Experimental study on the purification of HIx phase in the iodine–sulfur thermochemical hydrogen production process

Purification of hydriodic phase (HIx) plays an important role in avoiding the undesirable side reactions between HI and H₂SO₄ in the sulfur–iodine thermochemical cycle. In this paper, a series of experiments on HIx phase purification were conducted by means of a stirred reactor using N₂ as stripping gas. The effects of the iodine concentration, reaction temperature, and the striping gas flow rate on HIx phase purification were investigated systematically in terms of the conversion of H₂SO₄ and the reaction types during purification. It was observed that the iodine concentration played a significant role in dictating the reactions during purification. The quantitative analysis of the compositions of the initial and purified HIx phases showed that not only the conversion of H₂SO₄ was enhanced but also the side reactions were effectively impeded by increasing the iodine concentration, temperature and the stripping gas flow rate. Based on the experimental data, the suitable operating conditions for HIx phase purification were proposed.     

Decomposition of hydriodic acid by electrolysis in the thermochemical water sulfur–iodine splitting cycle

Although several technologies, such as reactive distillation and catalytic membrane reactor, have been proposed to improve HI conversion efficiency, they still experience several challenges for the application in HI section. In this study, an electrochemical cell was employed for hydriodic acid decomposition under the presence of iodine. Several commercial proton-exchange membranes (PEMs), namely, Nafion 117 and Nafion 115, were used as separators for the electrochemical cell. Anodization of iodide anion occurred at the graphite electrode in the anode compartment. Hydrogen was generated by the reduction reaction of hydrogen cations, which migrated from anolyte to catholyte. In electrolysis experiments, PEM showed good performance in terms of high transport number of proton and low iodine permeation. Several parameters, such as operating temperature, HI molarity, and I₂ molarity in anolyte, which affected current efficiency, iodine permeance, and electric resistance of test cell, were investigated. High operating temperature and I₂ molarity in anolyte enhanced the permeability of iodine, which had several negative influences on electrochemical cell performance. Although current efficiency was negatively affected by increasing temperature and I₂ molarity, it still remained above 0.85 in the range of 30 °C–75 °C. Ohmic resistance, which is a component of cell resistance, offered by PEM was investigated with Nafion 117 and 115. Apart from graphite plates, activated carbon papers were adopted as electrodes to reduce the overpotentials due to their high specific surface characteristic.     

A sulfur-iodine flowsheet using precipitation,electrodialysis, and membrane separation to produce hydrogen

The preliminary flowsheet of an electrodialysis cell (EDC) and membrane reactor (MR)-embedded SI cycle has been developed. The key components consisting of the preliminary flowsheet are as follows: a Bunsen reactor having a mutual separation function of sulfuric acid and hydriodic acid phases, a sulfuric acid refined column for the purification of the sulfuric acid solution, a HI_x-refined column for the purification of the hydriodic acid solution, an isothermal drum coupled to a multi-stage distillation column to concentrate the sulfuric acid solution, a sulfuric acid vaporizer, a sulfuric acid decomposer, a sulfur trioxide decomposer, a sulfuric acid recombination reactor, a condensed sulfuric acid solution and sulfur dioxide/oxygen gas mixture separator, a precipitator to recover excess iodine dissolved in the hydriodic acid solution, an electrodialysis cell to break through the azeotrope of the HI/I₂/H₂O ternary solution, a multi-stage distillation column to generate highly concentrated hydriodic acid vapor as a top product of the column, a membrane reactor to decompose hydrogen iodide and preferentially separate the hydrogen, and a hydrogen scrubber. The material and energy balance of each component was established based on a computer code simulation using Aspen Plus™. The thermal efficiency of the EDC and MR-embedded SI process has also been evaluated and predicted as 39.4%.     

