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Petrol from air?

November 19, 2008
Too much CO2? Suck it out

Too much CO2? Suck it out

Update 18 October 2012: Now another UK group has made petrol from air.

The heady combination of a global crisis and a ticking time bomb presented by climate modellers is proving an irresistible challenge, drawing some of the world’s best minds along with a growing band of experimental speculators, entrepreneurs and mavericks.

Distinguishing which among their offerings carries a technologically and economically viable solution to the jigsaw of climate and energy problems is tricky, and time is running out for policymakers.

One ingenious plan was described to me with great passion the other day by David Benton, a retired English chemist. His scheme would extract carbon dioxide directly from the air using a series of gas-diffusion membranes and “could help wean the world off fossil fuels”. The CO2 would be used in a chain of reactions to produce synthetic gasoline, diesel and aviation fuel, keeping the carbon used and produced in a virtuous cycle.

The technique advocated by Benton, who used to work at the UK’s Atomic Weapons Establishment at Aldermaston, is not new. It was used during the Manhattan Project in the 1940s to ‘enrich’ uranium for the bomb that would fall on Hiroshima and end World War II. More than 99% of naturally occurring uranium is present as uranium-238, whereas the atomic bomb requires the rare uranium-235 isotope. The two isotopes had to be physically separated and this was done by creating a gas of uranium hexafluoride and forcing it through a semi-permeable membrane. Since uranium-235 is lighter, molecules of this isotopologue diffuse faster through the membrane and the gas that emerges is enriched in uranium-235. The process is repeated many times with a cascade of membranes until more than 90% of the gas contains uranium-235.

And the method is used by the gas industry to remove CO2, a common impurity in natural gas that lowers its heat value. But CO2 occurs at concentrations of 10-15% in natural gas; Benton is talking about enriching the CO2 in air from less than 0.04% (380 parts per million) to around 99% purity. “It’s perfectly possible to do it,” he says, although his calculations to date reveal frighteningly expensive pressures to force the air through. He is now planning to try the process under low pressure, by creating a vacuum at one end so that the pressure gradient is just 1 atmosphere, and using large, cheap membranes made from polyimide, silicon rubber or cellulose acetate, which he believes will keep the (renewable) energy costs down enough to make a synthetic fuel that is competitive in price to petrol.

“The UK uses 140,000 tonnes of liquid fuels per day, so we need to capture 434,600 tonnes of CO2 to make this. That means pumping 712 million tonnes of air through the membranes per day. On an industrial scale there is nothing unusual in pumping this much air, it’s perfectly possible,” he repeats frequently.

It’s not me that needs convincing, however. I don’t have the money to invest in trials of such schemes even if I wanted to.

I called up someone who should know whether it’s possible: Kang Li, a membrane diffusion expert at Imperial College London. He says the amount of energy required to force the air through the membranes wouldn’t make commercial sense. Even at the low pressure Benton plans, the membrane size would have to be scaled up by 10 or 20 times, Li says, rendering such modules uneconomical. “If you want to get CO2 for your synthetic fuels, there’s plenty available from biogas or through flue-gas removal,” he points out.

The engineers at the UK’s Centre for Alternative Technology in Wales also think that extracting CO2 from the air is not feasible and that there is no longer time to indulge such ideas. “These technologies are still at a very early stage and they won’t be ready in time. We have just two decades to reduce CO2 emissions and sucking it out of the atmosphere isn’t cheap enough,” Alex Randall, a renewables expert at the centre, told me. “The problem with these ideas is they capture the imagination of the public and policymakers; but we have to spend money on what is going to deliver real reductions first.”

UK climate minister Joan Ruddock would probably agree. Yesterday, she told a select committee: “The concern is that people who don’t want to enter into agreements that mean they have to reduce their emissions might see [radical technologies] as a means of doing nothing, of being able to say, ‘science will provide, there will be a way out’,” she said, “it could be used politically in that way which would be extremely unfortunate.”

The idea that troublesome CO2 might be physically removed from the air does have a seductive simplicity – ordinarily, the gas persists for at least 100 years. Add Richard Branson’s promise of $25 million to the first person who removes a billion tonnes of CO2 from the atmosphere every year for a decade, and the quest has become one of the hottest new fields of research, with potent of rivalry.

Klaus Lackner of Columbia University in New York is probably the best known of the CO2-from-air advocates and has put forward with several methods over the past five years. The technique he is currently advocating involves scrubbing CO2 from the air using ion-exchange, in a similar way to how a water softener works. I called up Lackner, who was visiting the new Kiel Earth Institute in Germany. He says that the amount of compression required for Benton’s diffusion scheme is likely to make it too expensive.

Benton is undeterred. “I’d need 33,000 megawatts a year for my process, which is 1,000 wind turbines. People will always laugh at the energy needed for the whole process, whatever it is. 16% of the energy we get from oil is used for refining, but it’s built into the final cost. It wouldn’t matter if that figure was 50%, so long as the price on the forecourt is affordable,” he says.

And of course that’s what it comes down to: how much are we willing to pay for energy that doesn’t cost the Earth?

What do you reckon – worth a try?


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