Brewing fuels in a solar furnace
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Energy Sector Analysis
Improved techniques to create liquid fuels out of carbon dioxide and water hinge on finding the right redox materials and designing efficient solar reactors.
Brewing fuels in a solar furnace By Arthur L. Robinson, with contributions by Corinna Wu Feature Editor Aldo Steinfeld
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elying on existing concentrated solar power (CSP) technology for the thermal energy to drive it, solar thermochemical production of synthesis gas (syngas) anchors a promising solar route to liquid hydrocarbon fuels. Cyclical two-step oxidation-reduction (redox) reactions with water and carbon dioxide as feedstocks and a metal oxide as the redox material yield the syngas constituents hydrogen and carbon monoxide. Syngas itself is the precursor to the final energy product, conventional liquid fuels such as diesel, kerosene, and gasoline. Subsequent production steps rely on proven conversion technologies such as the Fischer–Tropsch and methanol-to-gasoline (MTG) processes, although purified hydrogen could itself fuel an alternative hydrogen economy. The impact of solar thermochemical fuels could be spectacular. With renewable liquid fuels derived from the sun, water, and carbon dioxide, many countries would benefit from dramatically increased energy security and financial stability. And hydrocarbon fuels are the only potential product family that could consume CO2 feedstock at a level comparable to current emissions, making the entire fuel production-to-combustion cycle, in principle, carbon-neutral. The ability to use the existing global storage and distribution infrastructure for liquid fuels would add an important economic and logistical plus. “Renewable liquid fuels would be pretty special,” said James Miller of Sandia National Laboratories’ Sunshine-to-Petrol project. While researchers seeking to produce fuels from the sun have a full menu of options (electrochemical, photochemical, and thermochemical), proponents of the two-step, redox approach cite the favorable thermodynamics arising from high-temperature operation and the full utilization of the solar spectrum that in principle could lead to solar-to-fuel energy conversion efficiencies—defined as the ratio of the calorific value of the syngas produced to the solar energy input—above 30%. In addition, there is no need for separation of a combustible fuel–oxygen mixture, a tricky problem for one-step methods. On paper, the two-step cyclic process seems simple: (1) Concentrated solar radiation heats the metal oxide to well above 1000°C, thereby driving its endothermic reduction and releasing oxygen as the product; (2) the reduced oxide is cooled to
Aldo Steinfeld, ETH Zürich and the Paul Scherrer Institute, Switzerland Arthur L. Robinson, [email protected] Corinna Wu, [email protected]
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MRS BULLETIN
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VOLUME 38 • MARCH 2013
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www.mrs.org/bulletin • Energy Quarterly
1000°C or below while steam or carbon dioxide flows through, re-oxidizing it and liberating hydrogen or carbon monoxide as the respective products. The net reaction is water or carbon dioxide
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