Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy

Chemical heat pumps—A basic thermodynamic analysis

Solar thermochemical processes have the potential to efficiently convert high-temperature solar heat into storable and transportable chemical fuels such as hydrogen. In such processes, the thermal energy required for the endothermic reaction is supplied by concentrated solar energy and the hydrogen production routes differ as a function of the feedstock resource. While hydrogen production should still rely on carbonaceous feedstocks in a transition period, thermochemical water-splitting using metal oxide redox reactions is considered to date as one of the most attractive methods in the long-term to produce renewable H2 for direct use in fuel cells or further conversion to synthetic liquid hydrocarbon fuels. The two-step redox cycles generally consist of the endothermic solar thermal reduction of a metal oxide releasing oxygen with concentrated solar energy used as the high-temperature heat source for providing reaction enthalpy; and the exothermic oxidation of the reduced oxide with H2O to generate H2. This approach requires the development of redox-active and thermally-stable oxide materials able to split water with both high fuel productivities and chemical conversion rates. The main relevant two-step metal oxide systems are commonly based on volatile (ZnO/Zn, SnO2/SnO) and non-volatile redox pairs (Fe3O4/FeO, ferrites, CeO2/CeO2−δ, perovskites). These promising hydrogen production cycles are described by providing an overview of the best performing redox systems, with special focus on their capabilities to produce solar hydrogen with high yields, rapid reaction rates, and thermochemical performance stability, and on the solar reactor technologies developed to operate the solid–gas reaction systems.

https://www.mdpi.com/2305-7084/3/3/63/pdf

Metal Oxides Applied to Thermochemical Water-Splitting for Hydrogen Production Using Concentrated Solar Energy

A chemical heat pump for upgrading low-level thermal energy has been proposed by adopting a reversible organic reaction couple, endothermic liquid-phase dehydrogenation of 2–propanol at low temperature and exothermic gas-phase hydrogenation of acetone at high temperature, where thermodynamical work is done by separating condensed 2–propanol from the gaseous mixture of 2–propanol, acetone and hydrogen in a fractionation column.



In the system constitution of the continuous type, the overhead vapour of the fractionation column is fed through the heat exchanger into the exothermic reactor, where acetone and hydrogen in excess are changed at 200°C into the equilibrium mixture, from which condensable 2–propanol is separated in the column by cooling at 30°C. The reverse reaction of 2–propanol decomposition into acetone and hydrogen proceeds in the endothermic reactor, i.e. the reboiler of the column, absorbing heat at 80°C. On the contrary, acetone and hydrogen in the overhead vapour of the fractionation column are stored at 30°C as liquid and metal hydride, respectively, in the system constitution of the storage type; when necessary, metal hydride is decomposed by heating at 80°C, with hydrogen at high pressure evolved and fed through the heat exchanger into the exothermic reactor, giving the equilibrium mixture at high pressure and temperature. Product condensates are transferred through a valve into the fractionation column in order to separate 2–propanol and acetone, the former of which is dehydrogenated in the endothermic liquid-phase reactor, regenerating acetone and hydrogen at 80°C and atmospheric pressure.



Energy efficiencies were evaluated for the system constitutions of both the continuous and storage types; the 80°C heat supplied was convertible into the 200°C heat continuously at the enthalpy efficiency or coefficient of performance (COP) of 0·36 in the former, whereas the 270°C heat was obtainable with the aid of metal hydride from the same heat source at COP of 0·21 in the latter.

Catalyst‐assisted chemical heat pump with reaction couple of acetone hydrogenation/2–propanol dehydrogenation for upgrading low‐level thermal energy: Proposal and evaluation

Driven by the need to develop technologies for converting CO2, an extraordinary array of chemical looping based process concepts has been proposed and researched over the past 15 years. This review aims at providing first a historical context of the molecule CO2, which sits at the center of these developments. Then, different types of chemical looping related to CO2 are addressed, with attention to process concepts, looping materials, and reactor configurations. Herein, focus lies on the direct conversion of carbon dioxide into carbon monoxide, a process deemed to have economic potential.

/pdf/110th-anniversary-carbon-dioxide-and-chemical-looping-5adqzheny8.pdf

110th Anniversary : carbon dioxide and chemical looping : current research trends

At present, much thermal energy exhausted from industrial processes has been unrecoverably lost. In this paper a new method, a chemical heat pump is proposed for recovering such low-level energy by use of chemical reaction cycles. Through this method the low-level energy can be converted to higher and more useful energy. Thermodynamical study of this method and the practical example, Triangle Process 1, 2 and 3 are described.

Chemical reaction cycles for the recovery of low-level thermal energy

A method for screening possible thermochemical decomposition processes for water using ΔG0 -T diagrams

A thin-layered catalyst supported on a thermo-conductive metal surface makes it possible to design a compact reactor that doubles in function as a heat exchanger. A Pt/Al2O3/Al catalyst was prepared by anodic oxidation (AO) of commercial aluminum in oxalic acid solution followed by hot water treatment in choloroplatinic acid solution. During and after AO, the produced alumina layer was dissolved into the oxalic acid electrolyte. The porosity of the layer increased with the treatment time (pore widening). The final thickness of the alumina layer was controlled by balancing the formation and dissolution rates. The maximum thickness and maximum porosity of the alumina layer were 350μm and 0.79, respectively. The platinum content increased as the thickness and porosity of the alumina layer increased, the maximum Pt content was 15.8g-Pt/m2. Platinum dispersion was about 0.5 for all catalysts. The catalytic activity for cyclohexane dehydrogenation increased with the platinum content. The maximum steady state reaction rate was 12mol/(h·m2) at 473K under 0.1atm of cyclohexane.

https://www.jstage.jst.go.jp/article/sfj1989/47/2/47_2_173/_pdf

Hideo Kameyama

Papers

Chemical reaction cycles for the recovery of low-level thermal energy

A method for screening possible thermochemical decomposition processes for water using ΔG0 -T diagrams

Preparation of the Anodized Alumina Layer as a Catalyst Material