CO2 Splitting via the Solar Thermochemical Cycle ... - ACS Publications

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Chapter 3

CO2 Splitting via the Solar Thermochemical Cycle Based on Zn/ZnO Redox Reactions Peter G. Loutzenhiser,1 Anton Meier,2 Daniel Gstoehl,2 and Aldo Steinfeld1,2,* 1Department

of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland 2Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland *[email protected]

A two-step thermochemical cycle for splitting CO2 and processing into solar fuels via ZnO/Zn redox reactions is considered. The first, solar step is the endothermic dissociation of ZnO to Zn and O2. The second, non-solar step is the exothermic reduction of CO2 with Zn to CO and ZnO; the latter is recycled to the solar step. The thermodynamics and kinetics of the pertinent reactions are examined and the reactor technology for both steps of the cycle is described and an outlook for the future is provided.

Introduction A project currently underway that provides a promising and sustainable alternative to CO2 sequestration is the splitting of CO2 into separate streams of CO and O2 using solar energy. CO can be either used as combustion gas or further processed to synthetic liquid fuels for transportation. The direct thermolysis of CO2 at atmospheric pressure occurs at ultrahigh temperatures, i.e. 30% dissociation is theoretically obtained above 2700 K. Further complications arise from the need to separate the product gases at these high temperatures in order to avoid recombination upon cooling. The operating temperature can be reduced and the separation problem bypassed by making use of thermochemical cycles. Of special interest is the two-step thermochemical cycle based on ZnO/Zn redox reactions, shown schematically in Figure 1, comprising: 1) the solar endothermic © 2010 American Chemical Society In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


dissociation of ZnO(s) and 2) the non-solar exothermal reduction of CO2 with Zn into CO and ZnO(s), and represented by:

The net reaction is CO2 → CO + ½ O2, with products formed in different steps, thereby eliminating the need for their high-temperature separation. A combination of theoretical and experimental work that includes thermodynamic and kinetic analyses and reactor technology is described to provide a project overview. Previous work using Zn/ZnO redox reactions in a two-step solar thermochemical cycle has focused on splitting H2O. A second law analysis indicates a maximum solar-to-chemical conversion efficiency of 29% (1). A comparison of the Zn/ZnO H2O splitting cycle with other metal redox reaction cycles is given by (2). Kinetic analyses for determination of the rate law for reducing H2O to H2 with Zn were performed by (3, 4). This reaction has been realized in aerosol reactors designed for in-situ Zn nanoparticle formation followed by a subsequent reaction with H2O to produce H2 and ZnO (4–8). A different type of reactor where steam was bubbled through molten Zn was designed by (9). CO2 splitting in two-step solar cycle has been examined for spinel ferrites (10, 11) and splitting H2O and CO2 with ceria for synthesis gas production (12).

Thermodynamics and Kinetic Analyses Thermodynamic equilibrium compositions were computed as a function of temperature and pressure. The thermal dissociation of ZnO, eq 1, proceeds endothermically and at reasonable rates at above 2000 K and 1 bar (13). A kinetic rate law was derived from experimental measurements in a solar-driven thermogravimeter for directly irradiated ZnO samples (14). Rapid quenching of the products was required to avoid product recombination (15). The reaction of CO2 with Zn, eq 2, proceeds exothermically below 1000 K (16). Kinetic studies with thermogravimetry indicated an initial fast surface-controlled regime followed by a slow diffusion-controlled regime, described using a shell-core kinetic model (17). A second law thermodynamic analysis was performed for the CO2 splitting flow diagram shown in Figure 2. The solar-to-chemical energy conversion efficiency is defined as the portion of solar energy that is converted into chemical energy, given by the Gibbs free energy of the products, i.e. the maximum possible amount of work that can be extracted from the products when transformed back to the reactants at 298 K in a reversible, ideal fuel cell:

For the CO2 splitting cycle with ZnO/Zn redox reaction operating at a solar concentration ratio of 5000 suns, ηsolar-to-chemical= 39% (16). High efficiencies 26 In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Scheme of the two-step solar thermochemical cycle for CO2 reduction via Zn/ZnO redox reactions.

Figure 2. Model flow diagram of the two-step CO2-splitting solar thermochemical cycle applied for the Second-Law analysis.

27 In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 3. Scheme of the solar reactor configuration, featuring a rotating cavity-receiver lined with ZnO particles that are directly exposed to concentrated solar radiation and serve simultaneously the functions of radiant absorbers and chemical reactants (20). directly translate to lower solar collection area and associated reduced costs of the heliostat field, which amount to about half of the capital cost for the entire solar CO2 splitting plant.

Reactor Technology A 10 kW solar reactor prototype for the thermal dissociation of ZnO has been designed, fabricated, and tested at the solar furnace at Paul Scherrer Institute (PSI) (18). Its configuration is shown in Figure 3. It features a rotating cavity-receiver lined with ZnO particles that are directly exposed to concentrated solar radiation and serve simultaneously the functions of radiant absorbers and chemical reactants. A transient heat transfer model was developed for analyzing the reactor thermal performance (19). This model couples radiation, convection, and conduction heat transfer to the reaction kinetics for a shrinking domain and simulates a transient ablation regime with semi-continuous feed cycles of ZnO particles. The second step of the cycle was experimentally investigated in a hot-wall quartz aerosol flow reactor, shown in Figure 4 (20). It was designed for quenching of Zn(g), formation of Zn nanoparticles, and in-situ oxidation with CO2. The effects of varying the reactants stoichiometry and reaction temperatures were investigated. Chemical conversions of Zn to ZnO of up to 88% were obtained for a residence time of ~3.0 s. For all experiments, high Zn conversions corresponded to large Zn depositions in the reaction zone thought to be a result of heterogeneous nucleation of the Zn of the reactor surfaces followed by a reaction with CO2. 28 In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 4. Scheme of the hot-wall aerosol flow reactor consisting of evaporation, cooling, and reaction zones for reducing CO2 with Zn.

Summary and Outlook A two-step thermochemical cycle for CO2 splitting via Zn/ZnO redox reactions has been examined. In-depth descriptions are given in the cited references for the thermodynamics analysis, the reaction kinetics, and the reactor technology for both steps of the cycle. For the solar dissociation of ZnO, a scaleup of the reactor technology from 10 kW to 100 kW is currently underway. For the second non-solar step, a cycle that combines the CO2 and H2O splitting two-step cycles to produce synthesis gas into one cycles is being investigated. Preliminary thermodynamic and experimental works have demonstrated that reactions with mixtures of CO2 and H2O with Zn are possible (21). Kinetic modeling of the competitive reaction with varying concentrations of CO2 and H2O with Zn are being studied to optimize H2/CO ratios for further processing to liquid fuels. A novel new fixed bed reactor concept is also being investigated aimed at immobilizing Zn particles on a substrate below the melting point to reduce particle sintering and increase the overall reaction extent.

Acknowledgments This work is being financially supported by the Swiss Federal Office of Energy, the Swiss National Science Foundation, and the Baugarten Foundation. We acknowledge the assistance of our colleagues Alwin Frei, Markus Haenchen, 29 In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Lothar Schunk, Anastasia Stamatiou, and Daniel Wuillemin in performing the experimental campaigns at PSI’s high-flux solar furnace/simulator and ETH’s thermogravimetric system.


References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19. 20. 21.

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30 In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

CO2 Splitting via the Solar Thermochemical Cycle ... - ACS Publications

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