The big question mark over gasoline from air

In a shipping container on a British industrial park, not far from where George Stephenson launched the world's first steam railway in 1825, another transport revolution might be beginning. Every day the machinery inside produces half a litre of purified gasoline. It sounds humdrum until you realise one thing: the only raw material used is air.

Last week, Air Fuel Synthesis (AFS), a company in Stockton, UK, revealed the first successful demonstration of an idea that dates back to the oil crisis of the 1970s: that carbon, hydrogen and oxygen can be plucked from carbon dioxide and water in air to be converted into methanol and then morphed into gasoline.

However, amidst the headlines, some media coverage overlooked the key point: the energy efficiency of the process has yet to be demonstrated. This matters because the technique uses electricity for key stages. It should not require more energy input than is gleaned from burning the fuel it produces.

The big idea is to capture atmospheric CO₂ and turn it into fuel so there's no net increase in CO₂ from cars and trucks fuelled by such gasoline. As long as the process is powered by renewable electricity sources such as solar, wind or tidal, using the gasoline is carbon neutral.

Snagging carbon dioxide

The AFS plant comprises a CO₂ capture unit in one shipping container, with a methanol reactor and miniature gasoline refining system in another. Air is blown into a sodium hydroxide mist, snagging CO₂ as sodium carbonate. A condenser collects water from the same air. To make methanol ? formula CH3OH ? hydrogen is generated by electrolysing the water while the carbon and oxygen come from electrolysing the sodium carbonate. The methanol is then converted to gasoline.

Following tests over the last three months, AFS chief executive Peter Harrison says the demonstrator reliably produces half-a-litre of gasoline a day. Peter Edwards ,an inorganic chemist at the University of Oxford whose team is working with a Saudi firm on similar ideas, is impressed: "I take my hat off to Air Fuel Synthesis. They have taken a concept that has been around for 35 years and gotten the process going."

But he points out the demonstrator, funded with a ?1.2 million, two-year investment from private backers, was built to make gasoline, "not to prove its net efficiency or energy balances".

Douglas Stephan, a chemist at the University of Toronto, Canada, also researching fuel production from CO₂, describes AFS's demonstrator as "an engineering tour-de-force". But he too warns efficiency is the key. "Until a detailed assessment of the energy efficiency is enunciated, I would remain sceptical about this technology," he says.

Andrew Bocarsly, chief science advisor at Liquid Light Inc, a company in Monmouth Junction, New Jersey, aiming to synthesise chemicals like methanol from CO2, points out that many researchers worldwide have so far failed to find cost-effective and efficient ways to split hydrogen from water.

Going to need a bigger plant

"I do wonder about the cost efficiency of their chemical conversion processes," he says, noting energy is required to back convert carbonate to gaseous CO₂, to liberate hydrogen from water, to convert the hydrogen and CO₂ to methanol and to transform methanol to gasoline.

AFS says demonstrating efficiency will have to wait for a bigger plant, which will fit into three shipping containers that can be dropped anywhere fuel is needed and produce 1200 litres of gasoline a day. Harrison says motorsport venues, keen to reduce their fossil fuel dependence, and some remote islands have expressed an interest in these ?5 million units. "The demonstrator has given us the confidence that this next level of gasoline plant will be efficient enough," says AFS marketing manager Graham Truscott.

Harrison says the ultimate goal is to build refinery-sized plants that could compete with oil ? but he says they could cost ?10 billion and need serious government aid. That in turn would need serious proof of energy efficiency. Bocarsly adds: "This issue will be the test for commercialisation."

There's one more factor to consider, says Edwards: "The efficiency of this process would also have to be balanced against the cost of alternative measures like burying or dumping CO₂ underground."

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