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Fuels from water, CO2 and solar energy
Accepted Manuscript News & Views Fuels from Water, CO2 and solar energy Yong Hao, Aldo Steinfeld PII: DOI: Reference:
S2095-9273(17)30411-5 http://dx.doi.org/10.1016/j.scib.2017.08.013 SCIB 198
To appear in:
Science Bulletin
Please cite this article as: Y. Hao, A. Steinfeld, Fuels from Water, CO2 and solar energy, Science Bulletin (2017), doi: http://dx.doi.org/10.1016/j.scib.2017.08.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical Abstract
Fuels from Water, CO2 and solar energy Yong Hao1,* and Aldo Steinfeld2,* 1
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China.
2
Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland
*
Corresponding authors: [email protected] (Y Hao); [email protected] (A steinfeld)
Storage of the vast, yet intermittent, diffuse and unevenly distributed solar energy resource is essential for the transition away from fossil fuels. Liquid hydrocarbon fuels offer exceptionally high energy densities and are convenient for the transportation sectors, especially for aviation, without changes in the current global infrastructure. Sustainable production of liquid hydrocarbon fuels can be realized with the help of solar technologies. However, their production from H2O and CO2 using solely solar energy input has remained a grand challenge. Concentrated solar energy provides a virtually unlimited source of clean, non-polluting, hightemperature heat. Solar flux concentration ratios exceeding 2000 suns (1 sun = 1 kW/m2) are attainable with large-scale solar tower and dish systems. These solar concentrating systems have been proven to be technically feasible for electricity generation in concentrated solar power (CSP)
plants. Solar thermochemical applications, although not as far developed as CSP, can employ the same solar concentrating infrastructure already established for commercial CSP plants. Solar thermochemical reduction-oxidation (i.e. redox) approaches to splitting CO2 and H2O inherently operate at high temperatures and utilize the entire solar spectrum, and as such provide an attractive thermodynamic path to solar fuels production with high energy conversion efficiencies [1]. Fig. 1 shows schematically the proposed 2-step thermochemical cycle based on metal oxide redox reactions, encompassing: (1) an endothermic reduction of the metal oxide using concentrated solar process heat; and (2) an exothermic reaction of the reduced metal oxide with H2O/CO2 to generate H2/CO.
Fig. 1 (Color online) Schematic of a two-step solar thermochemical cycle for H2O/CO2 splitting based on metal oxide redox reactions. MOox denotes a metal oxide, and MOred the corresponding reduced metal oxide. In the first, endothermic, solar step, MOox is thermally dissociated into MOred and oxygen. Concentrated solar radiation is the energy source for the required high-temperature process heat. In the second step, MOred reacts with H2O/CO2 to produce H2/CO (syngas). The resulting MOox is then recycled to the first step, while syngas is further processed to liquid hydrocarbon fuels [1].
The net reactions are H2O=H2+½O2 and/or CO2=CO+½O2, with O2 and H2/CO released in separate steps. The syngas mixture H2/CO can be further processed to liquid hydrocarbon fuels – via Fischer-Tropsch and other catalytic processes – such as diesel, kerosene, methanol, and gasoline using existing conventional technologies. Amongst candidate redox materials, ceria (CeO2) has emerged as a highly attractive redox active material choice because it displays rapid reaction kinetics [2]. The CeO2-based redox cycle is represented by: Reduction:
CeO 2 → CeO 2 −δ + δ2 O 2
Oxidation with H2O:
CeO2 −δ + δ H 2O → CeO2 + δ H 2
(2a)
Oxidation with CO2:
CeO2 −δ + δ CO2 → CeO2 + δ CO
(2b)
(1)
(a)
(b)
Fig. 2 (Color online) A solar reactor for splitting H2O and CO2 via a 2-step thermochemical redox cycle. (a) Schematic of the solar reactor configuration. Red arrow: endothermic reduction generating O2 (Eq. (1)). Blue arrow: exothermic oxidation with H2O and CO2 generating H2 and CO (Eq. (2)). Inset: ceria RPC with dual-scale porosities in the mm and µm ranges for enhanced heat and mass transport. (b) Photographs of the solar reactor, showing the front face of the solar reactor with the windowed aperture and its interior containing the octagonal RPC structure lined with alumina thermal insulation. Reproduced from Ref. [3] with permission from The Royal Society of Chemistry. .
