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Tailored Approach Is Key for Accurate Lifecycle Assessments

Tailored Approach Is Key for Accurate Lifecycle Assessments

Although lifecycle assessments have long helped determine how much carbon a fuel emits from manufacturer to end user, researchers have found they may not be working as well as they should.
Transportation fuels are one of the largest sources of greenhouse gases. Lifecycle assessments (LCA) determine the total pollution emissions—from creation through transportation, use, and disposal—from any proposed low-carbon fuel. Government agencies and elected officials can use these assessments to make policy changes and create laws.
But LCAs also need to be assessed.
A new report from the National Academies of Sciences, Engineering and Medicine, “Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States,” offers suggestions on how LCA tools can better evaluate transportation emissions.
“If a new transportation fuel is meant to reduce greenhouse gas emissions, we need to be confident that emissions are indeed likely to be reduced,” said Valeria Thomas, a professor of industrial and systems engineering at the Georgia Institute of Technology who led work on the report. “Determining the total net emissions of alternative fuels requires an understanding of how they are made and how they affect markets.”

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Despite their “clean energy” moniker, alternative fuels do emit carbon, just like traditional petroleum-based fuels. Petroleum alternatives such as electricity, biofuels, synthetic fuels, and hydrogen result and vary widely in carbon dioxide and other greenhouse gas (GHG) emissions from the tailpipe, from the production processes, or from wider supply-chain contributions, Thomas said.
Assessments can’t account for every part of a fuel’s lifecycle, but in the report, a 16-member National Academies’ committee came to numerous conclusions and recommendations for various fuels and standards, from the fundamentals of LCAs, direct and indirect effects, fossil fuels, aviation fuels, biofuels, and electricity as a vehicle fuel, among others.

The team identified two broad approaches to LCA: attributional life-cycle assessment (ALCA), which evaluates emissions attributable to a fuel, and consequential life-cycle assessment (CLCA), or how emissions would change if a given policy or set of actions were followed. When using these LCA approaches, modelers should provide "transparency, justification, and sensitivity or robustness analysis for modeling choices," the report states.

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The committee examined general methodological approaches of LCA, key issues for evaluating GHG emissions, issues that arise for transportation fuels, and methodological issues that arise for characteristic types of transportation fuels.
In one recommendation, study authors note that policymakers should consider recognizing the variation in GHG emissions across different petroleum fuel pathways and include mechanisms to reduce these emissions in fuel policies. And all biogenic carbon emissions and carbon sequestration generated during the LCA of a low-carbon fuel should be accounted for in LCA estimates.
On the biofuels side, researchers recommend that policymakers should exercise caution in crediting biorefineries for GHG emissions sequestration because it risks over-crediting downstream producers for behavior that is not necessarily occurring. Any credits generated should be verified before being administered.

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Overall, the study concludes that different types of LCAs are better suited for answering different questions or achieving different objectives, “from fine tuning a well-defined supply chain to reduce emissions, to understanding the global, economy-level effect of a technology or policy change.” Instead of using one blanketed method, policymakers should use a tailored approach for each fuel type, ensuring that all assumptions and methods are documented for transparency.

While researchers found LCAs to be a vital tool to use in formulating and revising fuel emissions regulations going forward, the study's suggested revisions and additions would make them far more effective in determining a fuel’s carbon-emitting potential.

The complete set of recommendations is available in the report, “Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States,” which can be downloaded from the National Academies of Sciences, Engineering and Medicine's website.
Jean Thilmany is a science and technology writer in Saint Paul, Minn.

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