Both these applications would still face competition from existing industrial sources of carbon dioxide, however, and consumer pressure may be required to overcome the economic barriers.

"Energetically, it is much more difficult to separate carbon dioxide from the air than from flue gas – in fact, it is on the order of 300 times more difficult," Jennifer Wilcox of Colorado School of Mines told environmentalresearchweb. "However, it will take a portfolio of options to prevent 2°C of warming by 2100, if the indications from climate models are correct."

Wilcox stresses that the first order of business is to minimize carbon dioxide emissions in the first place. Solutions here include a wider adoption of renewable energy and conventional approaches to carbon dioxide capture and storage. Augmenting this strategy with direct air capture is likely to remain a tough proposition, but the group’s analysis provides food for thought.

Wilcox is in favour of putting more power in the hands of the consumer, for example by offering fuel options made by reacting carbon dioxide extracted from the air with hydrogen derived from renewable sources via electrolysis. "Some consumers may be willing to pay the additional cost compared with conventional gasoline, and I think this could be an interesting space to explore," she said.

Looking at other options, the know-how exists today on how to make oil from microalgae, which absorb low concentrations of carbon dioxide from the air, or potentially from the product of direct air capture, to produce biomass. As the scientists point out, however, the process is expensive and historically microalgae cultivation has been limited to high-value nutraceutical products, currently a very small market.

"Algae may play a niche role, but in general there is no one silver bullet," said Wilcox. Even if algae-derived oil or synthetic fuel proved to be popular with consumers, a reliable way of storing the carbon dioxide gas emitted on burning would be needed to maximize the environmental benefit.

Considering enhanced oil recovery, which sees carbon dioxide injected into depleted oil fields in order to decrease the viscosity of the remaining oil and ease its retrieval, the scientists comment that it would be necessary to locate the carbon dioxide direct capture plant near the reservoir to avoid prohibitive pipeline transport costs.

Wilcox and colleagues reported their analysis in Environmental Research Letters (ERL) .

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