Biofuels have been around longer than cars have, but cheap gasoline and diesel have long kept them on the fringe. Spikes in oil prices, and now global efforts to stave off the worst effects of climate change, have lent new urgency to the search for clean, renewable fuels.
Our road travel, flights, and shipping account for nearly a quarter of the world's greenhouse gas emissions, and transportation today remains heavily dependent on fossil fuels. The idea behind biofuel is to replace traditional fuels with those made from plant material or other feedstocks that are renewable.
But the concept of using farmland to produce fuel instead of food comes with its own challenges, and solutions that rely on waste or other feedstocks haven't yet been able to compete on price and scale with conventional fuels. Global biofuel output needs to triple by 2030 in order to meet the International Energy Agency's targets for sustainable growth.
Biofuel types and uses
There are various ways of making biofuels, but they generally use chemical reactions, fermentation, and heat to break down the starches, sugars, and other molecules in plants. The resulting products are then refined to produce a fuel that cars or other vehicles can use.
Much of the gasoline in the United States contains one of the most common biofuels: ethanol. Made by fermenting the sugars from plants such as corn or sugarcane, ethanol contains oxygen that helps a car's engine burn fuel more efficiently, reducing air pollution. In the U.S., where most ethanol is derived from corn, fuel is typically 90 percent gasoline and 10 percent ethanol. In Brazil—the second-largest ethanol producer behind the U.S.—fuel contains up to 27 percent ethanol, with sugarcane as the main feedstock.
Alternatives to diesel fuel include biodiesel and renewable diesel. Biodiesel, derived from fats such as vegetable oil, animal fat, and recycled cooking grease, can be blended with petroleum-based diesel. Some buses, trucks, and military vehicles in the U.S. run on fuel blends with up to 20 percent biodiesel, but pure biodiesel can be compromised by cold weather and may cause problems in older vehicles. Renewable diesel, a chemically different product that can be derived from fats or plant-based waste, is considered a "drop-in" fuel that does not need to be blended with conventional diesel.
Other types of plant-based fuel have been created for aviation and shipping. More than 150,000 flights have used biofuel, but the amount of aviation biofuel produced in 2018 accounted for less than 0.1 percent of total consumption. In shipping, too, adoption of biofuel is at levels far below the 2030 targets set by the International Energy Agency.
Renewable natural gas, or biomethane, is another fuel that potentially could be used not only for transportation but also heat and electricity generation. Gas can be captured from landfills, livestock operations, wastewater, or other sources. This captured biogas then must be refined further to remove water, carbon dioxide, and other elements so that it meets the standard needed to fuel natural-gas-powered vehicles.
What is biofuel made from?
A variety of materials, or feedstocks, can be used to make biofuels. Though corn and sugarcane are well-established ethanol feedstocks, the process of growing the crops, making fertilizers and pesticides, and processing the plants into fuel consumes a lot of energy—so much energy that there is debate about whether ethanol from corn actually provides enough of an environmental benefit to be worth the investment.
So scientists and startups are exploring other materials that have the potential to serve as fuel without the accompanying concerns about food supply and environmental impact. Cellulosic ethanol, for example, uses corn stover, wood waste, or other plant material that would not be used otherwise. Other potential biofuel feedstocks include grasses, algae, animal waste, cooking grease, and wastewater sludge, but research continues to find the most efficient and cost-effective ways to transform them into usable fuel.