Thermodynamic Analyses of Fuel Production Via Solar-Driven Ceria-Based Nonstoichiometric Redox Cycling: A Case Study of the Isothermal Membrane Reactor System

A thermodynamic model of an isothermal ceria-based membrane reactor system is developed for fuel production via solar-driven simultaneous reduction and oxidation reactions. Inert sweep gas is applied on the reduction side of the membrane. The model is based on conservation of mass, species, and energy along with the Gibbs criterion. The maximum thermodynamic solar-to-fuel efficiencies are determined by simultaneous multivariable optimization of operational parameters. The effects of gas heat recovery and reactor flow configurations are investigated. The results show that maximum efficiencies of 1.3% (3.2%) and 0.73% (2.0%) are attainable for water splitting (carbon dioxide splitting) under counter- and parallel-flow configurations, respectively, at an operating temperature of 1900 K and 95% gas heat recovery effectiveness. In addition, insights on potential efficiency improvement for the membrane reactor system are further suggested. The efficiencies reported are found to be much lower than those reported in literature. We demonstrate that the thermodynamic models reported elsewhere can violate the Gibbs criterion and, as a result, lead to unrealistically high efficiencies. The present work offers enhanced understanding of the counter-flow membrane reactor and provides more accurate upper efficiency limits for membrane reactor systems. © 2019 by ASME.Australian Research Council (Wojciech Lipiński, Future Fellowship, Award No. FT140101213, Funder ID. 10.13039/501100000923). China Scholarship Council (Sha Li, Grant No. [2015] 3022, 201506020092, Funder ID. 10.13039/501100004543)

Thermochemical generation of hydrogen and carbon dioxide

Mixing of carbon in the form of high sulfur coal with sulfuric acid reduces the temperature of sulfuric acid decomposition from 830.degree. C. to between 300.degree. C. and 400.degree. C. The low temperature sulfuric acid decomposition is particularly useful in thermal chemical cycles for splitting water to produce hydrogen. Carbon dioxide is produced as a commercially desirable byproduct. Lowering of the temperature for the sulfuric acid decomposition or oxygen release step simplifies equipment requirements, lowers thermal energy input and reduces corrosion problems presented by sulfuric acid at conventional cracking temperatures. Use of high sulfur coal as the source of carbon for the sulfuric acid decomposition provides an environmentally safe and energy efficient utilization of this normally polluting fuel

An Exploration of Perovskite Materials for Thermochemical Water Splitting

Two-step thermochemical water splitting is a promising technology for the hydrogen production of solar energy. This process possesses the advantages of utilizing the full solar spectrum, producing flexible fuels, and requiring no precious metal catalysts. It furthermore temporally separates the oxygen release and hydrogen production steps, eliminating the possibility of O2 and H2 recombination. Ceria, which undergoes non-stoichiometric changes in oxygen content, has been demonstrated as an effective material for solar-driven thermochemical fuel production, but the process requires extremely high temperatures (~ 1600 degrees C), leading to efficiency penalties and challenges in reactor design and construction. Accordingly, the objective of this work is the development of new thermochemical reaction substrate materials which enable operation at lower temperatures and ideally increase fuel productivity and efficiency. Here we explore perovskite systems, specifically La1-xSrxMnO3-δ, La0.8Sr0.2Mn1-yFeyO3-δ, and La0.8Sr0.2Mn1-yAlyO3-δ. The link between the solid-state chemistry, redox properties, hydrogen production, and reaction kinetic limitations will be discussed. This study aims to learn how to design and tailor the good catalytic oxides for solar-driven thermochemical water splitting application

Development of perovskite-like structures for hydrogen production via two-step thermochemical water splitting

Hydrogen powered technologies are proposed to help mitigate climate change as low carbonemitting technologies. Devices such as fuel cells convert the chemical energy stored within hydrogen molecules via electrochemical redox processes to electrical energy for work. These technologies have the primary benefit of not emitting carbon dioxide – one of the main contributing pollutants towards the greenhouse effect. However, current commercial hydrogen production technologies require fossil fuel reactants and emit carbon dioxide as a product. Therefore, research into ways of producing hydrogen from sustainable non-polluting sources has been of keen interest within the scientific community. One such technique is high temperature thermochemical water splitting. This process uses renewable concentrated solar power to heat up and thermally reduce metal oxide compounds and induce an oxygen nonstoichiometry within the lattice. The oxygen deficiency is then removed upon reoxidising with steam and producing hydrogen gas. Numerous thermochemical redox cycles have been proposed within the literature with the main aim to lower the reduction temperatures and increase the hydrogen production volumes. This has turned the attention of the field to investigate the ABO3 perovskite structures due to their ability to support a larger oxygen deficiency at lower temperatures compared to the benchmark material, cerium oxide, CeO2. This thesis combines theoretical first principle approaches and a wide range of experimental techniques to understand and discuss three different families of perovskite and perovskite-like metal oxide structures. The main findings of this thesis can be summarised as the following: Effect of antimony incorporation on the redox kinetics of SrCoO3-d - Thermal analysis techniques observe large oxygen production volumes onset between 300 and 400 °C under an inert gas flow with increased antimony content lowering total production. - Density Functional Theory (DFT) confirms the low reduction enthalpy in the region of 0.5 eV/O atom. Increased Sb concentration and proximity to the dopant increases vacancy formation energy. 6 - Low reduction enthalpy of the material was not favourable to drive thermochemical water splitting, however isothermal redox cycling demonstrated good performance for the alternative application of thermochemical oxygen separation compared to literature materials. - Antimony donor ions are postulated to lower the cobalt crystal field splitting to support an intermediate spin electron configuration with more favourable orbital filling for fast redox kinetics (eg=1). Effect of iron incorporation in (La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites for thermochemical water splitting - Thermal analysis used to observed increasing Fe content coincides with an increase the oxygen production volumes and rates - DFT used to confirm lower vacancy formation energy in positions neighbouring Fe cations. Further predicted to have favourable thermodynamic properties for thermochemical water splitting. - Thermochemical water splitting observed hydrogen production rates similar to literature materials, Ce0.75Zr0.25O2-d. - Surface analysis techniques novel to this research field revealed increased strontium segregation towards the surface that prevented cyclability of the compounds. - Strontium-enriched perovskite surfaces can undergo reconstruction to form derivative phases such as Ruddlesden-Popper oxides, An+1BnO3n+1. Computational screening of n=1 Ruddlesden-Popper oxides for thermochemical water splitting - Screening study uses a combination of well-known crystallographic principles and DFT simulations to narrow down the field of this underexplored metal oxide family for use in thermochemical water splitting. - From an initial 27,899 structures, this study outlines a potential 30 A2BO4 Ruddlesden- Popper structures that have favourable reduction thermodynamics and “synthesisable” under laboratory conditions. - A new simpler and better fitting descriptor based on the lattice enthalpy is proposed to assist future screening work of Ruddlesden-Popper oxides at significantly reduced computational expense. Investigating Ca2MnO4 Ruddlesden-Popper oxide for thermochemical water splitting - Outputted compound from the prior screening study is explored further due its abundant constituent elements and favourable reduction thermodynamics. - Thermal analysis techniques observe similar oxygen production behaviour to the (La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites investigated in a previous chapter. - Hydrogen was successfully produced via thermochemical redox reactions cycling between 1000 and 800 °C, thus experimentally verifying the screening study. - Further improvements are suggested by including doping ions to alter the thermodynamics or investigating the effect of perovskite/Ruddlesden-Popper heterostructures that have previously been observed to accelerate oxidation reactions.Open Acces