Optimization of liquid–liquid phase separation characteristics in the Bunsen section of the sulfur–iodine hydrogen production process

In order to optimize the sulfur–iodine thermochemical cycle, a series of experiments were conducted to investigate the separation characteristics of the liquid–liquid phase in the H₂SO₄/HI/I₂/H₂O quaternary solution produced by Bunsen reaction. The effects of the solution composition in the feed and the operating temperature on the separation characteristics were analyzed to determine the preferable operating conditions in the Bunsen section. The increases in both the iodine content and the operating temperature improved the separation characteristics of the liquid–liquid phase when the occurrence of secondary reactions was neglected. The amount of impurities in both phases obviously decreased as the iodine content increased. The effect of the iodine content was more significant relative to that of the operating temperature. The optimal operating conditions were proposed to achieve the concentrations of HI in the HIx phase in excess of the azeotropic composition.     

Thermodynamic and kinetic study of transport and reaction phenomena in gallium nitride epitaxy growth

An iodine vapor phase epitaxy (IVPE) system has been designed and built to grow high quality thick gallium nitride film at the growth rate up to 80 μm/h with the deposition temperature of 1050 °C and the pressure of 200 torr. Numerical and experimental studies have been performed to investigate heat and mass transport and reaction phenomena in a vertical reactor. Geometrical parameters and operating conditions are optimized to achieve high and uniform GaN deposition rate. Gas phase and surface reactions in the growth chamber have been analyzed thermodynamically and kinetically, and primary transport species and important reactions are identified. The rate expressions for different surface reactions are determined and their contributions to the GaN deposition rate are studied for different V/III ratios. The sticking probability of the main reactants and adsorption activation energy are calculated.     

Reforming of methanol to produce hydrogen over the Au/ZnO catalyst

Gold particle with an average size of d_Au ~ 4 nm was dispersed on ZnO by the deposition precipitation method. The fabricated Au/ZnO catalyst was used to produce hydrogen from reforming of methanol. Four reforming reactions, i.e., decomposition of methanol (DM), steam reforming of methanol (SRM), partial oxidation of methanol (POM) and oxidative steam reforming of methanol (OSRM), were evaluated in a fixed bed reactor. A reaction temperature of T_R > 623 K was required for catalyzing reactions of DM and SRM. Interestingly, high methanol conversion (C_MeOH > 90%) was found from reforming reactions of POM and OSRM at an amazing low temperature of T_R < 473 K. Besides, a presentable hydrogen yield (R_H2 ~ 2.4) and a low selectivity of CO (S_CO ~ 1%) were simultaneously attained from the reaction of OSRM. Therefore, the low temperature OSRM reaction over the Au/ZnO catalyst is suggested as a friendly reforming process for on-board production of hydrogen.     

Phase separation characteristics of the Bunsen reaction when using HIx solution (HI–I2–H2O) in the sulfur–iodine hydrogen production process

To continuously operate an integrated sulfur–iodine (SI) hydrogen production process, the HI_x solution (HI–I₂–H₂O) could be recycled from the HI decomposition section as a reactant in the Bunsen reaction section. In this study, the temperature, iodine content and water content were varied to identify the phase separation characteristics of products from the Bunsen reaction using the HI_x solution with SO₂. Increasing the temperature increased the volume of the H₂SO₄ phase solution and decreased the impurity content in each phase. Increasing the iodine feed concentration somewhat decreased the volume of the H₂SO₄ phase solution, although the density difference between the phases increased. The amount of H₂SO₄ that separated into the H₂SO₄ phase was very small under most of these conditions, which significantly hindered the continuous operation of the integrated SI process. The feed of additional water in the separation step was suggested to improve the separation performance of the H₂SO₄ phase solution while minimizing side reactions.     

Rice husk combustion evolved gas analysis experiments and modelling

Rice husk is a major agricultural waste which could be a major source of fuel for boilers and furnaces if its calorific value could be realized efficiently. The oxidation kinetics of rice husks combustion were investigated using an evolved gas analysis technique. Rice husk samples were heated from 100 °C to 500 °C at a constant rate inside a small pressurised reactor. An oxygen-containing gas was passed through the reactor at a controlled flow rate and the evolved gas was continually analysed for its oxygen, carbon monoxide and carbon dioxide contents after moisture had been removed. A model for the oxidation of the rice husks samples is proposed that considers that the many simultaneous and competing oxidation reactions may be adequately represented by grouping them into three overlapping and competing reaction regimes in which CO₂, CO and H₂O are the only reaction products. The activation energies, and peak oxygen consumption temperatures were all found to be linear functions of the oxygen partial pressure in the reactor. Increasing the oxygen partial pressure decreased the temperatures at which peak oxygen consumption occurred. The total system pressure had no effect on the combustion behaviour other than through the oxygen partial pressure. At a heating rate of 80 K h⁻¹ and a system pressure of 500 kPa values for E/R for the low temperature, medium temperature and high temperature oxidation reactions are 14.7, 19.2 and 17.4 respectively.     

