Fe3O4), while the reduced metal oxide is still capable of achieving the water splitting reaction. Both Co3O4/CoO and MnO2/MnO redox pairs could be proposed regarding their low reduction temperatures. But, thermodynamic calculations [14,19,20] showed H2 yields less than 1% for a two-step cycle involving these oxides. In the previous studies, reaction temperature of iron oxide reduction was determined from thermodynamics [21] and from experiments [22–24]. The decomposition is appreciable only above the melting point of Fe3O4. Tofighi et al. [22] showed that 80% conversion of a 0.8 g sample was reached after 5 min at 2000 1C under argon atmosphere (the conversion can reach 40% in air). Weidenkaff et al. [25] pointed out that the exothermic reaction of wustite with water is influenced by non-stoichiometry and morphology of the parent wustite phases and by the reaction temperature. However, no quantitative analysis of hydrogen produced is available in order to determine the kinetic and chemical conversion of wustite hydrolysis. The iron oxide-based cycle is particularly attractive since it involves less complex chemical steps and reactants than nuclear-based cycles (resulting in less irreversibility and potentially higher cycle efficiency). In addition, it uses noncorrosive materials, has solid–gas reactions, and avoids the problem of recombination reaction during quenching encountered with volatile metal oxides such as zinc or cadmium oxides [22,26]. A key feature is that non-volatile iron oxide systems allow the continuous removal of the evolved oxygen from the condensed phase during the solar reduction step, thus high reduction rates are expected. The theoretical energy required to produce 1 mol of hydrogen is the sum of the energy needed to heat 1 mol of magnetite from 600 to 2100 1C (446.51 kJ), the enthalpy of endothermic reduction (242.84 kJ), and the energy needed to heat water from 25 to 600 1C (64.9 kJ). Based on these temperature levels and on the HHV of hydrogen (286 kJ/ mol), the theoretical energy yield of Fe3O4/FeO cycle is 37.1% [27]. This study focuses on the two-step iron-oxide-based cycle. The objective of the study is to detail the operating conditions for carrying out the cycle and to report on quantitative laboratory experimental data in order to demonstrate the potential of this cycle. First, a detailed thermodynamic analysis of the two-step cycle was performed in this paper. Then, an experimental study was achieved to determine the effect of pressure and temperature on the endothermic solar reduction reaction. Large quantities of solar wustite were prepared and the hydrolysis reaction was studied in detail. The hydrogen produced was analysed quantitatively versus time to determine the reaction kinetics and the influence of temperature and particles size on hydrolysis reaction conversion.

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2. Thermodynamic analysis Thermodynamics of iron oxide systems including thermal reduction and subsequent re-oxidation with steam was performed. Calculations predicted the stable species at equilibrium versus operating conditions (T, P, chemical composition), the equilibrium degree of conversion, and the possible secondary reactions (producing unexpected byproducts). Results were compared with those obtained from experiments. 2.1. High-temperature reduction reaction The solar reduction of iron(III) oxide (Fe2O3) into Fe3O4 and FeO was studied. Thermodynamics was based on the minimisation of the Gibbs free enthalpy and was realised with the HSC Chemistry 5.11 software [28]. Using a thermodynamic database (enthalpy, entropy, heat capacity, etc.) of compounds (in condensed and gas phases), the equilibrium composition module calculates the quantities of stable compounds under given conditions (temperature, pressure, quantities of reactants). The equilibrium degree of conversion, defined as the ratio between the quantity of products obtained and the quantity of products which could be obtained if the reaction was complete (Eq. (4)), can be estimated from the equilibrium results (Fig. 1). The following definition of the degree of conversion was used Fe3 O4 ! 3 FeO þ 12 O2 , Zchemical ¼

nFeO , 3 nFe3 O4

(3) (4)

with nFeO ¼ mol of FeO obtained, nFe3 O4 ¼ mol of Fe3O4 introduced. The input conditions were 1 mol of metal oxide Fe2O3, a total pressure of 1 bar and an inert N2 atmosphere (100 mol) to prevent re-oxidation (low pO2 ). The chemical conversions (Zchemical) obtained at equilibrium versus

Fig. 1. Equilibrium composition results of the Fe/O system (100 mol of N2, 1 mol of Fe2O3, P ¼ 1 bar).


