Stéphane Abanades

Solar thermochemical conversion of CO2 into fuel via two-step redox cycling of non-stoichiometric Mn-containing perovskite oxides

Solar-driven two-step thermochemical splitting of carbon dioxide represents one of the most promising candidate processes for carbon-neutral fuel production and solar energy storage. This redox process yields CO, one of the main precursors for liquid hydrocarbon fuels, and inherently results in the recycling and valorization of the CO2 greenhouse gas. In this context, non-stoichiometric perovskites such as La1−xSrxMnO3−δ have emerged as attractive candidate redox materials because of their large oxygen storage capacities and exchange capabilities that are required for applications in high temperature thermochemical redox cycles for splitting CO2. This study addresses the investigation of reactive Mn-containing substituted perovskites derived from this La1−xSrxMnO3−δ series, where Ba2+, Ca2+ or Y3+ have been incorporated on the A-site. Conversely, introduction of Al3+ and Mg2+ on the B-site was also examined. The materials were synthesized and their structures were validated by powder X-ray diffraction. Their thermochemical redox performances were subsequently quantified by thermogravimetric analysis, while the microstructure of the polycrystalline materials was examined by scanning electron microscopy. High redox activity for CO2 splitting was observed for the majority of the substituted perovskites examined here, in which Mn was the single reducible cation and Mn4+/Mn3+ redox pair was activated in the oxygen-exchange process. The structural evolutions and thermochemical behavior are discussed with respect to the distinct chemistries of the additional cations incorporated in these Mn-based perovskites.

High redox activity of Sr-substituted lanthanum manganite perovskites for two-step thermochemical dissociation of CO2

The La1−xSrxMnO3−δ series of non-stoichiometric perovskites (x = 0.35, 0.50, 0.65, 0.80) was examined in the context of solar-driven two-step thermochemical dissociation of CO2. Powder X-ray diffraction and thermochemical performance characterization were performed in order to assess the redox activity of these materials toward thermal reduction under inert atmosphere followed by re-oxidation for CO generation from CO2. To a certain extent, controlled introduction of Sr2+ into LaMnO3 allowed tuning the redox thermodynamics within the series, thus resulting in high activity toward both thermal reduction and CO2 dissociation. La0.50Sr0.50MnO3−δ composition appeared to be the most suitable trade-off for thermochemical CO2 splitting. Maximum CO production of about 270 μmol g−1 was reached during the CO2 splitting step with an optimal re-oxidation temperature of 1050 °C (after thermal reduction under Ar at 1400 °C), although the re-oxidation yield was limited to around 50%. Decreasing the amount of substituted Sr enhanced the re-oxidation yield at the expense of a lower final reduction extent, thus lowering the global amount of produced CO. The evolution of the Mn oxidation state implied partial re-oxidation of Mn3+ into Mn4+, thereby confirming the activation of Mn4+/Mn3+ redox pair in the perovskites. An elevated electronic transfer occurred within the Mn4+/Mn3+ redox pair (superior to that involved in the case of ceria within the Ce4+/Ce3+ redox pair), showing that mixed valence perovskites have clear potential for displaying redox properties suitable for efficient solar-driven thermochemical CO2 dissociation.

Modelling of heavy metal vaporisation from a mineral matrix

This study deals with the fundamental aspects of the volatilisation of heavy metals (HM) during municipal solid waste (MSW) incineration. The thermal treatment of a model waste was theoretically and experimentally studied in a fluid-bed. A mathematical model was developed to predict the fate of metallic species according to the main phenomena controlling the process: heat and mass transfer (transport phenomena), chemical reactions involving HM, and mechanism of vapour metal species sorption inside the porous matrix. The model assumes local thermodynamic equilibrium between the vapour and the metal compound on the substrate in the pores of a particle. This approach permits to predict the extent of HM vaporisation from a mineral porous matrix when its physical properties are known. Experimental data concerning CdCl(2) release from an alumina matrix in a 850 degrees C fluidised bed are in good agreement with theoretical results.

