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

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.