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temperature and pressure are presented in Table 1. In addition to FeO, the formation of non-stoichiometric wustite phases Fe1yO (mainly Fe0.947O) is predicted by thermodynamics (Fig. 1). The amounts of Fe1yO and FeO were summed in the conversion calculation. The influence of pressure on the temperature of reaction producing FeO is noteworthy (Table 1). A total pressure decrease lowers the reaction temperature below 2000 1C, which leads to higher exergy efficiencies than at atmospheric pressure because of a reduction of radiation losses (T4 dependency). The thermodynamic results show the possible synthesis of iron(II) oxide from Fe2O3 thermal reduction under inert atmosphere. 2.2. Steam-hydrolysis reaction The hydrolysis step (steam reduction) is thermodynamically possible if the Gibbs free enthalpy change of the metal oxide redox pair is higher than that of water reduction. Thus, water splitting with FeO is only possible below 800 1C. Fe3O4 is not able to split water spontaneously, which means that a work input is necessary to run the reaction (DG40). Consequently, the reaction between FeO and water produces Fe3O4 which cannot react further with water to give Fe2O3. The theoretical conversion of the water splitting reaction decreases with temperature according to thermodynamics Table 1 Chemical conversions given by thermodynamics (N2 atmosphere) Reactant

Product

Reaction temperature (1C)

Total Chemical pressure (bar) conversion (%)

Fe2O3 Fe2O3 Fe2O3 Fe2O3 Fe2O3

Fe3O4 FeO FeO FeO FeO

1250 2100 1950 1770 1600

1 1 0.1 0.01 0.001

60 98 95 92 90

Fig. 2. Equilibrium composition results of the Fe/O/H system (3 mol of FeO, 1 mol of H2O, N2 atmosphere, P ¼ 1 bar).

(Fig. 2). This thermodynamic result can be strongly modified by kinetic limitations. Thus, thermodynamics must be validated by experiments.

3. Experimental study 3.1. High-temperature reduction 3.1.1. Experimental set-up The endothermic reaction of Fe3O4/FeO cycles occurs at high temperature. In solar thermal power, one concentrates direct solar radiation to produce high-temperature thermal energy. Commercial powder of pure Fe2O3 (Prolabo, 99.9%, size 100 mm) was used. This powder was directly processed or preliminary compacted into pellets. The sample (about 1 g weight) was placed on a water-cooled holder and was heated by direct concentrated solar irradiation. It was studied either in air or in an inert (nitrogen) atmosphere. For experiments requiring a controlled gas pressure, a glass vessel was placed on the support and an inert gas (static or flowing N2) was used to provide a controlled atmosphere (vacuum or reduced pressure) around the sample (Fig. 3). A vacuum pump was connected at the reactor outlet to control the total pressure in the vessel and to ensure a continuous inert gas flow inside the reactor. The gas flow drift the oxygen released from the sample to the reactor outlet. Thus, reoxidation of wustite into magnetite was prevented by eliminating oxygen released during reduction. The whole device was set at the focus of a solar concentrating system consisting of one reflector (flat heliostat) and a concentrator (1.5 m diameter, peak flux density 16 MW/m2). A sample quenching (about 102 1C/s [23]) was obtained by withdrawing rapidly the device from the focus. The heating was controlled by adjusting vertically the position of the reactive solid sample with respect to the focal plane.

Fig. 3. Experimental set-up at the focus of a solar furnace.

