Detailed kinetic modeling of homogeneous HI decomposition for hydrogen production,part I: Effect of H2O on HI decomposition

A new, detailed kinetic model was developed for the homogeneous decomposition of HI–H₂O solutions in vapor phase in the sulfur–iodine cycle. The kinetics of the process was represented by a reaction mechanism involving 32 reactions and 11 species. Comparisons between the kinetic calculations and experimental data showed that this model correctly predicted the hydrogen yield at the 500 °C–1000 °C temperature range under 1 atm. The effects of temperature, reaction time, and HI/H₂O ratio on HI decomposition and hydrogen sensitivity analysis were investigated in the modeling process. The model predicted that the effect of the addition of H₂O changed from inhibiting the decomposition ratio to promoting it with increasing temperature. The sensitivity analysis showed that elementary reactions (1) HI + HI = H₂+I₂, (4) HI + H = H₂ + I, (5) HI + I = H + I₂, and (8) HI + OH = H₂O + I played important roles in hydrogen production. The reaction path of HI decomposition with H₂O was constructed based on detailed kinetic modeling and sensitivity analysis results.     

Energy and economic assessment of an industrial plant for the hydrogen production by water-splitting through the sulfur-iodine thermochemical cycle powered by concentrated solar energy

The faster and faster global growth of energy consumption generates serious problems on its supply and about the pollution that may result. Through the use of thermochemical cycles it is possible to use renewable energy to produce hydrogen from water, with the dual purpose of having an unlimited source of energy without producing greenhouse gases.     

热化学硫碘循环制氢系统研究进展

热化学硫碘循环制氢是目前最有前景的制氢方法之一.为了提高产氢效率,众多学者对硫碘循环制氢系统进行了优化.对热化学硫碘循环制氢系统及其各单元研究进展进行了综述,讨论了本生反应中水和碘的用量问题、相分离问题,以及硫酸分解与碘化氢分解反应的反应环境和反应催化等问题,并对目前研究该循环的部分团队的实验结果和流程计算进行了对比.     

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.     

The feasibility of membrane separations in the HIx processing section of the sulphur iodine thermochemical cycle

One of the most promising routes currently under development for sustainable hydrogen production is the sulphur iodine thermochemical water splitting cycle. The most important stage in determining overall process efficiency and feasibility of the cycle is the separation of HI_x, a mixture of HI, H₂O and I₂. This paper investigates the scope for applying membrane separations to the HI_x processing section, with the aim of improving the overall efficiency of the process. Simulations using ProSimPlus indicate that this efficiency can be increased from a base value of 36.9% to over 39% by applying a membrane separation unit which achieves 8.25% dewatering to the feed of the reactive distillation column. Comparison with experimental flux and separation data for both pervaporation and membrane distillation membranes gives design limits for the cost and feasibility of such a membrane system.     

Experimental study of two phase separation in the Bunsen section of the sulfur–iodine thermochemical cycle

The Bunsen reaction for the production of hydriodic and sulfuric acids from water, iodine and sulfur dioxide, has been studied with the evaluation of the effect of some operative parameters on product phase behavior. Results show that operative temperature has a minor effect on the phase behavior. In contrast, both iodine and water loads can be adjusted to enhance the downstream operations of the sulfur–iodine thermochemical water-splitting cycle: the effect of iodine and water excess on resulting phases purity, side reaction occurrence and acid concentration was studied and, then, the most favorable operative conditions defined.     

Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production

As a critical subsection in the sulfur-iodine (SI) thermochemical cycle, HI concentration and separation must cope with the pseudo-azeotropy of HIx (HI-I₂-H₂O) and excess iodine in HIx solution. Although electro-electrodialysis (EED) coupled with conventional distillation is a validated method of HI separation from HIx solution in the SI process, the iodine deposition, resulting from changes in temperature and HI molality in HIx solution, can lead clogged flow channels of the EED anode and other tubes. A precipitator can address this problem by recovering excess iodine from HIx solution after the HIx purification column. The energy duty and required input flow rate per mole HI was investigated in this study using a process flowsheet simulation. A decrease in iodine concentration in the streams to EED could reduce cell duty effectively. An increase in HI molality in the EED cathode outlet resulted in an increase in EED duty; however, the amplitude was slight. The iodine molar concentration in the feed of the distillation column exhibited an appreciable influence on the distillation duty. However, with an increase in distillation column pressure, the effect of diminished iodine in feed on the HI distillate duty continued to decline. To assess the utilization of an iodine precipitator in the HI separation subsection, the energy demands and required input flow rates of three different flowsheets were calculated using Aspen Plus and Microsoft Office Excel. Results indicated that the flowsheet that only recovered iodine in the stream to the EED anode chamber exhibited the least HI separation duty and the lowest required input flow rate.     

