Hydrogen production via sulfur-based thermochemical cycles: Part 2: Performance evaluation of Fe2O3-based catalysts for the sulfuric acid decomposition step

The sulfuric acid dissociation reaction, via which the production of SO₂ and O₂ 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 SO₃ decomposition with iron(III) oxide-based catalysts were performed investigating the effect of temperature, pressure and space velocity on SO₃ 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 SO₃ 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.     

Theoretical modeling of a directly heated solar-driven chemical reactor

A theoretical formulation for calculating the performances of a solar-driven catalytic chemical reactor was developed. It accounts for the spatial distribution of the deposition of primary energy within the receiver, the heat transfer into the catalytic bed and the thermochemical endothermic reaction, chemical composition and flow distribution within the reactor. The theory set forth was applied to analyze results obtained in a solar furnace with a directly heated U-shaped tubular reactor, wherein catalytic carbon dioxide reforming of methane occurred. We find that the receiver/reactor assembly acts as a self-regulating system. Beyond a fractional catalytic bed length of 0.14, solar energy can be converted primarily into chemical enthalpy. The fluid temperature gradient monitors the heat balance by adjusting the overall rate of conversion to the rate at which energy is being transferred through the reactor walls. Under certain circumstances, the process may be heat-transfer limited or controlled by chemical thermodynamics. A good fit between theory and experiment and accountability of all the intricate details in the various calculated performances of the receiver/reactor system support the theoretical model set forth in this study. We offer it as a tool for simulating future experimental results and for designing solar-driven reactors.     

Catalytic thermochemical reactor/receiver for solar reforming of natural gas: Design and performance

The advantages of thermochemical conversion of concentrated solar energy using catalytic processes are discussed. The design of a solar volumetric thermochemical reactor/receiver (TCRR) with catalytic absorber, method for synthesis of catalytically activated ceramics, and preparation of catalytic absorber have been described. The prototype TCRR was tested in the high flux solar furnace at the DAC, Cologne by using the dioxide reforming of methane. The tests were performed to check the main concept of the TCRR design and catalytic absorber, to study the influence of solar flux distribution, the reagent flows and their ratio on the productivity or conversion, determine the reagent's conversion depending on the focal point disposition with respect to the absorber, and to study the efficiency of the thermochemical conversion. The chemical and total efficiencies of the CO₂–methane conversion were calculated using the experimentally measured concentrations of the reaction products. The highest overall efficiency achieved in these experiments was 30% with the Ni–Cr catalytic absorber.     

Constructal method to optimize solar thermochemical reactor design

The objective of this study is the geometrical optimization of a thermochemical reactor, which works simultaneously as solar collector and reactor. The heat (concentrated solar radiation) is supplied on a small peripheral surface and has to be dispersed in the entire reactive volume in order to activate the reaction all over the material. A similarity between this study and the point to volume problem analyzed by the constructal approach (Bejan, 2000) is evident. This approach was successfully applied to several domains, for example for the coupled mass and conductive heat transfer (Azoumah et al., 2004). Focusing on solar reactors, this work aims to apply constructal analysis to coupled conductive and radiative heat transfer. As a first step, the chemical reaction is represented by a uniform heat sink inside the material. The objective is to optimize the reactor geometry in order to maximize its efficiency. By using some hypothesis, a simplified solution is found. A parametric study provides the influence of different technical and operating parameters on the maximal efficiency and on the optimal shape. Different reactor designs (filled cylinder, cavity and honeycomb reactors) are compared, in order to determine the most efficient structure according to the operating conditions. Finally, these results are compared with a CFD model in order to validate the assumptions.     

CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification

For the solar thermochemical gasification of coal coke to produce CO + H₂ synthetic gas using concentrated solar radiation, a windowed reactor prototype is tested and demonstrated at laboratory scale for CO₂ gasification of coal coke using concentrated Xe light from a 3-kW_th sun simulator. The reactor was designed to be combined with a solar reflective tower or beam-down optics. The results for gasification performance (CO production rate, carbon conversion, and light-to-chemical efficiency) are shown for various CO₂ flow rates and ratios. A kinetics analysis based on homogeneous and shrinking core models and the temperature distributions of the prototype particle bed are compared with those for a conventional fluidized bed reactor tested under the same Xe light irradiation and CO₂ flow-rate conditions. The effectiveness and potential impacts of internally circulating fluidized bed reactors for enhancing gasification performance levels and inducing consistently higher bed temperatures are discussed in this paper.     