Design of a Lab-Scale Rotary Cavity-Type Solar Reactor for Continuous Thermal Dissociation of Volatile Oxides Under Reduced Pressure

A high-temperature lab-scale solar reactor prototype was designed, constructed and operated, allowing continuous ZnO thermal dissociation under controlled atmosphere at reduced pressure. It is based on a cavity-type rotating receiver absorbing solar radiation and composed of standard refractory materials. The reactant oxide powder is injected continuously inside the cavity and the produced particles (Zn) are recovered in a downstream ceramic filter. Dilution/quenching of the product gases with a neutral gas yields Zn nanoparticles by condensation. The solar thermal dissociation of ZnO was experimentally achieved, the reaction yields were quantified, and a first concept of solar reactor was qualified. The maximum yield of particles recovery in the filter was 21% and the dissociation yield was up to 87% (Zn weight content in the final powder) for a 5 NL/min neutral gas flow-rate (typical dilution ratio of 300).

Natural gas pyrolysis in double-walled reactor tubes using thermal plasma or concentrated solar radiation as external heating source

Abstract The thermal pyrolysis of natural gas as a clean hydrogen production route is examined. The concept of a double-walled reactor tube is proposed and implemented. Preliminary experiments using an external plasma heating source are carried out to validate this concept. The results point out the efficient CH4 dissociation above 1850 K (CH4 conversion over 90%) and the key influence of the gas residence time. Simulations are performed to predict the conversion rate of CH4 at the reactor outlet, and are consistent with experimental tendencies. A solar reactor prototype featuring four independent double-walled tubes is then developed. The heat in high temperature process required for the endothermic reaction of natural gas pyrolysis is supplied by concentrated solar energy. The tubes are heated uniformly by radiation using the blackbody effect of a cavity-receiver absorbing the concentrated solar irradiation through a quartz window. The gas composition at the reactor outlet, the chemical conversion of CH4, and the yield to H2 are determined with respect to reaction temperature, inlet gas flow-rates, and feed gas composition. The longer the gas residence time, the higher the CH4 conversion and H2 yield, whereas the lower the amount of acetylene. A CH4 conversion of 99% and H2 yield of about 85% are measured at 1880 K with 30% CH4 in the feed gas (6 L/min injected and residence time of 18 ms). A temperature increase from 1870 K to 1970 K does not improve the H2 yield.

The kinetics of vaporization of a heavy metal from a fluidized waste by an inverse method

Abstract This study addresses the emission of heavy metals during the incineration of municipal solid waste. A global method was developed to determine the vaporization rate of the metal from the on-line analysis of exhaust gas. This method differs from direct models, which predict the time course of the metal concentration in the gas from the vaporization rate profile, but which are not practicable because this vaporization rate cannot be measured in real incinerators burning real wastes. The method is based on the determination of the global rate of release of heavy metal from the combustion of model wastes in a fluidized bed. It is an inverse method, which involves only the measured concentration of heavy metal in the exhaust gases and a model developed at the reactor scale. The thermal treatment of model wastes spiked with a metal was performed in a laboratory- scale fluidized bed. In fact, a solid matrix derived from real waste was dosed with Cd, Pb, or Zn and burned to simulate the metal’s release during the incineration of municipal solid waste. An on-line analysis system was linked to the gas outlet of the reactor, and the metal’s vaporization was tracked successfully by continuously measuring by inductively coupled plasma optical emission spectroscopy (ICP-OES) the relative concentration of the metal in exhaust gases. On the theoretical front, a bubbling bed model was developed and validated to calculate the metal’s vaporization rate from its concentration-time profile in the outlet gas. The inverse method consists in identifying these vaporization rates at the particle level from only the on-line diagnostic results and using the model, whatever the waste considered. The data obtained may be used in any process, in which wastes are heated rapidly (several hundreds of degrees per second), as in fluidized beds.

