Hydrogen iodide decomposition over nickel–ceria catalysts for hydrogen production in the sulfur–iodine cycle

In this work, pure CeO₂ and three nickel–ceria catalysts prepared by different methods have been tested to evaluate their effect on hydrogen iodide (HI) decomposition in the sulfur–iodine (SI or IS) cycle at various temperatures. BET, XRD, HRTEM and TPR were performed for catalysts characterization. Indeed, the pure CeO₂ also strongly enhance the decomposition of HI to H₂ by comparison with blank yield. Nickel–ceria catalysts show better catalytic activity, especially Ni-doping-G sample. It is found that, through the sol-gel method, the Ni²⁺ ions have dissolved into the ceria lattice instead of the Ce⁴⁺ ions during the synthesis process of Ni-doping-G sample. Oxygen vacancies are formed because of the charge imbalance and lattice distortion in CeO₂. The presence of Ni during the CeO₂ synthesis process of Ni-doping-G also causes smaller average particle size, larger surface area, better thermal stability and better Ni dispersion than the Ni-loading samples. These provide nickel–ceria catalyst with a potential to be used in the SI cycle for HI decomposition.     

Bunsen section thermodynamic model for hydrogen production by the sulfur–iodine cycle

A model for the Bunsen section of the Sulfur–Iodine thermo-chemical cycle is proposed, where sulfur dioxide reacts with excess water and iodine to produce two demixing liquid aqueous phases (H₂SO₄ rich and HI rich) in equilibrium. Considering the mild temperature and pressure conditions, the UNIQUAC activity coefficient model combined with Engels' solvation model is used. The complete model is discussed, with HI solvation by water and by iodine as well as H₂SO₄ solvation by water, leading to a very high complexity with almost hundred parameters to be estimated from experimental data. Taking into account the water excess, a successful reduced model with only 15 parameters is proposed after defining new apparent species. Acids total dissociation and total H⁺ solvation by water are the main assumptions. Results show a good agreement with published experimental data between 25 °C and 120 °C.     

Ceria as a catalyst for hydrogen iodide decomposition in sulfur–iodine cycle for hydrogen production

In this work, ceria (CeO₂) prepared with different methods and at various calcination temperatures have been tested to evaluate their effect on hydrogen iodide (HI) decomposition in sulfur–iodine (SI) cycle at various temperatures. The CeO₂ catalysts' strongly enhance the HI decomposition by comparison with blank test, especially gel CeO₂ 300. TG–FTIR, BET, XRD, TEM and TPR were performed for catalysts' characterization. The results show that the CeO₂ catalyst synthesized by citric-aided sol–gel method and calcined at low temperature (<500 °C) shows more lattice defects, smaller crystallites, larger surface area and better reducibility. Oxygen can promote the significantly rapid surface reaction, but simultaneously consume hydrogen species (H) contained in HI. Lattice defects, especially the reduced surface sites, i.e., Ce³⁺ and oxygen vacancy, play the dominant role in surface reactions of HI decomposition. A new reaction mechanism for HI catalytic decomposition over CeO₂ catalyst is proposed.     

Hydrogen production by flue gas through sulfur–iodine thermochemical process: Economic and energy evaluation

The use of low quality fossil fuel with high sulfur content is becoming more frequent and probably will play a very important role in the future, due to the depletion of reserves.     

Sulfur trioxide electrolysis studies: Implications for the sulfur–iodine thermochemical cycle for hydrogen production

In this paper we describe our efforts to develop a sulfur trioxide (SO₃) electrolyzer that could lower the temperature of the SO₃ decomposition step in the sulfur–iodine and hybrid sulfur thermochemical cycles. The objective is to develop an alternative to the standard process of converting SO₃ to SO₂, which is thermal decomposition at 830 °C and above. Thermodynamic calculations show that high SO₃ conversions can be obtained at 590 °C if oxygen is removed during the SO₃ decomposition stage. One way of achieving oxygen removal during SO₃ decomposition is electrolysis, if suitable electrode and electrolyte materials can be found. Active oxygen electrode materials are already developed and we have demonstrated suitability of a thin doped-zirconia electrolyte in this study. The main difficulty came in the development of an active and stable SO₃ electrode. Using Ga–V–O/NbB₂/Au electrodes we demonstrated high catalytic activity, but could not achieve acceptable electrochemical performance.     

