Sulfuric Acid Decomposition Research Articles

Hydrogen production via sulfur-based thermochemical cycles: Part 2: Performance evaluation of Fe 2O 3-based catalysts for the sulfuric acid decomposition step

The sulfuric acid dissociation reaction, via which the production of SO 2 and O 2 is achieved, is the most energy intensive step of the so-called sulfur-based thermochemical cycles for the production of hydrogen. Efforts are focused on the feasibility and effectiveness of performing this reaction with the aid of a high-temperature energy/heat source like the sun. Such coupling can be achieved either directly in a solar reactor by concentrated solar radiation, or indirectly by means of a heat-exchanger/decomposer reactor using a suitable heat transfer fluid. Since a very limited amount of work regarding the potential formulations and sizing of such suitable reactors has been performed so far, the present work addresses further steps necessary for the efficient design, manufacture and operation of such reactors for sulfuric acid decomposition. In this respect, parametric studies on the SO 3 decomposition with iron(III) oxide-based catalysts were performed investigating the effect of temperature, pressure and space velocity on SO 3 conversion. Based on these results, an empirical kinetic law suitable for the reactor design was developed. In parallel, siliconised silicon carbide honeycombs coated with iron(III) oxide were prepared and tested in structured laboratory-scale reactors to evaluate their durability (i.e. activity vs. time) during SO 3 decomposition, with the result of satisfactory and stable performance for up to 100 h of operation. The results in combination with characterization results of “aged” materials can provide valuable input for the design of prototype reactors for sulfuric acid decomposition.

Modeling of a solar receiver-reactor for sulfur-based thermochemical cycles for hydrogen generation

Sulfur-based thermochemical cycles for hydrogen generation from water have one reaction step in common, which is the decomposition of sulfuric acid, which is one of the most energy-consuming steps. The present work deals with the development of a dynamic mathematical model of a solar reactor for this key step. One of the core parts of the model is a partial model of the reaction kinetics of the decomposition of sulfur trioxide, which is based on experiments investigating the kinetics of the used catalyst platinum coated on a ceramic solar absorber. Other partial models describe, e.g. the absorption of solar radiation, heat conduction in the absorber, convection between gas and the absorber walls and energy losses due to heat radiation. A comprehensive validation of the reactor model is performed using measured data, which is gained in experiments with a prototype reactor. The operating behavior of the real reactor is compared with the results of the numerical simulation with the model. The validation is, in particular, performed by reproducing the influences of individual parameters on the chemical conversion and the reactor efficiency. The relative deviations between the experimental data and the simulation results are mostly within the range of measurement accuracy. In particular, the good agreement of calculated values of the derived parameters, SO3 conversion and reactor efficiency with those determined from the experiments qualifies the model for optimization purposes. Copyright © 2010 John Wiley & Sons, Ltd.

Catalytic activities of cobalt, nickel and copper ferrospinels for sulfuric acid decomposition: The high temperature step in the sulfur based thermochemical water splitting cycles

The catalytic decomposition of sulfuric acid is the most endothermic step of the sulfur based water splitting thermochemical cycles, which are promising technologies for large scale hydrogen production in future. In the present study the catalytic activities of three ferrospinels AFe 2O 4 (A = Co, Ni, Cu) were evaluated for high temperature sulfuric acid decomposition reaction. Catalyst characterization by Mössbauer spectroscopy confirmed the occupancy of octahedral and tetrahedral sites by Fe 3+ ions in all three inverse spinels. The temperature programmed reduction studies revealed that the reducibility of Fe 3+ was greatly enhanced in CuFe 2O 4 as compared to other ferrites. Copper ferrite was found to be the most active catalyst for the reaction with ∼78% conversion at 800 °C. Presence of sulfate species on the spent catalysts was revealed by an ex situ analysis of the spent catalyst samples by FTIR, SEM and evolved gas analysis (EGA). FTIR spectra of all the three spent catalyst samples exhibit four prominent peaks in the region 950–1200 cm −1, which is an indicative of C 2v symmetry and bidentate sulfate coordination. A plausible mechanism for the sulfuric acid decomposition over spinel ferrites was proposed via the metal sulfate formation and then decomposition followed with an oxygen evolution step with the sulfate decomposition being the rate determining step at higher temperatures. EGA showed evolution of SO 2 as a decomposition product from existed sulfate of spent catalysts at high temperatures, with the rate of SO 2 evolution following the order: CuFe 2O 4 > NiFe 2O 4 > CoFe 2O 4. The enhanced rate of decomposition of the sulfates of copper ferrite can be attributed to the higher electronegativity of Cu 2+ as compared to Ni 2+ and Co 2+, which renders the S–O bond in the mixed metal sulfate weaker than others and thus more susceptible to dissociation. The lower thermal stability of sulfate and better reducibility are responsible for the improved catalytic properties of copper ferrite among the three ferrospinels investigated for sulfuric acid decomposition.