Fig. 2 shows the schematic of a solar reactor developed for this 2-step redox cycle. It consists of a cavity-receiver with a small aperture sealed by a transparent quartz window for the access of concentrated solar radiation. The cavity contains a reticulated porous ceramic (RPC) foam-type structure made of ceria. With this arrangement, the RPC is directly exposed to the high-flux irradiation, enabling volumetric absorption and uniform heating. This reactor concept has been applied for the production of H2 and CO from H2O and CO2, yielding total selectivity, good stability, high mass conversion, and promising values of the solar-to-fuel energy conversion efficiency [2, 3]. Within the framework of the EU-project SOLARJET, the entire production chain to solar jet fuel from H2O and CO2 was experimentally demonstrated [4]. It is possible to perform both steps of the redox cycle simultaneously and continuously at isothermal/isobaric steady-state conditions by means of a ceria membrane reactor that establishes a spatial separation between the reduction and oxidation sites and conducts oxygen ions and vacancies driven by the oxygen chemical potential gradient across the membrane [5-8]. Isothermal redox reactions produce fuels by taking advantage of partial thermal dissociation of H2O and / or CO2 at elevated temperatures (e.g. 1500 °C) and selectively removing oxygen from the gas phase, thereby eliminating the dependence on temperature swing conventionally required for redox cycles. Temperatures at which isothermal splitting operates are usually high enough to eliminate kinetic limitations, and reactions can be considered as purely thermodynamics-driven [9]. In particular, isothermal splitting of CO2 is preferred over H2O due to more favorable thermodynamics of CO2 thermal dissociation [8], which also carries particular significance for the synthesis of liquid fuels.
(a)
(c)
(b)
Fig. 3 (Color online) Isothermal splitting of H2O and CO2 for solar fuel production. (a) Alternating production of H2 and O2 via a 2-step isothermal redox cycle for H2O splitting with ceria at 1500 C, reproduced from Ref. [6] with permission from the PCCP Owner Societies; (b) Continuous and stable production of CO and O2 from CO2 at 1475 °C using a ceria-membrane solar reactor, reproduced from Ref. [8]; (c) Schematic of the ceriamembrane solar reactor for isothermal splitting of CO2, reproduced from Ref. [8].
Isothermal splitting of H2O and CO2 via membrane separation further offers the advantage of continuous operation, in contrast to alternating redox cycles (Fig. 3a). High solar-to-fuel efficiencies are potentially achievable at conditions of low oxygen partial pressures and highefficacy gas-phase heat recuperation. Recent experimental studies (Fig. 3b, 3c) have demonstrated the continuous and stable splitting of CO2 and in-situ separation of CO and O2 via a solar-driven ceria membrane reactor [8]. Promising developments of efficient, robust and scalable solar reactors are being pursued jointly by ETH and CAS. In summary, the experimental results and associated thermodynamic studies provide compelling evidence of the technical feasibility of solar thermochemical redox approaches for converting H2O and CO2 to fuels. The potential for high solar-to-fuel energy conversion efficiency points out to the economic competitiveness vis-à-vis alternative paths to renewable liquid fuels. Notably, the large-scale implementation can be integrated to the concentrating solar tower and dish systems already established commercially. The solar production of syngas from H2O and CO2 can be coupled to the synthesis of liquid hydrocarbon fuels for sustainable transportation. Conflict of interest The authors declared that they have no conflict of interest. References
1.
Romero M, Steinfeld A. Concentrating solar thermal power and thermochemical fuels. Energ Environ Sci 5:9234–9245 (2012).
2.
Chueh WC, Falter C, Abbott M, et al. High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria. Science 330:1797-1801 (2010).
3.
Marxer D, Furler P, Takacs M, et al. Solar thermochemical splitting of CO2 into separate streams of CO and O2 with high selectivity, stability, conversion, and efficiency. Energ Environ Sci 10:1142-1149 (2017).
4.
Marxer D, Furler P, Scheffe J, et al. Demonstration of the entire production chain to renewable kerosene via solar thermochemical splitting of H2O and CO2. Energ Fuel 29:3241-3250 (2015).
5.
Fletcher EA, Moen RL. Hydrogen and oxygen from water. Science 197: 1050-1056 (1977).
6.
Hao Y, Yang CK, Haile SM. High-temperature isothermal chemical cycling for solar-driven fuel production. Phys Chem Chem Phys 15:17084-17092 (2013).
7.
Wang H, Hao Y, Kong H. Thermodynamic study on solar thermochemical fuel production with oxygen permeation membrane reactors. Inter J Energ Res 39:1790-1799 (2015).
8.
Tou M, Michalsky R, Steinfeld A. Solar-driven thermochemical splitting of CO2 and in-situ separation of CO and O2 across a ceria redox membrane reactor. Joule, doi: 10.1016/j.joule.2017.07.015 (2017).
9.
