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Hydrogen production using the sulfur–iodine cycle coupled to a VHTR: An overview
Energy Conversion and Management 47 (2006) 2740–2747 www.elsevier.com/locate/enconman
Hydrogen production using the sulfur–iodine cycle coupled to a VHTR: An overview X. Vitart *, A. Le Duigou, P. Carles Commissariat a` l’Energie Atomique, CEA/Saclay, DEN/DPC, 91191 Gif sur Yvette, France Available online 17 April 2006
Abstract The sulfur–iodine thermo-chemical cycle is considered to be one of the most promising routes for massive hydrogen production, using high temperature heat from a Generation IV VHTR. We propose here a brief overview of the main questions raised by this cycle, along with the general lines of French CEA’s program. 2006 Elsevier Ltd. All rights reserved. Keywords: Hydrogen production; VHTR; Sulfur–iodine cycle; Thermo-chemical cycles; Water splitting
1. Introduction The growth of global energy demand during the 21st century, combined to the necessity to master greenhouse gas emissions, could lead to the introduction of a new and universal energy carrier: hydrogen. Today most of the hydrogen production comes from hydrocarbons: oil (18%), coal (30%) and natural gas (48%). Only about 4% of H2 comes from water through electrolysis. These processes are considered to be the cheapest in the short and medium term. In the long term, to support the development of a hydrogen economy, massive production means are needed. Given the prospect of a lack of fossil resources and limitations on the release of greenhouse gases, only water and biomass are the two candidate raw materials for hydrogen production. The two processes that have the greatest likelihood of successful massive hydrogen production using water as the raw material are electrolysis and thermo-chemical cycles. Alkaline electrolysis at room temperature is a mature technology, but the initial works on high temperature electrolysis (HTE) showed significant advantages, such as reduced electricity needs (35%) compared to conventional electrolysis, together with a better efficiency. The thermo-chemical cycles are processes where water is decomposed into hydrogen and oxygen via chemical reactions using intermediate elements which are recycled. The sum of all the reactions is equivalent to the dissociation of the water molecule. Because they only use heat without having to convert it to electricity, these cycles have the potential of a better efficiency than electrolysis and hence have the potential to significantly reduce the cost for hydrogen production from water. *
Corresponding author. Fax: +33169086802. E-mail address: [email protected] (X. Vitart).
0196-8904/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.02.010
X. Vitart et al. / Energy Conversion and Management 47 (2006) 2740–2747
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Fig. 1. Comparison of heat demand of the sulfuric acid section of the I–S cycle with heat available from a VHTR.
A very simplified evaluation of hundreds of thermo-chemical cycles was made [1–3]: over a nine year program, 200 distinct thermo-chemical cycles were examined in the United States (Gas Research Institute), and, in the end, eight were operated successfully with recycled materials to achieve proof-of-principle. Recent reviews [2,3] conducted an evaluation based on selected criteria (number of reactions, number of chemical steps, flow of solids, corrosion issues, . . .). Four cycles clearly emerge from this review: the hybrid sulfur cycle and three close challengers, the Ispra hybrid cycle, the UT3 cycle and finally the sulfur–iodine (S–I) cycle [1,4]. In the range of the different primary energy sources, Nuclear Energy (especially with GEN IV systems, and in particular HTR and VHTR) intends to play an important role in massive hydrogen production through the routes of the above mentioned water splitting processes [5]. In particular, the sulfur family of thermo-chemical cycles, which have in common a sulfuric acid concentration and decomposition section, appears as most promising for coupling to a VHTR: indeed, the heat requirements of the sulfuric acid section fit continuously with the heat available from the VHTR (Fig. 1). French Commissariat a` l’Energie Atomique (CEA) has launched an integrated program to choose by 2008 the most promising way to produce hydrogen using the high temperature heat available from a VHTR. This program comprises: • • • •
Development of a methodology for process comparison. Acquisition of basic thermodynamic data. Flowsheet analysis and development. Preliminary design of a hydrogen production plant coupled to a VHTR, including energy distribution and safety issues. • Efficiency and cost analysis based on the previous items. CEA has chosen to concentrate on a limited number of processes, namely the sulfur–iodine cycle and high temperature electrolysis, which have been selected as being the most promising. Other options are only evaluated on a more theoretical basis. The present paper concentrates on the sulfur–iodine cycle. After a general presentation of the cycle, we quickly describe the issues associated with each of its three sections, before coming back to less specific questions related to the process in general. 2. Presentation of the sulfur–iodine cycle The S–I cycle can be split into the following reactions, in which the temperatures between brackets are approximate and depend upon the pressure which is not necessarily uniform in the different parts of the cycle:
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ð9I2 Þl þ ðSO2 Þg þ ð16H2 OÞ1 ) ð2HI þ 10H2 O þ 8I2 Þl þ ðH2 SO4 þ 4H2 OÞl L2 ¼ ð2HI þ 10H2 O þ 8I2 Þl ) ð2HIÞg þ ð10H2 O þ 8I2 Þl ð2HIÞg ) H2 þ ðI2 Þl
½230 C
½330 C
1 ðSO3 Þg ) ðSO2 Þg þ O2 2
½300 C
½360 C
ðH2 SO4 Þg ) ðSO3 Þg þ ðH2 OÞg
ð1Þ ð2Þ ð3Þ
Ll ¼ ðH2 SO4 þ 4H2 OÞ ) ðH2 SO4 Þl þ ð4H2 OÞl ðH2 SO4 Þl ) ðH2 SO4 Þg
½120 C
ð4Þ ð5Þ
½400 C
½870 C
ð6Þ ð7Þ
The first reaction, named the Bunsen reaction, proceeds exothermically in liquid phase and produces two immiscible aqueous acid phases which compositions are indicated between brackets: L1 phase which is aqueous sulfuric acid and L2 phase which is a mixture of hydrogen iodide, iodine and water named HIx. In the second reaction, HI is separated from L2. This separation is the most critical phase of the cycle. Reaction (3) is the thermal decomposition of HI. Knoche [6] proposed to perform reactions (2) and (3) in the same reactive distillation column. Reactions (5)–(7) proceed in gas phase and produce H2O, SO2 and oxygen. These gases are cooled down before to bubble in the Bunsen reactor to separate oxygen from SO2 and H2O. The whole cycle can be divided into three sections according to the GA nomenclature [1]: we shall call Section I reaction (1), Section II reactions (4)–(7) and Section III reactions (2) and (3). A sketch of the cycle is shown on Fig. 2. We use the efficiency definition g¼
DH 0H2 O ðT ¼ 25 CÞ ; Q þ gWel
where DH 0H2 O ðT ¼ 25 CÞ is the enthalpy of formation of liquid water at ambient temperature, Q and W are the heat and work requirements of the cycle and gel is the efficiency of the heat to work conversion system (taken as 0.5 in our models) [5,7]. From the set of Eqs. (1)–(7), the upper bound of this efficiency, calculated from the reversible heat and work requirements of the reactions, can be estimated to 51%. However, a refinement of the estimation, taking
Fig. 2. Schematic representation of the I–S cycle.
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into account a more detailed flowsheet along with plausible values of components efficiencies such as pumps or compressors, leads to a value between 34% and 37%, depending on the pinch assumed for heat recovery in Section III [7]. Improvements are therefore clearly required, even if it must be borne in mind that such evaluations are based on often poorly known thermodynamic data. The detailed flowsheet analysis which has been conducted allows to identify the issues raised by the cycle, and the areas where more precise basic data are required. 3. Bunsen section (Section I) The Bunsen reaction, such as written in (1), involves an excess of both water (to make the reaction spontaneous) and iodine (to induce the phase separation which is a key point of the process). However, such excesses are quite unfavourable for the following HIx section (as will be described below). Hence, research and development efforts are devoted to find new operation points for the Bunsen reaction with lower amounts of I2 and H2O, in order to find the best compromise between completion of reaction, phase separation, limitation of side reactions and energy loss in this low temperature exothermic step [8]. The experimental study of the Bunsen reaction has required the development of analytical methods to allow a quantitative determination of the total amount of H+ ions (potentiometry), sulfur (ICP-AES), iodine (UV– visible spectrophotometry after iodine reduction) and total water (density) in each phase. Specific devices have also been designed (Fig. 3), ranging from simple glass devices to develop the analytical methods to a tantalum pressure vessel allowing to study the Bunsen reaction in actual process conditions, including the possibility of varying the SO2 pressure. 4. HIx section (Section III) In the S–I cycle, HI decomposition according to Eq. (3) must be achieved from the HIx mixture produced during the Bunsen reaction described above. Four main difficulties have to be overcome: • the extraction of HI from the HIx mixture is difficult because of the presence of an azeotrope (Fig. 4) in the mixture, which prevents simple distillation; • the extraction of HI from the HIx mixture requires very large heat exchanges, due to the large heat capacity induced by the high water content of the mixture; • the decomposition reaction (3) is incomplete; • the decomposition reaction (3) is slow. Three main options are currently being considered for the HI section (Fig. 5):
Fig. 3. Glass devices for the study of the Bunsen reaction.