Heat transfer analysis of thermal disassociation of ZnO for solar hydrogen production

Hydrogen production by the two-step solar thermochemical cycle has high cycle efficiency, low cost, and a great development space. Of special interest is the solar thermochemical cycle based on ZnO/Zn redox reactions since its high theoretical hydrogen yield and relatively low endothermic reaction temperature. In this paper, a steady heat transfer model for thermal ZnO dissociation in a solar thermochemical reactor is developed, coupling conduction, convection and radiation with chemical reaction. Accuracy was evaluated by comparison of results obtained from other references. Based on the new proposed reactor, the model is adopted to analyze the operating parameter effect on the conversion rate and fluid feature inside the solar reactor. The results show that the mass flow rate of ZnO and aperture gas temperature have a positive relation with ZnO conversion rate, however, the diameter of particles and aperture gas velocity has an inverse relation with ZnO conversion rate under specific condition. The results will provide useful foundation for improving the solar-to-fuel conversion rate in the near future.     

An experimental study of the CO2---CH4 reforming-methanation cycle as a mechanism for converting and transporting solar energy

The CO₂---CH₄ reforming-methanation chemical cycle provides an attractive means of transporting solar energy to a central station in accord with the Solchem concept. A number of receiver elements (chemical reactors) have been tested in the laboratory in an effort to optimize the catalyst parameters and the catalyst-reactor configuration. These tests led to the design and fabrication of both prototype and full scale production model Solchem receivers which were operated successfully at the White Sands Solar Furnace. The development of energy delivery methanation reactors is proceeding along with the design of both laboratory and field-model closed-loop Solchem systems.     

Photocatalytic hydrogen production under direct solar light in a CPC based solar reactor: Reactor design and preliminary results

In despite of so many types of solar reactors designed for solar detoxification purposes, few attempts have been made for photocatalytic hydrogen production, which in our option, is one of the most promising approaches for solar to chemical energy conversion. Addressing both the similarity and dissimilarity for these two processes and by fully considering the special requirements for the latter reaction, a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar reactor has been designed for the first time. The design and optimization of this CPC based solar reactor has been discussed in detail. Preliminary results demonstrated that efficient photocatalytic hydrogen production under direct solar light can be accomplished by coupling tubular reactors with CPC concentrators. It is anticipated that this first demonstration of concentrator-based solar photocatalytic hydrogen production would draw attention for further studies in this promising direction.     

Multiphase reactor scale-up for Cu–Cl thermochemical hydrogen production

This paper examines selected design issues associated with reactor scale-up in the thermochemical copper–chlorine (Cu–Cl) cycle of hydrogen production. The thermochemical cycle decomposes water into oxygen and hydrogen, through intermediate copper and chlorine compounds. In this paper, emphasis is focused on the hydrogen, oxygen and hydrolysis reactors. A sedimentation cell for copper separation and HCl gas absorption tower are discussed for the thermochemical hydrogen reactor. A molten salt reactor is investigated for decomposition of an intermediate compound, copper oxychloride (CuO·Cl₂), into oxygen gas and molten cuprous chloride. Scale-up design issues are examined for handling three phases within the molten salt reactor, i.e., solid copper oxychloride particles, liquid (melting salt) and exiting gas (oxygen). Also, different variations of hydrolysis reactions are compared, including 5, 3 and 2-step Cu–Cl cycles that utilize reactive spray drying, instead of separate drying and hydrolysis processes. The spray drying involves evaporation of aqueous feed by mixing the spray and drying streams. Results are presented for the required capacities of feed materials for the multiphase reactors, steam and heat requirements, and other key design parameters for reactor scale-up to a pilot-scale capacity.     

