SnO water-splitting cycle for

international journal of hydrogen energy 33 (2008) 6021–6030

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Novel two-step SnO2/SnO water-splitting cycle for solar thermochemical production of hydrogen Ste´phane Abanadesa,*, Patrice Charvina, Florent Lemontb, Gilles Flamanta a

Processes, Materials, and Solar Energy laboratory (PROMES-CNRS, UPR 8521), 7 Rue du Four Solaire, 66120 Font-Romeu, France Commissariat a` l’Eńergie Atomique (CEA), Rhoˆne Valley Research Center BP17171, 30207 Bagnols-Sur-Ce`ze Cedex, France

b

article info

abstract

Article history:

The production of hydrogen from a novel two-step thermochemical cycle based on SnO2/

Received 12 March 2008

SnO redox reactions is presented. This process targets CO2-free hydrogen production by

Received in revised form

using renewable solar energy and water in a high-temperature water-splitting cycle. The

7 May 2008

cycle consists of a solar endothermic reduction of SnO2 into SnO(g) and O2 followed by

Accepted 16 May 2008

a non-solar exothermic hydrolysis of SnO(s) to form H2 and SnO2(s). The objective of this

Available online 26 September 2008

study was to demonstrate this innovative concept for H2 production and to establish the potential of cycle implementation in an integrated solar chemical process. The reduction

Keywords:

and hydrolysis reactions were experimentally tested in order to define optimal operating

Hydrogen

conditions, chemical conversion and hydrogen yield. The thermal reduction occurs under

Thermochemical cycle

atmospheric pressure at about 1600 C and over. The solar step encompasses the formation

Water-splitting

of SnO nanoparticles that can be hydrolysed efficiently in the temperature range

Tin oxide

500–600 C with a H2 yield over 90%. A preliminary process design is also proposed for cycle

Solar energy

integration in solar chemical plants. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The promising energy carrier hydrogen is an environmentally attractive and sustainable transportation fuel, having the potential to displace fossil fuels. Therefore, the development of sustainable, low cost, and efficient technology for the largescale production of H2 without greenhouse gas emission currently mobilises strong research efforts worldwide. Hydrogen can be produced from either hydrocarbons (fossil fuels and biomass) or water. Current technological process achievements and economical considerations have mainly directed the industrial production toward steam reforming of natural gas. However, flue gases contain significant amounts of CO and CO2, leading to large CO2 emissions into the atmosphere (0.43 mol CO2 equiv./mol of net hydrogen produced [1]), thereby imposing additional purification treatments for

further H2 processing. This contributes to increased downstream process costs and complexity. Alternative clean and efficient pathways for the production of pure hydrogen are water electrolysis and thermochemical water-splitting cycles. A thermochemical cycle effects the multi-step decomposition of water into hydrogen and oxygen using only heat. Thus, thermochemical cycles are expected to be more efficient than electrolysis for H2 production as their energy efficiency is not limited by the conversion of heat to electricity. Cycles with overall energy efficiency above 20% are targeted to compete with electrolysis process (solar heat to electricity conversion 25% electrolysis efficiency 80%). Consequently, solar-powered thermochemical cycles constitute an attractive option for massive H2 production, avoiding greenhouse gas emission and allowing complete recycling of chemicals. This sustainable ‘green’ process also

* Corresponding author. Tel.: þ33 04 68 30 77 30; fax: þ33 04 68 30 29 40. E-mail address: [email protected] (S. Abanades). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.05.042