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X. Vitart et al. / Energy Conversion and Management 47 (2006) 2740–2747
Bubble Pressure of HI-H2O Mixture 6
Bunsen 5
HI rich vapor
HI poor vapor
P (bar)
4 150°C
3
125°C
2
100°C 1 0 0
0.05
0.1
0.15
0.2
0.25
HI molar fraction Fig. 4. Azeotropic lines of HIx mixture.
Fig. 5. Schematic representation of the main options for the HI section.
• Extractive distillation was proposed by General atomics [1]: the introduction of phosphoric acid induces first the separation of iodine, and then allows simple distillation of HI. HI is then decomposed in gaseous (or possibly liquid) phase around 450 C to yield H2, which has to be separated from the gaseous mixture using membranes. • The present Japanese scheme favors electrodialysis [4], which removes some water of the HIx mixture to concentrate it beyond the azeotropic limit. Excess HI is then removed by simple distillation. The final decomposition and extraction steps are the same as in the extractive distillation process. • Reactive distillation was proposed in the 80s by RWTH Aachen [6]. HIx distillation and HI decomposition are performed in the same reactor at 350 C. A liquid–gas equilibrium is obtained in the middle of the column, I2 is solubilised in the lower liquid phase and a mixture of gaseous H2 and water is recovered at the top of the column. Apart from the membranes required for H2 separation in the extractive distillation and electrodialysis schemes, membranes could be used at other places in the cycle. In particular, pervaporation membranes [9]
X. Vitart et al. / Energy Conversion and Management 47 (2006) 2740–2747
2745
could allow removing water from the HIx mixture, thus providing a low energetic cost alternative to bypass the azeotrope. Reactive distillation is the reference scheme chosen by CEA [7]. However, the amount of hydrogen produced during the process depends closely on the I2 and HI concentrations in the vapor, which means that the correct evaluation of the actual efficiency of reactive distillation requires the knowledge of liquid–vapor equilibrium data for the HIx mixture. The model currently used to represent this equilibrium is based on total pressure measurements performed at RWTH Aachen, and CEA has launched a program to measure the relevant partial pressures under process conditions (up to 300 C and 5 MPa). Like for the Bunsen section, this program involves specifically designed experimental devices as well as the development of suitable analytical methods. In particular, we have developed optical diagnostics (FTIR spectrometry, UV–visible spectrophotometry, spontaneous Raman scattering) to measure the composition of the vapor phase in the very concentrated conditions encountered in the process. 5. Sulfuric acid section (Section II) This section appears to be the best known one of the cycle, because of the experience gained in the sulfuric acid industry. In the proposed flowsheet [7], sulfuric acid is concentrated through a series of flashes starting from low pressure. It is then dehydrated, before SO3 is decomposed into SO2. This decomposition being only partial, undecomposed SO3 is recombined with water, which allows to recover its heat content. The main remaining questions concern the high temperature step of the process, namely SO3 into SO2 decomposition. The reaction, which requires a temperature in the 870 C range, will take place in a reactor and use the heat from a VHTR. This reactor will therefore also be a very high temperature heat exchanger, which will raise technological issues. Furthermore, a low pressure will be sought on the reaction side, whereas the heat transfer fluid will more likely be at high pressure. Finally, the temperature provided by a VHTR is not high enough to avoid the use of a catalyst for the reaction, and the long term resistance of this catalyst under the severe conditions that prevail will have to be ensured. 6. Corrosion of materials The sulfur–iodine cycle is very demanding on materials, which are exposed to very corrosive species at elevated temperatures and pressures. A literature review was conducted, which provided some materials as candidates for the Section I (Bunsen) of the process: ceramics (SiC, Si3N4, Al2O3), glass, fluocarbons, tantalum and zirconium or Ni alloys. However, corrosion tests are necessary to assess the maximum temperature and acidity acceptable conditions, the long term behaviour and the corrosion mechanisms (Fig. 6).