Experimental study on sulfur trioxide decomposition in a volumetric solar receiver–reactor

Process conditions for the direct solar decomposition of sulfur trioxide have been investigated and optimized by using a receiver–reactor in a solar furnace. This decomposition reaction is a key step to couple concentrated solar radiation or solar high‐temperature heat into promising sulfur‐based thermochemical cycles for solar production of hydrogen from water. After proof‐of‐principle a modified design of the reactor was applied. A separated chamber for the evaporation of the sulfuric acid, which is the precursor of sulfur trioxide in the mentioned thermochemical cycles, a higher mass flow of reactants, an independent control and optimization of the decomposition reactor were possible. Higher mass flows of the reactants improve the reactor efficiency because energy losses are almost independent of the mass flow due to the predominant contribution of re‐radiation losses. The influence of absorber temperature, mass flow, reactant initial concentration, acid concentration, and residence time on sulfur trioxide conversion and reactor efficiency has been investigated systematically. The experimental investigation was accompanied by energy balancing of the reactor for typical operational points. The absorber temperature turned out to be the most important parameter with respect to both conversion and efficiency. When the reactor was applied for solar sulfur trioxide decomposition only, reactor efficiencies of up to 40% were achieved at average absorber temperature well below 1000°C. High conversions almost up to the maximum achievable conversion determined by thermodynamic equilibrium were achieved. As the re‐radiation of the absorber is the main contribution to energy losses of the reactor, a cavity design is predicted to be the preferable way to further raise the efficiency. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance improvement of a solar volumetric reactor with passive thermal management under different solar radiation conditions

To alleviate the effect of solar radiation fluctuation on the solar volumetric reactor, phase change material (PCM) is applied to buffer the temperature vibration and improve the stability of thermochemical reactions. In this work, we analyzed the heat flow and distribution characteristics of the conventional double-walled volumetric reactor filled with PCMs (SVR1). We then proposed a novel solar volumetric reactor design (SVR2) to solve the problems of local high temperature, slow charging-discharging rate, and fluctuating methane conversion in various radiation conditions. The heat and mass transfer model coupled with thermochemical reaction kinetics was established to compare the performance of SVR1 and SVR2 under steady state, heat charging-discharging mode, and actual solar radiation fluctuation, respectively. The results show that compared to SVR1, the maximum temperature of SVR2 decreases by 106.3 K, and the minimum methane conversion rate increases from 77.4% to 93.6% under natural solar radiation fluctuation.     

Thermodynamic analysis of solar thermochemical energy absorption

Thermochemical energy conversion is analysed on a thermodynamic basis with particular interest in obtaining guidance as to solar thermochemical absorber design at an early stage of development when technical and cost information is often unreliable. An earlier-used thermodynamic equivalence technique which is equally applicable to both the endothermic and exothermic reactions has been developed to the point where it gives a clear insight into all sources of heat and work. The method is applied in particular to separating endothermic reactions of which ammonia dissociation is a prime example. A reversibility ratio is defined as the ratio of irreversible to reversible work and it is shown that in a practical solar thermochemical absorber design the reversibility ratio should be minimized, corresponding to reaction temperature minimization and therefore to tower energy losses and to potential for use of lower cost reactor materials and more active catalysts. Values of reversibility ratio are calculated for the ammonia system and are discussed in relation to solar thermochemical absorber design. In a final analysis employing the thermodynamic equivalence technique, it is shown that the apparent paradox between liquid and alternative gas pump work requirements in a liquid/gas thermochemical system is thermodynamically consistent with the internal generation of effective work from the heat source used to drive the endothermic reaction system.     

Kinetic and thermodynamic analyses of mid/low-temperature ammonia decomposition in solar-driven hydrogen permeation membrane reactor

It is a promising method for hydrogen generation without carbon emitting by ammonia decomposition in a catalytic palladium membrane reactor driven by solar energy, which could also store and convert solar energy into chemical energy. In this study, kinetic and thermodynamic analyses of mid/low-temperature solar thermochemical ammonia decomposition for hydrogen generation in membrane reactor are conducted. Hydrogen permeation membrane reactor can separate the product and shift the reaction equilibrium forward for high conversion rate in a single step. The variation of conversion rate and thermodynamic efficiency with different characteristic parameters, such as reaction temperature (100–300 °C), tube length, and separation pressure (0.01–0.25 bar), are studied and analyzed. A near-complete conversion of ammonia decomposition is theoretically researched. The first-law thermodynamic efficiency, net solar-to-fuel efficiency, and exergy efficiency can reach as high as 86.86%, 40.08%, and 72.07%, respectively. The results of this study show the feasibility of integrating ammonia decomposition for hydrogen generation with mid/low-temperature solar thermal technologies.     