ARTICLE IN PRESS P. Charvin et al. / Energy 32 (2007) 1124–1133 Calculated temperature

Temperature (°C)

1700 1600 1500 1400

Table 2 Determination of emissivities of solid and liquid phases using the melting point of Fe3O4 Phase

T measured (1C)

IBB(T) (W/ m2/sr1)

Melting point Tm (1C)

I BB (Tm) (W/m2/sr1)

Emissivity

Solid Liquid

1391.3 1277.1

7.74 109 6.68 109

1597 1597

9.72 109 9.72 109

0.79 0.69

Measured temperature

1300 1200 Heating due to lift

1100

Emissivity change due to melting

1127

1000 2.2

2.4

2.6 2.8 3 3.2 3.4 Heating time (minutes)

3.6

3.8

4

Fig. 4. Temperature measurements of a Fe2O3 pellet heated under an inert atmosphere by a solar furnace.

3.1.2. Temperature measurements The temperature of the sample surface was measured by an infrared pyrometer fixed at the centre of the parabolic mirror. To avoid solar reflectance, the operating wavelength of the pyrometer was 5.2 mm (bandwidth of the filter: 4.9–5.5 mm). At this wavelength, the optical mirrors act as a cutting filter so that their reflectivity is nil (low FIR reflectance of the concentration optics), which eliminates the solar contribution. Thus, the pyrometer measures only the IR radiation emitted from the sample and the temperature measurement can be considered as ‘solarblind’ [29]. The measurement was performed through a vertically centreed CaF2 window (fluorine) located at the top of the reactor (Fig. 3). The transmissivity of the fluorine window is 0.95 at 5.2 mm and a temperature correction must be made (+35 1C at 1500 1C). It is very difficult to control heat provided to the sample at a temperature level of about 1500 1C with a solar furnace which can easily reach higher temperatures (maximum concentrated solar flux density of 1600 W/cm2). First, the sample was placed 10 mm below the focal plane to be heated slowly and to avoid thermal shock. Then, the experimental set-up was progressively lifted to reach the focal point where the solar flux density is maximum. Temperature of the sample increased rapidly since heat in excess was provided and sample melting was obtained. Emissivity correction must be realised to determine the real surface temperature of the sample. In order to determine the emissivity of liquid and solid, the material melting point was used as a reference. When magnetite melts, a sharp temperature decrease was measured due to emissivity change (Fig. 4). At this time, the real temperature (constant and equal to melting point) and the blackbody temperature (measured by pyrometry) are well known. Respective spectral intensities were calculated with the Planck law (Table 2) I BB l ðTÞ ¼

2hc2 l5

, (5) ðhc=lTÞ ek 1 where h is the Plank’s constant (6.6262.1034 J/s), c is the velocity of light (2.9979.108 m/s), k is the Boltzmann’s

5000

FeO

4500 Intensity

2

Fe3O4

4000 3500 3000

N2 atmosphere

2500 2000

Air atm osphere

1500 1000 25

30

35

40

45

50

55

60

65

Angle 2θ (°) Fig. 5. XRD pattern of solar-reduced Fe2O3 under air or N2 atmosphere at 1700 1C.

constant (1.3807.1023 J/K), and l is the average wavelength of the pyrometer filter. Emissivities of liquid and solid phase, given in Table 2, can be estimated from Eq. (6) BB I BB l ðTÞ ¼ I l ðT m Þ ¼ l I l ðT m Þ,

(6)

where T is the temperature measured by pyrometry, el is the spectral emissivity of the sample, IBB l (T) is the spectral intensity of a black body at T, Il(Tm) is the spectral intensity of the sample at the melting point (Tm), and IBB l (Tm) is the spectral intensity of a black body at Tm. The temperature measured by pyrometry (for e ¼ 1) and the sample emissivity were used to calculate the real temperature (Fig. 4) 3.1.3. Thermal reduction of Fe2O3 Iron(III) oxide (hematite) was used as the initial raw material in the Fe3O4/FeO cycle. The solar reduction step aims at producing iron(II) oxide (FeO) as pure as possible. Then, FeO (wustite) is used to split water (H2 production), and FeO re-oxidation with steam produces Fe3O4 (oxidation to Fe2O3 cannot be reached). Next cycles will utilise the cyclic thermal reduction of Fe3O4 and water hydrolysis with FeO to produce H2 and regenerate Fe3O4. X-ray diffraction (XRD) analysis (CuKa radiation) shows that the thermal reduction of hematite (Fe2O3) under concentrated solar irradiation was obtained (Fig. 5). The mass fraction of wustite (Table 3) was determined from a calibration with known compositions of wustite and magnetite mixtures. Then, the chemical conversion into wustite was calculated from Eq. (4). The final products depend on atmosphere composition (air or N2). A mixture


