Sulfur-iodine Cycle Research Articles

파이롯 규모 SI 공정 시험 설비에서의 헬륨 가열 장치 설계

In this study, researchers performed preliminary design and numerical analysis for a pilot-scale helium heating system in- tended to support full-scale construction for a sulfur-iodine (SI) cycle. The helium heat exchanger used a liquefied petroleum gas (LPG) combustor. Exhaust gas velocity at the heat exchanger outlet was approximately 40 m/s based on computational thermal and flow analysis. The maximum gas temperature was reached with six baffles in the design; lower gas temperatures were observed with four baffles. The amount of heat transfer was also higher with six baffles. Installation of additional baffles may reduce fuel costs because of the reduced LPG exhausted to the heat exchanger. However, additional baffles may also increase the pressure dif- ference between the exchanger's inlet and outlet. Therefore, it is important to find the optimum number of baffles. Structural analy- sis, followed by thermal and flow analysis, indicated a 3.86 mm thermal expansion at the middle of the shell and tube type heat exchanger when both ends were supported. Structural analysis conditions included a helium flow rate of 3.729 mol/s and a helium outlet temperature of 910°C. An exhaust gas temperature of 1300°C and an exhaust gas rate of 52 g/s were confirmed to achieve the helium outlet temperature of 910°C with an exchanger inlet temperature of 135°C in an LPG-fueled helium heating system.

Detailed kinetic study of the electrochemical Bunsen reaction in the sulfur–iodine cycle for hydrogen production

The electrochemical Bunsen reaction has been proposed as an innovative and promising method compared with the traditional Bunsen reaction for the sulfur–iodine cycle, and its kinetics were studied in this work. Polarization curves were measured using platinum electrode in the different concentrations of SO2–H2SO4–H2O and I2–HI–H2O solutions at a temperature range of 303–353K. Fundamental kinetic parameters, namely exchange current density, Tafel slope, asymmetry factor, were determined for anodic SO2 oxidation reaction and cathodic I2 reduction reaction according to the Tafel plots. The activation energy and pre-exponential factor for the electrode reactions were also analyzed using Arrhenius relation. Results indicated that an increase in the SO2 concentration in the anolyte and the I2 concentration in the catholyte reduced the activation energy, contributing to the easier anode and cathode reaction. Take high electrode reaction rate and easier electrode reactions into account, the initial SO2 concentration of 1.509mol/L and the I2/HI molar ratio of 0.5 could be optimal.

Influence of calcination temperature on CuO–CeO2/SiC catalysts for SO3 decomposition in the sulfur–iodine cycle for hydrogen production

The copper-cerium composite oxides supported on silicon carbide (SiC) catalysts (CuO–CeO2/SiC) for sulfuric acid decomposition were studied in sulfur-iodine (SI or IS) cycle. The influence of calcination temperatures (800 °C–1000 °C) of CuO–CeO2/SiC catalysts on their activities and morphology were investigated in this study. The activity of CuO–CeO2/SiC was prominently higher than that of CuO–CeO2 composite oxides catalyst. The SO3 conversion ratio of CuO–CeO2/SiC catalyst increased with calcination temperature from 800 °C to 900 °C, while decreased with calcination temperature at 900 °C–1000 °C. XRD pattern, TEM (HRTEM) images and Energy Dispersive X-ray (EDX) spectra revealed that copper–cerium composite oxides were well dispersed and immobilized on the surface of SiC grains. XRD pattern showed that the accelerated transformation of SiC crystals to amorphous SiO2 and the rapid expansion of CeO2 sub-grains at the calcination temperature of 900 °C–1000 °C, as well as the accelerated amorphization of CuO crystals at 800 °C–900 °C. The agglomeration of catalysts with sintering temperature was also proven by the BET test. At a higher calcination temperature, X-ray photoelectron spectroscopy (XPS) of Cu2p spectra revealed the reduction of more Cu2+ in the process of SiC oxidation. According to TEM (HRTEM), XPS, and infrared radiation, the amorphous SiO2 was produced by the oxidation of superficial SiC particles, and the thickness of the SiO2 layers on SiC grains increased with the calcination temperature. A simple mechanism depicted the oxidation of CuO–CeO2/SiC was created.