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solves the problem of long-term efficient thermal energy storage, by converting solar radiation in storable and transportable chemical fuels. The produced solar H2 fuel can be processed for heat and power generation, supplied as a clean energy carrier to advantageously replace polluting fossil fuels. Previously, most of the studied thermochemical cycles were proposed in combination with nuclear energy as the primary energy source, imposing operation temperature below 900 C [2–5]. Compatible cycles (e.g., I-S and UT-3 cycles [6]) involve incomplete reactions, awkward separation steps, hazardous and corrosive compounds, which make difficult a commercially viable process implementation. Regarding the increasing progress in solar energy collection and concentration at power level up to several tens of megawatts, more efficient reducedstep cycles may now be operated in a broader temperature range from 900 to 2000 C. In particular, two-step solar-driven cycles are advantageous due to their ease of implementation at process scale, thus offering the potential for safe, low cost, and large scale supply of H2 as a clean energy carrier. The solar thermochemistry for two-step water-splitting involves reactions of metal-oxide redox pairs. In a first step (water-splitting), oxidation of the active redox material (reduced state of a metal oxide) by water steam produces hydrogen. In a second step driven by solar thermal energy, reduction of the metal oxide enables active material regeneration with release of oxygen (Fig. 1). Such a scheme presents particular interests: (i) the maximum temperature of the cycle (generally ranging from 1200 to 1800 C) is compatible with renewable solar thermal energy; (ii) water and heat are the only inputs, and hydrogen and oxygen the only outputs; (iii) H2 and O2 are produced separately; (iv) the other chemicals and reagents are recycled in a closed cycle; (v) the produced H2 is pure and can be directly processed, e.g., in polymer electrolyte membrane fuel cell (PEMFC). Due to their promising potential, increasing researches devoted to the development of two-step solar-driven cycles (ZnO/Zn, Mn2O3/ MnO, Fe3O4/FeO, ferrites) are currently conducted in Switzerland (ETHZ, PSI [7–10]), Germany (DLR, EC funded project ‘‘Hydrosol’’ [11]), France (CNRS-PROMES [12,13]), USA (DOE funded projects [14]) and Japan [15–17]. The two-step metal-oxide cycles are commonly based on volatile (ZnO/Zn) or non-volatile redox pairs (Fe3O4/FeO, ferrite cycles). ZnO/Zn cycle is designated as a volatile oxide

cycle because the reduced zinc species produced by the reaction exits the reactor as a gaseous product (ZnO(s) / Zn(g) þ 1⁄2 O2). It has been studied extensively and is considered as one of the best candidate for coupling with a solar energy source. ZnO is decomposed near 1800 C in a solar reactor [18] and Zn is recovered after quenching the product gases. The recombination of Zn and O2 is a parasitic reverse reaction limiting the Zn yield after the solar step. The gas quenching to minimize Zn recombination (e.g., by adding large amounts of inert gas) constitutes the major challenge of this cycle. A fast quench to cool the products to under 900 C (corresponding to Zn(g) condensation temperature) is suggested [19,20]. An effective Zn/O2 separation technique may also be considered (e.g., by in situ electrolytic separation [21]). The solid metallic zinc may be used in a fuel cell or battery, or in the water-splitting reaction producing H2 and regenerating ZnO that can be recycled to the solar step. The hydrolysis of Zn was performed with steam bubbling through molten zinc at 500 C [22], but the reaction rate was limited by the formation of a ZnO(s) layer. Then, the steam-hydrolysis of Zn nanoparticles (size 70–100 nm) produced by vapo-condensation was considered yielding up to 70% H2 [8]. Based on an endothermic reaction at 2000 C, the energy efficiency of ZnO/Zn cycle is about 45% (HHVH2/DHZnO(25 C) / Zn(g) þ 0.5O2(2000 C)) and the maximum exergy efficiency is 29% without heat recovery [7] (defined as the ratio of chemical energy stored in H2 at room temperature to the amount of solar power input, hexergy ¼ DGH2 þ0:5O2 /H2 O =Qsolar ). Consequently, ZnO/Zn is presumably considered as the most favourable cycle given its potential for reaching high energy and exergy efficiencies, but strong technical challenges are remaining and the development of alternative cycles is required. We have recently demonstrated the new SnO2/SnO twostep cycle, using concentrated solar energy as the primary energy source. Similar to ZnO/Zn, this cycle belongs to the category of volatile metal oxide cycles. The only referenced cycle based on tin oxides is known as the ‘‘Sn-Souriau’’ three-step cycle: A- SnO2 / SnO þ 1⁄2 O2; B- 2SnO / Sn þ SnO2; C- Sn þ 2H2O / SnO2 þ H2.

Fig. 1 – Schematic representation of the two-step water-splitting cycle based on the MxOy/MxOyL1 system.


The concept of this cycle patented in 1972 [23] has not been subjected to an experimental validation. Hence, we have checked that reaction B is complete at 600 C after 10 min of heating under inert atmosphere to avoid oxidation of SnO. However, tin metal cannot be separated from SnO2 by liquefying Sn as indicated by the authors, because the liquid phase of tin does not form. The reaction C producing hydrogen from the Sn/SnO2 mixture issued from reaction B is slow and partial at 600 C (conversion rate of 45% after 30 min). For these reasons, this three-step cycle was not selected as a promising candidate. Instead, the innovative two-step SnO2/SnO cycle is proposed and described in this study.

