(How) can one make fuel from waste products that are released when fuel is burned by a car?

Answer

Dear Benoit,

That’s an excellent idea and one of the great challenges of the 21st century. Let’s see how that’s possible.

Fuels are hydrocarbons made from petroleum. During combustion, mainly CO₂ and water free. Combustion also releases energy, which we use for driving. This means that the waste products, CO₂ and water, contain very little energy. So we have to put energy back into it to turn it into a fuel, similar to a battery that you have to recharge when it’s empty.

The whole earth is full of things that have been doing that for millions of years: plants take in CO₂ and water and use solar energy to convert it into biomass (eg wood) via photosynthesis. If we now use that biomass to make fuels, we have done what you suggest. This is called closing the loop, because we reuse waste materials. We therefore also call biofuels CO₂-neutral. In practice, however, biofuels are less CO₂-neutral than you might think. It often takes a lot of energy to make them, and the raw materials often come from countries such as Canada, so a lot of energy is used for transport. In addition, many plants grow too slowly to use them and there is competition with food crops, which causes food prices to rise.

That is why we are investigating an alternative at KU Leuven: artificial photosynthesis. We are trying to build a kind of artificial plant, which CO₂ and converts water into fuels using solar energy. By building a system with a higher efficiency than plants, we can produce fuels faster. Moreover, such an artificial system is less demanding than a living plant, and you could just put it on your roof: that way you get a solar panel that produces fuel. However, this system is still in the research phase, and dozens of scientists around the world are trying to make the system practically applicable.

So who knows, in the future you may be able to refuel your car with a panel on the roof…