Experimental Evaluation of Indirect Heating Tubular Reactors for Solar Methane Pyrolysis

Solar thermal pyrolysis of natural gas is studied for the co-production of hydrogen, a promising energy carrier, and Carbon Black, a high-value nano-material, with the bonus of zero CO2 emissions. A 10 kW multi-tubular solar reactor (SR10) based on the indirect heating concept was designed, constructed and tested. It is composed of an insulated cubic cavity receiver (20 cm side) that absorbs concentrated solar irradiation through a quartz window by a 9 cm-diameter aperture. The solar concentrating system is the 1 MW solar furnace of CNRS-PROMES laboratory. An argon-methane mixture flows inside four graphite tubular reaction zones each composed of two concentric tubes that are settled vertically inside the cavity. Experimental runs mainly showed the key influence of the residence time and temperature on the reaction extent. Since SR10 design presented a weak recovery of carbon black in the filter, a single tube configuration was tested with an external plasma heating source. Complete methane conversion and hydrogen yield beyond 80% were achieved at 2073K. Hydrogen and carbon mass balances showed that C2H2 intermediates affect drastically the carbon black production yield: about half of the initial carbon content in the CH4 was found as C2H2 in the outlet gas. Nevertheless, the carbon black recovery in the filtering device was improved with this new configuration. Data are extrapolated to predict the possible hydrogen and carbon production for a future 50 kW solar reactor. The expected production was estimated to be about 2.47 Nm3/h H2 and 386 g/h carbon black for 1.47 Nm3/h of CH4 injected.

Simulation of heavy metal vaporization dynamics in a fluidized bed

Abstract The aim of this study is to validate a two-dimensional Eulerian–Lagrangian simulation of heavy metal vaporization in a fluidized bed. Simulation was performed by combining a heavy metal vaporization model and a Lagrangian model developed for bubbling fluidized bed, they are coupled by including the heavy metal transport equation in the gas flow. Experimental data were obtained by fluidizing a mixture of sand and CdCl2-spiked alumina at 1123 K . A fair quantitative agreement was obtained between the predicted and the measured vaporization rate. Moreover, the simulation results address the heavy metal vaporization dynamics at the particle level, and the effect of the particle flow structure on both heavy metal vaporization and vapor distribution in the fluidized bed.

Radiative heat transfer analysis of a directly irradiated cavity-type solar thermochemical reactor by Monte-Carlo ray tracing

Radiative heat transfer in a 1 kW cavity-type solar reactor devoted to the thermal reduction of compressed ZnO and SnO2 powders is analyzed by a Monte Carlo ray tracing simulation. The developed model takes into account the radiative properties of the reactant particles and of the ceramic cavity walls, as well as the angular intensity distribution of the incoming concentrated solar irradiation. The model also includes the conduction heat losses through the lateral walls and the energy consumed by the endothermic chemical reaction. It is used to predict the temperature and the absorbed flux density profiles on the inner cavity walls for different main features of the reactor, concerning the dimensions of the cavity and the type of reactant. Results show that the absorbed flux density profile and the theoretical thermochemical efficiency change with the cavity aspect ratio and with the oxide reactant. The cavity with an aspect ratio of 3 and a SnO2 pellet undergoing dissociation presents the highest thermoc...

HIGH-TEMPERATURE SOLAR CHEMICAL REACTORS FOR HYDROGEN PRODUCTION FROM NATURAL GAS CRACKING

This study deals with the thermal cracking of natural gas for the coproduction of hydrogen and carbon black from concentrated solar energy without CO2 emission. A laboratory-scale solar reactor (1 kW) was tested and modeled successfully. It consists of a tubular graphite receiver directly absorbing solar radiation, in which a mixture of Ar and CH4 flows. A temperature increase or a gas flow rate decrease results in chemical conversion increase. Methane conversion higher than 75% was obtained. Reaction occurred near the wall where temperature is maximal and gas velocity is minimal due to the laminar flow profile. The work focused also on the design of a medium-scale tubular solar reactor (10 kW) based on the indirect heating concept. A reactor model including gas hydrodynamics and heat and mass transfers coupled to the chemical reaction was developed in order to predict the reactor performances. Temperature and species concentration profiles and final chemical conversion were quantified. According to the results, temperature was uniform in the tubular reaction zone and the predicted chemical conversion was 65%, neglecting the catalytic effect of carbon particles.