Thermodynamic Modeling of the Sulfuric Acid−Water−Sulfur Trioxide System with the Symmetric Electrolyte NRTL Model

Thermodynamic modeling of the sulfuric acid−water−sulfur trioxide system is of great interest to the industry. The recently developed symmetric electrolyte NRTL activity coefficient model is applied to develop a comprehensive thermodynamic model for the sulfuric acid system over the whole concentration range from pure water to pure sulfuric acid to pure sulfur trioxide with temperature up to 773 K. The model takes into account partial dissociations of sulfuric acid and bisulfate ion, hydration of hydronium ion, decomposition of sulfuric acid to water and sulfur trioxide, and formation of disulfuric acid from sulfuric acid and sulfur trioxide in the concentrated sulfur trioxide region. Excellent matches between model correlations and available literature data are achieved for vapor−liquid equilibrium, osmotic coefficient, liquid phase speciation, heat of dilution, heat of mixing, heat of solution, and liquid heat capacity of the sulfuric acid−water−sulfur trioxide system.

Efficient catalytic decomposition of sulfuric acid with copper vanadates as an oxygen-generating reaction for solar thermochemical water splitting cycles

Copper pyrovanadate (Cu(2)V(2)O(7)) was found for the first time to achieve the high catalytic activity for the decomposition of sulfuric acid to evolve O(2) at moderate temperatures around 650 °C, which is essential for the development of solar thermochemical water splitting cycles.

Hydrogen production via sulfur-based thermochemical cycles: Part 1: Synthesis and evaluation of metal oxide-based candidate catalyst powders for the sulfuric acid decomposition step

The present work concerns the synthesis of various single and mixed oxide materials and their study as catalysts for the sulfuric acid dissociation reaction via which the production of SO 2 and O 2 is achieved. This is the most energy intensive step of sulfur-based thermochemical cycles for the production of hydrogen. Commercial (i.e. FeO, Fe 3O 4, Fe 2O 3, CuO, Cr 2O 3, γ-Al 2O 3, Pt/γ-Al 2O 3) and in-house binary and ternary compositions of the Cu–Fe–Al system as well as Fe–Cr mixed oxide materials prepared by the Solution Combustion Synthesis (SCS) technique were comparatively tested. The materials were studied in powder form, in a fixed bed reactor at 850 °C and ambient pressure. The feedstock was concentrated liquid sulfuric acid (95–98% wt) and all tests were performed at a Liquid Hourly Space Velocity-LHSV of 50–52 h −1. It was found that systems of both in-house synthesized mixed oxide families (Cu–Fe–Al and Fe–Cr) with proper composition could achieve SO 3 conversions comparable to that of the reference Pt/γ-Al 2O 3 system, being at the same time much less expensive. The results, in combination with characterization results of fresh and spent catalysts are employed to identify possible mechanisms of the reaction and streamline the synthesis of more efficient catalytic systems.

