Solar UT-3 thermochemical cycle for hydrogen production

The UT-3 thermochemical hydrogen producing cycle is a four step process developed at the University of Tokyo. In the process, only solid and gas reactants/products are used and the maximum temperature is 1033 K. The process has been developed to be coupled with gas cooled nuclear reactors (HTGR). In this article, a new UT-3 process is conceived to be coupled with a solar heat source. In the new process, all four reactions are carried out in a adiabatic equipment where steam (or steam + nitrogen) can be used as a vector. The operation of the process is done continuously. During sunshine hours, the energy to the process is supplied directly from the solar receiver. During cloudy periods and nights, it is supplied from a thermal storage system where the necessary high temperature heat is stored during sunshine hours. The solar UT-3 process has been evaluated using the ASPEN-PLUS code. It is found that the overall thermal efficiency is 49.5%, the exergetic efficiency is 52.9% and the process can be realized using conventional materials. Sizing of a solar hydrogen plant producing 2000 Nm³ per hour hydrogen has been carried out and operation of various equipment has been discussed.     

A new thermochemical process for hydrogen production

A new thermochemical water-splitting cycle using the oxydo-reduction couples Ag⁺/Ag⁰ and Cu²⁺/Cu⁺ is described. It includes four main reaction steps, some of them requiring a solvent. At the maximum temperature of 570°C the thermal efficiency is about 41%.     

MASCOT—a bench-scale plant for producing hydrogen by the UT-3 thermochemical decomposition cycle

A bench-scale plant for producing hydrogen has been constructed on the basis of the thermochemical water-decomposition process, UT-3, consisting of Br, Ca and Fe compounds. This plant is named MASCOT (Model Apparatus for Studying Cyclic Operation in Tokyo) and is designed to be capable of producing 3 l/h of gaseous hydrogen at standard conditions. During several test runs, the continuous production of hydrogen was successfully achieved. In the present paper, the construction of the MASCOT plant is described.     

SbICa process for thermochemical hydrogen production

A new thermochemical water-splitting process named “Sb-I-Ca Process” is described with relevant thermochemical data and preliminary experimental results. The process, which is an improved version of the “Sb-I Process” previously proposed, consists of basic steps of five reactions and three phase-transitions. It was found that all reaction- and separation-steps in this process could be conducted without serious difficulties. From a flow diagram made on the basis of the experimental results, the overall efficiency was estimated to be about 40% with an assumption of 70% heat recovery.     

Reactor development for a biosolar hydrogen production process

When exposed to sunlight purple bacteria produce hydrogen from water and organic substrates by photosynthesis. In this paper a technical approach to utilize this effect as an energy source is presented including the development of a suitable reactor concept. Phototrophic bacteria have special requirements concerning pH-value, temperature, light intensity etc.. Therefore natural or daily fluctuations of weather conditions cause severe problems for outdoor cultivation of this species. A new bioreactor concept with very low need of external energy was tested overcoming many of these problems and easing application of biological hydrogen evolution on a technical scale.     

Operational strategy of a two-step thermochemical process for solar hydrogen production

A two-step thermochemical cycle process for solar hydrogen production from water has been developed using ferrite-based redox systems at moderate temperatures. The cycle offers promising properties concerning thermodynamics and efficiency and produces pure hydrogen without need for product separation.     

Immobilization of calcium oxide solid reactant on a yttria fabric and thermodynamic analysis of UT-3 thermochemical hydrogen production cycle

UT-3 cycle has been considered as one of the most promising thermochemical processes for hydrogen production. In order to make the cycle practical, however, the solid reactants in the cyclic reactions must have adequate lifespan and better kinetics. In this paper, hydrolysis reaction of calcium bromide, the slowest process in the cycle, was investigated theoretically and experimentally. A new type of calcium oxide reactant was fabricated by dispersing and fixing it on a yttria fabric via a comparatively straightforward and inexpensive immobilization process. The characteristics and cyclic performance of the prepared fabric samples were evaluated and compared with the conventional calcium oxide pellets. It was shown that the calcium oxide immobilized on the yttria fabric had continuous higher reactivity and comparable hydrolysis rate compared with the conventional calcium oxide pellets. A theoretical analysis of the thermodynamic cycle was also conducted. The effect of excess steam on the equilibrium conversion was significant; however, the reaction temperature was limited due to the melting point of calcium bromide. By continuously removing the product gas, the conversion in the hydrolysis reaction which is the slowest reaction in the cycle could be completed theoretically. This hypothesis was found to be true based on the experimental tests.     

