Dominant dynamics of n-butanol/air autoignition and the influence of additives

Abstract

The algorithm of Computational Singular Perturbation is used in order to study autoignition dynamics and formaldehyde emission during isochoric, adiabatic n-butanol oxidation. It is shown that n-butanol autoignition has a significant chemical runaway for low initial temperatures and a limited one for high initial temperatures. Autoignition is supported mainly by the dissociation of H2O2 to OH and, to a lesser extent, by the attack of HO2 on the parent fuel that produces H2O2. The algorithmic analysis points to the pivotal role of mainly H2O2 and secondarily CH2O in the process and it is shown that addition of these two species to the initial mixture can shorten the ignition delay time substantially. The strong effect of H2O2 is based on the near elimination of the chemical runaway, because of the generation of a substantial OH pool very early in the process. The formation of formaldehyde in the products is shown to be practically insensitive to a very wide variety of additives for a given overall equivalence ratio, even when these additives had a substantial effect on ignition delay. This is attributed to the fact that, for combustion with air at a given equivalence ratio, the relative proportion of C/H/O atoms in the initial mixture does not change much, independently of the precise configuration of the molecules that constitute the fuel mixture and the additive. As a result, methodologies for the control and reduction of aldehyde and ketone emissions should be based on equivalence ratio, pressure, and temperature control, rather than the use of additives.