The Corrosion Behavior of Ni3(Si,Nb) Alloys in Boiling 70 wt.% Sulfuric Acid

Corrosion-resistant Ni3(Si,Nb) alloys are promising materials of construction for hydrogen-production systems based on the sulfur-iodine thermochemical cycle. In this work, the corrosion rates of three different Ni3(Si,Nb) alloys were measured in boiling 70 wt.% sulfuric acid and a three-stage corrosion mechanism was identified, based on the composition and morphology of surface scale that developed. The α(Ni) + β(Ni3Si) eutectic constituent of the alloy microstructure was selectively attacked by acid and, when present, is detrimental to corrosion resistance. The G-phase (Ni16Si17Nb6) is more passive than the β-matrix and seems to contribute to a lower steady-state corrosion rate.

A detection method for HIx composition in thermochemical water splitting iodine–sulfur cycle

For the HIx section of iodine–sulfur cycle, the sampling and chemical analysis of HI–H2O–I2 solution (HIx) are very difficult, because of the heavy corrodibility and volatility. In this work, density and electro-electrodialysis (EED) cell voltage were selected as two substitute variables to determine the complete composition of HIx, since HIx is considered as a system with 2 degrees of freedom at given temperature and pressure, according to Gibbs phase rule. Densities and EED voltages of a series of prepared HIx solutions were measured at 1 atm and 80 °C. Then multiple regressions were carried out to obtain equations describing HIx's density–composition relationship and EED voltage–composition relationship. As verified, using the consociation equation set consisting of above 2 equations, concentration of HI and I2 of a certain HIx could be worked out by inputting the solution's density and EED voltage, suggesting this newly developed estimation technique could give HIx's composition information by density test and voltage measurement, instead of traditional titrations.

Start-up dynamic behaviors of a HI–I2–H2O distillation column for the sulfur-iodine hydrogen production cycle

The sulfur-iodine (SI) cycle to produce hydrogen from water requires a HIx multistage distillation column to obtain hyperazeotropic hydriodic acid vapor as a top product that can be afterward decomposed into hydrogen and iodine. To design a HIx multistage distillation column that can be applied to the cycle, its static and dynamic simulation is essentially demanded. A 50NL-H2/h scale SI test facility to be operated under a pressurized environment has been constructed in Korea. This study focuses on the HIx multistage distillation column (HIxMDC-50L) for the 50NL-H2/h SI test facility. The HIxMDC-50L was designed and installed in 2013. Based on the design specifications and operation method, the start-up behavior of the HIxMDC-50L has been analyzed using the simulation code “KAERI-DySCo”. As a result of the start-up dynamic simulation, it is confirmed that the HIxMDC-50L will advance to the steady state value within 1050 s to fulfill a hydrogen production rate of 50NL-H2/h. On the other hand, not only the effects of the column design parameters on the progress to a steady state but also the effects of several operation variables on the top product have been evaluated for safely operating the HIxMDC-50L.

Experimental validation of an optimal operating window in the Bunsen reaction section of the iodine–sulfur cycle

The Bunsen reaction section of I–S cycle has two parts, reaction and phase separation. Some previous work focused on phase separation using a simulated reaction mixture with four components (H2SO4, HI, H2O and I2). Based on phase separation data an operating window was suggested. This previous work had the limitation that reaction was not included. The present work includes both reaction and separation. We demonstrated that the Bunsen reaction ratio is higher than 2, and the azeotropic ratio(mol of HI/mol of HI + H2O) is higher than 0.4 at 293 K and 2 mol of I2 in the feed. Bunsen reaction was promoted at a lower temperature and higher I2 content, while phase separation became worse since the generated H2SO4 and HI remained in the HIx phase without separation. We validated the previously suggested optimal window and examined phase separation at the lower temperature region.

