Energy Blog: Biofuels Under the Microscope

Energy Blog: Biofuels Under the Microscope

Biofuels are controversial. Some recent research results haven't settled the issue.
As a pathway to reducing carbon emissions, biofuels have always been tantalizing. They are by definition net-zero, since all the carbon from in their feedstocks—plant sugars or oils or animal fat—were pulled recently from the atmosphere. And some classes of biofuels, such as renewable diesel or sustainable aviation fuel (SAF), are drop-in replacements for petroleum-based fuels.

But biofuels have always been controversial. While the idea of growing fuel sustainably rather than extracting it from a depleting reservoir is attractive, many experts who have studied the corn ethanol industry in the United States have determined that the energy inputs—from diesel-burning tractors and the facilities that ferment the corn to make alcohol—can approach the energy content of the fuel produced.

Recently, another factor has started to weigh on the promise of biofuels: the efficiency at which plants convert sunlight to biomass. When a patch of ground with a crop on it was the only type of farm, the effectiveness of photosynthesis wasn’t questioned. Today, that ground can be used to grow biofuels or support photovoltaic panels in a solar “farm.” With PV cells that can convert around 20 percent of sunlight to electricity, solar farms generate a couple orders of magnitude more energy per acre each year than a biofuels farm can.

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“It takes about 100 acres worth of biofuels to generate as much energy as a single acre of solar panels,” wrote Michael Grunwald in the New York Times last year. “Worldwide, a land mass larger than California was used to grow under 4 percent of transportation fuel in 2020.”

Even so, biofuels are seen as a viable alternative fuel, especially in applications such as heavy machinery and aviation where battery electric vehicles are not considered practical as of yet.

In California, renewable diesel—a biofuel that is chemically identical to petroleum and can produced in conventional petroleum refineries that have special equipment—has become a growing market, with 40 million barrels consumed in 2023. That’s about a fourfold increase since 2018. Last year, the California Air Resources Board announced that half the diesel consumption in the state had been replaced by clean alternatives, much of it renewable diesel and biodiesel.

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Due to demand from California and elsewhere, renewable diesel production has increased rapidly in just the past couple years. Between 2021 and 2023, renewable diesel production tripled, while biodiesel, which typically has to be blended in order to be used in conventional diesel engines, fell by 13 percent. Ethanol still has the largest share of the biofuels market, with 78 percent of U.S. biofuel production capacity, according to the EIA. Biodiesel accounted for 9 percent and renewable diesel and other biofuels accounted for 13 percent.

And SAFs got a boost this month with the release of a report that an experiment involving running an Airbus A350 on the biofuel produced less soot and fewer contrail-forming ice crystals than jets burning conventional fuel. According to Airbus, it was the first experiment involving use of 100 percent SAF in both engines of a commercial aircraft. The reduction in soot and ice crystals is important because those byproducts of high-altitude aviation are implicated in global warming, meaning that the benefits of SAFs go beyond simple carbon emissions reductions.

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Other recent pollution news with regards to biofuels wasn’t so positive.

For instance, despite the hopes that biofuels would lead to energy independence, the United States imports about one-quarter of the biodiesel consumed. Much of the fuel comes from Europe, where prices are lower than they are in the U.S.

In addition, this month, the nonprofit Environmental Integrity Project released a report on air pollution facilities that produce ethanol and other biofuels in the United States. In 2022, the report states, biofuels manufacturing facilities emitted some 12.9 million pounds of hazardous air pollution, not much less than the 14.5 million pounds released by all U.S. oil refineries.

According to “Farm to Fumes: Hazardous Air Pollution from Biofuels Production,” one of the causes for the pollution is an exemption from emissions regulations. “Ethanol manufacturing plants enjoy some exemptions from air pollution permitting requirements, making it easier for companies to expand or build new facilities without installing or upgrading pollution controls,” the report states. “In 2007, EPA removed corn-based ethanol plants from a list of industrial facilities subject to more stringent emission thresholds under the Clean Air Act.”

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The group makes a series of recommendations for reining in emissions from biofuels facilities, including better—and more accurate—monitoring of emissions from the plants and tougher penalties for violations. “So far, much of the commentary surrounding biofuels has centered around greenhouse gas emissions,” the group concluded. “Often overlooked are their toxic emissions and lack of compliance with air pollution laws. Biofuel plants are worse than petroleum refineries when it comes to emitting the hazardous air pollutants acetaldehyde, acrolein, formaldehyde, and hexane.”

In spite of decades of production, there is still no clear consensus of whether biofuels are a boon or a boondoggle. They might well be a bit of both: a good replacement for fossil fuels in applications where nothing else works well, but not a one-for-one substitute for the outsized role petroleum plays in the modern economy.

Jeffrey Winters is editor in chief of Mechanical Engineering magazine.

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