A new thermochemical cycle for H
2 production based on CeO
2/Ce
2O
3 oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO
2
→
Ce
2O
3
+
0.5O
2; (2) ...hydrolysis, Ce
2O
3
+
H
2O
→
2CeO
2
+
H
2. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (
T
=
2000
°C,
P
=
100–200
mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600
°C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO
2), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production.
•Differences between solar and classical pyrolysis are emphasized.•It is shown how operation conditions govern the product distributions.•Main experimental data obtained during last 40years are ...reported.•Modeling approaches are described and discussed.•Values of upgrading factor are presented.
Solar pyrolysis of a carbonaceous feedstock (coal, biomass and wastes) is a process in which carbon-containing feedstocks are used as chemical reactants and solar energy is supplied as high-temperature process heat. This process has the potential to produce higher calorific value products with lower CO2 emissions than conventional pyrolysis processes. As a consequence, the intermittent solar energy is chemically stored in the form of solar fuels. Solar pyrolysis was first demonstrated in an indoor environment using a solar simulator (image furnace) for exploring the fundamental mechanisms of carbonaceous feedstock pyrolysis under severe radiative conditions (high temperatures and heating rates) in comparison to conventional pyrolysis. More recently, low-temperature solar pyrolysis has been demonstrated to be a good technology for bio-oil production. Our high-temperature solar pyrolysis process produces more combustible gas products than other processes. This paper reviews developments in the field of solar pyrolysis processing by considering fundamental mechanisms, experimental demonstrations, models and challenges.
Biomass represents a renewable source for transport fuels when processed by gasification, followed by catalytic conversion of the syngas to liquids. The efficiency of biomass gasification can be ...improved by supplying process heat from concentrated solar systems, which can attain the required temperature of 900 °C. Various chemical routes and contacting configurations are reviewed. The challenges related to biomass-based processes are discussed. Heat and material balances are then deduced. The area of land required for growing biomass can be reduced using the application of thermal solar to one half of that needed for a standard gasification system. If hydrogen is generated by solar means in order to reduce carbon dioxide emissions to zero, the figure becomes one third. Examples of the land requirements for three different biomass materials are presented.
► Review and new figures on production of synthetic fuels from biomass. ► Synthetic fuels from biomass using gasification. ► Benefits of using concentrated solar energy for gasification of biomass.
► A numerical model for photovoltaic–thermoelectric hybrid systems was developed. ► The performance of the GaAs–CoSb3 hybrid system is predicted by this model. ► Relations between critical parameters ...of the PV–TE hybrid system were studied. ► System optimization is carried out by analyzing the calculation results. ► Guidelines for PV–TE hybrid system design were provided.
This paper presents the numerical modeling and optimization of a spectrum splitting photovoltaic–thermoelectric (PV–TE) hybrid system. In this work, a simulation model is established in consideration of solar concentration levels and several heat dissipation rates. Exemplarily, the performance of a hybrid system composed of a GaAs solar cell and a skutterudites CoSb3 solar thermoelectric generator (TEG) is simulated. Analysis under different conditions has been carried out to evaluate the electrical and thermal performance of the hybrid system. Results show that the cutoff-wavelength of the GaAs–CoSb3 hybrid system is mainly determined by the band gap of solar cell, when the solar concentration ratio is ranged between 550 to 770 and heat transfer coefficient h=3000–4500W/m2K, the hybrid system has good electrical performance and low operating temperatures. Based on the analysis of the GaAs–CoSb3 hybrid system, guidelines for the PV–TE system design are proposed. It is also compared with a PV-only system working under the same cooling condition; results show that the PV–TE hybrid system is more suitable for working under high concentrations.
The solar thermochemical splitting of CO2 and H2O with ceria and Zr-doped ceria for CO and H2 production is considered. The two-step process is composed of the thermal reduction of the ceria-based ...compound followed by the oxidation of the nonstoichiometric ceria with CO2/H2O to generate CO/H2, respectively. As a reference, the reactivity of pure undoped ceria was first characterized during successive thermochemical cycles using a thermobalance. Then, Zr0.25Ce0.75O2 was synthesized using different soft chemical synthesis routes to evaluate the influence of the powder morphology on the reactivity during the reduction and the oxidation steps. The reduction yield of ceria was significantly improved by doping with Zr as well as the CO/H2 production yields, but the kinetic rates of the oxidation step for doped ceria were lower than for pure ceria. CO and H2 production of 241 and 432 μmol/g, respectively, have been measured. A kinetic analysis of the CO2-splitting step allowed one to estimate the activation energy that ranged between 83 and 103 kJ/mol depending on the synthesis route of Zr0.25Ce0.75O2. The powder morphology played an important role on the materials cyclability. In contrast to pure ceria, Zr-doped ceria showed possible deactivation when cycling at 1400 °C, and the influence of the synthesis route on the thermal stability was evidenced. The thermally resistant powders with porous morphology ensured stable reactivity during cycling. The Zr-doped ceria synthesized via pechini process produced the largest amounts of CO/H2 during successive cycles.
