Evaluation of Bunsen reaction at elevated temperature and high pressure in continuous co-current reactor in iodine-sulfur thermochemical process

Bunsen reaction is one of the three reaction steps of iodine-sulfur process. In present study, Bunsen reaction is carried out in co-current reactor to identify effect of different operating conditions on concentrations of Bunsen reaction product mixture. Bunsen reaction studies have been done in tubular reactor, which is made of tantalum tube and stainless steel jacket, in 50–80 °C temperature range, 2–6 bar (g) pressure range. Feed flow rates are varied for HIx (mixture of hydroiodic acid, water and iodine) 1.2 l/h - 3 l/h, SO₂ 0.02 g/s – 0.24 g/s and O₂ 0.008 g/s ?0.016 g/s. It has been observed that, increasing SO₂ feed flow rate and pressure results in increased mole fraction of HI in HIx phase and H₂SO₄ in sulfuric acid phase. Increase in temperature increased the mole fraction of HI in HIx phase but decreased the mole fraction of H₂SO₄ in sulfuric acid phase. Increase in feed I₂/H₂O ratio and HIx feed flow rate, decreased the mole fraction of HI in HIx phase. Higher pressure improved the conversion of Bunsen reactants to products.     

Flowsheet study of the thermochemical water-splitting iodine–sulfur process for effective hydrogen production

The Japan Atomic Energy Agency (JAEA) is performing research and development on the thermochemical water-splitting iodine–sulfur (IS) process for hydrogen production with the use of heat (temperatures close to 1000 °C) from a nuclear reactor process plant. Such temperatures can be supplied by a High Temperature Gas-cooled Reactor (HTGR) process. JAEA's activity covers the control of the process for continuous hydrogen production, processing procedures for hydrogen iodide (HI) decomposition, and a preliminary screening of corrosion resistant process materials. The present status of the R&D program is reported herein, with particular attention to flowsheet studies of the process using membranes for the HI processing.     

H2 production by thermochemical water splitting with reticulated porous structures of ceria-based mixed oxide materials

In this work, we present for the first time the preparation and evaluation of Ceria-based mixed oxides reticulated porous ceramic (RPC) structures for H₂ production by thermochemical water splitting. After appropriate screening of the powder materials, ceria-based materials modified with Co, Mn and Zr were discarded due to their low cyclability and/or hydrogen productivity, derived from segregation of active phases or sintering during the thermal reduction and reoxidation. Sponge replica method has been optimized to allow obtaining a Ce_0.9Fe_0.1O_y RPC sponge structure with an outstanding hydrogen production of 15 STPcm³/g_material·cycle at a maximum temperature of 1300 °C. This better performance, comparing to the powder, can be attributed to the open macroporosity of the reticulated porous structure which enhances both heat and mass transfer. The H₂ production is maintained along several consecutive cycles without loss of activity, reinforcing the favorable prospects for large-scale hydrogen production.     

Commercial activated carbon for the catalytic production of hydrogen via the sulfur-Iodine thermochemical water splitting cycle

Eight commercial activated carbon catalysts were examined for their catalytic activity to decompose hydroiodic acid (HI) to produce hydrogen; a key reaction in the sulfur-iodine (S-I) thermochemical water splitting cycle. Activity was examined under a temperature ramp from 473 to 773 K. No statistically significant correlation was found between the measured catalyst sample properties and catalytic activity. Four of the eight samples were examined for one week of continuous operation at 723 K. All samples appeared to be stable over the period of examination.     

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.     

Study of the first step of the Mn2O3/MnO thermochemical cycle for solar hydrogen production

In this work, a complete thermodynamic study of the first step of the Mn₂O₃/MnO thermochemical cycle for solar hydrogen production has been performed. The thermal reduction of Mn₂O₃ takes place through a sequential mechanism of two reaction steps. The first step (reduction of Mn₂O₃ to Mn₃O₄) takes place at teomperatures above 700 °C, whereas the second reaction step (reduction of Mn₃O₄ to MnO) requires temperatures above 1350 °C to achieve satisfactory reaction rates and conversions. Equilibrium can be displaced to lower temperatures by increasing the inert gas/Mn₂O₃ ratio or decreasing the pressure. The thermodynamic calculations have been validated by thermogravimetric experiments carried out in a high temperature tubular furnace. Experimental results corroborate the theoretical predictions although a dramatically influence of chemical kinetics and diffusion process has been also demonstrated, displacing the reactions to higher temperatures than those predicted by thermodynamics. Finally, this work demonstrates that the first step of the manganese oxide thermochemical cycle for hydrogen production can be carried out with total conversion at temperatures compatible with solar energy concentration devices. The range of required temperatures is lower than those commonly reported in literature for the manganese oxide cycle obtained from theoretical and thermodynamic studies.     