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Table 3 Results obtained from XRD after heating Fe2O3 pellets at 17001C with a solar furnace Reactant

Atmosphere

Pressure (bar)

Heating time

Product

Mass fraction of wustite (%)

Chemical conversion (%)

Fe2O3 Fe2O3 Fe2O3 Fe2O3 Fe2O3 Fe2O3

N2 N2 N2 N2 N2 Air

0.1 0.1 0.1 0.8 0.8 0.8

30 s 1 min 2 min 2 min 5 min 15 min

Fe3O4+FeO Fe3O4+FeO FeO Fe3O4+FeO Fe3O4+FeO Fe3O4+FeO

85 93 100 91 95 82

95 98 100 97 98.5 93.5

(Fe3O4+FeO) was obtained under air whereas FeO alone (target product) was produced under nitrogen. The decomposition can be divided in two successive reactions in which magnetite (Fe3O4) appears as an intermediate according to the following reactions: 3Fe2 O3 ! 2Fe3 O4 þ 12 O2 ,

(7)

Fe3 O4 ! 3 FeO þ 12 O2 .

(8)

For each tested condition, the final product obtained was free of hematite. Reaction (7) was complete when hematite was heated over 1500 1C. The reduction of magnetite is more complex and the product obtained was never pure FeO but a mixture of non-stoichiometric wustite, which is in agreement with thermodynamic previsions presented previously (Fe0.909O, Fe0.945O, Fe0.947O). Previous studies about the iron oxide cycle [22–24] report the presence of Fe0.905O and Fe0.982O. As shown in Table 3, the reduction of magnetite under nitrogen at 0.1 bar requires about 2 min to reach complete conversion. For experiments carried out in air (atmospheric pressure), several heating durations were tested from 1 to 15 min. A weak fraction of the sample mass was lost because of vapourisation (0.35% after 5 min), which is not detrimental for the cycle. The conversion increased with time but it was impossible to reach a complete conversion into wustite in air (Table 3). The re-oxidation on the surface during quenching is responsible for the formation of Fe3O4 [23]. Thus, the mass fraction of FeO increases when surface to volume ratio decreases (up to 82% for a 5 mm diameter particle and decreasing for smaller particles because of surface re-oxidation). An inert atmosphere allows to operate with a slow quenching and to obtain a high conversion into wustite because of the low partial pressure of oxygen. 3.2. Water splitting reaction producing hydrogen 3.2.1. Experimental set-up and method The reduced iron oxide prepared with a solar furnace was used to test the water splitting reaction producing hydrogen in the cycle: 3FeO+H2O-Fe3O4+H2. Because of the low temperature of the hydrolysis reaction (below 800 1C according to thermodynamics), an electrical furnace was preferred to study the hydrogenproducing step. The experimental set-up was composed of

Fig. 6. Experimental set-up used for water-splitting reactions.

