Oxford Team Outlines Progress and Potential in CO2 Capture and Conversion to Synthetic Transportation Fuels
26 June 2010
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| Natural photosynthesis uses solar energy to recycle CO2 (and H2O) into new plant life (biomass) and ultimately fuels (biofuels). Sustainable tri-reforming uses CO2, renewable energy and CH4 (or biogas) to yield syngas and, ultimately, synthetic fuels and commodity chemicals. Jiang et al.Click to enlarge. |
In an open-access paper published in Philosophical Transactions of the Royal Society A, researchers from the University of Oxford, led by Dr. Peter Edwards, provide an overview of progress in the area of the conversion of carbon dioxide to synthetic transportation fuels (Carbon Capture and Conversion, CCC), its potential, and barriers to future progress.
The authors highlight three possible strategies for CO2 conversion by physico-chemical approaches: sustainable (or renewable) synthetic methanol; syngas production derived from flue gases from coal-, gas- or oil-fired electric power stations; and photochemical production of synthetic fuels.
Major reductions in emissions from the transportation sector will necessitate a change in vehicle fuels. The three leading alternatives generally advanced at present are electric (battery), hydrogen and biofuels; the first two options require fundamental large-scale changes in our energy infrastructure, while the latter will not meet the exceptionally high and ever-growing demand for transportation fuels.
...In order to take full advantage of the high ‘tank-to-wheel’ efficiency of electric vehicles, critical steps will also be needed to decarbonize the upstream energy (electricity) supply. In addition, batteries are fundamentally limited by their very low net gravimetric and volumetric energy densities...while the net on-board density of liquid hydrogen comfortably exceeds that of batteries, it is still extremely low when compared with carbonaceous liquid fuels such as diesel, gasoline, ethanol and methanol. In addition, the provision of hydrogen production, distribution and refuelling facilities will necessitate enormous investments for this completely new infrastructure base.
This same analysis (Pearson et al. 2009) concludes:
A fourth option, the focus of this review, has emerged, namely, the idea of using captured, anthropogenically produced CO2 to synthesize liquid renewable or sustainable hydrocarbon and carbonaceous fuels. This approach offers the intriguing possibility of using primary energy from renewable, carbon-free sources (such as electricity derived from solar, wind, wave or nuclear) to convert CO2, in association with hydrogen (or indeed methane), into high-density vehicle fuels compatible with our current transportation infrastructure. Its real attraction is that this approach offers the prospect of decarbonizing transport without the paradigm shift in infrastructure required by electrification of the vehicle fleet or by conversion to a hydrogen economy.The fundamentals of physics and electrochemistry dictate that the energy density of batteries and molecular hydrogen is unlikely ever to be competitive with liquid fuels for transport applications.
—Jiang et al.
The authors note that with carbon capture and storage becoming a key element in worldwide efforts to control/minimize emissions, large amounts of CO2 could likely become available as feedstock for innovative conversions to synthetic fuels. Whole process energy balances and economics remain a critical issue.
Realizing any or all of the approaches for CCC will require major advances in the science and engineering of materials, as well as significant socio-political commitments to CO2 capture and utilization, they note.
International collaborative research between developed, and developing countries is also absolutely critical in such a venture. Our hope is that this present summary helps to catalyse such a worthwhile development.
—Jiang et al.
Resources
Z. Jiang, T. Xiao, V. L. Kuznetsov and P. P. Edwards (2010) Turning carbon dioxide into fuel. Phil. Trans. R. Soc. A vol. 368 no. 1923 3343-3364 doi: 10.1098/rsta.2010.0119
The needs of energy supply and emissions reduction rule this out. Stabilizing CO2 levels (let alone reducing them to the 350 ppm which Hansen et al. believe is the maximum safe level) requires at least an 80% reduction in total emissions. This cannot be achieved if even transport emissions alone are maintained at the current level, whether from petroleum or recycled CO2 from coal and gas.
Electrification of rail transport, local trucking and the first 20-40 miles of car trips is quite feasible with current technologies. If the bulk of it goes zero-carbon, the demands on the liquid fuel system shrink proportionately. The size of the liquid fuel system also shrinks drastically, which I suppose is the "paradigm shift" the authors are afraid of. However, it's much easier to shift to electricity than to build an enormous infrastructure to make synthetic fuels, whether hydrocarbons or hydrogen.