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.     

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.     

Influence of the initial HI on the multiphase Bunsen reaction in the sulfur–iodine thermochemical cycle

In the sulfur–iodine cycle flowsheet, HI may exist in the feeds of Bunsen reaction. The effects of the initial HI and the operating temperature on the kinetic process and thermodynamic equilibrium of the multiphase Bunsen reaction were investigated. Increasing initial HI concentration (HI/H₂O = 0–1/18) or temperature (303 K–358 K) amplified the reaction kinetic rate, and led to the earlier appearance of liquid–liquid separation and less time to reach the thermodynamic equilibrium. But the separation became difficult for further increase of the initial HI content. The liquid–liquid equilibrium (LLE) phase separation was enhanced with rising temperature. An increase in the initial HI content slightly weakened the LLE phase separation at a lower temperature, while at 345 K and 358 K, the LLE phase separation characteristics showed little variation in the HI/H₂O molar ratio range of 0–1/18. A hyper-azeotropic HI concentration in the HIx phase was obtained with feeding HI. The conversion of SO₂ lowered as the initial HI content and the temperature increased.     

Numerical simulation of hydrogen production by gasification of large biomass particles in high temperature fluidized bed reactor

Biomass as a renewable fuel compared to fossil fuels usually contains high moisture content and volatile release. Hydrogen production by large particle biomass gasification is a promising technology for utilizing high moisture content biomass particle in the high temperature fluidized bed reactor. In the present work, simulation of large particles biomass gasification investigated at high temperature by using the discrete phase model (DPM). Combustible gases with homogeneous gas phase reactions, drying process with a heterogeneous reaction, primary and secondary pyrolysis with independent parallel-reaction by using two-competing-rate model to control a high and low temperature were used. During the thermochemical process of biomass, gaseous products containing of H₂, H₂O, CH₄, CO and CO₂ was obtained. The effects of concentration, mole and mass fraction and hydrodynamics effects on gaseous production during gasification were studied. The results showed that hydrodynamic effect of hot bed is different from cold bed. Concentration and molar fraction of CO and H₂ production by continually and stably state and small amount of CO₂, H₂O, and CH₄ was obtained. The hydrodynamic of bed plays the significant role on the rate of gaseous products.     

Partial oxidation of ethanol using a non-equilibrium plasma

Partial oxidation of ethanol with air was carried out in a pulsed discharge plasma reactor at low temperature and atmospheric pressure. Effects of O₂:ethanol ratio, ethanol flow rate, and discharge current were investigated. H₂ and CO are the major products (>86%). Increases of O₂:ethanol ratio promote CO formation at the expense of C₂ hydrocarbons. H₂ selectivity and H₂ + CO selectivity are maximized at O₂:ethanol ratios of 0.3 and 0.5, respectively. Increases of feed flow rate accompanied by current increases allow the reactor to operate with high throughput. The LHV energy efficiency is increased with increasing feed flow rate, reaching 85% at high ethanol flow rates, conditions that also increase throughput. In contrast to catalytic and homogenous reactions, not all O₂ is consumed at high O₂:ethanol ratio for the low temperature plasma reaction. A radical reaction pathway of H abstraction from –OH and the α-H in ethanol to form CH₃CHO followed by C–C scission is proposed. The produced hydrogen rich gas can be potentially used in fuel cells and engines.     