a tubular fixed-bed reactor placed in a vertical furnace. The reactor was made of AISI 316L stainless-steel. The bed of solid reactant was supported by a stainless steel grid (sieve void 100 mm) settled in the middle of the furnace (Fig. 6). The bed was composed of inert sand particles (diameter 200–300 mm) at the bottom and of the reactive powder of reduced iron oxide above. The sand layer served as a thermal buffer. It also stopped the fine particles of reactive iron oxide which could be entrained by the carrier gas through the grate. The solar reduced oxide was milled into a fine powder in order to increase its specific surface area and to enhance the contact with steam. Then, the powder was sieved and experiments were conducted with different samples of about 2 g with defined ranges of particle size. The temperature of the furnace was controlled and regulated precisely. The temperature of the bed of reactive particles was measured by a K-thermocouple and recorded by a data-acquisition system. Steam was generated upstream in a first electrical furnace fed with liquid water by a stainless-steel capillary connected to a peristaltic pump. The water input was known precisely with such a device and it was fixed at 12.5 mmol/min (0.225 ml/min). The steam produced was carried by an argon flow (220 Nml/min at 0 1C, 1 atm) through the reactive fixed bed. The outlet gas was cooled

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The total volume of hydrogen produced during an experiment was calculated by integrating the production curve from the starting time (t0) to the time for which the hydrogen mol fraction was down to 0.5% (t0.5) Z t0:5 RT V H2 ¼ F H2 dt, (10) P t0 where yH2 is the mol fraction of H2 in the outlet gas, Fi is the molar flow rate of i and V H2 is the volume of H2 produced at normal conditions (T ¼ 0 1C, P ¼ 1 atm). Before each experiment, the whole equipment was purged with argon to eliminate air which could react with the reactive wustite during the temperature rising (O2 oxidises FeO at 200 1C). Then, the fixed bed of particles was heated to the desired temperature under the fixed flow rate of argon. During the heating period, a slight amount of hydrogen was produced and detected by the onlineanalysis system. This weak H2 release was the result of the reaction between the metal oxide and trace of moisture adsorbed on the reactor walls or at the surface of the reactive particles. Once the fixed bed at the operation temperature (steady state), water was injected in the steam generator and the evolution of hydrogen was measured and recorded via a data-acquisition system. A slight temperature increase was measured in the fixed bed due to the exothermic reaction between iron(II) oxide and steam. Few seconds later, the hydrogen mol fraction in the off-gas increased rapidly to a maximum. The time lag between water injection and H2 detection corresponded to the addition of the sampling time and of the gas analyser response time (t ¼ 5 s). Then, the hydrogen mol fraction decreased during several minutes until it became negligible (Fig. 7). In any experiment, the hydrolysis reaction producing hydrogen was rapid during the first moments. After about 30 min, the hydrogen production became weak, which means that the reaction stopped because of the lack of reaction sites in the solid iron oxide. The same kind of global hydrogen-production profile was observed in [14].

14

1129

550

exothermicity

12

500

10 Hydrogen mole fraction Temperature

8 6

450 400

4 350

2 0 0

200

400

Temperature (°C)

down and sent to a gas-treatment device composed of a bubbler (water trap) and a gas dryer in order to obtain only a dry mixture of argon and hydrogen. The hydrogen mol fraction in the outlet gas was measured online and continuously by a gas analyser (catharometer ARELCO Catarc 10P, detection limit: 100 ppm; precision: 1% of full scale). The analyser used argon as reference gas since Ar was also used as carrier gas in the experiments. The thermal conductivities of the reference gas (Ar) and of the gas to analyse (Ar+H2) were compared to determine the mol fraction of H2 (the quantitative analysis requires a calibration which was previously achieved). The molar flow rate of hydrogen (FH2 ) was calculated from the measured mol fraction of H2 (yH2 ), knowing the molar flow rate of argon (FAr constant): yH2 F H2 ¼ F Ar . (9) 1 yH 2

Mole fraction of H2 in outlet gas (%)

P. Charvin et al. / Energy 32 (2007) 1124–1133

300 600 800 1000 1200 1400 1600 Time (seconds)

Fig. 7. Mol fraction of hydrogen and temperature versus time recorded during a reaction between 2 g of FeO (30odpo50 mm) and H2O (FAr ¼ 220 Nml/min).

