Source: Michael Fielding, MEATINGPLACE, 8/30/16


A new study from University of Michigan researchers challenges the widely held assumption that biofuels such as ethanol and biodiesel are inherently carbon neutral.

Contrary to popular belief, the heat-trapping carbon dioxide gas emitted when biofuels are burned is not fully balanced by the CO2 uptake that occurs as the plants grow, according to research professor John DeCicco, a long-time critic of the biofuel industry, and his co-authors at the U-M Energy Institute. The study was underwritten by the American Petroleum Institute.

"This is the first study to carefully examine the carbon on farmland when biofuels are grown, instead of just making assumptions about it," DeCicco said in a news release. "When you look at what's actually happening on the land, you find that not enough carbon is being removed from the atmosphere to balance what's coming out of the tailpipe."

The use of liquid biofuels in the transportation sector has expanded over the past decade in response to policies such as the US Renewable Fuel Standard (RFS) and California's Low-Carbon Fuel Standard (LCFS). Those policies are based on the belief that biofuels are inherently carbon neutral, meaning that only production-related greenhouse gas emissions need to be tallied when comparing them to fossil fuels.

That assumption is embedded in the lifecycle analysis modeling approach used to write related policy
.. In other words, because plants absorb carbon dioxide as they grow, crops grown for biofuels should absorb the carbon dioxide that comes from burning the fuels they produce. Using that approach, crop-based biofuels such as corn ethanol and biodiesel offer at least modest net greenhouse gas reductions relative to petroleum fuels.

Field data for assessing the net carbon dioxide emission effect of biofuels has been available since the Renewable Fuel Standard was passed in 2005. DeCicco's team evaluated the data up to 2013, using the Annual Basis Carbon (ABC) accounting method he previously developed. It takes a circumscribed look at the changes in carbon flows directly associated with a vehicle-fuel system, and does not treat biofuels as inherently carbon neutral.

Instead, the ABC method tallies carbon dioxide emissions on the basis of chemistry in the specific locations where they occur. The system takes into account motor fuel consumption, fuel processing operations and resource inputs, including the use of cropland for biofuel feedstocks. Unlike lifecycle analysis, ABC accounting reflects the stock-and-flow nature of the carbon cycle, recognizing that changes in the atmospheric stock depend on both inflows and outflows.

The researchers found that the gains in carbon dioxide uptake by feedstock, such as corn, were enough to offset biofuel-related biogenic emissions by only 37 percent, rather than 100 percent, during the period 2005 to 2013.

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