Effect of Antioxidant Addition on NO<i>_x</i> Emissions from Biodiesel

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 (NO x ), 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 NO x 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 NO x . We proposed that if these radical reactions could be terminated, NO x production from biodiesel combustion would decrease. To test this hypothesis, we investigated the ability of antioxidants, which are capable of terminating these kinds of radical reactions, to reduce NO x 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 NO x emissions. The engine used for these studies was a single cylinder, direct-injection, air-cooled, naturally aspirated Yanmar engine. The NO and NO 2 in the exhaust stream were quantified using electrochemical sensors, and differences in NO x emissions from the combustion of B20 with and without antioxidant were compared. The addition of butylated hydroxyanisole or butylated hydroxytoluene reduced NO x emissions, but the other antioxidants tested did not have this effect.