Fig. 6. Device for the study of partial pressures around 120 C.
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Fig. 7. Electrochemical tests on metallic materials.
Fig. 8. Corrosion rates of metallic materials.
As a first step, electrochemical and dipping tests have been conducted in each acid produced by the Bunsenreaction (aqueous sulfuric acid and a mixture of hydriodic acid, iodine and water named HIx), up to 95 C, so that first operating ranges are given for the candidate materials. Immersion tests performed in separate acids up to 140 C showed that tantalum and zirconium seem to be the most metallic-relevant materials; nevertheless localized corrosion has been observed on zirconium in liquid Bunsen condition (H2SO4–HI–I2 10 wt.%– 10 wt.%–70 wt.%), (cf. Figs. 7 and 8). The next step will be the study the long term behaviour of selected materials in separate and both acids, under the standard temperature and pressure conditions given by the flowsheet of the process, in a pressurised reactor. 7. Iodine questions The use of iodine, a not so abundant and rather expensive material, in the process may raise questions about the economic viability of the cycle. The first question relates to the availability of large enough quantities of iodine in the world. First estimates of the iodine hold-up in a 600 MW VHTR coupled hydrogen production plant, designed using a detailed flowsheet of the cycle, are on the order of 3000 t. This amount seems realistic when compared to the world yearly production of 20 000 t and the estimated world reserves of 15 · 106 t [10]. Another concern relates to the iodine losses. With the stoichiometry given above and an average cost of $15 per kg of iodine [10], the production of 1 kg of H2 requires the handling of $20 000 of iodine. This implies that
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iodine molar losses must be well below 104 to reach the hydrogen production cost goal (on the order of $2 per kg of H2). 8. Conclusion As the sulfur–iodine cycle is investigated worldwide, the problems that have to be solved appear more and more clearly. However, the basic advantages of this cycle remain valid: • it only involves the handling of fluids; • it is purely thermo-chemical, which is associated with low electricity need and therefore high potential efficiency; • its coupling to a VHTR seems promising. CEA has chosen to concentrate its efforts on a scientific approach based on data acquisition (development of devoted devices and specific analytical methods) and modeling (physical models, flowsheet analysis, systemic approach) in order to develop its own expertise on thermo-chemical cycles assessment. This work is conducted in synergy and collaboration with international programs, which allow to share the cost of the studies, especially in the costly field of demonstration loops. Experience gained on the evaluation of the sulfur–iodine cycle will be built on to perform a theoretical assessment of other potentially interesting cycles. References [1] Besenbruch GE et al. GA-A18257 report, 1982. [2] Funk JE. Thermochemical hydrogen production: past and present. Int J Hydrogen Energy 2001;26:185–90. [3] Brown LC, Funk JF, Showalter SK. Initial screening of thermo-chemical water-splitting cycles for high efficiency generation of hydrogen fuels using nuclear power. GA-A23373 report, 2000. [4] Miyamato Y et al. R&D Program on hydrogen production system with high temperature cooled reactor. In: Proc int hydrogen energy forum 2000, Munich, Germany; vol. 2, 2000, p. 271–8. [5] Vitart X. Thermochemical production of hydrogen. Nuclear production of hydrogen. In: Hori M, Spitalnik J, editors. Current issues in nuclear energy series. International Nuclear Societies Council; 2004. p. 45–67. [6] Roth M, Knoche KF. Thermochemical water splitting through direct HI decomposition from H2O–HI–I2 solutions. Int J Hydrogen Energy 1989;14:545–9. [7] Goldstein S, Borgard JM, Vitart X. Upper bound and best estimate of the efficiency of the iodine sulfur cycle. Int J Hydrogen Energy 2005;30:619–26. [8] Vitart X et al. Investigation of the I–S cycle for massive hydrogen production. In: ANL symposium, Argonne, 2004. [9] Elder RH, Borgard JM, Priestman GH, Ewan BC, Allen RWK. Use of membranes and reactive distillation for the separation of HIx in the sulphur–iodine cycle. In: Proceedings of the AIChE 2005 Annual meeting, in press. [10] http://minerals.usgs.gov/minerals/pubs/commodity/iodine/.