2. Thermochemical reactions and process features The study addresses the production of hydrogen from the novel two-step thermochemical cycle based on the SnO2/SnO redox pair [24]: Solar reduction step : SnO2 ðsÞ/SnOðgÞ þ 1=2O2 ðendothermic; 1600 CÞ

(1)

Hydrolysis step : SnOðsÞ þ H2 OðgÞ/SnO2 ðsÞ þ H2 ðexothermic; 550 CÞ

(2)

The overall reaction corresponds to the splitting of water: H2O / H2 þ 1⁄2 O2 (DH ¼ 285.83 kJ/mol). The first endothermic solid–gas reaction (DH ¼ 557 kJ/mol SnO2 at 1600 C) consists in reducing at high temperature tin(IV) oxide (stannic oxide) into tin(II) oxide (stannous oxide). The reaction product SnO (Tm ¼ 1042 C, Tb ¼ 1527 C) is gaseous, and SnO(g) is released with O2. When the gas temperature decreases, the gaseous SnO is condensed (formation of nuclei and particle growth).

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The required temperature for reaction (1) is suitable for the coupling with a solar thermal energy source. In that case, the reduction of stannic oxide could be carried out in a solar chemical reactor associated with a solar concentrating system. The second solid–gas reaction producing hydrogen consists in hydrolysing SnO particles by steam. This moderately exothermic reaction (DH ¼ 49 kJ/mol at 500 C) occurs above 450 C at atmospheric pressure. The recovered stannic oxide can then be recycled into reaction 1, which closes the cycle. Thus, the metal oxide is not consumed by the process, and O2 and H2 are produced separately. The cycle productivity assuming a complete conversion is 166:3 mLH2 =gSnO at normal conditions, which represents about 14:8 mgH2 =gSnO (i.e., the storage mass capacity of SnO is 1.48%). The potential applications are related to the stationary or portable generation of hydrogen. SnO can be stored and transported more easily and surely than hydrogen, which constitutes a potential hydrogen tank. Moreover, the reactivity of tin monoxide with water is not altered after a long storage period at ambient air. Therefore, H2 can be generated on the delivery site on demand. The cycle does not contain any side reaction or compound responsible for the decrease of reaction efficiency. Only the reverse reaction between SnO and O2(g) is noticeable, which requires a gas quenching commonly encountered for volatile metal oxides cycles (e.g., ZnO/Zn cycle). The global thermochemical process that integrates the two chemical steps (solar reactor and hydrolyser) and the fuel cell is represented in Fig. 2. The produced hydrogen is injected in an ideal fuel cell generating at 25 C an electrical power (W ¼ 237 kJ/mol) and heat (Q ¼ 49 kJ/mol). The intrinsic energy efficiency of the SnO2/SnO cycle is about 42% based on the high heating value of hydrogen (HHVH2 ¼ 286 kJ=mol). The maximum absorption efficiency of a perfectly insulated blackbody solar receiver is about 86% at 1600 C (for

Fig. 2 – Block diagram of the global thermochemical process and energy analysis of the SnO2/SnO cycle based on thermochemical data.

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a concentration ratio C of 5000 and a direct normal irradiation I of 1000 W/m2). Then, the amount of necessary solar energy (Qsolar) to effect the endothermic reduction reaction is about 795 kJ/mol, which allows to estimate the process efficiencies of SnO2/SnO cycle. As a result, the global exergy efficiency of the process is about 29.8% at 1600 C, which is similar to ZnO/ Zn cycle at 2000 C [7].

3. Theoretical and experimental investigations Both theoretical studies and proof-of-concept experiments were conducted to establish reactions feasibility and operating conditions, and to quantify reaction yields and chemical kinetics. A thermodynamic analysis was used to predict the system composition as a function of the operating parameters. Then, the experimental testing and characterization of tin oxides reactivity was achieved to validate the cycle.

3.1.

