Development and test of a solar reactor for decomposition of sulphuric acid in thermochemical hydrogen production

Decomposition of sulphuric acid is a key step of sulphur based thermochemical cycles for hydrogen production by thermal splitting of water. The Hybrid Sulphur Cycle (HyS) consisting of two reaction steps is considered as one of the most promising cycles: firstly, sulphuric acid is decomposed by high temperature heat of 800–1200 °C forming sulphur dioxide, which in a second step is used to electrochemically split water. Compared to conventional water electrolysis only about a tenth of the theoretical voltage is required making the HyS one of the most efficient processes to produce hydrogen by concentrated solar radiation. As a result, this thermochemical cycle has the potential to significantly reduce the amount of energy required for water splitting and to efficiently generate hydrogen free of carbon dioxide emissions. The European research project HycycleS aims at a technical realisation of the HyS. One objective of the project is to develop and qualify a solar interface, meaning a device to couple concentrated solar radiation into the endothermal steps of the chemical process. Therefore, a test reactor for decomposition of sulphuric acid by concentrated solar radiation was developed and tested in the solar furnace of DLR in Cologne. Tests in concentrated solar radiation were carried out for temperatures of the honeycomb up to 950 °C decomposing sulphuric acid of 50 and 96 weight-percent. Mass and energy flow of the process were calculated in order to determine energy efficiency and chemical conversion. The influence of process parameters like temperature, flow rates and space velocity on chemical conversion and reactor efficiency was analysed in detail. If catalysts like iron oxide (Fe₂O₃) and mixed oxides (i.e. CuFe₂O₄) were used a conversion of SO₃ to SO₂ of more than 80% at a thermal efficiency of over 25% could be reached.     

Long-term corrosion tests of materials for thermal decomposition of sulphuric acid

The corrosion resistance of tubes of Incoloy 800 and AISI 310 SS has been studied in a mixture of air and H₂SO₄ vapours at 1 bar pressure. The duration of the test was one year (8500 h). The temperatures of the tubes ranged from 400 to 700°C for SS AISI 310 and from 400 to 900°C for Incoloy 800. The extent of the attack was determined by measuring the scale thickness and the penetration depth on cross-sections of samples taken from several zones of the tubes. X-ray diffraction and S.E.M. X-ray microanalysis techniques were used.     

Kinetics of the catalytic decomposition of hydrogen iodide in the thermochemical hydrogen production

The decomposition rates of hydrogen iodide over platinum supported γ-alumina were measured in the range from 480 to 700 K by the use of a flow method. It was found that Pt/γ-alumina catalyst was effective for the decomposition of hydrogen iodide. According to the Langmuir-Hinshelwood model, an overall rate equation was obtained on the basis of the mechanism where the rate-determining step was a surface reaction. Experimental data were well correlated with the rate equation.     

Analysis of sulfur–iodine thermochemical cycle for solar hydrogen production. Part I: decomposition of sulfuric acid

The sulfur–iodine (S–I) thermochemical water splitting cycle is one of the most studied cycles for hydrogen (H₂) production. S–I cycle consists of four sections: (I) acid production and separation and oxygen purification, (II) sulfuric acid concentration and decomposition, (III) hydroiodic acid (HI) concentration, and (IV) HI decomposition and H₂ purification. Section II of the cycle is an endothermic reaction driven by the heat input from a high temperature source. Analysis of the S–I cycle in the past thirty years have been focused mostly on the utilization of nuclear power as the high temperature heat source for the sulfuric acid decomposition step. Thermodynamic as well as kinetic considerations indicate that both the extent and rate of sulfuric acid decomposition can be improved at very high temperatures (in excess of 1000 °C) available only from solar concentrators. The beneficial effect of high temperature solar heat for decomposition of sulfuric acid in the S–I cycle is described in this paper. We used Aspen Technologies' HYSYS chemical process simulator (CPS) to develop flowsheets for sulfuric acid (H₂SO₄) decomposition that include all mass and heat balances. Based on the HYSYS analyses, two new process flowsheets were developed. These new sulfuric acid decomposition processes are simpler and more stable than previous processes and yield higher conversion efficiencies for the sulfuric acid decomposition and sulfur dioxide and oxygen formation.     

