Without new breakthroughs, producing much more biofuel than we do today might be a net-negative for the climate—and would definitely raise our fuel prices.

When we burn fossil fuels, we take carbon that had once been trapped safely underground and release it into the atmosphere. But when we burn biofuels, we're simply recycling carbon through our atmospheric system: the carbon begins in the air, is absorbed by plants as they grow, and then goes back into the air when we burn fuel made from those plants. Burning biofuels, therefore, doesn’t increase the amount of CO2 in the atmosphere and has the potential to help slow climate change.

In 2005, Congress wanted to take advantage of these benefits, while also reducing dependence on foreign oil and supporting US farmers. They passed the Renewable Fuel Standard, which required U.S. gasoline manufacturers to include a certain amount of biofuels into the mix of commercial gasoline. Today, most gas sold at gas stations includes about 10 percent bioalcohol fuel. This is usually ethanol, which is produced from corn or other vegetable byproducts and is, by far, the most prevalent biofuel in the country.

But there are limits to how much biofuel vehicles can handle.

"We currently use as much biofuels as a supplement into the gasoline system as we possibly can, given our current vehicle industry," said Kristala Jones Prather, a chemical engineering professor at MIT. "Anything higher would radically change the way vehicle engines perform."

The 10 percent ethanol threshold for fuels is approaching what is known as the “blend wall”: the percentage of biofuels that can be used in a conventional vehicle engine without reducing its fuel economy. Many newer cars can run 15 percent ethanol in their engines, while older cars have a lower blend wall. Adding a higher percentage of ethanol beyond this point would cancel out any of the environmental benefits of using it in the first place, because vehicles would use more fuel. While some vehicles, even conventional ones, can run on 100 percent biofuel, they never run as efficiently as they would with petroleum.

Other types of biofuels also have drawbacks. Biodiesel, which is usually made from soybeans, could replace conventional diesel in areas like aviation or long-haul trucking, but, for now, producing it at scale is cost-prohibitive.

"The feedstock inputs you need for biodiesel are more expensive than petroleum is," Jones Prather said. "On top of that, the processes for producing the fuel aren't yet efficient enough so that you can produce it very cheaply."

These feedstocks for biofuels tend to include food crops like corn, soybeans and other vegetable oils, and because these have other uses as food for people and livestock, the prices tend to be higher than that of oil. And with today's technology for making biofuels, it's not even clear that producing them on a large scale would be a good thing for the climate. Conventional crops like corn or soybeans require massive amounts of land to produce. Cultivating extra crops for fuel drives changes in land use that can diminish or wipe out the environmental benefits of using biofuels in the first place.

This is because when farmers cut down forest or grassland to convert it to cropland, it can release more carbon than those crops will eventually sequester. Even when existing cropland is used to plant feedstock for biofuels, that means that land elsewhere will likely need to change to grow more food.

Work is underway on more advanced biofuels that could be made with non-food crops. Cellulosic biofuels, for example, can be made from things like wood, algae and grasses that could be harvested sustainably. While some of those studies show promise, their slow progress comes back, again, to economics.

"What we're lacking is sufficient advances in the science, technology and engineering to actually give us material that's going to be able to compete with what you can go get at your local ExxonMobil," Jones Prather said.

Thank you to Dawson Gunn of Fort Collins, Colorado, for the question. You can submit your own question to Ask MIT Climate here.

Published September 20, 2021.