Formation kinetics of nitric oxide of a biodiesel surrogate relative to n-heptane under comparable oxygen equivalence ratio in a homogeneous reactor
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abstract
Interest in biodiesel has piqued with advent of stringent emissions regulations. Biodiesel is a viable substitute for petroleum diesel because biodiesel produces significantly lower particulate and soot emissions relative to petroleum diesel. Higher nitric oxide (NO) emissions for biodiesel, however, are of primary concern in biodiesel-fueled engines. Search for an in-cylinder technique to reduce NO emissions for biodiesel has motivated studies to gain an improved understanding of fundamental factors that drive increase in NO emissions with biodiesel. Potential factors include fuel-bound oxygen, fuel-bound nitrogen and post-flame gas temperature. The role of fuel-bound oxygen, however, is debated in the literature. The research objective of this study is to computationally determine if a biodiesel surrogate and n-heptane yield equivalent concentrations of NO with the same oxygen equivalence ratio in a 0-D homogeneous reactor, to explain the role of fuel-bound oxygen in biodiesel on increases in NO emissions with biodiesel. The results from this study indicate that one of the reasons the biodiesel surrogate (composed of equal parts of n-heptane, methyl-decanoate, and methyl 2,4 dimethyl-9-decenoate on molar basis) yields higher NO emissions than the n-heptane is because of its lower oxygen consumption efficiency. The lower oxygen consumption efficiency for biodiesel is likely because of the slower decomposition of the individual components and the blending ratios of the biodiesel surrogate blend. The relative differences in instantaneous fuel conversion percentage of individual components of the biodiesel blend suggest this conclusion. The quicker burning of the methyl esters relative to the n-heptane in biodiesel surrogate perhaps indicates the favorable role of fuel-bound oxygen in the fuel's combustion. The low utilization of oxygen by the biodiesel surrogate could not be explained in this study. On kinetics level, the dominance of NO 2 + H NO + OH and N + NO N 2 + O mechanisms during biodiesel combustion however lead to the high NO emissions for the biodiesel surrogate relative to the n-heptane. The biodiesel may yield lower NO emissions than the petroleum diesel if the blending ratios for the biodiesel are adjusted such that speed of fuel conversion of biodiesel and petroleum diesel is same or the NO 2 + H NO + OH and N + NO N 2 + O mechanisms are suppressed during biodiesel combustion. 2011 Elsevier Ltd. All rights reserved.