Hydrogen production using the sulfur–iodine cycle coupled to a VHTR: An overview X. Vitart *, A. Le Duigou, P. Carles Commissariat a` l’Energie Atomique, CEA/Saclay, DEN/DPC, 91191 Gif sur Yvette, France Available online 17 April 2006
Abstract The sulfur–iodine thermo-chemical cycle is considered to be one of the most promising routes for massive hydrogen production, using high temperature heat from a Generation IV VHTR. We propose here a brief overview of the main questions raised by this cycle, along with the general lines of French CEA’s program. 2006 Elsevier Ltd. All rights reserved. Keywords: Hydrogen production; VHTR; Sulfur–iodine cycle; Thermo-chemical cycles; Water splitting
1. Introduction The growth of global energy demand during the 21st century, combined to the necessity to master greenhouse gas emissions, could lead to the introduction of a new and universal energy carrier: hydrogen. Today most of the hydrogen production comes from hydrocarbons: oil (18%), coal (30%) and natural gas (48%). Only about 4% of H2 comes from water through electrolysis. These processes are considered to be the cheapest in the short and medium term. In the long term, to support the development of a hydrogen economy, massive production means are needed. Given the prospect of a lack of fossil resources and limitations on the release of greenhouse gases, only water and biomass are the two candidate raw materials for hydrogen production. The two processes that have the greatest likelihood of successful massive hydrogen production using water as the raw material are electrolysis and thermo-chemical cycles. Alkaline electrolysis at room temperature is a mature technology, but the initial works on high temperature electrolysis (HTE) showed significant advantages, such as reduced electricity needs (35%) compared to conventional electrolysis, together with a better efficiency. The thermo-chemical cycles are processes where water is decomposed into hydrogen and oxygen via chemical reactions using intermediate elements which are recycled. The sum of all the reactions is equivalent to the dissociation of the water molecule. Because they only use heat without having to convert it to electricity, these cycles have the potential of a better efficiency than electrolysis and hence have the potential to significantly reduce the cost for hydrogen production from water. *
Corresponding author. Fax: +33169086802. E-mail address: [email protected] (X. Vitart).
0196-8904/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.02.010
X. Vitart et al. / Energy Conversion and Management 47 (2006) 2740–2747
2741
Fig. 1. Comparison of heat demand of the sulfuric acid section of the I–S cycle with heat available from a VHTR.
A very simplified evaluation of hundreds of thermo-chemical cycles was made [1–3]: over a nine year program, 200 distinct thermo-chemical cycles were examined in the United States (Gas Research Institute), and, in the end, eight were operated successfully with recycled materials to achieve proof-of-principle. Recent reviews [2,3] conducted an evaluation based on selected criteria (number of reactions, number of chemical steps, flow of solids, corrosion issues, . . .). Four cycles clearly emerge from this review: the hybrid sulfur cycle and three close challengers, the Ispra hybrid cycle, the UT3 cycle and finally the sulfur–iodine (S–I) cycle [1,4]. In the range of the different primary energy sources, Nuclear Energy (especially with GEN IV systems, and in particular HTR and VHTR) intends to play an important role in massive hydrogen production through the routes of the above mentioned water splitting processes [5]. In particular, the sulfur family of thermo-chemical cycles, which have in common a sulfuric acid concentration and decomposition section, appears as most promising for coupling to a VHTR: indeed, the heat requirements of the sulfuric acid section fit continuously with the heat available from the VHTR (Fig. 1). French Commissariat a` l’Energie Atomique (CEA) has launched an integrated program to choose by 2008 the most promising way to produce hydrogen using the high temperature heat available from a VHTR. This program comprises: • • • •
Development of a methodology for process comparison. Acquisition of basic thermodynamic data. Flowsheet analysis and development. Preliminary design of a hydrogen production plant coupled to a VHTR, including energy distribution and safety issues. • Efficiency and cost analysis based on the previous items. CEA has chosen to concentrate on a limited number of processes, namely the sulfur–iodine cycle and high temperature electrolysis, which have been selected as being the most promising. Other options are only evaluated on a more theoretical basis. The present paper concentrates on the sulfur–iodine cycle. After a general presentation of the cycle, we quickly describe the issues associated with each of its three sections, before coming back to less specific questions related to the process in general. 2. Presentation of the sulfur–iodine cycle The S–I cycle can be split into the following reactions, in which the temperatures between brackets are approximate and depend upon the pressure which is not necessarily uniform in the different parts of the cycle:
2742
X. Vitart et al. / Energy Conversion and Management 47 (2006) 2740–2747
ð9I2 Þl þ ðSO2 Þg þ ð16H2 OÞ1 ) ð2HI þ 10H2 O þ 8I2 Þl þ ðH2 SO4 þ 4H2 OÞl L2 ¼ ð2HI þ 10H2 O þ 8I2 Þl ) ð2HIÞg þ ð10H2 O þ 8I2 Þl ð2HIÞg ) H2 þ ðI2 Þl
½230 C
½330 C
1 ðSO3 Þg ) ðSO2 Þg þ O2 2
½300 C
½360 C
ðH2 SO4 Þg ) ðSO3 Þg þ ðH2 OÞg
ð1Þ ð2Þ ð3Þ
Ll ¼ ðH2 SO4 þ 4H2 OÞ ) ðH2 SO4 Þl þ ð4H2 OÞl ðH2 SO4 Þl ) ðH2 SO4 Þg
½120 C
ð4Þ ð5Þ
½400 C
½870 C
ð6Þ ð7Þ
The first reaction, named the Bunsen reaction, proceeds exothermically in liquid phase and produces two immiscible aqueous acid phases which compositions are indicated between brackets: L1 phase which is aqueous sulfuric acid and L2 phase which is a mixture of hydrogen iodide, iodine and water named HIx. In the second reaction, HI is separated from L2. This separation is the most critical phase of the cycle. Reaction (3) is the thermal decomposition of HI. Knoche [6] proposed to perform reactions (2) and (3) in the same reactive distillation column. Reactions (5)–(7) proceed in gas phase and produce H2O, SO2 and oxygen. These gases are cooled down before to bubble in the Bunsen reactor to separate oxygen from SO2 and H2O. The whole cycle can be divided into three sections according to the GA nomenclature [1]: we shall call Section I reaction (1), Section II reactions (4)–(7) and Section III reactions (2) and (3). A sketch of the cycle is shown on Fig. 2. We use the efficiency definition g¼
DH 0H2 O ðT ¼ 25 CÞ ; Q þ gWel
where DH 0H2 O ðT ¼ 25 CÞ is the enthalpy of formation of liquid water at ambient temperature, Q and W are the heat and work requirements of the cycle and gel is the efficiency of the heat to work conversion system (taken as 0.5 in our models) [5,7]. From the set of Eqs. (1)–(7), the upper bound of this efficiency, calculated from the reversible heat and work requirements of the reactions, can be estimated to 51%. However, a refinement of the estimation, taking
Fig. 2. Schematic representation of the I–S cycle.
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2743
into account a more detailed flowsheet along with plausible values of components efficiencies such as pumps or compressors, leads to a value between 34% and 37%, depending on the pinch assumed for heat recovery in Section III [7]. Improvements are therefore clearly required, even if it must be borne in mind that such evaluations are based on often poorly known thermodynamic data. The detailed flowsheet analysis which has been conducted allows to identify the issues raised by the cycle, and the areas where more precise basic data are required. 3. Bunsen section (Section I) The Bunsen reaction, such as written in (1), involves an excess of both water (to make the reaction spontaneous) and iodine (to induce the phase separation which is a key point of the process). However, such excesses are quite unfavourable for the following HIx section (as will be described below). Hence, research and development efforts are devoted to find new operation points for the Bunsen reaction with lower amounts of I2 and H2O, in order to find the best compromise between completion of reaction, phase separation, limitation of side reactions and energy loss in this low temperature exothermic step [8]. The experimental study of the Bunsen reaction has required the development of analytical methods to allow a quantitative determination of the total amount of H+ ions (potentiometry), sulfur (ICP-AES), iodine (UV– visible spectrophotometry after iodine reduction) and total water (density) in each phase. Specific devices have also been designed (Fig. 3), ranging from simple glass devices to develop the analytical methods to a tantalum pressure vessel allowing to study the Bunsen reaction in actual process conditions, including the possibility of varying the SO2 pressure. 4. HIx section (Section III) In the S–I cycle, HI decomposition according to Eq. (3) must be achieved from the HIx mixture produced during the Bunsen reaction described above. Four main difficulties have to be overcome: • the extraction of HI from the HIx mixture is difficult because of the presence of an azeotrope (Fig. 4) in the mixture, which prevents simple distillation; • the extraction of HI from the HIx mixture requires very large heat exchanges, due to the large heat capacity induced by the high water content of the mixture; • the decomposition reaction (3) is incomplete; • the decomposition reaction (3) is slow. Three main options are currently being considered for the HI section (Fig. 5):
Fig. 3. Glass devices for the study of the Bunsen reaction.