Davenport TC, Yang CK, Kucharczyk CJ, et al. Implications of exceptional material kinetics on thermochemical fuel production rates. Energ Technol 4:764-770 (2016).
S2095-9273(17)30411-5 http://dx.doi.org/10.1016/j.scib.2017.08.013 SCIB 198
To appear in:
Science Bulletin
Please cite this article as: Y. Hao, A. Steinfeld, Fuels from Water, CO2 and solar energy, Science Bulletin (2017), doi: http://dx.doi.org/10.1016/j.scib.2017.08.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical Abstract
Fuels from Water, CO2 and solar energy Yong Hao1,* and Aldo Steinfeld2,* 1
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China.
2
Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland
*
Corresponding authors: [email protected] (Y Hao); [email protected] (A steinfeld)
Storage of the vast, yet intermittent, diffuse and unevenly distributed solar energy resource is essential for the transition away from fossil fuels. Liquid hydrocarbon fuels offer exceptionally high energy densities and are convenient for the transportation sectors, especially for aviation, without changes in the current global infrastructure. Sustainable production of liquid hydrocarbon fuels can be realized with the help of solar technologies. However, their production from H2O and CO2 using solely solar energy input has remained a grand challenge. Concentrated solar energy provides a virtually unlimited source of clean, non-polluting, hightemperature heat. Solar flux concentration ratios exceeding 2000 suns (1 sun = 1 kW/m2) are attainable with large-scale solar tower and dish systems. These solar concentrating systems have been proven to be technically feasible for electricity generation in concentrated solar power (CSP)
plants. Solar thermochemical applications, although not as far developed as CSP, can employ the same solar concentrating infrastructure already established for commercial CSP plants. Solar thermochemical reduction-oxidation (i.e. redox) approaches to splitting CO2 and H2O inherently operate at high temperatures and utilize the entire solar spectrum, and as such provide an attractive thermodynamic path to solar fuels production with high energy conversion efficiencies [1]. Fig. 1 shows schematically the proposed 2-step thermochemical cycle based on metal oxide redox reactions, encompassing: (1) an endothermic reduction of the metal oxide using concentrated solar process heat; and (2) an exothermic reaction of the reduced metal oxide with H2O/CO2 to generate H2/CO.
Fig. 1 (Color online) Schematic of a two-step solar thermochemical cycle for H2O/CO2 splitting based on metal oxide redox reactions. MOox denotes a metal oxide, and MOred the corresponding reduced metal oxide. In the first, endothermic, solar step, MOox is thermally dissociated into MOred and oxygen. Concentrated solar radiation is the energy source for the required high-temperature process heat. In the second step, MOred reacts with H2O/CO2 to produce H2/CO (syngas). The resulting MOox is then recycled to the first step, while syngas is further processed to liquid hydrocarbon fuels [1].
The net reactions are H2O=H2+½O2 and/or CO2=CO+½O2, with O2 and H2/CO released in separate steps. The syngas mixture H2/CO can be further processed to liquid hydrocarbon fuels – via Fischer-Tropsch and other catalytic processes – such as diesel, kerosene, methanol, and gasoline using existing conventional technologies. Amongst candidate redox materials, ceria (CeO2) has emerged as a highly attractive redox active material choice because it displays rapid reaction kinetics [2]. The CeO2-based redox cycle is represented by: Reduction:
CeO 2 → CeO 2 −δ + δ2 O 2
Oxidation with H2O:
CeO2 −δ + δ H 2O → CeO2 + δ H 2
(2a)
Oxidation with CO2:
CeO2 −δ + δ CO2 → CeO2 + δ CO
(2b)
(1)
(a)
(b)
Fig. 2 (Color online) A solar reactor for splitting H2O and CO2 via a 2-step thermochemical redox cycle. (a) Schematic of the solar reactor configuration. Red arrow: endothermic reduction generating O2 (Eq. (1)). Blue arrow: exothermic oxidation with H2O and CO2 generating H2 and CO (Eq. (2)). Inset: ceria RPC with dual-scale porosities in the mm and µm ranges for enhanced heat and mass transport. (b) Photographs of the solar reactor, showing the front face of the solar reactor with the windowed aperture and its interior containing the octagonal RPC structure lined with alumina thermal insulation. Reproduced from Ref. [3] with permission from The Royal Society of Chemistry. .