Thermodynamic analysis

The SnO2/SnO system was simulated using the computer program HSC Chemistry [25] and the equilibrium compositions were determined for the reduction and hydrolysis reactions. While the calculation approach assumes a closed system, an excess of inert carrier gas was considered to simulate the continuous gas flow for an opened system. - Influence of temperature on SnO2 reduction: The temperature for which DG equals zero is 2056 C for SnO2/SnO and 2068 C for ZnO/Zn. Hence, the reduction behaviours of SnO2 and ZnO are very similar. The higher the temperature, the higher the dissociation rate of SnO2 because of the endothermic reaction. Thermodynamics predict complete SnO2 dissociation occurring just under 1600 C with a 100:1 ratio of inert N2 carrier gas to SnO2 (Fig. 3). For an N2 to SnO2 ratio of 1:1, the necessary temperature for complete conversion rises just above 1900 C with the start of dissociation at 1500 C. Thus, the addition of inert gas reduces the

Fig. 3 – Equilibrium composition versus temperature for reduction of 1 mol SnO2 at 1 atm (100 mol N2).

necessary temperature for complete SnO2 reduction. This is a result of the reduced partial pressure of the product gases, shifting the equilibrium toward the products. - Influence of the amount of N2 on SnO2 reduction: The ratio N2/SnO2 was set to 100:1 in Fig. 3. The dissociation rate drops when this ratio decreases. When the ratio is less than 70:1, the dissociation of SnO2 at 1600 C is partial. For instance, the dissociation rate is about 70% for a ratio of 50:1 (Fig. 4). An excess of inert gas or a temperature increase is thus required to reach reaction completion. - Influence of total pressure on SnO2 reduction: The lower the pressure, the higher the dissociation rate, because the total mole number of gaseous species increases during the reaction. Fig. 5 shows that the dissociation rate is about 40% at 1500 C and 1 atm. When the total pressure decreases, the formation of gaseous SnO and O2 is favoured. At 1500 C, the reaction is complete below 0.4 atm. - Hydrolysis reaction: The DG of hydrolysis is negative below 618 C. SnO2 and H2 are the stable species at low temperature, whereas SnO and H2O become the major species when temperature increases (Fig. 6). Thus, thermodynamics predict that the hydrolysis reaction must be carried out at moderate temperature (below 600 C) in order to achieve significant hydrogen yields. As the reaction kinetics usually increase with temperature, an optimum must be found experimentally.

3.2.

Experimental validation

The SnO2/SnO cycle was experimentally demonstrated to validate the reactions. Quantitative performance data were acquired for the identified reactions: SnO2 / SnO þ 1⁄2 O2 (solar-driven reduction step) and SnO þ H2O / SnO2 þ H2 (hydrolysis step). The results regarding the chemical yields and kinetics of the reactions are promising and will be further improved by optimizing the operating parameters.

Fig. 4 – Equilibrium composition versus amount of N2 for reduction of 1 mol SnO2 at 1 atm (T [ 1600 8C).


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Fig. 5 – Equilibrium composition versus total pressure for reduction of 1 mol SnO2 at 1500 8C (100 mol N2).

Fig. 7 – Kinetics of SnO2 reduction versus temperature and pressure (measurements performed in a thermobalance).

3.2.1.

99.9%) was placed on a water-cooled holder enclosed in a transparent glass vessel to control the atmosphere and pressure. Nitrogen (1.3 L/min) was used as carrier gas and a vacuum pump connected to the vessel allowed the inert gas circulation and reduced pressure operation. The sample was directly heated by concentrated solar energy supplied by a solar furnace composed of a sun-tracking mirror and a concentrator (1.5-m diameter) delivering high-flux solar irradiation. The position of the sample was moved toward the focus of the concentrator to increase the temperature above 1600 C. For a given sample position (thus a given temperature), gaseous products (SnO and O2) were released during the reduction reaction at a rate controlled by the above kinetics. Then, the products started to cool down when entrained by the inert gas. The condensation of SnO led to the formation of nanoparticles with a mean diameter in the range 10–50 nm. The reaction product was collected as powder in a filter set at