3.2.2. Influence of the material-preparation mode The study of hydrogen-production step requires a great amount of FeO milled into fine particles. First, commercial FeO (supplied by CERAC, purity 99.8%) was used. Then, large quantities of wustite were prepared at the focus of a solar furnace to study the influence of the solar reduction on wustite reactivity. Results about hydrogen-production profile obtained with solar and commercial wustite were significantly different (Fig. 8). For solar wustite, the hydrogen mol fraction decreased sharply after reaching a maximum, whereas it decreased slowly and regularly for commercial FeO. Therefore, the final conversions obtained with commercial FeO were higher than for solar FeO under the same conditions (Table 4). At 575 1C, the final conversion reached 80% for commercial FeO and 50% for solar FeO (Fig. 9). However, the advantage of solar wustite lies in the high initial reaction rate (14% conversion per minute for solar sample and 10% per minute for commercial FeO), thereby lowering the necessary reaction time compared to that required for commercial FeO. As the main part of hydrogen is formed during the first moments of reaction (rapid oxidation of the surface area), the use of activated solar wustite is preferable even if the material is partially converted. Reaction kinetics observed for the two kinds of wustite are different, which suggests that process-controlling phenomena depend strongly on the material processing that governs properties. In all cases, the high reaction rate at the beginning of hydrolysis is due to the rapid oxidation of the surface area. The hydrogen evolution is temperature dependent and it is governed by a kinetic-limiting step. The reaction forms an oxide layer (magnetite) in the particle surface layers, which may cause a clogging of diffusion channels for the gaseous species (H2O and H2). Then, a mass-transfer limitation by diffusion occurs in the solid for commercial FeO because the mol fraction decreases slowly (and independently of temperature). For solar wustite, the hydrogen-production reaction runs faster initially and drops rapidly after reaching a maximum (11% for solar FeO and 6.5% for commercial FeO at 525 1C), which can


H2 molefraction inoutlet gas (%)

1130

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

FeOc, T= 480°C FeOc, T= 525°C FeOc, T= 575°C FeOs, T= 525°C FeOs, T= 575°C

0

50

100

150

200 250 Time (sec)

300

350

400

Fig. 8. Hydrogen-production profiles during the hydrolysis of FeO (particle size: 30–50 mm).

Table 4 H2 volumes and conversions obtained for different hydrolysis experiments Exp. no

Reactant

Mass (g)

Particles size (mm)

T (1C)

Reaction time (min)

Volume H2 (ml)

Chemical conversiona (%)

mmol H2/g of FeO

1 2 3 4 5 6 7 8

FeOs FeOs FeOc FeOc FeOc FeOc FeOs FeOs

2.35 2.35 2 2 2 2 2 2

100o dp o125 80o dp o100 125o dp o200 30o dp o50 30o dp o50 30o dp o50 30o dp o50 30o dp o50

675 675 575 480 525 575 575 525

32 40 28 29 50 44 28 18

17.3 72 71 70 138 191.2 112 60.9

7 29.5 34.1 33.6 66.4 92 53.9 29.3

0.33 1.37 1.58 1.56 3.08 4.27 2.5 1.36

FeOc, commercial FeO; FeOs, solar-made FeO. a Based on stoichrometric FeO.

100

FeOc, T = 480°C, size 30-50μm FeOc, T = 525°C, size 30-50μm FeOc, T = 575°C, size 30-50μm FeOc, T = 575°C, size 125-200μm FeOs, T = 525°C, size 30-50μm FeOs, T = 575°C, size 30-50μm

90

Conversion rate (%)

80 70 60 50 40 30 20 10 0 0

5

10

15

20

25

30

Time (min) Fig. 9. Evolution of chemical conversions versus time during hydrolysis reactions.

