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Energy from the air

Rather than seeing excess carbon dioxide in the atmosphere as a problem to be dealt with, by, for example, expensive carbon capture and underground storage, why not make use of it to produce fuel? The basic chemistry is simple, assuming you have some spare hydrogen: CO₂ + H₂ = CO + H₂0. The CO can then being converted to hydrocarbon fuel, for example by via the Fischer–Tropsch process. That of course needs more hydrogen. Fortunately, not only do we have plentiful supplies of CO₂ from the air, air movements can also supply the energy, via wind turbines, to make hydrogen, via the electrolysis of water. Problem solved! Except of course each stage in this process is difficult, with conversion efficiency’s of around 60%, unless the waste heat can be recycled/used.

Carbon capture techniques are of course being developed for use with power stations emissions. But there has been a parallel idea of ‘air capture’ – for example the ‘absorption tower’ approach developed by Prof. David Keith from Calgary University, in which a fine mist of strong sodium hydroxide solution is brought into contact with an air flow. The big advantage of that is that you can do it anywhere – not just at power plants. But rather than just storing the resultant mix, the CO₂ can be recovered, ready for conversion into a fuel.

That is what a team led by Prof. Tony Marmont are planning to do, using an electrochemical process based on a patented design developed by Prof. Dereck Pletcher, formally of Southampton University.

Prof. Marmont is a long time proponent of renewable energy in the UK and funded the set-up of the CREST organisation at Loughborough University. His team at Beacon Energy in Leicestershire has been working on hydrogen generation, storage and use for some while – with the electricity supplied by their own wind and PV systems, so the next stage should be a bit easier.

In the ‘air fuel synthesis’ (AFS) approach being developed by Marmont, the recovered carbon dioxide will then be reacted with electrolytic hydrogen; either directly to make methanol and thence to petrol via the Mobil Methanol-to-Gasoline route; or via the Reverse Water Gas Shift reaction (as above) with hydrogen, to make carbon monoxide, which in turn will be reacted with more hydrogen in a Fischer–Tropsch reaction to make hydrocarbons. In the latter case, variation of the reaction conditions could enable petrol, diesel or aviation fuel to be made.

It’s a fascinating idea, essentially using renewable energy to do what nature does with photosynthesis – convert atmospheric carbon dioxide back into organic molecules. But it does rely on multiple stages, each with significant energy losses. In terms of road transport applications, you would presumably get a much better return on the wind-generated energy if it was just used in battery electric cars – with the overall conversion efficiency then being 90% or so. So the AFS approach may only be an interim option while electric cars are improved. But liquid fuels have a much higher utility/energy storage density than batteries, and there may well be some application (e.g. for heavy goods vehicles and, crucially, for aircraft) where liquid fuels will have major advantages.

So there could well be a future for ideas like this, and the wind power resource could be up to it in time. The Marmont team calculate that ‘to make all UK oil – 140,000 tons a day – as synthetic, would take a windfarm area 175 miles by 175 miles in the North Sea. To make only aviation, marine and military fuel as synthetic, would take an area 72 miles by 72 miles.’

www.airfuelsynthesis.com/

The Los Alamos Lab in the US has proposed something similar, but with the energy supplied by a nuclear reactor. They gave their idea the somewhat cringeworthy label ‘Green Freedom’.

For good measure they suggested that conventional power station cooling towers could be used for the carbon dioxide trapping NaOH spay system. But as with the wind-AFS approach, it would probably be more efficient to use the nuclear electricity directly in electric cars, a point made elegantly at http://ergosphere.blogspot.com/2010/01/revisiting-green-freedom.html.

Nevertheless, with there being no easy aviation fuel substitutes, Air Fuel Synthesis does still seem worth exploring, as are the various other novel ‘Green Chemistry’ ideas for fuel production being developed around the UK and elsewhere, including the use of biomass as a feed stock for hydrogen production. See for example: www.claverton-energy.com/wp-content/uploads/2010/07/Tetzlaff_Birmingham2010.pdf.

For more on new renewable-energy developments, visit www.natta-renew.org from which some of the above was drawn. Thanks to Dave Benton for his input on AFS.

Posted by Dave Elliott on September 11, 2010 12:54 PM | Permalink