Study reviews pathways for recycling CO2 into fuels using renewable or nuclear energy; concludes co-electrolysis with FT production of fuels could be cost-competitive with diesel or gasoline
In a paper published in the journal Renewable and Sustainable Energy Reviews, researchers from Columbia University and the Risø National Laboratory for Sustainable Energy (Denmark) review the possible technological pathways for recycling CO2 into fuels using renewable or nuclear energy, considering three stages: CO2 capture; H2O and CO2 dissociation, and fuel synthesis.
The new review paper analyzes dissociation methods including thermolysis, thermochemical cycles, electrolysis, and photoelectrolysis of CO2 and/or H2O, and then identifies co-electrolyzing H2O and CO2 in high temperature solid oxide cells to yield syngas, and then producing gasoline or diesel from the syngas in a catalytic reactor (e.g. Fischer–Tropsch) as one of the most promising, feasible routes. It further analyzes this pathway in terms of energy balance and economics.
Based on the energy balance and economics estimates presented for this particular co-electrolysis based cycle, the state-of-the-art technologies at each stage of the cycle can be combined to work together efficiently today with an electricity-to-liquid fuel conversion efficiency of about 70%, and with mass production of the components, economic viability is feasible.
With an electricity price of less than 3 U.S. cents/kWh from a constant power supply (e.g. geothermal, hydroelectric, or nuclear), the synthetic fuel price could be competitive with gasoline at around U.S.D$ 2/gal ($ 0.53/L). If a higher gasoline price of $3/gal ($ 0.78/L) is competitive, the price of electricity driving the synthetic fuel process must be 4–5 U.S. cents/kWh, which is a similar range to recent average wholesale electricity prices in the U.S. Intermittent power sources would significantly increase the capital cost of the electrolyzer.
Intermittent power sources would significantly increase the capital cost of the electrolyzer. With intermittent operation, economical fuel production most likely requires additional technology development on the electrolysis system to reduce the capital cost (via achieving durable high current density operation and/or lower manufacturing cost).—Graves et. al.
|Map of the possible pathways from H2O and CO2 to hydrocarbon fuels. “Fischer–Tropsch” represents any of a variety of catalytic fuel synthesis processes similar to the original Fischer–Tropsch processes. Graves et al. Click to enlarge.|
Members of the team earlier this year published a paper in the journal Solid State Ionics on their investigation of the high-temperature co-electrolysis of CO2 and H2O using solid oxide electrolysis cells (SOECs) to produce a syngas for conversion into liquid hydrocarbon fuels. (Earlier post.)
They conclude that several developments could enable competitive fuel production using any inexpensive sustainable power sources, including:
Further development of the CO2 air capture process and full-scale demonstration, followed by cost reductions from mass production. In the near term, however, CO2 collected from industrial sources rather than the atmosphere could be used in the non closed- loop version of the synthetic fuel process.
Demonstration of durable operation of solid oxide electrolysis cell stacks at high current densities (≥1 A/cm2). Since the existing cells can be efficiently operated at such high current densities at the thermoneutral voltage, operating at this point would be a straightforward way to improve the economics, if performance at this operating point can be maintained over long-term operation.
Demonstration of intermittent cell operation, which may require development of specialized power management and heat management schemes.
In the review, Grave et al. examine the status of the enabling technologies for each stage, with special focus on the various thermochemical, electrochemical and photochemical energy conversion technologies that could be used for dissociation of H2O and CO2, the stage with the highest energy consumption. They noted that combining more than one stage into a single unit is possible, but there may be benefits to optimizing each stage separately.
With feasible technology development and mass production of the process components, CO2-recycled hydrocarbon fuels can be produced at the scale needed to replace transportation fuels at a price competitive with more conventional fossil-derived hydrocarbons, especially if oil and CO2 sequestration costs are high. The potentially greater sustainability of CO2-recycled fuels over fossil or biomass derived fuels, as well as independence from the geographic and supply related issues of conventional fuels, could also give CO2-recycled fuels a market advantage.—Graves et al.
Christopher Graves, Sune D. Ebbesen, Mogens Mogensen and Klaus S. Lackner (2011) Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy. Renewable and Sustainable Energy Reviews, Volume 15, Issue 1 Pages 1-23 doi: 10.1016/j.rser.2010.07.014