CO2 and H2O reduction by solar thermochemical looping using SnO2/SnO redox reactions: Thermogravimetric analysis

https://doi.org/10.1016/j.ijhydene.2012.02.158Get rights and content

Abstract

The thermochemical dissociation of CO2 and H2O from reactive SnO nanopowders is studied via thermogravimetry analysis. SnO is first produced by solar thermal dissociation of SnO2 using concentrated solar radiation as the high-temperature energy source. The process targets the production of CO and H2 in separate reactions using SnO as the oxygen carrier and the syngas can be further processed to various synthetic liquid fuels. The global process thus converts and upgrades H2O and captured CO2 feedstock into solar chemical fuels from high-temperature solar heat only, since the intermediate oxide is not consumed but recycled in the overall process. The objective of the study was the kinetic characterization of the H2O and CO2 reduction reactions using reactive SnO nanopowders synthesized in a high-temperature solar chemical reactor. SnO conversion up to 88% was measured during H2O reduction at 973 K and an activation energy of 51 ± 7 kJ/mol was identified in the temperature range of 798-923 K. Regarding CO2 reduction, a higher temperature was required to reach similar SnO conversion (88% at 1073 K) and the activation energy was found to be 88 ± 7 kJ/mol in the range of 973-1173 K with a CO2 reaction order of 0.96. The SnO conversion and the reaction rate were improved when increasing the temperature or the reacting gas mole fraction. Using active SnO nanopowders thus allowed for efficient and rapid fuel production kinetics from H2O and CO2.

Highlights

► A thermochemical-looping process for H2 and CO production from H2O and CO2 splitting. ► The solar process recycles and upgrades H2O and captured CO2 to high-value solar fuels. ► SnO reactive nanopowder was synthesized in a high-temperature solar chemical reactor. ► The nano-sized SnO powder allowed for efficient and rapid fuel production kinetics. ► SnO conversion up to 88% was measured by thermal analysis of H2O and CO2 reduction.

Introduction

The increasing concentration of CO2 in Earth's atmosphere is a matter of great concern with respect to energy sustainability. Human activities produce an annual excess of CO2 to the carbon cycle resulting in an excess of several Gt of carbon/year. Any attempts to reduce the anthropogenic CO2 emissions is thus of primary importance. In addition, the dwindling reserves of fossil fuels and the countries dependence on imported oil are immediate and urgent matters to address. One approach that copes with both problems is the CO2 valorization and conversion into carbon-neutral fuels [1], [2], [3], and one of the most attractive approaches being the splitting of the CO2 molecule into CO and O2 using solar thermal energy [4], [5]. The obtained CO can then be converted (with a suitable addition of H2) into synthetic liquid fuels, using available catalytic technologies. Consequently, hydrogen is also a key compound that can be produced separately from solar H2O splitting via thermochemical cycles [6], [7], [8], [9] with potential large-scale applications at competitive costs using concentrating solar technologies [10], [11]. Combined with water, CO can also be further processed to H2 by water-gas shift reaction for additional H2 supply. The synthesis of liquid fuels such as methanol or gasoline that contain far more energy by volume than hydrogen is a suitable solution to transport hydrogen and to store/distribute it through the already compatible existing infrastructures.

The common approach widely developed to reduce CO2 emissions includes the collection of CO2 as close as possible to the source, transfer by pipeline to adequate locations and underground injection for storage [12]. The proposed solar CO2-valorizing process is thus a sustainable alternative to CO2 sequestration since it considers CO2 as a raw material that can be recycled in a synthetic fuel, rather than a waste with a cost of disposal [1]. The expected positive impacts are (1) the recycling of captured CO2 from various industries (e.g., fossil fuel burning power plants, waste burning, cement factories, refineries, metal industries), thereby avoiding CO2 emissions, and (2) the long-term conversion and storage of solar energy into high-value chemical fuel.

Direct solar CO2 and H2O splitting may be achieved at high temperatures (above 3000 K) [5], [6], which is not viable on a practical viewpoint mainly because of low energy conversion efficiencies due to prevailing re-radiation losses and hardly thermally-resistant materials at such high temperatures. The proposed process for producing solar fuels focuses on an alternative pathway that occurs at lower temperatures to split CO2 and H2O separately. This pathway relies on solar thermochemical looping including: oxidation of materials by CO2 or H2O and regeneration of these materials for releasing O2 using concentrated solar energy (Fig. 1). The metal oxide, although reacting in each individual reaction, is not consumed in the overall thermochemical looping process due to its recycling and thus, it can be considered as a catalyst for the CO2- and H2O-splitting reactions. The efficient chemical storage of intermittent sunlight can be achieved by using concentrated solar energy as the high-temperature heat source to drive the endothermic reaction. The CO2 and H2O splitting via thermochemical looping is advantageous because it divides an unfavourable reaction (direct thermolysis) in two distinct steps that are thermodynamically more favourable.