be explained by the diffusion limitation combined to a high non-stoichiometry, Fe1yO [25]. Chiefly, the hydrogen production is hampered because of the steam-transfer limitation through the growing layer of magnetite formed at the external particle surface. The transfer of hydrogen from the reaction sites to the particle surface may not be a

limiting step due to the high molcular diffusion coefficient of hydrogen. These results prove that material characteristics influence the reactivity of the reduced iron oxide in the water splitting reaction. XRD analysis of the two reactants shows different stoichiometries: Fe0.95O and Fe0.9O for solar


wustite, whereas commercial wustite composition is close to FeO. The angle of the main diffraction peak is higher for solar wustite than for commercial wustite (Fig. 10). The crystallographic parameter of the cubic lattice is calculated in solar wustite (a ¼ 4.285 A˚) and the calibration curve of Touzelin [30] gives the oxygen mol fraction. With this method, a mean stoichiometry Fe0.9O is obtained. Thus, solar wustite is highly non-stoichiometric, which explains kinetic results. The solar activated wustite displays the highest initial reaction rates, which allows a bulk production of hydrogen when reaction is chemically controlled. 3.2.3. Influence of temperature The maximum hydrogen mol fraction was highly temperature dependent, thus the reaction was chemically controlled (kinetic limitation) in the first period, corresponding to a particle surface reaction (no mass-transfer limitation). A bed temperature increase led to a higher maximum hydrogen mol fraction because the reaction kinetic was improved (Fig. 8), which was also observed in [14]. Similarly, the final conversion increased with temperature (Fig. 9). However, after reaching a maximum, the reaction rate decreased with time corresponding to internal mass transfer (diffusion) limitations, as described in the previous section. 3.2.4. Influence of particle size Two different ranges of particle size were tested (Fig. 9). The results show that both the kinetic of hydrogen production and the final conversion increase when particle size decreases. This observation is related to the following: (1) the solid specific surface area increases when particle diameter decreases, which enhances contact between reactants, heat and mass transfers, thus the reaction kinetic rate; (2) the lower the particle diameter, the higher the final conversion, because the higher the surface-to-volume ratio, thus the higher the amount of available material for reaction. Furthermore, the effect of particle size on hydrogen evolution strengthens the diffusion-limitation statement.

1400

Intensity

1200 1000 800 600

Commercial FeO

400 Solar FeO + Fe3O4

200 0 30

35

40

45

50

55

60

65

Fig. 10. XRD analysis of solar and commercial wustites.

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4. Discussion Kinetics and final conversion of the hydrolysis reaction with FeO depend strongly on temperature, reaction time, particle size, and sample composition. The hydrogen production increases strongly with temperature especially at the beginning of the reaction, which indicates a chemical control of the reaction. The reaction rate decreases with time because a low-permeable oxide layer (Fe3O4) is formed at the surface of the particles and grows with time, which hinders the reaction. This phenomenon was also observed in [25]. At that time, the steam reactant must pass through the forming oxide layer to reach the fresh layer of reactive wustite. The process is controlled by diffusion inside the porous outside layer of magnetite, and the hydrogen production drops. However, results about the water splitting reaction show that high chemical conversions (450%) can be achieved with the iron oxide system. The influence of particle size on hydrogen production was studied. The results show that particle diameter is a key parameter. The advantages of using small particles are: (1) their high specific surface area that augments the reaction kinetics, heat transfer, and mass transfer; (2) their large surface-to-volume ratio that favours nearly complete oxidation and (3) their possible entrainment in a gas flow that allows for either particle fluidisation or continuous reactants feeding and products removal. Mechanical crushing spends energy, which results in lower cycle efficiency. The design of a solar reactor enabling the granulation/dispersion of the reduced oxide from the liquid state into micro-particles would be of primary interest. The highest initial reaction rates were obtained with solar wustite. The main part of hydrogen (90% of total volume evolved) is produced during the first 3 min of reaction, which is a positive feature for a large-scale process implementation since the reaction time should be minimal. The difference observed between the two kinds of wustites tested may be the result of non-stoichiometry effects. Normally, wustite phases with a low non-stoichiometry (y-0) in Fe(1y)O present more Fe(II) ions available to produce hydrogen. However, the reaction runs faster with wustite phases of high non-stoichiometry, probably because of the large amount of defect clusters (formed from octahedral vacancies and tetrahedral iron sites), which can serve as nuclei for the magnetite formation [25]. High initial reaction rates are obtained with solar wustite because solar synthesis results in the formation of a nonstoichiometric wustite phase. This assumption is strengthened by thermodynamics which predicts the formation of wustite phases with different non-stoichiometries (Fig. 1). The calculated conversion of hydrolysis reaction depends on the reactant stoichiometry. Conversions given in Table 4 have been calculated for stoichiometric FeO. However, the high non-stoichiometry of solar wustite (Fe0.9O) has an impact on chemical conversion calculations because less iron(II) atoms can react with water. The maximum