Preparation of Copper Chromite by Vaccum Freezing Drying Method and Its Catalytic Activity for Sulfuric Acid Decomposition

AbstractIodine sulfur (IS) process is one of the most promising routes for massive and CO2‐free hydrogen production. Among the three reactions IS process composed, decomposition of sulfuric acid is the most energy intensive step and has to be conducted in the presence of catalyst. In this work, copper chromite (CuCr2O4) was prepared, characterized, and used as the catalyst for sulfuric acid decomposition reaction. The co‐precipitation preparation method of CuCr2O4 which featured in vacuum freezing drying (VFD), was applied, and the main factors which influence the structure of CuCr2O4 such as pH values of the co‐precipitation solution and calcination temperature were investigated. Results showed that pH value affected the size of copper chromite granule, when pH was between 7 and 8, the size of prepared CuCr2O4 was in nanometer scale. Calcination temperature influenced the purity of the crystal. The samples were characterized by XRD, TEM and BET. The catalytic activity of CuCr2O4 prepared by VFD was evaluated and compared with CuCr2O4 prepared by usual co‐precipitation method and Pt/Al2O3. Results showed that the activity of CuCr2O4 was close to that of Pt catalyst, but apparently higher than that of CuCr2O4 prepared by usual method. In addition, it presents good stability in the 70 h test. Therefore, it was promising to be used in the sulfuric acid decomposition step of IS process.

Development of the once-through hybrid sulfur process for nuclear hydrogen production

The Once-through Hybrid Sulfur (Ot-HyS) process, proposed in this work, produces hydrogen using the same Sulfur dioxide Depolarized water Electrolysis (SDE) process found in the original Hybrid Sulfur cycle (HyS). In the process proposed here, the Sulfuric Acid Decomposition (SAD) process in the HyS procedure is replaced with the well-established sulfur combustion process. First, a flow sheet for the Ot-HyS process was developed by referring to existing facilities and to the work done by the Savannah River National Laboratory (SRNL) under their reasonable assumptions. The process was then simulated using Aspen Plus with appropriate thermodynamic models. It was demonstrated that the Ot-HyS process has higher net thermal efficiency, as well as other advantages, over competing benchmark processes. The net thermal efficiency of the Ot-HyS process is 47.1% (based on LHV) and 55.7% (based on HHV) assuming 33.3% thermal-to-electric conversion efficiency of a nuclear power plant with no consideration given to the work for the air separation. Hydrogen produced through the Ot-HyS process would be used as off-peak electricity storage, to relieve the burden of load-following and could help to expand applications of nuclear energy, which is regarded as a ’sustainable development’ technology.

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 H 2 and I 2 with the conversion of 20%. In sulfuric acid section, sulfuric acid was continuously purified, concentrated by distillation, and catalytically decomposed to SO 2, O 2 and H 2O with the conversion of 75%. In Bunsen section, water, including recycled water, reacted with I 2 and SO 2 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.

A DYNAMIC SIMULATION OF THE SULFURIC ACID DECOMPOSITION PROCESS IN A SULFUR-IODINE NUCLEAR HYDROGEN PRODUCTION PLANT

In order to evaluate the start-up behavior and to identify, through abnormal operation occurrences, the transient behaviors of the Sulfur Iodine(SI) process, which is a nuclear hydrogen process that is coupled to a Very High Temperature Gas Cooled Reactor (VHTR) through an Intermediate Heat Exchanger (IHX), a dynamic simulation of the process is necessary. Perturbation of the flow rate or temperature in the inlet streams may result in various transient states. An understanding of the dynamic behavior due to these factors is able to support the conceptual design of the secondary helium loop system associated with a hydrogen production plant. Based on the mass and energy balance sheets of an electrodialysis-embedded SI process equivalent to a 200 <TEX>$MW_{th}$</TEX> VHTR and a considerable thermal pathway between the SI process and the VHTR system, a dynamic simulation of the SI process was carried out for a sulfuric acid decomposition process (Second Section) that is composed of a sulfuric acid vaporizer, a sulfuric acid decomposer, and a sulfur trioxide decomposer. The dynamic behaviors of these integrated reactors according to several anticipated scenarios are evaluated and the dominant and mild factors are observed. As for the results of the simulation, all the reactors in the sulfuric acid decomposition process approach a steady state at the same time. Temperature control of the inlet helium is strictly required rather than the flow rate control of the inlet helium to keep the steady state condition in the Second Section. On the other hand, it was revealed that the changes of the inlet helium operation conditions make a great impact on the performances of <TEX>$SO_3$</TEX> and <TEX>$H_2SO_4$</TEX> decomposers, but no effect on the performance of the <TEX>$H_2SO_4$</TEX> vaporizer.