Posted by: Engineer-Poet | 27 June 2010 at 08:39 AM
All new cars FFV and creating biomass to methanol plants could happen in the next ten years with a price tag below making all cars HEV/PHEV/EV. Ten years from now we could have more than 50% of cars running methanol, where less than 2% would be EV.
Posted by: SJC | 27 June 2010 at 09:45 AM
Right the EV/PHEV, with smart biofuels to a limited extend and let's not forget natural gas is the least resistance past, compared to other options, though it won't be an easy and fast transition, especially if we to decarbonate our electricity. But at least we can see a light in the tunnel, even if it won't be neither easy cheap or fast, it will takes decades. No energy transition has been fast before and fossil fuel offer a high quality of energy storage portability and density so they are difficult to replace.
Posted by: Treehugger | 27 June 2010 at 03:03 PM
CO2 is the final fully oxidized state of Carbon. Un-oxidizing Carbon will consume as much or more energy than was obtained in burning it in the first place. Since carbon oxidation is the prime source of Man's current and presently price-constrained energy supply, undoing the oxidation is a Fool's errand, today.
Only when Mankind has access to lots of surplus energy, does the un-oxidizing of Carbon make any sense, whatever.
However, that situation is coming. Fission has been perfected, becoming what it should have been all along; and Fusion experimentation is building the first pre-protype of a commercial power station in Cadarache, France, financed by an international consortium. Either or both will provide the energy surplus required to undertake Carbon de-oxidation if anyone is fool hardy to actually want to do so.
Posted by: Stan Peterson | 28 June 2010 at 01:25 PM
E-P, assuming fossil-free energy production both vehicle electrification and artificial fuels produced from atmospheric CO2 result in no further net additions of manmade CO2 to the atmosphere. Why do you think that one approach is worse than the other? Indeed the artificial fuels approach results in the temporary yet continuous sequestering of CO2 in stocks of artificial fuel in the distribution pipeline (although this would be a relatively small amount in the best case, no more than a billion tonnes or so at any one time).
SJC, the stumbling block is in growing enough biomass to make all the methanol required. The fundamental problems are (1) Sunlight is relatively diffuse and (2) plants do not efficiently turn Sunlight into biomass. We would have to turn the entire land surface of Earth into a fuel farm, and that would be bad.
SP, your facts are correct, so why do you conclude that making artificial fuel is a fools errand?
Posted by: richard schumacher | 29 June 2010 at 05:08 AM
It's the capital cost of energy supply.
Optimistically, the efficiency of producing liquid fuels from CO2 and some source of energy is 50%. With conventional vehicles at around 20% efficiency, the net efficiency is close to 10%. The grid-to-wheels efficiency of an electric vehicle can be close to 70%. If you need 7 times as much investment in the energy source, plus all the chemical plant, you've got a permanent and probably fatal disadvantage.
Posted by: Engineer-Poet | 29 June 2010 at 06:14 AM
The company POET says that the U.S. can create 5 billion gallons of fuel per year just from corn cobs. When you count stalks and straw from farm lands and forest waste from lumber mills you end up with quite a bit of fuel.
Combine that with switch grass grown on marginal land and you could have enough to run half the cars on methanol. The Billion Ton Study projects 100 billion gallons of fuel per year.
When you say "grid to wheels" it should be fuel to wheels. If you take a therm of natural gas, convert it to electricity, transmit it, convert it, store it and discharge it through a controller an motor, it is a more realistic picture.
Posted by: SJC | 29 June 2010 at 10:34 AM
E-P, your analysis of 6/29 is flawed. You need to account for the costs of storing your 70% electrical energy compared with a tank or compressed tank carrying some kind of fuel. It's also the capital cost of energy STORAGE. You need to consider both, especially for vehicle applications.
It would be cheaper to buy and own a PHEV with 40 miles of electrical range than an EV with 250 miles of range, even if the fuel for the PHEV was entirely synthetically produced (elec.-> h2-> carbon fuel).
Why? Because batteries are so expensive and fuel use on a PHEV is so low.
And this doesn't even take into account the time-to-recharge problem that would plague a full EV.
Posted by: Jim | 29 June 2010 at 12:36 PM
Yes, it's possible that some relatively small amount of liquid fuel for PHEVs would hit the point of minimum total cost compared to the status quo; going all-electric would require more batteries which aren't used often enough to justify the extra. The point remains that the bulk of energy would be electric rather than chemical if electricity accounted for as little as the first 20 miles of everyone's driving each day.
Posted by: Engineer-Poet | 29 June 2010 at 09:38 PM