Air quality impacts of biodiesel in the United States
Although the majority of the on-road vehicle fleet in the United States is fueled by gasoline, diesel combustion makes up an overwhelming share of vehicle air pollution emissions. Air pollution emissions can be affected by blending biodiesel into diesel fuel. Studies have found that biodiesel combustion results in lower emissions of particulate matter (PM), carbon monoxide (CO), and hydrocarbon (HC), but increases the formation of nitrous oxide (NOx). The U.S. Environmental Protection Agency concluded in a 2002 report that, overall, biodiesel combustion does not worsen air quality compared to conventional diesel and reaffirmed that conclusion in a 2020 proposal and subsequent rulemaking.
This study presents a meta-analysis of air pollution changes from vehicles and engines running on biodiesel blends in the United States relative to a conventional diesel baseline. It draws from a comprehensive literature review of exhaust emission testing and performance studies, dozens of them published after studies conducted by the U.S. EPA, and analyzes changes in NOx, PM, CO, and HC for U.S.-relevant feedstocks. Unlike previous analyses, the study also assesses the impacts of fuel injection systems, engine horsepower, and emission control technologies on biodiesel exhaust emissions.
When analyzing the entirety of the available literature, the study finds that a 20% biodiesel blend (B20) increases NOx emissions by 2% compared to conventional diesel, in agreement with the U.S. EPA’s 2002 finding. However, this estimate includes many literature studies that are no longer relevant due to evolving developments in the industry. When the analysis is restricted to only studies reflecting modern conditions, we find that the biodiesel NOx effect for B20 increases to 4% due to the introduction of ultra-low sulfur diesel (ULSD) and common-rail fuel injection systems.
Under modern conditions, the analysis also finds that a 20% biodiesel blend (B20) increases HC and CO by 7% and 10%, respectively, and does not reduce PM compared to conventional diesel. This new finding presents a striking contrast with the conclusions in EPA’s 2002 meta-analysis that biodiesel sharply reduces emissions of all these pollutants.
These findings illustrate that our understanding of the air quality impacts of biodiesel should change in response to the large volume of new evidence. These updated results should inform the U.S. EPA’s future rulemakings pursuant to the Renewable Fuel Standard.