Thermochemical water-splitting cycle using iodine and sulfur

Abstract

Research and development on the thermochemical water-splitting cycle using iodine and sulfur, a potential large-scale hydrogen production method, is reviewed. Feasibility of the closed-cycle continuous water splitting has been demonstrated by coupling the Bunsen reaction, thermal decomposition of hydrogen iodide and that of sulfuric acid. Studies are in progress to realize efficient hydrogen production. Also, development of chemical reactors made of industrial materials has been carried out, especially those used in the corrosive process environment of sulfuric acid vaporization and decomposition.

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Kaoru Onuki

Dr Kaoru Onuki is Group Leader of IS Process Technology Group at Nuclear Science and Engineering Directorate of the Japan Atomic Energy Agency. He received a BS from Tohoku University, MS from the University of Tokyo, both in Applied Chemistry, and a PhD in Chemical Engineering from Yokohama National University. Dr Onuki has been working on hydrogen production research for the last 25 years.

Shinji Kubo

Mr Shinji Kubo is assistant principal researcher of JAEA. He received a BS and MS in Mechanical Engineering from Utsunomiya University. He was engaged in high density heat transport technology research and received a prize from the Japan Society of Mechanical Engineers in recognition of his work on micro capsule phase change materials. In 1999, Mr Kubo joined the thermochemical cycle development research at JAEA and led the closed-cycle demonstration study.

Atsuhiko Terada

Mr Atsuhiko Terada received a BS in Physics from Shizuoka University. From 1992 to 2004, Mr Terada worked at Ishikawajima Harima Heavy Industry Co. Ltd. where he was engaged in design and development related to vacuum systems for beam accelerators and, also, in conceptual design of target systems (neutron spallation source, proton beam window, etc.) for a neutron scattering facility which JAEA and KEK constructed in the J-PARC project. His main research interests are in thermal hydraulics analyses and experiments such as flow visualization, he is now engaged in component development research for thermochemical processes as an assistant principal researcher of JAEA.

Nariaki Sakaba

Dr Nariaki Sakaba is an assistant principal researcher of HTGR Development Promotion Office of Policy Planning and Administration Department in JAEA. He received a BS from Waseda University Tokyo in Applied Chemistry, and PhD from Tohoku University in Quantum Science and Energy Engineering. He is currently working on a high-temperature gas-cooled reactor and hydrogen production system for one of JAEA's heat application systems.

Ryutaro Hino

Dr Ryutaro Hino is Director of the Research Co-ordination and Promotion Office of the Nuclear Science and Engineering Directorate (NSED) of the Japan Atomic Energy Agency. He arranges and promotes all research carried out in NSED, particularly in nuclear hydrogen production technology with high-temperature gas-cooled reactors. He is a visiting professor of Utsunomiya University.

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Broader context

Thermochemical water-splitting cycles have a history of study of over 40 years since the pioneering work of Funk and Reinstrom. Amid mounting concern about the decreasing availability of fossil fuels caused by the oil crisis, the concept of a hydrogen energy system attracted interest and, in searching for a low-cost and efficient hydrogen production process, a number of studies were carried out from the 1970's until about the mid-1980's on thermochemical cycles considering mainly high temperature gas-cooled reactors that can supply nuclear heat of about 1000 °C as a heat source. The study in this period resulted in the invention of some promising chemical reaction cycles such as the so-called “sulfur family cycles”. Then, since around 2000 there has been growing interest in hydrogen energy mainly due to concerns about global warming, so the study of thermochemical cycles has revived worldwide, the main subject of which has been on R&D of the most promising cycles. The cycle using iodine and sulfur is representative of such a promising cycle and researchers are now working to bring forward technology developments from laboratory-scale to engineering-scale.

1. Introduction

The “thermochemical water-splitting cycle” offers a method for transforming heat energy into hydrogen energy, the energy carrier. Although direct thermal decomposition of water requires high temperatures, a few thousand Kelvin, it is possible to decompose water at lower temperatures by combining high temperature endothermic chemical reactions and low temperature exothermic chemical reactions, where the net chemical change resulting from the sequence of chemical reactions is water decomposition. The cycle of chemical reactions produces the free energy of water splitting.

Thermochemical water-splitting cycles were first studied by Funk and Reinstrom,^1,2 and an actual example was proposed by De Beni and Marchetti in the early 70's.³ Since then, a number of thermochemical cycles have been proposed and studied assuming utilization of large-scale high temperature heat sources such as solar or nuclear, in particular the High Temperature Gas-Cooled Reactor (HTGR) which can supply heat at temperatures close to 1000 °C.^4,5

The concept of thermochemical water-splitting cycle represents carbon-free hydrogen production. Compared with present counterparts, like water electrolysis, this method is in the early stages of development. However, it has potential merit for economical large-scale hydrogen production because of its expected economies of scale, which can meet the large hydrogen demand expected from a future hydrogen society. This paper summarizes state-of-the-art research in one of the most promising thermochemical cycles.