Reaction of thermal dissociation

According to thermodynamics, the reduction of SnO2 into SnO is complete at 1600 C at atmospheric pressure under an inert gas flow (Fig. 3). The kinetics of the dissociation reaction was determined in a thermobalance. The mass loss recorded during sample heating was only due to the reaction producing gaseous SnO. Indeed, SnO2 vaporisation cannot be encountered because the boiling point of SnO2 is high (Tb ¼ 2500 C). The kinetic parameters of SnO2 reduction into SnO were determined on the basis of the Arrhenius expression: k ¼ k0exp(Ea/RT ), assuming a first order reaction. At atmospheric pressure, Ea ¼ 424 kJ/mol and k0 ¼ 1.4 108/s. A pressure decrease or gas flow-rate increase results in a faster reaction kinetic. Actually, k0 ¼ 7.31 108/s at 0.1 atm and k0 ¼ 1.24 1010/s at 0.01 atm. Fig. 7 shows that the kinetic of SnO2 reduction is enhanced with a temperature increase or a pressure decrease. Tests of reduction of stannic oxide were performed in a solar reactor represented in Fig. 8. SnO2 powder (1 g, purity:

Fig. 6 – Equilibrium composition versus temperature for hydrolysis of 1 mol SnO at 1 atm (1 mol H2O, 100 mol N2). SnO(R) refers to the tetragonal mineral phase of SnO (Romarchite).

Fig. 8 – Experimental set-up for solar reduction of volatile metal oxide.

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the outlet. This powder was analysed by X-ray diffraction (XRD) to evaluate the fraction of SnO in the final product (the recombined SnO2 was then deduced). The presence of a refrigerant on the gas outlet path allowed a significant products quenching, which improved the reaction chemical yield. The mass fraction of SnO obtained at atmospheric pressure was 54%. The amount of SnO in the final product was increased sharply with a decrease in total pressure because of the decrease in the oxygen partial pressure. A total pressure of 0.2 atm led to a mass fraction of SnO of 86%. Similarly, the mass fraction of SnO was improved when the inert gas flow-rate increased because the recombination reaction was hindered. Two main effects of the inert gas can be noticed. First, the inert gas introduced at 25 C cools down rapidly the gas products by convective heat transfer. Second, the added inert gas flow-rate dilutes the gas products, which decreases the partial pressure of O2. The lower the partial pressure of O2 the higher the displacement of chemical equilibrium towards the formation of SnO.

3.2.2.

Hydrolysis reaction

The reduced species SnO was used in the second reaction to produce hydrogen (SnO þ H2O / SnO2 þ H2). The reaction was studied in a tubular stainless steel reactor (Fig. 9) that involved a fixed bed of reactive particles (0.2 g). The reactor was heated by an electrical furnace set vertically. The temperature of the bed was measured by a K-thermocouple. The reactor was first swept by a flow of inert carrier gas (Ar, 0.18 NL/min) to eliminate air, and then heated to the desired operating temperature. Once the temperature was stable, water steam was injected with argon through the bed. The steam (10.5 mmol/ min) was produced by injecting demineralised water in an upstream tubular furnace, thanks to a capillary connected to a peristaltic pump. The hydrogen was analysed continuously as a function of time, thanks to a specifically devoted H2 analyser (catharometer, precision: 1% of full scale), in order to

quantify the hydrogen produced by integration of the production curves. The analysis method is based on thermal conductivity detection by comparing the response signal between a reference gas (Ar) and the gas to analyse (Ar/H2 mixture). Hydrogen was detected few seconds after steam injection and reached a maximum. Before gas analysis, the outlet gas was cooled down and flowed through a set including a bubbler to eliminate steam in excess and a gas drying unit (desiccant column) to protect the analytical instrument. The maximum temperature of hydrolysis reaction was fixed at 600 C in order to avoid the dismutation of stannous oxide (2SnO / SnO2 þ Sn). The hydrolysis reaction of SnO was efficient above 470 C. It was nearly complete at 525 C with a final conversion rate up to 98% (Fig. 10), and the kinetics were favoured by the high specific surface area of SnO nanoparticles. A peak of hydrogen production was measured as soon as steam was injected (Fig. 11). Then, the reaction rate decreased slowly. The reaction was ended after about 20–30 min in a fixed bed and the agglomeration of particles was observed during water injection, which made difficult a homogeneous reaction in the whole bed. An improved reaction rate may be observed with a moving bed reactor that would increase heat and mass transfers. The experimental data were fitted to a kinetic rate law applied to the hydrolysis reaction [26]: dX/dt ¼ k(1 X )n, where X is the chemical conversion, k is the temperaturedependant kinetic constant given by the Arrhenius expression and n is the order of reaction. The experimental conversion was best described by a first-order reaction with k ¼ 2.1 103/ s (Fig. 10). The case n ¼ 2/3 corresponding to a contracting sphere model did not match the data as well. The real mechanism seems more likely intermediate between these two extreme cases, approaching a shrinking core model in which a growing layer of SnO2 forms around the unreacted core of SnO and limits the internal diffusion of water.

