• Energy Research

Thermodynamic cycles requiring high operating temperatures (≥750 °C up to 1200 °C) are currently being explored to improve the sun-to-electricity conversion efficiency of Concentrating Solar Power (CSP) plants. This is calling for the design of new efficient high-temperature (≥750 °C) Thermochemical Energy Storage (TCES) systems, which are fundamental for supplying power on demand during off-sun periods. Recently, Fe-doped CaMnO3 oxides have been proposed as TCES candidate materials, and the determination of their thermodynamics properties via thermogravimetric (TG) analysis allowed evaluation of their heat storage capacity at a very small scale (mg scale). A 10 % Fe-doped CaMnO3 composition (CaMn0.9Fe0.1O3-δ – CMF91) emerged as optimum candidate material for TCES application due to its large heat storage capacity complemented by enhanced thermal stability over multiple oxidation/reduction cycles. To advance in the thermal characterization of these materials at a multigram scale, here we carried out bench-scale reactor tests using CMF91 under conditions considered representative of future CSP plants. The redox-active material has been extruded in the form of porous pellets through a simple production method that required the use of carboxymethylcellulose as a removable binder and water. With the bench-scale reactor tests, the CMF91 pellets showed fully reversible reduction-oxidation in cycles between 500 and 1100 °C under relevant operating pO2 conditions without any deterioration of the pellet's structural integrity. Remarkably, the material exhibited the same δ(T, pO2) profile at this significantly larger scale (~40 g) than the one derived from thermodynamics. Nevertheless, slight differences in oxygen release/uptake profiles between cooling and heating branches can be tracked down to an excess heat generation in the perovskite bed not efficiently extracted by the carrier gas. These results demonstrate that CMF91 oxide is ideally suited for thermal energy storage applications with a large total (thermochemical and sensible) heat storage capacity (~ 916 kJ/kgABO3 or ~ 400 kWh/m3) and good scalability. © 2022 This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska- Curie grant agreement N◦ 74616. Support of the ACES2030-CM from “Comunidad de Madrid” and European Structural Funds to (P2018/EMT-4319), and the Spanish “Ministerio de Economía y Competitividad” through Research Challenges project ARROPAR-CEX (ENE2015-71254-C3-1-R) are also fully acknowledged. M. S´anchez is grateful to Spanish “Ministerio de Economía y Competitividad” by funding through internship FPI (BES-2016-077031). It is greatly acknowledged the Technical Research Support Unit of the Institute of Catalysis and Petroleum Chemistry (ICP-CSIC). The authors fully appreciate the advice provided by Prof. Pedro Avila and Dr. Raquel Portela from the SpeICat group of ICP-CSIC, about the procedure for pellets preparation. Supporting Information Peer reviewed

Abstract MCM-22 zeolite samples, having different Si/Al ratios, have been studied for the fast-pyrolysis of acid-washed wheat straw at two catalytic pyrolysis temperatures aimed to the production of partially upgraded bio-oil. The best combination of bio-oil deoxygenation activity and energy yield is obtained when the catalytic bed was operated at 450 °C using the MCM-22 sample with the lowest Al content (Si/Al = 40). Interestingly, the increase in the reaction temperature results in a lower amount of coke deposited over the zeolite. On the other hand, reducing the zeolite Si/Al ratio had a negative effect as a higher concentration of acid sites promotes non-desired reactions: severe cracking of the bio-oil vapours, leading to the enhanced production of gaseous hydrocarbons, and coke formation. Coke produced over MCM-22 zeolite exhibits high oxygen content, whereas the bio-oil fraction presents a high concentration of oxygenated aromatics. These results denote the limited aromatization activity of MCM-22 zeolite for producing aromatic hydrocarbons, in particular when compared with ZSM-5, being of interest for the selective production of phenolic compounds by biomass catalytic pyrolysis.

descripción no proporcionada por scopus AC thanks the support of a fellowship from “la Caixa” Foundation (ID 100010434). The fellowship code is LCF/BQ/PI20/11760015. JC acknowledge financial support from ACES 2030 (P2018/EMT-4319) from “Comunidad de Madrid” and European Structural Funds.

Among renewable energies, wind and solar are inherently intermittent and therefore both require efficient energy storage systems to facilitate a round-the-clock electricity production at a global scale. In this context, concentrated solar power (CSP) stands out among other sustainable technologies because it offers the interesting possibility of storing energy collected from the sun as heat by sensible, latent, or thermochemical means. Accordingly, continuous electricity generation in the power block is possible even during off-sun periods, providing CSP plants with a remarkable dispatchability. Sensible heat storage has been already incorporated to commercial CSP plants. However, because of its potentially higher energy storage density, thermochemical heat storage (TCS) systems emerge as an attractive alternative for the design of next-generation power plants, which are expected to operate at higher temperatures. Through these systems, thermal energy is used to drive endothermic chemical reactions, which can subsequently release the stored energy when needed through a reversible exothermic step. This review analyzes the status of this prominent energy storage technology, its major challenges, and future perspectives, covering in detail the numerous strategies proposed for the improvement of materials and thermochemical reactors. Thermodynamic calculations allow selecting high energy density systems, but experimental findings indicate that sufficiently rapid kinetics and long-term stability trough continuous cycles of chemical transformation are also necessary for practical implementation. In addition, selecting easy-to-handle materials with reduced cost and limited toxicity is crucial for large-scale deployment of this technology. In this work, the possible utilization of materials as diverse as metal hydrides, hydroxides, or carbonates for thermochemical storage is discussed. Furthermore, special attention is paid to the development of redox metal oxides, such as Co3O4/CoO, Mn2O3/Mn3O4, and perovskites of different compositions, as an auspicious new class of TCS materials due to the advantage of working with atmospheric air as reactant, avoiding the need of gas storage tanks. Current knowledge about the structural, morphological, and chemical modifications of these solids, either caused during redox transformations or induced wittingly as a way to improve their properties, is revised in detail. In addition, the design of new reactor concepts proposed for the most efficient use of TCS in concentrated solar facilities is also critically considered. Finally, strategies for the harmonic integration of these units in functioning solar power plants as well as the economic aspects are also briefly assessed.

