Understanding the Carbon Balance of Biofuel Production

In 2011, the USDA awarded $40 million to the Advanced Hardwood Biofuels Northwest (AHB) consortium to develop a system to convert poplar trees into liquid biofuels. Led by the University of Washington and the School of Environmental and Forest Sciences (SEFS), the AHB team is developing various strategies to create a renewable, direct replacement for existing fossil fuels that can be used in conventional cars, trucks and jet engines. The long-term vision is to produce 400 million gallons of biofuel per year from 400,000 acres of hybrid, sustainably-grown poplars.

Poplar Plantation

Poplar plantation in Oregon.

Four poplar demonstration plantations in the Pacific Northwest are being established as part of the AHB project to optimize production of biomass feedstock. At these poplar plantations in California, Idaho, Oregon and Washington, AHB researchers are thoroughly assessing the plantation environmental impacts on a number of factors, such as the carbon cycle, soil, wildlife and water usage.

Part of this research includes life cycle assessment (LCA) to determine total carbon emissions associated with production and use of biofuels. One question to be resolved by the LCA is the magnitude of greenhouse gas emissions associated with the production of biofuels, especially compared to petroleum-based fuels.

“The life cycle greenhouse gas emissions depend on many factors,” says SEFS Professor Rick Gustafson, who is leading the AHB research. He says preliminary results show that poplar-derived biofuels unquestionably lead to substantially lower greenhouse gas emissions compared to gasoline, but the precise magnitude of the reduction has yet to be worked out. These reduced emissions result from carbon sequestration of growing poplar feedstock balancing emissions from conversion of biomass into fuel and from use of the fuel product.

As a result, producing ethanol from plantation-grown poplar trees can be nearly carbon neutral. Research by Erik Budsberg, a SEFS Ph.D. student involved in the AHB program, shows that carbon emissions from fermenting the lignocellulosic sugars directly into ethanol, and burning the residual biomass to create electricity, is balanced out by the carbon sequestered by the poplar trees and by the displacement of fossil fuel-based electricity. The downside to this process, however, is that the total product yield—80 gallons of biofuel per ton of biomass used— is somewhat low, resulting in inferior process economics and greater feedstock demands. In addition, the ethanol fuel product is not compatible with our current transportation infrastructure, making its use somewhat limited.

Erik Budsberg

Erik Budsberg standing in front of year-old poplar trees at a GreenWood Resources poplar plantation in Boardman, Ore.

By using a different process, ethanol can be produced with a yield of 130 gallons per ton of biomass used. This process uses a different fermentation pathway but requires the addition of hydrogen to produce the fuel. While the yield is high—resulting in superior process economics and low biomass demand—this method has greater life cycle carbon emissions since it requires pumping natural gas, a fossil fuel, into the system. Even so, the process still results in a 60-percent reduction of greenhouse gases compared to gasoline.

A challenge of using bioethanol is that current infrastructure in the United States—most vehicles, and the fuel distribution network—is not built to handle fuels with high concentrations of ethanol, and that’s not likely to change any time soon, says Gustafson. To produce biofuels that are fully compatible with existing infrastructure, the ABH research program is developing processes that convert the poplar trees all the way to hydrocarbons, which are the molecules found in gasoline, diesel and jet fuel.

“By producing hydrocarbons, we end up with greater carbon emissions when compared to producing ethanol,” says Gustafson. The process the AHB team is developing, however, will produce infrastructure-compatible hydrocarbons with good yields while still reducing greenhouse gas emissions by more than 50 percent compared to gasoline, which is a big advancement.

It’s therefore clear that producing fuels from biomass like poplar trees leads to significant greenhouse gas emission reductions compared to petroleum-based fuel. The exact amount depends on many factors, such as the conversion process used and the choice of final products. The value of the research under way in the AHB project is that environmental benefits and impacts can be quantified before the factories are built and the feedstock plantations are established. Their research will also identify early on areas where environmental performance can be improved, enabling us to construct the most sustainable biofuels production enterprise possible.

Photo of poplar plantation © GreenWood Resources; photo of Budsberg © Renata Bura.

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