Air, can we have our carbon back?

Sucking carbon dioxide out of the atmosphere to reduce greenhouse gas emissions is outlandishly expensive. But a US scientist who has just worked out how to improve its efficiency predicts it will be necessary before the end of the century.   

Frank Zeman, of Columbia University, New York, has calculated how to drastically cut the energy demands of capturing carbon dioxide from air by tweaking the set of chemical reactions usually suggested for the feat.   

Zeman readily concedes that taking carbon dioxide out of the atmosphere can’t rival cheaper technologies for cutting greenhouse gas levels: for example, carbon capture and storage (CCS) from power plant emission streams, which are much richer in carbon dioxide. But even if CCS was deployed at all large industrial facilities, more than half of global greenhouse gas emissions would remain, Zeman said - thanks to the sum of emissions from cars, houses and planes. ’The long-term goal is to start removing these around 2050 to 2070,’ he said.   

In CCS, amine solvents or porous zeolites and membranes are commonly used to separate carbon dioxide from a mixture of power plant exhaust gases. But the concentration of carbon dioxide in the atmosphere is so low - just 380 parts per million - that only chemical reactions can capture the dilute trace.   

The reactions Zeman adapted trap carbon dioxide in artificial limestone (calcium carbonate) - an approach popularised a decade ago by Klaus Lackner, also at Columbia University. Huge volumes of air are blown through sodium hydroxide solution, which reacts with the carbon dioxide to form dissolved sodium carbonate. Mixing this with slaked lime (Ca(OH)₂) precipitates out flakes of limestone (CaCO₃). Finally the limestone is decomposed in a hot (900?C) kiln to release a pure stream of CO₂ gas.   

The problem with this idea is that drying out and decomposing the limestone flakes is a very energy intensive process. Zeman’s central trick is to get this heat from other chemical reactions (recycling slaked lime). By coupling two processes together, he calculates that the huge amounts of electricity required would be drastically reduced.   

’It’s the most effective system we’ve thought of,’ he said, while admitting that some assumptions - such as the amount of moisture that has to be wrung from the limestone flakes - were optimistic. He hopes to start a field project in May next year.   

Putting it to the test

For every tonne of carbon dioxide extracted by the new process, up to 0.6 tonnes would be created if it were run on coal-sourced electricity - mainly due to the huge volumes of air that have to be pushed around. But solar or nuclear sources of electricity would cut that carbon penalty.   

Yet Zeman’s ideas are not the only options. Lackner has set up his own company, Global Research Technologies, which in April announced a successful industrial trial for carbon capture from air. GRT president Allen Wright told Chemistry World that though his team began with the hydroxide/carbonate chemistry, since it was so well known, they discarded it early on. GRT instead converts sodium carbonate to sodium bicarbonate. ’Our current technology uses significantly less energy,’ Wright said.   

The key to success, Zeman and Wright agreed, is to clearly set out the baseline number for energy demand and the cost of the technology. Realistically, carbon capture from air will be doing well to creep below US$200 per tonne of CO₂ captured, and that doesn’t include the extra costs of transporting and burying the pure CO₂ gas stream created. By comparison, the price tag for CCS is currently an estimated $80, and expected to halve. ’Still, the difficulty is not about technical feasibility,’ Zeman said. ’It is about cost, and how to squeeze the energy usage down.’ 

Stuart Haszeldine, an expert in CCS from the University of Edinburgh, UK, said he was entirely in favour of Lackner’s efforts, and incentives such as entrepreneur Richard Branson’s ’Earth Challenge’, which promises to award $25 million to the best way of removing atmospheric greenhouse gases. ’Most countries are so slow at introducing CCS to powerplants that carbon dioxide levels may be over 400ppm by 2030,’ Haszeldine said. If that’s the case, governments may be forced to implement these schemes despite the high costs. 

For the moment, there are much better ways of cutting greenhouse gas levels - not the least of which is improving energy efficiency and reducing demand. But for supporters of carbon capture from air, there’s nothing like a drastic technological fix to quicken the heartbeat - and to raise awareness of the gravity of the problem.   

Richard Van Noorden

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