Abstract The Na–Mn thermochemical cycle is a three step process that has recently attracted renewed attention due to its potential for efficient hydrogen production. In this study, the two low temperature stages have been investigated in order to establish the factors determining the efficiency of both hydrogen production and recyclability of the different solid phases involved. The obtained result reveal that the influence of MnO particle size distribution is crucial for the solid–liquid reaction with NaOH and, therefore, for hydrogen production. Lower particle size and relatively high crystallinity causes a two-fold increment of the conversion, with respect to commercial MnO with very large particles. On the other hand, the influence of reaction conditions on the hydrolysis step has been analyzed in this study. Na extraction from the sodium manganese oxide is favored by performing the process at temperatures around 100 °C, in excess of water; during relatively longer periods and in inert gas. Nevertheless, it has been observed that the structure of the mixed oxide formed during the hydrogen production stage is the most relevant factor determining the efficiency of the Na–Mn oxide hydrolysis. This work reveals that α -NaMnO 2 presents the best ion exchange properties for the hydrolysis reaction, leading to more than 80% of Na recovery.

This study describes MFC operation with a pulse-width modulated connection of the external resistor (R-PWM mode) at low and high frequencies. Analysis of the output voltage profiles acquired during R-PWM tests showed the presence of slow and fast dynamic components, which can be described by a simple equivalent circuit model suitable for process control applications. At operating frequencies above 100 Hz a noticeable improvement in MFC performance was observed with the power output increase of 22-43% as compared to MFC operation with a constant external resistance.

Abstract Lamellar and pillared ZSM-5 materials modified with Mg and Zn oxides were synthesized and tested for in-situ catalytic upgrading of eucalyptus woodchips fast-pyrolysis vapors. The introduction of silica pillars into the lamellar ZSM-5 support led to a higher BET area, but also reduced the overall catalyst acidity. The incorporation of MgO and ZnO occurred with a high dispersion over the zeolitic supports, causing also a significant reduction in the value of their textural properties due to a partial blockage of the zeolite pores. Likewise, the acid features of the zeolitic supports underwent sharp changes by the addition of both MgO and ZnO with a strong decrease in the concentration of the Bronsted and Lewis acid sites present in the parent zeolite, as detected by pyridine adsorption followed FTIR spectroscopy. However, additional Lewis acid sites were created associated to the metal oxides deposited onto the zeolitic supports. Pyrolysis tests were accomplished using a lab-scale downdraft fixed-bed reactor working at atmospheric pressure and a temperature of 500 °C. The use of zeolitic catalysts increased the gas yield, mostly due to the formation of CO and CO 2 , to the detriment of bio-oil production. However, the so obtained bio-oils presented higher quality in terms of H/C and O/C ratios, and larger heating values. The incorporation of MgO and ZnO allowed tailoring the zeolite activity to avoid an excessive cracking of the bio-oil, which in turn resulted in a higher yield of the organic compounds present in the bio-oil, and decreasing the formation of undesired polyaromatic hydrocarbons and coke.

Abstract The effect of both indigenous (mineral components) and external (HZSM-5 zeolite) catalysts on bio-oil production by biomass fast-pyrolysis has being isolated and compared for two herbaceous and two woody biomass samples. Thereby, a variety of lignocellulosic biomasses (in both raw and de-ashed forms) have been subjected to fast-pyrolysis tests. Mineral components present in the raw biomasses were removed by an acid-washing treatment. The results obtained showed that both types of catalysts decreased the bio-oil* yield (water-free basis). However, whereas the indigenous catalysts almost did not affect the bio-oil* oxygen content, this parameter was significantly reduced when using the HZSM-5 zeolite. This finding denotes that mineral components are not really effective for bio-oil deoxygenation since they mainly promote the formation of additional char, which retains about 40% of the chemical energy contained in the raw biomass. In contrast, the external catalyst does favour oxygen removal from the bio-oil. Likewise, the deoxygenation route was strongly dependent on the type of catalyst. In the non-catalytic process dehydration was predominant, the indigenous catalysts favoured decarboxylation, whereas for the external HZSM-5 catalyst decarbonylation became the major deoxygenation pathway. Regarding the bio-oil* composition, both indigenous and external catalysts promoted the conversion of sugars and the formation of carboxylic acids, aldehydes and oxygenated aromatics. However, aromatic hydrocarbons were only produced over the external HZSM-5 catalyst, with a high proportion of alkyl-substituted benzenes and naphthalenes.