Air Quality Impacts of Biodiesel in the United States

Air pollution emissions can be affected by blending biodiesel, composed of fatty acid methyl ester (FAME), into diesel fuel. Biodiesel increases the efficiency of fuel combustion due to its high oxygen content and high cetane number. Studies have found that biodiesel combustion results in lower emissions of PM, CO, and HC, likely for this reason. However, studies have consistently found that biodiesel blending increases NOx formation.

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. We draw from a comprehensive literature review of exhaust emissions testing and performance studies, dozens of them published after EPA’s studies, and analyze changes in NOx, PM, CO, and HC for U.S.-relevant feedstocks. Authors assess the impacts that feedstocks, test cycles, and diesel quality have on the exhaust emissions from biodiesel combustion. Unlike previous meta-analyses, they also assess the impacts of fuel injection systems, engine horsepower, and emission control technologies on biodiesel exhaust emissions.

When analyzing the entirety of the available literature, authors find that a 20% biodiesel blend (B20) increases NOx emissions by 2% compared to conventional diesel, in agreement with EPA’s 2002 finding. However, this estimate includes many literature studies that are no longer relevant due to evolving developments in the industry. Authors find that the biodiesel NOx effect has increased in recent years with the introduction of ultra-low sulfur diesel (ULSD) and common-rail fuel injection systems. When they restrict our meta-analysis to only studies reflecting these conditions, they find that the biodiesel NOx effect for B20 increases to 4%. Under modern conditions, authors also find that a 20% biodiesel blend (B20) increases HC and CO by 7% and 10%, respectively, and does not reduce PM compared with 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.