Effect of sulphur during the catalytic partial oxidation of ethane over Rh and Pt honeycomb catalysts

The impact of sulphur addition (2–58 ppm) during the catalytic partial oxidation (CPO) of ethane was investigated on Rh- and Pt-based honeycomb catalysts tested under self-sustained high temperature conditions. Both steady state and transient operation of the CPO reactor were investigated particularly with regards to poisoning/regeneration cycles. A detailed analysis of products distribution in the effluent and a heat balance of the CPO reactor demonstrates that sulphur reversibly adsorbed on Rh selectively inhibits the ethane hydrogenolysis and, to a lower extent, steam reforming reaction. A further, simultaneous adverse effect of S on the kinetics of the reverse water gas shift reaction on Rh catalyst operating at temperatures < 750 °C can cause an unexpected increase in the H₂ yield above its equilibrium value for low concentrations of the poison. Pt catalyst is less active for those reactions but in turn is more S-tolerant.     

Reactivity investigation on chemical looping gasification of biomass char using nickel ferrite oxygen carrier

Chemical looping gasification (CLG) involves the use of an oxygen carrier (OC) which transfers oxygen from air to solid fuel to convert the fuel into synthesis gas, and the traditional gasifying agents such as oxygen-enriched air or high temperature steam are avoided. In order to improve the reactivity of OC with biomass char, facilitating biomass high-efficiency conversion, a compound Fe/Ni bimetallic oxide (NiFe₂O₄) was used as an OC in the present work. Effect of OC content and oxygen sources on char gasification were firstly investigated through a TG reactor. When the OC content in mixture sample attains 65 wt.%, the sample shows the maximum weight loss rate at relatively low temperature, indicating that it is very favorable for the redox reactions between OC and biomass char. The NiFe₂O₄ OC exhibits a good performance for char gasification, which is obvious higher than that of individual Fe₂O₃ OC and mechanically mixed Fe₂O₃ + NiO OC due to the Fe/Ni synergistic effect in unique spinel structure. According to the TGA experimental results, effect of the steam content and cyclic numbers on char gasification were investigated in a fixed bed reactor. Either too low steam content or too high steam content doesn't facilitate the char gasification. And suitable steam content of 56.33% is determined with maximum carbon conversion of 88.12% and synthesis gas yield of 2.58 L/g char. The reactivity of NiFe₂O₄ OC particles shows a downtrend within 20 cycles (～64 h) due to the formation of Fe₂O₃ phase, which is derived from the iron element divorced from the Fe/Ni spinel structure. Secondly, the sintering of OC particles and ash deposit on the surface are also the reasons for the deactivation of NiFe₂O₄ OC. However, the carbon conversion and synthesis gas yield at the 20th cycle are still higher than those of the blank experiment. It indicates that the reactivity of NiFe₂O₄ OC can be maintained at a relatively long time and NiFe₂O₄ material can be used as a good OC candidate for char gasification in the long time running.     

Demonstration of the I–S thermochemical cycle feasibility by experimentally validating the over-azeotropic condition in the hydroiodic acid phase of the Bunsen process

Conventional I–S cycle flowsheets suffer from low thermal efficiency and highly corrosive streams. To alleviate these problems, KAIST has proposed the optimal operating condition for the Bunsen reaction and devised a new flowsheet that produces highly enriched HI through spontaneous L–L phase separation and simple flash processes under low pressure. A series of experiments were performed at KAIST to validate the new flowsheet and extend its feasibility. The experimental procedure, measurement method with a rich iodine condition, and results of experiments are discussed in this paper. When the molar ratio of I₂/H₂SO₄ in the feed increased from 2 to 4, the molar ratio of HI/(HI + H₂O) in the HIx phase improved from 0.157 to 0.22, which is high enough to generate highly enriched HI gas through flashing. An inverse Bunsen reaction and a sulfur formation were observed when the temperature was increased from 313 K to 343 K and the molar ratio of I₂/H₂SO₄ was decreased from 4 to 1. 10–50 wt% of HI in the feed turned into I₂ when an inverse Bunsen reaction and a sulfur formation occurred. The experimental data utilized in the previous parametric study of KAIST has been validated.     

Dissociation of H2S in non-equilibrium gliding arc “tornado” discharge

The process of hydrogen sulfide, H₂S, dissociation was studied in a non-equilibrium gliding arc “tornado” (GAT) plasma discharge. Utilizing GAT ensured uniform H₂S gas treatment in the reactor. In addition, it created a low temperature zone near the cylindrical wall of the reactor, while maintaining a high temperature zone near the reactor axis. An energy cost of 1.2 eV per H₂ molecule was achieved in this plasma system at atmospheric pressure. These results are particularly important for the oil industry, which consumes large amounts of hydrogen in oil hydro-treatment, and for gas industry because of the high H₂S content in “sour” gas.     

