Recycling carbon dioxide into petrol

A new reactor could make chemically recycling carbon dioxide back into petrol a worthwhile endeavour, US scientists say.

Researchers at Sandia National Laboratories, New Mexico, are to test a prototype device this spring, which will use concentrated solar energy to drive chemical reactions that split carbon dioxide molecules to get carbon monoxide. The same system was originally designed to split water to form hydrogen; and these two products can then be combined to synthesise liquid hydrocarbon fuels - such as methanol or petrol.

Splitting the stable carbon dioxide molecule is so tough that many researchers think the most economic course of action is simply to bury the greenhouse gas underground. And solar plants usually generate electricity, rather than split CO₂.

But the Sandia team led by Jim Miller, Nathan Siegel and Richard Diver, who work on the 'Sunshine to Petrol' (S2P) project, think their device's chemical reactions are efficient enough to make it a worthwhile way of producing liquid fuels from CO₂ . Ellen Stechel, manager of Sandia's fuel and energy transitions department, explained to Chemistry World that the ultimate aim is to have a series of solar-powered reactors, each collecting around 22kg of carbon dioxide and 18kg of water daily, and churning out some 2.5 gallons of petrol, based on target conversion efficiencies. 'Liquid fuels can be stored in trucks or piped using existing infrastructure,' Stechel pointed out.

Lord of the rings?

A complete demonstration system is three to five years away, said Stechel, and to prove its commercial value will take much longer. But one key sticking point - CO₂ splitting - is what the S2P researchers hope to crack.

The Sandia reactor consists of rotating rings, made of a cobalt-doped ferrite (Fe₃O₄) ceramic. Concentrated sunlight is directed onto a ring, heating it up to around 1500°C and driving off oxygen gas. The reduced material (FeO) rotates into a second chamber containing carbon dioxide, from which it takes back oxygen at a lower temperature, leaving carbon monoxide behind. It then cycles back into the sunlight again, so that the CO₂ splitting should be a continuous process.

This simple chemical cycle also splits water into hydrogen and oxygen. But it is only plausible on a larger scale because of an engineering trick which conserves the heat entering the system. The reactor holds a series of stacked rings rotating in opposite directions; so that a heated ring moving out of the sun will heat up cooler rings about to face the sun. This arrangement limits the total energy input required.

So far, Stechel said, the researchers have shown this works for a batch process, but need to speed up their reactions to allow the more efficient series of continuous cycles. The final system, christened the Counter Rotating Ring Receiver Reactor Recuperator (CR5), should be about four times larger than the beer-keg sized prototype.

The Sandia team reckon their system is one of the most promising approaches to splitting CO₂ for fuel. They have a few competitors, such as the company Los Alamos Renewable Energy (LARE), who claim to use solar power to directly split CO₂ at very high temperatures; chemists who are taking catalytic approaches to split CO₂ with hydrogen; or the alternative of electrolytic splitting. But if splitting CO₂ is worthwhile at all, 'it's hard to imagine anything that will show better thermodynamics or kinetics,' said Stechel.

Richard Van Noorden

S2P: Sunlight to Petrol

Researchers at Sandia National Laboratories in New Mexico have found a way of using sunlight to recycle carbon dioxide and produce fuels like methanol or gasoline.

The Sunlight to Petrol, or S2P, project essentially reverses the combustion process, recovering the building blocks of hydrocarbons. They can then be used to synthesize liquid fuels like methanol or gasoline. Researchers said the technology already works and could help reduce greenhouse-gas emissions, although large-scale implementation could be a decade or more away.

“This is about closing the cycle,” said Ellen Stechel, manager of Sandia’s Fuels and Energy Transitions department. “Right now our fossil fuels are emitting CO2. This would help us manage and reduce our emissions and put us on the path to a carbon-neutral energy system.”

The idea of recycling carbon dioxide is not new, but has generally been considered too difficult and expensive to be worth the effort. But with oil prices exceeding $100 per barrel and concerns about global warming mounting, researchers are increasingly motivated to investigate carbon recycling. Los Alamos Renewable Energy, for example, has developed a method of using CO2 to generate electricity and fuel.

S2P uses a solar reactor called the Counter-Rotating Ring Receiver Reactor Recuperator, or CR5, to divide carbon dioxide into carbon monoxide and oxygen.

“It’s a heat engine,” Stechel said. “But instead of doing mechanical work, it does chemical work.”

Lab experiments have shown that the process works, Stechel said. The researchers hope to finish a prototype by April.

The prototype will be about the size and shape of a beer keg. It will contain 14 cobalt ferrite rings, each about one foot in diameter and turning at one revolution per minute. An 88-square meter solar furnace will blast sunlight into the unit, heating the rings to about 2,600 degrees Fahrenheit. At that temperature, cobalt ferrite releases oxygen. When the rings cool to about 2,000 degrees, they’re exposed to CO2.

Since the cobalt ferrite is now missing oxygen, it snatches some from the CO2, leaving behind just carbon monoxide — a building block for making hydrocarbons — that can then be used to make methanol or gasoline. And with the cobalt ferrite restored to its original state, the device is ready for another cycle.

Fuels like methanol and gasoline are combinations of hydrogen and carbon that are relatively easy to synthesize, Stechel said. Methanol is the easiest, and that’s where they will start, but gasoline could also be made.

However, creating a powerful and efficient solar power system to get the cobalt ferrite hot enough remains a major hurdle in implementing the technology on a large scale, said Aldo Steinfeld, head of the Solar Technology Laboratory at the Paul Scherrer Institut in Switzerland, in an e-mail.

He and Stechel said the technology could be 15 to 20 years from viability on an industrial scale.
The Sandia team originally developed the CR5 to generate hydrogen for use in fuel cells. If the device’s rings are exposed to steam instead of carbon dioxide, they generate hydrogen. But the scientists switched to carbon monoxide, so the fuels they produce would be compatible with existing infrastructure.

The Sandia team envisions a day when CR5s are installed in large numbers at coal-fired power plants. Each of them could reclaim 45 pounds of carbon dioxide from the air daily and produce enough carbon monoxide to make 2.5 gallons of fuel. Coupling the CR5 with CO2 reclamation and sequestration technology, which several scientists already are pursuing, could make liquid hydrocarbons a renewable fuel, Stechel said.

“It’s certainly technology that can be developed,” she said. “It’s not that it’s challenging, it’s that the ideas aren’t economically viable yet.” via WIRED