Using off-Peak Wind to Recycle CO2 Into Transportation Fuels

The absence of satisfactory energy-storage solutions has recently led to the increasing availability of negative-priced off-peak grid energy in areas of substantial build-up of wind energy. Putting the excess off-peak wind energy into stable synthetic liquid fuels should permit complete grid stabilization and at the same time address the need for carbon-neutral fuels for transportation.

It has long been accepted that it should theoretically be possible to convert CO2 and water into standard liquid hydrocarbon fuels at high efficiency. The problem has been that prior proposals for doing this conversion have had efficiencies of only 25% to 35%. That is, the chemical energy in the liquid fuels produced (gasoline, ethanol, etc.) would be less than the 30% of the input energy required. Our work shows that a factor-of-two gain in efficiency should be possible.

Doty Energy is developing novel processes that will allow the production of fully carbon-neutral ethanol, gasoline, jet fuel, diesel, and most plastics from waste CO2 and off-peak wind or other low-carbon energy at high efficiency and at prices that should soon be competitive with fossil-derived products. Converting CO2 into fuels can eliminate the need for CO2 sequestration and reduce global CO2 emissions by 40%.

The US transportation-fuel market is about $1T, and it is based mostly on imported oil. A better alternative for future transportation fuels is needed than those that have previously been advocated ? such as biofuels, hydrogen, and methane. It is in our economic and security interests to produce carbon-neutral transportation fuels domestically at the scale of hundreds of billions of gallons annually. Most biofuels are only 5% to 25% carbon neutral; and hydrogen has daunting cost challenges with respect to distribution, storage, and end use. Windfuels will be over 85% carbon neutral and will flow seamlessly within our current infrastructure.

The combination of the major technical advances we have simulated over the past two years should permit conversion of CO2 to fuels to be done at up to 60% efficiency, which is about twice what was expected by most researchers just three years ago. When the input energy is from off-peak wind or nuclear and reasonable consideration is included for climate benefit, WindFuels will sometimes compete even when petroleum is $45/bbl, and most experts are predicting oil will stay over $140/bbl after 2014. Our analysis shows windfuels production per gross land area in good wind regions should exceed biofuels production density in productive farming areas by factors of 4 to 20. Moreover, 99.9% of the land required for the wind farms is generally available for dual use. Switching 70% of global transportation fuels from petroleum to WindFuels should be possible over the next 35 years.

The WindFuels production system (disclosed in detail in pending patents) will be presented. It is based largely on the commercially proven technologies of wind energy, water electrolysis, and Fischer Tropsch (FT) chemistry. Wind energy is used to split water into hydrogen and oxygen. Some of the hydrogen is used in a process, the so-called reverse water gas shift (RWGS) reaction, that reduces CO2 to carbon monoxide (CO) and water. The CO and the balance of the hydrogen are fed into an FT reactor similar to that commonly used to produce fuels and chemicals from coal or natural gas.

Windfuels are truly sustainable. The needed CO2 could ultimately come from biofuels refineries, cement factories, ore refining, and possibly even the atmosphere. The water requirements are an order of magnitude less than for biofuels, and the wind will always blow.

The biggest part of the increase in our projected efficiency comes from improved separations processes in the fully recycled FT loop. Conventional processes for separation of CO2 from the other syngas components have typically required over 6 MJ/kg of CO2. Our simulations indicate a high-pressure cryogenic process can achieve sufficient separation in the FT recycle loop (under 15 molar-%) at under 0.8 MJ/kg. The next largest gain is expected from an order-of-magnitude advance in cost-effectiveness of gas-to-gas recuperation, which is expected to make 97% effectiveness practical where 75% was previously practical. The third largest gain may be from an optimized RWGS process which promises over 90% efficiency (implying under 1.55 MJ per kg of CO) compared to about 50% for prior demonstrations (about 2.8 MJ/kg-CO). Another significant gain comes from a novel thermodynamic cycle for conversion of the waste heat from the electrolyzer and the FT reactor. A novel approach to RFTS system integration and optimization leads to additional efficiency gains. All the processes have been simulated in detail, and key experiments will soon be carried out to help optimize process conditions.

References

1. FD Doty et al, ?WindFuels ? Competitive Fuels from Wind Energy and Waste CO2?, presented at AIChE annual meeting, Philadelphia, 2008 http://windfuels.com/PDFs/WindFuels_Sci_Engr_Basics_doc1.pdf.

2. SE Park, JS Chang, KW Lee editors, Carbon Dioxide Utilization for Global Sustainability, Studies in Surface Science and Catalysis 153?; OS Joo, KD Jung and J Yonsoo, ?CAMERE Process for methanol synthesis from CO2 hydrogenation?, pp 67-72, Elsevier, 2004.

3. Mark Z Jacobson, ?Review of Solutions to Global Warming, Air Pollutions, and Energy Security?, Energy Environ. Sci., 2009.

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5. O.S. Joo, K.D. Jung J. Yonsoo in. Carbon Dioxide Utilization for Global Sustainability, Studies in Surface Science and Catalysis 153, S.E. Park, J.S. Chang, K.W. Lee, Eds., pp 67-72, Elsevier, 2004.

6. M Xiang, D Li, H Qi, W Li, B Zhong, Y Sun, ?Mixed alcohols synthesis from CO hydrogenation over K-promoted b-Mo₂C catalysts?, Fuel 86, 1298-1303, 2007.

7. R. Zubrin, B. Frankie, T. Kito, AIAA 97-2767, (1997).

8. S Phillips, A Aden, J Jechura, and D Dayton, ?Thermochemical Ethanol via Indirect Gasification and Mixed Alcohol Synthesis...?, NREL/TP-510-41168, 2007.

9. FD Doty and S Shevgoor, ?Compact, High-Effectiveness, Gas-to-gas Compound Recuperator with Liquid Intermediary (CRLI)?, HT2009-88372, ASME Joint Conferences, San Francisco, 2009.

10. FD Doty and S Shevgoor, ?A Dual-source Organic Rankine Cycle (DORC) ...? ES2009-90220, ASME Joint Conferences, San Francisco, 2009.

11. GN Doty, FD Doty, LL Holte, D McCree, S Shevgoor, ?...Efficiently Recycling CO2 into Transportation Fuels ? Driving the Off-peak Wind Market?, Proceed. WindPower 2009, paper 175, Chicago, 2009.