2. Thermochemical IS process

Among the many cycles proposed so far, those that utilize thermal decomposition of sulfuric acid as the highest temperature endothermic reaction have been categorized as “sulfur cycles” and have attracted much interest.^5–7 The thermal decomposition of sulfuric acid, reaction (1),

H₂SO₄ → H₂O + SO₂ + 0.5O₂ (1)

actually proceeds in the following two steps.

H₂SO₄(aq) → H₂O(g) + SO₃(g) 300∼500 °C

SO₃(g) → SO₂(g) + 0.5O₂(g) 800∼900 °C

Both steps are highly endothermic and proceed smoothly without side reactions and with a high equilibrium conversion ratio at the temperature range indicated. The endothermic characteristics match well with the temperature distribution of the heat source, HTGR. The heat produced by HTGR is transferred to the chemical process through the sensible heat of helium gas, the temperature of which varies, e.g. 400∼900 °C. These characteristics make the reaction quite suited as a high temperature endothermic reaction for thermochemical water-splitting cycles.⁸

The Iodine–Sulfur cycle (or Sulfur–Iodine cycle, or Ispra Mark 16 cycle, hereafter termed as the IS process) utilizes the following two chemical reactions which combine with the sulfuric acid decomposition reaction to close the thermochemical water-splitting cycle (Fig. 1).

SO₂ + I₂ + 2H₂O → 2HI + H₂SO₄ (2)

2HI → H₂ + I₂ (3)

Fig. 1 Scheme of a thermochemical water-splitting cycle using iodine and sulfur.

Here, reaction (2), known as the “Bunsen reaction”, is a low-temperature exothermic reaction, where the raw material, water, reacts with iodine and sulfur dioxide producing spontaneously an aqueous solution of hydriodic acid and sulfuric acid. The thermal decomposition of hydrogen iodide (3) can be carried out either in gas phase or in liquid phase with the help of a catalyst. The IS process is a pure thermochemical cycle and can be an all fluid process that enjoys easy scale up.

3. Process chemistry and process design

For the combined operation of reactions (1)–(3), it was necessary, at first, to find a separation method for HI and H₂SO₄ produced by the Bunsen reaction, since conventional distillation of the mixed acid solution resulted in decomposition of the acids due to the occurrence of the reverse reaction of the Bunsen reaction. So far, the following ideas have been examined on the separation issue. De Beni et al.⁹ studied utilization of a solvent extraction method. Dokiya et al.¹⁰ proposed carrying out the Bunsen reaction using an electrochemical cell equipped with an ion exchange membrane. Introduction of a third element such as Ni,¹¹ Mg¹² was attempted to replace the acid separation with a separation of the corresponding salts, which could be carried out utilizing the difference in their solubility. However, the most intensive work has been carried out using a liquid–liquid separation phenomenon (LL separation), which researchers at General Atomics (GA) found and proposed to use for the HI/H₂SO₄ separation.^8,13 The LL separation occurs spontaneously in the presence of excess iodine. The heavier phase produced is mainly composed of HI, I₂, and H₂O, and is called “HIx” solution. The main components of the lighter phase are H₂SO₄ and H₂O. Fig. 2 shows the density of the phase-separated solution.¹⁴

Fig. 2 Relationship between density and H₂O molar fraction in H₂O + HI + H₂SO₄ of the mixed acid solution (H₂O + HI + H₂SO₄ + I₂ system) at 25 °C under iodine saturation conditions.¹⁴

The LL separation offered an easy way for the separation; however, it presented another problem, “How to separate HI from HIx solution?” In the studies carried out up to now, the composition of HIx solution produced by the Bunsen reaction hardly exceeded those of the quasi-azeotrope, and conventional distillation of the HIx solution resulted in a distillate of azeotropic hydriodic acid, the H₂O/HI molar ratio of which was ca. 5. Therefore, the distillation requires large reboiler duty, which reduces the conversion efficiency of heat to hydrogen energy. To overcome this the researchers at GA proposed an extractive distillation using phosphoric acid.⁸ Later, Engels et al.¹⁵ proposed the interesting concept of reactive distillation operated at elevated pressure, this enabled the separation of HI from the HIx solution and the decomposition of HI into hydrogen and iodine to be carried out in one column by utilizing the pressure shift of azeotropic and quasi-azeotropic composition. Recently, the application of membrane techniques such as electro–electrodialysis,¹⁶ pervaporation¹⁷ and membrane distillation¹⁸ have been examined to facilitate HI separation. However, at present, it is not clear which one is the best solution to the separation problem. At the same time, in pursuing a deeper understanding of the Bunsen reaction, parametric studies on the reaction system have been carried out in the LL separation condition by Giaconia et al.,¹⁹ Lee et al.,²⁰ and Nakajima et al.²¹Fig. 3 shows the effect of sulfur dioxide pressure on the concentration of HIx solution produced by the Bunsen reaction.²¹ These studies will be useful for constructing an efficient process scheme including the discussed separation of HI from the HIx solution.