The separate reactions of CO2 and H2O reduction and the combination of gas products in appropriate amounts allow tuning the syngas composition and the global net reaction is analogous to reverse combustion:xCO2+(1x)H2O+SolarEnergyxCO+(1x)H2+1/2O2

The produced syngas may be further processed to produce hydrocarbon liquid fuels using established catalytic techniques (Fischer-Tropsch).Liquidfuelproduction:nCO+(2n+1)H2CnH2n+2+nH2O

The reduction of CO2 to C/CO and H2O to H2 via ZnO/Zn, SnO2/SnO, Fe3O4/FeO and mixed metal oxides (ferrites or ceria) was experimentally studied [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Of these redox systems, those based on the successive thermal dissociation and reoxidation of volatile metal oxides (such as ZnO and SnO2) have been shown to exhibit the highest chemical reactivity because the involved reduced species are produced as nanoparticles during the solar step [24], which facilitates the subsequent reaction with CO2 and H2O. The beneficial effect of using nanoparticles synthesized from concentrated solar power was thus highlighted [13], [14], [19]. Indeed, their high specific surface area increases the reaction kinetics along with heat and mass transfers, while their large surface-to-volume ratio favours their nearly complete oxidation and limits surface passivating effects that were encountered for larger particles like commercial powders (with micronic particle size [17], [18]). For that reason, instead of using commercially available materials that may be not representative of the real solar powder morphology, the reduced metal oxide species (SnO) need to be first synthesized in a high temperature solar chemical reactor by solar thermal dissociation of the respective oxide (stannic oxide) and the reactivity of the solar nanopowders can then be investigated. This study focuses on the SnO2/SnO thermochemical system for H2O and CO2 reduction. This system was initially proposed for hydrogen production from two-step thermochemical water-splitting [13], [14]. The main advantage of this system over ZnO/Zn was the higher thermal dissociation yield during the solar step above 1900 K. Indeed, the reverse recombination reaction of SnO and Zn with O2 was investigated and the SnO reactivity with O2 was shown to be lower than Zn reactivity in the high-temperature dissociation zone [25]. This behaviour can be explained by a much higher boiling point of SnO (1800 K vs. 1180 K for Zn at 1 atm). SnO vapours thus condense rapidly when the gas temperature decreases because the gap between reaction temperature and SnO condensation temperature is narrow, which makes SnO quenching easier as compared with Zn. Thus, the dissociation rate of SnO2 is high and it is less dependent on the quenching rate than the dissociation rate of ZnO. Regarding the non-solar exothermal step, H2 production tests were performed up to 853 K in a fixed bed of SnO particles [15]. Hence, there is a scarcity of kinetic data concerning the CO2 and H2O splitting reactions using SnO solar nanopowders.

In the present work, a thermodynamic and a thermogravimetric analysis (TGA) of the oxidation step (i.e., CO2 and H2O reduction) in the SnO2/SnO thermochemical process are conducted. SnO was first synthesized from solar thermal dissociation of SnO2 and was used as an oxygen carrier during the CO2 and H2O reduction reactions. The results of thermodynamic equilibrium predict a rather wide temperature range in which significant conversion of CO2 and H2O to CO and H2 occurs. The experimental results of a thermogravimetric study are reported to verify the predictions and to derive kinetic data.

Section snippets

Synthesis of reduced SnO-rich materials

The SnO-rich nanopowders were first synthesized in a solar chemical reactor prototype of 1 kWth power (Fig. 2). This reactor was developed for carrying out the first solar step that consists in producing the active reduced species (stannous oxide) for the next CO2- and H2O-splitting steps. The reactant was injected continuously as compressed pellets inside a cylindrical alumina cavity (30 mm long and 30 mm i.d.). The cavity was insulated by a layer of alumino-silicate and closed at the front by

Chemical thermodynamic equilibrium

A thermodynamic analysis at chemical equilibrium was realized with HSC Chemistry 5.1 software [26] for determining the theoretical SnO conversion (reaction extent) during the H2O and CO2 reduction as a function of the temperature. The reaction extent is calculated as the ratio of the amount of H2 or CO produced at equilibrium to the initial amount of SnO in the system (1 mol). This analysis assumes a closed system without any mass transfer limitation. Gas flows through the system are not

Conclusion

This study focused on the kinetic characterization of the H2O and CO2 reduction reactions using reactive SnO nanopowders synthesized from solar thermal dissociation of SnO2. The solar process targets the production of H2 and CO from H2O and captured CO2 via thermochemical-looping using SnO as an oxygen carrier. The ultimate objective is the production of synthetic liquid fuels and high-temperature solar heat is applied to drive the endothermic reaction and to up-grade chemical feedstocks of low

Acknowledgments

This research has received funding from ANR, France (project contract ANR-09-JCJC-0004-01) and CNRS (Interdisciplinary Energy Program, DISCO2 project). The author thanks the technical support of R. Garcia from PROMES technical staff for the manufacturing of the solar chemical reactor.

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