Open-loop thermochemical cycles for the production of hydrogen

The concept of open-loop thermochemical cycles (cycles which have additional or other feedstocks than water and produce materials in addition to hydrogen and oxygen) is introduced. Preliminary analysis of possible feedstocks available indicates substantial quantities of hydrogen could possibly be produced through open-cycles. The advantages of open-cycles include the conversion of unwanted waste products to useful products while producing hydrogen. A compilation of open processes which would have SO₂ in addition to water as feedstock and which would produce sulfuric acid in addition to hydrogen and oxygen is given.     

Review of thermochemical hydrogen production

The periodical World Hydrogen Energy Conference (WHEC) is an opportunity to make an updated state of the art in the development of the thermochemical method for the production of hydrogen from water. The present paper summarizes the main evolution of this new technology giving also some comments.     

A plasmochemical concept for thermochemical hydrogen production

This paper discusses plasmochemical methods of water decomposition to produce hydrogen. Both the direct decomposition of water and a cycle involving the decomposition of carbon dioxide are discussed. A comparison of the relative merits of several methods of hydrogen production using a nuclear energy source is presented.     

Flowsheet study of the thermochemical water-splitting iodine–sulfur process for effective hydrogen production

The Japan Atomic Energy Agency (JAEA) is performing research and development on the thermochemical water-splitting iodine–sulfur (IS) process for hydrogen production with the use of heat (temperatures close to 1000 °C) from a nuclear reactor process plant. Such temperatures can be supplied by a High Temperature Gas-cooled Reactor (HTGR) process. JAEA's activity covers the control of the process for continuous hydrogen production, processing procedures for hydrogen iodide (HI) decomposition, and a preliminary screening of corrosion resistant process materials. The present status of the R&D program is reported herein, with particular attention to flowsheet studies of the process using membranes for the HI processing.     

Development studies on thermochemical cycles for hydrogen production

The experimental work in the field of the development of cycles of the Fe/Cl family is described. It appeared that application of a homogeneous support material for the gas-solids reaction was not feasible due to unexpected side reactions.The hydrolysis of ferrous chloride has been found to give good results and the development and operation of a continuously operating bench-scale reactor for this reaction has been described.The chlorination of the resulting magnetite could not be performed according to the originally proposed scheme. A satisfactory alternative has been found, i.e. chlorination at 150–200C with only hydrogen chloride and introduction of the reverse Deacon reaction as a fourth reaction in the cycle.     

Efficiency of thermochemical production of hydrogen

The overall thermal efficiency of several processes in the chemical industry is calculated from production units. The individual efficiency of a step was found to be 80% or less. Assuming an average thermal efficiency for each step to be 70%. the overall efficiency of two-, three- and four-step processes are estimated to be 50, 35 and 25%, respectively. Extrapolation of these results to the thermochemical decomposition of water leads to the conclusion that the process must consist of three steps or less in order for its efficiency to exceed that of electrolysis.     

Thermal efficiency of the magnesium-iodine cycle for thermochemical hydrogen production

The magnesium-iodine cycle, consisting of the redox reaction of iodine with magnesium oxide, the thermal decomposition of magnesium iodate, the hydrolysis of magnesium iodide and the thermal decomposition of hydrogen iodide, for thermochemical hydrogen production has been evaluated. The thermal efficiency of the process was calculated based on material and energy balances for three proposed flow-sheets. The three flow-sheets varied according to the method used (quenching, selective absorption of HI by magnesium oxide and the two-step chemical decomposition of HI by magnetite) to separate the products of hydrogen iodide decomposition. Values of 9–34% thermal efficiency were given as a function of the heat recovery of 65–85% for the quench method, 19–37% for the MgO method and 16–35% for the Fe₃O₄ method. In order to obtain more than 25–30% of the thermal efficiency, heat recovery must be more than 80%.     

