Sensitivity of hazardous air pollutant emissions to the combustion of blends of petroleum diesel and biodiesel fuel

Emission rates and composition of known hazardous air pollutants in the exhaust gas from a commercial agriculture tractor, burning a range of biodiesel blends operating at two different load conditions were investigated to better understand the emission characteristics of biodiesel fuel. Ultra-Low Sulfur Petroleum Diesel (ULSD) fuel was blended with soybean oil and beef tallow based biodiesel to examine fuels containing 0% (B0), 50% (B50) and 100% (B100) soybean oil based biodiesel, and 50% (B50T) and 100% (B100T) beef tallow biodiesel. Samples were collected using a dilution source sampler to simulate atmospheric dilution. Particulate matter and exhaust gases were analyzed for carbonyls, Volatile Organic Compounds (VOCs), and Polycyclic Aromatic Hydrocarbons (PAHs) to determine their respective emission rates. This analysis is focused on the emissions of organic compounds classified by the US EPA as air toxics and include 2,2,4 trimethylpentane, benzene, toluene, ethylbenzene, m-, p- and o-xylene, formaldehyde, acetaldehyde and methylethyl ketone. Emission rates of 2,2,4 trimethylpentane, toluene, ethylbenzene, m-, p- and o-xylene decreased more than 90% for B50, B100 and B100T blends; decreases in emission rates of benzene, formaldehyde and acetaldehyde were more modest, producing values between 23 and 67%, and methyl ethyl ketone showed decreases not exceeding 7% for the studied biodiesel blends. PAHs emission rates were reduced by 66% for B50, 84% for B100, and by 89% for B100T. The overall emissions of toxic organic compounds were calculated and expressed as benzene equivalents. The largest contributors of toxic risk were found to be formaldehyde and acetaldehyde. Reductions in formaldehyde emissions were 23% for B50 and 42% for B100 soybean, and 40% for B100T beef tallow compared to B0. Similarly, acetaldehyde reductions were 34% for B50 and 53% for B100 soybean biodiesel and 42% for B100T beef tallow biodiesel.