Fig. 9 – Experimental set-up for the production of hydrogen from hydrolysis of SnO(s).


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- The hydrolysis reactor enabling optimal solid–gas exchange area to improve kinetics and to minimize the reaction duration (e.g., fluidized bed or suspended particles in a steam flow). The energy needed for the reactant heating during hydrolysis is supplied by process heat recoveries from the high-temperature step combined with the exothermal effect of the reaction. - A quenching unit to avoid recombination of SnO and O2 by the mean of, e.g., a heat exchanger that recovers the sensible energy of products. This energy can then be transferred to a fluid for the production of steam that is injected in the hydrolysis reactor. - A solid–gas separation device for the recovery of SnO nanoparticles, and means for recycling SnO2 back to the solar reduction step including SnO2 recovery after hydrolysis and transportation to the solar reactor. Fig. 10 – Experimental chemical conversion rate of the hydrolysis reaction versus time at 525 8C and comparison with predictions of kinetic models.

Additional experimental data are required to characterize this process and to quantify the kinetic parameters (preexponential factor k0 and activation energy Ea).

4.

First process design

The thermochemical solar process features the following main components:

The process flow sheet is represented in Fig. 12. The first endothermic step is performed in the reactor R1 heated by concentrated solar energy. This reactor allows operation under inert controlled atmosphere (preferably at reduced pressure). The second exothermic step producing hydrogen is performed in the hydrolysis reactor R2 at 450–600 C. Two different modes of operation may be retained for reactor R1: continuous or semi-batch metal oxide processing. In the first case, the reactor enables the continuous injection of SnO2 particles, followed by the extraction of the gas products and the recovery of the condensed SnO reactive particles (opened reactor as considered in Fig. 12). In the second case, a given load of SnO2 is processed (semi-batch reactor) and the reaction occurs until total solid reactant consumption. One option would be to feed continuously the reactor (e.g., using

- A solar concentrating system supplying high solar flux densities at the focal plane. - The high-temperature solar reactor swept by an inert gas that carries the volatile oxide formed. This reactor can be based on the cavity-type receiver concept absorbing solar radiation. Either semi-batch operation with reactant consumption or continuous operation with reactant powder injection and product recovery can be considered for the reactor.

Fig. 11 – Time evolution of H2 flow rate and temperature during SnO hydrolysis.

Fig. 12 – Example of process flow diagram.