Chemical aspects of the water-splitting thermochemical cycle based on sodium manganese ferrite

Water thermolysis by means of the sodium manganese ferrite cycle for sustainable hydrogen production is reviewed, with particular focus on known elementary chemical processes taking place on solid substrates in the 600–800 °C temperature range. For the purpose, in-situ high temperature x-ray diffraction technique has been utilized to observe structural transformations produced by both temperature and reactive environment. The water-splitting reaction and the regeneration of initial reactants are described as multi-step reactions, in which the role of carbon dioxide, through carbonation and de-carbonation reactions is highlighted. A thermodynamic phase stability diagram is reported for the system MnFe₂O₄/Na₂CO₃/CO₂.     

Detailed kinetic modeling of homogeneous HI decomposition for hydrogen production—Part II: Effect of I2 on HI decomposition

The kinetic modeling of homogeneous decomposition of hydrogen iodide (HI) and HI/H₂O vapors with the addition of diatomic iodine (I₂) using the mechanism proposed in the companion work (part I) in the sulfur–iodine cycle was investigated in this paper. Thermodynamic results calculated by FactSage and the kinetic experiment verified the applicability of the mechanism. The effect of temperature, residence time, pressure, HI/H₂O/I₂ molar ratio, HI/I₂ molar ratio, and sensitivity analysis on the HI conversion was observed in the modeling process. The addition of small amount of diatomic iodine greatly decreases the HI conversion, and the overall pressure could promote the HI decomposition rate in the kinetic process. Sensitivity analysis shows that hydrogen yield was most sensitive to reactions (4) HI + H = H₂ + I, (1) HI + HI = H₂ + I₂, (5) HI + I = I₂ + H, and (8) HI + OH = H₂O + I. The existence of diatomic iodine increases the reverse reaction of (1) and (5).     

Sulphonated (PVDF-co-HFP)-graphene oxide composite polymer electrolyte membrane for HI decomposition by electrolysis in thermochemical iodine-sulphur cycle for hydrogen production

In overall iodine-sulphur (I-S) cycle (Bunsen reaction), HI decomposition is a serious challenge for improvement in H₂ production efficiency. Herein, we are reporting an electrochemical process for HI decomposition and simultaneous H₂ and I₂ production. Commercial Nafion 117 membrane has been generally utilized as a separator, which also showed huge water transport (electro-osmosis), and deterioration in conductivity due to dehydration. We report sulphonated poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) (SCP) and sulphonated graphene oxide (SGO) composite stable and efficient polymer electrolyte membrane (PEM) for HI electrolysis and H₂ production. Different SCP/SGO composite PEMs were prepared and extensively characterized for water content, ion-exchange capacity (IEC), conductivity, and stabilities (mechanical, chemical, and thermal) in comparison with commercial Nafion117 membrane. Most suitable optimized SCP/SGO-30 composite PEM exhibited 6.78 × 10^?2 S cm^?1 conductivity in comparison with 9.60 × 10^?2 S cm^?1 for Nafion® 117. The electro-osmotic flux ofSCP/SGO-30 composite PEM (2.53 × 10^?4 cm s^?1) was also comparatively lower than Nafion® 117 membrane (2.75 × 10^?4 cm s^?1). For HI electrolysis experiments, SCP/SGO-30 composite PEM showed good performance such as 93.4% current efficiency (η), and 0.043 kWh/mol-H₂ power consumption (Ψ). Further, intelligent architecture of SCP/SGO composite PEM, in which hydrophilic SGO was introduced between fluorinated polymer by strong hydrogen bonding, high efficiency and performance make them suitable candidate for electrochemical HI decomposition, and other diversified electrochemical processes.     