Fig. 2 shows the schematic of a solar reactor developed for this 2-step redox cycle. It consists of a cavity-receiver with a small aperture sealed by a transparent quartz window for the access of concentrated solar radiation. The cavity contains a reticulated porous ceramic (RPC) foam-type structure made of ceria. With this arrangement, the RPC is directly exposed to the high-flux irradiation, enabling volumetric absorption and uniform heating. This reactor concept has been applied for the production of H2 and CO from H2O and CO2, yielding total selectivity, good stability, high mass conversion, and promising values of the solar-to-fuel energy conversion efficiency [2, 3]. Within the framework of the EU-project SOLARJET, the entire production chain to solar jet fuel from H2O and CO2 was experimentally demonstrated [4]. It is possible to perform both steps of the redox cycle simultaneously and continuously at isothermal/isobaric steady-state conditions by means of a ceria membrane reactor that establishes a spatial separation between the reduction and oxidation sites and conducts oxygen ions and vacancies driven by the oxygen chemical potential gradient across the membrane [5-8]. Isothermal redox reactions produce fuels by taking advantage of partial thermal dissociation of H2O and / or CO2 at elevated temperatures (e.g. 1500 °C) and selectively removing oxygen from the gas phase, thereby eliminating the dependence on temperature swing conventionally required for redox cycles. Temperatures at which isothermal splitting operates are usually high enough to eliminate kinetic limitations, and reactions can be considered as purely thermodynamics-driven [9]. In particular, isothermal splitting of CO2 is preferred over H2O due to more favorable thermodynamics of CO2 thermal dissociation [8], which also carries particular significance for the synthesis of liquid fuels.
(a)
(c)
(b)
Fig. 3 (Color online) Isothermal splitting of H2O and CO2 for solar fuel production. (a) Alternating production of H2 and O2 via a 2-step isothermal redox cycle for H2O splitting with ceria at 1500 C, reproduced from Ref. [6] with permission from the PCCP Owner Societies; (b) Continuous and stable production of CO and O2 from CO2 at 1475 °C using a ceria-membrane solar reactor, reproduced from Ref. [8]; (c) Schematic of the ceriamembrane solar reactor for isothermal splitting of CO2, reproduced from Ref. [8].
Isothermal splitting of H2O and CO2 via membrane separation further offers the advantage of continuous operation, in contrast to alternating redox cycles (Fig. 3a). High solar-to-fuel efficiencies are potentially achievable at conditions of low oxygen partial pressures and highefficacy gas-phase heat recuperation. Recent experimental studies (Fig. 3b, 3c) have demonstrated the continuous and stable splitting of CO2 and in-situ separation of CO and O2 via a solar-driven ceria membrane reactor [8]. Promising developments of efficient, robust and scalable solar reactors are being pursued jointly by ETH and CAS. In summary, the experimental results and associated thermodynamic studies provide compelling evidence of the technical feasibility of solar thermochemical redox approaches for converting H2O and CO2 to fuels. The potential for high solar-to-fuel energy conversion efficiency points out to the economic competitiveness vis-à-vis alternative paths to renewable liquid fuels. Notably, the large-scale implementation can be integrated to the concentrating solar tower and dish systems already established commercially. The solar production of syngas from H2O and CO2 can be coupled to the synthesis of liquid hydrocarbon fuels for sustainable transportation. Conflict of interest The authors declared that they have no conflict of interest. References
1.
Romero M, Steinfeld A. Concentrating solar thermal power and thermochemical fuels. Energ Environ Sci 5:9234–9245 (2012).
2.
Chueh WC, Falter C, Abbott M, et al. High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria. Science 330:1797-1801 (2010).
3.
Marxer D, Furler P, Takacs M, et al. Solar thermochemical splitting of CO2 into separate streams of CO and O2 with high selectivity, stability, conversion, and efficiency. Energ Environ Sci 10:1142-1149 (2017).
4.
Marxer D, Furler P, Scheffe J, et al. Demonstration of the entire production chain to renewable kerosene via solar thermochemical splitting of H2O and CO2. Energ Fuel 29:3241-3250 (2015).
5.
Fletcher EA, Moen RL. Hydrogen and oxygen from water. Science 197: 1050-1056 (1977).
6.
Hao Y, Yang CK, Haile SM. High-temperature isothermal chemical cycling for solar-driven fuel production. Phys Chem Chem Phys 15:17084-17092 (2013).
7.
Wang H, Hao Y, Kong H. Thermodynamic study on solar thermochemical fuel production with oxygen permeation membrane reactors. Inter J Energ Res 39:1790-1799 (2015).
8.
Tou M, Michalsky R, Steinfeld A. Solar-driven thermochemical splitting of CO2 and in-situ separation of CO and O2 across a ceria redox membrane reactor. Joule, doi: 10.1016/j.joule.2017.07.015 (2017).
9.
Davenport TC, Yang CK, Kucharczyk CJ, et al. Implications of exceptional material kinetics on thermochemical fuel production rates. Energ Technol 4:764-770 (2016).