Continuous Bunsen reaction and simultaneous separation using a counter-current flow reactor for the sulfur–iodine hydrogen production process

The sulfur–iodine (SI) process, which consists of three chemical reactions of the Bunsen reaction, a H₂SO₄ decomposition and a HI decomposition, is an important potential method for hydrogen production among thermochemical water splitting methods. For steady-state operation of the SI process, it is very important to provide information on the composition of each phase that passes from the Bunsen reaction section to the following H₂SO₄ and HI decomposition sections. In this study, the Bunsen reaction was carried out using a counter-current flow reactor, the Bunsen reaction and product separation steps were shown capable of being performed simultaneously, and the composition variation of each phase discharged at the top and bottom of reactor was investigated. The process variables were the SO₂ feed rate, temperature, I₂/H₂O molar ratio. As a result of constant reactant feed and continuous product discharge operation, it was found that the composition remained constant after 120 min of reaction time, indicating steady-state operation. The phase separation characteristics of the Bunsen reaction were minimally affected by the SO₂ feed rate. As the amount of I₂ introduced increased with increasing temperature, the volume of the H₂SO₄ phase discharged from the upper phase was unchanged, while that of the HI_x phase discharged from the lower phase increased proportionally. The average molar composition of the H₂SO₄ phase (H₂SO₄/H₂O/HI) obtained at a typical operation condition (353 K, I₂/H₂O molar ratio of 0.406) was 1/5.30–5.39/0.02–0.04, and the composition of the HI_x phase (HI/I₂/H₂O/H₂SO₄) was 1/2.81–3.09/5.67–6.40/0.04–0.06. These results could be used for the design and operation of H₂SO₄ and HI decomposition sections of the SI process.     

Numerical analysis of the electrochemical Bunsen reaction for hydrogen production from sulfur–iodine cycle

The sulfur–iodine (S-I) water-splitting cycle is one of the most promising hydrogen production methods. The Bunsen reaction in the cycle affects the flowsheet complexity and thermal efficiency, but an electrochemical technique has recently been applied to make the S-I cycle more simplified and energy efficient. However, the performance of the electrochemical Bunsen reaction, especially the electrode reactions inside the electrolytic cell (EC) are not clear at present. In this work, a two-dimensional numerical model of EC was developed. The detailed reaction process was numerically calculated with considering the coupling of mass transfer and electrochemical reactions, and was verified using experimental data. The effects of various operating parameters on the reactions were investigated. The results showed that the increase of current density significantly improves the conversion rates of reactants. A higher temperature is unfavorable for concentrating H₂SO₄ and HI. Increase in the inlet flow rate reduces the conversion rates of reactants, but the impact declines with further rising flow rate. An optimal operating condition is also proposed. The theoretical simulation study will provide guidance for the improvement of experimental work.     

Activated carbon catalysts for the production of hydrogen via the sulfur–iodine thermochemical water splitting cycle

Seven activated carbon catalysts obtained from a variety of raw material sources and preparation methods were examined for their catalytic activity to decompose hydrogen iodide (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. Within the group of lignocellulosic steam-activated carbon catalysts, activity increased with surface area. However, both a mineral-based steam-activated carbon and a lignocellulosic chemically activated carbon displayed activities lower than expected based on their higher surface areas. In general, ash content was detrimental to catalytic activity while total acid sites, as determined by Boehm's titrations, seemed to favor higher catalytic activity within the group of steam-activated carbons. These results suggest that activated carbon raw materials and preparation methods may have played a significant role in the development of surface characteristics that eventually dictated catalyst activity and stability as well.     