HI Decomposition over Carbon-Based and Ni-Impregnated Catalysts of the Sulfur–Iodine Cycle for Hydrogen Production

To find highly active and relatively low-cost catalysts for HI catalytic decomposition in the sulfur–iodine thermochemical cycle particularly for hydrogen production, four carbon-based catalysts, namely, activated carbon (AC), multiwalled carbon nanotubes (MWCNT), carbon molecular sieve (CMS), and graphite nanosheets (GNS), were investigated before and after modification treatment with Ni impregnation. Their catalytic activities in HI decomposition were tested from 573 to 773 K. AC showed the highest activity, followed by CMS, MWCNT, and GNS. Evidently, the catalytic activities of all these catalysts were significantly improved with Ni support. The Ni-impregnated AC (Ni/AC) showed the highest catalytic activity. The HI conversion over Ni/AC at 773 K was 22.3%, even equivalent to the thermodynamic yield. Characterization results indicated that carbon supports played a significant role in Ni dispersion on the catalyst surface. A hypothetical mechanism of the HI decomposition aided by the Ni-impregnated catalysts was also established. Metal Ni, structural defects, and edge sites of carbon influenced catalytic activity. The high activity of Ni/AC can be explained by its catalytic property.

Apparent kinetics of a gas–liquid–liquid system of Bunsen reaction with iodine-toluene solution for hydrogen production through H2S splitting cycle

A chemical splitting cycle of H2S to produce H2 for sustainable oil sands bitumen upgrading was recently developed in our research group which includes three reactions (Wang, H. Hydrogen production from a chemical cycle of H2S splitting. International Journal of Hydrogen Energy 2007; 32:3907–14):H2S + H2SO4 → S + SO2 + 2H2O (H2S Oxidation) (1)2H2O + I2+SO2 → H2SO4 + 2HI (Bunsen Reaction) (2)2HI → H2 + I2 (HI decomposition) (3)With iodine provided by iodine-toluene solution, the Bunsen reaction can be conducted at room temperature. Compared with the General Atomics stoichiometry and operation condition for the sulfur–iodine cycle, this modification can minimize or avoid problems such as side reactions, corrosion and iodine deposition. In addition, it reduces the unnecessary transportation of the excessive cycling agents in the cycle. This work studies the apparent reaction rate of the Bunsen reaction in a gas–liquid–liquid multiphase system resulted from the presence of toluene in a closed, fixed-volume batch reactor using the initial rate analysis method. The reaction rate was found to be the first order with respect to SO2 and I2, respectively. The temperature effect on the reaction rate showed activation energy of 6.02 kJ/mol, which suggests that the rate-controlling step of the Bunsen reaction in the gas–liquid–liquid system is the step of SO2 absorption into the liquid phases. The variation of operating conditions including toluene/water volume ratio, agitation speed, and temperature confirmed the mass transfer rate-limiting step.

Experimental modeling of hydrogen producing steps in a novel sulfur–sulfur thermochemical water splitting cycle

One of the more well-known examples of thermochemical water splitting cycles is the sulfur–iodine cycle, which utilizes iodine and sulfur dioxide to transform water into hydrogen and oxygen. A novel sulfur–sulfur cycle has been developed based on the original sulfur–iodine cycle, seeking an overall more favorable chemical process. The initial stages of the original and novel cycles are the same, in using sulfur dioxide and iodine with an excess of water to form two strong acids, hydrogen iodide and sulfuric acid. The secondary reactions differ greatly and are based on a previously unwanted side reaction where the two strong acids regenerate iodine and water along with hydrogen sulfide, as shown in the equations below. The hydrogen sulfide can then be steam reformed to hydrogen and sulfur dioxide, completing the cycle. Excess sulfuric acid can also be treated to regenerate water and sulfur dioxide.I2(l)+SO2(g)+2H2O(l)→2HI(l)+H2SO4(l)8HI(l)+H2SO4(l)→4I2(l)+4H2O(l)+H2S(g)This study seeks to explore the reaction kinetics of this two-step reaction by altering the various reaction temperatures and the initial concentration of water and to develop a robust and simple predictive model based on the resulting experimental data. The results suggest that an increase in the initial concentration of water will increase the rate of both reactions substantially and that it is beneficial to carry them out in the same vessel. A predictive model was successfully developed that allows for monitoring the progression of iodine through the reaction system in a batch setting. The results of the experimental and modeling work warrants further development of this novel sulfur–sulfur thermochemical water splitting cycle.