Fig. 3 Effect of SO₂ partial pressure on the Bunsen reaction under iodine saturation condition at 50 °C. HI/(HI + H₂O) denotes the molar ratio in a hypothetical HIx solution that can be obtained by performing an ideal purification of the product heavy phase solution using back reaction of the Bunsen reaction.²¹

In the thermal decomposition of SO₃ and of HI, catalysts are required to achieve engineeringly feasible reaction rates, and these have been studied.^8,22,23 Also, the utilization of hydrogen separation membranes has been attempted as a way to purify the product hydrogen or to enhance HI decomposition (this equilibrium conversion ratio is limited to a low value of 20–30%.)²⁴

In parallel with these studies on process chemistry, the heat/mass balance of the water-splitting process has been studied by various plausible flowsheets. Researchers at GA published the results of a total flowsheet analysis, where the LL separation and the extractive distillation were considered.²⁵ A “process thermal efficiency”, defined as the ratio of the Higher Heating Value (HHV) of hydrogen produced and the net thermal energy input to the process, was estimated to be 47%. In their design, intensive heat recovery was assumed to include application of a steam recompression method to recover the vaporization heat of solvent water in the concentration of sulfuric acid and phosphoric acid. The application of a multi-effect evaporator was proposed to concentrate sulfuric acid by Knoche and co-workers.²⁶ Also, two different versions of the oxygen generation step were presented by Bilgen and co-workers.²⁷ As for the processing of HIx solution, reactive distillation has been attracting a lot of interest because of its simplicity.⁶

Precise thermodynamic data concerning the concentrated process solutions is desired for accurate evaluation and optimization of the heat/mass balance.²⁸ Especially, knowledge of the vapor–liquid equilibrium of the HI–I₂–H₂O system is insufficient in spite of its importance, and measurements to enhance the database are in progress at Commissariat à l'énergie atomique (CEA),²⁹ Ente per le Nuove tecnologie, l'Energia e l'Ambiente (ENEA)³⁰ and Japan Atomic Energy Agency (JAEA).³¹Fig. 4 shows the T–x–y curves at elevated pressures reported by JAEA.³¹

Fig. 4 Isobaric VLE data of the binary system HI(1) + H₂O(2). (■, □; 0.58 MPa), (▲, △; 0.30 MPa), (●, ○; 0.15 MPa), (◆, ◇: 0.016 MPa at 386 K, and 0.059 MPa at 353 K from Ref. 29. Here, filled keys denote bubbling point, whereas open keys denote dew point. A broken line with ◆ keys indicates the azeotrope calculated by Berndhäuser from Ref. 15b. [thick line, graph caption] (bubbling point curves) and ⋯ (dew point curves) were drawn by eye.³¹

4. Demonstration of closed-cycle continuous water-splitting

GA built a test apparatus named the “Closed-Loop Cycle Demonstrator (CLCD)” with a designed hydrogen production rate of 1.2 L h⁻¹.²⁵ The glass-made CLCD comprised a Bunsen reactor, HIx purifier, HI decomposer, H₂SO₄ purifier, H₂SO₄ decomposer, and was operated in batch mode to demonstrate the feasibility of water splitting. Following CLCD, GA built a bench-scale glass apparatus featuring the extractive distillation with a designed hydrogen production rate of 60 L h⁻¹. The unit operation was performed successfully in the Bunsen reaction step and in the sulfuric acid decomposition step, although it was not in the HI decomposition step.²⁵

Closed-cycle continuous operation of the water-splitting process featuring the LL separation was attempted by JAEA. Key points in the continuous operation were the selection and the control of the Bunsen reaction condition, by which possible side reactions forming sulfur and/or hydrogen sulfide should be avoided and also the composition of LL separated solution should be kept constant. With the help of an optical sensor for monitoring the composition fluctuation of the Bunsen solution, the continuous water-splitting was carried out with a hydrogen production rate of 1 L h⁻¹ using a small glass apparatus comprising Bunsen reactor, HIx purifier, HIx distillation column, HI decomposer, H₂SO₄ purifier/concentrator, H₂SO₄ decomposer.³² Later, a scaled-up glass apparatus equipped with an automatic control system featuring an ultrasonic sensor was built. Continuous and stable water-splitting was demonstrated for one week using the apparatus with a hydrogen production rate of ca. 30 L h⁻¹.³³Fig. 5 shows a simplified scheme of the apparatus. Although the basic technique for process control was developed by this work, improvement in composition monitoring is necessary to realize water-splitting under more efficient conditions. Recently, Kubo et al³⁴ derived a set of empirical equations for the densities and the composition of LL separated solutions, which will be a good basis for more versatile and convenient monitoring of the Bunsen reaction system.

