Making diesel out of thin air sounds like something from science fiction.
But small companies in Germany and Canada are doing precisely this - capturing carbon dioxide (CO2) and finding ways to sell it.
German company Sunfire produced its first batches of so-called e-diesel in April. Federal Minister of Education and Research, Johanna Wanka, put a few litres in her car, to celebrate.
And the Canadian company Carbon Engineering has just built a pilot plant to suck one to two tonnes of carbon dioxide from the air daily, turning it into 500 litres of diesel.
The process requires electricity, but if the start-ups use renewable electricity they can produce diesel that is carbon neutral.
In other words, burning it in your car only returns to the atmosphere the CO2 removed in the first place.Fossil fuels, on the other hand, add to the total amount of CO2 in the atmosphere.
And halting the growth of CO2 and other greenhouse gases has become of paramount importance given the many threats posed by climate change.
The concentration of CO2 in the air reached 400 parts per million in 2012-2013 - the highest since scientific measurements began. And the year July 2014-June 2015 was the warmest on record, says the US National Oceanic and Atmospheric Administration.
Elemental chemistry
The chemistry to make fuel from CO2 isn't especially hard - split water into hydrogen and oxygen through electrolysis, add the hydrogen to CO2 to make carbon monoxide and water, then bung in more hydrogen to build up hydrocarbon chains.
This last bit's called the Fischer-Tropsch process, and dates back to the 1920s.
But it's the technologies capturing the CO2 straight from the air that are new and now becoming cheap enough to be viable.
The biggest technological challenges have centred on the high-temperature furnaces, says Adrian Corless, chief executive of Carbon Engineering.He says there is still "a month of hard work" to get these to work as the company would like.
But these also have been his company's chief innovation, he says - precipitating captured CO2 into solid calcium carbonate pellets that can be easily washed and dried.
These pellets are then heated to 800-900C, whereupon they release a pure CO2 stream. As a residue, they leave calcium oxide which, handily, can be fed back in to the first air capture stage.