Efficient Conversion of Thermal Energy into Hydrogen: Comparing Two Methods to Reduce Exergy Losses in a Sulfuric Acid Decomposition Reactor

Two methods for increasing the exergy efficiency of thermochemical processes for the production of hydrogen from water and high temperature thermal energy are presented and compared. Increasing the exergy efficiency is equivalent to reducing the entropy production. Starting from a reference reactor for the decomposition of sulfuric acid, two new reactor designs are developed that both reduce the entropy production by 26%. The first design uses optimal control theory to obtain a more uniform distribution of the entropy production. As a result of this functional optimization we obtain optimal temperature profiles over the reactor length. This optimized design is found to perform the best, but it requires significant changes in the heating equipment in order to approximately realize the optimal temperature profiles. A second design is obtained by increasing the reactor length. This leads to a higher pressure drop and requires additional compressor duty.

Energy Efficiency Limits for a Recuperative Bayonet Sulfuric Acid Decomposition Reactor for Sulfur Cycle Thermochemical Hydrogen Production

A recuperative bayonet reactor design for the high-temperature sulfuric acid decomposition step in sulfur-based thermochemical hydrogen cycles was evaluated using pinch analysis in conjunction with statistical methods. The objective was to establish the minimum energy requirement. Taking hydrogen production via alkaline electrolysis with nuclear power as the benchmark, the acid decomposition step can consume no more than 450 kJ/(mol of SO2) for sulfur cycles to be competitive. The lowest value of the minimum heating target, 320.9 kJ/(mol of SO2), was found at the highest pressure (90 bar) and peak process temperature (900 °C) considered, and at a feed concentration of 42.5 mol % H2SO4. This should be low enough for a practical water-splitting process, even including the additional energy required to concentrate the acid feed. Lower temperatures consistently gave higher minimum heating targets. The lowest peak process temperature that could meet the benchmark of 450 kJ/(mol of SO2) was 750 °C. If the decomposition reactor were to be heated indirectly by an advanced gas-cooled reactor heat source (50 °C temperature difference between the primary and secondary coolants, 25 °C minimum temperature difference between the secondary coolant and the process), then sulfur cycles using this concept could be competitive with alkaline electrolysis provided that the primary heat source temperature is at least 825 °C. The bayonet design will not be practical if the (primary heat source) reactor outlet temperature is below 825 °C.

Simulation of Sulfur-Iodine Thermochemical Hydrogen Production Plant Coupled to High-Temperature Heat Source

The sulfur-iodine (SI) cycle is one of the leading candidates in thermochemical processes for hydrogen production. In this paper a simplified model for the SI cycle is developed with chemical kinetics models of the three main SI reactions: the Bunsen reaction, sulfuric acid decomposition, and hydriodic acid decomposition. Each reaction was modeled with a single control volume reaction chamber. The simplified model uses basic heat and mass balance for each of the main three reactions. For sulfuric acid decomposition and hydriodic acid decomposition, reaction heat, latent heat, and sensible heat were considered. Since the Bunsen reaction is exothermic and its overall energy contribution is small, its heat energy is neglected. However, the input and output streams from the Bunsen reaction are accounted for in balancing the total stream mass flow rates from the SI cycle. The heat transfer between the reactor coolant (in this case helium) and the chemical reaction chamber was modeled with transient energy balance equations. The steady-state and transient behavior of the coupled system is studied with the model, and the results of the study are presented. It was determined from the study that the hydriodic acid decomposition step is the rate-limiting step of the entire SI cycle.

