Biodiesel Vehicle Emissions
When used as a vehicle fuel, biodiesel offers some tailpipe and considerable greenhouse gas (GHG) emissions benefits over conventional gasoline and diesel. The GHG emissions benefits of biodiesel are especially significant, because carbon dioxide (CO2) released during fuel combustion is offset by the CO2 captured by the plants from which biodiesel is produced.
A U.S. Environmental Protection Agency (EPA) analysis compiled in 2002 provides data on biodiesel tailpipe emissions from heavy-duty engines. The findings were published in A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions, a report that analyzes data from 39 studies. It concluded that biodiesel decreases the tailpipe emissions of particulate matter (PM), carbon monoxide (CO), and hydrocarbons (HC) commensurately with its blend level, as shown in the graph.
The study found a slight increase in oxides of nitrogen (NOx) emissions—approximately 2% for B20. However, Effects of Biodiesel Blends on Vehicle Emissions, published by the National Renewable Energy Laboratory (NREL) in 2006, presented data and analysis suggesting that B20 has no statistically significant net impact on NOx emissions. An SAE report published in 2009—Biodistillate Transportation Fuels 2-Emissions Impacts—corroborated NREL's results and found the NOx increase at B100 to be in the 2% to 3% range.
These results may not accurately reflect the improved emissions performance of today's high-tech diesel vehicles, however. Integration of sophisticated engine controls—as well as fueling and exhaust aftertreatment devices—are the determining factors governing how clean a vehicle or engine will be.
Life Cycle Emissions
Life cycle analysis is a technique used to assess the environmental impacts of all stages of a produc's life, including raw material extraction, processing, manufacturing, distribution, use, and disposal or recycling. When comparing fuels, a life cycle analysis may focus on particular portions of a fuel's life cycle, such as from extraction-to-use or well-to-wheels, to determine the merits or problems associated with each fuel.
Life cycle analysis completed by the National Renewable Energy Laboratory, and later by Argonne National Laboratory, found that greenhouse gas emissions for 100% biodiesel (B100) could be more than 52% lower than those from petroleum diesel. These analyses also showed that biodiesel may reduce petroleum use by more than 88% throughout its life cycle.