Manganese oxide based thermochemical hydrogen production cycle

A MnO/NaOH based three-step thermochemical water splitting cycle was modified to improve the hydrolysis of α-NaMnO₂ (sodium manganate) and to recover Mn(III) oxides for the high-temperature reduction step. Sodium manganate forms in the reaction of NaOH with MnO that releases hydrogen. The hydrolysis of α-NaMnO₂ to manganese oxides and NaOH is incomplete even with a large excess of water and more than 10% sodium cannot be removed prior to the high-temperature reduction step.When mixed oxides of manganese with iron were used in the cycle, the NaOH recovery in the hydrolysis step improved from about 10% to 35% at NaOH concentrations above 1M. Only 60% sodium was removed at 0.5M from the mixed oxides whereas more than 80% can be recovered at the same NaOH concentration when only manganese oxides are used. A 10:1 Mn/Fe sample was cycled through all steps three times to confirm that multiple cycles are possible. The high-temperature reduction was carried out for 5h at 1773 K under vacuum and the conversion was about 65% after the 3rd cycle.Since sodium carryover into the high-temperature reduction cannot be avoided, even with the energy intensive hydrolysis step, a modified two-step cycle without low-temperature sodium recovery is proposed where α-NaMnO₂ is reduced directly to MnO at 1773 K under vacuum. On a laboratory scale, about 60% of the sodium that volatilized at the high temperatures was trapped with a water-cooled cold finger and conversions were stable at about 35% after three completed cycles.     

Likely near-term solar-thermal water splitting technologies

Thermodynamic and materials considerations were made for some two- and three-step thermochemical cycles to split water using solar-thermal processing. The direct thermolysis of water to produce H₂ using solar-thermal processing is unlikely in the near term due to ultra-high-temperature requirements exceeding 3000 K and the need to separate H₂ from O₂ at these temperatures. However, several lower temperature (<2500 K) thermochemical cycles including ZnO/Zn, Mn₂O₃/MnO, substituted iron oxide, and the sulfur–iodine route (S–I) provide an opportunity for high-temperature solar-thermal development. Although zirconia-based materials are well suited for metal oxide routes in terms of chemical compatibility at these temperatures, thermal shock issues are a major concern for solar-thermal applications. Hence, efforts need to be directed towards methods for designing reactors to eliminate thermal shock (ZrO₂ based) or that use graphite (very compatible in terms of temperature and thermal shock) with designs that prevent contact of chemical species with graphite materials at high temperatures. Fluid-wall reactor configurations where inert gases provide a blanket to protect the graphite wall appear promising in this regard, but their use will impact process efficiency. For the case of S–I up to 1800 K, silicon carbide appears to be a suitable material for the high-temperature H₂SO₄ dissociation. There is a need for a significant amount of work to be done in the area of high-temperature solar-thermal reactor engineering to develop thermochemical water splitting processes.     

Environmental evaluation of hydrogen production via thermochemical water splitting using the Cu-Cl Cycle: A parametric study

Variations of environmental impacts with lifetime and production capacity are reported for nuclear based hydrogen production plants using the three-, four- and five-step (copper-chlorine) Cu-CI thermochemical water decomposition cycles. Life cycle assessment is utilized which is essential to evaluate and to decrease the overall environmental impact of any system and/or product. The life cycle assessments of the hydrogen production processes indicate that the four-step Cu-Cl cycle has lower environmental impacts than the three- and five-step cycles due to its lower thermal energy requirement. Parametric studies show for the four-step Cu-Cl cycle that acidification and global warming potentials can be reduced from 0.0031 to 0.0028 kg SO₂-eq and from 0.63 to 0.55 kg CO₂-eq, respectively, if the lifetime of the system increases from 10 to 100 years.     

Analysis of solar chemical processes for hydrogen production from water splitting thermochemical cycles

This paper presents a process analysis of ZnO/Zn, Fe₃O₄/FeO and Fe₂O₃/Fe₃O₄ thermochemical cycles as potential high efficiency, large scale and environmentally attractive routes to produce hydrogen by concentrated solar energy. Mass and energy balances allowed estimation of the efficiency of solar thermal energy to hydrogen conversion for current process data, accounting for chemical conversion limitations. Then, the process was optimized by taking into account possible improvements in chemical conversion and heat recoveries. Coupling of the thermochemical process with a solar tower plant providing concentrated solar energy was considered to scale up the system. An economic assessment gave a hydrogen production cost of 7.98$ kg⁻¹ and 14.75$ kg⁻¹ of H₂ for, respectively a 55 MW_th and 11 MW_th solar tower plant operating 40 years.     

