A new study led by Colorado State University predicts significant climate benefits stemming from the use of advanced biofuel technologies. Accounting for all of the carbon flows in biofuel systems and comparing them to those in grasslands and forests, the team found that there are clear strategies for biofuels to have a net carbon benefit. The paper is published in Proceedings of the National Academy of Sciences (PNAS).

The climate benefits of cellulosic biofuels have been challenged based on carbon debt, opportunity costs, and indirect land use change, prompting calls for withdrawing support for research and development. Using a quantitative ecosystem modeling approach, which explicitly differentiates primary production, ecosystem carbon balance, and biomass harvest, we show that none of these arguments preclude cellulosic biofuels from realizing greenhouse gas mitigation.

Our assessment illustrates how deliberate land use choices support the climate performance of current-day cellulosic ethanol technology and how technological advancements and carbon capture and storage addition could produce several times the climate mitigation potential of competing land-based biological mitigation schemes. These results affirm the climate mitigation logic of biofuels, consistent with their prominent role in many climate stabilization scenarios.

—Field et al.

This is one of the first studies to look at both current and future carbon-negative biofuels.

John Field, research scientist at the Natural Resource Ecology Labat CSU, said that it has been a challenge for the biofuel industry to demonstrate commercial viability for cellulosic biofuels, created using nonedible parts of plants. Switchgrass, a native grass that grows in many parts of North America, is a leading candidate for the sustainable production of plant material.

The research team used modeling to simulate switchgrass cultivation, cellulosic biofuel production and carbon capture and storage, tracking ecosystem and carbon flows. Scientists then compared this modeling to alternative ways to store carbon on the land, including growing forest or grassland.

Carbon capture and storage technology is being used by at least one facility in Illinois that is processing corn as a conventional biofuel to create ethanol, but these systems are not yet widespread. As part of the study, researchers created models to simulate what this would look like at a cellulosic biofuel refinery.

What we found is that around half of the carbon in the switchgrass that comes into the refinery becomes a byproduct that would be available for carbon capture and storage.

—John Field

The resulting byproduct streams of high-purity carbon dioxide would not require much separation or clean-up before being stored underground.

The research team analyzed three contrasting US case studies and found that on land where farmers or land managers were transitioning out of growing crops or maintaining pastures for grazing, cultivating switchgrass for cellulosic ethanol production had a per-hectare mitigation potential comparable to reforestation and several-fold greater than grassland restoration. (A hectare is about two-and-a-half times the size of an average football field.)

Using switchgrass can be particularly helpful in parts of the country where planting more trees is not an option.

In the Great Plains, prairie is the more natural cover. Those systems don’t suck up as much carbon as a forest system does. If you start putting biofuels in the mix, they have two-and-a-half times the carbon benefits over grasslands. If you’re in an area where grassland would be the native cover, there’s a clear advantage to using biofuels.

—John Field

Field said that the team’s motivation for the study comes on the heels of several prominent critiques of biofuels.

This analysis shows a quantitatively reasoned case as to why the biofuel industry should advance, not simply as a means to provide a truly renewable source of biofuel but—when combined with carbon capture and storage—a means to actually remove carbon dioxide from the atmosphere at scale and in a viable manner.

—Stephen Long, a co-author and the Stanley O. Ikenberry Chair Professor of Plant Biology and Crop Sciences at the University of Illinois

Moving forward, the research team hopes to expand on its modeling, scaling it up nationally rather than looking at a few specific sites across the country.

This research was funded in part by the National Institute of Food and Agriculture - US Department of Agriculture, the US Department of Energy via the Center for Bioenergy Innovation, and the São Paulo Research Foundation in Brazil.

Additional study co-authors including Tom Richard and Erica Smithwick (The Pennsylvania State University), Hao Cai and Michael Wang (Argonne National Laboratory), Mark Laser (Dartmouth College), David LeBauer (University of Arizona), Stephen Long (University of Illinois at Urbana-Champaign, Lancaster University), Keith Paustian (CSU), Zhangcai Qin (Argonne National Laboratory, Sun Yat-sen University, Southern Marine Science and Engineering Guangdong Laboratory), John Sheehan (University of Campinas, CSU) and Pete Smith (University of Aberdeen).

Resources

• John L. Field, Tom L. Richard, Erica A. H. Smithwick, Hao Cai, Mark S. Laser, David S. LeBauer, Stephen P. Long, Keith Paustian, Zhangcai Qin, John J. Sheehan, Pete Smith, Michael Q. Wang, Lee R. Lynd (2020) “Robust paths to net greenhouse gas mitigation and negative emissions via advanced biofuels ” Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1920877117