Novel two-step thermochemical cycle for hydrogen production from water using germanium oxide: KIER 4 thermochemical cycle

This paper proposes a novel two-step thermochemical cycle for hydrogen production from water using germanium oxide. The thermochemical cycle is herein referred to as KIER 4. KIER 4 consists of two reaction steps: the first is the decomposition of GeO₂ to GeO at approximately 1400–1800 °C, the second is hydrogen production by hydrolysis of GeO below 700 °C. A 2nd-law analysis was performed on the KIER 4 cycle and a maximum exergy conversion efficiency was estimated at 34.6%. Thermodynamic analysis of GeO₂ decomposition and hydrolysis of GeO confirmed the possibility of this cycle. To demonstrate the cycle, the thermal reduction of GeO₂ was performed in a TGA with mass-spectroscopy. Results suggest GeO₂ decomposition and oxygen gas evolution. To confirm the thermal decomposition of GeO₂, the effluent from GeO₂ decomposition was quenched, filtered and analyzed. SEM analysis revealed the formation of nano-sized particles. XRD analysis for the condensed-filtered particles showed the presence of Ge and GeO₂ phases. The result can be explained by thermodynamic instability of GeO. It is believed that GeO gas disproportionates to ½Ge and ½GeO₂ during quenching. 224 ml hydrogen gas per gram of reduced GeO₂ was produced from the hydrolysis reaction.     

Continuous purification of H2SO4 and HI phases by packed column in IS process

Iodine–sulfur (IS) thermo-chemical water-splitting process is a promising technology to produce hydrogen using solar or nuclear energy. To avoid the undesirable side reactions between HI and H₂SO₄ phases formed in Bunsen reaction of IS cycle, it is necessary to purify the two phases. The purification process could be achieved by reverse reaction of Bunsen reaction. In this paper, the purification process in continuous mode by reacting sulfuric acid and HI in a packed column was experimentally studied; the influences of operational parameters, including the reaction temperature, the flow rate of nitrogen gas, and the flow rate of the raw material solutions, on the purification efficiency, were investigated in detail. Based on the results, the suitable conditions for continuous purification process of two phases were proposed.     

Synthesis,characterization and corrosion performance evaluation of DDR membrane for H2 separation from HI decomposition reaction

An all silica DDR (deca dodecasil rhombohedral) zeolite membrane with dense, interlocked structure has been developed for separation of H₂ from HI/I₂ mixture of HI decomposition reaction. In this work, the DDR zeolite membrane was synthesized on the seeded clay-alumina substrate within 5 days. The seeds were synthesized by sonication mediated hydrothermal process within short crystallization time which enhanced the nucleation for the membrane growth. The synthesized membranes along with seed crystals were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDAX). The selectivity of hydrogen with respect to CO₂ and Ar was evaluated by single gas permeation studies at room temperature. The tests for corrosion resistance were carried out upto 120 h with both support and DDR membrane at 130 °C which confirmed the stability of membrane under the harsh HI/I₂ environment.     

Chemical reactor network modeling of a microwave plasma thermal decomposition of H2S into hydrogen and sulfur

The conventional treatment method for H₂S is the Claus process, which produces sulfur and water. This results in a loss of the valuable potential product hydrogen. H₂S treatment would be more economically valuable if both hydrogen and sulfur products could be recovered. Based on standard heats of formation analysis, the theoretical energy required to produce hydrogen from H₂S dissociation is only 20.6 kJ/mol of H₂ as compared to 63.2 kJ/mol of H₂ from steam methane reforming and 285.8 kJ/mol of H₂ from water electrolysis. Among the many thermal decomposition methods that have been explored in the literature, Micro-wave plasma dissociation of H₂S prevails as the method of choice to attain the best conversion and energy efficiency. Chemical kinetics simulations using an ideal flow reactor network have been carried out on the CHEMKIN-PRO software package and they support these findings. The reactor network simulates the thermal plasma behavior in the plasma torch, the plasma reactor, and the sulfur condenser. Two chemical kinetics mechanisms have been used and the results show an almost complete conversion of H₂S into hydrogen and sulfur in the plasma reactor at an optimum temperature of about 2400 K at atmospheric pressure. While the most challenging task of the process is found to be the plasma cooling rate at the sulfur condenser where very fast quenching is required to conserve the hydrogen product from converting back to H₂S.     