Kinetic modelling of the first step of Mn2O3/MnO thermochemical cycle for solar hydrogen production

This work reports the kinetic study of the first step of the Mn₂O₃/MnO thermochemical cycle for hydrogen production by water splitting. The reaction kinetics of Mn (III) oxide thermal reduction has been evaluated using dynamic thermogravimetric analysis at constant heating rate under nitrogen flow. This way the reaction rate can be described as a function of temperature and different kinetic models were fitted to the experimental data obtained from thermogravimetric experiments. A good fitting can be observed for each experiment, although a significant disparity in the values estimated for the Arrhenius parameters has been found (activation energies and pre-exponential factors). Unique values for the kinetic parameters have been calculated by application of a multivariate non-linear regression method for the simultaneous fitting of data from all the experiments carried out at different heating ramps. However, also in this case the values of the Arrhenius parameters are significantly different depending on the chosen kinetic equation. Optimal kinetic parameters have been finally calculated through the estimation of activation energy values by model-free isoconversional methods and using a rigorous multivariate nonlinear regression for the calculation of the model-dependant pre-exponential factors.     

Study of the hydrogen production step of the Mn2O3/MnO thermochemical cycle

In this work, a complete study of the second step of the Mn₂O₃/MnO thermochemical cycle for solar hydrogen production has been performed. It includes a complete thermodynamic calculation of the equilibrium phases between MnO, NaOH and H₂, which shows that the reaction takes place theoretically at temperatures above 75 °C. However, the experimental results demonstrate that it is necessary at least 450 °C to achieve a satisfactory reaction rate. It indicates a dramatic influence of chemical kinetics and diffusion process, displacing the reaction to higher temperatures than those predicted by thermodynamics. The resultant solid of the reaction exhibits a phases distribution highly dependent on the temperature and the NaOH:MnO ratio and this is of great influence in the overall rate of the process. The kinetic study shows that the overall process involves not only the chemical reaction between MnO and NaOH, but also a number of physical processes (heat and mass transfer) and solid phase transformations. The apparent activation energy calculated is a composite value determined by the activation energies of those elementary processes.     

Exergy analysis of hydrogen production by thermochemical water decomposition using the Ispra Mark-10 Cycle

An investigation is reported of the thermodynamic performance of the Ispra Mark-10 thermochemical water decomposition process for hydrogen production. Thermochemical water decomposition has been identified as a potentially important future process for the production of hydrogen, which is currently an important industrial commodity and has significant future potential as a fuel. Exergy analysis is used since energy analysis on its own does not pinpoint true process inefficiencies, and often does not provide rational efficiencies. The analysis indicates that the principle thermodynamic losses occur in the primary water decomposition reactors and are mainly due to internal irreversibilities associated with chemical reaction and heat transfer across large temperature differences, and that the losses associated with effluents (particularly cooling water) are not that significant. Energy and exergy efficiencies are provided and are observed to depend strongly on the main external process inputs, i.e., electricity and process heat, or heat, or the raw resource from which heat and electricity are produced.     

Thermal efficiency evaluation of the thermochemical H2S splitting cycle for the hydrogen and sulfur production

A flowsheet of the thermochemical H₂S splitting cycle was designed and simulated for hydrogen and sulfur production. The heat and mass balance, as well as the thermal efficiency of the process, were calculated. A thermal efficiency of 40.865% for hydrogen production was obtained by optimizing the heat exchangers and the EED cell considering waste heat recovery. The effects of five calculation parameters, namely, the sulfuric acid concentration, hydrogen iodide (HI) conversion ratio, molar flow rate of HI_x phase, pressure, and reflux ratio at the distillation column, on thermal efficiency were evaluated. The results indicated that further research on the membrane reactor is needed. The optimized conditions for the over-azeotropic HI solution yield should be prioritized. Furthermore, an H₂SO₄ concentration system should be reasonably designed to reduce the complexity of the process and equipment settings, as well as to improve thermal efficiency.     