Catalytic decomposition of sulfuric acid over CuO/CeO2 in the sulfur–iodine cycle for hydrogen production

The present study is a follow-up of a previous one on a detailed kinetic modeling of the homogeneous decomposition of SO3–H2O vapor in the sulfur–iodine cycle for hydrogen production. In this paper, the activity and stability of complex metal oxides CexCu1−xO2−δ prepared by a sol–gel method with x values ranging within 0.2–0.8 were studied for SO3–H2O vapor decomposition having a feed rate of space velocity of 5000 ml g−1 h−1 at 727–877 °C. Sample Ce0.8Cu0.2O2-900 showed even higher activity than Pt catalyst at >800 °C and good stability at 850 °C for 60 h of continuous operation. The physicochemical properties and redox process of CuO/CeO2 catalysts for SO3 decomposition were characterized by temperature programmed reduction, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analyses. A redox mechanism was then proposed based on the characterization results and our previous homogeneous kinetic model. In this mechanism, both ceria-support and copper oxide clusters were reduced, oxidized, and interacted with each other. The overall effect was that CuO/CeO2 catalyst promoted the reaction of SO3 + O ↔ SO2 + O2, which was the limiting step of SO3 decomposition, by providing reactive oxygen.

A unique approach to the vapour phase of the HIx feed of the sulfur iodine thermochemical cycle: A Raman spectroscopy study

The Sulfur Iodine cycle has great potential for large scale hydrogen production from water. The HIx processing stage (Section III) of the cycle exhibits the major challenges. Among them, experimental vapour liquid equilibrium data is scarce, mainly due to inherent hurdles to monitor the system under operating conditions of this section, i.e. temperature, acidity, and corrosion. Until now, UV–Visible and FTIR spectroscopies are the online monitoring techniques of choice, however due their selection rules, there is no experimental spectroscopic evidence of H2 during direct decomposition of HIx. Bearing this in mind, here we demonstrate the feasibility of exploiting two different Raman spectroscopic techniques as in situ monitoring tools, to gain substantial knowledge on the mechanisms of hydrogen production in Section III. Whereas resonance Raman spectra revealed the occurrence of HI·(H2O)3 in the vapour phase; coherent anti-Stokes Raman spectra of H2 allowed us to estimate kinetic data such as the activation energy for HI decomposition of a given hyper-azeotropic ternary mixture (i.e. xHI: 0.144; xI2:0.308; xH2O:0.548) is 149.15 kJ/mol.

Pt/zirconia catalyst for hydrogen generation from HI decomposition reaction of S-I cycle

Hydrogen energy is considered as one of the ideal solutions for the fulfilment of the ever increasing energy demand. It is mainly due to the following two reasons: firstly, it can be produced from a very abundant source, that is, water; and secondly, it does not leave any harmful effect on the environment. Thermochemical cycles are amongst the most promising ways to generate hydrogen from water in an environment-friendly manner. Sulfur–iodine cycle is one of the most efficient thermochemical cycles. In this paper, we discuss synthesis of Pt/zirconia catalysts for HI decomposition reaction, which is one of the important steps of S–I thermochemical cycle. The catalysts were characterized by X-ray diffraction, scanning electron microscopy (SEM), field emission gun-SEM, transmission electron microscopy, N2 adsorption and H2 chemisorption. The catalytic activity and stability of these catalysts, for liquid phase decomposition of hydriodic acid was evaluated. Conversion is found to be dependent on the noble metal loading, with 18.7% conversion for 2% Pt/ZrO2 catalyst as compared with 2.7% of without catalyst, although the specific activity is highest for 0.5% Pt/ZrO2 catalyst. The catalyst was found to be stable under liquid phase HI decomposition reaction conditions. Copyright © 2014 John Wiley & Sons, Ltd.