Thermal efficiency of the magnesium-iodine cycle for thermochemical hydrogen production

The magnesium-iodine cycle, consisting of the redox reaction of iodine with magnesium oxide, the thermal decomposition of magnesium iodate, the hydrolysis of magnesium iodide and the thermal decomposition of hydrogen iodide, for thermochemical hydrogen production has been evaluated. The thermal efficiency of the process was calculated based on material and energy balances for three proposed flow-sheets. The three flow-sheets varied according to the method used (quenching, selective absorption of HI by magnesium oxide and the two-step chemical decomposition of HI by magnetite) to separate the products of hydrogen iodide decomposition. Values of 9–34% thermal efficiency were given as a function of the heat recovery of 65–85% for the quench method, 19–37% for the MgO method and 16–35% for the Fe₃O₄ method. In order to obtain more than 25–30% of the thermal efficiency, heat recovery must be more than 80%.     

Design of systems for hydrogen production based on the Cu-Cl thermochemical water decomposition cycle: Configurations and performance

In this study, we analyze several Cu-Cl cycles by examining various design schemes for an overall system and its components, in order to identify potential performance improvements. The factors that determine the number and effective grouping of steps for new design schemes are analyzed. A thermodynamic analysis and several parametric studies are presented for various configurations. The energy efficiency is found to be 44% for the five-step thermochemical process, 43% for the four-step process and 41% for the three-step process, based on the lower heating value of hydrogen. Also, conclusions regarding implementation of these new configurations are discussed and the potential benefits ascertained.     

Advances in hydrogen production by thermochemical water decomposition: A review

Hydrogen demand as an energy currency is anticipated to rise significantly in the future, with the emergence of a hydrogen economy. Hydrogen production is a key component of a hydrogen economy. Several production processes are commercially available, while others are under development including thermochemical water decomposition, which has numerous advantages over other hydrogen production processes. Recent advances in hydrogen production by thermochemical water decomposition are reviewed here. Hydrogen production from non-fossil energy sources such as nuclear and solar is emphasized, as are efforts to lower the temperatures required in thermochemical cycles so as to expand the range of potential heat supplies. Limiting efficiencies are explained and the need to apply exergy analysis is illustrated. The copper–chlorine thermochemical cycle is considered as a case study. It is concluded that developments of improved processes for hydrogen production via thermochemical water decomposition are likely to continue, thermochemical hydrogen production using such non-fossil energy will likely become commercial, and improved efficiencies are expected to be obtained with advanced methodologies like exergy analysis. Although numerous advances have been made on sulphur–iodine cycles, the copper–chlorine cycle has significant potential due to its requirement for process heat at lower temperatures than most other thermochemical processes.     

Modeling of a direct solar receiver reactor for decomposition of sulfuric acid in thermochemical hydrogen production cycles

Hydrogen production thermochemical cycles, based on the recirculation of sulfur-based compounds, are among the best suited processes to produce hydrogen using concentrated solar power. The sulfuric acid decomposition section is common to each sulfur-based cycle and represents one of the fundamental steps. A novel direct solar receiver-reactor concept is conceived, conceptually designed and simulated. A detailed transport phenomena model, including mass, energy and momentum balance expressions as well as suitable decomposition kinetics, is described adopting a finite volume approach. A single unit reactor is simulated with an inlet flow rate of 0.28 kg/s (corresponding to a production of approximately 11 kg_H2/h in a Hybrid Sulfur process) and a direct solar irradiation at a constant power of 143 kW/m². Results, obtained for the high temperature catalytic decomposition of SO₃ into SO₂ and O₂, demonstrate the effectiveness of the proposed concept, operating at pressures of 14 bar. A maximum temperature of 879 °C is achieved in the reactor body, with a corresponding average SO₂ mass fraction of 27.8%. The overall pressure drop value is 1.7 bar. The reactor allows the SO₃ decomposition into SO₂ and O₂ to be realized effectively, requiring an external high temperature solar power input of 123.6 kJ/molSO2 (i.e. 123.6 kJ/mol_H2).     

