Simulation of biodiesel and petrodiesel pollutant kinetics

Abstract

Biodiesel is considered as an attractive alternative to petrodiesel for transportation applications. Substituting petrodiesel with domestically produced biodiesel increases energy independence, reduces the carbon footprint, and offers a viable path toward biomass utilisation. It has been found that biodiesel-fuelled engines emit up to 70% lower particulate matter (PM) compared to petrodiesel-fuelled engines, although they emit up to 20% more nitric oxides (NOx). In this study, simulations are employed to improve the fundamental understanding of biodiesel and petrodiesel combustion under pressure and temperature conditions in engines. n-Heptane is used as the surrogate for petrodiesel fuel and a ternary mixture of methyl decanoate, methyl-9-decenoate, and n-heptane as the surrogate for biodiesel fuel. The choice of chemical kinetic mechanism is a compromise between computational time and accuracy. Four mechanisms are evaluated and a 160-species skeletal chemical kinetic mechanism is employed to model the oxidation of n-heptane. The computational time is shown to increase dramatically as the complexity of the mechanism increases. Soot kinetics is represented using a chemical mechanism that models the growth of soot precursors starting from a single aromatic ring and growing by hydrogen abstraction and carbon (acetylene) addition. NOx kinetics is represented using the thermal, prompt, and intermediate mechanisms. In the case of the ternary biodiesel surrogate, a 211-species reduced mechanism is employed to model the chemical kinetics. This mechanism was derived as part of this work by combining reactions from the 160-species n-heptane mechanism with reactions from a skeletal 115-species mechanism proposed in the literature. The influence of turbulence is modeled through an imposed strain rate in the simulations. The computations are carried out using a strained laminar flamelet code (SLFC). In addition to exploring the effect of strain rate (turbulence) on the ignition, extinction, and pollutant formation characteristics, the fundamental chemical pathways that lead to PM and NOx formation are studied by considering the evolution of precursor species. In general, ignition and extinction of flames in biodiesel combustion are more sensitive to turbulence than in petrodiesel flames. Soot volume fraction is lower in biodiesel combustion compared to petrodiesel combustion. This is consistent with measurements reported in engines. NO concentration is, however, lower for biodiesel combustion when considering kinetics alone, suggesting that the volume phasing and operating parameters of engines influence observed NO results in engine emissions where NO is observed to be higher in biodiesel-fuelled engines. It is shown that while increasing strain reduces soot formation for both fuels, the reduction is significantly greater for biodiesel suggesting that increased mixing has a greater effect on PM emissions in biodiesel-fuelled engines.