•A cross-flow fluidized-bed solar reactor for continuous calcination processes was demonstrated.•A nominal thermal power of 25 kW enabled to reach a particle temperature of 800 °C.•The half ...decomposition of dolomite (CaMg(CO3)2 → CaCO3 + MgO + CO2) was performed.•An MgCO3 conversion degree of 100% was obtained with a 9.4 kg/h feed flow rate of dolomite.•A model assuming a cascade of continuous stirred tank reactors was successfully compared with experimental data.
A laboratory-scale solar reactor prototype dedicated to calcination processes of non-metallic mineral particles is tested and characterized. The prototype consists of an indirect heating shallow cross-flow fluidized-bed reactor-receiver. It is composed of 4 compartments in series in which the particles are thermally treated with solar power in order to drive the endothermic calcination reaction. The particles are fluidized in the reactor with preheated air and are heated up to 800 °C through the front wall of the reactor receiving the concentrated solar flux (about 200 kW/m2). The tests are carried out at the 1-MW Odeillo’s solar furnace (France). The thermal decomposition of a continuous stream of 9.4 kg/h of dolomite (CaMg(CO3)2) is investigated in this paper. The half decomposition of dolomite (CaMg(CO3)2 → CaCO3 + MgO + CO2) is performed with a degree of conversion of 100%. The complete decomposition of dolomite (CaMg(CO3)2 → CaO + MgO + 2CO2) is not reached because, with respect to the CO2 partial pressure in the reactor, the temperature of particles is not high enough to decompose the calcium carbonate. The calculated thermochemical efficiency (i.e. the energy absorbed by the endothermic calcination reaction compared to the solar energy provided to the system) is 6.6%. This low efficiency is neither surprising nor critical since the reactor design was not optimised with respect to energy efficiency but designed to the control of particle flow and front wall solar flux distribution. A numerical model considering the 4 compartments of the reactor as 4 ideal continuous stirred tank reactors in series is developed. The model accounts for the mass and the energy balances, as well as the reaction kinetics of the half decomposition of dolomite. The model gives consistent results compared to the experimental data. These results are a proof of concept of continuous calcination reaction using concentrated solar energy in a cross-flow fluidized-bed reactor.
This study focuses on the use of cerium-based mixed oxides for hydrogen production by solar-driven thermochemical two-step water-splitting. Mixed cerium oxides are proposed in order to decrease the ...reduction temperature of ceria and to avoid material sublimation occurring above 2,000 °C during the high-temperature solar step. Ceria-based nanopowders were synthesized by soft chemistry methods including the modified Pechini method. The influence of the synthesis method, the type of cationic element mixed with cerium, and the content of this added element was investigated by comparing the reduction temperatures of the derived materials. The synthesized powders were characterized by X-ray diffraction, thermogravimetric analysis, SEM, and Raman spectroscopy. Results showed that the synthesized pure cerium oxide is more reactive toward reduction than a commercial powder. Among the different elements added to ceria that were screened, the addition of zirconium significantly improved the reduction of ceria at temperatures below 1,500 °C. Increasing zirconium content further favored cerium reduction yield up to 70%. Water-splitting tests were performed to demonstrate the reactivity of the developed materials for H
2
production. The amount of H
2
evolved was enhanced with a temperature increase, the maximum H
2
production from Ce
0.75
Zr
0.25
O
2−δ
was 0.24 mmol/g at 1,045 °C, and the powder reactivity upon cycling was demonstrated via thermogravimetry through two successive reduction–hydrolysis reactions.
This paper presents a process analysis of ZnO/Zn, Fe
3O
4/FeO and Fe
2O
3/Fe
3O
4 thermochemical cycles as potential high efficiency, large scale and environmentally attractive routes to produce ...hydrogen by concentrated solar energy. Mass and energy balances allowed estimation of the efficiency of solar thermal energy to hydrogen conversion for current process data, accounting for chemical conversion limitations. Then, the process was optimized by taking into account possible improvements in chemical conversion and heat recoveries. Coupling of the thermochemical process with a solar tower plant providing concentrated solar energy was considered to scale up the system. An economic assessment gave a hydrogen production cost of 7.98$
kg
−1 and 14.75$
kg
−1 of H
2 for, respectively a 55
MW
th and 11
MW
th solar tower plant operating 40 years.
The upwards flow of particles in an Upflow Bubbling Fluidized Bed (UBFB) is studied experimentally and modelled from pressure drop considerations and energy loss equations. For Geldart group A ...powders tested, the upward solid flux, G
, in the tube can be expressed in terms of the applied superficial gas velocity, the free fall (terminal) velocity of the particles during their hindered settling, KU
, the pressure exerted at the base of the conveyor tube, and the tube length. The model expression Formula: see text can be used for design purposes, with K, the correction factor for hindered settling of the particles, approximately equal to 0.1 at high G
-values, but a function of the solids fraction in the upward conveying. The energy efficiency of the system increases with increasing U and G
. The model equation was tentatively applied to predict the effects of particle size, tube length and operation in Circulating Fluidized Bed mode. It is demonstrated that the UBFB is an efficient and flexible way of transporting particles upwards, with limited particle attrition or tube erosion due to the low gas velocity applied.
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