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Bubble Pressure of HI-H2O Mixture 6
Bunsen 5
HI rich vapor
HI poor vapor
P (bar)
4 150°C
3
125°C
2
100°C 1 0 0
0.05
0.1
0.15
0.2
0.25
HI molar fraction Fig. 4. Azeotropic lines of HIx mixture.
Fig. 5. Schematic representation of the main options for the HI section.
• Extractive distillation was proposed by General atomics [1]: the introduction of phosphoric acid induces first the separation of iodine, and then allows simple distillation of HI. HI is then decomposed in gaseous (or possibly liquid) phase around 450 C to yield H2, which has to be separated from the gaseous mixture using membranes. • The present Japanese scheme favors electrodialysis [4], which removes some water of the HIx mixture to concentrate it beyond the azeotropic limit. Excess HI is then removed by simple distillation. The final decomposition and extraction steps are the same as in the extractive distillation process. • Reactive distillation was proposed in the 80s by RWTH Aachen [6]. HIx distillation and HI decomposition are performed in the same reactor at 350 C. A liquid–gas equilibrium is obtained in the middle of the column, I2 is solubilised in the lower liquid phase and a mixture of gaseous H2 and water is recovered at the top of the column. Apart from the membranes required for H2 separation in the extractive distillation and electrodialysis schemes, membranes could be used at other places in the cycle. In particular, pervaporation membranes [9]
X. Vitart et al. / Energy Conversion and Management 47 (2006) 2740–2747
2745
could allow removing water from the HIx mixture, thus providing a low energetic cost alternative to bypass the azeotrope. Reactive distillation is the reference scheme chosen by CEA [7]. However, the amount of hydrogen produced during the process depends closely on the I2 and HI concentrations in the vapor, which means that the correct evaluation of the actual efficiency of reactive distillation requires the knowledge of liquid–vapor equilibrium data for the HIx mixture. The model currently used to represent this equilibrium is based on total pressure measurements performed at RWTH Aachen, and CEA has launched a program to measure the relevant partial pressures under process conditions (up to 300 C and 5 MPa). Like for the Bunsen section, this program involves specifically designed experimental devices as well as the development of suitable analytical methods. In particular, we have developed optical diagnostics (FTIR spectrometry, UV–visible spectrophotometry, spontaneous Raman scattering) to measure the composition of the vapor phase in the very concentrated conditions encountered in the process. 5. Sulfuric acid section (Section II) This section appears to be the best known one of the cycle, because of the experience gained in the sulfuric acid industry. In the proposed flowsheet [7], sulfuric acid is concentrated through a series of flashes starting from low pressure. It is then dehydrated, before SO3 is decomposed into SO2. This decomposition being only partial, undecomposed SO3 is recombined with water, which allows to recover its heat content. The main remaining questions concern the high temperature step of the process, namely SO3 into SO2 decomposition. The reaction, which requires a temperature in the 870 C range, will take place in a reactor and use the heat from a VHTR. This reactor will therefore also be a very high temperature heat exchanger, which will raise technological issues. Furthermore, a low pressure will be sought on the reaction side, whereas the heat transfer fluid will more likely be at high pressure. Finally, the temperature provided by a VHTR is not high enough to avoid the use of a catalyst for the reaction, and the long term resistance of this catalyst under the severe conditions that prevail will have to be ensured. 6. Corrosion of materials The sulfur–iodine cycle is very demanding on materials, which are exposed to very corrosive species at elevated temperatures and pressures. A literature review was conducted, which provided some materials as candidates for the Section I (Bunsen) of the process: ceramics (SiC, Si3N4, Al2O3), glass, fluocarbons, tantalum and zirconium or Ni alloys. However, corrosion tests are necessary to assess the maximum temperature and acidity acceptable conditions, the long term behaviour and the corrosion mechanisms (Fig. 6).