Numerical modeling of a bayonet heat exchanger-based reactor for sulfuric acid decomposition in thermochemical hydrogen production processes

Sulfur-based thermochemical hydrogen production cycles represent one of the most appealing options to produce hydrogen from water on a large scale. The Hybrid Sulfur is one of the most advanced thermochemical cycles. The high temperature section of the process, common to all sulfur-based cycles, operates the sulfuric acid thermal decomposition reaction at temperatures on the order of 800 °C. The paper shows and discusses the modeling results obtained for a bayonet heat exchanger based high temperature reactor that decomposes the sulfur compounds into sulfur dioxide and oxygen. A detailed transport phenomena model, including suitable decomposition kinetics, has been set up using a finite volume numerical approach. A preliminary configuration of the reactor, established based on process simulation results and on the initial reactor prototype developed at Sandia National Laboratory, has been examined and simulated. Results, obtained for a reactor driven by thermal power provided by helium flow, demonstrate the effective decomposition performance at maximum temperatures on the order of 800 °C and pressures of 14 bar. For a laminar flow configuration a sulfur dioxide production yield of about 28 wt% (with sulfur trioxide reduction from 69 wt% to approximately 33 wt%) has been achieved, representing decomposition rates practically equal to the corresponding equilibrium values. Limited pressure drops of approximately 2500 Pa have also been achieved in the sulfur mixture region.     

Investigations on the hydrolysis step of copper-chlorine thermochemical cycle for hydrogen production

Detailed investigations of CuCl₂ hydrolysis step of Cu–Cl thermochemical cycle were carried out on various aspects: (a) characterization and thermal properties of reactants/products using X-ray diffraction (XRD), thermogravimetry–mass spectrometry (TG-MS), scanning electron microscopy (SEM), temperature-programmed desorption (TPD), and extended X-ray absorption fine structure (EXAFS); (b) performance evaluation of fixed bed hydrolysis; (c) parametric optimization with respect to S/Cu, flow rate (gas hourly space velocity, GHSV), reaction duration, temperature, and particle size; and (d) monitored hydrolysis using isothermal TG experiments at 360°C, 370°C, 380°C, 390°C, and 400°C to derive kinetic parameters rate constant (k) and activation energy (E_a) on the basis of the shrinking-core model. 97% conversion to Cu₂OCl₂ at 17 630 h⁻¹ of GHSV, 400°C was achieved using ball-milled CuCl₂ (BM6), as compared with that of 55% over commercial un–ball-milled reactant, CuCl₂ (UBM). Correspondingly, higher k value of 2.84 h⁻¹ over BM6 as compared with 0.97 h⁻¹ over UBM reactant at 400°C was achieved. E_a for hydrolysis of BM6 was 93 kJ/mol, while it was 106 kJ/mol for UBM as derived from the Arrhenius plot. A probable pathway for CuCl₂ hydrolysis is proposed here. It was found to be diffusion controlled, and the particle size of reactant molecules affects the packing and diffusion length. Based on our investigations, it is very unlikely to get >99% phase pure product (Cu₂OCl₂). Cu₂OCl₂ is labile in nature and tends to transform into structurally similar and stable compounds CuO and CuCl₂.     

Thermal efficiency evaluation of open-loop SI thermochemical cycle for the production of hydrogen,sulfuric acid and electric power

Total thermal efficiency of a new open-loop SI thermochemical cycle for the production of hydrogen, sulfuric acid and electric power was investigated. The new system without _CO2${\mathrm{CO}}_2$ emission is composed of sulfuric acid industry process which provides the chemical reactant _SO2${\mathrm{SO}}_2$ and heat, electric energy demands of the system and SI open-loop cycle separated into the Bunsen reaction system, the _HIx${\mathrm{HI}}_x$ system and the _H2SO4${\mathrm{H}}_2{\mathrm{SO}}_4$ concentration system. The selection of SI cycle to run in an open-loop fashion for China is tied-up with two important facts: (1) sulfur iron ore as _SO2${\mathrm{SO}}_2$ source is inexpensive and abundantly available; (2) the product sulfuric acid, in addition to hydrogen, is valuable and marketable. The mass and heat balance of the process were calculated with the optimized conditions. Thermal efficiency for hydrogen production was 66.3% with ideal operating conditions of the EED cell and heat exchangers in the case of generating electric power without waste heat and was 70.9% in the case of high performance waste heat recovery.     