Preliminary process analysis for the closed cycle operation of the iodine–sulfur thermochemical hydrogen production process

In the iodine–sulfur thermochemical hydrogen production process, a separation characteristic of 2-liquid phase (H₂SO₄ phase and HI_x phase) in the separator at 0°C was measured. Two-phase separation began to occur at about 0.32 of I₂ molar fraction and over. The separation characteristic became better with the increase in iodine concentration in the solution. The effect of flow rate variations of HI solution and I₂ solution from the HI_x distillation column on the process was evaluated. The flow rate increase in HI solution from the distillation column did not have a large effect on the flow rate of HI solution fed to the distillation column from the separator. The decreasing flow rate of I₂ solution from the distillation column decreased the flow rate of I₂ solution fed to the distillation column from the separator. The variation of I₂ molar fraction in the H₂SO₄ phase in the separator was sensitive to the variation in flow rate of both solutions from the distillation column. The tolerance level of the variation was investigated by considering I₂ solubility, 2-liquid phase disappearance and SO₂ reaction amount.     

Three-dimensional electrochemical Bunsen reaction characteristics in the sulfur–iodine water splitting cycle for hydrogen production

Hydrogen production from the sulfur–iodine water splitting cycle integrated with solar or nuclear energy has been proposed as a promising technique. Bunsen reaction is one of the three main steps in the cycle and electrochemical method has been applied to this reaction. In present work, a three-dimensional numerical study of the electrochemical Bunsen reaction was conducted. A three-dimensional, steady state, laminar and isothermal mathematical model of electrolytic cell was developed and verified by experiments. The spatial maldistribution of species concentration was found between electrodes and proton exchange membrane (PEM). The electric power drives most H₂SO₄ and I₂ to the anode and cathode surface, respectively, while the proton attraction contributes to HI enrichment on the surface of PEM. At the high inlet H₂SO₄ concentration of 50 wt%, the transformation of flow channel from single serpentine to single entry & double serpentine with the same inlet flow rate cannot solve the insufficient problem of SO₂. But the increase of the overall inlet flow rate in the double entry & double serpentine flow channel make SO₂ sufficient for anode reaction. Further decreasing the inlet H₂SO₄ concentration to 40 wt% and 30 wt% make the initial SO₂ sufficient for overall reactions. The single serpentine channel gives the highest SO₂ conversion rate, followed by the single entry & double serpentine and double entry & double serpentine flow channels. The single serpentine flow channel at the H₂SO₄ inlet concentration of 40 wt% is found optimal for achieving a high electrochemical Bunsen reaction performance.     

Equilibrium potential for the electrochemical Bunsen reaction in the sulfur–iodine cycle

The electrochemical Bunsen reaction was carried out in an electrochemical cell, where the anodic and cathodic compartments were separated by a Nafion 117 membrane. The equilibrium potential of the cell was experimentally measured and theoretically modeled. The effect of electrolyte concentration and temperature was explored. An increase in SO₂ or I₂ concentration reduced the equilibrium potential, whereas increasing H₂SO₄ or HI concentration had a contrary effect. The cell equilibrium potential decreased with increasing temperature. The derived theoretical equilibrium potential model was verified by the experimental data. The regression parameters M and Z in the model were independent of electrolyte concentration, but M decreased and Z kept constant with increasing temperature. An empirical equilibrium potential formula was proposed based on the theoretical and experimental results. The good reproducibility of this formula for measured data indicated its feasibility to estimate the equilibrium potential and also its guidance for optimizing the electrochemical Bunsen reaction.     