Hybrid photo-thermal sulfur-ammonia water splitting cycle: Thermodynamic analysis of the thermochemical steps

Solar driven hybrid sulfur-ammonia water splitting cycle (HySA) integrates a solar-photocatalytic hydrogen, H₂, production step (H₂ sub-cycle) with a high-temperature solar thermochemical oxygen, O₂, evolution step (O₂ sub-cycle), implementing efficient thermal energy storage as part of the cycle operation. Previous studies of the cycle omitted intermediate products, such as ammonium bisulfate, from the O₂ sub-cycle and, thus, neglected their potential impact on the cycle's chemistry. Also, there are discrepancies in reported literature for the thermodynamic properties of ammonium sulfate, (NH₄)₂SO_4, and ammonium bisulfate, NH₄HSO₄. In this study, thermal analysis experiments were conducted in order to determine the phase transition temperatures and enthalpies, and the heat capacity temperature dependence of the ammonium sulfate, (NH₄)₂SO_4, and ammonium bisulfate, NH₄HSO₄. Our experimentally determined values for these parameters agree well with the data reported in DIPPR Project 801 database. Moreover, an exploratory thermodynamic analyses was performed using AspenPlus^© and FactSage^©, that included all potential reaction products, in order to identify critical parameters for an optimum O₂ sub-cycle. A methodology is proposed and evaluated to mitigate AspenPlus^©'s deficiency to handle solid phase changes. The thermodynamic analyses demonstrate that the NH₄HSO₄ inclusion in the O₂ sub-cycle reduces the overall process energy requirements, and allows its use as an energy storage medium. Finally, we show that the use of molten salts, in combination with their interactions, significantly affects the efficiency and the operating conditions of the process, as well as the state of the mixtures.     

A comparative thermodynamic analysis of samarium and erbium oxide based solar thermochemical water splitting cycles

This paper reports a thermodynamic comparison between the samarium and erbium oxide based solar thermochemical water splitting cycles. These cycles are a two-step process in which the metal oxide is first thermally reduced into the pure metal, and the produced metal can be used to split water to produce H₂. The metal oxides can be reused for multiple cycles without consumption. The effect of water splitting temperature on various thermodynamic parameters which are essential to design the solar reactor system for the production of H₂ via water splitting reaction using the samarium and erbium oxides is studied in detail. The total amount of solar energy needed for the thermal reduction of samarium and erbium oxides is estimated. The amount of heat energy released by the water splitting reactor is calculated. Also, the cycle and solar-to-fuel energy conversion efficiency for both cycles are determined by employing heat recuperation. Obtained results indicate that the efficiencies associated with these cycles are comparable to the previously studies thermochemical cycles. It is observed that higher water splitting temperature favors towards higher efficiencies. At constant thermal reduction temperature = 2280 K, by employing 50% heat recuperation, the solar-to-fuel energy conversion efficiency for the samarium cycle (30.98%) is observed to be higher than erbium cycle (28.19%).     

Thermochemical two-step water splitting by ZrO2-supported NixFe3−xO4 for solar hydrogen production

A thermochemical two-step water-splitting cycle using a redox metal oxide was examined for Ni(II) ferrites or Ni_xFe_3−xO₄ (0  x  1) for the purpose of converting solar high-temperature heat to hydrogen. The Ni(II) ferrite was decomposed to Ni-doped wustite (Ni_yFe_1−yO) at 1400 °C under an inert atmosphere in the first thermal-reduction step of the cycle; it was then reoxidized with steam to generate hydrogen at 1000 °C in the second water-decomposition step. Although nondoped Fe₃O₄ powders formed a nonporous, dense mass of iron oxide by the fusion of FeO and its subsequent solidification after the thermal-reduction step, Ni(II)–ferrite powders were converted into a porous, soft mass after the step. This was probably because Ni doping in the FeO phase raised the melting point of wustite above 1400 °C. Supporting the Ni(II) ferrites on m-ZrO₂ (monoclinic zirconia) alleviated the high-temperature sintering of iron oxide; as a result, the supported ferrites exhibited greater reactivity and assisted the repeatability of the cyclic water splitting process as compared to the unsupported ferrites. The reactivity increased with the doping value x, and was maximum at x = 1.0 in the Ni_xFe_3−xO₄/m-ZrO₂ system.     

Oxygen-releasing step of nickel ferrite based on Rietveld analysis for thermochemical two-step water-splitting

We investigated the thermal reduction (T-R) of NiFe₂O₄, either supported by m-ZrO₂ or unsupported, as the oxygen-releasing step of a solar thermochemical water splitting cycle based on a ferrite/wustite redox system, by performing the Rietveld analysis using powder X-ray diffraction. The solid materials obtained after the T-R step at 1300–1400 °C were subjected to Rietveld analysis. The amounts and chemical compositions of the wustite phase produced by the T-R step and the remaining ferrite phase were identified quantitatively. Chemical reaction formulas for the different T-R temperatures were determined from the results. Consistency for the chemical reactions of the thermal reduction was discussed and evaluated comparing the O₂ amounts predicted by the chemical reaction formulas and measured experimentally by mass spectrometry.