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quantity of hydrogen which can be produced from 1 mol of Fe1yO is (1–4y)/3 mol of H2, whereas it is 1/3 mol from 1 mol of FeO 3 Fe1y O þ ð1 4yÞ H2 O ! ð1 yÞ Fe3 O4 þ ð1 4yÞ H2 (11) Thus, a correction factor must be applied to conversions in order to take into account the reactant non-stoichiometry (0oyo0.1), which improves conversion given in Table 4 for solar wustite. For instance, the corrected conversion is up to 82.6% for solar wustite hydrolysis at 575 1C (instead of 53.9% based on FeO composition). Therefore, hydrolysis reaction of wustite can reach nearly completion. Finally, the solid–gas reaction was carried out in a fixed bed in which the circulation of steam may not be evenly distributed. It must be noticed that the reaction extent should depend strongly on the chemical-reactor technology. Although external transfers may not be the prevailing controlling phenomena, the use of a fluidised bed reactor is expected to enhance further the kinetics of hydrogen production (while the final conversion would probably stay unchanged) thanks to optimised heat and mass transfers and to additional phenomena such as attrition. In other words, the time required to reach a given conversion should be lowered in fluid bed. 5. Conclusion Solar hydrogen production from water by the use of solar-driven two-step thermochemical cycles is potentially one of the long-term alternatives to fossil fuels. This work focussed on the iron oxide cycle, which is based on cyclic thermal reduction and water-hydrolysis reaction with wustite phase to split water. The reactions were experimentally studied using small-scale reactors to acquire accurate data. Operating conditions, chemical conversions, reaction kinetics were determined because they are of major importance when designing chemical reactors or when estimating process efficiencies and cost. The total reduction of hematite into wustite was obtained in a hightemperature solar reactor operating at 1700 1C and 0.1 bar under inert atmosphere. The reaction releasing oxygen proceeds with molten oxide and requires about 2 min to reach completion, which validates previous results. Solar wustite was able to split water in a two-step process with a maximum conversion of 54% for FeO milled into fine powder (30–50 mm particle size). The hydrolysis reaction of solar wustite reached nearly complete conversion (480%), accounting for the non-stoichiometric composition of wustite (Fe1yO) in the conversion calculation. The kinetics of hydrolysis reaction depends on the material properties, temperature, and particle size. The particle hydrolysis is governed by two consecutive controlling phenomena: the rapid oxidation of the external particle surface area by steam followed by diffusion of steam inside the porous particle. The highest initial reaction rates were obtained for solar-made wustite, which allows a bulk

production of hydrogen at the beginning of the reaction corresponding to the rapid oxidation of the surface area. Based on this new set of favourable results, the two-step Fe3O4/FeO cycle displays an attractive potential for large-scale H2 production if the solar reactor is efficiently scaled-up. Future work will be related to the design and testing of a solar reactor that allows the continuous molten oxide processing under controlled atmosphere. In addition, this reactor should include a granulation/reaction of the oxide from the liquid state to avoid mechanical crushing (energy spending).

Acknowledgements The authors thank R. Garcia and M. Hominal for their help in the construction of experimental reactors. Financial support of CNRS (Department of Engineering) is gratefully acknowledged.

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