Potential Methods for NOx Reduction from Biodiesel

Interest in biodiesel is increasing in the United States because it is a renewable fuel source that decreases carbon monoxide, unburned hydrocarbon, and particulate matter emissions. Although it is more expensive than petroleum based diesel fuel, it is a cost-effective fuel for government agencies to obtain EPAct alternative fuel credits. However, a 20% biodiesel blend in diesel fuel (B20) causes an average increase in NOx emissions of 2-5%. The emissions of NOx are critical, especially in ozone non-attainment areas, making the increase with biodiesel problematic to its widespread use. Using cetane improving additives and modifying feedstock composition are two possible methods to reduce NOx emissions from biodiesel. This study further explores the feasibility of these methods. First, the storage stability of the cetane improvers 2-EHN and DTBP are investigated in B20 by measuring the additive concentration by GC/MS and differences in the flash point, viscosity, and peroxide number over the duration of a short-term storage stability test (35 days). Secondly, the bulk modulus of biodiesel is higher than that of diesel fuel, a fuel property that leads to an advance in fuel injection timing, which in turn causes an increase in NOx emissions. The bulk modulus of some of the major methyl ester constituents of biodiesel are measured, as is that of a synthetic blend representing an achievable high-methyl oleate biodiesel. The results from the storage stability test show no deleterious effects caused by 2-EHN and DTBP addition. The bulk modulus measurements indicate that only marginal reduction of the bulk modulus can be achieved with high methyl oleate biodiesel.