Sulfuric acid decomposition on Pt/SiC-coated-alumina catalysts for SI cycle hydrogen production

Sulfuric acid decomposition was conducted at atmospheric pressure and a GHSV of 72,000 mL/g_cat h in the temperature ranges from 650 to 850 °C. The Pt–Al (1wt% Pt/Al₂O₃) and and Pt–SiC–Al (1wt% Pt/SiC-coated-Al₂O₃) catalysts were prepared by an impregnation method. The Pt–Al catalyst rapidly deactivated at 650 and 700 °C, but was stable at 750 and 850 °C. The aluminum sulfate was observed on the spent Pt–Al catalyst by an FT-IR, an X-ray spectroscopy and a TGA/DSC analyzer, which was suggested to be a cause of the deactivation at lower reaction temperature. The alumina support was coated with SiC by a CVD method with methyltrichlorosilane (MTS) to get a non-corrosive support (SiC–Al) with high surface areas. The thermal analysis of the spent Pt–SiC–Al showed that the aluminum sulfate formation was suppressed during the sulfuric acid decomposition. The Pt–SiC–Al catalyst was not only active higher than the Pt–Al catalyst, but was also stable at all the tested reaction temperature.     

Simulation and experimental study on the sulfuric acid decomposition process of SI cycle for hydrogen production

This work is concerned with customization of the existing electrolyte thermodynamic model for process simulation and experimental studies on the sulfuric acid decomposition process of the SI cycle, using a commercial steady state simulator. An electrolyte thermodynamic model for the sulfuric acid–water system was tailored with four candidates available in commercial software, utilizing data from Perry's Handbook. Simulation of the sulfuric acid decomposition process comprising a flash separator, distillation column and decomposer was validated with the experimental results. To facilitate the lumped-parameter steady-state model-based simulation of sulfuric acid decomposition, the decomposer was conceptually decoupled into three sections: evaporation, H₂SO₄ dissociation, and SO₃ catalytic reduction to SO₂. The process simulation results exhibited good agreement with experimental data. This work contributes to future work on simulation and experimental study of a scaled-up process system and exergy analysis for an optimal energy-efficient sulfuric acid decomposition process in the SI cycle.     

Comprehensive comparative analysis of open-loop and closed-loop iodine-sulfur thermochemical cycle for hydrogen production

The Iodine-Sulfur (IS) or called Sulfur-Iodine (SI) thermochemical water-splitting cycle is one of the most promising hydrogen production methods through heat. For future commercial application, the closed-loop cycle coupled to nuclear power plant and the open-loop cycle coupled to sulfuric acid plant are the best solutions. In this study, comprehensive comparative analysis between four different hydrogen production cases is investigated from the aspects of thermal efficiency calculation, economic evaluation and life cycle assessment. With reasonable assumptions, the processes of IS closed-loop and open-loop cycle are designed and optimized through the Aspen Plus software. The corresponding stream results, specific parameters of heat exchangers and reactors and power demand of the cycle are presented in detail. With sufficient internal heat exchange, the calculated thermal efficiency is 50.94% and 81.9% respectively. The levelized cost of Case A, B, C and D is 2.26, 1.82, 1.33 and 1.64 US$/kg H₂ respectively with market electricity and sulfuric acid price, so Case C and D seem more competitive. With life cycle assessment (LCA) evaluation, the environmental impacts of Case A and Case D are smaller, followed by Case B and Case C. Through comprehensive consideration of the levelized cost and environmental impacts, Case B and Case D are more promising.     

