Study finds PM from biodiesel blends may be 50-80% less toxic per unit PM mass than from petroleum diesel
In a study published in the ACS journal Energy & Fuels, a team from the University of Vermont reports that particulate matter from the combustion of biodiesel blends may be 50–80% less toxic per unit PM mass emitted than PM from petroleum diesel, depending on feedstock.
There is growing consensus that PM toxicity is linked to the formation of reactive oxygen species (ROS) on PM and the subsequent oxidative stress induced in cells. However, the relative toxicity of biodiesel emissions compared to petroleum diesel remains unclear. In the study, the team examined the relationships between biodiesel fuel blend, exhaust particle oxidative potential (OP), and PM composition.
OP in oxidized aerosols, including engine exhaust, is commonly quantified using the DTT (dithiothreitol) assay. The researchers analyzed samples of PM obtained from light-duty diesel engine transient cycle emission tests with two biodiesel feedstocks—soybean (SOY) and waste vegetable oil (WVO)—blended with ultralow sulfur petrodiesel at five different volume percentages of biodiesel, Bxx (B0, B10, B20, B50, and B100).
They found that DTT activity per mass of PM sampled generally decreased as the percent biodiesel increased in the fuel, for both feedstocks.
Mean DTT PM activity (±1 std dev) for SOY decreased from 20.9 ± 4.2 to 13.6 ± 3.8 nmol/min/mgPM for B0 and B100, respectively, and from 22.6 ± 4.5 to 8.5 ± 2.8 nmol/min/mgPM for the WVO feedstock.
Findings also suggested different combustion products between feedstocks only for the more highly oxygenated biodiesel blends (B50 and B100) used in the study.
Further, the organic composition of WVO exhaust particles as measured by GC-MS showed positive correlations between DTT PM activity and particle-phase polycyclic aromatic hydrocarbons (PAHs), n-alkanes, aromatic aldehydes, aromatic ketones, and quinones, but not aliphatic aldehydes.
The results of this study point to the importance of aromatic polar organic combustion products to the redox cycling potential of PM derived from biodiesel fuel combustion. Of the redox-active metals (Fe, Cu, and Zn), only Zn showed positive correlation with OP. The decreasing trend in WVO OP points to recent improvements in waste oil biodiesel fuel production technology that may have beneficial effects on exhaust emissions toxicity. Here, WVO feedstock preprocessing steps to remove free fatty acids and the relatively high (2000 ppm) fuel antioxidant concentration may partially explain the decreasing trend of OP with increasing Bxx.
Future biodiesel emissions studies should combine PM toxicity assays with detailed fuel, lubrication oil, and exhaust particle composition to better elucidate compositional factors contributing to toxicity and identify alternative biodiesel fuel blend compositions that minimize biological response from exposure to exhaust PM. This may be possible using fuel additives beyond antioxidants. Future studies should also quantify the sensitivity of biologic responses to blends commonly used in real-world engines (B0 to B20) given the variability observed in this study at low blend ratios.—Holmén et al.
Britt A. Holmén, Benjamin Rukavina, John Kasumba, and Naomi K. Fukagawa (2017) “Reactive Oxidative Species and Speciated Particulate Light-Duty Engine Emissions from Diesel and Biodiesel Fuel Blends” Energy & Fuels doi: 10.1021/acs.energyfuels.7b00698