Overview of nuclear hydrogen production research through iodine sulfur process at INET

Thermochemical water-splitting cycle is a promising process to produce hydrogen using solar or nuclear energy. R&D on hydrogen production through iodine sulfur (IS) thermochemical cycle was initiated in 2005 at INET. Fundamental studies on the three reactions of IS cycle, i.e., Bunsen reaction, HI decomposition reaction, sulfuric acid decomposition reaction, and related techniques, such as separation, concentration and purification, were carried through. In Bunsen section, the reaction kinetics and separation characteristics of H 2SO 4 and HI x phases were studied. In HI section, Pt catalysts were loaded on different supporters by various methods and used for HI decomposition; and electro-electrodialysis(EED) was developed for concentration of HI acid. In sulfuric acid section, non-Pt catalysts were developed for SO 3 decomposition. Based on fundamental researches, a closed-loop test apparatus of 10 NL/h H 2 was designed and established. The current status of IS process research is summarized in this paper. In addition, R&D plan of IS process at INET is presented.

Transient Analysis of Sulfur-Iodine Cycle Experiments and Very High Temperature Reactor Simulations Using MELCOR-H2

MELCOR is a thermal-hydraulic code used by the United States and the international nuclear community for the modeling of both light water and gas-cooled reactors. MELCOR was extended in order to model nuclear reactors that are coupled to the sulfur-iodine (SI) cycle for cogeneration of hydrogen. This version of the code is known as MELCOR-H2, and it includes modular secondary system components (e.g., turbines, compressors, heat exchangers, and generators), a point-kinetics model, and a graphical user interface. MELCOR-H2 allows for the fully coupled, transient analysis and design of the nuclear thermochemical SI cycle for the purpose of maximizing the production of hydrogen and electricity. Recent work has demonstrated that the hydrogen generation rate calculated by MELCOR-H2 for the SI cycle was within the expected theoretical yield.In order to benchmark MELCOR-H2, we simulated a set of sulfuric acid decomposition experiments that were conducted at Sandia National Laboratories during 2006. We also used MELCOR-H2 to simulate a 2004 Japan Atomic Energy Research Institute SI experiment.The simulations compared favorably with both experiments; most measured and calculated outputs were within 10%. The simulations adequately calculated O2, SO2, and H2 production rate, acid conversion efficiency, the relationship between solution mole percent and conversion efficiency, and the relationship between molar flow rate and efficiency.We also simulated a 6-stage turbine and a 20-stage compressor. Our results were mostly within 1 or 2% of the literature. Then, we simulated a pebble bed very high temperature reactor (VHTR) and compared key MELCOR-H2 results with the literature. The comparison showed that the results were typically within 1 or 2%. Finally, we compared the MELCOR-H2 point-kinetics model with the exact Inhour reactivity solution for various cases, including a 1.0 $ step reactivity insertion. We were able to employ a large time step while successfully matching the theoretical power level. These comparisons demonstrate MELCOR-H2’s unique ability to simulate fully coupled VHTRs for the production of hydrogen.

Optimal synthesis of heat exchanger network for thermochemical S-I cycle

In this paper, a brief survey of hydrogen production methods is presented with a focus on S-I cycle. Based on heat duty data of sulfuric acid decomposition in S-I cycle, optimization models are developed to explore the minimum utility consumption and the minimum number of heat exchangers. Finally an optimal heat exchanger network for S-I thermochemical cycle is defined by a mixed integer optimization model.

Solar Thermochemical Generation of Hydrogen: Development of a Receiver Reactor for the Decomposition of Sulfuric Acid

A critical step of sulfur based thermochemical cycles for hydrogen production from water is the endothermic decomposition of sulfuric acid. The necessary heat can be provided by concentrated solar radiation. A solar receiver-reactor has been developed and built to investigate this process. Experiments with the test reactor were carried out in the DLR solar furnace in Cologne and confirmed the technical feasibility of a receiver-reactor containing porous ceramics for the decomposition of sulfuric acid. The receiver-reactor and strategy of operation were iteratively optimized with respect to chemical conversion and reactor efficiency. Parametric studies were conducted with varying partial pressure of SO3, residence time, absorber temperature, the presence of catalyst, and the performance of different catalysts to quantify their influence on chemical conversion and reactor efficiency. The absorber temperature distribution was found to be the most crucial process parameter. Conversions close to the equilibrium—in some cases exceeding 90%—were achieved with a platinum catalyst. Thermal efficiencies of up to 35% for the foam vaporizer and 30% for the overall reactor were achieved. Enclosing the absorber in a cavity and using separate chambers for the evaporation and the SO3-decomposition were identified as potential measures to improve the reactor.