Solar hydrogen production using two-step thermochemical cycles

In this paper, a thermodynamic study is presented on solar hydrogen production using concentrated solar energy and two-step thermochemical cycles. After discussing the temperature availability from solar installations and temperature requirements, two-step water decomposition processes using metal/metal oxide cycles are studied in detail. Some hybrid metal/metal oxide, purely thermochemical and hybrid, metal oxide/metal sulfate cycles are also discussed. The solar high temperature heat source is briefly analyzed and interfacing problems are discussed.     

Manganese oxide based thermochemical hydrogen production cycle

A MnO/NaOH based three-step thermochemical water splitting cycle was modified to improve the hydrolysis of α-NaMnO₂ (sodium manganate) and to recover Mn(III) oxides for the high-temperature reduction step. Sodium manganate forms in the reaction of NaOH with MnO that releases hydrogen. The hydrolysis of α-NaMnO₂ to manganese oxides and NaOH is incomplete even with a large excess of water and more than 10% sodium cannot be removed prior to the high-temperature reduction step.When mixed oxides of manganese with iron were used in the cycle, the NaOH recovery in the hydrolysis step improved from about 10% to 35% at NaOH concentrations above 1M. Only 60% sodium was removed at 0.5M from the mixed oxides whereas more than 80% can be recovered at the same NaOH concentration when only manganese oxides are used. A 10:1 Mn/Fe sample was cycled through all steps three times to confirm that multiple cycles are possible. The high-temperature reduction was carried out for 5h at 1773 K under vacuum and the conversion was about 65% after the 3rd cycle.Since sodium carryover into the high-temperature reduction cannot be avoided, even with the energy intensive hydrolysis step, a modified two-step cycle without low-temperature sodium recovery is proposed where α-NaMnO₂ is reduced directly to MnO at 1773 K under vacuum. On a laboratory scale, about 60% of the sodium that volatilized at the high temperatures was trapped with a water-cooled cold finger and conversions were stable at about 35% after three completed cycles.     

Solar hydrogen production by hybrid thermochemical processes

In this study, a solar chemical process has been conceived and evaluated. The process is based on the sulfur family cycles in which the thermochemical decomposition of sulfuric acid at high temperature is a common reaction in various processes. The decomposition of sulfuric acid using oxygen as a vector was studied earlier in conjunction with nuclear hydrogen production. In the present study, the process is adapted to couple with a high temperature solar heat source and the problem of intermittent operation has been solved. An assessment is presented on hydrogen production using a dedicated central receiver solar system coupled to the chemical process and the Mark 11 cycle. For 10⁶ GJ per year solar plant, it is found that the overall efficiency of solar hydrogen production is about 38% and the solar hydrogen cost is from 15 to 70$/GJ hydrogen depending on the cost parameters.     

Uranium thermochemical cycle: hydrogen production demonstration

In hydrogen production industry, thermochemical cycle technology for converting thermal energy into chemical storage energy of hydrogen owns absolute advantages. Compared with other thermochemical cycles, thermochemical cycle technology based on uranium (UTC) is safer and more efficient. This technology consists of three steps, where only the hydrogen production step is unique. In this paper, the verification has been done for this step. Solid products were characterized by XRD and Raman spectroscopy, which were confirmed to be α-Na₂U₂O₇. Gas chromatographic analyses were performed for gas samples, in which hydrogen output was obtained using an internal standard method.     

生物质热化学过程制氢技术

生物质是世界上最丰富的可再生资源之一,氢能源是未来理想的能源载体.生物质生长周期短,产量巨大,作为能源利用时,其CO2排放量几乎为零,因此被视为非常有潜力的清洁能源之一.生物质制氢技术主要包括热化学过程和生物过程,其中热化学过程主要是将生物质气化或生成生物油,再进行重整和水气置换反应,从而获得较高产量的氢气.文章介绍了利用生物质热裂解和气化(包括超临界水条件下气化)制氢技术,并对其未来的发展做了展望.