Fig. 6. Device for the study of partial pressures around 120 C.
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Fig. 7. Electrochemical tests on metallic materials.
Fig. 8. Corrosion rates of metallic materials.
As a first step, electrochemical and dipping tests have been conducted in each acid produced by the Bunsenreaction (aqueous sulfuric acid and a mixture of hydriodic acid, iodine and water named HIx), up to 95 C, so that first operating ranges are given for the candidate materials. Immersion tests performed in separate acids up to 140 C showed that tantalum and zirconium seem to be the most metallic-relevant materials; nevertheless localized corrosion has been observed on zirconium in liquid Bunsen condition (H2SO4–HI–I2 10 wt.%– 10 wt.%–70 wt.%), (cf. Figs. 7 and 8). The next step will be the study the long term behaviour of selected materials in separate and both acids, under the standard temperature and pressure conditions given by the flowsheet of the process, in a pressurised reactor. 7. Iodine questions The use of iodine, a not so abundant and rather expensive material, in the process may raise questions about the economic viability of the cycle. The first question relates to the availability of large enough quantities of iodine in the world. First estimates of the iodine hold-up in a 600 MW VHTR coupled hydrogen production plant, designed using a detailed flowsheet of the cycle, are on the order of 3000 t. This amount seems realistic when compared to the world yearly production of 20 000 t and the estimated world reserves of 15 · 106 t [10]. Another concern relates to the iodine losses. With the stoichiometry given above and an average cost of $15 per kg of iodine [10], the production of 1 kg of H2 requires the handling of $20 000 of iodine. This implies that
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iodine molar losses must be well below 104 to reach the hydrogen production cost goal (on the order of $2 per kg of H2). 8. Conclusion As the sulfur–iodine cycle is investigated worldwide, the problems that have to be solved appear more and more clearly. However, the basic advantages of this cycle remain valid: • it only involves the handling of fluids; • it is purely thermo-chemical, which is associated with low electricity need and therefore high potential efficiency; • its coupling to a VHTR seems promising. CEA has chosen to concentrate its efforts on a scientific approach based on data acquisition (development of devoted devices and specific analytical methods) and modeling (physical models, flowsheet analysis, systemic approach) in order to develop its own expertise on thermo-chemical cycles assessment. This work is conducted in synergy and collaboration with international programs, which allow to share the cost of the studies, especially in the costly field of demonstration loops. Experience gained on the evaluation of the sulfur–iodine cycle will be built on to perform a theoretical assessment of other potentially interesting cycles. References [1] Besenbruch GE et al. GA-A18257 report, 1982. [2] Funk JE. Thermochemical hydrogen production: past and present. Int J Hydrogen Energy 2001;26:185–90. [3] Brown LC, Funk JF, Showalter SK. Initial screening of thermo-chemical water-splitting cycles for high efficiency generation of hydrogen fuels using nuclear power. GA-A23373 report, 2000. [4] Miyamato Y et al. R&D Program on hydrogen production system with high temperature cooled reactor. In: Proc int hydrogen energy forum 2000, Munich, Germany; vol. 2, 2000, p. 271–8. [5] Vitart X. Thermochemical production of hydrogen. Nuclear production of hydrogen. In: Hori M, Spitalnik J, editors. Current issues in nuclear energy series. International Nuclear Societies Council; 2004. p. 45–67. [6] Roth M, Knoche KF. Thermochemical water splitting through direct HI decomposition from H2O–HI–I2 solutions. Int J Hydrogen Energy 1989;14:545–9. [7] Goldstein S, Borgard JM, Vitart X. Upper bound and best estimate of the efficiency of the iodine sulfur cycle. Int J Hydrogen Energy 2005;30:619–26. [8] Vitart X et al. Investigation of the I–S cycle for massive hydrogen production. In: ANL symposium, Argonne, 2004. [9] Elder RH, Borgard JM, Priestman GH, Ewan BC, Allen RWK. Use of membranes and reactive distillation for the separation of HIx in the sulphur–iodine cycle. In: Proceedings of the AIChE 2005 Annual meeting, in press. [10] http://minerals.usgs.gov/minerals/pubs/commodity/iodine/.