Considering the costs of building new infrastructure and adapting to unfamiliar power sources, we aren’t likely to stop using fossil fuels anytime soon. What’s the next best solution? Make existing fuels greener and renewable.
That’s the idea behind new work from scientists at Imperial College London and the University of Turku in Finland, who aim to eventually coax photosynthetic bacteria to turn sunlight into propane gas. The technology has a long way to go before it’s commercially viable. But as a first step, the team has managed to trick E. coli, a bacteria found in our digestive system, into creating small amounts of engine-ready propane.
Traditionally, propane is created as a by-product of natural gas and petroleum processing. It's removed from natural gas to make transport along pressurized pipelines safer, and oil refineries produce it when they break down petroleum into either gasoline or heating oil.
In a three-step process, the scientists used enzymes to first free up fatty acids in E. coli that are normally used in the creation of cell membranes. One of these, butyric acid, was then converted with another enzyme into butyraldehyde—a derivative of butane. Finally, the team transformed the butyraldehyde into propane. Stimulating the converting enzyme with electrons enhances the process, the team found.
Recently described in the journal Nature Communications, the project is in its early stages. But Patrik R. Jones, one of the paper’s authors, says the method is simpler than similar attempts at creating fuel with living organisms. Yeast or bacteria play a role in producing ethanol from sugar or corn, and engineered photosynthetic bacteria create diesel from crops as well. Ethanol is now commonly added to gasoline in the United States, thanks mostly to government subsidies and incentives. But bacteria-derived biodiesel hasn't yet seen widespread use, due largely to continued issues with costs and efficiency.
“In the case of [photosynthetic] biodiesel, there are many steps in the process, and each of these steps has a penalty in terms of efficiency,” says Jones. “If we could cut down the number of steps, at least theoretically, we could then have a more efficient process.”
The focus on propane as opposed to other fuels also simplifies the process, because propane separates from the organisms' cells easily due to its compact chemical structure. Ethanol, which can be created from corn, sugar and other crops, needs to be physically separated from water in a process that is energy intensive. Current methods for harvesting diesel fuel from algae involve breaking open their cells and, in doing so, killing the organisms that are making the fuel. With propane, the fuel can be separated without destroying E. coli.
Propane is simple to collect as a gas, and yet easier to safely store than hydrogen, which is very dangerous as a gas, especially when mixed with air. It was also chosen, Jones says, because it's easy to liquefy for transportation, and it's compatible with the existing infrastructure. Propane is mostly associated with outdoor grills in the United States, but it’s also used to power forklifts and boat motors. Cars can even be converted to run on propane; the process is fairly common in the United Kingdom, where gas prices are much higher than in the United States.
The team is using E. coli at this stage because it’s simple to work with, Jones says. But eventually, the researchers hope to transplant the process from E. coli into photosynthetic bacteria so that sunlight provides the energy to power the cells, rather than the diet of nutrients that E. coli requires. This will again cut down the number of steps in the process, but there’s a lot of work left to be done before the scientists get to that point.
“Only theoretically perfect or near-theoretically perfect systems will ever have a chance of being commercialized,” says Jones. “That’s why it’s important to try and reach [a process] that works as well as possible.” At the moment, Jones estimates they’ll have to produce 1,000 to 5,000 times more fuel from their process before industry will show an interest. And from that point, more engineering and refinement would have to take place before it could be commercially viable as an alternative to existing fossil fuels.
“Some issues lie in the enzymes we use,” says Jones. “So there will need to be some search for alternative enzymes, or improvement of the enzymes we have, and these will be big projects on their own.”
It’s clear that we won’t be driving cars or grilling burgers using propane produced by bacteria and the sun anytime soon. But in an Imperial College London article, Jones said that he hopes the process will become commercially viable in the next 5 to 10 years.
Even if that estimate is generous, solar-powered propane production may be ready in time to help speed up the switch from dirty fuels to more environmentally friendly alternatives.