Chemical Kinetic Study of Nitrogen Oxides Formation Trends in Biodiesel Combustion

Junfeng Yang,J Javier López Sánchez,Pau Redón Lurbe,Valeri I Golovitchev

doi:10.1155/2012/898742

Abstract

The use of biodiesel in conventional diesel engines results in increasedNOxemissions; this presents a barrier to the widespread use of biodiesel. The origins of this phenomenon were investigated using the chemical kinetics simulation tool: CHEMKIN-2 and the CFD KIVA3V code, which was modified to account for the physical properties of biodiesel and to incorporate semidetailed mechanisms for its combustion and the formation of emissions. Parametric ϕ-T maps and 3D engine simulations were used to assess the impact of using oxygen-containing fuels on the rate of NO formation. It was found that using oxygen-containing fuels allows more O2molecules to present in the engine cylinder during the combustion of biodiesel, and this may be the cause of the observed increase in NO emissions.

Highlights

Biodiesel fuels consist of long-chain monoalkyl esters derived from vegetable oils and expected to be increasingly important alternatives or supplements to conventional diesel fuel for use in diesel engines
The normalized sensitivity coefficients for NO with respect to important reactions rate generated by perfectly stirred reactor (PSR) modeling are shown in Figure 6 for (a) diesel oil surrogate (DOS) and (b) rapeseed methyl ester (RME), focusing on the most sensitive reactions in the NO formation process
Chemical equilibrium and kinetics calculations show that similar flame temperatures are achieved with RME and DOS, giving rise to comparable NO mole fractions under equilibrium conditions

Summary

Introduction

Biodiesel fuels consist of long-chain monoalkyl esters derived from vegetable oils and expected to be increasingly important alternatives or supplements to conventional diesel fuel for use in diesel engines. Two classes of explanation have been put forward: engine calibration effects [3] and combustion effects, such as higher flame temperatures [4, 5], injection timing shifting due to high bulk modulus [6], shorter autoignition delays, or combinations of these factors [2] While these studies indicate that the increased NOx emissions have multiple causes, it is generally accepted that the specific combustion chemistry of biodiesel is probably a major factor. On the basis of our observations and analyses, we propose the following alternative hypothesis: the use of oxygen-containing fuels such as biodiesel (rather than a pure hydrocarbon fuel such as conventional diesel oil) generates a leaner fuel/air mixture, favoring NO formation Future studies in this area will focus on analyzing the physical processes involved in the combustion of biodiesel in order to allow for the combustion of this important fuel under lean conditions without the drawback of elevated NOx emissions

Formulation of the Model

The Model Formulation

C17 H34 O2 β-scission

Modeling Results and Discussion

Conclusion