An entropy production and efficiency analysis of the Bunsen reaction in the General Atomic sulfur-iodine thermochemical hydrogen production cycle

An entropy production and efficiency analysis of the first reaction in the General Atomic sulfur-iodine thermochemical hydrogen production cycle has been carried out by simulating the reaction including the mixing of reactants and separation of the resulting phases. The reaction:

was simulated at 388 K, which is slightly above the melting point of I₂. Analysis of only this reaction shows that the reaction should be run at 15–25% I₂ reacted and the greatest excess of H₂O which will produce two product phases. Actual operating conditions are however dependent on the total processing scheme. An entropy production and efficiency analysis along with economic factors for the entire process is necessary to obtain these conditions.     

MASCOT—a bench-scale plant for producing hydrogen by the UT-3 thermochemical decomposition cycle

A bench-scale plant for producing hydrogen has been constructed on the basis of the thermochemical water-decomposition process, UT-3, consisting of Br, Ca and Fe compounds. This plant is named MASCOT (Model Apparatus for Studying Cyclic Operation in Tokyo) and is designed to be capable of producing 3 l/h of gaseous hydrogen at standard conditions. During several test runs, the continuous production of hydrogen was successfully achieved. In the present paper, the construction of the MASCOT plant is described.     

Clean hydrogen production with the Cu-Cl cycle - Progress of international consortium, II: Simulations, thermochemical data and materials

This second of two companion papers presents the latest advances of an international team on the thermochemical copper-chlorine (Cu-Cl) cycle of hydrogen production. It specifically focuses on simulations, thermochemical data, advanced materials, safety, reliability and economics of the Cu-Cl cycle. Aspen Plus simulations of various system configurations are performed to improve the cycle efficiency. In addition, simulations based on exergo-economic and exergy-cost-energy-mass (EXCEM) methods for system design are presented. Modeling of the linkage between nuclear and hydrogen plants demonstrates how the Cu-Cl cycle would be integrated with an SCWR (Super Critical Water Reactor; Canada’s Generation IV reactor). Chemical potentials, solubilities, formation of Cu(I) and Cu(II) complexes and properties of Cu₂OCl₂, Cu(I) and Cu(II) chloride species are reported. In addition, the development of new advanced materials with improved corrosion resistance is presented. In particular, the performance of new anode electrode structures and thermal spray coatings is presented. This companion set of two papers presents new advances in a range of key enabling technologies for the thermochemical copper-chlorine cycle.     

Electrochemical investigation on the corrosion and hydrogen evolution rate of mild steel in sulphuric acid solution

Corrosion and hydrogen evolution rate of mild steel alloy have been investigated using various electrochemical techniques. Mild steel was polarized vs. saturated calomel electrode (SCE) in naturally aerated 0.1 M H₂SO₄ solution containing three newly synthesized heterocyclic compounds in different concentrations. The data obtained from polarization technique showed that the corrosion current density, i_corr, and the hydrogen evolution rate decrease with increasing concentration of heterocyclic inhibitors in 0.1 M H₂SO₄ medium, indicating a decrease in the corrosion rate of mild steel as well as an increase in the inhibition efficiency (IE) of the newly synthesized inhibitors. The impedance measurements confirmed well the polarization behaviour. Increasing the temperature leads to an increase in corrosion or hydrogen evolution rate of the mild steel and a decrease of the total resistance value (R_T) or the relative thickness (1/C_T) of the film. The obtained results were confirmed by surface examination using scanning electron microscope.     

The calcium-iodine cycle for the thermochemical decomposition of water

A new four-step thermochemical cycle based on calcium and iodine is proposed for the decomposition of water. The chemical reactions comprising the cycle are: the redox reaction of iodine with calcium hydroxide; the thermal decomposition of calcium iodate; the hydrolysis of calcium iodide; and the thermal decomposition of hydrogen iodide. Experimental verification of each of these reactions and an evaluation of the thermal efficiency of the process based on the material and energy balances for a proposed flow sheet are also presented.     

Corrosion resistance of Si-containing NiFeCr alloys used as materials for the thermal decomposition of sulphuric acid

The corrosion resistance of RA330 and NICRAL C tubes has been studied in a mixture of air and sulphuric acid vapours at temperatures ranging from 400 to 900°C. The tests were conducted at a pressure of 1 bar for 8500 h. The results are compared with those obtained in Incoloy 800 previously tested in the same conditions.