Thermochemical water-splitting cycle using iodine and sulfur

Research and development on the thermochemical water-splitting cycle using iodine and sulfur, a potential large-scale hydrogen production method, is reviewed. Feasibility of the closed-cycle continuous water splitting has been demonstrated by coupling the Bunsen reaction, thermal decomposition of hydrogen iodide and that of sulfuric acid. Studies are in progress to realize efficient hydrogen production. Also, development of chemical reactors made of industrial materials has been carried out, especially those used in the corrosive process environment of sulfuric acid vaporization and decomposition.

High-temperature sulfuric acid decomposition over complex metal oxide catalysts

Abstract Activity and stability of FeTiO 3 , MnTiO 3 , NiFe 2 O 4 , CuFe 2 O 4 , NiCr 2 O 4 , 2CuO·Cr 2 O 3 , CuO and Fe 2 O 3 for the atmospheric decomposition of concentrated sulfuric acid in sulfur-based thermochemical water splitting cycles are presented. Catalyst activity was determined at temperatures from 725 to 900 °C. Catalytic stability was examined at 850 °C for up to 1 week of continuous operation. The results were compared to a 1.0 wt% Pt/TiO 2 catalyst. Surface area by nitrogen physisorption, X-ray diffraction analyses, and temperature programmed desorption and oxidation were used to characterize fresh and spent catalyst samples. Over the temperature range, the catalyst activity of the complex oxides followed the general trend: 2CuO·Cr 2 O 3 > CuFe 2 O 4 > NiCr 2 O 4 ≈ NiFe 2 O 4 > MnTiO 3 ≈ FeTiO 3 . At temperatures less than 800 °C, the 1.0 wt% Pt/TiO 2 catalyst had higher activity than the complex oxides, but at temperatures above 850 °C, the 2CuO·Cr 2 O 3 and CuFe 2 O 4 samples had the highest activity. Surface area was found to decrease for all of the metal oxides after exposure to reaction conditions. In addition, the two complex metal oxides that contained chromium were not stable in the reaction environment; both leached chromium into the acid stream and decomposed into their individual oxides. The FeTiO 3 sample also produced a discoloration of the reactor due to minor leaching and converted to Fe 2 TiO 5 . Fe 2 O 3 , MnTiO 3 and NiFe 2 O 4 were relatively stable in the reaction environment. In addition, CuFe 2 O 4 catalyst appeared relatively promising due to its high activity and lack of any leaching issues; however it deactivated in week-long stability experiments. Complex metal oxides may provide an attractive alternative to platinum-based catalyst for the decomposition of sulfuric acid; however, the materials examined in this study all displayed shortcomings including material sintering, phase changes, low activity at moderated temperatures due to sulfate formation, and decomposition to their individual oxides. More effort is needed in this area to discover metal oxide materials that are less expensive, more active and more stable than platinum catalysts.

Design of a composite sulfuric acid decomposition reactor, concentrator, and preheater for hydrogen generation processes

The Hybrid Sulfur Process, as well as similar sulfur cycles for the production of nuclear hydrogen, requires the decomposition of sulfuric acid into sulfur dioxide, oxygen, and water at temperatures above 800 °C and at pressures up to 9 MPa. The design of a reactor for this process presents numerous challenges in terms of maintaining small pressure differentials and utilizing currently available materials of construction. This paper focuses on design calculations for a composite reactor that preheats, concentrates, and decomposes sulfuric acid for use in the production of hydrogen. The decomposition reaction takes place within individual tubes of a multitube reactor.