In a grey building squeezed in an industrial corner of Brooklyn, New York, shows me a jar full of black metallic pellets. 鈥淭his is the special sauce,鈥� he says. They don鈥檛 look like much, but he says they could help produce billions of litres of fossil-free liquid fuel.
Sheehan鈥檚 start-up, Air Company, is one of a growing set of manufacturers trying to use captured CO2 to replace products now made with fossil fuels, which can help to reduce emissions. But Sheehan says his company鈥檚 process is simpler and more energy efficient than competitors鈥� thanks to rethinking a century-old process to turn CO2 into fuel.
Next to us, a sleek system of tubes and pipes fills the room. On one side is a tank of carbon dioxide the size of a van and two boxy electrolysers, which make hydrogen using clean electricity to split water molecules. The hydrogen and CO2 can react with each other thanks to a metal catalyst 鈥� the aforementioned 鈥渟pecial sauce鈥�. The resulting liquid, after a refining step, flows into two tanks labelled AROMATICS and PARAFFINS. These are 鈥渢he building blocks for almost all fuels,鈥� says Sheehan.
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We are standing in the company鈥檚 largest plant yet, capable of producing a few thousand litres of liquid fuels each year, most of which goes to the US military to be tested in aircraft. Sheehan says the facility is the blueprint for additional plants under development that could produce millions of litres a year. The company is also working on a NASA grant that might one day help astronauts make from Mars鈥檚 CO2-rich atmosphere.
Here on Earth the aim is to reduce emissions. If the fuel manufacturing process uses clean electricity and CO2 captured from the atmosphere, the result will be near carbon neutral. If CO2 is captured from a source of emissions, the fuels won鈥檛 be carbon neutral, but the product might still replace fossil fuels that emit even more CO2 elsewhere.
Sustainable aviation fuels that can power existing jets are in especially high demand, and the shipping industry is also seeking cleaner sources of power, such as e-methanol. However, such synthetic fuels are expensive and require lots of energy to produce, most of which goes towards making hydrogen.
Normally, this hydrogen is first used to convert CO2 into carbon monoxide (CO) in a reactor kept at high temperature and pressure. The CO and more hydrogen are then combined to make syngas, which is finally converted to hydrocarbon fuels using a metal catalyst in a second reactor. This last step is a mainstay of industrial chemistry known as the Fischer-Tropsch process, invented in the 1920s.
Sheehan鈥檚 company squeezes all this activity into a single reactor by directly combining CO2 with hydrogen, along with a solid catalyst that enables them to react under mild conditions to make hydrocarbons; a different catalyst is used to make ethanol. He says this simplifies the process and uses around five times less energy per litre of liquid fuel, compared with the conventional operation.
The composition of the catalysts is secret, and Air Company hasn鈥檛 published enough information for outside researchers to assess its energy efficiency claims. But Sheehan sees the process replacing the one that relies on conventional Fischer-Tropsch. 鈥淚t鈥檚 about time we innovated on this 100-year-old technology,鈥� he says.
Air Company isn鈥檛 alone in rethinking how fuels are made from carbon. A California-based company called Twelve, which is constructing a large jet fuel plant in Washington state, produces its CO with a CO2 electrolyser, rather than using hydrogen. Illinois-based LanzaTech just a plant in Soperton, Georgia, to make jet fuel out of ethanol made from waste.
These synthetic fuels could offset emissions, but making them will require lots of clean energy that could reduce emissions more if used elsewhere, says at Pacific Northwest National Laboratory in Washington state. Based on what Air Company has , he is sceptical that their process saves much energy.
at New York University, who is collaborating with Sheehan on the rocket fuel project, also doubts there are large energy efficiency benefits. 鈥淵ou still need the same number of hydrogen molecules,鈥� he says.
But he says the process with just one reactor could have the advantage of simplicity. That could lower the barriers to making fuel from CO2, whether in Brooklyn or on Mars.