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a screw feeder) in order to keep constant the bed of particles. In any cases, the volatility of the product (SnO) facilitates its transportation from the reaction zone to the reactor outlet connected to a gas–solid separation unit. As the reduction reaction requires a high-temperature energy supply, the reactor R1 may benefit from the high-flux solar radiation provided by a solar concentrating system. Such heating system is suited to fast gas–solid reactions involving a particulate phase. A solar receiver must be implemented for solar radiation absorption and for energy transfer to the reactive media. The absorber may be either an opaque surface that radiates towards the reaction zone separated from the primary radiation source (indirect irradiation), or a cloud of particles that is directly irradiated and heated up (direct irradiation). The main drawback of the direct irradiation option is that if a window is needed in order to provide a closed atmosphere, deposition of particles on the window may occur. However, when a solid reactant is used, it can serve simultaneously as direct radiant absorber and chemical reactant. With indirect irradiation, heating rates are lower but window problems that may be encountered for volatile oxide systems such as SnO2/SnO are avoided. The direct heating option can be used with a fixed bed of particles consumed by the reaction (semi-batch reactor) or with a continuous injection of radiation absorbing particles (Fig. 12). In the latter case, the solar reactor is lined with a porous ceramic cavity that absorbs the solar radiations. The seeded particles of SnO2 dissociate when reaching the cavity walls, and the gaseous products (SnO and O2) are swept by a flow of inert gas through the ceramic walls with the support of a pumping device. Then, SnO vapours condense as nanoparticles during gas cooling, which allows their separation from the gas flow and their collection by a filtering device. This solid–gas separation device at the outlet of reactor R1 must have a cutting threshold of about 10 nm (ultrafiltration range) for efficient nanoparticles retention. Then, SnO particles are stockpiled in a storage tank S1 for their subsequent hydrolysis producing H2. A heat exchanger (E1) is set on the extracting path of stannous oxide and oxygen. This exchanger has two main functions: (1) quench of the reaction products to optimise conversion rate and (2) recovery of the sensible heat of the gaseous products (about 414 kJ/mol neglecting the inert carrier gas) to supply the necessary energy for producing water steam fed into the hydrolysis reactor R2. The energy supplied by the exothermic reaction also contributes to heat the reactants in reactor R2. The hydrolysis reactor R2 features a solid–gas contactor with preferably a moving bed favouring mass and heat transfers in order to minimize the reaction duration. Suspending particles in the form of aerosol can be carried along a tubular reactor by a steam flow. The exothermal enthalpy of hydrolysis allows the heating of reactor R2: the heat of reaction is recovered and employed to heat SnO(s) and H2O(g) at the required temperature in an auto-thermal reactor. When exiting reactor R2, the tin oxide is totally converted and reoxidized as SnO2, which can be recycled after being separated from the gas flow. A solid conveyer can be used to transport SnO2 particles from the storage tank S2 to the reactor R1. The gas flow exiting reactor R2 is composed of water steam in excess and of hydrogen. Hydrogen is separated by

condensing water and then stored. Liquid water is pumped towards reactor R2 and it is preheated in a heat exchanger E2 by the gas stream exiting reactor R2 (75 kJ/mol assuming an equimolar flow of H2/H2O). A supply of water is necessary to complete the flow rate consumed by the hydrolysis reaction.

5. Advantages of the two-step SnO2/SnO cycle As a general remark, the thermal dissociation of tin(IV) oxide into metallic tin was not considered as part of a cycle because it is impractical with current technologies since ultra high temperatures are required (DG ¼ 0 at 2647 C for SnO2/Sn). If metallic Sn is targeted, a reducing compound (such as C or CH4) must be used to lower the reaction temperature. However, the reactant is consumed, CO/CO2 is released as by-products and liquid Sn is obtained instead of solid particles, which is not convenient for subsequent hydrolysis. Consequently, the only feasible two-step cycle involving tin species relies on the SnO2/SnO system. The SnO2/SnO cycle exhibits attractive characteristics that may be compared to the most studied ZnO/Zn cycle, taken as reference because of its outstanding energy efficiency. They are chiefly related to cycle temperature, high chemical conversion rates and fast reaction kinetics. - The reduction temperature can be kept at 1600 C and below for a reduced pressure to insure a sufficient reaction rate. A decreased temperature makes the process more efficient and the cycle implementation in H2 production plant more feasible, which favours the commercial viability of this emerging solar-driven technology. - The dissociation rate of SnO2 is high and it is less dependent on the quenching rate than the dissociation rate of ZnO. This can be explained from the melting and the boiling point of Zn (Tm ¼ 420 C, Tb ¼ 907 C) and SnO (Tm ¼ 1042 C, Tb ¼ 1527 C). SnO(g) homogeneously condenses rapidly when the gas temperature decreases because the gap between reaction temperature and SnO condensation temperature is narrow. Inversely, Zn remains longer in the gaseous phase, which explains why a fast quench must be used to cool Zn(g) below the condensation temperature [19]. - The hydrolysis reaction producing H2 from SnO(s) is fairly rapid with a high conversion rate due to the use of nanoparticles produced in situ during the reduction step. Internal diffusion of gases inside the solid is not a limiting factor. Gas-particle external mass transfer is the only controlling step. Therefore, there is still some room to optimize the hydrolysis kinetics, mainly by increasing mass transfers between gas and particulate phases. In addition, several general advantages may be noticed: - The chemical process is safe and clean since H2 is produced from water and solar energy only. Then, the produced hydrogen is pure, and it is not contaminated by carbon oxides poisoning the catalyst of low temperature