System efficiency analysis of dual interconnected bubbling fluidized bed reactors for solar fuel production

Chemical looping syngas production is a two-step syngas fuel production process that produces CO and H 2 . The process is composed of two fluidized bed reactors (oxidation reaction and reduction reactor), oxygen carriers (metal oxides) circulating between the two reactors. A comprehensive model is developed to simulate the chemical looping water and carbon dioxide splitting in a dual fluidized bed reactors interconnected with redox cycling between these two reactors through metal oxides (non-stoichiometric ceria). An extensive FORTRAN subroutine is developed and hooked into Aspen plus V8.8 to appropriately model the complexities of the bubbling fluidized bed reactor including the reaction kinetics. The model developed has been validated for its hydrodynamics and kinetics level and individual correlation was quantified for its validity. The reduction reactor is maintained between the temperatures 1300-1500°C. The heat to attain this high temperature can be achieved with solar beam down tower. The oxidation reactor is supplied with a mixture of CO 2 and H 2 O with different mixture composition combining 60% and remaining N 2 . The oxidation reactor temperature is varied between 700-1000°C to identify the maximum efficiency achieved. It is found that the maximum efficiency achieved is 67.4% corresponding to highest temperature difference between the reactors.Preprin

Survey of hydrogen production and utilization methods. Volume 1: Executive summary

The use of hydrogen as a synthetic fuel is considered. Processes for the production of hydrogen are described along with the present and future industrial uses of hydrogen as a fuel and as a chemical feedstock. Novel and unconventional hydrogen-production techniques are evaluated, with emphasis placed on thermochemical and electrolytic processes. Potential uses for hydrogen as a fuel in industrial and residential applications are identified and reviewed in the context of anticipated U.S. energy supplies and demands. A detailed plan for the period from 1975 to 1980 prepared for research on and development of hydrogen as an energy carrier is included

Carbon promoted water electrolysis to produce hydrogen at room temperature

The objective of the work was to conduct water electrolysis at room temperature with reduced energy costs for hydrogen production. The electrochemical gasification of carbons consumes only 9.6 kcal/mol H2O compared to 56.7 kcal/molH 2O for conventional water electrolysis. In this work, carbon-assisted hydrogen production and the reaction energetics/kinetics at applied potentials |E0| between 0.1 and 1.8 V are studied. The carbon promoted water electrolysis could be performed at applied potentials as low as |E0|=0.21 V as opposed to conventional water electrolysis which requires |E0|\u3e1.25 V. The study reveals that the H2 produced per W h is higher at the lower voltages, but longer times are required to produce the same amount of H2. The following parameters were considered for evaluating the process: time taken, potential applied, current required and amount of carbon to be added. Based on such an evaluation, practical parameters of |E0| ≃ 0.5 V and carbon concentration (0.08 g/cm3) are suggested

Recent Progress and Approaches on Carbon-Free Energy from Water Splitting

Sunlight is the most abundant renewable energy resource, providing the earth with enough power that is capable of taking care of all of humanity’s desires—a hundred times over. However, as it is at times diffuse and intermittent, it raises issues concerning how best to reap this energy and store it for times when the Sun is not shining. With increasing population in the world and modern economic development, there will be an additional increase in energy demand. Devices that use daylight to separate water into individual chemical elements may well be the answer to this issue, as water splitting produces an ideal fuel. If such devices that generate fuel were to become widely adopted, they must be low in cost, both for supplying and operation. Therefore, it is essential to research for cheap technologies for water ripping. This review summarizes the progress made toward such development, the open challenges existing, and the approaches undertaken to generate carbon-free energy through water splitting.[Figure not available: see fulltext.]. © 2019, © 2019, The Author(s).Different approaches for efficient carbon-free energy from water splitting are summarized.Step-wise evolution of water splitting research is highlighted with current progress.It describes the open challenges of charge transport properties and future research direction. © 2019, The Author(s)