Numerical Study of NOx Emissions from n-Heptane and 1-Heptene Counterflow Flames

Several engine studies have observed higher amounts of nitric oxide (NO) during the combustion of biodiesel fuels than from regular diesel. One hypothesis for the increased NO formation is that unsaturated components in biodiesel fuels produce higher amounts of acetylene, especially during fuel-rich oxidation, which results in higher prompt NO. In this study, we examine this hypothesis by considering partially premixed flames (PPFs) in an opposed jet configuration, burning prevaporized n-heptane and 1-heptene fuels, which represent the hydrocarbon side chain of the two surrogate biodiesel esters, methyl octanoate (C9:0) and methyl octenoate (C9:1), respectively. The configuration involves two opposing jets, one containing a fuel−air mixture issuing from the bottom nozzle and the other containing air issuing from the top nozzle. It provides a nearly one-dimensional flat flame for detailed measurements and simulations and is well-suited for fundamental investigations of kinetics and transport processes. Using a comprehensive chemistry and transport model, PPFs are simulated for a range of equivalence ratios (φ). Results indicate that the β-scission and oxidation reactions related to the CC double bond lead to a higher amount of C2H2 and, thus, increased NO through the prompt mechanism in 1-heptene flames compared to that in n-heptane flames. However, differences in the NO formation between the two fuels become less noticeable as the level of partial premixing is reduced or as φ is increased toward the diffusion flame limit. In addition, analysis of the various NO production pathways indicate that the total NO formed is mainly due to the prompt NO and intermediate N2O mechanisms, followed by the NNH and thermal NO mechanisms.