Stabilized three-stage oxidation of DME/air mixture in a micro flow reactor with a controlled temperature profile

Ignition and combustion characteristics of a stoichiometric dimethyl ether (DME)/air mixture in a micro flow reactor with a controlled temperature profile which was smoothly ramped from room temperature to ignition temperature were investigated. Special attention was paid to the multi-stage oxidation in low temperature condition.Normal stable flames in a mixture flow in the high velocity region, and non-stationary pulsating flames and/or repetitive extinction and ignition (FREI) in the medium velocity region were experimentally confirmed as expected from our previous study on a methane/air mixture. In addition, stable double weak flames were observed in the low velocity region for the present DME/air mixture case. It is the first observation of stable double flames by the present methodology. Gas sampling was conducted to obtain major species distributions in the flow reactor. The results indicated that existence of low-temperature oxidation was conjectured by the production of CH₂O occured in the upstream side of the experimental first luminous flame, while no chemiluminescence from it was seen.One-dimensional computation with detailed chemistry and transport was conducted. At low mixture velocities, three-stage oxidation was confirmed from profiles of the heat release rate and major chemical species, which was broadly in agreement with the experimental results.Since the present micro flow reactor with a controlled temperature profile successfully presented the multi-stage oxidations as spatially separated flames, it is shown that this flow reactor can be utilized as a methodology to separate sets of reactions, even for other practical fuels, at different temperature.     

Three-dimensional simulation and optimization of an isothermal PROX microreactor for fuel cell applications

Three-dimensional numerical simulations of the reacting flow in rectangular micro-channel PROX reactors are performed. To solve the set of governing equations, a finite volume method is applied using an improved SIMPLE algorithm. A three-step surface kinetics for the chemical reactions is utilized that includes hydrogen oxidation, carbon monoxide oxidation, and water–gas shift reaction. The kinetics chosen are for a Pt–Fe/γ-Al₂O₃ catalyst and operating temperatures of about 100 °C. The PROX reactor is expected to remove the carbon monoxide content in a hydrogen-rich stream from about 2% to less than 10 ppm. Effects of the inlet steam content, oxygen to carbon monoxide ratio, reactor wall temperature, aspect ratio of the channel cross section, and the channel hydraulic diameter are investigated. It is found that increasing the steam content, oxygen to carbon monoxide ratio, or wall temperature may improve the performance of the microreactor. It is also shown that the rate of water–gas shift reaction or its reverse is much lower than the oxidation reactions. Finally, it is revealed that based on a modified CO yield definition, the optimum channel geometry is a square shape.     

A thermodynamic study of propanol reforming in presence of hydrazine for hydrogen production

Thermodynamic analysis of hydrogen production from propanol reforming reactions, by decomposition and steam reforming, in presence of hydrazine was evaluated as a function of temperature (300–900 K) at a constant pressure of 1 atm. The molar ratio of reactants were varied to identify the conditions leading to hydrogen rich product stream with low carbon formation. Steam reforming of propanol displayed higher hydrogen production and a gradual decrease in carbon content with an increase in the steam/propanol ratio. Addition of hydrazine leads to a further enhancement in hydrogen amount along with a suppression in coking. A similar trend was observed in case of propanol decomposition reaction. Addition of hydrazine leads to a favorable condition for hydrogen production along with a decrease in carbon formation. In both, steam reforming and decomposition, methane and water seem to be the stable products at low temperature, which react together at elevated temperatures following steam reforming of methane to generate CO and hydrogen. Hydrazine, on the other hand diminishes carbon at low temperature and produces ammonia, which decomposes at higher temperature to generate hydrogen and nitrogen. It is clear that steam assists in eliminating carbon at higher temperature whereas hydrazine is helpful in removing carbon formation at lower temperature. Also, a considerably high ratio of H₂/CO can be maintained in both the reactions, propanol steam reforming and propanol decomposition, by introducing a hydrazine stream in the feed.