Understanding the Carbon Balance of Biofuel Production

Developing biofuels from poplar trees can reduce the amount of carbon dioxide in the atmosphere compared to burning fossil fuels.  Poplar trees take in carbon dioxide from the atmosphere and store carbon in their stems, branches, and roots.  When poplar biomass is converted to fuel and that fuel is then burned for energy, the stored carbon is released into the atmosphere.  Meanwhile, the trees grow back after harvest and begin to re-capture the carbon back from the atmosphere.

The advantage of this system is that the carbon released by burning poplar biofuels came from the atmosphere in the first place rather than from deep in the ground as is the case with fossil fuels. If there were no other energy inputs to consider, this process would be carbon neutral, meaning that the carbon released by burning biofuels equals the carbon sequestered by growing poplar and thus no net change to atmospheric carbon. However, the process of converting poplar biomass into biofuels requires additional energy inputs, some of which may come from fossil fuels, to harvest and transport the wood to the biorefinery and for the refining process itself.  Even with these additional energy inputs, the net increase in atmospheric carbon is less for biofuels than for fossil fuels, and the AHB research is figuring out exactly how much.

Poplar-Carbon-CycleFin_web

The cycle of growth, harvest and regrowth leads to continued carbon storage.

The Sustainability Team is doing a life cycle assessment (LCA), which tracks all of the carbon inputs and outputs across the entire cycle of producing and using poplar-based biofuels.  The exact amount of carbon depends on many factors, such as the conversion process used and the choice of final products.  To produce biofuels that are fully compatible with existing infrastructure, the AHB 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.

A key question to be resolved by the LCA is how the magnitude of carbon emissions from biofuel production compares to that of petroleum-based fuels.  University of Washington (UW) Professor Rick Gustafson, who is leading the LCA research, said preliminary results show that poplar-derived biofuels unquestionably lead to substantially lower carbon emissions compared to petroleum-based fuels, but the precise magnitude of the reduction has yet to be worked out.

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

The value of the research under way in the AHB project is that the changes in greenhouse gas emissions can be quantified before the biorefineries are built and the bioenergy farms established. The research will also identify areas where reductions in carbon and other greenhouse gas emissions can be improved, enabling us to construct the most sustainable biofuels production enterprise possible.