Fig. 5 Scheme of continuous hydrogen production test apparatus.³³

The next step will be a demonstration of the continuous operation including an energy-efficient scheme for HI separation from HIx solution such as the extractive distillation, the reactive distillation, or the membrane processes. Recently, Sandia National Laboratory (SNL), GA and CEA collaborated to build a water-splitting test apparatus featuring the extractive distillation with a designed hydrogen production rate of 100–200 L h⁻¹, and operational tests are underway.³⁵

5. Development of components

Since sulfuric acid and iodine are very corrosive, the selection of construction materials is an important issue for the development of any large-scale plant. Corrosion tests have been carried out on commercially available materials at GA³⁶ and JAEA.³⁷ The knowledge obtained so far may be summarized as shown in Fig. 6. In the highest temperature process environment of gas-phase SO₃ decomposition, some refractory alloys that have been used in conventional industrial plants show good corrosion resistance. In the gaseous environment of HI decomposition at 450 °C, a Ni–Cr–Mo–Ta alloy was found to show good corrosion resistance. As for the Bunsen reaction step operating at around 100 °C, glass, Ta, Nb, SiC etc. showed good corrosion resistance, so be used as lining materials. In the environment of HIx distillation operating at around 230 °C, Ta showed excellent corrosion resistance. The most severe environment is in the boiling of concentrated sulfuric acid under high pressure (e.g. 2 MPa) at temperatures of up to about 400 °C, where materials containing Si, such as Si–SiC, SiC, Si₃N₄ and Fe–Si were the only materials that showed excellent corrosion resistance.

Fig. 6 Candidate construction materials which exhibited good corrosion resistance in simulated IS process environments.

Based on this knowledge, development of key components has been implemented. Researchers at JAEA, in co-operation with Toshiba Corp., have proposed the concept of a sulfuric-acid decomposer, in which the sulfuric acid of concentration more than 90 wt% is evaporated and, simultaneously, part of the H₂SO₄ spontaneously decomposes into gaseous SO₃ and H₂O using the heat supplied by high temperature helium gas.³⁸Fig. 7 shows a cut-away view of the sulfuric acid decomposer designed for a future test plant with a hydrogen production rate of 30 m³ h⁻¹. The decomposer is equipped with multi-hole heat exchanger blocks made of SiC. Two blocks of 0.25 m in outer diameters and 0.75 m in height are piled vertically with gold gaskets and tie-lods. Helium gas flows through the inside channels of the ceramic blocks, and exchanges heat with the counter-current sulfuric acid flow. Test fabrication of the ceramic heat exchanger was successfully completed which confirmed its fabricability. Also, helium leak tests using the test-fabricated heat exchanger under simulated seismic load confirmed its good seal performance.³⁹

Fig. 7 A Sulfuric Acid Decomposer. Left: concept of 30 m³ H₂ h⁻¹ scale model. Right: test-fabricated ceramic heat exchanger made of SiC.³⁸

Recently, researchers at SNL developed an interesting bayonet-type reactor made of SiC for the vaporization and decomposition of sulfuric acid for use in the hydrogen production test apparatus mentioned in Section 4. The reactor was composed of two concentric tubes and the outer tube had a hemispherical upper end. The SO₃ decomposition catalyst is set at the upper part of the annular space. Sulfuric acid is fed to the bottom of the annular space and vaporized by heating from outside of the outer tube. The vaporized sulfuric acid is superheated and decomposed into SO₂ and O₂ in the catalyst zone at a temperature of 800 °C. Then, the decomposed gaseous mixture flows down the inner tube while exchanging the heat with the gases flowing up the annular space. Sulfuric acid processing was successfully tested with the reactor.⁴⁰

Development of ceramic components for the sulfuric acid processing step is also in progress in Korea⁴¹ and in the EU.⁴²

6. Conclusions

The present status of research and development on thermochemical IS processing for hydrogen production is briefly reviewed. It should be noted that an international cooperation project has started on the development of a thermochemical water-splitting process, as one of the important research subjects in the framework of Very-High Temperature Reactor (VHTR) systems in the Generation IV International Forum. All of the research subjects mentioned in this paper will be studied in this project.

It is desirable that these activities produce fruitful results that will bring about a demonstration of large-scale economical hydrogen production.

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