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.

Influence of Ir content on the activity of Pt-Ir/C catalysts for hydrogen iodide decomposition in iodine–sulfur cycle

This study aimed to determine the influence of Ir content on the behavior of Pt-Ir/C catalysts for hydrogen iodide decomposition in iodine–sulfur thermochemical water-splitting cycle. A series of bimetallic xPt-yIr/C catalysts (x=4, 3, 2.5, 2, 1wt.%; y=1, 2, 2.5, 3, 4wt.%) were prepared by co-impregnation and reduction method. For comparison, monometallic 5%Pt/C and 5%Ir/C catalysts were also prepared. The catalysts were characterized by X-ray diffraction and transmission electron microscopy. Changing the Ir content in the bimetallic Pt-Ir/C catalysts can modify the particle size and dispersion state of the supported metal nanoparticles. The results of the characterizations of the fresh and used catalysts indicated that the size of the Pt nanoparticles has significantly increased after HI decomposition whereas that of the Pt-Ir nanoparticles is more resistant to particle growth. Among the Pt, Ir, and binary Pt-Ir catalysts, the bimetallic 2.5%Pt-2.5%Ir/C exhibited the highest catalytic activity toward HI decomposition. The 70h durability test showed that the Pt-Ir bimetallic catalyst possessed good activity maintenance.

Electrolysis of the Bunsen Reaction and Properties of the Membrane in the Sulfur–Iodine Thermochemical Cycle

The Bunsen reaction of the sulfur–iodine cycle was carried out using an electrolysis cell separated by a Nafion 117 membrane. The evolution of cell voltage, which indicates the variation of cell resistance and the dynamics of electrode reactions, the concentration of acids after electrolysis, and the membrane properties were studied. The effects of current density, anolyte and catholyte flow rate, operating temperature, initial H2SO4 concentration, HI concentration, and I2/HI molar ratio were explored. Higher H2SO4 and HI concentrations and cell voltage could be obtained at higher current densities. A flow rate of 20 mL/min, a temperature of 343 K, an initial H2SO4 concentration of 5.71 mol/kg, a HI concentration of 2.51 mol/kg, and an I2/HI molar ratio of 0.2 at a current density of 5 A/dm2 were optimal considering the concentration of acids and the cell voltage. The characterization of the membrane by scanning electron microscopy and Brunauer–Emmett–Teller method, as well as the electrolysis tests, showed that the activation favored the membrane recovery and thus contributed to its repeatability.

Reaction parameters of the Bunsen reaction under simulated recycling conditions

The iodine–sulfur (IS) thermochemical process is one of the most prospective, efficient, CO2-free, massive hydrogen production approaches that use nuclear or solar energies. Among the three reactions composed of the IS cycle, the Bunsen reaction is crucial to the smooth operation of the continuous closed cycle. In the Bunsen reaction, sulfuric and hydriodic acids are produced by the reaction of recycled products of the decomposition of these two acids, In this work, a Bunsen reaction under simulated closed-cycle conditions, i.e., reaction between I2/HI/H2O solution and SO2, was investigated. The effects of reaction conditions such as SO2 flow rate, HI acid concentration, I2/HI molar ratio, and temperature on the characteristics of the Bunsen products were examined. These characteristics were phase separation, phase volume ratio, and compositions of HIx and H2SO4 phases. The concentration of HI acid and the I2/HI molar ratio of the initial solution, but not the flow rate of SO2, were found to affect the phase states and compositions of Bunsen reaction products. The phase states and compositions of the products were predicted or calculated with self-built empirical models. Results well agreed with the experimental results for phase states and HIx composition. All these findings validated the reliability of the models and offered crucial reference and guidance for the operation of the closed-cycle IS process.

