Submitted to: Energy and Fuels
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/17/2005
Publication Date: 7/20/2005
Citation: Hess, M.A., Haas, M.J., Foglia, T.A., Marmer, W.N. 2005. The effect of antioxidant addition on nox emissions from biodiesel. Energy and Fuels. 19(4):1749-1754. Interpretive Summary: Biodiesel is a diesel fuel made from vegetable oils or animal fats. When biodiesel made from soybean oil is blended with diesel fuel and used in a diesel engine, it reduces many of the exhaust emissions, especially soot and carbon monoxide. However, the use of biodiesel elevates oxides of nitrogen (or NOx) emissions, which are strictly regulated, as NOx is a precursor to acid rain. Using an additive to reduce NOx emissions is an attractive hypothesis to control NOx emissions. We tested several different antioxidant additives in a single cylinder Yanmar diesel engine to determine if they reduced NOx emissions. Of the additives tested, two reduced the NOx emissions from biodiesel significantly, though the emissions observed were still higher than those from conventional diesel. These two additives will be examined in greater detail in the future. This research validated the concept that NOx emissions can be reduced by antioxidant additives. Widespread adoption of biodiesel will be promoted by every environmental advantage it offers over petroleum diesel.
Technical Abstract: Biodiesel is a renewable, domestically produced fuel that has been shown to reduce particulate, hydrocarbon, and carbon monoxide emissions from diesel engines. Biodiesel produced from certain feedstocks, however, has been shown to cause an increase in nitrogen oxides (NOx), which is of particular concern in urban areas that are subject to strict environmental regulations. There are several pathways proposed that try to account for NOx formation during the combustion process, one of which is the Fenimore mechanism. In the Fenimore mechanism, it is postulated that fuel radicals formed during the combustion process react with nitrogen from the air to form NOx. We proposed that if these radical reactions could be terminated, NOx production from biodiesel combustion would decrease. To test this hypothesis, we investigated the ability of antioxidants, which terminate these radical reactions, to reduce NOx levels in biodiesel exhaust. Several antioxidants added to a 20% soy biodiesel/80% diesel fuel blend (B20) at a concentration of 1000 ppm were screened using a small, minimally instrumented diesel engine to test their ability to reduce NOx emissions. The engine used for these studies was a single cylinder, direct injection, air-cooled, naturally aspirated Yanmar engine. The NO and NO2 in the exhaust stream were quantified using electrochemical sensors, and differences in NOx emissions from the combustion of B20 with and without antioxidant were compared. The addition of butylated hydroxyanisole or butylated hydroxytoluene reduced NOx emissions, but the other antioxidants tested did not have this effect.