High temperature oxygen separation for the sulphur family of thermochemical cycles - part I: Membrane selection and flux testing

This work investigates the feasibility of applying high temperature oxygen separation to the sulphuric acid decomposition process, a common step in the Sulphur Iodine and Hybrid Sulphur thermochemical cycles for massive scale hydrogen production. Process simulations indicate that the decomposition yield can be increased from 62.3% to 90.1% by using a catalytic membrane reactor. Equilibrium calculations show yttria-stabilised zirconia (YSZ), combined with platinum electrodes, to be a candidate membrane for the separation process. Bespoke experimental apparatus was designed and commissioned in order to investigate the oxygen flux through YSZ membranes in the presence of sulphur dioxide, during external voltage application. Results from an experiment at 900 °C show that oxygen permeation through YSZ membranes occurs under these conditions. X-ray diffraction and impedance spectroscopy are used to investigate the reason for the oxygen flux decrease with time.     

Low voltage H2O electrolysis for enhanced hydrogen production

In this study, we report the ability to split H₂O into hydrogen at a reduced voltage by the influence of sulfur dioxide (SO₂) and anode tolerance materials. This will improve the energy consumption for the production of hydrogen. Hydrogen is produced at the cathode while the anode electrode is bathed in sulfur dioxide and water to form sulfuric acid by the application of potential in the form of electrical energy. In the presence of SO₂, the theoretical equilibrium voltage requirement is 0.19 V, thereby reducing the thermochemical free energy to less than one-sixth of its initial value, that is, from 56 to 9.18 kcal/mole. By using SO₂ to scavenge the anode we have in practice reduced the equilibrium voltage to 0.6 V. Based on different electrode configurations, ruthenium oxide (RuO₂) electrocatalyst deposited on silicon (Si) electrode exhibited superior performance for the low voltage H₂O electrolysis.     

Catalytic decomposition of hydrogen iodide over pre-treated Ni/CeO2 catalysts for hydrogen production in the sulfur–iodine cycle

The Ni/CeO₂ catalysts with successive oxidization, reduction and re-oxidization have been tested for hydrogen production in sulfur–iodine (SI or IS) cycle. The samples were characterized by BET, XRD, TEM, TPR and XPS. The oxidative/reductive atmosphere affected the structure and performance of the catalysts. It was suggested that a migration of Ce⁴⁺ from the bulk to the surface occurred during the reductive treatment. The diffusion process was reversed when the atmosphere was changed to an oxidative one. The reduced and re-oxidized samples seemed to be similar all the time and showed better catalytic activity in comparison with the as-received and oxidized samples. For the re-oxidized sample, the strongest interaction compared with other samples occurred between Ni and CeO₂ and oxygen vacancies transferred from bulk to surface, which led to form more surface sites and oxygen vacancies. The metal Ni was found only on the surface of the reduced sample. The active site of metal Ni besides the surface site and oxygen vacancy were assumed to play an important role for hydrogen production.     

Aspects of the metabolism of hydrogen production by Rhodobacter sphaeroides

Photosynthetic bacteria are favorable candidates for biological hydrogen production due to their high conversion efficiency and versatility in the substrates they can utilize. For large-scale hydrogen production, an integrated view of the overall metabolism is necessary in order to interpret results properly and facilitate experimental design. In this study, a summary of the hydrogen production metabolism of the photosynthetic purple non-sulfur (PNS) bacteria will be presented.Practically all hydrogen production by PNS bacteria occurs under a photoheterotrophic mode of metabolism. Yet results show that under certain conditions, alternative modes of metabolism—e.g. fermentation under light deficiency—are also possible and should be considered in experimental design.Two enzymes are especially critical for hydrogen production. Nitrogenase promotes hydrogen production and uptake hydrogenase consumes hydrogen.Though a wide variety of substrates can be used for growth, only a portion of these is suitable for hydrogen production. The efficiency of a certain substrate depends on factors such as the activity of the TCA cycle, the carbon-to-nitrogen ratio, the reduction-state of that material and the conversion potential of the substrate into alternative metabolites such as PHB.All these individual components of the hydrogen production interact and are subject to strict regulatory controls. An overall scheme for the hydrogen production metabolism is presented.