Detailed kinetic modeling of homogeneous H2SO4 decomposition in the sulfur–iodine cycle for hydrogen production

This study presents a detailed kinetic mechanism conducted by CHEMKIN for the homogeneous decomposition of SO3–H2O vapor in the sulfur–iodine cycle. The kinetic mechanism involving 27 reactions and 11 species was validated by experimental results. The effects of temperature, SO3/H2O ratio, residence time, and pressure on the decomposition rate of SO3 were studied by modeling. SO3 conversion rapidly increased with increasing temperature but only moderately increased with the addition of H2O in the kinetic process. The SO3 conversion ratio was slightly promoted with residence time at 1000K and 1100K. Meanwhile, SO3 conversion sharply increased with reaction time at 1300K to 1400K. The results of sensitivity analysis showed that elementary reactions (2) SO3=SO2+O, (3) SO3+O=SO2+O2, (12) 2HO2=H2O2+O2, and (13) HO2=H+O2 had important functions in SO3 decomposition. The reaction path of homogenous H2SO4 splitting was constructed based on detailed kinetic modeling and sensitivity analysis.

Effects of Ce/Zr composition on nickel based Ce(1−x)ZrxO2 catalysts for hydrogen production in sulfur–iodine cycle

Abstract In this study, 5%Ni/Ce(1−x)ZrxO2( x = 0, 0.2, 0.5, 0.8, 1) catalysts were prepared by sol–gel method and tested for hydrogen iodide (HI) decomposition in sulfur–iodine (SI) cycle. The effects of zirconia incorporation were subsequently examined by a series of characterization methods. In comparison with the blank test, all of the catalysts, particularly Ni/Ce0.8Zr0.2O2, remarkably enhance the HI decomposition. Therefore, adding a small amount of zirconia results in the highest catalytic activity, which could be attributed to the following three effects: increase in oxygen mobility and oxygen vacancy, stronger interaction between NiO and the supports. The oxygen mobility and oxygen vacancy are dominant in the HI decomposition. The strong interaction between NiO and the supports accelerates the oxygen transfer from the bulk to surface, which could also enhance the decomposition. However, excessive zirconia content has negative effects, which is due to the decrease in oxygen mobility and surface area. The poor thermal stability in zirconia-rich supports also restricts catalytic performance. In addition, a hypothetic mechanism of HI catalytic decomposition over Ni/Ce(1−x)ZrxO2 is proposed.

Hydrogen production by HI decomposition over nickel–ceria–zirconia catalysts via the sulfur–iodine thermochemical water-splitting cycle

Effects of Ni/Ce0.8Zr0.2O2 catalysts with different calcination temperatures were investigated for HI decomposition in the sulfur–iodine cycle. Calcination temperature affected the properties and performance of the catalysts. The catalytic activity of Ni/Ce0.8Zr0.2O2 catalysts was higher than that of catalysts based on pure ceria. The Ni/Ce–Zr300 demonstrated the best catalytic activity for temperature below 425°C, while the Ni/Ce–Zr700 demonstrated the best catalytic activity above 450°C. The Ni/Ce0.8Zr0.2O2 catalysts were formed with Zr4+ replacing Ce4+ in the ceria lattice and the Ni2+ ions dissolved into the ceria lattice. The NiO species are highly dispersed in Ni/Ce–Zr300, Ni/Ce–Zr500 and Ni/Ce–Zr700, but the sintering and agglomeration occurred obviously in Ni/Ce–Zr900. Interfacial NiO sites that strongly interacted with ceria and oxygen vacancy were regarded as the main active sites in HI catalytic decomposition. The incorporation of Zr improved the catalytic performance, textual features, thermal resistance and oxygen storage/transport properties of Ce–ZrO2 mixed oxides and the corresponding catalysts.