Study on a lab-scale hydrogen production by closed cycle thermo-chemical iodine–sulfur process

The Iodine–Sulfur (IS) thermo-chemical process for the production of hydrogen is one of the most promising approaches for use of the high temperature process heat supplied by high temperature reactor, which was developed in the Institute of nuclear and new energy technology (INET) of Tsinghua University, China, and INET initiated the fundamental studies on IS cycle since 2005. Based on the experiment results obtained by fundamental researches, a lab-scaled closed cycle loop (IS-10), which featured in electro-electrodialysis (EED) for hydriodic acid (HI) concentration, was designed and built at INET. The loop was composed of three sections, i.e., Bunsen section, HI section and sulfuric acid section. The closed cycle experiment on the loop was successfully carried out recently. In HI section, HI_x produced by Bunsen reaction was continuously purified through reverse Bunsen reaction, concentrated by EED, and then HI solution was obtained by distillation. Finally HI was catalytically decomposed to H₂ and I₂ with the conversion of 20%. In sulfuric acid section, sulfuric acid was continuously purified, concentrated by distillation, and catalytically decomposed to SO₂, O₂ and H₂O with the conversion of 75%. In Bunsen section, water, including recycled water, reacted with I₂ and SO₂ recycled from HI section and sulfuric acid section to form two separated acids phases, thus to form a closed cycle. The closed cycle experiment lasted for 7 h with the hydrogen production rate of 10 NL/h, with Pt loaded on activated carbon and copper chromite used as the catalysts for HI and sulfuric acid decomposition, respectively. This paper summarizes the main features of IS-10 and the main results of the closed cycle experiment. So far IS-10 is the second reported facility on which closed experiment was carried out, and the first one with EED embedded to perform a closed cycle operation.     

Global warming potential of the sulfur–iodine process using life cycle assessment methodology

A life cycle assessment (LCA) of one proposed method of hydrogen production – thermochemical water-splitting using the sulfur–iodine cycle couple with a very high-temperature nuclear reactor – is presented in this paper. Thermochemical water-splitting theoretically offers a higher overall efficiency than high-temperature electrolysis of water because heat from the nuclear reactor is provided directly to the hydrogen generation process, instead of using the intermediate step of generating electricity. The primary heat source for the S–I cycle is an advanced nuclear reactor operating at temperatures corresponding to those required by the sulfur–iodine process. This LCA examines the environmental impact of the combined advanced nuclear and hydrogen generation plants and focuses on quantifying the emissions of carbon dioxide per kilogram of hydrogen produced. The results are presented in terms of global warming potential (GWP). The GWP of the system is 2500 g carbon dioxide-equivalent (CO₂-eq) per kilogram of hydrogen produced. The GWP of this process is approximately one-sixth of that for hydrogen production by steam reforming of natural gas, and is comparable to producing hydrogen from wind- or hydro-electric conventional electrolysis.     

Experimental study and development of an improved sulfur–iodine cycle integrated with HI electrolysis for hydrogen production

The sulfur–iodine (SI or IS) thermochemical cycle assembled with solar or nuclear energy has been proposed as a large-scale, clean and renewable hydrogen production method. In present work, an improved SI cycle integrated with HI electrolysis for hydrogen production was developed according to experiments and simulation. The mathematical models of HI electrolysis using proton exchange membrane (PEM) electrolytic cell was developed, and then the user-defined module of HI electrolysis was set up through Aspen Plus and verified by experimental data. After designing and simulating the new flowsheet of the SI cycle based on HI electrolysis, 10 L/h of H₂ and 5 L/h of O₂ were obtained. The theoretic thermal efficiency of flowsheet reached 25–42% in terms of the utilization of waste heat. An ideal thermal efficiency of 33.3% through the proper internal heat exchange in the flowsheet was determined. Sensitivity analyses of parameters in the system were conducted. Increasing proton transfer number of PEM electrolytic cell in HI section improved the thermal efficiency of SI cycle. The ratio of distillate to feed rate and the plate number of distillation column in H₂SO₄ section were the most sensitive factors to the heat duty of overall SI cycle. The proposed new flowsheet for SI cycle is competitive to the flowsheets previously proposed in the field of flowsheet simplification.     