Ammonia dissociation for solar thermochemical absorbers

A prototype ammonia dissociator has been constructed and operated over a wide variety of conditions, and its performance has been examined in relation to solar thermochemical absorber operation. High values of energy storage efficiency approaching unity are obtained for high values of reaction extent, corresponding to the use of high activity ammonia dissociation catalysts. There is a need for further development of such catalysts designed specifically for solar absorber operating conditions. It is shown that in order to make optimum use of the available catalyst volume, the power density profile and specific absorber design should be matched to give an essentially isothermal temperature profile along the catalyst chamber length. There is potential for a solar thermochemical absorber based on ammonia dissociation to be constructed using simple tube technology and thus within the severe cost constraints associated with solar absorber design.     

Research and development on membrane IS process for hydrogen production using solar heat

Thermochemical hydrogen production has attracted considerable interest as a clean energy solution to address the challenges of climate change and environmental sustainability. The thermochemical water-splitting iodine-sulfur (IS) process uses heat from nuclear or solar power and thus is a promising next-generation thermochemical hydrogen production method that is independent of fossil fuels and can provide energy security. This paper presents the current state of research and development (R&D) of the IS process based on membrane techniques using solar energy at a medium temperature of 600 °C. Membrane design strategies have the most potential for making the IS process using solar energy highly efficient and economical and are illustrated here in detail. Three aspects of membrane design proposed herein for the IS process have led to a considerable improvement of the total thermal efficiency of the process: membrane reactors, membranes, and reaction catalysts. Experimental studies in the applications of these membrane design techniques to the Bunsen reaction, sulfuric acid decomposition, and hydrogen iodide decomposition are discussed.     

Gasification of biomass: comparison of fixed bed and fluidized bed gasifier

Gasification as a thermochemical process is defined and limited to combustion and pyrolysis. A systematic overview of reactor designs categorizes fixed bed and fluidized bed reactors. Criteria for a comparison of these reactors are worked out, i.e. technology, use of material, use of energy, environment and economy. A utility analysis for thermochemical processes is suggested. It shows that the advantages of one of the reactor types are marginal. An advantage mainly depends on the physical consistency of the input. As a result there is no significant advantage for the fixed bed or the fluidized bed reactor.     

太阳能热化学能量贮存与转化研究

分析了以氨为工质对闭式循环工况下太阳能量收集、贮存、恢复转化过程,建立了闭式太阳能热化学反应热力循环过程二维均质催化反应传热传质数学模型,并对模型计算中相关参数的选择进行了分析说明.太阳能可逆化学反应能量贮存与转化过程可有效地结合到太阳能热力发电及空调制冷之中,解决太阳能量传输受气候间隙性的影响.     

Experimental and theoretical study of a solar thermochemical receiver module

A prototype test module of a solar thermochemical receiver using an reaction system is designed, built, and tested to establish a technical data base for future subsystem design efforts. Emphasis is placed on experimental verification of the computer simulation to establish a reliable design tool to predict the thermochemical performance of the receiver with a reasonable degree of confidence. The computational results are compared with experimental results obtained from the module tested at New Mexico State University. Reasonable agreement is found over a range of test conditions. It is concluded that the present design offers satisfactory conversion performance and operational flexibility for the construction of a complete reactor/receiver unit for use in a 10- to 15-kW dish collector system. However, further development works are required to resolve the problems associated with the catalyst contamination and the reactor construction material compatibility.     

Use of solar energy for direct and two-step water decomposition cycles

In this study, the feasibility of using concentrated solar energy at high temperatures to decompose water is experimentally demonstrated. The preliminary studies show that direct decomposition of water at 2000–2500°C is possible and the main development should be directed to reactor design and the separation of product gases. On the other hand, it is shown that two step thermochemical cycles for hydrogen production are feasible when the reactions are carried out at appropriate high temperatures in a solar furnace. The thermal decomposition of zinc oxide, suitable for such a two step cycle, is studied in detail.