fuel cells (PEMFC). The chemical products are non-toxic, and raw materials (H2O and SnO2) are abundant and available at moderate cost (cassiterite is the primary ore of tin as SnO2 and it costs about 5 US$/kg on the international market). Furthermore, the chemical reactions do not require any catalyst and the reduced tin oxide (SnO) reacts readily with water above 500 C. SnO is stable at ambient atmosphere and it can thus be transported to generate hydrogen on demand. Concerning SnO stability under air, thermogravimetric experiments showed that SnO reoxidation into SnO2 occurs only when heating and it remains weak (20% to 30%) at 500 C. - The thermal dissociation of SnO2 into SnO(g) followed by SnO condensation produces nanoparticles with a diameter less than 100 nm. The use of SnO nanoparticles has several advantages regarding the hydrolysis reaction: (i) their high specific surface area increases the reaction kinetics, heat and mass transfers with respect to larger particles, (ii) their large surface/volume ratio favours their oxidation and limits any surface passivating effect that may be encountered for larger particles, (iii) their simple entrainment in a gas flow allows continuous reactants feeding (SnO and H2O) and products removal (SnO2 and H2). - The formation of a volatile product during the hightemperature reduction step allows its natural transportation by a carrier gas flow from the reaction zone to the reactor outlet. Thus, the transport of solids in the process is restricted to the recovery of particles in the filter after hydrolysis and their injection into the solar reactor. - The possible integration of both reactors in a single chemical process operating continuously or not (storage of the solar-produced SnO reactive particles for subsequent H2 generation). Although the number of feasible metal oxide cycles is not numerous, some non-volatile metal-oxide cycles have also been demonstrated and their performances can be compared to those of the SnO2/SnO cycle. One of the most significant cycle regarding the hydrolysis reaction lies on the cerium oxides system [27]. We showed that the main interest of CeO2/Ce2O3 cycle is the pronounced reactivity of Ce2O3 with steam, resulting in high hydrogen yields and fast chemical kinetics. However, the main barrier was the high temperature required for the reduction of Ce(IV) to Ce(III) oxide above 2000 C. As the reaction proceeds with the melting of CeO2, partial vaporization of CeO2 during reduction also occurs. Consequently, the SnO2/SnO cycle can be a promising candidate considering the advantage of significant lower temperatures required for the solar reduction of SnO2. In spite of the fact that the hydrogen production rate was lower compared to Ce2O3 at the same temperature, high conversion rates and hydrogen yields were obtained in controllable reactions.

6.

Conclusion

This study focused on the development of a clean and efficient H2 production process based on an innovative two-step metal-

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oxide cycle, using non-toxic SnO2/SnO redox pair as chemical intermediates. Thermodynamics and proof-of-concept experiments were conducted to investigate and characterize the reactive chemical system for hydrogen production. The results showed that SnO2(s) can be reduced efficiently into SnO(g) under atmospheric pressure from 1600 C and over. A pressure decrease lowers the reduction temperature and improves the SnO2 dissociation rate, which was also supported by thermodynamics. SnO rapidly condenses as particles when the gas cools down. The maximal content of targeted SnO in the final solid product exceeds 80%, while the remainder corresponds to recombined SnO2. The reduction reaction forms SnO nanoparticles that can be hydrolysed above 500 C with both a satisfactory reaction rate and a final yield of hydrogen over 90%. Eventually, the SnO2/SnO cycle exhibits noteworthy advantages compared to other metal oxide cycles currently investigated extensively. Thus, it could be considered as an alternative cycle given its attractive characteristics such as lower temperature, high chemical yields and rapid kinetics for both the dissociation and hydrolysis reactions. Such specific features may be decisive when selecting the most competitive cycles. As a further step for ongoing work, a first design of the hydrogen production process was proposed gathering the main operation units required to operate the cycle continuously. The prospects for this novel SnO2/SnO cycle are encouraging. There is a need now to develop innovative solar receiver and appropriate chemical reactor technologies, while addressing materials stability at high temperature and upon successive cycling. Then, a particular focus must be given to the integration of the cycle in an efficient solar-powered chemical process. This requires the design of fully integrated solar chemical plants based on cost-effective solar concentrating technology.

Acknowledgements Financial support of the CNRS department of Engineering Science (ST2I) is gratefully acknowledged. The authors also thank R. Garcia for the technical support during the manufacturing of the experimental devices.

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