Hydrogen production with a solar steam–methanol reformer and colloid nanocatalyst

In the present study a small steam–methanol reformer with a colloid nanocatalyst is utilized to produce hydrogen. Radiation from a focused continuous green light laser (514 nm wavelength) is used to provide the energy for steam–methanol reforming. Nanocatalyst particles, fabricated by using pulsed laser ablation technology, result in a highly active catalyst with high surface to volume ratio. A small novel reformer fabricated with a borosilicate capillary is employed to increase the local temperature of the reformer and thereby increase hydrogen production. The hydrogen production output efficiency is determined and a value of 5% is achieved. Experiments using concentrated solar simulator light as the radiation source are also carried out. The results show that hydrogen production by solar steam–methanol colloid nanocatalyst reforming is both feasible and promising.     

Use of PSA for design of emergency systems in a sulfur–iodine cycle

A methodology is proposed to design emergency systems using Probabilistic Safety Assessment (PSA). It was used to design mitigation systems in the case of the formation of a toxic cloud due to an uncontrolled leakage of concentrated sulfuric acid in the second section of the General Atomics S–I cycle of a hydrogen production plant. Mitigation systems based on the isolation of a possible leak, the neutralization of a puddle of sulfuric acid and finally the flushing of that puddle were proposed and later analyzed with PSA. Many scenarios were taken into account to determine design changes and their impact on the probability of failure of the systems. Finally, the information produced in the PSA was used to provide feedback to optimize the design of the toxic cloud mitigation systems. The specific recommendations from the study suggest several design changes based on the PSA sensitivity runs. The results include optimized isolation and neutralization systems that will maintain the frequency of toxic cloud formation below 1.0E − 09 per year, which is only 16% of the frequency calculated for the original design based only on process engineering.     

Energy and exergy analyses of a novel sulfur–iodine cycle assembled with HI–I2–H2O electrolysis for hydrogen production

A novel sulfur–iodine (SI or IS) cycle integrated with HI–I₂–H₂O electrolysis for hydrogen production was developed and thermodynamically analyzed in this work. HI–I₂–H₂O electrolysis was used to replace the conventional concentration, distillation, and decomposition processes of HI, so as to simplify the flowsheet of SI cycle. And then the new cycle was divided into Bunsen reaction, H₂SO₄ decomposition and HI–I₂–H₂O electrolysis sections. Through incorporating the user-defined module of HI–I₂–H₂O electrolysis with Aspen Plus, the cycle was simulated and 0.448 mol/h (10 L/h) of H₂ was produced. The overall energy and exergy efficiencies of the novel SI system were estimated to be 15.3–31.0% and 32.8%, respectively. Most exergy destruction occurred in the H₂SO₄ decomposer and condenser for H₂SO₄ decomposition and Bunsen reaction sections, which accounted for 93.0% and 63.4%, respectively. A high exergy efficiency of 92.4% for HI–I₂–H₂O electrolysis section with less exergy destruction was determined, mostly due to the transformation of the overall electricity in electrolytic cell to exergy. Appropriate internal heat exchange and waste heat recovery will favor improving the energy and exergy efficiencies.     

Sulfuric acid decomposition in the iodine–Sulfur cycle using heat from a very high temperature gas-cooled reactor

Using the heat of high-temperature gas-cooled reactor to drive the thermochemical iodine-sulfur cycle is an important way to produce hydrogen on a large scale. The sulfuric acid decomposition is the key reaction affecting the hydrogen production efficiency in this method, so efficient sulfuric acid decomposition is needed. The present study focuses on the temperature rise, phase change and chemical reactions of sulfuric acid in a bayonet heat exchanger. The evaporation-condensation model in a steady temperature field was used to simulate the different stages of the phase change process with the species transport model used to calculate the product proportions. Finally, the effects of two catalyst arrangements on the decomposition fraction were analyzed. The results show that the temperature in the catalytic core area reaches 1100 K which provides the heat required for the decomposition reaction. When the sulfuric acid flow rate is 0.36 kg/h, the two-phase flow section is about 0.22 m long to promote better heat transfer. The simulations show that square and circular catalysts give sulfuric acid decomposition fractions of 65% and 57%. The pressure drop of the two catalyst arrangements is almost the same, while the square catalyst has a higher decomposition fraction, which can improve the economics. This study provides a reference for optimizing catalyst selection based on the flow and heat transfer rates.