Impact of Fuel Oxygenation on NOx Formation in Biodiesel Flames

Abstract

The increasing demand for energy and the need for reduction of greenhouse gases has necessitated the development of renewable sources such as biodiesel fuel. Methyl ester fuel burns more efficiently and has lower emissions of particulate matter, unburned hydrocarbon and carbon monoxide than fossil fuels. However, combustion of a methyl ester fuel results in increased nitrogen oxides (NOx) emissions relative to fossil fuels. This study is concerned with the formation of NOx in combustion of methyl formate, the simplest methyl ester molecule, under different flame conditions. Homogeneous ignition, freely propagating and diffusion flames of methane, methanol and methyl formate have been numerically simulated. To this end, recently developed chemical kinetic mechanism for methyl formate (Dooley 2009) has been identified and further developed to capture the production processes of pollutants. This is particularly important given that kinetics of combustion of methyl esters have not received much attention in existing literature. NOx concentration profiles for methyl formate in all configurations studied have been compared to those of methane/air and methanol/air flames which are well understood. It has been established that, the thermal NO in the three fuels are produced in nearly the same amount (within the same order of magnitude), while the prompt NO production in methane is observed to have a significant difference (one order of magnitude higher) compared to those of methanol and methyl formate flames. For thermal NO, the ratelimiting step: N2 + O −→ NO + N, which has a high activation energy is the decisive reaction. N2 and O are readily available in all the three fuels, hence the thermal NO production is nearly the same. In prompt NO, reaction: CH + N2 −→ HCN + N is the determining step. A small amount of CH and subsequently N atoms in CH3OH and CH3OCHO explain the low values of NO concentration as compared to that for methane. In addition, NO concentration showed a high sensitivity to reaction: NNH + O −→ NH+ NO in oxygenated fuels (CH3OH and CH3OCHO) as opposed to high sensitivity of reaction: CH + N2 −→ HCN + N seen in CH4 flames. The low concentration of N atoms in oxygenated fuels makes the contribution through the reaction path that results in NNH being significant. The NO formation in freely propagating and diffusion flames is mostly through prompt NO since the maximum flame temperatures attained are relatively low (approximately ≤2000 K). While the NO formation in a homogeneous system is mostly through thermal NO mechanism (Zel’dovich mechanism) since they attained high flame temperatures (approximately between 2800 and 3200 K) due to high initial temperatures. It is observed that, NO concentration in a homogeneous system is significantly higher (by three orders of magnitude) than those in freely propagating and diffusion flames.